xvEPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA 832-F-99-065
September 1999
Wastewater
Technology Fact Sheet
Fine Bubble Aeration
DESCRIPTION
In wastewater treatment processes, aeration
introduces air into a liquid, providing an aerobic
environment for microbial degradation of organic
matter. The purpose of aeration is two-fold: 1) to
supply the required oxygen to the metabolizing
microorganisms and 2) to provide mixing so that
the microorganisms come into intimate contact with
the dissolved and suspended organic matter.
The two most common aeration systems are
subsurface and mechanical. In a subsurface system,
air is introduced by diffusers or other devices
submerged in the wastewater. A mechanical system
agitates the wastewater by various means (e.g.,
propellers, blades, or brushes) to introduce air from
the atmosphere.
Fine pore diffusion is a subsurface form of aeration
in which air is introduced in the form of very small
bubbles. Since the energy crisis in the early 1970s,
there has been increased interest in fine pore
diffusion of air as a competitive system due to its
high oxygen transfer efficiency (OTE). Smaller
bubbles result in more bubble surface area per unit
volume and greater OTE.
APPLICABILITY
Saukville Wastewater Treatment Plant,
Saukville, Wisconsin
The Saukville Wastewater Treatment Plant (S WTP)
uses fine bubble aeration to increase treatment
efficiency and oxygen transfer, as well as reduce
power costs. Initially, the plant used stainless-steel
coarse bubble diffusers requiring two centrifugal
blowers. Although these diffusers required little
maintenance, oxygen transfer was inadequate to
meet process needs, and power requirements were
excessive.
In an attempt to reduce power costs and increase
treatment plant efficiency, the plant was retrofitted
with a combination of coarse bubble and fine
bubble ceramic diffusers in 1985. One of the two
treatment cells was retrofitted with fine bubble
ceramic diffusion, and the second retained its
coarse bubble diffusion system. This reduced
power requirements by eliminating the need for one
blower.
After start-up, it was noticed that the mixed liquor
dissolved oxygen (DO) levels were not as high in
the fine bubble cell as in the coarse bubble cell, due
to uneven air distribution between cells. To correct
this, a positive displacement blower was added to
the fine bubble cell. To reduce fouling of the
ceramic diffusers, an in-place gas-cleaning system
was installed to inject anhydrous hydrochloric acid
into the air stream while the system was in
operation.
In 1990, the plant switched to fine bubble
membrane diffusers, which were interchangeable
with the ceramic diffusers. The ceramic and
membrane efficiencies were comparable, so few
adjustments in air rate were needed. The SWTP
was awarded the U.S. Environmental Protection
Agency Award of Excellence in 1991 based on
overall energy savings and optimized operations
and maintenance practices.
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Renton Wastewater Treatment Plant, Renton,
Washington
The Renton Wastewater Treatment Plant serves the
urban and suburban areas east, south, and north of
Lake Washington, just east of Seattle. Rising
power costs created a need for modification of its
coarse bubble aeration system.
Perforated membrane fine bubble diffusers were
selected for an in-plant study in 1982. These
diffusers were placed hi the first two passes of one
aeration tank, while the other tanks retained their
coarse bubble units. DO was compared in the two
systems, and it was determined that the perforated
membrane diffusers required 30 to 40% less air
than coarse bubble diffusers to maintain a
comparable mixed liquor DO. Total energy
consumption decreased from 390 to 355 kW/1,000
m3 after installation of the fine bubble diffusers.
Ridgewood Wastewater Treatment Plant,
Ridgewood, New Jersey
The Ridgewood Wastewater Treatment Plant
(RWTP) was retrofitted from coarse bubble
diffusion to a fine pore diffusion aeration system in
1983. Fine pore aeration would allow the use of
one blower and maintain the same oxygen
utilization rate as provided by the coarse bubble
system. Oxygen transfer studies were performed on
the fine pore ceramic dome diffusers in order to
compare results with the coarse bubble diffusers.
The results showed that the coarse bubble diffuser
had an average standard oxygen transfer efficiency
(SOTE) under field conditions of 4.8% with an
average alpha (a) of 0.55. In contrast, with two
tanks in operation, the fine pore system had an
average SOTE under field conditions of about 9.5%
and an average a of 0.4 during normal daytime
high-load operation. Alpha is defined as the ratio
of KLa (volumetric mass transfer coefficient) of a
clean diffuser in process water to the KLa of the
same diffuser in clean water.
