EPA-600/2-82-003
February 1982
TECHNOLOGY ASSESSMENT
of
FINE BUBBLE AERATORS
by
Je*
-emiah J. McCarthy
Wastewater Research Division
USEPA Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
Project Officer
Robert P.G. Bowker
Municipal Environmental Research Laboratory
Office of Research and Development
U. S. Environmental Protection Agency
Cincinnati, Ohio 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Mention of trade names of commercial products does not constitute endorsement or
recommendation for use.
n
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FOREWORD
The U.S. Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health and
welfare of the American people. Noxious air, foul water, and spoiled land are
tragic testimonies to the deterioration of our natural environment. The
complexity of that environment and the interplay of its components require a
concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution;
it involves defining the problem, measuring its impact, and searching 'for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems to prevent, treat, and manage wastewater and
solid and hazardous waste pollutant discharges from municipal and community
sources, to preserve and treat public drinking water supplies, and to minimize
the adverse economic, social, health, and aesthetic effects of pollution. This
publication is one of the products of that research and provides a most vital
communications link between the researcher and the user community.
Increasing power costs and the potential for relatively high oxygen
transfer efficiency has generated renewed interest in fine bubble aeration
performance. This report evaluates fine bubble aeration technology and
discusses its development status.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
m
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ABSTRACT
The technology assessment addresses design and evaluation of fine bubble
aeration equipment. It discusses the associated gas transfer theory used as the
basis for measuring water and wastewater oxygenation efficiency. Mixing
requirements are also discussed.
While bubble aeration is not new technology, increasing power costs and the
potential for relatively high oxygen transfer efficiency has generated renewed
interest in fine bubble wastewater aeration performance. The many interrelated
variables affecting measurement and efficiency of fine bubble aeration systems
are identified and discussed. Comparison with other aeration methods is made
and an estimate of the national impact fine bubble aeration can have on
wastewater treatment energy savings is presented. Research and development
efforts which are needed to improve fine bubble aerator performance are
identified.
This report evaluates fine bubble aeration technology and discusses its
development status. The report is liberally referenced so the reader can obtain
details about a particular aeration question if desired.
IV
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CONTENTS
Forward..
Abstract.
Figures..
Tables...
Acknowledgement '.
1. Introduction and General Technology Description
2. Development Status
Summary of Research Findings
Full Scale Facilities in Use
Equipment/Hardware
3. Technology Evaluation
Process Theory
Capabilities and Limitations
Design Considerations
Energy Utilization
O&M Requirements
Costs
4. Comparison with Equivalent Technologies
5. Assessment of National Impact
6. Recommendations
Research and Development Requirements
Process/Technology Improvements
References
. .m
.. iv
,. vi
, .vi.i
. v i i i
3
3
5
6
11
11
15
21
31
33
36
38
41
43
43
44
.45
v
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FIGURES
Number
Page
1. The Norton/Hawker-Siddley dome diffuser . 8
2. The EPI/Nokia disc diffuser 8
3. The Clevepak jet aerator 9
4. The Aeration Industries aspirating propeller pump . . 9
5. The FMC tube diffuser 10
6. Schematic of gas transfer mechanism showing
pressure/concentration gradients for gases of
low solubility 12
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TABLES
Number
Pac
1. Preliminary Trends of Submerged Aeration Equipment
Efficiencies ..... . . 4
2. The Bubble Aeration Plants Surveyed 5
3. Major Submerged Aeration Equipment Manufacturers 10
4. References Discussing Sampling and Measurement Considera-
tions for Various Wastewater Parameters 21
5. Comparative Clean Water Oxygen Transfer Information
for Air Aeration Systems Under Standard Conditions 24
6. Information Required to Select and Verify Aeration
Equipment Performance 25
7. Blower Application Chart 28
8. Summary of Common Data Analysis Methods. ........... 32
9. Aeration Energy Requirements 34
10. Maintenance Data Summary 39
11. Cost Effectiveness Comparison for Several Activated
Sludge Aeration Systems 4Q
12. Summary of Wastewater Treatment Plants and Flows Using
Air Activated Sludge Treatment Processes Nationwide 42
13. Potential National Energy Savings Using Fine Bubble
Aerators in Air Activated Sludge Treatment Processes 42
vii
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ACKNONLEDGEMENT
The author would like to thank Richard C. Brenner, USEPA, for generously
sharing information and insight about fine bubble aerator technology. His
assistance has made this assessment a significantly better document.
vm
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1.
INTRODUCTION AND GENERAL TECHNOLOGY DESCRIPTION
There are two basic methods presently employed in existing wastewater
treatment plants for aerating wastewater. The first method is to transfer
oxygen into solution from air bubbles produced with submerged porous
diffusers or air nozzels. The second method is to agitate the wastewater
mechanically to promote solution of oxygen from air in the atmosphere." This
technology assessment addresses the transfer of oxygen into wastewater by
gas transfer. In particular it concentrates on the specification and
evaluation of fine bubble aeration equipment and on the associated gas
transfer theory used to explain the introduction of the oxygen in the air
into solution.
Fine bubble aerators are defined as those which produce a 2.0-2.5 mm
diameter bubble U). Coarse bubble aerators may produce a bubble up to 25
mm. Aerators are used in wastewater treatment processes where introduction
of air into a liquid is required. Examples are preaeration of raw
wastewater, aerated grit chambers, activated sludge aeration tanks, aerated
lagoons, aerobic digesters and post aeration (2).
Aeration of wastewater performs multiple functions. It supplies oxygen
required for the metabolic processes of the living organisms. It provides
sufficient mixing of the wastewater so that the organisms can receive
adequate dissolved oxygen and come into intimate contact with the dissolved
and suspended organic matter. It scrubs out of the water various metabolic
waste products such as C02- The first and third of these functions are gas
transfer processes. The second function is a mechanical energy transfer
process. In most activated .sludge processes, transfering adequate oxygen
into solution is the function which establishes minimum requirements for the
input of air or mechanical energy (2) (3). Thus this assessment of fine
bubble aerators emphasizes transfer of gas (oxygen) into solution. Mixing
requirements are not regarded lightly, however. There must be adequate
mixing for the gas transfer model to be valid.
Experiments on aeration of wastewater began in England about 1882 and
the activated sludge process was first introduced in 1914 (2). The volume of
reference material on wastewater aeration and fine bubble aeration accord-
ingly very large. This technology assessment is based on a limited selection.
of significant papers representing three broad areas: early pioneering and
theoretical work which comes mostly from "classical" or "bench mark"
published papers; state-of-the-art information which comes from published
papers, text books, WPCF Manual of Practice No. 5, and proceedings from an EPA
workshop titled "Workshop Toward an Oxygen Transfer Standard"; and plant
scale research efforts and information which come from EPA sponsored project
reports or related papers as well as the Workshop proceedings.
Clean water oxygen transfer efficiencies for fine bubble aerators fall
into a fairly broad range, depending on water depth and diffuser configura-
tion, with maximum efficiency being about 50%. There is considerable room
for improvement. In addition, evaluation of diffuser efficiency itself is not
an exact science, especially under activated sludge process conditions with a
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respiring biological system. There are many factors impacting on aerator
design and evaluation. This assessment identifies the major factors,
discusses their importance, and lists areas which need further process
development or research. It is liberally referenced so the reader can obtain
specific details if he desires.
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2. DEVELOPMENT STATUS '
Summary of Research Findings
A detailed summary of significant research about bubble aeration in
general and fine bubble aeration in particular is incorporated as part of the
Technology Evaluation (Section 3). This section summarizes present and
projected EPA sponsored research.
From November 1977 to March 1979, the Los Angeles County Sanitation
Districts (LACSD) supported in part by EPA conducted a series of clean water
oxygen transfer tests under closely controlled conditions comparing various
generic types of submerged air aeration equipment. Six types of devices were
selected which represented typical methods of dispersing various sizes of
bubbles:
Fine Bubbles
Jet Aerator
Dome Diffuser
Tube Diffuser
Medium Bubbles
Static Aerator
Coarse Bubbles
Fixed Orifice
Variable Orifice
All devices were compared using the same test tank and identical test
procedures throughout. The test tank was 20 ft x 20 ft x 25 ft maximum
variable depth. Clean water dissolved oxygen uptake was carried to
equilibrium using water chemically deoxygenated with sodium sulfite and
cobalt chloride catalyst. Three runs at four different depths (10, 15, 20,
and 25ft) were made. Each run had a different input power level (varying
from 0.008-.04 kW/m3) delivered to the water. Diffuser configurations were
selected by aerator manufacturers who were allowed different configurations
for different depths but had to maintain a constant configuration over the
series of three runs at any given depth. The configuration or geometric
pattern selected was one-intended to be economically feasible at full scale
and over the range of input powers evaluated.
Preliminary findings from this study were presented in a slide summary
at the 1980 Water and Wastewater Equipment Manufacturers Association Indus-
trial Pollution Conference (4). The official report will be issued by EPA's
Office of Research and Development in early 1982. In general, the ceramic
dome diffuser was found to transfer oxygen most efficiently. Its efficiency
was followed by that of the tube diffuser, then the jet aerator. Effi-
ciencies of the static aerator and coarse bubble diffusers were less than
the fine bubble diffusers and their results were mixed, depending on test
conditions. Table 1 summarizes the general trends found for the testing
period November 1977 to March 1979. The reader is urged to review the
pending EPA report or Reference (4) for details and qualifications appli-
cable to the general trends summarized.
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TABLE 1
PRELIMINARY TRENDS OF SUBMERGED AERATION EQUIPMENT EFFICIENCIES 111
STANDARD OXYGEN TRANSFER EFFICIENCY (%) VS. DELIVERED POWER DENSITY
(hp/1000 ft3)
• efficiency decreased for the fine bubble diffusers, remained fairly
constant for the static aerator, and increased for the coarse bubble
diffusers as power density increased
STANDARD WIRE AERATION EFFICIENCY (Ibs Oe/wire hp-hr) VS. DELIVERED
POWER DENSITY (hp/1000 ft3)
• efficiency decreased significantly for the fine bubble dome and
tube diffusers, decreased slightly for the static mixer or remained
fairly constant for the coarse bubble equipment as power density
increased
• the jet aerator had a local maximum in the middle of the power
density range tested
STANDARD OXYGEN TRANSFER EFFICIENCY (%) VS. DEPTH (ft)
• efficiency increased as depth increased
STANDARD WIRE AERATION EFFICIENCY (Ibs 02/wire hp-hr) VS. DEPTH (ft)
A
• efficiency increased or remained fairly constant as depth increased
STANDARD OXYGEN SATURATION CONCENTRATION (mg/1) VS. DEPTH (ft)
• oxygen saturation concentration increased as depth increased
In order to determine the alpha and beta factors associated with some
of these devices, the LACSD in conjunction with USEPA is conducting full
scale wastewater oxygen transfer tests. A decision was made to test three
promising types: 1) porous disc diffusers applied in a total floor coverage
configuration; 2) porous tube diffusers applied in a wide band dual aeration
configuration; and 3) directional jet aerators arranged along one longitu-
dinal wall and aimed at the opposite longitudinal wall. Tests began in
May 1981 and will run until the spring of 1982. In non-specific terms,
the proposed scope of work is the following (5):
1) To concurrently evaluate the oxygen transfer capabilities of the
three aeration systems in a municipal wastewater;
2) To concurrently evaluate the clogging potential of the three
aeration systems under field operating conditions;
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3) To evaluate the process performance of the most cost-effective
of the three aeration systems at different aeration times and
organic loadings;
4) To run laboratory scale tests (including clean water runs) to
evaluate alpha and beta for all three of the aeration systems;
5) To run plant scale tests (including clean water runs) to
evaluate alpha and beta for all three aeration systems.
