Influence of Mixing Parameters and Water Quality on the Flocculation of Kaolinite With Aluminium Sulphate

273

INFLUENCE OF MIXING PARAMETERS AND WATER QUALITY ON THE FLOCCULATION OF KAOLINITE WITH ALUMINIUM SULPHATE

K.J - FKANCOIS, and N. V. BEKAERT Research Center, B-8850 Zuievegem, BELGlllM

ABSTRACT Floc distributlons are calculated from a sedimentation curve. Together with the experimental set-up and procedure, the calculation of the floc diameters and the evaluation of the methodology are given. From the change of the average floc diameter of the suspension as a function of the coagulant dose, a reference dose can be deduced. With the reference dose, the largest flocs posiblem the given process conditions are obtained. This reference dose is a characteristic of the process, an objective parameter and useful as a basis for further research. The flocs formed with the reference coagulant dose are not only the largest flocs possible in the corresponding conditions, but they have also proved to be the strongest. The influence of the different mixing and water quality parameters on the reference dose. sludge production, floc buildup time, and zeta-potential is given and discussed. The zeta-potential of the formed hydroxide flocs does not yield any information for a feed-back control of the coagulant dose. The adsorption behaviour of salts dissolved in the water is investigated.

1. INTKODUCTION A fundamental study of the mechanisms of a coagulation-flocculation process requires knowledge about t h e characteristics of both t h e formed flocs and the process. To investigate the influence of the different process parameters o n the different characteristics, an objective coagulant dose must be used. T h e majority

274

of investigators use an arbitrary coagulant dose as the basis for their research. This explains the sometimes contradictory results and conclusions found in literature. In an attempt to find an objective coagulant dose imposed by the process, we have measured the influence of the dosage on the floc dimensions and some other characteristics. None of the commercial available particle sizers could fulfil the requirements of the investigation. The e1ectrot;ensitive techniques were not useful because the particles usually have to be dispersed in an electrolyte solution. A change of the ionic strength will affect the formed flocs. The commercial available apparati are based on sedimentation work with small sample volumes. N o controllable process conditions can be established in such a small sample. Sampling a suspension from another flocculator was impossi1)le because of the change of the sample during sampling. A third important group of particle-sizing techniques are the optical techniques. Apart from the microscopic or photographic techniques which destroy the sample, are time consuming, and are unable t o measure small flocs, two important measuring techniques are available. Although these techniques, the light blocking technique and the technique using diffraction patterns, flocs are to small. have advantages the maximum detectable A sedimentation experiment has been established as a commercial apparatus is not available. The sedimentation takes place in the flocculator itself. This way, the flocs can be formed under accurately controlled circumstances.

2. IMPORTANT PARAMETERS IN SIMULATING A COAGULATION-FLOCCULATION PROCESS

A coagulation-flocculation process consists of a rapid mixing and a slow mixing step. In the fiist phase, the coagulant has to be mixed homogeneously with the suspension. This promotes the coagulation. The slow mixing promotes the flocculation by creating collisions between the destabilized particles, giving rise to the growth of flocs. In addition to the mixing intensities, the duration of the mixing phases are important. The slow mixing is continued until the flocs are completely formed. Apart from the kinetic process parameters, the raw water quality plays a dominating role. The water parameters to control are pH, temperature, quantity and type of suspended particles, and electrolytes. Table 1 gives the conditions used in the experiments mentioned in this text.

275 Tab. 1 . Conditions of the different process parameters during the experiments Process conditions of the experiments Water quality parameters

75 mg/l suspension of kaolinite in distilled water temperature: 25 OC pH:

7 .O

Mixing conditions duration of the rapid mixing:

60 s

shear rate during rapid mixing:

389 s-l

shear rate during slow mixing:

34 s-l

3. EXPERIMENTAL SET-UP The flocculator is a cylindrical 5-1 jar (16.50 cm diameter) with a grid paddle (d . d = 7 . 7 cm), the lower side 4 cm above the bottom of the flocculator. To obtain a small energy spectrum during the mixing, no baffles were used. This way, the average velocity gradient (G) during the mixing is a good measure for the mixing intensity (1,2). The relation between rotations per minute and average velocity gradient for 4-1 suspensions in the above mentioned geometry is:

G = 0.07. RPM1.46

(20OC)

Fig. 1 gives the flow sheet of the combined experiment. During flocculation, samples of the flocculating suspension are taken by means of 2 pipettes. Through the detection cell of a flow-through turbidimeter they are recirculated to the flocculator. During the second part of the experiment, samples of the flocculated fluid are taken only by means of 1 pipette. This pipette is graduated so that the depth of the aspiration point of the sampler can be read. As a quick drop of the water level in the jar has an unfavourable influence o n the quiscent sedimentation, sampling is done using a lower flow rate and increasing the total flow rate by adding a stream of distilled water before being led to the turbidimeter. This is necessary from the experimental point of view. In order to have a swift response of the recorded signal, a high flow rate ( > 130 ml/min) through the detection cell is preferred. To prevent

276

interference with sedimentation, measurement of t h e flow rate. Before and

the

complete

stream

is

discharged

after

after t h e experiment, the tubing is rinsed with distilled water.

