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Bubble size in flotation thickening
War. Res. Vol. 28, No. 2, pp. 465-473, 1994 Printed in Great Britain.All rights reserved
0043-1354/94$6.00+ 0.00 Copyright© 1993PergamonPress Ltd
BUBBLE SIZE IN FLOTATION THICKENING SANDERE. De RIJK, JAAP H. J. M. VAN DER GRAAF and JAN G. DEN BLANKEN* Delft University of Technology, Faculty of Civil Engineering, Department of Sanitary Engineering and Water Management, Stevinweg 1, 2628 CN Delft, The Netherlands (First received November 1992; accepted in revisedform April 1993) Abstract--The performance of flotation thickeners for waste activated sludge in The Netherlands does not meet expectations; without the addition of chemical aids the dry solids content of the floated sludge is 3-4% instead of the expected 4.5%. An investigation into the improvement of the flotation thickening of waste activated sludge has been made. Experiments were carried out on the size and concentration of the produced air bubbles, both important parameters in the flotation process. Laboratory experiments showed that bubble size increased from 50 to 100-150#m when a tube was placed behind the nozzle. These results indicated the need to check bubble formation at the full-scalethickener of the wastewater treatment plant at Zwolle, The Netherlands. However, the negative effects which occurred at laboratory-scale were not found in the full-scale experiments, which resulted in bubble sizes of 45-60 #m. Key words--sludge treatment, dissolved-airflotation, thickening, performance, bubble formation, bubble size
INTRODUCTION Sludge treatment is an important process in wastewater treatment. The objective is to reduce the volume as much as possible by reducing the water content (dewatering). The thickening process is the first step in sludge dewatering. It is important to carry out the thickening process as well as possible in order to obtain the best results for the subsequent operations. In sewage treatment the activated sludge process is mostly used. Gravity thickening does not always result in sufficiently high suspended solids concentrations. Dissolved-air flotation units are generally used as an alternative thickening process. In dissolved-air flotation (DAF) a partial water flow, the dispersion water, is saturated with air under high pressure and consequently transferred with the excess activated sludge to the flotation unit (Fig. 1). In the flotation chamber reduction of the overpressure to atmospheric pressure causes air to precipitate as small bubbles from the solution. The air bubbles adhere to the sludge particles and produce bubblesludge agglomerates with a density lower than that of water. These agglomerates rise to the top of the flotation unit and accumulate as a float. During the flotation process the float rises still further above the water level because more lower-density particles are rising. The float solids concentration increases by draining the float; thus thickening is effected. Parameters which affect thickening have been identified as: ----solids loading rate - - a i r solids ratio *Author to whom all correspondence should be addressed.
----depth of float above water level (float removal) --sludge volume index --use of chemicals. The Department of Sanitary Engineering and Water Management of the Delft University of Technology has carried out a study as to what factors are important for design and possible improvement of flotation-thickening of excess activated sludge (other than adding polymers). One parameter that could have been insufficiently studied in practice till now is the bubble size (distribution). It is expected that this factor will certainly affect the flotation process. In order to get more insight into this parameter further studies have been carried out on laboratory scale as well as on full scale in practice. PRACTICAL RESEARCH IN THE NETHERLANDS
In The Netherlands, DAF is used to thicken excess activated sludge in four treatment plants. All plants were constructed in the period 1983-1985. More details are presented in Table 1. On account of results obtained in Sweden and the United Kingdom, a total suspended solids concentration of 4.5% dry mass could be expected without addition of polymers. After the disappointing results in the first period (Table 1), operation parameters were studied at the various treatment plants, among other things, the average scraper velocity and the air solids ratio. Furthermore, adaptations to the inlet and overflow system of the sludge were tested. However, most studies did not really improve the drymass percentage. Finally, the addition of polymer to the sludge appeared to be the only method to achieve acceptable results.
465
466
SANDER E. DE RIJK et al. I
I
1
I
m
Effluent
Waste act. sludge tank
Zll
Nozzle
I
Bottom sludge
Compressor
© 12"Z2"2 Flotation sludge tank Water for dispersion
Saturation unit
Fig. 1. Diagram of a flotation thickener. IMPORTANT ASPECTS OF THE FLOTATION PROCESS
A literature study of the factors affecting the flotation process has resulted in the following findings.
Saturation o f the dispersed water The dissolved-air flotation process makes use of the greater solubility of air in water at higher pressure. The quantity of air which can be dissolved in water is based on Henry's law:
where Cs = saturation concentration of the gas in water (g/m 3) k H = Henry's coefficient (g/m3*Pa) P = partial gas pressure (Pa). Henry's coefficient depends on temperature and the type of gas. Table 1. Results of practical studies at four plants in The Netherlands
Treatment plant Bath Zwolle Horstermeer 's-Hertog~mbosch
400,000 150,000 140,000 400,000
*A low solids loading rate. tWith dose of polymers.
--the saturation efficiency, this depends, among other things, on the type of saturator used --the air composition in the saturator. Because of the rise of the partial pressure of nitrogen in the air the theoretical quantity of air which can be dissolved is reduced by about 9% --the air-release efficiency.
Adhesion o f air bubble and floc
Cs = kH*P
Size in population equivalents
Besides pressure, the volume of air which can precipitate from a volume of dispersion water depends on:
Total suspended solids in percentage dry mass in 1983-1985
1991
2.3 3.9* 3.0 3.0
4.7-5.0t 4.0-4.5t 4.0 4.0t
When the excess sludge has been mixed with the dispersion water in the sludge pipeline just before the inlet into the flotation unit, it is important that the air bubbles should adhere to the sludge flocs, thus producing agglomerates with a density lower than that of water. There are two possbile mechanisms for adhesion between air bubble and floc: --inclusion of air bubbles into the sludge flocs. Adhesion forces are not important in this case, because the bubbles are included in the structure of the flocs. The occurrence of this mechanism will increase if the flocs are larger and have a more irregular shape --adsorption of the bubbles on the outside of the flocs. The adhesion can be effected because the floc serves as a nucleus to the bubble production or through interaction between the bubble and the floc.
467
Bubble size in flotation thickening
Liquid(1) ~
o
l
Chemicals
g
osl
osg Solid phase (s) ~ / / / / ~
Fig. 2. Adsorption of an air bubble to a solid surface. In both cases a three-phase system has been effected, which can be characterized by surface tensions and a contact angle (Fig. 2); the air bubble assumes such a shape that the potential energy in the three interfaces has reached a minimal value. It depends, among other things, on the size of the bubbles and the size and the nature of the sludge flocs whether, after contact between bubble and floc, a stable aggregate is actually produced. Furthermore, adhesion can be stimulated by means of chemicals. Bubble size
It is important that the air bubbles produced are small ( < 100-120 #m), because: - - w i t h small bubbles a smaller contact angle is possible than with large bubbles (Hanisch, 1959) --small bubbles are included into the floc more easily - - t h e possibility of collision and adhesion between a bubble and a particle increases more than proportionally to the number of bubbles and is independent of the bubble size (Flint and Howarth, 1971; Reay and Ratcliff, 1973); with a known quantity of air the bubbles should be as small as possible in order to increase the bubble concentration --small bubbles have a lower rising velocity than large bubbles, so that the residence time in the flotation unit is longer; because of this the possibility of collision between a bubble and a sludge floc is increased --because of the high rising velocity of large bubbles (t>2 ram) the shear forces on a rising bubble-floc agglomerate are so strong that the flocs may break (Jedele, 1984).
