SEPA
United States
Environmental Protection
Agency
Water Engineering Research
Laboratory
Cincinnati OH 45268
Technology Transfer
EPA/625/8-85/010
Summary Report
Fine Pore
(Fine Bubble)
Aeration Systems
,3 * ">
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Technology Transfer EPA/625/8-85/010
Summary Report
Fine Pore (Fine Bubble)
Aeration Systems
October 1985
This report was developed by the
Water Engineering Research Laboratory
Cincinnati OH 45268
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Contents Page
Introduction and Overview 1
Description of Devices 2
Types of Fine Pore Diffusion 2
Ceramic Materials 2
Plastic Materials 3
Flexible Sheaths 3
Types of Fine Pore Diffusers 3
Plate Diffusers 3
Tube Diffusers 3
Dome Diffusers 5
Disc Diffusers 6
Diffuser Layout 7
Plate Diffusers 7
Tube Diffusers 7
Disc and Dome Diffusers 8
Characteristics of Fine Pore Media 8
Permeability 8
Uniformity 9
Dynamic Wet Pressure 9
Strength 9
Other Characteristics 10
Performance Characteristics 11
Background 11
Clean Water Peformance 11
Process Water Performance 14
Test Methods 15
Factors Affecting Performance 16
Process Water Data Base 16
Operation and Maintenance Considerations 27
Impact of System Design and Installation on O&M 27
Process Design 27
Aeration Basin Design 28
Air Supply System Design 28
Materials Selection and Specification 28
System Installation 29
Impact of Fouling Phenomena on O&M 29
Background 29
Air Side 29
Liquor Side 29
Ex-Situ 29
In-Situ 29
Fouling Processes 30
Fouling Observations 30
Process Monitoring 32
Preventive Maintenance .- 34
Diffuser Cleaning 34
Cost Tradeoff Analysis 35
Retrofit Considerations 37
System Design Factors 37
Wastewater Characteristics 37
Existing Facilities 37
Aeration Tanks 37
Air Supply and Distribution 38
Air Filtration 38
Diffuser Selection 38
Economic Analyses 38
General 38
Determining System Cost 40
Determining Annual Savings 40
Determining Additional O&M Costs 40
Determining Economics Viability 40
Example Evaluation 40
Ongoing Studies 43
References 46
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Introduction and
Overview
Aerobic biological processes continue
to be one of the more popular
methods employed to treat municipal
and industrial wastewaters. The
supply of oxygen to the biomass in
activated sludge systems and aerated
lagoons represents the single largest
energy consumer in wastewater
treatment facilities. Recent studies
indicate that from 50 to 90 percent of
the net power demand for a
treatment plant lies within the
aeration system.1 A general survey of
data made available in 1982 on
municipal and industrial wastewater
treatment installations suggests that
on the North American Continent
there are approximately 1.3 million
kw (1.75 million hp) of aeration
equipment in place at an installed
value of 0.6 to 0.8 billion dollars.2
Operating costs for these systems
may be expected to be about 0.6
billion dollars/yr.
Originally, oxygen was diffused into
wastewater through perforated pipes
located at the bottom of the aeration
tank. The development of the porous
plate was considered an important
advance in the diffused aeration
process because of the higher oxygen
transfer efficiency offered by this fine
pore device.3 Porous diffuser plates
were used as early as 1916 and
became the most popular method of
aeration in the 1930s and 1940s.4'5 It
was clear shortly after the
development of porous diffusers that
clogging could be a problem. Early
work on clogging led to the use of
coarser media6 and eventually to
large orifice devices.7 The use of
mechanical aeration devices was
another answer to the clogging
problem, although these devices
were normally applied to small
treatment facilities and industrial
waste applications.7
The energy crisis of the early 1970s
rekindled interest and awareness
within the sanitary engineering
community relative to the efficiency
of oxygen transfer systems. As a
result, the fine pore diffusion of air
has gained renewed popularity as a
very competitive system. Yet,
considerable concern has been
registered regarding the performance
and maintenance of fine pore
diffusion systems owing to their
susceptibility to clogging. Diffuser
clogging, if severe, may lead to
deterioration of aeration efficiency
and corresponding escalation of
power costs. Furthermore,
troublesome maintenance of
diffusers may consume considerable
amounts of operator time and plant
operating budget.
The purpose of this summary report
is to provide current information on
the performance, operation and
maintenance, and retrofitting of fine
pore aeration systems in municipal
wastewater treatment service. It is
not intended to be a design manual,
but rather to provide a general
conceptual framework for practicing
engineers to assist them in the
selection, specification, design, and
control of fine pore aeration systems.
"Fine bubble" diffused aeration is
elusive and difficult to define. The
term "fine pore" is used hereafter
instead of "fine bubble" to more
nearly reflect the porous
characteristics of the diffusers
themselves. Typically, fine pore
diffusers will produce a headless due
to surface tension in clean water of
greater than about 5 cm (2 in.) water
gauge. For the purposes of this
report, fine pore diffusers are defined
as including the following diffusion
devices:
Porous ceramic plates, discs,
domes, and tubes
Porous plastic plates, discs, and
tubes
Flexible sheath tubes
This summary report is divided into
five major sections: Description of
Devices; Performance
Characteristics; Operation and
Maintenance Considerations; Retrofit
Considerations; and Ongoing
Activities. It will be followed in
approximately 2 years with a
comprehensive design information
manual on fine pore diffused aeration
based on studies being conducted
here and abroad to fill gaps in the
current state-of-the-art.
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Description of Devices
Since the introduction of the
activated sludge process in the early
1900s, a number of different types of
diffused aeration devices have been
designed and developed to introduce
air into liquids. These have ranged
from individual orifices (holes or
slots) drilled in a section of pipe to
more elaborate devices made up of
small diameter particles fused
together. Today, although the same
types of generic devices exist,
diffusers are commonly classified as
either fine or coarse bubble.
The demarcation between fine and
coarse bubbles is not well
differentiated. Coarse bubble
diffusers will typically produce a
bubble diameter of 10 to 20 mm in
clean water. So-called fine bubble
(fine pore) diffusers, when new, will
produce bubbles with a diameter of 2
to 4 mm in clean water.8'10 Some
references also describe a medium
bubble diffuser,11 which can be
assumed to produce a bubble
diameter somewhere in between.
Types of Fine Pore
Diffusion Media
A number of porous materials
capable of serving as effective
aeration devices are marketed today.
In general, a wide range of products
that were initially developed to filter
air have also been found to act as
satisfactory air diffusion devices.
Because of cost and specific
characteristics, only a few of these
materials are actually being used in
the wastewater treatment field.
Ceramic Materials
The oldest and still the most common
type of porous material on the market
is the ceramic type. It consists of
rounded or irregular-shaped mineral
particles bonded together to produce
a network of interconnecting
passageways through which
compressed air flows. As the air
emerges from the surface pores, pore
size, surface tension, and air flow
rate interact to produce a
characteristic bubble size.12
Ceramic diffusers manufactured
from glass- or resin-bonded silica or
alumina are available. The two most
popular materials are glass-fused
silica and glass-fused alumina.
The silica material is produced from
naturally occurring sand particles.
After screening to obtain the desired
uniform particle size, an amorphous
glass binder is added. The aggregate
and binder mix is then pressed in a
mold to produce the desired shape.
After pressing, the material is fired at
approximately 980°C (1800°F). At
this temperature, the binder material
encapsulates the sand particles.
When the mix is cooled, a glass bond
is formed by the binder material at
the contact points between the
individual particles.
The alumina material is made from
aluminum oxide. The actual grains
are produced by melting bauxite ore
at approximately 2050°C (3720°F) to
form large pigs. The pigs are then
crushed and the resulting particles
screened to select the desired size.
For the alumina, an elaborate binder
resembling porcelain is used. After
pressing, the grit and binder mix is
fired at 1425°C (2600°F), which
upon cooling creates the glass bond
at the contact points. The final
product is typically 80 to 90 percent
aluminum oxide.
A few minor differences exist
between the two types of material.
Because of the crushing process, the
alumina grains are more angular and
jagged in shape than the silica
particles. Silica is a mined material
with a limited particle size range, and
the pore size that can be produced
with it is limited by naturally
occurring grain sizes.
In general, for wastewater treatment
applications, the performance of both
silica and alumina is expected to be
approximately the same. It has been
claimed that the silica material,
because of its shape, may be more
resistant to fouling and more easily
cleaned.10 This claim has not been
well demonstrated based on
controlled experiments.
Today, the majority of ceramic
diffusers being marketed are
manufactured from aluminum oxide.
The alumina material is harder and
possibly somewhat stronger than
silica, but this probably is not the sole
reason for its widespread use. Just
as important may be the fact that
essentially all ceramic media are
manufactured by large companies
whose major product line is abrasive
materials (aluminum oxide grinding
wheels). Because the air diffusion
market is relatively small in
comparison, it is difficult to justify the
use of different raw materials for the
manufacture of air diffusion
equipment.
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Plastic Materials
A more recent development in the
fine pore diffuser field is the use of
porous plastic materials. As with the
ceramics, a material is created
consisting of a number of
interconnecting channels or pores
through which compressed air can
pass. Advantages of the plastic
material over aluminum oxide are its
lighter weight (which makes it
especially well suited to lift-out
applications), lower cost, better
durability, and, depending on the
actual material, greater resistance to
breakage. Disadvantages include its
reduced strength and susceptibility to
creep.
Porous plastics are made from a
number of thermoplastic polymers
including polyethylene,
polypropylene, polyvinylidene
fluoride, ethylene-vinyl acetate,
styrene-acrylonitrile, and
polytetrafluoroethylene.13 Probably
the two most common types of
plastic materials in use are high
density polyethylene (PE) and
styrene-acrylonitrile (SAN). PE is
used because it is relatively easy to
process when compared with other
thermoplastics. Shrinkage is low, a
uniform quality product can be
obtained, and small pore sizes can be
produced. The actual material is
manufactured by a proprietary
process, and, thus, little information
is available on it. One manufacturer14
did indicate that the PE media
contains no binders or additives, is
non-polar, and is made from a
straight homo-polymer (not a blend).
A European manufacturer produces a
double-layer PE material.15 It consists
of a grainy, open-pore structure
covered with a thin film layer. The
manufacturer claims that the double
layer results in a filtering effect that
decreases the required maintenance.
Presumably the lower maintenance
would result from a reduction in air-
side fouling of the diffuser. If the air
supply is properly filtered, air-side
fouling will likely not be a problem so
the savings in maintenance costs
would be minimal. The thin outer
layer is, however, potentially
beneficial in helping to produce a
small diameter bubble uniformly over
the diffuser surface. The
corresponding increased tendency for
external fouling to occur, if any, is
unknown.
The major advantages of the PE
media compared with the other
plastic alternatives are that it is
lighter in weight (approximately 560
kg/m3(35 Ib/cu ft)), essentially inert,
and will not break, even under
freezing conditions. In addition to the
disadvantages previously mentioned,
the PE material is also a relatively
new product (at least as an air
diffusion device) and all of the long-
term effects may not be known.
The second most common type of
thermoplastic material is SAN
copolymer. The raw material is a
mixture of four different molecules.
Physically, the media is made up of
very small resin spheres that are
fused together under pressure. The
SAN media has a density only slightly
greater than PE. The presence of the
styrene, however, makes the material
brittle, and the media can break if
dropped, even at room temperature.
A major advantage of the SAN
material is that is has been in use for
approximately 15 years without
known deleterious effects.
Flexible Sheaths
Flexible diffusers have been in use
for approximately 40 years. They
initially were referred to as "sock"
diffusers and were made from
materials such as plastic, synthetic
fabric cord, or woven cloth. Because
of the woven type sheaths, a metallic
or plastic core material was
necessary for structural support.
Although sock diffusers were capable
of achieving relatively high oxygen
transfer rates, fouling problems were
often severe. Today, there is
essentially no market for the early
sock design.
Within the last several years, a new
type of flexible diffuser has been
introduced. It consists of a thin
flexible sheath made from soft plastic
or rubber. Air passages are created
by punching minute slots in the
sheath material. When the air is
turned on, the sheath expands. Each
slot acts as a variable aperture
opening; the higher the air flow rate
the greater the opening. The sheath
material is supported by a tubular
frame.
This new generation of flexible
diffusers has been in operation at a
number of facilities for the last
several years. The new sheath
material has reduced the severe
fouling problems associated with the
earlier woven fabric design. The
manufacturer estimates sheath life to
be about 5 years.16
Types of Fine Pore Diffusers
Today, there are four general shapes
of fine pore diffusers on the market:
plates, tubes, domes, and discs. Each
will be discussed in detail in the
subsections that follow.
Plate Diffusers
The original fine pore diffuser design
was a flat rectangular plate. Plates
are typically 30 cm (12 in.) square
and 2.5 to 3.8 cm (1 to 1.5 in.) thick.
They are manufactured from either
glass-bonded silica or glass-bonded
aluminum oxide. The plates are
installed in the tank by grouting them
into recesses in the floor, cementing
them into prefabricated holders, or
clamping them into metal holders. Of
the three, the metal holders are the
least attractive because corrosion of
the holders tends to foul the
underside of the diffusers. A
chamber underneath the plates acts
as an air plenum. The number of
plates fixed over a common plenum
is not standard and can vary from
only a few to 500 or more. In current
U.S. designs, individual control
orifices are not provided on each
plate.
Fine pore plates were used almost
exclusively as the method of air
diffusion in the early activated sludge
plants through the 1920s. Today,
other than in some of the original
plants, fine pore plates are not often
specified and installed. Some
possible explanations for their
decline in popularity include 1)
problems obtaining uniform air
distribution with a number of plates
attached to the same plenum, 2) the
inconvenience of removing plates
when they are grouted in place, and
3) the difficulty in adding diffusers to
meet future increases in plant
loading.
Tube Diffusers
Like the plates, fine pore tubes have
been used in wastewater treatment
for a number of years. The early
tubes, Saran wound or made from
aluminum oxide, have been followed
by the introduction of SAN
copolymer, porous PE, and, most
recently, the new generation of
flexible media.
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All the tube diffusers on the market
are of the same general shape.
Typically, the media portion is 50 to
60 cm (20 to 24 in.) long and has an
O.D. of 6.4 to 7.6 cm (2.5 to 3.0 in.).
The thickness of the media is
variable. Flexible sheaths are very
thin, commonly in the range of 0.5 to
1.3 mm (0.02 to 0.05 in.). The PE
media is usually supplied with a
thickness of 6.4 mm (V* in.), the SAN
media at approximately 15.2 mm (0.6
in.), and fused ceramic material in
the range of 9.5 to 12.7 mm (% to 1/2
in.).
The holder designs for the ceramic
and porous plastic media are very
similar. Most consist of two end caps
held together by a connecting rod
through the center. The rod is
threaded into the feed end of the
holder, the media and outer end cap
installed, and a hex nut placed on the
threaded rod to secure the assembly.
In another version, the feed end cap
and inner support are one piece with
the assembly held together by a bolt
installed through the outer end cap
and threaded into the support frame.
For both designs, gaskets are placed
between the media and the end caps
to provide an air-tight seal. In some
cases, a gasket or O-ring is also used
in conjunction with the retaining bolt
or hex nut. Typical porous plastic
tube diffuser assemblies are shown
in Figure 1.
For the flexible sheath diffusers, the
end caps and support frame are one
piece. The sheath is installed over
the support frame and clamped on
both ends. In this design, no gaskets
are required. A typical flexible sheath
diffuser assembly is illustrated in
Figure 2.
To prevent corrosion, all components
of the various tube assemblies are
either stainless steel or durable
plastic. The gaskets are usually of a
soft rubber material.
Tube diffusers are designed to
operate in the air flow range of 1 to 5
L/s (2 to 10 scfm). Because of their
inherent shape, it is sometimes
difficult to obtain air discharge
around the entire circumference of
the tube. The air distribution pattern
will vary with different types of
diffusers. In general, however, the
extent of inoperative area will be a
function of the air flow rate and the
headloss across the media. Because
dead areas can provide sites for slime
growth and other foulant
End Cap
%-in. Threaded Nipple
Attachment Bolt
Polyethylene
Media
One Piece Endcap
and Center Support
ENVIREX
(used with permission of Rexnord, Inc.)
Control Orifice
Acrylonitrile Styrene j
Copolymer /
Gasket and Washer
End Cap
Nut
7
34-'m. Threaded Nipple
Threaded Connecting Rod
FMC
Figure 1.
Typical Porous Plastic Tube Diffusers
Diffuser Header Pipe
Stainless Steel Clamp
Flexible Sheath
24.5 in. Including Threaded Nipple
WYSS
(used with permission of Parkson Corp.)
Figure 2.
Typical Flexible Sheath Tube Diffuser
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development, it would be beneficial
prior to selecting a particular tube
design to observe its performance on
a laboratory- or pilot-scale basis.
Most tube assemblies are fitted with
a control orifice inserted in the inlet
nipple to aid in air distribution.
Typically, the orifice is approximately
13 mm (0.5 in.) in diameter, although
different sizes can be used for
various design flow rates. Also, some
assemblies include check valves to
prevent the backflow of liquid into
the air piping.
Dome Diffusers
The fine pore dome diffuser was
developed in Europe in the 1950s
and introduced in the U.S. market in
the early 1970s.17 Long considered
the standard in England and some
parts of Europe, domes are now
installed in a number of U.S. plants.
The dome diffuser is essentially a
circular disc with a downward-turned
edge. Currently, these diffusers are
18 cm (7 in.) in diameter and 3.8 cm
(1.5 in.) high. The media is
approximately 1 5 mm (0.6 in.) thick
on the edges and 19 mm (% in.) on
the top or flat surface. Domes
presently are being made only of
aluminum oxide.
