Microplastic accumulation in sewer sediments and its potential entering the environment via combined sewer over�ows: a study case in Paris

During wet weather events, combined sewer over�ows (CSOs) transfer large amount of particulate matter and associated pollutants into surrounding water bodies, thereby deteriorating the recipients’ ecological health. Resuspension of sewer sediments during these events contribute signi�cantly to pollution level of these discharges. However, how much this in-sewer process contributes to CSOs’ quality regarding microplastic (MP) pollution is little known. Therefore, an investigation on sewer deposits inside the Parisian combined sewer network was carried out. The study found high MP concentrations stored in this matrix, ranging from 5×10 3 to 178×10 3 particle/kg dry weight. Polymer composition is similar to what found in raw wastewater, containing a high proportion of polyethylene and polypropylene. Thus, the results indicated the retainment of MPs in sewer network during transport during dry weather periods to treatment facilities. Once resuspension of sewer deposits happens, MPs can be released into water �ow and get discharged along with CSOs. This highlights another potential pathway of MPs into freshwater environment.


Introduction
Sewer sediments are bed deposits found inside sewer networks, formed by particulate matter which detaches from water phase and settles down during transport of sewage.This phenomenon often occurs during low-activity intervals and dry-weather periods when ow rate and turbulence levels are moderate.Sewer sediments also mount wherever water ow decelerates, for instance, abrupt changes in shape or dimension of pipes, divergent or low slope sectors, etc.Thus, the accumulation of sewer sediments are temporal and spatial dependent, directly linked to water velocity inside sewer network (Seco 2014).
Sewer sediment is moist, heterogeneous and complex in composition.However, they can be subdivided into gross bed sediment (GBS), organic layer and bio lm based on the nature of materials (Rocher et al. 2004).While GSB has a high mineral content, the other types contain mainly organic matter.The main components in inorganic fraction of sewer deposits are particles swept from different surfaces of impervious areas by runoff water, surrounded soil coming along with groundwater in ltration and rusted metal of the sewer, etc. Whilst, organic fraction mostly stems from sanitary wastewater which contains high load of suspended solids (Ashley & Crabtree 1992; Ashley et al. 2004;Arthur et al. 2015).During wet weather events, sewage inside combined sewer systems accelerates, accompanied by an increasing turbulence level, which leads to the transport of sewer sediments in different modes (Butler et al. 2003).Some tend to roll, slide or leap as bed-load inside sewer network (e.g., grit and gross solids), some get resuspended and carried in suspension (e.g., organic solids, silt and ne sand).Thus, sewer sediments' materials (including pollutants) can be released and added into wet weather ows (WWFs) during these occurrences, known as erosion/resuspension of sewer sediments.
During intensive events, WWFs can be discharged into the environments without any treatment.These discharges are coined as combined sewer over ow (CSO) in case of combined sewer systems.Previous studies highlighted CSO pollution and its adverse impacts on the health of the receiving water bodies (Hvitved-Jacobsen 1982; Phillips et al. 2012;Rechenburg et al. 2006), as well as indicated remarkable contribution of sewer sediments to quality of CSOs (Chebbo et al., 1995;Gromaire et al., 2001;Ashley et al., 2004;Gasperi et al., 2010).The accumulation of these in-sewer deposits also causes more frequent CSOs due to reduction in conveyance e ciency of the sewer network (Crabtree 1989;Seco 2014;Veliskova & Sokac 2019).Therefore, sewer sediments can act as a sink-source of pollutants that contribute to rising pollution level in wet weather ows and receiving water bodies.
Microplastics (MPs) are found abundant in wastewater as a consequence of using and overusing plastics in modern daily life.Whilst the evolution of MPs in wastewater treatment plants (WWTPs) have been well studied in the last decade, it was not the case for their occurrence and fate before reaching these treatment facilities.During transport inside sewer network, MPs may integrate with mineral and organic particles in wastewater, settle down and get trapped in sewer sediments.Once these sediments erode, MPs may get released into water ow along with other pollutants.However, very little is known about the contribution of sewer sediments to the quality of CSOs regarding microplastic pollution.This study will investigate MP content in sewer sediments for the rst time.The obtained results will be used to improve the assessment of CSO contribution into plastic pollution.These two objectives aim to increase the knowledge on MPs occurrence and fate in the sewer network -WWTP continuum and their pathways in urban waters before entering the environment.

