Composted urban refuse and primary sewage sludge as a fish-pond manure

India produces per day about 44 000 tonnes of urban refuse having a pollution load of 59·4–68·2 tonnes. The annual nutrient potential of this waste is 0·084, 0·036 and 0·043 million tonnes of N, P and K, respectively. The refuse from Calcutta alone represents 0·005 million tonnes of N and 0·002 million tonnes each of P and K out of the total. The waste, in combination with sewage sludge, on aerobic incubation produces a compost with 1·34% nitrogen and a C/N ratio of 12·17 within 90 days. The compost contains 1·64 ppm copper and 0·84 ppm zinc, but compost-manured water contained only 0·08 and 0·03 ppm of the metals, respectively. The mineralisation rate of 30·51% on the twentieth day and the highest BOD 5 load of 18·93 mg/litre on the tenth day indicate a moderate transformation rate of the compost and low toxic effects of copper and zinc. The impact of the compost on the diurnal oxygen changes in the pond water was found to be non-detrimental to aquatic life. The compost-manured water showed an overall increase of 33% over the control in its nutrient levels, primary productivity and plankton production.