EPA-600/2-77-017a
December 1977
Environmental Protection Technology Series
Mental Researc
..........
uiifflflfflenfi Protection Agoncj
Imcmnati, Ohio
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II IP III I p II II III 1111 III ill IP 11
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are: \' "." " ,' ..",'.'.,'," .„.,,., '..,.
1. Environmental Health Effects Research
2. Environmental Protection technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6, Scientific and Technical Assessment Reports (STAR)
7, Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
this report has been assigned to the /ENVIRONMENTALPROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology"required for the control and treatment
of pollution sources to meet environmental quality standards.
Ill fl I Mill I 11 III II
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-017a
December 1977
ECONOMIC ANALYSIS, ROOT CONTROL, AND
BACKWATER FLOW CONTROL AS RELATED TO
INFILTRATION/INFLOW CONTROL
by
Richard H. Sullivan
Robert S. Gemmell
Lawrence A. Schafer
William D. Hurst
American Public Works Association
Chicago, Illinois 60637
Grant No. 803151
Project Officer
Anthony N. Tafuri
Storm and Combined Sewer Section
Wastewater Research Division
Municipal Environmental Research Laboratory (Cincinnati)
Edison, New Jersey 08817
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the views and
policies of the U.S. Environmental Protection Agency, nor does mention of trade
names or commercial products constitute endorsement or recommendation for use.
ii
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FOREWORD
concentrated and integrated attack on the problem require a
and qlP-M Guide (EPA-600/2-77-017O , £d the MenS S
Francis T. Mayo
Director
Municipal Environmental Research
Laboratory
iii
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ABSTRACT
tenance and often do not properly close.
The results of the study are presented in four volumes. This report
System cleaning and rehabilitation, and guides for new construction.
The study updates a similar effort conducted in 1970.
„
under the sponsorship of the U.S. Environmental Protection Agency.
was completed on this report in July 1976.
iv
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CONTENTS
Page
Foreword
Abstract iv
Figures v±
Tables vi
Acknowledgements ..,..
SECTIONS
I. Introduction, Conclusions, and Recommendations 1
II. The Study 5
III. Analysis of Surveys 10
IV. Economic Guidelines ..... 10
••••••••• io
V. Root Control Practices 41
VI. Tide and Backwater Gates 77
VII. Recent Research and Development Activities 95
References 101
v
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FIGURES
Number rage
1 Cost-effectiveness solutions for primary and secondary treatment . 32
2 Cost-effectiveness solutions for primary, secondary, and
10
11
12
13
14
15
16
17
tertiary treatment
portation costs in cost-effective analysis . 34
Cost-effectiveness solutions for primary, secondary, and tertiary
treatment based on peak flow capacity ......... ..... 36
Reported causes of sewer collection system problems ....,,, 50
Evolution of typical modern sewer system to divert dry-flow
to treatment ............... • ...... ..... ''
Typical large check valve .................... 80
Side-pivoted tide gate ...................... 80
Typical flap gate or flap valve . ......... . . , ..... 81
Typical cast tide gates ..................... 81
Circular pontoon tide gate ............. ....... 82
Typical timber tide gate .................. • • • 82
Inflatable fabric dam . . . . .................. 83
Schematic tide gate .................. ..... 84
Theoretical tide gate behavior ...... ......... ... 85
Typical urban tide gate installation ............ ... 86
Cross section of concrete sewer pipe under typical
corrosion conditions . ................... ... 97
TABLES
1 Infiltration/Inflow study — ranked major sources of I/I ., ..... 11
2 Results of I/I studies summary ........... ...... 12,13
3 Root control ....................... .... 13
4 Percentage of system subject to and requiring maintenance for
root intrusion ...................... .... 14
5 Inspection/acceptance testing .......... ... ..... 16
6 Community profile and explanatory key ...... . ....... 22
7 Summary of treatment costs ............. • ...... 28
8 Infiltration/Inflow: quantity and correction costs . . ...... 30
9 Herbicide tested in root control experiments ..... ....... 48
10 Relative importance of root problem by type? of sewer ...... 51
11 Nature of chemical maintenance program for root control ..... 51
12 Size of collector sewers affected by root intrusion ....... 52
13 Point of intrusion by roots ........... • ....... 53
vi
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ACKNOWLEDGEMENTS
The American Public Works Association wishes to thank the following
persons and their organizations for the services they have rendered to the
APWA Research Foundation in carrying out this study for the U.S. Environ-
mental Protection Agency.
Project Director
Richard H. Sullivan
Consultants
Morris M. Cohn., P.E. (deceased)
William D. Hurst, P.E.
Robert S. Gemmell, Ph.D., Northwestern University
G. E. Maguire, Inc. Engineers
Lawrence A. Schafer
APWA Staff
William F. Henson, P. E.
George J. Hinkle
Maurice L. Kimbrough, P. E.
Pro.ject Steering Committee
Timothy M. Kipp
Cecelia E. Smith
Dr. Shanka Banerji
Associate Professor
University of Missouri
Columbia, Missouri
Stuart H. Brehm, Jr.
Executive Director
Sewerage & Water Board
New Orleans, Louisiana
Leland E. Gottstein
President
American Consulting Services
Minneapolis, Minnesota
Shelley F. Jones
Director
Department of Public Works
Ventura, California
James M. MacBride
Manager, Regional Operations
City of Winnipeg
Winnipeg, Manitoba
A. E. Holcomb,
Manager
Wastewater Collection Division
Dallas, Texas
vii
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SECTION I
INTRODUCTION, CONCLUSIONS, AND RECOMMENDATIONS
INTRODUCTION
This report and the accompanying Appendices, Product and Equipment
Guide, and Manual of Practice represent a joint effort by the USEPA, the
American Public Works Association (APWA), and industry representatives to
compile needed information for local authorities and consulting engineers
on the control and elimination of infiltration/inflow flows to sanitary
sewer systems.
In 1970 an earlier study was sponsored by 39 local authorities and
USEPA to prepare a report and Manual of Practiced)- In the intervening years,
much has been learned. Federal recognition of the problem has resulted in
extensive regulations and guidelines. The practices outlined in 1970 have
been improved and simplified to make practical the analysis of conditions in
large sewer systems.
