Effect of liquid digestate recirculation on the ethanol-type two-phase semi-continuous anaerobic digestion system of food waste
Introduction
Food waste mainly comes from household kitchens, restaurants, canteens and other food processing-related industries, accounting for approximately 30–60% of municipal solid wastes (Ma and Liu, 2019). It is unstable and degrades rapidly, causing serious environmental and public health problems, such as odour, pest spread and soil and groundwater pollution (Gu et al., 2020). Current treatment methods of food waste include crush and direct discharge, landfill, incineration and anaerobic digestion. Amongst them, anaerobic digestion is considered as a potential technology, because it can recover biogas energy and achieve reduction of food waste and has huge economic and environmental advantages. In addition, its extensive adaptability of fermentation material is convenient for engineering applications (Xiong et al., 2019, Ye et al., 2018).
However, anaerobic digestion process often occurs the large accumulation of volatile fatty acids (VFAs) due to the rapid acidification rate and the slow methane generation rate, which is referred to as organic acid inhibition (Ren et al., 2018). VFAs are important intermediate metabolite in the anaerobic fermentation of organic matter, mainly including acetic acid, propionic acid and butyric acid and small amounts of valeric acid and isovaleric acid (Neshat et al., 2017). At high concentrations, VFA will penetrate into cell membrane, causing the acid–base imbalance of intracellular environment, inactivating the cells, reducing the degradation rate of organic acids and H2/CO2 and eventually leading to the failure of anaerobic fermentation (Wang et al., 2018).
Organic acid inhibition is one of the biggest challenges for the stable operation of food waste anaerobic digestion system, which limits the increase of organic load rate (OLR), affects the processing capacity of reactor and increases the construction cost (Yuan and Zhu, 2016). Upon investigating other literature using similar substrate (i.e. food waste) as this study, it has not been reported that its maximum OLR was higher than 5 g-VS·L−1·d−1 when using food waste as a single substrate for mesophilic anaerobic digestion organism (Feng et al., 2020, Ganesh et al., 2014, Voelklein et al., 2016, Wu et al., 2018). To this end, researchers have tried many ways to solve this problem. Zhang and Jahng (2012) increased the OLR to 6.6 g-VS·L−1·d−1 by supplementing trace elements, such as Co, Mo, Ni and Fe. Jabeen et al. (2015) increased the OLR to 6.0 g-VS·L−1·d−1 by adding cellulose substrates (co-digestion of rice husk and food waste). In the author’s previous research, the addition of yeast to food waste effectively alleviated VFA inhibition and increased the OLR to 5.5 g-VS·L−1·d−1. The main principle is that the addition of yeast promotes the hydrolysis of substrates and converts the degradable organic matter into neutral ethanol instead of propionic acid or butyric acid, which reduces the load of VFA in the system and provides more potentially available energy to methanogens, thereby improving the system’s stability (Ma et al., 2020, Wu et al., 2015). Other researchers have proposed a strategy by recirculating the liquid digestate of the methanogenic stage to the acidogenic stage to increase the pH of the acid-generating reactor and solve the intractable problem of digestate treatment (Wu et al., 2018).
In this study, a novel anaerobic digestion system was constructed that combined the advantages of conventional anti-acidification methods whilst reducing operating costs. In this system, an ethanol-type hydrolysed acidified phase was first formed by inoculating yeast, and the liquid digestate produced by the subsequent methane phase was then recirculated to the ethanol-type hydrolysed acidified phase. To explore the feasibility of this scheme, this study set up two liquid digestate recirculation groups (ethanol-type (R1) and traditional-type (R2)) and two non-recirculation control groups (ethanol-type (C1) and traditional-type (C2)) for 150-days semi-continuous experiment. The differences in stability indicators under the same OLR, such as VFA, alkalinity, pH value, ammonia nitrogen, etc. were then compared and analysed. Moreover, the reasons behind these differences were further explored using microbiological methods. This study provided a scientific basis for the construction and the potential industrial application of the recirculation-based ethanol-type two-phase anaerobic digestion system.
Section snippets
Substrate and inoculum
Food waste was collected from a university canteen in Haidian District, Beijing, China. It was drained, shredded and stored at −20°C after removing large bones, plastics and paper product. Anaerobic digestion sludge was collected from a rural biogas station in Shunyi District, Beijing, China and domesticated with food waste for two weeks before the experiment. The chemical characteristics of food waste and anaerobic sludge were shown in Table 1.
Table 1 Chemical properties of food waste and
Analysis of methane production performance
Using food waste as the substrate, a two-phase semi-continuous anaerobic digestion experiment was performed in accordance with the design of Section 2.2. During the whole process, the OLR and duration were discontinuously changed based on the stability of reactor operation, and the whole process took 150 days from the beginning to the end. The four experimental groups were ethanol-type liquid digestate recirculation group (R1), traditional-type liquid digestate recirculation group (R2)),
Conclusions
The ethanol-type liquid digestate recirculation anaerobic digestion system could increase the maximum OLR to 6.0 g-VS·L−1·d−1. The operation of liquid digestate recirculation improved the alkalinity and stability of the system. Approximately 64.5% of the bacterial species in the hydrolysed acidified phase during stable operation came from the recirculated digestates, which increased the relative abundance of VFA-producing bacteria and enriched diversity of bacterial communities in the
CRediT authorship contribution statement
Xinxin Ma: Conceptualization, Formal analysis, Writing - original draft, Visualization. Miao Yu: Methodology, Investigation. Min Yang: Visualization, Writing - review & editing. Shuang Zhang: Writing - review & editing. Ming Gao: Resources, Funding acquisition. Chuanfu Wu: Supervision, Project administration. Qunhui Wang: Writing - review & editing, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (grant numbers 2018YFC1900903, 2018YFC1900904). The support from Sino-US-Japan Joint Laboratory on Organic Solid Waste Resource and Energy Technology of University of Science and Technology Beijing is appreciated.
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Both authors contributed equally to this work.