EPA-600/2-78-051
March 1978
A COMPARISON OF OXIDATION DITCH PLANTS TO
COMPETING PROCESSES FOR SECONDARY AND
ADVANCED TREATMENT OF
MUNICIPAL WASTES
by
William F. Ettlich
Culp/Wesner/Culp-Clean Water Consultants
El Dorado Hills, California 95630
Contract No. 68-03-2186
Project Officer
Francis L. Evans, III
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 publi-
cation. 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.
11
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FOREWORD
The 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 testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its com-
ponents require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem
solution and it involves defining the problem, measuring its impact, and
searching for solutions. The Municipal Environmental Research Laboratory
develops new and improved technology and systems for the prevention,
treatment, and management of wastewater and solid and hazardous waste
pollutant discharges from municipal and community sources, for the preser-
vation and treatment of public drinking water supplies, and to minimize
the adverse economic, social, health, and aesthetic effects of pollution.
This publication is one of the products of that research; a most vital
communications link between the researcher and the user community.
This report includes information relating to oxidation ditch plant
equipment, design and application, operational problems and advantages,
operation and maintenance requirements, construction costs, and nitri-
fication and nitrogen removal applications. Much of the information is
based on visits to and analysis of data from actual operating installations,
In addition, the oxidation ditch plant characteristics are compared to
those of competing biological treatment processes. Nitrification and
nitrogen removal capabilities of the oxidation ditch process are also
compared to various biological and physical-chemical processes.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
iii
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EXECUTIVE SUMMARY
INTRODUCTION
The purpose of this study is to present state-of-the-art information
regarding oxidation ditch plant design, costs, operation, performance/ and
reliability. This information is used to compare oxidation ditch plants
to other competing biological processes. Design criteria and operational
conditions necessary to maximize single stage nitrification and total
nitrogen removals in oxidation ditch plants is also presented. Costs of
competing nitrification and nitrogen removal processes are compared to
those developed for oxidation ditch plants.
DESIGN FEATURES OF OXIDATION DITCH PLANTS
An oxidation ditch plant is typically an extended aeration type of
activated sludge process that uses a continuously recirculating closed
loop channel or channels as an aeration basin. The aeration basin is
normally sized for a 24-hour hydraulic retention time, but may be designed
for any other detention time. Mechanical aerators are commonly used for
mixing, oxygen supply, and for circulation of mixed liquor. Generally,
these are horizontal brush, cage, or disc-type aerators designed speci-
fically for oxidation ditch plants. Secondary clarifiers similar to
those used in other activated sludge processes are normally provided.
Primary clarification is not usually included in oxidation ditch plant
design.
The typical oxidation ditch aeration basin is a single channel or
multiple interconnected concentric channels. An oxidation ditch plant
normally consists of one or more basins of either type operated in paral-
lel depending on the flow and operation mode required. Channel geometry
can vary to include many possible configurations, however, the oval
configuration is the most common. The multiple concentric channel basin
can have any number of interconnected channels with three to five being
typical and provides some process flexibility since it can be changed to
other activated sludge modes with minor modification. Typically, the
outer channel is used for aerobic digestion of the waste activated sludge.
Shallow channels are typically four to six feet deep with 45 degree
sloping side walls. Deep channels have vertical side walls and are normally
10-12 feet deep. The channels are usually lined to prevent erosion and
leakage. Ditch lining can be reinforced concrete, gunite, asphalt, or thin
membranes. Typically, shallow channels with sloped side walls are con-
structed of concrete poured against earth backing with welded wire mesh
reinforcing. Deep vertical wall channels require reinforced concrete walls.
iv
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Several manufacturers supply oxidation ditch brush or disc type
mechanical aerators. These units may be either fixed or floating. The
aerators normally span the channel width and may be installed in one or
more locations around the channel. The aerators must supply the required
oxygen to the channel and impart a sufficient velocity in the channel
(>1.0 FPS) to keep the channel contents in suspension. Oxygen transfer
capabilities of an aerator will vary depending on the particular design,
rotational speed and submergence. Most units operate in the range of 60 RPM
to 110 RPM with a submergence of 2 to 12 inches and produce oxygen trans-
fer rates of from 3 to 5 Ibs of oxygen per hour. The number of aerators
provided depends on the size, configuration and oxygen requirements of the
plant. A minimum of two aerators should be installed so that at least
partial aeration can be provided when problems occur.
Generally, final clarifier design is consistent with other activated
sludge processes. A surface overflow rate of 400 to 500 gpd/sq ft is recom-
mended for average daily flows and 1000 to 1200 gpd/sq ft at peak 'flows.
Many plants are constructed with 8 foot deep clarifiers, but depths of
10 to 14 feet provide greater process reliability.
COMPARATIVE PERFORMANCE & RELIABILITY
Performance data for the 29 oxidation ditch plants studied are
summarized as follows:
SUMMARY PERFORMANCE OF 29 OXIDATION DITCH PLANTS
BOD5
High plant
Average
Low plant
Suspended solids
High plant
Average
Low plant
Effluent, mg/1
Removal
Average
Winter Summer annual Winter Summer
55
15.2
1.9
26.6
13.6
3.1
34
1.2
1.0
19.4
9.3
1.9
41
12.3
1.5
22.4
10.5
2.4
87
92
99
81
93
98
86
94
99
82
94
98
Average
annual
87
93
99
82
94
98
The performance of competing biological treatment processes was also
evaluated and the data are summarized as follows:
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PERFORMANCE - COMPETING BIOLOGICAL PROCESSES
Effluent, mg/1 Removal, %
TSS BOD5 TSS BODg
Activated sludge
(1.0 mgd) 31 26 81 84
Activated sludge
(Package Plants) 28 18 -
Trickling filters 26 42 82 79
Rotating biological
contactor 23 25 79 78
Analysis of data from 12 operating oxidation ditch plants showed the
reliability for meeting various BOD5 and TSS effluent standards as follows
RELIABILITY - OXIDATION DITCH PLANTS
% of time effluent concentration (mg/1) less than
10 mg/1 20 mg/1 30 mg/1
Best plant
Average all plants
Worst plant
Of the plants analyzed, the effluent BODs and TSS seldom exceeded a
maximum of 60 mg/1.
The reliability of competing biological treatment processes was
evaluated on the same basis and is summarized as follows:
AVERAGE RELIABILITY - COMPETING BIOLOGICAL PROCESSES
% of time effluent concentration (mg/1) less than
10 mg/1 20 mg/1 30 mg/1
T£
Activated sludge
(1.0 mgd)
Activated sludge
(Package Plants)
Trickling filters
Rotating biological
contactor 22% 30% 45% 60% 70% 90%
An oxidation ditch plant is capable of 95% to 99% nitrification with-
out design modifications. This high degree of nitrification even at waste-
water temperatures approaching 0°C is possible due to the 24-hour hydraulic
retention time in the channel(s) and the capability of operating at a high
solids retention time (SRT) of 10 to greater than 50 days.
Nitrogen removal by single-stage biological nitrification-denitrifica-
tion has also been achieved at properly designed and well operated
vi
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oxidation ditch plants. Nitrogen removal is achieved by producing both
aerobic and anoxic zones within the same channel. These zones are created
by controlling the aerator oxygen transfer rate so that mixed liquor dis-
solved oxygen is depleted within a portion of the aeration channel.
The carbon source for the anoxic zone (denitrification) is provided by
feeding the raw sewage into the channel upstream of the anoxic zone. With
careful operation, 80% nitrogen removal has been achieved in a single
channel oxidation ditch plant.
OXIDATION DITCH PLANT OPERATION
Oxidation ditch plants can be operated by average personnel to produce
above average performance results. Assuming no mechanical malfunction, oxi-
dation ditch plants are capable of performing well for several days at a
time with minimal operator attention.
Waste activated sludge handling requirements depend on the plant de-
sign and operation. Plants operated with a 24-hour channel hydraulic
retention time at 20 to 30 day SRT produce a biologically stable waste
sludge that can be handled without causing significant odor problems. Plants
operated with 6 to 8 hour channel hydraulic retention time and less than 10
day SRT require additional sludge treatment, typically aerobic digestion.
Some problems have been noted in dewatering activated sludge directly on
sand beds from plants operated at a 24-hour hydraulic retention time.
These sludges tend to dewater very slowly requiring significantly increased
sludge drying bed area. In areas where wet and cold weather are common,
sludge drying bed area should be even larger. Some plants are operated in-
definitely without formal sludge wasting allowing solids to build up in the
aeration channel. This type of operation is considered marginal because
the plant is prone to periodic clarifier upsets resulting in high final
effluent solids.
Aerators and drives have typically required major corrective mainten-
ance every 2-5 years for the following type problems.
1. Bearing and seal failure due to improper selection, constant
water splashing on the bearings and seals, and settlement of the
aerator support structure causing misalignment.
2. Loss of aerator elements due to corrosion.
3. Aerator torque tube failure or excessive deflection of very long
units.
COMPARATIVE COSTS
Costs associated with oxidation ditch plants were determined. Con-
struction costs for 44 plants were escalated by EPA Treatment Plant Index
262.3 for the third quarter of 1976. These costs include all facilities
except land, engineering, legal and financing during construction. Annual
VI1
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0 & M costs include labor, utilities, chemicals, maintenance materials, and
miscellaneous. These costs are as follows:
OXIDATION DITCH PLANT CONSTRUCTION AND ANNUAL 0 & M COSTS
$1000, 1976
Plant capacity, mgd 0.1 l.Q IQ.Q
Construction 195 600 3350
0 & M normal 22.1 62.4 446.6
O & M nitrification 22.6 63.1 467.5
0 & M N-removal 28.1 67.4 453.7
There are generally no increased construction costs for nitrification
or nitrogen removal.
Construction and 0 & M costs were developed for competing activated
sludge processes. These costs were developed on the same basis as the
oxidation ditch costs. The construction and annual O & M costs are as
follows:
CONSTRUCTION COSTS - COMPETING ACTIVATED SLUDGE PROCESSES
$1000, 1976
Capacity, mgd 0.5 1.0 5.0 10.0
Extended aeration
(package plants) 390 -
Contact stabilization
(package plants) 320 475 -
Conventional activated
sludge - 1045 2645 4138
ANNUAL O & M COSTS - COMPETING ACTIVATED SLUDGE PROCESSES
$1000, 1976
Capacity, mgd 0.5 1.0 5.0 10.0
Extended aeration
(package plants) 64.4
Contact stabilization
(package plants) 57.3 93.9
Conventional activated
sludge - 80.9 187.7 308.1
Construction and annual O & M costs were developed for competing
biological nitrification. These are summarized as follows for 20 mg/1
influent NH -N.
4
Vlll
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CONSTRUCTION COSTS FOR COMPETING BIOLOGICAL NITRIFICATION PROCESSES
$1000, 1976
Capacity, mgd 1 5 10
Activated sludge, 1 stage
(20 mg/1 NH -N) 1,210 3,203 5,107
Activated sludge, 2 stage
(20 mg/1 NH4-N) 1,448 3,830 6,031
ANNUAL 0 & M COSTS FOR COMPETING BIOLOGICAL NITRIFICATION PROCESSES
$1000, 1976
Capacity, mgd 1 5 10
Activated sludge, 1 stage
(20 mg/1 NH -N) 89.4 219.3 375.5
Activated sludge, 2 stage
(20 mg/1 NH4-N) 102.9 245.4 416.9
Construction and annual O & M costs were developed for competing
biological denitrification processes. These are summarized as follows
for 20 mg/1 influent NH -N.
CONSTRUCTION COSTS FOR COMPETING BIOLOGICAL DENITRIFICATION PROCESSES
$1000, 1976
Capacity, mgd 1 5 10
Mixed reactor denitrification
(20 mg/1 NH4-N) 539 1,357 2,291
Fixed film denitrification
(20 mg/1 NH4-N) 636 1,192 2,298
Capacity, mgd 1 5 10
Mixed reactor denitrification
(20 mg/1 NH4-N) 54.1 140.6 244.6
Fixed film denitrification
(20 mg/1 NH4-N) 51.3 115.3 195.0
Construction and annual 0 & M costs were also developed for selected
physical-chemical nitrogen removal processes. These are summarized as
follows for 20 mg/1 influent NH -N.
IX
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CONSTRUCTION COSTS FOR COMPETING PHYSICAL-CHEMICAL NITROGEN
REMOVAL PROCESSES
$1000, 1976
Capacity, mgd 1 5 To"
Breakpoint chlorination
(20 mg/1 NH4-N) 114.8 377.1 696.7
Selective ion exchange
(20 mg/1 NH4-N) 442.6 1,557.4 2,704.9
Ammonia stripping
(20 mg/1 NH4-N) 245.9 1,065.6 1,967.2
ANNUAL 0 & M COSTS FOR COMPETING PHYSICAL-CHEMICAL NITROGEN
REMOVAL PROCESSES
$1000, 1976
Capacity, mgd
Breakpoint chlorination
(20 mg/1 NH -N)
Selective ion exchange
(20 mg/1 NH -N)
Ammonia stripping
(20 mg/1 NH -N)
Construction costs for competing extended aeration and contact stabil-
ization plants were less than for oxidation ditch plants in the flow range
of 0.01 mgd to 2 mgd. Oxidation ditch plant construction costs were less
than for conventional activated sludge plants within the range of 0.01 to
10 mgd. Operation and maintenance costs for oxidation ditch plants were
less than for the competing processes in 0.1 mgd to 2 mgd range and the
total annual costs for oxidation ditch plants were less than for all other
competing processes in the range of 0.1 to 10 mgd. Within the flow range
of 0.01 to 0.1 mgd the total annual costs for extended aeration package
plants was less than for oxidation ditch plants. The total annual costs
for all competing denitrification processes were higher than for oxidation
ditch plants.
CONCLUSIONS
The results of this study show that oxidation ditch plants are capable
of consistently achieving high levels of BOD and TSS removals with
minimum operation. High levels of nitrification (95%-99%) are possible
with proper operation. Nitrogen removals as high as 80% can be achieved in
a single channel plant with careful operation of the aeration equipment to
produce aerobic and anoxic zones within the channel. Increased operator
attention is. required to produce the high levels of nitrogen removal.
Cost data developed in this report show that oxidation ditch plants
are competitive with other biological processes. Total annual costs for
oxidation ditch plants were less than for all competing biological
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processes within the flow range of 0.1 to 10 mgd. Oxidation ditch plants
were also shown to have lower total annual costs than competing biological
and physical-chemical nitrogen removal processes.
This report was submitted in partial fulfillment of Contract No.
68-02-2186 by Culp/Wesner/Culp - Clean Water Consultants under sponsorship of
the U.S. Environmental Protection Agency. This report covers the period of
July, 1976 to December, 1976, and work was completed as of August, 1977.
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CONTENTS
Foreword ill
Executive Summary iv
Figures xv
Tables xviii
1. Introduction 1
Study objectives 1
Study procedure 1
History 1
Current trends in the United States 2
2. Designs and Applications 6
Pretreatment 6
Oxidation ditch plant and arrangements
and designs 9
Sludge handling 14
Oxidation ditch unique equipment suppliers. . 15
Evaluation of designs 29
3. Performance and Reliability 47
General 47
Performance 48
Reliability 48
Oxygen uptake rate 57
Mixed liquor characteristics 57
Solids production 59
Effect of oxidation ditch configurations
on process performance 59
4. Nitrification and Nitrogen Removal 61
General 61
Nitrification 61
Design parameters for nitrification 68
Operational parameters for nitrification. . . 70
Design parameters for nitrogen removal. ... 72
Operational parameters for nitrogen removal . 75
Summary 76
5. Operation. , 78
Oxidation ditch plant problems 78
Operation and maintenance requirements. ... 82
Nitrogen removal 86
6. Construction 87
Construction cost 87
Nitrification construction costs 87
Nitrogen removal construction costs 87
Xlll
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Equipment pricing 91
Ditch configuration 91
Plant area 91
Cold climate 96
Other factors 95
7. Competing Processes 97
General 97
Capability and reliability of competing
biological processes 97
Performance summary 115
Costs of competing biological processes .... 115
Biological nitrification 129
Biological denitrification 136
Physical chemical nitrogen removal 136
Summary and comparison 145
8. Discussion and Evaluation. . . , 158
Process and design 158
Process equipment 158
Performance 159
Construction 159
Operation and maintenance 160
Sludge handling 162
Nitrification and nitrogen removal 163
Effective application 165
References 166
Bibliography .* ." 169
List of Metric Conversions 171
xiv
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FIGURES
Number Page
1 Municipal oxidation ditch plant installations
in the United States 5
2 Single channel oxidation ditch plant and typical
channel configuration 7
3 Multiple concentric channel oxidation ditch plant 8
4 Typical brush type aerator 17
5 Typical disc type aerator 18
6 Lakeside Cage and Mini-Magna aerator characteristics 20
7 Lakeside Magna-Rotor characteristics 21
8 Passavant Series 5200 aerator performance, 90 rpm 22
9 Passavant Series 5300 aerator performance, 70 rpm 23
10 Envirex 4'-6" aeration disc characteristics 24
11 Cherne "OTA aerator" aerator configuration 26
12 Cherne "OTA aerator" characteristics 27
13 Envirotec "Carrousel" schematic 28
14 Horizontal aerator efficiency 31
15 Carbonaceous BOD oxygen requirements 35
16 Effect of organic variations 37
17 Effect of hydraulic variations 41
18 Biological process mixed liquor relationships and
solids production 45
19 BODs and TSS removal performance 52
xv
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Number
- Page
20 Oxidation ditch plant BOD5 reliability ........... 53
21 Oxidation ditch plant suspended solids reliability ...... 54
22 Oxidation ditch plant COD reliability ........... 55
23 Oxidation ditch plant total nitrogen reliability ....... 55
24 Concentric channel nitrogen removal flow
diagram (Drews) <17) (18) .... cc
'•••••••••••••• OD
25 Oxidation ditch plant average construction cost ....... 89
26 Effluent quality, trickling filters ............. 98
27 Trickling filter effluent quality, two Texas plants ..... 100
28 Activated sludge effluent quality .............. 104
29 Activated sludge effluent quality, Dallas, Texas
nitrification pilot plant and El Lago, Texas ........ 106
30 Filtered activated sludge plant BOD5 quality
based on four plants ................. 1Q8
31 Activated sludge package plant reliability, BOD5 .......
36
38
32 Activated sludge package plant reliability,
suspended solids ................ ,.,,
33 RBC effluent quality, Gladstone, Michigan .......... 113
34 RBC effluent quality monthly data with chemical
coagulation, Gladstone, Michigan .............. 114
35 Activated sludge process schematic ............ 123
Biological treatment process construction cost, 1976 ..... 147
37 Biological treatment process operation and maintenance
cost, 1976 _ ,_
148
Biological treatment process total annual cost, 1976 149
39 Incremental construction cost for biological
nitrification, 1976
152
40 Incremental operation and maintenance cost for
biological nitrification, 1976 153
xvi
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Number
41 Incremental construction cost for biological and
physical-chemical denitrification, 1976 154
42 Incremental operation and maintenance cost for
biological and physical-chemical denitrification
1976 155
43 Incremental total annual cost for biological and
physical-chemical denitrification, 1976 156
xvi i
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TABLES
Number
Pa3e
1 Municipal Oxidation Ditch Plant Installations .... 3
2 Typical Single Channel Designs - Oxidation Ditch
Plants ^ 12
3 Typical Multiple Concentric Channel Design » Oxidation p
bitch Plants . 13
4 Comparative Aerator Characteristics ....... 16
5 Source of Performance Data 49
6 Oxidation Ditch Plant Performance Summary. . 50
7 Sources of Reliability Data and Plant Characteristics. 51
8 Oxidation Ditch Plant Dissolved Oxygen and Mixed
Liquor Suspended Solids 58
9 Typical Mixed Liquor Settleability 60
10 Ammonia and Total Nitrogen Performance Data 62
11 Multi Channel Oxidation Ditch Nitrogen Removal
Results (Drews) (1?) (18) .. . ,.-
12 Sources of Oxidation Ditch Plant O & M Information . . 84
13 0 & M Requirements, Oxidation Ditch Plants, 1976 ... 85
14 Sources of Oxidation Ditch Plant Construction Cost
Information. ....... 00
**••**.«...... oo
15 Oxidation Ditch Plant Construction Cost, 1976 90
16 Incremental Construction Costs For Nitrogen Removal. . 92
17 Oxidation Ditch Plant Major Equipment Cost, 1976 ... 93
18 Comparative Construction Cost Of Oxidation Ditch
Configurations
xvi 11
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Number Page
19 Approximate Oxidation Ditch Plant Area Require-
ments 95
20 Trickling Filter Summary, Plant Visits 101
21 Trickling Filter Summary, EPA Region Data 102
22 Activated Sludge Summary, EPA Region Data 103
23 Summary of Competing Process Performance 116
24 Construction Cost of Contact Stabilization Plants,
1976 118
25 Operation and Maintenance Costs, Contact Stabilization
Plants, 1976 119
26 Construction Cost of Extended Aeration Plants, 1976. . 120
27 Operation and Maintenance Costs, Extended Aeration
Plants, 1976 121
28 Design Parameters, Activated Sludge ... 124
29 Unit Process Sizes, Activated Sludge 125
30 Construction Costs, Activated Sludge, 1976 126
31 Operation and Maintenance Costs, Activated Sludge,
1976 I27
32 Design Parameters, Single Stage Activated Sludge
Nitrification 130
33 Unit Process Sizes, Single Stage Activated Sludge
Nitrification 131
34 Construction Cost, Single Stage Activated Sludge
Nitrification 132
35 Design Parameters, Two Stage Activated Sludge
Nitrification . 133
36 Unit Process Sizes, Two Stage Activated Sludge
Nitrification 134
37 Construction Cost, Two Stage Nitrification, 1976 . . . 135
38 Design Parameters, Mixed Reactor Denitrification . . . 137
xix
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Number ^
Page
39 Unit Process Sizes, Mixed Reactor Denitrification. . . 138
40 Construction Costs, Mixed Reactor Denitrification,
1976
139
41 Operation and Maintenance Costs, Mixed Reactor
Denitrification, 1976 140
42 Construction Costs, Fixed Film Denitrification .... 141
43 Operation and Maintenance Costs, Fixed Film
Denitrification, 1976 142
44 Construction Costs, Physical-Chemical Nitrogen
Removal, 1976 143
45 Operation and Maintenance Costs, Physical-Chemical
Nitrogen Removal, 1976 144
46 Operating Plant Operation and Maintenance Costs. ... 150
47 Characteristics of Biological Treatment Processes
(For Range of Sizes Considered in This Study).... 151
48 Characteristics of Nitrogen Removal Processes 157
xx
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SECTION 1
INTRODUCTION
STUDY OBJECTIVES
There are two major objectives to this study of oxidation ditch plants.
First, is to compare the cost, performance, and reliability of oxidation
ditch plants with conventional activated sludge plants in the size range
of 1-10 mgd* and contact stabilization, and extended aeration activated
sludge package plants within the size range of 0.01 to 1 mgd. Various
manufacturers' oxidation ditch equipment and design recommendations will be
studied and compared.
The second major objective is to summarize the design criteria and
operational conditions necessary to maximize single stage nitrification
and total nitrogen removal in oxidation ditch plants and to compare the
total costs to competing processes in the size range 0.01 to 10 mgd.
This study is intended to provide useful and state of the art informa-
tion on oxidation ditch plants to a wide audience of designers responsible
for planning, evaluating, selecting and designing municipal wastewater
treatment plants.
STUDY PROCEDURE
The work reported in this study is based on manufacturers literature,
published literature, information from consultants and others knowledgeable
in the subject and information from operating installations. The informa-
tion from operating installations was obtained by visiting approximately
20 operating plants by letter and telephone contacts to 20 others.
HISTORY
The oxidation ditch was developed during the 1950's at the Research
Institute for Public Health Engineering (TNO) in the Netherlands as an
easily operated and low-cost method of treating raw sewage emanating from
small communities and industries.
*English units are used uniformly in this report because many of the Eng-
lish measures are common in the sanitary field. Conversion factors are
contained in the List of Metric Conversions.
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The first oxidation ditch plant was reportedly placed in service in
1954 at Voorshoten, Holland. This plant was designed in accordance with
the principles of Dr. Ir. A. Pasveer of TNO for a population equivalent
of 360 persons. This plant was an intermittent flow type in which the
ditch also served as the final clarifier.
Since that time, oxidation ditch plants of many different configura-
tions and designs have been placed in operation throughout the world.
The first major USA installations were made in the early 1960's. The
equipment for these early installations was furnished by Lakeside Equipment
Corp. Since that time the number of installations has increased and the
following companies now manufacture or market oxidation ditch plant equip-
ment in this country.
Lakeside Equipment Corporation
Passavant Corporation
Envirex, Incorporated
Walker Process
Cherne Industrial, Inc.
Envirotech Corporation (Carrousel License)
In addition, several firms market equipment for oxidation ditch plants
used specifically in treatment of animal wastes including Fairfield Engin-
eering and Manufacturing Company and Thrive Centers, Inc.
As of late 1975, manufacturers literature indicated the following
number of municipal oxidation ditch plant installations in the United
States.
Lakeside 467
Passavant 56
Envirex 20
Cherne l
There are many "rotor" type aerators installed in other aeration appli-
cations such as aerated lagoons, extended aeration reactors, and aerobic
digesters. These applications are not considered as oxidation ditch plants.
CURRENT TRENDS IN THE UNITED STATES
The approximate distribution of oxidation ditch plant installations
by State and Canadian Province is shown in Table 1. The approximate
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TABLE 1. MUNICIPAL OXIDATION DITCH PLANT INSTALLATIONS
STATES
LAKESIDE
ALABAMA
ALASKA
ARIZONA
ARKANSAS
CALIFORNIA
COLORADO
CONNECTICUT
DELAWARE
FLORIDA
GEORGIA
HAWAII
IDAHO
ILLINOIS
INDIANA
IOWA
KANSAS
KENTUCKY
LOUISIANA
MAINE
MARYLAND
MASSACHUSETTS
MICHIGAN
MINNESOTA
MISSISSIPPI
MISSOURI
MONTANA
NEBRASKA
NEVADA
NEW HAMPSHIRE
NEW JERSEY
NEW MEXICO
NEW YORK
NORTH CAROLINA
NORTH DAKOTA
OHIO
OKLAHOMA
OREGON
PENNSYLVANIA
RHODE ISLAND
SOUTH CAROLINA
SOOTH DAKOTA
TENNESSEE
TEXAS
UTAH
VERMONT
VIRGINIA
WASHINGTON
WEST VIRGINIA
WISCONSIN
WYOMING
TOTAL USA
3
1
16
13
2
21
4
4
12
4
1
1
13
23
3
7
2
17
5
1
8
4
5
7
7
21
2
6
7
20
1
3
165
10
29
14
5
467
1
1
1
14
1
3
3
1
1
1
1
1
2
2
3
1
2
16
1
56
PASSAVANT ENVIREX CHERNE TOTAL
1
3
0
1
17
13
0
0
3
3 38
0
4
2 6
13
7
1
1
1 17
23
1
3
0
1 8
2
18
0
6
1
9
5
7
9
1 8
0
1 25
2
6
8
0
8 30
1
3
17 198
0
10
0
30
14
1
5
1
34
558
PROVINCES
ALBERTA 10
BRITISH COLUMBIA 24
MANITOBA 1
NEW BRUNSWICK 12
NEWFOUNDLAND 3
NOVA SCOTIA 22
ONTARIO 17
PRINCE EDWARD ISLAND 0
QUEBEC 0
SASKATCHEWAN 1
TOTAL CANADA
90
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number of United States installations by year are shown in Figure 1. All
of the installation data were developed from manufacturers published in-
stallation lists covering the1 period through 1975.
All information indicates that the current trend is toward increased
numbers of oxidation ditch plants especially in the size range up to 1.5
mgd. Only a few larger plants are presently in operation. There are only
a few states with no actual or planned oxidation ditch plant installations.
Increased installation of oxidation ditch plants is related to some
or all of the following considerations. These factors are discussed in
detail later in the study.
1. Construction cost equal to or less than competitive treatment
processes.
2. Plants require a minimum of mechanical equipment.
3. Plants appear to perform reasonably well even with minimum oper-
ator attention, primarily due to conservative design.
4. Waste sludge is relatively nuisance free and is readily disposed
of at most plants.