The dome diffuser is mounted on
either a PVC or mild steel saddle-type
base plate. The PVC saddles are
solvent welded to the air distribution
piping at the factory. This minimizes
adhesive problems that could occur
in the center of the dome. The bolt
can be made from a number of
materials, including brass and
stainless steel. Care must be taken
when installing the dome to prevent
over-tightening of the center bolt.
Applying too much force can lead to
immediate diffuser breakage and/or
future air leakage due to bolt
stretching if a nonmetallic bolt is
used. A soft rubber gasket (neoprene)
is placed between the diffuser and
the base plate. A washer and gasket
are also used between the bolt head
and the top of the diffuser.
Schematics of two dome diffusers
are shown in Figure 3.
The slope of the headloss vs. air flow
rate curve for a ceramic diffuser is
very flat. It has been reported that a
variation from the average of .±10
percent in the specific permeability of
a diffuser can result in a 200-percent
change in air flow rate for the same
headloss under wet operating
Pipe Strap
Adjustable Pipe Support
, Retainer Bolt and Washer
Dome Diffuser
Base Plate
Control Orifice
4-in. PVC Pipe
Anchor Bolt
GRAY ENGINEERING
Control Orifice-
Pipe Strap
Retainer Bolt and Washer
Base Plate
Gasket
Adjustable Pipe Support
NORTON
Figure 3.
Typical Ceramic Dome Diffusers
conditions.18 To better distribute the
air throughout the system, control
orifices are placed in each diffuser
assembly to create additional
headloss and balance the air flow.
The fastening bolt is hollowed out
and a small hole drilled in the side, or
the orifice is drilled in the base of the
saddle. The size of the orifice is
typically 5 mm (0.2 in.).
Dome diffusers are usually designed
to operate at an air flow rate of 0.5
L/s (1 scfm) with a range of 0.25 to 1
L/s (0.5 to 2 scfm). In designing a
system, careful consideration should
be given to the desired air flow
range. Testing has shown that
oxygen transfer efficiency (OTE) is
dependent on air flow rate per
diffuser, increasing as the air flow
rate decreases (refer to the next
major section, Performance
Characterisitcs). This performance
characteristic may tempt engineers
to design dome systems to operate at
air flows of 0.2 to 0.25 L/s (0.4 to 0.5
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scfm)/diffuser. Although favorable in
terms of oxygen transfer, this
practice can lead to operational
problems. At low air flow rates,
uniform air distribution across the
entire diffuser surface may be
difficult to obtain. Also, at 0.25 L/s
(0.5 scfm), the headloss across the
control orifice will be less than 25
mm (1 in.) water gauge. At this low
rate, a different size orifice will be
needed to balance air flow
throughout the system. In any case, if
either the entire surface or portions
of individual diffusers are not
discharging air, foulant deposition
can begin, which could then lead to
premature fouling of the entire
system.
The upper limit for air flow rate for a
dome diffuser is usually considered
to be 1 L/s (2 scfm). Operation above
this level is possible, but is not very
economical (refer to the next major
section. Performance
Characteristics). Increasing the air
flow rate above the recommended
upper limit results in a continuing
decrease in OTE and may require a
larger control orifice.
Disc Diffusers
Disc diffusers are a relatively recent
development. Discs are flat, or
relatively so, and are differentiated
from dome diffusers in that they do
not include a downward-turned
peripheral edge. While the dome
design is relatively standard,
currently available disc diffusers
differ in size, shape, method of
attachment, and type of material.
Schematics of two disc diffusers are
presented in Figure 4.
Disc diffusers are available in
diameters that range from
approximately 18 to 24 cm (7 to 9.5
in.) and thicknesses of 13 to 19 mm
(Vz to Vt in.). With the exception of
one design, all discs consist of two
flat parallel surfaces. For the one
exception, a raised ring slopes
slightly downward toward both the
outer edge and center of the disc. Not
only is the center not quite as thick
as the remainder of the disc, but it
also has a lower permeability. The
nonuniform profile is claimed to aid
in producing uniform air flow across
the entire disc surface.19 Two of the
discs also include a step on the outer
edge that is impervious to air flow.
This is done to reduce the area of the
Aluminum Oxide Disc
O-Rina \ Contoured Surface
Compressed Edge
Control Orifice
Threaded
Retainer Ring
Base Plate Solvent
Welded to Pipe
4-in. PVC Pipe
SANITAIRE
Polyethylene Disc
Gasket
Control Orifice and
Check Valve
Threaded
Retainer Ring
Base Plate
Mechanical Wedge Section
for Attaching Base
NOKIA
Figure 4.
Typical Disc Diffusers
vertical edge20 and is also of benefit
in attaching the media to the holder.
Although the majority of disc
diffusers are made from aluminum
oxide, a porous PE disc is also
available.
Like the dome diffusers, the disc is
mounted on a plastic (usually PVC)
saddle-type base plate. Two basic
methods are used to secure the
media to the holder: a center bolt or a
peripheral clamping ring. The center
bolt method is similar to that used
with the domes. A soft flat rubber
gasket is placed between the diffuser
and base plate. The bolt assembly
itself includes a washer and a gasket.
The more common method of
attaching the disc to the holder is to
use a screw-on retainer ring. With
the threaded collar, a number of
different gasket arrangements are
used. They include a flatgasket
placed below the disc, a U-shaped
gasket that covers a small portion of
the top and bottom and the entire
edge of the disc, and an 0-ring
gasket placed between the top of the
disc and the retainer ring. The base
plate typically includes small raised
ribs to aid in obtaining an air tight
seal between the gasket and the base
plate.
In general, the retainer ring method
of attaching the diffuser to the holder
has two potential advantages over a
center bolt. It has been reported that
as diffusers become fouled, excessive
amounts of air are discharged from
the edges and the area around the
center bolt washer.17 Although not
specifically documented under
controlled conditions, this
nonuniform air flow could reduce the
OTE of the system. The retainer ring
will tend to minimize these problems.
The second advantage is that
breakage of diffusers from over-
tightening the bolt or air leakage
problems from stretching a
nonmetallic bolt can be eliminated.
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There are two methods of attaching
disc diffusers to the air piping. The
first is to solvent weld the base plate
to the PVC header prior to shipment
to the job site. To avoid future
additional costs associated with
replacing sections of pipe, the
original design should include all the
base plates that may be needed to
meet future design requirements for
the system. During the early life of
the plant, not all the diffusers are
installed and plugs are simply
inserted in the unused base plates.
The second disc diffuser attachment
method uses mechanical means of
attachment. The mechanical
attachment can be either a bayonet-
type holder that is forced into a
saddle on the pipe or a wedge section
that is placed around the pipe and
clamps the holder to the pipe. With
the exception of one manufacturer
that employs the wedge clamp
method of attachment and ships
units preassembled, the pipe arrives
at the job site with only the holes
drilled. The latter technique makes
shipping the pipe somewhat easier
(less bulky) and can reduce damage
that may occur during shipment or
installation. With these types of
designs, holes for additional diffusers
can be prednlled and plugged or
drilled at a later date.
Disc diffuser assemblies also include
individual control orifices in each
assembly. Designs employing the bolt
method of attachment usually use a
hollow bolt with an orifice drilled in
its side. The other designs use either
an orifice drilled in the bottom of the
diffuser holder or a threaded inlet in
the base where a small plug
containing the desired orifice can be
inserted. The diameter of the orifice
is similar to that used with the dome
diffusers.
Disc diffusers have a design air flow
range of 0.25 to 1.5 L/s (0.5 to 3
scfm)/diffuser. The most economical
operating range will, however, be
somewhat dependent on diffuser
size. The 18-cm (7-in.) diameter discs
are usually operated in the range of
0.25 to 1 L/s (0.5 to 2 scfm), similar
to the dome diffusers. For the larger
discs, with diameters of 22 to 24 cm
(8.5 to 9.5 in.), typical lower and
upper limits are 0.3 to 0.45 L/s (0.6
to 0.9 scfm) and 1.25 to 1.5 L/s (2.5
to 3 scfm), respectively. Prolonged
operation at flow rates less than 0.3
L/s (0.6 scfm) is not desirable with a
large disc because insufficient air is
available to ensure good distribution
across the entire surface of the
media. In those applications where
operation above 1 L/s (2 scfm) is
desirable, the control orifice should
be sized accordingly so that the
headloss produced does not
adversely affect the economics of the
system.
Clean water testing has shown that
OTE is related to diffuser size.19'21'22
A fewer number of large-diameter
discs than small diameter-discs are
required to achieve equivalent
oxygen transfer. If the same air flow
rate is applied to equal numbers of
large- and small-diameter discs, the
rsulting lower flux rate on the larger
units will yield a slightly higher OTE.
There is, however, no generally
accepted ratio for comparing various
size diffusers. One 23-cm (9-in.)
diameter disc has been found to be
approximately equivalent to 1.1 to
1.4 18-cm (7-in.) diameter discs
when comparing media of a given
pore size. The actual ratio is related
to air flow rate and diffuser
submergence.
Diffuser Layout
Plate Diffusers
Fine pore plates are most often
grouted into the basin floor.
Downcomer pipes deliver the air to
open concrete channels below the
plates. The channels act as
distribution manifolds.
Plate diffusers can be installed in
either a total floor coverage or spiral
roll pattern. Total floor arrangements
may include closely spaced rows
running either the width (transverse)
or length (longitudinal) of the basin or
incorporated into a ridge and furrow
design. Spiral roll arrangements
include rows of plates typically
located along one or both walls of
long narrow tanks. The total floor
layout will produce a higher OTE,
whereas the spiral roll pattern will
produce more effective bulk mixing of
mixed liquor.
Tube Diffusers
Most tube diffuser assemblies
include a 19-mm (%-in) threaded
nipple (stainless steel or plastic) for
attachment to the air piping system.
This design makes the tubes
especially well suited for retrofit
and/or upgrade applications since
many coarse bubble diffuser systems
use the identical method of
attachment.
The air headers to which the tubes
are mounted are usually fabricated
from PVC, stainless steel, or
fiberglass reinforced plastic. Carbon
steel is sometimes used but is less
desirable because corrosion inside
the pipe can lead to fouling of the
media. In most cases, the wall
thickness of the pipe is not sufficient
to structurally support the diffuser.
Thus, threaded adapters or saddles
are either glued, welded, or
mechanically attached to the pipe at
the points where the tubes are to be
connected. The actual diameter of air
headers will vary depending on the
number of diffusers to be installed
and the design air flow rate.
The depth of tube submergence in
the basin will vary. In new
installations, the tubes are usually
placed as close to the floor as
possible, typically within 30 cm (1 ft).
In retrofit applications, the discharge
pressure of the existing blowers will
control the submergence. The tubes
will either be installed at the same
elevation as the original system or
possibly at a somewhat greater
distance off the floor to compensate
for any increase in headloss through
the fine pore media as opposed to the
coarse bubble device it is replacing.
The air headers may be secured to
the basin floor with adjustable
height, stainless steel pipe supports.
Tube diffusers are most often
installed along one or both long sides
of the aeration basin (single or dual
spiral roll pattern, respectively). In
some cases, the headers are
mounted on mechanical lifts. Using
this concept, the air headers and
diffusers can be removed for
inspection and cleaning without
dewatering the basin. On the header
itself, the tubes can be installed
along either one side (narrow band)
or both sides (wide band) of the pipe.
Tubes can also be installed in either
a cross roll or total floor coverage
pattern. In the cross roll design, the
headers are placed across the tank
width and the spacing between
diffusers, 0.3 to 0.9 m (1 to 3 ft), is
small in comparison to the spacing
between headers, 3 to 9 m (10 to 30
ft). In the total floor coverage pattern,
the distance between headers and
the spacing between diffusers on the
-------
headers approach the same value. In
general, total floor coverage will
provide the highest OTE. The spiral
roll configurations will provide better
bulk mixing throughout the tank than
either total floor coverage or cross
roll. One potential disadvantage of
the cross roll and total floor coverage
designs is that the location and
amount of piping required usually
makes the use of mechanical liftouts
impractical.
Disc and Dome Diffusers
Although their shape and operating
characteristics may differ, the typical
air piping and diffuser layout is
identical for both disc and dome
diffuser systems. The air distribution
manifold should preferably be made
of PVC, the compounds of which are
described in ASTM D-1784 or D-
3915, cell classifications 12454B and
124524, respectively (the latter is a
stress-rated compound and hence a
better choice). It is also
recommended that the PVC be UV
stabilized with 2-percent minimum
Ti02, or equivalent. The
specifications, dimensions, and
properties of the pipe itself conform
to either ASTM D-2241 or D-3034.
The piping network is usually a
nominal 10 cm (4 in.) in diameter,
with the actual O.D. ranging from
10.7 to 11.4 cm (4.2 to 4.5 in.). The
wall thickness is also variable,
typically ranging from approximately
3.0 to 3.6 mm (0.12 to 0.14 in.).
Sections of pipe are connected with
gasketed, mechanical expansion
joints to allow for expansion and
contraction of the PVC over a
temperature range of approximately
37°C (100°F). Pipe supports, usually
made from PVC, are provided to
secure the system to the tank floor.
The support consists of a cradle or
saddle and a holddown strap. The
strap is either secured with a bolt or
snaps into place. The pipe supports
are adjustable so variations in the
tank floor elevation can be
compensated for. The pipe support is
attached to the basin floor with a
single stainless steel bolt and a
concrete anchor. The PVC strap and
pipe support have in the past
experienced some breakage
problems. To eliminate these
problems, or in cases where the
diffusers are to be mounted a
significant distance above the tank
floor, e.g., 0.6 m (2 ft), stainless steel
pipe supports can be used.
Discs and domes are generally
installed in a total floor coverage or
grid pattern. In some cases where
oxygen demand is low and mixing
may control the design (near the end
of long narrow tanks), the diffusers
can be placed in tightly spaced rows
along the side or middle of the basin
to create a single spiral roll or center
roll mixing pattern, respectively. The
diffusers are usually mounted as
close to the tank floor as possible,
within 23 cm (9 in.) of the highest
point of the floor being typical. As
mentioned in the discussion of tube
diffusers, the submergence in some
retrofit applications may be
controlled by the available blower
discharge pressure.
Characteristics of Fine Pore
Media
The following parameters have been
used to characterize fine pore
media:20'23'24
permeability
uniformity
dynamic wet pressure
strength
chemical stability
resistance to heat
density (weight)
These characteristics, discussed
below primarily in conjunction with
porous ceramic materials, are also
applicable in most cases to porous
plastic materials. The flexible sheath,
however, is a very different type of
material. Some of the above
characteristics are important in
designing a flexible sheath diffuser
system, while others, e.g., strength
and density, are irrelevant.
Permeability
Permeability is a measure of a porous
medium's frictional resistance to
flow. It is an empirical rating that
relates flux rate to pressure loss and
pore size and/or pore volume. The
permeability test procedure was
developed by the ceramic
manufacturing industry as a simple
method of characterizing diffuser
units. Permeability is usually defined
as the amount of air at standard
conditions that will pass through
0.09 m2 (1 sq ft) of dry porous media
under a differential pressure
equivalent to 5 cm (2 in.) water
gauge when tested at room
temperature. The flow value obtained
(scfm) under these conditions is
referred to as the permeability (perm)
rating.
Permeability measurement does not
provide a true basis for comparison of
media performance because the
same permeability rating could be
obtained from a diffuser with a few
relatively large pores or a multitude
of small pores.5 Also, two diffusers
with exactly the same pore structure
would have different ratings if of
different thickness.
Currently, permeability is included in
specifications for porous diffusers. As
best can be determined, however, the
ceramic industry has not
"standardized" this test procedure.
The early specifications were
developed for 30-cm x 30-cm (12-in.
x 12-in.) plates 2.5 cm (1 in.) and 3.8
cm (1.5 in.) thick. Today,
specifications are needed for
products of various shapes, densities,
and wall thicknesses, often of ill-
defined effective area. Attempts have
been made to apply the principles of
the test through a parameter known
as specific permeability.25 In its
determination, an applied air flow
rate is measured through a diffuser
mounted on a fixture similar to the
fixture used in service at a pressure
differential of 5 cm (2 in.) water
gauge. From this measurement and
the geometry of the diffusers,
estimates are then made as to what
the air flow (scfm) would have been
at 5 cm (2 in.) water gauge
differential had the dimensions of the
test diffuser been 30 cm x 30 cm x
2.5 cm (12 in. x 12 in. x 1 in.).
The specific permeability procedure
has served to improve the utility of
this test, but does not overcome the
following remaining deficiencies:
Clamping and sealing details are
not well enough defined to provide
acceptable precision.
Effective diffusion area cannot
always be easily defined.
Correction factors to account for
pressure, temperature, and
humidity of the air have not been
developed.
-------
Uniformity
Uniformity of individual diffusers and
the entire aeration system is of
extreme importance if high OTE is to
be attained. On an individual basis,
the diffuser must be capable of
delivering uniform air distribution
across the entire surface of its media.
If dead spots exist, chemical or
biological foulants may form and
eventually lead to premature fouling
of the diffuser. Also, if small areas of
extremely high air flux rate are
present, larger bubbles may form and
OTE will increase.
A porous diffuser specification
should include a requirement for
testing to assure that the media will
distribute air uniformly. The common
practice is to select random samples
from each batch during the
manufacturing run. The diffusers are
placed in water for a fixed period to
ensure that they are saturated, then
tested in a shallow basin at a
predetermined air flow rate. In most
cases, a visual observation is the
basis for the test. This type of
qualitative method is unacceptably
arbitrary. Two individuals are likely to
have very different definitions for
what constitutes uniform air flow. In
other cases, a high air flow from
around the diffuser periphery may
tend to mask the center of the
diffuser, which may be completely
dead. The diffusers tested for
uniformity are usually the same ones
used for permeability testing. A pass-
fail criteria is established for
acceptance or rejection of the batch.
