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Introduction to Membrane Technology in WW treatment No. 12203-0904/KOSICE03

Petr Hlavínek Institute of Municipal Water Management, Faculty of Civil Engineering, Brno University of Technology

This project has been funded with support from the European Commission. This presentation/publication reflects the views only of the authors, and the Commission cannot be held responsible for any use which may be made of the information contained therein.

BRNO UNIVERSITY OF TECHNOLOGY

Content Benelux 16%

• CASP x MBR • The MBR market

UK and Ireland 19%

Iberia 19%

Germany 18% Italy 16%

France 12%

• Historical perspective • Membrane technology fundamentals • MBR configurations

• Fouling • Cleaning • Summary 2

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What´s wrong with CASPs? Nothing much, unless you count • large footprint • two or three stage process

• settlement problems from „bulking“ • further polishing required for disinfection/clarification • equalization of hydraulic and organic loadings required

so, all right as long as you: • have enough space and • don't want to recycle!!! 3

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Conventional process engineering in municipal waste water treatment screen

Influent

grit Preliminary chamber settling tank

activated sludge tank

secondary tertiary settling tank treatment

Effluent

Downstream arrangement of the membrane stage in waste water treatment

Integration of the membrane stage in municipal waste water treatment

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and what about MBRs? An MBR combines aerobic biotreatment with membrane separation, and so: • • • • •

clarified, disinfected product provided small footprint plant bulking problems become less relevant HRT and SRT are uncoupled and higher than at CASP intensive biotreatment provided, especially nitrification

that is why MBRs are: • capable of meeting strict environmental standards • KEY CANDIDATE IN WW RECYCLING TECHNOLOGY • opening up opportunities for decentralized treatment

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MBR operation MBRs can run at: • long SRTs (an MBR behaves biokinetically as an ASP with good

clarification) • lower F:M ratios • reduced sludge production

Long SRTs desirable in that : • MLSS increases (up to 18 g/L) and reactor size is reduced • sludge production is lower, BUT

• oxygen transfer efficiency is reduced, and • sludge viscosity increased, increasing clogging propensity

Use of membrane produces high-quality effluent!!! • ideal for meeting EC Bathing Water Directive, irrigation...etc 6

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Sludge and MBRs

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The MBR market Growing more rapidly that: • that larger market for advanced wastewater treatment equipment • the markets for other types of membranes systems

Global market: • $10m in 1995, $105m in 2000, $217m in 2005 • expected to reach $360 million in 2010

EU total market: • €25,3 m in 1999, €32,8 million in 2002, €57 m in 2004 • expected to rise annually by 6,7%, and double over seven years

US market, membrane water purification • $ 750 million in 2003 • Projected to reach $1,3 billion in 2010

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EU MBR Market Frost & Sullivan, 2005

Benelux 16%

UK and Ireland 19%

Iberia 19%

Germany 18% Italy 16%

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France 12%

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So far excellent, but everything has cons MBRs are technically limited by: • membrane surface fouling • membrane channel clogging • process complexity

Also have: • high capital and operation costs, but high value • relatively new technology (submerged configuration applied in

1990) • high degree of process complexity - requirement for piloting

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Drivers to MBR uptake Legislation getting stricter • implementation of WFD • Bathing water treatment directive • fresh water becoming scarcer

Decreasing investment costs (both membrane and process) Increasing confidence in and acceptance of MBR technology • experience and knowledge increasing • perceived process risk decreasing

Interest in technology  11

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Historical perspective MBRs brief history Relatively new technology, rapid expansion: • Sidestream configuration commercialised in 1970´s - little market penetration • first immersed MBR installed in 1990 • exponential growth in installed capacity over last 15 years

ssMBR use MT and FS modules - still used for niche and/or small-scale industrial applications

More recent iMBRs use FS or HF modules - use for large scale-municipal applications - dominated by two suppliers: Kubota (FS) and Zenon (HF) 12