5. Plants generally do not generate odors even under poor operating
conditions.
Discussions with plant operators, public works officials, consulting
engineers, and others who have had direct experience with oxidation ditch
plants indicate a high level of satisfaction with and acceptance of oxida-
tion ditch plants. There are a few exceptions and there are a few oxida-
tion ditch plants that have been removed from service for various opera-
tional reasons, but these are a small minority.
There are no apparent factors that would cause the rate of application
of oxidation ditch plants in the United States to decrease in the near
future. There will be cyclic variations from year to year, but the trend
should be increasing. Major changes in the EPA Construction Grants Program
or regulatory requirements could affect these trends.
-------
Prepared from th« following manufacturer's literature:
1001
tc
s
I
IL
i
IL
o
80
60
40
20
Lakssldt
Pastavant
Envirex
Chtrnt
1962
64
68
70
70
72
74
76
YEAR
Figure 1. Municipal oxidation ditch plant installations
in the United States.
-------
SECTION 2
DESIGNS AND APPLICATIONS
This section will review various process arrangements historically used
for oxidation ditch plants, arrangements used in the United States, design
parameters recommended by equipment suppliers and a technical review of the
designs. Present preliminary treatment and sludge handling and digestion
practices will be discussed. Each manufacturers' equipment will be des-
cribed and differences will be determined. Nitrification and nitrogen
removal is considered in Section 4.
There are many possible oxidation ditch plant configurations and oper-
ating modes. Oxidation ditch plants can be adapted to almost any variation
of the activated sludge process. Oxidation ditch plants are constructed to
provide a continuous ring shaped circuit(s) or channel(s) in which an
aerator (or aerators) is mounted to provide oxygen transfer and circulation
of the mixed liquor. The mixed liquor passes by the aerator at regular
intervals. The dissolved oxygen profile around the channel can vary signi-
ficantly which is important to certain operational modes, especially nitro-
gen removal as discussed in a following section.
Generally, oxidation ditch plants in the United States are of the
single channel design as shown in Figure 2 or of the multiple concentric^
channel design as shown in Figure 3.
PRETREATMENT
Generally, oxidation ditch plants contain some form of pretreatment
prior to the raw sewage entering the ditch. The following hap been used
with oxidation ditch plants.
1. Coarse Screening
a. Manually cleaned bar rack (typical opening size % to 1 in )
b. Barminutor w/bypass manual bar rack
2. Comminuter with bypass manual bar rack
a. Continuous
b. Oscillating
3. Grit Removal
a. Manually cleaned grit chamber
(1) Non aerated
(2) Aerated
-------
CIHCULAR
ELL
TYPICAL CHANNEL CONFIGURATIONS
TYPICAL PLANT
( Photo courtesy Lakeside Equipment Corporation)
Figure 2. Single channel oxidation ditch plant.
-------
RAW SEWAGE
TRANSFER PORTS
(betw*«n channels)
DECANT
DIGESTED
SLUDGE
TO FINAL
CLARIFIER
RETURN SLUDGE
TYPICAL SCHEMATIC (WITH AEROBIC DIGESTION)
TYPICAL PLANT
( Photo courtesy Envirex, Inc. )
Figure 3. Multiple concentric channel oxidation ditch plant,
-------
b. Mechanical grit chamber
(1) Non aerated
(2) Aerated
4. Raw sewage flow measurement, generally following other pretreat-
ment.
a. Orifice
b. Weir
c. Parshall Flume
Typical arrangements generally conform to the following.
1. Coarse screening followed by flow measurement.
2. Coarse screening, grit removal, and flow measurement.
3. Comminutor followed by flow measurement. If the comminutor is
not proceeded by a grit chamber, a gravel trap and floatable
trap is generally provided prior to comminution.
4. Grit removal, comminutor, and flow measurement.
The most common arrangement observed during the study was gravel trap,
comminution, and flow measurement.
Primary sedimentation is practiced at very few oxidation ditch plants
and would be considered an unusual variation.
OXIDATION DITCH PLANT ARRANGEMENTS AND DESIGNS
Most manufacturers have developed plant design criteria and suggested
plant layouts with a great deal of flexibility in design of plants so
that many design variations are possible to meet special treatment require-
ments or other special conditions.
The simplest arrangement is a single channel plant which is operated
to serve as an aeration ditch and final clarifier. Raw sewage flows into
the ditch with the aerator operating, and the effluent weir or valve is
positioned so no effluent flows out. When the maximum ditch operating
level is reached, the aerator is stopped. The raw sewage continues to
enter the ditch or is stored in a holding tank. During the time that the
aerator is off the ditch is used as a final clarifier and the solids in
the ditch settle. When a preset period of time has elapsed the super-
natant (effluent) is withdrawn until the ditch level drops to minimum. The
supernatant drawoff is completed, the aerator is restarted, raw sewage
flows into the plant, and the cycle is repeated.
A variation on this single ditch, intermittent effluent flow arrange-
ment is two ditch intermittent operation. The operation is identical to
the single ditch described previously except when one ditch is in the
settling and supernatant withdrawal mode, raw sewage is flowing into the
other ditch and the aerator is operating. There are many variations to
-------
this two ditch intermittent operation but the result is essentially the
same.
The configuration most common in the United States is an oxidation
ditch followed by a final clarifier. This configuration is operated on a
continuous flow basis rather than on a batch basis. Generally, the rate of
raw sewage flow into the ditch determines the hydraulic flow through the
plant; however, in some cases the rate of mixed liquor withdrawal from the
ditch is controlled so the ditch can provide some equalization storage.
When the ditch is used to provide equalization storage, the level varies,
and a floating or disc type aerator is normally used. This type of flow
equalization does not provide a constant flow rate through the plant, but
does dampen influent flow variations to the final clarifier. Most oxidation
ditch plants in the United States utilize the flow through configuration
where the raw sewage flow rate is passed on to the final clarifier. The
final clarifier is operated as in any other activated sludge process with
settled sludge returned to the oxidation ditch and a portion of the plant
sludge is periodically wasted from the system.
The ditch itself can be arranged in almost any configuration as long
as it forms a closed circuit. The most common is a single channel circular
or oval configuration as shown in Figure 2. The channel can be formed into
ells, horseshoes or other configurations to fit the site available. Typi-
cal forms are shown inF igure 2. Other plants are designed to utilize
multiple concentric, interconnected channels to form the treatment path
as shown in Figure 3. In some cases the "island" area on the inside of
the ditch is used for the final clarifier in order to conserve space and
use "common wall" construction. In some cases this "island" space is re-
duced in width to a single dividing wall. When a single, vertical dividing
wall is used, flow return baffles must be installed around all channel
bends. These baffles are vertical walls installed near the center of the
channel with a radius of approximately one half the channel width and ex-
tending in length around the bend.
Ditch lining can be of reinforced concrete, gunite, asphalt, thin mem-
brane, or unlined. Thin membrane linings have not been used to any extent
because the continuous velocities in the ditch have a tendency to lift or
move the thin membranes. One ditch lined with what appeared to be % inch
thick asphalt impregnated felt has been giving satisfactory service for 6
years. Some asphalt lined ditches in Canada are giving reasonably satis-
factory service. Unlined ditches are probably non-existant in municipal
applications because of the difficulty in stabilizing such ditches from
the effects of erosion. The most common ditch construction, based on
information available from the study, is reinforced concrete. Typical con-
struction for sloped side ditches consists of 4 in. of concrete poured
against earth backing. The concrete is typically reinforced with welded
wire mesh. The sloped side is usually at a 45 degree angle and the con-
crete is poured relatively dry and carefully finished so that face forms
are not required. Gunite can be used to form the ditch lining, but ex-
perience indicates the cost to be higher than reinforced concrete for this
application. Vertical ditch sides are generally reinforced concrete and
10
-------
must be designed for applicable structural conditions.
Normally, the single channel oxidation ditch is operated as an extended
aeration plant. However, more than one ditch could be constructed on one
plant site to form several process modifications such as standard activated
sludge plus aerobic digestion, two stage nitrification-denitrification, or
a three ditch contact stabilization plant.
The multiple concentric channel plant can have any number of channels,
but generally from two to five are used with four being common. Channel
depth is typically from 4 to 7 feet. These multiple channels are inter-
connected by submerged ports permitting flow from the outer ring to the
inner ring and then to the secondary clarifier. On larger designs, the
outer channel(s) can be used as an aerobic digester. In these cases, the
raw sewage influent is piped and valved so it can enter any of the first
three channels. The return sludge is pumped to either the first or second
channel. In all cases the flow of mixed liquor to the final clarifier is
from the inner channel. A typical schematic is shown in Figure 3.
The multiple channel arrangement may provide some flexibility over the
single channel for certain applications. For instance, some 4 channel mul-
tiple channel plants.are designed so that they can be operated in the
extended aeration mode, standard activated sludge plus aerobic digestion
mode, or contact stabilization mode simply by resetting gates and valves.
It is also possible to tailor the aeration to the requirements of each
channel. This could provide single stage biological nitrification-
denitrification as will be discussed in Section 4. The multiple channel
arrangement divides a large basin into efficient sized channels to minimize
turbulence and short circuiting and provides a maximum sized basin within a
minimum land area.
The drawoff of mixed liquor effluent from the aeration system of some
oxidation ditch plants is controlled by a submerged port or a narrow weir
which provides some equalization of short duration flow variations by
allowing a varying level in the aeration channels with varying raw sewage
flows. The effectiveness of this flow equalization depends on the duration
of the flow surges because as the channel level increases the flow through
the orifice or over the weir will increase. If the flow persists, the level
will eventually rise high enough to allow the full flow from the channel,
but short duration flows will be partially stored in the channel because
of the rising water level. Six to twelve inches of level variation are
possible depending on the plant and aerator design.
Aerators are marketed by several manufacturers. Aerators provide
oxygen transfer to the wastewater and also impart the horizontal velocity
to the channel. This horizontal velocity is imparted by the rotation of
the aerator in the upper part of the channel contents much like a horizontal
paddlewheel. The "Carrousel" system is slightly different because it uses
vertical aerators which impart channel velocity by centrifugal action. The
detailed construction of aerators varies by manufacturer, but typically
consists of an electric motor drive, speed reducer, and rotor with supports
11
-------
TABLE 2. TYPICAL SINGLE CHANNEL DESIGNS -
OXIDATION DITCH PLANTS
Average
Daily Flow
mgd
SHALLC
0.05
0.1
0.2
gpm
o
W, 45
35
70
139
SHALLOW, 45°
0.5
1.0
1.5
2.0
348
694
1,041
1,389
c
0)
3
rH
C
£H
SrH
"V^
fc 01
a) e
Pn
Q O
O XI O
OQ rH CN
SLOPED S
83
167
334
fc
i
rH
£
rH
0)
C ^j
C *W
id
6 8
CDE & ISLA1
6,640
13,280
26,560
SLOPED SIDE & VERT]
835
1,670
2,505
3,340
DEEP, VERTICAL SIDE Wi
0.20
0.50
1.00
2.00
5.00
10.00
139
348
694
1,388
3,470
6,944
334
835
1,670
3,340
8,350
16,700
66,400
132,800
199,200
265,600
LLLS
26,560
66,400
132,800
256,600
664,000
1,328,000
0
H
cr
H
H" -P
H
0) -
C X
£j ~P
id 04
ss
ID WA
4
5
5
CAL
6
6
6
6
12
12
12
12
12
12
O
•P
•P
m -P
rH
> -P
O •w
rH «.
o> x;
H tr>
•d c
6 3
92
101
188
276
374
552
537
80
161
209
248
473
918
rH
rH
&
0>
O -P
rH
0) -
C XJ
C -P
XJ -H
U *
42
52
54
49
69
69
93
31
37
57
81
125
125
*
0)
|£
25 "
M 4->
0 Cr>
•M C
S3.
1-4'
1-7'
2-8'
2-14'
2-24'
3-24'
3-32'
1-11 '
2-14'
2-24'
3-32'
4-54'
8-54'
Total
Drive Motor,
Horsepower
-P
O\ 0)
c e
-H 01
-P h
a -H
D D1
£3
2.5
5.2
10.3
25.0
50.8
76.2
101.7
10.4
25.0
50.8
101.7
259.0
518.0
0)
rH
rH
id
•P 0)
CO N
C -H
H CO
3
Ih
15
40
60
90
120
15
40
60
120
300
600
e
St
6 -P
(X U
CO O<
2 u
70
140
278
696
1,041
1,561
2,082
278
696
1,041
2,082
5,205
10,410
Channel Design:
Detention: 24 hr
Loading 12.5 Ib BOD/1,000 cu ft /day
Aeration Equipment:
OC: 2.35 Ib oc/lb BOD applied below 2,000' elevation
Source: Lakeside Equipment Corporation
12
-------
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-------
and bearings. One manufacturer uses an electro-hydraulic drive to provide
easy speed variation.
Most aerators operate within the rotational speed range of 50 to 110
rpm. Detailed construction of each manufacturer's aerator is described in
a following portion of this section. Aerators are normally installed with-
out protective covers, but in severe and moderately cold areas where spray
from aerators will freeze and accumulate, the aerators are generally fitted
with covers or complete enclosures which may be heated. A major considera-
tion is the accumulation of chunks of ice in the ditch which might damage
the aerator blades or brushes as the chunks pass through. A baffle at
water level upstream of each aerator may be helpful to keep ice chunks out
of the aerator.
Final clarifier design and application for oxidation ditch plants is
the same as for other activated sludge applications. The design overflow
rate should be in the range of 400 to 500 gpd/sq ft at average plant flow
and 1,000 to 1,200 gpd/sq ft at peak flows. Clarifier design solids
loadings should be 30 Ib/day/sq ft with provisions for continuous and
relatively uniform removal of sludge. Side wall depths should be at least
12 feet and preferably 12 to 14 feet. Clarifiers should be provided with
scum baffles and automatic skimmers. It is desirable to make piping
provisions so skimmings can be returned to the aeration ditch.
As a general rule, most small oxidation ditch plants (up to 0.5 to 1.5
mgd) are designed based on a 24 hour aeration period. The larger plants
may be designed for shorter aeration periods in the range of conventional
activated sludge plants. In these cases, some form of supplemental sludge
digestion is usually provided with aerobic digestion being most common.
Assuming a BOD loading of 0.17 Ib per capita per day, a detention time
of 24 hours, and a flow of 100 gallons per capita per day, the ditch load-
ing would be 12.7 Ib BODs/day/1,000 cubic feet. Assuming a volatile mixed
liquor suspended solids (MLVSS) range of 2100 mg/1 to 6300 mg/1 the food
to micro organism ratio (F/M) would be in the range of 0.10 to 0.034 Ib of
BOD per day per Ib MLVSS respectively. Sludge age (defined as the ratio by
weight of aeration system solids to influent solids) under these conditions
would be in the range of 10 to 33 days respectively. These criteria result
in a conservative design for treatment of domestic sewage.
SLUDGE HANDLING
Sludge handling is required at oxidation ditch plants when a portion
of the return sludge is wasted in order to maintain the systems solids
balance. Generally, this wasted sludge is highly stabilized when the plant
is operated in the extended aeration mode (24 hour aeration detention time).
If the plant is operated in the conventional activated sludge mode (6 to
8 hour aeration detention time), additional sludge stabilization such as
aerobic digestion may be necessary.
Some extended aeration mode oxidation ditch plants are operated without
any formal sludge wasting. Several plants were visited where the operator
had not wasted sludge for several years and it was claimed that the plant
14
-------
was able to meet effluent standards of 30 mg/1 suspended solids and BOD.
Generally, this mode of operation is only marginally satisfactory because
the plant is prone to periodic upset of the final clarifier resulting in
high final effluent solids.
In most oxidation ditch plants where the extended aeration process is
used (16 to 24 hours aeration detention) sludge is wasted directly to open
drying beds. In a few cases sludge is wasted directly to tank trucks which
spread the liquid sludge on the plant grounds or on adjacent land.
When an oxidation ditch plant is designed and operated as a standard
activated sludge process/ some form of sludge digestion is normally provided.
As discussed later, the larger Envirex plants are designed with an extra
channel which can be used for aerobic sludge digestion or as an additional
aeration channel. The Paris, Texas plant (Envirex) was the only plant
visited where separate aerobic digestion was practiced. All of the other
plants were operated in the extended aeration mode without separate sludge
digestion.
OXIDATION DITCH PLANT UNIQUE EQUIPMENT SUPPLIERS
Several manufacturers supply equipment for oxidation ditch plants.
Much of the equipment is standard to most wastewater treatment applications,
however, some is unique to oxidation ditch plants as described in this
section.
Each manufacturer has developed design and application information for
oxidation ditch plants using their unique aeration equipment.
Characteristics of the various aerators are shown in Table 4. Photo-
graphs of typical brush and disc type aerators are shown in Figures 4 and 5.
These photographs are reproduced from manufacturer's published literature.
Lakeside Equipment Corporation
This company was the first to manufacture and market oxidation ditch
plant equipment in the USA. Lakeside presently markets a "cage" type
aerator and small and large diameter "brush" type aerators (Mini-Magna and
Magna Rotors) in addition to other equipment. The cage and Mini-Magna
aerators are designed for use in shallow ditches and the Magna Rotor for
either shallow or deep ditches. The Magna Rotor is also manufactured in a
floating configuration. The immersion depth and speed of the aerators can
be changed in order to vary the oxygen transfer rate. The mounting of
aerators is normally fixed and the immersion depth is determined by the
channel liquid level. This liquid level is adjusted by the weir or other
device used to draw off mixed liquor from the channel. Floating aerators
have a mechanical means for adjusting immersion by changing the relationship
of the pantoons to the centerline of the aerator. Aerator speed is normally
factory set by the drive ratio between the motor and the aerator shaft.
Variable speed motors can be used, but have seldom been used in the past.
The aerators are driven by electric motors through gear reducer drives.
15
-------
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16
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(Photo courtesy Passavant Corporation)
Figure 4. Typical brush type aerator.
17
-------
(Photo courtesy Envirex, Inc.)
Figure 5. Typical disc type aerator.
18
-------
The gear reducers are generally of the oil bath, double reduction type and
the concentric planetary gear type for cage aerators.
The oxygen transfer capabilities and power requirements for the cage
and Magna Rotor aerators are shown in Figures 6 and 7 respectively. All
information was taken from manufacturers published literature.
Passavant Corporation
Passavant markets a series 5200 and series 5300 Mammouth aerator both
being brush type. The two series are very similar except the outside
diameter of the series 5200 is approximately 27 in. and the series 5300
is approximately 39 in. The series 5300 aerator is also manufactured in a
floating configuration.
The immersion depth and speed of the aerators can be changed in order
to vary the oxygen transfer rate in the same manner as with the Lakeside
aerators.
The aerators are driven by electric motors through oil bath, double
reduction gear reducer drives.
The oxygen transfer capabilities and power requirements for the Passa-
vant aerators are shown in Figures 8 and 9. All information was taken
from manufacturers published literature.
Envirex, Incorporated
The Pacific Flush Tank product line of Envirex includes oxidation ditch
plant equipment conforming to the Huisman Orbital System as developed and
applied in South Africa. Envirex manufactures aeration discs for use in
oxidation ditch plants.
The aeration discs are 4.5 feet in diameter and 1/2 inch thick perfor-
ated plastic mounted on line shafts. The two piece discs clamp on to the
shaft and can be added or removed in the field. The discs normally rotate
at 58.5 rpm through a motor driven gear reducer and final chain drive or
at 56 rpm through a motor driven gear reducer directly connected to the
disc shaft. Gear reducers are typically, oil bath, double reduction type.
The oxygen transfer capabilities and power requirements for the Envirex
discs are shown in Figure 10. All information was taken from manufacturers
published literature.
Walker Process
Walker Process Division of CBI markets a horizontal cage surface
aerator called the class 6227 ReelAer. This is a brush type aerator with
a diameter of approximately 38 in.
The minimum length of the ReelAer is 12 feet and maximum length is
19
-------
2.0
1.75
- 7
1.5
1
r 1.25
&
s
8
oc
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ec
S 0.75
0.5
0.25
- 6
3
E
« 5
i,
•
ul
H
C
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. 2
I I |
The data Indicate the rate of oxygen transfer In pounds
per hour, In tap water at 20°C., when oxygen concentration
Is zero.
i i |
Data from published Lakeside literature: Test method not specified^
POWER REQUIRED
60
70
80
90
100
AERATOR SPEED, r.p.m.
Figure 6. Lakeside Cage and Mini-Magna aerator characteristics,
20
-------
1.5
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X 4"
The data Indicate the rate of oxygen transfer
in pounds per hour, in tap water at 20 C;
when oxygen concentration is zero.
i
' Data from published Lakeside literature:
Test method not specified.
O2 TRANSFER RATE
POWER REQUIRED
50
60 70
AERATOR SPEED, r.p.m.
80
Figure 7. Lakeside Magna-Rotor characteristics,
21
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ui*
i
oc
Ul
IL
/
X
/
1.2
1.1
1"° fi
s
o
0.9 |
75
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0.7 I
0.6
0.5
0.4
o
Ul
oc
c
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0.3
0.2
0.1
0.0
AERATOR SUBMERGENCE, inches
Figure 8.
The data indicate the rate of oxygen transfer in pounds per hour,
in tap water at 20*C., when oxygen concentration Is zero.
Data from published Passavant literature: Test method not specified.
Passavant series 5200 aerator performance, 90 rpm.
22
-------
(0
I
i
uf
S
OC
UJ
LL
(/)
i
Q
111
oc
5
o
UJ
tr
OC
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s
Q.
AERATOR SUBMERGENCE, inches
The data indicate the rate of oxygen transfer
in pounds per hour, in tap water at 20 C.t when
oxygen concentration is zero.
Data from published Passavant literature:Test method not specified.
Figure 9. Passavant series 5300 aerator performance, 70 rpm.
23
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PERFORMANCE DATA
AT 760 MvHs. 10° C. 3500 MLS S, -=C« 0.85
PERFORMANCE DATA
AT 760 MM He, 20"C,
OPERATING
CONDITIONS
CONDITIONS
OXYGEN AT ION
CAPACITY
NET POWER
CONSUMPTION
Q ? / HP.-HOUR
AT SHAFT
EFFCENCY
RPM
G5
( Normal range 56 to 58 RPM)
Data from published Envirex literature: Test method not specified.
Figure 10. Envirex 4'-6" aeration disc characteristics
24
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25 feet. Two aerators may be driven by a single double shaft drive unit.
The ReelAer is driven by a direct coupled oil bath, double reduction speed
reducer which is belt driven by an electric motor.
Cherne Industrial, Inc.
Cherne manufactures and markets a floating perforated blade type aerator
called the "OTA Aerotor", shown in Figure 11, which is suitable for use in
oxidation ditches. The aerator is driven by a variable speed hydraulic motor
and is capable of speeds up to 110 rpm. A 15 Hp electric motor drives the
hydraulic supply system. The unit is available in one size only. The aerator
is 30 inches in diameter and 7 feet long and is constructed of reinforced
fiberglass. Oxygen transfer varies from 450 pounds of oxygen per day at
the low speed range to over 1250 pounds of oxygen per day at the high speed
according to Cherne. The oxygen transfer capabilities are shown in Figure
12 based on published literature.
The submergence of the aerator is also adjustable by adding water to
the floats to aid in further balancing of oxygen transfer rate to oxygen
demand and ditch velocity.
Cherne also markets a device to control the mixed liquor drawoff rate
to the final clarifier to provide relative constant flow rate operation of
the final clarifier. This unit is called the "Cherne Steady State Control"
and normally mounts in a chamber adjacent to the aeration channel.
Ti-.e "Steady-State Control" consists of 2 circular weir plates with
common float mounting. The submergence of the weirs is adjustable and
thus the flow rate is adjustable by means of a remote control panel. Once
the flow rate is adjusted, the weir assembly elevation varies with the
channel water level so that the set flow rate is maintained. A special
overflow is provided for excessive flows beyond two times the average daily
flow.
Cherne applies the floating aerator so that the aeration ditch can
absorb the diurnal raw sewage flow variations. The flow to the final
clarifier is then controlled to a relatively constant rate.
Envirotech Corporation
On June 1, 1976, Envirotech acquired licensing rights to the patented
"Carrousel" system for the United States and Canada. These rights include
access to the technical staff of Dwars, Heederik en Verhey, B.V., Amers.oort,
Dutch engineers who developed the system and to operating data derived from
more than 100 installations. The system, as shown in Figure 13f use^^.s
stationary vertical mechanical non sparged aerators rather than horizontal
brush or disc type aerators and ditches up to 15 feet deep. The vertical
aerators are mounted at 180 degree bends in the channel near the dividing
wall. The installation is designed so this aerator can impart ^city in
the channel. The previous licensee, Envirobic Systems, Inc. did not market
plant equipment; but only the right to construct a Carrousel plant, access
25
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[CONCRETE SUPPORT SPANS CHANNEL|
IHYORAUUC POWER PACK!
SIDE VIEW
(Courtesy of Cherne Industrial, Inc.)
Figure 11. Cherne "OTA Aerotor" aerator configuration
26
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OXYGEN CAPABILITY
BASED ON TEST RESULTS
S
S 8
«
1
« 7
I
o01
& 4
u
oc
UJ
u.
2 2
4.5
4.0
3.0
2.5
2.0
1.5
1.0
.5
oc
5
o
UJ
ff
oc
UJ
70
80
90
100
110
AERATOR SPEED, rpm
Data from published Cherne literature: Test method not specified.
Figure 12. Cherne OTA aerator characteristics,
27
-------
Low Speed
Aerator
(Courtesy of Envirotech Corporation)
Figure 13. Envirotech "Carrousel" schematic
28
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to technical data, and Carrousel pilot plant equipment. Envirotech will
license the design and construction of Carrousel plants. This license fee
typically is paid to Envirotech as a bid item in the construction contract.
The equipment is procurred by open bid and can be supplied by a number of
different manufacturers.
Carrousel literature indicates approximately 139 plants <"**•* «
operating in Europe and 154 in the world as of October 1975. One Carrousel
plant treating tannery waste has been in operation in Winchester, New
Lopshire since January 1977. Approximately six others are under design
in the United States.
EVALUATION OF DESIGNS
Pretreatment
Screening seems to be the single most important pretreatment step. If
rags, boards, and other similar objects are not removed prior to the ditch
they will cause trouble with the aerators and will, in most cases, plug
sludge control valves (telescoping valves), sludge lines, sludge pumps, and
weirs. Few problems were observed or reported directly related to inade-
quate grit removal although it is sure that this grit accumulates in the
ditch and will have to be removed at some time. Grit was not a significant
operational consideration for oxidation ditch plants up to 1 mgd or more
in size.
Aerator Design and Applications
The characteristics of each manufacturers aerator is shown in Table 4.
Based on a review of the physical characteristics of the aerators it is
difficult to see any significant difference between the Lakeside Magna ,
the Passavant Series 5300, and the Walker ReelAer aerators The Lakeside
Mini-Lgna, Cage, and Passavant Series 5200 aerators are also very similar.
For evaluation purposes aerators are classified in the following
categories:
1. Small diameter horizontal aerators
a. Lakeside Cage
b. Lakeside Mini-Magna
c. Passavant Series 5200
d. Cherne OTA Aerotor
2. Large diameter horizontal aerators
a. Lakeside Magna
b. Passavant Series 5300
c. Walker ReelAer
3. Envirex 4'-6" disc aerators
All of the aerators will operate over a variable submergence, thus
29
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allowing some variation of level within the ditch. Cherne, Passavant, and
Lakeside supply floating aerators which would be applicable in cases where
substantial variations in level were expected, where it might be desireable
to maintain constant aeration submergence with a varying ditch water level,
or where the flexibility of a movable aerator is needed.
The choice between aerators in the small and large diameter horizontal
aerator groups is largely one of individual preference and, in most cases,
will be determined by the bidding situation. Generally, the small diameter
horizontal aerators are limited to use in the shallower ditches and the
large diameter are used in shallow or deep ditches.
The large 'diameter horizontal aerators are available in lengths up to
30 feet. Field experiences with deformation and failure of torque tubes
would suggest care in specifying the longer aerators.
The Envirex disc aerator is somewhat unique to other manufacturers
equipment and to date, in the U.S., has been used only in the multiple
concentric channel type plant as far as the authors can determine. Disc
aerators could be used in single channel type plants. The discs may offer
some advantage over the other types of aerators. Discs can easily be added
to or deleted from the shaft in the field because they come in two halves
and clamp to the shaft. This allows the aeration to be modified in the
field.
