activated sludge bioselector processes - orris e....
TRANSCRIPT
Activated Sludge Bioselector Processes
Source: 1987 Annual Report, Davenport Wastewater Treatment Facility, Davenport, Iowa. (Used with permission)
History ● Benefits ● Arrangement ● Design● Experiences ● Troubleshooting ● Research Needs
Orris E. AlbertsonEnviro Enterprises, Inc.
LaBarge, WY
May 2005
TABLE OF CONTENTS
LIST OF TABLES............................................................................................................. iLIST OF FIGURES .........................................................................................................iiiDEFINITIONS/ACRONYMS.......................................................................................... vEXECUTIVE SUMMARY .......................................................................................... E-1
1.0 PROCESS DESCRIPTION ...............................................................................1-11.1 Definitions/Objectives ...........................................................................1-11.2 History of Bioselector Development .....................................................1-31.3 Theory of Bioselection.........................................................................1-121.4 Filamentous vs. Non-filamentous Bulking ..........................................1-151.5 Measurement of the Sludge Volume Index .........................................1-201.6 Dilute Sludge Volume Index Procedure ..............................................1-261.7 Conversion of uSVI to DSVI or SSVI.................................................1-28
2.0 BENEFITS OF BIOSELECTION .....................................................................2-1
3.0 GENERAL DESIGN ARRANGEMENT OF BIOSELECTORS .....................3-13.1 F/M Cascade Design ..............................................................................3-13.2 Contact Loading Analysis....................................................................3-113.3 Sludge Age...........................................................................................3-123.4 Design Features....................................................................................3-16
4.0 TYPES OR DESIGNS OF BIOSELECTORS ..................................................4-14.1 Aerated, High DO Bioselectors (SXAH).................................................4-74.2 Aerated, Low DO Bioselectors (SXAL)................................................4-124.3 Anoxic Bioselectors (SXAXM).............................................................4-184.4 Anaerobic Bioselectors(SXANM).........................................................4-12
5.0 PROCESS EXPERIENCES WITH BIOSELECTORS.....................................5-15.1 Davenport, IA – Aerated, Low DO (SXAL) and Anoxic (SXAXM)........5-15.2 Columbus Southerly, OH – Aerated, Low DO (SXAL)..........................5-35.3 Columbus Jackson Pike, OH – Aerated, Low DO (SXAL) ....................5-95.4 Santa Fe, NM – Aerated, Low DO (SXAL) ..........................................5-135.5 Gig Harbor, WA – Aerated, Low DO (SXAL) .....................................5-165.6 Phoenix 23rd Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL)...........5-185.7 Phoenix 91st Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL) ...........5-245.8 Tri City, Clackamas County, OR – Anaerobic (SXANM)
and Anoxic (SXAXM and SXAXAL)......................................................5-285.9 Upper Occoquan Sewage Authority, VA –
Aerated, High DO (SXAH) ...................................................................5-335.10 Hamilton, OH – Anoxic (SXAXAL and SXAXM) .................................5-365.11 Middletown, OH – Anaerobic (SXANM) .............................................5-385.12 Star Valley Cheese Coop, Thayne, WY – Anaerobic (SXANM)..........5-39
5.13 Tree Top, Selah, WA–Anaerobic (SXANM) andAerated, High DO (SXAH) ...................................................................5-39
5.14 Fibra, America, Brazil – Anaerobic (SXANM).....................................5-40
6.0 TROUBLESHOOTING BIOSELECTORS ......................................................6-16.1 Low F/M in the ICZ...............................................................................6-26.2 High F/M in the ICZ ..............................................................................6-46.3 Air Rate to the Bioselectors ...................................................................6-56.4 Limited Oxygenation Capacity in the Oxic Zones ................................6-66.5 Secondary Bulking (Oxic Zone) ............................................................6-66.6 A Very Low SVI – High Turbidity........................................................6-76.7 Toxic/Inhibitory Compounds.................................................................6-76.8 Limited Nitrogen and Phosphorus Supply/Availability.........................6-86.9 Soluble Organics Breakthrough.............................................................6-86.10 Summary Comments..............................................................................6-9
7.0 RESEARCH NEEDS......................................................................................... 7.17.1 General Introductory Comments............................................................ 7.27.2 Research Target ..................................................................................... 7.47.3 Recommended Areas of Research ......................................................... 7.4
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List of Tables
1.1 Activated Sludge Solids-Liquid Separation Problems............................................... 1-2
1.2 Comparison of Elements of Early Anaerobic/Aerobic Bioselector Processes ........ 1-12
1.3 Probable Causes of Hydrous Sludge Bulking and Filamentous Bulking ................ 1-16
1.4 Major Occurrences of Viscous/Hydrous Bulking.................................................... 1-19
2.1 Benefits of Bulking Control by Bioselection............................................................. 2-3
2.2 Effectiveness of Bioselectors in Controlling Filamentous Organisms .................... 2-10
3.1 General Design Guidelines for Bioselector Sizing
(after Albertson 1987,1992, 1994)............................................................................. 3-6
3.2 Prior Art Batch and Continuous Flow Experience with Bulking Sludge Control
Concepts and the DO and BOD5 Mass Loading in the ICZ ...................................... 3-7
3.3 Summary of the Design and Operation of Bioselectors
by Daigger and Nicholson (1990).............................................................................. 3-8
3.4 Summary of the Design and Operation of Bioselectors
by Marten and Daigger (1997)................................................................................... 3-9
3.5 Calculation of Baffle Wall Height ........................................................................... 3-19
4.1 Characteristics of Initial Contact Zone of Bioselectors ............................................. 4-3
4.2 Design FS/M Criteria for Anoxic (SXAXAL or SXAXM) Bioselectors
to Account for Daily Peaking Factors........................................................................ 4-4
4.3 Wastewater Characteristics Used in an Equation
to Establish Criteria for the Bioselector Design ........................................................ 4-5
4.4 Technical Parameters and Results of the Bioselector Activated Sludge Process
in the Leopoldsdorf Sugar Mill Obtained During the Campaign in 1984 (Kroiss) ... 4-9
4.5 Design Recommendations Aerated, High DO (SXAH) Bioselectors........................ 4-11
4.6 Results of 1973a Chudoba Laboratory Staged Aeration Studies............................. 4-12
4.7 Design Recommendations for Aerated SXAL and SXAXAL Bioselectors................. 4-18
4.8 Design Recommendations for Mechanically Mixed Anoxic
(SXAXM) Bioselectors.............................................................................................. 4-21
4.9 Deer Island, Boston, MA, Pilot Study Results ........................................................ 4-24
4.10 Phase 3 Secondary Treatment Performance at Deer Island WWTP........................ 4-26
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4.11 Design Recommendations for Three-Stage Anaerobic (SXANM) Bioselectors....... 4-30
5.1 Summary of the Columbus, OH Southerly Operating Results .................................. 5-8
5.2 Summary of the Columbus, OH Jackson Pike Operating Results ........................... 5-12
5.3 Summary of the Santa Fe, NM Operating Results................................................... 5-15
5.4 Summary of the Gig Harbor, WA Operating Results .............................................. 5-19
5.5 Prototype NdeN Performance Data, Monthly Average, Phoenix 91st Ave.............. 5-29
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List of Figures
1.1 Davidson’s Anaerobic-Aerobic Bulking Sludge Control Process .....................1-5
1.2 Relationship of SVI and sCOD in First Compartment of Reactors ...................1-6
1.3 1974 Heide and Pasveer Flowsheet: Bulking Control and TN Removal...........1-8
1.4 1975 The Five Stage Phoredox Process (Modified Bardenpho)........................1-9
1.5 NdeN Process Flowsheets of Ludzak-Ettinger, Drews and Barnard ...............1-10
1.6 Three and Five Stage UCT NdeN Processes ...................................................1-11
1.7 General Response of Activated Sludge to ICZ Food/Mass Ratio....................1-18
1.8 SVI vs Initial MLSS @ SSVI 80-85 mL/g ......................................................1-21
1.9 Settling Tests for SVI – Settlometer ................................................................1-24
1.10 Settling Tests for SVI – Graduated Cylinder...................................................1-25
1.11 Calculated DSVI from SVI Employing Merkel’s (1971) SVI to DSVI
Equation ...........................................................................................................1-30
2.1 The Effect of Extended Filament Length on the MLSS SVI (Palm et al.)........2-2
2.2 Secondary Clarifier Operation Diagram SSVI3.5 ...................................................................... 2-4
2.3 Secondary Clarifier Operating Diagram – DSVI...............................................2-6
2.4 Secondary Clarifier Operating Diagram – SVI (uSVI)......................................2-7
2.5 Secondary Clarifier Operation Diagram – DSVI...............................................2-8
3.1 Semi-Aerobic Process for Bulking Control and Nutrient Removal...................3-2
3.2 General Arrangement of Bioselectors in U.S. Facilities....................................3-4
3.3 Guideline of the Design/Operable Contact Loading in ICZ ............................3-13
3.4 Controlling the Contact Loading with Internal Recycle ..................................3-14
3.4.1 Relationship between Biomass Retention Time, SRT, and Occurrence of
Filamentous Microorganisms (after Wanner, 1994)........................................3-15
3.5 Design of Submergence Depth and Baffle Wall Arrangement........................3-17
3.6 Scum-Free Bioselector Zones with Submerged Baffle Walls .........................3-18
4.1 Effect of ICZ F/M on SVI of MLSS (after Chudoba et al., 1973a 1973b)......4-14
4.2 Relation between SVI and Theoretical Sludge Loading (MLSS = 3.5 g/l)
in the First Aeration Compartment of Plants Included in the Survey
(after Tomlinson, 1976) ...................................................................................4-16
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4.3 Relationship between SVI and Phosphorus to Organic MLSS Ratio ..............4-29
5.1 Davenport, IA – Wastewater Treatment Facility Annual Report, 1987 ............5-2
5.2 Davenport, IA – Historical SVI Data.................................................................5-4
5.3 Columbus, OH, Southerly – General Process Concept/Semi-Aerobic ..............5-6
5.4 Columbus, OH, Southerly – Results ..................................................................5-7
5.5 Columbus, OH Jackson Pike – Modified Aeration Basins
with SXAL Bioselectors ....................................................................................5-10
5.6 Columbus, OH Jackson Pike – Results............................................................5-11
5.7 Santa Fe, NM – Aerated Anoxic Bioselector Modifications (16.5 ft WD) .....5-14
5.8 Gig Harbor, WA – Bioselectors in a Contact-Stabilization Process................5-17
5.9 Phoenix 23rd Ave. – Modified Aeration Basin with Bioselection ...................5-21
5.10 Phoenix 23rd Ave. – Flow, SVI and MLSS......................................................5-22
5.11 Phoenix 23rd Ave. – Bi-weekly Range and Average DSVI .............................5-23
5.12 Phoenix 91st Ave. – Bioselector in the NdeN Aeration Basins........................5-26
5.13 Phoenix 91st Ave. – NdeN Demonstration Data ..............................................5-27
5.14 Tri-City, OR – Configuration of Anoxic Selector System
(Daigger and Nicholson, 1990)........................................................................5-31
5.15 Tri-City, OR – Performance of an Anoxic Selector
Daigger and Nicholson, 1990) .........................................................................5-32
5.16 UOSA, VA – Aeration Basin and Selector System
(Daigger and Nicholson, 1990)........................................................................5-34
5.17 UOSA, VA – Effect of Selector Operation on SVI
(Daigger and Nicholson, 1990)........................................................................5-35
5.18 Hamilton, OH – Monthly Average SVI...........................................................5-37
5.19 Fibra, Brazil – Sludge Volume Index Control .................................................5-42
6.1 True Nature of Hydrous (Viscous) Bulking Organisms ....................................6-3
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Definitions of Acronyms
alpha F - a characteristic of wastewater, ratio of oxygen transfer to wastewater vs.
clean water
AN - non-selector anaerobic zone for Bio-P removal
AX - non-selector anoxic denitrification zone for nitrate removal
AOTR - actual oxygen transfer rates, mg/L⋅hr, kg/hr
Bio-P - biological removal of phosphorus by a specialized group of organisms.
BOD5 - five-day biochemical oxygen demand, mg/L, kg/d
C - Fteley and Stearns submerged weir coefficient, Table 3.2
C-S - contact-stabilization activated sludge process
CBOD5 - five-day inhibited biochemical oxygen demand, mg/L, kg/d
CL - contact loading, mg sCOD/g TSS (RSS + IR MLSS)
COD - chemical oxygen demand, mg/L, kg/d
CMAS - complete mix activated sludge process
DF - dilution factor for DSVI test procedure
DO - dissolved oxygen, mg/L
DSVI - see SVI
ECP - exo-cellular protoplasm, sometimes improperly generalized as
polysaccharides unless specifically identified as polysaccharides
F/M - food (COD, BOD5)/mass, kg COD/kg MLSS⋅d, kg BOD5/kg MLSS⋅d
F/ΣM - food/ sum of the mass, kg/kg⋅d
FS/M - soluble food/mass, kg sCOD/kg MLSS⋅d, kg sBOD5/kg MLSS⋅d
IR - internal recycle flow from oxic zone to anoxic bioselector and/or anoxic
zone, L/s, m3/hr, ML/d
M - mechanically mixed anoxic or anaerobic bioselector
MCRT - mean cell residence time includes all solids in the process, days
ML - activated sludge mixed liquor, mg/L
MLSS - mixed liquor suspended solids, mg/L or g/L (also megaliter, ML/d)
MLVSS - mixed liquor volatile suspended solids
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N - nitrogen, any form
NA - not available
NH4-N - ammonia nitrogen, mg/L, kg/d
NO3-N - nitrate nitrogen, mg/L, kg/d
NO2-N - nitrite nitrogen, mg/L, kg/d
OX - oxic zone with typical DO level of > 2 mg/L
OUR - oxygen uptake rate, mg/L⋅hr, mg/g⋅hr
P - phosphorus, any form
Q, Qi - influent (raw or primary effluent) wastewater, L/s, m3/hr, ML/d
RAS - return activated sludge flow from clarification, L/s, m3/hr, ML/d
RSS - RAS suspended solids concentration, mg/L, g/L
sBOD5 - soluble BOD5, usually filtered at 0.45 or 1.5 µm, mg/L, %BOD5, kg/d
sCOD - soluble COD, usually filtered at 0.45 or 1.5 µm, mg/L, %COD, kg/d
SLR - solids loading rate in the secondary clarifier, kg/m2⋅d
SOTR - standard oxygen transfer rates, kg/m3⋅hr, mg/L⋅hr
SRT - solids retention time includes all solids in the biological reactors, days
SRTOX - aerobic or oxic SRT in zones with target DO > 2 mg/L, days
SVI - sludge volume index, mL/g
DSVI - diluted SVI at 160 to 240 mL/L SSV30, mL/g
SSVI - stirred SVI, mL/g
SSVI2.0 - stirred SVI at 2000 mg/L MLSS, mL/g
SSVI3.5 - stirred SVI at 3500 mg/L MLSS, mL/g
uSVI - undiluted SVI, mL/g
SSV - settled sludge volume at 30 minutes, mL/L
SX - bioselector zone, any type
SXAH - aerated, high DO bioselector
SXAL - aerated, low DO bioselector
SXANM - anaerobic bioselectors with mechanical mixing
SXAXAL - aerated, low DO anoxic bioselector
SXAXM - anoxic bioselectors with mechanical mixing
TKN - total Kjeldahl nitrogen (organic + ammonia), mg/L, kg/d
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TN - total nitrogen, mg/L, kg/d
TSS - total suspended solids, mg/L, kg/d
VSS - volatile suspended solids, mg/L, kg/d
WRF - water reclamation facility
WWTP - wastewater treatment plant
ZSV - zone settling velocity, m/hr
ZSV5 - ZSV for first 5 minutes, m/hr
E-1
Executive Summary
It has been over 50 years since the concept of bioselection was first uncovered
(Davidson, 1949) and nearly 30 years since a fundamental approach to bioselection was
published (British Ministry, 1969; Chudoba et al, 1973a, 1973b, 1974; Heide and Pasveer,
1974). The process of bioselection is now widely employed for new and retrofitted activated
sludge facilities. However, there is neither a common nor accepted methodology for bioselector
design, and as a result, the volumetric requirements of individual bioselector basins may vary by
50 to 1100% of the recommendations set forth in this study. While bioselection is successful in
reducing the sludge volume index (SVI) in nearly every case, the level of performance, as
measured by the average and especially the maximum SVIs, can vary widely. Maximum SVIs
govern biological process design – including clarification.
The common U.S. practice is to use the undiluted SVI (uSVI or SVI) to define sludge
settling/compaction characteristics. Unfortunately, uSVI is a very inaccurate method and, in
many cases, will prevent an acceptable comparison of data among plants with and without
bioselectors. This is particularly true when the settled sludge volume (SSV30) exceeds 300
mL/L, typical of facilities operating at >3000 mg/L mixed liquor suspended solids (MLSS) or
when the SVIs are >120 to 150 mL/g. This problem hampered this analysis and will cause
difficulties with any future study. The input data for an objective analysis of the sludge settling
characteristics must be diluted SVI (DSVI) or stirred SVI (SSVI), the European standard
procedure.
There are excellent case histories of multi-staged (three or four zones) bioselectors with
settling/compaction characteristics well within the target area of an average SSVI or DSVI of 60
< 80 mL/g and a maximum DSVI value of 80 < 100 mL/g. When the DSVI is controlled within
this range, wastewater processes are stable, operate at high efficiency, and result in maximum
utilization of aeration and clarifier capacity.
E-2
The body of technical literature on fundamental research and field studies of bioselectors
is large and increasing. There is the aforementioned problem of reducing the data to correlate
design and process factors to the activated sludge settling characteristics, unless DSVI or SSVI
measurements are employed. Currently, there are many designs of bioselectors ranging from one
up to six zones; they may be aerated or not, high DO, low DO, anoxic or anaerobic, and with
food/mass (F/M) ratios of 0.70 to 14.9 kg BOD5/kg MLSS.d in the initial contact zone (ICZ).
The number of independent variables influencing the bioselector performance could exceed 15
separate items of input data/information.
This document presents a portion of the pertinent literature and numerous references on
the control of bulking sludges by the natural biological method. Case histories are included to
support design recommendations. The bioselector design recommendations are provided for:
• Aeration, high DO (SXAH), secondary and advanced wastewater treatment
• Aeration, low DO (SXAL), secondary and advanced wastewater treatment
• Anoxic (SXAXAL and SXAXM), nitrification and denitrification
• Anaerobic (SXANM), secondary and biological nutrient (N & P) removal
The technology has evolved since the first noted ‘sludge bulking’ (Calvert, 1927) – from
Donaldson’s (1932) apt description of filamentous organisms as ‘the weeds of activated sludge’
and Davidson’s (1949) discovery of bioselectors and bulking control with the anaerobic-aerobic
treatment sequence to the 1969-1974 innovators who broke the ground for current bioselector
design. Future clarification of best design and operating modes is expected when the many full-
scale operating systems are scrutinized from a research point of view and their historical data are
fully analyzed.
This document is an initial step toward defining the design and effectiveness of
bioselection to improve the settling rate and compaction characteristics of activated sludge. A
more comprehensive study of the hundreds of bioselectors currently in operation will provide the
data necessary to expand the technology and refine the recommended design criteria.
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Chapter 1.0
Process Description
1.1 Definitions/Objectives
The term bioselection is employed to describe processes where natural (non-chemical)
selection governs or limits the growth of undesirable bacterial species. The specific objective of
bioselectors is to prevent the growth of filamentous organisms, which causes sludge bulking or
high sludge volume indices (SVIs). Improper design of bioselectors can also be the cause of a
different form of sludge bulking, viscous or hydrous bulking, which may or may not be of a
filamentous character.
There are a number of bioselector types employed in activated sludge treatment. The
definitions are qualitative in that they describe the physical environment within the bioselection
zone(s) as opposed to the biological functions that could be taking place. The bioselector zone
(SX) designs presented in this report are aerated – low DO (SXAL) and high DO (SXAH); anoxic
with (SXAXAL)and without aeration (SXAXM); and anaerobic (SXANM). The term oxic (OX) is
not employed for bioselector characterization in this report, although it does occur in the
literature. Oxic conditions would be included under the SXAH mode of operation since the
dissolved oxygen (DO) could range from >0.5 to >2.0 mg/L. The process conditions for each
mode will be further defined in this report.
While the settling properties can only be adequately characterized by stirred SVI (SSVI)
or diluted SVI (DSVI) procedures, SVI is employed in this text for simplicity. Whenever data are
presented, the measurement technique will be noted; SVI is used whenever the procedure
employed is unknown.
1-2
Non-filamentous (hydrous, viscous) bulking can be an operating problem and is
addressed in this review. Conditions that favor hydrous bulking may or may not favor
filamentous growth.
Bulking sludges historically have been the major cause of malfunctioning or inefficient
activated sludge processes. Design and operating practices evolved to compensate for bulking
sludge problems such as larger aeration basins and clarifiers with reduced mixed liquor
suspended solids (MLSS). In the 1960-1980s, chemical toxicants, peroxide and chlorine were
utilized to selectively control filamentous growths of organisms. The use of toxicants controlled
bulking sludges but also reduced effluent quality and could limit nitrification efficiency. The
operational need for higher solids retention times (SRTs) in biological nutrient removal
processes and the economical need to increase MLSS begged for a more practical method to
control bulking and foaming organisms. The bioselector has proven to be the answer for many of
these problems.
The problems of solids-liquid separation in activated sludge are classified in Table 1.1
(IAWQ, 1992a).
Table 1.1 Activated Sludge Solids-Liquid Separation Problems.
Problem Nature of Problem Characterization of Problem
Dispersed growth Flocs are dispersed, Turbid effluent. Poor flocforming only small clumps formation. Lack of zoneor single cells. settling of sludge.
Pin floc Small, compact, dense Low SVI, cloudy, turbidflocs that settle rapidly, effluents. Low quantity ofleaving lighter flocs in filamentous organisms.suspension.
Filamentous bulking An excessive filamentous High SVI, clear supernatant,population, which interferes low RAS concentration.with compaction and Increasing sludge blanketsettling of activated sludge depth. Poor sludge handling.characteristics.
1-3
Problem Nature of Problem Characterization of Problem
Hydrous zoogleal or The sludge flocs become High SVI, lower RASviscous bulking more hydrated and lose concentration. High sludge(non-filamentous) density. blankets.
Rising sludge Gas entrainment and/or Flocs or clumps of sludgegas release caused by rise rapidly to the surface.denitrification/ Surface effervescence.solids carryover.
Foaming and scum Aeration basin and clarifier Lightly colored frothy foamaccumulation of floating forms when aerated.materials, froth and scum Persistent, dark heavy foamof activated sludge. with high solids content.
Excessive sludge blankets: Diffuse sludge blankets Loss of solids and unstableNon-bulking sludge approaching weirs; operation. Solids
excessive solids loading or accumulation in clarifier.transport problems.
Control of bulking sludge as defined by the SVI results in a more stable process, reduces
operational requirements, and will generally produce a better effluent quality. The advantages
gained from SVI control are reflected in an increased capacity in both the aeration basin and
secondary clarifier as well as reduced waste activated sludge (WAS) volumes consistent with
maintaining a higher SRT. WAS processing costs generally decline as the SVI is reduced.
Control of the filamentous organisms can also eliminate most or all of the serious foaming
problems caused by filamentous organisms.
1.2 History of Bioselector Development
While Chudoba et al., (1973a, 1973b, 1974) must be given considerable credit for the
studies that defined environmental conditions that discouraged the growth of bulking organisms,
earlier researchers’ and practitioners’ efforts played an important role. Donaldson (1932a,
1932b), who characterized filaments as the ‘weeds of activated sludge,’ correctly postulated that
backflow mixing of long, rectangular (plug flow) basins contributed to the growth of filaments
and his proposal to baffle the aeration basin into compartments was a valid approach to assist in
the control of filamentous growths.
1-4
Engineers and scientists bent on improving the activated sludge process converted the
original Aldern and Lockett (1914) batch concept to a continuous flow process and with these
modifications introduced problems of bulking sludge. The batch feeding approach provided the
food/mass (F/M) gradient, described much later by Chudoba et al., as necessary to control
filamentous growth. Many problems of bulking occurred in the intervening years due to the
batch-fed lab studies, which led to the design of full-scale continuous flow plants that did not
have an F/M gradient. Sequencing batch reactors (SBRs), which are continuously fed, have had
serious sludge bulking problems.
