Abstract Experiments carried out in bioreactor landfill simulators demonstrated

that more than 40% of the total N was transferred into the liquid and gas phases during the incubation period of 380 days. Besides minimal contributions from other N-removal processes, Anammox bacteria were found to be active within the simulators. Anammox is considered to be an important contributor to remove N from the solid matrix. However, it was unclear how the necessary nitrite for Anammox metabolism was produced. Ammonium, an end product of protein degradation and important parameter to consider during landfill closure, tends to accumulate up to inhibitory levels in the leachate of landfills especially in landfills with leachate recirculation. Most efforts to remove ammonium from leachate have been focused on ex-situ and partial in-situ methods such as nitrification, denitrification and chemical precipitation. Nevertheless, little is known about the nature of residual nitrogen in the waste mass and possible mechanisms to remove it. Intrusion of small quantities of O2 is believed to be beneficial for the degradation process of MSW in bioreactor landfills but not sufficient to remove significant amounts of ammonium via nitrification. Volatilisation

Submitted for publication at Environmental Science & Technology as: Ammonium release and Anammox as an option for its removal in bioreactor landfill simulators. Valencia R., van der Zon W., Woelders H., Lubberding H.J., Gijzen H.J. January 2008.