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OutlineOutline
•• IntroductionIntroduction
•• Membrane IssuesMembrane Issues
•• Other IssuesOther Issues
OutlineOutline
•• IntroductionIntroduction
•• Membrane IssuesMembrane Issues
––General foulingGeneral fouling
––Impact of SRT or F/MImpact of SRT or F/M
––Impact of MLSSImpact of MLSS
––Impact of Impact of flocfloc structurestructure
•• Other IssuesOther Issues
Principle Disadvantages of Principle Disadvantages of MBR ProcessMBR Process
•• Hydraulic Hydraulic
throughputthroughput
•• Membrane Membrane
foulingfouling
•• High MLSS High MLSS
concentrations concentrations
can result in can result in
rapid foulingrapid fouling
Some Perspective on High MLSS Some Perspective on High MLSS Concentrations in Concentrations in MBRsMBRs
•• Yamamoto et al., 1989 published the Yamamoto et al., 1989 published the
first paper on the SMBR configurationfirst paper on the SMBR configuration
•• Yamamoto, operation was possible up to Yamamoto, operation was possible up to
MLSSMLSSmaxmax= 40g/L= 40g/L
•• Ross et al., Ross et al., 1990 showed 1990 showed that that MLSSMLSSmaxmax→→→→→→→→60 g/L for an 60 g/L for an EMBREMBR
•• So, why are we So, why are we at 8 to 10 g/L?at 8 to 10 g/L?
Membrane TheoryMembrane Theory•• Membrane flux controls the rate of Membrane flux controls the rate of
material transported to the membrane material transported to the membrane
surface, Jsurface, JSSSS
•• The shear force controls the rate at The shear force controls the rate at
which rejected material is which rejected material is
resuspendedresuspended to the bulk solution, Vto the bulk solution, VLL
•• Simple modelSimple model
J =∆P
ηW ⋅RT
RM
RF
RC
––JssJss = to membrane= to membrane
––VVLL = away from membrane= away from membrane
––JssJss ≥≥ VVL L (rapid fouling)(rapid fouling)
Sustainable FluxSustainable Flux
•• Normal steady state operationNormal steady state operation
–– JssJss ≤≤ VVLL–– i.e. Operating at a sustainable fluxi.e. Operating at a sustainable flux
•• For a given flux and aeration rateFor a given flux and aeration rate
––Increasing the MLSS Increasing the MLSS
increases increases JssJss
––Increasing the MLSS Increasing the MLSS
reduces Vreduces VLL
––So, increasing the So, increasing the
MLSS has two MLSS has two
compounding effects compounding effects
that increase Rthat increase RCC
Effect of High MLSSEffect of High MLSS
•• Conditions that Conditions that
result in the result in the
accumulation of Raccumulation of RCC
–– Increase MLSSIncrease MLSS
–– Increase flux Increase flux
((JssJss))
–– Decrease VDecrease VLL»» Change in viscosityChange in viscosity
»» Reduced aeration Reduced aeration
intensity (plugged intensity (plugged
ports)ports)
•• Generally, high MLSS results in Generally, high MLSS results in membrane membrane ““sludgingsludging”” due to due to excessive Rexcessive RCC
•• Common claim or differentiator between Common claim or differentiator between
manufacturers is, manufacturers is, ““Our system can operate at higher Our system can operate at higher
MLSS concentrations than X can.MLSS concentrations than X can.””
