Industrial Reverse Osmosis

A process that applies pressure to water to force it through a semi-permeable membrane that retains most of the dissolved salts.

In natural osmosis, water passes through a membrane from the more dilute solution to the more concentrated one. If a pressure higher than the osmotic pressure is applied on the concentrated side, the flow is reversed: the water passes through the membrane and most of the salts, organic matter and microorganisms are retained. Two streams leave the unit: the permeate, water with low conductivity and very low hardness, and the reject or concentrate, which carries away the separated salts and is discharged or reused.

An industrial unit includes pretreatment to protect the membranes: particle filtration, removal of free chlorine (polyamide membranes are degraded by it) and, depending on the water, softening or antiscalant dosing. Then come the high-pressure pump, the membranes and, often, a treated water tank and continuous measurement of the permeate conductivity.

With use, the membranes build up mineral scale, organic matter and biofilm, which reduces the permeate flow and increases the pressure difference. They are cleaned with specific products, usually acids for scale and alkalis for organic soiling, following the membrane manufacturer's instructions.

Osmosis-treated water is used as process water, for boilers, for the final rinse when limescale marks are to be avoided and in dishwashers or washing tunnels. Because it is so soft, it changes the behaviour of detergents and rinse aids formulated for hard water, so products should be chosen and dosed according to the water actually used.

More information: Electrical conductivity of water · Water microfiltration · Removing biofilm