Upgrading biogas produced in anaerobic digestion: Biological removal and bioconversion of CO2 in biogas

Biogas produced in anaerobic digestion contains energetically useable methane (CH4) and unavoidable unwanted carbon dioxide (CO2). To increase the calorific value of this environmental-friendly renewable fuel recovered from wastewater, an upgrading process is necessary to reduce the high concentration of CO2 and increase the associated CH4 content. The pipe-line quality biomethane concentration can be achieved after biologically converting CO2 by either microorganisms or algae. Over the contemporary reviews published on the biogas upgrading, no paper has ever comprehensively covered the emerging biological methods for converting or reducing CO2. Thus, the biotechnologies for CO2 bioconversion such as H2-assisted chemoautotrophic reactor, gas fermentation, microbial electrochemical cells (MEC) and microalgae-based photosynthetic technique are comprehensively reviewed from the aspects of mechanisms, configurations, bottlenecks and efficiencies in this article. The strategies towards improving the performance of each technique regarding CO2 conversion are systematically analysed. The feasibility of each method from economic and environmental perspectives is also outlined. The outlook for biotechnologies with larger scalability and better economic or technical feasibility are then put forward to facilitate their applications for more efficient biogas upgrading.