•• Nobody is defying physics and the basic principles Nobody is defying physics and the basic principles
outlined in this talkoutlined in this talk
•• Either one of the following things is occurring:Either one of the following things is occurring:
–– The system has a higher aeration intensity (e.g. total aeration The system has a higher aeration intensity (e.g. total aeration
required for membranes/total membrane area)required for membranes/total membrane area)
–– The system is at a reduced flux rate (e.g. membrane The system is at a reduced flux rate (e.g. membrane
thickeners)thickeners)
•• Specific system hydraulics cannot totally be neglected, Specific system hydraulics cannot totally be neglected,
but generalizations can be madebut generalizations can be made
Operating at Higher MLSS Operating at Higher MLSS Is PossibleIs Possible
Experiments to Understand
High MLSS Fouling
Equipment and ApparatusEquipment and Apparatus
•• PilotPilot--scale SMBRscale SMBR
•• Capacity of 11,000 Capacity of 11,000
gpdgpd or 43 mor 43 m33/d/d
•• Treating primary Treating primary
effluent from the effluent from the
City of San City of San
FranciscoFrancisco’’s SEPs SEP
–– COD = 325 mg/LCOD = 325 mg/L
–– TSS = 98 mg/LTSS = 98 mg/L
Membrane Operation and Membrane Operation and CharacteristicsCharacteristics
•• Zenon 500C ModuleZenon 500C Module
•• Pore sizePore size
–– Nominal = 0.035 Nominal = 0.035 µµµµµµµµmm
–– Absolute = 0.1 Absolute = 0.1 µµµµµµµµmm
•• HydrophilicHydrophilic
•• Membrane fluxMembrane flux
30 L/m30 L/m2.2.h or 18 gfdh or 18 gfd
•• Coarse bubble airCoarse bubble air
14 L/s or 30 scfm14 L/s or 30 scfm
•• Intermittent aerationIntermittent aeration
•• 9 min operating cycle9 min operating cycle
with 30 sec relaxwith 30 sec relax
Experimental MethodsExperimental Methods
•• ShortShort--term and Longterm and Long--term experimentsterm experiments
•• ShortShort--termterm
–– Achieved steady state condition at Achieved steady state condition at MCRTsMCRTs of 10, 20 of 10, 20
and 30 d (MLSS = 12and 30 d (MLSS = 12--15 g/L)15 g/L)
–– Performed MLSS thickening experiments at constant Performed MLSS thickening experiments at constant
flux (30 LMH or 18 gfd)flux (30 LMH or 18 gfd)
»» Monitored membrane permeability, MLSS concentration and Monitored membrane permeability, MLSS concentration and
ML propertiesML properties
–– Shut off influent 15 minutes prior to experimentShut off influent 15 minutes prior to experiment
–– Performed 1 experiment per dayPerformed 1 experiment per day
–– Performed the experiment with different coarse bubble Performed the experiment with different coarse bubble
aeration rates (20, 30, 40 scfm or 9.4, 14.2, 18.9 aeration rates (20, 30, 40 scfm or 9.4, 14.2, 18.9
L/s)L/s)
Experimental MethodsExperimental Methods
•• LongLong--termterm
–– Used the previously achieved steady Used the previously achieved steady
state condition as starting points state condition as starting points
»» MCRTsMCRTs of 10, 20 and 30 dof 10, 20 and 30 d
»» MLSS 12MLSS 12--15 g/L15 g/L
–– Ceased ML wastingCeased ML wasting
–– Continue operationContinue operation
»» constant flux = 30 LMH (18 gfd)constant flux = 30 LMH (18 gfd)
