Nutrient conditions influence size distribution and settling properties of activated sludge flocs

The study aimed to investigate the relationship between activated sludge (AS) particle (floc) size distribution and nutrient conditions in different bioreactor configurations. Size distribution profiles of fl ocs that formed in continuous (B1), continuous with clarifier and return sludge (B2) and SBR (B3) reactors were investigated in parallel under identical nutrient conditions. An eight-fold dilution of the influent COD of a synthetic dairy processing wastewater resulted in a “feast and fast” regime that triggered significant effects on the biomass and flocculation characteristics. Floc size analysis of reactor MLSS revealed a shift in floc sizes when reactors were fed with the mi nimum COD wastewater feed (0.61 g L-1 ). Increasing floc size distributions were detected for all reactors during the low COD feed though different size patterns were observed for the different reactor configurations. These increases corresponded with variations in aggregation indices and EPS quantities. The SBR yielded comparatively larger flocs when operated under both maximum and mi nimum COD feeds as indicated by d(0.9) values (90% of particles ? d in size). Overall the results indicated that floc for mation and floc size are mediated by nutrient concentrations and represents an important step towards improved process control .