Anaerobic Bioconversion of Carbon Dioxide to Biogas in an Upflow Anaerobic Sludge Blanket Reactor

Abstract

The increasing concentration of carbon dioxide (CO₂)—the most dominant component of greenhouse gases—in the atmosphere has been of growing concern for many years. Many methods focus on CO₂ capture and storage and there is always the risk of CO₂ release to the environment. In this study, a new method to convert CO₂ to biogas with a high content of methane (CH₄) in an anaerobic system with a lab-scale upflow anaerobic sludge blanket reactor at 35 °C was developed. In a series of experiments, the reactor was run with and without CO_2- saturated solutions including volatile fatty acids (VFAs) as sources of hydrogen. The concentration of dissolved CO₂ in the influent solutions was 2.2–6.1 g/L, with corresponding chemical oxygen demand (COD) values of 2.6– 8.4 g/L for the solutions. Overall CO₂ removal values of 2.7–20 g/day (49–88% conversion) were obtained for the organic loading rates (OLR) and CO₂ loading rates of 8–36 gCOD/L • day and 6–26 gCO₂/L • day, respectively with CH₄ purity of above 70%. Also, VFA and COD removal were in the range of 79–95% and 75–90%, respectively. Methanogenic activities of the cultures with the concentrations measured as volatile suspended solids (VSSs) were 0.12–0.40 L CH₄/gVSS • d with the highest value for the system containing acetic acid. This anaerobic method can be applied to reduce CO₂ emitted to the atmosphere from a wide variety of industrial point sources with a value-added product, CH₄.