&EPA
           United States
           Environmental Protection
           Agency
            Office Of Water
            (WH-547)
430/09-91-O09
September 1991
Hydrogen Sulfide Corrosion
In Wastewater Collection And
Treatment Systems

Report To Congress
                                     
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            REPORT TO CONGRESS

HYDROGEN SULFTOE CORROSION IN WASTEWATER
     COLLECTION AND TREATMENT SYSTEMS
        U.S. Environmental Protection Agency
             Office of Water (WH-595)
              Washington, DC 20460
                   May, 1991

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                        ACKNOWLEDGEMENT
This report to Congress was prepared by the U.S. Environmental Protection Agency
(EPA) with the assistance of J.M. Smith & Associates, PSC, Consulting Engineers
(JMS) under subcontract to HydroQual, Inc. (EPA Contract No. 68-C8-0023). IMS
employees who made major contributions to the document included
Robert P.O. Bowker, John M. Smith, and Hemang J. Shah.

Previous reports on hydrogen sulfide corrosion were prepared by EPA in 1988 with
the assistance of E.G. Jordan Co. under EPA Contract No. 68-03-3412. Under
subcontract to E.G. Jordan Co., Brown and Caldwell staff, including Perry Schafer,
Robert Witzgall, Roy Fedotoff, and Walt Meyer, prepared five case studies of
corrosion.

Many people provided valuable assistance during this study.  However, special
acknowledgment is appropriate for the staff of the County Sanitation Districts of
Los Angeles  County, especially the support from John Redner and Calvin Jin.

Ms. Irene M. Suzukida [Homer] was the EPA Work Assignment Manager for this
report
 This report is dedicated to all the individuals who work to preserve the
 wastewater systems of this country, whose contributions are too
 numerous  to identify.

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                               DISCLAIMER
This report contains discussions of several proprietary products and processes used
for the control and prevention of corrosion induced by hydrogen sulfide.  Mention
of trade names or commercial products does not constitute endorsement by EPA or
recommendation for use.

For this report, information was not collected for all products and processes, and
omission of products or trade names from this report does not reflect a position of
EPA regarding product effectiveness or applicability.
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                            TABLE OF CONTENTS

                                                                      Page

ACKNOWLEDGEMENT	. .    i

DISCLAIMER	  ii

EXECUTIVE SUMMARY	    E-l

1.0    BACKGROUND AND OVERVIEW	 .    1-1

      1.1  Legislative Charge  	,	    1-1
      1.2  Los Angeles County System History	• • • •     1-1
      1.3  Scope of Study		    1-2

2.0    CORROSIVE EFFECTS OF HYDROGEN SULFIDE	    2-1

      2.1  Consequences of Corrosion	  .  2-1
      2.2  Basic Mechanism of Hydrogen Sulfide Corrosion	  2-1
      2.3  Factors Affecting Corrosion 	  2-3
      2.4  Findings of Study	  2-6
      2.5  Results and  Conclusions	  2-16

3.0    EFFECTS OF INDUSTRIAL PRETREATMENT	  .  3-1

      3.1  Overview .	  3-1
      3.2  Reduction of Sulfide by Precipitation with Metals	  .  3-2
      3.3  Biological Inhibition by Metals and Toxic Compounds  	  3-4
      3.4  Comparison of Metals Levels at Los Angeles County with
            Other Cities Before Pretreatment	  3-4
      3.5  Site Visits to Industrialized Cities  	  3-5
      3.6  Beneficial Effects of Local Industrial
          Pretreatment Projects	  3-7
      3.7  Conclusions	  .  3-7

4.0    DETECTION, PREVENTION AND REPAIR OF HYDROGEN SULFIDE
      CORROSION DAMAGE	  4-1

      4.1  Detection of Hydrogen Sulfide Corrosion	  4-1
      4.2  Prevention of Hydrogen Sulfide Corrosion in Existing
            Systems  	  4-1
      4.3  Prevention of Hydrogen Sulfide Corrosion in the Design of
            New  Systems  	  4-4


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                      TABLE OF CONTENTS (cont)
     4.4 Repair of Damage Caused by Hydrogen Sulfide
         Corrosion . .' ....... ...... | ............... ........ • .....
     4.5 Findings and Conclusions . . . .............................   4-6
5.0   RECOMMENDATIONS  ...... .............................   5-1

6.0   REFERENCES ..... ........ ....... ......................

     GLOSSARY ............... ---- . ........................
                                   IV

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                               LIST OF TABLES
1     Theoretical Increase in Dissolved Sulfide Based
      on Metal Precipitation; Los Angeles County	  3-3

2     Comparison of CSDLAC Metals Levels Before and After
      Pretreatment with Metals Levels of 50 Cities in  1978-1979  .	  3-6

3     Beneficial Impacts of Controlling Industrial Discharges
      on Hydrogen Sulfide Corrosion	  3-8

4     Summary of Sulfide Control Techniques	  4-2

5     Approaches to Prevent Hydrogen Sulfide Corrosion During Design	  4-5

6     Principal Methods for Pipeline Rehabilitation	  4-7

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                              LIST OF FIGURES

                                                                         Page

1     Process of Sewer Failure Due to Hydrogen Sulfide Corrosion 	   2-2

2     Mechanism of Sulfide Generation  knd Corrosion
      in Sewers	•	   2-4

3     Severely Corroded Sewer in Casper, WY	   2-8
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4     Severely Corroded Sewer in Seattle, WA	   2-8

5     Non-Corroded, 50 Year Old Sewer in Milwaukee, WI	   2-9
                                     i

6     Brick Manhole with Loose and Miksing  Bricks	   2-9
                                     I
                                     I
7     Frequency of Corrosion Problems  kt Wastewater
      Treatment Plants	   2-11

8     Severely Corroded Metal Gate and Concrete Channel
      in Tampa, FL	.;	   2-12
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9     Severely Corroded Concrete Channel in Los Angeles, CA	   2-12

10    Corroded Electrical Control Panel in Tampa, FL	   2-13

11    Corroded Steel I-Beam in St Petersburg,  FL	   2-13

12    States Having Severe Corrosion Problems in Wastewater
      Systems of Four or More Municipalities	   2-14

13    Use of Proprietary PVC Lining to Prevent Corrosion of
      Concrete Pipe	1	•   2-15
                                       VI

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                             EXECUTIVE SUMMARY
1.0   BACKGROUND AND OVERVIEW

      Between the early 1970's and mid 1980's, the County Sanitation Districts of Los
Angeles County (CSDLAC) observed that the rate of sewer corrosion in their system
had increased dramatically. Subsequent studies showed a high correlation between the
reduction in certain wastewater constituents of industrial origin and the increase in
hydrogen sulfide responsible for corrosion.  This raised the question as to whether
implementation of industrial pretreatment standards was related to increased sewer
corrosion rates.

      Section 522 of the Water Quality Act of 1987 requires the U.S. Environmental
Protection Agency (EPA) to conduct a study and prepare a report on corrosion in
wastewater systems. As defined in the Act, the objectives are to determine the
following:

      •     the corrosive effects of hydrogen sulfide in wastewater collection and
             treatment systems.

      •     the extent to which uniform imposition of categorical pretreatment
             standards exacerbates this corrosion problem.

      •     the range of available options to deal with such  effects.

      Corrosion due to the presence of hydrogen sulfide is a well known phenomenon
in wastewater systems.  Its effects can range from poor reliability and premature
replacement of electrical systems to sewer pipe failures and street collapses.  Several
manuals have been prepared which discuss the mechanisms of hydrogen sulfide
corrosion, design procedures to avoid corrosion in  new systems, and options to control
corrosion in existing systems.  Although the rate and severity of corrosion may vary
widely depending on wastewater characteristics and environmental conditions, CSDLAC
is the first to provide documentation of accelerated corrosion whereby the rate of
corrosion has significantly  increased.  Accelerated corrosion leads to significantly  .
reduced lifetimes or premature failure of pipes, structures, and equipment; and costly
repairs and replacement of such components.                       .    .       .

      Attempts to gain an understanding of 1) the extent of hydrogen sulfide corrosion
problems in the U.S., and  2) the impact of industrial pretreatment standards on
corrosion rate were thwarted by the lack of available data and information.  Most
municipalities have little or no documentation of corrosion problems, and some respond
to such  problems only in the event of a catastrophic failure such as a pipe collapse. No
entities  other than CSDLAC were found to have sufficient historical data to establish a

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correlation between implementation of industrial pretreatment standards and an
increase in corrosion rate.  Research on this relationship appears to be limited to that
conducted or sponsored by CSDLAC/   |
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The Report to Congress generally follows the objectives established by the Water
Quality Act, and is organized as follows:  [
                                      i
       •      Background and Overview  j
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       •      Corrosive Effects of Hydrogen Sulfide

       •      Effects of Industrial Pretreajtment

       •      Detection, Prevention and Repair of Corrosion Damage

       •      Recommendations to Congress

       A significant amount of technical data and information was collected as part of
this study. Such information has been included in a separate 'Technical Report on
Hydrogen Sulfide Corrosion  in Wastewater Treatment Systems."
2.0    SUMMARY OF FINDINGS      i

       2.1    Corrosive Effects of Hydrogen Sulfide

       Very few municipalities hi the U.S. have good documentation of hydrogen sulfide
corrosion problems, much less monitor corrosion rate.  Many municipalities are unaware
of the severity of their corrosion problems, and sulfide  control measures are generally
instituted for the purpose of controlling odor emissions, not corrosion.  An analysis of
the information collected as part of this study yielded the following conclusions
regarding the corrosive effects of hydrogen sulfide:

       •     Severe problems are not limited to CSDLAC.  Extensive corrosion damage
             requiring immediate repair or rehabilitation has been observed in sewers
             and treatment plants in other cities.