A better approach to measuring
uniformity is to use a quantitative
technique. One such procedure
actually measures the rate of air
release from different areas of the
diffuser.28 With the diffuser
submerged in 5 to 20 cm (2 to 8 in.)
of water and at an air flow rate of
approximately 10 L/s/m2(2 scfm/sq
ft), the rate of air release is
determined by measuring the
displacement of water from an
inverted cylinder. Based on the air
volume, time, and area of the
collection cylinder, a flux rate is
determined, A comparison of the flux
rates from various points on the
diffuser surface will provide a true
indication of media uniformity.
Although procedures have been
presented, no guidelines have yet
been developed in regard to the
variations between points that could
be allowed before the diffuser would
be rejected as nonuniform.
Development of such guidelines
should be undertaken.
In addition to uniformity among
individual diffusers, air flow
characteristics for individual grids
must also be uniform. Nonuniformity
of air flow in individual grids is not
likely to be a problem if an adequate
piping distribution system and
suitably sized individual control
orifices are provided.
Uniformity tests are conducted on
only a relatively small number of
diffusers. Even if all the random
samples pass, it is still possible that
some nonrepresentative diffusers
may be installed in the tank. As a last
check, the diffusers can be operated
in the aeration tank and submerged
under only a few centimeters of
water. If a visual observation
indicates any abnormal diffusers,
they can be replaced prior to putting
the basin into service if spares have
been furnished.
Dynamic Wet Pressure
Dynamic wet pressure (DWP) is an
important consideration in evaluating
and selecting a porous media. DWP
is defined as the operating headless
across diffuser media submerged in
water at a specified air flow rate per
diffuser.26 As a general rule, the
smaller the bubble size, the higher
the DWP. While smaller bubbles may
increase OTE, the additional power
required to overcome the higher
headloss may negate any potential
savings.
The porous media currently in use
have a DWP of 5 to 36 cm (2 to 14
in.) of water when operated within
typical or specified air flow ranges.
The specific value depends on the air
flow rate, type of material, diffuser
thickness, and surface properties. For
ceramic and porous plastic materials,
the headloss vs. air flow rate curve is
linear over the typical operating
range and the slope is relatively flat.
DWP for these materials can vary
from 5 to 30 cm (2 to 12 in.) over the
same range. A fourfold increase in
air flow rate of 0.25 to 1 L/s (0.5 to 2
scfm) per unit for some diffusion
elements will result in only a 2.5- to
5-cm (1 - to 2-in.) increase in
headloss across the media itself. For
the flexible sheath material, the
small holes act as an orifice.
Consequently, the headloss vs. air
flow rate curve is steeper than for the
ceramic and porous plastic media.27
Over the typical air flow operating
range of 1 to 3 L/s (2 to 6 scfm) per
unit, the DWP for flexible diffusers
may increase from 13 to 36 cm (5 to
14 in.).
For the ceramic and plastic materials,
the majority of the DWP is associated
with the pressure required to form
bubbles against the force of surface
tension. Only a small fraction of the
DWP is required to overcome
frictional resistance.26 Thus, the
thickness of the material is only a
minor contributor to DWP.
DWP may be measured in the
laboratory or the field.26 It is
important that porous diffusers be
allowed to soak for several hours
(plastic materials may require much
longer) prior to testing to ensure that
they are completely saturated. Since
the actual headloss will be a function
of the degree of water saturation in
the diffusers, a slightly different
curve will be obtained if the air flow
is started at a low rate and is
increased or vice versa. Standard
practice is to purge the media at the
upper flow value for a predetermined
time interval (5 to 10 min), then
record subsequent headloss values
as the flow rate is decreased.
Because of the relationship between
standard and actual air flow rates
(scfm vs. acfm), the headloss in the
field will be a function of diffuser
submergence. If a field measurement
of DWP is made, different media
must be compared based on acfm.
Strength
Diffusion media must be strong
enough to withstand 1) the static
head of the water above the diffusers
(in cases where the air supply is shut
off), 2) the forces applied when
attaching media to diffuser holders,
and 3) stresses and shocks of
reasonable handling and shipping.
-------
Slightly different techniques have
been developed to evaluate the
strength of diffuser material. For
discs and domes, this usually
involves supporting the diffuser in a
fashion similar to the final assembly,
then applying a load to an area 2,5
cm (1 in.) in diameter area in the
center of the diffuser. Using this
method, developed primarily for the
dome diffuser, acceptable
compressive loads for the ceramic
material range from 270 to 455 kg
(600 to 1000 Ib). Diffusers that use a
peripheral clamping method do not
require the same strength as those
that employ the center bolt method of
attachment.
Other Characteristics
Other characteristics of porous media
are their chemical stability, heat
resistance, and density. All the
materials discussed (ceramic, plastic,
and flexible) are resistant to the
normal concentrations of chemicals
used or encountered in wastewater
treatment. This includes periodic
exposure to strong acid solutions
such as hydrochloric and formic acids
used in cleaning fouled diffusers. For
applications where unusual
concentrations of certain chemicals
are to be encountered, it is suggested
that the media manufacturer be
consulted and/or a testing program
undertaken.
The maximum allowable operating
temperatures for the various plastic
materials used in the manufacture of
diffusers have not been well defined.
In addition to the specific compounds
employed, other factors should be
considered: anticipated maximum
stress, environmental exposure,
allowable deformation (including
creep), and tolerance to changes in
characteristics while in service.
Further definitive work in this area is
needed. In general, however, the
maximum allowable operating
temperature for plastics will fall in
the range of 38 to 93°C (100 to
200°F).
On the other hand, ceramic materials
can safely tolerate temperatures up
to 815°C (1500°F). The maximum
allowable operating temperature for
ceramics, therefore, is not a concern
in wastewater treatment
applications.
Density is of importance mainly in
situations where the diffusers are to
be lifted out of the tank. The ceramic
material has a density that ranges
from approximately 1600 kg/m3(100
Ib/cu ft) for silica to 2325 kg/m3
(145 Ib/cu ft) for aluminum oxide.20
The porous plastic material in
contrast has a typical density of only
560 to 640 kg/m3 (35 to 40 Ib/cu
ft).13
10
-------
Performance
Characteristics
The purpose of this section is to
provide the reader with information
and reference sources regarding
clean water and process water
performance of the aeration devices
described in the preceding section. In
some cases, performance
characteristics of devices not
described in the preceding section
are included here for comparative
purposes.
Background
In the late 1 960s and early 1970s,
consulting engineers began
specifying that clean water
performance tests be conducted by
the aeration equipment suppliers as
a means of verifying aerator
performance. Various engineers
developed their own testing criteria.
In April 1978, a Workshop Toward
An Oxygen Transfer Standard28
cosponsored by the U.S.
Environmental Protection Agency
(EPA) and the American Society of
Civil Engineers (ASCE) was held in
an effort to obtain consensus
standards for the evaluation of
aeration devices in both clean and
process waters. The outcome of the
workshop was the formation of an
Oxygen Transfer Standards
Committee under ASCE.
Between 1978 and 1984, this
Committee developed and adopted an
ASCE Standard for the Measurement
of Oxygen Transfer in Clean Water29
and evaluated several process water
test methods.30 Progress with respect
to the development of standardized
test methods for the evaluation of
aeration devices in clean water has
been substantial. Due to the wide
variety of experience in clean water
testing and the desirability of
incorporating that experience into the
Standard, several years passed prior
to its publication in July 1984.
Clean Water Performance
The following discussion summarizes
clean water performance data on fine
pore diffusion devices. Some but not
all of the data were generated using
the current ASCE recommended
clean water standard.29 Thus, the
oxygen transfer results summarized
in this subsection reflect the
utilization of the current nonlinear
least squares method of analysis as
well as a prior procedure using the
linear least squares log deficit
analysis.29 The latter method
permitted data truncation. Both
methods produce comparable results
under ideal testing conditions. Every
effort has been made to screen the
data reported herein and to omit data
of questionable validity.
The results of clean water oxygen
transfer tests are reported in a
standardized form as either standard
oxygen transfer efficiency (SOTE),
standard oxygen transfer rate (SOTR),
or standard aeration efficiency (SAE)
as shown in Table 1. The standard
conditions for reporting clean water
tests are also delineated in Table 1.
All data reported in this section are
given as standard transfer values
unless otherwise noted.
Examination of Table 1 indicates that
one of the critical parameters
required for the calculation of oxygen
transfer rates is the equilibrium DO
saturation concentration, CS. For
submerged aeration applications, CS
is significantly greater than the
surface saturation value, CI,
tabulated in most standarad tables.30
It is, therefore, necessary to either
calculate30 CS or measure29 it during
clean water tests. The value of CS is
primarily dependent on diffuser
submergence, diffuser type, tank
geometry, and gas flow rate. One of
the more comprehensive evaluations
of CS in clean water tests was
reported by Yunt et al.31 Typical
results for a variety of diffuser types
at selected submergences are
presented in Figure 5.
The performance of diffusers under
clean water test conditions is
dependent on a number of factors in
addition to those standardized in the
calculations of SOTE, SOTR, and
SAE. Among the important factors
are:
diffuser type (material, shape, and
size),
diffuser placement and density
(area served per diffuser),
gas flow rate per diffuser, and
diffuser submergence and tank
geometry.
Typical SOTEs for fine pore diffused
air systems are presented in Table 2.
These data are reported for a diffuser
submergence of 4.6-m (15-ft). The
effect of diffuser type, placement,
and air flow rate per diffuser are
clearly delineated from this summary
11
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Table 1.
Standard Equations for Clean Water Oxygen Transfer Tests29
Standard Conditions:
DO = 0.0 mg/L a - 1.0
Temperature = 20°C /J = 1.0
Pressure = 1.00 atm
Standard Oxygen Transfer Rate (SOTR) - mass/time
SOTR = KtaaoC^V
Standard Oxygen Transfer Efficiency (SOTE)
SOTE = Mass Transferred x 100
Mass Supplies
= SOTR
Standard Aeration Efficiency (SAE) - mass/time, power
SAE =
SOTR
Power Input (specified as delivered, brake, wire, or total wire)
Ki.a = apparent volumetric mass transfer coefficient in ctean water, time"'
KLa2o = KLa @ 20°C, time"1
V = volume of water, length3
YR = mole fraction of oxygen delivered
A = density of oxygen at actual temperature and pressure, mass/length3
q, = volumetric air flow rate, lengthVtime
C°°2o ~ equilibrium DO saturation concentration attained at infinite time for given diffusion
device at 20°C and 1 atm, mass/length3
of eight different clean water studies.
In general, it can be observed that
ceramic domes and discs
demonstrate slightly higher clean
water transfer efficiences than
typical porous plastic tubes or flexible
sheath tubes in a grid placement.
Both tubes and discs/domes are
significantly superior to all coarse
bubble placements. Within a given
diffuser type, spreading the diffusers
more uniformly along the tank
bottom area (moving from single
spiral roll to dual spiral roll to grid)
tends to improve clean water
performance.39 The effects of tank
and diffuser geometry on diffuser
performance have been reported by
numerous investigators. One of the
early, notable studies by Bewtra and
Nicholas32 in a 1.2-m (4-ft) wide x
7.3-m (24-ft) long test tank using
coarse bubble spargers and fine pore
Saran tubes demonstrated similar
effects of geometry.
Figure 6, derived from the data
contained in the Table 2 references,
demonstrates the effect of air flow
rate per diffuser on SOTE. SOTEs for
domes and discs in a grid placement
decrease significantly with increased
air flow. A somewhat smaller effect
is evident for porous plastic media
and flexible tubes, while coarse
bubble patterns are relatively
unaffected by gas flow rate with
some indication of increasing SOTE
at the higher gas flows. Very similar
patterns were reported in 1964 by
Bewtra and Nicholas32 for coarse
bubble spargers and fine pore tubes.
The effects of water depth on oxygen
transfer performance for several
types of diffusers are illustrated in
Figures 7 and 8. Although these data
are for one specific test tank and air
flow rate,31 they are representative of
the typical effects of depth on
performance. In general, SOTE
values will increase with increasing
depth since mean oxygen partial
pressure is higher (thereby resulting
in a greater driving force) and
opportunity is present for longer
bubble residence time in the aeration
tank. The SAE, however, remains
relatively constant (or may decrease)
for fine pore diffusers as depth
increases since power requirements
to drive the same volume of air
through diffusers at the greater
depths will increase. In contrast, the
coarse bubble diffusers exhibit a
gradually increasing SAE with
increasing depth, while not reaching
the overall efficiencies demonstrated
by the fine pore systems.
The clean water SOTE performance
data in Table 3 are for PE plastic
tubes. The data are typical of fine
pore diffusers, exhibiting increasing
OTE with increasing diffuser density
(moving from single spiral roll to dual
spiral roll to grid). Popel35 observed
that increased diffuser density in
grids decreases upward flow
velocities and, therefore, increases
the retention time of bubbles. He
reported on one field test of a
countercurrent aeration system with
a rotating bridge. The aeration
channel had a width of 11 m (36 ft)
and a depth of 3.2 m (10.5 ft). SOTEs
of 5.6 to 6.9 percent/m (1.7 to 2.1
percent/ft) of submergence were
reported at air flow rates of 1.5 to 3
L/s (3 to 6 scfm)/diffuser.
Typical performance of a flexible
sheath diffuser37 is summarized in
Table 4. This diffuser also exhibits a
decreasing transfer efficiency with
increasing air flow rate. The effect of
diffuser placement is also evident.
The increase for quarter-point
placement vs. single spiral roll
placement in a rectangular basin is
greater than for the mid-width
placement. Bewtra and Nicholas32
found that a dual spiral roll
placement was more efficient than a
mid-width placement in a rectangular
tank.
The clean water SOTEs of disc/dome
grid systems are illustrated in Table
5. This type of system has produced
the highest transfer efficiencies
reported for fine pore devices. The
density of placement is greater than
in the tube grid systems, and the air
flow rates per diffuser are lower.
Huibregtse et al.21 reported a slightly
increased transfer efficiency with a
24-cm (9.4-in.) diameter disc vs. an
18-cm (7-in.) diameter dome. The
12
-------
12.0
11,5
11.0
O>
i. 10.5
10.0
9.5
Tank: 20 ft x 20 ft
Power: ~1 hp delivered/1,000 cu ft for tubes
and coarse bubble diffusers
~0.5 hp delivered/1,000 cu ft for ceramic domes
1 1
10 15
Diffuser Submergence (ft)
20
I
25
Figure 5.
Effect of Diffuser Submergence on C^20 for Three Diffuser Types
Table 2.
Clean Water Oxygen Transfer Efficiency Comparison for Selected Diffusers
Diffuser
Type & Placement
Ceramic Discs-Grid
Ceramic Domes-Grid
Porous Plastic Tubes
Air Flow Rate
(scfm/diffuser)
0.6-2.9
0.5-2.5
SOTE (%)
at 1 5-ft
Submergence
25-36
27-39
Reference
21
21,31,33,34
Grid 2.4-4.0
Dual spiral roll 3.0-9.7
Single spiral roll 2.0-12.0
Flexible Sheath Tubes
Grid 1-4
Quarter points 2-6
Single spiral roll 2-6
Coarse Bubble Diffusers
Dual spiral roll 3.3-9.9
Mid-width 4.2-45
Single spiral roll 10-35
28-32
18-28
13-25
22-29
19-24
15-19
12-13
10-13
9-12
35
21,31,36
21,36
37
37
37
31,38
31,38
31,38
increase was in the range of 5 to 15
percent, varying with depth. He
attributed the increase to a 70-
percent increase in effective surface
area with the discs. He also indicated
that the larger surface area limits the
degree of bubble coalescence. Houck
and Boon17 and Yunt and Hancuff27
have also reported a similar
relationship between dome/disc
diameter and oxygen transfer per
diffuser.
The data from the references in Table
5 are plotted in Figure 9 to show the
general trend between SOTE and
diffuser density and air flow rate per
diffuser. Increasing diffuser density
increases SOTE, and increased air
flow rates for a given diffuser density
decreases SOTE.
13
-------
o
CO
40
35
30
25
20
15
10
Ceramic Disc/Dome Grid
(References 21,31,33,34)
Higher SOTE values for one
diffuser type at any given flow
rate indicates increased diff user
density or dual placement.
Diffuser Submergence = 15 ft
Porous Plastic Tubes
(References 21,31,36)
Coarse Bubble Diffusers
(References 31,38)
6 9
Air Flow Rate per Diffuser (scfm)
12
15
Figure 6.
Effect of Air Flow Rate per Diffuser on SOTE for Four Diffuser Types
Process Water Performance
Development of a clean water
standard was the springboard from
which additional studies were
undertaken by the ASCE Oxygen
Transfer Standards Committee to
evaluate a number of test procedures
used for estimating oxygen transfer
under process conditions. Substantial
process oxygen transfer data have
been collected using these
procedures over the past few years.
The standard clean water transfer
rate (SOTR), as measured at 20°C
with a zero residual DO
concentration, may be related to
actual field conditions (OTRf)
according to the following
equations:40
OTR, = cSOTR
- C
where: a =
KLa,
KLa
C* (field)
C* (clean water)
T =
Q =
de
PS + de - P»
flT-20 _
KLaT
K ta 20
KL3f = apparent volumetric mass
transfer coefficient in
process water
C = equilibrium DO saturation
concentration corresponding
to a given partial pressure of
oxygen, temperature, and
volume
CsT = surface DO saturation at 1
atm total pressure, 100
percent relative humidity,
and temperature T
CS2o = surface DO saturation
concentration at 1 atm total
pressure, 100 percent
relative humidity, and a
temperature of 20°C
Pb = base pressure
Ps = standard pressure of 1 atm
Pv2o = vapor pressure of water at a
temperature of 20°C
de = effective saturation depth at
infinitive time
Before 1981, the methods used to
evaluate aerator performance under
process conditions were inconsistent
and coherent data on process water
performance were extremely limited.