Full Scale Facilities in Use
Houck (3) has made a plant survey of 19 activated sludge wastewater
treatment plants using fine bubble diffuser aeration as part- of an EPA
effort designed to define full scale aeration experience. The list of
plants visited is in Table 2. The plants were selected primarily on the
basis of long term operating experience with fine bubble diffusers (five
years or more). All were basically municipal wastewater treatment plants
with varying industrial contributions and all coincidentally used fine
bubble dome diffusers. Overall objectives of the survey were to better
define full scale plant aeration efficiency (i.e., oxygenation power
economy) operation and maintenance requirements, and proper design
approaches for fine bubble aeration systems.
TABLE 2
FINE BUBBLE AERATION PLANTS SURVEYED (3)
United Kingdom
Holland
Holten-Markelo
Steenwijk
United States
Glendale, CA
Madison, WI
Fort Worth, TX
Tallman Island,
NYC, NY
Basingstoke
Beckton (New Plant)
Beddington
Dartford
Mogden (Batt B)
Oxford
Ryemeads (Stage III)
Coolport
Coleshill (Stage III)
Finham (South)
Hartshill
Minworth
Strongford (New)
Basic conclusions of the Houck study about oxygen transfer performance
were that dome/disc fine bubble diffusers can (relatively) efficiently
transfer large amounts of dissolved oxygen into the water if they are
designed properly and good operation and maintenance procedures are
routinely practiced. The principal factors affecting plant performance
were found to be, in order of significance: 1) mixed liquor dissolved
oxygen (D.O.) concentration (maintenance of high D.O. levels decreased
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aeration efficiency); 2) the oxygen transfer factor, alpha (which signifi-
cantly affected the correct specification of aeration equipment for
wastewater basins); and 3) basin geometry (which significantly influenced
the value of- alpha and the D.O. profile for the basin). Aeration
efficiency (computed using a BOD and oxidized nitrogen mass balance
technique) varied considerably among plants because of differences in
design and operation. It averaged 1.5 kg 02/kWh and ranged from 0.8 to 2.1
kg Og/kWh. Houck concluded that with enhanced design and operating.
techniques and with no unusual alpha-depressing wastes present, it would
not be unreasonable to expect routine achievement of aeration efficiencies
25 to 75 percent higher than the average value observed for the 19 plants
surveyed. Houck's design, energy, and operation and maintenance obser-
vations about fine bubble diffusers are discussed in Section 3 under the
appropriate subject area.
Equ i pment/Hardware
Fine bubble aerators have been historically designed as permeable
structures formed by bonding near spherical or blocky particles at their
contact points which leaves a labyrinth of interconnecting passageways
through which air flows. As the air emerges from the surface pores, pore
size, surface tension, and flow rate interact to produce the char-
acteristic bubble size which is released at the diffusers1 surface. As the
bubble rises through the "head" of the liquid, oxygen from the air of the
bubble is continuously dissolved (diffused) into the liquid (6).
Ceramic diffuser media best typify the fine bubble diffusers. Most
common ceramic diffuser media compositions are: ceramically bonded grains
of fused, crystalline aluminum oxide; vitrous-silicate-bonded grains of
pure silica; and resin bonded grains of pure silica. Other diffuser media
consist of modified acrylonitrile-styrene copolymer and polyethylene
plastic, which is reportedly cleanable in soap and water (3).
With regard to shape, manufacturers offer plates, usually 12 x 12 x 1
or 1-1/2 inches thick, and tubes, usually 2-1/2 inches outer diameter x 1-
3/4 inches inner diameter x 24 inches long.
A third shape, ceramic "dome" or "bell" has become an accepted
standard in England. This report focuses on long term operation and
maintenance (O&M) and oxygen transfer performance of fine bubble dome
diffusers because the 19 fine bubble aeration plants surveyed by Houck
coincidentally all had dome diffusers (3). Criteria for plant selection
was that they be in operation at least five years and they employ well mixed
aeration basins to maximize oxygen transfer characteristics. The plants
chosen to meet these characteristics were in England, the Netherlands and
the United States. Houck and Boon who conducted the survey concluded that
the data they evaluated in the study indicated some parity of performance
among the ceramic dome and disc diffusers presently marketed in the United
States. Disc diffusers are generically similar to domes, but are flat or
nearly so without the turned down domw periphery and are not equipped with
a center hold down bolt.
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Aeration tanks using fine bubble diffusers frequently have a diffuser
grid configuration covering the entire aeration tank floor with u'nplasti-
cized polyvinyl chloride (UPVC) air supply piping and appurtenant hard-
ware. Air is metered to each diffuser disc or 'dome through a control
orifice.
Diffusers are not the only type devices which generate fine bubbles.
Jet aeration equipment consists either of radial jet clusters, each with a
distribution chamber or directional jet assemblies in which all the jet
nozzles are aligned in the same direction. The distribution chamber
receives recirculated mixed liquor from submersible pumps and low pressure
air from centrifugal blowers through separate submerged manifold piping.
Air and mixed liquor are combined within the jet nozzle where vortex-mixing
results in shearing of the air into small bubbles. The bubbles are dis-
charged horizontally with recirculated mixed liquor as a jet plume at the
bottom of the basin. An important advantage of this type of aeration
device over diffusers is that mixing action is independent of air flow
rate. This permits oxygen supplied to match process conditions without
compromising mixing requirements of the basin. Jet aeration devices are
particularly desirable for aerated lagoons where mixing and circulation
often control aeration design.
Another aeration device that is particularly applicable to aeration
situations where mixing and circulation may control design is the motor
driven propeller aspirator pump. This device basically consists of a 4-ft
hollow tube with an electric motor on one end and a propeller at the other.
The propeller end of the tube is equipped with a quide to direct underwater
air flow. The pump draws air from the atmosphere at high velocity and
injects it underwater where both velocity and propeller action create
turbulence and diffuse the air as bubbles into the water. Pumps can be
positioned at various angles depending on basin depth, aeration, and
mixing/circulation requirements. The pump is portable and can be mounted
on booms or floats in lakes and ponds. Degree of mixing, vector (initial
bubble direction), and speed of aspiration can be controlled. A new
aspirator pump with a dis'c rather than a propeller at the end to create a
finer bubble and disperse bubbles at a 90 degree angle to the shaft has been
introduced.
Figures 1 to 5 show examples of typical fine bubble aeration equipment
including some mounting arrangements. Table 3 lists names and addresses
of aeration equipment manufacturers.
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ORIFICE BOLT
POROUS
ALUNOUM
DOME
100mm uPVC
PIPING
uPVC
ADJUSTABLE
SADDLE
FINISHED
FLOOR
LEVEL
ELASTOMER
DOME JOINT
ACETAL NUT AND BOLT
Notes:
1. 1 mm = 0.039 in.
2. u-PVC = Unplasticized
Polyvinyl Chloride
uPVC
FLOOR FIXING
Figure 1. The Norton/Howker-Siddley dome diffuser
HKL 210 or MKL 210 diffuser,
side and front
Figure 2. The EPI/Nokia disc diffuser
8
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-Shore-located
air-supply
manifold
Butterfly valve
Retaining cable
Air piping float
7
Basin wall
Hi-density
polyethylene
air piping
(shore to
\f lexible hose)
Figure 3. The Clevepak jet aerator
Cable float
with lifting eye
-Liquid surface
Submersible
electrical cable
7x19 type 304
stainless steel
lifting cable
Self-cleaning
intake screen
Lagoon or
basin floor
lom resting
'support bracket
Figure 4. The Aeration Industries aspirating propeller pump,
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Figure 5. The F.MC tube diffuser.
TABLE 3
MAJOR SUBMERE6ED AERATION EQUIPMENT MANUFACTURERS
Norton Co.
Control -Industrial Ceramics Division
1 New Bond Street
Worcester, MA 01606
617-853-1000
FMC Corporation
Environmental Equipment Division
1800 FMC Dr. West
Itasca. IL 60143
Kenics Corporation
Kenics Park
North Andover, MA 01845
617-687-0101
Ajax International Corporation
P.O. Box 26607
San Diego, CA 92126
805-966-1796
Aeration Industries, Inc.
Hazel tine Gates
Chaska, MN 55318
612-448-6789
Sanitaire-Water Pollution Control Corp.
P.O. Box 744
Milwaukee, WI 53201
Envirex Inc.
1901 S. Prairie Ave.
Waukesha, WI 53186
414-547-0141
Infilco Degremont Inc.
Box K7
Richmond, VA 23288
804-285-9961
Enviroquip, Inc.
P.O. Box 9069
Austin, TX 78766
512-836-1614
Aeracleve Dei vision of
Clevepak
1075 Airport Road
Fall River, MA 02720
617-676-8571
10
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3. TECHNOLOGY EVALUATION
Process Theory
Gas Transfer in Water
All solutes tend to diffuse through solutions until there is a stable_
and homogenous state of uniform concentration throughout (equilibrium)."
According to Pick's first law of diffusion, the rate of such molecular
diffusion of a gas through a liquid depends on characteristics of the gas
and liquid (diffusivity), the cross sectional area through which diffusion
occurs, temperature, and most importantly, magnitude of change of concen-
tration with distance (concentration gradient) of the gas being diffused.
One theory advanced to explain the gas transfer process is the two film
theory of gas transfer proposed by Lewis and Whitman (7). The film theory
has no physical basis and no film has been observed, however it has
practical value in that two fictitious films at the interface are widely
used for the correlation and interpretation of mass transfer data. Lewis
and Whitman addressed gas absorption into a liquid not saturated with the
gas (supersaturation is considered negative absorption). The rate of
absorption or transfer of the gas from the gas phase (gas bubble) to the
liquid phase (water) is considered limited by two thin layers each side of
a gas-liquid interface which are essentially free of turbulent mixing.
These layers, or films, always persist regardless of turbulence in the
liquid or gas bulk although turbulence may reduce film thickness. The
films, one gas and one liquid, are assumed to offer all resistance to gas
transfer into the liquid bulk. The gas-liquid interface itself is
considered to offer no resistance and the two phases are considered at
equilibrium at that point even though there may be rapid diffusion (high
concentration gradients) on each side of the interface. All gas diffusion
proceeds through both films in series. Figure 6 is a schematic of the gas
transfer mechanism according to the two film theory of gas transfer.
Considering that the amount of gas transfer is proportional to the
interfacial area and that gas diffuses through the gas and liquid films in
series, the amount of gas absorbed per unit time and unit inter-
facial area is:
dW.TI = kg(Pg.Pi) =kL
[1]
where W = weight of gas, grams
t = time, hours
A = interfacial area through which transfer takes place, c
P = pressure of gas in gas phase, atmospheres
C = concentration of gas in liquid phase, gm/ml
subscript g applies to conditions in the bulk gas phase
subscript i applies to conditions at gas-liquid interface
subscript L applies to conditions in the bulk liquid phase
11
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kg = transfer coefficient through gas film, gm/hr-cm2-atm
RL = transfer coefficient through liquid film, cm/hr
BULK GAS PHASE
(TURBULENT MIXING)
GAS FILM
(LAMINAR FLOW
LU
OO
I/)
a;
ex.
o;
o
GAS-LIQUID
INTERFACE
Pi
LIQUID FILM
(LAMINAR FLOW)
1
BULK LIQUID
PHASE
(TURBULENT
MIXING)
CL
o
C_3
Figure 6.
