OutlineOutline

•• IntroductionIntroduction

•• Membrane IssuesMembrane Issues

•• Other IssuesOther Issues

OutlineOutline

•• IntroductionIntroduction

•• Membrane IssuesMembrane Issues

•• Other IssuesOther Issues

––AerationAeration

––Foam and ColloidsFoam and Colloids

––PretreatmentPretreatment

––Research needsResearch needs

Aeration

For Biomass

Oxygen TransferOxygen Transfer

•• Most commonly cited problem for operating Most commonly cited problem for operating at high MLSS conditionsat high MLSS conditions

–– Really because this issue is a still a Really because this issue is a still a design engineerdesign engineer’’s concerns concern

–– Membrane risks or issues are transferred Membrane risks or issues are transferred to manufacturer to manufacturer

•• Currently, aeration basin designs for Currently, aeration basin designs for MBRsMBRsare generally between 8 and 10 g/Lare generally between 8 and 10 g/L

•• Why 8 to 10 g/L and not 20 to 30 g/L?Why 8 to 10 g/L and not 20 to 30 g/L?

•• Because there is great concern and Because there is great concern and discussion about the alpha factor for MBR discussion about the alpha factor for MBR designsdesigns

DefinitionsDefinitions• Oxygen transfer rate (OTR) represents

the transfer rate into water

• OTR depends on the mass transfer

coefficient kLa and the concentration

difference:

• αααα factor relates kLa sludge/ kLa cleanwater

• ββββ factor relates C*sludge/C*cleanwater

• αααα factor is main concern, although this is still a very active area of MBR

research

OTRcleanwater

= kLa C

* −C( )

Wide Variation in Wide Variation in αααααααα ValuesValues

Adapted from Germain et al., 2007 Water Research

MLSS concentrationsMLSS concentrations

below 10 g/L helps to eliminatebelow 10 g/L helps to eliminate

concerns with low concerns with low αααααααα valuevalue

Not The Whole Story!Not The Whole Story!

•• We do not understand what impacts We do not understand what impacts αααααααα yetyet–– Wastewater and mixed liquor constituents are Wastewater and mixed liquor constituents are extremely importantextremely important

–– Common for ASP to have Common for ASP to have αααααααα values vary along the values vary along the length of a plug flow reactor (increasing length of a plug flow reactor (increasing αααααααα with with

increasing length of the reactor)increasing length of the reactor)

•• Researchers have only been able to correlate Researchers have only been able to correlate αααααααα with MLSS concentration to datewith MLSS concentration to date–– But there is a very wide variation for the same But there is a very wide variation for the same MLSS concentrationMLSS concentration

•• Continued research will be help define Continued research will be help define important factors contributing to important factors contributing to αααααααα and and thus, help rethus, help re--define MLSS designsdefine MLSS designs

Dissolved Oxygen LevelsDissolved Oxygen Levels

•• Some manufacturers market a lower reactor Some manufacturers market a lower reactor DO concentrationDO concentration–– An attempt to balance out the extra air costs An attempt to balance out the extra air costs for the membranesfor the membranes

–– There are negative impacts to low DO conc.There are negative impacts to low DO conc.

•• There is some literature that indicates DO There is some literature that indicates DO concentrations impact membrane foulingconcentrations impact membrane fouling–– Higher DO leads to a lower sludge fouling Higher DO leads to a lower sludge fouling propensity (Kang et. Al, 2003)propensity (Kang et. Al, 2003)

–– Anaerobic conditions are known to Anaerobic conditions are known to deflocculatedeflocculate((WilenWilen et. Al, 2004)et. Al, 2004)

–– Anoxic conditions have a higher fouling Anoxic conditions have a higher fouling proprensityproprensitythan aerobic (Jang et. Al, 2006)than aerobic (Jang et. Al, 2006)

•• Low DO concentrations encourage bulking Low DO concentrations encourage bulking filamentsfilaments

Aeration for

Membranes

Oxygen TransferOxygen Transfer

•• High Oxygen Transfer Efficiency High Oxygen Transfer Efficiency

–– Desired for cost effectively providing Desired for cost effectively providing dissolved oxygen to biomass dissolved oxygen to biomass

–– Requires small bubbles for high Requires small bubbles for high KKLLaa

•• Coarse bubble aeration used by most MBR Coarse bubble aeration used by most MBR manufacturers has 50% the OTE of fine airmanufacturers has 50% the OTE of fine air

•• US FilterUS Filter’’s s JetTechJetTech technology should technology should allow improved oxygen transfer allow improved oxygen transfer -- getting getting close to the OTE of fine air (75close to the OTE of fine air (75--80%)80%)–– But you lose the effectiveness of coarse bubble But you lose the effectiveness of coarse bubble air scourair scour

