A cooperative project to study the performance of Dowpac HCS, a gas-liquid contact tower packing, as a biological slime attachment surface for secondary treatment of sewage at the City of Battle Creek, Michigan, was initiated in December, 1955. The report contains a summary of data obtained and calculations based upon the data. An equation was developed from the data relating performance as a function of depth and the hydraulic application rate. From the available data, it was not possible to distinguish a relationship between performance and organic application rate. Indications were that within the narrow range of organic application rates studied, variations resulting from daily fluctuations in waste strength, efficiency was independent of organic loading.