COARSE BUBBLE DIFFUSERS
FOR AERATED LAGOONS IN
COLD CLIMATES
U. S. ENVIRONMENTAL PROTECTION AGENCY
NATIONAL ENVIRONMENTAL RESEARCH CENTER
ARCTIC ENVIRONMENTAL RESEARCH LABORATORY
College, Alaska 99701
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COARSE BUBBLE DIFFUSERS FOR AERATED LAGOONS
IN COLD CLIMATES
by
Conrad D. Christiansen
Environmental Protection Agency
Arctic Environmental Research Laboratory
College, Alaska 99701
Working Paper No. 17
January 1973
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TT.irviEOKMEUTAl PROTECTION AGENCY
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A Working Paper presents results of investigations which are, to some
extent, limited or incomplete. Therefore, conclusions or recommendations,
expressed or implied, are tentative. Mention of commercial products or
services does not constitute endorsement.
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INTRODUCTION
The Arctic Environmental Research Laboratory (AERL) constructed an
experimental aerated lagoon as part of a pilot facility at Eielson Air
Force Base, Alaska, in 1968. The lagoon was 16 ft wide by 12 ft deep by
82 ft long and divided into six cells. Perforated tubing (Hinde Engineering
Co., Highland Park, Illinois) was used originally for aeration. The tubing
is 5/8 inch OD with short slits cut every 1-1/2 inches on the top and a lead
keel on the bottom to keep it submerged.
Because of clogging problems encountered with the perforated tubing in
the experimental lagoon, and similar experiences reported by others, the
lagoon was modified in January 1970, by replacing the perforated tubing with
Aer-0-Flo (Aer-0-Flo Corporation, Florence, Kentucky) non-clog diffusers.
This was done to determine the feasibility of using coarse bubble or non-
clog diffusers in aerated lagoons. The Aer-0-Flo diffuser consists of a
cap which rests on a 1/8-inch pipe orifice (see detail of Figure 3). When
air is flowing the cap is forced up about 1/16 inch and air flows under the
cap and theoretically up through the small holes in the cap. When air is
shut off the cap falls back against the orifice and prevents solids from
backing up in the system.
The lagoon was operated for 2 years with no problems which could be
associated with these diffusers. Because of the success with the Aer-0-Flo
diffusers in the pilot facility, it was decided they should be demonstrated
in a full scale lagoon. This led to an agreement with the Army in the
summer of 1971 which allowed AERL to modify part of the waste treatment
lagoon at Ft. Greely.
The Ft. Greely lagoon has two principal cells which can be operated in series
or in parallel. Each cell is separated into two smaller cells by a baffle which
1
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LAGOON PERFORMANCE
The two principal cells of the lagoon were operated in paralled and
monitored over the 1971-1972 winter. Table 1 presents a summary of the per-
formance of the lagoon through this period. Nearly all of the influent
samples were collected using a composit sampler; however, some problems were
encountered with the collection equipment due to large solid particles fouling
the line. All effluent samples were grab.
The difference in detention times was due to an imbalance of flow to
the two sides. BOD removals for both sides appear to be typical of a cold
region aerated lagoon with removals of 81 percent and 82 percent, respectively,
for detention times of 25 and 38 days.
Figure 4 gives further information on the perforated tubing aerator
performance. Lagoon dissolved oxygen (D.O.) levels, taken between 10:00 AM
and 1:00 PM on the dates indicated,,were obtained with a YSI D.O. meter (Model
54) and probe. Air flow data was obtained through the use of a venturi meter
on the coarse bubble side, and this value was subtracted from the total
blower output for the fine bubble side.
Some of the variability of the data is due to clogging problems encoun-
tered with the perforated tubing. That is, as the tubing began clogging,
more air was diverted to the coarse bubble aerator side to reduce the
discharge pressure of the compressors. The trend begain in December or
January. Although the discharge pressures ranged around 9 psi in May,
the air flow through the perforated tubing was so low that the lagoon went
anaerobic. The cubing was cleaned with HC1 gas around the first of June.