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Commercial developments, history Late 1960s – Dorr Oliver developed first sidestream FS MBR Mid 1980s – University of Tokyo experiments with immersed hollow fibre MBR

Early-mid 1990s – Kubota commercialises immersed flat sheet

MBR in Japan – Zenon commercialises vertical immersed hollow fibre membrane module – Wehrle commercialises sidestream multitube Biomembrat system – Mitsubishi Rayon commercialises MBR based on immersed fine HF membrane, (horizontal)

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The future There are three factors with cost implications Far East manufactured membrane modules: • currently have insufficient integrity, but getting better

• potentially significantly cheaper than now available

products

Standardization: • MBR modules could potentially go the same way as RO

Further improvements in design.... COSTS LIKELY WILL CONTINUE TO FALL 14

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Membrane technology fundamentals Three elements to consider: • the membrane material and its geometry • the membrane separation process • fouling phenomena and their limitation

Main principle is to produce material: • of reasonable mechanical strength, and • high throughput of desired permeate

Optimum physical structure always comprises: • thin layer of material • narrow range of pore size, and • high porosity 15

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Membrane material types Dense and porous Polymeric and Metallic/Ceramic • Dense membranes (polymeric only) - achieve separation through physicochemical interactions between permeating components and membrane material - Relate to separation processes of highest selectivity • Porous membranes (all material) - Operate by simple sieving/surface filtration

• hydrophilicity is GOOD and hydrophobicity BAD • Commercial materials mainly limited to PVDF, PES and PS

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Membrane material types • Ceramic materials have: - Much higher membrane resistance - very low fouling propensity (low fouling rates) - High price!!!!

...and so are hardly used at all in practice

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Material structure Generally anisotropic: (symmetry in one direction)

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Membrane process types

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Separation mechanisms UF/MF Sieving/surface filtration • particle size of contaminants >

pore size of membrane

• membrane can foul with

suspended or precipitating matter

• fouling can improve rejection of

the membrane

Salts, Sugars and Low Mol. Weight Compounds pass through the membrane 20

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Separation mechanisms: RO/NF

Solubility/diffusion differences: • based on the principle that the membrane has a greater affinity

for the solvent than the solute

• also possible that nanopores exist for selective passage of

smaller molecules

• membrane can foul with precipitated matter 21

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Membrane configurations Impossible for any one configuration to satisfy all requirements Available configurations based on either a planar or cylindrical geometry: •

Planar: - FS (flat sheet) or P&F (late-and-frame) - SW (spiral wound)

• Cylindrical: - MT (tubular/multitube) - CT (capillary tube) - HF/HFF (hollow fibre/hollow fine fibre) 22

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Flat sheet

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Capilary tube/Hollow fibre • Membrane fibres are

thin and selfsupporting

• Simple, inexpensive

and backflushable design

• High packing density

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(Multi-) tube (MT) • Single or multitubes • Ceramic materials usually monoliths - cylindrical channels in block of ceramic material • good hydrodynamic control

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Spiral-wound

www.doctorh2o.ca

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MBR membranes configurations

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MBR membrane configurations

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Membrane and module summary Material • Ceramic or polymeric

Geometry

Submerged

• Cylindrical (HF or tube) or planar (FS)

Flow direction • Out-to-in or in-to-out (MT)

Pore size

Sidestream

• Microfiltration or ultrafiltration - The boundary between these is indistinct

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Process configuration Recirculated stream

Out

In

In Membrane

Air

Air Bioreactor

Sidestream

sMBR 30

Immersed Out

iMBR

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Process configuration Submerged

Sidestream

• more recent

• longest history

• membrane placed

• membrane placed

development

in bioreactor • permeate removed

either under pressure or suction

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external to bioreactor

• permeate removed

under pressure by crossflow filtration

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Process configuration feed

100% conversion (during operational cycle) referred to as dead-end or full flow

filter cake

membrane permeate

membrane

Sidestreams operate at partial conversion in cross-flow mode

concentrate

permeate

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support

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Dead end X cross-flow Dead end operation • leads to declining, non-equilibrium flux due to cake

formation • demands frequent washing/cleaning cycle • applies to all MF/UF HF

Crossflow operation • limited conversion per membrane element

• pseudo equilibrium flux attained • less frequent cleaning required • applies to all RO/NF, MF/UF FS and MT 33

