Co-digestion of sewage sludge and food waste : reactor operation and genome-centric microbial interaction analysis

Abstract

Food waste (FW) with 3,337 tonnes daily generation is a huge challenge in Hong Kong. Compared with traditional approaches for food waste disposal, such as incineration, landfilling, and compositing, anaerobic co-digestion of food waste with feeding sewage sludge (FSS) is a promising solution in increasing waste stabilization efficiency and improving energy recovery rate. A series of lab-scale experiments were conducted to investigate the effects of operating parameters, i.e. FW:FSS ratio, FW composition, and agitation rate, on the performance of co-digestion. These parameters were optimized to maximize volatile solid reduction and biogas production, and to improve process stability by minimizing intermediary products accumulation.

The pathogenic indicator bacterium inactivation kinetics in two anaerobic digestion modes (namely, mono-digestion of FSS and co-digestion of FW and FSS) was studied, and a wild type Escherichia coli (E. coli) was spiked in mesophilic digesters. The inactivation efficiency of E. coli profiled by combining the lab results with the model-fitting approach would be useful for evaluating the potential pathogen-inducing risks in different anaerobic digestion modes.

The effect of solid retention time (SRT) on both the performance of co-digesters and microbial community structure was investigated. 16S rRNA gene amplicon sequencing analysis revealed the divergency of microbial communities in SRT-differentiated digesters, and genome-centric analysis was conducted on the recovered metagenome-assembled genomes (MAGs) to classify populations into different functional guilds. The results revealed that functional gene redundancy was one of the reasons of complementary of functional roles in the microbial community to proceed hydrolysis, and varying sugar uptake and transporting mechanisms were the niche differentiation of competitive members to co-exist in the community.

Five long-term operated and well-functioned thermophilic digesters laid foundation in studying the microbe-mediated engineered system. 16S rRNA gene amplicon sequencing analysis revealed the complexity of microbial communities in FW:FSS ratio-differentiated digesters and gene-centric analysis showed that FW:FSS ratio-differentiated feedstocks did affect the abundance of functional pathway in different digesters. Additionally, genome-centric analysis unraveled that the shifting of functional microbe clusters was the response to changes of environmental variables caused by varying FW:FSS ratios. Lastly, SRT of 15 days was sufficient in ensuring the stability of the thermophilic co-digestion of FW and FSS, and acetoclastic methanogenesis was the dominant pathway. The prevalence of propionate was observed in all well-functioned thermophilic co-digesters, but this acid was quickly degraded in the mesophilic counterparts. Temperature and SRT largely shaped microbial community structure and the clear clustering was identified based co-occurrence pattern of microbial populations in temperature- and SRT-differentiated digesters.

In this thesis, the operating parameters, were optimized to achieve the satisfactory performance of a co-digestion system. The long-term reactor operation and high-resolution study on microbial community shed light into the “black box” of anaerobic reactor to view ecological roles of microbes, metabolic interspecies interaction, niche differentiation in the microbial community, and so on. The results of this thesis will provide clues for researcher and engineers to realize fine-tuned control of an anaerobic co-digestion system.