The culture of silver carp, bighead, grass carp and common carp in secondary effluents of a pilot sewage treatment plant

Silver carp, bighead, grass carp and common carp were stocked at a total density of 3000 fish/ha in the proportion of 7:5:6:6 and cultured in two oxidation ponds at the Shek Wu Hui pilot sewage treatment plant. One pond received secondary effluents from the activated-sludge process and the other a mixture of effluents from both the activated-sludge and high-rate biological filter systems. The silver carp and bighead grew at a faster rate in the oxidation ponds than in a commercial pond as control. The growth of the common carp was satisfactory but that of the grass carp was poor. The stocking density of 3000 fish/ha could be increased further provided that the final effluent quality can be controlled. The mixture of effluents from the activated-sludge and the biological filter systems enhanced fish growth as compared with the activated-sludge system alone. However, the increase in BOD, chlorophyll a, and ammonia levels, together with the decrease in dissolved oxygen in the effluents from the biological filter system in the later stage of the trial caused heavy mortality of fish. Bacteria and heavy metals found in the fish were within safety limits for human consumption. More investigation is necessary to explain the exceptionally high mercury level.