A high quality nitrate reduction cartridge.
Can be regenerated to some degree with salt water. It contains high capacity nitrate selective anion exchange resin. A cartridge this size will typically remove 5000 ppm / gallon (meaning, for example, if you have 10 parts per million nitrates in your water, you should change or regenerate it at about 750 gallons). Performance can vary considerably according to flow rate and other water constituents.