United States
Environmental Protection
Agency
Office Of Water
(WH-547)
EPA 832-R-92-001
September 1992
&EPA
Detection, Control,
Anri f^niTPPtinn Of
/~\l Ivl vxwl I wVsllvsi I V-/I
Hydrogen Sulfide Corrosion
In Existing Wastewater Systems
Printed on Recycled Paper
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832R92001
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF WASTEWATER ENFORCEMENT AND COMPLIANCE
WASHINGTON, D.C. 20460
DETECTION, CONTROL, AND
CORRECTION OF HYDROGEN
SULFIDE CORROSION IN EXISTING
WASTEWATER SYSTEMS
September, 1992
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NOTICE
Mention of trade names or commercial products does not constitute an endorsement by
EPA. Omission of certain products from this document does not reflect a position of
EPA regarding product effectiveness or applicability.
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ACKNOWLEDGMENTS
This document was prepared by J.M. Smith & Associates, PSC, Consulting Engineers.
Administrative and technical support was provided by the EPA Office of Water, Office of
Wastewater Enforcement and Compliance
Authors:
Reviewers:
Robert P.G. Bowker, J.M. Smith & Associates
Gerald A. Audibert, J.M. Smith & Associates
Hemang J. Shah, J.M. Smith & Associates
Neil A. Webster, Webster Environmental Associates
Lam K. Lim, EPA Office of Water
Robert E. Lee, EPA Office of Water
John A. Redner, County Sanitation Districts of Los Angeles County
Perry L. Schafer, Brown and Caldwell Consulting Engineers
Mike Bealey, American Concrete Pipe Association
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TABLE OF CONTENTS
DETECTION, CONTROL, AND CORRECTION OF
HYDROGEN SULFIDE CORROSION
IN WASTEWATER SYSTEMS
ACKNOWLEDGEMENTS i
1. INTRODUCTION 1-1
1.1 Significance of Hydrogen Sulfide Corrosion ... 1-1
1.2 Purpose of Handbook 1-1
2. PROCESS OF HYDROGEN SULFIDE CORROSION 2-1
2.1 Basic Mechanism of Hydrogen Sulfide Corrosion 2-1
2.2 Factors Affecting Corrosion 2-4
2.3 Effects of Industrial Pretreatment 2-7
2.4 References 2-9
3. MAJOR HYDROGEN SULFIDE CORROSION TARGET AREAS ... 3-1
3.1 Gravity Sewers 3-1
3.2 Pump Stations and Force Mains 3-3
3.3 Treatment Facilities ..' 3-4
3.4 References 3-7
4. DETECTION AND MEASUREMENT OF HYDROGEN
SULFIDE CORROSION 4-1
4.1 Approach for Identifying Existing or
Potential Corrosion Problems 4-1
4.2 Identifying Potential Problem Areas 4-1
4.3 Preliminary Inspection 4-4
4.4 Measurement of Corrosion 4-14
4.5 Comparing Measured and Predicted Corrosion 4-19
4.6 References 4-25
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TABLE OF CONTENTS (cont)
Page
5. ALTERNATIVES FOR CONTROLLING HYDROGEN
SULFIDE CORROSION IN SEWERS 5-1
5.1 Prevention of Hydrogen Sulfide
Corrosion in Existing Sewer Systems 5-1
5.2 Sulfide Control Systems Used in the United States 5-13
5.3 Procedure to be Followed in Selecting
Corrosion Control Method(s) 5-21
5.4 References 5-24
6. ALTERNATIVES FOR CONTROLLING HYDROGEN SULFIDE CORROSION
AT PUMP STATIONS AND TREATMENT FACILITIES 6-1
6.1 Control of Hydrogen Sulfide 6-1
6.2 Protective Coatings and Linings .6-5
6.3 Replacement with Corrosion-Resistant Materials 6-13
6.4 Case Studies 6-17
6.5 References 6-23
7.0 REHABILITATION OF CORRODED SEWERS . . 7-1
7.1 Rehabilitation and Repair Techniques 7-1
7.2 Approach to Selecting a Rehabilitation Technique 7-22
7.3 References . 7-25
APPENDIX A: CASE STUDIES
A.1 Sewer Systems
A2 Wastewater Treatment Facilities
Hi
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LIST OF TABLES
Page
2-1 Factors Affecting Sulfide Generation and Corrosion in Sewers 2-5
2-2 Beneficial Impacts of Controlling Industrial Discharges
on Hydrogen Sulfide Corrosion 2-8
5-1 Summary of Hydrogen Sulfide Control Techniques 5-3
5-2 Hydrogen Sulfide and Corrosion Control Systems Used
by Selected Cities in the U.S 5-14
6-1 Installment Society of America Classification of
Reactive Environments Based on H2S Concentrations 6-3
6-2 Los Angeles County Sanitation Districts Coating
Study: Coating Test Performance 6-9
6-3 Summary of Corrective Actions to Combat Hydrogen
Sulfide Corrosion at Hooker's Point WWTP 6-19
6-4 Summary of Corrective Actions to Combat Hydrogen
Sulfide Corrosion at Hyperion WWTP 6-20
6-5 Summary of Corrective Actions to Combat Hydrogen
Sulfide Corrosion at Terminal Island WWTP 6-21
7-1 Summary of Applicable Pipe Rehabilitation Methods
for Pipe Damaged by Hydrogen Sulfide Corrosion 7-2
7-2 Advantages and Disadvantages of Sliplining 7-3
7-3 Advantages and Disadvantages of Cured-in-Place Lining 7-11
7-4 Advantages and Disadvantages of Internal Liners 7-15
7-5 Advantages and Disadvantages of Specialty Concretes 7-17
7-6 Advantages and Disadvantages of Coatings 7-21
7-7 Factors to Evaluate for Selection of Rehabilitation
Method 7-23
IV
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LIST OF FIGURES
2-1 Mechanism of Sulfide Generation and Corrosion in Sewers ...•-.: 2-3
4-1 Approach to Identifying Existing and Potential
Corrosion Problems 4-2
4-2 Sewer Map Showing Locations of Potential Corrosion Problems ....... 4-5
4-3 Corrosion Inspection Checklist 4-7
4-4 H2S Probe/Liquid Sensor Device 4-9
4-5 Quick Method of Inspecting Sewer Lines 4-11
4-6 Sewer Crown pH Probe 4-13
4-7 Typical Sonic Caliper Longitudinal Plot 4-17
4-8 Relationship of Dissolved Sulfide Equilibrium to pH 4-21
4-9 Hydraulic Elements of Circular Sewers Running Partly Full 4-22
5-2 Ventilation Problems of Filled Sections of Sewers 5-11
5-4 Map Showing Locations of H2O2 Dosing Stations 5-17
5-5 Dissolved Sulfide Levels (Using Caustic Slugging), Main
Interceptor - Cedar Rapids, Iowa 5-19
7-1 Examples of Basic Sliplining Techniques . 7-6
7-2 Example of Deformed Pipe Insertion Technique 7-9
7-3 Cured-In-Place Method 7-12
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1. INTRODUCTION
1.1 Significance of Hydrogen Sulfide Corrosion in Wastewater Systems
Hydrogen sulfide corrosion is a well known and documented problem in wastewater
collection and treatment systems throughout the world. The presence of hydrogen sulfide
can lead to rapid and extensive damage to concrete and metal sewer pipe and tanks,
mechanical equipment used in the transport and treatment of wastewater, and electrical
control and instrumentation systems. Unfortunately, such problems are rarely brought to
the attention of the municipality until a catastrophic failure occurs such as street collapses
or sewer blockages resulting from sewer pipe failure, or complete deterioration of structural
and mechanical components at wastewater treatment facilities. Wastewater systems
suffering from hydrogen sulfide corrosion often require costly, premature replacement or
rehabilitation of pipes, manholes, tanks, pump stations and other mechanical equipment
Electrical components, instrumentation systems, and ventilation units are particularly
vulnerable to attack by hydrogen sulfide.
1.2 Purpose of Handbook
This handbook provides a comprehensive guide to detecting and monitoring corrosion,
controlling corrosion rate, and rehabilitating pipes or structures damaged by corrosion. The
handbook is intended for use by municipalities as well as engineers faced with existing or
potential corrosion problems. Techniques are presented to allow determination of whether
a corrosion problem exists, and the severity of the problem.
Subsequent chapters of this handbook are organized as follows:
Chapter 2: Process of Hydrogen Sulfide Corrosion
Chapter 3: Major Hydrogen Sulfide Corrosion Target Areas
Chapter 4: Detection and Measurement of Hydrogen Sulfide Corrosion
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Chapter 5: Alternatives for Controlling Hydrogen Sulfide Corrosion in
Sewers
Chapter 6: Alternatives for Controlling Hydrogen Sulfide Corrosion at
Pump Stations and Treatment Facilities
Chapter 7: Rehabilitation of Corroded Sewers
Case studies are included as Appendix A.
1-2
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2.0 PROCESS OF HYDROGEN SULFIDE CORROSION
2.1 Basic Mechanism of Hydrogen Sulfide Corrosion
2.1.1 Sulfuric Acid Corrosion
Hydrogen sulfide corrosion occurs in sewers, tanks, and equipment conveying or treating
wastewater that contains dissolved sulfide. The biological activity of anaerobic bacteria
results in the formation of sulfide which is released to the surrounding atmosphere as
hydrogen sulfide gas. In the presence of moisture, the hydrogen sulfide gas is biologically
converted to sulfuric acid. The sulfuric acid attacks exposed concrete and unprotected
surfaces of iron, steel and copper, resulting in corrosion and deterioration of the exposed
vulnerable materials. The process of sulfide generation and sulfuric acid corrosion is as
follows(l):
• In aquatic environments lacking dissolved oxygen, strict anaerobic bacteria colonize
the slime layer that coats the submerged surfaces of pipes and tanks. These bacteria
reduce sulfate (SOf), one of the most common anions in water and wastewater, to
sulfide (S2-).
• The sulfide ion combines with hydrogen ions in the wastewater to form hydrogen
sulfide. Depending on pH, the hydrogen sulfide dissociates to dissolved hydrogen
sulfide gas (H2S), hydrosulfide ion (HS~), and sulfide ion (S2"). At neutral pH of 7,
the distribution is approximately 50 percent H2S and 50 percent HS*. At pH 6, the
distribution is approximately 90 percent dissolved hydrogen sulfide gas and 10
percent hydrosulfide ion.
• Dissolved hydrogen sulfide gas is the only form of dissolved sulfide which can be
released from the wastewater to the atmosphere. The H2S produces the "rotten egg"
odor characteristics of stagnating sewage. The release of H2S from solution is
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accelerated under turbulent conditions. Since equilibrium conditions are rarely
observed, it is often difficult to predict atmospheric H2S concentration based on the
quantity of sulfide in solution.
* The released H2S combines with moisture on the non-submerged surfaces of pipe,
tanks and other non-submerged surfaces and is biologically oxidized to sulfuric acid.
Progressively acid-tolerant species of aerobic bacteria will successively colonize the
surfaces as additional sulfuric acid is produced and the pH drops. While new
concrete has an alkaline surface pH, weathered concrete has a surface pH of about
6, and concrete which is subject to active sulfuric acid corrosion may have a surface
pH of 1 to 3.
• The hydrogen ions of the acid attack the calcium hydroxide in the hydrated Portland
cement of concrete sewer pipes, channels and tanks while the sulfate combines with
the calcium ions to form gypsum (CaSO4), a soft corrosion product In the early
stages of corrosion, the concrete swells, making it difficult to measure concrete loss
due to corrosion.
Figure 2-1 summarizes the corrosion process which occurs in concrete sewers and structures
(2).
2.1.2 Direct Hydrogen Sulfide Attack of Metals
Another significant mechanism of corrosion is direct attack of metals such as iron, steel,
and copper by hydrogen sulfide gas. Electrical and instrumentation systems are particularly
vulnerable to low levels of hydrogen sulfide gas. H2S readily attacks copper contacts to
form copper sulfide, a poor conductor of electricity. This can cause equipment failure or
poor reliability of control systems. In addition, hydrogen sulfide attack can result in
substantial damage to wastewater processing equipment, steel structures and components,
and iron and steel pipe.
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SOA= anaerobic ,__ 3 =
bacteria
Anaerobic Slime Layer
(typically 1 mm thick)
(A) Sulfate is biologically reduced to
sulfide in the anaerobic slime layer
on the submerged pipe wall.
Condensate;
Location of H S Oxidizimj
Bacteria
Anaerobic Slime Layer
(typically 1 mm thick)
(B) H2S formed in the wastewater is
released from solution as a gas and
enters the sewer atmosphere.
Corroded
Mofst Pipe Surface
H2S + °2 aerobic *-
bacteria
(C) H2S is oxidized to sulfuric acid by
aerobic. Thiobacillus bacteria living on
moist, non- submerged surfaces. Acid
attacks concrete, causing corrosion.
S0
Figure 2-1
Mechanism of Sulfide Generation and Corrosion in Sewers
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23, Factors Affecting Corrosion
Hie rate of hydrogen sulfide corrosion is affected by the characteristics of the wastewater,
the collection system, the type of unit processes used in the transport and treatment of
wastewater, and materials of construction. Many variables directly or indirectly affect
sulfide generation, H2S release, and sulfuric acid corrosion. The factors that affect corrosion
are indicated in Table 2-1. Odor and corrosion problems associated with the collection,
handling and treatment of domestic wastewater is primarily due to the result of the
reduction of sulfate to H2S under anaerobic conditions, as shown by the following equation
(1):
anaerobic
+ organic matter — - > S2- + H2O + CO2
bacteria
HS.
Biological activity within wastewater results in the formation of a slime; layer on the
submerged walls of pipes or tanks. The slime layer need only be a few cell diameters thick
to support bacterial colonization. That portion of the slime layer furthest from the liquid
becomes increasingly devoid of dissolved oxygen. Within a dissolved oxygen range of 0.1
to 0.5 mg/1, anaerobic bacteria utilize sulfate and organic matter present in the wastewater
at an approximate ratio of 2 to 1, respectively, to form sulfide. The thickness of the slime
layer is controlled by velocity, which in turn is governed by pipe diameter, flowrate, and
energy gradient The proliferation of sulfate-reducing bacteria is dependent on dissolved
oxygen concentration, temperature, humidity, sulfate content and availability of organic
matter. The formation and release of hydrogen sulfide gas to the atmosphere is dependent
primarily upon the pH of the wastewater and the degree of turbulence.
For sewer systems having low velocities, pump station wet wells, treatment tanks and other
quiescent wastewater environments, deposition of organic solids may occur, promoting
anaerobic conditions. Also, as the depth of flow in sewers increases, so does the surface
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TABLE 2-1
FACTORS AFFECTING SULFIDE GENERATION
AND CORROSION IN SEWERS(2)
FACTOR
Wastewater Characteristics
Dissolved oxygen
Biochemical oxygen demand
(organic strength)
Temperature
pH
Presence of sulfur compounds
Humidity of sewer atmosphere
Sewer System Characteristics
Slope and velocity
Turbulence
Intermittent surcharging
Force main discharges
Sewer pipe materials
Concrete alkalinity
Accumulated grit and debris
EFFECT
Low DO favors proliferation of anaerobic
bacteria and subsequent sulfide generation
High BOD encourages microbial growth and
DO depletion, and increases sulfide generation in
proportion to BOD concentration
High temperature increases microbial growth
rate and lowers DO solubility
Low pH favors shift to dissolved H2S gas
Sulfur compounds required for sulfide generation
Biological growth requires high humidity level
Affects degree of reaeration, solids deposition,
H2S release
Same effect as slope/velocity
Reduces oxygen transfer and promotes sulfide
generation. May inhibit acid production by
flushing and dilution.
H2S generated under anaerobic conditions in the
force main is released from solution at the
discharge.
Corrosion resistance of pipe materials varies
widely
Higher alkalinity reduces corrosion rate
Slows wastewater flow, traps organic solids and
increases sulfide generation
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area available for the development of a slime layer below the water surface. As the
detention time increases, oxygen consumption also increases, organic matter becomes
increasingly solubilized and the oxidation-reduction potential (ORP) decreases. These
conditions favor the sulfate-reducing organisms.
The presence of dissolved oxygen in the wastewater stream encourages growth of an
aerobic portion of the slime layer. In such cases sulfide produced in the anaerobic
portion of the slime layer is likely to be oxidized in the aerobic zone.
Temperature also has a significant impact on the biological activity of the sulfate reducing
bacteria. It has been reported that the rate of sulfide production is increased seven percent
for every Celsius degree increase up to 40°C (4). This is equivalent to doubling the
reaction rate for every 10°C increase in temperature.
Atmospheric hydrogen sulfide gas concentrations appear to be proportional to the rate of
sewer corrosion that occurs. The proportionality may not be linear, however, as there may
be a limiting hydrogen sulfide level above which the oxidation reaction rate does not
increase. At that point, sulfur is stored as yellow deposits (3).
Finally, pH of the sewage governs the ratio of H2S gas and HS" ions in solution. This is
important in determining the potential for H2S gas release into the atmosphere, and,
consequently, the potential for the formation of sulfuric acid and corrosion. The CSDLAC
study also found the pH of the concrete surface to increase as relative humidity of the
sewer atmosphere dropped below 80 - 90 percent indicating that thiobacilli need moisture
to biologically convert hydrogen sulfide to sulfuric acid.
The CSDLAC study found that even when sewage contains oxygen, and dissolved sulfide
levels are less than 0.1 mg/1, there still remains a significant concentration of hydrogen
sulfide in a sewer atmosphere. As a result, concrete corrosion can be expected to continue
even when dissolved sulfide levels are quite low (3).
2-6
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23 Effects of Industrial Pretreatment
Another factor which may affect the rate of corrosion is the concentration of metals and
other wastewater constituents resulting from industrial discharges. Studies conducted by
the County Sanitation Districts of Los Angeles County (CSDLAC) have demonstrated that
the reduction in metals and other industrial constituents has apparently caused an
acceleration in corrosion rate, possibly due to biological inhibition of sulfate-reducing
bacteria and/or chemical precipitation by iron and other metals (2)(3). The compounds
which showed the highest correlation between reduction in concentration and an increase
in dissolved sulfide included nickel, chromium, zinc, cadmium, copper, cyanide, lead, and
iron (5). The CSDLAC study also noted that detergent manufacturers have made changes
over the past 20 years in the sulfonated compounds which occur in surfactant and
brightener formulations. These changes may allow for easier biodegradation of the sulfur
compounds. This same study cites a 1,000% increase in dissolved sulfide when comparing
1987 to 1971-1974 data. However, site visits conducted by EPA to compare corrosion in
residential versus industrial sewers were inconclusive regarding the impacts of metals and
other industrial constituents on hydrogen sulfide corrosion. No wastewater systems other
than CSDLAC have been found to have sufficient historical data to establish a relationship
between industrial pretreatment and hydrogen sulfide corrosion.
It should be noted that there are several aspects of industrial pretreatment that can lead
to reduced sulfide generation. A reduction in certain industrial discharges can directly
impact sulfide generation and corrosion. These are listed in Table 2-2.
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TABLE 2-2
IMPACTS OF CONTROLLING
INDUSTRIAL DISCHARGES ON HYDROGEN SULFIDE CORROSION
Type of Discharge Controlled
Sulfide-bearing wastes
Benefit
Lowers sulfide levels, corrosion
potential
High organic strength
wastes
Sulfide generation rate
proportional to organic strength;
reduction in organic strength
reduces oxygen uptake and
depression of dissolved oxygen in
wastewater
High temperature wastes
Lower temperature reduces sulfide
generation rate; increases solubility
of H2S, reducing release of H2S;
increases solubility of oxygen
Wastes containing fats,
.oils, and grease
Reduces potential for sewer
clogging, reduced velocities, solids
deposition, and sulfide generation
Acidic wastes
Maintaining pH at or above
neutral decreases amount of H2S
available for release to the sewer
atmosphere
2-8
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2.4 References
1. Bowker, R.P.G., J.M. Smith, and N.A. Webster, Odor and Corrosion Control in
Sanitary Sewage Systems and Treatment Plants. Design Manual. U.S. Environmental
Protection Agency, Center for Environmental Research, Cincinnati, OH, 1985.
2. Sulfide Corrosion in Wastewater Collection and Treatment Systems. Report to
Congress. U.S. Environmental Protection Agency, Office of Water, Washington, DC,
1991.
3. Morton, R., R. Caballero, Ching-Lin Chen, and J. Redner, Study of Sulfide
Generation and Concrete Corrosion of Sanitary Sewers. Sanitation Districts of Los
Angeles County, Carson, California, October, 1989.
4. Pomeroy, R., F.D. Bowlus, "Progress Report on Sulfide Control Research," Sewage
Works Journal 18 (4), pp. 597-640, 1946.
5. Morton, R., W.A. Yanko, D.W. Graham and R.G. Arnold, "Relationships Between
Metal Concentrations and Crown Corrosion in Los Angeles County Sewers,"
Research Journal Water Pollution Control Federation. Volume 63, Number 5,
July/August 1991, pp. 789-798.
2-9
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3. MAJOR HYDROGEN SULFIDE CORROSION TARGET AREAS
Hydrogen sulfide corrosion can damage pipes, structures, and equipment used in the
collection and treatment of wastewater. The degree of corrosion that results is impacted
by the design and construction of the facilities, existing loading conditions, and operation
and maintenance practices. Certain unit processes are more susceptible to corrosion
damage than others, generally due to the condition of the wastewater or sludge and the
characteristics of the process. For example, even if wastewater entering a treatment facility
is aerobic and devoid of hydrogen sulfide, sulfide generation can occur in a primary clarifier
due to the quiescent conditions and long detention times. The H2S gas can be readily
released from solution at the turbulent effluent weirs. Hydrogen sulfide which is generated
in primary clarifiers, sludge thickeners, and sludge holding tanks is typically released in
significant quantities during sludge dewatering operations.
Turbulence is a major factor in the generation and release of H2S from solution. In cases
where dissolved sulfide is not present, turbulence can be beneficial in promoting the
transfer of oxygen into solution, maintaining aerobic conditions. However, if dissolved
sulfide is present in concentrations as low as 0.1 to 0.2 mg/1, turbulence can dramatically
increase the rate at which the dissolved hydrogen gas is released to the atmosphere to cause
potential corrosion problems.
This chapter describes the various components of a wastewater collection and treatment
system which are most likely to result in generation and release of hydrogen sulfide gas.
Upon release from solution, the hydrogen sulfide can cause odor and/or corrosion
problems.
3.1 Gravity Sewers
Gravity sewers form an integral part of the entire wastewater collection infrastructure and,
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as such, are generally designed to accommodate growth well into the future (50 years is not
an uncommon design practice). Often, sewer designers will utilize larger pipe at shallower
slopes to minimize the sewer depth and construction cost Both of these practices often
result in lower wastewater velocities during the initial years when the sewer carries
substantially lesser volumes.
Wastewater velocity directly impacts wastewater detention time within the sewer, the
amount of grit and organic solids deposition (both of which tend to farther reduce
wastewater velocity and increase depth of flow), and the extent of slime layer buildup
within the submerged portion of the sewer. Velocity thus affects formation of dissolved
sulfide and also the release of hydrogen sulfide gas into the sewer atmosphere. The
hydrogen sulfide gas is biologically converted to sulfuric acid in the presence of moisture
and oxygen and attacks cementitious materials such as concrete pipe and mortar. In
addition, hydrogen sulfide gas and sulfuric acid directly attack ferrous meital and copper
surfaces. The most vulnerable pipe areas prone to sulfide corrosion are the crown of the
pipe and wall areas normally above the wastewater level. If allowed to continue over a long
period of time, the hydrogen sulfide corrosion can eventually lead to pipe failure and
trench collapse.
Other gravity sewer components affected by sulfide corrosion are sewer manholes, junction
chambers, metering vaults, and other structures. Manholes often serve as junctions for two
or more sewers. Typically, the smaller sidestream sewer will enter the manhole at an
elevation higher than the mainline sewer invert. This is to prevent surcharging of the
smaller sewer during periods of sustained high flow in the larger sewer. There can also be
substantial elevation differences in deep sewer situations, where, due to ground surface
topography, the sidestream sewer does not need to be installed as deeply as the mainline
sewer. These vertical drops can cause significant turbulence. A drop connection installed
upstream of the connecting manhole will help minimize hydrogen sulfide release. Also,
unless manhole inverts are properly formed to promote smooth hydraulic conditions,
excessive turbulence can occur due to abrupt changes in horizontal or vertical alignment.
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While turbulence allows introduction of air (oxygen) into the wastewater, it also causes the
release of hydrogen sulfide gas. The hydrogen sulfide corrosion process can then
deteriorate manhole walls, ladder rungs and any other corrosion-prone material. Some
manholes are being provided with corrosion-resistant ladder rungs rather than iron and
steel used in past decades, but the inserts can potentially become structurally deficient if
the surrounding concrete is allowed to corrode.
3.2 Pump Stations and Force Mains
Pump station wet wells can contribute to sulfide corrosion problems by promoting the
generation of sulfide within the wet well itself. Further, hydrogen sulfide gas from the
upstream sewer atmosphere may enter the wet well. Excessive turbulence in the wet well
caused by vertical drops from the influent pipe to the wastewater surface causes release of
hydrogen sulfide gas from the solution. This release of hydrogen sulfide gas to the
atmosphere likely will lead to odor complaints in populated areas.
