The formation of bulking sludge is abetted whenever there is a limitation in a parameter that is essential for the development of the sludge. Often, there is a lack of organic carbon which, due to the demand for nitrification requires a low specific sludge loading rate and thus a high solids retention time (or sludge age).

However, it may also be a lack of nitrogen or phosphorus or micronutrients, especially in industrial wastewater. This is resulting in the high growth of filamentous organisms. In such a shortage scenario, the filaments have an important advantage over the floc-forming bacteria because of their large surface area. They maintain higher access to the limiting substances that the floc-forming bacteria are able to do. This also applies to in case of a lack of oxygen supply.

Bulking sludge became a widespread problem after the demand for nitrogen elimination was passed on to the general majority of all wastewater plants. In the past, when it was all about the elimination of organic carbon, the phenomenon of bulking sludge was virtually unknown.

If the activated sludge has a granular sludge structure (S::Select^® process), then the filaments have no opportunity to establish themselves due to the morphology of the granules. Even in the suspended fraction of such a sludge, the filamentous organisms can hardly a niche for growth because the non-granular solids have a very low sludge retention time (3-4 days).


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