The adhesion between bubble and floc can be stimulated by adding chemicals which act as collectors (Hanisch, 1959). A collector is a long molecule with a hydrophilic (chemically active) and a hydrophobic (chemically non-active) part. The hydrophilic part turns to the solid phase and the hydrophobic part to the liquid phase. In this way the particle becomes hydrophobic. Sludge is often conditioned with organic polymers (polyelectrolytes). The polyelectrolyte stimulates the production of bubble-particle agglomerates in two ways. It links the sludge particles so that bigger floes are produced and it also acts as a collector so that the t o e ' s surface becomes hydrophobic. Both effects stimulate adhesion between bubble and floe.
BUBBLEraZE ANDBUBBLEFORMATION When the large-scale thickeners in practice are operated without polymers, their disappointing results may be due to insufficient adhesion of the bubbles to the sludge flocs. The bubble size, which is an important factor in adhesion, is determined to a high degree by bubble formation; regarding bubble formation, some aspects will be further discussed. Bubble production
When pressure suddenly drops in the nozzle or the valve, bubbles are probably formed due to cavitation. Cavitation is the growth (explosion) of a bubble because of a pressure drop, followed by implosion and bursting of the bubble into dozens of small bubbles. Cavitation results from a major pressure drop in the narrow opening of a nozzle or a valve (Fig. 3): l 2 Po +~pvo
=
l 2 (Bernoulli), Pk d-~pVk
and because Vk >>VO Pk = Po -- ~pvk I 2
where P0, v0 are respectively pressure and velocity before the narrowing; Pk, Vk are respectively pressure and velocity at the narrowing. Behind the orifice a highly turbulent mixing zone is formed. The turbulences in this zone cause severe pressure fluctuations and because the pressure in the jet stream Pk is lOW (Vk is very high), negative pressure
Nature and size o f the particle (floc)
The nature and structure of the particle determine what kinds of adhesion are possible and--with ads o r p t i o n - w h a t the maximum bubble size is when adhesion is still possible (Hanisch, 1959). The collision efficiency is (at least) proportional to the square particle size (Reay and Ratcliff, 1973). Furthermore, the t o e ' s firmness is also important because the shear forces which are exerted during rising of the bubble-floe agglomerate can cause breakage of the floc (Jedele, 1984).
Fig. 3. Pressure loss along a nozzle.
SANDERE. DE RIJK et al.
468
zones may appear. Because the resistance of the water against these stress forces is very good, cavities (bubbles) are only produced in so-called "weak spots" (for example, pollutants, irregularities in the wall).
[ Column ~
I
- - u p t a k e of air from the (supersaturated) main flow. As the bubble formation is less complete the main flow remains more supersaturated and the possibility of gas transport into the bubbles will increase ---decrease of the hydrostatic pressure. This effect on the bubble size is negligible at the water-depth used in practice (Jedele, 1984) ----coalescence. This is the merging of bubbles. In practice it appears that coalescence occurs mostly in the turbulent zone near the nozzle and that coalescence in the flotation unit itself is negligible, because the total air content in the flotation unit is low and, because the bubbles follow the flow and therefore cannot collide (van Bennekom, 1979). The possibility of coalescence can be reduced by adding chemicals that decrease the surface tension (Hanisch, 1959).
Effect o f pressure drop on bubble formation The bubble size decreases as the pressure drop near the air-releasing cell increases, e.g. when the saturation pressure is increased. However, the minimum diameter, measured as an average value, appears to be near 40 or 5 0 p m (van Bennekom, 1978; Takahashi et al., 1979; Jedele, 1984). Besides the effect on the bubble size, the pressure naturally determines the released air volume because the satu-
.1 "ogC~o O0~P 0
v,,i •
T .1.
Saturated water
/ I
~ 7 v valve Saturated water Fig. 4. Diagram of the flotation column.
]
V Cuvet
'
Bubble t a J
Bubble growth When the dispersion water is released the bubbles will generally be small, but the size of these bubbles can increase due to the following factors:
Camera
~'3
J Background (q~ 6, 5 cm)
/ /
Reflection
/
,, /
/ / /
/ /
Flash
Fig. 5. The set-up of the photographic equipment. ration concentration of air in water increases in proportion with the pressure.
Effect o f nozzle on bubble .formation Investigations into the operation of nozzles, which are applied in drinking water treatment, indicate the importance of a good nozzle construction, particularly of the part behind the release opening (orifice of jet pipe). A badly constructed nozzle will cause incomplete air release and coalescence (van Bennekom, 1978; Takahashi et al., 1979).
METHOD
FOR MEASURING THE BUBBLE SIZE
The flotation unit in the Laboratory of Sanitary Engineering of the Delft University of Technology has been used for the determination of bubble size. The pilot plant consists of a flotation column and an air-saturation unit. The saturation of tap water takes place with a venturi aerator resulting in a saturation efficiency of about 97%. The column is made of perspex, is 150 cm high and has an inner diameter of 29 era. During the measurements the valve or the nozzle can be placed on a support in the flotation column and be adjusted from the top of the column with a rod. The valve can also be placed outside the column. In this case the inlet is just above the bottom of the column via point A in Fig. 4. There is a tapping point in the wall near the top of the column from which a small flow of water saturated with bubbles can be tapped and led through a cuvet. The bubbles in the curet can be photographed so that later the bubble size and bubble concentration can be determined. The tapping point consists of a perspex tube with a diameter of 7 ram, which extends 7.5 cm into the column. The tap is 1 m above the inlet at the bottom of the column (see Fig. 4). When the valve is placed inside the column the distance between the valve and the tapping point is 60 cm. Figure 5 shows how the photographic equipment has been set up. Determination of the bubble size and the bubble concentrations from the photo negatives has been carried out with the aid of automatic image analysis equipment, the Magiscan (Joyce Loebl). For large numbers of measurements the use of image analysis equipment is an attractive alternative to manual measurements. However, the photographs have to be of good quality (sharp and have good contrast) and the bubble concentration must not be too high, because agglomerated bubbles are dLfficult to separate.
EXPERIMENTS ONLABORATORY SCALE The objective of the laboratory experiments is to predict the effects which may occur in practice.