       •     The geographical distribution of severe corrosion problems is widespread,
             and is not limited to areas having warm climates.  Severe corrosion was
             observed in Wyoming, Idaho, Wisconsin,  and Washington, as well as
             California,  New Mexico, Louisiana, Texas, and Florida.

       •     Hydrogen sulfide corrosion problems hi operating systems are often not
             recognized early enough to jtake corrective action before considerable
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            damage has occurred.

      •     Severe hydrogen sulfide corrosion may reduce the 50- to 100- year life
            expectancy of infrastructure to less than ten years.

      •     In a 1984 survey, of 89 cities, 32 cities reported sewer collapses, 26 of
            which were judged to be due to hydrogen sulfide corrosion.

      •     In two independent surveys, 60 to 70 percent of the municipalities
            reported corrosion problems at their wastewater treatment plants. In one
            of the surveys, 14  percent of the plants reported corrosion as being severe.
            EPA projects that more than 14 percent of the collection systems
            experienced severe corrosion.

      •     Hydrogen sulfide corrosion problems in sewers have been reported by at
            least 20 foreign countries.

      •     Due  to lack of historical data, average corrosion rate is often estimated
            based on depth of corrosion and age of pipe.  This may not reflect the
            true corrosion rate, which may be substantially higher at a given time and
            condition.

      •     No entities other than CSDLAC  had sufficient data on corrosion rate to
            establish whether  the rate of corrosion had changed over time.

      2.2   Effects of Industrial Pretreatment

      The national effects of industrial pretreatment on hydrogen sulfide corrosion are
very difficult to ascertain since no sanitation districts other than CSDLAC were found
to have sufficient data to establish a correlation.  Based on theoretical  analysis, review
of full scale and pilot scale research data from CSDLAC, and a series of site
investigations,  the  following conclusions  are presented.

      •     The reduction in metals and other industrial constituents in CSDLAC
            wastewater may have caused an acceleration in corrosion rate, possibly due
            to decreased biological inhibition and/or chemical precipitation.

      •     Two pilot studies conducted by CSDLAC demonstrated that sulfide
            generation was  reduced when metals were added to the wastewater at
            levels approximating those in the early 1970's.  (This is consistent with the
            known toxic effects of metals on other microorganisms.).

      •     When compared with data from 50 other wastewater treatment plants in
            the 1970's, total metals and cyanide levels in the CSDLAC wastewater

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            were, higher than levels in wastewater entering 47 of the 50 facilities.
            While 32% of the cities had!total metals and cyanide levels higher than
            CSDLAC levels after pretre^tment, it is difficult to project how many
            cities could potentially have a corrosion problem affected by industrial
            pretreatment since it is not known at what levels industrial constituents
            begin to suppress sulfide generation.

      •     Data comparing corrosion hi residential vs. industrial sewers were
            inconclusive as to whether metals suppressed hydrogen.sulfide corrosion.

      •     Local regulation of certain non-toxic constituents  hi industrial waste
            discharges (BOD, sulfide, temperature, pH) has had a beneficial impact in
            reducing the potential for hydrogen sulfide corrosion.
                                       !
      •     Additional research is necessary to establish the constituents and their
            associated levels at which sulfide generation is suppressed or accelerated.

      23   Detection, Prevention, and Repair of Hydrogen Sulfide Corrosion Damage

      Based on current information, the following findings and conclusions  are
presented for the detection, prevention and control of hydrogen sulfide corrosion, and
for repairing damage by hydrogen sulfide corrosion:

      •     Some municipalities are  not aware of sewer corrosion problems until street
            collapses or sewer blockages occur.

      •     No standardized technique exists for measuring corrosion to obtain
            accurate corrosion  rate data.
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      •     Educational programs are njecessary to disseminate information on
            corrosion detection and monitoring to municipalities.

      •     Although design procedures have  been developed to assist in controlling
            sulfide generation and corrosion, these procedures are not universally
            practiced. Some observed corrosion could have been foreseen and
            avoided using existing design principles that minimize sulfide generation
            and corrosion.

      •     A large variety of chemicals and techniques are used to control sulfide in
            sewers and treatment plants, often for odor control.  However, their costs
             and effectiveness for corrosion control vary widely based on site-specific
            conditions.

      •     Based on a 1984 survey of 89 cities, 34 percent take measures  to reduce


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             sulfide in sewers, and 63 percent provide corrosion protection of sewers or
             use one or more techniques to rehabilitate corroded sewers.

             The current national expenditure for controlling sulfide generation in
             sewers is  on the order of tens of millions of dollars per year. CSDLAC
             alone is spending approximately two million dollars per year on chemicals
             to control sulfide.  Some cities have discontinued chemical sulfide control
             measures due to the high costs.

             National  expenditures for rehabilitation of sewers and structures damaged
             by hydrogen sulfide corrosion is very difficult to estimate. Although
             municipalities maintain records of operation and maintenance activities,
             often the cost of corrosion-related rehabilitation and replacement activities
             are not readily retrievable.

             Many options are available to rehabilitate pipe which has been damaged
             due to corrosion.  Some, such as sliplining and cured-in-place inversion
             lining, have been widely used with satisfactory results. Others, such as
             application of "corrosion-resistant" coatings, have often experienced early
             failure.
3.0   RECOMMENDATIONS

      Based on the findings of this study, additional emphasis needs to be given to
information dissemination and education regarding hydrogen sulfide corrosion.  There is
a need to inform municipal political officials, design engineers, construction contractors,
and .operating staff of methods to minimize corrosion in new installations and detect
corrosion in existing structures.

      •     Municipalities should incorporate corrosion detection and monitoring
             strategies into their collection system operating and maintenance
             procedures, in order to protect their infrastructure  investment and
             preclude catastrophic failures.

                   In order to assist them in their efforts, EPA is developing a
                   guidance manual and educational material for detecting,
                   monitoring,  and correcting hydrogen sulfide corrosion problems.

      •     Municipalities should maintain records of the extent of corrosion,
             wastewater characteristics, and corrosion rates in different parts of their .
             systems. These records will assist in identifying factors that contributed to
             changes in rate over time.  As monitoring of corrosion rate becomes more
             established, the relationship between corrosion rate and other factors such

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            as water conservation, regionalization of wastewater treatment, and
            combined sewer separation can be studied.

      •     Programs to educate engineers regarding design procedures to minimize
            corrosion  must continue, and should be incorporated into academic
            curricula.
                                     i
      •     Other agencies should be encouraged to disseminate information on
            corrosion  issues.

                  The Department of Housing and Urban Development and the
                  Department of Agriculture's Farmer's Home Administration should
                  be encouraged to issue corrosion prevention design information to
                  municipalities obtaining funding from them for collection and
                  treatment systems.
                                     i
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      In spite of numerous previous and on-going efforts, corrosion is not entirely a
controllable phenomenon.  Therefore, additional research should be done in order to
reduce the high costs of correcting corrosion in existing infrastructure.
                                     i
      •     Additional research should be conducted on the effect of metals and
            cyanide on sulfide generation, and to establish threshold levels at which
            sulfide generation is inhibited.

      •     Research should be conducted to find a reliable method of monitoring the
            rate of corrosion.

      •     Microbial research should be encouraged to increase the understanding of
            the specific microbes contributing to the corrosion process as well as  to
            study the  relationship between these organisms and other microbial
            populations in a dynamic system.

      •     Applied research should be conducted on methods which offer low-cost
            approaches to controlling sulfide generation and hydrogen sulfide
            corrosion in sewers.
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1.0   BACKGROUND AND OVERVIEW

      1.1    Legislative Charge

      This report presents the results of the Hydrogen Sulfide Corrosion Study, a study
conducted by the U.S. Environmental Protection Agency (EPA) in response to a
specific Congressional mandate in the Water Quality Act of 1987 (Public Law 100-4).
Section 522 of the Act specified that:

      The Administrator shall conduct a study of the corrosive effects of sulfides in
      collection and treatment systems, the extent to which the uniform imposition of
      categorical pretreatment standards will exacerbate such effects, and the range of
      available options to deal with such effects (1).

      1.2    Los Angeles  County System History (2)

      The. sewer system of Los Angeles County serves over 4 million people in a 640
square mile area containing some 70 cities.  Over 500 million gallons per day of
wastewater is collected from residential, commercial, and industrial sources and
conveyed through 9,000 miles of sewers to six wastewater treatment plants.
Approximately 1,000 miles of sewers are owned and maintained by the County
Sanitation Districts of Los Angeles County (CSDLAC),  and the remaining 8,000 miles
are owned and maintained by local cities or Los Angeles County.

      The large sewers, typically constructed of reinforced concrete pipe with no
protective coatings or linings, range in size from 54 to 144 inches in diameter.  The
oldest of these sewers have been in service for over 65 years. During design it was
recognized that corrosion resulting from the presence of sulfide generated in the
wastewater was a potential problem, and in-house research on the subject began in the
1930's. It was believed that proper design could minimize the problem by preventing
the conditions which favor sulfide generation. As the size of the collection system
increased, it became apparent that sulfide generation was occurring.

      In 1968, a three-year research project was initiated to better understand the
processes of sulfide generation and hydrogen sulfide  corrosion.  Through measurements
made at monitoring stations throughout the sewer system, an empirical equation was
developed to allow prediction of sulfide generation and corrosion.  Detailed inspections
and monitoring in the early 1970's indicated the equation to be valid and able to predict
corrosion rates with reasonable accuracy.  Based on observed corrosion rates, the useful
structural lifetimes of the sewers were estimated to range from several decades for the
oldest sewers, up to hundreds of years for most of the sewers constructed after World
War II.