The Cherne OTA Aerotor does have a standard built-in variable speed
feature which may be of advantage in some cases. The other manufacturers
could supply variable speed motors or drives in all probability, *but this
feature was not found at any operating facilities.
There are many available configurations of plants and aerators and it
is recommended that care be taken to select the most energy efficient con-
figuration because aerator energy is a major operating expense for oxidation
ditch plants. There should be little actual difference between manufac-
turers aerator power requirements for given size plants. Some variation was
noted, however, and this may be because the illustrative plant designs were
not completely optimized for each manufacturers equipment and design recom-
mendations .
Analysis of energy requirements from actual operating plants showed no
consistent advantage for a particular manufacturers equipment.
A composite range of aerator oxygen transfer efficiency under standard
conditions is shown in Figure 14. These transfer tests were conducted by
various manufacturers and laboratories at different times, in different
facilities, but under similar stated test conditions. There is no easy way
to evaluate the comparability of the results between aerator manufacturers,
so the design engineer must use his judgement or will have to run indepen-
dent tests.
Figure 14 is based on manufacturers published performance information
30
-------
i
o
ui
u
\L
\L
ENVELOP INDICATES RANGE OF VARIOUS
AERATORS BASED ON MANUFACTURER'S
PUBLISHED LITERATURE.
SUBMERGENCE, inches
Assumptions:
1. Tapwater, 20* C, starting 02 concentration a 0
2. Based on manufacturer's published literature shown in figures 6 through 11
Figure 14. Horizontal aerator efficiency.
31
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and is non specific; being intended to show the range for horizontal type
aerators in general. At typical operating depths of six to nine inches for
the brush and cage type aerators, the efficiency varies from 3 to 5 pounds
oxygen per horsepower hour per lineal foot of aerator length as shown in
Figure 14. The data are not consistent between manufacturers probably due
to differences in test conditions.
There seems to be a great deal of inconsistency among the manufacturers
data relating to performance of aerators. The field investigation did
indicate that most plants were able to maintain 2 to 6 mg/1 of dissolved
oxygen in the aeration channel and that mixing was adequate. The informa-
tion and design recommendations provided by manufacturers for application
of aerators is generally adequate based on observations of operating
plants. There was no indication that aeration capacity was undersized for
any of the plants.
The site visits confirmed that the real problems (an* differences) are
mechanical; the drive unit, line bearings, seals, aerator torque tube,
materials of construction and similar considerations. These are the fea-
tures that will have the greatest impact on long term operation and main-
tenance of the plant.
The mechanical features should receive careful consideration in design
and preparation of specifications. Most of the bearings associated with
aerators are of the ball or roller type; grease lubricated with seals.
Therefore, bearings and drive units should be protected from splashing
water both to keep the water off and to provide convenient access for main-
tenance personnel. Extension of bearing grease fittings up to convenient
locations on catwalks or easily accessable locations will certainly contri-
bute to more satisfactory maintenance. Means for field alignment of
bearings should be provided and bearings should be carefully aligned prior
to operation. The bearing supports for horizontal aerators may be up to 30
feet apart and differential settling with eventual loss of alignment may
occur if this is not considered in the structural design of the aerator
support foundation. Generally, bearings with some self alignment capability
will provide more satisfactory long term service, but this feature does not
reduce the need for proper structural design and initial alignment.
Some means must be provided for removal of the aerator; either a per-
manently installed lifting mechanism, a portable lifting mechanism, or
access for mobile lifting equipment.
Many problems were caused by the aerator "slinging" mixed liquor onto
adjacent structures and onto aerator bearings, couplings, seals, and drive
units. Most aerators have "slingers" at each end, but they only compound
the problem when the wind is blowing as liquid flying off the "slinger" is
blown by the wind. Mixed liquor which lands on adjacent walkways causes
ice formation when it is cold and algal growths cause slippery conditions at
other times. Bearings and drive units should be adequately shielded from
the inevitable liquid spray.
32
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Consideration should be given to installation of at least two aerators
in each ditch so that a standby is available when troubles occur.
An adequate stock of spare parts for aerators and other equipment
should be maintained or readily available to plant personnel so down time
is minimized. This includes aerator bearings, seals, couplings, and belts.
Additional information is provided in Section 5.
Evaluation of Oxidation Ditch Plant Design Procedures
The oxidation ditch is an unusual process because of the aeration basin
geometry, but results from operating plants indicate consistently high per-
formance results without any more than average operator attention. Costs
are equal to or less than competing treatment processes.
The long narrow aeration channel of the oxidation ditch plant provides
a complete mix activated sludge process. Even though a plug flow mode
would seem to be applicable, the minimum velocity of 1.0 fps results in a
cycle time of less than 15 minutes in the longest channels used. Compared
to the typical 8 to 24 hour detention time design, the channel circuit time
becomes insignificant. Therefore, the oxidation ditch may be considered a
completely mixed activated sludge process (CMAS) with respect to organic
load. It is not completely mixed with respect to dissolved oxygen profile
because this profile can vary significantly around the channel and verti-
cally within the channel. The mixed liquor can pass through varying oxygen
rich or deficient zones as it passes around the channel, however, which is
important when considering nitrogen removal as discussed in Section 4.
The procedure for design of an oxidation ditch plant is basically the
same as used for an extended aeration process with emphasis given to the
hydraulic considerations imposed by the basin geometry. The following is
a rational design procedure which is consistent with manufacturers design
procedures. The manufacturers can provide assistance in design for specific
applications.
Some low alkalinity wastewaters may require pH adjustment when sub-
jected to extended aeration.
Oxidation ditch plants are designed for long sludge retention times
and nitrification will occur if sufficient oxygen is provided. This
design procedure assumes that sufficient oxygen will be provided for com-
plete nitrification in addition to satisfying the carbonaceous BOD require-
ments. Complete mix activated sludge plant design procedures have been
described by McKinney(1), MonodU) and Eckenfelder ^' and presented in a
unified model by Goodman(4'5). It is frivolous and time consuming to
compare the nuances of these various models; proper use of any of these
models results in sufficiently accurate results. General relationships
derived from the unified model1 } will serve the purposes of this discus-
sion,
Carbonaceous BOD stabilization requires oxygen for conversion of the
33
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organics to CO , water, and bacterial cells. In addition, oxygen for endo-
genous respiration of the cells is required, depending on the solids reten-
tion time (SRT) which is defined as the ratio by weight of aeration system
solids to wasted solids (waste sludge plus effluent solids) . A generalized
relationship between Ib oxygen/lb BOD versus SRT for a 24 hour detention
basin is shown in Figure 15 .
As the SRT approaches 20 days the oxygen requirement becomes asymptotic
to the value, 1.25 Ib oxygen/lb BOD applied and the oxygen for nitrifica-
tion is approximately equal to 4.5 Ib of oxygen per Ib of ammonia nitrogen
oxidized. Therefore, the total oxygen requirement may be calculated as
follows.
Ib oxygen = 1.25 x Ib BOD5 + 4.5 x Ib NH3-N
. For example, the oxygen requirements for removal of 200 mg/1 of BOD
and 30 mg/1 of ammonia nitrogen from a one mgd flow of wastewater are:
BOD = 200 x 8.33 x 1 = 1667 Ib/day
NH3-N = 30 x 8.33 x 1 = 250 Ib/day
Oxygen Required = 1667 x 1.25 + 250 x 4.5 = 3209 Ib/day
The manufacturers typical design procedure for circulation would pro-
vide a 1 million gallon basin and 48 lineal feet of aerator. The aerator (s)
would be driven by a combined power of 60 horsepower and would operate at
approximately 51 horsepower. This is comparable to conventional activated
sludge requirements.
If the following design conditions were established:
Minimum Basin Dissolved Oxygen 2.0 mg/1
Elevation 500 ft
Alpha a 0.8
Beta 6 ' 0.95
cs oxygen saturation concentration, mg/1
T 20C
KLa mass transfer coeff.
The calculated oxygen transfer capability of this aerator to pure
water at standard conditions would be:
do (T-20}
- U ZU)
s
-
T- = a Ka x 1.024U ZU) (3C -C)
at L
34
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1.5
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IU
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1.0
0.75
0.5
6 8 10 12 14 16 18 20
SRT, days
Based on calculations using tht unifltd model (*).
Figure 15. Carbonaceous BOD oxygen requirements
35
-------
do . . 3209 Ib/day
-r- required = / * = 16.05 mg/l/hr
24 x
16.05 = 0.8 K a x 1 (0.95 x 9.1 - 2.0)
L
KTa = 3.03
L
do
37- STD = K a x C
dt L s
do
— STD = 3.03 x 9.2 =27.9 mg/l/hr = 5578 Ib/day
Manufacturers literature indicates a typical average oxygen transfer
rate of 5 pounds per hour per lineal foot of aerator or a total transfer
rate of 5760 Ib of oxygen per day.
This calculation indicates that for the average design condition the
48 lineal feet of aerator will provide the required oxygen. If the oxygen
supply is not adequate to satisfy both the carbonaceous BOD and nitrifi-
cation demands under certain operating conditions, less than complete
nitrification will take place during these conditions.
The design basis of 24 hours detention is not sacred. It appears the
origin of this detention time is from the extended aeration activated
sludge process and represents a convenient convention for municipal appli-
cations with a typical loading of 12.7 Ib BOD per 1,000 cu ft of aeration.
The process needs for oxidation ditch or extended aeration plants
include:
1. A stable operating process
2. Conservative supply of oxygen for process
3. Stabilized waste activated sludge
The stable operating process requirement is served by providing a
sufficiently large aeration basin that peak flows will not cause extreme
variations in oxygen demand or unusual shifts in solids inventory between
the aeration basin and sedimentation basin.
For small plants, variations in organic and hydraulic loading are gen-
erally more extreme. However, aeration basin detention time will have only
a modest effect on the percent increase in oxygen uptake rate caused by
short duration organic load variations. A comparison between a conven-
tionally designed 6 hour detention aeration basin and a 24 hour detention
aeration basin is shown in Figure 16 for an average loading condition and
a short term peak load condition. The short term peak load imposed
36
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represents a sudden doubling of flow, and mass BOD. and ammonia. The follow-
ing illustrates the assumptions and calculations in developing Figure 14.
For a 1 mgd flow and 24 hour detention
Mass
BOD,
200 mg/1 =
NH -N = 30 mg/1
Hourly uptake rate
1,667 Ib/day x
250 Ib/day x
16 mg/1
Oxygen Demand
1.25 = 2,084 Ib/day
4.5 = 1,125 Ib/day
3,209 Ib/day
When the load to plant is doubled (Q = 2 mgd, BOD = 220 mg/1,
NH3-N = 31 mg/1) the immediate oxygen demand will increase due to the in-
cremental increase in load consisting of the conversion of ammonia to
nitrate and synthesis of organics. The short term load change will have
little immediate effect on the basin active bacterial fraction because this
fraction was based on average loading and can not increase immediately.
Therefore, the oxygen demand will reflect only synthesis of the incremental
BOD. The immediate increased oxygen demand for the incremental increase
in mass load is calculated as follows:
Nitrification = 250 Ib/day more x 4.5
Synthesis
«• 1,670 Ib/BOD /day more x 0.5
Total oxygen demand: Base 3,209 Ib/day
Increment 1,960 Ib/day
5,169 Ib/day
Assume 4 mg/1 dissolved oxygen at 16 mg/l/hr
16
= 1,125 Ib/day
'• 835 Ib/day
1,960^ Ib/day more
26 mg/l/hr
KLA =
a (0Cs-4)
16
0.8 (0.95 x 9.1-4)
= 4.3
At 26 mg/l/hr
C = 9.1 8 -
(0.8) (15)
0.7 mg/1
38
-------
A - 3.3 mg/1
10 mg/l/hr
= 0.33 hr or 19 min
For a 1 mgd flow and 6 hour detention
SRT = 5 days
Ib 02/lb BOD5 =1.0
Oxygen Demand
BOD = 200 mg/1 = 1670 Ib/day x 1.0 = 1670 Ib/day
NH -N = 30 mg/1 = 250 Ib/day x 4.5 = 1125 Ib/day
2795 Ib/day
Hourly uptake rate = 56 mg/l/hr
Incremental doubling of load
Oxygen Demand
Nitrification = 250 Ib/day x 4.5 = 1,125 Ib/day
Synthesis = 1670 Ib/day x 0.5 = 835 Ib/day
1,960 Ib/day
Total oxygen demand: Base 2,795 Ib/day
1,960 Ib/day
4,755 Ib/day = 95 mg/l/hr
Assume 4 mg/1 dissolved oxygen at 56 mg/l/hr
KLA = 56
a (304)
56
0.8 (0.95 x 9.1-4)
= 15
At 95 mg/l/hr
C = 9.1 f 95
a KLA
(0.8) (15)
= 0.3 mg/1
39
-------
A * 3.7 mg/1
39 mg/l/hr
= 0.095 hr or 5.7 min
The 24 hour detention basin (at average flow) would experience a 63
percent increase in oxygen demand, from 16 mg/l/hr to 26 mg/l/hr. If the
oxygen concentration in the basin were 4 mg/1, it would eventually drop to
0.7 mg/1 at the higher uptake rate and have a 3.3 mg/1 buffer, or at least
18 minutes at the increased uptake rate for the excess basin dissolved
oxygen to absorb the added load.
The 6 hour detention basin (at average flow) experiences a 70 percent
increase in oxygen demand, from 56 mg/l/hr to 95 mg/l/hr. If the oxygen
concentration in the basin were 4 mg/1, it would eventually drop to 0.3
mg/1 and would have a 3.7 mg/1 buffer which represents 6 minutes to absorb
the added load at the increased uptake rate.
The greater detention period will result in a slightly more stable
system for short term variations in organic load.
Most plant upsets are caused by loss of solids from the final clari-
fier either by poor solids inventory management or marginal designs. The
use of longer detention periods provides significant advantages in main-
taining good quality effluent with variations in hydraulic load. An
example is shown in Figure 17. The calculations for Figure 17 are simpli-
fied by assuming no sludge wasting, no loss of solids in the effluent
during the transition period, and a constant sludge return rate. It is
also assumed that the flow increase occurs after a period of stable opera-
tion at average flow. In reality, most such increases would be after a
period of stable operation at substantially less than average flow.
Most small plants are operated with a set, or fixed, return sludge
flow rate. Typically, small plants are designed for a maximum return
sludge flow rate of 100 percent of the average daily plant flow and oper-
ated at an average of 30 to 50 percent return sludge flow rate.
The comparison shown is between an aeration basin having 24 hours
detention versus an aeration basin having 6 hours detention both with con-
ventionally designed sedimentation basins. At night, when influent rates
are low, the system solids tend to shift to the aeration basin since the
solids flux to the final sedimentation basin is low and the recycle rate
is constant. When the daily peak flows occur, the solids shift to the
final sedimentation basin. The critical consideration is preventing the
filling of the final sedimentation basin with solids and then spilling
over into the effluent. The example in Figure 17 depicts the percentage
of the final sedimentation basin which is used for solids storage.
The 24 hour detention aeration basin under typical operating condi-
tions will result in only 18 percent of the volume of the final sedimenta-
tion basis occupied by sludge. A sudden increase in flow (2 times average)
40
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41
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will cause a greater concentration of solids in the final sedimentation
basin and a dilution of the solids concentration in the aeration basin.
After about 1 hour under this condition the solids in the aeration basin
will decrease from 16,660 pounds to 16,000 pounds.
The loss of solids from the aeration basin will, of course, be added
to the final sedimentation basin, increasing the inventory from 740 pounds
to 1400 pounds. The volume occupied by the solids will approach 34 percent
of the final sedimentation basin volume. The solids loading rate will
increase from 13 to 24 Ib/day/sq ft.
In the conventionally designed plant, the same circumstances will
cause the volume occupied by the sludge in the final sedimentation basin
to increase from 36 percent to 63 percent. The solids loading rate in-
creases from 27 to 43 Ib/day/sq ft. Therefore, as the extended aeration
plant remains within reasonable operating parameters for high quality
treatment, the conventionally designed plant approached marginally accept-
able conditions. This is a major reason for the apparent reliability of
oxidation ditch plants especially if the final clarifier is conservatively
designed. In effect the conventionally designed plant would require oper-
ational procedures to adjust for the change in hydraulic load, such as
varying the sludge recycle rate.
The shift in solids inventory is actually more pronounced than the
example depicts because the peak daily hydraulic load does not occur after
establishment of equilibrium conditions under the average hydraulic load,
but occurs after the night time minimum hydraulic conditions. During the
night the solids inventory shifts to the aeration basin. The greater
solids concentration in the aeration basin at onset of peak hydraulic load
causes a higher final sedimentation basin solids influx than depicted.
The management of solids inventory for the conventionally designed plant is
as important during minimum flows as during maximum flows to compensate
for this effect.
Final Clarification Evaluation
Many of the final clarifiers observed during the field visit program
were operating within the design range outlined in this section and were
producing a satisfactory effluent. Those final clarifiers that were not
performing properly were generally deficient in physical design such as
sidewater depth or were operating at excessive overflow rates or solids
loading rates. It is felt that the application of final clarifier design
parameters outlined in Section 2 will generally provide for satisfactory
operation.
If possible, two clarifiers should be considered although most plants
visited had only one clarifier. It is nearly impossible to service a single
final clarifier without discharging partially treated wastewater. Clari-
fiers should be inspected and serviced at least annually.
42
-------
Solids Production and Treatment
Any biological plant design must consider the quantity of sludge pro-
duced, the nature and stability of the sludge, and suitable disposal
procedures. An oxidation ditch plant, operated in the extended aeration
mode, has certain inherent advantages relative to sludge handling and dis-
posal. Oxidation ditch plants can be operated at a 20 to 30 day SRT
resulting in a sludge having characteristics similar to a well stabilized
aerqbically digested sludge. A conventional activated sludge plant operated
at a 4 to 10 day SRT will produce a sludge with high residual biodegradable
organic content. If placed on drying beds or on the land, it will become
odorous and objectionable. Aerobic digestion of this sludge for 7 to 15
days is normally required to produce a stable product suitable for disposal
on drying beds or the land. For a properly operated conventional activated
sludge plant plus aerobic digestion the total SRT prior to disposal will be
15 to 20 days. In effect, the oxidation ditch extended aeration process
provides sludge stabilization equivalent to conventional activated sludge
plus aerobic digestion.
In most oxidation ditch plants where the extended aeration process is
used (16 to 24 hours aeration detention) sludge is wasted directly to open
drying beds. In a few cases sludge is wasted directly to tank trucks
which spread the liquid sludge on the plant grounds or on adjacent land.
During this study, no odor problems were encountered at plants using either
method. The author visited plants using both methods and detected no
noticeable odors or any other indication of nuisance complaints. The field
inspections consistently confirmed the lack of odor problems with sludge
from the extended aeration oxidation ditch plants. Several plants com-
plained of insufficient drying bed capacity. Because of poor dewaterability
especially during periods of cold or wet weather. Adequate drying bed
capacity must be considered carefully in design.
Some design engineers and regulatory authorities may require additional
sludge stabilization for oxidation ditch plants operated in the extended
aeration mode. Part of the consideration may be the possibility of periodic
poor operation of the process. Also, future flow increases may force oper-
ation of the plant at shorter SRT's to the point where additional sludge
stabilization is required.
The quantity of sludge produced is related to the characteristics of
the incoming wastewater solids. Normally, wastewater solids contain frac-
tions that are inert, volatile and nonbiodegradable, and volatile and bio-
degradable. The inert (nonvolatile) and volatile/nonbiodegradable will
accumulate in the system solids inventory in proportion to the SRT. Normally
20 to 25 percent of the raw waste suspended solids are inert. The remaining
75 to 80 percent are volatile solids with, typically, 30 to 40 percent
nonbiodegradable. Therefore, about half of the incoming suspended solids
are not subject to biological action and will accumulate in the mixed liquor
in proportion to the SRT. The effect is illustrated by assuming the
following conditions:
43
-------
Raw wastewater BOD = 200 mg/1
Raw wastewater suspended solids = 200 mg/1
Nonbiodegradable suspended solids =100 mg/1 (50% of raw suspended
solids)
Aeration detention = 24 hours
The nonbiodegradable suspended solids in the raw sewage are 0.5 Ib/lb
BOD .
The remaining (biodegradable) suspended solids and soluble organics
are synthesized into bacterial cells and loose their original identity.
The 100 mg/1 of biodegradable suspended solids in the assumed waste will
produce approximately 0.7 Ib of bacterial cells per pound of BOD at low
SRT. As the SRT increases the biodegradable solids are reduced until at
very high SRT only about 0.15 Ib of bacterial cells per Ib of influent
BOD5 remain. This remaining portion of the bacterial cells is nonbiode-
gradable. The total solids production at low SRT would be 1.2 Ib/lb 8005
(0.5 Ib/lb 8005 nonbiodegradable influent plus 0.7 Ib/lb BOD5 of bacter-
ial cells) and at high SRT would be 0.65 Ib/lb BODs (0.5 plus 0.15). For
various SRT, based on these example conditions and the absence of primary
sedimentation, the solids production has been calculated^) an(j is shown
in Figure 16.
The solids production is also shown with primary sedimentation pre-
ceeding aeration using the same general assumptions plus an assumed 50
percent suspended solids and 30 percent BOD removal in primary sedimenta-
tion.
If the effluent suspended solids concentration averaged 25 mg/1, the
remaining sludge production would normally be removed from the system by
the waste activated sludge (WAS). For example, with an SRT of 20 days,
the suspended solids production for the raw wastewater condition is 0.68
Ib/lb BOD5. At a final effluent solids concentration of 25 mg/1 the
effluent solids would represent 0.125 Ib/lb of influent BOD5. Therefore,
sludge wasting would be based on 0.56 Ib/lb BOD applied.
The SRT, aeration detention time and MLSS are related as shown in
Figure 18 for the conditions assumed. This relationship would apply to
a given plant (where aeration detention time would normally be a physical
constant) and also illustrates the interrelationship of these parameters
for various activated sludge plants. The SRT is a direct function of MLSS
and aeration detention time. If aeration detention time is a constant,
the SRT and MLSS are directly related. This graphical relationship assumes
steady state operation with sludge wasting rate equal to incoming solids.
Though wastewater solids are continually being synthesized, it is
possible to store activated sludge in the aeration system by allowing the
mixed liquor suspended solids to increase. Solids can then be wasted on
44
-------
\s
sooa qi/qi 'aaonaoud sanos Auvanooas
s
lids production
iquor relationships and
xed
process
Biologica
00
iH
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fc
CM oCft0^- «o in « m CM
*JH '3WI1 NOI1N313Q NOIlVd3V
45
-------
a periodic schedule convenient with other plant operations or with sludge
disposal operations. It is recommended that sludge be wasted on a regular
schedule, however, to assure that sludge solids do not accumulate and cause
operating difficulties. Adequate and simple sludge return should be pro-
vided. Sludge control (telescoping) valves should be large enough so they
do not plug with material passing the plant screening equipment. The sludge
return rate should be measured, adjustable and of minimal mechanical complexity,
Generally, non-clog centrifugal pumps have proven satisfactory. The re-
turn pumps can be eliminated if sludge can be returned back to the wet well
of the raw sewage lift pumps.
Some extended aeration plants do not formally waste sludge at all,
however because of the nonbiodegradable content of the wastewater suspended
solids it is obvious that the mixed liquor suspended solids will continue
to increase. Eventually, some solids must be released from the plant
either through high effluent suspended solids levels or a major plant
upset.
46
-------
SECTION 3
PERFORMANCE AND RELIABILITY
GENERAL
As a group, the oxidation ditch plants appeared to perform consistently
well in spite of limited operation and maintenance in a number of cases.
All plants appeared to have adequate aeration capacity, adequate velocity
in the ditch, an acceptable mixed liquor and a lack of odor or other nuis-
ance. In general, the operators were able to obtain good treatment results
in almost all cases. Sludge handling and disposal was relatively simple
and trouble free, however, in some cases operators reported difficulties
in dewatering the sludges on open drying beds due to lack of proper bed
area or due to slow dewatering of the sludge. The plants were easy to keep
in service and would operate for long periods of time with little operation
and maintenance. Many plants operated for periods of time unattended (even-
ings and weekends) without significant problems. No particular type of
oxidation ditch plant seemed to stand out as superior or substandard; the
relative performance depended on many more factors such as original design
criteria (clarifier surface overflow rate for instance) and operational
procedures. There are some exceptions to these comments and a few oxida-
tion ditch plants have been removed from service because of operational
problems. These cases are the exception.
Performance and reliability data for oxidation ditch plants were
developed from actual plant operating records, obtained from published
literature, telephone and letter contact with operating plants, visits to
operating plants, EPA records, special studies, and the contractor's files.
There were very few plants where complete data could be obtained for all
desired parameters. In addition, reliance had to be placed on the sampling
and analysis methods used by various plant personnel. It is recognized
that these procedures are not consistent from plant to plant and therefore,
not always directly comparable. Nevertheless, the results presented herein
represent the data as obtained without modification. Inordinately high or
low readings were not removed from the data during compilation because
most plants experience these variations at one time or another. It is
felt that these variations do occur in oxidation ditch plants as in most
other activated sludge plants. In some cases a very limited number of
data points was available and this is noted in the supporting tables to
follow.
47
-------
PERFORMANCE
Oxidation ditch plant performance was developed primarily from monthly
average data. Generally, the data from a plant were not used unless several
data points were available. Plant daily performance data, when available,
were converted to monthly averages, which were then analyzed for each plant.
The sources of data and number of monthly average data points used are
indicated in Table 5 along with some pertinent plant parameters. Where
possible, the performance is calculated for both summer and winter. Winter
is arbitrarily determined to be the months of November through March. The
"average" performance was determined by averaging the performance of all of
the individual plants. The high and low individual plant performance is
shown to establish the range limits of individual plants.
Performance averages are summarized in Table 6. Performance data
points as a function of plant size are shown in Figure 19. Both BODs and
suspended solids removal appear to be relatively independent of plant
capacity. There is some indication for BODs removal that small and large
plants perform slightly better than mid-sized plants, but there are few
data points for plants over 1 mgd in size. The small plants are performing
well for BODs removal in comparison to other size plants.
RELIABILITY
The reliability curves were developed from actual daily plant data.
Averaged data was not used because averaging would remove the data peaks
and minimums. In general, the same comments relating to quality of data
apply here as with the performance data. Readings below 5 mg/1 BODs were
considered to be 5 mg/1 because of analytical limitations inherent in the
0 to 5 mg/1 range. This assumption has a minimal effect on the reliability
curves and would affect only the low end of the curves. Reliability curves
were plotted for summer, winter, and total year for the average of all
plant data. In addition, the best and worst plant reliability data is
plotted. The sources of data used for the reliability curves are shown in
Table 7. The reliability is shown in Figures 20 through 23.
There is a slight trend of somewhat improved 8005 and suspended solids
reliability in the summer over winter conditions. There is also a signi-
ficant variation in reliability between the best and worst plants. The
reliability of the best plants is outstanding and approaches that of acti-
vated sludge plus effluent filtration as shown in Figure 30. Even the
worst plants are able to meet a 20-20 standard 50 percent of the time and
a 30-30 standard 70 to 80 percent of the time.
The COD and total nitrogen reliability is based on a limited avail-
ability of data, primarily from the Dawson, Minnesota plant (23). The
Dawson plant was operated in a unique mode during the period represented
by these data and, therefore, any use of these data should be preceded by
a study of the referenced report. Various considerations relating to
nitrification and nitrogen removal are covered in Section 4.
48
-------
TABLE 5. SOURCES OF PERFORMANCE DATA
Number of monthly average data points
used in performance calculations
m m
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Average 0.73 0.54
Lakeside
Passavant
Envirex
Grab samples - not monthly averages - these were averaged by month.
Data not available
CNCNCNCNCNCNCNCNCNCN
49
-------
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100
90
80
too
70
90
60' 8(
, SUSPENDED SOLIDS
0.1
0.2
0.4 0.6 0.8 1
6 8
PLANT CAPACITY, mgd
Figure 19.- BOD and TSS removal performance,
52
-------
90
SEE TABLE 9 FOR SOURCES OF DATA
2 5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 20. Oxidation ditch plant BOD5 reliability.
53
-------
to
CO
w
u.
u.
