SrEPA
                                  United States
                                  Environmental Protection
                                  Agency
                                  Municipal Environmental Research
                                  Laboratory
                                  Cincinnati OH 45268
                                  Research and Development
                                  EPA-600/S2-81-222  Oct. 1981
Project  Summary
                                  Survey and  Evaluation  of
                                  Fine  Bubble  Dome  Diffuser
                                  Aeration  Equipment
                                  D. H. Houck and A. G. Boon
                                    This research project was initiated
                                  with  the overall objective  of better
                                  defining the oxygen transfer perfor-
                                  mance, operation and maintenance
                                  requirements, and proper design
                                  approaches  for fine  bubble dome
                                  diffuser aeration systems used in
                                  activated sludge wastewater treat-
                                  ment.
                                    Working with the  British  Water
                                  Research Centre of Stevenage, Eng-
                                  land,  the Association of Metropolitan
                                  Sewerage Agencies  surveyed 19
                                  wastewater  treatment plants with
                                  dome diffuser aeration equipment and
                                  reviewed the related literature. Thir-
                                  teen of the plants were in the United
                                  Kingdom, two in The Netherlands, and
                                  four in the  United States. The U.K.
                                  plants were selected primarily on the
                                  basis  of long-term experience (5 yr or
                                  longer) and were all municipal waste-
                                  water treatment plants with varying
                                  industrial flows. The Netherlands and
                                  U.S. plants were chosen on the basis
                                  of availability rather than longevity.
                                    As  nearly as  possible,  data on
                                  influent and effluent wastewater
                                  characteristics, power demand, air
                                  supply, and process parameters were
                                  compiled for a 5-yr period. Mainte-
                                  nance personnel were interviewed to
                                  develop a summary  of long-term
                                  operation and maintenance (O&M)
                                  experience. Specific designs and plant
                                  equipment for aeration, air cleaning,
                                  and diffuser maintenance were studied.
                                  Discussions were held with designers,
                                  equipment manufacturers, and re-
                                  search scientists to develop a better
                                  understanding of  design and per-
                                  formance.
                                   Although this survey clearly shows
                                  the need for optimized  design  and
                                  operating control strategies to realize
                                  the full energy saving potential of this
                                  type of equipment, dome diffuser fine
                                  bubble aeration systems were provid-
                                  ing relatively efficient, low-mainte-
                                  nance service in the plants visited.
                                   This Project Summary was developed
                                  by  EPA's Municipal Environmental
                                  Research Laboratory, Cincinnati, OH,
                                  to  announce key findings of the
                                  research project that are fully docu-
                                  mented in a separate  report of the
                                  same title (see Project Report ordering
                                  information at back).

                                  Introduction
                                   As with other energy-intensive  in-
                                  dustries, energy-conserving design and
                                  operation is  receiving increased  em-
                                  phasis in the wastewater treatment
                                  field. Aeration equipment employed in
                                  activated sludge service is usually the
                                  single largest energy consumer in a
                                  wastewater treatment plant, normally
                                  accounting for 60 to 80 percent of total
                                  power demand. Because  fine bubble
                                  aeration equipment has the potential for
                                  markedly  higher oxygen transfer effi-
                                  ciencies  than the  more  traditional
                                  coarse bubble spiral roll design, its use
                                  is rapidly expanding in new or retrofitted
                                  treatment plants.
                                   Historically, fine bubble aeration
                                  equipment was widely used in the
                                  United States before 1950. It gradually
                                  fell into disfavor because  of its fairly

-------
intensive maintenance requirements
and  was replaced by the very  low
maintenance coarse bubble equipment
during the period  of  relatively  cheap
power prior to 1972. Rapid escalation in
U.S. power costs since the 1974 Arab oil
embargo has renewed interest in  fine
bubble aeration.
  Because power costs have traditionally
been much higher in the United Kingdom
and Western Europe than in the United
States, fine bubble aeration equipment,
along with mechanical  surface aerators,
continued to be  widely used  and
improved there. The ceramic  dome
diffuser,  which is the  main subject of
this study, was first developed in 1954
and  refined into its  present  form by
1961. In 1972, it became  available in
the United  States under  a  licensing
agreement. Although there are presently
only a handful of U.S.  installations, the
dome  diffuser  is  in  use in several
hundred  treatment plants  around the
world and the last few years have seen
the evolution of  competing devices in
either dome or disc form.
  The  purpose  of  this study was to
assess the long-term  oxygen transfer
performance and O&M history of dome
diffuser aerators. A total of 19 treatment
plants were studied—13 in the United
Kingdom because of the large number
of major municipal treatment  works
with 5 yr or greater operating experience
in that  country. The British Water
Research Centre (WRC) cooperated in
the U.K. study  and was  able to  add
substantially to  the  data base. Two
plants in The Netherlands were studied,
and the  considerable  Dutch research
effort on the various types of dome/disc
diffuser  aerators was reviewed. Four
plants were visited in the United States;
three of  these were running side-by-
side  comparisons with other  types of
aeration equipment. A  literature review
was carried out in conjunction with the
WRC and EPA. Acorollaryactivity inthis
project was a review of  the process
design of dome diffuser aeration systems
and the  formulation of design recom-
mendations.


