INTRODUCTIONhe purification of water isone of the most importantenvironmental tasks today.

This includes the physico-chemicaltreatment of natural water for thesupply of drinking water and thebiological treatment of wastewaterbefore its discharge into nature.The status of wastewater treatmentvaries from country to country.Most of the industrialized countrieshave a very good coverage inwastewater treatment plants withmore than 90 % of the inhabitantsconnected; whereas less developedcountries continue to suffer from alack of sewage collection systems,water-, and wastewater treatmentplants. The quality of the wastewatertreatment can be segmented into thefollowing types of treatment:

1. Primary Treatment:Mechanical treatment usingscreens and primary sedimentationto separate solid matter.

2. Secondary Treatment:Includes the oxidation of biode-gradable organics, usually referredto as BOD1 and COD2 by meansof additional oxygen. The mostcommonly applied process is the so-called activated sludge process.

3. Tertiary Treatment:Includes the biological reduction ofBOD and COD plus full nutrientremoval. In this case nitrates andammonia are also removed by usinganaerobic processes. In these pro-cesses the wastewater needs to bemixed without the addition of oxygen.

In the EU tertiary treatment for fullnutrient removal is obligatory for allmember states since May 1991(EEC, 1991). A standard whichcurrently has not been reached inall member states. Many plants stillhave to be built and many have tobe upgraded - a situation which istypical for the status of wastewatertreatment in many countries in theworld. This is the reason why theactivated sludge process includingmixing and aeration have becomea key topic in wastewater treatment.

he activated sludge processstands for a large variety ofcontinuous, space and time-

oriented periodic processes. Asmentioned before, the activatedsludge process is used for BOD-removal and complete nitrogenremoval. Well-known variants arethe preliminary nitrification/denitri-fication and the post-denitrification(Metcalf and Eddy,1991). Well-known batch process variants arethe Sequencing Batch ActivatedSludge Reactor (SBASR), the CyclicActivated Sludge System (CASS) andthe Intermittent Cycle ExtendedAeration System (ICEAS) which arecovered in detail in the literature:Irvine, 1971; Wilderer andSchroeder, 1986 or Metcalf andEddy, 1991. All these biologicalpurification processes as well as theknown standard activated sludgeprocess include two major oper-ations: mixing and aeration. Withoutmixing and aeration an effectivereduction of BOD and nitrogen isimpossible. Since approximately70% of the total energy consumptionis used for these two processes(Höfken et al., 1995) a properchoice and design of such systemsis indispensable. Hence, onlyinnovative and energy-efficientequipment should be used.

The above reasons clarify theimportance of discussing the roleand impact of mixing and aerationprocesses on the biological treatmentprocess. The paper at hand focuseson mixing and presents the basicdesign rules for the selection andlayout of mixers for activated sludgetreatment plants. Using the exampleof the design of mixers for a largewastewater treatment plant it isshown how an intelligent selectionand design of mixers can help toimprove the reactor behavior andreduce the investment and operatingcosts of a plant.

ABOUT MIXINGGENERAL

review of the mixingsystems available on themarket for wastewater

treatment reveals an impressivenumber of different products (Höfken,1993). In order to somewhat classifythe large variety it is possible torefer to the definitions of the basicmixing tasks as they are used in thefields of chemical and processengineering. A distinction is madein process engineering between thefollowing basic mixing tasks:

➣ HomogenizationCompensation of differences inconcentration or temperature

➣ SuspensionWhirling up and suspendingof solid particles

➣ DispersionLiquid/liquid > Emulsions, po-

lymerizationLiquid/gas > Aeration,

mass transfer➣ Heat transfer

Intensifying of the heat trans-mission (cooling, heating)

This breakdown leads to a classifi-cation of all the different types ofmixing systems available on themarket today that are used forwastewater treatment and enablesan example of application inwastewater treatment to be foundwithout difficulty for each mixingtask shown in table 1.

Mixing is one of the most importantunit processes used in the treatmentof water- and wastewater. Figure 1shows a typical flow diagram of awastewater treatment plant andillustrates in which treatment stepsmixing is, or can be involved.

