The legal limit for nitrates is 50mg/l. A nitrate reduction system is made up of a vessel filled with nitrate selective resin with a multi-port valve on the top. As the water passes through the resin the nitrate is trapped. Periodically this nitrate needs to be removed by regenerating with salt. As the nitrate levels vary from month to month a time based regeneration system is recommended. Smaller systems can be cabinet based, larger ones come with a separate brine tank to hold the salt.