The anaerobic co-digestion of fruit and vegetable waste and horse manure mixtures in a bench-scale, two-phase anaerobic digestion system

In this study, the anaerobic digestion of mixtures of food waste (FW) and horse manure was investigated using a bench-scale two-phase reactor system. Both phases were maintained at 35 degrees C for the duration of the 30-day study period. The first phase reactors were prepared with biomass mixtures in deionized water such that each mixture had an initial total solids (TS) concentration of 6wt%. The second phase reactors were inoculated with cow manure in water two weeks prior to the study period at 3wt% TS. The biogas from all second phase reactors contained greater than 60vol% methane in the biogas before they were used in the study, thus indicating the presence of active methanogens. Filtrate (5mL) from the first phase was used as feed to the second phase reactor. The chemical oxygen demand (COD), total organic carbon, and volatile solids (VS) of the feed from Phase 1 increased with FW content in the biomass mixture, and so the organic loading rates (OLRs) to the Phase 2 reactors also increased. Accordingly, the volume of biogas and methane generated from Phase 2 also increased with FW content. The low OLR (<0.2gVS/L/day), the use of a two-phase system, and the use of filtrate from Phase 1as feed to Phase 2 allowed for high utilization of the feed; the observed specific methane yields (mL/g COD) were greater than 80% of the theoretical yields for all mixtures. The methane yields were statistically similar to within a 95% confidence interval.