Assessment and accumulation of microplastics in sewage sludge at wastewater treatment plants located in Cádiz, Spain

Sludge from Wastewater treatment plants (WWTPs) have been determined as a sink of microplastics (MP) removed from wastewater. The aim of this research work has been to evaluate the presence of these pollutants in the sludge of seven WWTPs (five urban and two industrial), located in southern Spain. Samples were collected in the primary, secondary and digested sludge matrixes, MPs were extracted following wet peroxide oxidation and the removal of cellulose, finally the samples were analyzed according to their abundance, size (from 100 μm to 5 mm), shape, colour, and polymer type. Subsequently, the data obtained on the WWTPs were compared, the main difference among the WWTPs and different sample points showed high heterogeneity in terms of abundance of microplastics, due to the differences in the sludge loaded, the processes and the type of sludge. The results from this study established that the most abundant shape was fibers; regarding the size, 100-355 μm fraction was the most abundant, showing that the amount of MPs increased when the size decreased. Regarding the type of polymers, 23 were identified by ATR-FTIR. Further, Acrylate, PE, EAA and PP were the most abundant found polymers. The presence of MPs in the digested sludge varied from 0.02 ± 0.006 MP g DW to 57.18 ± 20.69 MP g DW in the WWTP 6 (food industry) and WWTP 3 (urban city over 212,000 inhabitants), respectively; higher abundance of MPs found in the primary sludge in respect to secondary sludge; in concordance with the removal from wastewater line reported in other studies. The results obtained showed that MPs were widely present in sludge, becoming a sink of these pollutants, estimating that among 8.05 · 10 and 1.77 · 10 MPs · day were loaded to sludge; therefore, these facilities act as a significant source of MPs into agriculture when sludge is used as soil amendment.