4 mark can be given by means of a contract-breaker hetween t h e turbidimeter and t h e recorder.

11 d i s t i 11e d water

171

.

L31

F i g . 1. Flow sheet of the experiment: 1 - mixer; 2 - pH control; 3 - temperature control; 4 - pump; 5 - turbidimeter; 6 - switch; 7 - recorder; I - coagulation-flocculation; z - sedimentation; 3 - cleaning

4. EXPEKIhlENTAL PROCEDURE T h e procedure is illustrated with t h e help of an idealized registered curve

(Fig.

a).

First t h e turbidity of the water which the suspension is prepared is measured (A + B) to yield a baseline. O n moment B (signal) t h e actual sampling of the stirred suspension is started using both pipettes. O n point D t h e turbidity of the suspension is measured. Time BC is t h e residence time of t h e fluid from t h e pipettes to t h e sampling cell. O n moment D (signal) the coagulant is added, followed by NaOH to adjust pH. The flocculating suspension is pumped in closed circuit through t h e turbidimeter (D -+ F). When flocculation is complete the cell is rinsed with distilled water ( F -+ G). After having

277

aspirated the flocculated suspension (G -t H), 1 pipette is cut off and the suspension is mixed with distilled water (H + I). The turbidity of the suspension is registered in point I. On moment I (signal) the mixer is stopped and the depth under water level of the sampling point is noted. After 15 to 20 minutes of sedimentation (I -t J) the cell is rinsed again with distilled water (K-t J).

h

.r( U

'cl

.d

P

3

U

A

F i g . 2. Idealized turbidity path

5 . INTERPRETATION OF THE REGISTERED TURBIDITY CURVE 5 . 1 . FOULING OF THE SAMPLING CELL

Turbidities in G and K after substraction of the turbidity in €3 give the fouling of the sampling cuvette. During the calculations, the registered turbidities are corrected in a linear way between the points B and G and the points G and K. 5.2. FLOCCULATION

The turbidity of the suspension depends on the character of the reflecting surface. One can compare turbidity values only when the method of measuring

278

is the same and when the particles have similar reflecting properties. Path + F in Fig. 2 gives the change in turbidity during the flocculation. The initial turbidity degradation D - t E can be explained as an adsorption coagulation. The kaolinite lamellae are grouped into card-house type of flocs by an edge-to-face aggregation. The area of the reflecting surface decreases and the reflecting properties of the surface are not significantly changed. This phase ends within a minute after dosage. In phase E + F the aluminium hydroxide flocs are formed. These flocs have an average sphericity of 0.8 (3--5). The surface of gelatinous hydroxide flocs have completely different reflecting properties. Apart from that, the turbidity increases because of the enormous increase of reflecting area caused by the formation of insoluble hydroxides. Flocs are considered as full-grown when they reach a constant value point F.

D+ E

5 . 3 . SEDIMENTATION

The start of sedimentation t = 0 equals point I + BC o n the curve. The knowledge of the total flow rate through the sampling cell and of the turbidities in H and I enables us to calculate the flow rate of sample drained away from the jar. Knowing the geometry of tht: jar, the drop of water level in the flocculator can be calculated. In other words, the sedimentation depth as a function of sedimentation time is known and the sedimentation velocity in each point of the sedimentation curve can be calculated. The decrement of turbidity between two points o n the curve is a measure for the quantity of surface with equivalent diameters between these corresponding to both limits. The conversion from surface fraction to volume fraction and weight fraction is plausible.

6. CALCULATION OF FLOC CHARACTERISTICS Newtons law for sedimentation can be used as basis for the calculations (6):

H'=

4

(.-.

g

D '

.

