United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-88/001 Sept. 1988
v°/EPA Project Summary
Survey and Evaluation of Fine
Bubble Dome and Disc Diffuser
Aeration Systems in North
America
Daniel H. Houck
A study of 19 North American
municipal activated sludge plants
equipped with either ceramic fine
bubble dome or disc diffuser
aeration systems was carried out to
better define the oxygen transfer
performance and operation and
maintenance (O&M) requirements of
these systems and the proper
approaches to their design. Two of
the plants were located In metro-
politan Toronto, Ontario. The re-
maining 17 were located in the
United States. The plants were
selected on the bases of size and
age of the system, location, and
quality of available data from
installation lists provided by the
principal manufacturers of dome and
disc diffuser equipment. All treat
predominantly domestic wastes,
though some have significant
industrial flows as well.
Data on process design, influent
and effluent wastewater char-
acteristics, aeration power and air
flow, and O&M experiences were
requested from each plant. These
were supplemented as needed by
on-site investigations and Inter-
views of plant personnel.
The results of this work indicate
that, although the North American
experience has not been as
uniformly satisfactory as that of
overseas users, ceramic fine bubble
aeration technology can be
successfully implemented here.
Those plants that have avoided major
design flaws and are operated
conscientiously are performing quite
well. Most of the problems encoun-
tered would require little money or
time to correct. Better training of
plant operators and improved design
practices are urgently needed.
This Project Summary was
developed by EPA's Water Engineering
Research Laboratory. Cincinnati, OH,
to announce key findings of the
research project that is fully
documented in a separate report of
trie same title (see Project Report
ordering information at back).
Introduction
Interest remains high in the
wastewater treatment industry in reduc-
ing power consumption and costs of
energy-intensive treatment processes.
Aeration for secondary and tertiary ac-
tivated sludge treatment, often account-
ing for 50% or more of total plant energy
consumption, continues to be a primary
focus in the effort to reduce energy
costs. Consequently, expanded use of
-reportedly more efficient aeration equip-
ment has been experienced in North
American plants in recent years. It was
decided that enough new ceramic dome
and disc fine bubble aeration systems
had been installed and operated for a
sufficient period by late 1982 to justify
undertaking a domestic survey and
evaluation of the technology.
The study's primary objectives were
to assess the oxygen transfer per-
formance and O&M history of ceramic
-------
dome and disc diffused aeration systems
in North America and to enumerate and
discuss the principal design factors
affecting that performance. To allow
comparison with an earlier foreign study
of U.K. and European ceramic dome
systems (Houck, D.H. and A.G. Boon.
Survey and Evaluation of Fine Bubble
Dome Diffuser Equipment. EPA-600/2-
81-222, September 1981), the study
approach and assessment methodology
used were quite similar to that employed
previously.
Characteristics of Aeration
Systems
Genera/
All 19 plants evaluated were
equipped with either ceramic dome or
disc diffusers supplied by one of the
following manufacturers:
"Envirex, Inc., Milwaukee, Wl
Gray Engineering Group, Ltd.,
Markham, Ontario, Canada
Norton Company, Worcester, MA
Sanitaire-Water Pollution Control
Corp., Milwaukee, Wl
The Gray and Norton systems featured
18-cm (7-in.) diameter dome diffusers
of the type studied in the earlier U.K.
survey. Envirex and Sanitaire manu-
facture disc diffusers. The Sanitaire disc
diffuser is 22 cm (8.7 in.) in effective
surface diameter; the Envirex disc is
slightly larger. A list of the surveyed
plants along with background information
is given in Table 1.
Design and Operation
Aeration system design and
operating data for the 19 plants visited
are summarized in Table 2. Thirteen of
the systems inspected were being
operated in the plug flow mode. Another
four were utilizing the step feed
configuration, while one was using both
the plug flow and step feed operating
regimes in different tanks. One plant was
employing the complete mix operating
mode.
Several of the plants had aeration
tanks described by their designers as
complete mix that were clearly
functioning in the plug flow mode (e.g.,
Riverside). Only three plants - West
Bend, North Buffalo, and Coulton - were
being operated in multiple-pass, plug
flow configurations that resulted in
length-to-width (L/W) ratios greater
"Mention of trade names or commercial products
does not constitute endorsement or recom-
mendation for use.
than 15. In contrast, over half of the U.K.
and Dutch plants evaluated in the first
survey project had aeration basin L/W
ratios of more than 15. High L/W ratios
create design problems in attempting to
match oxygen demand with a diffuser
layout of appropriate tapered density that
will not yield zones of either under or
overaeration.
