Leachate pre-treatment strategies before recirculation in landfill bioreactors

Nitrified leachate recirculation represents a promising strategy for a more sustainable landfill management. Our objective was to determine the reactions involved in nitrate reduction in municipal solid waste batch biodegradation tests. Anaerobic digestion of waste in the three control reactors showed a good reproducibility. In two test reactors, nitrate was added at various moments of the waste degradation process. We observed that: (1) H₂S concentration controlled the nitrate reduction pathway: above a certain threshold of H₂S, dissimilatory nitrate reduction to ammonium (DNRA) replaced denitrification. (2) N₂O/N₂ ratio varied with the organic carbon concentration: the lower the easily biodegradable carbon concentration, the higher the N₂O/N₂ ratio. (3) N₂ was consumed after denitrification. The possibility of a nitrogen fixation reaction in the presence of NH₄⁺ is discussed. Nitrified leachate recirculation during acidogenesis should be avoided because of higher H₂S production which could induce DNRA.