      A second series of inspections hi the early 1980's revealed that the rate of


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corrosion had increased dramatically since the previous inspections hi the 1970's, and
was no longer predictable using the empirical formulas.  In some sewers corrosion rates
had increased from 0.01 niches per year to 0.25 inches per year.  This would reduce the
life expectancy of those sewers from 100 years to four years.

      Subsequent studies showed a high'statistical correlation between the reduction in
the levels of certain metals and other specific wastewater constituents and the increase
in levels of hydrogen sulfide responsible for the corrosion.  These constituents had been
reduced through implementation of industrial pretreatment standards in 1975-1977
(ocean discharge requirements) and in 1983 (EPA categorical pretreatment standards).
This raised the question of whether implementation of industrial pretreatment standards
had resulted hi an increase  in corrosion rate, which would have significant economic
implications.                          ;

      Two of the theories being researched by CSDLAC are that the high levels of
metals and toxic constituents present in the  early 1970's 1) inhibited the biological
generation of sulfide, or 2) reduced release  of hydrogen sulfide  to the sewer atmosphere
where it can be converted into sulfuric acid  to cause corrosion.  When concentrations of
these compounds were reduced upon implementation of industrial pretreatment
standards, CSDLAC reported that the rates of sulfide generation  and corrosion
increased.  Indeed, CSDLAC found a high correlation between the drop in such
constituents and the increase in sulfide generation.  Between 1971 and 1986, average
total sulfide levels entering  the main  wastewater treatment plant increased from 0.4 to
3.0 mg/1.

      CSDLAC has estimated that at lekst $130  million will be needed to-replace or
repair approximately 25. miles of sewers that are severely corroded. An additional 16
miles will likely require repair or replacement within five years.  Almost $2 million per
year is currently being  spent for chemicals to control corrosion in the CSDLAC sewer
system.  CSDLAC is conducting corrosion-related research as well as supporting
research at the University of California -j Los Angeles, University of Southern
California, California Institute of Technology, and University of Arizona.
                                     j
      13    Scope of Study

      Hydrogen sulfide corrosion is a well-known phenomena that occurs in many
wastewater collection and treatment systems throughout the world. Several design
manuals have been published on the subject (3)(4)(5)(6)(7).

      The objectives of this study were to 1) document the extent and severity of
hydrogen sulfide corrosion problems  hi wastewater collection and treatment systems in
the United States, 2) investigate the effects  of implementing industrial pretreatment
standards on sulfide generation and corrosion, and 3) study options available to prevent
corrosion and rehabilitate corroded structures.
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      Achieving the first two objectives was limited by the lack of existing data and
information.  Specifically, many cities are unaware  of the existence of severe corrosion
problems, and have little or no documentation of corrosion. Corrosion problems often
are addressed only in the event of a catastrophic failure, as with a cave-in or pipe
collapse. No standardized technique is available to accurately measure the extent of
corrosion, resulting in poor reproducibility and questionable accuracy. Those
municipalities that have attempted  to quantify corrosion in sewers have done so
recently. Thus, the estimated corrosion rate represents an average rate over the lifetime
of the pipe, and does not reflect changes  in the  rate of corrosion which might have
occurred in the past No entities other than CSDLAC were found to have  sufficient
historical data on sulfide levels or corrosion rates to establish a correlation  between
implementation of industrial pretreatment standards and increased rate of corrosion.
Research on  the relationship between wastewater characteristics and potential corrosion
rates appears to be limited to that conducted or sponsored by CSDLAC.

      The study concentrated on the three areas mandated by the Act (see Section
1.1). Many factors influence corrosion besides the implementation of pretreatment
requirements, such as solids deposition, turbulence, temperature, and so on.  The lack
of an accurate corrosion-measuring technique and the limited data base on hydrogen
sulfide corrosion would have limited the ability of EPA to ascertain the effects of these
factors.  In addition, the study did not explore the impacts of transporting sewage
further to regional treatment plants, constructing separate sewers for sanitary
wastewater and storm water, or implementing water conservation programs. This report
does not discuss the problems caused by the toxicity of hydrogen sulfide gas or the odor
nuisance associated with its presence, as these issues were not mandated by the Act.

This report addresses the following topics:                                 ;

      1.     The Corrosive Effect; of Hydrogen Sulfide

             - consequences of corrosion
             - mechanisms of corrosion
             - factors affecting corrosion
             - extent of corrosion problems in wastewater systems in the U.S.

      2.     Effects of Industrial Pretreatment

                   theories of potential  effects of industrial wastewater constituents on
                   corrosion rate
                   results of CSDLAC research
                   comparison of metals and other industrial constituent levels in
                   wastewater in  the U.S.
                   results of EPA site visits to industrial cities
                   beneficial effects of industrial pretreatment on corrosion rate


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      3.     Detection. Prevention and Repair of Corrosion Damage
                                     i
                  options for corrosion; control in existing systems
                  techniques for corrosion prevention during design
                  rehabilitation techniques
                                     i
      4.     Recommendations        !
                                     i                             •          •
                  guidance to municipalities
                  continuing education to design  engineers
                  additional research |

      A separate document entitled, "Technical Report on Hydrogen Sulfide Corrosion
in Wastewater Collection and Treatment jSystems" has been prepared as a companion
document to the Report to Congress.  The Technical Report contains additional data
and technical detail to support the findings  and conclusions presented in the Report to
Congress.                                                                   .
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2.0   CORROSIVE EFFECTS OF HYDROGEN SULFIDE

      2.1   Consequences of Corrosion

      Corrosion of wastewater conveyance and treatment systems induced by the
presence of hydrogen sulfide can cause rapid and extensive damage to concrete and
metal sewer pipe, equipment used in the transport and treatment of wastewater, and
electrical controls and instrumentation systems. Such problems are rarely brought to
the attention of the public until  a catastrophic failure occurs such as street collapses or
sewer blockages resulting from sewer pipe failure.  However, sewer systems suffering
from hydrogen sulfide corrosion generally require costly, premature replacement or
rehabilitation of pipes, manholes, lift stations and pump stations. Figure 1 shows the
mechanism of sewer failure due to hydrogen sulfide corrosion.

      Equipment used in the treatment of wastewater is often subject to hydrogen
sulfide corrosion, resulting in equipment malfunctions,  poor reliability, increased
maintenance, and premature replacement  Electrical components (e.g. brushes,
switches, relays) process instrumentation, air conditioning and ventilation units, and
computer systems are particularly vulnerable to attack by hydrogen sulfide  at pumping
stations, lift stations, and treatment plants. This can cause poor reliability  of control
systems, increased maintenance  requirements, and premature replacement of costly
electrical components and computer equipment

      Hydrogen sulfide corrosion can also compromise structural integrity by corroding
equipment (bar screens, conveyors, etc.), pipe and  equipment supports, wastewater and
sludge storage tanks, and guard  rails, walkways, and grating at the treatment plant

      2.2    Basic Mechanism  of Hydrogen  Sulfide Corrosion

      Hydrogen sulfide corrosion may result from two mechanisms: 1) acid attack
resulting from the biological conversion of hydrogen sulfide gas to sulfuric acid in the
presence of moisture and 2) the direct attack of metals such as copper, iron, and steel
by hydrogen sulfide gas.  The first mechanism is responsible for corrosion of sewers and
structures used in the conveyance and treatment of sewage.  The second mechanism is
generally responsible for corrosion of electrical contacts, copper pipe, and other metallic
components in pumping stations, lift stations, and treatment plants.

      In properly designed gravity sewers the velocity of the sewage promotes surface
aeration helping to replenish any losses of oxygen due  to microbial activity. Under
certain conditions oxygen is consumed by microorganisms faster than it is supplied,
causing a change from aerobic to anaerobic (devoid of oxygen) conditions. Such
conditions  can occur in gravity sewers with low sewage velocities or long detention
times, in pressurized or surcharged mains which convey wastewater through a full pipe
with no opportunity for aeration, and in pumping stations or retention basins having


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                       K^y^%^.«STi-V:oV:hV^>&:tt.%
                       0>:CgqckfiII (gravel, soil J^^-tns^aO.'ov?^

                       "cMi^^^M^

                 (A) Corrosion reduces structural integrity
                                of  pipe crown.

                                                   Accumulation  of
                                                      Debris
                 (B) Crown collapses  and  void  forms  from
                         backfill washing  into  sewer.

                                                   O-^.-p
                                               3-:f\b?/;f

                                            <46^;
                                            i:'<~&.::o--'<
                     Backfill continues to wash into sewer
                     eventually  leading  to sewer  blockage
                         and/or street   collapse.
                                   Figure   I

Process   of  Sewer   Failure   due  to   Hydrogen  Sulfide   Corrosion

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long detention times.  In such cases, high velocities and turbulence actually promotes
release of dissolved hydrogen sulfide gas.

      The process of sulfide generation and sulfuric acid corrosion is as follows
1.     Under anaerobic conditions, anaerobic bacteria reduce sulfate, one of the most
      common constituents in water and wastewater, to sulfide. Wanner wastewater
      favors increased bacterial growth and metabolic activity. In large sewers this
      occurs primarily in slimes (0.25-3 mm, but typically 1 mm thick) attached to the
      submerged portion of the interior pipe surface (Figure 2a).

2.     The sulfide ions combine with hydrogen ions to form hydrogen sulfide, which
      exists as a gas dissolved in the water (Figure 2b).

3.     Hydrogen sulfide gas is released from the wastewater to the sewer atmosphere.
      The escape of hydrogen sulfide gas from solution increases with temperature due
      to decreased solubility in the wastewater, and is greatly accelerated under
      turbulent conditions (Figure 2b).

