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Microplastics and nanoplastics in water: Improving removal in wastewater treatment plants with alternative coagulants
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Conventional water treatment plants that use alum as a coagulant become significantly less effective at removing microplastics and nanoplastics as water pH rises above 7.8, which is common in municipal wastewater. Switching to alternative coagulants — particularly aluminum chlorohydrate and cationic polyamine blends — maintained high removal rates at elevated pH, with nanoplastic removal reaching 71% and microfiber removal staying above 95%. The findings offer practical guidance for upgrading treatment plants to better capture plastic particles before they are discharged into waterways.
Due to growing concerns of plastic pollution release into water bodies, it is imperative to understand and assess the removal of nanoplastics and microplastics during wastewater treatment processes. Although earlier studies have shown high removal of plastic contaminants during coagulation, flocculation and settling, a limited number of experiments have examined the removal of nanoplastics in relevant water chemistries including higher pH values and in the presence of organics. In this study, the removal of nanoplastics and microfibers is assessed using conventional (alum) and alternative coagulants including aluminum chlorohydrate (ACH) and cationic polyamines (pDADMAC) in a synthetic wastewater matrix for pH conditions between 7 and 8.6. Our results show that plastic removal is considerably reduced at higher pH values when alum is used as a coagulant. At the shorter settling time of 30 s, removal of pristine and aged nanoplastics declined from 64 ± 3% and 76 ± 3% respectively (pH 7) to less than 20% at pH values higher than 7.8. Similarly, polyester microfiber removal was observed to decline from 97 ± 1% at pH 7 to 85 ± 3% at pH 8.6 for samples collected after 3 min of settling. Replacing alum with polynuclear aluminum coagulants resulted in greater plastic contaminant removal at pH values above 8 with a maximum observed microfiber removal of 95 ± 1% (ACH; pH 8.6; 3 min settling time) and a maximum observed nanoplastic removal of 71 ± 5% (ACH+pDADMAC; pH 8.2; 30 s settling time). Quartz crystal microbalance with dissipation monitoring (QCM-D) measurements revealed that ACH coagulant species results in a thicker deposition layer on negatively charged surfaces compared to alum. The addition of pDADMAC to ACH led to a more favorable interaction with a model negatively charged surface, indicated by a faster and more rigid deposition of the alternative coagulant. Taken together, these results show that environmental conditions, including water pH and interactions with wastewater colloids, affect plastic contaminant removal during primary wastewater treatment. Alternative coagulants, particularly ones that contain stable cationic species, offer municipalities improved removal of plastic contaminants under these challenging treatment conditions.
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Microplastics and nanoplastics in water: Improving removal in wastewater treatment plants with alternative coagulants
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Laboratory tests showed that conventional aluminum sulfate (alum) coagulant becomes much less effective at removing micro- and nanoplastics from water at pH above 7.8—a common condition in municipal wastewater—but switching to aluminum chlorohydrate largely restores removal efficiency. This matters because wastewater treatment plants are a critical barrier preventing microplastics from entering rivers and oceans, and many currently use alum. The study gives water utilities a practical, drop-in solution to significantly improve microplastic capture under challenging water chemistry.
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