Variable sediment methane production in response to different source-associated sewer sediment types and hydrological patterns: Role of the sediment microbiome
Graphic abstract
Introduction
With a global warming potential 34 times that of carbon dioxide (CO2) over 100 years, methane (CH4) has been recognized as a highly potent greenhouse gas (GHG) that significantly contributes to climate change. Regarding anthropogenic CH4 sources, concerns about municipal sewer systems have intensively grown in recent decades due to mounting evidence of ubiquitously high CH4 yields from the sewers of many cities worldwide (Chaosakul et al., 2014; Chen et al., 2020; Liu et al., 2014). These emissions can contribute a GHG budget of roughly 48~60% above that from a wastewater treatment plant (WWTP) (Guisasola et al., 2008). The global sewer CH4 emissions are conservatively projected to reach 1~10 Mt CO2-e according to an estimation carried out in Australia (during 2011–2012) (Short et al., 2017). Furthermore, methanogenesis depletes carbon sources in sewage, which is detrimental to the downstream nutrient removal process of WWTPs. Gaseous CH4 concentration of up to 50,000 ppmv has a low explosive limit of approximately 5% vol. and can cause an explosion when in contact with air in a confined sewer space (Spencer et al., 2006).
Current research aims to improve knowledge on the estimation of sewer CH4 emissions impacted by sewage quality, hydrodynamics and sewer attributes (Liu et al., 2015c; Short et al., 2017; Willis et al., 2018). Notably, CH4 produced by sewer wall biofilm has been an intense area of focus (Guisasola et al., 2009; Sun et al., 2018). Methanogenesis is a surface area-dependent process (Liu et al., 2016). Accumulated sediment in extensive sewer networks, which is deposited at a reported rate of 30~500 g/m of sewer length per day (Ashley et al., 2003), represents a problem deserving greater attention due to its extremely large combined surface area, as well as the substantial total network-scale CH4 generation (Liu et al., 2015b) and methanogenic efficiency (Vollertsen and Hvitved-Jacobsen 2000). Moreover, sewer sediment is substrate-rich and biologically active, contributing more than 50~60% chemical oxygen demand (COD) and >80% suspended solids (SS) and biological oxygen demand (BOD5) (on average) to storm sewage overflows (Chebbo et al., 2001; Gasperi et al., 2010), which constitutes a robust endogenous potential for sediment methanogenesis.
In recent decades, a widespread view focused on sewage transport force based municipal sewer classification has guided the studies on sewer CH4. Considering the essential but still unclear biochemical influence of the diverse influent pollution sources from the sediment deposition process, it is vitally important to further profile the sewer sediment CH4 production via pollution source classification. In this regard, sanitary sewers, storm sewers and illicit discharge-associated (IDA) storm sewers collect heterogeneous influent substrates from different pollution sources. Generally, sanitary sewers receive domestic sewage containing highly bioavailable organics that can be preferentially utilized by microorganisms. Multi-site surveys have confirmed recalcitrant humic substances, petroleum derivatives and solid particles flushed via stormwater as the dominant influent substrates in storm sewers (Chen et al., 2017; Durand et al., 2004). IDA storm sewers, a type of defective sewer that broadly exists worldwide (Pitt 2004), comprise of both substrate input types mentioned above. Long-term input of these heterogeneous substrates could greatly shape the sediment substrate composition and microbiome differentiation to reflect influent source characteristics, as well as promote the adaptive tendency of the microbial functional capabilities in mediating anaerobic bioreactions including fermentation, hydrolytic acidification, methanogenesis, sulfate-reduction and denitrification (Hvitved-Jacobsen et al., 2013; Jin et al., 2018). This link could ultimately dominate the carbon stream for microbial growth or stoichiometric biogas productions and alter the efficacy of methanogenesis throughout the whole process (Guillemette et al., 2016).
