United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/2-81-222
September 1981
Research and Development
vvEPA
Survey and
Evaluation of
Bubble Dome Diffuser
Aeration Equipment
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EPA-600/2-81-222
September 1981
SURVEY AND EVALUATION OF
FINE BUBBLE DOME DIFFUSER
AERATION EQUIPMENT
by
Daniel H. Houck
Association of Metropolitan Sewerage Agencies
Washington, D.C. 20036
and
Arthur G. Boon
British Water Research Centre
Stevenage, England SGI 1TH
Grant No. R806990
Project Officer
Richard C. Brenner
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
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FOREWORD
The U.S. Environmental Protection Agency was created because of
increasing public and government concern about the dangers of pollution to
the health and welfare of the American people. Noxious air, foul water,
and spoiled land are tragic testimonies to the deterioration of our natural
environment. The complexity of that environment and the interplay of its
components require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solu-
tion; it involves defining the problem, measuring its impact, and searching
for solutions. The Municipal Environmental Research Laboratory develops new
and improved technology and systems to prevent, treat, and manage wastewater
and solid and hazardous waste pollutant discharges from municipal and commu-
nity sources, to preserve and treat public drinking water supplies, and to
minimize the adverse economic, social, health, and aesthetic effects of pol-
lution. This publication is one of the products of that research and pro-
vides a most vital communications link between the researcher and the user
community.
As part of these activities, a survey project was undertaken in August
1979 to review, document, and evaluate power requirements, design practices,
and operating and maintenance characteristics for 19 fine bubble dome
diffuser aeration systems. Thirteen of these systems are located in the
United Kingdom, two in The Netherlands, and four in the United States. The
information documented in this report should be of particular interest to
design engineers and municipal officials who are considering utilizing fine
bubble air aeration equipment in new activated sludge treatment plants or
switching to such equipment in existing plants.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
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ABSTRACT
»
This research project was initiated with the overall objective of better
defining the oxygen transfer performance, operation and maintenance require-
ments, and proper design approaches for fine bubble dome diffuser aeration
systems used in activated sludge wastewater-treatment.
Working in conjunction with the British Water Research Centre of
Stevenage, England, a survey of 19 wastewater treatment plants with dome
diffuser aeration equipment was carried out along with a review of the rela-
ted literature. Thirteen of the plants were located in the United Kingdom
(U.K.), two in The Netherlands, and four in the United States (U.S.). The
U.K. plants were selected primarily on the basis of long-term experience
(5 yr or longer) and were all municipal wastewater 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 charac-
teristics, power demand, air supply, and process parameters were compiled
for a 5 yr period. Maintenance personnel were interviewed to develop a
summary of long-term operation and maintenance experience. Specific designs
and plant equipment for aeration, air cleaning, and diffuser maintenance
were studied. Discussions were held with designers, equipment manufacturers,
and research scientists to develop a better understanding of design and per-
formance.
Results of this work indicate that, relative to other devices, fine
bubble dome diffuser aeration systems can perform at high efficiency when
proper design, operation, and maintenance practices are followed and when
strong performance-depressing industrial wastes are absent. Operation and
maintenance experience with this equipment has generally been very good,
with 6-10 yr, and sometimes longer, between cleanings. Operation and main-
tenance performance was found to be closely related to conscientious adher-
ence by plant operators to simple but necessary operating guidelines for this
equipment. Oxygen transfer performance was found to be highly variable be-
tween the plants. The oxygen transfer rates of at least two of the aeration
systems in the United Kingdom were apparently quite adversely affected by
industrial wastes. Many plants carried excessive mixed liquor dissolved
oxygen levels in the aeration basins. Plants with multiple-channel plug
flow tanks generally performed less efficiently, from an energy standpoint,
than those with single-pass systems.
This report was submitted in fulfillment of Grant No. R806990 by the
Association of Metropolitan Sewerage Agencies and the British Water Research
Centre under the partial sponsorship of the U.S. Environmental Protection
iv
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Agency. This report covers the period August 1, 1979, to February 1, 1980,
and work was completed as of September 15, 1980.
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CONTENTS
Foreword iii
Abstract [ \ -jv
Figures '.'.'.'.'.'.'.'. vii
Tables -jx
Abbreviations xl-
Acknowledgements xiii
1. Introduction 1
2. Conclusions 3
3. Recommendations 7
4. Study Approach 9
5. Methods of Analysis 12
6. Results 16
7. Dome and Disc Fine Bubble Aerators 50
8. Design of Dome Diffuser Activated Sludge Systems 57
References 75
Appendices
A. Sample U.K. Plant Survey Form 77
B. Plant Survey Data
Basingstoke 86
Beckton 93
Beddington 103
Long Reach 110
Mogden 114
Oxford 121
Ryemeads 129
Coal port 135
Coleshill 139
Finham 144
Hartshill 148
Minworth 152
Strongford 157
Plants in The Netherlands 159
Madison, Wisconsin 162
Other U.S. Plants Visited 169
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FIGURES
Number Page
1 Relationship of Oxygen Demand to F/M Loading 15
2 U.K. Thickener-Style Secondary Clarifier 19
3 Bag House Air Filters At Beddington 25
4 Disposable Cartridge Filters At Finham 26
5 Electrostatic Air Filter At Basingstoke 28
6 Impact of Mixed Liquor D.O. on Aeration Efficiency 35
7 Impact of Length-to-Width Ratio on Aeration Efficiency. . . 39
8 The Norton/Hawker-Siddeley Dome Diffuser 41
9 Slime Growths on Domes at Beddington 43
10 Brushing Domes 'At Beddington 43
11 Clean and Slimed Domes From WRC Tests 44
12 Formation of Scale on Basingstoke Domes 45
13 Beckton Dome Firing Facility 47
14 The Ames Crosta-Babcock Diffuser. . 52
15 The Envirex Disc Diffuser 52
16 The EPI/Nokia Disc Diffuser 53
17 The Infilco/Degremont Disc Diffuser 53
18 The Sanitaire Disc Diffuser 54
19 The W. E. Farrer Dome Diffuser 54
20 Dome Diffuser Headless vs. Air Flow Rate 58
21 Relationship Between Specific Oxygen Transfer and Air Flow
per Dome 60
22 Effect of Diffuser Density on Oxygen Transfer Efficiency. . . 62
23 Variation of Diffuser Oxygenation Efficiency with Depth. . . 64
24 Blower Brake Horsepower Requirements vs. Aeration Tank Depth. 65
25 Tapered Aeration at Oxford 67
26 Variation of Alpha with Degree of Treatment 68
27 Ridge and Furrow Tank at Ryemeads 70
vii
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FIGURES (continued)
Number Page
28 Replaceable Air Filter System 73
B-l Basingstoke Site Plan 87
B-2 Beckton Site Plan 94
B-3 Beckton D.O. Profile Comparison 101
B-4 Beddington Site Plan 104
B-5 Long Reach Site Plan Ill
B-6 Mogden Site Plan 116
B-7 Oxford Site Plan 122
B-8 Ryemeads Site Plan (Secondary System) 130
B-9 Coalport Site Plan 136
B-10 Coleshill Site Plan 140
B-ll Finham Site Plan 145
B-12 Hartshill Site Plan 150
B-l3 Minworth Site Plan 155
B-14 Holten-Markelo Site Plan 160
B-l5 Madison Site Plan 163
vm
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TABLES
Number Page
1 Surveyed Plant Characteristics . 17
2 Clarifier Data 18
3 Aeration System Design Data 21
4 Aeration Process Performance Data 22
5 Anoxic Zone Design Data 23
6 Blower/Air Cleaner Data. . 27
7 Oxygen Transfer Performance Data Summary 29
8 Los Angeles Fine Bubble Retrofit Analysis 32
9 Maintenance Data Summary 40
10 Clean Water Oxygen Transfer Efficiency Comparison 56
B-l Basingstoke Design Data 88
B-2 Basingstoke Influent-Effluent Data Summary 90
B-3 Basingstoke Aeration Efficiency Calculations 91
B-4 Basingstoke Mixing Data 92
B-5 Beckton Design Data 96
B-6 Diffuser Configuration In Beckton 1975 Activated Sludge Plant. 97
B-7 Beckton Influent-Effluent Data Summary 99
B-8 Beckton Aeration Efficiency Calculations 100
B-9 Beckton Mixing Data 102
B-10 Beddington Design Data 106
B-ll Beddington Influent-Effluent and Process Loading Data Summary. 107
B-12 Beddington Aeration Efficiency Calculations 109
B-l3 Beddington Mixing Data 110
B-14 Long Reach Influent-Effluent Data Summary 112
B-l5 Long Reach Mixing Data 114
B-l6 Mogden Design Data 117
B-17 Mogden Influent-Effluent Data Summary 118
B-18 Mogden Aeration Efficiency Calculations 119
B-l9 Mogden Mixing Data 120
B-20 Oxford Design Data 123
B-21 Oxford Influent-Effluent Data Summary 126
B-22 Oxford Aeration Efficiency Calculations 127
B-23 Oxford Mixing Data 128
B-24 Ryemeads Design Data 131
B-25 Ryemeads Influent-Effluent Data Summary 133
B-26 Ryemeads Aeration Efficiency Calculations 134
B-27 Ryemeads Mixing Data 135
B-28 Coalport Influent-Effluent Data Summary 137
B-29 Coalport Mixing Data 138
B-30 Coleshill Design Data 141
B-31 Coleshill Aeration Efficiency Calculations and Mixing Data . . 143
ix
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TABLES (continued)
Number Page
B-32 Finham Design Data 146
B-33 Finhan Influent-Effluent Data Summary .. 147
B-34 Finham Aeration Efficiency Calculations 148
B-35 Hartshill Design Data 151
B-36 Hartshill Aeration Efficiency Calculations and Mixing Data . . 153
B-37 Minworth Design Data 154
B-38 Minworth Influent-Effluent Data Summary 156
B-39 Minworth Aeration Efficiency Calculations and Mixing Data . . 158
B-40 Holten-Markelo Mixing Data 159
B-41 Madison Design Data 165
B-42 Madison Influent-Effluent Data Summary 167
B-43 Madison Aeration Efficiency Calculations 168
B-44 Madison Mixing Data 170
B-45 Glendale Aeration Efficiency Calculations and Mixing Data . . 172
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ABBREVIATIONS
-- 5-day biochemical oxygen demand
BHP -- brake horsepower
COD — chemical oxygen demand
D.O. -- dissolved oxygen
DWF -- dry weather flow
EPA -- U.S. Environmental Protection Agency
F/M -- food-to-microorganism ratio
kn -- kilonewton
kWh -- kilowatt hour
LACSD ~ Los Angeles County Sanitation Districts
L/W -- length-to-width ratio
mgd — million gallons per day (U.S. gallons)
MLSS -- mixed liquor suspended solids
MLVSS -- mixed liquor volatile suspended solids
N/HS — Norton/Hawker-Siddeley
NHg-N — ammonia nitrogen
N03-N -- nitrate nitrogen
O&M — operation and maintenance
PI — principal investigator
SWD -- side water depth
TSS -- total suspended solids
U.K. — United Kingdom
xi
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ABBREVIATIONS (continued)
uPVC -- unplasticized polyvinyl chloride
U.S. -- United States
W/m3 -- watts per cubic meter
WRC -- British Water Research Centre
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ACKNOWLEDGEMENTS
The cooperation of the Thames Water Authority and the Severn Trent
Water Authority in the United Kingdom, and the Zuiveringschap - West
Overijssel Authority in The Netherlands is gratefully acknowledged. The
generous assistance given to the principal investigator by the equipment
manufacturers whose equipment is discussed herein is also greatly apprecia-
ted. Special thanks go to Alistair Stratton, Eric Branton and Ray Hull of
Hawker-Siddeley Water Engineering Ltd. in the United Kingdom, and Paul Cum-
mings of the Norton Company, Worcester, Massachusetts, for their fine efforts
in support of this work. The considerable contributions of Lloyd Ewing, and
Dave Redmon, Ewing Engineering Co., and Jerome Wren of the Sanitaire Corp.,
both of Milwaukee, Wisconsin, are also gratefully acknowledged.
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As with other energy-i
operation is receiving increas
Aeration equipment employed in
gle largest energy consumer in
counting for 60-80 percent of
tion equipment has the potential
ciencies than the more
is rapidly expanding in new or
intensive industries, energy-conserving design and
ed emphasis in the wastewater treatment field.
activated sludge service is usually the sin-
a wastewater treatment plant, normally ac-
total power demand. Because fine bubble aera-
for markedly higher oxygen transfer effi-
traditional coarse bubble spiral roll design, its use
retrofitted treatment plants.
950
Historically, fine bubble
United States (U.S.) before 19
of its fairly intensive maintenance
low maintenance coarse bubble
power prior to 1972. Rapid es
Arab oil embargo has resulted
aeration equipment was widely used in the
It gradually fell into disfavor because
requirements, being replaced by the very
equipment during the period of relatively cheap
calation in U.S. power costs since the 1974
in renewed interest in fine bubble aeration.
Because power costs have
Kingdom (U.K.) and Western
aeration equipment, along with
widely used and improved there
main subject of this study,
present form by 1961. In 1972
der a licensing agreement. Al
U.S. installations, the dome
plants around the world and th
competing devices in either
The purpose of this study
performance and operation and
aerators. A total of 19 treatment
Kingdom because of the large
5 yr or greater operating experience
search Centre (WRC) cooperated
tially to the data base. Two
considerable Dutch research efjfort
aerators was reviewed. Four
of which were running side-by-
equipment. A literature review
and the U.S. Environmental Protection
SECTION I
INTRODUCTION
traditionally been much higher in the United
Europe than in the United States, fine bubble
mechanical surface aerators, continued to be
The ceramic dome diffuser, which is the
first developed in 1954 and refined into its
it became available in the United States un-
though there are presently only a handful of
diffuser is in use in several hundred treatment
last few years have seen the evolution of
or disc form.
dome
was to assess the long-term oxygen transfer
maintenance (O&M) history of dome diffuser
plants were studied, 13 in the United
number of major municipal treatment works with
in that country. The British Water Re-
in the U.K. study and was able to add substan-
plants in The Netherlands were studied, and the
on the various types of dome/disc diffuser
plants were visited in the United States, three
side comparisons with other types of aeration
was carried out in conjunction with the WRC
Agency (EPA).
A corollary activity in this project was a review of the process design
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of dome diffuser aerators and the formulation of design recommendations. The
principal conclusions of this work are given in Section 2. Recommendations
including areas of further study are summarized in Section 3. The study ap-
proach and the method of oxygen transfer performance estimation are described
in Sections 4 and 5, respectively. The results of this investigation are
evaluated in Section 6, currently available dome and disc fine bubble aerators
are described in Section 7, and a discussion of approaches to dome diffuser
aeration system design is presented in Section 8. A sample U.K. plant survey
form and the detailed plant survey data are provided in Appendices A and B,
respectively.
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SECTION 2
CONCLUSIONS
In general, dome diffuser fine bubble aeration systems were providing
relatively efficient, low maintenance service in the surveyed plants. How-
ever, the plant visits and related study clearly indicated a need for opti-
mized design and operating control strategies if the full energy saving po-
tential 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 vary-
ing estimates of field oxygen transfer performance for the dome dif-
fuser. Generally, field performance was lower than might be ex-
pected from clean water oxygen transfer data. Using the mass balance
technique described in Section 5, based on empirically derived oxy-
gen consumption values for 6005 removed and ammonia nitrogen oxi-
dized and a similarly derived oxygen credit for nitrate nitrogen de-
nitrified, the process (i.e., dirty water or mixed liquor) aeration
efficiency for the 16 of 19 plants with adequate data to make pre-
dictive estimates averaged 1.48 kg 02 transferred/kWh consumed (2.43
Ib 02/wire hp-hr). The highest and lowest aeration efficiencies ob-
served were 2.13 kg 02/kWh (3.50 Ib Op/wire hp-hr) and 0.78 kg Og/
kWh (1.28 Ib Op/wire hp-hr) at Oxford and Steenwijk, respec-
tively. For the three plants (Finham, Madison, and Glendale ) with
a reasonably sufficient comparative data base, fine bubble dome dif-
fuser process aeration efficiency was approximately 1.65 times higher
than for side-by-side coarse bubble diffuser 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 were frequently found to be contributing
to less-than-optimum oxygen transfer performance.
• In the U.K. plants particularly, volumetric and F/M loading
rates were often lower than required for nitrification and/or
high levels of BOD removal. The least efficient plants with
two exceptions were underloaded volumetrically.
• A number of the plants were also overaerating the mixed liquor
and had taken no steps to monitor dissolved oxygen (D.O.) con-
centrations and reduce air flows to more efficient operating
levels. The two most efficient plants, Oxford and Beckton,
closely monitored mixed liquor D.O. and adjusted air flows
accordingly.
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3. Lowered oxygen transfer efficiency could also be traced to design
practices that make it very difficult for operators to run treatment
plants effectively.
• The use of multiple-pass plug flow systems results in poor
matching of air supply capability with oxygen demand, particu-
larly in the second and subsequent aeration channels, leading
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 aeration dome configuration was also of limited
value in suppressing overaeration in the second and subsequent
passes of multiple-pass systems. However, it was of signifi-
cant value in suppressing diffuser sliming. Tapered aeration
did not exhibit any apparent advantage in overall oxygen
transfer performance over non-tapered systems in the plants
surveyed.
• The full practical operating range attainable with the equip-
ment, in terms of air flow per dome, is not properly utilized
in the selection of 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, m3/min or 0.5 cfm, defined by con-
trol orifice headless characteristics) for large portions of
the day, producing extended periods of overaeration. The rec-
ommended maximum unit dome air flow rate of 0.057 m-Vmin (2.0
cfm) is consequently rarely approached in operation.
• Many of the plants had shallow aeration tanks, 3.7 m (12 ft)
or less, reducing attainable oxygen transfer efficiency.
• Most of the plants surveyed lacked air flow monitoring capabi-
lity 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 correcting overaeration conditions be-
cause of equipment limitations.
4. Significant industrial waste fractions in municipal wastewater may
substantially lower dome diffuser oxygenation efficiency via a re-
duction in the alpha factor. Alpha is especially affected in the
first segment of long, plug flow aeration tanks (to values reported-
ly as low as 0.3-0.4) where detergents and other surfactants haven't
had sufficient contact time to be biodegraded. As these surfactants
are oxidized in passing through the aeration process, alpha increas-
es to values of 0.8 or higher at the effluent end of the tank. Bed-
dington and Hartshill are two examples of plants that are adversely
affected by industrial waste discharges.
5. It is the opinion of the authors that with enhanced design and ope-
rating techniques, aeration efficiencies (by the method of Section
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5) of dome diffuser plants with no unusual alpha depressing wastes
present could be increased 25-75 percent over the average value of
1.48 kg 62 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 be-
ing marketed in the United States. There appears to be a definite
correlation between dome or disc diameter (of the horizontal sur-
face) 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 experience with dome diffusers ranged from
good to excellent. Both of the plants reporting significant main-
tenance problems, Beddington and Basingstoke, had developed opera-
ting strategies which were effectively controlling the problems
without excessive costs or downtime. It is concluded that the gen-
erally quite good maintenance experience is directly attributable to
two principal factors:
• Conscientious (though not labor intensive) attention to aera-
tion system operation, particularly as related to air cleaning
and repair of infrequent equipment failures.
• Steady improvement and refinement of the dome diffuser equip-
ment and its application over the course of its history, par-
ticularly in piping and air cleaning.
8. Diffuser sliming, causing external fouling, is apparently produced
by conditions of high F/M loading and/or low dissolved oxygen, such
as can occur as a result of the introduction of strong industrial
wastes 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 recommended by the manufac-
turer, will improve operational flexibility and thereby improve
overall system efficiency. Aeration efficiency is only one parame-
ter of diffuser performance; high reliability 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. Manufacturer's recommendations in this
area should be followed carefully. When dome diffuser systems are
retrofitted into existing plants, existing air piping should be
carefully checked for rusting or scaling and cleaned or coated as
needed to avoid particle shedding from its pipe walls into the air
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stream where it can cause internal diffuser fouling,
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SECTION 3
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 per-
formance under field operating conditions of dome/disc diffusers vs.
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 diffus-
ers (in wide band spiral flow) exists at three U.S. treatment
plants: Madison, Wisconsin; Tallman Island (New York City); and
Fort Worth, Texas. In conjunction with pending 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. It is quite possible that such studies could
be rapidly implemented in cooperation with the WRC. In addition,
one or more major tests in U.S. plants, using the design concepts
discussed in Section 8, should be initiated in the near future, pos-
sibly under EPA's Innovative Technology Program.
4. The Nokia and Degremont diffusers, which have experienced signifi-
cant overseas application, are now being marketed in the United
States. A follow-up effort to 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.
5. 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 to the
smaller Norton/Hawker-Siddely dome. However, available data are too
limited for final judgment and further evaluation is strongly recom-
mended.
6. Diffuser cleaning is a labor intensive and costly process that can
usually be forestalled by careful O&M. However, provision for dif-
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fuser cleaning was the usual practice in the United Kingdom and
appears prudent in light of British experience. Alternatives to re-
firing, notably ultrasonic cleaning, need further development. Fur-
ther study of ultrasonic cleaning might be carried out in coopera-
tion with the Fort Worth, Texas plant to document labor requirements,
cleaning effectiveness, and equipment reliability.
It is recognized that these recommendations have been stated in terms of
urgency; however, in view of the increasing number of dome and disc diffuser
systems being designed and bid in the United States, it is believed that ex-
pedited research is necessary to avoid repeating the deficiencies observed at
the surveyed plants.
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SECTION 4
STUDY APPROACH
This section discusses the approach used in carrying out the survey and
evaluation of fine bubble aerated plants. The approach used in analyzing da-
ta to estimate oxygen transfer performance is detailed in Section 5.
SELECTION OF PLANTS FOR VISITATION
The principal criteria utilized in selecting the plants to be visited
were as follows:
1. Size: Plants with average flows in excess of 18,925 m3/day (5 mgd)
were of prime interest. A range of plant sizes from very large down
to this minimum flow rate could provide insight into system design,
performance, and O&M as a function of size.
2. Age: A principal concern the study was to address was the long term
O&M requirements and system reliability of fine bubble dome diffus-
er currently being marketed in the United States. Consequently,
plants with 5 or more yr of operating experience were emphasized.
3. Process type: In selecting plants, a range of process types, rang-
ing from low-to high-rate and including nitrification constituted
another primary criterion.
4. Industrial wastes: The study was to be concerned mainly with typi-
cal municipal wastewaters. However, plants that were receiving var-
ying amounts of wastes from industries were also investigated.
5. Water characteristics: The effect of hard and/or alkaline waters on
system performance and O&M requirements represented another area of
interest.
6. Geographic location: Plants were to represent, as much as possible,
the different geographic conditions of the host country. Variation
in design and O&M practices with geographic location could then be
assessed.
7. Availability of data: Plants that had adequate records and scienti-
fic staff were favored, for obvious reasons.
Selection of plants for visitation in the United Kingdom was principally
carried out by the WRC. U.S. plants were selected by the principal invest!-
-------
gator (PI) after consultation with equipment vendors and others. Dutch
plants were selected by the Zuiveringschap-West Overijssel Authority.
DEVELOPMENT OF SURVEY FORMS
The survey forms shown in Appendix A were developed first by the PI and
the EPA project officer and later substantially modified by the WRC to re-
flect data availability at U.K. plants. The initial study plan called for
completing the survey forms at the time of the site visits. However, the WRC
was able to carry out this activity in advance, working with the Thames Water
Authority and the Severn Trent Water Authority. This enabled a substantial
increase in the number of plants that could be visited, from the originally
planned seven up to 13. The PI refined and added to the data in the survey
forms as needed during the site visits.
In developing the survey forms, the goal was to document an adequate da-
ta base over a period up to 5 yr. Emphasis was given to annual and monthly
average data as opposed to daily or weekly analyses, as the purpose of the
study was to define long-term average performance histories rather than
short-term or seasonal perturbations. Provision was made in the survey forms
and during site visits to identify non-typical conditions, such as unusual
flows or loading variations. The survey forms also included fairly specific
data for plant design, emphasizing the aeration system, air cleaning and
blowers, and secondary clarifiers. Considerable additional design data were
generated by the plant visits. In many cases, the PI was given as-built
drawings of the aeration system, detailing air supply grid and D.O. control
design.
Very little O&M data were requested in the survey forms; rather, the in-
formation was developed through direct interviews with plant personnel.
PLANT VISITS
A total of 19 plants were surveyed, as listed below.
United Kingdom
Thames Water Authority:
Beckton (London)
Mogden (London)
Beddington (London)
Ryemeads
Basingstoke
Oxford
Long Reach (London)
10
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Severn Trent Water Authority:
Minworth (Birmingham)
Coleshill (Birmingham)
Coal port
Hartshill
Finham (Birmingham)
Strongford (Stoke on Trent)
The Netherlands
Zuiveringschap-West Overijssel Authority:
Hoi ten-Markelo
Steenwijk
United States
Fort Worth, Texas
Glendale, California
Tallman Island (NYC), New York
Madison, Wisconsin
Generally, 1-2 days were spent at each plant plus travel time. Survey
forms for the British plants were completed in advance by the scientific
staffs of the U.K. water authorities. Data for the other plants visited were
developed by the PI at the time of the site visit, with the exception of Ma-
dison, Wisconsin, where plant personnel completed the survey forms in advance
and Steenwijk for which data were provided by the Zuiveringschap-West Over-
ijssel Authority in lieu of a plant visit.
LITERATURE SURVEY
In support of the field studies, a limited literature survey was carried
out. A References section is provided at the conclusion of the body of the
report. This activity was substantially assisted by the WRC and the EPA pro-
ject officer.
11
-------
SECTION 5
METHODS OF ANALYSIS
The approach used in estimating mixing and oxygen transfer performance
from the data collected in the plant surveys is presented in this section.
A discussion of the individual plant surveys along with a description of each
plant is provided in Appendix B and includes tabulation of all calculated ox-
ygen transfer and mixing performance results. Section 6 summarizes and com-
pares these results.
ANALYSIS OF MIXING DATA
Mixing data analysis is based on minimum air flow rates per diffuser,
normally 0.011-0.014 m3/min/unit (0.4-0.5 cfm/unit). Emphasis was placed on
documenting minimum air flow rates per unit surface area and per unit volume
of the aeration tank. When correlated with direct observation by plant per-
sonnel and others, this technique can help define the lower threshhold of
mixing requirements for dome diffusers. In most cases, the surveyed plants
operated at minimum air flow rates for substantial portions of the day,
month, and year.
Power per unit volume estimates were developed from power and air rate
data obtained at each individual plant. No attempt was made to factor out
energy losses in the air distribution grid or account for varying blower ef-
ficiencies. Equal air flow to each dome was assumed as was generally the
case in the visited plants. Where this was not true, the plants were not
monitoring actual air flows rates to subsections of the aeration systems.
Therefore, in these plants, a minimum air flow rate had to be assumed.
ANALYSIS OF OXYGEN TRANSFER EFFICIENCY
Currently, there are many methods for measuring oxygen transfer effi-
ciency, including steady and non-steady state procedures. The reader is
referred to Reference 1 for a detailed discussion of current EPA efforts to
develop a unified oxygen transfer testing standard.
For purposes of this study, oxygen transfer efficiency was estimated
using influent and effluent 6005, NH3-N, and N03-N data. The method de-
scribed below was developed by Boon and Hoy!and of the WRC based on the work
of Eckenfelder2 and has an estimated +_ 20 percent accuracy. Several of the
plants and other researchers have measured performance using the non-steady
state sulfite reaeration method. These results are discussed in Section 8.
12
-------
Oxygen Required by Activated Sludge Process
The net rate, G^., of oxygen consumption by the microorganisms in an aera-
tion tank is given by:
Gt = rate of oxygen consumption by the heterotrophs (Gn)
+ rate of oxygen consumption by the nitrifiers (Gn)
- rate of oxygen production by denitrification (G
-------
Gj - rate of conversion of ammoniacal nitrogen to nitrified nitrogen
- rate of loss of nitrified nitrogen in the final effluent
= 1CT3 f [(Ns-Ne) (l-0.06)-Nel (4)
where Ng is the concentration of nitrified nitrogen in the final effluent.
Assuming 1 kg nitrified nitrogen provides 2.83 kg oxygen for biochemical oxi-
dation, it follows from Equation 4 that:
Gd = 2.83 TO'3 f[0.94 (Ns-Ne)-Ng] (5)
From Equations 1, 2, 3, and 5 it may be deduced that:
Gt = 10-3 f[R(Bs-Be) + 1.64 (Ns-Ne) + 2.83 N*] (6)
where R, kg 02/kg BOD5 removed, is given by:
R = a + bp MLSS T/(Bs-Be) (7)
Substituting the typical values for a, b, and f in Equation 7 yields:
R = 0.75 + 2 x 10-3 x 24 MLSS T/(B$-Be) (for 0.1 - F/M - 0.5) (8)
where T, days, is the aeration tank volume divided by the wastewater flow rate.
F/M, the food-to-microorganism ratio, can be defined as B.-Be/MLSS x T,
on a total sludge mass basis (volatile + nonvolatile suspended solids). Sub-
stituting in Equation 8, the ratio, R, is expressed as follows:
R = 0.75 + 0.05/(F/M) (9)
A plot of Equation 9 is presented in Figure 1 and was used to estimate R in
making oxygen transfer efficiency computations in this report.
For purposes of computation, Equation 3 can be expressed in terms of mass
flow in kg/day:
Gt = R (Bs-Be) + 1.6 (Ns-Ne) + 2.83 (N*) (10)
where Bs-Be, N?-Ne, and Ng are expressed in kg/day. Equation 10 is used here-
in to compute total oxygen supplied on a daily basis by the aeration systems
evaluated.
14
-------
1.0
-o
QJ
>
o
O)
LO
O
O
CQ
CD
C\J
o
0.5
R = 0.75 + 0.05/(F/M)
(0.1 f F/M f 0.5)
J.
1
0.1 0.2 0.3
F/M Loading (kg BOD5/day/kg MLSS)
Figure 1. Relationship of oxygen demand to F/M loading.
0.4
0.5
-------
SECTION 6
RESULTS
This section summarizes the results of the plant surveys and the find-
ings of the supportive literature search with regard to general design char-
acteristics, oxygen transfer performance, and O&M performance. Suggested re-
vised design approaches based on direct observation, consultation with manu-
facturers and designers, and the literature review are presented in Section 3.
GENERAL DESIGN CHARACTERISTICS OF SURVEYED PLANTS
A total of 19 plants were surveyed during the study -- 13 in the United
Kingdom, four in the United States, and two in The Netherlands. Because the
emphasis of the study was on long-term performance and O&M data, all of the
surveyed plants were equipped with dome diffusers manufactured by Norton/
Hawker-Siddeley. Other, similar equipment is now available and is discussed
in Section 7.
Most of the visited plants had aeration systems of the plug flow confi-
guration, using long, narrow channels with one or more passes. Several used
step feeding for better load distribution. 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.
Clarification Systems
Although this project focused on aeration system performance, it was re-
cognized early in the study that primary clarifier design and performance
differed markedly between plants in the United States and United Kingdom.
Table 2 summarizes clarifier loading parameters for the visited plants. With
several exceptions, U.K. primary clarifiers are designed for hydraulic load-
ings that average 50-60 percent of those used in the United States and The
Netherlands. Discussions with designers in the United Kingdom indicated that
their prevailing philosophy is to maximize removal of BOD in a low energy
process, primary settling, to save energy in the aeration systems. Also,
most of the U.K. systems are designed to fully treat flows up to three times
the average dry weather flow (DWF) and most had large stormwater holding/set-
tling systems in addition to very conservative primary clarifiers.
Also of interest is the thickener style of design applied to most secon-
dary clarifiers. Figure 2 depicts a typical secondary clarifier encountered
at most of the plants visited in the United Kingdom. The steep floor slope,
normally 30 degrees, greatly facilitates desludging, and the scraper mechanism
16
-------
TABLE 1. SURVEYED PLANT CHARACTERISTICS
Plant Location/Name
United Kingdom
Basingstoke
Beckton (New Plant)
Beddington
Long Reach
Mogden (Battery B)
Oxford (1969 Plant)
Ryemeads (Stage III)
Coal port
Coleshill (Stage III)
Finham (South)
Hartshill
Minworth
Strongford (New Plant)
The Netherlands
Hoi ten-Markelo
Steenwi jk
United States
Glendale, Calif.
Madison, Wise.
Fort Worth, Tex.
Tallman Island, N.Y.
*Key: A = average
+ = better than
- = worse than
**10 ITO data.
*** 3 mo data.
Aeration System Description
Nitrifying, 1-pass plug flow, symmetrical aeration
Nitrifying, 1-pass plug flow, tapered aeration
Nitrifying, 2-pass plug flow, tapered aeration
Non-nitrifying, 4-pass plug flow, tapered aeration
Nitrifying, 4-pass plug flow, some aeration taper
Nitri/denit, 1-pass plug flow, tapered aeration
Nitri/denit, 4-pass plug flow, tapered aeration
Nitrifying, 2-pass step feed, symmetrical aeration
Nitri/denit, 1-pass plug flow, tapered aeration
Non-nitrifying, 1-pass plug flow, symmetrical aeration
Non-nitrifying, 1-pass plug flow, tapered aeration
Nitri/denit, 1-pass plug flow, tapered aeration
Nitrifying, 1-pass plug flow, some aeration taper
Nitri/denit, 2-pass plug flow, tapered aeration
Nitrifying, 2-pass plug flow, tapered aeration
Non-nitrifying, 1-pass plug flow, tapered aeration
Non-nitrifying, 3-pass step feed, tapered aeration
Non-nitrifying, 1-pass plug flow, tapered aeration
Non-nitrifying, 2 pass plug flow, step feed
average
average
1978/1979
Average Flow
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
3.0**
14.5
99***
68
0 & M
Average Performance Exper-
m3/sec %(BOD5)R
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
0.13**
0.64
4.3***
2.98
97
95
96
94
97
98
98
95
96
90
94
96
95
93
96
90
88
—
86
%TSSR ience*
97 A
94 +
97
91 +
97 +
96 +
98 +
95 +
96 +
92 +
94 ^
96 +
\-
92 +
95 +
90 +
92 +
—
-
-------
TABLE 2. CLARIFIER DATA
00
_
Primary System
Plant Location / Name
United Kingdom
Basingstoke
Beckton (New Plant)
Beddington (New Tanks)
Long Reach
Mogden (Battery B)
Oxford (1969 Plant)
Ryemeads (Stage III)
Coal port
Coleshill (Stage III)
Finham (South)
Hartshill
Minworth
Strongford (New Plant)
The Netherlands
Holten-Markelo
Steenwijk
United States
Glendale, Calif.**
Madison, Wise.
Fort Worth, Tex.
Tallman Island, N.Y.
* 1 aod/ft2 = 0.041 m3/da
Type
Circ.
Rect.
Circ.
Rect.
Circ. /Rect.
Circ. /Rect.
Rect.
Rect.
Rect.
Circ.
Circ.
Rect.
Circ.
Circ.
Circ.
Rect.
Rect.
Circ.
Rect.
v/m2
Surface
Loading
(gpd/ftz)*
348
490
306
670
1235/583**
141 overall
300
217
260
--
346
180
327
730
700
-
1000
-
--
Detention
Time
(hr)
6.3
4.1
6.2
3.0
1.8+3.7**
6.85/10.9"1"1"
6.0
6.0
8.0
—
6.3
7.5
8.0
1.65
2.0
—
1.4
—
—
Average 8005
Removal Type
45
55
52
_46
58
47
54
N.Av.
N.Av.
50
N.Av.
N.Av.
N.Av.
27
31
-
25
—
30
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Circ.
Rect.
Circ.
Circ.
Rect.
Secondary System
Surface
Loading
(gpd/ft2>
410
500
368
380-760+
619
260
400
414
325
724
323
271
446
300
300 (est.)
442
454
--
820
Detention
Time
(hr)
5.0
5.0
5.3
4.9-2.4
4.1
8.65
7.5
6.0
6.0
3.5
5.9
6.0
4.5
5.0
5.0 (est.)
6.5
3.4
—
2.7
Based Stormwater
On Retention
DWF
DWF
DWF
1979 Avg. Flow
DWF
DWF
DWF
DWF
DWF
DWF
DWF
DWF
DWF
Avg. Flow
Avg. Flow
Avg. Flow
Avg. Design
Avg. Flow
Avg. Flow
Yes
No
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
No
<• Unequally loaded.
** Two-stage primary tanks.
•H- Two types of primary tanks.
