Characterization and sources apportionment of overflow pollution in urban separate stormwater systems inappropriately connected with sewage

Monitoring (micro-)pollutants in wastewater treatment plants: comparing discharges in wet- and dry-weather

Municipal wastewater treatment plants (WWTPs) are crucial for maintaining good quality of surface water, limiting environmental pollution. However, during wet-weather events, WWTPs become an important point-source discharge due to the activation of the bypass, which releases a mix of untreated wastewater and stormwater. This work aims to assess how the WWTP discharges (effluent and bypass) impact on the receiving surface water body during dry- and wet-weather, monitoring 78 pollutants (7 conventional pollutants, 19 heavy metals, and 52 micropollutants) in each stream (effluent during dry-weather, effluent and bypass during wet-weather), including the influent in dry-weather for comparison. The occurrence, concentration levels and variability, and environmental risk were addressed, with a specific focus on high-resolution (up to 20-minutes) sampling of the bypass, based on the expected relevant temporal dynamicity. A wider range of pollutants occurred in the bypass, included undetected compounds in the dry-weather influent. Besides, a greater inter-events variability in bypass concentrations was observed, but smaller intra-event variability, with only some pollutants exhibiting a distinct first-flush effect. To address the challenge of a cost-effective bypass monitoring, we explored the applicability of readily measurable water quality parameters (total suspended solids and electrical conductivity) as proxies for micropollutants. Correlations between these parameters and specific pollutant groups suggest a promising path for further investigation and broader application. The magnitude of the rain event also affected concentration levels, with event volume clearly affecting pollutants dilution. The environmental risk assessment revealed a significantly higher risk associated to bypass discharge compared to the effluent, especially for conventional pollutants and metals, highlighting the urgency of improved bypass management strategies. Overall, this study highlights the contribution of wet-weather discharges from WWTPs, emphasizing the importance of high-frequency bypass monitoring to capture peak pollutant concentrations and accurately assess the environmental risk.

Bibliometric analysis of global performance and trends of research on combined sewer overflows (CSOs) from 1990 to 2022

Combined sewer overflows (CSOs) are one of the main sources of pollution in urban water systems and significantly impede the restoration of water body functionalities within urban rivers and lakes. To understand the research and frontier trends of CSOs comprehensively and systematically, a visual statistical analysis of the literature related to CSOs in the Web of Science core database from 1990 to 2022 was conducted using the bibliometric method using HistCite Pro and VOSviewer. The results reveal a total of 1,209 pertinent publications related to CSOs from 1990 to 2022, and the quantity of CSOs-related publications indicated an increasing trend. Investigations of the distribution and fate of typical pollutants in CSOs and their ecological effects on receiving waters and studies on pollution control technologies (source reduction, process control, and end-of-pipe treatment) are the current focus of CSOs research. CSOs pollution control technologies based on source reduction and the monitoring and control of emerging contaminants are at the forefront of scientific investigations on CSOs. This study systematically and comprehensively summarized current research topics and future research directions of CSOs, thus providing a reference for CSOs control and water environment management research.

Reducing particle accumulation in sewers for mitigation of combined sewer overflow impacts on urban rivers: a critical review of particles in sewer sediments

Sewer sediments contain various hazardous compounds, leading to significant pollution risks when combined sewer overflows (CSOs) occur without appropriate controls. This paper presents a comprehensive review of the issues associated with particles in sewers, specifically focusing on the non-negligible contribution of particulate matter to CSOs, which leads to pollution in urban rivers. Therefore, the sources of particulate matter in sewers, their contributions to the overflow particles, and the specific areas of concern when it comes to managing particulate matter during particle transportation are outlined. Overall, carefully considering the goal of avoiding sedimentation during the drainage system design is the most effective prevention and control method for pipeline sediment, where minimum velocity and minimum shear stress are the core parameters. The establishment of a flexible and adaptive particle simulation method in drainage pipelines requires reliable simulation of particle sedimentation and erosion, the development of sediment prevention facilities with strong adaptability, and a comprehensive evaluation of economic and environmental benefits. With the ongoing enhancement of urbanization in developing countries, such studies will have more practical significance.

