Separation and identification of microplastics from primary and secondary effluents and activated sludge from wastewater treatment plants

Although wastewater treatment plants can retain a high percentage of microplastics (MP) arriving at the facilities, no method for extracting and characterizing these microparticles has been still standardized in these units. This study investigated three protocols of chemical digestion, prior to analysis of microplastics, one directed to the effluents, using peroxidation, and two for activated sludge (peroxidation and Fenton). The samples (primary effluent, secondary effluent and activated sludge) were collected from a wastewater treatment plant (WWTP) located in Valencia (Spain). In addition, four common types of polymers (Low density polyethylene-LDPE, Polypropylene-PP, Polystyrene-PS and Polyethylene terephthalate-PET) were used to assess the influence of reagent exposure on microparticle integrity. Peroxidation was effective in treating the studied effluents (primary and secondary) and was also identified as the ideal protocol for activated sludge. The analysis showed that the use of H2O2 does not compromise the identification of the polymers evaluated by FTIR and also significantly reduced the concentration of suspended solids, resulting in an efficient visual separation of the microparticles. After been properly separated, the microparticles were characterized according to their size, colour and shape, and a fraction submitted to identification by µ-ATR-FTIR/ATR-FTIR. In all samples, a high presence of microfibers (MF) was observed, corresponding to more than 90% of the microparticles. However, in relation to secondary effluents, only 9% of these MF were identified as plastics, the remaining ones corresponded to cotton. The fragments found in the samples were classified as secondary in origin, and were mainly PE and PP, lower than 1 mm size.