The process starts in the mixing andequalization basin where mixers are

used to homogenize temperatureand load peaks. Flash-mixing whichmeans fast and intense mixing isneeded in the neutralization cham-ber. Precipitation and flocculationprocesses also strongly depend onthe correct design of the mixingequipment. During precipitationmixers shall mix the precipitant fastand thoroughly with the wastewaterwhereas during flocculation themixers shall supply gentle mixingwith low shear to allow for a goodsuspension and build up of flocs. Inthe anaerobic or anoxic processesmixers for "Biological PhosphorusElimination" and "Denitrification"have to supply a flow-field whichtakes care that no sedimentationoccurs at the bottom and whichtotally suspends and homogenizesthe sludge floc. An additional criteriais the retention time and reactor typebehavior of the tanks which isinfluenced severely by the choiceand the design of the mixers. Thisis one very important applicationfor mixers which will be the focussedon in this paper and discussed inmore detail in the following chapters.Mixing also plays a major role inthe aerobic steps in which BOD is

removed and nitrification is achieved.Either aeration is realized usingmixers which disperse the air inbubbles which then transfer theiroxygen into the water or membraneaerators are used to produce bubbles.In the latter case the airflow itselfhas to provide sufficient mixing toavoid sedimentation of sludge flocand to prevent short-circuiting. Theimportant role of mixing in aeratedtanks is highly underestimated inmany designs.

Furthermore mixing is needed in thetreatment of waste sludge. This showsthat in almost all treatment stepsmixing plays an important role andcan decide about the quality of theoverall treatment result.

The critical point can be defined intwo ways: by selecting a point onthe bottom or on the wall outsidethe area of influence of the corner.Own investigations (Höfken, 1994)have shown that the axial speedsnear the wall directly after thechange of direction are even slightlyhigher than the maximum measuredradial bottom velocities.The influence of the corner flow canbe eliminated by selecting a distanceof a=L/10 from the wall and adistance b=L/30 from the bottomfor both critical points. There is nodanger of deposits forming in thecorner vortex as higher velocitiesand turbulences occur here, whichprevent particle deposits providedthe minimum velocity calculatedbelow is observed. The possibilityexists here, however, to reduce to aminimum the losses that occur asa result of vortex formation byfurnishing the corners with concretehaunches. Both definitions of thereference point can be used to thesame equivalent extent. For the initialtheoretical calculation of theminimum bottom velocity givenbelow, however, it is simpler to selecta point on the bottom at a distanceof L/10 because boundary layerobservations are easier here.

Two limit cases can be formulatedto estimate the forces exerted by thefluid on the particles contained inthe boundary layer at the bottom:

1. Only the wall shear stress actson the particles

2. A tractive force is active with velocity v as a result of the flow.

According to Latzel (1986), limitcase 2 leads to a condition for thelimit case of the minimum suspensionof a single particle (ϕp→0). Sincethis case is not of any technicalrelevance, it is not considered here.We shall consider case 1, the effectof the wall shear stress. The behaviorof the turbulent velocity profile nearthe wall can be derived from thePrandtl-mixing length, which can bestated in general as follows basedon Levich (1962):

where uτ is the so-called wall shearstress velocity and y+ is the dimen-sionless wall distance. The followingdefinitions apply:

(2)

(3).

In the case of an even or slightlyarched bottom, the following formulaapplies for the local wall shear stressaccording to Schlichting (1965):

(4).

For boundary layers that areturbulent from the start the followingformula can be obtained by recal-culation in contrast to the proposalby Latzel (1986):

(5).

This then leads to an implicitcalculation method for uoo. Equation(5) is used for further calculation.The Reynolds number is formed herewith development length l of theboundary layer. Length l is depend-ent on the mixer design and on thetank size.

(6)

If (3), (4) and (5) are used in (2),an equation is obtained after con-version for determining the flowvelocity at the critical point, whichcorresponds to the minimum bottomvelocity.

(7).

In order to calculate this explicitly,an equilibrium of forces must nowbe created between the shear stressforces and the gravity forces actingon the particles.

(8).

Use of (2) in (8) results afterconversion in an equation betweenthe dimensionless units Reτ and Ar:

(9)

(10)

By calculating the Archimedes'number it is possible to calculate uτ

and use it in (7) as the missingvariable. Various equations can beapplied for length l of the boundarylayer depending on the mixingsystem and the method used to createthe flow. Figure 3 shows theminimum bottom velocity for variousparticle diameters as a function oflength l=f (tank size, method usedto create the flow).

