Reverse Osmosis UK

Reverse Osmosis

What is a Reverse Osmosis Membrane

Derwent Water Services use only the best Reverse Osmosis membranes available. This enables us to design each RO specifically for each application maximising the efficiency of our RO systems and saving customers names. We use this film composite spiral wound membranes.

The most important factors of a Reverse Osmosis membrane are as follows:

1) It must comprise of a very thin layer of material ( cellulose acetate ).

This thinner of this membrane the better the efficiency. The overall membrane resistance is directly proportional to its thickness.

2) It must have a narrow range of pore sizes to be 'selective' in the removal of salts etc.

3) It must have a high parosity Most membranes in particular Reverse Osmosis membranes comprise of 2 layers.

The first layer being the 'active' filtering layer and the second layer comprising of a very porous supporting layer which is bonded onto the membrane purely for mechanical strength.

Other types of membrane found mostly in other membrane processes - ie Nano filtration and microfiltration include:

• Pleated filter cartridge
• Plate and frame
• Tubular
• Hollow fine fiber

It is necessary to encourage turbulence on the surface of a membrane to encourage the flux value.

Below is a table which highlights Turbulence Promotion

Configuration	Area / volume ratio	Cost	Turbulence promotion
Pleated cartridge	high	low	Very poor
Plate and frame	Low	High	Fair
Spiral wound	High	Low	Poor
Tubular	Low	Very high	Very good
Capillary tube	High	Low	Good
Hollow fibre	Very high	Very low	Very poor
Configuration	Advantages	Disadvantages	Applications
Pleated cartridge	Robust construction compact design	Easily fouled cannot be cleaned	Dead end MF
Plate and frame	Can be dismantled for cleaning	Large installation cannot backflush	ED, UF, RO
Spiral wound	Low energy cost robust and compact	Not easily cleaned ­ cannot backflush	RO. UF
Tubular	Easily mechanically cleaned tolerates high TSS waters	High capital and membrane replacement cost	Cross-flow filtration, high TSS waters
Capillary tube	Characteristics between tubular and hollow fibre		UF
Hollow fibre	Can be backflushed compact design		MF, RO