Four of the 13 plants with plug flow
basins were designed with uniform
diffuser configurations; the other 9 were
designed with tapered aeration. A
uniform diffuser density substantially
increases the difficulty of accurately
matching oxygen demand with oxygen
(air) supply in a plug flow aeration basin.
Zones of over and/or underaeration are
virtually impossible to avoid in such a
situation. The problem becomes acute in
multiple-pass plug flow basins with very
long L/W ratios.
The recommended ranges of spe-
cific air flow rates for dome and disc
diffusers are 0.24 to 0.94 L/sec (0.5 to
2.0 scfm) and 0.24 to 1.42 L/sec (0.5 to
3.0 scfm), respectively. Headless across
the media becomes very small at
specific air flows less than the recom-
mended lower limits, making it difficult to
obtain uniform air distribution across the
entire diffuser surface. Power costs
generally become uneconomic if the
recommended upper operating limits are
exceeded for substantial periods
because of decreased oxygen transfer
efficiency and increased pressure on the
blowers. The average air flow per diffuser
was within the recommended ranges for
13 of the 17 plants with available air flow
operating data. Four facilities were op-
erating below their recommended
ranges.
Diffuser density and air flow rates
per diffuser varied widely, reflecting the
lack of any standardized approach for
designing dome and disc diffuser aera-
tion systems in North America. Minimum
power levels were generally much higher
than those found in the U.K. plants. No
problems with solids settling in the aer-
ation tanks were reported by any of the
plants evaluated.
Process Performance
Aeration system process per-
formance data are presented in Table 3
for the 19 plants surveyed. Most of the
plants were not designed for nitrification,
though it was occurring in a number of
them because they were underloaded or
as a result of the mode of operation
selected by plant personnel. Several
plants featured two-stage activated
sludge treatment. Most of the plants
were operating well below design flows
and were producing very high quality
effluents.
Air flow varied from 22 to 112 m3/kc;
total 5-day biochemical oxygen demanc
(TBOD5) applied (350 to 1,800 ft3/lb) jr
the North American plants but generally
averaged less than that for the U.K
plants, even where nitrification was being
practiced. In general, the non-nitrifying
plants averaged less than 62 m3 aii
supplied/kg TBOD5 applied (1,000 ft3/lb]
unless there were problems with the
aeration equipment. Nitrifying plants
averaged much higher with the exception
of the Village Creek plant, where the aii
flow data may have been questionable,
Volumetric loadings in the North
American plants were similar to those
found in the United Kingdom, but food-
to-microorganism (F/M) loadings were
somewhat higher here, ranging from 0.03
to 0.59 kg TBODs/day/kg mixed liquor
suspended solids (MLSS) vs. 0.05 to 0.45
in the United Kingdom. MLSS levels in
the North American plants were usually
less than 3,000 mg/L. Very little
consistency was noted in basic process
parameters among the North American
plants, even between similar nitrifying or
non-nitrifying plants.
Several disc-equipped plants had
been originally designed and specified
for the smaller dome diffusers. Sub-
sequently, disc units were purchased and
substituted for the domes on a one-to-
one basis. At West Bend, this resulted in
substantial overdesign of the aeration
system such that it could not be operated
efficiently at current loadings. Plant
operators reported that they could not
turn down air flow sufficiently to reduce
the mixed liquor dissolved oxygen (DO)
level below 6 to 9 mg/L and still maintain
recommended minimum diffuser specific
air flow rates.