Davidson (1949) studied several process alternatives to control filament growth in the
treatment of distillery wastewater and found success with an integrated anaerobic-aerobic
flowsheet with the sludge recycle from the aerobic back to the initial anaerobic zone. The unique
process mode, shown in Figure 1.1, demonstrated capability to reduce the undiluted SVI (uSVI)
of 300 to 1000 mL/g to less than 50 mL/g on this difficult-to-treat wastewater. This novel
anaerobic-aerobic concept was patented (Davidson, 1957), but not exploited nor understood by
wastewater researchers and practitioners until more than two decades later.
Bhatla (1967), British Water Pollution Laboratory (1969), Garber (1972) and Ryder
(1973) all noted that low DO (SXAL) in the initial zone of aerated, long, rectangular or staged
basins would provide bulking control. Koller (1966) and Pasveer (1969) rediscovered that
intermittent feeding of batch reactors would limit the growth of filamentous organisms. Chudoba
et al., (1973a, 1973b), in the first two papers of a series, demonstrated that compartmentalization
(staging) of the aerated reactors was a key factor to controlling sludge bulking. This idea was
first postulated by Donaldson (1932a, 1932b). The Czech researchers further recognized that the
organic loading gradient in the initial compartments of the treatment process was the key to
bulking sludge control. The effect of compartmentalization on SVI from their studies is shown in
Figure 1.2. As the number of compartments was expanded in the biological reactor, the
1-5
1-6
1-7
maximum level of soluble chemical oxygen demand (sCOD) in the MLSS increased in the first
compartment. The presence of 70 to 120 mg/L sCOD as a result of an increased F/M in the
smaller initial contact zones (ICZs) was the significant factor in the control of bulking
organisms. Rensink (1974) confirmed the observations of earlier investigators regarding the
benefits of staging the reactor zones to reduce sludge bulking.
The studies of Heide and Pasveer (1973) also revealed that an initial stage of raw
sewage-return sludge (or MLSS) contact at a high F/M ratio was critical to the control of bulking
organisms. They correctly postulated that organic matter, defined by the soluble biochemical
oxygen demand (sBOD5), was removed by faculative bacteria in the absence of an oxygen
source and, in this manner, would limit filamentous growth. The flowsheets representing the
pilot and full-scale operations evaluated by the Dutch researchers are presented in Figure 1.3.
Nicholls (1975) proposed the use of an anaerobic stage ahead of the four-stage
Bardenpho process (Figure 1.4) to enhance the biological phosphorus removal. Spector (1977)
was awarded a U.S. patent for an anaerobic stage as the first contact between the return sludge
MLSS, a process similar to that of Davidsons’ patent (1957) and Nicholls’ studies. Nicholls’
work evolved into the 5-stage Bardenpho and 3-stage Phoredox systems and the Spector
patented process was marketed as the A2O process. Other modifications to the alternating
anaerobic-oxic (AN-OX), anoxic-oxic (AX-OX), and anaerobic-anoxic-oxic (AN-AX-OX ) systems
to remove phosphorus, nitrogen and control bulking sludge were developed in the period before
and after 1975 (Figures 1.5 and 1.6).
Further studies by Lee et al., (1982) and Chudoba et al., (1985) confirmed earlier
conclusions regarding the need for an F/M gradient to control bulking. Albertson (1987, 1992)
summarized the efforts of these earlier innovators and those investigators who followed and
added to the understanding of mechanisms, design and operation of the several types of
bioselectors in use today. The design embodiments of these early innovators shared many
common features. The major difference identified was whether or not the initial zone of the
biological process was mechanically mixed or aerated (Table 1.2).
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1-9
1-10
1-11
1-12
Table 1.2 Comparison of Elements of Early Anaerobic/Aerobic Bioselector Processes
HeideComponent Davidson Garber Chudoba Pasveer Nicholls Barnard Spectoror Mode 1952 1972 1973 1974 1975 1976 1977Mix InitialZone with Air No Yes Yes No No No No
Oxic Zone Yes Yes Yes Yes Yes Yes Yes
Clarifier Yes Yes Yes Yes Yes Yes Yes
Recycle OxicSludge to InitialZone (a) Yes Yes Yes Yes Yes Yes Yes
Mix RawWastewaterWith ReturnSludge Yes Yes Yes Yes Yes Yes Yes
Claim BulkingSludge Control Yes Yes Yes Yes No No Yes
(a) This zone may be either anoxic or anaerobic depending on the plant design temperature,
wastewater characteristics and the definition employed by the author of the publication.
The focus of this discussion is the control of filamentous organisms and hence the
elimination of bulking sludges. Because bulking sludge control and biological phosphorus
removal often occur in a common environment, these processes and data are interwoven in the
literature. Important general references in this regard are Tomlinson and Chambers (1978),
Jenkins et al., (1993) and Wanner (1994). The effectiveness of bioselectors is undisputable, but
there is much to be learned about the process and optimization.
1.3 Theory of Bioselection
The fundamental basis for the natural selection of non-filamentous organisms is the
control of environment during the initial contact of the influent wastewater (or primary effluent),
where most of the soluble substrate (sBOD5, sCOD) is removed from solution to the biomass
1-13
with or without limited oxidation. The key environmental conditions are the presence or level of
DO and the F/M in the selector zone(s). The addition of nitrates via return sludge or an internal
recycle (IR) from the nitrifying zone may also play a positive role in suppressing the growth of
filamentous organisms.
Filamentous organisms, one cause of bulking sludges, are considered to be relatively
primitive. They lack the capability to produce the necessary enzymes to hydrolyze complex
organic compounds into low molecular weight molecules, which they could then oxidize as a
food source. Thus, their primary energy source is small, soluble molecules such as sugars,
organic acids and alcohols, which are in the raw wastewater or products of biological hydrolysis.
In order to metabolize these soluble organics, filamentous organisms require the availability of
molecular oxygen for oxidation and production of cell matter. Combined oxygen, such as nitrate
nitrogen, cannot be efficiently utilized by filamentous organisms, and perhaps most importantly,
they lack the ability to store substrates for later use when molecular oxygen is available.
Heterotrophic organisms are more complex and adaptable to a wide variety of
environmental conditions. These organisms form large, dense flocs, and in the absence of an
excessive number of filaments, settle rapidly and compact well. Bioselection favors those
heterotrophic species that can readily sorb and store or modify soluble substrates in the absence
of molecular or combined oxygen. Bioselection may or may not enrich the MLSS with
organisms that also have the capability to remove and store excess levels of phosphorus (Bio-P)
in the cells. The type of bioselector employed will, in part, determine the level of Bio-P removal
by the system.
Thus, the bioselective mechanism is to contact the return activated sludge (RAS) – and
an internal recycle when employed – with the influent wastewater in an initial contact zone
(ICZ) of the biological reactor. The initial contact zone typically consists of three or four zones
with a high to lower F/M and having limited or no molecular oxygen present. In these zones,
heterotrophs remove the majority (75-90%) of the low molecular weight, soluble substrates from
the wastewater. Since the favored substrate (small, soluble organics/molecules) of the
filamentous bacteria is now limited in the heavily aerated oxic zones following the bioselectors
1-14
and anoxic (denitrification) zones (if employed), their growth – and reproduction – is thus
inhibited. The presence of a limited quantity of filamentous bacteria is generally desirable as
they can help produce a stronger and larger floc structure (Palm et al., 1980), which will readily
settle and compact well in the secondary clarifier.
While bioselectors will cause the removal of the bulk of the simple organics (alcohols,
volatile acids, sugars and amino acids) from solution prior to anoxic or oxic zones, the
hydrolysis of colloidal and particulate organics in the following oxic zones may provide an
opportunity for filamentous organisms to feed and grow. Wanner and Grau (1988) referred to
this cause of bulking as "secondary bulking" and recommended a high to low F/M gradient
(compartmentalization) in the oxic zones. This postulation of Wanner and Grau may be the
reason why bioselectors are very effective when treating highly soluble industrial wastewater
flows. The bulk of the influent COD (BOD5) is removed in the bioselectors and the by-products
of hydrolysis of the low level of particulates in the oxic zone are minimal. Thus, secondary
bulking cannot occur and this could be the reason that very low SVIs (20-50 mL/g) have been
reported (Davidson, 1957; Cranston Print Works, 1992; Tree Top Apple Juice, 1993; Okey,
1997) when treating highly soluble industrial wastewaters containing mostly low molecular
weight organics.
In order to maintain the bioselection capacity of the biomass, it is necessary that the
stored organics be oxidized prior to returning to the initial contact zone. That is, the stored food
must be aerobically processed to carbon dioxide (CO2), water and cell matter by the organism
prior to returning to the bioselector for the process to be effective. If the bacterial storage
capacity is not regenerated in the oxic zone, the soluble substrate can enter the aeration zone and
cause proliferation of filamentous organisms. Also, this condition can cause the development of
viscous sludge bulking (see Section 1.4). Chudoba et al., (1982) correctly referred to the need to
fully oxidize the ‘sorbed’ organics before clarification and sludge recycle as ‘sludge
regeneration.’
The removal of soluble components by the bioselector is a very rapid process. In the
Phoenix 23rd Ave Water Reclamation Facility (WRF) and 91st Ave Wastewater Treatment Plant
1-15
(WWTP), the retention time in the three-stage bioselectors is 5-8 minutes based on the total flow
of primary effluent, return sludge and mixed liquor recycle (Albertson and Hendrix, 1992;
Albertson and Stensel, 1994). After this short contact time, the sCOD in the bioselector effluent
approaches the final effluent sCOD of 25-30 mg/L or over 85% removal of the primary effluent
sCOD defined by 1.2-1.5 µm filtration. These results are typical of other plants with staged
bioselectors.
1.4 Filamentous vs. Non-filamentous Bulking
Historically, the emphasis on bulking sludge has been focused on the study and
identification of the various filamentous organisms. However, when the concept of bioselection
was employed, another cause of bulking sludge surfaced: hydrous or viscous bulking conditions.
It is not uncommon to have both types of bulking present in a wastewater treatment plant
(WWTP).
The first published experiences of viscous bulking began appearing in the 1950s
(Wanner, 1994). The condition was related to the overflowing of slowly settling sludge from the
clarifier and a dilute underflow concentration. Often it has been described by operators and
experienced personnel as a condition where there were two distinct sludge blankets – a light,
fluffy, unstable blanket above a denser blanket. The upper, lighter layer of suspended solids was
susceptible to washout during the higher diurnal or wet weather flow periods, increasing the
effluent TSS to 30 to 150 mg/L during these periods.
Non-filamentous bulking is also known as hydrous, viscous or zoogleal bulking. In this
condition, the biomass accumulates and stores excess food as a slime surrounding the cell or
floc. This storage product, called exo-cellular protoplasm (ECP), results in a lower density,
slower settling floc due to the higher volume of water intimately associated with the floc. The
MLSS and return sludge will feel slick or slippery, and the MLSS will be viscous at a relatively
low solids concentration (Wanner, 1994). Often these hydrous sludges are referred to as having
an excessive level of polysaccharides. However, viscous sludges may also contain loosely bound
organic nitrogen compounds, and thus the term ‘excess polysaccharides’ is not the correct
generalized definition of a non-filamentous bulking condition. Instead exo-cellular protoplasm
1-16
(ECP) is preferred as a proper terminology for excess storage products in hydrous sludges.
Hydrous sludges can be characterized by India ink staining (Jenkins et al., 1993) as the
organically bound water is not stained and is thus visible under microscopic examination.
Jenkins et al., (1993) discussed the relationship of total carbohydrates as glucose, which
acts as an indicator of settling problems, and provided test procedures for staining and
carbohydrate measurements.
There is a lack of experimental documentation on the causes of viscous bulking. It is
probable that some of the environmental conditions that encourage filamentous bulking also
generate hydrous bulking. Hydrous bulking, while unrecognized, has been present as long or
longer than filamentous growths in activated sludge. It can develop in all types of biological
reactors including the batch systems of Aldern and Lockett (1914). However, complete mix
processes are the least susceptible to hydrous bulking, unless toxicity is a problem or the
wastewater is deficient in nutrients, such as nitrogen and phosphorus, as well as oxygen. Some of
the likely causes of non-filamentous bulking and opinion regarding filamentous bulking are set
forth in Table 1.3 (Albertson, 1994).
Table 1.3 Probable Causes of Hydrous Sludge Bulking and Filamentous Bulking
Causative Factor Hydrous Filamentous
Nutrient/Micronutrient Deficiency X X
High Contact Loading X
Inadequate O2 Supply X X
Toxicants X
Uncouplers X
Low F/M Loading X
Soluble Wastewater X X
Excess F/M Loading X X
Excess Storage Products in Return Sludge X X
1-17
The conditions within the bioselectors, such as low DO and high F/M gradient, are also
conducive to generating a hydrous bulking sludge if there is organic overload or the storage
capacity of the return sludge has not been adequately regenerated. Thus, an overloaded
bioselector can be a cause of bulking (filamentous and hydrous) sludges. There is a general
relationship of the F/M (Chudoba et al., 1973a, 1973b) in the initial contact zone to the SVI of
the activated sludge. However, there are too many site-specific aspects of biological growth
kinetics to make this statement in other than a qualitative term as shown in Figure 1.7. The width
of the ‘window’ of low SVI sludge production is not fully developed at this time. However, it
appears to be in an F/M range of 3-6 kg sCOD/kg TSS·d (1.5 to 3.0 kg sBOD5/kg TSS·d) in the
first contact zone of return sludge and influent wastewater. This is an area in need of quantitative
research, both laboratory and field studies. There is also a possible impact of the specific
wastewater components on the selector design criteria that is not understood at this time.
Biological reactors of similar design may or may not have bulking problems with or without a
selector. Operational conditions must also be considered.
Lack of nutrients (N and P) or micronutrients are considered to be a cause of bulking and
the addition of N, P and iron compounds have been found to be beneficial in these cases.
Supplements of magnesium and potassium, which are involved in the Bio-P formation, have also
improved the SVI. High contact loadings occur when the quantity of soluble substrate exceeds
the level of normal bacterial metabolism and storage capacity, and excessive levels of ECP are
produced. These conditions can encourage the growth of filaments. An inadequate supply of
oxygen to support fully aerobic metabolism in the succeeding oxic zone(s) can be a cause of both
types of bulking.
Toxicants and uncouplers can reduce filaments, but may also cause hydrous bulking by
limiting normal phosphate uptake for metabolism. Chlorine, as a toxicant, is used to control
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1-19
filaments by destruction of the filamentous growth. However, this practice, while effective, can
increase turbidity in the effluent and upset sensitive processes. While complete mix activated
sludge (CMAS) systems are more resistant to toxicants, the process favors the growth of some
types of filamentous organisms.
Viscous/hydrous bulking is not limited to the activated sludge processes. Albertson
(1994) reported on a number of facilities with fixed and suspended growth systems that had
severe bulking problems. While most of the facilities in Table 1.4 are industrial, there are
municipal plants processing mostly domestic wastewater that have also experienced hydrous
bulking problems.
Under specific environmental conditions, soluble substrates can be a source of both
hydrous and filamentous bulking. However, bioselection is most effective with soluble
wastewaters provided there is not an excessive F/M loading and there is adequate sludge
regeneration. If the aeration system is incapable of oxidizing the stored substrate in the cells, the
return sludge may be unable to rapidly metabolize the new soluble substrates in the initial
contact zone. This operating deficiency may lead to hydrous and/or filamentous bulking
conditions. Thus, it is necessary to identify the type of bulking, filamentous and/or non-
filamentous, in order to initiate an investigation into the cause(s). While CMAS systems are the
most common cause of filamentous bulking (but not hydrous bulking) in municipal and
industrial wastewater treatment plants, floc overload, aeration capacity, and nutrient levels must
be considered. The ‘cause and effect’ aspect of bulking sludges is still being developed.
Table 1.4 Major Occurrences of Viscous/Hydrous Bulking
Wastewater Location Mode/Operation
Candy Wastes Puebla, MX Sequencing Batch Reactor (SBR)
Bakery Wastes New Jersey Equalization Basin
Pharmaceutical Ireland Trickling Filter
Domestic Phoenix, AZ ICZ Nitrification-Denitrification Activated
Sludge
1-20
Brewery Fulton, NY Mix Box ICZ Activated Sludge
Leachate Seattle, WA Submerged Fixed-Film Reactor (FFR)
Starch Iowa Trickling Filter
Domestic Michigan Trickling Filter
Apple Juice Washington SBR
Domestic & Dairy Iowa Trickling Filter
Wastewater Location Mode/Operation
Brewery Maine Trickling Filter/Activated Sludge
Domestic & Ind Waste Tennessee Trickling Filter/Activated Sludge
Domestic Wastewater Colorado Trickling Filter
Domestic Wastewater Pennsylvania Trickling Filter
1.5 Measurement of the Sludge Volume Index
Due to the lack of a universally accepted procedure to determine the settling
characteristics of activated sludge, it is difficult to fully quantify differences in this property.
The most common measurement in the U.S. of the SVI of the mixed liquor is the unstirred
procedure (uSVI). Unfortunately, it is the most unreliable measurement of the true
settling/compaction characteristics of the mixed liquor in a clarifier.
Rachwal et al., (1982) presented the relationship of unstirred settled sludge volume
(SSV30) of an activated sludge with a stirred sludge volume index (SSVI) of 80-85 mL/g over a
MLSS range of 1000 to 10,000 mg/L. Figure 1.8 displays the results of uSVI vs. SSVI
procedures and reveals that the uSVI method was invalid to define settling characteristics above
approximately 2000 mg/L MLSS. That is, when the MLSS was <1500 to 2000 mg/L, the two
procedures produced approximately the same values. At 4000 mg/L MLSS, the uSVI was about
225 mL/g with an SSVI of 80 mL/g. The SSVI (and the DSVI) tests are a more reliable and
accurate predictors of full-scale settling/compaction characteristics of activated sludge.
The term ‘SVI’ has many different meanings in the international technical community
since data have been reported as uSVI30 (and uSVI60), SSVI30, SVI 2.0, SSVI2.0, SSVI3.5, DSVI30
and perhaps other definitions. The relationships between these various SVI tests are not constant,
1-21
1-22
and plant-to-plant comparisons are difficult or impossible when the uSVI exceeds 100 to 125
mL/g or when the SSV30 exceeds 300 mL/L.
The universal need to standardize the SVI procedure is obvious. The question in debate is
which procedure to use. A brief description and comment on each procedure follows:
(1) uSVI30 (unstirred). The MLSS settling test is conducted in a 1 L graduated cylinder
or a 2 L settlometer for 30 minutes. The recorded 30-minute volume is converted to
SVI as a function of the MLSS. When the settled sludge volume (SSV30) exceeds
250-300 mL/L, the relative value of this test to full-scale settling characteristics is
limited and unpredictable. At SSV30 above 400 mL/L, the values produced may be
of little worth and unpredictable and, worse, often misleading. Sometimes this test,
and the following tests, are continued for 60 minutes and reported as SVI60, etc.
(2) SSVI30 (stirred). This test is conducted in a 1 L graduated cylinder or a 2 L
settlometer. A stirrer operating at about 1 rpm is used to disrupt floc particle
bridging and assist consolidation of the solids. This test will generally produce a
lower 30-minute sludge volume than the uSVI if the SSV30 is > 200 mL/L. It is
considered more representative of full-scale sludge compaction, but may not define
solids-liquid separation rates.
(3) SVI2.0 (unstirred) and SSVI2.0 (stirred). These tests are conducted for 30 minutes
at a MLSS of 2000 mg/L (2.0 g/L) and represent an effort to standardize the
procedure for comparison. The uSVI2.0 has the same limitations as the uSVI30 tests,
and the SSVI2.0 is an attempt to eliminate the effects of varying MLSS on both the
settleability rate as well as the solids-liquid separation rate. The SSVI2.0 procedure
has an advantage over the uSVI and SVI2.0 tests for plant-to-plant comparisons.
(4) SSVI3.5 (stirred). This test is similar to the SSVI2.0 procedure except that it is
conducted at 3500 mg/L (3.5 g/L) MLSS. This procedure is best suited for the range
of MLSS concentrations typically employed in full-scale advanced wastewater
treatment plants. As the SVI increases, the SSVI3.5 test results will be increasingly
higher than those of the SSVI2.0.
1-23
(5) DSVI30 (unstirred, diluted). This procedure involves dilution of the MLSS until the
resulting 30-minute settled volume is about 200 mL/L. A tolerance of ±40 mL/L
does not have a large effect on the resulting DSVI value. In comparison to the uSVI
test, this procedure will provide a more representative value of the solids-liquid
separation rate and a better projection of the potential concentration characteristics
of the settled solids due to minimization of bridging and MLSS concentration
effects. This procedure is excellent for plant process control but would not be as
effective for plant-to-plant comparison as SSVI2.0 or SSVI3.5.
Many treatment facilities operate with MLSS of 2500 to 4000 mg/L, a range in which the
uSVI test is meaningless when the SSVI is > 80 mL/g. Thus, it is necessary to employ either the
SSVI or the DSVI procedure in order to accurately define solids-liquid separation characteristics.
The studies of Koopman and Cadee (1983) indicate that the DSVI and SSVI procedures produce
reasonable, comparable results. Lee et al., (1983) correlated total extended filament length with
uSVI, uSVI1.5, uSVI2.5, DSVI and SSVI2.5 and judged that DSVI was the most quantifiable
procedure for use in judging clarifier settling and compaction characteristics. This conclusion
was also supported by the work of Pitman (1984) and Yamamoto and Matsui (1988).
Test vessels employed for the SVI procedures have been one or two liter cylinders and
one or two liter settlometers These vessels will produce different values for SVI with the same
sludge. Keinath and Wahlberg (1990) conducted tests in 1 L cylinder and 2 L settlometers using
the SVI and SSVI procedures. A portion of their results is presented in Figures 1.9 and 1.10. A 2
L settlometer is the preferred settling vessel for all types of SVI tests due to reduced wall effects
and thus is more analogous to full-scale settling. However, as shown by Rachwal (Figure 1.8),
the uSVI test is invalid above SSV30 ≥ 150 to 200 mL/L (% volume). The DSVI test uses 160 to
240 mL/L SSV30 as an acceptable measure to define the DSVI.
The presence of hydrous bulking organisms is not necessarily reflected in a high SVI.
The SVI tests may hide the presence of the hydrated organisms due to the bulk settling
characteristics of the mixed liquor in test vessels. A small hydrated fraction (<10%) can be
1-24
1-25
1-26
entrapped by the more rapidly settling fraction of denser floc particles. However, in the clarifier
there is an opportunity for the slower settling, lower density, hydrated floc particles to segregate
and accumulate in the area above the more concentrated settled sludge being transported to the
return sludge hopper. These floc form fluffy sludge blankets of low total suspended solids (TSS)
concentrations, which are susceptible to being flushed into the effluent during diurnal or wet
weather flows.
The portion of the MLSS that is hydrated but causing problems could be quite small. If
the MLSS is 3000 to 3500 mg/L, 10 % of these solids, or 300 to 350 mg/L TSS, could form the
dilute blankets and easily contribute to a 30 to 150 mg/L increase in TSS, which may overflow
the clarifier during these periods.
1.6 Dilute Sludge Volume Index Procedure
The DSVI procedure involves adding a known volume of secondary effluent (diluent) to
the MLSS sample to reach 1000 mL (cylinder) or 2000 mL (settlometer). For example, if the
undiluted MLSS settled to 500 mL/L (SSV30), then a 0.5 dilution factor (DF) will likely result in
a settled volume of 180 to 220 mL/L or within the target range of 200 mL/L ± 40 mL/L (160 to
240 mL/L SSV30).
Initially, for training experience, the operators will usually set up two or three settling
vessels to target the correct dilution factor (DF) and usually within a week or two of experience
will rely on just one test vessel. The dilution factor and settled volume from the previous shift is
the best guide to determine the required DF for the next shift. For example, if in the previous
shift, the DF was 0.5 at an SSV30 of 240 mL/L, the next shift should try a 0.4 DF. The lower DF
of 0.4 would have produced (0.4/0.5)(240 mL/L) = 192 mL/L or close to the 200 mL/L target
volume had a 0.4 DF been employed on the shift reporting 240 mL/L SSV30.
Generally, the adjustments of the dilution factor from one shift to the next is not more
than 0.1 DF. The DF values and calculations for DSVI are provided below.
