Bethpage Water District uses ion exchange for the removal of Nitrate which is naturally occurring and also impacted from years of fertilizing the local farms. We all know that opposites attract and ion exchange works on this principle. The negatively charged Nitrate ion adheres itself to the positively charged resin media within the filter. Once the resin within the filter is exhausted with nitrate ions, it is regenerated with a backwashing of brine or salt water to bring the resin back to its full capacity.