Hydrothermal pretreatment of sewage sludge for enhanced anaerobic digestion: Resource transformation and energy balance

Abstract

The objective of this study was to evaluate the effect of hydrothermal pretreatment (HTP) at 90, 125 and 155 °C on the extent of anaerobic digestion (AD) of sewage sludge, energy balance, and the distribution and availability of N and P. AD was evaluated with four bench-scale mesophilic, semi-continuous digesters with 20-d (Phase 1) and 10-d (Phase 2) solids retention time (SRT), fed with raw sludge (R1), 90 °C (R2), 125 °C (R3) and 155 °C (R4) HTP sludge. The two sludge mixtures used in this study had high ultimate biodegradability (58.3 for Phase 1 and 56.9% for Phase 2). With the exception of Phase 2 R4, HTP increased organic matter destruction and methane production. HTP of Phase 2 sludge mixture at 155 °C resulted in the formation of high propionate levels, which led to a lower methane production. HTP significantly changed the digesters’ bacterial communities, but had a minor effect on the archaeal communities. Abundance of well-known propionate degraders in all digesters was very low; however, Proteiniphilum, which is involved in the degradation of intermediates in the propionate degradation pathway, was found in relatively high abundance in Phase 2 R4. The highest net energy gain (ΔE) was obtained with the control (i.e., raw sludge AD without HTP) for both phases. HTP heat recovery greater than 85% is required to attain the same net energy as the control or higher. HTP at 155 °C followed by AD led to increased solids reduction, overall crude protein removal and release of ammonium N. HTP-AD decreased P availability. Thus, P and N recovery is recommended before and after HTP-AD, respectively.