, however, be more fruitful to try to improve the Aerobic
biological methods have been successfully used to treat livestock and
poultry wastes (Kargi et al., 1980; Bicudoe et al., 1995; Carta, et al.
1999; Tiquia et al., 2002; Juteau et al. 2004; Mohaibes and
Heinon-Tanski, 2004; Zhu et al., 2004; Zhang et al., 2006). The main
functions of any aeration device are to: (a) supply a sufficient
quantity of oxygen to the liquid medium to maintain aerobic conditions,
(b) circulate the liquid to keep solids in suspension and avoid
settling, (c) distribute the oxygenated liquid throughout the liquid
body to avoid anaerobic zones and (d) keep good contact between the
microbial cells, nutrients and dissolved oxygen to ensure efficient
biodegradation of the organic matter (Carta et al., 1999; Fritsche and
Hofrichter, 2008). From the viewpoint of the livestock producer, the
aeration process should be accomplished at the least cost per kilogram
of waste treated. The oxidation ditch system has been successfully used
for treating animal waste (Loehr, 1971; Murray et al., 1975; Ghaly,
1982; Ushikubo et al., 1991; Otawa et al., 2006). The ditch essentially
consists of a closed loop, open channel in which liquid circulation and
oxygen input are maintained by a mechanical device such as paddle,
brush, or disc aerators. The disc aerator described by Drews et al.
(1972) and Ghaly (1982) has certain advantages over the brush and paddle
wheel rotors with regard to foam generation. However, the oxygenation
capacity of an individual disc is limited and the use of a number of
discs on the rotating shaft was suggested in order to allow for a wider
range of oxygenation capacities (Drews et al., 1972; Ogilvie and
Kingsbury, 1974; Ghaly, 1982; Nemerow, 2007). It wouldoxygenation
capacity of a single disc aerator rather than merely use large numbers
The objectives of this study were to investigate the effects of
various disc design parameters and system operation parameters on the
oxygenation capacity of the disc aerator.
Experimental Apparatus: A bench scale oxidation ditch system (Fig.
1) equipped with a disc aerator was used to gain an understanding of the
phenomenon of oxygen transfer and to study the effects of the disc
design parameters and system operation parameters on the rate of oxygen
transfer in deionized water. The equipment included a ditch, a hood,
aerator discs, a motor and a speed controller.
[FIGURE 1 OMITTED]
A race track type oxidation ditch was constructed of acrylic
plastic (Fig.2). The thickness and height of the ditch walls were 0.4
and 16.0 cm, respectively. The walls of the ditch were gluedonto a 76 *
38 cm acrylic plate of 0.6 cm thickness. The inner and outer diameters
of the circular part of the ditch were 15 and 30 cm, respectively. The
length of theditch side was 31.35 cm, the ditch area was 1000 [cm.sup.2]
and the volume was 16000 [cm.sup.3].
[FIGURE 2 OMITTED]
The aerator discs were fabricated of acrylic plastic. The diameter
of the discs was 29 cm. Holes were drilled in the outer 7 cm, parallel
to the disc axis. The independent variables considered in the design of
the discs were: disc thickness, hole diameter and number of holes. Five
disc thicknesses (0.32, 0.64, 1.28, 1.92 and 2.55 cm) and five hole
diameters (0.00, 0.32, 0.64, 1.28 and 1.29 cm) were studied. A total of
twenty five discs were fabricated. The number of holes drilled in the
discs were such that all discs had the same perforated area (123
[cm.sup.2] out of660 [cm.sup.2] i.e. 19%). This resulted in 1536, 384,
96 and 43 holes per disc for the discs having 0.32, 0.64, 1.28 and 1.92
cm hole diameters, respectively. The aerator was covered with a hood
made of acrylic plastic to avoid loss of the ditch contents by
The disc was mounted on a rotating shaft so that it was partially
submerged in the ditch contents. The disc shaft was driven by an
adjustable speed (0-500 rpm) electric motor (Steadi-Speed Stirrer No.
