Guides
Guides
5 short guides, each answered from the same data as the rest of the site: ro waste water, when to change a filter, nsf standards explained, pfas limit, what's in my water.
How much water does a reverse osmosis system waste?
The reverse osmosis systems we track send between 0.25 and 3.03 gallons to the drain for each gallon of filtered water (median 0.5, across 24 systems that publish it; lower is better). For 2 people drinking and cooking with it, that is about 102 gallons a year, roughly $2.15 of water and sewer. For comparison, EPA estimates a typical reverse osmosis unit sends at least 5 gallons to the drain per gallon treated, and WaterSense-labeled models no more than 2.3.
How often should you change a water filter?
By the brands' own ratings: pitcher filters about every 2 months, faucet-mount filters about every 3 months, under-sink filters about every 12 months, reverse osmosis filter stages about every 12 months, whole-house filters about every 6 months (medians across the filters we track). Change a filter at its gallon rating or month limit, whichever comes first; a busy household reaches the gallon limit sooner.
What do NSF/ANSI 42, 53, 58, 401 and 473 mean?
NSF/ANSI 42: certifies that point-of-use or whole-house filters reduce non-health (aesthetic) contaminants such as chlorine, taste and odor, chloramine, particulate, iron, manganese, zinc and TDS; NSF/ANSI 53: certifies that point-of-use or whole-house filters reduce specific health-related contaminants (over 50 claims, e.g., lead, Cryptosporidium, VOCs, chromium, PFOA/PFOS); NSF/ANSI 58: covers reverse osmosis systems that push water through a semi-permeable membrane to reduce EPA/Health Canada-regulated contaminants; NSF/ANSI 401: certifies reduction of up to 15 trace 'emerging' contaminants (e.g., certain pharmaceuticals, herbicides, pesticides, BPA) not regulated by EPA/Health Canada; microplastics claim also sits here; NSF/ANSI 473: NSF P473 was a 2016 protocol testing filter reduction of PFOA/PFOS to EPA's 2016 70 ppt health advisory; its methods were folded into NSF/ANSI 53 and 58, and all P473 listings were replaced by 53/58. A product is certified only if a certifier lists it; "tested to NSF standards" is not a certification.
What is the EPA limit for PFAS in drinking water?
The EPA's limit is 4 ppt (ng/L) for PFOA and for PFOS. All six limits set in April 2024 remain in effect as of October 2026: PFOA and PFOS 4.0 ppt each; PFHxS, PFNA and HFPO-DA (GenX) 10 ppt; and a Hazard Index of 1 for mixtures. Public water systems must comply by April 26, 2029. In May 2026 EPA proposed letting utilities extend the PFOA and PFOS deadline to April 2031 and rescinding the other four limits; those proposals are not final. Filter certifications for PFAS (NSF/ANSI 53 and 58) currently test reduction to below 20 ppt combined, not to the 4 ppt limits; EPA says current certifications do not yet show removal down to those levels.
How do I find out what's in my tap water?
Community water systems must deliver a Consumer Confidence Report by July 1 each year, covering the previous calendar year's data. EPA recommends testing a private well every year for total coliform bacteria, nitrates, total dissolved solids, pH. EPA does not regulate private wells.