Pf -- Pw

-___

.

d f p

pw

This formula is valid for Reynolds numbers up to 50 and for sphericities from 1 to 0.8. As the gelatinous hydroxide flocs have an avarage sphericity of 0.8 (3--5), the drag coefficient can be approximated in laminar conditions (Re < 0.1) by ( 7 - 4 ) :

with:

Re = -

df . v

. Pw

7)

For the calculation of the floc density, Tambo’s model has been chosen (3--5):

Pe = -

with:

pe =

a

(df) pf

(4)

KP

- pw

a = 0.0013

--

0.0011 . (In(ALT) + 2)

Kp= 0.9 + 0.528 -(ln(ALT) + 2) ALT = --

aluminium ion concentration dosed suspended particle concentration

This model is valid for pH values between 6.5 and 8.0 and for normal alkalinity values. The sedimentation velocity can be corrected for repulsive forces because of the surface charge of equal sign of the flocs. The relation between the reduced sedimentation velocity of the particles and their zeta-potential is, after conversion to MKSA units (10, 11):

For practical reasons the zeta-potential is expressed in mV. After combination equations 1--9 the following is obtained:

.of

280

a This non-linear equation can be solved easily for the equivalent floc diameter using numerical Newton-Raphson iteration. With the determined diameter, the effective density and porosity of the flocs can be calculated:

An example of t h e calculated results obtained for flocculation under t h e conditions mentioned in Table 1 and with a coagulant dose of 3.738 mg(AI)/I is given in Figs.

3-6. In Fig. 3, t h e dependence of floc density and floc porosity with floc dimension is clearly demonstrated. Flocs larger than 80 pm have porosities of more than 90%. Fig. 4 shows how t h e reflecting surface is distributed over t h e different flocs

-

c

c

Hl

0

2.65 2.50

; 0

a

$

v1

rR

2.00

w

r

09

\

1.50

gU

W

”1’400 200

0

h-

L 600

f l o c diameter Fig. 3. Floc density

(0)

and floc porosity

(0)

8

1.oo 0

(urn)

as a function of the floc diameter

281

in the suspension. From this diagram the distribution on volume basis is calculated. This distribution is shown in Fig. 5. With the use of the floc density relation, the distribution on weight basis (Fig. 6) is deduced from the volume-based distribution.

floc diameter

(urn)

Fig. 4. Distribution of reflecting surfaces

Fig. 5. Distribution of floc volumes

f l o c diameter (urn).

282

floe diameter (pm) Fig. 6. Distribution of the flocs o n weight basis

7. EVALUATION OF THE METHODOLOGY 7.1. INTRODUCED SIMPLIFICATIONS

7.1.1. Constant scattering coefficient By using turbidities for calculating the light scattering surface between 2 limiting dimensions, the scattering coefficient is assumed to be constant. In reality, the scattering properties depend on the structure of the reflecting surface, and thus on floc structure. A relation between scattering coefficient and floc structure is unknown. A link with floc structure is provided by the floc porosity. The porosity of flocs increases steeply with increasing floc diameter (Fig. 3). Consequently, only small flocs are expected to have deviating scattering coefficients. Although tendencies noted with weight-based and volume-based average floc diameters are the same, volume-based for the rest of the investigation. The impact of the averages will be used fractions of the smallest flocs o n a volume basis is much smaller than on a weight basis.

7.1.2. Hindered sedimentation The sedimentation of the particles is assumed to be free. The correction of the measured sedimentation velocity because of the concentration of sedimentating

283

particles is neglectable. This is demonstrated by the equation of Richardson and Zaki (12):

. df

with n

= n' + 19.5

and

n'

= 4.65

for

R e < 0.2

n'

= f(Re)

for

0.2 < R e < 500

n'

= 2.39

for

500< Re

.-

D

The influence of the second part of the right-handside of the equation increases with floc diameter. However the Reynolds number during sedimentation increases and thus n' decreases. Calculations for sedimentating hydroxide flocs in the conditions of the experiments have demonstarated that n = 4.65 is a good average value, indifferent of floc size. Michaels and Bolger (13) also accepted an average value of 4.65 for n. They also proved the validity of the of Richardson and Zaki for flocculated suspensions. equation The porosity of the flocculated suspensions in the experiments is always smaller than 0.005. This value is measured with a Malvern Particle Sizer type 2200 when very high coagulant doses were used. Therefore, the correction for the sedimentation velocity is always smaller than 1.02.

7.1.3. Sedimentation flocculation The influence of sedimentation flocculation on the final average floc diameter is small. The rate of collisions between 2 groups of particles because of a difference of sedimentation velocity can be calculated with (14): Ni

. N. . J

(Vi

--

In Table 2 some calculated results are taken together. When larger and smaller flocs collide with each other, they can form a new floc which is not much larger. When a floc of 2360 p m agglomerates with one of 680 p m , the new floc has a diameter of only 2379 p m , Only aggregations of large flocs have an impact on the average floc diameter. However the number of large flocs

284

is reduced, as is t h e difference between their sedimentation velocities. Therefore, t h e number of collisions per unit of volume and per unit of time is reduced. Because of their fast sedimentation, t h e time available for contacts is also very small. Finally, not every collision gives rise to a n agglomeration. The collision efficiency factor for t h e process conditions given in Table 1 is about 0.06. (15,16). From t h e results in Table 2 it is clear that t h e sedimentation flocculation may be neglected, or there exists a real chance of agglomeration with a neglectable impact o n the average floc diameter, or, for large flocs, t h e chance of agglomeration is rather small.