UJ
90
80
70
60
50
40
30
20
10
SEE TABLE 7 FOR SOURCES OF DATA
2 5 10 20 30 40 50 60 70 80 90 95 98
PERCENT OF TIME VALUE WAS LESS THAN
Figure 21. Oxidation ditch plant suspended solids reliability.
54
-------
8
Z
UJ
-I
IL
IL
Ul
160
140
120
100
80
60
40
20
SEE TABLE 7 FOR SOURCES OF DATA
10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 22. Oxidation ditch plant COD reliability.
55
-------
90
80
- 70
ff
«
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C 60
<
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50
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3
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ui 40
30
20
10
SEE TABLE 7 FOR SOURCES OF DATA
2 5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 23. Oxidation ditch plant total Nitrogen reliability.
56
-------
Typical dissolved oxygen and mixed liquor data are shown in Table 8.
These data indicate that adequate dissolved oxygen levels are being main-
tained by the aeration equipment, however a number of the plants are
operating below design flows as shown in Table 5. The low dissolved oxygen
reported for the Dawson plant was intentional because of the special mode
of operation. The mixed liquor suspended solids data are typical for acti-
vated sludge plants. At the time of data collection two of the plants
were being operated at abnormally high MLSS, but operation was satisfactory
in both cases.
OXYGEN UPTAKE RATE
It was nearly impossible to develop meaningful oxygen uptake rate
information from data available from the plant visits. Detailed on site
uptake rates.were measured at Berthoud, Colorado under EPA Contract
68-03-2224T . These tests indicate a mixed liquor uptake rate of 12
mg/1/hour or a total of 1,375 Ib/day for the plant at an average flow of
0.69 mgd during the measurement period. Average BOD load to the plant
during the period of measurement was 816 Ib/day which indicates a total
rate of 1.68 Ib 0 /lb BOD5. The Berthoud Study indicates the following
estimated distribution of the oxygen demand.
BOD Requirement - 56% of total
Nitrification Demand - 33% of total
Endogeneous Respiration - 11% of total
The rate without nitrification would be 1.12 lb 02/lb BOD or a sludge
age of about 16 days according to Figure 13. The calculated sludge age
based on average plant data during the measurement period was 21 days.
Typically, for normal domestic sewage the oxidation ditch equipment
manufacturers recommend a design oxygen input of 2.35 to 2.5 lb 02/lb
BOD .
The Lakeside Cage aerator used at Berthoud, operating at 66 rpm with
9 inches submergence provides approximately 2.06 lb 0 /hr/lineal foot of
aerator or 2,175 lb 0,/day for the 44 lineal feet of aerator. This is
approximately 2.66 lb 0,/lb BOD and appears to provide a margin of safety
over the on site measured plant uptake rate of 1.68 lb 02/lb BOD5.
MIXED LIQUOR CHARACTERISTICS
The settling characteristics of the mixed liquor in oxidation ditch
plants is good in most cases. In some cases, poor settling has been
observed and it is felt that it may be due to over aeration or
filamentous growths. Data from the Clayton County Water Authority, Georgia
illustrates two typical cases; one with average MLSS and the other with
57
-------
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58
-------
very high MLSS. Both report good effluent performance. These data are
shown in Table 9. Most plants do not develop detailed mixed liquor perfor-
mance data, however most plants visited during the field visit phase of
this study produced a mixed liquor typical to the N.E. Plant shown in
Table 9.
SOLIDS PRODUCTION
Few oxidation ditch plants maintain adequate records to calculate the
solids production. There is no reason that this parameter for oxidation
ditch plants should be any different from the parameter for other extended
aeration plants. Typically this parameter should be approximately 0.7
Ib/lb BOD5 removed when the plant is operated at a long SRT. This relation-
ship is shown in Figure 16.
The Wymore, Nebraska and Glenwood, Minnesota oxidation ditch plant
records are adequate for calculation of this parameter. Wymore is a Passa-
vant plant operating at less than one half of design flow and at a SRT
of approximately 24 days. The calculated solids production for this plant
is 0.67 Ib/lb BOD5. Glenwood(15)(16) is a Lakeside plant operating at
design flow. The calculated solids production for this plant is 0.6 Ib/lb
BOD5 and the approximate SRT is 27 days. Wymore raw BOD5 and suspended
solids are above the assumed 200 mg/1 and Glenwood raw BOD5 and suspended
solids are about 100 mg/1 each. These calculations relate to the informa-
tion developed in Figure 16 and the corresponding theoretical discussion
of solids production
EFFECT OF OXIDATION DITCH CONFIGURATIONS ON PROCESS PERFORMANCE
There are two basic oxidation ditch plant configurations presently in
use in the United States; the single channel and the multiple concentric
channel. Performance data are available for both configurations. A com-
parison of suspended solids and BOD performance data from a limited number
of plants of each configuration shows no significant differences.
59
-------
TABLE 9. TYPICAL MIXED LIQUOR SETTLEABILITY
N.E. PLANT, CLAYTON COUNTY WATER AUTHORITY, GEORGIA
Month
Oct 1975
Nov 1975
Dec 1975
Jan 1976
Feb 1976
Mar 1976
Apr 1976
May 1976
June 1976
July 1976
Aug 1976
Sept 1976
Average
R.L. JACKSON
Oct 1975
Nov 1975
Dec 1975
Jan 1976
Feb 1976
Mar 1976
Apr 1976
May 1976
June 1976
July 1976
Aug 1976
Sept 1976
Average
Number
of tests
5
8
6
7
8
9
8
8
9
9
7
7
91
PLANT, CLAYTON
4
4
3
4
4
2
4
3
4
5
3
5
41
MLSS,
mg/1
2540
2940
3667
3945
3800
2576
3722
4512
3933
3756
3964
4172
3294
COUNTY
7375
7210
8010
9215
9238
7650
9587
8907
8312
7442
7987
7658
7658
30 min
settleable
solids,
mg/1
637
66
580
510
368
172
369
392
309
271
273
290
353
WATER AUTHORITY
980
980
980
987
980
985
980
990
987
983
980
972
982
Basin
D.O. ,
mg/1
3.1
2.1
4.3
4.4
3.9
2.7
2.8
2.6
2.9
3.1
3.2
3.1
3.2
, GEORGIA
5.6
6.3
6.7
8.3
5.8
5.0
4.1
4.7
5.6
5.7
5.7
6.0
9
Effluent
quality,
mg/1
SVI
252
211
163
135
99
64
94
89
77
69
69
69
116
133
127
122
107
106
129
102
111
119
140
123
127
121
SS
11
11
15
11
29
67
22
18
22
29
15
19
22
24
13
8
9
12
11
8
8
10
19
17
17
13
6
7
6
8
13
28
13
8
6
6
4
5
9
8
4
5
3
3
3
3
2
2
3
3
4
4
60
-------
SECTION 4
NITRIFICATION AND NITROGEN REMOVAL
There is documented evidence that some degree of biological nitrifica-
tion and nitrogen removal can be accomplished in oxidation ditch plants.
PasveerT T has claimed 90 percent removal of nitrogen and others report
nitrogen removals up to the 80 percent level reported by Mulbarger^ for
a three-stage system. Reports from other oxidation ditch plants indicate
varying degrees of nitrification and nitrogen removal as shown in Table 10.
Nitrification
The oxidation ditch is capable of substantial to complete nitrifica-
tion without design modifications. This is assuming that it is designed
for extended aeration operation. Only operational modifications are re-
quired to assure maintenance of optimum conditions for nitrification. The
factors which affect nitrification are:
1. Aeration detention time
2. Specific concentration of ammonia related to the MLVSS
3. Mean cell residence time
4. Temperature
5. pH
6. D.O. Level
7. BOD concentration
8. Toxic materials
If a high MLVSS level is maintained in the ditch then only the fore-
going factors 4, 5, 6, 1, and 8 must be considered in operating for high
degrees of nitrification.
As long as pH is in a nominal range (approximately 7.5 to 9.5) it
should have little effect. During cold weather it may be desireable to
maintain pH near the theoretically optimal level of 8.0 to 9.0 and some
61
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chemical addition may be required to accomplish this depending on ammonia
levels and alkalinity of the wastewater. The first choice would be to
increase the mixed liquor suspended solids if possible.
The effect of factors 6 and 7 are not critical as long as adequate
aeration capacity is available to maintain a dissolved oxygen level greater
than 1 mg/1. Variation of influent BOD concentration of typical municipal
wastewater should have no effect on nitrification in an oxidation ditch
plant provided adequate aeration capacity is available.
Certain toxic materials will affect nitrification as with any biologi-
cal process. Toxic materials in normal domestic sewage will normally be
well below the toxic concentration. An indication of the effect of heavy
metals and organics is shown in the EPA Nitrogen Manual(2 , Section 3.2.9.
Nitrification rates are significantly inhibited at low temperatures.
With long hydraulic detention times, high mean cell residence times, and
the maintenance of a reasonable MLVSS level in the oxidation ditch high
nitrification can be achieved even in cold weather. These relationships
are shown in the EPA Nitrogen Manual(27) and need not be repeated in this
study. In addition, the pH can be maintained in the range of 8.0 to 9.0
during cold weather as a further step in optimizing cold weather nitrifi-
cation although this would normally be done only when absolutely necessary.
There is evidence from many oxidation ditch plants as shown in Table 10
that high levels of nitrification can be obtained year around. The Wymore,
Nebraska oxidation ditch plant averaged 91 percent reduction in ammonia
nitrogen during the year 1975-1976. This average was 96.7 percent for the
months of March through December and dropped to 56 and 73 percent for the
months of January and February respectively. Nitrification in the oxida-
tion ditch plant is a biological process and is subject to upset from time
to time. Care and attention in operating the plant should minimize the
periods of upset. Significant populations of nitrifying organisms develop
slowly, therefore, more time is required to establish or reestablish a
population of these organisms compared to typical activated sludge organ-
isms. These establishment times would be greater when liquid temperatures
are cold. This is shown in Figure 3-7 of the EPA Nitrification Manual( '.
Nitrogen Removal
A limited amount of work has been done with single stage biological
nitrification-denitrification in oxidation ditch plants. Nitrification
has already been discussed and has been demonstrated to occur in standard
extended aeration oxidation ditch plants and is a function of operational
parameters more than design modifications to standard plants. It is
expected that substantial nitrification occurs in most oxidation ditch
plants.
The oxygen profile around a single channel continuous oxidation ditch
can vary considerably depending on the aerator locations, ditch length,
point of introduction of raw sewage and return sludge, and the submergence,
size, and speed of the aerators (oxygen transfer rate). The vertical
63
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oxygen profile within an oxidation ditch can also vary considerably especi-
ally in deep ditches. Theoretically, it is possible to control the dis-
solved oxygen level at any point in the ditch to a set point by varying
the oxygen transfer rate of the aeration equipment (speed, submergence, or
number in operation). In practice, this is not easy to accomplish either
manually or automatically. The necessary monitoring equipment requires
continual maintenance and the velocity imparted to the ditch contents may
be inadequate at low aerator submergence or speed. Solutions to these
problems are discussed in this section.
Because it is possible to create and maintain oxygen rich and oxygen
deficient (aerobic and anoxic) zones around the ditch, it is then possible
to create conditions for biological nitrification and denitrification within
the same ditch. The nitrification takes place in the aerobic zones and the
denitrification in the anoxic zones. The velocity in the ditch should be
approximately 1.0 foot per second. In a typical ditch which may have a
total channel length of 325 feet, the contents make a circuit about every
4 to 6 minutes. Thus, the mixed liquor and the biological contents are
subjected to rapid alternation between aerobic and anoxic conditions. This
rapid alternation enables the entire spectrum of organisms to survive * .
It is further believed, in this case, that the nitrate concentration in the
ditch remains low because the alternating aerobic-anoxic cycles quickly
denitrify the nitrates produced during nitrification in the aerobic zone(18).
The denitrification process was probably not rate limited. If the denitri-
fication process is rate limited (by lack of an adequate carbon source, for
instance) the nitrates will be high as shown at Dawson, Minnesota<23).
A hydrogen donor is required in the anoxic zone and can be provided in
part by distribution of the raw sewage into the channel near the start of
the anoxic zone. The return sludge would also be introduced at this point.
This is bas^c,aj.ly the operation used at the Dawson, Minnesota oxida-
tion ditch plant . This type operation produced sustained total nitro-
gen removals above 80 percent with controlled operation and average removals
of 51 percent through the 10 to 12 month demonstration period.
Drews, et al reported on nitrogen removal work performed in
South Africa at a four-channel Huisman type oxidation ditch plant. The
channels were operated essentially in series with four different aeration
configurations as shown schematically in Figure 24.
Mode 1 was conventional configuration with a positive oxygen level
maintained in all parts of all channels. Sludge return rate for this mode
was 50 percent of the raw flow.
Mode 2 was arranged for denitrification-nitrification-denitrification-
nitrification. The first channel would denitrify any nitrates returned
with the return sludge. The single aeration disc in the denitrifying
channels (1 and 3) is to keep the mixed liquor moving to prevent settling.
Mode 3 was the reverse of Mode 2 and produced extremely poor results.
64
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Oxidation ditch plant channel number
with number of aeration discs in
channel shown (typical for all modes)
FINAL
CLARIFIER
RAW
SEWAGE4
FINAL
R.S.
\V1^/EFFL
WASTE
MODES 1 and 4
Conventional multiple channel operation
De-N
De-N
MODE 2
Anoxic—aerobic—anoxic-aerobic channel operation
De-N
De-N
MODE 3
Aerobic—anoxlc—aeroblc-anoxic channel operation
SLUDGE
FINAL
CLARIFIER
AW
»•
IWAGE '
I
R.S.
^1^31
Xi>
1 ^-
EFFLUENT
WASTE
SLUDGE
FINAL
CLARIFIER
kW
WAGE*1
k
R.S.
.
*J
\
FINAL
EFFLUENT
_ WASTE
SLUDGE
Figure 24. Concentric Channel Nitrogen Removal Flow Diagram (Drews)
(17) (13)
65
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Sludge was continuously rising in the final clarifier producing high sus-
pended solids in the effluent. This was probably due to denitrification in
the final clarifier. This mode was dropped from further consideration and
no performance data were provided in the paper.
Mode 4 was the same as Mode 1 except the aeration was limited so that
each channel experienced both aerobic and anoxic zones. Each of these
zones comprised approximately half of each channel.
Averaged results for Modes 1, 2 and 4 are shown in Table 11.
Ammonia and total nitrogen removal were 82 to 100 and 69 to 79 percent
respectively in all three modes of operation as shown in Table 11. Only
the data shown in Table 11 were published in the referenced article. The
total nitrogen removal experienced in Mode 1 would not normally be expected
if a positive dissolved oxygen level were maintained in each of the channels
during the test. In fact, positive oxygen levels could not be maintained
in the channels and at times the Mode 1 operation reverted to Mode 4. This
probably explains the total nitrogen removal observed for Mode 1 operation.
Generally, a turbid effluent was produced in Mode 2 operation and a
floating scum layer was always present in the final clarifier. The report
did not indicate the magnitude of the effluent suspended solids.
Mode 4 produced a clear effluent, but incomplete nitrification. The
efficiency of the process varied considerably over a day as might be ex-
pected when attempting to maintain aerobic and anoxic zones in each channel.
Results of the study indicate:
1. Single stage nitrogen removal of 70-90 percent could be achieved
without the aid of methanol or other carbon sources.
2. When maximum nitrogen removal was obtained, a clear effluent low
in COD was produced.
3. Energy costs for the aerators were reduced because of close con-
trol of aeration.
4. Skilled attention is required.
5. The oxidation ditch plant is a suitable configuration for this
type operation.
6. The study reported that "in order to produce a good quality
effluent at winter temperatures a somewhat lower sludge concen-
tration will have to be maintained than at summer temperatures".
The validity of this conclusion can not be confirmed.
Another operational variation for nitrogen removal was used success-
fully at Dawson, Minnesota(Z3> for a short time and has been used in
actual operation at the Baldwin, Georgia oxidation ditch plant. The
66
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TABLE 11. MULTI CHANNEL OXIDATION DITCH NITROGEN REMOVAL
RESULTS (Drews)(1?)(18)
Parameter
o
Wastewater Temp, C Ave.
Return Sludge Rate
Detention Time, Hours
Number of Discs
MLSS, Channel 4, mg/1 Ave.
Ammonia -N Removal, % Ave.
COD Removal, % Ave.
Total N Removal , % Ave .
Mode 1
18.4
1.5:1
13.9
11
3,853
95.0
95.5
69.0
Mode 2
16
2:1
18.5
10
4,828
100.0
94.5
79.3
Mode 4
18
1.5:1 to 2:1
13.9 to 24.0
5 to 11
3,998
82.0
95.3
79.0
67
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aerator(s) are operated on an adjustable on-off cycle; such as 1 hour on
and 1 hour off. In practice, the cycle could be fully adjustable and con-
trolled by an automatic timer. The Baldwin, Georgia oxidation ditch plant
is presently using this mode and is attaining very satisfactory operation
with substantially decreased electrical energy, however, they do not mon-
itor any nitrogen parameters. This mode of operation would be applicable
especially to underloaded plants. The solids settled during the off time,
but were resuspended when operation resumed. It was a very easy way to
adjust the oxygen input to the ditch and it could also, perhaps, provide
alternating aerobic and anoxic cycling. Several other oxidation ditch
plants were considering this mode of operation, but only because they had
excessive oxygen levels in the ditch and final clarifier and felt they
could substantially reduce their electrical energy usage.
In summary, there are documented results from both single channel and
multiple concentric channel oxidation ditch plants and other extended
aeration plants relating to nitrification and nitrogen removal. The follow-
ing design and operation parameters properly applied are capable of pro-
viding essentially complete nitrification in an oxidation ditch plant at
least down to mixed liquor temperatures of 1°C or, essentially, freezing.
DESIGN PARAMETERS FOR NITRIFICATION
Channel Physical Dimensions and Features
The physical length, width, and depth dimensions do not have any signi-
ficant effect on nitrification as long as proper detention time is provided.
There is no evidence to suggest that the single channel or the multiple
concentric channel configurations possess any unique characteristics in
this regard.
For a single channel plant the raw sewage and return sludge should be
introduced into the ditch immediately upstream of an aerator. The mixed
liquor should be withdrawn far enough upstream of the point of introduction
of raw sewage to prevent any possibility of short-circuiting raw sewage
directly to the final clarifier.
In a multiple concentric channel plant the raw sewage and return sludge
would be introduced into the first channel and the mixed liquor withdrawn
from the last channel.
Aeration Equipment
The aerator type, sizing, or spacing is not critical to nitrification
as long as adequate oxygen is provided to the mixed liquor. A positive
oxygen level should be maintained in all parts of the ditch under all
treatment conditions. When mixed liquor temperatures are increasing,
there will be additional oxygen demand because of increased volatilization
of solids due to higher temperatures. This extra demand should be met to
assure good nitrification. At Dawson, Minnesota a completely nitrified
effluent was maintained with as little as 0.5 mg/1 dissolved oxygen
68
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(23)
immediately downstream of the aerato'r . It is suggested that the goal
be a positive oxygen level in all parts of the ditch and this will be
achieved for normal municipal sewage if the rotors are designed for proper
channel velocity.
Auxiliary Velocity Devices
The need for auxiliary devices to increase velocity in the oxidation
ditch should not arise for cases involving nitrification. The aerators
will impart proper velocity unless they are undersized or the ditch is
improperly designed for the aeration equipment used.
Sludge Storage, Return, and Wasting
Oxidation ditch plants designed for nitrification should be designed
to operate within a range of MLSS of at least 4,000 to 6,000 mg/1. Posi-
tive sludge removal should be provided from the final clarifier along with
a means for returning this sludge at a known and controllable rate to the
ditch. A sludge return capacity of at least 100 percent of the average
flow should be provided. Proper sludge wasting is necessary including
provisions for measuring the volume of sludge wasted.
It is recommended that sludge be removed from the final clarifier
continuously and at a rate adequate to maintain a positive oxygen l^vel in
the return sludge. Otherwise, denitrification may take place in the
clarifier which could lead to substantial carryover of solids to the
effluent.
The need for auxiliary in-plant sludge storage capacity depends on a
number of factors many of which are local in nature. Adequate sludge
handling facilities are required so that sludge can be wasted from the
plant as dictated by biological system operation rather than by sludge pro-
cessing and disposal considerations. It is possible to store solids in
the aeration system by allowing the mixed liquor suspended solids to
increase, but jthis practice can lead to operational problems.
Flow Modulation
There is nothing to indicate that flow modulation or equalization is
necessary to satisfactory nitrification provided the final clarifier is
properly designed for expected flow variations. In cases of anticipated
excessive diurnal flow variations it may be desireable to provide flow
equalization storage or a method to stabilize the flow to the final clari-
fier such as the method used at Dawson, Minnesota'^3).
Aeration Detention
If a high degree of nitrification is required all year in a relatively
cold climate, the oxidation ditch plant should be designed for a detention
time of 24 hours at average flow assuming a normal domestic sewage. In
warmer climates where the mixed liquor temperature is expected to remain
69
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above about 15°C, a detention time of 13 hours was shown to be adequate by
Drews, et al <17' <18'.
Final Clarifier Sizing and Design
The final clarifier(s) should be designed for activated sludge appli-
cations with means for continuous and relatively uniform removal of sludge.
The final clarifier overflow rate should be in the range of 400 to 500
gpd/sq ft at average plant flow and 1,000 to 1,200 gpd sq ft at peak flows.
Design for clarifier solids loadings should be 30 Ib/day/sq ft. Side wall
depths should be at least 12 feet and preferably 12 to 14 feet. Deep scum
baffles and double skimmer arms will be helpful to retain any rising solids
resulting from denitrification in the final clarifier. It is desireable
to make piping provisions so skimmings can be returned to the aeration
channel.
Instrumentation
Special instrumentation should not be necessary for proper nitrifica-
tion control. Therefore, only conventional instrumentation typical to an
oxidation ditch plant as follows is required.
Raw sewage or effluent flow measurement, recording & totalizing
Return sludge flow measurement
Waste sludge measurement
Chlorine feed pacing to flow
Normal laboratory instruments and equipment including a portable
D.O. instrument
This instrumentation represents the minimum necessary for proper
operation and recordkeeping.
OPERATIONAL PARAMETERS FOR NITRIFICATION
Oxygen Profile
No special oxygen profile is necessary in the ditch except it is
desireable to have a positive oxygen level throughout the ditch except
when denitrification is required.
MLSS and MLVSS
The Dawson, Minnesota plant was able to achieve essentially complete
nitrification over the MLSS range of 2,600 to 10,000 mg/1* 3'. Typical
MLSS reported by Drews, et al^1^ was 3,600 to 5,500 mg/1 with essentially
complete nitrification reported. Theory would indicate for a 200 mg/1
BOD5 sewage and 24 hour detention time, a MLSS of approximately 3,000 mg/1
would be adequate for at least 95% nitrification down to a mixed liquor
70
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temperature of 5°C. The corresponding MLVSS will vary with plant conditions,
but at Dawson, Minnesota it varied from 51 to 78 percent by weight of the
MLSS. Volatile solids less than 60 percent were normally obtained when the
MLSS concentration was greater than 6,000 mg/1 and the liquid temperature
above 20°c'23)i The most satisfactory MLSS for any given plant and season
will have to be determined by actual operation, but a target of at least
3,000 to 5,000 mg/1 is reasonable and should be achieveable consistently.
Carbon-Nitrogen Ratio
f 26 )
Literature i
cant factor for plants operated at a 10 day or longer SRT.
f 26 )
Literature indicates that carbon-nitrogen ratio is not a signifi-
The nitrification process destroys alkalinity and this may have a
depressing effect on system pH if the wastewater alkalinity is not high
enough to provide the residual needed for nitrification.
The rate of nitrification is pH sensitive and this characteristic is
well documented. Maximum rate of nitrification occurs at a pH of about
8.5, but high rates can be expected within the pH range of 7.5 to 9.5 at a
temperature of 20°C. Normally, even in a cold climate, pH control is not
necessary if the MLSS is maintained at reasonably high concentration thus
providing a high MLVSS.
Sludge Settleability
If a positive oxygen level is maintained in the aeration ditch and
sludge is removed from the final clarifier at a proper rate, there should
be little reason to expect any effect on sludge settleability as a result
of the nitrification process. Over- aeration can lead to sludge settling
problems because of air bubble release in the final clarifier, but this is
not particularly related to nitrification. If oxygen levels are allowed to
decrease to zero or if sludge is not properly removed from the final clari-
fier, denitrification can occur in the final clarifier. The resulting re-
lease of nitrogen gas will cause sludge to float.
Sludge Recycle Ratio
This is a variable which is difficult to generalize. The sludge re-
turn should be continuous and flow measurement and control are desire-
able. Flow proportional sludge return is uncommon at small plants.
Normally, sludge return flow rates are maintained at about 30 to 100 percent
of the average daily flow. Very high return rates up to 200% were used by
Drews, et al.(18) in South Africa but this is not common practice in the
United States. The sludge return rate should be adequate to maintain good
clarifier performance.
71
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Sludge Wasting Rate
Sludge wasting should be carried out to maintain the desired MLSS in-
ventory in the biological system. When the plant is operating under
relatively stable conditions, regular sludge wasting should be carried out
to maintain the stability. The mixed liquor should be checked at least
daily and sludge can be wasted daily or every few days. During some per-
iods at Dawson, Minnesota, sludge did not have to be wasted for a week or
more particularly in winter<23) It is better to waste nominal amounts of
sludge on a regular schedule than to make large periodic adjustments. Typi-
cal sludge production is discussed in a previous section.
Solids Retention Time
Solids retention time is very important to nitrification, however, if
the aeration detention is 24 hours and the raw sewage is typical domestic
(200 BODs), operation at a solids retention time of 16 days can be achieved
at a MLVSS of 1,922 mg/1 and 24 days at 2,880. These are extremely conser-
vative, should provide excellent nitrification even in very cold climates,
and should be relatively easy to achieve.
Temperature
The rate of nitrification is very much a function of liquid temprea-
ture. In cold climates anythingvthat can be done to conserve the liquid
heat will be beneficial. Excessive aeration will tend to dissipate liquid
heat and therefore should be avoided. As an example, the rate of nitri-
fication at 5°C is about 20 percent of the rate at 20°C. However, as long
as the MLSS is maintained in the recommended range, very complete nitrifi-
cation can be obtained even at liquid temperatures approaching freezing.
Hydraulics
No special hydraulic considerations are required over and above those
dictated by proper design of an oxidation ditch plant and sizing of the
final clarifier. Placement and design of the aerator structure should
consider the water spray produced by the aerator. Baffles or other protective
devices should be provided so this spray does not fall on bearings and gear
reducers. Walkways and other accesses should be placed so the spray will
not wet these areas.
DESIGN PARAMETERS FOR NITROGEN REMOVAL
Channel Physical Dimensions and Features
The channel width, depth, and length are functions of the type of
aerator to be used, its length and total detention time. The Dawson,
Minnesota report<23) recommends a channel length of 600 feet (for a single
channel plant) in order to increase the time during which anoxic conditions
are maintained. With this channel length the total time for one "revolu-
tion" of the mixed liquor would be 10 minutes and approximately 5 minutes
72
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could be provided for the anoxic zone based on a channel velocity of 1 foot
per second.
The multiple concentric channel type plant should also be considered.
This configuration might offer some advantages in denitrification because,
conceiveably, it should be easier to establish and maintain the aerobic
and anoxic zones using separate channels with separate aeration.
For a single channel plant the raw sewage and return sludge should be
introduced in the vicinity of the transition between the aerobic and anoxic
zones to act as a carbon source. The best location for removal of mixed
liquor is open to question. One consideration would be at the end of the
anoxic zone just prior to the aerator. The disadvantage of this arrange-
ment is that denitrification will occur in the final clarifier causing
rising sludge and solids carryover in the effluent. This was confirmed by
Drews, et al^ ' and is comparable to Mode 3 shown in Figure 24. Drawoff
of the mixed liquor downstream of the next aerator and prior to reaching
the anoxic zone might provide better results. This would be comparable
to Modes 2 or 4 in Figure 24. Mode 4 produced a clear effluent and a total
nitrogen reduction of 79 percent. Flexibility should be provided in the
physical and process plant design so that field changes can be made in the
operation of the plant.