Study site
Paris Centre -a part of the Parisian metropolitan area -is chosen for this case study.This catchment has a high population density (20 000 inhabitants per km 2 ) with intense commercial and service activities.Impervious surface constitutes about 70% of the total area.The sewer systems serving Paris Center is fully combined; in which, Seine river is the recipient of treated wastewater from WWTPs or CSOs in wet weather conditions.The Parisian sewer network has experienced the formation and accumulation of sewer deposits.Therefore, sand chambers, which behave as sediment traps, were installed to reduce adverse impacts of these deposits on hydraulic conveyance e ciency.About 100 sand chambers were located over the network (Rocher et al. 2004).Most of them have the same width as the sewer, but they are deeper.This induces abrupt decrease in the owrate of wastewater when passing through, thereby allowing particles to detach from water phase and settle down.Sand chambers are regularly cleaned up as a part of the system maintenance.Extracted sediments are then sent to specialized centers for proper treatment.
Twelve sediment samples were collected from sand chambers inside the Parisian sewer network during regular cleaning-up activities between March and August 2021.Samples from primary sewer pipes were named P1, P2 and P3, with letters 'a' and 'b' indicating samples from the same sewer pipe.Samples from secondary sewer pipes were listed as S1 to S7.The map of sampling points is shown in Fig. 1, and the exact addresses with the coordinates of each site are provided in Table S1.After collection, samples were stored in glass jars at 5°C until further analysis.Sediments in sand chambers showed similar characteristics to sewer deposits found along the network (Rocher et al. 2004).

Sample preparation & analysis
Samples were rst freeze-dried to remove water content and then sieved through 5 mm metallic mesh for removal of large items.A subsample of 0.5g dry-weight (dw) sediment underwent chemical oxidation with H 2 O 2 30% at 45°C for 24 hours.Sample was then ltered through 10µm metallic lters to remove degraded organic matter.Remaining material was subjected to density separation using sodium iodide solution (> 1.6 g/cm 3 ) for 24 hours before going through another oxidation step with H 2 O 2 .The pretreatment protocol for MP isolation is summarized in Fig. 2. Three replicates were carried out for Site 1.After treatment, samples were deposited on Anodisc lters (Ø25mm; pore size of 0.2µm) before being analyzed with the automated µ-FTIR mapping in transmission mode (Nicolet iN10 MX, Thermo Scienti c, 25×25µm pixel resolution).Data processing was carried out later with the siMPle software (version 1.1.β,developed by Aalborg University, Denmark and Alfred Wegener Institute, Germany) to identify the polymer composition of the particles.Besides, particulate organic carbon (POC) was measured in all samples.

Microplastic content in sewer sediments
MPs were encountered in all samples analyzed in this study.The highest concentration up to 178×10 3 particle/kg dw was found in S3.It is 30 times higher than the lowest concentration detected in S4 with 5×10 3 particle/kg dw.The median concentration of all samples was 44×10 3 particle/kg dw.It can be observed that MP contamination level in sewer sediments varied among different sampling sites, regardless of their localization on primary or secondary pipes.The variability between samples from the same pipe (i.e., P1a-P1b and P2a-P2b in Fig. 3) increased with the distance between sampling points.The characteristics of wastewater can be considered homogenous in a large-scale sewer system like Paris, thus, MP content in insewer deposits may mostly be related to the accumulation of these sediments inside the system.The accumulation depends generally on different factors such as water owrate, sewer structure and cleaning frequency.MP content found in sewer sediments and sewage sludge are in the same order of magnitude (Table 1).This indicates that a considerable amount of MPs in wastewater remains within the sewer network instead of reaching WWTPs.In other words, the ndings in this study highlight a major stock of MPs in sewer sediments and the associated risk of downstream transfer during wet weather events due to the resuspension of these sediments.Polymer composition divers from sample to sample (Fig. 4).While some samples (e.g., P2a, P3, S3) contained 8 to 9 different polymers, S2 was composed of PE exclusively.Researches on sewage sludge also found polyole n abundant in their samples, which is consistent with the results in this study (Vollertsen &  The variability between different samples was high, both in terms of MP number concentration and polymer composition. Additionally, MP content showed no relationship with POC (R 2 < 0.01, Fig S1).This re ects the heterogeneity of sewer sediments as an environmental matrix.High variability was also frequently observed for different substances in urban water pollution studies, for example, PAHs and metals in stormwater and combined sewer over ows (Gasperi et al. 2011;Zgheib et al. 2011).
For more accurate insights into MP occurrence and its potential emission during wet weather, future studies are recommended to have a larger sample size across the entire sewer network.This will help identify hotspots and temporal variations of MP pollution, thereby enabling the formulation of mitigation strategies.
Particle size distribution of MPs found in this study is illustrated in Fig. 5.More than 20% of total detected particles were smaller than 100µm and most of them are smaller than 300µm.These ndings are in agreement with previous studies (