Oxygen Transfer Performance
Method of Measuring Oxygen
Transfer Performance
Considerable development work has
been conducted in recent years for
measuring oxygen transfer performance,
including steady and non-steady state
methods and off-gas analysis. For this
project, since no direct oxygen transfer
field measurements were made, oxygen
transfer performance was estimated
using empirically derived oxygen
consumption values based on TBODs
removal and ammonia nitrogen (NH4-N)
oxidized. This oxygen mass balance
technique was developed by Boon anqj
Hoyland of the British Water Research
-------
Table 1. Characteristics of Surveyed Plants
Plant Location
(Plant Name)
United States
Coulton, CA
Greensboro, NC
(North Buffalo)
Howard County, MD
(Little Patuxent)
Levittown, PA
(Lower Bucks County)
Rialto, CA
Riverside, CA
West Bend, Wl
Whither, CA
(Whither Narrows)
Berlin, NH
Berlin, Wl
Fort Worth, TX
(Village Creek)
Ltitz, PA
Meriden, CT
Montpelier, VT
Houston, TX
(Park Ten Municipal
Utilities Dist.)
Ridgewood, NJ
Seymour, Wl
Canada
Toronto, Ontario
(Highland Creek)
Toronto, Ontario
(Humber-North plant)
Aeration System Description
Partially nitrifying, concentric step feed basins with sludge
reaeration, uniform diffuser layout, Gray domes
Nitrifying, 2-pass plug flow basins following ist-stage
roughing biofilters, tapered diffuser layout, Envirex discs
Nitrifying (summer), two-stage system, 2-pass step feed
1st stage basins, 1-pass plug flow 2nd-stage basins
(operated in summer only), uniform diffuser layout both
stages, Norton domes
Non-nitrifying, 1-pass plug flow basins, tapered diffuser
layout, Norton domes
Nitrifying, 1 -pass step feed basins, uniform diffuser
layout, Gray domes
Partially nitrifying, 1-pass plug flow basins, tapered
diffuser layout, Norton domes
Nitrifying, 5-pass plug flow basins following ist-stage
roughing biofilters, uniform diffuser layout, Sanitaire discs
Non-nitrifying, 1 -pass plug flow basins, tapered diffuser
layout, Sanitaire discs
Unknown nitrifying, 1-pass plug flow basins, tapered
diffuser layout, Norton domes
Partially nitrifying, 1-pass step feed basins, uniform
diffuser layout, Sanitaire discs
Partially nitrifying, i -pass plug flow basins, tapered
diffuser layout, Norton domes
Nitrifying, two-stage system, i-pass plug flow basins
both stages, tapered diffuser layout both stages, Norton
domes
Nitrifying, two-stage system, complete mix basins both
stages, uniform diffuser layout both stages, Sanitaire discs
Non-nitrifying, J-pass plug flow basins, uniform diffuser
layout, Sanitaire discs
Unknown nitrifying, 2-pass step feed basins, uniform
diffuser layout, Norton domes
Partially nitrifying, 1-pass plug flow basins, tapered
diffuser layout, Gray domes
Nitrifying, concentric plug flow basins, uniform diffuser
layout, Sanitaire discs
Nitrifying, 1 -pass plug flow basins, uniform diffuser
layout, Norton domes
Partially nitrifying, 1-pass plug flow basins, tapered
diffuser layout,Norton domes
WW Flow (mgd)'
Design
5.4
16.0
15.0
12.0
2.0
13.8
9.0
15.0
2.2
1.6
40.0
3.5
11.6
3.97
1.0
4.5
0.61
4.8
31.2
Average"
3.2
12.0
8.9
8.0
2.35
9.0
4.5
12.5
1.7
0.8
54.5
0.9
7.1
1.5
0.2
3.0
0.54
3.0
24.5
Avg. % Removal
T8ODS
96
95
97
93
94
98
98
90
94
96
95
98
95
92
U
90
98
98
94
TSS
94
95
97
90
93
98
98
90
94
98
96
98
95
95
U
90
99
96
94
= Unknown
1 mgd = 0.044 m3/'sec
At time of plant visits from late-1982 to mid-1983
Centre based on the work of Eckenfelder
and 0'Conner (Biological Waste Treat-
ment. Pergammon Press, New York, NY,
1961) for use on the earlier survey. It has
an estimated accuracy of ± 20% if
reliable influent, effluent, and mixed
liquor concentration data are available
over a meaningful operating period along
«ith dependable records of wastewater
Jow and air supply. The limits of
accuracy become much broader if his-
torical data are questionable or unreliable
and/or if air flow control is poor.
