Reverse osmosis is a cross-flow filtration process that uses water pressure to push water through a semipermeable membrane, leaving dissolved contaminants behind in a "reject" stream that goes to the drain (Water Quality Association). Pressure forces water to flow in the reverse direction of natural osmosis — from concentrated to dilute — leaving dissolved material behind on the membrane's feed side.
A typical under-sink RO system runs water through a sediment pre-filter, an activated carbon pre-filter (protects the membrane from chlorine), the RO membrane itself, and often a carbon post-filter for taste, usually storing finished water in a small pressurized tank until needed (WQA).
RO membranes block a wide range of dissolved contaminants that carbon filtration alone typically can't touch. The table below shows the minimum reduction a system must hit to earn NSF/ANSI 58 certification for a given claim, alongside the EPA's enforceable drinking water limit.
| Contaminant | NSF/ANSI 58 Minimum Required Reduction | EPA Drinking Water Standard (MCL) |
|---|---|---|
| Arsenic (pentavalent) | 96.7% | 0.010 mg/L (10 ppb) |
| Lead | 96.7% | Action Level 0.015 mg/L (15 ppb) |
| Chromium | 66.7% | 0.1 mg/L |
| Nitrate/nitrite (as N) | 66.7% | 10 mg/L (nitrate) |
| PFOA/PFOS | 95.3% | 0.004 ppt (4 parts per trillion) each |
Sources: EPA WaterSense RO Specification; EPA National Primary Drinking Water Standards. These NSF minimums are pass/fail certification thresholds, not a performance ceiling — and certification is claim-specific, so a system must be tested and certified for the exact contaminant you care about.
Yes — reverse osmosis is one of a small number of technologies EPA identifies as effective at removing PFAS, alongside granular activated carbon and ion exchange resins. EPA describes high-pressure membranes like RO as "extremely effective," typically exceeding 90 percent removal across a wide range of PFAS compounds, including shorter-chain types that are harder for carbon filters to catch (EPA). NSF added a specific PFOA/PFOS reduction claim to NSF/ANSI 58 in 2020; certified systems must achieve at least 95.3 percent reduction (EPA WaterSense).
Rhode Island's own PFAS standard — a 20 ppt combined limit for six PFAS compounds — applies only to public water systems, not private wells. If PFAS is a concern for a private well, RO has real evidence behind it, but a RIDOH-certified lab result, before and after treatment, is the only way to confirm it's needed and working.
Yes, for both, when the system is certified for that specific contaminant. Arsenic occurs naturally in New England's bedrock aquifers: an estimated 20–30% of private wells in eastern New England exceed the federal arsenic standard of 10 µg/L, per joint USGS/National Cancer Institute research (USGS). NSF/ANSI 58-certified systems must remove at least 96.7% of pentavalent arsenic; arsenic sometimes needs pre-oxidation to that form for RO to work effectively.
Nitrate is more localized, usually tied to agricultural land use, septic systems, or fertilizer near a wellhead. RO systems certified for nitrate/nitrite must remove at least 66.7%, generally enough to bring moderately elevated nitrate under EPA's 10 mg/L standard. Neither contaminant can be detected by taste, smell, or appearance.
Under-sink (point-of-use) RO treating one faucet is by far the more common residential setup — RO is a relatively slow, low-flow process impractical at whole-house volumes. For most households, the real decision is RO at the kitchen tap for drinking water, paired with whatever whole-house treatment (softener, iron filter) actually matches the raw water — not whole-house RO.
Every RO system sends some water to the drain. NSF/ANSI 58 measures this as an "efficiency rating" (tank systems) or "recovery rating" (tankless/gravity): the percentage of feed water that becomes usable treated water (WQA). Commercial RO units typically recover 50–75% of feed water; EPA's WaterSense program introduced a 2024 certification specification for point-of-use RO requiring an automatic shut-off and a verified efficiency/recovery rating so products can be compared (EPA WaterSense). Permeate-pump and tankless designs generally waste less than older pressurized-tank designs.
Yes — RO removes most dissolved minerals, including calcium and magnesium, along with contaminants; the membrane doesn't distinguish between the two. The World Health Organization's 2005 report on demineralized water found a plausible connection between very low-mineral water and certain health effects in specific contexts, and recommended remineralization be considered there (WHO) — but for a typical household on a normal diet, water is a small fraction of total mineral intake compared to food. This is why many RO systems add an optional remineralization stage; it's worth knowing about, not worth being alarmed about.
Look for NSF/ANSI 58 first — the specific standard for point-of-use RO, covering material safety, structural integrity, and contaminant claims like TDS, arsenic, lead, nitrate, and (since 2020) PFOA/PFOS (NSF). Systems may also carry NSF/ANSI 42 (taste/odor) and NSF/ANSI 372 (lead-free materials). Certification is claim-specific — check the actual certification listing for the contaminant that matters to you rather than assuming "NSF certified" covers everything.
RO is a drinking-water finishing step, not a whole-home fix. A property with hard water and iron generally still needs a softener or iron filter ahead of an RO system, both to protect the membrane and because RO alone doesn't address whole-house hardness or iron. Know what's actually in your specific water before choosing equipment — a certified lab result, not a guess based on geography, is what treatment decisions should be built around. AquaTrust's free on-site evaluation (hardness, iron, pH, TDS) is a useful starting point, but it is not a certified test for PFAS, arsenic, or nitrate — those require a RIDOH-certified laboratory.
Estimated 2021 winter population using domestic self-supply wells. These are population estimates, not well counts.
Yes. EPA identifies reverse osmosis, along with granular activated carbon and ion exchange, as an effective PFAS treatment technology, typically removing more than 90% of PFAS compounds. Systems certified under NSF/ANSI 58 for PFOA/PFOS must achieve at least 95.3% removal.
Yes, for the general population. RO removes most dissolved minerals along with contaminants. WHO has noted a theoretical concern about very low-mineral water in specific contexts, but for a typical diet, water is a minor source of dietary minerals compared to food.
It depends on the system's design and efficiency rating. Commercial RO units typically recover 50–75% of feed water; EPA's WaterSense program now certifies point-of-use RO systems specifically for water efficiency.
Yes, when the system is certified for arsenic reduction. NSF/ANSI 58-certified systems must remove at least 96.7% of pentavalent arsenic — a documented issue in New England's bedrock aquifers.
For most homes, under-sink RO at the kitchen tap is the practical choice. Whole-house water issues like hardness and iron are typically handled by a softener or iron filter instead.
No. AquaTrust's free on-site evaluation checks hardness, iron, pH, and TDS — not PFAS, arsenic, or nitrate. A RIDOH-certified laboratory result is the right starting point for those.
We’ll collect a sample, have it professionally screened for common treatment conditions, and explain the results in plain English.
No pressure. No obligation. If treatment doesn’t make sense, we’ll tell you.
Local Rhode Island & Southeastern Massachusetts service. Certified laboratory testing recommended when health contaminants require it.
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