Investigating hydrothermal pretreatment of food waste for two-stage fermentative hydrogen and methane co-production
Introduction
With the rapid development of economy and improvement of living standard in China, increasing amounts of municipal solid wastes (MSWs) are being generated. In 2015, the MSWs generated in 246 large and medium-sized cities reached up to 186 million tons (MEP of China, 2016a), which was higher than the decontamination and processing quantity (180 million tons) of MSWs for the entire country (MEP of China, 2016b). The accumulation of MSWs exerts considerable pressure on the environment.
In 2015, 63.9% and 33.9% of the processed and decontaminated MSWs were disposed through sanitary landfill and incineration, respectively (MEP of China, 2016b). However, food waste (FW), which accounts for more than half of the generated MSWs (Dou, 2015), is unsuitable for either landfill disposal or incineration. Although the landfill disposal of FW presents some advantages, such as low investment and facile operation, this approach is associated with numerous problems, such as large land occupation and environmental pollution caused by the underground leaching of hazardous substances and the excessive emission of landfill gas when improperly managed (Dou, 2015). The high moisture content of FW renders the use of incineration energy inefficient. Alternatively, anaerobic digestion, which combines organic waste reduction and energy recovery through methane production (Ariunbaatar et al., 2014a), can serve as an ideal strategy for FW disposal.
Anaerobic digestion of FW has been extensively investigated because of its environmental and energetic benefits, (Browne and Murphy, 2013, Zhang et al., 2014). However, complex organics in FW, such as lipids from animal fats and vegetable oils, insoluble proteins, and large-molecular-weight carbohydrates, always limit the hydrolysis step, hence leading to the inhibition and even failure of anaerobic digestion (Ariunbaatar et al., 2014a, Xia et al., 2016). Moreover, the dietary customs of Chinese contribute to the high lipid contents of FW (Li et al., 2017). In addition to hindering hydrolysis, high lipid content can inhibit the metabolism of microbial cells because of the absorption of lipid derivatives (i.e., long-chain fatty acids) on cell surfaces (Hanaki et al., 1981). To address these issues and promote methane production, researchers have subjected different substrates to various mechanical, chemical, thermal, and biological methods prior to anaerobic digestion (Ariunbaatar et al., 2014a). Hydrothermal pretreatment (HTP), a thermal treatment method with a history of successful industrial applications because of its high efficiency and environmental friendliness, has been widely explored as a pretreatment method for methane production (Ariunbaatar et al., 2014a, Tekin et al., 2014). By disintegrating cell membranes, HTP facilitates the dissolution of recalcitrant organic compounds and further promotes the hydrolysis of dissolved macromolecular organics under suitable temperatures and retention times (Ariunbaatar et al., 2014a, Yin et al., 2014). Most studies have focused on the HTP of sewage sludge and lignocellulosic substrates, whereas reports on the HTP of FW are limited. Yin et al. (2014) and Yu et al. (2016) performed the HTP of FW for fermentative volatile fatty acid (VFA) production. They reported that HTP simultaneously enhances organic compound dissolution and VFA production. Similarly, Jin et al. (2016) and Li et al. (2017) applied HTP on kitchen waste and found that solubilization of organics and the corresponding biodegradability in subsequent anaerobic digestion increased.
As an alternative to pretreatments, a two-stage process involving the dark hydrogen fermentation of FW and the anaerobic digestion of fermentation effluent has been suggested to facilitate the hydrolysis of FW substrates and stabilize anaerobic digestion (Kobayashi et al., 2012, Lee et al., 2010, Voelklein et al., 2016). Methanogenesis is blocked during dark hydrogen fermentation. Hydrolysis and acidogenesis can be remarkably strengthened through the optimization of process parameters (e.g., pH value and retention time) of dark hydrogen fermentation (Liu et al., 2013, Voelklein et al., 2016). Voelklein et al. (2016) observed that compared with one-stage anaerobic digestion, dark hydrogen fermentation facilitates the hydrolysis of FW, allows higher loading rates, and increases the subsequent methane production. In addition to VFAs, hydrogen is produced through the dark hydrogen fermentation of FW. Hydrogen is considered as an extremely clean energy carrier and an important chemical feedstock. Kobayashi et al. (2012) established a two-stage system for FW disposal and obtained hydrogen and methane yields of 147.3 and 383.0 mL/g volatile solids (VS), respectively.
Rafieenia et al. (2017) combined aerobic pretreatment and two-stage fermentation to energetically valorize synthetic protein-rich FW. However, to our best knowledge, a study on two-stage fermentative hydrogen and methane co-production using FW subjected to HTP has yet to be reported in literature to date. Thus, to fill in this knowledge gap, this study specifically aims to:
- (1)
Evaluate the effects of temperature and time of HTP on the solubilization and acidification of FW.
- (2)
Investigate HTP conditions (temperature and time) on FW for two-stage fermentative hydrogen and methane production.
- (3)
Assess the energy conversion efficiencies (ECEs) from FW to hydrogen and methane following the two-stage process.
Section snippets
Substrate
FW was collected from Hangzhou Environmental Group, Co., Ltd., Zhejiang Province, China. Raw FW originated from restaurants, campus canteens, and company dining halls in the urban areas of Hangzhou City. Large non-biodegradable wastes, such as cans, plastic bags, and bottles, were first discarded through an automatic separation system and FW was subsequently blended into pulp, which was cryopreserved at −20 °C prior to experiments.
Inocula
The seed inoculum for dark hydrogen fermentation was obtained
Food waste characterization
The characteristics of FW are outlined in Table 1. The diverse sources of FW all over the city and loose classification criteria for FW led to a VS/TS ratio of 79.70%, which was lower than the reported ratios (Browne and Murphy, 2013, Lee et al., 2010). Despite the low VS/TS ratio, the FW used in this study possessed a suitable C/N ratio of 22.1, which lies within the suggested optimum C/N range of 20–30 (Xia et al., 2016), for anaerobic digestion. The high heating value (23.9 MJ/kgVS),
Conclusions
This study demonstrated that HTP is a suitable approach in pretreatment of FW to promote fermentation performance. Subjecting FW to HTP at 140 °C for 20 min facilitates the solubilization of organic components (i.e., carbohydrates and proteins), thus benefiting the two-stage fermentative hydrogen and methane co-production and consequently enhancing the overall ECEs. However, new methods are still required to process rich lipids that remain stable in hydrothermally pretreated FW. Additionally,
Acknowledgements
This study was supported by the National Key Research and Development Program-China (2016YFE0117900), and Zhejiang Provincial Key Research and Development Program-China (2017C04001).
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