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Dilution Factor (DF) Volumes of MLSS and Secondary Effluent
Dilution MLSS EffluentFactor(1) mL/L(2) mL/L(2)
1.0 1000/2000 0
0.9 900/1800 100/200
0.8 800/1600 200/400
0.7 700/1400 300/600
0.6 600/1200 400/800
0.5 500/1000 500/1000
0.4 400/800 600/1200
0.3 300/600 700/1400
0.2 200/400 800/1600(1)DF = the dilution factor is a decimal fraction of MLSS in total volume(2)1000 mL cylinder / 2000 mL settlometer
Calculation Procedure:
DSVI =)/,)((
)/,( 30
LgMLSSDF
LmLSSV(1.6-1)
Example:
If MLSS was 3200 mg/L (3.2 g/L), SSV30 was 225 mL/g after diluting 600 mL of mixed liquor
with 400 mL of effluent (DF = 0.6), the DSVI is:
DSVI =)/2.3)(6.0(
/225
Lg
gmL(1.6-2)
= 117 mL/g
Wastewater treatment plant operators are strongly encouraged to employ SSVI30 or
DSVI30 procedures since they will accurately identify changes in the settling properties of the
MLSS. The uSVI procedure, as shown in Figure 1.8, is unreliable and can provide misleading
information, which can result in the employment of an improper mode of operation for SVI
control in the plant.
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1.7 Conversion of uSVI to DSVI or SSVI
The conversion of uSVI to DSVI or SSVI poses some problems due to the inaccuracies
of the SVI test. Keinath and Wahlberg (1990) presented data showing the relationship of uSVI
and SSVI in 1L and 2L cylinders (Figures 1.9 and 1.10). The SVI and DSVI sludge loadings vs.
SVI and return sludge concentration graphs of Daigger (1995) illustrate the differences between
the SVI, DSVI and SSVI3.5 values. The SVI, SSVI3.5 and DSVI secondary clarifier graphs of
Daigger are reproduced in Section 2.0
As shown in Figures 1.9 and 1.10, the relationship between SVI procedures is poor and
conversion of uSVI to SSVI has limitations when the uSVI > 300 mL/L (Figure 1.8).
An excellent reference on the influence of sludge bulking on clarifier design and
performance is the 1997 IAWQ (now IWA) publication Secondary Settling Tanks. This
document provides background information on the various measurement techniques for SVI and
support for the DSVI procedure. It was noted that it is necessary to define the MLSS
concentration at which the SSVI procedure is conducted: SSVI2.0 or SSVI3.5. Even the SSVI
procedure at different MLSS concentrations will provide different SSVI values. The SSVI
increases with higher MLSS as does the uSVI, but to a much lesser degree.
The relationship between SVI and DSVI was investigated by Merkel (IAWQ, 1992b)
employing a large data base. The resulting equation for the SSV30 range of 300 to 800 mgL/L for
the SVI test was
DSVI = SVI (300/ SSV30)0.6
In the IAWQ (1997) review and discussion of the Merkel equation, it was concluded that
the limits for SSV30 were appropriate and the equation is considered to be the best representation
of the relationship. This document summarized three sets of Western Cape (South Africa) full-
scale UCT data and developed the plot of measured DSVI vs. calculated DSVI from the Merkel
equation. This plot from IAWQ (1997) is reproduced as Figure 1.11. Although the level of
1-29
variance from actual to the measured DSVI is appreciable, DSVI is a better measurement. The
best procedure is SSV3.5 which is used widely overseas.
The IAWQ document provides additional and important background data and
observations on the bulking organisms and the development of measurement and control
methods for limiting the mixed liquor SVI. The emphasis of the control modes is natural
bioselection although it includes information generated by European researchers – some of
which, but not all, have been included in the citations of this report. The significance and
interrelationship of secondary clarifier capacity and performance have been fully developed in
the IAWQ document.
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2-1
Chapter 2.0
Benefits of Bioselection
The process benefits of a lower and stable sludge volume index (SVI) are well known to
plant operators. Control of the activated sludge process is simplified and the effluent quality is
more stable with minimal sludge blankets in the clarifier. In addition, the biological average and
peaking capacities of the biological system are increased – that is, BOD5 loading and hydraulic
flow. Sludge bulking conditions limit clarifier solids and liquid processing capacity. They also
limit the mixed liquor suspended solids (MLSS) concentration of the aeration basins due to a
lower return sludge solids concentration and a lower solids handling capacity of the clarifier.
Control of SVI is important to the reliability of the activated sludge system. Palm et al.,
(1980) documented the effect of the total extended filament length (TEFL) on the SVI. His
results of their studies (Figure 2.1) are important to understanding the value of controlling the
DSVI to the range of 60 to 100 mL/g. Historically, an SVI of ≤ 150 mL/g has been considered an
acceptable value for a non-bulking sludge. However, as shown by Palm et al., the SVI at 150
mL/g is in an area of instability and a small increase in TEFL can result in an escalating SVI and
reduced clarifier and aeration capacity. This figure, like that of Rachwal (Figure 1.8), also
reveals the lack of precision of the SVI test. The equivalent diluted SVI (DSVI) for the SVI at
150 mL/g would be 100 to 120 mL/g.
An uncontrolled increase in the SVI often results in a need to change the operating modes
of the aeration basins and clarifiers, which can upset the biological process. Since sludge bulking
has been the most common cause of reduced capacity and failures of activated sludge facilities,
the concept of bioselectors and its attendant benefits have been welcomed and widely accepted
as a standard practice.
Daigger (1995) developed a series of clarifier loading graphs as a function of undiluted
SVI (uSVI), stirred SVI (SSVI) and DSVI of the MLSS and return sludge concentration. These
2-2
2-3
relationships and graphs, Figures 2.2, 2.3 and 2.4, and similar graphs developed by Pitman
(1984), Daigger and Roper (1985), Keinath (1990) and Mines et al., (2000) provide a basis to
define the benefits of bioselection in a wastewater facility. Using Figure 2.2, the aeration and
clarification requirements for a plant with and without bioselection and the maximum SVIs are
as set forth in Table 2.1. The bioselector reduced the aeration volume by 14%, clarification area
by 38% and RAS flow by 40%. Furthermore, at ≤ 120 mL/g SVI the process will be more stable,
produce a higher quality effluent and be easier to control. There is a greater potential for the SVI
to increase from 150 to above 250 mL/g without bioselectors, but with bioselectors the more
probable uSVI is ≤ 100 mL/g. Further, as indicated by Figure 2.1, there is no rational method of
predicting the range of SVI when it exceeds 150 mL/g. It is not known how much of the uSVI
results are impacted by the problems of poor reproducibility of this SVI procedure. Historically,
the bulking problem has required designers to oversize the aeration and clarification facilities in
order to handle higher SVIs. Employing bioselectors will reduce both the aeration and
clarification volume requirements.
Table 2.1 Benefits of Sludge Bulking Control by Bioselection
Parameter Units w/o Bioselection w/ Bioselection
Flow (L/s) mgd 438 (10.0) 438 (10.0)
MLSS mg/L 3000 3500
Max uSVI (DSVI) mL/g 250 (130) 120 (80)
Aeration Mass 103 kg (103 lb) 29.5 (65) 29.5 (65)
Max. Clarifier SLR kg/m2.d (lb/ft2.d) (18) (30)
Return Sludge Conc.(1) mg/L(1) 7200 11,600
RAS Flow L/s (mgd) 313 (7.14) 189 (4.32)
Aeration Volume 103m3 (103ft3) 9.84 (347.2) 8.43 (297.6)
Clarifier Area 103m3 (103ft3) 1.94 (20.85) 1.21 (12.97)
(1) Design set at 80% of the RAS suspended solids (RSS) concentration in Figures 2.3 and 2.4 toallow for operational factors.
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2-5
The bioselector reduced the required aeration volume and clarifier area. Without
bioselectors, the aeration volume would need to be 16.7% larger and the clarifier surface area
would increase by 60%. It is expected that the smaller plant with bioselectors would also produce
a better effluent quality.
It is not recommended that the clarifier solids loading rates (SLR) and RSS
concentrations (Xu) in Figures 2.2, 2.3 and 2.4 be employed without an allowance (or safety
factor) to account for the normal load variations occurring in a typical wastewater treatment plant
as well as the inaccuracy of SVI measurements. With a good floor slope (≥ 1:12), center sludge
withdrawal mechanism and properly sized collection equipment, the RSS concentration will
exceed 80% of the values on the graph. Rapid sludge removal equipment may achieve only 60-
65% of the RSS concentrations. The SLRs set forth and the RAS/Q ratios will be proprotionally
higher. The predicted RSS and operating RSS of 9000 and 7200 mg/L, respectively, were
generated from the Daigger’s Figures 2.3 and 2.4.
Graph OperatingFigure uSVI DSVI SLR RSS RSSNo. mL/g mL/g lb/ft2.d mg/L mg/L 2.4 250 – 18 9,000 7,200
2.3 120 80 30 14,000 11,600
The procedure for employing the Daigger (1995) graphs is described below and
illustrated in Figure 2.5 using DSVI input data.
1. Locate the line of the maximum operating DSVI: 80 mL/g
2. Draw a horizontal line from the operating maximum SLR to the maximum DSVI and
note equivalent underflow solids concentration (RSS) and draw vertical line: 14,000
mg/L
3. Multiply the RSS value by 0.80 (14000 mg/L x 0.8) to determine operating RSS value of
11,600 mg/L to use to define the RAS flow:
2-6
4.
2-7
2-8
2-9
)/,()/,/,(
)/,(sLQ
LmgMLSSLmgRSS
LmgMLSS
−
)/438()/3500600,11(
)/3500(sL
Lmg
Lmg
− = 189 L/s
The graphs are developed such that all conditions to the left of the DSVI (SSVI3.5 and
SVI) line are considered operable clarifier designs. However, the design engineer should
recognize that there are optimal SLRs as a function of the RSS concentration. That is, an
underflow based on 18,000 mg/L at 80 mL/g DSVI would require a 300% larger clarifier. Thus,
judgment is required in the final selection of a SLR consistent with the overall process design
and operation and the configuration of the clarifiers (IAWQ 1992b, IAWQ 1997, WEF 1998).
While the Daigger DSVI and SSVI3.5 curves project similar underflows at the SLRs and
RSS concentrations selected, there are differences at other conditions. The curve employed must
be the same as the settling test procedure. The DSVI or SSVI3.5 test should be employed due to
the greater inaccuracy of the uSVI test as noted earlier.
As of the year 2002, bioselectors have been employed in hundreds of U.S. and overseas
facilities and are considered to be standard practice for new and renovated activated sludge
plants. Bioselectors have allowed the upgrading and rerating of plants for higher capacity and/or
for biological nutrient removal (BNR) by operating at much higher MLSS and solids retention
times (SRTs) in existing basins and increasing sludge handling capacity of clarifiers. Some of
these facilities will be reviewed in Chapter 5: Process Experiences with Bioselection.
While bioselectors provide control of many filamentous and foaming organisms, they do
not fully control some organisms. Anoxic bioselectors can control Nocardia growth in the
laboratory studies and full-scale facilities, but anaerobic selectors have not been successful
(Jenkins et al., 1993). Microthrix parvicella has been shown to be resistant to control in anoxic
bioselectors. Kucman (1987) reported high denitrification rates in activated sludge dominated by
M. parvicella, raising the possibility that this organism could use oxygen derived from nitrates.
2-10
Most filamentous organisms are inefficient users of combined oxygen (NO_
2 and NO_
3) and thus
can be controlled in mixed or aerated bioselectors with low or no DO.
In 1993 Jenkins et al., summarized the experiences regarding the effectiveness of
bioselectors and that information is reproduced in Table 2.2. New information from the
increasing number of operating facilities and ongoing studies will continue to expand that
information as well as the content of this study.
Thus, while there are some organisms that resist control by bioselectors, the overall
experiences are a marked improvement (a lowering) in the SVI. In general, the higher the
operating SVI before the installation of bioselectors, the greater the improvement that will be
realized in the plant operating capacity and performance.
Table 2.2 Effectiveness of Bioselectors in Controlling Filamentous Organisms
Effective Not Always Effective
S. natans type 0041
type 1701 type 0675
type 021Na type 0092
Thiothrix spp.a M. parvicella
N. limicola
H. hydrossis
Type 1851
Nocardia spp.b
aNot effective when caused by nutrient deficiencybAerobic selectors not always effective; anoxic selectors effective (Cha et al., 1992).
In many cases, bioselectors can be installed within existing basins as a series of small
compartments using 6 to 10% of the total aeration volume. The loss of this oxic volume is easily
offset by the increased MLSS and SLRs resulting from the modified operation of the system at a
low SVI. In denitrification systems, internal recycle of nitrified mixed liquor to the bioselector
zones results in highly efficient denitrification with no measurable loss of operating capacity.
3-1
Chapter 3.0
General Design Arrangement of Bioselectors
Since the bulk of the soluble biochemical oxygen demand (sBOD5) – or soluble chemical
oxygen demand (sCOD) – removal from solution occurs in the first 5 to 10 minutes of contact of
wastewater and return sludge, the bioselectors must be installed in the initial (first) contact zone
(ICZ) of the influent wastewater and recycled oxidized activated sludge. In most cases, the
recycled solids will be in the return activated sludge (RAS) from the secondary clarifiers.
However, the recycled solids also can be mixed liquor suspended solids (MLSS) from the oxic
zone or the discharge from the aeration basin. Heide and Pasveer (1974) demonstrated successful
bulking sludge control by recycling oxidized MLSS from an existing oxidation ditch and
contacting the MLSS with raw sewage in a six-stage external bioselector (Figure 1.3). The
food/mass (F/M) gradient as defined by Chudoba et al., (1973a, 1973, 1974) was also noted in
the Heide and Pasveer study and it followed the earlier staged aeration of the British Ministry
(1969). All staged system (F/M cascade) modes were successful in controlling the SVI.
While there are cases where single-stage bioselectors have been effective, the multi-
staged concepts of Chudoba et al., and Heide and Pasveer are recommended to ensure that the
bioselector design will be fully effective in controlling filamentous organism growths.
3.1 F/M Cascade Design
Bioselectors will most often comprise three or four complete mix activated sludge
(CMAS) reactors (zones) in a series for the initial contact of the return sludge and the wastewater
to be treated. These small compartments will provide a F/M gradient, which has been
demonstrated to be the most important process factor in the environmental conditions favoring
the production of a non-bulking sludge. The general arrangement of bioselectors for an advanced
wastewater treatment plant is shown in Figure 3.1. In this flowsheet, there can be an option for
biological phosphorus removal (Bio-P) and the anoxic zone allows for removal of the nitrates
(denitrification) in the internal recycle (IR) stream. When nitrification (N) and denitrification
(deN) are not required, the anoxic zones after the bioselectors and internal recycle are not
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3-3
employed. However, the bioselectors are equally effective in controlling bulking in conventional
secondary treatment.
When rectangular, longitudinal flow reactors are employed, the bioselectors are often
incorporated into the influent end of the aeration basins. For complete mix and orbital basins, it
is more common to construct the bioselectors (and anoxic zones, if required) externally to the
aeration basin. Some typical bioselector arrangements in U.S. installations are shown in Figure
3.2. The term ‘semi-aerobic’ (Albertson et al., 1987) has been employed for bioselectors that are
aerated at a low rate, but oxygen deficient (Albertson et al., 1991), or where there is a recycle of
nitrate-containing oxidized MLSS to the initial contact zone employing mechanical mixing. The
environment of all types of bioselectors and the terminology are now more definitive. The
method and degree of aeration, mixing and internal recycle of (or lack of) nitrate-containing
return sludge and mixed liquor are employed to define the type of bioselector.
The staging volume of the bioselector zones is similar for the U.S. and Czech designs
except that the U.S. recommendation (Albertson, 1987, 1992) is three stages of 25, 25 and 50%
of the total volume, while the Czech design (Wanner and Chudoba, 1988; based on Kroiss, 1985)
employs four equal volumes. Both concepts may or may not employ air for mixing when Bio-P
removal is required. The best performance for Bio-P removal is with mixing only and without
internal recycle. The question of whether the low DO aeration or anoxic mode results in a lower,
more consistent SVI is yet to be answered. The unaerated volume requirements for Bio-P
removal are larger than required for bulking sludge control. However, Bio-P designs are not a
focus of this report. Bioselectors designed for bulking control, with and without denitrification,
will generally reduce the effluent total phosphorus (TP) to 1.5 to 3.0 mg/L, depending on
incoming wastewater characteristics and the nitrogen inventory in the system.
The F/M gradient of the staged bioselector involves consideration of the COD, sCOD,
BOD5, and sBOD5 of the wastewater discharged to the initial contact zone of the bioselector. If
only inhibited or carbonaceous BOD5 (cBOD5) is available, the cBOD5 can significantly
understate the true strength of the wastewater, and comparable COD and sCOD should be
collected to define COD and sCOD to the cBOD5 and soluble cBOD5 ratios. When the
COD/cBOD5 ratios exceed 2.1, employ the COD and sCOD values for design. The COD and
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3-5
sCOD are the most accurate measurements of the wastewater’s available ‘food’ and these are the
favored input data for bioselector and aeration design. Since the sCOD (or sBOD5) is the
component involved in the bioselection mechanisms, these data should be employed when
available.
Municipal facilities and industrial facilities may receive significantly higher organic
loading in 8- or 16-hour periods. Because the biological growth characteristics is a function of
the load over a short period (5-15 minutes in the initial contact zone), it is necessary to consider
diurnal or peaking load (COD, BOD5) periods when they exceed 140% of design. The design
recommendation of this report will allow for typical municipal wastewater diurnal peaking
conditions, and if the peaking factors exceed 140% of the average load, then the cascade F/M
values must be adjusted (lowered) and the bioselector volumes increased.
Albertson (1987, 1992, 1994) and Jenkins et al., (1993) have suggested general design
criteria for aerated and anoxic bioselectors treating typical municipal wastewater and the
loadings are set forth in Table 3.1. However, there is neither general agreement by researchers
and practitioners on the F/M loading profile nor the number of profiles at this time. There is also
a question whether anaerobic bioselectors (SXANM), anoxic bioselector (SXAXM and SXAXAL),
and aerated (SXAH and SXAL) bioselectors should use the same F/M values. This will be
reviewed in Chapters 4 and 5. Currently, bioselector design criteria are most often different from
plant to plant and the results also differ, which may or may not be due to the bioselector design.
3-6
Table 3.1 General Design Guidelines for Bioselector Sizing(after Albertson 1987,1992,1994)
Zone F/ΣM
Sx-1 Sx-2 Sx-3
COD kg/kg.d 10-12 5-6 2.5-3
sCOD kg/kg.d 5-6 2.5-3 1.25-1.5
BOD5 kg/kg.d 5-6 2.5-3 1.25-1.5
sBOD5 kg/kg.d 2.5-3 1.25-3 0.63-0.75
Notes: (1) The sCOD or sBOD5 (f1.5μm) is the best basis for design of bioselectors.
(2) Use parameter (COD, etc) that generates the smallest bioselector volume.
(3) The COD/BOD5 and sCOD/sBOD5 range is presumed to be 1.8-2.2. Use COD and
sCOD only if outside this range. No criteria are presumed for cBOD5. Data for sBOD5
and sCOD should be collected when plant data are cBOD5.
(4) The diurnal peak load is assumed to be ≤ 1.4 average, 8-hour loading ≤ 1.3 average
and 16-hour loading ≤ 1.3 average. Reduce F/M criteria if loadings are higher.
The compartment sizing of the bioselectors to generate a F/M gradient has evolved
considerably. Still, the general mechanistic concept of bioselection using a F/M gradient in three
or four stages that was initially presented by Chudoba et al., (1973a, 1973b, 1974) and Heide and
Pasveer (1974) is still considered valid.
Wanner (1994) reported that the aerated mode was successful at the suggested initial
contact zone loadings in Table 3.1. His results are included in Table 3.2 with additional data.
3-7
Table 3.2 Prior Art Batch and Continuous Flow Experience with Bulking SludgeControl Concepts and the DO and BOD5 Mass Loading in the ICZ
Selector Feed DO F/M SVI (Test)Author Mode Mode mg/L kg/kg⋅d mL/g
Davidson, 1959 AN Continuous 0.0 1.0 34 (u)
Bhatla, 1967 AL Continuous 0.0 >2.5 <120 (u)
British Ministry, 1969 A Continuous Unkn 0.8 <75 (u)
Milbury, 1971 AL Continuous 0.0 >2.0 <100 (u)
Chudoba, 1973 AL Continuous <0.5 >2.5 <100 (u)
Heide & Pasveer, 1973 AN/Ax Continuous 0.0 >5.0 <100 (u)
Batch 0.0 infinity 50 (u)
Rensink, 1974 A Continuous Unkn 3.6 <100 (u)
Tomlinson, 1976 A Continuous Unkn >2.0 <100 (u)
Spector, 1977 AN Continuous <0.7 >3.0 <100 (S)
Chudoba & Wanner, 1988 AH Continuous ~1.0 12.0 <50 (S)
Daigger and Nicholson, 1990 AH Continuous 2.0 14.7 <75 (D)
Albertson et al., 1992 AL Continuous <0.3 5.0 <100 (D)
Albertson and Hendricks, 1992 AL Continuous <0.3 7.0 <100 (D)
Linne et al. AL Continuous Unkn >5.0 <80 (D)
A – aerated, DO level unknown
Daigger and Nicholson (1990) reported on the performance of four wastewater treatment
plants employing selectors. The selectors employed different modes and configurations. The
design and performance characteristics of the bioselectors at Upper Occoquon Sewage Authority
(UOSA), VA; Northside WWTP, Tulsa, OK; Fayetteville, AK; and Tri-City, OR are summarized
in Table 3.3.
3-8
Table 3.3 Summary of the Design and Operation of Bioselectors by Daigger andNicholson (1990)
Item Units UOSA Northside Fayetteville Tri-CiyFlow (% of Design) mL/d (%) 80 (79) 84 (80) 60 (94) 11.9 (37)
Bioselector Type ANSX Aerated ANSX AXSX
SX Stages (L/W) No. 1 (5) 2 (17) 6 (1) 1 (?)
ICZ F/M (BOD5) kg/kg.d 14.8(1) 25.6(2) 4.4 0.8
Total SX F/M kg/kg.d 4.9 3.2 0.7 0.8
SX Retention Time (Q) minutes 14 20 93 86
SX DO mg/L 2-3 1.0 0 0
NO3-N Recycle ? No No Yes(3) Yes
Aeration Design No.-Type 2-CMAS 1-CMAS 4-CMAS 1-CMAS
Aeration Operation Mode Series – Series –
Average SVI (month) mL/g 80 150 90 70
Maximum SVI (month) mL/g 150 >300 180 180
Effluent BOD5 mg/L 6.2 5.5 7.4 7.7
Effluent NH4-N mg/L 0.4 1.4 1.1 1.3
(1) Estimated by dye testing – equivalent to three stages/compartments(2) Estimated by this author to exceed eight equivalent stages(3) Not employed during evaluation phase.
The only selector that was not considered effective by the authors was at Northside in
Tulsa, OK. This was a two-stage bioselector aerated channel with a 17:1 length:width, which
would likely have the equivalent stages equal to eight or more compartments. This would
produce an F/M in the initial contact zone of about 25 kg BOD5/kg MLSS⋅d. At this loading,
there is a high potential for hydrous bulking. The bioselectors were also aerated at 1.0 mg/L DO.
This range of DO is known to encourage filamentous growth in bioselectors. In fact, if the SVI is
too low and causing excess turbidity due to rapid settling sludge, increasing the DO to 0.5 to 1.5
mg/L is recommended to encourage growth of filamentous organisms. Thus, it is possible that
bulking is a result of generating both filamentous and hydrous bulking organisms in the aerated
channel.
3-9
The F/M in the Tri-City bioselector is considered to be low and a single-stage unit is less
than optimum. Large, first-stage anoxic zones common to the early South African and American
Bardenpho process plants also had bulking problems with maximum SVIs of 150 to over 200
mL/g.
A rigorous comparison of the four bioselectors is not possible because the SVI (uSVI)
test was employed. Once the SSV30 value exceeds 250 to 300 mL/L, the SVI fails to define the
true settling characteristics of the MLSS. That is, at MLSS of 3500 to 4000 mg/L, a uSVI of 140-
150 mL/g could be a DSVI value of 80-110 mL/g and good settling would occur in the clarifier.
The MLSS data were not available to make any approximation of the relative DSVI values.