In conjunction with the influence of the source-associated sewer sediment type, the hydrological pattern of the total sewer system, e.g., hydrological connection, disconnection (influent cut-off caused by inadequate slope, mini flow or intermittent influent) (Chaosakul et al., 2014) and turbulence (causing sediment suspension in durable hydraulic), further promotes a shift in the exogenous substrate input mode and eventually contributes to variability in the sediment methanogenesis efficacy. Particularly, hydrological disconnection has been reported to amplify the heterogeneity of substrate pools and the biochemical kinetics in fluvial systems (Casas-Ruiz et al., 2015), whereas in-sewer turbulence has been suggested to induce ‘first flush’ peaks in wastewater CH4 emission due to mobilization of methanogenic sewer sediment (Short et al., 2017). The current related studies about sewer CH4 have only assessed the effects of gradient changes in hydraulic shear force or hydraulic retention time (HRT) during stable sewage flow (Liu et al., 2016).
Collectively, we still do not know the mechanism or the extent to which the source-associated sewer sediment type and hydrological pattern affect sewer sediment CH4 production, as these issues have long been overlooked. Bridging the knowledge gap to better understand the mechanistic underpinnings involving the differentiation of microbiome and gene functional capability in sewer sediment, as well as their key roles in mediating methanogenesis-related metabolisms, is essential for establishing links between sediment CH4 production and sewer sediment type and hydrological patterns.
This study aimed to provide a detailed assessment of the variability of sediment CH4 production in response to the different source-associated sewer sediment types (sanitary, storm and IDA storm sewers) and hydrological patterns (hydrological connection, disconnection and turbulence) based on long-term laboratory batch tests, and addressed the role of the sediment microbiome. The endogenous CH4-production capability of the initial sewer sediment was determined by large-scale sampling and five-day anaerobic digestion tests. Sediment microbiome was profiled using 16 s rRNA gene sequencing. The enzymes involved in methanogenesis, sulfur reduction and oxidation, and denitrification in sediments cultivated under hydrological connection and disconnection conditions were analyzed by metagenomic sequencing and subsequent KEGG annotation.
Section snippets
Sampling campaigns
Sediment samples were collected from seven separate sewer systems located in Shanghai, China. The catchments of these systems were mainly associated with residential land-use. Sampling sites were selected based on the sewer location, configuration, age and characteristics of the influent wastewater (Tables S1 and S2). The in-field criterion included (Crabtree 1989): 1) adequate sediment quantity in the sewer channel; 2) sediment depth of 100~300 mm; 3) no flow-locking effects of tide and high
CH4-production capability of sewer sediment
Consistent with the initial properties of the sediments and their associated overlying wastewater (Table S2), significant discrepancies (ANOVA, p = 0.05) were also observed in their endogenous CH4-production capabilities during the five-day degradation tests (Fig. 1a). Storm sewer sediments (sediment COD = 124±83 mg/L, VS/TS = 0.068±0.020; overlying reserved storm runoff COD = 43±11 mg/L) showed the minimum endogenous CH4 flux (mean 4.48 × 103 mg/m2), whereas the endogenous CH4 yield of
Sanitary sewer sediment: microbiome phenotype 1
The feeding of sufficient highly biodegradable organic substrate conveyed by domestic sewage in sanitary sewers promotes the sinking and formation of sediment that is biologically active as well as acetate precursor-rich, e.g., proteins, carbohydrates, long-chain aliphatic hydrocarbon and short-chain fatty acids (SCFAs). In this context, this kind of sediment presents a high capability for CH4 production. Methanosaeta, which is significantly prevalent in sanitary sewer sediments (microbiome
Conclusions
This study provides new insights into the crucial links among sewer sediment CH4 production and the impacts of source-associated sewer sediment type and hydrological pattern. Taking into account the increasing uncertainty of urban hydrology caused by climatic change and intense construction of sewer networks fueled by global urbanization and improved sanitation, this work will help to stimulate and guide the resolution of ‘bottom-up’ system-scale carbon budgets and GHG sources, as well as the
Declaration of Competing Interest
The authors declared that they have no conflicts of interest to this work.
Acknowledgments
This study was funded by the Natural Science Foundation of Shanghai (grant numbers 19ZR1443800, 19ZR1443700), the Key Technologies Research and Development Program (grant numbers 2017YFE0135500, 2018YFD1100502-02), the China National Critical Project for Science and Technology on Water Pollution Prevention and Control (grant number 2017ZX07207001-02), the National Natural Science Foundation of China (grant number 51908355), the Scientific and Innovative Action Plan of Shanghai (grant number
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