Enhancement of hydrogen production by recycling of methanogenic effluent in two-phase fermentation of food waste

The feasibility of operational strategies was investigated for hydrogen and methane production from food waste. Food waste was heat-treated at 70 °C and fed to a two-phase anaerobic sequencing batch fermenting system. Maximum hydrogen productivity of 1.19 m ³ H ₂/m ³ d was observed at a food waste concentration of 30 g carbohydrate/L, a hydraulic retention time of 2 d, and a solids retention time of 3.4 d. The effluent from hydrogenesis was efficiently converted to methane at an organic loading rate of up to 3.6 kg COD/m ³.d. The methanogenic effluent was then recycled to the hydrogenesis reactor without any pretreatment. The recycled effluent not only successfully replaced external dilution water and decreased alkaline dosage by 75%, but also increased hydrogen production by 48%, resulting in hydrogen productivity of 1.76 m ³/m ³ d. The two-phase digestion with recycling would convert 91% of organic pollutants in food waste to hydrogen (8%) and methane (83%) without any external dilution water. Highlights: Recycle of methanogenic effluent to upstream hydrogenesis enhanced H ₂ production. Even there was no pretreatment on the effluent, recycle did not disturb hydrogenesis. Recycle reduced alkaline dosage and enabled optimum feedstock concentration. Internal biomass retention guaranteed hydgenesis at an HRT of 2 d. Two-phase digestion with recycle recovered 91% of COD in food waste as H ₂ and CH ₄.