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.



Results on microlitter in STP recipient water

The outlet for effluent water from Ryaverket is located in the river Göta älv where it runs through Gothenburg, the second largest city in Sweden. In an attempt to separate microlitter particles from STP effluents from litter particles that had reached the river by other pathways e.g. storm water or windblown litter, trawling was done not only in the waste water plume but also at other locations away from the plume but still in the recipient area (=reference site 1 in Figure 4). A location in the Gullmar Fjord, 100 km north of Gothenburg was selected as a “clean” reference site (reference site 2). Sampling in the recipient area (in figure 4 marked as “Recipient, STP plume” and “Reference 1, outside plume”) was done at two occasions whereas sampling at reference site 2 was done only once. The first sampling occasion in the STP recipient took place during a period of dry weather and the second on a day with heavy rainfall. Sampling at the reference site took place during rainfall and should hence be compared to STP recipient data from the second sampling occasion.


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