In a plant for purifying contaminated air, the contaminants are transferred from the air to a liquid flowing through a spray booth through which the contaminated air also flows in intimate contact with the liquid. The liquid is then collected in a container, from which it is recirculated to the spray booth. The closed liquid circulation circuit in flow communication with the spray booth is connected to a biological cleaning step in which the contaminants dissolved and/or suspended in the liquid are decomposed and then separated. The biological cleaning step comprises a bioreactor in which the main portion of the bio-degradation occurs, and a separator, for separation of the biosludge coming from the reactor. At least part of the contaminated liquid passing through the spray booth is continuously transferred to the biological cleaning stage from which substantially the same quantity of the cleaned liquid is recirculated to the spray booth circulation circuit.