4.     The released H2S is  oxidized to sulfuric acid by aerobic bacteria of the genus
      Thiobacillus on moist,  non-submerged surfaces of the  pipe (Figure  2c).

5.     The acid attacks  the Portland cement and calcareous aggregate (limestone) of the
      concrete sewer pipes to form soft corrosion products such as gypsum.  These
      products are washed away or fall out of the concrete matrix,  exposing fresh
      concrete and aggregate to corrosion processes (Figure 2c).

      Concrete has been one of the most widely used pipe materials for large diameter
pipe, traditionally used for  interceptor sewers that carry large amounts of wastewater
contributed from smaller collector sewers. Results of one survey indicated that over 90
percent of the cities used unlined, reinforced concrete sewer pipe in portions of their
collection systems.  Certain pipe materials such as polyvinyl chloride (PVC) and vitrified
clay, which are immune to  hydrogen sulfide corrosion, have been used frequently for
pipes less than approximately 36 to 42 inches in diameter (see Section 3 for further
discussions of corrosion protection).

      2-3    Factors Affecting Corrosion

      Many factors affect the presence and rate of sulfide generation in wastewater
systems and the corrosion which may occur as a result of the presence of hydrogen
sulfide gas.

      Sulfide generation occurs in a sewer when dissolved oxygen (DO) levels in

                                        2-3

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                                                           =  anaerobic mr_ 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  HS  Oxidizing
                                                  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
                                             Moist  Pipe  Surface
                                                   H_S + 0_ aerobic __ H_ SO
                                                     2       2 ,            &
                                                              bacteria
           (C)  H2S is  oxidized  to  sulfuric acid  by
                aerobic. Thiobacillus   bacteria  living on
                moist,  non-submerged  surfaces.  Acid
                attacks  concrete, causing corrosion.
                            Figure  2
Mechanism   of  Sulfide  Generation  and  Corrosion   in   Sewers

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 the wastewater are nearly depleted.  Dissolved oxygen is reduced when the uptake of
 oxygen by bacteria present in the wastewater exceeds the replenishment of oxygen by
 natural aeration.  Factors that affect the depletion of dissolved oxygen include:

       1.    Velocity of sewage (low velocities reduce turbulence which decreases
             surface aeration).
       2.    Detention time (long detention times allow for oxygen depletion and
             subsequent sulfide generation).
       3.    Temperature (higher temperatures reduce the solubility of oxygen and
             increase growth rates of oxygen-consuming microorganisms).

       Once dissolved oxygen levels have been depleted, factors that affect the rate of
 sulfide generation are:

       1.    Concentration of organic materials and nutrients (higher concentrations
             increase bacterial growth and metabolism).
       2.    Temperature of the wastewater (higher temperatures increase bacterial
             growth rates).
       3.    Presence of toxic materials, including metals (reduces  bacterial activity).

       The subsequent release of hydrogen sulfide gas to the atmosphere of a sewer,
 retention basin,  or other structure is affected by:

       1.    Relative acidity (pH) of the sewage (pH values below  7 favor the
             formation of dissolved hydrogen sulfide gas, which can be released to the
             atmosphere).
       2.    Level of turbulence (turbulence promotes release of H2S gas).
       3.    Temperature of the sewage (higher temperatures reduce solubility of H2S
             in  the wastewater).
       4.    Presence of metals (metal ions react with sulfide to  form insoluble metallic
             sulfide precipitates which reduce the amount of H2S released), (see
             Section 3.2).

       Finally, the corrosion of the concrete or metal pipe or structure is affected by:

       1.     Presence of moisture (needed for bacterial activity).
       2.     Temperature of the pipe (higher temperatures increase bacterial activity).
       3.     Alkalinity of the concrete and its aggregate (higher alkalinity increases
             resistance to acid attack).

       Thus, it is clear that many interacting factors affect the conditions in which
hydrogen sulfide corrosion occurs,  and the rate of corrosion. In summary, the major
conditions which must be satisfied in order for hydrogen sulfide corrosion to occur  are:
                                        2-5

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       1.     Absence of, or very low levels of dissolved oxygen in the wastewater.
       2.     Release of hydrogen sulfide gas from solution.
       3.     Presence of moisture on a material that is subject to attack by sulfuric
             acid.
                                        1                                 .
       2.4    Findings of Study
                                        i
             2.4.1  Sites with Severe Corrosion

       Hydrogen sulfide corrosion may be found in many wastewater collection and
treatment systems throughout the world. Such corrosion varies widely with respect to
severity and rate. To distinguish levels of severity and rate, the following arbitrary
definitions were developed for this study:  ;

       Severe corrosion - loss of one inch or more of concrete, loose or missing
       aggregate, exposed reinforcing steel.
                                        i
                                        i
       High-rate corrosion - rate of corrosion which would cause a loss of at least one
       inch of concrete in twenty years.  This rate  is significant since reinforcing steel is
       generally about one inch below the interior concrete surface of large pipes
       constructed according to  industry standards (see glossary).  Exposure of
       reinforcing steel to corrosion can lead to structural impairment

       Accelerated corrosion - an increase in the rate of corrosion with time.
                                        i
       During EPA's site investigations, the estimated depth of corrosion was divided by
the age of the pipe to yield a lifetime average corrosion rate.  However, it is impossible
to determine from these  data whether the corrosion rate has changed with  time. Such
inspections merely offer a "snapshot" of the  corrosion processes and provide no
information on the history of corrosion, i.e;, whether accelerated corrosion  had
occurred.

       Site visits conducted by EPA revealed that the most severe cases of corrosion
generally occurred at areas of high turbulence, where hydrogen sulfide gas  was released
from the wastewater in large quantities.  Examples of sites where severe corrosion was
observed are:

       1.     Discharges of pressurized pipes (force mains) into manholes or other
             structures.

       2.     Junction  boxes, metering stations and transition structures.
                                        |
       3.     "Wet wells"  of pumping stations from which  the pumps draw the sewage.
                                        2-6

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       4.     "Drop" manholes, where wastewater cascades through a drop of several
             feet, creating high turbulence.

       5.     Treatment processes at wastewater treatment plants, where anaerobic
             sewage is subjected  to screening, aeration, discharge over weirs, arid other
             processes which impart turbulence.

       6.     Shallow slope collector sewers, large diameter or low-flow interceptors
             conveying wastewater containing relatively high levels of sulfide.

       7.     Structures, equipment, and instrumentation near areas where hydrogen
             sulfide is released to the atmosphere due to turbulence.

             2.4.2  Biological Mechanism  of Corrosion

       EPA field studies included sampling and analysis of sewer slimes from corroded
and non-corroded sites in  CSDLAC and Seattle for microbiological analysis.  The
Thiobacillus microbial community, responsible for production of corrosive sulfuric acid
at the pipe crown, was significantly different at corroded sites in CSDLAC. Much lower
numbers of viable, aerobic organisms were found on pipe crowns with low pH in both
systems.  However, all the crowns contained  high numbers of the Thiobacillus bacteria
responsible for the production of sulfuric acid. As would be expected, higher numbers
of sulfate-reducing (sulfide-producing) bacteria were found in the submerged sewer
slimes at corroded sites. Although relatively large numbers were also found in the bulk
wastewater, the sewer slimes are still considered to be the predominant site for sulfide
generation.

       A review of the literature and discussions with microbiologists active in the field
of hydrogen sulfide corrosion revealed that the body of knowledge on  the role of other
microorganisms in sulfide generation and corrosion is limited.  Because of the genetic
diversity and adaptability of the bacteria responsible for hydrogen sulfide corrosion,
results of laboratory studies are not directly applicable to the field.

            2.43  Extent of Corrosion

       Site visits conducted by EPA revealed that severe corrosion problems are not
limited to areas having warm or semi-tropical climates. Severe corrosion was observed
in Seattle, WA; Milwaukee, WI; Boise, ID; and Casper, WY; in  addition to
Albuquerque, NM; Baton  Rouge,  LA; Fort Worth, TX; Los Angeles County, CA; New
Orleans, LA; and Tampa, FL.  Figure 3 is a photograph of a severely corroded sewer in
Casper, Wyoming.  Figure 4 depicts a severely corroded, 108-inch diameter sewer in
Seattle. Corrosion processes had removed  at least one inch of concrete and had
exposed the reinforcing steel.  For comparison, Figure 5 shows a 50 year old uncorroded
sewer in Milwaukee, WI. Figure 6 shows a brick manhole in Baton Rouge, LA, in


                                       2-7

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FIGURE 3. SEVERELY CORRODED SEWER IN CASPER, WY

                                            £.;:^3*#
FIGURE 4. SEVERELY CORRODED SEWER IN SEATTLE, WA

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FIGURE 5.  NON-CORRODED, 50 YEAR OLD SEWER IN MILWAUKEE, WI
   FIGURE 6. BRICK MANHOLE WITH LOOSE AND MISSING BRICKS

-------

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 which bricks were loose or missing due to corrosion of the mortar.

       In the 1984 survey, 32 of 89 cities reported sewer collapses.  Based on review of
 the responses, it is estimated that 26 of the 32 cities had experienced collapses due to
 hydrogen sulfide corrosion.  Overall, almost 30 percent of the cities surveyed had
 experienced one or more pipe collapses that were judged to be due to hydrogen sulfide
 corrosion.

       Corrosion of sewers due to the presence of hydrogen sulfide is an international
 problem. At least twenty countries have reported hydrogen sulfide corrosion problems
 in their sewer systems.

