Separate production of hydrogen and methane from biodiesel wastewater with added glycerin by two-stage anaerobic sequencing batch reactors (ASBR)

The objective of this study was to investigate hydrogen and methane production from biodiesel wastewater with added glycerin by using two-stage anaerobic sequencing batch reactors (ASBR) under 37 °C with a recycle ratio of the effluent from the methane ASBR unit-to-the feed flow rate of 1:1. Hydrogen ASBR unit was operated at a constant pH of 5.5 whereas the pH of methane ASBR unit was not controlled. Glycerin was added into biodiesel wastewater having 2330 mg/l COD to obtain a constant feed chemical oxygen demand (COD) of 45,000 m/l. At optimum COD loading rate 11.3 kg/m3d (based on feed COD load and methane ASBR volume), the system provided the highest overall hydrogen and methane production performance (75.15 ml H2/g glycerin removed) which was consistent with the maximum overall COD removal of 76.7% and the highest overall glycerin uptake of 90%. Most of added glycerin was converted to gaseous products with small amounts of organic acids and 1,3 propanediol. The low volumetric ratio of hydrogen ASBR unit-to-methane ASBR unit of 1:6 resulted in low hydrogen yields with high methane yields. Process performance of the two-stage ASBR system was limited by the inhibition of total volatile organic acids produced in both bioreactors.