14-498A, Fisher Scientific, Montreal, Quebec, Canada). The motor shaft
was attached to the disc shaft through a chuck assembly. The disc
immersion depth was adjusted by lowering or raising both the drive and
disc shafts to the required height using two pieces of Plexiglas of the
same thickness (2.5 and 5.0 cm) under the bearings. A generator
tachometer (Servo-Tek Model ST-9540-20, Fisher Scientific, Montreal,
Quebec, Canada) was connected to the disc shaft by means of a Tygon tube
to measure the speed of rotation.
A general purpose filtering funnel (Kimble 28950 No 10-322E, Fisher
Scientific, Montreal, Quebec, Canada), connected to the ditch by a tygon
tube, was used to gradually add sodium sulfite and cobalt chloride
solutions to the ditch. An impeller of 31 cm length and 4.4 cm propeller
diameter, connected to a stirring apparatus (Dyna-Mix Model 43, Fisher
Scientific, Montreal, Quebec, Canada) with the propeller at the
mid-depth of the ditch, was used to circulate and mix dissolved oxygen
was measured by a polarographic electrode (Beckman 39553 [O.sub.2]
Sensor, Fisher Scientific, Montreal, Quebec, Canada) connected to a
dissolved oxygen meter (Beckman Fieldlab Oxygen Analyzer Model 1008,
Fisher Scientific, Montreal, Quebec, Canada).
A Locam 16 mm High Speed Motion Picture Camera (500 frames per
second) was used to film the movement of bubbles during the aeration
Experimental design: Three independent disc design variables (disc
thickness, hole diameter and number of holes) and two operational
variables (disc immersion depth, disc rotational speed and number of
coaxially mounted discs) were considered for optimization of the
oxygenation capacity of the system. However, a complete factorial
experiment including all six factors was considered impractical in view
of the scope and time available for the completion of this study.
Therefore, three sets of experiments were carried out. In the first set
of experiments, the oxygen transfer coefficient [(K.sub.L]a) was
determined at all combinations of three immersion depths (2.5, 5.0 and
7.5 cm) and five disc speeds (50, 100, 150, 200 and 250 rpm) for one
disc having a thickness of 0.64 cm and 48 holes of 1.28 cm diameter. The
results were compared with those obtained when using a nonperforated
disc of the same thickness.
After having eliminated the immersion depth as an independent
variable, a second set of experiments was conducted to study the effects
of disc speed, disc thickness and hole diameter on [K.sub.L]a. In this
set, a 5 x 5 x 5 factorial experiment, laid down in a completely
randomized block design, was used. Split-split-plot design in two blocks
was utilized. The five levels of the hole diameter (0.00, 0.32, 0.64,
1.28 and 1.92 cm) were randomly assigned into the main plot units in
each block. Then, the five levels of the disc thickness (0.32, 0.64,
1.28, 1.92 and 2.55 cm) were assigned at random into the sub-plot units
in each main plot unit. Finally, the five levels of disc speed (50, 100,
150, 200 and 250 rpm) were assigned at random into the sub-sub-plot
units in each sub-plot unit. The two blocks were used as replicates
resulting in a total of 250 treatments.
In the third set of experiments, the effect of using more than one
aerator disc on the rotating shaft on [K.sub.L]a was studied. Because of
the limited width of the ditch (7.5 cm), it was only feasible to install
a maximum of two discs, spaced at 2.5 cm on the rotating shaft. This was
done for two disc thicknesses (0.32 and 0.64 cm). The immersion depth of
7.5 cm was maintained during the experiments. [K.sub.L]a values were
obtained for these two discs at five disc speeds (50, 100, 150, 200 and
250 rpm). The results were compared with those obtained the ditch
contents during the deoxygenation process. when using single discs of
the same thickness and single discs of double the thickness.
