Research articleNutrients balance for hydrogen potential upgrading from fruit and vegetable peels via fermentation process
Introduction
Developing sustainable alternative energy sources is urgently needed in order to substitute the conventional fossil fuels, particularly in low income countries (Elsharkawy et al., 2019). Hydrogen gas has recently given a great attention because of its potential as a successful alternative of classical sources of energy from economic and environmental point of view (Bajaj and Winter, 2013, Elsamadony and Tawfik, 2018). H2 gas provides heating value ranged from 120 to 142 kJ/g compared to maximum heating values of 45.0 and 47.5 kJ/g harvested from conventional hydrocarbon fuels such as diesel and gasoline, respectively (Ghimire et al., 2015, Wazeri et al., 2018a). Besides, H2 is a clean energy source with only water as a combustion by-product (Elreedy et al., 2019, Robledo-Narváez et al., 2013). Moreover, H2 can be easily produced from bio-wastes via dark fermentation process (Mohamed, 2019, Mostafa et al., 2016). Egypt annually produces of approximately 3.14, 1.28, 8.29, 0.18 and 0.03 million tons of orange, banana, tomatoes, green peas and spinaches (FAOSTAT, 2017). As a matter of fact, agro-industry is the backbone economy in most of developing countries including Egypt (Ismail et al., 2019a, Yasin et al., 2013). Hence, huge amounts of fruit and vegetable peels (FVPs) are produced creating severe environmental problems (Elsamadony et al., 2015b, Pitara et al., 2016). Since the most common way for the disposal of FVPs, in Egypt and other developing countries, is the direct dumping in the landfills; this process forms leachate that has serious environmental problems such as attracting the insects and rodents, as well as the emitting of greenhouse gases (El-Fadel et al., 1997, Tawfik et al., 2019). Fortunately, FVPs are rich of biodegradable organics, nutrients (N&P) and trace elements, which represented promising substrate for hydrogen fermentative process (Soltan et al., 2019, Tawfik and Elsamadony, 2017). For instance, considerable values of hydrogen yield (HY) were achieved from relevant substrates, i.e., 9.9–12.8 mL H2/gCOD from vegetable waste (Lee et al., 2008), 100.8 mL H2/gCOD from mixture of Fruit (43%) and vegetable (38%) (Chen et al., 2006), 209.9 mL H2/gVS from banana peel (Nathoa et al., 2014), as well as, 459.9 mL H2/gVS from mixed fruit peel waste (Vijayaraghavan et al., 2007). This discrepancy in HY values is mainly attributed to the composition of substrates, which critically affect bacterial activity and metabolic pathways (Anzola-rojas et al., 2015). The presence of macronutrients (C, N, and P), in addition to, micronutrients (trace elements and vitamins) in FVPs does vitally affect the behavior of anaerobes, bacterial counts, enzymes’ activities, metabolite by-products and hydrogen production (Namsree et al., 2012, Paone and Komilis, 2018). However, insufficient data were recorded for assessing the detailed impact of the presence of such macronutrients and micronutrients in FVPs during sole, dual and multi-fermentation processes.
In particular, at low C/N ratio, the substrate is mostly utilized for cell growth, while at high C/N ratio, nitrogen becomes insufficient for cell growth, a deterioration of hydrogen production and yield occurs as a result (Hendriks et al., 2017). C/N ratio of 26–31 was the optimum for co-fermentation of food waste with primary and secondary sludge. Whereas, C/N/P ratio of 500/15/5 was the ideal value for hydrolysis and acidogenesis processes (Deublein and Steinhauser, 2008). Micronutrients such as Fe2+, Ca2+, Na+, Mg2+ and Zn2+ are, in addition, necessary for hydrogen fermentation process. In particular, Fe2+ ions contribute to the active sites of [Fe-Fe], [Ni–Fe] and [Ni–Fe–Se] hydrogenase enzymes, which are responsible for H2 evolution (Elreedy et al., 2019, Tawfik et al., 2019). Likewise, Ca2+ ions act as coagulant during fermentation process to agglomerate and propagate the anaerobes. Moreover, Ca2+ ions represent bio-catalytic ions for microbial consortium for enhancing the biodegradation process. Nevertheless, high concentrations of Ca2+ and Fe2+ would inhibit the anaerobes due to bacterial cells dehydration (Elsamadony and Tawfik, 2015). Mg2+ ions act as mediator and assist substrate utilization rate since they react with adenosine triphosphate (ATP) to form (Mg-ATP), which supplies substrate by the phosphate group to form substrate-X-phosphate as a step for promoting bacterial metabolic pathway. Moreover, Mg2+ ions support the reaction medium by providing protons and electrons, which in turn, enhances H2 productivity (Srikanth and Mohan, 2012). Na+ actively transport substrate inside the bacterial cell, as result of unbalanced ions gradient (Lee et al., 2012). Contrary to their critical importance, the optimum concentrations of micronutrients, to accomplish advanced production of hydrogen and subsequent reusable digestate while using FVPs as substrate, were not investigated so far.
Therefore, the novelty of this work is to introduce the most balanced mixture of fruit and vegetable peels for food industry in order to simultaneously ecofriendly waste disposal and valorize energy production in terms of hydrogen and ethanol. This was figured out by monitoring also the balanced nutrients at each mixture, as well as, hydrolytic enzymes activity, count of H2 producers and metabolic pathways. Finally, energetic and economic analyses were performed to determine the optimal mixture for industrial application.
Section snippets
Characteristics of fruit and vegetable peels (FVPs)
Fruit [banana (B), orange (O)] and vegetable peels [spinach (S), pea (P), tomato (T)] were daily collected from FARAGALLA manufacturing company (Alexandria, Egypt). The facility produces frozen and canned vegetable (okra, spinach, peas, and green beans). The main products are concentrated natural fruit pulp, tomato paste, bottled natural fruit juices and jams. The factory daily produces of approximately 20 ton of vegetable and fruit peels which are collected and dumped into the western desert,
Sole fermentation of fruit and vegetable peels
Fermentation of separated FVPs provided significant differences in terms of volumetric hydrogen production (VHP) and H2 content (HC) as presented in Fig. 1. The highest values of VHP and HC of 1.12 ± 0.08 L/L and 51.8 ± 4.4% were harvested from banana peels (S-B). This is due to nutrients' balance, where C/N and C/P ratios were 38.6 ± 4.5 & 85.4 ± 6.4, respectively. Bouallagui et al. (2003) reported that the proper C:N:P ratio of 100–128:4:1 was necessary to optimize hydrogen production from
Conclusions
Sole fermentation of spinach, pea, tomato, banana and orange peels revealed limited net energy gains (NEGs) of −0.42, 0.05, −0.11, 0.34 and −0.62 kJ/gfeedstock, respectively. This can be assigned to improper concentrations of nutrients and metals, dwindled bacterial reproduction and limited activities for the vital enzymes. However, multi-peels fermentation batch M-PTBO (25% pea +25% tomato + 25% banana +25% orange) balanced macro- and micro-nutrients to their ideal levels. This was reflected
Acknowledgements
The first author would like to thank the Egyptian Ministry of Higher Education (MoHE) for providing him the financial support (Ph.D. scholarship) for this research as well as the Egypt-Japan University of Science and Technology (E-JUST) for offering facility and tools needed to conduct this work.
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