Effects of Initial pH on Two-Stage Biogas Production and Hydrolysis in a Co-Digestion Process of Food Waste and Waste Activated Sludge

Abstract

Batch tests were performed to investigate the effects of initial pH on the two-stage hydrogen and methane production process by co-digestion of food waste and waste activated sludge. Results show that the two-stage process had good pH adaptive capacity. Hydrogen production was similar at initial pH of 5.5–11.0 with hydrogen yields of 37.3–42.0 mL-H₂/g-VS_added, but was inhibited at initial pH of 4.0. The highest hydrogen production rate was obtained at a stable pH of 4.5–5.7. The dominant acidification products at the end of hydrogen production was acetate at initial pH of 4.0 and butyrate at initial pH of 11.0, and the subsequent methane production was likely to be affected by the component of volatile fatty acids (VFA) at the end of hydrogen fermentation. The residuals produced from hydrogen fermentation process at initial pH of 5.5–10.0 had similar methane yields (MY) of 313.6–318.7 mL-CH₄/g-VS_added at a fermentation time of 24.8 days. By contrast, methane production in the second stage was enhanced at extreme alkaline initial pH of 11.0 in the first stage, but partly inhibited at extreme acidic initial pH of 4.0. In the test of initial pH of 11.0, the lowest content of VFA and propionate throughout methane production stage were beneficial for MY and methane production rate. The hydrolysis efficiencies were also influenced by the initial pH in hydrogen production stage, which followed the descending order of alkaline, neutral, and acidic condition for hydrogen fermentation and the whole two-stage process, but followed the opposite orders for methane fermentation.