Hi

The most economical way to remove nitrate from water is to use the self-purifying capacity of river.

There is no cheaper way .

Well,

Reverse osmosis systems are cost-effective, and when invented they could only produce a few gallons of treated water each day. Significant recent improvements in membrane elements allow for systems that produce 100 or more gallons per day.

Nitrate-Selective Ion Exchange Filters work by removing nitrate ions by adsorbing them onto anion-exchange resins. These systems adsorb sulfate and nitrate ions from the water. Because these resins preferentially adsorb sulfate ions, their effectiveness at removing nitrate will be negatively impacted if sulfate concentrations are high. Care must be taken to replace or regenerate a filter before it becomes saturated. Otherwise, sulfate ions will replace nitrate ions already adsorbed to the filter, leading to an increase in the nitrate concentration of the water instead of a decrease. The removal of nitrate takes place in filter cartridges that may be part of entry systems that treat all water entering the house.

Regards

I agree with @Namour's and @Bayan's answer!