Microlitter in sewage treatment systems

The occurrence of microscopic litter particles in the sea is a problem that has received considerable attention over the past decade. There are numerous possible sources to these microparticles and also numerous ways by which they may reach the marine environment. In order to take efficient measures to reduce the concentrations important sources and entrance routes have to be identified. Effluent water from sewage treatment plants (STPs) is one entrance route for microlitter to the sea and other aquatic environments. The purpose of the present study has been to quantify the amount of litter particles being discharged into the sea this way and also to investigate whether elevated microlitter concentrations could be detected in water, sediment and biota in the STP recipient areas.

The study was limited to particles ≥300 μm in water and particles ≥100 μm in biota and sediment.The microlitter content was analysed in influent and effluent water at two STPs in each of Sweden, Finland and Iceland. Analyses of microlitter concentrations in water, sediment and biota were done in the recipient to one of the STPs in each country. Two major groups of microlitter were registered; microplastics and anthropogenic non-synthetic fibers (e.g. cotton).The study showed that in the Swedish and Finnish STPs more than 99.7% of the microlitter particles ≥300 μmThe plume of waste water coming from the discharge tube was fairly easy to detect in the Swedish and Finnish STP recipients. In both these recipients the microlitter concentrations in the plumes were found to be significantly higher than in water at the reference sites.

The Swedish STP recipient water was localized in a river mouth in a heavily urbanized and industrialized area but the microlitter concentration in the waste water plume was still distinctly elevated compared to the water unaffected by the waste water.Microlitter concentrations in the recipient to the Swedish STP were found to be considerably lower when sampling during a period with no precipitation compared to sampling during a heavy rainfall, 1.9 microplastics and 1.5 non-synthetic fibres per m3 compared to 10.5 microplastics per m3 and non-synthetic fibers too numerous to be counted. In the Finnish recipient the concentrations were on average 12.7 microplastics and 11.3 non-syntheic fibres per m3 on the first sampling occasion and 0.7 microplastic and 6.7 non-synthetic fibres per m3 on the second. The difference could not be explained by any obvious climatolocial factors since both samplings were carried out during periods of dry weather. The microplastic concentrations in the Icelandic STP recipient water were slightly elevated compared to the reference site, and the difference was larger for the non-synthetic fibres than for the microplastics.

Concentrations of microplastics varied between 2 and 5 litter particles per m3 at both sites. The small difference between the STP recipient and the reference site was probably at least partly due to the fact that the discharge point was localized in an area with an open coastline and a very good water circulation.Microplastics and anthropogenic non-synthetic fibres were detected in biota and sediment from the waste water recipient areas in all countries but it was not possible to trace them with any certainty to the waste water effluents.The study shows that STP effluents are entrance routes for microplastics and other microlitter particles to the aquatic environment. If the plants are equipped with chemical and biological treatment most of the litter particles in influent waste water will be retained in the sewage sludge. This reduces the impact on the recipient water, but if the sludge for example is to be used as an agricultural fertilizer the microlitter will still be spread to the environment. Efforts to reduce the microlitter concentrations in waste water should therefore preferably be done in households and other locations where the waste water is originally being formed.

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