Hi Carlos, as others have ...

Hi Carlos, as others have suggested, biological denitrification is a relatively inexpensive process. Rather than using organic matter as the energy source for the bacteria involved, I would suggest using sulphur. That way, potential pathogens and e.g.toxin-producing fungi will be unable to thrive. Essentially, at low or zero dissolved oxygen concentrations, many bacteria can use nitrate for respiration in place of oxygen. If sulphur is used to supply the energy, a sub-set of autotrophic bacteria will oxidise it to sulphate and reduce the nitrate to nitrogen gas. Thus, you end up with sulphate in the water rather than nitrate. The WHO guideline limit for sulphate is 500 mg/L but that is for taste rather than toxicity reasons, and many mineral waters contain a high concentration of sulphate. Furthermore, autotrophic metabolism produces less biomass than heterotrophic metabolism (organic matter as energy source), so there will be less biological sludge to dispose of. Rather than treating the entire flow, a proportion can be treated and blended with the untreated portion, to supply water with nitrate less than the WHO guideline concentration (50 mg/L, as the nitrate ion = 11.3 mg/L as N, which is maybe where your value of 10 mg/L came from). Best wishes, Mike