Wastewater treatment plants remove microplastics effectively. Advanced filtration and settling processes capture 95-99% of microplastic particles larger than a few micrometers. The effluent is cleaner than the influent. The treatment works.
The treatment also creates nanoplastics. Mechanical stress, UV exposure, chemical oxidation, and biological degradation within the treatment process fragment larger plastic particles into smaller ones. Particles that entered the plant as microplastics — tens of micrometers across — exit as nanoplastics, tens of nanometers across. The concentration of particles counted by number can increase even as the concentration measured by mass decreases.
The distinction matters toxicologically. Nanoplastics cross biological membranes that microplastics cannot. They penetrate cell walls, pass through gill epithelia, cross the blood-brain barrier in fish. The smaller the particle, the larger the surface-area-to-volume ratio, the greater the capacity to adsorb and transport hydrophobic contaminants. A nanoplastic particle carries proportionally more toxic cargo per unit mass than the microplastic it came from.
The treatment plant converts a large, relatively inert particle into many small, relatively bioactive particles. The mass is removed. The hazard is multiplied. The metric that says the plant is working — mass removal efficiency — does not capture the metric that matters — number concentration of bioavailable particles in the effluent.
The treatment doesn't fail. It succeeds at the measured objective (remove mass) while producing an unmeasured outcome (create fragments). The gap between what we measure and what matters is the gap where the problem lives.