       Corrosion problems are also prevalent in wastewater treatment plants.  In two
 independent surveys, approximately 60 to 70 percent of the respondents indicated the
 existence of corrosion problems at their wastewater treatment plants.  In one of the
 surveys,  14 percent of the 1,003 cities reported severe corrosion problems at their
 wastewater treatment plants, (see Figure 7).  Corrosion at the wastewater treatment
 plant may be an indication of corrosion in the collection system (6), and is more easily
 detected. On the  other hand, half of the jurisdictions EPA visited with severe corrosion
 problems in the collection systems had minor or no corrosion problems at the
 wastewater treatment plants. Therefore,  it is  likely that more  than  14 percent of the
 surveyed systems may have some serious corrosion problems in their collection systems.

       Site visits were made to two plants in the city of Los Angeles, two in New
 Orleans, and one in Tampa where severe corrosion problems were observed.  Problems
 ranged from severe deterioration of concrete structures to corrosion- of equipment,
 electrical contacts  and instrumentation systems. Severe corrosion of wastewater
 treatment plant components was also observed. Figure 8 shows a corroded metal slide
 gate and concrete  channel at Tampa, FL.  Figure 9 depicts a severely corroded concrete
 channel at the Hyperion wastewater treatment plant in Los Angeles.  Figure 10 shows
 the badly corroded exterior of an electrical control panel at the Hookers Point plant in
 Tampa.  Figure 11 depicts a severely corroded steel I-beam in an influent screen
 chamber that was previously covered  (St  Petersburg, Florida).

       Figure 12 shows relative frequency of severe corrosion problems in sewer systems
 or treatment plants throughout the United States.  This is based on EPA site
 investigations, surveys conducted by other organizations,  and the experiences of
 professionals active in the field of hydrogen sulfide corrosion control.  This does not
 represent all the cities experiencing severe corrosion problems.

       Figure 13 is a map which shows the frequency of use of a proprietary, corrosion-
resistant  liner for concrete pipe.  This type of liner is specified during design for
concrete  pipes which may be subjected to hydrogen sulfide corrosion.  The map does
not represent actual corrosion problems, but rather indicates where corrosion prevention


                                       2-10

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                                        DON'T
                                        KNOW
                                         2%
            CORROSION
             PROBLEM"
               32%.
FIGURE 7.  FREQUENCY OF CORROSION PROBLEMS
          AT WASTEWATER TREATMENT PLANTS
              (FROM WPCF SURVEY, 1989)
                   2-11

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 FIGURE 8. SEVERELY CORRODED METAL GATE AND
          CONCRETE; CHANNEL IN TAMPA, FL
FIGURE 9. SEVERELY CORRODED CONCRETE CHANNEL

         IN LOS ANGELES, CA

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FIGURE 10. CORRODED ELECTRICAL CONTROL PANEL
          IN TAMPA, FL
      FIGURE 11.  CORRODED STEEL I-BEAM IN
                ST. PETERSBURG, FL

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                                            2-15

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was considered appropriate for one or moire projects in these localities.

      High-rate corrosion (> 1 inch in 20 years) was found in many of the cities visited
by EPA, including Los Angeles, Albuquerjque, Baton Rouge, Boise, Casper, Fort Worth,
and Seattle. In one Albuquerque sewer the corrosion rate was estimated to be one inch
in seven years.  In Boise, corrosion rate in one .manhole was approximately one inch in
3Vz years (see the Technical Report). The literature reveals corrosion rates of up to one
inch in two years being reported hi Venezuela, Egypt, and Iraq (8)(9).
                                      i
                                      !
            2.4.4 Accelerated Corrosion
                                      !

      The term "accelerated" corrosion refers to an increase in the rate of corrosion.
This phenomenon was experienced by CSJDLAC, where corrosion rate increased from
approximately 0.01 inches per year (1 inch in 100 years) prior to 1975 to 0.17 inches per
year (1 inch in 6 years) during 1983-1987.  Total sulfide levels in the wastewater during
this same period increased  from about 0.5(  mg/1 to between 2 and 4 mg/1 (2).  Clearly,
the increase in observed sulfide levels and corrosion rate over the past 20 to 25 years is
dramatic.                             [

      EPA was unable to document othey cases of accelerated corrosion during the site
investigations and during discussions with other municipalities, consulting engineers, and
vendors of equipment and materials used for sulfide and corrosion control. One of the
major problems is lack of data, as CSDl^C is perhaps the only entity with extensive
historical data that quantify the sulfide  leyels and the extent of corrosion over a long
time period.  Thus,  CSDLAC has been able to document the change in rate at which
corrosion is occurring, as well as the levels of sulfide in the wastewater.  None of the
municipalities contacted had such historical data.

      During EPA's site investigations, the estimated depth of corrosion was divided by
the age of the pipe  to yield a lifetime average corrosion rate. However, it was
impossible to determine from these data yvhether the corrosion rate had changed with
time. Such inspections merely offer  a "snapshot" of the corrosion process and provide
no information on the history of corrosion.

             2.5    Results and Conclusions

      Attempts to gain a thorough understanding of the severity and extent of
hydrogen sulfide corrosion problems in U.S. were thwarted by the lack of historical data
on sewer corrosion, the lack of a standardized technique to measure corrosion, and the
poor documentation by municipalities of jsewer corrosion and the expenditures for sewer
rehabilitation  or replacement   Upon review of information gathered,  the following
findings and conclusions are presented:

       •     Severe hydrogen sulfide corrosion problems are not limited to CSDLAC.
                                      i
                                      I
                                        2-16

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            Extensive corrosion damage requiring immediate repair or rehabilitation
            can be observed in sewers in other cities. In some cases, corrosion
            damage is so extensive as to compromise the structural integrity of the
            pipe, which could lead to collapse.

      •     Hydrogen sulfide corrosion problems in operating systems are often not
            recognized early enough to take corrective action before considerable
            damage has occurred.

      •     In a 1984 survey, approximately 30 percent of the  89 cities reported sewer
            collapses that were judged to be due to hydrogen sulfide corrosion.

      •     In two independent surveys, 60 to 70 percent of the municipalities
            reported hydrogen sulfide corrosion at their wastewater treatment plants.
            Of those plants experiencing corrosion, about 20 percent are considered to
            have severe problems.

      •     Hydrogen sulfide corrosion problems have been documented in the
            literature of at least 20 foreign countries.

      •     Due to lack of historical data,  corrosion rate is estimated based on depth
            of corrosion and age of pipe.  This may not reflect the true corrosion rate,
            which may be substantially higher at a given time-and condition.

      Evidence of severe corrosion may be  found in cities throughout the United States
and other countries. Cases of "high  rate"  corrosion are also common.  However, at this
time EPA has been unable to document other cases of "accelerated" corrosion of the
type that has been experienced in the sewers of CSDLAC.
                                       2-17

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3.0   EFFECTS OF INDUSTRIAL, PRETREATMENT

      3.1   Overview

      EPA's industrial pretreatment program regulates the discharge of certain
constituents such as metals and toxic materials into municipal sewer systems.  The
County Sanitation Districts of Los Angeles County (CSDLAC) implemented industrial
pretreatment standards in 1975-1977 to meet ocean discharge requirements, and in 1983
to comply with the EPA-mandated industrial pretreatment program.

      Metals and other constituent levels in CSDLAC wastewater dropped substantially
between the early 1970's and the mid 1980's.  A concomitant rise in sulfide levels in the
wastewater occurred over this same time period, and CSDLAC observed an increase in
the rate of corrosion in their concrete sewers.  One theory proposed  is that the high
levels of metals and other toxic constituents present in the early  1970's inhibited the
biological generation of sulfide.  A second theory  is that these materials prevented
release of hydrogen sulfide to the sewer atmosphere where it could be converted into
corrosive sulfuric acid.

      A thorough search of the literature and contacts with municipalities throughout
the U.S. revealed that no data existed from other  cities to show a correlation between
implementation of industrial pretreatment standards and increased sulfide generation
and corrosion.  Municipalities simply do not have  historical data on corrosion rates or
sulfide levels that would allow establishing a correlation such as  was  found in CSDLAC.
(See Section 2.4.4).

      Given the unavailability of full-scale data to support the theory proposed by
CSDLAC,  the study objectives were determined to be  as follows:

      1.    Investigate the theoretical impacts of metals on sulfide  levels.

      2.    Review and analyze research conducted or supported by CSDLAC.

      3.    Compare metals levels of CSDLAC  with other cities to assess whether
            other municipal sewer systems could potentially experience a similar
            phenomenon (decrease in industrial wasfewater constituents and increase
            in corrosion).

      4.    Review data from  site visits to industrial cities to determine if corrosion
            rates differed in sewers with high industrial contributions vs. those with
            predominately residential contributions.

      5.    Review other potential impacts of implementing pretreatment standards.
                                       3-1

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      3.2    Reduction of Sulflde by Precipitation with Metals

      It is well known that many metals '[bind" with sulfide to produce a precipitate
which is insoluble, effectively preventing release of hydrogen sulfide gas to the sewer
atmosphere and preventing formation of corrosive sulfuric acid.  As discussed in Section
4.2, salts of metals such as iron and zinc tire routinely added to wastewater to prevent
odors and corrosion associated with hydrogen sulfide.

      The weight of metal required to precipitate a given weight of sulfide can be
predicted theoretically using chemical reaction equations.  However, in wastewater
containing  a complex  mix of organic and inorganic compounds which interfere with such
reactions, the amount of metal required to precipitate a given weight of sulfide may be
much greater than what would be predicted from the equations.
                                      I.
      Table  1 shows  the theoretical increjase in sulfide levels which could be accounted
for by the reduction in metals available to precipitate the sulfide. Based on  the
measured decrease in metals in CSDLAC; wastewater between the periods 1971-1974
and 1983-1986, the theoretical increase in dissolved sulfide is in excess of 4 mg/1.
However, the actual increase measured in dissolved sulfide in CSDLAC wastewater
during that same period was approximately  1 mg/1.

