simultaneous nitrification denitrification to meet low
TRANSCRIPT
Simultaneous Nitrification�Denitrification to Meet Low
Effluent Nitrogen Limits
Jose Jimenez1*
, Derya Dursun1
1Brown and Caldwell, USA (E�mail: [email protected])
Abstract
Simultaneous nitrification�denitrification (SND) has been referred to as a biological process for nitrogen
removal where nitrification and denitrification occur concurrently in the same aerobic reactor. SND facilities,
investigated in this study, have been shown to remove 80 to 96 percent nitrogen without additional carbon and
alkalinity. Carbon availability and bulk dissolved oxygen (DO) concentrations were found to be important
process parameters for SND activity. Data suggest that a chemical oxygen demand�to�nitrogen (COD:N) ratio
of at least 10.0 is required to achieve significant denitrification; and an optimum bulk DO concentration
ranging from approximately 0.3 milligrams per liter (mg/L) to 0.7 mg/L appears to maximize SND activity.
However, this low DO condition required for SND provides an environment more susceptible to sludge
bulking as observed by the SVI data from the selected SND facilities in this study.
Keywords
Nutrient removal, nitrogen removal, simultaneous nitrification�denitrification, low DO bulking, carbon
INTRODUCTION
Biological reduction of nitrogen in the activated sludge process relies primarily on two
mechanisms: aerobic nitrification and anoxic denitrification. Generally, in conventional
biological nutrient removal (BNR) facilities, the two processes are carried out in physically
separated aerobic and anoxic zones with internal recycles (Barnard, 1975; Ju et al., 1995).
However, nitrogen losses in excess of that required for biomass synthesis from aerated facilities
have been observed frequently when the right environmental conditions are in place (Applegate
et al., 1980; Rittmann and Langeland, 1985; von Münch et al., 1996; Pochana and Keller, 1999a;
Daigger and Littleton, 2000, Trivedi and Heinen, 2000; Strom et al., 2004). This phenomenon
has been referred to as simultaneous nitrification�denitrification (SND) because both biological
reactions occur concurrently in the same reactor.
SND is a well�known phenomenon in BNR activated sludge systems. It largely depends on the
bioreactor configuration (macro environment that is related to mixing), bulk oxygen
concentration, and floc size (micro environment that affects oxygen diffusivity on flocs)
(Pochana and Keller, 1999a; Daigger and Littleton, 2000; Kaempfer et al., 2000; Stensel, 2001;
Littleton et al., 2002; Littleton et al., 2003a; Littleton et al., 2003b Satoh et al., 2003; Ju et al.,
2007). In addition to the environmental factors affecting SND, some studies have also indicated
that the organic carbon available for denitrification plays a major role in SND activity (Barnard,
1992, Isaac and Henze, 1994; Pochana and Keller, 1999a; and Peng and Qi, 2007).
Even thought the mechanisms responsible for SND are well understood; SND is difficult to
control since it depends on limited controlled aspects of the process such as floc sizes, internal
storage of COD and DO profile within the flocs (Pochana and Keller, 1999a; and Daigger et al.,
2007). Therefore, SND process is not a simple matter to design and operate such systems.
However, if the operating mechanisms could be better characterized, SND could be used in a
wider range of applications thereby reducing the cost of using BNR and making it possible to use
it more easily and reliably at existing facilities.
A key to understanding SND activity in aerated facilities is to understand how process design
and operating parameters affect the performance of SND. The objective of this study is to
evaluate the treatment performance of SND facilities including an aerobic activated sludge pilot
plant treating synthetic wastewater. Operating data from selected treatment facilities using SND
were also evaluated to understand the factors affecting its performance.
METHODOLOGY
Evaluation of SND Plants Performance at Selected Treatment Facilities
Historical data collected from selected wastewater treatment plant facilities performing SND
were analyzed to understand how process design and operating parameters affect the SND
performance. Table 1 summarizes the facilities evaluated during this study. The plants are
mainly located on the southeast region of the United States, and they serve principally municipal
areas and receive domestic and commercial wastewater. The design capacities for these facilities
range in size from 50 cubic meters per hour (m3/hr) to 630 m
3/hr.