»» Coarse bubble air = 14 L/s (30 scfm)Coarse bubble air = 14 L/s (30 scfm)
–– Monitored membrane performance, MLSS Monitored membrane performance, MLSS
concentration and ML propertiesconcentration and ML properties
Experimental MethodsExperimental Methods
•• ML ViscosityML Viscosity
–– Brookfield Rotational Brookfield Rotational Viscometer (LV2 Viscometer (LV2 spindle)spindle)
•• Additional ML Additional ML PropertiesProperties
–– Soluble CODSoluble COD
–– Soluble Microbial Soluble Microbial Products (SMP)Products (SMP)
–– Extracellular Extracellular Polymeric Substances Polymeric Substances (EPS)(EPS)
–– Colloidal materialColloidal material
Membrane PerformanceMembrane Performance
0
10
20
30
40
50
60
70
80
90
100
0 50 100 150 200
Days of Operation
0
10
20
30
40
50
60
70
80
90
100
Flux, LMH
Specific Flux @ 20C Flux
10-d MCRT 20-d MCRT 30-d MCRT
Long-term
Experiments
Long-term
Experiment
Poor Sludge Filterability
Unable to Maintain Flux
at 30 LMH
Mixed Liquor ViscosityMixed Liquor Viscosity
y = 14676x-
0.7747
y = 2406.6x-
0.7019
y = 4617x-
0.6733
y = 6788.4x-
0.6958
10
100
1000
10000
100000
0.1 1 10 100
Spindle Speed, rpm
14.4 g/L 16.4 g/L 18.4 g/L 20.2 g/L
Mixed Liquor ViscosityMixed Liquor Viscosity
y = 6.357e0.1726x
R2 = 0.9393
y = 5.3776e0.2142x
R2 = 0.9348
y = 7.4013e0.1656x
R2 = 0.9885
0
100
200
300
400
500
600
700
10 12 14 16 18 20 22 24 26 28
MLSS, g/L
10-d MCRT 20-d MCRT 30-d MCRT
LV Spindle #2, 100 rpm
10-d MCRT:
20-d MCRT:
30-d MCRT:
Eiler’s EquationEilerEiler’’s Equations Equation
ηη o
= 1 +
η i
2φ
1 −φ
εMax
2
Where,
η = viscosity of suspension
ηο = viscosity of pure suspending fluid
ηI = intrinsic viscosity
φ = volume fraction≈MLSS/MLSSmax
εMax = maximum packing density
Eiler’s Equation vs. MLSSEilerEiler’’s Equation vs. MLSSs Equation vs. MLSS
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30
MLSS, g/L
MLSS max =40 g/L
MLSS max =120 g/L
MLSS max =80 g/L
MLSS max =60 g/L
MLSS max =50 g/L
ShortShort--term term -- 20 scfm20 scfm
0.5
0.6
0.7
0.8
0.9
1.0
1.1
10 12 14 16 18 20 22 24 26 28
MLSS, g/L
10-d MCRT 20-d MCRT 30-d MCRT
Coarse Airflow Rate = 20 scfm (9.4 L/s)
ShortShort--term term -- 30 scfm30 scfm
0.5
0.6
0.7
0.8
0.9
1.0
1.1
10 12 14 16 18 20 22 24 26 28
MLSS, g/L
10-d MCRT 20-d MCRT 30-d MCRT
Coarse Airflow Rate = 30 scfm (14.2 L/s)
ShortShort--term term -- 40 scfm40 scfm
0.5
0.6
0.7
0.8
0.9
1.0
1.1
10 12 14 16 18 20 22 24 26 28
MLSS, g/L
10-d MCRT 20-d MCRT 30-d MCRT
Coarse Airflow Rate = 40 scfm (18.9 L/s)
ShortShort--term term -- 1010--d MCRTd MCRT
0.5
0.6
0.7
0.8
0.9
1.0
1.1
10 12 14 16 18 20 22 24 26
MLSS, g/L
20 scfm 30 scfm 40 scfm
MCRT = 10 d
Importance of ViscosityImportance of Viscosity
0.5
0.6
0.7
0.8
0.9
1.0
1.1
0 50 100 150 200 250 300 350 400 450
Viscosity, mPa.s
1.5E-4 m/s 2.3E-4 m/s 3.0E-4 m/s
Critical Flux IllustrationCritical Flux Illustration
•• Normal steady state operationNormal steady state operation
–– JssJss ≤≤ VVLL–– i.e. Operating below critical fluxi.e. Operating below critical flux
•• ShortShort--term experimentsterm experiments
––JssJss increased as MLSS increased as MLSS
increasedincreased
––Membrane permeability Membrane permeability
varied with MLSS Conc.varied with MLSS Conc.
