A comparative study on biogas production, energy balance, and nutrients conversion with inter-stage hydrothermal treatment of sewage sludge

Abstract

As an alternative to pre-stage hydrothermal treatment (HT) before anaerobic digestion (AD), inter-stage HT (i.e., AD-HT-AD) has been proposed to increase biogas production and to further reduce the residual organic matter. The goal of this study was to evaluate the effect of inter-stage HT at 155 °C on the ultimate biodegradability and AD extent of sewage sludge mixture (i.e., primary and waste activated sludge). The sludge ultimate biodegradability was evaluated through biochemical methane potential tests. AD-AD and AD-HT-AD configurations were investigated in semi-continuously fed bench-scale digesters in terms of methane production, solids reduction, nutrients transformation, and energy balance. Results were compared with those of AD and HT-AD configurations from our previous study. Inter-stage HT increased the ultimate biodegradability of the sludge mixture; however, pre- and inter-stage HT resulted in comparable overall specific methane production. Compared to AD and HT-AD, AD-AD and AD-HT-AD had comparable methane production, higher VS destruction (by 3.4–9.3%), but lower overall crude protein removal (by 4.0–7.5%) and soluble orthophosphate concentration decrease (by 32.5–60.8%). There was minimal difference in net energy production by AD and AD-AD (single digester vs. two digesters; 1.4 GJ/d), as well as by HT-AD and AD-HT-AD (pre-HT vs. inter-HT; 0.4 GJ/d). High HT heat recovery is needed for HT-AD and AD-HT-AD to obtain energy balance comparable to AD and AD-AD. Compared to single-step AD, the two-step AD process is more complex and thus less attractive for the digestion of sewage sludge with a relatively high ultimate biodegradability as was the case in this study. However, AD-HT-AD may be more beneficial considering post-AD sludge handling processes.