We have examined the optimum conditions for the growth of ciliated protozoa that feed on filamentous microorganisms, and the development of a method of mass culture and the fixation of this type of ciliated protozoa in activated sludge, in this paper. At first we determined that the ingestion method of filamentous microorganisms by ciliated protozoa differs according to the type but that they have in common a special small oral organ called a cytopharyngeal basket. Then we succeeded in separating and subculturing Trithigmostomacucullulus and Trochililioidesrecta as protozoa that feed upon filamentous microorganisms such as Type 021N, Sphaerotilusnatans causing the bulking from biological sewage treatment plants. By using these predacious protozoa it was clarified that SE medium (activated sludge extract solution) was most effective as a growth factor of T.cucullulus, one of the successfully separated protozoa, and that the optimum growth temperature of T.cucullulus in the presence of SE medium is about 20°C, and that stigmasterol, Vitamin B12 and thiamine are very effective in increasing the predacious protozoa as another growth factor. Furthermore it was made clear that the T.cucullulus and T.recta differed in the rate of ingesting filamentous microorganisms, and the rates ranged from about 1 to 30 microns per second. From these results it can be understood that bulking might be eliminated in a very short period of time, if these protozoa exist in large quantities and ingest filamentous microorganisms at about the same rates.

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