-------
•
Fiaure 2. U.K. thickener-style secondary clarifier.
-------
usually consists of a simple dragging chain mechanism. Several plants were
equipped with this type of unit along with clarifiers using blade scraper
mechanisms and relatively flat floors. Without exception, plant operators
expressed a preference for the design shown in Figure 2. Typically, second-
ary clarifier diameter did not exceed 18-24 m (60-80 ft). U.K. designers in-
dicated this was a common practice, even in the larger installations, for old-
er plants. Newer secondary clarifiers in the United Kingdom generally have a
floor slope of 7-10 degrees and have scrapers. Most of these newer clarifiers
are 30 m (98 ft) in diameter.
The combination of highly nitrified effluents and long final clarifier
detention time has often led to denitrification and associated flotation of
clarifier sludge blankets in a number of U.K. plants. To alleviate this
problem, four of the U.K. plants have created anoxic zones in the first sec-
tion of the plug flow aeration channels, with variable results. Most of the
plants reported improvement in settling but not to the point of fully solving
the problem, partcularly during warm weather. Design of the anoxic zones is
discussed later.
Aeration Systems
Aeration system design and process data for the surveyed plants are sum-
marized in Tables 3 and 4, respectively, and discussed below.
Plug flow aeration systems were in use at all of the plants visited dur-
ing this study. Approximately 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 pass-
es were considered. The U.K. plants, again, exhibited very conservative de-
sign approaches, owing principally to very stringent discharge requirements.
Only three plants were visited there that did not fully nitrify. Most achieved
treatment levels exceeding 95 percent removal of 6005, suspended solids, and
ammoniacal nitrogen. Food-to-microorganism (F/M) loadings in U.K. plants
typically ranged from 0.1-0.2 kg BOD5/day/kg MLSS, and volumetric loadings
ranged from 0.16-0.40 kg BOD5/day/m3 (10-25 lb/day/1000 ft3) except in the
higher rate plants or those receiving strong industrial wastes. Similarly,
the nitrifying U.K. plants consumed two to three times more air per unit of
BODs removed than did the conventional activated sludge, non nitrifying, U.S.
plants. However, because volumetric loading rates were lower, air flow rates
per unit volume of aeration tank were similar to those in U.S. plants. Dif-
fuser density and air flow rates per diffuser were also quite similar, re-
flecting the commonality of dome diffuser aeration design in both countries.*
Tapered aeration, full or partial, was used in all but four of the 19 plants
surveyed.**
* The dome diffuser was introduced in the United States under license by
the U.K. manufacturer, Hawker-Siddeley, who provided most of the initial
design expertise to the U.S. vendor, Norton Company.
** Discussed in Section 8.
20
-------
TABLE 3. AERATION SYSTEM DESIGN DATA
Aeration Basin
Dimensions
Plant Location/Name
United Kingdom
Basingstoke
Beckton (Hew Plant)
Beddinqton (New Tanks)
Long Reach
Mogden (Battery B)
Oxford (1969 Plant)
Ryemeads (Stage III)
Coal port
Coleshill (Stage III)
Finham (South)
Hartshill
Minworth
Strongford (New Plant)
The Netherlands
Holten-Markelo
Steenwijk
United States
Glendale, Calif.
Madison, Wise.
Fort Worth, Tex.
Tallman Island, N.Y.
Lgth
M*
79.2
223
67
80
122
37.8
70
65
64
61
27.4
178
108
30
100
73.2
41.2
83.8
110
Wdth
W*
6.7
41.2
7.3
6.0
4.6
6.9
4.3
4.6
18.3
3.0
9.2
18.3
9.3
6.6
6.75
9.75
9.1
36.6
28
Dpth
(•)*
2.5
3.1
2.4
3.8
3.7
2.4
3.0
4.3
2.9
3.6
3.2
3.0
3.0
4.0
4.0
4.9
4.7
4.3
4.9
L/W
12
5.4
18.4
53
106
5.5
65
27.8
3.5
20.3
3.0
9.7
46.4
9.1
29.6
7.5
13.6
23
7.9
Diffuser
Density
(domes/m )
3.9
2.8-1.9
2.7-1.1
7.8-3.5
5.0-3.1
3.8
4.6-2.3
2.8
3.9-2.0
4.3
5.9-4.1
0.4/1..9-0.9
2.3-1.9
1.9-0.9
2.8-1.5
3.0-0.9
9.1-3.6
5.4-3.0
1.3
Minimum Mixing
Aeration Taper Power Level
(*) (W/m3)**
none
46/31/23
34/28/23/15
35/27/23/15
34/22/22/22
43/28
21/33/28/18
none
See Appendix B
none
59/41
See Appendix B
See Appendix B
34/25/25/16
34/25/25/16
57/43
48/29/23
34/27/21/18
none
20.8
13.6-6.8
16.1-6.4
58.7-25.7
29-18.5
18.8
29-13.8
25-16.7
36.9-16.7
36.0
89.0-62.0
26.8-13.4
—
—
20-10
24-7.5
33.7-13.4
—
Avg. Air Flow/
Min. Air Flow
1.5
1.5
1.8
2.0
1.0
1.5
2.4
1.2
2.0
1.5
—
1.25
--
—
—
1.4
1.5
1.4
1.2
Remarks
Large basin. Small basin L/W = 24
Basin No. 6. L/W = 98 (old plant)
New basin Nos. 1, 8, 9, and 16
Mixing power at operating air flow
Mixing power at operating air flow
1 basin tapered, 7 step feed
First pass anoxic for first 29 m
Air split 60/40 between passes
5.7 W/m3 in anxoic zone
High rate plant
Mixing power at operating air flow
2.6 W/m 3 in anoxic zone
Air supplied by gas engines
1 tank test
Tank Nos. 1-6
Tank Nos. 1 , 2, and 4
Tank Nos. 1 and 2
* 1 m = 3.28 ft ,
+ 1 dome/rr = 9.29 domes/100 ftS
** 1 W/m3 = 0.038 wire hp/1000 ft
-------
TABLE 4. AERATION PROCESS PERFORMANCE DATA
ro
ro
Plant Name/Location
United Kingdom
Basingstoke
Beckton (New Plant)
Beddington
(New tanks)
Long Reach
Mogden (Battery B)
Oxford (1969 Plant)
Ryemeads (Stage III)
Coal port
Coleshill (Stage III)
Finham (South)
Hartshill
Minworth
Strongford (New
Plant)
The Netherlands
Holten-Markelo
Steenwijk
United States
Glendale, Calif.
Madison, Wise.
Fort Worth, Tex.
Tallman Island, NY
Average Flow
& Data Year
(nr/sec)*
0.22/78-79
7.6/78-79
1.12/78-79
2.31/78-79
1.98/78-79
0.23/78-79
0.45/78-79
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
Design
,DWF
(irr/sec)*
0.26
8.8
0.96
1.97
1.53
0.17
0.42
0.20
0.62
0.26
0.28
2.11
0.77
0.15
0.62
4.2
3.5
Raw
281
169
320
334
238
367
310
--
—
321
500-700
--
250
400
312
220
?13
91
BODs (mg/1)
Primary
157
96
149
180
99
165
144
157
158
162
400-500
142
50-100
182
102
158
156
64
Effluent
4
8
12
20
8
7
5
9
12
32
20-40
6
10
21
12
11
19
13
Volumetric
Loading
(IbBODs/day/lOOOft3)**^
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 ,,
BOD5/day/kg^)
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
Average
ft3/lb BOD^+
1910
1110
1785
612
1392
1046
1416
1402
1000
693
747
689
--
748
732
Air Flow
cfm/1000 ft3*
28.9
16.8
13.4
16.6
20.8
26.2
23.0
11.0
15.8
34.4
43.8
10.1
—
15.4
80.0
r Remarks
Large basin
Basin No. 6
Non-nitrifying
Non-nitrifying
Initial anoxic
Initial anoxic
Initial anoxic
High rate, non-
nitrifying
1 mo data
Initial anoxic
Figures approx.
Non-nitrifying
Partial
nitrification
Non-nitrifying
Non-nitrifying
Non-nitrifying
Non-nitrifying
zone
zone
zone
zone
* 1 m3/sec = 22.8 mgd
** 1 lb/day/1000 ftJ = 0.016 in /day/i/
+ 1 ft3/lb = 0.062 m3/kg
•H- 1 cfm/1000 ft3 = 0.017 l/m3/sec
-------
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 occuring in the lightly mixed anoxic zone. Signif-
icantly, all of the lightly mixed plants had very effective primary sedimen-
tation. Mixed liquor suspended solids (MLSS) at all of the plants except Ox-
ford were less than 3500 mg/1. Oxford compensates for higher-than-average
volumetric loadings by carrying 4500-5000 mg/1 MLSS, maintaining low F/M
loadings to promote 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 aera-
ted 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 (see Section 8).
Single-stage nitrification (BOD removal and nitrification in the same
tank) was being achieved in most of the U.K. plants surveyed. To combat de-
nitrification in the final clarifiers, four plants have been experimenting
with partial denitrification using anoxic zones. Table 5 provides basic de-
sign parameters for these zones.
TABLE 5. ANOXIC ZONE DESIGN DATA
Plant
Oxford
Description •
Maintain low D.O. in
first 25 percent of
Zone
Volume
(m3)*
160
Detention
Time (hr)
2
Mixing
Power
Level (W/m3)+
--
Remarks
Minimal ef-
fectiveness
tank length, prior to
introduction of first
50% of wastewater flow
Ryemeads First 50 percent of
first pass (four total
passes) anoxic zone.
Five mechanical stir-
ers, three of which
are in operation
452
1.5
29 Removes 30-
40 percent
of oxidized
nitrogen;
mechanically
mixed
Minworth
Coleshill
First 17 percent of 1629 2
single-pass tank has
dome density reduced
to 0.4 dome/m2**
First 28 percent of 950 1.5
single-pass tank has
dome density reduced
to 0.7 dome/m2
2.9 Removes 10-
20 percent
of oxidized
nitrogen
5.7 Removes 40-
50 percent
of oxidized
nitrogen
1 m3 = 35.3 ft3
1 W/m3 = 6.038 wire hp/1000 ft3
** 1 dome/m2 =9.29 domes/100 ft2
23
-------
Experimental denitrification studies have been conducted at Ryemeads by
the WRC and the Thames Water Authority.3 It was determined that 50 percent
removal of nitrate nitrogen was the practical upper limit of the process as
used at Ryemeads. Parallel laboratory studies suggested that the degree of
denitrification might be increased by a further 10-20 percent by adding a se-
cond anoxic zone at the beginning of the third pass at Ryemeads. This has
not been fully supported by the experimental results at Ryemeads.
Process modifications have been undertaken at Coleshill to optimize
overall activated sludge performance and reduce settling problems in the fi-
nal clarifiers caused by denitrification.4 In the period June-December 1978,
nitrate removal through the process (including that occurring in final clari-
fiers) ranged from 42-57 percent. Dramatic improvement in the problem of ris-
ing sludge in the clarifiers was reported. A change in the clarifier de-
sludging schedule, decreasing detention time during low flow periods, also
helped to alleviate the problem.
Blower/Air Cleaning Equipment
Out of 19 visited plants, ten had centrifugal-type blowers and nine had
positive displacement types. There was no apparent preference for the selec-
tion of one type over the other and most were designed to operate in the
range of 28-55 kN/m2 (4-8 psi).
The three types of air cleaning equipment encountered included bag house
collectors, electrostatic precipitators, and disposable cartridges. Of these,
bag house collectors were most commonly used in the U.K. plants visited. The
units are constructed as steel enclosures which house sets of cloth stocking
tubes that are precoated with filter aid before being placed in service and
after each cleaning. Historically, an asbestos-based precoat was used. How-
ever, because of health and safety considerations, it is rapidly being sup-
planted with a cellulose-based precoat. Figure 3 shows three of the large
bag house air filters "at Beddington. Currently, six units are in service con-
tinuously and another one is on standby.
The size, expense, and precoat requirements of bag filters have dimin-
ished their selection for newer plants. Rather, replaceable filters (Fi-
gure 4) or electrostatic precipitators (Figure 5) are increasingly the tech-
nologies of choice in the newer plants. The precipitators can be installed
in a third to half the space required for bag houses and have relatively sim-
ple maintenance needs. Simplest of all are the replaceable cartridge fil-
ters. As discussed in Section 8, these units are relatively expensive to re-
place but take up minimal room and require very little maintenance.
Table 6 summarizes blower/air cleaner data for the surveyed plants. Air
cleaning design and cost data are provided in Section 8 of this report.
24
-------
rv>
01
Photo courtesy of
Thames Water Authority
Figure 3. Bag house air filters at Beddington.
-------
to
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-------
TABLE 6. BLOWER / AIR CLEANER DATA
ro
Blower Data
Plant Location/Name
United Kingdom
Basingstoke
Beckton
Old Plant
New Plant
Beddington
Long Reach
Mogden (Battery B)
Oxford
1969 Plant
Renov. Plant
Ryemeads (Stage III)
Coal port
Coleshill (Stage III)
Finham (South)
Hartshill
Minworth
Strongford (New Plant)
The Netherlands
Holten-Markelo
Steenwijk
United States
Glendale, Calif.
Madison, Wise.
Fort Worth, Tex.
Tallman Island, N.Y.
Type*
PD
C
C
C
C
C
PD
PD
C
C
PD
PD
PD
C
C
C
C
C
PD
C
PD
No.
5
5
16
6
3
10
3
6
3
4
16
4
3
8
4
2
3
3
5
5
5
Size Range
(cfm)+
1362-3373
— —
21,000
6357-10,383
18,750
18,000-27,000
2000-3000
1400-2400
7400
2145
1600
1650
3000-5000
11,500-17,500
11,400
--
2500
10,000-20,000
5,840-11,000
--
20,100
Design
Pressure
(psi)**
4.8
— -.
6.0
5.5
5.8
6.2
6.0
8.6
7.1
5.5-7.0
6.0
6.0
—
—
—
--
60
7.5
•9.5
--
8.25
Type
Bag
Elect.
Elect.
Bag
Bag
Repl.
Bag
Elect.
Elect.
Bag
Bag
2 Stage Fil
Elect.
Bag
Bag
Elect.
Elect.
Repl.
Repl .
Elect.
Elect.
Air Cleaner Data
No.
3
8
16
5
—
--
2
3
5
2
2
ter 4
3
—
4
1
1
2
5
2
1
Rated Air
Flow/Unit
(cfm)
8200
—
—
10,000
—
--
3000
5000
5000
6300
12,800
1650
8200
—
12,000
—
__
24,000
12,500
--
25,000
* PD = positive displacement, C - centrifugal
+ 1 cfm = 0.472 I/sec
** 1 psi = 6.90 kN/m2
-------
Figure 5. Electrostatic air filter at Basingstoke.
OXYGEN TRANSFER PERFORMANCE
The results of the plant surveys are discussed in this subsection along
with factors that affect field (i.e., mixed liquor) oxygen transfer perfor-
mance. A summary of oxygen transfer performance data from literature sources
is also given.
Results of Plant Surveys
Table 7 summarizes the oxygen transfer performance developed as a result
of the plant surveys. 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. The values of aeration efficiency
shown in Table 7 for the various plants were estimated using the methodology
presented in Section 5.
A wide range of performance levels are apparently occurring at the^dome
diffuser plants surveyed, even where process conditions are seemingly^simi-
lar. Plants with long-term performance 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.
Coarse Bubble vs. Fine Bubble Comparison
Five plants had coarse bubble aeration tanks in parallel with dome dif-
fuser equipped tanks:
• Finham, United Kingdom • Glendale, California
• Madison, Wisconsin • New York City, Tallman Island
28
-------
TABLE 7. OXYGEN TRANSFER PERFORMANCE DATA SUMMARY
rv>
Plant
Beckton
Basingstoke
Mogden
Oxford
Ryemeads
Coal port
Coleshill
Minworth
Strongford
Beddington
Hartshill
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
(mq/1)
2900
4900
2300
5500
4700
2500
3000
3200
5000
2300
3000
1700
2000
3300
2000
2000
Percent
Mi xed
Range
10-80
10-60
10-100
10-40
20-100
--
20-50
--
20-100
—
reported
10-40
--
—
10-30
10-30
Saturation of
Liquor D.O.
Average
.40
30
50
20
60
—
35
--
80
15
low
20
--
—
20
20
Aeration Efficiency*
High Low Average Average
(kg/kWh) (kg/kWh) (kg/kWh) (Ib/hp-hr)
1.95
1.20
1.62
2.34
1.14
--
--
--
--
1.25
—
--
--
--
--
1.99
1.54
1.08
1.12
1.93
1.04
—
--
--
--
1.05
—
—
--
—
--
1.56
Average:
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
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 02 transferred per unit of power input as measured by the line draw.
-------
t Fort Worth, Texas.
Of these, four had comparative data of value. Operational problems at the
Fort Worth plant prevent meaningful comparisons at this time. The New York
City plant had data from a one-time test performed in 1978. Results from
this plant and the other three comparisons are discussed below.
Finham --
Finham employs a trickling filter plant with activated sludge pretreat-
ment. The North works has four 563-m3 (19,880-ft3) basins providing 1 hr de-
tention time for a DWF of 54,550 nvVday (14.4 mgd). The South works has six
656-m3 (21,160-ft3) basins providing 3.5 hr detention time for a DWF of
22,730 m3/day (6.0 mgd). F/M loadings are 1.80 and 0.45 kg
MLSS, respectively.
The North works has coarse bubble aerators of British design that pro-
vide essentially symmetrical tank floor coverage. The South works has dome
diffusers symmetrically distributed on the basin floor at a density of 4.3
domes/Fir (39.9 domes/100 ft2).
Using the method of Section 5, aeration efficiency was estimated at 1.50
kg O^/kWh (2.46 Ib/wire hp-hr) for the North works and 1.76 kg 02/kWh (2.89
Ib/wire hp-hr) for the South works. This was the smallest difference noted
in side-by-side coarse bubble and fine bubble system comparisons. Two fac-
tors may account for this:
1. The coarse bubble aeration system covered the tank floor, a configu-
ration that is known to improve efficiency.5
2. Detergent concentrations at Finham were quite high, often exceeding
10 mg/1. Fine bubble diffusers may be more adversely affected by
surfactants than coarse bubble diffusers.
It is believed that the comparisons from the other three plants, discussed
below, are more representative of the typical performance difference between
coarse and fine bubble aeration in municipal wastewater service.
Madison, Wisconsin --
In 1977, Madison retrofitted six of 16 activated sludge basins at its
Nine Springs treatment plant with fine bubble dome diffusers. The purpose
was twofold: to increase oxygenation capacity of the plant and to study the
performance and O&M requirements of the equipment for possible later retrofit
in the remaining aeration basins. Madison's coarse bubble system consists of
wide band aerators placed along one side of the tanks. The fine bubble sys-
tem is comprised of dome diffusers in a tapered configuration with full floor
coverage (See Appendix B for description of facilities).
Three assessments of oxygen transfer performance have been carried out at
Madison. Rollins and Kocurek, graduate students at the University of Wiscon-
sin (Madison), used gas capture analysis in 1978 to estimate performance in
30
-------
Tank No. 4 (fine bubble) vs. Tank No. 7 (coarse bubble) and found, with lim-
ited data, that the fine bubble system was approximately 2.8 times as effi-
cient as the coarse bubble system.6 Coarse bubble oxygen transfer efficiency
averaged 5 percent vs. 14 percent for the fine bubble system. Both tanks
were being used for sludge reaeration at the time, and loading rates were
somewhat lower on the coarse bubble system.
Later in 1978, the staff of the Madison plant repeated the experiment
when both sides of the plant were being operated in the contact stabilization
mode, but using only the sludge reaeration tanks for the tests. Their stu-
dies indicated that the fine bubble aerators were approximately 3.3 times as
efficient as the coarse bubble units. Oxygen transfer efficiency averaged
5.6 percent for the coarse,bubble diffusers vs. 18.6 percent for the fine
bubble diffusers.
Finally, as part of this study, performance for both systems, now being
operated as step feed activated sludge units, was estimated by the method of
Section 5. Coarse bubble aeration efficiency averaged 0.66 kg 02/kWh (1.09
Ib/wire hp-hr); fine bubble aeration efficiency averaged 1.77 kg 02/kWh (2.91
Ib/wire hp-hr), or apparently 2.7 times more efficient than the coarse bubble
system.
The agreement between the different tests at Madison is excellent. The
tests conducted by the plant staff show the widest performance gap, not un-
expectedly since these tests were run when the tanks were reaerating return
sludge and were not receiving influent wastewater. Under this condition, it
can be expected that much of the influent detergent, which particularly af-
fects fine bubble performance, had already been removed in the contact part
of the process. The other two tests were performed with the systems opera-
ting in the step feed mode.
It should be noted, however, that operating conditions at Madison tend
to make comparative analyses of the type discussed above somewhat difficult.
Firstly, there is not complete system separation between coarse and fine bub-
ble units. Mixed liquor is settled in a common secondary settling system, and
return sludges are intermixed. Secondly, to provide the higher pressure re-
quired by the fine bubble system and to distribute air flow, the coarse bub-
ble system is throttled, decreasing its efficiency. The quantitative effect
of both factors could not be evaluated during this study.
Glendale, California --
One aeration tank at the Glendale plant was converted to fine bubble
aeration and a 10-mo comparative study initiated in July 1978.7 Using a
steady state analytical approach, the fine bubble system was found to be ap-
proximately twice as efficient as the wide band coarse bubble system. Aera-
tion efficiency was estimated at 1.14 kg 02/kWh (1.87 Ib/wire hp-hr) for the
fine bubble system, using data from Reference 7, and 0.7 kg 02/kWh (1.15 lb/
wire hp-hr) for the coarse bubble system.
Based on these results, fine bubble retrofits were proposed for both the
Glendale and Terminal Island plants in the Los Angeles City system. Table 8
31
-------
TABLE 8. LOS ANGELES FINE BUBBLE RETROFIT ANALYSIS
Glendale lerminal Island
Parameter Treatment Plant Treatment Plant
Air Flow, ft3/gal wastewater flow*
Coarse Bubble 1.9 5
Fine Bubble 1.0 2.6
Air Compressors
Coarse Bubble (hp)** 960 3665
Fine Bubble (hp)** 510 1900
Power Saved (kWh/yr x 106) 2.94 11.53
Cost Savings ($/yr)+ 97,000 415,000
Construction Costs
Equipment++
Misc. (20%)
Contingencies (20%)
Profit (15%)
Subtotal
Inflation (20%)
TOTAL COST
Payback (yr)
$420,000
84,000
84,000
63,000
651,000
130,200
$781,200
8.07
$450,000
90,000
90,000
67,500
697,500
139,500
$837,000
2.02
* 1 ft3/gal =7.48 m3/m3
** Assumes an air compressor efficiency of 75 percent and a discharge pressure
of 7 psi (48.3 kN/m2); 1 hp = 0.746 kW.
+ At $0.033/kWh for Glendale and $0.036/kWh for Terminal Island.
++ Includes diffusers, in-tank aeration piping, connection to existing piping,
air filtration system, and appurtenant controls and meters.
32
-------
summarizes the data and computations from these analyses. Los Angeles pro-
jects an 8-yr payback on a fine bubble retrofit at Glendale and a 2-yr payback
at Terminal Island.
New York City (Tallman Island) --
The Tallman Island plant, located in the New York City Borough of
Queens, has recently been upgraded, and a side-by-side test of fine bubble
dome diffusers, wide-band coarse bubble diffusers, and wide-band fine bubble
tubular diffusers will be conducted in the near future. Full-scale long term
testing is awaiting the acceptance of air metering equipment. However, a
limited field analysis of fine bubble vs. coarse bubble equipment was carried
out in early 1979 by students of Manhattan College. That work indicated an
oxygen transfer efficiency of about 6-8 percent for course bubble aeration
and 18-20 percent for fine bubble aeration.^ When correlated with blower re-
quirements, the installed power for a fine bubble system was estimated at ap-
proximately one-half that for a coarse bubble system, based on the results of
this single test.
Factors that Affected Oxygen Transfer Performance at the Surveyed Plants
The relationship between oxygen transfer rate as measured under field
operating conditions vs. clean water conditions can be expressed as follows:
OTR - SOTR (oC) \B C* - C)/C*] 9T~20 (11)
y y
where: OTR = field oxygen transfer rate, kg/hr
SOTR = standardized oxygen transfer rate in clean water at stated con-
ditions of temperature, D.O., mixing, geometry, etc., kg/hr
= Alpha factor = ratio of K.a in wastewater application to Kia in
clean water at equivalent conditions of temperature, geometry,
mixing, etc.
= Beta factor = ratio of D.O. saturation concentration in waste-
water to D.O. saturation concentration in clean water at equiv-
alent conditions of temerature and partial pressure
C* = D.O. saturation concentration in clean water corresponding to a
given partial pressure and temperature, mg/1
C = desired average mixed liquor D.O. concentration, mg/1
0 = temperature adjustment factor defined so that (K.a at temp. TO
/(KLa at temp. T2) = 9 (T]-T2)
K^a = volumetric mass transfer coefficient, given conditions of geo-
metry, mixing, temperature, etc., based on the liquid film,
hr'1.
This seemingly simple generalized relationship is subject to numerous
modifiers and corrections, which will not be enumerated here. The reader is
33
-------
referred to Boyle et al.' for an extensive discussion of the art of oxygen
transfer testing and analysis. However, the above equation can be used to
illustrate the relationships between oxygen transfer rate and the principal
factors that were observed to markedly affect plant performance at the visit-
ed plants. These are listed below in order of anticipated significance:
• Mixed liquor D.O. (C)
• Alpha factor (CxL)
• Aeration system geometry
Tank depth
Tand length-to-width ratio (degree of plug flow)
The relationship of these and other factors to plant design is discussed in
Section 8 as well. There are other variables which can markedly affect oxy-
gen transfer efficiency, including diffuser type and configuration and air
flow rate. These variables were quite similar at the visited plants, and it
is concluded that they did not significantly enter into the efficiency varia-
tions observed.
Mixed Liquor D.O. --
With the exception of the two plants that were receiving large industri-
al waste loads, Beddington and Hartshill, control of mixed liquor D.O., or
lack of it, seemed to be the principal determinant of aeration efficiency.
The inverse relationship between mixed liquor D.O. and aeration efficiency is
evident in the data of Table 7 and the plotted band of Figure 6. Many of the
less efficient plants had large portions of the total aeration volume in
which D.O. approached saturation, particularly in the third and subsequent
tanks of multi-channel plug flow plants.
Holding all other factors constant, Equation 11 can be expressed as a ra-
tio for comparison of two cases of mixed liquor D.O.:
OTR]/OTR2 = (^C* - C^/^C* - C2) (.12)
where: C] = mixed liquor D.O. concentration in situation 1, mg/1
C2 = mixed liquor D.O. concentration in situation 2, mg/1.
This can be further simplified by estimating C-| and C2 as follows:
G! = % Sat} (C*/100) (13)
C2 = % Sat2 (CJ/100) (14)
Combining Equations 12, 13, and 14 yields:
OTRT/OTR2 = [^ - (% Sa^/lOO)]/^ - (% Sat2/100)] (15)
34
-------
Saturation of Mixed Liquor D.O.
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At 20°C (68°F) and assuming a typical ft value for domestic wastewater of 0.95,
it is seen that an aeration process operating at 20 percent of saturation has
an oxygen transfer rate five times greater than one operating at 80 percent
of saturation:
OT^/OTRg = [0.95 - (20/100)]/[0.95 - (80/100)] = 5 (16)
Comparing Oxford and Ryemeads, for example, averaging 60 and 20 percent
of saturation, respectively, this simplified analysis would indicate that Ox-
ford would have an oxygen transfer rate 2.1 times higher than Ryemeads. This
agrees closely with the average aeration efficiency results shown in Table 7.
Similarly, the ratio between Oxford and Mogden would be 1.67, again close to
the average data summarized in Table 7. It is recognized that this highly
simplified comparison is valid only where conditions of temperature, alpha,
and D.O. saturation are similar. However, in the case of the plants compared
above, this is a reasonable assumption. All of the plants in this comparison
were quite similar with respect to these variables.
Overaeration seemed to be present in all of the U.K. plants to some de-
gree, with the exception of Beddington and Hartshill. In most cases, plant
personnel noted that they were constrained by minimum required air flow rates
of 0.014 m3/min/dome (0.5 cfm/dome) and relatively low volumetric loading
rates, the latter needed in most cases to maintain appropriate sludge reten-
tion times for single-stage nitrification. (Design approaches to avoid this
problem are discussed in Section 8). Most of the plants were not equipped
with adequate D.O. and air flow monitoring equipment or valves to control air
flow to tank sections and/or individual aeration channels. D.O. levels were
highest in the latter half of the aeration process, particularly in those
plants with multiple-pass plug flow configurations.
Alpha Factor Variation --
The alpha (0C) factor is a dimensionless conversion factor that relates
the oxygen mass transfer coefficient expected for a given aerator when opera-
ted in wastewater (i.e., respiring biomass) to that measured with the same
aerator in clean water under similar conditions of temperature, geometry,
mixing, etc. In effect, it is used to derate the clean water data to field
conditions. Alpha is most affected by the presence of surfactants (surface
active agents), which interfere with the transfer of oxygen across the film
of liquid surrounding the bubbles. It has been known for some time that al-
pha varies with mixing power level, surfactant concentration, basin geometry,
and aerator type.9,10,11,12 The surfactant effect appears to be more pro-
nounced with fine bubbles than coarse bubbles. Boon11 reported on experiments
with dome diffusers where alpha varied from 0.3-0.8 from the inlet to the
outlet of a plug flow system treating domestic wastes and producing a highly
nitrified effluent. For purposes of comparison, he states that a mechanical
aeration system can have an alpha of 1.2 vs. 0.4 for fine bubble diffused air
on the same wastewater.
Alpha variation.provides a likely explanation for two apparent inconsis-
tencies observed during the plant visits:
1. Air flow requirements in the plug flow systems seemed to decrease
more rapidly along the length of the aeration channel than would be
36
-------
expected from process oxygen demand gradients as the wastewater
passed from inlet to exit.
2. Although clean water test comparisons between fine bubble dome dif-
fusers and coarse bubble (single wide band) diffusers indicate that
the fine bubble unit is 3-4 times more efficient,I3'14 data from side-
by-side field comparisons indicate that fine bubble diffusers are
only 1.5-2.5 times more efficient.
The first observation can be explained by the increase in alpha as the
detergent initially present in the raw wastewater (5-10 mg/1) is removed in
the plug flow aeration process. As alpha increases, oxygen transfer effic-
iency increases and substantially less air flow is required to meet the pro-
cess oxygen demand.
Observation No. 2 seems to suggest that coarse bubble aerators are some-
what less "alpha sensitive" owing to larger bubbles and more turbulence re-
sulting from greater air flow per unit volume of tank. At the visited plants,
the fine bubble aerators still exhibited a substantial aeration efficiency edge
over the coarse bubble units, but it was considerably less than clean water da-
ta would indicate. However, data from extensive side-by-side clean water
testing, submitted by the Sanitaire Corporation, does not support this obser-
vationJS The Sanitaire data*, comparing the company's fine bubble disc
aerator to a single wide band spiral flow coarse bubble diffuser suggest that
alpha, with 5 mg/1 of surfactant added, is the same for either system. In
view of this apparent contradiction, the variation of alpha for different
types of aerators merits further research.
The relatively poor oxygen transfer performance of the Beddington and
Hartshill plants, even with reasonably low mixed liquor D.O., can probably be
attributed to the presence of strong industrial wastes, which significantly
depressed alpha and caused marked reduction in oxygen transfer efficiency.
Beddington personnel reported that the date of the introduction of the waste-
water flow from organic chemicals manufacture coincided with a severe loss in
apparent oxygen transfer efficiency. Hartshill has always treated the ren-
dering waste it receives and has adequate oxygenation capacity, but plant
personnel were aware of the relatively low level of oxygen transfer efficien-
cy being achieved by their dome diffuser aeration system.
Aeration System Geometry —
Aeration system geometry affects mixed liquor D.O. control, and the al-
pha factor, as discussed above. Systems that had the greatest D.O. control
problems, as evidenced by overaeration and lower aeration efficiencies, were
usually those with multiple-pass aeration basins. Conversely, with the ex-
ception of Long Reach and Basingstoke, plants with single pass plug flow sys-
tems tended to be more efficient. The three most efficient plants, Oxford,
Beckton, and Coleshill,all had a length-to-width (L/W) ratio of less than 12.
* Available on request from Sanitaire Corporation, Milwaukee, Wisconsin.
37
-------
As shown in Figure 7, the data suggest a correlation between L/W and aeration
efficiency. The only single-pass plug flow plant with relatively low aeration
efficiency was Basingstoke, for reasons which are discussed in Appendix B.
Long Reach, a four-pass plug flow plant, was more efficient than other multi-
ple-pass plug flow systems due to stringent D.O. control by plant operators.
As discussed in Section 8, overall aeration efficiency should improve
with increasing tank depth. However, the plant data showed no clear correla-
tion between mixed liquor depth and oxygenation efficiency. It is likely that
the impact of tank depth was overshadowed by other factors, notably mixed li-
quor D.O. and alpha, in the aeration systems of the surveyed plants.
OPERATION AND MAINTENANCE
With a few exceptions, the surveyed plants reported minimal problems with
their dome diffuser aeration systems. This subsection summarizes the overall
maintenance history encountered, including special maintenance problems en-
countered at some plants.
General Maintenance Experiences
Maintenance observations at the 19 survey plants are summarized in Table
9. 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 diffusers
mounted on a cast iron air distribution grid. Rusting of the interior suf-
faces of the air lines led to deposition of rusts and scale on the interiors
of the domes, causing plugging after 5-6 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 pulling out of
the anchors that hold the plastic pipe saddles to the tank floor (refer to
Figure 8). The cause of this seems to be spall ing of concrete around the
mounting holes in the floors. This has not been reported as a problem in
systems where tank concrete is new and apparently less vulnerable to spelling.
Several plants have also reported scattered failures of other plastic
parts, notably the pipe coupling straps and orifice bolts. Beckton had major
problems 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 supervision 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 previously used brass bolts.
38
-------
co
to
100
80
60
4J
O1
OJ
(O
CO
c:
o
4J
ro
CD
40
20
1.0
O
Notes: 1. Beddington and Hartshill data
excluded due to excessive in-
dustrial contribution.
2. 1 Ib/hp-hr = 0.608 kg/kWh
O
2.0 3.0
Aeration Efficiency (Ib 02/wire hp-hr)
4.0
Figure 7. Impact of length-to-width ratio on aeration efficiency.
-------
TABLE 9. MAINTENANCE DATA SUMMARY
Plant Name/Location
United Kingdom
Basingstoke
Beck ton
New Plant
Old Plant
Beddington (New Tanks)
Long Reach
Mogden (Battery B)
Oxford
Ryemeads
Coal port
Coleshill (Stage III)
Finham (South)
Hartshill
Minworth
Strongford (New Plant)
The Netherlands
Holten-Markelo
Steenwijk
United States
Glendale, Calif.
Madison, Wise.
Fort Worth, Tex.
Talltnan Island, M.Y.
_S_tarted JJjD Startup Experience
Cleaned
JlitiLIS Experience
1964-71 Some problems with plastic
tank bottom mounts
1970 Problems with plastic holddowns
1959 No significant problems
1969 No significant problems
1978 No significant problems
1961 No significant problems
1969 Some problems with plastic
tank bottom mounts
1956-70 Some problems with retrofitted
plastic piping
1970 No significant problems
1968 No significant problems
1974 No significant problems
1973 No significant problems
1971 No significant problems
1972 No significant problems
1978 No significant problems
1977 No significant problems
1978 Several blowoff lines failed
1977 No significant problems
1978 Some problems with blowoffs
1979 No significant problems
Every 5 yr Fair, scale problems (see discussion)
Every 8 yr Good after initial problems
Twice in 15 yr Gradual plugging due to rust in cast iron
pipes
Every 4 yr* Poor but improving major slime problem
Not yet Good, new plant
Every 6 yr Plastic retrofit in Battery B (1968) has
not yet required cleaning
Not yet Good, no apparent loss of effluenty quality
after 10 yr
Every 6 yr Fair, plugging due to rust in older lines.
Plastic system good
Not yet Good
Not yet Good, tanks cleaned once/year and domes
brushed
Not yet Good, only have had to repair several small
line leaks
Not yet Fair, some slime growth
Not yet Good, tanks cleaned once/year and domes
brushed
Not yet Good
Not yet Good
Not yet Good
Not yet Good, small evidence of slime
Not yet Substantial sliming problem in mid-1980
after 3 yr of operation
Not yet Some line breaks and problems evident, but
overall performance stable
Not yet Good
*Initially. Cleaning has not been required for the last 6 yr.