Martin and Daigger (1997) reported on bioselector performance in four wastewater
treatment plants. The results achieved by the bioselectors were inconsistent, but there was an
appreciable difference in the design criteria. The operational characteristics of the facilities are
provided in Table 3.4
Table 3.4 Summary of the Design and Operation of Bioselectors by Marten andDaigger (1997)
Beloit Green Bay Green Bay Landis Tri-CityItem Units WI North, WI South, WI NJ OR
Flow (Design) m3/d 30,100 NA NA 31,000 51,000
Bioselector Type AXSX AXSX AXSX AXSX AXSX
SX Stages No. 1 1 1 3 1
ICZ F/M (BOD5) kg/kg⋅d 0.7-1.2 1.0-1.6 1.2-2.2 0.63 0.5-1.4
Total SX F/M kg/kg⋅d 0.7-1.2 1.0-1.6 1.2-2.2 0.21 0.5-1.4
SX Retention Time (Q) minutes 162 60 72 528 48
Air Added ? No No No No Yes/No
SX DO mg/L 0 0 0 0 0
NO3-N Recycle ? Yes Yes Yes(3) Yes Yes
Aeration Design No.-Type PF(1) PF(1) PF(1) 4-CMAS 1-CMAS
3-10
Beloit Green Bay Green Bay Landis Tri-CityItem Units WI North, WI South, WI NJ OR
Aeration Operation Mode – – – Series –
Aeration SRT Days 8-12 8-11 8-11 14-33 4-11
Average SVI mL/g 100 110 125 160 100
Maximum SVI mL/g 120 250(2) 250(2) 280(2) 160
Nitrifying ? Yes Yes Yes Yes Yes
(1) No estimate of the equivalent number of stages(2) Chlorine employed to control filamentous organisms
In three of the five activated sludge systems, it was necessary to use chlorine to control
filamentous growth. That is, the filamentous growths could not be controlled by the bioselector
and also scBOD5 breakthrough occurred in the Beloit and Green Bay plants, even at the low F/M
ratios. Thus, the question arises whether the bioselectors, with the exception of the Beloit units,
failed to control bulking due to the design and operation of the bioselector or due to wastewater
characteristics. Based on the literature surveyed and recommendations of other researchers and
practitioners, possible reasons why the bioselectors were sometimes ineffective (maximum
month SVI > 150 mL/g) were:
• The single-stage bioselector did not have F/M cascade – after Chudoba (1973a, 1973b),
Albertson (1987, 1991) and Jenkins et al. (1993).
• The low F/M in the ICZ, less than 2-2.5 kg BOD5 /kgMLSS⋅d, did not provide adequate
stress (Wanner, 1994) in the initial contact zone necessary to control filamentous growth –
after Chudoba (1973a, 1973b), Albertson (1987, 1992), and Wanner (1993).
• The single-stage bioselectors are less effective in the removal of sCOD than multi-stage units
and sCOD breakthrough can cause filamentous growths.
• Biological volumes necessary for denitrification are too large for bioselectors, which need to
be designed on a higher F/M gradient in three to four stages followed by the anoxic zones.
• In the case of one facility, the F/M in the long, rectangular bioselector (Northside) would be
very high (at > 25 kg/kg.d) in the equivalent volume of the initial contact zone. This F/M
could cause hydrous bulking. However, this bioselector was aerated at 1.0 mg/L DO, a DO
that is known to produce filamentous organisms. This document discourages use of aerated
3-11
bioselectors at a DO range of 0.3 to 2.0 mg/L unless the objective is to raise the SVI by
growing filaments when the SVI is too low for good clarification.
• Lack of adequate oxygen in the oxic zones to fully regenerate the return sludge.
3.2 Contact Loading Analysis
Contact loading (CL) is the ratio of soluble organics (sCOD) mass to the mass of
biological solids in the initial contact zone and is defined by mg sCOD/g TSS. The contact
loading in the initial contact zone will not be a design consideration for most municipal
wastewaters with sCODs of 125 to 250 mg/L. At 3000 to 3500 mg/L MLSS in the initial contact
zone, the contact loading is 25 to 60 mg sCOD/g TSS when the zone is anaerobic. If the ICZ also
receives an internal recycle flow, the CL will be lower. However, it can be a definite
consideration for those municipal and industrial facilities that have an above average sCOD in
the influent to the initial contact zone. There will be a point where the soluble food/mass (FS/M)
loading criteria will result in a higher value than the recommended limiting contact loading.
Under these conditions, the contact loading criteria govern the bioselector design.
In the initial contact of return sludge (RAS and/or MLSS), the majority of the sCOD is
removed in a few minutes with minimal oxidation (Kroiss, 1985). This conversion to cell storage
has a limited capacity for healthy (non-bulking) sludge generation. Excessive levels of food will
result in exo-cellular protoplasm (ECP) accumulation (hydrous/viscous bulking) and possibly
breakthrough of soluble substrates to the oxic zone where filamentous growths can develop.
Thus, the contact loading is the ratio of the amount of soluble substrate per contacting unit of cell
weight in the ICZ without the time function of the F/M ratio.
While there has been successful bulking sludge control at contact loadings up to 200 mg
sCOD/g TSS, the recommended limiting value is 100 mg/g (10% of cell weight) for the average
daily value. It is recognized that the peak loading periods could result in CL values up to 140
mg/g. The limiting sCOD into the initial contact zone as a function of the MLSS is shown in
Figure 3.3. As discussed earlier, bioselection is favored by higher MLSS and the resulting
increased oxygen demand in the ICZ.
3-12
When the influent (Qi) sCOD is higher than indicated in Figure 3.3, then an additional
source of activated sludge solids must be added to the initial contact zone. The solids must be
fully oxidized (regenerated) MLSS from the effluent end of the aeration basin via an internal
recycle (IR). The IR (Figure 3.4) requirements are defined by:
CL ≤ 100 mg/L ≤ )/,)(/,()/,)(/,(
)/,)(/,(
LmgMLSSsLIRLmgRSSsLRAS
LmgsCODsLQ ii
+(3.2-1)
When the contact loading is greater than 100 mg/g, the above equation suggests that there
are two possible responses to reduce the contact loading.
• Increase the RAS flow of RSS (higher MLSS) within the capability of the secondary
clarifiers to process the (Q + RAS)(MLSS) solids loading.
• Turn on or increase the IR flow. The IR flow is preferred due to lower power
requirements, better control, and no adverse impact on the secondary clarifiers. If the
system is nitrifying, denitrification will occur in the bioselectors.
When treating strong municipal and industrial wastewater, it is necessary to check the
contact loading of a bioselector design to be assured there is not a problem of high contact
loading that could result in hydrous sludge (excess ECP) production.
3.3 Sludge Age
Washout of filamentous organisms is difficult because their growth rate (sludge age)
spans such a large range as shown in Figure 3.5 (Wanner, 1994). This procedure has been
employed for secondary level of treatment using 1- to 2-day solids retention time (SRT), but it is
not practical for biological nutrient removal (BNR) processes requiring a 5- to 15-day SRT.
There is probably a minimal sludge age (defined by SRT) where bioselectors lose their
effectiveness. The exact SRT resulting in washout of preferred biomass (rapid settling) is not
known and may be both site and temperature dependent. However, bioselection has consistently
proven effective at SRTs of > 3 to 4 days and as low as 1.6 days at the Deer Island pilot plant
(Bowen et al., 1992 and MWRA, 1995).
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3-14
3-15
3-16
3.4 Design Features
It is necessary to eliminate uncontrolled backmixing between the bioselector zones. The
design should employ submerged walls (Figure 3.6) to prevent the entrapment of floating
material in the compartments. Floating material is unsightly, can be odorous and
counterproductively act as a seed for increased filamentous growth. The design of the submerged
baffle walls should allow for about 1.2 mm (0.5 inch) head loss per baffle at the maximum
month combined flow of wastewater, RAS and IR at each stage. An opening at the bottom of the
baffle to allow filling and drainage of the component is necessary and eliminates the need for the
baffle walls to structurally withstand the hydraulic load.
A sizing procedure for the submerged baffle walls, described by Albertson (1999), has
been successfully employed in many facilities including those at Columbus, OH (Jackson Pike
and Southerly); Phoenix, AZ (91st and 23rd Ave); Santa Fe, NM; Baltimore, MD (Back River);
Pierce Co, WA; Lakeland, FL; and Wash, DC. (Seneca).
The submerged wall height is designed to produce about a 1.2 mm (0.5-inch) water
column (WC) head loss (ΔH) as shown in Figure 3.6. The head loss equations of Fteley and
Stearns (King, 1939) with the coefficient (C) provided can be employed to generate the data
necessary to calculate the information for the wall configuration. The flow over the baffle would
include maximum influent flow and RAS as well as the internal recycle flow. A sample
calculation is provided in Table 3.5.
A slot at the end of the partition wall has been employed, but it is not as effective unless
it is designed to have sufficient headloss to prevent backmixing. With aeration on one side and
mixing on the other side, backflow at the bottom of the slot will occur if the head loss is too low.
The submerged baffle eliminates backmixing and does not trap solids. The surface of the
aeration basin is clear of foam as shown in the photograph (Figure 3.7) of the four-pass aeration
basin at the 23rd Ave Water Reclamation Facility in Phoenix, AZ.
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3-18
3-19
Table 3.5 Calculation of Baffle Wall Height
Input Data
Avg/Max Month Flow – 8/11 mgd Baffle Wall Length(1) – 30 ft
Max. Month RAS – 8 mgd Basin Depth – 16 ft
IR Flow @ 4 QAMF – 32 Head Loss (ΔH = 0.5 in) – 0.0417 ft
Total Flow – 51 mgd (79.1 ft3/sec)
F&S Eq Qt = CL(ΔH)0.5(H + d/2)
= 0.2042 CL(1.5H – 0.0208) (@ ΔH = 0.0417 ft)
H = 0139.0))()(3063.0(+
LC
QT
1st Estimate H = 2.25 ft
d/H =25.2
0417.025.2 − = 0.9815
Fteley and Stearns C = 3.304
∴H = 2.62 ft from Fteley and Stearns Equation
2nd Estimate H = 2.60 ft
d/H = 0.9840
C = 3.310
∴H = 2.61 ft OK = 2.60 ft submergence
(1) Basin width is also 30 ft.
Values of Fteley and Stearns Submerged-Weir Coefficient C
d/H 0.0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
0.0 0.000 3.330 3.331 3.835 3.343 3.360 3.368 3.371 3.372 3.370
.1 3.365 3.359 3.353 3.343 3.335 3.327 3.318 3.310 3.302 3.294
.2 3.286 3.278 3.271 3.264 3.256 3.249 3.241 3.231 3.227 3.220
.3 3.214 3.207 3.201 3.194 3.188 3.182 3.176 3.170 3.168 3.159
.4 3.155 3.150 3.145 3.140 3.135 3.131 3.127 3.123 3.119 3.116
.5 3.113 3.110 3.107 3.104 3.102 3.100 3.098 3.096 3.095 3.098
.6 3.092 3.091 3.090 3.090 3.089 3.089 3.089 3.090 3.090 3.091
.7 3.092 3.093 3.095 3.097 3.099 3.102 3.105 3.109 3.113 3.117
.8 3.122 3.127 3.131 3.137 3.143 3.150 3.156 3.164 3.173 3.181
.9 3.190 3.200 3.209 3.221 3.233 3.247 3.262 3.280 3.300 3.325
Source: King, H.W. Handbook of Hydraulics, 2d ed. p. 98 McGraw Hill Book Company, Inc., New York, 1939.
4-1
Chapter 4.0
Types or Designs of Bioselectors
Nomenclature employed to define the type of bioselectors (SX) for bulking sludge control
and the mode of mixing are anaerobic (AN), anoxic (AX), and aerated, where mixing is provided
by low air rate (AL) with DO < 0.3 mg/L, high air rate (AH) with DO > 2.0 mg/L or mechanical
(M) means.
• Aerated, High DO (SXAH): The SX zones are heavily aerated to maintain a
dissolved oxygen (DO) > 2.0 mg/L for bulking sludge control. This process is
employed for both secondary and/or nitrification modes of operation with or
without limited denitrification. That is, denitrification is not a design requirement.
• Aerated, Low DO (SXAL): The SX zones are aerated at a rate whereby the DO is
< 0.3 mg/L during off-peak loadings and zero DO during average to peak
loadings for secondary and/or nitrification processes with or without limited
denitrification.
• Anoxic with Mechanical Mixing (SXAXM): An anoxic environment at zero DO
is maintained by mechanical mixing. Nitrates are recycled from the oxic zones for
enhanced levels of denitrification in the bioselectors and subsequent anoxic
zones.
• Anoxic, Aerated, Low DO (SXAXAL): An anoxic environment at near zero DO
with aerated zones as set -forth in SXAL. Nitrates are recycled for enhanced levels
of denitrification.
• Anaerobic with Mechanical Mixing (SXANM): An anaerobic environment is
maintained by employing mechanical mixing and the design limits the average
concentration of nitrates to 0.5 mg/L in the total flow entering the initial contact
zone (ICZ). That is, internal recycle is not discharged into the bioselectors. This
mode of operation is preferred for maximum levels of Bio-P removal.
4-2
-
While a bioselector may have two to four compartments (zones), the terminology ‘initial
contact zone’ (ICZ) refers only to the first compartment of the bioselector where influent and
return sludge are mixed. All bioselectors should be staged (three or four compartments) and will
thus create a food/mass (F/M) gradient (cascade). This does not mean that a single-stage
bioselector will not work, but it is less effective and thus not reliable. For those reasons and the
history of successful bioselectors, only three- or four-stage designs are recommended for this
study.
The recommendations in this section will not reflect the full range of successful operating
conditions for bioselectors. These are site-specific conditions that are not fully understood at this
time. Thus, the recommendations reflect what is considered to be the most reliable design criteria
and process approach with the available data. There can exist numerous successful systems that
are not within the suggested guidelines. In fact, there are, for unknown reasons, complete mix
activated sludge (CMAS) systems that do not bulk (Tomlinson, 1976), while many others,
seemingly with similar operational conditions, that do bulk. Tomlinson’s survey of 65 English
plants clearly revealed the benefits of multistage aeration basins for producing lower SVIs.
However, there were CMAS plants with SVIs <100 mL/g as well as plants at >500 mL/g.
It is recognized that the ‘anaerobic’ bioselector is not actually anaerobic (methane
producing) as facultative organisms are the dominant group present. However, it is a convenient
definition and provides differentiation as described in a Bio-P and bioselector patent (Spector,
1977). This patent is the source of distinguishing an anaerobic zone (< 0.5 mg/L nitrate level in
the total flows entering the initial contact zone) vs. an anoxic zone, where anoxic is defined as
>0.5 mg/L NO3-N. The term ‘anaerobic’ was also used earlier by Davidson (1957) to describe
the first contact zone of his bioselection process which was also patented. Albertson (1987,
1992,1994) differentiated the type of bioselectors in quantitative terms as shown in Table 4.1.
This terminology has been employed throughout this document.
4-3
Table 4.1 Characteristics of Initial-- Contact Zone of Bioselectors
Environment Criteria
Aerated, High DO (SXAH) - air or oxygen added, DO > 2.0 mg/L, without nitrates present
Aerated, Low DO (SXAL) - air added, DO = 0.0 to 0.3 mg/L,NO3-N < 0.5 mg/L in mixed liquor formed
Anoxic (SXAXM) - no added air or oxygen, DO = 0.0 mg/L,NO3-N > 0.5 mg/L in mixed liquor formed
Anoxic, Aerated, - air added, DO < 0.0-0.3 mg/LLow DO (SXAXAL) NO3-N > 0.5 mg/L in mixed liquor formed
Anoxic, AeratedHigh DO (SXAXAH) - not recommended at this time
Anaerobic (SXANM) - no added oxygen or air, DO = 0.0 mg/L, mechanically mixed,NO3-N < 0.5 mg/L in mixed liquor formed
F/M Cascade - 2.5 to 3 kg sBOD5/kg MLSS⋅d in ICZ with 3-4 stages
The environmental conditions set forth above are somewhat arbitrary and are not
exclusive. Prediction of bulking conditions in a system is problematic and sometimes conditions
that should control filamentous growths have not done so, and the opposite situation can be true.
The objective must be to develop an environment where there is the maximum opportunity to
control growth of undesirable species of bacteria. In the author’s 20-plus years of experiences
with multistage bioselectors (1980-2005), the process has been 90-95% successful in the control
of filamentous organisms. Some failures are due to inappropriate designs, lack of understanding
of the control mechanisms, and/or changes in or inadequate definition of the influent
characteristics.
The following sections present alternative bioselector designs with and without aeration
and mechanical mixing for aerated, anoxic and anaerobic bioselectors. The recommendations
may agree or conflict with other cited references. This is not unusual for a developing
technology that does not have a sufficient quantity of readily available and reducible full-scale
data.
4-4
Because bioselector zones probably involve only the soluble (defined by filtration at 1.2-
1.5 μm) fraction of the wastewater organics, it is more correct to use the soluble chemical
oxygen demand (sCOD), or soluble biological oxygen demand (sBOD5), fraction to define the
soluble food/sum of the mass (FS/ΣM) gradient. It is also necessary to protect against
overloading of the biomass in plants with higher than normal peaking loads to prevent periodic
overloading and subsequent hydrous sludge production. In small wastewater plants, the higher
peaking factors dictate use of a lower (FS/M) gradient. As an example, the recommended FS/M
criteria in a three-stage anoxic (SXAXM or SXAXAL) bioselector are set forth in Table 4.2 for 1.4
and 2.0 organic load peaking factors. When the peaking factors are higher than 1.4, the F/M in
the initial contact zone should be reduced, as shown in this table.
If the COD/BOD5 and sCOD/ BOD5 ratios are not about 2.0 ± 0.1, sCOD values should
be employed for the design or the sBOD5 FS/M adjusted to account for the difference. The sCOD
is a better measure of the loading than sBOD5 and should be employed when data are available.
It is recognized that the influent sCOD is reduced at each stage. However, it is convenient to
express the design criteria for each stage as the influent soluble value (food) divided by the sum
of the accumulated mass (Fs/ΣM) in the zones. It is a bookkeeping procedure for design and is
not representative of the conditions within the bioselection zones.
Table 4.2 Design FS/M Criteria for Anoxic (SXAXAL or SXAXM) BioselectorsTo Account for Daily Peaking Factors
sBOD5 sCOD
Peaking Load Factor (8 hr)(1) 2.0 1.4(2) 2.0 1.4(2)
Zone Average Loading
Sx-1 FS/ΣM – kg/kg⋅d 2.2 3.0 4.4 6.0
Sx-2 FS/ΣM – kg/kg⋅d 1.1 1.5 2.2 3.0
Sx-3 FS/ΣM – kg/kg⋅d 0.55 0.75 1.1 1.5(1)Interpolate FS/M loadings for intermediate or higher peaking factors.(2)Typical peaking factor for domestic wastewater treatment plants.
4-5
A typical set of calculations for bioselectors for a hypothetical wastewater is defined in
Table 4.3. These wastewater characteristics will be employed for all bioselector designs in this
section, but the peaking load factor must be considered on a site-specific basis. Calculations that
use similar design methodology will refer to the first set of calculations developed for bioselector
designs. Because the calculations are soluble (f1.5µm) substrate quantities, the origin (raw or
settled) of the wastewater is not of consequence. However, if soluble BOD5 and COD data are
available, it is strongly recommended that these data be collected prior to finalizing the design of
the bioselectors.
Table 4.3 Wastewater Characteristics Used in an Equation to EstablishCriteria for the Bioselector Design
Parameter Units Maximum Month Load 8-hr Peak
Flow L/sec 1,000 1,400
COD kg/d 43,200 69,100
sCOD kg/d 17,000 27,200
BOD5(1) kg/d 19,400 31,000
sBOD5 kg/d 7,200 11,700
TKN kg/d 3,600 --
TP kg/d 650 --
Temperature °C 15/25 --(1)cBOD5 data are not acceptable input values since they can be 0.70 to 0.90 of the BOD5.
A review of Table 4.3 reveals a sCOD/sBOD5 ratio of 2.3, or higher than 2.1; thus the
sCOD value should be employed for the design of the bioselectors. The 8-hour peaking factor is
1.6 (> 1.4) and factoring the F/M according to Table 4.2 instructions, a FS/ΣM gradient of 5.5,
2.75 and 1.38 kg sCOD/kg MLSS⋅d will be employed with the average maximum month sCOD
loading.
4-6
The calculations for the bioselector design would be as set forth below.
Zone SX-1 FS/ΣM = 5.5 kg sCOD/kg⋅d
1−XSM =dkglb
dkg
⋅/5.5
/000,17
= 3091 kg
1−XSV =3/5.3
/091,3
mkg
dkg
= 883 m3 @ 3500 mg/L MLSS
Zone SX-2 FS/ΣM = 2.75 kg sCOD/kg⋅d
2−XSM = kgdkglb
dkg3091
/75.2
/000,17−
⋅
= 3091 kg
2−XSV =3/5.3
/091,3
mkg
dkg
= 883 m3
Zone SX-3 FS/ΣM = 1.38 kg sCOD/kg⋅d sCOD/kg●d
3−XSM = kgkgdkglb
dkg30913091
/38.1
/000,17−−
⋅
= 6182 kg
3−XSV =3/5.3
182,6
mkg
kg
= 1766 m3
The aeration design for the system produced a basin average actual oxygen transfer rate
(AOTR) of 40 mg/L⋅hr for the oxic volume. Thus, the maximum aeration AOTR design for the
bioselectors would be 32 to 40 mg/L⋅hr (80 to 100%). However, the normal diffuser turndown
will generally provide 40 to 100% of the overall basin average AOTR to account for startup
conditions and lower than design loadings. The conversion to standard oxygen transfer rate
4-7
(SOTR) would be made using standard practice equations. Most often, coarse bubble diffusers
have been employed in bioselectors, but membrane diffusers have provided satisfactory service.
It is recommended that an alpha value of 0.7 be used for coarse bubble and 0.35 for membrane
diffusers in the bioselector zones. The type of diffuser employed will be dependent on the ability
to remove a basin from service while maintaining adequate facilities in operation. When there is
a mixture of coarse bubble diffusers and fine bubble diffusers, there should be a separate air
service with a control valve and flow meter to all for a constant air flow to the selector zones.
The total volume (3532 m3) of the bioselectors sized in the previous example would be
allocated based on the number of aeration basins. Generally each aeration basin has a
bioselector, but one bioselector can serve two or more basins. When one bioselector serves two
or more basins, the aeration design should allow the bioselector to remain in service at all times.
4.1 Aerated, High DO Bioselectors (SXAH)
The first published reference to a cascade of heavily aerated zones was Chudoba (1973a,
1973b, 1974), wherein the initial stage of the 2-, 4-, 8- and 16-stage reactors was targeted at a
DO level of 2 mg/L. This target level proved to be difficult to maintain in the initial contact zone
of the 8- and 16-stage reactors. The ICZ oxygen uptake rate (OUR) would be very high in both
the 8- and 16-stage systems.
Casey et al., (1975) were issued a U.S. patent (No. 3,654,147) on a heavily aerated or
oxic bioselector that set forth specific criteria for the control of filamentous bacteria. The design
criteria had a F/M component wherein the mass employed was the active mass fraction. This
fraction could be from <30 to >70% of the mixed liquor volatile suspended solids (MLVSS). The
critical parameters for the initial contact zone in the Casey patent were:
1.55 BOD5/active mass ≥ 4 kg/kg⋅d
DO ≥ 2 mg/L
OUR ≥ 100 mg O2/L⋅hr
4-8
For the most part, the patent and the above criteria were established for pure oxygen
systems. The initial contact of the influent wastewater was controlled to a specific BOD5:active
mass ratio by compartmentalization. Pure oxygen systems were designed to provide sufficient
driving force to maintain the first compartment DO at 2-8 mg/L, even with high oxygen uptake
rates. However, in practice, the DO in the first stage of pure oxygen plants was often below 2
mg/L when the process was at design load.
In full-scale pure oxygen operations, control of filamentous growth and the resulting
sludge volume index (SVI) were not always attained. In many cases, there was severe bulking
and it was necessary to operate the pure oxygen aeration basins at low (1000 to 1500 mg/L)
mixed liquor suspended solids (MLSS) in order to prevent overloading the clarifiers. Often the
installed oxygenation capacity in the first stage was not adequate to meet the oxygen demand
necessary to produce the required DO at the design MLSS. Conversely, some systems have
operated at high MLSS and zero or near zero DO successfully in the first stage with a low SVI.
While the expired patent suggests maintaining a DO greater than 2.0 mg/L is necessary, it
is probable that a higher DO was necessary to fully maintain the entire floc aerobic. That is, in
the high-rate regime common to pure oxygen activated sludge, a liquid DO of 4-8 mg/L may be
required to provide the gradient necessary for oxygen to fully penetrate the floc. It has been
reported from tests at San Francisco, CA, that a low DO in the initial stage of the pure oxygen
reactor was more effective than higher DO in the high-rate oxygen reactors. However, the
Boston, MA, pure oxygen plant with a four-stage bioselector has determined that 10 mg/L DO in
the initial contact zone is the best operating mode (Tyler, 2002).
