Reverse Osmosis plants for ships & offshore platforms

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 (8) 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. 

RO plants typically have a single or a double pass configuration. In double pass plants the treated water passes through the membranes twice. The permeate from the first pass becomes the feed water for the second membrane-pass via another high-pressure pump (7). Single pass RO plants typically supply freshwater under 500 ppm total dissolved solids (TDS). When high quality water with a TDS under 20 ppm is required (for example in boilers), a double pass RO plant is necessary.