      Actual dosages needed to precipitate sulfide are considerably higher than the
theoretical dosage ratio of 1.7 to 1. Field studies by CSDLAC on the addition of iron
to control sulfide showed that when the dissolved sulfide levels were between 1 and 4
mg/1,  a dosage ratio of six to seven parts iron to 1 part dissolved sulfide was  required to
achieve 90 percent removal. When dissolved sulfide was less than 1 mg/1, a  dosage ratio
of 44 to 1 was required (10).  Thus, four jto 25 times the theoretical dosage was required
in the CSDLAC system.  Other studies have shown that five to seven  times the
theoretical dosage is required to remove sulfide using zinc. The last column in Table  1
indicates the predicted increases in sulfide based on the decreased zinc and  iron.  Based
on these field studies, the expected increase in dissolved sulfide expected from
decreased zinc and iron concentrations is; 0.1 to 0.8 mg/1, vs. the 1 mg/1 observed.

      It is unlikely that decreased precipitation alone could account for the changes in
sulfide levels. Decreasing metals concentrations would  be expected to increase the
amount of dissolved sulfides without affecting the total sulfide concentration. This is
because the total sulfide test measures dissolved sulfides and metal-sulfide precipitates.
However, between 1971  and 1986,  average total sulfide  levels entering the main
CSDLAC wastewater treatment plant increased dramatically from 0.3  to 3.0 mg/1.
Therefore, metal precipitation principles jdo not explain the rise  in total sulfides
experienced in CSDLAC.              j
                                        3-2

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                                 TABLE 1

             THEORETICAL INCREASE IN DISSOLVED SULFIDE
                BASED ON METAL PRECIPITATION; CSDLAC
Metal

Chromium
Copper
Lead
Zinc
Nickel
Iron
Cadmium
TOTAL
Reduction
in Metals1
mg/1
0.68
0.38
0.17
1.34
0.14
4.92
0.01
Theoretical
Increase in Dissolved
Sulfide Concentration2
mg/1
0.42
0.10
0.03
0.66
0.08
2.83
0.00
4.12
Expected
Increase Based
on Field Studies3
mg/1
NFD4
NFD
NFD
0.06 - 0.1
NFD
0.1 - 0.7
NFD
0.16 - 0.8
1  Difference in average values for the periods 1971 - 1974 and 1983 - 1986.

2  Based on stoichiometry of chemical precipitation reactions

3  Based on field dosages required to precipitate dissolved sulfide;
  CDSLAC research data.

4  No field data on dosage performance are available
                                    3-3

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33   Biological Inhibition by Metals arid Toxic Compounds

      Another potential mechanism to explain the increased sulfide generation in the
CSDLAC sewer system is that the high levels of metals present in the 1970's inhibited
the bacteria responsible for the biochemical generation of sulfide. CSDLAC set up
several laboratory and pilot-scale experiments to investigate this theory.

      The first experiments involved laboratory-scale investigations to determine the
acute toxicity of selected metals and cyanide on cultures of sulfate-reducing bacteria.
While the results were inconclusive at the levels of metals present in CSDLAC
wastewater in the 1970's, the experiment^ showed some bacterial inhibitions at various
levels of constituents.  In general, field conditions are difficult to reproduce in
laboratory "bench scale" experiments.
                                      i
      The next experiment used a series; of columns designed to provide conditions
appropriate for the generation of sulfide. [ Wastewater "spiked" with individual metals
and cyanide as well as a "cocktail" of multiple metals and cyanide was added to the
columns.  The cocktail containing multiple metals and cyanide was added to the
wastewater at levels approximating those in the early 1970's, as well as at five times
those levels.  At the 1970's levels, total and dissolved sulfide generation was reduced by
34 percent At five times the 1970's levels, sulfide generation virtually ceased (11).

      The most recent experiment involved the construction of small diameter piping
systems to more closely simulate conditions in a sewer.  At metals and cyanide levels
approximating those in the early 1970's, results similar to the column experiments were
obtained:  Total sulfide levels were reduced by 34 percent and dissolved sulfide levels by
25 percent (11).

      The results of these experiments strongly suggest that the generation  of hydrogen
sulfide in  the wastewater of CDSLAC was suppressed due to the presence of
constituents associated with industrial discharges of the early 1970's. Higher levels of
sulfide in  the wastewater would be expected to result hi higher concentrations of
hydrogen  sulfide gas in the sewer atmosphere and higher sewer corrosion rates.
However, the relationship  between waste|water sulfide levels and corrosion rate is not
well established.                       j    .

      3.4    Comparison  of Metals Levels at CDSLAC with Other Cities Before
             Pretreatment
                                      i
      Using  available data, levels of meials and cyanide for CDSLAC wastewater
entering the main treatment plant during the periods 1971 - 1974 and 1986 were
compared with levels hi the wastewater of 50 municipal treatment plants across the U.S.
in 1978 - 1979.  Data were analyzed for these 50 cities from the EPA report, "Fate of
Priority Pollutants in Publicly Owned Treatment Works" (12)(13). These data were
                                      i
                                       3-4

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collected in 1978-1979 prior to any significant implementation of industrial pretreatment
standards. The fifty cities had estimated industrial flow contributions ranging from ten
to fifty percent of the total flow.  Thirty-two (64%) of the 50 cities listed industries with
metal wastes. Analysis of these data allowed determination of the number of cities with
metals and cyanide levels similar to those of CDSLAC prior to pretreatment, and
assessment of whether other cities may have had the potential to experience suppression
of sulfide generation and corrosion due to the presence of these constituents.

      Table 2 shows a ranking of the 50 cities plus CSDLAC based on the
concentrations of selected metals and cyanide in the wastewater.  This was developed
from the sum of the equivalent weight concentrations of each of the constituents, and
does not consider the toxicity  of the constituents (alone or in combination) on sulfate-
reducing bacteria.  Of the 50 other municipalities, only three (six percent) are ranked
higher than CDSLAC, while 47 (94 percent) are ranked lower. The total concentratipn
of metals and cyanide in CDSLAC wastewater was approximately three times the
median concentration  for the  51 cities.

      The total metals levels  in CSDLAC wastewater in 1986 are also shown  in Table
2. On an equivalent weight basis, 1986 levels were 42 percent of 1971 - 1974 levels.
Comparing 1986 CSDLAC levels with 1978 - 1979  levels of 50 other cities, 16  cities (32
percent) were higher than CSDLAC,  and 34 cities  (68 percent) were lower.  Current
concentrations of metals in these cities, with pretreatment standards, is not available.

      Clearly, sulfide generation and corrosion in  CSDLAC sewers increased
dramatically between 1971 and 1986, and metals and cyanide levels dropped   :
significantly.  Unfortunately, it is not known what levels of specific metals, cyanide, and
combinations cause suppression of hydrogen sulfide corrosion.  Although CSDLAC
appears to have passed a threshold level of metals  and cyanide which resulted in
increased sulfide levels and corrosion, it is difficult to predict whether other cities could
experience a similar increase in sulfide generation  and corrosion upon reduction in
metals levels resulting from industrial pretreatment

      3.5   Site Visits to Industrialized Cities

      EPA conducted site visits to three cities having portions of the sewer system with
high industrial contributions.  Initially, it was believed that comparison of corrosion in
residential vs. industrial sewers might show differences attributable  to the metals and
other constituents present in the wastewater.  The cities were Charlotte, NC,
Milwaukee, WI, and Tempe, AZ. Results of the inspections were inconclusive, as
corrosion was observed in both residential and industrial sewers.  Further efforts to
inspect residential and industrial  sewers were abandoned due to the multitude of factors
and conditions which affect sulfide generation  and  corrosion, which could easily mask
any effects associated with the presence of industrial constituents.
                                       3-5

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                                           TABLE 2
COMPARISON OF CSDLAC METALS LEVELS BEFORE AND AFTER
PRETREATMENT WITH METALS LEVELS OF 50 CITIES IN 1978-1979
'
CADHIUM CHROMIUM COPPER CYANIDE LEAD MERCURY NICKEL
PLANT
1
2
3
LA County
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      3.6    Beneficial Effects of Local Industrial Pretreatment Programs
                                    '<*••..           ••.•
      It is important to recognize that several aspects of local industrial pretreatment
regulations can lower the potential for hydrogen sulfide corrosion in sewer systems.
Among  the more important of these are reduction in:  1) sulfide-bearing wastes, 2) high
strength organic waste  discharges, 3) high temperature discharges, 4) fats, oils, and
grease, and 5) acidic wastes. Because of the complex interaction of all the factors that
affect hydrogen sulfide corrosion it is very difficult to quantify these effects for a broad
base of sewer systems.  Beneficial impacts of local regulation of industrial waste
discharges on sulfide generation in municipal sewers are summarized in Table 3.  In this
table, sulfide is the only parameter specifically regulated by the EPA Categorical
Pretreatment Standards.

      3.7    Conclusions

      The national effects of industrial pretreatment on hydrogen sulfide corrosion are
impossible to ascertain since no municipalities other than CSDLAC were found to have
sufficient data to establish a correlation.  Based on theoretical analyses, review of full
scale and pilot scale  research data from CSDLAC,  and a series' of site investigations, the
following conclusions are presented.

      •     The reduction  in metals and other industrial constituents in CSDLAC
             wastewater apparently caused an acceleration in corrosion rate, possibly
             due to  biological inhibition and/or chemical precipitation.

