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Chapter No. 11. Bulking sludge


Activated sludge, the most commonly applied wastewater treatment technology, consists of two stages: (1) a biochemical stage, where organic matter, nitrogen and phosphorus are removed by biological means, and (2) a physical stage, generally composed of a secondary clarifier, where bacteria are removed by settling in order to produce a clarified effluent. Since bacteria form flocs which can be separated from the treated wastewater by gravity forces, settling in secondary clarifiers is the standard technology applied for solid-liquid separation. Bacteria removed in the secondary clarifier are retained in the system and recirculated to the biochemical stage in order to ensure and keep certain biomass concentration to perform the biological removal processes. Overall, a good separation (settling) and compaction (thickening) of activated sludge in the secondary clarifier are necessary conditions to guarantee a good effluent quality having a strong impact on the effectiveness of the activated sludge process.

Under certain conditions, the excessive growth of filamentous bacteria severely reduces the settling properties of the sludge. This phenomenon, described as bulking sludge, is a common and long standing problem in activated sludge systems where suspended solids cannot be maintained in the settler affecting the overall efficiency of the plant.

This course consists of 3 video lectures recorded by Prof. Mark van Loosdrecht from Delft University of Technology, in Delft, The Netherlands, that cover fundamental aspects and different theories formulated to explain the occurrence of filamentous bulking sludge, as well as potential remedial design and control aspects to suppress the growth of filamentous organisms.

Time Framework for the Course: April 6th to 12th, 2015.

Aims of the Course

  • To discuss and describe the filamentous bulking sludge phenomena, its overall effects and consequences on the process performance of activated sludge wastewater treatment systems.
  • To present an overview of relevant historical aspects on the development of activated sludge systems and their relationship with the occurrence of filamentous bulking sludge.
  • To analyze the fundamentals on the relationship between morphology and ecophysiology of filamentous microorganisms as well as on their identification.
  • To discuss different theories formulated to explain the filamentous bulking sludge phenomenon on the basis of lab- and full-scale observations and potential remedial actions.
  • To introduce recent developments on mathematical modelling of bulking sludge and on aerobic granular sludge as an opposite phenomenon to bulking sludge.

Learning Objectives

  • To understand the bulking sludge phenomenon in activated sludge systems and apply potential design and control measures to suppress the occurrence of filamentous bacteria in activated sludge wastewater treatment systems.
Last modified: Tuesday, 3 March 2015, 1:12 PM