One of the important aspects of this study was to determine the oxygen transfer relationships of the mechanical aerator. Routine determinations were made for dissolved oxygen in the mixed liquor and for oxygen uptake rates. The data given in Table 2, show the routine operation of the aerator. The dissolved oxygen in the aeration unit was consistently high until January 29, 1961. An extended cold spell caused ice to build up on the aerator which was mounted on a floating platform and caused the entire platform to sink lower in the water. The added resistance to the rotor damaged the drive belts and reduced the oxygen transfer capacity. It was approximately one month before the belt problem was noticed and corrected, but at no time was there a deficiency of dissolved oxygen.

A series of eight special tests were conducted at different rotor speeds to determine the oxygen transfer rate. Five of the tests were conducted with a polyethylene cover to simulate an ice cover. The rate of oxygen transfer at 1.0 -- mg/l dissolved oxygen concentration and 10 ` C for various rotor speeds is given in Table 3,. The maximum rate of oxygen transfer at 1.0 mg/l dissolved oxygen was calculated as 220 lb/day at a maximum rate of 9.3 mg/l/hr. The actual power requirements indicated 2 lb oxygen transfer/hr/hp. The polyethylene cover reduced the oxygen transfer rate by 10 per cent, indicating that the maximum oxygen transfer is at the rotor rather than through the surface.