Enhancing bioenergy production from food waste by in situ biomethanation: Effect of the hydrogen injection point

Abstract

The increasing rate of food waste (FW) generation around the world is a growing environmental concern, notwithstanding, its valorisation through anaerobic digestion (AD) makes it a potential resource. Moreover, there is a growing demand to optimise the biomethane from AD for gas-to-grid (GtG) and vehicular applications. This has spurred researches on hydrogen gas (H2) injection into AD systems to enhance the biological conversion of H2 and carbon dioxide (CO2) to methane (CH4), a process known as biomethanation. A simplistic approach for biomethanation is to add H2 directly into working AD reactors (in situ biomethanation). However, a competition for the injected H2 towards other biological reactions besides H2/CO2 conversion to CH4 could follow, thus, reducing the efficiency of the system. Hence, this study was conducted to understand how different H2 injection points would affect H2/CO2 conversion to CH4 during FW in situ biomethanation, to identify an optimal injection point. Experiments were designed using H2 equivalent to 5% of the headspace of the AD reactor at three injection points representing different stages of AD: before volatile fatty acids (VFA) accumulation, during VFA accumulation and at depleted VFA intermediates. Lower potential for competitive H2 consumption before the accumulation of VFA enabled a high H2/CO2 conversion to CH4. However, enhanced competition for soluble substrates during VFA accumulation reduced the efficiency of H2/CO2 conversion to CH4 when H2 was added at this stage. In general, 12%, 4% and 10% CH4 increases as well as 39%, 25% and 34% CO2 removal were obtained for H2 added before VFA accumulation, during VFA accumulation and at depleted VFA intermediates, respectively. For immediate integration of biomethanation with existing AD facilities, it is suggested that the required H2 be obtained biologically by dark fermentation.