Tab. 2. Influence of sedimentation flocculation on the flocs formed under the conditions given in Table 1 (coagulant dosage = 5.02 mg(AI)/I; destabilization constant

a.

=

0.06)

Diameter of the fractions

N

(Pm)

(-)

(s)

(cm/s)

(-)

24QO ...2360 ...2320

23

24

0.398

9.4

720

... 680 ... 640

984

55

0.174

1.2

3480

...3440 ...3400

3

9

1.111

38

...3280

5

15

0.666

22

3360 ...3320

t

Re

V

Diameter

No. collisions ((I .s)-')

No. aggre ations

5

new floc

(1- )

(Pm)

368

9

2379

1.3

4260

2.4

7.1.4. Flow regime of sedimentating particles

Using t h e linear relation between drag coefficient and Reynolds number, a pure laminar flow pattern o f the scdimentatirig particles is implied. For spherical particles, flow is considered as laminar for Reynolds number smaller than 0.1. For Reynolds numbers u p t o 1, t h e linear relation with drag coefficient is generally accepted. Several investigators accepted the linear relation as far as Re = 10 (17). For particles with a sphericity of 0.8, t h e linear relation is valid for Reynolds numbers up to 0.6 (9). In our calculations we use the linear relation for all t h e particles although some large flocs sedimentate with Reynolds numbers situated in the transition region (Table 2). This results in an underestimation of t h e drag coefficient, thus in a n underestimation of tho particle size. This simplification seems to be allowed or necessary because of several reasons. One rea.wn is hecause of tlic uncertainly of the relation

285

between drag coefficient and Reynolds number for non spherical particles (7,8) and also because of the small number of flocs sedimentating with Re > 0.6 (Table 2) the error on the drag coefficient is not absolutely large and the impact of a few underestimated diameters on the average diameter of the suspension is small. The use of the linearization was decided after a study of the work of Tambo and Watanabe (3--5). They also used the same linearization for determining the flocs. The density of photographed sedimentating aluminium hydroxide constants a and Kp in their model have no physical meaning. The values are obtained by pure mathematical fittings. To be sure of the validity of the model, not only the experimental constraints but also the mathematical background has to be respected. 7.2. POSSIBLE SYSTEMATIC ERROR

No influence on floc dimensions due to transportation through the tubes could be noted. Six equal experiments with pump rates varying from 110.3 ml/min to 149.6 mI/min (1.46 . 1 0 3 m/s to 1.99 . m/s in the sampling pipette) were carried out without giving rise to a detectable difference in the end results.

7.3. REPRODUCIBILITY

The reproducibility of the experiment is very good. For 9 experiments under conditions given in Table 1, with a coagulant dose of 5.02 mg(Al)/l, the average of the maximum detected floc diameter was 3495 p m with a standard deviation of 134pm, i.e. 3.8%of the average.

8. EXPERIMENTAL RESULTS

If the coagulant dose changes from experiment to experiment and the floc dimensions are calculated each time, then, for a small dosage range very large flocs are detected (Fig. 7). The dosage which yields the largest flocs for a given set of process parameters will be called the reference dose.,, If one or more of the mixing parameters is changed or if water quality changes, then the reference dose will also change. This dosage is optimal from the point of view of the flocculation mechanism. It is not an economical optimum for running an industrial installation, but a property of the coagulation/flocculation process

28 6

carried out under the given process conditions in water of a given quality. Thus, the reference dose can be considered as an objective coagulant dose. This dosage is a useful basis for further research. If experiments are carried out with the reference doses corresponding with the experimental conditions or with dosages which are the same fraction of the reference doses, only then may their results be compared with each other. A complete investigation based on the use of reference doses has been carried out already (15).