General Design Characteristics
of Surveyed Plants
  Most of the visited plants had aeration
systems of the plug flow configuration,
using long, narrowchannels with one or
more passes. Several used step feeding
for better load distribution. All of the
surveyed plants  were equipped with
dome diffusers manufactured by Norton/
Hawker-Siddeley.* Most of the plants in
the  United  Kingdom produced fully
nitrified effluents of high  quality;
several practiced denitrification as well.
A  list of the surveyed plants  and
background data are provided in Table 1.

Aeration Systems
  Aeration systems design data for the
surveyed  plants  are  summarized in
Table 2. Average process performance
data for 1978-79 are presented in Table
3.
  Plug flow  aeration systems were in
use at all of the plants visited.  Approxi-
mately one-half of the plants had two or
more passes per  aeration tank.  The
majority of the plants were operated in
the full plug flow mode with  effective
length-to-width ratios up to 106 when
multiple passes were  considered. The
U.K. plants exhibited very conservative
design approaches, owing principally to
very stringent discharge requirements.
Only  three  U.K.  plants did not fully
nitrify. Most achieved treatment levels
exceeding 95 percent removal  of BOD5,
suspended solids, and ammonia nitro-
gen.  Food-to-microorganism (F/M)
loadings in U.K. plants typically ranged
from 0.1 to 0.2 kg BOD5/day/kg mixed
liquor suspended solids (MLSS),  and
volumetric loadings ranged from 0.16 to
0.40 kg BODs/day/m3 (10 to 25 Ib/
day/1000 ft3) except in the higher rate
plants or  those receiving strong in-
dustrial wastes. Similarly, the nitrifying
U.K. plants  consumed two to three
times more air per unit of BOD5 removed
than  did the conventional activated
sludge, non-nitrifying, U.S. plants.
Because volumetric loading rates were
lower, however, air flow rates per unit
volume of aeration tank were similar to
those in  U.S. plants.  Diffuser density
and air flow rates per diffuser were also
quite similar;  this reflects the com-
monality of dome diffuser aeration design
in both countries. Tapered aeration, full
or partial, was used in all but four of the
19 plants surveyed.
  Mixing power levels at minimum air
flow rates were relatively low in most of
the plants. Only one plant, Minworth,
reported any deposition of mixed liquor
solids; that occurred in the lightly mixed
anoxic zone. Significantly,  all of the
lightly mixed plants had very  effective
primary sedimentation. MLSS at all of
the plants except Oxford were less than
'Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
3500 mg/l.  Oxford compensates for
higher-than-average volumetric loadings
by  carrying 4500-5000  mg/l MLSS,
maintaining low F/M loadings to pro-
mote nitrification. The range of power
levels given reflects the practice  of
tapered aeration, whereby air input (and
hence power input) is front loaded in the
plug flow plants. Often, mixing in the
lightly aerated  section of plug flow
plants with tapered  aeration  was
enhanced by central placement of the
diffusers,  along  the tank length axis,
carrying a double spiral mixing pattern.

Prevention of Denitrification in
Final Clarifiers.
  Single-stage nitrification (BOD re-
moval and nitrification in the same tank)
was being achieved in most of the U.K.
plants surveyed. To combat denitrifica-
tion in the final clarifiers, four plants
have been experimenting with partial
denitrification using anoxic zones in the
front ends of their respective aeration
tank batteries.
  Experimental denitrification  studies
have been conducted at Rye Meads by
the  WRC  and  the Thames Water
Authority. It was determined that 50
percent removal of nitrate nitrogen was
the practical upper limit of the process  |
as used at Rye Meads. Parallel labora-
tory studies suggested that the degree
of denitrification might be increased
another 10 to 20 percent by adding a
second anoxic zone at the beginning of
the third pass at Rye Meads. This has
not been fully supported by the experi-
mental results at Rye Meads.
  Process modifications have  been
undertaken at Coleshill to optimize
overall activated sludge  performance
and reduce settling problems in the final
clarifiers caused by denitrification.  In
the  period June-December 1978, ni-
trate removal through  the  process
(including that occurring  in final clari-
fiers) ranged from  42  to 57  percent.
Dramatic improvement in the problem
of rising sludge in the clarifiers was
reported.  A change in  the  clarifier
desludging schedule, decreasing deten-
tion time during  low flow periods, also
helped alleviate the problem.


