Kinetics of thermophilic batch anaerobic digestion of thermal hydrolysed waste activated sludge

Abstract Enhancing waste activated sludge (WAS) anaerobic digestion by thermal pretreatment has become of high interest. However, thermal treatment has been mainly combined to mesophilic anaerobic digestion. This paper presents the combination of sludge thermal pretreatment (110, 165 and 220 °C) and batch thermophilic anaerobic digestion (55 °C). Optimal conditions of thermal pretreatment were shown to be 165 °C, involving a chemical oxygen demand (COD) and volatile solids (VS) solubilisation of 18 and 15% and a biodegradability increase from 47 to 61%. Treatments at 165 °C were carried out in electric and steam modes and no significant difference on the impact of heating mode on sludge anaerobic biodegradability was observed. Moreover, it may be recommended not to carry out successive batch experiments to assess thermophilic BMP of sludge as accumulation of volatile fatty acids (VFA), particularly propionate, and a decrease of VFA uptake rates may occur. However, thermal pretreatment at 165 °C allowed the decrease of propionate accumulation and an higher methane production.