1250 1000

0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

dosage coagulant (mg [All /I) F i g . 7. Influence of the dosage on the floc dimensions (Process conditions as given in Table 1 )

Tables 3 and 4 give the influence of mixing conditions and water quality o n the reference dose. In the different experiments, the experimental conditions are those as mentioned in Table 1; except the variable which is investigated. This value is given in column 1 of Tables 3 and 4. Some relations between a process parameter and the reference dose are monotonously increasing or decreasing, other relations pass through a maximum (pH, duration of rapid mixing). The salt content in the raw water especially has an important influence on the reference dose. If salts are dissolved in the water, the peak as shown in Fig. 7 is much broader and lower. Also the quantity of solids suspended in the water has a significant impact. The impact of the mixing parameters o n the reference dose is rather small.

287 Tab. 3. Influence of the mixing conditions on the reference dosage, the sludge index floc build-up time, and o n the electroforetic mobility

0

5.017

0.0515

1320

1.043

5

5.013

0.0705

9 60

1.069

30

5.013

0.0749

700

0.986

60

5.021

0.0746

840

1.020

120

5.041

0.0764

900

1.062

E -2 150

5.050

0.0753

900

0.970

180

5.052

0.0808

1050

1.063

240

5.035

0.0837

1200

0.987

300

5.009

0.0862

1230

1.016

360

4.983

0.0889

1260

0.948

2 80

5.013

0.0722

900

1.020

389

5.02 1

0.0746

900

1.049

542

5.026

0.081 3

870

1.064

696

5.032

0.0837

720

1.065

843

5.037

0.0837

690

1.051

1018

5.041

0.0847

660

1.016

21

5.037

0.0677

1000

1.049

27

5.032

0.0723

940

1.038

34

5.02 1

0.0737

900

1.049

46

5.018

0.0735

850

1.059

54

5.018

0.0739

640

1.025

62

5.017

0.0761

520

1.042

3 a 4-

z a E

0

& i n

'm

-

g -

3 7;

288 Tab. 4?Influence of the water quality on the reference dosage, the sludge index, the floc build-up time and the electroforetic mobility Reference dosage (mg(Al)/l)

Variable

-

5 J

-Q u1

Sludge index ) ml sludge ( mg clay . 5 mg(Al)/l

Floc build-up time (S)

E.M. ( ( w IS)/( V /cm)

6.5

4.971

0.0665

174Q

1.145

6.7

4.998

0.0703

1270

1.lo2

7 .O

5.021

0.0746

900

1.020

7.3

5.018

0.0732

710

0.948

7.5

5.013

0.0717

600

o .a98

7.7

5.002

0.0696

570

0.807

8.0

4.977

0.0660

540

0.780

'EI

25

4.780

0.2706

ioao

1.071

B -a 5

50

4.869

0.1253

9 60

1.063

62

4.860

0.0900

9 30

1.055

75

5.021

0.0746

900

1.049

100

5.100

0.0548

870

1.041

150

5.187

0.0345

840

1.034

200

5.175

0.0268

780

1.026

5

4.809

0.0724

900

1.023

10

4.879

0.0794

950

0.995

15

4.910

0.0801

970

0.998

20

4.980

0.0761

950

1.026

25

5.021

0.0746

900

1.020

lo4

3.92

0.0614

1900

1.310

3.08

0.0624

1540

1.714

2.40

0.0639

940

2.083

4.06

0.0683

1690

1.ooa

2.54

0.0778

1090

0.549

z c

Q

n

-0 -3 x

.-

1 1

9

0

1

2 s

B E

,0

i? 2 -m

U

m 0

U

5 5n

z

lo4

289

The sharp small peak is caused b y a combination of elements giving rise to the formation of very strong flocs when t h e reference dose is used. T h e fact that those flocs are large is already an indication of their strength. A floc is the result of constructing internal forces and disrupting external shear forces. For t h e same shear rate in t h e flocculator t h e largest flocs must have t h e largest internal forces and are therefore t h e strongest. This reasoning is also used b y Tambo et al. (5). The statement that t h e strongest flocs are formed if t h e reference dose is used, is experimentally confirmed (Fig. 8). Full-grown flocs formed under conditions as indicated in Table I , but in a solution of 103M CaCI2, were ruptured with a n average velocity gradient of 1398 s- 1 . After 1.5 minutes t h e velocity gradient was lowered to t h e same value before t h e rupture i.e. 34 s-'. With a Malvern Particle Sizer type 2200 the floc distributions were measured. This was possible because in the conditions mentioned, t h e maximum floc diameter was always smaller than 1879.9 p m, t h e upper limit of t h e instrument. From t h e diameter distributions, t h e volume median floc diameters during t h e experiment could be calculated. With t h e measured diameter of t h e full-grown flocs, t h e ruptured flocs and t h e regrown flocs, 2 factors related with floc strength can be calculated:

40

0 -

80

70

90

100

110

120

130

dosage vs. reference dosage(%)

environment : Fig. 8. Strength factor

(0)

