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Water Environment-Membrane Technology Lab.Seoul National University
Let’s open the biological black-box in Membrane BioReactor :
MBR for the next generation
Director & Prof. Chung-Hak LEE,
Institute of Environmental Protection and SafetyWater Environment – Membrane Technology Lab.School of Chemical Engineering,
Seoul National University, KOREA
Water Environment-Membrane Technology Lab.Seoul National University
MBR : Worldwide Buisiness
~ thousands of installations of MBRs in the World
Vivendi (France), Memcore (USA)Zenon (Canada), Mitsubishi, Kuboda (Japan), etc..
~ 750 MBRs in Korea since last 6 years
Water Environment-Membrane Technology Lab.Seoul National University
Commercial MBR plants in Korea *2003(2002)
CROSSFLOWCROSSFLOWFILTRATIONFILTRATION
200(200(--))13(13)13(13)19961996PSFPSF(30,000Da)(30,000Da)--ZENIX ENG.ZENIX ENG.
TUBULAR TUBULAR MEMBRANEMEMBRANE
RUSSIARUSSIA
734(559)734(559)TOTALTOTAL
2,000(2,000)2,000(2,000)50(50)50(50)19951995PESPES(40,000Da)(40,000Da)
BIOSUFBIOSUFAQUATECHAQUATECHMEMBRATEKMEMBRATEK(SOUTH AFRICA)(SOUTH AFRICA)
350(250)350(250)20(2)20(2)20022002CPVCCPVC(0.25 )(0.25 )--PUREPURE
ENVIENVI--TECHTECH
PUREPUREENVIENVI--TECHTECH(KOREA)(KOREA)
4,000(900)4,000(900)68(67)68(67)19991999PolyolefinPolyolefin(0.4)(0.4)NIXNIX--MBRMBR
ZENIX ENG ZENIX ENG & JIN WOO & JIN WOO
ENV.ENV.PLATEPLATE
MEMBRANEMEMBRANE
YUASAYUASA(JAPAN)(JAPAN)
--101020022002PVDFPVDF(0.1(0.1--0.4)0.4)--RAPAHRAPAH
TECHTECH天津膜天社天津膜天社
((CHINA)CHINA)
--10(10)10(10)19981998PSFPSF(0.1)(0.1)
KIMAS KIMAS ⅠⅠ,,ⅡⅡKOLONKOLON
SKC,SKC,E.N.E.E.N.E.
(KOREA)(KOREA)
600(225)600(225)150(100)150(100)20022002PE PE (0.4)(0.4)--KMSKMSKMSKMS
(KOREA)(KOREA)
1,000(300)1,000(300)10(7)10(7)20002000PVDFPVDF(0.035)(0.035)
ZENOGEMZENOGEMSAESAE--HANHANZENONZENON(CANADA)(CANADA)
DEADDEAD--ENDENDFILTRATIONFILTRATION
4,000(1,400)4,000(1,400)403(300)403(300)19971997PEPE(0.4)(0.4)
SMAS & SMAS & HANTHANT
KEC &KEC &HECHEC
HOLLOWHOLLOWFIBERFIBER
MEMBRANEMEMBRANE
MRCMRC(JAPAN)(JAPAN)
FitrationFitrationModeMode
Highest Highest capacitycapacity
(M(M33/d)/d)Number of Number of InstallationInstallation
Starting Starting YearYear
MembraneMembraneMaterialMaterial
(PORE SIZE, (PORE SIZE, ㎛㎛ ))
Trade Trade markmarkCompanyCompanyModule typeModule typeMembraneMembrane
ManufacturerManufacturer
Water Environment-Membrane Technology Lab.Seoul National University
Biofouling
: Membranes in contact with the broth of activated sludge reactor will be colonized within short time by microorganisms, leading to the formation of a composite layer known as biofilm.
