Characterization of HDPE microparticles in sludge aerobic digestion and their influence on the process.

The occurrence of microplastics (MPs) in wastewater has been studied in the last years. The high efficiency of their removal from wastewater is linked to their transfer to the sludge. In this work, the effect of high-density polyethylene (HDPE) on aerobic digestion was evaluated and these MPs were monitored, characterizing them by three different techniques. Two parallel batch digesters were monitored. AD-Control (meaning Aerobic Digester) operated as a reference, with no external HDPE particles, whereas these polymeric fragments were introduced to the second aerobic digester (AD-HDPE) using ring pulls as microplastic support. FTIR, Raman spectroscopies and fluorescence analysis of these microparticles showed some relevant results that should be highlighted. Higher fluorescence appeared after 7 days in the digester. It coincided with an increase of active volatile suspended solids (AVSS) in the AD-HDPE, which means that an increase of the microbial activity took place. Despite the presence of HDPE particles in the sludge, the digester performance was not compromised. Besides, the HDPE particles did not affect the microbial diversity (Shannon index) of the bacterial community at the end of the experiment compared to the bacterial community of the aerobic digester control tank. Based on the analysis of the relative abundances of microbial taxa, it was concluded that HDPE had selective effects on sludge microbial community, increasing the relative abundance of Bacteroridota phylum.

Effect of polystyrene nanoplastics on the activated sludge process performance and biomass characteristics. A laboratory study with a sequencing batch reactor.

The fate and presence of nanoplastics in wastewater treatment systems is a topic of increasing interest. Furthermore, challenges related to their quantification and identification have made it difficult to set up experimental conditions and compare results between studies. In this study, the effect of 100 nm polystyrene nanoplastics on activated sludge was evaluated. A concentration of 2 µg/L was used to continuously feed a sequencing batch reactor (SBR-NPs). Under the experimental conditions used in this study, no changes were observed in the process performance of the SBR-NPs compared to the reactor used as a control. Neither nitrification nor organic matter removal efficiency, which was 96% for both SBRs, were affected by the presence of 100 nm polystyrene nanoplastics, which suggests that the tested nanoplastics were not sufficiently toxic to the biomass. Although no significant differences in the relative abundances of predominant phyla between SBR-Control and SBR-NPs were observed, a slight shift in the relative abundance of Patescibacteria (1.5 ± 0.6% and 3.7 ± 0.8% in SBR-Control and SBR-NPs, respectively, at the end of the test) occurred. The higher abundance of this phylum in SBR-NPs compared to SBR-Control may suggest that these bacteria have some sensitivity to the presence of 100 nm polystyrene nanoplastics. Furthermore, even with the absence of nitrification inhibition, it was observed stagnation of the growth of Nitrotoga bacteria in SBR-NPs, which also suggests that the polystyrene nanoplastics could have an inhibitory effect on these cells and an impact on nitrification in the long term.

Methodology for removing microplastics and other anthropogenic microparticles from sludge dewatering system.

Anthropogenic microparticles (e.g., microplastics) are present in sewage plants, especially in sludge streams. However, the lack of standardized protocols to scrutinize the presence of anthropogenic microparticles in sludge makes the comparison between studies unfeasible. To tackle the knowledge gap regarding the efficiency of methodologies on the extraction of anthropogenic microparticles from the complex organic matrix, dewatered sludge, and digested sludge was treated with peroxidation and density separation, and the recovery of microparticles from these samples was investigated. The results showed that with the use of a higher density solution (NaI, 1.5 g/cm3) a much better recovery of anthropogenic microparticles from sludge samples (approximately 1000 microparticles/g-dw and 2000 microparticles/g-dw, from dewatered and digested sludge, respectively) was achieved in comparison with the use of a lower density solution (NaCl, 1.2 g/cm3) (200 microparticles/g-dw and 600 microparticles/g-dw from dewatered and digested sludge, respectively). Moreover, although the use of peroxidation is an essential step to break down the sludge structure and to release microparticles to the liquid phase, the use of peroxidation after or before density separation did not affect the overall recovery of microparticles. Polyethylene, polypropylene, and copolymer ethylene-ethyl-acrylate were the main microplastic fragments identified in digested sludge and dewatered sludge. However, no relation was observed between the method applied and the polymer recovered. Regarding the presence of anthropogenic microparticle in centrifuge effluent, 450 ± 212 microparticles/L were counted, and although little is known about this stream, in can be a relevant source of anthropogenic microparticles.