Microplastics are synthetic solid particles ranging from 1 nanometer to 5 millimeters, derived from the breakdown of larger plastic debris or manufactured to be small. These tiny particles, composed of various polymers like polyester, nylon, and polyethylene, are now ubiquitous in the global environment, contaminating water sources and air. Primary sources include fibers shed from synthetic textiles, tire dust, and microbeads used in cosmetics, while secondary sources arise from the slow degradation of plastic waste exposed to natural elements.
Filtering Microplastics from Drinking Water
The effectiveness of household water filtration depends directly on the particle size a system is designed to intercept. Reverse osmosis (RO) systems offer the highest level of filtration, utilizing a semipermeable membrane with extremely fine pores, typically around 0.0001 microns, which is significantly smaller than most microplastics. This ultra-fine pore size allows RO to achieve removal rates as high as 99.99% for particles ranging from 0.001 to 5 microns. Ultrafiltration (UF) systems also employ a membrane technology but with slightly larger pores, usually between 0.01 and 0.1 microns, still providing a high-efficiency physical barrier with removal rates often exceeding 98.5%.
Activated carbon filters, commonly found in pitcher and refrigerator systems, rely on different mechanisms depending on their design. Granular Activated Carbon (GAC) filters may have limited success against the smallest particles. However, high-quality carbon block filters with a sub-micron rating, such as 0.5 or 0.2 microns, utilize physical interception to remove larger microplastic fragments and fibers, generally those greater than 1 micron. Filters that incorporate microfiltration (MF) membranes, particularly those rated at 0.2 micrometers, have demonstrated superior performance, showing removal efficiencies between 94% and 100% for specific microplastic fragments.
Large-Scale Removal: Municipal and Industrial Systems
Wastewater treatment plants (WWTPs) serve as a final barrier to prevent microplastics from entering rivers and oceans. Conventional wastewater treatment, which includes primary screening and secondary biological processes, typically removes a significant portion of microplastics, with average removal efficiencies ranging from 65% to 72%. The microplastics are generally captured in the settled sludge during these initial stages. More advanced purification methods, often termed tertiary treatment, are required to achieve higher removal rates for the remaining particles.
Membrane Bioreactors (MBRs) represent a highly effective tertiary technology, combining biological treatment with a membrane filtration step. MBRs have demonstrated exceptional removal efficiencies, with some facilities reporting up to 99.9% elimination of microplastics from the effluent water. Chemical processes like coagulation and flocculation, followed by rapid sand filtration, also prove highly successful. Coagulation uses chemical agents to bind microplastic particles together, forming larger clusters that can be more easily filtered out, resulting in up to 98% removal. While these advanced treatments are highly effective at cleaning the water, the captured microplastics are concentrated in the sewage sludge, which creates a new challenge for safe disposal or management.
Air Filtration and Emerging Technologies
Filtration is also an effective strategy for managing microplastics suspended in the air, particularly within indoor environments. High-Efficiency Particulate Air (HEPA) filters are the standard for capturing airborne microplastics, which often originate from synthetic fabrics and dust. HEPA filters are designed to capture 99.97% of particles that are 0.3 microns in size, effectively trapping most microplastic fragments and fibers that fall within this size range. These particles are mechanically captured as air is forced through a dense mesh of fibers, a process that significantly lowers the concentration of airborne microplastics indoors.
Beyond commercial air and water filtration, researchers are exploring innovative technologies to address the smallest microplastics. One promising experimental method is magnetic separation, which involves treating the water with tiny magnetic nanoparticles that adhere to the plastic fragments. Applying a magnetic field then allows for the highly efficient separation and removal of the microplastic-nanoparticle clusters from the water, with some studies demonstrating a high separation efficiency. Bio-based filters are also being developed, utilizing natural materials such as lignocellulose flocculants derived from plants. These bio-based materials promote the aggregation of microplastics into larger masses, which can then be easily filtered out, offering a sustainable approach to water purification.