The suspended solids discharged in the effluent were found to be the major source of the BOD. Removal of the suspended solids by a membrane filter yielded an average effluent containing only 20 mg/l BOD. The BOD in the drainage ditch receiving the pilot plant effluent averaged 12 mg/l. This low BOD was due to removal of the excess suspended solids by sedimentation since the only dilution was surface runoff which was very low during this study.

Routine microscopic examinations were made of the mixed liquor as indicated by McKinney and Gram for the various types of protozoa. It was found that the aerated lagoon was an activated sludge system rather than an oxidation pond. At no time were algae found in the mixed liquor. The bacteria formed typical activated sludge floc. The floc particles were all small as the heavier floc settled out.

Initially, the flagellated protozoa predominated, but they soon gave way to the free swimming ciliated protozoa. As the temperature decreased, the number of free swimming ciliated protozoa decreased. Very little protozoa activity existed below 40 ` F. When the temperature reached 32 ` F all protozoan activity ceased; but as the temperature rose, the numbers of protozoa increased rapidly. Only once were stalked ciliates found in the mixed liquor. The predomination of free swimming ciliated protozoa is indicative of a high bacterial population.