Kinetic study of two-step mesophilic anaerobic-aerobic waste sludge digestion: Focus on biopolymer fate

Kinetics of the sequential anaerobic-aerobic digestion operated under mesophilic conditions on waste activated sludge of a full-scale wastewater treatment plant was investigated. Special focus was given to the fate of proteins and carbohydrates, given the influence of biopolymers on dewatering properties of the sludge. Kinetic tests were performed to characterize the suspended solid degradation and the trend of biopolymers in both digestion steps. Volatile solid degradation rates were 0.93 and 0.52 kgVSm-3 d-1 in anaerobic and aerobic conditions, respectively. Different models (1st order, Michaelis-Menten, Valentini and Contois) have been compared for VS degradation: Contois equation provided the best data fitting (correlation coefficients >=0.99). Evolution of biopolymers during two-step process exhibited a similar pattern: during the anaerobic phase, an increase of about one order of magnitude was observed for carbohydrates and of 100% for proteins, while in the aerobic bioreactor both decreased of 29 and 73%, respectively. Data from kinetic tests were employed to model the biopolymer patterns taking into account their production from the hydrolysis of particulate organic substrate and their biodegradation in the different anaerobic and aerobic reaction environments. Michaelis-Menten equation gave satisfactory predictions of the biopolymer fate with correlation coefficients ranging from 0.92 to 0.97, for both carbohydrates and proteins.