Anaerobic digestion of food waste under non-conventional operating conditions: enhancing process stability while reducing energy and chemical inputs

Decentralized anaerobic treatment presents a sustainable alternative for managing the substantial generation of food waste. However, the associated high costs may render this option less feasible for small- to medium-scale systems. To explore a novel strategy for cost reduction in anaerobic digestion systems used for food waste treatment, this study applied organic loading rates (OLR) ranged from 0.6–1.9 kg VS m⁻³ d⁻¹ in a reactor under non-conventional conditions combined in anaerobic reactors: non-controlled temperature and pH, intermittent mixing, semi-continuous feeding, and biomass sedimentation. To evaluate the reactor's performance for 194 days, key parameters such as process stability, volatile solids (VS) removal, methane production, and biomass accumulation were monitored. Throughout the operation, the reactor temperature fluctuated between 19 and 36 °C, while the sludge retention time (SRT) values were up to 140% higher than the hydraulic retention time (HRT), indicating effective decoupling compared to the equal SRT and HRT observed in the conventional continuous stirred tank reactor (CSTR) model. Compared to conventional CSTR operation, the intermittent mixing approach still achieved an 80% reduction in mixing energy requirements. The results demonstrated that the optimal conditions were achieved at an OLR of 1.5 kg VS m⁻³ d⁻¹ and an HRT of 36 days. Under these conditions, a VS removal efficiency of 82% was obtained, with a methane production rate of 0.645 m³ CH₄ m⁻³ d⁻¹. Overall, the anaerobic reactor demonstrated stable performance under simplified operational strategies, reaching efficiencies comparable to conventional mesophilic systems with temperature control and alkalinity supplementation.