The aim of the present study was to investigate whether sewage treatment plants (STPs) are important entry routes for small litter particles, so called microltter, to the marine environment. Whether or not this is the case depends of course on the abundance of litter particles reaching the STPs, which in turn is the result of all the activities that create the waste water. But the amount of litter particles that are actually being discharged from the STP into the recipient water also heavily depends on how the waste water is treated in the plant. In this study microlitter is defined as particles <5 mm which is the definition most frequently used. STPs are primarily designed to reduce the amount of organic matter and the nutrients nitrogen and phosphorous in waste water. The retention of particles like microlitter in the STPs is hence unintentional, but a positive side-effect of the waste water treatment. There are within the Nordic countries large variations both in the origin of waste water being treated in STPs and in the techniques applied. In the present report, which is based on collaboration between scientist in Sweden, Finland and Iceland, the implications of these differences on the release of microlitter to the environment via STP effluents will be investigated. Two STPs were selected from each country, one representing the highest standard in the country regarding techniques for waste water treatment, and the other a representative for an “ordinary” level. The level of technology of the plants were found to vary significantly. Sweden and Finland, whose geographical positions are in densely populated areas and in enclosed sea areas, have invested in advanced treatment techniques where mechanical, chemical and biological treatment is standard. Iceland with its low density of people and the open Atlantic sea surrounding the country has at present only mechanical treatment of waste water, even at the largest STP serving the residents of Reykjavik. This large variation in waste water treatment opened up the opportunity to also study how the use of different techniques affects the content of microlitter in effluent water.
This report describes the results of a two year project called “Microlitter in sewage treatment systems – A Nordic perspective on waste water treatment plants as pathways for microscopic anthropogenic particles to marine systems”.