Fermentative H2 production from food waste: Parametric analysis of factor effects
Introduction
Sustainable management of bio-waste is being more and more regarded as a key issue in both industrialized and emerging countries, on account of the need to reduce the potential environmental impacts from natural uncontrolled degradation and the energy exploitation perspectives that may be opened. European waste statistics indicate that ∼245 Mt of municipal solid waste were generated in the EU-28 in 2016, out of which food waste (FW) is estimated to account for ∼35% by weight. The environmental policies on bio-waste in most industrialized countries prescribe specific reduction targets to final disposal, promoting materials and energy recovery from bio-waste.
In this framework, biological treatment of FW is one of the key options for the environmentally sound management of biodegradable residues. More specifically, dark fermentation aimed at H2 production ahead of further biological treatment has been widely studied for a variety of organic waste materials. In addition to the well-known positive environmental features of H2 as an energy carrier, particularly if generated from renewable non-fossil sources, a first dark fermentation stage in AD may produce further environmental and economic advantages. Separate optimization of the acidogenic and methanogenic phases in the two-stage configuration has been reported to significantly enhance energy recovery (10–25% (Lee and Chung, 2010); 20% (De Gioannis et al., 2017); 8–43% (Schievano et al., 2014); 38% (Massanet-Nicolau et al., 2013)) compared to the conventional single-stage layout.
Various FW and kitchen waste components as well as the organic fraction of municipal solid waste (OFMSW) are recognized to be suitable and relatively inexpensive sources of biodegradable organic matter for H2 production, mainly due to their high carbohydrate concentration, adequate moisture content and wide availability (Alexandropoulou et al., 2018, Alibardi and Cossu, 2016, Alibardi and Cossu, 2015, Dong et al., 2009, Kim et al., 2011b, Kim et al., 2004, Kobayashi et al., 2012, Liu et al., 2006, Nazlina et al., 2011, Tawfik and El-Qelish, 2012, Wang and Zhao, 2009, Zhu et al., 2008).
Fermentative H2 production depends on several factors, acting either synergistically or antagonistically. Factors include substrate-related characteristics (substrate composition, concentration and pre-treatment methods), microorganisms-related characteristics (inoculum type [pure/mixed cultures], inoculum pre-treatment and selection methods, inoculum-to-substrate ratio [ISR]), and control and operating parameters (temperature, pH, organic loading rate, hydraulic and cell residence time, reactor type and operation regime) (Alexandropoulou et al., 2018, Alibardi and Cossu, 2016, Ghimire et al., 2016, Tawfik and El-Qelish, 2014, Van Ginkel et al., 2001). Therefore, prediction and optimization of the fermentation pathways requires both the individual effects of the relevant parameters and their mutual interactions to be described and quantified accurately. To this aim, since investigations based on a “one variable at a time” approach are considered to be inadequate to provide a reliable understanding of the process (Akhlaghi et al., 2017), alternative experimental design and data analysis methods should be adopted to pick the complex interrelations among the relevant factors.
Under batch conditions and for a given substrate type, the fermentation process is chiefly governed by the operating pH and the availability of microorganisms. In particular, the operating pH is recognized to govern the substrate hydrolysis yield, the activity of hydrogenase, the energy utilization yield by the biomass as well as the metabolic pathways (Kim et al., 2011a, Rodríguez et al., 2006). The availability of microorganisms in the system is measured through either the ISR or its reciprocal, the food-to-microorganisms (F/M) ratio. The relative amounts of substrate and biomass in the system can determine a variety of conditions ranging from substrate-limited to substrate-sufficient growth (Liu, 1996), in turn affecting the yield of substrate conversion into the metabolic products (Cappai et al., 2015). For a more detailed review of the individual effects of pH and ISR on the fermentation process, the reader is referred to previous literature studies (see e.g. (Akhlaghi et al., 2017, Cappai et al., 2018, Cappai et al., 2014, Cappai et al., 2018, De Gioannis et al., 2014, De Gioannis et al., 2013, De Gioannis et al., 2009, Ghimire et al., 2015) and references therein). Although the individual effects of pH and ISR on the kinetics and yield of fermentative H2 production from organic residues have been widely investigated, so far relatively few studies (Ghimire et al., 2016, Pan et al., 2008, Van Ginkel et al., 2001) have been conducted on their combined influence, particularly for food waste. It is also emphasized that most studies have focused on the influence of the initial pH only, while that of the operating pH (which, on the other hand, is by far more relevant for the biochemical reactions) has been largely overlooked. The present work attempts to fill in the gaps on the joint effects of pH and ISR on hydrogenogenic fermentation of organic waste by means of a dedicate experimental campaign on a food waste sample that was deemed to be representative of the typical composition of the food fraction of Italian municipal solid waste (Andreasi Bassi et al., 2017). The main novel contribution to the knowledge in the field lies in the identification of the relationships and mutual interactions among the operating pH, ISR and the response variables of the fermentation process of the food waste of concern. To this aim, a systematic approach based on factorial experiments was adopted, followed by the identification of hidden relationships among the factors and the response variables by means of statistical analysis tools and empirical modelling of parameters effects. All such tools were combined to interpret the complex biochemical transformations involved in the process, identify the optimal conditions for hydrogenogenesis and provide indications for further enhancing the process yield.
Section snippets
Feedstock and inoculum
The substrate used in this study was source-separated OFMSW coming from door-to-door collection of municipal waste in a medium-size city located in central Italy. The OFMSW was manually sorted to select food components and then homogenised to ensure the reproducibility of sub-samples used for the characterization and fermentation tests. The obtained waste was deemed to be representative of the typical composition of the food fraction of Italian municipal solid waste (Andreasi Bassi et al., 2017
H2 production yield
The specific hydrogen production yield (SHPY) per unit of initial TOC of FW in the mixture is reported in Fig. 1. Individual data points are direct H2 production measurements, while continuous lines represent the two-stage Gompertz production curves derived from model (1). The results of fitting of H2 production data with model (1) are reported in Table 3 in terms of parameter values and related statistics. It is worth mentioning that the degree of data fitting by the two-stage Gompertz model
Conclusions
pH and ISR affected the process yield, kinetics and biochemical pathways, implying careful optimization to maximize H2 production.
The hydrogenogenic process was mirrored by carbohydrate degradation. Changes in waste composition would imply different H2 yields, requiring specific investigation of the fermentation process.
The metabolic pathways involved both hydrogenogenic and competing reactions. Enhancing organic matter conversion into H2 beyond the observed maximum (1.45 mol H2/mol hexose)
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2022, International Journal of Hydrogen EnergyCitation Excerpt :The importance of H2 production as an alternative fuel focuses on its high energy content and that it does not generate greenhouse gases when it is burned [4,5]. However, H2 gas is not readily available in nature like fossil fuels and natural gas [6], but H2 can be produced through an intermediate anaerobic digestion process known as dark fermentation (DF) [7]. In this process, anaerobic microorganisms degrade carbohydrate-rich substrates in the absence of light [8,9].