Activated sludge process combined with biofilm cultivation

Abstract The effect of secondary fixed-film biomass on the occurrence of filamentous microorganisms in the biocenosis of activated sludge and on nitrification has been studied in four laboratory systems. These sytems consisted of completely mixed tanks in which dispersed plastic foam particles, partly submerged rotating discs and fully submerged plastic discs served as support material for the attached growth. The last system was operated as a control unit with suspended biomass only. In comparison with the simple activated sludge system, where filamentous bulking occurred, the presence of a biofilm positively affected the activated sludge settling properties, especialy in the system with plastic foam particles. Besides the effect of decreasing the activated sludge loading, the anoxic zone formed in biofilms contributes to suppressing the growth of filamentous microorganisms in the mixed liquor. It has been demonstrated that the greater the portion of total biomass in the system fixed in the biofilm, the lower the SVI-values of the suspended biomass. It has been also found that in the systems where biofilm was present, nitrification became independent of the solids retention time of the suspended biomass because nitrifying bacteria were predominantly attached on support materials.