A series of oxidation ponds and subsequent sand filtration experiments was performed to improve the water quality of secondary effluent containing organics and nutrients. The results show that the main reaction mechanism in the oxidation pond is the fixation of inorganic nutrients by algae. T-N and T-P concentrations in pond effluent decrease to 80 % and 77 % of influent respectively owing to the sedimentation of the particulate fraction. A sand filtration experiment on the pond effluent was conducted. At a filtration rate higher than 5 m/day, physical filtration mainly takes place, and clogging of the sand filter occurs within 1–3 days. However, at a rate lower than 1 m/day, the process of decomposition of deposits is also observed, and the run length becomes longer (> 20 days). The removal efficiency of SS is very high, ranging from 97 to 100 %. The total removal efficiency by an oxidation pond with sand filtration treatment is 24 to 37 % in T-N and 32 to 75 % in T-P. The most rational filtration rate for good performance seems to be 0.5 m/day because of its long run length. The effect of fish (Rhinogobius brunneus) on this treatment becomes obvious, prolonging the run length by 20 % or more due to feeding on the deposits.

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