Experimental procedure: During each experimental run, the ditch
contents were aerated, deoxygenated and then reaerated. The unsteady
state method with sulfite oxidation described by Ghaly et al. (1988) was
used. Initially, the ditch was cleaned thoroughly with tap water and
then flushed several times with deionized water before each run. Eleven
litres of deionized water were then added to the ditch. The dissolved
oxygen meter, recorder and aerator motor were started simultaneously and
the temperature and barometric pressure were recorded.
When the saturation concentration [(C.sub.S]) was reached (the
oxygen concentration curve became flat), the aerator motor was switched
off and 13.8 ml of 0.10 M [[Na.sub.2]SO.sub.3] solution (173.8 mg sodium
sulfite) and 1.6 ml of 0.01 M [[CoCl.sub.2]*6H .sub.2]O solution (2.2 mg
cobalt chloride) were added. These solutions were distributed throughout
the oxidation ditch by the impeller. When the water was completely
deoxygenated, the impeller motor was shut off and the aerator motor was
started. The dissolved oxygen concentration [(C.sub.L]) then increased
until it reached the saturation concentration [(C.sub.S]). During a
typical experiment, [C.sub.L] remained at zero until the excess sodium
sulfite had been oxidized, rose rapidly at the beginning and then slowly
approached its saturation value again. A typical recorded curve of the
entire process of saturation-deoxygenation-reaeration is shown in Fig.
3. Data from the reaeration curves were plotted on semilogarithmic study
to determine the [K.sub.L]a values as shown in Fig. 4. [K.sub.L]avalues
obtained under laboratory conditions were converted to their equivalent
values at 20 101.3 kPa using the following equations:
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[C.sub.sp] = [C.sub.s] [[P.sub.ab]/[P.sub.at] (1)
[K.sub.L][a .sub.(20)] = [[K.sub.L][a .sub.(T)] [(1.24).sup.20-t]
Slope = [-K.sub.L]a
[C.sub.s] = Concentration of [O.sub.2] in water at laboratory
barometric pressure (mg L [.sup.-1])
[C.sub.sp] = Concentration of [O.sub.2] in water at standard
atmospheric pressure (mg L [.sup.-1])
[P.sub.ab] = Absolute laboratory barometric pressure (kPa)
[P.sub.at] = Standard atmospheric pressure (kPa)
[K.sub.l][a.sub.(20)] = The overall volumetric oxygen transfer
coefficient at 20[degrees]C
[K.sub.l][a.sub.(t)] = The measured overall volumetric oxygen
T = The water temperature ([degrees]C)
Equation 1 was used to correct the value of [C.sub.s] and equation
2 was used to correct the value of [K.sub.L]a. This left [K.sub.L]a
exclusively a function of the design parameters of the disc aerator in
the oxidation ditch.
Immersion depth experiments: The effect of immersion depth on
[K.sub.L]a was studied at various disc speeds. The perforated disc of
0.64 cm thickness having 48 holes of 1.28 cm diameter and the
nonperforated disc of the same thickness were used. The results of these
experiments are reported in Table 1 and shown in Fig. 5.
[FIGURE 5 OMITTED]
Factorial experiment: A 5X5X5 factorial experiment, laid down in a
completely randomized split-split-plot design in two blocks, was
utilized. The immersion depth of 7.5 cm was maintained throughout these
experiments. The analysis of variance was performed on the
[K.sub.L]adata using the statistical computer program ANOVA (Sigmaplot,
Version 11, Systat Software Inc., California). The results are shown in
Table 2. To test the differences among the levels of each factor,
Duncan's Multiple Range Test was performed on the [K.sub.L]adata.
The results are shown in Table 3-5. The effects of each variable
(disc speed, hole diameter or disc thickness) on Ka, while holding all
other variables constant, was investigated. The effects of disc speed
(using discs of 2.55 cm thickness, having 28 holes of 1.92 cm diameter),
hole diameter (using discs of 2.55 cm thickness at a disc speed of 250
rpm) and disc thickness (using perforated discs having holes of 1.92 cm
diameter at a disc speed of 250 rpm) are shown in Fig. 6. Changing the
values of the fixed parameters yielded similarly shaped curves for the
disc speed, hole diameter and disc thickness.