The cleaning plus algal activity accounts for the high D.O. levels in June.
Air flow data was not obtained in early July.
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TABLE 1
FT. GREELY WINTER DATA SUMMARY
November 1971 through May 1972
Coarse Bubble Fine Bubble
Influent1 Diffuser1 Diffuser2
Detention Time (Days) --- 25 38
Loading
(#BOD/day/1000 ftd) — 0.45 0.30
BOD
Percent Removal
COD
Percent Removal
SS
Percent Removal
182
343
153
35
81
118
66
39
74
33
82
114
67
36
76
1 - Average of 10 samples
2 - Average of 9 samples
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Lagoon
D.O.
(mg/1)
Air
Supply
Lagoon
Temp .
co
12
10
8
6
4
2
0.6
0.4
(SCFM/ o.2
1000ft3)
100 +
Ice
Cover 50
Ot
20
-| 0
0.
point
by COE (9)
1
5^-Sampling
Dates
Oct Nov Dec Jan Feb Mar Apr May June July
Figure 4. FT. GREELY FINE BUBBLE AERATOR
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The low D.O. which occurred in July was not due to low air flow but
was due to bottom sludge turnover. This turnover is caused by heavy sludge
layers which undergo increasing anaerobic action as the temperature increases
until the resulting gas production causes the sludge to rise and release
sulfides. The result is a drastic decline in algae and a great increase in
D.O. uptake which reduces the lagoon D.O. level to 0. This lasts for a few
days at which time the D.O. level begins to increase again. This phenomenon
usually occurs once each year after spring breakup in an aerated lagoon
in which sludge accumulates with little decomposition over the winter months.
Figure 5 presents the same information for the coarse bubble diffuser
side. The lagoon temperature and ice cover was about the same except the ice
did not reach 100 percent. The air flow increased during the period of
clogging of the perforated tubing until the cleaning in June. Most of the
air was diverted to the fine bubble aerator side after the cleaning for an
oxygen transfer test; however, the algae had created supersaturated conditions
and the attempt was abandoned. The low D.O. point in early June was again
due to the sludge turnover phenomena.
Some questions have been raised concerning ice fog generation by the
coarse bubble diffusers. Visual observation indicated open water over all
four clusters throughout the winter period. These open areas tended to
shrink as the air temperature decreased and would almost ice over completely
above the Aer-0-Flo clusters at -40°F. The open area above the Shearfuser
cluster near the influent always maintained a larger open area, shrinking
to about 25 feet in diameter at -40°F. Significant ice fog generation seered
to occur only over this cluster. No ice fog blanket was ever observed
around the lagoon as the ice fog that was formed was either not enough to
become a nuisance or dissipated rapidly. It should be noted that the influent
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Lagoon
D.O.
(mg/1)
Air
Supply
(SCFM/
1000ft3)
12
'10
8
6
4
2
0.8"
0.6
0.4t
100 t
Ice
Cover 50 "1"
Lagoon 20 ••
Temp. 10..
o--
Data point
by COE (9)
Sampling
Dates
Oct Nov Dec Jan Feb Mar Apr May June July
Figure 5. FT. GREELY COARSE BUBBLE AERATOR
10
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sewage temperatures to this lagoon range around 20°C even in winter and
that lower influent temperatures would reduce ice fog generation. During
the colder period of the year the perforated tubing side of the lagoon was
completely ice covered and no ice fog was observed.
Ice fog generation could be reduced by providing more diffuser clusters
which would increase the spacing between diffusers. The increased spacing
would reduce agitation over the diffusers which would increase the ice cover,
thereby reducing the ice fog generation. The diffuser efficiencies would not
be affected as transfer rates are independent of diffuser spacing, provided
the spacing is sufficient to minimize interfering bubble patterns (6). In-
creased spacing may provide an improvement over the very close coarse
bubble diffuser arrangement of the Ft. Greely clusters which may have resulted
in an interfering bubble pattern.