Two methods of cleaning were used at the RWTP:
anhydrous hydrochloric acid brushing and water
hosing. Installation of the fine pore aeration system
achieved the oxygen utilization desired, reduced
power consumption by approximately 28%, and
resulted in a significant improvement in effluent
quality with respect to nitrification.
ADVANTAGES AND DKSADVANTAGES
Some advantages and disadvantages of various fine
pore diffusers are listed below:
Advantages
Exhibit high OTEs.
• Exhibit high aeration efficiencies (mass
oxygen transferred per unit power per unit
tune).
• Can satisfy high oxygen demands.
Are easily adaptable to existing basins for
plant upgrades.
• Result in lower volatile organic compound
emissions than nonporous diffusers or
mechanical aeration devices.
Disadvantages
• Fine pore diffusers are susceptible to
chemical or biological fouling, which may
impair transfer efficiency and generate high
head loss. As a result, they require routine
cleaning. (Although not totally without cost,
cleaning does not need to be expensive or
troublesome.)
• Fine pore diffusers may be susceptible to
chemical attack (especially perforated
membranes). Therefore, care must be
exercised in the proper selection of
materials for a given wastewater.
• Because of the high efficiencies of fine pore
diffusers at low airflow rates, airflow
distribution is critical to their performance
and selection of proper airflow control
orifices is important.
• Because of the high efficiencies of fine pore
diffusers, required airflow in an aeration
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basin (normally at the effluent end) may be
dictated by mixing - not oxygen transfer.
• Aeration basin design must incorporate a
means to easily dewater the tank for
cleaning. In small systems where no
redundancy of aeration tanks exists, an
in-situ, nonprocess-interruptive method of
cleaning must be considered.
DESIGN CRITERIA
Diffusers
In the past, various diffusion devices have been
classified based on their OTE as either fine bubble
or coarse bubble. Since it is difficult to clearly
demarcate or define between fine and coarse
bubbles, diffused aeration systems have been
classified based on the physical characteristics of
the equipment. Diffused aeration systems can be
classified into three categories:
Porous (fine bubble) diffusers: Fine pore
diffusers are mounted or screwed into the
diffuser header pipe (air manifold) that may
run along the length or width of the tank or
on a short manifold mounted on a movable
pipe (lift pipe). These diffusers come in
various shapes and sizes, such as discs,
tubes, domes, and plates.
Nonporous (coarse bubble) diffusers: The
common types of nonporous diffusers are
fixed orifices (perforated piping, spargers,
and slotted tubes); valved orifices; and
static tubes. The bubble size of these
diffusers is larger than the porous diffusers,
thus lowering the OTE.
Other diffusion devices: These include jet
aerators (which discharge a mixture of air
and liquid through a nozzle near the tank
bottom); aspirators (mounted at the basin
surface to supply a mixture of air and
water); and U tubes (where compressed air
is discharged into the down leg of a deep
vertical shaft).
Types of fine pore diffusers
Fine pore diffusers (discs, tubes, domes, and plates)
are usually made from either ceramics, plastics, or
flexible perforated membranes. Although many
materials can be used to make fine pore diffusers,
only these few are being used due to cost
considerations, specific characteristics, market size,
and other factors.
Ceramic media diffusers have been in use for many
years and have essentially become the standard for
comparison since, in the past, they were the primary
media in the fine pore aeration market. .Ceramic,
plastic, and flexible materials are resistant to the
chemicals used in wastewater treatment. Discussed
below are common types of fine bubble diffusers.
However, recent advances in technology have.
resulted in modifications to these types, which are
shown in Figure 1.
Orifice
Baseplate
' .Retainer
Ring
Diffuser Holder-
Membrane
Air Distribution Pipe
a) Membrane Disc Diffuser
Diffuser Bott
Washer & Gasket
Ceramic Dome Diffuse
Dome Gasket
b) Dome Diffuser Assembly
Source: SANITAIRE brand products, reprinted with
permission by the Water Pollution Control Corporation
Brown Deer, Wisconsin, 1999.
