The invention relates to a method for processing an aqueous effluent (E) containing in solution at least one type of metal (M) nitrite or nitrate whose oxide is stable in an aqueous medium consisting (I) in introducing cations NH.sub.4+ in aqueous solution, generally in the form of an aqueous ammonia solution, into the aqueous effluent and, subsequently (II) in processing the obtainable medium at a temperature ranging from 100 to 700.degree. C. and a pressure of 1-50 MPa (i.e. from 10 to 500 bar), thereby converting the metal (M) nitrite or nitrate into metal (M) oxide, nitrogen and water.