The current study has also shown major upgrading of local authority
specifications. The 1970 report suggested an infiltration allowance of
3
200 gal/in-diam/mi/day (0.18 m /cm-diam/km/day) as achievable without
increased construction cost. This limit appears to be the generally accepted
specification today.
With the increased attention to I/I conditions, manufacturers have
provided joints and sewer pipe appurtenances capable of meeting very exacting
infiltration standards. In addition, auxiliary equipment and products have
been developed for flow measurement, sewer repair, and rehabilitation and
sewer inspection.
Most important, however, has been the-number of local authorities
who have used the available techniques and have successfully, and often
dramatically, reduced I/I flows in their system. At the start of this study,
there was great concern for the value in terms of cost and time required to
study the I/I conditions prior to the USEPA Construction Grants Program. In
the past year, the value of such studies has become apparent and many local
authorities have been able to make significant reductions in their flows in
a cost effective manner, reducing the cost of construction and operation of
wastewater treatment facilities and the maintenance of their sewer system.
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CONCLUSIONS
Infiltration/Inflow
Many local authorities have utilized an I/I investigation to
accomplish major flow reductions, thus allowing smaller wastewater treatment:
facilities and lower operational costs.
At the time of the APWA survey, few local authorities had completed
I/I studies in accordance with USEPA guidelines.
The magnitude (time and cost required) and complexity of the I/I
survey efforts for major systems requires improvement and development of
better methods.
A lack of trained reviewers in many states and USEPA regions
coupled with a lack of completed studies to act as benchmarks, appear to
have severely delayed completion of many I/I studies. Recent USEPA actions
appear to be correcting this situation.
Rehabilitation of sewer systems must be a continual program to
maintain design levels of I/I. Generally this will require increased
funding.
Major sources of infiltration were found to be pipe defects, manholes,
service connections, and poor initial construction.
Major sources of inflow were found to be open manholes, roof leaders,
illegal connections, catch basins, and yard and ground drains.
As a result of the surveys conducted correlations were not evident
relating infiltration or inflow to climate, geographical region,, or
population.
Infiltration design allowances have been reduced in recent years.
Pipe and joint manufacturers have responded with, greatly improved materials
allowing a wide choice of pipe systems to be used. To achieve the low levels
3
of infiltration, 200 gal/in-diam/mi/day (0.18 m /cm-diam/km/day), competent
inspection is required and rigid adherence to construction specifications is
required.
It is seldom cost-effective to correct all points of infiltration and
some types of inflow. Only major conditions should be corrected prior to
determining the economics of rehabilitation versus transport-treatment.
Correction of inflow conditions will generally be.dependent upon
political support of community leadership.
Many sources of inflow are on private property. Many local author-
ities reported that they could pay for such correction work.
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Many important factors for economic analysis must be specific to
the site. Sample calculations as developed in this report may be used only
as a guide.
The cost of water conservation devices, such as flow-reducing shower
heads and lavatory fixtures, may be cost-effective when the entire system
costs are considered.
Roots
A survey of local authorities to determine the extent and effect of
tree roots in sewers revealed that:
a. roots are a major sewer maintenance problem;
b. roots in conjunction with grease and sand contribute to clogged
sewers reducing flow characteristics;
c. conventional root control practices such as cutting appear to
encourage regrowth;
d. chemical methods of control have been developed and are being
used very successfully by many local authorities;
e. the efficiency of the chemical control methods has not yet been
tested with all types of plants; and
f. root penetration is a major problem in house laterals where sewer
depth is less and construction may not have been adequately
inspected.
Tide Gates
A survey of tide and backwater gates revealed that:
a. few local authorities or manufacturers have attempted to improve
the design of tide gates, other than by changes in materials;
b. malfunctions of gates are frequent and often lead to large
amounts of unnecessary flow to the treatment facility. Units
were reported to malfunction as often as 29 days per year and
3
allow up to 100 mgd (263 m /min);
c. maintenance practices were generally reported as minimal, in
part due to the inaccessible location of many such facilities;
d. sensing facilities to determine when units are closed or open
have been installed by some authorities with success. Such
systems must be carefully designed and adequately maintained
or they will be of no value;
e. corrosion is a major problem with most facilities. Choice of
materials must be carefully made. Changes in levels of pollutants
in the receiving water may affect the type and extent of problems
which will be encountered; and
f. many facilities were found to have been abandoned due to lack of
resources for maintenance, pdor initial design, and inaccessibility.
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General
The Manual of Practice and Product and Equipment Guide developed
by this study should be of assistance to local officials and consultants.
However, the effect of USEPA guideline changes and rapid developments in
the improvement of infiltration/inflow (I/I) evaluation must be considered
by each user.
RECOMMENDATIONS
The benefits of several successful I/I control programs should be
documented and published for the benefit of those consulting engineers and
local authorities who have not had experience with studies.
Local authorities should be encouraged to establish ongoing sewer
maintenance programs following sewer rehabilitation in order that the
treatment facilities will not be overloaded.
Local authorities should conduct flow monitoring activities after re
habilitation in order to evaluate the effectiveness of correction activities.
Local authorities should conduct public information programs high-
lighting the need for elimination of inflow sources from private property.
Where such connections were legally installed, the local authority should
consider paying for all or part of the correction work required.
«h«,-M faC1t°rs which may directly influence the economic analysis
luch factorreCt^ Stud±ed/nd quantified to improve economic projections.
and sfwP^ T 5 C°S* °f StreSt cave-lns» increased cleaning required,
quantificat?on?PS ^ ? * pr°perty are examPles of «eaa which Jequire
Local authorities should annually evaluate the cost-effectiveness of
requiring water conservation devices for all new plumbing installations.
Rapidly changing financial conditions for both wastewater treatment and
drinking water may make it advantageous to require such devices .
Controlled testing and evaluation should be conducted of the two
major sealants to better define the conditions under which both can best
be used.
Controlled testing of Vaporooter should be conducted to determine
its effectiveness as a root growth inhibitor with trees and shrubs found
troublesome in northern climates.
As a part of any program for the upgrading of combined sewer overflow
regulators, tide and backwater gates should be refurbished and a monitoring
system installed to allow remote sensing of the position of its gate.
