Ammonia can be removed from landfill leachate through aerobic biological processes. The biological aerated filter (BAF) combines biological treatment and subsequent biomass separation in one reactor providing a small footprint alternative to conventional systems. Leachate from an operational landfill was found to be aerobically treatable using the OECD recommended Modified Zahn- Wellens test. This leachate was used as feed to a pilot-scale BAF at influent COD and ammoniacal-nitrogen concentrations of 765 mg/l and 568 mg/ l respectively. During an initial period of stable operation without pH control, 33 % of influent ammonia was removed. The reactor pH was 9.2 with little conversion to total oxidized nitrogen (<45 mg/l). Therefore this removal was accounted for primarily by air stripping. In a second period of stable operation, the reactor pH was reduced to pH 7.2 by addition of hydrochloric acid. Ammonia removal was increased to 97 % with a concomitant increase in effluent nitrite concentration to an average of 524 mg/l. Biological aerated filters can therefore be used to nitrify landfill leachates
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