It can be seen that bottom flowvelocities of 15cm/s are certainlyadequate enough to stir up activatedsludge and keep it suspended. Therecommended design particle sizeis dp=100 µm (middle curve), thereal value for samples from a greatnumber of wastewater treatmentplants was dp = 60 µm as depictedin the bottom curve. Bottom velocitiesof 10cm/s therefore should also besufficient, however without any safetyagainst variations. This statementdoes not contradict the recommend-ation of the ATV3 to maintain bottomvelocities of 10 - 30 cm/s, but clearlyshows that a demand for minimumbottom velocities greater than30cm/s is unjustifiable. Evenmaximum particle sizes do notdemand bottom velocities greaterthan 20cm/s. It is fully adequate,therefore, even taking possibledensity fluctuations into account, toapply bottom velocities greater than15cm/s.

Fluid Mechanical Considerations

Basic Considerationsenitrification and biologicalphosphorus elimination areanaerobic processes, i.e.

oxygen-transfer via the free surfaceis undesirable and negatively affectsthe biological processes. A certaindegree of turbulence promotes thework of bacteria since the floc re-mains limited in size and is providedto a greater extent with substrate asa result of a larger surface area andthe occurrence of periodic stressconditions. The main requirement isto distribute the activated sludge flocas evenly as possible and to avoiddead zones and short circuiting inthe reactors.

These preliminary observations raisethe question of the optimum flowconditions in the activated sludgetank. In order to prevent the activatedsludge floc from settling on thebottom of the tank, it makes senseto produce the maximum possibleflow rates together with a certaindegree of turbulence near to thebottom. If we look at the macro-scaleflow pattern it is sufficient here tocirculate the tank volume uniformlyin order to achieve the most evendistribution; too much turbulence inthis case would have a negativeeffect on efficiency. It is especiallyimportant to avoid surface turbu-lence, since this would increaseoxygen-transfer capacity via the freesurface.

If we look at the micro-scale flowpattern, higher turbulent fluctuationswhich can whirl up sedimentationsare desirable at the bottom of thetank.

Since the majority of sewagetreatment plants are built withrectangular tanks, the flow in thistype of tank will be examined herein more detail. The design of mixingsystems in carousel and circular tanksis described in Höfken, 1993. Thedesign rules are similar to the onespresented below.

It has been deduced that the mostfavorable energy conditions areobtained for suspension using tankbottom mixing systems arrangedcentrally in the tank. Clarification isneeded on the question of theminimum flow velocity required atthe bottom of the tank.

Analytical ApproachIn order to arrive at a practicalmethod for calculating theminimum bottom velocities in a

reactor it is necessary to initiallydefine the critical point for depositsin the reactor under question. It isshown that the critical area isnormally close to the edge of thetank at the point where the horizontalflow is changing to the verticaldirection. Here the bottom boundarylayer reaches its full length and hasnormally increased to such an extentthat a greater number of particlescan penetrate the viscous sublayerdue to the zones of lower flow ratespresent close to the bottom (wall). Itis known from experience that thecorner flow itself is not a criticalarea with respect to flow patternsproduced by radial mixing systemsnear to the bottom because an eddyflow with a strong upward compo-nent occurs as a result of slowingdown the rotation flow, preventingthe formation of deposits at this point.The flow patterns shown in Figure 2are obtained from an examinationof the change of direction of the flowat the wall and on the right handside a single particle in the bottomboundary layer is shownschematically.

Figure 2: Change of direction of flow near the walland a single particle in the bottom boundary layer

A b o u t t h e D e s i g no f M i x i n g S y s t e m sf o r A n a e r o b i c a n dA n o x i c B a s i n s f o rL a r g e W a s t e w a t e rT r e a t m e n t P l a n t s

Marcus Hoefken, Walter Steidl, and Peter Huber

re

se

ar

ch

&d

ev

el

op

me

nt ABSTRACT

In the field of wastewater treatment efficient mixers

are needed to suspend solids and to homogenize in large mixing

and equalization tanks, and especially in anaerobic and anoxic

tanks for full nutrient removal in the activated sludge process.

The layout and design of such mixing systems has to

take into account process and the reactor design.