Construction of house laterals must receive the same degree of concern and
inspection as other sewers.
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SECTION II
THE STUDY
To provide for the reevaluation of conditions in the I/I field,
the U.S. Environmental Protection Agency authorized APWA to undertake
Grant 803151 as described in this section.
Objectives
The scope of the investigation was as follows:
Task 1 - Conduct a complete literature review covering the I/I
field, including detection, sewer system evaluations, technological
developments, maintenance programs, design and improved construction
practices, and rehabilitation of defective sewer systems.
Task 2 - Conduct a general survey of new and existing materials,
methods and mechanisms used for I/I detection, evaluation, rehabilitation,
and construction processes and procedures.
Task 3 - Conduct in-depth field investigations, interviews, surveys
and studies of policies, practices and performance in I/I control achieved
by municipal and regional sewer system agencies, state regulatory agencies,
consulting engineers and I/I-related service companies, for the purpose of
recording and evaluating present-day practices.
Task 4 - Provide actual field information on the implementation of
cost-effective analytical procedures for determining necessary investments
for sewer system rehabilitation vis-a-vis the cost of providing transporta-
tion and treatment of infiltration/inflow wastevater flows, by whichever
means such economic alternatives are being evaluated in establishing the
excessiveness or nonexcessiveness of I/I conditions in analyzed-evaluated
systems.
Task 5 - Prepare a final report on the study to interpret and
clarify practices and policies which have been used or should be used to
analyze I/I conditions, evaluate the most effective and economical means
for resolving the problems of intrusion water piracy, and the procedures
by which sewer system rehabilitation, maintenance, and operation can be
performed.
The purpose of this section is to describe the specific steps by
which these tasks were planned and executed in order to focus on information
about I/I control practices and to develop the most helpful guidelines on
improved procedures by which conformity with the requirements of PL 92-500
-------
regulations and EPA rules can be achieved by applicants for federal grants
to wastewater handling and treatment works.
Literature Search in the I/I Field
Under a subcontract, Indiana University conducted a comprehensive
literature search and review of recorded documents relating to all phases
of the infiltration/inflow problem and solutions thereof. Significant
references were summarized in capsule form to present the pertinent informa-
tion and are included in the appendices published separately by NTIS.
Products and Equipment
With increasing interest in investigation, evaluation, and correc-
tion of infiltration/inflow conditions in sewer systems on the part of
owners, designers, constructors, and regulatory agencies, it was apparent
that guidance was needed in the field concerning the equipment, methods,
and materials available to carry out studies of this problem and to initiate
corrective measures.
Staff representatives interviewed exhibitors at the APWA 1974
Congress at Toronto, and the Water Pollution Control Federation 1974 Conference
at Denver, who offered materials, mechanisms, and methods for I/I control
purposes. Literature was solicited and interpreted and personal interviews
with exhibitors disclosed additional information. In addition, manufacturers
and service organizations were contacted by mail and catalog data, brochures,
reports, and other documents were requested.
The product and service information was collated under six categories
of I/I control functions for clarity and guidance purposes: Detection and
Analysis of I/I; Sewer System Evaluation; Sewer System Cleaning; Sewer
System Rehabilitation; Construction; and Safety.
The need for a comprehensive consolidation of product and service
information, in the form of an overall equipment and product manual, including
interpretive data on how materials, mechanisms, and methods can be used for all
facets of'I/I investigative and corrective work, was recognized. As a result,
the development of a comprehensive document was authorized and has been published
under separate cover as EPA-600/2-77-017c, Product and Equipment Guide.
Field Investigations
To determine the policies and performance of I/I survey-evaluation-
rehabilitation activities, field investigations were conducted at municipal-
ities, consulting engineering offices, state regulatory agency headquarters
and sewer service firms.
The consulting engineering organizations surveyed were chosen to
provide a representative cross section of design and I/I study experience
in terms of geographical location of their practices, sizes of municipal
systems served, and reported experiences disclosed by inquiries. The firms
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covered by on-site investigators were: Wallace and Associates, Virginia,
Minnesota; Camp Dresser and McKee, Boston, Massachusetts; Elston T.
Killam Associates, Millburn, New Jersey; Consoer, Townsend & Associates,
Chicago, Illinois; and Gannett, Fleming, Corddry and Carpenter, Harrisburg,
Pennsylvania.
In addition to these office interviews, other consulting engineering
firms (C.E. Maguire, Inc., Waltham, Massachusetts, for Fall River, Mass.;
and Mingus Associates, Farmington, Connecticut, for Norfolk, Conn.) were
visited because they possessed key information on I/I studies conducted for
some of the municipalities chosen for that portion of the national on-site
investigations.
The state water pollution control agencies chosen for on-site surveys
were intended to provide a revealing mix of geographical locations and
problems, sizes of state involved, and unusual policies on I/I control. For
these reasons the following state entities were investigated; Florida
Water Pollution Control Department; New Hampshire Water Supply and Pollution
Control Commission; and Wisconsin Water Pollution Control Agency. These
investigations served a dual purpose; they disclosed state policies and
practices, and they provided a broad-gauged evaluation of the status of I/I
studies in their states and their experiences in dealing with EPA regional
offices.
States were requested to "nominate" communities within their
boundaries which were known to have engaged in the control of I/I through
surveys of sewer system conditions or through routine preventive maintenance
procedures. In addition, the Technical Advisory Committee members were
polled concerning outstanding I/I studies conducted at the local level.
Supplementary choices were based on early responses to a nationwide I/I
mail survey questionnaire described below.
Out of these and other sources, sixteen municipal or multi-
municipal systems were chosen for on-site investigation. The surveyed
authorities were: Minneapolis-Saint Paul Metropolitan Waste Control Commis-
sion; Des Moines, Iowa; Oakland County, Michigan; Conway, Arkansas; Little
Rock,. Arkansas; South Tahoe Public Utility District; Fort Lauderdale, Florida;
Emmetsburg, Iowa; Fall River, Massachusetts; Knoxville, Tennessee; Portland,
Oregon; Longview, Texas; Richmond, California; Norfolk, Connecticut; Tampa,
Florida; and New York, New York.
