Distribution of microplastics present in a stream that receives discharge from wastewater treatment plants

The presence of microplastics (MPs) in freshwater systems that receive discharge of urban effluent implies a great environmental impact. In order to be able to generate proposals that solve this problem, it is necessary to know in detail the contributions of different MPs sources. The aim of this work was to study the contribution of urban sewage discharge to MPs pollution in a stream that runs through a medium-sized city. The spatial distribution of MPs with sizes between 100 mu m and 1.5 mm present in surface water was measured and their characteristics, dimensions, shapes and identification were determined. Physical-chemical parameters of the stream water were measured, and a decrease in water quality was found due to wastewater treatment plants. The main source of MPs was effluent from the plants (97% of the total MPs), while the rest came from storm drains and discharge of tributaries. The maximum concentration of MPs found was around 72,000 MP/L (equivalent to 53 million MPs/ s), at a point after discharge from both plants. Around 70% of MPs correspond to microfibers with a mean length of around 300 mu m and a mean width of around 15 mu m, and they are mainly polyethylene fibers. The remaining 30% of MPs are particles with lengths of around 140 mu m. The transport of MPs between a point located after discharge of the plants and another point located about 3 km further on was studied, and no significant variation was found in the concentration of MPs. Electrical conductivity was used as a conservative tracer of MPs con-centration. This work presents for the first time a detailed analysis of different contributions of MPs to a freshwater system in South America, which receives discharge of wastewater treatment plants, evidencing its important role in pollution.

Automated summary

This study provides a detailed analysis of the different contributions of microplastics (MPs) to a freshwater system in South America, with urban sewage discharge identified as the main source of pollution. The study found that 97% of the total MPs in the system came from effluent from wastewater treatment plants. The study also revealed that microfibers, primarily polyethylene fibers, accounted for a significant portion of the MPs.