DISTANCE
Schematic of gas transfer mechanism showing pressure/
concentration gradients for gases of low solubilityU)
Equation [1] is the fundamental gas absorption equation given by
Lewis and Whitman and serves as the basic model applicable to the addition
of oxygen to water. It applies to gas transfer under constant temperature
conditions, when the liquid bulk is not saturated with the gas, and in the
absence of appreciable chemical or biochemical oxygen demand.
Oxygen is a slightly soluble gas in water. As a result, it will
diffuse slowly through the liquid film which will offer the most resistance
and limit the rate of transfer. Because diffusion is slow, only a small
pressure difference is needed across the gas film to transfer it to the
liquid phase. This difference in gas film partial pressures is considered
negligible so that Pg~Pi« Furthermore, at the interface, P-j is in
equilibrium with C-j and in proportion according to Henry's Law. For these
special conditions where the interfacial conditions are practically the
same as those existing in the main body of the gas, the value of Ci is
essentially the same as that of a liquid saturated with oxygen at Pg.and may
be expressed as Cs. When the concentration gradient is taken as a straight
12
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line as in Figure 6, the rate of gas transfer into solution can be written
as an ordinary differential equation in time:
dW 1 dC „ ,r r v
— — = — = K| a (Cc--Ci )
dt M dt L s L
where V = volume of liquid phase, cm3 or ml
a = A/V
[2]
Cs = equilibrium concentration of the gas in the liquid
phase corresponding to its pressure in the gas
phase (saturation concentration, constant for
given temperature), mg/1
K[_a = overall gas transfer coefficient (assumed constant), hr~l
This simpler concept of gas transfer modeled by equation [2] is
sometimes termed the stationary liquid film theory. Equation [2] is the
basic aeration equation employed in aeration equipment evaluation. The
overall (both films) gas transfer coefficient K[_a can be considered as an
overall conductance: when the resistance to gas transfer is large, K|_awi]l
be small, and vice-versa.
As the liquid bulk approaches saturation with respect to the gas being
transferred into it, the rate of gas transfer (dC/dt) is not constant
because C|_ is changing with time and, therefore, concentration deficit
(Cs-C|_)is changing. For this non-steady state case, a transfer rate
expression can be derived by integrating equation [2] between the limits of
time equal to t]_, and t2- This has been done below after some rearranging:
KLa = In
where
[3] or
[4]
and C2 = concentration of solute in liquid phase at time
tl and t2 respectively, mg/1
(t2-tl)
KLa = 2.3 log [(Cs-Ci)/(Cs-C?)]
~
Equations [3] and [4] imply that a semilog plot of (Cg-Ci)/(Cs-C2)
versus (t2-t^) will give a linear trace with slope equal to Kia/2.3
enabling the overall gas transfer coefficient to be evaluated for
different aeration systems. This mathematical approach for non-steady
state evaluation of aeration systems represented a real contribution to
understanding aeraton systems and is generally attributed to Haney (8).
Haney's detailed evaluation of the principles of aeratiorr and the
characteristics of liquids and gases resulted in a cogent description of
fundamental and theoretical advantages and disadvantages about the aeration
of water. His main points are summarized briefly below. Haney's original
paper should be read for proper appreciation of his conclusions:
13
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1. The rate of gas transfer at any instant is proportional
to the concentration deficit at that time.
2. The rate of gas transfer is proportional to the area/volume ratio, a.
3. The rate of gas transfer is porportional to the gas transfer
coefficient which in turn is a function of diffusivity and film
resistance.
4. Changes in temperature are important. An increase in temperature
makes the gas less soluble in the liquid, but increases its rate of
absorption into the liquid.
5. Agitation and mixing decrease film resistance and minimize the
concentration gradients in the liquid bulk.
6. Film "thickness" may be considered an overall measure of resistance
to gas transfer. (Haney is referring to the liquid film. The gas film
may be thicker but offers negligible resistance which is ignored as
discussed in the derivation of equation [2]).
7. Gas partial pressure influences the saturation value of the gas and
therefore gas absorption.
8. Depth of the basin affects gas pressure and bubble area/ volume ratio
and therefore gas absorption.
Haney also evaluated in detail the reciprocal relationship of A/V and
time with respect to gas transfer efficiency. He noted that for a given
bubble volume, surface area increases as bubbles get smaller and
emphasized the importance of obtaining as much uniformity in (small)
bubble size as possible. Haney's evaluation of bubble aeration effectively
outlined the basic controlling parameters for subsurface aeration design.
They are 1) bubble size; 2) relative velocity; and 3) residence time.
Gas Transfer in Wastewater
Strictly speaking, mathematical models for gas transfer into waste-
water are not theoretically derived as was the case for pure water because
of wastewater's varying composition and biological activity. Instead, the
approach has been to take these differences into account and modify gas
transfer equations obtained for pure water accordingly.
In wastewater the value of K|_a is usually less for wastewater than for
tap water. This is because of the presence of soluble organic compounds,
particularly surface active materials. The surface active materials, such
as short chain fatty acids and alcohols, create a concentration of
molecules or additional "film" at the air/water interface which retards
molecular diffusion and decreases K^a. The effect of waste constituents on
oxygen transfer was studied in detail by Barnhart (9). He hypothesized
that the film's effect depended on the type of surface active agent, the
number of carbon atoms, molecular configuration, and the time necessary to
14
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reach adsorption equilibrium. Barnhart defined a coefficient, alpha (°0
to relate the oxygen transfer rate in waste to the transfer rate in
water.
K^a of wastewater
I
-------
Molecular diffusion theory indicates that gas transfer capability is
limited by the magnitude of the driving force (saturation deficit) pushing
it into solution and the diffusion potential of the gas entering the liquid
(Pick's Law). The simplicity is deceiving. Identifying, quantifying, and
minimizing the many factors which contribute to these limitations have
resulted in a large volume of work from many different approaches which is
beyond the scope of this assessment. The intent of this section is to
outline in a convenient form the major topics which should be addressed
when considering bubble aeration systems and to provide some key
references for more detailed study if desired.
Clean Water Considerations
The work of Bewtra and Nicholas using tap water and a full scale
aeration tank expanded on Haney's earlier fundamental observations about
bubble size, residence time, and bubble velocity (TO. They noted that for
fine bubble diffusers most oxygen transfer occurs during bubble formation
when the interfacial area exposed to the liquid is constantly being
renewed. Remaining diffusion occurs during the bubble's ascent to the
surface and at the water surface itself (the least). They explored the
relationship between air flow rate and bubble formation and release. They
investigated the relative velocity of the rising bubble to that of the
surrounding, water and its effect on liquid film thickness. They
experimented with different diffuser arrangements, submergences, and
differing tank geometries, bubble sizes and air flow rates. Numerous
conclusions about the behavior of variables affecting: 1) the rate of
oxygen transfer into solution as predicted by their oxygen transfer
equation (a special version of equation [2]; 2) the effect of diffuser
submergence on oxygenation; and 3) the effect of aeration tank width on
oxygenation added further insight to diffuser design considerations. The
reader is urged to review the detailed conclusions in the paper.
Bubble size and aeration capacity was studied in detail by
Barnhart(9). He evaluated data from his and other studies to show that
despite theoretical considerations about increasing interfacial area per
unit volume with decreasing bubble size, the overall g-as transfer
coefficient, Ki a, increased as the bubble diameter approached 0.22 cm
then decreased" as the diameter got smaller. He explained this by
considering the forces acting on the bubble surface and obtaining a
coefficient of drag which he correlated with the rate of bubble surface
renewal and the liquid film coefficient.
The influence of water temperature on aerator testing is quite
significant-. If, for example, temperature increases from 10 to 20°C, the
gas transfer coefficient can increase by more than 50 percent and the
dissolved oxygen saturation concentration will decrease about 20 percent
(12). Equation [2] gives the relationship of these parameters on the rate
of gas transfer into solution (dC/dt). Present practice is to evaluate the
overall gas transfer coefficient at standard temperature conditions
(20°C) or convert it to standard conditions using the following empirical
relationship:
16
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(KLa)T = (KLa)20e(T-20)
where T = actual temperature, °C
0 = temperature correction factor
(9)
The temperature correction factor has been reported to vary from less
than 1.01 to more than 1.05 (13). A common value used is 1.024(14).
Oxygen saturation values also vary with pressure, and the depth of the
aerator must be taken into account. Because most aeration units use
oxygen from air under pressure, generally the average of saturation at the
surface and saturation value for diffused aeration is more nearly found at
the one-third depth point (14). The Process Equipment Manufacturers
Association recommends the following formula:
^(-gl
\ ij —' • I.
0.
[10]
\
where CP = oxygen saturation at the surface, mg/1
= air pressure at release for the bubble aerator,
inches of mercury
0-t = percentage of oxygen in the gas leaving the tank
surface
Wastewater Considerations
All the parameters which effect oxygen transfer in water affect them
in wastewater. Wastewater composition adds an additional complicating
factor to attempts to consistently measure wastewater aeration efficiency.
Alpha and beta determinations are intended to minimize water and
wastewater test differences and have been defined earlier. It is
reasonable to state that there is a lack of consensus among researchers
regarding the influence and significance of these parameters (13).
Conversely, most investigators agree it is difficult to obtain true
values of beta, and especially alpha that are representative of process
conditions (10)(13)fl4).
In general, the important variables in alpha determination are mixing,
air flow rate, temperature, wastewater composition and aeration device
type and geometry. The effect of these variables is minimized by
following certain techniques. Stukenberg (14) recommends adjusting the
air flow rate so that the ^a in the test unit is the same as that expected
in the full-scale aeration tank. This procedure is intended to minimize
the differences in mixing between the bench scale aeration tank used to
determine alpha arid the full-scale unit itself. Barnhart (9) notes that
bubble size differences between bench- and full-scale units significantly
affect alpha values, and cautions that the bench-scale diffuser should
produce the same size bubbles as in the actual aeration tank. Temperature
effects can be minimized by running all tests at standard 20°C or at the
expected waste temperature (preferred). Surfactants in wastewater are
generally acknowledged to be the wastewater components that have most
17
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effect on oxygen transfer (9) (14) (15). Surfactants may increase or
decrease alpha depending on the aeration system. In a mechanical surface
aeration system, for example, the large number of small bubbles formed
(due to the decreased surface tension) have surfaces which are continually
renewed with respect to the bulk liquid and aeration efficiency is
reportedly increased. In fine bubble diffusion these surfaces are not
renewed and the detergent forms a stationary film or boundry layer,
decreasing oxygen transfer despite increased surface area. High suspended
solids concentrations mav also have an effect on alpha values, but this is
still being debated (14) (15) (16). Experiments with a fine bubble
aeration system operating in an essentially plug flow aeration tank have
shown variations in alpha from 0.3 at the start of treatment (when the
wastewater first comes into contact with recycled sludge) to 0.8 at
tnpletion of treatment (when a fully nitrified effluent is produced
'. Thus alpha can vary throughout the length of a plug flow tank. For
such cases (compared to completely mixed conditions) wastewater samples
must be taken at several points throughout the length of the tank for
alpha determination.