Influent sewers at wastewater treatment plants may become surcharged if inlet screens or
comminutors are present but not properly maintained. Sewers discharging to a pump
station may also be surcharged if the wet well level is allowed to rise above the influent
sewer.
As with sewers, pump station wet wells are normally designed for future flows. While this
practice makes economic sense, it does allow for excessive detention times during the initial
years of operation. The longer the detention time, the greater the likelihood that organic
matter will settle to the wet well bottom, and the wastewater will become septic. Unless
the pump suction pipes and wet well geometry are appropriately designed, the accumulation
of organic matter will aggravate the generation of dissolved sulfide and hydrogen sulfide
gas. Options to consider in reducing the long detention times in wet wells include utilizing
a smaller portion of the wet well, adjusting the "pump-on" and "pump-off1 levels, or using
variable speed pumps. Wet well cleaning should be performed on a routine basis.
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Wet well corrosion can include the concrete or masonry structure, access covers, ladders
and stairs, bar racks and comminutors, control devices such as sluice gates, slide gates and
weirs, heating and ventilating systems, and electrical components. Electrical contacts are
particularly susceptible to corrosion failure. ,
Force mains, inverted siphons and other surcharged pipes are normally completely full of
wastewater, and because this condition does not allow reaeration from the sewer
atmosphere, dissolved oxygen levels in the wastewater become depleted, and significant
quantities of dissolved sulfide can be generated. Since the pipes are generally full of
wastewater, corrosion will not occur within surcharged pipes unless they contain an air
pocket If an air pocket exists, corrosion may occur very quickly. This situation has been
reported at several locations throughout the U.S.. Upon reentry to a gravity sewer or
treatment plant headworks, an immediate release of hydrogen sulfide gas occurs due to the
high level of turbulence. Force main and inverted siphon terminus locations are therefore
highly susceptible to hydrogen sulfide corrosion and should be routinely monitored. Other
areas of concern with regard to hydrogen sulfide corrosion include special structures such
as those utilized for flow measurement, and valves used for flow isolation and air release.
33 Treatment Facilities
• i
Corrosion damage can be extensive at the headworks of wastewater treatment facilities,
particularly if long influent force mains or large diameter gravity sewers constructed at flat
slopes discharge to the facility. These influent sources typically convey wastewater which
is septic and already has a significant dissolved sulfide concentration. Headworks. areas
most susceptible to corrosion attack include influent channels, flow measurement facilities,
comminutors and bar racks, grit chambers and areas where vertical drops occur. All of
these components promote turbulent conditions and the resultant release of hydrogen
sulfide gas. In addition, septage or other high-strength wastewater and sidestream returns
may contribute to the generation and release of hydrogen sulfide. All components used
in headworks areas should be inspected regularly due to the high potential for hydrogen
3-4
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sulfide corrosion.
Septic conditions are likely to occur in primary clarifiers due to the quiescent conditions
and long detention times. The most critical areas with regard to the release of hydrogen
sulfide are launders, troughs, effluent weirs and outlet channels, especially where large
drops in elevation occur. Because these components are normally open to the atmosphere,
the corrosion potential is lessened somewhat, though the odor potential remains very high.
Secondary biological treatment processes are not normally considered to be sources of
sulfide generation due to the presence of aerobic conditions. Localized problems can
develop, however, in areas having either poor wastewater distribution or poor air
distribution, which results in anaerobic zones. Also, trickling filters which are heavily
loaded can generate sulfide. Secondary clarifiers and subsequent downstream facilities
normally are not areas of hydrogen sulfide corrosion due to the aerobic conditions and high
wastewater quality at these stages. Enclosed secondary clarifiers should be monitored,
however, due to the minimal ventilation provided. Catwalks, railings and other structural
elements and fasteners should be constructed of corrosion-resistant materials.
Sludge storage tanks, sludge thickeners, sludge dewatering systems, and solids processing
operations in general are particularly susceptible to hydrogen sulfide corrosion. This is due
to the rapid generation of hydrogen sulfide in sludge storage tanks and gravity thickeners.
Introduction of turbulence, such as from dewatering equipment, can substantially increase
H2S release and corrosion potential.
Within any enclosed space containing H2S, most concrete and metallic surfaces are
susceptible to corrosion, particularly in poorly ventilated areas. Structural elements such
as gratings, railings, platforms, structural inserts, clamps and beams are prone to hydrogen
sulfide attack, either directly by chemical reaction, or by acid attack. In addition, electrical
components are particularly sensitive to hydrogen sulfide corrosion, because copper, iron
and silver are all directly attacked by hydrogen sulfide gas. Other affected processes
3-5
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include heating and ventilating systems and wastewater processing equipment.
Appendix A includes case studies describing hydrogen sulfide corrosion problems at
wastewater collection and treatment facilities.
3-6
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3.4 References
1. Sulfide in Wastewater Collection and Treatment Systems. ASCE Manual of Practice
- No. 69, American Society of Civil Engineers, 1989.
2. Bowker, R.P.G., J.M. Smith, and N.A. Webster, Odor and Corrosion Control in
Sanitary Sewage Systems and Treatment Plants. Design Manual. U.S. Environmental
Protection Agency, Center for Environmental Research, Cincinnati, OH, 1985.
3. Hydrogen Sulfide Corrosion in Wastewater Collection and Treatment Systems.
Technical Report U.S. Environmental Protection Agency, Office of Water,
Washington, DC, 1990.
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4. DETECTION AND MEASUREMENT OF HYDROGEN SULFIDE CORROSION
4.1 Approach for Identifying Existing or Potential Corrosion Problems
Figure 4-1 shows a flow diagram of the basic steps involved in identifying existing or
potential corrosion problems. These steps are:
1. Reviewing existing information to identify potential problem areas.
2. Conducting preliminary inspection of potential problem areas.
3. Conducting detailed inspections and measuring corrosion at known problem
areas.
4. Estimating corrosion rates and prioritizing areas for further monitoring
and/or correction.
These four steps are discussed in subsequent sections of this chapter.
42. Identifying Potential Problem Areas
A review of the layout and design of the existing collection and treatment system and other
information is useful to identify areas that are susceptible to corrosion problems. These
areas should be highlighted on sewer maps and other available plans for subsequent
inspections in the field.
Areas where septic wastewater conditions are likely to develop are also areas with corrosion
potential. Force mains are a major source of sulfide generation. Where force mains
discharge into gravity sewers, wet wells, or junction chambers, substantial quantities of H2S
can be released due to turbulent conditions, creating conditions ripe for corrosion. Force
main discharges and inverted sewer ("siphon") outlets should receive priority for inspection.
Gravity sewers with low velocities or long detention times should also be identified as
4-1
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FIGURE 4-1
APPROACH TO IDENTIFYING EXISTING
AND POTENTIAL CORROSION PROBLEMS
REVIEW EXISTING INFORMATION
TO IDENTIFY POTENTIAL
PROBLEM AREAS
- Sewer maps
- Records of pipe manufacture
- Locations of pipe
replacement/rehabilitation
- Odor complaints
-TV logs
- O & M personnel interviews
CONDUCT PRELIMINARY INSPECTION
AT POTENTIAL PROBLEM AREAS
- Visual inspection
- Atmospheric H2S
• Wastewater sulfide
- pH of pipe crowns, channels, tank walls
above sewage level
MEASURE CORROSION AT KNOWN PROBLEM
AREAS
- Depth of penetration estimates
- Core sampling
- Sonic caliper inspection
- TV inspection
I
ESTIMATE CORROSION RATES AND
PRIORITIZE AREAS FOR FURTHER
MONITORING AND/OR CORRECTION
4-2
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potential problem areas due to the possibility of sulfide generation. Problems often occur
in pipes sized for future development that has not yet occurred. Pipes installed at flat
slopes with low sewage velocities are subject to potential corrosion problems.
Pumping stations should be earmarked for inspection, since hydrogen sulfide is often
generated in wet wells. Septic wastewater entering metering stations, junction chambers,
drop manholes, and wet wells may be subjected to sufficient turbulence to release H2S that
has formed upstream. In general, any areas of high turbulence in the collection system
should be targeted for inspection.
Locations where odor complaints have originated should be identified on the map. Odor
complaints often result from hydrogen sulfide gas escaping from manholes or vents.
Hydrogen sulfide gas is responsible for the "rotten egg" odor associated with septic sewage.
Records of pipe manufacturers should be reviewed. For instance, the corrosion rate of
spun concrete pipe can be as little as one-third of that found in cast concrete pipe, given
the same H2S level and acid production rate. This phenomenon occurs due to the presence
of an alkali-rich layer on the spun pipe. Once this layer is lost to corrosion, however, the
corrosion rate increases. Slower initial rates of corrosion can be similarly expected with
concrete pipe containing calcarious aggregate or sacrificial layer. Without information
pertaining to pipe manufacture, it is extremely difficult to predict corrosion rates.
Alternatively, if no records exist, core samples can be taken to determine the materials of
construction.
Sewer maintenance staff should be consulted to help identify potential trouble spots in the
system. Such discussions can be very valuable. Records of sewer and treatment facility
rehabilitation or replacement should be reviewed to determine if corrosion was a factor in
requiring rehabilitation work. If TV tapes and/or logs are available for a portion or all of
the system, these should be reviewed to determine if corrosion was noted. Poor resolution
with early generation TV systems may make identification of corrosion difficult, especially
4-3
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to the untrained observer.
Corrosion at the headworks of the wastewater treatment plant may be indicative of
upstream corrosion problems. Bar screens, concrete channels, grit chambers and electrical
equipment at the headworks should be identified for subsequent inspection. Primary
clarifiers, sludge handling systems and other unit processes discussed in Chapter 3 should
also be evaluated.
Figure 4-2 shows a typical sewer map with potential corrosion problem areas targeted for
inspection.
4.3 Preliminary Inspection
A preliminary inspection of areas having the potential for corrosion problems can
determine 1) the presence or absence of corrosion, 2) the extent of the problem, and 3) the
severity of the problem. The inspection will also indicate the need for further inspections
and/or corrective actions, and may lead to a regular program of inspection and monitoring.
Because the preliminary inspection involves measurement of certain parameters that are
affected by wastewater temperature, it is recommended that the inspection be conducted
during, summer months when hydrogen sulfide is most likely to be present
The preliminary inspection generally involves the following elements:
• Measurement of atmospheric H2S
• Visual inspection
• Measurement of temperature and pH in wastewater
• Measurement of total and dissolved sulfide in wastewater
• Measurement of crown pH
4-4
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4-5
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A preliminary inspection checklist is shown in Figure 4-3. Each of the major elements of
the inspection are described below.
43.1 Atmospheric
An electrochemical H2S analyzer or simple detector tubes can be used to detect and
measure hydrogen sulfide gas in the atmosphere of manholes, junction chambers, metering
stations, headworks structures, etc. This technique can and should be used without
entering the structure. (Measurement of H2S, lower explosive limit, and percent oxygen is
standard procedure before entering a confined space).
With electrochemical sensors, internal or external sampling pumps in combination with an
extension hose should be used to allow measurement of H2S near the water surface, since
H2S is heavier than air. Detector tubes can use either a manual or electric pump which can
be fitted with an extension hose for remote sampling. Such hoses can be lowered through
the pickholes of manholes or through access hatches in junction chambers and other
structures without significantly disturbing the conditions in the sewer atmosphere. Opening
a manhole or structure cover completely will often destroy the ambient atmospheric
conditions in the sewer gas space, thus providing inaccurate H2S measurements.
It is critical that H2S is measured within 6 to 12 inches of the liquid surface, and that the
wastewater atmosphere not be disturbed. A simple battery-powered sensor can be
fabricated to serve the purpose of locating the liquid level without removing manhole
covers, as depicted in Figure 4-4.
433. Visual Inspection
A visual inspection of the condition of manholes, metering stations, wet wells, headworks,
and other structures is essential to identify corrosion problems. Areas that are accessible
should be viewed without entering the structure. Hazardous atmosphere can exist in
4-6
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FIGURE 4-3
CORROSION INSPECTION CHECKLIST
Type of Structure:
Date:
Manhole.
Other
Structure No.
Location:
Pipe Orientation
and Sizes:
Age of Pipe/Structure:
Pipe Material:
Structure Material:
Approx. Depth of Flow:
Headspace H2S:
Sewage pH:
Temp:
Turbulence:
Manhole
Structure
_yr
in.
_ppm
units
°C/°F
Total Sulfide:
Dissolved Sulfide:
mg/I
mg/1
.Quiescent
Moderate
.High
4-7
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General conditions
Structure:
Pipe:
Evidence of corrosion -
Manhole barrel:.
Pipe crown:
Other:
Bottom debris:
Comments:
Signature
4-8
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AMMETER
BATTERY
NOTE: INSERT INTERCONNECTED PROBE
& ELECTRICAL LEADS THROUGH
MANHOLE PICK HOLE OR VENT
ELECTRICAL LEADS
(Connected to Probe)
WATER SURFACE
H2S
ANALYZER
•H2S PROBE
8"-12"
FIGURE 4-4. H2S PROBE/LIQUID SENSOR DEVICE
4-9
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sewers, manholes and other confined spaces. Workers entering such areas must receive
safety training. Proper safely procedures for confined space entry must be strictly followed
Items noted in a visual inspection include:
1. Condition of ladder rungs, bolts, conduit, and other metal components.
2. Presence of protruding concrete aggregate.
3. Presence of exposed reinforcing steel.
4. Development of black coating (copper sulfide) on copper pipes and electrical
contacts.
5. Evidence of loss of concrete from pipe crown or walls.
6. Condition of equipment such as bar screens, grit removal systems, sludge
thickening and dewatering equipment
A quick method of inspecting the general condition of sewers can be performed with a
telescoping rod onto which are attached a halogen light and adjustable mirror at one end,
and a 4x sight scope at the other end, as indicated in Figure 4-5. The rod is inserted into
a manhole, and by slightly tilting the rod and flashing the light beam down the sewer, its
condition can be observed. This procedure is useful when small-diameter sewers are
involved. Also, because entry into a confined space is not required, there is minimal risk
of being overcome by potentially harmful sewer gas.
433 Measurement of Sulfide
For screening purposes, wastewater sulfide can be estimated using field test kits. Although
dissolved sulfide is the best indicator of potential corrosion problems, it requires an
additional flocculation and decant step prior to analysis for sulfide. For purposes of
preliminary inspection, total sulfide determinations are adequate, although both total and
dissolved sulfide should be determined for several samples to show the relationship between
4-10
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FIGURE 4-5. QUICK METHOD OF INSPECTING SEWER LINES
4X sight scope
adjustable 7-inch
polished steel mirror-
extension bar
4-11
remote
halogen light-
-------
the two. Dissolved sulfide normally comprises 70 to 90 percent of total sulfide.
43.4 Measurement of Surface pH
One of the most useful indicators in the determination of the potential for hydrogen sulfide
corrosion problems is the pH of the pipe crown or structure walls and roof. A simple test
using color-sensitive pH paper is applied to the moist crown wall or roof to measure the
pH. New concrete has a pH of 11. After aging, the pH under non-corrosive conditions
may drop to near neutral, though the presence of carbon dioxide in the sewage atmosphere
can further reduce surface pH below 7. Concrete experiencing severe hydrogen sulfide
corrosion may have a pH of 2 or lower. Color-sensitive pH paper is available for many
ranges of pH to yield fairly accurate results. Ranges should be selected to allow accuracy
to ±, 0.5 pH unit Figure 4-6 shows a simple device for obtaining surface pH without
entering a confined space.
43.5 Measurement of Sewage Temperature and pH
Sewage temperature and pH should also be measured during the preliminary survey. pH
can be measured using a laboratory pH meter, or using a simple, hand-held probe.
43.6 Review of Data
Data from the preliminary inspection program should be tabulated and reviewed to gain
perspective on the overall magnitude of the problem. If corrosion appears severe or was
identified at most sites inspected, further inspections may be justified for other areas of the
collection and treatment system, and a more detailed physical measurement of the extent
of corrosion should be performed.
4-12
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Threaded PVC, aluminum, or other
lightweight material with extensions
as required
Rubber pad with attached pH paper
FIGURE 4-6. SEWER CROWN PH PROBE
4-13
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4.4 Measurement of Corrosion
Measuring the relative extent of corrosion is useful to 1) quantify the severity of the
problem, 2) estimate the rate of corrosion, 3) project the remaining useful lifetime of the
structure, and 4) develop a database to calibrate predictive corrosion rate models.
Measuring corrosion depth and estimating corrosion rate are often difficult and not highly
accurate due to the variation in corrosion rate within a system. Consequently, data
collected as part of an assessment of corrosion should be carefully reviewed, and sufficient
data should be collected to allow reasonable assessments to be made.
The following are several techniques used to estimate the depth of corrosion penetration.
4.4.1 Manual Methods
The simplest but least accurate technique is to remove the soft corrosion product using a
screwdriver or sharp tool until sound, uncorroded concrete is reached, and directly measure
the depth of penetration. Obviously, this method has substantial limitations, since the
depth is measured from a point where concrete loss has likely already occurred. Often,
depth of penetration is measured from the top of protruding aggregate to exposed
uncorroded concrete.
Another technique is to use extendable rods to effectively measure inside diameter of
corroded pipe and compare this with the measured inside diameter of an uncorroded
section. Diameters can be measured horizontally (parallel to the waterline), vertically
(from crown to invert), and at various angles to yield a range of values. As with the
manual technique, corrosion product must first be removed where the measurements are
being taken. The problem with this approach is that pipes are likely to be slightly out-of-
round, making it impossible to get accurate measurements of corrosion depth.. In addition,
the depth of concrete over reinforcing steel may vary within the same pipe.
4-14
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Both of these techniques are most applicable to large diameter, man-entry size sewers. To
estimate "average" corrosion penetration, corrosion measurements should be taken away
from manholes or structures where turbulence could result in higher, localized corrosion
rates. The most severe corrosion observed in gravity sewers will typically be within a
distance of three to ten feet from manholes and other turbulent areas.
4.4.2 Concrete Coring
Taking a concrete core of a pipe or concrete tank wall can be useful for close examination
of corrosion damage, and for estimating the depth of corrosion penetration. For measuring
corrosion, coring is beneficial only if the original thickness of concrete is known. Cores can
be taken from uncorroded sections or from other locations in the same structure or pipe
for comparison. Cores are especially helpful when attempting to determine remaining
concrete cover over reinforcing steel, which can be used to determine the remaining
effective life of the pipe or structure.
Any pipe in an area with corrosion potential may be fitted with non-corroding vitrified clay
plugs or stainless steel rods to establish a reference point from which future corrosion can
be measured. The plugs or rods can be installed at any time, regardless of the current
stage of corrosion.
4.43 Sonic Caliper
A recent development in remote sewer inspections is the use of "sonic caliper" technology
to detect and measure the internal conditions of pipes. The sonic caliper technique uses
the travel-time measurement of a sonic signal to determine the distance from the sonic
transmitter to the target The sonic caliper operates in the pulse-echo-sonar mode. Each
sonic transducer acts as both a transmitter and a receiver of sonic signals. Distance
measurements are made from the transducer to the walls of the pipe. The distances to the
walls and the position of the transducer on a floating raft are used to calculate the vertical
4-15
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diameter of the pipe (1).
The sonic caliper system is comprised of an instrument raft inserted into the sewer line, a
computer at the surface which controls the system and displays data, and a conductor cable
which connects the two.
Figure 4-7 shows a typical sonic caliper plot In the plot the crown loss averages two
inches, and the depth of bottom debris varies from three to six inches. The actual crown
loss may be somewhat greater than reported because the signals will be reflected from the
first surface that they strike. This may be a corrosive crust, protruding aggregate, part of
the reinforcing cage, or in some cases roots or gaskets hanging below the crown. Studying
the overall trend of the longitudinal plots and also TV inspection of the line prior to sonic
investigation can help to resolve any uncertainties.
In Tampa, approximately 40,000 feet of line were surveyed using this technique as part of
an equipment development and demonstration project The Tampa project yielded the
following results and conclusions:
1. The equipment developed during the project was able to read dimensions in
pipes from sizes 36" to 60".
2. Accuracy of the equipment was within one-half inch of the actual pipe wall
thickness.
3. Completed dimension surveys were used to identify those lines in critical
condition and assist in prioritizing which lines should be rehabilitated,
replaced or continued in service until rescheduled for inspection.
4. The bottom transducers are useful in determining cleaning requirements for
either restoration of original capacity or providing information for
4-16
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rehabilitation by sliplining.
4.4.4 Television Inspection
Television inspection of a sewer line is not a true means of measuring corrosion. However,
it can provide a relative indication of how rapidly corrosion is progressing by comparing
conditions in the same line over a period of several years. It is also useful in identifying
problem areas between physical access points.
TV inspection involves use of a closed-circuit television camera to observe the conditions
in the sewer lines. The results are shown on a monitor, and documentation can be made
by a videotape. The cameras used to inspect sewers are specially designed for sewer
conditions. The camera is mounted in a casing and is pulled or pushed through the sewer
with cables. Light sources are provided for illumination.
4.4.5 Other Techniques
In cases of severe corrosion and subsequent loss of portions of the pipe crown, soil and
backfill may wash into the pipe, creating a void. Such conditions may eventually lead to
pipe collapse.
The identification of voids can be made using infrared thermographic inspection. This
thermographic system produces a temperature map displayed on a portable computer
monitor. The voids show up as different colored areas on the screen, and allows the
inspection crew to locate the problem areas. Concurrent with the infrared inspection, a
standard visual from a video camera is recorded. This identifies any obvious deterioration
(2)-
The technique is useful for identifying areas where sewer collapses are likely. However, it
is applicable only after substantial damage has been done to the pipe, and the subsurface
4-18
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void is large. For instance, work performed by the County Sanitation Districts of Los
Angeles County found that upon excavation, none of the anomalies (temperature variations
suggested as being possible voids) observed were related to actual sewer deficiencies.
Rather, it was determined that the voids were generally within one foot of ground surface
and reflected backfill density variations of little or no significance to sewer system structural
integrity. The technology is, however, in its infancy and improvements may lead to a
reliable means of locating sewer problems.
4.5 Comparing Measured and Predicted Corrosion
4.5.1 Introduction
The ability to identify, locate and define areas susceptible to corrosion is necessary for
effective corrosion control. This ability can be enhanced through the use of predictive
models. Equations have been developed to allow prediction of both sulfide generation rate
and corrosion rate. Because it is imperative that reasonable assumptions be made in order
to accurately predict rates of corrosion throughout a system, it is recommended that actual
sulfide concentrations be obtained at all points of interest Once this information is
available, corrosion rates can be predicted. It is important to remember, however, that the
predictive corrosion rates thus determined must be verified with field monitoring where
possible to confirm the accuracy of the results.
Model for Predicting Corrosion Rate
The model for predicting corrosion rate consists of two basic equations which determine
1) the rate of flux of hydrogen sulfide to sewer walls, and 2) average and peak rates of
corrosion. Based on predicted corrosion rate, a third equation predicts anticipated service
life of the sewer or structure being investigated. These empirical equations are addressed
in greater detail in publications by the U.S. Environmental Protection Agency, the
American Society of Civil Engineers, and the American Concrete Pipe Association
4-19
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The rate of flux of hydrogen sulfide to the sewer pipe wall can be calculated by the
following equation:
0^ = 0.45(sv)3/8j[DS]b/p'
where: 0m = rate of flux of hydrogen sulfide to the sewer wall, in g/m2-hr
0.45 = conversion factor from meters to feet
s = energy grade line slope of the stream
v = stream velocity, in ft/sec
j = factor relating the fraction of dissolved sulfide present as H2S to
pH (see Figure 4-8)
[DS] = dissolved sulfide concentration, in mg/1
b = surface width of stream, in feet
p' = perimeter of pipe exposed to atmosphere, in feet
Refer to Figure 4-9 for relationships of b and p' with sewers flowing partly full.
Once the rate of H2S flux is known, the average rate of corrosion can be estimated as
follows:
CW8= 0.45 k 0ro
A
where:
Qwg = average corrosion rate of exposed pipe perimeter, in inches/year
0.45 = conversion factor from meters to feet
k = incomplete acid reaction factor, ranging from 0.3 to 1.0. The k
factor estimates the efficiency of the acid reaction considering the
estimated fraction of acid remaining on the wall. The k factor
4-20
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NOTES
1. RELATIONSHIP SHOWN IS FOR AVERAGE VALUE OF k,« I0~7; k, VARIES SOMEWHAT WITH SALINITY
AND TEMPERATURE.
2. CONCENTRATION OF Ss IS NEGLIGIBLE IN pH RANGE SHOWN.
Figure 4-8. Relationship of Dissolved Suffide Equilibrium to pH
4-21
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0.1 02 as 0.4 as o.c a? • as as 1.0 la .-u
Figure 4-9. Hydraulic Elements of Circular Sewers Running Partly Full
4-22
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approaches unity for complete acid reaction.
A = concrete alkalinity, expressed as calcium carbonate, equivalent
decimal fraction. For granitic aggregate concrete, A is 0.17 to 2.0.
For calcareous aggregate, A is 0.9 to 1.1. The A value for mortar
lining is 0.4, and 0.5 for asbestos cement For ferrous pipe, a value
of 0.5 should be used to account for direct attack by H2S (4).