Table 1. Summary of selected treatment plants performing SND
Plant1 Location
Capacity
(m3/hr)
Process SRT
(days)
Effluent
TN (mg/L)
N Removal
(%)
SVI
(mL/g)2
Iron Bridge3 Orlando, FL 6,420 Bardenpho 15 2.0 96 115/165
Eastern
Reg.3
Orange Co., FL 4,010 Bardenpho 12 2.6 89 120/160
Snapfinger3 DeKalb Co., GA 2,410 Single�Stage 20 3.8 80 200/300
Central3 Ft. Myers, FL 1,765 Single�Stage NA 5.5 84 140/180
Winter
Haven3
Winter Haven, FL 1,205 Bardenpho 25 2.4 93 130/190
Mandarin5 Jacksonville, FL 1,205 MLE 18 4.0 90 150/180
Marlay
Taylor3
St. Mary’s Co., MD 965 Single�Stage 25 4.5 86 170/280
Northwest
Reg.3
Hillsborough Co.,
FL 805 Bardenpho 12 2.7 93 NA
Tarpon
Springs3
Tarpon Springs, FL 645 Bardenpho NA 2.2 92 NA
Stuart6 Stuart, FL 645 Single�Stage 18 5.5 86 212/350
Smith
Creek7
Raleigh, NC 545 A2O 25 4.5 90 200/245
1 Plants do not have supplemental carbon for denitrification
2Average and 90�percentile values
3 Average based on 2007�2008 data
4 Average data based on 2005�2006 data
5 Average data based on 2001�2003 data
6 Average data based on 2004�2005 data
7 Average data based on 2003�2005 data NA – Data not available
RESULTS AND DISCUSSION
Performance of SND Facilities
There are several plants that achieve higher SND performance and do not require additional
carbon and alkalinity to meet low effluent total nitrogen (TN) concentrations. One example is the
Iron Bridge Wastewater Treatment Plant (WWTP) (City of Orlando, Florida). This facility uses a
five�stage Bardenpho configuration as presented in Figure 1, with the main aerobic zone being
an oxidation ditch operating in SND mode. Figure 2 presents long�term effluent nitrogen
concentrations from this facility in the form of 30�day moving average concentrations. As
depicted heretofore, the Iron Bridge WWTP is able to meet very low effluent TN concentrations.
It should be noted this facility, as well as the other facilities presented in Table 1, do not have
supplemental carbon for denitrification. Dissolved oxygen, ammonia and nitrate profiles were
developed at the Iron Bridge WWTP and the average results are presented in Figure 3. During
this period, the average influent BOD5 and ammonia concentrations were 203 mg/L and 25
mg/L, respectively. The average DO concentration in the oxidation ditch was 0.3 mg/L. As
indicated in Figure 3, nitrification was essentially complete in the oxidation ditch at very low DO
concentrations with values typically less than 0.2 mg/L. Despite the complete nitrification
occurring in the oxidation ditch, the nitrate levels in the oxidation ditch averaged only 0.95
mg/L. Yearly average effluent nitrate values, leaving the oxidation ditch, as low as 1 mg/L have
been recorded at most of the facilities listed in Table 1, which is surprisingly low. Historical TN
removal for the Iron Bridge WWTP has been in the 92 to 98 percent range, with an average
value of 96 percent. Data for other SND facilities presented in Table 1 indicate that significant
total nitrogen removal was occurring at each facility with average removal efficiencies ranging
from 80 to 96 percent. Facilities with BNR configurations such as Iron Bridge, Eastern Regional,
Northwest Regional, Tarpon Springs, Winter Haven, Mandarin and Smith Creek exhibited TN
removal efficiencies of 89 to 96 percent; whereas, facilities using a single�reactor configurations
(without explicitly defined anoxic zones) for nitrogen removal such as Snapfinger, Central,
Marlay Taylor, and City of Stuart realized removal efficiencies in the order of 80 to 86 percent.
Anaerobic Anoxic Oxidation Ditch operated in SND
Influent and
RAS
Post
Anoxic
Post
Aerobic
Mixed Liquor Recycle
To FST
Figure 1. Schematic configuration for the Iron Bridge WWTP, City of Orlando, Florida
0
1
2
3
4
1/1/2005 7/1/2005 1/1/2006 7/1/2006 1/1/2007 7/1/2007 1/1/2008 7/1/2008
Eff
luen
t (m
g/L
)
Date
30�d TN 30�d TKN 30�d NOx�N
Figure 2. Effluent nitrogen concentrations at the Iron Bridge WWTP, Orlando, Florida
0 00.3
0
1
15.5
7.1
0.20.5
0.140 0
0.95
0.35 0.4
0
2
4
6
8
10
12
14
16
18
Anaerobic Anoxic Oxidation Ditch Post�Anoxic Post�Aerobic
Con
cen
trati
on
(m
g/L
)
DO NH3�N NO3�N
Figure 3. Dissolved oxygen, ammonia and nitrate profiles at the Iron Bridge WWTP,
Orlando, Florida
Factors Affecting SND
From previous studies, it has been found that two principal factors predominately influence
SND. These are the available carbon and the bulk DO concentration.
