United States Officeof Research and Center for Environmental Research
Environmental Protection De#£*$nnpt Information
Agency Washington DC 2O460 Cincinnati OH 45268
Technology Transfer EPA/625/1 -85/018
«s>ERA Design
Manual
Odor and Corrosion
Control in Sanitary
Sewerage Systems and
T reatment Plants
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EPA/625/1-85/018
October 1985
Design Manual
Odor and Corrosion Control in
Sanitary Sewerage Systems and
Treatment Plants
Center for Environmental Research Information
U.S. Environmental Protection Agency
Office of Research and Development
Cincinnati, OH 45268
i
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Notice
This document has been reviewed in accordance with the U.S. Environmental
Protection Agency's peer and administrative review policies and approved for
publication. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
/>'
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Contents
Chapter Page
1 Introduction
1.1 Background 1
1.2 Purpose 1
1.3 References 1
2 Theory, Prediction, and Measurement of Odor and
Corrosion .......; 3
2.1 Introduction 3
2.2 Compounds Causing Odor and Corrosion ... 3
2.3 Mechanisms for the Generation of Hydrogen Sulfides 8
2.4 Mechanisms of Corrosion 17
2.5 Predicting Sulfide Buildup and Corrosion in Sewars 20
2.6 Approach to Investigating Odor and Corrosion 25
2.7 Measurement and Monitoring of Corrosion and Odor 29
2.8 Toxicity and Safety Practices .31
2.9 References 33
3 Odor and Corrosion Control in Existing Wastewater
Collection Systems 35
3.1 Introduction 35
3.2 Improving the Oxygen Balance 35
3.3 Chemical Addition 53
3 4 Case Histories 61
3.5 References 66
4 Odor and Corrosion Control in Existing Wastewater
Treatment Plants 69
4.1 Introduction 69
4.2 Sources of Odors in Wastewater Treatment Plants 69
4.3 Control of Odors in Existing Wastewater Treatment Plants 71
4.4 Corrosion in Wastewater Treatment Plants 94
4.5 Corrosion Control Techniques at Existing Wastewater
Treatment Plants 95
4.6 Case Histories 96
4.7 References .100
5 Designing to Avoid Odor and Corrosion in New Wastewater
Collection Systems. 103
5.1 Introduction..... 103
5.2 Hydraulic Design 103
5-3 Ventilation of Sewers 110
5.4 Selection of Materials 112
5.5 References... 116
Hi
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Contents (continued)
Chapter Page
6 Designing to Avoid Odor and Corrosion in New Wastewater
Treatment Facilities ^ ^
6.1 Introduction 117
6.2 Common Sites of Odor Generation 118
6.3 General Design Considerations for Avoiding Odor
Generation and Release 1 119
6.4 Design Procedures for Specific Odor-Producing
Unit Processes 121
6.5 General Design Considerations for Avoiding Corrosion .... 126
6.6 Paint and Coatings ....127
6.7 Selection of Materials 130
6.8 References 132
iv
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Figures
Number Page
2-1 The Sulfur Cycle 5
2-2 Effect of pH on Hydrogen Sulfide Equilibrium 6
2-3 Proportions of H2S and HS~ in Dissolved Sulfide 7
2-4 Relationship of Dissolved to Total Sulfide Concentration in the
Sacramento, CA Central Trunk Sewers 8
2-5 Processes Occurring in Sewers with Sufficient Oxygen to
Prevent Sulfide from Entering the Stream 9
2-6 Processes Occurring in Sewers Under Sulfide Buildup Conditions .. 10
2-7 Reaeration Rates in Sewers Flowing Half Full 12
2t8 Relative Reaeration Rates in a Sewer 12
2-9 Equilibrium Concentration of HZS in Air 14
2-10 Effect of Velocity and Pipe Size on Sulfide Flux to Pipe Wall
Under Specified Conditions 16
2-11 Factor to Apply to 08v»from Figure 2-10 to Calculate
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Figures (continued)
Number Page
3-20 Dissolved Oxygen Increase in the Wastewater Across
U-Tube vs. Head Loss Comparing Oxygen and Air Injection 51
3-21 Typical Design Curve for Oxygen U-Tube 52
3-22 Typical Chlorinator Installation ,55
3-23 Typical Chlorine Diffusers for Gravity Sewers and
Force Mains 55
3-24 Typical Reaction Profile of Hydrogen Peroxide in Wastewater ...... 56
3-25 Typical Package H2O2 Dosing Installation 57
3-26 Typical Bulk H202 Storage and Injection System ........ 58
3-27 Effects of Sodium Nitrate on Sulfide Generation in Bluff Cove
Force Main; Los Angeles, CA .60
3-28 Impact of Shock. Dosing with NaOH on Recovery of Sulfide
Buildup Capacity; Los Angeles, CA 61
3-29 Sewer Service Area; Delta Diablo Sanitation District 7A,
California 63
3-30 Wastewater Collection System with H2O2 Dosing Stations;
Palm Beach County, FL 65
3-31 Average Wastewater Temperature, Flow, and H2O2
Used at Lift Station No. 229; Palm Beach County, FL 66
4-1 Sulfide Removal from Wastewater Using Potassium
Permanganate 74
4-2 Results of Pilot Studies Using Potassium Permanganate for
Removal of Dissolved Sulfides 74,
4-3 Sulfide Removal from Wastewater Sludge Using Potassium
Permanganate 74
4-4 Effectiveness of Potassium Permanganate Addition for
Controlling HjS Generation from Sludge Centrifugation 75
4-5 Effect of Nitrate Addition on HZS Emissions from a
Trickling Filter 75
4-6 Typical Wet Scrubber System 78
4-7 Odorous Gas Removal Efficiency as a Function of Chlorine
Concentration at Top of Packed Tower 79
4-8 Typical Activated Carbon Filter for Odor Control 82
4-9 Performance of KMn04-!mpregnated Activated Alumina
Adsorbers on Hydrogen Sulfide 84
4-10 Typical Iron Oxide Filter for Odor Control 84
4-11 Performance of an Iron Oxide Filter for Odor Control 85
4-12 Typical Ozone System for Odor Control 86
4-13 Ozone Requirements for Various Air Flows and Ozone Dosages .... 87
4-14 Direct Flame Oxidation Systems 88
4-1 5 Catalytic Oxidizer with Heat Recovery 88
4-1 6 Flow Diagram for a Direct Flame Oxidation System 89
4-17 Soil/Compost Filter for Odor Control 90
5-1 Sulfide Occurrence in Small Sewers 103
5-2 Minimum Scour Velocity Based on Boundary Shear Stress 104
5-3 Flow-Slope Relationships as Guides to Sulfide Forecasting 105
5-4 Alternative Sewer Grading Designs 106
5-5 Drop Manhole Design 106
5-6 Streamlined Junction....... 107
5-7 Pump Station with an Air Bypass 109
5-8 Air-Lift Pump Station 109
5-9 Forced Draft Ventilation for Odor Control; Austin, TX 112
5-10 T-Lock PVC Liner for Concrete Pipe 115
6-1 Typical Design of a Septage Receiving Station 125
VI
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Tables
Number Page
2-1 Odorous Sulfur Compounds in Wastewater 4
2-2 Logarithmic Ionization Constants (pK,) for Hydrogen Sulfide 6
2-3 Solubility of H2S in Water at a Pressure of 1
Standard Atmosphere 8
2-4 Physical and Chemical Properties of Hydrogen Sulfide . 9
2-5 Expected Oxygen Adsorption in Wastewater Falls 12
2-6 Dissociation of Hydrogen Sulfide ,..,,15
2-7 Comparison of Predicted and Measured Total Sulfides in
Sacramento Central Trunk ,, 25
2-8 Comparison of Measured vs. Predicted Corrosion Penetration,
Sacramento, CA Central Trunk Sewer . 25
2-9 Interpretation of Results of Wastewater Survey 27
2-10 Hazardous Characteristics of Hydrogen Sulfide Gas 32
3-1 Selected Studies of Direct Injection of Compressed Air into
Force Mains for Sulfide Control 36
3-2 Performance of Aspirated Air U-Tubes at Jefferson Parish, LA 38
3-3 Performance of Compressed Air U-Tubes at Port Arthur, TX 39
3-4 Performance of Compressed Air U-Tubes at Sacramento, CA 39
3-5 Performance of Pressure Tank Air Injection at Port Arthur, TX 40
3-6 Suggested Oxygen Reaction Rates 42
3-7 Typical Costs for Direct Compressed Air Injection into a
Force Main for HZS Control 45
3-8 Performance of Direct Oxygen Injection into Force Mains at
Port Arthur, TX 47
3-9 Performance of Direct Oxygen Injection into Force Mains at
Sacramento, CA 47
3-10 Performance of Oxygen U-Tube for Sulfide Control at
Sacramento, CA 47
3-11 Performance of Multiple U-Tube Oxygen Injection 46
3-12 Typical Costs for Direct Oxygen Injection into Force Main for
H2S Control 53
3-13 Performance of Chlorination for Sulfide Control at
Sacramento, CA 54
3-14 Effects of Chlorination on H2S Concentrations in Force Main at
Tampa, FL 54
3-15 Commercially Available Forms of Chlorine for
Wastewater Applications 55
3-16 Typical Costs for Chlorine Injection for H2S Control 56
3-17 Physical Properties of Hydrogen Peroxide 56
3-18 Performance of H2O2 for Sulfide Control in Wastewater
Collection Systems 57
3-19 Typical Costs for Hydrogen Peroxide Injection for H2S Control 58
3-20 Performance of FeSO* Addition for Sulfide Control • 59
3-21 Typical Costs for FeS0< Injection for H2S Control 59
3-22 DO Depletion in Raw Wastewater Westville Force Main,
Woodbury, NJ ...... 62
vii
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Tables (continued)
Number Page
3-23 Results of Air Injection Into Westville Force Main,
Woodbury, NJ .... 62
3-24 Costs for Sulfide Control in Westville Force Main Using
HaOz and H2O2 with Air Injection, Woodbury, NJ 63
3-25 Force Main Characteristics at Pure Oxygen Injection Points,
Antioch, CA 63
3-26 Effectiveness of for Sulfide Control at Palm Beach
County, FL .66
4-1 Use of HgOa to Control H2S Odors at Baltimore, MD 73
4-2 Use of H2O2 for H?S Odor Control in Sludge Handling
System, Pittsburgh, PA 73
4-3 Performance of Nitrate Addition Compared with Other
Measures for Sulfide Control in Activated Carbon Columns 76
4-4 Effectiveness of Hypochlorite Wet Scrubbers for Removal of
Several Odorous Gases 78
4-5 Performance of Pilot-Scale Wet Scrubbers Using KMnO* and
NaOH for H2S Removal 79
4-6 Typical Costs for Wet Scrubbers for Odor Control 81
4-7 Physical Characteristics of Activated Carbon for Odor Control ...... 81
4-8 Pilot Study on Sewer Odor Control Using Activated Carbon at
Sacramento, CA ..... 82
4-9 Typical Costs for Activated Carbon Adsorbers for
Odor Control 83
4 10 Typical Costs for Ozone Systems for Odor Control 87
4-11 Agents that Adversely Affect Catalysts of the
Platinum-Group Metals 88
4-12 Typical Costs for Thermal Incinerators for Odor Control 90
4-13 Typical Costs for Catalytic Incinerators ...... 90
4-14 Performance of Compost Filter for HjS Control at
Moerewa, New Zealand 91
4-15 Estimated Costs of Compost Filters for Odor Control 92
4-16 Results of Wet Scrubber Performance Tests for H2S Control,
Santa Cruz County, CA 97
4-17 Design and Performance of Wet Scrubber/Activated Carbon
Odor Control System at Tampa, FL 98
5-1 Oxygen Absorption in a Sewer With and Without a Drop 106
6-1 Potential for Odor Generation from Common Unit Processes
in a Wastewater Treatment Plant 118
6-2 Matrix of Potential Odor-Producing Processes and
Recommended Odor-Control Methods 122
6-3 Surface Preparations and Coatings for Various
Environmental Exposures 128
mi
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A cknowiedgements
Many individuals contributed to the preparation and review of this manual.
Contract administration was provided by the U.S. Environmental Protection
Agency, Center for Environmental Research Information, Cincinnati, Ohio.
AUTHORS: Robert P. G. Bowker and John 1M. Smith - J. M. Smith &.
Associates, PSC, Cincinnati, Ohio
Ne i I A. Webster - FW Consultants, Louisville, Kentucky
CONTRACT PROJECT OFFICER:
Denis J. Lussier - EPA-CERI, Cincinnati, Ohio
TECHNICAL PEER REVIEWERS:
Michael Bealey - American Concrete Pipe Association,
Vienna, Virginia
Daniel Glasgow - Whittier, California
James F, Kreissl - EPA-WERL, Cincinnati, Ohio
Lam K, Lim - EPA-OMPC, Washington, DC
OTHER CONTRIBUTORS AND REVIEWERS:
Scott W, Duggan, Diane Jesion - Interox America,
Houston, Texas
Michael E. Garrett - British Oxygen Co., Ltd., London, England
George H. Hollerbach, Jr. - Airco Industrial Gases,
Murray Hill, New Jersey
Carlie L. McGinty - Calgon Carbon Corp., Pittsburgh, Pennsylvania
Harold J. Rafson - Quad Environmental Technologies Corp.,
Highland Park, Illinois
E. David Smith - Wise Chemical Co., Pittsburgh, Pennsylvania
OTHER REVIEWERS:
H. Forbes Davis - Davis Water and Waste Industries, Inc.,
Tallevast, Florida
Percey R. Dykes - Ameron Protective Coatings Division,
Brea. California
Kenneth J. Ficek - Carus Chemical Co., LaSalle, Illinois
Cameron B. Gray - Gray Engineering, Markham, Ontario, Canada
Jack Prince - Hydro-Vac, Inc., Port Arthur, Texas
ix
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Chapter 1
Introduction
1.1 Background
Wastewater is known to the public for its potential to
create odor nuisance. Sometimes it is the odors
escaping from sewer manholes that cause com-
plaints; more commonly, the odor source is a waste-
water treatment facility. Yet there are wastewater
treatment facilities that are free from this stigma, and
techniques to prevent odor nuisances are available,
to those committed to construct odor-free treatment
works.
A major cause of odors in wastewater treatment
systems is hydrogen sulfide (H2S), a gas detectable in
very low concentrations. H2S is also notable for its
toxicity and its ability to corrode various materials
used in sewer and treatment plant construction.
In the last three decades, much research has been
done on various aspects of the sulfide problem, and
important contributions have been made by engineers
in the United States, Australia (1} and South Africa
(2).
Traditional sanitary sewer design practice has not
fully acknowledged the importance of corrosion and
odor control, as evidenced by the widespread occur-
rence of sulfide and odor control problems throug hout
the United States for sanitary sewers serving both
small and large tributary areas. The 1984 EPA Needs
Survey estimates the backlog cost of major sewer
rehabilitation to be $3.2 billion (3). This cost is in
addition to the costs for correcting infiltration/inflow
problems and is for major structural repair or
replacement of sanitary sewers, a significant part of
which may be attributed to sulfide-induced deter-
ioration. The same survey further estimates the
construction costs for new collectors and interceptors
through the year 2000 to be $38.8 billion. These cost
estimates reflect the importance of adequately con-
sidering sulfide control in the design of new sanitary
sewer systems.
Since publication of the Process Design Manual for
Sulfide Control in Sanitary Sewerage Systems in
1974, substantial information on odors and corrosion
in municipal sewerage systems has been reported. In
addition, significant developments have evolved for
the control of odors and corrosion in wastewater
treatment plants. In particular, use of chemicals for
odor and corrosion control has increased substan-
tially. Inclusion of these advances is the primary
reason for revising this manual. To further the
understanding of odor and corrosion control in
sewerage systems and treatment works, many case
histories and examples have been added also.
1.2 Purpose
The need exists for a comprehensive design manual
that brings together available information in a form
convenient for those designing new systems or
applying odor and corrosion control procedures in
existing systems. This manual is intended to satisfy
this need.
While sulfide control is now a well developed
technology, continuing advances in basic knowledge
and in control procedures are to be expected.
Application of the art in its present state, however, as
set forth in this manual, can overcome sulfide-
producing tendencies in existing systems and help
minimize future problems.
1.3 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1. Thistlethwayte, D.K.B. Control of Sulphides in
Sewerage Systems. Ann Arbor Science, Ann
Arbor, Mi, 1972.
2. Stutterheim, N., andJ.H.P.Van Aardt. Corrosion
of Concrete Sewers and Some Remedies. South
African Industrial Chemistry, No. 10, (1953).
3. Assessment of Needed Publicly-Owned Waste-
water Treatment Facilities in the United States.
EPA 430/9-84-011, U.S. Environmental Pro-
tection Agency, Washington, DC, 1985.
J
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Chapter 2
Theory, Prediction, and Measurement of Odor and Corrosion
2.1 Introduction
Evaluation of existing or potential odor or corrosion
problems requires knowledge of the types of com-
pounds likely to cause such problems and the
mechanisms for their formation in wastewater sys-
tems. Prediction of where such problems will occur in
new systems is necessary so that designs can be
tailored to minimize odor and corrosion.
Where odor and/or corrosion problems exist, a
monitoring program should be developed to charac-
terize the severity of the problems and to identify the
sources of odorand corrosion. Such a program would
involve careful sample collection and analysis, fol-
lowed by interpretation of the data. Because collec-
tion of samples or inspection of sewers and wet wells
can be hazardous, plant operators and sewer workers
must be familiar with the potential dangers of
confined spaces in contact with wastewater, and
must strictly observe appropriate safety practices.
Once sufficient data have been collected to fully
characterize the problems, control options can be
evaluated and a suitablecontrol system implemented.
This chapter reviews odor and corrosion-causing
compounds, and describes the mechanisms of sulfide
generation. Methodologies for predicting sulfide
generation and corrosion are also presented, as well
as measurement and monitoring techniques to
characterize new and existing wastewater collection
and treatment systems.
2.2 Compounds Causing Odor and
Corrosion
Odor-producing substances found in domestic waste-
water and sludge are small, relatively volatile mole-
cules with a molecular weight of 30 to 150(1). Most of
these substances result from the anaerobic decompo-
sition of organic matter containing sulfur and nitro-
gen. Inorganic gases produced from domestic waste-
water decomposition commonly include hydrogen
sulfide, ammonia, carbon dioxide and methane. Of
these gases, only hydrogen sulfide and ammonia are
malodorous. Often, odor-producing substances in-
clude organic vapors such as indoles, skatoles,
mercaptans and nitrogen-bearing organics.
H2S is the most commonly known and prevalent
odorous gas associated with domestic wastewater
collection and treatment systems. It has a charac-
teristic rotten egg odor, is extremely toxic, and is
corrosive to metals such as iron, zinc, copper, lead
and cadmium, H2S is also a precursor to the formation
of sulfuric acid, which corrodes lead-based paint,
concrete, metals and other materials.
The conditions required for HzS corrosion are (2):
1. Presence of dissolved sulfides in the waste-
water.
2. Release of HgS gas from the water phase to the
gaseous phase.
3. Biological oxidation of H2S to sulfuric acid above
the wastewater surface in a pipe or basin
4. Acid attack on the moistened surfaces of
cement itious or metallic surfaces exposed to the
atmosphere.
The conditions leading to formation generally
favor the production of other malodorous organic
compounds. Investigations of the conditions favoring
H2S formation can also help to quantify the potential
for odor generation from other compounds. Thus,
solving H2S odor problems can often solve other odor
problems as well.
Many of the odors detected in wastewater collection
and treatment systems result from the presence of
sulfur-bearing compounds. A list of common mal-
odorous sulfur-bearing compounds is shown in Table
2-1. The lower the molecular weight of a compound,
the higher the volatility and potential for emission to
the atmosphere. Substances of high molecular
weight are usually not perceptibly odorous and are
neither volatile nor soluble. Mercaptans are com-
monly found in wastewater and are analogous to
alcohols with a substitution of sulfur for oxygen in the
[OH] radical. Mercaptans are a reduced form of
organic sulfur compounds. They are malodorous and
can contribute to odor problems due to their extremely
low threshold odor numbers (concentration below
which a substance is no longer detectable by the
human nose), as shown in Table 2-1 (3).
2.2.1 Sources of Sulfur in Domestic Wastewater
Sulfur is present in human excreta and sulfates are
Preceding page blank
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Table 2-1. Odorous Sulfur Compound* in Wactewstar (3)
Substance Formula
Allyl Mercaptan
CH^CH-CHj-SH
Amy! Mercaptan
GHrfCH2)3-CHrSH
Benzyl Mercaptan
C^sCHj-SH
Crotyl Mercaptan
CHrCH=CH-CHrSH
Dimethyl Sulfide
CHj-S-CHa
Ethyl Mercaptan
CHjCHj-SH
Hydrogen Sulfide
HjS
Methyl Mercaptan
CH3SH
Propyl Mercaptan
CHs-CHy-CHa-SH
Sulfur Dioxide
S02
Tert-butyl Mercaptan
(CHj)jC-SH
Thiocresoi
CH,-C8H4-SH
Ttiiophenol
CaHsSH
found in most water supplies. Sufficient sulfur is
normally available in domestic wastewater in the
form of inorganic sulfates and sulfides such as
mercaptans, thioethers, and disulfides for the produc-
tion of odorous gases by anaerobic and facultative
bacteria.
The sulfate ion {SOi) is one of the most universal
anions occurring in natural waters. It occurs fre-
quently in rainfall, particularly from air masses that
have encountered metropolitan areas. Sulfate con-
centrations in wastewater can vary from only a few
milligrams per liter (mg/l) to hundreds of milligrams
per liter (4).
Organic sulfur compounds are present in excreta,
with domestic wastewater containing 1 to 3 mg/l. All
sulfur compounds in oxidized or reduced forms,
organic or inorganic, represent a potential for sulfide
production. Generally, for domestic wastewater, the
main source of sulfide is sulfate. The sulfur cycle is
shown in Figure 2-1 (5).
2.2.2 Nature of Sulfide Compounds
2.2.2.1 Sulfate Reduction
The serious odor and corrosion problems associated
with the collection, handling and treatment of domes-
tic wastewater are primarily the result of the reduc-
tion of sulfate to H*S under anaerobic conditions, as
shown by the following reactions:
anaerobic
SOi + organic matter *¦ S= + H20 + C02
bacteria j2-1)
S= + 2H+ (2-2)
Odor
Threshold
Molecular
Weight
ppm
0.00005
74,15
0.0003
104,22
0,00019
124.21
0.000029
90.19
0,0001
62.13
0-00019
62.10
0.00047
34.10
0,0011
48,10
0.000075
76.16
0,009
64.07
0.00008
90.10
0.000062
124.21
0.000062
110.18
in the biochemical oxidation of organic matter,
bacteria remove hydrogen atoms from the organic
molecule and, in the process, gain energy. Through a
series of biochemical reactions, the hydrogen atoms
are transferred to a hydrogen acceptor. The hydrogen
acceptor may be an inorganic or organic substance.
Under aerobic conditions, free oxygen is the final
acceptor for hydrogen, the oxygen being reduced to
water. In the absence of free oxygen, combined
oxygen may be used as a final acceptor of hydrogen.
The following reactions indicate the hydrogen ac-
ceptors and subsequent reduced products (6):
Hydrogen
Acceptor
Hydrogen
Atoms
Added
Reduced
Product
(A)
O2
+ 4H+
2 HaO
(B)
2 NOi
+ 12 H*
N2 + 6 HsO
(C>
so;
+ 10 H*
H2S +¦ 4 H20
(D)
Oxidized
+ xH+
Reduced
Organics
Organics
IE)
CO 2
+ 8H*
CH4 + 2 HaO
Reactions (A), (B) and (E) result in odorless products.
Reaction (C) results in the malodorous HaS, and
reaction (D) often results in odorous products, such as
mercaptans, The anaerobic reactions (B-E) occur only
when oxygen is either absent or limited. Bacteria will
utilize the hydrogen acceptors preferentially in the
order given in the reaction list; i.e., oxygen first,
nitrate second, sulfate third. However, not all micro-
organisms can use any hydrogen acceptor, as some
are strictly aerobic bacteria. Others, such as obligate
Characteristic
Odor
Strong garlic-coffee
Unpleasant-putrid
Unpleasant-strong
Skunk- like
Decayed vegetables
Decayed cabbage
Rotten eggs
Decayed cabbage
Unpleasant
Pungent, irritating
Skunk, unpleasant
Skunk, rancid
Putrid, garlic-like
4
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Figure 2-1. The sulfur cycle (6)
H2S
Sulfide
S
SOj
Sulfites
SO
Waste
Org. Matter
Organic S
Urine
S04
Elementa
S
SO,
Sulfates
S04
Animal
PrDtein
Organic S
Plant
Protein
Organic S
Reproduced with permission of McGraw-Hill Book Co
anaerobic bacteria, can use only combined forms of
oxygen(NOi SOJ), while a large number of others are
facultative and can use either free Dr combined
oxygen as a hydrogen acceptor.
In the absence of dissolved oxygen (DO) and nitrates,
sulfates serve as the hydrogen acceptor for bio-
chemical oxidation by obligate anaerobic bacteria as
expressed in reaction 2-1. The most important
sulfate-reducing organism is the species Desulfo-
vibrio. These bacteria are found both in the digestive
tract of man and animals and in mud containing
organic matter, and are normally present in domestic
wastewater. The source of organic matter for this
microorganism is quite restricted, and ammonia is
the sole source of nitrogen,
2.2.2.2 Organic Compound Reductions
Proteins consist of amino acids, some of which
contain sulfur. H2S can be produced by the anaerobic
decomposition of amino acids such as cysteine,
cystine and methionine. This fermentation process is
carried out by many species of proteolytic bacteria,
including Veilfonefla, Clostridia, and Proteus(l). Al-
though organic decomposition can contribute to HzS
production, sulfate reduction is the most significant
mechanism for H2S generation in wastewater.
5
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2.2.2.3 Dissolved Sulfides
Molecular HjS, formed from sulfate reduction, dis-
solves In watBr and dissociates in accordance with
reversible ionization reactions, expressed as:
Table 2-2.
Logarithmic Ionization Constants (pKt) for
Hydrogen Sulfide (8)
H2S *- HS" + H+
HS~ •< *- S= + H"
(2-3)
(2-4)
The relative proportions of the species are related by
the following expressions:
. [HS"]
log = pH - pK,
[HaS]
, [SI
log
[HS1
where.
= pH - pKz
(2-5)
(2-6)
[H2S3, [HS ], [S~] = molar concentrations of (he
respective constituents
pKi, pKa
negative logarithms of the
ionization constants
The distribution of the above species as a function of
pH is shown in Figure 2-2 (5). It is apparent that the
concentration of S° species is insignificant within the
normal pH range of municipal wastewater (6,0 to 8,0).
Table 2-2 gives values for pKi as a function of specific
electrical conductance (representing ionic strength)
andtemperature(8). For municipal wastewater, pKi =
7.0 is a reasonable approximation.
Figure 2-2. Effect of pHon Hydrogen sulfideequilibrium|5)
100
Specific
Electrical
Conductance
@ 25' C
Temperature. °C
micromhos/cm
0
25
100
200
400
700
1,200
2,000
3,000
4,000
5,200
7,200
10,000
14,000
22,000
50,000*
10 15 20 26 30 35
40
7.24 7.17 7,10 7.03 6.9S 5,69 6.82
7.23 7.16 7.09 7.02 6-95 6,88 6.81
7.22 7.15 7,08 7.01 6.94 6.87 6.80
7.21 7.14 7.07 7.00 6.93 S.86 6.79
7.20 7.13 7.06 6.99 6-92 8.85 6.78
7.19 7.12 7-05 6,98 6.91 6.84 6.77
7.18 7 11 7.04 6,97 6.90 6.83 6.76
7.17 7.10 7.03 0.96 6.89 6.82 6,75
7.16 7 09 7.02 6.95 6.68 6.81 6,74
7.15 7.08 7,01 6.94 6,87 6.80 6.73
7.14 7.07 7.00 6,93 6.86 6.79 6.72
7.13 7,06 6.99 5,92 6.85- 6.79 6,71
7.12 7.05 6.98 6,91 6.B4 6.77 6,70
7.11 7,04 6.97 6.90 6.83 6.76 5.69
7,10 7.03 6.96 S.89 6.82 6.75 6,68
7.09 7.02 6.95 5,88 6.81 6.74 6.67
'Approximates ssa water.
Figure 2-3 shows the distribution of H2S and HS"
species as a function of pH-pKi or, assuming pK, =
7.0, as a function of pH (8). The relative H2S
concentration increases with decreasing pH. At a pH
of 7.0, HzS represents 50 percent of the dissolved
sulfides present, while at a pH of 6.0, HzS represents
90 percent of the dissolved sulfides. If part of the
dissolved H2S escapes to the atmosphere, the remain-
ing dissolved sulfide will be divided between H2S and
HS" in the same proportion as before because the
equilibrium re-establishes itself almost instantly.
The distinction between the types of sulfide com-
pounds is significant because only the HaS can
escape from solution and create odor and corrosion
problems. It is important, therefore, to quantify the
total and dissolved sulfides present and the pH of the
wastewater. The amount of total sulfides occurring in
the soluble form varies considerably in domestic
wastewater, but most frequently appears to be 70 to
90 percent. The percentage of dissolved sulfides
present varies with the pH of the wastewater and the
amount of metals present.
6
-------
Figure 2-3,
Proportions of HrS and HS" in dissolved sulfide 16).
50
6.0 6.2 6.4
6.6
pH if pK = 7.0
6.8 7.0
7.2
7.4
7.6 7.8 8,0
100
90
80
70
60
50
40
30
20
10
9.0
\
. k
\
\
\
\
s,
\
s
\
y
\
\
•k.
\
\
\
\
\
V
\
\
\
>
s
s
\
10
20
30
w
I
40 -
CL
0>
50 |
3
05
60
70
80
90
100
-2.0 -1.0 -.6
-.2
+.2
+.4
+.6 +.8 +1.0 +2.0
pH ™ pK
Total and dissolved sulfides from samples collected
over a 15-year period from The Central Trunk Sewers
in Sacramento, California are shown in Figure 2-4 (9).
The dissolved sulfides in the Sacramento system are
approximately 70 percent of the total sulfide. Based
on a survey conducted on the River Oaks Plant
collection system in May, 1982, Hillsborough County,
Florida, reported dissolved sulfides averaging 88
percent of the total sulfides (10).
2.2.3 Physical and Chemical Properties of Hydro-
gen Sulfide
H2S is a colorless gas that has a foul odor (rotten egg
smell) and is slightly heavier than air. Human
exposure to small amounts of H2S in air can cause
headaches, nausea, and eye irritation, and higher
concentrations can cause paralysis of the respiratory
system, which results in fainting and possible death.
Concentrations of the gas of 0,2 percent are fatal to
humans after exposure for a few minutes (11). The
physiological effects of HaS gas are discussed further
in Section 2.8. H2S gas is explosive at concentrations
from 4.3 to 45.5 percent in air.
HjS is moderately soluble in water, ranging between
3,000 and 4,000 mg/l at the normal temperatures
found in wastewater. The solubility of H2S decreases
with increasing temperatures. Table 2-3 presents
solubility data for H2S at various temperatures (1).
7
-------
Figure 2-4, Relationship of dissolved to total sulfide con-
centration in the Sacramento, CA central trunk
sawers (9).
Table 2-3, Solubility of H?S in Water at a Pressure of 1
Standard Atmosphere j11
2.0
x
a
E
S>
¦c
3
w
~a
>
o
«
U5
a
2 0
30
Total Sulfide, mg/l
Table 2-4 summarizes the physical and chemical
properties of H2S.
2.3 Mechanisms for the Generation of
Hydrogen Sulfide
The occurrence of sulfide in municipal wastewater
results principally from the biochemical reduction of
inorganic sulfur compounds. Although sulfide may be
found in high concentrations in some industrial
wastes or occasionally in ground water infiltrating
sewers, its presence in municipal sewers and treat-
ment plants is largely due to the bacteriological
reduction of sulfate in the absence of oxygen and in
the presence of organic matter. The sulfur reactions
that are of particular interest in controlling odors and
corrosion in wastewater collection and treatment
systems are:
1. Reduction of sulfate or sulfur-containing matter
to sulfide
2. Release of H2S gas to the atmosphere
3. Oxidation of H2S to sulfuric acid on the exposed
walls of pipes and other structures
2.3.1 Sulfate Reduction
Many bacteria reduce sulfate to sulfide, including:
1. Assimilatory microbes—those that assimilate
inorganic sulfur and reduce it to sulfide within
their protoplasm
2. Proteolytic bacteria—several of which can hydro-
lyze proteins and amino acids under anaerobic
conditions, resulting in the release of sulfides
3. Sulfate-reducing bacteria—specialized bacteria
that use inorganic sulfate as the hydrogen
acceptor in their energy cycle
Te mperature
Solubility
"C
mg/l as S
0
6,648
1
6,434
2
6,227
3
6,028
4
5,834
5
5,646
6
5,465
7
5,291
8
5,124
9
4,964
10
4,810
11
4,667
12
4,529
13
4,398
14
4,271
15
4,150
16
4,033
17
3,922
18
3,816
19
3,714
20
3,618
21
3,523
22
3,432
23
3,344
24
3,258
25
3,175
26
3,095
27
3,018
28
2,945
29
2,874
30
2,806
35
2,491
40
2,221
The sulfate-reducing bacteria, primarilyDesulfovibrio
desulfuricans (also called Desulfatomaculum desut-
furicans), are the principal mechanism of sulfate
reduction in municipal wasterwater collection and
treatment systems. These are obligate anaerobes
which utilize sulfate as the oxygen source (hydrogen
acceptor) and various forms of organic matter as a
food supply (hydrogen donor), including amino acids,
carbohydrates, organic acids, etc. Using C to repre-
8
-------
Table 2-4. Physical and Chemical Properties of Hydrogen
Sulfide
Molecular Weight
34.08
Boiling Point, °C
-60.2
Melting Point, °C
-83,8 to 85,5
Vapor Pressure, -0.4°C
10 atm
25.5°C
20 atm
Specific Gravity
(compared to air)
1,192
Auto Ignition Temperature, °C
250
Explosive range in air, percent
4.5 to 45.5
sent organic matter, the reaction can be expressed as
follows:
bacteria
SO4 + 2 C + HjO — 2 HCOj + H2S (2-7)
Sulfate, organic matter, and sulfate-reducing bacteria
are present in virtually all wastewaters, yet sulfide
generation does not always occur. Proper design and
maintenance can often prevent odors and corrosion
associated with sulfide generation.
Most of the sulfate reduction in sewers occurs in the
biological slime layer on the pipe wall or in sludge and
silt deposits on the pipe invert (8). These slimes are a
matrix of filamentous organisms and gelatinous
material (zoogleae) embedding smaller bacteria.
Typically, slime layers are 0.3-1.0 mm (0.01 -0.04 in)
thick, although this varies depending on velocity and
abrasive content of the wastewater and other envi-
ronmental conditions.
Oxygen in wastewater diffuses into the slime layer.
The extent of diffusion into the film is limited by the
rapid oxygen utilization by aerobic bacteria near the
surface of the layer. Beneath the aerobic zone,
anaerobic conditions may prevail, providing condi-
tions for sulfate reduction to occur. Closest to the pipe
wall, the slime layer is anaerobic but largely inactive
due to limited diffusion of nutrients. As long as an
aerobic zone is present in the slime layer, sulfide
diffusing out of the anaerobic zone will be oxidized
and will not enter the wastewater stream. This
condition is shown in Figure 2-5.
The relative thickness of the aerobic and anaerobic
zones is determined by oxygen supply. The depth to
which oxygen will penetrate is dependent on the
oxygen concentration in the wastewater as well as on
temperature and concentration of organic matter. If
the oxygen concentration in the stream approaches
zero (i.e., < 0,1 mg/l) insufficient oxygen will be
present in the slime to oxidize all of the sulfide
Figure 2-5. Processes occurring in sewers with sufficient
oxygen to prevent sulfide from entering the
stream (8).
Air
Oxygen Entering the Water
IPipe
!Wall
Wastewater
Dissolved Oxygen >1 rrtg/I
Dissolved Sulfide Zero orTrace
Diffusion of Os and Nutrients
Diffusion of S0« and Nutrients
Diffusion and Oxidation of Sulfide
diffusing out of the anaerobic zone, and sulfide will
enter the stream. This condition is shown in Figure
2-6.
If the stream is stationary or moving slowly, local
anaerobic conditions may occur near the pipe wall
and some sulfide may escape, even though the DO
concentration in the bulk liquid may be several
milligrams per liter. However, completely anaerobic
conditions must be approached for all of the sulfide to
pass into the wastewater.
The two other reactions previously mentioned, the
escape of H2S into the sewer atmosphere and the
oxidation of HjS to sulfuric acid, are equally important
in odor control.
2.3.2 Rate of Sulfide Production
The rate at which sulfide is produced by the slime
layer depends on the following environmental condi-
tions:
9
-------
Figure 2-6.
Processes occurring in sewers under sulfide
buildup conditions (8).
HjSFluxto Pipe Wall
Oxidation to H2SO4
HjS Entering the Air
Oxygen Entering the Water
Slime
Layer
(typically
Wastewater
Dissolved Oxygen <0.1 mg/l
Dissolved Sulfide Present, HS
Depletion of Oi in the Laminar Layer
Diffusion of SO4 and Nutrients
Diffusion of Sulfide Into the Stream
• Concentrations of organic material and nutrients
• Sulfate concentration
• DO
¦ pH
¦ Temperature
• Stream velocity
• Surface area
• Detention time
2.3.2.1 Concentrations of Organic Material and
Nutrients
Organic matter and nutrients must diffuse into the
slime layer to be utilized by the sulfate-reducing
bacteria. Little is known about specific nutrients
utilized by the Desulfovibrio bacteria, but unless
sufficient quantities are present, sulfide generation
may be limited by their availability. Pomeroy has
assumed that the concentration of these nutrients is
proportional to the BOD in most municipal waste-
waters, and that the rate of sulfide generation by the
slimes is proportional to the BOD if excess sulfate is
available (8). Thistlethwayte has postulated that the
rate of sulfide production is proportional to both BOD
and sulfate concentration, varying as [BOD]0 8 and as
[SO4]04 (2).
2.3.2.2 Sulfate Concentration
Sulfate and organic matter will be utilized by the
sulfate-reducing bacteria in the ratio of approximately
2:1, depending on the nature of the organics. It is
unlikely that both will diffuse into the sulfate-reduc-
ing zone in ideal proportions. If sulfate is abundant,
the sulfide generation rate will be proportional to the
organic matter and/or nutrient concentrations. If
sulfate is limiting, sulfide generation will be propor-
tional to the sulfate concentration. Pomeroy has
determined that if sulfate is in excess, the rate of
sulfide production will be relatively independent of
sulfate concentration (12). It has been estimated that
sulfate will cease to be a limiting factor in sulfide
production in most wastewaters at concentrations of
20 to 100 mg/l (8).
Thistlethwayte has developed an empirical equation
which links sulfide generation directly to sulfate
concentration and which predicts increased sulfide
generation with increasing sulfate concentration (2).
However, this has been shown not to hold true when
excess sulfate is available (12).
2.3.2.3 Dissolved Oxygen
The critical dissolved oxygen concentration in the
wastewater below which sulfate reduction can occur
is 0.1 to 1.0 mg/l. Above 1.0 mg/l, sulfate reduction
will be eliminated because of increased redox poten-
tial and inhibition of Desulfovibrio. Presence of DO in
the stream will also encourage growth of the aerobic
portion of the slime layer, increasing the distance
through which organic matter and sulfate must
diffuse to reach the sulfate reducers. Any sulfide
produced in the active anaerobic zone is likely to be
oxidized as it passes back through the aerobic zone.
Early studies on sulfate reduction showed that H2S
would not be produced until the bacteria utilized all of
the DO and reduced all of the nitrates (8). Oxygen is
gained primarily through reaeration at the stream
surface and through turbulence induced byjunctions,
drops, hydraulic jumps and other places where air
and wastewater mix. Oxygen is lost through con-
sumption by microorganisms present in the waste-
water and in the slime layer during biochemical
oxidation of organic matter. The rate of change of
oxygen in the stream due to consumption in the slime
layer can be estimated by:
R0 = 5.3 [OJIsu^FT1
where,
Re = loss of oxygen from the stream by
reaction with the slime layer, mg/l-hr
5.3 = empirical coefficient
(2-8)
10
-------
(Oz]= oxygen concentration, mg/l
s = slope of the energy grade line, m/l
u = stream velocity, m/s
R = hydraulic radius of the stream, m
The reaeration rate, R,, is the rate of change of oxygen
concentration due to absorption from the atmosphere,
and is related to other factors by the following;
(2-9)
Rt = —=
dm dm
where,
Rt = reaeration rate, mg/l-hr
= flux of oxygen per unit area, g/m2-hr
dm= mean hydraulic depth (defined as the cross
sectional area of the stream divided by its
surface width), m
f = exchange coefficient, m/hr
D = oxygen deficit, mg/l or g/m3
It should be noted that the reaeration coefficient, K2,
used in the Streeter-Phelps equation and typically
expressed in u nits of days" , is related tof and to Rf by
the following equations:
f
K2 = — (2-10)
R, = K2D
where,
12-11]
-1
K2= reaeration coefficient, hr
Pomeroy and Parkhurst found that for wastewater
flowing in a partially filled sewer, the exchange
coefficient, f, can be predicted by the following
equation (13):
f = 0.96(CaHT)(su)3/®
where,
0.96= empirical coefficient applicable to
wastewater streams
12-12)
Ca = factor representing the effect of turbulence
in creating additional air-water interface
compared to slow moving streams
T = temperature coefficient, equal to unity at
20°C
CA can be approximated by:
0.17 u2
1 +
where.
g d„
(2-13)
0.17= empirical coefficient
g = gravitational constant = 9.8 m/s2
Substituting the empirical equation for f
(Equation 2-12) into the equation for Rf
(Equation 2-9) yields:
R, = O.gBCAKsul^Dld^r1
(2-14)
where,
Rt
Ca
T
s
u
D
dm
reaeration rate, mg/l-hr
turbulence factor
temperature coefficient
slope of energy grade line, m/m
stream velocity, m/s
DO deficit mg/l
mean hydraulic depth, m
Based on the preceding equations, Figure 2-7 was
developed to show estimated reaeration rates for
various size sewers flowing half full as a function of
stream velocity. These curves assume a DO deficit of
7 mg/l, 1 atmosphere of pressure, an oxygen content
in the sewer atmosphere of 20.9 percent, and a
wastewater temperature of 20°C. In calculating the
deficit, the solubility of oxygen must be corrected for
pressure, oxygen content of the sewer atmosphere,
and temperature. An increase in temperature in-
creases K2 and the rate of oxygen absorption, if the
same DO deficit is assumed. However, the deficit
would likely be less due to the decreased solubility of
oxygen.
11
-------
Figure 2-7. Reaeration rates in aewers flowing haH full (8). Figure 2-8. Relative reaeration rates in a sewer (8).
Pipe Size
8"
12"
18"
Wastewater Temp. = 20°C
DO deficit = 7 mg/l
24"
2 4 6
Velocity, fps
cr
?
n
«
Kh
40 60
Depth of Flow, percent
waterfall reaeration coefficient, m
100
Hi and Ha = elevations of the hydraulic grade
line upstream and downstream from
the jump, m
Figure 2-8 shows the relative values of Ka and Rt in a
sewer. These factors can be applied to thB values
obtained fmm Figure 2-7 to yield a value corrected for
depth. However, if these two figures are used together
in calculating reaeration rates for a sewer flowing
other than half full, the input value used for velocity in
entering the curve in Figure 2-7 must be the half-full
velocity, not the actual velocity at other than half-full
conditions.
Oxygen may also be added through turbulence
induced by junctions, drops, hydraulic jumps, etc.
Pomeroy and Lofy found that in simple drops or falls,
the oxygen concentration approaches saturation
logarithmically with the height of the fall according to
the following equation (14):
In = Kh IH1-H2) (2-15)
Dz
where,
Di and D2 = oxygen deficits upstream and
downstream from the drop, mg/l
The average value of Kh for wastewater is approx-
imately 0.41 m~1 (8J. Using this value, Table 2-5
shows the percentage of the oxygen deficit that may
be expected to be satisfied for various heights of fall.
Table 2-B. Expected Oxygen Absorption in Wastewater Falls
(Kh = 0.41 m )
Oxygen Deficit
Hi-Hz Satisfied
ft
percent
1
12
2
22
3
31
4
39
5
46
6
53
8
63
10
71
15
87
20
92
30
98
12
-------
In junctions where two or more sewers having
different" energy grade lines meet, one should use the
difference between the average elevation of the
energy grade lines entering the junction and the
elevation of the outlet line for the combined flows to
calculate H.
For large trunk sewers, reaeration rates may be quite
slow due to the gentle slopes and large depth of flow
in the pipe. However, a drop in a large sewer will have
approximately the same effect on DO concentration
as a drop in a small sewer. This follows from Equation
2-15, which shows reaeration from drop-induced
turbulence to be independent of pipe size or flow rate.
Oxygen is consumed by microorganisms both within
the stream and within the slime layer. Short-term
oxygen consumption rates may vary according to
distance travelled by the wastewater, and bear no
correlation to standard BOD tests. Samples of waste-
water collected near the upper ends of small sewers
show relatively low oxygen consumption rates, on the
order of 2 to 3 mg/l-hr. Oxygen consumption rates of
the wastewater increase with distance travelled in
the sewer due to sloughing of biologically active
slimes from the pipe wall.
In larger trunk sewers, slopes are generally flatter
and depths of flow greater. This results in a decrease
in surface reaeration rates. At the same time, oxygen
consumption rates are high (5 to 10 mg/l-hr) due to
the distance travelled through the collector lines
feeding the trunk. In trunks with diameters greater
than 0.6 m (2.0 ft) flowing half full at a velocity of 0.6
m/s (2.0 ft/s), DO concentrations will approach zero
if surface reaeration is the only mechanism of oxygen
supply. Oxygen consumption rates will then be limited
by oxygen supply.
In small sewers with low oxygen consumption rates
and relatively high DO, a significant amount of
oxygen consumption will occur at the slime layer. In
larger sewers with high oxygen consumption rates in
the wastewater, flatter slopes, and smaller surface
area-to-volume ratios (greater hydraulicradii), oxygen
consumption by the slime layer is insignificant
When sulfide produced in the slime layer diffuses into
a wastewater stream containing DO, sulfide may be
oxidized chemically or biochemically. In typical mu-
nicipal wastewaters, biological oxidation to thiosul-
fate is the prevalent mechanism, as follows:
bacteria
202 + 2HS" *- S205 + HzO (2-16)
Another mechanism, the oxidation of sulfide to
elemental sulfur, does not occur to a great extent
under normal conditions in municipal sewer systems.
The ratB of biochemical sulfide oxidation varies with
the degree of biological activity, and may range from 1
to 2 mg/l-hr in fresh wastewater to 10 to 15 mg/l-hr
for wastewater retained in the sewer for several
hours. This rate is independent of sulfide and DO
concentration in the wastewater as long as the
concentrations of each are 1 mg/l or greater,
2.3.2.4 pH
The relative proportions of H2S andHS aredependent
on pH, which is of particular importance in assessing
the potential for H2S gas release into the sewer
atmosphere, as discussed later in Section 2,3.3. The
sulfate-reducing bacteria, however, are tolerant to
changes in pH, being able to exist in a pH of 5.5 to 9.0
(8). The optimum pH for sulfate reducers is 7.5 to 8.0
(12).
2.3.2.5 Temperature
Temperature has a significant impact on the biological
activity of the sulfate-reducers. It has been reported
that the rate of sulfide production is increased 7
percent/°C up to 30°C (12). This is approximately
equivalent to a doubling of the reaction rate for every
10°C increase in temperature. It has been theorized
that, with increasing temperature, sulfide generation
is further enhanced by the reduction in thickness of
the laminar flow layer and by increase in nutrient
supply to the sulfate reducers (2). However, as
discussed in Section 2.3.2.6, this theory is incon-
sistent with other findings that reduction in laminar
flow layer thickness by increased velocity did not
affect the rate of sulfide production (8).
2.3.2.6 Stream Velocity
The rate of sulfide production is not directly altered by
wastewater velocity (2)(12). Although a decrease in
wastewater velocity is generally thought to increase
the thickness of the slime layer, the thickness of the
active sulfide-producing layer may remain un-
changed, since the overall increase is possibly due to
an increase in the inactive layer adjacent to the pipe
wall (see Figure 2-8). Reduction in velocity would also
tend to increase the thickness of the laminar flow
layer, lengthening the distance through which nutri-
ents must pass to reach the sulfate reducers.
However, nutrients must also diffuse through the
aerobic slime layer, and the overall transport rate is
controlled by the layer of greater resistance.
Increased velocity will tend to reduce the thickness of
the slime layer due to increased shear. However, the
minimum thickness that will impair the rate of sulfide
generation is not known. It has been shown that the
rate of sulfide buildup in a pressure main flowing at
1.2 m/s (4.0 ft/s) was not significantly different from
that in other mains flowing at lower velocities (8). It
13
-------
should be noted that when the velocity is less than
scouring velocity, deposition of organic solids on the
pipe invert may occur, and the deposited solids will
serve as sites for sulfate reduction if anaerobic
conditions develop.
In gravity sewers flowing less than full, higher
velocities will result in turbulence-induced reaeration
(see Equation 2-14). While this may not affect the rate
of sulfide production from the anaerobic sulfide-
producing zone in the slime layer, the DO caused by
turbulence-induced reaeration will be used for the
oxidation of sulfides in the aerobic zone of the slime
layer or in the wastewater stream.
Figure 2-9 shows the partial pressure of HZS gas as a
function of temperature for a range of concentrations
of dissolved H2S. At 1 atmosphere of pressure, the
partial pressure in millionths of an atmosphere is
equal to volumetric concentration in ppm. Thus, at
20°C, 3.0 mg/l of dissolved H2S will be in equilibrium
with approximately 780 ppm by volume of gaseous
H2S. Increasing the temperature decreases the
solubility of the gas, and more will be present in the
atmosphere.
2.3,3.1 Rate of Hydrogen Sulfide Gas Release
The extent and rate of HgS gas release to the sewer
atmosphere are controlled by the following factors:
2.3.2.7 Surface Area
Factors which affect the pipe surface available for
sulfate reduction in a wastewater collection system
are flow rate, pipe diameter and energy gradient.
These elements control depth of flow in the sewer. It
follows that, as the depth increases, so does the
surface available for development of slimes below the
water level. Treatment plant structures such as wet
wells, interprocess piping, and junctures that permit
the accumulation of solids provide possible sites for
sulfide generation.
2.3.2.8 Detention Tims
As detention time in sewers, force mains, and non-
aerated holding basins increases, the oxygen con-
sumption Increases, the oxidation-reduction potential
(ORP) decreases, and organic matter becomes in-
creasingly solubilized. These conditions favor the
activity of the sulfate-reducing organisms. Thus, in
the design of collection and treatment systems,
minimizing detention time can limit the activity of the
Desulfovibrio bacteria and thus the rate of sulfide
production.
2.3,3 Hydrogen Sulfide Release
In enclosed vessels containing dissolved H2S at
equilibrium, the concentration of H2S gas in the
atmosphere will vary with the dissolved H2S con-
centration according to Henry's Law:
x
K
whBre,
(2-17)
K = Henry's Law constant, atm
p = partial pressure of the gas phase over the
solution, atm
x = mole fraction of the dissolved gas in the
liquid phase, dimensionless
a. Dissolved Oxygen
Sulfides generated in the sulfate-reducing slime
layer are likely to be oxidized in the aerobic layer or in
the wastewater stream iftheDO concentration is 1.0
mg/l or greater. If, however, the ORP and DO are low,
some of the sulfide produced will diffuse into the
stream.
b. pH
The dissociation of H2S is dependent upon pH. The
dissociation of HS'to H~ and S= is of minimal concern,
as this is significant only at high pH values. The un-
ionized H2S is the only form of sulfide which can be
Figure 2-9. Equilibrium concentration of HjS in air,
1,600
Atmosphere
1,600
1,400
I 1,200
t 1,000
10 20 30
Temperature, °C
14
-------
released to the sewer atmosphere. Lower pH values
favor the un-ionized HZS, and thus result in greater
potential for release of the gas from the liquid. Table
2-6 illustrates the relative proportions of H2S and HS~
as a function of pH, assuming a pK( of 7.0 and total
dissolved sulfide content of 4.0 mg/l.
Table 2-6. Dissociation of Hydrogen Sulfide (pKt = 7.0;
[DS] = 4.0 mg/l)
pH
H ?S rt(]
HS"
mg/l
mg/l
6.0
3.6
0.4
6 5
3.0
1.0
7,0
2.0
2,0
7.5
10
3.0
8.0
0,4
3.6
c. Metal Concentration
Several metals typically found in municipal waste-
water form insoluble metallic sulfides upon reaction
with dissolved sulfide. Such metals include iron, zinc,
copper, lead and cadmium. Given a typical range of
pH for municipal wastewater, these metallic sulfides
will continue to flow through the sewer without
further reaction. The typical range of concentrations
of insoluble metal lie sulfides in domestic wastewater
is 0.2 to 0.3 mg/l (8). However, industrial contribu-
tions of metal-bearing wastes may significantly
increase the metals availablefor sulfide precipitation.
d. Velocity
Velocity is a factor in release of H2S to the sewer
atmosphere for two reasons:
1. Increased velocity induces turbulence, which
increases the water surface area for gas transfer
and increases H2S release to the atmosphere.
2. Turbulence will likely increase the DO concen-
tration in the stream due to surface reaeration,
which may reduce H«S gas release due to
oxidation of sulfides (see Section 2.3.2.3).
a. Depth of Flow
Depth of flow in a sewer of a given size determines
the cross-sectional area of flow and the water surface
area, both of which affect gas transfer. These factors
can be represented by the mean hydraulic depth, dm,
defined as the cross-sectional area of the stream
divided by its surface width.
t Temperature
Increased temperature increases the reaeration
coefficient for a fixed oxygen deficit. However,
increased temperature also decreases the oxygen
deficit since solubility is reduced. The result is an
offsetting of these two factors, the net effect of
temperature on HjS release being minimal (15).
2,3,3.2 Predicting Hydrogen Sulfide Gas Release
Pomeroy has developed an equation to predict the
decline of sulfide in a stream due to loss of H2S to the
atmosphere:
where,
RSf = depletion of sulfide in the stream due to
escape of H2S, mg/l-hr
0ef - flux of HzS from the stream surface,
grams of sulfide per m2-hr
dm = mean hydraulic depth (defined as the cross-
sectional area of the stream divided by its
surface width), m
= 0.69 Cfl T (su)3 8 (1 -q) j [DS] (2-19)
where,
Ca = factor representing the effect of turbulence
in comparison to a slow stream (see
Section 2.3.2.3)
T = temperature coefficient, equal to unity at
20°C
s = slope of the energy grade line of the
stream, m/m
u = stream velocity, m/s
q - relative H2S saturation in the air compared to
equilibrium concentration (typically 2 to 20
percent), expressed as decimal fraction
j = proportion of dissolved sulfide present as
HzS (from Figure 2-3)
[DS] - dissolved sulfide concentration in the
wastewater, mg/l
Under typical sewer conditions, excluding shallow,
high velocity streams or any points of high turbulence.
Equation 2-19 can be approximated by:
0af = 0.69 (su)3 8 j [DS] (2-20)
If it is assumed that all of the H2S escaping to the
sewer atmosphere is oxidized on the pipe wall, the
15
-------
average sulfide flux to the wall can be calculated by
multiplying the sulfide flux from the surface by the
ratio of surface width to exposed wall perimeter;
03W = 0.69 (su)38 j [OS] {b/P'J (2-21)
where,
#»w = flux of HzS to the pipe wall, g/m2-hr
(b/P') = ratio of width of wastewater stream at
surface to exposed perimeter of the pipe
wall above the water surface
Figure 2-10 shows estimated sulfide flux to the pipe
wall as a function of stream velocity for pipes flowing
half full. The smaller pipe diameter yields higher flux
rates due to increased turbulence. Figure 2-11
provides correction factors to be applied to the values
from Figure 2-10 in order to calculate sulfide flux in
pipes other than half full. Under these conditions, the
Figure 2-10. Effect of velocity and pipe size on sulfide flux
to pips wall under spacified conditions (B).
Pipe Diameter
0.24 r
Conditions:
1) Pipss flowing half full
2) Un-ioni»d HsS = 1.0 mg/f
O.20
24"
E
CD
1
¦e
46'
72"
x
£ °°8
x
144"
0.04
0
2
4
8
6
Velocity, fps
value of velocity used to enter the curve in Figure
2-10 must be the half-full velocity, not the actual
velocity at other than half-full conditions.
2.3.4 Hydrogen Sulfide Oxidation
The bacterial reduction of sulfate to sulfide, and the
subsequent release of H2S gas to the sewer atmos-
phere has been described. Corrosion of exposed
concrete or metal surfaces occurs from the bacterial
oxidation of H2S to sulfuric acid under aerobic
conditions. This is described by the following reaction:
bacteria
HzS + 20s H2S0« (2-22)
This reaction is brought about by the action of
Thiobacitlus bacteria. Under acidic conditions, the
principal bacteria are ThiobacMus concretivorus.
These organisms are very tolerant to low pH, remain-
ing active at sulfuric acid concentrations of 7 percent.
They are autotrophic aerobes, which require a sulfur
Figure 2-11. Factor to apply to ,„ from Figure 2-10 to
calculate ,„ for other than half-full flow 18).
0 20 40 60 BO 1W
Depth of Flow, percent
76
-------
source (H2S, elemental sulfur, or thiosulfate) for
energy. Carbon dioxide provides the carbon source.
Moisture must be present on the exposed surfaces to
support the bacterial metabolism necessary for the
production of H2SO4. For new concrete, condensed
moisture will be highly alkaline, with pH values
ranging from 11,0 to 13.0. Thiobacillus are unable to
survive under these conditions. Weathering of the
concrete converts calcium hydroxide to calcuim
carbonate, which then dissociates to bicarbonate,
resulting in a decrease of surface pH, In addition,
presence of carbon dioxide in the sewer atmosphere
(approximately 1 percent by volume) drops the pH to
about 7.4, Thiobacillus thioparus and similar bacteria
will then establish themselves in the pH range of 5 to
9. These autotrophic bacteria oxidize the H2S to
thiosulfuric and polythionic acids, further reducing
the pH of the condensate. Thiobacillus thiooxidans
and Thiobacillus concretivorus then become estab-
lished at pH < 5. These organisms are able to oxidize
HzS, elemental sulfur, thiosulfate and polythionates
to H2SO4, a strong acid. This further reduces the pH,
often to values below 2.0.
Sufficient moisture must be present on the pipe wall
both for weathering to occur and for prevention of
dessication of the sulfur bacteria. Thus, moisture is a
primary factor affecting H2S oxidation to sulfuric acid.
In some cases, sewer ventilation is usedto reducethe
humidity in collection systems, thereby reducing the
amount of condensate formed on the pipe wall, this
may also increase the rate of surface reaeration
2.4 Mechanisms of Corrosion
Corrosion may be broadly defined as the destruction
or deterioration of materials by the direct chemical or
electrochemical reaction with their environment|16).
Various types of corrosion are discussed in the
"following sections,
2.4.1 Direct Chemical Corrosion
2.4.1.1 Oxidation
Oxidation is the mostfamiliartypeof corrosion and is
readily observable in the form of rust. Oxidation
involves an exchange of electrons between the metal
and free oxygen present in the environment. Ex-
amples of the reactions which occur during the
oxidation of iron are shown in Figure 2-12. Metal
oxides formed during oxidation are more electro-
chemically stable than the original metal. Thus,
buildup of the oxide on a metal surface acts as
insulation to reduce the rate of reaction. Oxides of
chromium, aluminum, and nickel form a barrier of
microscopic thickness that will effectively prevent
further corrosion, However, some oxides, such as
iron oxide, are not effective barriers, and the oxide
itself may be porous or subject to chemical attack by
such wastewater constituents as chlorides and
sulfates (17).