Otherwise the plant cannot perform properly

and according to the specification.

The paper at hand discusses the basic demands

on a mixing system. It presents the basic design rules

for the layout and design of mixing systems for

anaerobic and anoxic basins. Using the example of a large

wastewater treatment plant it is shown that the mixer

design can influence the reactor behaviour positively

if the design is chosen correctly. Finally some mechanical

aspects of a mixer design are discussed.

KEYWORDS

Mixing; Plug-flow reactor; Complete-mix reactor

Activated Sludge Process, Wastewater treatment.

i n n o v a t i o n f o r n a t u r e

intr

od

uc

tio

n

ab

ou

t m

ixin

g

T A

I

D

T

1 BOD: Biological Oxygen Demand2 COD: Chemical Oxygen Demand „The important role of mixing in aerated tanks

is highly underestimated in many designs.”

3 ATV: German Association for Waterand Pollution Control

biological phosphorusremoval, denitrification

Storm water tanks,sludge tanks

Biological phosphorusremoval, denitrification

(Mainly in carouselbasins)

Biological phosphorusremoval, denitrification,BOD-removal, nitrifi-cation, intermittentprocesses, sludgetreatment

BOD-Removal,Nitrification

BOD-Removal,Nitrification, sludgetreatment

BOD-Removal,Nitrification, sludgetreatment

Sludge digesters

HorizontalPropeller mixers

(slow-speed)

HorizontalPropeller mixers

(high-speed)

Blade mixer

Hyperboloidmixers andmixer/aerators

Surface aerators

Self-aspirating sub-merged aerators

Self-aspiratinghelical aerators

Digested sludgemixers

Homogenization,Suspension

Homogenization,Suspension(higher-viscosityfluids)

Homogenization,Suspension,Dispersion

Homogenization,Suspension,Dispersion,Heat transfer

Suspension,Dispersion

Dispersion

Dispersion

Homogenization

Mixer type Schematic Mixing task(s) Applications

u m w e l t u n dv e r f a h r e n s t e c h n i k

INVENT Umwelt- und Verfahrenstechnik GmbH & Co. KG • Am Weichselgarten 36 • D-91058 Erlangen, GermanyPhone +49 (0) 9131 / 6 90 98 - 0 • Fax (0) 9131 / 6 90 98 - 99 • www.invent-uv.de

Table 1: Overview of mixing systems

Note: The mixing tasks marked in bold type should preferably be performed using the mixing system assigned to themin the above table. In more than a few cases this may be inconsistent with normal applications in wastewater treatment.

Figure 1: Mixing applications in wastewater treatment

Figure 3: Minimum bottom velocity as a functionof tank size

u m w e l t u n dv e r f a h r e n s t e c h n i k

INVENT Umwelt- und Verfahrenstechnik GmbH & Co. KG • Am Weichselgarten 36 • D-91058 Erlangen, GermanyPhone +49 (0) 9131 / 6 90 98 - 0 • Fax (0) 9131 / 6 90 98 - 99 • www.invent-uv.de

Process Considerations

he two reactor types mostcommonly used in thebiological treatment of

wastewater are the plug-flow reactorand the continuous-flow stirred-tankreactor or complete-mix reactor.

Figure 6: Plug-flow reactor

In a plug-flow reactor the fluidparticles pass through the tank andare discharged in the same sequencein which they enter. The particlesretain their identity and remain inthe tank for a time equal to thetheoretical detention time. This typeof flow is approximated in long tankswith a high length-to-width ratio inwhich longitudinal dispersion isminimal or absent.

Figure 7: Continuous-flow stirred-tank reactor

Complete mixing occurs when theparticles entering the tank aredispersed immediately throughoutthe tank. The particles leave the tankin proportion to their statisticalpopulation. Complete mixing can beaccomplished in round or squaretanks if the contents of the tank areuniformly and continuouslydistributed.

Figure 8: Continuous-flow stirred-tank reactor in series

A series of complete-mix reactors isused to model the flow regime thatexists between the hydraulic flowpatterns corresponding to thecomplete-mix and plug-flow reactors.If the series is composed of onereactor, the complete-mix regimeprevails. If the series consists of aninfinite number of reactors in seriesthe plug-flow regime prevails.