Beta is commonly referred to as a salinity correction factor because
dissolved salts reduce oxygen solubility in wastewater. Dissolved
organics and gases also reduce oxygen solubility and, unfortunately, can
affect dissolved oxygen measurements as well (13). Like alpha, well
designed and standardized tests can minimize beta errors. Conducting
measurements at equal barametric pressure and at field design temperatures
will reduce variations to those caused by wastewater constituents alone.
Again, wastewater samples should be taken at several points throughout
the tank for plug systems to minimize differences in wastewater
composition.
Biological oxygen uptake measurements (dO/dt) must be made with care
because the waste sample being analyzed is changing as it is stablized.
Fresh wastes do not enter the sample during this test and the rate of
oxygen uptake decreases to the point where endogenous respiration is the
sole cause of oxygen use. One method proposed to reduce the error in
trying to measure a changing uptake rate is to stop fresh wastewater flow
to the aeration basin at least 60 minutes (longer if nitrification is
occuring) prior to testing and run the aeration tests under endogenous
respiration conditions. Oxygen uptake rates will be low (F 60 mg/l-hr is
desired) and their rate of change will be at a minimum resulting in less
'chance for error in the oxygen uptake test (14). Other investigators feel
such externally determined values are artificial and not representative of
what is actually going on in the aeration basin. Indirect dO/dt
calculation methods have been proposed to get a "true" oxygen uptake rate
ithout having to directly measure it from samples taken from the basin
' •
Knowledge of dissolved oxygen (D.O.) concentrations throughout the
aeration basin is important for several reasons. The fundmental gas
absorption equation (equation [1]) shows that the rate of transfer of oxygen
into the wastewater will decrease as the saturation deficit decreases. The
important D.O. measurement point in this case is in the liquid approaching
18
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the aerator since that value determines the driving force accross the
aerator. There must also be a minimum dissolved oxygen concentration
throughout the tank so that D.O. transfer from the liquid to the microbes
will not be limiting. It is important to recognize that it is possible to
have a residual D.O. in the mixed liquor and still be deficient in oxj
This minimum concentration is usually considered to be about 2 mg/1
however, this is an area which needs additional research (14J. Finally,
determining a dissolved oxygen profile around the aeration tank will also aid
in understanding tank fluid flow patterns and aid determining if the fliud is
short circuiting.
Mixing and Tank Geometry
Mixing is defined as the circulation which conveys the oxygen enriched
fluid throughout the basin and provides the degree of agitation necessary to
maintain solids suspension (18). A precise process model would seperate gas
transfer effects and fluid convection effects, but a simplified first order
differential equation which varies with time only (and not distance) is gen-
erally used to describe the overall process (equation [2]). Thus, mixing
effects are intrinsic to the gas transfer model (adequate mixing is assumed)
and the gas transfer coefficient is indicative of both mixing and gas
transfer interactions.
Practically speaking, there must be adequate mixing to keep the micro-
organisms in suspension in uniform contact with the dissolved oxygen and oxi-
dizable waste. Bottom velocities (magnitude and direction) are good
indicators of solids suspension capabilities and are often specified in a
velocity profile diagram for a given aerator under certain conditions U8)
(19). WPCF Manual of Practice (MOP) No. 5 recommends a minimum velocity of
fps across the bottom of the aeration tank to keep solids in suspension
The MOP summarizes the effect- of diffuser placement on mixing
velocities for a full scale sprial flow tank as follows:
1. Increasing tank width decreases surface and bottom velocities.
2. Increasing diffuser band width adjacent to the side of a
tank decreases surface and bottom velocities.
3. Moving the air diffusion band toward the center of a tank
(within the outer third of the tank width) decreases surface
and bottom velocities.
Dissolved oxygen gradients are to be expected in a well mixed basin and
all other conditions being equal, will be "typical" or characteristic of well
mixed conditions. Dissolved oxygen uptake rates and/or suspended solids
concentrations should be uniform throughout the basin and should be used in
conjunction with D.O. gradient information £0 ascertain if a basin is well
mixed (I4) (I8). For tests such as these, at least four to six sample points
in the aeration basin must be analyzed. Desirable sample locations have been
recommended for diffused aeration systems U4) (2°). Indirect indicators of
mixing such as aerated wastewater turnover time, pumping capacity, and power
per unit volume do not alone assure adequate solids suspension. They must be
19
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used in conjunction with other measurements for a given tank geometry.
Basin geometry affects the mixing regime established by a specific aera-
tion device and, therefore, the oxygen transfer rate. This is the principal
reason mixed results have been obtained using manufacturers shop test tanks
to specify field aerator performance, even though the shop tests are closely
controlled. Substitution of shop tests for field tests have not been without
problems (W) (21).
There are inconclusive data about the relationship between velocity of
circulation and aeration efficiency (2). Higher velocities improve mixing
but may decrease time of bubble contact ("hang time"). One study evaluating
coarse bubble diffusers in tap water concluded that tank geometry and
diffuser placement configurations were most significant to oxygen transfer
efficiencies at depths over 15 feet (21). Another study evaluating fine
bubble diffusers in tap water with 5 mg/1 anionic detergent found that basin
geometry and diffuser placement influences were most significant at depths
of 10 feet and less (15). Although specific conclusions vary, it is
generally agreed that changes in tank geomerty and diffuser placement result
in changes in mixing patterns and hence the relative velocities of bubbles
and water, all of which affect oxygen transfer. It is important to note,
however, that for any given aeration device, the influence of basin geometry
is a definable parameter (22).
Houck discusses how dome diffuser operating characteristics are
influenced by mixing and tank geometry (3). As discussed earlier, alpha in
plug flow systems can approximately double (Houck reported ranges from 0.4
to 0.8) as wastewater is progressively oxidized from inlet to outlet. The
situation encourages biological fouling (sliming) tendencies which primar-
ily occur in regions of high organic loading and low dissolved oxygen.
Plug flow further exacerbates such tendencies because of the localized
high organic loadings experienced in the first pass. In situations where
there are long narrow tanks in multiple pass series, oxygen demand is lowered
to the point where it is virtually impossible to decrease diffuser density
adequately to prevent overaeration and still maintain sufficient mixing.
Houck's data suggested a correlation between length to width ratio (L/W) and
aeration efficiency. The three most efficient plants visited all had L/W
less than 12:1.
In addition to poor matching of air flow capability with oxygen require-
ments, lack of control to adequately adjust the air flow capacity available
and basin geometry poorly suited to the operating characteristics of the
diffuser equipment were noted as other factors contributing to low aeration
efficiency. Houck concluded that aeration tank design and operation is
easier in a system where alpha is averaged and localized high volumetric
organic loadings which can occur in the influent zone of the first pass of
multiple pass plug flow tanks are avoided. High localized loadings can lead
to low D.O. and biological fouling of dome exteriors followed by the onset of
coarse bubbling and reduced oxygen transfer efficiency. He recommended
consideration of completely mixed tanks (as opposed to plug flow) whenever
possible and noted that such completely mixed systems could probably be
20
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operated at volumetric and sludge loadings in excess of those currently
used. Where plug flow geometry is utilized, Houck recommended the design of
single-pass tanks with L/W limited to 12:1.
A better understanding of optional design and operating parameters is
required. Current, largely empirical knowledge is inadequate according to
observations from Houck's survey. EPA has recently co-sponsored a project
with the United Kingdom and Canada at Rye Meads Wastewater Treatment Plant
near London, England to investigate and/or document optimal and limiting
aeration tank geometry, particularly tank L/W, aeration taper, diffuser
density, and air flow per diffuser for two activated sludge process varations
(23). Several operating strategies will be explored. Process performance
efficiency and economics will be documented. Field evaluation is scheduled
to run until July 1982 or longer, depending on the severity of the 1981/82
winter.
Sampling and Measurement Considerations
Sampling and measurement techniques following quality assurance guide-
lines serve little purpose if the samples taken are not representative or the
measurements made are erroneous. Considerable work has been done to take
into account outside effects on aerator test procedures. Detailed discus-
sions about representative sampling and the effects of interferences on
key measurements used to determine aeration equipment capacity and efficiency
are found in the references listed in Table 4 below.
TABLE 4
REFERENCES DICUSSING SAMPLING AND MEASUREMENT CONSIDERATIONS FOR VARIOUS
WASTEWATER PARAMETERS
Parameter
sampling
oxygen
temperature
pH, Fe, Mn
gas flow and power
general test procedures
Reference Number
24
25
12
26
27
20
Design Considerations
Characteristics of Fine Bubble Aerators that Affect Design (2) (3)
Major design factors affecting fine bubble aerator performance
efficiency are air flow range, aerator density and configuration, depth and
tank geometry.
Use of a wider air flow range for peak load periods will allow specifi-
cation of fewer aerators for the aeration basin. Rarely will oxygen demand
require more than three to four times the minimum air flow rate in a municipal
wastewater treatment plant. For example, a range of 0.5-2 cfm/dome for the
21
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Norton Hawker-Siddeley dome was found to be desirable in Houck's study. A
suggested design procedure is to determine the number of domes for 0.5 cfm
air flow per dome to meet the minimum oxygen demand' and then check the air
flows for maximum demand. Minimum air flow rates are controlled by the
headless across the control orifice. Maximum rates are controlled by their
relationship with oxygen transfer efficiency which decreases with increas-
ing air flows. Hawker-Siddeley currently recommends that their diffusers
be designed for a 5:1 maximum:minimum air flow ratio.
Aerator density should be maximized within the constraints of minimum air
flows and economic costs. Denisity should also be tapered in plug flow tanks
concomittant with decreasing oxygen demand to avoid potentially extreme
overaeration and reduction of power economy in the middle and latter sections
of aeration tanks. There appears to be a definite correllation between dome
or disc diameter (horizontal surface) and specific oxygen transfer per
diffuser. Data from clean water tests suggests that fewer of the larger
diameter disc units may be required to transfer equivalent ammounts of
oxygen at the same oxygen transfer efficiency as the smaller diameter dome
units.
The nearly linear correlation between increased oxygen transfer and
aerator depth to at least 20 ft overcomes increased hydrostatic pressure
power requirements. The net result is a decrease in blower brake horsepower
with depth for a given oxygen demand. Tapering off of oxygen transfer
efficiency at higher depths ( 20 ft) is caused by oxygen depletion in the
bubble. Within limits imposed by the treatment plant site and economic
considerations, maximization of aeration tank depth up to 30 ft is
recommended. (This assumes well mixed conditions exist in the basin and that
oxygen transfer, not mixing controls diffuser placement. Other researchers
suggest that a diffuser depth between 8 and 16 ft usually gives the optimum
balance between mixing and oxygen transfer rate (1)).
Reference 2 summarizes aeration practices in wastewater treatment as of
1971. It is an important background reference which discusses in detail the
design considerations affecting aeration equipment selection and addresses
most of the itmes discussed in this Technology Evaluation Section. Reference
3 contains the results of a 1979 full size activated sludge plant survey
designed to review, document, and evaluate power requirements, design
practices and operating and maintenance characteristics for 19 fine bubble
dome diffuser aeration systems. The information documented in Reference 3
should be of particular interest to design engineers and municipal officials
who are considering utilizing fine bubble aeration equipment in new
activated sludge plants or switching to such equipment in existing plants.
Specifying and Evaluating Wastewater Aeration Equipment (2) (14) (28)
Two major areas require specification: Mechanical aspects and equipment
performance. This discussion concentrates primarily on performance
requirements.