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OXYGEN TRANSFER STUDIES
The oxygen transfer equation is as follows:
d| = 0KLa(CCs-C)-r
d£ Change in concentration (mg/l/hr). For
dt steady state conditions: dc/dt = 0.
Ki_a = Oxygen Transfer Coefficient (hr~1).
a = Ratio of Ki_a of lagoon contents to K|_a of tapwater.
Cs = Oxygen saturation concentration (mg/1).
C = Oxygen concentration in liquid (mg/1).
3 = Ratio of Cs in liquid to Cs in tapwater.
r = Oxygen Demand Rate (mg/l/hr).
The ^a value is the overall oxygen transfer coefficient and can be
used for rating aerators. CS-C represents the driving force in the transfer
of oxygen into the liquid.
When using the K|_a coefficient, it must be remembered that the constant
applies only to a given aeration system (1,8). The value of the constant
will be influenced by tank geometry and diffuser type. Air flow rates also
affect the l^a value. For a constant oxygen uptake rate, as the air flow
rate is increased, the D.O. level in the liquid increases, which in turn
reduces the driving force and increases the K|_a value.
It should also be remembered that the efficiency of aerators is related
to the oxygen uptake rate in the reacting liquid. That is, under steady
state conditions, the amount of oxygen transferred cannot be greater than
that utilized in the liquid, and the aerator effeciency will decrease with
increasing D.O. levels.
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The K, a value is influenced by temperature which may be accounted for
through the Arrhenius temperature correction equation as follows:
KLa (T) = KLa(20)eT-20
6 = Temperature Coefficient
T = Temperature, °C
A value of 1.02 was used in adjusting the Kia values obtained in these
studies (5).
In order to compare the performance of the coarse and fine bubble aerators,
oxygen transfer studies were conducted at the Ft. Greely lagoon.
K^a values were calculated from the oxygen transfer equation for steady
state conditions. Values of 0.85 and 0.9 were used for a and p respectively, for
all the tests. Cs was found from the following equation (7):
Cs = Cw (3! + ^)
Cw = Oxygen saturation concentration at the actual
barometric pressure.
Pfc = psia at the aerator depth.
Ot = percent oxygen in the air
leaving the tank
D.O. uptake samples were obtained at mid-depth from the three docks on
each half of the lagoon and the values reported are an .average of the three
samples.
D.O. uptakes were determined by bringing samples into a laboratory
building, shaking the samples for aeration, placing the samples in BOD
13
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bottles, placing the BOD bottles in a water bath at the same temperature
as the lagoon, and reading the D.O.'s with a YSI D.O. meter (Model 54) and
YSI BOD probe (Model 542A). This procedure was accomplished as quickly as
possible, usually within a few minutes. The bottles were agitated periodi-
cally during the D.O. uptake period. The uptake for May 18 was adjusted
for algae production and bottom sludge demand. Algae production was deter-
mined by hanging light and dark BOD bottles in the lagoon as per Camp (2).
Bottom sludge demand was determined by means of a bottom sludge respirometer
(3). The higher D.O. uptakes for the coarse bubble diffusers are a result
of the higher influent flow to that side as mentioned previously.
D.O. levels in the lagoon were also determined with a YSI meter and
probe. D.O.'s were measured at various points throughout the lagoon at the
beginning of the sampling season and found to be reasonably uniform. There-
after, D.O.'s were determined from the three docks on each half of the lagoon
and the D.O. levels reported are an average of these.
Table 2 presents the oxygen transfer data collected and is grouped by
type of aerator. Also shown is one data point for a lagoon serving the
Northway, Alaska, FAA station which at that time had air gun aerators (Aero-
Hydraulics Corp., Montreal, Canada). The last data point is taken from the
literature and represents an evaluation of an air gun installation at
Brampton, Ontario (12). Air gun aerators are designed for use in deeper
lagoons (15 feet) and consist of a tube with a chamber at the bottom. Air
is pumped into the chamber and builds up until released by a siphoning effect
through a tube in one large bubble. Water is drawn into the tube and a
pumping action occurs. The units are designed to provide mixing as well as
oxgenation.
