Chapter No. 12. Final settling
In activated sludge reactors, a concentrated mixture of sludge and wastewater is produced and maintained in the biochemical stage for the biological treatment of wastewater. Once sufficient biological treatment is achieved, the sludge is separated from the wastewater in the physical stage, producing a clarified flow stream (secondary effluent). The sludge in the reactor consists of microorganisms (mainly bacteria) and cell debris of a micrometer size that would be normally difficult to separate from the liquid phase. However, the sludge is flocculent in nature and given the right conditions, flocs much larger than the individual cells can be readily formed, allowing the sludge to settle out of the wastewater in settlers located downstream of the biochemical stage. This process is called final settling and takes place in sedimentation tanks widely identified as secondary settlers. Most of the mixture concentrated in the secondary settlers is re-circulated to the biochemical stage and a fraction is disposed of to ensure and keep a desired biomass concentration for the biological treatment. As a consequence, the design, operation and performance of secondary settlers become essential not only to achieve an effluent of good quality but also for the operation of the activated sludge system.
This chapter contains 3 videolectures on "Final Settling" recorded by Dr. Imre Takács, formerly at Envirosim and since 2011 at Dynamite Process Modelling, in Pessac, France. It provides an overview of the processes, design and operating aspects that influence the solid-liquid separation in secondary settling tanks. Special emphasis is given to provide guidance for the design and operation of the phase separation units.
Time Framework for the Course: 1 week
Aims of the Course
- To provide an overview of the (secondary) processes, design, operating and practical aspects that affect the solid-liquid separation that takes place in secondary settling tanks in activated sludge wastewater treatment systems.
After the successful completion of this chapter, the student will be able to:
- To design a secondary settler based on the settling properties of an activated sludge mixed liquor sample following the most widely applied guidelines for the design and operation of secondary settlers from activated sludge systems.