Odorous H2S and NH3 gases were effectively biodegraded in a horizontal biotrickling filter (HBTF) packed with H2S-exhausted carbon. During the process, the operational characteristics including the distribution of degradation products, biomass accumulation and biological activity, carbon surface characteristics and pressure drop were investigated. The results show that the content of biodegradation products on carbon was low, i.e. 0.9-2.8wt% S and 0.3-1.0wt% N. The low content benefited the stable operation of the HBTF, due to preventing a toxic concentration of degradation products on packing bed over a long-term operation. The biomass was distributed evenly along the HBTF. This avoided the problems of bed clogging and activity loss. On the other hand, the deteriorated performance was observed due to the biomass accumulation over a long-term operation. Carbon surface characteristics in the HBTF remained almost unchanged. pH values of carbon were neutral and micropore structure of carbon remained relatively stable. In addition, the pressure drop in the HBTF was very low. These operational characteristics of the HBTF system significantly contributed to efficient co-removal of H2S and NH3 in the HBTF over a long-term operation. (C) 2009 Elsevier B.V. All rights reserved.