What is a reverse osmosis water system?

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Reviewed by Water Treatment Answers Editorial Team · Last updated August 2026

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• Installed-cost ranges are typical U.S. figures compiled from published 2025–2026 industry averages (HomeGuide, Angi, HomeAdvisor, and Forbes Home). Your price varies by water quality, system type, and plumbing.
• Water hardness classifications (as calcium carbonate) and the roughly 85% of U.S. homes with some hard water: U.S. Geological Survey (usgs.gov). Grains per gallon equal milligrams per liter divided by 17.1.
• Drinking-water filter certifications (NSF/ANSI 42, 53, 58, 44, and 401): NSF (nsf.org). Certification is contaminant-specific, so confirm the claim for the contaminant you need to remove.
• Public water systems are regulated under the federal Safe Drinking Water Act; private wells are the owner's responsibility. Testing guidance (at least yearly for coliform bacteria and nitrate): U.S. EPA (epa.gov/privatewells) and the CDC (cdc.gov).
• In 2024 the EPA set the first federal drinking-water limits for the PFAS chemicals PFOA and PFOS in public water systems: U.S. EPA (epa.gov/sdwa/pfas). Confirm current requirements, which continue to be updated.
• Cost, interval, and performance figures here are educational ranges, not an offer or a promise, and vary by product, water quality, and region.