Membrane filtration: reverse osmosis (RO)

Purpose

• Separation of salts by concentration

• Production of demineralised water

Principle

In reverse osmosis techniques the feed water is passed through a semipermeable membrane by applying pressure, just like in other membrane filtration applications. Contrary to other membranes a reverse osmosis membrane has no actual pores. Water and other neutrally loaded molecules migrate through the membrane by diffusion through the molecular structure. During reverse osmosis established by pressure the water is forced in the opposite direction of the natural osmotic gradient.

Reverse osmosis retains all dissolved organic molecules, viruses and nearly all salts. The quality of the treated effluent is nearly equivalent to demineralised water. A concentrate with a relatively high salt concentration is created as a byproduct.

Most of the time the membrane configuration consists of spirally wound polyamide membranes in standardised modules.
In order to protect the membranes the reverse osmosis modules are often preceded by other techniques such as sand filtration and/or ultra-filtration.

Scheme

Applications

Reverse osmosis is used to produce process water with a very low conductivity, close to deionised water. Applications can be found in the production of drinking water, production of demineralised water for industrial applications, wastewater treatment, reuse of purified wastewater, …

Example of realisation

Operational costs

Reverse osmosis is a pressure driven technology requiring a relative high energy consumption. Also a chemical cost for the periodical maintenance cleaning of the membranes needs to be considered. Moreover, the membrane lifetime is limited and in most applications the membranes have to be replaced every 5 to 10 years.