Alpha is probably the most
controversial and reasearched
parameter used in translating clean
water oxygen transfer data to actual
field performance. Variables affecting
the value of alpha include aerator
type, nature of wastewater
contaminants, position within the
treatment scheme, process loading
rate, bulk liquid DO, water depth, and
air flow rate. Coherent data on alpha
values for various aeration devices
are limited. Alpha values of 0.2**^o
1 .SS^nave been published. Because
much of the reported alpha data was
obtained from bench-scale units
(which did not properly simulate
mixing and Kua levels, aerator type,
water depth, and/or the geometry
effects of their full-scale
counterparts), these data are of
questionable value. Reliable full-
scale test procedures for use under
process conditions, coupled with
clean water performance data are
required to overcome these
deficiencies. Several references on
this subject provide useful
information.40'43"45
14
-------
50
40
30
20
10
Tank: 20 ft x 20 ft
Power: ~1 hp delivered/1,000 cu ft for tubes
and coarse bubble diffusers
0.5 hp delivered/1,000 cu ft for
ceramic domes
I
10 15
Water Depth (ft)
20
25
Figure 7.
Effect of Water Depth on SOTE for Three Diffuser Types31
Test Methods
In 1981, the ASCE Oxygen Transfer
Standards Subcommittee undertook
a seven-site study to evaluate, in
parallel, four principal methods for
estimating oxygen transfer under
process conditions.46 These methods
include the steady state method, the
nonsteady state method, an off-gas
analysis procedure, and two inert gas
tracer techniques.
Those test methods requiring that the
rate of DO change be zero at any
given point in the test volume are
referred to as steady state methods;
those depending on a rate of DO
change with time are called
nonsteady state tests. In both cases,
however, it is necessary that the
wastewater influent flow rate and
characteristics, as well as the test
volume, oxygen uptake rate, and field
oxygen transfer coefficient, Ki_at, be
constant. In addition, basin DO
values must be in excess of 1 mg/L
for carbonaceous oxidation and 2
mg/L for nitrification to obtain valid
uptake data.
To overcome the problem of
maintaining steady load and KL3(
conditions and to ensure better
precision, wastewater flow may be
discontinued during a test. This
method of testing is referred to as the
batch or batch endogenous technique
as contrasted with continuous flow
methods. Batch testing, however,
suffers the critical disadvantage that
it does not realistically measure true
field transfer rates of alpha values
under normal loading conditions. For
this reason, it was not included as
one of the methods studied by the
Oxygen Transfer Standards
Committee.
In contrast to the steady state and
nonsteady methods, the off-gas and
inert gas tracer procedures do not
require steady process loads, do not
need positive basin DO, and do not
require the measurement of oxygen
uptake rate. Accordingly, these
procedures can be more effective in
measuring oxygen transfer in the
field under actual process conditions
than the steady state and nonsteady
state methods.
Of the four methods discussed, the
off-gas procedure is unique in that it
measures the fraction of oxygen
transferred from the gas stream
directly, whereas the other methods
(the liquid phase methods) determine
KL3f. For those methods relying on
Ki_a determinations, errors in applied
air flowrates proportionally affect
computations of OTE and oxygen
transfer rate. Since the off-gas
procedure measures OTE directly as
well as the rate of gas flow leaving
the liquid surface at each test
position, accurate plant air flow
measurement is not critical. Accurate
plant air flow measurement is
desirable, however, to validate that a
representative gas sampling was
obtained. Where off-gas OTE results
must be converted to standard
conditions, accurate measurements
of basin temperature and DO must be
made.
Selected factors affecting the
estimation of OTE are identified in
Table 6. The inert gas tracer methods
have the broadest applicability since
they may be used for both
mechanical and diffused air devices.
The steady state and nonsteady state
methods generally are not applicable
to plug flow reactors where alpha,
DO, backmixing, and applied air flow
rate vary throughout the basin. The
tracer techniques, though less
affected by the above factors, also
are not ideally suited for use in plug
flow regimes. The questionable
impact of variable gas flow rate and
KL3f throughout the basin, along with
the need for accurate knowledge of
local air flow rates throughout the
basin, can adversely affect the
accuracy of the tracer techniques.
The off-gas procedure, which is
capable of measuring localized
performance throughout the basin
with respect to OTE, air flow rate.
15
-------
10
8
CD
ra
i
o
i 6
a
CM *
0 4
.D
LLJ
2
o-
<
" Tank: 20 ft x 20 ft
Power: 1 hp delivered/1 ,000 cu ft for tubes
and coarse bubble diffusers
0,5 hp delivered/ 1,OOO cu ft for ceramic domes
-
Ceramic Domes - Grid Placement
-
Porous Plastic Tubes - Dual Spiral Roll Placement
"
^»TT- ,.
^^^^ff/^/ZZZZz^
Coarse Bubble Diffusers
1111
3 10 15 20 25
Water Depth (ft)
Figure 8.
Effect of Water Depth on SAE for Three Diffuser Types3'
affected by a myriad of factors, some
of the more important of which are
wastewater characteristics,
process type and flow regime,
loading conditions,
basin geometry,
diffuser placement and
performance characteristics,
changes in performance due to
fouling,
mixed liquor DO control and air
supply flexibility,
mechanical integrity of the
system,
operator expertise, and
quality of preventive maintenance.
Previous Water Pollution Control
Federation (WPCF) Manuals of
Practice (MOP) on Aeration,5'7 along
with the upcoming revised MOP No.
5 are good general references on the
above factors. To minimize life cycle
costs of an aeration system, all of
these factors must be considered
during design.
The areas of greatest concern in
process water oxygen transfer
performance are wastewater
characteristics, process type and flow
regime, and loading conditions. They
all have a significant effect on the
alpha profile of a system, DO control,
and changes in aerator performance
with time due to diffuser fouling.
These factors are discussed after the
following process water data base
presentation.
Table 3.
Clean Water Oxygen Transfer Efficiencies of Porous Plastic Tubes
Placement
Grid*
Dual Spiral Roll
Single Spiral Roll
Air Flow
(scfm/diffuser)
2.4-4.0
3.2-6.3
9.0-9.7
2.0-6.7
8.0-12.0
SOTE (%) at Water
1 0 ft 1 5 ft
28-32
11-16 17-24
10-14 15
12-15 15-20
10-15 10-17
Depth
20ft
22-32
21-26
22-25
22
Reference
35
21,31,36
21,31,36
21,31,36
21,31,36
Placement density = 7.7 sq ft/tube. Tank is 14.4 ft x 108.2 ft.
and driving force, is well suited for
evaluating the process water
performance of diffused aeration
systems in plug flow as well as
complete mix tanks. As with all test
methods, representative sampling of
the test basin is essential with this
procedure if an accurate appraisal of
system performance is to be
obtained.
Factors Affecting Performance
The performance of diffused aeration
systems under process conditions is
Process Water Data Base
As indicated earlier, a substantial
data base exists for the clean water
performance of the diffused aeration
systems considered in this report.
The process water oxygen transfer
data base is much more limited.
Since oxygen transfer performance
under field conditions is really the
ultimate goal, expansion of the latter
data base is needed. The following
summarizes and discusses available
data for the fine pore diffusion
systems previously described. A few
aeration systems not previously
described are also examined for
comparative purposes.
Process water oxygen transfer
performance data from 13
evaluations at various sites
employing a variety of aeration
16
-------
Table 4.
Clean Water Oxygen Transfer Efficiencies of Flexible Sheath Tubes37
Placement
Floor Cover (Grid)
Quarter Points
Mid-Width
Single Spiral Roll
Air Flow
(scfm/diffuser)
1-4
2-6
2-6
2-6
SOTE (%) at Water Depth
10ft
14-18
13-15
9-11
7-11
15ft
21-27
18-22
15-18
14-18
20ft
29-35
24-29
23-17
21-28
Table 5.
Clean Water Oxygen Transfer Efficiencies of Ceramic Disc/Dome Grid Systems
Diffuser Density
(sq ft/diffuser)
Air Flow
(scfm/diffuser)
SOTE(%) at Water Depth
10ft
15ft
20ft
Reference
Disc-9,4 in,
6.4
4.1
3.2
Dome-7 in.
0.9-3.0 20-22 31 34-37 21
0.8-2.9 21-24 30-34 35-41 21
0.7-2.6 22-25 31-34 38-41 21
5.6
4.2-4.4
3.2-3.3
2.2-2.5
1.5
0.5-2.0
0.5-2.5
0.5-2.0
0.5-2.5
0.5-2.5
i f
16-23
20-24
17-23
18-26
25-31
25-32
27-37
27-35
27-34
28-40
30-41
31-44
33-47
33
33,34
21,31,33
33,34
34
systems are presented in Table 7.
Each data set represents the
observed performance of a particular
system over a period of several hours
only and is not suitable for the design
of similar systems. The intent of this
table is to give the reader a general
feeling for the range in performance
of the systems listed under a variety
of operating conditions.
The oxygen transfer data were all
collected using the off-gas test
procedure. Apparent values of alpha
were estimated from clean water
performance data for similar tank
geometry, air flow rate per diffusion
unit, and diffuser placement. Since
the performance of most porous
diffusion devices is likely to change
with time, the term "apparent
alpha," aa, has been adopted to
distinguish between differences in
clean and process water performance
for cases where the diffusers are
process tested at a condition of
undetermined fouling (aa) vs. those
where they are process tested'new or
just after cleaning (a). The latter
condition measures the alpha value
due to wastewater characteristics
only. In all cases, the OTE< (field
results) values have been converted
to aaSOTE values, i.e., to 20° and
zero residual DO.
The first three data sets originated
from off-gas testing at Madison,
Wisconsin."7"149 The ceramic grid data
(the first data set) represent the
overall performance of a three-pass
system. OTEf, air flux rate (air flow
per unit surface area of tank), and
residual DO profiles are shown in
Figure 10 as a function of tank
length. Values of apparent alpha with
position in the basin are plotted in
Figure 11. Apparent alphas were
estimated from clean water data
having the same diffuser density, air
flux rate, and liquid submergence as
the grids from which the off-gas data
were collected. Due to variable flux
rates and diffuser densities, the OTEf
values in Figure 10 do not accurately
reflect changes in apparent alpha
along the tank. At this facility, the
apparent alpha varies from about 0.4
at the tank inlet to near 1.0 at the
tank outlet. A reduction in apparent
alpha occured at each point of
primary effluent addition.
The second Madison data set for
ceramic and SAN plastic tubes,
applied in a dual spiral roll
configuration, represents
performance for the first pass of a
three-pass system (Figure 12).
Passes two and three, represented by
the third data set, are equipped with
wide-band, fixed-orifice, coarse
bubble diffuser, also oriented in a
dual spiral roll placement. The higher
relative alpha values of the latter two
passes are strongly influenced by the
favorable position of these passes at
the middle and effluent end of the
process train compared to the lead-
pass position of the ceramic and SAN
plastic tubes. This relationship of
alpha to tank position or degree of
treatment is similar to that observed
for the first Madison data set (Figure
11).
The two data sets for Whittier
Narrows, California
represent approximately 9 months of
operation.47'48 The two systems for
which data are presented were part
of a three-system field oxygen
transfer evaluation conducted by the
Los Angeles County Sanitation
Districts (LACSD) for EPA in parallel
trains.30
These data sets compare the
performance of a ceramic grid system
to that of a jet aeration system where
the jets were installed on one side of
the basin along the entire tank length
with the nozzles being directed
across the basin floor in a reverse
spiral roll. Figure 13 is a plot of
apparent alpha for each system vs.
position from the inlet end. The ratio
of apparent alpha values varies with
tank length, decreasing toward the
effluent end of the tank.
17
-------
o
C/J
45--
40--
35--
30--
25--
20
Water Depth = 15 ft
Increasing Density of Placement
5.4 sq ft/diffuser
0 0.5 1.0 1.5 2.0 2.5
Air Flow Rate per Diffuser jscfm)
Figure 9.
Effect of Diffuser Density on SOTE for Ceramic Disc/Dome Grid Configurations21
Of particular interest are the
apparent alpha values near the
discharge end of these basins as
contrasted to those observed at
Madison, Wisconsin. The terminal
apparent alpha of the ceramic grid
system at the Whittier Narrows
facility was approximately 0.6 vs.
almost 1.0 for Madison's ceramic
grid system. The presence of
nonbiodegradable surfactants is one
explanation for the low apparent
alphas at Whittier Narrows. Other
possibilities include high process
loadings, low DO operation, and
significant diffuser fouling. This
observation illustrates the dangers of
extracting data from specific sites for
general design purposes. Each
treatment facility has unique
characteristics that must be
considered individually.
The Brandon, Wisconsin, data set
depicts performance of a
9.l-m (30-ft) long x 4.6-m (15-ft) wide
x 4.6-m (15-ft) deep completely
mixed aeration tank using jet
aerators at two different air flow
rates.48'49 This municipal facility
treats a combination of domestic and
industrial wastewaters.
In contrast, the Orlando, Florida,
aeration system,51 which employs
wide-band, fixed-orifice coarse
bubble diffusers, treats domestic
wastewater only. This system is
currently being retrofitted with a
ceramic grid system in an effort to
improve aeration efficiency and
increase aeration capacity.
The data shown for
Seymour, Wisconsin,52 a site
analyzed by Houck53 in his North
American survey of disc and dome
diffuser systems and also studied by
Vik et al.,54 were collected on a lightly
loaded system with a sludge
retention time in excess of 25 days at
the time of the tests.
The two data sets from Lakewood,
Ohio ,55 demonstrate the
relative performance of two parallel
basins, one whose diffusers had
recently been cleaned and the other
which had been operating for
approximately I year with no diffuser
cleaning. The entire system was
retrofitted with ceramic disc diffusers
in a grid configuration during 1 982
and 1983. In this instance, the
uncleaned system as found was
performing at a mean weighted
aaSOTE of 8.9 percent vs. 14.5
percent for the cleaned system.
During the 1 -year operating period, it
appears that oxygen transfer
performance deteriorated by roughly
40 percent. Part of this reported
performance deterioration may have
been due to fouling resulting from
several periods of air interruption to
make necessary modifications to the
air supply piping during the retrofit
process.
The last two data sets
provide information on new ceramic
grid and static aerator systems that
were tested on brewery wastes
within the same completely mixed
basin.56 Of interest is the relative
performance of both systems as
measured by oxygen transfer and
apparent alpha data. The ratio of the
mean weighted apparent alphas of
the static aerator system to that of
the ceramic grid system was
observed to be 0.50/0.37 = 1.35.
This ratio is not considered
appropriate for design since, among
other things, the levels of alpha
observed in this industrial
wastewater application are lower
than usually encountered. The
respective ratio of craSOTE values
was 7.4/14.2 = 0.52.
Data are presented in Table 8 from a
Madison, Wisconsin, oxygen transfer
field evaluation for the last pass of a
three-pass system employing
selected tubular diffusers in a dual
spiral roll configuration.47 The aerator
layout is depicted in Figure 14. The
off-gas test procedure was used in
collecting the OTEt data. Since alpha
approached unity at this tank
location, direct use of the data is not
suitable for design purposes. The
relative performance of the new and
used ceramic tubes and the used
SAN plastic tubes indiactes the
significance of fouling. The used
ceramic and SAN plastic tubes were
in service continuously for about 3
years in a different tank prior to
relocation for this test. It should be
noted that analysis of multiple
systems within a given tank cannot
be conducted by any other technique
than off-gas analysis. Known
18
-------
Table 6.
Selected Factors Affecting Oxygen Transfer Field Testing for Estimation of Oxygen
Transfer Efficiency46
Oxygen Transfer Test
Factors
Steady
State
Nonsteady
State
Off-Gas
Inert
Gas
Tracers
Sensitivity To
Variations in
Influent wastewater flow rate
Oxygen uptake rate
Alpha
DO concentration
Product of air flow rate x KLa
ccurate measure of
Oxygen uptake rate
DO concentration
DO saturation value
Air flow rate
Other
Costs
Manpower +
Analytical +
Capital investment +
Calculations +
Estimated Precision
0
+/0
0
0
1 Calculate OTE directly.
2 Requires accurate measurement of CDs in gas.
3 Requires accurate estimate of the ratio of Kt,ac8r/KLa.
+ Positive response (e.g., not sensitive, less costly, more precise,
0 Intermediate response.
- Negative response.
0
0
0
0
f
easier).
0
0
available data on tube and flexible
sheath systems are scant, and
additional reliable information would
be of significant value.
On the basis of the data presented in
Table 7, it appears that the
differences between apparent alpha
values for ceramic grid fine pore
diffusers and apparent alpha values
for other more turbulent systems
such as jet aerators and static
aerators may not be as great as
previously reported in the literature.57
In addition, the overall average
apparent alpha values presented in
Table 7 and elsewhere58"60 for a
variety of aeration devices are lower
than many alpha values historically
used for design purposes.
A recent study at Rye Meads, United
Kingdom,61 demonstrates the impact
of process goals relative to aeration
efficiency. In this study, optimization
of the nitrification process in
conjunction with an initial anoxic
zone, tapering diffuser density to
meet oxygen demand, and the use of
automated DO control resulted in an
overall aeration efficiency of 2 kg
Oa/kWh (3.3 Ib/wire hp-hr) vs. 1.2
kg Oa/kWh (2.0 Ib/wire hp-hr) for an
unmodified control basin. A third
parallel train employing tapered air
and DO control in a non-nitrifying
operational mode averaged about 1.4
kg O2/kWh (2.3 Ib/wire hp-hr) during
the study phase. Proper placement of
the ceramic dome diffusers,
automated DO control, and level of
treatment were identified as
essential elements of Rye Mead's
aeration efficiency optimization.