The layout of the multiple concentric channel type plant would be the
same as for the nitrification case; only the configuration of the aeration
discs would change.
Aeration Equipment
The oxygen transfer characteristics and operating power requirements of
competing aeration devices should be varified (by actual testing if neces-
sary) in order to select the most satisfactory equipment.
It is important to avoid oversizing the aerator and some means must
be provided to allow adjustment of oxygen transfer rate over a fairly
wide range. This can be accomplished by adjusting speed, submergence, num-
ber in operation, and perhaps using an on-off operating cycle.
The disc type aerator used in the multiple concentric channel type
plants would seem to have additional advantages for this application be-
cause the number of discs in any channel can be changed at will by adding
or deleting discs on the shaft.
Spacing of aerators aroung the channel in single channel plants should
provide clearly defined aerobic and anoxic zones. Specific criteria can not
be provided to define this spacing. Approximate spacing can be calculated
based on expected oxygen uptake rate of the specific wastewater arid then,
the actual capacity of each aerator can be field adjusted as previously dis-
cussed. Use of floating aerators may be beneficial because they can be
moved in the field as required.
73
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Auxiliary Velocity Devices
Conceiveably, in nitrogen removal applications, the aeration require-
ments may be less than the mixing or velocity requirements. In this case,
the velocity in the ditch would be inadequate and solids would settle.
Auxiliary flow devices were installed in the ditch at Glenwood, Minnesota
(15) (16) because the aerators could not impart adequate velocity to the
ditch contents at the low aeration levels required. The auxiliary flow
devices were submerged propeller-like devices driven by electric motors and
mounted on floating platforms. They are designed to provide velocity to
the mixed liquor without adding any oxygen; hence, the reason for complete
submergence of the propellers. Similar devices may be required in other
plants practicing nitrogen removal where the oxygen requirements are rela-
tively low. Plants using brush or cage aerators may experience problems
with sludge settling within the channel where aerator submergence is re-
duced to produce lower dissolved oxygen levels within the channel. In this
case, auxiliary flow devices may be required.
Sludge Storage, Return, and Wasting
These considerations should be the same as for nitrification.
Flow Modulation
One of the major problems in nitrogen removal is providing the con-
trolled aeration required to create the proper aerobic and anoxic zones in
the channel. This should be accomplished in the simplest possible manner
consistent with the plant design. Diurnal variations in either flow or
loading, tend to upset the stability of the aerobic and anoxic zones.
Stabilization of the plant influent conditions, with processes such as flow
equalization, will ease the control problem. On the other hand, perfect
oxygen control as described under Instrumentation could easily accommodate
a wide range of load variations satisfactorily.
Aeration Detention
Considerations related to aeration detention time are essentially the
same as described for nitrification.
Final Clarifier Sizing and Design
These considerations are essentially the same as described for nitri-
fication.
Ins trumentation
Along with the basic plant instrumentation, additional requirements
are needed in nitrogen removal to create and maintain aerobic and anoxic
zones in a portion of the ditch. Theoretically, a dissolved oxygen probe
could be immersed at the desired start of the anoxic zone and this probe
74
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used to control a variable aeration parameter(s) such as speed and/or
submergence. This is not a simple task because of the need for almost
constant maintenance of the probe, the lag in the system, and the range of
aeration control needed. Considering normal diurnal flow changes plus
normal BOD5 load variations, both speed and submergence of the rotor would
have to be varied to obtain a 4:1 range in oxygen transfer rate. It is
questionable whether a satisfactory automatic control scheme meeting these
requirements is available. Perhaps this control can be approximated manu-
ally with careful operator attention. It is not known how much the anoxic
zone size can vary and still achieve good nitrogen removal. The experience
at Dawson, Minnesota demonstrates that the required control can be accom-
plished manually under carefully controlled conditions . A high degree
of operator attention may be required to achieve consistently high removal
of nitrogen at typical oxidation ditch plants. Additional full scale
plant experience is needed regarding the application of automatic controls.
OPERATIONAL PARAMETERS FOR NITROGEN REMOVAL
Oxygen Profile
Theory indicates that for nitrogen removal a zone of the ditch must
be maintained under anoxic conditions. In addition, a source of carbon
should be added to this zone, which can be raw sewage or methanol. The
optimum arrangements have not been demonstrated, but it is recommended
that as a first trial approximately 50 percent of the ditch be maintained
under anoxic conditions. Again, flexibility is needed in the aeration
system for nitrogen removal applications.
MLSS and MLVSS
These considerations should be about the same as for conventional oxi-
dation ditch plants and as outlined for nitrification. Some observation
and experimentation will be required at each plant to develop operational
guidelines for optimum nitrogen removal.
Carbon-Nitrogen Ratio
The biodegradable COD:N requirement for denitrification may be deter-
mined based on research conducted using methanol as an organic source.
The methanol requirement essentially is that amount which will permit
depletion of the oxygen in the dissolved nitrate and nitrite forms. The
equation representing the methanol requirement is:
Cm = 2.47N + 1.53N, + 0.87D
o 1 o
Where Cm = required methanol concentration mg/1.
N = initial NO -N concentration mg/1.
N = initial NOp-N concentration mg/1.
75
-------
D = initial DO, mg/1.
o
Methanol (CH3OH) has a COD of 1.41 Ib/lb methanol. Therefore, for a
strictly nitrate nitrogen conversion a biodegradable COD to nitrate nitro-
gen ratio of 3.5 will be required for complete conversion.
The denitrification process increases the alkalinity concentration, but
normally will not totally offset the alkalinity lost by nitrification. The
rest of the considerations are essentially the same as for nitrification.
Sludge Settleability
The comments for nitrification apply although some data suggest denit-
rified effluent does not settle as well as nitrified. In addition, if
denitrification takes place in the final clarifier, gas will be released
and sludge will tend to float and be carried over the effluent weir. There-
fore, it is desireable to carry a positive oxygen level in the final clari-
fier and to remove sludge promptly from the final clarifier.
Sludge Recycle Ratio
The considerations are the same as for nitrification.
Sludge Wasting Rate
The considerations are the same as for nitrification.
Solids Retention Time
The considerations are the same as for nitrification except there is
some indication that denitrification is rate limiting at cold temperatures.
Under these conditions a higher SRT may be necessary for denitrification
than for nitrification.
Temperature
The considerations are the same as for nitrification except for
possible higher SRT during cold weather operation as discussed in the pre-
vious paragraph.
Hydraulics
If the plant hydraulic flow can be stabilized to some degree (for
instance, with flow equalization) it will be easier to maintain the aerobic
and anoxic zones necessary for nitrogen removal.
SUMMARY
Design of the plant for long aeration detention time and operation at
76
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relatively high MLSS of 4,000 to 6,000 mg/1 should provide the long SRT
necessary for year around nitrification and nitrogen removal. For winter
operation the mixed liquor should be kept at the highest possible concentra-
tion and pH should be kept in the range of 7.5 to 9.0.
For nitrification only, adequate aeration should be provided to main-
tain a positive oxygen level in all parts of the ditch.
For nitrification plus nitrogen removal the aeration must be controlled
so that at least one anoxic zone is maintained in the ditch. This will
require a high degree of operator attention. It is questionable whether
an automatic control system could satisfactorily control the formation of
the required anoxic zone with any degree of reliability.
The multiple concentric channel type plant with aeration discs prob-
ably possesses some advantages for nitrogen removal applications, but this
has not be demonstrated.
A properly designed oxidation ditch plant is capable of attaining
essentially complete nitrification except when the mixed liquor temperature
is near freezing.
Nitrogen removal of 70 to 80 percent can be achieved in oxidation
ditch plants operating at temperatures above 15°C with proper design and
operator control. At lower temperatures, 40 to 70 percent nitrogen removal
is possible. This has been demonstrated at several oxidation ditch plants,
which were properly designed and operated.
77
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SECTION 5
OPERATION
OXIDATION DITCH PLANT PROBLEMS
As shown in Tables 5, 7, and 8, over 30 oxidation ditch plants were
visited, contacted by letter or telephone, or reviewed in literature
articles during this study. In each case particular attention was directed
toward identifying various operational, maintenance, and process features
and problems. It is felt that the comments in this section represent an
excellent cross section of actual plant experience as related by respon-
sible plant personnel.
Process and Process Operation
The most serious process operation difficulties resulted from equipment
related problems.
Observations based on this study indicate that, as a group,
oxidation ditch plants can be operated by average personnel to
produce above average performance results.
Many of the plants visited were manned only during a single shift and,
in many cases, for only a portion of a single shift. A number of plants
received little or no attention during weekends. Assuming no mechanical
malfunctions, the plants perform well for long periods (days to weeks at
a time) with little or no operator attention. Most plants practiced regular
or periodic sludge wasting. In some cases sludge wasting was not prac-
ticed, but this caused eventual or regular carryover of excess solids in
the effluent.
Almost without exception, operators and administrative personnel were
well satisfied with the plants. In most cases the plants were meeting
state discharge requirements.
(22)
A study by EPA Region VII of winter performance of secondary waste-
water treatment facilities concluded " — in general, the facilities were
not meeting the secondary treatment effluent definition on the average
except for the oxidation ditch subset".
All plants visited exhibited good mixed liquor characteristics (by
visual observations). Some plants reported more than desired carryover of
solids in the effluent and it is believed that the cause was either
78
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excessive oxygenation or poor management of sludge return or wasting.
Many plants could reduce the level of oxygenation in the aeration basin
without detrimental effect and reduce energy cost. There were no signs of
lack of oxygen. One plant was cycling the aerator one hour on and one hour
off and reported excellent results for over a year. Several other plants
were considering this scheme to lower operating costs and reduce oxygen
levels.
Where screening or comminution is not regularly and properly carried
out, rags and debris tend to cause problems in the plant especially in the
return sludge system.
One deep type (10 feet) single channel plant reported poor mixing in
the ditch.
A number of plants lacked sufficient drying bed capacity which may
have led to erratic sludge wasting practices. This problem was also re-
ported by EPA Region VIII personnel.
A number of plants lacked proper laboratory facilities and equipment.
Some even lacked a building. These facilities are necessary for proper
operation and maintenance of a wastewater treatment plant.
Equipment Problems
As with any plant containing mechanical equipment, oxidation ditch
plant equipment was subject to problems, malfunctions, and failures. The
level of preventative maintenance is important; however, this study did
not attempt to relate these problems to the level of preventative mainten-
ance.
As a general observation, oxidation ditch plants are capable of long
periods of operation without mechanical problems and appear to operate
with a very high mean time between equipment failures. There are some
things that can be done to increase reliability of these plants as dis-
cussed later.
The following is a general summary of mechanical problems as related
by plant personnel.
1. Some plant personnel reported that drives trip out on momentary
electrical failures and do not restart upon restoration of power.
This causes problems with unattended operation and consideration
should be given to maintained-contact type electrical control
for drives such as lift pumps, aerators, sludge return pumps,
and other items in cases where plants operate unattended. Time
delays can be provided so all drives do not start at the same
time.
79
-------
2. Some troubles were reported with return sludge pumping. In gen-
eral, centrifugal, non-clog pumps seemed to give good service
especially when a separate potable grade seal water source was
used. Air lift pumps were not common as would be expected at
plants with mechanical aeration. Where possible, it worked well
to return the sludge by gravity to the raw lift station and
pump it to the ditch with the raw sewage. This increased the
size of the lift pumps, but eliminated one set of pumping.
3. Comminutors were a continuing maintenance problem. They required
regular cleaning and care. Some operators had so many problems
they stopped using the comminutor and allowed unscreened sewage
to flow into the ditch. Almost without exception this caused
problems in the return sludge system by clogging the sludge return
rate adjustment valve (generally a telescoping valve) or pumps.
This altered the sludge return rate and caused process problems
if not detected early. Oscillating type comminutors appear to
give best service, but still require regular maintenance.
4. Problems have been experienced with continuous dissolved oxygen
controls because probes require almost continuous cleaning.
5. A hole is drilled in the scum box at some plants to keep the box
and scum pipe flushed continuously and to provide some dilution
water into the scum pit.
6. At one plant a non-reinforced concrete block intermediate wall
in a multiple concentric channel plant collapsed and had to be
replaced. Such walls should be reinforced.
7. One plant operator suggested that sludge return pumps should be
variable speed, however, there are less expensive methods to
adjust the return activated sludge flow rate.
8. Access walks should be designed to avoid receiving spray from
aerators even under various wind conditions. This spray creates
hazardous conditions because of algal growths and freezing.
This oversight created continuing maintenance problems and should
be considered in all designs.
9. Sludge reportedly settled in channels at some plants where flow
control walls were not installed as recommended by manufacturers.
10. There was some reported corrosion in final clarifiers which re-
sulted in materials failures (bolts in particular). These units
should be drained and inspected annually. Any corrosion should
be removed and the area recoated.
11. Weirs are a maintenance problem and must be cleaned frequently;
in some cases as often as daily.
80
-------
12. Aerators and aerator drives account for a major portion of the
mechanical problems. Most plants experienced the following
aerator related problems every two to five years per aerator unit.
a. Loss of some "teeth" from brush type aerators due to cor-
rosion of bolts or damage sustained while handling the
aerators is a common occurance. This generally is not a
serious problem and can be repaired during periodic shut-
downs. Some manufacturers have redesigned their aerators
to minimize this problem.
b. Bearing problems were reported in gear drives, line shafts,
and aerator shafts. Experience would seem to indicate a
bearing problem every 2 to 5 years per aerator. These
problems result from poor selection of bearings, constant
splashing of water onto the bearing, improper initial align-
ment, differential settlement of bearing support structures
and similar problems. Some manufacturers have taken steps
to reduce bearing problems by using self aligning bearings,
double seals, and by providing water shields, but bearing
problems can still be expected. The magnitude of the bear-
ing problem is not excessive and normal plant maintenance
programs can handle this problem.
c. Flexible couplings between line shafts caused problems on
multiple concentric ring type plants.
d. At one plant some of the disc aerators loosened from the
shaft and had to be shimmed and reclamped.
e. Gear reducer output shaft seals need replacing about once
a year at some plants.
f. There are a number of plants were gear reducer failure was
experienced within a year of plant startup. This was
probably due to improper initial alignment or differential
settling of the aerator support structure.
g. A couple of plants have experienced aerator torque tube
failure or excessive deflection with very long aerators. At
one plant the tube failed, collapsing in the middle. One
consultant now requires solid shafts on all aerators rather
than hollow torque tubes. It may be well to avoid the use
of very long aerators. If a wide ditch is necessary, the
width can be spanned using two shorter aerators driven by
a common drive.
h. Protective covers around bearings, couplings and drive units
are not provided at most installations. Spray from the
aerators keeps these components wet and possibly contributes
to short life. The "slingers" at each end of the aerator
81
-------
are ineffective because wind blows this "tail" of water
sideways onto adjacent components such as bearings and
drive units. Corrosion and grit on shafts probably con-
tributes to short seal life. For maximum service life and
minimum maintenance these components should be shielded from
water spray. Some manufacturers are taking steps to modify
their standard designs to provide spray baffles and shields.
i. Some drive configurations require the aerator to be lifted
out of position to remove the gear drive. This is a diffi-
cult operation requiring a crane. Access for a mobile
crane should be provided to all aerators or other lifting
provisions designed into the plant.
Cold Weather Operations
Cold weather problems related to oxidation ditch plants appear to be
minimal. Those which have been identified are listed. These comments are
based on visits and literature^9'12'13'14'22).
1. Final clarifiers should be covered where this is typical practice
for other types of plants.
2. In moderately cold areas the spray from aerators will freeze on
adjacent structures, bearings, gear reducers, and like equipment
making maintenance difficult. Drive components should be covered
to provide shielding from spray or these drives mounted in
isolated compartments.
3. In moderately cold areas some problems are reported from ice
build-up on clarifier scum collection boxes and eventual jamming
of skimmer mechanisms.
4. Problems were reported with freezing of spray around aerators.
The problem is solved by covering aerators in moderately cold
areas, providing heated covers in very cold areas, and providing
heated buildings over the aerators or over the whole ditch in
extremely cold areas. When installed inside buildings, a shield
should be provided over aerators to control spray.
5. In areas with periods of very cold weather all equipment requir-
ing regular maintenance or service should be housed.
6. Poor mixed liquor settling was experienced during winter months
at several plants in cold weather locations. This was probably
due to filamentous growths resulting from the low winter loadings.
OPERATION AND MAINTENANCE REQUIREMENTS
Actual operation and maintenance information was obtained from a number
of operating oxidation ditch plants. This information was analyzed in
82
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order to develop 0 & M requirements as a function of plant size and to
determine if any significant variations are related to specific manufac-
turers equipment. A summary of the O & M requirements was obtained from the
plants listed in Table 12. O&M requirements for normal plant operation, nitrifi-
cation, and nitrogen removal are shown in Table 13. These requirements include
sludge handling and drying on outdoor drying beds. Note that labor and
electric energy costs are shown in two forms; the actual field costs and
calculated costs based on assumed unit rates. This additional information
is used for purposes of comparisons in Section 7.
The O&M requirements were developed by the following categories:
1. O&M labor
2. Energy (essentially electrical)
3. Chemicals (primarily chlorine)
4. Maintenance materials
5. Other (miscellaneous supplies, training, and other incidentals)
Labor
Labor requirements include operation, maintenance, sampling, and
laboratory analysis. Actual O&M labor rates varied from $3.00 to $10.00
per hour including fringe benefits with the smaller facilities generally
experiencing the lower costs. Typically, 15 to 20 hours per week were
required to perform the sampling and laboratory analysis. Some plants
contract for outside sampling and/or laboratory services.
Field investigations indicated a wide range of O & M labor require-
ments but the information in Table 13 was developed based on average
practices. Information was available from very few plants over 1.0 mgd
in size, and from none over 5 mgd, therefore, portions of the curve are
extrapolated using graphical methods.
Energy
The electrical energy requirements include electric drives for aerators
and pumps, heating and other miscellaneous plant requirements as reported
by operating plants. Energy cost varied from $0.02 to $0.03 per kwh for
the plants surveyed. Energy requirements were also calculated based on
the aerator sizes shown in Table 2 plus pumping, heating, lighting, and
other uses as a cross check on actual field data. These calculated usages
correlated very well with the actual usages reported from the field.
Chemicals
Chemical costs, principally for chlorine, varied widely. Chlorine
requirements and costs were calculated as a cross check on the field data.
83
-------
TABLE 12. SOURCES OF OXIDATION DITCH
PLANT 0 & M INFORMATION
Facility
Battle Creek, NB
Bolivar, MS
Nixa, MS
Seymour, MS
Birch Tree, MS
Eaton, CO
Morrison, CO
Berthoud, CO
Dillon, SC
Pee Dee
Maple Swamp
Clayton County, GA
Jackson Plant
N.E. Plant
Clarksville, TX
Huntsville, TX
Paris, TX
Daingerfield, TX
Whitewright, TX
Susanville, CA
Burney, CA
Kershaw, SC
Wymore, NB
Design capacity, mgd
0.21
1.34
0.41
0.26
0.07
0.22
0.07
1.10
0.80
0.36
1.00
1.00
0.70
0.80
4.62
0.70
0.25
0.80
0.44
0.50
0.40
Reference
EPA Region VII
EPA Region VII
EPA Region VII
EPA Region VII
EPA Region VII
EPA Region VIII
M & I Report(20)
M & I Report(21)
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Letter
CA WRCB
Plant Visit
Letter
84
-------
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The calculated costs were based on a feed rate of 10 mg/1 and a chlorine
cost ranging from $0.11 to $0.22 per pound varying with plant size from
large to small respectively.
Maintenance Materials
The cost for maintenance materials varied widely from plant to plant
and little information was available for the large plants. The information
in Table 13 is quite reliable in the range of plant size from 0.1 to 1.0
mgd and is extrapolated outside these limits using graphical methods.
Other
The "other" category includes miscellaneous costs related to O & M
including minor supplies, training, and similar items.
NITRIFICATION
Only slight incremental increased O&M costs are required for nitri-
fication optimization over those for a normal extended aeration oxidation
ditch plant. Some low alkalinity wastewaters may require continuous pH
adjustment, but these are considered special cases.
The total incremental addition to plant O&M costs would be 6 to 12
additional days of operator time per year for monitoring pH. This is
shown in Table 13.
NITROGEN REMOVAL
The primary incremental O&M considerations for nitrogen removal
would be additional operator attention to maintain the anoxic zone and
some possible electrical energy savings due to more controlled aeration
and prevention of over aeration. There is no actual plant information to
serve as a basis for the determination so the incremental O&M modifi-
cations are based on estimates and calculations. It is estimated that
aerator energy use could be decreased by 20 percent with the careful con-
trol of aeration required by single stage nitrogen removal operation. The
savings might be much higher in some special cases. Additional operator
attention is required to reliably maintain the anoxic zone and this is
reflected by higher O&M labor requirements. The additional time will
vary somewhat with plant size because the larger plants generally have
more aerators and are spread out more. It is estimated that the annual
additional operator time will vary from 90 man days for small plants to
130 man days for the larger plants including any nitrification consider-
ations. The incremental O&M requirements are shown in Table 13. The
incremental labor requirements shown may seem excessive in relation to
basic requirements.- but operating the plant for consistent nitrogen removal
in the 70 to 80 percent range will require significant operator attention
both during the week days and on weekends. Operation and maintenance
requirements assume that no methanol feed is required for nitrogen removal.
86
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SECTION 6
CONSTRUCTION
CONSTRUCTION COST
Oxidation ditch plant construction costs were determined from costs of
recent plants constructed as shown in Table 14 and include a building with
appropriate laboratory.
The construction costs are shown in Figure 25 with an estimated break-
down of construction costs shown in Table 15. The breakdown of costs
was determined from bid tabulations obtained by letter and during plant
visits. All costs are referenced to the EPA treatment plant index of 262.3
for third quarter of 1976. Construction costs do not include land, engin-
eering, legal, or financing during construction.
NITRIFICATION CONSTRUCTION COSTS
The construction costs for an oxidation ditch plant designed for
nitrification should be essentially the same as the standard oxidation
ditch plant.
NITROGEN REMOVAL CONSTRUCTION COSTS
Any additional construction costs related to nitrogen removal in a
single oxidation ditch channel would be only for permanent ditch dissolved
oxygen monitoring and control equipment, provisions for easy adjustment of
the ditch oxygen level, any necessary auxiliary flow velocity devices,
and, perhaps, a more conservative final clarifier rating because of the
possibility of some nitrogen gas release in the clarifier.
It is doubtful whether an automatic dissolved oxygen control loop will
give satisfactory performance in this application and, therefore, question-
able whether a permanent installation should be made. The best arrangement
may be the use of a portable instrument, manual measurements, and manual
adjustment of the aeration.
The aeration equipment should be readily adjustable in order to facil-
itate changing the oxygen transfer rate. The effluent weir can be designed
so the ditch level will change with flow rate and thereby change the oxygen
transfer rate to partially compensate for flow variations. It is suggested
that the aerator be equipped with a variable speed drive to provide easy
manual adjustment of the aerator oxygen transfer rate. It is also possible
87
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TABLE 14. SOURCES OF OXIDATION DITCH PLANT
CONSTRUCTION COST INFORMATION
Facility
Bossier City, LA
Tivoli, TX
Bernie, MO
Glen Rose, TX
Beevilie, TX
Del Rio, TX
Burkburnett, TX
Henrietta, TX
Woodsboro, TX
Shiner, TX
Glenpool, OK
Crofton, NB
Diboll, TX
Tolar, TX
Mexia, TX
Del Rio, TX
Wylie, TX
Seymour, MO
Marion, S.C.
Greensboro, GA
Cleveland, GA
Baldwin, GA
Clayton Co., GA
Jackson Plant
NE Plant
Clarksville, TX
Huntsville, TX
Paris, TX
Daingerfield, TX
Whitewright, TX
Big Lake, CA
June Lake, CA
Bar Harbor, ME
Indian Island, 1
Newtown Co., GA
Conyers, GA
Kershaw, S.C.
Nixa, MO
Salem, MO
Rushsylvania, OH
Wheelersburg, OH
Lucasville, OH
Greenville, OH
San Antonio, TX
Vernon, TX
Design
capacity, mgd
A 8.00
0.08
0.44
0.35
2.50
1.00
1.10
0.28
0.20
0.60
0.25
0.25
1.00
0.10
0.45
0.40
1.00
0.25
1.00
0.35
0.33
0.30
1.00
1.00
0.70
0.80
4.62
C 0.90
0.25
0.15
1.00
0.07
IE 0.07
0.50
1.00
0.50
0.37
0.74
I 0.10
I 1.20
2.20
3.30
0.16
1.00
Year
constructed
1976
1976
1976
1976
1976
1975
1975
1975
1975
1975
1974
1974
1973
1972
1972
1972
1972
1971
1969
1971
1969
1974
1974
1971
1975
1971
1972
1971
1971
1973
1973
1973
1974
1974
1975
1974
1970
1970
1970
1970
1970
1970
1970
1970
Original
cost, $1000
3,275
179
326
243
1,568
534
369
207
118
300
220
209
225
128
150
150
223
94
412
322
80
165
1,102
431
311
916
1,236
198
130
246
408
139
382
344
569
385
95
187
57
560
607
610
40
185
Reference
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
Plant Visit
State
State
Letter
Letter
Plant Visit
Plant Visit
Plant Visit
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
Lakeside
88
-------
10.C
s
= 1,000
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2 34 56789 2 34 56789 2 34 56789
3.01 0.1 1-0 10
PLANT CAPACITY, mgd
Cost includes building, laboratory outdoor sludge drying beds, but
excludes land, engineering, legal and financing during construction.
All costs referenced to EPA treatment plant index 262.3, fall 1976.
Figure 25. Oxidation ditch plant average construction cost
89
-------
TABLE 15. OXIDATION DITCH PLANT CONSTRUCTION
COST, 1976
Construction Costf $1,000
Plant Capacity, mgd 0.1 i.o IQ.Q
Manuf. equipment & install. 49 138 770
Site work & general 27 96 536
Reinf. concrete & misc. steel 55 180 1,005
Building 16 42 234
Electrical & instrumentation 19 48 268
Piping and valves 29 96 537
Total 195 600 3,350
Costs include outdoor sludge drying beds, a building, and laboratory.
Costs do not include land, engineering, legal, or financing during
construction.
All costs referenced to EPA treatment plant index of 262.3
90
-------
to apply on-off operation of the aerator, but this technique has not been
widely demonstrated in the field and, therefore, would be experimental.
The need for auxiliary flow velocity devices would depend on the
application. The use of these devices has been demonstrated at Glenwood,
Minnesota using a custom built floating device. It is possible that
commercially available propeller mixers could be adapted to this service.
It is recommended that the final clarifier be rated for an overflow
rate of 300 to 400 gpd/sq ft in nitrogen removal applications. The clari-
fiers should also have deep scum baffles and double skimmer bars.
The estimated cost effect of these recommendations is shown in Table
16.
EQUIPMENT PRICING
Pricing was obtained from the manufacturers for aerators, clarifier
mechanisms, and effluent weirs and is shown in Table 17. These costs
include freight, but not installation. Costs are shown for two cases; with
minimum equipment and with at least dual aerators and clarifiers. Where
dual units are used each is sized for one half of design flow.
As a general rule, the small and large diameter aerators are very
similarly priced per foot of length with the small diameter being less
than 10 percent below the price of the large diameter aerator.
DITCH CONFIGURATION
Construction cost estimates were made for various single channel oxi-
dation ditch configurations for comparative purposes. The configurations
include shallow sloped and straight sides and deep straight side ditches.
These estimates were all based on the same assumptions using the Richardson
Estimating Guide<27> and, therefore, should be readily comparable. The
results are shown in Table 18. Costs do not include land, engineering,
legal, or financing during construction, but include 25 percent contractor
profit and overhead.
The effect of configuration is not highly significant but indicates
the shallow sloped side ditch is cost effective for small plants and either
the shallow sloped side ditch or deep ditch is suitable for intermediate
plants. The deep configuration would probably be used for large plants.