[FIGURE 6 OMITTED]
Two disc experiment: The effect of using two discs on [K.sub.L]a
was investigated at an immersion depth of 7.5 cm. Two discs of 0.32 and
0.64 cm thickness were used individually and in pairs and the results
were compared with those obtained from individual discs of double the
thickness. The results are reported in Table 6 and presented in Fig. 7.
[FIGURE 7 OMITTED]
Immersion depth: The immersion depth had a significant effect on
[K.sub.L]a. Increasing the immersion depth increased [K.sub.L]afor both
the perforated and nonperforated discs (Table 7). Increasing the
immersion depth from 2.5 to 5.0 cm (100 % increase) increased
[K.sub.L]aby 30-93 % for the nonperforated disc and by 40-193% for the
perforated one, depending on the rotational speed. A further increase in
immersion depth from 5.0-7.5 cm nearly doubled these values.The increase
in [K.sub.L]awas due to several factors. First, increasing the immersion
depth increased the disc area passing through the liquid. About 32, 57
and 77 % of the disc area passed through the liquid for 2.5, 5.0 and 7.5
cm immersion depths, respectively (Table 8).
Second, increasing the immersion depth increased the number of
holes passing through the liquid. For immersion depths of 2.5, 5.0 and
7.5 cm, there were 38, 70 and 96 holes passing through the liquid per
revolution, respectively. Third, increasing the immersion depth
increased the average residence time of the holes and, therefore,
presumably the residence time of the bubbles.
Fourth, increasing the immersion depth increased the extent of the
oxygenated and thoroughly mixed layer. The oxygenated layer was 23, 46
and 69 % for the immersion depths of 2.5, 5.0 and 7.5 respectively.
The number of holes = 48
The hole diameter = 1.28 cm
In this experiment, the effect of disc speed on [K.sub.L]awas also
extensive. Increasing the disc speed increased [K.sub.L]afor both the
perforated and nonperforated discs. The rate of increase was dependent
on the immersion depth; the greater the depth the more rapid was the
increase in [K.sub.L]a (Table 9).
[FIGURE 8 OMITTED]
For all disc speeds, the perforated disc had much values compared
to those of the nonperforated disc. Increasing the number of holes
passing through the liquid increased the number of bubbles in the liquid
per unit time, although it decreased the average residence time.
Moulick et al. (2002) found the immersion depth of a paddle wheel
aerator to have a significant effect on aeration. Through a series of
tests, performed while holding other variables constant, they were able
to determine the optimum depth for the paddle wheel system. Thakre et
al. (2008) assessed the effects of immersion depth on the oxygenation
capacity and power consumption of paddle wheel aerators and [K.sub.L]a
[(min.sup.-1]) found that increasing immersion depth allowed for a
marginal increase in oxygenation capacity and a dramatic rise in power
consumption. Thakre et al. (2009) tested the effects of immersion depth
on the oxygenation capacity of curved blade aerators and found that as
immersion depth was increased from 4.8-7.2 cm (50% increase), the oxygen
transfer coefficient [(K.sub.L]a) rose from 8.41-10.93 [h.sup.-1] (30%
increase) and the power consumption almost doubled (from 69.9 W to 136
Pasveer (1953) investigated the effect of immersion depth of a 42
cm diameter brush aerator on the oxygenation capacity and found that
when the immersion depth was increased from 5 cm to 14 cm (180%
increase), the oxygenation capacity rose from 147 g/hr [m.sup.3] to 391
g/hr [m.sup.3] (166% increase). Al-Ahmady (2006) investigated the oxygen
transfer capacity in a bench scale subsurface aerator and found that by
increasing the water depth the average bubble residence time increased
allowing more time for oxygen transfer to occur. As water depth rose
from 0.5 m to 4.6 m (820% increase) the oxygen transfer capacity rose
from 18-170 g [m.sup.3] [*.sup.-1] h (844% increase). Gillot et al.
