MBR CourseMembrane Fouling & Cleaning

October 16 & 17, 2012

Hamid Rabie

Effect of Fouling on Permeability

initial permeability

Irreversible fouling

Restored permeability

Reversible fouling

Permeability after time t

Time

Per

mea

bilit

y

Irreversible fouling is a relative terminology

Fouling Loss of productivity due to various factors (physical/chemical):

membrane compaction

adsorption

membrane deposits and solid build up

pore plugging and precipitation

gel compaction

concentration polarization (less important for MBR)

Fouling prevention is major design/operational issues

Fouling can be controlled by Membrane Modification (pore size, porosity, structure, nature)

Pre-treatment (screening, pre-oxidation, )

Process Considerations (flux, recovery, sludge age, )

Maintenance Cleaning (back pulse, regular chemical cleaning, relaxation, ...)

Hydraulic Improvements (tank design, air distribution, ...)

Structure of an Asymmetric Membrane

Fouling can occur on surface or inside membrane structure.

Membrane compaction

changes membrane structure

and reduces porosity resulting

in higher resistance to flow.

15

20

25

30

0 5 10 15 20Time (day)

Wat

er p

erm

eabi

lity

(gfd

/psi

)

With 15min initial permeation

Adsorption Capacity

Fibres were soaked in bioreactor and clean water permeability was measured at different times

Rate of Decline in Permeability

Without initial permeation

0.05 gfd/psi/day

With 15min initial permeation

0.24 gfd/psi/day

Adsorbed material may be removed by extensive water rinsing or chemical cleaning.

Cake Formation and Concentration Polarization

High molecular weight solutes

Membrane

JW

CP

sub-layer

CS

Cb

JW

Gel layer or cake

Bulk flow

Note: C s > Cb

1. Decrease in back diffusiondue to established sub-layer(concentration polarization)

2. New condition at membranemay exceed the limit (e.g., 0.02% for silica)

3. Increase in C P (due to Cs > Cb )

4. Cake layer resistance

At Steady State:Rate of solutes coming towards

membrane due to filtration =

Rate of migration of solutes away from membrane due to diffusion,

shear and back pulse

All components in feed can foul the membrane

In many systems, the main foulant exists in trace amount

Common Foulants:

Scaling: CaCO3, CaSO4, BaSO4

Metal Oxides: iron, aluminum

Inorganic Colloids: silt

Silica

Organics: oil, NOM

Biofilms: bacteria and EPS (protein, nucleic acid, lipopolysaccharides,

DNA, )

Foulant is a mixture of chemicals usually in a complex

matrix

What Fouls the Membrane?

Membrane Biofouling

Accumulation and attachment of microorganisms (e.g.., bacteria, fungi, microalgae)

Complete growth of biofilm: bacteria is hidden in EPS matrix. Note fibrous structure.

Biofilm is in multilayer of living and dead cells a nd associated EPS.

Biofouling is more severe on MF and UF since nutrie nts can pass through.

Biofouling may result in degradation of membrane an d joint glue.

Cleaning Fundamentals

1. Competitive adsorption

2. Solubilization Changing solubility (e.g., increasing T)

Emulsifying

Dispersing

3. Chemical Modification

Hydrolysis of fats and oils (e.g., at high pH)

Oxidation (e.g., organics)

Degradation of proteins

Chelating (e.g., divalent cations)

Reaction of metal oxides and acids

Cleaning Mechanism

1. Acids: HCl, HNO3, H3PO4, C3H4(OH)(COOH)3

2. Alkalis: NaOH

3. Surfactants:anionic (sulfonates, sulfates, phosphates), cationic (quaternary ammonium

salts), nonionic (PEO: -O-CH2), competitive adsorption, emulsifying, micelle

4. Chelators: ethylenediamine tetraacetic acid (EDTA)

5. Oxidizers: NaOCl, ClO2, HOBr, H2O2, ozone, UV

6. Enzymes:degrade proteins, starch, fat, oil, cellulose, cleave peptide linkage in proteins

in specific sites, it is very selective and bacteria can adopt, very slow reaction

Common Cleaners

CaCO3 + 2HCl CaCl2 + H2O + CO2

Fe2O3 + 6HCl 2FeCl3 + 3H2O

Acid Cleaning

HCl cheap, high rate of reaction and yield, corrosive

H2SO4 cheap, moderate rate of reaction and yield, corrosi ve

HNO3 also oxidant for organics and biological, moderaterate of reaction and yield, too corrosive

H3PO4 chelating, pH buffering, less corrosive, uses too m uchfor pH < 2.3, Ca phosphates has limited solubility, tooexpensive, not recommended for P removal processes

Citric acid good chelating with Ca, its complex with ferrous io nhas low solubility, low kinetic, high yield

1. Dissolves: silicasome inorganic colloids (dispersion)many biological and organic foulant

2. Sanitizer

3. Neutralizes fatty acids and humic acids (R-COOH)

4. Hydrolysis fats and oils

C3H5-(OOCR)3 + 3NaOH C3H5(OH)3 + 3NaOOCRester soap

NaOOCR has emulsification properties

NaOOCR is insoluble at low pH, also precipitate with Ca, Mg, add in

presence of chelating agent (EDTA) to remove Ca, Mg

5. Hard to maintain pH, measure pH during cleaning

Alkalis Cleaning (NaOH)

1. Extremely effective specially for pore fouling

2. Disinfection property (biofouling control)

damage to cell wall

alteration of cell permeability

inhibition of enzyme activity

3. Alkali cleaning property (increases pH)

NaOCl + H2O HOCl + NaOHactive component

widely used and inexpensive

more effective at low pH and also more corrosive

harmful by-products (e.g., THM)

NaOCl (Oxidizer/Disinfectant)

Depends greatly on types of foulant and membrane and the

way membrane has been fouled (see examples)

Cleaning Sequence/Condition

1. Biofilm (dominant) + Inorganic

Alkalis/Oxidizer/Acid (initial acid cleaning increases adhesion of humic material, initial caustic cleaning can remove significant org anic so oxidizer cleaning becomes more efficient.)