In comparison to the standard caseFigure 19 shows the resulting axialand radial forces on a hyperboloid-mixer. In this case only downwarddirected forces act on the shaft whichcannot bend the shaft and almost noradial forces occur due to the closedmixer-body with a high number ofsingle vanes. This is a significantadvantage since the forces actingon the shaft, gear-drive and bridgeare much lower and - even moreimportant - they are directed in theharmless direction. Therefore thedesign for the shafts, gear-drives andbridges can be much lighter forhyperboloid-mixers.

Figure 19: Axial and radial forces acting on ahyperboloid-mixer

CONCLUSIONSo achieve optimum purificationpower in wastewater treatmentplants, advanced and efficient

mixing and aeration systems haveto be used because mixing andaeration are the major processesneeded in the biological treatmentof wastewater. Approximately 70%of the total energy demand of awastewater treatment plants isneeded for these processes (Höfkenet al., 1995).

A study of systems available on themarket showed that numeroussystems are offered which can beclassified and compared using somesimple considerations. It was shownthat efficient mixing systems shouldbe bottom mounted and fluidmechanically optimized. Minimumbottom velocities of approx. 15 to20 cm/s are sufficient in most of theapplications.

The design of mixing systems alsoinfluences the overall reactor be-havior of a wastewater treatmentplant. Therefore it is crucial to selectthe mixing system in compliance withthe initial process design.

For the mechanical design of mixersit is important to understand the mainforces acting on a mixing systemand the bridges. There are severedifferences between standard pro-peller mixers and modern hyper-boloid mixers since the resulting axialforces on the mixing system aresignificantly less harmful in the caseof hyperboloid mixers. This resultsin a lighter design and longerlifetime.

ACKNOWLEDGEMENTSWe would like to thank the Berlin WaterAuthorities, who awarded us with thecontract for the engineering studies andthe supply of 96 HYPERCLASSIC ®-Mixersplus bridges, for the professional supportand the excellent cooperation.

Mechanicalhe mechanicaldesign of mixers mainlyconcerns

1. Mechanical forces on the mixerelement, e.g. propeller blades

2. Mechanical forces on the shaft

3. Mechanical forces on thedrive, mainly gear-reducer andbearings

4. Mechanical forces on the bridgeor support for the mixer

The mechanical forces on the mixer-element or the blades are causedby the flow-field around the mixer-element. The resulting force normallycan be split in a normal-componentFn and a tangential-component Fz.These two components are relevantfor the structural design of the mixer-element. Since this design is mixer-dependent we will not further discussit at this point.

For the shaft design it is morecommon to split the resulting mainforce into an axial-component FAXand a circumferential component FU.The sum of all circumferential forces- the number is dependent on thenumber of single blades or vanes ofthe mixer-element - times the validradius results in the total torqueacting on shaft and gear-drive.Unfortunately the single forces actingon each blade or vane do not actregularly on the blades but are, dueto the turbulent flow stochasticallyacting on the blades. This is thereason why for all kind of mixer-elements a resulting radial force FRoccurs. FR is a very important forcefor the design of the shaft and thegear-drive. FR is the bigger thestronger the turbulent trailing vorticesbehind the blades are and itdecreases with the number of bladesor vanes. This means that a two-blade propeller mixer has a muchhigher resulting radial force than afour-bladed propeller mixer or ahyperboloid-mixer with 8 transport-vanes.

Figure 18 shows the resulting axialand radial forces on a standardpropeller-mixer. The induced flow-field results in an upwards directedaxial force and in a high radial forcewhich tends to bend the shaft. Thismeans high mechanical loads onthe shaft, the gear-drive and thebridge. These type of mixers demandfor very heavy design of shafts, gear-drives and bridges.

Figure 18: Axial and radial forces acting on astandard propeller-mixer

igure 10 shows one whole tankin a cut through the symmetryplane. The color contour-plot

visualizes the magnitude of thevelocities in z-direction (upwards/downwards-flow). By this measurethe velocity distribution between themixers can be illustrated very nicely.It can be clearly shown that there isa strong upwards flow between eachpair of mixers which clearly indicatesthe division of the long tank intosingle cells. If one compares theflow-pattern at the overflow of thereal separating wall and at the sidewall to the flow between each pairof mixers, this impression becomeseven stronger. This unique flowpattern is called the production ofvirtual walls between two mixerswhich can only be achieved withhyperboloid mixers. This was onereason why HYPERCLASSIC ®-Mixerswere chosen for this project.