It must be stressed that the value obtained in the above-noted equation results in an
average value for the corrosion rate. Studies indicate that peak corrosion rates up to 1.5
to 2.0 times the average rate may occur at the sewer crown. High turbulence levels
increase the release of H2S and subsequent corrosion rate. The "crown corrosion factor"
and "turbulence corrosion factor" are used to account for these conditions. The peak rate
is determined by the following equation:
Q«« = C.vgxCCFxTCF
where:
QMI ^ Maximum rate of corrosion, in inches/year
Cwg = Average rate of corrosion, as previously determined
CCF = Crown corrosion factor, ranging from 1.5 to 2.0
TCF = Turbulence corrosion factor, which typically varies from 1.0 to 2.5
for well-designed junction structures or other areas with
nonuniform flow conditions, to 5.0 to 10.0 for drops and other
turbulent junctions.
Finally, the useful life expectancy of the sewer can be approximated by the equation:
L = Z
C
where:
L = useful services life, in years
4-23
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Z = thickness of allowable concrete loss over the reinforcing steel, in
inches
C = corrosion rate, in inches/year. The maximum value should be
used.
In pipe systems where corrosion has already occurred, the value for Z should be diminished
to indicate actual allowable concrete loss. When dealing with other pipe materials, such
as asbestos cement, non-reinforced concrete pipe, or ferrous pipe, the estimate of useful
life should be based on the amount of remaining pipe wall which can be corroded before
failure occurs. It is important to remember that all pipe is designed to support the soil load
plus a live load and an appropriate factor of safely. Rigid pipe such as asbestos cement,
reinforced concrete, non-reinforced concrete, cast iron and vitrified clay does not require
side support, but flexible pipe such as ductile iron, steel, polyvinyl chloride, polyethylene,
fiber reinforced plastic and reinforced plastic mortar pipe do require lateral soil support
As with other types of failure analysis, suitable factors of safety should be utilized in
determining useful life expectancy prior to failure.
4.5.3 Reliability of Predictive Models
Two studies were reviewed to assess the accuracy of corrosion modeling: the San Diego -
West Point Loma interceptor in the City of San Diego, California, and the Sacramento
Central Trunk Sewer in Sacramento County, California (4). The results of these two
studies are indicative of the current status of the Pomeroy predictive model equations: The
San Diego - West Point Loma analyses estimated the reinforcing steel in the 114-inch
diameter sewer pipe would be exposed in six years, and the reinforcing steel was actually
exposed in a period of seven years; for the Sacramento Central Trunk Sewer, however, the
actual maximum crown corrosion was two to five times less than the model predicted in the
upper reaches, but only one to two times less in the lower sewer reaches.
Predictive models have been found to be accurate in some cases, particularly when peak
4-24
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corrosion and turbulence factors are utilized and the effect of sidestreams are considered.
A factor of safely may also be incorporated.
Due to the possibility of several factors which can compound the difficulty in predicting
sulfide generation, such as sewer blockages, grit and slime layer buildup, only actual
measured sulfide concentrations should be used as input to the model to estimate the
corrosion rate. As with all predictive models, actual measurements of corrosion penetration
should be taken to confirm and calibrate corrosion model equations over time for any area
under investigation.
4.6 References
1. Cronberg, A.T., Morriss, J.P., and T. Price, "Determination of Pipe Loss Due to
Hydrogen Sulfide Attack on Concrete Pipes," paper presented at the 62nd Annual
Conference of the Water Pollution Control Federation, San Francisco, 1989.
2. Weil, G.J. , and K.L. Coble, "Infrared Scanning Finds Sewer Weak Spots,"
Operations Forum, November, 1985.
3. Bowker, R.P.G., J.M. Smith, and N.A Webster, Odor and Corrosion Control in
Sanitary Sewerage Systems and Treatment Plants. Design Manual. U.S.
Environmental Agency, Center for Environmental Research, Cincinnati, OH, 1985.
4. Sulfide in Wastewater Collection and Treatment Systems. ASCE Manual No. 69,
American Society of Civil Engineers, New York, NY, 1989.
5. Sulfide and Corrosion Prediction and Control. American Concrete Pipe Association,
Vienna, VA, 1984.
6. A Guide to Safety in Confined Spaces. National Institute for Occupational Safety
and Health (NIOSH), No. 87 - 113, Morgantown, WV, 1987.
7. Safety and Health in Wastewater Systems - Manual of Practice No. 1. Water
Pollution Control Federation, Washington, DC, 1983.
4-25
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5. ALTERNATIVES FOR CONTROLLING HYDROGEN SULFIDE CORROSION
IN SEWERS
5.1 Prevention of Hydrogen Sulfide Corrosion in Existing Sewer Systems
5.1.1 Introduction
This chapter presents the available techniques for controlling sulfide in sewage collection
systems. A step-by-step approach to the analysis of alternatives is suggested in this
document for a municipality experiencing corrosion or odor problems in the wastewater
collection system. Hydrogen sulfide corrosion problems in wastewater treatment plants are
addressed in Chapter 6. Control techniques presented in these two chapters may be
applicable to either collection systems or treatment plants.
In sewer systems, hydrogen sulfide corrosion control can be achieved by the following
techniques:
1. Oxidation of hydrogen sulfide in the wastewater, involving air or oxygen
injection or addition of oxidizing chemicals such as hydrogen peroxide
(H2O2), chlorine (C12) or potassium permanganate (KMnO4).
2. Precipitation with metallic salt, such as ferrous chloride (FeCl2) ferrous
sulfate (FeSO4). The dissolved sulfide is converted to an insoluble
precipitate, thus preventing release of H2S gas.
3. Elevation of pH through shock treatment with caustic to inactivate the
sulfate-reducing bacteria in the slime layer.
The mechanisms of hydrogen sulfide corrosion were described in Chapter 2. Most control
methods presented in this chapter are oriented toward reducing dissolved sulfide in
solution, and therefore, minimizing the release of hydrogen sulfide gas into the sewer
atmosphere. It is important to understand that reducing dissolved sulfide levels by some
5-1
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percentage does not necessarily reduce the rate of corrosion by the same proportion.
No single control system is the most cost-effective in all situations. Selection of a sulfide
control technology for a particular collection system depends upon site specific conditions,
such as dosage rate, availability of chemical, wastewater characteristics, and sewer system
characteristics. In fact, multiple types of control processes are often used within one
collection system.
Basic goals and treatment levels for any collection system should be established after
identifying the extent of the problem. Treatment objectives may be developed for each
system, but from experience, these guidelines are offered:
1. Maintain dissolved oxygen greater than 0.5 mg/1.
2. Keep dissolved sulfide less than 0.1 to 0.3 mg/1. While 0.1 mg/1 is preferred,
it may be difficult and costly to achieve.
3. Maintain hydrogen sulfide in the air at less than 3 to 5 ppm.
4. Increase pipe crown pH to 4.0 or higher.
Techniques available to control hydrogen sulfide corrosion have been categorized as
follows:
• Oxidation Systems
• Precipitation Systems
• pH Elevation
• Other Methods
These techniques are summarized in Table 5-1, and are discussed below (1):
5-2
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TABLE 5-1
SUMMARY OF HYDROGEN SULFIDE CONTROL TECHNIQUES
Technique
I. OXIDATION
Air injection
Oxygen injection
Hydrogen peroxide
Chlorine
Potassium
permanganate.
II. PRECIPITATION
Iron salts
Frequency
of Use
Low; limited
by application
Low in US.;
high in U.K.,
Australia
High
High
Low
High
Cost
Factors
Power only
Liquid oxygen and
storage
Dosages for
oxidation of all
components
Dosages for
oxidation
Chemical cost
Availability of
low cost material;
freight charges
Advantages
Low cost, adds
DO to wastewater
to prevent further
sulfide generation
5 times solubility of
air, high DO possible;
economical in force
jnainf
Effective for odor/
sulfide control in
gravity sewers or ,
force mains; simple
installation
Applicable to gravity
sewers or force mains
Effective, powerful
oxidant
Economical for
sulfide control in
gravity sewers or
force mains
Disadvantages
Applicable only to
force mains; potential
for air binding. Limited
rate of O2 transfer
Applicable only to force mains;
achieving good O2 transfer may
be difficult
Costs can be high if
dosages much greater
than stoichiometric amount
for sulfide oxidation
Safety considerations
High cost, difficult to
to handle
Does not control non-HjS
odors; sulfide control to
low levels may be difficult;
dosages variable
Zinc salts
Low
Chemical avail-
Lower solubility
Zinc discharge is regulated
III. pH ELEVATION
Sodium hydroxide
(shock dosing)
Medium
- Chemical cost
- Storage
- Frequency of use
Intermittent appl.
may be acceptable.
Very simple, little
equipment required
Special handling of high
pH slug may be required
at treatment plant.
IV. OTHERS
Nitrate formulations Low
Sewer ventilation Very low
Chemical cost
Power
Odor Control
Provides source of O2
Helps maintain a safe,
sewer environment
May not be practical except on
interceptors; not proven
5-3
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5.1.2 Oxidation Systems
5.1.2.1 Air Injection
As discussed in Chapter 2, sulfide is produced by anaerobic bacteria that reduce the sulfate
which is normally present in sewage. Dissolved oxygen levels above 0.5 mg/8 can generally
prevent sulfide formation. Sulfide can still be produced within the slime layer, but if
dissolved oxygen is present, sulfide will be oxidized as it passes into an aerobic
environment If dissolved oxygen is not present, dissolved sulfide will enter the bulk
wastewater where it can be present as dissolved hydrogen sulfide gas (H2S).
Aeration can be a cost effective method for controlling sulfide generation, but unless air
is introduced by passive means, such as the presence of turbulent conditions in the system,
equipment must be provided to compress and to introduce it into the sewage. Advantages
include a reduction of BOD in sewage and non-toxicity. In pressurized lines, a gas pocket
may develop and cause localized problems if inadequate oxygen transfer has occurred, even
when well-protected by air release valves. Air injection is most commonly applied to force
mains and wet wells.
When using an air injection system, it is necessary to:
1. Estimate the oxygen required for the bulk wastewater (mg/1 of O2) by
measuring oxygen uptake rate in the laboratory at the expected detention
times.
2. Estimate oxygen required for the slime layer.
3. Determine air flow needed.
4. Select type of air injection, such as direct injection into the force main or
dissolution in a U-tube.
5-4
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5.1.23 Oxygen Injection
Because pure oxygen is five times more soluble in water than air, it is possible to achieve
higher DO levels in sewage by injecting pure oxygen instead of air. Pure oxygen may
therefore a more effective method of sulfide control for cases where the total oxygen
requirement exceeds that which can be transferred using air injection. Use of pure oxygen
as a sulfide control measure is particularly advantageous in pressurized systems, because
dissolution of oxygen is greater at higher pressures. Since less oxygen gas is required than
air to achieve the desired DO levels, the potential for gas pocket generation in force mains
is substantially reduced.
Pure oxygen systems typically include a liquid oxygen storage vessel, vaporizer, pressure
regulator, oxygen feed and injection systems, and a control system. Most of these
components can be leased from an oxygen supplier. Oxygen can be injected: 1) in a
pressurized side stream, which is mixed with the main flow; 2) through a U-tube oxygen
dissolve! which increases dissolution of oxygen at the greater pressures; or 3) directly at a
pump discharge or force main.
Studies have shown that oxygen will oxidize dissolved sulfides (DS) depending on the
dosage. For example, a typical design guide is to provide 5 mg O^mg DS for oxidation and
then enough additional oxygen to meet the oxygen uptake rate of the wastewater and slime
layer (2). The location of oxygen injection systems is usually at the pump discharge.
Addition of Hydrogen Peroxide
When hydrogen peroxide (H2O2) is added to wastewater, it oxidizes dissolved sulfide and
decomposes to water and oxygen, thus keeping conditions aerobic. Dosage rates range
from 1 to 5 Ib H2O2/lb sulfide, depending upon degree of control, wastewater
characteristics, sulfide levels and length of time involved between the injection and sulfide
control point (3).
5-5
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Equipment used for H2O2 addition is relatively simple, consisting mainly of storage and feed
components. It must be made of corrosion-resistant material since H2O2 will react
vigorously with contaminants such as iron and organic materials. It is usually fed from 50
gallon drums or from bulk storage tanks directly into the sewage and is typically available
as 35% and 50% solutions. It may be necessary to add the chemical at several points along
the sewer. Generally more H2O2 is required if sewage remains in the line for more than
90 minutes. Time required for reaction with dissolved sulfide is about 15-30 minutes and
is slower at lower sulfide concentrations, particularly below 1 mg/1. Protective gear must
be worn when handling hydrogen peroxide.
5.1.2.4 Addition of Potassium Permanganate
Potassium permanganate is a strong oxidizing agent which oxidizes sulfide. It is normally
supplied in a dry state, and is fed as a 6% solution in water. Therefore, equipment for
dissolving and feeding it must be supplied. Protective gear must be worn by personnel
handling the material. Because it is a strong indiscriminate oxidizing chemical, the dose
ratio required to achieve sulfide control can be higher than the stoichiometric weight ratio
of 6.5:1.
5.1.2.5 Addition of Chlorine
Chlorine will oxidize sulfide to sulfate or to elemental sulfur, depending on pH. It is added
at a dosage rate of 10 to 15 Ib Cl2per Ib H2S removed (3). Its effectiveness is frequently
reduced because of reactions with other components in sewage. It may be added as an
aqueous solution (sodium hypochlorite) or directly as a gas, using equipment similar to that
installed in wastewater treatment plants for effluent disinfection.
Wastewater treatment personnel are well acquainted with chlorine, and since it is already
being purchased, there may be a tendency to use it in collection systems, as well as in the
wastewater treatment plant However, application sites will often need to be located in
5-6
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residential or commercial areas; therefore, safety in these areas should be considered.
5.13 Precipitation Systems
5.13.1 Iron Salts
Iron salts such as ferrous chloride and ferrous sulfate react with sulfide to produce an
insoluble precipitate, and are added to wastewater to prevent the release of H2S into the
sewer atmosphere. Dosages are usually dependent on initial sulfide levels, but will
generally range from 4 -15 Ib Fe/Ib sulfide (4). Iron salts may be received as dry chemicals
and dissolved in water for ease of injection, but more commonly they are purchased as a
solution.
Ferrous chloride and ferrous sulfate are often purchased in bulk, usually in a 40% solution,
and being acid in nature, they must be stored in corrosion resistant tanks and fed through
corrosion resistant lines. A typical feed system involves feeding the iron solution at
multiple rates in relation to diurnal fluctuations in dissolved sulfide and flow rate.
Ferrous salts will also precipitate phosphorus compounds. When used in sewage systems,
the insoluble phosphates will be removed in the treatment plant settling tanks. Use of iron
salts may be particularly suitable for those systems where limitations exist for phosphorous
discharged in the plant effluent
5.13.2 Other Metallic Salts
Other metallic salts will also produce insoluble sulfides. In general they are more costly
than iron salts Zinc salts have been used, but since effluent standards and sludge disposal
regulations frequently include zinc limitations, these salts are not usually recommended.
5-7
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5.1.4 pH Elevation
The amount of hydrogen sulfide gas in solution is negligible at a pH above 9.0, since the
sulfide present is nearly entirely in its ionic form (HS~). However, continuous feeding of
sodium hydroxide (NaOH) to maintain an elevated pH is expensive and may disrupt
downstream treatment processes.
The most effective use of sodium hydroxide (caustic soda) is shock treatment of sewers to
produce a pH of 12.5 to 13.0 in the wastewater for a period of 20 to 30 minutes (3). Such
a high pH inactivates sulfate reducing bacteria in the slime layer for a period of a few days
to one to two weeks. The high pH slug may "have to be isolated at the WWTP and fed
slowly into the system if it is not diluted in the collection system.
Shock treatment with sodium hydroxide requires little equipment It is necessary only to
send a tank truck to the injection point (upstream as far as possible) from which liquid
caustic soda is added by gravity over a short period of time at a rate sufficient to keep the
pH elevated. As with acidic materials, protective gear should be worn by personnel
handling sodium hydroxide to avoid skin and eye contact
In some cases, it may be important to mechanically scrape the slime layer in the sewer
before performing the first treatment The caustic slugging method of treatment usually
provides less direct control of H2S because of the cyclic build-up and destruction of sulfide,
but can be cost-effective.
Another approach to corrosion control using NaOH is one developed by the County
Sanitation Districts of Los Angeles County. This involves direct spraying of the pipe crown
with caustic soda to neutralize the sulfuric acid. The process, which was still experimental
at the time this document was prepared, consists of a floating raft to which is mounted a
spray head. Caustic soda is pumped from a truck though a hose to the spray head, which
applies the chemical to the pipe crown. Preliminary data indicate that the process is
5-8
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effective and economical. After application of caustic, pH of the crown surface decreased
by an average of 0.1 pH unit per day. The Districts are proceeding with procurement of
a trailer-mounted, self-contained caustic spray delivery system (5).
5.1.5 Other Methods
5.1.5.1 Chemical and Physical Techniques
Use of sodium nitrate and formulations containing nitrate have been successfully used for
sulfide control. The presence of nitrate suppresses sulfide generation because anaerobic
bacteria preferentially use the nitrate ion before sulfate as a source of oxygen (1). A
proprietary formulation containing nitrate has been shown to be a cost-effective alternative
for sulfide control at several locations in the United States (6).
Treatment with special bacteria may suppress the action of sulfate reducing bacteria or may
promote the destruction of the slime layer. While successful in the laboratory, such
cultures have shown only limited success in the field.
Limited success has been reported in physically removing the slime layer. At high flows,
the slime layer erodes, but this is seldom relied upon as a practical control method in
existing systems. Likewise, submerging sewers may prevent corrosion in the submerged
reaches, but generation of sulfide is likely to increase due to increased detention times and
less opportunity for reaeration.
5.1.5.2 Sewer Ventilation
Two of the requirements for hydrogen sulfide corrosion to occur are the presence of
hydrogen sulfide within the sewer atmosphere and damp conditions along the sewer walls
to support microbiological activity. Sewer ventilation can minimize the potential for
hydrogen sulfide corrosion in two ways:
5-9
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1. Ventilation can reduce hydrogen sulfide concentrations in the sewer
atmosphere by dilution of the sewer air.
2. Ventilation can reduce humidify levels along sewer pipe and structure walls.
Natural sewer ventilation is provided in all gravity sewer systems, whether the ventilation
mechanism is through house vents, as is prevalent in the United States, or when building
sewers contain house traps, through special ventilation stacks as in the United Kingdom
and Australia (7)(8). Provisions for separate venting must also be provided for inverted
siphons and often flooded sewer sections (Refer to Figure 5-2). Natural sewer ventilation
occurs through any of the mechanisms listed below.
• Relative difference in air density between the sewer atmosphere and outside
air.
• Frictional drag of the wastewater at the air/liquid interface.
• Rise and fall of wastewater level within the sewer .
• Changes in barometric pressure along the sewer.
• Induced air currents caused by wind velocity past vents.
The extent of natural ventilation available within a given sewer is difficult to predict and
highly variable due to the number of factors which can affect it Thistlethwayte has
recommended a minimum of 0.05 cubic feet per minute of natural air flow for each square
foot of sewage surface to control sewer ambient humidity levels to less than 85%, though
he states that ventilation alone may not prevent corrosion (8). In addition, the discharge
of hydrogen sulfide-laden sewer air will likely lead to complaints unless significant odor
treatment is provided. For these reasons, natural ventilation is not a likely means by which
to effectively control either hydrogen sulfide concentration or humidity within a given
sewer.
Mechanical ventilation has been used with claims of at least some success at various
installations, including Austin, Texas (9), Los Angeles, California (10) and Sydney,
5-10
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X
r\
^ OTJ
fe -c S
K -1
III
"2
3-O
ti'i'i'i'i'r I'i'i1
. I'l'i'ri'i'i'tM
5-11
-------
Australia (7). At the Hyperion wastewater treatment facilities in Los Angeles, an 8-mile
long sewer was provided with fans to provide 17,000 cfm air flow to control corrosion and
maintain negative pressure within the sewer. The Austin, Texas sewer system originally
contained two separate mechanical ventilation systems which were installed in 1965; one
system serviced approximately 19,600 ft of a 42-inch influent gravity sewer to a pump
station, and the other system serviced an inverted siphon. The air from both systems was
originally discharged into aeration tanks at the treatment facilities.
Subsequent discussions with City of Austin staff indicated that the gravity sewer system was
abandoned many years ago. The siphon, commonly referred to as the Walnut Creek/Cross-
Town Tunnel, was recently discovered to have severe corrosion problems, likely as the
result of the installation of several large interceptors within the past few years. The
successful results reported prior to that may have been more the result of good engineering
practice than due to forced ventilation. The tunnel project was bored; consequently, any
dips in grade or changes in alignment were gradual. In addition, the upper reaches of the
sewer contained a cunette for self-cleansing velocities at low flow. The sewer was also lined
for a distance of 200 feet each way from drop shafts. Finally, energy dissipators were
installed at major intersections, along with air exclusion gates in combination with
adjustable air orifices.
Mechanical ventilation may have a limited effect in sewers which contain numerous house
vents, influent sideline sewers or other air sources. Also, as fresh air travels along a sewer
and becomes saturated, the rate of absorption will become less and deposition of moisture
will occur. Properly sized, a mechanical ventilation system might be capable of reducing
the relative humidity to less than 85%, but if the humidity of the outside fresh air source
is greater than 70%, more frequent air changes will be required to attain the same result
(8).
5-12
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5.2 Sulfide Control Systems Used in the United States
5.2.1 Summary of Control Systems Used by Selected Cities
The sulfide control techniques used by selected communities in the U.S. are listed in Table
5-2. In most cases, personnel in these cities are willing to provide more information to
another community to help them solve their hydrogen sulfide corrosion problems, or to
better understand the available technologies.
5.2.2 Case Histories
1. Sulfide Control with Ferrous Chloride {FeCl2) m Large Diameter Sewers -
County Sanitation Districts of Los Angeles County (4)
Ferrous chloride addition was partially effective in reducing corrosion in large diameter (54-
144 in.) sewers based on full scale testing and an interim installation for three outfalls
leading to the Joint Water Pollution Control Plant in Los Angeles County, California.
FeCI2 dosages varied for each outfall, depending on the concentration of dissolved sulfide
present as shown below: ,
FeCl? Dosage for 90% Dissolved Sulfide Removal
(November. 1988)
Estimated Ratio of
Dissolved Sulfide Level Ferrous Chloride to Dissolved Sulfide
4 mg/1 or greater 7:1
1 - 4 mg/1 15:1
less than 1 mg/1 100:1
Testing performed during full scale demonstrations included monitoring dissolved sulfide,
pH, temperature, dissolved oxygen and iron. Hydrogen sulfide levels in the sewer
atmosphere were reduced, but not in proportion to dissolved sulfide removals.
5-13
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TABLE 5-2
HYDROGEN SULFIDE AND CORROSION CONTROL SYSTEMS
USED BY SELECTED CITIES IN THE U.S
CITY
Albuquerque, NM
Altamonte Springs, FL
Antioch, CA
Austin, TX
Baton Rouge, LA
Battle Creek, MI
Bayvffle, NJ
Broward County, FL
Casper, WY
Cedar Rapids, IA
Charlotte, NC
Colorado Springs, CO
Dallas, TX
Denver, CO
Duluth, MN
El Paso, TX
Farifax, VA
Fayettevffle, AR
Fort Lauderdale, FL
Fort Worth, TX
Greensboro, NC
Hilton Head Island, SC
Honolulu, HI
Indianapolis, IN
Jefferson Parish, LA
Keene, NH
Knoxville, TX
Lake Worth, FL
Lakeland, FL
Little Ferry, NJ
Los Angeles, CA
Louisville, KY
Mesa, AZ
Milwaukee, WI
Mineola, NY
Myrtle Beach, SC
Nashville, TN
Omaha, NE
Orlando, FL
Phoenix, AZ
Pine Bluff, AR
CONTROL METHOD USED
C12 Inject, H2O2 Inject., O2 inject., KMNO4 Inject.
FeSO4 Injection
O2 Injection
Sewer Ventilation
C12 Injection
Air Injection, KMnO4 Injection
Air injection, C12 Injection
C12
NaOH Injection
NaOH Injection
H2O2 Injection
H2O2 Injection
Min. Pipe Slope. Corr. Resist. Mat'ls.
H2O2 Injection
Corr. Resist. Materials
O2 Inject, KMnO4 Inject., Corr. Resist. Mat'ls
H2O2 Injection
NaOH Addition, FeC12 Injection
Air Injection, H2O2 Injection
H2O2 Injection, KMnO4 Injection
Air Injection
FeSO4 Injection
Corr. Resist. Materials, C12 &H2O2 Injection
FeS04
O2 Injection
O2 Injection (U-tube)
C12 Injection
FeSO4 Injection
Corr. Resistant Materials
H2O2 Injection
Air Inject, NaOH Slugging, FeC12 Inject, Sewer Ventilation
FeC12 Injection
FeC12 Injection
NaOCl Injection
Flooding lines
Ozone Injection
Corr. Resistant Materials, FeSO4 Injection
FeC13 Injection, NaOH Slugging
H2O2 Injection
C12 Injection, H2O2 Injection, Air Injection
Unspecified Chemical
REFERENCE*
3
11
12
9
3
3
13
3
20
3
3
3
3
3
14
3
11
3
3
11
15
4,10,12
12,16
3
3
3
3
3
3
5-14
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TABLE 5-2 (Cont.)
CITY
Raleigh, NC
Sacramento, CA
San Antonio, TX
San Diego, CA
Seattle, WA
St. Louis, MO
Tampa, FL
Tempe, AZ
Virginia Beach, VA
West Palm Beach, FL
Yuma, AZ
CONTROL METHOD USED
C12 Inject., Corr. Resist. Mat'ls.