-------
ORIFICE BOLT
POROUS
ALUNDUM
DOME
100mm uPVC
PIPING
ELASTOMER
DOME JOINT
uPVC
ADJUSTABLE
SADDLE
ACETAL NUT AND BOLT
Notes:
1. 1 mm = 0.039 in.
2. uPVC = Unplasticized
Polyvinyl Chloride
FINISHED
FLOOR
LEVEL
uPVC
FLOOR FIXING
Courtesy Hawker-Siddeley Ltd. and Norton Company
Figure 8. The Norton/Hawker-Siddeley dome diffuser.
Formation of Biological Slimes on Diffusers
The major operational problem associated with the dome diffusers was the
formation of biological slimes on diffusers operating in zones of high volu-
metric loading and/or low D.O. Beddington continues to have major problems
with slime formation, which manifests itself as coarse bubbling at the sur-
face of the aeration tank. The slime growth, shown on Beddington domes in
Figure 9, does not cause an increase in air pressure; rather, it induces an
apparently wholly external surface fouling which causes the air bubbles to
coalesce after exiting the surface of the diffuser domes. The resulting
41
-------
coarse bubbling lowers oxygen transfer efficiency, thereby lowering mixed li-
quor D.O. and further encouraging slime growth.
When first confronted with the problem, Beddington removed and refired
their fouled domes. However, upon startup of a cleaned tank, the problem
quickly recurred and it was soon obvious that other, less costly solutions
were needed. In further tests, it was found that a vigorous brushing of the
dome surface accompanied by high air flow rates would return the dome to
nearly new performance levels. Periodic tank cleaning and dome brushing (de-
picted in Figure 10)have allowed Beddington to control (not eliminate) the
problem at moderate cost.
While Beddington's sliming problem was intensified by the presence of
strong industrial wastes, which depressed oxygen transfer efficiency causing
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 visited plant exhibited
some signs of coarse bubbling, likely 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 parti-
cularly severe in the first 20-50 percent of the first pass of two-to four-
pass plug flow systems. Tapering the aeration helped somewhat but did not
fully cure the problem.
The WRC has conducted experiments to verify the causes of diffuser slim-
ing. The photograph in Figure 11 was taken during one of those experiments
and shows diffuser fouling as a result of very high loading rates. It was
found that the diffusers could be returned to normal service by brushing the
growths off while at the same time allowing air to flow through the units from
the inside.
Several plants observed that mild slime growth could be reversed by
greatly increasing air flow and reducing primary effluent flow for 24-48 hr.
Degremont, a manufacturer of disc ceramic aerators (Section 7), recommends
this procedure as a routine O&M activity.
Following the plant visit to Madison in early 1979, but prior to comple-
tion of the final report for this study, the Madison plant experienced a
major episode of diffuser sliming in all three dome diffuser aerated tanks.
Madison personnel reported that sliming was most pronounced in the first pass.
To summarize, slime growths appeared to occur in zones of heavy organic
loading and/or low D.O. The occurrence of these growths was exacerbated by
extreme plug flow aeration tank design and the presence of strong industrial
wastes. Section 8 discusses design approaches that might ameliorate or elim-
inate the problem.
Scale Formation on Domes
One plant, Basingstoke, had substantial problems with formation of cal-
cium carbonate scale on the exteriors of their domes (Figure 12). The scale
is thickest toward the center of the domes, immediately around the dome hold-
down bolts. Cleaning, required every 5 yr, consists of scrubbing, acid soak-
42
-------
*,» -
»• * «3r •* "
Figure 9. Slime growth on domes at Beddington.
.« >'"4
Figure 10. Brushing domes at Beddington.
Photos courtesy of Beddington works.
43
-------
Photo courtesy of British Water Research Centre.
Figure 11. Clean and slimed domes from WRC tests.
-------
Figure 12. Formation of scale on Basingstoke dome.
45
-------
ing in a 10 percent hydrochloric acid solution for 24 hr, and steam cleaning.
The domes are not refired, although the manufacturer (Hawker-Siddeley) ex-
pressed a belief this should be done at least after every other cleaning.
Some evidence of light calcium carbonate scaling was also seen at Mogden
and Beckton. It did not appear to affect oxygen transfer performance or in-
crease O&M requirements at these plants. A review of the water quality data
for the various plants in and around London indicates that all had hardness
in the range 250-300 mg/1 (as CaC03), but only Basingstoke had alkalinity in
the same range. Basingstoke was receiving industrial effluents from light
manufacturing as well.
Removal, Cleaning, and Replacement of Domes
Plants in the United Kingdom and The Netherlands virtually without ex-
ception scrupulously maintain their aeration equipment, minimizing interrup-
tion of air flow, and maintain a minimum air flow per dome of 0.014 m3/min
(0.5 cfm).
These practices undoubtedly have contributed substantially to the low
incidence of maintenance-related problems encountered with domes in Europe.
With several exceptions, most of the plants had never undertaken a major dome
removal, cleaning, and replacement operation. Three plants, Beckton, Mogden,
and Basingstoke, however, have accrued substantial experience with dome re-
newal. Beckton and Mogden have dome refiring furnaces. Data on the mainten-
ance experiences of all three plants are summarized below.
Beckton Dome Cleaning Program --
In 1971, a diffuser dome refiring furnace, shown in Figure 13, was con-
structed at Beckton. The facility has a capacity of 100,000 domes/yr and was
designed to service domes from other Thames Water Authority plants as
well as Beckton. It was constructed at a cost ofdt66,130 ($132,260 U.S.) in
1971, and its current estimated replacement cost is three-and-a-half times
its initial cost.
The furnace is capable of firing 650 domes per cycle with a fuel oil
consumption of 318£(84 gal). Each cycle lasts about 24 hr and is divided as
follows:
• Heat up to 1000°C (1832°F) - 10 hr
t Hold at 1000°C (1832°F) - 4 hr
• Cool down to 200°C (366°F) - 10 hr.
The firing operation is the principal cleaning method used by Beckton
personnel. It is augmented by acid washing of the domes after firing, using
hydrochloric acid (HC1) to remove light carbonate deposits.
Significant labor is involved in loading and unloading the domes, main-
taining the furnace, and carrying out the complete dome cleaning cycle.
46
-------
~S
tt>
co
fD
O
O
3
Q.
O
fD
-h
_j.
-S
-h
Q)
O
-------
Beckton reported the following labor data:
1. Unload 650 domes from transport pallet, stack in furnace, and start
furnace - 2 man-hr (unskilled).
2. Remove 650 domes from furnace and place in storage - 1.5 man-hr
(unskilled).
3. Load 650 domes into acid cleaning bath, remove, and place in storage-
8 man-hr (unskilled).
4.
Furnace maintenance
a. Every 3 wk - 2.
b. Every 3 mo - 7.
c. Every 6 mo - 7.
5
5
5
(skilled).
hr
hr
hr.
The Beckton furnace is currently processing about 40,000 domes/yr, or 40 per-
cent of maximum capacity.
Mogden Dome Firing Facility --
Mogden has a much older dome firing plant used for cleaning domes at
Mogden only. It is fueled by natural gas and consumes 340 nr (12,000 ft3)
per 900 domes cleaned. Mogden estimated a cost of approximately 20 pence
(40
-------
Routine Dome System Maintenance --
The plant operators reported a variety of approaches, but all could be
summarized thusly:
1. Careful attention to avoiding loss of air flow and maintaining mini-
mum air flow rates per dome.
2. Periodic, usually weekly, operation of the air vents to vent off any
trapped moisture.
3. Prompt repair of air leaks and daily surveillance of aeration pat-
terns to detect abnormalities.
All plant operators stressed that good preventive maintenance and careful at-
tention to air flow rates were much less costly in the long run than the more
frequent full-maintenance cycles (removal, cleaning, replacement) that would
result without such precautions.
Maintenance of Air Cleaning Equipment
All of the plants reported very low maintenance requirements for their
air cleaing equipment, no matter which type was used. Bag filters seemed to
require the least attention, with cleaning cycles of 1-2 yr. Electrostatic
units required three to four cleanings/yr, but each cleaning operation is sim-
ple and takes less than half a man-hr. Every 2 yr, the electrostatic units
require a more thorough workdown, consuming half a man-day. Disposable fil-
ters were the simplest of all, maintenance wise, but replacement filters
are costly (see Section 8) and care must be taken when locating air intakes
to avoid premature fouling from heavily contaminated air.
49
-------
SECTION 7
DOME AND DISC FINE BUBBLE AERATORS
A brief description of dome and disc fine bubble aerators currently
available from U.S. sources at the time that this report was prepared is gi-
ven in this section. Clean water oxygen transfer data for some of the equip-
ment is also discussed here.
THE NORTON/HAWKER-SIDDELEY DOME DIFFUSER
The Norton/Hawker-Siddeley (N/HS) dome diffuser, shown previously in Fi-
gure 8, was in use in all of the plants surveyed in this study. This device,
developed in 1954, has been installed in activated sludge plants world wide.
It is available in the United States from the licensee, Norton Company of
Worcester, Mass. It is 17.8 cm (7 in.) in diameter and constructed of alumi-
num oxide (alundum), which is fused at high temperature into a rigid, dome
shape. The manufacturer specifies an average pore diameter of 150 microns.
It is normally installed in a grid configuration covering the entire aeration
tank floor with unplasticized polyvinyl chloride (uPVC) air supply piping and
appurtenant hardware.
OTHER FINE BUBBLE DIFFUSION EQUIPMENT
Because this study focused on long-term O&M and oxygen transfer perfor-
mance data, N/HS dome diffusers were emphasized. However, during the course
of the work, the number of similar competing dome or disc aerators available
in the United States expanded rapidly. As of June 1980, five U.S. vendors of
this equipment were identified:
t Ajax International Corporated San Diego, California
• Envirex, Incorporated
t EPI Corporation
• Infilco/Degremont
• Sanitaire, Incorporated
t W.E. Farrer, Limited
Milwaukee, Wisconsin
Richmond, Virginia
Richmond, Virginia
Milwaukee, Wisconsin
Birmingham, England*
Disc diffuser
Disc diffuser
Disc diffuser
Disc diffuser
Disc diffuser
Dome diffuser
* Presently not sold in the United States^
50
-------
This equipment is discussed in turn below.
Ajax International Corporation
Ajax is the U.S. vendor for the ceramic disc diffuser system manufac-
tured by Ames Crosta Babcock, Ltd., of the United Kingdom. The discs are
18.5 cm (7.3 in.)in diameter and are secured in a uPVC holder with a threaded
collar (Figure 14). Air is metered to each disc through a control orifice
similar to the N/HS diffuser. A grid diffuser configuration is employed.
Envirex, Incorporated
Envirex markets a ceramic disc, 23 cm (9 in.)in diameter, which are
mounted on uPVC pipe in a grid arrangement. The diffuser housing snaps onto
the pipe grid using a snap-on attachment developed by Envirex for thin wall
pipe (Figure 15). The discs are flat and are molded of fused aluminum oxide.
EPI Corporation
EPI has recently been licensed to market the Nokia diffuser line in the
United States. Nokia, a Finnish manufacturer, supplies the Nokia disc in ei-
ther ceramic or porous polyethylene plastic (Figure 16). The latter is
provided in either a medium bubble or fine bubble configuration. The geome-
try of the device is similar to that of the N/HS dome aerators in that a con-
trol orifice is used to meter the air into a conical housing, which is sup-
plied mounted on a uPVC grid.
The polyethylene plastic diffuser element differs markedly from the ce-
ramic units provided by other U.S. sources. EPI reports that it is cleanable
in soap and water and that there are approximately 30 installations world-
wide, mostly in Western Europe.
Infilco/Degremont
Infilco/Degremont is the U.S. subsidiary of the French firm, Degremont,
a major European manufacturer of water and wastewater equipment. Degremont
has marketed an alumina disc diffuser (Figure 17) on the continent for some
time. As discussed later, the device has been tested by Dutch investigators
and found to be similar in performance to the N/HS domes. It is designed as
a flat plate, 22.9 cm (9 in.) in diameter, 19 mm (3/4 in.) thick, and is sup-
plied on a uPVC piping grid which uses some brass hardware.
Sanitaire, Incorporated
The Sanitaire disc diffuser, shown in Figure 18, measures 22 cm (8.7
in.) in diameter and is installed in a grid layout on uPVC piping. The dif-
fuser disc is constructed of fused alumina and is retained on the conical
housing by a threaded collar. Air is metered to each disc through a control
orifice.
51
-------
Figure 14. The Ames Crosta-Babcock diffuser.
X
CPOLYPROPYLJEME)
WPE
SUPPORTfPVCJWIfH
RAWL OR EQUAL
Figure 15. The Envirex disc diffuser.
52
-------
HKL210or MKL 210 diffuser,
side and front
Figure 16. The EPI/Nokia disc diffuser.
Figure 17. The Infilco/Degremont disc diffuser.
53
-------
Figure 18. The Sanitaire disc diffuser.
Figure 19. The W. E. Farrer dome diffuser.
54
-------
W.E. Farrer, Limited
The W.E. Farrer diffuser is quite similar to the N/HS unit, using a 17.8-
cm (7-in.) dome diffuser mounted on uPVC pipe, in a grid system. The device,
shown in Figure 19, features a fully sealed dome unit which is affixed to the
manifold directly, eliminating the conical dome plate used on the N/HS dome.
The Farrer diffusers are currently not directly marketed in the United States;
however, it is quite possible that they will be sold here in the future.
CLEAN WATER OXYGEN TRANFER PERFORMANCE DATA
Table 10 summarizes some recent water test data for the fine bubble aera-
tors listed above. All tests were conducted using the non-steady state sul-
fite reaeration (cobalt catalyzed) test method. All of the tests were con-
ducted within 5°C (9°F) of the standard 20°C (68°F) test temperature and were
carried out by experienced investigators.
Recognizing the dangers inherent in comparing clean water test data from
widely divergent sources, Table 10 nevertheless exhibits remarkable consis-
tency relative to the different tests for the N/HS dome. Additional data not
shown in this table parallel these results closely. It appears that clean wa-
ter aeration efficiency for this device will fall in the range of 4.5-5.0 kg
02/kWh (7.4-8.2 Ib/wire hp-hr) for submergence depths of 4.0-4.5 m (13-15 ft).
The various dome or disc diffusers appear to perform similarly when test-
ed under the same conditions. At Los Angeles County Sanitation Districts
(LACSD), the N/HS and Sanitaire units tracked closely, even though there were
fewer of the Sanitaire units (98 vs. 126) in the tank. At Holten-Markelo, the
Nokia and N/HS tracked closely; the Degremont dome, at first glance, performed
better. However, unlike the LACSD tests, the number of diffusers in the tank
was not adjusted to reflect the larger (22.9 cm vs. 17.8 cm, 9 in. vs. 7 in.)
diameter of the Degremont disc. The Dutch researchers who conducted these
tests stated that they considered the performance of the three units nearly
equivalent, with the ceramic units perhaps having a slight advantage over the
Nokia units. The Nokia disc and the N/HS dome are the same diameter.
At the air flow rates that would be found in activated sludge systems,
0.014-0.042 m3/min/diffuser (0.5-1.5 cfm/diffuser), the air will primarily
exit the horizontal surfaces of either dome or disc diffusers. Thus, a diffu-
ser with a larger surface area would have a higher transfer rate per dome and
fewer diffusers would (theoretically) be needed to tranfer a given amount of
oxygen. The N/HS vs. Sanitaire data from LACSD (17.8 cm vs. 22 cm or 7 in.
vs. 8.7 in. diameter) and N/HS vs. Degremont data from Holten-Markelo (17.8 cm
vs. 22.9 cm or 7 in. vs. 9 in. diameter) would seem to bear this out. The re-
lationship between disc/dome area and oxygen transfer capability per dome
should be studied further. A 20-25 percent differential in diffuser units re-
quired in a large activated sludge plant could be quite significant in terms
of equipment costs.
55
-------
TABLE 10. CLEAN WATER OXYGEN TRANSFER EFFICIENCY COMPARISON
cr>
Device
N/HS Dome
N/HS Dome
N/HS Dome
N/HS Dome
Sanitaire
Disc
N/HS Dome
Nokia Disc
Degremont
Disc
Submergence
Data Depth
Source Description (m)*
Paulson, 16 37.8-tn3 (10,000-gal) 4.4
treated river water supply 2 8
from municipal system
Paulson & 250-m3 (66,000-gal) treated 4.1
McMullen, 17 lake water from municipal 3.5
system
WRC #537R, 18 9.6-m3 (2530-gal) 2.0
lab-scale test tank at WRC 2.8
LASCD+++.14
LASCD++f-, 14
170 m3 (45,000 gal)
municipal water
6.1-m (20- ft) square tank
4.3
4.3
4.3
^ <^"
Hoi ten-Ma rkelo,
19
Ref. 19
Ref. 19
/
Cylindrical 9.5-m (2500-gal)
test tank at Holten-Markelo
(Netherlands) wastewater
treatment plant
3.8
3.8
3.8
3.8
3.8
k ^-J 3.8
Diffuser
Density
(domes/n^)**
4.8
2 6
3.6
3.6
2.5
2.5
3.4
3.4
2.6
2.8
2.8
2.8
2.8
2.8
2.8
Air Flow
Dome
(cfm)+
1.0
1.0
0.5
0.5
1.0
1.0
0.6
1.0
1.1
0.6
1.2
0.6
1.2
0.6
1.2
Oxygen Transfer
Efficiency
(*)
31.5
19.4
35.9
25.1
17.4
10.0
31.3
29.7
34.0
36.2
33.7
33.8
32.0
49.7
40.1
Remarks
Lab-scale tests
Equivalent to 4.9
kg 02/kWh++
Second test with
6 mg/1 of detergent
Equivalent to 4.92
kg 02/kWh++
Equivalent to 4.52
kg 02/kWh++
* 1 m = 3.28 ft
** 1 dome/m2 = 9.29 domes/100 ft2-
+ 1 cfm = 0.028 m3/min
++ 1 kg/kWh = 1.64 Ib/wire hp-hr
+++ LACSD - Los Angeles County Sanitation Districts
-------
SECTION 8
DESIGN OF DOME DIFFUSER ACTIVATED SLUDGE SYSTEMS
In visiting and studying a large number of dome diffuser activated
sludge plants, it was observed that O&M played a large part in oxygen trans-
fer efficiency. However, it was also apparent that design was as much or
more a factor in efficient use of electrical power. This section discusses
the principal design factors that affect oxygen transfer performance and aera-
tion efficiency and offers suggested approaches for more efficient design.
CHARACTERISTICS OF DOME DIFFUSERS THAT AFFECT DESIGN
Like any process equipment, dome diffusers have performance characteris-
tics that define design boundaries. These include the following:
• Range of air flow per diffuser
• Diffuser density
• Diffuser depth.
It was observed that dome diffusers have flexibility in design that has not
been fully exploited by existing design practices, particularly as related to
these three factors.
Air Flow Range
Any given dome diffuser will have a minimum air flow below which it
should not be operated and a maximum air flow above which oxygen transfer ef-
ficiency decreases rapidly. The minimum air flow rate for the N/HS dome is
defined by the headless across the control orifice (see Figure 20). At an
air flow rate of 0.014 m3/min/dome (0.5 cfm/dome), the headloss across the
control orifice is essentially zero and the headloss across the dome is about
12.7 cm (5 in.) of water, which is also equal to the distance from the invert
of the air supply pipe to the inside surface of the dome (Figure 20). Thus,
in operation, the air pressure in the pipe counterbalances the outside pres-
sure, minimizing the infiltration of mixed liquor into the pipe. In addition,
plant operators at Mogden reported that mixed liquor appeared to infiltrate
through the lower part of the sides of the dome at lower air flow rates.
Based on these and similar observations, it would appear that Norton/Hawker-
Siddeley's suggested minimum air flow rate of 0.014 m-Ymin/dome (0-5 cfm/dome)
is not overly conservative and should be followed in design of N/HS dome ae-
rated plants.
57
-------
3.5 m3/hr = MAXIMUM VOLUME
178 mm DOME WITH
CONTROL ORIFICE
178 mm DOME ONLY
i
100
0-5 I-O I-5 2-0 2-5 3-0
AIR FLOW (m3/hr) THROUGH 178 mm DIA. DOME
Notes:
1. 1 m3/hr =35.3 ft3/hr
2. 1 mm = 0.039 in.
Courtesy of Hawker-Siddeley, Ltd.
Figure 20. Dome diffuser head loss vs. air flow rate.
58
-------
Maximum air flow rates per dome are selected on the basis of their rela-
tionship with oxygen transfer efficiency, which decrease with increasing air
flow. Clean water studies of the N/HS diffuser at LACSD14 produced the results
given in Figure 21. Wire-to-water efficiency at the 4.6-m (15-ft) water depth
decreased by about 14 percent in the air flow range of 0.017-0.56 m^/min/dome
(0.6-2.0 cfm/dome). A similar study at Steenwijk (The Netherlands) showed
a 13 percent loss in oxygen transfer efficiency over the same air flow range.
The Steenwijk study was performed as a full-scale test, using a portion of the
plant's aeration tank which had been baffled off to produce a test area mea-
suring 6.4 m x 6.8 m x 4 m deep (20.9 ft x 22.3 ft x 13.1 ft).
Most of the U.K. plants visited were designed using an air flow range of
0.014-0.028 m3/min/diffuser (0.5-1.0 cfm/dome)*. Several U.K. designers ex-
pressed the belief that 0.021 m3/min/dome (0.75 cfm/dome) was the optimum op-
erating air flow rate. This rather narrow design air flow range is apparently
justified in the United Kingdom by the above described decrease in clean water
oxygen transfer efficiency exhibited by the N/HS diffuser with increasing air
flow rate.
The plant survey identified a fairly consistent pattern of aeration sys-
tem overdesign and lack of operational flexibility, at least partly attribu-
table to the aforementioned use of narrow air flow ranges for dome systems.
The number of domes required was based on very conservative loading considera-
tions as well. In actual operation many of the plants found it necessary to
operate at a minimum total plant air flow rate dictated by the number of in-
stalled domes x 0.014 nr/min (0.5 cfm) during periods of reduced flow and load
and low oxygen demand. The result was that the plants overaerated during much
of the 24-hr diurnal cycle. Strongford operated at minimum allowable air flow
for 18-20 hr/day. Mogden operated for at least 12-16 hr/day at minimum allow-
able air flow. At Beckton, an ongoing program of experimental dome removal is
addressing the overaeration problem (see Appendix B).
Use of a wider air flow range, up to 0.057 m3/min/dome (2.0 cfm/dome) dur-
ing peak load periods, would allow the use of fewer domes, lowering the manda-
tory total plant minimum air flow rate. Rarely would the maximum oxygen demand
be greater than three to four times the minimum demand in a municipal treatment
plant. If the number of domes is derived equating the anticipated minimum oxy-
gen demand during a typical day to the minimum recommended air flow rate of
0.014 m3/min/dome (0.5 cfm/dome), the system should be more than capable of
meeting maximum oxygen demand without causing overaeration during much of the
daily treatment cycle. Hawker-Siddeley currently recommends that their diffus-
ers be designed for a 5:1 maximum:mini mum air flow ratio.
In advancing this argument, it is recognized that the diffusers would be
operated at less than optimal efficiency during a portion of the daily treat-
ment cycle. However, as discussed in Section 6 of this report, the alpha
The air flow rates cited above are directly applicable to the N/HS diffuser
only. The larger Sanitaire and Degremont discs could be expected to have
higher minimum and maximum air flow rates. However, if the number of dif-
fusers is adjusted to reflect the larger surface area of these discs, the
overall effect should be the same.
59
-------
o
m
u
u.
IL
Ul
oe
ui
u.
a
I
X
o
o
K
O
Z
50
45
40
35
30
25
20
15
10
5
• 10 ft DEPTH
• 16 ft DEPTH
• 20 ft DEPTH
A 25 ft DEPTH
0 I 0.2 I 0.4 I 0.6 I 0.8 I I.O I I.2 I I.4
O.I 0.3 0.5 0.7 0.9 I.I I.3 I.5
DELIVERED POWER DENSITY (hp/1000 ft3)
120 FINE BUBBLE CERAMIC DIFFUSERS APPLIED IN A TOTAL
FLOOR COVERAGE CONFIGURATION
Notes:
1. 1 ft = 0.305 m
2. 1 hp/1000 ft3 = 0.026 kW/m3
Source: Reference 14
Figure 21. Relationship between specific oxygen
transfer and air flow per dome.
60
-------
factor is affected by mixing power level, tending to higher values as mixing
intensity increases. It is possible in an operating activated sludge tank
that a three-or four-fold increase in the mixing power level could increase
alpha sufficiently to offset much of the oxygen transfer efficiency lost at
higher air flow rates. Also, the net daily treatment efficiency, expressed
as units of BODg removed per kWhof power could well be improved significant-
ly by reducing or eliminating the long periods of overaeration.
This question should be researched as part of an overall study to optimize
the design of dome/disc systems. It is the authors' belief that design of dome
diffuser systems should be predicated on a wider air flow range, possibly
0.014-0.056 nrVmin/diffuser (0.5-2
would considerably improve overall
Diffuser Density
area. Theoretically, there would
oxygen demand
when the tank
the perimeter. Practically speaki
allow for cleaning
dome centers is a 5
ty of 10.8 domes/m'
.0 cfm/diffuser),and that this approach
system efficiency.
Diffuser density is defined as the number of aerators per unit floor
36 no lower limit except as dictated by
and mixing considerations and the upper limit would be reached
floor was covered wi
and other maintenance.
ractical minimum limit,
(100.3 domes 100/ft2).
th diffusers, each touching the other at
ng, some spacing between domes is needed to
Several researchers have noted
efficiency with increasing diffuser
tests on N/HS domes in a bench-scale
over a range of depths. Figure 2
those tests, run using the non-
of detergent added. Oxygen
than would be predicted by the i
estingly, these data tend to
with higher air flow (power level]
efficiency per dome with
the apparent increase in oxygen transfer
density. Lister and Boon2! conducted
tank of 2.25-m2 (24.2-ft2) floor area
(A) summarizes the results at one depth of
stelady state reaeration procedure with 5 mg/1
transfer efficiency increased somewhat faster
ncrease in total gas flow rate alone. Inter-
support the possibility of alpha enhancement
as a possible offset to lower clean water
air flow.
increasing
Paulson16 evaluated N/HS dome!s
employing the non-steady state sul
in Figure 22(B), depict an almost
with increasing diffuser density.
Therefore, when designing ne
mi zed within the contraints of mi
and economic costs. It is expect
provement possible with optimized
than those which would accrue froiln
tors being equal. It is possible
dome diffuser plants might be sub
posed by process requirement, by
designed in this manner should reft
associated with increased diffuse
at the same time still alowing thi
volumetric loadings for dome (or
studied to determine practicabili
A spacing of 30 cm
corresponding to a
(1 ft) between
diffuser densi-
in clean water (no detergent added), also
fite reaeration method. His results, shown
linear rise in oxygen transfer efficiency
systems, diffuser density should be maxi-
iimum allowable air flows (discussed above)
d, however, that the aeration efficiency im-
air flow ranges would be substantially higher
increased diffuser density alone, other fac-
that allowable volumetric organic loadings of
stantially increased, within constraints im-
jtilizing greater diffuser densities. A plant
lize the higher oxygen transfer efficiency
density and greater turbulence levels, while
use of an optimized air flow range. Higher
disc) fine bubble diffuser systems should be
ty and limits.
61
-------
ro
O
c:
,
X
O
36
35
34
33
32
31
30
29
28
27
26
25
24!
Notes:
1. PAULSON (Ref. No. 16)
2. 4.6-m (15.1-ft) test depth
3. No detergent added
0.5 cfm/dome—
-1.5 cfm/dome
(B)
0246
Diffuser Density (domes/m2)
Figure 22. Effect of diffuser density on oxygen transfer efficiency.
-------
Diffuser Depth
A review of the clean water research work cited earlier reveals that all
of the studies noted a nearly linear correlation between oxygen transfer ef-
ficiency and depth, up to about 6.1 m (20 ft), with or without detergent ad-
detion. The LACSD tests on the N/HS dome, plotted in Figure 23 (B), are typi-
cal of these results. The tapering off of oxygen transfer efficiency at great-
er depths is caused by oxygen depletion in the bubble* which places an overall
limit on the efficiency/depth relationship. Paulson'" found transfer efficien-
cy to be essentially linear up to a test depth of 4.6 m (15 ft) if data scatter
is taken into account. Figure 23 (A) summarizes several years of testing by
Wheat!and and Boon22 at the WRC, who concluded that oxygenation capacity, with
5 mg/1 detergent added, is essentially linear in the depth range of 1-8 m (3-26
ft). The oxygenation rate does not taper off at greater depths in their plot
because bubble oxygen depletion occurs less rapidly due to the surfactant ef-
fect of the added detergent.
Assuming that oxygen transfer efficiency is linear up to 6.1 m depth
(20 ft), a comparison of power required vs. depth to transfer an equivalent
amount of oxygen can be developed, using the blower equation:
BHPadiabatic = [WRT1/(33,000 nE)] [(P2/Pl)n-l .0] (17)
where:
W = weight flow of air, Ib/min
R = gas constant, 53.5
n =0.283 for air
E = combined efficiency of blower, coupling, and motor
T] = inlet air temperature, degrees Rankine
P! = absolute inlet air pressure, psia
?2 = absolute outlet air pressure, psia.
The weight of air flow, W, can be related to the inlet air flow, as follows:
W = ICFM x Pairj°F (18)
where:
ICFM = inlet air flow, actual cfm at inlet conditions P] and TI
Pai>T°F = density of air flow at inlet conditions, lb/ft3.
The density of air is proportioned to temperature and pressure. Thus, air
density at inlet conditions can be related to those at standard conditions
as follows:
PairSTD/PairToF = (PsTD/Tso)/^!/1"!) (19)
63
-------
No.OFDlFFUSERS NOMINAL AIRFLOW RATE PER DOME
0
1 234567
Depth of Water Above Diffusers (m)
Notes:
1. 1 m = 3.28 ft
2. 1 kg/m2/hr = 0.205 Ib/ffYhr
3. 1 m3/min =35.3 ft3/min
4. Floor area = 2.25 m2 (24.2 ft2)
5. 5 mg/1 detergent added.
6. Source: Reference 22
LU
i-
CO
c
0)
CD
50
c
OJ
o
I 40
O
c:
OJ
o
30
20
10
10
LEGEND (power delivered to the water)
o 0.3 hp/1000 ft3
A 0.5 hp/1000 ft3
O 1.0 hp/1000 ft3
(B)
15
Tank Depth (ft)
20
25
Notes:
1. 1 ft = 3.28 m
2. 1 hp/1000 ft3 = 0.026 kW/m3
3. Floor area = 36.5 m2 (400 ft2)
4. No detergent added.
5. Source: Reference 14
Figure 23. Variation of diffuser oxygenation efficiency with depth.
-------
Substituting in Equation (17):
BHPadiabatic = (4.2965 x lQ-4/e)(T1 )(SCFM)[(P2/P1) 0.283_-| <0] (20)
This relationship is used to develop a comparison of blower efficiency with
depth, based on the following assumptions:
e = combined efficiency of motor, coupling, and blower = 0.65
P2 = hydrostatic pressure + 1 psia system headless
P-j = inlet pressure at standard temperature of 68°F (20°C) = 14.7 psia
The results, plotted in Figure 24,show a 15.4-percent reduction in blower
power required as depth is increased from 2.4 to 4.6 m (8 to 15 ft). A further
increase to a 9.1-m (30-ft) depth reduces the calculated power demand by an
overall 28.7 percent. Of course, any power savings at increased depths may be
offset by non-linearity of oxygen transfer at higher depths, and by deviations
of actual blower systems from the idealized conditions of this analysis. Never-
theless, it is probably realistic to expect some efficiency gains with increas-
ing depths, up to 9.1 m (30 ft). Further study in mixed liquor of the rela-
tionship of depth to oxygen transfer efficiency (OTE), and the relationship
of blower power to depth (with linear OTE), is recommended.
30 r
25
20
15
(O
§ 10 L
(O
i-
o>
0
Notes:
5 I I- 1 ft = 0.305 m
2. 1 BMP = 0.746 brake kw
30
35 40
Blower BHP Requirement
45
Figure 24. Blower brake horsepower requirement vs. aeration tank depth.
65
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PRIMARY AND SECONDARY CLARIFIER DESIGN
As noted in Section 6, primary and secondary clarifier design was ob-
served to be a significant factor in the impressive pollutant removal perfor-
mance of the U.K. plants. Primary clarification consistently removed 45-55
percent of the influent BODr and 55-70 percent of the influent suspended sol-
ids. Primary clarifier configuration was basically the same ,as that used in
the United States. However, surface loading rates were substantially less.
Several British design engineers noted that the philosophy in the United
Kingdom is to remove as much of the influent suspended solids and BOD as
practicable to minimize power requirements in the aeration process and maxi-
mize gas production in the anaerobic digesters.
The tradeoff between expenditure for larger primary tanks vs. aeration
power is highly plant specific and will not be pursued further here. However,
the British approach suggests that primary clarifier design should be given
more careful consideration in future new plant design in the United States.
Larger primary clarifiers that might remove 40-45 percent of the influent
BOD5 could be economically justifiable in light of sharply higher power costs.
Likewise, U.K. secondary clarifier designs were quite conservative and
appeared to give plant operators more reserve capability in the event of pro-
cess upsets and episodes of poor sludge settleability. However, the long de-
tention times used could be a source of problems with sludge blanket denitri-
fication as well, as noted in Section 6. The U.K. design configuration that
utilizes steeply sloped floors (30° slope) may merit further attention in the
United States in view of the very good operating experience with this simple
design (see Figure 2, Section 6).
TAPERED AERATION
The term "tapered aeration" is used to describe systems which have pro-
gressively fewer diffusers as the tank is traversed from the inlet to the out-
let end. Figure 25 shows a schematic of a plug flow tank at Oxford with ta-
pered aeration. The diffuser distribution shown in Figure 25 is quite typical
for this design approach. The diffusers in the last quarter section are con-
centrated in the center of the tank, causing a double spiral roll mixing
effect in this zone of the tank.
The basis for tapered aeration is derived from the two principal mechan-
isms that affect oxygen transfer in a plug flow tank:
f Rate of oxygen demand decreases with distance from the inlet end
• As surfactants (surface active agents) are oxidized in the process,
alpha values increase; hence (all other factors unchanged) oxygen
transfer efficiency increases along the tank length (Figure 26).
Seeking to create a relatively flat D.O. profile (with distance from the inlet
end), tapered aeration was empirically developed by Hawker-Siddeley and first
applied in the late 1960's. It has been used since that time, with variable
success, on most of the plug flow systems equipped with Norton/Hawker-Siddeley
66
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dome diffusers. Most of the tapered aeration plants visited during the course
of this study still exhibited rising mixed liquor D.O. as the flow passed from
inlet to exit; however, the rise may have been more moderate than it would have
been with a symmetrical diffuser configuration. Qualitatively, it appears that
tapered aeration may be more effectively used in single-pass systems such as
Oxford. Further study of this possibility is suggested.
AERATION BASIN GEOMETRY
Dome diffusers have several operating characteristics that are markedly
affected by aeration basin geometry:
• Alpha sensitivity and variation
• Sliming tendency in zones of high localized loading and/or low D.O.
• Minimum allowable air flow requirements
• Depth vs. oxygen transfer efficiency (discussed above)
• Other factors.
As noted in Section 6, all of the surveyed plants had varying degrees of ex-
cess aeration in multi-pass systems and slime growth on domes in zones where
primary effluent was introduced. These problems were largely caused by aera-
tion basin geometry that is poorly suited to the optimum operating character-
istics of dome diffusers. Discussed below is each major characteristic and
how it is impacted by current basin design practices.
71
1
1
1
1
1
1
•
<« 1
Grid 1
425 domes
M rows
i
Grid 2
270 domes
9 rows
i
I
1
1
1 .)
1
1
I
|
Grid 3
198 domes
9 rows
Grid 4
90 domes
3 rows
Figure 25. Tapered aeration at Oxford.
67
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Alpha Sensitivity
As discussed in Section 6, fine bubble dome diffusers exhibit alpha sen-
sitivity, i.e., they are greatly affected by the presence and concentration
of surfactant contaminants in the wastewater. Wheatland and Boon22 summariz-
ing the investigations of the WRC over a period of years note that alpha in
plug flow dome diffuser systems can double as wastewater is progressively oxi-
dized in passing from the inlet to the outlet end (Figure 26). Thus, all other
things being equal, a dome diffuser in the outlet section of a plug flow sys-
tem will be about twice as efficient as one at the inlet. Tapering of the ae-
ration system, used to deal with this inbalance (Figure 25), is only partly
successful because of the high localized volumetric loading and sliming that
occurs.