To accomplish denitrification in any process, the availability of readily biodegradable organic
carbon has been found to be the most essential factor (Barnard, 1992). Long term operation data
from the Winter Haven WWTP show the effect of the influent BOD5:TKN ratio on the effluent
nitrate concentrations from an activated sludge process operating in SND mode (Figure 4�A).
Data presented in Figure 4�B show the effect of the influent COD:N ratio on denitrification at the
pilot plant facility treating synthetic wastewater. Data from the full�scale facility show that
complete denitrification can be achieved consistently when the influent BOD5:TKN ratio is at
least 6.0; however, when this ratio falls below 2.0, limited denitrification was achieved at the
facility. Similar observations were obtained at the pilot plant where denitrification performance
increased as the influent sCOD:N ratio increased. Very limited denitrification was observed at
the pilot plant at sCOD:N ratios of 6.0 or lower; however, optimum denitrification occurred at
sCOD:N ratios in the 12�15 range. However, it should be noted that in the case of the pilot plant,
the sCOD also corresponded to the total COD available; therefore, in reality somewhat lower
sCOD:N ratios could be expected to work as well since other COD fractions will also be partly
used in the process via hydrolysis or fermentation. In addition, it should be noted here that the
higher sCOD:N ratio needed for denitrification in the pilot plant could be the result of the low
operating HRT of the process. These results indicate that the soluble carbon for denitrification
strongly influence the SND performance. This confirms results by others (Isaacs and Henze,
1994; Pochana and Keller, 1999a; Insel et al., 2003).
0
2
4
6
8
10
12
14
16
0 2 4 6 8 10 12
Eff
luen
t N
O3�N
(m
g/L
)
Influent BOD:TKN Ratio (mg BOD5/mg TKN as N)
(A) Winter Haven WWTP, City of Winter Haven, Florida
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20
Eff
luen
t N
O3�N
(m
g/L
)
Influent rbCOD:N Ratio (mg COD/mg N)
(B) Pilot Plant, Marrero, Louisiana
Figure 4. Effect of available organic carbon on the effluent nitrate during SND activity
The control of bulk DO concentration in the system is an essential part of achieving a higher
degree of SND. It is well known that denitrification rates are the highest when the DO in bulk
liquid is close to zero (Lie and Welander, 1994). On the other hand, several studies have shown
that DO concentrations for nitrification should be higher than 1.0 mg/L with 2.0 mg/L often
recommended to achieve higher nitrification rates. Therefore, the success of the SND process
relies on the balance of the bulk DO concentration so both kinetic reactions, nitrification and
denitrification, can occur simultaneously in the same reactor. von Münch et al. (1996) found in
batch reactor experiments operated at 15 days SRT and approximately 20oC, that a bulk liquid
DO concentration of 0.5 mg/L was suitable to achieve a nitrification rate equal to the
denitrification rate, which would therefore lead to complete SND. Bliss and Barnes (1986) found
that a DO concentration of at least 0.2 mg/L was critical for nitrification. The graph presented in
Figure 5 shows the effect of bulk DO concentration on the overall SND activity at the Stuart
facility operating in SND mode. During the full�scale experiment, the bulk DO concentration in
the aerobic tank was manually modified to understand the impact of the DO concentration on the
overall SND process. During this experiment, the facility was operated at a constant SRT of 15
days and the influent total COD:N ratio ranged from 12 to 20. Based on the data collected at this
facility, an optimum bulk DO concentration ranging from approximately 0.3 mg/L to 0.7 mg/L
appears to maximize SND activity in the aerobic reactor. When the bulk DO concentration drops
below 0.3 mg/L, the SND performance is limited by nitrification; whereas, higher bulk DO
concentrations of 0.7 mg/L affect denitrification. However, it should be noted that these bulk DO
levels are dependent on the floc size in the system; therefore, it is difficult to draw a general
conclusion with these data. Larger floc size might require higher bulk DO levels than smaller
flocs due to the mass�transfer limitations within activated sludge flocs.