The reported use of a heavily aerated (SXAH) ICZ was not actively pursued until 1985
(Kroiss) when an aerated four-stage bioselector was employed to treat sugar mill wastewater.
The Czechoslovakian bioselector design employed a SOTR up to 167 mg/L⋅hr in the initial
contact zone (Grau, 1991) in order to maintain >1 mg/L, preferably ≥ 2.0 mg/L DO in the initial
contact zone. The bioselector at the sugar mill was designed on the criteria set forth in Table 4.4.
4-9
Table 4.4 Technical Parameters and Results of the Bioselector (SXAH) Activated SludgeProcess in the Leopoldsdorf Sugar Mill Obtained During the Campaign in1984 (Kroiss)
Parameter Dimension Value
Flow m3/d 45,000
Selector Volume m3 400
Total Volume m3 16,400
Selector Detention Time h 0.21
Total Detention Time h 8.71
COD Loading - System kg/m3⋅d 1.23
BOD5 Loading - System kg/m3⋅d 0.93
MLSS mg/L 3,300
SRT d 8
SVI mL/g 50
Energy Consumption kWh/kg BOD5 0.7
COD Removed in Selector % of CODtot Removed 72
Oxygen Consumed in Selector % of CODtot 8.5
Influent sCOD g/m 450
Effluent sCOD g/m 45
Influent sBOD5 g/m 340
Effluent sBOD5 g/m 10
COD Removal % 90
BOD5 Removal % 97
The four-stage bioselector F/ΣM loadings were 12, 6, 4 and 3 kg BOD5/kg MLSS⋅d,
respectively. Since the COD/ BOD5 ratio is 1.32 kg/kg, the equivalent COD loading is 15.9, 7.9,
5.3 and 4.0 kg/kg⋅d. The ‘equivalent’ BOD5 at COD/ BOD5 = 2.0 kg/kg reduces the BOD5
loading effect by 34% to 7.9, 4.0, 2.6 and 2.0 kg/kg⋅d. Due to the low COD/ BOD5 ratio for the
sugar wastewater, the design based on BOD5 can be misleading. However, the COD and BOD5
in the sugar mill wastewater is mostly soluble and the equivalent soluble loadings are higher than
4-10
recommended in this document. The oxygenation (SOTR) capacity was 4 kg/m3⋅d (167 mg/L⋅hr)
for the bioselectors and 2 kg/m3⋅d (83 mg/L⋅hr) for the overall system, and the reported actual
oxygen transfer rate (AOTR) would be about 109 mg/L⋅hr based on CODR.
The Upper Occoquan Sewage Authority (UOSA) plant in Virginia employs an aerated,
high DO (SXAH) three-stage bioselector that can maintain the DO at ≥ 2 mg/L. The overall F/M
is 4.9 kg BOD5/kg MLSS⋅d and a loading of 14.7 kg BOD5/kg MLSS⋅d in the initial contact
zone. The bioselector has proven capable of reducing the SVI to an average of 74 mL/g from
SVIs exceeding 600 mL/g prior to the bioselector installation (Daigger and Nicholson, 1990).
The UOSA plant has operated at equivalent COD F/Ms exceeding those set forth by Kroiss
(1985) and about 250% higher than recommended by Jenkins et al., (1993). There has not been
any reported problem (Sellman, 2001) of exo-cellular protoplasm (ECP) accumulation after 10
years of operation. Process data from the UOSA wastewater facility are included in Chapter 5.
Jenkins et al., (1993) and Czechoslovakian (now Czech Republic) researchers Chudoba
and Wanner’s (1988) recommendations for F/M loadings of an aerated, high DO (SXAH)
bioselector are:
Jenkins et al. Chudoba and Wanner
Zone SX-1 12 kg COD/kg MLSS⋅d 12 kg BOD5/kg MLSS⋅d
Zone SX-2 6 kg COD/kg MLSS⋅d 6 kg BOD5/kg MLSS⋅d
Zone SX-3 3 kg COD/kg MLSS⋅d 4 kg BOD5/kg MLSS⋅d
Zone SX-4 – 3 kg BOD5/kg MLSS⋅d
The Chudoba and Wanner organic loadings are similar to those in the UOSA and
Hamilton, OH, (Wheeler et al., 1984) bioselectors, which have also operated successfully but
about 100% higher than the more recent Jenkins et al., reference.
The oxygen supply in the initial contact zone may need to meet up to 30 to 35 mg O2/g
MLSS⋅hr in warm wastewaters at 3500 to 4000 mg/L MLSS. Thus, SOTR values of 140 to 160
mg/L⋅hr are necessary and may not meet the target DO values of 2 mg/L. An SOTR of 160
4-11
mg/L⋅hr is about the practical transfer limit of diffused aeration employing air feed. Thus,
maintaining 1 to 2 mg/L DO in the initial contact zone of a high-rate nitrification (SRT 4-6 days)
system may be at the limit of air-supplied SOTR capabilities. Further, the proportion of sCOD in
the influent will be important in defining the maximum value of AOTR and SOTR.
The BOD5 loading in the initial contact zone of the reported operating plants is 12 to 14.7
kg BOD5/kg MLSS⋅d. Experience is limited and the reduced loadings suggested by Jenkins et
al., (1993) form the basis for the aerated, high DO (SXAH) bioselector design criteria. The
primary reason for employing loadings lower than the cited references is concern for production
of hydrous sludges from floc organic overload. Further, there is evidence that the lower loadings
will still provide good bulking sludge control. The SXAH bioselector design recommendations
are set forth in Table 4.5.
Table 4.5 Design Recommendations for Aerated, High DO (SXAH) Bioselectors
Relative Design LoadingsBioselector AOTR Volumes COD sCOD(1) BOD5 sBOD 5
(1)
Zone mg/g⋅hr % kg/kg⋅d kg/kg⋅d kg/kg⋅d kg/kg⋅d
SX-1 30-35 25 16 8 8 4
SX-2 25-30 25 8 4 4 2
SX-3 25-30 50 4 2 2 1
(1) Preferred wastewater characteristic for design.
The three-stage design shown in Table 4.2 can also be used for the four-stage Czech
design by dividing the third zone (SX-3) of Table 4.5 into two compartments creating a
bioselector with four equal zones.
If the operating SXAH bioselector was producing a higher than target SVI (stirred SVI,
diluted SVI < 90 mL/g) and the DO was 0.5 to 1.0 mg/L at a maximum air rate, the air rate
should be reduced to cause the DO to be 0.0 mg/L and retested (i.e., convert aerated to low DO
bioselector).
4-12
4.2 Aerated, Low DO Bioselectors (SXAL)
In the 1960s, some wastewater researchers and practitioners recognized that the DO, or
lack of it, in the initial length of long, rectangular basins had an impact on the treatment
effectiveness. Their observations (Vacker et al., 1967; Wells, 1969; Scalf et al., 1969; Milbury et
al., 1971, Garber et al., 1972) related not only to the biological removal of phosphorus (luxury
uptake) but also to the control of SVI. Due to the high BOD5 loading and oxygen demand,
coupled with diffuser fouling, low or no DO was present in the initial portions of the long,
rectangular basins. That is, the low DO in the initial 20-30% of the basin length encouraged
biological phosphorus removal, later called Bio-P removal, as well as producing reduced SVIs.
Tomlinson (1976) found that English wastewater treatment plants with long, rectangular basins
generally had SVIs less than 100 mL/g. Additional findings in the 1970s by Chudoba et al.,
(1973a, 1973b and 1974) and Heide and Pasveer (1974) recognized the significance of the high
organic loading in the initial contact of return sludge and influent wastewater as it related to the
control of bulking organisms. The results of Chudoba’s studies in 1973 are shown in Table 4.6.
Table 4.6 Results of 1973a Chudoba Laboratory Staged Aeration Studies
Reactor No. I II III IV
Volume – L 4 4 4 4
Retention Time – hr 8 8 8 8
Compartments – No. 1 4 8 16
COD – mg/L 650 650 650 650
BOD5 – mg/L 230 230 230 230
MLSS - mg/L 1900 3280 2940 3100
SRT – days 3.0 4.8 4.7 4.5
COD F/M – kg/kg TSS⋅d 1.03 0.595 0.663 0.629
BOD5 F/M – kg/kg TSS⋅d 0.44 1.02 2.29 4.36
Settled Effluent
TSS – mg/L 53 8.9 13.1 15.0
COD – mg/L 107.8 30.6 43.0 50.7
BOD5 – mg/L 40.8 11.0 10.2 10.8
4-13
Reactor No. I II III IV
NH4+ – mg/L 2.9 0.4 3.4 3.6
NO_
2 – mg/L 29.0 7.9 17.9 22.0
NO_
3 – mg/L 16.3 33.5 23.1 25.9
Org N – mg/L 4.9 4.0 3.9 4.9
PO4–3 – mg/L 3.1 0.9 0.2 2.0
SVI – mL/g 517 300 91 51
ICZ COD F/M – kg/kg TSS⋅d 1.03 2.38 5.30 10.06
ICZ BOD5 F/M – kg/kg TSS⋅d 0.44 1.02 2.29 4.36
Chudoba et al., experiments with 1, 2, 4, 8 and 16-stage and 1, 4, 8 and 16-stage reactors
furthered the British Water Pollution Control Laboratories’ work (1969) with 1, 2 and 4-stage
reactors and correlated the SVI to the F/M gradient in the reactors. The 8 and 16-compartment
aerated reactors with the highest F/M in the initial contact stage (ICZ) produced the lowest SVI
(Figure 4.1). While the target DO in these studies was 2.0 mg/L, the operating DOs in the 8 and
16-stage ICZs were less than 0.5 mg/L (Chudoba, 1985b) due to the high oxygen demand in the
small compartments. Thus, the work of the Czech investigators was consistent with the U.S. and
British investigators (Tomlinson, 1976; Tomlinson and Chambers, 1978b) who had earlier found
SVI control in activated sludge systems with long aeration basins that provided an F/ΣM
gradient. This gradient was better maintained when the basin was baffled into compartments as
was first noted by Donaldson (1932) and later by the British Water Pollution Laboratory (1969).
Chudoba et al., (1973a, 1973b) studies confirmed these earlier observations.
In 1976, Tomlinson reported on an extensive survey of 65 English activated sludge
facilities. The focus of the study was the bulking sludge history of each facility. It was noted that
the larger plants tended to have less bulking problems. These same plants would normally have
long, rectangular tanks with more of a plug-flow mode of operation. The mathematically
4-14
4-15
calculated dispersion index to define the number of compartments was translated into the
theoretical F/M loading in the first compartment (ICZ or SX-1).
The results of this analysis, Figure 4-2, supports the position that the F/M loading must
be sufficiently high to ensure biological stress, and this F/M is >2 and most likely above 3 kg
BOD5/kg mass⋅day in the initial contact zone. However, some plants, even in a complete mix
(CMAS), will not have bulking problems. As shown, at F/Ms of 0.1 to 0.5 kg/kg⋅d, the SVI
ranged from about 70 mL/g to 680 mL/g, but only four of 25 plants with an F/M < 0.5 kg/kg⋅d in
the initial contact zone had SVIs < 150 mL/g and the average value exceeded 300 mL/g SVI.
With an F/M > 1.8 kg/kg⋅d in the initial contact zone, none of the plants had an SVI > 150 mL/g.
The DO in the initial portion of the long basins was not reported. However, typically fine
bubble diffusers in the initial portion of a long basin blinded more easily and alpha values were
lower. As the result, the DO was low and the low SVIs were due to operating in a high F/M,
aerated, low DO mode (SXAL) of bioselection in the initial contact zone of the long basin.
In the aerated, low DO bioselectors, the AOTR will be much less than the oxygen uptake
rate and the AOTR needed for the MLSS in the oxic zones. For example, the 20°C OUR of a
3500 mg/L MLSS can be 30 to 40 mg O2/g⋅hr, but the design AOTR would usually not be more
than 30 to 35% of this rate. This will result in a zero DO in the initial contact zone at design
loadings. The recommended AOTR for the bioselector zones is about 80 to 100% of the average
AOTR for the aeration basin when Bio-P reactions are not required. As a consequence, the DO
will be at or near zero except during low BOD5 loading periods when biological growth is
minimal.
Earlier bioselector criteria (Albertson, 1987) employed a three-stage design with 5.6, 2.8
and 1.4 kg BOD5/kg MLSS⋅d F/ΣM gradient for the primary effluent at the Southerly plant in
Columbus, OH. Later this was modified for a COD design at a 12, 6 and 3 kg/kg⋅d F/M gradient
for primary effluent. However, it is the soluble fraction of the wastewater that is involved in the
bioselector mechanisms and this should be employed for design. The soluble fraction of raw
4-16
4-17
domestic wastewater is 30-40% and primary effluent normally has 45-55% sBOD5/BOD5
(sCOD/COD). Industrial content can modify the values; thus, a design based on soluble fraction,
preferably sCOD, would be a more reliable approach. This approach is consistent with the
contact loading (CL) criteria of mg sCOD/g TSS set forth earlier in Chapter 3, Section 3.2.
The aerated, low DO bioselectors may or may not be anoxic, but both modes will be
equally effective in controlling the growth of filamentous organisms. A bioselector in secondary
treatment mode may not be as effective at less than three days solids retention time (SRT).
However, the Deer Island, Boston, MA, bioselectors (Bowen et al., 1992) produced low SVIs at
1.6- to 2.6-day SRT. Because nitrification can occur at SRTs greater than 2-3 days, a contact-
stabilization (C-S) mode can be employed to provide a 3- to 5-day SRT, minimize nitrification,
and maintain an adequate SRT for the bioselection process and control nitrification if it occurs in
the stabilization zone. The nitrates produced would be reduced to nitrogen gas (N2) in the
bioselectors prior to the contact zone. The C-S mode is employed at Gig Harbor, WA, and the
flowsheet with operating results are provided in Chapter 5.
The aerated, low DO bioselectors operating in an anoxic mode have been successfully
applied in many facilities including large facilities at Southerly and Jackson Pike in Columbus,
OH (Albertson et al., 1992), Santa Fe, NM, the 23rd and 91st Ave plants in Phoenix, AZ
(Albertson and Hendricks, 1992 and Albertson and Stensel, 1994) and Baltimore Back River
wastewater treatment plants (WWTPs). Operating data are provided for several facilities in a
later section. When denitrification is desired, the air rate is more limited in the bioselectors and
about 30-35% of the internal recycle (IR) is discharged to the initial contact zone (Figure 3.1)
through Valve V1. If partial (65-80%) phosphorus removal is an objective, then the internal
recycle is discharged either to the third bioselector stage or to the anoxic zone after the
bioselector. The denitrification rates did not show any adverse effects of aeration at 40-50
mg/L⋅hr AOTR at Phoenix 23rd and 91st plants, which have warm wastewaters (20-33°C)
(Albertson and Stensel, 1994).
Albertson et al., (1992, 1994) reported on successful SVI control with aerated
bioselectors (SXAL) at the Jackson Pike and Southerly WWTPs in Columbus, OH, and 23rd Ave
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and 91st Ave plants in Phoenix, AZ. The bioselector ICZs were operated with a F/M of 5 to 6 kg
BOD5/kg MLSS⋅d (12-15 kg COD/kg MLSS⋅d). The air rate was low and DO was essentially
zero. Selector effluent sCOD approached the concentration (25 to 30 mg/L) present in the final
effluent. At 23rd Ave WWTP, the combination of a 25% increase in BOD5 input and an
improperly placed ICZ baffle resulted in a COD F/M of 16-19 kg/kg.MLSS⋅d (BOD5 of 7.5-9
kg/kg⋅d) in the initial contact zone, and hydrous bulking occurred until the baffle was moved to a
new position that reduced the COD F/M to 10-12 kg/kg⋅d (BOD5 at 4.8.-5.7 kg/kg⋅d).
The design criteria for the aerated, low DO bioselector are the same for processes without
denitrification (SXAL) and anoxic (SXAXAL) processes. At this time, the solids retention time
(SRT) of the aeration basin is assumed to be > 3 days. The 8-hour peaking load factor is assumed
to be < 1.4 average loading. Adjust the design F/M as noted earlier for higher peak load factors
(see Table 4.5) and if the COD/BOD5 ratios vary from 1.9-2.1. The sCOD data will provide the
most reliable design criteria. The design criteria are summarized in Table 4.7.
Table 4.7 Design Recommendations for Aerated SXAL and SXAXAL Bioselectors
Relative Design Loadings(1)
Bioselector AOTR Volumes COD sCOD(1) BOD5 sBOD5
Zone mg/g.hr % kg/kg.d kg/kg.d kg/kg.d kg/kg.d
SX-1 < 20 25 12 6 6 3
SX-2 < 20 25 6 3 3 1.4
SX-3 < 20 50 3 1.5 1.5 0.75(1) Adjust loadings if either the 8-hour load peaking factor is > 1.4 or the sCOD/sBOD5 ratio
differs from the range of 1.9 to 2.1 in the case where BOD5 data are employed.
4.3 Anoxic Bioselectors (SXAXM and SXAXAL)
The anoxic bioselector design often employs mechanical mixers rather than air mixing to
maintain the MLSS in suspension. About 30-40% of the internal recycle (IR) is discharged to the
initial contact zone and the balance to the third stage of the bioselector. A significant portion of
the nitrates formed in the aerated or oxic zones will be removed in the bioselectors and the
balance reduced in the one or more anoxic stages following the bioselectors.
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The design of the anoxic bioselectors with mixers employs a three- or four-stage F/M
gradient – the same employed for the aerated, low DO mode of operation. Thus, the bioselector
volumes in each stage will be determined in the same manner as established in the example for
the aerated, low DO process. The concerns for the potential to overload the biomass, causing
viscous bulking, exist and the same criteria for the 8-hour peaking/average loading and
sCOD/sBOD5 ratio need to be considered for site-specific designs.
At this time, it is not possible to determine whether there are conditions that favor the
mechanically mixed anoxic bioselector over the aerated, low DO mode of operation. No
advantage of curtailing the air flow was found at Phoenix 91st Ave (Albertson and Stensel,
1994), which has a warm (20-33°C) and relatively strong primary effluent (320-450 mg/L COD,
200-220 mg/L sCOD) wastewater. It is possible that aeration in the bioselectors would be more
detrimental for cold and relatively weak wastewater and with low MLSS when there would be
reduced oxygen uptake and, hence, less stress. The level of biological stress reflected by the F/M
would also be exhibited by the oxygen uptake rate of the MLSS. Under conditions of reduced
oxygen demand (mg/L⋅hr), the effectiveness of bioselection could be diminished and the rate of
denitrification reduced. Comparative studies under these conditions have not been reported in the
literature.
Marten and Daigger (1997) studied five facilities with anoxic bioselectors: Beloit, WI;
North and South WWTP in Green Bay, WI; Landis, NJ; and Tri-City, OR. Their conclusion from
the study was that the F/ΣM should be in the range of 0.7 kg cBOD5/kg MLSS⋅d in cold
wastewaters and up to 1.2 kg/kg⋅d in warm wastewaters to produce improved SVI control. The
equivalent BOD5 and COD values to the cBOD5 values were not reported. Also, the ICZ
loadings were not provided and ICZ loadings are probably the most significant criteria.
However, there were still excursions to higher SVIs as indicated by the reported results in the
earlier Table 3.4.
The level of aeration in the Columbus and Phoenix facilities (see data in Chapter 5)
would result in only a small fraction of the sCOD removal by oxidation, probably 5-15%, in the
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bioselectors. Thus, aeration would seem to be an unlikely reason for the higher operable F/M in
these facilities. The range of SVI was significantly lower than reported for unaerated anoxic and
anaerobic zones of plants operating at a lower F/M in the initial contact zone. Additional full-
scale data analysis of the anoxic bioselector F/M operating range is needed.
Jenkins et al., (1993) suggested that oversized anoxic bioselectors will not produce
unacceptable results. However, South African and U.S. orbital systems with single-stage lower
F/M anoxic zones have had bulking (SVI >200 mL/g) problems. There is the distinct probability
that large differences in bioselector design can cloud the comparison of data from several plants.
The staging of the bioselectors can be critical and currently the overall professional opinion and
results support a three- to four-stage bioselection zone. Chudoba et al., (1973a, 1973b), the
British Water Pollution Laboratory (1969), Tomlinson (1976), Heide and Pasveer (1974),
Chambers and Tomlinson (1978a, 1978b), and Albertson (1987, 1992) premise that a high F/M
cascade is critical to achieve a low SVI biomass is valid. Designs by this author using these
references have produced low SVIs in many installations.
In the Jenkins et al., publication, the overall F/M in the bioselector is provided, not the
F/M in the initial contact zone. At Beloit, WI, the F/M is 2.1 to 3.6 kg/kg⋅d in the initial contact
zone and the biological train with the higher F/M produces the lowest range of SVI. The three
anoxic stages at Landis WWTP in Vineland, NJ, have only an F/M of 0.45 to 0.9 kg/kg⋅d and
may not exert sufficient selective pressure on the system. The single-stage bioselectors at Green
Bay, WI, do not have a cascading F/M, which may be the cause of periodic higher SVI
excursions.
At this time, the information on the F/M design for mechanically mixed anoxic
bioselectors is conflicting. Jenkins et al., (1993) recommended a COD F/M profile for anoxic
bioselectors. No design criteria for a sCOD nor BOD5 F/M gradient were provided.
Zone SX-1 6 kg COD/kg MLSS⋅d
Zone SX-2 3 kg COD/kg MLSS⋅d
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Zone SX-3 1.5 kg COD/kg MLSS⋅d
During the plant startup or lower BOD5 (COD) loading periods, the F/M in the first
selector zone (SX-1) may be too low to produce the level of biological stress that Wanner (1994)
considered necessary to promote (select) the growth of rapid settling floc formers. A heavily
aerated SX-1 has proven operable at BOD5 F/M loadings up to 14 kg/kg⋅d in the initial contact
zone (Jenkins et al., 1993; Kross, H., 1985, Grau and Wanner, 1988). Loading recommendations
for both aerated, low DO and anaerobic bioselectors have been 5.5 to 6.0 kg BOD5/ kg⋅d in the
initial contact zone. The addition of oxygen, DO in the influent, DO in the recycle, DO by the
aeration, or the recycle of nitrates to the initial contact zone may reduce the biological stress
necessary to select the preferred biomass in the biological reactor. However, this aspect has not
been researched and both lowly and highly aerated bioselectors have been effective.
By introducing oxygen into the initial contact zone, the loss of selectivity is apparent
from the reduction in the level of phosphorus removal in aerated (high or low) and anoxic
bioselectors. At the same time, the bioselectors with oxygen added in any form have worked well
with substantially higher F/M ratios than the criteria set forth by Jenkins et al., (1993) and noted
earlier. The recommended anoxic bioselector loadings of this document are presented in Table 4-
8. The preferred and most reliable criteria will be defined by sBOD5 and/or sCOD data.
Table 4.8 Design Recommendations for Mechanically Mixed Anoxic (SXAXM)Bioselectors(1)
Relative sCOD sBOD5
Zone Volumes Loading Loading COD BOD% kg/Σkg mass⋅d kg/Σkg mass⋅d kg/kg⋅d kg/kg⋅d
SX-1 25 5.0 - 6.0 2.5 - 3.0 10 -12 5 - 6
SX-2 25 2.5 - 3.0 1.3 - 1.5 5 - 6 2.5 - 3
SX-3 50 1.3 - 1.5 0.63 - 0.75 1.5 - 3 0.75 - 1.5
(1) Adjust loadings if either the 8-hour load peaking factor is > 1.4 or the sCOD/sBOD5 ratiodiffers from the range of 1.9 to 2.1 in the case where BOD5 data are employed.
4-22
Mixing in the anoxic bioselectors can be provided by either vertical-mounted turbine
mixers or slide rail-mounted axial flow mixer pumps. The general power requirement guidelines
for the two types of mixing systems are:
Turbine Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)
Axial Flow Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)
The mixing power requirements can be reduced by using lower shaft speeds (larger
impellers) at a higher capital cost. The installed cost of the slide rail-axial flow mixer is less than
the bridge-mounted, vertically submerged turbine mixer. As noted in the section discussing
aerated, low DO bioselectors, coarse bubble aeration has been employed successfully in many
nitrification denitrification (NdeN) systems, but could interfere with Bio-P removal.
4.4 Anaerobic Bioselectors (SXANM)
In order to optimize biological removal of phosphorus, the introduction of free and
combined sources of oxygen into the bioselector zone must be minimized. This maximizes the
availability of sBOD5 (sCOD) for the Bio-P reactions discussed in Chapter 1 on process theory.