0.8,-
0.4
Ol
3
D
D
D
Inlet
J.
_L
Outlet
J
400
100 200 300
Cumulative Oxygen Absorbed by Wastewater (mg/1)
Notes: waste Depth (m)
37 °
1. 1 m = 3.28 ft
6.1 ^
2. Data from nitrifying plants 8 -j n
3. Source: Reference 22
Figure 26. Variation of alpha with degree of treatment.
68
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Sliming and Plug Flow
Boon and Burgess23 reported on studies that indicated dome diffusers
could effectively transfer oxygen into activated sludge systems that were ex-
periencing high volumetric loading rates. However, a side effect of this
condition was the heavy slime growth shown earlier in Figure 11. Periodic
brushing maintained oxygen transfer efficiency in this test case. Otherwise,
the slime growth caused the fine bubbles emerging from the diffuser stone to
coalesce into medium and coarse bubbles, markedly reducing oxygen transfer
efficiency. A similar phenomenon has occurred with regularity at Beddington
(discussed in Section 6), apparently caused by the strong industrial wastes
being treated there.
Recently, the Madison, Wisconsin plant also experienced extensive dif-
fuser sliming related to increased loading on the dome diffuser tanks. A
combination of a 40-percent increase in BODs loading from a dairy waste and
decreased air flow to the dome tanks caused rapid slime growth which began at
the head of the two three-pass step feed tanks and progressed to the effluent
end of the first pass over a period of 2 mo. Oxygen transfer efficiency mea-
surements by plant personnel indicated that aeration efficiency decreased by
nearly 50 percent as a result of the sliming. Draining the tanks and steam
cleaning were used by Madison to restore aerator performance. The Madison ex-
perience, coming after 3 yr of stable performance with little apparent sliming,
further strengthens the suspected relationship of dome sliming to zones of low
D.O. and/or high volumetric loading.
As noted earlier, sliming was apparently occurring to some degree in most
of the visited plants and was most noticeable in those plants with a high de-
gree of plug flow. Plug flow exacerbates volumetric loading by effectively
causing an initial zone in the inlet area of the aeration tank of relatively
high F/M loading with associated high oxygen demand. At the same time, the
alpha factor in this zone is likely to be low as a result of surfactants in
the incoming raw wastewater. With increasing L/W, all of these effects become
more pronounced, with a resulting increase in the number of diffusers affected
by slime growth. Tapering the aeration is a partial solution, limited by the
aeration/mixing requirements of the lightly loaded back end of the plug flow
system. Step feeding helps distribute oxygen demand and alpha depression more
equally; however, its use is limited in single-stage nitification systems.
Hawker-Siddeley reports that some plants have had good experience with feeding
raw wastewater downstream of the return sludge feed point, effectively creat-
ing a zone of sludge reaeration in the first section of the aeration tank.
Commensurate with the process advantages of plug flow, new dome (or disc)
diffuser systems should be designed at the lowest practicable L/W. Tapered
aeration and provision for step feeding at least to mid-length should be pro-
vided. D.O. monitoring and provision for independent air flow control should
be provided for each aeration grid and/or pass (in multiple channel) systems.
In designing plug flow systems, the oxygen demand for each distinct segment
of the aeration process should be calculated and the aeration system should be
sized taking into account the variation of alpha from inlet to outlet.
69
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An optimal system configuration, providing the benefits of plug flow
with more efficient utilization of dome (disc) diffusers, should be the subject
of further study, preferably in a full-scale operating facility.
Air Flow Requirements
The rationale behind minimum allowable air flow was discussed above. It.
is believed that the lower limit is valid and should be observed in operation.
Careful adherence to minimum allowable levels was the standing rule at virtu-
ally all of the U.K. plants, and it is believed that this practice played an
important part in the excellent aeration system maintenance history at most
of these plants.
However, the adherence to minimum air flow per diffuser was also appar-
ently a main reason for the mediocre energy efficiency of many of the plants.
Plant operators at most of the plants were aware of the connection between
minimum air flows that greatly exceeded process air requirements much of the
time and higher than necessary electrical costs. Better matching of process
and aeration tank design to diffuser system design constraints will probably
be the most effective solution to the problem of overaeration.
Other Factors
Depending on the age of the plant, some aeration basins were constructed
with ridge and furrow floor designs. This configuration, shown in Figure 27,
was originally developed as an aid in mixing and tank circulation. It has
since been determined in the United Kingdom that it is costly to construct
and adds little to performance; consequently, it was not seen in the newer
plants included in the survey.
Figure 27. Ridge and Furrow tank at Ryemeads.
70
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Filets along the bottom of the tank where the floor meets the wall were
observed in about half of the plants. Where whole tank coverage of diffusers
is used, it is believed that fillets are not totally necessary, though they
probably prevent the deposition of small amounts of solids in the corners of
the aeration basin.
EFFECT OF PROCESS DESIGN
It is believed by the authors that dome diffuser systems in complete mix
basins might be operated at volumetric and F/M loadings in excess of those
currently used. Except for lab-scale studies at the WRC, no real test of this
belief has been made, either by the manufacturers of this equipment or in ac-
tual plant designs. The highest F/M loading rates observed were in the U.S.
plants at Glendale (0.35 kg BOD5/day/kg MLSS) and Madison (0.3 kg BOD5/day/kg
MLSS). F/M loadings up to 0.5-0.6 kg BOD5/day/kg MLSS might be achievable
with this equipment. If "practicable, this would allow operation at loading
rates approaching that of pure oxygen systems (0.6-0.8 kg BODc/day/kg MLVSS)
and could significantly affect economic comparisons between the two alterna-
tives. Recognizing that sludge settling characteristics and diffuser sliming
may impose the ultimate limit on loading, research along these lines is strong-
ly recommended.
Single-stage nitrification appears to be practical, and potentially ef-
ficient, based on the results from Oxford. The use of anoxic zones in the
aeration tank, as typified by Ryemeads, has definite merit as an aid to clar-
ifier performance but may pose problems for the downstream aerobic zones.
The first aerobic zone following the anoxic zone always exhibited severe
coarse bubbling, indicative of slime growth. A change in tank geometry, a
higher concentration of diffusers, and/or higher air flow rates may be needed
to alleviate this problem. Anoxic zones should be mechanically mixed. Full
scale study of anoxic zone optimization is also suggested.
MIXING
Minimum and average mixing power levels in use at the surveyed plants
often were quite low (refer to Table 3). Usually, the power imput required
to satisfy oxygen demand exceeds that needed to prevent solids deposition,
i.e., to sustain adequate mixing. The data in Table 3 indicate that power
levels as low as 10 W/m^ (0.38 wire hp/1000 ft3) are satisfactory for suspen-
sion of biological solids. Concentrating the aerators along the center of the
tank in tapered aeration systems, creating a spiral flow situation, enhances
mixing at the expense of oxygen transfer and can be avoided by avoiding the
long multi-pass plug flow designs that require excessive diffuser taper in back-
end passes to prevent overaeration.
DESIGN OF AIR CLEANING SYSTEMS
Of the three types of air cleaning systems observed, the replaceable
filters are the simplest to construct and operate. For example, a replace-
able air filter assembly for an air flow of 283 m3/min (10,000 cfm), adequate
for a 75,700-m3/day (20-mgd) municipal wastewater treatment plant, costs a-
bout $1200 for the enclosure and $1000 for the filter elements. Only one en-
closure is required as system backup is provided by stocking a set of re-
placement air filters at a cost of $1000 more. By comparison, an electrosta-
71
-------
tic air filter system for the same size plant would require two units, each
sized for an air flow of about 198 m3/min (7000 cfm), allowing for temporary
operation on one unit when the other is shut down for maintenance. Equipment
costs would be at least $25,000 for this approach.
Replaceable air filter systems are generally provided with a coarse fi-
berglass prefilter, followed by fine final filters. The prefilter normally
consists of a roll of fiberglass cloth mounted on a frame across which it is
automatically advanced by an opacity sensor. The final filters are housed
in racks behind the prefilter (Figure 28). This system will remove 95 per-
cent of the particles in excess of 0.3 microns and larger, the required stan-
dard for N/HS dome diffusers.
Bag house dust collectors are bulky and expensive, though apparently not
maintenance intensive. None of the U.S. installations visited use bag house
systems; rather, most prefer replaceable units. Four of the newer U.K.
plants utilize bag houses, and the Dutch plants all employ electrostatic pre-
cipitators.
Replaceable air filters are the recommended approach except where poor
air quality requires replacement of the fine filtration elements more fre-
quently then once per year. In this case, electrostatic units may be cost
effective and should be considered.
DIFFUSER CLEANING SYSTEMS -
A variety of techniques and equipment for diffuser cleaning were evident
during the plant surveys. Organic fouling was generally removed by dome re-
firing. Acid washing combined with clean water and steam cleaning was used
to deal with the inorganic calcium carbonate fouling observed at Basingstoke
and to a lesser degree at Beckton.
As detailed in Section 6, ceramic diffuser firing systems are costly to
build and use and are probably not justified except for the very largest
plants. Their advantage is the wide range of fouling problems that can be
successfully remedied by this approach and the number of diffusers that can
be cleaned at a time.
Ultrasonic cleaning has been experimented with in the United Kingdom and
installed at Fort Worth, Texas, in the United States. The units at Fort
Worth operate at 25 kilocycles/sec frequency and can clean six domes per 8-
min cycle per unit. The diffusers are presoaked in a hot bath (40°C, 104°F)
of clean water to which a wetting agent (e.g., detergent) has been added. An
estimated 600 diffusers can be cleaned per operator work shift.
Currently, no U.S. or foreign plant has operated ultrasonic cleaners
over a long enough period of time to establish their suitability for this ap-
plication. In view of the high cost of the equipment, caution is suggested
in relying on ultrasonic cleaning for diffuser cleaning. Firing and/or acid
washing have been well established as effective techniques and should be used
where possible.
72
-------
OJ
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o
o
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a;
i.
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DESIGN TO FACILITATE O&M
Plant designers can ease the operator's work by building in aids to fa-
cilitate dome diffuser system maintenance. The following items were observed
to be needed and/or were mentioned most frequently by operators:
• Provision for rapid tank draining and cleaning. -- Tanks should be con-
structed with longitudinal drain channels feeding into a sump. Con-
nection of the sump to plant drains/pumping is preferred; however, a
convenient-to-use portable pump is satisfactory.
• A simple portable hoist for lifting baskets of domes out of the aera-
tion basin, particularly where tanks are large (many domes) and/or
very deep.
• A convenient clear water source, such as plant effluent, connected to
the tank so that it can be readily added to the tank during startup
after cleaning.
• Either provision of facilities for on-site dome cleaning or a determi-
nation that cleaning service can be obtained conveniently elsewhere.
-- Providing a central facility for cleaning domes from several plants
proved to be cost effective for the Thames Water Authority and should
be considered where appropriate.
• Spare parts stocked at the plant. -- At minimum, enough domes to fully
retrofit one aeration tank, along with gaskets, spare holddown bolts,
and spare air supply piping should be provided.
a Provision of individual air valves for each grid of the aeration system.
-- In many cases, plant operators were unable to correct overaeration
problems due to the lack of air flow control to sections of the aeration
tanks. Air valves should be provided for each grid and should be of a
type that can be adjusted with some sensitivity and repeatability.
74
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REFERENCES
1. Boyle, W. C., Ed. Workshop Toward An Oxygen Transfer Standard. Proceedings
Subcommittee on Oxygen Transfer Standards, American Society of Civil
Engineers, Pacific Grove, California, 1978. EPA-600/9-78-021. 271 pp.
2. Eckenfelder, Jr., W. W. and D.J. O'Connor. Biological Waste Treatment.
Pergammon Press, New York, 1961.
3. Cooper, P. F., E. A. Drew, D. A. Bailey, and E. V. Thomas. Recent
Advances in Sewage Effluent Denitrification: Part I. Water Pollution
Control, p. 287-296, 1977.
4. Fryer, B. and P. Musty. Optimization of Activated Sludge Plant Operating
Conditions. Severn Trent Water Authority, Coleshill Works, 1978. 17 pp.
5. Schmit, F. L., J. D. Wren, and D. T. Redmon. The Effect of Tank Dimen-
sions and Diffuser Placement on Oxygen Transfer. Journal Water Pollution
Control Federation, Vol. 50, pp. 1750-1767, 1978.
6. Rollins and Kocurek. Gas Flow Rates and Oxygen Transfer Efficiency
Field Studies. Student Report CEE629, University of Wisconsin, Madison,
1978. 18 pp.
7. Egan, J. T. Norton DDAS Fine Bubble Pilot Plant Final Report. Los
Angeles - Glendale Water Reclamation Plant, City of Los Angeles Public
Works Department, Los Angeles, California, July 1979. 14 pp.
8. Ghirardi, S., M. Nicodema, and J. Rotondo. Tallman Island's Aeration
System Evaluation. Student Report, Manhattan College, New York, New
York, 1979. 73 pp.
9. Salzman, R. and M. B. Lakin. Influence of Mixing in Aeration. In:
Proceedings: Workshop Toward an Oxygen Transfer Standard, ASCE & EPA,
Pacific Grove, California, 1978. EPA-600/9-78-021. 13 pp.
10. Gilbert, R. G. Measurement of Alpha and Beta Factors. Ibid. 16 pp.
11. Boon, A. G. Oxygen Transfer in the Activated Sludge Process. Ibid.
8 pp.
12. Eckenfelder, Jr., W. W. Oxygen Transfer: A Historical Perspective -
The Need For a Standard. Ibid. 2 pp.
13. Brenner, R. C. Philosophy of and Perspective on an Oxygen Transfer
75
-------
Standard - The EPA View. Ibid. 4 pp.
14. Yunt, F. W. and T. 0. Hancuff. JWPCP Research Report - Aeration Equip-
ment Evaluations. Los Angeles, California, April 1978.
15.
Wren, J. and L. Ewing. Clean Water Oxygen Transfer Test Data, With and
Without Surfactant Added. Personal Communication, April 1980.
16. Paulson, W. L. Oxygen Absorption Efficiency Study - Norton Co. Dome
Diffusers. Report to Norton Company, March 1976.
17. Paulson, W. L. and L. D. McMullen. Oxygen Transfer Study of Norton Dome
Diffuser System. Ontario Ministry of the Environment, Brampton, Ontario,
July 1977.
18. Boon, A. G. Rate of Transfer of Oxygen Into Water Using Fine Bubble
Diffused Air Systems. Water Research Centre, WRCS Report No. 537 R,
November 1975.
19. Verhaagen, J. and M. W. Ilsink. (Oxygen Transfer Studies of the Nokia
HKL 210 and Degremont DP 230 Diffuser at Holton-Markelo STP, The Nether-
lands). Zuiveringschap - West Overijssel Waste Treatment Authority,
Zwolle, The Netherlands. 1979 (Translated).
20. IIsink, M. W. and F. A. Brandse. (Fine Bubble Aeration by Means of
Aeration Domes at the Steenwijk Sewage Treatment Works, The Netherlands).
H20, No. 6/78, pp. 107-113, 1978 (Translated).
21. Lister, A. R. and A. G. Boon. Aeration In Deep Tanks: An Evaluation of
a Fine - Bubble Diffused Air System. Journal of the Institute of Water
Pollution Control, No. 5, 1973. 18 pp.
22. Wheatland, A. B. and A. G. Boon. Aeration and Oxygenation In Sewage
Treatment - Some Recent Developments. Progress In Water Technology, Vol.
11, No. 3, pp. 171-180, 1979.
23. Boon, A. G. and D. R. Burgess. Treatment of Crude Sewage In Two High
Rate Activated Sludge Plants Operated In Series. Water Pollution Control,
73, 1974.
76
-------
APPENDIX A
SAMPLE U.K. PLANT SURVEY FORM
PLANT PERFORMANCE DATA FOR YEAR .ASii":
Name of plant ..BASINGSTOIffi .SWAGS. TEEAWEIff_¥OEKS.
1. RAW SEWAGE
Any significant industrial contribation ...-11.0.
If yes. what type of industry
Average percentage industrial contribution:
4-6
by flow (#
by BOD, mass loading (%)
Define any liquors recycled to head of works: .^«B.*»??|F.?*f J?:.
activated sludge: Supernatant from sludge treatment:
treatment waste.
, .
r tlotation
BOD total
BOD, soluble
COD total
Anionic surfactant as Manoxol OT
(Detergent)
Total suspended solids (TSS)
Volatile suspended solids CVSS)
Total'Kjeldahl nitrogen (TKN)
Anmoniacal nitrogen (NH -N)
Annual
average
(mg/1)
328
Not
assessed
700
20.5
378
Not
assessed
54.1
36.9
Monthly average (mg/1)
Max*
381
Not
assessed
817
2J.6
403
Not
assessed
61.0
41.6
Min*
296
Not
assessed
651
20.7
392
NOT;
assessed
51.2
36.8
Max and Min denote months when the organic loading rates (kg BOD/day) are at
the maximum and minimum respectively. Please define the months:
Max
January
u;n August
Min ..........
77
-------
PLANT PERFORMANCE DATA FOR YEAR
2. SETTLED SEWAGE
BOD total
BOD, soluble
COD total
Detergent
TSS
vss
TKN
NH -N
Dissolved oxygen (DO)
pH
3. MIXED LIQUOR
SS
VSS
DO
pH
4. FINAL EFFLUENT
BOD total
BOD soluble
COD total
Detergent
TSS
.5r.l276 (contd)
Annual
average
Ug/1)
174
Not
assessed
366
13.8
121
Hot
assessed
47.1
36.1
Hot
assessed
7.75
3847
Not
s s^p^pjprl
tt
it
3.46
Not
assessed
30.3
0.2?
6.1
Monthly average (mg/1)
Max
216
Hot
assessed
434
17-0
153
Hot
assessed
55.2
41.0
Hot
assessed
7.85
6197
Not
assessed
!f
It
5.73
Hot
assessed
36.5
0.34
9.1
Min
132
Not
assessed
314
12.9
90.1
Not
assessed
47.5
34.4
Not
assessed
7.60
4646
Hot
assessed
it
tt
1.51
Not
assessed
21.5
0.35
2.76
78
-------
PLANT PERFORMANCE DATA FOR YEAR ..r?7<7i2I§...(Contd>
vss
TXN
NH -N
Nitrate nitrogen (N03-N)
DO
5. UNDERFLOW SLUDGE
SS
VSS
6. DRINKING WATER*
Hardness as CaC03
Alkalinity as CaCO
Total dissolved solids (TDS)
Sulphates
Chlorides
Iron
Manganese
Annual
average
(rag/1)
Hot
assessed
2.96
0.97
33.0
Hot
assessed
9293
Hot
assessed
Monthly average (mg/1)
Max
Hot
assessed
3.0
0.95
34.7
Not
assessed
9482
Not
assessed
Min
Not
assessed
2.4
0.04
53.9
Not
assessed
7909
Not
assessed
* The drinking water is supplied to the area served by the works.
79
-------
PLANT PERFORMANCE DATA FOR YEAR .. .12Z5-.12JA... (contd)
7. HYDRAULIC DATA
Dry weather flowrate (m /d)
Average flowrate of influent
(raVd)
Recycle rate (.% of influent)
Flowrate of waste sludge (m /d)
8. LIQUOR TEMPERATURE (°C)
Raw sewage
Primary effluent
Mixed liquor
Final effluent
Annual
average
13,290
13,690
143
69
Hot
assessed
M
16.05
15-86
Monthly average
Max
12,080
12,160
143
137
*
Not
assessed
"
20.3
21.5
Min
12,130
11,940
140
62
*
Not
assessed
M
12.9
13.5
* Max and Min for the temperature data denote the months when the
temperature is the maximum and minimum respectively. Please define
the months:
Max ..
Min
/ Experiments being conducted to reduce solids from an average
of 10,000 mg/1 to 3,500/4,000 rag/1.
80
-------
CONSTRUCTION AND PERFORMANCE DATA FOR AERATION SYSTEM
Name of plant BAS.IHG.STpH3 .SEWAGE T^RffiNT. WORKS,
1. TANK DIMENSIONS
Length (m)
Width (m)
Total depth (m)
Operating depth (m)
2. DIFFUSES LAYOUTt
No of diffuses in row
No of rows
Spacing in rows (m)
Spacing between rows (m)
Distance from floor (m)
3. AIR FLOW DISTRIBUTION (TOTAL)
minimum air flow (ra /s)
minimum power (kW)
average air flow (m /s)
average power (kW)
maximum air flow (m /s)
maximum power (kW)
Section*
1
3 No.
@
79.25
3.35
2.36
2.36
275
4
0.3
0.76
0.22
2
1 No.
@
79-25
3.35
2.51
2.51
275
4
0.3
0.76
0.22
3
4 No.
(9
79-25
6.71
2.51
2.51
275
8
0.3
0.76
0.22
4
2.60
37
4.04
116
5.56
160
* A section is each part of the aerator basin having a different diffuser
arrangement or air flow per diffuser.
t Explanatory sketches showing diffuser layout may be drawn on the blank
page overleaf.
81
-------
AERATION SYSTEM DATA (cont)
4. DIFFUSER DATA:
Are ail diffusers the same type (yes/no)
Manufacturer(s) . .N0.1^.0.11. Model No(s)
Description ("round dome, 12" square plate)
Granulation or pore size ^.O .microns
-1 Q
Time in service (years) ..
5. BLOWER SYSTEM DATA: 2 So> 10"
,. ,n 1 No. 14"
Manufacturer .?eo.. waller Model No ^ ^..
Type .fffPtef '.. Number of units . . }. ;I.°:. }?" '}...
Rated air flowrate . .3.3.4...m.3./.m.i.n. at P;.3-.4. .k-f /?? pressure
Power demand at: Maximum air flow . .?ZZ .^{^ total elec. power
Minimum air flow total elec. power
Average air flow total elec. power
6. AIR CLEANIn« SYSTEM:
Please describe type (cyclone, electrostatic, etc)
Tilghian Ultra Filtration
7. CAPITAL COSTS:
Cost of air diffuser system, including in tank piping and fittings
Hot available year
Cost of air cleaning equipment
Not available ,,_.,
• *•••••-•••••••••••••*• 7 c3.i •••••*••••••••••••••
* 2 No. @ 2302 mVkour
1 No. @ 4605 "
1 No. @ 5098 "
1 No. @ 5709 "
82
-------
AERATION SYSTEM DATA (cont)
Cost of air compressor equipment and appurtenances
Not available y»ar
Total Wastewater Plant cost
8. ELECTRICITY CONSUMPTION
year
Consumption
kWh/year
YEAR
1
Sst.
900,000
2
Sst.
900,000
3
Est.
900,000
4
Est.
900,000
5
ictual
963, ooc
83
-------
PLANT DESIGN DATA
Name of plant .343i¥}S1;QKS .SWQfi .WR-WOT. WWS, ..................
Address Department of Water Pollution Control, Reading Eoad, Chineham,
......................................... STCX2 ', Hants .
Approximate year of construction . . 1,9.6J. . .&. . 197
Year fine bubble aeration system installed . . M .aP.°7f ;
1 . FLOWRATES (ACTUAL)
Dry-weather flowrate (m3/d) ... A6.'J.8.5.
Annual-average flowrate (raVd) ,1.8.'.5.8.°.
•7
Peak monthly-average flowrate (m /d) .. .2.8>.%/.
Peak daily-average flowrate (m /d) 41>5.QP.
Peak instantaneous flowrate to ,
treatment (x DOT) §00 .litres/sec .= .DOT ^ £. 3__
2. AERATION DESIGN CRITERIA
Definition of design flowrate
(average, DWF, etc)
Type of plant (tapered aeration, etc) .
Hydraulic conditions in tanks
(plug flow,, complete-mix etc) ....?."!. ...w.
Nominal retention time (h) of sewage in aeration tanks
at design flowrate 8/5
Recycle flowrate (%) of design flow ..WQ.
Settled sewage:
Average BOD, (mg/1) ... .^7
Average NH3-N (mg/1) ..^4.-.8
Mixed liquor:
Average SS (mg/1) A27A
Average VSS (rag/1) .... .1°.* .a.s.s.e.s.s.e.d;
Were any tanks designed for sludge reaeration (yes/no) ...Y?S...
If yes, what is the volumetric proportion of tK&
reaeration tanks ?^°
84
-------
3. CLARIFIED DESIGN CRITERIA
Definition of design flow
(average, DWF, etc) I A . Pi'T.
Nominal retention time (h) of sewage in clarifier
at design flowrate ^. :*....
Upward velocity (m/h)
at design flowrate C.:°.S.l,
Total length of weir(s) per tank (m) . A0.-JA
Number of tanks \ -
Wall depth (n) i:5?
Diameter or length (m) .!<:? A1.3;:
Width (m) ~
Location of inlet (centre or rim) . .Qwti1^
Location of weir(s) BetWlSt^?
Type of sludge removal equipment . .QinUW.Hs. J%dr.o.st.a.t.1.c.
Clarified effluent:
Average BOD5 (mg/1)
Average SS (mg/1)
Underflow sludge:
Average SS (mg/1)
Average flowrate of waste
sludge (m3/d) ??§.
85
-------
APPENDIX B
PLANT SURVEY DATA
BASINGSTOKE
History and Background
Prior to the initiation of activated sludge treatment on the present site,
wastewater from Basingstoke was treated at a 40.5-ha (100-ac) land treatment
plant. Initially, it was planned to use the existing site and continue the
practice of pumping wastewater to it. However, in 1960 it was decided to
merge wastewater flows from several communities in the area. A downstream
site, which would drain the service area by gravity, was selected adjacent to
the River Loddon.
The activated sludge plant was constructed in three stages beginning in
1964. The first and second stages were designed to serve populations of
40,000 and 80,000, respectively. The second stage was completed in 1971. A
final expansion, increasing plant size to a population equivalent of 96,000,
was completed in 1976. The population currently being served is approximately
70,000. Figure B-l shows the site plan at Basingstoke.
Plant Description
The Basingstoke wastewater treatment plant is a fine bubble diffused air
activated sludge plant, with primary and secondary settling. In addition, the
plant is provided with drum-type microstrainers and stortnwater retention fa-
cilities owing to the very high discharge requirements of the River Loddon, a
trout stream. Sludge is digested and disposed on land in liquid form. Facil-
ities for sludge thickening and drying are also available at the plant. Ta-
ble B-l provides design details on the major plant elements.
In total, there ?.re eight aeration basins in the activated sludge plant,
the four smaller of which are 3.4 m (11 ft) wide; all are 79.2 m (260 ft) in
length. Three of the smaller basins are designed to allow for flow in either
direction. The other small basin is used exclusively for return sludge aera-
tion. The four larger basins are for mixed liquor aeration only.
Dome distribution in all eight basins is symmetrical, with 1032 domes in
each of the smaller basins and 2068 in each of the larger. A total of 708
domes are used for aerated primary effluent and mixed liquor channels. Each
row of diffusers is laid lengthwise in the basin and has 275 domes; there are
four rows in the smaller tanks and eight rows in the larger basins. Spacing
86
-------
CO
^« O *
O
nuiuiimri
OOP
OOP
KEY: A. INLET WORKS
B. PRIMARY SEDIMENTATION TANKS
C. SLUDGE TREATMENT
D.
E.
F.
G.
Y" G &
AERATION TANKS
MICROSTRAINERS
BLOWER AND POWER HOUSE
AMENITY AND ADMINISTRATION BUILDING
Figure B-l. Basingstoke site plan.
-------
TABLE B-l. BASINGSTOKE DESIGN DATA*
Stormwater Tanks
2 Circular Tanks
Diameter:
Sidewater depth:
Floor slope:
27.4 m+
3.7 m
2.5 degrees
Flow to tanks automatically controlled by No-flote controls and Rotork
driven penstocks. Duty tank filled before it is shut down and flow di-
verted to standby tanks. Should tank fill and overflow, the first 4,546
m3/day (1.2 mgd) is fed back to the aeration tanks. Any flow above 4,546
m3/day (1.2 mgd) is irrigated on prepared grassland, collected, and dis-
charged to the river, subject to the same standards as the final effluent.
Primary Sedimentation
5 Circular
Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
Aeration Basins
3 Dual Purpose Mixed Liquor/
Reaeration Tanks
1 Mixed Liquor Tank
4 Mixed Liquor Tanks
Detention Time
Dome Distribution
Small tanks:
Large tanks:
Final Sedimentation
5 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
20.3 m
3.3 m
5 degrees
6.3 hr @ DWF
13.4 m3/m2/day @ DWF (330 gpd/ft2)
3.4 m x 79.2 m x 2.4 m SWD
(each tank)
3.4 m x 79.2 m x 2.4 m SWD
6.7 m x 79.2 m x 2.4 m SWD
(each tank)
8.5 hr @ DWF
1032
2068
19.2 m
1.5m
30 degrees
5 hr @ DWF
16.3 m3/m2/day @ DWF (400 gpd/ft2)
* DWF = 22,700 m3/day (6 mgd)
+ 1 m = 3.28 ft
88
-------
between domes is 0.30 m (1 ft) and between rows 0.76 m (2.5 ft). The domes
are installed 17 mm (8 in.) off the basin floor. Minimum air flow to the
domes in the mixed liquor channels is 0.011 m3/min (0.4 cfm).
Air supply is provided by five Rootes-type positive displacement blowers
driven by electric motors. All are fixed speed but are sized to cover a range
of outputs at 33.1 kN/m2 (4.8 psi) as summarized below:
Power Output
Blower No. BHP J
-------
tions in 6005 and 94-95 percent reductions in suspended solids. Detergent re-
movals have consistently averaged 97-99 percent.
Monthly average hydraulic and influent 6005 loadings varied about +_ 25
percent from the annual average. Of the total plant flow, 4-6 percent is de-
rived from light industrial activities in the service area.
Reductions in ammonia nitrogen concentrations have averaged 96 percent,
and the level of nitrate nitrogen in the final effluent has been constant to
+_ 10 percent.
TABLE B-2. BASINGSTOKE INFLUENT-EFFLUENT DATA SUMMARY
Parameter
Flow: mgd
m3/day
Raw Wastewater: (mg/1 )
BOD5
COD
TSS
TKN
NHo-N
Detergent +
Primary Effluent: (mg/1)
BOD5
COD
TSS
TKN
NHo-N
Detergent +
Final Effluent: (mg/1)
BODr
COD°
TSS
TKN
NHrN
NOo-N
Detergent +
78/79
4.91
18,600
281
658
372
45
35
13
157
323
114
43
35
11
3
33
6
2.1
1.3
27
0.3
77/78
4.25
16,100
271
634
356
45
35
13
156
342
112
45
38
11
3
33
5
3.5
2.4
31
0.3
Year*
76/77
3.97
15,000
326
695
363
54
38
19
169
366
126
46
37
14
3
34
6
3.0
1.1
31
0.3
75/76
3.62
13,700
328
700
378
54
37
21
174
366
121
47
36
14
3
30
6
3.0
1..0
33
0.4
74/75
4.65
17,600
275
573
310
45
30
18
149
308
111
43
30
12
4
28
6
3.8
1.7
28
0.2
* Year begins April 1 and- ends the following March 31.
+ Detergent as manoxol OT (a proprietary ABS type compound).
MLSS levels have varied over the period of record from 4000-5000 mg/1.
The F/M loading rate has averaged 0.08 kg BOD5/day/kg MLSS. Based on Figure 1
in Section 5, 1.23 units of oxygen are required to remove each unit of 8005.
Aeration efficiency, as calculated by the method of Section 5 and summarized
90
-------
in Table B-3, ranged from 1.08-1.20 kg 02/kWh (1.78-1.97 Ib/wire hp-hr).
Based on data provided by plant personnel, a 6005 of 12 mg/1 from the second-
ary clarifier, before microstraining, is assumed in computing BOD5 removals.
Mixed liquor D.O., under most conditions, ranges from less than 10 per-
cent of saturation in the first one-quarter of the aeration basin to 70-80
percent of saturation at the outlet end. The return sludge channel is aerated
only to about 2 percent of saturation to promote denitrification.
TABLE B-3. BASINGSTOKE AERATION EFFICIENCY CALCULATIONS
Parameter
BOD5 Removed ( kg/day )*+
NH3-N Removed (kg/day)*
NOa-N Produced (kg/day)*
02 Required0 (kg/day)*
Power Consumed (kWh/day)00
78/79
2694
623
496
5992
4989**
77/78
2317
566
491
5283
4440++
Year
76/77
2358
544
464
5240
4440++
75/76
2218
481
452
4796
4440++
74/75
2410
504
498
5131
4440++
Aeration Efficiency:
kg
Ib
02/kWh
02/wire
hp-hr
1
1
.20
.97
1.19
1.97
1.18
1.94
1.08
1.78
1.16
1.91
* 1 kg= 2.21 Ib
+ Assumes secondary effluent BODg averaged 12 mg/1 before microscreening.
0 Calculated by the method of Section 5, assuming 1.23 units 02 required/
unit BODc removed.
** Corrected for 5 percent of power used to aerate influent and effluent
channels and to air lift return sludge.
++ Estimated value.
00 1 kWh/day =1.34 wire hp-hr/day.
The minimum mixing intensity or power input level at Basingstoke is 20.8
(0.80 wire hp/1000 ft-^), as shown in Table B-4. Maximum levels range to
106 percent above minimums. Plant personnel have not observed instances of
inadequate mixing in any of the aeration basins.
Operation and Maintenance
Basingstoke is the only plant visited that operates a planned maintenance
schedule (every 5 yr or less) for dome cleaning. Observation of the tanks in
operation revealed that three of the eight tanks were experiencing possible
dome plugging, as indicated by coarse bubbling. Pressure increase, if any,
was not discernable, and plant personnel used D.O. levels to indicate the on-
set and degree of plugging.
91
-------
TABLE B-4. BASINGSTOKE MIXING DATA
Parameter _ Small Basin _ Large Basin
Surface Area (m2)* 265.5 531
Wetted Volume (m3)+ 627 1254
Diffuser Density (domes/m2)0 3.89 3.89
Air Flow/Basin:**
cfm 410 823
m3/min 11.6 23.3
Air Flow/Area:**
cfm/ft2 0.144 0.144
m3/m2/min 0.044 0.044
Air Flow/Volume** (cfm/1000 ft3)++ 18.6 18.6
Minimum Power Imput:
wire hp/1000 ft3 0.80 0.80
W/m3 20.8 20.8
* 1 m2 = 10.8 ft2
+ 1 m3 = 35.3 ft3
1 dome/m2 =9.29 domes/100 ft2
** Based on a minimum air flow rate of 0.011 m3/min/dome (0.4 cfm/dome).
++ Same as m3/1000 m3/min.
The PI inspected fouled domes that had recently been removed for cleaning.
It was determined that a white scale, apparently calcium carbonate, was build-
ing up on the domes and penetrating approximately 0.6 cm (0.25 in.) into the
outer surface. Drinking water data for the plant revealed that hardness at
285 mg/1 as CaC03 is similar to that of Oxford where the deposition problem
has not been experienced. However, alkalinity at Basingstoke is 245 mg/1 as
, over two times the 111 mg/1 level at Oxford.
To clean the carbonate fouled domes, Basingstoke personnel immerse the
domes in a 10 percent solution of hydrochloric acid for 24 hr and follow with
steam cleaning of each dome. Each batch of domes is checked after cleaning
by testing a sample (2-4 domes) to determine headless as compared to a new,
unused dome.
Although plant personnel expressed satisfaction with this procedure, a
subsequent discussion with the manufacturer of the domes cast some doubt on
the validity of the cleaning process. It was the manufacturer's opinion that
the procedure used at Basingstoke does not adequately clean the domes. They
noted that the 3.4-6.9 kN/m2 (0.5-1.0 psi) differential in pressure between
one of the cleaned domes at Basingstoke and a new dome indicates that the
domes are not fully cleaned by the process. It was their opinion that the
92
-------
domes should be refired to be fully cleaned.
Other problems with dome fouling have occurred in older sections of the
plant that use cast iron manifolds on the tank floor. Rust formation in these
mains, and resultant deposition on the interior surfaces of the domes, has
caused some fouling in the past.
The maintenance supervisor at Basingstoke outlined the following proce-
dure and manpower requirements for cleaning the domes in one of the larger
tanks:
1. Empty and Clean Tank: 4 man days of work over a 3-4 day period.
Work is hampered by lack of tank drains or cleanout sumps.
