The culture of tilapia (Sarotherodon mossambica) in secondary effluents of a pilot sewage treatment plant

Juvenile tilapia, Sarotherodon mossambica, was cultured in two oxidation ponds at the Shek Wu Hui pilot sewage treatment plant. One pond received the 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. S. mossambica grew and survived well in the oxidation ponds. The respective yield rates were 1114 kg/ha-y and 1527 kg/ha-y without feeding at stocking densities of 2000 fish/ha and 1000 fish/ha, respectively. The stocking density could be considerably increased if the water quality of the culture medium could be controlled. Only male tilapia should be used for culturing, so as to avoid wild breeding. The mixture of effluents from the biological filter and activated sludge systems of the pilot plant contained higher nutrient contents than the effluent from the activated-sludge system alone. The former was found to be more productive. Bacteria and heavy metals in fish muscle were found to be within the safety limit for human consumption. More detailed investigation is recommended if large-scale production is to be introduced.