Nutrients balance for hydrogen potential upgrading from fruit and vegetable peels via fermentation process

Highlights

• Maximum H₂ yield recorded at multi fermentation of pea, tomato, banana and orange

• Multi-fermentation of FVPs balanced the nutrition hierarchy for anaerobes.

• Harmonizing of macro and micro-nutrients augmented enzymatic and bacterial activity.

• Maximum profit from fermentation process was 4.11 $/kg_feedstock.

Abstract

The sole, dual and multi-fermentations of fruit and vegetable peels (FVPs) were investigated in order to balance nutrition hierarchy for maximizing hydrogen potential via Batch experiments. The highest volumetric hydrogen production of 2.55 ± 0.07 L/L and hydrogen content of 64.7 ± 3.7% were registered for multi-fermentation of M-PTBO (25% pea +25% tomato + 25% banana +25% orange). These values outperformed sole and dual fermentation. The multi-fermentation of FVPs provided sufficient nutrients and trace elements for anaerobes, where C/N and C/P ratios were at levels of 24.7 ± 0.2 and 113.2 ± 9.4, respectively. In specific, harmonizing of macro and micro-nutrients remarkably maximized activities of amylase, protease and lipase to 4.23 ± 0.42, 0.035 ± 0.002 and 0.31 ± 0.02 U/mL, respectively, as well as, substantially incremented counts of Clostridium and Enterobacter sp. up to 5.81 ± 0.23 × 10⁵ and 2.17 ± 0.09 × 10⁶ cfu/mL, respectively. Furthermore, multi-fermentation of M-PTBO achieved the maximum net energy gain and profit of 1.82 kJ/g_feedstock and 4.11 $/kg_feedstock, respectively. Nutrients balance significantly develops bacterial activity in terms of hydrogen productivity, anaerobes reproduction, enzyme activities and soluble metabolites. As a result, overall fermentation bioprocess performance was improved.

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/g_COD from vegetable waste (Lee et al., 2008), 100.8 mL H₂/g_COD from mixture of Fruit (43%) and vegetable (38%) (Chen et al., 2006), 209.9 mL H₂/g_VS from banana peel (Nathoa et al., 2014), as well as, 459.9 mL H₂/g_VS 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.