2. Remove Domes: Two men take 1 day for removal. A hoist or some other
means of lifting the batches of domes should be provided.
3. Cleaning Domes: 2 man days to soak domes in 10 percent HC1, steam
clean, and place in clean water bath.
4. Replacing Domes: 4 man days, over 2 days, to replace domes and check
air manifolds and "hold-down" hardware. The large rubber gasket is
replaced at each cleaning. 0-rings in the dome hold-down bolts are
checked and cleaned.
5. Final Checking: 1 man day over 1 day. Final effluent water is
pumped into the basin to a depth of 15 cm (6 in.) over the domes and
the air valve is partially opened. Domes are observed for leaks and
pattern irregularity and repaired as needed.
The need for careful supervision of the reinstallation was emphasized by
the supervisor to avoid over tightening of the dome hold-down bolts. About 2
percent of the domes are lost during the removal, cleaning, and replacement
operations, mainly due to breakage during handling. The plant stocks 2000
spare domes plus rubber gaskets, plastic piping, hold-down bolts, and other
hardware associated with the domes and air supply manifolds.
The bag filters are operated without a precoat and are cleaned by vigor-
ous shaking of the bags once per year. Air quality in the vicinity of this
plant is very good, resulting in only light loading of the filters. No prob-
lems have been experienced with this maintenance procedure or the bag filters.
BECKTON
History and Background
In 1864, wastewater handling at the site of the present Beckton treatment
works began with the construction of a holding reservoir system. Wastewater
collected from London was piped to the site and held in four large storage re-
servoirs, being discharged without treatment on the outgoing tide. This
served as the principal wastewater disposal system for London until 1889 when
manually cleaned primary sedimentation basins were added. In 1939, work was
93
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©••••O
sewage
inlet
RNAL SEDIMENTATION
TANKS
O PRIMARY SLUDGE
DIGESTION TANKS
DIGESTED SLUDGE
TANKS
effluent
Figure B-2.
Beckton site plan.
94
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completed on a mechanical aeration plant that treated 272,500 m3/day (72 mgd),
only a part of the flow. By 1955, screening and grit handling facilities had
been added, along with eight mechanically cleaned primary sedimentation basins.
In 1959, a second 272,500-m3/day (72-mgd) activated sludge plant was
added, aerated by fine bubble diffusers. This equipment, dome diffusers
mounted on cast iron piping manifolds that extended to all portions of the
tank, was the precursor to the present day dome diffuser systems. The domes
were manufactured by Activated Sludge Ltd., predecessor to Hawker-Siddeley
Water Engineering Ltd., the current supplier in the United Kingdom.
At the conclusion of 1959 modifications, the plant provided secondary
treatment to 50 percent of the DWF, the remainder being discharged to the
Thames River with only primary treatment. In 1968, a program to provide se-
condary treatment for all flows received at Beckton was initiated, with com-
pletion coming in 1975. Presently, the plant is providing full secondary
treatment for flows ranging from 999,200 m3/day (264 mgd) up to 2,725,000 m3/
day (720 mgd). The presently served population is 2.4 million over a service
area of 295 km2 (114 mi2). The service area encompasses most of London north
of the Thames River. The site plan of the current facilities at Beckton is
given in Figure B-2.
Plant Description
Table B-5 provides an overview of the design details of this plant, one
of the largest activated sludge plants in the world. The activated sludge
plant is divided into two portions:
t The 1959 plant, comprising aeration tank Nos. 9-14 and 24 secondary
clarifiers.
• The 1975 plant, comprising aeration tank Nos. 1-8 and 48 secondary
clarifiers.
The 1959 plant consists of full plug flow systems with eight separate channels
for each tank. Wastewater flows through each channel in turn, for a total
distance of 951 m (3120 ft). This plant was originally equipped with 15,000
diffusers per tank (1875 per channel). As part of the 1968 program, the 1959
plant was refitted with plastic air distribution piping and the number of
domes per tank was increased to 18,000. The rehabilitated 1959 plant is rated
at 454,200 m3/day (120 mgd).
The 1975 activated sludge plant, rated at 1,854,600 m3/day (490 mgd),
uses a single-pass modified plug flow process. Originally, each tank was
equipped with 36,000 domes, distributed equally over the tank floor. In oper-
ating the plant, it was determined that maintenance of the minimum recommend-
ed air flow per dome, 0.014 m3/min (0.5 cfm), led to extensive overaeration
and substantial power wastage. D.O. profiles of the tanks under the normal
operating conditions revealed that mixed liquor D.O. levels were near to satu-
ration over about one-half of the tank length. Accordingly, a formal program
to study the problem and make modifications was undertaken. The basic ap-
proach has been to remove domes from the tanks, thereby reducing required to-
95
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TABLE B-5. BECKTON DESIGN DATA
Primary Sedimentation
16 Rectangular Tanks
Tank Nos. 1-8:
Tank Nos. 9-16:
Detention time:
Surface loading:
Aeration Basins
45.7 m+ x 79 m+ x 3.4 m+ SWD
45.7 m x 77.4 m x 3.4 m SWD
4.1 hr @ DWF
19.4 m3/m2/day @ DWF (477 gpd/ft2)
14 Rectangular Tanks
Tank Nos. 1-8: 41.2 m x 223 m x 3
Tank Nos. 9-14: 39 m x 119 m x 3.7
Detention time: 7 hr @ DWF
Note: Tank Nos. 1-8 are single-pass plug flow.
Tank Nos. 9-14 are eight-pass plug flow.
Final Sedimentation
1 m .SWD
m SWD
72 Circular Clarifiers
Tank Nos. 1-48:
Tank Nos. 49-64:
Tank Nos. 65-72:
Average detention time:
Average surface loading:
Diameter
Sidewater depth
Floor slope
Diameter
Sidewater depth
Floor slope
Diameter
Sidewater depth
Floor slope
34.4 m
3.7 m
1:24
33.5 m
3.5 m
1:24
29 m
2.9 m
1:12
hr @ DWF
19.9 m3/m2/day @ DWF (489 gpd/ft2)
Blower System
Aeration Tank Nos 1-8:
4 blower houses, each with four 595-m3/min (21,000-cfm) blowers
driven by 582-kW (780-hp) motors. Rated pressure - 41.4 kN/m2
(6 psi). Air cleaned by 4 electrostatic filters with fiberglass
prefilters.
Aeration Tank Nos. 9-14:
5 blowers, each driven by a 716-kW (960-hp) gas turbines.
8 electrostatic air cleaners.
DWF = 1,135,000 m3/day (300 mgd)
1 m = 3.28 ft
96
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tal plant air flows.
There are six aeration grids in each tank of the 1975 plant, occupying
one-sixth of the tank length per grid and each supplied separately from the
air header. A total of 5000 domes was removed from each of tank Nos. 3-8 ini-
tially. Because D.O. levels were still quite high, an additional 5000 domes
were recently removed from tank No. 6 as a further experiment. Table B-6 pro-
vides the current dome configuration in the 1975 plant. Results of the exper-
imentation are discussed below under "Performance".
TABLE B-6. DIFFUSER CONFIGURATION IN BECKTON 1975 ACTIVATED SLUDGE PLANT
Original
Number
of Domes
Grid per Tank
1 6000
2 6000
3 6000
4 6000
5 6000
6 6000
36000
(Effluent)
Tank Nos.
6000
6000
6000
6000
6000
6000
36000
Current Configuration per
1,2 Tank Nos. 3-5,7,8
6000
6000
5000
5000
4500
4500
31000
Tank
Tank No. 6
6000
6000
4000
4000
3000
3000
26000
Air is supplied to the 1959 plant by five centrifugal blowers, each rated
for 708 m3/min (25,000 cfm) at 51.7 kN/m"2 (7.5 psi). Each blower is driven by a
716-kW (960-hp) gas turbine, which is normally fueled by methane qas from the
plant's anaerobic digesters. Normally, three blowers are operated to supply
2226 m3/min (78,600 cfm], drawing the equivalent of 1629 kW (2185 hp) against
a pressure of 41.4 kN/m3 (6 psi). Air cleaning is provided by eight electro-
static air cleaners.
The 1975 plant has four blower houses, each equipped with four electric
motor driven centrifugal blowers and four electrostatic air filters. Each
blower is driven by a 582-kW (780-hp), single-speed motor and is rated for.
595 m3/min (21,000 cfm) at 41.4 kN/m3 (6 psi). At the minimum air flow of
0.017 m3/min/dome (0.6 cfm/dome), the six blowers normally in service for six
aeration tanks deliver 3738 m3/min (132,000 cfm),drawing 3580 kW (4800 hp).
The plant makes extensive use of air lift pumping, which requires approx-
imately 15 percent of the blower capacity of both the old and new plants.
Minimum air flows to the domes are held at 0.017 m3/min (0.6 cfm) to both
the 1959 plant and the 1975 plant. During much of 1979, two of the newer
activated sludge basins were not in service, being maintained as standby
units.
Final clarifiers are of the circular type with rotory scrapers. The sur-
face loading rate at design flow is 19.0 m3/m2/day (465 gpd/ft2) for Nos. 1-
48. Clarifier Nos. 49-64 are sized at 22.8 m3/m2/day (560 gpd/ft2), and Nos.
97
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65-72 are rated at 16.8 m3/m2/day (412 gpd/ft2). Design flow for the clarifi-
ers is 1,135,000 m3/day (300 mgd), and peak flow is calculated as 2.4 x design
flow. The design recycle rate is 70 percent of the average influent flow.
Detention time is 5.2 hr at design DWF flow for clarifier Nos. 1-48 and 4.6 hr
for clarifier Nos. 49-72. Sludge return for Nos. 1-64 is via air lift pumps;
the older clarifiers (Nos. 65-72) use centrifugal pumps.
Beckton is one of the two plants visited that has a diffuser dome clean-
ing facility, consisting basically of an oil fired kiln, dome storage space,
and handling equipment. The system, discussed further in Section 6, is in-
tended to serve the dome cleaning needs of a number of Thames Water Authority
plants.
Costs of the most recent plant modification, in 1969, stated at the rate
of $2 =Sl sterling, were $2,472,000 for the aeration equipment and piping out
of a total of $46,000,000 for the wastewater treatment plant. Expansion of
sludge digestion facilities added $6,250,000 to the cost.
Performance
Table B-7 is a summation of the Beckton influent and effluent data for
the years 1976 through 1979. Data for the 1975 plant was supplied by Beckton
in response to the survey. Data for the 1959 plant is for 1977-1979 only and
was developed from other material supplied. The data year at Beckton spans
the period from April 1 to March 31 of the following year.
Primary sedimentation at this plant performed well, removing 55-59 per-
cent of the influent BOD5 and 70-73 percent of the influent suspended solids,
with a detention time of about 4 hr and a surface loading rate that averaged
22.1 m3/m2/day (542 gpd/ft2). With an F/M loading of 0.13 kg BOD5/day/kg MLSS
and a volumetric loading of 0.37 kg BODc/day/m3 (23 lb/day/1000 ft3), the to-
tal plant averaged 96 percent removal of 6005 and 94 percent removal of sus-
pended solids over the 3-yr period. Influent detergent averaged 11.3 mg/1 as
maroxyl OT and was 94 percent removed through the process. Both sections of
the plant performed similarly under similar loading conditions. Monthly aver-
age flows varied approximately ± 20 percent from annual averages. Daily
flows can range up to 3 x DWF because of the extensive combined sewer system
discharging to the plant. Only about 10 percent of the flow is industrial in
origin.
Using the method discussed in Section 5 of this report, the aeration ef-
ficiency of the aeration equipment has been computed in Table B-8. For pur-
poses of comparison, a similar computation made by the Beckton scientific
staff is also shown in Table B-8. The new activated sludge plant (aeration
tank Nos. 1-8) averaged 1.75 kg 02/kWh (2.881b/wire hp-hr), with correspond-
ing values of 2.14 and 3.52, respectively, for the old plant (aeration tank
Nos.9-14). Because power to the old plant cannot be measured directly, the
accuracy of these data is not expected to be on a par with the new plant.
Based on observations at other plants, it is not anticipated that the old
plant would be significantly more efficient in oxygen transfer than the new
plant. Rather, the data in this report indicate that multi-pass plug flow
systems are less efficient as a whole than single-pass systems.
98
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TABLE B-7. BECKTON INFLUENT-EFFLUENT DATA SUMMARY
Year*
Aeration Tank Nos. 1-8
Aeration Tank Nos. 9-14
Parameter
Flow:
mgd
1000 m-Vday
78/79
176
666
77/78
178
674
76/77
181
685
78/79
104
394
77/78
88
333
Raw Wastewater: (mg/1)
BODr
COD
TSS
TKN
NH3-N
Detergent+
Primary Effluent:
BOD5
COD
TSS
TKN
NH3-N
Detergent"1"
(mg/1
169
428
268
35
22
14
96
204
78
32
23
11
230
501
297
40
24
9
105
233
77
36
25
7
237
546
303
42
26
12
115
243
82
36
27
8
169
428
268
39
22
14
96
78
32
24
230
501
286
38
24
9
100
233
77
35
25
Final Effluent: (mg/1)
BOD5
COD°
TSS
TKN
NH3-N
N03-N
Detergent"1"
8
55
17
3.6
1.5
18
0.5
6
49
11
3.1
0.9
23
0.2
7
53
15
3.2
1.1
21.2
0.3
9
--
19
3.9
1.6
17
—
7
49
16
3.2
0.8
22
—
* Year begins April 1 and ends the following March 31.
+ Detergent as manoxyl OT (a proprietary ABS type compound).
Table B-9 lists minimum mixing power levels for the aeration tank config-
urations at Beckton. MLSS in all tanks is held in the range between 2600-3400
mg/1. The lowest power input is 6.8 W/m3 (0.26 wire hp/1000 ft3) in the last
section of aeration tank No. 6. The highest power input at minimum air flow
is 13.6 W/nr* (0.52 wire hp/1000 ftj) in all parts of aeration tank Nos. 1 and
2. The average air flow is about 30 percent higher than the minimum.
Surveys by plant personnel have verified that suspended solids are evenly
distributed in all parts of the tank. Further verification will be carried
out on the recently modified Tank No. 6.
Removal of domes to introduce an aeration taper has had some impact on
the D.O. profile of the original configuration. Figure B-3 is a plot of D.O.
99
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TABLE B-9. BECKTON MIXING DATA
Aeration Tank Surface Area
& Grid Nos. (1000 m'-)*
Tanks 1, 2
(each tank):
Tanks 3-5,7,8
(each tank):
Grids 1,2
_, Grids 3,4
o Grids 5,6
ro
Tank 6:
Grids 1,2
Grids 3,4
Grids 5,6
Tanks 9-14
(each tank)
(8 passes/tank)
9.2
9.2
3.07
3.07
3.07
9.2
3.07
3.07
3.07
4.64
Wetted
Volume
(1000 m3)+
28.5
28.5
9.5
9.5
9.5
28.5
9.5
9.5
9.5
17.2
Diffuser
Density
(domes/iii)
3.91
3.34
3.91
3.26
2.93
2.83
3.91
2.60
1.95
3.88
Air Flow/Section**
3 (cfm) 1
23,400
20,150
7,800
6,500
5,850
16,900
7,800
5,200
3,900
15,000
[m3/min}
663
571
221
184
166
479
221
147
no
425
Air Flow/Area**
(cfm/ ft2)
0.24
0.20
0.24
0.20
0.18
0.17
0.24
0.16
0.12
0.30
(m^/m /min)
0.072
0.062
0.072
0.060
0.054
0.052
0.072
0.048
0.036
0.092
Air Flow/Volume** Minimum Power Input
(cfm/ 1000 ft3)++
23.
20.
23.
19.
17.
16.
23.
15.
11.
24.
3
0
3
4
4
8
3
5
6
7
(wire hp/1000 ft3) (V'/m3)
0.52
0.44
0.52
0.43
0.39
0.37
0.52
0.34
0.26
0.41
13.6
11.7
13.6
11.3
10.2
9.8
13.6
9.0
6.8
10.9
* 1 m2 = 10.8 ft2
•+ 1 m3 = 35.3 ft3
o I dome/m2 = 9.29 domes/100 ft2
** Based on minimum air flow rates of:
0.018 tn3/min/dome (0.65 cfm/dome) for Tank Nos. 1-8
0.017 m3/inin/dnnie (0.6 cfm/dome) for Tank Nos. 9-14
++ Same as m3/1000 rrr/min.
-------
pliances such as those in common use in the United States. However,
total flow at Beckton has remained relatively.
3. Beckton employs two basic final clarifier configurations. The older
clarifiers, Nos. 65-72, have a floor slope of 1:12; the newer units,
Nos. 1-64, have a floor slope of 1:24. Plant personnel indicated
that the older units generally performed better and desludged more
readily.
Operation and Maintenance
Experience with dome diffusers at the Beckton plant spans a period of 20
yr. The original domes, installed on cast iron mains in aeration Tank Nos. 9-
14,performed satisfactorily, without cleaning. Minimum air flow was held at
0.017 m3/min/dome (0.6 cfm/dome), and special care was taken to maintain posi-
tive air flow at all times and keep air distribution piping free of moisture.
Pressure increased 4.2 kN/m2 (0.6 psi) over the period before the plant was
rehabilitated, but was increasing rapidly in the 2-yr period prior to rehabi-
litation.
In the newer plant, there have been a large number of failures of the
uPVC strap which holds the piping to the floor mount. This problem was appar-
ently caused by inadequate strength of the strap, a problem which has been
corrected by the manufacturer. There have been no failures of the heavier
hardware now supplied by the manufacturer.
Some dome cleaning has been carried out in the new activated sludge plant
(aeration tank Nos. 1-8). The domes have been cleaned successfully in the
plant's dome cleaning facility, and plant management is estimating a cleaning
frequency not to exceed once every 10 yr.
The electrostatic air filters have generally performed satisfactorily.
However, when fogs or freezing rains occur concurrent with near-freezing temp-
eratures, the moisture freezes onto the prefilters, gradually diminishing air
flow. The prefilters are now routinely removed when such conditions occur,
and no further problems of this type have been encountered. The electrostatic
filters require light cleaning (1 man hr) about every 8 days in the form of a
washdown with hot water. Every 3 yr, an alkaline cleaning solution is sprayed
on the dust collecting elements and washed off with hot water (2 man hr).
BEDDINGTON
History and Background
The site of the Beddington plant has been used for wastewater treatment
for over 100 yr. From 1860-1902, the site was used for land treatment. From
1902-1912, primary sedimentation and trickling filters were added. By 1932,
the flow had increased to 43,100 m3/day (11.4 mgd) and an activated sludge
plant and additional trickling filters were constructed.
In 1966, construction began on an entirely new activated sludge plant on
the site. The plant, shown in Figure B-4, has 12 aeration tanks and 16 final
103
-------
EFFLUENT - RECiRCULATION P S
ACTIVATED SLUDGE
RETURN PS
ACTIVATED SLUDGE
RETURN PS
Figure B-4. Beddington site plan.
-------
clarifiers. In mid-1978 it was necessary to add four additional aeration
channels to reduce volumetric loading rates.
Plant Description
Plant design data are presented in Table B-10. Primary settling is pro-
vided by eight circular clarifiers, which have a retention time of 6.2 hr at
the design DWF of 82,500 m3/day (21.8 mgd). Flows up to 3 x DWF are passed
directly through the plant. Excess flows are diverted to the stormwater re-
tention tanks where solids are removed,and the settled overflow is returned to
the process when flows decrease.
Of the 16 aeration basins, 12 have 3915 domes each equally distributed
among two plug flow passes and four employ tapered aeration. The last half of
each tapered second pass has 523 domes placed on four centrally located rows,
causing a double spiral flow hydraulic regime to be set up (tapered aeration
is discussed in Section 8). Primary effluent and mixed liquor channels are
also dome aerated, consuming 11 percent of the air flow.
Air is supplied by six centrifugal blowers rated for 236 m3/min (8330
cfm) at 37.9 kN/m2 (5.5 psi). Normally, five units are operating, supplying
1081 m3/min (38,163 cfm) at 27.6 kN/M2 (4 psi). Air is cleaned by five bag
filters using mixed cellulose and asbestos precoating.
The 16 final clarifiers are mated in sets of four to four sets of aera-
tion basins. The clarifiers have steeply sloped hopper bottoms (30°) and
shallow sidewater depth characteristic of many U.K. plants. Maximum surface
overflow rate at 3 x DWF is 43.9 m3/m2/day (1078 gpd/ft2).
Performance
Over the past 10 yr, the performance of this plant has been highly vari-
able, due chiefly to problems with sludge handling and the impact of indust-
rial wastes containing high strength organics and metal plating liquors. The
industrial wastes caused several immediate problems:
1. An apparent depression of the alpha factor, resulting in lowered
oxygen transfer efficiency.
2. Slime formation rapidly appeared on many of the domes in the 12 ori-
ginal tanks, leading to coarse bubbling and subsequent loss of oxy-
gen transfer capacity.
In an effort to deal with these problems which span several years, step feed-
ing of wastewater, refiring of domes in several of the tanks, and periodic
draining and dome brushing were tried, with only limited success. In 1974,
two additional, identical blowers were added, to allow for blower maintenance
and supply peak air needs. To lower volumetric loadings from an average of
0.5 kg BOD5/day/m3 (31 lb/day/1000 ft3) to 0.4 kg BODr/day/m3 (25 lb/day/1000
ft3) and to provide additional oxygen transfer capability, four additional
aeration tanks were added in 1978. A summary of the annual data, from start-
up to the present was provided by plant personnel as shown in Table B-ll. The
105
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TABLE B-10. BEDDINGTON DESIGN DATA*
Stormwater Tanks
8 Rectangular Tanks
Total volume:
Detention time:
Primary Sedimentation
8 Circular Clarifiers
Diameter:
Sidewater depth:
Detention time:
Surface loading:
Aeration Basins
16 Rectangular Tanks
(2 passes per tank)
Detention time:
Dome Configuration:
Aeration Tank & Section Nos.
Tank Nos. 1-12
(2 passes/tank)
15.4 m+ 76.2 m+ x 2.8 m+ SWD
25,900 m3
7.6 hr @ DWF
32 m
2.9 m
6.22 hr @ DWF
12.2 m3/m2/day @ DWF (300 gpd/ft2)
7.3 m x 67 m x 2.4 m SWD
(each pass)
10.7 hr @ DWF
Description
3915 domes, 9 rows, spaced 0.76 m
(2.5 ft) between rows, 0.30 m (1.0)
between domes, 0.25 m (0.8 ft) off
floor
Tank Nos. 13-16
Section
1
2
3
4
5
6
7
Section Length (m)
16.
16.
16.
16.
16.
16.
16.
75
75
75
75
75
75
75
16.75
No. Domes
637
676
534
545
449
434
269
254
No. Rows
13
13
10
10
8
8
4
4
Note: Sections 7 and 8 have 4 rows centered in the tanks,
Final Sedimentation
16 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
20.4 m
1.37 m
30°
5.3 hr @ DWF
14.6 m3/m2/day (359 gpd/ft2)
DWF = 82,500 m-Vday (21.8 mgd)
1 m = 3.28 ft
106
-------
TABLE B-ll. BEDDINGTON INFLUENT-EFFLUENT AND PROCESS LOADING DATA SUMMARY
Primary Effluent
(mg/1 )
Year
69/70
70/71
71/72
72/73
73/74
74/75
75/76
76/77
77/78
78/79*+
BOD5
206
228
165
174
161
124
138
175
156
149
COD TSS
— 136
-- 169
-- 134
-- 146
480 153
392 123
371 121
433 133
373 120
-- 105
NH3-N
37
36
34
38
37
30
29
36
31
29
Final Effluent
(mg/1)
BOD5
28
35
13
11
12
9
12
17
16
12
TSS
21
37
n
11
13
11
10
8
9
7
N03-N
16
10
17
25
27
23
23
21
23
24
NH3-N
29
30
6.5
4.6
4.2
3.5
5.3
9.7
5.7
4.3
MLSS BOD5 Loading
(mo/1) 1000 kg/day
(1000 Ib/day)
3651
2466
2391
2495
2398
2452
2346
2275
2270
2199
12.1
(26.8)
18.1
(39.9)
13.3
(29.3)
13.1
(29.0)
13.0
(28.7)
12.0
(26.5)
12.4
(27.3)
13.4
(29.5)
13.8
(30.4)
14.4
(31.8)
Air Supply
i
m3/kq B005
(ft3/lb)
5.1
(82)
4.4
(70)
7.8
(125)
8.0
(128)
7.9
(127)
8.4
(135)
8.0
(128)
8.3
(133)
7.8
(125)
7.1
(114)
1/1 (kg Bl
(ft3/gal)
12.3
(1.64)
16.4
(2.19)
21.5
(2.87)
22.9
(3.06)
21.8
(2.91)
17.7
(2.37)
18.3
(2.45)
21.7
(2.90)
20.3
(2.71)
17.4
(2.33)
Sludge Nominall
Volumetric Return Aeration
F/M Loading Loading Rats Time
305/day/kg MLSS) kg BOD5/ (%) (hr)
day/m3
(lb/day/1000 ft3)
0.12
0.27
0.20
0.19
0.20
0.18
0.19
0.21
0.22
0.18
0.44
(27.5)
0-66 g3_2 8.3
(°30480) 89'2 8'2
0.48 „, , po
(30.0) yblb a'8
0.47 q. q „ 2
(29.3) 94'9 ^
0.43 -., . , ,
(26.8) 76'4 6'7
0.45 -, , ? j
(28.1) 75'4 1'i
0.49 gz 0 05
(30.6)
0.50 Rn -, 7 s
(31.2) 80'3 7'5
0.41 7n Q o q
(25.6) 70'8 8'9
* Extensions commissioned - aeration tank volume increased to 36,000 m3 (1,271,000 ft3) (May/June).
+ Sludge lagoon supernatant liquor returned to works inlet from August 1978.
-------
plant has produced a partially nitrified effluent since 1971 even with the ra-
pid increase in oxygen required to satisfy BODc applied, which occurred in
that year. Data are for April 1 to March 31 of following year.
Primary clarifier performance was reported by plant personnel for the
last 5 yr. BOD5 removal through the primaries has consistently averaged 50-55
percent; SS removal averaged 70-75 percent.
Based on plant performance data, with a power demand of approximately 50
kW (67 wire hp) at an air flow of 60 m3/min (2128 cfm) per channel, average
plant aeration efficiencies can be calculated as shown in Table B-12. The F/M
loading has routinely been held to 0.2 kg BODs/day/kg MLSS, resulting in an esti
mated oxygen requirement of 1.06 kg 02/kg BODc removed. Over the nearly 10 yr
of record, aeration efficiency has been remarkably consistent, if low, averag-
ing 1.12 kg 02/kWh (1.84 Ib/wire hp-hr). The likely cause of the 'low efficien-
cy is the impact of the industrial waste contribution on the oxygen transfer
coefficient alpha (C(]. Also, the very shallow tanks (for diffused aeration)
tend to reduce overall aeration efficiency (see Section 8).
Even though Beddington uses a high air flow rate per unit of organic
loading in order to counter the poor oxygen transfer efficiency, the energy
used for mixing is in the lower range among the plants visited (Table B-13).
The relatively long detention time and large aeration tank volume in this
plant offset the high air flows, resulting in relatively low power levels.
Plant personnel reported all tanks were well mixed with no solids deposition
problems. Under current operating conditions, air flow per dome averages
0.017 m3/min (0.6 cfm).
Operation and Maintenance
Formation of slime on the surface of the aeration domes in zones of low
D.O. has been the major maintenance problem at this plant since startup. The
low oxygen transfer efficiency leads to low mixed liquor D.O., which then ap-
parently causes slime to grow on the exterior of domes. Coarse bubbling re-
sults, further lowering mixed liquor D.O. and exacerbating the slime formation
problem. At the time of the plant visit, coarse bubbling, indicating the pre-
sence of slime, was very much in evidence in the first one-half pass in the 12
basins with equally distributed domes. Only the first one-quarter pass in the
tapered aeration basins exhibited coarse bubbling, and plant personnel noted
that the tapered aeration process was less susceptible to the problem.
A review of the wastewater characterization data provided by the plant
operator does not illuminate the source of the problem. COD/BOD5 ratios aver-
age 2.5, not unusual in the United Kingdom. Influent BOD5 strength, at 275-
325 mg/1, is not excessive for English plants. The efficiency of primary sed-
imentation is normal. Surfactant levels, at 11-13 mg/1, are typical. Visual-
ly, the milky white industrial flow was markedly different from conventional
municipal wastewater. Even after dilution with domestic flows from another
sewer, the presence of this milky white waste was very apparent. Also, an ir-
ridescent oily slick was very much in evidence in the grit facility and prima-
ry clarifiers.
108
-------
TABLE B-12. BEDDINGTON AERATION EFFICIENCY CALCULATIONS
Parameter
Flow: mgd
m3/day
Air Supply:
1000 ft3/day
1000 m3/day
Power Consumed (kWh/day)+
B0n5 Removed (kg/day)0
NH3-N Removed (kg/day)o
02 Required** ( kg/day )o
Aeration Efficiency :++
kg 02/kWh
Ib 02/wire hp-hr
* Partial year, October
+ 1 kWh/day = 1.34 wire
o 1 kg - 2.21 Ib
** Calculated by the met!
78/79
25.5
96,500
58,020
1639
21,500
13,220
2374
24,220
1.19
1.96
- March only
hp-hr/day
icd of Sectio
77/78
23.4
88,600
61,380
1734
?2,760
12,395
2250
22,815
1.05
1.73
n 5, assun
76/77
20.5
77,800
63,190
1785
23,420
12,985
2022
22,460
1.01
1.66
ning 1 .06 uni
75/76
24.4
92,500
56,530
1597
20,950
11,668
2230
21,960
1.11
1.82
ts Oo reqi
Ye;
74/75
26.3
91,400
57,383
1621
21.270
11,390
2614
23,310
1.16
1.91
ui red/unit BODc
jr
73/74
20.2
76,600
58,870
1663
21,820
11,400
2500
22,830
1.09
1.79
: removed .
72/73
20.0
75,500
60,030
1695
22,240
12,310
2552
24,020
1.14
1.87
—
71/72
21.3
80,700
59,366
1677
22,000
12,240
2226
22,540
1.08
1.76
70/71
21.0
79,500
45,454
1284
16,850
15,380
453
18,250
1.25
2.05
69/70*
15.5
58,500
35,040
990
12,9^0
10,444
456
13,030
1.05
1 . 73
++ Assumes 11 percent of air (and power) used in aeration channels.
-------
Initially, when slime occurred, domes were removed from the aeration ba-
sin and refired in the dome kiln at Mogden. It was soon determined that vi-
gorous brushing with a concurrent high rate of air flow would remove the most-
ly external growths. Periodic draining of the tanks followed by brushing is
now the procedure used to deal with this problem. The anticipated dome kiln-
ing cycle is once every 5-6 yr under current procedures.
TABLE B-13. BEDDINGTON MIXING DATA
Tank No.
2-7, 10-15
(each tank)
1, 8, 9, 16
Parameter
First
Quarter
Second
Quarter
Third
Quarter
Final
Quarter
Diffuser Density
(domes/m2)* 2.0
Air Flow/Section:
cfm 2260
m3/min 64
Air Flow/Area:
cfm/ft2 0.11
m3/m2/min 0.033
Air Flow/Volume
(cfm/1000 ft3)+ 13.4
2.7
759
21.5
0.14
0.044
18.0
2.2
621
17.6
0.12
0.036
14.8
1.1
508
14.4
0.10
0.029
12.1
1.1
304
8.6
0.06
0.017
7.2
Average Power Input:
wire hp/1000 ft3 0.47
W/m3 12.4
0.61
16.1
0.50
13.2
0.41
10.8
0.24
6.4
* 1 dome/m2 = 9.29 domes/100 ft2
+ Same as m3/1000 m3/min.
LONG REACH
Plant Description
At Long Reach, a new activated sludge plant was constructed on the site
of an older plant and was commissioned in 1978 (see Figure B-5). The new
plant has eight rectangular primary tanks, each one 20.4mx64mx3.4m SWD
(67 ft x 210 ft x 11 ft). At the current average flow of 208,900 m3/day (55.2
mgd), surface loading is 26.4 nr/m2/day (648 gpd/ft2)., The detention time is
4.2 hr.
Six, four-pass, plug flow aeration basins comprise the activated sludge
plant. Each pass is 6 m x 80 m x 3.8 m SWD (20 ft x 262 ft x 12.5 ft). The
dome diffusers are symetrically distributed in the first three basins. In the
last basin, they are placed toward the center causing a double spiral roll
110
-------
7
\
ITERATION
TANKS
liJ
,^~^\
FINAL \
SEDIMENTATION
TANKS I
^**~~~—^^^*r
STORMWATER OUTFALL
SEWER-UNDER CONSTRUCTION
?3
m m
>
o
m
iTFALL
Figure B-5. Long Reach site plan.
-------
mixing effect.
follows:
Each four-pass aeration tank has 11,086 domes, distributed as
Tank Pass
First Pass
Second Pass
Third Pass
Last Pass
No. of
Domes
3836
3068
2502
1680
No. of
Rows
14
11
9
6
Eight circular final clarifiers provide final settling. Each is 38.1 m
(125 ft) in diameter with a sidewater depth of 3.1 m (10 ft) and 5 percent
floor slope. Flow is unequally distributed to the clarifiers. The plant is
divided into two sections, each with four clarifiers. However, there are four
aeration basins on one side and two on the other. Thus, four clarifiers re-
ceive one-third of the flow; the other four receive two-thirds of the flow.
Surface loadings are 15.1 m3/m2/day (371 gpd/ft2) on one side and 30.2 m3/m2/
day (742 gpd/ft2) on the other. Detention times are 4.9 hr and 2.4 hr, re-
spectively.
Air is provided by three 634-kW (850-hp) centrifugal blowers, capable of
supplying 1593 m3/min (56,250 cfm) at 40 kN/m2 (5.8 psi). Coated bag filters
precede the blowers.
Performance
One year's performance data, April 1, 1978-March 31, 1979 were reported
by Long Reach, as follows:
TABLE B-14. LONG REACH INFLUENT-EFFLUENT DATA SUMMARY
Raw Wastewater: (mg/1 )
BOD5
TSS
TKN
NH3-N
334
341
53
33
Primary Effluent: (mg/1)
TSS
TKN
NH3-N
Detergent (manoxyl OT)
Final Effluent: (mg/1)
BODr
TKN
NHs-N
180
113
45
31
15
20
30
35
33
1-2
112
-------
Influent flow averaged 200,000 m3/day (52.8 mgd) over the period of re-
cord. Power consumption averaged 15,000 kWh/day (20,115 wire hp-hr/day). The
F/M loading averaged 0.3 kg BOD5/day/kg MLSS and ranged from 0.27-0.34, with
MLSS averaging 1700 mg/1. BODr and SS removals through the primary clarifiers
averaged 46 and 67 percent, respectively. BOD5 and SS removals by secondary
treatment averaged 89 and 73 percent, respectively. NH3-N removal was
negligible.
Based on the method of Section 5, with 0.97 units 02 required/unit 8005
removed, aeration efficiency was calculated to be 2.07 kg 02/kWh (3.40 Ib/wire
hp-hr). Mixed liquor D.O. is typically quite low, near 0.5 mg/1 at the inlet,
rising to 1-3 mg/1 at the effluent.
Normally, Long Reach is able to generate all of its plant power needs
from digester gas burned in dual fuel engines. Plant policy is to avoid the
use of outside power; consequently, considerable effort has been devoted to
optimizing the aeration process as a means of reducing power consumption. Air
flow has been individually adapted to each pass of the aeration tank based on
D.O. levels in that section.
Table B-15 provides mixing data for Long Reach, given an average air flow
rate of 0.013 m3/min/dome (0.45 cfm/dome). Because air flow per section has
been adjusted to D.O., it is quite possible that domes in some sections have
lower or higher air flow rates. Mixing power levels are more than adequate
for suspension of solids based on experiences at the other plants visited.
Operation and Maintenance
This plant is the newest facility visited in the United Kingdom. Conse-
quently, little O&M experience has been accrued. For the period of operation,
there have been no apparent problems with either the aeration system or the
air cleaning equipment. Observation of the tanks in operation indicated some
coarse bubbling in the first 25 percent of the first pass, but it was not se-
vere.
MOGDEN
History and Background
Wastewater was first treated in the new treatment plant at Mogden in late
1935. Full operation was commenced in mid-1936 with the completion of the
plant, now the present day East works (and part of the West works), and an ex-
tensive collection system serving 427 km2 (165 mi2) and 1.05 million people.
