Seawater is pumped into the RO system by a seawater pump (1). Incoming water is subjected to double filtration. In the first prefilter stage (2) the seawater is filtered from all suspended solids up to 40-50 micron for large RO plants, and 5 microns for small standard plants. A second prefilter stage (4) further filters the water down to 5 microns for large RO plants, and 1 micron for small standard plants. In large RO plants a UV reactor (3) is installed between the two filtration stages to disinfect the filtered water and avoid bacterial growth on the RO membranes.
The filtered seawater is then pumped through the RO membranes by a high-pressure pump (5). Semi-permeable RO membranes (6) remove ions, particles, and bacteria from seawater to generate permeate/freshwater. The permeate leaves the membranes, while the brine is diverted for discharge. Optional post-treatment (9) consists of pH correction, re-hardening and remineralisation. Large RO plants offer automatic flushing with low salinity water to remove the salty water from the RO housing.