C12 Injection, H2O2 Injection, Air Injection
Air Injection, H2O2 Injection
Corr. Resist. Materials
H2O2 Injection, Air Injection, Corr. Resist. Mat'ls.
Air Injection, H2O2 Injection
Unspecified Chemical
Caustic slugging
C12 Inject., NaOH Inject., H2O2 Inject., Corr. Resist. Mat'ls.
H2O2 Injection
FeC12 Injection
REFERENCE*
3
12
3
3
3,17
3
3
3
12
18
j* REFERENCES - AT CONCLUSION OF THIS CHAPTER
5-15
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The District measured a one unit increase in pH on the pipe crown and estimated
corrosion at the rate of 0.2 in/yr, which was a three fold decrease in corrosion rate due to
FeCl2 addition. The District also estimated that dissolved sulfide was reduced by 70-90%
to a range of 0.2-0.5 mg/1, and H2S was reduced by 50-70% in the sewer atmosphere.
A cost of $700,000 was estimated for three installations to cover 75 miles of sewer. An
annual cost of $2.5 million was projected for 90% sulfide control.
Sulfide Control with Hydrogen Peroxide - City of Clearwater. Florida (19)
The City of Clearwater, Florida uses hydrogen peroxide (H2O2) in the collection system to
control odor and corrosion. The long force mains from the beach area to the main pump
station and wastewater treatment plant have a detention time of greater than four hours.
H2O2 is dosed at three locations as shown on Figure 5-3.
H2O2 has been used in this system since 1984. The long force mains allowed significant
amounts of dissolved sulfide (8.5 mg/1) to be generated. Corrosion also occurred in
sections of the force mains that were not flowing completely full.
The results of the sulfide control system have been to maintain dissolved sulfide at 0.4 - 0.8
mg/1 over the past several years (1986 - 1989) and H2S in atmosphere of the Bay Front
Pump Station at about 1 ppm. Dosages are reported to be averaging 1.5-1.6 Ib
dissolved sulfide.
Sulfide Control with Pure Oxygen - Jefferson Parish, Louisiana
Jefferson Parish uses direct oxygen injection systems in a large collection system comprised
of over 500 pump stations. The oxygen is injected to maintain aerobic conditions in the
system tributary to the treatment plant, to reduce sulfide levels, and to control H2S at the
plant to 5 ppm as inlet air to the odor control scrubbers.
5-16
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I i3 f
O »- C
1
75
03
OJ
1
C
3
CM
IE o a. z
/
I
u_
Si
o
5-17
-------
Each direct injection installation includes a liquid oxygen storage tank, vaporizer, control
unit and injection diffusers in the force mains. Oxygen is fed continuously, based on the
pumps running, and the dosage is increased using a timer to meet peak demands. The
systems were started up in 1988 and 1989 and are still being tested and optimized. The
current cost (August, 1990) for liquid oxygen is about $0.03/lb and the City spends about
$200,000/year on oxygen for the entire system. The largest installation is being expanded
from a 600 cfm to a 1200 cfm oxygen injection rate. One problem currently experienced
is diffuser plugging.
In the upstream areas of the collection system, ferrous chloride is added to remove existing
sulfide to supplement the control program using pure oxygen. Other corrosion control
techniques used at this location include corrosion-resistant liners in the collection system.
Sulfide Control with Caustic Slugging - Cedar Rapids. Iowa (12)
The City initiated caustic slugging in 1985 on the 5.9 mile, 84 in. and 94 in. gravity
interceptor sewer to the Cedar Rapids Water Pollution Control Facility, and has continued
with the operation through 1990. A tanker load of 50% caustic is dumped during a 20 -30
minute period in the early morning at a point well upstream of the plant
The objective is to inactivate the slime layer in the pipe to reduce sulfide levels. The pH
is elevated to about 12.5 and arrives as a slug at the plant During the early years of
operation, a primary clarifier was set aside to receive and store the material. However, now
only the pH through the plant is monitored and no negative effects have been reported.
In 1985, data were collected on the sulfide levels at the plant. The cyclic nature of the
sulfide reduction/build-up is seen in Figure 5-4. Currently, the operation is repeated every
2-3 weeks or is slugged with caustic when sulfide levels reach 2 mg/1 at the treatment plant.
The City reports that the cost of caustic has doubled since 1985 (current price is $0.19/lb
5-18
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2.5
b
s.
uj 1.5.
.V)
Q 1.0
en
3
•Caustic
injection
MAY
JUNE
JULY
AUGUST
AUGUST
SEPTEMBER
OCTOBEF
NOVEMBER
1985
RGURE5-5
DISSOLVED SULF1DE LEVELS (USING CAUSTIC SLUGGING),
MAIN INTERCEPTOR - CEDAR RAPIDS, IOWA
5-19
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as 100% NaOH), and therefore, it is considering other sulfide control strategies in
conjunction with caustic slugging, or in place of slugging. The City will initiate field
inspections for corrosion in the collection system in 1991 but reports that corrosion is not
a significant problem. The City does not store NaOH because of the need to control
temperature to prevent crystallization of the product, which occurs at approximately 40°F.
5.23 Chemical Costs
The following are typical costs for chemicals commonly used for sulfide control:
Range of Reported
Range of 1990 Dosage Rate: (Ib
Costs for Chemical per Ib
Chemical Chemicals ($/lb) H7S Removed)
Ferrous Chloride 0.08 - 0.36(*> 4-15
Pure Oxygen 0.03 - Q.W 5-10
Hydrogen Peroxide 0.34 - 0.75 1-5
Potassium Permanganate 1.23 - 1.28(d) 6-7
Chlorine 0.20 - 0.33 10-15
Sodium Hydroxide 0.18 - 0.25(0 pH = 12.5
NOTES
(a) Costs are per Ib as FeQ2 which will vary from 25-35% (specific gravity 1.3 - 1.44)
and iron content will vary from 10-15%. The range in prices is due to freight
charges as a commodity chemical. Costs will be quoted from suppliers by iron
content or per gallon.
(b) Costs are per Ib of pure oxygen. Prices are quoted per 100 cu ft and there are 8.28
Ibs O2/100 cu ft The range of prices is from very large to very small quantity of
purchases. Rental of liquid oxygen storage tanks can cost from about $500 -
$2,000/mo. (1,500 - 13,000 gal. storage tanks).
5-20
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(c) Reported as 100% H2G>2, but is purchased as 35 or 50% solution. Cost range is due
to freight charges.
(d) Reported per Ib of KMnO4 and is based on 24,000 Ib minimum annual usage. Costs
do not include freight charges which are variable.
(e) Reported per Ib of chlorine for one ton cylinders, depending on quantity purchased
and freight Chlorine costs are very low in some states.
(f) Costs are per Ib 100% NaOH, but is delivered as 50% solution. Range is dependent
upon freight charges, and is reported for bulk deliveries. Fifty-five gallon drums cost
about $0.40-$0.50/lb as NaOH.
It must be emphasized that material cost is only one element of total treatment cost. It is
necessary to add labor, capital costs, utilities and maintenance to determine total treatment
cost
S3 Procedure to be Followed in Selecting Corrosion Control Method(s)
53.1 Bench Scale Testing and Preliminary Cost Analysis of Alternatives
Using actual wastewater samples containing typical sulfide levels, jar tests can be conducted
in the laboratory to estimate pounds of chemical needed per pound of H2S removal. Such
results will not always correspond to quantities needed in the field because other reactions
are taking place, such as natural introduction of oxygen. Moreover, the slime layer which
exists in a sewer cannot be reproduced accurately in the laboratory. The laboratory tests
will serve to eliminate candidates which perform poorly and will help in sizing equipment
to be used in field tests.
As mentioned previously, reviewing control methods used in similar collection systems is
a good start for evaluating control alternatives. In discussing the problem with personnel
from other facilities, or in reviewing the literature, some systems may be initially eliminated.
Selection of viable alternatives will then depend on cost, unless other factors such as
5-21
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introduced hazards may eliminate others.
'While some chemical costs are listed in Section 5.2.3, actual cost in a particular location
depends upon such factors as quantity required, distance from a source of supply, etc.
Chemical supply representatives should be contacted for current quotations. The supplier
can often provide information on the type of equipment recommended for storage and
feeding of chemicals. A rough estimate of investment costs should be made which should
include chemical cost, capital carrying charges, labor, miscellaneous supplies, utilities, and
an allowance for maintenance.
532. Field Demonstration of Best Control and Lowest Cost .Alternatives
A field test of likely alternatives is generally mandatory. Frequently, suppliers can advise
on how their product can be tested economically, and they will lease equipment A test
site(s) should be selected for an injection point upstream of a section of the system where
hydrogen sulfide corrosion is known to be taking place.
The reaction time required for the chemical should be estimated. For example, FeCl2
requires about 20 minutes and H2O2 about 30 minutes. Select the location for chemical
addition and assess if two or more points of injection are required, because of downstream
distances or sulfide generation that is occurring downstream. Add more chemicals to
account for predicted downstream generation.
The full scale tests should be set up to provide sufficient monitoring of performance.
Therefore, analyses of dissolved sulfide, total sulfide, DO, pH, temperature, and
atmospheric H2S should be tested. In some cases, such as when using iron salts, the
wastewater should be tested for residual iron levels. Offset testing can be considered where
a theoretical slug of sewage is monitored as it travels between test locations to get a
"snapshot" of the effectiveness of the product over time and distance. Dyes are used to
verify that a sample is taken of the same slug as it travels downstream.
5-22
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Record dosages of the chemical, and using the data, prepare tables that relate each location
on the reach to (1) the dosage ratio of product to dissolved sulfide, (2) dissolved sulfide
without treatment, (3) dissolved sulfide with treatment, and (4)'sulfide reduction. Assess
the dissolved sulfide:total sulfide ratio before and after treatment Plot dissolved sulfide
and atmospheric H2S for each location with and without treatment
Perform tests of crown pH after one week to determine any changes to background levels.
Consider follow-up testing to assess rates of corrosion (in/tyr) by the sampling techniques
previously presented.
533 System Selection
Performance criteria need to be established, but are sometimes difficult to achieve or verify.
For example, to control hydrogen sulfide corrosion, the following objectives may apply to
a particular system or sulfide control program:
• Maintain DO greater than 0.5 mg/1 to eliminate sulfate reduction, or increase
ORP to + 100 mv
• Reduce dissolved sulfide to 0.1 to 0.3 mg/1.
• Reduce H2S (in sewer air) to less than 3 to 5 ppm to reduce corrosion rate.
• Increase crown pH to 4.0 or greater.
Field tests of dosages and local chemical costs are used to determine the cost per pound
of dissolved sulfide removed. Also, the performance of each product according to the
desired performance standard, is required in this evaluation. The selection of a
performance standard is site specific.
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5.4 References
1. Bowker, R.P.G., J.M. Smith, and N.A. Webster, Odor and Corrosion Control in
Sanitary Sewerage Systems and Treatment Plants. Design Manual. U.S. Environmental
Protection Agency, Center for Environmental Research Information, Cincinnati, OH,
1985.
2. "Interim Odor Control Report", report prepared by Webster Environmental Associates
for Indianapolis Department of Public Works, Indianapolis, IN, April, 1987.
3. "Hydrogen Sulfide Corrosion in Wastewater Collection and Treatment Systems",
Report to Congress: Technical Report, U.S. EPA, Office of Water, Washington, DC,
1991.
4. Won, Donna L., "Sulfide Control with Ferrous Chloride in Large Diameter Sewers",
County Sanitation Districts of Los Angeles County, November, 1988.
5. Badia, J., C-L. Chen, E. Esfandi, and W. Kimbell, "Caustic Spray for Sewer Crown
Corrosion Control," paper presented at the 64th Annual Conference of the WPCF,
Toronto, 1991.
6. Hunniford, D.J., "Control of Odors and Hydrogen Sulfide Related Corrosion in
Municipal Sewage Collection Systems Using a Biochemical Process: Bioxide,"
presented at the 63rd Annual Conference of the WPCF, Washington, DC, 1990.
7. Pescod, M.B. and Price, A.C., "Fundamentals of Sewer Ventilation as Applied to the
Tyneside Sewerage Scheme," Water Pollution Control. 1981.
8. Thistlewayte, D.K.B., The Control of Sulphides in Sewerage Systems. Ann Arbor
Science, Ann Arbor, MI, 1972.
9. Ullrich, A.H., "Forced Draft Ventilation Protects Concrete Sewer Pipe, Controls
Odor," Water & Wastes Engineering. April, 1968.
10. Joint Committee of the American Society of Civil Engineers and Water Pollution
Control Federation, Design and Construction of Sanitary and Storm Sewers. 1969.
11. Tatum, R. and DJ. Hunniford, "The Application of Odophos for Control of
Hydrogen Sulfide and Removal of Phosphorus in Wastewater Treatment Systems",
report prepared by Davis Water and Waste Industries, Inc., November, 1986.
12. "Sulfide in Wastewater Collection and Treatment Systems", Manual of Practice No.
69 ASCE, New York, NY, 1989.
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13. Baker, Jerry L., "Study of Hydrogen Sulfide Control in Wastewater Force Mains by
Chlorination".
*
14. "Odor Control Study for the City of Fayetteville, Arkansas Sewage Collection System",
report prepared by Metcalf &, Eddy, Inc., October, 1988.
15. Boyer, Karl W. and Caballero, Virgil, "Rehabilitating Lakeland's Western Trunk
Sewer", Operations Forum, July, 1990.
16. Jameel, Pervez, "The Use of Ferrous Chloride to Control Dissolved Sulfides in
Interceptor Sewers at Mesa, Arizona", report prepared by Brown and Caldwell.
17. Chu, Eddy T.H., "Corrosion Corrections", Operations Forum, July, 1990.
18. Van Dunne, Gayle P. and Berkenpas, Karla J., "Comparison of Sulfide Control
Products for a Collection System in Yuma, Arizona", 61st Annual Conference WPCF,
Dallas TX, 1988.
19. Robinson, P., "The City of Clearwater, H2S Control in a Long Force Main System,"
paper presented at meeting of South Carolina WPCA, April, 1991.
20. Schafer, P.L., and D. Rockwell, "Sulfide Control in Omaha and Cedar Rapids", ASCE
National Conference on Environmental Engineering, July 8-10, 1986.
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6. ALTERNATIVES FOR CONTROLLING HYDROGEN SULFIDE CORROSION
AT PUMP STATIONS AND TREATMENT FACILITIES
•*
There are primarily three methods by which hydrogen sulfide corrosion can be controlled;
by minimizing the generation and release of hydrogen sulfide, by the use of paints and
other protective coatings and linings and by the use of corrosion-resistant materials. These
alternatives and some of the more common examples which fall under each category are
described in the following sections.
6.1 Control of Hydrogen Sulfide
Control of hydrogen sulfide can be accomplished by chemical addition as discussed in
Chapter 5. This consists primarily of introducing materials to the wastewater to prevent
sulfide generation, to oxidize or precipitate existing sulfide compounds, or to temporarily
adjust pH to discourage sulfide generation.
Ventilation of corrosive atmospheres is generally not practiced on gravity sewer systems
(except in the case of inverted siphons). However, ventilation is common in pump station
wet wells, covered tanks and channels, though it is normally provided to minimize the
potential buildup of flammable gases and to ensure a safe working environment for human
exposure rather than for protection from hydrogen sulfide corrosion. Many of these
systems are not of sufficient capacity to maintain a fresh air supply on a continuous basis,
because human activity within the confined area is minimal and infrequent Often, only a
vent stack or pipe is provided for release of noxious gases, and no positive air inlet is
provided other than from the sewers entering and exiting the tank. In other cases where
forced mechanical ventilation is provided, the equipment operates only when a door is
opened. Even though ventilation (and particularly forced mechanical ventilation) can offer
an effective method to minimize hydrogen sulfide accumulation, this is rarely practiced, and
excessive corrosion may occur along the walls, and ceilings of these structures. Ventilation
systems should be provided to ensure positive air changes occur continuously and at a
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moderate rate. The air flows collected may in turn be treated to reduce offsite odor
impacts. In addition to providing adequate ventilation within enclosed areas, measures
should be taken to minimize turbulence by providing drop pipes or other means for
obtaining submerged inlets, thus preventing vertical liquid free fall. To prevent
accumulation of grease and scum, outlets should not be submerged in quiescent tanks
without skimmers. Grease is particularly offensive in odor, and some floating solids can
contribute to the generation of hydrogen sulfide gas. Steps should also be taken to limit
the amount of solids and debris allowed to collect on bar racks, screens, in wet wells, tanks,
and channels, as discussed in Chapter 3.
Electrical enclosures can become severely corroded by hydrogen sulfide, either by virtue
of their presence in a corrosive atmosphere, or by hydrogen sulfide migrating to the
enclosures via unsealed electrical conduit According to the Instrument Society of America
(ISA), active sulfur compounds rank with inorganic chlorides as the predominant cause of
atmospheric corrosion in the process industries (7). ISA has classified gaseous airborne
contaminants based on the rate at which corrosive contaminates react with copper. The
ISA Classification of Reactive Environments is presented in Table 6-1. The ISA
classification indicates that only the Gl - Mild rating for contaminant levels (less than 3
ppb H2S) presents an environment sufficiently well-controlled to preclude corrosion from
affecting equipment reliability. It should be noted that the classifications are listed based
on less than SO percent relative humidity. The classification should be increased one
severity level for each a) 10 percent increase in relative humidity over 50 percent, or b)
relative humidity rate of change greater than 6 percent per hour.
Four methods are frequently utilized to minimize corrosion within electrical enclosures; 1)
using gasketed enclosures to prevent entry of corrosive gases, 2) maintaining positive air
pressure within the enclosures, 3) purging with nitrogen gas, and 4) using vapor corrosion
inhibitors.
Gasketed electrical enclosures, such as NEMA 4X, which are rated watertight, dust tight
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TABLE 6-1
INSTRUMENT SOCIETY OF AMERICA
CLASSIFICATION OF REACTIVE ENVIRONMENTS
BASED ON HjS CONCENTRATIONS
Severity Level Gl (H2S < 3ppb)
Mild - An environment sufficiently well-controlled such that corrosion is not a factor in
determing equipment reliability.
Severity Level G2 (H2S < 10 ppb)
Moderate - An environment in which the effects of corrosion are measurable and may
be a factor in determing equipment reliability.
Severity Level G3 (H2S < 50 ppb)
Harsh - An environment in which there is a high probability that corrosive attack will
occur. These harsh levels should prompt further evaluation resulting in environmental
controls or specially designed and packaged equipment
Severity Level GX (H2S >. 50 ppb)
Severe - An environment in which only specially designed and packaged equipment
would be expected to survive. Specifications for equipment in this class are a matter of
negotiation between user and supplier.
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and corrosion-resistant, are commonly used in corrosive areas. NEMA 4X enclosures
require the use of conduit hubs or equivalent provision for watertight connection at the
conduit entrance, and mounting provisions external to the equipment cavity. The
enclosures must be closed properly in order to retain their rating. Commonly due to
personnel inattention, the enclosure doors are not properly secured and internal corrosion
results.
In order to offset the potential for leakage due to inattention or for added precaution, fans
with an external clean air source can be used to maintain a positive air pressure within the
enclosure. In this manner corrosive substances are much less likely to enter the enclosure.
The key elements of this type of electrical enclosure corrosion protection include a constant
power supply to ensure positive pressure is maintained by the fan, and a fresh air supply.
At wastewater treatment facilities, air free from hydrogen sulfide may not be available.
As an alternative to maintaining pressure with clean ah*, nitrogen gas can be used. With
this option, each panel, enclosure, or instrument case is connected to a low-pressure
nitrogen supply, which serves to maintain positive pressure within the enclosures and thus
*
minimizes exposure of the electrical equipment to corrosive ambient air.
Vapor corrosion inhibitors (VCI) consist of fogs, sprays, impregnated foams, plastic
emitters, strips, powders and other vehicles by which ions are dispensed to create a
molecular corrosion inhibition film. The VCI dissolves in the presence of moisture to form
a water electrolyte. It vaporizes and condenses on exposed metal surfaces, and is claimed
to be self-healing and self-replenishing. By effectively coating exposed metal surfaces
through ionic attraction, the VCI protects metals from corrosive attack. The VCI are
stated to have a useful life of up to 24 months, are compact and require no special
installation tools or activation procedures. Data on then* effectiveness against hydrogen
sulfide attack is limited to case studies provided by the manufacturer. In one instance, a
wastewater pump station was experiencing monthly call-outs to a sewage pumping station
due to corrosion occurring on the contactor armatures. Upon installing VCI, the corrosion
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stopped, provided the cabinet doors were properly closed, as protection depends on vapor
being trapped within the cabinet
Protective Coatings and Linings
Protective coatings include paints, sacrificial coatings and chemical conversion coatings,
while linings consist of polymeric materials (usually pre-cured) physically attached to the
material being protected.
6.2.1 Protective Coatings
Protective coatings can be subdivided into three generic types:
1. Metallic Coatings
2. Non-Metallic Coatings
3. Chemical Conversion Coatings
All paint and coating systems require care in the selection of type of coating system to be
used, surface preparation requirements, methods of application, wet film thickness, curing
requirements and total dry film thickness. Of particular importance with paint systems are
curing temperature and curing time prior to immersion.
All paint and coating systems are susceptible to corrosion attack. Paint and coating systems
can be attacked by any of the mechanisms listed below.
• Water, in the presence of salts, is an electrolyte and hydrolyzes certain paint
components. The paint strength and adhesive properties are reduced.
• Oils, greases and soaps contain solvents which can soften the paint or make
it more susceptible to abrasion.
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• Physical forces such as alternate wetting and drying, heating and cooling and
freezing and thawing, especially at the water surface.
• Sunlight and abrasion can similarly degrade coating surfaces.
• Hydrogen sulfide gas may be able to penetrate paints and protective coatings
which have been partially eroded by the wastewater stream and biological
activity.
• Once hydrogen sulfide gas reaches steel, the buildup of corrosion products
and gas can form blisters which rupture, thus providing a larger area for
corrosion attack.
• Hydrogen sulfide in moisture can be biologically oxidized to sulfuric acid,
which can attack through pinholes and other surface defects.
6.2.1.1 Metal Coating Systems
Metallic coating systems can be subcategorized further into galvanizing and electroplating
systems. Galvanizing provides sacrificial protection of the base metal and is commonly
performed by dipping the metal to be protected into a molten zinc or cadmium bath. The
effective service life of zinc galvanizing is directly proportional to the thickness of the
galvanized coating. Inorganic zinc coatings have also been developed and include sodium
silicate, which provides good bonding with the base metal while producing stable insoluble
corrosion byproducts.
Other metallic coating systems fall under the electroplating sub-category, whereby a thin
copper layer is first applied to the base metal, and an impervious physical barrier is
electrically bonded to the copper. Metals used in the electroplating process include
aluminum, tin, lead, nickel/copper alloys, stainless steel, nickel, chromium and silver. Of
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these, nickel, chromium and silver coatings are frequently used for corrosion protection.
6.2.1.2 Non-Metallic Coating Systems
Non-metallic coating systems consist of vinyl, epoxy and silicone resin primers and paints.
Paints can be further classified as either 1) thermoplastic, including asphaltic/coal tar and
polyethylene, which are applied hot and cure by cooling, and 2) thennosetting, such as
polyurethane and epoxy, which set by chemical reaction caused by a separate curing agent
All painting systems require great care in selection of paint type, surface preparation, wet
film and total dry film thicknesses, and curing requirements (particularly temperature and
time prior to immersion). In addition, recent volatile organic compound (VOC) regulations
have been promulgated by the Clean Air Act amendments for 1992 and beyond. This has
resulted in the modifications of paint and primer formulas. Its effect on corrosion
protection, if any, likely will not be known for several years.
The most common painting systems used for submerged environments and corrosive
atmospheres are coal tar epoxies and epoxy paints. There are probably more coal tar epoxy
coatings on concrete at wastewater treatment facilities than any other coating system
currently available. However, the County Sanitation Districts of Los Angeles County
(CSDLAC) experience has been that coal tar epoxy coatings fail within a few years when
subjected to sulfuric acid attack (5). Some have argued that the reason for failure of coal
tar epoxies are twofold: 1) that microorganisms utilize the organic sulfur within the coal
tar epoxy itself, and 2) that because coal tar epoxy has a significantly greater coefficient of
thermal expansion, high induced stress in the concrete lead to premature failure (6).
Epoxy paint systems have been shown to be effective for submerged applications on steel
and concrete. They are durable and provide excellent resistance to acids, alkalis, solvents,
abrasion and impact Upon exposure to sunlight, epoxy coatings chalk, but the integrity of
the coating system is unaffected. Epoxies can be cured by heat or by internal
polymerization with amines. Polyamides are more frequently used as a curing agent,
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however. They provide better adhesion, moisture tolerance and flexibility, and have a
higher solids content than do the amines. A disadvantage of the polyamides is less
chemical resistance than provided by the organic amine curing agents.