The original plant included a pumping station, screening and degritting, pri-
mary sedimentation, activated sludge treatment, final sedimentation, and
stormwater tanks. Sludge was treated by anaerobic digestion, and a large pow-
er station was constructed to generate power and provide compressed air for
the plant.
By 1961, the plant had been expanded with further construction of the
West works to serve a population of 1.5 million and the activated sludge aera-
tion basins, originally equipped with square diffuser stone aerators set in
the basin floor, had been converted to dome diffusers mounted on cast iron
113
-------
TABLE B-15. LONG REACH MIXING DATA
Aeration Surface Area
Basin (m2)*
First Pass
Second Pass
Third Pass
Last Pass
480
480
480
480
Wetted
Volume
(m3) +
1752
175?
1752
1752
Diffuser
Density
(domes/m2)0
7,
6.
5.
3.
.8
.4
.2
,5
Air Flow/Section**
(cfm)
1719
1374
1119
752
(m3/min)
48.7
38.9
31.7
21.3
Air Flow/Area**
(cfm/ ft2)
0.33
0.27
0.23
0.13
(m3/m2/min)
0.10
0.08
0.07
0.04
Air Flow/ Volume**
(cfm/ 1000 ft2)++
27.8
22.3
18.2
12.2
Average nixing Power
(wire hp/1000
2.23
1.78
1.45
0.98
ft3) (W/m3)
58.7
46.9
38.2
25.7
* 1 m2 = 10.8 ft2
+ 1 m3 = 35.3 ft3
o 1 dome/in2 = 9.29 domes/100 ft2
** Based on an average air flow rate of 0.013 m3/min/dome (0.45 cfm/dome).
++ Same as m3/1000 nP/min.
-------
mains. The entire plant, comprising the East and West works as presently con-
structed, is shown in Figure B-6.
Plant Description
The East works includes 12 primary settling basins (eight first-stage and
four second-stage), 12 plug flow aeration basins each with four passes and di-
vided into Battery A and Battery B, and 40 final clarifiers. Eight storm wa-
ter retention basins are also located on the East works site. The West works
includes four primary clarifiers, six plug flow aeration basins with four
passes each, comprising Battery C, and 24 final clarifiers. Table B-16 summa-
rizes the design data for both sides of the plant.
All of the aeration basins are equipped with dome diffusers mounted ei-
ther on cast iron or the newer plastic air manifolds. A program of refitting
basins with plastic piping is underway. Because Battery B has been fully
changed over for 6 yr, Mogden has provided performance data for this battery
covering the period 1974-1979.
Dome diffuser configuration in Batteries A, B, and the first three tanks
of C is similar. The first pass of each tank has 2800 domes in five rows laid
lengthwise along the 122-m (400-ft) long tanks. Spacing between domes is 20.3
cm (8 in.) and between rows 0.76 cm (2.5 ft). The other three passes of each
aeration tank have 1750 domes each laid lengthwise in three rows with 20.3 cm
(8 in.) between domes and 1.22 m (4 ft) between rows. Some of the three-row
tanks have the original "ridge and furrow" tank floor, as described in Section
8. All tanks are operated in the plug flow mode with return sludge and prima-
ry effluent inlets at the head of the first pass.
Battery C has six tanks, four with dome configurations similar to Batter-
ies A and B, one with all flat floors and one with all flat floors and a ta-
pered dome configuation for experimentation. Battery C receives flow from the
high level sewer only and thus tends to treat wastewater of different composi-
tion from that treated by the East works.
Final clarifiers at Mogden feature the steeply sloped (30°) floors and
shallow side water depth (2.5 m = 8.2 ft) in widespread use in the United
Kingdom. The 1978/79 flow to Battery B averaged 170,300 m3/day (45 mgd) and
varied between a maximum monthly flow of 196,800 m3/day (52 mgd) and a minimum
monthly flow of 143,800 m3/day (38 mgd). Peak flow is 3 x dry weather flow.
Air for the entire plant is supplied by 10 centrifugal air compressors,
six rated at 510 m3/min (18,000 cfm) and four rated at 765 m3/min (27,000 cfm).
All blowers are driven by dual fuel digester gas/diesel fuel engines. Maximum
system pressure is 42.7 kN/m^ (6.2 psi). Air is cleaned by replaceable dual
element filters. The outer filters are coarse fiberglass units. The inner
filters are labyrinth box-type fine filters.
Mogden reports a construction cost of d? 32,937 ($65,874) for new domes
and fittings andtf? 22,100 ($44,200) for piping and labor, indexed to 1979, to
retrofit a four-pass activated sludge unit. Cost of changing the air cleaning
filters for Battery B isc£ 700 ($1400) /yr.
115
-------
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116
-------
TABLE B-16. MOGDEN DESIGN DATA
EAST WORKS (DWF = 264,000 m3/day = 70 mgd'
Stormwater Tanks
Flows exceeding 659,000 m3/day (174 mgd) are diverted to adjacent Stormwater tanks.
8 Rectangular Tanks
Detention time:
Primary Sedimentation - Two Stage
8 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
4 Rectangular Clarifiers
Detention time:
Surface loading:
Aeration Basins
12 Rectangular Tanks
(4 passes per tank)
(6 tanks per battery)
Detention time:
Final Sedimentation
40 Circular Clarifiers
(20 per battery)
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
WEST WORKS (DWF = 132,000 m3/day = 35 mgd)
Preaeration
2 Rectangular Tanks
Detention time:
Primary Sedimentation
4 Circular Clarifiers
Diameter:
Sidewater depth:
Detention time:
Surface loading:
Aeration Basins
6 Rectangular Tanks
(4 passes per tank)
Detention time:
Final Sedimentation
24 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
46 m* x 70 m* x 3.7 m* SWD
6.2 hr @ DWF (6 tanks in use)
27.4 m
3.4 m
1:10
1.9 hr @ DWF
55.2 m3/m2/day (1355 gpd/ft2)
46 m x 61 m x 3.7 m
3.8 hr @ DWF
24.0 m3/m2/day (589 gpd/ft2)
4.6 m x 122 m x 3.7 m SWD (each pass)
9.0 hr @ DWF
18 m
2.5 m
30°
4.5 hr Q DWF
22.3 m3/m2/day (548 gpd/ft")
6 m* x 40.8 m* x 3.7 m* SWD
30 min @ DWF
42.6 m
0.9 m
1.8 hr @ DWF
23.0 m3/m
-------
Performance
Performance data discussed herein are for Battery B only, although all
sections of the plant performed similarly. Battery B data are the most repre-
sentative as it was fully retrofitted with plastic piping between 1974-1979.
Primary sedimentation serving Battery B has averaged 53 percent removal
of BODg and 68 percent removal of suspended solids over the period of record
(Table B-17). Secondary treatment averaged 91 percent removal of BOD5, 91
percent removal of suspended solids, and 94 percent conversion or removal of
ammonia nitrogen. Overall, the plant achieved average removals of 96 percent
for BOD5, 97 percent for suspended solids, and 93.8 percent for ammonia nitro-
gen through Battery B. Detergent was reduced an average of 97 percent through
the process. Battery B flow averaged 166,500 m^/day (44 mgd). Monthly aver-
age flow and BOD varied +_ 15 percent from the annual average. Approximately
15 percent of the flow is derived from a variety of industrial sources.
TABLE B-17. MOGDEN INFLUENT-EFFLUENT DATA SUMMARY*
Year+
Parameter
78/79
77/78
76/77
75/76
74/75
Flow:
mgd
1000
nvVday
45.2
'171
Raw Wastewater: (mg/1)
BOD5 238
COD 542
TSS 311
NH3-N 24
Detergent0 9.3
Primary Effluent: (mg/1)
BOD5 99
COD 228
TSS 96
NH3-N 24
Detergent0 5.3
42.3
160
225
511
292
24
7.5
97
228
90
26
4.3
40.2
152
208
291
25
6.3
93
99
26
3.4
* For Battery B only.
+ Year begins April 1 and ends the following March 31
0 As manoxyl OT (a proprietary ABS type compound).
118
41.5
157
225
299
27
8.5
118
92
30
6.2
46.5
176
215
281
26
10.6
112
94
27
7.9
Final Effluent: (mg/1)
BOD5
COD
TSS
NH3-N
N03-N
Detergent0
8
--
8
1.3
20
0.3
12
--
8
1.7
20
0.2
10
--
8
1.6
21
0.2
8
--
9
1.9
19
0.2
6
--
10
1.3
20
0.3
-------
Aeration efficiency is calculated for the 5 yr shown in Table B-18 and
averaged 1.37 kg 02/kWh (2.25 Ib/wire hp-hr). In computing these efficiencies,
constant power consumption in kWh was assumed over the 5-yr period. Plant
personnel stated that the assumption was most accurate for 76/77, 77/78, and
78/79. They were unsure of 74/75 and 75/76 power consumption. The average
aeration efficiency for the last 3 yr of 1.25 kg C^/kWh (2.05 Ib/wire hp-hr)
is probably most representative of typical performance.
TABLE B-18. MOGDEN EFFICIENCY CALCULATIONS*
Year
Parameter
BOD5 Removed ( kg/day )+
NH3-N Removed ( kg/day )+
N03-N Produced ( kg/day )+
02 Required0 ( kg/day )+
Power Consumed**
78/79
15,560
3830
3437
32,030
77/78
13,600
3600
3216
29,080
76/77
12,620
3480
3162
27,580
75/76
17,270
3956
2936
34,280
74/75
18,660
4365
3538
37,430
(kWh/day)++ 23,140 23,140 23,140 23,140 23,140
Aeration Efficiency:
kg
Ib
02/kWh
02/wire
hp-hr
1.
2.
38
27
1.
2.
26
07
1.12
1.84
1.
2.
48
43
1.
2.
62
66
* For Battery B only.
+ 1 kg = 2.21 Ib
0 Calculated by the method of Section 5, assuming 1.08 units 02 required/
unit 8005 removed.
** One-half of power consumption of Batteries A and B. Power assumed con-
stant over 5 yr per plant operators.
++ 1 kWh/day =1.34 wire hp-hr/day.
The relatively low efficiency at this plant is probably attributable to
the substantial degree of excess aeration being practiced. Lack of hydraulic
(wastewater flow) control and air flow control are principal contributors to
this problem. Also, the extreme plug flow configuration and lack of tapered
aeration mitigates against efficient use of the aeration process. D.O. reach-
es 70 percent of saturation in the third pass of the four-pass basins and can
reach 100 percent at the exit of the last pass.
Mixing power levels at Mogden (Table B-19) are similar to those at the
Beckton plant at 29 W/m3 (1.10 wire hp/1000 ft3) in the first pass and 18.5
W/mJ (0.70 wire hp/1000 ft3) in succeeding passes. Plant personnel stated
that the tanks were adequately mixed, with the exception that a small snail
which grows in the tanks at Mogden is deposited in large quantities under the
domes and must be removed about once every 5 yr during routine tank cleaning.
119
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TABLE B-19. MOGDEN MIXING DATA*
Parameter First Pass Other Passes
Surface Area (m2)+ 557.5 557.5
Wetted Volume (m3)° 2174 2174
Diffuser Density (domes/m2)** 5.02 3.14
Air Flow/Section:++
cfm 2083 1306
m3/min 59 37
Air Flow/Area:++
cfm/ft2 0.348 0.220
m3/min/m2 0.016 0.067
Air Flow/Volume++ (cfm/1000 ft3)00 27.3 17.0
Average Power Input: ***
wire hp/1000 ft3 1.10 0.70
W/m3 29.0 18.5
* For Battery B only.
+ 1 m2 = 10.8 ft2
1 m3 = 35.3 ft3
** 1 dome/m2 =9.29 domes/100 ft2
++ Based on an operating air flow rate of 0.021 m3/min/dome (0.75 cfm/dome).
op Same as m3/1000 m3/min.
*** Assumes power apportioned between tanks in proportion to number of domes
in each.
Operation and Maintenance
Mogden has operated 17.8-cm (7-in.) dome diffusers continuously since 1961,
initially using domes mounted on cast iron mains. Rusting of the iron mains
caused deposition of iron oxides on the inside of the domes. Prior to the re-
trofit of the tanks with plastic lines, which began in 1972, it was necessary
to clean the domes about once each 6 yr. Battery B, which was fully converted
by 1979, has not required cleaning since and is performing well. Very little
coarse bubbling is occuring, mostly limited to the first one-third of the
first pass, and there has been no pressure increase. There was some evidence
of calcium carbonate deposition in domes that had been in service for 8 yr,
but the amount was very light compared to that observed at Basingstoke.
In general, plant personnel expressed satisfaction with the equipment and
its performance. Some failures of the plastic dome hold-down bolts had
occurred, and the scientific staff was investigating alternative materials.
The air filtration units are performing satisfactorily, requiring replacement
of the filter elements once per year.
120
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OXFORD
History and Background
Wastewater treatment for the City of Oxford began at Sanford-on-Thames,
8 km (5 mi) south of the City in the 1870's. Initially, the City's wastewater
was land disposed on a "sewage farm" adjacent to a tributary of the Thames Ri-
ver. Increasing population progressively overloaded the facility, and, in 1957,
it was replaced with an activated sludge system that employed mechanical sur-
face aeration of the Simplex type.
In 1969, the 1957 plant was expanded with the provision of two additional
primary clarifiers and four additional final clarifiers. A new treatment
plant was constructed alongside comprised of three rectangular primary clari-
fiers, three rectangular storm/balancing tanks, eight diffused air activated
sludge tanks, and three final clarifiers. The new works, sized for 15,000
nryday (3.96 mgd), was operated in parallel with the upgraded existing plant
(Figure B-7).
By the early 1970's,it became apparent that the original activated sludge
plant was reaching the end of its useful life and required substantial mech-
anical rehabilitation. Based on performance and maintenance studies, the de-
cision was made to install diffused air aeration equipment. The work, cur-
rently under way, also includes refurbishment of the four original flat bot-
tomed final clarifiers. Modifications will be completed by mid-1980. Sepa-
rate contracts for further improvements in sludge handling and disposal are
scheduled to follow this work.
Plant Description
Table B-20 summarizes basic design data for both sections of the Oxford
plant. Wastewater flow is divided between the two activated sludge plants
with 67 percent being treated in the 1957 plant and 33 percent in the 1969
plant. The 1957 plant was originally constructed with four aeration tanks of
10 aeration cells each. These tanks are presently being converted to diffused
aeration with the addition of 168 dome diffusers per cell or 1680 per tank.
The hopper bottoms of the individual aeration cells are being partially filled
to provide a base for installing the dome diffusers, and the final operating
depth will be 5 m (16.4 ft).
The 1969 plant was constructed with eight aeration tanks, each with 990
domes per tank. Seven of the units employ an equally distributed grid dome
layout of nine rows spanning the length of the tank with 110 domes per row.
The eighth unit was designed in the tapered aeration mode (still experimental
in 1969) for the purpose of research. Dome configuration in tanks Nos. 2-8 is
0.30 m (1 ft) between domes and 0.76 m (2.5 ft) between rows. Dome diffuser
configuration in tank No. 1 is as follows:
121
-------
ro
ro
CD
CD
I
X
-h
O
CX-
I/I
-o
13
-------
TABLE B-20. OXFORD DESIGN DATA
1969 PLANT (DWF = 15,000 m3/day = 3.96 mgd)
Stormwater Tanks
3 Rectangular Tanks
Detention time:
Primary Sedimentation
2 Circular Clarifiers
Diameter:
Sidewater depth:
Detention time:
3 Rectangular Tanks
Detention time:
Overall surface loading:
Aeration Basins
8 Rectangular Tanks
Detention time:
Final Clarifiers
7 Circular Tanks
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
1957 nl_ANT (DWF = 30,OCO m3/day = 7.92 mgd)
Primary Sedimentation
6 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
Aeration Basins
4 Rectangular Tanks
(10 cells each)
Wetted volume:
Detention time:
Final Sedimentation
4 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
4 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention tiire:
Surface loading:
15.2 m* x 45.6 m* x 3.3 m * SWD
11 hr @ DWF
18.3 m
3.6 m
6.8 hr (? DWF
15.2 m x 45.6 m x 3.3 m SWD
10.9 hr @ DWF
5.5 m3/m2/day (135 gpd/ft2)
6.9 m x 37.8 in x 2.4 m SWD
8.0 hr 9 DWF
24.4 m
1.5 m
30°
8.6 hr @ DWF
10.3 mW/day (253 gpd/ft2)
18.3 m*
3.6 m
20°
6.8 hr I? DWF
18.5 m3/m2/day (454 gpd/ft2)
9.2 m x 9.2 m x 4.4 m SWD (each eel V
14,750 rn3+
11.7 hr @ DWF
24.4 m
3.6 m
12°
10 hr @ DWF
8.4 nrW/day (206 gpd/ft2)
24.4 m
1.5 m
30°
8.6 hr I? DWF
10.3 m3/m2/day (253 gpd/ft2)
* 1 rn = 3.28 ft
+ 1 m3 = 35.3 ft3
123
-------
Tank Section Description Percent of Domes
First quarter of tank 425 domes, 17 rows, 43
8.3 m (27.25 ft) 0.30 m (1 ft) between domes,
0.40 m (1.3 ft) between rows.
Second quarter of tank 270 domes, 9 rows, 28
9.4 m (31.0 ft) 0.30 m (1 ft) between domes,
0.76 m (2.5 ft) between rows.
Third quarter of tank 198 domes, 9 rows, first 1/3 20
10.0 m (33.0 ft) at 0.30 m (1 ft) centers, last
2/3 at 0.61 m (2 ft) centers,
0.76 m (2.5 ft) between rows.
Last quarter of tank 90 domes, 3 rows in center, 9
9.9 m (32.5 ft) 0.30 m (1 ft) between domes,
0.76 m (2.5 ft) between rows.
The diffusers are placed 20 cm (8 in.) above the basin floor in all eight
tanks. Air is distributed via a single main that enters the tank at mid-length
and is split into feeders for each of the four grids.
Primary effluent is step fed, 50 percent at 8.2 m (27 ft) from the head
of the aeration tank and 50 percent at 14.6 m (48 ft), into the seven tanks
with equal dome spacing. The tapered tank is fed the full flow at 8.2 m
(27 ft) from the head of the tank. Recycled activated sludge enters at the
influent end of all eight tanks.
The renovated 1957 plant will be served by six Rootes-type positive dis-
placement blowers, two of which are provided for standby. The standby blowers
are fixed-speed, 93-kW (125-hp) units, operated at a maximum load of 78 kW
(104 hp). The remaining four blowers are driven by variable-speed, direct-
current thyristor drives and operate in the range of 48-75 kW (64-100 hp).
The maximum operating pressure is 59.3 kN/m2 (8.6 psi), and air flow ranges
from 40-68 m3/min (1400-2400 cfm) per blower.
Three positive displacement Rootes-type blowers, one fixed and two varia-
ble speed, provide air to the 1969 plant. The fixed-speed blower, used prin-
cipally for standby, draws 57 kW (77 hp) at 41.4 kN/m2 (6 psi) and delivers 78
m3/min (2750 cfm). The variable-speed blowers are rated from 31-45 kW (42-60
hp) with an output ranging from 42-59 m3/min (1500-2100 cfm).
The renovated 1957 plant will be equipped with a D.O. monitoring and con-
trol system. Four sensors will be provided for each tank, and a controller
will blend the signals and control blower operation.
Air filtration in the renovated 1957 plant will be provided by three
electrostatic air filters, each sized to precede two blowers. These filters
were selected based on two factors:
1. The electrostatic units are substantially smaller than bag filters.
124
-------
2. Use of the D.O. control system will cause wide variations in air flow
rate. Bag filters are poorly suited to varying and cyclic (on/off)
air flow conditions.
Air filtration in the 1969 plant is provided by two 85-m3/min (3000-cfm)
Tilghman-Wheelabrator bag filter units. The units require the use of a pre-
coat, which is added immediately after cleaning. Formerly, an asbestos pre-
coat was used, but a cellulose-based precoat is now applied when the units are
cleaned.
Final clarifiers for the 1969 Oxford plant are of the circular, center-
feed type and feature steeply sloped floors (30°) and shallow sidewater depth of
1.5 m (5 ft). Detention time at the design flow of 15,000 m3/day (3.96 mgd) is
8.6 hr. The surface loading rate is 10.3 m3/m2/day (253 gpd/ft2), not includ-
ing a recycle flow of 75-100 percent of design flow. Settled sludge flows to
the center of the clarifier aided by a simple rotating chain mechanism and is
normally concentrated to 9000 mg/1 when withdrawn.
Construction costs of the 1969 works totaled £l.3 million ($2.6 million),
of which £47,000 ($94,000) was spent on diffusers, piping, blowers, and con-
trols. The current 1957 plant renovation is costing £ 550,000 ($1.1 million),
of which £42,700 ($85,400) is required for the diffusers and in-tank piping
and installation.
Performance
Performance data were provided by the City of Oxford for the years 1974-
75 to 1978-79 and are summarized in Table B-21. The data are for the 1969
works only and do not include the 1957 plant.
Primary treatment removed an average of 46.6 percent of the influent 8005
and 71.3 percent of the influent suspended solids. Secondary plant perfor-
mance averaged near 93 percent 6005 removal and 85 percent suspended solids
removal, with an average MLSS concentration of 5000-5500 mg/1 and an F/M load-
ing of 0.1 kg BODs/day/kg MLSS. Overall plant performance has consistently
exceeded 95 percent removals of BODs and suspended solids. Consistent nitri-
fication has been achieved, with ammonia nitrogen removals exceeding 92 per-
cent. Approximately 13 percent of the flow to this plant is derived from a
variety of industrial sources. Monthly averages of volumetric loading vary
approximately +_ 20 percent from annual averages.
Aeration efficiency (Table B-22) as calculated by the method detailed in
Section 5, ranged from 1.93-2.34 kg 02/kWh (3.17-3.85 Ib/wire hp-hr) over the
5-yr data base. Mixed liquor D.O. at Oxford is maintained at 20 percent of
saturation at the outlet end of each aeration tank, a relatively low value
when compared to the other plants visited. The purpose of this practice is
twofold: first, to maximize aeration efficiency and, second, to create con-
ditions at the tank inlet that are conducive to denitrification in the first
25 percent of the tank length. Substantial denitrification has been consis-
tently achieved at Oxford over the last 2 yr. Plant operators expressed a
strong belief that the creation of conditions conducive to denitrification in
the first quarter of each tank improves effluent quality by suppressing deni-
125
-------
trification in the final clarifiers and increases aeration efficiency.
TABLE B-21. OXFORD INFLUENT-EFFLUENT DATA SUMMARY
Year*
Parameter 78/79 77/78 76/77 75/76 74/75
Flow:
mqd
nH/day
5.34
20,200
4.61
17,400
3.75
14,200
4.19
15,900
5.07
19,200
Raw Wastewater: (mg/1)
BOD5 367 353 299 248 277
COD 927 881 695 608 641
TSS 480 475 376 364 352
TKN -- - - 59 62
NH3-N 36 43 34 27 31
Primary Effluent: (mg/1)
BOD5
COD
TSS
TKN
NH3-N
Final Effluent:
BOD5
COD
TSS
TKN
NHo-N
N03-N
165
349
112
—
29
(mg/1 )
7
53
20
__
2.6
19
170
364
130
__
40
13
66
31
3.3
18
177
363
112
_ _
34
14
64
27
— _
2.8
32
157
352
119
45
27
10
44
13
45
1.4
26
142
317
106
50
28
13
40
15
50
2.4
25
* Year begins April 1 and ends the following March 31.
Table B-23 summarizes mixing conditions in the 1969 plant. Tank Nos. 1-4 and
6-8 have a uniform diffuser density. The minimum power level utilized is
18.8 W/m3 (0.71 wire hp/1000 ft3). At the maximum air flow rate currently em-
ployed, the mixing power level is 29.2 W/m3 (l.ll wire hp/1000 ft3). Corre-
sponding air flow rates are 0.013 m3/min/dome (0.45 cfm/dome) and 0.020 m3/
min/dome (0.70 cfm/dome), respectively. The tapered aeration tank (No. 5) has
a minimum mixing power level of 6.4 W/m3 (0.24 wire hp/1000 ft6) in the final
section. This low power input is compensated for by placing the four diffuser
lines in the center of the tank lengthwise creating a spiral roll condition in
each half of the final section. Plant personnel stated that all of the tanks
have been checked for adequate mixing and found to be satisfactory.
The renovated 1957 plant will also have a uniform distribution of domes,
with a projected minimum air flow of 0.024 m3/min/dome (0.83 cfm/dome). The
126
-------
maximum air flow rate will be 0.039 m3/min/dome (1.38 cfm/dome).
TABLE B-22. OXFORD AERATION EFFICIENCY CALCULATIONS
Parameter
BOD5 Removed
(kg/day)*
COD Removed
(kg/day)*
NHo-N Removed
(kg/day)*
NOo-N Produced
(kg/day)*
02 Required"1"
(kg/day)*
Power Consumed
(kWh/day)°
Aeration Efficiency:"
kg 02/kWh
78/79
3195
5797
535
380
4420
2290
f+
1.93
Ib 02/wire hp-hr 3.17
77/78
2737
5200
642
321
4949
2108
2.34
3.85
Year
76/77
2317
4247
438
460
4663
2180
2.14
3.52
75/76
2330
4889
413
411
4501
2331
1.93
3.17
74/75
2475
5313
491
482
5029
2200**
2.29
3.76
* 1 kg = 2.21 Ib
+ Calculated by the method of Section 5, assuming 1.20 units 02 required/
unit BOD5 removed.
0 1 kHh/day =1.34 wire hp-hr/day
** Estimated value.
++ Corrected for 10 percent of air supply used in aerated channels and else-
where.
Operation and Maintenance
Oxford has operated the dome diffusers in the 1969 plant continuously for
over 10 yr with no apparent loss of aeration efficiency and no dome cleaning.
In the first 9 mo of operation, several domes had to be regasketed, but there
have been no other failures of the domes, dome mountings, or plastic air pip-
ing in the tanks. Operating personnel at this plant have followed the follow-
ing procedures since starting the 1969 plant:
1. Maintain minimum dome air flow rates near 0.014 m3/min (0.5 cfm).
2. Scrupulously maintain air cleaning equipment.
127
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TABLE B-23. OXFORD MIXING DATA
ro
00
& Section Nos.
Tanks 1-4, 6-8
Tank 5:
Section 1
Section 2
Section 3
Section 4
1957 Plant
(each cell)
* 1 m2 = 10.8
+ 1 m3 = 35.3
o 1 dome/m2 =
(m2)*
261
58
65
70
68
83.7
ft2
«3
9.29 domes/100
Wetted
(n,3) +
639
141
161
170
167
369
ft2
, pLns^tv" Air F1°w/Section** Air Flow/Area** Air Flow/ Volume** Minimum Power Input
(domes/m2)o (cfm) (m3/min) (cfm/ft2) (m3/m2/min) (cfm/1000 ft3)++ (wire hp/1000 ft3) (W/m3)
3.8 445 12.6 0.16 0.048 19.9 0.71 18.8
7.3 191 5.4 0.31 0.093 38.5 1.38 36.3
4.1 120 3.4 0.17 0.051 21.6 0.77 20.4
2.8 88 2.5 0.12 0.036 14.9 0.53 14.0
1.3 40 1.14 0.06 0.018 6.8 0.24 6.4
2.0 74 2.1 0.15 0.047 10.7 0.38 10.1
** Based on a minimum air flow rate of 0.013 m3/inin/dome (0.45 cfm/dome).
++ Same as m3/1000 m3/min.
-------
3. Immediately repair air leakage to prevent infiltration of mixed li-
quor into aeration piping.
4. Operate aeration blowoff manifold valves once per month to clean
lines of condensation.
Oxford uses coated bag filters for air cleaning. The 85-m3/min (3000-cfm)
units require cleaning approximately every 5 yr. Cleaning is scheduled on the
basis of headless across the filters. When clean, the headless averages 5.1-
7.6 cm H20 (2-3 in.). Cleaning is scheduled when headless exceeds 12.7 cm (5
in.). Mechanical performance of the two bag filters has been satisfactory, and
there have been no breakdowns.
RYEMEADS
History and Background
The treatment plant at Ryemeads discharges to the River Lee and is part
of the Middle Lee Regional Drainage Scheme. The plant receives wastewater
from existing communities and New Towns to the north of London. Because
treated effluent is discharged to the river only 16 km (10 mi) above the in-
takes to a substantial portion of London's drinking water supply, effluent
standards are very high and the treated wastewater is passed through a series
of lagoons prior to final discharge to the river.
The Ryemeads plant was built in three stages, beginning in 1956 with the
startup of the first stage (Figure B-8). Stage 2 was completed in 1965. The
final stage, consisting of four, four-pass plug flow aeration tanks and six
circular final clarifiers, was completed in 1970.
Plant Description
All three stages of the plant are designed on a similar basis. Common
primary treatment, with 6 hr detention time at DWF and a 12-m3/m2/day (295
gpd/ft) overflow rate, is provided by 12 rectangular clarifiers measuring 51.2
m x 17.4 m x 3.3 m SWD (168 ft x 57 ft x 10.75 ft) (DWF = 36,300 m3/day = 9.6
mgd). Seven of the clarifiers handle the normal plant flow. Two are used for
daily flow equilization and three for stormwater retention. Flow is then
split among the three stages of the plant. Because the third stage is fully
modernized, employing domes on plastic air distribution lines in a tapered
configuration, performance data have been provided for this section only.
Other parts of the plant are currently being modernized, replacing cast iron
air lines with plastic piping (Table 8-24).
In 1976, denitrification in the form of an anoxic zone in the first 40
percent of the first pass of each aeration tank was added. Mixing was provid-
ed by five 2.2-kW (3-hp) paddle stirrers in this zone. (Later investigations
proved that mixing could be maintained by operating three of these.) Table
B-24 summarizes design data for Stage 3 before and after this modification.
Basically, domes removed from the first 40 percent of the first pass were in-
stalled in the second and third passes.
129
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Administration
and
Laboratory
Final Clarifiers
Final Clarifiers
CO
o
Stage 3
Aeration Tanks
Sludge
Thickener
Drain
Blower
House
CZD
Final Clarifiers
Figure B-8. Ryemeads site plan (secondary system)
-------
TABLE B-24. RYEMEADS DESIGN DATA*+
Aeration Basins
4 Rectangular Tanks
(4 passes per tank)
Detention time:
Oiffuser Configuration:
First Pass
Second Pass
Third Pass
Fourth Pass
TOTAL DOMES
First Pass
Second Pass
Third Pass
Fourth Pass
4.27 m° x 70 m° x
9.6 hr @ DWF
PRE-JULY 1976
Symmetrical, 9 rows,
150 domes/row, 0.47 m
between domes, 0.43 m
between rows.
Two sections, each 35 m
long. First has 9 rows,
75 domes/row, spaced like
first pass. Second section,
5 rows, 120 diffusers/row,
0.3 m between domes, 0.7 m
between rows.
Symmetrical, 5 rows, 120
domes/row, 0.6 m between
domes, 0.7 m between rows.
Same as third pass.
1350
675 +
600
600
600
3825
m° SWD (each pass)
CURRENT
First 28.7 m, no domes
(5 equally spaced
stirrers). Last 41.3 m.
9 rows, 90 domes/row,
same spacing as before.
Same as pre-July 1976.
Symmetrical, 5 rows,
220 domes/row, 0.3 m
between domes, 0.7 m
between rows.
Symmetrical, 5 rows,
HO domes/row, 0.5 m
between domes, 0.7 m
between rows.
0 + 810
675 + 600
1100
700
3885
Final Sedimentation
6 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
22 m
1.3 m
30°
7.5 hr 9 DWF
15.8 m3/m2/day (389 gpd/ft?)
* For Stage 3 only.
+ DWF = 36,300 m3/day (9.6 mgd)
0 1 m = 3.28 ft
131
-------
Air supply for Stage 3 is provided by three variable-vane centrifugal
blowers with a shaft outout that varies from 89-242 kW (120-325 BHP). The
units will deliver 210 m3/min (7400 cfm) at the normal operating pressure of
49 kN/m2 (7.1 psi). Air cleaning is provided by five electrostatic air fil-
ters out of a total of 12 connected to a common duct. Approximately 6 percent
of the air supply is used for the domes that are mounted in the primary efflu-
ent and mixed liquor channels.
Final clarifiers are of the type recommended by the diffuser manufacturer
and feature a steep floor slope and a shallow sidewater depth. A single chain
scraper moves sludge to the center hopper. Sludge recycle rates average 75-
100 percent.
Performance
Data was provided for Stage 3 for the period January 1974-December 1978.
However, considerable modification of Stage 3, with variable operation of two
to four aeration tanks, took place in 1975 and 1976. Accordingly, these data
have been deleted in this analysis. All four units were operated in 1974,
1977, and 1978, and influent and effluent data for these 3 yr are summarized
in Table B-25.
The wastewater received by the Ryemeads plant is fairly strong and con-
tains about 10 percent (by flow) industrial wastes from metal finishing, malt-
ing, and pharmaceutical industries. Over the period 1974-1978, the influent
BODs concentration declined by approximately 25 percent, reflecting growth in
the contributing New Towns and the use of more water by the new inhabitants.
8005 removal has consistently averaged 53-56 percent in the primary system and
96-97 percent in the secondary system. Approximately 75-77 percent of the in-
fluent suspended solids are removed in the primary plant and 88-91 percent in
the activated sludge process. Overall, the plant removed an average of 98
percent each of the BOD5 and suspended solids in the raw wastewater. Ammonia
nitrogen removal exceeded 99 percent. Monthly average flows and BODr loadings
varied about ± 20 percent from annual averages. Influent detergent 'as manox-
yl OT) averaged 10-15 mg/1 and was 95 percent removed through the process.
The stable flow and 6005 loadings to the Ryemeads plant reflect the relatively
new collection system serving the plant.
Aeration efficiency computations are shown in Table B-26, based on data
for Stage 3 only. MLSS averaged 4500-5000 mg/1, and the F/M loading rate av-
eraged 0.08 kg BODc/day/kg MLSS. Using the method of Section 5, 1.22 units 02
were estimated to be required per unit of BODc removed and the aeration effi-
ciency ranged from 1.04-1.14 kg 02/kWh (1.71-1.87 Ib/wire hp-hr). D.O. levels
in the aeration tanks average 20 percent of saturation in the first pass,
reaching 50-60 percent of saturation by the end of the second pass and 90-100
percent of saturation in the effluent from the final pass. These elevated le-
vels are likely the main cause of the relatively poor aeration efficiency at
this plant. Plant personnel indicated that these D.O. concentrations were the
by-product of maintaining total plant air flows to keep dome air flow rates
from falling below 0.014 m3/min (0.5 cfm). Unlike Beddington and Hartshill,
there is no indication that alpha factors are depressed at this plant.
132
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TABLE B-25. RYEMEADS INFLUENT-EFFLUENT DATA SUMMARY
Year
Parameter
Flow:
mgd
m^/day
1978
10.45
39,600
1977
11.9
44,880
1974
10.1
38,200
Raw Wastewater: (mg/1)
Primary
BOD5
COD
TSS
NH3-N
Effluent: (mg/1)
BOD5
COD
TSS
TKN
NH3-N
310
698
490
32
144
299
117
44
33
341
749
483
33
152
308
109
51
32
403
--
430
35
178
—
105
48
36
Final Effluent: (mg/1)
BOD5
COD
TSS
NHs-N
N03-N
5
42
10
0.2
22
6
45
13
0.3
21
5
--
12
0.2
28
Mixing power levels have been calculated for the various pass configura-
tions at Ryemeads and are shown in Table B-27. These data are calculated for
minimum air flows and indicate that mixing power levels are well into the
middle range of the plants surveyed. Maximum air flow rates range up to 100
percent higher than minimums. The nitrification section is mixed at a power
level similar to the balance of the first aeration pass. Plant personnel re-
ported no settling of solids in any of the aeration tanks.
Operation and Maintenance
Ryemeads has operated dome diffuser systems continuously since 1956 when
Stage 1 was constructed. Stage-! and-2 diffusers have required cleaning every
5-6 yr. Problems have been experienced with rust fouling of the diffusers in
Stage 1, owing to the use of cast iron air distribution mains in the original
plant. These mains are now being converted to uPVC pipe.
Considerable coarse bubbling was observed in the first pass of the aera-
tion tanks in Stage 3 at the time of the plant visit. Plant personnel were
uncertain as the causes of this, but it appeared to be a manifestation of the
slime growth problem experienced at other plug flow plants.
