FOOD WASTE POTENTIAL AND FEEDING RATES ON BIOGAS PRODUCTION IN A TWO-STAGE DIGESTION WITH EFFLUENT RECIRCULATION

Abstract

Two experiments were conducted succeedingly to investigate food waste potential and feeding rates on biogas production. Anaerobic digestions of food waste and glucose were carried out in the first experiment to evaluate their biochemical methane production potential. Results indicated that the high biogas production yield was obtained with the presented microbial inoculum (seed) and the used food waste. The accumulated biogas yields produced from the food waste and glucose were 0.98 and 5.82 m³/kg.COD with 55.4 and 49.2% methane contents, respectively. Effects of food waste feeding rates (0.25, 0.50, 1.0, 2.0, and 3.0 kg.COD/m³.day) using a two-stage anaerobic digestion with effluent recirculation on biogas production were investigated in the second experiment. Results revealed that the 1.0 Kg.COD/m³.day feeding rate with effluent recirculation increased biogas and methane yields approximately 31.3% higher as compared to that without recirculation. The increased rates from 0.25 to 0.50, 1.0, and 2.0 Kg.COD/m³.day, though slightly decreased the biogas production rate (per COD unit), the accumulation of biogas yields were increased from 0.065 to 0.123, 0.221, and 0.408 m³, accordingly. However, the highest feeding rate of 3.0 Kg.COD/m³.day with effluent recirculation dropped the accumulation biogas yield sharply to 0.019 m³. This high feeding rate would hinder the processes of acidogenic digestion and resulted in high concentration of VFA in the tested effluent. A low pH (5.2) and high ratio (1.35) of VFA/alkalinity as noted in the effluent would inhibit methanogens for methane production.