Microplásticos en lodos de edarevaluación de tecnologías para su eliminación y análisis de efectos tóxicos en el medio ambiente

Abstract

Consumer behaviour has undergone significant transformations throughout the last 70 years as a result of the massive use of plastics, whose consumption has exponentially increased to the present day. This transformation has improved the quality of life, facilitating the development of multiple daily activities. However, these materials are characterized by their short shelf life and scarce degradability, leading to the generation of large amounts of residues whose management has not undertaken sustainably, leading to the accumulation of these waste products in the environment. The problem of plastic pollution has gained increasing attention and concern not only in the scientific community but also in society, mainly due to the degradation and fragmentation of macroplastics into plastic particles of a size smaller than 5 mm (Microplastics). Wastewater treatment plants (WWTPs) have been identified as one of the main routes of entry of microplastics into the environment. The wastewater reaching the treatment plants has a high concentration of these pollutants due to the high use of plastics in daily activities. The current Doctoral Thesis addresses the impact of microplastics on wastewater treatment processes, assessing their performance and the final accumulation of microplastics in sludge. Additionally, a study on the possible toxic effects that microplastics may pose on the biota existing in agricultural soils was conducted. Firstly, to determine the behaviour of microplastics in WWTPs, a survey was conducted in different facilities receiving wastewater from heterogeneous sources located in the province of Cádiz. This study revealed the elevated contribution of microplastics to the receiving environment (the concentration in the effluent of the WWTPs fluctuated between 4.16 ± 1.17 MPs/L and 131.35 ± 95.36 MPs/L) despite the 90% removal rate of these micropollutants reached in the water line. Since there is no specific treatment to eliminate microplastics in these facilities, these micropollutants were expected to accumulate in the sludge. To confirm this hypothesis, a screening was conducted at WWTPs in the provinces of Cadiz and Seville to assess the presence of microplastics in the sewage sludge throughout the treatment line. The results obtained in this study confirmed the above hypothesis and showed that sewage sludge acts as a sink for microplastics previously removed from the water line (the concentration of microplastics identified in the digested sludge ranged from 0.02 ± 0.01 MPs/g dry weight to 57.18 ± 20.69 MPs/g dry weight). Anaerobic digestion is one of the most commonly applied technologies in the sewage sludge line of a WWTP. Based on the confirmed accumulation of microplastics in the sludge, the influence of microplastics on anaerobic digestion processes was investigated to determine the possible impact of these micropollutants on the operational parameters and biomethane production. For that purpose, biochemical methane potential (BMP) tests at the laboratory scale were conducted with defined concentrations of microplastics in mesophilic and thermophilic temperature regimes. The results indicated no significant differences in the mesophilic conditions test, whereas, in the thermophilic regime experiment biomethane production was inhibited as the concentration of microplastics increased. Finally, considering the application of biosolids as a soil amendment as one of the principal destinations of sewage sludge, it was agreed to execute an acute toxicity test to study the possible physiological and biochemical effects of microplastics at environmental concentrations previously established in the WWTP sludge from the province of Cadiz. The organism selected was Eisena Foetida. The mentioned earthworm has been considered a model for acute toxicity tests in soils according to the OECD guideline n°207. The results showed that under the test conditions, no effects were observed at the physiological level or in the enzyme biomarkers analysed (cholinesterase, carboxylesterase, and Glutathione S-transferase). This Doctoral Thesis is presented as a compendium of five publications in which the main results of the presence and impact of microplastics in the treatment of wastewater, sewage sludge, and soil are addressed.