Design and operation of the Laayoune sea water reverse osmosis plant

The largest destination plant currently in operation in Morocco is the Laayoune sea water reverse osmosis (RO) plant. It is operated by the Office national de 1'eau potable and has a capacity of 7000 m3/d. Pretreatment consists of chlorination; the acidification (H2SO4) required for adequate coagulation; coagulation using ferric chloride; pressure filtration through a sand filter; acidification (H2SO4) to reduce the precipitation of calcium carbonate at the membrane level; injection of an anti-scalent agent (Flocon 100) to reduce the precipitation of sulphates at the membrane level; microfiltration using 5 μm cartridge filters; and dechlorination using sodium metabisulphite. The plant uses a brine staging design concept, and has four trains, each with a capacity of 1750 m3/d. The RO section of the plant uses Dupont polyanide hollow fine fibre membranes. The design specifications are as follows: sea water total dissolved solids (TDS) of 40,000 ppm; a silt density index of <3; a maximum pressure of 70 bar, a recovery ratio of 45%; and a desalinated water TDS (product) of 1000 ppm. The plant was commissioned in November 1995 and has been in operation ever since, operating at a constant production of 1750 m3/d per train. After 7000 hours of operation, the key operating conditions are as follows: an operation pressure of 60 bar; a product salinity of 750 ppm; and a power consumption of 5.52 kW·h/m3. The main design features of the plant and the experience gained in the first year of operation are outlined. (author)