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

Introduction

Microplastics, a type of plastics generally accepted as measuring smaller than 5?mm in size, are widely distributed in the water environment (Rochman,?2018). Microplastics have posed a threat to most parts of ecological system and even transport through the?food chain?into human bodies (Ross?et?al., 2021;?Wright?and Kelly,?2017). Researchers have found microplastic particles in human blood samples, and deep in the lungs of living humans (Jenner?et?al., 2022;?Leslie?et?al., 2022). Annual global plastic production is estimated to reach 500 million tons in 2025 (Plastics?Europe,?2019). 80% of marine microplastics originate from terrestrial systems (Andrady,?2011;?Yonkos?et?al., 2014). Fluvial system are an essential pathway for land-based inputs (Huang?et?al., 2021;?Niu?et?al., 2021;?Xu?et?al., 2021). Due to the high correlation with human activities, urban areas may be considered of an integral source for microplastics emissions into rivers (Wagner?et?al., 2019).

The major sources of microplastics in urban agglomerations include?sewage?discharge,?atmospheric deposition, and surface runoff. These sources are all related to the collection and discharge of drainage systems, which are the important channels for pollution transfer between land and water bodies (Browne?et?al., 2011;?Eriksen?et?al., 2013;?Mak?et?al., 2020). In general, generated microplastics are transported via sewers to the?wastewater treatment plants?(WWTPs). The removal rates is influenced by a combination of service area characteristics and treatment processes (Blair?et?al., 2019;?Carr?et?al., 2016;?Long?et?al., 2019). Although appreciable removal can be achieved depending on the WWTP types, due to the continuous discharge of treated effluent, high concentrations of microplastics are still detected in both the water column and sediment downstream of the WWTP (Conley?et?al., 2019;?Murphy?et?al., 2016). Also, rapid and dense urbanization has contributed to increased abundance of microplastic in urban rivers (Esquinas?et?al., 2020). The sources and pathways of microplastics are closely related to land use and population density. Microplastics are generated from the use of personal care products, washing, illegal waste dumping, etc. (Choi?et?al., 2021;?ó Briain?et?al., 2020;?Piehl?et?al., 2018). There is great spatial heterogeneity in microplastic emissions due to differences in production patterns (Jang?et?al., 2020;?Su?et?al., 2020). However, most previous studies revealed the influence of land use types through the variation of microplastic abundance in water environment in different regions (Dikareva?and Simon,?2019;?Wagner?et?al., 2019). While few have reported the abundance of microplastics in directly generated wastewater from drainage plots. Understanding microplastics produced by different drainage plots is critical for identifying sources and managing?wastewater treatment?facilities.

Another key source of impact to pollution discharge from urban drainage systems is sewer overflow pollution during wet weather. High pollution load caused by sewer overflow has been widely demonstrated, such as COD, nutrients, heavy metals, persistent organic compounds, etc. (Launay?et?al., 2016;?Pilotti?et?al., 2021;?Zhang?et?al., 2022). During storms or intense rainfall, surface scouring causes microplastics to transport into sewers with runoff and eventually discharge into rivers (Lange?et?al., 2021;?Liu?et?al., 2019a). The major sources include the form of tire wear particles, road paint plastic waste, etc. Large amounts of runoff cause the water volume to exceed the operational load in the drainage sewers, resulting in overflow pollution. Direct input of untreated wastewater containing high microplastic abundance might occur, which have been detected in overflow effluent in Paris, Italy, and Shanghai (Chen?et?al., 2020;?Di?Nunno et?al., 2021;?Dris?et?al., 2015;?Treilles?et?al., 2020). In addition, the continuous transport of wastewater in sewers during dry weather results in the settlement or adsorption of large amounts of microplastic particles into the sediments (Sang?et?al., 2021;?Shahsavari?et?al., 2017). While the increased flow during storm or intense rainfall continuously flushes the sediments at the bottom of the sewers to the receiving water. Sewer sediments may be considered a prominent source of overflow pollution.