CSDLAC is conducting continuing studies to evaluate the effectiveness of various coating
used for protecting concrete from sulfuric acid attack. The pilot scale study involves
subjecting the coating, which is applied by the vendor to damp concrete, to a solution of
10 percent sulfuric acid. The study cited several formulations of polyester resin, vinyl ester,
coal tar epoxy, epoxy and specialty concrete as having excellent bonding characteristics to
both corroded and uncorroded concrete, as well as to themselves, and also excellent
resistance to sulfuric acid attack (5). It must be noted that all of the successful coating
systems consisted of sand-extended resin systems (polyester, vinyl ester, epoxy and coal tar
epoxy) that were applied as a three-step process (primer, sand-extended thickened coating
and top coat) to a thickness of 120 - 150 mils. For instance the same product which scored
an excellent rating when used as, an extended sand system scored very poorly when used
without the sand (reference test numbers 17 and 57 in Table 6-2.) Table 6-2 summarizes
the coating systems evaluated by CSDLAC, and their performance.
When dealing with metals, consideration of coating systems must also include proper
selection of primers. Two basic types of primers exist; 1) inhibitive primers and 2) barrier
primers. Inhibitive primers include zinc chromate and red lead. Zinc chromate is not
recommended for use in acidic or immersed environments. Red lead primers are alkaline
and thus can neutralize acid agents. They also react with oils to form dense, low-
permeability films. Inorganic zinc primers are very effective and an excellent choice for use
in immersion service.
6.2.13 Chemical Conversion Coatings
Chemical conversion coatings cause a chemical reaction to occur between the coating
material and the base metal such that the coating becomes an integral part of the metal.
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Phosphate coatings are produced by reaction of the base metal with a phosphoric acid
solution containing iron phosphates and zinc, iron or manganese. These coatings are
normally used to provide better adhesion of paint
6JL2 Linings
Linings typically consist of pre-cured polymeric materials such as polyethylene, polyvinyl
chloride, polypropylene and glass-reinforced plastic. Liners generally can be supplied in
either of two forms; sheet goods or thin-walled forms.
Segmented polyvinyl chloride, polyethylene, fiberglass-reinforced plastic, fiberglass-
reinforced cement and interlocking polyvinyl chloride strips are all fully described in
Chapter 7.
According to a study performed by the County Sanitation Districts of Los Angeles County,
one polyvinyl chloride sheet liner demonstrated no reaction to acid, but the non-welded
jointing systems allowed acid to seep behind the liners and corrode the concrete. Where
this type of liner is to be used, the CSDLAC recommends the use of welded PVC joints,
detailed construction inspection and spark testing (5).
As an alternative to mechanically anchoring liners to concrete and relying on suitable joints,
the use of an acid resistant mastic that bonds the liner material to concrete can be used.
This type of system was tested by CSDLAC and is now specified by the Sanitation Districts
for rehabilitation projects. It has also been test applied in a gravity sewer by the Seattle
Metropolitan Sewer District
New construction or replacement allows for the use of PVC liners that are formed in place
with the concrete. Proper welding of the liner joints is critical in eliminating the potential
for seepage to occur between the liner and concrete. The use of PVC liners is more fully
discussed in Chapter 7.
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63 Replacement with Corrosion-Resistant Materials
Materials often found in wastewater treatment facilities and pump stations that are prone
to hydrogen sulfide corrosion include cast iron, ductile iron, steel, copper, asbestos cement
and concrete. Section 6.2 discussed the types of protective coatings and liners available to
extend the useful life of materials subject to corrosion. However, many of these
components will eventually fail or otherwise require rehabilitation or total replacement
When replacement does occur, the use of corrosion-resistant materials should be strongly
considered. The use of such materials will likely increase the beneficial life of these
components substantially. The following paragraphs describe the types of corrosion
resistant materials available.
63.1 Corrosion-Resistant Metals
Several metals are available that can provide effective corrosion-resistant service. These
metals range from coated materials such as are obtained by galvanizing and electroplating
(see Section 6.2.1.1), to stainless steel, aluminum and more exotic metals.
63.1.1 Copper Alloys
Copper can be readily attacked by H2S, which turns the surface of the metal black (copper
sulfide). However, copper alloys, such as brass and bronze, can offer good protection.
Brass is a copper alloy containing from 5 to 45 percent zinc, and protects the copper by
selective corrosion of the zinc. However, this leads to increased porosity and a loss of
structural strength of the alloy.
Bronze can contain either tin (up to 12%), aluminum (up to 10%), or silicon (up to 4%).
Because bronze does not undergo selective corrosion, it is generally stronger and harder
than brass. Aluminum bronze is the most resistant to H2S and acid attack, and silicon
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bronze is resistant, particularly to hydrochloric and sulfuric acid, alkalies and some organic
compounds.
63.1.2 Nickel Alloys
Nickel and nickel alloys provide superior resistance to corrosion, and are stronger and
harder than copper or aluminum alloys. Depending on desired properties and corrosion
resistance, nickel alloys may contain other metals such as copper, silicon, chromium, iron,
and molybdenum.
63.13 Stainless Steel
Stainless steels are alloys containing over 11.5 percent chromium and possibly nickel (6 to
22 percent). They provide excellent corrosion resistance as well as resistance to organic
and inorganic acids and alkalies. Stainless steels are susceptible, however, to halides,
seawater and oxidizing chlorides. Stainless steels are classified by three basic crystal
structures; martenistic, ferretic and austenitic.
Of the three crystalline structures, austenitic stainless steel is used in corrosive
environments. More specifically, Type 304L and Type 316L are low carbon steels (less than
0.03 percent carbon) routinely used in corrosion applications. The main difference between
these two stainless steel types is that Type 316L contains 2-3 percent each of molybdenum
and nickel, and has less chromium than 304L. Type 316L is more resistant and better
suited to sulfuric acid environments.
63.1.4 Aluminum
1
Aluminum is used widely in the wastewater treatment field because of its light weight,
strength and resistance to hydrogen sulfide corrosion. It is not affected by methane, carbon
dioxide or sulfur dioxide. However, aluminum is susceptible to galvanic corrosion, and
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must be physically separated from dissimilar metals and concrete. Frequently the physical
separation consists of an asphaltic coating, stainless steel or neoprene washers. Aluminum
is also subject to attack by acids, salts and aggressive water.
Corrosion-Resistant Concrete
Concrete can be rendered more resistant to sulfide corrosion attack in the following ways:
1. through the use of high alumina or silica cements in lieu of Portland cement,
or
2. through the use of calcareous aggregate instead of granitic aggregate.
When cements containing aluminum are used instead of the more common calcium
hydroxide, the alumina gels and coats the hydrates. The alumina does not dissolve readily
until pH drops below 4.0, so a concrete mixed with high alumina cement can provide an
increased level of sulfuric acid corrosion protection in those instances where the pH will
not be too acidic. However, pH of concrete walls may often drop to 1.0 or 2.0 in enclosed
and unventilated areas, such as sewers and enclosed tanks. High alumina cement content
concrete can actually be attacked more quickly than Portland cement at these very low pH
levels.
Sodium silicate has also been used as an inorganic cement substitute. These are presumed
to fail under two conditions. First, the sodium, which is alkaline, reacts with acidic sulfur
compounds to form sodium sulfate. Upon hydration, the cement expands and failure
occurs. Secondly, when used as a coating over Portland cement concrete, microorganisms
enter the porous lining and continue to attack the original concrete substrate (6).
Potassium silicate is claimed to be superior to other inorganic cements due to its resistance
to acids and its resistance to spelling in the presence of sulfuric acid. Surface preparation
consists of water or abrasive blasting, application of a moisture-tolerant primer (if the
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concrete cannot be dried sufficiently), crack repair by epoxy injection, and pH analysis of
the original concrete surface by petrographic examination. pH adjustment of the concrete
may be required prior to application of the new coating system (the pH of unattacked
concrete should be 11 or greater). Fresh concrete can be applied to obtain a good base,
and then top coated. Anchors are preferred over wire mesh for the new concrete substitute
onto which two coats of a urethane-based membrane is applied. The 100% potassium
silicate-bonded concrete is then usually applied by the gunite method. The urethane-based
membrane wet film thickness should be in the range of 63 to 130 mils (60 to 125 mils dry),
applied by airless spray at a pressure of approximately 4,000 psi. The potassium silicate
cement concrete consists of a binder, a catalyst and the aggregate. Some suppliers provide
the binder and catalyst combined as a powder. Because the dried potassium silicate powder
is very slowly soluble in water, concrete from these one-part mixes are generally of a low
strength. Application thickness is generally 1% to 3 inches. Rate of application is on the
order of 1,000 square feet per 8-hour shift, and curing time can be as short as 24 hours
(when temperature is in excess of SOT). The cities of Orlando and Jacksonville have
utilized this concrete product for a period of up to 7 years in grit chambers, as has Tampa,
Florida for a junction chamber (6). It should be noted that potassium silicate concrete is
actually quite porous, and that the corrosion protection may be from the urethane coating
rather than the concrete itself.
Changing the aggregate from granitic to calcareous can also be used as a deterrent to
hydrogen sulfide corrosion by providing additional alkalinity of the concrete in order to
neutralize the sulfuric acid formed.
633 Corrosion-Resistant Synthetic liners
Synthetic materials generically include plastics, ceramics and elastomers. Many of these
products are unaffected by sulfuric acid.
Plastics are identified as either thermoplastics or thermosetting plastics. Thermoplastics
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can be heated to a plastic state, molded, cooled, reheated and remolded. Thermoplastics
are not normally used at temperatures greater than 65°C (150°F). Polyvinyl chloride,
polyethylene and vinyl are all thermoplastics. Unlike thermoplastics, thermosetting plastics
cannot be remolded due to the chemical reactions that occur upon initial heating.
Polyesters, epoxies and phenolics are all considered thermoplastics. Some thermoplastics
can be used at temperatures up to 150°C (SOOT).
Fiberglass reinforce plastic (FRF) is fabricated by impregnating interwoven continuous glass
filaments with a thermosetting resin, and heat curing. Typical FRP uses at wastewater
treatment facilities are piping, grating and stair treads. In addition fiberglass reinforced
with a thermosetting polyester or vinyl ester resin is manufactured to produce numerous
corrosion-resistant, non-conductive structural shapes and sheets. These materials are
actually stronger on a pound-for-pound basis than is structural steel. Uses include
structural elements, railings, weirs and baffles.
Ceramics are also excellent for use in corrosive areas, but they remain brittle.
Elastomers such as neoprene, butyl, isoproprene and others are commonly used as sealants.
Neoprene is resistant to oils, grease and other contaminants, and is often used at
wastewater treatment facilities.
6.4 Case Studies
The following paragraphs illustrate corrective actions taken at several wastewater treatment
facilities. Refer to Appendix A.2 (case studies) for more information concerning
rehabilitation/replacement techniques employed at selected wastewater treatment facilities.
The Hooker's Point Wastewater Treatment Plant, located in Tampa, Florida, currently
processes 54 mgd and has a design flow of 60 mgd. The facility was expanded in 1978, and
a sludge dryer and pelletizer were started up in 1990. The treatment plant has experienced
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severe corrosion and expends significant resources to combat hydrogen sulfide corrosion,
including a fine-mist scrubber installed to treat H2S-laden air emissions at the influent
junction box. The scrubber was installed at a capital cost of $1,000,000, and annual
operating cost is estimated at $400,000. Table 6-3 summarizes the corrective actions taken
by the City of Tampa to combat corrosion at the Hooker's Point WWTP.
The Hyperion Wastewater Treatment Plant, which serves the City of Los Angeles,
California, is designed for 150 mgd through secondary treatment and 400 mgd through
primary treatment The treatment facility is approximately 40 years old and is currently
undergoing massive reconstruction due to changes in regulations which have eliminated
ocean sludge disposal and mandated secondary treatment Many of the facility's unit
processes are being upgraded or replaced, but severe corrosion remains evident in several
areas. The corrective actions at Hyperion are summarized in Table 6-4.
The Terminal Island Wastewater Treatment Plant, which also serves the City of Los
Angeles, provides secondary treatment for 30 mgd. This facility was constructed in 1935
and upgraded in 1977. Its flows and loads are 50 percent and 70 percent industrial,
respectively. Similar to the Hyperion facility, most of the preliminary treatment area is
covered and not easily observable. The corrective actions taken to combat corrosion at
Terminal Island are summarized in Table 6-5.
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TABUE 6-3
SUMMARY OF CORRECTIVE ACTIONS TO COMBAT
HYDROGEN SULFIDE CORROSION AT HOOKER'S POINT WWTP
ITEM
INFLUENT JUNCTION BOX
Concrete walls
Carbon steel parts
Electrical components, outlets
Aluminum parts
Atmospheric H2S
CORRECTIVE ACTION
Plastic liner
Replaced with stainless steel where
possible
Covered or replaced with plastic
Replaced with fiberglass
Installed scrubber
PRIMARY CLARIFIERS
Moving parts, including scraper
mechanism
Electrical/mechanical components
Concrete walls
Replaced with plastic
Covered with corrosion-resistant materials
No action yet, but considering plastic liner
SLUDGE HANDLING
Metals
INSTRUMENTATION & CONTROLS
Instruments
Cabinets
Control Rooms
Contacts and Relays
Transformer housings
Replaced with galvanized and stainless
steel
Covered
Purged with clean air
Air conditioned
Cleaned regularly
Periodically replaced
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TABLE 6-4
SUMMARY OF CORRECTIVE ACTIONS TO COMBAT
HYDROGEN SULFIDE CORROSION AT HYPERION WWTP
ITEM
HEADWORKS TANKS & CHANNELS
(COVERED)
Concrete covers and walls above
waterline
Building enclosures
Piping
Electrical conduit
CORRECTIVE ACTION
Channels to be rehabilitated or replaced
using PVC liners. Tanks will be coated
with coal tar epoxy or other material.
Negative pressure maintained by fans and
discharged to the secondary process
blowers.
Mostly PVC; air handling ducts are
fiberglass
PVC or aluminum
PRIMARY CLARIFIERS (COVERED)
Upper concrete walls
Channels
Steel chains
Wood sludge rake boards
Grouting only (earlier epoxy coatings have
failed)
Walls to be lined with PVC. Covers are
aluminum plate
Replaced with plastic
Replaced with fiberglass
INSTRUMENTATION & CONTROLS
Field-mounted instrumentation
Control room atmosphere
Purged with nitrogen
Scrubbed, filtered and air conditioned
6-20
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TABLE 6-5
SUMMARY OF CORRECTIVE ACTIONS TO COMBAT
HYDROGEN SULFIDE CORROSION AT TERMINAL ISLAND WWTP
ITEM
HEADWORKS
Concrete walls
Metallic covers
Handrails
Conduit
Bar screen sheet metal
Steel bar screen frame
Building enclosures
Piping
Electrical conduit
CORRECTIVE ACTION
No action (negligible corrosion)
Aluminum deck plate
Aluminum
Aluminum
Replaced with sheet PVC
To be replaced with stainless steel
Negative pressure maintained by suction
through the secondary process blowers
and discharge to aeration basins
Mostly PVC; air handling ducts are
fiberglass
Aluminum or PVC
PRIMARY CLARIFIERS
Clarifier covers
Influent/effluent channel cover plates
Steel chain
Wood sludge rake boards
Aluminum
Aluminum
Replaced with plastic, but due to
problems with chain jumping sprockets,
reverting back to steel
Replaced with fiberglass
6-21
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TABLE 6-5 (cent)
SUMMARY OF CORRECTIVE ACTIONS TO COMBAT
HYDROGEN SULFIDE CORROSION AT TERMINAL ISLAND WWTP
ITEM
ANAEROBIC DIGESTERS
External gas collection pipe
CORRECTIVE ACTION
Replaced with welded stainless steel
Motorized valves, copper grounding wire
(bare) and elevator
INSTRUMENTATION & CONTROLS
Field-mounted instrumentation
Control room atmosphere
Corrosion may be due to salt air
Purged with nitrogen
Scrubbed, filtered and air conditioned
6-22
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6.5 References
1. "Cost Effective Corrosion Protection," and "VCI Emitting Systems," Cortec
Corporation, 310 Chester Street, St Paul, Minnesota.
2. Sulfide in Wastewater Collection and Treatment Systems. ASCE Manual of Practice
• No. 69, American Society of Civil Engineers, 1989.
3. Bowker, R.P.G., J.M. Smith, and N.A. Webster, Odor and Corrosion Control in
Sanitary Sewage Systems and Treatment Plants. Design Manual. U.S. Environmental
Protection Agency, Center for Environmental Research, Cincinnati, OH, 1985.
4. Hydrogen Sulfide Corrosion in Wastewater Collection and Treatment Systems.
Report to Congress. U.S. Environmental Protection Agency, Office of Water,
Washington, DC, 1990.
5. Redner, J.A., R.P. Hsi, and E J. Esfandi, "Progress Report -Evaluation of Protective
Coatings for Concrete," paper presented at EPA Technology Transfer Seminar on
"Sewer System Infrastructure Analysis and Rehabilitation," 1991.
6. Hall, G.R., "Potassium Silicate Concrete for Restoring Wastewater Treatment
Systems," Journal of Protective Coatings and Linings. Volume 4, No. 8, Steel
Structures Painting Council/Technology Publishing Company, August, 1987.
7. Standard - Environmental Conditions for Process Measurement and Control
Systems: Airborne Contaminants. Instrument Society of America, ANSI/ISA-
S71.04, 1985.
6-23
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7. REHABILITATION OF CORRODED SEWERS
7.1 Rehabilitation and Repair Techniques
Rehabilitation of corroded sewers can be implemented by many techniques. This chapter
examines each of the generic methods and provides guidance on the conditions under
which each method is applicable. For each rehabilitation method, a general description is
provided, and procedures and equipment are discussed. It should be noted that new
techniques for sewer rehabilitation are continually being developed. While all major
generic rehabilitation techniques have attempted to be included, specific proprietary
systems have not been evaluated. ..
Table 7-1 lists common corrosion problems in sewer pipes and applicable rehabilitation
method(s) for each.
7.1.1 Insertion Renewal (Sliplining)
7.1.1.1 Description
Insertion renewal, or sliplining, is used to rehabilitate sewers by pushing or pulling a
flexible liner pipe of slightly smaller diameter into an existing circular pipeline and then
reconnecting the service connections to the new liner. By fusing sections together during
installation, the liner forms a continuous, water tight length within the existing pipe.
Pipe insertion techniques can be used to rehabilitate sewer, water and other lines that may
have severe structural problems such as extensively cracked lines, lines in unstable soil
conditions, deteriorated pipes hi corrosive environments, and pipes with massive and
destructive root intrusion problems. Advantages and disadvantages of sliplining as a
method of rehabilitation can be found in Table 7-2 (1).
7-1
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TABLE 7-1
SUMMARY OF APPLICABLE PIPE REHABILITATION METHODS
FOR PIPE DAMAGED BY HYDROGEN SULFIDE CORROSION
Problem
Rehabilitation Method
1. Pipe is seriously damaged or collapsed
a. Excavation and replacement
2. Poor structural integrity caused by
corrosion
a. Excavation and replacement
b. Insertion renewal
c. Some specialty concretes
3. Severe corrosion but reinforcing steel
not exposed
a. Insertion renewal
b. Cured-in-place inversion lining
c. Excavation and replacement
4. Damaged pipes under structures, large
trees, or busy streets
a. Insertion renewal
b. Cured-in-place inversion lining
5. Severe corrosion in noncircular pipes
a. Cured-in-place inversion lining
b. Excavation and replacement
6. Mildly deteriorated structure
a. Insertion renewal
b. Cured-in-place inversion lining
c. Liners
d. Specialty concrete
Note: When replacement is the only feasible option, consideration of new alignment may
be warranted to avoid excavation of existing utilities.
7-2
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TABLE 7-2
ADVANTAGES AND DISADVANTAGES OF SLIPLINING (1)
ADVANTAGES
Minimal disruption to traffic and urban
activities (as compared to replacement)
Minimal disturbances to other
underground utilities; affects only those in
the vicinity of access pits
Less costly than replacement
Rapid installation
Good protection against acid corrosion
May not require bypassing
Wide, range of pipe sizes (i.e., 3 to 144
inches)
Can be used" to rehabilitate pipelines with
severe corrosion
Stops leaks and root intrusion
DISADVANTAGES
Possible reduction in pipe capacity,
depending on wall thickness required
Requires excavation of an access pit
Not applicable to sewers with numerous
curves, bends, offset joints or protruding
service taps
Requires obstruction removal prior to
sliplining
Grouting of annular space may be
required
7-3
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Materials frequently used to slipline sewers include polyethylene, thermoset plastic,
pofyvinyl chloride and ductile iron pipe.
Non pressure-rated polyethylene pipe is primarily used for sewer relining applications. PE
pipe is available in sizes ranging from 4 through 48 inch diameter, and jointing is commonly
by butt-fusion, though fittings can be used.
Thermoset plastic pipe, which includes reinforced thermosetting resin (RTR) and
reinforced plastic mortar (RFM), is manufactured with reinforcement such as fiberglass
embedded in a thermosetting resin. Thermoset plastic pipe is available in sizes ranging
from 8 through 144 inches, and a number of jointing methods are available.
Polyvinyl chloride pipe is manufactured by extrusion of plastic. PVC pipe Is available in
sizes ranging from 4 thorough 27 inch diameter, and jointing is commonly elastomeric seal
gasket with bell-and-spigot connection.
Ductile iron pipe is manufactured by adding cerium or magnesium to cast iron prior to
casting. DIP is available in sizes ranging from 3 through 54 inch diameter, and various
jointing methods are available. Interior and exterior coatings are available.
7.1.1.2 Procedures and Equipment
Prior to sliplining, the sewer or water main should be first be inspected by closed circuit
TV to identify all obstructions such as displaced joints, crushed pipes and protruding service
laterals. The inspection also should locate all service connections that will need to be
connected to the new liner pipe. The pipe must be thoroughly cleaned. Proof testing the
existing pipe by pulling a short piece of liner through the sewer section should be
performed prior to sliplining. Sliplining generally will require excavation of an access pit
from which to work from, though some sliplining techniques can operate from a manhole.
Protection of sliplining access pits is critical to prevent trench collapse which could occur
7-4
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as a result of sewage flooding or groundwater pressure.
Sliplining is performed by either a push or a pull technique; both methods are illustrated
in Figure 7-1. In the pull method a pulling head is attached to the end of the pipe. A
cable is run from the termination point to the access point and is connected to the pulling
head. The slipline pipe is then pulled through the existing pipe with the cable by a truck
mounted winch assembly.
The push technique is similar, except a backhoe or other suitable equipment is used to push
the liner through the pipe. For most insertion projects it is not necessary to eliminate the
entire flow stream within the existing pipe structure. Actually, some amount of flow can
assist positioning of the liner by providing a lubricant along the liner length as it moves
through the deteriorated pipe structure. The insertion procedure should be timed to take
advantage of cyclic periods of low flows that occur during the operation of most gravity
piping systems. During the insertion process, which often takes a period of 30 minutes or
less per length of pipe, the annular space between the sliplining pipe and the existing pipe
can usually carry sufficient flow to maintain a safe level in the operating section of the
system being rehabilitated during low flow periods (2). Sewer flow gauging and a
calculation of the hydraulic capacity of the annular space should be performed to ensure
that no upstream flooding will occur during the insertion process.
Once the slipline pipe has been installed in the existing deteriorated pipe, it is grouted in
place. Grouting at each manhole connection is required, and grouting of the entire length
of the pipe may be required if the liner cannot support loads resulting from the collapse
of the original pipe. The severity of structural deterioration and anticipated hydrostatic and
structural loadings must be considered in evaluating the need for grouting. Grouting
provides the following advantages:
- provides structural integrity
- increases hydrostatic and structural loading capacities of the pipe
7-5
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"PULL" INSERTION TECHNIQUE
.WINCH ASSEMBLY
RAMP FOR TWO-WAY
INSERTION'
MINIMUM OF
12 X LINER
DIAMETER
, LINER PIPE
/ EXISTING PIPE-' ' ' "XcMLE ATTACHED
DEMOTE MANHOLE TO GUIDE CONE
OR ACCESS PIT
PIPE SUPPORT ROLLER'
•TUSH" INSERTION TECHNIQUES
WINCH ASSEMBLY
EXISTING PIPE
JOINING MACHINE V MINIMUM OF STANDARD PIPE LENGTH
//
REMOTE MANHOLE
OR ACCESS PIT
TO PUSH PLATE
PUSH PLATE
—*.
CONE "^ CABLE PASSING ^*-PIPE
THROUGH LINING SUPPORT
EXISTING PIPE «« ANCHORED
LINER PIPE
ROLLER
FIGURE 7-1. EXAMPLES OF BASIC SLIPLINING TECHNIQUES
" (from Reference 2)
7-6
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- prevents liner from moving
- locks in service connections
- extends life of pipe from uncertain to 50 years plus
- provides support to liner when cleaning
Equipment required for the insertion of the sliplining pipe are: joining equipment, pulling
or pushing head, winch, rollers, proofing tool, grout tank and pump. The joining
equipment is used to join segmented pipe length to a continuous pipe of desired length.
This is done by aligning the two pipes together, heating the ends and butting the ends
together. The pulling head is used to facilitate the pulling of the pipe into the sewer. One
end of the pulling head is attached to the pipe to be pulled while the other end is attached
to the pulling cable. The winch, consisting of a power operator and a pulling cable, is used
to pull the pipe. The rollers are used to hold the liner in place in order to grout the
annular space between the pipe and manhole connections.
7.1.2 Deformed Pipe Insertion
This technique involves use of a thermoplastic polyethylene pipe extruded as a round pipe.