133
-------
TABLE B-26. RYEMEADS AERATION EFFICIENCY CALCULATIONS*
Year
Parameter
BOD5 Removed ( kg/day )+
NH3-N Removed ( kg/day )+
N03-N Produced (kg/day)+
62 Requi red °( kg/day )+
Power Consumed (kWh/day)**
1978
5483
1258
870
10,010
9640
1977
6548
1423
942
11,594
10,732
1974
4923
1328
1068
9892
8640
Aeration Efficiency:
kg 02/kWh 1.04 1.08 1.14
Ib 02/wire hp-hr 1.71 1.78 1.87
* For Stage 3 only.
+ 1 kg = 2.21 Ib
0 Calculated by the method of Section 5, assuming 1.22 units 02 required/
unit 6005 removed.
** 1 kWh/day =1.34 wire hp-hr/day
Plant maintenance workers estimated a cost of & 1.50 ($3.00) each to re-
move, fire in a kiln, and replace domes in the tanks at Ryemeads, including
5-10 percent handling loss. They noted that the domes did not return to full
oxygen transfer efficiency after the cleaning operation, but apparently lost
10-25 percent efficiency. Ryemeads domes are refired in the kiln at Beckton.
The plant spends approximately ,£ 12,000/yr ($24,000/yr) for diffuser mainten-
ance and cleans diffusers about once every 6 yr. A total of £ 35,000/yr
($70,000/yr) is being spent on the diffuser system, including retrofitting of
cast iron air supply piping when a tank is removed from service for cleaning.
The electrostatic air filters have performed satisfactorily, requiring
hot water washing every 3-4 mo. Approximately once every 2 yr, each air
cleaner is dismantled and washed in a mild phosphoric acid solution. The job
requires two men working 8 days to complete.
COALPORT
History and Background
The Coalport treatment plant was completed in 1970, serving a combination
of existing villages and New Town areas. The plant was constructed on a for-
mer industrial site and is designed for expansion in step with New Town deve-
lopment. The construction cost in 1970 was £l.3 million ($2.6 million) for
the facilities shown in Figure B-9.
134
-------
TABLE B-27. RYEMEADS MIXING DATA
We tted
pass Surface Area Volume
(in2)* (m3)*
CO
01
First Pass:
First Sectionoo
Second Section
Second Pass:
First Section
Second Section
Third Pass
Fourth Pass
122
177
150
150
299
299
372
538
455
455
910
910
(domes/in*)0 (cfm)
0
4.6
4.6
4.6
3.7
2.3
0
403
335
300
547
350
(in3/min)
0
11.4
9.5
8.5
15.5
9.9
Air
/•~cf..7~t
\iodo~Ttzy
0
213
210
187
171
108
Flow/Area**
\iOOO ni^-min/
0
65
64
57
52
33
Air Flow/Volume** Minimum Power Input
(cfru/1000 ft3)++ (wire hp/1000 ft3) (W/m3)
0
21.3
20.9
18.6
17.1
10.9
0
1.12
1.12
0.99
0.90
0.52
0
29.5
29.5
26.1
23.7
13.8
* 1 in2 = 10.8 ft2
t- 1 ni3 - 35.3 ft3
o 1 dome/m2 = 9.29 domes/100 m2
** Cased on a minimum air flow rate of 0.014 m3/iiiin/clorne (0.5 cfm/dome) and current tank configurations in Stdye 3 only.
++ Same as ni3/1000 m3/min.
oo With three mixers operating at 2.2 kW (3 hp) each.
-------
n TROWK pOOL SEWERS
PIPE/ &eioeE
R.1VER, 5EVER.N
Q 50BV\EBSEO OOTFALL
INLET
BROSELEl'
TieUNIC SEWER.
CO
01
_i COMAAHOUTORS
ooo
j,
VELOCITY
PARA50LIC SE'T
t*ODER)
( FLOVM RECORDERS
SEDIMENTATION
TANK*
&DILDIN6
MAIN PUMPHOUSE
LABOBATORTV
CONTBOL ROOM
TANKS
COMPRESSOR
PUMP HOUftE
SLUDGE
TMICKE.NING
TANK-,
SCRE^"^
MOUSE, su6 STATION
'5U3DSE SCREEN
BOUC LIME EXPERIMENTAL
LIME. TANKS TANKS
SILO
AIR
WORKSHOP
Figure B-9. Coal port site plan.
-------
Plant Description
The Stage I plant at Coalport includes two rectangular primary sedimenta-
tion tanks, one stormwater overflow tank, four two-pass plug flow aeration
tanks, and six circular final clarifiers. Designed for a DWF of 17,700 m3/day
(4.68 mgd), it is currently receiving about 67 percent of that flow. The pri-
mary tanks measure 53.2 m x 18.8 m x 2.8 m SWD (175 ft x 62 ft x 9 ft). The
surface loading at DWF is 8.6 m3/m2/day (212 gpd/ft2) with a current average
loading of about 5.8 m3/m2/day (141 gpd/ft2).
The aeration tanks each measure 65 m x 4.6 m x 3.4 m SWD (213 ft x 15 ft
x 11 ft) and have 830 domes, symmetrically distributed. Air is distributed
60 percent to the first pass and 40 percent to the second pass. Twenty per-
cent of the wastewater flow is fed at the head of the first pass, along with
the return sludge. The balance is fed 21.7 m (71 ft) from the head of the
first pass. The final clarifiers are each 14.9 m (49 ft) in diameter with a
sidewater depth of 2.7 m (8.9 ft). The floor slope is 30°. The detention
time and surface loading at DWF are 6 hr and 16.9 m3/m2/day (414 gpd/ft2), re-
spectively.
Air is supplied by four Waller two-speed centrifugal blowers after fil-
tration through two bag type air filters, each of which is sized to handle the
full air flow. The bag filters are precoated with an asbestos coating and re-
quire cleaning every 2 yr.
Performance
Staff of Severn Trent Water Authority at Coalport provided plant perfor-
mance data for 1978/79, as shown below:
TABLE B-28. COALPORT INFLUENT-EFFLUENT DATA SUMMARY
Average Flow: mgd 3.24
m3/day 12,300
BOD5: (kg/day)*
Load imposed 1927
Load removed 1820
NH3-N: (kg/day)*
Load imposed 354
Load removed 341
TSS: (kg/day)*
Load imposed 1198
Load removed 1125
* 1 kg/day =2.21 Ib/day
137
-------
Influent flow averaged 12,300 m3/day (3.24 mgd). BOD5 to the aeration basin
averaged 157 mg/1 and influent Nh^-N averaged 29 mg/1. The F/M loading aver-
aged 0.14 kg BODs/day/kg MLSS and ranged from 0.11-0.20. MLSS averaged 2500
mg/1. Monthly loadings varied about +_ 25 percent from annual averages. Flow
to this plant is mostly domestic with some light industrial contributions.
BODs and suspended solids were removed on an average of 94 percent each
through the combined primary and secondary processes.
Based on an average aeration power consumption of 3047 kWh/day (4086
wire hp-hr/day), aeration efficiency averaged 1.08 kg 02/kWh (1.78 Ib/wire hp-
hr), with 1.13 units oxygen required/unit 6005 removed. This is on the low
side of the plants surveyed but can probably be accounted for by high mixed
liquor D.O. The plant operators noted that, even with the uneven air distri-
bution, aeration D.O. was normally quite high in the second pass of the two-
pass system. Previously, the plant had been operated at even lower flows
with all four aeration basins in operation. A change in management resulted
in cutting out one basin to improve aeration efficiency.
Based on an average air flow rate of 158,640 m3/day (5600 x 103 ft3/day),
air flow per dome is 0.9 cfm (0.025 m3/min) in the first pass and 0.6 cfm
(0.017 m3/min) in the second. Plant operators limit minimum air flows to
0.5 cfm (0.014 m3/min) in the second pass. Based on this minimum air flow
rate, mixing data for the Coalport plant are given in Table B-29. Coalport's
power input at minimum air flow is typical among the surveyed plants.
TABLE B-29. COALPORT MIXING DATA
Parameter First Pass Second Pass
Air Flow/Basin:*
cfm 777 519
m3/min 22.0 14.7
Air Flow/Area:*
cfm/ft2 0.23 0.16
m3/min/m2 0.07 0.05
Air Flow/Volume* (cfm/1000 ft3)+ 13.2 8.8
Minimum Power Input:
wire hp/1000 ft3 25 16.7
W/m3 0.95 0.63
* Based on a minimum air flow rate of 0.014 m3/min/dome (0.5 cfm/dome).
+ Same as m3/1000 m3/min.
Operation and Maintenance
Coalport has accrued 9 yr of continuous operating experience with very
few problems. Aeration tanks are routinely cleaned once every year. Inspec-
tion of the empty tanks revealed no evidence of scale build-up or solids de-
138
-------
position. Some coarse bubbling was noticeable at the inlet and one-third
point feed zone, being most severe in the latter. This can most likely be
attributed to the growth of slime on the domes in the feed zones. Air filters
have performed satisfactorily, with no non-routine maintenance required.
COLESHILL
History and Background
The Coleshill treatment plant, shown in Figure B-10, was constructed in t
three stages. The first stage was commissioned between 1931 and 1934 and was
one of the first activated sludge plants in the world. The second stage, a
nearly identical duplicate of Stage 1, was constructed between 1939 and 1942,
along with additional final clarifiers for the first stage. The capacity of
the two stages then was 11,400 m3/day (3 mgd).
The original concept was to develop this plant as five identical stages.
Rapid increases in flow due to population growth and industrial expansion
forced reconsideration of this approach, and the final stage was designed as
a 54,500 m3/day (14.3 mgd) dome diffuser activated sludge plant. The first
two stages were then removed from service to be renovated by 1971 and reopen-
ed when flows increased further.
The third stage was constructed and opened by sections in the period
1968-1971. Excess sludge from the new plant was initially handled in exist-
ing facilities. In 1976, construction of a sludge incineration plant serving
both Coleshill and Minworth was started on an adjacent site.
Plant Description
Described herein and detailed in Table B-30 are the basic design parame-
ters for the third stage plant at Coleshill.
Primary settling is provided by eight rectangular clarifiers providing
8 hr of detention time at the DWF of 54,100 m3/day (14.3 mgd). Primary set-
tling is sized to accept flows up to 3 x DWF, with excess flows going to the
stormwater overflow tanks.
The eight aeration tanks are coupled individually to the eight final
clarifiers. Return sludge from each clarifier is individually pumped to its
associated aeration basin, with provision for cross connection should either
an aeration basin or a clarifier be taken out of service. Detention times at
DWF in the aeration basins and final clarifiers are 12 hr and 6 hr, respec-
tively. The maximum surface loading (3 x DWF) is 38.9 m3/m2/day (954 gpd/ft2).
Initially, the aeration tanks were equipped with 3100 domes each. In an
effort to improve plant performance by introducing intentional denitrification,
domes have been removed from five of the tanks, Nos. 1, 2, and 6-8, to create
anoxic zones at the influent ends, similar to those at Minworth. Tank Nos. 1
and 2 now have 2186 domes each; Tank Nos. 6-8 have 2153 domes each. The re-
maining tanks have 3100 domes each and were not in service at the time of the
plant visit. These tanks remain in the initial configuration whereby 50 per-
139
-------
its
'a.
-------
TABLE B-30. COLESHILL DESIGN DATA*+
Stormwater Tanks
3 Rectangular Tanks
Total volume:
20 m° x 105 m° x 3.2 m° SWD
192,650 m3**
Primary Sedimentation
8 Rectangular Clarifiers
Detention time:
Surface loading:
10.7 m x 61 m x 3.5 m SWD
8 hr @ DWF
10.3 m3/m2/day (253 gpd/ft2)
Aeration Basins
8 Rectangular Tanks
Detention time:
Diffuser Configuration:
Aeration Tank
& Section Nos.
Tank Nos. 1,2,6-8:
Section 1
Section 2
Section 3
Tank Nos. 3-5:
Section 1
Section 2
Tank Section
Length (m)
16.5
4.9
42.7
21.3
42.7
18.3 m x 64 m x 2.9 m SWD
12 hr @ DWF
No. of
Domes
206
394
1586
1538
1562
Diffuser Density
(domes/m2)"1"1"
0.7
4.4
2.0
3.9
2.0
Final Sedimentation
8 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
26 m
2.4 m
20°
6 hr @ DWF
13.0 m3/m2/day
DWF (318 gpd/ft2)
* For Stage 3 only.
+ DWF = 54,100 m3/day (14.3 mgd)
1 m = 3.28 ft
** 1 m3 = 35.3 ft3
++ 1 dome/m2 =9.29 domes/100 ft2
141
-------
cent of the domes are concentrated in the first one-third of the tank, with
the remaining 50 percent distributed equally across the last two-thirds of the
tank floor.
Air supply is provided by 16 56-kW (75-hp) Rootes type blowers, each ra-
ted at 45.3 m3/min (1600 cfm). Two blowers are matched to each aeration tank,
with provision for cross connection when units are out of service. Two bag
filters, located at either end of a common feed duct for the blowers, provide
filtered air.
Performance
In an effort to improve plant performance and reduce secondary settling
problems, this plant has been operated experimentally in several modes over
the last few years. A problem with denitrification occurs in the secondary
clarifiers. This is caused by high concentrations of ammonia in the plant in-
fluent and production of a nitrified effluent, resulting in frequent episodes
of poor effluent quality, particularly during the summer months. To improve
energy efficiency, as well as reduce denitrification problems, Tank Nos. 1, 2,
and 6-8 were modified by removing domes from the first 15+ m (50+ ft) (Table
B-30), creating anoxic zones where denitrification could occur. Tank Nos. 3-5
were placed on a standby and the loading increased to the other tanks to re-
duce the detention time and increase the F/M loading from 0.05 to 0.1 kg BOD5/
day/kg MLSS. MLSS were maintained in the range of 3000-3500 mg/1.
Performance data were provided by plant personnel for the period Septem-
ber 1, 1978 to August 30, 1979, as follows:
Average Flow 51,200 m3/day (13.5 mgd)
BOD5 Removed 146 mg/1
NH3-N Removed 30 mg/1
Denitrification 1000 kg/day (2205 Ib/day) NOs-N removed
Air Flow/Dome 0.025 m3/min (0.88 cfm)
Power Consumed 6130 kWh/day (8220 wire hp-hr/day)
Based on the above figures, aeration efficiency and mixing parameters have
been computed as shown in Table B-31. In the present configuration, the plant
is one of the more energy efficient visited and is significantly more energy
efficient than the Minworth plant, even though both plants have similar con-
figurations and similar volumetric loadings. Plant personnel indicated a be-
lief that aeration efficiency was increased substantially when flow was con-
centrated in five of the eight tanks. Mixed liquor D.O. ranges from 0.0 mg/1
in the anoxic zones to 2-4 mg/1 at the effluent end of the aeration tanks.
Operation and Maintenance
Plant personnel reported no major problems with the dome diffuser equip-
ment or air filters. .Similar O&M procedures to those at Minworth are followed.
Tanks are cleaned and domes are checked annually. Air filters require clean-
ing once per year as well. Coleshill reported no significant settling of sus-
pended solids in the aeration basin, even in the lightly mixed anoxic zones.
Coarse bubbling of the air from partially blocked domes was most noticeable in
142
-------
TABLE B-31. COLESHILL AERATION EFFICIENCY CALCULATIONS AND MIXING DATA
AERATION EFFICIENCY CALCULATIONS
BOD5 Removed (kg/day)*
NH3-N Removed (kg/day)*
N03-N Produced (kg/day)*
03 Required"1" (kg/day)*
Power Consumed (kWh/day)0
Aeration Efficiency:
kg 02/kWh
Ib 02/wire hp-hr
MIXING DATA
Air Flow (Tanks 6-8)**
Anoxic Zone
First Aeration Zone
Second Aeration Zone
Air Flow/Area**
Anoxic Zone
First Aeration Zone
Second Aeration Zone
Air Flow/Volume**
Anoxic Zone
First Aeration Zone
Second Aeration Zone
Average Power Input
Anoxic Zone
First Aeration Zone
Second Aeration Zone
cfm
191
307
1391
cfm/ft2
0.056
0.37
0.17
7480
1537
1000
13,005
6130
2.12
3.49
m
m3/min
5.4
8.7
39.4
3/m2/mir
0.017
0.113
0.052
cfm/1000 ft3++
6.0
38.6
17.5
wire hp/1000 ft3
0.22
1.40
0.63
* 1 kg = 2.21 Ib
+ Calculated by the method of Section 5, assuming 1.20 units 02 required/
unit BOD5 removed.
0 1 kWh/day =1.34 wire hp-hr/day
** Based on an average air flow rate of 0.025 m3/min/dome (0.88 cfm/dome).
++ Same as m3/1000 m3/min.
143
-------
the anoxic zones and the immediately downstream aeration zones.
FINHAM
History and Background
The Finham Plant, shown in Figure B-ll, was constructed over a period of
years, beginning in 1932 with the startup of a 13,600-m3/day (3.6-mgd) trick-
ling filter plant. Numerous additions were made over the years, including ac-
tivated sludge pretreatment systems for both the North and South works. The
South activated sludge plant began operation in 1955 with fine bubble aeration
using 10-cm (4-in.) ceramic domes. No air filtration was provided, and con-
siderable problems with diffuser clogging were encountered. In 1963, the ae-
ration system was converted to coarse bubble diffusers. The coarse bubble
system performed satisfactorily but was replaced in 1974 by a dome diffuser
fine bubble aeration system to improve oxygen transfer and remedy mechanical
failures. Air filtration, employing two-element air filters, was added to the
blower facility.
The North activated sludge plant was added in 1963 as a pretreatment
stage to the North works trickling filters. It was initially equipped with
coarse bubble aeration, which has remained in service to the present. Plans
are being made to retrofit this plant with fine bubble equipment in the future.
Plant Description
Influent wastewater enters at opposite ends of the site, where it is
screened and degritted. Stormwater overflow tanks are located at each inlet
works as well. Both lines feed to a common splitter box for diversion to ei-
ther the North or South works. Each plant has three primary clarifiers, each
35 m (115 ft) in diameter and with a sidewater depth of 3.7 m (12 ft). All of
the flow receives primary treatment; part of the flow is then diverted to ac-
tivated sludge treatment to reduce organic loading on the trickling filter
beds.
Table B-32 summarizes design details for the North and South activated
sludge plants. The South works has six aeration basins, each with 800 dome
diffusers distributed evenly on four longitudinal rows. Domes are spaced 0.30
m (1.0 ft) on centers with 4.9 m (1-6 ft) between rows. The normal operating
configuration is one tank reaerating return sludge, four tanks aerating mixed
liquor, and one tank on standby. The detention time at DWF is 3.5 hr. Four
Rootes type air compressors suoolv air to the South plant. The blowers each
deliver 46.7 m^/min (1650 cfm) at 41.4 kN/m^ (6.0 psi). Air cleaning is pro-
vided by two-element Vokes air filters mounted on a common intake manifold
for each pair of blowers. Four prefilters and four high efficiency filters
are provided for each paired intake manifold.
Three final clarifiers serve the South works, averaging 3.5-hr detention
time at DWF. The clarifiers are equipped with steeply sloped (30°) floors and
simple peripheral drive scrapers.
The North plant has four aeration basins, providing 1-hr detention time
144
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BO O 100 100 300 400 500
O1
Figure B-ll. Finham site plan.
Inlet Screens
Detritors
Storm Overflow
4 Storm Water Station
5 Sedimentation Tank
6 Activated Sludge Channels
7 Activated Sludge Settlement
Tank
8 Bacteria Beds A.D F.
9 Bacteria Beds A.S.
10 Primary Humus Tanks
11 Secondary Humus Tanks
12 Filtration Pumping Station
13 Sand Filters
14 River Outfall
15 Washwater Settlement Tank
16 Low Level Transfer Pumping
Station
17 High Level Pumping Station
18 Thickeners for Secondary Sludge
19 Sedimentation Tanks Pumping
Station
20 Sludge Thickening & Dewatering
Tanks
21 Sludge Digesters
22 .Sludge Pumping Station
23 Power House
24 Administration Block
25 Experimental Plant
26 Workshops, Stores & Welfare
-------
at DWF. Air flow to the coarse bubble diffusers is tapered along the length
of the tanks. Three of the tanks aerate mixed liquor; one is used for sludge
reaeration.
Seven Rootes-type blowers provide air to the North works. Two units are
rated at 28.3 m3/min (1000 cfm); five are rated at 36.8 m3/min (1300 cfm).
Pressure at rated air flow for all seven blowers is 41.4 kN/nr (6.0 psi). No
air cleaning is provided for this coarse bubble aeration system.
Four final clarifiers serve the North activated sludge plant with a de-
tention time at DWF of 3.5 hr.
TABLE B-32. FINHAM DESIGN DATA
South Plant* North Plant*
Aeration Basins
Number 6 4
Dimensions:
Length (m)° 61.0 49.4
Width (m)° 3.0 3.0
Sidewater depth (m)° 3.6 3.8
Volume** (m3)++ 656 563
Detention Time00 (hr) 3.5 1.0
Final Clarifiers
Number
Dimensions:
Diameter (m)
Sidewater depth (m)
Detention Time00 (hr)
Surface Loading00 m3/rrr/day
(qpd/ft2)
3
18.5
3.0
3.5
28.8
(707)
3
23
3
3.5
33.1
(813)
* DWF = 22,700 m3/day (6.0 mgd)
+ DWF = 54,500 m3/day (14.4 mgd)
0 1 m = 3.28 ft
** Each tank.
++ 1 m3 = 35.3 ft3
00 At DWF.
Performance
Data for the year beginning April 1, 1978, and ending March 31, 1979, was
provided for both sections of this plant, as shown in Table B-33. Because the
activated sludge plants are used for pretreatment at Finham, F/M loading rates
are high; 0.45 kg BOD5/day/kg MLSS in the South works and 1.80 kg BOD5/day/kg
MLSS in the North plant. Finham South averages 50 percent removal of BODs in
the primary and 72 percent removal in secondary for an overall removal averag-
146
-------
ing 90 percent. Finham North removes 42 percent of the 8005 in the primary
section and 65 percent in the secondary for an overall removal of approximate-
ly 80 percent. Suspended solids removal is similar in the two plants, and the
North plant removes about 18 percent of the influent ammonia nitrogen.
TABLE B-33. FINHAM INFLUENT-EFFLUENT DATA SUMMARY*
Parameter
Finham
South
Finham
North
Flow:
mqd
rrp/day
Raw Wastewater: (mg/1)
BOD5
COD
TSS
NH3-N
Detergent+
Primary Effluent: (mg/1)
BOD5
COD
TSS
NHo-N
Detergent"1"
Final Effluent: (mg/1)
BOD5
COD
TSS
NH3-N
Detergent"1"
MLSS (mg/1)
7.5
28,300
321
812
475
29
10
162
348
128
28
10
32
118
40
26
5
2500
11.4
43,000
258
632
315
28
11
150
352
133
28
11
53
147
50
23
8
2000
* For the data year beginning April 1, 1978, and ending March 31, 1979.
+ Detergent as manoxyl OT (a proprietary ABS type compound).
Calculated aeration efficiencies (Table B-34) seem to indicate that the
fine bubble diffuser system (South plant) is performing only marginally better
than the coarse bubble diffuser system (North plant). However, lacking mixed
liquor D.O. data, and in view of the rough estimates used for power, it is
possible that figures for both plants are seriously in error. Also, the me-
thod of analysis, derived for more lightly loaded plants, may seriously over-
state oxygen requirements for the heavily loaded North plant.
At an average total system air flow rate of 140 m3/min (4960 cfm), the
average air flow rate per dome in the South plant approximates 0.035 m3/min/
dome (1.24 cfm/dome). This is well within the range required for efficient
147
-------
performance. Mixing power input is moderately high at 36 W/m3 (1.37 wire hp/
1000 ft3). Diffuser density is more typical at 4.3 domes/m2 (39.9 domes/100
ft2). The air flow per unit volume is 42.7 m3/lOOO m3/min in the South plant.
By comparison, mixing power input in the North plant is 51.5 W/m3 (1.96 wire
hp/1000 ft3) and the air flow per unit volume is 50.4 m3/1000 rrr/min.
TABLE B-34. FINHAM AERATION EFFICIENCY CALCULATIONS
Finham Finham
Pa r ame ter South North
BOD5 Removed (kg/day)* 4680 4171
NH3-N Removed (kg/day)* 56.6 215
02 Removed+ (kg/day)* 4920 4180
Power Consumed0 (kWh/day)** 2800 2784
Aeration Efficiency:
kg Oo/kWh 1.76 1.50
Ib 02/wire hp-hr 2.89 2-47
* 1 kg = 2.21 Ib
+ Calculated by the method of Section 5, assuming 0.88 units Oo required/
unit BODr removed for Finham South and 0.78 units 02 required/unit BOD5
removed for Finham North.
0 Estimated data.
** 1 kWh/day = 1.34 wire hp-hr/day.
Operation and Maintenance
The Finham plant staff reported very few maintenance problems with the
dome diffuser plant. Since 1974, it has only been necessary to repair several
small air distribution line leaks and to change the air filter elements every
18 wk at a total cost of less than £ 1500 ($3,000/yr). An inspection of the
air filters indicated that they are trapping exhaust mists from the dual fuel
engines that are located in the blower room along with the air intakes. It is
likely that providing an outside intake might significantly reduce fouling of
the air filters, lowering the annual cost of changing filter elements.
HARTSHILL
History and Background
The Borough of Nuneaton established treatment works in 1871, opening a
primary sedimentation plant, assisted by chemical coagulation, on the site of
the present day St. Mary's Road pumping station. In 1901, steam driven pumps
were installed at St. Mary's Road to pump wastewater 4 km (2 1/2 mi) to the
Hartshill site. There, the wastewater was given primary treatment followed by
148
-------
trickling filtration. Subsequent expansions were carried out at Hartshill in
1920, 1939, and 1973, when the fine bubble dome diffuser activated sludge
plant was added. A site plan for the current works at Hartshill is presented
in Figure B-12.
Plant Description
Screening and grit removal are carried out at the St. Mary's Road pumping
station, which is also provided with three stormwater overflow basins, total-
ling 7592 m3 (268,100 ft3) in volume.
The design DWF is 24,500 m3/day (6.46 mgd) based on a population of
80,500. The treatment works are designed to treat 3 x DWF, and extensions
can be made to treat 104,500 m3/day (27.6 mgd) from a population of 100,000,
the works pipelines, channels, etc., having been sized for this flow. The
preliminary treatment works at St. Mary's Road is designed to receive a maxi-
mum flow of 227,000 m3/day (60 mgd) and to pump to Hartshill Sewage Treatment
Works a maximum of 78,700 m3/day (20.8 mgd), the remaining storm sewage being
given partial treatment before discharge to the River Anker. The partial
treatment is designed to produce an effluent with a suspended solids concen-
tration not exceeding 150 mg/1.
Table B-35 provides design details for the settling basins, aeration pro-
cess, and blower system at Hartshill. The plant is comprised of three primary
clarifiers, six aeration basins, two sludge reaeration basins, and eight final
clarifiers. Each aeration basin has 740 domes in the inlet half and 520 domes
in the outlet half. The reaeration basins have about 400 domes apiece.
Aeration basin dome configuration consists of 16 longitudinal rows equal-
ly spaced on the tank floor, with 30.5 cm (1 ft) between domes in the first
half of the tank and 42.7 cm (1.4 ft) in the second half.
Air supply is provided by three positive displacement (Rootes-type) air
blowers, two variable and one fixed speed. Air is drawn from a common air
duct supplied by three electrostatic air cleaners, any two of which can supply
the maximum air flow demand.
The final clarifiers have steeply sloped floors (30°) and shallow side-
water depths, 1.37 m (4.5 ft),characteristic of many dome diffuser activated
sludge plants in the United Kingdom.
Performance
Meaningful long-term performance data were not available for this plant
due to the effects of variable industrial loading, principally rendering
wastes. However, performance can be estimated from current monthly average
loading conditions, as shown below:
Flow 21,700 m3/day (5.73 mgd)
BOD5 Removed 330 mg/1
149
-------
v^ — iff^
15 .' " r- -j
jl
~t~n
'/ V
1 1'
KEY
i, WORKS INLET
2 PRIMARY SETTLEMENT TANKS
3. ACTIVATED SLUDGE PLANT
4. FINAL SETTLEMENT TANKS
5. EFFLUENT PUMPING STATION
6 SLUDGE PUMPING STATION
7. PRIMARY SLUDGE DIGESTION TANKS
8. CONSOLIDATION TANKS
9. SECONDARY DIGESTION TANKS
IO. GAS HOLDER
II. COMPRESSOR HOUSE AND
WORKS DRAINAGE PUMPING
STATION
12. ELUTRIATION TANKS
13. SLUDGE PRESS HOUSE
14. ADMINISTRATION BUILDING
15. SLUDGE STORAGE LAGOON
Figure B-12. Hartshill site plan.
150
-------
TABLE B-35. HARTSHILL DESIGN DATA*
Stormwater Tanks
3 Circular Tanks
Diameter:
Sidewater depth:
Floor slope:
Total volume:
Detention time:
Primary Sedimentation
3 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
Aeration Basins
6 Aeration Tanks
2 Reaeration Tanks
Overall detention time:
Diffuser Configuration:
Aeration Tanks:
First 1/2
Second 1/2
Reaeration Tanks:
Final Sedimentation
8 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
25.3 m+
4.4 m+
6°
7952 m3°
7,5 hr @ DWF
27.4 m
3.0 m
7.5°
6^3 hr @ DWF
14.2 m3/m2/day (348 gpd/ft2)
9.2 m x 27.4 m x 3.2 m SWD
3.0 m x 27.4 m x 3.2 m SWD
5.5 hr @ DWF
No. of
Domes
740
520
400
Diffuser Density
(domes/m2)**
5.9
4.1
4.9
17.4 m
1.4 m
30°
59 hr @ DWF
13.0 m3/m2/day (318 gpd/ft2)
* DWF = 24,500 m3/day (6.46 mgd)
+ 1 m = 3.28 ft
0 1 m3 = 35.3 ft3
** 1 dome/m2 = 9.29 domes/100 ft2
151
-------
NH3-N Removed 40 mg/1
MLSS 4000 mg/1
Air Flow 211 m3/min (7440 cfm)
Power Consumed 9,600 kWh/day (12,874 wire hp-hr/day)
Aeration efficiency calculations and mixing data based on these figures are
presented in Table B-36. The influent wastewater strength at Hartshill is
quite high, ranging from 500-700 mg/1 8005 due to the varying contribution of
the rendering waste, which is often shock loaded into the system. Nitrifica-
tion, as a result, tends to be variable. Because of a high loading rate of
ammonia nitrogen, 50-70 mg/1, the plant experiences periodic episodes of deni-
trification with resultant sludge blanket flotation in the final clarifiers.
The apparent aeration efficiency at this plant falls in the lower range
among the plants visited at 1.11 kg 0?/kWh (1.82 Ib/wire hp-hr). Possible
causes for the low efficiency may include the variable strength of the waste-
water, which could depress the alpha factor, and the relative shallowness of
the tanks.
The relatively low oxygen transfer efficiency at this plant necessitates
relatively high air flow requirements, placing mixing power levels in the up-
per range of the plants surveyed. Observation of the tanks, verified by plant
personnel, indicated that the aeration basins were well mixed. D.O. ranged
from 20-80 percent of saturation, tending toward the higher level at the aera-
tion tank exits. Plant operators try to keep D.O. levels fairly high as an aid
in suppressing denitrification and absorbing shock loads from the rendering
plant.
Operation and Maintenance
The dome diff users at Hartshill have been in service since 1973 with no
instances of failure or cleaning. Observation of the tanks, however, revealed
substantial coarse bubbling in the front third of the aeration tanks, probably
caused by the high volumetric loading of this area of the plug flow tanks and
concomitant biological fouling of the dome exteriors (see Section 6). Blower
pressure has remained steady over the period at about 37.9 kN/m^ (5.5 psi).
The electrostatic air filters have performed well, requiring light wash-
ing every several weeks and a more thorough caustic wash annually. At start-
up, problems with deterioration of the air filter grids caused by vibration
from the air compressors were experienced. Pulsation of the air compressors
set up a pulsing action in the air flow that damaged the air filter internals.
To solve the problem, individual air dampeners were added at the inlet of each
compressor.
MINWORTH
History and Background
152
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TABLE B-36. HARTSHILL AERATION EFFICIENCY CALCULATIONS AND MIXING DATA
AERATION EFFICIENCY CALCULATIONS
Removed (kg/day)*
NH3-N Removed (kg/day)*
02 Required+ (kg/day)*
Power Consumed (kWh/day)°
Aeration Efficiency:
kg Op/kWh
Ib 02/wire rip-hr
MIXING DATA
Air Flow**
Aeration Tanks:
First 1/2
Second 1/2
Reaeration Tanks
Air Flow/Area**
Aeration Tanks:
First 1/2
Second 1/2
Reaeration Tanks
Air Flow/Volume **
Aeration Tanks:
First 1/2
Second 1/2
Reaeration Tanks
Average Power Input
Aeration Tanks:
First 1/2
Second 1/2
Reaeration Tanks
cfm
657
583
353
cfm/ft2
0.53
0.46
0.39
7164
868
10,682
9600
1.11
1.82
m
46.4
41.1
38.6
wire hp/1000 ft3
3.35
2.36
2.73
18.6
16.5
10.0
0.16
0.14
0.12
cfm/1000 ft3++
88.2
62.0
71.8
* 1 kg = 2.21 Ib
+ Calculated by the method of Section 5, assuming 0.97 units 02 required/
unit BOD,, removed.
0 1 kWh/daj? = 1.34 wire hp-hr/day
** Based on an average air flow rate of 0.025 m3/min/dome (0.88 cfm/dome).
++ Same as m3/1000 rrr/min.
153
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Wastewater treatment at Minworth began in the late 1800's with the utili-
zation of the site for land application. During the period from 1905-1921,
six large rectangular trickling filters were constructed on the site, providing
a total bed area of 17 ha (42 ac).
By the end of World War II, the treatment system, comprising Minworth
and four other plants, was overloaded and it was decided to concentrate ser-
vice at Minworth, modernizing that plant and gradually shutting down the other
plants. By 1952, 3.2 ha (8.0 ac) of the trickling filters had been upgraded
along with the provision of improved sludge handling facilities. The period
1960-1970 saw extensive construction to improve interceptor sewers and major
rehabilitation and expansion of the Minworth plant. In 1971, the activated
sludge plant was completed and the plant was substantially completed as shown
in Figure B-13.
Plant Description
Primary settling for the entire plant is provided by 10 rectangular tanks,
each 76.2 m x 48.8 m x 3.0 m SWD (250 ft x 160 ft x 10 ft) with 7.5 hr deten-
tion time at the current average flow of 272,500 m3/day (72 mgd). Surface
loading at DWF is 7.2 m3/m2/day (177 gpd/ft2). These are supplemented by six
stormwater tanks with a total volume of 69,000 m3 (2.44 million ft3).
Twelve aeration tanks and 24 final settling tanks comprise the activated
sludge plant at Minworth. Design parameters are shown in Table B-37.
TABLE B-37. MINWORTH DESIGN DATA
Aeration Basins
12 Rectangular Tanks
(3 groups of 4 each)
Detention time:
18.3 m* x 178 m* x 3.0 m* SWD
14.5 hr @ DWF with 1 tank out of service
Diffuser Configuration:
Section Length (m) No. of Domes Diffuser Density (domes/m2)+
First 1/6 29.7
Second 1/6 29.7
Final 2/3 118.6
Final Sedimentation
24 Circular Clarifiers
Diameter:
Sidewater depth:
Floor slope:
Detention time:
Surface loading:
756
3375
6750
1.4
6.2
3.1
27.4 m
2.1 m
19.5°
6 hr at DWF
10.8 m3/m2/day (265 gpd/ft2)
* 1 m = 3.28 ft
+ 1 dome/m2 = 9.29 domes/100 ft2
154
-------
en
A -
B -
C -
D -
E -
F -
G -
H -
R -
Saltley to Minworth Mam
Sewers
Screenhouse
Grit Channels
Measuring Flumes
Sedimentation Tanks
Stormwater Tanks
Main Control Chamber
Secondary Control
Chamber
Bacteria Beds
Humus Tanks
Mam Effluent Conduit
Activated Sludge Plant
Activated Sludge Plant
Outfall
Primary Sludge Digestion
Plant
Secondary Sludge Diges-
tion Plant
Sludge Drying Beds
Generating Station
Maintenance Workshops
Laboratory
Figure B-13. Minworth site plan.