Thus, it is desirable to have minimal DO in the influent to the initial contact zone and low
nitrates in the return activated sludge (RAS). There is no internal recycle of nitrates to the
bioselector zones whenever Bio-P is to be optimized.
There are not sufficient data to define whether the anaerobic bioselector should have the
same F/M gradient as employed for the aerated, low DO and aerated anoxic bioselectors.
Bulking sludges have been reported as a problem in the A2O process and South African WWTPs
that have low F/M single-stage anaerobic ICZs. Since the single-stage processes typically had 2-
to 4-hour retention time in the first anoxic zones, the F/M loadings were low – less than 1 to 1.5
kg BOD5/kg⋅d – and provided inadequate biological stress for bioselection.
4-23
Further, it is not known whether the best process design for bioselection and bulking
control will permit optimization of phosphorus removal. The emphasis of this document is the
control of bulking sludges; thus F/M gradient recommendations set forth for aerated, low and
high DO bioselectors would not necessarily be valid for Bio-P removal. If the Bio-P removal
requirements result in a longer period than necessary for bioselection, then the volume of the
anaerobic zone could be expanded by adding a fourth zone, where the required volume and the
recycle of nitrate-containing mixed liquor from the oxic zone would be discharged downstream
of the anaerobic zone.
Wanner et al., (1987) concluded that SVI control would be achieved if nearly all of the
soluble organics were removed in the initial contact zones. They also concluded that a cascading
F/M gradient was a more effective design but did not provide numerical criteria. Results of
Rensink and Donker et al., (1985) suggested that in completely mixed anaerobic, anoxic and
oxic zones, filamentous organisms were suppressed only if there was P release (polyphosphate
depolymerization) in the anaerobic zone. However, good bulking control was achieved at Tree
Top (1993), Cranston Print Works (1992), and other studies on industrial waste where excess
phosphorus was not available. An alternative pathway of glucose storage resulted in the removal
of sCOD and, hence, bulking control.
A two-stage anaerobic bioselector was piloted for the Deer Island project (Bowen et al.,
1992) and compared to a pure oxygen process without a bioselector. This control study
demonstrated the benefits of bioselection to control SVI. The initial or zone settling velocity
(ZSV5) increased more than 100% and the concentration of the return sludge solids (RSS) more
than doubled. The significant findings are set forth in Table 4.9.
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Table 4.9 Deer Island, Boston, MA, Pilot Study ResultsICZ F/M
Period Mode SRT ZSV5 RSS SVI F/M TotalDays m/hr mg/L mL/g kg/kg⋅d kg/kg⋅d kg/kg⋅d
1 SXAN-COAS 1.0 5.5 14,222 62 5.2 0.58
2 COAS 2.1 1.8 6,312 96 2.8 0.71
3 COAS 2.6 2.3 5,685 190 2.5 0.62
4a SXAN-COAS 3.3 3.4 13,124 64 3.1 0.34
4b SXAN-COAS 1.6 5.1 10,046 50 4.0 0.50
SXAN-COAS – Anaerobic bioselector; COAS – conventional oxygen activated sludge
Note: Two-stage anaerobic bioselector at 568 L and four-stage aerobic reactor at 1987 L totalvolumes, respectively. The ICZ is 284 L in Periods 1, 4a and 4b and 497L in four-stage aerobicreactor.
The anaerobic bioselector in the pure oxygen mode was effective in controlling the
growth of filamentous or bulking sludge as evidenced by the higher zone settling velocity
(ZSV5), lower SVI and higher return sludge solids concentration (RSS). The first stage of a pure
oxygen reactor has been considered an aerobic selector (Tracy, 1975) but was not as effective.
The significant difference could be the lower F/M in the initial contact zone of the COAS
reactor.
The net sludge solids for the bioselector arrangement (activated sludge) was 14% higher
than for that of the conventional activated sludge system (without bioselector zones). An
increase of net yield has been noted in other studies with bioselectors.
The increase in the net biomass yield would, in part, be due to accumulation of excess
phosphorus (Bio-P) in the bioselector arrangement. While the anaerobic mode provides the
highest level of removals, Bio-P removals in anoxic and low DO bioselectors are typically 65-
80%. For each additional kg of TP removed, about 4 to 4.5 kg of waste sludge will be produced
and will increase total waste sludge proportionally.
Since the bioselector process involves conversion–storage–oxidation of soluble organics,
it is possible that a portion of the stored product is not oxidized and thus it increased the net
4-25
sludge yield. Net yields, when hydrous sludge is being produced, increase due to the difficulty of
oxidizing this fraction of exo-cellular protoplasm (ECP) storage. Reported poor SVI control with
bioselection could be in part due to excess ECP content.
Further Deer Island, WWTP studies (Phase 3) of anaerobic vs. aerobic bioselectors were
undertaken in 1994-95. The test operation was designed to simulate design capacity operation in
the pilot-scale reactors and clarifiers. The four-stage aerobic and anaerobic bioselectors have a
volume of 17.85 m3/stage (630 ft3/stage) and the four-basin pure oxygen activated sludge
bioselectors have a volume of 58.02 m3/basin (2048 ft3/basin). Total system volume is 303.46
m3/train (10,712 ft3/train). The results of these tests are summarized in Table 4.10 (MWRA,
1995).
In these studies, the SVI was controlled by the anaerobic and aerobic bioselector at the
lowest F/M loadings. The F/M loadings in the initial contact zone of the four-stage bioselector
were very high, 50 to 100% higher than recommended in this document. The SVIs were
generally good with the anaerobic bioselector producing lower SVIs. It appeared that the
anaerobic bioselector did result in a higher net yield although the data were not consistent. At the
higher loadings and flow, the aerobic bioselector effluent quality was better than that from the
anaerobic bioselector system.
The anaerobic bioselector did not release a significant amount of phosphorus as would be
expected. The evaluation attributed this fact to the low SRT and colder temperatures. This
supports the position that the bioselector(s) was not operating in a true bioselection mode, and
with the low SRT, the sludge may not have been adequately regenerated.
The full-scale bioselectors at the Deer Island WWTP have operated better in an aerobic,
high DO mode (Tyler, 2002) than in a mixed anaerobic mode. In recent years, the aerated
(SXAH) mode has operated at about 10 mg/L DO from pure oxygen aeration at a MLSS of 1000
to 1500 mg/L (Tyler, 2002). The SRT is 1.2 to 1.4 days and the SVI is 120 to 160 mL/g. Tyler
reported that converting to a two-stage bioselector by removing walls in the model-scale unit
4-26
appeared to improve operation. The F/M would be 50% of the full-scale design and of the data
reported in Table 4.10.
Table 4.10 Phase 3 Secondary Treatment Performance at Deer Island WWTP
Aeration(1) F/M Loading(2) Net Yield Sec. EffluentPeriod Flow MLSS SRT SVI BOD5 COD BOD5R BOD5 TSSTrain m3⋅d mg/L days mL/g kg/kg⋅d kg/kg⋅d kg/kg mg/L mg/L
12/24/93-12/23/94
Train 1 (AH) 2273 1244 2.3 152 7.1/0.42 21.3/1.25 0.92 5 9
Train 2 (AN) 2064 1643 2.2 152 6.0/0.35 18.5/1.09 1.31 5 8
12/24/94-1/3/95
Train 1 (AH) 2651 1075 2.1 92 10.5/0.62 23.8/1.40 0.76 8 10
Train 2 (AN) 2693 1045 2.2 95 10.5/0.35 18.5/1.09 0.75 5 8
1/9-25/95
Train 1 (AH) 3788 1766 2.0 112 10.9/0.64 25.5/1.50 0.83 9 9
Train 2 (AN) 3841 1944 2.0 90 9.7/0.57 23.8/1.40 1.10 17 23
1/29-2/14/95
Train 1 (AH) 4546 2143 1.7 121 10.9/0.64 25.5/1.50 0.83 9 9
Train 2 (AN) 4523 2177 1.2 80 11.4/0.67 34.0/2.00 1.75 22 43
2/15-3/3/95(3)
Train 1 (AH) 4546 2272 1.5 78 12.2/0.72 32.3/1.90 1.27 9 8
Train 2 (AN) 4496 2208 1.9 61 12.4/0.73 32.3/1.90 1.30 14 16
3/4 -13/94
Train 1 (AH) 4546 2127 2,8 71 11.9/0.70 28.9/1.70 1.48 29 40
Train 2 (AN) 4364 1590 1.7 63 12.6/0.74 30.6/1.80 1.49 35 53
3/14-31/95(3)
Train 1 (AH) 4167 2655 2.6 114 9.9/0.58 25.5/1.50 0.96 11 15
Train 2 (AN) 4201 2685 1.9 72 8.7/0.51 25.5/1.50 1.22 17 28
All Data Averages
Train 1 (AH) 3689 1957 2.1 117 10.7/0.63 27.2/1.60 1.05 12 15
Train 2 (AN) 3580 1942 1.9 94 8.5/0.50 25.5/1.50 1.26 16 22
4-27
(1) Aerobic SRT included solids in the bioselector volume (Train 1) and only the aeration volume
solids in the anaerobic (Train 2) bioselector system.(2) Bioselector ICZ F/M /Overall system F/M.(3) Polymers employed at the same dosage in both systems for clarification.
The pilot-scale tests were conducted at the lower range of the recommended F/M in the
initial contact zone and were successful in limiting the SVI to <100 mL/g when the F/M in the
initial contact zone was >2.8 kg/kg⋅d. Since the tests were using cBOD5 at a cBOD5/COD ratio
of 0.4, the equivalent BOD5 F/M would have been >3.5 kg BOD5/kg MLSS⋅d. The model-scale
tests were conducted at cBOD5 loadings of 6 to 12.6 kg/kg⋅d and COD loadings of 18.5 to 30.6
kg/kg⋅d in the initial contact zone. These higher F/M loadings in the initial contact zone at
Phoenix 23rd Ave Water Reclamation Facility (WRF) caused hydrous bulking and the baffle was
moved to decrease the loading. The hydrous bulking resulted in poorer clarification although the
DSVI was only marginally affected, increasing from 60-90 mL/g to 110-120 mL/g.
The Fayetteville, AR, WWTP (Jenkins et al.,1993) employs a six-stage anaerobic
bioselector prior to a four-stage oxic zone. The system is arranged so that it can operate in an
AN/OX (A/O) or an AN/AX/OX (A2O) mode. The SVI prior to the bioselector installation could
only be maintained below 150 mL/g with nearly continuous use of chlorine to the RAS. Since
the bioselector was installed, the SVI has averaged 86 mL/g and has not exceeded 185 mL/g.
The highest SVIs occur during wet weather with dilute sewage and influent DOs of 8-10 mg/L.
In this mode, it would be difficult to achieve a truly anaerobic state in the bioselector. The
operating characteristics of the Fayetteville WWTP are:
Bioselector Activated Sludge SystemICZ Total SX Total Volume
F/M – kg/kg⋅d 1.6 0.27 0.045
Retention – hrs 0.25 1.5 9.0
Jenkins et al., (1993) do not provide specific design criteria for anaerobic selectors, but
noted that the required liquid retention time is 0.75 to 2.0 hours – much longer than the typical
15-30 minutes employed for aerated and anoxic bioselectors. They do advise that the anaerobic
4-28
zone may be divided into zones with the same gradient as they set forth for anoxic bioselectors.
Because the retention time for the anaerobic bioselectors is longer than required for bioselection,
there would then be three initial stages of 6, 3 and 1.5 kg COD/kg MLSS⋅d zones followed by a
fourth anaerobic zone to produce the total required retention time. The anaerobic zone volume is
often defined by criteria for Bio-P removal, which requires a longer retention time as opposed to
bulking sludge control. However, lower SVIs are associated with higher phosphorus content in
the MLSS (Bio-P) as shown in Figure 4.3.
Wanner (1994) noted that the aerated ICZ F/M of 1.0 kg BOD5/kg MLSS⋅d for a brewery
wastewater was not high enough to produce selective stress to control filamentous growth.
However, the removal of the air supply controlled SVI to the 100-120 mL/g range; this
modification converted the bioselector from aerobic to an anaerobic mode. Based on experiences
with soluble wastewaters at other sites, a higher ICZ F/M would likely have further depressed
the SVI to the range of 40 to 70 mL/g.
Historically, the Newark, OH, wastewater treatment plant had experienced a problem of
foaming and bulking problems (Albertson, 1987; IAWQ 1992). A three-stage anaerobic
bioselector was installed and operated with a F/ΣM of 1.59, 0.79 and 0.59 kg BOD5/kg MLSS⋅d,
respectively. The SVI decreased from an operating range of 100 to 400 mL/g to 80 to 200 mL/g
and averaged 124 mL/g. While this was a significant improvement, the higher SVIs would still
limit plant clarification capacity. Based on data presented in this document and Wanner’s (1994)
experience, the F/M in the initial contact zone was too low to produce the stress necessary for
full bioselection – that is, eliminate most filamentous bacteria.
The reported F/M in the anaerobic ICZ designs are much lower than employed for
aerated (SXAH and SXAL) and anoxic (SXAX) bioselectors. The SVIs of the SXAH, SXAL and
SXAX aerated or mechanically mixed systems at higher ICZ F/M ratios appear to be more stable
and averaged a lower SVI. There are only minor excursions in SVIs for these systems with F/Ms
≥ 3-6 kg BOD5/kg MLSS⋅d in the initial contact zone. Thus, it is possible that in the cited
system,
4-29
4-30
ICZ F/Ms were too low to provide the minimum selective stress to fully control the filamentous
growth. This appears to be the case for some anoxic (SXAX) systems. At the Fibra WWTP (Okey,
1997), the anaerobic (SXANM) bioselector produced very low SVIs treating a highly soluble
wastewater, but the operating ICZ loading was not available (see Chapter 5).
From a technical standpoint, all bioselectors are operating on the same basis – providing
an environment where soluble substrates can be removed and stored in floc formers prior to
aerobic zones, where filamentous organisms would otherwise use the same soluble substrate to
proliferate. Therefore, there should be similar F/M gradients for anaerobic (SXANM)
bioselectors. Based on this analogy, the criteria set forth in Table 4.11 are proposed for SXANM
bioselectors:
Table 4.11 Design Recommendation for Three-Stage Anaerobic (SXANM) Bioselectors
AN Zone Fs/ΣM Zone (sCOD) Fs/ΣM (sBOD5)kg/kg⋅d kg/kg⋅d
SX-1 6 ≥3
SX-2 3 1.5
SX-3 1.5 0.75
Contact Loading <100 mg sCOD/s MLSS
If additional anaerobic volume is required for Bio-P release and subsequent removal, it
should be added as a fourth zone to maintain the F/M cascade necessary for bioselection.
Due to the minimal and conflicting amount of available data, it is recommended that the
design engineer keep current with operating experiences of anaerobic bioselectors and adjust the
loadings as required to achieve best results.
Mixing in the anaerobic bioselectors can be provided by either vertical-mounted turbine
mixers or slide rail-mounted axial flow mixer pumps. The general power requirement guidelines
for the two types of mixing systems are:
4-31
Turbine Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)
Axial Flow Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)
5-1
Chapter 5.0
Process Experiences with Bioselectors
The following data presentations provide background information on the use of bio-
selectors in U.S. facilities. The mode of operation in the initial contact zone (ICZ) of the
bioselector is noted.
5.1 Davenport, IA – Aerated, Low DO and Anaerobic (SXAL and SXANM)
The Davenport wastewater treatment plant (WWTP) plant has two aeration trains of four
complete mix activated sludge (CMAS) zones in each train. The zones can be arranged to
operate as three and up to eight zones in a series (staged aeration). Prior to the time of modifying
the operation to bioselector-staged aeration (1987), the SVI ranged from 100 to over 400 mL/g
without any basis to define causative factor(s). A submerged turbine aerated each zone and the
air flow was adjusted to maintain the desired DO in each zone. Normal practice during sludge
bulking occurrences was a target DO of 1 to 2 mg/L in the ICZ.
The plant was designed to process an average flow of 1140 L/s (26 mgd), a peak flow of
1754 L/s (40 mgd) and process 3420 L/s (78 mgd) through primary clarification. In the initial
nine years of operation, the presence of bulking conditions limited the plant’s secondary
treatment capacity to 887-1096 L/s (20-25 mgd) due to rising sludge blankets in the secondary
clarifiers.
In 1987, the plant modified the operation (Davenport 1987) of the first zone (of six zones
in operation) to be in a low DO mode by restricting the air flow to the submerged turbine. The
impact of the 0.0-0.3 mg/L DO in the first zone on the SVI is displayed in Figure 5.1; the cartoon
in the figure reflects the plant operating staff’s reaction to the bioselector’s reduction of the SVI.
The reduction in the SVI allow for secondary treatment of flows up to 1750 L/s (40 mgd)
without incurring excessive depths of sludge blankets in the secondary clarifiers.
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In subsequent periods of time, the plant staff returned to the normal practice of 1-2 mg/L
DO in the first stage and the SVI increased beyond acceptable levels in 1987 and 1988. In 1989,
a process control procedure was instituted where the air rate was increased (higher DO) when the
SVI was < 70 mL/g and decreased (lower DO) when the SVI > 90 mL/g. This action produced a
stable operating SVI as shown in Figure 5.2.
By 2001, the BOD5 loadings to the Davenport WWTP had increased significantly and the
effectiveness of the single-stage bioselector had diminished. In 1987, the first stage F/M was 1.2
to 3.0 kg/kg⋅d, averaging about 1.8 kg BOD5/kg MLSS⋅d. The loading increases were offset by
operating eight zones, but SVIs had reached as high as 150 to 200 mL/g while the annual
average SVI was <100 mL/g. The waste characterization had changed due to increased soluble
organic wastewaters and in 2002 the bioselectors operated in an anaerobic mode (SXM). A three-
stage bioselector (SXAH) with 3.0 kg sBOD5 kg MLSS⋅d in the initial contact zone (ICZ or SX-1)
was under design in 2002 with plans to modify the staged aeration to contact-stabilization to
increase the BOD5 capacity.
Davenport is an example of the range of responses to bioselectors. In this case, a single-
stage zone with a low F/M was operable for a number of years. Whether the addition of a new
industrial waste input was the cause of the increased SVI cannot be determined at this time, since
similarly designed facilities may or may not have bulking problems without bioselectors.
5.2 Columbus Southerly, OH – Aerated, Low DO (SXAL)
The Southerly WWTP had a historical problem of uncontrolled bulking of activated
sludge prior to mid-1988 when the plant was expanded to 4990 L/s (114 mgd) and the aeration
basins were modified to include a three-stage bioselector. Prior to modification to a 10-stage
reactor, two-pass basins, which were 274.4 ml x 7.92 mw x 4.45 mwd (900 ft x 26 ft x 14.6 ft),
had an SVI varying from 150 to 500 mL/g in a plug-flow mode. In the step-feed mode of
operation, bulking problems increased. Due to the strong, highly soluble influent, the initial
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aeration stages had a low DO. The wastewater was a mixed domestic and industrial wastewater
with about 40% of the influent loading from a large brewery.
The three-stage bioselectors (Figure 5.3) were designed (Albertson et al., 1992) with an
F/ΣM gradient of 5.6, 2.8 and 1.4 kg BOD5/kg MLSS⋅d. The COD gradient was about 13, 6.5
and 3.3 kg/kg⋅d. The bioselectors and first anoxic zone were equipped with jet aerators, which
could either provide mixing, aeration-mixing or low DO (AL) aeration only. The aerated
bioselectors were designed to operate at 0.0-0.3 mg/L DO. An internal recycle of 1.25 Q to the
anoxic selectors was provided but not always employed. This produced an aerated (or unaerated)
anoxic environment with denitrification in the bioselectors and in the first anoxic/oxic zone
(Ax/Ox-1). A total of six 61 m∅ x 4.57 m (200 ft∅ x 15 ft) SWD mechanical flocculating
secondary clarifiers with central sludge withdrawal using spiral scrapers replaced the existing
rectangular basins.
Figure 5.4 displays the average monthly SVI profile for the first 14 months of operation
of the upgraded wastewater treatment plant. The low and stable SVI and daily SVI range reflect
the effectiveness of the anoxic bioselectors. The bioselectors operated with aeration (jets off)
only to minimize power usage. Maximum and minimum daily SVIs were generally ≤ +15 mL/g
of the monthly average value. The average MLSS was 3250 mg/L with an average SVI of 78
mL/g. The settled secondary effluent quality averaged 2 mg/L cBOD5, 6 mg/L TSS, 1.5 mg/L
TKN, 0.13 mg/L NH4-N and 1.1 mg/L TP. The plant treated monthly flows up to 5739 L/sec
(131 MGD) for the first 2.5 years of operation as shown in Table 5.1.
For flexibility in operating a SXAH, SXAL, SXANM and anoxic modes (AX), the
bioselectors were equipped with jet aerators. The plant staff has operated the system with jets
only (anaerobic), air only (low DO) and air plus jets (low DO) without internal recycle. The
treatment performance, bulking sludge control and effluent quality were unchanged. There was
5-10 mg/L of NO3-N in the return sludge at 0.4 RAS/Q or 1.4 to 2.8 mg/L NO3-N in the total
flow to the initial contact zone (1a, Figure 5.3). However, no NO3-N was present in the initial
contact zone if the internal recycle pumps were not operating. Still, the system has operated
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efficiently at MLSS of 4000-5000 mg/L due to the low SVIs. Based on the criteria set forth in
the BNR patents, the initial contact zone would still be considered anoxic (>0.5 mg/L NO3-N in
the total influent flow) with the 5-10 mg/L NO3-N in the RAS flow.
The Southerly plant has continued to produce low SVIs and similar effluent quality with
increasing loads. The aeration basins and final clarifiers have since been expanded to process
additional flow and loadings. The flexibility offered by the more costly and power intensive jet
aeration in the bioselector zones is no longer considered necessary.
5.3 Columbus Jackson Pike, OH – Aerated, Low DO (SXAL)
The Jackson Pike WWTP historically processed about 4380 L/s (100 mgd) in secondary
treatment with periodic bulking incidents that limited treatment capacity. The construction of the
12 aeration basins was similar to that of the Southerly WWTP – each 274.4 ml x 7.92 mw x 4.45
md (900 ftl x 26 ftw x 14.6 ftd). The 12 basins (Figure 5.5) were being operated in a plug-flow
mode on a mixed domestic and industrial wastewater.
The sBOD5 content of the wastewater was low at about 25% of the influent BOD5. Based
on a requirement to minimize investment, a two-stage bioselector with a 5.5/1.4 kg/kg⋅d F/ΣM
gradient was employed. Mixing/aeration of the bioselectors and the oxic zones was modified
from coarse bubble to tubular membrane diffusers. No provision for an internal recycle for
denitrification was provided for this plant, which is slated for retirement in the future.
The treatment facility was renovated to nitrify a flow of 2630 L/s (60 mgd) to an effluent
NH4-N of less than 1.0 mg/L. The SVI (Figure 5.6) in the first 11 months of operation (1988-
1989) ranged from 52 to 100 mL/g, averaging about 75 mL/g. The average effluent quality over
this period was 3.1 mg/L cBOD5, 7.3 mg/L TSS, 1.5 mg/L TKN and 0.09 mg/L NH4-N, while
treating slightly more than design capacity. The treatment capacity was later further increased
25% above the original design flow without loss of treatment efficiency. The summary data for
the first 2.5 years of operation are shown in Table 5.2.
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The two-stage bioselector did not produce as stable an SVI as did the Southerly three-stage unit.
However, in itself, this is not a reason to establish that the three-stage design would have
produced a more stable SVI. The Jackson Pike wastewater had a low soluble BOD5 content,
while the brewery wastewater resulted in Southerly’s wastewater having a higher than normal
sBOD5 content. As noted earlier, the bioselection process is most effective on soluble
wastewaters. Further, as Wanner and Grau (1988) suggested, there could be secondary bulking
organism growth in the oxic zones due to the high BOD5 particulate content at Jackson Pike.
With a maximum monthly SVI of 108 mL/g, the bioselector was effective regardless of the
daily-monthly fluctuations.
5.4 Santa Fe, NM – Aerated, Low DO (SXAL)
The 263 L/s (6 mgd) Santa Fe WWTP consists of two 1932 m3 (0.51 mg) Barrier
Oxidation ditches. The available internal recycle flow rate was up to 300% and primary clarifiers
were not employed. The four rectangular final clarifiers were 3.72 m (12.2 ft) deep with a total
area of 2007 m2 (21,595 ft2) and employed traveling sludge siphons to remove the settled sludge.
