A treatment plant which removes air pollutants using microorganisms contained in a support material. The incoming air passes upwards through the filter bed, which consists of particles of organic material, on the surface of which forms a thin aqueous layer, or «biofilm». In the biofilm resident microorganisms, which metabolize the pollutants contained in the air inlet, through a reaction of biological oxidation, which uses the oxygen content in the air. The support material for biomass also provides other essential nutrients. The degradation products are constituted by water vapor, carbon dioxide and biomass generated by bacterial metabolism. With the use of suitable organic support materials, such as compost or plant substrates. The air humidity is essential to ensure the permanence of the biofilm and consequently the microbial population. The nutrients that are necessary to supplement the diet required for the metabolism of the biomass, are supplied directly to the support material. Finally, it is essential to ensure adequate ventilation of the support material in all its volume, to facilitate the diffusion of oxygen of the biofilm.