Occurrence of Microplastics in Waste Sludge of Wastewater Treatment Plants: Comparison between Membrane Bioreactor (MBR) and Conventional Activated Sludge (CAS) Technologies

Abstract

In this study, the presence of microplastics in the sludge of three wastewater treatment plants (WWTPs) was examined. The investigated WWTPs operated based on a conventional activated sludge (CAS) process, with (W1) or without (W2) primary clarification, and a membrane bioreactor process (MBR) (W3). The microplastics (MPs) concentration in the samples of W3 was approximately 81.1 ± 4.2 × 103 particles/kg dry sludge, whereas MPs concentrations in W1 and W2 were 46.0 ± 14.8 × 103 particles/kg dry sludge and 36.0 ± 5.2 × 103 particles/kg dry sludge, respectively. Moreover, MPs mainly consisted of fragments (66–68%) in the CAS plants, whereas the fractions of MPs shapes in the MBR sludge were more evenly distributed, although fiber (47%) was the most abundant fraction. Furthermore, samples from the MBR showed a greater diversity in MPs composition. Indeed, all the main polyesters (i.e., textile fibers and polyethylene terephthalate), polyolefins (i.e., polyethylene and polypropylene) and rubber (i.e., polybutadiene) were observed, whereas only polybutadiene, cellulose acetate and polyester were detected in the CAS plants. These findings confirmed that MPs from wastewater are transferred and concentrated in the waste sludge. This is a critical finding since sludge disposal could become a new pathway for microplastic release into the environment and because MPs might affect the fouling behavior of the membrane.

Detection and Analysis of Microfibers and Microplastics in Wastewater from a Textile Company

“…The highest MPs removal efficiency (99.9%) was reported for membrane bioreactor (MBR) processes [36,37]. A recent Italian study [38] analyzing three WWTPs demonstrated that MBR technology allows to greatly improve (up to 76%, compared to active sludge with or without primary settling) the concentration of MPs in the sludge. Considering the consequent phases of state-of-the-art WWT, mostly based on active sludge processes, it was reported [19,31] that 35-59% of MPs are removed through preliminary treatments, 50-98% via primary settling, up to 20% through secondary settling, and that about 2% residual MPs can be detected in the effluent.…”

Enhancing Biogas Production Amidst Microplastic Contamination in Wastewater Treatment Systems: A Strategic Review

“…MPs originate from various sources, including the breakdown of more extensive plastic materials and intentional discharge into the environment. Studies have shown varying concentrations, colors, and shapes of MPs in sludge [16][17][18][19]. Most MPs discovered in wastewater consist of fibers, pieces, and granules, indicating their varied sources.…”

Unaccounted Microplastics in the Outlet of Wastewater Treatment Plants—Challenges and Opportunities

“…Currently, conventional wastewater treatment procedures are used, while modifications to adapt for MP removal are continuously being explored and implemented. In general, removal of MPs in WWTPs is carried out in primary, secondary, and tertiary treatments, using one or a combination of the following procedures: skimming and sedimentation [68], dissolved air flotation filtration [69], sand filtration [70], biofiltration [71], membrane filtration [72,73], coagulation [74], ozonation [75], adsorption [76], magnetic extraction [77], membrane bioreactor (MBR) [78], conventional activated sludge [16], and degradation using microorganisms [79,80].…”

“…This mismatch results in the underestimation of MPs in the sludge and effluent that are released into the environment. Therefore, sludge and effluent were identified as primary carriers of 'unaccounted' MPs in our ecosystems [13][14][15][16][17][18][19][20].…”