The oxygen mass balance technique
used in this study is represented by the
following equation:
Oygen consumed (Ib/day) (1 )
= R(BODS- BODe)
where:
4.3 (Ns-Ne)
R = units of oxygen consumed
by heterotrophs per unit of
TBOD5 removed in Ib/day
and is described by the
equation:
R = 0.75 + 0.05/(F/M) (2)
with an assumed maximum
R value of 1.5
-------
BODS = reactor influent TBOD5
Ib/day
BODe = secondary effluent TBOD5,
Ib/day
Ns = reactor influent NhU-N,
Ib/day
Ne = reactor effluent NH4-N,
Ib/day
F/M = food-to-microorganism
loading, day1, based on
MLSS under aeration
In contrast to the U.K. experience, no
North American plants were equipped
with lead-stage anoxic zones for
promoting nitrate reduction and oxygen
recovery using the denitrification pro-
cess. Consequently, the third term of
Boon and Hoyland's equation, which
accounts for the oxygen credit (chemical
oxygen released to the mixed liquor that
lessens the amount of DO needed)
derived from denitrification, was not
needed in this study and is omitted from
Equation 1.
An adiabatic compression equation,
with corrections for equipment
efficiencies, was used to estimate blower
power consumption when only air flow
data were available. Compressor effi-
ciency was assumed at 70%, coupling
efficiency at 95%, and motor efficiency
at 92%. Factoring in these assumptions
yields the following relationship:
Wire Power = 0.276 Q
Req'd (kW)
(3)
P
.
1
where:
Q = air flow, scfm
Pa = ambient air pressure, psi
PI = piping system headless,
psi
DI = diffuser headloss, psi
SH = static head above diffuser,
psi
Pi = inlet pressure, psi
Diffuser stone headloss was assumed to
be 0.3 psi, and total piping system
headloss was assumed to be 0.3 psi.
Ambient pressure was assumed to be
14.7 psi, and inlet pressure was taken as
14.6 psi.
Aeration Efficiency Estimates
Oxygen transfer performance is
typically expressed in terms of aeration
efficiency, which is defined as the mass
transfer of oxygen per unit of line (or
wire) power input. Mass balance
estimates of oxygen consumption and
either measured or estimated blower
power consumption, as described in the
previous section, were utilized to
calculate estimated aeration efficiency
values for each plant visited except Lititz
and Park Ten as shown in Table 4.
A wide variation is evident in the
estimated aeration efficiencies of the
North American plants, ranging from 0.63
kg O2/kWh (1.03 Ib/wire hp-hr) for
Humber to 2.52 kg 02/kWh (4.15 Ib/wire
hp-hr) for Ridgewood. The average for
the 17 plants for which aeration
efficiencies could be calculated was 1.51
kg 02/kWh (2.49 Ib/wire hp-hr). This
compares favorably with the average
estimated aeration efficiency of 1.48 kg
02/kWh (2.43 Ib/wire hp-hr) for the 16
plants from the earlier survey for which
adequate information was available to
prepare estimates.
Of the above 17 North American
plants, six were totally nitrifying at the
time of the study (North Buffalo, Rialto,
West Bend, Meriden, Seymour, and
Highland Creek), six more were partially
nitrifying (Coulton, Riverside, Village
Creek, Ridgewood, Humber, and Berlin,
Wl), four were not nitrifying at all (Little
Patuxent, Lower Bucks County, Whittier
Narrows, and Montpelier), and no
nitrogen data were available for one plant
(Berlin, NH). The estimated average
aeration efficiency was 1.59 kg 02/kWh
(2.62 Ib/wire hp-hr) for the six nitrifying
plants, 1.45 kg 02/kWh (2.38 Ib/wire hp-
hr) for the six partially nitrifying plants,
and 1.32 kg 02/kWh (2.17 Ib/wire hp-hr)
for the four non-nitrifying plants.
The above results suggest that
nitrifying systems are more energy
efficient than non-nitrifying systems. A
possible reason for their better oxygen
transfer performance is their lower
organic loading rates and longer sludge
retention times (SRT's) contrasted with
typical non-nitrifying systems. Longer
SRT's are generally believed to promote
higher alpha values and higher oxygen
transfer rates in wastewater, thereby
resulting in higher system aeration
efficiencies provided the SRT's are not
substantially longer than necessary to
sustain nitrification.
