Long-term operation of anaerobic digestion of food waste under simplified conditions: effect on reactor performance.

Anaerobic digestion under simplified conditions can contribute to food waste decentralised management. However, there is an absence of knowledge on the effect of long-term operation under these conditions. This study aimed to evaluate the operational conditions of an anaerobic reactor treating food waste after long-term operation without temperature control and low-intensity mixing. For this, a demonstration-scale reactor (18.8 m3) was operated for 160 days, while stability parameters were used to control the applied organic loading rate (OLR). Stability parameters indicated that the reactor was operated at stable conditions with an OLR of 0.3 kg VS m-3 d-1, while it was overloaded at an OLR of 0.4 kg VS m-3 d-1. This was observed through high value of intermediate and partial alkalinity ratio (IA/PA ratio), 2.5, and low values of bicarbonate alkalinity and pH, 1800 mg CaCO3 L-1 and 6.8, respectively. Moreover, there was a change in the concentration of intermediated metabolites, with a higher content of propionate and acetate, 1080 and 3775 mg L-1, respectively. Consequently, the methane production rate was decreased from 0.12 to 0.08 m3 CH4 m-3 d-1 and methane yield from 0.43 to 0.15 m3 CH4 kg VS-1. The reactor instability at a relatively low OLR was most probably due to hydrodynamic factors caused by the accumulation of recalcitrant material. Therefore, this material reduced the reactor performance and requires attention for a sustainable long-term operation.