Oxygen Transfer Performance

Method of Analyzing Oxygen
Transfer Efficiency
  Currently, there are  many  methods
for measuring oxygen transfer efficiency,
including  steady and nonsteady states

-------
Table 1.    Surveyed Plant Characteristics
1978/1979 O&M
Average Flow Average Performance Expert-
Plant Location/Name Aeration System Description
United Kingdom
Basingstoke Nitrifying, 1 -pass plug flow.
symmetrical aeration
Beckton (New Plant) Nitrifying, 1 -pass plug flow, tapered aeration
Beddington Nitrifying, 2-pass plug flow, tapered aeration
Long Reach Non-nitrifying, 4-pass plug flow.
tapered aeration
Mogden (Battery B) Nitrifying, 4-pass plug flow, some aeration taper
Oxford (1 969 Plant) Nitri/denit, 1 -pass plug flow, tapered aeration
Rye Meads Nitri/denit, 4-pass plug flow, tapered aeration
(Stage III)
Coalport Nitrifying, 2-pass step feed.
symmetrical aeration
Coleshill (Stage III) Nitri/denit, 1 -pass plug flow, tapered aeration
Finham (South) Non-nitrifying, 1 -pass plug flow.
symmetrical aeration
Hartshill Non-nitrifying, 1 -pass plug flow.
tapered aeration
Minworth Nitri/denit, 1 -pass plug flow, tapered aeration
Strongford Nitrifying, 1 -pass plug flow, some aeration
(New Plant) taper
The Netherlands
Holten-Markelo Nitri/denit, 2-pass plug flow, tapered aeration
Steenwijk Nitrifying, 2-pass plug flow, tapered aeration
United States
Glendale, Calif. Non-nitrifying, 1 -pass plug flow.
tapered aeration
Madison, Wise. Non-nitrifying, 3-pass step feed,
tapered aeration
Fort Worth, Tex. Non-nitrifying, 1 -pass plug flow.
tapered aeration
Tallman Island, N.Y. Non-nitrifying, 2-pass plug flow, step feed
mgd

4.9

174
25.5
52.8

45.2
5.3
10.4

3.2

13.5
7.5

5.7

72.4
10.6


4.7
11.8

3.0t

14.5

99$

68
m3/sec

0.22

7.6
1.12
2.31

1.98
0.23
0.46

0.14

0.59
0.33

0.25

3.17
0.46


0.21
0.52

0.1 3}

0.64

4.3$

2.98
%fBODs)n

97

95
96
94

97
98
98

95

96
90

94

96
95


93
96

90

88

--

86
Q£7"CO_ on/*o*
fOI WOR trl flsC

97 A

94 +
97
91 +

97 +
96 +
98 +

95 +

96 +
92 +

94 +

96 +
+


92 +
95 +

90 +

92 +

..

+
*A - average: B - better than average; - = worse than average.
^10-mo data.
%3-mo data.
procedures.  For this study, oxygen
transfer efficiency was estimated using
a  mass balance technique based on
empirically derived oxygen consump-
tion  values for BOD5 removed and
ammonia  nitrogen  oxidized and on a
similarly  derived  oxygen credit for
nitrate nitrogen denitrified. The method
was developed by Boon and Hoyland of
the WRC based  on  the work of Ecken-
felder and has an estimated  ± 20
percent accuracy.

  The oxygen mass balance technique
used in this project is represented by the
following equation:
        G,

where:  Gt
= 10'3f[R(B,-Be)+ 1.64
  (N8-Ne) + 2.83Ne»]
= overall rate of oxygen
  consumption by
       microorganisms in an
       aeration tank, kg/sec
f     = average wastewater
       flow rate, mVsec
Bs    = primary effluent
       BOD5, mg/l
Be    = final effluent BOD5
       mg/l
N8    = primary effluent
       NH4+-N, mg/l
Ne    = final effluent NH4+-N,
       mg/l
Ne*    = final effluent NOa'-N,
       mg/l
R     =0.75+0.05/(F/M),for
       0.1 
-------
Table 2.    Aeration System Design Data
                                          Aeration Basin
                                            Dimensions
 Plant Location/Name
                              Lgth
Wdth
(m)*
Dpth
fm)*
                                                                L/W
  Diffuser
  Density   Aeration Taper
Idomes/rrfft       1%)
 Minimum
  Mixing
Power Level Avg. Air Flow/
             Min. Air Flow
 United Kingdom
  Basingstoke                   79.2        6.7        2.5         12         3.9          none          20.8          1.5
  Beckton (New Plant)          223        41.2        3.1          5.4       2.8-1.9      46/31/23     13.6-6.8         1.5
  Beddington (New Tanks)        67         7.3        2.4         18.4       2.7-1.1    34/28/23/15   16.1-6.4         1.8
  Long Reach                   80         6.0        3.8         53        7.8-3.5    35/27/23/15  58.7-25.7        2.0
  Mogden /Battery B)            122         4.6        3.7        106        5.0-3.1    34/22/22/22    29-18.5        1.0
  Oxford (1969 Plant/             37.8        6.9        2.4          5.5        3.8         43/28         18.8          1.5
  Rye Meads (Stage III)           70         4.3        3.0         65        4.6-2.3    21/33/28/18    29-13.8        2.4
  Cos/port                      65         4.6        4.3         27.8        2.8          none         25-16.7        1.2
  Coleshill(Stage III)             64        18.3        2.9          3.5       3.9-2.0          §        36.9-16.7        2.0
  Finham (South)                61         3.0        3.6         20.3        4.3          none          36.0          1.5
  Hartshill                      27.4        9.2        3.2          3.0       5.9-4.1       59/41      89.0-62.0
  Minworth                    178        18.3        3.0          9.7     0.4/1.9-0.9        §        26.8-13.4        1.25
  Strongford (New Plant)         108         9.3        3.0         46.4       2.3-1.9          §

