Seite - 75 - in Freshwater Microplastics - Emerging Environmental Contaminants?

maximum of 473 fibers L 1. In comparison, another study found similar values about 180 textile fibersL 1 [19]. Unlikefibers, fragmentswereonly rarelycollected,probablybecauseof the low volumes of the samples (100–400 mL). In the 20 samples studied, 16 did not contain any fragments. For the four remaining samples, three contained one frag- ment, while the other contained two fragments. These small values indicate a potential upper limit for the concentration of these particles. No fragments in a 400 mL sample, for instance, imply a concentration<2.5 particles L 1. Larger valueswere observed in other studies: 430 particles L 1 [19] or 13 particles L 1 [20]. If this variation in fragment concentrations is site dependent or is related to differentmethodologies, it remainsunknownat themoment.Fragmentscould, inan oversized sewage network, settle inside the sewer during dryweather periods and reach theWWTP only at low concentrations. It is known that Paris wastewater sewerage system is oversized leading to a very lowflowvelocity and therefore a high sedimentationof particles inside the sewer. TwoWWTPswere selected inorder to estimate the treatment efficiency. Seine CentreWWTPwasconsidered for thecaseofa treatmentbybiofilters,whileSeine Aval WWTP is a conventional activated sludge plant. For Seine Centre, three sampling campaigns on three consecutive working days were carried out. Raw wastewater after pretreatment, settledwastewater (after the primary clarifier), and treated water (after the biofilters treatment) were all considered. Only limited volumeswere consideredhere.As the disposal channel of SeineAval is large and longenoughtobenavigable,amantatrawlwitha330μmmeshsizewastowedbya motor boat in theupstreamdirection inorder to sample highvolumes (68m3) and count fragments. In thecaseofSeineCentre, onlyfiberswereencountered.The results suggested a removal from83 to95%of thefiber contamination.Otherworks showedhigher removal efficiencies ofMPsbyWWTPs, i.e., between95and99%[20,21].How- ever, all these results are hardly comparable since methods used vary from one study to another. Fragmentswere detected at the outlet of theWWTPSeineAval because larger volumeswere integrated. Irregular fragmentswereobservedprimarily, and foams, films,andsphereswerefoundpunctually.Thenumberof fragmentsobserved in the treatedwater varies between 6 10 5 and 3 10 4 fragmentsL 1. The concen- trations of fragments are much lower than the levels of fibers observed in the automatic sampler samples (105more fibers). Being able to observe fragments in themanta trawl samples is also an additional indication that fragments are present inWWTPeffluents but just too scarce to beobserved in small sample volumes. A combined sewer system collects rainwater runoff and domestic sewage. However, during heavy rainfall events, the volume of water may exceed the capacity of the sewer and can cause flood events. In this situation, untreated water is dischargeddirectly into thewater bodies. In this context, 500mLsamples were collected atLaBricheCSOoutlet during three events. Both fragments and fibers were encountered in the CSO samples. For fibers, levelsof190,898and1,046fibersL 1weredetected.Thepresenceoffibers ismost Sources andFate ofMicroplastics inUrbanAreas:AFocusonParisMegacity 75