2.4.1.2 Hydrogenation
Hydrogenation may occur when a metal is immersed
in non-aerated water or non-oxidizing acid. Some of
the water is reduced to its ions, H+ and OH". Under
high temperatures, pressures, and stress conditions,
hydrogen penetrates the lattice structure of the metal
and reacts with its interna! structure. The internal
changes which occur can cause loss of ductility
(hydrogen embrittlement) and creation of internal
pressures and splitting (hydrogen cracking). In more
malleable metals, surface blistering results (17).
Separation of structural boundaries in a metal can be
caused by increasing the temperature, roughening
the surface, working the metal, or subjecting the
metal to stress. These allow the hydrogen to penetrate
the metal and attack exposed faces on interior
surfaces. As the ions build up, they slowly join to form
molecules of free hydrogen which are unable to
escape and generate internal pressures (17).
2.4.1.3 Other Direct Chemical Reactions
Corrosive chemicals such as chlorine, various acids
and alkalies, and ferricchloride are commonly used in
wastewater collection and treatment operations. In
addition, byproducts of sewage treatment processes,
such as sludge supernatant liquors, are very cor-
rosive. Gases such as HzS and S02are also corrosive,
both in the gaseous state and after reaction with
water and oxygen to form sulfuric acid. In coastal
areas, salt (sodium chloride) in water vapor can be
very damaging. Dew can also be quite corrosive in
industrial areas due to the absorption of corrosive
gases from manufacturing operations (17),
HgS gas can directly attack metallic components of
wastewater systems such as steel tanks, structural
members, gratings and walkways, and equipment
(grit collectors, bar screens, conveyors, etc.). In
addition, H2S reacts directly with copper electrical
components to form black copper sulfate, a poor
conductor.
In collection systems, a major causative agent of
corrosion is sulfuric acid formed from the oxidation of
H2S in the presence of moisture, the mechanism of
which is described in Section 2.3. Corrosion of the
pipe wall of a sewer is not uniform. This is due to
several factors, including air currents, migration of
sulfuric acid down the pipe wall, and exposure to
water. The pipe wall is normally cooler than the
wastewater, particularly during the summer. Air
cooled by the walls moves downward along the walls,
and is replaced by slightly warmer air that rises from
the center of the stream surface. As a result,
-------
Figure 2-12, Chemical reactions in the corrosion of iron.
Liquid
Iron Going Into Solution
as Iron Ions
Fe(OH):
OH"
Fe(OH)s
OH"
Electron Flow Away From
Area of Dissolving Iron
Iron or Steel Structure
Reproduced with permission of Ameron Protective Coatings Division
maximum rate of H2S transfer to the pipe wall occurs
at the crown. Acid formed on the pipe wall as a result
of HzS oxidation migrates down the wall toward the
stream. Where corroded pipe has been removed for
inspection, the effects of this migration can be seen
as irregular vertical grooves in the pipe wall. Cor-
rosion at the waterline is often severe. This is due to
intermittent washing to the pipe wall in this zone,
which cleans away the pasty decomposition products
of concrete. This exposes new concrete, which is
su bject to rapid attack by the acid. Typical distribution
of corrosion in the interior of a sewer is shown in
Figure 2-13.
2,4.2 Bacteriological Corrosion
Bacteria play an important role, either directly or
indirectly, in corrosion of materials in wastewater
systems. For example, anaerobic sulfate-reducing
bacteria can attack the protective sulfate coatings on
metal and concrete and leave them vulnerable to
corrosion by sulfuric acid resulting from oxidation of
H2S. H2S oxidation to sulfuric acid in the presence of
moisture also results from action of bacteria which
colonize the moist slimes above the water surface.
Other types of bacteria can destroy asphaltic coatings
that are normally resistant to chemical attack, leaving
the parent surface exposed and vulnerable to attack
by chemical agents.
Another potential result of bacterial action is the
formation of localized galvanic cells in an electrolyte
such as wastewater. Bacterial colonization on surface
slimes or in the liquid results in a depression of the pH
in the immediate area of the bacteria, ThepH change
may result in a lower electrical potential, with the
localized area acting as a cathode. The adjacent metal
becomes an anode, and electrochemical corrosion
may result (17).
2.4.3 Fatigue Corrosion
Virtually all ductile metals have a limit to the number
of times they can be stressed or bent before cracking
or breaking. When these metals are subjected to such
stresses under corrosive conditions, this limit may be
Figure 2-13. Distribution of corrosion in a sewer.
J J
18
-------
reduced substantially. The process by which the
stress limit is reduced is called fatigue corrosion. The
actual corrosion mechanism that is responsible for
this phenomenon may be oxidation, hydrogenation,
direct chemical corrosion, or galvanic corrosion due
to the heat and stresses generated within the metal.
Oxidation, hydrogenation, and direct chemical cor-
rosion may occur due to the separation of the grain
boundaries, allowing penetration of the corrosive
agents to the internal surfaces of the metal. Galvanic
cells may be formed from electrical currents and
differential pressures that result from frictional heat
generation during slippage of the grain boundaries
(17).
2.4.4 Stress Corrosion
Stress corrosion is similar to fatigue corrosion, except
that in stress corrosion, the stress is generally pre-
applied and may result from temperature or strains
induced by working. However, the mechanism (slip-
ping or separation of grain boundaries) is much the
same. The result is usually splitting or cracking of the
metal.
Straining of metal also produces electrical currents
which polarize the metal and increase its attraction to
oxygen and other corrosion-inducing agents. Metals
can retain this polarity and may thus be subject to
much more corrosion than if straining had not
occurred (17).
2.4.5 Fretting Corrosion
Fretting corrosion is a combination of wear (erosion)
and the oxidation of the wear products on the freshly
exposed metal. A good example of fretting corrosion
in a wastewater treatment plant occurs due to the
action of flights along the rails in the bottom of a
clarifier. The wearing action of the metal shoes is
accelerated by the presence of grit and abrasive
material in the wastewater. The fresh metal surfaces
exposed by this action are subjected to corrosion by
the corrosive agents in the wastewater (17).
2.4.6 Cavitation Erosion
Cavitation erosion is normally associated with pump
impellers, although it can occur at any point where
high liquid velocities and sudden, violent reductions
of fluid pressure exist. This can occur even in non-
corrosive fluids. Several theories exist regarding this
mechanism, including the following (17):
1. Sudden and extreme changes in pressure distort
the metal surfaces, allowing penetration of
oxygen or hydrogen into the lattice structure
during periods of high pressure. Gas molecules
combine and literally explode during periods of
low pressure, breaking off minute sections of
the metal surface.
2. Penetration of the lattice structure of the metal
by the corrosive agent results in oxidation or
hydrogenation of the interior surfaces, with
subsequent erosion of the corrosion products
due to the velocity of the liquid.
3. Galvanic cells form in the metal as a result of
differential pressures in the liquid.
2.4.7 Filiform Corrosion
Filiform corrosion may occur on metal surfaces with
organic coatings and is induced by pinpoint penetra-
tion of moisture at numerous points on the surface.
Through chemical and electrochemical processes,
the corrosion progresses in narrow lines beneath the
coating. Oxygen and moisture penetrating the coating
support corrosion and the subsequent growth of
these filament-like grooves in the metal surface (17).
2.4.8 Electrochemical Corrosion
2.4.8.1 Bimetallic or Galvanic Corrosion
Galvanic corrosion occurs from the electrical current
created when two or more dissimilar metals are
immersed in an electrolyte. Although water or
wastewater is usually the electrolyte of interest,
moist soils or moist gases may also serve as
electrolytes. The resulting current-generating cells
are referred to as "dissimilar electrode cells," and
may occur under a wide range of conditions. Examples
of such conditions include use of brass or bronze
valves with iron pipe, variation in chemical composi-
tion and moisture content of backfill materials
surrounding a pipe, differences in dissolved gas
concentrations in the electrolyte, and differences in
temperature within a pipe.
The tendency of metals to enter into this type of
reaction is due to a property referred to as electro-
motive force or electric potential. The following
metals are listed in the order of decreasing electro-
motive force: magnesium, aluminum, zinc, chrom-
ium, iron, cadmium, nickel, tin, lead, hydrogen,
copper, mercury, silver, platinum, and gold (43),
When two metalsform a dissimilar electrode cell, the
metal with the highest electromotive force serves as
the anode (negative polarity), while the other metal
acts as the cathode ( positive polarity). Factors which
affect the rate of reaction include proximity of the two
metals, conductivity of the electrolyte, temperature,
and pH (17).
2.4.8.2 Parting
Parting occurs in alloys immersed in an electrolyte
when dissimilar electrode cells are formed between
the various metals in the alloy. When the corrosion
products are eroded by the velocity of the liquid,
certain metals may be removed from the alloy. This
19
-------
can substantially change the alloy's property, such as
reduce its strength or ductility (17)
2,4.8,3 Electrolysis or Stray Current Corrosion
This type of corrosion is caused by stray or external
currents of electricity passing through soil or water in
which a metal object is submerged. Current enters
the metal, travels along the metal as the path of least
resistance, and leaves at what becomes the anode,
where corrosion occurs. Stray current corrosion is
not a significant concern at wastewater treatment
plants, although it may be a problem with steel pipe
used in wastewater collection systems (17).
2.5 Predicting Sulfide Buildup and
Corrosion in Sewers
Prediction of the rate of sulfide buildup and corrosion
potential is an essential element in the design of new
sewer systems as well as in the evaluation of existing
systems. Equations are presented in this section that
can be used for this purpose. It should be noted,
however, that several of the coefficients are empir-
ically determined, and will vary significantly from one
condition tothe next. For existing systems, assumed
coefficients can be used in the predictive models to
estimate sulfide buildup or corrosion penetration.
2.5.1 Predicting Sulfide Buildup
2.5.1.1 Pipes Flowing Less Than Full
Several equations have been developed to predict the
buildup of sulfides in both gravity sewers and force
mains. The first, applicable only for lines flowing less
than half full, determines the "marginal velocity"
above which sulfide generation will not occur (12),
Another equation incorporates wastewater flow
depth and velocity (18), A further modification yields
the "Z" equation, which can be used for gross
estimates of sulfide generation potential (19).
The following equation was developed by Pomeroy
and Parkhurst to account for other factors affecting
sulfide buildup in a pipe flowing less than full (20).
This equation applies only to pipes flowing less than
full and in which little or no DO exists.
d[S] _ M' EBOD m [S](su)3/8
dt R dm
(2-23)
where,
d[S]
dt
M'
rate of change of total sulfide, mg/l-hr
effective sulfide flux coefficient for
sulfide generation by the slime layer in
gravity sewers, experimentally determined
empirical constant, m/hr
EBOD = effective BOD = B0Ds x 1,07T~20, mg/l
T = wastewater temperature, °C
R = hydraulic radius, equal to area of flow
divided by wetted perimeter (P), m
m = empirical coefficient to account for sulfide
losses by oxidation and escape to
atmosphere, dimensionless
[S] = total sulfide concentration, mg/l
dm = mean hydraulic depth, equal to area of
flow divided by surface width (b), m
u = mean sewage velocity, m/s
s = slope of energy grade line, m/m
The first term on the right-hand side of the equation
accounts for sulfide generation by the slime layer,
and assumes that this is the sole source of sulfide.
This assumption is fairly accurate, as sulfide genera-
tion within the stream of a gravity sewer is usually
negligible. The second term accounts for losses of
sulfide due to oxidation in the stream and emission to
the sewer atmosphere.
At equilibrium, d[S]/dt ~ 0. Therefore:
M' EBOD M[SJsu)3'B
Solving for [S], the theoretical upper limit at equil-
ibrium, yields:
(S],™ = (M /m) EBOD (sup'" (P/b)
(2-24)
This limit will never be reached theoretically, but will
be approached asymtoticaliy.
The downstream sulfide concentration, S2, at time t2
can be predicted directly from the following:
S2
where,
{S«m-Si>
log"1
¦ mjsu)
L 2-31
5
dm J
(2-25)
predicted sulfide concentration at time
t2, mg/l
20
-------
Si = sulfide concentration at time ti, mg/l
S[,m - limiting sulfide concentration from
Equation 2-23, mg/l
s = slope, m/m
u = stream velocity, m/s
t = (t2 - ti) = flow time in a given sewer
reach with constant slope, diameter, and
flow, hr
m = empirical coefficient for sulfide losses
dm - mean hydraulic depth, equal to area of flow
divided by surface width, m
This equation allows the input of initial sulfide
concentration Si. (Slim - Si) being the initial sulfide
deficit. Solving the equation for S2 requires an
estimate of flow time, t. This can be calculated from
an estimate of the stream velocity, u. Velocities can
be estimated using dyes, floating objects, velocity
meters, or hydraulic computations. In one study,
velocities were estimated using dyes, floating objects,
and hydraulic computations. Most probable velocity
values were then selected based on the tests
conducted and the condition of the lines (21).
Two sets of assumed values for the coefficients M"
and m have been suggested, with the following
interpretation (20):
Moderately conservative: Low DO, sulfide buildup
in progress
M' = 0.32 x 10 3
m = 0.96
More conservative: Observed sulfide buildup
generally less than
predicted
M' = 0.32 x 10~3
m = 0,64
These coefficients can be used as first estimates for
Equations 2-24 and 2-25. Where possible, the
equations can be calibrated based on data collected
from existing systems. This will allow adjustment of
coefficients to Improve correlation between meas-
ured and predicted values. This is often done when it
is desired to predict sulfide levels in proposed
expansions to existing collection systems.
2.5.1.2 Pipas Flowing Full
In a force main or surcharged sewer trunk line, the
pipe is flowing full, thus minimizing or eliminating
surface reaeration, sulfide oxidation, and sulfide
losses to the sewer atmosphere, The term on the far
right of Equation 2-23 can be eliminated. The first
term on the right-hand side of Equation 2-23 has
been modified to account for sulfide generation in the
stream, and assumes zero DO in the wastewater
(20):
d[S] 4
—= M[EBOD(—+ 1.57)] (2-26)
dt d
where,
d[S]
= rate of change of total sulfide, mg/l-hr
dt
M = sulfide flux coefficient for filled pipe,
experimentally determined empirical
constant, m/hr
EB0D = effective BOD = BOD x 1.07(T-M\ mg/l
d = pipe diameter, m
Integrating and solving for S2 yields:
S2 = S,+(MHt)[EBOD(-+1.57)] (2-27)
d
where,
S2 = predicted sulfide concentration at time
ta, mg/l
Si = sulfide concentration at time ti, mg/l
t _ t2 ti = flow time in a given sewer reach with
constant slope, diameter, and flow, hr
Figure 2-14 shows the distribution of empirically
determined values for M. In general, a value of 1 x
10 3 m/hr is reasonable for force mains in which
conditions are favorable for sulfide buildup (8).
Given the initial sulfide concentration, hydraulic
radiusofthepipe(d/4), EBOD, andanassumedvalue
for M, it is possible to predict the sulfide buildup at any
point downstream (at time t) in a pressure main.
2.5.1.3 Consideration of Junctions and Tributaries
in Estimating Sulfide Buildup
Junctions and tributary sewers can cause changes in
wastewater quality that affect the generation of
sulfide in sewer systems. For example, a force main
or pressure sewer discharging into a gravity line may
significantly increase the sulfide concentration in the
27
-------
Figure 2-14. Specific sulfide flux coefficients from ftlled-
pipe data (8).
s,
Si
St
(Qtrunk) ([DS]trunk) + (Qlrib) {[DSjtr
Qtrunk + Qtrib
(4.0) <2.0)+ (1.0) (1.0)
4.0 + 1.0
1.8 mg/l
M x 10s
gravity sewer. In addition, turbulent junctions may
strip Hj>S to the sewer atmosphere and add DO to the
wastewater. In general, where sulfide is likely to be
present, junctions should be designed to minimize
turbulence, and thus releasing of H2S, in order to
avoid odor and corrosion problem at that point.
In estimating sulfide buildup in a sewer trunk, the
affect of tributary flows can be handled in the
following manner:
1. Calculatethe estimated sulfide buildupfor each
of the tributary sewers using Equation 2-25
(gravity mains) or Equation 2-27 (force mains).
2. Calculate the starting sulfide concentration, Si,
in the trunk line at each junction using simple
mass balance relationships.
For example, at a given junction:
Qtrunk = 4.0 cfs (before junction)
[DSJtiunk = 2.0 mg/l (before junction)
Qirib = 1.0 Cfs
[DS]t,ib = 1.0 mg/l
3. After calculating a new starting sulfide con-
centration, Si, at each junction, calculate the
sulfide concentration for each downstream
reach.
2,5.1.4 Example
The following example illustrates the methodology
for predicting downstream sulfide concentrations in a
gravity sewer,
DATA: Single gravity sewer trunk with no other
contributing flows.
d = diameter = 0.91 m
s = slope - 0.001
depth of flow = 0,45 m
u
M'
m
Si
bod5
T
velocity - 0.61 m/sec
0 32 x 10 3. assumed
0 96, assumed
0-5 mg/l
200 mg/l
25 °C
Calculate sulfide concentration for a t of 5 hrs.
[S],im - (M'/m) EBOD (su| 3 8 (P/b)
.32 x 10~3
(2-24)
[S],i
[S],i
S2
. 0.96
[(0.001 K0.61 )]~3 8
[200 (1 07>2S 2°]
1.43
0.91
2.36 mg/l
(S|im - Si)
log"1
m(su)
Ta
2.31 dr
(2-25)
2.36
(2.36-0.5)
I -1
log
(0.96) [(0,001) (0.61 )f/a(5)
(2.31) (0.36)
22
-------
2.36
1.86
log" (0.360)
S2= 2.36 - 0.81
S2 = 1 -55 mg/l
2.5,2 Predicting Rates of Corrosion
2.5.2.1 Corrosion Rate Predictive Model
The rate of corrosion of cementitious pipes can be
presented theoretically by equations. The corrosion
rate depends upon the rate of sulfuric acid production
and the alkalinity of the pipe material. Sulfuric acid
production is related to the mass emission of sulfide
from the wastewater. The corrosion rate of cement
bonded pipe can be estimated by assuming the rate at
which H2S will reach the pipe wall and the amount
reaching the wall that will be oxidized and available
for reaction. Thirty-two grams of sulfide is required to
produce the sulfuric acid to dissolve 100 grams of
alkalinity expressed as CaCC>3 The corrosion rate
equation developed by Pomeroy is (8):
11,5 k 0s
'AVG '
(2-28)
where,
Cavg = average rate of penetration, mm/yr
k = Coefficient of efficiency for acid
reaction considering the estimated
fraction of acid remaining on the
wall. May be as low as 0.3 and will
approach 1.0 for a complete acid
reaction.
0BW = flux of H2S to the pipe wall, gm2-hr
A = Alkalinity of the cement bonded
material, expressed as CaC03
equivalents. Approximately 0,18 to
0.23 for granitic aggregate concrete,
0.9 for calcareous aggregate, 0.4 for
mortar linings, and 0.5 for asbestos
cement.
11.5 = constant
0aw = 0.69 (su)3'* j [DS] (b/P'J
where,
s = energy gradient of wastewater
stream, m/m
u = stream velocity, m/s
j = fraction of dissolved sulfide present
as H2S as a function of pH (see
Figure 2-3)
(2-21)
[DS] = average annual concentration of
dissolved sulfide in the wastewater,
mg/l
b/P' = ratio of width of wastewater stream
at surface to exposed perimeter of
the pipe wall above the water
surface
Rates of corrosion are usually expressed in milli-
meters of penetration per year measured inwards
from the original interior profile. Areas where the rate
of corrosion reaches a maximum may show greater
rates of penetration than the average condition, It has
been suggested that the most rapid attack may be 1.5
times the average (8). Note that the dissolved sulfide
concentration used in Equation 2-28 is the annual
average sulfide concentration, not the peak or
climactic concentration. Average annual dissolved
sulfides may be only 25 to 50 percent of the climactic
values. A thorough discussion of average annual vs.
peak dissolved sulfide concentrations may be found
in reference 22.
The choice of the coefficient of efficiency for the acid
reaction, k, is a matter of engineeringjudgment. K will
approach unity when the rate of acid production is
very slow, and may be as low as 0.3 to 0.4 if acid
production is rapid and if much condensate is formed,
as with warm wastewater flowing in a cold pipe. In
large pipes with moderate rates of acid formation,
most of the acid will react, and k will most likely be in
the range of 0.9 to 1.0.
2.5.2.2 Example
DATA: Reinforced concrete pipe with granitic
aggregate; 25-mm cover over reinforcing
steel
d = diameter = 1.07 m
s = slope = 0.00088
y0 - depth of flow = 0.214 m
pH = 7.0
k = coefficient of efficiency for acid
reaction, assumed to be 0.8
A = measured alkalinity of concrete-0.2
(granitic aggregate)
[DS] = average annual dissolved sulfide
concentration = 2.0 mg/l
CALCULATE;
1. Corrosion rate
2. Expected lifetime of pipe
23
-------
First, calculate hydraulic factors:
V
Area = 3,14r2 (0/180) - (b/2) (r-yc)
where,
1 (r-yo)
6
e
b
b
= cos
COS
(0.535 -0.214)"
0.535 J
2[r2-(r-y0)2]os
2[(0.535)2 - (0.535-0.214)2]05
53.1°
0.86 m
Area - [3.14 (0.535)2 (53.1 /180)]
-[(0.86/2) (0.535-0.214)] = 0.127 m2
P
P
P'
P'
R
V
2 (3.14) (x)\B/180)
2 (3.14) (0.535) (53.1/180) = 0.99 m
2 (3.14)r - P
(2) (3.14) (0.535) - 0.99 = 2.37 m
= 0.127/0.99 = 0.128 m
r^3S1/2
n '
(Manning equation; assume n = 0.01 3)
24
(0.1 28)^ (0.00088)1
0.013
0.58 m/s
(Alternatively, V can be based on actual measured
values.)
Next, calculate 08W and Cavg:
0SW = 0.69(su)3/a j[DSJb/P')
From Figure 2-3, at pH = 7.0, j = 0.5
03W
0.69 (0.00088 x 0.58)3'8 (0.5) (2.0)
(0.86/2.37)
0sw
Cevg
0.015 g/m2-hr
11.5 K 0SW
(Assume K = 0.8)
0.69 mm/yr
r - 11-5 (0.8) (0.015)
Lavg ~ q £
Expected life of pipe(based on exposure of reinforcing
steel)
= 25 mm/0.69 mm/yr = 36.2 years
The preceding calculation is based on the assumption
that the life of the pipe is equal to the time required for
acid corrosion to reach the reinforcing steel. If the
expected life is less than the desired design life,
changes to such parameters as pipe slope, alkalinity,
cover thickness, etc. should be considered.
2.5.3 Case Study for Predicting Sulfide Buildup
and Corrosion
In 1976, the county of Sacramento, California,
conducted an extensive sulfide investigation of 13
miles of gravity sewer trunk (Central Trunk). The
sewer was placed into operation in 1963, and total
and dissolved sulfides have been continuously moni-
tored since 1965. Visual inspections were conducted
in 1964, 1968, 1969, and 1976. Severe corrosion
was not apparent until 1974, which spurred a more
extensive sampling program to establish the cause
and extent of sulfide buildup and corrosion.
Table 2-7 provides a comparison of predicted and
measured total sulfide concentrations in the Sacra-
mento Central Trunk (9). The predicted values were
calculated to account for gains and losses of sulfide
from tributary streams. Note that the correlation is
good when the coefficient m in Equation 2-25 is
assumed to be 0.96.
A comparison was made between the actual meas-
ured corrosion penetration and that predicted by the
corrosion rate equation (Equation 2-28). Alkalinity
was measured to be 0.16 and k was assumed to be
1.0. The sulfide flux rate, 0BW, was calculated based
on 15 years of operating data, which included
dissolved sulfide concentrations, flows, pH values.
-------
Table 2-7. Comparison of Predicted and Measured Total
Sulfides in Sacramento Central Trunk (9)
Figure 2-15. Observed and predicted corrosion penetration,
Sacramento, CA central trunk sewer.
Distance Upstream
Total Sulfides
Irom CWTP*
Predicted6
Measured"
ft
mg/l
8.000
1.5
1.4
22,000
1.8
1.5
35,000
19
1.8
55,000
1.4
0.9
"Sampling stations in the Central Trunk located upstream from the
Central Wastewater Treatment Plant (CWTP).
"Predicted using Equation 2-25 with m = 0.96 and considering the
effect of junctions.
^Represents the average of 48 samples.
and sewer gradients at manholes. Expandable rods
were used to measure pipe diameters. Soft concrete
was chipped away to hard concrete to determine the
depth of acid penetration. The measured corrosion
penetration for the various reaches of the trunk sewer
are shown in Table 2-8 (9).
Table 2-8, Comparisonof Measured vs. Predicted Corrosion
Penetration, Sacramento, CA, Central Trunk
Sewer (9!
Pipe Pipe
Avg. Corrosion Penetration
Diameter Length
Measured
Predicted*
m m
cm
cm
0,69 823
0
0.3
0.76 1,458
0.1
0.4
0.91 2,792
0.8
0.6
0.99 1,778
0.8
0.8
1.07 14,838
1.5
1.1
1.22 3,703
2.0
1.4
1.37 2,765
1.9
1.8
1.52 3,738
2.1
2.0
2,5
Observed Avg, Corrosion
Predicted Avg. Corrosion
•Using Equation 2-29 with A = 0.16
k = 1.0
The predictive equation (Equation 2-28) compared
well with the actual measured corrosion in the trunk
sewer. The results are shown graphically in Figure
2-15.
The utility in comparing existing sulfide levels and
corrosion rates to predictive models for sulfide
generation and corrosion is best demonstrated when
an existing system in the same city or area is
undergoing expansion. For design purposes, an
analysis can be conducted of the existing system to
0.6 0 7 0,8 0.9 1.0 1 1
Pipe Diameter, m
Increasing Detention Times
obtain sulfide levels and corrosion rates. Data from a
system in close proximity to, or part of, the proposed
system are preferred since climate, terrain, and
wastewater characteristics are likely to be similar.
The measured corrosion rates can be used to calibrate
the predictive models for subsequent use in the
design and evaluation of new or expanded systems,
as well as for the design of corrective measures for
the existing systems. The models yield only estimates
and should be used accordingly. The accuracy of the
model is dependent upon the choice of empirical
coefficients which, when possible, should be based
on historical data. The use of predictive models in
design of new collection systems is described in detail
in Chapter 5.
2.6 Approach to Investigating Odor and
Corrosion
A survey of a wastewater collection system and
treatment works is necessary to identify the sources
and causes of odor generation. A number of moni-
toring stations should be selected within the collec-
tion system based on area served; length, diameter,
flow and hydraulic gradient of the sewers; location of
force main discharges; and other site-specific factors.
Treatment plants should be monitored at the plant
head works, at points of sidestream returns, and at
other locations where sulfide generation or release is
likely to occur. Collection systems should be moni-
tored at lift stations, force main discharge points,
junctions of major collectors and interceptor sewers
and at points where the sewer system undergoes
major changes in average slope.
Wastewater quality and water levels should be
monitored at each selected station. Samples should
be taken and analyzed both in the field and in the
laboratory for selected parameters. Measurements of
atmospheric H2S at force main discharge points, at
transition manholes and sewage lift stations, and in
25
-------
enclosed areas exposed to wastewater and sludge
should be included as critical elements of tha survey.
All conditions favoring the formation of sulfides
throughout the system should be identified as a
function of varying flow rates, wastewater temper-
atures, and other seasonal conditions that affect odor
generation potential.
2.6.1 Preliminary Monitoring Program
Normally, repeated odor complaints in a community,
either from nearby residents or from inspectors
during routine sewer system checks, are the first
indicators of potentially damaging sulfide generation
within a system. In more extreme cases, the problems
are manifested by deteriorated conditions in man-
holes and pipes or by structural failures.
Early evidence of sulfide generation in existing
systems warrants the implementation of at least a
preliminary program to assess the overall potential
for sulfide generation. Such a preliminary program
should include a thorough investigation of odor
complaints, and a systematic investigation of the
collection and treatment system to identify major
potential contributors such as force main discharge
points, critical sewer reaches and juncture points,
and possible industrial sources. The preliminary
investigation will require knowledge of past com-
plaints as well as an up-to-date record of sewer maps
and flow information.
2.6.1.1 Collection System and Treatment Plant
Schematics
A collection system map should be available, includ-
ing sizes and types of pipes, slopes of lines, flows and
accurate manhole locations. This information can be
reduced to a one-line schematic diagram for easy
reference and for recording subsequent data. The
diagram should provide information on major pump-
ing stations; trunk, tributary and collector sewers;
pipe diameters; pipe lengths; flow rates; and gravity
and force main locations. The frequency of pump
operation and wet-well sizes should also be deter-
mined at this stage of an investigation. Similarly,
schematic diagrams and site maps of the treatment
plant may allow preliminary identification of potential
sources of odor and corrosion.
2.6.1.2 Preliminary Sampling
The survey of odor generation should begin at the
wastewater treatment works and proceed upstream
throughout the system. The preliminary survey
should consist of a simplified field analysis of sulfide
levels to determine the actual trouble areas. A field
survey crew can use a portable kit in which a sample
is collected, an effervescent agent is added to liberate
the H2S, and lead acetate paper discs are used to
detect the presence of sulfides in the wastewater
(23). A comparison chart is used to determine the
approximate concentrations up to 5 mg/l. With a 1 to
1 dilution, the analysis is acceptable up to 10 mg/l of
H2S. This technique takes about 5 minutes per
sample so that the preliminary survey can progress
rapidly. Normally, odor problems will occur when
dissolved sulfide levels are 1 .Oto 1.5 mg/l or greater,
although some communities have reported odor
problems with dissolved sulfide levels of only 0.1
mg/l.
Working upstream in the collection system using the
preceding gross quantitative sulfide measuring tech-
nique is a rapid and effective method to isolate
problem areas requiring further investigation. The
following locations should be checked:
1. Pump stations—sample wet-well influents,
pump discharges, and ends of pressure mains to
determine sulfide balance.
2. Force main discharges into gravity sewers—
turbulence may release HZS gas and cause
corrosion of manhole chambers.
3. J unctions and tributaries—sample tributaries to
determine contribution of sulfides. Tributary
flows with little or no sulfides and some DO may
be beneficial in reducing sulfide problems,
4. Areas of turbulence and long detention times—
sample locations of turbulence where sulfide
may be released. Measurement of dissolved H2S
before and after the point of turbulence can give
an indication of the quantity of H2S released to
the atmosphere. Also, check sewer reaches and
wet wells with long detention times where
sulfide generation is favored.
Ata wastewater treatment plant, a preliminary survey
would entail collection of samples from upstream of
the headworks, downstream of preliminary treatment
facilities, and downstream of primary clarifiers.
Recycle streams such as supernatants from thicken-
ers, digesters, and other sludge treatment processes
should also be sampled, as these can contain high
concentrations of H2S that can result in severe odor
and/or corrosion problems at the point of sidestream
return. Air samples from enclosed spaces exposed to
wastewater may also be collected to determine the
severity of odors,
2.6.2 Detailed Evaluations
2.6.2,1 Sampling
The preliminary program will identify the locations for
further sampling points and, in some cases, such as
in very small communities, limited additional sam-
pling may be all that is needed to quantify sulfide
levels. In larger systems, more information would
likely be required. Sulfide levels vary with diurnal
flow rate so it is important to sample at different times
26
-------
of the day. Design of odor control systems is based on
maximum and minimum conditions, not just average
sulfide levels. Ideally, 24 discrete hourly samples
should be collected over a period of 2 to 5 days, and
the results plotted to reflect sulfide levels with a time
cycle, flow cycle, and day-to-day cycle. Samples
should be collected so that no aeration of the sample
occurs. Field analyses should be performed immed-
iately upon sample collection. Samples that are to be
transported to the laboratory for sulfide analyses
should be preserved immediately upon sample cot-
lection by adding a zinc acetate solution and by
sealing the sampling container when completely full
of liquid.
A mass sulfide profile should be prepared for each
interceptor sewer entering the treatment plant as
shown in the example in Figure 2-16 for the Orange
County Sanitation District Plant No, 2, Orange
County, California (24). The mass profile is used to
design proper systems to control odor and corrosion.
2,6.2.2 Analyses
The wastewater should be analyzed for the following;
1. Sulfates (SOi), mg/l
2. pH
3. Dissolved Sulfides [DS], mg/l
4. Total Sulfides [TS], mg/l
5. Biochemical Oxygen Demand (BOD), mg/l
6. Dissolved Oxygen (DO), mg/l
7. Temperature, °C
8. Oxygen Depletion Rate(ODR), mg/l-hr.
9. Suspended Solids (TSS), mg/l
These parameters are all needed for properly assess-
ing potential odor and corrosion problems and for
utilizing the predictive models presented in Section
2.5 of this manual. In addition to these parameters,
the pH of the crusty moist area on pipe walls or
structures should betaken to determine if a corrosive
environment exists. This can be accomplished by
pressing litmus paper against the interior wall of the
pipe (21). It is also useful to measure the ORP of the
wastewater. A 1981 H?S control study for the Kailua
sewage collection system on the island of Oahu,
Hawaii used DO and ORP as criteria to evaluate the
effectiveness of preventing sulfide generation, and
dissolved sulfides were used to evaluate the effective-
ness of the odor abatement measures to remove
sulfide from the wastewater(25). ORP is a measure of
relative concentrations of oxidants (oxygen, nitrate,
sulfate, etc.) and reductants (ammonia, sulfides,
organics, etc.) in a system. An anaerobic biological
system displays an ORP lower than that of an aerobic
system. A value of +100 millivolts was set as the
minimum ORP required at the various Kailua sewage
pumping stations, transition manholes and end-of-
line manholes to effectively prevent sulfide genera-
tion throughout the downstream portions of the
Kailua collection system. The critical value for ORP is
dependent on the character of the wastewater and
the configuration of the collection system, and it may
be different for each system.
Figure 2-1 B. Composite H2S mass profile entering Plant No. 2, Orange Co., CA (24).
200
100
Total
Imerplant
Miller Holder
Bushard
Trunk A & B ____
Coast
ll^llll ¦¦¦ r m«B| .
e 9 10 11 Noon 1
AM
10 11 Mid- 1
night
2 3
AM
Time of Day
27
-------
2.6.2,3 Analytical Tachniques
Samples of wastewater should be analyzed for total
and dissolved sulfides. Total sulfide analysis mea-
sures the H2S and HS" species present, and any acid-
soluble metal sulfides contained in the suspended
solids, Dissolved sulfide analyses determine the
sulfide remaining in the wastewater upon removal of
the suspended solids by filtration. The samples may
be analyzed in the field using commercially available
test kits, or may be transported to the laboratory for
analyses.
Qualitative field analysis is useful for determining the
presence of H2S in wastewater samples. Several
methods of field analysis are applicable for sulfide
detection. The antimony test, silver sulfide-silver
electrode test, lead acetate paper, and silver foils are
all described in Standard Methods for the Examina-
tion of Water and Wastewater (14th Edition). Field
tests are also available that will give quantitative as
well as qualitative sulfide results. These kits use a
colorimetric sulfide determination and can be used
for either dissolved or total sulfide, depending on
sample preparation.
Laboratory analysis of wastewater samples for sulfide
can be accomplished using either the methylene blue
colorimetric method(Qto 20 mg/l range)orthe iodine
titration technique (1 to 20 mg/l range). There are
substances that react with iodine, such asthiosulfate,
sulfite and various organic compounds, that will
interfere unless removed. The iodine titrimetric
method is used to standardize the colorimetric
method.
A novel method of sulfide analysis that uses an
indicator tube and a sealed sample container has
recently been developed (26). In this method, the
sample is added to a buffering solution with a pH of
5.0 in a flask fitted with a two-holed stopper. An
HaS-indicating tube is inserted into one hole of the
stopper and a glass tube extending below the sample
surface is inserted into the other hole of the stopper.
After agitating the sample for 1 minute, a known
volume of sample is pulled through the indicator tube.
The concentration of total sulfides is determined by
comparing the scale reading on the indicator tube to a
pre-established calibration curve made by applying
the same procedures to standard solutions of known
sulfide concentration. This technique can be per-
formed in the field once a standard calibration curve
has been prepared.
Most professional surveys incorporate the use of the
methylene blue technique for sulfide analysis. This
procedure, which takes about 30 minutes per sample
either in the laboratory or in the field, is simple,
inexpensive, and accurate from 0 to 20 mg/l of
sulfide. It is based on the reaction of sulfide, ferric
chloride and p-aminodimethylaniline under condi-
tions that produce methylene blue. A commercially
available portable test kit is commonly used that is a
variation of the methylene blue technique and which
employs a matched set of test tubes. The blue color is
developed in one test tube using the test water, which
is then matched to a control solution in the other. The
color in the second tube is developed with drop-by-
drop addition of two standardized methylene blue
solutions. This method substitutes N-diethyl-p-phen-
ylene diamine oxalate (DPD) for N, N-dimethyl-p-
phenylenediamine, which yields a more sensitive
result (27).
The monitoring of atmospheric conditions in man-
holes, pumping stations, enclosed preliminary treat-
ment works, and sludge handling buildings should
include the identification of H2S gas. The gas con-
centration can be determined directly by several
portable methods:
1. Photoionization
2. Colorimetric detection tubes
3. Metal oxide semiconductor
4. Electrochemical sensor
These methods are described in Section 2.8.3.
2.6.3 Interpretation of Results
Serious odor or corrosion problems can be identified
and isolated from the survey of collection systems
and treatment works. This survey primarily identifies
sources of sulfide generation; areas of release of H2S
which cause odor complaints; toxicity problems and
direct corrosion of metals; and, through oxidation to
sulfuric acid, corrosion of sewage collection system
pipes, treatment works and equipment. Guidelines
for interpreting the survey results are presented in
Table 2-9.
The data compiled are used in odor control design
decisions. Three general catagories of odor control
are usually considered;
1. Source Control Source controI requires prevent-
ingtheentry of materials not commonly found in
typical domestic wastewater that produce odors
during transport in the collection system, or
eliminating one or more conditions necessary
for H2S generation (e.g., low DO, high temper-
ature, favorable pH). For example, management
of the discharge of septic tank pumpings (sep-
tage) into upstream manholes or at the treat-
ment plant may be considered as a source
control measure.
2. Inhibition of W2S Formation (tn-Stream Treat-
ment). Alternatives for controlling the formation
and release of HjS in the collection system or at
the treatment plant include:
28
-------
a. Improving the oxygen balance
-• compressed air injection
• pure oxygen injection
b. Chemical treatment,including the addition
of:
• chlorine
• hydrogen peroxide
• nitrates
• metallic ions
¦ lime
Table 2-9. Interpretation of Results of Wastewater Survey
Constituent
Interpretation
Sulfate
pH
BODs and DO
Dissolved Sulfide and
Total Sulfide
Temperature
Atmospheric H2S
Oxidation Reduction
Potential (OHP>
Sulfate will be reduced to sulfide under
anaerobic conditions. Potential for sul-
fide generation rarely affected by sul-
fate concentration.
At lower pH values, a greater proportion
of moleculartiydrogen sulfide ispresent
that can be released to the atmosphere.
A pH of 1 to 6 is conducive to sulfide
generation and H2S release.
Wastewater with a high oxygen demand
rapidly takes up available DO and can
create anaerobic conditions favoring
sulfide generation, DO of at least 1.0
mg/l is desirable to prevent sulfide
generation.
Dissolved sulfides of 1.0 to 1.5 mg/l
(lower in some cases) will normally
contribute to odor and corrosion prob-
lems Dissolved sulfides are usually 70
to 90 percent of total sulfides present.
Sulfide levels are used as a criterion to
determine effectiveness of odor control
techniques.
Higher temperature favors the biochem-
ical generation of sulfides and lowers
their solubility.
H2S hasaverylowodorthresholdand is
a toxic gas. Liberation of HjS may be
extremely variable depending upon con-
ditions. HtS gas emissions lead to corro-
sion problems. Concentration ot atmos-
pheric HzS determines design param-
eters for air scrubbers and oxidizers for
odor control.
Only at ORP's below zero.does reduction
of sulfates take place. Anaerobic sys-
tems display ORP's lower than aerobic
systems.
ORP
Condition
millivolts
+50
No action by anaerobic
bacteria
0 Poor anaerobic activity
-100 to-200 Maximum efficiency for
anaerobic activity
-50 to-300 Favored by sulfate-reduc-
ing bacteria for produc-
tion of sulfides
Collection and Treatment of Foul Air. Exhaust
air odor control systems are commonly designed
to create negative air pressures in an enclosed
area and to treat the air before exhausting to the
atmosphere. Atmospheric H2S should be re-
duced to less than 1 ppm before discharge.
Specific treatment methods are discussed in
subsequent chapters of this manual. The cost
effectiveness of This approach vs. in-stream
treatment is dependent upon pumping station
locations, collection system layout, pipe sizes
and wastewater flow rates, and the concentra-
tions of H2S in the wastewater and in the
atmosphere of enclosed spaces.
2.7 Measurement and Monitoring of
Corrosion and Odor
2.7.1 Corrosion
Corrosion is an ever-present problem in many
wastewater collection and treatment systems, and
prevention of corrosion can result in significant cost
savings. Acid attack of collector or interceptor sewers
is a significant cause of early pipe failure (28). The
corrosion process discussed in this chapter is that
which affects the surfaces of concrete, mortar, some
metals, and other structural materials when they are
exposed to a humid atmosphere containing H2S.
2.7,1.1 Corrosion of Concrete
Characteristic features of corroding concrete surfaces
are high sulfuric acid concentrations with low pH
values. Acid attack is confined to the interior,
unsubmerged portion of the sewer pipe and is
heaviest at the crown and just above the liquid level.
Concentrations of sulfuric acid can reach 5 percent,
and a high percentage of this acid will react with the
exposed surfaces and be neutralized by the alkalinity
of the concrete. An acid attack situation can easily be
demonstrated by testing the wall crust for pH.
An existing system can be examined for corrosion
with proper planning and analysis. A sewer system
map should be prepared for detailed assessment. The
information should include age, size, and types of
pipes; slopes of the lines; wastewater flows; and
accurate manhole locations. Manholes should be
examined for corrosion effects. An expandable prob-
ing rod can be used to measure internal diameters for
comparison with originally installed diameters. Soft
deteriorated concrete should be first chipped away to
determine the depth of corrosion penetration. Core
samples should be taken to determine wall thickness,
alkalinity, and compressive strength at various loca-
tions along the sewer. The core samples need to be
taken above and below the normal water line for
comparison of thickness, because below the water
line corrosion would not be expected. If proper core
29
-------
samples cannot be removed, the inside of the sewer
pipe can be measured for thickness with a probing rod
placed inside the drilled hole in the pipe.
2.7.1.2 Corrosion of Metals
Metallic components of wastewater treatment plants
should be periodically inspected to detect the pres-
ence and severity of corrosion, since corrosion can
adversely affect the properties of the metal. For
example, the structural integrity of iron and steel
deteriorates in the moist, oxidizing atmospheres of
wastewater treatment plants (29). Copper contacts
and components of electrical systems may be rapidly
oxidized to black copper sulfate in the presence of
only small amounts of HaS (4 to 8 ppm), resulting in a
weakened, poorly conducting material.
Structural components such as bridge work, bolts in
concrete, gratings, wet-well steps and ladder rungs,
and walkways should be carefully inspected for the
effects of corrosion. The severity of the corrosion
should be noted. Other metallic components such as
bar screens, conveyor mechanisms, railings, and
steel tanks should also be inspected as part of the
investigation. Enclosed areas exposed to wastewater
such as wet wells, preliminary treatment works, and
sludge handling buildings generally harbor conditions
favorable to corrosion due to the presence of moisture
and corrosive gases such as H2S. Corrosion of metal
surfaces has been noted to be extremely severe for
some plants in which preliminary treatment facilities
have been enclosed without proper ventilation and
dehumidification,
2.7.2 Odors
Measurement and characterization of odors is not
only important in assessing the magnitude of the
problem; it is also crucial for the proper design of odor
control systems. For systems which remove odorous
compounds from the air, such as wet scrubbers and
activated carbon processes, accurate data on atmos-
pheric concentrations of odorants are essential.
Numerous cases exist in which failure to accurately
measure peak concentrations of odorants led to
underdesign of the treatment unit and poor per-
formance.
2.7.2.1 Odor Threshold
An odor concentration level below which malodorous
substances are no longer detectable by the human
nose is defined as the threshold odor number (TON).
The sensitivity to odors varies from individual to
individual and, as such, odor thresholds are more
subjective than objective in nature. When the in-
organic gases or organic vapors, such as those
described in Section 2.2, contact the human olfactory
fibers, the sensation known as odor is created (3).
All odor threshold values are determined using
several persons on an odor panel to decide when the
odor is no longer detectable, Low odor thresholds, low
molecular weights and high vapor pressures suggest
that a compound, if present in a malodorous atmos-
phere, can be expectedto contribute to the objection-
able odor. Odors can be contributed from a mixture of
compounds.
After the odor concentration reaches and then
exceeds the odor threshold, the odor intensity in-
creases rapidly at first, then changes only slowly with
further odor concentrations (Figure 2-17). Finally, the
nose becomes insensitive and the person may no
longer be conscious of the odor. This is especially
critical in the case of H?S, since (oss of sensitivity to
the odor can mean that the gas has reached a
dangerous level.
Figure 2-17. Typical concentration—intensity relationship
for odors.
|
c
o
(13
c
<0
u
c
o
o
o
¦O
o
2.7.2.2 Odor Measurement
To control odors, it is necessary to identify the
odorous components of the vapors and gases causing
a nuisance. However, positive identification is a
difficult taskand manytimes requires the use of a gas
chromatograph (GC). With the GC, the procedure is to
separate the gaseous components of air, make
educated judgments as to the nature of the com-
pounds, and then inject standards in order to match
the compound in question with the pure compound.
Because an odor is usually defined in physiological or
psychological responses, the human nose is still used
to identify and measure the intensity of odor. Al-
though HzS can be monitored effectively with instru-
mentation, other malodorous substances cannot be.
Detectable by Human Nose
Odor Threshold
Not Detectable by Human Nose
Odor Intensity
30
-------
Therefore, the odor panel isthe most common method
used to identify odor nuisances.
The odor panel normally is made up of a given number
of persons, usually eight or more. A sample is
collected in a glass sampling bulb(25 to 1000 ml size)
and delivered immediately to the odor panel for a
series of dilutions and sniffings. The most widely
accepted technique for odor measurement is the
triangle olfactometer method, in which three samples
are presented to the panelist from a series of glass
sniffing ports. Two are test room air (blanks), and the
third is odorous air diluted with test room air. The
olfactometer supplies six dilution levels. The volu-
metric amount of the odorous gas which is detectable
by only half the odor panel (of eight or more persons)
in 0,03 m3 (1 cu ft) of odor-free air is called an odor
unit. The strength of an odor is determined by the
number of dilutions with odor-free air needed to
reduce an odor to a barely detectable level (30). The
odor unit can be used as a referencepoint to compare
to other systems, or to compare on a routine basis
with a particular odor nuisance. Some state regula-
tions are based on the odor unit as well. If odor control
measures are instituted, the odor unit can be used to
determine effectiveness of the odor control system by
comparison with pre-control odor units. Caution must
be observed in using an odor panel, and local air
pollution control agencies should be consulted as to
their required practices.
2.8 Toxicity and Safety Practices
H2S is an acutely toxic material and has been
responsible for the death of a number of sewer
system workers. H2S is heavier than air and therefore
can be found in the lower portion of manholes. This
deadly gas, whose toxicity has been ranked with
hydrogen cyanide, is colorless and has a character-
istic rotten egg smell at low concentrations. But as
the levels of HZS increase, workers are generally
unaware of its presence. A person's ability to sense
dangerous concentrations by smell is quickly lost. If
the concentration is high enough, unconsciousness
will come suddenly, followed by death if there is not a
prompt rescue. Many times, rescue attempts are ill-
fated since a person may not consider his own safety
in trying to save a co-worker's life.
2.8.1 Dangers of Hydrogen Sulfide
The physiological effects of HZS are summarized in
Figure 2-18(11). Very low concentrations of this gas
can cause serious health hazards. Death has resulted
from concentrations of 300 ppm by volume in air {28).
Such concentrations can be obtained in an enclosed
chamber with high turbulence, from wastewater
containing 2 mg/l of dissolved sulfide at a pH of 7.0.
Based on Henry's Law, Figure 2-9 was developed to
Figure 2-18. Hydrogen sulfide toxicity spectrum (11).
Rotten Egg .
Odor Alarm
Threshold of
Serious Eye
Injury
Loss of
Sense of Smell
Imminent
Life Threat
Immediate
Collapse
with —
Respiratory
Paralysis
Odor Threshold
Offensive Odor
Headache
Nausea
Throat and Eye Irritation
Eye Injury
Conjunctivitis
Respiratory Tract Irritation
Olfactory Paralysis
Pulmonary Edema
Strong Nervous System Stimulation
Apnea
Death
ppm
• 0 1
0.2
-10
-50
-100
-300
- 500
-1,000
-2,000
show HjS levels in the atmosphere in equilibrium
with the given concentrations of HjS in the water at
the respective wastewater temperatures.
Concentrations of toxic gases to which a worker may
be exposed can be expressed in several ways;
1. Eight-Hour Time Weighted A verage (TWA). The
maximum average concentration to which a
worker can be exposed for 8 hours a day, 40
hours a week. This is normally called the
threshold limit value (TLV).
2. Celling Value, A limit generally not to be
exceeded.
3. Acceptable Maximum Peak. A concentration
limit which is not acceptable for specified
maximum duration.
The Occupational Safety and Health Administration
(OSHA) has established limits for exposure to H2S of
20 ppm (15-minute exposure) for an acceptable
ceiling concentration and 50 ppm for a maximum
peak during an 8-hour shift if no other measurable
exposure occurs. The National Institutes of Occupa-
tional Safety and Health (NIOSH) established an H2S
exposure level of 10ppm(10 minutes) as a maximum
permissible limit (once per 8-hour shift), with con-
tinuous monitoring required where H2S concentra-
tions could be 50 ppm or greater (31). OSHA is
currently revising their standards in a comprehensive
guideline document applicable to confined spaces.
31
-------
H2S is an explosive as well as toxic gas. The lower
explosive level for H2S is 4.3 percent by volume in air
and the upper explosive limit is 45 percent by volume
in air. When the H?S concentration is within this
range, a spark can cause an explosion. Table 2-10
summarizes the hazardous nature of H2S.
Table 2-10. Hazardous Characteristics of Hydrogen Sulfide
Gas
Chemical Formula: H2S
General Properties:
• Irritant and poisonous volatile compound
• Rotten egg odor in small concentration
• Exposure lor 1 Vi to 2 minutes at 0.01% impairs sense of smell
• Odor not evident at high concentrations
• Colorless
• Flammable
Specific Gravity(compared to air = 1.0): 1.1 9
Physiological Effects:
• Impairs sense of smell rapidly as concenlration increases
• Death in few minutes at 0.2%
• Exposure to 0,07% to 0,1 % rapidly causes acute poisoning
• Paralyzes respiratory center
Maximum Safe 1 5 Minute Exposure: 20 ppm (OSHAt
Explosive Range, percent by volume in air:
• Lower Explosive Limit: 4.3
• Upper Explosive Limit: 45 0
Likely Location of Highest Concentration:
• Near bottom of confined space, but may be higher if air is heated
and highly humid
• Areas of turbulence in collection system
• Law-lying flat sewers
Most Common Source: Sewer gas or sludge gas resulting from
wastewater or westewater constituents that have undergone
anaerobic decomposition
2.8.2 Safety Practices for Confined Spaces
The dangers and toxicity of H?S are well documented.
The importance of following proper entry procedures
for confined spaces is now obvious and should never
be overlooked. Monitoring the air can be accom-
plished economically in a short time. The cardinal rule
for everyone planning to enter a confined area where
H2S may be present is "Never Trust Your Senses"
(32). What may look like a harmless situation may
indeed be a potential threat.
The most common atmospheric conditions that
constitute hazards are;
• Oxygen deficiency
• Combustible gases or vapors
• Toxic gases and vapors
One should always anticipate that any one or a
combination of the above atmospheric conditions
might exist within a collection system or confined
space within the wastewater treatment plant. Proce-
dures should be developed by each agency involved in
the collection and treatment of wastewater for
atmospheric monitoring andtesting. Asafety program
should include consideration of the proper ventilation,
entry, and extraction of personnel. Proper training of
personnel and maintenance of monitoring equipment
are mandatory for an effective safety program.
2.8.3 Monitoring for H^S
When testing underground structures for HaS, it is
important to remember that HZS is heavier than air,
and its presence will not likely be detected at ground
level. Therefore, one must lower the monitoring
device into underground structures such as manholes
and test at different levels. There are several types of
devices available to test for HaS:
1. Electrochemical sensor
2. Metal oxide semiconductor
3. Colorimetric detector tube or badge
4. Photoionization
5. Solid-state sensitized film (with ceramic chip)
Some of these devices are portable, continuous
monitors, such as metal oxide semiconductors or
electrochemical sensors that have visible or audible
alarms which alert the worker of an HaS concentra-
tion of 10 ppm or the OSHA ceiling limit of 20 ppm
(33), A 10-ppm or 20-ppm concentration can be used
as a go/no-go signal for entry or exit of a confined
space. These monitors range in price from about
$650 to $1,200 (1984). They can also be supplied
with digital indicators to display the concentration of
H2S gas within the working environment.
Another device is a badge that contains an H2S-
sensitive indicator that will change to a dark color at 5
ppm. These devices are available for about $30 per
dozen and are considered go/no-go type warnings.
In addition, colorimetric indicator tube testers are
marketed which will read a given concentration of the
gas. These devices are equipped with a bellows-type
pump and, when fitted with the proper tubes, can
detect other possibietoxic substances found in sewer
systems, such as carbon monoxide. The pumps draw
acalibratedvolumeofairthrough a detectortube.The
detector tube contains a reagent (in the H2S case, lead
acetate) which changes color in the presence of the
gas. The extent of the discoloration indicates the
amount of toxicgas present in parts per million. These
tube testers cost approximately $300 with 10 tubes
costing an additional $30 (1984), The tubes are
available in different scale readings and for long-term
duration usage to determine time-weighted average
values.
32
-------
A method has been developed for sampling heavier-
than-air gases. This method uses a J-tube arrange-
ment to sample near the water surface. A float is
placed on the bend in the "J" to suspend the
monitoring device above the water level. A color-
sensitive indicator badge or an indicator tube with
hose connection can be attached to the tube so that a
representative sample can be taken as close as
possible to the water surface. This will indicate if
entry is safe or if further monitoring is required.
2.9 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703)487-4650
1. Piscarcyzyk, K. Odor Control with Potassium
Permanganate. Presented at Ohio Water Pol-
lution Control Conference, Dayton, OH, June
16-18, 1982.
2. Thistlethwayte, D.K.B. (ed). The Control of
Sulphides in Sewerage Systems. Ann Arbor
Science, Ann Arbor, Ml, 1972.
3. Odor Control for Wastewater Facilities. Manual
of Practice No. 22, Water Pollution Control
Federation, Washington, DC, 1979,
4. Drinking Water and Health. National Academy
of Sciences, Washington, DC, 1977.
5. Sawyer, C.N., and P.L, McCarty. Chemistry for
Sanitary Engineers. McGraw-Hill, New York,
NY, 1 967.
6. Dague, R.R. Fundamentals of Odor Control.
JWPCF 44 (4): 583-594, 1972.
7. Harkness, N. Chemistry of Septicity. Effluent
and Water Treatment Journal 20 (1): 16-23,
1980.
8. Process Design Manual for Sulfide Control in
Sanitary Sewerage Systems. NTIS No. PB-
260479, U.S. Environmental Protection Agency,
Center for Environmental Research Information,
Cincinnati, OH, 1974.
9. Meyer, W.J. Case Study of Prediction of Sulfide
Generation and Corrosion in Sewers. JWPCF 52
(11): 2666-2674, 1980.
10. Sulfide Survey Report, Hillsborough County,
Florida. Interox America, Houston, TX, May,
1982.
11. Hydrogen Sulfide. Report by Committee on
Medical and Biologic Effects of Environmental
Pollutants (Subcommittees on Hydrogen Sul-
fide), Division of Medical Sciences, National
Research Council, Washington, DC, 1979.
12. Pomeroy, R., and F.D. Bowlus. Progress Report
on Sulfide Control Research. Sewage Works
Journal 18(4): 597-640, 1946.
13. Parkhurst, J.D., and R.D. Pomeroy. Oxygen
Absorption in Streams. Journal ASCE-SED 98
(SA1): 101-124. 1972.
14. Pomeroy, R.D., and R.J. Lofy. Feasibility Study
on In-Sewer Treatment Methods. NTIS No. PB-
271445, U.S. Environmental Protection Agency,
Cincinnati, OH, 1972.
15. Warren, G.D., and G. Tchobanoglous.,4 Study of
the Use of Concrete Pipe for Trunk Sewers in the
City of Delano. California. Prepared for the
California Precast Concrete Pipe Association,
September, 1976.
16. Speller, F.N. Corrosion: Causes and Prevention.
McGraw-Hill, New York, NY, 1951.
17. Paints and Protective Coatings for Wastewater
Treatment Facilities. WPCF Manual of Practice
No. 17, Water Pollution Control Federation,
Washington, DC, 1969.
18. Davy, W.J. Influence of Velocity on Sulfide
Generation in Sewers. Sewage and Industrial
Wastes 22 (9): 1132-1137, 1 950.
19. Pomeroy, R.D. Sanitary Sewer Design for Hydro-
gen Sulfide Control. Public Works, 101 (10): 93,
1970.
20. Pomeroy, R.D., and J.D. Parkhurst. The Fore-
casting of Sulfide Buildup Rates in Sewers.
Progress in Water Technology 9 (3): 621-628,
1977.
21. A Case Study: Prediction of Sulfide Generation
and Corrosion in Concrete Gravity Sewers.
Prepared by J.B. Gilbert and Associates for the
American Concrete Pipe Association, Washing-
ton, DC, 1979,
22. Design Manual: Sulfide and Corrosion Predic-
tion and Control. American Concrete Pipe
Association, Vienna, VA, 1984.
23. Keating, E.J. Regular Testing Can Control
Hydrogen Sulfide. Water and Sewage Works
125 (7): 68-70, 1978,
24. Hydrogen Peroxide Demonstration Report,
Orange County Sanitation District. Interox Amer-
ica, Houston, TX. 1982.
25. Yogi, D.R., R.L. Smith, and N.C. Burbank, Jr.
Hydrogen Sulfide. Control in Sewers Containing
33
-------
Brackish Water Honolulu, A Case Study. Pre-
sented at: 54th Annual Conference, Water
Pollution Control Federation, October 4-9,1981.
26. Ballinger, D„ and A. Lloyd. A Method for the
Determination of Sulphides in Water Sewage
and Effluents, JWPCF 80 (5): 648-654, 1981.
27. Kloster, M.B., and M.P. King. The Determination
of Sulfide With DPD, JAWWA, Water Tech-
nology/Quality, October; 544-546, 1977.
28. Duggan, S.W A Perspective on Hydrogen
Sulfide in Sewers. Presented at New Jersey
Water Pollution Control Association, November
19, 1980.
29. Shepherd, J.A., and M.F. Hobbs. Control of
Sewage Hydrogen Sulfide With Hydrogen Per-
oxide. Water and Sewage Works 120(8) 67-71,
1973.
30. Huang, J.C., G.E. Wilson, and T W. Schroepfer.
Evaluation of Sludge Odor Control Alternatives.
Journal ASCE-SED 104(6): 1135-1147, 1978.
31. Criteria for a Recommended Standard for the
Occupational Exposure to Hydrogen Sulfide.
U.S. Department of Health, Education and
Welfare, NIOSH, May, 1979.
32. Dally, K.A. Hazards Lurk in Innocent Looking
Manholes. Deeds and Data, WPCF Highlights 19
(2): 7-10, 1982.
33. Tomphin, F C. Jr., and J.H. Becker. An Evalua-
tion of Portable Direct Reading HzS Meters.
NIOSH, Contract No. 210-75-0037, July, 1976.