-3 10 ki C a C 1 2

and recovery factor ( 0 ) vs. coagulant dosage (100%= 3.85 mg(AI)/I)

290

strength factor

. 100

= - druptured flocs

'full-grown

recovery factor =

flocs

dre-grown flocs dfull-grown flOCS

.-

--

druptured flocs

- loo

drUptwed floes

The strength factor gives an indication of the resistance of the flocs against rupture. The recovery factor indicates the degree of recovery of the ruptured flocs after the original velocity gradient is restored. These measurements are discussed with many details elsewhere (18). From Fig. 8 it is clear that the highest recovery factor occurs when flocs formed with the reference coagulant dose. The relation between strength factor and coagulant dose is very smooth with a weak optimum for flocs formed with t h e reference dose. Fig. 9 shows the sludge index for the same experiments as in Fig. 7. The sludge index in the graph is expressed in ml sludge formed per mg suspended 1 liter flocculated suspension was measured in solids. The sludge volume of an lmhoff cone after 30 minutes of sedimentation. From Fig. 9 it is clear that a linear relation exists between the coagulant dose and the sludge index if the process conditions of the flocculation process remain constant. Tables 3 and 4 (column 3) give the sludge index obtained from different experiments during which the flocs were formed under a wide range of process conditions. The sludge index in the tables is recalculated to a constant coagulant dose (i.e. 5 mg(Al/l). This way only the influence of a change of a mixing parameter or of the water quality is expressed in the result. The difference between the minimum and the maximum sludge index is smaller than 15%. Without doubt the quantity of AI,(SOd:, used in the process is by far the most important factor which influences the sludge volume of t h e freshly formed hydroxide flocs. The effect o n sludge production of a change of floc structure because of a change of the experimental conditions is thus subsidiary to the influence of the dosage. Of course the floc structure (1920) plays an important role in the rate of reduction of the sludge because of floc ageing phenomena (21). The floc buildup times of the experiments given in Figs. 7 and 9 are pointed o u t in Fig. 10. The larger the coagulant dose, the smaller the floc build-up time. This is self-evident because the destabilization of the particles is more complete if the coagulant dose increases, this speeds-up t h e formation of flocculi (19, 20). Also the quantity of insoluble hydroxides increases with increasing A12(S04$3 dose. This promotes a fast enmeshment mechanism during floc formation. More details

291

about the kinetic of floc formation and on the influence of the coagulant dose o n it can be found elsewhere (1622).

-

0.10

v)

a

.rl

rl

0

v)

0.08

5

a,

a $a 0.06 a m

a 5 G m

.rl

0)

bo\

2

0.04

ma,

5 b o

rl5

a 5

rl v)

0.02

rl

E

v

0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6 3

dosage c o a g u l a n t (mg [All /1) Fig. 9. Influence of the dosage on the sludge index (process conditions as given in Table 1)

Tables 3 and 4 (column 4) give the influence of the different mixing parameters and of the water conditions on the total floc build-up time. From the results it is clear that a more intense mixing during coagulation (Grapid) and during flocculation (Gdow) accelerates floc formation. The choice of the duration of the rapid mixing is more critical. A minimum time of rapid mixing is necessary, but there also exists a critical rapid mixing time which may not be exceeded (23, 24). Also an increase of the concentration of salts i n - the water, of the quantity of suspended particles, and of pH will speed-up floc formation. Notable is the fact that the temperature has little or no effect on the floc build-up time. During the interpretation one has to bear in mind that for the different experiments, the corresponding reference ' doses were used. Lastly, an important floc property discussed in this paper is the zeta-potential. The zeta-potentials were measured with a Rank Brothers Mark I1 zeta-potential meter. The electroforetic mobility (E.M.) of hydroxide flocs formed under different experiment conditions are given in Tables 3

292

4 (column 5). The zeta-potentials in mV can be calculated from the E M . by multiplication of the E. M. by 12.9. The E.M. are almost independent of the process conditions except in the case where the pH was changed or if salts were dissolved in the water. and

3500

m

3000

2500

v

g

2000

.d

u

9

1 'd .rl +-I

3

P

u

0

1500 1000

500

+-I

w

0

.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

d o s a g e c o a g u l a n t (mg [All / I ) Fig. 10. Influence of the dosage o n the floc build-up time (process conditions as given in a b l e 1 )

Table 5 gives another list of results. In a kaolinite suspension prepared in distilled water, different salts were added together with the aluminium sulphate 14. The coagulant. The salt concentration in the suspension was always different cations had a chlorine counter-ion and the anions had a sodium counter-ion. In jar tests, the coagulant dose needed for a complete turbidity removal was determined, as was the corresponding E.M. of the flocs. In the list of Table 5 , only ions which can dissolve easily in the water are mentioned. At the given coagulant dose, the EM. all had reached their constant values. From the results in Table 5 it is clear that a correlation exists between the diameter of the ions dissolved in the water and the E.M. of the hydroxide flocs formed in that water. The relation is linear for cations or anions with the same valency. This means that the adsorption of ions o n hydroxide flocs

293

decreases linearly with their diameter for cations or anions of the same valency. It is also important that no correlation exists between the coagulant dose for complete turbidity removal and the E.M.