Another factor of concern was
observed in a recent, long-term study
of ceramic grid systems62 where the
slope of log OTE vs. log applied air
flow rate under process conditions
had a significantly steeper negative
slope with increasing air flow rate
than observed under clean water
conditions. An illustration of this
observation is shown in Figure 15.
Other investigators,61 however, have
not observed this phenomenon.
Figure 15 shows that the slope of the
log OTE vs. log applied air flow rate
curve of a new or clean porous
diffuser is very different from that of
a fouled or partially fouled unit.
Danley63 has observed that, as
diffusers become biologically fouled,
the effective pore area of the
diffusers decreases drastically.
Typical plant practice is to operate
diffusers at the same or greater
specific air flow rate over a period of
time. Since the effective pore area
has been greatly reduced by the
accumulated foulants, the actual air
flow rate per operating pore
correspondingly increases.
Generalizations on this relationship
cannot be made at this time,
however, due to the lack of an
adequate data base.
The data described above represent a
diverse cross section of process
performance for selected oxygen
transfer devices under a variety of
operating conditions. No attempt has
been made here to correlate oxygen
transfer performance to process type,
process loading, wastewater
characteristics, and other factors. It is
evident that gaps in the current data
base still exist for which additional,
in-depth study is needed to address
designer concerns.
19
-------
Table 7.
Process Water Oxygen Transfer Efficiency Comparison for Selected Aeration Systems
Site
Madison, Wl
Madison, Wl
Madison, Wl
Whittier
Narrows, CA
Whittier
Narrows, CA
Brandon, Wl
Brandon, Wl
Orlando, FL
Seymour, Wl
Lakewood, OH
Lakewood, OH
Brewery
Brewery
System
Ceramic grid
Ceramic & SAN
plastic tubes
Wide-band, fixed-
orifice coarse
bubble diff users
Ceramic grid
Jet aerators
Jet aerators
Jet aerators
Wide-band, fixed-
orifice coarse
bubble diff users
Ceramic grid
Ceramic grid
Ceramic grid
Ceramic grid
Static aerators
Flow
Regime
Step feed
Step feed
Step feed
Plug flow
Plug flow
Complete mix
Complete mix
Complete mix
Plug flow
Plug flow
Plug flow
Complete mix
Complete mix
a. SOTE (%) s
Mean
17.8
11.0
10.0
11.2
9.4
10.9
7.5
7.6
16.5
14.5
8.9
14.2
7.4
Range
12.6-26.2
7.5-13.4
7.9-10.9
9.3-15.2
7.8-10.8
9.7-12.1
7.4-7.7
6.8-8.4
12.0-18.8
12.4-15.9
7.0-11.1
12.5-15.2
5.7-8.8
Diffuser
ubmergence Variation
(ft) in aa SOTE
14.8
15.0
15.0
13.5
13.5
12.5
12.5
13.0
13.8
13.3
13.3
19.2
19.8
Rising from
inlet to outlet
Rising from
inlet to outlet
Random
Rising from
inlet to outlet
Rising from
inlet to outlet
Random
Random
Random
Random
Rising from
inlet to outlet
Rising from
inlet to outlet
Uniform
Uniform
Estimated a.
Mean
0.64
0.62
1.07
0.45
0.58
0.45
0.47
0.75
0.66
*^
0.31
0.37
0.50
Mean Air
Flux Rate
Range (scfm/sq ft)
0.42-0.98
0.46-0.85
0.83-1.19
0.35-0.60
0.48-0.72
0.40-0.50
0.46-0.48
0.67-0.83
0.49-0.75
0.44-0.57
0.26-0.37
0.32-0.37
0.36-0.51
0.28
0.53
0.53
0.21
0.37
0.13
0.39
0.92
0.07
0.14
0.09
0.30
0.53
20
-------
16 -
14
12
10
Gridl
Tapered air, step aeration system with ceramic dome grid layout
OTEt values measured at indicated field conditions
of DO and air flux rate
PE = Primary effluent
RAS = Return activated sludge
834 Domes
GridS
7
505
Domes
Grid 4
410 Domes
f Grid 5
7
392
Domes
Grid6
332 Domes
<
-X
0.5
-0.3
4. 0.14-0
o>
.2 £
O
Q
100
200
Tank Length (ft)
300
Figure 10.
Gas Transfer Analysis Along Tank Length, Madison, Wl47
21
-------
2.
a
a.
<
c
ffi
1.0
0.8
0.6
o
0.4
0.2
\i
Ceramic dome grid system
PE = Primary effluent
RAS = Return activated sludge
LU
O_
100
200
Tank Length (ft)
300
Figure 11.
Estimated Change in Apparent Alpha with Tank Length, Madison, Wl47
22
-------
Pass 1
Fine Pore Tubes
Passes 2 and 3
Coarse Bubble Diffusers
OTEi values measured at indicated field conditions
of DO and air flux rate
PE = Primary effluent
RAS = Return activated sludge
-|0.45
100
200
300
Tank Length (ft)
400
500
0.4
0.35
0.3
-4
o>
O
Q
-2
Figure 12.
Gas Transfer Analysis Along Tank Length, Madison, Wl47
Table 8.
Process Water Oxygen Transfer Comparison for Selected Tubular Diffusers at
Madison47
Diffuser Type
Wide-Band, Fixed-Orifice
Coarse Bubble Diffusers
New Flexible Sheath Tubes
Used SAN Plastic Tubes
Wide-Band, Fixed-Orifice
Coarse Bubble Diffusers
New Ceramic Tubes
Used Ceramic Tubes
Wide-Band, Fixed-Orifice
Coarse Bubble Diffusers
Location*
Station 8
Station 9
Station 9
Station 10
Station 1 1
Station 1 1
Station 1 2
OTE, (%)
8.0
11.6
9.2
8.0
12.9
9.1
7.0
DO fmg/L)
0.9
1.7
1.7
2.0
1.7
1.7
1.2
a.SOTE (%)
8.3
14.2
11.3
10.3
16.0
11.0
8.3
'Refer to Figure 14.
23
-------
0.8
1
10
r.
a.
5 0.6
c
0)
<0
Q.
Q.
0.4
0.2
Jet Aeration System
Ceramic Disc
Grid System
I
100 200
Tank Length (ft)
300
Figure 13.
Estimated Change in Apparent Alpha with Tank Length, Whittier Narrows, CASO
24
-------
Y-Wall
\ .
Wide-Band j
Fixed-Orifice |
Coarse Bubble j
Diffuser
New Flexible
Sheath Tubes
Wide-Band
Fixed-Orifice
Coarse Bubble
Diffuser
New Ceramic
Tubes
Wide-Band
Coarse Bubble
Diffuser
L
A
v i
r""~""~ - n^* 1| -».
1 8A 8B 8C I
\ 9-1A 9-1B 9-1C J
f 1[~~ -Jl " ^Ti1
I
r ir Ji -,
I 9-2A 9-2B 9-2C
| 10-1A 10-1B 10-1C
H -J[ II - .
L w. ^
f- Jf^1 IP J
J10-2A 10-2B 10-2C fi
Jl1-1A 11-1B 11-1C
11 ^\ -Jl J
F -
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1 12A 12B 12C
^ 1P~ Jl -,
I Outlet
25.5ft
32.5ft
<
e
to
in
u>
T
I
Header
mf
Wide-Band
Fixed-Orifice-
Coarse Bubble
Diffuser
Used SAN
m T i-
Fixed-Orifice
Coarse Bubble
Diffuser
Used Ceramic
Tubes
Wide-Band
Coarse Bubble
Diffuser
Figure 14.
Diffuser and Off-Gas Hood Sample Site Layout for Comparative OTEf Analysis of
Table 8"
25
-------
Clean Water
(New D iff users)
Dirty Water
(New Diffusers)
Dirty Water
(Fouled Diffusers}
Log Air Flow Rate per Diffuser
Figure 15.
Change in OTE with Fine Pore Diffuser FoulingA Hypothetical Case
26
-------
Operation and
Maintenance
Considerations
Preceding sections of this report have
described the various fine pore
diffused aeration systems available
and discussed their performance
characteristics. Considerations
relative to the operation and
maintenance (O&M) of fine pore
diffusers are addressed in this
section. Design and installation
practices are examined specifically as
they relate to O&M and operating
problems commonly encountered.
Techniques are reviewed for dealing
with these problems, the most
frequently occurring of which is
diffuser fouling. The causes of, and
approaches to coping with, diffuser
fouling are given special emphasis in
this discussion.
Impact of System Design and
Installation on O&M
The principal objective in the design
of fine pore aeration systems should
be to provide a system with the
lowest possible life cycle cost,
maintaining an optimum balance
between capital and long-term O&M
expenditures. Since long-term O&M
characteristics will generally be
determined by the capabilities and
constraints originally designed into
such systems, it is important that due
consideration be given during design
to anticipated O&M requirements. In
some cases, relatively minor capital
expenses can lead to significant
reductions in overall cost by reducing
those requirements.
This subsection assesses the impacts
of process design, materials
selection, aeration basin design, air
supply design, and aeration system
installation on O&M. Emphasis is
placed on identifying key areas
where additional consideration
during design can lead to significant
long-term benefits.
Process Design
The operating characteristics of fine
pore diffusers are different than
those of other oxygen transfer
devices, and these differences affect
process design. While diffused
aeration systems can produce strong
vertical mixing components,
horizontal components will generally
either be unidirectional (as where the
diffusers are located along only one
wall of the basin as in a spiral roll
configuration) or largely nonexistent
(as in full floor coverage
applications). Consequently, the
wastewater flow pattern is likely to
approach plug flow in character
above certain length-to-width
(aspect) ratios, establishing a
gradient in process oxygen demand
(high to low) from the aeration basin
inlet to the outlet.
Wastewater constituents may also
affect fine pore diffuser OTEs to a
greater extent than they do other
oxygen transfer devices, resulting in
lower alpha factors. Diffuser layout
design is strongly affected by this
phenomenon since the region in the
aeration basin where oxygen demand
is likely to be the highest (at the tank
inlet) is also the region where the
alpha value is likely to be the lowest.
This combination of practical
considerations usually requires that
fine pore diffuser density be
substantially increased in the inlet
portion of a plug flow basin to avoid
DO deficiencies and potential process
upsets during high-load operation.
For most fine pore diffusers, the
practical maximum-to-minimum
operating range in air flow rates per
diffuser is roughly 5 to 1, providing
an approximate 4 to 1 range in
oxygen transfer capacity. The lower
air flow limit per diffuser, which is
set by the manufacturer, is the gas
rate required to maintain a uniform
flow of air across the surface of the
diffuser. Operation below this limit
for an extended period of time has
been found to result in accelerated
diffuser fouling rates.17 The upper
limit corresponds to the air flow rate
beyond which headloss across the
diffuser control orifice increases
substantially and/or OTE decreases
significantly.
Taken together, the above factors
generally dictate that fine pore
diffused aeration systems be
designed with tapered aeration
capabilities in tanks with high aspect
ratios. At a minimum, the diffuser
density, i.e., the effective basin floor
area per diffusion unit, should vary
with the highest density near the
tank inlet and the lowest at the tank
outlet. The design should be capable
of meeting expected variations in air
flow requirements, considering both
variations in process oxygen
requirements and alpha factors along
the length of the aeration basin. It
may also be desirable to section the
diffusion system into grids, with
independent air supply control to
each grid. For example, a total of
27
-------
three grids might typically be
provided in an aeration basin with a
length-to-width ratio of 3 to 1 or
greater.
Failure to provide proper aerator
tapering in tanks with high aspect
ratios or in staged tanks can result in
inadequate oxygen transfer capacity
and low DO concentrations at the
inlet end. Such conditions have been
found to accelerate biofouling of fine
pore diffusers (discussed in more
detail below) and contribute to other
process-related and/or operational
problems.53 On the other hand,
overdesign, particularly insufficient
tapering of diffuser density in the
middle and latter stages of a long
plug flow tank, can lead to inefficient
use of energy when the air flow rate
to meet the minimum operating
requirement per diffuser exceeds that
to meet process oxygen
requirements.
In some cases, it may not be possible
to accurately forecast future
variations in oxygen demands and
alpha values along an aeration basin.
If the aeration system is designed
with sufficient oxygen transfer
capacity and turndown capability,
however, process operation can be
adjusted to meet actual demands. For
example, assuming that adequate
flexibility has been designed into the
air supply system, overaeration
caused by the maintenance of
minimum air flow rates per diffuser
can be combatted by removing
diffusers from the area of the
aeration basin that is overaerated.
A eration Basin Design
The mixing characteristics of a
particular oxygen transfer system
may necessitate the use of inlet and
outlet flow distribution schemes to
prevent short-circuiting in the
aeration tank. Weirs, baffles, multiple
gates, or favorable mixing patterns
induced by diffuser placement may
be used for this purpose.
Consideration should also be given to
the point of entry of flow into an
empty aeration basin. For example,
entry over a weir may be acceptable
when the basin is full but may cause
damage to the diffusers and/or
header system below when the basin
is empty. In this case, an alternate
means of filling the basin must be
provided (perhaps through the basin
drainage system).
As previously discussed, many fine
pore aeration systems are
nonretrievable while in service; this
necessitates that the aeration basin
be drained to gain access to the
diffusers. Because of the fouling
characteristics of these diffusers
(discussed below), periodic access via
basin drainage will be required. A
reliable and easy-to-operate aeration
basin drainage system capable of
completely draining the basin in a
convenient time interval (say 8 to 24
hours) is recommended. The required
frequency of drainage will vary
depending on the rate of fouling and
the cleaning methods used.
Adequate volumes of nonprocess
water should also be provided,
including hydrants or faucets at
frequent intervals along the aeration
basin, to assist in basin washdown.
Air Supply System Design
The air supply system must be
designed to meet both the process
oxygen needs and the operational
requirements of the selected fine
pore diffusion system. Air
requirements will vary as process
oxygen requirements vary, and the
air supply system must have
sufficient flexibility to meet these
demands if the full energy benefits of
the fine pore diffusion are to be
realized. Achieving this flexibility will
generally require the use of multiple
blowers, each provided with
appropriate turndown capability, i.e.,
variable-speed motors on positive
displacement blowers or various
means of control on centrifugal
blowers. The air flow meter(s) and
flow control valve(s) must also be
sized properly for the range of air
flow rates anticipated. Flexibility can
be enhanced with instrumentation
and automated controls that should
be integrated with the air supply
system design. These control
features will be discussed in the
upcoming WPCF MOP No. 5.
The small air passage orifices in fine
pore diffusers that cause them to be
more efficient in transferring oxygen
also make them more prone to
plugging by paniculate matter in the
air supply. Proper air filtration must
be provided to prevent atmospheric
dirt or blower oil from entering the
air distribution system and causing
air-side plugging of the diffusers.
Removing these potentially harmful
particles requires efficient air
filtration. Manufacturers of ceramic
fine pore diffusers have historically
recommended a minimum removal of
95 percent of all particles 0.3 micron
and larger to avoid air-side plugging
of the diffusers. Alternatively,
specifications on filtered air quality
require paniculate concentrations
less than 0.1 mg/28 m3 (1,000 cu
ft).5'7
Air filters can be located on the inlet
to the blowers or in-line in the air
distribution system. One drawback of
in-line filters is the incremental
increase in blower discharge
pressure required to overcome losses
in the filter. This consumes some
power, but the effects can be
minimized by properly sizing and
maintaining the filters. Large plants
may wish to investigate baghouse
filters or electrostatic precipitators as
economic alternatives to in-line
filters. Unique problems such as the
filtering of wet air will require special
attention.
Uniform distribution of air among
individual diffusers in an aeration
grid is also an important
consideration. The use of fixed-size
orifices on individual diffusers is a
common practice today to achieve
uniform air distribution.
A final consideration is the reliability
of the power supply. Interruptions in
service allow mixed liquor to enter
the air header system through the
diffusers and any leaks in the system.
Some suspended solids will be
filtered out by the diffusers, but some
will also enter the system piping.
When air supply is resumed, a
properly designed purge system
should be used to clear the system so
that the suspended solids will not be
trapped and retained on the inner
surfaces of the diffusers. Suspended
solids filtered out by the diffusers
during a power outage may be
retained within the media on
resumption of air delivery. These
retained solids will result in higher
headlosses across the diffusers and
may lead to a change in OTE.
Consequently, extra care should be
taken during design to provide a
reliable power supply with
appropriate backup to minimize the
occurrence of power outages.
Materials Selection and
Specification
O&M of fine pore diffused air
systems is facilitated when the
materials of construction of the
diffusion system components are
28
-------
properly selected and specified.
These tasks are typically the
responsibility of the engineer. Unless
the aeration system is provided with
the necessary degree of mechanical
and structural integrity, the potential
energy economies of fine pore
diffuser operation will not be realized
due to air leaks. Leaks may also allow
entry of mixed liquor into the
diffusion piping, resulting in plugging
of the diffusers from the air side. The
detailed aspects of selecting
materials for fine pore diffusion vary
among manufacturers and with
diffuser type. Additional details will
have to be considered as new
equipment is developed.
Nevertheless, some general
principles are relevant.
Special precautions must be taken to
select materials that will contribute
to trouble-free operations of fine pore
aeration systems. Factors such as
freeze/thaw and sunlight exposure
are important. Due consideration
should be given to the use of
stainless steel appurtenances.53
Specifying stainless steel for items
such as anchorage straps and bolts
will add little to overall system cost,
but may significantly increase the
mechanical and structural integrity of
the system. The selection of
corrosion-resistant materials is also
recommended since corrosion
products can be transported to
diffusers through the air supply
system, producing air-side diffuser
plugging.
Many fine pore diffusion systems use
plastic header piping to reduce
system capital cost. Although such
use has proven successful, the
resulting system is more fragile than
the steel or cast iron piping systems
previously used.