22
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Very generally, the aeration system must maintain microbial solids in
suspension so they can come into contact with dissolved oxygen in" the waste-
water, and it must transfer enough oxygen to the wastewater to satisfy
microbial metabolism requirements. Mixing requirements are normally
specified by minimum wastewater horizontal flow velocities which must be
maintained in the basin. Oxygen transfer requirements are specified by an
oxygen transfer coefficient KLa. Activated sludge systems are designed to
be food limiting so that metabolism rather than a limiting oxygen concent-
ration sets the rate of oxygen demand. In doing so, the transfer of
oxygen through the films to the bulk liquid to maintain the dissolved oxygen
concentration desired becomes the controlling factor for the aeration system
design.
It is important that mixing power requirements be checked for each appli-
cation. In the design of activated sludge basins, adequate mixing usually
occurs if metabolic oxygen demands are met. In the design of aerated lagoons
for the treatment of domestic wastes, the mixing power requirement will most
often be the controlling factor. Typical air requirements for diffused air
systems to insure good mixing vary from 20 to 30 cfm/1000 ft^ of tank volume
(29). Mixing by jet aerators is independent of air flow since they
recirculate the wastewater as well as aerate.
Designing aeration systems based on oxygen transfer requirements makes
it possible in theory to use either steady or non-steady state tests to
determine aeration equipment characteristics. Some practical difficulties
with this approach are discussed in the section below. In any case, manufac-
turers' commonly used criteria for aeration performance are aeration
capacity (weight of oxygen absorbed into solution per unit time) and aeration
efficiency (aeration capacity per unit of energy supplied). Results are
normally given for clean water using the non-steady state test at standard
conditions (20°C, 1 atmosphere pressure, and 0 rng/1 initial dissolved
oxygen). Table 5 gives estimated ranges of comparative clean water oxygen
transfer and aeration efficiencies for several generic devices. It is an
update of original compilations by Brenner (30).
Aeration requirements for the bio-oxidation process under consideration
are a function of the measured or design uptake rate (dO/dt) of that process.
In addition, the expected oxygen deficit (C$ - C[_) as well as alpha and beta
must be estimated or determined. Once this is done, the value of K|_a can be
calculated (equation [8]), oxygen requirements determined, and aeration
equipment selected from manufacturers aeration capacity and efficiency
information discussed earlier.
In practice, selection of aeration equipment involves more than oxygen
requirement considerations. Selection of aeration systems also involves
consideration of climate; mixing flexibility; diurnal flow variations;
mechanical complexity and reliability; capital, operating and maintenance
costs; aesthetics; and perferences of the owner. Table 6 outlines consider-
ations which must be addressed to select any type of .aeration equipment.
Fisette provides additional insight into the many tangible and intangible
considerations which must be taken into account (31).
23
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TABLE 5
COMPARATIVE CLEAN WATER OXYGEN TRANSFER INFORMATION FOR
AIR AERATION SYSTEMS UNDER STANDARD CONDITIONS
(44)
(a)
Range of
Clean Water 0
Type of Aeration Device Transfer (%)
Mechanical Aerator
Low speed surface
High speed surface
Turbine sparger (b) 14-18
Fine Bubble Aerators (c)
Fine Bubble Diffuser
Total floor coverage 20-32
Side wall mounted 15-20
Jet Aerator (b) 15-26
Coarse Bubble Diffuser (c)
Static aerator 10-16
Coarse bubble dual aeration 10-13
Coarse bubble single side
aeration 8-10
Range of Clean Energy
Water Efficiencies Requirement
kg 02/kwh kwh/kg Q£
(Ib 02/hp-hr) (kwh/lb 02)
1.5-2.2
(2.5-3.5)
1.2-1.8
(2.0-3.0)
1.2-1.8
(2.0-3.0)
3.0-4.6
(5.0-7.5)
1.8-3.3
(3.0-5.5)
1.6-2.3
(2.7-3.8)
1.4-1.9
(2.3-3.2)
1.4-1.6
(2.3-2.7)
1.2-1.5
(2.0-2.5)
0.46-0.66
(0.21-0.30)
0.55-0.82
(0.25-0.37)
0.55-0.82
(0.25-0.37)
0.22-0.33
(0.10-0.15)
0.31-0.55
(0.14-0.25)
0.44-0.62
(0.20-0.28)
0.51-0.71
(0.23-0.32)
0.62-0.71
(0.28-0.32)
0.60-0.71
(0.30-0.32)
(a) Compiled using a combination of manufacturers' company
bulletins, technical reports, and historically accepted
data ranges.
(b) Includes energy requirements for two prime movers.
(c) Based on clean water test at 15 ft. water depth; submergence
varies depending on device.
24
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TABLE 6
INFORMATION REQUIRED TO SELECT AND VERIFY
AERATION EQUIPMENT PERFORMANCE
Treatment Process Description
• design flow
• tank geometry and configuration to give
- aeration basin volume
- hydraulic detention time
• operating conditions
- flow rate
- mean cell residence time for activated sludge processes
- environmental parameters (temperature, pH, altitude)
- mixing requirements
- dissolved oxygen concentration at steady state
Nastewater Characterization
expected influent and required effluent BOD
oxidation and nitrification metabolic requirements
dissolved oxygen saturation concentration
oxygen uptake rate in the aeration system
suspended solid concentrations in the aeration system
Oxygen Transfer Coefficient KLa Determination
. alpha
• beta
Aerator Performance and Design Requirements
equipment operating flexibility
allowable power variations and limitations
aerator placement and configuration
mixing capacity - basin horzontal liquid velocities
aeration capacity
oxygen transfer efficiency
Method of Aeration Equipment Testing
. steady state
• non-steady state
• power measurement
Data Analysis Method
Health and Welfare Aspects
• spray
• mist
• noise
25
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• The aeration system selected should be field tested to determine if it
meets expectations. Each application of aeration equipment is sufficiently
site specific such that the field performance of the aeration equipment
cannot be predicted with confidence. The compexity of the interrelated
variables affecting aeration performance and its measurements is the subject
of this assessment. Testing aeration equipment, even though adding
considerably to the cost of the installation is necessary and justified
Basin Geometry and Mixing Considerations (3)
Recognizing the process advantages of plug flow systems and considering
the wastewater characteristics which contribute to sliming, Houck recom-
mended in his 19 plant study that new dome or disc diffuser systems should be
used in a plug flow aeration tank having the lowest practicable L/W. Tapered
aeration was also recommended; however it was noted that it is only a partial
solution, limited by the aeration/mixing requirements of the lightly loaded
back end of the plug flow system. It was reconized that step feeding helps
distribute oxygen demand and alpha depression more equally; however its use
is limited in s'ingle-stage nitrification systems. It was also reported that
some plants have had good experience with feeding raw sewage down stream of
the mixed liquor feed, effectively creating a zone of sludge reaeration in
the first section of the aeration tank. In any case it was emphasized that
the oxygen demand for each distinct segment of the plug flow aeration
process should be calculated and the aeration system sized appropriately,
taking into account the variation of alpha from inlet to outlet. Dissolved
oxygen monitoring and provision for independent air flow control should be
provided for each aeration grid and/or pass in a multiple channel tank.
The rationale behind minimum allowable air flow requirements has been
discussed previously. Houck found that for the plants he surveyed, the
practice of adhering to minimum specific air flow rates promoted good
maintenance history but contributed to mediocre energy efficiency at many of
the plants because oxygen demand requirements were exceeded. Better
matching of process and aeration tank design to diffuser system design
constraints was considered the most effective solution to the problem of
overaeration.
Clean water studies show a nearly linear correllation between oxygen
transfer efficiency and depth up to at least, 20 ft. Furthermore, increases
in blower efficiency can be expected up to about 30 ft using a blower
equation comparing blower power required versus depth to transfer into
solution an equivalent amount of oxygen. Beyond this, oxygen depletion in
the bubble clouds the analysis. Thus, overall aeration efficiency should
improve with increasing tank depth. Significantly, however, Houcks data
showed no clear correlation between mixed liquor depth and oxygenation
efficiency at depths greater than 12 ft. Plants that had shallow aeration
tanks, 12 ft or less, had lowered oxygen transfer efficiency. It is likely
that the relatively modest improvement in efficiency with depth is over-
shadowed by other factors in the aeration system when tanks are 12 ft of
deeper.
26
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Depending on the age of the plant, Houck found that some aeration basins
were constructed with ridge and furrow floor design. This configuration was
originally developed as an aid in mixing and tank circulation. Houck reports
that the concensus in England is that the ridge and furrow configuration is
costly to construct and adds little to performance; consequently, it was not
seen in the newer plants in his survey.
Specifying Air Supply Equipment
The type of blower used for a particular situation depends primarily on
economics, space and air flow. Generally speaking, both positive dis-
placement and centrifugal blowers are normally considered for air volumes to
15,000 cubic feet per minute (CFM) of air. Overall economy favors
centrifugal blowers for units larger than 15,000 CFM. The Axial compressor,
ideally suited for large flows, should be considered for volumes over
100,000 CFM. Szczensy discusses the many types of compressors used in sewage
aeration applications v-^J- The trend seems to be toward single-stage
centrufugal compressors with their lower first cost and efficiency con-
parable to multistage centrifugal compressors. Table 7 gives a summary of
general application information on types of units and their volume range of
application (3*1.
There are three basic types of air cleaning systems: viscous
impingment, dry barrier, and electrostatic precipitation. In a viscous
inpingment system, filtrate particles strike an oil-coated surface of a
filter and become trapped until the filter is cleaned. A large portion of
low specific gravity particles, however, can pass trough. This type of air
cleaning system is most suitable for primary filtering. In a dry carrier
system, the filter material if generally quite fine. Bag house dry barrier
systems are most commonly used. Their efficient is greater after they are
partially dirty or precoated because retained particles increase effect-
iveness of the straining mediun. Bag house collector size, expense and
precoat requirements have diminished their selection in many newer plants.
Replaceable filter assemblies are an easy method to filter the air but can
be costly. The electrostatic precipitator gives particles and electric
charge so that they are subsequently removed by attraction to elements of
opposite polarity. Electrostatic precipitators can remove small particles
at a constant high efficiency. Ashe discusses wastewater treatment plant
air filter design considerations in some detail (33).
Cleaning efficiency is the primary filter design characteristic and is
determined by the equipment it is designed to protect. For fine bubble
diffusers, the common standard recommended for effluent air quality is 0.1
milligrams or less of dirt per thousand cubic feet of air.
In addition to particulates in the air, diffusers can be clogged
externally by fine sand in the tank liquor, excessive calcium carbonate
hardness in the water supply, and reduced iron salts in the incoming waste.
When retrofitting fine bubble diffusers into existing plants, the air
piping should be carefully checked for rusting or scaling. Consideration
should be given to cleaning or coating existing piping to avoid particle
27
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shedding from its walls where it can cause fouling. Morgan discusses plant
experience and proposes corrective measures for causes of diffuser
fouling other than dirty air (34). It is important to keep in mind however
that a properly filtered air supply is the most important single
consideration for minimizing diffuser fouling.
Methods of Aeration Equipment Testing
There are two general methods for determining the oxygen transfer
capicity of aeration equipment selected. The non-steady state or clean
water test measures oxygen transfer capacity from the gas to the liquid
phase (tap water) and does not address the effect of wastewater consti-
tuents. In this test, the concentration of the dissolved oxygen in the
water is constantly changing over time (dC/dt is not constant) as the
liquid approaches saturation. . The steady state test is conducted under
process conditions, which for an activated sludge treatment plant means
after the plant is in operation and microbial suspension has developed to
the design value. In this case, the concentration of oxygen in the
wastewater is constant over time (dC/dt=0).