Due to mechanical failure of the ditch aeration system, it was necessary to replace the aeration
system. This necessitated removing 50% of existing treatment capacity for several months. The
remaining oxidation ditch could not meet more than 40% of the total oxygen demand.
While the interim construction permit relaxed the TN from 10 mg/L to 25 mg/L, these
levels still required nitrification and 40-45% TN removal until the first ditch was modified and
put into operation. The TN limit was then set at 17 mg/L until the second ditch was also
modified to diffused aeration.
Because the plant had a history of bulking sludge and foaming problems, a two-stage
aerated bioselector was installed in each of the existing lead anoxic basins (Figure 5.7). All
bioselector and anoxic basins were retrofitted with a high density of 9"∅ membrane diffusers
designed to operate at 0.0-0.3 mg/L DO in the bioselector and first anoxic stage. The single
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oxidation ditch was operated to generate the minimum TN value and maximum oxygen transfer
capacity, but the ammonia oxidation capacity of the single oxidation ditch was limited.
The aerated zones were operated at 0-0.5 mg/L in the bioselectors (SXAX-1, SXAX-2, and
SXAX-3) and generally at 4-6 mg/L DO in the second zone (AX-4) with an internal recycle of
250% Q. The influent flow averaged 265 L/s (6 mgd) with a temperature range of 16-19°C in the
5.5 months of operation in 1993. The summary of the operating results is provided in Table 5.3.
The aeration capacity of the oxidation ditch was further limited by the loss of an
additional aeration pump during the conversion. While the loss further inhibited the ability to
nitrify fully, it decreased NO3-N to less than 0.5 mg/L during the last 30 days of operation.
However, the modified aeration anoxic facility was able to provide treatment to meet final
criteria with 50% of the oxic volume off-line and the balance of the oxygenation capacity
impaired. The SVIs averaged 103 mL/g and 10-day averages ranged from 79-132 mL/g. The
1993 COD, TSS and TKN loadings in the raw wastewater were 132%, 123% and 117%,
respectively, of the 1991 data used for the design of the failed aeration system. The bioselectors
and aerated anoxic basins with one aeration basin on-line met the effluent requirements at 110 to
115% of original design loadings.
5.5 Gig Harbor, WA – Aerated, Low DO (SXAL)
The secondary treatment was expanded from 26.3 to 70.2 L/s (0.6 to 1.6 MGD) in 1997.
The activated sludge process was modified from complete mix (CMAS) to contact-stabilization
(C-S) with bioselectors. The return sludge after the stabilization period was mixed with the
influent wastewater and processed through a three-stage bioselector prior to the contact zone
(Figure 5.8). In the ultimate expansion of the plant to 153 L/s (3.5 mgd), the influent flow will be
reversed and primary treatment will be provided by the two secondary clarifiers. Two new 19.8
m∅ (65 ft∅) secondary clarifiers will be constructed. With primary treatment, the C-S mode of
operation will be able to process the full flow of 153 L/s.
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At a flow of about 60-80% of design capacity, the original facility averaged 5 mg/L
cBOD5 and 13 mg/L TSS from mid 1988 through 1991. Maximum monthly effluent TSS was
approaching the effluent limits of 30 mg/L. A variable and unpredictable SVI limited the
aeration MLSS and the clarification capacity and was also the cause of higher effluent TSS.
The operation and performance of the expanded wastewater treatment plant is presented
in Table 5.4. The plant was loaded at 40-50% of its hydraulic design capacity and the first-stage
bioselector zone (Sx-1) had an average F/M of 3.4 lb cBOD5/lb MLSS⋅d. The average SVI was
71 mL/g and the monthly averages ranged from 55 to 81 mL/g.
The design criteria for the low DO (SXAL) bioselectors incorporated a gradient of 14.4,
7.2 and 3.6 kg COD/kg MLSS⋅d (6.8, 3.4, 1.7 kg BOD5/kg⋅d). The basis of design was 5.0 kg
sCOD/kg⋅d (sBOD5 = 2.4 kg/kg⋅d) in the initial contact zone.
5.6 Phoenix 23rd Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL)
The Phoenix 23rd Ave Water Reclamation Facility (WRF) was designed for 1622 L/s (37
mgd) of secondary treatment capacity. Historically, the plant had operated at 658 to 1096 L/s (15
to 25 mgd) capacity due to a chronic sludge bulking problem with SVIs of 150 to > 500 mL/g.
Due to clarifier limitations from the bulking sludges, the MLSS concentration was maintained in
the range of 500 to 1000 mg/L. Flow capacity not processed at the 23rd Ave WRF was
transferred to the 6750 L/s (154 mgd) plant at 91st Ave. However, the operating philosophy was
to maximize the flow treated at the 23rd Ave plant.
The 23rd Ave plant had two-four pass aeration basins, each 378 ml x 7.62 mw x 4.63 md
(1240 ft x 25 ft x 15.2 ft). The basins were aerated with ceramic fine bubble diffusers and the
basins were operated in a plug-flow mode. It was difficult to maintain DO in the first pass as the
ceramic diffusers easily fouled and the fouling alpha (αF) value of the mixed liquor was about
0.27.
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During a severe bulking (SVI > 800 mL/g) period, steps were taken to rapidly implement
bioselection. Initially, canvas baffles were placed in the front portion of the basin to effect a
three-stage bioselector. The bioselection volumes were based on a F/ΣM of 6, 3 and 1.5 kg
BOD5/kg MLSS⋅d. While successful in reducing the SVI, the initial baffles deteriorated due to
bacterial decay and were replaced with submerged wooden baffles.
There was also a need to provide advanced treatment with an effluent of ≤ 10 mg/L total
nitrogen, low BOD5 and TSS. The arrangement of the bioselectors and the internal recycle for
denitrification is shown in Figure 5.9. The fine bubble diffusers were replaced by coarse bubble
diffusers in the three-stage bioselectors (SXAX-1, SXAX-2 and SXAX-3) and membrane diffusers
were installed in the first anoxic/oxic zone (AX/OX-1). The total internal recycle flow was
discharged in the initial contact zone and the bioselectors were operated in an aerated, low DO
anoxic mode.
The results of the first nine months of the plant operation (Albertson & Hendricks,1992)
after the startup of the bioselectors on February 15, 1990, and the preceding 13 months are
displayed in Figure 5.10. There was an increase in SVI30 for the first two weeks, which was
mostly an artifact of the SVI30 test (Rachwal, 1985) as the MLSS was increased from 550 to
1100 mg/L. That is, the increased SVI30 was due to higher MLSS, but the DSVI30 was
decreasing.
The DSVI decreased to ≤ 100 mL/g after 14 weeks, the MLSS was increased to 2200
mg/L and the flow to 1400 L/s (31.9 mgd). After permanent modifications were completed, the
MLSS was further increased to 3000-3300 mg/L and the flow rate to 1500-1750 L/s (34-40
mgd). The 20-month review (Figure 5.11) of DSVI data reveals an average monthly range of 60
to 120 mL/g with relatively low daily range. Subsequently, it was found that the combined effect
of a 25% increase in BOD5 loading and a misplaced baffle in the initial contact zone caused floc
overloading and some hydrous bulking. The baffle was moved downstream to reduce ICZ F/M to
about 6 kg BOD5/kg.d.
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The fully modified plant operating at an average rate of 1450 L/s (33 mgd) produced a
monthly average secondary effluent of 7.6 mg/L BOD5, 8.2 mg/L TSS, 1.3 mg/L NH4-N, 4.1
mg/L NO3-N and 2.9 mg/L TP. The plant clarification capacity was limited at this flow due to
the shallow sidewater depth (2.74 m, 9 ft) of the final clarifiers.
Subsequently, the 23rd Avenue WRF has been expanded to 2758 L/s (63 mgd) using four
aeration basins and the aerated anoxic (SXAXAL) bioselectors. The SVI ranges from 60-90 mL/g
and the effluent quality has further improved due to the newly expanded clarification capacity
with four 54.9 ∅ x 4.88 m SWD (180 ft ∅ x 16 ft) final clarifiers with spiral scrapers and central
sludge drawoff.
5.7 Phoenix 91st Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL)
The 91st Ave WWTP provides service for seven cities in the Phoenix, AZ, area: Glendale,
Mesa, Phoenix, Scottsdale, Tempe, Tolleson, and Youngtown. The plant design capacity was
6.75 m3/s (154 mgd) when operating in a secondary treatment mode. The balance of the City of
Phoenix’s wastewater was processed by the 23rd Ave WRF, a 1.62 m3/s (37 mgd) secondary
treatment plant operating at 660–1100 L/s (20-25 mgd), which was converted to nitrification-
denitrification (NdeN) mode of operation in 1991 (Albertson and Hendricks, 1992) and was then
operating at 1400–1500 L/s (32-35 mgd) producing an effluent total nitrogen (TN) of ≤8 mg/L.
There are several other water reclamation plants in the service area that remove and treat
wastewater and discharge their waste activated sludge to the interceptors feeding the 91st Ave
WWTP.
The 91st Ave WWTP treatment plant consisted of three separate primary clarification
systems and then the flow was subdivided into five independent secondary treatment plants.
These plant facilities had been constructed separately over a 30-year period as flows increased in
the metropolitan area. The design capacity in secondary treatment (≤ 30 mg/L BOD5/30 mg/L
TSS) for each plant was Plant IA, 1320 L/s (30 mgd); Plant IB, 1320 L/s (30 mgd); Plant IIA,
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1050 L/s (24 mgd), Plant IIB, 1580 L/s (36 mgd); and Plant IIIA 1320 L/s (30 mgd). The
existing sludge handling facilities included in-situ primary sludge thickening, dissolved air
flotation of waste activated sludge (WAS), anaerobic digestion with sludge decanting, and solar
drying in lagoons.
The 91st Ave WWTP had a history of bulking sludge and the SVI generally ranged from
150 to over 400 mL/g. Chlorination of the return sludge and low MLSS (500-1,000 mg/L) had
been used to maintain clarification capacity. Wastewater was primarily of domestic origin
including the waste sludges from wastewater reclamation plants. New effluent requirements had
established 10 mg/L as a TN limit. Plant IIIA was chosen for a full-scale process demonstration
of nitrification-denitrification.
Plant IIIA, employed for the full-scale pilot demonstration of bioselectors in the NdeN
mode, had two 378 ml x 7.62 mw x 4.63 md (1250 ft x 25 ft x 15.2 ft) aeration basins with eight
57.9 ml x 12.2 mw x 3.90 md (190 ft x 40 ft x 12.8 ft) secondary clarifiers. The conversion to
NdeN at 3500 mg/L MLSS required extensive modifications to the aeration basins and final
clarifiers. The cost of both the bioselector and NdeN modifications was about $2,000,000 U.S. or
$16.20/m3 ($0.06/gal).
The design concept tested in Plant IIIA (Figure 5.12) was a high-rate (SRT of 4.8-7.5
days) aerated, low DO anoxic bioselector mode of operation. Due to the limited aeration volume,
the anoxic zones were also aerated at an actual oxygen transfer rate (AOTR) of 30 to 50 mg/L
with a DO of < 0.3 mg/L. During average to peak loading periods, the DO was zero. The full-
scale operation at 920 to 1540 L/s (20 to 35 mgd) with AOTRs of 40-50 mg/L⋅hr to the
bioselectors and anoxic zones was successful in limiting the DSVI to 57 to 81 mL/g at 3000-
3800 mg/L MLSS as shown in Figure 5.13.
The lowly aerated anoxic (SXAXAL) mode was successful in meeting the TN effluent of
10 mg/L except when 50% of the internal recycle capacity was lost due to pump failures.
Aeration of the anoxic zones at AOTRs of 40-60 mg/L⋅hr following bioselection did not impair
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the ability of the system to reduce nitrates. The oxygen uptake rate (OUR) values were 100-130
mg/L, thus the DO was still zero at these AOTR values. That is, the kinetic rates were about the
same with or without aeration. Average monthly effluent quality in the 12-month study period at
full capacity is provided in Table 5.5.
Based on the success of the full-scale study, the five plants in the 91st Ave complex were
converted to the low DO, aerated bioselectors and aerated anoxic zones for nitrification-
denitrification to achieve an effluent TN of 6-8 mg/L. A summary of the 91st Ave WWTP 2004
operating data was provided by J Coughenhour (2005) for the six plants within the complex.
Essentially all six plants had a monthly DSVI of 72 + 10 mL/g for the full year. The initial
success of bioselectors piloted in 1992 has continued.
5.8 Tri City, Clackamas County, OR – Anaerobic (SXAXM) and Anoxic
(SXAXAL and SXAXM)
The Tri-City WWTP is an advanced secondary treatment system designed to remove
nitrogen from a flow of 591 L/s (13.5 mgd) in the summer. While an effluent of 20 mg/L BOD5
and TSS was required in the summer, the process nitrified, and an internal recycle for
denitrification was incorporated to recover alkalinity needed due to the low alkalinity wastewater
(Figure 5.14). In the winter, the plant operated in an aerobic step-feed mode (Daigger and
Nicholson, 1990).
The anoxic selector was a single-stage zone with an F/M of 0.72 kg BOD5/kg MLSS⋅d
and retention time of 86 minutes. During the summer period, the anoxic bioselector was effective
in reducing the SVI to an average value of 79 mL/g for 1986, 1987 and 1988. When the system
was operated in a step-feed mode for the winter period, the SVI increased (Figure 5.15).
A test of employing aeration in the bioselector in the summer of 1987 resulted in a large
increase in the SVI. The F/M was about 15% of the recommended F/M in the initial contact zone
of an aerated, low DO (SX AL) bioselection. The anoxic bioselector had occasionally reduced the
SVI to 20 to 30 mL/g, which causes a turbid effluent. Similar to the experiences at Davenport,
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IA, addition of air to the initial contact zone with a relatively low F/M increased the SVI because
there was inadequate stress for full bioselection.
5.9 Upper Occoquan Sewage Authority, VA – Aerated, High DO (SXAH)
The UOSA Regional Water Reclamation Plant (Daigger and Nicholson, 1990) is a 1183
L/s (27 mgd) facility employing primary clarification, single sludge nitrification system and
secondary clarification. The secondary effluent was chemically treated prior to discharge to a
drinking water reservoir.
The existing biological facility was upgraded because the existing CMAS system
historically had a severe bulking problem with SVIs as high as 600 mL/g. Due to the high SVIs,
the mean cell residence time (MCRT) was reduced by lowering the MLSS to satisfy the limited
clarifier capacity. The dominant problem organism was the filament M. parvicella, especially in
colder weather. Following successful bench-scale tests, full-scale bioselectors were incorporated
into the expanded biological system.
The modified plant (Figure 5.16) produced a low, stable SVI averaging 74 mL/g. Four
years of SVI data, before, during construction and after construction showed significant
improvement in sludge settleability (Figure 5.17). The high DO bioselectors operated above 2
mg/L DO and removed 60% of the sBOD5 and 40% of the sCOD with a residence time of 11
minutes based on influent flow.
Evaluation of the aerated bioselector oxygen transfer characteristics and the oxygen
uptake rate (OUR) indicated that about 0.1 mg O2/mg sBOD5 removal occurred. Because the
synthesis value would normally be 0.5 to 0.6 mg O2/mg sBOD5 removed, most (70-80%) of the
sBOD5 removed was converted to storage products. These results are similar to those reported by
Kroiss (1985).
The F/ΣM in the total volume of the aerated, high DO bioselector (SXAH) was 4.9 kg
BOD5/kg MLSS⋅d and the F/M in the initial contact zone was 14.8 kg/kg⋅d. While this value of
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F/M was higher than recommended by Chudoba and Wanner (1987) as well as in this document
for an aerated, high DO bioselector (12 kg/kg⋅d in ICZ), the SVI results were very good and are
shown in Figure 5.17. However, the experiences with the aerated, low DO (SXAL) bioselector at
the 23rd Ave WRF in Phoenix, AZ, revealed that hydrous bulking occurred at an F/M of 9-10 kg
BOD5 /kg⋅d in the initial contact zone. Thus the recommended loadings for the heavily aerated
bioselector were reduced for inclusion into this document. The heavily aerated bioselectors have
continued to produce a good operating SVI (Sellman 2001). The DO in the initial contact zone is
maintained at 2.0-2.5 mg/L.
5.10 Hamilton, OH – Anoxic (SXAXAL and SXAXM)
The City of Hamilton’s activated sludge WWTP has a design capacity of 1095 L/s (25
mgd) and operated at about 80% of that capacity in 1993 (Jenkins et al). The plant treats a
mixture of domestic wastewater (about 38%) and the discharge from a fiber recover plant (paper
mill).
Modifications and expansion of the existing plant in 1987 included a new aerated MLSS
channel to the existing (T3) and two new diffused aeration (T2A and T2B) basins, which had a
hydraulic retention time of 8-12 minutes (Q + RAS). Initially, only basin T3 received MLSS
from the channel and the SVI was 80-150 mL/g. Basins T2A and T2B received primary effluent
and RAS separately in a step-feed mode and the SVI was usually over 200 mL/g. In 1982, the
system was repiped to contact primary effluent and RAS in a gently aerated channel for 7
minutes prior to entering the three basins. The SVI profile of the three basins in the facility from
1982 through 1989 is shown in Figure 5.18.
During 1985-1986, it was necessary to remove Basin T3 from service. As a result the
bioselector retention time was reduced to 5 minutes and the SVI became unstable. It was
presumed that the bioselector retention time was too short and overloaded to a point where
sBOD5 was passing through the bioselector and into the aeration basins. It was not established
whether the increased SVI was caused by filamentous organisms or hydrous bulking from excess
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exo-cellular protoplasm (ECP) storage. Once T3 was returned to service, the SVI decreased to
the earlier values.
The air compressor to the inlet channel failed in May 1985 and was not replaced. In this
period, T3 was also out-of-service. However, the SVI from 1986-1989 was generally low
without air but operating in an anoxic condition from the NO3-N recycle. The anoxic bioselector
mode performed as good or better than the aerated anoxic mode. No analyses of the biology
during the SVI excursions of 1984-85 (aerated) and 1988 are available. In both cases, only one
or two of the three trains had elevated SVIs.
5.11 Middletown, OH – Anaerobic (SXANM)
The existing Middletown WWTP had a history (Jenkins et al., 1993) of chronic bulking
problems. Extensive pilot studies were carried out using the plant’s fully aerated configuration in
an anoxic-oxic (AX-OX) flowsheet for NdeN. This mode produced a lower SVI with an aerated
bioselector design. Further pilot studies were then carried out using the four-stage, aerated, high
F/M bioselector (SXAH) concept advocated by Chudoba and Wanner (1988) and based upon
Kroiss (1985).
The four-stage, high to low F/M (12, 6, 4 and 3 kg BOD5/kg MLSS⋅d, respectively) with
a medium DO ( ≤ 1.0 mg/L) selector was able to reduce the SVI to less than 100 mL/g within
three weeks after startup and averaged 65 mL/g SVI at 3000-4000 mg/L MLSS. In the following
month, the SVI averaged 47 mL/g. However, there were SVI excursions to 160 to 190 mL/g,
which can disrupt plant operations. This bioselector operated at least part-time in the range of 0.3
to 2.0 mg/L DO, which can generate filamentous organisms. The microscopic analysis showed
that the filamentous species in the main plant were dominated by N. limicola II and type 1851
and, to a much lesser extent, types 0675 and 0041. The pilot plant had similar species, but lesser
quantities, and much larger and better settling flocs.
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5.12 Star Valley Cheese Coop, Thayne, WY – Anaerobic (SXANM)
The cheese production facility employs a sequencing batch reactor (SBR) to process the
strong wastewater (sBOD5 of 600-2000 mg/L) in an anaerobic or anoxic-oxic (ANOX or AXOX )
mode, depending on whether NH4-N is available for nitrification. While over 90% of the dairy
wastewater treatment facilities have reported bulking problems, the Star Valley SVI is generally
40-80 mL/g and as low as 25 mL/g with operation at design loadings. The SVI increased to 300
mL/g when the plant was overloaded 200-400% for three to five days and the DO was zero to 1.0
mg/L at the end of the aeration cycle. An increase in the SVI was generally observed when
organic overloading prevented the DO from reaching 4-5 mg/L at the end of the oxic cycle. That
is, the sludge was not fully regenerated (Chudoba et al, 1982) and thus unable to remove a
sufficient level of sBOD5 in the unaerated period after feeding – prior to the oxic cycle. The
sBOD5 passing into the oxic period promoted filamentous as well as hydrous growth conditions.
Bioselection was promoted in SBRs when the units are batch fed (Heide and Pasveer,
1974). Batch feeding with mixing or low DO mode of operation produces the high to low F/M
condition necessary for bioselection.
The effluent BOD5 and TSS were 6-8 mg/L and 10-15 mg/L, respectively. Phosphorus
accumulation in the MLSS was noted and would be available for nutrient shortage periods.
Phosphorus and nitrogen content of MLVSS, not effluent concentrations, are the best measures
of nutrient sufficiency since there is Bio-P removal and NdeN occurs if NO3-N is present.
Nocardia sp. was a continuing problem but foaming had been reduced significantly by a change
in the plant’s cleaning detergent.
5.13 Tree Top, Selah, WA – Anaerobic (SXANM) and Aerated, High DO
(SXAH)
The apple and other fruit juice wastes produced by the plant have been treated in a 12-
day flow-through CMAS aerated lagoon for several years. The resulting MLSS of 500-1200
mg/L did not settle and the SVI was always greater than 1000 mL/g. Dominant filaments were
types 1701, 021N and 0041. Nutrient supplemented pilot studies were conducted using two
5-40
anaerobic-oxic (ANOX) batch-fed reactors with one and two hours anaerobic and one semi-
continuously fed AHOX reactor. All units were able to produce an SVI of 40-70 ml with the SX
ANOX unit producing the best clarified effluent. There was 90% removal of the sBOD5 in the
unaerated initial contact zone without phosphorus release during the anaerobic period.
Phosphorus was a limiting nutrient in the wastewater. For reasons unknown, some dispersed floc
was present in all supernatants.
Initial operation of the modified, full-scale CMAS aerated lagoon with four-hour
unaerated mixing zone and a 5:1 recycle (MLSS:Q) reduced the SVI to less than 150 mL/g. This
resulted in two phases of sludge: one of which settled very poorly, and the other that settled very
rapidly. The F/M ratio was 1.8 kg/kg⋅d in the initial contact zone and nitrification did occur. No
reasons for the dispersed solids were found. This level of treatment eliminated problems with the
effluent discharge to a municipal wastewater treatment plant.
5.14 Fibra, America, Brazil – Anaerobic (SXANM)
This WWTP treats a soluble high sugar wastewater from a Nestle plant processing
wastewater from cocoa production with a two-stage CMAS process. Each basin was fully
aerated to ≥ 2 mg/L (Okey, 1997) using mechanical aerators. In this mode, the SVI ranged
between 500 and 1000 mL/g. When one, then two, of four first-stage aerators were shut down
and the DO was reduced, the SVI decreased to the range of 250-400 mL/g (Figure 5.19). A
single-stage bioselector, which is mechanically mixed and maintained in an anaerobic mode, was
constructed and precedes the aeration basin.
The bioselector was designed on the basis of zero-order assimilation of a monosaccharide
based on 60% removal in the bioselector. Depending on the flow, the retention time in the
bioselector is one to two hours. The bioselector was started up on Day 19 (Figure 5.19) and there
was an immediate decrease in the SVI; thereafter the SVI decreased to less than 100 mL/g within
6 days. In some periods, the SVI decreased to less than 50 mL/g and the effluent turbidity
increased. The process control mode to stabilize the SVI was to bypass more or less flow around
5-41
the bioselector into the first oxic zone. This flow resulted in a controllable growth of filamentous
organisms, which stabilized the floc structure and improved effluent clarity.
5-42
6-1
Chapter 6.0
Troubleshooting Bioselectors
The actual performance of a bioselector may be less than optimal for various reasons. If
the bioselectors are constructed per design and the operation is per the plan, then the analysis
must consider a number of possible reasons for bulking conditions. The causes can be:
• Low food/mass (F/M) in the initial contact zone (ICZ) – filamentous
• High F/M ICZ – hydrous
• Air rate (DO = 0.3 to 1.5 mg/L) to bioselector – filamentous
• Limited oxygenation (regeneration) capacity in oxic zone – hydrous / filamentous
• Secondary (oxic zone) bulking – filamentous
• Very low sludge volume index (SVI) (diluted SVI) – high turbidity
• Toxic/inhibitory compounds – hydrous
• Limited nitrogen or phosphorus supply – hydrous/filamentous
• Soluble organic breakthrough – hydrous/filamentous
• Single bioselector stage
The current state of the art of bioselection is mostly results of laboratory and field
experience than a fundamental understanding of the mechanisms involved. There is a general
understanding of how filamentous and non-filamentous growths can be controlled. Further,
research and full-scale experiences have shown that soluble wastewaters will be more amenable
to bioselection than wastewaters with a high degree of particulate and colloidal solids. Lastly, it
is understood that the bioselector should remove the bulk (>80%) of the available soluble
chemical oxygen demand (sCOD) prior to leaving the bioselector and entering the oxic zone.
