Relationships between biological solids retention time and settling characteristics of activated sludge

Abstract It was the objective of this study to delineate the relationships between the settling characteristics of a non-filamentous activated sludge grown on a soluble waste, and a rational kinetic based parameter used to characterize the growth or loading rate of the system. The parameters used to characterize settling were Sludge Volume Index (SVI), zone settling velocity, and percentage dispersion. Biological Solids Retention Time ( θ c ), the reciprocal of mean cell growth rate, was used to characterize the metabolic activity level of the activated sludge. The chosen parameter ( θ c ) can be related to food to microorganism ratio and other widely used organic loading expressions. A series of bench scale laboratory aerobic reactors of the Ludzack type were operated as continuous flow systems with internal microbial solids recycle at a constant temperature of 20°C. A 6-h hydraulic retention time was maintained in all units. The values of Solids Retention Times studied varied from 12 days to 0–25 days which corresponds to a range of F/M (lb. BOD 5 lb. −1 MLSS-day) of 0·38–10·2. Operational parameters measured included COD, MLSS, dissolved oxygen, pH, oxygen uptake rates, and nitrogen content of the MLSS. Once steady state operation of the reactors was achieved, settling characteristics, i.e., SVI, zone settling velocity and percentage dispersion, were determined on mixed liquor samples withdrawn from each reactor. In addition, microscopic examination of the activated sludge floc was performed at each value of θ c . Experimental results were analyzed graphically to assess the nature of the relationships between each of the three measured settling parameters and θ c . Based on a criterion of minimal time rate of solids lost to the effluent, it appears that θ c values should be maintained in the range of 4–9 days. The implications of results of this study for design and operation of activated sludge systems are discussed.