 The Netherlands
  Holten-Markelo                30         6.6        4.0          9.1       1.9-0.9    34/25/25/16
  Steenwijk                    100         6.75       4.0         29.6       2.8-1.5    34/25/25/16    20-10
United States
Glendale, Calif.
Madison, Wise.
Fort Worth, Tex.
Tallman Island. N.Y.

73.2
41.2
83.8
110

9.75
9.1
36.6
28

4.9
4.7
4.3
4.9

7.5
13.6
23
7.9

3.0-0.9
9.1-3.6
5.4-3.0
1.3

57/43
48/29/23
34/27/21/18
none

24-7.5
33.7-13.4
-.
--

1.4
1.5
1.4
1.2
*; m = 3.28 ft.
t/ dome/m* - 9.29 domes/100 ft".
t/ W/m3 = 0.038 wire hp/1000 ft3.
§See Appendix B of main report.

Table 3.    Aeration Process Performance Data
Average Flow Design
& Data Year DWF
Plant Name/Location (m3/sec)* (of/sec)"
United Kingdom
Basingstoke
Beckton (New Plant)
Beddington
(New Tanks)
Long Reach
Mogden (Battery B)
Oxford (1969 Plant)


0.22/78-79
7.6/78-79
1. 12-78-79

2.31/78-79
1.98/78-79
0.23/78-79

Rye Meads (Stage III) 0.45/78-79

Coalport
Coleshill (Stage III)

Finham (South)

Hartshill
Minworth

Strongford (New
Plant)
The Netherlands
Holten-Markelo
Steenwijk

United States
Glendale. Calif.
Madison. Wise.
Fort Worth. Tex.
Taltman Island. NY

0 14/78-79
0.42/78-79

0.32/1979

0.25/1979
3.17/1978

0.47/1979


0.21/1978
0.52/1978


0.13/78-79
0.63/1979
4.3/pt. 1979
3.0/78-79

0.26
8.8
0.96

1 97
1.53
0.17

0.42

0.20
062

0.26

0.28
2.11

0.77


0.15
0.62


-

4.2
3.5
BODs (mg/l)
' Raw

281
169
320

334
238
367

310

—


321

500-700
-

250


400
312


220
213
—
91
Primary

157
96
149

18O
99
165

144

157
158

162

400-500
142

50-100


182
102


158
156

64
Volumetric
Loading
(Ib BODi/day/
Effluent

4
8
12

20
8
7

5

9
12

32

20-40
6

10


21
12


11
19
--
13
1000 ft3n

22.4
20.6
11.7

44.0
9.8
41.0

24.3

36.0
22.9

70.0

112
22.2

-


30.4
24.2


31.9
27.0

29.6
F/M
Loading
A verage Air Flow
(kg BODs/day/kg2) ft3/* SODst cfm/1OOOft3§ Remarks

0.08
0.13
0.20

0.30
0 18
0.10

0.08

0.14
0.10

0.45

0.30
0.09

0.05


0.18
0.11


0.35
0.30

0.24

1910
1110
1785

612
1392
1046

1416

1402
1000

693

747
689

--


--
--


748
732
--
--

28.9
16.8
13.4

16.6
208
26.2

23.0

11.0
15.8

34.4

43.8
10.1




—
—


15.4
80.0
—
-


Non-nitrifying


Non-nitrifying

Initial anoxic
zone
Initial anoxic
zone

Initial anoxic
zone
High rate, non-
nitrifying
1 mo data
Initial anoxic
zone
Figures
approx.

Non-nitrifying
Partial
nitrification

Non-nitrifying
Non-nitrifying
Non-nitrifying
Non-nitrifying
*1m3/sec = 22.8 mgd.
t/ lb/day/1000 ft3 = 0.016 m3/day/m2.
tl ff/lb = 0.062 rr?/kg.
§1 cfm/1000 ft3 = 0.017 1/m3/sec.