Figure 11 shows the virtual wallbetween one pair of hyperboloidmixers in more detail. To clearlyshow the similarity between theoverflow at the real wall and theflow-pattern at a virtual wall in Figure11 and Figure 12 the color-contourplots are shown together with thecorresponding velocity vectors.These graphs illustrate the typicalflow pattern caused by hyperboloid-mixers with the radial floworiginating from the mixer body overthe bottom, going up the wall andreturning to the center of the mixedcell. Both graphs, for the real wallsimulation and for the virtual wallsimulation look identical. This provesthat hyperboloid mixers can createa series of complete-mix reactorswithout building separating walls.This effect cannot be achieved withany other type of mixer and is verybeneficial to the plant design. Realseparating walls normally cause highoverflow velocities which areundesirable because they increasethe probability of short-circuits onthe downstream side of the wall.Real separating walls also contributeto higher total hydraulic losses ofthe plant.

It is very important to also knowwhether the detention time reachesthe theoretical detention time andwhether short circuits occur in sucha tank. To examine this a particletracing study was done to derive thedetention time distributions. In totalmore than 20,000 particles wereadded to the inflow of the tank andtraced until they left the tank throughthe outlet. Figure 14 shows 25trajectories which enter the basinfrom the left side. The color of thetrajectories indicate the detentiontime. Figure 15 shows one singleparticle on its way through the tank.It very nicely shows how the virtualand the real separation walls retainthe particle and increase thedetention time.

The statistical analysis of all particletrajectories finally results in theoverall detention time behavior ofthe reactor which in this case is veryclose to the theoretically expectedvalue. This shows how the properdesign of a reactor and the selectionof mixers can be achieved usingmodern fluid mechanical engineeringtools. Additionally tremendousinvestments in civil works could besaved because no separating wallshad to be built.

Example Iigure 4 shows the velocity fieldof a hyperboloid mixer in a400mm x 400mm lab-scale

tank. The measurements wereexecuted using ultrasound-Doppler-anemometry. Similar results can beobtained with laser-Doppler-anemometers. Such diagrams canbe used to determine the mixerspeed needed to create a givenvelocity at a given point in the basin.The velocity vectors show themagnitude of the velocity and thedirection of the flow. In the turbulentregime they are directly proportionalto the tip speed of the mixer andcan therefore be used for designpurposes.

It is important to validate scale-upswhich are based on experiments inmodel-scale. Figure 5 shows acomparison of the results obtainedin the model-tank with results froma real wastewater treatment plantin the Eiger-Northface region inSwitzerland, which was equippedwith geometrically similarhyperboloid-mixers.

The comparison shows excellentagreement between model and largescale. More than 5 years trouble-free operating experience of 24HYPERCLASSIC ® -Mixers support theexact design and layout of thesemixers.

F T

i n n o v a t i o n f o r n a t u r e

F

TThe above considerations also showthat it is unrealistic to specifyparticular power inputs, since thesecannot guarantee particular bottomvelocities. The power input requiredis dependent on the mixing systemand on the type of flow generationas described above. To try to ensureadequate bottom velocities byspecifying minimum power inputs isonly possible if very high andunnecessary safety factors areaccepted. Just how drastically thedemand for excessive bottomvelocities affects the overall powerconsumption and thus the operatingcosts of a plant is described below.

The following equation can be usedto describe the relationship betweenthe bottom velocity at the criticalpoint and the rotational speed ofthe mixing element

(11)

where constant C is dependent onthe type of flow generation and onthe mixing system. At this point it isimportant to note that the minimumbottom velocity consists of theaverage velocity and the turbulentfluctuations according to

(12)

If the turbulent fluctuations areneglected at the design stage, safetyis assured but the powerconsumption will increase. Thefollowing equation applies to thepower:

(13)

where Ne is the power or Newtonnumber (a variable dependent onthe mixing element, see table 2).Table 2 shows Newton numbers forvarious mixers for the Reynoldsnumber range Re>104.

