Progress and kinetics of aerobic digestion of secondary sludges

Abstract

Present investigations were undertaken to further explore the endogenous decay in wide ranges of total solids concentration of activated sludge (1000–80,000 mg l −1, initial pH range of 4.5–14.0 and digestion temperature range of 5–60°C. Batch aerobic digestion studies on waste activated sludge indicate that BOD 5 and COD of activated sludge undergoing aerobic digestion can be used for expression of aerobic digestion kinetics. Digestion period depended on and increased with initial solids (biomass) concentration. Percent solids reduction (and not the digestion period) was greatly affected by the digestion temperature. Data analysis offers predictive models for digestion period and progress during aerobic digestion in 5–35°C temperature range. Kinetic rate constants were observed to decrease with increasing concentrations of initial solids and BOD 5/COD values and to follow exponential relationship with them. Kinetic rate constants increased with digestion temperature. Initial pH around 8.0 was the optimum pH value yielding minimum digestion time and maximum values for kinetic rate constants. The efficiency of removal, digestion time and the kinetic decay coefficient have been modelled individually as well as compositedly.