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Behavioral Dynamics of Primary Microplastics in Sand Filtration
Summary
Researchers found that when tiny plastic particles build up in the sand filters used to clean drinking water, they actually form a dense layer that traps even more contaminants—making the water clearer. But there's a catch: this plastic buildup clogs the filters faster and could release trapped microplastics and other pollutants back into the water when the filters are cleaned. This matters because it shows water treatment plants may need to rethink filter design to keep our drinking water both clean and free of plastic contamination.
Primary microplastics (MPs) pose significant risks in drinking water treatment plants, yet their removal behavior and hydraulic implications during sand filtration remain poorly understood. This study investigates the fate, retention mechanisms, and operational implications of industrially derived polyvinyl chloride and high-density polyethylene primary MPs during sand filtration, one of the most widely applied drinking treatment processes. More than 300 filtration runs were conducted with sands of varying grain sizes and specific filter depths. The results showed that cake filtration dominated MP removal, with an efficiency of over 95%, while the contribution from depth filtration was minimal. MP retention promoted the formation of a dense cake layer at Dsand/DMP ratios of 3.22–10.67, which enhanced suspended solid capture and improved effluent turbidity to <1 nephelometric turbidity unit (NTU), compared to conventional sand filtration. However, cake formation induced progressive head loss by up to 150%, particularly in filters with finer media, highlighting trade-offs between enhanced particle removal and operational sustainability. Scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDX) analysis revealed that suspended solids were immobilized on MP layers via straining, entrapment within surface depressions, and adsorption, confirming MPs as active sites for retaining secondary particles. Modeling results showed that filtration dynamics were well described by a declining rate (DR) model (R2=0.96), suggesting that sand filters (SF) exposed to primary MP-rich effluents should be designed as DR filters. Although these findings show that primary MP accumulation enhances particle removal and alters filter hydraulics, they also highlight the risk of MPs as long-term contaminant carriers and the potential for their release during backwashing.