469
Bubble size in flotation thickening 120 ~20
110 _ 30¢m
o 50 I/h
+ 75 I/h
,.,100 - ~
Sludge
¢ 100 I/h
90
ValV~A
A
~50
~
+80ram
.~ 70
/ Top view
Saturated
water
°°lOI
Distance A : 12 cm at H'meer 64 cm at Zwolle
0e---
--t:~f
Fig. 6. The air-release system in practice; sewage treatment plants at Horstermeer, Zwolle (Bath). In the flotation units o f Horstermeer, Zwolle and Bath the depressurization o f the dispersion water is effected with the aid o f flap valves. After air release, dispersion water is added to the sludge pipe from two sides (4 pipes per flotation tank). This is shown in Fig. 6. The flotation unit o f Bath differs from the others in that the sludge is added at ] o f the length o f the tank. It was presumed that the presence o f the pipe or tube behind the valve, before intensive mixing of the dispersion water with the sludge, would cause bubbles to grow as a result o f coalescence and uptake o f air from the liquid. It has been studied in the laboratory if this type o f phenomenon can indeed occur. This has been done by measuring, at various pressures and flow rates, the bubble formation obtained with the valve only and comparing it with the bubble formation obtained under the same circumstances when tubes of various lengths and diameters have been placed behind the valve. The flow rates and dimensions o f
Pressure (bar)
Flow rate (l/h) 50 75 100 50 75 100 50
6.2
50 4o 30
I
I
3.5
5.0
I 6.2
S a t u r a t i o n p r e s s u r e (bar) Fig. 7. Relation between bubble size (median) and saturation pressure (laboratory experiment, needle valve without tube, flow rate 50, 75 and 1001/h). the tubes behind the valve have been selected in such a way, that the retention times and the flow rates inside the tube correspond to the design o f the flotation units at Horstermeer and Zwolle. During the measurements a needle valve (Econosto) was used because a sufficiently small normal valve was not available. For comparison, measurements have also been carried out with a normal valve (the smallest available) and a scale model o f the Hague nozzle. The laboratory experiments produced the following results (see also Table 2). RESULTS
Bubble size Figure 7 shows the relation between the median b u b b l e size a n d t h e s a t u r a t i o n p r e s s u r e f o r t h e needle valve ( w i t h o u t t u b e ) at different flow rates. It a p p e a r s f r o m t h e figure t h a t t h e b u b b l e size decreases w i t h a n increase in t h e s a t u r a t i o n p r e s s u r e a n d w i t h a n increase in t h e flow rate. T h e effect o f t h e flow rate o n t h e b u b b l e size d e c r e a s e s as t h e s a t u r a t i o n p r e s s u r e increases a n d is h a r d l y p r e s e n t at 6.2 bar.
Table 2. Median bubble size for various situations Needle valve Normal valve Nozzle -10 10 60 60 ---4 10 4 10 ---
Tube Length (cm) Diameter (ram)
5
6o
m
25 cm
Side view
3.5
~
° O°oo.i.o c °o¢o o~ o o o •
~- . . . . .
8o
75 100
Median bubble size (pro) [Standard deviation (pro)] 107 [6.0 84 [4.2 57 [I.6 74 [I.9 52 [I.0 51 [I.1 43 [I.O 49 [1.2 38 [0.7
96 2.1 93 3.4 109 3.6 86 !.6 107 2.9 92 2.8 75 3.0 80 2.6 76 2.3
94 2.6 114 6.1 113 3.7 121 4.2 103 2.2 110 3.8 97 3.8 92 3.5 91 2.4
143 11 121 12 152 14 135 8.6 119 8.0 146 17 150 19 129 10 102 8.4
122 8.8] 131 5.8] 112 7.2] 72 5.4] 86 4.4] 116 ! 1] 84 3.7] 75 2.0] 86 2.9]
---
63 1.5 --
58 1.5 --
75 2.4 40 1.2 --
47 1.1
39 0.8
-
-
-
-
-
-
39 0.9
-
-
-
-
33 0.6
470 30 28 26 24 22 20 18 ~ 16 ~ 14 ~ 12 ~ 10 8 6 4 2 0
SANDERE. DE RUK et al. +
+
\ +
\
÷ I I
0
,~/
J 40
+
x+,
I
1+7+'+.+.++. LIt ,J,l . . . . . .
80 120 160 Bubble size (gin)
200
240
Fig. 8. Bubble size distribution (laboratory experiment, needle valve without tube, flow rate 100 l/h, pressure 5 bar).
Figure 8 shows a representative example of the bubble size distributions obtained with the needle valve. This distribution has been measured at 5 bar and at a flow rate of 100 l/h.
x.J co
LZ
Table 2 shows that the bubble size increases in most situations when a tube is placed behind the valve. Particularly when " g o o d " bubbles ( ~ 60 #m) are produced without a tube behind the valve, the deteriorating effect of the tube is significant and this then results in larger bubbles. Figure 9 shows how the bubble size distribution changes when different tubes are placed behind the valve. Figure 9(a) shows the distributions obtained with both the needle valve itself and the valve with the two short tubes (10 cm) at a pressure of 5 bar and a flow rate of 1001/h. When a tube is placed behind the valve the bubble size distribution moves to larger bubble diameters. In Fig. 9(b) the distributions obtained with the two long tubes (60cm) behind the valve are shown. The uneven curves of the distributions are due to the limited number of measurements which were available (due to low bubble concentration). Figure 9(c) shows the cumulative distributions. With increasing tube length the curves become more flat, indicating a shift of the distribution to larger diameters.
26 24 22
o nv and tube 10em/4mm
20 18 16
v nv
4 2 0
~
+ nv and tube lOcm/lOmm
101 f.1 "+x.h
I0
50
90
130 170 210 Bubble size (gm)
250
290
3024262820-I22 (b) Anv and tube 60cm/10mm
o"
0 nv and tube 60cm/4mm
18 16 14 12 10
-
6 4 28
o ----~ lO
50
90
130 170 210 Bubble size (~,rn)
Fig. 9 (a) and (b). Caption opposite
250
290
901
Bubble size in flotation thickening
471
r
8O
7O
i
40 3o
/
/
I-
/ ~ 4
A nv and tube 60cm/10mm + n , and
20[-
~ / "
l
V/~
//
10~-
ff / ' f :~
+'l 10
V nv 0 nv and tube 60cm/4mm
_.j..+/'
o 50
tube10cm/10mm
I
I
I
90
I
130
I
I
170
I 210
I
I
I
250
I 290
B u b b l e size (I.tm) Fig. 9(c)
Fig. 9. Bubble size for various situations (laboratory experiment, flow rate 100 l/h, pn~sure 5 bar). (a) and (b) Distributions and (c) cumulative. Bubble concentration
The bubble concentration is inversely proportional to the bubble size, so that the relations found for the bubble size also apply for the bubble concentration. Increases in the pressure and the flow rate lead to a higher bubble concentration in the valve without a tube. With a tube behind the valve the bubble concentration decreases and there is no longer a significant relation between the flow rate and the saturation pressure. A relation between the bubble concentration and the dimensions of the tube has not been found. Released air volume
An estimate of the released air volume has been made from the bubble size distribution. When comparing the calculated volumes with the theoretical maximum volumes, the former appear to be only 0.03-2.2% of the maximum volumes. The difference between these values can be explained by the dilution of the bubble concentration in the flotation unit caused by the rise and disappearance of the bubbles at the sur- face. The extent of the dilution depends on the rising velocity, and therefore the size, of the air bubbles. The diluted concentrations are calculated as a
percentage of the bubble concentration immediately behind the valve. The results of these calculations are shown in Table 3. In the calculations the column has been approached as a completely mixed system. The rising velocities have been determined from Stokes' equation: S = 1 ,g*d 2
18
s = rising velocity (m/s) d = bubble diameter (m) g = 9 . 8 1 m/s 2 v = kinematic viscosity; 1.1 x 10 -6 (m2/s). During the experiments it has also been observed that large bubbles with an estimated diameter of 1-5 mm were not tapped from the column. Therefore the method used is selective for smaller bubble diameters, so that larger bubbles, with a considerable share in the air volume, are measured less often or not at all. This is why it is unknown in what way the measured air volumes compare with the actual released air volumes. Due to this, the measured values of the median bubble sizes will also be lower than the actual median values of the released bubbles. Because the effect of the dilution is stronger as the bubbles are larger, the
Table 3. Dilution of the bubble concentration Flow rate (l/h)
Bubble diameter (~m)
Rising velocity (m/h)
Residencetime in the column (min)
Concentration with respect to maximum (%)
100
10 50 100 150 10 50 100 150
0.2 4.6 17.8 40.1 0.2 4.6 17.8 40.1
50 11 3.2 !.5 86 12 3.3 !.5
83 18 5.3 2.5 71 10 2.7 1.3
50
WR 28/2----O
v
where
SANDERE. DE RUK et al.
