Aged refuse enhances anaerobic fermentation of food waste to produce short-chain fatty acids
Graphical abstract
Introduction
Food waste is an important component of municipal solid waste (MSW), accounting for about 30–50% (Troschinetz & Mihelcic, 2009). It is reported that the production of food waste was about 1.6 billion tons worldwide in 2012, and with the rapid increase of population and economy, the production of food waste is expected to continue to increase (Ma et al., 2017). Food waste has the characteristics of high moisture content, high organic matter content and easy to spoil and deteriorate (Yong et al., 2015, Yin et al., 2016). If food waste is not treated properly, it will cause secondary pollution to the environment and even break the ecological stability (Yin et al., 2014, Yu et al., 2016). Reduction, harmlessness and resource utilization of food waste have become the focus of attention.
Generally, anaerobic fermentation (or anaerobic digestion) was considered a preferable technology for food waste treatment, because it can reduce the amount of food waste, make it harmless and recover energy materials such as short chain fatty acids, hydrogen and methane (Zhao et al., 2015, Xu et al., 2018). The biodegradable organic matter in food waste is fermented by microorganisms and converted into energy sources such as hydrogen, methane and short chain fatty acids (SCFA) (Zhao et al., 2017b, Hobbs et al., 2018). Meanwhile, food waste has been reduced and stabilized. Recently, SCFA production from food waste anaerobic fermentation has attracted increasing attention, because the value added SCFA was a crude material for microbial production of biodegradable plastics and a preferred substrate for biological nutrient removal microorganisms (Chen et al., 2013, Zhao et al., 2016).
Anaerobic fermentation of food waste generally consists of four sequential bioconversion processes: hydrolysis, acidogenic, acetogenic, methanogenic phases. During the hydrolysis stage, solid or macromolecule organic matter in food waste are enzymatically converted into soluble small molecule organic matter by the microbial secreted extracellular hydrolase (such as protease and amylase) (Zhang et al., 2017). Hydrolysis process is the key step of anaerobic fermentation of food waste. The hydrolysis efficiency has an important influence on the stability and effectiveness of food waste fermentation (Lin et al., 1997). Hence, improving the hydrolysis rate of anaerobic fermentation of food waste has become a focus of scientists' attention. In previous studies, heat, alkaline and/or mechanical pre-treatments were applied to enhance the hydrolysis process of food waste (Zhang et al., 2014, Karthikeyan et al., 2018). Although the above pretreatment strategies can significantly accelerate the hydrolysis process, the above pretreatment will consume a large amount of energy to a certain extent, resulting in a relatively poor economy (Qiao et al., 2018, Zhao et al., 2017a). Therefore, an economical, feasible and efficient strategy to promote anaerobic fermentation of food waste needs to be developed urgently.
Aged refuse (AR) refers to the landfill refuse when the landfill reaches a stable state. Aged refuse contains a large number of degradable microorganisms (especially anaerobic microorganisms), and also contains rich enzymes, such as urease, which can promote the hydrolysis of organic matter (Zhao et al., 2017c). The degradable organic matter in AR has been almost exhausted after years of storage (e.g., 8 years). The special formation conditions and heterogeneous structure make AR enriched with different microbial communities, especially those that degrade refractory pollutants (Zhao et al., 2007). The above characteristics provide high-quality conditions for the wide use of AR. For example, Zhu et al. (2012) applied AR to the treatment of landfill leachate and livestock and poultry wastewater by using the porous characteristics of AR, and achieved satisfactory experimental results. In addition, AR is also widely used in solid waste treatment, Li et al. (2008) reported the addition of AR (50% in weight) can considerably increase the hydrogen yield in the biogas to over 26.6% with pH ascending from 4.36 to 5.81, further statistical analysis showed that the ultimate hydrogen production potential (Hp) and hydrogen production rate (R max) in the presence of 50% AR were 193.85 mL/gVS and 94.35 mL/(h gVS), respectively. Bioremediation of petroleum-contaminated soils using aged refuse from landfills has achieved satisfactory results (Liu et al., 2018). Recently, Zhao et al (2017b) applied AR to improve anaerobic fermentation of waste activated sludge, and confirmed that AR accelerated the hydrolysis and acidification processes during sludge anaerobic fermentation, resulting in shorter fermentation time. In view of the above excellent characteristics of AR, the addition of AR into food waste fermentation system was predicted to promote anaerobic fermentation to produce SCFA. However, this hypothesis has not been verified by experiments, and the mechanism of AR enhanced anaerobic fermentation of food waste has not been revealed. In addition, carbohydrate and protein are the main organic components of food waste. The crude protein content of food waste in China was relatively high, accounting for about 15–26%. Proteins have a unique three-dimensional structure, and their folding structure makes it difficult for them to be hydrolyzed by protease (Herman et al., 2006, Carbonaro et al., 2012). Proteins in food waste can be divided into plant protein and animal protein according to their composition. How the presence of AR affects the degradation of the two main proteins to produce SCFA remains unclear.
Therefore, this study explored the feasibility of AR-enhanced anaerobic fermentation of food waste to produce SCFA on a laboratory scale. Then, the details of how AR affected the disintegration, hydrolysis, acidification, and methanogenesis of organic matter during food waste anaerobic fermentation were described systematically. Additionally, the results of degradation of main proteins (animal protein and plant protein) in food waste to produce SCFA enhanced by AR were compared. Finally, the effects of AR on key enzyme activities and microbial communities during anaerobic fermentation of food waste were also analyzed in order to reveal the mechanism of AR enhancing anaerobic fermentation of food waste. The results obtained in this study are helpful to provide data support and theoretical basis for the resource management of food waste.
Section snippets
Experimental materials
- (I)
Sources of food waste: Food waste used in this study was collected from a student canteen of Qingdao University of Technology. First, the non-digestible impurities such as plastic bags, chopsticks, bones, etc were manually removed from the food waste. Second, food waste was separated into 1–3 mm small substances by an electric grinder. Food waste contains a certain amount of lipid. In practice, the lipid would be separated and collected before anaerobic fermentation to produce high value-added
Effects of AR on SCFA yield and components in anaerobic fermentation of food waste
SCFA is an important intermediate product in the anaerobic fermentation process of food waste, and its yield and components play an important role in the stability of the food waste anaerobic fermentation system (Zhao et al., 2015). Fig. 1 showed the variations of SCFA content with AR addition during the anaerobic fermentation of food waste. The content of SCFA in all the reactors increased rapidly and then decreased steadily with the fermentation time. In the blank, the maximum SCFA appeared
Conclusion
In this work, AR was first applied to improve the accumulation of SCFA from food waste anaerobic fermentation and the underlying mechanism of AR enhancing anaerobic fermentation of food waste was also revealed. The optimal dosage of AR was 300 mg/g, and the corresponding SCFA yield was 32.5 g/L. AR improved both hydrolysis and acidification efficiencies but inhibited the methanogenesis process. In addition, AR can promote the degradation of two kinds of proteins (animal and plant proteins) in
Acknowledgements
This research was financially supported by the project of National Natural Science Foundation of China (NSFC) (No. 51678315), the Key research and development plan of Shandong Province (2018GSF117030, 2018GSF117042), the Shandong Province Natural Science Foundation (ZR2018BEE037), the China Postdoctoral Science Foundation funded project (2018M632642).
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