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

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