34
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Chapter 3
Odor and Corrosion Control In Existing Wastewater Collection Systems
3.1 Introduction
This chapter presents alternatives for control of odor
and corrosion in existing wastewater collection
systems. Since odor and corrosion problems are
typically related to the presence of HzS, the alterna-
tives are aimed at preventing sulfide generation or
removing sulfides through chemical or biological
action. In-stream sulfide control alternatives fall into
two major categories: 1) improving the oxygen
balance (operation and maintenance, air injection or
entrapment, oxygen injection); and 2) chemical
addition (chemical oxidation, inhibition of sulfate
reduction, precipitation, pH control),
3.2 Improving the Oxygen Balance
As previously discussed, presence of greater than 1.0
mg/l DO in the wastewater stream is sufficient to
prevent sulfide buildup, since any sulfide that might
be produced in the slime layer will be aerobically
oxidized to thiosulfate. With no dissolved sulfides
present in the stream, H2S emission to the sewer
atmosphere will not occur. Several techniques are
available for improving the oxygen balance in waste-
water collection systems, as discussed in the follow-
ing sections.
3.2.7 Operation and Maintenance
Proper operation and maintenance of a sewer system
can minimize unnecessary oxygen depletion. Partial
blockages in the sewer can cause a backup of flow,
resulting in lower velocities and deposition and
accumulation of organic solids and debris. Such
conditions favor the reduction of sulfate to sulfide,
and the subsequent release of H2S to the sewer
atmosphere. These conditions can be minimized,
however, by instituting a regular program of sewer
inspection and cleaning. Inspections can be made
manually or remotely, for example, by using television
cameras. Several different techniques are available
for sewer cleaning, including hydraulic methods and
mechanical systems. Regular cleaning has been
shown to temporarily reduce the rate of sulfide
buildup, particularly where deposition of organic
solids is a problem (1).
Since substantial sulfide generation can occur in a
short time period in sewer lines used for flow
equalization, constant-speed pump stations should
be operated with start-stop cycles that are short
enough to avoid backup of wastewater into influent
lines and to avoid excessive wet-well detention times.
The effect of sluggish sewer flows, including in-
creased depth of flow and prolonged surcharged
conditions, can increase sulfide buildup due to
decreased reaeration opportunity. However, shorter
duration surcharged conditions caused by infiltra-
tion/inflow are often characterized by weaker waste-
water and colder temperatures that may result in
reduced sulfide generation due to higher flow veloc-
ities, scouring of accumulated solids, and reduced
biological activity.
3.2.2 Air Injection
Improvement in the oxygen balance in collection
systems can be realized by the addition of air into the
flowing wastewater. Addition of sufficient DO can
prevent or significantly reduce further sulfide genera-
tion from occurring and allow biochemical oxidation
of existing dissolved sulfides. Methods of air addition
include:
1. Direct injection of compressed air into force
mains
2. Use of Venturi aspirators in force mains or lift
stations
3. Use of air lift pumps at lift stations
4. U-tube dissolution using either compressed air
or Venturi aspirators in either gravity lines or
force mains
5. Pressure tank air dissolution systems for gravity
lines.
3.2.2.1 Air Injection Alternatives
a. Direct Injection of Compressed Air
Direct injection of compressed air into force mains
has been practiced at several locations in the United
States. Increased pressure in a force main allows
greater dissolution of oxygen into the stream. At
atmospheric pressure and 21 °C (70°F), water will
dissolve approximately 2 percent air by volume; this
increases to 4 percent at 103 kPa (15 psig), and 6
percent at 207 kPa (30 psig),
35
-------
Direct injection of compressed air into force mains
has been studied in California (3)(7), Texas (2)(5),
Louisiana (4), and New Jersey (6). A typical direct air
injection system is shown in Figure 3-1 and data from
selected studies of compressed air injection into force
mains for sulfide control are summarized in Table
3 1. Note the wide variation in air injection rates
necessary to achieve sulfide control. Required air
injection rates vary depending on wastewater charac-
teristics (oxygen uptake rate), detention timB in the
force main, temperature and pressure of the system,
hydraulic profile of the force main, and degree of
sulfide control desired. "Rules of thumb" suggest
providing air at the rate of 0.75 to 2.25 m3/m3(0.1 to
0.3 standard cu ft/gal) of wastewater or providing an
airflow of 0.7 to 1.3 m'/hr/cm (1 to 2 cfm/in) of pipe
diameter (5). In practice, full characterization of the
wastewater and information on hydraulic character-
istics of the pipe are necessary before air require-
ments can be accurately estimated.
Figure 3-1. Typical system for injecting air into force main.
' Force Main
Controls
Flexible I GauBe £Xe01' Valve*
Coupling!^ j
Air
Compressor/^ Valve
Pressure i
Relief
Table 3-1.
Diam.
Selected Studies Df Direct Injection of Com-
pressed Air into Force Mains for Sulfide Control
Total Sulfide
Length
Air
Input
Pump
Station
Discharge of
Force Main
cm m
Los Angeles, CA(3)
m3/ri
mg/l
15
15
636
340
0
Avg.
0.32
16.3
0.2
0.06
32-6
0.06
0
020
24.5
Avfl.
0.31
36.7
0.1
0.78
53,0
Trace
53.0
0.05
53.0
0.10
Table 3-1. (continued)
Total Sulfide
Air
Pump
Discharge of
Diam. Length
Input
Station
Force Main
cm m
mVd
mg/l
25 525
0
16,4
53,0
12.4
61.2
14.8
61.2
12.9
61,2
Avg.
12.3
61,2
2.1
3,4
65.2
3.2
73.4
8.0
77.6
7.5
81.6
4.5
30 455
0
4.6
139
Avg,
0.2
171
0.7
0.2
30 600
0
5.7
57.1
5.6
B9.7
3.8
110
Avg.
1.9
163
0.5
3 5
428
0.25
428
0 2
428
0 25
Port Arthur, TX (2)
41 1,170
0
2 3
489
0,7
979
2,1
1,350
Avg.
2.7
1,470
14
0.6
1,960
0.3
2,450
0.1
2,690
1.5
4,078
0.1
76 1,340
0
Avg.
5.6
4,490
2,3
1,4
Trinity River Authority, TX (5)
76 1,550
0
Range
5 to 8
3,260
0 to 3
<0.3
Sacramento, CA (7)
60 1,700
0
0.45
410
Avg.
0.39
550
0.3
0.26
860 to 920
0.19
b. VenturiAspirators
Venturi-type aspirators have been employed for
sulfide control in force mains and lift stations. A
Venturi aspirator operates on the principle that liquid
flowing at high velocity through a nozzle of decreasing
diameter creates a negative pressure at the discharge
36
-------
side of the restriction. Provision of an opening to the
atmosphere at this point allows air to be drawn into
the device. Turbulence at the discharge provides for
intimate mixing of air and water. A typical Venturi
aspirator is shown in Figure 3-2.
Figure 3-2. Typical Venturi aspirator.
Irvlet
Air Inlet
Flow
Outlet
A Venturi aspirator was installed in a 20-cm {B-in)
force main in Jefferson Parish, Louisiana as part of a
demonstration project on sulfide control. This device
aspirated approximately 200 to 245 m3/d (5 to 6 cfm)
of air at a wastewater flow of 5,440 mVd (1,000
gpm), resulting In an air-water ratio of about 4
percent (4).
One proprietary system that has been employed for
sulfide control in lift stations draws wastewater from
the wet well, pumps it at a pressure of 138 kPa (20 psi)
through the Venturi aspirator, and recirculates the
aerated sewage back to an upstream manhole (B)(9).
Depending on the magnitude of the sulfide problem,
recirculation ratios can be varied. Typical recirculation
ratios vary from 0.75:1 to 1 1 .This system is depicted
in Figure 3-3.
c. Air Lift Pump
An air lift pump was installed in a 75-cm (30-in)
diameter gravity sewer in Jefferson Parish, Louisiana
to control sulfide by upstream aeration (4). It was
installed because of its ability to provide simultaneous
aeration and pumping. A diagram of the air lift pump
used in Jefferson Parish is shown in Figure 3-4.
Several different aeration techniques werestudied at
various points in the Jefferson Parish collection
system. At the location of the air lift pump, 60 percent
of the entering wastewater was unaerated, while 40
percentwassubjectedto upstream aeration by one or
more alternative techniques. Prior to any aeration,
dissolved sulfides averaged 0.63 mg/l at a point 255
m (835 ft) downstream of the air lift pump. After
system aeration was begun, dissolved sulfides at the
sampling point were reduced to zero (4).
d. U Tube Aeration
The use of U-tubes for in-stream aeration to control
sulfides in collection systems has been evaluated at
Jefferson Parish, Louisiana (two Venturi-aspirated
U-tubes) and Port Arthur, Texas (two parallel com-
pressed air U-tubes) (2)(4)(10). In addition, U-tube
aeration was investigated at Sacramento, California
[11
A diagram of a typical U-tube aerator is shown in
Figure 3-5. Air (or oxygen) is introduced at the top of
the downleg, from which the air-sewage mixture
flows downward through an expanded pipe section.
This allows reduced velocity and greater residence
Figure 3-3. Application of Venturi aspirator for sulfide control in wet well.
venturi
Aspirator
Manhole
Approximately 100'
Wet Well
Dry Wei
rrfluent Line
Main Sewage Lift Pumps Not Shown
Reproduced with permission of Hydro-Vac, Inc.
37
-------
Figure 3-4. Air lift aerator installed at Jefferson Parish, LA.
From Compressor
Discharge to Sewer
Wastewater
Compressed Air Line
3 m
Aeration Air Injection
6 m
Lift Air Injection
Figure 3-5. Typical U-tube installation (2).
Aeration Device:
Aspirator
Compressed Air
Gaseous Oxygen
Discharge to
Gravity Sewer
Upleg
Expanded Section
Down leg
Pump
Discharge
38
time to promote oxygen transfer under conditions of
increasing hydrostatic pressure. The oxygen enriched
wastewater then continues through an upleg of
reduced diameter to increase the velocity and prevent
deposition of solids.
The larger capacity, 12.8-m (42-ft) deep Venturi-
aspirated U-tube at Jefferson Parish was installed at
the discharge of a 30-cm (12-in) force main into a
53-cm (21 -in) gravity sewer. The other U-tube, 16.5-
m (54-ft) deep, was installed at the discharge of a
25-cm (10-in) force main and 25-cm (10-in) gravity
line into a 45-cm (18-in) gravity sewer (4)( 10). The
overall sulfide control scheme at Jefferson Parish
actually employed four aeration devices located at
various points in the collection system; two Venturi-
aspirated U-tubes, an air lift pump, and an in-line
Venturi aerator. Data were collected at numerous
sample stations before and after simultaneous opera-
tion of all aeration devices. This made evaluation of
the performance of each individual device difficult.
Data in Table 3-2 were taken from samples collected
at downstream stations which best represent the
performance of the air aspirated U-tubes (4).
Table 3-2, Performance of Aspirated Air U-Tubes at Jeffer-
son Parish. LA (4)
Air-Water Dissolved Sulfides
Sample Wastewater Ratio Before After
Location" Flow (Volume) Aeration Aeration6
mVd mg/l
A 9,267 0,08 0.51 0,05
B 3.543 0,05 0.42 0.11
'Location A; 113m downstream of U tube 1
Location B: 122 m downstream of U-tube 2
'Peformance reflects partial contribution from other aeration
devices upstream.
Two parallel 6.1 -m (20-ft) deep compressed air U-
tubes installed at a lift station in Port Arthur, Texas
discharged into a 75-cm (30-in) gravity sewer (2X10).
Although the intended design of the Port Arthur U-
tubes provided for an air flow of 1,630 mVd (40 cfm)
to yield an air:water volumetric ratio of 0.21, the
maximum air flow the compressor could deliver
resulted in an ainwater ratio of 0.082. As a result,
only minor reductions in sulfide concentrations were
achieved. These data are presented in Table 3-3 (2).
In Sacramento, California, a 12.2-m (40-ft) deep U-
tube was installed at the end of a 1,720-m (5,650-ft)
long 60-cm (24-in) diameter force main which
discharged into a 69-cm (27-in) diameter gravity
sewer. Average dry weather flows at the site were
approximately 7,600 m3/d (2 mgd). Ainwater ratios
investigated ranged from 0,06 to 0.16 by volume.
-------
Table 3-3. Performance of Compressed Air U-Tubes at Port Arthur, TX (2)
Sulfide Concentrations
Air:Water
Wastewater Ratio In jPump Discharge) Out(U-tube Discharge;
Flow (Volume) Total Dissolved Total Dissolved
m3/d mg/1
7,179
0.034
4.3
4.0
2.6
2.4
7,190
0-045
5.3
4.2
2.7
2.3
7,223
0.045
1.7
1.6
0.9
O.S
7,086
0.057
2.3
2.1
1.5
1.1
7,163
0.063
5.0
4.8
2.8
2.3
6,890
0.071
18
1.5
1.5
1.1
7,086
0.081
4.2
3,8
3.8
3.0
6,945
0.082
2.4
2.0
1.7
1.2
Results from this study are shown in Table 3-4 (7), It
was found that variation in airwater ratios over the
ranges studied had little observable effect on the
degree of sulfide reduction. However, increased
airwater ratios did result in increased DO levels
downstream. At ainwater ratios above 0.15, disper-
sion of the air was so poor that air blocks occurred in
the downleg above the air injection collar.
Table 3-4,
Performance of Compressed Air U-Tubes at
Sacramento, CA (7)
Air-Wat«r Total Sulfide Concentration
Wastewater
Ratio
1,000 m
Flow
(Volume)
In
Out
Downstream
m3/d
mg/l
8,740
0
-
-
0.60
9,050
0.06-0.08
0.61
0.46
0.17
8,740
0.11 0,13
0.76
0.52
0.17
8,630
0.15-0.16
0.69
0.56
0,13
e. Pressure Tank Air Injection
Pressure tank air injection is a technique that takes
advantage of the increasing solubility of oxygen with
increasing pressure. Such a system is applicable to
sulfide control at lift stations discharging to gravity
sewers or in short force mains where detention or
contact time does not allow adequate oxygen dissolu-
tion and sulfide oxidation. Pressure tank air dissolu-
tion systems were installed and evaluated at two lift
stations at Port Arthur, Texas (2). A diagram of a
typical system is shown in Figure 3-6. After problems
were noted with deposition of solids in the pressure
vessel, a recirculation line from the bottom of the
pressure tank to the lift station wet well was installed.
The use of pressurized tank injection systems at Port
Arthur resulted in airwater volumetric ratios consid-
erably higher than with other aii* injection methods
Figure 3-6. Pressure tank dissolution system (2j.
Biowoff Valve
Effluent to
Gravity Sewer
or Force Main
Influent
Air Supply
Line
Diffusers
Drain and Recirculation
Line to Wet Well
evaluated. Airwater ratios were 3.9 for one installa-
tion and 3.5 for the other. These ratios were
considered excessive for adequate sulfide control.
Performance data for the two pressurized tank air
injection systems are summarized in Table 3-5 (2).
3.2.2.2 Air Injection Equipment
Equipment requirements for an air injection system
vary with the type of system selected. For applications
requiring small quantities of air, less than 14 mVmin
(500 cfm), equipment requirements may be minimal
and may consist, for example, of merely a pump and a
Venturi aspirator. Larger applications will probably
require multiple compressors, more sophisticated
control systems, and possibly separate dissolution
systems.
39
-------
Table 3 5 Performance of Pressure Tank Air Injection at
Port Arthur. TX (2)
Average Sulfide
Concentration
Location
Total
Dissolved
DO
mg/l
mg/l
A:
Upstream manhole
2.3
Z.O
0
Wet Well
0.6
0.5
2.1
Pressure tank effluent
0.3
0.2
6.3
Force main discharge {180 m)
0.1
0
4.8
B:
Wet Well
3.9
3.1
0
Pressure tank influent
1.1
09
01
Pressure tank effluent
0.1
0
4.9
Force main discharge (30 m) 0-2 O.05 3.9
Many air injection systems are installed in existing lift
stations or pump stations in the collection system.
Some site modifications will generally be required to
accommodate the additional equipment, depending
on specific site conditions. Such modifications may
include provision of a concrete pad, paved access
roads, fencing, and electrical supply, although electric
power is generally available to operate compressors,
pumps, and control systems at the site. If sufficient
area is available within the pump station, equipment
can often be located inside thefacility. If not, separate
housing for compressors and associated controls may
be required.
For direct injection of compressed air into force
mains, hardware would consist of compressors) and
receiver or reservoir tank, air piping and valves,
injection nozzles, and a control system. Control
options are discussed in Section 3.2.2.3. Air-cooled,
single-stage compressors are generally recommend-
ed because they develop adequate capacity at low
pressure (11). Compressors should be designed for
continuous duty and be sized to handle peak aeration
demands with a reasonable duty cycle. Most standard
compressors are rated at 690 kPa (100 psig). At lower
pressures, more air can be delivered at reduced
power. Piping not in contact with wastewater should
be constructed of standard weight galvanized steel
(up to 15-cm diameter) or galvanized spiral steel (20
to 60 cm). For submerged piping, the following
materials are recommended: less than 7.5-cm (3,0-
in) diameter—standard weight galvanized steel,
painted outside; greater than 7.5-cm (3.0-in) diam-
eter—cast iron, galvanized steel (12). Several different
nozzles and diffusers have been used in force main
aeration. If the point of injection is at the bottom of a
vertical riser, diffusers may enhance dissolution.
However, for typical applications of air injection into a
sloping force main, air bubbles coalesce into large
bubbles within a short distance downstream, regard-
less of the manner of air injection. For these
applications, the usual practice is to merely pipe the
air into a suitable connection in the main.
For air injection by Venturi aerators, the major
equipment required is the aeration device and a
pump. Most Venturi aerators require a minimum
pressure at the inlet and a minimum wastewater flow
rate to achieve the desired rate of air aspiration.
Bypass valves and piping are recommended to allow
diversion of flow. Alternatively, the aspirator and
pump can be located in a recirculation tine that allows
the flow to bypass the device when the pump is not
operating.
Air lift pumps can be used to provide both pumping
and aeration. Equipment required for an air lift pump
consists of a compressor, air feed piping, and the air
lift device. Af Jefferson Parish, Louisiana, "lift air"
was injected at the bottom of the 6-m (20-ft) riser, and
"aeration air" was-added at a point approximately
3-m (10-ft) deep in the airlift casing (4).
U-tube dissolution systems employ either compres-
sors or Venturi aerators for air supply. U-tubes can be
of any depth, but little advantage is gained at depths
exceeding 30 m (100 ft), as total cost per unit of
oxygen dissolved increases due to the need for
specialized drilling equipment (13). U-tubes may be
fabricated from steel or fiberglass-reinforced plastic
pipe and secured within standard steel well casing.
To avoid plugging, a pair of vertical pipes connected
by a 180-degree return bend is recommended. Use of
concentric pipes or a rectangular trough with center
divider may be less expensive, but such configurations
are more subject to plugging when used for raw
wastewater aeration (10). If sufficient head is not
available, pumps may be required to force the air-
water mixture through the U-tube. With aspirated air
systems, little control over air flow can be exercised,
the amount of air aspirated being a function of the
aspirator design, inlet pressure, and wastewater flow
rate. For compressed air U-tubes, air injection rales
can be regulated to some degree by programmed
control of the compressor to inject air only when
required on an intermittent basis. At Port Arthur,
Texas, compressed air was introduced into an air
injection collar located near the top of the downleg.
Thirty-two0.3-cm (0.12-in) holes were spaced equally
around the circumference of the line through which
compressed air was introduced (10).
Equipment requirements for a pressure tank dissolu-
tion system consist of an air compressor, pump,
pressure vessel, air piping and diffusers, pressure
relief valve (piped to discharge line), and air flow-
meters and valves. Wastewater bypass piping and
valves should also be provided. Provision of periodic
or continuous recycle from the bottom of the vessel to
40
-------
an upstream location may be necessary to control
solids accumulation due to deposition.
3.2.2.3 Design of Air Injection Systems
a. Air Requirements
The first step in design of an air injection system is to
estimate air requirements to achieve the desired
objectives of sulfide control. This is based largely on
expected oxygen uptake rates in the sewer. The
overall oxygen balance in a sewer can be described by
the following equation (3):
^ - H, R, Re (3 1)
dt
where,
d02 _ rate of change of oxygen concentration as
dt the stream moves down the sewer, mg/l-hr
R» = rate of gain of oxygen by surface reaeration,
mg/l-hr
R, = rate of loss of oxygen due to reaction in the
stream (oxygen uptake rate of wastewater),
mg/l-hr
Ra = rate of loss of oxygen due to reaction on the
slime layer, mg/l-hr
In force mains, the surface reaeration rate. Re can be
assumed to be zero. In gravity sewers, R| can be
estimated using equation 2-14. Values for Rr, oxygen
uptake in the wastewater, should be experimentally
derived from sampling at several locations along the
sewer reaches of interest. The sampling period should
fully cover normal diurnal flow variations. R. can be
estimated using Equation 2-8, as experimental
determination is difficult and time-consuming.
Several different approaches can be employed to
estimate air requirements for sulfide control in a
particular sewer reach. One approach involves the
following procedures:
1. Determine typical initial oxygen uptake rates, Rr,
in wastewater from sampling programs and
develop oxygen uptake curve, as shown in
Figure 3-7.
2. Calculate wastewater detention time in the
sewer reach at minimum, average, and peak
flows.
3. Measure oxygen uptake rate at 1 -hour incre-
ments on samples collected during periods of
maximum oxygen uptake.
4. Plot oxygen uptake rate with time, as shown in
Figure 3 8.
5. Calculate area under each curve in Figure 3-8
for cumulative increments of time to yield
oxygen demand in mg/l.
6. Plot ratios of oxygen demand to initial oxygen
uptake rate, as shown in Figure 3-9.
Figure 3-7. Example of determination of oxygen uptake
rats.
7 r
C
o
s
w
c
m
u
c
o
(J
c
©
CP
>-
X
O
*o
9
_>
O
4,0 mg/Mv
a>
m
a
30
lO
0
Time, mill
Figure 3-8. Example of variation in oxygen uptake rate writh
time.
15|—
It a.m. Sample
w
a
cc
a>
W
-O
1
4
5
0
3
2
Time, hr
7. Multiply design oxygen uptake rate by above
ratios at detention times corresponding to
minimum, average, and daily peak flow to yield
oxygen requirements in mg/l. For design pur-
poses, use ratios from curve (Figure 3-9) which
represents the greatest potential for oxygen
usage.
8. Calculate oxygen uptake rate by slime layer, Ra:
R, = S.SlOsJsuJ^R-1 (2-8)
where.
41
-------
Figure 3-9. Example ratio of cumulative oxygen demand to
initial oxygen uptake vs. time.
6 r—
From 5 a.m. Samples
Time, hr
Re = loss of oxygen from the stream by
reaction with the slime layer, mg/l-hr
[0Z] - oxygen concentration, mg/i
s = slope of the energy grade line, m/m
u - stream velocity, m/s
R = hydraulic radius of the stream, m
9. Multiply oxygen uptake rate in slime layer by
ratios obtained in Step 7 to yield oxygen required
by slime layer (mg/l) at various detention times
(flow rates).
10. Add results from Steps 7 and 9 to yield total
oxygen required in mg/l at various flow rates.
11. Determine mass flow rate of oxygen required
(flow x concentration), kg/d.
12. Calculate air requirements in m3/hr for mini-
mum, average, and maximum flows.
For direct air injection into force mains, an alternative
approach is to use the following equation (14):
Qfl = (6 x 10'4)VR, x Ua+Uw (3-2)
P, log [(P, + P2)/Pj] X ua
where,
Qa = required air flow at ambient force main
pressure, m3/min
V = volume of pressure main, m3
42
Rr = rate of loss of oxygen due to reaction in the
stream (oxygen uptake rate of wastewater),
mg/l-hr
Pi = ambient atmospheric pressure, atm
P2 = gage pressure at injection point, atm
ua = velocity of the air relative to the wastewater,
m/s = 1.66 d'/2, where d = pipe diameter, m
uw = velocity of the wastewater, m/s
This equation generally predicts more than enough
air for sulfide control, since it is based on maintaining
an excess of oxygen. In actual practice, adequate
sulfide control can often be achieved using one-third
or one-half as much air as would be required to satisfy
the oxygen-consuming capability of the wastewater.
It should also be noted that Equation 3-2 is not useful
at pressures greater than 276 kPa (40 psi) due to the
large impact on both oxygen and nitrogen dissolution.
This equation also requires knowledge of the oxygen
reaction or uptake rate, Rr, of the wastewater. This
should be determined through sampling and analysis.
If this cannot be done, Table 3-6 can be used as a
general guide to selecting oxygen reaction rates.
Table 3-6. Suggested Oxygen Reaction Rates (16)
Age of Wastewater Suggested R, (conservative)
mg/i-hr
1 hr @ 20°C or Vi hr @ 30X 5
2 hr @ 20°C or 1 hr @ 30°C 10
Over 3 hr @ 20°C or 1 Vi hr @ 30°C 15
b. Control Systems
Since air requirements vary with diurnal flows and
oxygen reaction rates, it is often desirable to program
air injection rates to more closely match air require-
ments, thereby avoiding excess air use and associated
higher expenditures of energy. Several different
control approaches with varying levels of sophistica-
tion are possible. For single compressor installations,
the compressor controls can be interlocked with
pump starter circuits. A time-delay circuit may be
employed to delay air injection until after the pumps
have started in order to avoid gas locking. A common
technique is to use a pressure switch on the receiver
tank, which operates a device that lifts the air intake
valves so that the compressor idles (6)(7). Although
an idling compressor still utilizes energy, it is at a
reduced rate. Air injection at a pump station in
Gloucester County, New Jersey was controlled with a
pressure switch on the receiver using the following
strategy (6):
-------
Condition Response
Pressure < low Compressor operates
pressure switch setting
Pressure > high
pressure switch setting
1. Pump on Compressor continues to
operate
2. Pump off Compressor idles for
minimum run time, then
shuts down unless
pressure switch calls for
compressor operation
Another simple control scheme is the use of two
compressors of different capacities to provide three
rates of air flow. For example, if diurnal air require-
ments varied between 10 and 30 mVhr, a 10-m3/hr
unit and a20-m3/hr unit could be specified,providing
possible air flow rates of 10, 20, and 30 m vhr (5.9,
11.8, and 17.7 cfm). A timer circuit would actuatethe
compressors on a pre-programmed schedule based
on diurnal variations in oxygen requirements.
A more sophisticated approach is the use of cam-
operated controllers which actuate solenoid valves
on metering lines. The lines are equipped with
interchangeable flow-controlling orifices (16). Still
another approach involves use of variable-speed
compressor drives.
An important consideration in determining air require-
ments for sulfide control in a force main is the loss of
air through air release valves. Air losses at each relief
valve can be as much as 30 percent, depending on
force main pressure and actuation pressures of the
valve. Accounting for such losses may yield signifi-
cantly higher design air flow rates than calculations
based on oxygen uptake would indicate (16).
Location of the air injection point is critical in
designing a direct air injection system. Maximum
dissolution occurs in the vertical riser near the
discharge of the pump, since this is the point of
maximum velocity, pressure, andturbulence. In some
cases, as with submersible pumps, this may not
be practical, and alternative injection locations must
be considered. Selection of an alternative injection
point may, however, result in reduced dissolution
efficiencies.
Evaluation of the plan and profile of the force main,
location of air release valves, and materials of
construction is necessary in the design of injection
facilities to avoid accumulation of large gas pockets
that can result in gas locks in the line or in pumps.
When injecting air into a force main, the amount of air
fed into the stream will often exceed the solubility. As
a result, gas pockets will form. This is not necessarily
detrimental, as the gas pockets will be carried with
the flow and will dissolve further downstream in the
main, primarily in descending legs. However, in
mains with low pump-head pressure and undulating
profiles, gas pockets can form at high points between
pump cycles, and may result in greater resistance to
flow.
Gas locking of pumps can also occur, particularly
wherea downhill gradient exists immediately follow-
ing the discharge riser from the pumps. When the
pumps are operating, undissolved gas will form a long
pocket in the downhill portion of the force main if
conditions are not favorable to complete dissolution.
When the pumps and air injection are stopped, the
pockettraveis upgradient and collects in the discharge
riser. If the gas pocket is of sufficient volume and the
check valves following the pumps are not watertight,
gas may leak back into the pump to cause gas locking
(17).
Operation of pumps while dry causes packing gland
failure due to overheating. Problems associated with
gas locking of pumps can be avoided by following
these guidelines (17):
1. Avoid down grades immediately after the dis-
charge riser.
2. Inject air after the down grade.
3. Control the size of the gas pocket by more
efficient dissolution techniques.
4. Check static head to determine if it is sufficient
to push air pocket back to pumps, and check
volume of gas pocket.
Location of gas pockets in the discharge riser can be
monitored by installing a "boiler glass" on the riser.
The level of the gas pocket in the sight glass should
never be more than one pipe diameter below the
crown of the horizontal section of the main.
Where proprietary equipment is to be used for
aeration, as in the case of Venturi aspirators and air
lift pumps, manufacturers' representatives should be
contacted to assist in equipment specification. Venturi
aspirators have air capacity ratings based on a specific
fluid flow rate and inlet pressure. Deviations from
these conditions will result in fluctuations in quanti-
ties of air delivered by the device. This should be
considered during design.
Air lift pumps are low in efficiency as pumps, but
since they can serve as aeration devices, the com-
bined efficiency is improved. At the Jefferson Parish,
Louisiana, installation, the air lift was designed to
raise the wastewater above the top of the sewer and
let it fall back into the sewer. However, objectionable
odors resulted from exhalation of air from the sewer.
43
-------
This can be avoided by allowing the compressor to
draw air from the sewer atmosphere. H?S should not
have any effect on the compressor because the
temperature is high enough to prevent condensation,
but droplets of moisture must be removed from the
inlet air,
c. U-Tube Dissolution Systems
Design of U-tubes can be somewhat difficult due to
the number of design and operating variables. A
computer program has been used to allow optimiza-
tion of design through parametric calculations {10).
Air entrainment can be accomplished using an
aspirator device or a compressor and injector. Use of a
compressor allows greater flexibility in feed rates,
permits use of higher air-water ratios, and results in
lower head fosses than air aspirated systems. How-
ever, the Venturi aspirator requires little or no
maintenance and requires no power other than that
required to overcome head losses.
The U-tube flow configuration can take many forms,
including: 1) a pair of vertical pipes connected by a
180-degree return bend; 2) a pair of concentric or
eccentric pipes; 3) a single pipe with a flat, vertical
partition; and 4) a rectangular trough with a vertical
partition to separate the downward and upward flow
passages Concerns with pipe plugging or fouling
have led to the use of two pipes connected by a return
bend. For large U-tubes, two concentric pipes may be
used if the cross-sectional areas of flow are suffi-
ciently large to allow passage of solids.
C ross-sectional areas of the downleg are greater than
the upleg to allow greater detention time for oxygen
transfer to occur in the downleg, and to prevent
"deposition of solids by increased velocity in the upleg.
Typical velocities in the downleg are about 0 5 m (1.5
ft)/s, while velocities in the upleg should reach 1.2m
(4.0 ft)/s at least once a day to prevent accumulation
of solids.
When the inlet pipe is raised above the outlet from the
U-tube, relatively little change i n performance can be
expected, but the head loss is reduced Raising the
inlet pipe may be desirable for ease of construction or
operation.
Head loss is an important consideration in the design
of U-tubes. Head loss in an operating U-tube varies
primarily with theair.water ratio, head loss increasing
with increasing air injection rates (18). Figure 3-10
shows the relationship of head loss in the downleg to
air-water ratio for the Jefferson Parish, Louisiana,
and Sacramento, California, U-tubes (7). Unit head
loss increases dramatically at air:water ratios be-
tween 0.1 and 0.3.
Location of the'air injection point should be near the
top of the downleg. This provides maximum contact
Figure 3-10.
1.0
Head loss in downleg of U-tubes at Jofferson
Parish, LA and Sacramento, CA {7|.
i oja
Best hit Curve for
Jefferson Parish (42' Downleg!
Best Fit Curve lor
Sacramento (45* Down leg)
Jefferson Parish Data
Sacramento Data - 10/2/74
icaV
= Sacramento Data 11/4/74
~S 0 .2
02 03 0.4 0.5
AirWater input Ratio
time with the wastewater to allow dissolution to
occur. Lowering of the injection point will reduce
head loss through the system but at the expense of
lower DO concentrations at the outlet.
Figures 3-11 and 3-12 show typical design curves for
compressed air U-tubes and aspirated air U-tubes,
respectively. These curves were generated using a
computer design program for U-tube aerators |10f.
For the specified flow rate and pipe sizes, these
curves show expected outlet DO and head loss
through the system as a function of oxygen supplied.
d. Pressure Tank Dissolution Systems
Design criteria for pressure tank air dissolution
systems are not well established due to limited
experience with the application of this technique for
sulfide control. At Port Arthur, Texas, the volumetric
air-to-water ratio was 3.91 at nominal conditions,
corresponding to an oxygen application of 949 mg/l.
At design conditions, detention time in the pressure
tank was 17 minutes, and operating pressures were
between 1 and 2 atmospheres. The design of the
device required continuous operation of wastewater
pumps and air compressors. As a result, maintenance
and power costs were estimated to be higher than
with other aeration alternatives (2).
3.2.2.4 Cost of Air Injection Systems
Typical costs of systems for direct injection of
compressed air into a force main are shown in Table
3-7. These costs are budget level estimates (+30
percent, -15 percent). If a separate dissolution system
is required, as for gravity sewer applications, addi-
tional capital costs would be incurred. Due to lack of
definitive cost information, as for pressure tank
dissolution, and the wide variability in site-specific
44
-------
Figure 3-11. Typical compressed-air U-tube parametric
designs (10).
Figure 3-12.
- Q = 3,472 gpm
Pipe Sizes: 23.75 in ID Down leg
Casing Depth, ft
100
40 60 80 100
Oxygen Supplied, mg/l
140
x
<
a>
>
o
e
c
«
o
u
O
o
design criteria, as for U-tubes, these costs have not
been included.
3.2.3. Oxygen injection
Injection of pure oxygen into force mains and gravity
sewers is a demonstrated technique for sulfide
control. Over 100 installations have been completed
in the United Kingdom and Australia, and about 10 in
the United States (19). Pure oxygen addition has
several advantages over air injection:
1. Oxygen is five times more soluble in water than
is air
2. Smaller quantities of gas need to be injected,
reducing the likelihood of gas pocket formation
3. Compressors are not required
4. Greater solubility results in improved pumping
efficiency and oxidation of existing sulfides, and
maintenance of a higher residual DO to prevent
further sulfide generation.
The efficiency of oxygen injection is dependent on the
energy expended in dissolving the gas, the tempera-
ture and pressure of the stream into which the oxygen
Typical as pirated-air U-tube parametric da-
signs (10).
Q - 3,472 gpm
• Pipe Sizes: 23.25 in ID Down leg
23.0 in ID Upleg
Casing Depth, ft
Casing Depth, ft
20 40 GO
Oxygen Supplied, mg/l
TOO
Table 3-7.
Typical Costs for Direct Compressed Air I njsctkin
into a Force Main for HjS Control (1984 i)
Condition
Capital Cost*
Flow = 3,785 mVd
Diameter = 25.4 cm
Length = 1,600 m
Pressure = 158 kPa
Air flow = 2 5 mVmin
Flow = 37,850 mVd
Diameter = 61.0 cm
Length = 1,600 m
Pressure = 158 kPa
Air flow = 15.3 mVmin
S
21,000
54,000
Includes concrete pad, compressor, piping, valves, start-stop
controls, pre fab building, and installation.
45
-------
is introduced, and the method used for injection.
Maximum oxygen dissolution is obtained in zones
with sufficiently high turbulence to produce additional
bubbles. Bubble size is also an important considera-
tion. Smaller bubbles provide greater surface area for
oxygen transfer.
As shown in Figure 3-13, dissolution of oxygen
improves with increasing pressure (19). Higher
pressures can be obtained by increased discharge
heads of force mains, or use of pressurized sidestream
systems, U-tubes, or other dissolution systems. It is
desirable to inject oxygen at the lowest possible
location in the pump discharge piping. Application
points are located in the collection systems such that
downstream conditions provide for maximum main-
tenance of residual DO.
Figure 3-13.
160
Saturation concentration of oxygen in water
at different pressures <19).
Pressure, atm
3.2.3.1 Oxygen Dissolution Alternatives
a. Direct Oxygen Injection Into Pipes Under Pressure
Direct injection of oxygen is the simplest and most
common means of oxygen dissolution but is only
applicable to force mains or pipes under pressure. In
gravity systems, and in some force mains, insufficient
pressure and turbulence exist to achieve desired
dissolution, requiring consideration of alternatives to
artificially create these conditions.
Figure 3-14 shows a direct oxygen injection system at
a pump station. Oxygen is injected at a point of high
turbulence, favoring the formation of small bubbles
which will readily dissolve. Injection at the bottom of
the vertical riser improves dissolution due to in-
creased pressure and reduced coalescing of the
bubbles. However, undissolved bubbles will tend to
coalesce in the horizontal portions of the main, where
a gas-liquid interface forms. Turbulent conditions
downstream promote transfer of oxygen from the
gaseous to the liquid phase.
Figure 3-14. Typical direct oxygen injection system (19).
From Oxygen
Source
Wet
Wei!
Force Mam
Check Valve
Oxygen Injection Point
Results from a study of direct oxygen injection into
force mains at Port Arthur, Texas are summarized in
Table 3-8 (2). These data show that direct injection of
pure oxygen into force mains can be successfully
used to control sulfides. Resu Its of field tests of direct
oxygen injection into a force main in Sacramento,
California are presented in Table 3-9 (7). As can be
seen, oxygen injection effectively controlled sulfides
at the lowest oxygen dosage studied.
Siphons are often associated with sulfide generation
due to long retention times during periods of low flow.
Direct oxygen injection has been successful in
controlling sulfides in three siphons of 99-cm (39-in)
diameter in the Tyneside Sewage System in the
United Kingdom (20).
The ideal location for oxygen injection into a siphon is
the bottom elbow of the descending leg where high
turbulence and greater pressures promote good
oxygen transfer. For efficient dissolution, a minimum
wastewater velocity of 0.6 m/s (2 ft/s) should be
maintained. The lower pipe in a siphon must be
horizontal or sloped upwards to prevent gas collection
at the crown. The final ascending leg should also have
vertical rise to promote gas-liquid intermixing.
b. Single U-Tube Injection
There have been several investigations of the single
U-tube (Figure 3-5) for dissolving pure oxygen in
wastewater (2)(10)( 18). A U-tube aeration system will
absorb pure oxygen with an efficiency of 60 to 80
percent when the volumetric rate of pure oxygen
injection is 0.5 to 1 percent of the wastewater flow
rate (18).
46
-------
Table 3-8. Performance of Direct Oxygen Injection into Force Mains at Port Arthur, TX (2)
Sulfide Concentration
Oj Injection Wet Weil Force Main Discharge
Force Main Rate Total Dissolved Total Dissolved
ma/d mg/l
Diameter = 40.6 cm
0
0 5
0.4
1.2
1.1
Length = 1,174 m
82
1.5
0.9
0.1
0
Flow = 6,540 mVd
122
1,7
0.9
0.1
-
163
1.0
0.7
0.2
0.1
184
0.8
0.5
0.2
0
204
1.9
1,3
0.1
0
245
2.0
1.2
0,1
0
Diameter = 20.3 cm
0
2.3*
1.9
6.4
4.6
Length = 960 m
204
2-3"
1.9
0.9
0,5
Flow = 3,270 m'/d
245
2.3"
1.9
0.5
0.3
285
2.3'
1.9
0.6
0.3
Diameter = 20.3 cm
0
0.9
0.8
10.7
9.8
Length = 1,654 m
41"
12
•1.0
2,2
1,7
Flow = 1,907 mVd
"Weighted averages,
"Maximum injection rate limited by system hydraulics.
Table 3-9. Performance of Direct Oxygen Injection into
Force Main at Sacramento, CA (7)
Total H2S Concentration
Dosage
Wet Well
Downstream Sample Tap
mg/l
mg/l
0
0,16
2.14
18.8
0.12
0.22
37.5
0.07
0.13
45.6
0 09
0
Force main characteristics:
Diameter = 53 cm and 66 cm
Flow = 11,920 mVd
Distance to sampling tap = 2,930 m
A U-tube oxygen injection system was evaluated at
Port Arthur, Texas. Oxygen transfer efficiencies were
lower than expected. This was attributed to errors in
estimating pump capacity, resulting in a detention
time in the U-tube of only 17 seconds compared to the
design detention time of approximately 30 seconds
(2),
Results of limited testing conducted on an oxygenated
U-tube in Sacramento, California are summarized in
Table 3-1 0(7). Use of the oxygen U-tube resulted in a
60- to 70-percent reduction in total sulfide concentra-
tion at a point 1,000 m (3,300 ft) downstream (7).
c Multiple U Tube Dissolution
A proprietary multiple U-tube oxygen dissolution
system has been successfully applied in Australia for
H2S control in force mains and gravity sewers, and for
in-sewer wastewater treatment (19). A three-stage
U-tube dissolver is shown in Figure 3-15 (21),
Wastewater flows through the U-tubes creating a
series of waterfalls which generate oxygen bubbles.
Bubbles tend to reform at the top of each column in
gas pockets. Wastewater flowing through the pockets
under turbulent conditions results in partial dissolu-
tion of the accumulated oxygen. Travel through
successive gas pockets increases the amount of
oxygen dissolved in the wastewater. The oxygen
entrainment rate (oxygen flow divided by liquid flow)
through a three-stage U-tube is approximately 10
Table 3-10. Performance of Oxygen U-Tube for Sulfide Control at Sacramento, CA (7J
Total Sulfide DO
1,000 m 1.000 m
Flow Supplied In Out Downstream In Out Downstream
mVd
m3/d
mg/i
mg/l
mg/l
8,300
0
0
0.61
0,70
0.67
0
0
0.24
10,300
130
16.8
0,55
0,43
0.28
0
9.1
1.0
9,000
240
35.5
0,53
0.45
0,21
0
12.4
4.0
10,640
355
44,3
0.66
0,59
0,23
0
>20
9.0
47
-------
percent by volume. Because this exceeds the oxygen
solubility, undissolved oxygen is collected in a phase
separator and recycled back to the first stage of the
tlissolver
Figure 3-15. Multiple Utube dissolution system (1).
Bubbles Coalesce lo
Form Separate Gas Phase
O2 Recycle
O2 Injection
Phase
Separator
/// AW/// >
,V/// v.w
Oullet Valve
Limited performance data for a multiple U-tube
oxygen injection system installed near the end of a
60 cm (24-in) force main in Australia are presented in
Table 3-T t (19). Because of the limited detention time
prior to discharge and the resulting inefficiency in
dissolution, the pressurized multiple U-tube dissolver
was recommended.
Table 3-11.
PttrfoiuianCM of Multiple (J-Tube Oxygen Injec
Hon [19)
Winter
Trial
Summer
Trial
Flow. mVd
16.2
15.9
Injected 0a mg/1
Day
34
54
Night
185
2 B0
Sulfide removed, mg/1
20
40
Total sulfide removed, kg/d
364
637
Ratio of Os used to sulfide
removed
2.3:1
1.8:1
DO at headwords, mg/1
10
10
of the oxygenated liquid discharging from the nozzle
results in good mixing and dilution with the main
stream. This keeps bubble size to a minimum,
allowing good oxygen dissolution.
There are two main types of pressurized sidestream
dissolvers: the "basic" system in which all the gas
added is dissolved in the sidestream; and the propri-
etary "Vitox" system in which, after equilibrium is
reached, only part of the gas is dissolved in the
sidestream, the remainder being dissolved when the
stream is returned via a submerged nozzle or jet to the
main flow. Both types can operate over a wide
pressure range, although the basic sidestream dis-
solution system requires a higher pressure for
equivalent oxygen dissolution. The high velocity of
the returning oxygenated liquid results in good mixing
and dilution with the main stream in both systems.
The basic system normally operates at pressures up
to 700 kPa (100 psig) at which 250 mg/1 of oxygen
can be dissolved in the sidestream. However, the
turbulence and pressure drop at the return nozzle
causes effervescence and gas loss. Operation at
lower pressures reduces effervescence problems, but
oxygen dissolution is reduced proportionately.
The sidestream from the proprietary Vitox process
reportedly requires only 210 kPa (30 psig) to dissolve
220 mg/1 of oxygen. Dissolution of the gas remaining
undissolved in the sidestream is achieved by creating
a shock wave in the nozzle which returns the
sidestream to the main stream. The shock wave
generates clouds of microbubbles which are rapidly
dispersed in the main stream by the turbulence of the
jet. This system has been effective on shallow trunk
lines and channel inlets. It requires less energy than
the basic sidestream dissolution system to dissolve
an equivalent amount of oxygen (22). A typical
sidestream flow diagram is shown in Figure 3 16.
Figure 3-16. Typical sidestream dissolution system (22|.
Oxygen
\
*"* It" .'".vr" -
d. Pressurized Sidestream Injection
One technique for injecting oxygen involves satu-
rating a portion of the flow under pressure with
oxygen, then introducing the oxygen-enriched side-
stream back into the main flow through a submerged
nozzle or jet. Using a pressurized sidestream disso-
lution system ensures that little or no oxygen comes
out of solution in the sidestrea m.andthehigh velocity
Venturi
Injector
Sidestream
Pump
Wastewater
Flow
Sump
' Expansion
Nozzle
48
-------
e. Hydraulic Fall Infection
Oxygen can be injected efficiently into a hydraulic fad
due to the intense mixing and turbulence resulting
from such structures. This can be an effective
technique for oxygenation of force main discharges
prior to entrance into gravity sewers. A diagram of
oxygen injection at a hydraulic fait is shown in Figure
3-17 (7).
Figure 3-17,
System for oxygen injection into a hydraulic
-fall (7).
I—Off Gas
High Flow
Resistant
Low Flow
Profile
Lining
pressurization systems, may he required. A typical
layout of an oxygen injection station is shown in
Figure 3-18 (23).
Figura 3 18 Typical liquid oxygen station {23}.
Perlif e Insulation
Evacuated to a
High Vacuum
Rupture Disc /vapor Space 2SOpsig
Working Pressure
Inner Tank: 9% Nickel.
, Steel. Aluminum, or
Stainless Steel
^ Outer Tank Carbon Steel
Liquid Level and
, Pressure Gauges
High Pressure
Ambient Air
The turbulence caused by such a fall promotes
oxygen transfer, but at the same time releases any
HzS gas to the wet-well atmosphere, thus requiring
consideration of use of corrosion-resistant materials
and collection and treatment of exhaust air. In one
hydraulic fall oxygen injection system installation in
Sacramento, California, HzS and methane concentra-
tions in an enclosed hydraulic fall reached levels
considered to be hazardous to human health and
constituted a potential explosion hazard. In an attempt
to mitigate these problems, chlorine was added
upstream of the fall to reduce sulfide levels. However,
the oxygen injection system had not yet been
commissioned as of January, 1984.
3.2.3.2 Oxygen Injection Equipment
Applications requiring less than approximately BOO
kg (2,000 lb)/d of oxygen typically use purchased
liquid oxygen. Where oxygen requirements exceed
900 kg (2,000 lb)/d, generation on-site using a
Pressure Swing Absorption system may be more
economical.
Equipment used for oxygen injection systems employ-
ing liquid oxygen includes a liquid oxygen storage
vessel, a vaporizer, a pressure regulator, oxygen feed
and injection piping, and a control system. For other
than direct oxygen injection into force mains, separate
dissolution equipment, such as U tubesor sidestream
Final Lme
Regulator
Gaseous Oxygen
Relief Valve
Concrete Pad
A liquid oxygen storage tank, which can be purchased
or leased from the supplier, consists of an inner and
outer vessel separated by an insulated annular space
evacuated to 10 to 100 mm Hg. The inner tank is
constructed of aluminum, stainless steel or high-
nickel steel alloys. The outer vessel is carbon steel.
Most tanks arc designed to regulate pressure auto-
matically. Excess gas pressure is relieved by release
of vaporized oxygen into the discharge line, while low
pressures are increased by vaporization of a portion of
the liquid oxygen and recycle to the head space of the
vessel.
The vaporizer is typically constructed of304stainless
steel or aluminum with extruded fins, allowing
vaporization of the liquid oxygen through exchange of
heat from the ambient air. In some high-use applica-
tions. a low-temperature shut-off device is installed
downstream of the vaporizer to prevent discharge of
liquid oxygen and to provide protection of downstream
equipment.
Oxygen piping is typically copper or stainless steei.
although copper is more commonly used due to lower
cost and ease of installation. These materials do not
embrittle at low temperatures.
49
-------
Selection of a control system is based on the type and
sophistication of control desired. To prevent injection
of excess oxygen, oxygen injection can be interlocked
with pump starter circuits. A time delay can be
designed to delay injection once pumping begins, and
a timer used to limit the amount of oxygen injected
during a pump cycle. Alternatively, rate of oxygen
injection can be automatically controlled based on
actual flows encountered in the system (proportional
control).
Injector design and bubble size are important con-
siderations, In cases where the rate of oxygen
dissolution is not critical, as in force mains with
velocities greater than 0,67 m/s (2.0 ft/s) and
pressures greater than 15 m (49 ft) of head, a nozzle is
frequently sufficient to achieve desired dispersion
and local turbulence, For velocities less than 0,67
m/s (2,0 ft/s) or for lower pressures, a porous plate
diffuser will achieve smaller bubble size and greater
interfacial oxygen transfer surface. However, these
devices are susceptible to biofouling and provision
must be made for cleaning, Presence of sufficient
mixing energy is critical for efficient oxygen dissolu-
tion. Unfortunately, current knowledge on oxygen
injection into wastewater force mains does not allow
precise calculation of the mixing energy required for
complete oxygen dissolution.
For applications requiring greater than approximately
900 kg (2,000 lb)/d of oxygen, generation on-site
using a Pressure Swing Adsorption (PSA) system may
be more economical than purchasing liquid oxygen. A
flow diagram for a PSA system is shown in Figure
3-19 (24). Pressure Swing Adsorption employs a
cycle which operates between two pressures, adsorb-
ing and separating the gas stream at the higher
pressure and desorbing and exhausting waste prod-
ucts at the lower pressure. Separation of nitrogen and
other contaminants occurs in a vessel by selective
adsorption using a zeolite molecular sieve. This is a
batch process, but cycling between multiple adsorp-
tion vessels allows reasonably constant oxygen flow,
A product storage vessel is normally incorporated for
further attenuation of the cyclic oxygen generation,
A timer and pressure switches control valves on a
manifold, which directs pressurized feed air from a
compressor to the adsorption vessels. The only
moving parts are the valves and air compressor,
which require periodic maintenance. The compressor
operates either fully loaded (100-percent flow) or
unloaded (no flow) and repressurizes a bed in
approximately 1 minute.
Equipment requirements for a PSA oxygen-gener-
ating station consist of a single-stage reciprocating,
two-stage centrifugal, or screw compressor. Addi-
tional equipment includes inlet air filter-silencer,
piping and valves, adsorbent vessel designed to meet
Figure 3-19. Pressure swing absorption—basic flow dia-
gram.
Backfill Oxygen
To Oxygen
Product
Compressor
Vessel l|
Vessel s|
Vessel 3
on Stream
Repressur-
Cleaning
I
izing |
Oxygen
Air
Nitrogen Rich
Waste Gas
Nitrogen Rich
Waste Gas
to Vacuum
AS ME Code requirements using SA-516 Firebox,
Grade 65 steel or equivalent, an integrated valve
assembly interfaced with feed air, adsorbent vessels,
an oxygen injection system, and appropriate instru-
mentation and controls. Some processes also use a
vacuum pump. An evaporative cooling tower is
provided to lower the temperature of the recirculated
cooling water used to cool the air discharged from the
compressor. In addition, liquid oxygen storage is
normally provided as a back-up oxygen supply.
For oxygen injection into a gravity sewer or at
transitions from pressure to gravity conditions, a
separate oxygen dissolution system may be required
to improve dissolution efficiency and prevent de-
gassing. Systems previously described include U-
tubes (single and multiple), and pressurized side-
stream (high or low pressure) oxygen dissolution
systems. Equipment requirements will vary depend-
ing on the specific application and the dissolution
system selected.
3.2.3.3 Design of Oxygen Injection Systems
The design of an oxygen injection system issimilarto
the design of an air injection system in that the first
step is to estimate oxygen requirements. Collection of
sufficient data to estimate oxygen reaction rates is
necessary, 0 ne design approach follows the stepwise
50
-------
procedures outlined in Section 3.2.2.3 for air injection
systems, where the oxygen requirements are based
on oxygen uptake in the wastewater stream and
within the slime layer. This procedure can be applied
to the design of oxygen injection facilities for both
force main and gravity sewer applications.
An alternative design approach for direct oxygen
injection into force mains is to use the following
equation, which yields the initial DO level required to
maintain aerobic conditions (25).
C0 = Rr + ^(0.785 d2) x -txlO""4
d F
(3-3)
where,
C0 = required oxygen concentration, mg/1
R, ~ oxygen reaction rate, mg/1
280 = factor corresponding to an assumed oxygen
reacting rate on the slime layer equal to 0.7
g/m2/hr
d - pipe diameter, cm
L = pipe length, m
F = discharge, m3/hr
10"4 = conversion factor, cm2 to m2
The total daily oxygen requirement can then be
determined as:
kg Os/day =
- F x Co x 24
1,000
(3-4)
Many of the design considerations that apply to air
injection also apply to pure oxygen injection. These
include location of oxygen injector, control of gas
pocket formation, and options for control systems.
Gas pockets are likely to form in any two-phase
system, and the potential always exists for gas
locking of pumps. However, problems associated with
gas pocket formation are less likely to occur with
oxygen injection, since the volume of oxygen gas
necessary to add 1 kg of oxygen is approximately
one-fifth the volume of air required to add an
equivalent amount of oxygen. However the same
precautions should be observed as those outlined in
Section 3.2.2.3 in order to prevent such occurrences.
a. Single U-Tube
Design criteria for single U-tube installations have
been describedfor air injection applications in Section
3.2.2. In general, design of an oxygen U-tube is
approached in the same manner. Oxygen is injected
at the top of the downleg to maximize contact time
with the wastewater to allow dissolution to occur.
Since the volume of oxygen is approximately one-fifth
the volume of air to achieve the same mass of oxygen
transferred (assuming complete dissolution), head
losses with oxygen U-tubes are considerably less
than with compressed air or aspirated air U tubes.
Figure 3-20 shows the relationship between DO
increase across a U-tube and head loss for both air
and oxygen U-tubes (7).
Because oxygen must be purchased unless generated
on-site, it is desirable to make efficient use of the
oxygen. Oxygen U-tubes should, therefore, be de-
signed for greater depths to promote maximum
dissolution. Volumetric gas:water ratios for oxygen-
ated U-tubes are generally about 0.02, corresponding
to an injection rate of 20 to 30 mg/1.
Figure 3-21 shows an expected performance curve
for an oxygenated U-tube designed to handle a flow of
6,540 m3/d (1.7 mgd). This shows the relationship
between oxygen supplied and oxygen transferred for
U-tubes of various depths, as well as expected head
losses vs. oxygen supplied for various U-tube depths.
The family of curves at the top of Figure 3-21 shows
the ratio of oxygen transfer with pure oxygen to
oxygen transfer with air, the ratio ranging from 3 5 to
4.5.
b. Multiple U-Tube
The proprietary multiple U-tube system is different
from the conventional U-tube and can be considered
as much a mixing device as a dissolver, As opposed to
conventional U-tubes, the length of the vertical legs
in the multiple U-tube is only 1.8 to 2.4 m (6 to 8 ft).
Velocities through the uniformly sized pipe are
normally 0.5 to 1,2 m (1.5 to 4.0 ft)/s, although they
may be as high as 2.4 m(8.0ft)/s. Figure 3-15 shows
the application of the system for force main oxygen-
atiori, where the existing pressure in the force main
Figure 3-20.
0.5
0.4
I 0-3
Q
di
0.2
i
0.1
Dissolved oxygen Increase in the wastewater
across U-tube vs. head loss comparing oxygen
and air injection (7|.
• Air Injection
X Oxygen Injection
4 8 12 16 20
Dissolved Oxygen Increase in the Wastewater
Across U-tube, mg/l
24
57
-------
Wigum 3-21, Typical design cwvs for oxygen U-tube (7).
<
SS
5 0
i-
4.5
35
.100
SO
- TO
,54.25
4-
^ 35-
o
E 30
£ 25 -
¦£20-
c
¦ 15 -
EIO-
100
. 80
• 70
. 54-26
40
20
60 BO 100 120 140
Oxygen Supplied, rog/l
drives the wastewater through the dissolution sys-
tem. However, the system can also be designed as a
pressurized sidestream dissolution system for appli-
cation to gravity sewers or at transitions from
pressure to gravity conditions by employing high
pressure pumps. Design head loss across the multiple
U-tube dissolver is 2 to 3 m (6.5 to 10 ft). Expected
oxygen dissolution will be equivalent to that of a
water fall of this height
c. Pressurized Sidestream Dissolution
Sidestream dissolution systems can be of the basic or
proprietary Vitox type. Basic systems are only appli-
cable to force mains in which the profile and ambient
pressure are sufficient to prevent excessive oxygen
degassing after injecton. Vitox systems avoid the
degassing problem at equivalent oxygen addition
rates and are also appropriate for gravity sewers. The
injection of a high pressure oxygen enriched side-
stream from the basic sidestream system into waste-
water at or near atmospheric pressure would likely
result in high gas loss.
Basic sidestream dissolution systems operate at
pressures from 105 to 700 kPa (15 to 100 psi), while
Vitox systems usually operate at pressu res of 210 kPa
(30 psi) (22). Sidestream flow rates will depend on
oxygen requirements of the wastewater and incoming
sulfide concentration.
A Vitox sidestream system approaches 40- to 60-
percent oxygen transfer efficiency at an injection rate
of 10 to 30 mg/l in a shallow channel (22}.
d Hydraulic Fall Injection
A hydraulic fall is an excellent location for oxygen
injection due to the intense mixing and turbulence
resulting from dissipation of the kinetic energy of the
wastewater. Often oxygen can be injected into a
hydraulic fall at the transition from a force main to a
gravity sewer.
Dissolution efficiencies for hydraulic falls can be
estimated using the following relationship:
In K„{H, -H2) (2 15)
Ds
where,
Di and D2 = oxygen deficits upstream and down-
stream, respectively, from the drop,
mg/l
Kh = waterfall reaeration coefficient, m1
(approx. 0.41)
H, and Hz - elevations of the hydraulic energy lines
upstream and downstream, respectively,
from the drop, m
This equation was used to develop Table 2-6, which
predicts the percent of the oxygen deficit that would
be satisfied for falls of various elevations. This table
can be used to generate a first estimate of the
efficiency of dissolution if pure oxygen is injected.
However, an assumption must be made regarding the
fraction of oxygen that is wasted in such an injection
scheme. For example, for a drop of 3 m {10 ft), a
71-percent oxygen dissolution efficiency might be
expected {Table 2-6). If 30 mg/l of oxygen is applied,
of which 20 percent is assu med to be lost or wasted,
the resulting DO concentration can be estimated to be
30x0.71 x{1 -0.20)= 17 mg/l. In general, whenever
DO levels are raised above the equilibrium value with
air (8 to 10 mg/l). then oxygen gas is lost in removing
some of the dissolved nitrogen which is in excess
An important design consideration in any hydraulic
fall is the release of HtS resulting from the intense
turbulence. If sulfides are present in the incoming
fiow, significant odor and corrosion may result. Thus,
it may be necessary to provide for corrosion protection
of the structure and for removal and scrubbing of the
HzS-laden atmosphere. Consideration should be
given to the accumulation of potentially hazardous or
explosive gases, and to whether the enclosed struc-
52
-------
ture can be adequately ventilated to prevent such
problems.
3.2.3.4 Cost of Oxygen Injection Systems
Typical costs of systems for direct oxygen injection
into a force main are shown in Table 3-12. These
costs are budget level estimates (+30 percent, -15
percent). Additional costs would be incurred if
separate oxygen dissolution systems were employed.