Operation and Maintenance
Maintenance observations at the 19
plants surveyed are summarized in Table
5. Over one-half of these plants had
significant problems with the diffuser
systems at startup or within the first few
years of operation. Two plants required
complete replacement of the initially
installed equipment. Plant operators on
the job during initial installation reported
that installing contractors were given little
supervision and often did not fully check
out the system after installation.
It was observed that some plant
operators did not comply with the
recommended minimum air flow rates
given in literature provided by all the
equipment suppliers. Four of the plants
were operated at air flows below
recommended minimums much of the
time. In one case, the operator
overloaded the aeration system in lieu of
putting a second basin on stream, greatly
exacerbating problems caused by failure
of diffuser hardware. Installers at this
same plant had overtightened much of
the system's hardware, causing extensive
dome hold-down bolt failure and air
leakage.
About one-half of the plants were
doing an adequate maintenance job.
Several, such as Berlin (NH), Montpelier,
and Seymour, were highly aware of the
benefits of preventive maintenance and
had set up and followed routine cleaning
and checking schedules much like those
observed in the United Kingdom. These
plants reported excellent O&M experi-
ences with their diffuser systems.
Conclusions
Unlike the generally favorable O&M
performance observed overseas, the
North American plants visited were more
likely to have experienced significant
problems with their fine bubble aeration
systems. It appeared that many of the
same design deficiencies noted in plants
overseas have been repeated here.
Problems with equipment had occurred
in about one-half of the plants
evaluated. Those plants that had
experienced significant equipment
problems tended also to exhibit relatively
poor aeration efficiencies.
Overall, estimates of oxygen transfer
performance for the North American
plants were on a par with those estimated
previously for the U.K. plants. In both
surveys, however, several plants were
producing aeration efficiencies well below
the potential capabilities of ceramic
diffusion technology. The sub-standard
oxygen transfer performance of those
U.K. plants exhibiting below normal
aeration efficiencies could be tied in most
cases to long tank L/W ratios, non-
tapered diffuser configurations, and
associated overaeration and wasted
energy. On the other hand, the
contributing factors for those North
American plants with below-average
aeration efficiencies appeared to be
linked more closely to wastewater char-"
acteristics (i.e., greater contributions from
-------
Table 2. Aeration System Design and Operating Data
Aeration Basin Dimensions
Plant Name
Coulton: Unit 1
Unit II
North Buffalo
Little Patuxent
Lower Bucks
County
Rialto
Riverside
West Bend
Whittier
Narrows:
Tank 1
Tanks 2&3
Berlin (NH)
Berlin (Wl)
Village Creek:
Tanks 1,2, & 4
Utitz: Stage 1
Stage II
Meriden: Stage 1
Montpelier
Park Ten
Ridgewood
Seymour
Highland Creek
Humber
Length
(ft)"
1241174
153.5
260
185
200
100
250
113
300
300
100
80
239
114
139
100
39
92.3
116
201
115
246
Width
(ft)"
8.25
14
20
30.25
30
20
40
19.8
30
30
25
20
104
25
30
56
39
30
24
26
58
58.3
SWD
(ft)"
10
14.4
14.5
15.3
15
15
17.6
18
14.4
14.4
15
15
13.8
15
15
18
18
14.5
15
14.7
25
24
Effect. Basin
L/W
726.5*
32.9ft
26.0
12.2
6.7
10.0
6.3
28.5
10.0
10.0
4.0
4.0
2.3
4.6
4.6
5.4
1.0
6.2
4.8
7.7
2.0
4.2
Diffuser Density
(No./fP)-
0.30-0.25
0.41
0.23-0.14
0.39
0.28-0.16
0.47
0.54-0.45
0.17
0.26-0.15
0.33-0.19
0.27-0.15
0.21
0.50-0.28
0.49-0.26
0.41-0.22
0.10
0.18
0.31
026-0.14
0.12
0.54
0.56-0.28
Diffuser Taper
(%)
Uniform
Uniform
33/26/22/19
Uniform
64/36
Uniform
26/26/26/22
Uniform
39/38/23
39/38/23
45/32/23
Uniform
34/27/21/18
48/26126
48/26/26
Uniform
Uniform
Uniform
33/29/19/19
Uniform
Uniform
47/29/24
Air Flow per Unit
Volume
(cfm/1,000 ft3)?
