Screening of microplastic particles in and down-stream a wastewater treatment plant

This project aims at evaluating the role of waste water treatment plants (WWTPs) as entrance routes for microplastic particles to the marine environment, and is completed within the scope of the Swedish EPA screening programme 2014. Sampling was done at Långeviksverket in Lysekil, a relatively small WWTP with a load of 14 000 population equivalents (pe), and with effluent water being discharged into the sea. Analyses were performed on microplastic particles collected on a filter with a 300 μm mesh size, both in the actual WWTP (incoming and effluent water, and sewage sludge) and in the recipient water. Background levels of microplastics were obtained by analysing seawater from a location not directly affected by the effluents from Långeviksverket. All the sampling was done during the course of one day. It is not obvious to give an exact definition of the size of the collected particles, e.g. plastic fibres with a diameter smaller than 300 μm might either be caught on the filter or slip through it. Still, in this report the collected material is referred to as microplastics ≥300 μm, even though it would have been more accurate to define it as the microplastics collected on a 300 μm filter. WWTP incoming water was found to have a mean concentration of 15 000 microplastic particles ≥300 μm per m 3, which resulted in an inflow of 3 200 000 microplastic particles per hour. More than 99 % of the particles were retained in the WWTP sludge and the concentration in effluent water was 1 770 microplastic particles per hour. The retention rate was affected by the shape of the particles, and plastic fibres were retained to a higher degree than particles of other shapes. The microplastic concentration in the recipient of the effluent tube was elevated compared to an area presumed to not be directly affected by the effluent; 1.1 - 1.8 plastic particles m -3 were found in the effluent plume compared to 0.45 m-3 in the reference area. Higher particle concentrations were found close to the mouth of the tube compared to 200 m ...