Rapid quantification of the surface overflow and underground infiltration in sewer pipes based on computer vision and continuous optimization

The overloading of the sewer network caused by unwarranted infiltration of stormwater may lead to waterlogging and environmental pollution. The accurate identification of infiltration and surface overflow is essential to predict and reduce these risks. To retrieve the limitations of infiltration estimation and the failure of surface overflow perception using the common stormwater management model (SWMM), a surface overflow and underground infiltration (SOUI) model is proposed to estimate the infiltration and overflow. First, the precipitation, water level of the manhole, surface water depth and images of the overflowing point, and volume at the outfall are collected. Then, the surface waterlogging area is identified based on computer vision to reconstruct the local digital elevation model (DEM) by spatial interpolation, and the relationship between the waterlogging depth, area and volume is established to identify the real-time overflow. Next, a continuous genetic algorithm optimization (CT-GA) model is proposed for the underground sewer system to determine the inflow rapidly. Finally, surface and underground flow estimations are combined to perceive the state of the urban sewer network accurately. The results show that, compared with the common SWMM simulation, the accuracy of the water level simulation is improved by 43.5% during the rainfall period, and the time cost of the computational optimization is reduced by 67.5%. The proposed method can effectively diagnose the operation state and overflow risk of the sewer networks in real time during rainfall seasons.

Microplastics discharged from urban drainage system: Prominent contribution of sewer overflow pollution

Urban drainage system is an important channel for terrigenous microplastics (<5 mm in size) to migrate to urban water bodies, especially the input load caused by overflow pollution in wet weather. Investigating how they transport and discharge is essential to better understand the occurrence and variability of microplastics in different water ecosystems. This study evaluated the abundance and distribution characteristics of microplastics in the drainage systems of typical coastal cities in China. The impacts of meteorological conditions and land use were explored. In particular, the prominent contribution of drainage sewer overflow pollution during storm events were investigated. The results showed that the microplastics abundance in daily sewage discharge from different drainage plots ranged between 13.6 and 30.8 items/L, with fibers as the dominant type of microplastics. Sewer overflow discharge can greatly aggravate microplastic abundance to 83.1 ± 40.2 items/L. Road runoff and sewer sediment scouring were the main pollution sources. Systematic estimates based on detailed data showed that the average microplastics emitted per capita per day in household wastewater was 3461.5 items. A quantitative estimation method was proposed to show that the annual emissions load of microplastics via urban drainage system in this research area was 5.83×10¹⁰ items/km², of which the proportion of emissions in wet weather accounted for about 60%. This research provides the first full-process of assessment and source apportionment of the microplastic distribution characteristics in old drainage system. The occurrence of storm events is an important marker of increased microplastic abundance in urban rivers, with a view to urgent need for interception of surface runoff and purification of sewer overflow pollution.

Isotopic and hydrochemical analyses reveal nitrogen source variation and enhanced nitrification in a managed peri-urban watershed

Watershed management practices (WMPs) alter the sources and transformation of reactive nitrogen (N) in peri-urban watersheds, but a precise description of how WMPs impact N cycling is still lacking. In this study, four sampling campaigns were conducted in the wet and dry seasons of 2019 (before WMPs) and 2020 (after WMPs) to determine the spatiotemporal variations in nitrate isotopes (¹⁵N-NO₃^- and ¹⁸O-NO₃^-) and hydrochemical compositions in the Muli River watershed. The results showed that the WMPs could significantly reduce the N load in the middle and lower reaches, but substantial improvements were not observed in 2020. Manure and sewage (M&S, 36.2 ± 15.8-55.0 ± 19.4%) was the major source of nitrate (NO₃^-) in the stream water, followed by smaller-scale wastewater treatment plants (WWTPs, 14.0 ± 10.9-25.6 ± 11.5%). The WMPs were effective in controlling M&S, resulting in an approximately 16.7% (p < 0.01) lower M&S contribution during the dry season in 2020 compared to that in 2019. However, the smaller-scale WWTP input increased by approximately 5.4% (p < 0.01) after the WMPs. During the study period, the assimilation of NO₃^- by phytoplankton was important for NO₃^- loss, but the WMPs promoted nitrification in the watershed because of the elevated redox potential (Eh). Overall, the present study provides a better estimate of the variations in nitrogen sources and transformation in a peri-urban watershed after WMPs and provides an approach for developing timely nitrogen management solutions.