Microplastics Are Concentrated in the Waste Line
RO membranes have pores roughly 0.0001 microns wide — smaller than most virus particles. Because virtually all microplastics and nanoplastics are far larger, they physically cannot pass through. Instead, the pressurized system pushes pure water through the membrane while continuously flushing rejected contaminants — including microplastics — down the wastewater (reject) line into your household drain.
Because a standard under-sink RO system wastes 1 to 4 gallons of water for every gallon of purified water it produces, the microplastics from the source water are densely packed into that rejected stream. The result: your drinking water is cleaner, but your drain is receiving a highly concentrated dose of the same particles you were trying to avoid.
A standard under-sink RO system wastes 1–4 gallons for every gallon of clean water produced.
The System Itself Can Shed Plastic Over Time
Here is where it gets paradoxical. The RO unit that protects you from microplastics can, over time, become a source of them.
Most under-sink RO systems are built with plastic housings, plastic fittings, rubber seals, and polymer-based structural components. The RO membrane itself — the heart of the system — is typically a thin film composite made of polyamide, a type of plastic.
As the system ages and experiences repeated pressure changes and internal friction from water flow, these polymer components slowly degrade. The result: microscopic plastic fragments shed directly from your filter and are swept out through the wastewater drain. This is not a reason to avoid RO filtration — it is a reason to maintain and replace components on schedule, and to choose systems with NSF/ANSI 58 certification that have been tested for material integrity.
The RO membrane is made of polyamide — a plastic. Aging and pressure cause slow shedding of microscopic fragments.
Where Do the Concentrated Microplastics Go?
When your RO wastewater flows down the sink drain, it enters the municipal sewer system and travels to a Wastewater Treatment Plant (WWTP). This is actually good news.
Modern municipal treatment plants are highly efficient at capturing these concentrated particles. Through sedimentation, filtration, and biological treatment stages, WWTPs typically remove up to 99% of microplastics before releasing treated water back into local rivers, aquifers, or the ocean. The particles that are captured end up in sewage sludge (biosolids), which has its own management pathway.
This does not mean the problem disappears — biosolids are often land-applied as fertilizer, which can return microplastics to soil and eventually groundwater. But for your immediate household water supply, the RO system is doing its intended job.
Modern WWTPs remove up to 99% of microplastics — but particles that are captured end up in biosolids spread on farmland.