25.6-22.1
24.9-24.5
22.7-13.2
32.1
23.3-13.3
19.3
11.1-9.3
3.5
23.3-14.2
23.0-13.4
7.2
10.4
20.5-11.3
U
U
11.9
3.6
U
10.7-6.0
5.4
7.2
30.2-15.1
Avg. Air Flow
per Diffuser
(cfm)tt
0.87
087
1.43
1.27
1.25
0.62
0.36
0.37
1.14
0.93
0.71
0.74
0.56
U
U
1 73
0.37
U
0.62
0.64
0.34
1 29
U = Unknown
' 1 ft = 0.305 m
" 1 dome/ft2 = 10.76 domes/m2
i 1 cfm/1,000 ft3 = 0.017 Um3/sec
tt 1 cfm = 0.472 Usec
t Based on six plug flow aeration sections of 174 ft each
tt Based on three plug flow aeration sections of 153.5 ft each
industry with lower concomitant alpha
values), equipment failure, and a higher
incidence of diffuser sliming or fouling.
The principal conclusions of this
study follow:
1 Estimates of system aeration
efficiency varied widely for the visited
plants but seemed to be linked to pro-
cess configuration and loading con-
ditions, wastewater characteristics, and/or
O&M problems. Plants using higher rate
processes seemed to have lower
aeration efficiencies with one exception
(Whittier Narrows) where O&M practices
were rigorous and effective. Within the
limits of the accuracy of the mass
balance technique employed in this
study, the estimated aeration efficiencies
for the non-nitrifying activated sludge
systems averaged 1.32 kg Oa/kWh (2.17
Ib/wire hp-hr). The average estimated
aeration efficiency of those plants where
complete or a significant degree of
nitrification was occurring was 1.52 kg
02/kWh (2.50 Ib/wire hp-hr). In general,
it appears that the lower F/M and
volumetric loadings and longer sludge
ages necessary to sustain nitrification
result in improved oxygen transfer
performance and reduced rates of
diffuser fouling.
2. Inadequate or inappropriate O&M
procedures were found to be a principal
contributor to less-than-optimum ox-
ygen transfer performance and/or major
equipment maintenance problems ob-
served at some plants.
For the most part, operators had been
provided little or no literature or
training for diffuser system operation,
troubleshooting, or maintenance.
Several of the plants visited had
experienced major equipment failure,
but the operators were not aware of
this until it was pointed out to them. In
general, plant maintenance mechanics
did not know the correct procedures
for checking, tightening, and replacing
diffuser hardware, though several had
developed effective procedures by
trial and error.
With only two exceptions, plant
operators did not understand that fine
bubble ceramic diffusers would
probably require cleaning after 6 mo
to 2 yr of operation, depending on the
rate of diffuser media fouling and
headless buildup. Advance provisions
for diffuser cleaning had been made
only at the Village Creek plant
(ultrasonic cleaning) and the Seymour
plant (acid gas cleaning) and there
was general ignorance of the time,
manpower and equipment
requirements, and costs associated
with diffuser cleaning.
-------
Table 3. Aeration System Process Perforrr
Average TBOD5 (mg/L)
Plant Name
Coulton
North Buffalo
Little Patuxent
Lower Bucks
County
Rialto
Riverside
West Bend
Whittier Narrows
Berlin (NH)
Berlin (Wl)
Village Creek
Utitz
Meriden
Montpelier
Park Ten
Ridgewood
Seymour
Highland Creek
Humber
Raw
WW
244
200
150
220
256
160
150
325
195
485
274
177
264
128
U
140
360
145
200
Primary
Erf.
J80
720~
115
220
(est.)
185
80
62"
142
60
242
175
119
90
66
-
90
--
-
100
Final
Eff.