All normal and abnormal forces
should be considered during the
design of the header piping
anchorage system. This can be
especially critical if a new fine pore
aeration system is to be used in
conjunction with another aeration
system in the same basin. For
example, expansion and contraction
of the header system must be taken
into account during both header
layout and detailed design. The
anticipated temperature range should
consider both in-service and out-of-
service conditions, e.g., a drained
tank on a hot summer day. Long-term
structural/mechanical integrity and
long-term maintainability will be
greatly affected by the consideration
given during design to the static and
dynamic forces that the air
distribution system is expected to
withstand.
System Installation
Special precautions are required with
regard to certain aspects of fine pore
aeration system installation. As an
example, all construction debris and
dust should be removed from the air
supply system before the diffusers
are installed. If not removed, these
materials can be transported to the
diffuser during operation, resulting in
plugging. Flushing can be
accomplished with either air or water
and should be followed by inspection.
Care also must be exercised in the
installation of the more fragile
components of the air header and
diffusion system. For example,
structural failures resulting from
overtightening of the retaining bolt
have occurred with one type of
ceramic dome diffuser.
Overtightening during installation
resulted in failure of either the plastic
retaining bolt or the plastic saddle
that it was inserted into.53 The
problem may be addressed by the use
of a properly-set torque wrench to
install the bolt. This example
illustrates the need for increased
concern with some fine pore
systems, particularly those using
plastic components. These
considerations are more crucial to a
fine pore than to a coarse bubble
aeration system because air leaks
can lead to proportionally higher air
flow and energy requirements, thus
negating the major advantage of the
fine pore system.
Impact of Fouling
Phenomena on O&M
Background
Porous ceramic plate diffusers,
introduced in the United States in the
1920s, had become the predominant
air diffusion device by mid-century.5'7
Various types of foulants were
identified by early investigators, and
the list has been expanded by recent
studies to include the following:26
Air Side
Dust and dirt from unfiltered air
Oil from compressors or viscous
air filters
Rust and scale from air pipe
corrosion
Construction debris due to poor
cleanup
Wastewater solids entering
through diffusers or pipe leaks
Liquor Side
Fibrous material attached to sharp
edges
Inorganic fines entering media at
low or zero air pressure
Organic solids entering media at
low or zero air pressure
Oils or greases in wastewater
Precipitated deposits, including
iron and carbonates
Biological growths on diffuser
media
A number of different cleaning
procedures have been developed,
identified, and applied including the
following:5'7
Ex-Situ
Refiring
Silicate-phosphate washing
Alkaline washing
Acid washing
Detergent washing
In Situ
Acid washing
Flaming
High pressure water hosing
Withholding influent (creating
endogenous conditions)
Sandblasting
Chlorine washing
Air bumping (air turned off and on)
Steam cleaning
Gasoline washing
Drying
The rate of fouling has historically
been gauged by the rise of back
pressure while in service. Since
significant biological fouling can take
place with little attendant rise in
backpressure, this provided a crude
and qualitative measure at best.
It was common practice in earlier
times to operate a number of
diffusers from a common plenum.
This practice resulted in less
uniformity of air distribution than is
obtained today with the use of
restrictive flow control orifices on
individual diffusers. The lack of air
29
-------
flow uniformity probably augmented
the rate of biological fouling
experienced in the past.
In the 1960s and early 1970s, the
relative cost of energy to
maintenance labor was low. As a
consequence, many of the ceramic
plate installations were replaced with
less efficient, fixed-orifice coarse
bubble diffusers. In the middle
1970s, this trend was reversed and
many of those installations are now
being replaced by porous media
diffusers with individual air flow
control.
In the early 1980s, better methods of
measuring the degree of fouling and
the effects of cleaning became
available. These methods include
dynamic wet pressure (DWP), bubble
release vacuum (BRV), the ratio of
one to the other, and chemical, as
well as microbiological, analysis. The
practice of employing pilot diffusers
that could be removed from the tank
and individually analyzed also came
into use.26
Concurrently, better methods were
being developed to measure the
performance of operating aeration
systems, which permitted better
appraisal of the effects of fouling and
facilitated better preventive and/or
corrective maintenance scheduling.
These methods include inert gas
tracers, off-gas analysis, a dual
nonsteady state technique that uses
hydrogen peroxide, and DO and
respiration rate profiles.46'47'64"66 Off-
gas equipment has been effectively
used to evaluate the adverse effects
of fouling on both full-scale systems
and on individual diffusers.47
Fouling Processes
Recent work has contributed
measurably toward an increased
understanding of some of the
mechanisms of fouling.67 For
example, it has become apparent that
flux rate is a parameter that can have
a significant influence on the rate of
fouling and its consequences. Flux
rate may be expressed in several
ways. For the purposes of this
discussion, apparent flux rate is
defined as the air flow rate per
diffuser or diffusers divided by the
effective diffuser area involved. Local
flux rate is defined as the air flow
rate per unit area of a small defined
segment of a given diffuser. Effective
flux rate is defined as the weighted
average flux rate for one or more
diffusers. This value may be obtained
for a given diffuser by measuring the
local flux rates on representative
sample positions of a diffuser and
dividing the sum of the products of
flux rate and air flow rate of the
individual sample points by the sum
of the measured air flow rates.
Types of fouling may be differentiated
on the basis of the effects of flux rate
on them. For one classification of
foulants, fouling rates are increased
by high local flux rates and reduced
by low local flux rates. Included in
this classification are air-side fouling
from air-borne particulates and
liquid-side fouling by precipitates
such as metal hydroxides and
carbonates. In the process of fouling,
the areas of the diffusers with the
highest local flux rate foul more
rapidly, which serves to reduce the
flux rate in high flow areas and to
increase it in low flow areas, the
combined effect of which is to
improve uniformity of air distribution.
The effective flux rate approaches the
apparent flux rate as fouling
progresses. In the case of chemical
precipitate fouling, the accumulation
of foulants in the pores reduces the
effective pore diameter and the
backpressure or DWP rises
correspondingly. Due to the reduced
effective pore diameters and the
smaller bubbles produced, OTE does
not decline and can actually increase.
At the same time, the increase in
DWP can exceed the capabilities of
the air supply system and process air
delivery may fall short of
requirements. An idealized
representation of OTE and
backpressure (DWP) changes with
time under fouling conditions of this
type is shown in Figure 16a.
Another classification of foulants
causes fouling rates to increase with
low local flux rates and decrease
with high local flux rates. Included in
this category are microbiological
slimes. Examples of this type of
fouling may be observed on the
underside of fine pore tubular
diffusers and on the less pervious
portions of planar diffusers.
As fouling progresses with the latter
type of foulant, low local flux rates
tend to further decrease, high local
flux rates tend to increase, and air
distribution becomes progressively
less uniform. A condition can be
reached where the flux rates in the
small remaining working areas of the
diffusers are so high that subsequent
fouling is nearly completely arrested.
The net result of high effective flux
rates for individual diffusers or
groups of diffusers is a substantial
reduction in OTE. Figure 16b is an
idealized representation of OTE and
backpressure (DWP) changes with
time under fouling conditions of this
type.
Process variables that appear to
affect the rate of biofouling are not
fully understood. Experience and test
data67 provide some indications that
the rate of biofouling is increased by
operation at high organic loading
rates and/or low air flow rates. Other
data indicate that biofouling rates
may be accelerated by the presence
of certain types of industrial wastes,
particularly high-strength, readily-
biodegradable, and/or nutrient
deficient wastes.68 It is believed that
under service conditions all of the
types of fouling discussed above, and
some others in addition, can occur
singly or in combination with variable
dominance from plant to plant and
within the same plant from time to
time.
Fouling Observations
Substantial data are available and in
the process of being assembled
regarding fine pore diffuser fouling
and its effects. Unfortunately,
consistent methods of reporting have
not as yet been developed. Table 9 is
a tabulation of fouling rate data from
a number of ceramic diffuser-
equipped municipal treatment plants
that are considered to be
representative of the data base from
which it was selected. A similar
selection of fouling rate data is
presented in Table 10 from various
treatment plants using ceramic
diffusers in which a significant
fraction of the wastewater is of
industrial origin. A parameter called
BRV (bubble release vacuum) is
introduced in Tables 9 and 10. BRV is
defined as the negative pressure
required to form and release bubbles
in tap water at a given location or
locations on a given diffuser at an air
flux rate of 5.1 L/m2/s(1 scfm/sq
ft).62 Fouling rates in terms of
ABRV/yr and ADWP/yr were
calculated from actual
measurements of pressure difference
assuming a linear increase with time.
Since fouling rates usually are not
30
-------
Q.
Q
LU
o
Time
High local flux rate fouling (indicative of air-side paniculate fouling and/or
water-side chemical precipitate fouling) produces a significant increase in
DWP and a slight increase in OTE.
CL
Q
Time
b. Low local flux rate fouling (indicative of water-side biological fouling)
produces a slight increase in DWP and a significant decrease in OTE.
Figure 16.
Effect of two types of fine pore diffuser fouling on DWP and OTE
constant with time, these values are
not directly comparable, but are
believed to be useful in generating
approximate estimates.
Foulants in plants A, M, N, HI, Jl, and
Kl were known to contain higher-
than-usual fractions of inorganic
constituents. Diffusers in these
plants also exhibited higher ADWP/
ABRV ratios with the exception of
plant Kl where the inorganic
constituents appeared to be very
loosely attached to the diffusers.
A number of tentative observations
may be proposed from Tables 9 and
10 as well as other ongoing work:
Fouling rates appear to be widely
variable for a given plant as well
as at a given location within a
tank.
Industrial plant fouling rates
appear to be higher than those of
municipal plants.
Air-side fouling due to particulates
in the air has not been found to be
a significant adverse factor in any
of the installations cited in Table 9
or10 or in the data base of the
approximate 50 plants from which
they were drawn.
The visual appearance of foulants
has failed to consistently provide
reliable bases for identifying the
nature or origin of the foulants.
In plug flow regimes, the fouling
rate is greatest at the influent end
of the tank. If the influent end air
flux rate is reduced below the
level required to maintain positive
DO, fouling can progress down the
tank.
Predominately biological and/or
inorganic fouling may reduce
OTEs up to one-half of their
original values in a matter of
weeks.74 Yet, in other situations,
very little reduction in OTE may be
noted in over a year.
Pilot plant fouling tests appear to
correlate reasonably well with
full-scale tests that employ the
same diffusers at comparable air
flow rates.74
Ferrous sulfate (and probably
other metallic salts) added ahead
of or to the aeration tank for
phosphorous control may
aggravate fine pore diffuser
fouling.74
Silica has frequently been found
as a major constituent of fine pore
diffuser foulants.
The present data base is not
sufficient to develop firm conclusions
on the above observations. A current,
pressing need exists to analyze and
expand the data base being
assembled for fine pore diffuser
systems with respect to their
tendencies to foul and the
consequences thereof. Insufficient
information is available to evaluate
the fouling tendencies of flexible
sheath diffusers. Relative fouling
rates for glass-bonded ceramic
diffusers and porous plastic diffusers
are not known. The comparative
overall fouling rates of plug flow and
complete mix systems also are not
well documented. Various biological,
physical, and chemical factors
31
-------
Table 9.
Representative Ceramic Diffuser Fouling Data for Municipal Treatment Plants
Plant
A
B
C
D
E
G
H
K
L
M
N
Diffuser
Type
Discs
Discs
Discs
Discs
Discs
Discs
Discs
Domes
Discs
Discs
Discs
Discs
Domes
Domes
Domes
Domes
Domes
Domes
Discs
Discs
Exposure
Time
(Days)
120
120
133
360
90
365
365
1100
210
93
93
100
210
210
360
360
350
350
900
900
BRV (in.'W.G.)
Initial
5.9
5.6
5.7
6.3
5.8
6.0
5.7
6.2
4.9
(5.0)
(5.0)
5.0
4.0
4.3
4.3
(5.6)
(5.6)
After
Exposure
9.0
11.6
8.5
10.7
27.4
16.3
23.0
36.8
18.0
11.8
13.0
11.2
9.8
11.7
10.3
17.5
68.9
199
DWP(in. W.G.)
Initial
6.6
6.5
(5.8)
5.3
6.0
6.5
6.2
5.5
5.8
6.0
5.8
(5.0)
5.0
5.0
5.0
5.0
4.0
4.0
(5.6)
(5.6)
After
Exposure
8.6
8.4
(7.0)
9.6
9.3
8.7
11.2
16.8
10.5
14.3
10.8
9.8
9.2
11.4
7.3
6.8
6.5
9.1
18.6
130
ABRV
(in./yr)
I9-?
17.3
8.2
7.6
20.0
10.5
17.0
10.3
20.6
25.1
19.9
10.8
4.8
6.7
6.0
13.5
25.5
77.8
ADWP
(in./yr)
6.1
5.8
3.3
3.8
17.4
2.2
4.0
3.8
8.2
32.6
19.6
17.5
7.3
11.1
2.3
1.8
2.5
5.1
13.0
50.3
ADWP
ABRV
0.67
0.33
0.40
0.51
0.87
0.21
0.29
0.36
0.40
0.70
0.53
1.01
0.48
0.27
0.42
0.38
0.51
0.65
Reference
69
69
69
70
69
69
69
69
69
69
71
Numerical values in parentheses represent estimated values.
influencing the rate of fouling need
further investigation and
quantification. Finally, standardized
O&M procedures need to be
developed to address fouling for
different types of fine pore diffusers.
Process Monitoring
Fouling phenomena can induce
changes in process performance.
Proper process monitoring is
necessary to define system
performance, identify process
problems, and determine system
O&M requirements. Recommended
process monitoring measurements
are listed in Table 11.
Air-side and liquid-side fouling of the
type produced by high local air flux
rates causes an increase in diffuser
headloss at constant air flow rates.
Such increases in wet headloss may
be detected by operating conditions
within the air supply system.
Depending on the specific design
approach, an increase in air supply
system pressure (monitored, for
example, in the blower discharge
header or by increased opening of
the flow control valves) can indicate
an increase in diffuser headloss.
Significant increases in blower
pressure may be indicative of
extensive fouling of major portions of
the diffuser system. For this reason,
blower pressure along with air flow
rate should be monitored on a daily
basis.
While overall system pressure
monitoring serves as a potential
indicator of extreme fouling, it does
not provide a very sensitive indication
of increased diffuser headloss, nor
will it necessarily reveal significant
fouling of the type inversely affected
by air flux rate. For example, a 10-
percent increase in system air
pressure (an apparently minor
increase) may represent a much
higher percentage increase in
diffuser headloss. Even more
importantly, this buildup in headloss
may be having a significant
detrimental effect on OTE. In
addition, fouling of only a portion of
the diffusion system may lead to a
substantial redistribution in air flow
but little increase in overall system
pressure. Consequently, use of a
monitoring technique more sensitive
than system pressure is desirable
and often necessary.
Increased headloss sensitivity is
provided by measuring DWP with a
device of the type illustrated in Figure
17. Individual diffusers are outfitted
with an array of manometers that
allow measurement of the headloss
across the air distribution control
orifice and across the diffuser.
Headloss across the orifice is used in
determining the rate of air flow
through the diffuser, while headloss
across the diffuser media indicates
the degree to which the diffuser has
fouled. By outfitting selected
individual diffusers throughout the
aeration system with DWP
measuring setups, the condition of
various portions of the diffusion
system can be monitored.
32
-------
Table 10.
Representative Ceramic Diffuser Fouling Rates for Industrial Treatment Plants
Plant
Al
Bl
Cl
Dl
El
Gl
HI
Jl
Kl
LI
Diffuser Plant
Type Type
Domes
Discs
Discs
Discs
Discs
Discs
Discs
Discs
Discs
Discs
Discs
Discs
Plates
Plates
Plates
Domes
Pulp & Paper
and Domestic
Pulp & Paper
and Domestic
Pulp & Paper
Pulp& Paper
Pulp & Paper
Domestic &
Industrial
Pharamaceutical
Pharamaceutical
Food Processing
Food Processing
Brewery
Brewery
Domestic &
Industrial
Domestic &
Industrial
Domestic &
Industrial
Domestic &
Industrial
Exposure
Time
(Days)
720
120
16
92
218
21
34
31
420
420
90
90
30
77
58
110
BRV(in. W.G.)
Initial
5.7
9.5
5.8
5.6
6.3
5.8
5.8
5.8
5.6
5.6
6.2
5.9
6.0
6.8
7.0
6.5
After
Exposure
28.5
29.1
48.0
(92)
27.4
28^4
(31.9)
12.0
50.6
61.0
54.0
66.8
129
50.0
12.1
31.6
DWP(in. W.G.)
Initial
5.5
8.9
6.2
5.7
6.0
5.3
6.8
5.8
5.6
5.6
5.1
5.2
7.5
6.0
After
Exposure
50
15.0
12.0
20.0
9.3
12.7
«*»
7.8
42.4
43.0
45.9
48.0
12.4
12.5
ABRV
(in./yr)
141
59.3
962
341
35.3
383
Hah
73.0
39.1
53.0
219
270
1470
208
33.0
83,4
ADWP
(in./yr)
22.6
18.6
132
56.5
5.3
128
t*£ri
24.1
32.0
37.4
186
194
17.2
18.3
ADWP
ABRV
0.16
0,31
0.14
0.17
0.16
0.33
&J&
0.33
0.82
0.71
0.85
0.72
0.10
0.22
Reference
69
72
69
69
69
69
69
69
73
69
Numerical values in parentheses represent estimated values.
Table 11.
Recommended Process Monitoring
Measurements for O&M of Fine Pore
Diffusers
System Pressure
Dynamic Wet Pressure
Bubble Release Vacuum
Specific Air Flow (Air Volume per Unit of
Oxygen Demand Satisfied)
OTE Measurement
Air Distribution Assessment
Visual Observations
Boils
Coarse bubbling
Poor air distribution
Air Source
Liquid Surface
j
Diffuser^ ' .