472
Table 4. Measurementsof the bubble size at the sewage treatment plant at Zwolle Bubble size(# m) Sludge flow rate
(m3/h) 6 3 3 3 3
Dispersion water flow rate (rn3/h)
Median
Standard deviation
4 4 2.75 2.25 1.9
59 61 62 47 46
0.8 0.9 0.8 -0.6
real differences in bubble size will be larger than has been measured.
Effect of the type of valve Table 2 shows that the size of the bubbles produced with the needle valve and the normal valve differ little. At a saturation pressure of 5 bar and a flow rate of 50 l/h the bubbles produced with the needle valve are approx. 25% larger. With the two other adjustments the bubble diameters are practically equal. Therefore the exact shape of the orifice seems to have little effect on the bubble formation and it is justified to make use of the needle valve for the small-scale experiments. Table 2 shows that the bubbles produced with the Hague nozzle are generally smaller than those produced with the needle valve.
laboratory experiments at 5 bar and 1001/h, an adjustment which best approaches practice, a bubble size of about 120-150#m was measured with the same tube length as the one at Zwolle, viz. 60 cm; without the tube the median was about 50 #m. The measured bubble sizes ( 4 6 - 6 0 # m ) approach the m i n i m u m value which can be obtained according to literature. Therefore it may be concluded that the bubble formation in the flotation plant at Zwolle cannot be improved much. The bubble growth which occurred in the laboratory experiments is probably caused by scaling effects. Due to the limited capacity of the available saturation unit it was, however, impossible to work on a larger scale. CONCLUSIONS It appears possible to study the bubble size distribution both on laboratory scale and in practice. With the measuring method used the performance of an air-release system can be indicated fairly quickly after a trial period. When only the operation of the valve is considered (i.e. without a tube), an increase in the saturation pressure leads to the formation of smaller bubbles. 3o
28 -Measurements in practice The laboratory experiments have shown, among other things, that the bubble size increases significantly in the tube behind the valve, but a relation between the residence time or the flow rate in the tube and the size and the concentration of the bubbles has not been found. Therefore it is difficult to apply these results to a large flotation unit in practice. As bubble growth may, in practice, also occur in the tube behind the valve it has been decided to measure the bubble size in practice. These measurements have been carfled out at the sewage treatment plant at Zwolle. The bubble-saturated water was tapped from the sludge pipe at a distance of 15 cm behind the mixing point (of the dispersion water with the "sludge"). Since the bubbles were photographed, effluent was now pumped through the sludge pipe instead of sludge. It appears from laboratory experiments that direct tapping of a partial flow from a pipe to the cuvet led to an irregular flow through the cuvet. For this reason the tapped bubble flow was first led through the column of the laboratory flotation unit and then to the cuvet so that a proper flow through the cuvet was obtained. Furthermore, with this method the bubbles in the column could already be assessed visually and compared with the laboratory results. The results of the measurements in practice are presented in Table 4 and Fig. 10. The median of the bubble size distribution is about 6 0 # m under normal operating conditions. In the
e~
L~
26-24-22-20-18-16 14 12 10 8 6 4 2 0
(a)
10
50
90 130 170 210 Bubble size (ptm)
250
290
100
@. ~ ~ ~" N
80' 70 60 5O 4O 3O 20 10 0 ~1 10
t I I t i i I I I I I I l 50 90 130 170 210 250 290 Bubble size (l~m) Fig. 10. Bubble size for the situation in practice (sewage treatment plant at Zwolle, sludge flow rate 6m3/h, dispersion water 4 m3/h). (a) Distribution and Co) cumulative.
Bubble size in flotation thickening This corresponds with theory. With low saturationpressure values ( < 5 bar) a higher flow rate also stimulates the formation of small bubbles, whereas with higher pressure no significant effect of the flow rate can be observed. Bubble size increases with the presence of a tube behind the valve when the pressure values and the flow-rate values are the same. The effect of the tube geometry on bubble formation is not clear; a significant relation between bubble size and residence time or flow rate has not been found. The bubble formation through the needle valve and the normal valve are practically equivalent, whereas the Hague nozzle produces slightly smaller bubbles. The median size of the bubbles produced in the flotation thickener of the sewage treatment plant at Zwolle is approx. 60/zm under normal working conditions. The bubble growth, which occurred in the laboratory experiments in the tube behind the valve, is a very significant phenomenon, but is probably a result of the scale used. Considering the measured bubble size it is not expected that the flotation results of the sewage treatment plant at Zwolle can be improved by adaptations to the bubble formation. Acknowledgements--The authors wish to thank Mr R. Neef (Zulveringsschap Amstel- en Gooiland), Mr F. A. Brandse (Zulveringsschap West-Overijssel), Mr J. Mandemaker
473
(Sewage Treatment Plant at ZwoUe), Mr J. Kruit 0Vaterschap De Dommel), Mr R. E. M. van Oers (Hoogheemraadschap West-Brabant), Mr A. M. J. Pel (Sewage Treatment Plant at Bath) and Mr P. L. Buyink (Drinkwater-bedrijf Zuid-HoUand) for their collaboration. Appreciation is also given to Mr C. S. van der Reijden (Delft University of Technology, Department of Sanitary Engineering and Water Management) who assisted with the image analysis equipment.
REFERENCE~ van Bennekom C. A. (1978, 1979) The effect of different types of nozzles on the flotation process. KIWA reports SWE-202 and SWE-213 (in Dutch). Flint L. R. and Howarth W. J. (1971)The collisionet~ciency of small particles with sphericM air bubbles. Chem. Engng Sci. 26, 1155-1168. Hanisch B. (1959) The scientific application of flotation with very small air bubbles for purification of sewage water. Stuttgart University of Technology (in German). Jedele K. (1984) Application of dissolved-air flotation for segregation of the activated sludge from the water. Stutt. Ber. Siedlungswass. wirtsch. 84 (in German). Reay D. and Ratcliff G. A. (1973) Removal of fine particles from water by dispersed-air flotation: effects of bubble size and particle size on collection efficiency. Can. J. Chem. Engng 51, 178-185. de Rijk S. E. (1991) Thickening of sewage sludge by dissolved-air flotation. Delft University of Technology, Faculty of Civil Engineering, Department of Sanitary Engineering and Water Management (in Dutch). Takahashi T., Miyahara T. and Mochizuki H. (1979) Fundamental study of bubble formation in dissolved-air pressure flotation. J. chem. Engng Jap. 12, 275-280.
