"Every year, about 5 million vehicles pass the Sundsvall Bridge. For us, it is important that the polluted runoff from the bridge does not reach Sundsvallsfjärden untreated," Anna Maria Kullberg says, Stormwater Coordinator, MittSverige Vatten & Avfall (MSVA).
Stormwater is a diffuse source of microplastics, since it often contains microplastic particles from car tires and wear and tear on roads as it runs off the roadway. Microplastics, which are small pieces of plastic of a few micrometers or millimeters, are gaining more and more interest, as it takes a long time for them to break down in the natural environment. Increasing resources are therefore being invested in reducing the spread of microplastics, but it is important to choose the right method.
At the foot of the high-traffic Sundsvallsbron, a unique stormwater treatment facility has been built by the Swedish Transport Administration in collaboration with MSVA, and with input on the design from Luleå University of Technology. The treatment effectiveness of the facility has been investigated through a unique research study. The facility consists of several parts - partly a pre-sedimentation chamber in combination with oil separators, partly so-called stormwater biofilters that have been built with different types of design in order to enable research on which type of biofilter works best. Biofilter is a plant-covered infiltration surface where the stormwater infiltrates and is treated by the plants and the soil. One of the investigated biofilters in Sundsvall was built without vegetation to be able to evaluate the importance of the plants in the treatment process. Biofilters are a relatively new treatment technology, but more and more such facilities have been built around Sweden in recent years.
"As society's investments in stormwater treatment plants increase, it is of course important that investments are made in facilities that really work, which has not always been the case," Godecke Blecken says, assistant professor in Urban Water Engineering, at Luleå University of Technology.
Although it is considered important to remove microplastics from stormwater, there are very few studies on this. To date, only two studies on microplastic removal in stormwater biofilters have been carried out in the world with regard to treatment of stormwater from low-traffic roads and parking lots, respectively. The study at Sundsvallsbron, which was carried out within the research projects DRIZZLE and GreenNano by researchers at Luleå University of Technology in collaboration with IVL and MittSverige Vatten & Avfall (MSVA), is world-unique, as it examines runoff from a high-traffic motorway. It is also unique in that the ability of a pre-sedimentation chamber to clean microplastics has been investigated.
"As expected, the results show that the pre-sedimentation chamber had a limited ability to remove microplastics. However, the two investigated types of biofilters had a good ability to remove microplastics," Katharina Lange says, PhD candidate in Urban Water Engineering at Luleå University of Technology, who conducted the study under the supervision of Professor Maria Viklander and Assistant Professor Godecke Blecken at Luleå University of Technology.
The vegetated biofilter showed better removal efficiency compared to the non-vegetated filter.. This is good news according to the researchers in the study, as vegetated biofilters are generally considered to be more aesthetically pleasing. They also require less maintenance than biofilters without plants, as they suffer from a lesser degree of clogging.
Results from the study will partly be presented in a Swedish Water Development (SVU) report and in the journal Water Research, with Katharina Lange as lead author and co-authors Maria Viklander, Godecke Blecken, all of Luleå University of Technology, and Kerstin Magnusson, IVL.