––Strong function of Strong function of
VVLL, not , not JssJss
Mixed Liquor ViscosityMixed Liquor Viscosity
Adapted from Cui et al., 2003 Journal of Membrane Science
Membrane Permeability RecoveryMembrane Permeability Recovery
Percent Post Experiment
Initial After Change Recovery Time, min
9.4 70.4 62.4 11% 0
9.4 same 69.7 1% 180
14.2 69.6 59.3 15% 5
14.2 same 68.6 1% 120
18.9 71.1 67 6% 2
18.9 same 70.5 1% 180
9.4 66.2 66.7 -1% 0
14.2 65.5 67 -2% 180
18.9 65.4 64.4 2% 0
9.4 79.8 75.7 5% 0
14.2 81.3 80.5 1% 0
18.9 77.2 76.4 1% 0
30
20
Coarse
Aeration Rate,
L/s
Specific Flux@20oC, LMH/bar
MCRT, d
10
•• No chemical cleans were performedNo chemical cleans were performed
•• Collected permeate and diluted ML Collected permeate and diluted ML to original MLSS concentrationto original MLSS concentration
Mixed Liquor PropertiesMixed Liquor Properties
1.5x10-4
2.3x10-4
3.0x10-4
1.5x10-4
2.3x10-4
3.0x10-4
1.5x10-4
2.3x10-4
3.0x10-4
t0 98 72 73 23 20 27 26 29 21
tF 86 77 - 27 32 23 46 38 48
Total SMP 29 45 105 41 30 46 33 32 35
SMPc 12 26 26 19 20 19 21 21 21
SMPp 17 19 79 22 10 27 12 11 14
SMP P/C - 1.4 0.7 3.0 1.2 0.5 1.4 0.6 0.5 0.7
Total SMP 45 53 166 45 35 48 33 35 39
SMPc 16 27 26 20 22 19 21 22 20
SMPp 29 26 140 25 13 29 12 13 19
SMP P/C - 1.8 1.0 5.4 1.3 0.6 1.5 0.6 0.6 1.0
Total EPS 81 87 102 115 104 114 103 103 105
EPSc 24 24 27 26 27 27 29 28 29
EPSp 57 62 74 88 77 87 74 75 76
EPS P/C - 2.4 2.6 2.7 3.4 2.8 3.2 2.6 2.6 2.6
t0 73 82 75 47 50 50 37 74 63
tF 101 115 113 70 62 57 71 59 92
t0
Colloidal
Material
tF
t0
Units
10-d MCRT
mg/g VSS
sCOD mg/L
mg/L
mg/L
NTU
Aeration Intensity, m/s
20-d MCRT 30-d MCRT
Sample*
Mixed
Liquor
Property
Mixed Liquor PropertiesMixed Liquor Properties
y = 4.1402x + 17.96
R2 = 0.7552
y = 1.79x + 35.777
R2 = 0.2364
y = 0.6978x + 48.644
R2 = 0.166
20
40
60
80
100
120
140
10 12 14 16 18 20 22 24 26 28
MLSS, g/L
10-d MCRT 20-d MCRT 30-d MCRT
LongLong--term Experimentsterm Experiments
0.5
0.6
0.7
0.8
0.9
1.0
1.1
10 15 20 25
MLSS, g/L
10-d MCRT 20-d MCRT 30-d MCRT
Coarse Airflow Rate = 30 scfm
What happened?What happened?•• LongLong--term experimentsterm experiments
–– JssJss is increasing with timeis increasing with time
–– VVLL is decreasing with time as coarse is decreasing with time as coarse
bubble aeration efficacy decreases with bubble aeration efficacy decreases with
increasing ML viscosityincreasing ML viscosity
•• Importance of TimeImportance of Time
–– Small difference Small difference between between JssJss and Vand VLLbecomes more important becomes more important with timewith time
––Slower MLSS thickening Slower MLSS thickening results in fouling at results in fouling at lower MLSS conc.lower MLSS conc.
Exposure Time to High MLSSExposure Time to High MLSS
0.5
0.6
0.7
0.8
0.9
1.0
1.1
10 12 14 16 18 20 22 24 26 28
MLSS, g/L
Short-term Exp Extended Short-term Exp No Wasting Exp
Data from 20-d and 30-d MCRTs
Aeration Intensity = 2.3E-4 m/s (14.2 L/s)
Membrane Flux = 30 L/(m2h)
Increasing Exposure
Time to High MLSS
Conditions
ConclusionsConclusions
•• High MLSS concentrations reduce High MLSS concentrations reduce
membrane permeabilitymembrane permeability
•• Increased coarse bubble aeration Increased coarse bubble aeration
rates improved permeabilityrates improved permeability
•• ML viscosity is important for ML viscosity is important for
backtransportbacktransport of rejected materialof rejected material
•• Exposure time to high MLSS is Exposure time to high MLSS is
importantimportant
ConclusionsConclusions
•• High MLSS concentrations reduce High MLSS concentrations reduce
membrane permeabilitymembrane permeability
•• Increased coarse bubble aeration Increased coarse bubble aeration
rates improved permeabilityrates improved permeability
•• ML viscosity is important for ML viscosity is important for
backtransportbacktransport of rejected materialof rejected material
•• Exposure time to high MLSS is Exposure time to high MLSS is
importantimportant
Questions?Questions?