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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