FIGURE 1 SCHEMATIC OF VARIOUS
FINE BUBBLE DIFFUSERS
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3Disc diffusers
Disc diffusers are relatively flat and range from
approximately 18 to 24 cm in diameter with
thicknesses of 13 to 19 mm. Materials for discs
include ceramics, porous plastics, and perforated
membranes. Therefore, thicknesses vary, as do
construction features. Currently, manufacturers
provide plenums or base plates that will accept all
materials.
The disc is mounted on a plastic saddle-type base
plate, and either a center bolt or a peripheral
clamping ring is used to secure the media and the
holder together. More commonly, the disc is
attached to the holder via a screw-on retainer ring.
Disc diffiisers are designed to have an airflow range
of 0.25 to 1.5 L/s per diffuser.
Tube/flexible sheath diffusers
A typical tube diffuser is either a rigid ceramic or
plastic hollow cylinder (tube) or a flexible
membrane secured by end plates in the shape of a
tube. A tube difruser has a media portion up to 200
cm long. The thickness of the difruser varies, but
the outside diameter is approximately 6.4 to 7.6 cm.
The various components of a tube diffuser are made
of stainless steel or durable plastic. Threaded rods
are used with ceramic or porous plastic. The rod is
threaded into the feed end of the holder with a
hexagonal nut secured on the rod to hold the
" assembly in place. Air flows through tube diffusers
in the range of 1 to 5 L/s.
Dome diffusers
Made from ceramics or porous plastics, dome
diffusers are typically circular, 18 cm in diameter,
and 3.8 cm high. The media is about 15 mm thick
on the edges and 19 mm on the horizontal or flat
surface. The dome diffuser is mounted on either a
poly vinyl chloride or a steel saddle-type base plate.
The airflow rate for dome diffusers is usually 0.5
L/s with a range of 0.25 to 1 L/s.
Plate diffusers
Plate diffusers are flat and rectangular,
approximately 30 cm2 in area, and 2.5 to 3.8 cm
thick. They are normally made from ceramic or
membrane materials. Installation involves either
grouting the plates into recesses in the floor,
cementing them into prefabricated holders, or
clamping them into metal holders. Air plenums run
under the plates and supply air from headers. Plate
diffusers have largely been replaced by porous
discs, domes, and tubes in new installations.
PERFORMANCE
The performance of diffused aeration systems under
normal operating conditions is directly related to
the following parameters:
• Fouling.
Wastewater characteristics.
• Process type and flow regime.
• Loading conditions.
• Basin geometry.
Diffuser type, size, shape, density, and
airflow rate.
Mixed liquor dissolved oxygen (DO)
control and air supply flexibility.
Mechanical integrity of the system.
Operator expertise.
• The quality of the preventive operation and
maintenance (O&M) program.
The wastewater characteristics that establish the
oxygen demand placed on a fine pore aeration
system are the influent flow rate, biochemical
oxygen demand load, and ammonia-nitrogen
(NH3-N) load.
Fouling is generally classified as one of two types:
Type I and Type II. Characteristics of Type I
fouling are clogging of the diffuser pores, either by
airborne particulates clogging the air side, or metal
hydroxides and carbonates clogging the liquid side.
Type II is characterized by a biofilm layer forming
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and growing on the surface of the diffuser. In
practice, it can be difficult to distinguish between
the two types because they occur together, with one
or the other dominating.
Historically, the rate of fouling was measured by
monitoring the rise in backpressure. However, this
proved to be a crude and qualitative method
because significant fouling can occur without much
increase in backpressure, but with great reductions
in OTE.
The presence of constituents such as surfactants,
dissolved solids, and suspended solids can affect
bubble shape and size and result in diminished
oxygen transfer capability. In general, ceramic
domes and discs yield slightly higher clean water
transfer efficiencies than typical porous plastic
tubes or flexible sheath tubes in a grid placement.
Other key parameters that have an effect on the
performance characteristics of a fine pore media
diffuser are permeability, uniformity, dynamic wet
pressure, and strength.
Effective long-term process control depends on
appropriate selection and integration of the solids'
retention time, the food-to-microorganisms loading,
and the wastewater flow regime. Short-term,
day-to-day variables at the disposal of the operator
include control of diffuser airflow rate and mixed
liquor DO concentration. It is essential to
understand how each of these parameters affects
aeration efficiency in order to develop optimum
short- and long-term operating procedures.
