For drinking water, certified reverse osmosis is one of the most practical nitrate-reduction options. Private-well owners should test regularly because nitrate levels can change.
Best point-of-use category: reverse osmosis
NSF/ANSI 58 includes optional nitrate/nitrite reduction claims for RO systems.
Verify the exact model rather than assuming every RO system has the same certified scope.
Why carbon is not enough
Activated carbon is excellent for many taste, odor and organic-compound problems but does not generally solve nitrate in the way RO or specialized ion exchange can.
Well-water testing matters
Nitrate can be associated with agriculture, septic systems and groundwater conditions. Treatment does not eliminate the need for periodic source-water testing.
Whole-house treatment is a different project
Whole-house nitrate treatment requires careful sizing and regeneration economics. For many homes, treating only drinking/cooking water is more practical.