Using a thermochemical deforming process, the liner, which is manufactured in various wall
thicknesses, is deformed into a U-shape. The deformed pipe is then pulled through the
existing sewer pipe with minimal friction. Once the liner has been set in place, a patented
process involving use of heat and pressure restores the liner to its original round shape for
tight fit, leaving no annular space. Virtually all of the original pipe's flow capacity is
restored (1)(4)(5).
The liners can be installed in continuous lengths. Lateral connections are cut from within
the pipe using a video-monitored milling apparatus. The operation of inserting the liner
is as follows:
First the corroded line is cleaned, then the deformed line is pulled inside the
7-7
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pipe.
The liner is cut at such a length that it can be shaped to form its own integral
seal at each manhole joint or terminal.
A thennomechanical process is used to reform the compressed liner to its
original round extruded shape. This may be done with heated liquid or in
combination with a thermally controlled pig.
Once in place and property reexpanded and flanged, the inserted liner is
tested in conformity with A.P.I./D.N.V. standards.
The following advantages are offered by the use of deformed pipe insertion techniques:
Long continuous sections are possible (lengths may range up to 1,000 feet).
No annular spaces are formed; no grouting is required.
Fast installation and thus minimum downtime.
Improved hydraulic characteristics possible.
A variety of wall thicknesses are available to suit differing needs.
Figure 7-2 indicates a typical deformed pipe insertion technique.
7.13 Cured-in-Place Inversion Lining
7.13.1 Description
Inversion lining is formed from a resin-impregnated felt tube which is inverted under
pressure against the inner wall of an existing sewer and allowed to cure. The pliable nature
of the resin-saturated felt prior to curing allows installation around curves, filling of cracks,
bridging of gaps, and maneuvering through pipe defects. After installation, the fabric cures
to form a new pipe of slightly smaller diameter, but of the same shape as the original pipe.
The new pipe has no joints or seams and has a very smooth interior surface which may
actually improve flow capacity despite the slight decrease in diameter (1)(2)(3)(6).
7-8
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Winch Pulls Pipe
Steam, Pressure,
Temperature
Instrumentation
Power. Steam/Pressure Generation.
Studio Operating Room
Hydraulic Advantage
Gradual Transition
Lateral recognized
tor Remote Cutting
Steam,
Pressure
Injector
Restored Broken Pipe
End Restrained
FIGURE 7-2. EXAMPLE OF DEFORMED PIPE INSERTION TECHNIQUE
7-9
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Two resin types (polyester and epoxy) are widely used in this method of pipe rehabilitation.
Both these resins are liquid thermosetting resins, and have excellent resistance to domestic
sewage and sewage gases.
Inversion lining is successful in dealing with a number of structural problems*, particularly
in sewers needing minor structural reinforcement Inversion lining can be accomplished
relatively quickly and generally without excavation, though insertion pits may be required
for larger pipe sizes. This method of pipeline rehabilitation is thus particularly well suited
for repairing pipelines located under existing structures, large trees, or busy streets or
highways where traffic disruption must be minimized. This method is also effective in
correcting corrosion problems and can be used for misaligned pipelines or in pipelines with
bends or non-uniform cross sections. Table 7-3 provides a summary of the advantages and
disadvantages of sewer rehabilitation by cured-in-place lining (1).
7.13.2 Procedures and Equipment
Installation of cured-in-place inversion lining is carried out by inserting the resin
impregnated fabric tube (turned inside out) into the existing pipe line. It is then cured in
place through the use of heated water or air/steam. Prior to the installation of the liner,
the pipeline section must be cleaned to remove loose debris, roots and solids. The
installation procedures are illustrated in Figure 7-3. The pipeline segment must be isolated
from the system by bypassing flows during the installation of the inversion lining. The
inversion felt tube liner is usually inserted from existing manholes and valve structures, but
for larger pipe sizes, insertion pits may be required. Following curing of the liner, the ends
are cut and sealed. Service connections are restored by a remotely-operated cutting tool
and video camera (1)(2).
7-10
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Table 7-3
ADVANTAGES AND DISADVANTAGES OF CURED-IN-PLACE LINING (1)
ADVANTAGES
DISADVANTAGES
Applicable to all shapes
Rapid installation
Minimum traffic disruption
Excavation normally not required
In-line lateral reconnections can be made
without excavation
Improved hydraulics
Bridges gaps and misaligned joints
Custom designed wall thickness
Adds some structural integrity
Stops leaks and root intrusion
Pipe grouting not required
Bypassing required
Post-installation remote camera inspection
required
Largest size pipe rehabilitated to date is
96"
Maximum unrestrained pressure of 75 psi
Not recommended for pressure gas
applications using current material and
designs
Curing time can approach or exceed 24
hours
7-11
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FIGURE 1
RESIN
IMPREGNATED
mSITUTUBE'
INVERSION TUBE
/ I I
t/MANHOLE
FIGURE 7-3. CURED-IN-PLACE METHOD
7-12
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7.1.4 Liners
3 it- & s
7.1.4.1 Description
Liners include prefabricated panels or flexible sheets fastened to existing structures with
anchor bolts or concrete penetrating nails, continuous plastic strips which interlock to form
a liner, or liners which are applied to the external portion of the pipe and then encased in
concrete. The following types of liners are available for rehabilitation purposes
• Segmented PVC liners
• Segmented PE liners
• Segmented fiberglass reinforced plastic liners
• Segmented fiberglass reinforced cement liners
• Continuous interlocking PVC strips
PVC liners are manufactured from acid-resistant, rigid unplasticized PVC which has
excellent resistance to acids and better hydraulic characteristics than concrete. The liners
are pin-hole free, forming an effective barrier to gaseous penetration. PE liners are also
similar to PVC liners but are made of polyethylene resins. Fiberglass reinforced plastic
liners consist of a composite of fiberglass and acid-resistant resin. The resins are specified
according to the degree of acid resistance required. Polyethylene liners are tough, rigid,
acid-resistant, smooth and relatively inexpensive, and are manufactured in a wide range of
wall thicknesses.
A relatively new development in liner technology for small diameter (<30 inch) pipe
involves use of a winding machine to interlock continuous PVC strips into a circular tube.
The winding machine is positioned in a manhole or other access point and the tube is
inserted directly into the pipe. The annular space is then grouted.
7-13
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liners do not provide any structural support but they provide an adequate corrosion barrier
and smooth lining for structurally sound sewers. These liners have little absorption
capability and no apparent permeability. These types of liners can be used in non-circular
sewers and can be segmented to fit the diameter required. Advantages and disadvantages
of internal liners as a sewer rehabilitation technique are listed in Table 7-4 (1).
7.1.4.2 Procedures and Equipment
After a thorough line cleaning and dewatering, segmented liners are installed in four foot
lengths which overlap at the joints. The flanges on the segments may be predrilled for
screws or concrete nails. Space is allowed between the existing surface and the liner for
grouting. Joints are coated with an adhesive, and sealed with an acid resistant resin. After
all the panels are set in place, the entire section is cement pressure grouted in place to
prevent sagging and deformation.
Another lining technique involves excavation to the damaged pipe, application of external
PVC sheets around that portion of the pipe above the waterline, and encasement of the
liner and upper portions of pipe in concrete. This approach allows the existing pipe to
remain in service during rehabilitation. However, excavation requirements may be
significant
Installation of strip-type liners requires a manhole to accommodate an access pit that would
fit the special winding machine that joins a male and female PVC strip within the access
pit to form the liner pipe. Another liner installation method uses an acid resistant mastic
to fasten the sheets directly to the sewer concrete surface. This technique does not require
grouting but requires through cleaning prior to installation (4)(S).
7-14
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TABLE 7-4
ADVANTAGES AND DISADVANTAGES OF INTERNAL LINERS (1)
ADVANTAGES
Material cost inexpensive
Linear materials have very good acid
resistance
No disruptions to traffic as installation is
performed entirely in-line
Smooth surfaces provide good hydraulics
DISADVANTAGES
Applicable only to man entry size sewers
(i.e., 2.5 feet or greater)
Susceptible to leakage due to number of
joints
Timely to install. Thus total project cost
maybe uneconomical because of
installation
Prolonged bypass required
Surface preparation required
PE can crack in areas of turbulent flow
7-15
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7.1.5 Specialty Concrete
7.1.5.1 Description
Specialty concretes containing additives such as potassium silicate have shown greater
resistance to acidic attack on sewer pipes and manhole structures than standard concrete
mixes. Specialty concrete is used to repair deteriorated concrete pipes and structures by
applying an acid resistant coating over the original surface. Concretes containing potassium
silicate are unique in that their matrix is not formed by a hydration reaction. Rather, they
are the result of the reaction of an acid reagent with an alkaline solution of a ceramic
polymer of potassium silicate (7). Specialty cements can resist attack by many substances
like mineral salts, mild solutions of organic and mineral acids, sugar solutions, fats and oils.
Applicability of specialty concrete depends on the degree of corrosion-related deterioration
and the structural integrity of the unit in question. Thin film specialty concrete is
applicable to mildly deteriorated pipes or structure, whereas an elastic membrane concrete
system is applicable to all cases. After curing, the specialty concrete bond;; firmly to the
original surface. The new acid-resistant layer, if applied and cured properly, extends the
useful life of the structure. Advantages and disadvantages of specialty concretes are listed
in Table 7-5 (1).
7.1.5.2 Procedures and Equipment
Specialty concretes are available in three types: cement mortar, shotcrete, and cast
concrete. These are briefly described below (2)(6).
Acid resistant mortars have been used in industries as linings in tanks or as mortar bricks.
7-16
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TABLE 7-5
ADVANTAGES AND DISADVANTAGES OF SPECIALTY CONCRETES (1)
ADVANTAGES
Mortar Lining
Minimal sendee interruption
Improved structural integrity
DISADVANTAGES
Excavation required for sharp bends or
curves
Bypass required
Applicable for wide range of pipe sizes
May improve flow capacity
Shotcrete
Minimal excavation required.
Can restore structural integrity to a pipe
that might otherwise require replacement
Minimum traffic interruptions
Applicable to all shapes of man-entry size
pipes
Extensive surface preparation required
May not provide adequate corrosion
resistance
Requires complete diversion of flow and
interruption of service
Extensive surface preparation required
Extended downtime period of 3 to 7 days
or longer required for cleaning,
application, and curing
Some reduction in hydraulic capacity
Limited to man-entry size structures
May not provide adequate corrosion
resistance
7-17
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TABLE 7-5 (cant)
ADVANTAGES AND DISADVANTAGES OF SPECIALTY CONCRETES
ADVANTAGES
Cast Concrete
Established procedure
Simple to design
Applicable to all shapes of pipes
DISADVANTAGES
Surface preparation required
Bypass required
Seldom applicable to pipes less than 48
inches in diameter
May not provide adequate corrosion
resistance
7-18
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Development of mechanical in-line application methods (centrifugal and mandrel) has
established mortar lining as a successful and viable rehabilitation technique for sewer lines,
manholes and other structures. Mortar lining is applied using a centrifugal lining machine.
The machine has a revolving, mortar-dispensing head with trowels on the back to smooth
the mortar immediately after application. A variable speed winch pulls the lining machine
by its supply hose. Reinforcement can also be added to the mortar with a reinforcing
spiral-wound rod. The reinforcing rod is inserted into the fresh mortar and a second coat
is applied over it For man entry structures the mortar can be applied manually with a
trowel.
Shotcrete is used in man entry size sewers (30 inches or greater) and manholes. Prior to
shotcreting, reinforcing steel is set into place. The shotcrete lining machine is self
propelled and controlled by a person riding it An electrically driven supply cart conveys
mortar from the access hole to the feeder, which is attached to the lining machine. The
dry cement and aggregate is mixed with water in a specially designed spray nozzle, and the
resulting mixture is shot into place under pressure. Curing occurs under moist conditions
for the first 24 hours and an additional six days at a temperature above 40°F.
Cast concretes such as potassium silicate are bonded, cast or hand placed structural
concretes. They typically have half the in-place density, or strength value, of shotcrete.
Solids to liquid mix ratios are generally 2:1, similar to cement mortar.
Cast concrete is placed over prefabricated or custom built interior pipe forms that can be
removed and reused section by section. Reinforcing steel is added between the original
surface and the form, setting within the cured thickness.
Each of the three application techniques require prior cleaning to remove oils, greases,
foreign objects, and loose materials, as well as sewage bypass during application and initial
curing.
7-19
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7.1.6 Coatings
7.1.6.1 Description
Coatings include a myriad of proprietary materials such as coal tar epoxy, concrete sealers,
epoxy, polyester, silicone, urethane, and vinylester. These materials, can be applied by
spray machine or brush to concrete surfaces. They are intended to form an acid resistant
layer that protects the substrate concrete from corrosion. Coatings have been applied to
sewer pipes and manholes since the 1960's, with mixed success. The inconsistency of success
is largely due to the specification of coating materials based on manufacturer claims
without actual field testing (8). As a result, engineers are recommending standard field
testing of new products prior to their inclusion in specifications. Some of the advantages
and disadvantages associated with the use of coatings are listed in Table 7-6 (1). Further
discussions of coatings as a means of preventing hydrogen sulfide corrosion is found in
Section 6.2.
7.1.6.2 Procedures and Equipment
Application of coatings usually includes the following steps:
1. Bypassing of sewage
2. Preparation and cleaning of concrete surface
3. Allowing concrete surface to dry
4. Application of coating by brush or spray
5. Allowing the coating to cure
6. Resuming sewage flow
Most coatings can be brush or spray applied. Spray application requires pressures of up
to 3,000 psi, which is double the pressure used for conventional airless spraying. Spraying
is excellent for coating uneven surfaces and is much faster than brush application methods
for some products.
7-20
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TABLE 7-6
ADVANTAGES AND DISADVANTAGES OF COATINGS
ADVANTAGES
DISADVANTAGES
Economical
No disruption to traffic or other utilities
work is performed in-line
Most are fast curing, some cure in less
than one hour
Quick to apply
Can be applied to uneven surfaces
Applicable only to man-entry sewers and
manholes
Bypassing required
Application difficulties
blowholes
pinholes,
Poor bonding to vertical or overhead
surfaces
Surface preparation required
Contractor inexperience with products
Surface repairs often required prior to
application
Developing technology, with history of
failure under corrosive conditions
7-21
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7.2 Approach to Selecting a Rehabilitation Technique
Selection of a proper sewer rehabilitation technique is very important for the successful
rehabilitation of corroded sewers. The methods employed for repairing or rehabilitating
a corroded or failing pipe are contingent upon several parameters. The type of application
or service also has an impact on procedures and/or methodology employed in doing the
repair. The correct interpretation of the cause of the problem is imperative. Some of the
base parameters that must be considered in selecting the right approach for the sewer
rehabilitation program are listed below and in Table 7-7 (9):
Site Conditions
Location
State roads, arterial roads, or similar high volume thoroughfares frequently
have construction permit time restrictions and stringent restoration
requirements. These conditions favor trenchless, short term reconstruction
techniques such as cured-in-place and other lining systems using existing
manhole access. Backyard locations frequently require techniques that utilize
remote reopening of lateral connections due to inaccessibility for excavation
equipment
Surface
High costs associated with paving restoration favor trenchless techniques.
Rigid pavement replacement becomes even more cost prohibitive. In
contrast, pipes in the shoulder of roads or turf covered rights of way tend to
minimize potential cost savings of trenchless techniques.
Pipe Condition
Diameter
In their current state of development, some technologies are applicable for
only a limited range of diameters. Knowing these limitations allows for the
proper selection of method and specifications for the scope of work. Policy
7-22
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TABLE 7-7
FACTORS TO EVALUATE FOR SELECTION
OF REHABILITATION METHOD (6)
TANGIBLE
Inspection
Engineering
Rights-of-way
Easements
Other Utilities
Business Cost
Government Costs
Construction Time
Construction Cost
Business Disruption
Mobilization
O & M Savings
Detour Costs
Flow Bypassing Cost
As-Built Drawings
Existing Mapping
Rehabilitation Options
Service Laterals
Dewatering Cost
Material Storage
INTANGIBLE
Noise
Dust-Dirt
Bus Rerouting
Parking
Pedestrian Inconvenience
Road Settlement & Potholes
Bidding Environment
Safety (Pedestrians and Auto)
Road Surface Replacement
City Reputation
Traffic Inconvenience
Complaints
Liability
7-23
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and technical decisions can also pre-determine allowable reconstruction
techniques. For instance, it may be preferable to upsize deteriorated 6"
sewers to 8" sewers for future maintenance and inspection. Likewise,
sliplining techniques may also be eliminated from 6", 8", and 10" sewers due
to hydraulic and access consideration.
Pipe Conditions
Severely damaged pipe or pipes with protruding connections may require
significant line preparation (excavation), thus minimizing tiie economic
advantages of trenchless methods. This has little to no impact on techniques
where excavation is already required. Areas of soil voids can be a concern
where adequate soil support is required for proper performance of flexible
pipe. Slope, alignment or offsets also have an impact
AdjacentJLJtilities
Minimum clearance between water, sewer and/or gas pipes may rule out
sewer replacement or encasement in concrete.
Rehabilitation Design Life
Pipeline insertion with materials that are also used for replacement pipes
should last as long as a new pipe installation, and perhaps longer because the
old pipeline soil environment has usually stabilized.
Cured-in-place-pipe should last as long or longer than new pipe when the
resin characteristics have been properly identified and utilized in the design.
Procedures that coat the pipe, such as cement mortar lining, cannot be
expected to last as long as a new pipe but can have a significant useful life
when the design compensates for the corrosive conditions in the pipe.
These are some of the criteria and conditions to be evaluated in selecting an approach to
identify appropriate reconstruction methods. The final objective is to select a method
which appropriately addresses the problems in a cost-effective manner for the life of the
facilities.
7-24
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Appendix A.1 includes Case Studies which describe rehabilitation/replacement techniques
employed in wastewater collection systems.
73 References
1. Hydrogen Sulfide Corrosion in Wastewater Collection and Treatment Systems.
Technical Report. U.S. Environmental Protection Agency, Office of Water, 1991.
2. Utility Infrastructure Rehabilitation, prepared by Brown and Caldwell for the
Department of Housing and Urban Development, Washington, D.C., 1984.
3. Inspector Handbook for Sewer Collection System Rehabilitation. The National
Association of Sewer Service Companies, 1988.
4. No-Dig Technology Report. National Association of Sewer Service Companies, May
5,1989.
5. No-Dig Technology Report. National Association of Sewer Service Companies, June
15, 1990.
6. Schrock, B.J., Pipeline Rehabilitation Seminar Handout August 8, 1988, Portland,
ME.
7. Hall, G.R., "Potassium Silicate Concrete for Restoring Wastewater Treatment
Systems," Journal of Protective Coatings and Linings, Vol. 4, No. 8, August, 1987.
8. Redner, J.A., R.P. Hsi, and EJ. Esfandi, "Progress Report -Evaluation of Protective
Coatings for Concrete," paper presented at EPA Technology Transfer Seminar on
"Sewer System Infrastructure Analysis and Rehabilitation," 1991.
7-25
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APPENDIX A
CASE STUDIES
A.1 Sewer Systems
A.2 Wastewater Treatment
Plants and Pump Stations
-------
CASE STUDIES
Several wastewater collection, conveyance and treatment systems were visited to document
the location, nature and extent of hydrogen sulfide corrosion. These facilities were pre-
selected based on earlier surveys conducted by the Water Pollution Control Federation and
County Sanitation Districts of Los Angeles County, both in 1984, and a 1987 survey
conducted by the Association of Metropolitan Sewerage Authorities. Of the 131 candidate
cities compiled, 66 reported sewer corrosion problems. From this, those communities
showing the greatest potential for significant corrosion problems were selected and visited.
The following sections summarize the information thus obtained.
A.1 Sewer Systems
Eight cities were visited to gather specific information regarding the extent of sewer
corrosion. High-rate corrosion was observed in six of these cities (Charlotte, NC and
Milwaukee, WI did not). A summary of the conditions observed in each city is listed in
Table A-l.
A.1.1 Albuquerque, New Mexico
The City of Albuquerque maintains approximately 1,400 miles of sewer which serve
approximately 450,000 people and transport an average of 49 million gallons per day (mgd)
of wastewater to the city's treatment facility. Separate storm sewers are used throughout
most of the city, but some combined systems do exist
Albuquerque experiences 90 to 100 collapses per year that are attributed to
hydrogen sulfide corrosion in its approximately 400 miles of 8-inch-diameter concrete pipe.
These collapses are mostly in residential areas, and each typically involves two to four pipe
sections (20 feet). The problem of pipe collapse is widespread in the city, but seems
concentrated in North Valley, an older part of town that has the most concrete pipe, and
in pipe 40 to 60 years old. The rest of the collectors are mostly clay pipe.
A-l
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Corrosion seems to be worst at locations where a force main discharges to a manhole, at
lift stations in gravity sewers (the city is beginning to use polyvinyl chloride [PVC] liners
at those locations), near interceptors where hydrogen sulfide moves back into laterals, and
at locations midway between manholes. Albuquerque does not have much industrial
discharge, and pipe failures are not related to the presence of industrial discharges. There
are only about three small electroplaters in the area.
In the past, Albuquerque has not had a formal program to identify corrosion. The city now
has a television inspection program for small-diameter sewers (8 inches). The city replaces
about 18,000 feet per year of 8- to 10-inch pipe. The goal is to replace 30,000 feet per
year. The city has replaced up to 12-inch concrete lines with clay or PVC to prevent
further corrosion. A total of about 40 miles of mostly 8-inch-diameter pipe has been
sliplined since 1978.
Albuquerque experiences a summertime odor problem, and injects chlorine gas and
hydrogen peroxide at several locations for odor control during the summer. Untreated
wastewater has had total sulfide concentration of up to 4.3 mg/1. The city will be switching
some of the chlorine units to hydrogen peroxide in the future, because of longer lasting
effects and safety concerns.
Sewers 24 inches or less in diameter are cleaned at intervals ranging from three months to
two years. Larger sewers are not cleaned.
Corrosion at the wastewater treatment facility is limited primarily to metal components.
Ventilation is used to help control corrosion inside the treatment buildings.
The city identified six sites scattered throughout the area to exemplify the corrosion
problem in Albuquerque: Arno and Wesmeco streets; Marquette Avenue and Edith Street;
Iron and 14th streets; Coors Boulevard and Churchill Road; Atrisco Street; and Rossmoor
Street
A-5
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An example of high-rate corrosion exists at the Atrisco Street site. The site iis seven years
old and near the upstream end of the system. The slope in this reach of pipe is very flat,
and sewage velocity was estimated to be 0.5 feet per second (fps). The manhole at this site
was installed with a bituminous coating that has separated almost entirely from the
concrete. In seven years, the concrete on the walls and soffit of the manhole has corroded
up to an estimated depth of 1.0 inch. The inlet and outlet pipes at this manhole are PVC-
lined and in good condition. Measurements of pH on the manhole and pipe walls ranged
from 1 to 5.
Three other sites (i.e., Arno and Wesmeco streets, Iron and 14th streets, and Coors and
Churchill streets) have experienced severe corrosion. Measurements of pH on the walls
of manholes and pipe ranged from 2 to 5 at Arno Street, and from 1 to 2 at Coors Street,
and were 4 at Iron Street Depth of friable concrete or corrosion product ranged from 0.50
to 2 inches. However, the pipe and manholes at these sites are considerably older than the
Atrisco Street site, reflecting a lower rate of corrosion over the life of the installation. The
current rate of corrosion at these sites cannot be determined from available information.
The manhole at Marquette Avenue and Edith Street has experienced some corrosion;
however, manhole access problems prevented quantification. The flow in this manhole is
turbulent The Rossmoor Street site is not corroded badly, although pH ranged from 2.5
to 4 at this site.
Except for the Marquette Avenue and Edith Street site, release of hydrogen sulfide gas is
not believed to be accelerated by turbulence or drops at the Albuquerque sites. Long
detention times, fiat slopes, and warm sewage temperatures are thought to promote
hydrogen sulfide corrosion of concrete system-wide in Albuquerque, as reflected by low pH
readings at all sites.
A-6
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A.1.2 Baton Rouge, Louisiana
The City of Baton Rouge maintains approximately 250 miles of sewer which transport an
average of 36 mgd of wastewater to the city's three treatment facilities. The sewer system
serves the entire East Baton Rouge Parish except for two small communities. The system
serves 375,000 people. Baton Rouge officials estimate that they have approximately 75
miles of unlined reinforced concrete pipe larger than 24 inches in diameter.
Industry contributes less than 5 percent of the total sewered flow. The major industries,
including a large oil refinery, treat their own waste and do not discharge industrial effluent
to the sewers. Those industries that do discharge to the Baton Rouge system are generally
in compliance with the established pretreatment program. Industry is not concentrated in
any one area of the system, and city engineers do not correlate corrosion in their system
with industrial discharge.
The Baton Rouge sewer system is completely separate. Corrosion of concrete pipe is
system-wide. Baton Rouge experienced its first sulfide-related pipe collapse about five
years ago. This collapse was the first indication to the city of the severity of its corrosion
problem. A consultant's report to the city on preventative maintenance of the system made
reference to odor control, but did not focus on corrosion. The city did try chlorine addition
in the mid-1970s, but abandoned the program in less than one year because of high costs.
The city does some television inspection of the system, but does not have a system-wide
hydrogen sulfide corrosion prevention program.
The city has experienced multiple problems in some pipe reaches. Repairs made with
fiberglass or plastic pipe appear to be holding up well; however, one repair done with
concrete pipe experienced corrosion and needed subsequent replacement Baton Rouge
acknowledges that turbulent flow conditions due to changes in grade or direction, pump
station discharges, or drop connections are usually prevalent at problem areas.
