Abstract
AbstractThe 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 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 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.
Highlights
A third of the food crops cultivated annually for human consumption is reportedly wasted or lost at some level within the food supply chain from production to consumption (FAO, 2011); accounting for 44% of global waste (Kaza et al, 2018)
A relatively low amount of H2 was used because this study aims to understand the adjustment of the system to in situ biomethanation using food waste (FW) anaerobic digestion (AD) systems
The low solubility of H2 in water means that changes in the head-space H2 would be a consequence of gas–liquid H2 transfers mediated by active microorganisms associated with H2 consumption or production
Summary
A third of the food crops cultivated annually (about 1.3 billion tonne—Bt) for human consumption is reportedly wasted or lost at some level within the food supply chain from production to consumption (FAO, 2011); accounting for 44% of global waste (Kaza et al, 2018). It is reported that annual global solid waste generation will increase from the 2.01 billion tonnes estimate of 2016 to 2.59 billion tonnes in 2030 and 3.40 billion tonnes in 2050 (Kaza et al, 2018), FW increases are expected This increasing rate of FW generation around the world is a growing environmental concern, its valorisation through anaerobic AD makes it a potential resource. Physicochemical technologies for biogas upgrade separate CO2 in the biogas from CH4 and includes processes such as absorption, adsorption and cryogenic and membrane separation (Angelidaki et al, 2018) These technologies are associated with a 20%–72% elevation in CH4 production cost due to high energy, chemical and water demand, and up to 8% CH4 losses (Linville et al, 2016). Impurities in the biogas such as ammonia, carbon monoxide and hydrogen sulphide (H2S) can be harnessed by the microorganisms during CH4 enrichment (Aryal et al, 2018)
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