OPERATION AND MAINTENANCE
The main operational objective is to achieve
acceptable effluent quality while maximizing the
aeration efficiency. It is essential that diffusers be
kept clean through cost-effective preventive
maintenance procedures. Preventive maintenance
can virtually eliminate air-side (blower filtration
system) paniculate fouling of fixed fine pore
diffusers.
Filtration equipment maintenance entails cleaning
and changing filter media. Calibration and/or
zeroing of meters is necessary as part of preventive
maintenance because accurate airflow and DO
measurements are a critical part of monitoring
aeration systems.
Preventive maintenance is needed to keep an
aeration system operating at the required level of
performance and to decrease the need for corrective
maintenance. In addition, preventive maintenance
will reduce the number of interruptions in the air
supply, thus preventing solids from entering the air
distribution system.
The cleaning methods used to restore diffuser
efficiency are either process interruptive (aeration
basin out of service) or process noninterruptive
(access to basin not needed). Diffusers can be
cleaned by removing them from the basin (ex-situ)
or onsite inside the basin (in-situ). Some cleaning
techniques used are acid washing, alkaline washing,
gas injection, high-pressure water jetting, and air
bumping.
When placing an empty aeration basin into service,
all recommended operational steps for start-up and
shutdown should be followed. If a basin is put into
service during cold weather, care must be exercised
to prevent any damage from buoyant forces exerted
by ice trying to float. Aeration basins must not be
drained during freezing weather unless absolutely
necessary because ice and frost can cause serious
damage. In the event that an aeration basin should
stand idle for more than 2 weeks, it should be
drained and cleaned thoroughly.
COSTS
The aeration system consumes approximately 50 to
65% of the net power .demand for a typical
activated sludge wastewater treatment plant.
Therefore, the designer is responsible for selecting
a system that will meet the mixing and oxygen
requirements for the process at the lowest cost
possible. Once the requirements for aeration are
determined, comparative costs for different types of
aeration systems can be estimated and the final
equipment configuration selected to best match the
requirements of the job.
Construction cost items mainly consist of aeration
basins, air piping and headers as appropriate,
aeration devices and their supports, air cleaning
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equipment, blowers, and buildings to house these
items. O&M costs are primarily for power,
cleaning, and replacement.
Operational costs are determined in part by the OTE
of the fine bubble aeration system being used, as
well as the characteristics of the influent
wastewater. Aerator cleaning costs depend on the
aerator type; how easily the aerators can be
removed, cleaned, or replaced; and the plant's O&M
procedures.
REFERENCES
1. Egan-Benck, K.; G. McCarty; andW.
Winkler. 1993. "Choosing Diffusers."
Water Environment and Technology, vol. 5.
no. 2. pp. 54-59.
2. McCarthy, J. 1982. "Technology
Assessment of Fine Bubble Aerators." U.S.
Environmental Protection Agency (EPA)
Wastewater Research Division. Municipal
Environmental Research Laboratory.
Cincinnati, Ohio.
3. U.S. EPA, 1995. Technological Assessment
of Fine Bubble Aerators.
EPA-600/2-82-003.
4. Metcalf & Eddy, Inc. 1991. Wastewater
Engineering: Treatment, Disposal, and
Reuse, 3d ed. The McGraw-Hill
Companies. New York, New York.
5. U.S. EPA, 1985. Summary Report: Fine
Pore (Fine Bubble) Aeration Systems. EPA
Water Engineering Research Laboratory.
Cincinnati, Ohio. EPA/625/8-85/010.
6. U. S. EPA, 1989. Design Manual: Fine,
Pore Aeration Systems. EPA Center for
Environmental Research Information.
Cincinnati, Ohio. EPA/625/1-89/023.
ADDITIONAL INFORMATION
Mike McGee
President
EP Aeration
2615 Meadow Street
San Luis Obispo, CA 93401*
Gordon F. Pearson
Vice President
International Ecological Systems & Services, IESS
P.O. Box 21240
B-l Oak Park Plaza
Hilton Head Isl, SC 29925
Leroy C. Reid, Jr., Ph.D
1273 Annapolis Drive
Anchorage AK 99508-4307
1 I ' ' '
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for use by the U.S. Environmental
Protection Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, D.C., 20460
IMTB
Excellence In compliance through optimal technical solutions
MUNICIPAL TECHNOLOGY B R A iT
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