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NSF P231: Microbiological water purifiers

Official title: Microbiological Water Purifiers

An NSF protocol for purifiers that claim to reduce bacteria, viruses and cysts. It is based on EPA's 1987 Guide Standard and Protocol for Testing Microbiological Water Purifiers.

What it can cover

  • Bacteria (listing claim: "Bacteria Reduction")
  • Viruses (listing claim: "Virus Reduction")
  • Cysts (listing claim: "Live Cyst Reduction")

What it does not

  • Lead, PFAS or other chemicals, unless the product also holds those claims under 53 or 58
  • Chlorine taste and odor

The usual mistake

Mixing it up with NSF/ANSI 244. P231 covers purifiers for bacteria, viruses and cysts. 244 is only a backup for water that is already treated or shown to be safe.

The fine print

  • NSF calls P231 a protocol, not an NSF/ANSI standard.
  • WQA lists it as "NSF Protocol P231" and words the claims "Bacteria," "Virus" and "Cyst."

Sources

  1. NSF Standards for Water Treatment Systems, NSF. Checked Oct. 4, 2026.
  2. NSF listings for NSF P231 Microbiological Water Purifiers, NSF. Checked Oct. 4, 2026.
  3. Contaminant Reduction Claims Guide, NSF. Checked Oct. 4, 2026.
  4. Find WQA Certified Products (Gold Seal listings by standard and claim), Water Quality Association. Checked Oct. 4, 2026.