-------
 Table 4.    Oxygen Transfer Performance Data Summary
Percent Saturation of
Plant
Beckton
Basingstoke
Mogden
Oxford
Rye Meads
Coalport
Coleshill
Minworth
Strongford
Beddington
Harts/?///
Long Reach
Finham
Steenwijk
Glendale
Madison

Aeration
Tank L/W
5.4
12
106
5.5
65
27.8
3.5
9.7
46.4
18.4
3.0
53
20.3
29.6
7.5
13.6

MLSS
(mg/l)
2900
4900
2300
5500
4700
2500
3000
3200
5000
2300
3000
1700
2000
3300
2000
2000

Mixed Liquor D. O.
Range
10-80
10-60
10-100
10-40
20-100
-
20-50
—
20-100
--
reported low
10-40
--
--
10-30
10-30

Average
40
30
50
20
60
--
35
--
80
15
--
20
—
-
20
20

High
(kg/kWh)
1.95
1.20
1.62
2.34
1.14
--
—
--
--
1.25
--
--
--
—
--
1.99

Aeration Efficiency*
Low
(kg/kWh)
1.54
1.08
1.12
1.93
1.04
--
--
--
--
1.05
--
-
—
—
--
1.56
Average:
Average
(kg/kWh)
1.75
1.16
1.37
2.13
1.09
1.08
2.12
1.71
1.49
1.11
1.11
2.07
1.76
0.78
1.14
1.77
1.48
Average
(Ib/hp-hr)
2.88
1.91
2.25
3.50
1.79
1.78
3.49
2.81
2.45
1.83
1.83
3.40
2.89
1.28
1.87
2.91
2.43
Years
of
Data
3
5
5
5
3
1
1
1
1 wk
10
1 mo
1
1
1
10 mo
2

 *Defined as mass of Oi transferred per unit of power input as measured by the line draw.
 data, such as Beddington, exhibit fairly
 constant performance data over the
 period of record. The two U.S. plants for
 which performance could be estimated
 seem to be  similar in both process
 design and  performance. The  Dutch
 plants are more closely related to U.S.
 plants in design; however, the estimated
 performance at Steenwijk is somewhat
 less for unknown reasons.
 Operation and Maintenance

 General Maintenance
 Experience
   Maintenance observations at the 19
 survey plants are summarized (Table 5).
 Generally, the plants  have had good,
 and often exceptional, reliability from
 dome diffuser equipment. After initial
 shakedown,  the plastic pipe  mounted
 systems have performed well. Earlier
 plants used dome diff users mounted on
 a cast iron air distribution grid. Rusting
 of the interior surfaces of the  air lines
 led to rust and scale  deposits on  the
 interiors of  the domes and  caused
 plugging after 5 to  6  yr.  Most of  the
 plants with iron pipe are retrofitting to
 plastic pipe with generally good results.
 Several  of the retrofitted plants have
 experienced minor problems with some
 of the anchors that hold the plastic pipe
 saddles to the tank floor coming loose
 and pulling out. The cause of this seems
 to be spalling of concrete around  the
 mounting holes in the floors. This  has
i not  been  reported  as a problem in
systems where tank concrete is new
and apparently  less  vulnerable to
spalling.
  Several plants have also reported
scattered failures of other plastic parts,
notably the  pipe  coupling straps and
orifice bolts. Beckton had major prob-
lems on  startup  with  the coupling
straps. Mogden has had considerable
problems with failure of the orifice bolts,
probably  related to over tightening
during installation. Most of the plants,
however,  reported few or no  startup
problems of this nature. Careful super-
vision of installation to avoid over-
tightening of plastic parts was cited as
the key to trouble free startup by most of
the plant personnel. It was also noted
that the  plastic parts were much less
costly  to replace than  the  previously
used brass bolts.
Formation of Biological Slimes
on Diff users
  The major operational problem asso-
ciated with the dome diff users was the
formation of biological  slimes on
diffusers operating  in  zones of  high
volumetric loading and/or low dissolved
oxygen (D.O.). Beddington continues to
have major problems with slime forma-
tion, which  manifests itself as coarse
bubbling at the surface  of the aeration
tank. The slime growth  does not cause
an  increase in air pressure;  rather,  it
induces an apparently wholly external
surface fouling  that causes the air
bubbles to coalesce after exiting the
surface of the  diffuser domes.  The
resulting coarse bubbling lowers oxygen
transfer efficiency, thereby lowering
mixed liquor D.O. and further encourag-
ing slime growth.
  When first confronted with  the
problem,  Beddington removed  and
refired their fouled domes. On startup of
a  cleaned tank, the  problem quickly
recurred, however, and  it was soon
obvious that other,  less costly solutions
were needed. I n further tests, a vigorous
brushing of the dome surface accom-
panied by high air flow rates was found
to return the dome to nearly new perfor-
mance  levels. Periodic tank  cleaning
and dome brushing  have  allowed
Beddington to control (not eliminate) the
problem at moderate cost.
  Although Beddington's sliming prob-
lem was intensified by the presence of
strong industrial wastes, which
depressed oxygen transfer efficiency
and caused low D.O. in the first passes
of the multi-pass plug flow tanks, it was
not the only plant that exhibited sliming.
Indeed, every plant visited  showed
some signs of coarse bubbling,  which
was probably attributable  to  slime
growth on domes.  Without exception,
the phenomenon occurred at the primary
effluent feed points or at the transition
from anoxic to aerobic treatment. It was
particularly severe  in the first 20 to 25
percent of the first  pass of two-to-four-
pass plug flow systems. Tapering the
aeration helped somewhat but did not
fully solve the problem.

