The condensate is removed via a membrane condensate drain (5). Via the lower shuttle valve (8) the air is led into desiccant cartridges (1) in which the air is dried down to a pressure dew point of -40°F (equivalent to a remaining water content of 110 ppm). In the following activated carbon purification stage (9) oil vapours, hydrocarbons, taste and odours are adsorbed to a level far below 0.003 ppm.