Annual estimates of microplastics in municipal sludge treatment plants in southern Spain

Abstract

Billions of microplastics (MPs) eliminated during wastewater treatment are prone to be found in sewage sludge, which would lead to dynamic redistribution of these particles in agricultural soils and consequently to the uptake and remobilization of various pollutants. As far as we know, there is limited information on MPs in sludge from Spanish wastewater treatment plants (WWTPs). In this study, their abundance during primary and secondary treatments in four WWTPs was investigated in two samplings, in a southern nationwide area. Combining shape, size, and chemical composition, we discussed the reasons for the existence of MP sizes (> 100 μm), as well as an estimate of the annual emission of each polymer from land application. MP concentrations in sludge samples ranged from 96 to 769 particles/g dry sludge, with an average of 392.4 ± 72.01 particles/g dry sludge. Microparticles in the shape of fibres are the most common type ranged between 100 and 355 μm. Fourier-transform infrared spectroscopy (FT-IR) showed that most MPs belonged to thermoplastics, ethylene derivatives, polyethylene terephthalate and polyamide. Based on the total sewage sludge production in Spain, the average amount of MPs derived from sewage sludge that could enter the soil was estimated to be 2.80 × 1014 MPs/year. The results confirmed that dry sludge is one of the potential sources of MPs contamination in the terrestrial environment.