-------
Table 5.    Maintenance Data Summary
Plant Name/Location
United Kingdom
Basingstoke

Beckton
New Plant
Old Plant

Beddington
(New Tanks)
Long Reach
Mogden (Battery B)

Oxford

Rye Meads

Coalport
Coleshill (Stage III}

Finham /South)

Warts/7///
Minworth

Strongford (New Plant)
The Netherlands
Holten-Markelo
Steenwijk
United States
Glendale, Calif.
Madison. Wise.

Fort Worth, Tex.

Tallman Island. N.Y.
Started Up

1964-71


1970
1959

1969

1978
1961

1969

1956-70

1970
1968

1974

1973
1971

1972

1978
1977

1978
1977

1978

1979
Startup Experience

Some problems with plastic
tank bottom mounts

Problems with plastic holddowns
No significant problems

No significant problems

No significant problems
No significant problems

Some problems with plastic
tank bottom mounts
Some problems with retrofitted
plastic piping
No significant problems
No significant problems

No significant problems

No significant problems
No significant problems

No significant problems

No significant problems
No significant problems

Several blowoff lines failed
No significant problems

Some problems with blowoffs

No significant problems
Cleaned

Every 5 yr


Every 8 yr
Twice in 15 yr

Every 4 yr*

Not yet
Every 6 yr

Not yet

Every 6 yr

Not yet
Not yet

Not yet

Not yet
Not yet

Not yet

Not yet
Not yet

Not yet
Not yet

Not yet

Not yet
Operating Experience

Fair, scale problems


Good after initial problems
Gradual plugging due to rust in cast iron
pipes
Poor but improving major slime problem

Good, new plant
Plastic retrofit in Battery B 11968) has
not yet required cleaning
Good, no apparent loss of effluent quality
after 10 yr
Fair, plugging due to rust in older lines.
Plastic system good
Good
Good, tanks cleaned once/year and domes
brushed
Good, only have had to repair several small
line leaks
Fair, some slime growth
Good, tanks cleaned once/ year and domes
brushed
Good

Good
Good

Good, small evidence of slime
Substantial sliming problem in mid-1980
after 3 yr of operation
Some line breaks and problems evident, but
overall performance stable
Good
*Initially. Cleaning has not been required for the last 6 yr.

  To summarize,  slime  growths
appeared to  occur  in zones of heavy
organic loading, or low D.O., or both.
The occurrence of these growths was
exacerbated by extreme plug flow
aeration tank design and the presence
of strong industrial wastes.
Conclusions
  In general, dome diffuser fine bubble
aeration systems were providing relative-
ly efficient, low-maintenance service in
the surveyed plants. However, the plant
visits and related study clearly indicated
a need for optimized design  and  oper-
ating control strategies if the full energy
saving potential of the equipment is to
be   realized.  Listed below are the
principal  conclusions  resulting  from
this study.
    1. Assessment  of  data  from the
      surveyed plants resulted in widely
      varying estimates of field oxygen
      transfer performance for the
      dome diffuser.  Generally, field
      performance was  lower  than
      might be expected from  clean
      water oxygen transfer data. With
      the use of a mass balance  tech-
                                  6
nique (based on empirically de-
rived oxygen consumption values
for BOD5 removed and ammonia
nitrogen oxidized and a similarly
derived oxygen  credit for nitrate
nitrogen denitrified), the process
(i.e., dirty water or mixed liquor)
aeration efficiency for the 16 of
19 plants with adequate data to
make predictive estimates aver-
aged 1.48 kg 02 transf erred/kWh
consumed (2.43 Ib 02/wire hp-
hr). The highest and lowest
observed aeration efficiencies
were  2.13  kg 02/kWh (3.50  Ib
02/wire hp-hr) and 0.78 kg 02/
kWh (1.28  Ib 02/wire  hp-hr) at
Oxford and Steenwijk, respec-
tively. For the three plants (Fin-
ham,  Madison, and Glendale)
with  a  reasonably  sufficient
comparative data base, fine
bubble dome diffuser process
aeration efficiency was approxi-
mately 1.65 times higher than for
side-by-side coarse bubble dif-
fuser  systems:  1.56 kg 02/kWh
(2.56  Ib 02/wire hp-hr) vs. 0.95
kg 02/kWh (1.56 Ib 02/wire hp-
hr).
2.  Methods of plant operation fre-
   quently contributed to less-than-
   optimum oxygen transfer per-
   formance.
     • In the U.K. plants particu-
       larly,  volumetric and F/M
       loading rates were often
       lower than required for
       nitrification, or high levels
       of BOD removal, or  both.
       The  least energy efficient
       plants, with two exceptions,
       were  underloaded volumet-
       rically.
     • A number of the plants were
       also overaerating the mixed
       liquor and had taken  no
       steps to monitor D.O. con-
       centrations and reduce air
       flows to more efficient op-
       erating levels. The two most
       energy efficient plants,
       Oxford and Beckton, closely
       monitored mixed liquor D.O.
       and adjusted air flows ac-
       cordingly.
3.  Lowered oxygen transfer effici-
   ency could also be traced to
   design practices that make it very M