PLANT AREA
The space requirements for various oxidation ditch plants will vary
widely, however the typical designs shown in Table 2 were used to develop
the average area requirements as shown in Table 19. These do not repre-
sent the absolute minimum size, but the area needed for a reasonable and
workable layout. The area requirements include space for the raw sewage
lift station, headworks, oxidation ditch, final clarifier(s), return sludge
91
-------
TABLE 16. INCREMENTAL CONSTRUCTION COSTS
FOR NITROGEN REMOVAL
Construction cost, $1000
Plant capacity, mgd 0.1 0.5 l.Q IQ.Q
Aerator variable speed drive 2.0 10.0 14.0 112.0
Auxiliary flow devices 2.0 4.0 6.0 32.0
Final clarifier modifications 3.0 5.0 9.0 40.0
Total incremental addition 7.0 19.0 29.0 184.0
% of standard plant construction
costs 3.6 5.0 4.8 5.5
These costs would be additive to the construction costs for a standard
oxidation ditch plant shown in Table 15 and are based on the same assump-
tions.
92
-------
TABLE 17. OXIDATION DITCH PLANT MAJOR
EQUIPMENT COST, 1976
Plant capacity, mgd
™^—1"•^^™—"^^™^^ • •- --
Clarifier mechanism (s)
Number (minimum number units)
Size, diameter, ft.
Cost, $
(**)
Number (dual)
Size, diameter, ft.
Cost, $
Aerator (s)
Number (minimum number units)
Length, ft.
Cost, $
(*)
Number (with at least dual units)
Length, ft.
Cost, $
Adjustable effluent weir
Cost, $
Total, minimum equipment
Total, dual equipment
(**)
0.1
1
16
13,000
2
12
23,000
1
6
9,000
2
4
16,000
1,700
1.0
10.0
1 2
50 112
30,000 230,000
2 2
36 112
40,000 230,000
2 10
24 50
36,000 320,000
2 10
24 50
36,000 320,000
3,000
$23,700 $69,000 $550,000
$40,700 $79,000 $550,000
(*)Based on minimum of one unit
(**)Based on minimum of two units
Prices include an allowance for freight, but no installation
93
-------
TABLE 18. COMPARATIVE CONSTRUCTION COST OF
OXIDATION DITCH CONFIGURATIONS
Design capacity, mgd
Shallow sloped side (6 feet deep)
0.1
0.5
1.0
Shallow straight side (6 feet deep)
0.1 '
0.5
1.0
Deep straight side (12 feet deep)
0.1
0.5
1.0
Cost,$ Design assumptions
30,621 4" thick poured concrete
85,368 with welded wire reinfor-
133,589 cing. Concrete poured
against undisturbed earth.
Sloped sides poured with-
out use of face forms.
35,516 4" thick concrete bottom
98,249 with welded wire rein-
144,770 forcing poured against
undisturbed earth. Ver-
tical walls 8" concrete
with rebar and concrete
footings. Center wall
common to both sides of
ditch.
33,699 6" thick reinforced con-
76,605 crete bottom poured
132,464 against undisturbed earth.
Vertical walls 12" con-
crete with rebar and
concrete footings. Center
wall common to both sides
of ditch.
Estimates based on Richardson Estimating Guide*27) and contain'25%
contractor profit and overhead, but exclude land, engineering, legal, and
financing during construction. Assumed dimensions are shown in Table 2.
94
-------
TABLE 19. APPROXIMATE OXIDATION DITCH
PLANT AREA REQUIREMENTS
Plant
capacity, Area,
mgd
0.05 -
0.1 1-2
0.5 2.8
1.0 4.2
5.0 10.0
10.0 17-0
Area includes space for raw sewage lift station,
headworks, oxidation ditch, final clarifier(s), return
sludge pumping, sludge drying beds, access roads, control
building, and a 40 foot buffer around plant perimeter.
Space is not provided for future expansion.
95
-------
pumping, sludge drying beds, access roads, control building, and at least a
40 foot buffer around the plant perimeter. Area requirements do not provide
space for plant expansion.
COLD CLIMATE
In extremely cold climates it may be necessary or desireable to enclose
the final clarifier. In addition, all equipment requiring periodic main-
tenance or service should be enclosed including aerators. In very severe
climate it may be desirable to cover the entire oxidation ditch, but exper-
ience in Fairbanks, Alaska^12'13'14)indicates that ditches can operate
in extremely cold climates without an overall cover.
OTHER FACTORS
There are few other factors which have an impact on oxidation ditch
plant construction costs because the plants are generally conservatively
designed. Influent nitrogen concentration above 60 mg/1 would require
additional aeration capacity, but nitrogen concentrations will seldom be
that high.
96
-------
SECTION 7
COMPETING PROCESSES
GENERAL
This section contains process performance and cost information on
treatment processes which are competitive to oxidation ditch plants. These
competitive processes include biological treatment and physical-chemical
nitrogen removal. The biological treatment also includes biological nitri-
fication and denitrification.
The performance and cost information was developed based on actual
plant data to the extent possible. Portions of the cost data are based on
other published cost information as referenced in the text.
CAPABILITY AND RELIABILITY OF COMPETING BIOLOGICAL PROCESSES
Treatment capabilities of the following processes are covered in this
section:
1. Trickling Filters
2. Activated Sludge - with and without effluent filtration
3. Rotating Biological Discs - with and without chemical coagulation
Performance data is presented in two forms. The first form is expected
average process performance which is simply an averaging of plant perfor-
mance over a period of time.
The second form is the return frequency or probability of occurance of
specific effluent qualities. This form presents a more complete and real-
istic assessment of the capabilities of a specific plant or process because
it includes the high and low peaks. Oxidation ditch plant data as previous-
ly developed are plotted on the same figures for comparative purposes.
Trickling Filters
Average data reported in the Deeds and Data section of the Journal of_
Water Pollution Control Federation (JWPCF) between 1960 and 1965 are
summarized on Figure 26. Organic loading for these plants ranged from 5.7
to 100 pounds of BOD5/1,000 cu ft/day.
97
-------
I
c?
s
t-
z
111
130
120
110
100
90
60
70
60
SO
40
30
20
10
10 20 30 40 50 60 70 80 90 100
- POUNDS PER 1.000 CUBIC FEET/DAY
Figure 26. Effluent quality, trickling filters.
98
-------
Typical variations in effluent quality from two Texas trickling filter
plants, one low rate and one high rate, are shown in Figure 27. The data
for trickling filter plants in Figure 27 were selected from several reports
and are a fairly typical representation of the capability of this process
in a relatively warm climate. In specific instances, an effluent quality
of 25 mg/1 and nitrification to levels of 2 mg/1 NH3~N have been achieved.
(28)
Benzie, et al , showed decreasing BODs removal efficiencies due to
cold weather operation of 17 trickling filter plants in Michigan. The loss
in efficiency varied considerably; depending on recirculation practices.
Averaging the data of all 17 plants, the loss in BOD5 removal efficiency was
12%. The decrease in efficiency for those plants employing recirculation
was 21%. Those plants without recirculation showed a 6% decrease in effi-
ciency. The analyses show the 21% difference to be statistically signi-
ficant but the 6% difference was not.
fCulp/Wesner/Culp visited 13 trickling filter plants of various designs
as part of the work under EPA Contract 68-01-4329. One to three years
data were obtained from each of these plants and are shown in Table 20.
Trickling filter plant data summarized from EPA Region inspection and
technical assistance reports are shown in Table 21.
Activated Sludge
The activated sludge process is capable of converting essentially all
influent soluble organic matter to solids. It is necessary to efficiently
remove these solids in order to attain high quality effluents. Therefore,
the final clarification unit process is extremely important to production
of a high quality effluent. Careful operation is necessary to attain good
effluent quality consistently.
a
The data from activated sludge processes reflect the problems in attain-
ing consistently good effluent quality. The deeds and data section of the
JWPCF reports data from 20 plants during the period from 1960 to 1965.
Plant BOD loadings ranging from 18 to 74 pounds BODs/1,000 cu ft/day resulted
in average effluent BODs values of 3 to 86 mg/1 with 8 of the 20 plants
reporting average BOD5 values of less than 20 mg/1.
Activated sludge plant data summarized from EPA Region inspection and
technical assistance reports are shown in Table 22. Data are shown for
conventional, contact stabilization, and extended aeration process varia-
tions .
Daily data presented in the form of return frequency are shown on
Figure 28 and iere developed from 4 Austin, Texas contact stabilization
plants and activated sludge plants at Grand Island, Nebraska; Dallas,
Texas; High Point, North Carolina; Chicago, Illinois; and Ypsilanti, Michi-
gan. The data, in each case, represent an entire year. The plants
selected for data presentation were those where good analyses are known to
be performed, a variety of activated sludge processes are used and a range
99
-------
80
70
^ 60
i
50 IbAOOO Cu ft
u>
Q
8
UJ
_J
LL
U.
LU
50
40
30
20
10
2016/1,000 Cu ft
AVE. OXIDATION
DITCH (FIG. 18)
12.7 lb/1,000 Cu ft
5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 27. Trickling filter effluent quality, two Texas plants.
100
-------
TABLE 20. TRICKLING FILTER SUMMARY, PLANT VISITS
Average effluent
Location
Iowa:
Shellsburg
Center Pt.
Monticello
Cascade
Independence
Lake view
Georgia:
Westside,
1975
Westside,
1976
Kennesaw,
1974
Sand town,
1976
Newman,
1975
Newman ,
1976
Intr. Cr.,
1975
Intr. Cr. ,
1976
College Pk. ,
1976
Athens #1,
1975
Athens #1,
1976
Athens #2,
1975
Athens #2 ,
1976
Cedartown,
1974
Cedartown,
1975
Cedartown ,
1976
Design
flow,
mgd
0.0825
0.200
0.800
0.220
0.750
0.175
1.00
1.00
0.30
1.00
0.40
0.400
20.0
20.0
1.2
5.00
5.00
2.00
2.00
1.00
1.00
1.00
Av
Average
flow,
mgd
0.0609
0.141
0.412
0.081
0.889
0.153
1.059
0.971
0.27
1.222
0.328
0.346
13.9
13.1
1.36
5.60
5.14
2.90
2.60
0.82
0.92
1.06
erage removal
BOD5
mg/1
47
43
39
48
85
69
25
35
24
51
30
23
40
35
43
80
64
46
47
46
17
23
BOD5
Removal ,
%
70
72
80
76
87
69
75
72
85
70
88
88
82
83
90
69
73
73
75
77
89
88
79
TSS,
mg/1
—
—
-
—
-
30
38
28
50
28
29
26
31
35
64
47
58
40
40
18
22
TSS
removal , %
—
—
—
—
—
70
68
87
60
88
86
78
74
76
73
80
68
78
80
90
88
78
Data obtained during plant visits under EPA Contract 68-01-4329
by Culp/Wesner/Culp.
101
-------
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50
45
CONTACT STABILIZATION
40-50 lb/1,000cuft
CONVENTIONAL
ACTIVATED SLUDGE
20-80 lb/1,000cu
AVE. OXIDATION
DITCH (FIGS. 181. 19)
12.7 lb/1,000cuft
5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 28. Activated sludge effluent quality.
104
-------
of loadings are experienced. The conclusions which may be made from these
data are:
1. Two of the activated sludge plants treat significant industrial
waste flows. The High Point, North Carolina, Eastside plant
receives textile dye wastes and the Grand Island, Nebraska plant
receives slaughterhouse wastes. Both plants perform as well as
the other domestic waste plants. Therefore, activated sludge
plants can be designed and operated to treat industrial wastes
and perform as well as plants treating little or no industrial
wastes.
2. The plant loadings range from 20 to 80 pounds of BOD5/day/l,000
cu ft of aeration tank volume. The performance of the plants is
not related to unit aeration basin organic loading.
3. Whereas all of the plants from which data are used are considered
to have good operational control and design, the Grand Island
plant, for one year, produced an effluent 6005 significantly
better than 10 mg/1, 70 percent of the time. Four of the plants
produced BODs effluent better than 35 mg/1, 90 percent of the
time.
The potential for the activated sludge process is exemplified by the
Grand Island plant which produced a BOD5 better than 5 mg/1, 50 percent
of the time and 20 mg/1, 90 percent of the time.
Biological nitrification of ammonia to nitrate is a well established
phenomenon and several bench scale processes and demonstration processes
have shown virtually complete conversion is possible if sufficient oxygen
transfer is available. Several activated sludge plants having excess
oxygen transfer capability do nitrify; however, until the past few years,
few plants routinely monitored effluent ammonia. ^
A source of good data suitable for probability analysis on activated
sludge nitrification is available from the Dallas demonstration pilot
plant as shown in Figure 29. The plant was a constant flow plant receiving
trickling filter effluent having an average BOD5 of 60 mg/1. The aeration
basin was loaded at 20 pounds/1,000 cu ft/day and had an average hydraulic
detention time of 4 hours. Solids retention time (SRT) varied from 7 to
20 days. The final clarifier had an overflow rate of 350 gpd/sq ft and a
3.5 hour detention time.
The median effluent BOD5 was less than 20 mg/1 and 50 percent of the
time the effluent contained zero ammonia nitrogen. Seventy percent of the
time the effluent ammonia was less than 2 mg/1. Poorer results were ob-
tained when the SRT was in excess of 15 days. Clarifier solids buildup
associated with attempting to thicken sludge in the clarifier resulted in
nitrification and poorer effluent quality. Thfe pilot plant was monitored
continually and the operators were highly skilled individuals who reacted
quickly to ill effects.
105
-------
30 40 50 60 70
PERCENT OF TIME VALUE WAS LESS THAN
Figure 29. Activated sludge effluent quality, Dallas, Texas
nitrification pilot plant and El Lago, Texas.
106
-------
The data show that the activated sludge process may produce an effluent
quality of 2 mg/1 NH -N 70 percent of the time.
The low rate trickling filter plant at El Lago, Texas was modified in
1973 to provide advanced waste treatment. A second stage suspended growth
reactor was added down stream of the trickling filter to provide essenti-
ally two stage nitrification. Small media biological denitrification
towers were added to provide denitrification using methanol as a carbon
source. Performance results for this plant are shown in Figure 29. Al-
though the El Lago treatment process is not totally activated sludge, the
data do provide a good indication of biological nitrification-denitrifica-
tion performance.
Polishing of activated sludge effluent using filtration can produce
a high effluent quality. Soluble BOD5 from the activated sludge process
is low and the majority of the remaining BODs results from the solids
escaping the final clarifier.
When no coagulants are used, the filterability of solids in a biolog-
ical plant effluent is dependent upon the degree of flocculation achieved
in the biological process. A trickling filter achieves a poor degree of
flocculation and efficient filtration of the effluent from a trickling
filter plant will usually provide only about 50 percent removal or less of
the suspended solids normally present. The activated sludge process is
capable of much higher degrees of biological flocculation than is the
trickling filter process. Gulp and Hansen^8* found that up to 98 percent
of the suspended solids in an extended aeration plant effluent with 24 hr
aeration treating domestic sewage could be removed by filtration producing
turbidities as low as 0.3 Formazin Turbidity Unit (FTU) without the use of
coagulants. These authors later reported that pilot plant studies showed
that the degree of biological flocculation achieved in an activated sludge
plant was directly proportional to the aeration time and inversely pro-
portional to the ratio of the amount of organic material added per day to
the amount of suspended solids in the aeration chamber (F/M ratio) . Vari-
ation of mixed liquor suspended solids in the normal operating range of
1,500-5,000 mg/1 did not significantly affect the filterabiltiy of the
effluent at a given aeration time and load factor. For domestic wastes,
aeration times of 10 hr or more were found to provide flocculation ade-
quate to permit an efficient downstream filter to remove 90-98 percent of
the effluent suspended solids. The flocculation provided by aeration times
of 6-8 hr with domestic wastes enabled 70-85 percent suspended solids
removal from the secondary effluent.
Data from four activated sludge plants with effluent filtration are
shown in Figure 30. The data represent periods from 20 days to one year
of operation. The average and best oxidation ditch unfiltered effluent
are shown for comparison.
The data are fairly consistent, indicating an effluent BOD5 of less
than 5 mg/1, 50 percent of the time, and less than 10 mg/1 90 to 95 per-
cent of the time.
107
-------
AVE. OXIDATION DITCH
BOD. (FIG. 18)
(UNFILTERED)
SPRING VALLEY, IL
BEST OXIDATION
DITCH BOD5 (FIG. Ql)
(UNFILTERED)
30 40 50 60
PERCENT OF TIME VALUE WAS LESS THAN
Figure 30. Filtered activated sludge plant BOD quality based
on four plants.
108
-------
The current Dallas effluent criteria require an effluent quality of
10/10 (BOD5/TSS). Since the effluent filtration of activated sludge pro-
vided better quality than the criteria, effluent filters were installed at
Dallas to treat only a portion (70%) of the flow and the effluent was
blended to meet effluent quality requirements. This resulted in cost
savings over providing filtration of the entire flow.
A survey of small privately operated extended aeration and contact
stabilization package plants was carried out in the Cincinnati, Ohio, area
and in Dade County, Florida. These plants were treating domestic sewage
and were in the size range of 0.05 to 1.0 mgd.
The median final effluent quality was reported as follows:
Dade Co. Cincinnati
No. of Plants 46 20
BOD5, mg/1 13 29
Suspended Solids, mg/1 26 42
The reported reliability is shown in Figures 31 and 32 along with com-
parative data for oxidation ditch plants. These data show that the oxida-
tion ditch plants perform better than conventional package plants.
Rotating Biological Contactors
Rotating biological contactors (RBC), as a secondary treatment alter-
native, are relatively new and only a few plants have been in operation
for more than one year. Very little full scale data are available.
Recently, the data from the Gladstone, Michigan plant have become
available affording a detailed analysis of the RBC process capability at
one plant.
The Gladstone, Michigan plant is a 1 mgd plant and consists of primary
sedimentation, RBC's designed for 1.94 gpd/sq ft of effective disc surface
area, chemical addition, and final sedimentation. The plant started up in
March of 1974 and reached stable operation by June of 1974. The manufac-
turer's literature would predict the following effluent quality based on
the operating data when chemicals were not added. The actual results are
in parenthesis.
109
-------
90
80
60
50
40
30
20
10
CINCINNATI AREA PLANTS
DAOE CO. FLORIDA PLANTS
AVERAGE
OXIDATION DITCH PLANT (FIG. 18)
2 5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 31. Activated sludge package plant reliability, BOD.
110
-------
180
160
140 -
120
r
8 100
o
(A
O
111
o
3
80
60
40
20
CINCINNATI AREA PLANTS
DADE CO. FLORIDA PLANTS
5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 32. Activated sludge package plant reliability, Suspended Solids,
111
-------
Q
gpd/sq
1.5
1*7
1.5
Removal ,
Predicted
(Actual)
BOD NIL
f t % D
92.9(82.8)
92.4(81.9)
92.2(88.2)
C
99
97
99
Effluent
Quality,
Predicted
(Actual)
BOD NH.
rog/I me
7(17)
8(19) 0.61
8(12) 0.21
s
(<1
[<1
BOD
in,
Month mg/1
June, 1974 99
July, 1974 105
Aug., 1974 102
Work by the University of Michigan at the Saline, Michigan RBC plant
showed ammonia nitrogen removals of 95 to 98 percent at hydraulic loadings
between 1.0 and 3.0 gpd/sq ft and temperatures between 11 and 21°C.
For the three months of operation, the effluent BOD5 averaged 16 mg/1,
whereas a BOD5 of 8 mg/1 would be predicted by the manufacturer's liter-
ature. The nitrification, according to the manufacturer's literature
should be anticipated to be near complete, and the results (6 analyses)
support the prediction for the months shown.
Therefore, the Gladstone plant BODs results indicate that the manu-
facturer's design bases should be used cautiously when high quality
effluents are required.
Further analysis of the Gladstone data on a daily basis was made for
the three months of stable operation where chemical additions were not made.
These results are shown on Figure 33.
These data are comparable to well operated conventional activated
sludge plants. At low unit flows the effluent quality that might be
anticipated is less than 20 mg/1, 50 percent of the time and less than
30 mg/1 90 percent of the time.
In September of 1974, chemical treatment of the RBC effluent prior to
final clarification was initiated. About 80 mg/1 of alum and 0. 5 mg/1
of polymer were added. The results of the chemical addition proved to
increase effluent quality as shown on Figure 34. A period of nonchemical
treatment during the latter part of December, 1974 through the early part
of February, 1975 showed a concomitant increase in effluent BOD_. Ammonia
nitrogen removal remained good during summer months when wastewater tem-
peratures were in excess of 13 C, but lower nitrification rates were
exhibited during the colder winter months.
The conclusions which may be reached based on the Gladstone, Michigan
data are as follows:
1. At low unit flow rates (1.0-2.0 gpd/sq ft of effective disc area)
effluent BOD5 values from the RBC, will be comparable to con-
ventional activated sludge processes.
112
-------
5 2.0
;g
=> O
111
IL
u.
Ul
1.0
80
40
UL
IL
111
10
AVE. OXIDATION
DITCH PLANT SUSPENDED
SOLIDS (FIG.
AVE. OXIDATION
DITCH PLANT BOD5
(FIQ. IS)
5 10 20 30 40 50 60 70 80 90 95 98 99
PERCENT OF TIME VALUE WAS LESS THAN
Figure 33. RBC effluent quality, Gladstone, Michigan.
113
-------
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114
-------
2. With chemical coagulation additions, effluent BOD,, values from
the RBC will be consistently less than 10 mg/1, at the loadings
used at Gladstone, Michigan.
3. Ammonia nitrogen concentrations in the Gladstone, Michigan efflu-
ent exceeded 2 mg/1 consistently; however, good nitrification was
experienced during the warmer summer months.
The RBC plant at Pewaukee, Wisconsin is a 0.46 mgd plant with a design
disc hydraulic loading of 2.45 gpd/sq ft and final clarifier overflow rate
of 500 gpd/sq ft. Average operating results without chemical addition for
the years 1972, part of 1974, and 1976 are as follows.
Final effluent, Removal accross
mg/1 RBC, %
Year BODc; TSS BODg TSS
1972 20.3 15.8 84 83
1974(9 months) 31.9 24.0 69 73
1976 25.0 30.0 86 88
The RBC plant at Edgewater, New Jersey operated at an average flow of
0.51 mgd, disc hydraulic loading of 2.5 gpd/sq ft, and final clarifier
overflow rate of 518 gpd/sq ft produced the following results in 1973-1974.
Removal across
Final effluent, mg/1 RBC, %
BODs TSS BODtj TSS
37.6 21.6 75 69
PERFORMANCE SUMMARY
A review of effluent data for various biological waste treatment pro-
cess indicates that realistic effluent 6005 and NH3-N criteria for average
to well operated plants may be assigned as shown in Table 23.
COSTS OF COMPETING BIOLOGICAL PROCESSES
Contact Stabilization
Aeration requirements are based on 30 cfm per thousand cubic feet of
aeration volume and 20 cfm per thousand cubic feet of aerobic digester
volume, plus any air lift pumps within the plant. Costs are based on
centrifugal blowers with stand-by for the largest unit. All plants in-
clude internal chlorine contact and sludge stabilization chambers. The
smaller sizes are of all steel construction, based on above-grade install-
ation. Below-grade installation would be somewhat more expensive, due to
the additional reinforcing required for external soil loads when the tanks
are empty. Concrete pad costs were based on the diameter of the package
plant plus 10 feet (5 feet extension beyond side of package plant). The
larger sizes are based on steel internal equipment installed in poured
reinforced concrete tankage. A contingency allowance of 15 percent was
115
-------
TABLE 23. SUMMARY OF COMPETING PROCESS
PERFORMANCE.
(*)
Ammonia-N,
BODg, mg/1 mg/1
50% of time 90% of time 50% of time
Trickling Filter (Average
for various plant
loadings). 25 40 2
Activated Sludge 15 40
1
Activated Sludge with
Effluent Filtration 5 10 l
20 35 2
RBC With Chemical Coagulation 10 20 2
Activated Sludge Package
Plants 20 50 1
Oxidation Ditch 8 21 1
*Assuming system is designed for nitrification at the lowest anticipated
operation temperature for the location.
116
-------
added to the manufacturer's estimate of the equipment and erection costs.
In addition, electrical and instrumentation was calculated at 15 percent
of equipment costs and contractors overhead and profit at 25 percent of
equipment costs.
O & M costs were based on a labor rate of $9/hour including fringe
benefits with seven day/week staffing as follows:
Capacity, mgd Annual Labor, Hours
0.1 1,800
0.25 2,600
0.50 3,900
0.75 5,000
1.00 6,000
3.00 10,000
This amount of operator attendance is greater than many package plants
have received in the past, but is the minimum felt by the authors to be
consistent with satisfactory plant performance. Maintenance materials are
based on 3% of the equipment costs.
Construction costs for contact stabilization plants are shown in Table
24. Operation and maintenance costs are shown in Table 25.
Extended Aeration
Air requirements are based on 2,100 cubic feet per pound of BOD re-
moved (2 Ib BOD/1,000 gallons/day). Positive displacement blowers with 100%
standby are provided. As with contact stabilization plants, a contingency
allowance of 15 percent was added to the manufacturer's estimate of the
equipment and erection costs. In addition, electrical and instrumentation
was calculated at 15 percent and contractors overhead and profit at 25
percent of equipment costs.
0 & M costs were based on a labor rate of $9/hour including fringe
benefits with seven day/week staffing as follows:
Capacity, mgd Annual Labor, Hours
0.01 500
0.02 - 650
0.05 1,000
0.07 1,422
0.09 1,670
0.5 5,200
The amount of labor is based on CWC experiences with small package
extended aeration plants and the labor required for proper O & M.
Construction costs for extended aeration package plants are shown in
Table 26. Operation and maintenance costs 'are shown in Table 27.
117
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TABLE 24. CONSTRUCTION COST OF CONTACT
STABILIZATION PLANTS, 1976
Construction cost, $1,000
Plant capacity, mgd
Tankage & equipment
in place
Chlorination equipment
Concrete work
Subtotal
Yardwork
Total construction cost
0.1
132
6
7
145
20
165
0.25
191
6
14
211
30
241
0.50
250
6
24
280
40
320
0.75
298
10
36
344
48
392
1.0
355
12
50
417
58
475
3.0
972
33
137
1,142
158
1,300
Costs do not include land, engineering, legal, or financing during
construction, but include 25 percent contractors profit and overhead.
118
-------
TABLE 25,
Annual cost, $1,000
Plant capacity, mgd
Labor
Energy
Maintenance materials
Chlorine
Total annual
Total, $/l,000
gal 0.638 0.403 0.314 0.278 0.257 0.167
0.1
16.2
2.4
4.0
.7
23.3
0.25
23.4
5.7
6.0
1.7
36.8
0.50
35.1
11.4
7.5
3.3
57.3
0.75
45.0
18.0
9.0
4.0
76.0
1.0
54.0
24.0
10.6
5.3
93.9
3.0
90.0
65.0
15.0
13.0
183.0
Costs include labor at $9.00 per hour including fringe benefits and
electrical energy at $0.03 per kwh.
119
-------
TABLE 26.
Construction cost, $1,000
Plant capacity, mgd
Tankage & equipment
in place
Chlorination equipment
(cylinder mounted)
Concrete work
Subtotal
Yardwork
Total construction cost
0.01
31
0.02
40
0.05
89
0.07
109
0.09
126
0.5
268
1
_4
36
5
41
1
J5
47
6
53
1
_2
99
9
108
1
11
121
17
138
1
14
141
19
160
2
60
330
60
390
Costs do not include land, engineering, legal, or financing during
construction, but include 25 percent contractors profit and overhead.
120
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TABLE 27. OPERATION AND MAINTENANCE COSTS,
EXTENDED AERATION PLANTS, 1976
0.01
4.5
0.4
0.9
0.1
5.9
0.02
5.8
0.6
1.2
0.1
7.7
0.05
9.0
1.2
2.7
0.4
13.3
0.07
12.8
2.1
3.1
0.5
18.5
0.09
15.0
2.7
3.5
0.6
21.8
0.5
46.8
7.0
7.5
3.3
64.6
Plant capacity, mgd
Labor
Energy
Maintenance materials
Chlorine
Total annual
Total $/l,000
gal 1.616 1.055 0.729 0.724 0.664 0.354
Costs include labor at $9.00 per hour including fringe benefits and
electrical energy at $0.03 per kwh.
121
-------
Conventional Activated Sludge
The oxidation ditch, extended aeration, and contact stabilization
plant approaches provide an aerobically stabilized sludge for final dis-
posal. In order to provide a comparable sludge, the conventional plants
incorporate aerobic digesters. Sludge handling costs beyond aerobic
stabilization are not included. Design of the aerobic digesters is based
on criteria and procedures developed under EPA Contract 68-03-2186, Task 7
by CWC.
