The Fishy Business of Waste; April 1995; Scientific American Magazine; by Mukerjee; 1 Page(s)
Fish love sewage, and the inhabitants of Calcutta love fish. These city dwellers also produce sewage: 700 million liters of it a day. By growing fish on this waste, fisherfolk to the east of the Indian metropolis discovered how to link these needs at the turn of the century. But this natural recycling system--the largest of its kind and possibly the oldest--is threatened by a state government that sees profitable real estate in the expanse of shallow waters.
The fisheries flourish on elaborate folk technology. Diverted into a series of ponds, the sewage sheds its putrescent solids. Air-breathing fish and water plants survive in pools nearest the sewage inlet. Further downstream, algae bloom on nutrients in the clear water. Tilapia and carp, in turn, thrive on the algae. More than 20 tons of these fish feed the Calcutta market every day. (Bioassays of these fish show that their levels of heavy metals and coliform bacteria are low.) The algae also provide much needed oxygen to the heavily polluted city.