0
5
10
15
20
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Eff
luen
t Q
uali
ty (
mg
/L)
Bulk DO Concentration (mg/L)
NH3�N NO3�N
Figure 5. Effect of bulk dissolved oxygen concentration on SND activity at the City of
Stuart WWTP, Florida
Low DO environments required for SND processes are conventionally considered more
susceptible to sludge bulking, primarily because of the excessive growth of filamentous bacteria
(Jenkins et al., 2003; Martin et al., 2004). This has been considered one of the main
disadvantages for SND processes. Many facilities being operated in SND mode produce mixed
liquor with marginal settling characteristics as presented in Table 1. Conventional BNR facilities
equipped with anaerobic or anoxic selectors often produce SVI values (90 percentile) of less than
120 and 150 mL/g (Parker et al., 2004). However, in the case of the selected SND facilities listed
in Table 1, SVI values (90 percentile) ranging from 160 to 245 mL/g (with selectors) and 180 to
300 mL/g (without selectors) have been recorded.
In many instances, severe low DO bulking conditions in many SND facilities have limited the
applicability of these processes due to their negative effects on the overall secondary clarification
capacity of the plants. This has driven many SND plants to convert to more conventional BNR
processes with high DO levels in the aeration tanks. Table 3 presents effluent TN and SVI data
from three facilities converted from SND to conventional BNR facilities. The main process
upgrade to the facilities listed in Table 3 included additional aeration capacity in the main
aerobic reactor to increase the bulk DO concentration to limit the growth of filamentous bacteria.
Based on 2008�2009 data, the conventional BNR mode of operation at the Mandarin and Smith
Creek facilities has led to a slightly decrease in the overall effluent TN removal. In the case of
the Winter Haven facility, higher TN removal efficiencies were achieved as a result of adding
external supplemental carbon to increase the post denitrification in the Bardenpho process.
Despite the slightly lower TN removal efficiencies at the Mandarin and Smith Creek plants, the
sludge settling characteristics improved tremendously, which allowed rerating of the facilities
without additional tankage requirements.
SND limited by
denitrification
SND limited by
nitrification Optimum SND
DO Regime
Table 3. Total nitrogen and SVI data comparison for SND plants upgraded to conventional
BNR facilities
Treatment Plant SND Operation Conventional Operation
1
TN Removal (%) SVI (mL/g) TN Removal (%) SVI (mL/g)
Mandarin 90 180 86 100
Smith Creek 90 245 82 110
Winter Haven 93 190 952 120
1 Average data based on 2008�2009 data
2 Upgrades included supplemental carbon addition for denitrification
Figure 6 presents long term SVI data from the City of Stuart facility. SVI data from January
2004 through December 2006 include operating SVI data when the facility was operating in
SND mode with average and 90�percentile SVI values of 212 and 350 mL/g, respectively due to
excessive growth of filamentous bacteria. As a result of the extremely high SVI data produced at
the Stuart SND facility, the treatment plant was upgraded in 2007 to incorporate anaerobic
selectors and to increase the aeration capacity of the process with the goal of improving the
settling characteristics of the mixed liquor to restore plant capacity. Figure 6 presents the SVI
data after the plant’s upgrade with average and 90�percentile SVI values of 90 and 110 mL/g,
respectively.
