Enhanced methane production from food waste: A systematic comparison between conventional single-stage and lactate-based two-stage anaerobic digestion processes

The increasing generation of food waste (FW) poses significant environmental and management challenges, requiring efficient and sustainable treatment methods. This study presents the first systematic comparison between a conventional single-stage anaerobic digestion (AD) process and a lactate-based two-stage AD process using food waste (FW) as the substrate. Both AD configurations were operated in parallel under identical operating conditions, i.e., 37 °C, 20 days hydraulic retention time, 2.3 g volatile solids (VS)/L-d organic loading rate, and pH 8. The two-stage AD system exhibited a methane productivity of 959 NmL CH4/L-d and a methane yield of 398 NmL CH4/g VSfed, which were 32.0 ± 5.6 % and 35.9 ± 0.6 % higher than those of the single-stage AD process, respectively. The two-stage AD system also showed significant lactate accumulation in the acidogenic stage, which was almost completely oxidized in the methanogenic stage. Furthermore, molecular analysis of the acidogenic stage revealed diverse bacterial communities, with a prevalence of lactate-producing bacteria such as Lactobacillus. In the methanogenic stage, various bacteria and archaea, including Methanobacterium and Methanothrix, were identified as major contributors to methane production. The enhanced methane production performance of the two-stage AD system was attributed to the physical separation of the acidogenic stage from methanogenesis and the occurrence of lactate-type fermentation in the acidogenic stage.