Far some of the dissolution alternatives (e.g., U-
tubes), costs are largely dependent on site conditions
and resulting site-specific design criteria. Other
dissolution techniques, such as multiple U-tube
systems, have not been used in the U nited States, and
as a result, few definitive cost data are available.
Thus, cost estimates are not given for these alterna-
tives.
It should be noted that the capital cost estimates
shown in Table 3-12 assume purchase of equipment
by the user. In many cases, oxygen suppliers will
lease the necessary storage tanks, vaporizers, etc. at
rates that will result in equipment lease being
economically advantageous over direct purchase. In
estimating costs for an oxygen injection system,
oxygen suppliers should be contacted to determine
costs for leasing of equipment
3.3 Chemical Addition
Numerous chemicals have been employed for control
of sulfides in wastewater collection systems. Chemi-
cal addition can control sulfides by: 1) chemical
oxidation (Cla H2O2); 2) sulfate reduction inhibition by
providing an additional oxygen source (NOD; 3)
precipitation (metal salts}; or 4) pH control {strong
alkalies).
3.3.1 C Marine
Chlorine may be added to wastewater either as
hypochlorite or chlorine gas. Hypochlorite may be
used where applications are occasional or dosages
are small, but economies of scale dictate use of
chlorine gas where larger quantities, i.e., >2.3 kg (5
lb)/d, are necessary.
Chlorine combines with water to form hypochlorous
and hydrochloric acids, as follows [26);
CI2 + H2O *¦ HOCI + H* + CI (3-5)
The dissociation of HOCI is shown as;
HOCI H+ + OCr (3-6)
The equilibrium constant for this reaction at 20°C is
pK>7.57.
When calcium hypochlorite is added to water, it
ionizes to yield hypochlorite ion:
Ca(0CI)2 >¦ Ca" + 20CI" (3-7)
Table 3-12. Ty|rical Costs l«r Direct Oxygen Injection into
Force Main for H«S Control (1984 S)
Annual Chemical
Condition Capital Cosf (Qj Costs
S S/yr
Flow = 3,785 inVd
Pipe: diam = 36 cm
length = 1.600 m 20.000 9,000"
O2 req'd = 83 kg/d
Flow = 37,850 m3/d
Pipe: diam = 76 cm
length = 1,600 m 50.000 20,000'
O2 req'd = 310 kg/d
"Includes concrete pad. liquid O2 storage vessel, vaporizer, piping,
start-stop controls, and installation.
bBased on Ox cost of S0.31/kg.
'Based on Oncost of SQ.IB/kg.
The OCF ion establishes an equilibrium with hydro-
gen ions in accordance with Equation 3-6. The same
equilibria are established whether chlorine is added
as a gas or as hypochlorite, although pH may be
affected. Chlorine gas tends to reduce the pH, while
hypochlorite tends to increase the pH.
If excess chlorine is added to a wastewater containing
sulfide, sulfide is oxidized to sulfate according to the
following reaction (14);
HS~ + 4CI2 + 4HzO SO: + 9H+ + 8CI" (3 8)
For this reaction, 8.87 parts by weight of chlorine are
required to oxidize each part of sulfide. If chlorine is
added slowly to a pure sulfide solution under vigorous
mixing conditions, sulfide is oxidized to sulfur as
described by the following reaction (14);
HS~ + CI? *- S + H+ 2CI" (3-9)
Chlorine consumption in this reaction is 2.22 parts
chlorine per part of sulfide. In laboratory tests
conducted by the Los Angeles County Sanitation
District, the chlorine requirement for elimination of
sulfides was three to nine times the sulfide concen-
tration. Actual observations in full-scale studies
showed somewhat higher dosages, possibly due to
inefficient mixing (27).
Table3-13 shows data from a field testing program in
Sacramento, California, where chlorine was added to
a pump station located at the beginning of a 1,520 m
(5,000-ft) long, 61 -cm (24-in) diameter force main (7).
Average dry weather flow at the pump station was
7,570 m3/d (2.0 mgd). Data were collected at chlorine
application rates of 500 kg (1.100 lb)/d. 270 kg (600
lb)/d, and 135 kg (300 lb)/d. Based on actual flow
rates, this was approximately equivalent to dosages
of 46 mg/l, 28 mg/1. and 14 mg/l, respectively. As
shown byTable3-13, all three application rates were
effective in reducing total sulfide concentrations to
zero.
S3
-------
Table 3-13.
Performance of Chlorination for Sulfide Control at Sacramento, CA [7)
Distance from
Injection Point
Approximate Flow
Time from Cb
Injection Point
Average Cl2 Residual
Cl2 Dosage of:°
46 28
with
14
Average HsS Concentration
Without With
Chlorination Chlorination1'
m
hours
mg/l
mg/l
1,700
1.4
38 5
17.5
8.0
0.50
0
2,700
1.8
42.0
17.4
4,3
0.60
0
3,300
1.9
3.6
.05
0
0.32
0
4,100
2.1
2.2
0
0
0.24
0
6,000
2.8
0.3
O
0
0.19
0
"Values for Cl2 residua! are an average of two to five measurements,
'The three feed rates were studied over a total of five days. At each downstream sampling site there were 15 to 20 sulfide determinations
over this time and no sulfide was found.
A study was conducted in Tampa, Florida to evaluate
the effectiveness of chlorination for sulfide control in
a force main (28). Prior to chlorine addition, total
sulfide levels in the raw wastewater were as high as
14 mg/I. Detention times in the section of force main
studied were 10 to 17 hours. Average year-round
temperature of the wastewater was 28°C. Flows in
the force main typically ranged from 18,900 to
22,700 m3/d(5 to 6 mgd).Table 3-14summarizesthe
results of this study. It can be seen that at average
chlorinedosagesof35 to40 mg/l, sulfide concentra-
tions were reduced to zero at a point 6.5 km (4 0 mi)
downstream of the injection point, and were reduced
by about 60 percent at a point 13.7 km (8.5 mi)
downstream of the injection point.
3.3.1.1 Equipment Required
Table 3-1 5 summarizes the characteristics of chlorine
available as liquid or gaseous chlorine and as
hypochlorite (29).
Hypochlorite feed systems are generally used when
the requirement for chlorine is less than 2.3 kg (5.0
lb)/d. If the dosage were 10 mg/l, this would
Table 3-14. Effects of Chlorination on H?S Concentrations
in Force Main at Tampa, FL (28)
H2S Concentrations
Chlorine
Feed Rate
Average
CI2 Dose
Injection
Point
4.6 m 6.5 km 1 3.7 km
kg/d
mg/l
mg/l
0
0
0.4
1.8
7.0
9.0
455 (constant)
21.6
0.7
0
4.6
6.8
455(12 a.m.-7 a.m.)
35.0
0.8
0
0
3,7
910(7 a.m.-12a.m.)
682(12 a.m.-7 a.m.)
3B.0
0.8
0
0
3.7
910(7 a.m.-12 a.m.)
910 (constant)
40.0
0,6
0
0
3.5
correspond to chlorinating a flow of about 230 m3
(60,000 gal)/d. Equipment required for hypochlorite
addition includes: 1) a hypochlorite storage vessel; 2)
a metering pump; and 3) a diffuser or injector.
Most applications of chlorination for sulfide control
employ gaseous chlorine systems. A typical chlorina-
tion system, shown in Figure 3-22, consists of a
chlorine storage facility chlorine scale, a water supply,
a chlorinator, and a diffuser for injecting the chlorine
solution into the stream (29). Figure 3-23 presents a
typical design of chlorine diffusers for gravity sewers
and force mains (30). Booster pumps are generally
required when chlorine is injected into a force main.
For installations requiring greater than 230 kg (500
lb)/d, a separate evaporator is used.
Several control options are available for chlorine feed
systems. The simplest and most inexpensive is a
manually controlled system in which chlorinedosages
are adjusted manually and the feed rate is constant.
Control systems that are particularly useful for
installations lacking flow measuring devices are: 1)
varying dosage by a preset cam controller based on a
diurnal chlorine requirement; and 2) setting chlorin-
ators to dose in increments based on the number of
pumps operating (30). Somewhat more sophisticated
control systems include proportional control, Cl2
residual control, and compound loop control. Propor-
tional control adjusts chlorine feed based on an
electrical signal from a flowmeter. Chlorine residual
control adjusts the chlorine feed as necessary to meet
a desired chlorine residual immediately downstream
of the injection point. Finally, compound loop control
adjusts chlorine feed based on both flow rate and
residual chlorine concentration.
3.3,1.2 Design
Design of a chlorination system for sulfide control is
dependent upon objectives of sulfide control (target
levels of sulfide at a given point downstream), initial
sulfide concentration, characteristics of the waste-
54
-------
Table 3-15. Commercially Available Forms of Chlorine for Wastewater Applications (29)
Chemical
Shipping
Containers
Handling
Materials
Available
Form
Commercial
Strength
Characteristics
Liquid, gaseous
chlorine (Cl;)
Sodium hypochlorite
(NaOCI)
45-, 70-, and
900-kg containers;
tank cars
20- and 50-1
carbovs; 4,900- to
7,500-1 tank trucks
Calcium hypochlorite Small cans(7 kg);
(CaOCy 45-, 135-, and
365-kg drums;
190-kg bbl.
Steel
Ceramics, glass,
plastics, rubber
Glass, rubber,
stoneware, wood
Liquid, gaseous
100
Light yellow liquid 12 to 15 (avail.
chlorine)
White granular
powder
70 (avail, chlorine)
Liquid, vapor at
atmospheric
conditions
Deteriorates with time
1 to 3 percent
available chlorine
solution used
Figure 3-22. Typical chlorinator installation (29),
,Safety Vent
jtr
^Chlorinator
Chlorine Gas
Chlorine Cylinder
Strainer
Shutoff Valve
' PLmL
Check
Valve
Weighing Scale
From Water
"Supply
Overflow
to Drain
Chlorine Solution To Wastewater
Figure 3-23, Typical chlorine drffusert for gravity sewer*
and force mains.
Chlorine
Solution
Provide Support
Manhole Wall
Water Level
Anchor * Flow
Diffuser
Valve
Chlorine
' Solution
water (presence of other chlorine-demanding consti-
tuents), and the degree of chlorine feed control
desired.
At a minimum, laboratory studies should be con-
ducted to estimate the chlorine dosage required to
produce the desired sulfide reduction. Field applica-
tion rates have generally been 10 to 15 kg Ch/kg H?S
oxidized. Since laboratory data are not easily applied
to full-scale design due to variations in flow rate,
sulfide concentration, wastewater characteristics,
and mixing energy at the point of injection, flexibility
in feed rate is necessary. Ideally, full-scale studies
should be performed over a period of several weeks to
determine the effectiveness of chlorine in achieving
the specific objectives in sulfide control for the
locations of interest.
Provision of intense mixing at the point of chlorine
injection is critical for efficient chlorination. It is likely
that failure to provide adequate mixing in full-scale
applications accounts for the discrepancies between
laboratory and full-scale chlorine requirements. One
method used to achieve good mixing is to inject the
chlorine solution into a hydraulic fall of 0.3 m (1 ft) or
greater, or into a hydraulic jump. Poor mixing and
sluggish, shallow flows are likely to cause fuming
from localized chlorine overdose.
Chlorine is a hazardous material. Design of a chlo-
rination station must recognize the need to make
appropriate provisions for the safe handling and
storage of chlorine (29). Another concern which has
been raised is the potential formation of toxic or
carcinogenic chlorinated hydrocarbons during waste-
water chlorination. These factors should be consid-
ered in the evaluation of sulfide control alternatives.
3.3.1,3 Costs
Typical costs for a chlorine injection system are
presented in Table 3-16, These costs are budget level
55
-------
Tabls 3-16, Typical Costs for Chlorine Injection for H2S
Control (1984 $)
Condition
Capital Cost"
Annual Chemical
(Cl2 Costs')
$
$/yr
Flow = 3,785 mVd
[HjS] = 5 mg/l
Clj dose = 30 mg/l
18,000
14,000
Flaw = 37,850 mVd
[HzS] = 5 mg/l
Cl2 dose = 30 mg/l
40,000
137,000
'Includes concrete pad, chlorinator, booster pump, scale, piping,
safety equipment, start-slop controls, building and vaporizer
(larger system only), and installation.
bBased on typical cost of chlorine in 900 kg (1 ton) cylinders of
60.33/kg.
cost estimates (+30 percent;-15 percent). A simple
control scheme, consisting of chlorinator interlock
with pump starter circuits at a pump station, has been
assumed for simplicity. More sophisticated control
schemes, such as flow proportional or chlorine
residual control, were not considered appropriate for
application to a remote pumping station in a collection
system. Other simple control schemes, such as timer
control or preset cam control, would be equally
appropriate for such applications.
3.3.2 Hydrogen Peroxide
Hydrogen peroxide chemically oxidizes H2S according
to the following reactions:
pH < 8.5: H2O2 + H2S > S + 2H20 (3-10)
pH > 8.5: 4H2O2 + S= > SO; + 2H20 (3-11)
At pH <8.5, the stoichiometric H202 requirement is 1
g H2Oz/g HzS. In practice, a somewhat greater weight
ratio may be employed, depending on whether the
application is only for oxidizing existing sulfides or for
preventing additional sulfide formation. For the latter,
dosage rates will vary with BOD and temperature of
the wastewater, and with hydraulic characteristics of
the sewer.
The reaction of H202 with H2S is rapid. Figure 3-24
shows a typical reaction profile for H2O2 in waste-
water. Generally, 90 percent of the peroxide is
reacted within 10 to 15 minutes, with the reaction
completed in 20 to 30 additional minutes (31).
Hydrogen peroxide is commercially available as
solutions of 35-, 50-, and 70-percent H2O2 by weight.
The physical properties of hydrogen peroxide are
summarized in Table 3-17. Bulk shipments of hydro-
gen peroxide are as 50- or 70-percent H2O2 by weight.
For storage, the 70-percent solution is diluted to 50
percent, since at the higher concentrations, hydrogen
peroxide becomes more hazardous to handle. For
safety reasons, 50-percent solutions are typically
Figure 3-24, Typical reaction profile of hydrogen peroxide
in wastewater [31).
10-0
8.0
Weight Ratio of HjOj HjS = 1.5:1
v
ex
E
6.0
c
o
(0
Residual
Hydroge n Peroxide
I 4.0
c
o
o
Dissolved Oxygen
2.0
Total Sulfide
purchased for sulfide control applications in populated
areas. For small package installations, 230-kg (500-
lb) drums are purchased, while larger installations
rely on shipments in bulk by tank truck or tank car.
Storage tank capacities vary depending on anticipated
peroxide usage.
Hydrogen peroxide has certain advantages over other
sulfide control alternatives (31):
1. It can be used for either gravity sewer or force
main applications.
Table 3-17. Physical Properties of Hydrogen Peroxide
H2O2 Concentration
Parameter by Weight, Percent
35
50
70
Volume strength, 0"C and 1 atm
130
197
300
Active oxygen content.
percent by weight
16.5
23.5
32.9
Specific gravity, 20°C
1.13
1 20
1.29
Density, kg/m3 @ 20°C
1,126
1,198
1,294
Boiling point, °C
108
114
126
Freezing point, °C
-33
-52
-40
Viscosity at 25°C, centipoise
(mPa/s)
1.0
1,06
1.12
Refractive index, 25°C
1.355
1.366
1.381
Dielectric constant, 20°C
83
83
82
Total vapor pressure, 30°C (mm Hg)
23.3
18.3
10 1
Partial pressure af H2O2, 30°C
(mm Hg)
0.28
0.56
1.17
Heat of dilution, cal/g mole of
H2O2, 25°C and 1 atm
-84
-178
-381
Surface tension, dynes/cm, 20°C
74.6
75.6
77.3
Appearance: colorless, odorless liquid
SG
-------
2. The chemical feed system is relatively simple
and inexpensive.
3. The reactions with sulfide or other wastewater
constituents produce harmless by-products.
4. The decomposition of excess H2O2 results in
addition of DO to the stream.
5. With proper dosage, H2S generation will be
suppressed for 3 to 4 hours after H2O2 addition.
Use of hydrogen peroxide for sulfide control has been
successfully demonstrated at numerous locations in
the United States, the United Kingdom, Australia, and
elsewhere (32-41). Table 3-18 summarizes data from
five locations in the United States where hydrogen
peroxide has been used for sulfide control.
3.3.2.1 Equipment
Equipment required for hydrogen peroxide addition is
relatively simple, consisting of a storage vessel,
metering pumps, appropriate valving and transfer
piping, and injection nozzle. For small or intermittent
applications, a package dosing system as shown in
Figure 3-25 can be used. In this system, hydrogen
peroxide is withdrawn directly from containers de-
livered by the supplier. Figure 3-26 illustrates thB
design of a bulk H202 feed installation for a force
main. Such larger installations generally employ
pneumatically or electrically activated ball valves
operated in unison with the metering pump. The valve
is open only when the metering pump is operating,
preventing backflow of wastewater into the H202 feed
piping. A pressure relief valve is provided in the event
of failure of the ball valve in the closed position. In
addition, a check valve must also be used for
additional assurance against backflow of wastewater
and contamination of the H2O2 feed piping.
Because hydrogen peroxide is a strong oxidant,
specification of materials for storage and metering of
the chemical is important. Bulk storage tanks are
Figure 3-26. Typical package H2O a dosing Installation (32).
Delivery Tube
Support in Outlet Flexible or Rigid PVC Pipe
Check
Valve
Metering Pump
Chemical Container
Sewer
typically of high purity aluminum alloy construction.
Smaller drums, 230-kg (500-lb), are polyethylene-
lined steel, polyethylene with steel overpack, or self-
supporting plastic barrels. Aluminum, stainless steel,
or PVC is used in piping. Diaphragm-type metering
pumps are generally used, and must be constructed
of materials resistant to H2O2 exposure, such as
aluminum alloys and Teflon. Hydrogen peroxide is
very sensitive to contamination by many materials,
which may cause rapid degradation of the Hj02. A
field cleansing procedure known as passivation is
essential for new equipment installations, or where
subsequent maintenance work has potentially con-
taminated the storage or metering systems. Passiva-
tion consists of successive washings with a detergent,
a solvent, water, nitric acid, and water (38).
3.3.2.2 Design
Ratios (by mass) of peroxide addition to incoming
sulfides have typically ranged from 0.9 to over 3.0,
Table 3-1 8. Performance of HjOa for Sulfite Control in Wastewater Collection Systems"
Plant
Location
Average
Daily Flow
Untreated
Influent
HjS
Average
Hs02 Dose
Time - H2O2
Dosed Upstream
From Headworks
Residual HjS
Primary
Headworks Weirs
mVd
mg/l
mg/l
kg/d
mg/i
Fort Worth, TX
227,100
7.9
9.3
2,120
10-15 minutes
0.7
1.7
Orange County, CA
662,400
5.2
7,8
5.160
6-9 minutes
1.1
0.5
5.2
5,5
3,650
6-9 minutes
1.6
1.1
Baltimore, MD
673,700
0.95
2.0
1.360
4 minutes
NT"
0.5
673,700
1.47
3.0
2,020
4 minutes
NT6
0,7
Gainesville, FL
24,600
19.2
16
390
60 minutes
1.9
NT0
West Palm Beach, FL
45,400
11
15 total°
690
7 hours total c
1.0
NT"
'Data compiled from internal data and demonstration reports, Interox America, Houston, TX.
°NT: No testing was conducted al these points.
~2 dosing sites: 1) 7 hours upstream f'om plant influent; 2) 3.5 hours from plant influent.
57
-------
Figure 3-26. Typical bulk H ;>02 storago and injection system.
Continuous Floating
Sightglass yent Manwvay Cover
Dilution
Water Inlet
(Optional)
High Purity
Aluminum
Tank
Mixing
Eductor i
HjO
Fill Line
(Optional)
HjO? Discharge Valva
3m
Dike Area
/tl^ for Spill
_Safety Shower Containment
Eyewash Station
(a) Bulk Storage System
9 A
washdown
Water Hose
50% HaOi Irom Storage
Access Manhole
4" Sch. 00 PVC Sleeve
Metering
Pump
<0
Dilution Water
A
or 1" Polyethylene Tubing, 250 psi
Force Main
Ihemieal
Application
Point
A Block Valve
B Back Pressure Vallve
C Pressure Relief Valve
D Check Valve
E Backflow Preventer
(t>) Injection System
depending on the objectives for sulfide control
(removal of existing sulfides or prevention of sulfide
buildup), wastewater BOD, and pipe characteristics.
Because of the variations in flow and incoming
sulfide mass, metering pump capacities should be
selected to provide adequate flexibility in feed rates. It
is good practice to specify a pump having a maximum
discharge of twice the anticipated feed rate, and to
specify a pump adjustable over a range of 10 to 100
percent of its maximum capacity(37). Multiple pumps
integrated into a manifold system can be operated on
a simple timer circuit to increase feed rate during
periods of high influent sulfides.
In order to properly specify the number and capacity
of metering pumps, it is necessary to thoroughly
characterize the diurnal flow rates and fluctuations in
mass of sulfides in the system. It is prudent to
optimize dosages in order to minimize chemical costs.
Mass sulfide profiles similar to that shown in Figure
2-16 shou'd be developed for each sewer reach
where sulfides are to be controlled.
Laboratory tests should first be conducted to generate
an initial estimate of the kinetic rate of the Hz02
reaction with sulfide. The reaction rate will be
dependent on the wastewater characteristics and
iron content. Testing is necessary to estimate the
time required for reaction completion in order to
select a suitable injection point.
In some cases, H2O2 suppliers will provide a range of
professional services, including pilot studies, start-up
services, and post construction monitoring. In addi-
tion, arrangements can be made for lease of equip-
ment rather than direct purchase.
Adequate safety precautions must be observed in the
handling of hydrogen peroxide. Protective clothing,
including face shields, must be worn during bulk
storage filling or during repair or maintenance work
where contact or spillage might occur. Any spills
should be immediately washed down with water to
prevent spontaneous combustion of organic materials
in the presence of H2O2. Emergency eye wash and
shower facilities are necessary for maintenance
personnel.
3.3.2.3 Costs
Typical costs of hydrogen peroxide injection systems
are shown in Table 3-19. These are budget level
estimates (+30 percent; -15 percent). It is assumed in
this case that all the necessary equipment is pur-
chased by the user. In practice, however, it is often
economically advantageous to lease the equipment
from a supplier. Some suppliers offer design, start-up
and monitoring services for the municipality. Such
organizations should be consulted in determining the
cost effectiveness of hydrogen peroxide injection for
sulfide control.
Table 3-19. Typical Costs lor Hydrogen Peroxide Injection
for HzS Control (1984 $)
Condition
Capital Cost*
Annual Chemical
!HjOs) Costs
9
S/yr
Flow = 3,785 mVd
[HaS] = 5 mg/l
HjOj dosa = 10 mg/lb
25,000
21,000c
Flow = 37,850 mVd
IH*S] = 6 mg/l
H2O2 dose = 10 mg/l
50,000
189,000d
Includes concrete pad, HjOj storage tank, metering pumps,
piping, valves, timer controls, safety equipment, and installation.
"Dosage to control 5 mg/l H2S at dosing station plus 2 mg/l HaS
formed downstream {2-hr detention).
Uased on H2O2 cost of 81,50/kg as 100-percent HiOi,
dBased on HjOj cost of S1.36/kg as 100-percent HjOj,
58
-------
3.3.3 Metal Salts
The salts of many metals will react with dissolved
sulfide to form metallic sulfide precipitates, thus
preventing HZS release to the atmosphere. For
effective removal of dissolved sulfides, the metallic
sulfide formed must be highly insoluble.
Iron salts have been used for sulfide control at several
locations in the United States (42). The ferrous ion
reacts with sulfide as shown below:
Fe++ + HS~ > FeS + H+ (3-12)
Pomeroy found that the reaction of a mixture of iron
salts with a molecular ratio of one part ferrous to two
parts ferric was superior for sulfide control compared
to the reaction of either one alone(27). The reaction of
the mixed iron salts was hypothesized to occur as
follows:
Fe++ + 2 Fe~ + 4HS~ > Fe3S4 + 4H* (3-13)
Zinc salts have also been used for sulfide control. Zinc
sulfide is much less soluble than iron sulfide, allowing
theoretical reductions of sulfide concentrations to
less than 0.1 mg/l. The effects of adding zinc solutions
to reaches of force mains and gravity sewers have
been studied. It was concluded that 10 to 15 parts of
zinc would be required for every part of sulfide
removed (3). The stoichiometric requirement is
approximately 2:1. The city of Los Angeles, California,
used zinc for sulfide control in a large trunk sewer.
Solutions were prepared by dissolving scrap zinc in
waste acid. This practice has since been discontinued
(14).
Other metals, such as lead and copper, could also be
used for sulfide control. However, the high costs and
possible detrimental impact on downstream biological
treatment processes generally eliminate their use
from further consideration.
A mixture of ferric sulfate and nitric acid was
successful in controlling sulfides in a force main in
the United Kingdom, and was found to be more cost-
effective than hydrogen peroxide or sodium nitrate
addition. The mixture had a strength equivalent to
42,000 mg/l nitrate nitrogen and 125,000 mg/l
ferric iron (43).
Of all the metal salts which have been or are being
used for sulfide control, ferrous sulfate is the most
common. One commercially available FeS04 solution
is derived from the manufacture of titanium dioxide,
which results in the production of ferrous sulfate
crystal, and is sold for both sulfide control and
phosphorus removal. Waste pickle liquor, resulting
from the reaction of scrap iron with sulfuric acid, has
also been used. However, such products may contain
a high free acid content, which may result in
detrimental impacts on wastewater pH and alkalinity.
Results from use of FeS04 for sulfide control are
shown in Table 3-20.
Table 3-20. Performance of FeSOj Addition for Sulfide
Control (42)
Location
Average
Wastewater
Flow
Average
FeSO<
Dosage9
Dissolved
HzS
mVd
mg/l
mg/S
Clearwater, FL
18,900
0
6 to 8
30
<1
Naples, FL
20,820
0
5 to 20
25
1 to 2
Boyton Beach, FL
45,420
0
10
25
0.5 to 2,0
"FeSOi solution used was a proprietary product derived from the
manufacture of titanium dioxide, resulting in ferrous sulfate
crystal.
3.3.3.1 Equipment
Equipment required for a FeS04 feed system is
simple, and includes a storage tank, chemical meter-
ing pumps, piping and valves, control system, and
injector. Materials suitable for a FeS04 storage tank
include: 1) polyolefin; 2) concrete lined with poly-
urethane; 3) fiberglass; 4) steel lined with rubber; or
5) stainless steel.
Ferrous sulfate is only mildly corrosive, but safety
precautions must be observed in its handling.
3.3.3.2 Design
The overall reaction of FeS04 with HgS can be
expressed as:
FeS04 + H2S > FeS + H2S04 (3-14)
Based on this reaction, removal of 1 g of HzS would
require approximately 4.5 g of FeSO* (1.6 g as Fe).
Actual dosage requirements for a particular condition
are determined by field application.
3.3.3.3 Costs
Typical costs for ferrous sulfate injection systems are
shown in Table 3-21. Due to variability in site
conditions, injection points, etc., capital costs are
budget level estimates (+30 percent, -15 percent).
Table 3-21. Typical Coat* for F*SO< Injection for H iS Control
(1984 «)
Annual Chemical
Condition Capital Cost* [FeS04 Costsb
$ $/yr
Flow = 3,785 mVd
¦ [HjS] = 5 mg/1 10,000 13,000
FaS04 dose = 23 mg/l
Flow = 37,850 mVd
[HbS] = 5 mg/l 21,000 130,000
FbSO< dose = 23 mg/l
Includes concrete pad, storage tank, metering pumps, piping,
valves, controls, and installation.
bBased on typical cost of FeS04 of $0,67/liter for solution
containing 163 g/l FbSO*.
59
-------
3.3.4 Nitrate
Nitrate can be added as a supplemental oxygen
source to control sulfides in wastewater collection
systems. Certain bacteria can utilize nitrate as an
oxygen source during the biochemical reduction of
nitrate to nitrogen gas under anaerobic conditions.
Bacteria will utilize hydrogen acceptors preferentially
inthe order: 1) O2; 2) NOj; and3)SO<. Thus, if nitrate
is present, no sulfate reduction will occur until all of
the nitrate has been reduced.
Laboratory tests conducted to show the inhibitory
effect of nitrate addition on sulfide production con-
cluded that addition of sodium nitrate to furnish
enough oxygen to satisfy 50 percent of the 5-day BOD
gave complete protection against odors (44).
Full-scale tests have been conducted on the use of
sodium nitrate to control sulfide buildup (3). Several
key observations resulted from these tests. First, a
population of nitrate-reducing bacteria hadto become
established in the slime layer. This required several
days of nitrate addition before the sewer became
"conditioned." Second, nitrate reaction was incom-
plete, with only about half of the nitrate reacting, the
remainder showing up in the force main discharge.
Figure 3-27 shows the effect of nitrate dosage on
sulfide buildup in a force main. In general, about 10
parts of nitrate were required for every part of sulfide
eliminated. At the point of nitrate addition, total
sulfide averaged 0.59 mg/l. Before nitrate addition,
total sulfide concentration at the force main discharge
was 4-5 mg/l. Addition of 27 mg/l of nitrate reduced
sulfide in the force main discharge to about 2 mg/l,
while dosages of 50 mg/l or greater reduced sulfides
to approximately 1 mg/l. It was concluded that nitrate
addition has limited utility in controlling sulfides in
wastewater collection systems, and may be of
practical value only when sulfide concentrations are
high (3).
Figure 3-27. Effects of sodium nitrate on sulfide generation
in Bluff Cove force main, Los Angeles, CA(3).
Avg. Temp, = 23°C
Detention Time = 90 min
Avg. total sulfide at pump sta. = 0,69 mg/
+2.0
70
0
20 30 40 50
10
60
Sodium Nitrate Dosage, mg/l
Nitrate addition can be effected by a simple low-cost
system consisting of a storage vessel, metering
pump, piping, valves, and appurtenances. For a
solution containing 240 g/l (2 lb/gal) of nitrate, the
cost is approximately $0.13/1 ($0.50/gal).
Nitrate addition has been successfully used for odor
control in anaerobic lagoons, trickling filters, and
carbon columns (45-48). Use of nitrate for odor
control at existing wastewater treatment facilities is
discussed in Chapter 4,
3.3.5 Strong Alkalies
Increasing the pH reduces the proportion of dissolved
H2S in the H2S - HS~ equilibrium. For example, at a pH
of 7.0, equal concentrations of dissolved HZS and HS"
exist at equilibrium, while at a pH of 8.0, only about 10
percent of the dissolved sulfide exists as H2S. Since
dissolved HjS is the only form which can be released
to the atmosphere, it follows that increasing the pH
would reduce odors and corrosion by maintaining the
dissolved sulfides in the HS" form.
Continuous addition of strong alkalies for mainte-
nance of a high pH is generally not practical in
collection systems. Tributary flows and production of
C02 and organic acids from biological action will tend
to lower the pH. A drop of 0.5 pH units could result in
substantial release of H2S to the atmosphere.
Both sodium hydroxide and lime have been used for
shock dosing of sewers in an attempt to inactivate the
sulfide-generating slime layer (3) (14). Figure 3-28
shows the recovery of sulfide buildup capacity with
time after shock dosing with sodium hydroxide
(NaOH). Depending on the NaOH dosage and the
initial pH achieved, times for recovery to 100 percent
of normal sulfide buildup ranged from several days to
two weeks. The one run at pH 13.2 should not be
considered conclusive, since it was not supported by
confirmatory observations (3). No detrimental down-
stream effects were observed with this practice.
However, use of large quantities of lime could result
in accumulation of calcium carbonate incrustations
on the pipe.
Although shock addition of strong alkalies has been
shown to be effective in temporary inactivation of the
slime layer, continuous addition to prevent H2S
release would not appear to be practical due to
potential downstream pH depression from 1) biochem-
ical production of organic acids and C02, and 2)
tributary flows of normal pH.
3.3.6 Potassium Permanganate
Potassium permanganate is a strong oxidizing agent,
and reacts with H2S according to the following:
Acidic
conditions: 3 HZS + 2 KMnO« 5*
3 S + 2 H20 + 2 KOH + 2 Mn02 (3-15)
60
-------
Figure 3-28. Impact of shock dosing with NaOH on recovery of sulfide buildup capacity. Los Angeles. CA (3).
•Approximate Equivalent Concentration of NaOH, mg/l
pH 10.5
1 run
pH 12 0-12 5
Avg, 6 runs
pH 12.5-13.2
Avg. 4 runs
pH 13 2
1 run
6 8 10
Elapsed Time Afler Treatment, days
Aklaline
conditions: 3 H2S + 8 KMnO* >
3 K2SO4 + 2 H20 + 2 KOH + 8 Mn02 (3-16)
Several reactions ranging between these extremes
may take place, yielding not only elemental sulfur
and/or sulfate, but also thionates, dithionates, and
manganese sulfide as possible end products. The
actual reaction will depend on specific conditions.
Therefore, dosages are case-specific and difficult to
predict.
Potassium permanganate has been employed for
sulfide control in collection systems and lift stations,
generally for small or intermittent applications. In
practice, six to seven parts of KMnO«are required per
part of sulfide to be oxidized. The relatively high cost
of the chemical, $2.26/kg ($1 03/lb), makes it
economically unattractive for continuously treating
wastewater streams at high flow rates or wastewater
streams that contain substantial concentrations of
sulfide. For example, given a flow rate of 378 m3/d(1
mgd) of wastewater containing 5 mg/l of dissolved
sulfide, permanganate requirements based on 6:1
KMnO^HzS would be approximately 110 kg (250
lb)/d. Costs for chemicals alone would be in excess of
$90,000/year, Metering equipment, however, is
simple and easily installed. Total installed cost of a
potassium permanganate metering system with a
capacity of 45 kg (100 lb)/d is approximately $ 10,000.
3.4 Case Histories
3.4.1 Force Main Aeration, Gloucester County
Utilities Authority {GCUA), Woodbury. New Jersey
For several years, odor problems were noted at the
GCUA Wastewater Treatment Plant and at pump
stations throughout the collection system. The GCUA
undertook an extensive testing program to develop
force main aeration design criteria at its Westville
pump station, and to establish the cost effectiveness
of force main aeration plus HjOa addition compared to
the addition of H2Oz alone (6)(16).
Analysis of air addition to the Westville force main
included development of design parameters, includ-
ing minimum and maximum detention times; aeration
efficiency; pressure relief valve losses; oxygen deple-
tion rates; and operational costs of aeration vs. the
addition of hydrogen peroxide.
61
-------
During the testing program, the average flow to the
pump station was 1,385 m3/d (0.37 mgd). The pump
Station was originally equipped with constant speed
pumps which were later converted to variable speed
pumps with a maximum pumping capacity of 5,336
m3/d (1.41 mgd) and a peak discharge pressure of
510 kPa (74 psig). The Westville force main has a
storage volume of 177 m3 (46,800 gallons) that
results in a detention time of 3.12 hours at the
minimum flow of 1,870 mVd (0.36 mgd).
Oxygen depletion data from the Westville test main
are presented in Table 3-22.
Table 3-22. DO Depletion in Raw Wastewater Westville
Force Main, Woodbury, NJ (16)
Time
DO
hr
mg/l
0
26.5
1
25.4
2
23.2
3
21.2
4
19 1
5
15.3
6
8.1
7
0.3
8
0
The design air supply rate for the Westville pump
station was based on respirometric data that indicated
an oxygen demand of 7-12 mg/l-hr, which resulted
in a design injection rate of 37.4 mg/l. The air
compressor was sized to deliver a maximum of 50
mg/l DO with an assumed 40-percent oxygen transfer
efficiency within the force main. This resulted in a
design compressor capacity of 0.44 m3 (15.4 cu
ft)/min. For the Westville pump station, dual 3-HP
830 rpm compressors were selected, each with a
capacity of 0,25 to 0.28 m3 (9 to 10 cu ft)/min at a
receiver pressure of 655 kPa (95 psig). Each com-
pressor had a duty cycle of 0.49.
In the Westville installation, the compressor controls
were installed for both start-stop and dual-mode
operation. In the start-stop cycle, the compressor
operates only when the pump is on. In the dual-mode
cycle, the compressor operates based on a low-
pressure switch in the receiver tank. When the
receiver high-pressure switch is reached, the com-
pressor continues to operate if the pump is on until
the pump cycle is over. If the pump is not on, the
compressor shuts down after operating for a preset
time. Time delays are also provided in this design to
delay air injection after pump starting.
The air injection rates for the Westville installation
were 0.08 to 0.4 m3 (3 to 14 cu ft)/min. Without
aeration, the Westville force main discharged a dark
odorous wastewater with a dissolved sulfide concen-
tration of approximately 0.6 mg/l. Background sulfate
concentrations were 40 to 80 mg/l. The results of the
force main aeration testing are presented in Table
3-23.
Table 3-23. Results of Air Injection into Westville Force
Main, Woodbury, NJ (6)
Force Main Discharge
Air Feed
Oxygen
Injected
DO
Dissolved*
Sulfide
pH
HjS
mVmin
mg/l
mg/l
mg/l
mg/l
0.08
18.0
0.0
-
-
0.1
0.10
21.0
0.7
1.3
-
0.6
0.11
24.0
0.0
0.6
-
0.26
0.13
27.0
0.15
-
6.8
-
0.14
30.0
0.0
0.2
-
0.09
0.14
30.0
-
0.0
7.5
0.0
0.17
36.0
1.0
0.4
6.7
0.2
0.22
47.0
1.0
0.0
6.5
0.0
0.23
50.0
1.4
0.2
6.5
0.12
0.24
50.0
1.6
0.1
6.7
0.05
.20 to 0.40 42.0 to 84.0
-
0.3 to 3.0
6.8
0.13 to 3.0
•Concentrations prior to aeration were typically 0.6 mg/l.
The results of the preceding tests indicate that air
injection into the force main at rates of 0.08 to 0.4 m3
(3 to 14 cu ft)/min successfully reduced sulfide to
levels as low as 0,05 mg/l, and provided DO levels of
up to 1.6 mg/l.
The average design aeration rate for the Westville
force main was 0,17 m3 (6 cu ft)/min, which was
approximately three times the theoretical aeration
rate based on the oxygen demand of the wastewater.
This was due to mixing efficiency in the force main
and the loss of air through air relief valves. In the
Westville force main, air losses through the relief
valves were determined to be 30 percent with the
valves operating at 414 kPa (60 psig) and a receiver
pressure of 655 kPa (95 psig) (16).
The installed cost of the Westville force main aeration
system was $13,500 (1979) with an estimated
operation and maintenance cost of $750/year. For a
20-year design lifetime and an interest rate of 8
percent, the total amortized cost was $3,025/year.
The use of air injection was found to reduce hydrogen
peroxide addition by 70 percent, which amounted to
an annual savings of $7,770 in hydrogen peroxide
costs.
A comparison of the annual costs for peroxide addition
alone and peroxide addition with force main aeration
is presented in Table 3-24 for the Westville pump
station. This comparison indicates a net savings of
$4,780/yr for force main aeration plus hydrogen
peroxide addition over hydrogen peroxide addition
alone at the Westville pump station.
62
-------
Table 3-24. Costs for Sulfide Control in Westville Force Main Using HsOs and H^Oz with Air Injection, Woodbury, NJ (G)
H2O2 Aeralion Total
Without With Without With
Aeration* Aeration" Capital" Electrical Maintenance Aeration Aeration
$/yr 9/yr S/yr
11,100 3,330 1,375 865 750 11,100 6,320
*Hb02 @ 50.083/kg.
"Amortized capital cost assuming installed cost of $13,500 (1979), 20-year life, 8-percent interest.
'Electrical energy cost = $0.04/kWh.
3.4.2 Oxygen Injection. Delta Diablo Sanitation
District 7A, Antioch, California
In 1976, an Environmental Impact Report compiled by
the East/Central Contra Costa County Wastewater
Management Agency concluded that the existing
treatment facilities for the cities of Pittsburg and
Antioch, California, and the unincorporated commun-
ity of West Pittsburg, California, could not meet EPA
discharge requirements, and that a new regional
facility would be required. Design was begun in 1976,
and the new treatment facility went on-line in 1982.
The three existing plants were abandoned, and
wastewater from these communities was conveyed
through a series of new force mains to the new
36,000-mVd (9.5-mgd) trickling-fiIter, activated-
sludge secondary treatment plant (49).
Because the length of the force mains was as much
as 11.3 km (7 miles), substantial sulfide generation
was anticipated. Evaluation of sulfide control alterna-
tives led to the selection of pure oxygen injection into
the force mains at three locations to maintain
sufficient dissolved oxygen to prevent generation of
sulfides. Figure 3-29 is a map of the sewer service
area showing locations of the pump stations where
pure oxygen is injected into the force mains. The new
force mains are coated steel, cathodically protected to
minimize electrochemical corrosion. Pipe sizes and
wastewater flows for the three force mains where
oxygen is injected are presented in Table 3-25.
Figure 3-29. Sewer service area. Delta Diablo Sanitation
District 7A, California.
Suisun Bay
Outfall
|to|)(wistP^r£) Plttsbura
Antioch
O Wastewater Treatment Plant
~ Pump Station
Force Main
Table 3-25. Force Main Characteristics at Pure Oxygen
Injection Points, Antioch, CA
Location
Pipe Diameter
Design Flow
Actual Flow
cm
m3/d
mJ/d
Antioch
61
20,820
15,140
Pittsburg
61
32,360
7,570
Shore Acres
41
14,570
1,890
Total flows to the plant were approximately 30,280
m3/d (8.0 mgd) in the winter and 28,390 m3/d (7.5
mgd) in the summer of 1983. The plant is presently
being expanded to a capacity of 47,690 m3/d (12.6
mgd). The target level of dissolved sulfide entering the
plant is 0.1 mg/l.
Total sulfides entering the plant before oxygen
injection was initiated were 3.0 to 6.0 mg/l. Dissolved
sulfides were typically 1.5 to 5.0 mg/l.
Oxygen injection rates are automatically varied in
proportion to the flow. At the Pittsburg pump station
oxygen is injected at the discharge side ofthepumpat
an average rate of 490 kg (1,080 lb)/d. The oxygen
feed system went on-line in October, 1981. At the
Antioch pump station, average oxygen injection rates
are 345 kg (760 lb)/d in the winter and 605 kg (1,330
lb)/d in the summer. The oxygen injection system
was commissioned in 1979. Because ofthe low flows
in the Shore Acres force main, refilling of the pure
oxygen storage tank has not yet been required since
commissioning in October 1981. Thus, the tank has
not been calibrated to allow estimation of oxygen
injection rates at that site.
Raw wastewater entering the plant is analyzed for
sulfides on the average of three times per week. If
sulfides begin to exceed target levels of 0.1 mg/l, two
options can be initiated—increase the oxygen dosage
or clean the line of accumulated slimes by the use of a
"pig." Pig launch sites were designed into the force
main system to facilitate line cleaning. In the Antioch
force main, actual oxygen requirements to maintain
aerobic conditions sometimes exceed the capacity of
the oxygen injection unit, particularly in the summer.
Periodic cleaning ofthe line (as much as once every 3
63
-------
weeks in the summer) has been found to be very
effective in reducing sulfides to acceptable levels at
the end of the main. This procedure requires approxi-
mately4 hours of operational staff timeper cleaning.
Oxygen injection into the three force mains in the
Delta Diablo Sanitation District has been successful
in controlling sulfides in the collection system and at
the headworks of the treatment plant. Total sulfides
are 0.0 to 1.5 mg/l at manholes just upstream from
the headworks. Dissolved sulfides entering the plant
are 0.0 to 0.7 mg/l during oxygen injection. Typical
dissolved H2S (gas phase) levels are 0.0 to 0.1,
depending on pH.
Total installed cost for the three oxygen injection
systems was approximately $ 18,000, broken down as
follows: Antioch—$8,000 (1979), Pittsburg—$5,000
(1981), and Shore Acres—$5,000 (1981). As the
oxygen storage vessel and evaporator are leased from
oxygen suppliers, these costs include concrete pad,
piping, valves, controls, appurtenances, and instal-
lation. Annual operational costs for the three systems
amounted to approximately S31,200 in 1983. Of this
figure, about $20,000 was expended for rental of
equipment from the oxygen supplier, the remainder
for purchase and delivery of liquid oxygen. Any
maintenance problems with the oxygen storage
vessel and evaporator are the responsibility of the
oxygen supplier. According to plant operational staff,
any such problems are promptly attended to by the
supplier.
Occasionally, odors are generated from the four
trickling-filter towers. These units are 6.4-m (21-ft)
deep employing modular plastic media. Forced draft
ventilation by reversible fans was included in the
design. Under normal conditions, the trickling filters
are naturally ventilated. However, during certain
times of the year when the temperature differential
between the ambient air and the wastewater is
insufficient to induce a natural draft, the ventilating
fans are brought on-line in an attempt to control odor
emissions. Air quality surrounding the plant and
pump stations in the collection systems is monitored
regularly by the State Air Quality Regulatory Agency.
A planned residential housing development in close
proximity to the plant may require greater control of
odor emissions in the future.
3.4.3 Hydrogen Peroxide Addition. Palm Beach
County. Florida
In 1979, the county of Palm Beach, Florida, began
operation of new wastewater transmission facilities
and regional treatment plant to serve the east-central
region of the county. The collection system consisted
of 39 km (42 mi) of force mains, ranging in diameter
from 10 cm (4 in) to 91 cm (36 in), and 25 lift stations.
Almost immediately upon commissioning the system,
the county began experiencing severe concentrations
of H2S around the pump stations which caused
numerous citizen complaints. Attempts at controlling
sulfide included chlorine addition, air injection, and
ferrous sulfate addition. None of these techniques
proved satisfactory for the reasons cited (34):
• Chlorine - reacted too fast, odor problems
• Air - poor dissolution, air binding
• Ferrous sulfate - did not provide needed residual
DO
The failure of the preceding alternatives to provide
satisfactory HZS control, in conjunction with court
action by affected citizens, led the county to investi-
gate use of hydrogen peroxide for sulfide control,
A diagram of the key components of the collection
system is presented in Figure 3-30. This figure
indicates length and diameters of force mains,
location of pumping stations, and locations of H202
dosing stations. In general, dosing stations were
located 20 to 40 min (wastewater travel time)
upstream of the respective pump stations to provide
sufficient reaction time.
Dates of commissioning of the H2O2 dosing stations
are shown below:
Pump Station Commissioning Date
241 June,1980
229 June, 1980
236 June, 1981
15 August, 1982
204 December, 1982
56 December, 1982
Few data are available indicating H2S levels prior to
H2O2 dosing. Figure 3-30 presents calculated sulfide
buildup in each force main reach using the Pomeroy
and Parkhurst predictive equation, assuming an
average annual wastewater temperature of 28°C
(82°F) and an average BOD of 150 mg/l. Generally,
predicated sulfide levels were in the range of 2 to 15
mg/l.
Table 3-26 summarizes the average wastewater
flows and H202 dosing rates at each of the six pump
stations, and gives measured HjS concentrations at
these locations after addition of H2O2. As can be seen
from the data, injection of H202 was effective in
reducing H2S concentrations in the wastewater to
below 0.5 mg/l at all six pump stations. In addition,
further sulfide generation was inhibited by maintain-
ing DO levels greater than 1.0 mg/l. Table 3-26
indicates average annual dosing rates. In practice,
dosages were varied depending on the rate of sulfide
generation as affected by temperature, flow rates,
wastewater characteristics, etc. Figure 3-31 shows
wastewater flows and temperatures and H2O2 usage
from January, 1982 through March, 1983 (50). Note
64
-------
Figure 3-30.
Wastewater collection system with Hj02 dosing stations. Palm Beach County, FL,
To West Palm Beach WWTP
54" FM
241
15 mgd
(4.8 mg/
10,560'
46" FM
2.1 hr
Sb = 2.2 mg/l
11.5 mgd
229
i7 3 mg/l
9 mgd
4,200*
30" FM
8,980'—42" FM
236
1.75 hr S0 - 1 9 mg/l
2.3 hr
Sa = 4,8 mg/l
.8.4 mg/
1 3 mg/l
12,800'
30" FM
7.0 hr
14.6 mg/l
14,780'
42" FM
227
201
6,400'
24" FM
2.3 hr
SB = 4.8 mg/l
204
2.3 mgd
Pumping Station
19 mg/l
HsOg Dosing Station
7,700'
30" FM
4.0 hr
Sb = 7,4 mg/l
The calculated sulfide buildup
in each force main; it is not
cumulative
5,700'
30" FM
265
___ 5.0 hr
1 mgd Sa = 8.6 mg/l
10,600' 4.4 hr
FM Force Main
Detention Time
0.86 mgd
.40 mg/!
that the greatest rate of Hz02 usage generally
corresponds to periods of high temperatures and low
wastewater flows.
The first dosing stations weretemporary installations,
and were commissioned under a leasing arrangement
with a supplier by which the cost of the dosing
stations was incorporated into the unit cost of
purchased H2Oj. This cost also covered all start-up
and monitoring costs by the supplier. Palm Beach
County has since constructed permanent dosing
stations.
65
-------
Table 3-26. Effectiveness ot H2O2 for Sulfide Control at
Palm Beach County, FL (50}
Pump
Dosing*
HaS After*
DO After*
Station
Flow*
Rate
HjO 2 Addition
H2O2 Addition
mVd
mg/l
mg/l
mg/l
15
1,890
40.0
0.5
3.3
204
6.430
19.0
0.4
3,4
56
6,430
8.4
0.1
4.5
236
33,310
13.0
0.4
1.4
229
41,640
7.3
0.2
2.3
241
45,420
4.8
0.1
1.4
'Average values.
Figure 3-31. Average wastewater temperature, flow and
HzOz used at Lift Station No. 229, Palm Beach
County, FL (50).
c
a.
8
r- "O
. O) U.
C o
1 c
30 -30 -8S
-20
20
-81
Flow
77
10
L73
J FMAMJ JASOND J FM
1932 1 M983
3.5 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1. Thistlethwayte, D.K.B. The Control of Sulphides
in Sewerage Systems. Ann Arbor Science
Publishers, Inc., Ann Arbor, Ml, 1972.
2. Sewell, R.J. Sulfide Control in Sanitary Sewers
Using Air and Oxygen. NTIS No. PB-243894,
U.S. Environmental Protection Agency, Cincin-
nati, OH, 1975.
3. Pomeroy, R.D., J.D. Parkhurst, J. Livingston,
and H.H Bailey. Sulfide Occurrence and Control
in Sewage Collection Systems. U.S. Environ-
mental Protection Agency, EPA 600/X-85-052,
Cincinnati, OH, 1985.
4. Condon, R.L., R.A. Cooper, and A.J. Englande.
Instream Aeration to Control Dissolved Sulfides
in Sanitary Sewers. NTIS No. 223342, U.S.
Environmental Protection Agency, Cincinnati,
OH, 1973.
5. Laughlin, J.A. Studies in Force Main Aeration.
Journal ASCE-SED 90 (SA 6): 13-24, 1964.
6. Vivona, M.A. and G.W. Whalen, Controlling
Sulfides and Odors in Sewers - Part 1. Public
Works 113(3): 73-76, 1982.
7. Control of Odors and Corrosion in the Sacra-
mento Regional Wastewater Conveyance Sys-
tem. Sacramento Area Consultants, September,
1976.
8. Mullins, W. H. Aerators Control Lift Station Odor
and Corrosion. Water and Sewage Works 1 24
(3): 75, 1977.
9. Hydro-Vac Jet Wastewater Conditioner. Bulletin
No. HVC 8-84, Hydro-Vac, Inc., Port Arthur, TX,
1984.
10. Mitchell, R.C. U-Tube Aeration. NTIS No. PB-
228127, U.S. Environmental Protection Agency,
Cincinnati, OH, 1973.
11. Vivona, M.A. Designing for Force Main Odor
' Control. Public Works 111 (7): 74-76, 1980.
12. Aeration in Wastewater Treatment. Manual of
Practice No. 5, Water Pollution Control Federa-
tion, Washington, DC, 1 971.
13. Speece, R.E., J.W. Eheart. and C.A. Givler. U-
Tube Aeration Sensitivity to Design Parameters.
JWPCF 55 (8): 1,065-1,069, 1983.
14. Process Design Manual for Sulfide Control in
Sanitary Sewerage Systems. NTIS No. PB-
260479, U.S. Environmental Protection Agency,
October, 1 974.
15. Vivona, M.A. Force Main Odor Control by Air
Injection. Public Works 110(12): 70-72, 1979.
16. Vivona, M.A. and G.A. Whalen. Controlling
Sulfides and Odors in Sewers - Part Two. Public
Works 113 (4): 69-72, 1982.
17. Shaw, R.G. Avoiding Gas Locked Pumps When
Sewer Sweetening by Oxygen Injection. Com-
monwealth Industrial Gases, Enviroshietd In-
terchange Index No. 1, 108, Sydney, Australia,
1978.
66
-------
18. Speece, H.E. and R. Orosco. Design of U-Tube
Aeration Systems. Journal ASCE-SED 96 (SA
3): 715-725, 1970.
19. Hollerbach, G.H. The Use of Molecular Oxygen
for Hydrogen S ulfide Control in Sanitary Sewage
Systems. Presented at 56th Annual Conference
of the WPCF, Atlanta, GA, October 4, 1983.
20. Cocksey, J., and W.B. Norgrove. Control of
Hydrogen Sulfide at the Howdon Sewage Treat-
ment Works. Presented at the Regional Meeting
of the Institution of Water Engineers and
Scientists, Durham, England, September 21,
1979.
21. Commonwealth Industrial Gases, Ltd. Dissolv-
ing Techniques for Pumped and Gravity Sewers.
Sydney, Australia, 1980.
22. Kite, O.A., and M,E. Garrett. Oxygen Transfer
and Its Measurement. Journal ofthelnstituteof
Water Pollution Control 82 (1), 1983.
23. Airco Industrial Gases, Inc. Industrial Gases
Data Book, 6th Edition. Murray Hill, NJ, 1982.
24. Aldred, M.I., and B.G. Eagles. Hydrogen Sulphide
Corrosion of the Baghdad Trunk Sewerage
System. Water Pollution Control 81 (1): 80-93,
1982.
25. Boon, A.G., and A.R. Lister. Formation of
Sulphide in Rising Main Sewers and Its Preven-
tion by injection of Oxygen. Progress in Water
Technology 7 (2), 1975.
26. Sawyer, C.N. and P.L. McCarty. Chemistry for
Sanitary Engineers. Second Edition, McGraw-
Hill, New York, NY, 1967.
27. Pomeroy, R.D. and F.D. Bowlus. Progress Report
on Sulfide Control Research. Sewage Works
Journal 18(4): 597-640, 1946.
28. Baker, J.L. Study of Hydrogen Sulfide Control in
Wastewater Force Mains by Chlorination. Report
to Broward County Utilities Division, Broward
Co., FL, 1979.
29. Chlorination of Wastewater. Manual of Practice
No. 4, Water Pollution Control Federation,
Washington, DC, 1976,
30. Design and Construction of Sanitary and Storm
Sewers. Manual of Practice No. 9, Water
Pollution Control Federation, Washington, DC,
1969.
31. Duggan, S.W. A Perspective on Hydrogen
Sulfide in Sewers. Presented at New Jersey
Water Pollution Control Association Technology
Transfer Seminar, November 19, 1980.
32. Sims, A.F.E. Odor Control with Hydrogen Per-
oxide. Progress in Water Technology 12 (5):
609-620, 1980.
33. Anon. Decentralize Your Odor Control. Water
and Sewage Works 121 (10): 60-62, 1974.
34. Anon. Technology and Teamwork Key to Palm
Beach Odor Control. The Overflow, Florida
Water and Pollution Control Operators' Assn.,
September-October, 1982,
35. Matthews, D.G. Hydrogen Peroxide in Collection
System Odor Control. WPCF Deeds and Data,
April 1977.
36. Newton, L.C. Peroxide Stops Odor and Corrosion
in Beachfront Force Main. Pollution E ngi neeri ng
12(7): 36-37,1980.
37. Shepherd, J.A. and M.F, Hobbs, Control of
Hydrogen Sulfide With Hydrogen Peroxide.
Water and Sewage Works 120(8): 67-71,1973.
38. Burgh, J.A. and A.N. Gaume. Attacking Odors
WithHiOz Water Engineering and Management
129(11): 26 28, 1982.
39. Lindstrom, S R. Hydrogen Peroxide Solves
Hydrogen Sulfide Problem. Pollution Engineer-
ing 7 (10): 40-41, 1975.
40. Hydrogen Peroxide Demonstration Report,
Orange County Sanitation Districts. Interox
America, Houston, TX, 1982.
41. Hydrogen Peroxide Demonstration Report, City
of Baltimore. Interox America, Houston, TX,
1982.
42. Davis, H.F, J.P. Harshman, and T.P. Powers.
Techniques for Odor Control. Presented at Expo
'81, sponsored by The Florida Specifier, Tampa,
FL, July 22-24, 1981.
43. Griffiths, I.W. Sulphide Control in Rising Mains.
Water Pollution Control 80; 644-647, 1981.
44. Heukelekian, H. Effect of the Addition of Sodium
Nitrate to Sewage on Hydrogen Sulfide Produc-
tion and BOD Reduction. Sewage Works Journal
15 (2): 255-261, 1943.
45. Dague, R.R. Fundamentals of Odor Control.
JWPCF 44 (4): 583-594, 1972.
46. Moss, W.H.,R E.Schade,andS.S Sebesta.Fi///
Scale Use of Physical/Chemical Treatment of
Domestic Wastewater at Rocky River, Ohio.
JWPCF 49 (11): 2,249-2,254, 1977.
47. Lorgan. G.P., J.D. Hill, and S.M. Summers.
Nitrate Addition for the Control of Odor Emis-
sions From Organically Overloaded Super Rate
Trickling Fitters. Proceedings of the 31 st Purdue
Industrial Waste Conference, 1976.
67
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48. Directo, L.S., C.L. Chen, and I.J. Kugelman./V/of
Plant Study of Physical-Chemical Treatment.
JWPCF 49 (10): 2,081-2,098, 1977.
49. Delta Diablo Sanitation District Annual Report,
1983-1984. Contra Costa County Sanitation
District 7A, Antioch, CA, 1984.
50. 1982 Annual Audit for Palm Beach County,
Florida. Interox America, Houston, TX, 1983.
68
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Chapter 4
Odor and Corrosion Control in Existing Wastewater Treatment Plants
4.1 Introduction
Odor and corrosion are significant problems at many
wastewater treatment facilities. In a 1973 survey of
500 treatment plants in the United States, 40 percent
of the plant superintendents responded that they had
received complaints about odors (1). In addition,
superintendents at 37 percent of these plants indi-
cated that odor abatement measures, either through
process modification or installation of odor control
equipment, have been, or were planned to be,
instituted.
Corrosion of concrete and steel in wastewater
treatment plants can result in significant mainte-
nance and replacement costs over the lifetime of the
plant. Corrosive substances in purely domestic waste-
waters are principally H2S, chlorine, aggressive
water, ammonia, and salt (2). Industrial wastes
containing acids, alkalies, and various organic chem-
icals discharged into a municipal sewer system may
aggravate the problem.
This chapter describes likely sources of odors at
wastewater treatment plants, alternatives for control
of odor emissions, techniques for treatment of
odorous air, odor masking and counteraction agents.
In addition, a section on corrosion addresses the
mechanisms of corrosion of concrete and metals,
corrosion control alternatives, and selection of
corrosion-resistant coatings and materials.
4.2 Sources of Odors in Wastewater
Treatment Plants
Most unit processes in wastewater treatment plants
are potential sources of odor. This section will discuss,
in flow chronology, each unit process and the
conditions conducive to odor generation.
4.2.1 Headworks and Preliminary Treatment
If odorous gases such as HZS are dissolved in waste-
water entering the headworks of a wastewater
treatment plant, turbulence induced by drops, flumes,
aerated grit chambers, or similar structures will
cause the gases to be released from solution. High
organic strength septic sidestreams from sludge
processing operations, such as wet oxidation decant
liquors, filtrates, digester supernatants, filtrates and
centrates, may also release malodorous gases under
such conditions.
