Removal of microplastics during drinking water treatment: Linking theory to practice to advance risk management

Abstract

Microplastics (MPs) have emerged in the past decade as widespread contaminants that are harmful to human and ecosystem health. While their removal from water may be similar to those of other particulate contaminants, its characterization is complicated because MPs can undergo weathering, photolysis, and microbial degradation in the natural environment, resulting in the presence of functional groups (e.g., carbonyl, hydroxyl) on their surfaces, which may affect their removal during drinking water treatment. Given that studies using seeded polystyrene microspheres/MPs as surrogates for oocysts have shown good (but sometimes variable) removals through conventional drinking water treatment composed of coagulation, flocculation and sedimentation (CFS) followed by filtration, MPs are likely to be well removed in optimized conventional drinking water treatment plants. While many studies have focused on the removal of larger (i.e., >50 µm sized microplastics), investigations of the removal of smaller sized (<10 μm) microplastics by drinking water treatment processes have been limited largely to case studies in which foundational mechanisms necessary for maximizing treatment performance have only been superficially investigated, if at all. To address this gap, the study focused on whether MPs removal by conventional chemical pretreatment (i.e., coagulation, flocculation, and sedimentation) with alum aligns with the removal of other particles, including Cryptosporidium oocysts, for which particle destabilization is essential for removal. The study aimed to advance knowledge through three main objectives: (1) characterize MPs removal by CFS with different particle destabilization mechanisms and compare it to other important particulate contaminants (i.e., Cryptosporidium spp. oocysts), (2) evaluate the effect of particle size on MPs removal by CFS, and (3) assess the influence of weathering on MPs removal by CFS. To evaluate MPs removal by chemical pretreatment reliant on (1) adsorption and charge neutralization and (2) enmeshment in precipitate (i.e., sweep flocculation) particle destabilization mechanisms, bench-scale investigations of alum-based CFS (i.e., jar tests) were conducted with synthetic water using pristine and weathered PS microplastics of 1, 5 and 10 μm diameter. Several synthetic raw water matrices were explored to identify scenarios in which both particle destabilization mechanisms were clearly discerned. The final synthetic raw water was composed of deionized water spiked with sodium carbonate and kaolin (70 NTU) at pH 7.0. To demonstrate that MPs removal by CFS aligns with coagulation theory, sixteen alum doses between 0–38.8 mg/L were used to evaluate MPs removal by CFS. Turbidity reduction was also evaluated, and zeta potential was analyzed to identify maximal particle destabilization. MPs removal increased with particle size, aligning with gravitational settling theory. MPs removal during CFS with optimized particle destabilization was generally consistent with reported removals of other particles, including Cryptosporidium spp. oocysts during optimized chemical pretreatment, thereby suggesting that similar approaches for risk management may be relevant to MPs. Notably, differences in pristine and weathered MPs removal by CFS were not significant under the conditions investigated, thereby suggesting that weathering does not affect MPs removal when particle destabilization by coagulant addition is optimized. This study bridges the gap between the theories of conventional drinking water treatment and concerns regarding the potential passage of MPs through drinking water treatment plants, demonstrating that MPs can be removed in the same manner as other colloidal particles using conventional chemical pretreatment and—by well-recognized theory-based extension—physico-chemical filtration.