Table 1
Microplastic contamination in different types of sewage sludge reported in literature.Values in the table are presented as min-max or average with standard deviations depending on the available data.The unit is particle per gram of dry-weight sample matrices.
Twelve different polymer types were found in sewer deposit samples.Polyethylene (PE) is the most common, present in all samples, ranging from 11% in P1b to 100% in S2.Polypropylene (PP) and polystyrene (PS) are also common, found in 10 and 9 out of total samples, respectively.Cellulose acetate was found abundant in S3 up to 21%, whist being absent in most samples.The similar phenomenon was observed for Acrylic, Polyvinyl acetate (PVAC), Polyvinyl chloride (PVC).Other polymers including Acrylonitrile butadiene styrene (ABS), Polyamide (PA), Polyesters (PEST), Polyurethane (PU) and Vinyl copolymer (VC) were present with much less frequency, only accounting for 7% of all detected plastic particles (N = 424).
Hansen 2017; Li et al. 2018; Liu et al. 2019; El Hayany et al. 2020).However, they also found a large amount of PA or nylon in sludge which might stem from laundry activity of textile products.PA was present in sewer sediments in this study, however, only in 4 out of 12 samples, accounting for less than 2% of total detected particles.Apart from PP, PE and PA that are considered as low-density polymers, high-density types such as PEST, physicochemical properties of these particles(Kelly etal.2021; Martínez-Campos et al. 2021; He et al. 2022).The exposure to different forces during conveyance with wastewater inside sewer systems can also in uence the sinking behavior and subsequent transport modes of plastic particles (Aghilinasrollahabadi et al. 2021; Jiang et al. 2023).
(Li et al. 2018;Liu et al. 20198etected in sewer sediments, which is different from polymer composition of sewage sludge.This can be attributed to lack of harmonization in methodology.For instance, some studies only perform chemical characterization for a small part of suspected particles due to limit of contemporary technological advance(Gies et al. 2018;Li et al. 2018).Some applied NaCl (1.2g/cm 3 ) in density separation which did not allow to recover all polymer types(Li et al. 2018;Liu et al. 2019).Besides, the diversity of polymers found in this study suggests that different and complex mechanisms might contribute to the accumulation of MPs in sewer deposits rather than gravity sedimentation only.For example, the presence of organic particulate matter in wastewater might facilitate the development of bio lms on the surface of plastic debris, thereby altering the (Chebbo et al. 1995Hayany et al. 2020).The smaller particles are, the more likely they attach on the surface of suspended solids or organic particulate matter in wastewater during transport and then accumulate in sewer sediments(Liu et al. 2019).3.Estimated contribution of sewer sediments to MP contamination in CSOsPrevious studies have indicated the signi cant contribution of suspended particulate matter from in-sewer remobilization to high contamination levels in wet weather ows.Given the abundance of MPs in sewer sediments, water ow during these intensive occurrences is expected to exhibit high concentrations of MPs(Chebbo et al. 1995; Ashley et al. 2004; Gasperi et al. 2010).Eq. 1 was applied to estimate the amount of MPs released from sewer sediments and discharged along with CSOs into surrounding water bodies.Ka et al. (2008) reported that suspended solids in water samples collected at Clichy, one of two major CSO outfalls in Paris, ranged from 203 to 343 mg/L.Whilst, it is estimated that 20-80% of suspended solids in CSOs originate from sewer sediments (Gromaire et al. 2001; Passerat et al. 2011; Hannouche et al. 2014).The estimation showed that the resuspension of sewer sediments can result in a minimum concentration of 203 particle/m 3 up to 48×10 3 particle/m 3 in CSOs.Stormwater, a main source of pollutants in CSOs, also contained an equivalent amount of MPs, 3×10 3 to 12.9×10 4 particle/m 3 according to Treilles et al. (2021).These values are in the same range as MP level reported in e uents of WWTPs worldwide, from 250 to 22.5×10 4 particle/m 3 (Leslie et al. 2017; Ziajahromi et al. 2017; Gies et al. 2018; Gündoğdu et al. 2018; Akarsu et al. 2020).Thus, CSO discharges can act as a potential pathway transporting MPs from urban area into the environment besides treated wastewater.
ConclusionPart of MPs in urban waters entering sewer systems is not transported to treatment facilities, but temporarily stored in sewer sediments.These sediments once resuspended might release trapped MPs into water ow.Due to intensive rain events, particulate matter and associated pollutants stemmed from the erosion of sewer sediment can be carried by CSOs into