BACKGROUNDAnaerobic ammonium oxidation (anammox) is a promising approach for removing nitrogen from landfill leachates and other nitrogen-laden wastewater. Although heavy metals, which are known for their microbial toxicity, are commonly present in those wastewater streams information on their inhibitory impact on anammox activity is still limited. The main aim of this study is to investigate the inhibitory effects of six common metal ions (Cu2+, Cd2+, Ni2+, Zn2+, Pb2+, and molybdate) on anammox activity. RESULTSThe soluble concentrations of cationic metals decreased due to precipitation reactions. Cu, Zn, Cd, and Ni were the most toxic metals with 50% inhibiting soluble concentrations of 4.2, 7.6, 11.2, and 48.6mgL(-1), respectively. Molybdate and Pb2+ were not or only moderately inhibitory at the highest soluble concentrations tested (22.7mg Mo L-1 and 6.0mg Pb L-1, respectively). A noncompetitive inhibition model successfully described the relationship between microbial inhibition and metal concentrations for all tested metals except for Pb. CONCLUSIONCu, Zn and Cd were the most toxic heavy metals for anammox bacteria. The soluble concentration of these heavy metals needs to be monitored carefully before applying anammox processes. (c) 2014 Society of Chemical Industry