Analyzing a Malfunctioning Clarifier With COMSOL

8/16/2019 Analyzing a Malfunctioning Clarifier With COMSOL

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Analyzing a Malfunctioning Clarifier with

COMSOL’s Mixture Model

Arie C. de Niet ∗,1, Arjen F. van Nieuwenhuijzen 1, and Arthur J. Geilvoet 11Witteveen+Bos, Consulting Engineers ∗Corresponding author: PO box 233, 7400 AE Deventer, The Netherlands, [email protected]

Abstract: In wastewater treatment plantsclarifiers are used to separate sludge andwater. In this paper we analyze a malfunc-tioning clarifier with the mixture model of COMSOL Multiphysics. Because the de-scription of sludge by wastewater engineersdoes not match with the mixture model, we

propose a translation. With proper transla-tion we are able to get model results that arereasonably close to measurements from thereal clarifier. With the model we can explainthe bad seperation of water and sludge inthe clarifier. Engineers proposed severalmeasures to improve the performance of the clarifier. All measures were applied inthe model. It appears that a slight increaseof the flock diameter of the sludge is themost effective measure.

Keywords: mixture model, sedimentation,active sludge, wastewater treatment

1 Introduction

In wastewater treatment plants (WWTP)the influent wastewater is treated with theso-called active sludge process in combina-tion with sedimentation in clarifiers. Activesludge consists of biological micro-cultures

(bacteria) that is able to remove nitrogenand phosphorous from the water. Aftertreatment the active sludge needs to beseparated from the water before the wa-ter leaves the plant, such that the activesludge can be re-used and clear water flowsinto the environment. Separation of ac-tive sludge and water happens in a clari-fier, usually a large circular tank with lowflow velocities that allow the sludge, whichis heavier than water, to settle on the bot-tom, where it is removed for re-use. Clearwater floats over the top at the outer bound-aries of the tank.

Figure 1: The clarifier at the wastewatertreatment plant.

A certain Dutch wastewater treatmentplant has serious problems with one of itsclarifiers. Large amounts of sludge leave

the clarifier with - what is supposed to be -clear water at the outer boundaries of tank.The number of suspended solids exceedsthe desired number with a factor 4 and theactive sludge that is tapped for re-use con-tains too much water. Witteveen+Bos has been asked to detect the causes for this mal-function and to propose measures to im-prove the performance of the clarifier. Tofind out what causes the problems and toverify the proposed measures, the flow inthe clarifier was analyzed with COMSOLMultiphysics 3.4. The malfunctioning clari-fier at study is depicted in Figure 1.

In this paper we describe the model thatwe used and the outcome of the analysis.

2 Modeling the Clarifier withCOMSOL Multiphysics

The clarifier in the WWTP is circular. How-ever, the inner part of the tank is in use fora different step of the process of wastewatertreatment. The actual clarifier therefore hasthe shape of a torus. The bottom has a slightslope towards the centre of the tank.

Excerpt from the Proceedings of the COMSOL Conference 2008 Hannover


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The natural way to do this is

φin = SV I · DS in

≈ 60 [ml/g ds] · 2.5 [g ds/l]

= 0.09 [1].

2.4 Uncertainties in PhysicalQuantities

Unfortunately there is lack of informationon the exact physical conditions in the clar-ifier. Not all relevant physical quantities areknown and few measurements are availableto calibrate the model. We have knowledgeabout the controlled variables like the in-

flow (Qin = 2, 667m3/h) and the return-sludge flow (Qret = Qin/4). Further-more a time-average value is given for theamount of sludge at the outflow: DS out =80 mg ds/l (φout = 0.0028). The de-sired value is four times smaller: DS out =22 mg ds/l (φout = 0.00079).

Since there is no information aboutthe density and the flock diameter of thesludge, we use reasonable values from liter-ature (ρsludge = 1, 025 kg/m3, d = 0.8 mm).Moreover we assume that sludge and wa-

ter have the same viscosity (both ν = 10−3

).In the mixture model we use a volume av-erage viscosity, hence there is no maximumpacking density involved in the computa-tion. The Schiller-Naumann model is usedto compute the slip velocity between sludgeand water.