•• Bubble rise velocity, UBubble rise velocity, ULL, is proportional , is proportional to its sizeto its size–– But Large bubbles provide better lift and more But Large bubbles provide better lift and more effectlyeffectly resuspendresuspend rejected materialrejected material

RM

RF

RC

––JssJss = to membrane= to membrane

––VVLL = away from membrane= away from membrane

––JssJss ≥≥ VVL L (rapid fouling)(rapid fouling)

Operating ConditionsOperating Conditions

Reactor MCRT, d HRT, h

Target

MLSS, g/L G Value, 1/s

2-5

1.1-3.6SMBR

CMAS Lo

CMAS Hi

63282-10

2-10 8.0 1-3 72

8.3 1-3 250

G =P

Vµ

0.0

0.1

0.2

0.3

0.4

0.5

0.6

2-4 4-6 6-8 8-10 10-20 20-40 40-100 100-2000

Fre

qu

ency

CMAS Lo

Particle Size DistributionParticle Size Distribution-- CMAS Lo vs. SMBRCMAS Lo vs. SMBR

CMAS Lo

SMBR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

2-4 4-6 6-8 8-10 10-20 20-40 40-100 100-2000

Characteristic Length, µµµµm

SMBR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

2-4 4-6 6-8 8-10 10-20 20-40 40-100 100-2000

Fre

quen

cy

CMAS Hi

Particle Size DistributionParticle Size Distribution-- CMAS Hi vs. SMBRCMAS Hi vs. SMBR

0.0

0.1

0.2

0.3

0.4

0.5

0.6

2-4 4-6 6-8 8-10 10-20 20-40 40-100 100-2000

Characteristic Length, µµµµm

SMBR

Colloidal MaterialColloidal Material

0

10

20

30

40

50

60

70

80

2 3 4 5

MCRT, d

CMAS Hi

CMAS Lo

SMBR

Activated Sludge DispersionActivated Sludge Dispersion

Extracellular Polymeric Substances

0

50

100

150

200

250

300

350

400

2 3 4 5 10

MCRT, d

CMAS Hi

CMAS Lo

SMBR

FilamentousMicroorganisms

CMAS Hi CMAS Lo SMBR

Total Filamentous Microorganisms

(intersections/gTSS) 8.1x105

2.2x106

1.9x107

Nocardioform Fraction 13% 26% 85%

Sludge Properties Sludge Properties ConclusionsConclusions

•• Particle Size Distribution (2Particle Size Distribution (2--2000 2000

micron)micron)

–– CMAS Lo had a bimodal distributionCMAS Lo had a bimodal distribution

–– CMAS Hi and SMBR had similar distributionsCMAS Hi and SMBR had similar distributions

•• Colloidal Material (<10 micron)Colloidal Material (<10 micron)

–– Higher levels in SMBRHigher levels in SMBR

•• Extracellular Polymer SubstancesExtracellular Polymer Substances

–– Lower levels in SMBRLower levels in SMBR

•• Filamentous MicroorganismsFilamentous Microorganisms

–– Higher levels in SMBRHigher levels in SMBR

Biological Foaming Biological Foaming -- NocardioformNocardioform

Illustration of Mechanical Foam Illustration of Mechanical Foam ControlControl

ML Recycle Line

ML Recycle

Pump

Membrane

TankAeration

Tank

Coarse

Aeration On

Coarse

Aeration OffIntermittent

Aeration

(10s on/10s off)

D = 0.787 m

D = 0.1 m

Foam

Foam

Foam

Foam

Liquid

Liquid

Liquid

Liquid

Foaming EventsFoaming Events

Sprayer SystemSprayer System

Biological FoamingBiological Foaming

•• Much more than a process Much more than a process ““nuisancenuisance””

–– Removes biomass from the reactor Removes biomass from the reactor

–– Dramatically shifts F/M in the reactorDramatically shifts F/M in the reactor

–– Often correlates with membrane foulingOften correlates with membrane fouling

•• MBRsMBRs are the perfect trapping environmentare the perfect trapping environment–– Recommend ALL sludge wasting be done from the Recommend ALL sludge wasting be done from the surfacesurface

–– Only place foaming filaments can survive is on Only place foaming filaments can survive is on the surfacethe surface

•• Membrane tanks are particularly prone to Membrane tanks are particularly prone to biological foaming due to the aerationbiological foaming due to the aeration–– Recommend overflowing the membrane tank Recommend overflowing the membrane tank continuouslycontinuously

•• Controlling foaming filaments by surface Controlling foaming filaments by surface wasting does not control bulking filamentswasting does not control bulking filaments

Pretreatment

PretreatmentPretreatment•• Adequate pretreatment is essential Adequate pretreatment is essential

–– Protects the membranes from damage Protects the membranes from damage

–– Protects the membrane from Protects the membrane from requiring manual cleaningrequiring manual cleaning

–– Allows membrane fouling to be Allows membrane fouling to be controlled as designedcontrolled as designed