The activated sludge system schematic is shown in Figure 35. The
major unit processes are primary sedimentation, activated sludge aeration,
secondary sedimentation, aerobic digestion, and chlorination.
Using the primary effluent data and McKinney's complete mix model(1),
the activated sludge system design criteria shown in Table 28 were devel-
oped. Unit process sizing is shown in Table 29. The aeration system
design was limited to a peak hour oxygen uptake rate of 70 mg/l/hr. An
SRT of 5 days was used. The return activated sludge pumps were sized for
a 1 percent sludge concentration and completing a system solids balance.
The secondary clarifiers were sized based on hydraulic overflow rate of*
600 gpd/sq ft of average flow.
The chlorine contact basins are sized for a 30 minute detention time
at peak dry weather flow (PDWF, 1.5 times design flow). A dosage rate
of 10 mg/1 was applied to the PDWF for sizing feed equipment.
General—
The construction and O & M costs for conventional activated sludge
plants were developed by unit process through a review of the costs of
actual plant construction and operation, equipment cost data from manu-
facturers, and published cost data. Generalized cost curves developed
under EPA Contracts 68-03-2186 and 68-03-221 were used for estimating
unit process costs. These generalized curves should not be used for com-
paring alternative processes as in this report. Individual plant costs
must be developed based on the specific wastewater treatment plant design,
local labor and material costs, and local climatic and site conditions.
Some of the limitations, in addition to the general local conditions
discussed above, include no standby provisions, no specific modular sizing
other than minimum available sizes, and no adjustments for local regulatory
agency design restrictions.
Estimated construction costs are shown in Table 30 and O & M costs
in Table 31.
Construction Costs—
Sedimentation—The source of the construction cost curve for sedimen-
tation was the report to EPA, "Costs of Chemical Clarification of Waste-
water", January, 1976, EPA Contract 68-03-2186. These cost data were
developed from quantity takeoffs and equipment manufacturer's estimates
122
-------
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123
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TABLE 28. DESIGN PARAMETERS, ACTIVATED SLUDGE
Raw wastewater:
Suspended solids, mg/1 200
Volatile content, % 75
BOD5, mg/1 200
Temperature, C 20
Peaking factor (dry weather) 1,5
Alkalinity, mg/1 as CaCO 200*
Primary sedimentation:
Surface loading, gpd/sq ft @ ADWF (*) 800
Suspended solids removal, % 65
Sludge concentration, % 5
BOD5 removal, % 30
Effluent BOD5, mg/1 140
Activated sludge:
F/M Ib BOD5/lb MLVSS/day 0.355
MLSS, mg/1 3 ^50
Hydraulic detention time, hr ' 4
Solids retention time (SRT), days 5
Final clarifiers:
Surface loading, gpd/sq ft @ ADWF(*) 600
Solids loading, Ib/sq ft/day @ PDWF (**) <35
Return activated sludge, percent of
influent flow 46
Return activated sludge concentration, % i
Chlorination:
Detention time @ PDWF, minutes 30
Dosage, mg/1 10
Aerobic digestion, conventional air:
Influent solids, percent
1 mgd -^
5 and 10 mgd (automated decant cycle) 2
Detention time, days 15<6
*ADWF - Average dry weather flow
**PDWF - Peak dry weather flow
124
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TABLE 29. UNIT PROCESS SIZES, ACTIVATED SLUDGE
Plant capacity, mgd
Unit process or component
Primary sedimentation tanks
surface area, sq ft
Aeration tank volume, cu ft
Aerators, hp
Final clarifier surface
area, sq ft
RAS pumps, mgd
WAS pumps, gpm
Sludge pumps, gpm
Chlorination, cu ft
Chlorine feed equipment,
tons/year(average/peak)
Aerobic digestion
Tank volume, cu ft
Aerators, hp
15.2/22.8
51,000
58
(*)
76/114
127,750
145
10
1,250
22,300
40
l'<*7
.46/.69v '
45
15
4,180
6,250
111,500
200
8,335
2.30/3.45
225
75
20,900
12,500
223,000
400
16,670
4.60/6.90
450
150
41,800
152/228
225,500
290
Average/peak - Average flow is used to determine the power requirement
and maintenance materials cost. Peak capacity is used to determine
construction cost and labor requirement.
125
-------
TABLE 30. CONSTRUCTION COSTS, ACTIVATED SLUDGE, 1976
Plant capacity, mgd
Primary sedimentation tanks
Aeration basins
Aeration equipment
Secondary sedimentation tanks
Return activated sludge pumping sta.
Waste activated sludge pumping sta.
Primary sludge pumping station
Chlorine contact basins
Chlorination equipment
Aerobic digestion
Basins
Aerators
Yardwork
Total construction cost
Construction cost, $1,000
_JL 5 10
$1,045
$4,138
Costs do not include land, engineering, legal, or financing during
construction, but include 25 percent contractors profit and overhead.
126
-------
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Aeration Basins—Historical aeration basin cost data has been updated
with results of other detailed cost studies by CWC and recent costs ob-
tained from Black & Veatch and CH2M Hill,
These costs include excavation, concrete, walkways, in-basin process
piping, handrails, and attendant costs. The construction cost data apply
more closely to circular or square tanks used for complete mix activated
sludge design than to long, narrow tanks as used for a plug flow mode.
Mechanical Aeration Equipment—Cost data for installed mechanical
equipment have been derived from experienced cost data and equipment costs
supplied by manufacturers.
Return Activated Sludge Pumping Station—The cost relationships for
recycle pumping developed by Black and Veatch(29) were adjusted and used
as a basis for estimating the cost of the return activated sludge pumping
stations in this study. The costs shown by Black and Veatch have approxi-
mately doubled due to inflation, stricter OSHA requirements, and regula-
tory agency reliability standards.
The pumping stations are assumed to employ vertical diffusion vane
pumping units with attendent valves, piping, and control facilities. The
pump is suspended in the wet well and motors and motor control centers are
housed in a superstructure.
Waste Sludge Pumping Stations—Waste sludge pumping equipment costs
are based on the use of intermittent sludge pumping with positive displace-
ment pumps. The cost data presented in the Black and Veatch cost curves
were updated for this study.
Included in the pump station cost is an underground structure which
houses the pumps and piping and is constructed adjacent to and in conjunc-
tion with the sedimentation basin. Also included is a superstructure which
houses electrical control equipment.
Chlorination—Chlorine contact basin costs are based on the same con-
struction used for the aeration basin costs. Chlorine feed equipment costs
are based on chlorine gas feed and are taken from the draft report by CWC
for the EPA "Estimating Initial Investment Costs and Operating and Mainten-
ance Requirements of Stormwater Treatment Processes", EPA Contract
68-03-2186.
Operation and Maintenance Costs—
The operation and maintenance costs consist of labor, power and main-
tenance materials. The individual costs were developed through a variety
of resources including recent CWC work for the EPA and the Black and Veatch
study . in some instances, operating plants were consulted for informa-
tion on labor requirements.
Sedimentation Basins—O & M requirements are based on the Black and
Veatch report(29>.
128
-------
Mechanical Aeration—Operation and maintenance requirements for the
aeration systems are expressed in terms of the installed aerator horsepower.
Labor requirements are based on the Black and Veatch report(29>. The power
requirements were calculated on the basis of an assumed oxygen transfer
of 2.0 Ib 02/hp-hr or 3.0 Ib Oo/KWH. Maintenance material costs are based
on the Black and Veatch report*29).
Return Activated Sludge Pumping Station—The return activated sludge
pumping station labor requirements are based on the Black and Veatch report
The power requirements are based on the Black and Veatch report<29) assuming
a head of 10 feet. The maintenance material costs are an update of the
Black and Veatch report(
Waste Sludge Pumping Station—Labor requirements for the waste sludge
pumping stations are based on the Black and Veatch report1 9'. The power
requirements were based on a pumping head of 25 feet and a pumping effi-
ciency of 40 percent (progressing cavity pumps). Maintenance material
costs were updated from the black and Veatch report * '.
Chlorination—Labor requirements and maintenance material costs for
chlorination are based on the Black and Veatch report . The chlorine
costs are based on recent quotes for ton cylinders and tank car lots.
BIOLOGICAL NITRIFICATION
Single Stage Nitrification
The design of this system is based on a mean cell residence time of
10 days to achieve nitrification in a single stage activated sludge system.
A complete discussion of nitrification is contained in Section 4. Tables
32 and 33 show design parameters for the nitrification system and the
resulting unit process sizes. Construction and 0 & M costs are based on
the same sources described for the conventional activated sludge system.
The effects of different average ammonium nitrogen concentrations (10, 20,
and 30 mg/1) were estimated based upon providing 4.6 Ib oxygen per Ib of
ammonium nitrogen. Peak hourly ammonium nitrogen concentrations of twice
the average were assumed with peak hourly BOD concentrations of 1.5 times
the average. An oxygen transfer efficiency of 2 Ib /hp-hr was used.
Estimated construction costs are shown in Table 34 and 0 & M costs in
Table 31.
Two Stage Nitrification
Tables 35, 36, and 37 show the design parameters used for this system
and resulting costs. The same basic assumptions used for the single stage
nitrification system on ammonium concentrations, oxygen transfer, and
similar parameters were used for two stage.
129
-------
TABLE 32.
Raw wastewater:
Suspended solids, mg/1 200
Volatile content, % 75
BOD5/ mg/1 200
Temperature, C 20
Peaking factor (dry weather) 1.5
Alkalinity, mg/1 as CaCO 200
Primary sedimentation:
Surface loading, gpd/sq ft @ ADWF*** 800
Suspended solids removal, % 55
Sludge concentration, % 5
BOD5 removal, % 30
Effluent BOD5, mg/1 140
Activated sludge:
F/M, Ib BOD5/lb MLVSS/day 0.20
MLSS, mg/1 3,270
Hydraulic detention time, hr ' 7
Solids retention time (SRT), days 10
Ammonium concentrations, average 10,20,30 mg/1
Peak hourly ammonium concentration 2 x average
Peak hourly BOD5 1.5 x average
Final clarifiers:
Surface loading, gpd/sq ft @ ADWF(** 600
Solids loading, Ib/sq ft/day @ PDWF 35
Return activated sludge, percent of
influent flow 59
Return activated sludge concentration,
percent 0>8
Chlorination:
Detention time @ PDWF, minutes 30
Dosage, mg/1 10
Aerobic digestion, conventional air:
Influent solids, percent
1 mgd ^
5 and 10 mgd (automated decant cycle) 2
Detention time, days 15.6
(*)
ADWF - Average dry weather flow
(**)
PDWF - Peak dry weather flow
130
-------
TABLE 33. UNIT PROCESS SIZES, SINGLE STAGE ACTIVATED SLUDGE NITRIFICATION
Plant capacity, mgd
Unit process or component
Primary sedimentation tanks surface
area, sq ft
Aeration tank volume, cu ft
Aerators, hp
10 mg/1 NH -N
20 mg/1 NH -N
30 mg/1 NH4-N
Final clarifier surface area,
sq ft
RAS pumps, mgd
WAS pumps, gpm
Sludge pumps, gpm
Chlorination contact tank volume,
cu ft
Chlorination feed equipment,
tons/year (average/peak)
Aerobic digestion
Tank volume, cu ft
Aerators, hp
1
1,250
39,000
50
70
85
1,667
69/1.03(*)
36
15
5
6,250
195,000
270
350
425
8,335
3.45/5.18
180
75
10
12,500
390,000
540
700
850
16,670
6.90/10.3
360
150
15.2/22.8
4,180 20,900
(*)
41,800
76/114 152/228
51,100 127,750 255,500
58 145 290
Average/Peak - Average flow is used to determine the power requirement
and maintenance materials cost. Peak capacity is used to determine
construction cost and labor requirement.
131
-------
TABLE 34.
CONSTRUCTION COST, SINGLE STAGE ACTIVATED SLUDGE
NITRIFICATION
Construction cost/ $1,000
_!_ . 5 10
Plant capacity, mgd
Primary sedimentation tanks
Aeration basins
Aeration equipment
Secondary sedimentation tanks
Return activated sludge pumping sta.
Waste activated sludge pumping sta.
Primary sludge pumping station
Chlorine contact basins
Chlorination equipment
Aerobic digestion
Basins
Aerators
Yardwork
Total construction cost
at 20 mg/1 NH -N
at 10 mg/1 NH ~N
at 30 mg/1 NH^-N
Table based on 20 mg/1 NH4-N. Construction costs of aeration equipment
at '
10 mg/1 NH4-N: 1 mgd = $105,000; 5 mgd = $270,000; 10 mgd = $550,000
20 mg/1 NH4-N: 1 mgd = $160,000; 5 mgd = $530,000; 10 mgd = $900,000
The costs do not include land, engineering, legal, or financing during
construction, but include 25 percent contractor profit and overhead.
$1,210
$1,185
$1,240
260
570
450
330
200
190
93
140
37
380
160
393
$3,203
$3,023
$3,283
440
850
800
590
300
270
140
200
60
550
280
627
$5,107
$4,957
$5,207
132
-------
TABLE 35. DESIGN PARAMETERS, TWO STAGE ACTIVATED SLUDGE
NITRIFICATION
Raw wastewater:
Suspended solids, mg/1 200
Volatile content, % 75
BOD5, mg/1 200
Temperature, C 20
Peaking factor (dry weather) 1.5
Alkalinity, mg/1 as CaCO_ 200
Primary sedimentation:
Surface loading, gpd/sq ft @ ADWF 800
Suspended solids removal, % 65
Sludge concentration, % 5
BODtj removal, % 30
Effluent BOD5, mg/1 140
Activated sludge:
First stage complete mix aeration
F/M, Ib BOD5/lb MLVSS/day 0.34
MLSS, mg/1 3,000
Hydraulic detention time, hr 3
Solids retention time (SRT), days 4
Second stage plug flow aeration
F/M, Ib BOD5/lb MLVSS/day 0.12
MLSS, mg/1 2,400
Hydraulic detention time, hr 4
Solids retention time (SRT), days 10
Ammonium concentrations, average 10,20,30, mg/1
Peak hourly ammonium concentration 2 x average
Peak hourly BOD5 1.5 x average
Final clarifiers - first & second stage: ...
\ 7
Surface loading, gpd/sq ft @ ADWF, 600
Solids loading, Ib/sq ft/day @ PDWF ( ' 35
Return activated sludge, percent of
influent flow 69
Return activated sludge concentration,
percent 0.8
Chlorination:
Detention time @ PDWF, minutes 30
Dosage, mg/1 10
Aerobic digestion, conventional air:
Influent solids, percent
1 mgd 1
5 and 10 mgd (automated decant cycle) 2
Detention time, days 15.6
(*) ADWF - Average dry weather flow
(**) PDWF - Peak dry weather flow
133
-------
TABLE 36. UNIT PROCESS SIZES, TWO STAGE ACTIVATED SLUDGE NITRIFICATION
Unit process or component
Primary sedimentation tanks surface
area, sq ft
First stage aeration tank
Volume, cu ft
Aerators, hp
Second stage aeration tank
Volume, cu ft
Aerators, hp
10 mg/1 NH -N
20 mg/1 NH*-N
30 mg/1 NH4-N
Final clarifier surface area,
sq ft
RAS pumps, mgd
WAS pumps, gpm
Sludge pumps, gpm
Chlorination contact tank volume,
cu ft
Chlorination feed equipment,
tons/year (average/peak)
Aerobic digestion
Tank volume, cu ft
Aerators, hp
Plant capacity, mqd
1
1,250
16,700
80
22,300
15
30
50
1,667
(*)
.69/1. 03 v '
36
15
4,180
(*)
15. 2/22. 8V '
51,100
58
5
6,250
83,400
400
111,500
75
150
250
8,335
3.45/5.18
180
75
20,900
76/114
127,750
145
10
12,500
167,000
800
223,000
150
300
500
16,670
6.90/10.3
360
150
41,800
152/228
255,500
290
(*)
Average/Peak - Average flow is used to determine the power requirement
and maintenance materials cost. Peak capacity is used to determine
construction cost and labor requirement.
134
-------
TABLE 37. CONSTRUCTION COST, TWO STAGE NITRIFICATION, 1976
Plant capacity, mgd
Primary sedimentation tanks .^.
Aeration basins, first stage .^
Aeration basins, second stage
Aeration equipment, first stage
Aeration equipment, second stage
Secondary sedimentation tanks
Nitrif. sedimentation tanks
Return activated sludge pumping sta,
Nitrif. RAS station
Waste activated sludge pumping sta.
Primary sludge pumping station
Chlorine contact basins
Chlorination equipment
Aerobic digestion
Basins
Aerators
Yardwork
Total construction cost
at 20 mg/1 NH4-N
at 10 mg/1 NH4-N
at 30 mg/1 NH4-N
Construction cost, $1,000
1 5 10
$1,448
1,413
1,490
260
360
340
230
210
330
330
160
160
170
93
140
37
380
160
470
$3,830
3,730
3,942
440
530
480
390
360
590
590
230
230
220
140
200
60
550
280
741
$6,031
5,821
6,171
*Table based on 20 mg/1 NH4-N. Construction costs of both stages of
aeration equipment at:
10 mg/1 NH4-N: 1 mgd = $235,000; 5 mgd = $600,000; 10 mgd = $800,000;
20 mg/1 NH*-N: 1 mgd = $312,000; 5 mgd = $812,000; 10 mgd = $1,150,000
These costs do not include land, engineering, legal, or financing during
construction, but include 25 percent contractor profit and overhead.
135
-------
BIOLOGICAL DENITRIFICATION
Mixed Reactor
Tables 38 and 39 show the design basis for denitrification in a mixed,
uncovered reactor and the unit process sizing. The anoxic denitrification
reactor is followed by an aerobic stabilization reactor for removal of any
excess methanol. Solids are then removed in a clarifier and recycled to
the denitrification reactor. Estimated construction costs are shown in
Table 40 and 0 & M costs in Table 41.
Fixed Film Denitrification
The costs of fixed film denitrification were estimated based upon the
following criteria: 6 ft deep bed, gravity system, 2-4 mm sand, 2.7
gpm/sq ft at average flow, backwash 15 min/day at 25 gpm/sq ft and 25 ft
TDK with auxiliary air scour, 3:1 methanol to N03-N ratio. Costs for the
fixed film system were based on the work conducted on filtration system
costs by CWC under EPA Contract 68-03-2186.
Construction costs are shown in Table 42 and operation and maintenance
costs in Table 43.
PHYSICAL-CHEMICAL NITROGEN REMOVAL
Cost information was developed for the physical-chemical alternatives
of breakpoint chlorination, selective ion exchange, and ammonia stripping.
Construction costs are shown in Table 44 and operation and maintenance
costs in Table 45.
Breakpoint Chlorination
The basic design criteria used were as follows:
Provide 30 seconds of rapid mixing, G = 900
Peak NH3 concentration = 2 x average NH concentration
Chlorine feed capacity = 10 x peak NH -N concentration @
average flow
Costs of chlorine contact facilities were not included because such
facilities would normally be provided for disinfection purposes even with-
out the need to remove nitrogen.
Costs were estimated for average NH4-N concentrations of 10, 20 and
30 mg/1. Construction and 0 & M cost information was derived from reports
prepared under EPA Contract 68-03-2186 by CWC. Chlorine usage in the
various size facilities is as follows:
136
-------
TABLE 38. DESIGN PARAMETERS, MIXED REACTOR
DENITRIFI CATION
Denitrification reactor:
Type
Nitrate removal rate,
Ib NO--N/lb MLVSS/day
MLVSS, mg/1
Mixer type
Mixer size, hp/1,000 cu ft
Aerated stabilization reactor:
Detention, minutes
Aeration, hp/1,000 cu ft
Final clarification:
Overflow rate, gpd/sq ft
Sludge recycle, %
Methano1 feed:
Methanol: nitrogen ratio
NO.-N concentrations, mg/1
Suspended growth
0.1
1500
Submerged turbine
0.5
50
1
700
50
3:1
10, 20, 30
137
-------
TABLE 39. UNIT PROCESS SIZES, MIXED REACTOR DENITRIFICATION
Plant capacity, mgd
Unit process or component
Denitrification reactor
\
Volume, cu ft/mixing, hp
10 mg/1 N03-N
20 ing/1 N03-N
30 mg/1 N03-N
Aerated stabilization reactor
Volume, cu ft
Aeration, hp
Clarifier
Area, sq ft
Return sludge , mgd
Methanol feed, Ibs/hr
10 mg/1 N03-N
20 mg/1 N03-N
30 mg/1 N03-N
1
8,900/5
17,800/10
26 , 700/15
4,500
5
1,500
*
0.5/0.75
9
17
26
5
44,500/22.5
89,000/45
133,500/67.5
22,500
22.5
7,500
2.5/3.75
44
87
131
10
89,000/45
178,000/90
267,000/135
45,000
45
15,000
5/7.5
87
174
261
Average/Peak - Average flow is used to determine the power requirements
and maintenance materials cost. Peak capacity is used to determine
construction cost and labor requirement.
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TABLE 42. CONSTRUCTION COSTS, FIXED FILM
DENITRIFICATION, 1976
Construction cost, $1,000
Plant capacity, mgd 1 5 10
Structure 336 776 1,506
Media 12 40 80
Air/water backwash 160 150 280
Methanol feed & storage 50 80 150
Yardwork 78 146 282
Total construction cost
at 20 mg/1 N03~N $636 $1,192 $2,298
at 10 mg/1 NO3~N $619 $1,138 $2,218
at 30 mg/1 NO3-N $653 $1,238 $2,355
These costs do not include land, engineering, legal, or financing during
construction, but include 25 percent contractor profit and overhead.
141
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142
-------
TABLE 44. CONSTRUCTION COSTS, PHYSICAL-CHEMICAL NITROGEN REMOVAL,
1976
Construction cost, $1,000
NH.-N concentration
Process & plant capacity, mgd
Breakpoint chlorination
0.01
0.1
1.0
5.0
10.0
Selective ion exchange
0.01
0.1
1.0
5.0
10.0
Ammonia stripping
0.01
0.1
1.0
5.0
10.0
10 mg/1
4.6
17.2
70.5
229.5
377.1
98.4
147.5
442.6
1,557.4
2,704.9
3.9
31.1
245.9
1,065.6
1,967.2
20 mg/1
7.1
25.4
114.8
377.1
696.7
98.4
147.5
442.6
1,557.4
2,704.9
3.9
31.1
245.9
1,065.6
1,967.2
30 mg/1
12.3
36.1
176.2
623.0
1,475.4
98.4
147.5
442.6
1,557.4
2,704.9
3.9
31.1
245.9
1,065.6
1,967.2
Costs do not include land, engineering, legal, or financing during
construction, but include 25 percent contractor overhead and profit.
143
-------
TABLE 45.
Annual cost, $1,000
NH^-N concentration
Process & plant capacity, mgd
Breakpoint chlorination
0.01
0.1
1.0
5.0
10.0
Selective ion exchange
0.01
0.1
1.0
5.0
10.0
Ammonia stripping
0.01
0.1
1.0
5.0
10.0
10 mg/1
3.7
11.0
52.0
170.0
210.0
13.0
18.0
40.0
130.0
210.0
2.8
6.2
18.0
57.0
170.0
20 mg/1
4.7
17.0
100.0
280.0
380.0
14.0
19.5
47.0
150.0
250.0
2.8
6.2
18.0
57.0
170.0
30 mg/1
5.4
23.5
140.0
400.0
600.0
15.0
21.0
55.0
180.0
320.0
2.8
6.2
18.0
57.0
170.0
Costs include labor at $9.00 per hour including fringe benefits
and electrical energy at $0.03 per kwh.
144
-------
10 mg/1 NH -N 20 mg/1 NH -N 30 mg/1 NH -N
0.01 mgd 1.5 ton/yr 3.1 4.5
0.1 15 31 45
1 150 310 450
10 1500 3100 4500
Costs of chlorine were based on use of ton cylinders for quantities up
to 450 tons/year ($0.11/lb) and on tank cars over 450 tons/year. Between
1 and 10 mgd, the demurrage cost on the rail cars will result in an effec-
tive chlorine cost in excess of $0.05/lb and a gradually decreasing cost
down to $0.05/lb at 10 mgd. To determine O & M costs, a labor rate of
$9.00 per hour and energy cost of $0.03 per kwh were used. The same unit
costs for labor and power were used for all alternatives.
Selective Ion Exchange
The costs for this process are based on use of clinoptilolite exchange
media in gravity structures with recovery of the regenerant in closed-loop
stripping towers. A minimum of four exchangers was provided for each
capacity. A four foot deep clinoptilolite bed loaded at 5.25 gpm/sq ft
was used. Exchanger construction costs are based on cost of gravity filtra-
tion structures developed by CWC under EPA Contract 68-03-2186. Costs
include the exchange structure, backwash facility, influent pumping, clari-
fication-softening facility for the spent regenerant, and closed-loop
stripping tower for regenerant recovery. Construction costs are essentially
unaffected by ammonia concentration but regeneration frequency and operating
costs increase as ammonia concentration increases. Costs for the closed-
loop tower modules (such as illustrated on pages 9-75 and 9-76 of EPA's
Technology Transfer Manual on Nitrogen Control) are based on the estimated
cost of such units for the Upper Occoquan, Virginia plant. Influent pumping
costs are based on 15 ft TDK with regenerant recovery pumping at 35 ft
TDK. Chemical costs are based on those projected for the Upper Occoquan
plant.
Ammonia Stripping
Construction costs are based on a tower loading rate of 1 gpm/sq ft
with a tower packing of the type used in the Orange County, California
plant (see page 9-90, EPA's Technology Transfer Manual on Nitrogen Con-
trol) with 24-foot packing depth. Construction and O & M costs include
influent pumping (50 ft TDK). The costs do not include elevating the pH
of the wastewater to an adequate level for stripping nor of subsequent
downward pH adjustment following stripping. The costs of the stripping
process to provide a given percentage removal of ammonium-nitrogen are
independent of influent concentration (at a given temperature).
SUMMARY AND COMPARISON
A complete discussion of the data developed herein is contained in
Section 8 based on the following comparisons of competing unit processes.
145
-------
Biological Treatment Processes
Comparative construction, operation and maintenance, and total annual
costs for the various biological treatment processes considered is shown
in Figures 36, 37, and 38 . The total annual costs are the O & M costs
plus the amortized construction costs at 7 percent over 20 years. A summary
of operation and maintenance costs from a number of operating plants is
shown in Table 46 for comparative purposes. This information was summarized
from EPA Region inspection and technical assistance reports.
A summary of biological treatment process characteristics is shown in
Table 47. Some of the factors are difficult to define specifically, there-
fore, they are covered by general ranges.
The relative performance of various biological treatment processes is
illustrated in Figures 26 through 34 . These data will allow some general
comparisons between the various biological treatment processes in use.
Nitrification Processes
A comparison of the incremental construction and O & M costs (over
the basic biological treatment process) is shown in Figures 39 and 40 .
All processes will provide essentially complete nitrification with proper
design.
Nitrogen Removal Processes
Comparable incremental construction, operation and maintenance, and
total annual costs for the various biological and physical chemical
nitrogen removal processes are shown in Figures 41, 42, and 43. The total
annual costs are the incremental O & M costs plus the incremental amortized
construction costs at 7 percent over 20 years.
The biological nitrogen removal processes include costs for the re-
quired nitrification and denitrification steps.
Some comparative characteristics of the nitrogen removal processes are
shown in Table 48.
146
-------
10,000
8
u
1,000
100
0.01
PLANT CAPACITY, mgd
Data from Tables 15, 24, 26 and 30
Figure 36. Biological treatment process construction cost, 1976,
147
-------
1,000
i
o
100
0.01
PLANT CAPACITY, mgd
Figure 37. Biological treatment process operation and maintenance
cost, 1976.
148
-------
1,000
100
i
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0.01
PLANT CAPACITY, mgd
Figure 38. Biological treatment process total annual cost, 1976,
149
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100,000
NOTE: NO INCREMENTAL COST
— DIFFERENCE FOR OXIDATION
100
0.01
0.1
PLANT CAPACITY, mgd
Data from Tables 34 and 36 at 20 mg/l influent NH4-N,
Figure 39. Incremental construction cost for biological nitrification,
1976.
152
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nitrification, 1976.