Microplastic abundance in rivers and estuaries can surge several or even tens of times following intense rainfall and storm events (Chen?et?al., 2020). Given accelerated urbanization process and progressively larger paved areas, the risk of microplastic generation and emission is likely to rise (Dalu?et?al., 2021;?Esquinas?et?al., 2020). Meanwhile, in China, as well as other countries or regions, there are numerous problems with drainage systems, such as broken, blockages, and separation of rainwater and sewage drains (Schilperoort?et?al., 2013;?Ellis?and Butler,?2015;?Tan?et?al., 2019;?Xu?et?al., 2020,?2019). These exacerbate the?leakage?or overflow of untreated wastewater and sediment transmitted, which also meant that more land-based microplastics would migrate into the nearby water bodies. There is consensus that the most effective way to reduce microplastics is to control the sources and migration pathways (Woodward?et?al., 2021). However, this microplastic migration pathway has not attracted enough attention.

The aim of this study was to determine the migration and emission of microplastics in urban drainage systems during normal and storm flows. The urbanized catchments of typical coastal cities in southern China are selected. Representative microplastic samples were collected from surface runoff, overflow effluent, sewer sediment, and receiving river during multiple rainfall events. The objectives of this study were to (1) determine the microplastics abundance in the daily sewage discharge from drainage plots of different land use types; (2) evaluate the microplastics abundance of sewer overflows, and investigate the migration characteristics of microplastic in receiving river after storm events; (3) propose a quantitative evaluation method for microplastics emission load in normal and wet weather. Understanding the sources of microplastics in these catchments is crucial for deciding priorities for management interventions and reducing inputs load to waterways.

Climate change impact on infection risks during bathing downstream of sewage emissions from CSOs or WWTPs

Introduction

Concentrations of human?pathogens?in surface waters are determined by a several processes. The pathogen input is important, but once released in the aquatic environment, pathogens are diluted, reduced by die-off and (temporarily) reduced by sedimentation.

Domestic wastewater is a major source of human pathogens to surface waters. Commonly, wastewater is treated by?WWTPs?before it is discharged into the surface waters. However, in the case of?combined sewer systems, the capacity of the sewer systems and WWTPs may be exceeded during periods of high rainfall, and, untreated wastewater will be discharged directly into the surface waters. Since climate change predictions show an increase in intensive precipitation events (KNMI, 2014b), an increase in?CSOs?is expected in the Netherlands. Recent research on the?effects of climate change?on the frequency, duration and volume of CSOs supports this (Abdellatif et?al., 2015,?Bi et?al., 2015,?Nie et?al., 2009,?Semadeni-Davies et?al., 2008). An increase in CSOs could result in increased influx of microbial pathogens and other pollutants into receiving waters.

Besides the changes in influx, climate change is also expected to influence river flow rates. In winter time, precipitation will increase river flow rates (Middelkoop et?al., 2001). During dry summers, periods of low discharge will occur more often. This is mainly true for surface water-dominated rivers, such as the river Meuse (de Wit et?al., 2007), as they cannot rely on a relatively stable groundwater fed base flow. Based on parameters, including general water quality variables, nutrients, heavy metals and?metalloids, a case study on the impact of summer droughts on the water quality of the Meuse river (Van Vliet and Zwolsman, 2008) indicates a general degeneration of the water quality of the Meuse river during droughts. They concluded that the reduction of the dilution capacity of point source effluents was one of the reasons for the decline in water quality. The effects of changing flow rates may amplify or counteract the change in pathogen influx through CSOs.

Most of the 715 official bathing water locations in the Netherlands are not in contact with wastewater from outlets of WWTPs or CSOs, as shown by completing bathing water profiles (EEA, 2015). An inventory of the accessible bathing water profiles (86%) shows that, of the bathing water locations in the Netherlands, only 11% and 10% could be influenced by wastewater discharges from WWTPs and CSOs respectively (Anonymous, 2016). At some of the locations with high wastewater discharge, additional?wastewater treatment?is applied to the emission or high emissions are monitored and communicated to the bathers.

About two-third of recreational activities in the Netherlands takes place at official bathing sites (Schets et?al., 2011), so the implication is that people frequently bathe at unofficial sites as well. In Amsterdam,?Greven and Jakobs (2015)?found that 5%, of the people surveyed with regard to their swimming habits, occasionally swim in the canals. Another possibility is that people jump, fall or get pushed into the canal (Schets et?al., 2008). Depending on the exact location of bathing, this section of the bathing population may experience increased exposure risks to surface water contaminated with pathogens from outlets of WWTPs and CSOs. Exposure to surface water contaminated with human pathogens, including viruses, bacteria or parasites, may lead to infection and subsequent illness, such as gastroenteritis or skin, ear and eye infections (Brunkard et?al., 2011,?Schets et?al., 2010).

