The presence of an ever increasing amount of plastic in the Italian river system makes it necessary to understand the contribution of their different sources. We focus on the contribution from the wastewater treatment plants to the microplastics (MPs), size less than 5 mm, conveyed to the fluvial system, and on the development of methods for their detection in this matrix. This study, one of the first in Italy, is aimed to investigate the content of MPs present in the effluent of the main wastewater treatment plant in Florence (Italy). We sampled wastewater during dry season to mainly quantify the contribution from civil and municipal activities to the MPs release. The samples were continuously collected over a period of 24 h at the exit of the water line using a series of 8 sieves with different mesh sizes (almost 1000 L filtered volume). The sampled material was analyzed by optical microscopy and micro-Raman spectroscopy by use of low-cost, portable instruments. The spatial resolution of the spectrometer matches the minimum dimension of the mesh size in use (38 μm). The analysis detected an average concentration of 5 MPs per liter in the 38-1000 μm diameter range, corresponding to a daily release of about 35 kg/day into the River Arno, a result in line with other studies carried out on Europe's major rivers. We provide a classification of the polymer composition showing the predominant presence of Polypropylene (29%), Polyethylene (18%) and Polyester (14%). The MP shape classification reveals the relevance of fibers in effluents. The number of sieves used provided an accurate size distribution curve of the sampled MPs allowing to estimate, by extrapolation, a relevant quantity of MPs finer than 38 μm whose determination would otherwise require much more sophisticated methods.
Keywords: Continuous sampling method; Effluents concentration; Microplastics; Raman spectroscopy; Wastewater treatment plants.
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