CHEMICAL DENITRIFICATION OF LANDFILL LEACHATES UNDER FREEZING/THAWING CONDITIONS / Kemisk denitrifikation av lakvatten från deponier vid frysning/upptining
Chemical denitrification has been proposed (Nikolic and Hultman, 2003) as the second part in a new treatment system for landfill leachates, where ammonium or urea is oxidized with nitrite to yield nitrogen gas and water. Concentrated aqueous solutions of ammonium nitrite or urea nitrite are unstable. One of possible methods to concentrate solution and in that way accelerate the reaction rate is freezing/thawing. Several expe-riments were performed with high and equal concentrations of ammonium or urea and nitrite (from 200 mg N/l to 3200 mg N/l of each component) in water solution and in leachate at pH 4, and under freezing/thawing conditions. Inorganic nitrogen compounds and conductivity were measured in all experiments before and after freezing/thawing. Results showed that freezing had an influence on the reaction rate and that is probably due to a concentration effect. Furthermore, it was shown that chemical denitrification is possible and that the percentage of removed nitrogen increased with increasing initial concentration of solution. 68% of total inorganic nitrogen (or 4347 mg/l) was removed from the water solution of 3200 mg/1 of ammonium nitrogen and 3200 mg/1 of nitrite nitrogen, and about 56 % (or 3549 mg/1) from leachate solution with the same concent-ration. Leachate acted as an inhibitor to chemical denitrification, but this behaviour decreased with increasing concentration. Solution with urea and nitrite decomposes more slowly than ammonium nitrite solution. Nitrogen removal was followed with conductivity, and it was shown that this could be a good method to follow ammonium nitrite decomposition.