Membrane Technologies
These technologies are continuous processes using a variety of synthetic membrane materials and configurations. The pore size of the membranes varies continuously over a range and the divisions given below are arbitrary and used for convenience. In all cases, the water to be treated is pumped at various set pressures, depending on the dissolved solids and membrane type, over or through the membrane and this gives rise to two streams, the permeate, that has passed through the membrane and is the purified stream and a concentrate, that has failed to pass the membrane and is the concentrated stream that has a majority of the dissolved/suspended material in it. The ratio of these stream flows, the pre-treatment necessary to prevent scaling or fouling of the membranes and chemical dosing requirements, if any, all give rise to the membrane system being just one of a series of unit processes, each of which must be carefully selected. The following are specific types of Membrane Technologies:-

Reverse Osmosis
The typical size range employing this class of elements is greater than 0.0001 micron and larger with operating pressures of typically up to 20-60 bar. Essentially, only water can pass through the membrane and so dissolved matter can be separated.

Microfiltration
The typical size range employing this class of elements is greater than 0.05 micron and larger with operating pressures of typically up to 3 bar. No dissolved molecules are retained on these units

Ultrafiltration
The typical size range employing this class of elements is greater than 0.005 micron and larger with operating pressures of typically up to 10 bar. Some very large molecules are retained on these membranes

Nanofiltration
The typical size range employing this class of elements is greater than 0.001 micron and larger with operating pressures of typically up to 30 bar. These membranes will only allow low molecular weight organics and monovalent ions to pass through. Bivalent ions can pass depending on the membrane.

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