Insight into the Mechanism of Biogas Upgrading with Hydrogen Addition in Thermophilic Anaerobic Digestion of Sewage Sludge

Integrating power-to-gas system with anaerobic digestion is an innovative approach towards sustainable energy generation where biogas production is enhanced through the addition of clean hydrogen. However, concerns regarding its stable performance and inhibition control remain particularly in thermophilic conditions. The objective of this research is therefore to develop a feasible and energy-efficient operation of biomethanation by assessing the hydrogen dissolution rate and biogas productivity in a thermophilic anaerobic digestion system. Two digesters (2 L) were fed with sewage sludge and operated simultaneously under 55.0 ± 0.1°C. Results revealed low equilibrium hydrogen gas concentration (< 1%) in the biogas implying an effective hydrogen gas-to-liquid mass transfer at a hydrogen injection rate of 0.54 NL L-reactor∁E d∁E. At this loading rate, high methane concentration (81 ± 1%) was observed with biogas yield up to 0.67 NL g-(volatile solids added)∁E. However, a sudden increase in loading resulted in a significant decrease in biogas production where possible inhibition due to high pH and volatile fatty acids accumulation had occurred. The findings from this study provide fundamental information to understand the underlying mechanism and performance of exogenous hydrogen addition in the digestion process.