Enhancing energy recovery of wastewater treatment plants through hydrothermal liquefaction

Sewage sludge (SS) management constitutes both a challenge and an opportunity for the sustainability of wastewater treatment plants (WWTPs). Standalone anaerobic digestion (AD) stabilizes the biodegradable organics contained in SS but recovers only a fraction of the chemical energy stored therein and produces large amounts of un-stabilized sludge. Hydrothermal liquefaction (HTL) coupled with AD can enhance the treatment and energy recovery of SS. Standalone AD was compared against (1) an HTL-AD configuration, with SS as HTL input, and the generated aqueous product (AP) as AD input; and (2), an AD-HTL-AD configuration, with SS as AD input, the digestate as HTL input, and the generated AP as AD input. Both configurations decreased the SS' COD from 27.5 to 0.6 g L⁻¹, while the overall energy recovered was increased up to 2.2-fold relative to conventional SS treatment using only AD. Under the HTL-AD configuration, biocrude yields were higher (i.e., 26.4 vs. 15.8) and the AP generated was more biodegradable (0.78 vs. 0.65), than those obtained under the AD-HTL-AD configuration. Monte Carlo uncertainty analyses confirmed that overall energy recoveries would follow the order AD-HTL-AD > HTL-AD > AD; with energy recoveries (95% confidence) between, 63.5-94.7%, 54.6-91.2%, and 33.2-71.1%, respectively. This study shows that, by implementing HTL as a standalone SS treatment, WWTPs can recover more energy than using AD alone. Furthermore, WWTPs with existing AD would recover additional energy through HTL of the generated digestate, significantly reducing the environmental impacts and costs of conventional solids management.