Salinity Range

The salinity range for electrodialysis (ED) is generally less than 5,000 mg/L because

higher concentrations increase the electricity cost for ion removal. ED selectively

removes dissolved solids, based on their electrical charge, by transferring the brackish

water ions through a semi permeable ion exchange membrane charged with an

electrical potential. So ED is one of the desalination processes. It is not a pre-

treatment process.

Reverse osmosis (RO) and electrodialysis (ED) are membrane processes for the

removal of dissolved constituents from a water source. Both technologies are

employed for drinking water desalination because of their ability to remove dissolved

salts.

Microfiltration (MF) membrane (filter) has a pore size around 0.1 micron, so when

water undergoes microfiltration, many microorganisms are removed, but viruses

remain in the water.

Ultrafiltration (UF) membrane (filter) has a pore size around 0.03 micron, i.e. UF

nominal pore diameter is around 0.03 µm. Ultrafiltration would remove larger

particles, and may remove some viruses. UF membranes show low rejection of oil,

reactive silica, dissolved organic compound (DOC) and hardness which could pass

through the UF. UF could protect RO elements and remove suspended solids, bacteria

and colloids effectively. UF membranes can provide feed water for RO with low silt

density index (SDI) and turbidity, even in difficult applications where raw water

quality fluctuates.

Neither microfiltration nor ultrafiltration can remove dissolved salts. There are no the

salinity range for MF and UF. Both MF and UF can handle high saline water since

they will not remove or reduce the salinity caused by dissolved salts. The main

advantages of UF membrane technology as pretreatment of RO are:

• Stable quality of UF permeates independent of raw water quality.

• Low silt density index (SDI) and turbidity of the UF permeate. UF can reliably

provide RO feed water with SDI <3 and Turbidity <0.1 NTU, even in difficult

feed water.

• Reliable removal of bacteria and viruses by UF, reducing bio-fouling of the RO

membranes.

• UF systems are flexible in their performance and can handle changing qualities

and flows.

Reverse osmosis filters have a pore size around 0.0001 micron. Reverse osmosis

removes dissolved salts down to monovalent ions, which means that it desalinates the

water.

Definitions:

Desalination: any process used for the removal of dissolved salts.

Pretreatment: any process used for the removal of suspended solids (salts); or

physical, chemical or biological treatment of raw water to be fed to desalination

process.