The general mathematical models which have been adopted to describe the
transfer of oxygen to a liquid have been developed earlier for both the
non-steady state and steady state tests (equations [3] and [8], respect-
ively). There are two basic assumptions common to the use of both these
models. First, it is assumed that oxygen uptake and transfer are occuring
in an adequately (homogeneously) mixed fluid. Secondly, it is assumed
that Henry's Law applies and the ratio kL/kg remains constant throughout
the contacting device ('). in practice, this ratio may not remain constant
(and KLa will vary) when aeration capacity is inadequate to satisfy BOD
and dissolved oxygen concentrations in the bulk liquid do not follow
Henry's Law predictions. From a practical standpoint, correlation between
steady state wastewater tests and non-steady state clean water tests are
difficult because of these and other inherent problems discussed earlier.
The need to accurately correlate clean water and wastewater test results to
minimize costs of field modifications of aeration equipment has been
recognized by EPA as an important area of research (3°).
Considerable literature is available describing various approaches
used to conduct steady state and non-steady state tests. There is no
commonly accepted procedure, and engineering apecifications outlining test
methods vary. Paulson (20) has summarized the non-steady and steady state
procedures cited in the literature as well as those currently in use by
owners, consultants, and manufacturers. He discusses significant dif-
ferences among them. Other more recent procedures for field evaluation of
both fine and coarse bubble aeration devices are discussed in references 35
and 36.
Neither the steady state nor non-steady state aeration equipment test
is free of problems. However the problems are not insurmountable and the
tests are valid. Major problems in the steady state test are determination
of the correct values of dO/dt, alpha, Cs, and C[_ to be used. Major
29
-------
problems in the non-steady state test are determination of the correct
value of Cs to be used and possible interferences in the dissolved oxygen
analysis. Measures to minimize these problems have been discussed or
referenced elsewhere in this paper. From a theoretical standpoint the
steady state test is the preferred method of aeration equipment evaluation
because it takes into account wastewater composition. However, practi-
cally speaking, the non-steady state method is more commonly used because
it has less interferences and possibilities for error and in many design
situations field testing with biomass is not feasible. The value of clean
water testing is enhanced when it is conducted in the actual aeration tank
with continuous dissolved oxygen measurement and recording.
Data Analysis
The basic mathematical model describing the rate of gas transfer into
solution is defined in equation (2). The model allows estimation of the
gas transfer coefficient Ki a by analysis of data obtained from experi-
mental measurement of dissolved oxygen concentration with time. The KLa is
characteristic of the aeration equipment and process conditions producing
it and allows equipment efficiencies to be calculated. Three forms of this
basic mathematical model are commonly used for estimation of KLa.
Equation [2] is called the differential or general form and is repeated
below:
dC
KLa (CS-CL)
[2] DIFFERENTIAL FORM
By specifying the initial condition that C[_=C0 at ti=0 and C|_=C at
t2=t, equation [2] can be integrated and rearranged to become a more
specific equation [3] which is termed the integrated of log deficit form:
In (CS-C) = ln(Cs-C0) - KLat
[11] LOG DEFICIT FORM
Finally, transforming the logarithmic form to base 10 numbers allows
equation [11] to be expressed in terms of dissolved oxygen concentration:
c=cs - (Cs-C0)e -KLat
where e=2.71828
[12] EXPONENTIAL FORM
A variety of graphical and numerical procedures have been proposed to
to analyze oxygen transfer data. Most procedures deal with the non-steady
state test. The conventional approach graphs semi-log plots of oxygen
saturation deficits versus time according to Haney (°) using the log deficit
form of the equation. The slope of the line is the negative of the gas
transfer coefficient K[_a. Cs, the oxygen saturation value, may be assumed to
be the value at the surface, corrected for depth, or experimentally measured.
A newer data evaluation procedure has been proposed by Stukenberg U*U which
uses the differential form of the basic equation and plots oxygen transfer
rate versus oxygen concentration directly. This method of analysis may be
used on results from steady state tests (where the oxygen saturation
concentration is 3 Cs) or used for non-steady state tests. Still another
30
-------
procedure fits the exponential form of the basic equation to the experimental
data. In this analysis, values of dissolved oxygen concentration are used
directly and equation [12] is fit to the data using non-linear least squares
procedures. Brown has discussed these and other oxygen transfer parameter
estimation methods in detail (37).
Data analysis methods are influenced by the form of the fundamental mass
transfer equation they follow and by how they attempt to account for limita-
tions in the experimental data. As a result there is a relatively large
number of them. Differences among then result from the use of a variety of
values for Cs: some calculated and some determined from experimental data.
Linear models commonly use the least squares method to fit the equations to
the expermental data. Non-linear models use other iterative regression
analysis techniques. Data truncation (below 20 and above 80 percent of
saturation) is often required because of dissolved oxygen measurement
limitations, especially as dissolved oxygen saturation is reached. Brown
and Yunt have summarized and presented a general review of data analysis
techniques along with information about each (37) (38). The five common
methods noted above are presented in Table 8.
Energy Utilization
The costliest item in the activated sludge process is the aeration
system because of its high energy consumption during operation. Aeration
equipment power consumption for secondary activated sludge normally accounts
for 60-80% of total power demand (3) (39).
Electrical power consumption can be estimated for diffused air equipment
which consumes most of the power in the activated sludge process I39):
kWh/lb 02 = (0.39 + 0.318 GP)/OTE [13]
where: GP = compressor exit pressure, psig
OTE = oxygen transfer efficiency in percent =
mass air dissolved in aeration basin x ,00
mass air supplied to diffusers
It can be seen that electrical power consumption per unit weight of oxygen
required is inversely proportional to oxygen transfer efficiency and
directly proportional to compressor exit pressure.
Clean water aeration energy requirements can be estimated using Table
5. For example, the average estimated energy requirement for fine bubble
aerators is 0.41 kwh/kg 02 from the data in Table 5. For an oxygen
requirement of 1000 Ibs/yr, electrical energy required is 67,868 kWh/yr.
Table 9 summarizes the average energy requirements from the ranges given in
Table 5 for the three major aeration devices in clean water. It then
estimates these requirements for wastewater. Assumptions are given in the
table.
31
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Note that the estimation of aeration efficiency is very important when
calculating energy requirements. In Houck's survey, aeration efficiency was
estimated for 16 operating activated sludge systems using dome diffuser fine
bubble aeration \*). The highest and lowest yearly average aeration
efficiencies observed were 3.5 Ib 02/wire hp-hr and 1.3 Ib Op/wire hp-hr.
For the three plants with a reasonably sufficient comparative data base,
fine bubble dome diffuser systems were approximately 1.7 times higher than
for side-by-side coarse bubble diffuser systems (2.56 vs 1.56 Ib Oo/wire hp-
hr). It was the opinion of the authors that with enhanced design and
operating techniques, aeration efficiencies of dome diffuser plants with no
unusual alpha depressing wastes present could be increased 25-75 percent
over the average value of 2.43 Ib 02/wire hp-hr estimated from the survey.
Energy utilization is only one parameter of aerator performance.
Mixing capability, reliability and flexibility of operation should also be
considered in conjunction with operational and capital costs when selecting
the aerator type.
O&M Requirements (3)
t
Historically., fine bubble aeration equipment was widely used in the
United States prior to 1950. Because of fairly intensive maintenance
requirements it was gradually replaced with low maintenance coarse bubble
equipment. The increase in power costs since 1974 has resulted in renewed
interest in fine bubble aeration operation because of its more efficient
oxygen transfer potential.
The plants surveyed by Houck exhibited few maintenance problems with
the dome diffuser aeration systems. The Norton/Hawker-Siddeley dome
diffuser was in use in all of the plants surveyed in this study. The dome
diffuser was developed in 1954, and its clean water aeration efficiency
normally runs 7.4-8.2 Ib/wire hp-hr at 13-15 ft water depths. Houck
concluded that the good mainteance experience was directly attributable to
two principal factors:
. Concientious (though not labor intensive) attention to aeration
system operation, particularly as related to air cleaning and
repair of infrequent equipment failures. Minimizing interruption
of air flow and maintaining an air flow per dome of 0.5 cfm also
contributed substantially to the low incidence of maintenance
problems.
• Steady improvement of dome diffuser air piping and air cleaning
equipment over the course of its history.
A major operational problem encountered was formation of biological
slime on the external surface of the diffuser. Diffuser sliming is
apparently produced by conditions of high F/M loading and/or low-dissolved
oxygen and manifests itself as coarse bubling at the aeration tank
surface. One possible explanation for the coarse bubbling is that slime
causes the bubbles to coalesce during formation. Another theory proposed
33
-------
TABLE 9
AERATION ENERGY REQUIREMENTS
Aeration Device
In Clean Water
kWh/kg 02*
In Wastewater
kWh/kg 02** kWh/106 gal***
Mechanical aerator
Fine bubble aerator
Coarse bubble aerator
0.64
0.41
0.64
1.02
0.65
1.02
546
348
546
* From Table 7
** Assume wastewater energy requirements = (Clean water energy
requirements)/(ax3) where a = 0.7, 3 = 0.9
*** Assume A soluble BOD = (136-20) mg/1, ANH4-N = (20-17) mg/1 and
unit oxygen requirements per unit of BOD and NH4-N are 1.1 and
4.6 respectively resulting in 535 kg 02 needed per million gallons
wastewater (1180 Ib 02/mgd) for oxidation
34
-------
is that the slime gradually blocks off air flow through the cermie media,
forcing air to take the path of least resistance up through the dome
orifice that surrounds the center hold-down bolt and finally out through
the poor-sealing bolt gasket. If this latter explanation proves to be
valid, the recently developed disc diffuser would probably remedy the
situation as it has no other potential avenue for the air to escape other
than through the media. The coarse bubble phenomenon deserves increased
investigation. Regardless of the cause(s), coarse bubbling is undesirable
because larger bubbles result in less oxygen transfer efficiency. Sliming
was observed to occur most frequently at tank locations where organic
loading was highest and dissolved oxygen the lowest. It was found that
mild sliming could be reversed by greatly increasing the air flow and
reducing raw sewage flow to the affected tank area for 24-48 hours.
Routine tank cleaning (for example, yearly) and in-place dome brushing
manually or with high pressure air was found desirable for long term
control of sliming at some plants.
Calcium carbonate scaling was not a major problem at the plants
surveyed by Houck, with one exception. For that case, domes had to be
removed and cleaned every 5 years. Cleaning consisted of scrubbing, acid
soak in 10 percent hyrochloric acid for 24 hours, and steam cleaning. The
domes were not refired, although the manufacturer recommended refiring
after every other cleaning. In general, intervals between major cleaning
efforts (removal and refiri.ng or equivalent) for all plants varied from 4
to over 9 years. Table 10 summarizes maintenance experience found by
Houck.
Monitoring and maintaining a desired mixed liquor dissolved oxygen
concentration is necessary to optimize plant aeration efficiency. If
hydraulic and/or organic loading rates decrease and air flow rates do not
respond accordingly, the dissolved oxygen deficit in the activated sludge
basin decreases, resulting in less than optimum oxygen transfer perform-
ance. Houck found a number of plants were overaerating their mixed liquor
and had taken no steps to monitor dissolved oxygen concentrations and
reduce air flows to more efficent operating levels.