: Biofouling has restricted the widespread application of MBR,because i) it limits the maximum flux obtainable,
ii) it leads to substantial cleaning requirements, iii) it shortens membrane life time
Water Environment-Membrane Technology Lab.Seoul National University
Overview of factors leading to membrane biofouling
Environmental & Operating Factors
• Substrates• DO• Air flow rate &
bubble size• pH• Temperature• Growth mode
(attached or suspended)
• Growth phase (log or endogenous)
• Cyclic format in SBR
• etc
MicroorganismsEPSOrganic debrisInorganic particlesIons
Size
Shape
Density
Porosity
Stickiness
Membrane
Biofouling
( )
Biofilm composition and structure
Biofilmproperties( )
Bulk phase composition ( )
cR
fR
J
?
?
?
?
Water Environment-Membrane Technology Lab.Seoul National University
Biofilm in resistance in series model
η⋅++∆
=)( cfm RRR
PJ
Rm : Intrinsic Membrane Resistance
Rf : Fouling layer resistance (specific membrane-solute interactions, either by surface deposition or pore fouling) Bulk phase Compositions
Rc : Cake layer resistance Biofilm (e.g., floc properties)
.
Water Environment-Membrane Technology Lab.Seoul National University
Overview of factors leading to membrane biofouling
Environmental & Operating Factors
• Substrates• DO• Air flow rate &
bubble size• pH• Temperature• Growth mode
(attached or suspended)
• Growth phase (log or endogenous)
• Cyclic format in SBR
• etc
MicroorganismsEPSOrganic debrisInorganic particlesIons
Size
Shape
Density
Porosity
Stickiness
Membrane
Biofouling
( )
Biofilm composition and structure
Biofilmproperties( )
Bulk phase composition ( )
cR
fR
J
?
?
?
?
Water Environment-Membrane Technology Lab.Seoul National University
Highlights of the past on MBR
Raw Wastewater
Treated water
MF/UF(<0.2um)
Bioreactor
Black box ?Flux !!!(Engineering, Materials)
Water Environment-Membrane Technology Lab.Seoul National University
TMP profile of (a) MSBR with anoxic phase of 10 min. and (b) MSBR without anoxic phase.
0
5
10
15
20
25
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
Time (day)
TMP
(kPa
)
(a) (b)
anoxic fill aerobic filtration0 10min 3hrs 50min
10min 10min 2hrs 50min 50min
Cycle format
(a)(b)
Water Environment-Membrane Technology Lab.Seoul National University
Membrane Fouling
?
Membrane Fouling
PhysicochemicalApproach
HydrodynamicApproach
MicrobiologicalApproach
Flow regime: (side stream vs. submerged )Module type:(tubular, plate,hollow fiber)Critical flux
Membrane materialsMembrane surface modificationHybrid systemChemical additivesSurface chemistry
Water Environment-Membrane Technology Lab.Seoul National University
Membrane Fouling
Membrane Fouling
PhysicochemicalApproach
HydrodynamicApproach
MicrobiologicalApproach
Flow regime: (side stream vs. submerged )Module type:(tubular, plate,holow fiber)Critical flux
Biofilm formation mechanism(Quorum sensing, etc.)
Cell physiology& morphologyMicroorganism population dynamics
Membrane materialsMembrane surface modificationHybrid systemChemical additivesSurface chemistry
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )Cell physiology (Growth phase)Cell physiology (DO concentration)Cell physiology (Cycle Format in SBR)Cell physiology (Pump Shear)Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
Effect of Floc Morphology on Membrane Fouling
• Pin point floc (left)• Bulking sludge (right)• Normal activated sludge (center)
Water Environment-Membrane Technology Lab.Seoul National University
Effect of Floc Morphology on Membrane Fouling
0
10
20
30
40
50
60
70
80
90
100
1 2 3 4 5Concentration Factor
J/Ji
w(%
)
Normal Sludge
Pin point Sludge
Bulking Sludge
Fig. 3b Flux declines according to floc structures during ultrafiltration of activated sludge with PM30 membrane
Water Environment-Membrane Technology Lab.Seoul National University
Analysis of resistances in MBR
∆Pt; Pressure dropµ ; Fluid viscosityε ; PorosityL ; Cake layer thicknessψ ; sphericityk ; Kozeny constantdp ; Particle diameter
=mR f(Membrane Material, Pore size, etc.)