-------
diff icu It for operators to ru n treat-
ment plants effectively.
  •  When multiple-pass plug
     flow systems are used, the
     air supply capability is poorly
     matched with  the oxygen
     demand, particularly in the
     second  and   subsequent
     aeration channels. This leads
     to overaeration in the latter
     passes and localized organic
     overloading and  diffuser
     sliming  in  the  first pass.
     Step feeding only partially
     alleviated the overaeration
     problem. Tapering the aera-
     tion dome configuration
     was also of limited value in
     suppressing overaeration in
     the second and subsequent
     passes of  multiple-pass
     systems;  however the  ta-
     pering significantly  helped
     suppress diffuser sliming.
     In terms of overall  oxygen
     transfer performance, tapered
     aeration apparently had no
     apparent advantage over
     the nontapered systems.
  •  The full practical operating
     range  attainable with the
     equipment,  in terms of air
     flow per dome, is not properly
     used in  selecting  diffuser
     density. Providing too many
     domes creates a  situation
     where the  minimum total
     aeration system air  flow is
     controlled by the minimum
     allowable air flow rate per
     dome (0.014 mVmin or 0.5
     cfm, defined  by control
     orifice headless character-
     istics)  for large portions  of
     the day; this produces ex-
     tended periods of overaera-
     tion. The recommended
     maximum  unit dome  air
     flow rate of 0.057 mVmin
     (2.0 cfm) is consequently
     rarely approached  in opera-
     tion.
  •  Many  of the plants  had
     shallow aeration tanks, 3.7
     m (12 ft)  or less,  which
     reduces  attainable oxygen
     transfer efficiency.
  •  Most of the plants lacked air
     flow monitoring capability
     for individual aeration grids,
     and air control valves, where
     provided, were usually too
     coarse in their adjustability
     to be of use in controlling air
       flows. Plant operators were
       often prevented from  cor-
       recting overaeration  condi-
       tions because of equipment
       limitations.

4. Significant industrial waste frac-
   tions  in  municipal wastewater
   may substantially lower dome
   diffuser oxygenation efficiency
   via  a  reduction  in  the  alpha
   factor. Alpha is especially affected
   in the first segment of long, plug
   flow aeration tanks  (to values
   reportedly as  low as  0.3 to 0.4)
   where detergents and  other
   surfactants haven't  had suffi-
   cient contact  time to be  biode-
   graded. As these surfactants are
   oxidized in  passing through the
   aeration process, alpha reportedly
   increases to  values of 0.8 or
   higher at the  effluent end of the
   tank.  Beddington and  Hartshill
   are  two examples of plants that
   are  adversely  affected by  indus-
   trial waste discharges.
5. The authors  believe  that, with
   enhanced design  and operating
   techniques, aeration efficiencies
   of dome diffuser plants with no
   unusual alpha depressing wastes
   present could  be increased 25 to
   75  percent over the average
   value of 1.48  kg Oz transferred/
   kWh (2.43 Ib 02/wire hp-hr)
   estimated from the survey.
6. The limited data evaluated in this
   study indicate  some parity of
   performance among the ceramic
   dome and disc diffusers presently
   being marketed in the United
   States. There appears  to  be a
   definite  correlation between
   dome  or  disc diameter (of the
   horizontal surface) and specific
   oxygen transfer  per diffuser.
   Data from clean water tests
   suggest that fewer of the larger
   diameter  units may be  required
   to transfer equivalent amounts of
   oxygen  at  the  same  oxygen
   transfer efficiency.
7. Generally, maintenance experi-
   ence with dome diffusers ranged
   from good to  excellent. Both of
   the  plants reporting  significant
   maintenance problems. Bedding-
   ton and Basingstoke,  had  devel-
   oped operating strategies  that
   were effectively controlling the
   problems  without excessive costs
   or downtime. It is concluded that
   the generally quite good mainte-
      nance experience is directly attri-
      butable to two principal factors:
        • Conscientious (though  not
           labor intensive) attention to
           aeration system  operation,
           particularly that relating to
           air cleaning and repair of
           infrequent equipment fail-
           ures.
        • Steady  improvement and
           refinement of the dome
           diffuser equipment and its
           application over the course
           of its history, particularly in
           piping and air cleaning.
    8. Diffuser sliming, causing exter-
      nal fouling, is apparently produced
      by conditions of high F/M loading,
      or low D.O., or both—conditions
      that  can  occur when strong
      industrial wastes are introduced
      into a plant. Three plants, Beckton
      (temporary reduction of loading),
      Beddington (brushing),  and
      Madison (steam cleaning) have
      developed somewhat effective
      responses to sliming.
    9. In designing new  plants,  close
      attention should  be given to
      required air flow  at  minimum
      loading. Use of a wider range of
      air flows in the design of dome
      diffuser systems, as now recom-
      mended by the manufacturer,
      will improve operational flexibil-
      ity and  thereby improve overall
      system efficiency. Aeration effi-
      ciency is only one parameter of
      diffuser performance;  high reli-
      ability and flexibility of operation
      should  also be considered in
      conjunction with operational and
      capital costs.
   10. Careful attention should be given
      to air cleaning to avoid  internal
      fouling of dome diffusers. Manu-
      facturer's recommendations in
      this area  should  be followed
      carefully.  When dome diffuser
      systems are retrofitted into exist-
      ing plants,  existing air piping
      should be carefully checked for
      rusting or scaling and should be
      cleaned or coated  as needed to
      avoid particle shedding from the
      pipe walls into the air stream
      where  it  can  cause internal
      diffuser fouling.


