A foaming problem in an industrial-size deacidifier (packed column) within the ammonia-hydrogen sulfide circulation scrubbing process (AS process) on a specific site is experimentally investigated. The foamability of solutions is successfully assessed in a small test cell, and the Bikermann coefficient is calculated to characterize the foaminess behavior. The investigations revealed that filtration of the real sour water taken from the specific site does not alter foaming characteristics. Impurities, especially phenol and its derivatives, cresols, are found to highly contribute to foaminess behavior. Experiments in a pilot plant revealed higher pressure drop and lower flood point data. Critical spots of foam formation are identified, and design suggestions for column internals are given.