R.O is a membrane process that acts as a filter to remove up to 95 - 99 % of all dissolved minerals, 95 - 97 % of most dissolved organics, and more than 98 % of biological and colloidal matter from water

The water is passed over a membrane under pressure. Depending on raw water quality a larger or smaller part of water goes through membrane, leaving most of the dissolved solids behind. These solids and leftover water (reject) leaves membrane surface and is piped to drain. The water which goes through membrane and gets purified is called permeate. The equipment which accomplishes the process is called a Reverse Osmosis (R.O) Unit

An R.O Unit only passes some of Total Dissolved Solids (TDS) through membranes. The percentage which gets through is called TDS-passage or salt-passage. At a given value of salt-passage TDS, or quality of permeate will change as feed TDS changes.

An approximation of TDS in water can be attained by using a conductivity meter.

Along with the amounts of solids passing through membranes, to be considered is the amount of purified water retrieved from feed water. This is called recovery which is usually stated as a percentage. Normal recoveries are in the range of 40 - 80 % for brackish water and 10 - 40 % for sea water.

The recovery of R.O system is preset during commissioning. When recovery is changed, so is the salt-passage. On the other hand, if system is run at a lower recovery, there is a better permeate water quality.

The amount of water passed through membranes is directly proportional to the net pressure on them. Increasing system pressure increases permeate flow rate. Thus, if pressure is raised and reject flow rate is held fixed, the recovery goes up.

Similarly, if system pressure is held fixed and reject flow rate is made to decrease, the recovery also goes up. If, at a constant pressure, reject flow were stopped altogether, the recovery would be 100 %. There must always be enough reject flow to flush out rejected salts. The temperature of feed water can change the output by as much as 50 % in the range from nearly freezing to 40??? C. Unfortunately, it is not always possible to control temperature of the water, which varies depending on the location and time of the year.