0043-1354/94$6.00+ 0.00 Copyright© 1993PergamonPress Ltd
BUBBLE SIZE IN FLOTATION THICKENING SANDERE. De RIJK, JAAP H. J. M. VAN DER GRAAF and JAN G. DEN BLANKEN* Delft University of Technology, Faculty of Civil Engineering, Department of Sanitary Engineering and Water Management, Stevinweg 1, 2628 CN Delft, The Netherlands (First received November 1992; accepted in revisedform April 1993) Abstract--The performance of flotation thickeners for waste activated sludge in The Netherlands does not meet expectations; without the addition of chemical aids the dry solids content of the floated sludge is 3-4% instead of the expected 4.5%. An investigation into the improvement of the flotation thickening of waste activated sludge has been made. Experiments were carried out on the size and concentration of the produced air bubbles, both important parameters in the flotation process. Laboratory experiments showed that bubble size increased from 50 to 100-150#m when a tube was placed behind the nozzle. These results indicated the need to check bubble formation at the full-scalethickener of the wastewater treatment plant at Zwolle, The Netherlands. However, the negative effects which occurred at laboratory-scale were not found in the full-scale experiments, which resulted in bubble sizes of 45-60 #m. Key words--sludge treatment, dissolved-airflotation, thickening, performance, bubble formation, bubble size
INTRODUCTION Sludge treatment is an important process in wastewater treatment. The objective is to reduce the volume as much as possible by reducing the water content (dewatering). The thickening process is the first step in sludge dewatering. It is important to carry out the thickening process as well as possible in order to obtain the best results for the subsequent operations. In sewage treatment the activated sludge process is mostly used. Gravity thickening does not always result in sufficiently high suspended solids concentrations. Dissolved-air flotation units are generally used as an alternative thickening process. In dissolved-air flotation (DAF) a partial water flow, the dispersion water, is saturated with air under high pressure and consequently transferred with the excess activated sludge to the flotation unit (Fig. 1). In the flotation chamber reduction of the overpressure to atmospheric pressure causes air to precipitate as small bubbles from the solution. The air bubbles adhere to the sludge particles and produce bubblesludge agglomerates with a density lower than that of water. These agglomerates rise to the top of the flotation unit and accumulate as a float. During the flotation process the float rises still further above the water level because more lower-density particles are rising. The float solids concentration increases by draining the float; thus thickening is effected. Parameters which affect thickening have been identified as: ----solids loading rate - - a i r solids ratio *Author to whom all correspondence should be addressed.
----depth of float above water level (float removal) --sludge volume index --use of chemicals. The Department of Sanitary Engineering and Water Management of the Delft University of Technology has carried out a study as to what factors are important for design and possible improvement of flotation-thickening of excess activated sludge (other than adding polymers). One parameter that could have been insufficiently studied in practice till now is the bubble size (distribution). It is expected that this factor will certainly affect the flotation process. In order to get more insight into this parameter further studies have been carried out on laboratory scale as well as on full scale in practice. PRACTICAL RESEARCH IN THE NETHERLANDS
In The Netherlands, DAF is used to thicken excess activated sludge in four treatment plants. All plants were constructed in the period 1983-1985. More details are presented in Table 1. On account of results obtained in Sweden and the United Kingdom, a total suspended solids concentration of 4.5% dry mass could be expected without addition of polymers. After the disappointing results in the first period (Table 1), operation parameters were studied at the various treatment plants, among other things, the average scraper velocity and the air solids ratio. Furthermore, adaptations to the inlet and overflow system of the sludge were tested. However, most studies did not really improve the drymass percentage. Finally, the addition of polymer to the sludge appeared to be the only method to achieve acceptable results.
465
466
SANDER E. DE RIJK et al. I
I
1
I
m
Effluent
Waste act. sludge tank
Zll
Nozzle
I
Bottom sludge
Compressor
© 12"Z2"2 Flotation sludge tank Water for dispersion
Saturation unit
Fig. 1. Diagram of a flotation thickener. IMPORTANT ASPECTS OF THE FLOTATION PROCESS
A literature study of the factors affecting the flotation process has resulted in the following findings.
Saturation o f the dispersed water The dissolved-air flotation process makes use of the greater solubility of air in water at higher pressure. The quantity of air which can be dissolved in water is based on Henry's law:
where Cs = saturation concentration of the gas in water (g/m 3) k H = Henry's coefficient (g/m3*Pa) P = partial gas pressure (Pa). Henry's coefficient depends on temperature and the type of gas. Table 1. Results of practical studies at four plants in The Netherlands
Treatment plant Bath Zwolle Horstermeer 's-Hertog~mbosch
400,000 150,000 140,000 400,000
*A low solids loading rate. tWith dose of polymers.
--the saturation efficiency, this depends, among other things, on the type of saturator used --the air composition in the saturator. Because of the rise of the partial pressure of nitrogen in the air the theoretical quantity of air which can be dissolved is reduced by about 9% --the air-release efficiency.
Adhesion o f air bubble and floc
Cs = kH*P
Size in population equivalents
Besides pressure, the volume of air which can precipitate from a volume of dispersion water depends on:
Total suspended solids in percentage dry mass in 1983-1985
1991
2.3 3.9* 3.0 3.0
4.7-5.0t 4.0-4.5t 4.0 4.0t
When the excess sludge has been mixed with the dispersion water in the sludge pipeline just before the inlet into the flotation unit, it is important that the air bubbles should adhere to the sludge flocs, thus producing agglomerates with a density lower than that of water. There are two possbile mechanisms for adhesion between air bubble and floc: --inclusion of air bubbles into the sludge flocs. Adhesion forces are not important in this case, because the bubbles are included in the structure of the flocs. The occurrence of this mechanism will increase if the flocs are larger and have a more irregular shape --adsorption of the bubbles on the outside of the flocs. The adhesion can be effected because the floc serves as a nucleus to the bubble production or through interaction between the bubble and the floc.
467
Bubble size in flotation thickening
Liquid(1) ~
o
l
Chemicals
g
osl
osg Solid phase (s) ~ / / / / ~
Fig. 2. Adsorption of an air bubble to a solid surface. In both cases a three-phase system has been effected, which can be characterized by surface tensions and a contact angle (Fig. 2); the air bubble assumes such a shape that the potential energy in the three interfaces has reached a minimal value. It depends, among other things, on the size of the bubbles and the size and the nature of the sludge flocs whether, after contact between bubble and floc, a stable aggregate is actually produced. Furthermore, adhesion can be stimulated by means of chemicals. Bubble size
It is important that the air bubbles produced are small ( < 100-120 #m), because: - - w i t h small bubbles a smaller contact angle is possible than with large bubbles (Hanisch, 1959) --small bubbles are included into the floc more easily - - t h e possibility of collision and adhesion between a bubble and a particle increases more than proportionally to the number of bubbles and is independent of the bubble size (Flint and Howarth, 1971; Reay and Ratcliff, 1973); with a known quantity of air the bubbles should be as small as possible in order to increase the bubble concentration --small bubbles have a lower rising velocity than large bubbles, so that the residence time in the flotation unit is longer; because of this the possibility of collision between a bubble and a sludge floc is increased --because of the high rising velocity of large bubbles (t>2 ram) the shear forces on a rising bubble-floc agglomerate are so strong that the flocs may break (Jedele, 1984).
