Relative kinetics of anaerobic digestion under thermophilic and mesophilic conditions

With several advantages over the conventional mesophilic anaerobic digestion, such as better sludge quality and higher biogas production, thermophilic anaerobic digestion is regarded as a promising alternative for sludge digestion. Primary and activated sludges are complex materials, and historically, analysis of kinetics has been largely on whole sludge, without analysis of individual components. This paper analyses relative digestion kinetics of pure substrates designed to target main stages of sludge digestion under thermophilic and mesophilic conditions. Hydrolysis rate of cellulose was significantly influenced by temperature with hydrolysis coefficients of – at 55 °C (0.7 ± 0.1 day⁻¹), 60 °C (0.8 ± 0.2 day⁻¹), 65 °C (1.1 ± 0.2 day⁻¹) and 70 °C (1.2 ± 0.2 day⁻¹) over 38 °C (0.4 ± 0.1 day⁻¹). This strongly follows the Arrhenius relationship, with an activation energy (E_A) of 31 ± 4 kJ mol⁻¹, corresponding to an increase of 1.5x for each 10 °C of temperature increase. Glucose uptake was rapid with a wide variety of fermentation products detected under mesophilic conditions, while uptake was slower under thermophilic conditions with acetate and propionate being dominant products. Propionate acetogenesis and acetate-utilizing methanogenesis kinetics were not influenced by temperatures. Hydrolysis is widely regarded as a rate-limiting step in sludge digestion, thus improvements in hydrolysis rates as measured during this study have the potential for significant improvements in overall apparent sludge digestion rates.