Because of long detention times, flow equalization
basins may become septic if not aerated. Inadequate
mixing in such basins may also result in increased
deposition of organic material, which can aggravate
the problem.
Accumulation of organic debris in influent channels,
on bar screens, comminutors, and fine screening
devices can result in odor generation if regular
cleaning and flushing is not practiced. Grit chambers
and grit conveyance systems can also be serious
sources of odors due to the organic coating on grit
particles. This is especially true in smaller plants
where grit may be stored for long periods of time
before disposal (3).
4.2.2 Primary Clarifiers
Primary clarifiers can be a source of odors if improper-
ly designed and maintained. If scum removal mecha-
nisms are inadequate, resulting scum accumulation
and subsequent putrefaction will result in odor
generation. Infrequent or incomplete withdrawal of
settled solids can result in septic conditions and
generation of odorous gases that can also result in
sludge rising to the surface due to buoyancy from
trapped gases (3), Discharge over the effluent weirs
can release odorous gases dissolved in the primary
effluent.
4.2.3 Fixed Film Reactors
Fixed film reactors, such as trickling filters and
rotating biological contactors, can be sources of odors
when the air supply to the biological film is inade-
quate. This often occurs during hydraulic overload
conditions. Plugging or improper sizing of underdrains
in trickling.filters reduces air circulation and promotes
anoxic conditions. Poor distribution of the wastewater
onto the media results in discontinuous wetting and
excessive slime buildup, which can lead to mcreased
presence of anaerobic zones and subsequent odor
generation.
4.2.4 Activated Sludge Basins
Aeration basins are not normally significant sources
of objectionable odor. However, the existence of poor
69
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mixing characteristics can result in deposition of
organic solids in corners or along the edges of the
tank (3), Such sludge deposits may generate odorous
gases at a rate greater than they can be oxidized by
the overlying aerated liquid. Clogging of diffusers
causes uneven distribution of air and may result in
anoxic zones and solids deposition. Aeration tank
walls that are intermittently wetted by wastewater
spray may develop putrescible slimes that can
generate odors.
4.2.5 Final Clarifiers
Final clarifiers are not normally sources of odors if
upstream aerobic stabilization processes are properly
designed and operated. The major consideration in
preventing odor generation in clarifiers is maintaining
adequate rates of sludge withdrawal to prevent septic
conditions. In addition to being a source of foul odors,
septic sludge will create additional oxygen demand
when returned to the aeration basins.
4.2.6 Sludge Thickening, Conditioning, and
Holding
Sludge handling systems are normally the most
significant source of odors in wastewater treatment
plants. Unit processes which allow exposure of the
sludge to the atmosphere, such as holding tanks and
thickeners, will generate odors with intensities
rangingfrom mildly offensive to nauseating. Virtually
all sludges emit odors, but fresher sludges generate
less intense, less offensive odors. Septic sludges emit
highly offensive and persistent odors. Sludge thick-
eners are often the cause of odor complaints from
neighborhoods surrounding municipal wastewater
treatment plants due to exposure of raw sludge to the
atmosphere |4). Wet oxidation processes operated at
temperature and pressure regimes needed for condi-
tioning sludges are a major source of odors unless
special precautions have been taken to contain and
treat the odorous discharges. One of the major
sources of odor from this process is decant tanks that
are generally located outside of the housed sludge
handling facilities and are often uncovered.
4.2.7 Sludge Dewatering, Stabilization, and
Storage
Sludge dewatering processes are often sources of
odors. Such processes include vacuum filtration,
plant and frame and moving belt pressure filtration,
centrifugation, and gravity and vacuum drying beds.
The extent of odor generation will vary depending on
the type and characteristics of the sludge, the method
used for dewatering, and the chemicals used for
conditioning. For example, vacuum filtration of a high
pH digested sludge will often result in release of
ammonia.
Sludge stabilization processes include anaerobic
digestion, aerobic digestion, lime stabilization, com-
posting, and chlorine oxidation. In most cases, odors
generated during sludge stabilization are not highly
offensive if the system is properly designed and
operated. "Sour" anaerobic digesters and overloaded
aerobic digesters, however, will often generate
offensive odors. Lime stabilization processes may
generate large quantities of ammonia gas resulting
from the high pH. This has been noted to be a serious
problem in physical-chemical treatment plants em-
ploying lime precipitation. Properly designed aerated
pile composting systems exhaust odorous air through
piles of finished compost. Windrow composting
systems may generate significant odors during
turning of the piles, particularly if insufficient quanti-
ties of bulking agent are employed to allow proper air
circulation. Properly operated chlorine oxidation
systems will generate medicinal or chlorine odors.
However, insufficient chlorine dosages or long-term
storage of the chlorinated sludge may result in
putrefaction and release of objectionable odors.
Sludge storage tanks, basins, and lagoons are
principal sources of odor at wastewater treatment
facilities. The problem is difficult to control since
storage vessels are often uncovered, and the large
surface areas provide high exposure of the sludge to
the atmosphere. Wind action on the surface of
storage lagoons can compound the problem.
4.2.8 Sludge Incineration and Solids Reduction
Processes included in this category include multiple
hearth and fluidized bed incineration, starved air
combustion, pyrolysis, wet oxidation and flash drying.
Odor problems in combustion processes result from
incomplete oxidation of odorous gases, or from
spillages during sludge transfer operations. In many
cases, gas scrubbers or direct flame oxidation systems
employed to meet air pollution emission control
requirements are effective in reducing odorous
discharges.
4.2.9 Process Sidestreams
Liquid streams resulting from sludge processing
operations have a variety of names depending on the
type of process they arise from and include super-
natants, centrates, filtrates, elutriates, thermal pro-
cess decant liquors, and filter backwash waters. In
many cases, these sidestreams exhibit very high
COD, BOD and ammonia-nitrogen concentrations
and are often major sources of odor.
Sidestreams are often returned directly to the head-
works of the plant where they may cause odor
problems due to turbulence and release of odorous
gases, or due to BOD overloading and rapid depletion
of DO. In many instances, high strength sidestreams
70
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require pretreatment before return to the wastewater
treatment processes.
4.2.10 Septage Handling
Septage receiving and handling facilities are major
sources of odor at municipal wastewater treatment
plants, since these wastes are almost always septic
and are often handled on a periodic and unscheduled
basis, Septage is composed of highly putrescible
materials which cause very persistent and objection-
able odors. Odors are often generated during transfer
from septage hauling trucks to holding tanks at the
plant site and during discharge to manholes, lift
stations, headworks or sludge processing facilities.
Turbulent conditions result in release of odorous
gases to the atmosphere. Uncontrolled addition of
septage to the main wastewater stream may also
result in rapid DO depletion and subsequent odor
generation.
4.2.11 Physical-Chemical Treatment Plants
Odor generation and corrosion problems have been
noted to be especially severe in physical-chemical
treatment systems, sincethese systems provide little,
if any, opportunity for oxidation of dissolved sulfides
thay may be present in the influent waste stream. The
problem is compounded, especially in the smaller
facilities, because the unit processes such as pre-
treatment, lime clarification, filtration and carbon
adsorption are often housed, thus creating a confined
atmosphere that extends the corrosive influence
beyond the original source. Single or two-stage lime
precipitation may reduce sulfide release potential,
but can liberate ammonia, which is especially corro-
sive to instrumentation and control systems. Carbon
adsorption systems often become anoxic and release
significant amounts of HzS when opened to the
atmosphere, such as during backwashing and carbon
transfer operations.
4.3 Control of Odors in Existing
Wastewater Treatment Plants
There are three general categories of odor control:
• Prevention of odorous emissions
• Collection and treatment of odorous air
• Odor modification, counteraction and masking
In many cases, odor emissions can be reduced or
eliminated through improved operation and mainte-
nance practices. Regular, frequent cleaning of prelim-
inary treatment devices such as comminutors, bar
screens, and grit chambers; flushing of tank walls;
removal of sludge deposits from influent and inter-
process channels; and increased rate of withdrawal
of settled solids are examples of routine operation
and maintenance techniques that are necessary to
control odors. Where high sulfide concentrations are
present in the influent wastewater, treatment in the
collection system through the injection of air or
oxygen, or the addition of chemicals such as hydrogen
peroxide or metal salts, can reduce or eliminate odors
at the headworks. Addition of chemicals, such as
sodium nitrate, at the headworks can also be effective
for odor control in follow-on unit processes, such as
trickling filters. Potassium permanganate and hydro-
gen peroxide have been used effectively for odor
control in sludge processing operations (e.g., de-
watering). Odors from many wastewater treatment
operations are released due to air stripping, which
can be reduced by subsurface discharge of liquids
such as sidestreams and septage, which contain
dissolved odorous compounds.
Where odors are generated in enclosed spaces, such
as sludge processing buildings, covered holding
tanks, and wet wells, the odorous air can be effectively
treated prior to release to the atmosphere by a variety
of techniques, including wet scrubbers, activated
carbon, chemical adsorbers, and soil or compost
filters.
Finally, odors can be made less objectionable through
the use of odor masking and counteractive agents.
Since this often involves merely replacing an ob-
jectionable odor with a more pleasant one, this
approach is generally the least preferred of the
available techniques for odor control and should not
be considered for a permanent solution.
4.3.1 Prevention of Odorous Emissions
4,3,1.1 Operation and Maintenance
Good housekeeping is always essential to the pre-
vention of. odors being generated. Many odors
associated with wastewater treatment operations
can be controlled or eliminated by ensuring that
process components are kept clean and free of
accumulated grease, solids and debris.
Bar screens and preliminary treatment processes
should be cleaned daily to remove any accumulated
organic debris that can putrify and cause odors. Grit
and screening conveyance systems should beflushed
with water to remove organic debris and grit, and the
materials should be transferred to closed containers
to minimize escape of odors.
Scum scrapers, pits and wet wells on primary
clarifiers should be cleaned frequently and chemi-
cally treated, if necessary, to remove accumulated
grease and scum and reduce the potential for
biological degradation. Scum and grease collection
wells and troughs should be emptied and flushed
regularly to prevent putrefaction of accumulated
71
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organics. Settled solids should not be allowed to
accumulate in the bottom of clarifiersfor long periods,
since septic conditions can develop within 2 to 6
hours. Pumping frequency should, therefore, be
adjusted to prevent settled solids from being detained
for more than 1.5 to 2 hours (3).
Trickling filter media should be continuously wetted
and kept free from plugging to allow adequate air
circulation for the fixed film to remain aerobic and
odor free. During periods of low flow, this can be
accomplished by increasing recirculation rates. Dis-
tribution nozzles should be kept clear at all times to
allow uniform application of wastewater to the media.
Filter underdrains and drain lines should be checked
periodically to ensure that they are not plugged,
thereby reducing air circulation through the media
which causes anaerobic conditions and odor genera-
tion (5). Interior walls of trickling filters and walls
surrounding rotating biological contactors should be
cleaned and flushed regularly.
In activated sludge basins, sufficient and complete
mixing is essential to prevent deposition of solids.
Clogging of diffusers results in poor mixing, and is
manifested by a laGk of turbulence and an accumu-
lation of foam, bubbles, or scum at the surface. Air
piping and diffusers should be inspected and cleaned
periodically. Tank walls subjected to intermittent
wastewater spray should b&cleaned regularly.
Scum scrapers, troughs, weirs and interior walls of
final clarifiers should be cleaned and flushed regularly
to remove putrescible organics. Settled solids should
not be allowed to accumulate long enough for
anaerobic conditions and the resultant odor genera-
tion to begin.
Since wastewater sludge is a significant source of
odors, special care should be taken to ensure that
sludge transfer systems such as bucket conveyors,
screw pumps, belt conveyors, and conduits be kept as
clean as possible. Spillages should be cleaned and
flushed immediately to prevent unnecessary odor
generation. Elutriation water or dilution water for
gravity thickeners should contain maximum DO to aid
in odor control. Sludge blankets in flotation thickeners
should be removed at regular frequent intervals or
continuously.
Gas from poorly operating anaerobic digesters is
odorous and, if possible, should be burned in the
waste gas burner. An auxiliary fuel source may
sometimes be necessary to ensure complete oxida-
tion.
Septage received at wastewater treatment plants
should be transferred from the hauler truck into a
closed tank or subsurface receiving basin by using
quick-disconnect, watertight fittings. This prevents
splashing, turbulence, and release of odors. Spills
should be immediately flushed with water. Provision
should be made for control of the rate of addition of
the septage into the wastewater stream to avoid
excessive DO depletion. Allowable loadings to main-
stream processes will depend on the aeration and
solids handling capacity of the plant and the charac-
teristics of the septage. One reasonable guideline is
to limit the volatile solids loading from the septage to
10 percent of the volatile solids entering in the raw
wastewater over the same time period.
Extraneous odor generation in wastewater treatment
plants can be minimized through a regular inspection
and maintenance program that involves frequent
removal of accumulated solids and organic debris,
and regular cleaning of tanks, unit process equipment,
and hardware that come in contact with wastewater
or wastewater sludges. Such a program can usually
be implemented at little cost, often with substantial
reduction of odor generation.
4.3.1.2 Upstream Treatment
Odor problems at wastewater treatment plants are
often caused by the release of HaS gas at the
headworks. If this is the case, it is likely that odors are
being released in the collection system as well.
Rather than attempt to collect and treat the odorous
gas at each point where it is being released, it is often
more cost effective to control the sulfides by one or
more of the techniques discussed in Chapter 3. These
include:
1. Air injection or entrainment
2. Pure oxygen injection
3. Chemical addition
• chlorine
• hydrogen peroxide
• metal salts
• nitrates
The point of injection gas or chemical addition is
dependent on how far upstream odor control is
desired, and the reaction time required to minimize or
eliminate odors at the plant headworks. In many
cases, multiple injection points are necessary. For a
detailed discussion of in-sewer odor control, the
reader is referred to Chapter 3.
4.3.1.3 Chemical Addition
Direct chemical addition to wastewater, wastewater
sludge, or process sidestreams can be a simple and
effective technique for odor control. Chemicals used
for this purpose incl ude hydrogen peroxide, potassium
permanganate, sodium nitrate, and chlorine.
a. Hydrogen Peroxide
Under conditions typically found in municipal waste-
water, hydrogen peroxide reacts with H2S according
to the following reaction:
H202+H2S •> S + 2 H2O (4-1)
72
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In addition to the oxidizing sulfide, excess hydrogen
peroxide decomposes to yield oxygen and water.
Thus, it has the added benefit of increasing the DO of
the stream. Use of hydrogen peroxide for sulfide
control in collection systems has been described in
Chapter 3. If odor emissions from manholes, lift
stations, etc. are not a problem, but the wastewater
entering the plant is high in dissolved sulfides,
hydrogen peroxide can be injected upstream of the
headworks to minimize or prevent odor generation
when the wastewater enters the plant. An injection
point should be selected that will provide 15 to 45
minutes reaction time, depending on H2S concentra-
tion and wastewater characteristics.
A similar application for odor control with H202 has
been injection upstream of the primary clarifiers.
Long wastewater residence times in the clarifiers due
to operation at flows significantly less than design
flows, or inadequate sludge withdrawal rates, can
result in substantial sulfide generation in the clari-
fiers. Rapid DO depletion problems may also result
from return of anaerobic sidestreams to the head-
works. Turbulence promoted by the fall of the
wastewater from the effluent weirs into the collection
trough can release H2S, resulting in odor generation.
In 1982, the city of Baltimore, Maryland conducted a
demonstration using for control of HZS being
released at the primary effluent weirs at the 674,000
m3/d (178 mgd) Back River Wastewater Treatment
Plant. Table 4-1 shows the results of the demonstra-
tion (6). In comparing the data for H2S concentrations
before and after the primary effluent weirs prior to
H2O2 dosing, it can be seen that substantial H2S
losses were occurring during discharge over the
weirs into the effluent trough. This was attributed to
stripping of the HZS from solution (6). Average mass
ratio of applied H2O2 to H2S at the dosing manhole,
located 150 m upstream of the headworks, was
2.14:1 in order to oxidize existing sulfides and to
prevent additional sulfide formation. The data in
Table 4-1 show that maximum HjS levels before the
primary weirs were reduced from 2.5 to 1.0 mg/l.
Prior to H2O2 addition, an averageof 0,32 mg/l of HZS
was being released from solution during discharge
over the primary weirs, but after H2O2 addition, an
average of only 0.11 mg/l H8S was being released.
Hydrogen peroxide has also been employed for odor
control in sludge handling systems (7-9). At the
757,000-m3/'d (200-mgd) secondary treatment plant
operated by the Allegheny County Sanitary Authority
(Pittsburgh, PA), major odor problems occurred in the
sludge handling system (9). The unit processes of
concern were the constant head tanks, the sludge
holding/mixing tanks, and the vacuum filters. Air
from the vacuum filter building was exhausted
through an activated carbon system for odor control.
However, odors generated and released in the
building were greater than anticipated and far beyond
the capacity of the carbon filters. Trials were con-
ducted with H2O2 addition to the constant head tanks.
Results are shown in Table 4-2. A dosage of40ppmto
the sludge was sufficient to reduce atmospheric HZS
levels above the mixing tanks and in the building
exhaust to zero, and to reduce dissolved sulfide
concentrations in the vacuum filter filtrate to 0,1
mg/t. Based on these trails, a permanent system was
Table 4-2. Use of H2O2 for H2S Odor Control in Sludge
Handling System, Pittsburgh, PA (9!
Atmospheric HaS Dissolved Sulfides
Over Mix
Building
Vacuum Filter
HjOs Dose
Tank
Exhaust Duct"
Filtrate
ppm
ppm
ppm
0
8
2
1 to 30
2
2
1 to 50
20
0
0
0.35
0
0
0.25
0
0
0.25
40
0
0
0.10
50
0
0
0,10
0
0
0,10
*Carbon filters were in use at all times fortreatment of exhaust air,
but were not completely effective, as shown by data collected
prior to HjOj injection.
Table 4-1. Use of HjOj to Control H2S Odors at Baltimore, MD (6)
HjS Concentration
Without HjOj With H2O/
Location Maximum Minimum Average Maximum Minimum Average
mg/l mg/I
Dosing Manhole" 1.8 0.3 0.92 NRC NR NR
Before Primary Weirs 2,5 0.2 0.96 1.0 0.2 0.55
After Primary Weirs 0 9 0,4 0.64 1.0 0.1 0.44
"HsQa dosing schedule: 8 AM - 8 PM: 2.8 mg/l
B PM - 8 AM: 1.1 mg/l
'Located approximately 150 m (500 ftt upstream of headworks.
'Data not reported.
73
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constructed for injection of 40 ppm of H2O2 at a point
46 m(150 ft) upstream of the constant head tanks (9).
b. Potassium Permanganate
As discussed in Chapter 3, potassium permanganate
(K.MnO«) is a strong oxidizing agent which reacts with
H2S as follows:
Acidic
conditions:
Alkaline
conditions:
->
3 HzS + 2 KMn04 -
3 S + 2 H2O + 2 KOH + 2 MnOa
(4-2)
3 HzS + 8 KMn04
(4-3)
3 K2SO4 + 2 HjO + 2 KOH + 8 Mn02
KM n0< reacts with many odor-producing compounds,
including aliphatic, aromatic, nitrogen-containing,
sulfur-containing, and inorganic compounds. How-
ever certain compounds in these categories do not
react readily with KMn04. For lists of compounds
which do and do not react with KMn04, the reader is
referred to References 3 and 10.
KMn04 has been applied to various points in the
liquid stream of a wastewater treatment plant as well
as to sludge processing operations such as dewater-
ing. Required KMn04:H2S weight ratios to achieve
sulfide control in wastewater generally range from
2.5:1 to 6:1. Figure 4-1 shows percent sulfide
removed as a function of thB KMn04:HzS weight ratio
for a wastewater with pH = 6.8 (11). Figure4-2 shows
a plot of data collected at a Florida wastewater
treatment plant where influent dissolved sulfides
were 12 to 15 mg/l. KMn04 was added to the
headworks for sulfide control. Dosages of 25 to 35
mg/l were generally sufficient to reduce dissolved
sulfides to below 2 mg/l (12). Although this repre-
sents a sulfide removal efficiency of 80 to 90 percent,
a wastewater stream containing 2 mg/l of dissolved
Figure 4-1.
Sulfide removal from wastewater using potas-
sium permanganate (11),
sulfide still holds significant potential for release of
gaseous HaS to the atmosphere.
KMn04has been used successfully for odor control in
sludge handling applications, particu larly dewatering,
where it is added to the suction side of sludge pumps
feeding the dewatering unit. Figure 4-3 shows the
relationship between dosage of KMnCU and the
fraction of sulfide removed(11). Total sulfide removal
is achieved at dosages of 100 to 120 ppm. From a
survey of 45 plants using KMn04, for sludge odor
control, the average dosage was 37 ppm (13).
At one California plant, H2S concentrations were
measured above centrifuges used for dewatering
anaerobically digested sludge(14). KM n04 was added
immediately upstream of the centrifuges at various
dosages to determine the impact of HZS emissions. As
Figure 4-2. Results of pilot studies using potassium per-
manganate for removal of dissolved sulfides
(12).
Initial Concentration
of Dissolved Sulfide
Sulfide readings taken 6 a.m. to noon
and 6 p.m. to midnight jA)
Sulfide readings taken
noon to 6 p.m. and
midnight to
.. ^'6a.m.(«)
10 15 20 25 30 35
Potassium Permanganate Dosage, mg/l
Figure 4-3.
Sulfide removal from wastewater sludge using
potassium permanganate (11).
pH = 6.8
Z:1 3:1 4:1 5:1 6:1 7:1
Permanganate:Suifide Weight Ratio
40 BO 80 100 120 140
Permanganate Added to Sludge, ppm
160
74
-------
can be seen in Figure 4-4, KMn04 dosages of 110 to
120 mg/l were successful in reducing atmospheric
HzS levels to below about 5 mg/l,
c. Sodium Nitrate
Sodium nitrate (NaN03) has been successfully used
for odor control in anaerobic lagoons, carbon columns,
trickling filters, and sludge storage lagoons (16-20).
As discussed in Chapter 3, bacteria will utilize
hydrogen acceptors preferentially in the order; 1)02;
2) NOj; and 3) SOI Thus, theoretically, in the absence
of oxygen, no sulfide will be generated until all of the
nitrate has been reduced to nitrogen gas.
Nitrate was employed for odor control at a 36-mgd
industrial wastewater facility treating wastes gener-
ated from the manufacture of photographic paper,
film, and chemicals (16). The plant consisted of
primary settling and neutralization followed by two
parallel, plastic media, super-rate trickling filters
(identified as the source of the odors) which preceded
a completely mixed activated sludge system. Earlier
studies had indicated that the natural draft design
was inadequate to ensure uniform distribution of air
Figure 4-4.
Effectiveness of potassium permanganate addi-
tion for controlling H2S generation from sludge
centrifugation (14).
"I
I
E
<
300
270
240
210
180 -
150
r Centrifuge
No. 4
Sludge
Total Flow = 2.7 m3 (715 gal)/min
Centrifuge
No. 5
J
/
Centrifuge
No. 3
120
through the trickling filters. However, installation of
forced draft ventilation improved air distribution but
did not prevent odor generation. Atmospheric HzS
concentrations above thetrickling filters were as high
as 20 ppm, and were found to vary directly with the
strength of the influent, measured as Total Oxygen
Demand (TOD). H2S emissions were most significant
at influent TOD concentrations above 500 ppm.
Average influent sulfate concentration was approxi-
mately 200 mg/l.
It was found that addition of approximately 5 mg/l
NOa-N to the trickling filter influent was sufficient to
control most H2S emissions from the trickling filter.
Figure 4-5 shows the relationship between influent
TOD and atmospheric H2S measured above the filters,
as well as the impact of nitrate addition on atmos-
pheric HzS emissions (16).
NaN03 was also effective for odor control in a sludge
storage lagoon receiving sludge from an industrial
activated sludge plant (17). The four lagoons had
liquid depths of 4,5 to 6.1 m (15 to 20 ft), with surface
areas ranging from 1.1 to 2.8 ha (2.7 to 7 acres).
Results from the field application of a waste NaNOs
solution indicated an initial dose of 20,000 mg/l NO3-
N to satisfy the initial nitrate demand and provide a
nitrate residual. The waste NaN03 solution (40
percent NaNOs) was distributed onto the lagoon
surface using a floating boom equipped with spray
nozzles. Oxidation reduction potential (ORP) was
used as a measure of the potential for odor emissions.
After 3 months of nitrate addition, ORP values
gradually rose from -200 mV to over +200 mV and
sulfide odors were eliminated (17).
Nitrate was also successfully used to control H2S
generation in both pilot-scale and full-scale tertiary
Figure 4-5.
Effect of nitrate addition on H?S emissions from
a trickling filter (16).
20
18
16
E
14
Q.
cl.
12
<
10
c
a
in
«
I
6
4
2
Without NO3-N Addition-
• •
1.2 pp"1
20 40 60 80 100 120
Potassium Permanganate Added, ppm
2-3 PP"1
With NOj N Addition
L_ 1 L
200 300 400 500 600 700 800 900 1,000
Influent TOD, mg/l
75
-------
activated carbon columns. Anaerobic conditions,
which may develop in carbon columns due to
biological growth stimulated by soluble organics in
the influent, are conducive to sulfide generation. In
the pilot studies, several sulfide control schemes
were investigated, including routine backwashing,
intermittent oxygen addition, sodium hypochlorite
injection, sodium nitrate addition, and increasing the
DO in the influent to the carbon columns. Chemical
addition was to the influent wastewater immediately
upstream of the carbon columns. Results from this
investigation are summarized inTable4-3 (1 8). It was
found that addition of 5.4 mg/l N03-N, in conjunction
with routine backwashing, reduced total sulfide
concentrations in the carbon column effluent to zero.
A greater rate of headloss development was observed,
however, due to the enchanced biological activity
within the carbon column during nitrate addition.
At the full-scale physical/chemical wastewater
treatment system at Rocky River, Ohio, formation of
H2S in the carbon columns was the most evident
day-to-day operating problem (19). Initially, copper
sulfate at 6 mg/l Cu** was added to the backwash
water as a bactericide. Not only did this fail to control
odors, it nearly doubled the turbidity in the effluent.
Addition of NaN03to the column influent at concen-
Tabla 4-3. Performance of Nitrate Addition Compared with
Other Measures for Sulfide Control in Activated
Carbon Columns (19)
Total Sulfide in Carbon
Column Effluent
Sulfide Control Method Average Range
mg/l
2,86 1.0 to 5.7
1,85 1.4 to 2.5
1.87 0.8 to 3,0
1.74 0 to 4.3
113 0,1 to 2.6
0.30 0 to 0.95
0.13 0 to 0.60
0.05 0 to 0.26
0 02 0 to 0,10
0 0 to 0.05
"As sodium hypochlorite solution.
"As sodium nitrate solution.
trations of 4 to 6 mg/l NO3-N effectively reduced HZS
levels in the carbon column effluent from 9.5 mg/l to
less than 1 mg/l during the trial (19).
d Chlorine
Chlorine can be an effective means of odor control
from wastewater unit processes. Its applications in
wastewater treatment facilities have been primarily
aimed at preventing odor generation from the liquid
stream. Chlorine addition at the headworks is a
common odor control technique for many plants. The
amount of chlorine required for odor control is
typically less than 80 percent of the wastewater
chlorine demand. Because of the high chlorine
demand of wastewater sludges, chlorine has not
been used for controlling odors from sludge handling
operations. Use of chlorine for sulfide control in
wastewater collection systems is discussed in detail
in Chapter 3.
e. Equipment, Design, and Costs
Because equipment requirements are similar to these
for chemical addition to wastewater collection sys-
tems, the reader is referred to Chapter 3 for a detailed
discussion of equipment and construction materials
for various chemical feed systems in treatment
facilities. For odor control in sludge handling opera-
tions, such as dewatering, field testing is recom-
mended due to the variability in sludge characteristics,
dewatering equipment, and conditioning agents.
Cost estimates are given in Chapter 3 for chemical
addition to collection system flows of 3,785 m3/d (1
mgd) and 37,850 m3/d (10 mgd). These would
generally be applicable to chemical addition for odor
control to the headworks of a similarly sized waste-
water treatment plant. Costs for chemical addition for
odor control in sludge handling operations are difficult
to estimate due to the wide variability in sludge
characteristics, physical conditions, and plant opera-
tions. Costs will be largely dependent on the unit cost
of the chemical and its dosage requirements (per-
formance). Since performance of chemicals cannot
be estimated for a "hypothetical" sludge, attempts at
estimating costs are strictly subjective; thus, costs for
sludge odor control by chemical addition were not
developed.
4,3.1.4 Covering of Odor Producing Units
In cases where odors are generated from wastewater
treatment unit processes, such as primary clarifiers,
sludge thickeners, and septage holding tanks, it is
often possible to construct covers or domes over the
odor generating units for the purpose of containing
the odors. The contained air is then passed through
an air pollution control device, such as a scrubber,
filter, or absorptive media, for odor removal prior to
release to the atmosphere.
1. Surface wash—air/water back-
washing technique
2. No. 1 + intermittent 02 addition to
carbon column at D O. level =
4 mg/l
3. Surface wash + air/water back-
wash + oxygenation of influent to
D O. = 2 to 6 mg/l
4. No, 3 + 20 mg/l Cla" to carbon
influent
5. No. 3 + 40 mg/l CI2 to carbon
influent
6. No. 3 + 2.9 mg/l NO»-Nsto
. carbon influent
7. No. 3 + 5.1 mg/l N0a-N to
carbon influent
8. No. 3 + 5.3 mg/l NOa-N to
carbon influent
9. No. 1 + 5.3 mg/l NOa-N to
carbon influent
10. No, 1 +5.4 mg/l N03-N to
carbon influent
76
-------
Domes are generally constructed of fiberglass, alu-
minum, .or styrofoam. Inflatable domes have also
been employed. Aluminum domes are available with
clear spans of up to 120 m (400 ft), while fiberglass
domes can be used for covering tanks having
diameters up to 27 m (90 ft). Flat, low profile covers
can be used for covering tanks which do not require
frequent access for maintenance, such as holding
tanks with little or no mechanical equipment. These
may be less costly and have lower ventilation
requirements, although access for cleaning or main-
tenance may be limited.
a. Design
Covers should be designed so as to minimize con-
densation problems within the dome. They must be
designed to withstand wind loadings as well as static
loadings resulting from snow and ice accumulation.
Materials should be of sufficient thickness to prevent
damage by hail. Normally, negative pressures are
maintained under the domes to prevent escape of
odors through openings and cracks, and to allow
continuous exhausting of odorous air to subsequent
treatment units.
Domes can be designed with any number of access
hatches, doors, and translucent panels. Manufac-
turers should be contacted to determine design
criteria for specific applications.
b. Costs
Several manufacturers of both aluminum and fiber-
glass domes were contacted during 1984 to deter-
mine a range of installed costs. Unit costs (dollars per
square meter of covered surface area) varied de-
pending on diameter, specific site conditions, and
number of vents, access doors, translucent panels or
other appurtenances desired. For standard domes
covering tanks with diameters of 15 to 30 m (50 to
100 ft), installed costs are generally $107 to $160/m2
of covered surface area ($10 to $15/sq ft).
4.3.2 Collection and Treatment of Odorous Air
If prevention of odor generation is not feasible or
cost-effective using the various techniques previously
discussed, odorous air within a confined space can be
removed and treated before being released to the
surrounding atmosphere. Wet scrubbers, activated
carbon and other adsorptive or absorptive processes
can be used to remove odor compounds from the air.
Following is a discussion of these odor control
processes.
4.3,2.1 Wet Scrubbers
Wet scrubbing involves contact of odorous gas with a
scrubber solution, typically in a countercurrent or
cross-flow fashion, to allow transfer of the odorants
from the gas stream to the scrubber liquid by one or
more of the following mechanisms:
• Condensation of odorous vapors
• Removal of odorous particulates
• Odor absorption into the scrubbing solution
• Odor reaction with an oxidizing scrubbing solution
• Emulsification of odorous gases in a chemical
reagent
Wet scrubbing isoften ideallysuitedforthetreatment
of large air flows, greater than 1 mVs (2,000 cfm),
contaminated with low odor threshold compounds,
such as mercaptans and HzS, at levels greater than
100 odor units per liter (20).
Scrubber designs may be of the vertically oriented,
countercurrent type, or of the horizontally oriented,
cross-flow type. A typical countercurrent system
employs spray nozzles for injection of the scrubbing
solution and an inert packing material to provide
gas/liquid contact surface. The gas stream enters the
bottom of the scrubber unit as evenly distributed as
possible and passes through the packing material,
which is irrigated with the scrubber liquid. The gas
stream then passes through a mist eliminator to
remove any liquid droplets, and is exhausted to the
atmosphere by a fan. Wet scrubber systems are also
available which generate very fine fogs or mists of
scrubber liquid to achieve large surface areas for gas-
liquid contact, thereby precluding the need for packing
material. These are often referred to as spray
chambers. Several designs of wet scrubber systems
are shown in Figure 4-6.
Selection of a scrubbing liquid is dependent largely on
the specific odorants to be removed. Water soluble
gases such as H2S, ammonia, and organic sulfur
gases; organic nitrogen compounds such as amines;
organic acids; and chlorine compounds may be
removed by scrubbing with water. It is common
practice, however, to use a reactive compound such
as chlorine, potassium permanganate, hydrogen
peroxide, or ozone in the scrubbing liquid for chemical
reaction with the odorous compounds in the incoming
air. In some cases, acidic or alkaline solutions can be
used to neutralize the odorous compound, or to adjust
the pH for better performance when used in combina-
tion with another additive. Proprietary scrubber
solutions have also been developed for removal of
high concentrations of specific odorous compounds
such as H2S (21). One proprietary wet scrubber
system generates a dilute hypochlorite solution on-
site for use as a scrubbing liquid. Wet scrubbers may
employ a single pass of the scrubbing liquid with no
recirculation, such as for water systems, or, more
commonly, may collect and recirculate the scrubbing
liquid to reduce the costs for chemical additives. Wet
scrubber systems may employ single or multiple
stage units depending on the nature and severity of
77
-------
Figure 4-6. Typical wet scrubber systems.
Mist Eliminator
Spray Systsm
Packing
Media Support
Odorous Air
¦ Clean Air
Fan
Scrubber Liquid
(a) Countercurrent
Packed Tower
Drain Sump
Mist Elimin ~C_/|-an
Spray System
Odorous Air-
Recycle Pump
Clean Air
Scrubber Liquid
(b) Spray Chamber
Absorber
! m \ " Recycle Pump
Drain Sump
Spray System . ¦— Scrubber Liquid
I\ K A K A A if
Odorous
Air —
nator
Packing
' Drain
Clean Air
(c) Crossflow
Recycle Pump Scrubber
Table 4-4,
Gas
Effectiveness of Hypochlorite Wet Scrubbers for
Removal of Several Odorous Gases (3)
Expected Removal Efficiency
HjS
Ammonia
SOj
Mercaptans
Other oxidizable compounds
percent
98
98
95
90
70 to 90
the odor, and may utilize different scrubber solutions
for each stage. Reaction times in wet scrubbers may
vary from several seconds to 1 minute.
The most commonly used oxidizing scrubbing liquids
are chlorine (particularly sodium hypochlorite) and
potassium permanganate solutions. Hypochlorite
scrubbers can be expected to remove oxidizable
odorous gases when other gas concentrations are
minimal. Table 4-4 indicates the expected perform-
ance of hypochlorite scrubbers for removal of odorous
gases (3). Although the removal efficiencies appear
high, concentrations of odorous components in
the exhaust gas may still be above desirable levels,
possibly requiring additional treatment. Figure 4-7
shows the dependence of effective chlorine concen-
tration at the top of the scrubber tower on removal
efficiencies of various malodorous gases (22). It
should be noted that exhaust air from hypochlorite
scrubbers often has medicinal, chlorine odors which
may be objectionable in residential areas.
Multistage scrubber systems are often employed for
odor control. Number of stages and choice of scrubber
liquids depend on the characteristics and intensity of
the odor, and the effectiveness of the particular
chemical additives in the scrubber water. At a location
experiencing H2S odors, pilot studies were conducted
using scrubbing liquids containing KMnCUand NaOH.
Results are shown in Table 4-5. As a result of pilot
testing with various scrubber liquids, two systems
were proposed; a two-stage scrubber system using
NaOH in the first stage and KMnO
-------
Figure 4-7. Odorous gas removal efficiency as a function of chlorine concentration at top of packed tower (22].
100
E
g>
CC
CHaJaS
CHaSH
tCH3)aS
ch33n
150
200
Effective Chlorine Concentration, ppm
Table 4-5, Performance of Pilot-Scale Wat Scrubbers Using KMnO< and NaOH for HiS Removal (23)
H*S Concentration
Air flow Hate
Retention Time
Scrubbing Liquid
In
Out
H*S
Removal
m3/min
sec
ppm
%
7.1
2.1
2% KMnOj; pH = B.6
150
18
87
3.5
. 4,0
2% KMnO<; pH = 8.6
after 3 hrs
130
B
94
3.5
4.0
2% KMnOi; pH = 8.6
after 6 hrs
130
30
77
3.5
4,0
2% KMnOi; pH = B.6
after 9 hrs
1B0
140
22
7.1
12 7
12.7
2.1
1.2
1.2
2% NaOH; fresh
2% NaOH; fresh
2% NaOH; after 4 hrs
190
190
90
2
4
80
99
98
12
4. Design a full-scale system based on resu Its from
pilot tests.
Full characterization of the contaminated air is an
important first step in design. Diurnal fluctuations in
odor intensity or concentrations of odorous com-
pounds should be recorded continuously, if possible,
in order to select peak design values. Fluctuations in
air volumes should be similarly recorded. Over-
ventilation increases total air volume and decreases
79
-------
intensity (concentrations, which results in increase in
the size and cost of the system to maintain required
efficiency.
Selection of scrubber liquid(s) is critical in designing
an efficient system. It is based on the chemical and
physical characteristics of the contaminated air.
Hypochlorite and potassium permanganate have both
been used widely for control of odors from wastewater
treatment plants. Hypochlorite has some advantage
because it can be generated electrically on-site,
precluding the need for chemical handling and
Storage. Potassium permanganate is effective, but
scrubbers require additional maintenance to remove
manganese dioxide (Mn02), a precipitate that coats
the packing. Other scrubbing solutions which have
been used include water, acids, alkalies, ozone,
chlorine, chlorine dioxide, and sodium bisulfite. Other
agents and catalysts have been employed for removal
of specific contaminants.
The ratio of liquid to gas flow is an important design
consideration, since increasing the liquid-gas ratio
will reduce the theoretical height of the scrubber (or
the number of scrubber units). The limiting velocity of
the gas is called the flooding velocity, which depends
on the physical properties of the gas and the tower
packing. Usually, the optimum gas velocity for
contaminant removal is 50 to 70 percent of the
flooding velocity (22).
The contact area between gas and liquid is important,
since absorption is directly proportional to the amount
of liquid surface area exposed to the gas stream. One
method of increasing this area is to introduce the
scrubber liquid to the tower through the use of high
pressure spray nozzles which generate a fine mist or
fog. Spray chambers employing high pressure nozzles
can be very effective odor removal devices without
the need for packing material. Another approach is to
select a packing with a high specific surface area
(area per unit volume). Such factors as increased
pressure drop and susceptibility to clogging must be
taken into account when considering such packings.
Corrosion prevention is an important consideration in
design of a wet scrubber system. Usually, thermo-
plastics and fiber-reinforced thermoplastics are cost-
effective corrosion resistant materials for small to
medium size scrubbers and for most of the piping and
duct work. For large units, fiberglass-reinforced
plastics, stainless steel, and resin-coated mild steel
are common construction materials.
Scrubber water may have to undergo treatment
during use or before disposal. For example, Mn02
particles in KMnO« scrubbers can clog nozzles and
valves, and should be removed by filtration or other
means if continuous operation is contemplated. If
operation is intermittent, accumulated sludge should
be drained periodically. Scrubber water disposal is
generally not a problem at a wastewater treatment
plant, since the spent scrubber water can be intro-
duced back into the wastewater stream at a rate
which will not upset the processes.
Design of a wet scrubber system may be complex
depending on the intended application, characteris-
tics of the gas stream, scrubber liquid used, and
design objectives. Pilot testing is almost always
recommended prior to full-scale design. Equipment
representatives can be of significant help in deter-
mining such design factors as tower height, packing
materials, and scrubber liquid selection.
An example of a wet scrubber system application for a
municipal treatment plant sludge processing opera-
tion is at the 378,500 m'Vd(100 mgd) Southerly Plant
in Cleveland, Ohio. This facility also handles sludge
from the 568,000 m3/d (150 mgd) Easterly Plant and
uses low pressure oxidation (LPO) and vacuum
filtration to condition and dewater the sludge. The
oxidized sludge from the LPO system is discharged
into a wet well, where dissolved gases are allowed to
escape from solution. Air exhausted from this well is
burned in a dual fuel (gas/oil) fume incinerator at
760°C (1,400°F).
Air from the four LPO decant tanks is exhausted to the
odor control system, which is a two-stage packed bed
scrubber. Chlorine gas is injected directly into the
first-stage scrubber. This scrubber provides the
contact media and contact time for the chlorine and
odorous air. The second-stage scrubber uses water to
remove any heavy chlorine odor. It was determined
that the compound causing the odor was an acid
aldehyde and that the chlorine scrubber system
removed 39 percent of this substance. Air from the
vacuum filter room and vacuum pump exhaust is also
treated with a wet scrubber, this one using a
potassium permanganate scrubbing solution.
c. Costs
Typical costs for wet scrubber systems for odor
control are presented in Table 4-6. Note that an inlet
H2S concentration of 20 ppm has been assumed.
Although H2S concentrations well in excess of this
value have been observed, H2S concentrations can be
expected to fluctuate widely, depending on the size of
the building, types of equipment housed, character-
istics of wastewater or sludge being processed, time
of day, and scheduling of treatment operations. For
purposes of this analysis, an outlet concentration of
<1 ppm has been assumed, representing a required
removal efficiency of 95 percent. In actual practice,
lower outlet concentrations may be required, since 1
ppm is well above the odor threshold.
These costs are budget level estimates and are
accurate to within +30 percent, -20 percent for a
80
-------
Table 4-6.
Typical Costs for Wat Scrubbers for Odor Control
(1S84 »|*
Design
Air Flow
Capital Costs"
Annual
Chemical Costs
mVmin
$
$/yr
28
39,000
2,000
280
77,000
19,500
'Assumed conditions:
1. Continuous operation
2. Inlet HjS = 20 ppm
3. Outlet HsS = < 1 ppm
4. Scrubber liquid = NaOCI + NaOH solution
5. Chemical Costs:
NaOCI: $1.41/kg
NaOH: S0.44/kg
"Including scrubber tower, packing, recirculation pump, fan and
ductwork, appurtenances and controls, installation.
typical case. In practice, detailed estimates of capital,
operating, and total present worth costs should be
developed when comparing costs of alternative odor
control technology for a specific application. In
addition, expected performance and reliability should
be assessed and ranked for each of the various
alternatives investigated in order to select the most
cost-effective and reliable system that will meet the
desired objectives.
4.3.2,2 Activated Carbon Adsorption
Activated carbon adsorption is a commonly used
method for treatment of malodorous air. It has been
used in wastewater treatment plants as a primary
odor control system and as a polishing step following
other alternatives such as scrubbers. Adsorption is
the phenomenon whereby molecules adhere to a
surface with which they come in contact. Activated
carbon has a high surface-to-volume ratio; thus, a
large surface area is available for adsorption in a
relatively small volume. The physical characteristics
of activated carbon are shown in Table 4-7.
Due to the non-polar character of the surface,
activated carbon adsorbs organic, and some inorganic
compounds, in preference to water -vapor. The
quantity of materials adsorbed is partially dependent
Table 4-7,
Physical Characteristics of Activated Carbon for
Odor Control
Parameter
Value
Suface area, mVg
Surface area, mVcm3
Pore volume, cmVg
Pore volume, cmVcm3
Mean pore diameter, angstroms*
950
380 to 600
0.6 to 1.0
0.24 to 0.50
1 5 to 20
on the physical and chemical characteristics of the
compound. In general, organic compounds with
molecular weights greater than 45 and boiling points
over 0°C will be readily adsorbed, Adsorption of
organic compounds is relatively nonselective; that is,
it is not strongly affected by solubility or chemical
class of the compounds. Under normal conditions,
adsorptive capacity of activated carbon can reach 5 to
40 percent of the weight of the activated carbon (24).
The quantity of materia) that can be adsorbed in a bed
of activated carbon depends on the following factors:
7.
8.
9.
Concentration of material in the space around
the activated carbon
Total surface area of the activated carbon
Total pore volume
Temperature
Presence of other competing contaminants
C haracteristics of the compounds to be adsorbed
(molecular weight, boiling point, polarity, size,
shape)
Polarity of the activated carbon
Relative humidity of vapor stream
Contact time of vapor stream within the acti-
vated carbon bed
"Refers to micropore volume (<25 angstrom diameter); Macropores
{>25 angstrom} not included.
Maximum adsorbing capacity is favored by a high
concentration of the substance surrounding the
activated carbon, large surface areas, freedom from
competing contaminants, low temperature, and ag-
gregation of the contaminant in large molecules that
fit and are strongly attached to the receiving sites on
the adsorbent (22).
The nonselectivity of activated carbon has an advan-
tage due to the ability to remove complex mixtures of
odorous compounds. However, nonselectivity can
present a disadvantage in that the capacity of the
carbon can be exhausted prematurely by the adsorp-
tion of nonodorous hydrocarbons. A pilot study
conducted in Sacramento, California attempted to
investigate the possible occurrence of this phenom-
enon in an evaluation of activated carbon adsorption
for treatment of sewer off-gases (25). It was found
that the useful life of the carbon prior to odor
breakthrough was 1.5x1 05to4x 105 air volumes per
carbon volume, or 276 to 735 m3/kg (4,420-11,800
ftVlb) of activated carbon. Data indicated greater
than 90 percent removal of hydrogen sulfides and
total hydrocarbons prior to odor breakthrough. After
odor breakthrough, the activated carbon was still
removing up to 37 percent of the H2S and 71 percent
of the hydrocarbons. These results are .shown in
Table 4-8.
Based on data collected, carbon bed life was estimated
for hydrocarbon saturation andHzS saturation. It was
found that the useful life based on hydrocarbon
saturation, 75 mVkg (1,200 ftVlb), was two orders of
81
-------
Table 4-8. Pilot Study on Sewer Odor Control Using Activated Carbon at Sacramento, OA (25!
Day 1 Day 2* Day 3* Day 4*
Parameter
In
Out
%
Rem.
In
Out
%
Rem.
In
Out
%
Rem.
In
Out
%
Rem.
CHU, ppm
265
270
0
460
440
4
165
161
2
41
50
0
CO2, percent
0.4
0,3
0
0,6
0.7
0
0.2
0.2
0
0,1
0.1
0
HsS, ppm
1.1
0.1
91
11,0
7.0
36
15
9.4
37
<0.1
<0.1
0
Total hydrocarbons, ppm
59.1
2.8
95
531,4
117.0
67
76.1
52.1
32
12.9
3.8
71
Odor conc., odor units
15
<2
1 6,700
18,800
75
75
66
75
•Sample taken after odor breakthrough.
magnitude lower than for H2S saturation, 8,800
m3/kg(141,000 ftVlb). However, useful life based on
odor breakthrough governs practical design of an
activated carbon system (25).
An interesting observation in this and other pilot
studies was that H2S odor in sewer off-gas is altered
by antagonistic or inhibiting effects of other sub
stances in the exhaust gas such that there is little
direct relationship between the actual HaS concentra-
tion and the odor concentration as measured by an
olfactometer. Removal of antagonistic or inhibitory
hydrocarbons by the activated carbon can result in
significantly higher perceived odor concentrations
from a given chemical concentration of H2S. This has
been observed on activated carbon units treating off-
gases from headworks and primary clarifiers, where
the gas odor characteristics can change through the
carbon column. Thus, odor breakthrough can occur
while H2S and hydrocarbons are still being removed
due to reductions in antagonistic or inhibitory hydro-
carbons that can modify the odors. The practical
result is that useful carbon life based on odor
breakthrough may be significantly less than if based
on hydrocarbon saturation (25).
A special activated carbon impregnated with caustic
(NaOH or. KOH) is often specified for odor control
applications in wastewater collection and treatment
works. HZS is adsorbed on the carbon surface, and
reacts to form elemental sulfur and sulfates.
Deep-bed adsorbers are generally used where odor
concentrations > 5 ppm are present, or when on-site
carbon regeneration is used. Bed depths range from
0.3 to 1.8 m (1 to 6 ft). Design capacities range up to
1,130 mVmin (40,000 cfm) Experience has shown
that a 0.9-m (3-ft) deep carbon bed offers sufficient
depth to provide reliable treatment efficiency without
causing excessive pressure drop.
Relatively little mechanical equipment is required
with an activated carbon odor control system other
than the vessel containing the adsorbent. In some
cases, contaminated air may bepretreatedby using a
grease filter and a condensing unit, which results in
temperature reduction of the air stream and more
efficient operation. A typical activated carbon ad-
sorber is shown in Figure 4-8.
A typical activated carbon odor control facility would
consist of air pretreatment units (optional), activated
carbon adsorber unit(s), exhaust fan(s), and associ-
ated piping and ductwork. Back-up carbon storage is
not normally required.
Activated carbon is typically regenerated thermally,
although special activated carbon, designed for odor
control applications and impregnated with caustic
Figure 4-8. Typical activated carbon filter for odor control.
a, Equipment
Thin-bed (about 2-cm) carbon adsorbers can provide a
useful service life if odor concentrations are low«5
ppm) and the effective mass transfer zone for
adsorption is very short (rapid adsorption kinetics).
Thin-bed adsorbers have an advantage of low resis-
tance to air flow. Activated carbon is retained between
perforated metal plates in flat, cylindrical or pleated
shapes. Cylindrical canisters are commercially avail-
able to handle about 0.7 mVmin (25 cfm), while
larger pleated cells can handle 21 to 28 mVmin (750
to 1,000 cfm), and systems comprising aggregates of
flat-bed components can handle 57 mVmin (2,000
cfm) (22).
Clean Air
r
Activated^
Carbon
(Dual Bed)
Odorous
82
-------
(NaOH or KOH), can be regenerated chemically using
a solution of NaOH or KOH. Thermal regeneration
using multiple hearth furnaces requires removal of
the carbon from the adsorption system. However,
chemical regeneration of NaOH- orKOH-impregnated
carbon is conducted in situ using a 50-percent NaOH
solution. Due to its lower cost per unit weight and its
ability to dissolve more sulfur per unit weight than
KOH impregnate, NaOH-impregnated carbon and
NaOH solution use for regeneration can result in
significant savings in operation and maintenance
costs.
b. Design
The design of activated carbon adsorbers involves
four fundamental steps:
1. Characterize the contaminated air {volumes,
concentrations of constituents) and the desired
effluent characteristics.
2. Select the adsorbent.
3. Conduct pilot studies to determine expected
performance, useful life of carbon, design
criteria, etc.
4. Apply pilot data to full-scale design.
The importance of full characterization of the con-
taminated air cannot be overemphasized. Of partic-
ular importance is the diurnal variation in air volumes
and odor intensity. Failure to accurately estimate
these parameters may result in poor performance or
higherthan anticipated carbon replacement or regen-
eration frequency, Characterization of specific consti-
tuents may also be important in design, since special
impregnated activated carbons now available may
demonstrate improved performance over standard
carbons. For example, activated carbon impregnated
with sodium hydroxide is often recommended for use
when sulfur-based compounds, such as H2S and
mercaptans, are the principal odor-causing materials.
In addition to claims of its superior performance, the
carbon can be regenerated in situ using a commercial
grade caustic in 50-percent solution.
Since exhaust air from sludge handling buildings, wet
wells, and covered process tankage often contains a
complex mixture of odorous compounds, pilot testing
is essential prior to full-scale design. Objectives in
pilot testing include defining expected performance
{removal efficiency), estimating the useful life of the
carbon, determining effectiveness and ease of carbon
regeneration, and developing design criteria for the
full-scale system. In addition to analytical determina-
tion of specific compounds such as H2S and methyl
mercaptan, an olfactometer should be used as a
measure of total odor removal through the system.
Odor breakthrough curves should be developed to
assist in determining frequency of carbon regenera-
tion or replacement.
An initial approximation of carbon life before odor
breakthrough can be gained by using Turk's equation,
(3):
t = (WHS) (4-4)
(E)(R)(C)
where,
t ~ carbon life, days
W = mass of adsorbent, g
S = proportion of maximum adsorption of
adsorbent, 0.16 to 0.5
E = average efficiency of carbon adsorber
R = gas flow rate, mVd
C = concentration of odorant, g/m3
c. Costs
Table 4-9 contains estimated costs for activated
carbon adsorbers for odor control at air flow rates of
28 mVmin (1,000 cfm) and 280 mVmin (10,000
cfm). It is assumed that under continuous operation,
HZS concentration at the inlet to the adsorber would
be 20 ppm average, and at the outlet, less than 1 ppm.
In practice, use of wet scrubbers may be recom-
mended ahead of the carbon adsorbers for this inlet
H2S concentration. Costs are also provided for carbon
adsorbers for an inlet H2S concentration of 10 ppm.
A specially manufactured activated carbon impreg-
nated with NaOH is normally specified for H2S
removal applications. This carbon can be regenerated
within the vessel using a 50-percent NaOH solution.
Table 4-9, Typical Costs for Activated Carbon Adsorbers
for Odor Control" (1984 9)
Design
Air Flow Capital Costs" Annual Costs"
mVmin
9
$/yr
28
29,800
6,200
280
128,000
48,000
"Assumed conditions:
1. Continuous operation
2. Inlet HjS = 20 ppm
3. Outlet HjS - <1 ppm
4. Caustic impregnated carbon
5. Carbon costs:
a. 28 m'min system: 56.67."kg (93 03/lb)
b. 280 m3min system: 65.48/kg ($2 49/lb)
6. Regeneration chemical (NaOH) cost = 90.1 7/L ($0.65/gal)
"Including adsorber vessel, fan, ductwork, appurtenances and
controls, installation.
including initial and replacement carbon plus regeneration
chemicals.
NOTE: Costs are for activated carbon adsorption alone; in practice,
use of wet scrubbers ahead of carbon may be recommended
for inlet HZS concentrations of 20 ppm.
83
-------
The 28-mVd (1,000-cfm) system would consist of a
single-vessel single-bed unit, while the 280-mVd
system would employ two dual bed units.
The costs shown are budget level estimates (+30
percent, -15 percent), presentedto show typical costs
of activated carbon adsorbers for odor control. In
practice, detailed cost estimates would be required
for site-specific analysis of alternative odor control
technologies.
4.3.2.3 Other Adsorption Processes
Other adsorptive media besides activated carbon
have been used for odor control applications. Two
such media are activated alumina impregnated with
potassium permanganate and wood chips mixed with
iron oxide. These alternatives are primarily suited for
small installations with relatively low volumes of
malodorous air requiring treatment.
Several commercial products are available which
consist of dry pellets of activated alumina impreg-
nated with potassium permanganate. Odorous com-
pounds are adsorbed into the surface of the pellets
and are subsequently oxidized by the potassium
permanganate. Pellet diameters are typically 3 to 9
mm (V's to 3/a in), and they contain approximately 5-
percent KM n04 by weight. Contaminated air is passed
through a deep bed or series of shallow beds
containing the media, and exhausted to the atmos-
phere. Prefabricated package systems are availableto
handle up to 280 m /min (10,000 cfm) of contami-
nated air.
Packaged odor control systems using KMn04-
impregnated alumina are typically horizontal or
vertical flow units employing a prefilter and/or a mist
eliminator for removal of particulates and moisture, a
series of 7.6-mm (3-in) deep beds containing the
media, and a blower. For some applications, a final
filter may be used.
The potassium-permanganate-impregnated activated
alumina pellets have a finite capacity for removal of
odorous compounds. When the capacity is exhausted,
it is discarded, as this media cannot be regenerated.
The useful lifetime of the media is dependent on the
total mass throughput of the odorous contaminant.
For H2S, it has been estimated that 1 kg of media will
remove 0.076 kg of HaS gas before it is exhausted.
Typical performance of a 3,2-cm (1,25-in) deep bed of
KMn0<-impregnated alumina for removal of H2S is
shown in Figure 4-9 (26). Such a system would not be
practical for treating air contaminated with high
concentrations of H2S, and is more suited for relatively
clean air applications such as control rooms, com-
puter rooms, etc.
The use of iron oxide filters for odor control has been
investigated at several locations, including the U.S.
Figure 4-9. Performance of KMnO^-impragnated activated
alumina adsorbers on hydrogen sulfide (28).
Influent HjS
Concentration:
TOO
0.5 ppm H*S
80
1.0
2.0
3.0
£ 40
5.0-
20
200
250
150
300
100
50
Air Velocity Through 1W Bed of Ve" Pellets, ft/min
and Norway. Such systems typically incorporate a bed
of wood chips mixed with iron oxide(Fe203). Contam-
inated air is passed upward through the bed, where
odorous contaminants are adsorbed by the media.
The postulated mechanism for H2S removal is de-
scribed by the following reaction;
Fez03 + 3 H2S > FezSa + 3 H20 (4-5)
A schematic diagram of an iron oxide filter is shown in
Figure 4-10.
Figure 4-10. Typical iron oxide filter for odor control.
Mixture of Wood Chips and Iron Oxide
Dense Top
Fan
Clean
Air -
Odorous
— Air
Bottom
Distribution Pipes
Wire Mesh
Drainage
A pilot system was studied in Norway, where wood
chips were mixed with iron oxide particles in the
proportion of 0.2 kg Fe2Oa/kg wood chips. The bed
was 0.4-m (1,3-ft) deep and was loaded at a rate of 16
to 18 m3/m2/hr (0.9 to 1.0 cfm/ft2). Figure 4-11
shows the results of the study. H2S concentrations in
the incoming air ranged from 10 to 380 //g/m3, while
concentrations in the exhaust air ranged from 0 to 3.3
//g/rn3, H2S removal efficiencies were from 90 to 100
percent. Ammonia removal was somewhat less,
ranging from 70 to 100 percent (27).
84
-------
Figure 4-11,
^ 30
c
i 20
1 10
~.
E 0
i—
-10
£
„• 30
E
> 20
E
rf, 10
c
-a 0
.3 400
g 300
O.200
^3.
yj 1 00
n
x o
soo
- 400
E
"^300
^200
^100
0
Performance of an iron oxide filter for odor
control (27).
-J 1 L
> •* *• ~ ^
Oct. Nov. Dec. Jan. Feb. Mar. Apr.May June July Aug. Sept.
I I 1
-1975—' i
-1976-
Month/Year
4.3,2.4 Ozone Contactors
Ozone is a powerful oxidant that has been used for
odor control, particularly in industrial applications.
The principle of ozone treatment of odorous gases is
that, given sufficient time for contact of the odorous
compounds with ozone, the odors will be eliminated
by chemical oxidation. Ozone can be utilized for the
oxidation of numerous odor-causing compounds.
Several examples of ozone reactions in odor control
applications are given below (28).
Ozone is a very unstable gas which requires genera-
tion on-site. For applications requiring greater than
0.9 kg (2 !b)/d of ozone, ozone is generated using the
corona discharge principle (29). This involves passage
of a parent gas such as oxygen or pre-treated air
through a discharge gap, across which a highvoltage
is applied. The resulting gas contains ozone at
relatively low concentrations of 1 to 2 percent by
weight. Package ozonators are available which
employ the corona discharge principle.
For economical operation of an ozone generator, heat
must be efficiently removed from the system, and the
feed gas must be clean, cool, and dry. Ozone
decomposes more readily at high temperatures, thus
requiring cooling systems using air or water. Water
and impurities in the parent gas can cause generation
of fouling agents which may coat the dielectrics, thus
lowering ozone production and increasing power
consumption. Moisture in the feed gas may also
cause formation of nitric acid in the corona, which is
corrosive and may result in reduced efficiency and
increased maintenance.