A mail survey of approximately 200 agencies was also conducted.
It was found that few correlations could be developed concerning the
experiences and practices of the interviewed and surveyed communities. This
was due in part to the state of knowledge at the time of the survey. Field
interview reports are contained in the appendices to this volume published by
NTIS. Limited results of the survey are contained herein under Section III.
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Cost-Effective Analysis
The definition of "excessive I/I" by USEPA is based, not on volume
of wastewater intrusion in sewer systems, but on the comparative costs of
eliminating all or part of this I/I, vis-a-vis the cost of transporting
and treatment. Thus, the decision as to sewer system rehabilitation or
treatment works enlargement is based on economic factors. Rational cost-
effective determinations must be made. The study plan included the
development of data on a typical I/I problem and the evaluation, of cost-
effective factors for either sewer system rehabilitation or treatment and
transportation in collector and interceptor sewers of the excess flows.
Particular emphasis was placed on the costs-effectiveness facet of
the on-site investigations of municipal agencies, consulting engineering
firms and state regulatory agencies. It was deemed most productive to
utilize this broad-based approach to the cost .analysis problem rather than
utilize a single community problem as the basis for a typical cost-
effectiveness analysis. The investigations demonstrated that many factors
can affect the cost-effectiveness determination procedures and accordingly,
no single approach can be considered representative of all analytical
methodologies.
Section IV of this report contains various examples of studies of
economic alternatives of various kinds, applicable to several conditions,
as found in the field.
Root Control Practices
A mail questionnaire on root control practices was sent to older
cities where it might be expected that roots were a problem. On the basis
of the replies, 20 follow-up field investigations were conducted to explore
in depth the unique problems or methods used to minimize root problems.
The results and conclusions of the survey are given in Section V.
Tide Gate and Backwater Flow Control Facilities
Tide gates or backwater flow controls are used on many systems where
combined sewers or bypasses discharge to tidal waters or major rivers.
Previous studies have indicated that malfunctioning facilities contribute
large amounts of extraneous flow to their system. Such conditions are
often noted where salt waters enter the system and disrupt secondary waste-
water treatment processes.
The consulting engineering firm of C. E. Maguire was retained to
prepare a report based upon a mail survey of 100 communities, field visits
to six communities known to have taken action to improve their systems and
their own study. The results are presented in Section VI.
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Technical Advisory Committee
Much of the progress in the elimination of future infiltration into
new sewer systems and the minimization of existing infiltration conditions
will depend on materials of construction; equipment for installations;
instrumentation; mechanisms for seeking out infiltration and inflow;
methods for evaluating I/I conditions and materials and devices for
rehabilitating defective sewers, joints, manholes, and other entry points
of unwanted waters.
In recognition of the important role which manufacturers of such
facilities and the part which engineering designers play in I/I control, a
Technical Advisory Committee (TAG) was created, composed of representatives
of involved manufacturing and sewer service companies and consulting
engineering firms closely identified with this field. Membership is
designated in the Acknowledgements.
The TAG body was divided into specialized subgroups, covering Sewer
System Evaluation; Sewer System Rehabilitation; and Construction. These
groups met independently to develop guidance information for the study.
The data they produced is presented in the Manual of Practice.
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SECTION III
ANALYSIS OF SURVEYS
For the purpose of obtaining general information on I/I causes and
effects in representative sewer systems in the United States and evaluating
the preventive, corrective, and regulatory practices in effect, a mail
survey was conducted in January 1975.
The questionnaire was sent to approximately 200 communities recom-
mended by state water pollution control agencies as having experienced I/I
problems and having information available. A total of 117 replies were
received, of which 99 contained usable information.
Status of I/I Investigation
Fifty-six communities reported that preliminary I/I surveys had been
completed. Of these, 16 were carried out by in-house staffs and consultants.
Generally, the entire system had been studied at least in preliminary form.
Some of the systems that had commenced preliminary I/I studies prior
to requirements of PL 92-500 and USEPA continued them thereafter. Percentages
of the systems studied varied between 7 and 100 percent. Of those answering
the specific question, 14 stated that the work was carried out to comply with
local requirements, four because of lack of sewer system or treatment plant
capacity, and four for both these reasons. Zonal differences were not
significant.
Twenty communities indicated that they had extended system analyses
(Phase I), to evaluation surveys, Phase II. Of these, 15 performed the
studies themselves, four through consultants, and one was a joint effort.
Four communities reported that they carried out evaluations for legal reasons,
seven for reasons of capacity, and six for both reasons. , The portion of the
systems studied varied between 25 and 100 percent.
Of the 17 communities reporting inflow reduction studies, only one
assigned the work to consultants; all the others were carried out in-house.
Seven communities reported that the work was performed under the terms of
PI 92-500; eight communities reported nonconformity with federal guidelines.
The extent of the systems studied varied between 25 and 100 percent. The
reasons given for inflow investigations were as follows: two of the systems
were studied for legal reasons, eight for capacity, and four for both reasons.
10
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Sources of I/I
Table 1 lists the responses received concerning the sources of I/I.
Table 1. Infiltration/Inflow Study-Ranked Major Sources of I/I
Infiltration Sources Total %
(mentioned more than once)
A
B
C
D
E
F
G
. Pipe defects,
deterioration, also
broken pipe, open
joints 39 39
. Leaky manholes 27 25
. Defective service
connections 25 23
. Root Intrusions 2 1.8
. Poor construction
practice including
porous pipes 27 25
. Soil conditions 7 6
. Oakum sewer joints 1 0.9
Inflow Sources
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
Foundation drains
(footer drains)
Total
8
Roof leaders , drains
including cellar drains 15
Manhole covers of
"open" type
Cleanouts on private
property
Illicit and illegal
connections
Cross connections
(bypasses)
Overflow structures
Catch basins
Street Inlets
Yard drains
24
4
13
7
4
12
6
10
%
8
14
23
4
12
7
4
12
6
10
Cost of I/I Corrections
Table 2 lists the basic information received concerning the cost of
the solution and methods considered. Only a small number of those answering
the questionnaire provided cost information.
11
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Table 2. Results of I/I Studies Summary
City and Infll.
Population Sorved Correction
Oakland, Co. Cost $6,000,000
(362,000)
Truckee, Ca.