14
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Mixing over the center baffle was too great to permit separate evalua-
tion of the two types of non-clog aerators at Ft. Greely. As a result, the
diffusers performance has been lumped together for evaluation. This should
not detract substantially from the results as any difference between the two
coarse bubble aerators will be small compared to that between the coarse
bubble and fine bubble aerators.
The progressive drop in D.O. levels for the fine bubble diffuser is a
result of the clogging mentioned previously. Data for the fine bubble
aerator was not obtained on May 18 because the lagoon had gone anaerobic at
that time. Data for November and December was not obtained because the fine
bubble diffusers had been renovated during the summer by manually punching
larger holes in the tubing which changed the diffuser characteristics.
Based on the 02 transfer efficiencies for comparable lagoon D.O. levels,
the coarse bubble diffuser efficiencies appear to be 75-90 percent of those
for the fine bubble diffuser. An important point to note, however, is that
although the fine bubble diffuser efficiencies are generally higher, the
pounds of oxygen transferred per hp-hr are lower due to the higher compressor
discharge pressures associated with the clogged tubing. The low 02/hp-hr
values would indicated that although the fine bubble diffuser is more
efficient in oxygen transfer, it is not necessarily more economical because
of the power requirements.
Figure 6 presents a plot of the K[_a value at 20°C times the aeration
basin volume per diffuser (K^a ' V) vs. the air flow rate per diffuser
(SCFM/diffuser). The coarse bubble djffusers were evaluated as shearfusers.
This was done because, situated in the first cell where the oxygen demand
was greatest, the shearfuser received the major portion of the total air
flow. Each Aer-0-Flow diffuser cluster was considered as one shearfuser
which made a total of 10 shearfusers for the calculations.
16
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10000
9000
8000
7000
6000
3000
4000
3000 - -
2000--
KLa-V
1000
900
800
700
600
500 +
400
300 4-
200--
100
T
i
X
*
/o'
Estimated limit of accuracy
Shearfuser-From Eckenfelder (7)
Extension of Eckenfelder Curve
Ft. Greely Coarse Bubble Diffuse
Ft. Greely Fine Bubble Diffused
Northway Air Gun
Brampton Air Gun (12)
** Values/100 ft of tubing
5 6 7 8 9 10
Gs - SCFM/Diffuser
20
30
40 50 60
FIGURE 6
OXYGEN TRANSFER CHRACTERISTICS FOR DIFFUSERS IN AERATED LAGOONS
17
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The solid line shown was obtained from the literature and relates the
KLB value for a certain diffuser and tank configuration to the tank volume
per diffuser. The curve is based on data obtained in a tank 24 ft. long by
4 ft wide by 15 ft deep, using the shearfuser diffuser. Other curves for
similar coarse bubble diffusers were shown but are not presented here.
Because of the variability of the shearfuser data obtained at Ft.
Greely, maximum and minimum values of K[_a based on possible errors in proce-
dures and equipment were calculated and a range of K|_a * V values plotted
as shown. The accuracies used in the calculation were as follows:
C = ±0.5 mg/1 - Standard Methods (11) indicates an
accuracy of ±0.1; however, ±0.5 was used because
of the slow instrument response due to the cold
conditions during the studies.
r^= ±15 percent - Sawyer (10) indicates the BOD test
accurately is considered to be 5 percent. The 15
percent value was used to account for sampling error.
i
$ = ±0.05
a = ±0.01
The uppermost point shown represents the data for May 18 which is probably
the most questionable because of the need to account for algae D.O. produc-
tion and bottom sludge demand. The algae D.O. and bottom sludge demand
values were varied by +50 percent and -50 percent for the error calculations.
