Holistic Assessment of Sewage Sludge Co-digestion With Glycerol: Process Performance, Priming Effect, and Microbial Dynamics

Abstract Mono-digestion of sewage sludge (SS) is limited due to low C/N ratios, a low organic matter content and poor biodegradability of the organic fraction. Several studies recommend its co-digestion with crude glycerol (CG), a by-product of biodiesel production, to overcome these limitations. As previous studies primarily focused on process performance, this study aimed to conduct in-depth investigations into the co-digestion of SS with glycerol, combining microbial, biochemical, and process-related aspects, with a focus on the potential priming effect induced by glycerol. To avoid cross-effects, the co-digestion of SS with pure glycerol (PG) instead of CG was investigated at concentrations from 0% to 35% based on chemical oxygen demand (COD). Co-digestion with 35% PG increased methane yield by up to 148% compared to the control. Additionally, the reaction kinetics improved, with production rates reaching 85 mL h⁻¹ at 20% PG compared to 49 mL h⁻¹ in the control, enabling more targeted and demand-oriented biogas production. The lag phase was also reduced by 80%. However, no priming effects were observed. At higher PG concentrations (≥ 10%), COD degradation of PG was incomplete, while at lower concentrations (≤ 5%), COD was fully degraded. Further, co-digestion with PG did not change the dewatering properties of the digestate. PG altered microbial diversity and evenness without affecting richness, though no clear patterns emerged. Metaproteomic analysis showed that higher PG concentrations boosted methanogenesis, increasing it by 87% at 35% PG compared to the control. Overall, PG co-digestion is effective for boosting methane production, and low to moderate PG loadings (< 5%) are most efficient.