*\ f
-
Mane
j r
^
m
^
eters
Tap 2
''.': ''.;.';
I] >:','..:; .,'-:x;.'">:i-;c\'rAlJ
, Bubbler
' Pipe
Tap 3
; Bubbles
Orifice
Figure 17.
On-Line Device for Monitoring DWP of Fine Pore Diffusers
33
-------
Since diffuser fouling can
substantially decrease OTE without
significant attendant increases in
backpressure, effective process
monitoring should include other
parameters in addition to diffuser
headloss and system pressure
measurements. Savings in power
obtainable by optimizing diffuser
cleaning schedules will usually
justify the modest equipment and
labor costs required for such
additional monitoring. Candidate
parameters for additional monitoring
include OTE and BRV. Another
candidate parameter is specific air
flow, which is the volume of air
supplied per unit of oxygen demand
satisfied. Air flow rate is measured in
standard m3 (standard cu ft), while
the reduction in oxygen demand can
be measured in terms of BOD or COD
removed plus ammonia nitrogen
oxidized to nitrate nitrogen. The ratio
m3 air supplied/kg 02 demand
satisfied (cu ft/lb) should be routinely
monitored along with aeration basin
DO. An increase in the specific air
flow, a decrease in mixed liquor DO,
or both may indicate a decrease in
OTE, although other conditions such
as an increase in system organic
loading can produce similar effects.
OTE can be measured using a variety
of techniques described in the
previous section. The easiest and
most direct is the off-gas method.
These quantitative measures of
oxygen transfer performance should
also be combined with visual
observations of the system. The
surface pattern on the aeration basin
should be smooth with no air "boils."
Boils arise because of breaks in the
air supply piping that allow large
quantities of air to leak from the
distribution system at one or more
point sources. Such leaks should be
repaired as quickly as possible, both
because of the decrease in OTE that
will occur due to maldistribution of
air along with the release of coarse
bubbles and because of the
possibility of further damage to the
diffusion system.
Nonuniformity of the surface pattern
can signify that portions of the
diffusion system are becoming
plugged. For example, an unusually
low degree of surface turbulence in a
segment of the aeration basin may
indicate restriction of air flow to that
portion of the basin resulting from
fouled diffusers. Cleaning of the
affected diffusers may be required.
The size of the air bubbles evident on
the aeration basin surface can also
provide an indication of fouling.
Loosely adherent biomass on fine
pore diffuser media causes the
formation of large bubbles. Some
degree of "coarse bubbling" is
typically observed at the inlet end of
an aeration basin that may not be a
result of biofouling. It is believed that
the observed phenomenon may
sometimes be due to high local air
flux rates caused by surfactants
contained in the influent wastewater.
These surfactant materials are
quickly adsorbed and/or degraded by
the activated sludge, which restricts
the size of the "coarse bubble" zone.
On the other hand, if biofouling
occurs, the coarse bubble zone can
expand until, in the worst cases, it
covers the entire surface of the
aeration basin. It is recommended
that the surface of the aeration basin
be inspected and photographed when
initially placed in service to become
familiar with the size and appearance
of the bubbles at the inlet and outlet
ends of the basin. This familiarity will
provide a basis for recognizing more
extreme coarse bubbling should it
occur later.
Once problems are identified
qualitatively by visual observation,
quantitative measurements should
be made to confirm the type and
extent of fouling and the type of
cleaning required. Experience
indicates that qualitative
observations can be a valuable tool
when used in conjunction with
quantitative measures of system
oxygen transfer performance.
Preventive Maintenance
A major finding of a survey
conducted by Houck and Boon17 on
dome diffuser plants in the United
Kingdom and Holland was that the
historically excellent O&M
performance of these grid systems
was due to both the knowledge and
diligent care of treatment plant
operators. Routine draining, tank and
grid washdown, and hardware
inspection were standard operating
procedures at all plants surveyed.
Operators were also aware of the
symptoms of problems in the diffuser
system and were quick to respond.
Preventive maintenance is necessary
to keep a fine pore aeration system in
proper working order and at an
optimum level of performance and to
minimize the rate of diffuser fouling.
It should also eliminate the need for
emergency maintenance resulting
from system failure.
Preventive maintenance on the air
filtration and supply system can
virtually eliminate air-side dust and
paniculate fouling of fine pore
diffusers. The guidance provided by
the equipment manufacturer is
generally sufficient for this purpose.
Proper maintenance procedures will
also decrease the frequency of
interruptions in air supply that can
lead to the entry of solids into the
distribution system as discussed
previously. The deposition of solids
on the liquid side of the diffusers and
subsequent penetration into the
upper pores will also decrease with a
decrease in air supply interruptions.
Operation at or above minimum
allowable air flow rates per diffuser
will assist in preventing the
deposition of solids on diffuser
media."'7
Experience indicates that the above
preventive maintenance steps will
help reduce the rate of liquid-side
fouling of fine pore diffusers. Fouling
will still occur (although at a lower
rate), however, and the diffusers will
have to be cleaned periodically.
Diffuser Cleaning
A variety of fine pore diffuser
cleaning techniques are currently
available. They can be broadly
classified as process interruptive or
process noninterruptive. Process
interruptive cleaning techniques
require that the aeration basin be
taken out of service to provide access
to the diffusers, while process
noninterruptive techniques do not
require such access. A further
distinction in cleaning techniques
can be made between those that do
not require removal of the diffusers
from the basin (in-situ) and those
that do require diffuser removal (ex-
situ). All ex-situ techniques are
process interruptive, while only some
in-situ techniques are process
interruptive.
Among the important in-situ cleaning
methods in use today are water
hosing, steam cleaning, and acid
cleaning, all of which are process
interruptive, and gas cleaning, which
is process noninterruptive. Hosing
with either high pressure or low
pressure sprays and/or steam
cleaning will effectively dislodge
loosely adherent, liquid-side
34
-------
biological growths. The application of
14-percent HCI (a 50-percent
solution of 18° Baume inhibited
muriatic acid) with a portable spray
applicator to each ceramic diffuser
following hosing or steam cleaning
and, then, rehosing the spent acid is
effective in removing both organic
and inorganic foulants.67'74'75
Gas cleaning consists of the injection
of an aggressive gas (HCI or formic
acid) into the air feed to the fouled
diffusers. The cleaning agent is
transported to the diffuser by the air
flow where it may dislodge most
foulants. The exceptions are
atmospheric dust deposited on the air
side of the diffuser, which has not
been found to be a significant source
of fouling as previously reported, and
granular material such as silica
deposited on, or incorporated in, a
gelatinous slime adherring to the
liquid side of the diffuser.
Refiring is the most expensive
cleaning technique used and applies
only to ceramic diffuser elements. It
involves removal of the diffuser from
the aeration basin, placing it in a kiln,
and heating it in the same fashion
originally used in its manufacture.
The result is removal of most
foulants from, or incorporated in, the
diffuser element and restoration of
the element to essentially its original
condition.
The quantitative effectiveness of the
various cleaning methods being used
today for the variety of foulants
encountered on different fine pore
media is not well documented.
Furthermore, costs for these methods
are not generally available. Current
research being conducted by
EPA/ASCE78 is attempting to develop
a sound data base on cleaning
technology. As a general premise at
this point in time, it is believed that
most fine pore diffusers (including
virtually all those cited in Tables 9
and 10) can be restored to
substantially original conditions by
one or combinations of the following
in-situ cleaning methods: water
hosing, steam cleaning, gas cleaning,
and acid soaking.69'7*7^
The EPA/ASCE research program*^
may ultimately identify the factors
affecting diffuser cleaning rates and
costs and provide an adequate data
base from which to develop typical
cleaning procedures and schedules
for various generic devices. Until that
occurs, the effects of various
cleaning methods and their required
frequencies should be considered
site specific and should be developed
specifically for each system.
Several methods are available to
measure the effects of diffuser
cleaning on the characteristics of the
diffuser. One approach is to apply the
process monitoring procedures
previously discussed. The effects are
measured as a decrease in system
pressure or diffuser DWP, an
increase in system OTE, or a
decrease in system specific air flow,
i.e., air flow per unit of oxygen
demand satisfied. Techniques can
also be applied to directly measure
the characteristics of individual
diffusers. These include OTE47
chemical analysis of foulants, and
measurements of air flow capacity of
individual diffusers. These latter
techniques include specific
permeability and BRV, which
measure, respectively, the air flow
rate at a specified applied diffuser
headless and the applied headloss
required to induce air flow through
the diffuser.25'87 Pilot-scale cleaning
tests have also been shown to
produce correlative data applicable to
full-scale cleaning situations.77
As discussed above, the
effectiveness and costs of the various
fine pore diffuser cleaning
techniques is an area of active
research. The development of
detailed information in this area
should be forthcoming. However,
there is little doubt that the
incorporation of an effective diffuser
cleaning schedule is a necessary and
justifiable component of any fine
pore diffused aeration preventive
maintenance program.
Cost Tradeoff Analysis
Diffuser cleaning may be
accomplished according to a regular
preventive maintenance schedule
that balances the cost of diffuser
cleaning against the power cost
savings resulting from higher system
OTE and lower system pressure. It
may be possible to generate a
relationship of the type illustrated in
Figure 18. System power costs
decrease with higher OTE due to
lower system air requirements. On
the other hand, the cleaning costs
required to maintain a certain
average OTE increase as the target
OTE increases. This occurs because
an increased cleaning frequency and.
perhaps, a more rigorous cleaning
method may be required to maintain
a higher OTE. The optimum OTE is
the OTE that minimizes the sum of
the power cost and the cleaning cost
required to maintain that OTE, thus
producing the lowest overall system
operating cost. This same concept
may be applied to system pressure.
It should be recognized that Figure
18 is an idealized plot. It presumes,
among other things, that the fouling
rate and its effects remain constant
with time and that the relationship of
cleaning vs. OTE does not
progressively change. A cost-
effective solution to overcoming
these assumptions could consist of
instituting routine monitoring
programs and initiating diffuser
cleaning at set-point changes in OTE
or DWP, whichever occurs first.
35
-------
(I)
o
o
Cleaning Cost to Maintain
Specified OTE
Optimum OTE
OTE
Figure 18.
Idealized Plot of Optimum OTE to Balance Power and Diffuser Cleaning
36
-------
Retrofit
Considerations
The aeration equipment used in
activated sludge service performs the
dual roles of supplying oxygen to the
process and maintaining the mixed
liquor solids in suspension. Retrofit
evaluations should be considered
whenever more efficient, reliable
aeration devices become
commercially available. The principal
benefit of retrofitting an existing
aeration system with fine pore
diffusers is to reduce the air flow
required to provide the oxygen
necessary for effective activated
sludge treatment. This reduction in
required air flow can result in
significant energy savings if proper
design and O&M attention are given
to all system components. Estimates
of electrical energy cost increases of
25 to 35 percent in excess of
inflation by the year 1989 have been
made78.
Other reasons for considering fine
pore retrofitting are to:
replace existing aeration
equipment that has reached the
end of its useful life,
increase treatment capacity to
handle higher influent flow
and/or organic load, and
improve process removals to meet
more stringent National Pollutant
Discharge Elimination System
(NPDES) Permit limits.
Increased oxygen transfer capability
in itself may not alter plant treatment
capacity. Adequate capacity must
also exist in all unit processes and
appurtenances to handle higher plant
loadings. If air supply capacity is the
limiting factor, however, replacing
existing coarse bubble aeration
'systems with fine pore diffusers can
increase plant treatment capacity.
System Design Factors
The design of a fine pore diffused
aeration system to replace an
existing coarse bubble diffused
aeration system or a mechanically
aerated system is essentially the
same as any aeration system design
with some notable exceptions:
The aeration tank dimensions are
fixed as the tanks are already in
existence.
The air supply and air distribution
systems are already in place in the
case of an existing diffused air
system.
The design and actual flows and
organic loadings to the aeration
system are known through review
of the design criteria for the
existing system and recent plant
records.
These "given" conditions must be
reviewed and evaluated during the
design phase of the retrofit project as
outlined in the discussions that
follow.
Wastewater Characteristics
The wastewater characteristics that
impact the design of a fine pore
aeration system are flow, BOD5 load
to the aeration system, and NH3-N
load to the aeration system if the
plant's NPDES Permit requires
nitrification all or part of the year.
These parameters establish the
oxygen demand placed on the
system. Other constituents present in
the influent may significantly affect
oxygen transfer rates and/or
promote the rapid plugging or fouling
of fine pore diffusers. For example,
very hard water, or other sources of
calcium from industrial wastes, may
contribute to the precipitation of
inorganic compounds within the
media of fine pore diffusers. In
addition, surfactants, or surface
active agents, are typically
detrimental to oxygen transfer rates
in mixed liquor.
The capabilities of the new aeration
equipment should be assessed for
satisfying current operating
requirements and future design
conditions of flow and load. Present
and anticipated future NPDES Permit
effluent limitations establish the
mass of oxygen demanding
substances that may be discharged
with the plant effluent.
Methods for determining actual
oxygenation requirements (AOR) are
well documented in the literature.79'80
Existing Facilities
Aeration Tanks:
Aeration tank dimensions and
configurations have a number of
important impacts on proper diffuser
selection. Minimum airflows
required to maintain adequate mixing
in the basins are dictated by tank
geometry. Minimum mixing
requirements with coarse bubble
diffusers in a cross roll pattern are
generally estimated at 0.7 to 1.2
mVhr/m3 (12 to 20 scfm/1000 cu ft)
37
-------
of tank volume.81 Mixing
requirements with fine pore tube
diffusers in the same arrangement
are comparable. Minimum air flow
rates for mixing with fine pore
diffusers in a grid configuration are
estimated in the range of 1.8 to 2.7
m3/hr/m2 (0.1 to 0.15 scfm/sq ft) of
tank surface area.**1''
The number of aeration tanks and the
interconnecting piping design
strongly influences the type of
process modifications that are
possible with a given activated
sludge system. The flexibility to
dewater single tanks or portions of
individual tanks must be carefully
considered in selecting fine pore
diffusion devices and methods for
cleaning them when they become
fouled. Finally, aeration tank depth
and geometry have a direct impact on
OTE for both the existing aeration
equipment and the proposed fine
pore retrofit system as described in
the section on Performance
Characteristics.
Air Supply and Distribution:
If the aeration system to be replaced
is either a diffused air system or
mechanical aerators with sparged
air, air blowers and distribution
piping will already be present.
Obviously, a plant with mechanical
surface aerators will require
installation of new blowers and air
piping to use fine pore diffusers.
Replacement blowers may be
required in some cases due to age or
lack of flexibility or capacity.
The energy savings available with
fine pore diffusers result from a
reduction in the volume of air
required to provide the process with
necessary oxygen. On the other
hand, savings may be partially offset
by the increased operating pressure
in fine pore diffusion systems. The
reduction in air flow, if achieved, will
result in operating fewer blowers
and/or operating the same blowers
at different points on their
performance curves.
Most air supply blowers in municipal
treatment plants are either single- or
multi-stage centrifugal types or
rotary positive displacement units.
The efficiency of both single- and
multi-stage centrifugal blowers can
vary from more than 80 percent to
less than 40 percent, depending on
the blower itself and the operating
combination of discharge volume and
discharge pressure. Estimating input
horsepower for these units should
always be done using actual
performance curves generated from
power factor and power consumption
measurements at the plant.
Potential power savings from
operating at reduced air flows with
fine pore diffusers can be completely
negated by a decrease in blower
operating efficiency resulting from
the reduction in air flow itself. It is
critical, therefore, to accurately
estimate blower horsepower for the
actual conditions that will prevail
with the retrofitted system and not by
using compression formulae that
require an estimate of blower
efficiency to determine power for a
given discharge condition.
Centrifugal blower capacity should be
regulated to the extent possible by
throttling on the inlet side, as
significant power savings are
available at any duty point. A method
for estimating blower brake
horsepower for a centrifugal blower
with inlet throttling is illustrated in
Figure 19. Rotary positive
displacement blower capacity cannot
be varied by throttling for all practical
purposes. Air flow to the aeration
system can be changed only by
operating more or less units, varying
speed, or by "blowing off" some of
the air to atmosphere. Wasting air to
the atmosphere may reduce the
actual air flow to the aeration
system, but will not reduce the power
consumption of the blowers. Here
again, it is crucial that blower
horsepower be estimated using
actual blower performance curves
and under the anticipated actual
conditions of operation because
efficiency for rotary positive blowers
is by no means constant from one
unit to another.
Blower peak capacity should be
checked at the maximum anticipated
inlet temperature and the minimum
expected pressure at the blower inlet
flange. The overall blower system
integrity should be evaluated in
terms of its turndown capability and
flexibility of operation.
The existing air distribution piping
can in general be reused with some
reservations. Because air flow rates
will decrease with enhanced OTE,
the size of the existing blower
discharge headers and air mains that
deliver air to the tanks will normally
be sufficient. Depending on the type
and arrangement of fine pore
diffusion equipment, the individual
drop pipes into the tanks may also be
large enough. The air distribution
system should be checked for
capacity, corrosion, and integrity.
For new systems that will employ full
floor coverage grid configurations
and fewer air drops per aeration tank,
each drop pipe should be sized such
that air velocities at average flow
rates are less than 900 m (3,000
fjXmin|to avoid excessive air
pressure drop in these reaches of
pipe.7
Air Filtration:
Blower inlet filters will effectively
remove contaminants from the
outside air but will not protect the
diffusers from dirt, rust, scale, or
other debris that might already be in
the downstream piping. It is
recommended, therefore, that careful
consideration be given to the use of
in-line filters. In some cases, it may
even be desirable to locate filters
adjacent to the air drops into the
aeration system so that new
corrosion-resistant pipe need only be
located between the filters and the
diffusers. Existing piping systems
composed of galvanized or stainless
steel will present little danger of
present or future rust or scale
particles plugging the diffusers if
blower inlet filters are selected.
