Bioaugmentation to enhance anaerobic digestion of food waste: Dosage, frequency and economic analysis
Introduction
Food waste (FW) is receiving increasing attention, not only for its contribution to environmental pollution, but also for its utilizability (Sahu et al., 2017, Yuan and Zhu, 2016). Approximately 1.3 to 1.6 billion tons of FW are generated globally every year (Braguglia et al., 2017). Anaerobic digestion (AD) is one of the main environmentally friendly and efficient methods for FW treatment (Parthiba Karthikeyan et al., 2018). Through the AD process, organic matter in FW can be metabolized into methane, which can be converted into energy using a combined heat and power system (Ayala-Parra et al., 2017, Barbot et al., 2016).
However, anaerobic digesters of FW often operate at a relatively low organic loading rate (OLR) to avoid AD failure. As FW is an easily biodegradable organic feedstock (Xiao et al., 2015), a high OLR anaerobic digester of FW is prone to deterioration caused by VFA accumulation and/or ammonia inhibition (Li et al., 2018b). Consequently, the susceptible methanogenesis phase is inhibited (Wang et al., 2016), leading to poor methane production and low efficiency of FW AD (Ye et al., 2018).
Operating at a higher OLR, the digesters can enhance the utilization of FW and reduce the cost of operation. Thus, improving the stability of the AD system running at a higher OLR is essential for increasing the economic benefits and reducing the possibility of economic losses owing to a system failure (Koo et al., 2019, Zhang et al., 2017). To this end, researchers have incorporated the use of additives (Ye et al., 2018), pretreated feedstock (Li et al., 2017b), and modified reactors (Xiao et al., 2015); further, they have facilitated co-digestion with nutrition mutual supply wastes (Ye et al., 2018), and explored optimized operational conditions (Zamanzadeh et al., 2016). However, very little information on bioaugmentation strategies for FW AD is available.
Bioaugmentation has been known to add supplementary microorganisms with relevant biodegradation capacities (Li et al., 2018d, Raper et al., 2018), and consequently improve the AD performance. It can obviously accelerate acetate and propionate degradation, thus augmenting the maximum OLR and influencing process stability (Li et al., 2018c, Lianhua et al., 2018, Town and Dumonceaux, 2016). The main components of VFAs accumulated from failures of AD systems for FW are acetate and propionate (Kim et al., 2017, Li et al., 2017a), which indicates that bioaugmentation with a suitable microbial culture may enhance the stability and efficiency of FW anaerobic digestion. Thus, an investigation of the feasibility of utilizing a bioaugmented consortium to improve the performance of AD for FW is necessary.
However, there are still unresolved issues in bioaugmentation. Economic cost, including additional and maintenance cost, has been considered as a barrier to successful bioaugmentation (Raper et al., 2018). Although relatively high addition dosages and frequencies are more conducive for the improvement of AD performance, they also increase the economic cost of bioaugmentation for long-term treatment of an unstable system (Abeysinghe et al., 2002). In contrast, a relatively lower addition dosage may reduce the relative cost but lead to an unacceptable improvement. These phenomena indicate that investigations of the application of an AD strategy should include evaluations of economic feasibility in addition to those of biochemical efficiency (Barbot et al., 2016, Ganesh Saratale et al., 2018). Therefore, an economic analysis is required to evaluate bioaugmentation strategies.
This study evaluates the suitability of bioaugmentation to AD of FW and determines the optimum dosage and frequency of bioaugmentation. Additionally, the effect of bioaugmentation on microbial communities and economic feasibility in an up-scale biogas plant were evaluated. The findings of the study provide an innovative way of improving the AD of FW, and explain the bioaugmentation mechanism in terms of microbial succession. Furthermore, the study offered insights into improving the economic feasibility of bioaugmentation, thus aiding in its large-scale application.
Section snippets
Substrates, inoculum and bioaugmentation culture
The food waste used in this study was collected from the canteens in Guangzhou Higher Education Mega Center (HEMC). Its total solid (TS) and volatile solid (VS) contents were 22.67 ± 0.09% and 20.95 ± 0.04%, respectively. The carbon, nitrogen and hydrogen element contents were 50.75 ± 0.57%, 2.33 ± 0.01%, and 7.38 ± 0.07%, respectively. The carbon to nitrogen ratio of the substrate was 21.78 ± 0.11%. Carbohydrate, lipid, and protein contents were 52.9%, 22.3%, and 14.9%, respectively.
The
Performance of food waste anaerobic digestion at different bioaugmentation conditions
The AD performance of semi-continuous digesters with and without bioaugmentation at different dosages and frequencies is shown in Fig. 2. Various operational parameters of the four digesters were compared, including volumetric biogas production (VBP), methane content, volatile fatty acids (VFA), acetate, propionate, TAN, pH and IA:PA.
As shown in Fig. 2, the four reactors collapsed successively with rising OLR. The maximum OLRs that R1 R2, R3 and Rc could reach were, respectively, 4.4 g L−1 d−1,
Conclusion
This study demonstrates that bioaugmentation enhanced the AD performance of FW regardless of the dose and frequency of bioaugmentation culture additions. When routinely adding bioaugmented cultures, AD reactors operated at a higher OLR and thus produced more biogas, which could be attributed to a higher density of acetoclastic methanogens. The economic feasibility and level of improvement resulting from bioaugmentation depends on the culture addition strategy. An optimal dosage and frequency
CRediT authorship contribution statement
Junfeng Jiang: Data curation, Formal analysis, Methodology, Writing - original draft, Visualization, Software. Lianhua Li: Supervision, Conceptualization, Investigation, Writing - original draft. Ying Li: Supervision, Funding acquisition, Project administration, Writing - review & editing, Visualization, Software. Yu He: Methodology, Formal analysis. Changrui Wang: Methodology, Formal analysis. Yongming Sun: Funding acquisition, Project administration.
Acknowledgments
This work was supported by the National Nature Science Foundation of China [No. 51708538], and the Strategic Priority Research Program of the Chinese Academy of Sciences [No. XDA21050400], Youth Innovation Promotion Association, CAS.
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