Control of Low-Density Microplastics in Wastewater Treatment Plants: The Effects of Process Configuration and Polymer Properties on Removal Efficiency

The growing concern over emerging contaminants in the urban water cycle has highlighted the role of wastewater treatment plants (WWTPs) as major pathways for their release into aquatic environments. The study investigates the fate of low-density microplastics (MPs) in two full-scale wastewater treatment plants (WWTPs) with moving bed biofilm reactor (MBBR, WWTP-A) and an activated sludge (AS) process coupled with ultrafiltration membrane bioreactor (MBR, WWTP-B). Polyethylene (PE) and polydimethylsiloxane (PDMS) were quantified through a mass-based analytical method using proton nuclear magnetic resonance (1H NMR) spectroscopy. These polymers are characterized by high prevalence in WWTPs influents and low-density characteristics, associated with their abundant presence in final effluents. Both plants exhibited high PE removal efficiencies, averaging 94.9 ± 1.0% in WWTP-A and 97.5 ± 3.5% in WWTP-B, confirming the effective retention of this polymer through WWTP processes. Conversely, PDMS removal resulted in more variable outcomes, with efficiencies of 67.4 ± 22.8% in WWTP-A and 87.8 ± 9.2% in WWTP-B. The lower PDMS removal was attributed to its high hydrophobicity and low surface energy, which hindered adsorption onto microbial biomass and extracellular polymeric substances. The results highlight the importance of design proper technologies for mitigating the discharge of highly hydrophobic polymers into aquatic environments.