Optimal deployment of thermal hydrolysis and anaerobic digestion to maximize net energy output based on sewage sludge characteristics

Abstract

Thermal hydrolysis (TH) is widely employed in combination with anaerobic digestion (AD) to efficiently treat primary sludge and waste-activated sludge in municipal wastewater treatment plants. In this study, four different scenarios-conventional AD (S1), TH-AD (S2), AD-TH-AD (S3), and characteristics-based AD-TH-AD (S4, primary AD only for primary sludge)-were evaluated to determine the optimal deployment of TH and AD for treating primary sludge and waste-activated sludge to maximize net energy output. The maximum net energy output of 4899 MJ/t-TS_fed (per ton total solids of sludge fed) was achieved in S4 when assuming the recovered heat was only used for AD heating and surplus heat was wasted, and the net energy output of S4 was 70.8 % higher than that of S1 and 48.6 % higher than that of S2. This remarkable improvement was attributed to a reduction of > 15.2 % in refractory compounds, resulting in a 17 % increase in methane yield. Importantly, this study provides the first comparison of refractory compounds between inter-thermal hydrolysis (inter-TH) and pre-thermal hydrolysis (pre-TH) using a simulated A²O process. Overall, this study provides innovative insights and strategies for enhancing the TH and AD process performance based on the specific characteristics of sewage sludge derived from wastewater treatment plants.

Keywords: Energy balance; Inter-thermal hydrolysis; Primary sludge; Refractory compounds; Waste-activated sludge.