Clarifier modelling (BOF-PostDoc)

Integrating a Population Balance Model (PBM) for AS flocculation in a hydraulic (CFD) model for secondary clarifiers

Conventional wastewater treatment plants are highly dependent on the functioning of the secondary clarifier and, hence, the sludge settleability in order to obtain a good performance. On the other hand, the more recent membrane technology for separation suffers from membrane fouling, which appears to be related to the size distribution of the sludge flocs. Moreover, floc size and settleability are closely related to the flocculation behaviour of activated sludge. Recently, progress was made in the study of activated sludge flocculation behaviour through the development of an on-line measurement technique based on laser diffraction, enabling the quantification of the time evolution of the floc size distribution (Govoreanu, 2004). This created perspectives for mathematical modelling of the floc size distribution by means of a population balance model (PBM) (Nopens et al., 2002), which typically is formulated in terms of an integro-differential equation and that needs to be solved numerically in most cases. In a recent NSF-project (G.0032.00), a PBM model was implemented in the modelling and simulation software package WEST (Hemmis N.V., Kortrijk, Belgium). The model calibration was performed with experimental data collected within the same project. However, a problem that occurs is the correct description of the interaction between the fluid dynamics and the aggregation- and breakage rates, which are typically assumed to be function of average shear rates. However, the latter can significantly differ from the local shear rates (factor 10-100). In order to improve the accuracy of the flocculation model, there is a need for a prediction of the local shear rates. Models describing the fluid dynamics could provide a solution. This type of models, also known as “Computational Fluid Dynamics” (CFD) models, are commercially available (e.g. Fluent, Fluent Inc., UK, available at the department) and are based on the Navier-Stokes equations that describe the mass and momentum balances. This set of equations can be expanded with an energy balance (turbulence) and a solids transport equation (solids transport and settling). The set of partial differential equations is discretised in space (using a grid) and solved using numerical techniques, which are available in the software. In a recent NSF-mandate (Bob De Clercq, UGent, 1998-2002), a CFD-model of a secondary clarifier was implemented in Fluent and calibrated and validated using in-situ measurements of velocity profiles and floc size distributions (De Clercq, 2003). One of the perspectives of this study was an improvement of the dynamics of the floc size distribution (especially in the low concentration region above the sludge blanket), expressing the need of a PBM. In summary, the aim of this project proposal is to build an integrated CFD/PBM model and apply it to several test cases. The individual expertise of both types of models was already developed in previous research projects. Noteworthy is that similar attempts in other scientific areas, also dealing with particle loaded fluid flows (e.g. crystallisation, fluidised beds), have proven the feasibility and usefulness of such integrated models (Zucca et al., 2004).
B. De Clercq (2003). Computational fluid dynamics of settling tanks: development of experiments and rheological, settling, and scraper submodels. PhD Thesis, Ghent University, Belgium.
A. Zucca, D. Marchisio, A. Barresi and R. Fox (2004). Implementation of the population balance equation in CFD codes for modelling soot formation in turbulent flames. In: Proceedings of the 2nd International Conference on Population Balance Modelling, May 5-7, Valencia, Spain.

Project duration: 2005-11-11 - 2008-11-10

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Ingmar Nopens
Department of Applied Mathematics, Biometrics and Process Control
Coupure Links 653
9000 Gent
Tel: +32 (0)9 264 59 35
Fax: +32 (0)9 264 62 20

Last update: 01 december 2008,

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