Model system of bulking and flocculation in mixed culture of Sphaerotilus sp. and Pseudomonas sp. for dissolved oxygen deficiency and high loading

A mixed continuous culture system was made up as a model for bulking and flocculation phenomena of the activated sludge to study the effect of dissolved oxygen (DO) deficiency and the effect of high organic loading. The system consisted of a floc forming bacterium and a filamentous bacterium which were isolated from the activated sludge and were identified as Pseudomonas sp. and Sphaerotilus sp., respectively. Sphaerotilus sp. had potential to cause a filamentous bulking phenomenon on the activated sludge. It was observed that the filamentous microorganism showed three kinds of growth form, filamentous form, pellet form and dispersed form, and that the floc former showed two kinds of growth form, good floc form and dispersed form. In the model system, these changes of growth form of two microorganisms, which could be thought as the cause of settling characteristics changes, depended on the DO level and the dilution rate (as a substitution for organic loading). The DO level also influence the aggregative ability of each microorganism and the maximum oxygen uptake rate, Q_O2max, of filamentous microorganism. The proportions of both microorganisms in model system were inverted reversibly by the DO level or the dilution rate changes.