Two‐stage anaerobic digestion for reduced hydrogen sulphide production

BACKGROUND This study evaluated a two‐stage process as an alternative to single‐stage anaerobic degradation, with the aim of separating sulphidogenesis and methanogenesis while treating a mixture of organic fraction municipal solid waste (OFMSW) and thin stillage. For full‐scale applications, no pH regulation was applied. The hydrolytic/acidogenic stage was evaluated with a hydraulic retention time (HRT) between 3 and 15 days, with and without recirculation of methanogenic stage reactor material. RESULTS Homoacetogenesis dominated in the hydrolytic/acidogenic stage. However, sulphate reduction was unsuccessful. Using a two‐stage approach with OFMSW at a HRT of 10 days in the first stage and 28 days in the second stage resulted in a 12% increase in specific methane production and a 6% increase in methane content. The two‐stage process with thin stillage was less effective, probably because of decreasing pH (<4) in the first stage. CONCLUSION Two‐stage operation resulted in a pH too low (~4) for successful sulphate reduction in the first stage. However the resulting pH and applied organic loading rate caused homoacetogenesis to dominate, producing acetate as a favourable energy carrier between the stages.