A method for discharging a contaminated layer floating on the liquid into the liquid reservoir of an air treatment unit for use in a cattle shed, comprising: - a housing provided with: - a stale air inlet opening for the admission of stale air (air dwelling in the shed), - a stale air outlet opening for discharging stale air to the environment outside the cattle shed, - a stale air duct extending in the housing between the stale air inlet opening and the stale air outlet opening, and - a plurality of openings (9, 10) in one or more walls of the housing, - a plurality of air ducts extending in the stale air duct and having open ends connected to the openings, of which air ducts one end forms an outside air inlet opening and ends up in the environment outside the cattle shed and the other end forms an outside air outlet opening and ends up in the cattle shed, - first ventilation means for sucking in or blowing outside air via the air ducts into the cattle shed, - second ventilation means for sucking in or blowing the stale air via the stale air duct into the environment outside the cattle shed, - spraying means (17, 19) for spraying and / or atomizing liquid onto and between the air ducts, which spraying means (17, 19) comprise first spraying means which are present in the housing above the air ducts for spraying liquid onto and against the air ducts for the purpose of cleaning and / or cooling down the air ducts, which first spraying means comprise a first liquid pump and a plurality of spray nozzles, the first liquid pump pumping the liquid from the liquid reservoir to the spray nozzles, and - a receptacle which is present underneath the air ducts for receiving liquid, characterized in that the receptacle is provided with a bottom which is sloping down to the stale air inlet opening, the liquid reservoir is provided on the stale air inlet side of the air treatment unit and the receptacle is connected with its sloping end to the liquid reservoir where a side wall of the liquid reservoir located opposite to the receptacle is lower than the remaining side walls and forms an overflow over which a layer of dust floating on the collected liquid flows into a collecting trough located next to the overflow and is discharged via an outlet pipe connected to the collecting trough, and in that during the operation of the first spraying means the first liquid pump, which pumps the liquid out of the liquid reservoir, is periodically briefly switched off, so that an extra quantity of liquid briefly flows over the overflow and thereby carries along the floating contaminated layer. - a housing provided with: - a stale air inlet opening for the admission of stale air (air dwelling in the shed), - a stale air outlet opening for discharging stale air to the environment outside the cattle shed, - a stale air duct extending in the housing between the stale air inlet opening and the stale air outlet opening, and - a plurality of openings (9, 10) in one or more walls of the housing, - a stale air inlet opening for the admission of stale air (air dwelling in the shed), - a stale air outlet opening for discharging stale air to the environment outside the cattle shed, - a stale air duct extending in the housing between the stale air inlet opening and the stale air outlet opening, and - a plurality of openings (9, 10) in one or more walls of the housing, - a plurality of air ducts extending in the stale air duct and having open ends connected to the openings, of which air ducts one end forms an outside air inlet opening and ends up in the environment outside the cattle shed and the other end forms an outside air outlet opening and ends up in the cattle shed, - first ventilation means for sucking in or blowing outside air via the air ducts into the cattle shed, - second ventilation means for sucking in or blowing the stale air via the stale air duct into the environment outside the cattle shed, - spraying means (17, 19) for spraying and / or atomizing liquid onto and between the air ducts, which spraying means (17, 19) comprise first spraying means which are present in the housing above the air ducts for spraying liquid onto and against the air ducts for the purpose of cleaning and / or cooling down the air ducts, which first spraying means comprise a first liquid pump and a plurality of spray nozzles, the first liquid pump pumping the liquid from the liquid reservoir to the spray nozzles, and - a receptacle which is present underneath the air ducts for receiving liquid, characterized in that the receptacle is provided with a bottom which is sloping down to the stale air inlet opening, the liquid reservoir is provided on the stale air inlet side of the air treatment unit and the receptacle is connected with its sloping end to the liquid reservoir where a side wall of the liquid reservoir located opposite to the receptacle is lower than the remaining side walls and forms an overflow over which a layer of dust floating on the collected liquid flows into a collecting trough located next to the overflow and is discharged via an outlet pipe connected to the collecting trough, and in that during the operation of the first spraying means the first liquid pump, which pumps the liquid out of the liquid reservoir, is periodically briefly switched off, so that an extra quantity of liquid briefly flows over the overflow and thereby carries along the floating contaminated layer.