The adhesion between bubble and floc can be stimulated by adding chemicals which act as collectors (Hanisch, 1959). A collector is a long molecule with a hydrophilic (chemically active) and a hydrophobic (chemically non-active) part. The hydrophilic part turns to the solid phase and the hydrophobic part to the liquid phase. In this way the particle becomes hydrophobic. Sludge is often conditioned with organic polymers (polyelectrolytes). The polyelectrolyte stimulates the production of bubble-particle agglomerates in two ways. It links the sludge particles so that bigger floes are produced and it also acts as a collector so that the t o e ' s surface becomes hydrophobic. Both effects stimulate adhesion between bubble and floe.
BUBBLEraZE ANDBUBBLEFORMATION When the large-scale thickeners in practice are operated without polymers, their disappointing results may be due to insufficient adhesion of the bubbles to the sludge flocs. The bubble size, which is an important factor in adhesion, is determined to a high degree by bubble formation; regarding bubble formation, some aspects will be further discussed. Bubble production
When pressure suddenly drops in the nozzle or the valve, bubbles are probably formed due to cavitation. Cavitation is the growth (explosion) of a bubble because of a pressure drop, followed by implosion and bursting of the bubble into dozens of small bubbles. Cavitation results from a major pressure drop in the narrow opening of a nozzle or a valve (Fig. 3): l 2 Po +~pvo
=
l 2 (Bernoulli), Pk d-~pVk
and because Vk >>VO Pk = Po -- ~pvk I 2
where P0, v0 are respectively pressure and velocity before the narrowing; Pk, Vk are respectively pressure and velocity at the narrowing. Behind the orifice a highly turbulent mixing zone is formed. The turbulences in this zone cause severe pressure fluctuations and because the pressure in the jet stream Pk is lOW (Vk is very high), negative pressure
Nature and size o f the particle (floc)
The nature and structure of the particle determine what kinds of adhesion are possible and--with ads o r p t i o n - w h a t the maximum bubble size is when adhesion is still possible (Hanisch, 1959). The collision efficiency is (at least) proportional to the square particle size (Reay and Ratcliff, 1973). Furthermore, the t o e ' s firmness is also important because the shear forces which are exerted during rising of the bubble-floe agglomerate can cause breakage of the floc (Jedele, 1984).
Fig. 3. Pressure loss along a nozzle.
SANDERE. DE RIJK et al.
468
zones may appear. Because the resistance of the water against these stress forces is very good, cavities (bubbles) are only produced in so-called "weak spots" (for example, pollutants, irregularities in the wall).
[ Column ~
I
- - u p t a k e of air from the (supersaturated) main flow. As the bubble formation is less complete the main flow remains more supersaturated and the possibility of gas transport into the bubbles will increase ---decrease of the hydrostatic pressure. This effect on the bubble size is negligible at the water-depth used in practice (Jedele, 1984) ----coalescence. This is the merging of bubbles. In practice it appears that coalescence occurs mostly in the turbulent zone near the nozzle and that coalescence in the flotation unit itself is negligible, because the total air content in the flotation unit is low and, because the bubbles follow the flow and therefore cannot collide (van Bennekom, 1979). The possibility of coalescence can be reduced by adding chemicals that decrease the surface tension (Hanisch, 1959).
Effect o f pressure drop on bubble formation The bubble size decreases as the pressure drop near the air-releasing cell increases, e.g. when the saturation pressure is increased. However, the minimum diameter, measured as an average value, appears to be near 40 or 5 0 p m (van Bennekom, 1978; Takahashi et al., 1979; Jedele, 1984). Besides the effect on the bubble size, the pressure naturally determines the released air volume because the satu-
.1 "ogC~o O0~P 0
v,,i •
T .1.
Saturated water
/ I
~ 7 v valve Saturated water Fig. 4. Diagram of the flotation column.
]
V Cuvet
'
Bubble t a J
Bubble growth When the dispersion water is released the bubbles will generally be small, but the size of these bubbles can increase due to the following factors:
Camera
~'3
J Background (q~ 6, 5 cm)
/ /
Reflection
/
,, /
/ / /
/ /
Flash
Fig. 5. The set-up of the photographic equipment. ration concentration of air in water increases in proportion with the pressure.
Effect o f nozzle on bubble .formation Investigations into the operation of nozzles, which are applied in drinking water treatment, indicate the importance of a good nozzle construction, particularly of the part behind the release opening (orifice of jet pipe). A badly constructed nozzle will cause incomplete air release and coalescence (van Bennekom, 1978; Takahashi et al., 1979).
METHOD
FOR MEASURING THE BUBBLE SIZE
The flotation unit in the Laboratory of Sanitary Engineering of the Delft University of Technology has been used for the determination of bubble size. The pilot plant consists of a flotation column and an air-saturation unit. The saturation of tap water takes place with a venturi aerator resulting in a saturation efficiency of about 97%. The column is made of perspex, is 150 cm high and has an inner diameter of 29 era. During the measurements the valve or the nozzle can be placed on a support in the flotation column and be adjusted from the top of the column with a rod. The valve can also be placed outside the column. In this case the inlet is just above the bottom of the column via point A in Fig. 4. There is a tapping point in the wall near the top of the column from which a small flow of water saturated with bubbles can be tapped and led through a cuvet. The bubbles in the curet can be photographed so that later the bubble size and bubble concentration can be determined. The tapping point consists of a perspex tube with a diameter of 7 ram, which extends 7.5 cm into the column. The tap is 1 m above the inlet at the bottom of the column (see Fig. 4). When the valve is placed inside the column the distance between the valve and the tapping point is 60 cm. Figure 5 shows how the photographic equipment has been set up. Determination of the bubble size and the bubble concentrations from the photo negatives has been carried out with the aid of automatic image analysis equipment, the Magiscan (Joyce Loebl). For large numbers of measurements the use of image analysis equipment is an attractive alternative to manual measurements. However, the photographs have to be of good quality (sharp and have good contrast) and the bubble concentration must not be too high, because agglomerated bubbles are dLfficult to separate.
EXPERIMENTS ONLABORATORY SCALE The objective of the laboratory experiments is to predict the effects which may occur in practice.
