Study on BOD Removal Efficiency in a Rural Sewerage Facility with a Submerged Biological Filter under Recirculation Operation

Abstract

BOD removal performance was investigated using observed data on rural sewerage facilities with a submerged biological filter under recirculation operation. It was found that the probability distribution of influent and effluent BOD in rural sewerage facilities agreed with the logarithmic normal distribution. BOD removal in an anaerobic filter tank was assumed to depend on the recirculating ratio, influent BOD concentration, and effluent SS concentration. The multiple regression equation between BOD removal and the three factors can be obtained from observed data. The predicted values of BOD removal in an anaerobic filter tank were approximately in agreement with the observed values. The deposited sludge on the bottom of an anaerobic filter tank have considerable influence on the effluent SS concentration from the anaerobic filter tank. The control of sludge thickness is effective in maintaining BOD removal efficiency in an anaerobic filter tank. The multiple regression equation between BOD removal and sludge thickness was effective in estimating the removal efficiency in an anaerobic filter tank. BOD removal in a contact aeration tank was also influenced by four factors, i.e., influent BOD concentration, hydraulic loading rates, effluent SS concentration, and effluent ATU-BOD concentration. It is possible that BOD removal in a contact aeration tank can be predicted by four factors. Effluent SS concentration from a sedimentation tank was approximately linear in the range of observed hydraulic loading rates in a contact aeration tank, so the effluent SS concentration was predicted from the hydraulic loading rates. The BOD removals in an anaerobic filter tank and contact aeration tank have limitations due to elution of accumulated biofilm in the filters.