Ambient air processing equipment includes compres-
sors, heat exchangers, and various size filter units. A
typical air filtration system may include an air condi-
tioning filter, a 50-micron filter for the compressor
intake, a 5-micron filter, and a 4-angstrom molecular
sieve. The molecular sieve is used for the removal of
water vapor, carbon monoxide, carbon dioxide, nitro-
gen, methane, andHzS. Conditioned air is delivered to
the ozone generator at a dry -51 °C (-60°F) frost point.
Alternatively, pure oxygen can be used as the parent
gas, precluding the need for pretreatment (29).
The resulting gas stream, containing 1 to 2 percent
ozone by weight, is then introduced into a baffled
contact chamber to allow mixing and contact with the
air to be deodorized. Movement of the odorous air
through the contact chamber is effected through the
use of fans. Contact times may vary from 3 to 60
seconds, although 15 seconds is generally recom-
mended as a minimum. A typical ozone generator and
contact chamber system is shown in Figure 4-12 (29).
Hydrogen
sulfide:
major path
H2S + 03 minor Path
Amine;
R3N+¦ Oa ~
Methyl
mercaptan:
CH3SH +03
-$»¦ S + H2O + O2
SO2 + h2o
-> R3N-O +o2
(amine oxide)
(4-6)
(4-7)
->
->CH3-S-S-CH3 + Oa
CH3-SO3H + 02 (4-8)
(methyl sulfonic acid)
a. Equipment
For many odor control applications, a package ozone
generator may be purchased. Most applications
requiring less than 45 kg (100 lb)/d of ozone employ
ambient air as the parent gas, while larger systems
may use pure oxygen, either supplied in containers as
a liquid, or generated on-site. For odor control
applications, use of oxygen as a feed gas is itiore likely
to be economically impractical, since it cannot be
recycled as it can for other applications, such as
disinfection of treatment plant effluents.
Equipment requirements for an air-fed ozone system
consist of an air pretreatment system (compressors.
85
-------
Figure 4-12. Typical ozone system for odor control (29).
Odorous
Air
Mixing Zone
Dispersion
Stack
Clean
Contact'Chamber
Ozone
Generator
Oxygen or
Pretreated Air
heat exchanger, air filters, molecular sieve); an ozone
generator and diffuser or injector; a baffled contact
chamber; and appropriate piping, valves, appurte-
nances and controls. Ozonators generally come
equipped with a number of safety features, including
a pressure switch on the air circuit, a safety relief
valve and flow switch on the cooling water circuit,
microswitches on cabinet doors to automatically shut
off power when the doors are opened, and audible
and visible alarms. Controls are electrically inter-
locked for shutdown in the event of malfunction (30).
Ozone dose may be controlled by a manual setting, or
by the use of an ozone residual meter, which by
monitoring ozone concentrations in the treated
exhaust air sends a signal back to the generation
equipmentthat increases ordecreasesthe amount of
ozone fed to the contact chamber.
Ozone is a powerful oxidizing agent, second only to
fluorine, and thus some care must be exercised in
selecting materials. Stainless steel is preferred for
piping materials in contact with ozone, although
unplasticized PVC has been used. Ozone will attack
the unsaturated portions of the plasticizer molecule,
and eventually soften normal PVC. Epoxy resin is not
suitable for joining PVC pipe exposed to ozone,
requiring either threading and use of Teflon tape, or
hot welding. Teflon tape is inadequate for sealing
stainless steel joints; Teflon tape with RTV cement
has been found to be satisfactory. Gaskets should be
manufactured from fluoroelastomers, as should the
diaphragms in an ozone metering pump (31). Ozone
contact chambers have been constructed of concrete,
fiberglass, PVC, transite, and stainless steel.
b. Design
Design criteria for an ozone odor control system
include: 1) type of odor; 2) concentration of the odor;
3) temperature of the exhaust gases; 4) humidity of
exhaust gases; 5) retention time within the contact
chamber; and 6) the distribution of ozone within the
contact chamber (32).
In many cases, the feasibility of ozone oxidation for
odor control can be established in the laboratory.
However, on-site pilot testing is often necessary to
accurately determine design parameters.
Normally, 3 to 4 ppm O3 by volume fed to the odorous
gas stream is sufficient to control odors. This may be
more or less depending upon the specific application.
An important consideration in designing an odor
control system using ozone is to size the equipment
based on peak requirements, since magnitude of the
odor problem often varies diurnally and seasonally.
Although 1 to 2 ppm is often cited as a sufficient
ozone dosage to handle odors from wastewater
treatment plants, this may not be adequate for odor
concentrations at peak conditions. Exhaust air from
sludge storage tanks and dewatering rooms may
require 10 ppm of ozone or more. A general guideline
for H2S oxidation is that one ppm O3 by volume will
oxidize 10 ppm H2S by volume (33).
Reaction times in the contact chamber may vary
considerably. Although contact times of as little as 7
seconds have been effective for odor control at
wastewater treatment plants, contact times of 30 to
40 seconds are more commonly recommended
(30)(34). The required detention time is dependent on
the type of odor and its concentration, and general
design criteria cannot be employed for specific odor
control applications. The contact chamber should be
designedfor complete, intimate mixing and contact of
ozone with the odorous air to ensure complete and
efficient oxidation.
Given the necessary dosage of ozone for oxidation of
odorous compounds and the air flow rate. Figure4-13
can be used to estimate the ozone requirement in
grams per hour (28). Thus, for an air flow of 140
m3/min (5,000 cfm) and a required dosage of 3 ppm,
the ozone requirement would be approximately 50
grams per hour, or 1.2 kg (2.6 lb)/d.
Other important design considerations when consid-
ering odor control by ozonation are the occupational
and environmental health and safety aspects. OSHA
has set a maximum 8-hour continuous occupational
exposure level of 0.1 ppm for ozone. Ozone can be
detected by the human nose at concentrations as low
as 0.04 ppm, which often allows detection of leaks
before ozone concentrations become hazardous.
Ozone dosage should be adjusted to minimize dis-
charge of unreacted ozone. Ozone monitors can be
installed at the discharge of the contact chamber to
either trigger an alarm or send an electrical signal to
the ozone generator to increase or decrease the rate
ofgeneration,
c. Costs
Typical costs for ozone systems are shown in Table
4-10 for air flow rates of 28 m3/min (1,000 cfm) and
86
-------
Figure 4-13. Ozone requirements for various airflows and
ozone dosages (28).
100
90
70 -
60 -
£ 50 -
-------
Figure 4-14, Direct flame oxidation systems (20).
Burner
Clean, Hot Gases
Odorous Fumes
Fuel
Combustion Air
Residence Chamber
a) Conventional Direct Flame Combustion
Burner
Clean, Hot Gases
Preheated Fumes
Fuel
Combustion Air
Heat Exchanger
Residence Chamber
Odorous Fumes
b) Direct Ftame Combustion with Heat Recovery
Figure 4-16. Catalytic oxidizer with heat recovery (22).
Burner
Clean, Hot Gases
Preheated Fumes
Fuel
Combustion Air
Heat Exchanger
Catalyst
Odorous
Fumes
and manganese, are used, Although the precise
mechanism of catalytic combustion is not well
understood, it proceeds through three steps: 1) ad-
sorption on the active surface; 2) chemical reaction
(oxidation) on the surface; and 3) desorption of the
reaction products. One major limitation of catalytic
oxidation is that it is generally recommended for use
only when the concentration of odorous gas is at least
1,000 ppm (3). Most applications of catalytic oxidation
systems have been for removal of solvents and
organic vapors from industrial processes.
One concern with catalytic combustion systems is the
potential poisoning or fouling of the catalyst from
contaminants in the incoming air. Table 4-11 lists
various materials that can result in poor performance
of the catalyst. Note that sulfur compounds are listed
as catalyst suppressants, which may limit the appli-
cability of catalytic oxidation systems for odor control
in municipal wastewater treatment systems.
Table 4-11. Agents That Adversely Affect Catalysts of the
Platinum-Group Metals (22)
Type of Agent Examples
Poison
Heavy metals
Phosphates
Arsenic
Suppressant
Halogens (free and combined)
Sulfur compounds
Fouling agent
Inorganic particles
Alumina and silica dust
Iron oxides
Silicones
Advantages of direct flame oxidation over catalytic
combustion include lower maintenance costs, less
downtime, and better odor control. A catalytic system
may require less fuel but may have higher overall
power requirements (3).
Choice of one system over another will depend on the
specific application and the characteristics of the gas
to be treated.
a. Equipment
There are many commercially available combustion
devices marketed for odor control applications. Each
design is somewhat different, and manufacturers
should be contacted to determine specific details
regarding design and construction of particular units.
Little additional equipment is required for a direct-
flame or catalytic combustion system. Ductwork is
required to convey the contaminated air to the unit,
and high temperature exhausts must be discharged
through a stack to prevent human contact. A suitable
source of fuel must be available. Temperature sensors
should be located in the influent air ductwork, in the
combustion chamber, and in the exhaust stack to
ensure proper temperatures are maintained for
complete combustion. Heat exchangers are often
used to defray fuel costs for preheating influent air.
88
-------
Materials of construction include high temperature-
resistant metals, insulating materials, and refractory
materials that are used in the combustion chamber to
withstand temperatures of 650° to 760°C (1,200° to
1,400°F).
b. Design
Major considerations in design of thermal combustion
systems for odor control include residence time,
temperature, and mixing. Although mixing is often
overlooked, it may be one of the most important
parameters in design of an efficient system. Ineffec-
tive operation can often be traced to poor mixing
characteristics (35).
Temperature required for complete combustion is
dependent on the compounds to be oxidized. Gener-
ally, in direct flame units, temperatures of 480° to
815°C (900° to 1,500°F) must be maintained.
Relatively small temperature drops can result in
substantial loss in oxidation efficiencies of specific
compounds.
Residence time is closely interrelated with tempera-
ture, Increased temperature results in expansion of
the gas and therefore lowers residence times in the
combustion chamber. Typical residence times re-
quired for odor destruction range from 0.25 to 0.6
second.
Figure 4-16 shows a flow diagram for a direct flame
combustion system. As shown, a portion of the
contaminated air is bypassed directly to the combus-
tion chamber, the remainder being mixed with air.
This proportion is generally about 50 percent. The
mixture of air and slipstream enters the combustion
chamber, and is then mixed with the bypass contami-
nated air. Achieving good mixing of these two streams
is critical for efficient operation. The mixture must be
retained for a sufficient time to allow complete
oxidation of the malodorous components.
Proper mixing can be accomplished through proper
selection and sizing of burners, combustion cham-
bers, and reactor configurations. One approach is the
use of burners distributed in the combustion chamber
Figure 4-16. Flow diagram for a direct flame oxidation
system.
Supplementa
Clean Air
Dilute Fume
if Required
Odorous Fumes
Bypass Stream
Slipstream
Mixing of
Fume and Hot
Combustion
Gases
~650-800°C
Retention of
Fumes at High
Temperature for
Sufficient Time
area (35). Location and size of burners is important.
They must be placed where the flow is fully turbulent
and where good mixing conditions prevail. Velocities
of 3 to 9 m/s( 10 to 30 ft/s) are generally sufficient for
molecular mixing (35). Baffles have also been effec-
tive for improved mixing.
An example of the use of combustion systems for odor
control is at the 760,000 m3/d (200 mgd) Metro Plant
in Minneapolis, Minnesota. The sludge handling
scheme consists of gravity and dissolved air flotation
thickening, sludge storage, low pressure oxidation
(LPO), plate and frame filter presses, and multiple
hearth incineration. There are four decant tanks for
the LPO system and odors from the tanks are
contained for destruction or treatment. The Metro
Plant has several options for treatment of the odors
from the decant tanks.
The first option is to use the odorous air as secondary
combustion air for the incineration process. Exhaust
air from the decant tanks is combined and diluted
with the air from the solids handling building and fed
to the incinerator. Experience has shown that the
higher the temperature in the hearths, the better the
removal of odors. The Minnesota Pollution Control
Agency restricts the plant to a n odor concentration of
less than 150 odor units at the plant boundaries, If the
incinerator is down, the plant can use the originally
designed wet scrubber followed by activated carbon
contactors. Problems have been experienced with the
carbon regeneration which did not remove all the
organic ketones contained in the decant tank vapors.
As a result, the ketones remaining on the carbon
caused spontaneous combustion in the carbon bed
during backwash with steam at 93° to 1 21 °C(200°to
250°F!.
The city tried using a spray chamber absorption
system but was not satisfied with the results. An
effluent gas with a sweet odor was produced,
apparently the result of the organics not being
completely removed.
C. Costs
Typical costs for direct flame incinerators and catalytic
incinerator systems for odor control are shown in
Tables 4-12 and 4-13, respectively (36). The cost of
thermal and catalytic incinerators is a function of the
volumetric air flow rate and heat content of the gas,
and various design factors, including materials of
construction, type of refractory, level of control and
required air residence times.
Budget level cost estimates(+30 percent, -15 percent)
have been developed to show typical costs of combus-
tion systems for odor control. In practice, detailed cost
estimates must be developed which take into account
the specific site conditions, actual concentrations and
89
-------
Table 4-12, Typical Costs for Thermal Incinerators for Odor
Conlrol" (1984 S)(37)
Figure 4-17, Soil/compost filter for odor control.
Capital Cost", S
Air Flow
Air Residence
Time
28
mVmin
280
sec
0.2
19,800
31,500
0.5
24,800
41,300
1.0
31,400
51,200
20
41,300
72, 600
"Assumed HZS inlat concentration = 20 ppm; outlet concentration
<1 ppm,
includes cost of incinerator, incinerator base, fan, motor, starter,
integral ductwork, controls, instrumentation, and refractory
linings. Does not include heat exchanger.
Table 4-13. Typical Costs for Catalytic incinerators°(1984
$)<37>
Capital Cost , $
Air Flow
Type of
mVmin
incinerators
28
280
Package units
26,100
42,500
Custom units without heat
exchange
35,300
70,300
Custom units with heat
exchange
60,900
97,700
"Assumed HaS inlet concentration ppm, outlet concentration <1
ppm,
"Includes catalyst bed, refractory lining, preheat burners, ductwork,
fan, controls, installation,
variability of odorous contaminants, necessary ven-
tilation rates, and objectives in odor control,
4.3.2.6 Soil/Compost Filters
Soil and compost filter beds have been successfully
used to remove odors from relatively small volumes of
air containing odorous compounds (37-40).
They can be used as a primary odor control system or
as a polishing step for exhausts from other odor
control systems such as wet scrubbers. Contaminated
gas is introduced Into a bed of soil or compost through
perforated pipe located near the bottom of the bed. As
the gas passes upward through the bed, the soil
substrate sorbs and oxidizes odorous gases such as
HaS, SOa, and NH3. The oxidation products are CO2,
Hz0, sulfate, and nitrate.
The soil provides sorptive surface area, structural
support, and nutrients and water to support bacteria
which can biologically oxidize certain odorous com-
pounds. Several different configurations of soil or
compost filters have been employed. A schematic
diagram of a soil/compost filter system is shown in
Figure 4-17.
Clean Air
t t t
Soil or Compost
t ~ ~ /
Odorous
Air
- >np\
Air Distribution Pipe
Soil filters require moisture for the sorption and
reactions of both organic and inorganic gases. To
maintain an optimum moisture content, watering of
the bed may be required. Too little water may cause
reduced microbial activity, lower sorption capacity,
and cracking of the soil which may lead to short-
circuiting and release of odors. Too much water may
cause development of anaerobic conditions and loss
of filter function. In very moist climates, it may be
necessary to provide an underdrain for removal of
excess water, or to cover the filter bed.
Temperature of the bed is important to filter per-
formance. Temperature of the bed is affected by the
temperature of the inlet gas and degree of microbial
activity. While an active microbial population is
desirable, very high soil temperatures decrease the
capacity for physical adsorption of the odorous gas.
Soil filters typically use sandy loam, compost, or
mixtures of soil and peat moss, with depths of 1 to 3 m
(3 to 10 ft). Performance is dependent on numerous
factors, including the type and concentration of the
odorous compou nds to be removed, characteristics of
the filter media (organic content, bulk density,
porosity, etc.), moisture content of the bed, tempera-
ture, bed depth, and time in service.
A full-scale compost filter system was constructed in
1978 at Moerewa, New Zealand to treat odors from a
treatment facility handling wastes from a rendering
plant (40). Approximately 1 m (3 ft) of sludge-derived
compost was used as the filter media in a 7 m x 6 m
(23 ft x 20 ft) bed. Perforated plastic pipe was used to
distribute the odorous air into the filter bed. The
bottom of the bed was sloped slightly to allow
collection and removal of excess water. The system
was designed to treat 15 mVmin (530 cfm) of air
containing H2S concentrations of up to 1,000 ppm by
volume. Results from 4 months of testing of the full
scale odor removal system are shown in Table 4-14.
As can be seen from the data, the filter bed was
successful in removing the high H2S concentrations
in the influent air.
90
-------
Table 4-14. Performance of Compost Filter for H?S Odor Control at Moerewa, New Zealand (40)
H2S Concentration
Ambient Air
Internal Temp,
of Filter Bed
Influent
Effluent*
Atmosphere"
Mverage n
Removal
Temperature
Max.
Min.
Max.
Min
Max.
Min.
(inft.-effl)
"C
°C
ppm
%
7 13
29-57
660
37
0,08
0.00
0.32
0,00
99.97
12-28
33-58
600
9
0.11
0.01
0.18
0.00
99.79
16-29
32-55
820
49
0.19
0.00
0.24
0.00
99.93
10-27
21-46
703
156
0.29
0.00
0,08
0.00
99.96
10-28
24-51
487
112
0.10
0.00
0.10
0.00
99,98
10-29
24-50
406
219
0.11
0.00
0.08
0.00
99.98
16-25
34-46
669
139
0,10
0,01
0.10
0.00
99,98
19-28
20-47
320
75
0.12
0.00
0.04
0,00
99,98
8-25
21-52
400
84
0.19
-
0.04
0,00
99.91
7-23
11-32
195
69
0.00
0.00
0.04
0,00
100.00
9-23
17-40
535
70
0.00
0,00
0.00
0,00
100.00
5-21
15-40
99
23
0.02
0.00
0.00
0.00
99.99
'40 mm above surface.
b2 m above surface.
Pilot studies on soil beds for HZS control were
conducted at the University of Washington (38).
Major conclusions from the study were:
1. Moist loam soils were found to have excellent
possibilities in the efficient and inexpensive
removal of undesirable odorous gases from air
streams. Sand and clay were less satisfactory
soils.
2. The effects of ion exchange, chemical composi-
tion, or oxidation and soil or water adsorption
were negligible, the primary odor reduction
mechanism being through the action of micro-
organisms.
3. Conditions should be maintained in an optimal
range for bacterial growth. The soil should be
kept moist and warm.
4. Over a 3-month test period, H2S gas concentra-
tions of 15 ppm at a loading rate of 0.11
m3/min/m2 (0.35 cfm/ft2) of soil surface were
reduced to imperceptible levels in 81 cm (32 in)
of soil.
5. Effectiveness of the'soil beds for removal of H2S
did not diminish duringthe 3-month test period.
a, Equipment
Equipment requirements for a typical soil/compost
odor filter are minimal. A simple system consists of a
perforated pipe network surrounded by gravel to
prevent plugging of the perforations, and overlaid
with a suitable media such as sandy loam or compost.
Air piping should be constructed of acid-resistant
materials such as PVC. Valves may be employed to
direct odorous air to certain portions of the filter bed,
or to bypass parts of the beds to allow maintenance.
For moist climates where soil saturation may be a
problem, an underdrain system may be used to
remove excess water, or the beds may be covered to
prevent direct infiltration from rainfall. In dry climates,
periodic watering may be required to maintain a
suitable moisture content in the bed. This may be
accomplished manually or through the use of an
automated watering system.
b. Design
Design of soil or compost filters depends on the
nature and concentration of odorous compounds, air
flow rates, target concentrations in exhaust gas, and
characteristics of the soil or compost media. Designs
vary from simple filter beds with networks of per-
forated PVC pipe to more elaborate systems with
concrete berms, underdrain systems to collect excess
water, and automated watering hardware. Most
designs employ a gravel pack surrounding the gas
conduit. Gravel suitable for this application might
have diameters ranging from 2 to 4 cm (0,8 to 1.6 in).
Depth of gravel depends on the size of the gas
conduit, but is typically 0.2 to 1 m (0.8 to 3 ft). A
permeable barrier such as marsh hay, screens, or
synthetic fabric is placed above the gravel to prevent
soil fines from infiltrating into the gravel and air
distribution system. The gravel is then covered with 1
to 3 m (3 to 10 ft) of loosely packed sandy loam or
compost.
The air loading rate (volumetric rate of air flow per
unit of horizontal filter area) to the filter bed should be
determined empirically. Loading rates for experimen-
tal and full-scale facilities have ranged from 0,11 to
over 1.0 ma/min/mz(0.35 to 3.3 cfm/ft2M37-40). For
high concentrations of odorous compounds or com-
pounds which oxidize slowly, loading rates on the low
91
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end of the above range are employed. Higher loading
rales would be appropriate for air contaminated with
only 10 to 15 ppm of H2S.
Because of the many variables affecting design and
performance, it is recommended that pilot studies be
conducted before designing full scale facilities. Such
studies can be conducted at little cost in order to
collect sufficient data to establish design criteria.
c. Example—Columbus Southwesterly Compost
Facility, Columbus, Ohio
This compost facility, which handles about 1 82 metric
tons (200 tons)/d of raw and waste-activated sludge,
had odor complaints from the community and a court
order to cease the odor nuisance. Sludge is hauled as
a 16-percent TS centrifuge cake to the facility from
the Southerly Wastewater Treatment Plant (WWTP)
in Columbus, Ohio.
Most of the odors emanated from the mixing opera-
tion, where sludge and wood chips were combined.
Therefore, a30mx60mx9m high (100 ft x 200 ft x
30 ft) pre-engineered metal building, with an air
handling system, was erected to contain this opera-
tion. The air handling system provides five to six air
changes/hour. The air is exhausted through a
network of plastic pipes into a "bio-filter" which
contains about 0.6 m (2.5 ft) of cured, composted
material.
This project has been successful, but the facility
generated odors from other sources, particularly the
composting and curing piles. It was identified that the
odors only occurred when insufficient air (oxygen)
was supplied to these operations. I n fact, plans have
been made to add air to the curing stages (30-days
storage). By keeping the piles aerobic, objectionable
odors will be reduced.
d. Costs
Table 4-15 presents estimated costs of compost
filters for odor removal at air flow rates of 28 mVmln
(1,000 cfm) and 280 imVmin (10,000 cfm).
The costs presented are budget level estimates (+30
percent, -15 percent). Actual costs would depend on
site conditions, availability and costs of compost, and
the degree of sophistication of the design. Provisions
for concrete berms, underdrains, covers, or sprinkler
systems would increase the costs.
4.3.2.7 Existing Biological Stabilization ProcBSSBS
In several cases, odorous air has been collected and
piped into the bottom sections of trickling filters and
activated sludge basins for effective odor control.
Introducing air into the bottom of a trickling filter
results in countercurrent flow similar to that in a wet
Table 4-15. Estimated Costs of Compost Filters for Odor
Control8 (1984 $(
Design Annual Cost of
Air Flow Capital Cosl£b Compost Replacement
mVmin
$
$/yr
26
9,700
300
260
89,000
3,000
"Assumed conditions:
1. Continuous operation
2. Inlet H2S = 20 ppm
3. Outlet HjS = <1 ppm
4. Loading rate of 0 2 m3/min/m! (0.66 cfm/ftJ)
5. Compost replaced after 5 years
"including excavation, piping and installation, fan, compost, backfill
and grading.
scrubber system, and can be a cost-effective tech-
nique for odor control. This method has been success-
fully used at Beaumont, California; Avila Beach,
California: Stockton, California; Sydney, Australia,
and elsewhere (39).
In some cases where the influent to the trickling filter
contains dissolved sulfides, introduction of odorous
air into the bottom of the filter may strip out HzS from
the liquid, and discharge malodorous gases at the top.
At Palm Springs, California, two perforated pipes
discharged waste digester gas and sewer air to a
point mid-depth in the 2.9-m (9.5-ft) deep rock filter
(39). Air was drawn down through the filter in
sufficient volumes to maintain a constant downward
air velocity that prevented escape of odors at the top.
Activated sludge basins have been used for odor
control by introducing malodorous air into the inlet
side ofthe blowers. Biological deodorization occurs in
the overlying aerated liquid in the aeration basin. This
method was tested at pilot-scale and subsequently
adopted for use at the Los Angeles Hyperion plant.
This method is most applicable for controlling HzS
odors, because sulfide is rapidly oxidized in the
aerobic mixed liquor. Other odor compounds may not
react this way, and although some dissolution may
occur, odors may not be removed completely (39).
a. Equipment
Equipment requirements are minimal, and consist
mostly of piping to convey the odorous gases to the
biological process. Where trickling filters are used for
biological odor control, an auxiliary blower may be
required to force the gas through the media. For an
activated sludge system, existing blowers can be
employed to inject the odorous gas into the basin.
Piping should be corrosion-resistant to prevent attack
by HzS gas and sulfuric acid which may form from the
oxidation of sulfides. H2S in the input air generally
does not harm blowers, since the high temperature
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prevents condensation. Downstream piping may be
affected, however, due to cooling, condensation of
moisture, and production of sulfuric acid. Underdrains
in trickling filters may also be affected, and should be
constructed of corrosion-resistant materials.
b. Design
Effectiveness of the technique depends on the nature
and concentration of the odor, wastewater character-
istics in the stabilization process, volume of air to be
treated, and the area and depth of the stabilization
units. For trickling filters, contact time of the gas with
the wastewater and media is important. Minimum
contact time of 8 to 10 seconds has been found
effective (39).
Piping of odorous air to activated sludge basins
should consider dispersal of the air over the entire
basin to achieve initial dilution and allow rapid
oxidation of sulfides in the mixed liquor.
Because of the lack of design criteria and the wide
variability in gas characteristics and process designs,
pilot studies are recommended prior to implementa-
tion of this odor control alternative. Such studies can
be conducted relatively inexpensively to determine
the effectiveness at various air flow rates.
c. Example—Kalamazoo WWTP, Kalamazoo,
Michigan
At the 204,000 m3/d (54 mgd) Kalamazoo WWTP,
odors from all of the covered plant processes are
collected in fiberglass piping and directed into the
activated sludge air blowers and diffused into the
powdered activated carbon secondary aeration
basins. The processes from which air is collected
include grit chambers, primary clarifiers, thickeners,
screen room, low pressure oxidation decant tanks,
vacuum filter building and vacuum pumps. The air is
directedto a block-constructed plenum and serves as
make-up air to the centrifugal air blowers.
Air from the decant tanks is first run through a wet
scrubber before being combined with the other
odorous gases. In addition, odorous air from the
decant tank will be used as combustion air for the
incineration process.
4.3.3 Odor Modification, Counteraction, and
Masking
There has been a great deal of confusion as to the
purpose and effectiveness of using various chemical
agents to disguise odors, reduce their intensity, or
render them less offensive. The following definitions
are therefore provided (41H42).
Odor modification is the name given to a process
whereby two substances of given concentration are
mixed, with the resultant odor of the mixture being
less intense than that of the separate components,
and in some cases imperceptible.
Odor counteraction is the name given to the phenom-
enon whereby odor intensity is reduced by adding
non-chemically reactive odor agents to a malodor.
Examples of pairs of odiferous gases that, in certain
proportions, result in the mixture being odorless or
nearly so are: ethyl mercaptan andeucalyptol; skatole
and coumarin; and butyric acid and juniper oil.
Odor masking is the namegiventoa method by which
the quality of a malodor is overcome by mixing it with
a substance having a strong, pleasant odor. Thus, the
pleasant odor effectively masks the malodor, resulting
in a less objectionable odor.
Counteraction agents are available to counteract the
following odor types (3):
1.
Phenolic
2
Aldehyde
3.
Amine
4,
Solvent
5.
Mercaptan
6.
Aromatic
7.
Organic, fatty acid
Typical masking agents include heliotropin, vanillin,
eugenols, benzyl acetate, and phenylethyl alcohol.
These are organic aromatic compounds that can be
effective as short-term or emergency solutions to
severe odor problems.
Odor modification, counteraction, and masking have
met with variable success at wastewater treatment
plants. They have often been applied as interim
solutions, or for short duration during periods of odor
emissions. In some cases where odor masking agents
have been used, the resulting odors have been
considered equally unpleasant. The problem is often
further complicated by the subjective human re-
sponse to odors. In general, prevention of odor
emissions and positive control and removal of the
odor-causing substance(s) are preferred alternatives
for effective odor control.
A more detailed discussion of the theory of odor
modification is contained in Reference 43.
4,3.3.1 Equipment
Equipment requirements for application of odor
modification, counteraction, and masking agents are
generally minimal and depend on the method of
addition. Application techniques include the following
(41):
1. Direct addition to the odorous material
2. Spraying
a. hand spraying
b. automated spraying
c. stack spraying
d. aerial spraying
93
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3. Metered feeding to the liquid stream
4. Dispenser feeding to an air stream
Oneof the most common application techniques used
at wastewater treatment plants is automated atom-
ized spray systems located around the periphery of
the plant for dispersion of masking agents.
4.3.3.2 Design
Designing for use of an odor modification agent
requires characterization of the odor and screening of
various chemical agents to determine their effective-
ness, This requires subjective testing, as with an odor
panel.
Important considerations when evaluating use of
odor modification, counteraction, and masking agents
are (3)(41):
1. ANY GAS OR VAPOR THAT MAY BE TOXIC
(SUCH AS HzS) SHOULD NOT BE DISGUISED.
2. Odor modification should not be used as a
substitute for good housekeeping.
3. If possible, odors should be controlled at the
source.
4. Odor modification, counteraction, and masking
agents must conform to local, state, and federal
specifications and regulations.
5. Odor modifying chemicals should always result
in a lowering of the olfactory intensity.
4,4 Corrosion in Wastewater Treatment
Plants
The physical and chemical mechanisms of corrosion
are fully discussed in Section 2.4. The following
discussion will focus on the conditions in wastewater
treatment plants that lead to corrosion of materials.
4.4.1 Continuous or Intermittent Submerged
Exposure
These conditions include those in which the compo-
nent is subjected to direct contact with wastewater,
wastewater sludge, or various process sidestreams
on either a continuous or intermittent basis.
Plant components or structures at the waterline are
subject to the most severe exposure in terms of
potential corrosion, since the structure is alternately
subjected to wet and dry conditions as well as
fluctuations in ambient temperatures. Water itself
may be destructive because, in the presence of salts,
it acts as an electrolyte. Water may hydrolyze certain
paint components, which decreases both the strength
and adhesion of the coating. In addition, it may
decrease the resistance of the coating to passage of
oxygen and other gases such as H2S, which may be
corrosive to the underlying material. Oils, greases,
andsoaps which typically accumulate at the waterline
of wet wells, holding tanks, and aeration basins may
also present a problem because these materials
contain solvents which may soften the paint or
coating and make it more susceptible to abrasion or
rupture.
Paints and protective coatings at the waterline are
also subjected to physical forces, such as those
stresses formed by alternately wetting and drying,
heating and cooling, and freezing and thawing. These
actions may be highly destructive. In addition, sun-
light may affect certain organic coatings and reduce
their effective life.
Iron and steel do not readily deteriorate when
completely submerged in wastewater due to the
formation of a protective glassy iron oxide film on the
surface. However, when exposed to the atmosphere,
the iron oxide coating quickly comes loose to expose
fresh metal, and the corrosion process is continued.
Metallic zinc coatings on steel aeration basins or on
metal components in aeration basins are subject to
deterioration, apparently due to the presence of
carbon dioxide resulting from biological oxidation,
and from the high DO content (44).
The presence of high DO concentration is also cited
as the reason for accelerated deterioration of mate-
rials in pure oxygen activated sludge plants. This was
the subject of an EPA-sponsored research project in
which the resistance of various materials was
evaluated under the conditions found in such plants
<45).
4.4.2 Moist Atmosphere Exposure
Enclosed wet wells, grit and screen chambers,
holding tanks, and buildings housing wastewater or
sludge processing equipment are often potential sites
for corrosion due to the presence of moisture as well
as corrosion-inducing gases such as HaS. In such
enclosures, moisture tends to condense in a film on
cold surfaces sucl] as walls, handrails, pumps,
conduits, and windows. The moisture absorbs oxygen
and other gases which may result in production of
very corrosive condensates.
It has been found that H2S gas can penetrate many
paints and coatings. Upon reaching steel, HzS reacts
directly to form iron sulfide. Buildup of corrosion
products and gas may blister and rupture the coating,
allowing more rapid corrosion of the freshly exposed
material. HZS may also be oxidized to sulfuric acid in
the presence of oxygen and moisture, allowing the
acid to attack the coating or penetrate the surface
through pinholes or defects. The acid will then
vigorously attack the underlying material, allowing
corrosion to spread laterally beneath the coaling.
Another problem with exposure to materials in moist
atmospheres is the physical effects of stresses in the
94
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paint or coating due to fluctuations in moisture and
temperature. Such variations result in expansion and
contraction of the coatings which may result in
rupture of the protective film.
Moist atmosphere exposure is perhaps the single
most destructive condition for protective coatings and
other materials encountered in a wastewater treat-
ment plant (45).
4.4.3 Dry Atmosphere Exposure
Relatively dry atmospheres inside buildings are
generally not conducive to corrosion unless HgS gas
is present. H2S may attack metal directly, and this can
present a problem with oxidation of electrical contacts
and other metal surfaces. In addition, exposure of
certain paint pigments to H2S may result in discolora-
tion.
4.4.4 Outside Weather Exposure
Normal exposure to ambient weather conditions can
result in corrosion and deterioration of paints and
protective coatings due to the effects of sunlight,
humidity, temperature fluctuations, salt, and abra-
sion. Although such problems may be found at any
building or structure, the effects may be accelerated
by the presence of HZS or other corrosive chemicals
associated with sewage treatment works.
Effects of exposure to ambient conditions include
aging, cracking, and discoloration of paints due to
sunlight; deterioration of metal surfaces by salt air;
wear of paints and coatings by abrasion from blowing
dust and sand; deterioration of concrete by freeze-
thaw cycles; and discoloration of paints and corrosion
of metal surfaces by H2S.
4.4.5 Miscellaneous Exposure
Materials used in certain unit processes or process
components may be subjected to special conditions
which foster corrosion. Examples include physical-
chemical treatment processes, anaerobic digester
heating systems, plant boilers and steam piping, and
thermal sludge conditioning systems.
Physical-chemical treatment system components are
often subject to corrosion and scaling from the use of
chemicals such as lime and ferric chloride. Scaling
may be a serious problem with the use of strong
alkalies due to precipitation of calcium carbonate
scale on metal or concrete surfaces at elevated pH.
Ferric chloride is a very corrosive material which will
depress the pH of wastewater, resulting in a more
aggressive water which may attack both concrete and
metallic components.
Boilers and heat exchangers used in digester heating
may also be subject to corrosion and scaling due to
calcium precipitation and galvanic currents estab-
lished from differences in temperature.
Stress corrosion and surface pitting of stainless steel
heat exchangers in thermal sludge conditioning units
have been reported at chloride concentrations ex-
ceeding 500 mg/I. Use of caustic soda was adopted at
Green Bay, Wisconsin to increase the pH of the
sludge from 4.2 to 5.5 in order to control the rate of
corrosion in the thermal sludge conditioning system.
Formation of calcium oxalate scale required flushing
of the system with a 5-percent nitric acid solution
every 500 operating hours (46).
4.5 Corrosion Control Techniques at
Existing Wastewater Treatment Plants
A variety of methods is available for control of
corrosion in wastewater treatment plants, Thefollow-
ing discussion presents the most common alterna-
tives employed for corrosion control.
4.5.1 Chemical Addition
Chemical addition has been employed to control
sulfide-induced corrosion in wastewater treatment
plants. The action of chemicals on HZS results in
oxidation, precipitation, or inhibition of sulfide forma-
tion. Chemicals previously discussed in this chapter
include hydrogen peroxide, metal salts, nitrates, and
chlorine. These chemicals have been found effective
for sulfide control; however, not all corrosion prob-
lems are sulfide-induced, and use of such chemicals
may not be appropriate for control of corrosion due to
other conditions or agents. For example, for low pH
waters, addition of alkalies such as caustic soda can
elevate the pH to a neutral range, greatly reducing the
rate of acid-induced corrosion of concrete and metal
surfaces.
4.5.2 Cathodic Protection
Cathodic protection has been successfully used for
corrosion control of iron and steel components, such
as clarifiers, aeration tanks and sludge digesters, in
both water and wastewater treatment systems.
Cathodic protection is defined as "the reduction or
prevention of corrosion of a metal surface by making
it cathodic, for example, by the use of sacrificial
anodes or impressed currents" (44). If an electrical
current from any source is impressed on a corroding
metal in sufficient amounts to neutralize the corrosion
currents, corrosion will cease in that particular area.
In normal applications of cathodic protection, the
metal to be protected is electrically connected to the
negative terminal of a current source, and the positive
terminal is connected to an anode in the corrosive
electrolyte. Current from the anode passes through
the electrolyte to the metal, making it cathodic and
reversing the current at the anodes of localized
galvanic cells which have been established in the
metal. The source of current is typically a rectifier
which supplies low voltage direct current of several
amperes.
95
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Cathodic protection systems using sacrificial anodes
must use anodes that are more active in the galvanic
series than the metal to be protected. For iron or steel
protection, aluminum, zinc, and magnesium could
theoretically be used; however, magnesium has the
highest electrical potential and is most often used
(44). Other anode materials include niobium, titan-
ium, and high-silicon cast iron.
The applied voltage required to give adequate cor-
rosion protection is determined by measuring the
electrical potential of the structure to be protected.
This is the key criterion in assessing the effectiveness
of cathodic protection and in designing an appropriate
system.
Because success of cathodic protection systems has
been variable, a competent corrosion control engineer
should be consulted prior todesign and installation of
such a system.
4.5.3 Ventilation and Heating
Ventilation and heating of enclosed spaces for
humidity control have been successfully used to
prevent condensation of corrosive vapors on compo-
nents and structures in wastewater treatment facili-
ties. In most cases, moisture must be present for
corrosion to occur. Adequate provision of heating and
ventilation systems in enclosed spaces such as screen
and grit buildings, pumping rooms, and pipe tunnels
can reduce or eliminate condensation of moisture,
and thus greatly reduce the rate of corrosion and
increase the life of protective paints and coatings.
Recommendations for design of ventilation and
heating systems for enclosed spaces are provided in
Chapter 6.
4.5.4 Materials Selection
When deteriorated components require replacement,
consideration should be given to the conditions which
resulted in the deterioration and the use of materials
which will be more resistant to those conditions.
Corroded appurtenances such as submerged sludge
collector chains in rectangular clarifiers, general
facility decking and walkways, handrails, ventilation
ducts and other components can generally be re-
placed with materials such as plastic, fiberglass or
stainless steel which are more resistant to the
corrosive exposures experienced in those applica-
tions. A detailed discussion of materials selection in
the design of facilities for corrosive environments is
presented in Chapter 6.
4.6 Case Histories
4.6.1 Odor Control Using Wet Scrubbers
The Aptos wastewater transmission system, operated
by the Santa Cruz County (California) Sanitation
District, is 13.7-km (8.5-mi) long, employing four
regional pump stations ranging in capacity from
11,000 mVd (2.9 mgd) to 21,000 m3/d (5.5 mgd).
Transmission lines are constructed of polypropylene,
with diameters ranging from 38 cm (15 in) to 122 cm
(48 in). Detention times in the system can be as long
as 24 hr. Two of the four pump stations are located
adjacent to beaches, one at the edge of a shopping
center, and one directly behind an exclusive restau-
rant. The Aptos wastewater transmission system was
completed in 1979 at a cost of $8,500,000, and
included odor control equipment at the lift stations
and in the force mains.
The odor control equipment at the lift stations
consisted of wet scrubber reactors utilizing sodium
hypochlorite (NaOCI) as the scrubbing solution. Each
0.6-m3 (20-cu ft) reactor contained a packed bed of
2.5-cm (1 -in) diameter random-dumped plastic rings.
The NaOCI solution was continually recirculated at a
preset, manual rate. Make-up NaOCI was required to
maintain the concentration at the desired level.
Design requirements called for 95 percent removal of
an anticipated maximum H2S concentration of 10
ppm in the wet well ventilation exhaust.
In addition to wet scrubbers, air injection equipment
was installed at various locations along the force
main to prevent anaerobic conditions and to minimize
sulfide buildup and odor release.
Within 1 year after start up of the transmission
system, both the air injection and wet scrubber
system experienced operating and performance
limitations, resulting in frequent odor complaints.
Excessive pressures developed in the force main
resulting from air injection, causing air relief man-
holes to blow off. In the wet scrubbers, higher than
anticipated H2S levels and insufficient contact of the
HjS with the NaOCI solution resulted in incomplete
reaction of the odorous gases, even at maximum
NaOCI concentrations. Clogging of nozzles and pump
impellers occurred, apparently due to formation of
salts at NaOCI concentrations greater than 5 percent
by weight. Daily monitoring of the chemical concen-
trations was required to maintain the desired NaOCI
levels.
The district chose to install a proprietary odor control
system as a demonstration unit at the New Brighton
Beach pump station. The system was wet scrubber
type of unit which eliminated the need for packing
material by generation of a fog of micron-sized
droplets in the contact chamber that provided much
greater surface area (6,000 m2/m3)than conventional
packing materials. This was accomplished by using
compressed air in conjunction with specially designed
nozzles. Water softeners were required to prevent
scale formation and nozzle clogging. The demonstra-
tion unit employed a 9.6-m3 (340-cu ft) stain less steel
96
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contact chamber, allowing contact times of up to 30
seconds. The scrubber liquid employed was sodium
hypochlorite (for HZS oxidation) and sodium hydroxide
(for pH control). Sodium hydroxide feed rates were
automatically adjusted based on the pH of the spent
scrubber liquid. As opposed to conventional wet
scrubbers which continually recirculate the scrubber
liquid, this system used a once-through process with
no recirculation of scrubber liquid.
The objective of the new odor control system called
for a removal efficiency of 90 percent of an anticipated
maximum influent HaS level of 30 ppm. The following
design criteria were developed for the demonstration
unit:
Removal efficiency
Air flow (min)
Chemical flow
Compressed air
Soft water flow (max)
Reaction time (min)
= 90 percent
= 28 m3/min (1.000 cfm)
= 7.6 l/min (2 gpm)
= 2,2 mVmin @ 550 kPa
(80 cfm @ 80 psi)
7.6 l/min (2 gpm)
10 sec
During the initial performance tests, inlet H2S con-
centrations up to 57 ppm were reported, which were
beyond the design capacity of the system. However,
relatively simple modifications to the system allowed
acceptable performance at the higher loadings. These
modifications consisted of reducing the exhaust fan
speed (increasing contact time), and increasing the
nozzle size.
Final performance tests were conducted at air flows
of 28 mVmin (1,000 cfm), liquid (chemical) flows of
2.6 l/min (0.7 gpm), and inlet H2S levels of 30 to 100
ppm. The results of these tests are summarized in
Table 4-16. As can be seen from the data, perform-
ance was excellent, with removal efficiency averaging
approximately 95 percent.
The outlet gas from the new unit is now introduced
into the older, packed scrubber (using clear water as
the scrubbing solution) for removal of any chlorine
odors. The system has operated satisfactorily for over
1 year. NaOCI consumption is approximately 76 I (20
gal)/day. Based on successful performance at the
New Brighton Beach pump station, additional units
have been planned for two other lift stations exper-
iencing odor problems1
4.6.2 Odor Control With Wet Scrubbers and
Activated Carbon
Severe odor problems existed at several pumping
stations in one of two major sewage collection
Table 4-16,
Test No.
R emits of Wet S crubber Performance Tests foi
H?S Control. Santa Cruz County, CA
Average
Average Inlet Outlet Removal
HzS H2S Efficiency
ppm
ppm
%
1A
31
1.0
97
IB
42
1.8
96
2A
40
1.6
96
2B
44
2.0
95
3A
56
2.8
95
3B
32
0.8
98
4
38
1.0
97
5
43
0.4
99
6
92
5.4
94
7
54
4.0
93
8
57
3 8
93
9
45
1.6
96
1 0
55
4.4
92
11
43
2.3
95
12
166
24
86
-
100
10
90
-
58
1.6
97
'Personal Communication; John V. Nutt and Martha Shedden. Santa Cruz
County Sanitation District, California.
systems serving the city of Tampa, Florida. Waste-
water flows in the system averaged only 10 to 15
percent of design flows, resulting in detention times
in the force mains of 5 to 6 hours. Gentle slopes and
warm temperature combined to make conditions
conducive to sulfide generation. H2S released from
pumping stations created a severe odor nuisance in
adjacent residential neighborhoods, H2S concentra-
tions in one pumping station were as high as 200 ppm
(47).
Chemical addition to the wastewater to oxidize or
inhibit sulfides was rejected as being too costly or
inappropriate for the specific conditions. Due to the
irregular profile of the sewer, air and oxygen addition
was ruled out because of the potential for gas locking.
It was decided, therefore, to investigate treatment of
the H2S-laden air. Design criteria for the treatment
alternatives were based on the following:
1. Continuous ventilation of wet wells at the rate of
12 air changes/hour.
1. Intermittent ventilation of wet wells at the rate
of 24 air changes/hour.
3. H2S concentrations to be treated:
a. 10 ppm H2S—low average concentration
b. 25 ppm HzS—moderate average concentra-
tion
c. 50 ppm H2S—observed average concentra-
tion
4. Design outlet HzS concentration of 1 ppm.
A wide range of alternatives was reviewed. Based on
a cost-effectiveness analysis of these alternatives, a
97
-------
system consisting of wet scrubbers using sodium
hypochlorite followed by NaOH-impregnated activa-
ted carbon adsorbers was selected. Four of these
systems were installed in 1981 and 1982.
Design criteria and operating data from one of these
installations are presented in Table 4-17. Due to
equipment malfunctions, including rectifier over-
loads, and recirculation pump and cooling fan failures,
the wet scrubber did not perform as designed,
removing an average of about 70 percent of the
incoming H2S, As a result, mass loadings of HsS to the
activated carbon units exceeded design values,
requiring more frequent regeneration and replace-
ment of the carbon than originally anticipated. It
should be noted that when all equipment was
operating satisfactorily, the wet scrubber system was
effective, removing 85 to 95 percent of the incoming
H2S.
Table 4-17. Design and Performance o4 WbI Scrubbsr/
Activated Carbon Odor Control System at
Tampa, FL (47)
Operating Parameter
Design
Value
Initial
Operating"
Data Value
NaOCI Wet Scrubber Inlet
Annual Average HaS
Concentration - ppm 50
NaOCI Wet Scrubber Outlet
Annual Average H2S
Concentration - ppm 5
NaOCI Wet Scrubber Average
Removal - Percent 90
NaOH Impregnated Activated
Carbon Annual Average
Hemoval - Percent 100
NaOH Impregnated Activated
Carbon Virgin Run - Days 730
49.1
14.5
70
100
180
'Includes abnormal conditions due to equipment startup variations
and malfunctions.
Although the combined wet scrubber-activated car-
bon system was effective in removing H2S from the
contaminated air when all equipment was function-
ing, additional problems developed. A sweet, ripe-
olive odor was detected at the outlet of all four odor
control systems. Although H2S was still being re-
moved, it was postulated that another organic odor
was breakingthrough the activated carbon adsorbers.
A program was initiated to determine the source of
this odor and to analyze alternatives for its removal.
Another problem which occurred was the presence of
hydrochloric acid vapors in the discharge ductwork of
the hypochlorite scrubbers. These vapors condensed
in the ductwork and the activated carbon columns,
resulting in pitting of the stainless steel dampers and
corrosion of the duct silencers. In addition, this
appeared to adversely affect the adsorption capacity
and expected life of the caustic-impregnated carbon.
The problem was analyzed and found to result from
excess NaOCI (fed at a constant rate) during periods of
low HjS concentrations, and from variation in pH due
to feeding NaOH at a constant rate. Automatic
controllers and pH monitors were proposed to control
NaOCI and NaOH feed rates (47).
4,6.3 Odor Control Using Activated Carbon
In Ocean County, New Jersey, odors were being
released from pumping stations in large collection
systems serving three regional treatment plants (48).
Some of the pumping stations were located as close
as 15 m (50 ft) from residences, and were the source
of numerous odor complaints.
To solve the odor problem, 14 activated carbon
adsorption systems were installed at various pump
stations where odor problems existed. Ozone systems
had been installed, but it was found that operation
and maintenance requirements of these highly
instrumented systems were too complex for the
operating staff. Wet scrubbers were used for the less
sensitive pump stations, but it was felt that activated
carbon would be more effective, particularly for those
pump stations handling strong industrial wastes.
The 14 adsorbers were all of reinforced fiberglass
construction, with diameters ranging from 1.2 to 3.7
m (4 to 12 ft). Air handling capacities of these units
varied from 28 to 250 m3/min (1,000 to 9,000 cfm).
The smallest adsorber contained 570 kg (1,250 lb) of
granular activated carbon, while the largest contained
4,770 kg (10,500 lb). Total carbon content of the 14
adsorbers was approximately 36,400 kg (80,000 lb).
Thirteen of the 14 adsorbers used a caustic-(NaOH~)
impregnated carbon specifically designed for H2S
removal. The other used a high surface area carbon
media that has a strong affinity for alcohols, chlori-
nated hydrocarbons, esters, ketones, hydrocarbons,
and aromatics. The activated carbon systems have
been found effective in controlling odors from the
pump stations.
At the Loxahatchee River Wastewater Treatment
Plant in Jupiter, Florida, an activated carbon system is
used to control odors from the master lift station.
Periodic checks of the influent HaS levels to the
system indicate influent H2S concentrations of 0to60
pptn.
A 1.22-m (4-ft) diameter, single bed adsorber is
employed using NaOH-impregnated carbon. Air flows
through the unit are approximately 0.33 m3/s (700
cfm). The unit began operating in December 1981.
98
-------
The installed capital cost of the system was approxi-
mately 525,500 in 1981. Operation and maintenance
costs are shown below.
Virgin carbon lasts approximately 10 months before
breakthrough occurs. The carbon is regenerated once
using 1,130 (300 gal) of 50 percent NaOH solution.
The unit contains 613 kg (1,350 lb) of activated
carbon. After regeneration, the carbon lasts approxi-
mately 6 months, at which time the spent carbon is
replaced with virgin carbon. Thus, the system oper-
ates on a 16-month cycle.
On a 16-month basis, the materials cost for carbon
regeneration and replacement is as follows:
Virgin carbon: S7.33/kg x 613 kg = $4,495
NaOH regeneration: $0.17/1x1,1301 195
Total for 16 months = $4,690
The city estimates that operation and maintenance of
the system requires 8 man-days/yrof labor. The total
annual operating and maintenance (O&M) costs are
estimated to be:
Carbon and NaOH (12 mo) $3,520/yr
O&M labor—8 days at $ 160/day 1,280/yr
Power costs @ $.07/kWh 600/yr
Maintenance mat'ls and supplies 180/yr
Total Annual O&M Costs $5,580/yr
The system has been successful in controlling odors
from the master lift station. Odors are not detectable
until breakthrough of the carbon occurs. The need for
regeneration of the carbon is determined based on
sensory perceptions from city personnel or frequency
of odor complaints from neighbors. Monitoring of
inlet and outlet H2S concentrations is not routinely
conducted.
4.6.4 Odor Control Using Direct Combustion
The city of North Olmstead, Ohio operated a Zimpro
low oxidation process for conditioning gravity-
thickened primary and waste-activated sludge from
its 34,000-m3/d (9-mgd) activated sludge plant. All
sludge from the plant was gravity thickened, heat
treated at 132°C (270°F) acid vacuum filtered prior to
disposal. Odors from the heat treatment processes
were a severe problem and created nuisance com-
plaints because of the proximity of the plant to
surrounding homes(50),The original Zimpro installa-
tion included a catalytic burner for treatment of
odorous gases collected from the oxidized sludge tank
and vacuum filters. This unit proved inadequate and
was replaced with a high temperature gas fired
afterburner which receives the odorous gas from the
oxidized sludge tank and vacuum filters. The original
catalytic burner was retained for treatment of the
decant tank off-gases and was equipped with an air
scrubber using effluent water. The oxidized sludge
tank and vacuum filters are fully enclosed. These
modifications were effective in controlling plant
odors.
The city of Bedford Heights, Ohio operated a Zimpro
low pressure oxidation heat conditioning system at its
13,600-m3/d (3.6-mgdj activated sludge plant from
1970 until 1975 when the heat treatment system
was temporarily taken out of service (49). The sludge
recycle streams caused odors when returned to the
main stream liquid processing units. Off-gas from the
oxidized sludge tank was identified as a major source
of odor. The originally installed catalytic incinerator
was not effective in controlling odors in its present
condition. The use of ozone, as well as acid and
caustic scrubbers was also found to be ineffective, At
the time of the shutdown, the city planned to replace
the existing odor control system with a high temper-
ature, 815°C (1,500°F) gas-fired afterburner. How-
ever, since the initial shutdown of the Zimpro units,
the city increased the operational staff, and repaired
leaks in the covered oxidized sludge tank. The original
catalytic incinerator was repaired and was reported to
be effective in controlling odors from the oxidized
sludge tank (49).
The Rockland County, New York Plant is a 38,000-
m3/d (10-mgd) activated sludge plant that utilizes an
intermediate 200°C (400°F) Zimpro oxidation sludge
conditioning process for treatment of primary and
secondary sludges (49). Control of odors from the
oxidized sludge tank was a difficult problem during
the first 4 years of operation. The district used several
gas burners to control odors from the oxidized sludge
tank, but experienced problems due to low incinera-
tion temperatures and installation problems. These
problems were overcome with the installation of a
new 815°C (1,500°F) (exhaust temperature), gas-
fired incinerator equipped with heat exchangers to
raise the incoming air temperature to 415°C (780°F)
and to recover 55 percent of the exhaust heat.
The cities of Muskogee, Oklahoma; Gresham, Oregon;
and Portland, Oregon utilize direct combustion at
649-760°C (1,200-1,400°F) for odor control of off-
gases from thermal sludge conditioning units.
The above case histories and examples indicate that a
major source of odorous air from thermal conditioning
of sludge is the oxidized sludge a nd decant tanks. The
most frequently used method to treat this air stream
has been either, catalytic or high temperature inciner-
ation. Based on a survey of odor treatment methods at
36 operating treatment facilities, the following con-
clusions were drawn (49).
1. The odorous off-gases can be successfully
treated by incineration at temperatures of 815°C
(1,500°F).
2. Low temperature catalytic incineration has not
been as successful as high temperature incin-
eration,
99
-------
3. Conversion of low temperature catalytic com-
bustion processes to high temperature incinera-
tion has not been successful because of
inadequate gas capacity and overheating of the
stack.
4. The fuel requirements for high temperature
incineration are costly, and use of effective heat
exchange systems should be considered,
4.7 References
When an NTIS number is cited in a reference, that
reference is available from;
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
(703) 487-4650
1. Lovett, WD. and R.L. Poltorak. Activated Carbon
Used to Control Odors. Water and Sewage
Works 121 (8): 74-75, 1974.
2. Lopata, J.R. and C. Leutwiler. Waste Plant
Corrosion Must be Treated With Care, Water
and Sewage Works 121 (5): 46-47, 1974.
3. Odor Control for Wastewater Facilities. Manual
of Practice No. 22, Water Pollution Control
Federation, Washington, DC, 1979.
4. Rains, B.A., M.J, DePrimo, and I.L.
Groseclose. Odors Emitted From Raw and
Digested Sewage Sludge. EPA-670/ 2-73-098,
NTIS No. PB-232369, U.S.
Environmental Protection Agency, Wash-
ington, DC, 1973.
5. Henry, J.G. and R. Gehr. Odor Control:
Operator's Guide. JWPCF 52 (10): 2,523-2,537,
1980.
6. Hydrogen Peroxide Demonstration Report, City
of Baltimore. Interox America Environmental
Services, Houston, TX, 1983.
7. Sims, A.F.E. Odor Control With Hydrogen Per-
oxide. Progress in Water Technology 12 (5):
609-620, 1980.
8. Miller, R.G, Hydrogen Peroxide Solves Sludge
Odor Problem. Water and Sewage Works 123
(5): 74-76,1976.
9. Brinsko, G.A. and J.A. Shepherd. Sludge Treat-
ment System Odors Controlled With Hydrogen
Peroxide. JWPCF, Deeds and Data, 14 (4); 1977.
10. Ficek, K.J. Potassium Permanganate for Odor
Control. In: Industrial Odor Technology Assess-
ment, edited by Cheremisinoff and Young, Ann
Arbor Science, Ann Arbor, Ml, 1975.
11. Pisarczyk, K.S and L A. Rossi. Sludge Odor
Control and Improved Dewatering With Potas-
sium Permanganate. Presented at 55th Annual
Conference of the Water Pollution Control
Federation, St. Louis, MO., October, 1982.
12. Internal Report, Carus Chemical Co., LaSalle,
IL„ 1983.
13. Pisarczyk, K.S. 1983 Cairox™ Potassium Per-
manganate Wastewater Customer Survey.
Carus Chemical Co., LaSalle, IL, January, 1984.
14. Internal Memorandum, Carus Chemical Co.,
LaSalle, IL, May, 1982.
15. Dague, R.R. Fundamentals of Odor Control.
JWPCF 44 (4): 583-594, 1 972.
16. Lorgan, G.P., J.D. Hill and S.M. Summers.
Nitrate Addition for the Control of Odor Emis-
sions From Organically Overloaded, Super Rate
Trickling Filters. In. Proceedings of the 31st
Purdue Industrial Waste Conference, 1976.
17. Poduska, R.A. and B.D. Anderson. Successful
Storage Lagoon Odor Control. JWPCF 53 (3):
299-310, 1981.
18. Directo, L.S., C. Chen, and I.J. Kugelman. Pilot
Plant Study of Physical-Chemical Treatment.
JWPCF 49 (10): 2,081 -2,098, 1977.
19. Moss, W.H., et al. Full Scale Use of Physical-
Chemical Treatment of Domestic Wastewater at
Rocky River, Ohio. JWPCF 49(11): 2,249-2,254,
1977.
20. Yang, M Y. and P.M. Cheremisinoff. Wet Scrub-
bing for Odor Control. In: Industrial Odor Tech-
nology Assessment, edited by P.N, Cheremisi-
noff and R.A. Young, Ann Arbor Science, Ann
Arbor, Michigan, 1975.
21. Hardison, L.C. and L.R. Steenberg. A New
Process for HsS Odor Control. Presented at the
79th National Meeting of the American Institute
of Chemical Engineers, Houston, TX, March 16-
20, 1975.
22. Odors from Stationary and Mobile Sources.
Committee on Odors From Stationary and
Mobile Sources, National Research Council,
National Academy of Sciences, Washington,
DC, 1979.
23. Boscak, V., N. Ostojic and D. Gruenwald. Odor
Problems? Don't Just Hold Your Nose. Water
and Wastes Engineering 12 (5): 62-67, 1975.
24. Lovett, W.D. and R.L. Poltorak. Activated Carbon
and the Control of Odorous Air Pollutants, in:
Industrial Odor Technology Assessment, edited
by Cheremisinoff and Young, Ann Arbor
Science, Ann Arbor, Ml, 1975,
100
-------
25. Huang, J.Y.C., G.E. Wilson andT.W.Schroepfer.
Evaluation of Activated Carbon Adsorption for
Sewer Odor Control. JWPCF 51 (5): 1054-1062,
1979.
26. Purafil Product Manual. Purafil, Inc., Chamblee,
GA, 1979.
27. Handbook: Septage Treatment and Disposal.
EPA-625/6-84-009, U.S. Environmental Pro-
tection Agency,Municipal Environmental Re-
search Laboratory, Cincinnati, OH, 1984.
28. Nebel, C.A, and N. Forde, Principles of Deodori-
zation With Ozone. In: Ozone; Analytical Aspects
and Odor Control, edited by R.G, Rice and M.E.
Browning, the Internationa! Ozone Institute,
Syracuse, NY, 1976.