Tab. 5. Influence of the type of dissolved ion on the coagulant dosage for complete turbidity removal and on the electroforetic mobility of the hydroxide flocs

Mg2+

<1

0.66

1.380

so;-

<1

4.33

0.658

lK0,-

1.143

4.12

0.878

so;-

1.870

5.58

0.715

2.364

16.36

0.456

s20;-

2.442

8.34

0.611

ClO,

2.805

4.30

0.656

N H ~

3.1 43

1.43

1.037

c1-

3.221

1.81

1.265

Na+

3.221

0.97

1265

3.616

-

0.943

3.688

1.33

1.043

3.766

0.99

1.423

NO;

3.922

2.80

1.038

NO3

4.130

4.09

0.918

I-

4.300

2.20

1.205

(F~(CN ) 5 )2-~

K+ Ca2

+

~

Taking the results mentioned in Tables 3, 4, and 5 together, some important conclusions about zeta-potential measurements can be drawn. The E.M. of the formed hydroxide flocs is no measure of the surface charge which plays an If hydrolyzing important role during the destabilization (coagulation). metal salts are used as the coagulant, the particles of the suspension are first destabilized and afterwards enmeshed in insoluble hydroxides. The measured E.M. is the mobility of the hydroxides in which the solids are

294

enmeshed. The value of the EM. is determined on one side by the degree of destabilization of the particles and by the number of destabilized particles enmeshed in the hydroxides, on the other side by the composition of the insoluble hydroxides and the quantity and type of ions which are adsorbed on the hydroxides. With this statement as a basis, the experimental results in Tables 3 and 4 can easily be explained. An increase of the quantity of suspended solids yields a small decrease of the E.M. because of the increasing amount of destabilized particles in the suspension. The increase of the aluminium sulphate dose decreases the E.M. because of the increasing amount of sulphate ions which decrease the E.M. of the flocs (Table 5). The addition of salts or polyelectrolytes (experiments not mentioned in this article -- see Reference 15) causes a change of the E.M. of the flocs because of the adsorption on the clay particles and on the hydroxides. A change of pH changes the structure of the aluminium complexes ( 2 5 ) , thus changes the E.M. A pH-increase increases the E.M. because of the increasing importance of the positive aluminium complexes. It is logical that a change of mixing conditions or a change in temperature does not influence the E.M. From this point of view it is self-evident that the zeta-potential does not yield any feed-back information for controlling the flocculation process.

9. GENERAL CONCLUSIONS For a given set of process conditions there exists a small dosage range yielding large flocs. The dosage in the center of the peak, in the relation between dosage and average floc diameter of the suspension, is called the reference dose. A change in one of the process conditions changes the reference dose. Only the results of experiments with the reference dose as a basis may be compared with each other. This reference dose is a characteristic of the process, imposed by the system itself. The reference dose is strongly influenced by the electrolytes dissolved in the water. The influence of the quantity of suspended solids in the water is also important. The flocs formed when the reference dose is used, are the largest and strongest flocs which can be formed under the corresponding process conditions. The sludge production is in linear relation with the quantity of coagulant used. The influence of the mixing conditions and of the water quality is of second importance. With an increase of the coagulant dose, the coagulation and flocculation is speeded-up. This results in shorter floc built-up times. All the process parameters,

295

except the temperature, have an important influence on the kinetic of the process. The zeta-potential of hydroxide flocs is mainly determined by the dissolved salts and by the pH. The adsorption of the salts on the hydroxide flocs decreases linearly with their diameter for cations or anions of the same valency. The zeta-potential of the hydroxide flocs does not yield any information for a feed-back control of the flocculation process.

NOMENCLATURE a -CD -d -df --

D Do g

G K K

n , n9

--

---

-----

N -r -Re -RPM -t -v -v -r,P v -r,s

a.