469
Bubble size in flotation thickening 120 ~20
110 _ 30¢m
o 50 I/h
+ 75 I/h
,.,100 - ~
Sludge
¢ 100 I/h
90
ValV~A
A
~50
~
+80ram
.~ 70
/ Top view
Saturated
water
°°lOI
Distance A : 12 cm at H'meer 64 cm at Zwolle
0e---
--t:~f
Fig. 6. The air-release system in practice; sewage treatment plants at Horstermeer, Zwolle (Bath). In the flotation units o f Horstermeer, Zwolle and Bath the depressurization o f the dispersion water is effected with the aid o f flap valves. After air release, dispersion water is added to the sludge pipe from two sides (4 pipes per flotation tank). This is shown in Fig. 6. The flotation unit o f Bath differs from the others in that the sludge is added at ] o f the length o f the tank. It was presumed that the presence o f the pipe or tube behind the valve, before intensive mixing of the dispersion water with the sludge, would cause bubbles to grow as a result o f coalescence and uptake o f air from the liquid. It has been studied in the laboratory if this type o f phenomenon can indeed occur. This has been done by measuring, at various pressures and flow rates, the bubble formation obtained with the valve only and comparing it with the bubble formation obtained under the same circumstances when tubes of various lengths and diameters have been placed behind the valve. The flow rates and dimensions o f
Pressure (bar)
Flow rate (l/h) 50 75 100 50 75 100 50
6.2
50 4o 30
I
I
3.5
5.0
I 6.2
S a t u r a t i o n p r e s s u r e (bar) Fig. 7. Relation between bubble size (median) and saturation pressure (laboratory experiment, needle valve without tube, flow rate 50, 75 and 1001/h). the tubes behind the valve have been selected in such a way, that the retention times and the flow rates inside the tube correspond to the design o f the flotation units at Horstermeer and Zwolle. During the measurements a needle valve (Econosto) was used because a sufficiently small normal valve was not available. For comparison, measurements have also been carried out with a normal valve (the smallest available) and a scale model o f the Hague nozzle. The laboratory experiments produced the following results (see also Table 2). RESULTS
Bubble size Figure 7 shows the relation between the median b u b b l e size a n d t h e s a t u r a t i o n p r e s s u r e f o r t h e needle valve ( w i t h o u t t u b e ) at different flow rates. It a p p e a r s f r o m t h e figure t h a t t h e b u b b l e size decreases w i t h a n increase in t h e s a t u r a t i o n p r e s s u r e a n d w i t h a n increase in t h e flow rate. T h e effect o f t h e flow rate o n t h e b u b b l e size d e c r e a s e s as t h e s a t u r a t i o n p r e s s u r e increases a n d is h a r d l y p r e s e n t at 6.2 bar.
Table 2. Median bubble size for various situations Needle valve Normal valve Nozzle -10 10 60 60 ---4 10 4 10 ---
Tube Length (cm) Diameter (ram)
5
6o
m
25 cm
Side view
3.5
~
° O°oo.i.o c °o¢o o~ o o o •
~- . . . . .
8o
75 100
Median bubble size (pro) [Standard deviation (pro)] 107 [6.0 84 [4.2 57 [I.6 74 [I.9 52 [I.0 51 [I.1 43 [I.O 49 [1.2 38 [0.7
96 2.1 93 3.4 109 3.6 86 !.6 107 2.9 92 2.8 75 3.0 80 2.6 76 2.3
94 2.6 114 6.1 113 3.7 121 4.2 103 2.2 110 3.8 97 3.8 92 3.5 91 2.4
143 11 121 12 152 14 135 8.6 119 8.0 146 17 150 19 129 10 102 8.4
122 8.8] 131 5.8] 112 7.2] 72 5.4] 86 4.4] 116 ! 1] 84 3.7] 75 2.0] 86 2.9]
---
63 1.5 --
58 1.5 --
75 2.4 40 1.2 --
47 1.1
39 0.8
-
-
-
-
-
-
39 0.9
-
-
-
-
33 0.6
470 30 28 26 24 22 20 18 ~ 16 ~ 14 ~ 12 ~ 10 8 6 4 2 0
SANDERE. DE RUK et al. +
+
\ +
\
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0
,~/
J 40
+
x+,
I
1+7+'+.+.++. LIt ,J,l . . . . . .
80 120 160 Bubble size (gin)
200
240
Fig. 8. Bubble size distribution (laboratory experiment, needle valve without tube, flow rate 100 l/h, pressure 5 bar).
Figure 8 shows a representative example of the bubble size distributions obtained with the needle valve. This distribution has been measured at 5 bar and at a flow rate of 100 l/h.
x.J co
LZ
Table 2 shows that the bubble size increases in most situations when a tube is placed behind the valve. Particularly when " g o o d " bubbles ( ~ 60 #m) are produced without a tube behind the valve, the deteriorating effect of the tube is significant and this then results in larger bubbles. Figure 9 shows how the bubble size distribution changes when different tubes are placed behind the valve. Figure 9(a) shows the distributions obtained with both the needle valve itself and the valve with the two short tubes (10 cm) at a pressure of 5 bar and a flow rate of 1001/h. When a tube is placed behind the valve the bubble size distribution moves to larger bubble diameters. In Fig. 9(b) the distributions obtained with the two long tubes (60cm) behind the valve are shown. The uneven curves of the distributions are due to the limited number of measurements which were available (due to low bubble concentration). Figure 9(c) shows the cumulative distributions. With increasing tube length the curves become more flat, indicating a shift of the distribution to larger diameters.
26 24 22
o nv and tube 10em/4mm
20 18 16
v nv
4 2 0
~
+ nv and tube lOcm/lOmm
101 f.1 "+x.h
I0
50
90
130 170 210 Bubble size (gm)
250
290
3024262820-I22 (b) Anv and tube 60cm/10mm
o"
0 nv and tube 60cm/4mm
18 16 14 12 10
-
6 4 28
o ----~ lO
50
90
130 170 210 Bubble size (~,rn)
Fig. 9 (a) and (b). Caption opposite
250
290
901
Bubble size in flotation thickening
471
r
8O
7O
i
40 3o
/
/
I-
/ ~ 4
A nv and tube 60cm/10mm + n , and
20[-
~ / "
l
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//
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ff / ' f :~
+'l 10
V nv 0 nv and tube 60cm/4mm
_.j..+/'
o 50
tube10cm/10mm
I
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I
90
I
130
I
I
170
I 210
I
I
I
250
I 290
B u b b l e size (I.tm) Fig. 9(c)
Fig. 9. Bubble size for various situations (laboratory experiment, flow rate 100 l/h, pn~sure 5 bar). (a) and (b) Distributions and (c) cumulative. Bubble concentration
The bubble concentration is inversely proportional to the bubble size, so that the relations found for the bubble size also apply for the bubble concentration. Increases in the pressure and the flow rate lead to a higher bubble concentration in the valve without a tube. With a tube behind the valve the bubble concentration decreases and there is no longer a significant relation between the flow rate and the saturation pressure. A relation between the bubble concentration and the dimensions of the tube has not been found. Released air volume
An estimate of the released air volume has been made from the bubble size distribution. When comparing the calculated volumes with the theoretical maximum volumes, the former appear to be only 0.03-2.2% of the maximum volumes. The difference between these values can be explained by the dilution of the bubble concentration in the flotation unit caused by the rise and disappearance of the bubbles at the sur- face. The extent of the dilution depends on the rising velocity, and therefore the size, of the air bubbles. The diluted concentrations are calculated as a
percentage of the bubble concentration immediately behind the valve. The results of these calculations are shown in Table 3. In the calculations the column has been approached as a completely mixed system. The rising velocities have been determined from Stokes' equation: S = 1 ,g*d 2
18
s = rising velocity (m/s) d = bubble diameter (m) g = 9 . 8 1 m/s 2 v = kinematic viscosity; 1.1 x 10 -6 (m2/s). During the experiments it has also been observed that large bubbles with an estimated diameter of 1-5 mm were not tapped from the column. Therefore the method used is selective for smaller bubble diameters, so that larger bubbles, with a considerable share in the air volume, are measured less often or not at all. This is why it is unknown in what way the measured air volumes compare with the actual released air volumes. Due to this, the measured values of the median bubble sizes will also be lower than the actual median values of the released bubbles. Because the effect of the dilution is stronger as the bubbles are larger, the
Table 3. Dilution of the bubble concentration Flow rate (l/h)
Bubble diameter (~m)
Rising velocity (m/h)
Residencetime in the column (min)
Concentration with respect to maximum (%)
100
10 50 100 150 10 50 100 150
0.2 4.6 17.8 40.1 0.2 4.6 17.8 40.1
50 11 3.2 !.5 86 12 3.3 !.5
83 18 5.3 2.5 71 10 2.7 1.3
50
WR 28/2----O
v
where
SANDERE. DE RUK et al.
