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 m3 H2/m3 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/m3.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 m3/m3 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.

► Recycle of methanogenic effluent to upstream hydrogenesis enhanced H2 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 H2 and CH4.