Evaluation of bulking and foaming problems in activated sludge – Identification of filamentous microorganisms

The activated sludge process, the most widely used in wastewater treatment, relies on the development of a complex ecosystem of microorganisms, consisting of numerous groups of bacteria and higher microbial species, which are in an often-fragile population balance. The main objective of such a system is, on the one hand, to achieve a satisfactory removal efficiency of the target pollutants contained in the incoming wastewater and, on the other hand, to form sludge flocs with appropriate sedimentation characteristics, such as, in the final stage of secondary sedimentation, sufficient separation of solids and treated effluent takes place.

Therefore, biomass composition plays an important role in the performance of an activated sludge system, and the most important and arduous operating problems are often caused by the unbalanced growth of different groups of microorganisms.

In the case of sludge bulking, the sludge that develops in the system settles very slowly and shows poor thickening at the bottom of the secondary sedimentation tank, while the phenomenon of foaming often causes operational problems both in the process and in the mechanical equipment of the plant. These two problems are the most important problems encountered in more than half of the world’s WWTPs.

The common cause of these undesirable phenomena is the growth of filamentous microorganisms, which are so called because their cells grow linearly resulting in the formation of filaments. Filamentous microorganisms can be identified by microscopic examination of biomass samples and distinguished by their morphological characteristics.

Criteria for their identification include the length of the filaments, the attachment mode on the sludge flocs and the response to specific tests in a microscopic field of view (Gram and Neisser staining, sulphur test etc.). The identification of filamentous bacteria is critical because the growth and dominance of each species in an activated sludge system is favored under specific operating and environmental conditions.

Therefore, through microscopic identification, the root causes of sludge bulking and/or foaming phenomena are determined, thus making their mitigation or elimination feasible.