29. Churchill, P.W. Ozonation of Septic Odors at a
Pretreatment Facility. JWPCF, Deeds and Data,
49 (7), 1977.
30. Diaper, E.W.J. Ozonation Systems for Odor
Control. In: Ozone: Analytical Aspects and Odor
Control, edited by R.G. Rice and M.E. Browning,
thelnternational Ozone Institute, Syracuse, NY,
1976.
31. Jain, J.S., N L. Presecan, and M, Fitas. Field-
Scale Evaluation of Wastewater Disinfection by
Ozone Generated From Oxygen. In: Progress in
Wastewater Disinfection Technology: Proceed-
ings of the National Symposium, Cincinnati, OH,
September 18-20, 1978, EPA-600/9-79-018,
NTIS No. 299338, U.S. Environmental Protection
Agency, Cincinnati, OH, 1979.
32. Nebel, C.A. and R.D. Gottschling. Industrial
Odor Control by Oxidation with Ozone. In:
Industrial Odor Technology Assessment, edited
by P.N. Cheremisinoff and R.A. Young, Ann
Arbor Science, Ann Arbor, Ml, 1975.
33. Unangst, P.C. and C.A. Nebel. Ozone Treatment
of Sewage Plant Odors, Water and Sewage
Works, Reference Number, R-42, R-43, 1971.
34. Bollyky, L.F. Pitfalls in Design Requirements for
Ozone Odor Control Systems. In: Ozone: Ana-
lytical Aspects and Odor Control, edited by R.G.
Rice and M.E. Browning, the I nternational Ozone
Institute, Syracuse, NY, 1976.
35. Bhatia, M.V. and P.N. Cheremisinoff. Combus-
tion Methods for Odor Control. In: Industrial
Odor Technology Assessment, edited by Cher-
emisinoff and Young, Ann Arbor Science, Ann
Arbor, Ml, 1975.
36. Neverit, R.B., J,U. Price, and K.L. Engdahl.
Capital and Operating Costs of Selected Air
Pollution Control Systems, Air Pollution Control
Association, Pittsburgh, PA, January, 1979.
37. Bohn, H.L. Soil and Compost Filters of Malodor-
ant Gases. Journal Air Pollution Control Associ-
ation 25 (9): 953-955, 1975.
38. Carlson, D.A., and C.P. Leiser. Soil Beds for the
Control of Sewage Odors, JWPCF 38 (5): 829-
840, 1966.
39. Pomsroy, R.D. Biological Treatment of Odorous
Air JWPCF 54(12): 1541-1545, 1982.
40. Rands, M.B., D.E. Cooper, Chee-Pan Woo, G.C.
Fletcher, and K.A. Rolfe. Compost Filters for H^S
Removal From Anaerobic Digestion and Ren-
dering Exhausts. JWPCF 53 (2): 185-189,1981.
41. Lauren, O.B. Odor Modification and Masking, In:
Specialty Conference on the State-of-the-Art of
Odor Control Technology, edited by the Air
Pollution Control Association, APCA, Pittsburgh,
PA, 1974.
42. Cheremisinoff, P.N., R.M. Bethea, T.M. Hellman,
and O.B. Lauren. Techniques for Industrial Odor
Control. Pollution Engineering 7 (10): 24-31,
1975.
43. Cox, J.P. Odor Control and Olfaction. Pollution
Sciences Publishing Co., Lynden, WA, 1975.
44. Paints and Protective Coatings for Wastewater
Treatment Facilities. Manual of Practice No. 1 7,
Water Pollution Control Federation, Washing-
ton, DC, 1969.
45. Uyeda, H.K., B.V. Jones, T.E. Rutenbeck and
J.W. Kaakinen. Materials for Oxygenated
Wastewater Treatment Plant Construction.
EPA-600/2-78-136, NTIS No. PB-286417, U.S.
Environmental Protection Agency, Municipal
Environmental Research Laboratory, Cincinnati,
OH, 1978.
46. Voegtle, J.A., W.E. Foster and D.W. Martin,
Start-up of a Municipal-Industrial Solids Pro-
cessing Facility. JWPCF 51 (5): 926-937,1979.
47. Bizzarri, R.E., J.R. Popeck, D.W. Pickardand J.E.
Drapp. HiS Odor Control on Tampa's Major
Sewerage Systems. Presented at the 55th
Annual Conference of the Water Pollution
Control Federation, St. Louis, MO, October,
1982.
48. Carbon Adsorption: A "Fail-Safe" Technology
for Sewage Odor Control. Calgon Corporation,
Pittsburgh, PA.
49. Effects of Thermal Treatment of Sludge on
Municipal Wastewater Treatment Costs. EPA-
600/2-78 073, NTIS No. PB-285707, U.S.
Environmental Protection Agency, Office of
Research and Development, Cincinnati, OH,
1978.
101
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Chapter 5
Designing to Avoid Odor and Corrosion in New Wastewater Collection Systems
5.1 Introduction
The potential for sulfide and odor generation in new
sewer systems must be fully evaluated in the design
stage based on the characteristics and properties of
odor-causing compounds and the principles of control
described in Chapters 2 and 3 of this manual.
Warm temperatures, flat topography and large sewer
service areas are physical conditions which, unless
specifically considered during design, are likely to
result in sulfide odors and sulfide-induced corrosion
within the collection systems. Proper selection of
slopes, rational design of hydraulic structures such as
drops and junctions, proper design of pumping
stations, wet wells, holding basins, and force mains
along with the provision of adequate ventilation are
all critical elements of a total sewer system design
that are necessary to minimize sulfide generation
potential.
Although designing to avoid sulfide generation may
increase the capital cost of a new sewer system, this
approach is technically and economically preferable
to having to control sulfides after they become a
problem.
This chapter provides guidance for eliminating or
minimizing the generation of sulfides in the design of
new wastewater collection systems. Specific refer-
ence is made to control of H2S, since this is the most
prevalent odor source associated with wastewater
conveyance systems.
Design procedures outlined for sulfide control will
often be applicable for control of other odor-producing
compounds present in municipal wastewater, since
many of the design concepts presented deal with
preventing the anaerobic conditions under which
undesirable odors are more likely to occur.
5.2 Hydraulic Design
5.2.1 Slope
Slope is the key criterion in designing a wastewater
collection system to avoid sulfide problems. Sewers
designed with long runs at minimum slope are prone
to sulfide generation due to long residence times,
poor oxygen transfer, and deposition of solids. Sulfide
generation can be a serious problem in new sewers,
where actual flows are much less than design flows
during the early lifetime of the system, and velocities
are inadequate to maintain solids in suspension.
In the 1950's, a study was made of small collecting
sewers in southern California. The sewers, all 15 and
20 cm (6 and 8 in) in diameter, were divided into four
slope classes. The results from this study are shown
in Figure 5-1 (1). This figure clearly shows the effect
of slope and sewer length on sulfide generation.
Steeper slopes increase turbulence and oxygen
transfer, thus maintaining aerobic conditions in the
wastewater and preventing significant sulfide gener-
ation. Although the values shown in Figure 5-1 for
average sulfide concentration appear relatively low,
peak sulfide concentrations were as much as four
times the average (1),
In designing a wastewater collection system to
minimize sulfide generation, velocities should be
Figure 5-1. Sulfide occurrence in small sewers (1 ].
0.8
131
£
c 06
o
Line
Slope (°
«>)
No. of
Sewers
Avg. Results
Range
Avg.
BOD
Temp. (°C)
A
0.20-0.25
0.23
18
253
24.7
B
0.32-0.46
0.40
22
212
24.4
¦ C
0.82-0.64
0.57
16
178
24.7
D
0.72-1.20
0.9
8
184
24.3
• A
5 0.2
400 800 1,200 1,600
Distance from Upper End of Sewer, ft
2,000
103
Preceding page blank
-------
sufficient to prevent deposition of solids. Current
conventional design practice recommends that a
minimum velocity of 0.6 m/s (2 ft/s) be achieved
regardless of pipe size to maintain a self-cleaning
action in sewers. Another approach is to maintain a
minimum boundary shear stress to prevent suspend-
ed particles from settling out on the invert.
The minimum horizontal velocity required to suspend
particles of known characteristics can be computed
using the following equation (2):
Vh = 8k(s-1)gd"» (5-D
f
where,
VH = horizontal velocity that will just produce
scour, m/s
k = constant which depends on type of material
being scoured (typically 0.04 to 0.06)
s = specific gravity of particles
g = acceleration due to gravity = 9.8 m/sJ
d = diameter of particles, m
f = Darcy-Weisbach friction factor (typically
0.02 to 0.03)
If required minimum velocity is established based on
maintaining a constant boundary shear stress, mini-
mum velocities deviate from the recommended 0.6
m/s (2 ft/s) as a function of pipe size. Figure 5-2
shows minimum velocities required to maintain a
constant shear stress as a function of pipe size (3). If a
boundary shear stress of T0= 0,15 kg/ms(0.03 lb/ft2)
Figure 5-2.
011 -
0.10 -
0.9 -
0.8 -
0.7 -
to
0.6 -
0.5 -
o
® 04 -
0.3 -
0.2 -
0.1 -
o L
0 15 20 30 40 50 100 150
Sewer Diameter, cm
is used, the minimum velocity requirement exceeds
0.6 m/s (2 ft/s) at pipe diameters greater than 35 cm
(14 in), but is less than 0.6 m/s(2 ft/s) at smaller pipe
diameters. This suggests that larger pipes require
greater slopes to maintain adequate scouring veloci-
ties. For sewers with Manning's n = 0.013 or less, a
design boundary shear stress in the range of 0.15 to
0.20 kg/m2 (0,03-0.04 lb/ft2) will likely keep self-
cleaning sewer systems free from sulfide problems.
For sewers with n = 0.015 or greater, a design shear
stress of 0.2 kg/mz(0.04 lb/ft2) should be used (3).
It should be noted that the often recommended 0.6
m/s (2 ft/s) is a minimum velocity. It is desirable to
have a velocity of 0.9 m/s (3 ft/s) or more whenever
practical (2).
Pomeroy has developed guidance regarding flow-
slope relationships for preventing sulfide buildup.
This is shown as Figure 5-3 (1). The curves are based
on an assumed effective BOD (EBOD) of 500 mg/1,
HerB, EBOD represents the BOD during the daily
maximum 6-hour flow period during the three hottest
months of the year. The calculation of EBOD for
Figure 5-3 is:
EBOD = 1.25 B0D6x 1.07,t"zd! (5-2)
where,
EBOD = effective BOD, mg/1
1.25 = factor to convert average daily BOD to
maximum 6-hour flow BOD
BOD5 = Standard 5-day BOD, mg/l
T = average wastewater temperature for three
hottest months, °C
In the development of Figure 5-3, it is assumed that
the depth of flow does not exceed two-thirds of the
pipe diameter, and that the effective slope is calcu-
lated upstream of the point of interest over distances
representing flow times of approximately 1 hour.
With an assumed EBOD of 500 mg/1, a system
designed with a flow-slope relationship falling on or
above Curve A may be expected to produce very little
sulfide, rarely more than 0.1 to 0.2 mg/l of dissolved
sulfide. Use of the flow-slope design points between
Curves A and B may result in moderate sulfide
concentrations which may cause odor and corrosion
problems at points of high turbulence. Flow-slope
design points falling below Curve B may result in
substantial sulfide generation.
Figure 5-3 is intended only as a guide to predict when
sulfide generation is likely to be a problem for certain
slopes and flows, and is not to be used for detailed
design purposes. Where wastewater characteristics
vary from the assumed EBOD of 500 mg/l, the flow-
Minimum scour velocity based on boundary
shear stress (3).
Manning's n = 0.013
104
-------
Figure 5-3. Flow—slope relationships as guides to sulfide
forecasting (1).
0.6
Effective BOD =
500 mg/1
0.5 -
0.4
Little Sulfide
Generation Potential
0.3
Moderate Sulfide
Generation Potential
0.2
Severe Sulfide
Generation Potential
0.1
o 0.1 1.0 5.0 10 20 40
0.5 15 30
Flow, cu ft/s
slope relationships will increase or decrease in
proportion to the square roots of the ratio of EBOD's.
For a design EBOD other than 500 mg/l the minimum
acceptable slope can be calculated according to (4);
SD = EBOPD1/2 x Sc (5-3)
500
where,
Sd = minimum acceptable slope for design
EBOD
Sc = minimum slope from Figure 5-3
EBODd = design EBOD (mg/l)
Figure 5-3 is based on the average slope of sewer
runs with lengths ranging from 365 to 580 m (1,200
to 1,900 ft). Even if the necessary average slope to
prevent sulfide generation is attained, specific
reaches should be checked to ensure that self-
cleaning velocities are maintained where feasible,
and that a minimum scouring velocity of 0.6 m/s (2
ft/s) is maintained during peak daily flow conditions.
The recommended scouring velocity of 0.6 m/s (2
ft/s) for pipes flowing one-half full at design flow can
result in velocities as low as 0.2 m/s (0.67 ft/s)
during low flow periods early in the design lifetime of
the system, thus allowing deposition of sewage
solids. While this is undesirable, it cannot be econom-
ically avoided in certain instances. Sulfide generated
from accumulated solids is generally much less
critical than that generated from the siime layer,
especially when the accumulated solids are flushed
from the system on a daily basis.
Choice of a design slope depends on several factors
other than flow and EBOD, including topography,
subsurface conditions, depth of service laterals, pipe
size and material, as well as overall economic trade-
offs between gravity flow vs. pumped systems. If
sewage pumping is required, a savings in pumping
head by minimizing slopes should not, by itself, be a
reason for using slopes that will result in significant
sulfide generation. If choice of slopes for use in Figurte
5-3 results in points falling on or below Curve B,
indicating high potential for sulfide generation.
Equation 2-25 should be used to calculate estimated
sulfide buildup. Equation 2-28 can then be used to
estimate the rate of corrosion of the pipe material. If
the corrosion rate is such that the expected lifetime of
the pipe is less than the design lifetime, several
options are available to the engineer. These include
use of steeper slopes or other means to promote
natural reaeration, injection of air or oxygen, addition
of chemicals, or selection of materials that are more
resistant to corrosion. In general, the last option
would be least desirable since, although rates of
corrosion of pipe materials may be reduced, sulfide
levels may still be high and may result in substantial
odor generation.
Design lifetime is an important parameter in consid-
ering sulfide generation and subsequent corrosion of
pipe materials in wastewater collection systems. EPA
cost-effectiveness guidelines recommend a useful
life of 50 years for wastewater conveyance structures,
including collection systems, outfall pipes, intercep-
tors, force mains, and tunnels. For special situations,
as with sewers designed for interim service, shorter
design lifetimes may be selected that are consistent
with the planning objectives of the municipality.
5.2.2 Pipe Size
If sulfide generation has been determined to be a
potential problem, larger pipe sizes may be selected to
improve the rate of reaeration. A larger pipe for the
same flow rate and slope reduces the mean hydraulic
depth (cross-sectional area of the stream divided by
surface area), which increases surface area available
for reaeration (Equation 2-14). Figure 2-9 shows that
reducing the relative depth of flow from 0.75 to 0.5
approximately doubles the reaeration rate. Adequate
scouring velocities must be maintained if larger pipe
is used, but this is not normally a problem since for a
given flow and slope, velocity is influenced very little
by pipe size.
Force mains have often been constructed of minimum
diameter pipe in order to reduce detention time and
105
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avoid sulfide buildup. However, the smaller pipe has a
greater ratio of slime-supporting pipe wall to volume
of wastewater, partially offsetting the benefit of
reduced detention time, Choiceof minimum pipe size
is, therefore, not considered to be of significant value
in reducing sulfide generation.
5.2.3 Drops and Falls
For wastewater containing little or no dissolved
sulfide, drop structures can result in the wastewater
stream picking up substantial amounts of oxygen,
helping to maintain aerobic conditions and preventing
sulfide generation. However, for wastewater contain-
ing dissolved sulfide, the turbulence associated with
drops or falls will release H2S from the stream,
resulting in odors and corrosion.
The benefits of reaeration through drops and falls
were discussed in Chapter 2, Figure 5-4 shows two
alternatives for grading a sewer. Alternative A
employs a lesser slope but allows free fall of the
wastewater at manhole 2, Alternative B shows the
more conventional approach of increasing the slope
of the line between manholes 1 and 2. Table 5-1 was
developed by Thistlethwayte to show the impact of
the two alternative designs on oxygen absorption (5).
Note that the expected oxygen absorption in the
sewer using a drop of about 1.2 m(4ft) is 50 times the
oxygen absorption without the drop. If some DO were
present in the influent to manhole 1, the ratio would
remain the same, but the a mounts of oxygen absorbed
would be reduced in proportion to the actual oxygen
saturation deficits. Whether or not such a drop could
be justified would depend on the DO levels upstream
and the desired DO level downstream. Considering a
DO increase of more than 3 mg/1 by the use of a drop,
it may be possible to lay the downstream sections at a
flatter grade without exhausting the DO added by the
drop (5).
Typical drop manhole designs are shown in Figure
5-5 (6). Drops may be subject to clogging or stoppages
Table 5-1.
Oxygen Absorption in a Sewer With and Without
a Drop (5)
Alternative A*
Alternative B*
Dry weather flow, m3/ mi n 3.4
jmax. 6 hr)
Sewer diameter, cm 61
Slope, percent 0.1
Wastewater velocity, m/s 0.53
Oxygen absorbed, mg/l
(assuming saturation
deficit of 10 mg/l)
In the sewer
At the drop
Total oxygen absorbed
0.038
3.1 a
3.22
•Refer to Figure 5-4,
Figure 5-5. Drop manhole designs (6).
Wastewater
3.4
61
05
0,96
0,069
0.07
Wastewater
a) Drop Located
Outside Manhold
Drop Located Inside
Manhold for Easier
Access
due to bridging of sticks or other debris over the drop
pipe. An alternate to the standard design places the
drop inside the manhole, allowing easy access to the
drop pipe. To avoid stoppages, the drop pipe may be of
a larger diameter.
Figure 5-4, Alternative sewer grading designs (5).
Manhole 1
Manhole 2
Alternative A
Elev. = 21
Elev. = 20
Elev, = 1 B
Alternative B
Drop
106
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For large flows and relatively targe drop distances,
vortex drops aresometimes used. In a vortex drop, the
flow is directed tangentially to produce a spiral flow
pattern. Advantages of vortex drops include: 1) main-
tenance of a continuous air core down the shaft; 2)
excellent conditions for oxygen uptake; 3) no accumu-
lation of solids or scum; 4) less likelihood of stoppages;
and 5) better energy dissipation (5).
A diagram of a hydraulic fall is presented in Figure
3-17. Such designs are unlikely to have stoppages
associated with sticks and debris, yet will provide
substantial reaeration of the wastewater.
Drop manholes generally have not been used where it
is economically feasible to steepen the sewer because
of potential maintenance problems and increased
construction costs (6). However, where sulfide gen-
eration potential exists, well designed drops and falls
are effective techniques for maintaining aerobic
conditions and preventing sulfide generation.
Drops or falls are generally not recommended when
appreciable amounts of dissolved sulfide are present
in the wastewater. Turbulence will release sulfide
from the stream, generating odors and potentially
deteriorating the structure. If drops must be used
under such conditions, construction materials must
be selected based on anticipated corrosion problems.
To avoid odors and downstream corrosion, mechani-
cal ventilation should be used to move air from
downstream sections back to the drop structure.
Sewer ventilation is discussed in Section 5.3.
5,2.4 Junctions and Transitions
Sewer line junctions and transitions require special
consideration because they offer an opportunity for
both solids deposition and the release of dissolved
sulfide. For aerobic wastewater, the major goal of
junction design is to provide smooth transitions with
minimum turbulence between incoming and outgoing
fines so as to prevent eddy currents or low velocity
points that will permit deposition of solids.
Design of junctions is more critical for sewers
conveying septic wastewater, and special precautions
must be taken to streamline the junction to minimize
turbulence. Major factors that create turbulent condi-
tions are:
1. Abrupt changes in grade between upstream and
downstream sewer lines.
2. Large differences in velocity between two or
more upstream sewer lines entering the same
manhole.
3. Acute angles between upstream and down-
stream lines.
4. Large changes in flow between two or more
upstream sewer lines that may be caused by
upstream pumping or daily flow variation be-
tween different sewer service areas.
5. Large differences in flow between a trunk line
sewer and tributary collector sewers.
Atypical streamlined junction is shown in Figure 5-6.
Figure 5-6. Streamlined junction (5j.
Downstream
Channel
v, Qi
Approach Channel
junction Zone
Downstream Channel
For one or more of the preceding conditions, turbu-
lence will be minimized when the energy loss through
the transition is minimum. This will be achieved
when the following conditions are met (5):
1. The angle of convergence of the channels within
the junction zone (ft and #2) is as small as
possible.
2. The channels are constructed so that the lateral
momentum (CIVS sin and Q2V2 sin 82) of each
of the incoming lines is reduced by the channel
geometry before convergence of the two
streams.
3. Velocity changes at the junction occur gradually.
5.2.5 Pumping Stations
The design of pumping stations is a critical element of
sanitary sewer collection systems. Ideally, pumping
stations should be designed so as not to increase the
total sulfide generation potential of the collection
system. This is often difficult, however, since con-
temporary design practice for pumping stations
requires some wet-well storage of wastewater plus
retention in the force main. When supplementary
aeration is not provided, both of the above conditions
will tend to increase the potential for sulfide genera-
tion by increasing the total residence time in the
system, and by increasing the contact time of the
wastewater with sulfide-generating slimes within
107
-------
the force main and the wet-well surfaces. Potential
also exists for sulfide generation from solids deposi-
tion in the wet welf if the wet-well design does not
contain adequate bottom slopes and suction piping
arrangements for their continuous removal.
Pumping stations may be generally classified as
continuous or intermittent, depending chiefly on the
size of the tributary sewer system and average and
maximum design flow rates. Of the two, the inter-
mittent pumping stations have a much greater
potential for sulfide generation than the continuous
stations, where at least the minimum flow is pumped
continuously and wet-well detention times are less.
For thB smaller intermittently pumped stations, the
most common design practice is the provision of a wet
well equipped with on off pumping controls whereby
a single pump is activated by a high level switch and
pumps at a constant rate until the level of sewage in
the wet well is reduced to a predetermined level.
Higher wet weather flows are accommodated by a
second pump activated by a level control. Since the
wet well storage and pumping schedules are gener-
ally established for average design flow conditions,
the residence time in both the wet welt and force
mains is often excessive during low flow periods,
especially in the early part of the system's design
lifetime.
The volume of the wet-well storage provided depends
on the peak and average flows and the minimum duty
cycle of the pumping system. Many of the smaller
pumping applications within a size range of 380 to
11,350 m3/d{0 1 to 3.0 mgd) utilize package pumping
stations. Two pumps are normally provided, with a
single pump sized to accommodate peak flow condi-
tions. Duty cycles (time between successive starts)
are typically 15 to 20 minutes, with minimum
pumping times of 2 to 5 minutes.This design approach
results in effective wet-well detention times of 5 to 15
minutes and total detention times of 7 to 20 minutes
under average flow conditions. This may lead to
excessive detention times and possible sulfide gen-
eration during low flow periods. Pomeroy indicates
that significant sulfide generation will not occur in
wet wells with detention times of less than 2 hours
(1).
Wet wells should be as small as possible to minimize
the potential for sulfide generation. A maximum wet-
well design detention time of 30 minutes or less for all
but the larger pump stations is recommended. The
wet well should further be designed to avoid the
accumulation of solids.
Wet-well detention times in larger pump stations
equipped with variable speed pumps or with a
combination of constant speed and variable speed
pumps are generally sufficiently short to avoid sulfide
generation.
Pump station wet wells are often the site of sulfide
release when upstream DO levels are inadequate.
Alternatives for sulfide control in pump stations are:
1) wet-well aeration; 2) chemical addition; 3) col-
lection and treatment of HsS-contaminated air; 4) air
bypassing to a downstream section of sewer; and 5)
injection of air or oxygen upstream in the force main.
Wet-well aeration is effective in oxidizing dissolved
sulfides, but can cause release of HZS by air stripping.
Short detention times in wet wells are insufficient to
achieve sulfide oxidation, while longer detention
times (>1 hour) may be adequate for complete
oxidation. Wet-well aeration has the added advantage
of temporarily increasing DO levels to prevent or
reducesulfidegeneration in downstream force mains.
Currently, package pump station manufacturers do
not include wet-well aeration as a part of their
standard design, but some provide a mixing valve
from the discharge side of the pu mpto the wet well to
provide increased mixing.
Wet-well aeration is a sulfide-control alternative that
should be considered when excessive wet-well
detention times must be provided and when the
incoming wastewater exhibits DO, BOD, and ORP
levels conducive to sulfide generation. This method is
not recommended where significant sulfide (>0.5
mg/l) is present in the incoming wastewater. An ORP
level of +100 millivolts has been used as the
minimum design ORP for pump stations in a system
design for Honplulu, Hawaii, to prevent sulfide
generation in the downstream portions of the system
(7). The target ORP level is dependent on the individual
wastewater characteristics and the downstream
collection system network.
All lift station designs should include an evaluation of
the influent wastewater conditions, and of the impact
of wet-well storage and force main sulfide-generation
potential on downstream segments. In many cases,
air or oxygen injection into the force main should be
considered as an alternative to wet-well aeration.
This method eliminates the problem of H2S release,
has the flexibility of providing the increased oxygena-
tion capability where it is needed, and offers a higher
and simpler level of control. Air and oxygen injection
for sulfide control in collection systems is discussed
in detail in Chapter 3.
The design of pump stations must also include an
analysis of the sulfide generation potential of force
mains. Sulfide generation within the force main is
related to the wastewater characteristics, including
DO present, EB0D, temperature, and sulfate concen-
tration. Thistlethwayte has postulated that the con-
centration of sulfide buildup in a force main is
proportional to (L/DKBODXSOJ for continuously
pumped systems, and that buildup for intermittently
pumped systems is proportional to (L/DKB0D)(S04)
(1,64)(VC/Vi) where L is the sewer length, D is the
108
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sewer diameter, Vc is the velocity of a continuously
pumped system, and M, is the average velocity for an
intermittently pumped system (5). It should be noted
that other researchers have indicated that sulfate
concentration does not limit sulfide buildup except at
very low concentrations. Based on the work of
Thistlethwayte, it was found that the total mass of
sulfide generation was approximately equal in a given
time period for a particular wastewater regardless of
pumping cycles, but that the concentration of sulfide
generated by intermittent pumping for certain pump-
ing cycles could be several times that of a continu-
ously pumped system. This finding is critical for
downstream situations where the intermittent dis-
charge of sulfide from force mains could create
significant localized odor and corrosion problems.
Since sulfide generation within force mains is due
primarily to surface slime, larger force main sizes
reduce the sulfide generation potential for a given
design flow and wastewater characteristics, since
they result in a smaller surface-area to cross-
sectional-area ratio. The selection of force main size
is normally made based on a cost analysis of increased
pumping cost vs. the capital cost of the force main. For
situations in which sulfide generation potential exists
within the force main, a larger size force main may be
warranted to reduce sulfide generation and subse-
quent sulfide control costs. Since most force mains
may sometimes operate under conditions that pro-
duce sulfide, the discharge should be designed to
minimize turbulence. Some circumstances may re-
quire special ventilation, sealing or collection and
treatment of the odorous air.
Figure 5-7. Pump station with an air bypass (1).
Tightly Sealed
Wet Weil
Air Bypass
Manhole
Force Main
Wastewater
WWW
Wastewater Pump
ally when considering that it serves two functions,
pumping and aeration. Figure 5-8 shows a schematic
diagram of a lift station employing an air lift pump
with an air bypass line around the lift station (1). To
eliminate a net discharge of air, the compressor takes
air from the bypass connecting the sealed wet well
and the downstream sewer.
The design of wet-well aeration systems, air or
oxygen injection into force mains, and chemical
addition is discussed in Chapter 2.
An air bypass can often be used to shunt air around a
pump station to control odor release from the wet
well. This involves the use of a sealed wet well as
shown in Figure 5-7 (1). An air bypass line is
constructed between the wet well and the closest
upstream gravity manhole. In some cases, this
distance may be so long as to make this approach
impractical. It is suggested that the air bypass line be
approximately two-thirds of the force main pipe
diameter (1). This approach would be especially
applicable when a downstream gravity flow segment
has significant reaeration potential and the force
main distances are short. When significant sulfide
generation is anticipated, separate air collection and
treatment may be warranted. Separate off-gas treat-
ment is discussed in Chapter 4.
In cases where pumps are designed for lift only and
the wastewater is not discharged into a pressure
main, it may be desirable to consider use of an air lift
pump. Air lift pumps are typically used only for low
flow applications, where their ease of maintenance
and reliable operation outweighs their low efficiency,
which is limited to about 15 percent. However,
economics of an air lift pump may improve substanti-
Figure 5-B, Air-lift pump station (1).
Compressed
Air
Air Intake
Air Bypass
Air
Wastewater
Air Injection Collar
109
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5.2.6 Siphons
Siphons, also called inverted siphons or depressed
sewers, are used to convey wastewater under
streams or highways, conduits or other obstructions
to the normal sewer grade line, and to regain as much
elevation as possible after passing the obstruction.
Siphons are normally limited to pipe sizes greater
than 20-cm (8-in) diameter. Sewage in siphons is
under pressure, since the conduits are below the
hydraulicgradeline. Because the siphon remainsfull
even during periods of no flow, it is a potential site of
significant sulfide generation and odor release.
Methods of controlling sulfide generation that would
be applicableto siphons were discussed in Chapter 3.
These techniques include improving the oxygen
balance by air or oxygen injection, or by addition of
chemicals to oxidize or precipitate the sulfide or
prevent its formation.
Siphon design must consider the potential for odor
release. Positive pressure develops in the atmosphere
upstream of the siphon due to the downstream
movement of air induced by the wastewater flow. Air
thus tends to exhaust from the manhole at the siphon
inlet and may escape in large amounts from small
openings, such as pick holes in manhole covers. At
less than maximum flow, wastewater dropping into
the inlet may cause turbulence and odor release.
One technique that has been successfully used to
minimize odor release at siphons is the use of air
jumpers. These are pipes that take the air off the top of
the inlet structure and convey it to the end of the
siphon. In most cases, air jumpers run parallel to the
siphon, although the pipe can be suspended above
the hydraulic grade line. Provisionsshouldbe made to
drain the air jumper to periodically remove accum-
ulated condensate. Usually, the diameter of the air
jumper pipe is approximately one-half that of the
siphon (6). Solids deposition is another potential
problem and siphons should normally be designed for
velocities of 0.9 m/s (3.0 ft/s) to prevent solids
deposition and subsequent odor generation. In some
cases, multiple siphon lines are installed to ensure
adequate velocities during the early design lifetime of
the system. In these instances, the unused sewer line
may be used as the air jumper line during the early
design period when flow is small and sulfide genera-
tion may be a problem.
5.3 Ventilation of Sewers
5.3.1 Objectives of Ventilation
Ventilation of sewers is often undertaken for a variety
of reasons. For the most part, only the control of odors
is practically achievable with ventilation, Some of the
reasons ventilation has been attempted are discussed
here.
5.3.1.1 Increasing the Oxygen Content of the
Sewer Atmosphere
The oxygen content of the sewer atmosphere does
not change significantly as a result of the septicity of
the wastewater, in partially filled sewers, riseandfall
of the liquid level results in displacement and
replacement of air, and there is normally a down-
stream flow of air due to a drag effect between the
air-sewage interface. Oxygen concentrations in such
sewers are rarely less than 90 percent of normal. If
oxygen concentrations are above 90 percent of
normal, ventilation is unlikely to make a significant
difference in the oxygen balance of the stream.
5.3.1.2 Reducing the Atmospheric
H2S Concentrations
Although it would seem feasible to ventilate sewers
to reduce the atmospheric sulfide concentrations and
thus control corrosion, this approach has little
practical value. In order to have measureable results,
complete replacement of the sewer atmosphere with
fresh air would be required at frequent intervals. Even
if this approach were economical, there would be the
problem of disposal of large volumes of malodorous
air.
5.3.1.3 Drying the Walts of Sewers and Other
Structures
The oxidation of hydrogen sulfide gas to sulfuric acid
does not occur if the surface is dry, since moisture
must be present for bacterial oxidation of H2S.
Ventilation has been used with the objective of drying
sewer walls. Thistlethwayte estimated that when the
relative humidity of the sewer atmosphere exceeds
80 to 85 percent, sufficient moisture will be present
on the walls to support bacterial activity (5). Thistle-
thwayte also proposes a design procedure for ventila-
tion of sewers to control humidity, but indicates that
in most cases this approach is not practicable. This is
due to the rapid increase in relative humidity of
ventilation air with flow along the sewer, the large
number of ventilation stations required, and the
significant increase in operation and maintenance
costs, Pomeroy also indicates that this approach is
impractical for year-round protection for even typical
sewer distances between manholes (1).
5.3.1.4 Preventing Lethal Atmospheres
Portable fans or blowers are often used to ventilate
manholes before workers enter. This is acceptable
practice for localized conditions, provided other
normal safety procedures are followed. However, it is
questionable as to whether this practice would
provide a safe environment between manholes. It is
not feasible to ventilate large sections of a sewer
system sufficiently to assure a safe environment for
sewer workers.
no
-------
5.3.1.5 Preventing Explosive Atmospheres
Explosions in sewers generally result from the
presence of large amounts of volatile hydrocarbons or
from leaking natural gas mains. Only under very
unusual conditions do explosions result from accum-
ulation of sewer gases. Because of the unpredictable
causes of explosions and the conditions under which
they occur, it is unlikely that ventilation could assure
protection from explosions in a wastewater collection
system.
5.3.1.6 Controlling Odor Emissions
Sewer ventilation can withdraw malodorous air at
one point in order to prevent odor emissions at other
locations. Normally, contaminated air must undergo
treatment by one or more of the techniques discussed
in Chapter 4. Ventilation is often practiced at waste-
water treatment plants, where air is withdrawn at the
downstream terminus of the sewer (plant headworks)
and either treated separately or piped to existing
biological stabilization processes for removal of odors.
Although most other possible objectives have not
been achieved by practical levels of ventilation alone,
control of odor emissions can be effectively served by
continuous ventilation.
5.3.2 Methods of Ventilation
Ventilation of a sewer can occur through both natural
and mechanical means. Virtually all sewers incor-
porate some method of natural ventilation. Mechan-
ical ventilation, on the other hand, is normally
employed only in response to complaints of odor
emissions from a portion of the collection system
following the original design. The two methods are
discussed below.
5.3.2.1 Natural Ventilation
Collection systems in the United States do not
normally incorporate special vents or hardware to
assist in natural sewer ventilation. Rather, manholes
and building vents are generally considered adequate
to keep sewers sufficiently ventilated (6).
Natural ventilation occurs from the following forces
(5X8).
1. Change in barometric pressure along the sewer
2. Wind velocities past vents
3. Frictional drag of wastewater on sewer air
4. Rise and fall of the wastewater level in the
sewer
5. Relative density differences of sewer air and
outside air
The degree of natural ventilation which occurs in a
sewer is difficult to predict, since fluctuations in the
above variables may change both the direction of
movement and velocity of the air contained in the
sewer.
Whereas no special provisions are normally made to
enhance natural ventilation of sewers in the United
States, special ventilation systems are routinely
incorporated into sewer designs in the United
Kingdom and Australia (5). The reason for this is that
collection systems designed in the United Kingdom
and Australia have typically incorporated "boundary
traps" or "running traps" at building sewers or house
laterals, which effectively prevent the transfer of air
between the sewer and building vents. Since the
building vent is no longer a source of ventilation air,
induct and educt stacks are placedat various locations
in the collection system to allow air movement into
and out of the sewer. Research on natural sewer
ventilation systems is discussed in References 7 and
8, and detailed design procedures for such ventilation
systems are presented in Reference 4.
5.3.2.2 Mechanical Ventilation
Mechanical ventilation may be employed where a
constant velocity and direction of air flow is desired.
This may be necessary where odor emissions from
sewers must be controlled, as in residential neigh-
borhoods, or where hydraulic conditions that occur in
siphons or surcharged sewers result in stagnant air
pockets with reduced oxygen contents. Mechanical
ventilation may also be employed at headworks of
wastewater treatment plants in order to convey
malodorous sewer gases to odor control systems.
Figure 5-9 shows two examples of the use of
mechanical ventilation for odor control in Austin,
Texas (10). At Williamson Creek, odors escaping from
septic wastewater entering the wet well necessitated
sealing of the wet well and upstream manhole to
allow withdrawal of air from 5,980 m (19,600 ft) of
106-cm (42-in) diameter concrete outfall line. A 7.1 -
mVmin (250-scfm) blower was used to remove
odorous gases from the sewer and discharge them to
an aerated stabilization pond. This approach was
successful in controlling odors from the system.
A similar approach was used for the Walnut Creek
system. This was a total gravity system which
included a siphon for conveying wastewater under
Walnut Creek. Two 14.2-mVmin (500-scfm) blowers
were used to remove odorous gases from 3,200 m
(10,500 ft) of concrete sewer at a sealed manhole
upstream of the siphon. The blower discharged the
gases through air lift pumps in the aeration basin of
the treatment plant to achieve better mixing of the
tank contents and absorption and oxidation of the
odorous components of the gas in the aerated liquid.
Ventilation of pumping stations is part of normal
design procedures for these structures. A minimum
of 12 air changes per hour is recommended for
continuously ventilated wet wells and 30 air changes
per hour for intermittently ventilated wet wells. A
minimum of 6 air changes per hour is recommended
for continuously ventilated dry wells and 30 air
111
-------
Figure 5-9, Forced draft ventilation far odor control, Austin, TX (9).
Sealed
Manhole
Air Inlet
or Vent
Blower
Manhole
Air in Upper Part
of Pipe
Aeration Tank
Flowing Sewage in
Bottom of Pipe
Sewage Pump
Sealed
Wet Well
-19,600 ft
a) Ventilation System at Williamson Creek, TX
Sealed
Manhole
Air Inlet
or Vent
Manhole
Air in Upper Part
of Pipe
Blower
Flowing Sewage in
Bottom of Pipe
Aeration Tank
10,500 ft
Siphon
b) Ventilation System at Walnut Creek, TX
changes per hour for intermittently ventilated dry
wells and other below grade structures (11).
5,4 Selection of Materials
Materials selection is a critical aspect in design of
wastewater collection systems in which sulfide
generation is likely to pose problems. The additional
expense of using materials with greater degree of
corrosion resistance may be justified by the cost
savings for replacement or rehabilitation of deterior-
ated structures at some later date. The following
discussion describes the various materials used in
collection systems, with particular emphasis on the
corrosion-resistant properties of each material.
5,4. f Pipe Materials
If sulfide is expected to be present in sufficient
quantities to cause corrosion, consideration must be
112
given tothe use of pipe materials with higher degrees
of corrosion resistance. Design considerations in
selecting such materials are (1 >:
1. Availability of the materials in the pipe sizes
required
2. Minimum and maximum levels of sulfide ex-
pected in the wastewater
3. Factors other than acid resistance of the pipe
(abrasion resistance, stress-corrosion resis-
tance, load-bearing strength, and other durabil-
ity considerations)
4. Hydraulic characteristics of the materials under
conditions of actual use
5. Other advantages or disadvantages of the
material (ease of installation, resistance to
infiltration, flexibility, etc.)
6. Expected future service requirements
7. Relative costs vs. expected service lifetimes of
various kinds of pipe
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5,4.1,1 Concrete Pipe
Concrete is one of the most common materials used
in construction of sewer pipe, and is virtually the only
material used for large diameter trunk sewers.
Several alternatives are available to extendthe design
lifetime of concrete pipe in corrosive atmospheres
found in sewers. These include; 1) specification of
calcareous aggregate, which increases the overall
alkalinity of the concrete; 2) specification of additional
wall thickness to serve as sacrificial material; and 3)
use of liners or coatings with high degrees of
corrosion resistance on the interior pipe walls.
Alkalinity of the concrete pipe and the thickness of
concrete cover over the reinforcing steel have been
used in the development of the "life factor" equation
(12);
(A)(z) = 0.45 k mv = flux of HaS to the pipe wall, g/mVhr
(see Equation 2-20)
L = desired design lifetime, years
This equation is useful in that, if the H2S flux is
calculated based on assumed conditions, the desired
servicelife Dfthe pipe can beenteredanda lifefactor
computed, which allows flexibility in selecting various
combinations of pipe thickness and alkalinity of the
concrete.
Example:
Assume L = 50 years
0sw = 0.03 g/m2/hr
k = 0.7
(A)(z) = 0.45 k 09w L = (0.45)(0.7)(0.03)(50)
= 0.47
This life factor could theoretically be met by numerous
combinations of alkalinity and wall thickness, ex-
amples of which are shown below.
Alkalinity
of Concrete
Aggregate Concrete(A) Cover (z) (A)(z)
Granitic
50-percent
Calcareous
100-percent
Calcareous
weight fraction
0.2
0.50
0.85
in
2.4
1.0
0.6
0.48
0.50
0,51
A manufacturer can thus meat the required lifefactor
by using the combination of alkalinity and wall
thickness that is most economical and suitable to the
expected use and to the production process.
The alkalinity of concrete varies with the cement
content and type of aggregate. Ranges of alkalinity
(weight fraction) for concrete pipe containing 352 kg
cement/m3 (594 Ib/cu yd) are shown below for
various aggregates;
Granitic aggregate: Alkalinity = 0.18 to 0.22
50-percent calcareous
aggregate; Alkalinity = 0.4 to 0.6
100-percent calcareous
aggregate: Alkalinity = 0.8 to 0.9
Procedures for obtaining interior wall cores of
concrete pipe and for determining alkalinity of the
samples are described in References 13 and 14,
Alkalinity of the interior wall of concrete pipe will also
vary with the method of manufacturing. Centrifugally
spun pipe generally has a higher interior wall
alkalinity than cast pipe due to the migration of
cement toward the interior wall during production
(14).
It should be noted that not all concrete pipe manu-
facturers have ready access to calcareous aggregates.
Most manufacturers should be able to meet life
factor, (A)(z) design specifications through a combina-
tion of concrete alkalinity and wall thickness.
5.4.1.2 Asbestos Cement Pipe
Asbestos cement pipe is subject to attack by sulfuric
acid. Because the cement content is higher than for
reinforced concrete pipe, the alkalinity may also be
higher, depending on the type of aggregate used.
However, corrosion of asbestos cement pipe immedi-
ately begins to degrade the structural section of the
pipe, as opposed to corrosion of reinforced concrete
pipe in which the concrete cover over the reinforcing
steel is degraded before the structural integrity of the
pipe is affected.
Although variability in the alkalinity of asbestos
cement pipe is limited, the life factor design approach
can be employed to determine required thickness.
Alkalinity of asbestos cement pipe is typically in the
range of 0.5 to 0.6 (12).
It should be noted that asbestos cement pipe is
banned in many areas, and is generally not available
in the United States because of the known health
effects of asbestos fibers.
5.4.1.3 Vitrified Clay Pipe
Vitrified clay pipe is immune to attack by sulfuric acid
and most volatile industrial waste products, and as
such is a suitable material for use where high sulfide
113
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concentrations are expected. Elastomeric jointing
materials, which are resistant to attack by many
corrosive" materials, should be used with the vitrified
clay pipe, Properly laid and jointed, vitrified clay pipe
will remain serviceable for a very long time if not
disturbed by external forces. Vitrified clay pipe is
available in sizes ranging from 10cm (4 in) to 107 cm
(42 in); the larger sizes may not be available in all
parts of the country.
5.4.1.4 Reinforced Plastic Mortar Pipe
Reinforced plastic mortar pipe is constructed of
polyester resin mixed with sand and reinforced with
fiberglass. The resulting product is not subject to
sulfuric acid attack unless the glass fibers are exposed
due to damage during handling or deflections of the
pipe. If the fibers are exposed, acid can creep along
the fibers and react with impurities in the fibers. The
oldest sewer constructed of reinforced plastic mortar
pipe is a trunk sewer installed in San Jose, California
in T966(1)(12).
5.4.1.5 Homogeneous Plastic Pipe
Polyvinyl chloride (PVC), acrylonitrile-butadiene-
styrene (ABS), and polyethylene (PE) are pipe mate-
rials resistant to sulfuric acid attack and thus suitable
for use where high sulfide concentrations are
expected. Care must be given to bedding and backfill
to keep pipe deflections to an acceptable minimum
(1)(12). ABS pipe is very susceptible to stress corro-
sion.
5.4.1.6 Steel Pipe
Steel pipe is subject to corrosion by sulfuric acid as
well as by HzS in the presence of oxygen when the
sewer is flowing partially full. The oxidation product,
iron sulfide, can accumulate to an extent that the
hydraulic capacity and the structural integrity of the
line are significantly reduced.
5.4.1.7 Ductile Iron Pipe
Ductile iron pipe has replaced gray cast iron pipe for
use in wastewater collection systems. Gr.ay cast iron
pressure pipe is no longer manufactured in the United
States.
Ductile iron pipe normally lasts longerthan steel pipe
due to the increased wall thickness. However, iron is
subject to corrosion in the presence of oxygen as is
steel, and the bulky corrosion products may accumu-
late and restrict the cross-sectional area of a pipe and
affect the structural integrity of the pipe.
When iron pipe is corroded by H^S gas, sulfuric acid,
or other agents, the process proceeds by graphitiza-
tion, which involves dissolution and removal of the
iron crystals, leaving behind non-metallic compo-
nents such as graphite, carbides, silicidesof iron, and
corrosion products. Although the pipe may appear to
be in good condition, its structural strength is often
greatly reduced (15).
5.4.2 Pipe Linings and Protective Coatings
Many different types of linings and coatings have
been used in attempts to protect pipe from corrosion
due to wastewaters containing sulfides. Unfortu-
nately, success with these materials has been quite
variable. The problem is in achieving a sealed lining
that is firmly affixed to the interior pipe wall and
which has no defects, pinholes, or construction
damage that would allow penetration of acid to the
pipe. Such defects can result in localized corrosion
occurring at a greater rate than if the acid attack were
distributed over the total pipe surface, Acid, pene-
trating through pinhole-sized defects, attacks the
underlying material, and the accumulation of expan-
sive corrosion products eventually ruptures the lining
or coating, allowing greater acid penetration and
progressive deterioration of the pipe. Coatings can be
painted, sprayed, or troweled onto the interior surface
of the pipe, and linings may be applied as preformed
sheets or panels during manufacture of the concrete
pipe.
5.4.2.1 PVC Liners
One of the few lining systems which has been used
successfully for long-term protection of concrete is a
PVC liner mechanically attached to the concrete. The
liner consists of sheets of plasticized PVC approxi-
mately 1,5-mm (1/16-in) thick with T-shaped keys
running longitudinally on one face. The sheets are
fastened to the forms, key side in, before pouring of
the concrete during manufacture of the pipe. In the
finished pipe, the keys are firmly imbedded in the
concrete, The PVC sheets are heat-welded at the pipe
joints to produce a completely sealed liner. A sche-
matic diagram of a T-lock PVC liner for concrete pipe
is shown in Figure 5-10(16).
Some problems have arisen in the installation of such
liners in cast-in-place concrete pipe because of the
difficulty in imbedding all the keys. No such problems
have been reported for factory made pipe. Although
PVC liners may be subject to damage from very high
turbulence or from mechanical cleaning tools, proper
design and operation can overcome such problems
(1). In California, PVC sheet lining has been success-
fully used for concrete sewer pipe protection for over
30 years (17).
5.4.2.2 Vitrified Clay Liner Plates
Vitrified clay liner plates mechanically locked to
concrete pipe have also been used. However, porosity
of the clay allowed acid to diffuse into the concrete,
softening and expanding it, which resulted in cracking
of the plates and breaking of the lugs which lock the
114
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Figure 5-10. T-lock PVC liner for concrete pipe.
Extruded T Anchors,
9,5 mm High
Concrete Poured
in Form
Flexible PVC Membrane,
1.6 mm Thick
plates to the interior wall of the pipe (1X18). This
product is no longer available in many areas.
5.4.2.3 Thick Film Coal Tar/Epoxy Coatings
One of the few pipe coatings for which long-term
performance data are available is the thick film coal
tar/epoxy coating. Such coatings are spray-applied
either during or after manufacture of the pipe, but
prior to pipe installation. Although many coatings are
applied in relatively thin films <2.5 mm (10 mil), thick
film coal tar/epoxy coatings are generally applied
with a minimum thickness of 10 mm (40 mil). In some
cases, film thicknesses of up to 25 mm (100 mil) are
specified. Coal tar/epoxy coatings are used for both
metal and concrete pipe.
For successful long-term performance of thick film
coal tar/epoxy coatings, the following conditions
must be met:
1. Adequate surface preparation—sandblasting of
the surface to remove all foreign materials and
contaminants; removal of dust.
2. Adequate film thickness—minimum film thick-
ness of 10 mm (40 mil).
3. Adequate quality assurance procedures, includ-
ing:
a. Checks on wet film as applied
b Checks on dry film thickness
c. Low voltage holiday detection on 100 percent
of barrel surface
d, Hanging-weight adhesion tests
5.4.2.4 Cement Mortar Liners
Cement mortar is often used as a liner for iron or steel
pipe in wastewater applications. For conditions in
which sulfide-induced corrosion may present a
problem, additional liner thickness and/or alkalinity
of the cement may be specified. The life factor design
approach can be used to achieve a desired lifetime of
the cement mortar lining. Alkalinities of mortar used
in lining of ferrous pipe are typically 0.4 to 0.5 (12).
5.4.2.5 Other Pipe Lining Coatings
Another alternative lining material is type 3 lb L
stainless steel sheeting with a thickness of 0.5 to 0.6
cm (0.18 to 0.25 in). These sheets may be used where
PVC sheet liners may be subject to mechanical
damage.
Numerous coatings are available for sewer pipe.
Some of the more common materials not previously
discussed include asphaltic compounds, polyethyl-
ene, and polyurethane. Asphaltic compounds have
not proved to be successful in sewers in which H2S is
present. Volatile materials present in the wastewater
can dissolve the coating, and scratches, defects, or
pinholes allow acid to migrate to the pipe. Long-term
field experience with polymeric materials, such as
polyethylene and polyurethane, is limited.
5.4.3 Construction Materials for Appurtenances
In designing a wastewater collection system to avoid
sulfide problems, selection of pipe materials is of
paramount concern. However, the design engineer
must also consider selection of materials for sewer
appurtenances such as manholes, transition struc-
tures, and drops.
If relatively high sulfide concentrations are expected
in the wastewater, such appurtenances may promote
turbulence and release of H2S, which can result in
H2S gas and sulfuric acid attack on both pipe and
appurtenances. An example of this type of occurrence
is described below.
In Port St. Lucie, Florida, a 10-cm (4-in) PVC pressure
pipe carrying septic tank effluent from approximately
115
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200 homes discharged intothe concrete manhole of a
larger diameter gravity sewer conveying raw waste-
water. The septic tank effluent contributed approxi-
mately 20 percent of the total flow, and contained
dissolved sulfide concentrations in excess of 10 mg/l.
After approximately 8 years of service, severe deteri-
oration of both the concrete manhole and the cast
iron manhole cover was observed. This was attributed
to the turbulence at the junction of the two streams,
which released H2S gas to the sewer atmosphere.
The concrete manhole was replaced with one fabri-
cated from fiberglass, and a drop pipe was installed to
reduce turbulence. Although no deterioraton of the
fiberglass manhole has been observed, evidence of
corrosion has been noted at the next concrete
manhole downstream.1
It is, therefore, necessary to carefully consider
materials selection for all components of a waste-
water collection system, including manholes, junc-
tions, and drops, in which the presence of H2S gas
and sulfuric acid poses a potential corrosion problem.
5.5 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1. Process Design Manual for Sulfide Control in
Sanitary Sewerage Systems. U.S. Environmen-
tal Protection Agency, Technology Transfer, PB-
260 479/AS, October, 1974.
2. Metcalf and Eddy, Inc. Wastewater Engineering.
McGraw-Hill, New York, NY, 1972.
3. Yao, KM.Functional Design of Sanitary Sewers.
JWPCF 48 (7): 1772-1 778, 1976.
4. Design Manual for Sulfide and Corrosion Pre-
diction and Control. American Concrete Pipe
Association, Vienna, VA, 1984.
5. Thistlethwayte, D.K.B. The Control of Sulphides
in Sewerage Systems. Ann Arbor Science, Ann
Arbor, Ml, 1972.
6. Design and Construction of Sanitary and Storm
Sewers. Manual of Practice No. 9, Water
Pollution Control Federation, Washington, DC,
1970.
7. Yogi, D.R., R.L. Smith, and N.C. Burbank, Jr.
Hydrogen Sulfide, Control in Sewers Containing
'Persona! Communication; Patricia H. Lodge, Genera! Development Utilities,
fnc.
Brackish Water—Honolulu, A Case Study. Pre-
sented at the 54th Annual Conference, Water
Pollution Control Federation, Las Vegas, NV,
October, 1981.
8. Pescod, M B. and A.C. Price. Major Factors in
Sewer Ventilation. JWPCF 54 (4): 385-397,
1982.
9. Pescod, M B. and A.C. Price. Fundamentals of
Sewer Ventilation as Applied to the Tyneside
Sewerage Scheme. Water Pollution Control 80
(1); 17-33, 1981.
10. Ullrich, A H Forced Draft Ventilation Protects
Concrete Sewer Pipe, Controls Odors. Water
and Wastes Engineering (4): 52-55, 1968.
11. Recommended Standards for Sewage Works.
Great Lakes-Upper Mississippi River Board of
State Sanitary Engineers, Health Education
Service, Albany, NY, 1978.
12. Kienow, K.K. and R.D. Pomeroy. Corrosion
Resistant Design of Sanitary Sewer Pipe. Water
and Sewage Works, Reference Number, R-8-
13, 1979.
13. Concrete Pipe Handbook. American Concrete
Pipe Association, Vienna, VA, 1981.
14. Kienow, K.K. Protecting Reinforced Concrete
Pipe Sanitary Sewers. Water and Sewage Works
122(10): 94-97, 1975.
15. Pomeroy, R.D. Corrosion of Iron and by Sulfides.
Water Works and Sewage, April, 1945.
1 6. Spindel, E. Methods of Preventing Corrosion in
Sewerage Systems. Corrosion 12 (3), 1956.
17. Graham, R.E. San Diego's PVC Sewer Liners
Fight Corrosion After 20 Years. Water and
Sewage Works 116(5): 168-171.
18. Pardee, L.A. and F.G. Studley. Concrete Sewer
Protection. Water and Sewage Works, April,
1957.
r is
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Chapter 6
Designing to Avoid Odor and Corrosion in New Wastewater Treatment Facilities
6.1 Introduction
Current design practices for wastewater treatment
facilities do not normally consider the potential for
odor generation and corrosion and the factors re-
quired to control and minimize these problems.
Failure to adequately consider odor and corrosion
control during design has been due to: 1) the
difficulties in predicting and quantifying sulfide
generation; 2) an over reliance on accepted criteria
for aeration, hydraulic, and ventilation design; and 3)
the absence of specific design information and
guidance for odor and corrosion evaluation and
control. It must be recognized that some odors will
occur at virtually all wastewater treatment plants and
that some background odor level cannot be totally
avoided. The objective of odor and corrosion evalua-
tion and control in new treatment plant design is to
prevent or minimize corrosion and the occurrence of
nuisance odor levels to the surrounding community.
An odor intensity of 176 odor units/m3 (5 odor
units/cu ft) at the boundary of the treatment facility
has been found satisfactory to avoid nuisance com-
plaints (1).
One of the more important considerations in new
plant design is the analysis of present and future
wastewater characteristics and flows as they affect
the hydraulic design of plant components, basin
detention times, loadings to biological processes, and
the sludge generation potential of the facility. Both
underloaded and overloaded unit processes have
potential for odor generation during the design
lifetime of thefacility. Significant odor generation has
been observed for underloaded facilities due to
excessive detention times in wet wells and holding
basins and solids deposition resulting from low
channel velocities within the plant. The more common
situation, however, is odor generation resulting from
organic overloading, inadequate supply of air, im-
proper ventilation, or simply the failure to recognize
that certain unit processes may require the imple-
mentation of special odor control technology.
An important design consideration is plant siting.
Designing for odor control is more critical if the plant
is sited close to residences, major highways, com-
mercial developments, or other populated areas.
Odors generated from a plant sited in a relatively
remote location may be considered acceptable unless
the severity is such that it is objectionable to plant
operators and maintenance personnel.
Certain unit processes in wastewater treatment
plants have increased the potential for odor genera-
tion and require special consideration. Physical-
chemical systems are often subject to severe odor
and corrosion problems due to the absence of aeration
and the subsequent low oxidation-reduction potential
(ORP), Septage and sludge handling systems are
common sources of objectionable odors. Sludge
treatment operations, especially those employing
thermal conditioning processes, may require special
attention to ensure adequate odor control.
The type and severity of odors vary significantly with
the plant size and type of treatment employed. For
example, thermal sludge conditioning units in large
plants generate odors that often cause complaints
from adjacent neighborhoods; these units may be the
dominant odor source from facilities where the
process is employed. Odors from preliminary treat-
ment processes in large plants are controlled by
enclosing the components in a building. However,
in small plants where facilities are not enclosed,
odors generated from preliminary treatment facilities
are dispersed into the atmosphere. Sludge drying
beds and unaerated storage lagoons commonly used
at small plants may also be odor sources, particularly
if the sludge is not well stabilized. The design of a
given facility must consider the relative magnitude of
all potential odor sources and the control methods
necessary to reduce odors to acceptable levels. The
more common odor-generating unit processes in
wastewater treatment facilities are presented in
Section 6.2,
Corrosion potential must also be considered during
design. Basic design considerations for minimizing
corrosion in treatment plants are presented in this
chapter. Where corrosive agents such as H2S or acids
are known to be present in the wastewater, special
provisions are required for effective corrosion control.
Although it may be difficult to predict with great
certainty odor and corrosion problems over the
lifetime of a wastewater treatment plant, cognizance
of potential problems and adherence to certain design
procedures can minimize their occurrence.
117
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6.2 Common Sites of Odor Generation
Table 6-1 lists the more common odor generating unit
processes in municipal wastewater treatment plants,
and ranks their potential for odor generation. In
general, the greatest potential for odor generation is
associated with preliminary treatment of the raw
wastewater, and with the storage and treatment of
sludge,
Table 6-1, Potential for Odor Generation from Common
Unit Processes in a Wastewater Treatment Plant
Odor Potential*
Liquid Stream Processes
Flow Equalization
H
Preaeration
H
Screening
H
Grit Removal
H
Primary Clarification
H
Stabilization
Suspended Growth
L
Fixed Film
M
Chemical
H
Secondary Clarification
L
Tertiary Filtration
L
Disinfection
L
Sidestream Returns
H
Sludge Stream Processes
Thickening/Holding
H
Aerobic Digestion
M
Anaerobic Digestion
M
Thermal Conditioning
H
Storage Lagoons
H
Dewatering
Vacuum Filter
H
Centrifuge
H
Belt Filter
H
Filter Press
H
Drying Beds
H
Composting
H
Septage Handling
H
*L = Low
M = Moderate
H = High
6.2.1 Liquid Stream Processes
Preliminary treatment works are potential sources of
odor since they process raw wastewater which
contains putrescibleorganics anddebris. Raw waste
water may also be septic and contain dissolved
sulfides and other odorous gases. Flow equalization
basins are generally aerated to maintain solids in
suspension and to prevent septicity. However, in-
coming dissolved sulfides or other odorous gases may
be released by such aeration. Preaeration basins, bar
screens, and aerated grit chambers may also induce
release of odorous vapors from the wastewater.
Storage and handling of raw screenings and organi-
cally coated grit can result in odor generation from
putrefaction of the organic materials. In small plants,
118
these materials may be allowed to accumulate and
decompose over periods of several days or more
before quantities are sufficient to warrant removal
and disposal.
