Some RO systems are certified for pentavalent arsenic reduction, but performance can vary with arsenic species and concentration. Test the source water and verify the treated water when arsenic is the reason for treatment.
NSF/ANSI 58 includes arsenic claims
The RO standard includes optional arsenic-reduction claims, including pentavalent arsenic. Certified model listings define the conditions under which the claim applies.
Arsenic species matters
Research on household RO has shown that arsenic removal can vary, particularly when trivalent arsenic is present. A system that reduces TDS well is not automatically proof that arsenic is below a health target.
Testing is not optional for a private well
If arsenic is suspected or known in well water, test the untreated water and verify the finished water after installation. Treatment should be designed around measured chemistry rather than generic RO marketing.
When another treatment may be better
Adsorptive media such as iron-based arsenic media can be used in some well-water applications. The right technology depends on concentration, pH, arsenic form and whole-house versus drinking-water scope.