-------
Diffuser domes are spaced on 0.61-m (2-ft) centers in the first one-third of
each aeration tank and 1.22-m (4-ft) centers in the final two-thirds of each
tank. To promote denitrification and reduce aeration power, most of the first
grid of 2600 domes in each tank have been turned off, essentially halving the
aeration zone in the first one-sixth of each tank.
Air is supplied by eight centrifugal blowers, as follows:
2 - 336 kW (450 hp), 326 m3/min (11,500 cfm)
2 - 410 kW (550 hp), 411 m3/min (14,500 cfm)
4 - 485 kW (650 hp), 496 nr/min (17,500 cfm)
Air filtration is provided by bag filters.
Performance
Performance data for the secondary system were provided by Minworth per-
sonnel for the year ending December 1978 and are summarized below:
TABLE B-38. MINHORTH INFLUENT-EFFLUENT DATA SUMMARY
Average Flow: 274,000 m3/day (72.4 mgd)
BOD5: (mg/1)
Influent 142
Effluent 6
COD: (mg/1)
Influent 353
Effluent 13
TSS: (mg/1)
Influent 144
Effluent 13
NHs-N: (mg/1)
Influent 21
Effluent 2.6
N03-N: (mg/1)
Influent 1.9
Effluent 16
MLSS (mg/1) 3400 (avg.)
Secondary removals in 1978 averaged 96 percent for BODc, 91 percent for sus-
pended solids, and 87 percent for NHo-N. Average air flow was 1189 m3/min
(42,000 cfm) for the 12 tanks with 8360 active domes in each for an air flow
rate of 0.011 m3/min/dome (0.4 cfm/dome). The average F/M loading for 1978
was 0.9 kg BOD5/day/kg MLSS. About 17 percent of the flow to this plant is
industrial in origin from chemical, metal finishing, and food processing sources,
156
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Table B-39 summarizes aeration efficiency calculations and mixing data
for the aeration process at Minworth. Aeration efficiency falls in the middle
range of the plants visited, at 1.71 kg 02/kWh (2.81 Ib/wire hp-hr). Mixing
power levels are similar to that of the Beckton plant with the exception of
the first section. Plant personnel stated that when the tanks are given an
annual cleaning, some solids deposition, 20-30 cm (8-12 in.), is evident in
this zone. Little deposition occurs in other parts of the tank,
Operation and Maintenance
Dome diffusers have been in continuous operation at Minworth since 1971.
The only maintenance procedure routinely used is an annual aeration tank clean-
ing and brushing of the domes. There have been a few failures of the air sup-
ply piping connections and hold-down bolts. Air cleaning equipment is rou-
tinely cleaned on an annual basis.
STRONGFORD
While visiting plants of the Severn Trent Water Authority, a brief visit
was made to the works at Strongford. The original plant, dating back to the
1930's, was recently expanded with the addition of a second parallel plant
using fine bubble dome diffusers in 1973. At that time, the original plant
was converted to dome diffusers as well. The new plant was equipped with
four single-pass basins per aeration tank, 1950 diffusers per basin. Each
new basin is 108 m x 9.1 m x 3.0 m SWD (354 ft x 30.0 ft x 10.0 ft). The
domes are uniformly distributed with the exception that the first 27 m (88 ft)
of each tank has 10 lines of domes on 0.46-m (1.5-ft) centers. The remaining
segment of each tank has eight lines with domes on 0.46-m (1.5-ft) centers.
The old plant has 10 two-pass plug flow aeration tanks, each sized
64 m x 2.7 m x 3.0 m SWD (210 ft x 9.0 ft x 10.0 ft). These extremely narrow
tanks are only moderately tapered with approximately 30 percent of the 800
domes (per two-pass tank) installed in three rows in the first half of the
first pass. The balance of each tank has two rows of domes on 0.37-m (1.2-ft)
centers.
Several problems have combined to severely reduce oxygen transfer effi-
ciency at this plant. Typically, the plant has been designed for future ra-
ther than present flows and, thus, was underloaded at startup. Adding to
this is the waste discharged from extensive pottery and china making opera-
tions at Stoke on Trent. The clay material in the effluent from these opera-
tions acts like a coagulating agent, significantly improving primary clarifi-
er performance. Primary BOD5 and suspended solids removals of 70-80 percent
are common at this plant. A plant performance evaluation conducted by Hawker-
Siddeley in 1976 and repeated in 1978 indicated that mixed liquor D.O. ap-
proached saturation in most of the plant for 16-20 hr/day. Using the data
provided in this Hawker-Siddeley evaluation, aeration efficiency in the new
plant was estimated at 1.49 kg 02/kWh (2.45 Ib/wire hp-hr) by the method of
Section 5 for the new plant. The average aeration efficiency could vary sub-
stantially from this estimate, however, as the computation was based on a sin-
gle day grab sampled analysis.
157
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TABLE B-39. MINWORTH AERATION EFFICIENCY CALCULATIONS AND MIXING DATA
AERATION EFFICIENCY CALCULATIONS
BODr Removed (kg/day)*
-N Removed (kg/day)*
N03-N Produced (kg/day)*
02 Required"1" (kg/day)*
Power Consumed (kWh/day)c
Aeration Efficiency:
kg 02/kWh
Ib 02/wire hp-hr
MIXING DATA
Air Flow**
First 1/6 of Tank
Second 1/6 of Tank
Final 2/3 of Tank
Air Flow/Area**
First 1/6 of Tank
Second 1/6 of Tank
Final 2/3 of Tank
Air Flow/Volume**
First 1/6 of Tank
Second 1/6 of Tank
Final 2/3 of Tank
Average Power Input
First 1/6 of Tank
Second 1/6 of Tank
Final 2/3 of Tank
cfm
109
1144
2291
:fm/ft2
0.020
0.197
0.098
37,182
4932
3726
65,577
38,360
1.71
2.81
nr-ymin
3.1
32.4
64.9
m
3/m?/
0.006
0.060
0.030
cfm/1000 ft3++
4.8
28.4
11.5
wire hp/1000 ft3
0.10
1.02
0.51
l/J/m3
2.6
26.8
13.4
* 1 kg = 2.21 Ib
+ Calculated by the method of Section 5, assuming 1.21 units 02 required/
unit BODc removed.
0 1 kWh/day = 1.34 wire hp-hr/day
** Based on an average air flow rate of 0.011 m3/rnin/dome (0.40 cfm/dome).
++ Same as m3/1000 m^/min.
158
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Plant personnel expressed satisfaction with the maintenance performance
of the dome diffuser systems and electrostatic air filters. The dome diffus-
ers have performed satisfactorily for 6 yr without requiring cleaning. There
was no evidence of coarse bubbling in any of the aeration tanks, not surpris-
ing when the very low F/M loading rate of 0.05-0.08 kg BOD/day kg MLSS is
considered.
PLANTS IN THE NETHERLANDS
The Dutch are best known for their development and application of oxida-
tion ditch treatment and horizontal rotor mechanical aeration. Until recent-
ly, there were no fine bubble aeration plants in Holland. A few plants had
medium bubble Brando! tube aerators, but virtually all Dutch plants are mech-
anically aerated. Recently, several activated sludge plants using dome dif-
fusers were constructed, including Holten-Markelo and Steenwijk.
Holten-Markelo
The Holten-Markelo plant shown in Figure B-14 is a small plant serving
about 25,000 people. Design population is 50,000, and the plant is currently
very underloaded. To compensate, only one of the two aeration basins is in
service. Each basin has two passes and operates in the plug flow mode. The
734 diffusers per tank are tapered with 34 percent in the first half pass, 25
percent each in the second and third half passes, and 16 percent in the final
half pass. For the one aeration basin (two passes) in use, air flow averages
30 m3/min (1064 cfm) distributed equally among the domes. Each pass is 30.0
m x 6.6 m x 4.0 m SWD (98.4 ft x 21.6 ft x 13.0 ft). Mixing data are shown
below in Table B-40.
TABLE B-40. HOLTEN-MARKELO MIXING DATA
Section*
1
2
3
4
Domes per Diffuser Density
Section* (domes/m2)+
368
276
276
184
1.9
1.4
1.4
0.9
Air Flow
' per Section*0
cfm
184
138
138
92
nrVmin
5.2
3.9
3.9
2.6
Air Flow
per volume0
(cfm/1000 ft3)**
13.3
10.0
10.0
6.7
* One section = one half pass
+ 1 dome/m2 =9.29 domes/100 ft2
0 Based on an assumed minimum air flow rate of 0.014 m^/min/dome (0.5 cfm/
dome).
** Same as m3/1000 m3/min.
The above data represent minimum air flow rates. In reality, air flow rates
are averaging 0.28-0.42 m3/min/dome (1.0-1.5 cfm/dome). Considerable research
159
-------
Effluent
Blower
Room
t
f
Aeral
Basi
•f
r
:ion
ns
.. j
Sludge
Drying
Beds
Primary
Sludge
Thickener
Admini strati
or
Digested Sludge Thickener
Sludge Digester
Digester Gas Storage Tank
Hoi ten Markelo and Laboratory
Figure B-14. Holten-Markelo site plan,
160
-------
has convinced Dutch designers that the domes should be operated at air flow
rates substantially higher than those recommended by Hawker-Siddeley. This
plant was designed on the basis that minimum nighttime flows (and oxygen de-
mand) would correspond to the minimum air unit dome flow rate of 0.014 m-Vmin
(0.5 cfm). Consequently, dome density is low and average air flow rates are
relatively high when compared to English practice.
Recently, plant personnel began experimenting with an anoxic zone by se-
verely restricting air flow to the first grid. The plant normally produces a
nitrified effluent, and it is believed that the anoxic zone will reduce ener-
gy costs as well as improve performance in the final settling tank.
Air supply is provided by a centrifugal blower driven by a gas engine
that burns a mixture of about 65 percent sludge gas and 35 percent pipeline
gas. Plant personnel could not estimate power consumption (kWh). Consequent-
ly, aeration efficiency could not be calculated.
Primary clarifier design is much more typical of U.S. practice than that
of the English plants. The diameter of the single unit is 24.0 m (78.7 ft),
and the sidewater depth is 2.0 m (6.6 ft). At the maximum influent flow rate
of 21,800 m3/day (5.76 mgd), the detention time is 2 hr and the surface load-
ing rate is 48.0 m-Vm^/day (1178 gpd/ft2). At the average influent flow rate,
the surface loading is 28.8 m3/m2/day (707 gpd/ft2).
The secondary clarifier is designed for a maximum hourly surface loading
of 24.0 ffi^/mVday (589 qpd/ft^) and a 2.5-hr detention time. It is circular,
34 m (112 ft) in diameter and has a sidewater depth of 2.5 m (8.2 ft). Both
the primary and secondary clarifiers are equipped with stainless steel wipers
and scraper mechanisms.
The plant was started up in 1978 and officially commissioned in 1979.
There have been no equipment failures in either of the aeration basins; how-
ever, at the time of the plant visit, there was considerable coarse bubbling
in the second aeration grid, probably due to the growth of slime in the zone
following the anoxic zone. The electrostatic air filters were providing sat-
isfactory service.
Steenwijk
The plant at Steenwijk was the first major dome diffuser plant construct-
ed in The Netherlands. It was commissioned in late 1976 and has two two-pass
aeration basins that are laid out in a tapered configuration. Design flow is
54,100 m3/day (14.3 mgd), and the influent flow in 1978 averaged 44,700 m3/
day (11.8 mgd) j^ 30 percent. The primary and secondary clarifier design ba-
sis is the same as that used at Holten-Markelo.
Three motor driven multi-stage centrifugal blowers, one for each aera-
tion tank and one standby, provide air for the process. The two-pass aera-
tion basins have four aeration grids with 800, 560, 560, and 438 domes, re-
spectively. This is the same percentage distribution as that of Holten-Mar-
kelo. The dimensions of each aeration pass are 100 m x 6.7 m x 4.0 m SWD
(328 ft x 22 ft x 13.1 ft).
161
-------
Steenwijk reported the following performance data for 1978:
Raw Wastewater
BOD5 : 312+_ 74 mg/1
NH3-N : 52+_ 12 mg/1
Primary Effluent
BODs : 102+_ 19 mg/1
NH3-N : 25+_ 3 mg/1
Final Effl
BODs
NH3-N
N03-N
uent
12+ 7 mg/1
12+12 mg/1
11 mg/1
An average of 4860 kg (10,714 lb) BOD5 and 702 kg (1548 Ib) NH3-N were
removed daily at this plant in 1978. The average power consumption was
14,100 kWh/day (18,908 wire hp-hr/day),and the F/M loading was 0.11 kg BODs/
day/kg MLSS. No air flow data were reported. Based on the method of Section
5, with 1.18 kg Op required/kg BODs removed, the aeration efficiency was 0.62
kg 02/kWh (1.02 IB/wire hp-hr). The computation was based on total power
consumed for all purposes at this plant. No breakdown of power use was avail-
able. If aeration consumes 80 percent of the electrical power, a typical le-
vel, the aeration efficiency would be 0.78 kg 02/kWh (1.28 Ib/wire hp-hr),
still quite low. Personnel from the Zuiveringschap-West Overijssel Authority,
the authority that operates the plant, noted that the plant is somewhat under-
loaded and mixed liquor D.O. is high.
Ilsink and Brandse reported clean water ?eration efficiencies of 3.3-3.7
kg 02/kWh (5.4-6.1 Ib/wire hp-hr) at a water temperature of 10°C (50°F) and a
pressure of 760 mm Hg (14.7 psi) for Steenwijk.20
The aeration equipment at Steenwijk has accumulated about 3 yr of opera-
ting time. There have been no failures of the equipment, and plant personnel
report no apparent loss of aeration efficiency over the period.
MADISON, WISCONSIN
History and Background
The Nine Springs Wastewater Treatment Plant serves the City of Madison,
Wisconsin, and many of the surrounding suburban communities. The original
plant was constructed in 1928 and provided primary treatment followed by
trickling filters. The plant has been expanded five times, the last in 1977.
By 1985, the plant will be further modified to provide nitrification and ef-
fluent polishing.
The current facilities, depicted in Figure B-15, are rated at 219,500
m3/day (58 mgd) and treated 132,500 m3/day (35 mgd) in 1978. Flow to the
trickling filter plant accounted for about 14 percent of the total flow
treated.
162
-------
,f To Sludge Storage Lagoons
I
9 / ~
ooi
!ooi L.LO..OJ
1 Influent
L ____
u
i
o o o
o o o 5 o
o
Effluent
KEY
1. GRIT CHAMBERS
2. PRIMARY CLARIFIERS
3. TRICKLING FILTERS
4. AERATION TANKS
5. FINAL CLARIFIERS
o o
6. CHLORINE CONTACT
7. SLUDGE THICKENERS
8. SLUDGE DIGESTERS
9. DIGESTED SLUDGE THICKENERS
Figure B-15. Madison site plan.
163
-------
Plant Description
Prior to the 1977 modification, the activated sludge plant at Madison
was wholly coarse bubble aerated. To increase organic loading capacity and
to study the performance of dome diffusers under actual operating conditions,
aeration tank Nos. 1-6 were retrofitted with dome diffusers arranged in a
grid fashion. Aeration tank No. 7 was later equipped with fine bubble tube
diffusers that could be installed utilizing the existing spiral roll manifolds
without need for major piping modifications. The remaining basins remained
in their original coarse bubble spiral flow aeration mode. No modifications
were made to primary or secondary clarifiers. Air filtration was added to
the blower system (see Table B-41 for design data).
The six aeration tanks with dome diffusers are divided into two sets of
three-pass units and are tapered as shown in Table B-41. These can be opera-
ted in either the contact stabilization mode, using one tank for contact and
two for reaeration, the plug flow mode using three passes, or a three-pass
step feed mode. Two aeration grids are provided for each tank, and the domes
are distributed fairly symmetrically in each grid. Spacing between domes is
either 30.5 or 61 cm (12 or 24 in.). Spacing between rows varies, with pair-
ing of six rows (12 total rows) plus two individual rows in the first pass,
ten individual rows in the second pass, and nine individual rows in the final
pass.
Five Rootes-type positive displacement blowers provide air for all 15 of
the aeration tanks. One is driven by a digestor gas fueled engine; the other
four are powered by electric motors. Normal operating pressure is 58.6 kN/m2
(8.5 psi). Air cleaning is provided by a two-stage air filtration system.
The first stage uses five automatically operated coarse fiberglass filters,
each rated at 354 m3/min (12,500 cfm). These roll-type units are automatical-
ly advanced by a photocell sensor and drive that, respectively, senses the
opacity of the filter and periodically advances fresh medium onto the filtra-
tion frame. Five sets of final filters follow the prefilters, as described
in Table B-41. A pressure gauge scaled between 0 and 5 cm h^O (0 and 2 in.)
reads headloss across the filters. They are to be changed when headless ap-
proaches 5 cm (2 in.). After 2 yr of operation, the current reading on the
gauge is 12.7 mm (0.05 in.).
Final settling is provided by 10 circular clarifiers of varying dimen-
sions. At an average influent flow rate of 111,7000 m3/day (29.5 mgd), the
surface loading is 18.5 m3/m2/day (454 gpd/ft2) and the detention time with
50 percent sludge recycle is 3.4 hr.
Madison reported installed costs of $282,000 for the dome diffusers and
air distribution piping and $28,450 for the air cleaning system.
Performance
For purposes of comparison between types of aerators, aeration tank Nos.
1-6 and 7, 8, and 9 have been isolated from the general plant flow and opera-
ted as a unit. These tanks are dosed by primary clarifier Nos. 1-16 and
final settling is carried out in clarifiers Nos. 1-10. Air flow to each tank
164
-------
TABLE B-41. MADISON DESIGN DATA
Primary Sedimentation
16 Rectangular Tanks
Tank Nos. 1-2:
Tank Nos. 3-4:
Tank Nos. 5-6:
Tank Nos. 7-16:
Detention time
(Tank Nos. 1-2 & 5-16):
Surface loading
(Tank Nos. 1-2 & 5-16):
Aeration Basins
15 Rectangular Tanks
Tank Nos. 1-6:
Tank Nos. 7-15:
Aeration Equipment
Tank Nos. 1-6:
Tank No. 7:
Tank Nos. 8-15:
9.7 m* x 26.2 m* x 3.3 m* SWD
(trickling filter plant)
9.7 m x 31.1 m x 3.3 m SWD
10.7 m x 26.8 m x 3.3 m SWD
1.44 hr @ 219,500 m3/day (58 mgd)
55.1 m3/m2/day (1354 gpd/ft2) @
219,500 m3/day (58 mgd)
9.1 m x 41.2 m x 4.7 m SWD
9.8 m x 57.9 m x 5.3 m SWD
Fine bubble dome diffusers
Fine bubble tube diffusers
Coarse bubble diffusers
Diffuser Configuration (Tanks Nos. 1-6)
Tapered aeration, 2 grids/tank.
3 tanks per unit. Can be operated as plug flow, contact
stabilization, or step feed.
First Pass:
Grid No. 1
Grid No. 2
Second Pass:
Grid No. 3
Grid No. 4
Third Pass:
Grid No. 5
Grid No. 6
No. Domes
834
709
505
410
392
332
No. Rows
14
14
10
10
9
9
(continued on next page)
165
-------
TABLE B-41. (continued)
Final Sedimentation (Activated Sludge Plant)
10 Circular Clarifiers
Tank Nos. 1-4: Diameter 23.7 m
Sidewater depth 3.8 m
Surface loading 18.5 m3/m2/day (454 gpd/ft2)
@ 111,700 m3/day (29.5 mgd)
Tank Nos. 5-6: Diameter 26 m
Sidewater depth 3.8 m
Surface loading same as 1-4
Tank Nos. 7-10: Diameter 32 m
Sidewater depth 3.8 m
Surface loading same as 1-4
Detention time (all clarifiers): 3.4 hr @ 111,700 m3/day
(29.5 mgd) and 50% recycle
Blower System
5 Rootes-Type Positive Displacement Blowers
No. 1: Engine driven, 312 m3/min (11,000 cfm) @ 65.5 kN/m^ (9.5 psi)
No. 2,3: Single speed, 373 kW (500 hp) motor driven, 306 m3/min
(10,800 cfm) @ 65.5 kN/m2 (9.5 psi)
No. 4: Dual speed, 261-410 kW (350-550 hp) motor driven, 220-307
m3/min (7760-10, 850 cfm) @ 65.5 kN/m2 (9.5 psi)
No. 5: Dual speed, 224-373 kW (300-500 hp) motor driven, 165-257
m3/min (5840-9070 cfm) @ 65.5 kN/m2 (9.5 psi)
Air Filter System
Primary Filters:
Automatic roll-type filter
5 sets
Each set rated 354 m3/min (12,500 cfm) @ 152 m/min (500 ft/min)
net velocity
Manufacturer - American Air Filter Company
Secondary Filters:
Dry disposable type - Biocell
5 sets
Each set has 8 filters each 0.3 m^ (3.2 ftO in area:
2 sets also have 8 filters each 0.3 x 0.6 m (1.0 x 2.0 ft)
Other 3 sets also have 4 filters each 0.3 x 0.6 m (1.0 x 2.0 ft)
Manufacturer - American Air Filter Company
166
-------
is measured periodically. Influent and effluent data summarized in Table
B-42 for 1978 and 1979 are for this portion of the plant only.
TABLE B-42. MADISON INFLUENT-EFFLUENT DATA SUMMARY*
Year
Parameter 1979 1978
Flow:
mgd
mVday
14.5
54,900
15.7
59,400
Raw Wastewater: (mg/1)
BOD5 213 184
TSS 163 157
TKN 32 35
NH3-N 17 18
Primary
Effluent: (mg/1)
BOD5
TSS
TKN
NH3-N
156
79
29
18
138
88
28
18
Final Effluent: (mg/1)
BOD-
TSS"
TKN
NH3-N
N03-N
19
12
14
12
1.5
23
13
16
11
2.3
* For that portion of the Madison plant comprised of primary clarifier Nos.
1-16, aeration tank Nos. 1-9, and final clarifier Nos. 1-10 only.
In 1978, the test plant was operated wholly in the contact stabilization
mode. This was continued until August 15, 1979, at which time the test plant
was briefly operated in the plug flow mode for 1 mo. After mid-September
1979, the test plant was changed over to step feed operation. The F/M load-
ing was held at 0.26 kg BODs/day/kg MLSS in 1979 and 0.3 in 1978 to avoid ni-
trification. MLSS levels averaged 2000 mg/1 in 1979 and 1500 mg/1 in 1978.
Peak monthly flows and BOD loads varied approximately +_ 25 percent from annu-
al averages. About 15 percent of the flow to the plant comes from food and
milk processing plants.
By comparison to the plants visited in the United Kingdom, Madison's
primary treatment system was considerably less effective, removing 25-27 per-
cent of the influent BODg and 44-48 percent of the influent suspended solids.
This is probably related to the surface loading rate of 28.5 m^/m^/day (700
gpd/ft2), which is considerably higher than the 12.2-20.4 m3/m2/day (300-500
gpd/ft2) rate more typical in the United Kingdom.
167
-------
Secondary treatment achieved average removals of 83 percent for BOD^ and
85 percent for suspended solids in 1978. 1979 performance was comparable,
averaging 88 percent and 76 percent removals, respectively. Overall removal
efficiencies were 87 percent (BODg) and 92 percent (suspended solids) in 1978,
with corresponding removals of 91 percent and 93 percent in 1979.
The two plants, coarse bubble and •firfe 'bubble, are operated in parallel,
with 44 percent of the flow (and BODs load) going to the coarse bubble
aeration tanks. Air flow is split with 14.4 percent of total plant air
directed to fine bubble tank Nos. 1-6 and 29.9 percent to coarse bubble
tank Nos. 7-9. Power consumption for the total aeration system averaged
16,900 kwh/day (22,660 wire hp-hr/day) in 1978 and 21,150 kWh/day (28,360
wire hp-hr/day) in 1979. Based on these data, comparative aeration
efficiencies for the two sections have been computed by the method of
Section 5 in Table B-43. The results indicate that the fine bubble system
TABLE B-43. MADISON AERATION EFFICIENCY CALCULATIONS
Fine Bubble*
Parameter
1979
1978
Coarse Bubble"1"
1979 1978
Flow:
mgd
m3/day
8.1
30,700
8.8
33,300
6.4
24,200
6.9
26,100
Removed (kg/day)0
NH3-N Removed (kg/day)0
02 Required** (kg/day)0
Power Consumed (kWh/day)++
4207
160
4768
3046
3847
231
4840
2431
3306
125
3744
6324
3022
154
3684
5047
Aeration Efficiency:
kg
Ib
02/kWh
02/wire
hp-hr
1.
2.
54
53
1.
3.
96
22
0.
0.
58
96
0.
1.
72
18
* Comprised of aeration tank Nos. 1-6.
+ Comprised of aeration tank Nos. 7-9.
0 1 kg = 2.21 Ib
** Calculated by the method of Section 5, assuming 1.01 units 02 required/
unit BODs removed in 1978 and 0.97 units 02 required/unit BODs removed in
1979.
++ 1 kWh/day = 1.34 wire hp-hr/day
was about 2.7 times more efficient than the coarse bubble system, with both
showing somewhat higher aeration efficiencies in 1978. In both years, plant
personnel operated the plant to hold maximum mixed liquid D.O. in the range
of 2-4 mg/1. Since the plant was operated in the contact stabilization mode
during all of 1978, with no operational changes, the 1978 data may be more
accurate. As discussed in Section 6, these data correlate well with the rela-
tive aeration efficiencies determined in separate tests conducted at Madison
168
-------
in September 1978.
Mixing power levels for a minimum air flow rate of 0.014 m3/min/dome
(0.5 cfm/dome) have been calculated and are shown in Table B-44. Mixing pow-
er levels are comparable to that at many of the plants in the United Kingdom,
reflecting similarities in design and operation procedures recommended by sup-
pliers of dome diffusers in both the United States and the United Kingdom.
Operation and Maintenance
Madison has accrued 3 yr of maintenance history on dome diffusers and
experienced little trouble until mid-1980. In 1978, it was necessary to
drain several tanks to fix small air leaks and several manifolds that had
pulled out of the concrete floor. A visual inspection in February 1980 of
the six aeration tanks equipped with dome diffusers revealed only small zones,
immediately adjacent to the raw wastewater feed points, where coarse bubbling
was occuring, probably indicative of light slime growth. Concurrent with an
increase in waste flow from a large dairy in early-1980, slime began to accu-
mulate on the domes in all of the aeration tanks, particularly in the first
and second passes. Efficiency decreased sharply in the late spring and it
became necessary to dewater the tanks in June 1980 and steam clean the domes
in place to remove the slime growths. Prior to steam cleaning, plant opera-
tors increased air flow and passed only return activated sludge through a set
of tanks in an attempt to promote self cleaning as observed at Beckton. This
did not significantly improve performance.
All of the aeration tanks had been cleaned and returned to service by
mid-July. However, there was some indication that the diffusers had not ful-
ly come clean as a result of steam cleaning. This possibility was being in-
vestigated at the time of this writing and was not fully resolved.
Madison operators expressed great satisfaction with the air filtration
equipment. Installed in 1977, it has performed well and required little main-
tenance since then. The coarse filter requires changing every 6 mo at a cost
of $58 and consumes less than 1 man-hr to remove and replace the filter roll.
OTHER U.S. PLANTS VISITED
Four U.S. plants were visited during the course of the study:
t Madison, Wisconsin
• Glendale, California
t Fort Worth, Texas
• New York City (Tallman Island)
The Madison results have been detailed previously herein as it was the only
U.S. plant visited that could provide complete performance data. Summarized
below is the information obtained at the other U.S. plants visited.
169
-------
TABLE B-44. MADISON MIXING DATA
- _
Aeration Tank
& Grid Nos.
Tank 1
(first pass):
Grid 1
Grid 2
Tank 2
(second pass) :
Grid 1
Grid 2
Tank 3
(third pass):
Grid 1
Grid 2
Surface Are
(rn2)*
91.8
91.8
91.8
91.8
91.8
91.8
Wetted
:a Volume
(1000 m3)+
0.433
0.433
0.433
0.433
0.433
0.433
Diffuser
Density
( domes /m2)0
9.1
7.7
5.5
4.5
4.3
3.6
Air Flow
(cfm)
417
352
252
205
191
166
Section**
(nr/min)
11.8
10.0
7.1
5.8
5.4
4.7
Air Flow/Area**
(cfm/ft2)
0.42
0.35
0.25
0.21
0.19
0.17
3 2
(m /m /min)
0.128
0.106
0.076
0.064
0.058
0.052
Air Flow Volume** Minimum Power Input00
(cfm/ 1000 ft3)++ (wire hp/1000 ft3) (W/m3)
27.3
23.0
16.5
13.4
12.5
10.8
1.
1.
0.
0.
0.
0.
28
08
78
63
59
51
33.7
28.4
20.4
16.6
15.4
13.4
_
* 1 m2 = 10.8 ft2
+ 1 it)3 - 35.3 ft3
o 1 dome/nr = 9.29 domes/100 ft
** Based on a minimum air flow rate of 0.014 m3/min/dome (0.5 cfm/dome).
++ Same as mj/1000 m-Vnrin.
oo Assumes compressor efficiency of 350 kWh per 10,000 cfm.
-------
Glendale, California
To evaluate dome diffusers on a large scale, a 1-yr test was set up at
the City of Los Angeles Glendale wastewater treatment plant. One aeration
tank and two final clarifiers were isolated, and the aeration tank was fitted
with 1570 dome diffusers on two grids. In operation, 57 percent of the air
was directed to the first grid; the balance of 43 percent went to the second
grid. This split resulted in equal air flows to individual domes in either
grid. The aeration basin measured 9.7 m x 73.2 m x 4.9 m SWD (31.8 ft x 240
ft x 16.1 ft).
The results of the pilot study were reported by Egan7 and have been dis-
cussed in detail in the body of this report. Basically, it was found that
the fine bubble system removed 85-90 percent of the influent BODg and sus-
pended solids using 63 percent of the power required for the parallel coarse
bubble, spiral roll aeration system. Reported operating conditions and per-
formance for the fine bubble test system are as follows for the 10 mo- begin-
ning August 1978.
Flow 11,360 m3/day (3.0 mgd)
BODs: Influent 158 mg/1
Effluent 11 mg/1
MLSS 1500 mg/1
F/M Loading 0.35 kg BOD5/day/kg MLSS
Sludge Recycle Flow Rate 50 percent of influent flow rate
Air Flow Rate 54.0 m3/min (1907 cfm)
Power Consumed 13,152 kWh/day (17,637 wire hp-
hr/day)
The performance of the coarse bubble system was similar, although the F/M
loading at 0.25 kg BOD5/day/kg MLSS was somewhat less than for the fine bub-
ble system. The average power consumption of the coarse bubble system was
20,740 kWh/day (27,810 wire hp-hr/day).
Based on these limited data, performance data can be calculated as pre-
sented in Table B-45. The fine bubble system aeration efficiency was about
45 percent higher than for the coarse bubble system in this test. Mixing da-
ta indicate air flow rates and power levels typical of other dome diffuser
plants.
A visual inspection of the dome diffuser system in operation revealed
significant coarse bubbling in the first 20 percent of the tank. Plant per-
sonnel indicated that some problems were experienced at startup with several
air blowoff lines that failed at the point of connection to the main grid.
Also, several of the uPVC dome holddown bolts failed. Finally, there was a
substantial problem with rust and scale from the plant's air supply lines be-
ing deposited in the air grid lines and within the dome housings. The test
unit uses two-element disposable filters of the type described in Sections.
Fort Worth, Texas
In 1978, the Fort Worth plant was expanded with the addition of four
171
-------
TABLE B-45. 6LENDALE AERATION EFFICIENCY CALCULATIONS AND MIXING DATA
AERATION EFFICIENCY CALCULATIONS
BOD5 Removed (kg/day)*
kg 02/kg BODc Removed
02 Required (kg/day)*
Power Consumed (kWh/day)+
Aeration Efficiency:
kg Op/kWh
1b 02/wire np-hr
Fine Bubble
System
1670
0.93
1553
1460
1.06
1.74
Coarse Bubble
System
1670
1.0
1670
2302
0.73
1.20
MIXING DATA0
Piffuser Density
(domes/m2)
**
First Grid
Second Grid
Air Flow
First Grid
Second Grid
Per Dome
Air Flow/ Area
First Grid
Second Grid
Air Flow/Volume
First Grid
Second Grid
Average Power Input
First Grid
Second Grid
3.0
1.6
cfm
1087
820
1.2
cfm/ft2
0.34
0.18
(cfm/ 1000
21.
11.
wire hp/1000 ft3
0.91
0.29
m3/min
30.8
23.2
0.034
m3/m2/min
0.104
0.055
ft3)++
2
4
W/m3
24.0
7.5
* 1 kg = 2.21 Ib
+ 1 kWh/day =1.34 wire hp-hr/day
0 For fine bubble system only.
** 1 dome/m2 = 9.29 domes/100 ft2
++ Same as m3/1000 nH/min.
172
-------
dome diffuser activated sludge basins and additional final clarifiers to
serve in parallel with an existing coarse bubble, spiral roll aeration system.
The plant expansion program, to be completed in 1980, includes major modifica-
tion of sludge handling facilities as well. The addition was designed for an
average influent flow of 363,400 m3/day (96 mgd) to be divided equally among
the four basins.
Initially, each tank, measuring 31.7 m x 72.7 m x 3.2 m SWD (104 ft x
239 ft x 10.5 ft), was equipped with 13,314 dome diffusers, distributed in
four equal-size grid areas as follows:
First Grid 4158
Second Grid 3276
Third Grid 3594
Fourth Grid 2286
In fitting the basins with domes, an experiment in optimization was set up.
All basins were equipped with the same manifolds and dome fittings. However,
basin No. 3 was fitted with only 7986 domes, distributed as follows:
First Grid 2178
Second Grid 2038
Third Grid 2004
Fourth Grid 1776
It was determined from this experiment that the modified aeration basin could
treat the same amount of wastewater with a 25 percent reduction in air flow
and with better control of mixed liquor D.O.
Several factors combined to make a performance evaluation of this plant
impossible at this time. First, the continued construction on the site, with
resulting system disturbance, cause the data to be highly scattered. In par-
ticular, major problems with sludge handling equipment have contributed to
continued process upsets. Secondly, there has been a substantial turnover of
key personnel at the plant. The new personnel have not had adequate time to
become fully trained in plant operation or instrument maintenance.
The plant visit revealed a number of potentially serious problems (if not
corrected) with the dome diffuser equipment. There were a number of substan-
tial air leaks apparent in aerations basin Nos. 2 and 3. Several of the air
blowoff lines were plugged, and one was apparently broken. The air leaks were
likely of sufficient size to admit mixed liquor to the interior of the air
distribution grid.
New York City (Tallman Island)
New York City's Tallman Island plant, located on the Hudson River, was
originally constructed in 1939 as a coarse bubble aerated activated sludge
plant. A major plant upgrading was carried out in 1977-1979, at which time a
side-by-side test of dome diffusers, ceramic tubes, and coarse bubble diffus-
ers was conducted. Influent flow, averaging 302,800 m3/day (80 mgd),was divi-
ded between four four-pass step feed basins. Two of these are equipped with
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coarse bubble units mounted on a dual spiral flow configuration. One tank was
equipped with fine bubble ceramic tube diffusers on the same manifolding as
the coarse bubble units, and one tank was equipped with dome diffusers laid
out in a symmetrical grid configuration with 6400 domes in the four passes.
In setting up this test, it was intended to gather side-by-side compari-
son data for use in all New York City plants. Accordingly, adequate instru-
mentation for performance measurement of each aeration tank set was provided
when the plant was upgraded. However, equipment problems have been exper-
ienced and the equipment had not been accepted by the City at the time of the
plant visit. Consequently, although influent and effluent data for the 1-1/2
yr of operation were available, they could not be correlated with either oxy-
gen transfer or mixing performance.
In early 1979, environmental engineering students at Manhattan College
carried out a short-term performance comparison between the coarse bubble and
fine bubble equipment. Their data indicated that the coarse bubble equipment
would require approximately twice the compressor power consumption to treat
the same quantity of wastewater under the conditions encountered at Tallman
Island. These findings correlate well with data from Madison, Misconsin, and
Glendale, California.
At the time of the plant visit, the aeration equipment was performing
satisfactorily. There was some evidence of coarse bubbling immediately adja-
cent to two of the three-step feed inlets; however, it was not severe. Plant
personnel indicated that the electrostatic air cleaning equipment was perform-
ing well.
System personnel report 1978 capital costs for the fine bubble dome diffuser
aeration system of $880,000, including installation, plus $40,000 for the
electrostatic air cleaner.
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6 US GOVERNMENT PRINTING OFFICE 1982 -559-09Z/3402
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