Reclamation of wastewater for polyculture of freshwater fish: Wastewater treatment in ponds

The main objective of the present study is to investigate the possibility of wastewater treatment in ponds for fish culture. A series of eight ponds (Au Tau pond system) were constructed at Au Tau Fisheries Office, Hong Kong Agriculture and Fisheries Department. Polluted river water was pre-treated in the first four ponds by providing sedimentation (S1, S2) and aeration (A1, A2), the effluent from A2 was introduced into the remaining four ponds (C1–C4) for fish culture. Fries of six species of freshwater fish including silver carp, big head, grass carp, tilapia, common carp and black bass were stocked in the ponds (C1–C4) during 11/93–2/94, and 1/4 of normal fish feed (peanut cake) were used in the fish ponds. The pond water physico–chemical and microbiological qualities were monitored during the pond operation period (7/93–9/95). The results showed that the average removal efficiencies of BOD, NH3–N and PO4–P in the ponds (S2–C4) were 73.5%, 93.4% and 82.7%, respectively, while more than 99% of pathogen indicators (total coliform, fecal coliform and E. coli) were removed. Up to 97% total coliform, 95% fecal coliform and 95% E. coli were removed in S2 (the 2nd sedimentation pond). The water treatment ponds (S2–A2) played an important role in removing pollutants (organic matter, nutrients and pathogenic bacteria) and algal promotion, while the fish culture ponds (C1–C4) also demonstrated the high ability in PO4–P and NO2−–N removal. The average fecal coliform concentrations in fish ponds C1–C4 (6.35×103 no. 100 ml−1) were within the range of WHO guideline (103–104 no. 100 ml−1), but a maximum of 2.3×104 no. 100 ml−1 was detected in pond C4 in May, 1995.