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

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 (H-2) injection into AD systems to enhance the biological conversion of H-2 and carbon dioxide (CO2) to methane (CH4), a process known as biomethanation. A simplistic approach for biomethanation is to add H-2 directly into working AD reactors (in situ biomethanation). However, a competition for the injected H-2 towards other biological reactions besides H-2/CO2 conversion to CH4 could follow, thus, reducing the efficiency of the system. Hence, this study was conducted to understand how different H-2 injection points would affect H-2/CO2 conversion to CH4 during FW in situ biomethanation, to identify an optimal injection point. Experiments were designed using H-2 equivalent to 5% of the head-space 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 H-2 consumption before the accumulation of VFA enabled a high H-2/CO2 conversion to CH4. However, enhanced competition for soluble substrates during VFA accumulation reduced the efficiency of H-2/CO2 conversion to CH4 when H-2 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 H-2 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 H-2 be obtained biologically by dark fermentation.