Stalked bacteria in activated sludge

Summary A laboratory-scale activated sludge unit was fed continuously with a simulated industrial wastewater, consisting of a dilute solution of inorganic salts, at a rate giving a mean retention time of about 3 days. The system produced a well-settling sludge which on examination by electron microscopy was found to contain considerable numbers of stalked bacteria. These were identified as Caulobacter, which have the ability to attach to surfaces and other organisms by means of a prosthecal holdfast and to flourish in waters with a low content of organic nutrients, and whose occurrence in activated sludge has not apparently been previously recorded. Conditions advantageous to Caulobacter generally prevail in activated sludge systems when these operate in growth phases tending to produce well-settling sludge. Since their holdfast gives Caulobacter the ability to initiate and enlarge microbial clusters by attachment, it is suggested that Caulobacter contribute to microbial floc-formation in activated sludge.