Reverse osmosis works by using a high-pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.

​

In simple terms, feed water is pumped into a Reverse Osmosis (RO) system. You end up with two types of water coming out of the RO system: good water (Purified) and bad water (Concentrated contaminants). The good water that comes out of an RO system has the majority of contaminants removed and is called permeate. Another term for permeate water is product water – they mean the same thing. Permeate is the water pushed through the RO membrane and contains very few contaminants.

​

The 'bad' water is the water that contains all of the contaminants that we're unable to pass through the RO membrane and is known as the concentrate, reject, or brine. All three terms (concentrate, reject, and brine) are used interchangeably and mean the same thing. Below is a simple schematic that shows how an RO system works. As the feedwater enters the RO membrane under pressure (enough pressure to overcome osmotic pressure), the water molecules pass through the semi-­permeable membrane. The salts and other contaminants are not allowed to pass and are discharged through the concentrate stream, which goes to the drain or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the dissolved salts removed from it.