A system with a submerged biofilter was employed for purifing pond water. The slime on the biofilm has a high ability of adsorption of pollutants when BOD loading is below one tenth of the ordinary. The circulating number, the ratio of the flow rate to the pond volume, is important for total purifing system. The number is 1-2d^-1 under the conditions in which the flow of the pond is complete mixing type, the specific growth rate of algae is 0.5-1d^-1, and the removal ratio of SS is 0.5.
The experimental biofilter bed for the pond water of 450m³ was 0.45m width with a depth of 0.6m. The bed packed with the media of 22mm pitch corrugated plates was elongated about 80m to introduce the adsorption ability and to deduce the excess sludge and was not aerated. The result showed that the quality of the pond water maintained BOD 1mg·l^-1, SS 0.9mg·l^-1, and transparency > 100cm. The removal ratio of SS on the bed was about 60%.
This is an energy saving system because the filtering head of the bed is lower than that of filters such as a sand filter or a filter cloth.