      •     Two pilot studies conducted by CSDLAC demonstrated that sulfide
             generation was reduced  when metals were added to the wastewater at
             levels approximating those in  the early 1970's.  (This is consistent with the
             known  toxic effects of metals  on other microorganisms.)

      •     When comparing 197Q's data  from 50 other cities having 10 to 50 percent
             industrial flow input, total metals and cyanide levels in CSDLAC
             wastewater were higher  than levels in 94 percent of 50 U.S. cities.

      •     If current (1986) CSDLAC data are compared with 1970's data from 50
             cities, CSDLAC  levels would  be lower than 32 percent of the cities.

      •     It is difficult to project how many cities could potentially have a corrosion
             problem  affected by industrial pretreatment since it is not known at what
             levels industrial constituents begin to suppress sulfide generation.

      •     Site visits to inspect corrosion in residential vs. industrial sewers were
             inconclusive regarding the impacts of metals and other industrial
             constituents on hydrogen sulfide corrosion.

                                        3-7

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                                    TABLES
                   BENEFICIAL 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
High temperature wastes
Sulfide generation rate
proportional to organic strength;
reduction in organic strength
reduces oxygen uptake and
depression of dissolved oxygen in
wastewater
Lower temperature reduces
sulfide generation rate; increases
solubility of H2S, reducing release
of H2S; increases solubility of
oxygen
Wastes containing fats,
oils, and grease
Acidic wastes
Reduces potential for sewer
clogging, reduced velocities, solids
deposition, and sulfide generation
Maintaining pH at or above
neutral decreases amount of H2S
available for release to the sewer
atmosphere
                                       3-8

-------
Local regulation of certain non-toxic constituents in industrial waste
discharges has likely had a beneficial impact in reducing the potential for
sulfide generation and corrosion.
Additional research is necessary to establish the constituents and their
associated levels at which sulfide generation is suppressed or accelerated.
                             3-9

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-------
4.0   DETECTION, PREVENTION, AND REPAIR OF HYDROGEN SULFIDE
      CORROSION DAMAGE
      Significant advances have occurred during the past twenty years in the area of
sewer rehabilitation.  More economical techniques are now available to rehabilitate
sewers damaged by corrosion, eliminating the need to excavate and replace the damaged
pipe.  Several design manuals have been prepared on the subject of preventing and
controlling sulfide generation in sewers, both for the design of new systems and for the
operation of existing systems. These include manuals by the Environmental Protection
Agency, the American Society of Civil Engineers, the American Concrete Pipe
Association, and others (3)(4)(5)(6)(7). Methods for the detection of corrosion are
resource intensive and not routinely used by cities.  Public education programs are
needed to make municipalities aware of the problem before considerable damage has
been done so that funding can be provided to correct the problems.

      4.1    Detection of Hydrogen Sulfide Corrosion

      Many municipalities are unaware of a corrosion problem until it manifests itself
in the form of a pipe collapse or other catastrophic failure.  With unproved resolution
of remote television cameras used in sewer inspections, corrosion can be detected by the
trained observer. In most cases,  however, considerable damage may already have been
done.  Sonic devices have also been successfully applied for sewer corrosion detection
and measurement, but damage must already have occurred for detection by this method.
Physical measurements of corrosion have  also been attempted, but the accuracy using
such techniques is poor.  No standardized technique is available for measuring and
quantifying the extent of corrosion.

       Detection of acid on the pipe crown and walls is probably the best "early
warning" that a potential or existing corrosion problem exists. However, most
municipalities are not aware of the pH test or its application (4).  An education
program is necessary to train municipal officials on proper techniques for the detection
and monitoring of corrosion.

       4.2    Prevention  of Hydrogen  Sulfide Corrosion  in Existing Systems

       A number of techniques have been used to control corrosion and odors
associated with hydrogen sulfide  generation in  existing systems.  The most common
techniques can be divided into the general categories of  oxidants, precipitants, or pH
elevators.  Oxidants control sulfide by chemically or biologically causing the oxidation of
sulfide to sulfur, thiosulfate  or sulfate.  Metals  form insoluble metal sulfide precipitates,
preventing release of gaseous H2S. Elevation of the pH  through shock dosing of caustic
controls sulfide generation by inactivation of sulfate-reducing slimes present on the wall
of the sewer pipe. A summary of sulfide  control techniques is provided in Table 4.

                                       4-1

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      All of the above control techniques are oriented towards reducing the levels of
dissolved sulfide in solution such that less hydrogen sulfide is released to the sewer
atmosphere. Work conducted by CSDLAC  under field conditions indicated that,
although significant reductions (75 to 95%)  in dissolved sulfide could be obtained with
chemical addition, only modest reductions (50 to 60%) in H2S levels in the sewer
atmosphere were realized. Thus, a 90% reduction in dissolved sulfide does not indicate
that the rate of corrosion will be reduced! by 90% (2).

      No one sulfide control technique can be generalized as being the most cost-
effective.  Dosages of chemicals to control sulfide vary widely from one wastewater
system to another, and are dependent on wastewater characteristics and other site-
specific factors.  Sulfide control options must be considered on a case-by-case basis.

      Controlling sulfide generation by the addition of chemicals is often a costly
proposition. Most municipalities add chemicals to control odors, not corrosion. Some
cities have discontinued sulfide control efforts because of the high cost
                                      i
      43   Prevention of Hydrogen Sulfide Corrosion in the Design of New Systems
                                      I
      Consideration of sulfide generation and corrosion is critical  in the design of
wastewater collection systems. While it is possible, and sometimes necessary, to
incorporate chemical addition stations for sulfide control as part of the overall system
design, the most cost-effective and rational engineering  approach is to develop a
hydraulic design that minimizes sulfide generation. In general, such an approach strives
to maintain aerobic conditions in the wastewater by providing adequate wastewater
velocities, preventing deposition, of solids] and by minimizing the use of force mains,
inverted  siphons, and surcharged sewers in which anaerobic conditions can develop.
                                      i
      Under certain conditions, sulfide generation may be unavoidable. Empirical
equations have been developed for prediction of the rates of sulfide build-up and
corrosion, but these require specific data \ input and field verification.  Where sulfide
generation is anticipated, corrosion resistant materials can be selected, or the alkalinity
and thickness of concrete pipe can be increased to help minimize the effects of
hydrogen sulfide corrosion. Table 5 summarizes various approaches used to minimize
sulfide generation and corrosion during the design of wastewater collection and
treatment facilities.                    !
                                      i
                                      I
      4.4   Repair of Damage Caused by Hydrogen Sulfide Corrosion
                                      i

      Once corrosion damage has occurred, it may be necessary to repair a structure to
reduce the potential for failure or collapse.  In the past, excavation and replacement  was
a common repair solution for corroded pjpes and structures.  However,  due to the
expense, the disruption to traffic, the potential for damage to other underground
utilities, and. the interruption to the  service  itself, in-line rehabilitation techniques have
                                        4-4

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become more prevalent (14). Rehabilitation techniques are methods and repairs
applied to an existing structure to prolong its useful life.  With such techniques,
municipalities can repair existing structures  at a somewhat lower cost than replacement,
and with less public inconveniences due to traffic disruptions and service interruptions.
New techniques are being developed and refined that offer additional solutions to the
problems of sewer rehabilitation.  However, sewer rehabilitation is still a very costly
operation.

       Numerous rehabilitation techniques exist, but not all are applicable to corrosion
repair.  The selection of a particular method of rehabilitation depends on many factors
such as economics, extent of damage to structural integrity, disruption of traffic, and
excavation requirements.   Corrosion rehabilitation techniques generally  focus on repairs
to protect structural integrity and provide a protective barrier against subsequent acid
attack.  Table 6 describes various methods of pipeline rehabilitation, indicating their
applications, advantages, and disadvantages (15).

       4.5   Findings  and Conclusions

       Based on current information, the I following findings  and conclusions are
presented for the detection, prevention and control of hydrogen sulfide  corrosion, and
for repairing damage by hydrogen sulfide corrosion:

       •    Many municipalities are nojt aware of corrosion problems until
            catastrophic failure  occurs. !

       •    No  standardized  technique [exists for measuring corrosion  to allow
            estimation of corrosion rate1.
                                      i
       •    Educational programs are necessary to disseminate  information on
            corrosion detection  and monitoring to municipalities.

       •    Although some design guidelines have been developed which should assist
            in minimising sulfide generation and corrosion, these guidelines  are not
            universally practiced. Some observed corrosion could have been foreseen
            and avoided using existing design  principles that minimize sulfide
            generation and corrosion.

       •    A large variety of chemicals and techniques are used to control sulfide in
             sewers.  However, their costs and  effectiveness for corrosion control vary
            widely based on site-specific conditions.

       •     Based on a 1984 survey of 89 cities, 34 percent take measures to reduce
             sulfide in sewers, and 63  percent provide corrosion  protection of sewers or
             use one or more techniques to rehabilitate sewers damaged by hydrogen
                                                 •
                                              .

                                        4-6

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             sulfide corrosion.
                                              fc* .                         i
      •     The current national expenditure for controlling sulfide generation in
             sewers is on the order of tens of millions of dollars per year.  CSDLAC
             alone is spending approximately two million dollars per year on chemicals
             to control sulfide.

      •     National expenditures for rehabilitation of sewers and structures damaged
             by hydrogen sulfide corrosion is very difficult to estimate.  Although
             municipalities maintain records of operation and maintenance activities,
             often the cost of corrosion-related rehabilitation and replacement activities
             are not readily  retrievable.