--

Ef

---

ES

Pe Pf Pm Pw 77

t

--.-

---

--

constant in Tambo's model [ kg dm-3 ] drag coefficient [ -- ] diameter [m 1 equivalent floc diameter [ m] diameter flocculator [ m ] dielectric constant in zero electric field [ -- ] gravitational acceleration [ ms-2 1 average velocity grsdient [ s-l ] conductance of bulk fluid [ S2-l mconstant in Tambo's model [ kg dm- 3 constants in the model of Richardson and Zaki [ --- ] number of particles [ dm-3 3 radius of a sedimentating particle [ m ] Reynolds Number [ -- ] number of rotations [ min-'1 sedimentation time [ s ] sedimentation velocity of an undistrubed particle [ ms-' ] sedimentation velocity of a hindered particle [ ms-' ] sedimentation velocity of a charged particle [ ms-' ]

2

collision efficiency [ -- ] porosity of the flocs [-- ] porosity of the suspension [ -- ] effective floc density [ kg dm-3] floc density [ kg dm-3 ] mineral density [ kg dm-3 ] water density [ kg dm-3 ] dynamic viscosity [ Pa m ] electrokinetic potential of the hydrodynamic surface [ mV ]

296

REFERENCES 1. T. R. Camp and P. C. Stein, J . Bos. SOC.Civ. Engrs., 30(1943), pp. 219-237. 2. T. R. Camp, J. Bos. SOC. Civ. Engrs., 56(1969), pp. 1-28. 3. N.Tambo and Y. Watanabe, J.Japan Wat. Wks.Ass., 397(1967), pp.2-10; pp. 14-17; 445 (,1971), pp. 2-9.

410 (1968),

4. N. Tambo and Y. Watanabe, Wat. Res., 13 (1979), pp. 409-419. 5. N.Tambo and H. Hozumi, Wat. Res., 13(1979), pp. 421-427. 6. R. H. Perry and C. H. Chilton, in Chemical Engineers Handbook, fifth edn., McGraw-Hill Kogakuska Ltd., .Tokyo, 1973, pp. 5.61 -5.65. 7. E. S. Pettyjohn and E. B. Christiansen, Chem. Eng. Progress, 44 (1948), pp. 157-172. 8. H. A. Becker, Can. J - Chem. Eng., 37 (1959), pp. 85-91. 9. A. S. Foust, L. A. Wenzel, C. W. Clump, L. Maus, and L. B. Andersen, in Principlesof Unit Operations, Lehigh University, Bethlehem, Pennsylvania, 1960, pp. 449-453. 10. F. Booth, J. Chem. Phys., 22 (1954), pp. 1956-1968. 11. Davies and Rideal, in Interfacial Phenomena, Academic Press, New York 1963, pp. 139-141). 12. J. F. Richardsonand W. N. Zaki, Trans. Inst. Chem. Engrs., 32(1954), pp. 35-53. 13. A. S. Michaels and J. C. Bolger, Ind. Eng. Chem. Fundamentals, 1(1962), pp. 24-33. 14. K . J. Ives, in The Scientific Basis of Flocculation, Sijthoff & Noordhoff, Alphen aan den R&, 1979,59-61. 15. R. J . Francois, in Study of Coagulation-Flocculation of Kaolinite Suspensions with Aluminium Sulphate, Ph.D. Thesis, University of Leuven (K.U.L.), (1985). 339 pages (in Dutch). 16. R. J. Francois, Kinetic of the growth of hydroxide flocs, submitted for publication in Wat.Res. (1985). 17. R. B. Bird, W. E. Steward and E. N. Lightfood, in Transport Phenomena, John Wiley &Sons Inc., New York/London, 1960, 190-195. 18. R. J. Francois, Strength of aluminium hydroxide flocs, submitted for publication in Wat. Res. (1985). 19. R. J . Francois and A. A. Van Haute, in Studies in Environmental Science 23 - Chemistry for Protection of the Environment, Elsevier, Amsterdam 1984, 221-234. 20. R. J. Francois and A. A. Van Haute, Wat. Res., 19(1985), pp. 1249-1254. 21. R. J. Francois, Ageing of Hydroxide flocs, submitted for publication in Wat. Res. (1985). 22. R. J. Francois and A. A. Van Haute, in Proc. 5th Asia Pacific Regional Water Conference & Exhibition, Seoul, September 15-20, 1985 (in press).

Supply

23. R. J . Francois and A. A. Van Haute, Choice of a rapid mixing time in a flocculation process, in Proc. 8th Int. Congress CHISA’ 84, Prague, September 1984. 24. R. J . Francois and A.A. Van Haute, Wat.Sci.Tech.,

17(1984), pp. 1091-1101.

25. AWWA-Committee Report, J . Am. Wat. Wks. Ass., 63(1979), pp- 99-108.