The major objectives of this study were to demonstrate the feasibility of the Deep Shaft biological treatment process and to evaluate its application for the treatment of municipal wastewater. A 757-cu m day (0.2-mgd) pilot plant facility was constructed at the existing wastewater treatment plant site in Ithaca, New York, for this purpose. Performance of the Deep Shaft process was evaluated under a variety of operating conditions including raw wastewater and primary effluent as influent sources, constant and diurnal (varying) flow patterns, with and without polymer addition as a flotation aid, and with alum addition for a short period for phosphorus removal. Pilot plant influent characteristics were not typical of domestic raw wastewater or primary effluent as had been anticipated in selecting Ithaca as the project site as a result of the practice of returning partially ground screenings and abnormally strong anaerobic digester supernatant to the main plant headworks.