A-7
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Baton Rouge selected eight sites for EPA to observe: a pump station, the Central
Treatment Plant, and six manholes located throughout the sewer system (two in the north
subsystem, one in the central subsystem, and three in the south subsystem). The sites
ranged in location from one within approximately 1 mile of a treatment plant and 10 miles
from the upstream end of a reach, to one located near the upstream end of a reach.
Corrosion was observed at each site, with varying degrees of severity. All sites visited were
constructed in the early 1960s. Pipe slopes ranged from 0.003 ft/ft to 0.00015 ft/ft
Pump Station No. 59, a 27-year-old structure, which is located 1 mile upstream from the
Central Treatment Plant and collects wastewater from about 10 miles upstream, was the
first site visited. A pH of 6 was measured on the wet well walls; shallow corrosion, 0.25 to
0.5 inches deep, was observed. The wet well often surcharges, washing the walls.
The Central Treatment Plant had shallow corrosion of some concrete structures. The force
main discharge structure at the plant headworks was corroded, and aggregate was exposed
in both the primary clarifier influent and effluent channels. Some corrosion of metal had
also occurred at the plant headworks. A 0.6 parts per million (ppm) total sulfide content
was measured in wastewater at the plant headworks. Plant influent pH was 6.
The first manhole visited is located at Front and North streets in the Central District, about
0.75 miles downstream of a pump station and within 1 mile of the Central Treatment Plant
Measurements of pH in the manhole and the 30-inch-diameter pipe ranged from 4 to 6.
Large aggregate, indicating up to 0.5 inches of pipe loss, was visible in the pipes above
normal water line. Flow at this location was turbulent due to the pump station upstream,
a change in slope about 100 feet downstream, and a 12-inch-diameter inlet with to 2- to 3-
foot drop. The wastewater had a trace of sulfide and pH of 6.
The next two sites are in the North District: Devall Lane off Blount Road, and Georgia
Street at Harding Boulevard. The Devall Lane site is directly downstream of a pump
station, and flow is made more turbulent by a 1.5-foot drop across the manhole. The site
A-8
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is located at the midpoint of a 12-mile-long drainage area. A total sulfide concentration
of 0.05 mg/1 and a pH of 6 were measured in the wastewater. Pipe surface pH
measurements ranged from 2 to 5.5. The pipe at this location was severely corroded above
the normal water surface (during pump discharge). Some mortar is missing between the
bricks in the manhole and some bricks were observed on the floor of the downstream pipe.
Observations revealed that as much as 1.5 inches of concrete may be corroded.
The Georgia Street site is in the upstream third of the same drainage area. The wastewater
had a sulfide concentration of 0.08 mg/1 and pH of 6.5. Pipe pH measurements ranged
from 5.5 to 6. Although this site is also less than 0.25 miles downstream of a pump station,
pipe corrosion was estimated to be minor. Only small aggregate was exposed, indicating
0.25 to 0.50 inches of concrete loss. Two drop pipes enter this manhole, and the pipe
changes direction about 100 feet downstream.
The final three sites visited are in the South District: Winbourne Street at East Brookstown
Drive, East Contour Drive, and Staring Lane. The Winbourne Street site had a wastewater
pH of 7 and sulfide of 0.17 ppm. Pipe surface pH measurements ranged from 5 to 6. The
36-inch pipe at this location is corroded severely and corrugations were visible at
reinforcing steel locations. There is a 2-foot drop across the manhole. Winbourne Street
is located near the upstream end of a 15-mile-long drainage reach.
The Contour Drive and Staring Lane sites are on the same 54-inch pipe in the middle and
near the downstream end of the reach, respectively. Wastewater sulfide content was 1.1
ppm at the Contour Drive site; wastewater pH averaged 6 at the two sites. Pipe surface
pH measurements were between 2.5 and 4 at the Contour site, and 2.5 and 5 at Staring
Lane. Corrosion at these sites was limited to about 0.50 to 1 inch of concrete loss,
exposing only the first layer of aggregate.
Hydrogen sulfide gas levels of 3 to 4 ppm were measured at the Contour Drive site. The
Staring Lane site is just downstream from a 36-inch-diameter force main terminus. In
A-9
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addition, the downstream pipe at the Staring Lane site had a broken invert near the
manhole, which has created a backwater condition and turbulence at the manhole.
A.13 Boise, Idaho
The City of Boise maintains approximately 325 miles of sewer which transport an average
of 24 mgd of wastewater to the city's three treatment facilities. Boise provides sewer
service to three sewer districts and to Garden City. Boise has recognized a hydrogen
sulfide corrosion problem in its system since 1983. Concrete sewers and manholes in at
least four areas have experienced severe corrosion. Some of their most seriously damaged
manholes have been coated recently with materials to resist further sulfide attack.
Hydrogen sulfide corrosion in Boise is system-wide. Boise officials feel their corrosion
problem can be correlated to low flows in hydraulically oversized sewers and to turbulent
flows created by force main discharges and drops in manholes. There is very little industry
in the area, and Boise operates a completely separate sewer system.
Boise, in consultation with CSDLAC, has tried Polymorphic resin and Zebron coalings and
Chrystallok and fiberglass liners in several of then* manholes. Insituform and sliplining
have been and are presently being used in Boise to rehabilitate corroded sewers.
After Boise discovered its problem in 1983, it realized that the 1977 television monitoring
tapes indicated previously overlooked signs of corrosion such as concrete swelling and
spatting. During the visit, Boise displayed over IS samples of 4-inch-diameter cores,
recovered from a 1984 coring program, which showed the extent of corrosion in different
pipe sizes, ages, and areas of the system.
Based on measurements of core thickness and the known age of the pipe, Boise has
calculated that lifetime corrosion rates are as high as 0.12 inches per year in the sewer pipe
at Glenwood and Chinden streets, and 0.15 inches per year in the sewer pipe at Canal and
A-10
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Columbus streets. Corrosion rates calculated similarly for pipe in the warm springs area
was 0.03 inches per year over a 37-year period, and 0.06 inches per year at Protest and
Federal streets.
About 30 homes in the Warm Springs area use a geothennal water source for home
heating. The water is extracted from the ground at about 175°F and discharged from
homes to the sewer at about 130°F. The sewage in this area of town averages between 90
and 100°F. The sulfate concentration of this water source is about 23 mg/1. Corroded
manholes were observed in this area.
The maintenance supervisor from the neighboring West Boise Sewer District (West Boise)
described a serious problem in his system. West Boise replaced six manholes after a 5-year-
old sewer collapsed due to hydrogen sulfide corrosion'in 1983. There were 10-foot-drop
laterals at some of these spun concrete, Type 2 concrete manholes. West Boise feels that
hydrogen sulfide conditions are worse at turbulent flow areas (e.g., drop manholes). In
addition, the supervisor cited uneven slope during installation of the system as contributing
towards solids deposition in the lines.
West Boise previously used chlorine and hydrogen peroxide dosing and experimented
unsuccessfully with bacterial seeding to control sulfide generation. The chemical treatment
program was successful once the proper dosing was defined, but very expensive. The West
Boise maintenance supervisor also feels that hydrogen sulfide conditions are worse at
turbulent flow areas resulting from drop manholes. He noted that Garden City, a nearby
area with high infiltration and inflow, has little corrosion.
The West Boise Sewage Treatment Plant, owned and operated by Boise City, has had air
scrubbing equipment installed to reduce odor emissions.
A tour of Boise's treatment plant revealed some concrete corrosion. Influent channels
covered for three years at the plant's headworks have experienced corrosion, particularly
A-ll
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the channel that formerly carried sludge. The covered wet wells had no corrosion, but the
air has been scrubbed since the 1970s to reduce odor complaints. The remainder of the
process tanks at the plant are not covered (except for the anaerobic digesters), and are not
experiencing any concrete corrosion.
The field team members made observations at 12 sites in Boise. Of these, five sites (i.e.,
one at Protest Avenue and Federal Way, and four along a segment of another sewer
between North Gary Street at West Baron Street and Glenwood Street at Chinden
Boulevard) showed a high rate of corrosion. The remaining sites, although they often have
acidic pH levels on walls, do not yet show evidence of corrosion.
The Protest Avenue site is located only 2 miles from the upstream end of the collection
area and has a 10-foot-drop inlet The long drop creates turbulence that is believed to
accelerate release of hydrogen sulfide and corrosion. A screwdriver could be pushed up
to 2 inches into the remaining concrete of the manhole wall. Measurements of pH were
2 on the manhole wall. This site is 14 years old.
Four sites along a single 12-year-old line between North Gary Street at West Baron Street
and Glenwood Street at Chinden Boulevard also have high-rate corrosion. Pipe at the
downstream end (Glenwood Street at Chinden Boulevard) showed deep corrugations at
reinforcing steel, indicating that corrosion had penetrated deeper than the reinforcement
Surface pH levels were 6 in the pipe and 1 in the manhole at North Gray Street, 3 in the
pipe and 1 in the,manhole at Bluebird, and 3 in the manhole at State Street The
wastewater sulfide concentration was 2.2S rng/1 near the upstream end.
A brick manhole and a previously corroded concrete manhole coated with Polymorphic
resin were inspected in the Warm Springs area of Boise. The surface of the brick manhole
had a pH of 5, and the coated concrete manhole had a pH of 2.0 - 3.0. Both the brick
manhole and the resin coating appeared to be in good condition.
A-12
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Additional observations at one unlined and two lined manholes did not reveal corrosion.
Shallow corrosion, zero to 0.50 inches deep, was observed at a pump station wet well.
A.1.4 Casper, Wyoming
Casper officials feel that a severe hydrogen sulfide corrosion problem exists in that city.
The problem first came to light in 1975 during reconstruction of the wastewater treatment
facility when a severely corroded influent line to the primary clarifier needed replacement
Since that time, the city has begun looking for corrosion in manholes as part of its manhole
inspection program. In addition, the city tried sodium hydroxide dosing once in 1986 and
once in 1987 to control the slime layer inside sewer pipes and has added clean water to
upstream portions of the system to increase flow rates and decrease detention times. The
sodium hydroxide treatments were effective for approximately three-week periods.
Generation of hydrogen sulfide is only a problem during summer months. Casper also has
a problem with hydrogen sulfide corrosion of the engines being fueled with digester gas in
its cogeneration plant A $16,000 rebuild was recently completed. City staff reported that
this digester gas cogeneration problem is shared with Billings, Montana, and Boulder,
Colorado.
Casper officials identified seven manholes for the visit The first observation was in a
manhole on a 29-year-old 36-inch sewer line about 0.75 miles from the wastewater
treatment facility. The remaining observations were along a 10-mile segment of a 6- to 7-
year-old sewer that transports wastewater from the western side of Casper to the
wastewater treatment facility.
Corrosion is clearly evident in the 29-year-old manhole. Aggregate is exposed and loose
in some instances. Up to 1.5 inches of pipe wall may have washed away. Corrosion
product was not observed at this location; however, a pH of 3 was measured on the
manhole wall and a pH of 4 to 5 was measured in the crown of the downstream pipe.
Corrosion is evident at all the manholes observed on the 6- to 7-year-old sewer.
A-13
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Furthermore, corrosive conditions appear to worsen the farther downstream that
observations were made. The farthest upstream observation was at a manhole located
about 200 feet below a force main river crossing. The sewer pipe appeared in very good
condition, except for 0.12S inch of erosion evident along the side of the outlet Pipe and
manhole surface pH was 6 at this location; there was no corrosion product
As the observers progressed to downstream locations, the presence of corrosion product
increased and pH levels on pipe and manhole surfaces decreased. Measurements at three
downstream locations showed pH levels of 2 or less. At the farthest downstream location,
Center and G St, approximately 1.5 inches of soft, mushy corrosion product was evident
on the walls of the manhole. Because of the short length of time that this sewer segment
had been installed, it was difficult to estimate the amount of concrete that had corroded.
However, corrosion was clearly occurring.
The effluent channel of the primary clarifiers at the wastewater treatment facility at Casper
had severe corrosion. Up to 2 inches of concrete may be missing from parts of the channel.
The facility superintendent believes that a major contributing factor to sulfide generation
in that city is excessive sewage detention time. This results from hydraulically oversized
sewers constructed in anticipation of growth that did not occur because of a regional
economic downturn. In addition, high sulfate concentrations in the local drinking water,
180 to 200 mg/1, may aggravate the problem.
A.1.5 Fort Worth, Texas
The City of Fort Worth maintains approximately 2,000 miles of sewer which transport
wastewater to a single treatment facility located adjacent to Village Creels; a tributary of
the Trinity River. A second facility, the Riverside facility, used to treat wastewater for the
city; however, flow to that facility was diverted to the Village Creek facility several years
ago.
A-14
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Fort Worth experiences hydrogen sulfide odor problems during warm weather and has had
a pipe collapse that is attributed to hydrogen sulfide corrosion. In particular, one of the
Village Creek collectors collapsed. The city now injects chlorine into the two main
interceptors (90 and 96 inches) to control sulfide and odor. The closing of the Riverside
Treatment Facility and concomitant shifting of flow to Village Creek have decreased
detention time and hydrogen sulfide levels in these two interceptors.
The industrial contribution of wastewater is a fairly uniform 10 to 20 percent throughout
. the collection system. The major sources are from electroplating, brewing, food processing,
and aircraft manufacturing.
The levels of metals in the wastewater have declined dramatically during the past five years.
However, levels of aluminum and iron are high because of the discharge of drinking water
treatment sludge to the wastewater collection system at several locations.
A pipe collapse was reported to have occurred at the end of a force main in the
neighboring City of Grand Prairie. The City of Pantego, also a neighbor, was said to have
a major problem.
Field team members entered five manholes in Fort Worth to assess the presence and
effects of corrosion in the city sewer system. The manholes are spread out across the city
and represent several sewer main subsystems. Two manholes manifested severe corrosion.
At Rosedale Street, a section from the crown of a 36-inch pipe is clearly visible lying on
the pipe floor. The pipe walls have corrugations 1.5 to 2 inches deep; an estimated 2 to
3 inches of pipe is missing. The city is aware of problems in this 30-plus-year-old line and
has rerouted wastewater to allow replacement of this sewer. This sewer has a steep, easily
observable slope that increases sewage velocity and could accelerate the release of hydrogen
sulfide gas. The pH of the pipe surface at this manhole was approximately 6, indicating
that conditions were not as corrosive at the time of the visit as in the past, probably
because of the rerouting of the wastewater.
A-15
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The second location with severe corrosion was a 65-year-old, 54-inch pipe on Bomar Street
At this location, the pipe upstream and downstream of the manhole had corrugations 1 to
2 inches deep. In addition, a section of pipe wall approximately 1 foot high by 6 feet long
is missing from the right side of the pipe approximately 15 feet downstream. An estimated
2 inches of concrete has eroded from the lower portion of the manhole, and the joint
between the manhole and outlet pipe has deteriorated. Two 15-inch laterals enter this
manhole, but do not appear to be very active. The upstream manhole has an active drop
lateral, and flow in the downstream manhole is very turbulent In both instances, these
factors could have contributed to release of hydrogen sulfide gas and an increased
corrosion rate. The pH of the pipe surface at this manhole was approximately 6. There
is no apparent indication of current or ongoing corrosion.
The other three manholes observed in Fort Worth are approximately 30 years old. Even
though these locations had lower pH levels of 4 to 5, corrosion is not as severe as at the
other two locations. Drop laterals were not observed at or near these three locations.
A.1.6 Seattle, Washington
The Municipality of Metropolitan Seattle (Metro) maintains approximately 247 miles of
sewer which transport 186 mgd of wastewater to Metro's treatment facilities. Metro has
had a hydrogen sulfide odor problem for many years. A large number of its concrete
sewers and sewage structures have experienced extensive corrosion damage. The most
serious identified cases of hydrogen sulfide corrosion have been replaced or repaired by
coatings or liners.
Construction of Metro's interceptor facilities began in 1963; corrosion is widespread in this
relatively new system. Local municipalities provide smaller-diameter sewage collection
systems which were not investigated during this study. The Seattle area is heavily
industrialized, and industrial flow represents about 25 percent of the total flow, however,
industrial discharges have not been correlated with sulfide generation or concrete corrosion.
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Areas served by Metro to the east and north of Lake Washington have separate sewer
systems for stormwater transport Areas to the west of Lake Washington are served
predominantly by a combined sanitary-stormwater sewer system.
Metro has an extensive sulfide monitoring program, and has had full-time staff working on
the problem since early 1987. Metro personnel look for hydrogen sulfide damage as part
of sewer inspections during which headspace hydrogen sulfide concentrations and pipe
surface pH levels are also measured. Hydrogen sulfide concentrations from 0.1 to over 50
ppm have been found along with pH readings as low as 2. Metro's records indicate lower
pH readings occur at sites with higher hydrogen sulfide gas concentrations.
Metro has tried various concrete liners and coatings in pipes and on structures to control
corrosion as well as chemical addition to control sulfide. Sliplining, epoxy, polyethylene
(PE), PVC, UPC (a polyurethane polyethylene copolymer), Ameron lining, polyurethane
(Sancon), C.T.E. coating, and Aquatapoxy all are being or have been tested by Metro since
1974. Both satisfactory and unsatisfactory performances have been observed. For example,
the PE liner on the East Bay Interceptor - Section 8 is in good shape and is protecting the
concrete behind it, but the UPC coating on the Lake Sammanish Interceptor failed and is
peeling off. Hydrogen peroxide addition to control sulfide was tried but abandoned for
monetary reasons. However, Metro did find that once a large shock dose of peroxide was
added, subsequent dosages could be reduced to control sulfide.
Metro has been involved in other activities related to hydrogen sulfide corrosion control.
Power cleaning of sewers, use of sacrificial concrete in its sewers, and sonar, radar and
ultrasonic measurement of pipe wall thickness have been tried. Metro has also tried to
monitor corrosion rate with concrete coupons and copper shavings hanging in pipes; but
found reactions too slow to provide useful data. In addition, the Renton Treatment Plant
has a $5,000,000 odor-control system employing scrubbers, activated carbon, impregnated
carbon, and chlorine addition. A facilities plan study by a consultant included sulfide-
control recommendations. Concrete corrosion at Metro's treatment plants is not a
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problem.
Seattle Metro personnel recommended five sites for observation. The sites are widely
distributed throughout the system and in parts of different subsystems. Three sites are
directly downstream of force main discharges: a manhole at East Marginal Way and South
112th Street, downstream of the Renton sludge force main; a manhole near 15th Avenue
W and W Raye streets, downstream of the Interbay Pump Station force main; and the
Hollywood Pump Station discharge structure. One of the remaining sites, a manhole at
15th Avenue NW and 188th Street NW, is a few blocks downstream from a force main.
The fifth site, a manhole on the Lake Sammanish Interceptor at NE Union HOI and
Avondale roads, is not downstream of a force main.
Concrete pipe downstream of both the Renton and the Interbay force main discharges has
experienced severe corrosion. Corrosion appears to have penetrated the second layer of
aggregate (1-inch loss) leaving only a short distance to reinforcing steel in the pipe
downstream of the Renton force main. The surface pH averaged 1.8. The sewer
downstream of the Interbay force main carries combined flow and occasionally surcharges.
The 21-year-old sewer pipe was PVC-lined in 1978 for about 200 feet downstream of the
Interbay force main; however, severe corrosion begins where the liner ends. Assuming that
the corrosion all occurred in the seven years following the lining, the corrosion rate at the
Interbay site is over 0.2 inches per year. Rust spots are visible on the unlined concrete pipe
wall, indicating that reinforcing steel wfll likely be exposed soon. One and one-half inch
is estimated to be missing. Measurements of surface pH average 13 at Interbay.
Exposed aggregate and corrosion were observed around the flap gates at the Hollywood
Pump Station discharge and on concrete not protected by a PVC lining. However, most
of this structure is PVC-lined. The exposed portions are probably exposed to erosional
forces when the pumps discharge.
The manhole at Union Hill and Avondale roads showed shallow corrosion, zero to 0 JO
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inches deep, and had a surface pH of 3. The inlet and outlet pipes to this manhole were
in good condition, even though the UPC lining wail in poor condition. The site at 15th
Avenue and 188th Street NW also showed only shallow corrosion, which was limited to the
outlet pipe. This sewer carries combined sanitary-stonnwater flows. The surface pH level
averaged 1.2 at this site. A wastewater total sulfide concentration of 0.6 mg/1 was measured
at the Union Hill Road site.
The frequent presence of force mains, required to overcome topographic barriers, appears
to increase the hydrogen sulfide corrosion problem in Seattle. Seattle feels industrial metal
bearing discharges have no correlation with corrosion, since that industry has always had
pretreatment standards.
A.1.7 Charlotte, North Carolina
In the Charlotte-Mecklenburg Utility District (CMUD) system, EPA compared corrosion
conditions in purely domestic sewers with conditions in sewers that carry industrial flow.
Approximately IS metal finishers and a large foundry are permitted for discharge into the
CMUD sewer system. The field team entered six sewers with a large flow contribution
from industry and four sewers with only domestic flow.
CMUD personnel were not aware of system-wide hydrogen sulfide corrosion problems,
although a failure occurred in the Briar Creek sewer sometime prior to 1973. Since that
time, CMUD has been specifying tricalcium phosphorus as an additive to its concrete pipe.
CMUD also currently specifies a 1-inch sacrificial layer of concrete in its pipe. In the late
1960s, CMUD had an odor study done on the Briar Creek Sewer, it implemented a
program of hydrogen peroxide addition for odor control in 1974. The hydrogen peroxide
was added to a point about 3 miles upstream of the Sugar Creek Treatment Plant to which
the Briar Creek sewer is tributary. This action was unrelated to the prior Briar Creek
failure. Strong odors at the Sugar Creek treatment facility prompted another odor study
in the late 1970s. The second study lead to the injection of hydrogen peroxide at a location
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0.50 miles upstream in both 54-inch influent lines to the plant
The Charlotte water supply is categorized as "soft" by CMUD and has a 8.0- to 9.0-ppm
total sulfate concentration.
Two of the domestic sites (Davidson Street at East 22nd Street, and Myers Street at East
12th Street) are in the Sugar Creek drainage area and two (Arborway near Sedley Road,
and Old Providence Road near Sharonview Road) are in the McAlpine Creek drainage
area. The Sugar Creek sites are 7 and 6 miles from the treatment plant, and 5 and 6 miles
from the upstream end of the same drainage area, respectively. The McAlpine Creek sites
are 7 and 8 miles from the treatment plant, and 3 and 10 miles from the upstream end of
their respective drainage areas. All pipe observed hi the CMUD system is 20 to 25 years
old. Wastewater sulfide concentrations at the four sites ranged from 0.2 to 0.6 mg/1.
Wastewater pH measurements were 6 at three sites, and 5.5 at the Old Providence Road
site. It was the only site in Charlotte with pipe and manhole surface pH measurements
below 6. Pipe surface pH measurements were 4.5 to 5. Corrosion product extended about
0.25 inches deep, exposing "peastone" aggregate at this site.
Two of the four domestic sites experience turbulent flows due to a bend and an obstruction.
Although the wastewater contained measurable concentrations of sulfide at each site (0.6
mg/1 and 0.4 mg/1), there was no measurable headspace hydrogen sulfide. The Old
Providence Road site has a 42-inch pipe and was flowing half full at about 2 fps when
observed. The three clean pipes ranged in size from 24 to 54 inches in diameter.
Three of the industrial sewers (Clanton Road at the Irwin Creek Bridge, Remount Road
at the municipal park, and Freedom Drive at Thrift Road) are in the Irwin Creek drainage
area, 1 to 6 miles from the treatment plant, and 5 to 10 miles from the upstream end of
the same drainage area. Two of the industrial sewers (Old Nations Ford Road near Ervin
Lane, and Granite Street near Continental Boulevard) are located in the McAlpine Creek
drainage area. The remaining industrial site is located next to Park Road near Moncure
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Drive in the Sugar Creek drainage area. The McAlpine Creek sites are located approxi-
mately 10 and 7 miles, respectively, from the farthest upstream points in their drainage
areas. The Granite Street site is about 1 mile downstream of a 12,000-foot, 24-inch-
diameter force main; the wastewater pH was 5.5 at this site. The Park Road site is located
about 7 miles from the farthest upstream point in its drainage area.
Two of the six industrial sites showed signs of very shallow hydrogen sulfide corrosion. The
Remount Road site had lost just enough concrete to expose aggregate and also had
turbulent flow. The Granite site had turbulent flow and an observed velocity of
approximately 10 fps. This site also had four consecutive drop manholes upstream. Pipe
wall and manhole surface pH measurements were pH 6, and some corrosion product was
observed. Wastewater pH measurements were 6 at four of the industrial sites, 5.5 at one
site, and 10 at the remaining site. Wastewater sulfide ranged from 0.0 to 03 mg/1. The
wastewater sulfide level was 0.05 mg/1 at the site where wastewater pH was 5.5, and 0.0 mg/1
at the site where wastewater pH was 10. There was no measurable headspace hydrogen
sulfide gas at any of the six industrial sites.
Pipe diameter at the industrial sites ranges from 21 to 54 niches, and all pipes are
approximately 20 years old. The observed flows range from one-third to two-thirds full,
from smooth to extremely turbulent, with velocities typically 2 to 4 fps.
