Urban pipe network systems, including water supply system and sewer system (Rabaey et al., 2020), can be regarded as the blood vessels of a city. These infrastructures are essential for urban life, production activities, and maintaining the urban water environment. The purpose of a sewer system is to collect and discharge wastewater to sewage treatment plants (Chen et al., 2024), and collect rainwater to mitigate urban flooding (Hesarkazzazi et al., 2022). Thus, maintaining the drainage capacity of sewer systems is essential. However, some substances such as particulate matter (PM) (Hu and Zhao, 2022), heavy metals (Schertzinger et al., 2018), and nutrients (Yu et al., 2022) can be deposited in sewer systems, which reduces the drainage capacity of the sewer system (Lv et al., 2024; Murali et al., 2019), while large volumes of water flow can flush sediments with high pollution loads from sewer systems (Schertzinger et al., 2019; Yu et al., 2022), resulting in serious environmental risks. Thus, sediments should be a key consideration when designing and maintaining sewerage systems.
Urban sewer systems comprise open and circular channels (OCs and CCs, respectively) and junctions. The sewer system collects large quantities of wastewater from buildings to OCs, and this wastewater, along with surface stormwater runoff, influences sediment in OCs, CCs, and junctions (Fig. S1). Generally, surface PM comes from dry and wet atmospheric deposition (Audoux et al., 2023; Wei et al., 2020), tire and road wear (Harrison et al., 2021), etc., and is washed into the sewer system during rainfall (Hu and Zhao, 2022). Thus, sediment accumulation in OCs, CCs, and junctions usually involves multiple processes, including the build-up and wash-off of surface sediments (Muthusamy et al., 2018; Zhao et al., 2018), exchange of sediments between the surface and the sewer system, and accumulation and transport of sediments in the sewer system (Campisano et al., 2019; Liu et al., 2023b; Murali et al., 2019), and some models were constructed based on these processes (Campisano et al., 2019; Liu et al., 2023b; Murali et al., 2019).
However, the generation and migration of sediments in the sewer system is a very complex process, which is influenced by various EFs. Recently, several field investigations showed that the build-up and wash-off processes of PM on the urban surface are influenced by drying time, human activities (Wang et al., 2020), traffic (Mahbub et al., 2010), rainfall characteristics (Da Silva et al., 2024), and particle size (Hu and Zhao, 2022). Furthermore, in the sewer system, the exchange of substances between the pollutants in the sewage and the sediments via physicochemical deposition and biochemical transformation processes (Shi et al., 2018) will affect the sediment content. However, these studies mostly explored sediment distribution in specific points (i.e. neighborhood scale or segment level) (Campisano et al., 2019; Montes et al., 2021), the influence of EFs at the urban scale is rarely considered (Delli Compagni et al., 2019; Liu et al., 2023b). In the extensive sewer system of an urban area, however, sediment accumulation at specific points cannot be explored in isolation from the rest of the system. Neglecting the interaction between EFs across the entire sewer catchment reduces the accuracy of sewer sedimentation models (Liu et al., 2023b).
Although several studies have made sporadic mention of the combined effect of EFs on sediments on urban surfaces or in sewer systems. For example, the positive effects of population and plants on sediment loads at the community scale (Liu et al., 2023b) and the complex effects of traffic and rainfall characteristics on heavy metals deposited on the surface (Mahbub et al., 2010). In addition, previous studies have shown from a theoretical analysis perspective that climate change will affect the long-term application of predictive models for deposition and wastewater quality in sewer systems (Langeveld et al., 2013; Murali et al., 2019). Therefore, considering the combined effects of multiple environmental factors can help construct more rational and long-term effective models in the future, which has rarely been addressed. Unfortunately, it is impractical to consider all EFs to construct the intrinsic equations, as the difficulty of constructing physical-mathematical models will increase with the number of dependent variables, and the increased need for monitoring indicators will increase the simulation costs. Therefore, screening more important EFs is necessary to reduce modeling difficulties and economic costs (Liu et al., 2023b).
Based on the above considerations, this study collected monthly sediment accumulation data in different structures of the Kyoto City sewer system (OCs, CCs, and junctions). It assessed the catchment-wide contribution of 25 EFs, representative of five EF categories (population, traffic, climate, air quality, and wastewater quality), to changes in sewer sediment content from 2014 to 2023. Based on the monthly variations of each EF and sediment data at the urban scale, this study hypothesizes that the interactions of the above five EF categories will lead to changes in the sediment content of the sewer system and that the influence of the different EFs is not consistent, and that some of the EFs should not be ignored in predicting the sediment content. The study aims to develop a conceptual framework for understanding the role of EFs in sediment content in the sewer system. Specifically, the objectives were, 1) to characterize the direct and indirect contribution and interactions of each EF category to sewer sediment accumulation, 2) to assess the significance of each EF as a sewer sedimentation driver, and, 3) to identify those EFs whose input should be invariably included in sewer system sedimentation models. An overall understanding of the role of these factors and their impact on sediment enhances understanding of sedimentation in urban sewer systems and informs sewer system design and maintenance, especially about sedimentation control, and will be beneficial to the decision-makers concerned in taking timely and effective measures to control the more important factors affecting sediments.
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