MEMBRANE PROCESSES

Reverse Osmosis

Ultrafiltration

Microfiltration

What Are Membranes?

Membranes are thin films of synthetic organic or inorganic (ceramic) materials, which can bring about a very selective separation between a fluid and its components. The fluid may be a gas or a liquid but in Environmental Engineering we are more concerned with water and wastewater.

Microfiltration

Membrane

Dead-end Filtration

Feed

Filter Cake

Permeate

Microfiltration

Crossflow Microfiltration

Feed Retentate

Permeate

Membrane

Notes: The retentate may be recirculated to the feedA “dynamic” membrane may be created by adding Lime orKMnO4 to the feed, to precipitate MnO2 on membrane

Membrane Operation

FeedSuspended Solids

ColloidsDissolved Solids

ConcentrateSuspended Solids

ColloidsDissolved Solids

PermeateLow molecular

weight dissolved solidsLiquid

Membrane

100% 50% !

50% !

To River, Sewer or Re-use

Applications of Membranes in Wastewater Treatment

• Pressure driven solid liquid separation processes• Tertiary treatment of wastewaters following

secondary sedimentation• Production of high quality effluent in re-use

schemes• Alternative to sedimentation tank for solid/liquid

separation in:– aerobic biological treatment

– anaerobic biological treatment

Pretreatment Sed Tank

Aerobic Biological

Process

Final Effluent

InfluentBOD 300SS 300TKN 50PO4 15

BOD <2SS 0

PrimarySludge

SecondarySludge

Aerobic Biological Treatment with Membrane Separation

MF

Membrane Anaerobic Reactor

Influent

Effluent

UF/MF UnitAnaerobic Reactor

WastageRecycle

Gas

Advantages of Membrane Processes

• They are usually continuous• Comparatively low energy utilisation• No phase change of contaminants• Small temperature change• Modular design• Minimum of moving parts• Physical separation of contaminants

Filtration Range

Reverse Osmosis

Nanofiltration

Ultrafiltration

Microfiltration

MWCO 0 100 200 20000 100000 500000

microns 0 0.001 0.01 0.1

Ionic rangemicro-molecular

microorganismscoarse particles

macro-molecular

APPLIED PRESSURES

Reverse Osmosis: 100-800 psi

1000-5500 kPa

Ultrafiltration: 5-100 psi

35-695 kPa

Microfiltration: Low

Microfiltration Design Considerations

• High flux

• Back-flushable

• High membrane area/volume ratio allowing low pressure drop whilst maintaining high tangential velocities

• Simple installation and continuous operation with minimum supervision

• Low operating pressure

• Easy maintenance and simple membrane replacement

• Low energy consumption

• Relatively low capital costs

Microfiltration Applications

Current FutureWine Drinking Water

Dextrose clarification Hydrocarbon Separation

Haze removal from gelatin Milk-fat Separation

Beer clarification Food and Beverage

Pharmaceutical/biological Industrial Wastewater

Municipal Wastewater Treatment Paint

Biotechnology

Ultrafiltration Applications

Current FutureJuice clarification Domestic wastewater

Increase cheese yield More industrial wastewaters

Gelatin concentration Protein harvesting

Electroplating wastewater Petroleum processing

Protein from cheese whey Wastewater re-use

Concentration of oil emulsions Potable water (THM concern)

Pharmaceutical industry Abattoir (blood recovery)

“Grey” water Pulp and paper industry

Industrial Wastewater etc etc

Reverse Osmosis

Very different process to MF of UF. It is a solubilisation diffusion technique that makes use of a semi-permeable membrane which in turn acts as a barrier to dissolved salts and inorganic molecules.

It also confines organics with molecular weights greater than 100

RO membranes do not have identifiable pores as in MF or UF (i.e. too small - atomic size)

Applications of Reverse Osmosis

Current FutureDesalination for potable water

,, from sea water Chemical process industries

,, from brackish water Metals recovery

,, for effluent re-use Food processing WWT

Ultrapure water for semiconductors Textile wastewater re-use

,, ,, for pharmaceuticals Pulp and paper WWT

,, ,, for medical use Contaminated land remediation

Boiler feed water Dairy industry WWT

Treatment of hazardous materials

Comparison of Membrane ProcessesUltrafiltration Reverse Osmosis Microfiltration

Operates on difficult Requires extensive pre- Rapidly fouled by colloids

colloidal water treatment of colloids giving high replacement costs

Low pressure (2-6 bar) High pressure (10-30 bar) Low pressure (2-4 bar)

Low energy consumption High energy Low energy

High recovery (up to 95%) Low recovery (50-80%) 100% recovery

Chemical tolerance pH 1-13 pH 2-11 pH 1-13

High temperature up to 80oC 45oC max. High temperatures possible

High resistance to oxidising Limited resistance to High resistance to oxidising

agents oxidising agents agents

Stream sterilisable Stream sterilisation not Stream sterilisation possible

membranes available possible

Hygienic module designs Modules not as hygienic Hygienic designs available

available

Some Properties of Typical CommercialUltrafiltration Membranes

Material pH Maximum Pressure Maximum Temp.

(bar) (bar)

Polysulphone 2-12 15 70

Polyarcylonitrile 2-10 10 60

Cellulose Acetate 3-6 25 30

Polyethersulfone 2-12 30 70

Fluoropolymer 2-12 10 60

Polyvinylidene fluoride 2-12 10 70

Poly vinyl chloride 2-12 10 50