Use of chemical denitrification for nitrogen removal from landfill leachates

Abstract: Landfill leachate usually has a high concentration ofammonium nitrogen and heavy metals, but too low alkalinitycontent for full biological nitrification, as well as a too lowconcentration of biologically degradable organic carbon forbiological denitrification, and at the same time very variablecomposition (quantity and quality) with time. These factorsresult in serious problems for treating leachate with existingsystems. A new treatment system that removes nitrogen fromlandfill leachate has been proposed. The first part entailsbiological nitritation where half of the amount of ammonium isoxidized to nitrite. The second part starts with concentratingequal amounts of ammonium and nitrite, followed by chemicaldenitrification that produces nitrogen gas. The concentrationof nitrite and ammonium plays the most important part for thechemical denitrification. The possibility to concentrate thesecomponents with freezing/thawing and evaporation/drying wasinvestigated. Conductivity was measured during all experiments,before and after freezing or evaporation, in order to test ifit is possible to follow the whole process with conductivity,and in that way to simplify the maintenance of the treatmentplant.Several series of experiments were performed using bothwater and leachate solutions. In the series equalconcentrations of ammonium and nitrite were varied uniformly atdifferent pH values for each of these two solutions. In thefirst series, freezing was investigated; the samples werefrozen and then thawed. In the second series, evaporation wasinvestigated; the samples were evaporated at differenttemperatures using different evaporation methods.For these methods, freezing and evaporation, the higher theinitial concentration of ammonium and nitrite in the solutionsof water and the leachate, the greater was the percentage oftotal nitrogen removed. During freezing, for the highestconcentrations and at pH 4, 64 % of ammonium was removed fromthe water solution and about 54 % from the leachate solution.During the evaporation/drying process, for all concentrations,nearly all ammonium was transferred to the gas phase at initialneutral pH-values. This occurs when 95 to 100 % of the liquidis evaporated. The removal of ammonium nitrite was very wellcorrelated with a decrease of conductivity.The proposed system, with partial nitritation followed bychemical denitrification, with natural freezing /evaporationprocesses can be applied as a pre-treatment to existingtreatment systems. It could also be applied as a full-scaletreatment system if technical evaporation with recovery ofmethane gas is used or if the natural climatic evaporation issufficient. Safety risks need to be studied and evaluated;crystals of ammonium nitrite may explode when heated above60-70 ºC, and there are possible side reactions such asthe formation of nitrogen oxides or release of ammonia. Furtherresearch is needed before the proposed system can be applied infull-scale.Keywords:Chemical denitrification; conductivity;evaporation/drying; freezing/thawing; leachate; nitrogenremoval