During reverse osmosis, pretreated water is pumped past the input surface of an RO membrane under pressure (typically 4-15 bar, 60-220 psi) in cross-flow fashion. RO membranes are typically thin film composite (polyamide). They are stable over a wide pH range, but can be damaged by oxidizing agents such as chlorine, present in municipal water. Pretreatment of the feedwater with microporous depth filters, softener and activated carbon is usually required to protect the membrane from large particulates, hardness and free chlorine. Typically 75%-90% of the feedwater passes through the membrane as permeate and the rest exits the membrane as concentrate, that contains most of the salts, organics, and essentially all of the particulates. The ratio of the volume of permeate to the volume of feedwater is referred to as the "recovery". Operating an RO system with a low recovery will reduce membrane fouling, especially that due to precipitation of low solubility salts. However, recoveries of up to 90% are possible, depending on the quality of the feedwater and the use of filtration and softening pretreatment.

The performance of the RO component of a water purification system is typically monitored by measuring the percent ionic rejection, which is the difference between the conductivities of the feed and permeate divided by the feed conductivity, calculated as a %. The "ionic rejection" and "recovery" will vary with the feedwater, the inlet pressure, the water temperature and the condition of the RO membrane.

Due to its exceptional purifying efficiency, reverse osmosis is a very cost-effective technology for the removal of the great majority of impurities. Reverse osmosis protects the system from colloids and organic fouling. It is often followed by ion exchange or electrodeionisation. Reverse osmosis units need periodic cleaning & sanitisation with acid and alkaline solutions. Specially constructed membranes are available for hot water sanitisation at 85°C.