0
50
100
150
200
250
300
350
400
450
1/1/2004 1/1/2005 1/1/2006 1/1/2007 1/1/2008 1/1/2009
SV
I (m
L/g
)
Date
Construction
Period
A/O Process � Anaerobic Selector
and New Fine�bubble aeration
SND Process � Extended Aeration with
Mechanical Aerators
Figure 6. Long term SVI data at the City of Stuart WWTP, Florida
Data provided by the Winter Haven WWTP (City of Winter Haven, Florida) was used to explore
the effect of DO concentration on the settling conditions of the mixed liquor. Long term
operating data from this facility was reviewed and is presented in Figure 7. To characterize the
settling conditions of the mixed liquor, SVI measurements were collected three times per day in
a 2�liter settlelometer. During the full�scale trial, two identical trains were operated in parallel
under similar operating conditions (influent flows and loads, SRT of 8.5 days and MLSS of
3,300 mg/L); however, Train 1 was operated at DO concentrations slightly higher (0.6 mg/L to
0.95 mg/L) than Train 2 (0.25 mg/L to 0.5 mg/L). Figure 7 presents the SVI data collected at the
facility and compares the settling characteristics from both trains. Overall, Train 1 produced
considerably higher SVI values than Train 2, despite both treatment trains experiencing SND
activity. Based on the difference on the quality of the settling conditions between both trains,
plant personnel selected to implement DO control in both treatment trains, resulting in lowering
the SVI conditions in Train 1. These results seem to indicate that DO values ranging from 0.25 to
0.5 mg/L appear to reduce the propensity for low DO bulking due to excessive growth of
filamentous bacteria. However, it is important to mention these results are inconsistent with the
other data presented in Table 1 and Figure 6. One possibility could be that the lower DO
environment in the SND reactor is acting as an anoxic selector improving the settling conditions
of the sludge. Another possibility could be the growth of phosphorus accumulating organisms
(PAOs) at the low DO environment which have been demonstrated to significantly improve
mixed liquor settling rates (Schuler and Jang, 2007). However, this needs to be confirmed
further with additional research and microscopic analysis of the biomass samples since these
results are unique and different than the other SND facilities included in this study.
0
50
100
150
200
250
300
3/16/07 4/16/07 5/16/07 6/16/07 7/16/07 8/16/07 9/16/07
Slu
dg
e V
olu
me
Ind
ex (
mL
/g)
Date
Figure 7. Effect of bulk dissolved oxygen concentration on SND activity at the Winter
Haven WWTP, Florida
CONCLUSIONS
SND is a well�known phenomenon in BNR activated sludge systems and is a point of interest for
designers and operators of wastewater treatment plants as it may have potential advantages over
conventional nitrogen removal systems. SND facilities have been shown to meet very low
effluent total nitrogen levels without additional carbon and alkalinity. Data from SND facilities
indicate that 80 to 96 percent total nitrogen removal can be realized. This paper addressed the
impact of two main factors on the performance of SND plants, organic carbon and dissolved
Train 1 – Operating DO
from 0.6 to 0.95 mg/L
Train 2 – Operating DO from
0.25 to 0.5 mg/L
Train 1 – DO Control
Implemented. Operating
DO from 0.25 to 0.5 mg/L
Train 2 – Operating DO from
0.25 to 0.5 mg/L
oxygen. Data presented herein suggest that a COD:N ratio of at least 10.0 is required to
maximize denitrification; and an optimum bulk DO concentration ranging from approximately
0.3 mg/L to 0.7 mg/L appears to maximize SND activity in the aerobic reactor. However, this
low DO condition required for SND provides an environment more susceptible to sludge bulking
as observed by the SVI data from the selected SND facilities.
SVI data presented in this study suggest that excessive growth of filamentous bacteria, resulting
in bulking sludge, is the main disadvantage on SND processes. This has created capacity issues
in many SND facilities and as a result, some of them have upgraded to conventional BNR
configuration without sacrificing performance as it is the case for the Mandarin and Smith Creek
WWTPs.
SND activity was evaluated in a continuous�flow aerobic activated sludge pilot plant treating
synthetic wastewater. The pilot plant revealed BNR in excess of that required for biomass
synthesis, including biological phosphorus removal with acetate addition, which has been rarely
documented in single stage aerated reactors.
Even though SND processes have some advantages over conventional BNR facilities, the
application of such processes may be limited by several factors, including:
• Relatively high influent COD:N ratio required for SND.
• SND processes may require larger reactors for nitrification than conventional BNR
processes because of the low DO and its impact on nitrification rates.
• Sludge bulking issues due to the excessive growth of filamentous bacteria. This would
limit it applicability in plants with limited secondary clarification capacity.
• The operator has limited control over important parameters impacting SND in the plant,
such as floc size, internal storage of COD and the DO profile within the flocs.
ACKNOWLEDGEMENTS
The authors thank the owners and operators of the wastewater treatment facilities that provided
the data included during this study. Special thanks are extended to Mr. Brian Smith (City of
Orlando, Florida), Mr. Tim Madanagopal (Orange County, Florida), Mr. Dwayne Phillips (City
of Stuart, Florida), Mr. Terry Carver (City of Winter Haven, Florida) and Mr. DuWayne Potter
(St. Mary’s County, Maryland). Pilot plant data used during this study was collected in
connection with graduate studies by Jose Jimenez and supervised by Dr. Enrique La Motta at the
University of New Orleans, New Orleans, Louisiana.
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