RO is based on the process of osmosis. Osmosis involves the selective movement of water from one side of a membrane (a plastic film that looks similar to cellophane) to the other. To make the process work, pressure is applied to the contaminated water, forcing water through the membrane. Since contaminants do not move with the water as it moves across the membrane, purer water collects on the other side of the membrane. The purified water that accumulates on one side of the membrane can then be used or stored. A specific amount of pressure is necessary to separate purified water and contaminants. This required pressure is based on the type and concentration of contaminants in the water. Supplying even more pressure to the contaminated water than is required provides better separation and a higher production rate.

The levels of most dissolved compounds and suspended matter present in water can be reduced by RO treatment. The efficiency with which membranes reject the contaminant molecules depends on the pollutant concentration and chemical properties of the pollutant. Membrane type and operating conditions will also affect the degree of pollutant removal.

Efficiency of removal is often described using the term “rejection percentage,” which is the percent of a particular contaminant that doesn’t cross the membrane, i.e., is rejected by the membrane. It is important to know not only rejection percentages, but also incoming pollutant concentrations to effectively reduce contaminant concentrations in the drinking water to safe levels. Basic components of an RO system should include a prefilter to remove fouling agents such as rust and lime; an RO module containing the membrane; an activated carbon postfilter to remove residual taste, odor and some compounds from the purified water; a storage tank; and various valves, including a shut-off valve that stops the water flow when the storage tank is full. The system must also provide for waste flow to drains. Prefilters containing activated carbon are commonly used to protect chlorine-sensitive membranes.

Reverse Osmosis System Technical Specifications