All of the plants reported low maintenance requirements for their air
cleaning equipment no matter which type was used. Bag filters required
least attention with one or two cleaning cycles per year. Electrostatic
units required three to four cleanings per year, but each cleaning
operation was simple and took less than half a man-hour. Every 2 years the
electrostatic units required more thorough maintenance, consuming half a
man-day. Disposable filters were simplest of all, but replacement filters
are costly.
Significant industrial waste fractions in municipal wastewaters may
substantially lower dome diffuser oxygen transfer efficiency via a
reduction in the alpha factor. This is especially true in the first
segment of long, plug flow aeration tanks. Houck reported alpha values as
low as 0.3-0.4 at the head of such tanks where detergents and other
35
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36
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surfactants haven't had sufficient contact time to be biodegraded. Alpha
increased to values of 0.8 or higher at the effluent end of the tank.
Diffuser cleaning is a labor intensive and costly process that can
usually be forstalled by careful O&M. MOP No. 5 discusses common methods
used for diffuser cleaning (*). Houck found diffuser cleaning frequencies
to vary from 4 to over 9 years in the plants surveyed (see Table 10). It
is prudent practice to have provisions for diffuser cleaning at any plant.
Methods of cleaning porous ceramic diffusers include kiln burning
(refiring), acid cleaning (diffusers removed or in place), and alkaline
cleaning. Ultrasound is a new alternative to conventional methods of
cleaning which needs further development. In the Sanitaire system, a
cleaning agent can be added along with process air. The cleaning agent
presently chosen in HCL gas, and gas consumption of 0.25 Ib/diffuser per
cleaning cycle is reportedly typical (40). The Vortex Jet Aerator
manufactured by the Aerocleve Division of- Clevepak Corporation contains an
automatic pneumatic backflush system which claims to virtually eliminate
all below the water maintenance and clogging problems (41).
Costs
It is the responsibility of the designer to choose an adequate
aeration system that will supply the mixing and oxygen requirements for
the process at minimum annual cost. Determining the requirements of an
adequate system is not simple and was discussed earlier under the Design
Considerations Section. Once they are determined, various fine bubble
aeration systems made up of a specific number, type, and equipment
configuration can be specified and costs for comparison among systems can
be estimated.
Major construction cost items are air piping and headers as appro-
priate, the aeration devices and their supports, air cleaning equipment,
blowers, and buildings to house the latter items. Operating and
maintenance costs are principally operational power costs, aerator
cleaning and replacement costs, and air cleaning costs.
Operational power costs not suprisingly depend on oxygen transfer
efficiency of the fine bubble aeration system chosen and influent
wastewater characteristics. Aerator cleaning costs depend upon the type
of aerator, its flexibility with respect to cleaning (removal) or
repalcement, O&M practices at the plant, and influent wastewater charac-
teristics. MOP no. 5 discusses in some depth how cleaning and replacement
costs_vary with air-passed-betweencleanings and pressure loss from
clogging (*). Air cleaning equipment costs represent only a small
percentage of total aerator system O&M costs; however, properly filtered
air is an important part of aeration system operation. Section 4 which
compares equivalent technologies gives some "typical" cost comparisons and
references other cost studies.
There are other factors besides cost which may preclude selection of
a certain type aeration system. Major ones are climate (winter)
37
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considerations affecting operation, control ability of the aeration system,
noise levels, and compatability with other aeration systems already on
site. The capability to increase aeration capacity in response to
potential future increases to oxygen demand or mixing should also be
considered.
38
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4. COMPARISON WITH EQUIVALENT CONVENTIONAL TECHNOLIGIES
Current methods used to transfer oxygen in aerobic biological
wastewater treatment processes include: 1) jet aeration, 2) compressed
air diffusion, 3) submerged turbine aeration, 4) static tube aeration, and
5) mechanical surface aeration. Stukenberg U4) has suggested that
compressed air diffusers "of the conventional design" should not be used
when oxygen demand exceeds 40 mg/l-hr. Surface aerators are recommerded to
meet oxygen demands up to 80 mg/l-hr. Compressed air diffusers and
submerged aerators are more suitable for areas experiencing long periods
of freezing weather. (Surface aeration is an effective heat disipation
process which may significantly lower the temperature of the liquid.)
Submereged turbine aerators involve more mechanical equipment but have the
most flexibility with respect to turndown.
Stukenberg's suggestions are a guide'to system selection. The specific
selection will depend on a careful review of many factors which were
discussed in the Technology Evaluation Section. In general, any compar-
ision of specific aerators involves defining the performance aspects the
aerator must meet (see Table 6), identifying the capabilities and
limitations of the equipment being considered (from manufacturers
brochures, published literature, and testing), and satisfying the pre-
ferences of the owner (including personal, pragmatic, and economic
considerations).
Langford made a detailed study of costs of several types of activated
sludge aeration systems in general use in the United States in 1972. His
purpose was not to make an unequivocable determination about the cost
effectiveness of any type aeration system, but rather to present a
comparative cost analysis within the framework of assumptions and
approximations adopted v^2). A wastewater composition with influent BOD
of 180 mg/1 was assumed and design prodecures outlined by Eckenfelder were
followed to compute needed detention times. Three types of fine bubble
diffused air sytems, one coarsebubble system and two mechanical aeration
system designs were evaluated. A complete listing of the prices for
various sizes of components was developed. These prices plus the cost of
the construction needed to put these materials together (basin struc-
tures, blowers and piping, air filters, etc.) were then used to develop a
range of construction cost estimates for plant sizes from 0.1 to 100 MGD.
Operation and maintenance costs for each of these systems were also
developed. A summary of the reported costs is presented in Table 11.
Even for the low power costs in effect at the time of this study
the ceramic plate diffusers turned out to be the most economical
system at 10 and 100 MGD, and the second most cost effective of the six
systems studied at 1 MGD. The study shows that fine bubble diffuser
systems are potentially cost effective over a wide range of activated
sludge plant sizes. References 35, 43, 44, and 45 contain other cost and
energy comparisons of different kinds of aerators for the activated sludge
process.
39
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TABLE 11.
COST EFFECTIVENESS COMPARISON FOR SEVERAL
ACTIVATED SLUDGE AERATION SYSTEMS
Aerator Types
Q, Construction Maintenance
MGD Costs,$/year Costs,$/year
Operating Total Costs,
Costs}$/year $/year
Mechanical
Low Speed
Mechanical
High Speed
Saran Tubes
Ceramic Tubes
Spargers
Ceramic Plates
0.1
1
10
100
0.1
1
10
100
0.1
1
10
100
0.1
1
10-
100
0.1
1
10
100
0.1
1
10
100
924
2,941
18,700
130,800
654
2,470
15,050
116,200
1,133
3,560
17,585
136,465
1,134
3,566
17,640
136,300
1,010
3,367
16,715
136,440
892
3,020
16,250
125,290
597
921
4,475
28,952
344
597
3,335
19,412
113
508
3,280
27,100
97
353
1,730
11,500
91
385
1,320
6,470
95
261
1,440
8,380
2,136
5,710
20,100
126,400
2,124
5,670
20,000
125,400
2,512
6,420
23,200
137,000
2,512
6,420
23,200
137,000
2,608
7,400
27,500
179,000
2,428
5,630
19,550
103,200
3,660
9,570
43,300
286,000
3,120
8,740
38,400
261,000
3,760
10,500
44,100
301,000
3,740
10,300
42,600
285,000
3,710
11,200
45,500
322,000
3,420
8,910
37,200
237,000
NOTE: Costs are 1972 dollars.
40
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5. ASSESSMENT OF NATIONAL IMPACT
Of the three major aerobic wastewater treatment processes (activated
sludge, trickling filter, aerobic stabilization ponds), activated sludge
is by far the most prevalent, both in number of plants and by volume of flow
t4b'. Houck reports only a "handful" of U.S. activated sludge facilities
using fine bubble aeration although he notes that the fine bubbles dome
aerators are in use at several hunderd other treatment plants around the
world (3)t More importantly, he and others have noted the fact that rapid
escalation of power costs and replacement of iron air distrubution
networks with plastic piping have made fine bubble diffuser O&M costs more
competitive with other aeration equipment. The 1978 Needs Survey compiled
a large amount of cost and technical data about present and future
municipal wastewater treatment needs (46). Table 12 summarizes-estimated
activated sludge unit process requirements to meet needs for the year
2000. It estimates the number of plants and wastewater flow (to be)
treated for plants now in use, under construction, or required but not
funded.
The large numbers of activated sludge plants where fine bubble
diffusers can potentially be used increase their potential impact on
treatment costs. Average energy required for activated sludge plants in
the United States is 1.07X106 kWh/yr for each plant (43). From Table 12 the
average air activated sludge plant size is 3.5 mgd resulting in a total
plant energy requirement of about 306,000 kWh/yr per mgd size plant.
Mechanical and coarse bubble aerator requirements are about 199,000
kWh/yr per mgd or 65 percent of total energy requirements according to
Table 9 estimates (546 kWh.million gallons). Fine bubble aerators use an
average of 127,000 kWh/yr per mgd (at 348 kwh/million gallons), saving
approximately 72,000 kMh per mgd yearly when used. This is a potential
energy savings of 24 percent of the total plant energy requirements because
of increased aeration efficiency.
If aeration efficiency were the only consideration in aeration device
selection, or if it were always the limiting design factor then fine bubble
aerators would be the simple choice. However, other considerations,
especially mixing requirements, total life cycle costs, aeration capacity,
and equipment flexibility must also be addressed (see Table 6) so that fine
bubble aerators will be chosen only part of the time. In any case, their
high aeration efficiency is a definite advantage. Table.13 summarizes this
advantage expressed as national potential energy savings when fine bubble
aerators are used 20, 40 or 60 percent of the time. These savings are one
element of the total cost effectiveness analysis.
41
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TABLE 12.
SUMMARY OF WASTEWATER TREATMENT PLANTS AND FLOWS USING
AIR ACTIVATED SLUDGE TREATMENT PROCESSES NATIONWIDE ^46^
Now in Use
Under Construction
Activated Sludge
Treatment Process
Conventional
High rate
Contact stabilization
Extended aeration
Plants
(number)
3816
34
873
1902
Flow
(mgd)
19,085
771
2,257
1,197
Plants
(number)
378
8
75
176
Flow
(mgd)
1723
221
102
173
Required, Not Funded*
Plants Flow
(number) (mgd)
3178
19
249
3177
6199
127
508
1157
6625
23,310 } averages 3.5 mgd/plant now in use
* Required by the year 2000
TABLE 13.
POTENTIAL NATIONAL ENERGY SAVINGS USING FINE BUBBLE AERATORS IN
AIR ACTIVATED SLUDGE TREATMENT PROCESSES*
Activated Sludge
Treatment Process
Conventional
High rate
Contact stabilization
Extended aeration
National Potential
Energy Savings £
(kwh/yr)X10°
Percent of Time Used in
Plants Under Construction
20%
24.9
3.2
1.5
2.5
40%
49.8
6.4
3.0
5.1
60%
74.7
9.6
4.4
7.6
Percent of Time Used in
Plants Required, Not Funded**
20%
89.6
1.8
7.3
16.7
40%
179.2
3.7
14.7
33.4
60%
268.8
5.5
22.0
50.2
32.1
64.3 96.3
115.4 231.0 346.5
* Using net energy savings of (546-348) - 198 kwh/million gallons wastewater
treated and flow information in Table 12.
