Stable and high-rate anaerobic co-digestion of food waste and cow manure: Optimisation of start-up conditions

Abstract

Food waste (FW) and cow manure (CM) were co-digested to achieve a stable and high-rate of methane production. The start-up conditions (substrate mixing (FW/CM) ratio, substrate to inoculum ratio, and initial pH) were optimised, and the optimised parameters were experimentally confirmed by batch operation under mesophilic temperatures. To further verify the effects of start-up conditions on the long-term co-digestion process, a semi-continuous dynamic membrane bioreactor was operated for over 300 days with an FW/CM ratio of 2.5. Following the optimised operation scheme, the organic loading rate gradually increased to 11.9 g COD/L/d. Thus, stable anaerobic co-digestion was maintained at FW/CM ratio of 2.5, and a high CH₄ production of 2.71 L CH₄/L/d and CH₄ yield of 441 mL CH₄/g VS was achieved. In the long-term operation, the digestate pH was stable at approximately 8.4, which indicated a very favourable anaerobic reaction condition without volatile fatty acid accumulation.

Keywords: Co-digestion; Initial pH; Optimisation; Start-up condition; Substrate mixing ratio; Substrate to inoculum ratio.