Recommendations
  This study has identified a number of
significant research needs that should
be addressed as soon as practicable:

-------
      1.  The question of alpha  sensitivity
         as it relates to the relative perfor-
         mance under field operating con-
         ditions of dome/disc diffusers
         versus other aeration devices
         should be a high priority research
         need.
      2.  The opportunity to develop useful
         side-by-side comparison data for
         dome  diffusers,  coarse bubble
         aerators, and fine bubble tube
         diffusers (in wide band spiral flow)
         exists  at three U.S. treatment
         plants: Madison, Wisconsin; Tall-
         man Island (New York City); and
         Fort Worth, Texas. In conjunction
         with ongoing process (dirty water)'
         testing at the Los Angeles County
         Sanitation  Districts, data should
         be developed from these plants.
      3.  Oxygenation performance studies
         of plants that have been modified
         to optimize application of dome or
         disc diffusers should be conducted
         as soon as possible. Such studies
         could possibly be rapidly  imple-
         mented  in cooperation with the
         WRC.  In addition, one or  more
         major tests in U.S. plants should
         be  initiated in the near future,
         possibly  under EPA's Innovative
         Technology Program.
      4.  The Nokia and  Degremont dif-
         fusers, which have  experienced
         significant overseas application,
         are now being marketed  in the
         United States. Afollow-upeffortto
         evaluate the O&M performance of
         this equipment is recommended.
         The Nokia dome, in  particular,
         represents  a  radical departure
             from conventional ceramic dome
             technology and should be of prime
             interest in further studies.
             Data evaluated during this project
             appear  to  predict  substantial
             performance equivalence between
             the Norton/Hawker-Siddely dome,
             the Sanitaire disc, the Degremont
             disc, and the Nokia disc. The larger
             diameter Sanitaire and Degremont
             units may transfer more oxygen
             per diffuser, allowing the use of
             fewer diffusers,  when compared
             with the smaller Norton/Hawker-
             Siddely  dome. Available  data are
             too limited  for  final judgment,
             however, and further evaluation is
             strongly recommended.
             Diffuser cleaning is a labor inten-
             sive and costly process  that can
             usually  be forestalled by careful
             O&M. Providing for diffuser clean-
             ing was the usual practice in the
             United  Kingdom, however,  and
     appears prudent in light of British
     experience. Alternatives to refiring,
     notably ultrasonic cleaning,  need
     further development. Further study
     of  ultrasonic  cleaning  might  be
     carried out in cooperation with the
     Fort Worth, Texas, plant to docu-
     ment labor requirements, cleaning
     effectiveness,  and equipment
     reliability.
  These recommendations have  been
stated in terms of urgency. Inviewofthe
increasing  number of dome and disc
diffuser systems being designed and bid
in the United States,  it is believed that
expedited research is necessary to avoid
repeating the deficiencies observed at
the surveyed plants.
  The  full  report  was submitted  in
fulfillment of Grant No. R806990 by the
Association of Metropolitan Sewerage
Agencies under the partial sponsorship
of the U.S. Environmental  Protection
Agency.
           D.  H.  Houck was formerly with the Association of Metropolitan Sewerage
            Agencies, Washington, DC 20036; A. G. Boon is  with the Water Research
            Centre, Stevenage, England SGI  1TH.
           Richard C. Brenner is the EPA Project Officer (see below).
           The complete report, entitled "Survey and Evaluation of Fine Bubble Dome
            Diffuser Aeration Equipment," (Order No.  PB 82-105 578; Cost: $15.50,
            subject to change) will be available only from:
                   National Technical Information Service
                   5285 Port Royal Road
                   Springfield, VA22161
                   Telephone: 703-487-4650
           The EPA Project Officer can be contacted at:
                   Municipal Environmental Research Laboratory
                   U.S. Environmental Protection Agency
                   Cincinnati. OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
               Postage and
               Fees Paid
               Environmental
               Protection
               Agency
               EPA 335
Official Business
Penalty for Private Use $300

RETURN  POSTAGE GUARANTEED
           MERL0063240
           LOU W  TILLEY
           REGION  V  EPA
           LIBRARIAN
           230 S  DEARBORN  ST
           CHICAGO  IL  60604
                                                                                                  Third-Class
                                                                                                  Bulk Rate
                                                                                       GflVFDNMFNT PPTNTTNC fl FF T TF • 1 Qft 1 - - ^ 50 -00? / '

-------