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Membrane fouling Three different ways of defining fouling: • practically - Reversible, irreversible or irrecoverable • mechanistically - surface coating, pore plugging, internal pore absorption, etc • by material type - chemical nature: inorganic or organic - Surface chemistry/charge: positive or negative

- particle size: molecular, macromolecular, colloidal or particulate

- origin: microbial, terrestrial or manmade

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Practical definition Based on permeability recovery: • Reversible/temporary: - removed by physical celaning

• Irreversible/permanent - removed by chemical cleaning • Irrecoverable/absolute - not removed by any cleaning regime

Diminuition of flux is normally referred to as fouling. Accumulation of solids in channels leads to clogging. 35

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Flux „J“ Flux is volume per unit area per unit time: • m3 pre m2 per s (SI units) • litres per m2 per hour or „LMH“ (Euro) • gallons per ft2 per day or GFD (US)

• m per day (used for MBRs)

Flux is determined by: • Membrane resistance • Operational driving force per unit membrane area • Hydrodynamic conditions at interface, and • Fouling and subsequent cleaning of membrane 36

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Fouling mechanisms Various mechanisms to explain permeability transients on theoretical basis:

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Flux sustainability 4 options available for sMBRs: • operation at low flux (J) - balance between capital and operational expenditure

• operation at high shear - high liquid cross-flows - air scouring - moving membrane surface

• (pre-)treatment to remove/transform foulants - effective screening - chemical modification of biomass

• effective cleaning - physical cleaning by backflushing or relaxation - chemical cleaning in-situ or ex-situ

All have cost implications 38

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Pretreatment SCREENING In general : • Sub-3mm screening recommended for FS iMBRs • Sub-1mm screening recommended for HF iMBRs • Step/slot/bar screens normally insufficient because

they allow hairs into system

• sMBRs are generally more tolerant, but have higher

energy consumption

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Membrane cleaning Chemical

Physical BACKFLUSHING • with air • without air RELAXATION

Combination Chemically

BASE • Caustic soda • Cirtic/oxalic ACIDS • Hydrochloric/sulphuric • Citrc/oxalic OXIDANTS • Hypochlorite • Hydrogene peroxide

enhanced backwash 40

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Membrane cleaning • Backflushing almost exclusively restricted to HF

MBRs • Relaxation and/or backflushing applied to submerged systems • CEBs applied- weekly, low reagent concentrations, contact time of few minutes, for maintenance • CIPs applied monthly to 6- monthly, high reagent concentration, contact time of hours, either for maintenance or recovery

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Operation of Immersed Membranes

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Operation • Plants normally run at constant flux • TMP increases over course of filtration cycle • Fixed-interval cleans comprise: - Backflushing/relaxation (usually 5-15 min intervals) - Maintenance chemical cleaning (2-30 day intervals) • Recovery cleaning applied on demand

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Good MBR design entails • Obtaining appropriate balance between operational flux,

aeration and cleaning, which means:

- Maximising impact of aeration (in terms of fouling reduction) - Faciliating cleaning with minimal downtime and chemicals consuption, and - Providing a high membrane area at low cost.

Most important of these is aeration efficiency

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Summary „Membrane bioreactors are today robust, simple to operate, and ever more affordable. (…) Because membrane processes make sanitation, reuse, and decentralization possible, water sustainability can become an achievable goal for the developed and developing worlds.“ Bellagio Statement from International Residency Team of 14 membrane experts, 23-26 April 2003 Journal of Membrane Science 228 (2004) 127-128.

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Summary

„Membrane bioreactor looks set to revolutionise numerous applications in municipal and industrial facilities and offers an exciting future for wastewater treatment..” Frost & Sullivan, 7/7/2003.

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