12
10
78ft
15
13
5
8
4
12
20
79
5ft
5/t
10
10
5
4
5
20
lance Data
Average Volumetric
Loading (Ib
TBOD5/day/l,OOOft3)«
22.7
19.9
27.7ft
40.8
60.4
8.5
5.8
38.9
7.6
76.8
58.3
10.4ft
17. en
7.5
U
27.0
10.5
10.9
29.7
Average MLSS
(mg/L)
2,500
2,300
2,800ft
2,800
6,450
2,700
600
7,053
7,750
7,400
3,500
U
3,900tt
2,000
U
2,000
5,800
2,500
4,300
Average F/M Loading (kg
TBOD5/day/kg MLSS)
0.74
0.14
0.24/t
0.23
0.15
0.05
0.75
0.59
0.07
0.79
0.27
U
0.07ft
0.72
U
0.22
0.03
0.07
0.11
Average Air Flow
ft3/lb TBOD5 applied)t
7,570
7,249
7,066ft
647
461
7,799
866
678
576
892
499
U
7S7tt
349
U
428
777
953
7,037
U=unavailable
11b TBOD5/day/l,000 ft3 =0.016 kg/day/m3
t 1 fP/lb TBOD5 applied =0.062 rn^/kg
TBOD5 of roughing biofilter effluent
n Based on first-stage aeration only
Plant operators were not aware of the
relationship between process
operation and aeration efficiency. Only
a few were aware of the need to
maintain minimum air flows, and
several of the underloaded systems
were being operated below
recommended air flow rates per
diffuser. None of the plant O&M
manuals inspected provided any
guidance for diffuser system
maintenance or efficiency monitoring.
3. Poor aeration system performance
and/or O&M problems were often
attributable to design inadequacies or
errors.
Typical design errors included lack of
aeration taper, poor inlet and outlet
design, too many or too few diffusers,
and lack of DO monitoring equipment.
The excessive aeration tank L/W
ratios common to many U.K. plants
were not observed in this study.
Little attention had been given to
facilitating periodic maintenance at
many of the plants studied. In most
cases, draining of aeration tanks
required the use of special pumping
equipment.
Most of the plants were not equipped
with the monitors necessary to check
aeration system performance.
Specifically, few had separate power
meters for aeration blowers and many
had no means of measuring air flow to
the aeration tanks. Provision of on-
line DO monitors was uncommon, and
those plants that had DO monitors
often did not maintain them properly.
Several plants had been designed for
28-cm (7-in.) dome diffusers but
were equipped with the larger 22-cm
(8.7-in.) disc diffusers because the
latter were low bid, However, design
engineers required that the same
number of the larger diffusers be
installed, resulting in oversizing of the
aeration systems in these plants.
Extensive research at Los Angeles
County Sanitation Districts has verified
that three 22-cm (8.7-m.) disc
diffusers are equivalent to four 18-
cm (7-in.) dome diffusers from an
oxygen transfer standpoint.
4. Poor quality installation was a major
cause of subsequent equipment
problems. Often, critical hardware was
over- or under-tightened, causing
leakage and/or breakage. Manufacturer
and/or design engineer supervision (
most installations was minimal, an
contractors often did not follow publishe
guidelines. In some cases, the fragility <
the plastic hardware contributed to th
problem. The equipment supplied by th
major manufacturers varied in sensitivil
to installer error. However, when correctl
installed, most of the equipment, with th
exception of some gasket materials, w£
relatively trouble free. Also, substanti,
improvements in product quality hav
been made in response to field problenr
and competitive pressures over the la
several years. Where problems hav
been experienced, all of the princip.
suppliers have promptly honore
equipment warranties, even wher
complete system replacement has bee
required.
5. Although diffuser sliming and foulin
were only clearly indicated at four of th
plants visited, zones of coarse bubblin
were evident in several other plant
Coarse bubbling may or may not b
indicative of fouling, but it definitely has
negative impact on oxygen transf<
efficiency. Based on these limite
observations, ceramic diffuser foulir
appears to become more prevalent wi
-------
Table 4.
