Anaerobic co-digestion of sewage sludge and food waste: staging and carriers enhance system performance and process stability

Sewage sludge (SS) generated from biological wastewater treatment requires adequate treatment. Anaerobic digestion of SS remains a preferred approach, but suffers from slow hydrolysis and low biogas production. Co-digestion with food waste (FW) could improve hydrolysis rate and nutrients balance, leading to improved biogas production. This study investigates the performance of eight semi-continuous anaerobic systems: single-stage (mono-and co-digestion) and two-stage (mono- and co-digestion). All the systems were operated with and without carriers (Anox Kaldness K5 for biofilm support) under mesophilic conditions for 205 days and at a total hydraulic retention time (HRT) of 20 days. In SS mono-digestion, biogas production averaged 230 ± 31 and 538 ± 40 mL_biogas L⁻¹ d⁻¹ in single- and two-stage systems with carriers, respectively. A maximum specific methane yield of 509 ± 54 and 367 ± 16 mL CH₄ g⁻¹_vs was measured in two-stage systems with and without carriers, respectively treating 20 % SS and 80 % FW (on wet weight basis) on day 134. Carriers improved biogas production and process stability by reducing volatile fatty acids (VFA) accumulation, with two-stage system with carriers achieves the highest cumulative biogas production and operational stability.