Urban drainage system is responsible for transferring and treating urban wastewater (Ai et al., 2016). It is considered as a critical infrastructure to provide services for the society (Zhang et al., 2021). The human activities and the long-term use of drainage systems have inevitably led to widespread sediment deposition in sewers (Fujita et al., 2014; Meng et al., 2019). Some areas have caused serious sediment deposition (Avila et al., 2016). In addition, sewage sediments can bring serious pollution problems (Tian et al., 2017). Specifically, the sewage sediment contains a large amount of pollutants, such as surfactants, heavy metals, antibiotics and other organic substances (Xu et al., 2018; Fan et al., 2018; Meng et al., 2019). The high concentration of organic substances will produce toxic gases such as methane and hydrogen sulfide after a series of reactions under microbial activities (Ai et al., 2016). These pollutants will then be discharged into the receiving water with the sediment during rainy weather, threatening aquatic environment (Banasiak et al., 2005). With the lengthen of rainy seasons and the arrival of heavy rainfall caused by global climate change, a large number of pollutants accumulated in sewer sediments are eroded and discharged into the receiving water (Xu et al., 2018). The construction of sewage treatment system is far from the rapid development cities, which facing the severe challenge brought by combined sewer overflow (CSOs) pollution. (Li et al., 2014).
According to the buried depth and special structure under different topographic conditions, a complex environment of aerobic, anaerobic and anoxic is formed in the sewer, which makes it possible to regard the sewer as a reaction vessel (Shahsavari et al., 2017; Hvitved-Jacobsen et al., 2002). Many studies have shown that sewer sediments have significant viscous characteristics (Nalluri et al., 1992; Berlamont et al., 1996; Zhou et al., 2015). Coupled with microbial activity, biofilms adhere to sediment particles and increase the bed critical shear stress (Chen et al., 2017). Vignaga et al. (2013) found that non viscous sediments are bounded by biofilms, which increases the difficulty of entraining them into surrounding fluid. The incipient movement of sediment is also affected by microbial activity (Gu et al., 2020). However, few studies have linked microbial activities to the assessment of sediment anti-scouribility. The physicochemical properties of sewer sediments will be changed due to microbial activities, which affects the anti-scourability of sediments (Meng et al., 2019), and it further determines the amount of sediment pollutants that will be washed into the receiving water during rainy weather (Ahyerre et al., 2001). The experiment by Hagardon et al. (2012) showed that the thickness of biofilm and the degree of sediment erosion vary with the incubation period, which indicates that the sediment may show different characteristics in terms of anti-scouribility with the increase of contact time.
In addition to affecting the anti-scouribility of sewage sediments, biological factors will also have a great impact on the release of contaminants from sediments to the surrounding environment (Cowle et al., 2018). Most microorganisms exist in the form of microbial aggregates, which are tightly bound by secreting highly viscous extracellular polymers (EPS) to form biofilms (Adessi et al., 2018; Desmond et al., 2018). EPS combines with cells through complex interactions to form a broad network structure, attaching microorganisms and organic matter to the sewer wall, so as to prevent erosion (Rocher et al., 2003; Meng et al., 2019). It was found that EPS can work with a large amount of water to protect cells from dehydration and toxic substances (Bridier et al., 2011; Ying et al., 2010). Microbial activity increased EPS secretion (Choi et al., 2002; Gao et al., 2013). Some scholars used activated sludge to replace sediment, and observed experimentally the change of sediment erosion intensity with time through (Lau and Droppo, 2000; Prochnov et al., 2001).
It was found that EPS could not only affect the absorption of pollutants such as hydrocarbons, phosphorus and metal elements by sediment particles, but also released pollutants to the surrounding environment (Rocher et al., 2003; Zhao et al., 2021; Fang et al., 2017; Lai et al., 2018). Moreover, the dissolved oxygen content in the water environment around the sediment will also affect the degree of pollutants released by the sediment into the fluid (Chen et al., 2003). This showed that the microbial activities are significantly affected by the changes of the surrounding environment (Shammi et al., 2017). According to the special structure, buried depth and seasonal factors of the sewer, the microbial activities in the sewer environment may be different. However, few studies had comprehensively considered the growth of biofilm (i.e., the incubation time) according to different environmental factors, and the impact of incubation time on the erosion behavior of pipe sewer sediments has not been clearly understood.
In view of the above considerations, this study explores the pollutant release and scouring characteristics of sediment under microbial activities, hoping to have certain engineering significance for the stable operation of the drainage system, and the effective control of water pollution. The following aspects were studied: (1) the activity characteristics and distribution forms of microbial communities under different sewer environments (e.g., incubation time, dissolved oxygen, accumulation thickness, temperature, flow conditions, etc.); (2) the effects of incubation time (the growth of microbial communities) on the physicochemical characteristics of sewer sediments under different sewer environments (layered bulk density and erosion resistance, etc.); (3) the release of pollutants (nitrogen, phosphorus, etc.) from microbial communities to the overlying water of sediments; (4) the effects of incubation time and dissolved oxygen on the anti-scourability of sewer sediments.