Thriving on sewage

both the public as well as the industry in Chennai frequently face acute water problems. Sometimes, industrial units are left with no option but to close down. Madras Fertilisers Ltd ( mfl ) and Madras Refineries Ltd ( mrl ), located at Manali on the outskirts of the state capital were among the units facing this problem. But, unlike others, they looked for an alternative source of water and found it in the form of the city's sewage.

The city sewerage system has its primary and secondary treatment plants at Kodungaiyur, about 4 km from Manali. Both mfl and mrl decided to build a plant capable of producing enough water from the secondary treatment plant, which would be used for cooling.

The Kodungaiyur plants treat effluents from two of the five city zones. The primary treatment involves removing floating matter followed by grit removal and sedimentation of solids. The secondary treatment level uses an "activated sludge' process consisting of floating aerators and purifiers. Effluents at this level have relatively high levels of chemical oxygen demand ( cod , which indicates the amount of chemicals in the water), besides quantities of ammonia, phosphorous, alkalinity, total salinity and chlorides