472
Table 4. Measurementsof the bubble size at the sewage treatment plant at Zwolle Bubble size(# m) Sludge flow rate
(m3/h) 6 3 3 3 3
Dispersion water flow rate (rn3/h)
Median
Standard deviation
4 4 2.75 2.25 1.9
59 61 62 47 46
0.8 0.9 0.8 -0.6
real differences in bubble size will be larger than has been measured.
Effect of the type of valve Table 2 shows that the size of the bubbles produced with the needle valve and the normal valve differ little. At a saturation pressure of 5 bar and a flow rate of 50 l/h the bubbles produced with the needle valve are approx. 25% larger. With the two other adjustments the bubble diameters are practically equal. Therefore the exact shape of the orifice seems to have little effect on the bubble formation and it is justified to make use of the needle valve for the small-scale experiments. Table 2 shows that the bubbles produced with the Hague nozzle are generally smaller than those produced with the needle valve.
laboratory experiments at 5 bar and 1001/h, an adjustment which best approaches practice, a bubble size of about 120-150#m was measured with the same tube length as the one at Zwolle, viz. 60 cm; without the tube the median was about 50 #m. The measured bubble sizes ( 4 6 - 6 0 # m ) approach the m i n i m u m value which can be obtained according to literature. Therefore it may be concluded that the bubble formation in the flotation plant at Zwolle cannot be improved much. The bubble growth which occurred in the laboratory experiments is probably caused by scaling effects. Due to the limited capacity of the available saturation unit it was, however, impossible to work on a larger scale. CONCLUSIONS It appears possible to study the bubble size distribution both on laboratory scale and in practice. With the measuring method used the performance of an air-release system can be indicated fairly quickly after a trial period. When only the operation of the valve is considered (i.e. without a tube), an increase in the saturation pressure leads to the formation of smaller bubbles. 3o
28 -Measurements in practice The laboratory experiments have shown, among other things, that the bubble size increases significantly in the tube behind the valve, but a relation between the residence time or the flow rate in the tube and the size and the concentration of the bubbles has not been found. Therefore it is difficult to apply these results to a large flotation unit in practice. As bubble growth may, in practice, also occur in the tube behind the valve it has been decided to measure the bubble size in practice. These measurements have been carfled out at the sewage treatment plant at Zwolle. The bubble-saturated water was tapped from the sludge pipe at a distance of 15 cm behind the mixing point (of the dispersion water with the "sludge"). Since the bubbles were photographed, effluent was now pumped through the sludge pipe instead of sludge. It appears from laboratory experiments that direct tapping of a partial flow from a pipe to the cuvet led to an irregular flow through the cuvet. For this reason the tapped bubble flow was first led through the column of the laboratory flotation unit and then to the cuvet so that a proper flow through the cuvet was obtained. Furthermore, with this method the bubbles in the column could already be assessed visually and compared with the laboratory results. The results of the measurements in practice are presented in Table 4 and Fig. 10. The median of the bubble size distribution is about 6 0 # m under normal operating conditions. In the
e~
L~
26-24-22-20-18-16 14 12 10 8 6 4 2 0
(a)
10
50
90 130 170 210 Bubble size (ptm)
250
290
100
@. ~ ~ ~" N
80' 70 60 5O 4O 3O 20 10 0 ~1 10
t I I t i i I I I I I I l 50 90 130 170 210 250 290 Bubble size (l~m) Fig. 10. Bubble size for the situation in practice (sewage treatment plant at Zwolle, sludge flow rate 6m3/h, dispersion water 4 m3/h). (a) Distribution and Co) cumulative.
Bubble size in flotation thickening This corresponds with theory. With low saturationpressure values ( < 5 bar) a higher flow rate also stimulates the formation of small bubbles, whereas with higher pressure no significant effect of the flow rate can be observed. Bubble size increases with the presence of a tube behind the valve when the pressure values and the flow-rate values are the same. The effect of the tube geometry on bubble formation is not clear; a significant relation between bubble size and residence time or flow rate has not been found. The bubble formation through the needle valve and the normal valve are practically equivalent, whereas the Hague nozzle produces slightly smaller bubbles. The median size of the bubbles produced in the flotation thickener of the sewage treatment plant at Zwolle is approx. 60/zm under normal working conditions. The bubble growth, which occurred in the laboratory experiments in the tube behind the valve, is a very significant phenomenon, but is probably a result of the scale used. Considering the measured bubble size it is not expected that the flotation results of the sewage treatment plant at Zwolle can be improved by adaptations to the bubble formation. Acknowledgements--The authors wish to thank Mr R. Neef (Zulveringsschap Amstel- en Gooiland), Mr F. A. Brandse (Zulveringsschap West-Overijssel), Mr J. Mandemaker
473
(Sewage Treatment Plant at ZwoUe), Mr J. Kruit 0Vaterschap De Dommel), Mr R. E. M. van Oers (Hoogheemraadschap West-Brabant), Mr A. M. J. Pel (Sewage Treatment Plant at Bath) and Mr P. L. Buyink (Drinkwater-bedrijf Zuid-HoUand) for their collaboration. Appreciation is also given to Mr C. S. van der Reijden (Delft University of Technology, Department of Sanitary Engineering and Water Management) who assisted with the image analysis equipment.
REFERENCE~ van Bennekom C. A. (1978, 1979) The effect of different types of nozzles on the flotation process. KIWA reports SWE-202 and SWE-213 (in Dutch). Flint L. R. and Howarth W. J. (1971)The collisionet~ciency of small particles with sphericM air bubbles. Chem. Engng Sci. 26, 1155-1168. Hanisch B. (1959) The scientific application of flotation with very small air bubbles for purification of sewage water. Stuttgart University of Technology (in German). Jedele K. (1984) Application of dissolved-air flotation for segregation of the activated sludge from the water. Stutt. Ber. Siedlungswass. wirtsch. 84 (in German). Reay D. and Ratcliff G. A. (1973) Removal of fine particles from water by dispersed-air flotation: effects of bubble size and particle size on collection efficiency. Can. J. Chem. Engng 51, 178-185. de Rijk S. E. (1991) Thickening of sewage sludge by dissolved-air flotation. Delft University of Technology, Faculty of Civil Engineering, Department of Sanitary Engineering and Water Management (in Dutch). Takahashi T., Miyahara T. and Mochizuki H. (1979) Fundamental study of bubble formation in dissolved-air pressure flotation. J. chem. Engng Jap. 12, 275-280.