(7,000)
Willets. Ca.
(3,600)
San Mntoo, Ca.
(80,000)
Athens, Al.
(16,000)
Ft. Worth, TSs.
(410.000)
Longview, Tx.
(52,300)
Hot Springs, Ar.
(37,000)
Jcsup, Ga.
(8,000)
Cocoa Beach, Fl.
(18,000)
S/Capita
Cost
S/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
S/Capita
Cost
$/Capita
St. Potorsburg.FI. Cost
(27S.OOO)
Tampa, Fi.
(280,000)
Trenton, NJ
(85,000)
Warren Twp.NJ
(6,000)
N. Anleboro,
Twp., Ma.
(13,500)
Pueblo, Ca.
(97,500)
Lovolond, Co.
(22,000)
E. Greenwich,
Co.
(3,500)
Ft. Wayne, In.
(213.000)
S/Capita
Cost
$/Capita
Cost
S/Capita
Cost
S/Capita
Cost
S/Capita
Cost
$/Capita
Cost
S/Capita
Cost
S/Capita
Cost
$/Capita
16.57
192,000
27.43
540,000
150.00
N/A
N/A
7,000,000
17.07
N/A
4,000,000
108.11
250,000
31.25
130,000
7.22
4,300,000
15.64
3,750,000
13.39
1,600,000
18.82
40,000
6.67
990,000
73.33
2,000,000
20.51
N/A
95,000
27.14
N/A
Inflow
Correction
WWT
Increase
Methods Considered
Total Storage/ WWT
Cost Surge Plant
$20,000,000 $102,000,000 $128,000,000
55.25
45,000
6.43
28,000
7.78
N/A
N/A
Incl. in Infil.
Column
546,000
10.44
N/A
100,000
12.50
N/A
N/A
100,000
0.36
N/A
N/A
N/A
N/A
10,000,000
102.56
N/A
40,000
11.43
N/A
281.78
470,000
6.7.14
740,000
205.56
5,000,000
62.50
3,500.000
218.75
12,000,000
29.27
3,353,000
64.11
N/A
300,000
37.50
N/A
N/A
N/A
N/A
N/A
260,000
43.33
N/A
1,800,000
18.46
3,000,000
136.36
1,000,000
285.71
9,000,000
42.25
353.59
707,000
101.00
1,308,000
363.33
5,000,000
62.50
3,500.000
218.75
19,000,000
46.34
3,899,000
74.55
4,000,000
108.11
650,000
81.25
130,000
7.22
4,300,000
15.64
3,850,000
13.75
1 ,600,OOO
18.82
300,000
50.00
990,000
73.33
13,800,000
141.54
3,000,000
136.36
1,135,000
324.29
9,000,000
42.25
Yes
Yes
Yes
No
Yes
No
Yes
Yes
No
No
No
No
Yes
No
No
No
No
No
Yes
Yes-
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
(continued)
12
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Table 2 (Continued)
City and Infil Inflow
Population Served Correction Correction
Norfolk, Va. Cost N/A N/A
(800,000) $/Capita
College PI,,Wa.
(4,800)
Moses Lake.Wa.
(14,OOO)
Gunnison, Co.
(5,000)
Jamestown, Rl
(2,200)
Honolulu, Hi.
(531,600)
Fairbanks, Ak.
(1,200)
Des Moines, !a.
(201,400)
Webster City, la.
(8,900)
Duluth, Mn.
(100,000)
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
Cost
$/Capita
10,000
2.08
50,000
3.57
N/A
N/A
N/A
N/A
N/A
725,000
7.25
10,000
2.08
50,000
3.57
N/A
308,000
140.00
N/A
N/A
•. N/A
N/A
46,000
0.46
WWT
Increase
1,500,000
1.88
2,000
0.14
105,000
21.00
1 ,400,000
636.36
17,700,000
33.30
3,500,000
2,916.67.
900,000
4.47
513,000
57.64
59,965,000
599.65
Total
Cost
1 ,500,000
1.88
20,000
4.16
1 02,000
7.28
1 05,000
21.00
1 ,708,000
776.36
1 7,700,000
33.30
3,500,000
2,916.67
900,000
4.47
513,000
57.64
60,736,000
607.36
Methods Considered
Storage/ WWT
Surge Plant
No
No
No
Yes
Yes
No
Yes
No
No
Yes
Yes
.No
Yes
Yes
No
Yes
Yes
Yes
Yes
Yes
Root Control
Table 3 summarizes the data from the 64 reporting communities. Most
communities do not maintain house laterals. Where house laterals are m'ain-
tairied, a large number of dig-ups can be expected. St. Petersburg, Florida
reported 1,200 dig-ups per year for the portion of the lateral in the public
right-of-way.
Table 3. Root Control
Number of
Communities
64
Root Control
Program
in Collection
System
Yes
32
No
32
Root Control
Program in
House Laterals
Yes
8
No
48
No Answer
8
Dig-Up s
Per Year in
Collection
System
3,909
Dig-Up s
Per Year
In House
Laterals
1,381
Table 4 summarizes the percentage of system subject to and requiring
maintenance for root intrusion. House laterals are shown to have more root
intrusion problems than collection sewers.
13
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Table 4. Percentage of System Subject to and Requiring Maintenance for
Root Intrusion —
Number
Communities
64
Average % of System
Subject to Root Problem
Collection
System
26.6
House
Laterals
42.2
Requiring Maintenance
Collection
System
16.4
House
Laterals
13.8
Forty-seven communities reported that they use rodding to control
roots; 27 employ chemicals; jetting, buckets, and flushing are used in only
a few communities. Few communities are satisfied with the procedures; the
general reaction is that "nothing better is available."
A supplementary study of the sewer root problem was conducted to
augment the information obtained by the subject mail survey and is contained
in Section VII.
Tide Gates
A few communities employ tide or backwater gates to prevent backwater
intrusion. One community reported 35 percent of inflow from this source;
another reported 2.5 percent due to backwater effects. Information received
on this subject is regarded as unsatisfactory and no conclusions can be
drawn. A separate survey of tide gate or backwater gate problems and prac-
tices was conducted, and the results are contained in Section VIII.
