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

The objective of this study was to evaluate the effect of hydrothermal pretreatment (HTP) at 90, 125 and 155 degrees 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 degrees C (R2), 125 degrees C (R3) and 155 degrees 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 degrees 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 (Delta E) 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 degrees 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.

Automated summary

The study evaluated the impact of hydrothermal pretreatment (HTP) on anaerobic digestion (AD) of sewage sludge at different temperatures. HTP increased organic matter destruction and methane production, but high temperature HTP led to increased propionate levels and lower methane production. The bacterial communities in the digesters were significantly affected by HTP, while the archaeal communities showed minor changes. The highest net energy gain was observed in the control group, and a heat recovery greater than 85% is needed to achieve a similar net energy gain with HTP.