=cR f(Deposited microbial flocs, biofilm)
3
22
)1(6εε
ψ−
⋅
=
pdLk)( fcm
t
RRRPJ++
∆=µ
=fR f(Membrane Meterial, EPS, SMP, etc.)
Water Environment-Membrane Technology Lab.Seoul National University
N- limited Activated sludge
0
20
40
60
80
100
120
1 2 3 4 5
Conce ntration Factor
Nitroge n-limite d Activate d S ludge
Contro l Activate d S ludge
YM30 Membranes
Water Environment-Membrane Technology Lab.Seoul National University
EPS contents under different nutrient condition
SludgeType
Rm(1011×m-1)
Rc(1011×m-1)
Rf(1011×m-1)
Rt(1011×m-1)
Rc/Rt(%)
EPS(VS mg/g MLSS)
ControlN-limitation
1918
9757
12
11777
8374
245151
Water Environment-Membrane Technology Lab.Seoul National University
Effect of SRT on Membrane Flux
0
20
40
60
80
100
120
1 2 3 4 5 6
Co nc e ntra tio n Fa c to r
S R T = 33 d a yS R T = 8 d a yS R T = 3 d a y
XM30 Membranes
Water Environment-Membrane Technology Lab.Seoul National University
EPS content of activated sludge flocs at different SRT
SRT EPS content
Range(VS mg/g MLSS)
Average(VS mg/g MLSS)
3 days 234∼276 268
8 days 228∼273 244
33 days 187∼242 213
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )
Cell physiology (Growth phase)Cell physiology (DO concentration)Cell physiology (Cycle Format in SBR)Cell physiology (Pump Shear)Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
Addition of PAC in MBR
Operation time (day)0 4 8 12 16 20 24
Suc
tion
pres
sure
( -c
mH
g)
0
5
10
15
20
25
30
Activated SludgeBAC Sludge
• Filtration PE 0.1 µm, 0.056 m2
Flux 15 L/m2/hrair flow : 2.0 L/min
• Bioreactor MLSS: 6,000~10,000HRT 4~6 hour SRT 25~40 dayLoading
2.2~2.4 (kgCOD/m3/day)
0.3 bar
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )
Cell physiology (Growth phase)Cell physiology (DO concentration)Cell physiology (Cycle Format in SBR)Cell physiology (Pump Shear)Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
Effect of Physiological states of Activated Sludge on Membrane Biofouling
Water Environment-Membrane Technology Lab.Seoul National University
EPS contents of activated sludge flocs at different growth phase
Rm(1011×m-1)
Rc(1011×m-1)
Rf(1011×m-1)
Rt(1011×m-1)
Rc/Rt(%)
EPS(VS mg/g MLSS)
YM30Log growth phaseEndogenous phase
1820
3363
0.10.1
5183
6576
200270
PM30Log growth phaseEndogenous phase
43
4282
1311
5996
7185
200270
Water Environment-Membrane Technology Lab.Seoul National University
Effect of Foaming on Membrane Biofouling
01020304050
60708090
100
1 2 3 4 5
Co nc e ntra tio n Fa c to r
No n-Fo a mingFo a ming 1Fo a ming 2
YM30 Membranes
Water Environment-Membrane Technology Lab.Seoul National University
Hydrophobicity of foaming and non-foaming activated sludge
Characteristic ofactivated sludge