2. Inorganic (dominant) + Organic + Biofilm

Mixture of Acids and Chelators/Oxidizer or Alkalis

Conditions:Type of Cleaner, Temperature, Concentration, Contac t TimeCertain relations exists, for example: bacteria kil l is related directly to dosage

The best sequence and condition can be obtained aft eridentifying foulant and preliminary tests.

Operational Considerations of Cleaning

Mechanical Cleaning

1. Scrubbing: solid removal

2. Back Pulse: reverse TMP and drive permeate backward

3. Relaxation: eliminate TMP, allow gel layer to dissipate

Chemical Cleaning

1. In-Situ/Empty Tank: back wash regularly with chemicals in CIP from top only,in pulses at moderate flow for uniform distribution

2. In-Situ/Full Tank: the same as empty tank except back wash from both ends

3. Soak: soak in chemical solution for extended period

Cleaning Methods

Backwash

Clean-In-Place Tank(Filtrate from membrane)

Process Tank Water

Backwash Cleaning(Reverse Flow with Filtrate)

Use clean filtrate to backwash membranes. A reverse flow from the CIPtank is fed to the inside of the membrane fibers cleaning from the inside out

Cleaning chemicals are optional and not always necessary.

X-section

Maintenance cleaning

Used for maintaining permeability (conditioning rate of decline)frequent, short contact time such as: back pulse, relaxation, in-situ/full tank

Recovery cleaning

Used for restoring permeability close to one of new membrane, not frequent, long contact time, such as: soak.

Cleaning Methods (Another Classification)

Flux Distribution in Fibres during Back Wash

0

0.4

0.8

1.2

1.6

0 0.5 1 1.5Fiber Length from top (m)

Nor

mal

ized

Flu

x

30 gfd

15 gfd

Empty Tank Back Wash(only from top)

0

0.4

0.8

1.2

1.6

0 0.5 1 1.5Fiber Length from top (m)

Nor

mal

ized

Flu

x

30 gfd

15 gfd

Full Tank Back Wash(from both sides)

Calculation is done based on a true permeability of 10 gfd/psi

Why using pulsed back wash athigher flow rates ?

1. uniform distribution

2. low chemical consumption

Calculation is done based on a true permeability of 10 gfd/psi

Long pipe lines reduces back wash performance

1. Membrane Limitations

pH

Temperature

Chemicals

2. Process Limitations

Bioreactor (e.g., nitrifiers sensitive to chlorine and pH)

Drinking Water (e.g., THM, residual chlorine)

Any Cleaning Should not Exceed:

Steps Towards Identifying Problem(s)

1. Solid build up

2. Aerator Status

3. Fibre appearance (black, brown, damaged, )

4. Fluctuations in designed operating conditions (flux, pH, )

Operating at, for example, high solid or flux can foul membranes in short time but it takes long before membranes recover.

5. Record any changes in operation, cleaning results, and any

suspected chemicals.

Check List

Fully characterise the feed

Cleaning of fouled fibres and analysis of extracts

Extraction of foulant and analysis (GS/MS, Pyrolysis, ...)

Autopsy of fouled membrane

ESEM (environmental scanning electron microscopy) for morphology

TEM (transmission electron microscopy) for biological fouling

EDX (energy dispersive x-ray) for elemental analysis

MWCO (molecular weight cut-off)

Pore size

Identifying Nature of Foulants

Membrane fouling can be controlled

Essential to diagnose what is the cause

Crucial to understand fluid dynamics and mass transfer

Crucial to understand chemical interactions with membrane

Develop rational pretreatment and cleaning protocols

Conclusions

Membrane Biofouling (STEM micro -graph)

1 micron

Note significant biofouling on the surface of membr ane

STEM micro -graph after Maintenance Cleaning

Note that the fouling has been controlled

1 micron

Fouled Membrane in Tap Water

Fouling is mainly due to fibrous organic matter; moderate microbial and

inorganic fouling.

STEM micro-graph of foulant

0.5 micron

Note the disperse/loose structure of foulant

Burlington tap water

Dead-end filtration

MBR Pilot with Ferric Addition

0.5 micron

ESEM Micro-graph

of fouled membrane

(Foulant Analysis)

EDX examination of foulant on membranes used in an MBR pilot

Foulant is mainly inorganic:iron and calcium precipitate

(almost 10 to 1 ratio)

Iron precipitates in different forms which have different optimum pH

values for precipitation

(Foulant Structure)

ProblemsStructure of inorganic fouling. Membrane pores were plugged with precipitates. Further analysis also showed bacteria growth typical of sewage treatment plants.

Conventional cleanings (chlorine or acid) did not recover permeability.

Solutions

A mixture of chelating agents and acid for cleaning

Regular maintenance cleaning with formulated chemical according to the Manual

Control of inorganic fouling: pH adjustment, flash mixer, low dosage

ESEM analysis of foulant at an MBR pilot