Existing painted or uncoated steel or
iron pipe, however, should be reused
only with extreme caution unless in-
line filters are installed downstream.
Diffuser Selection
Selection of the proper fine pore
diffuser for a given retrofit situation
will depend on the oxygen transfer
capabilities of the device within the
constraints of the existing aeration
tank geometry, existing blower
characteristics, wastewater
properties, desired O&M
requirements, and the desired air
flow control scheme. Refer to the
discussions in previous sections of
this report.
Economic Analysis
General
The factors used in the design of a
fine pore diffusion retrofit system
also comprise the basis for
determining its economic viability.
Installation of fine pore equipment
38
-------
Barometer: 14.3 psia
Inlet Temperature: 38°C (100°F)
Blower
Horsepower Curve
Pt. A: 3,030 icfm
at 21.2
_Qc = 4,180 icfm
Pt. B: 4,060 icfm
at 20.3 psia
Blower
Performance
Curve
HPT = Horsepower with inlet throttling
= HPdQr/Qc)
HPC = Horsepower from blower performance curve
QT = QA at throttled position
Qc = QA from blower performance curve
Pt. A: HPT = 1 40(3,030/3,1 50) = 1 35 bhp
Pt. B: HPT = 158(4.060/4,180) = 153 bhp
1000
2000 3000 4000
Blower Inlet Air Flow, QA (icfm)
5000
6000
Figure 19.
Method for Estimating Centrifugal Blower BHP with Inlet Throttling (Power
Computation per Reference 82)
39
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should be undertaken only if a
reasonable return on investment can
be foreseen. The cost effectiveness of
retrofitting is most appropriately
based on present day flow and
loading to the plant rather than
anticipated future increases and
should consider the total present
worth of the investment as well as
simple payback.
Determining System Cost
The cost of a fine pore retrofit
includes the new aeration
equipment, modifications to the
existing air filtration system, air
distribution piping modifications if
required, and replacement or
modification of the air supply
equipment as necessary. If the
existing system uses mechanical
aerators, a completely new air supply
and distribution system will obviously
be needed. The cost of installation,
either by a private contractor or the
owner, must also be included.
Determining Annual Savings
Net annual savings in operating costs
with fine pore diffusers consist of
annual electrical (or other) energy
savings resulting from the reduction
in air flow to the aeration tanks, less
any additional O&M costs associated
with the fine pore diffusers. Annual
power cost savings can be
determined by comparing the
average input power required for
operating the existing system and the
estimated average input power for
operating the new system. Power
determinations should be made using
actual blower performance curves.
Power requirements should also be
estimated with varying inlet
temperatures depending on the time
of year. One method of accounting
for the variables involved is to
develop average air flow
requirements for the existing and
proposed aeration equipment on a
monthly average basis and compute
horsepower input monthly. Minimum
air flow requirements for mixing
should be determined and compared
with estimates of air flow for
satisfying oxygen demand alone. This
is especially important in plants with
NPDES Permits that specify seasonal
nitrification. When mixing requires
more air than necessary to satisfy
oxygen demand, the higher air flow
rate should be used to determine
input power for the period in
question.
Determining Additional O&M
Costs
Additional O&M costs with fine pore
diffusers include diffuser monitoring
and cleaning and air filter
maintenance and/or replacement.
Liquid-side diffuser cleaning is the
subject of other sections of this
report, and the method used will
depend on the type of fine pore
device selected and the particular
operating characteristics of the plant
in question. The cost of the labor and
materials for diffuser cleaning must
be accounted for in the economic
analysis. In addition, labor and
material for monitoring diffuser
conditions and air filter maintenance
and/or replacement must also be
factored into the economic analysis.
Determining Economic Viability
Economic viability of a fine pore
diffuser retrofit project can be
evaluated by comparing the present
worth of all future energy savings
minus the O&M costs of the fine pore
diffuser system against its initial
capital cost. In determining the
present worth of both future savings
and costs in the following example, a
"real" (inflation-free) discount rate
will be used. This avoids the need to
inflate future prices. An inflation-free
discount rate, r, can be obtained from
a "market" discount rate, R, by
adjusting the latter for the inflation
rate, I:
For a planning period of n years, the
net present worth of project savings,
NPWS, can be calculated as:
NPWS = SPWF(r,n)(EAS) - ICC
where:
SPWF(r.n) = series present worth
factor at discount rate 4
over n years
SPWF = [(1+r)n-1]/[r(1+r)n]
EAS = estimated annual
savings
ICC = initial capital cost
For the project to be economically
viable, NPWS must be positive over
the planning period.
EAS can be written as follows:
EAS = AES - AROM - EACC
where:
AES = annual energy cost
savings
AROM = annual cost of routine
O&M on the fine pore
diffuser system, e.g., air
filter cleaning, air flow
monitoring, etc.
EACC = equivalent annual cost
of diffuser cleaning
If the diffusers are cleaned every t
years at a cost of DCC, the EACC can
be expressed as:
EACC =
In this manner, the effect of
alternative diffuser cleaning
frequencies can be analyzed.
An equivalent payback period can be
defined as the length of the planning
period that yields zero net present
worth savings. From the expression
above for NPWS, this occurs when:
SPWF(r,n)=ICC/EAS
A table of series present worth
factors at discount rate r can then be
consulted for the value of n with
entry closest to the computed SPWF.
This payback period should not be
interpreted as one based on actual
cash flow since it incorporates the
equivalency effects of discounting.
Example Evaluation
A generalized example of a simplified
economic evaluation of fine pore
diffuser retrofitting is presented
below. The energy savings analysis is
for one month only. For a complete
analysis, it is recommended that
appropriate costs be determined for
each month in a 12-month period.
40
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Flow
Present average 5,0 mgd
Future average 7.5 mgd
Primary effluent BOD5 120mg/L
Primary effluent NH3-N 15mg/L
NPDES Permit Limit
BOD5 30 mg/L
NH3-N 2.0 mg/L
Average Mixed Liquor
DO 2.0 mg/l
Average Water
Temperature 18°C(64°F)
Blower Inlet Pressure 14.3 psia
Blower Inlet
Temperature 38°C(100°F)
Aeration Tanks
Number 4
Dimensions 120 ft x 35 fix
14 ft SWD
Average Power Cost
(including demand
charge) S0.06/kWh
Existing Aeration System (Coarse Bubble Spargers)
ffaSOTE 5.0 percent
OTEt 4.0 percent (off-gas measurement)
C*2o 10.1 mg/L (see Performance Characteristics
Section)
AOR 7,500 Ib/d
Mass flow of air 804,400 Ib/d
Inlet air flow 8,120icfm
Air for mixing 3,530 icfm (15 icfm/1,000 cu ft)
Proposed Aeration System (Dome/Disc Diffusers, Full Floor Grid)
OTE( 10.8 percent
C*,2o 10.2 mg/L (see Performance Characteistics
Section)
AOR 7,500 Ib/d
Mass flow of air 300,000 Ib/d
Inlet air flow 3,030 icfm
Air for mixing 2,015 icfm (0.12 icfm/sq ft)
For this month, mixing does not
control, so power cost estimates will
be made using the air flow rates
required for process oxygen transfer.
System pressure required for blower
operation should be determined by
developing a system head curve.
However, pressures are estimated for
this example as follows:
Submergence
Diffuser head loss
System losses (estimated)
Total system pressure
Coarse Bubble
12ft-0. in.
Oft - 8 in.
1 ft -3 in.
13ft- 11 in.
= 6.0 psig
(20.3 psia)*
Fine Bubble
13 ft -3 in.
1 ft -4 in.
1 ft - 3 in.
15 ft -10 in.
= 6.9 psig
(21. 2 psia)"
*Local barometric pressure = 14.3 psia.
41
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Estimated Blower Input Horsepower
(from Figure 19):
Existing spargers = 306 bhp,
2 blowers at 4060 icfm each
Proposed fine pore = 1 35 bhp,
1 blower at 3030 icfm
Input power in kW must take into
account blower motor efficiency and
the blower/motor flexible coupling
efficiency. For this example, motor
efficiency is assumed at 90 percent,
coupling efficiency at 95 percent.
Estimated Power Costs:
Existing = $11,918
Proposed = $ 5,258
Estimated cost savings for month =
$6,660
If this month was typical of the entire
year, annual energy savings,^A
would be estimated at $79,920.
Actual savings, however, will vary
from month to month depending on
plant operating conditions.
For this example, no change in
blowers is recommended.
Development of detailed construction
costs and annual maintenance costs
is beyond the scope of this report.
The initial capital cost is estimated to
be $150,000, including the new fine
pore aeration system, new efficient
air filters, installation, engineering,
and contingencies. Additional annual
routine O&M costs, AROM, including
air filter cleaning and replacement
are estimated to be $2,620. Diffuser
cleaning is assumed to occur every 2
years at a cost, DCC, of $10,000
(2,000 diffusers at $5/diffuser). An
inflation-free discount rate, r, of 8
percent is used.
The estimated annual savings, EAS,
for this retrofit project are computed
as follows:
EAS = $79,920-2,620-(10,000)
(0.08)/[(1+0.08)2-1]
EAS = $72,492
The net present worth savings,
NPWS, for various project periods, n,
are summarized below:
Year
1
2
3
4
5
10
15
20
SPWF
(0.08,n)
0.926
1.783
2.577
3.312
3.993
6.710
8.560
9.818
Net Present Worth
Savings
$72,492 (SPWF) -
$150,000
-82,872
-20,747
36,812
90,094
139,461
336,421
470,531
561,726
As shown by the progressive growth
of the project's net present worth, ->av
the investment breaks even after
slightly longer than 2 years. If the
results of a simplified analysis of the
type presented above are favorable, a
more rigorous analysis considering
actual variations in daily air flow
requirements, DO and air flow
control schemes, and other operating
considerations should be undertaken.
42
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Ongoing Studies
Significant progress has been made
in the last 5 years to better delineate
the design, testing, maintenance, and
control requirements of fine pore
diffused aeration systems. The
discussions in this report clearly
indicate, however, that many gaps
still exist in our complete
understanding of these systems. For
example:
How do the various fine pore
aeration systems perform in
processwaters, both in absolute
terms and relative to each other?
How can clean water test data
best be translated to field
conditions?
What is the behavior of fine pore
diffusers with respect to long-
term, liquid-side fouling?
What strategies will cost
effectively control and maintain
these systems while still yielding
acceptably high field OTEs?
Several research projects have been
recently completed or are presently
underway in the United States,
Canada, and the United Kingdom,
addressing the above and other
questions related to fine pore
aeration system design, performance,
operation, maintenance, installation,
control, and costs. Among the
significant government-sponsored
projects are those briefly described
below.
In addition to the above-listed
government-sponsored studies,
numerous shop and field tests are
being conducted every day by
consultants, manufacturers, and
owners of wastewater treatment
facilities to better characterize fine
pore diffuser behavior. This
information also will be disseminated
into the public sector as it becomes
available.
In the preparation of a
comprehensive design information
manual on fine pore diffused aeration
systems, currently scheduled for
completion in late 1987 or early 1988,
the ASCE Committee on Oxygen
Transfer will make every effort to
review all pertinent technical data
and information in the literature as
well as in unpublished reports made
available from consulting
engineering firms, municipal
treatment plant owners and
operators, universities, and research
organizations.
Country/Performing
Organization:
Sponsors:
Topic/Dates:
Objectives/Scope:
Site:
United Kingdom - Water Research Centre
Department of Energy - United Kingdom; Environmental
Canada - Canada; EPA - United States
Full-Scale Optimization of Fine Pore Aeration to Produce
Energy Savings - 1982 to 1985
To modify the design of ceramic dome diffuser systems
in existing aeration tanks at a large treatment plant to
significantly reduce the energy used in treating waste-
water and to increase the throughput capacities of the
modified tanks while still meeting specific effluent
objectives.
To make an economic comparison of the modified vs. the
existing systems to produce a fully nitrified effluent of
high quality 95 percent of the time in one train and a
non-nitrified effluent of 30:20 mg/L (SS:BOD) 95
percent of the time in a second train.
To obtain design information that will enable new and
retrofitted plants equipped with ceramic dome diffuser
systems to achieve higher OTEs and greater throughput
capacities.
Rye Meads (England) Sewage Treatment Works (121,000
m3/d = 32 mgd); parallel process trains
43
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Country/Performing
Organization:
Sponsors:
Topic/Dates:
Objectives/Scope:
Site:
Canada - Wastewater Technology Centre
Environment Canada - Canada; EPA - United States
Full-Scale Demonstration of Energy Savings, Opera-
tional Benefits, and Instrumentation/Control in Auto-
mated Aeration Systems - 1 982 to 1987
To demonstrate energy savings and improved operation
with DO set-point maintenance through automated air
distribution and effective blower control.
To determine and optimize the true cost of maintaining
the integrity of on-line instrumentation and control
hardware.
To conduct cost/benefit analyses with respect to the
benefits of implementing automated DO control.
This project is currently evaluating coarse bubble
diffused aeration systems. It is anticipated that the
systems will be retrofitted with fine pore diff users before
the project is completed.
Tillsonburg (Ontario) Wastewateer Treatment Plant
(6,000 mVd = 1.6 mgd); parallel process trains
Country/Performing
Organization:
Sponsor:
Topic/Dates:
Objectives/Scope:
Site:
United States - Madison (Wisconsin) Metropolitan
Sewerage District
EPA
Investigation of Biological Fouling of Ceramic Fine Pore
Diffusers and the Effectiveness of Several Cleaning
Strategies - 1982 to 1985
To evaluate the effects of selected aeration tank control
parameters (DO, total and soluble organic load, and gas
flow per diffuser) on the biological fouling of glass-
bonded, ceramic fine pore diffusers.
To evaluate the effects of these parameters on diffuser
characteristics and OTE.
To evaluate the effectiveness of a number of cleaning
methods on fouled ceramic diffusers.
Madison (Wisconsin) Nine Springs Wastewater Treat-
ment Plant; 3,800- and 5,600-L (1000- and 1500-gal)
pilot aeration tanks; selected full-scale aeration trains;
laboratory studies of cleaning
44
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Country/Performing
Organization:
Sponsor:
Topic/Dates:
Objectives/Scope:
Site;
United States - Los Angeles County Sanitation Districts
EPA
Comparative Full-Scale Evaluation of Two Types of Fine
Pore Diffusion with Jet Aeration - 1980 to 1985
To compare over a 6-month period the field oxygen
transfer and fouling performance of ceramic dome
diffusers (grid configuration), porous plastic tube
diffusers (dual spiral roll configuration), and directional
flow jet aerators.
To evaluate the long-term process performance, oxygen
transfer performance, and fouling characteristics of the
most efficient of the above systems (which was deter-
mined to be the dome diffuser system in the 6-month
preliminary phase) at varying volumetric organic loads
and under two different flow regimes (plug flow and step
aeration).
Whittier Narrows (California) Water Reclamation Plant
(58,000 mVd = 15 mgd); parallel process trains
Country/Performing
Organization:
Sponsor:
Topic/Dates:
Objectives/Scope:
Site:
United States - ASCE
EPA
Design Information on Fine Pore Diffused Aeration -
1985 to 1988
To evaluate the existing data base on the performance of
fine pore diffused aeration systems in clean and process
waters.
To carry out field studies at a number of municipal
wastewater treatment facilities employing a variety of
fine pore aeration systems to fill perceived data gaps in
the design and performance of these systems.
To evaluate the field effectiveness of available fine pore
diffuser cleaning technologies.
To conduct economic analyses of the most promising
cleaning technologies and delineate factors affecting
the selection of best methods of cleaning for site-specific
conditions.
To prepare an interim guidance report on fine pore
diffused aeration systems.
To prepare a final comprehensive design information
manual on fine pore diffused aeration systems.
ASCE (New York, New York) administers project under
technical direction of the Steering Subcommittee of the
Committee on Oxygen Transfer with studies conducted
at a number of selected sites throughout the United
States and Europe.
45
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When an NTIS number is cited in a
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National Technical Information
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5285 Port Royal Road
Springfield, VA 22161
(703)487-4650
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This Summary Report was prepared by the American Society of Civil
Engineers (ASCE), Committee on Oxygen Transfer, New York, NY, under
Cooperative Agreement No. 812167 between U.S. EPA and ASCE. The report
was edited by Committee Chairman William C. Boyle, University of Wisconsin,
Madison, Wl. Principal contributors were Thomas A. Allbaugh and S. Joh Kang,
McNamee, Porter & Seeley, Ann Arbor, Ml; Glen T. Daigger, CHUM Hill, Denver,
CO; Lloyd Ewing and David T. Redmon, Ewing Engineering Company,
Milwaukee, Wl; Gregory L. Huibregtse, Rexnord, Inc., Milwaukee, Wl; and
Wayne L. Paulson, University of Iowa, Iowa City, IA. Review comments were
solicited from the entire Committee. Detailed peer review was provided by
Walter G. Gilbert, Office of Municipal Pollution Control, U.S. EPA, Washington,
DC; Henryk Melcer, Environment Canada, Burlington, Ontario; and H. David
Stensel, University of Washington, Seattle, WA. Co-project officers for U.S. EPA
were Richard C. Brenner, Water Engineering Research Laboratory, Cincinnati,
OH, and Denis J. Lussier, Center for Environmental Research Information,
Cincinnati, OH.
This document has been reviewed in accordance with U.S. Environmental
Protection Agency policy and approved for publication. Mention of trade names
or commercial products does not constitute endorsement or recommendation for
use.
50
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