(2005) Bayramoglu et al. (2000) developed models to determine oxygen
transfer rates in diffused air aeration tanks and found that as water
depth increased, the oxygen transfer rate increased. Groves et al.
(1992) found that perforated membrane diffusers provided better oxygen
transfer than coarse bubble diffusers due to increases in bubble surface
area to volume ratio.
Disc speed, thickness and hole diameter: The analysis of variance
performed on the data indicated that all three factors (disc speed, hole
diameter and disc thickness) had highly significant effects (at 0.0001
level) on [K.sub.L]a. All three factors were also significantly
interrelated (at the 0.0001 level). The five levels of each factor were
found to be significantly different from each other.
For any given combination of disc thickness-hole diameter,
increasing the disc speed increased [K.sub.L]arapidly. For example, at
the best combination (producing highest [K.sub.L]a) of disc
thickness-hole diameter (2.55 cm and 1.92 cm), increasing the disc speed
from 50 -250 rpm (400 % increase) increased [K.sub.L]a by 585 % (from
Although the effect of disc thickness on [K.sub.L]a was not as
profound as that of disc speed, it was significant. At the best
combination of disc speed-hole diameter (250 rpm and 1.92 cm),
increasing the disc thickness from 0.32-0.64 cm (100% increase)
increased [K.sub.L]aby 55 %. A further increase in the disc thickness up
to 2.55 cm yielded an additional increase in [K.sub.L]a of only 39 %.
The presence of holes and their diameters, showed considerable effects
on [K.sub.L]a. At the best combination of disc speed-disc thickness (250
rpm and 2.55 cm, respectively), the presence of small holes of 0.32 cm
diameter increased [K.sub.L]a by 116 % over that of the nonperforated
disc. Increasing the hole diameter from 0.32-1.92 cm (500% increase)
resulted in an additional increase in [K.sub.L]a of 223 %.
Paolini (1986) performed a study on a Rotating Biological Contactor
(RBC) aerator used in waste water treatment and found that as the disc
speed increased from 3 rpm to 25 rpm (733 % increase) [K.sub.L]a
increased from 0.11-0.33 [min.sup.-1] (200 % increase). Moulick et al.
(2002) obtained similar results when adjusting mixing speed in a paddle
Clarke et al. (2006) tested the effects of alkalinity on bioreactor
and found that the effects of alkalinity were tied to the speed of the
impeller. The addition of alkalinity increased [K.sub.L]a at speeds
above 300 rpm and reduced [K.sub.L]a at speeds below300 rpm.
Alkalinity is known to improve turbulence, surface reaction
properties and the oxygenation capacity.
Double disc: For all aerator discs (single and double), increasing
the disc speed increased [K.sub.L]arapidly. Increasing the disc speed
from 50 rpm to 250 rpm (400% increase) increased [K.sub.L]aby 583-1824 %
for single aerator discs and by 1990-2456 % for double aerator discs,
depending on the disc thickness. It was observed that [K.sub.L]a values
obtained from a double aerator disc of 0.32 cm thickness (two discs of
0.32 cm thickness each, spaced at 2.5 cm and operating in parallel) were
similar to those obtained with the single disc of 0.64 cm thickness over
most of the speed range; it was however, 19 percent greater at 250 rpm.
For the double aerator disc of 0.64 cm thickness, at lower speeds
(50-150 rpm) [K.sub.L]a values were lower than those of single discs of
1.28 cm thickness but they were considerably greater at higher speeds
(up to 46% higher at 250 rpm). No reports were found in the literature
about the use of multiple discs except that of Drews et al. (1972) in
which they were able to adjust the oxygenation capacity by varying the
number of discs on a single shaft.
Visual observations: High speed movies were taken during an
aeration test at various disc speeds in order to gain a better
understanding of the physical processes involved in the oxygen transfer.