•• In 1999, only 3 mm screens required In 1999, only 3 mm screens required for all for all MBRsMBRs

•• Now, MBR manufacturers are Now, MBR manufacturers are attempting to separate themselves attempting to separate themselves from the others by claiming reduced from the others by claiming reduced screenings requirements (2 screenings requirements (2 vsvs 3 mm)3 mm)

PretreatmentPretreatment•• Problem remains for all Problem remains for all -- this inert material this inert material

(hair, fibers, and grit) will accumulate in the (hair, fibers, and grit) will accumulate in the reactorreactor

–– Grit will settle at the bottom below the air Grit will settle at the bottom below the air distributors and form pilesdistributors and form piles

•• Best operational results are obtained with high Best operational results are obtained with high quality pretreatment (0.8 mm)quality pretreatment (0.8 mm)

–– Minimizes handling of the membranes and exposure to Minimizes handling of the membranes and exposure to cleaning/repair procedurescleaning/repair procedures

•• The need for such advanced screening has The need for such advanced screening has significant negativessignificant negatives

–– Labor intensiveLabor intensive

–– Require a lot of washing and compacting of this materialRequire a lot of washing and compacting of this material

–– Not pleasant to deal withNot pleasant to deal with

•• When it makes economic sense (> 5 MGD), primary When it makes economic sense (> 5 MGD), primary treatment is highly recommendedtreatment is highly recommended

–– Reduce the load on screenings Reduce the load on screenings -- eases operationeases operation

–– Prescreen becomes insurance policyPrescreen becomes insurance policy

PretreatmentPretreatment•• The standard prescreen type that is proven The standard prescreen type that is proven

successful are:successful are:

–– MeshMesh

–– Punch holePunch hole

•• Its imperative that there is no bypass or Its imperative that there is no bypass or opportunity for solids carryoveropportunity for solids carryover

•• Also imperative that the screen loading not be Also imperative that the screen loading not be too aggressivetoo aggressive–– Frequently results in the facility backing up sewage Frequently results in the facility backing up sewage and spillsand spills

•• Most commonly employed prescreen devices areMost commonly employed prescreen devices are–– Internally fed rotating drum screenInternally fed rotating drum screen

–– In channel rotating drum screenIn channel rotating drum screen

–– Rotating brush screenRotating brush screen

–– Traveling band screenTraveling band screen

Research Needs Research Needs

So much progress, but so much left to do!So much progress, but so much left to do!

Research NeedsResearch Needs•• Means of improving and controlling sludge Means of improving and controlling sludge

filterabilityfilterability

–– Sludge properties completely control foulingSludge properties completely control fouling

–– Investigate more thoroughly the benefits of Investigate more thoroughly the benefits of chemical additiveschemical additives

–– Focus on biological reactor design as wellFocus on biological reactor design as well

–– What are the measures of filterability? How do What are the measures of filterability? How do we measure filterabilitywe measure filterability

–– Although we donAlthough we don’’t need the sludge to settle, we t need the sludge to settle, we do need it to filterdo need it to filter

•• Improve our understanding of how to control Improve our understanding of how to control ααααααααvalues for aeration designsvalues for aeration designs

–– Are we using all of the tools we have availableAre we using all of the tools we have available

–– This commonly cited as the reasons This commonly cited as the reasons MBRsMBRs cannot cannot be more compact or carry higher MLSS be more compact or carry higher MLSS concentrationsconcentrations

–– I believe that Praxair and other companies like I believe that Praxair and other companies like this are focusing on this issue and will make this are focusing on this issue and will make good progressgood progress

Research NeedsResearch Needs•• Energy reduction designsEnergy reduction designs

–– Air scouring is imperative in the high solids Air scouring is imperative in the high solids environment of environment of MBRsMBRs

–– To date, this is what prevents conventional To date, this is what prevents conventional wastewater utilities from really embracing wastewater utilities from really embracing MBRsMBRs

–– ZenonZenon has made significant strides in this respect, has made significant strides in this respect, but total aeration demand is 75% of the energy for an but total aeration demand is 75% of the energy for an MBR and about 1/2 of that air is for the membranesMBR and about 1/2 of that air is for the membranes

–– Minimize shear and Minimize shear and flocfloc breakagebreakage

–– Koch Koch PuronPuron also has some unique ideas on how to also has some unique ideas on how to minimize membrane air requirementsminimize membrane air requirements

•• Management of peak flowsManagement of peak flows

–– Important for MBR manufacturers to be honest about Important for MBR manufacturers to be honest about the significance of this issuethe significance of this issue

–– Potentially, when a peak membrane flux is required, Potentially, when a peak membrane flux is required, the system could not maintain for even 1 hthe system could not maintain for even 1 h

–– Need to think about possible membrane related Need to think about possible membrane related solutionssolutions

Thank you Thank you