Studies of microplastics present in sewage sludge, effects on amended agricultural soils

Resumen

Intensive plastic usage aims at making life easier, with the highest value and the lowest input of resources. Global environmental alarm and social sensitivity revealed that microplastics (MPs) are the most notable type of plastic degradation and pollution that has attracted much attention among researchers, who have detected MP traces in marine and terrestrial environments. Facing the problem of MPs accumulation that will undoubtedly cause leaks clogging over time and that are still no proven methods by the scientific community, several harmonious procedures have been proposed to determine MPs in any type of matrix. This study presents a scoop of MPs extraction in dry-weighed sludge samples. Given a variety of protocols, differing retention times and digestion efficacy have been compared, using the principle of density separation followed by filtration with size fractioning, the results in each protocol have been different. Four protocols were intercompared to assert the major and apply it on a real mud. For the validation of the method, 30 g of sludge was used (ielab 0301-Spain) with 2 types of polyolefins and nylon. The sludge had undergone a division into 2 fractions according to four protocols, each protocol running triplicates and blanks. The first Hcl-based protocol was inspired by Cole et al., (2014) study. The second protocol of n-hexane. Instead of Hcl, the recommendation of Shim et al., (2016) to use n-hexane was followed to increase the potential for comparability. The third H2O2 enzyme two-step protocol was an alternative, stable against most of the mild chemical existing treatments. Two portions of H2O2 were used given its property of bleaching while rational heating, according to Sujathan et al., (2017). The last protocol was an enhanced version of Löder et al., (2017) study. Data collection depicted by an optical visualization and chemical identification, via microscopy and Infrared Spectroscopy (FT-IR) to record polymer spectra was established to interpret the potential retrieval. The results revealed that MP recuperation with the third protocol based on hemicellulase and H2O2 was more efficient, there was no large fluctuation in MP number nor interference caused by the reaction reagents. Then, the optimal extraction method for MPs in sewage sludge is suggested to be digested with hemicellulase at 50 ºC, followed by bleaching with H2O2. Conclusively, in the short run, it is recommended to optimize this protocol to claim a faster and more economical operational system while seeking to increase MPs' recovery rates. Moreover, a higher quantity of MPs in real sludges was found, ranging from 96 to 769 particles/g dry sludge (ds), with an average of 392.4 ± 72.01 particles/g ds, with the dominance of 100-355 m sizes, commonly fibres. Thermoplastics, ethylene derivatives, polyethylene terephthalate and polyamide were the dominant MPs. The estimation of MPs for the province of Cádiz (2.34 × 108 MPs/tonne) made it possible to estimate an average number of MPs for Spain at 2.80 × 1014 MPs per year, which can go to other terrestrial environmental compartments through the sludge generated by the WWTPs of the country. To know the possible impact of MPs on the use of sludge as agricultural fertilization, the presence of MPs in agricultural soils previously treated with sludge or compost from sewage treatment plants for fertilization, and their exposure to earthworms in southern Spain was evaluated. Composts were used, selected at different stages of the composting process (humid, semi-dried and fine composts), and different types of fertilised soils at ascending doses (S1, S2 and S3), taking samples at different depths (0-5, 5-10 and 10-20 cm) to study the shape, size, type and abundance of MPs using FT-IR. Two earthworm species have been also studied as bioindicators to check the level of ingestion at increasing doses of the common MPs encountered (0.9, 2.1 and 3% w/w). Research on MPs contamination in soils was not either surprising, as soils are a grave of fragmented agricultural mulch film, organic fertilizer, sewage irrigation, sludge/compost amendment, atmospheric deposition and surface runoff. So far, results indicated the presence of higher amounts of 408.3 ± 12.4 × 106 and 176.2 ± 15.5 × 106 MPs/tonne, in studied composts and soils respectively. The distribution of MPs types, most of them were fibres, differentiated by size and types (polytetrafluoroethylene, thermoplastic polyamide elastomer, polypropylene, polyvinyl chloride and polyethylene terephthalate) evolved differently, the predominant size was 250 m. Continuously, and in order to demonstrate the transition of the micropollutants to the biota, the results revealed significant ingestion of MPs in earthworms with increasing doses of the common MPs encountered ranging from 2.1 to 3% w/w, characterized mostly by polypropylene, as the most ingested polymer type. From a future perspective, the research topic highlights one of the most important problems at present, such as the contamination of MPs from daily use and the lives of people, which reach the terrestrial environment through the sludge of the treatment plants The utility of this doctoral thesis is twofold: on the one hand, it sheds light so that future regulations (both for wastewater treatment and the application of sewage sludge) take into account the need to control this pollutant, which has come to stay. On the other hand, it reveals the impact that it may be generating for decades and unknowingly on the terrestrial microbiota. From my point of view, further advanced toxicity tests are recommended that shed light on the impact it may have, in this case on earthworms.