Rehabilitation of Sewers
Thirty-five surveyed communities have employed chemical grouting for
infiltration control. Thirty communities have used AM-9 and eight com-
munities have used or are experimenting with 3M Elastomeric sealing compound.
Of those using AM-9, 13 reported good results, three excellent, two
fair, and four poor. Those using 3M reported one excellent, one good, and
one poor result.
Some communities reported use of nonchemical methods. Dallas, Texas
reported good results with slip linings; Portland, Oregon reported poor
results with grout lining; Seattle, Washington reported good results (90
percent effective) with cement grouting.
Of the 117 communities, 28 (24 percent) had active rehabilitation
work underway while four (3 percent) were in the planning stage. The balance
(73 percent) had not advanced to sewer system rehabilitation. Of the
communities having work underway, seven were conducting smoke or dye tests;
one was engaged in a relining program; 10 were rehabilitating manholes;
four were conducting a campaign to discontinue floor and roof drain
connections; one was installing backwater or flap valves; and five were
attacking the problem in miscellaneous other ways.
14
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House Laterals
The mail survey questionnaire posed a series of questions about
house lateral contributions to infiltration and house connection con-
tributions to inflow.
Insofar as house laterals are concerned, they vary in length from 15
to 150 ft (4.5 m to 45 m), with an average length of 51.5 ft (15.7 m), and
they are laid at an average depth of 5.1 ft (1.5 m).
Three communities reported that house laterals contribute 55 percent
of the infiltration, others stated that they contribute a substantial amount.
Factual information was limited.
As far as house connections are concerned, 22 communities reported
that they are enforcing, or are in the process of amending, their ordinances
to reduce this source of inflow. Several communities have assigned inspection
crews. Only two communities are requiring removal of roof leaders from
sanitary sewer lines.
Prevention of Infiltration
Proper pipe selection and good construction practices can minimize
infiltration. Pipe types such as asbestos-cement, plastic, ductile iron,
and truss pipe were reported to be increasing in use. The use of vitrified
clay and concrete is also widespread when used with rubber jointing rings
(gaskets) and neoprene "0" rings. Both conventional bell-and-spigot and
compression-type joints are used.
Bedding practices now stress the placement of granular material in
the trench, particularly in the pipe zone. Sand, fine gravel, and crushed
stone and stone dust are used. A few still use selected soil; one community
reported the use of lean concrete.
Precast concrete with rubber gasketing, and poured-in-place concrete
are both being used for manhole construction. Judging from the survey data,
brick construction appears to be declining.
Jointing practices reportedly favored mechanical joints, slip joints,
flexible joints, PVC couplings and adapters, fabricated polyurethane com-
pression joints and grouting (concrete and AM-9).
In Ohio, saddles are used and encouraged. With reference to service
connection construction, cast iron pipe was reported to be widely used, as
is vitrified clay pipe, PVC pipe, and asbestos-cement. Insofar as jointing
is concerned, rubber rings, neoprene "0" rings, mechanical compression and
chemical welded joints were reported in favor, to the exclusion of cemented
joints. Lead oakum joints are only occasionally employed.
15
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To minimize infiltration in building connection lines, inspection
is normally required. Of 68 communities reporting, 52 use visual inspec-
tion; seven employ smoke, air or water testing; one has TV inspection;
and three use miscellaneous methods. One community requires no testing;
four leave inspection and approval to building or plumbing inspectors.
As to the method of connecting building service lines to the sewer
system, a wye or tee connection is the most popular in 36 out of the 60
reporting, 14 require saddles, and 10 employ individually developed
methods.
The design allowable infiltration rate varied from 5,000 gal/in-diam/
mi/day (4.75 m3/cm-diam/km/day) down to nil.
The methods used for Inspection/Acceptance Testing in 73 communities
are listed in Table 5. Note: most of the communities use more than one
method.
Table 5. Inspection/Acceptance Testing
Infiltration
48
Exfiltration
41
Air.
Testing
36
TV
Photo
36
Other,
Mostly Visual
17
SUMMARY
A mail survey of 200 local authorities recommended by state water
pollution control authorities resulted in 99 usable replies. Data from the
replies indicated that the local conditions which have caused infiltration
and inflow problems are too complex and too localized to be able to make
broad characterizations of conditions. Although climate was used to analyze
the data it was found that it was not in itself a good indicator of conditions
which could be anticipated.
The authorities which responded do not constitute a statistical
sample as the interest of the survey was to obtain a broad cross section
of current practice,by knowledgeable agencies. Over half had completed
preliminary studies and one-fifth had begun evaluation surveys. Studies
of infiltration conditions had been generally conducted by consultants
whereas for inflow, the local authorities generally conducted the work.
The amount of infiltration reported ranged from 14 to 70 percent of
the total WWF with an average of 71 percent of peak DWF. Inflow averaged
144 percent of the peak DWF, highlighting the importance of such flows.
Sources and conditions resulting in 1 to 1 conditions were identified and
quantified. Manhole covers were mentioned by 25 percent of those who replied.
House laterals were identified often as a major source of infiltration.
Cost information concerning correction or control alternatives were sparse
and meaningful conclusions could not be drawn.
16
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Sewer maintenance activities appear to receive increased resources
and concern as the extent of the 1 to 1 problem becomes known. There is
general use of high pressure cleaning equipment. Responsibility for the
lateral either on private property or in the public way varies widely.
Where such lines are the public's responsibility they often constitute a
major maintenance problem.
Roots and tide gates were identified as problem areas.
surveyed had relatively minor experience with sewer grouting.
Authorities
Most authorities reported upgrading of specifications for new sewer
work along with more stringent infiltration allowances. Acceptance testing
methods varied with no one method predominantly used.
17
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SECTION IV
ECONOMIC GUIDELINES
The Economic Factors Involved
When excessive amounts of I/I waters enter sanitary or combined
sewer systems, the immediate effects are "physical." As important as
these physical effects on the capacities and capabilities of sewer conduits,
pumping facilities, treatment works, and regulator overflow structures may
be, the full impact of such extraneous waters cannot be known until the
"financial" factors are computed and evaluated.