If climate change increases pathogen contributions of CSO events and affects flow rates, an increase in risk of infection is expected during recreation in close proximity of a WWTP or CSO. The main aim of this study is to quantify this change in risk for the Netherlands.?Quantitative microbial risk assessment?(QMRA) is used to determine the risk of gastroenteritis when exposed to surface water contaminated with norovirus,?Campylobacter?and?Cryptosporidium?originating from wastewater under current and future scenarios. This selection of pathogens was based on human disease burden, data availability and pathogen characteristics.

In previous QMRA studies (Sterk et?al., 2015,?Sterk et?al., 2016), the assumption was that people swim randomly over the summer. However, as discussed in these papers, better estimates of probabilities of human exposure to surface waters could improve predictions of the infection risks. One improvement would be to weigh the chance that people will be swimming based on the conditions of a certain day. Intuitively, one could say that higher temperatures result in more water recreation. However, besides water temperature, factors like air temperatures, precipitation, sunshine and economic factors like amount of leisure time will largely determine whether or not people will be bathing. For example, data from Statistics Netherlands shows that recreation takes place more often during the weekends than on a weekday (Centraal Bureau voor de Statistiek 2009). In this study, change in risks of infection are not only based on changes in dose, but also on bathing behaviour.

Organic micropollutants discharged by combined sewer overflows – Characterisation of pollutant sources and stormwater-related processes

Introduction

The European Water Framework Directive (WFD) came into force in 2000 to achieve and maintain good chemical and ecological status of surface waters and groundwater. Decision 2455/2001/EC of November 2001 defined 33 priority substances, including metals, industrial chemicals, biocides and pesticides, which represent a significant risk for the aquatic environment. Directive 2008/105/EC adopted?environmental quality standards?(EQS) for these 33 substances, indicated as annual average (AA-EQS) and maximum allowable concentration (MAC-EQS). The European Commission has to re-examine and update this list at least every four years. In 2013, a new Directive (European Commission, 2013) amended the previous ones regarding priority substances. A new list of 45 substances was defined. Newly identified substances were added and EQS of some existing compounds were revised.

In order to apply measures for reducing the?pollutant emissions?into the aquatic environment, it is necessary to have a precise knowledge about the pollutant origin and behaviour in the urban catchment and in wastewater systems as well as about the main pathways to the environment. Several studies have shown that aside from?wastewater treatment plant?(WWTP) discharges, principal sources of?micropollutants?in urban surface waters are?stormwater runoff?and combined sewer overflows (CSOs) (Ellis, 2006,?Welker, 2007,?Phillips et?al., 2012,?Luo et?al., 2014). In particular CSO discharges contain pollutants originating from domestic?sewage,?industrial wastewater?and stormwater runoff. The need to minimise the environmental risk of CSO events represents a major challenge for public utilities because of the high number of CSOs in urban catchments. In fact in Germany, more than 71,700 CSO structures are currently listed (German Federal Statistical Office, 2015). Regarding environmental standards, inputs from CSOs with very high?pollutant concentrations?can affect the surface water quality (Phillips and Chalmers, 2009,?Weyrauch et?al., 2010,?Launay et?al., 2013). However, previous studies focused only on a few selected micropollutants or groups of pollutants, due to the high human and analytical efforts and the costly?pollutant analysis. To our best knowledge, only?Gasperi et?al. (2012)?dealt with a comparable number of substances as this study. However, the monitoring was focused on priority substances. In contrast to previous monitoring programs, this study assesses concentration levels of a vast variety of micropollutants including?pharmaceutical and personal care products?(PPCPs), X-ray contrast agents, biocides, herbicides, industrial chemicals,?flame retardants,?plasticisers?and polycyclic aromatic hydrocarbons (PAHs) in sewage, in CSO discharges as well as in the receiving water.

This monitoring study was undertaken in an urban catchment in southwest Stuttgart, Germany to characterise CSO emissions regarding organic pollutants. It aims at (1) assessing the occurrence of a wide range of organic micropollutants with different?physicochemical properties?and different origins in dry and wet weather flow, (2) evaluating the importance of the pollutant transport and in-sewer processes for micropollutant concentrations and fluxes discharged by CSOs, (3) improving the understanding of the impact of CSO discharges on water quality.