Extracellular hydrolytic potential drives microbiome shifts during anaerobic co-digestion of sewage sludge and food waste
Graphical abstract
Introduction
Anaerobic digestion is a biotechnological process that offers a sustainable solution for the management of sewage sludge (SS). It converts a large proportion of the biodegradable fraction into biogas, which can be used to generate electricity, and turns the nutrient-rich waste into a biologically stabilized product, which can be applied in soil to recycle nutrients back to agricultural land for crop growth. However, only a portion of the potential energy contained in the raw wastewater is recovered as biogas by the anaerobic digestion of mixed primary and secondary sludge (McCarty et al., 2011). Co-digestion with organic co-substrates may be used to boost the production of biogas from sewage sludge, ideally fulfilling the energy demands of wastewater treatment. Food waste (FW) meets the criteria for a suitable co-substrate to increase energy production. Furthermore, co-digestion is a more sustainable option than landfill disposal, as the latter contributes significantly to anthropogenic emission of greenhouse gases (Karthikeyan et al., 2018). Due to its high biodegradable organic matter content this resource is a useful substrate for biogas production through anaerobic digestion. however, co-digestion of food waste with sewage sludge could be more challenging than sewage sludge mono-digestion as regards to the operational management of the anaerobic process. A high load of easily biodegradable organic matter may lead to the rapid accumulation of volatile fatty acids (VFA), causing acidification of the digester, and thus the disruption of the concerted action of the various groups of microorganisms involved in the production of biogas (Carballa et al., 2015).
The type of substrate is a major driver of variations in the phylogenetic structure of microbial communities (Zhang et al., 2014). Previous studies have shown that microbial communities in sewage sludge digesters responded to changes in substrate composition and concentration (Patil et al., 2021). Addition of carbon-rich co-substrates caused substantial shifts in community composition and a reduction of diversity (Nguyen et al., 2018). The microbiota of anaerobic digesters of sewage sludge changes significantly when SS is co-digested with food waste. Compared to mono-digestion, co-digestion studies found an increase in the proportion of members of Bacteroides and Methanosphaera (Treu et al., 2019) and in the abundance of bacteria capable of degrading complex organic polymers, such as Thermotogales, Thermonema and Lactobacillus (Fitamo et al., 2017). Other 16S rRNA gene-based studies found Syntrophomonas and Methanosarcina as dominant genera in FW-enriched digesters (Kim et al., 2019, Xu et al., 2017).
However, the functional significance of the observed shifts in species composition cannot be directly determined using ribosomal marker genes. Shotgun sequencing allows the full characterization of the metabolic capacities of microbial communities (Vanwonterghem et al., 2016, Zhu et al., 2020) By applying genome-centric metagenomics specific microbial taxa can be surveyed in the context of the functional properties of the community (Fontana et al., 2018, Treu et al., 2016). In this work we applied metagenomics to investigate co-digestion of sewage sludge and food waste with the aim of understanding the dynamics of the digestion microbiome upon changes in substrate composition and concentration. We hypothesized that these changes are primarily driven by deterministic processes linked to microbial functional traits. The experimental design allowed us to compare communities in replicated digesters, as well as to determine the acclimation of microbial communities to the digestion of sewage sludge to increasing shares of food waste.
Section snippets
Operation of digesters
The experimental setup has been described in a previous article (Orellana et al., 2019) and is briefly summarized as follows. Four laboratory-scale anaerobic digesters with a working volume of 5 L were inoculated with a single sludge inoculum taken from an anaerobic digester of a municipal wastewater treatment plant (WWTP). During the first 3 weeks of acclimation, all reactors were fed with a mixture of settled primary sludge and thickened waste activated sludge collected from the same WWTP.
Digesters performance
The composition of the food waste used in this study was taken from a previous characterization of waste collected from restaurants in two of the most frequented quarters of the city of Buenos Aires (M.A. and R.P, unpublished results). It consisted of 74% carbohydrates, 18% proteins and 4% lipids. High proportion of carbohydrates and relatively low-fat content is typical of food waste generated in urban areas (Wang et al., 2020). No meaningful changes were observed in volatile fatty acids
Conclusions
By applying gene- and genome-centric metagenomic approaches we show that extracellular polysaccharides-hydrolyzing enzymes governed community changes when food waste was added to sewage sludge for co-digestion. The shift in abundance of various hydrolytic bacteria, notably Proteiniphillum, which has the capacity to degrade several complex carbohydrates and proteins, correlated with the increase in FW, suggesting that new microbial niches for hydrolytic bacteria were created in response to the
CRediT authorship contribution statement
Esteban Orellana: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing – original draft, Writing – review & editing, Visualization. Leandro D. Guerrero: Methodology, Formal analysis, Data curation, Visualization. Carol Davies-Sala: Methodology, Investigation. Melisa Altina: Conceptualization, Resources. Rodrigo M. Pontiggia: Conceptualization, Resources, Funding acquisition. Leonardo Erijman: Conceptualization, Methodology, Formal analysis, Resources, Data
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.
Acknowledgments
This work was partially supported by the Ministry of Science, Technology and Innovation of Argentina (FITS 12/2013, MINCyT-Argentina). L.E. and L.D.G are career members of CONICET.
Data availability
Raw sequence files were deposited in NCBI-SRA under the accession number PRJNA544497.
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