Thermophilic anaerobic acid digestion of biosolids: hydrolysis, acidification, and optimization of retention time of acid digestion

The objective of the study was the optimization of the retention time in a thermophilic acid digestion system. To achieve this optimization, acidification index defined as the ratio between acidification rate and hydrolysis rate was used. First-order reaction model developed by Eastman and Ferguson (1981) was used to describe hydrolysis of particulate matter in sludge. The first-order hydrolysis rate constant was found to be 2.36 d^–1, and the initial concentration of degradable particulate COD was found to be 10.4 g/L. A model of acid digestion was proposed that included acidogenic biomass growth and decay. The total acidogenic population was estimated and kinetic parameters of acidogenic population were specific growth rate 0.78 d^–1, net specific growth rate 0.25 d^–1, decay coefficient 0.52 d^–1, and observed yield 0.05 mg biomass / mg COD consumed. Based on the acidification index, the optimum retention time in a thermophilic acid digester was 3–3.5 d. Key words: wastewater sludge, biosolids, anaerobic digestion, hydrolysis, acidification, kinetics.