Relative Hydrophobicity(%)
Range Average
non-foamingfoaming 1foaming 2
54 ∼ 6065 ∼ 9362 ∼ 91
578082
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )Cell physiology (Growth phase)
Cell physiology (DO concentration)Cell physiology (Cycle Format in SBR)Cell physiology (Pump Shear)Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
TMP profiles at various DO concentrations under constant pneumatic mixing intensity
AirN2 gas
DO0.3mg/L
7.0mg/L
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Time (hr)
TM
P (k
Pa)
DO7.0mg/L
DO1.7mg/L
DO0.3mg/L
0.5hr 1.5hrs1.1hrs0
5
10
15
20
25
30
35
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Time (hr)
TM
P (k
Pa)
DO7.0mg/L
DO1.7mg/L
DO0.3mg/L
0
5
10
15
20
25
30
35
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Time (hr)
TM
P (k
Pa)
DO7.0mg/L
DO1.7mg/L
DO0.3mg/L
0.5hr 1.5hrs1.1hrs
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )Cell physiology (Growth phase)Cell physiology (DO concentration)
Cell physiology (Cycle Format in SBR)Cell physiology (Pump Shear)Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
Effect of Cycle Format on Membrane Fouling in Membrane-coupled Sequencing Batch Reactor(SBR)
0
5
10
15
20
25
0.0 5.0 10.0 15.0 20.0 25.0
Time (day)
TMP
(kPa
)
6.8day 21.5dayMSBR(A/O/-) MSBR(-/O/-)
Water Environment-Membrane Technology Lab.Seoul National University
Image analysis(-/O/-) (A/O/-)
x200x200MSBR(A/O/-); MSBR with anoxic phaseMSBR(-/O/-); MSBR without anoxic phase
Water Environment-Membrane Technology Lab.Seoul National University
EPS analysis in Membrane –coupled SBR
( ) : standard deviation
5.0( 0.2)
2.4( 0.2)
52.7( 1.8)
24.1( 1.4)
MSBR - ) (mg/L)
2.2( 0.2)
1.0( 0.1)
66.5( 1.9)
35.8( 1.7)
MSBR(- /O/ - ) (mg/L)
PolysaccharideProteinPolysaccharideProtein
EPS (soluble)EPS (bound)
5.0( 0.2)
2.40.2)
52.71.8)
24.1( 1.4)
MSBR(A/O/ - ) (mg/L)
2.20.2)
1.00.1)
66.5( 1.9)
35.8( 1.7)
MSBR(- /O/ - ) (mg/L)
PolysaccharideProteinPolysaccharideProtein
± ± ±
± ± ±
±
±(A/O/
Water Environment-Membrane Technology Lab.Seoul National University
Stickiness of Biofilm vs. cycle formatin SBR
Cake Disc
Single blade paddle mixer
05
1015202530354045
0 100 200 300 400 500
Time (min)
Turb
idity
(NTU
)
MSBR(A/O/-)
MSBR(A/O/F)
MSBR(-/O/-)
MSBR(A/O/-); MSBR with anoxic phaseMSBR(-/O/-); MSBR without anoxic phase
Water Environment-Membrane Technology Lab.Seoul National University
MSBR (O)MSBR (A/O)
S- A- A-S-
S- : Suspended
A- : Attached
Phylum. Proteobacteria phy.Class. BetaProteobacteria
Order. BurkholderialesFamily. Comamonadaceae
Genus. Diaphorobacter
Khan,S.T.,et al . Appl. Environ. Microbiol. 68 (7), 3206-3214 (2002) :
Comamonadaceae, are primary PHBV-degrading denitrifiersin activated sludge.
