Enhancing anaerobic co-digestion of municipal mixed sludge with fruit and vegetable waste: Insights from batch and CSTR trials

Anaerobic digestion is widely used to treat organic waste from the food industry and municipal wastewater treatment plants, converting organic matter into biogas. However, mono-digestion of municipal mixed sludge (a blend of primary and secondary sludge) faces challenges due to an imbalanced carbon-to‑nitrogen (C/N) ratio. Co-digestion has been proposed to address this issue, and fruit and vegetable waste provides a readily biodegradable carbon source that can enhance the anaerobic digestion of mixed sludge through synergistic effects. This study evaluated different mixtures of fruit and vegetable peel purées with municipal mixed sludge through batch trials, aiming to identify the optimal combination for full-scale application based on biogas yield and methane content. A blend of apple, banana, and carrot peel purées (40:30:30) was selected for further testing. Semi-continuous trials were conducted in a continuously stirred tank reactor (CSTR) with 10 and 15 % of the selected blend. The 10 % co-substrate addition improved biogas and methane production by 19.9 and 21.2 %, respectively, in batch tests. In CSTR trials, co-digestion reduced hydrogen sulfide (H₂S) levels by 50–71 % and increased specific methane production by 28–44 %, compared to mono-digestion. An energy gain of 0.877 kWh/kgTVS was achieved with the 10 % blend. • Best performance mixtures were the ones with 40 and 50 % APP (vol.). • FVPW addition increased biogas production of up to 21 % in batch trials. • H 2 S reduction between 50 and 71 % when comparing AcoD with AD in CSTR trials. • Increment on SMP between 28 and 44 % from AD to AcoD in CSTR trials. • Additional 0.877 kWh/kg TVS in AcoD with 10 % FVPP in CSTR trials.