Major nitrogen feeds for plant growth; they play a significant role as fertilizers. All natural waters normally contain nitrates at doses which vary with the season (about a few milligrams per litre). In many ground and surface waters, there is today an increase in the nitrate concentration from non-point (run-off of nitrates from inorganic or organic fertilizers that are not used by plants) or point sources (discharge of domestic, agricultural or industrial wastewater). The progressive nitrate enrichment of water can compromise the latter's use for drinking water production and leads, in some cases, to significant algae development. This eutrophication* phenomenon is accentuated by the presence of phosphorus.