Treatment of wastewater for irrigation in Mauritius

When complete, the St Martin Sewage Treatment Plant will serve nearly a third of Mauritius’ 1·1 million population. As well as protecting the marine environment, it will provide a valuable source of water to supplement irrigation supplies to west-coast sugar-cane plantations. This paper explains: (a) the treatment process selection which favoured conventional activated sludge over extended aeration; (b) the need for nutrient removal; and (c) the choice of ultraviolet (UV) irradiation for disinfection and problems of dyes from the substantial textile industry. It also describes the complexity of the effluent discharge control system designed to: (a) release a tertiary quality effluent to the irrigation canals blended with the present surface water supplies to avoid sodium accumulation in the soils; (b) avoid flooding the canals; and (c) provide an emergency storm flow discharge via the same common outfall pipe to the sea. Construction of the new treatment plant at St Martin began in January 2001 and is due to be completed in August 2004.