The films were analyzed frame by frame on a photo optical data analyzer
which gave a good picture of the liquid and air bubble movement. Three
mechanisms are believed to contribute simultaneously to the process of
oxygen transfer: (a) bubble aeration, (b) eddy aeration and (c) surface
The holes of the disc had a great effect on the addition of bubbles
and creation of turbulence in the liquid. The bubbles were formed when
the disc entered the liquid and the water replaced the air trapped in
the holes. The bubbles were then detached from the disc and continue
with the fluid as shown in Fig. 8a. The oxygen transfer then occurs from
the air bubbles into liquid phase through the gas-liquid interface.
Bubbles with a higher surface area to volume ratio will increase oxygen
transfer (Waites, 2008). Therefore, a large number of small bubbles is
more desirable than a small number of large bubbles of the same total
volume. Ippen et al. (1954) found that smaller bubbles, distributed in a
The disc also serves as an agitator. The liquid was caught by the
edges of the holes and began to form eddies. As the disc rotated and
holes left the liquid, a mass of eddies was brought above the surface of
the liquid as shown in Fig. 8b. These eddies are considered as
continually exposing fresh liquid surface to the air, then gliding
swiftly away and mixing into the bulk of liquid. During the exposure of
any portion of the liquid to air, transfer of oxygen occurs by molecular
diffusion. The rate of production of fresh eddies was a function of the
disc rotational speed. Deglon et al. (1998) stated that intermediate and
high frequency eddies are better able to cause bubble breakup. They
found that spinning nonperforated discs tend to generate low frequency
eddies and recommended that spinning discs should be modified by adding
grooves or cuts to the edges of discs to improve bubble breakup. In this
study, the larger hole diameter and higher speed created high frequency
eddies with improved bubble breakup and higher oxygen transfer. Oxygen
transfer, from the atmosphere to the liquid body, may have also taken
place at the liquid surface in the ditch due to the movement of water.
The dissolved oxygen concentration of the surface layer would be higher
than that of the bottom layer so that oxygen was transferred downwards.
Eckenfelder (1959) found that surface aeration is the result of bubble
breakup and the velocity gradients present at the liquid air interface.
Increasing the disc speed increased water velocity at the surface layer
and improved the oxygen transfer rate.
The effects of immersion depth, disc speed, disc thickness, disc
perforation and hole diameter on the oxygen transfer coefficient of disc
aerators in an oxidation ditch were investigated. It was found that disc
speed had the most significant effect on [K.sub.L]a with the immersion
depth and hole diameter both showing strong effects as well. While disc
thickness had a significant effect on [K.sub.L]a it was not as strong as
other parameters. There were also significant interactions between these
parameters. Increasing any of the parameters resulted in an increase in
[K.sub.L]a. In all cases, especially those at higher disc speeds,
perforated discs caused significant increases in [K.sub.L]a when
compared to nonperforated discs. Increasing the number of coaxially
rotating discs was also seen to have a significant effect on [K.sub.L]a.
However, the effect of adding a second disc was comparable to using a
single disc of double the thickness at lower disc speeds while at disc
speeds higher than 200 rpm doubling the thickness of a single disc had
less of an effect on [K.sub.L]a than adding a second disc. The visual
observations, made with the assistance of high speed film, indicated
that bubble aeration and eddy aeration, created by disc rotation, were
the prevalent oxygen transfer mechanisms. Surface aeration at the
liquid-air interface may also have played a smaller role in oxygenating
This research was supported by the National Science and Engineering
Research Council (NSERC) of Canada.
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Abdel Ghaly and Skai Edwards Department of Process Engineering and
Applied Sciences, Dalhousie University, Halifax, Nova Scotia, Canada
Corresponding Author: Abdel Ghaly, Department of Process
Engineering and Applied Sciences, Dalhousie University, alifax, Nova
Table 1: [K.sub.L]a ([min.sup.-1]) at various immersion depths and disc
speeds for perforated and non-perforated discs of 0.64 cm thickness
Immersion depth (cm)
Speed 2.5 5.0
Non-perforated Perforated Non-perforated Perforated
50 0.012 0.043 0.015 0.060
100 0.016 0.068 0.031 0.126
150 0.031 0.099 0.060 0.290
200 0.075 0.191 0.121 0.448
250 0.108 0.258 0.180 0.636
50 0.018 0.076
100 0.033 0.171
150 0.068 0.361
200 0.144 0.600
250 0.227 0.847
Table 2: Analysis of variance.
