Composite samples were collected at weekly intervals. The long retention period and the complete mixing concept prevented rapid changes in either the mixed liquor or in the effluent. Weekly samples would make any changes more readily discernible than daily samples. The composite samples were normally collected over a 6 -- hr period, but an occasional 24 -- hr composite was made. Examination of the operations of the shopping center permitted correlation of the 6 -- hr composite samples with 24 -- hr operations. The data indicated that the organic load during the 6 -- hr composites was essentially 50 per cent of the 24 -- hr organic load.

Grab samples were collected from the existing oxidation pond to determine its operating conditions. Efforts were made to take the grab samples at random periods so that the mass of data could be treated as a 6 -- hr composite sample. A single 24 -- hr composite sample indicated that the sewage flow pattern and characteristics were typical.

The BOD of the influent to the pilot plant varied between 110 and 710 mg/l with an average of 350 mg/l. This was equivalent to 240 mg/l BOD on a 24 -- hr basis. The BOD of the raw sewage was typical of domestic sewage from a subdivision. The BOD in the effluent averaged 58 mg/l, a 76 -- per cent reduction over the 24 -- hr period. Examination of the data in Table 1, shows that a few samples contributed to raising the effluent BOD. The periods of high effluent BOD occurred during cold periods when operational problems with the aerator resulted. Ice caused the aerator to overload, straining the drive belts. The slippage of the drive belts caused the aerator to slow down and reduce oxygen transfer as well as the mixing of the raw sewage.