Re ECF Project: 2014-75

Project Title: Enhanced Methane Production and Biogas Upgrading During Food Waste Anaerobic Digestion

Applicant: Professor Jonathan W C Wong, Hong Kong Baptist University

Total Approved Grant: $930,400

Duration: 01/11/2015 to 31/7/2017

Project Status/Remarks: Completed

The Government plans to divert food wastes to biological treatment at Organic Waste Treatment Facilities (OWTFs) with anaerobic digestion (AD) as the core technology. The biogas produced from the food waste AD reactors mainly contains methane and carbon dioxide (CO2), and the CO2 content reduces the heating value of the biogas. Therefore, the biogas must be upgraded. This project aims to develop a biological upgrading method for biogas, with a view to increasing its calorific value. The proposed approach could increase the methane content in the biogas to >95% and also increase the net methane production in a sustainable way. It also converts CO2 in the biogas into methane, thus reducing global warming.

Summary of the Findings/Outcomes:
Application of bioelectrochemical systems for electricity production from organic waste was intensively investigated in the past two decades. In the project, the effluents of anaerobic digesters were utilized to produce electricity in Microbial Fuel Cells (MFC) and methane generation would be upgraded through "electromethanogenesis" using in-situ hydrogen produced in Microbial Electrolysis Cell (MEC). The electroactive biofilms of electrodes in MFC were successfully formed and the results showed that the overall activity of MFC bioreactor at the optimum operating conditions of 0.6 ml/min (flow rate), 9 g/1 (Chemical Oxygen Demand (COD) Concentration) and 22 Ω (resistance) generated 0.43 V of stable voltage, 0.4 W/m2 of power density, 1.79 A/m2 of current density, 93.5% of COD removal and currents generation of 0.042/-0.018 A Oxidation Current (OC) / Reduction Current (RC). Enriched hydrogentrophic methanogens were found in cathode biofilm of MECs such as Methanomicrobia and Methanolinea which elevated the biogas upgradation ability by reduction of CO2 in the biogas of an anaerobic digester using MECs. The overall methane concentration increased to 93.4% and the efficiency of COD removal improved to 92.8% under the optimum operating condition of 0.5V voltage, 0.6 ml/min biogas flowrate and 6 g/l anodic COD concentration. The study successfully established a successful integration of MFC and MEC. The MFC fed with methanogenic effluents as the anode substrate efficiently generated the supplementary energy requirement for the MEC, while the MEC also operated with UASB effluent and biogas in integration with MFC bioreactor for potential application. Biogas upgrading in a MEC reactor showed high efficiency of CO2 reduction as indicated by the over 90% methane concentration in the biogas. The result of carbon mass balance calculation revealed that gas-liquid mass transfer along with other parameters is the rate limiting factors for efficient biomethane upgradation.