Assessomg the Performance of Secondary Settling Tank by Means of Flow Modeling

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ABSTRACT
A model of the double-decker rectangular settling tanks proposed for the Wastewater Treatment Plant of Besòs in Barcelona, Spain was prepared using Computational Fluid Dynamics (CFD). The modeling approach used was a three dimensional, multi-phase simulations with solids transport and removal included, and reflected the state of the art in clarifier modeling. Special attention was paid to the inlet baffles and outlet weirs in the model setup. The techniques used within and the general results of the model will be presented. The results include vector plots, sludge blanket levels, effluent solids concentrations and detailed residence time distribution (RTD) curves.

INTRODUCTION
The Wastewater Treatment Plant of Besòs in Barcelona, Spain is a covered treatment plant that incorporates compact technologies in its process train. One of the more important of these elements are the double-deck rectangular secondary settling tanks. Although there is design and operating experience with these tanks, it is limited and the Besòs installation is a large facility with a design flow of 590,000 m3/d (156 mgd). To verify the overall design of the settling tanks and to test specific aspects of their design, the Empresa Metropolitana de Sanejament S.A. (EMSSA), the owner-operator of the facility, decided to model the tanks using Computational Fluid Dynamics (CFD). The use of CFD in the modeling of secondary settling tanks is not new, but it is still in its infancy. To insure a state of the art model, EMSSA contracted Fluent France, S.A. to perform the actual modeling. The model was to be a complex, three dimensional, multi-phase simulations with solids transport and removal included. Special attention to the inlet distribution and outlet weirs was required in the model setup.

Of specific concern were the inlet structures with their energy dissipating and distribution baffles and the outlet weirs. Specifically, two weir arrangements were compared, a single end wall weir and a double weir arrangement.

GOALS AND OBJECTIVES
The main objective of the study was the comparison of the two outlet weir arrangements. To accomplish this it was felt that a three dimensional, multi-phase model of the settling tanks would be necessary. A model of this type would permit that many secondary objectives could also be achieved. Secondary objectives include being able to study in detail the hydrodynamics of the settling tanks, visualizing the effects of the inlet and outlet arrangements as well as flow path lines, sludge blanket levels, and detailed information on residence time distribution for all flows, treated water and returned activated sludge. To achieve these secondary objectives, which really guarantee that the main objective is achieved, many process and CFD issues needed to be addressed.

Principal among these issues are the following:

  • Modeling of inlet baffles (energy dissipating and distribution)
  • Modeling of solids settling and concentration
  • Modeling of sludge transport and removal
  • Residence time distribution (RTD)

How each of these issues was resolved is the main theme of this paper.

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