A typical RO plant onboard is a compact unit with a much smaller footprint than a flash evaporator. It has a capacity of 12-15 m3/hr depending upon seawater quality and the condition of the filters.
A cruise ship can have one or two RO plants, depending upon the requirement. A typical RO plant found onboard cruise ships is shown below.
Seawater is supplied by a low-pressure feed pump through a coarse strainer. This coarse strainer separates medium-sized impurities such as mud, some shells etc that might have escaped past the sea-suction strainers. From here the seawater is further filtered by the sand filters, which are usually two in number.
They are effective in the removal of solid particles and micro-organisms from the water. Sand filters themselves require backwashing by either seawater or fresh water at least once a day when the plant is operational or before shutdown. This is to ensure continued plant efficiency.
After the sand filters, the water is dosed/treated with an antiscalant chemical. As the name suggests, this is to prevent/inhibit scale-formation on the RO membrane surfaces which can result in fouling and loss of plant efficiency. Thereafter, the water undergoes fine filtration in the cartridge filter. This is a 5–10-micron filter which separates finer impurities before the water reaches the membranes.
The cartridge filter is replaced according to differential pressure which is when it exceeds 1 bar. The filtered water is now supplied to the membranes by the High-Pressure pumps which are usually 3 or 4 in number.
The HP pumps operate at a pressure of 50-70 bar and aid in the final step of the RO process, the separation of dissolved salts during the passage through the membranes. The membranes consist of 3-4 banks. Each bank has a set of membranes, one of which is a primary which receives water from the HP pumps as feed and produces clean freshwater as permeate and reject which is a solution of the separated salts from the feed.
The clean permeate goes out from the plant as produced freshwater whereas the reject goes to the secondary membrane as feed where it again splits up to permeate and reject. The permeate produced joins the clean permeate outlet whereas the final stage reject goes overboard.