2.5 Setup of NumericalExperiments

The mixture model for the clarifier is solvedvia a transient simulation. Initially thetank is filled with pure water and in ap-proximately 3 hours the volume fractionof sludge at the inflow is increased to thegiven value φin = 0.09.

Wastewater engineers at Witteveen+Bosproposed several measures to improve theperformance of the clarifier. The effectof the measures is verified in the mixturemodel by changing several parameters: thedensity of sludge ρsludge, the flock diameterd and the position and slope of the deflec-tion plates.

3 Results

In all model runs the tank reaches equi-librium after approximately 48 hours. With

the chosen values for density (1, 025 kg/m3

)and flock diameter (0.8 mm) we getan amount of sludge at the outflow of 460 mg ds/l, which is more than six timesthe measured average value of 80 mg ds/l.The flow pattern at equilibrium and the dis-tribution of sludge in the tank is depictedin Figure 3. The streamlines show a shortcircuit flow from the inflow to the return-sludge conduit. It is very likely that thisshort circuit flow causes the high levels of dry solids at the outflow. This is supported by the observation that the short circuitflow only occurs after 36 hours. The firstone-and-a-half day the flow is more likeFigure 4 (more about that figure later), thenin a short period of time the flow changesinto the given flow pattern. The transitionleads to a slight decrease of DS ret, howeverall particles eventually have to leave thetank, so the increase of DS out is sever.

It is very likely that the short circuitflow is caused by the torus-shape of thetank. In a more common circular clarifier

the mixture leaves the deflection plates witha higher velocity. The flow has more mo-mentum; hence it will not sink immediatelyas happens in the torus-shaped tank.

Another disadvantage in the design of the tank is the position of the return sludgeconduit. The transition of the flow patternas observed is probably triggered by theaccumulation in the corner nearby the con-duit.

Even though it would probably be

very effective, it is extremely expensiveto change the geometry of the tank. Thesame holds for adaptations to the return-sludge conduit. So other, cheaper measuresto change the flow pattern had te be found.Unfortunately the flow pattern appears to be quite persistent. Small changes to the ge-ometry did not help. The deflection plateswere moved in horizontal or vertical posi-tion, the length and slope was increased, but there was no significant reduction of DS out.

Fortunately the two other measures ap-


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peared to be effective in the model. An in-crease of the sludge density to 1, 050 kg/m3

or an increase of the flock diameter to d =1.0 mm diminishes DS out. Of these two

measures an increase of the flock diameteris most effective: DS out becomes 10 mg/lwhich is below the target value. One mightwonder if it is possible at all to change thesephysical properties of the sludge. The an-swer is: yes, flock forming can be altered by adding certain chemicals to the mixture before it enters the clarifier. Better pack-ing of sludge particles results in higher den-sities and stimulation of growth of flocksleads to an increase of the flock diameter.In Figure 4 one finds the flow pattern andthe distribution of sludge in the clarifier for

rho = 1, 025 kg/m3 and d = 1.0 mm. Theflow pattern differs a lot from the patternin Figure 3, especially the short circuit flowdisappeared.

4 Conclusions

Even though there was lack of informationon the exact physical parameters in the real

clarifier, it seems to be possible to builda proper model using the mixture modelof COMSOL Multiphysics. The proposedtranslation of ”sludge volume index” and”sludge content” to a volume fraction of sludge appears to be a reasonable choice. Atleast the modelled clarifier suffers from thesame high number of dry solids at the out-flow.

Accoording to the model, the most effec-tive measures are an increase of sludge den-sity and diameter. Changes to the deflectionplates do not help.

References

[1] Comsol multiphysics 3.4, chemical engi-neering module, user’s guide, 2007.

[2] E. Loth, Computational fluid dynamics of bubbles, drops and particles, CambridgeUniversity Press, 2008, in preparation.

Figure 3: Suspended solids (g/l) and streamlines in the clarifier at equilibrium with flock diameter0.8mm


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Figure 4: Suspended solids (g/l) and streamlines in the clarifier at equilibrium with flock diameter1.0mm

Analyzing a Malfunctioning Clarifier With COMSOL

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