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Microlitter in sewage treatment systems

A Nordic perspective on waste water treatment plants as pathways for microscopic anthropogenic particles to marine systems

image of Microlitter in sewage treatment systems

The report presents results from a study on the role municipal sewage treatment plants (STPs) have as entrance routes for microplastics and other microlitter particles to the marine environment. Microlitter concentrations were analysed in waste water before and after treatment in the STPs, and in the recipient waters where the treated waste water is discharged.

Municipal waste water was found to contain a substantial amount of microlitter, but in STPs equipped with chemical and biological treatment most of the litter particles were retained in the sewage sludge. This reduces the impact on the recipient water, but if the sludge is used as fertilizer on farm land the microlitter will still reach the environment. Efforts to reduce the microlitter concentrations should therefore preferably be done in households and other locations where the waste water is originally being formed.

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Results on microlitter in STP waste water

The concentration of microplastics and non-synthetic fibres (number of particles per m) in influent and effluent water is presented in Table 4. The concentrations were high in the influent water to the Finnish STPs, compared to incoming water to the Swedish and Icelandic plants. This might be explained by differences in the origin of the water, which in a varying degree could be e.g. residential settlements and services, industries or storm water. However, it should also be noted that all samples were snapshots of the situation and not integrated over a longer period of time, so the human activities representing the waste water at that specific time determines the sources of waste water coming into the plants.

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