Reverse Osmosis is a technology commonly used for the purification of water and aqueous solutions. There are lots of applications, including but not limited to:
- drinking water (including sea water desalination)
- cooling tower and boiler water treatment
- minerals process water
- mine effluent treatment
- landfill leachate treatment
- fruit juice concentration
- milk and whey concentration
The heart of reverse osmosis technology is a membrane barrier. This allows pressurised water to pass through, while resisting the passage of dissolved substances such as salts and organics. Some of the water is retained on the feed side to carry the dissolved substances out of the system. This is known as the “reject”, “concentrate”, or “brine” stream. The proportion of water that passes across the membrane is known as the “recovery“, and is expressed as a percentage of the feed flow.
The driving force of the reverse osmosis process is pressure. The feed solution (e.g. water) is pumped to the required pressure. The pressure of the solution overcomes the resistance of the membrane barrier to force the solution through the membrane. The membrane, however, has much higher resistance to the dissolved substances (salts, organics, etc) so they stay behind on the feed side (they are “rejected”).
Although most of the dissolved salts and organics will be rejected into the reject stream, there will always be a small proportion that passes through with the water. The amount passing through will be affected by the properties (such as size, charge, polarity, etc) of the dissolved substance, as well as reverse osmosis system properties such as membrane type, temperature, and flux.