Quick take
For drinking water, an RO system with an exact arsenic claim can be appropriate. For private wells with significant arsenic, test first and consider a treatment design specific to the measured chemistry.
Best drinking-water option: validated RO
NSF/ANSI 58 includes optional arsenic-reduction claims under defined conditions. The exact certified model and claim matter.
Why arsenic form matters
Trivalent and pentavalent arsenic can behave differently in treatment. A generic statement that “RO removes arsenic” hides that complexity.
Adsorptive media can be a whole-house path
Specialized iron-based and other adsorptive media can be used for arsenic depending on chemistry and treatment scope.
Verify the treated water
When arsenic is the reason for treatment, post-installation laboratory testing is the only responsible way to confirm the system is achieving the target.