Table 2: Newton numbers of standard mixers

Use of (11) in (13) taking (12) intoconsideration results in

(14)

The rotational speed and thus thebottom velocity are cubed in thepower equation (i.e. even smallincreases in specified bottom velocitycause the power consumption toincrease at very high rates). Equation(14) also shows that it is morefavorable to choose slow-speedmixing systems with large diametersthan small fast-running systems.A further advantage concerns thesmaller shear stress acting on thebacteria floc, but this point will notbe discussed here.

If the speed range and the turbulentfluctuation speed of a mixing systemas well as constant C are known, itis thus possible to implement acomplete design for the mixingsystem for activated sludge tanks.The exact procedure is described inHöfken, 1994. One way todetermine the constant C is to domeasurements in model and in largescale. The second possibility is touse numerical simulations like shownin the next paragraph. Ideal is thecombined application of bothmethods. In this case the experi-mental work is used to validate thenumerical simulations. The numericalsimulations ensure the exact scale-up.

The choice of the reactor type is onevery important design step whichdepends on several operationalfactors such as: reaction kinetics,oxygen-transfer requirements, natureof the wastewater, local conditionsetc.. This means that the activatedsludge tanks, which are biologicalreactors were at one stage of thedesign process consciously or atleast implicitly designed as a plug-flow or a complete-mix reactor or acombination of both. This has to betaken into account when the mixingequipment for the reactors is chosen,otherwise the selection and designof the mechanical equipment mayjeopardize the reactor design.

Case study Ihe following case study showsan example of a largewastewater treatment plant

with 6 parallel trains which eachconsist of again 2 parallel lines ofanaerobic, anoxic and oxic tanks.The anoxic zones were designed asso-called cascaded denitrificationtanks which consist of a series ofcomplete-mix reactors. In an ex-tensive engineering study whichincluded experimental and numericalfluid mechanics (CFD4) the optimaltype and number of mixers wereselected. One very importantadditional task was to clarify whetherthe series of complete-mix reactorscould be realized without buildingwalls between the mixers as wasinitially planned.

Figure 9 shows three of the six trainswith the chosen arrangement of themixing equipment. Each basin is110m long, 8m wide and 4.1mdeep (water depth). As the optimummixing system 8 HYPERCLASSIC ®-Mixers were chosen for each basinof which the last 2 units weresupplied as mixer and aerationsystems to allow for additionaloxygen supply during the coldseason. This so-called bivalent zonewas separated by a physicalseparating wall to avoid back-mixingof oxygen-rich wastewater into thedenitrification zone. In the left partof the drawing the anaerobic zonesin which biological phosphorusremoval takes place are shown.

The following graphs show resultsof the 3-dimensional computersimulation which had been executedin the design phase to find theoptimum design and to prove thereactor behavior. The simulationconcerned the flow created by therotation of the mixers as well as theinflow of wastewater and recircu-lation-flow. More than 2 million gridpoints were used to model the tankand the mixers. The rotation of themixers was modeled by using theso-called sliding grid technique.

T

Figure 14: Numerical simulation of an activatedsludge tank with 6 + 2 HYPERCLASSIC®-Mixers showingdetail of 25 trajectories of particles following the flow

Figure 15: Numerical simulation of an activatedsludge tank with 6 + 2 HYPERCLASSIC®-Mixers showingdetail of one trajectory following the flow

Figure 13: Numerical simulation of an activatedsludge tank with 6 + 2 HYPERCLASSIC®- Mixers showingdetail of virtual separation wall between one pair ofmixers

Figure 12: Numerical simulation of an activatedsludge tank with 6 + 2 HYPERCLASSIC® -Mixers showingdetail of real separation wall between one pair ofmixers

Figure 10: Numerical simulation of an activated sludge tank with 6 + 2 HYPERCLASSIC®-Mixers comparing virtualand real separation walls

Figure 11: Numerical simulation of an activatedsludge tank with 6 + 2 HYPERCLASSIC®-Mixersshowing detail of virtual separation wall betweenone pair of mixers

Figure 16: Numerical simulation of an activatedsludge tank with 6 + 2 HYPERCLASSIC®-Mixers -Detention time distribution

Figure 17: Numerical simulation of an activatedsludge tank with 6 + 2 HYPERCLASSIC®-Mixers -Picture of real plant with a total of 96 mixers

