Wastewater treatment by using a series of bioreactors with submerged net plates, on which microbial growths could be developed, was studied to evaluate its treatment performance and to find suitable operational conditions for sufficient BOD and nitrogen removal. A comprehensive experimental study was carried out by using a bench-scale plant which treated artificial wastewater and a pilot-scale plant which treated municipal wastewater.

This process treated well the wastewater with higher pollutant loads in comparison with the conventional activated sludge process. The wastewater containing 200 mg BOD/l was reduced to 20 mg BOD/l at the high BOD loading of 1,94 kg/m3 day and the retention time less than 5 hours. Nitrogen removal could be explained well by an index defined as the product of the water temperature and the retention time. When the ratio, in flow rate, of recycled water to influent was 4 and the index was more than 400, nitrogen removal more than 80 % was obtained.

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