4. membrane processes

MEMBRANE

PROCESSES

Ali Masduqi

A membrane is a film

A semi-permeable membrane is a VERY THIN film that allows some types of matter to pass through while leaving others behind

Advantages of membrane

technologies

1-The quality of the treated water is independent on the raw water quality

2-It produces high quality of treated water

3- physical and not a chemical process:

the solid/liquid phase separation & disinfection may be achieved without chemical addition

4- the amount of sludge is moderate

Disadvantages of membrane

technologies

1-Fouling (colloids, solids, dissolved organicmatter, microorganisms)

2-Scaling (CaSO4, CaCO3, CaF2)

3- Microorganisms may also damage the material of the membrane

4- For reverse osmosis: hydrolysis of the membrane may ocur

cellulose acetate cellulose+acetate

5- Cost (expensive technologies)

Colloids

Bacteria

Pollens Yeasts

Organic macromolecules

Organic compounds

Viruses Dissolved

salts

RO

NF

MF Sand filter

1 mm 0.1 mm 0.01 mm 0. 1 nm 0. 1 nm 10 mm 100 mm

hair visible to

naked eye

Red

globule Smallest

microorganisms

Polio

virus

UF

Membrane Classifications (Pore Size)

Microfiltration

Ultrafiltration

Nanofiltration

10 100 1000 10000 100000

Total dissolved solid concentration mg/L

Ion exchange

Electrodialysis

Reverse osmosis

Freezing

distillation

Membranes Classification (Driving Force)

Vacuum (Submerged Membranes)

• Compatible with higher solid concentration

• Can be used for retrofit

• High energy demand with air scouring

• Noise & evaporation concerns

Membranes Classification (Driving Force)

Pressure (Canister Membranes)

• More compact design

• Cannot handle high solid concentration (> 100 NTU) for a substantial period of time

Membranes Classification (Configuration)

Flat Sheet (Spiral-wound)

Mostly used in Reverse Osmosis

& Nanofiltration

Membranes Classification (Configuration)

Tubular Membranes (OD > 3 mm)

Mostly used in Industrial MF

Membranes Classification (Configuration)

Hollow Fiber Membranes (ID < 1.5 mm)

Mostly used in MF & UF

Membranes Classification (Location of Membrane )

Inside-out Membranes

• Outside-In Membranes

Raw Water

Filtered Water

APPLICATION OF MEMBRANE

TECHNOLOGIES IN

DRINKING WATER TREATMENT

1-Microfiltration: mostly for industrial water treatment to reduce turbidity.

2-Ultra filtration : mostly in industry

to separate macromolecules and to

reduce turbidity • PAC+ ultra filtration to remove :

- suspended solids.

- Organic matter.

- Micro pollutant.

- Taste and odors .

• Aeration +PAC+ ultra filtration to remove taste and odor

• Ultra filter membrane mainly applied at low pressure=10.5 kg/cm2

3- nanofiltration

to remove:

- Suspended & dissolved matter .

- Micro pollutants

- Microorganisms

- T H M precursors

- Water softening

4- electrodialysis • In Electrodialysis , the membrane allow ions to pass but not water. • The driving force is electrical current which carries the ions through the membrane. • By using stacks of alternate anion and cation permeable membranes ,it is possible to produce a treated and concentrate stream. The principle of Electrodialysis is illustrated on next figure:

Source: Hall & Hyde, 1992

Electrodialysis Uses electrical power to draw ions from

product water to the concentrate stream.

5- reverse osmosis

The applied pressure is higher than the pressure for nanofiltration 10.5 -- 28.1 kg/cm2

- Mainly used to remove salt from seawater.

- It can be also applied to remove organic matter

What is Osmosis

It is the transfer of water from regions of low

concentration to region of high one to

equilibrate between concentrations.

The process stops when hydrostatic pressure

on the high solute side counter acts the

osmotic pressure.

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REVERSE OSMOSIS Reverse osmosis consists of separating a solvent, such as water from a saline solution by the use of a semi permeable membrane and a hydrostatic pressure ( see this figure)

Reverse Osmosis The Osmosis process can be reversed by applying high

pressure to the high concentration (source water, reject) side through a selective semi-permeable membrane.

Membranes develop from natural pig bladder to synthetic materials (polyamides-PA) membranes highly efficient at rejecting contaminants.

Membranes are made tough enough to withstand the greater pressures necessary for efficient operation .

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RO component and definition

RO membrane is one of the most important component in water treatment system.

RO is a process in which water is purified using ion exclusion semi-permeable membrane.

Reverse Osmosis is the reversing the Osmosis process

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Reverse Osmosis vs. Osmosis

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Type of reverse osmosis

Spiral wound

Tubular

Hollow fiber

See figures

Source: http://twri.tamu.edu/reports/2002/2002-027/sr2002-027.pdf

Spiral wound

Source: The Management and implementation Authority of the Man-Made River Project,

Tubular Reverse Osmosis Equipment

Source: Reynolds & Richards, 1996

Hollow-Fiber Reverse Osmosis Equipment

Source: Reynolds & Richards, 1996

Source: http://twri.tamu.edu/reports/2002/2002-027/sr2002-027.pdf

Schematic of typical RO treatment system

Pre-filters in RO systems

RO systems require a carbon pre-filter for the

reduction of chlorine (as mentioned before), which

can damage an RO membrane

A sediment pre-filter is required to ensure that fine

suspended materials in the source water do not

permanently clog the membrane.

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Comparing RO membranes

Disadvantages Advantages

Membrane

• Chlorine sensitive, requires

carbon pretreatment

• Little higher reject ratio

• Operate at wider pH

range

Thin film

membrane (TF)

• Not sensitive to chlorine

• Lower reject ratio

• Operate on less pH

range

Cellulose Triacetate

membrane (CTA)

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RO membrane performance

RO membrane performance is measured by

percent rejection.

Final product water quality can be measured

by either conductivity in micro-siemens/cm or

total dissolved solids (TDS) displayed as mg/L

or parts per million (PPM).

AAMI recommends both percent rejection and

water quality monitors be used

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