A biotrickling filter with Nitrosomonas europaea immobilized on polyurethane foam is proposed for treating ammonia contaminated air. The effect of the surface velocity of the recirculation medium, nitrite concentration, pH, empty bed residence time (EBRT) and ammonia inlet load on the NH3 removal process was investigated. The total amount of biomass immobilized on the carrier was 3.29 +/- 0.52 x 1010 cells g(-1) dry carrier. The maximum elimination capacity of the biotrickling filter was 270 g N m(-3) h(-1) at pH 7.5, an EBRT of 11 s, and nitrite concentrations below 100 mM. These results show that system studied can be considered as a viable alternative for the treatment of gaseous emissions containing high concentrations of ammonia. (C) 2008 Elsevier Ltd. All rights reserved.