      •     Alternatives are available to rehabilitate pipe which has been damaged due
             to corrosion. Some, such as sliplining and cured-in-place inversion lining,
             have been widely used with satisfactory results. Others, such as
             application of "corrosion-resistant" coatings, have experienced early failure.
             Although in-situ sewer rehabilitation has become more prevalent due to its
             economic advantage over sewer replacement, it remains a very costly
             operation for municipalities.

      Design guidelines for  minimizing hydrogen sulfide corrosion in sewers have been
developed, although they are not universally  practiced and do not ensure absence  of
corrosion problems. Controlling  corrosion is difficult and costly. Procedures for
rehabilitating pipe damaged by corrosion are well established, but such  options are also
very costly.  Research on new, economical approaches to controlling existing sewer
corrosion appears  to be limited.
                                         4-9

-------

-------
5.0   RECOMMENDATIONS

      Based on the findings of this study, additional emphasis needs to be given to
information dissemination and education regarding hydrogen sulfide corrosion.  There is
a need to inform municipal political officials, design engineers, construction contractors,
and operating staff of methods to minimize  corrosion in new installations and detect
corrosion in existing structures.

      •     Municipalities should incorporate corrosion detection and monitoring
             strategies into their collection system operating and maintenance
             procedures, in order to protect their infrastructure  investment and
             preclude catastrophic failures.

                   In order to assist them in their efforts, EPA is developing a
                   guidance manual and educational material for detecting,
                   monitoring, and correcting hydrogen sulfide corrosion problems.

      •     Municipalities should maintain records of the extent of corrosion,
             wastewater characteristics, and corrosion rates in different parts of their
             systems. These records will assist in identifying factors that contributed to
             changes in rate over time. As monitoring of corrosion rate becomes more
             established, the relationship between corrosion rate and other factors such
             as water conservation, regionalization of wastewater treatment, and
             combined sewer separation can be studied.

       •     Programs  to educate engineers regarding design procedures to minimize
             corrosion  must continue,  and  should be incorporated into academic
             curricula.

       •     Other agencies should  be encouraged to disseminate information on
             corrosion  issues.

                   The Department of Housing  and Urban Development and the
                   Department of Agriculture's Farmer's Home Administration should
                   be  encouraged to issue corrosion prevention design information to
                   municipalities obtaining funding  from them for collection and
                   treatment systems.

       In spite of numerous previous and on-going efforts, corrosion is not entirely a
 controllable phenomenon.  Therefore,  additional research should be done in order to
 reduce  the high costs of correcting corrosion in existing infrastructure.

       •     Additional research should be conducted on the effect of metals and
             cyanide on sulfide generation, and to establish threshold levels at which


                                         5-1

-------
      sulfide generation is inhibited.
                                i
•     Research should be conducted to find a reliable method of monitoring the
      rate of corrosion.          i

•     Microbial research should b<5 encouraged to increase the understanding of
      the specific microbes contributing to tie corrosion process as well as to
      study the relationship between these organisms and other microbial
      populations in a dynamic system.

•     Applied  research should be [conducted on methods which offer low-cost
      approaches to controlling sulfide generation and hydrogen sulfide
      corrosion in sewers.        |
                                  5-2

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6.0    REFERENCES
1.     Water Quality Act of 1987, Public Law 100-4-Feb.4, 1987, Sec. 522.  Sulfide
      Corrosion Study.

2.     Stahl, J.S., Redner, J., and R. Caballero, "Sulfide Corrosion in the Sewer System
      of Los Angeles County," presented at llth U.S./Japan Conference on Sewage
      Treatment Technology, the Public Works Research Institute, Tsukuba, Japan,
      October, 1987; and ASCE Conference on Sulfide Control in Wastewater
      Collection and Treatment Systems, Tucson, AZ, February, 1989.

3.     "Process Design Manual for Sulfide Control in Sanitary Sewerage Systems,"
      USEPA, Cincinnati, OH, 1974.

4.     "Odor and Corrosion Control in Sanitary Sewerage Systems and Treatment
      Plants,"  EPA/625/1-85/018 USEPA, Cincinnati, OH 1985.

5.     "Sulfide and Corrosion Prediction and Control,"  American Concrete Pipe
      Association, Vienna, VA, 1984

6.     "Sulfide in Wastewater Collection and Treatment Systems, " ASCE Manual
      No.69, ASCE, New York, NY,  1989.

7.     Thistlethwayte, D.K.B., The Control of Sulphides in Sewerage Systems," Ann
      Arbor Science, Ann Arbor, MI, 1972.

8.     Prevost, R.C., "Corrosion Protection of Pipelines Conveying Water and
      Wastewater - Guidelines," World Bank Technical Paper No. 69, Water Supply
      Operations Management Series, The World Bank, Washington, D.C., 1987.

9.     Nadarajah, A., and J.  Richardson, "Prevention and Protection of Sewerage
      Systems Against Sulphide Attack with Reference to Experience in Singapore,"
      Prog. Wat Tech.  Vol  9: 585-598, Pergamon Press, Great Britain, 1977.

10.   Won, D.L., "Sulfide Control with Ferrous Chloride in Large Diameter Sewers,"
      internal report, County Sanitation Districts of Los Angeles County, November,
      1988.

11.   Morton, R., Caballero, R., Chen, C-L., and J. Redner, "Study of Sulfide
      Generation and Concrete Corrosion of Sanitary Sewers," for presentation  at
      WPGF Annual Conference, San Francisco, CA,  October, 1989.
                                       6-1

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12.    "Fate of Priority Pollutants in Publicly Owned Treatment Works - Volume I,"
      EPA 440/1-82/303, USEPA, Washington, D.C., Sept, 1982.
                                      i
13.    "Fate of Priority Pollutants in Publicly Owned Treatment Works - Volume 11,"
      EPA 440/1-82/303, USEPA, Washington, B.C., Sept, 1981

14.    "Fragile Foundations:  A Report on America's Public Works", National Council
      on Public Works Improvement, Washington, DC, February, 1988.

15.    "Utility Infrastructure Rehabilitatibn"[ U.S. Dept of Housing  and Urban
      Development, Washington,  D.C., Noyember, 1984.
                                      6-2

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                                   GLOSSARY

Aerobic wastewater - Wastewater containing elemental oxygen.

Aerobic bacteria - Bacteria that require free elemental oxygen for their growth.^5)

Anaerobk wastewater - Wastewater devoid of elemental oxygen.

Anaerobic bacteria - Bacteria requiring, or not destroyed by the absence of air or elemental
      oxygen. (6)

Accelerated Corrosion - An increase in the rate of corrosion over time.

BOD - Abbreviation for biochemical oxygen demand.  A standard test used in assessing
      wastewater strength, modified from (6)

Crown - The top interior surface of a pipe.

CSDLAC - County Sanitation Districts of Los Angeles County.

DO - Abbreviation for dissolved oxygen. The oxygen dissolved in wastewater, expressed in
      milligrams per liter or parts per million in this  report modified from (6)

Drop Manhole - A manhole in which the wastewater enters at a point higher in elevation
      than the exit point.

Equivalent weights - Measure of amounts of chemical available for reaction.

Force Main - A sewer flowing full under pressure imparted by a pump.

Gravity Sewer -  A sewer  sloped to flow partially full under the influence of gravity.

HjS - Hydrogen sulfide gas in the atmosphere or dissolved in wastewater.

High Rate Corrosion - Corrosion which results in the  loss of at least one inch of concrete
      in 20 years, as defined for this study.

Interceptors - Major sewers transporting large amounts of wastewater contributed from
      smaller collector sewers.

Lift station - A small wastewater pumping station that lifts wastewater to a higher elevation
      when the continuation of the sewer at reasonable depths would involve excessive
      depths of trench, or that raises wastewater from areas too low to drain into available
      sewers, modified from (6)

                                       G-l

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PE - Polyethylene

pH - The reciprocal of the logarithm of the hydrogen-ion concentration. Neutral water has
      a pH of 7, acidic water has a pH value of less than 7, and alkaline water has a pH
      value greater than 7. modified from (6)

Pilot Studies - Experiments conducted at small scale designed to simulate conditions at full
      scale.                            :
                                       i
Precipitation: - A chemical process whereby two soluble components combine to form a
      solid, insoluble product Metal sulfide precipitates formed during sulfide control are
      generally carried as suspended solids in wastewater until removed at the wastewater
      treatment plant

PVC - Polyvinyl chloride

Reinforced Concrete Pipe - Pipe composed of concrete formed over a framework of steel
             reinforcing  bars.   The  steel contributes to the strength of the pipe to
             withstand internal loads and external forces from soil, streets, and  traffic.
Septic - Devoid of oxygen.

Severe Corrosion - Corrosion in which at
least one inch of concrete is lost, as defined for
       this study.                       i

Shock dosing - The intermittent application of chemicals.

Sulfide - Commonly refers to a variety of! forms of sulfur, usually within the liquid phase,
       including soluble hydrosulfide (HS), molecular H2S, organic sulfide complexes, and
       inorganic metal sulfides  (FeS, Zn$, etc.).  Technically , sulfide should refer to the
       sulfide ion (S'2). (6)              i

Sulfide corrosion - Refers to hydrogen sulfide-induced corrosion, caused either directly by
       H2S gas or indirectly from biological conversion of gaseous H2S to sulfuric acid.
       Technically, there is no corrosion  caused by sulfide ions.

Surcharged Sewer - A gravity sewer in which sewage flow "backs up" and completely fills
      ' the pipe.                        ;

VCP - Vitrified clay pipe.

Weir - A fixed plate used to regulate  the flow of a liquid in an open channel.
                                       i
Wet Well - A reservoir from which  pumps draw liquid.

WWTP - Wastewater treatment plant   ;
                                       t
                                       ! G-2

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