Effect of aerobic pre-treatment on hydrogen and methane production in a two-stage anaerobic digestion process using food waste with different compositions
Introduction
The use of renewable energy sources is a critical issue worldwide due to the serious negative environmental consequences caused by the use of fossil fuels, in addition to the proximate depletion of the latter in the near future. Anaerobic digestion (AD) is one of the most widely investigated methods used in the production of energy from different kinds of organic waste. During this process, strictly anaerobic bacteria and archaea are utilized to produce biofuels such as hydrogen and methane when growing on organic substrates. Hydrogen has been indicated as one of the most promising fuels for the future (Ozkan et al., 2010, De Gioannis et al., 2013). However, subsequent to anaerobic hydrogen production substrate conversion remains incomplete, with the majority remaining as a residue after the process. A promising system is represented by a two-stage AD process combining H2 and CH4 productions. During the first stage, organic compounds are hydrolysed and utilized by hydrogen producing bacteria to produce H2 and volatile fatty acids (VFAs), whilst in the second stage, VFAs are used as substrates for CH4 production by methanogens. Two-stage AD provides a positive energy yield (40–90% available energy), thus underlining the highly important process sustainability (Ruggeri et al., 2010). Several studies have demonstrated the ability of two-stage AD to improve CH4 yields during the second stage, likely due to better hydrolysis (Liu et al., 2006, Pakarinen et al., 2011). Moreover, compared to one-stage AD, process control would be simpler and stability would be improved (Lim and Wang, 2013; Ariunbaatar et al., 2015).
During hydrolysis, the rate limiting step of anaerobic digestion, organic compounds including proteins, carbohydrates and lipids are broken down by hydrolytic bacteria into amino-acids, sugars and long chain fatty acids, respectively. Substrate pre-treatment methods are aimed at promoting and improving hydrolysis of high molecular weight compounds to readily-biodegradable constituents, and subsequently increasing the AD process product yields.
Hydrolysis occurs under both aerobic and anaerobic conditions; however, hydrolysis rates are significantly higher under aerobic conditions, likely due to the higher production of enzymes (Botheju et al., 2009). In addition, pre-aeration reduces accumulation of VFAs, resulting in a drop of pH during the process, thus improving the start-up stability of anaerobic digestion. Limited pre-aeration prior to anaerobic digestion has been shown to improve hydrolysis and biogas production (Charles et al., 2009, Zhu et al., 2009, Ahn et al., 2014, Cossu et al., 2016, Peces et al., 2016).
Composition of organic wastes varies according to the source from which the wastes are collected. Slaughterhouse wastes may be rich in proteins and lipids, while food wastes and organic fraction of municipal solid wastes (OFMSW) are rich in carbohydrates. An in-depth understanding of effective pre-treatment methods for each kind of waste is fundamental in improving biogas production.
To the best of the Authors’ knowledge, no scientific reports have been published to date on the effects of aerobic pre-treatment on food waste with different compositions for either H2 and/or CH4 production in a two-stage AD process. Moreover, the effect of carbohydrate, lipid and protein content of food waste on pre-aeration efficiencies has not been addressed before. Therefore, the present work aims to study the aerobic pre-treatment effect of carbohydrate rich (C), protein rich (P), and lipid rich (L) food waste prior to two-stage anaerobic digestion on both H2 and CH4 production.
Section snippets
Organic waste samples
Synthetic food waste samples were prepared in order to simulate industrial or municipal food waste with different compositions as indicated in a previous study (Alibardi and Cossu, 2016).
Three different substrates were prepared and classified as C, P, and L substrates. The composition of samples is shown in Table 1. The percentages are based on wet weight.
Food waste samples were shredded after preparation and characterized (Table 2) in order to have more detailed information for each substrate
Hydrogen production
The results obtained for hydrogen production potential from three different food waste samples are shown in Fig. 1. Data obtained through GC analysis revealed a lack of methane in the emitted gas, due to efficiency of the thermal pre-treatment of inoculum. In the first stage of AD, substrate C without aeration produced considerably more hydrogen (55.31 ml/gVS) compared to L (27.93 ml/gVS) and P (7.96 ml/gVS) substrates. This finding is in agreement with Alibardi and Cossu (2015), who concluded
Conclusions
The efficiencies of a two-stage AD treatment using organic wastes with different compositions in both the presence and absence of aeration as a treatment were compared by evaluating the H2 and CH4 production. This study suggested that pre-aeration of organic waste did not constitute an effective treatment for the purpose of improving H2 production potential during the first stage of the AD process. However, during the subsequent stage of AD, carbon conversion to CH4 was higher for pre-aerated P
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