How do sewage treatment plants remove microplastics, or pieces of plastic generally smaller than 1 millimeter? That is the main question that Deltares, the VU University Institute for Environmental Studies and Delft University of Technology are researching.

There is strong evidence that most of the plastic litter in our seas comes from land. Sewage treatment plants are thought to be one of the main sources of microplastics entering the surface water. The study is expected to be completed in early 2017.

There is increasing interest in microplastics in Dutch sewage treatment plants. Like the removal of endocrine-disruptive substances and traces of medicines in wastewater, they are confronting water managers with new challenges. Treatment plants are not designed to remove such small particles.

Larger pieces are sieved out of the waste water during the first treatment stage. However, due to the low density of microplastics, sedimentation would not appear to be an effective approach to removing them. The next step in the treatment process involves the biological removal of substances in solution, such as nitrogen and phosphate. The bacteria used in this process will not remove the microplastics.

How much microplastic and where does it go?

Measurements have been conducted recently at sewage treatment plants and in the IVM-VU laboratory looking at microplastics in sewage, sewage sludge, and the treated waste water that is discharged into the surface water. The results failed to provide an adequate picture of the dynamics of microplastics that enter sewage treatment plants or of the possible ways of removing them.

This new and more extensive study will help water managers to answer questions including how much microplastic is in the treated sewage and how much is left behind in the sewage sludge.

The study is also expected to establish an insight of the changes or new technologies required to remove plastics. The researchers will be looking at the inflow and removal of the microplastics in different conditions and at different times of day.

International potential

It should be kept in mind that microplastics captured in sludge in Dutch treatment plants are ultimately incinerated. That is often not the case in other European countries or in North America, where sewage sludge is used to produce biosolids that are spread on the land.

The microplastics they contain could have a negative impact on the ecological quality of water and land. The answers produced by this study may therefore also provide other countries with an understanding of the role and relevance of sewage treatment plants in terms of eliminating microplastics from the environment.

Microplastics harm people and the environment

Microplastics are a threat to environmental quality and human health. That explains why they are in the spotlight. Plastic pollutants come from sources such as litter, and the result is that 250,000 tons of plastic are already floating around our oceans and seas.

The weather, sunlight and the waves break the plastic down into minute particles. When marine animals ingest the particles, microplastics enter the food chain. The particles examined during this study will mainly come from the wear and tear of synthetic textiles during washing, from tires wearing, and from toothpaste and cosmetics.