Based on the plot of data in Figure 6, it would appear that the K. a • V
vs. air flow data that has been published may be used for coarse bubble
diffuser oxygen transfer determinations for aerated lagoons. This data should
be used with caution, however, as the published data is based on much smaller
aeration basin volumes per diffuser. At the larger basin volumes per diffuser
- 18
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the slope of the curve may be somewhat different and not reflected in the
limited data presented here.
Data points for the fine bubble and air gun diffusers shown in Figure
6 should be considered as providing approximate information only because of
the limited number of points and the possible scatter as reflected in the
shearfuser data.
19
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CONCLUSIONS AND RECOMMENDATIONS
The results of these studies indicate that, although the fine bubble
diffuser is 10-25 percent more efficient in oxygen transfer than the coarse
bubble diffuser, it is not more economical. Power requirements in terms
of Ib. D£ transferred/hp-hr are higher because of the restricted diffuser
openings which require higher blower discharge pressures. Maintenance
requirements are also higher because of clogging in the tubing which requires
periodic cleaning. The clogging also results in higher compressor mainte-
nance due to increased discharge pressures.
Regarding lagoon design with coarse bubble diffusers, it is suggested
that published K|_a • V vs. air flow/diffuser data may be used. A certain
amount of caution should be exercised, however. Although the Ft. Greely
coarse bubble diffuser data presented here indicated the published curve
could be extended for use in aerated lagoon design, the scatter prevented
defining a slope for the extended curve.
In areas where ice fog is a problem, consideration should be given to
using a large number of clusters (less aerators per cluster) which would
provide more space between the diffusers. This increased spacing would
reduce the generation of ice fog as the percent of ice cover would increase
with lower agitation above the aerator clusters. Spreading out the diffusers
would not reduce the oxygen transfer efficiencies and may provide an improve-
ment over the Ft. Greely cluster arrangement.
20
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BIBLIOGRAPHY
1. Busch, A.W., "Biological Factors in Aerator Performance," Presented at
the 25th Industrial Waste Conference, Purdue University, 1970.
2. Camp, Thomas R., Water and its Impurities, pp 306-307, Reinhold Book
Corporation, New York, 1963.
3. Christiansen, C.D., Unpublished Data, Environmental Protection Agency,
Arctic Environmental Research Laboratory, 1972.
4. "Design Analysis, Ft. Greely Aerated Lagoon," U.S. Army Corps of
Engineers, Alaska District, Anchorage, Alaska.
5. "Ekenfelder, W. Wesley, Jr., Water Quality Engineering for Practicing
Engineers, Barnes and Noble, Inc., New York, 1970.
6. Eckenfelder, W. Wesley and Ford, Davis L., "New Concepts in Oxygen
Transfer and Aeration," Advances in Water Quality Improvement, Water
Resources Symposium ito. 1, edited by Earnest F. Gloyna and W. Wesley
Eckenfelder, University of Texas Press, Austin, 1968.
7. Eckenfelder, W. W., Jr., and O'Connor, D.J., Biological Waste Treat-
ment, Pergamon Press, New York, 1961.
8. Pfeffer, John T. , e_t al_., "Field Evaluation of Aerators in Activated
Sludge Systems," Water and Sewage Works, Vol. 115, No. 11, November
1968.
9. Salisbury, J., Private Communication, U.S. Army Corps of Engineers,
Alaska District, Anchorage, Alaska, 1972.
10. Sawyer, Clair N. and McCarty, Perry L., Chemistry for Sanitary Engineers.
McGraw-Hill Book Co., 1967.
11. Standard Methods for the Examination of Water and Wastewater," 13th
Edition, American Public Health Association, 1740 Broadway, New York,
New York
12. Thon, J. "Oxygen Transfer Rate Determination on an Aero-Hydraulics 'Bubble
Gun' Aeration System at the Brampton Pilot Plant," Ontario Water Resources
Commission, November 1964.
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230 C-:,i..;i L*e*i-fcora Street
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