The hidden costs of I/I usurpation of system capacities and
capabilities generally have been overlooked when corrective action is
planned. Even the readily computable costs seldom have been evaluated and
properly interpreted in terms of the economics of urban services.
Where preventive measures have been taken to reduce infiltration
in new sewer systems, designers and utility officials have been concerned
about any added cost of projects because of specification requirements for
tighter lines. Little consideration has been given to immediate and long-
range savings that might accrue in terms of reduced size of new sewer
lines and longer service life of such systems.
If and when correction of infiltration in existing overtaxed sewer
systems has been considered or undertaken, the usual concern has been the
immediate and direct expenditure for sealing or replacing defective sewer
structures. Little thought has been given to the long term evaluation of
costs versus the benefits to be derived in sewer system service and in the
pumping and treatment of wastewater. Few jurisdictions actually have
evaluated the volumes of flow due to infiltration and- inflow or the marked
economic effect of such flows. Now PL 92-500 and USEPA guidelines require
studies of the cost-effectiveness of corrective actions, vis-a-vis the
costs involved in system rehabilitation and the treatment of intruded flows.
In terms of local effects of surcharged sewer systems, emphasis
has usually been placed on the adverse effects to the public which uses
thoroughfares and on property owners inconvenienced and injured by back-
flooding into their properties. Unfortunately, the monetary costs of
these adverse effects have seldom been identified.
No meaningful evaluation of this problem, in all of its ramifica-
tions and implications, can be made without considering the costs of such
18
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intruded flows and the capital investments required to eliminate or alleviate
the difficulties previously mentioned.
The evaluation of the economic factors of I/I has, in the past, been
given little attention. This stems partly from the unavailability of rational
fiscal guidelines that will permit the computation of the tangible costs of
handling excessive amounts of extraneous waters. Such costs must be balanced
against the cost of constructing relatively infiltration-free sewer systems
in the future and financing projects to correct I/I conditions in existing
systems.
It is true that present capabilities for developing such fiscal
comparison guidelines are not substantially better than in the past; however,
heightened environmental awareness on the part of the general public, in
concert with the manifestations of population growth and creeping urbanism,
has now made some practices unacceptable, that once were commonplace. For
example, when only primary treatment of municipal sewage flows was deemed
adequate, the periodic discharge to local receiving waters of sewage untreated
but extensively diluted by the presence of I/I was considered an allowable
practice. There is now widespread concern for the threats posed to receiving
waters from untreated discharges, consequently water quality standards have
become stringent, necessitating in most instances substantial increases in
the extent and cost of wastewater treatment prior to discharge. With this
increase has come the realization that the costs and benefits associated with
wastewater treatment and conveyance should be more fully explored and should
be the primary determinant in wastewater management decision making.
Thus it is that I/I poses problems of both environmental and economic
significance. They include, but are not limited to the following:
1,
2,
4.
5,
Increased size and cost of new sewers if excessive I/I are permitted.
Need for construction of relief or supplementary collection and in-
terceptor sewers earlier than the originally estimated economic life
of existing sewers.
Operation and maintenance costs for handling local sewer surcharges,
clean up of flooded areas, and damages to flooded private properties.
Increased operation and maintenance costs for pumping excess flows.
Cost of repairing pavement cave-ins and washouts of subsurface
utilities caused by infiltration and exfiltration.
6. Cost of removing soil, debris, and tree roots entering sewers through
defective sewer pipes and joints.
7. Cost of excessive wear on pumping station equipment and power
requirements.
8. Increased operation and maintenance costs at wastewater treatment
plants.
9. Need for increases in treatment capacity because systems are over-
loaded with excessive I/I volumes.
The USEPA requires that communities seeking matching funds for
wastewater treatment plant construction or renovation must engage in
facilities planning. Included in this planning is a description of all
elements of the system, from the service area and collection system, through
19
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treatment, to the ultimate discharge of treated wastewaters and disposal
of sludge. Applicants must demonstrate that each sewer system discharging
into the treatment works is not, or will not be, subject to excessive I/I.
The determination of whether excessive I/I exists may tsike into ac-
count, in addition to flow and related data, other factors as public health
emergencies, the effects of plant bypassing or overloading, and other rele-
vant economic or environmental factors. Of paramount importance is a deter-
mination of the cost-effectiveness of sewer system rehabilitation versus the
handling of I/I by means of enlarged sewers and treatment facilities.
Excessive I/I is defined primarily as that portion of the flow that
can, by corrective measures, be removed or excluded from the system more eco-
nomically than it can be transported to and treated at the treatment works.
The basic components of a sewer system evaluation survey and I/I
analysis are fully identified in the Construction Grant Regulations and
are not elaborated here. These regulations are updated or changed from time
to time and care must be used to determine that current regulations are
being adhered to. It is important to note, however, that the I/I analysis
is based principally on currently available information such as flows,
population served, age, and condition of the sewer system. It serves as a
preliminary indication of possible excessive I/I.
If the analysis reveals the possible existence of excessive I/I,
a detailed evaluation must be performed to ascertain the extent of I/I,
necessary remedial measures, and the associated costs. Thus, a two-step
process is involved in the identification and the quantification of any
sewer system's I/I problem.
The economic evaluation utilizes a cost-effectiveness approach to
I/I reduction and/or treatment. Its elements are detailed in the construc-
tion grants regulations and include suggested unit costs for system
rehabilitation and treatment plant facilities. In the following portions
of this section, the principal elements of the cost-effectiveness analysis
are discussed and illustrated in the context of an example community. The
cost assumptions and other factors used in the example may or may not
reflect conditions in any specific individual jurisdiction system. Each
community has its own cost experiences, sewer system needs, pumping, and
treatment requirements and other local or indigenous factors. Such
specific factors should be used to replace the arbitrarily chosen physical
and economic assumptions used in the following sample analysis.
Cost-Effectiveness Analysis
The purpose of a cost-effectiveness analysis is to determine which
of all feasible wastewater management systems (or component parts thereof)
would, if adopted and implemented, result in the minimum total resource
costs over time, and meet applicable federal, state, and local requirements
for pollution abatement. Included in the resource costs are those
expressible in monetary terms, such as capital and operating costs, and
20
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in nonmonetary terms , such as social and environmental costs . Monetary
costs are presented as present worth values (equivalent annual costs)
calculated on the basis of a 20 year planning period and an interest
(discount) rate as promulgated by the Water Resources Council for use
in water resource projects. Nonmonetary costs are accounted for des-
criptively.