A.1.8 Milwaukee, Wisconsin
The Milwaukee Metropolitan Sewerage District (MMSD) maintains approximately 305
miles of sewer and two treatment facilities which serve approximately one million people
in the Milwaukee area. The average daily wastewater flow is 190 mgd, of which industrial
flows represent over 25 percent MMSD estimates that 15 percent of the area served by
its system contributes storm flow. Wet weather flows at both treatment plants double dry
weather flows. The average biochemical oxygen demand (BOD) is 200 mgA, and total
suspended solids (TSS) is 250 mg/1 at the two plants.
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In the MMSD system, EPA compared corrosion conditions in purely domestic sewers to
conditions in sewers that carry industrial flow. Approximately 90 electroplaters and metal
finishers and about 15 tanneries are permitted for discharge into the MMSD sewer under
its pretreatment program. Some of the permitted tanneries have waivers to discharge
wastewater without pretreatment for sulfide, making data obtained from the MMSD system
particularly pertinent to this study. Observations covered five sewers with only residential
flow and five sewers with a heavy industrial contribution to the flow.
MMSD personnel were not aware of any hydrogen sulfide corrosion problems. The District
recently inspected (by television) 20 percent of its large-diameter pipe. Annually, it inspects
an additional 40,000 feet MMSD also manually inspects manholes and sewers during a
standard manhole step replacement program and a seasonal manhole cleaning program.
MMSD has some odor problems; however, these are located in parts of the system where
the odors do not generate public complaints.
Three of the residential sites are in the northern part of the service area, 4 to 6 miles from
the Jones Island Treatment Plant, and 3 to 5 miles from the upstream end of the system.
Pipe ages at these sites range from 50 to 70 years old. None of the three sites revealed any
wastewater sulfide. Wastewater pHs were all 6.5, and pipe and manhole surface pHs were
all 6.5. (According to carbonate chemistry, one would expect weathered concrete to be
about pH 63.) No corrosion or signs of corrosion of pipe or manhole concrete were
observed at these sites, even though one site is a junction structure and another site is
located just downstream of a pump station. In both cases, these locations often experience
turbulent flow and potential release of hydrogen sulfide gas.
The other two residential sites, located in the South Shore Treatment Plaint basins, are 8
to 10 miles from the treatment plant, and 3 to 5 miles from the upstream end of the basin.
The first site is less than 20 years old, and the second site, Kinnickinnic, is 50 years old.
Observations at the 20-year-old site were similar to those at the first three residential sites.
However, a wastewater sulfide content of 0.5 mg/1 was found at the Kinnickinnic site and
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pH of 3.5 was measured on the crown of the downstream pipe. Kinnickinnic had severe
corrosion from the water line up the pipe about 1 fool Up to 1 inch of concrete appeared
lost as estimated by aggregate exposure in this 36-inch-diameter pipe. A black slime growth
was observed from 1 inch above the normal water line to 2 inches below.
Five sites had large amounts of industrial flow. Three sites are about 2 to 3 miles north
of the Jones Island Treatment Plant and two are immediately upstream of the plant All
five sites are at least six miles from the upstream end of the system and are at least 40
years old. Corrosion was not observed at any of these sites.
Two of the industrial sites had measurable sulfide in the wastewater: 0.18 and 0.40 mg/1.
Wastewater pH ranged between 6.5 and 7.5. Pipe and manhole surface pH measurements
ranged between 6.0 and 7.0. One of the industrial sites was located less than 0.5 miles
downstream from a tannery. Two sites had initial hydrogen sulfide gas concentrations of
between 0.5 and 0.6 ppm in the pipe headspace. One site located in the downtown.
industrial area could not be entered because of a photoionization meter reading of greater
than 1,000 ppm. Two sites had abrupt changes in direction 20 to 30 feet upstream from
the manhole and 6 to 8 inches of bottom debris. Typical at these sites was a grease buildup
on pipe and manhole walls, calcium buildup, and slime, but solid concrete pipe underneath.
A3, Wastewater Treatment Plants and Pump Stations
Site investigations were conducted at five wastewater treatment plants in three cities. The
purpose of these investigations was to document the location, nature and severity of
hydrogen sulfide corrosion problems at these facilities. The wastewater treatment plants
included the Hookers Point facility in Tampa, FL, the East Bank and West Bank plants in
New Orleans, LA, and the Hyperion and Terminal Island plants in Los Angeles, CA
Pump station corrosion was also investigated as part of these site visits.
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The type and extent of information available from the various cities varied widely. Some
cities closely monitored hydrogen sulfide levels in the wastewater and in the atmosphere,
and maintained detailed records of corrosion repair and rehabilitation efforts. Others had
done little to monitor or control .corrosion.
The following provides a summary of the information collected from the site visits to cities
where corrosion was believed to be a problem in the wastewater treatment plant and pump
stations.
A.2.1 Tampa, Florida
The Hooker's Point Advanced Wastewater Treatment Plant was expanded in 1978 to
handle a design flow of 60 mgd. The plant is averaging approximately 57 mgd, and employs
advanced waste treatment (AWT) for biological nitrogen removal. Unit processes at the
plant include influent screens and grit chambers, primary clarification, two stage activated
sludge treatment, secondary clarification, denitrifying filtration, chlorination and
dechlorination. The plant achieves nitrification/denitrification before it discharges to
Tampa Bay. Sludge handling processes are varied, and consist of gravity, dissolved air
flotation, or belt filter thickening of waste activated sludge, anaerobic or aerobic digestion,
and belt press or drying bed dewatering. A new sludge dryer and pelletizer will come on-
line in the fall of 1990.
Hydrogen sulfide corrosion at the wastewater treatment plant is very severe. The walls of
the influent junction box were constructed with a corrosion-resistant plastic liner. H2S
corrosion is also severe in the screen and grit building and in the effluent chamber in the
grit building. Dissolved sulfide is approximately 10 mg/1 in the influent wastewater.
Concrete on the roof of the junction box had also corroded to an extent that the aggregate
was exposed. All mechanical equipment showed mild to severe corrosion. Hand rails,
platform, and other structures at the primary clarifiers were corroded.
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The plant expends significant resources to combat hydrogen sulfide corrosion. All carbon
steel parts have been replaced by stainless steel parts wherever possible. Electrical
components have been covered and electrical sockets replaced using plastic materials. A
very rigorous painting schedule is maintained on all equipment and parts at the junction
chamber. H2S levels in the atmosphere of the screen and grit building are as high as 20
ppm. A fine-mist scrubber was installed to treat the H2S-laden air emissions from the
junction box. Although designed to handle 50 ppm of H2S, levels entering the scrubber
range from 400 to over 1000 ppm. The capital cost of the scrubber system was
approximately $1,000,000. Annual operating cost is estimated to be $400,000/yr.
The primary clarifiers at the wastewater treatment plant are also at an advanced stage of
corrosion. Some clarifiers are 40 years old and the others were built during the expansion.
There is little corrosion at the influent end of the clarifiers but severe corrosion at the
effluent end. The wastewater has a fall of four feet in the effluent channel thereby creating
turbulence and releasing H2S to the atmosphere with the result that the concrete structure
at the effluent channel is severely corroded.
Most of the moving parts on the clarifiers have been replaced by plastic, including the
scraper mechanism. Gear motors and electrical/mechanical components are covered with
corrosive-resistant materials. Approximately 2 to 4 inches of the side walls at the effluent
channel in the primary clarifiers have been lost due to corrosion. At some locations,
reinforcing steel was visible. The rehabilitation of the clarifiers is now under contract and
includes the installation of a plastic liner on the walls. Hydrogen sulfide corrosion
downstream of the clarifiers is very limited. There is very little hydrogen sulfide corrosion
found at other treatment processes and sludge handling facilities.
Hydrogen sulfide corrosion of instrumentation and controls at the wastewater treatment
plant was severe at the transformer cabinets. All copper tubing and wiring corrodes
rapidly. Corrosion of electrical contacts was widely observed. Switchgear at the influent
junction chamber also corrodes rapidly. Corrosion prevention measures for instrumentation
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and control equipment includes covering the instruments, purging with clean air, and air
conditioning control rooms. All electrical equipment at the plant is on a preventative
maintenance and painting schedule. Contacts and relays are cleaned regularly.
Transformer housings must be replaced periodically.
Although corrosion of sludge handling components and structures has been a problem in
the past, such problems have largely been eliminated through gradual replacement with
corrosion resistant materials such as galvanized and stainless steel. Spare parts are stored
in an air-conditioned warehouse to prevent corrosion. Minor corrosion problems are still
evident where components such as conduit fittings are not available in corrosion resistant
materials.
There are 160 lift stations in the sewer system that collect and transport wastewater to the
treatment plant The more recent pump lift stations are built of concrete.
Medium to very high rate corrosion was found at many of the lift stations. Most of the
manholes, wet wells and interior control room walls in lift stations have sulfur (yellow)
deposits. There was severe corrosion near turbulent areas of the lift stations. The concrete
was corroded and reinforcing steel was visible. Most of the lift stations have mild to severe
corrosion present Steel sound enclosures over wet wells had to be replaced by fiberglass
buildings. Most of the larger pump stations have fine-mist scrubber systems. The City
tried a hydrogen peroxide dosing system, but it was judged to be too expensive to operate.
A few lift stations have used a ferrous sulfate dosing system to control H2S. The City also
tried packed tower air scrubbers. They were very high in maintenance. Carbon adsorption
systems were also installed on some lift stations.
Corrosion of instrumentation and control systems at the lift stations was not quite as severe
as at the plant This was primarily due to the active preventative maintenance program
imposed by the City. Copper tubing and exposed wiring were seen to be corroded. All
motor control centers and electrical equipment were covered.
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New Orleans
The East Bank and West Bank wastewater treatment plants of the City of New Orleans
were visited to document the extent of hydrogen sulfide corrosion at the facilities. The
East Bank plant treats the sanitary flows from downtown and the northeast part of the City.
The plant was originally built in 1963 for primary treatment and was later expanded for
secondary treatment in 1980. The original design flow at the plant was 30 mgd but the
facility has been expanded to handle 122 mgd. A total of 1500 miles of collection system
comprised of gravity and force mains collect and convey sewage to the plant The
treatment plant consist of screens and grit removal, pure oxygen activated sludge system
and secondary settling. Effluent is discharged to the Mississippi River. Secondary sludge
is dewatered and then incinerated. The ash, along with screenings and grit, are disposed
of in a sanitary landfill.
Plant headworks at the East Bank plant had severe corrosion in the screen and grit basins.
Some parts of the grit basins were built in 1963 and were then expanded to meet the new
design flows. Three force mains feed wastewater to these grit basins. One force main
conveying flows from the City has long detention times, and hence the wastewater is very
septic when it reaches the plant The color of the wastewater was very dark (black) and
was deficient in D.O.
The side walls of the grit chamber were severely corroded. Approximately 1 to 1% inches
of concrete was corroded away at some locations. Severe corrosion was also observed at
the effluent end of the grit box where the wastewater spills into a channel which led it to
the pure oxygen activated sludge tanks. The grit chambers were installed with screens on
each pass. These screens were in a deteriorated condition. Many of the components of
the screens had rusted and the metal frames on which they were attached were corroded
along with the concrete below the frames.
Corrosion of instrumentation and controls was found to be severe at the East Bank plant
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Contacts on electrical equipment were oxidized. The plant personnel replace small items
and clean contacts and equipment on an annual basis. They sometimes must take
equipment off-line for service and maintenance. As preventative maintenance, they use a
light coating of oil, and cabinets purged with cleaned air. The plant has entered into an
annual preventative maintenance contract They allocate two men 1 to 1-1/2 days/wk for
electrical equipment maintenance. The electrical contacts on indicator lights, pump relays,
and contacts operate intermittently because of oxidation problems at the contacts. The
instrument control room is fully air conditioned. Air cleaning is done through
permanganate beads which are replaced every month. The plant expends significant effort
for replacement and maintenance of the electrical and instrumentation components.
The plant does not have any control measures to prevent future corrosion. No efforts have
been made to rehabilitate the corroded structures. The plant has a limited budget and does
not plan to employ rehabilitation of structures as a corrective action until there is a failure.
The West Bank plant serves the population of the western side of the City of New Orleans.
The plant was originally built in 1971 for a design maximum flow of 15 mgd. The average
dry weather flow (ADF) to the plant is approximately 7 to 8 mgd. The plant is now under
design for expansion to 40 mgd. The treatment plant consists of influent bar screens, grit
removal, primary sedimentation, high rate trickling filters, secondary sedimentation,
chlorine contact and final discharge to the river. The sludge from the clarifiers goes to a
thickener and a vacuum filter and is then incinerated. The ash from the incinerator is
disposed of in a local landfill.
The West Bank plant also has severe corrosion at the influent head box where the screens
and grit chamber are located. Corrosion has degraded the sidewalls on the grit chamber
to a depth of 1 to 1% inches. Again, corrosion was found to be severe at areas of high
turbulence i.e. at the influent and effluent end of the grit basins. The metal grating and
handrails on the grit basins were also corroded. The wastewater entering the plant was
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septic and the dissolved oxygen was always found to be 0 mg/1 except during heavy rainfalls
when the D.O. would increase to 0.2 mg/1. As the plant is located adjacent to a golf
course, there are plans to cover the plant headworks, the sludge thickener and some other
tanks to control odor emissions.
There are no efforts being taken to rehabilitate the degraded structures. No rehabilitative
techniques have been employed to correct the odor and corrosion problems.
The vacuum filters at the West Bank Plant are located in a building that is equipped with
a passive air ventilation system. The mechanical and support parts of the vacuum filters
are in a severely corroded state. The plant had to replace grating over the filter supports.
When the filters are operating, high H2S levels are reported in the building. There is no
corrosion reported at other parts of the plant Corrosion at instrumentation and controls
is minimal. Corrective action at this plant is based primarily on minimizing odors which
are affecting the neighboring golf course.
There are a total of 87 lift stations and 1500 miles of sewers that serve both the East and
West Bank Treatment Plants in the City of New Orleans. The lift station wet wells are
made of brick and concrete. Force mains range in size from 42 to 52 inches and are
constructed of cast iron, steel or concrete. Ninety to 95 percent of the collection system
is 8 to 10 inch diameter pipes. Concrete pipes were laid in late 1930's. There are a few
older pipes made of clay. Since the 1970's, plastic pipe has been used where possible.
All of the 87 lift stations employed in the collection systems for the East Bank and West
Bank plants are in some stage of corrosion. The older lift station wet wells were built of
brick and are severely deteriorated. The pump base and supports have corroded and at
some places are on the verge of falling down into the wet well. Rehabilitation of brick wet
wells consists of coating by gunite. The New Orleans Sewage Board experimented with
pump cycle times to minimize detention times and decrease H2S levels. Continuous
ventilation is provided in the lift stations at six air changes per hour. At some places the
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Board has tried adding ferric chloride but found that it forms clinkers in the incinerator at
the plant H2S levels in the atmosphere of the wet wells average approximately 100 ppm.
The Board spends around $5.2 million per year for lift station maintenance. About 30
percent of total man hours is utilized for lift station maintenance. Electrical and
instrumentation equipment have minor corrosion problems. New electrical equipment has
been installed with clean ah* supplied by treatment through potassium permanganate.
There is reported to be more corrosion in lift station wet wells at the east side of town.
City of Los Angeles
A.23.1 Hyperion Wastewater Treatment Plant
The plant is designed for 400 mgd through primary treatment and 150 mgd through
secondary treatment Present day flows are 370 mgd and 200 mgd, respectively. The ability
of the secondary process to handle the additional flow is attributed to the addition of fine
bubble diffusers. The headworks, primaries, secondaries and anaerobic digesters are
approximately 40 years old. Regulations eliminating ocean sludge disposal and requiring
full secondary treatment, along with population growth, have resulted in 10 years of
construction at the plant The City foresees at least another 5 to 10 years at the same pace.
The latter includes replacement of the existing secondary process with a pure oxygen
process.
With the exception of the gravity degritter in the east headworks, all trash and grit removal
tankage are under cover, making direct observation of corrosion on these processes difficult
without considerable expenditure of staff manpower. The covers on the west aerated grit
chamber effluent channel were small enough to be managed by one person and were lifted
for observation. Corrosion of the concrete sewer at those points was observed to be severe,
with penetration to at least 12 inches at the water line diminishing to 1 to 2 niches in the
closed channel and 0 to 1 inch at ground level of the open tank. The plant carpentry
superintendent in charge of all in-house concrete repair indicated the observed areas were
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typical of all headworks tankage of the same age. The cost of these repairs are not
segregated from general plant maintenance costs.
The extent of corrosion below the water line in both tanks and channels was described as
minor (less than 1 inch) even in the oldest tankage. All covers (tank and channel) and
deck plates are made of aluminum, as were handrails, conduit and other hardware (some
stainless steel). No corrosion of these materials was apparent
The headworks processes are all contained in buildings. The ambient atmosphere of the
buildings is swept by fans and discharged to a collection point at the suction of the
secondary process blowers. Thus a slight negative pressure is maintained in each building.
This prevents noxious odors from escaping the plant and with normal infiltration plus some
outside air intakes, avoids the build up of corrosive gases in the atmosphere of the process
buildings. All windows in the aerated grit chamber building were sealed in order to reduce
escape of hydrogen sulfide, even though the tanks are covered. The few pieces of carbon
and galvanized steel found in the buildings were severely corroded. This was especially true
of steel doors. No maintenance program is in force for the doors other than repainting
when scratched or chipped. The ambient air removal system piping is fiberglass and most
other piping is PVC. Conduit is aluminum or PVC.
A short section of the force main entering the plant collapsed and was replaced in 1987.
The collapse was attributed to corrosion-weakened concrete pipe combined with the ground
vibration caused by heavy construction equipment
The decision to rehabilitate or replace all or part of the headworks has yet to be made.
There is obvious structural damage in some places and some doubt in the mind of staff as
to the structural integrity of a rehabilitation effort given the frequency of earthquakes in
the area. In either case, PVC liners with concrete slabs will be used in all channels and the
inside of all tankage will be at least coated with coal tar or an alternative coating material.
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Hie primary clarifiers are covered with concrete slabs so casual inspection was not possible.
The influent and effluent channels are covered by aluminum plates which can be easily
removed for inspection. Like the headworks, concrete exhibited deep corrosion penetration
from the water line to the surface, with some of the deepest penetration (6 to 8 inches) at
the surface adjacent to the channel covers. Most of the corrosion at the top has been
repaired by cutting back to good concrete, reforming to the original geometry and grouting.
These repairs are recent, and are not covered by any protective coating. An epoxy-type
coating had been applied to the early patches and began peeling almost immediately, so
coating was discontinued. The channels will be covered with PVC liners. The type of
coating for the inside of tank walls and covers is as yet undetermined. They are in the
process of converting from steel to plastic chain and from wood to fiber glass boards for
the sludge rakes. The existing primaries will be rehabilitated once new primary
construction is complete.
With the exception of anaerobic digestion, the sludge handling processes came on-line in
1985-1986. Ocean disposal of sludge ceased in 1987, and digested sludge is now either
dried and applied to power generation (Carver-Greenfield process) or dewatered by
centrifuges and transported to a Yuma, AZ land application site. Due to safely regulations
for construction at the site, the sludge handling facility was off-limits to visitors. The
addition of ferrous chloride (280 mg/1) for hydrogen sulfide reduction after sludge digestion
is to control sulfur emissions as opposed to corrosion control.
All instrumentation and control electronic equipment is conformably coated (a thin lacquer-
like coating applied to circuit boards and components to seal them from the atmosphere)
in the manufacturing process. This is standard practice in the industry for wastewater
treatment equipment suppliers. In addition, all field mounted instrumentation (sensors,
transmitters, etc.) are nitrogen purged. The case of each instrument is connected to a low
pressure nitrogen supply which maintains a slight positive pressure in the instrument
housing to prevent exposure of the components to ambient air. Inspection of the
equipment disclosed no sign of corrosion. All circuit boards, contacts, wire terminations
A-32
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and other exposed metal was bright and shiny. The annual cost of nitrogen is estimated
at less than $3,000. The only sensing elements immersed in liquid process streams are DO
probes. These are newly installed and as yet have no track record. The control room is
isolated from ambient atmosphere by scrubbing, filtering, and air conditioning. No
problems were reported or apparent with these systems.
Although not as severe, there is ample evidence of concrete corrosion in secondary
treatment The worst is at the aeration basin influent mixing channel where corrosion has
penetrated to the reinforcing steel (2 inches). Other areas of the reactors have exposed
aggregate. Steel hand rails and steel plate on the side of the reactors are pitted and rusted
where chipped or peeled paint allowed exposure to atmosphere.
v j
Since a new oxygen activated sludge system is planned, only those repairs necessary for the
existing system to remain operational will be made.
The scavenged air recovered from buildings and below tank covers is ducted to the aeration
basin blowers for scrubbing in the activated sludge mixed liquor. This air is not cleaned
by other than conventional blower inlet air filters, nor are the blowers constructed of
special corrosion resistant materials. This has not caused any additional blower
maintenance or reduced the useful life of the blowers. The only impact is on the carbon
steel linkage that moves the internal guide vanes and this impact is considered minor by
the maintenance staff.
A.23.2 Terminal Island Wastewater Treatment Plant
The original plant was constructed in 1935 and completely rehabilitated in 1977. The plant
is designed for full secondary treatment of 30 mgd. Present day dry weather diurnal flow
ranges were modified from 5 to 35 mgd, a 7:1 ratio, to 10 to 30 mgd, a 3:1 ratio, by
requiring (as part of pretreatment enforcement) local industries to shift discharges to off-
peak hours. Over 50 percent of the flow and 70 percent of the load is of industrial origin.
A-33
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With the exception of the bar screen, all trash and grit removal tankage is under cover,
making direct observation of corrosion on these processes difficult without considerable
expenditure of staff manpower. Corrosion of the concrete at the bar screens was negligible
at those points observed, with penetration barely to the aggregate at the water line. The
extent of corrosion below the water line in both tanks and channels was described as minor
(less than 1 inch) in the oldest tankage. All covers (tank and channel) and deck plates are
made of aluminum, as were handrails, conduit and other hardware (some stainless steel).
No corrosion of this material was apparent The bar screen frame and sheet metal is of
coated (coal tar) carbon steel, which was severely corroded. Most of the sheet metal has
been replaced with sheet PVC. The frame (Vi inch angle iron) will probably be replaced
with stainless steel.
The headworks processes are all contained in buildings. The ambient atmosphere of the
buildings is collected by the suction of the secondary process blowers (no fans). Thus a
slight negative pressure is maintained in each building. This prevents noxious odors from
escaping the plant and with normal infiltration plus some outside air intakes, avoids the
build up of corrosive gasses in the atmosphere of the process buildings. The few pieces of
carbon steel found in the buildings were severely corroded including galvanized steel
hardware. This was especially true of steel doors. No maintenance program is in force for
the doors other than repainting when scratched or chipped.
The ambient air removal system piping is fiberglass and most other piping is PVC. Conduit
is aluminum or PVC.
The primary clarifiers are fitted with aluminum covers. The influent and effluent channels
are also covered by aluminum plates which can be easily removed for inspection. Plant
staff had previously converted from steel to plastic chain and from wood to fiberglass
boards for the sludge rakes. Because of problems with the plastic chain jumping the
sprockets, they are converting back to steel chain.
A-34
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The egg shaped anaerobic digesters appear to be in good condition externally. An external
-£ ••
pipe that collects gas for mixing has been replaced with a welded stainless steel pipe. The
sacrificial anodes are replaced routinely as part of the maintenance program. The
motorized valves located on top of the digesters are also being replaced, but this is because
they do not have weather proof housings, although the problem may have been exacerbated
by hydrogen sulfide. The earth ground bonding wire (bare copper) in this location has
almost turned to dust and is being replaced with an insulated wire. This location is also
exposed to winds from the sea, and the corrosion observed may be the result of salt air.
The elevator at this location is a high maintenance item, since it is exposed to both sea air
and ambient hydrogen sulfide.
The addition of ferrous chloride (450 mg/1) for hydrogen sulfide reduction (10 fold) after
sludge digestion is to control sulfur emissions as opposed to corrosion control.
All instrumentation and control electronic equipment is conformably coated. In addition,
all field mounted instrumentation (sensors, transmitters, etc.) are nitrogen purged. The
case of each instrument is connected to a low pressure in the instrument housing to prevent
exposure of the components to ambient air. Inspection of the equipment disclosed no sign
of corrosion. All circuit boards, contacts, wire terminations and other exposed metal was
bright and shiny. The annual cost of nitrogen is estimated at less than $2,000. The only
sensing elements immersed in liquid process streams are DO probes. These are relatively
new yet have performed well to date. The control room is isolated from ambient
atmosphere by scrubbing, filtering, and air conditioning. No problems were reported or
apparent with this system.
The scavenged air recovered from buildings and below tank covers is ducted to the aeration
basin blowers for scrubbing in the activated sludge mixed liquor. This air is not cleaned
by other than conventional blower inlet air filters, nor are the blowers constructed of
special corrosion resistant materials. This has not caused any additional blower
maintenance or reduced the useful life of the blowers. The only impact is on the carbon
A-35
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steel linkage that moves the internal vanes and this impact is considered minor by the
maintenance staff.
A-36
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IMTB
Office of Wastewater Enforcement & <
MUNICIPAL TECHNOLOGY BRANCH'
DETECTION, CONTROL, AND
CORRECTION OF HYDROGEN
SULFIDE CORROSION IN
EXISTING WASTEWATER
SYSTEMS
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