ZweckverbandLandeswasser-versorgung
Optimization of a flocculation-sedimentationtreatment plant with lamellas (FLUENT)
Dipl.-Ing. Marian BrendaDresden-Rossendorf
Backround Information
The Zweckverband Landeswasserversorgung providesdrinking water for the north-east of Baden-Württemberg and parts of StuttgartWater delivery 90 Mio m³/aWithin the water treatment plant of Langenau raw water from the riverDanube is purified
Flow diagram of the flocculation-sedimentation
Clear Water
Sludge
Lime Milk
Flocculation Aid
Raw Water(Danube)
Flocculant
Pre-Mixer Accelator
Overview
Capacity 400 to 800 l/sDiameter Accelator: 36 mHeight Accelator: 9,5 m
Measuring the settling velocity
During the settling test the sludgelevel is measured over the timeWithin the first minutes linear settlingtakes placeThereafter hindered settling takesplaceWithin the phase of linear settling theconcentration and settling velocity isconstant settling velocity for a certain concentrationRelatively easy to measure (standardtest DIN 38 414)Height of the sludge level after 30 Minutes is called sludge volume(important parameter in waste watertreatment plants)
Settling curve
0
100
200
300
400
500
600
700
800
900
1000
0 5 10 15 20 25 30
Time (min)
Slud
ge le
vel (
ml/l
)hinderedsettling
linear settling
Settling velocity depending on concentration
Settling velocity depending on concentration
0
0,0005
0,001
0,0015
0,002
0,0025
0,003
0 1 2 3 4
Concentratin [g/l]
Sett
ling
velo
city
[m/s
]
Flow Curve
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
Shear Rate [1/s]
Shea
r St
ress
[Pa]
Rheology
Non Newton fluidShear thinningViscosity depending on concentration
19,4 g/l
14,8 g/l
12,6 g/l10,8 g/l7,2 g/l
Validation of sludge modell
The settling test was simulated withFLUENTThe results were compared with themeasurementsSimulation and measurements fit quite well according to sludge volume
Sludge Volume - Concentration
050
100150
200250
300350
400450
0 1 2 3 4
Concentration (g/l)
Slud
ge V
olum
e (m
l/l)
Accelator
Simulation
The sludge model
Sludge is modeled with a „user defined scalar“ (uds) and contains of following components:
Convection
Diffusion
Viscosity
Density
Settling velocity depends on concentration
Turbulent diffusion depends on density gradients („full buoyancy effect“)
Viscosity depends on concentration and shear rate
Density depends on concentration
The sludge model
Components which will be developed in further researchprojects:
Convection
Viscosity
Parameter Identification
Settling velocity, regarding low concentrations
Viscosity of low concentrated sludge,Yield stress,Time dependent viscosity
Mathematical Optimization of model parameters
Multiphase or Singlephase?
Even though the sludge model is a singlephase approach it interactsintensively with the flow regimeAdvantage: no particle size of flocs required to determine the settlingvelocityModelling of sludge with multiphase approachs also possible, but requirecomplex models to describe the variation and distribution of particle sizesSinglephase approach requires less computational effortsTo model the sedimentation process a relatively long flow time isrequired. Example for this case: 12 cores, 8 Mio cells, 3.600 seconds flowtime, ~30.000 iterations à 20 seconds/iter 1 week per calculation
4 Variations of lamella clarifier:different slope and distance between lamellas (5 or10cm)
Geometry
Grid: between 6 and 13 Mio cells
Scheme
OutletInlet Channel
Lamellaclarifier
Stirrer
(C) by Dr.-Ing. Alexander Sonnenburg
Pathlines
Velocity
Flow regime
Sludge concentration
Density
Coupling turbulent viscosity -diffusion coefficient (Schmidt Number 1.3)
Molecular viscosity
Molecular viscosity vs. turbulent viscosity in the sedimentation zone
Grid effects
13 cell rows betweenlamellas: backflow of highly concentrated sludge
7 cell rows betweenlamellas: no backflow of highly concentrated sludge
Results
Results can be seen only in comparison between the simulations. For validation on the real plant the lamella clarifier has to be built and measurements have to be taken to compare simulation with reality.Sludge concentration at the outlet of lamella clarifier are 2 (10cm distance) to 5 (5cm distance) orders of magnitude lower than withoutThe slope has no significant effect on the flow regime or the outletconcentrationThe grid quality is very important for the resultsFurther research is necessary to improve the sludge model and to verify itwith the real plant
ZweckverbandLandeswasser-versorgung
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