In the activated sludge process, the cause of filamentous bulking is often the filamentous bacteria Type 021N. At the Kitaminato sewage treatment plant, it was found that when the DO in the aeration tank decreased, filamentous bulking occurred. If the low DO condition is allowed to continue, anaerobic degraded organic matter is produced which creates a favorable condition for the multiplication of Type 021N. Entosiphon sp. is reported to show resistance to low DO; however, sometimes before filamentous bulking occurs, Entosiphon sp. itself multiples. Also if Entosiphon sp. increases and Cinetochilummargaritaceum, of the Ciliophora, multiples, then bulking by Type 021N has been seen not to occur. Cinetochilummargaritaceum has low resistance to hydrogen sulfide; therefore, hydrogen sulfide must not be present in the aeration tank and this means that bulking by Type 021N can not become serious. However, if filamentous bulking becomes serious, only increasing the DO level in the aeration tank will not cause the disappearance of the filamentous bulking. At this stage, if we allow 30% of the final effluent to flow back into the grit chamber, then Type 021N decreases. This is because Trithigmostomacucullulus, of the Ciliophora, increases to 3,000 number/ml, and it ingests Type 021N.

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