Mixing System Newton Number, Re > 104

Propeller mixer, 3-blade, α = 25° 0,35

Blade mixer, 6-blade 5,00

Hyperboloid mixer, 8 transport ribs 0,20

Hyperboloid mixer, 8 transport ribsand shear ribs for aeration 0,50

Figure 4: Velocity field of a hyperboloid mixer close to the bottom of a squared tank

Hyperboloid

Bottom velocity

Figure 5: Measured bottom velocities in modeland in large-scale

Figure 9: 60 of in total 96 HYPERCLASSIC®-Mixers in a large wastewater treatment plant

4 CFD: Computational Fluid Mechanics

T

................................................

HYPERCLASSIC ® is a registeredtrademark of INVENT Umwelt- undVerfahrenstechnik GmbH & Co. KG.

REFERENCES

EEC Directive of the 21st of May 1991 about the Treatmentof Municipal Sewage (91/271/EWG), Official Gazetteof the EEC, No. L135/40, Brussels.

Höfken, M. (1993). Die Bedeutung der Rührtechnik in derAbwasserreinigung. Abwassereinigung Alte Probleme -Neue Lösungen, LSTM-Seminar 23./24. June 1993 inSulzbach-Rosenberg, Oberpfalz, Germany.

Höfken, M. (1994). Moderne experimentelle Methoden fürdie Untersuchung von Strömungen in Rührbehältern undfür Rührwerksoptimierungen. Ph.D.-thesis completed at theLehrstuhl für Strömungsmechanik (LSTM) of the Friedrich-Alexander-University Erlangen-Nürnberg.

Höfken, M., Bischof, F. and Durst, F. (1995). Energy Savingsin the Biological Treatment of Sewage.The Future of the Baltic Sea, Metropolis Verlag.

Irvine, R. L. and Davis, W.B. (1971). Proc. 26th. Ann. Ind.Waste Conf., Purdue Univ., Ann Arbor Sc., Michigan.

Latzel, W. (1986). Mindestrührerdrehzahlen zumSuspendieren von Feststoffpartikeln. Ph.D.-thesis completedat the Institute for Mechanical Process-Technology of theFriedrich-Alexander-University Erlangen-Nürnberg.

Levich, V. G. (1962). Physicochemical Hydrodynamics.Prentice-Hall International, Inc., United Kingdom and Eire.

Metcalf and Eddy (1991). Wastewater Engineering -Treatment, Disposal and Reuse. McGraw Hill Inc.

Schlichting, H. (1965). Grenzschichttheorie. 5. erw. Aufl.,G. Braun Verlag, Karlsruhe.

Wilderer, P. A.; Schroeder, E. D. (1986). Anwendung desSequencing Batch Reactor Verfahrens zur biologischenAbwasser- reinigung. Hamburger Berichte 1986.

SYMBOLS

a [m] distance from the wallb [m] distance from the bottomcw [--] friction number (particle)dP [m] particle diameterdR [m] diameter of mixerg [m/s2] ground accelerationl [m] developing length of

boundary layern [Hz] number of revolutionsu [m/s] velocityu(y+) [m/s] velocity in boundary layeruTip [m/s] tip-speed of mixeruτ [--] wall shear stress velocityu [m/s] minimum bottom velocityu [m/s] mean value of minimum

bottom velocityu' [m/s] turbulent fluctuationsv [m/s] velocity which acts on an

particle inside the boundarylayer

y [m] wall distancey+ [--] dimensionless wall distance

Ar [--] Archimedes' numberC [--] constantFW [N] drag forceFrP [--] Froude numberH [m] water heightNe [--] Newton numberRe [--] Reynolds numberRel [--] Reynolds number (l)ReP [--] Reynolds number (dP)P [W] Power

δl [m] thickness of boundary layerϕP [%] volume fraction of particlesν [m2/s] kinematic viscosityρL [kg/m3] density of the liquidρP [kg/m3] density of particles∆ρ [kg/m3] difference in densityτW [N/m2] wall shear stress

88

8

re

se

ar

ch

&d

ev

el

op

me

nt

„The design of mixing systemsinfluences the overall reactor behaviourof a wastewater treatment plant.”

co

nc

lus

ion

sreal separating wall (design)

virtual separating wall (design)