The use of crossflow microfiltration to enhance the performance of an activated sludge reactor

Abstract

Crossflow microfiltration (CFMF) was investigated as a substitute for secondary settling in the activated sludge process, to retain a greater biomass concentration in the reactor and increase the reactor chemical oxygen demand (COD) degrading capacity.The system was operated at six different steady-state conditions. Sludge age was varied between 5 and 20 days and chemical oxygen demand (COD) loadings ranged between 7.5 and 45.0 g l−1. The reactor suspended solids concentration was increased from 0.95 to 19.47 g l−1, over three times the secondary settling maximum of 6.0 g l−1. Effluent quality was good with 0.013 g l−1 suspended solids and COD removal above 97%. With secondary settling the effluent suspended solids concentration is commonly 0.020 g l−1 although this figure increases as the maximum reactor solids concentration exceeds 6.0 g l−1.For CFMF it was necessary to pump sludge under pressure through the filtration system. The pumping resulted in a particle size of 2 μm, in comparison with floc sizes of 100 μm in activated sludge systems operating with secondary settling. The size reduction had no deleterious effect on the reactor COD removal capacity. It was noticed, however, that it was not possible to settle the sludge particles.