Not all feedwater passes through the membrane, however. Some is required to flow over the membrane, sweeping away rejected impurities in a crossflow filtration mode. Therefore, from one feedwater stream, an RO machine produces one purified water stream called permeate. It makes a second stream called concentrate, brine, or reject. Feedwater enters the machine at fairly low pressure.
Water flows through prefilter cartridges to remove suspended particles, such as silt.
The prefilters provide an inexpensive way to assure that the membrane is kept clean. An automatic valve opens when the machine is turned on, permitting water flow to the pump, which provides the pressure necessary for operation. Permeate exits the sepralator under low pressure once again, due to the pressure drop through the membrane. First, the machine is designed to run by itself. It requires only periodic operating data collection and routine maintenance.