Aeration System Oxygen Transfer Performance Data
Plant Name
Coulton
North Buffalo
Little Patuxent
Lower Bucks County
R/a/to
Riverside
West Bend
Whittier Narrows
Berlin (NH)
Berlin (Wl)
Village Creek
Lititz
Meriden
Montpelier
Park Ten
Ridgewood
Seymour
Highland Creek
H umber
U = Unavailable
Avg. WW Avg. Air Flow* Avg. Power
Flow* (mgd)t (cfm)* Usage (kW)
3.2 5,400 149
12.0 10,420 386
8.9 5,500 154
8.0 6,600 223
2.35 1,160 50.2
9.0 7,500 203
4.5 1,400 61.1
12.5 6,966 207
1.7 340 8.3
0.8 1,000 31.4
54.8 27,720 812
0.9 U U
7.1 2,800 102
1.5 200 7.3
0.2 U U
3.0 670 19.6
0.54 800 24.3
3.0 2,400 75
24.5 14,710 730
How Power Calc. Field Aeration
Usage Derived? Data Quality (Ib O?/wire hp-hr)
ca/c. poor 1.33
meas. good 1.36
meas. fair 1 39
meas. fair 1 84
ca/c. poor 2.90
meas. fair 1.89
meas. good 1 85
ca/c. good 1.94
meas. fair 3.74
meas. good 1.91
ca/c. fair 3.97
poor
meas. fair 3.80
meas. fair 3.49
poor
meas. short 4.15
meas. fair 3.22
meas. fair 2.57
meas. good 1.03
Average: 2.49
Efficiency
(kg/kWh)
0.81
0.83
0.85
1.12
1.75
1.15
7.13
1.78
2.27
1.16
2.41
--
2.31
2.12
--
2.52
7.96
1.56
0.63
1.57
* At time of plant visits from late-1982 to mid-1983
1 1 mgd =0.044 m3/sec
t 1 cfm =0.472 /./sec
Table 5. Aeration System Maintenance Summary
Aeration System
Plant Name Year Started Up Startup Experience
Coulton
North Buffalo
L/ttle Patuxent
Lower Bucks County
Rialto
Riverside
West Bend
Whittier Narrows
Berlin (NH)
Berlin (Wl)
Village Creek
Lititz
Meriden
Montpelier
Park Ten
Ridgewood
Seymour
Highland Creek
Humber
1981 Poor, entire system replaced
1982 OK, minor problems
1980 Some breakage, leaking
1982 OK
1981 OK
1982 OK
1980 OK
1981 OK
1979 OK, some contractor error
1981 OK
1978 Poor, contractor error
1981 Poor, entire system replaced
1982 OK, some contractor error
1981 OK
1978 OK
1983 OK, vendor's rep. msta/ted
1982 OK
1968 OK, few problems
1982 OK
Aeration System Operating Experience
Excellent, no problems since replacement
General disc gasket failure in 1 yr
Poor, frequent failure of plastic parts (particularly dome
Fair, slime growth from heat treatment recycle
Excellent
Excellent
Excellent
Some slime growth, cleaned periodically with hosing or
no mechanical problems
OK, a few small leaks
Some slime growth and possible plugging
retainer bolts)
gas injection,
Poor, significant leakage and periodic failures of plastic hardware
Excellent, no problems since replacement
Excellent
Excellent
Poor, system failed due to O&M error
Some slime growth, cleaned periodically with hosing or
acid brushing
Fair, some plugging, in-situ gas cleaning system works well
Excellent, no failures in 14 yr
No way to check system, possible failure
-------
increasing process load, particularly at
the influent end of plug flow reactors and
the multiple feed points of step feed
reactors. Where rapid diffuser fouling is
encountered, a recently-developed,
proprietary, in-situ, non-process in-
terruptive cleaning technique using
hydrochloric acid gas injection from the
air side may permit aeration efficiency to
be maintained at acceptable levels
between more rigorous process-inter-
ruptive cleaning cycles.
6. Although the O&M peTTbrmaw,v, ^
collected in this project__are
generally positive as ttibse" reported in
the earlier U.K. study, it should be noted
that several plants were visited where
ceramic diffusers are performing quite
well and have produced major energy
cost savings. These plants are
characterized by careful attention to
correct installation and O&M of their
diffuser systems. Where problems have
been experienced, they could normally
be diagnosed and corrected at
/ icdsuuaoie COST, basically, this stud
verified that "fine bubble ceramic diffusio
technology can work well in Nort
American plants and that improvei
design, installation, and O&M practice
are the primary ingredients needed t
maximize aeration performance am
potential cost savings.
The full report was submitted ii
fulfillment of Purchase Order No
C2667NASX by D.H. Houck Associates
Inc., under the sponsorship of the U.S
Environmental Protection Agency.
Daniel H. Houck is with D. H. Houck Associates, Inc., Silver Spring, MD 20901.
Richard C. Brenner is the EPA Project Officer (see below).
The complete report, entitled "Survey and Evaluation of Fine Bubble Dome and
Disc Diffuser Aeration Systems in North America," (Order No. PB 88-
243 886/AS; Cost: $19.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-88/001
0000329 PS
60604
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