153
-------
10,000
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S NQLE STAGE - FIXED FILM -
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0.01
PLANT CAPACITY, mgd
Data from Tablet 15, 34, 37, 40, 42 and 44 at 20 mg/l influent NH«-N.
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of .Ingle or two stage biological nitrification (over costs of conventional
activated sludge).
Figure 41. Incremental construction cost for biological and physical-
chemical denitrification, 1976.
154
-------
1.000
8
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TWO STAGE 7 MIXED REACTOR
TWO STAGE - FIXED FILM
SINGLE STAGE - MIXED REACTOR
SINGLE STAGE - FIXED FILM
OXIDATION DITCH
3 4 5 6 789
2 3456 789
345 6789
0.01
PLANT CAPACITY, mgd
Data from Tables 13, 31, 41, 43 and 45 at 20 mg/l influent NH4-N.
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single or two stage biological nitrification (over costs of conventional
activated sludge).
Figure 42. Incremental operation and maintenance cost for biological
and physical-chemical denitrification, 1976.
155
-------
1,000
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TWO STAGE - FIXED FILM -*
SINGLE STAGE - FIXED FILM
0.01
PLANT CAPACITY, mgd
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Construction coats amortized at 7 percent ovtr 20 years.
Mixed reactor and fixed film denltrlfication include
incremental costs of single or two stage biological
nitrification (over costs of conventional activated sludge).
Figure 43. Incremental total annual cost for biological and physical-
chemical denitrification, 1976.
156
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157
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SECTION 8
DISCUSSION AND EVALUATION
PROCESS AND DESIGN
PROCESS EQUIPMENT
sr;^.srs"iT^vss,TE^,^t".r;t
Sire srssKsssrifi-ssr ™£r
Plant, licensed by Envirotech, uses vertical aeration equipment
eouio-nt36^°r ™anufaoturers P«>vide application information for their
equipment. Even though a number of the aerators are very similar in
range of 3 to 5 Ib O2 per hp-hr-lineal foot of aerator length according
158
-------
to manufacturers published literature. Actual testing of aerators was
beyond the scope of this study, but information from actual installations
would indicate that manufacturers aerator recommendations provide adequate
oxygen in nearly all applications. A number of the plants were under-
loaded organically so the observations made do not positively indicate the
adequacy of aeration at design load, but the trend was clear.
Most of the other process equipment used in oxidation ditch plants is,
or can be, standard industry equipment as available from a number of manu-
facturers. Therefore, oxidation ditch plant equipment can be specified
and bid competitively as is common practice with municipal wastewater treat-
ment plants.
The Carrousel plant is patented and must be designed and operated under
license, however, the process equipment for the plant is not covered under
the patent and may be obtained from a number of manufacturers. The other
oxidation ditch plant configurations are not patented and may be used with-
out license fee.
PERFORMANCE
Analysis was made of extensive data from operating oxidation ditch
plants. The average performance of the plants studied was equal to or
better than a 20-20 effluent 85 to 90 percent of the time and a 30-30 efflu-
ent 95 percent of the time. The best of the plants studied met a 10-10
effluent 99 percent of the time. The reliability curves indicate slightly
better performance in summer than winter, but the difference is small. Of
the oxidation ditch plants analyzed, the worst met a 20-20 effluent 50
percent of the time and a 30-30 effluent 70 to 80 percent of the time with
the effluent seldom exceeding 60 mg/1 BOD5 and TSS.
Average removal of BODs is 93 percent and TSS is 94 percent with a
range of about 85 to 98 percent for each parameter among all oxidation
ditch plant data analyzed.
There is little difference in BOD5 and TSS performance, on the average
between plants of various sizes.
The performance of oxidation ditch plants was compared to various com-
peting processes such as standard activated sludge, contact stabilization,
extended aeration, rotating biological contactor, and trickling filter.
The average BODc and TSS performance of oxidation ditch plants exceeded
the corresponding performance of the competing processes; in some cases by
a wide margin. A summary of the comparative performance is shown in
Table 23.
CONSTRUCTION
Construction costs for oxidation ditch plants were determined from
actual plant experiences and compared to construction costs for other
biological treatment processes. A comparison of the costs is shown in
159
-------
Generally, the extended aeration package plant is the lowest
th m f Z6S/ the C°ntaCt sta*ili*ation plants are lowest cost in
the mid range of sizes, and the oxidation ditch is lowest cost for larger
sized plants. The costs of alternative plants are close enough in the
• ° 10 ^ ^^ that 10Cal factors and te ect process
d • oces
design will have a significant effect on the relative construction costs.
VAC,*.^! ^eCt ?f ditCh confi^rati°n on construction costs was also in-
vestigated for plants up to 1 mgd. This comparison is shown in Table 18.
For small plants the shallow sloped side ditch is most cost effective and
the deep ditch becomes more cost effective as the capacity increases. For
Plants less than one mgd in size, the effect of configuration is minor,
but for large plants over one mgd, the deep configuration is being used
frequently.
Space requirements for oxidation ditch plants are shown in Table 19
based on a reasonable layout of the required facilities including sludge
drying beds. These requirements are very comparable to similar require-
ments for other treatment processes as shown in Table 47.
OPERATION AND MAINTENANCE
The operation and maintenance requirements for oxidation ditch plants
were determined and compared to the corresponding requirements for com-
peting processes. The operation and maintenance requirements are summar-
ized in Table 13. These requirements are based on actual plant experiences
to the extent possible. The actual costs for labor and electrical energy
are shown to illustrate the extent of variation of unit costs. Labor unit
costs varied from about $2.00 to $10.00 per hour and were generally lower
fa'™ Plants. For purposes of comparison, a uniform unit cost of
59.00 per hour was used for labor and $0.03 per kwh was used for electri-
cal energy. A comparison of operation and maintenance total cost for
various biological treatment processes is shown in Figure 37 The follow-
ing comments relate to operation and maintenance costs only. Package
extended aeration plants are the lowest cost for small capacity applications
below about 0.1 mgd. This is probably due to the compact plant configura-
tion and the minimal amount of mechanical equipment. The standard activated
sludge process is the lowest cost for larger capacity plants generally
above 2 or 3 mgd although the difference between standard activated sludge
and oxidation ditch costs is small up to 10 mgd. The major variable relates
to the staffing practices of particular plants because at some point within
the capacity range of 1 to 10 mgd full time, 24-hour staffing is generally
desireable. *
Within the capacity range of 0.1 to 2 or 3 mgd the oxidation ditch
plant is very competitive in operation and maintenance cost. This is due
to the conservative design, process simplicity, and relatively easy sludge
handling associated with the oxidation ditch plant.
An analysis of total annual costs is shown in Figure 38 with construc-
tion costs amortized at 7 percent over 20 years. The oxidation ditch plant
160
-------
is cost effective over the range of 0.1 to almost 10 mgd although the
differences are small at some points. Therefore, it is important to com-
pare the major alternatives in each specific case and consider the local
factors. Package extended aeration plants are most cost effective for the
small capacity plants below 0.1 mgd.
As a group, oxidation ditch plants are simple to operate and reliable.
The process simplicity, conservative design, and use of standard equipment
contributes to the very satisfactory operating experience. These plants
perform well for extended periods with little or no operator attention.
With few exceptions, operators and administrators are pleased with their
plants.
Oxidation ditch plants are subject to periodic problems, primarily
mechanical. These problems are detailed in Section 5. The major problems
are as follows:
1. Problems can be expected every 2 to 5 years with each aerator
unit. These problems include bearings, seals, flexible coup-
lings, gear reducers, and loss of teeth on brush aerators.
Some failures of the aerator torque tube have been experienced,
but this is rare.
2. Comminutors require regular maintenance and cleaning for satis-
factory operation.
3. Spray from the aerators is a problem when it blows onto walkways
and other access areas. It results in slippery slime accumu-
lations or ice; both are hazards to personnel. In addition, this
spray blows onto bearings and drive units contributing to early
failure.
4. There are a number of other reported minor problems with return
sludge pumping, cleaning of weirs and scum boxes, clarifiers,
and similar items common to other types of plants.
Generally, the operational experience with oxidation ditch plants has
been very good and careful design and operation will overcome a number of
the problems mentioned.
Manufacturers are helping to provide solutions to some of the problems
by providing better seals, double sealed bearings, self-aligning bearings,
better designs for aerator support structure, and shields or "dry" compart-
ments for bearings and drive units. These considerations should contribute
to even better performance in the future.
Cold weather affects the operation and maintenance of oxidation ditch
plants depending on the severity of the cold. Areas where freezing is only
intermittent and of short duration require very little special considera-
tion. In moderately cold areas where spray from aerators will freeze and
accumulate, the aerators are generally fitted with covers which may be
161
-------
C0nsideratlon is the accumulation of chunks of ice in the
toua£ A h^nt ^^ th? aerat°r blades °r brushes as the cj™*s P^s
through. A baffle at water level upstream of each aerator may be helpful to
keep ice chunks out of the aerator. Severely cold areas may require
special design considerations for satisfactory operation and maintenance,
but this varies from case to case. An outdoor oxidation ditch plant is
operating quite satisfactorily in Fairbanks, Alaska with minimum protec-
have Sin"!"! h°Ter/ ±n °ther V6ry C°ld areas Special Protective features
have been used such as covering of ditches and final clarifiers and housing
of equipment requiring regular maintenance. Oxidation ditch plants gener-
ally require cold weather protective considerations similar to other pro-
cesses for a given climate.
SLUDGE HANDLING
Oxidation ditch plants are operated similar to other activated sludge
plants with the settled activated sludge returned from the final clarifier
to the oxidation ditch. The considerations are essentially the same as
for the standard activated sludge process. The sludge should be returned
^n^r517 £ a Tf generally about 30 to 50 percent of the plant average
daily flow. The sludge return rate should be adjustable and measureable.
The return sludge flow rate is generally not varied in proportion to the
plant flow rate, but can be if desired.
Normally, a portion of the return sludge is wasted to maintain the
plant solids balance. Some plants do not waste sludge at all, but this
practice is not recommended because of the buildup of inert solids and
the eventual uncontrolled wasting (plant upset) which almost invariably
results from this type of operation.
It is recommended that sludge be wasted on a regular schedule to main-
tain the desired MLSS in the ditch.
*•* ^Ty?ically' waste Slud9e is handled in one of two ways at oxidation
ditch plants. The most common is to utilize extended aeration (24-hour
aeration detention) and to waste sludge directly to sand drying beds
The second is to provide less aeration detention (8 to 15 hours) and
waste sludge to an aerobic digester then to sand drying beds. In some
cases the liquid waste sludge is spread on land without drying. There are
Variati°nS' but nost Plants waste sludge directly to sand dry-
At nearly all oxidation ditch plants visited, the waste sludge was
dewatered in open drying beds without any further treatment. Without
exception, plant personnel indicated an absence of odor and an absence
of nuisance complaints. No odors were detected during site visits to
approximately 20 plants of various sizes, locations, and types.
A number of plants reported a shortage of sand drying bed capacity and
slow dewatenng of the sludge on sand beds. This would indicate the need
for adequate design for the particular region. It is desireable to have
162
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adequate drying capacity so that wasting can be scheduled according to
process needs rather than by available drying bed capacity. It is possible
to store sludge in the oxidation ditch by allowing the MLSS to increase,
but this must be carefully monitored to prevent plant upsets. It is far
better to waste sludge at regular intervals.
If sludge is wasted directly to sand beds it is important that the
plant be properly operated so that a stable sludge is produced. If the
sludge is not well stabilized odors may be generated during the drying
process.
Some regulatory agencies may require additional sludge stabilization
prior to disposal and these requirements should be taken into consideration
in plant designs. Sludge storage may be required in some applications;
for instance, if sand drying beds are provided, but are not usable during
some periods of the year.
It is important in the design of any solids handling process to assess
the impact of the non-biodegradable portion of the plant solids. The non-
biodegradable solids will accumulate in the aeration process and will not
be further reduced bytextended aeration or aerobic digestion and eventually
must be wasted and disposed of. The non-biodegradable fraction of the
influent solids will vary widely, but may typically be 30 to 50 percent of
the raw sewage solids. This entire fraction of non-biodegradable solids
enters the aeration process when the plant has no primary settling.
NITRIFICATION AND NITROGEN REMOVAL
A standard oxidation ditch plant is capable of essentially complete
nitrification when designed for 24-hour aeration detention treating normal
domestic sewage. Generally, the only modifications required are operational
in nature provided adequate aeration capacity is available to maintain a
positive dissolved oxygen level in the ditch, normal return sludge capabil-
ity is provided, and an adequate final clarifier(s) is provided. If the
MLSS is maintained in the range of 2,600 to 5,000 mg/1 the plant should
achieve complete nitrification down to mixed liquor temperatures near
freezing. In some cases, the mixed liquor pH may vary enough to require
chemical adjustment to the optimum range of 7.5 to 9.5, but these cases are
rare. There are a number of design and operational parameters which effect
operation for optimal nitrification as outlined in Section 4, but for the
typical plant, normal operation will produce a very high degree of nitri-
fication.
Oxidation ditch plants are capable of substantial nitrogen removal,
however, the design and operation is rather critical if consistently high
levels of nitrogen removal are to be achieved. It has been demonstrated
that 40 to 80 percent nitrogen removal can be achieved and under favorable
conditions 70 to 80 percent removal should be possible with mixed liquor
temperatures above 15 C.
Oxidation ditch plants are able to achieve single stage nitrogen
163
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removal because alternating aerobic and anoxic zones can be maintained
h H ^.^ This "1""" ««ful control of the^eration
the deep ditch type plants it is also possible to produce aerobic
was
ue aero
and anoxic zones due to vertical stratification. This stratificatio
shown at the Vienna-Blumenthal plant (19). Work by Drews et al^"°
. IT* !tot multiple °°n«ntric <*annel type plants can be operated
single stage nitrogen removal.
Normally, an auxiliary source of carbon is required for the denitrifi
cation step and this can be provided by introducing thfrfw sewage in"to the
thetCreta "** **~* «" ™°^ ^ «"«* ~« •&?$£*
« f-hK e provided for easy »ntrol of the aeration
so that the aerobic-anoxic zones can be maintained under varying plant '
flow and loading. This can be accomplished by varying aerator Immersion
depth, aerator speed, number of aerators in operation, or operating the
aerators on an on-off cycle. Floating aerators may have some advantage
because their position in the ditch can be changed. Under some cases of
low oxygen demand the aerators may not impart sufficient velocity to main-
tain the solids in suspension in the ditch. In this case, an auxiliary
velocity device may be necessary to impart additional velocity without
adding oxygen such as a submerged' propeller.
Ideally, the use of an automatic dissolved oxygen control system would
assure maintenance of the required aerobic and anoxic zones with minimum
operator attention, but this has not been demonstrated. *
I
Generally, the operational considerations for nitrogen removal are
about the same as for nitrification except for the aerobic-anoxic zones.
The oxidation ditch operation and maintenance costs for nitrification
and nitrogen removal are shown in Table 13. The incremental oxidation
ditch operation and maintenance costs for nitrification are very small
The operation and maintenance costs for nitrogen removal are actually less
than for normal oxidation ditch plant operation in larger plant sizes
because of savings in electrical energy due to controlled aeration, in
essence, the incremental operation and maintenance costs for nitrification
or nitrogen removal over those for normal oxidation ditch plant operation
are nil .
Construction costs for nitrification are the same as for a normal
oxidation ditch plant and for nitrogen removal are about 4 to 6 percent
higher.
A comparison of the oxidation ditch to single and two-stage biological
nitrification is shown in Figures 39 and 40. Each process will provide
essentially complete nitrification, however, the oxidation ditch plant
requires no additional construction cost and very little additional opera-
tion and maintenance cost over the basic oxidation ditch treatment plant
164
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A comparison of various nitrogen removal alternatives is shown in
Figures 41, 42, and 43. Ammonia stripping and breakpoint chlorination
are the only close competitors (in small plants) to the oxidation ditch
plant, but both have serious process limitations. These limitations are
outlined in Table 48. The incremental costs for ammonia stripping do not
include raising the pH prior to or lowering the pH after stripping. If
very high nitrogen removal is required, the oxidation ditch plant would
not be suitable, but for removals up to 80 percent the oxidation ditch
plant is cost effective compared to the other alternatives.
EFFECTIVE APPLICATION
The effective application of oxidation ditch plants requires more than
consideration of the aeration process itself. Many other factors contribute
to overall satisfactory performance.
The raw sewage should be at least coarse screened or comminuted before
entering the ditch to prevent plugging of return sludge valves and pumps.
The screen should have openings no wider than 3/4 to 1 inch. A number of
plants have no grit removal facilities and have been in operation for
several years without any adverse effects from grit accumulation. There-
fore, removal of grit may be desireable, but lack of grit removal does not
seem to cause serious operating or maintenance problems. It is likely
that grit may have to be removed from these ditches after some period of
operation.
The final clarifier design and sizing is very important to overall
plant performance especially if the discharge requirements are stringent.
It is recommended that final clarifiers be designed for an overflow rate
of 400 to 500 gpd/sq ft at average plant flow and 1,000 to 1,200 gpd/sq ft
at peak flows. Clarifiers should be designed for a solids loading of
30 Ib/day/sq ft. Sidewall depths should preferably be 12 to 14 feet. A
minimum of two final clarifiers should be considered for all plants. Deep
scum baffles and double skimmer arms are helpful if rising solids are
expected because of denitrification in the final clarifier. Return sludge
should be withdrawn from the final clarifier continuously and the rate
measured. This sludge can be returned to the ditch or to the raw sewage
lift station.
Provisions should be made for easy sludge wasting and some means
should be available for measuring the quantity of sludge wasted.
Drying beds are the most common method of dewatering, but the beds
should be adequately sized for the quantity of sludge to be wasted (con-
sidering the non-biodegradable solids) and the climate. Additional sludge
storage may be necessary or desireable in some applications to assure that
sludge can be wasted and handled under all operating conditions.
165
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REFERENCES
1. McKinney, R. E., "Mathematics of Complete Mixing Activated Sludge",
Trans. Amer. Soc. Civil Eng., 128, Part III, Paper No. 3516 (1963).
2. Eckenfelder, W. W., Jr., and O'Connor, D. J., "Biological Waste
Treatment", Pergamon Press, Oxford, England (1961).
3. Monod, J., "Research on Growth of Bacteria Cultures", Herman et
Cie, Paris (1942).
4. Goodman, B. L., and Englande, A. J., "A Unified Model of the Acti-
vated Sludge Process", JWPCF, 46, 2, p. 312, February, 1974.
5. Goodman, B. L., "Monod Type Relationships Applied"to Complete Mixing
Activated Sludge", Unpublished, January 25, 1973.
6. Stenquist, R. J., et al, "Carbon Oxidation - Nitrification in Synthetic
Media Trickling Filters", JWPCF, 46, 10, p. 2327, Oct., 1974.
7. Trossey, DeFro, et al, "Tertiary Treatment by Flocculation and Filtra-
tion", JSED, ASCE, 96, SA1, p. 75, February, 1970.
8. Gulp, G. L., and Hansen, S., "Extended Aeration Effluent Polishing by
Mixed Media Filtration", Unpublished paper, June, 1967.
9. Long, Leroy W., "Shelters Boost Winter Treatment Efficiencies", Water
and Sewage Works, August, 1976, pp. 32-33.
10. Kaneshige, Harry M., "Performance of the Somerset, Ohio Oxidation
Ditch", Journal WPCF, 42:1370-1378, 1970.
11. Kampelmacher, E. H., and Jansen, L, M., "Occurrence of Salmonella in
Oxidation Ditches", Journal WPCF, 45:348, 1973.
12. Grube, Gareth, A., and Murphy, R. Sage, "Oxidation Ditch Works Well
in Sub-Arctic Climate", Water and Sewage Works, pp. 267-71, 1969.
13. Murphy, R. Sage, "Evaluation of an Oxidation Ditch Wastewater Treat-
ment Plant in Sub-Arctic Alaska", NTIS, PB-215-461, 1968.
14. Ranganathan, K. R., and Murphy, R. Sage, "Bio-Processes of the Oxida-
tion Ditch When Subjected to a Sub-Arctic Climate", Institute of Water
Resources, U. of Alaska, Report No. 1WR-27, 1972.
166
-------
15. Halvorson, H. 0., Irgens, Roar, and Bauer, Henry, "Channel Aeration
Activated Sludge Treatment at Glenwood, Minnesota", Journal WPCF,
44:2266, 1972.
16. Halvorson, H. 0., Irgens, Roar, and Bauer, Henry, "A Report on The
Channel Aeration Process at Glenwood, Minnesota - A Two Year Study".
17. Drews, R. J. L. C., Malan, W. M., Meiring, P. G. J., and Moffatt, B.,
"The Orbital Extended Aeration Activated Sludge Plant", Journal WPCF,
44:221, 1972.
18. Drews, R. J. L. C. and Greeff, A. M., "Nitrogen Elimination by Rapid
Alternation of Aerobic/"Anoxic" Conditions in "Orbital" Activated
Sludge Plants", Water Research, 7:1183-1194, Pergamon Press, 1973.
19. Matsche, N. F. and Spatzierer, G., "Austrian Plant Knocks Out Nitrogen",
Water and Wastes Engineering, January, 1975, pp. 18 .
20. M & I, Inc., Preliminary Survey Wastewater Treatment Facilities
Morrison Sanitation District, Morrison, Colorado, Contract No.
68-03-2224, 1976.
21. M & I, Inc., Preliminary Survey Wastewater Treatment Facilities,
Berthoud, Colorado, Contract No. 68-03-2224, 1976.
22. Draft Report, Winter Performance of Secondary Wastewater Treatment
Facilities, EPA Region VII.
23. Bullert, James M. and Grounds, Harry C., Demonstration Project, Dawson,
Minnesota Wastewater Treatment Plant, U.S. EPA MERL, Cincinnati,
Ohio, Grant No. S-803067-01-1.
24. Pasveer, A., "Developments in Activated Sludge Treatment in the
Netherlands", advances in Waste Treatment edited by W. W. Eckenfelder,
Jr. and Joseph McCabe, the MacMillan Co.
25. Mulbarger, M . C., "Nitrification and Denitrification in Activated
Sludge Systems", Journal WPCF, 43:3059-2070, 1971.
26. Stover, Enos L., "Effects of COD:NH3-N Ratio on a One-Stage Nitrifica-
tion Activated Sludge System", Water and Sewage Works, pp. 120,
September, 1976.
27. Process Plant Construction Estimating Standards, Richardson Engineering
Services, Inc., Solana Beach, California, 1976-1977 Edition.
28. Benzie, W., et al. Effects of Climatic and Loading Factors on Trick-
ling Filter Performance. Journal Water Pollution Control Federation.
35, No. 4. pp. 445-455. 1963.
167
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29. Black & Veatch, "Estimating Costs and Manpower Requirements for Con-
ventional Wastewater Treatment Plants", U.S. Environmental Protection
Agency Report, EPA - 17090 DAN, 1971. ui^raon
168
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BIBLIOGRAPHY
Aeration in Wastewater Treatment. WPCF Manual of Practice No. 5, 1971,
pp. 52, 53, 56-59.
Jacobs, Allan. Loop Aeration Tank Design Offers Practical Advantages.
Water and Sewage Works. October, 1975, pp. 74-75, November, 1975,
pp. 74-75.
Metcalf & Eddy, Inc. Wastewater Engineering. McGraw-Hill Book Company.
1972, pp. 502-503.
Clark, John W., Viessman, Warren Jr., and Hammer, Mark J. Water Supply
and Pollution Control. International Textbook Company, 1971,
pp. 518-519.
Bolton, R. L., and Klein, L. Sewage Treatment Basic Principles and Trends,
Ann Arbor Science Publishers Inc. 1973, pp. 190-191.
Busch, Arthur W. Aerobic Biological Treatment of Waste Waters. Oliga-
dynamics Press. 1971, pp. 231-232.
Parker, Dr. Homer W. Wastewater Systems Engineering. Prentice-Hall, Inc.
1975, pp. 235-271.
McDowell, P. E., and Goldman, Dr. Michael. Advanced Waste Treatment
Plant Has Physical-Chemical Option. Public Works. September,
1976, pp. 82-83.
Ray, William. Nitrates: What's Happening in Britain? Water and Wastes
Engineering. April, 1976, pp. 264.
Argaman, Y., and Spivak, E. Engineering Aspects of Wastewater Treatment
in Aerated Ring-Shaped Channels. Water Research. 8:317-322.
Pergamon Press, 1974.
Jones, P. H., and Patni, N. K. Nutrient Transformations in a Swine Waste
Oxidation Ditch. Journal WPCF. 46:366, 1974.
Bhide, A. D., Mathur, Dr. R. P., and Thiruchitrambalam, N. Performance
of an Oxidation Ditch at Different Rotor Speeds. Institute of
Engineering India. 49: pp. 62-67, 1969.
169
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Kshirsagar, S. R. Oxidation Ditches of Netherlands. Environmental Health
10: 97-105, 1968.
Brisbin, Sterling, G. Concentric Waste Treatment Plant Saves Land, Cuts
Cost. Civil Engineering, pp. 74, February, 1976.
Bogan, S. A. Eighteen Months of Nuisance Free Operation. The American
City, June, 1971.
Chatterjee, R. M., and Niyogi, S. A Rational Approach to Predict Sludge
Solids Accumulation in Oxidation Ditch. IE (I) Journal - PH
53:55, 1973.
Chandhuri, N. Technique of Evaluation of System Parameters Relating Solids
Accumulation in Oxidation Ditches Process. Advanced Water Pollution
Research Proc. Int. Conf. 5th. II-10/1.
170
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LIST OF METRIC CONVERSIONS
English Unit
acre
cfm
cfs
cfs/acre
cfs/sq mile
cu ft
cu ft
cu in.
cu yd
cu yd/mile
cu yd/sq mile
°F
ft
gal
gal
gpd/acre
gpd/cu yd
gpd/ft
gpd/sq ft
gpm
gpm/sq ft
hp
in.
Ib
lb/1,000 cu ft
Ib/day/sq ft
Ib/ft
Ib/mil gal
mgd
mile
pcf
psf
psi
sq ft
sq in.
Multiplier
0.405
0.028
1.7
4.2
0.657
0.028
28.32
16.39
0.765
0.475
0.29
0.555(°F-32)
0.3048
0.003785
3.785
0.00935
5.0
0.0124
0.0408
0.0631
40.7
0.7457
2.54
0.454
16.0
4,880
1.51
0.12
3,785
1.61
16.02
4.88
0.0703
0.0929
6.452
Metric Unit
ha
cu m/min
cu m/min
cu m/min/ha
cu m/min/sq km
cu m
1
cu cm
cu m/sq km
C
m
cu m
cu m/day/ha
1/day/cu m
cu m/day/m
cu in/day sq m
I/sec
1/min/sq m,
kw
cm
kg
g/cu m
g/day/sq m
km
g/cu m
cu m/day
km
kg/cu m
kg/sq m
kg/sq cm
sq m
sq cm
171
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
EPA-600/2-78-051
3. RECIPIENT'S ACCESSIOWNO.
A Comparison of Oxidation Ditch Plants to Competing
Processes for Secondary and Advanced Treatment of
Municipal Wastes
5. REPORT DATE
March 1978 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
William F. Ettlich
9.PERF
CulD
'MING ORGANIZATION NAME AND ADDRESS
p/Wesner/Culp
Clean Water Consultants
El Dorado Hills, California 95630
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
1BC611
11. CONTRACT/tBMdJCKNOT
68-03-2186
IG AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory--Cin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Francis L. Evans III
Task Director: Jon H. Bender
(513) 684-7610
This report includes information relating to oxidation ditch plant plant
equipment, design and application, operational problems and advantages, operation
and maintenance requirements, construction costs, and nitrification and nitrogen
removal applications. Much-of the information is based oh visits to and analysis
of data from actual operating installations. In addition, the oxidation ditch
plant characteristics are compared to those of competing biological treatment
processes. Nitrification and nitrogen removal capabilities of the oxidation ditch
process are also compared to various biological and physical-chemical processes.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COS AT I Field/Group
Activated Sludge Process
Construction Costs
Performance
Reliability
Oxidation Ditch Plants
Single Stage Nitrificatic
Denitrification
Nitrogen Removal
0$M Costs
0£M Problems
13B
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
192
EPA Form 2220-1 (9-73)
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
172
; U. S. GOVERNMENT PRINTING OFFICE: 1978-657-060/1523 Region No. 5-11
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