Many bioselectors will reduce the sCOD to the range of the final effluent in 12 to 20 minutes of
the retention time in the bioselectors based on raw or settled effluent flow.
6-2
However, this understanding does not extend to the point where we can define why one
facility will operate at 50 to 80 mL/g SVI and another with a similar bioselector design and
similar wastewater will operate at 90 to 120 mL/g. On the other hand, the six separate plants at
Phoenix 91st Ave WWTP all had SVIs about 60-80 mL/g in 2004.
It is often difficult to clearly identify hydrous bulking problems (Figure 6.1). The SVI
may or may not indicate a serious bulking condition. When hydrous bulking occurred due to floc
overload at the 23rd Ave WRF in Phoeniz, AZ, the DSVI only increased from the 60-90 mL/g
range to 100-120 mL/g range. However, the 5-minute zone settling velocity (ZSV5) was lower,
and large, fluffy floc (sometimes called straggler floc) appeared in the upper layers of the
clarifier. A two-phase sludge blanket developed in the clarifier: the lower one dense and the
upper one light, of low density and nearly buoyant. During higher flows these large, low density
flocs (hydrous) can be flushed into the overflow.
The hydrous biomass may not be visible in the bulk settling tests since the majority of the
floc can settle well, pulling down those hydrous flocs and producing a uniform settled mass.
However, this test procedure does not duplicate the dynamic conditions in the continuous
clarifier. The initial (5 min) ZSV will generally be significantly lower than the values predicted
by the Daigger and Roper (1985) or Daigger (1995) equations.
The good news is that a good bioselector design with three or four zones will maintain
the SVI at ≤ 120 mL/g (DSVI ≤ 95 mL/g) at all times. Most of the lowly aerated and anoxic
three- or four-stage bioselectors operate at < 100 mL/g SVI (DSVI < 80 mL/s). If the settling
characteristics are poorer (higher SVIs) than these values, then there can be problems with the
bioselector design, its operation, or constituents in the wastewater as will be discussed below.
6.1 Low F/M in the ICZ
The F/M gradient in a bioselector could be lower than the recommended design loadings
due to lower operating flow rate and/or lower influent BOD5 or COD concentrations or
combinations of these factors. Some of the early bioselectors were single stage and too large;
6-3
6-4
hence there was no gradient and the F/M in the initial contact zone was too low to effect good
bioselection. With the lower F/M, the mixed liquor suspended solids (MLSS) would be expected
to show a higher level of filamentous organisms. Generally, the bioselector should operate
efficiently over the range of 40 to 120% of the recommended loadings. In conditions less than
40% of these loadings, removal of an aeration train from service should be considered to
maintain a more economical operation. However, most bioselectors will be effective over the
range of 40 to 120% of the recommended design F/M gradient. Some units will be effective at
20-30% of the suggested loadings, although this situation is not predictable.
Lowering the MLSS to achieve a higher F/M may not have the desired effect. There is
some indication that the higher oxygen demand (mg/L⋅hr) produced by the higher MLSS is a
significant factor in promoting non-filamentous growth. This would also be true for weaker
wastewater in colder periods when the influent wastewater could have a higher DO. It is
suggested as the initial step of corrective action to increase the MLSS and minimize air flow (if
employed and ensure that the bioselector zones are at or near zero DO if aerated (SXAL) and > 2
mg/L if heavily aerated (SXAH).
If low F/M in the ICZ is a normal condition, add intermediate baffles to increase ICZ
F/M. If problems persist, it will be important to identify the causative organisms or other factors
that will assist in the identification of the problem.
6.2 High F/M in the ICZ
If the F/M is excessive in the initial contact zone, the biomass will accumulate and store
the soluble organics as exo-cellular protoplasm (ECP) slimes and the result is hydrous bulking.
The hydrous bulking condition can cause lowered return sludge solids (RSS) concentrations and
a fluffy, unstable sludge blanket which may overlay a more concentrated sludge layer of non-
hydrous sludge. Corrective actions include increasing the air rate to bioselector zones and
maximizing the mixed liquor concentration (lower F/M). Increasing the return activated sludge
(RAS) and internal recycle (IR) rates to the initial contact zone may not change the F/M ratio,
6-5
but it does decrease the contact loading, which is one possible cause of the accumulation of ECP.
The increased quantity of nitrates and TSS with a higher internal recycle will also help to reduce
bioselector overload due to the denitrification comsumption of soluble organics and lower
contact loading (mg sCOD/g MLSS).
If the overloading (high F/M) is a permanent situation, it is then necessary to enlarge the
volume in the initial contact zone by moving the baffle wall or removing the baffle to double the
ICZ volume. Baffle supports that are bolted to the wall and baffles that are easily moved to a
new position represent good design practice. The three- (or four-) zone bioselector design should
be retained. However, the volume in the third bioselector zone (SX-3) has less significance and
can be reduced if necessary.
6.3 Air Rate to the Bioselectors
The air rate to the bioselectors can influence the SVI. In general, higher air flow in the
aerated, low DO (SXAL) mode will increase SVI and vice versa. To minimize SVI, the air rate
should be limited to keep the DO at 0.0 mg/L during the diurnal peak BOD5 loading (and
bacterial growth) periods. If the air rate is set to maintain at ≤ 0.3 mg/L DO during the early
morning at low flows, the DO during the higher loading periods throughout the day will be zero.
In some applications, the SVI will be too low and the rapidly settling sludge leaves
turbidity (non-flocculated TSS) in the effluent liquid. In those cases, the growth of filaments can
be encouraged by increasing the air rate and DO. The alternative is to bypass the bioselector
(Okey, 1997) with a portion of the influent flow into the oxic zone(s).
In the aerated, high DO (SX AH) mode of operation, increased SVI can be due to the
inability to maintain > 2 mg/L in the initial contact zone during peak loading periods. If the air
rate cannot be increased to achieve 2 mg/L DO, then an aerated, low DO (SX AL) mode (DO =
0.0) could be evaluated in the initial contact zone of the bioselector. The balance of the
bioselector zones would be maintained at > 2 mg/L DO. If the F/M is too high (> 4 kg
6-6
sBOD5/kg⋅d), a low DO in the initial contact zone could result in the accumulation of ECP
leading to hydrous bulking.
6.4 Limited Oxygenation Capacity in the Oxic Zones
If the aeration capacity is limited – volume or actual oxygen transfer rate (AOTR) – in
the oxic zones (overloaded), then the stored organics from the bioselectors may not be oxidized,
i.e., sludge is not regenerated. The return of these regenerated solids can reduce the normal
sorption-storage capacity of soluble substrates in the bioselectors. However, the biomass will
then use an alternative means of storage producing ECP slimes and hydrous bulking will result.
The oxygen demand of the first oxic zone in a multistage reactor can be 80 to 130
mg/L⋅hr at 3.5 g/L MLSS. It is imperative to be able to maintain > 2 mg/L DO in this zone.
Lower DOs can result in excessive ECP production and hydrous bulking.
The corrective action is to reduce the loading (add aeration volume on-line, if available)
and/or increase the oxic zone oxygenation capacity if DO is limited. Short-term actions would
include maximizing internal recycle to the initial contact zone and an increased RAS flow may
be beneficial. Increasing MLSS (and the SRTOX) can also be beneficial as the diffused aeration
αF value of the mixed liquor may often increase the AOTR when the higher solids retention time
(SRT) is increased. The net effect is reduced air requirements (kg O2 supplied/kg BOD5).
6.5 Secondary Bulking (Oxic Zone)
Secondary bulking can result from the hydrolysis of particulate BOD5 to soluble
compounds. These compounds can support the growth of filaments, especially in a soluble
organic limited (low F/M) process. It is probable that low DO in the first oxic zone may
contribute to bulking situations – both filamentous and hydrous sludge production.
Compartmentalization of the initial portion of the oxic zone is beneficial. The DO should be
maintained above 2.0 mg/L, or if not possible, reduced to 0.0-0.3 in the initial oxic zone.
6-7
6.6 A Very Low SVI – High Turbidity
Bioselectors can sometimes produce very low SVIs (20 to 40 mL/g) and there can be
excessive turbidity in the overflow due to high zone settling velocity reaching 4 to 6 m/hr. This
can be due to excessive floc shear, high settling rates disrupting reflocculation, fragile flocs or
presence of toxic/inhibitory substances. The problem is more likely associated with highly
soluble industrial wastewaters where SVIs of 20-40 mL/g have been produced and the very high
rate of floc settling limits opportunity for floc growth.
The corrective action steps are:
• Evaluate shearing forces in the aeration basin to the final clarifier feedwell.
Conduct lab vs. field settling results to define the benefits of controlled settling.
Corrective actions would include checking the nutrient supplied, effluent nutrients
and the nitrogen (N) and phosphorus (P) content of the biomass. The N and P
content should be ≥ 8% N/VSS and ≥ 1.5% P/VSS in the MLSS.
• Maximize the DO in the bioselectors if the units are aerated. The aerated, low DO
(SXAL) mode should produce ≥ 1.0 mg/L DO at maximum air flow. This mode
will help generate filaments to strengthen floc.
• Provide a bypass of influent around the bioselector to the oxic zone.
• If toxic wastes are suspected, find and remove the culprit component(s).
6.7 Toxic/Inhibitory Compounds
Specific organic and inorganic compounds act similar to chlorine – that is, they suppress
the growth of filaments and reduce SVI. However, it is also possible that these compounds will
slow nutrient transport into the cells and slow normal growth. As a consequence, ECP
production can occur with hydrous bulking the result.
6-8
The corrective action is to find and remove the culprit compound. Maximizing the MLSS
and sludge inventory may have some short-term benefits. The culprit could be metals,
chlorinated hydrocarbons, complex phosphates, and a host of other organic compounds that
inhibit biological growth.
6.8 Limited Nitrogen and Phosphorus Supply/Availability
If nutrients are limited, it is possible that both filamentous and hydrous bulking
conditions could co-exist. In warm climates at high oxygen uptake rates (30 to 40 mg O2/g
MLSS⋅hr), it is possible that the cells are unable to sorb/process sufficient nutrients to match the
growth needs. Thus, the cells could bypass nutrient needs by ECP production if normal substrate
storage capacity is limited. The limiting oxygen uptake rate (OUR) value has not been defined
through research, but it may be in the area of > 35-40 mg/L⋅hr⋅g MLSS. The N and P content of
the MLSS should be in the range set forth in Section 6.6.
If the nitrogen and/or phosphorus content of the MLSS is low, then the possible causes
will need to be evaluated. They are:
• Inadequate influent nutrients. The N and P must be available and the mixed liquor should
contain at least 5 mg/L NH4-N and 1 to 2 mg/L ortho PO4-P prior to initiation of nitrification
(if occurring) and Bio-P removal. It is possible to have limited Bio-P removal with low
organic phosphorus/MLSS levels.
• Presence of toxic compounds. Generally nitrification will be adversely affected, but this
may not be obvious in high SRT systems. Conduct batch nitrification to determine if there is
a significant lag in the initiation of nitrification. Also, check the rate of nitrification (mg/mg
MLSS⋅hr) to determine if the rate is adversely affected.
6.9 Soluble Organics Breakthrough
It is possible that the bioselectors are not removing the readily bio-degradable or soluble
organics prior to the oxic zone for one or more of the reasons set forth earlier. If breakthrough
occurs, there is the opportunity for filamentous growths as well as the potential to produce
6-9
hydrous sludges in the bioselectors since the return sludge may not be fully regenerated.
Multistage bioselectors are more efficient in sCOD removal and should be employed with the
recommended F/M cascade design. If ICZ F/M is low, add intermediate baffles.
Generally, the sCOD (f1.5COD) in the effluent from the bioselector should not be more
than 125-175% of the secondary effluent sCOD for an aerated, low DO, high DO or anoxic
bioselectors. Depending on the design, the sCOD may be higher in the effluent of anaerobic
bioselectors employed to achieve Bio-P removal. The Bio-P design of a biological selector may
have anaerobic retention times of 1 to 2 hours vs. 12 to 20 minutes typical for a bioselector for
only bulking control.
A laboratory sequencing batch reactor (SBR) study is effective in defining the SVI that
can be achieved by optimizing bioselection. In this type of study, the reactor is operated on a
full-settle-draw basis, normally fed twice daily and wasting once per day. The reactor is batch
fed, mixed for a limited time if denitrification is required and then aerated at > 2 mg/L DO.
Heide and Pasveer (1974) and Albertson et al., (1992a) reported good agreement between
laboratory SBR and full-scale facilities with multistage bioselectors. This lab procedure has been
employed successfully in other facilities – Tree Top Apple Juice, WA; 23rd Ave WRF in
Phoenix, AZ; Mt. Pleasant Rifle Range Road WWTP, SC; Star Valley Cheese Co., WY; and
Southerly and Jackson Pike in Columbus, OH – to define the potential of bioselectors.
A bioselector design with staggered baffles extending partially (75-85%) across the basin
can encourage backmixing and disrupt the bioselection operation. Overflow walls with about 10-
12 mm head loss per stage is recommended.
6.10 Summary Comments
The causative organisms should be identified by microscopic examination as to whether
they are filamentous or non-filamentous (hydrous) by standard procedures. The specific
organism can be a guide to the source of the problem. An experienced, well-trained analyst is
often required to ensure accurate definition of the organisms and possible causes. The nitrogen
6-10
and phosphorus content of the MLSS should be checked to ensure that adequate quantities of N
and P are not only available but also being utilized by the organisms.
India ink staining and the total glucose content of the MLSS (Jenkins et al., 1993)
provide important data to evaluate the characteristics of the MLSS relating to settling
compaction and clarification of the mixed liquor. Hydrous sludge production can result in
significantly lower 5-minute zone settling rates (ZSV5) predicted for SVI and MLSS
concentrations using the Daigger and Roper equation (1985).
In the analysis of problem bioselectors, all tools should be considered, specifically:
• Define whether poor settling is the result of filamentous or hydrous bulking.
• Check the level of N and P in the mixed liquor volatile suspended solids (MLVSS).
• Determine whether the soluble F/M in the initial contact zone is in the proper range.
• Check the contact loading to ensure that the contact loading value is in the range
recommended.
• Evaluate the potential for toxicity.
• Determine the level of total glucose in the MLVSS.
• Determine the 5-minute ZSV5 at two or three concentrations.
• Evaluate the DO and oxygenation capacity of the oxic zone(s).
• Check the removal of sCOD by the bioselector.
• Evaluate whether the return sludge is fully regenerated and has up to 15% of the cell weight
as sCOD sorption capacity.
• Conduct a laboratory SBR study to determine the achievable SVI if full-scale SVI is high.
As the possible number of independent variables increases, the amount of data required
to define the impact on the dependent variable SVI increases geometrically. Thus, there is an
advantage to employing bioselector designs that have proved to be most effective. Based on
Czech and US experiences, bioselectors comprising three or four zones and effecting an F/M
cascade where 75-85% of the sCOD will be removed from solution have proven to be the most
successful.
7-1
Chapter 7.0
Research Needs
The technical effort required to document the rational design approach to bioselectors
and determine whether there is a common ground to designing aerated (SXAL and SXAH), anoxic
(SXAX and SXAXAL) and anaerobic (SXANM) is daunting. The fact that plants, of similar design
and receiving typical municipal wastewaters, may or may not bulk (Tomlinson, 1976) is
evidence of the magnitude of the problem faced in developing a comprehensive understanding of
the design parameters for bioselectors. The fact that an activated sludge plant may only
occasionally have bulking problems is a further measure of the difficulty faced by the
researchers.
This study revealed that there is no common ground for bioselector design whether
aerated, low DO (SXAL), aerated, high DO (SXAH), anoxic (SXAxM and SXAxAL) or anaerobic
(SXANM) mode. The initial contact zone (ICZ), often considered the most critical zone, may
have had an organic loading varying from:
BOD5: <1.0 to >14 kg/kg MLSS⋅d
COD: <2 to >25 kg/kg MLSS⋅d
and still be successful in reducing a high SVI to a more acceptable range. The question “How
good is the optimum performance?” cannot be answered by the data available in this document.
Also, bioselectors have been constructed from as few as one to up to six compartments in a
series. How many zones are required to provide optimum reduction in the SVI? This study has
provided some insight and perhaps some preliminary guidelines to direct research efforts as well
as some general comments for introducing the research needs.
7-2
7.1 General Introductory Comments
The following comments cannot be considered site-specific because there are probably too
many factors, known and unknown, involved in any adoption/acceptance of these comments.
That is, there is and may continue to be many contradictions until the biological mechanisms are
better understood. A rejection or acceptance of one or more of the following comments by
careful lab and field studies can be considered a step toward expanding our understanding of
bioselectors.
• Filamentous and hydrous bulking are on opposite ends of the floc loading and F/M
spectrum. There is a middle ground where the adverse effects of each type of bacteria
growth are minimized (Figure 1.7).
• The initial contact zone (ICZ) of the bioselector and its food/mass (F/M) and or its
contact loading will be the key to the definition of the optimum design of each type of
bioselector.
• The aerated, low DO (SXAL), anoxic (SXAXM and SXAXAL) and anaerobic (SXANM)
bioselectors should provide a F/M cascade using three or four compartments. Fewer
zones (two) would be acceptable if demonstrated effective by pilot studies. The design of
bioselectors should be based on soluble input (f1.5µm) of preferably soluble chemical
oxygen demand (sCOD), and alternately soluble biochemical oxygen demand (sBOD5), if
COD data are unavailable. The initial contact zone of the bioselector should have a
sCOD F/M >3 and <6 kg/kg MLSS⋅d (sBOD5 >1.5 and <3 kg /kg.d). The bioselector
volumes can be 25, 25, 50% or 25, 25, 25 and 25% of the total volume. The aerated, high
DO (SX AH) bioselector loading range is > 3 to < 9 kg sCOD/kg MLSS⋅d (>1.5 kg to 4.5
kg sBOD5/kg⋅d).
• The bioselector design and optimization should be based on the soluble BOD5 (or sCOD)
entering the initial contact zone of the bioselector. The success or failure of bioselection
may be wholly dependent on the ability of the bioselector to remove the majority (75-
7-3
85%) of the incoming removable soluble sCOD (sBOD5) prior to the anoxic and/or oxic
stages.
• Limited ability to remove a large fraction of the sCOD5 in the initial contact zone will be
a potential cause of variable and poor results. The oxic zone must be fully evaluated to
determine if the ‘sorptive’ capacity of the return sludge was restored (regenerated).
• Aerated (SX AL and SX AH) and anoxic (SX AX) bioselectors may prove to be more
flexible for controlling SVI than anaerobic designs and should be employed when Bio-P
is not required. When denitrification is required, the bioselector design is not modified
and additional anoxic volume as required for completion of denitrification is added as a
fourth, and if necessary, a fifth zone. The aerated bioselector may handle a higher F/M in
the initial contact zone and thus minimize the bioselector volume. This same approach –
additional zones after the bioselectors – should also be employed for anaerobic
bioselectors when Bio-P volume requirements are larger than necessary for bioselection.
• The performance of bioselectors should be compared when the design (ICZ F/M),
number of stages and operation are similar. That is, what are the performance
characteristics of a three-stage aerated, low DO anoxic (SXAXAL) bioselector loaded in
the range of 3.0 to 6.0 kg sCOD/kg MLSS⋅d (1.5 to 3.0 kg sBOD5/kg⋅d) to the initial
contact zone? What are the average and maximum DSVI (or SSVI) values? Standard
deviations? (e.g., Columbus Southerly: 80 + 15 mL/g; Jackson Pike: 69 + 17 mL/g; Gig
Harbor: 70 + 8 mL/g; Phoenix 91st Ave: 81 + 6 mL/g – based on monthly results.)
• There is an increasing body of evidence that supports staging in the bioselector, anoxic,
or oxic zones in order to achieve the lowest SVI.
• The U.S. wastewater industry should adopt as the standard the DSVI or SSVI procedure
as well as a standard size of test vessel. The traditional uSVI procedure is not always
responsive to changes in the settling properties when SSV is >250 to 300 mL. It can be
misleading and cause improper and upsetting operational steps. Further, plant-to-plant
comparisons cannot be made with any reliability.
• Bioselectors should not be designed on COD, c BOD5, or BOD5 data.
7-4
7.2 Research Target
Research can progress toward a better understanding of the basic mechanisms of
bacterial growth that cause filamentous and non-filamentous bulking and also undertake a
detailed analysis of operating installations. It is suggested that the target criteria for the DSVI
be:
Average Maximum
Month Month
DSVI – mL/g 60 < 80 80 < 100
These DSVI values will allow for the effective and economical utilization of aeration and
clarification capacity and thus were chosen for targets. While a bioselector DSVI range of 80 to
140 mL/g would be an improvement over a DSVI of 120-180 mL/g, the clarification capacity at
140 mL/g is still significantly less than <100 mL/g DSVI. The effectiveness of bioselectors must
be evaluated on the maximum DSVI of the MLSS, not the average value for a week or a month
of operation because clarification sizing is dependent on the highest expected short-term DSVI.
7.3 Recommended Areas of Research
This study revealed that there are a number of process and design factors that may or may
not affect the efficiency of bioselection. These factors include:
• The number of stages in the bioselector. Based on prior studies, a three- or four-stage
bioselector would appear to be the best design to ensure that the maximum SVI is limited
to the target range. An initial field search should determine whether the three- or four-
stage bioselector should be adopted as a basic design (Both may use same bioselector
volume).
• Define the minimum ICZ loading that could cause filamentous growths (insufficient
stress) and the maximum loading that could lead to excessive ECP (hydrous sludge)
production.
• Establish whether future bioselector design criteria should be based on sCOD (or sBOD5)
rather than total COD (or BOD5). Bioselector designs based on sCOD are the same for
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raw and primary effluent; if based on COD (or BOD5), they would be 30 to 45% larger
for raw (unsettled) wastewater of domestic origin. This document recommends use of
soluble (f1.5µm) values for design since soluble removal defines the effectiveness of the
bioselector.
• Determine the maximum design ICZ loading and operating parameters for aerated
secondary (SX AL, SX AH), anoxic (SX AX) and anaerobic (SX AN) bioselectors. Establish
the role aeration (SX AL, SX AH) plays in terms of improving the range/operability of
these bioselectors. Since there are now hundreds of bioselectors in operation, an
extensive survey of the field units and their performance would provide useful
information. Plants where the SVI is good, but doesn’t meet the above targets, should be
considered for minor modifications to enhance performance. For example, add/delete
baffles to modify the ICZ F/M ratio.
• While there is an extensive body of laboratory research on bioselectors of many designs,
these units don’t undergo the rigors of the full-scale operating plants. However, a format
to summarize both lab and field results is needed in order to compare similar
configurations and operating conditions to determine if there are guiding design criteria
common to both sets of data. Multiple regression analysis could be a useful tool for
analyzing similar designs and comparing different design concepts. Because SVI is the
dependent variable, the problem will often be the use of undiluted SVI (uSVI) instead of
DSVI or SSVI for the U.S. data. All of the necessary data defining independent variables
may not be available without further field study.
• The research should be directed to define the significance of independent variables on
DSVI. Independent variables to be considered would be:
1. Bioselector ICZ and overall F/M
2. ICZ contact loading
3. Removal (%) of sCOD (sBOD5) by bioselector zones
4. Number of Sx zones
5. Soluble/total COD (sBOD5/BOD5) ratio of wastewater
6. Aeration (AL, AH) level and DO in bioselector (SX) zones
7. Wastewater strength and variations
8. Effect of wet weather, dilution and DO
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9. Effect of mode of operation: SXAL, SXAH, SXAxM, SXAxAL, SXANM
10. Effect of system sludge age, SRT
11. Definition of the bacterial species accompanying an excursion in the DSVI in
a bioselector system
12. Oxygen demand in the ICZ as an indicator of bioselector stress
13. Process temperature
As the number of variables increases, the mass of data required is larger. Thus, there is an
advantage of pre-screening data for major variables such as: (1) number of stages; (2)
F/M in the initial contact zone; and (3) type of bioselector. Based on Czech and US
experiences, bioselectors with three- or four-zones have been most successful,
particularly in plants with long or staged oxic zones.