How Reverse Osmosis Works
A plain-language walkthrough of the 4–5 stage RO process, what it removes, waste ratios, pros and cons.
Reverse osmosis (RO) is one of the most thorough residential water treatment methods available. It forces water through a semi-permeable membrane that blocks contaminants down to approximately 0.0001 microns - far smaller than what carbon filters can achieve. Here is how the process works, what it removes, and who actually needs one.
The Multi-Stage Process
Most residential RO systems use 4 or 5 stages. Water passes through each stage sequentially:
Sediment Pre-Filter (5 micron)
Removes dirt, sand, rust, and larger particles. Protects the carbon filter and RO membrane downstream. Typically replaced every 6–12 months.
Carbon Pre-Filter
Activated carbon removes chlorine, chloramines, and organic compounds that would damage the RO membrane. Chlorine is the primary enemy of thin-film composite (TFC) membranes.
RO Membrane
The core of the system. Water is pressurized against a semi-permeable membrane with pores approximately 0.0001 microns. Dissolved solids, heavy metals, PFAS, nitrates, fluoride, and most other contaminants are rejected and flushed to drain. Only water molecules and some dissolved gases pass through.
Storage Tank
RO filtration is slow (50–100 gallons per day for most residential systems). Filtered water is stored in a pressurized tank so it is available on demand. Tank sizes typically range from 2 to 4 gallons. Tankless systems use a pump and higher-capacity membranes instead.
Carbon Post-Filter (Polish)
A final activated carbon stage removes any residual taste or odor before the water reaches your glass. Some systems add a remineralization stage here to restore calcium and magnesium for improved taste.
What RO Removes
An NSF 58-certified RO system is verified to reduce:
- Total dissolved solids (TDS) - typically 90–99% rejection
- Lead, arsenic, mercury, cadmium, chromium-6
- PFOA, PFOS, and other PFAS compounds
- Fluoride and nitrate
- Radium, uranium, and barium
- Microplastics and pharmaceuticals
- Cysts (Giardia, Cryptosporidium)
RO does not effectively remove dissolved gases (radon), some volatile organic compounds (VOCs), or chlorine - which is why carbon pre- and post-filters are included in the system.
Waste Ratio
Traditional RO systems produce wastewater (concentrate) as rejected contaminants are flushed to drain. Older systems waste 3–4 gallons for every 1 gallon of filtered water (a 3:1 or 4:1 ratio). Modern systems achieve 1:1 or even 2:3 waste ratios. Some newer systems recirculate reject water to improve efficiency further. Check the waste ratio specification before purchasing.
Pros and Cons
Pros
- Most comprehensive residential contaminant reduction
- Reduces TDS, heavy metals, PFAS, fluoride, nitrate
- NSF 58 certification available for verified performance
- Replacement membranes last 2–3 years
Cons
- Produces wastewater (check the ratio)
- Slow flow rate - requires storage tank or pump
- Removes beneficial minerals (calcium, magnesium)
- Higher upfront and maintenance cost than carbon filters
- Requires under-sink installation
Who Needs RO?
RO is typically the right choice if your water contains contaminants that carbon filters cannot adequately reduce - particularly fluoride, nitrate, TDS, or high levels of arsenic. It is also appropriate if you have multiple contaminants of concern and want a single system that addresses all of them. Use our RO vs Carbon vs UV tool to get a personalized recommendation.
If your main concerns are chlorine taste, lead, or PFAS, a quality carbon block filter (certified under NSF 53) may be sufficient and more practical.
Sources
Educational guidance only - consult your water utility or a certified lab for situation-specific recommendations.