** Required by the year 2000
42
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6. RECOMMENDATIONS
Research and Development Requirements
Research and developemnt requirements identified by most researchers
are those which address in some manner the more significant factors
affecting successful aerator performance. There are many approaches
depending on the point of view and scope of interest. From a historical
perspective, the major factors affecting overall aerator efficiency have
been summarized by Eckenfelder (47). Houck arid .Brenner have outlined
research needs from a more pragmatic viewpoint (3) (30). From whichever
perspective, significant research and development efforts which need to
be continued are summarized below. They are not necessarily independent
of each other.
1.
2,
3.
Efforts to develop a standard method to measure oxygen transfer
and evaluate aeration equipment efficiency in wastewater.
Efforts to correlate manufacturer aeration efficiency clean
water shop test results with wastewater field test results. '
Efforts to define alpha sensitivity for various types of
aerators and especially with respect to basin geometry, degree
of mixing, and concentration of surfactants.
4. Efforts to define the minimum dissolved oxygen concentration in
the aeration basin required to provide adequate oxygen transfer
to the wastewater biota including:
• the minimum air flow needed to attain it
• the minimum degree of mixing needed to uniformly disperse it
5. Efforts to define the relationship between the rate of oxygen
transfer into solution and the rate of biological oxygen uptake.
This involves an investigation into the relationship between the
oxygen transfer coefficient and minimum dissolved oxygen for
biological oxygen uptake.
6. Efforts to compare different types of aerators side-by-side
• oxygen transfer performance comparisons under identical
conditions
7.
8.
. O&M comparison over the long term
Efforts to evaluate different methods and techniques of d iff user
cleaning.
Efforts to identify the cause(s) and solutions(s) to the fine
bubble diffuser coarse bubble phenomenon due to biological fouling.
43
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Process/Technology Improvements
The maximum aeration capicity currently achieved in practice by full-
scale aeration systems is about 0.1 kg 02/M3 hr (l5). This can limit the
maximum rate of biological treatment for high biomass concentrations.
Brenner has noted that the key to efficient high rate-activated sludge
treatment is operation with high biomass concentrations allowing low
reactor detention times at high organic loadings (3°). Development of high-
rate air systems which can efficiently transfer increased amounts of oxygen
into solution and stand up to the repeated scrutiny of a testing procedure
is a necessary prerequisite for more efficient high-rate activated sludge
treatment. In summary, improvements in aeration capacity, in aeration
efficiency, and in the ability to reliably measure, reproduce, and predict
field wastewater aeration results are the most desirable process/technology
improvements.
44
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REFERENCES
1. Benefield, L.D. and C. W. Randall, Biological Process Design for Waste-
water Treatment, Prentice-Hall, Inc., Englewood Cliffs, NO, 1980.
2. WPCF Manual of Practice No. 5, Aeration in Wastewater Treatment, Water
Pollution Control Federation, Washington, DC, 1971 and 1952.
3. Houck, D.H. and Boon, A.G., "Survey and Evaluation of Fine Bubble/Dome
Diffuser. Aeration Equipment", EPA/MERL Grant No. R806990, September,1980.
4. Yunt, F.W.. T. 0. Hancuff, R. C. Brenner, G. W. Shell, "An Evaluation of
Submerged Aeration Equipment - Clean Water Test Results", A -Summary of
Photographic Slides Presented at the WWEMA Industrial Pollution Confer-
ence, Houston, TX, June 5, 1980. :
5. "Aeration Equipment Evaluation - Phase II", USEPA Contract 68-03-2906,
November 14, 1979.
6. Bartholomew, G.L., "Types of Aeration Devices", Aeration of Activated
Sludge in Sewage Treatment, Donald L. Gibbon, Editor, Pergamon Press,
Inc., New York, pp 25-27, 1974.
7. Lewis, W.K. and Whitman, W. G., "Principles of Gas Absorption", Indus-
trial and Engineering Chemistry, Vol.. 16, No. 12, pp 2433-2443,
December 1924.
8. Haney, P.O., "Theoretical Principles of Aeration", Journal American Water
Works Association, Vol. 46, No. 4, pp 353-376, April 1954.
9. Barnhart, Edwin L., "Transfer of Oxygen in Aqueous Solutions", Journal
of the Sanitary Engineering Division. ASCE, Vol. 95, No. 3, pp 645-661,
June 1969. ,.,
10. McKinney R.E. and J. R. Stukenberg, "On Site Evaluation: Steady State
vs. Non-Steady State Testing", Proceedings of the Workshop Toward an
Oxygen Transfer Standard, EPA 600/9-78-021, pp 195-204, April 1979.
11. Bewtra, J. K. and W. R. Nichols, "Oxygenation from Diffused Air in
Aeration Tanks", Journal of the Water Pollution Control Federation, Vol.
36, No. 10, pp 1195-1224, October 1964.
12. Hunter, J. S. Ill, "Accounting for the Effects of Water Temperature in
Aerator Test Procedures", Proceedings of the Workshop Toward an Oxygen
Transfer Standard, EPA 600/9-78-021, pp 85-90, April 1979.
13. Gilbert, R. G., "Measurement of Alpha and Beta Factors", ibid, pp 147-162,
14. Stukenberg, J. R., V. N. Wahbeh, R. E. McKinney, "Experiences in Evalu-
ating and Specifying Aeration Equipment", Journal Water Pollution Control
Federation, Vol. 49, No. 1, pp 66-82, January 1977.
45
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REFERENCES (continued)
15. Boon', A. 6., "Oxygen Transfer in the Activated Sludge Process", Pro-
ceedings of the Workshop Toward an Oxygen Transfer Standard,
EPA 600/9-78-021, pp 232-239, April 1979.
16. Weber, W. J., Physio-chemical Processes for Water Quality Control, John
Wiley and Sons, Inc., New York, NY 1972.
17. Kalinske, A. A., "Problems Encountered in Steady State Field Testing of
Aerators and Aeration Systems", Proceedings of the Workshop Toward an
Oxygen Transfer Standard, EPA 600/9-78-021, pp 205-209, April 1979.
18. Salzman, R. N. and M. B. Lakin, "Influence of Mixing in Aeration", ibid.
pp 59-71.
19. Kenics Aerator Bulletin AE-1 , Kenics Corporation, Andover, MA,
October 1973.
20. Paulson, W. L., "Review of Test Procedures", Proceedings of the Workshop
Toward an Oxygen Transfer Standard, EPA 600/9-78-021, pp 41-49,
April 1979.
21. Stukenberg, J. R. and V. N. Wahbeh, "Surface Aeration Equipment: Field
Performance Testing vs. Shop Performance Testing", ibid, pp 163-179.
22. Rooney, T. C., "Influence of Tank Geometry on Aerator Performance",
i.bi d. pp 50-57.
23. Boon, A. G., "Energy Saving: Optimization of Fine Bubble Aeration",
Application for Federal Assistance to EPA, November 10, 1980.
24.
25.
26.
27.
28.
29.
Shell, G.L., "Sampling Considerations," Proceed ings of the Workshop
Toward an Oxygen Transfer Standard, EPA 600/9-78-021, pp X2-/5, April
_
Stack, V. T. , "Analytical .Measurements and Saturation Values for Dis-
solved Oxygen in Water", ibid pp 76-84.
Naimie, H. and S. Nelson, "Influence of pH and Iron and Manganese Concen-
trations on the Non-Steady State Clean Water Test for the Evaluation of
Aeration Equipment", ibid, pp 91-104.
Yunt, F. W., "Gas Flow and Power Measurement", ibid, pp 105-127.
Sherrard, J. 'H. "Aeration: Proper Sizing is Critical", Water and Wastes
Engineering, Vol. 14, No. 4, pp 62-71, April 1977.
Metcalf & Eddy, Inc., Wastewater Engineering, McGraw-Hill Book Company,
New York, 1979.
46
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30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
REFERENCES (continued)
Brenner, R.C., "Philosophy of and Perspectives on an Oxygen Transfer
Standard-the EPA View," Proceedings of the Workshop Toward an Oxygen
Transfer Standard, EPA 600/9-78-021, pp 12-16, April 1979
Fisette, G.R., "An Aeration Equipment Manufacturer's View of an
Oxygen Transfer Testing Standard," ibid, pp 3-6.
Szczensy, F. F., "Air Supplies for Activated Sludge Aeration", Aeration
of Activated Sludge in Sewage Treatment. Donald L. Gibbon, Editors
Pergamon Press, Inc., New York, pp 31-47, 1974.
Ashe, J.T., "Air Filtration for Diffused Air Systems," Proceedings of
the Workshop Toward an Oxygen Transfer Standard, EPA 600/9-78-0217"
pp. 49-57, April 1979
Morgan, P. F.9 "Maintenance of Fine Bubble Diffusion", Journal of the
Sanitary Engineering Division, ASCE, Vol. 84, No. SA2, Paper 1609,
28 pages, April. 1958.
Ghirardi, S. A., M. A. Nicodema and J. A. Mueller, "Field Evaluation of
Diffused Aeration Equipment", Clearwaters, NYWPCA, Vol. 10, No. 4,
pp 20-26, December, 1980.
McKinney, R. E., "Testing Aeration Equipment in Conventional Activated
Sludge Plants", JWPCF, Vol. 53, No. 1, pp 48-58, January 1981.
Brown, L.C., "Oxygen Transfer Parameter Estimation," Proceedings of
the Workshop Toward an Oxygen Transfer Standard, EPA 600/9-78-021,
pp. 27-40, April 1979
Yunt, F. W. and T. 0. Hancuff, "Aeration Equipment Evaluation - Clean
Water Testing Techniques:, A Summary of Photographic Slides Presented
at the 51st Annual CWPA Conference, San Diego, CA, April 26, 1979.
USEPA, "Total Energy Consumption for Municipal Wastewater Treatment",
MERL Report No. 600/2-78-149, August 1978.
Unpublished Engineering Report, Sanitaire In-Place Cleaning System,
Sanitaire Water Pollution Control Corporation, Milwaukee, WI, 1981.
Aerocleve Vortex Jet Aeration Systems Manual, Clevepak Corporation, Fall
River, MA, 1981.
Langgford, D., "Cost-Effectiveness Comparison of Aeration Systems for
Use in Activated Sludge Treatment of Sewage", Aeration of Activated
Sludge in Sewage Treatment, Donald L. Gibbon, Editor, Pergamon Press,
Inc., New York, pp 65-88, 1975.
USEPA, "Energy Conservation in Municipal Wastewater Treatment", OWPO
Report No. 430/9-77-011 (MCD-32), March 1977.
47
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REFERENCES (continued)
44. USEPA, "Innovative and Alternative Technology Assessment Manual",
OWPO/ORD Report No. 430/9-78-009 (MCD-53), February 1980.
45. USEPA, "Areawide Assessment Procedures Manual", Vol. Ill, MERL Report
No. 600/9-76-014, July 1976.
46. USEPA, "1978 Needs Survey - Conveyance and Treatment of Municipal
Wastewater Summaries of Technical Data", OWPO Report No. 430/9-79-002,
February 10, 1979.
47. Eckenfelder, W. W. Jr., "Oxygen Transfer: A Historical Perspective -
The Need for a Standard", Proceedings of the Workshop Toward an Oxygen
Transfer Standard, EPA 600/9-78-032, pp 1-2, April 1979.
48
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