Source Df Ss Ms F Pr>f
Total 249 23.27185
Bet. all mpu (rd comb.) 9 5.64887
R 1 0.00003 0.00003 1.05 0.3665
D 4 5.64873 1.41218 52915.19 0.0001
Error-a (r*d) 4 0.00011 0.00003
within all mpu
(Ts comb. in ra comb.) 240 17.62298
spu in all mpu
tin ra comb.) 40 1.47002
T 4 0.71535 0.17884 719.65 0.0001
D*T 16 0.74970 0.04686 188.55 0.0001
Error-b (r*t(d)) 20 0.00497 0.00025
sspu in all spu 200 16.15296
S 4 13.62288 3.40572 16364.85 0.0001
D*s 16 2.08040 0.13002 624.78 0.0001
T*s 16 0.19872 0.01242 59.68 0.0001
D*t*s 64 0.23015 0.00360 17.28 0.0001
error-c (r*s in dt comb.) 100 0.02081 0.00021
Table 3: Mean values of as affected by disc speed
Speed (rpm) ([min.sup.-1]) N [K.sub.L]a Grouping
50 50 0.0557 A
100 50 0.1278 B
150 50 0.2711 C
200 50 0.4579 D
250 50 0.6999 E
Table 4: Mean values of [K.sub.L]a as affected by disc thickness
Thickness (cm) ([min.sup.-1]) N [K.sub.L]a Grouping
0.32 50 0.2417 A
0.64 50 0.2915 B
1.28 50 0.3293 C
1.92 50 0.3498 D
2.55 50 0.4001 E
Table 5: Mean values of [K.sub.L]a as affected by hole diameter
Speed (rpm) Disc thickness
0.32 0.32* 0.64 0.64 0.64* 1.28
50 0.033 0.046 0.050 0.050 0.073 0.105
100 0.092 0.140 0.148 0.148 0.263 0.291
150 0.200 0.339 0.354 0.354 0.588 0.562
200 0.410 0.620 0.639 0.639 1.146 0.887
250 0.635 1.176 0.986 0.986 1.526 1.047
* Two discs of the specified thickness
Table 6: The effects of disc speed and disc thickness on [K.sub.L]a
([min.sub.-1]) using hole diameter of 1.92 cm at 7.5 cm immersion depth
(cm) N [K.sub.L]a ([min.sub.-1]) Grouping
0.00 50 0.1045 A
0.32 50 0.2058 B
0.64 50 0.3436 C
1.28 50 0.4455 D
1.92 50 0.5129 E
Table 6: Percentage increase in KLa due to increase in immersion depth,
at various disc speeds
Increase in Immersion Depth
(from 2.5 to 5.0 cm) (from 5.0 to 7.5 cm)
(rpm) Non-perforated Perforated Non-perforated Perforated
50 30 40 56 77
100 92 85 108 150
150 93 193 120 264
200 61 134 92 213
250 67 147 111 229
Table 7: The effect of immersion depth on the disc area passing through
the liquid, the number of holes per revolution and the thickness of the
Disc area passing
Immersion through liquid Number of Oxygenated
depth holes per layer
(cm) ([cm.sup.2]) (%) revolution (%)
2.5 208 32 38 23
5.0 377 57 70 46
7.5 506 77 96 69
Table 8: Percentage increase in [K.sub.L]a due to the presence of holes,
at various immersion depths and disc speeds
Immersion depth (cm)
(rpm) 2.5 5.0 7.5
50 272 304 323
100 335 320 424
150 219 383 428
200 157 271 317
250 140 254 272