Abstract: A tower of contact media, having thereon bio-slimes kept wet and active by a recirculating nutrient liquid, and through which gases, such as foul air of a sewage treatment plant, pass for odor removal by biochemical action aided by liquid trickling downward through the media, and which may be conventional in many respects, is provided with:(a) A gas recirculation system, i.e., means for drawing off gas which has passed once upwardly through the media and reintroducing it for passage through the media with raw incoming gases so that on the average the gases have more than one passage through the media,(b) Means for distributing on a layer of the media higher than the usual recirculation of liquid a solution such as hypochlorite for removal of residual odors left or caused by the recirculating liquid, and(c) Successive sections of media of differing coarseness.