Reclamation of wastewater for polyculture of freshwater fish: Bioassays using Chlorella and Gambusia

The feasibility of using the secondary effluents from two sewage treatment plants in Hang Kang (Yuen Long and Shek Wu Hui) for fish culture was assessed. Total ammonia contents in the two sewage effluents surpassed the level of 2 mg L^-1 recommended for wastewater fish culture. The two sewage effluents both needed essential elements for supporting algal Chlorella vulgaris growth? whereas only sewage effluent from Yuen Long had contaminants at toxic levels. Total ammania in water explained more than 80% variations of toxicity of water samples to Gambusia patruelis, mosquito fish, according to regression analysis. Removal of ammonia from the sewage effluent is necessary before being used for fish culture. In addition, the rather high levels of heavy metals (Cu, Zn, Cd) in the effluent should be closely monitored.