Water Environment-Membrane Technology Lab.Seoul National University
Distribution of microorganismsbetween bulk phage and membrane
S1 S2 S3 S4 S5 S6 S7 S8 M5M1 M2 M3 M435%
60%
0 7 13 29 34 47 58 68 7 13 29 47 68
DGGE profile ( Yamamoto et al.,2004)
340047S6,M4
246034S5
220029S4,M3
204013S3,M2
364068S8,M5
386058S7
MLSS (mg/l)
Operation time(day)
Lane
21707S2,M1
17200S1
Membrane and sludge have different DGGE band patternsMicrobe on membrane surface was selectively grown
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )Cell physiology (Growth phase)Cell physiology (DO concentration)Cell physiology (Cycle Format in SBR)
Cell physiology (Pump Shear)Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
Effect of pumping device on flux in crossflow MBR
Operation time (hour)
0 24 48 72 96 120 144
Flux
(L/m
2 /hr)
0
50
100
150
200
centrifugal pump
rotary pump
Water Environment-Membrane Technology Lab.Seoul National University
Resistance Analysis
• Resistance-in-series model
J PR
PR R Rt m c f
=⋅
=⋅ + +
∆ ∆µ µ ( )
Pump typeResistance(1012m-1) Rotary Centrifugal
0.6317.917.9
0.6219.119.1
0. 6010.810.8
0.4411.811.8
Rm
Rc
Rf
Rt
* Resistance after 7 days’ crossflow operation
Water Environment-Membrane Technology Lab.Seoul National University
Particle Diameter(µm)
1 10 100 1000
Vol
ume
Freq
uenc
e(%
)
0
10
20
without recirculation recirculation with centrifugal pump - 7dayrecirculation with rotary pump - 7day
Water Environment-Membrane Technology Lab.Seoul National University
0.30.20.4~0.5Observed sludge
yield(gMLVSS/gCOD)
MBR with a centrifugal
pump
MBR with arotary pump
Conventionalactivated sludgePump type
Sludge Production in MBR system :Observed sludge yields in conventional activated sludge and in crossflow MBR
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )Cell physiology (Growth phase)Cell physiology (DO concentration)Cell physiology (Cycle Format in SBR)Cell physiology (Pump Shear)
Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
Membrane- coupled Fixed bed bioreactor
Pressure gauge
Water bath
Membrane module
Tap water
Support media (option)
Suction pump
Level sensor
Feeding pump
Electronic balance
Permeate Personal computer
Air diffuserConcentrated synthetic
wastewater
Water Environment-Membrane Technology Lab.Seoul National University
Suspended vs.Attached
Attached Growth Reactor(MLSS: 100~2,000 mg/L, attached
biomass: 2,000 mg/L)
Suspended Growth Reactor(MLSS: 2,000~5,000 mg/L)
Water Environment-Membrane Technology Lab.Seoul National University
Comparison of Filtration Performance :Suspended vs.Attached
0
5
10
15
20
25
30
35
0 50 100 150
Operation time (hr)
Tra
nsm
em
bra
ne p
ress
ure
(kPa)
Attached growthMLSS: 100 mg/L, attached biomass: 2,000 mg/L
Suspended growthMLSS: 3,000 mg/L
Water Environment-Membrane Technology Lab.Seoul National University
Filtration behaviors with varying MLSS concentration in attached and suspended growth MBR
Attached growth(mgMLSS/L)
Suspended growth(mgMLSS/L)
All attached growth reactors contain additional 2,000 mg/L of attached biomass.
0
5
10
15
20
25
30
35
0 50 100 150 200 250
Operation time (hr)
Tra
nsm
em
bra
ne p
ress
ure
(kP
a)
1001,000
1,5002,000
3,0002,000
5,000
Water Environment-Membrane Technology Lab.Seoul National University
Specific Resistances of Mixed Liquors
n = 0.79
n = 1.08
n = 1.04
n = 1.07
1.0E+11
1.0E+12
1.0E+13
1.0E+14
1 10 100 1000
Pressure (kPa)
Specific
cak
e resist
ance
(m/k
g)
Attached growthMLSS: 100 mg/L,attached biomass: 2,000 mg/L
Suspended growthMLSS: 3,000 mg/L
Attached growthMLSS: 2,000 mg/L,attached biomass: 2,000 mg/L
Suspended growthMLSS: 2,000 mg/L
•The mixed liquor of attached growth would have a higher fouling potential compared with that of suspended growth.
