A new approach for studying the operation of a secondary sedimentation tank (SST)

By Lucio Pezza

Contact: lucio.pezza [at] fastwebnet.it

Any technician who has acquired a considerable amount of experience in managing activated sludge waste water treatment plant (WWTP), even in the absence of specific measures, has no doubt been able to ascertain these two phenomena:

  • with equal loading parameters generally considered for a secondary clarifier (overflow rate and/or solid flux), those with a small diameter offer better results than those with a larger diameter;
  • beyond certain limits, the increase in the amount of return activated  sludge (RAS)  is counterproductive for the functioning of the secondary clarifier, inasmuch as it pumps an excessive turbulence into the system which hinders the sedimentation of the sludge.

These two simple observations led to the idea of studying secondary sedimentation on the basis of the physical phenomenon that represents: a loss of potential energy hindered by the turbulence found in the system. The measures undertaken, beyond the results obtained (which should be researched with additional studies), have demonstrated that said default setting is correct and can lead to a model of the phenomenon of the secondary sedimentation that is closer to reality than the theory of solid flux. The most significant practical results that appear definitively ascertained, for a circular secondary sedimentation tank (SST), are the following:

  1. The thickened sludge moves on the bottom of the SST like a fluid with its own hydro-dynamic characteristics and which, like all fluids, moves from those areas with higher energy potential (the sides of the thickener) to those areas with a lower energy potential (the center of the SST).
  2. In the central part of the SST, due to the high values of the velocity of the incoming flow (with centrifugal direction) and of the outgoing thickened sludge (with centripetal direction) the turbulence is so high as to prevent the phenomenon of sedimentation.
  3. The phenomenon of sedimentation is concentrated, instead, in the side part of the SST, where the ratio between the strength of dragging exercised by the clarified water on the sludge in suspension and the weight of the sludge itself is sufficiently low.
  4. With respect to the above, the effectiveness of a circular SST does not depend on the surface of same but on the cylindrical surface that develops its vertical side wall and, consequently, the capacity for treatment not increased with the squaring of the radius but only in proportion to said radius.
  5. The optimum value of the extent of the RAS flow is equal to 100% of the value of the incoming flow; ratios of higher recirculation worsen the situation because they insert a turbulence in the system which prevents the sedimentation of the sludge.
  6. The balance of mass of a SST (incoming solid mass = outgoing solid mass) is ensured exclusively by the system of sludge extraction; while the  sludge scraper only needs to remove that fraction of sludge that adheres to the bottom of the tank, in order that it no longer responds to the laws of hydro-dynamics.


For additional information about the developed research, see "A new approach for studying the operation of a secondary sedimentation  tank (SST)" Water Practice & Technology, March 2016, 11 (1) 187-197; DOI: 10.2166/wpt.2016.025.

For more information about the author, please visit: http://www.studiopezza.it/ 

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