Primary clarifiers may generate odors if the influent
wastewater is high in sulfides or if settled sludge
residence times are sufficiently long to allow septic
conditions to develop. In many cases, insufficient
sludge removal frequencies may allow septic condi-
tions to develop in the settled sludge. Turbulence
associated with the fall of primary effluent over the
weirs can release H2S and other odorous gases to the
surrounding atmosphere. Scum handling systems
can be significant sources of objectionable odors if
not regularly flushed and cleaned. It is recommended
that the sludge withdrawal schedule for primary
clarifiers be established to limit sludge residence
times to less than 1 hour under average flow
conditions. Scum handling equipment should be
designed to be easily flushed, degreased, and disin-
fected.
Liquid stream biological treatment processes, such as
aeration basins and trickling filters, are not normally
sites of significant odor generation if properly de-
signed and operated. In organic overloading, however,
DO depletion and septic conditions can develop.
These conditions can occur with organic overloading
of trickling filters, high seasonal waste loads, and first
stage overloading of rotating biological contactors.
Failure to provide for adequate mixing in an aeration
basin can result in deposition and accumulation of
organic solids in "dead zones," which then may
become septic and generate odors.
Secondary clarifiers normally do not generate odors,
since the incoming liquid is aerobic. However, sludge
withdrawal rates that provide for greater than 1.5- to
2-hour sludge residence times can allow septic
conditions to develop in the settled sludge.
Return of sidestreams to the headworks or the liquid
stream processes has high potential for odor genera-
tion, since sidestreams from sludge stabilization,
conditioning, thickening, and dewatering operations
often contain high concentrations of organic and
odorous materials that have high oxygen uptake rates
and can become septic very quickly. Return of these
streams under turbulent conditions allows odorous
gases to be released to the atmosphere.
Other potential sources of odors are unit processes
that have been permanently or temporarily taken out
of service. Failure to provide complete dewatering
and thorough cleaning of unused tankage can lead to
odorous putrefaction of remaining solids and organic
slimes.
6.2.2 Sludge Handling Processes
Sludge handling processes are normally the major
sources of odors at most municipal wastewater
-------
treatment facilities. Primary sludge, due to its raw,
unstabilized state, has a highly offensive odor and a
low ORP. Secondary sludges from activated sludge or
trickling filter systems are generally less objection-
able, but when stored in holding tanks or subjected to
thickening processes with residence times in excess
of 1 to 2 hours, they may generate odors due to the
reduction in ORP and development of septic condi-
tions. Sludge stabilization processes such as aerobic
and anaerobic digestion, are not normally significant
odor sources if properly designed and operated.
However, overloaded aerobic digesters or improperly
operated anaerobic digesters with uncontrolled re-
lease of gaseous products can be odorous. Thermal
sludge conditioning systems produce offensive odors
that require special attention. Sludge handling facil-
ities downstream of thermal conditioning processes,
such as decant tanks and blending tanks should be
covered and separate treatment provided for the
gaseous discharges. This is because of the greater
release of odor compounds at higher temperatures
and the odor-producing chemical reactions that take
place during the conditioning process.
Sludge dewatering units are also potential sites for
odor generation, since physical removal of water
releases odorous gases. Since dewatering units are
normally housed in buildings, odor concentrations
inside these facilities can easily reach objectionable
levels. Adequate ventilation is necessary to maintain
odor concentrations at an acceptable level in such
enclosed spaces; however, the exhaust air may also
have to be treated in many instances to prevent
release of objectionable odors to the atmosphere.
Sludge storage tanks and lagoons are other potential
odor generators due to long residence times and
development of septic conditions, Odor generation
from lagoons is more severe during sludge withdrawal
or lagoon loading due to increased turbulence, Where
lagoons must be located in highly populated areas,
designs incorporating an aerated top layer of water
have been successful in oxidizing sulfides released
from bottom sediments. Temporary odor masking
maybe used duringsludgeremoval.The major design
considerations for lagoons involve analysis of atmos-
pheric conditions and siting, including prevailing
wind direction, local weather patterns, and proximity
to populated areas.
6.3 General Design Considerations for
Avoiding Odor Generation and Release
6,3.1 Hydraulic Design
Proper hydraulic design is critical for minimizing the
potential for odor generation in a wastewater treat-
ment plant. In general, self-cleaning velocities should
be achieved in all channels and interprocess piping to
prevent solids deposition. Minimum self-cleaning
velocities of 0.45 m/s (1.5 ft/sec) at minimum flow
are recommended to prevent solids deposition.
Velocities through horizontal flow grit chambers are
normally maintained at a reasonable constant level of
about 0.2 m/s (0.75 ft/sec) through the use of Sutro
or proportional weirs at the channel outlets. Alterna-
tively, a parabolic channel, approximated by a trape-
zoid, can be used for velocity control (2). Velocities of
less than 0.2 m/s (0.75 ft/sec) will cause deposition
of organic solids with the grit and subsequent
generation of objectionable odors. Aerated grit
chambers will release sulfides, if present, due to
turbulence and short hydraulic detention times.
Severe corrosion has been noted for mechanically
cleaned and housed aerated grit chambers that have
not been properly ventilated.
Rectangular interprocess distribution channels
should be constructed with fillets at the bottom
corners to avoid solids accumulation in "dead" flow
zones that are difficult to clean (3). Channels with
trapezoidal cross-sections can also be constructed to
ensure self-cleaning velocities throughout the plant's
lifetime. A common approach is to use aerated
channels to maintain solids in suspension and to
provide DO. This ensures no deposition of solids over
a wide range of anticipated flows, Aeration systems
for such channels are recommended and should be
sized to provide air at the rate of 0.2 to 0.5 m3/min per
lineal meter of channel (2-5 cfm/lineal foot) (2).
Turbulence should be avoided for any process streams
which may be septic and contain malodorous gases
such as H2S, since this will tend to release these
gases to the atmosphere. For example, process side-
streams returned to the headworks, or other liquid
stream processes should be introduced below water
surface, since hydraulic drops will result in rapid
release of gases that can cause odors and induce
corrosion of metal and concrete.
Flow equalization, preaeration, and activated sludge
aeration basins must be designed to prevent solids
deposition and creation of "dead zones." This is
accomplished by proper selection of basin geometry,
and by appropriate sizing and placement of aeration
equipment. A general rule of thumb for achieving
adequate mixing is to provide a minimum air flow of
0.3 mVrnin per meter of tank length (3 cfm/ft) (2).
Triangular baffles or fillets should be provided in the
bottom corners to avoid accumulation of solids.
For plastic media trickling filters, minimum wetting
rates are normally specified to ensure that the entire
surface area is wetted, and to prevent filter fly
nuisances and odor generation. Depending on the
specific media employed, recommended minimum
wetting rates vary from 5.7 to 28.5 I/mi n/m2(0.14 to
0.7 gpm/ft2). Provisions should be made for periodic
119
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flooding of the filter to control larvae and organic
slimes.
All tanks and chambers should be designed to allow
complete dewatering and flushing to prevent septic
wastewater, slimes or sludge from remaining and
creating odors when the units are taken out of
service.
6.3.2 A eration
Aeration is a widely used and effective method of
preventing sulfide generation in wastewater treat-
ment plants. The proper design of aeration systems
requires an understanding of both the benefits of
preventing sulfide formation as well as potential
detrimental effects of release of sulfide that might be
present in solution. The following summarizes the
effects of aeration for various applications in the
design of new treatment facilities:
1. Aeration of waste streams containing appreci-
able dissolved sulfide {> 0.5 mg/l) will cause
sulfide release that may increase the H2S
concentration in enclosed air spaces to several
hundred times the odor threshold.
2. Aeration of sulfide-containing wastes in short
detention time basins {<30 min) will not reduce
dissolved sulfide levels sufficiently to prevent
release,
3. Aeration of sulfide-bearing wastes for long
detention periods or in the presence of active
biomass will reduce dissolved sulfides to the
point that stripping is not significant.
4. Aeration of wastes containing no appreciable
dissolved sulfide is recommended whenever
increased DO is desired to prevent low ORP's
and subsequent sulfide generation.
5. Aeration is recommended to prevent solids
deposition and to maintain adequate DO levels
throughout the plant.
For distribution channels, air should normally be
supplied at the rate of 0.2 to 0.5 m3/min per lineal
meter (2 to 5 cfm/lineal foot), Preaeration basins
should be designed to provide air at the rate of 0.7 to 3
m3/m3 (0.1 to 0.4 ft3/gal) of wastewater, while flow
equalization basins should be supplied with air at a
minimum rate of 9.4 ma/min/1,000 m3(1.25 cfm/
1,000 gal) of storage capacity (2)(3),
6.3.3 Covering or Housing of Odor-Producing
Processes
A number of unit processes in wastewater treatment
plants are usually housed in buildings. Housing is
normally done for climate protection of the equipment
and for ease of operation and maintenance rather
than for containing odors. Mechanical dewatering
equipment is almost always housed, regardless of
plant size. Preliminary treatment works, such as
screens and grit chambers, are often housed for
medium to large plants, but are normally sited
outdoors for small plants.
Larger plants may employ large buildings to house
sludge handling processes, such as thickeners,
conditioning processes and elutriation tanks, and
dewatering equipment. In severe northern climates,
many processes are either housed or covered to
protect against freezing. These include preliminary
treatment works, primary clarifiers, fixed film pro-
cesses such as trickling filters and rotating biological
contactors, secondary clarifiers, and sludge handling
processes. Where weather conditions or site con-
straints are severe, aeration basins may also be
covered. For cases in which normally unhoused
processes are enclosed for protection from extreme
climate conditions, special precautions must betaken
to ensure adequate ventilation to control humidity
and resulting corrosion potential within the enclosed
space.
The preceding discussion describes general design
practices for housing or covering unit processes in
wastewater treatment plants to protect equipment
from climatic exposures and to facilitate operation
and maintenance. It is often difficult during design to
predict the source and severity of odor generation and
to design covers or enclosures accordingly. In addi-
tion, covering of odor-producing units often requires
collection and treatment of exhaust air, the cost-
effectiveness of which must be weighed against
other odor control alternatives, such as air or oxygen
injection or chemical addition.
Certain unit processes normally sited outdoors have a
very high potential for odor generation. These include
gravity sludge thickeners, solids separation devices
for thermally conditioned sludges, sludge blending
tanks, and septage receiving/holding tanks. Mechan-
ical sludge thickeners, such as dissolved air flotation
and centrifugation units are normally housed. It is
also recommended that gravity sludge thickeners and
septage holding tanks sited outdoors be covered, and
that exhaust air be continuously removed and treated
to prevent escape of odors to surrounding neighbor-
hoods (4).
Scum pits and holding bins for screenings, grit, and
septic solids are major odor sources, and should
always be covered to prevent odor escape.
6.3.4 Ventilation
Adequate ventilation is required for any enclosed
areas in wastewater treatment plants where plant
employees may be present. The need for ventilation is
particularly critical where H2S is present, not only for
odor control, but for prevention of potentially hazard-
ous working conditions and control of relative humid-
ity. Recommended ventilation rates for sludge
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handling buildings range from 6 to 12 air changes per
hour with a relative humidity of not greater than 60
percent. In areas where climate permits, sludge
handling buildings with seasonal enclosures are
recommended; thus maximum benefit may be ob-
tained from natural ventilation for at least part of the
year.
6.3.5 Construction Materials
Many construction materials can physically absorb
odors by chemisorption, condensation, or chromo-
sorption. The degree to which materials may absorb
odors is dependent upon porosity, coarseness, color,
and composition. For buildings housing odor-pro-
ducing processes, such as raw wastewater screens
and grit chambers or sludge dewatering equipment,
particular care should be taken to select non-
absorbent construction materials. Dark, rough bricks,
gray, porous concrete, and dark plastics readily absorb
odorous compounds and then emit these odors for
long times thereafter. To minimize or prevent odor
absorption, the exposed surfaces should be dense,
smooth, light in color, chemically stable or inert, and
poor conductors of heat. Materials, such as glazed
vitrified clay or glazed ceramic, make excellent non-
absorbing surfaces.
6.3.6 Maintenance Provisions
Poor housekeeping practices are a secondary, but
important potential cause of odor generation at many
wastewater treatment plants. For this reason, plant
designs must include adequate provision for cleaning
and flushing of channels, scum pits, holding tanks,
screens, grit conveyors, and other unit processes in
which solids and slimes may accumulate and gener-
ate odors.
Hose bibs for pressurized process water should be
located throughout the plant such that less than 30 m
(100 ft) of hose is required to reach all unit processes.
All components should be readily accessible to
facilitate cleaning. Drainage systems should be
properly located and sized to allow easy removal of
flushing water. Floors should be sloped for easy
collection and removal of flushing water. These
requirements are particularly critical for sludge
handling buildings.
All tanks and process equipment should be designed
to allow complete dewatering and subsequent access
by plant personnel for cleaning and flushing.
6.4 Design Procedures for Specific Odor-
Producing Unit Processes
Table 6-2 presents a matrix of potential odor-
producing unit processes in wastewater treatment
plants and recommended methods for odor control.
Several of these processes, such as suspended
growth systems, secondary clarification, tertiary
filtration, and disinfection, are not normally major
sources of odors. Others, however, discussed in
greater detail below, often generate odors.
6.4.1 Headworks
Included under headworks are flow equalization
basins, screens, grit chambers, and preaeration
basins. The first consideration is whether or not the
incoming wastewater is septic. If this is the case, it is
likely to be more cost-effective to control sulfides
upstream of the headworks than to collect and treat
the odorous gases released during preliminary treat-
ment. Upstream treatment techniques that have been
successfully used to control odors at the headworks
include air injection, oxygen injection, and addition of
chemicals, such as hydrogen peroxide or chlorine.
These alternatives are discussed in detail in Chapters
3 and 4. If the wastewater entering the headworks
contains no sulfide, potential still exists for odor
release due to the presence of other volatile odorants
in the raw wastewater.
Flow equalization basins are likely candidates for
potential development of septic conditions and sub-
sequent odor generation. Design of equalization
basins should incorporate an aeration system to
maintain aerobic conditions and to keep solids in
suspension. The minimum air supply rate as recom-
mended by the "Ten State Standards" is 8.4 m3/min/
1,000 m3(1.25 cfm/1,000 gallon) of storage capacity
<3).
Screening and grit removal processes are significant
sources of odors due to the potential for accumulation
of putrescible organics and other debris. These
materials must be removed on a daily basis, and the
units must be cleaned and flushed regularly to
prevent odors. Grit and screening transfer systems
such as conveyors should be designed to prevent
spillage and to convey to the point of ultimatedisposal
with minimum detention time. Conveyor drip pans
should be sloped to drain at one end, and should be
accessible for cleaning and flushing. Accumulated
screenings and grit should be kept in closed con-
tainers before disposal. An adequate pressurized
water supply should be provided nearby to allow for
cleaning and flushing of all preliminary treatment
units (4).
Preaeration is sometimes used to prevent septicity, to
improve grease removal, and to promote flocculation
of wastewater solids prior to primary clarification.
Detention times vary from 15 to 60 minutes. The
value of preaeration in preventing septicity is ques-
tionable, due to the short detention times as previous-
ly discussed. One study showed the effects of 30
minute preaeration on ORP to be only temporary.
Although the ORP of the raw wastewater was
121
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Table 6-2. Matrix of Potential Odor-Producing Processes and Recommended Odor Control Methods
Recommended Control Methods
Unit Process
Chemical, Air
or O2 Addition
Upstream of
Plant
Aeration
Chemical
Addition
Covering With Improved
Collection and Hydraulics
Treatment of to Avoid
Air Turbulence
Improved
O &M
Flow Equalization
Preliminary Treatment
Screening
Grit removal
Preaeration
Liquid Stream Treatment
Primary clarification
Suspended growth systems
Fixed film systems
Phys/chem systems
Secondary clarification
Tertiary filtration
Disinfection
Sidestream returns
Sludge Stream Treatment
Gravity thickening
DAP thickening
Blending and storage
Aerobic digestion
Anaerobic digestion
Chemical stabilization
Thermal conditioning
Mechanical dewatering
Drying beds
Composting
Septage receiving/holding
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
increased from 10 mV to 170 mV, ORP was reduced
to 35 mV after primary sedimentation. Another study,
however, concluded that a 60-minute preaeration
time was sufficient to achieve an ORP that did not
decline during subsequent sedimentation (4). Pre-
aeration of sulfide-bearing wastewaters is not
recommended, as theturbulence induced by aeration
will release HjS and other odorous gases to the
atmosphere.
6.4.2 Liquid Stream Processes
This discussion focuses on liquid stream treatment
processes which have relatively high potential for
odor generation or release. Included in this category
are primary clarification, physical-chemical systems,
and sidestream returns. Although not considered to
be normally significant sources of odors, activated
sludge and trickling filter systems are also discussed,
since improper design of these processes can result
in odor generation.
Primary clarifiers are potential odor sources due to
the relatively long liquid residence times, particularly
when actual flows are considerably less than design
flows, which can lead to septic conditions. The
problem may be further compounded by the presence
of septic conditions in the raw wastewater. Unfortu-
nately, since clarifiers are normally sized to accom-
modate peak design flows, little can be done during
design of the clarifier to eliminate this problem.
Preaeration for a minimum 60-minute detention time
can be effective in preventing septicity in the primary
clarifier, and should be considered during design if
clarifier detention times will be excessive at less than
design flows, and if the raw wastewater contains no
sulfide. If sulfide is present, preaeration will release
sulfides and create severe odor problems. Sulfide
release from accumulated sludge and scum removal
was discussed in Section 6.2.1.
Design of covered primary clarifiers, with collection
and treatment of exhaust air, would only be consid-
ered during design if sulfides are known to be present
in the raw wastewater and if upstream treatment
with air, oxygen or chemicals is not practical. It is
recommended that generation of odors in primary
clarifiers resulting from excessive detention times at
low actual to design flows be controlled by temporary
solutions, such as prechlorination or addition of
hydrogen peroxide.
Odors from activated sludge basins generally result
from organic overload conditions (inadequate DO) or
722
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from poor mixing characteristics, For diffused air
systems, air requirements to ensure good mixing are
20-30 m3/min/1,000 m3 Power requirements to
ensure complete mixing with mechanical aerators
are 1 3 to 26 kW/1,000 m3 (0.5 to 1 0 hP/1,000 ft3)
(2), These will vary with actual basin and aerator
characteristics.
Trickling filters may produce odors due to organic
overload conditions, inadequate wetting rates, pond-
ing conditions, or improper ventilation. Design organic
loading rates, which are dependent on filter type (low,
intermediate, or high rate) and media type (rock or
synthetic), are 0.08 to 4.8 kg BOD/mVd (5 to 300 lb
BOD/1,000 ft3). For plastic media trickling filters,
minimum wetting rates are 5.7 to 28.5 I/min/m2
(0.14 to 0.7 gpm/ft2), depending on the particular
media employed (5). Ventilation is important in
trickling filter design to prevent odor generation.
Normally ventilation occurs naturally due to the
temperature differential of the wastewater and the
ambient air. If air temperature is higher than the
wastewater temperature, the flow is downward, and
vice versa. General design recommendations for
ventilation of trickling filters are as follows (2)(3)(5):
1. Underdrains and collection channels should be
designed to flow no more than half full at peak
design flows.
2. Underdrains should have a minimum slope of 1
percent, and effluent channels should be de-
signed for velocities of 0.6 m/s (2.0 ft/sec) at
average design flow.
3. Ventilating manholes with open-grate covers
should be installed at both ends of the central
collection channel.
4. Large diameter filters should be designed to
provide branch collection channels with ventila-
ting manholes or vent stacks at the filter
periphery.
5. Open .area of the slots in the top of underdrain
blocks should be a minimum of 15 percent of the
filter area.
6. One square meter of gross area of open grating
in ventilating manholes or vent stacks should be
provided for each 250 square meters of filter
area.
7. For plastic media trickling filters manufacturers
often recommend 0.1 m2 (1 ft2) of ventilating
area for each 3 to 5 m (10 to 15 ft) of filter
periphery.
8. Forced-air ventilation should be employed for
extremely deep or heavily loaded trickling filters.
Physical-chemical treatment systems are likely can-
didates for odor generation, since lack of aeration
results in a low ORP, and use of chemicals such as
lime raises the pH and can result in generation of
ammonia odors. Conversely, chemicals such as ferric
chloride can reduce the pH, which favors release of
H2S. Since turbulence will accelerate the release of
odorous gases, turbulence should be minimized
where possible. Mixing tanks should be covered to
avoid escape of odors. Depending on the chemical
treatment processes employed, chemical addition
might be used for odor control. Unfortunately, lack of
data from the limited number of operating physical-
chemical systems makes firm design recommenda-
tions difficult. It should be noted, however, that
common design practice has been to enclose the
major elements of the physical-chemical treatment
plant, which has resulted in severe corrosion prob-
lems in some cases.
Sidestream returns from sludge handling operations
such as thickening, digestion, and thermal condition-
ing are often significant sources of odors. Odor
release from anaerobic sidestreams usually results
from turbulence created when the sidestream enters
the main plant flow. For this reason, sidestreams
should always be returned below the surface of the
liquid. Adverse effects on primary clarification, such
as septicity and odors, can be avoided by returning the
sidestream to the biological process. Mixing of the
sidestream with waste activated sludge prior to
return to upstream processes has been used to
promote adsorption of odors by the activated sludge
particles. Chlorination of sidestreams at dosages of
100 to 300 mg/l has also been employed for odor
control, as has aeration in an enclosed conduit (4)(6).
Separate treatment facilities have been designed for
high strength sidestreams such as those from thermal
conditioning processes, although such systems are
generally used primarily to reduce BOD and SS
loadings to upstream processes.
6,4,3 Sludge Stream Processes
Sludge handling processes with high potential for
odor generation include gravity thickening, sludge
blending and storage, thermal conditioning, chemical
stabilization, mechanical dewatering, composting,
and septage receiving and holding.
Gravity thickeners are common sources of odors
because primary or primary/waste activated sludges
generally have low ORP values, and may become
septic during detention in the thickener. Odor prob-
lems result from feeding of thick, aging sludges to the
thickener, without providing sufficient aerobic dilu-
tion water to "freshen" the sludge. Increasing rates
of sludge withdrawal can sometimes avoid septicity,
although intermittent odor problems are still likely. It
is, therefore, recommended that gravity thickeners be
covered to contain odors. Covers may be of the flat,
low-profile type, or self-supporting aluminum or
fiberglass domes. Since flat covers do not allow ready
access for observation, equipment maintenance, or
cleaning, dome-type covers are usually recommended
for gravity thickener applications. To ensure person-
123
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nel safety, the enclosed space must be adequately
ventilated. Ten to twenty air changes per hour are
typical for such spaces.
For gravity thickeners, designs with covers, and
forced-air ventilation, the exhaust air must undergo
suitable treatment prior to discharge to the atmos-
phere (see Chapter 4), An exception to the recom-
mendation of covering gravity thickeners is when the
incoming sludge solids are fresh and aerobic, as
might occur from extended aeration plants (> 20 day
SRT) with no primary clarification (7).
Sludge blending and storage tanks also may be sites
of significant odor generation, particularly when raw
primary sludge is being blended or stored with other
plant sludges. For sludges that have undergone
aerobic digestion or that are derived from extended
aeration processes, odor generation is likely to occur
only if the sludge is stored for sufficiently long periods
(> 2 hours) to allow development of septic conditions.
Sludge blending and storage tanks should normally
be covered to contain odors. This is typically accom-
plished with a sealed, flat cover. Manways are
provided for access to equipment. The most positive
means of odor control from blending or storage tanks
is direct combustion of the vapors at approximately
760°C (1,400°F). Because gas production is low and
the enclosed air space is small with a flat cover,
energy requirements are moderate and electrical
heat can be used in combustion for simplicity [7). For
flat, tight-fitting covers, ventilation rates need only be
sufficient to maintain a slight, negative pressure
under the cover. Recommended ventilation rates for
such applications are four to six air changes per hour
(B), Treatment of exhaust air is discussed in detail in
Chapter 4.
Thermal sludge-conditioning systems are potentially
major sources of objectionable and complex odors.
The odorous gases produced are low molecular
weight volatile substances that include aldehydes,
ketones, organic acids and various sulfurous com-
pounds. The odor levels produced depend on the total
hydrocarbon content of the individual odor source. All
sludge processing operations downstream of the
reactor will produce odor because of the low vapor
pressure of the volatile odorous compounds and the
relatively high sludge temperatures, The most com-
mon sources of odor are: 1) vapors from treated
sludge storage tanks; 2) decant tanks; 3) thickeners;
4) exhaust air from vacuum filter pumps; 5) exhaust
airfrom other loaded or enclosed dewatering devices;
and 6) vapors released from further transport and
treatment of decant liquors.
Of the precedi ng, the odor levels are most severe from
decant tanks or thickeners immediately following the
reactor. Odorous air from sludge decant tanks,
thickeners, separate strong liquor pretreatment sys-
tems, sludge loading or transfer hoppers, and vacuum
dewatering equipment should be collected and
treated before being released to the atmosphere,
Vacuum filter pump exhaust must also be collected
for treatment to remove odors.
The more commonly used alternatives to control
odors in the exhaust air from thermal sludge treat-
ment systems include wet scrubbing, combustion,
and activated carbon adsorption. Masking, dilution,
and evaporation control also have been used, al-
though their effectiveness is limited. For the air
streams with high hydrocarbon content, such as
those produced by sludge thickeners and decant
tanks, the most effective odor control system is
incineration or wet scrubbing followed by incinera-
tion. These systems will reduce odor levels to a range
of 880 to 3,530 odor units/m3 (25 to 100 odor
units/scf). Wet scrubbers can be employed using
plant effluent as the scrubber liquid at rates of 2.7 to
4.0 l/min per m3/min of air flow(20 to 30 gpm/1,000
scfm), Incineration can be either by direct flame at
815°C<1,500°F) or by catalytic combustion at 425°C
(800°F).
Wet scrubbing plus activated carbon adsorption can
also be used for odor removal from the high hydro-
carbon content gas streams. Scrubber liquids com-
monly are solutions of potassium permanganate,
sodium hydroxide, or sodium hypochlorite. Activated
carbon adsorption normally uses a mutiple bed unit
sized to minimize regeneration requirements. Re-
generation of the carbon for such applications is
typically accomplished by steam stripping. For a 28-
mVmin (1,000 cfm) air flow, a typical carbon system
would use dual beds, each containing 820 kg (1,800
lb) of carbon and an adsorption cycle of 24 hours.
After 24 hours, the second bed would be placed into
operation, and the first bed would be regenerated for
1 hour using steam (9). The actual adsorption cycle is
a function of hydrocarbon content and should be
determined under actual plant conditions.
The third alternative for treating the high hydrocarbon
gas streams is use of multiple wet scrubbers. One of
the most effective multiple scrubber options is to use
three stages, the first employing plant effluent as the
scrubber liquid, the second using a 5-percent sodium
hydroxide solution, and the third using a 3-percent
potassium permanganate solution.
Other techniques for odor control from thermal sludge
treatment systems include the use of a nitrifying
trickling filter as a scrubber, discharge of the gases to
aeration basins, and discharge of gases to aerated
lagoons (9).
The most effective method of odor control from
thermal sludge conditioning processes depends on
the specific chemical composition of the odorous gas,
odor strength, volume of air to be treated and odor
124
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reduction required. The degree of odor control
depends on site characteristics, including topography,
climate, prevailing wind direction and proximity to
populated areas.
Based on a survey of 28 operating facilities, the most
effective methods of odor control were found to be
high temperature incineration, activated carbon
adsorption, and wet chemical scrubbing.
The costs for treating odorous gases from thermal
conditioning processes are 5 to 10 percent of the total
cost of thermal treatment. For systems treating 3 to
30 m3/min (106 to 1,060 cfm) of odorous air, wet
chemical scrubbing was found to be the most cost-
effective method, followed by incineration and carbon
adsorption. Detailed costs are presented in Chapter 4
(Si-
Chemical stabilization processes, particularly lime
stabilization, are potential sources of odors. In lime
stabilization systems, pH of the sludge is elevated to
12.0 for 2 hours in order to prevent biological
decomposition and generation of noxious odors.
However, elevation to pH values above 9.5 favors
release of ammonia to the atmosphere. In addition,
turbulence caused by mixing of lime and sludge
accelerates the rate of ammonia release. For this
reason, such mixing tanks should be covered to
minimize the escape of odors.
Sludge composting processes are likely sources of
odors, primarily due to the requirements for handling
and transfer of materials. These processes involve
mixing of raw sludge with bulking agents, transfer of
these materials to piles or windrows, periodic turning
of windrows, and removal and storage of compost.
Because non-mechanical sludgecompostlng systems
are land-intensive, relatively remote sites are often
selected with adequate buffer zones to allow dis-
persion of any odors that might be generated. Static
pile composting systems employ small blowers to
"pull" air through the pile for aeration; the exhaust air
is passed through a small pile of finished compost for
adsorption and removal of odors. Finished compost
has been found to be effective for this purpose.
Composting systems can be enclosed in buildings,
and odors controlled through proper ventilation and
treatment of the exhaust air. In most cases, however,
operations are conducted outdoors, making site
selection the key design criterion for odor control.
Septage is a putrescible organic material with a
highly objectionable and persistent odor. Special
consideration must be given to control odors from
septage receiving stations and holding tanks at
wastewater treatment facilities. Receiving stations
must be designed to minimize spills and turbulence
during discharge of septage from hauling vehicles. A
hose from the holding tank equipped with a quick-
disconnect, watertight fitting should be provided for
direct transfer of vehicle contents to the holding tank
or to pretreatment facilities, such as bar screens.
To allow for discharge of septage from vehicles not
equipped with compatible hoses or fittings, a hopper
should be provided which drains to the holding tank
(10). Such a design is shown in Figure 6-1. A
pressurized water supply must be provided for
flushing any spills and for cleaning the facilities.
Receiving tanks should be totally enclosed to prevent
escape of odors.
Although aeration times required for stabilization and
odor control are excessively long for application to
holding tanks, aeration can release odorous gases so
as to minimize odor release in downstream processes.
Forced-air ventilation should be provided to exhaust
the odorous gases, which must be treated before
discharge to the atmosphere. In Europe, the most
common technique is to use wet scrubbers, with
sodium hypochlorite as the scrubbing liquid. Use of
soil or compost filters, discharge of the gas to
activated sludge basins, and combustion have also
proven successful. However, use of activated carbon
for odor removal has not been very successful, due to
incomplete odor removal and operational expenses of
carbon replacement (10). Ozone has also been used
for treatment of odors from exhaust gases emanating
from septage-receiving tanks (11).
Figure 6-1,
Typical design of a septage receiving station.
Manholes
Holding Tank
Q
Drain to
Holding
Tank
Flushing
Water
Supply
* Concrete
Apron
Hose with Quick-
Disconnect Fitting
Plan
fel"
Vt
=FF
i i
i i
H
Holding Tank
Elevation
I VA.-'v.'
k
"Reproduced with permission ol Butterworth Publishers, Inc.
125
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6.5 General Design Considerations for
Avoiding Corrosion
Because of the characteristics of wastewater and the
unit processes employed in wastewater treatment
plants, corrosion is always a potential problem. Repair
or replacement of components due to corrosion can
be costly, and it is therefore important to prevent or
minimize potential for corrosion during design.
Additional costs incurred for specification of materials
with high degrees of corrosion resistance and for
designs that minimize corrosion are generally insig-
nificant compared to the costs of repair and replace-
ment of corroded components over the typical 20-
year design lifetime of the plant. The following
discussion presents general design considerations
for avoiding corrosion in wastewater treatment
facilities.
6.5.1 A voiding Moisture Retention and Ponding
Design of channels, angles, and structural beams
should be such that possible catchment areas for
liquids and moisture are avoided. In the use of
structural steel, areas that are difficult to clean and
maintain, such as back-to-back angles or structural
sections with inaccessible areas, should be avoided,
as should flat or dished sections that can collect or
retain moisture. Corners should be rounded where
passible to prevent accumulation of dirt and moisture
which may act as an electrolyte to induce corrosion. If
it is not possible to avoid catchment areas for
accumulation of liquid, drainage holes should be
provided. These must besized and sited carefully.and
maintained free from blockages (12).
For purposes of minimizing moisture and dirt accum-
ulation, butt-welded joints are preferable to lap-
welded joints. If lap-welded joints must be used,
exposed edges should be treated in such a way as to
prevent retention of moisture and dirt in the crevices
(12).
Steel storage containers and tanks should be sup-
ported on legs to allow free circulation of air over the
tank surface to prevent condensation. Insulation can
also be used to prevent condensation. Condensed
moisture can be retained on sheltered horizontal
surfaces, such as building eaves or undersides of
tanks. Where possible, breathing holes should be
provided to allow circulation of air and evaporation of
the moisture. Control of condensation is further
discussed in Section 6.5.3,
6.5.2 Avoiding Contact of Dissimilar Metals
Bimetallic or galvanic corrosion, described in Chapter
4, occurs when two or more dissimilar metals are
immersed in, or are conveying, an electrolyte; ex-
amples are: use of bronze or brass valves with iron
pipe, use of steel rivets to fasten aluminum sheets (or
vice versa), and use of steel and brass or copper pipe
in the same system. Contact of dissimilar metals
should always be avoided, where, for example, steel
rivets are used for fastening aluminum sheets,
corrosion of the aluminum sheets can result in
loosening of the rivets, slipping of the sheets, and
potential structural damage. Such corrosion can be
prevented by using an insulating, non-hardening
compound in the areas where the two metals come in
contact (12).
6.5,3 Ventilation and Heating for Condensation
Control
Ventilation and heating of enclosed spaces effectively
controls condensation and corrosion. Such control is
particularly important where enclosed areas are
exposed to open water surfaces such as in covered
grit chambers and screens, wet wells of pump
stations, covered sludge thickeners, sludge handling
buildings, other equipment areas, and pipe galleries.
As an example, at Yellow Springs, Ohio, freezing
problems occurred in thegrit removal mechanismsin
an uncovered, aerated grit chamber. The units were
covered in an attempt to control ice formation.
Although freezing problems were controlled, failure
to provide adequate ventilation resulted in accumula-
tion of condensates on mechanical components,
which led to rapid and severe corrosion of metallic
parts.
Enclosed spaces should be ventilated by forcing fresh
air into the enclosure in order to displace air
containing high levels of moisture. This has proved
successful at many locations, including Detroit,
Michigan; Massilon, Ohio; Winona, Minnesota;
Circleville, Ohio; and Pontiac, Michigan (12), Areas
that were ventilated in these cases included wet
wells, chambers and pump rooms, pipe tunnels,
screen and comminutor rooms, and the space be-
tween the roof and bottom plate of a floating cover
digester (12), "Ten State Standards" recommends
ventilation of wet wells at a rate of 12 air changes per
hour, and ventilation of dry wells at 6 air changes per
hour, if continuous. If intermittent, the recommended
rate for both wet and dry wells is a minimum of 30 air
changes per hour (3). Separate ventilation should be
employed for wet and dry wells. If intermittent,
ventilation equipment should be interlocked with the
lighting system. Effective ventilation is accomplished
by forcing fresh air into the bottom of the structure
and exhausting it through the roof (13).
The design of heating, ventilating, and air conditioning
systems must include consideration of both minimum
heating and ventilation requirements and humidity
control. Humidity control is especially important for
enclosed areas that house open tanks and for
subsurface structures. Unnecessary heating of the
air should be avoided, since warm air promotes
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condensation on tanks, pipes and basins that contain
cooler wastewater. The basic heat requirement is for
freeze protection, with recommended temperatures
of 4°C(40°F). Minimum ventilation requirements are
specified by Federal, state, and local building codes
for protection of health and safety. Ventilation rates
range from 2 to 30 air changes per hour, depending
on the occupancy classification, type of equipment or
process that is housed, and the potential for genera-
tion of objectionable or hazardous air.
6.6 Paint and Coatings
The adequate protection of steel and concrete in
wastewater treatment plants is important for mini-
mizing maintenance and/or replacement of corroded
components. Asa result, it is critical that construction
specifications contain provisions that will assure
proper techniques are used for surface preparations,
and that the appropriate primers, paints and coatings
are specified for the type of environmental conditions
to which these materials are subjected. Further, the
paints and coatings must be properly applied to
ensure a long life with minimal maintenance.
6.6.1 Types of Coatings
Corrosion of steel and concrete can be abated or
prevented by coating them with materials which have
greater resistance to corrosion. Two generic coatings
are used: physical barrier coatings, which provide a
barrier between the material to be protected and the
environment, and sacrificial coatings such as zinc
and cadmium, which corrode preferentially and save
the primary base metal from attack. Coatings are
further subdivided into metallic coatings, non-metallic
organic coatings, and chemical conversion coatings.
6.6.1.1 Metallic Coatings
Zinc and cadmium coatings have a higher electro-
motive force than steel, and can be used to cathod-
ically or galvanically protect iron and steel. Here, the
coatings are corroded preferentially, preventing attack
of the primary metal.
Application of zinc coatings (galvanizing) is normally
accomplished by dipping the component in a molten
zinc bath. It has been found that the effective service
life of a zinccoating varies directly with the thickness
of the coating. Service life of a galvanized coating also
varies with the severity of the exposure (12).
Nickel coatings do not provide sacrificial protection.
Rather, they must provide an impervious, non-porous
physical barrier to prevent attack of the primary
metal. Electroplated nickel coatings vary in thickness
from 0.5 to 10 mils depending on the exposure. Such
coatings are typically applied over a very thin layer of
copper to improve adhesion. Nickel can also be
applied by electroless plating and by cladding. Other
electroplates, such as chromium and silver, are also
useful for some corrosive environments.
Metallic coatings such as aluminum, tin, lead, monel,
and stainless steel are often used for corrosion
protection. Hot-dipped aluminum coatings have been
found to be useful in high temperature, corrosive
environments, since they have a high resistance to
corrosive condensates which form when the heated
component cools down.
Inorganic zinc coatings have been developed which
consist of metallic zinc particles in a vehicle such as
sodium silicate, A curing agent or hardener is
employed to complete the chemical reaction during
formation of the coating. Such coatings bond tightly
to the base metal surface, and protection is afforded
by the preferential corrosion of the zinc and by the
production of stable, insoluble corrosion products
such as hydroxides, oxides, and carbonates (12).
6.6.1.2 Non-Metallic Organic Coatings
Organic coatings provide a protective barrier between
the surface to be protected and the environment.
Organic vehicles such as thinners, drying oils, and
resins are used in such formulations. Synthetic
resins are commonly used to enhance the ability of
the coating to resist acids and alkalies (12). Vinyl
resins provide impervious surfaces that resist pene-
tration by water. Epoxy resins are becoming quite
popular, as they show good chemical resistance and
excellent surface adhesion. Silicone resins are used
for high temperature service.
6.6.1.3 Chemical Conversion Coatings
Chemical conversion coatings are so named because
of chemical reactions that occur between the coating
and the base metal; the coating becomes an integral
part of the original surface. Two common chemical
conversion coatings are phosphate coatings and
controlled oxidation coatings.
Phosphate coatings are produced by the chemical
reaction of the primary metal with a phosphoric acid
solution containing zinc, iron, or manganese along
with iron phosphates. A crystalline, non-metallic
layer is formed on the surface of the metal. Such
coatings have found greatest use as a base to provide
better adhesion of paints.
Controlled oxidation coatings are formed by exposing
metal components to hot oxidizing liquids or gases.
This results in theformation of athin (0.02 to0.2 mil)
black oxide coating that can provide protection against
corrosion or serve as a base for painting.
6.6.2 Surface Preparation
6.6.2.1 Steel Surfaces
Proper preparation of steel surfaces is important for
assuring good adhesion between the coating and the
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surface. Sand or shot blasting are methods commonly
used for thorough cleaning of the steel, both in
fabrication shops and in the field. Other cleaning
methods, such as solvent cleaning and hand tool
cleaning, are also discussed.
The Steel Structure Painting Council (SSPC) has
issued the fol lowing specifications for various degrees
of sand or shot-blasted preparation of metal surfaces
<12X14):
1. SSPC-SP5, white metal—Blast cleaning for
complete removal of all visible rust, mill scale,
paint, corrosion products, and foreign materials.
2. SSPC-SP10, near white metal—Blast cleaning
for 95 percent removal of visible residues;
minimum required for immersion service.
3. SSPC-SP6, commercial—Blast cleaning for 67
percent removal of visible residues.
4. SSPC-SP7, brush off—Blast cleaning for re-
moval of all but tightly adhering residues of rust,
mill scale, and coatings.
Other surface preparation specifications are:
1. SSPC-SP1, solvent cleaning—Removal of all oil,
grease, dirt, salts and contaminants by cleaning
with solvents, vapors, alkali emulsions, or
steam.
2. SSPC-SP2, hand tool cleaning—Removal of
loose rust, mill scale, and paint by hand chipping,
scraping, sanding, or wire brushing.
3. SSPC-SP3, power tool cleaning—Removal of
loose rust, mill scale, and paint by power tool
chipping, sanding, wire brushing, or grinding.
4. SSPC-SP8, pickling—Complete removal of rust
and mill scale by acid pickling. Iron phosphate
coating is produced which improves paint
adhesion.
Table6-3 summarizes the recommended preparation
technique for steel surfaces depending on the
conditions of exposure (14). For the most severe
exposures, blasting to near white or white metal
provides the best conditions for good adhesion of the
paints or coatings, while less severe exposures call
for less extensive surface preparation alternatives.
6.6.2.2 Concrete Surfaces
Concrete surfaces must be prepared for paints and
coatings by thoroughly removing all grease and oils,
dirt, scale, and loose and foreign materials to provide
good adhesion of the coating with the concrete.
Vinyls and chlorinated rubber coatings require good
surface preparation since they have relatively weak
bonding properties. One of the best preparations for
concrete floors is acid etching by swabbing with a
solution of muriatic acid followed by thorough rinsing.
For concrete walls, a zinc sulfate solution is often
used. The zinc sulfate combines with calcium hydrox-
ide to form zinc hydrate [Zn(0H)2] and calcium sulfate
Table 6-3. Surface Preparations and Coatings for Various
Environmental Exposures (14)
Surface Minimum
Preparation for Coaling Film Recommended
Exposure Steel Thickness Coating
mils
7 Epoxies
Vinyls
Coal-tar epoxies
5 Epoxies
Vinyls
Chlorinated
rubber
Coal-tar epoxies
5 Steel: Alkyds
Epoxies
Concrete:
Chlorinated
rubber
High-build
epoxy
3 5 Steel: Alkyds
Concrete:
Chlorinated
rubber
Epoxy
'Steal Structure Painting Council Specifications
(CaSO*), both of which are paint pigments. A 2-
percent zinc chloride, 3-percent phosphoric acid
solution is also effective for preparing concrete
surfaces. For concrete surfaces that are greasy or
oily, contaminants may be removed with trisodium
phosphate, solvents, or caustic lye (12).
6.6.2.3 Galvanized Iron Surfaces
Galvanized iron surfaces can be further protected by
application of paints. If painting is desired, the surface
must be adequately prepared to permit bonding of the
paint. Two methods are commonly used:
1. Weathering—Weathering is a natural phenom-
enon which produces a roughened surface due
to the oxidation of zinc to zinc oxide. This
changes the surface from a shiny finish to a dull
gray surface to which the paint will bond.
2. Application of primers—When it is not desirable
or practical to wait for natural weathering to
occur, primers can be used to improve the
adhesion of paints. These include a vinyl wash
coat (phosphoric acid solution) followed by a
zinc dust primer, acetic acid, and zinc dust—zinc
oxide primers.
6.6.3 Selection of Primers, Coatings and Paints
6.6.3.1 Primers for Steel
Primers can be divided into two types: inhibitive
primers, such as zinc chromate and red lead; and
Submerged Minimum of near-
white metal blast
SSPC-SP10*
Moist Commercial blast
Atmosphere SSPC-SP6
Outside Commercial blast
Weather SSPC-SP6
Inside, Dry Hand tool cleaning
Atmosphere SSPC-SP2
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barrier primers, which protect the surface by providing
a mechanical barrier to corrosive agents.
Red lead primers are alkaline, and can neutralize
acidic agents that may penetrate the film. Red lead
pigments react with oils to form dense, tough films
with low permeability.
Zinc chromate is an effective inhibiting agent due to
the slow release of chromate ions. Zinc chromate
primers are not recommended for acidic environ-
ments or for immersion service.
Inorganic zinc primers are very effective due to their
ability to provide sacrificial protection, as well as to
develop a resistant coating with time. These are
excellent primers for immersion service. In addition,
special inorganic zinc preconstruction primers are
available which need not be removed before welding.
Such primers offer excellent protection for steel
between arrival at the job site and application of
topcoats (15).
Wash coat primers are actually pretreatments, as
their purpose is to improve adhesion of subsequent
paints or coatings,
6.6,3.2 Paints and Coatings
a. Coal Tar Epoxy
Coal-tar epoxy coatings are often specified for
submerged surfaces such as clarifiers, digesters, and
process tanks. They do not crack when exposed to
sunlight, are not softened by oils and fats, and
demonstrate good adhesive properties and abrasion
resistance. Coal-tar epoxy can be applied to both steel
and concrete surfaces.
Coal-tar epoxies employ a two-component, cold
curing system using either amine or polyamide curing
agents. Many specifications for field applied coal-tar
epoxy call for two coats applied over near-white
blasted steel, or two coats over concrete, the first
applied at reduced viscosity to allow penetration and
improved bonding.
A disadvantage of coal-tar epoxy coatings is that, in
order to assure good bonding, the top coat must be
applied soon after the previous coat, generally within
24 to 72 hours, depending on temperature and
formulation (14). Table 6-3 shows some recommend-
ed applications and film thicknesses for coal tar
epoxies.
b. Epoxies
Epoxies have a multitude of applications as coatings
in wastewater treatment plants. They are extremely
effective for submerged service applications on steel
or concrete. Epoxies are durable, adhesive, and
provide excellent resistance to acids, alkalies, sol-
vents, abrasion, and impacts. They arethermosetting,
and can be cured by heat or by internal polymerization
using organic amines as curing agents. Polyamide-
cured epoxies do not provide the solvent and chemical
resistance of amine-cured epoxies, but have higher
solids contents, better adhesion, moisture tolerance,
and flexibility. Topcoats must generally be applied
within 72 hours. When exposed to sunlight, epoxy
coatings chalk and lose their gloss, although this does
not affect the integrity of the film (14). Epoxy coatings
have become quite popular, and seem to be replacing
coal tar epoxies for wastewater treatment plant
applications. Table 6-3 shows some recommended
applications and film thicknesses for epoxy coatings.
c. Vinyl Coatings
Vinyl-resin coatings can be used for submerged
service and moist atmosphere exposures of concrete
and steel. Because vinyl-resin coatings are high in
viscosity and low in solids content, films are thin,
requiring three to four or more coats depending on
the application. Vinyl coatings have very low perme-
abilities, and are resistant to oils and fats, alkalies,
and many chemicals, although they can be attacked
by acetic and other organic acids. Although they are
excellent for submerged service applications, the thin
film thickness requiring multiple coats has generally
favored use of high-build epoxy coatings. For corrosive
environments exposed to sunlight, vinyl coatings are
often preferred since they do not chalk and fade
rapidly. Applications and some recommended film
thicknesses for vinyl coatings are summarized in
Table 6-3.
d. Aikyd Resin Coatings
Alkyd resins form hard durable films with good
resistance to dulling and fading in outdoor exposures.
They are not suitable for submerged or moist atmos-
phere exposures or for coating of concrete. A common
application of alkyd resins (see Table 6-3) is for
protection against industrial exposures of interior and
exterior metal surfaces.
e. Phenolic Resin Coatings
Phenolic resins, made from phenol and formaldehyde,
are often used as primers. Although superior to
alkyds for chemical resistance.they are inferior to
vinyls, epoxies or chlorinated rubber coatings under
severe conditions. Certain types of phenolic resins
are suitable for submerged exposures. Because they
form hard, insoluble films, adhesion of topcoats may
be a problem (14).
f. Chlorinated Rubber Coatings
Chlorinated rubber compounds are easily applied,
and can be used for both steel and concrete. Although
chlorinated rubber has good resistance to HZS gas
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and moisture, it is not suitable for submerged
exposures because of poor resistance to oil, grease
and solvents.
g. Emulsion Coatings
Most emulsion-type coatings are made from acrylics,
polyvinyl acetate, butadiene styrane, or combinations
of the above. They are commonly used on concrete
and concrete block walls. They are easily applied, do
not have strong solvent odors, are not a fire hazard,
and have good gloss and color retention. Cleanup is
easily accomplished with water. They must be applied
at temperatures in excess of 10°C (50°F).
6.7 Selection of Materials
A large number of materials, both metallic and non-
metallic, are used in the construction of wastewater
treatment plants. It is important during design to be
cognizant of the potential corrosion problems that
may occur, and to carefully select materials that have
a high degree of corrosion resistance and require
little maintenance. Greater capital investments for
such materials are justified by the savings in mainte-
nance and replacement costs over the lifetime of the
plant. Materials typically used in wastewater treat-
ment plants are discussed below relative to their
durability and degrees of corrosion resistance.
6.7.1 Cast and Ductile Iron
Cast and ductile iron corrodes at about the same rate
of steel. However, because of their greater thickness
and the formation of a dense, tenacious oxide which
retards further corrosion, they hold up well in some
corrosive environments. Gray cast iron may be subject
to graphitization when immersed in salt water or
moist, sulfate-bearing soils. This involves dissolution
of ferrite in the cast iron, leaving the graphite intact,
but reducing the density and structural strength of
the component. White cast iron is not subject to
graphitization,
6.7.2 Low Alloy Steels
The composition of low alloy steels has no appreci-
able impact on corrosion resistance in submerged or
buried conditions. However, for atmospheric expo-
sures, addition of chromium, copper, or nickel in
small amounts (0.1 to 1 percent) results in formation
of a dense, adherent, protective film which, upon
oxidation, reduces the rate of corrosion. The type of
atmospheric exposure will affect the rate of corrosion.
6.7.3 Copper and Copper Alloys
Copper and copper alloys have low positions in the
electromotive series, and as such demonstrate good
resistance to corrosion. Copper exposed to the
atmosphere develops a thin, green protective coating
that is largely copper sulfate. Copper offers good
resistance to dilute, non-oxidizing acids and salt
solutions. However, copper and its alloys can be
readily attacked by HaS, which turns the surface
black. Copper is also sensitive to corrosion by high
velocity waters containing high DO levels. Copper is
not resistant to oxidizing acids (nitric, hot sulfuric
acid), ammonium hydroxide (plus oxygen), and oxidiz-
ing heavy metal salts [FeCU, Fe^SO^a].
Copper alloys such as brasses and bronzes, generally
offer good resistance to corrosion. Brass is a copper
alloy containing 5 to 45 percent zinc; white bronze
contains either tin (up to 12 percent), aluminum (up to
10 percent) or silicon (up to 4 percent). The major
corrosion process in brass is dezincification, or loss of
zinc from the alloy, resulting in increased porosity and
loss of structural strength. Tin or arsenic may be
added to inhibit dezincification.
Bronzes are not subject to corrosion processes which
remove one element such as in dezincification, and
are generally stronger and harder than brasses.
Aluminum bronze is the most resistant of any bronze
to attack by HZS and acids. Silicon bronzes are also
resistant to corrosive compounds, particularly hydro-
chloric and sulfuric acids, alkalies, and some organic
compounds.
6.7.4 Stainless Steel
Stainless steels are metal alloys containing chromium
(> 11.5 percent) and, for some types, nickel (6 to 22
percent). Stainless steels demonstrate excellent
corrosion resistance, and have been used in waste-
water treatment plants for many applications, includ-
ing flow control gates, aeration piping, handrails, and
gratings. There are three basic classes of stainless
steels:
1. Martensitic—These alloys have chromium con-
tents of 11.5 to 17 percent, and carefully
controlled carbon content. They may be harden-
ed by heat treatment to yield a martensite
structure. Applications include steam turbine
blades, tools, and cutlery.
2. Ferretic—These are low carbon alloys contain-
ing 17 to 27 percent Cr. They can be hardened to
some degree by cold working. The crystal
structure is ferretic, and demonstrates superior
atmospheric corrosion resistance than marten-
sitic alloys. Applications include autombiie trim
and components exposed to nitric acid.
3. Austenitic—These are low carbon alloys con-
taining 16 to 26 percent chromium and 6 to 22
percent nickel, with austeniticcrystal structures.
Addition of nickel improves corrosion resistance,
making high nickel austenitic alloys superior to
other types of stainless steels.
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Stainless steels show good resistance to inorganic
and organic acids and alkalies. However, they are not
resistant to halidesfBr, CI, F), seawater, and oxidizing
chlorides. Some stainless steels are subject to pitting
and intergranular corrosion. However, such problems
can generally be overcome by proper alloy selection,
heat treatment, and exclusion of certain chemicals
(12).
6.7.5 Nickel and Nickel Alloys
Nickel and high nickel (> 50 percent) alloys are
excellent materials for corrosion resistance. Nickel
alloys are stronger and harder than copper or
aluminum alloys and offer superior resistance to
corrosion. The six main types of high nickel alloys are
described below (12);
Group I, Nickel: 93.5-99.5 percent nickel
• Excellent mechanical properties
• High strength, malleable
• Resists hydrogen chloride, caustic soda, oxidation
and scaling, and stress corrosion in atmospheric
exposures.
Group II, Nickel-Copper: 63-70 percent nickel, 29-30
percent copper
• "Monel" type alloys
• More resistant than nickel under reducing condi-
tions
• More resistant than copper under oxidizing condi-
tions
• Not resistant to strong solutions of nitric or
sulfurous acid, ferric chloride
• Often used as wire mesh for vacuum filter cloth
support, applications involving high velocity sea-
water (pump shafts, impellers, piping)
Group III, Nickel-Silicon: 85 percent nickel, 10 percent
silicon
¦ Tough, strong, extremely hard
¦ Excellent resistance to corrosion by hot or cold
non-oxidizing acids
• Sometimes used for pump and valve parts
Group IV, Nickel-Chromium-lron: 54-78.5 percent
nickel, 12-18 percent chromium, 6-28 percent iron
• Excellent corrosion resistance at high tempera-
tures
• Withstands repeated heating and cooling
• Tough, strong, hard
Group V, Nickel-Molybdenum-Iron: 55-62 percent
nickel, 17-32 percent molybdenum, 6-22 percent
iron
a Excellent resistance to hydrochloric acid
• Expensive
• Exceptional cases of corrosion resistance require-
ments only
Group VI, Nickel-Chromium-Molybdenum-lron: 51-
62 percent nickel, 15-22 percent chromium, 5-19
percent molybdenum, 6-8 percent iron
• High corrosion resistance to oxidizing acids
• High resistance to thermal shock
• Hard and difficult to work
• Used for pump and valve parts, nozzles, piping
exposed to oxidizing agents
6.7.6 Silicon Cast Iron
Commercial grades of silicon cast iron alloys contain
14.5 percent silicon. Addition of silicon improves
corrosion resistance to strong, non-oxidizing acids.
Such alloys have been used for pipes to convey waste
chemicals, centrifugal pumps, valves, chlorine ejec-
tors, spray nozzles, and agitators (12).
6.7.7 Aluminum
Aluminum is widely used in wastewater treatment
plants due to its light weight, strength, and resistance
to corrosion. Aluminum is not affected by H2S,
methane, carbon dioxide, or sulfur dioxide. Formation
of a stable oxide coating on the surface by atmospheric
exposure or by anodizing provides excellent resis-
tance to corrosion. It may be attacked by acids, salts,
or aggressive waters.
Typical applications in wastewater treatment plants
include gratings, deck plates, railings, doors, window
frames, and ladders(12)(16). Since aluminum is high
in the electromotive series, care must be exercised to
prevent contact with iron, steel or other metals that
can result in galvanic corrosion.
6.7.8 Plastics
Plastics include a broad range of synthetic materials,
and are divided into two major categories. Thermo-
plastics such as polyvinyl chloride, polyethylene, and
vinyl can be heated to a plastic state, molded, cooled,
then reheated and remolded. Thermosetting plastics
such as polyesters, epoxies, and phenolics once
formed cannot be reheated to a plastic state due to
chemical changes which occur from the application
of heat and pressure during forming.
Plastics demonstrate excellent resistance to a broad
spectrum of corrosive materials such as acids and
oxidizing chemicals, including ferric chloride, ferric
and ferrous sulfate, and chlorine. The main disadvan-
tage of plastics is their loss of strength at high
temperatures. Thermoplastics are not normally used
at temperatures above 65°C (150°F), while certain
thermosetting plastics can be used at temperatures of
up to 150CC (300°F), Plastics have high coefficients of
expansion, are lower in strength than metals, and are
relatively costly.
The range of applications for plastic in wastewater
treatment plants is continually expanding. Plastics
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are used for pump impellers and casings, structural
members, weirs, flumes, fans, fasteners, and labora-
tory equipment.
6.7.9 Elastomers
Elastomers include natural rubber, neoprene, butyl,
isoprene, and others. The primary use of elastomers
for wastewater applications is for sealants and
gaskets, Neoprene has good resistance to oils and
greases and oxidation, and is commonly usedfor such
applications.
6.7.10 Ceramics. Glass, and Vitrified Clay
These materials are virtually immune to corrosion
due to their inert, impervious surfaces. The major
disadvantage with these materials is their brittleness,
6.7.11 Concrete
Portland cement concrete is the most widely used
construction material in wastewater collection and
treatment systems, in general, concrete is economical
and provides excellent resistance to corrosion under
both atmospheric and submerged exposures.
6.8 References
When an NTIS number is cited in a reference, that
reference is available from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
(703)487-4650
1. Treatment Plant Odors: Control and Mitigation.
California Association of Sanitation Agencies,
Agency Managers Meeting, January 27, 1984.
2. Wastewater Engineering: Collection, Treatment,
and Disposal. Metcalf and Eddy, Inc., McGraw-
Hill, New York, 1972.
3. Recommended Standards for Sewage Works.
Great Lakes-Upper Mississippi River Board of
State Sanitary Engineers, Health Education
Service, Inc., Albany, NY, 1978
4. Odor Control for Wastewater Facilities. Manual
of Practice No, 22, Water Pollution Control
Federation, Washington, DC, 1979.
5. Wastewater Treatment Plant Design. Manual of
Practice No. 8,American Society of Civil Engi-
neers, New York, NY, and Water Pollution
Control Federation, Washington, DC, 1977,
6. Process Design Manual for Sludge Treatment
and Disposal. NTIS No. PB-260479, U.S. Envi-
ronmental Protection Agency, Center for Envi-
ronmental Research Information, Cincinnati,
OH, 1979.
7. Sludge Thickening. Manual of Practice No. FD-
1, Water Pollution Control Federation, Washing-
ton, DC, 1980.
8. Kennedy, W. Odor Scrubbing. In: Treatment
Plant Odors—Control and Mitigation. Agency
Managers Meeting, California Association of
Sanitation Agencies, Sacramento, CA, January
27, 1 984.
9. Ewing, L.J., H.H. Almgren, and R.L. Gulp. Effects
of Thermal Treatment of Sludge on Municipal
Wastewater Treatment Costs. EPA-600/2-78-
073, NTIS No. PB-285707, U.S. Environmental
Protection Agency, Municipal Environmental
Research Laboratory, Cincinnati, OH, 1978.
10. Handbook for Septage Treatment and Disposal.
U.S. Environmental Protection Agency, EPA
625/6-84-009. Municipal Environmental Re-
search Laboratory, Cincinnati, OH, 1984.
11. Churchill, P.W. Ozonation of Septic Odors at a
Pretreatment Facility. JWPCF, Deeds and Data,
49 (7), 1977.
12. Paint and Protective Coatings for Wastewater
Treatment Facilities. Manual of Practice No. 17,
Water Pollution Control Federation, Washing-
ton, DC, 1969,
13 ¦ Design of Waste water and S tormwater Pumping
Stations. Manual of Practice No. FD-4, Water
Pollution Control Federation, Washington, DC,
1981.
14. Vivona, M.A. and T.P. Delany. A Guide to
Protective Coatings in Water/Wastewater
Treatment Facilities. Water and Sewage Works,
Reference Number, R 92,R 98, 1980.
15. Lopata, J.R. and C, Leutwiler. Waste Plant
Corrosion Must be Treated with Care, Water and
Sewage Works 1 21 (5): 46-47, 1974.
1 6. Rigdon, J H. Materials Selection and Corrosion
in Wastewater Systems. Materials Protection 7
(8) 32 36, 1968.
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