•At the same MLSS of 2,000 mg/L, mixed liquor from both attached and suspended growth revealed similar cake properties. similar filtration behavior at the same MLSS concentration
Water Environment-Membrane Technology Lab.Seoul National University
Formation of Dynamic Membrane
Low molecular weight or submicron colloidal particle
Microbial floc
Attached growth(without suspended solids)
Suspended growth
Membrane DYNAMIC MEMBRANE
Water Environment-Membrane Technology Lab.Seoul National University
Let`s Open the Black box in MBR
Morphology of activated sludge
Hybrid system (Biological activated carbon )Cell physiology (Growth phase)Cell physiology (DO concentration)Cell physiology (Cycle Format in SBR)Cell physiology (Pump Shear)
Growth mode (Suspended vs. Attached)Conclusion and Research on MBR in 21C
Water Environment-Membrane Technology Lab.Seoul National University
Conclusion
Environmental & Operating Factors
in MBR
Biofilm
Properties ( )
Bulk phase properties ( )
cR
fR
?
? ?
Membrane
Biofouling
( )J?
?
Membrane Biofouling is determined by the matrices of biofilm and bulk phase which are governed by environmental and operating factors.
The matrices of biofilm and bulk phase are in dynamic equilibrium with communities and physiology of microorganisns which are continuously changing.
Water Environment-Membrane Technology Lab.Seoul National University
Past research on MBR ?
Environmental &Operating Parameters
•Growth mode(attached or suspended)•Growth phase(log or endogenous)•Dissolved oxygen •SRT•Pumping shear•Air flow rate & bubble size•MLSS Conc.•Cyclic format inSBR
, etc.
.
.
MembraneFiltration
Performance
Higher Flux
Longer life time
Easy Cleaning
Lower EnergyConsumption
MicrobialProperties
(Biofouling)
BlackBox
•Microbial Floc Size•Specific Resistance& Compressibility•EPS type & conc.
•Morphology•Adhesion Strength
BlackBox
Water Environment-Membrane Technology Lab.Seoul National University
TMP profile of (a) MSBR with anoxic phase of 10 min. and (b) MSBR without anoxic phase.
0
5
10
15
20
25
0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0
Time (day)
TMP
(kPa
)
(a) (b)
anoxic fill aerobic filtration0 10min 3hrs 50min
10min 10min 2hrs 50min 50min
Cycle format
(a)(b)
Water Environment-Membrane Technology Lab.Seoul National University
Research on MBR in 21C
Using the tool of molecular biology ( FISH, PCR-DGGE, Quorum Sensing )
Biofilm formation mechanism
Cell Morphology & Physiology
Microorganism population dynamics
Innovative MBR processInnovative MBR process
Water Environment-Membrane Technology Lab.Seoul National University
Need for Molecular Biology to improve MBR
Molecular Biology provides high-level information
which is not attainable by traditional methods:
- Track critical groups of microorganisms
- Track specific metabolic reaction
Water Environment-Membrane Technology Lab.Seoul National University
Biofilm formation mechanism:Quorum Sensing
Autoinducer, low molecular weight, diffusible signaling molecules, can be involvedin the induction of various genes that are responsible for aggregation behavior, EPS-production, disaggregation, and so on.
Water Environment-Membrane Technology Lab.Seoul National University
Biofilm formation mechanism:Quorum Sensing
Cell-Cell communication :
Thus, cell-to-cell communication may be of fundamental importance to the dynamics of aggregation in flocs and biofilmsand needs more attention in the future.
The issue is how to identify and control autoinducers involved in biofilm formation in MBR consisting of heterogeneous mixture of microorganisms instead of pure culture.
Water Environment-Membrane Technology Lab.Seoul National University
Thank you for your attention !!