The most cost-effective alternative is the one determined from
the analysis to have the lowest present worth (or annual cost) without
overriding adverse nonmonetary costs and realizing at least identical
minimum benefits in terms of applicable federal, state, and local standards
for effluent quality, water quality, and other provisos.
Acceptable ranges of service ^life for treatment works are: land,
permanent-structures, 30-50 years; process equipment, 15-30 years; and
auxiliary equipment, 10-15 years. Other service life criteria are
acceptable if sufficiently justified. Salvage values are based on
straight-line depreciation except that land for treatment works shall
have a salvage value equal to the prevailing market value at time of
the analyses, and rights-of-way values are assumed to be not greater than
the prevailing market value.
Example of Community Profile
Example cost-effectiveness analyses are presented in the remainder
of this section. The calculations are based on an example community, as
outlined in Table 6. While similar information would be needed as back-
ground for an I/I analysis and/or any required system evaluation survey,
the profile as shown should not be regarded as a complete example of these
background requirements.
The cost-effectiveness analysis that follows is based on the premise
that the community is presently served by a 10 mgd (438 1/s) design flow
activated sludge plant. A plant expansion of 7 to 17 mgd (307 to 745 1/s)
design flow is proposed to meet existing flows (including I/I) and
provide for anticipated community growth. As proposed, the expanded plant
would provide secondary treatment for 10 years, until 1985, and tertiary
treatment thereafter. In all cases, peak flow capacity of the plant is
taken to be twice the design flow capacity.
Since the proposed plant expansion to 17 mgd (745 1/s) design flow
includes all existing I/I, the analysis must weigh the costs of I/I
reduction against the savings in capital and operating costs of treatment
gained through I/I flow reductions and the decreases in plant design
capacity they permit. The costs associated with transporting the excessive
I/I to the treatment facility, or the savings gained by no longer doing so
should also be included.
In general, all costs presented in the analysis should be itemized
to the greatest degree practicable. For the example only representative
itemization has been used.
21
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Table 6. Community Profile and Explanatory Key
A. Community Profile
Population 100,000
1. Area (acres) 12,000 (4856 ha)
2. Density (persons/acre) 8.3 (20.5 per/ha)
3. Housing Units
(3.5 persons/unit) 28,570
4. Housing Structures
(75% of units) 21,430
5. Mfg. Establishments 112
6. Business Establishments 1,470
7. Business Structures
(75%ofest.) 1,100
8. All Structure Connections
(ft/structure) 60 (18m/stru)
9. Diameter (inches) 6 (15.2cm)
10. House Connections
(feet) 1,285,800 (391,912m)
11. Mfg. Connections
(feet) 6,720 (2048 m)
12. Business Connections
(feet) 66,000 (20,117m)
1,368,520 (m)
TOTAL 6 inch
Building Sewer Connections
Municipal Systems
13. Sewer Miles/acre 0.022
14. Total Sewer Miles 264
15. Pipe Size as Percent of Total
System (feet)
6 to 8 in. @ 75% 1,045,440
10 to 12 in. @ 14% 195,149
15 to 18 in. @6% 83,635
21 to 27 in. @ 4% 55,757
30to42in.@1% 13,939
Total Length of Sewer Sys. 1,393,920 ft
16. Manholes 3,485
(087 km/ha)
(425 km)
(0.15 to 0.2m)
(0.25 to 0.3 m)
(0.4 to 0.69 m)
(0.53 to 0.69m)
(0.76 to 1 m)
(425 km)
B. Explanatory Key
1. Area — Average sewered area of cities in
this population range. Taken from a
survey summary prepared by APWA for
a study on combined sewer overflows.
Problems of Combined Sewer Facilities
and Overflows 1967. Federal Water
Pollution Control Administration, U.S.
Dept. of the Interior; December, 196>7.
2. Density - Based on average area and
population by city size, from overflow
report summary (Ibid).
3. Housing Units — Average for all SMSAs
in 1960. 1960 Census of Housing.
Bureau of the Census, U.S. Dept. of
Commerce.
4. Housing Structures — Average for all
SMSAs in 1960 (Ibid)
5. Manufacturing Establishments — Based
on average number for appropriate
SMSA size. Each establishment assumed
to occupy separate structures. 1963
Census of Manufacturers. Bureau of the
Census, U.S. Dept. of Commerce.
6. Other Business Establishments — Based
on average number of retail and service
establishments by appropriate SMSA
size. 1963 Census of Business, Bureau of
the -Census, U.S. Dept. of Commerce.
7. Other Business Structures — Assumed to
be 75 percent of business
establishments.
8/ All connecting sewers were assumed to
9. be 6 in. (15 cm) vitrified clay pipe with
a length of 60 ft (18 m) between the
structure and the municipal sewer.
10. Presented in feet. Number of structures
times 60 ft (18m).
11. Presented in feet. Number of structures
times 60ft (18m).
12. Presented in feet. Number of structures
times 60 ft (18m).
13. Sewer Miles/Acre — Based on overflow
survey cited in (1) above.
14. Total Sewer Miles — Sewer miles/acre,
times average area.
15. Pipe Size and Percent of System —
Average sizes as percent of system based
on U.S. totals estimated by BSDA.
Picton, Walter L.; "2.7 Billion Feet of
Sewer Pipe Will Serve Communities by
1975." Wastes Engineering. November,
1959.
16. Manholes — One manhole for each 400
ft (122 m) of municipal sewers. Merritt,
Frederick S. Ed. Standard Handbook for
Civil Engineers. New York: McGraw-Hill
Book Co. 1968
22
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Treatment Costs
The capital costs of primary and secondary treatment are based on a
study conducted by the U.S. Public Health Service in 1964(2) . The cost of
tertiary treatment has been estimated at 100 percent of the costs of primary-
secondary treatment in 1969 (3). Plant operating and maintenance costs are
based on a study carried out by P. P. Rowan, K. L. Jenkins, and D. H. Howells
of the Public Health Service in 1961(4). The operating costs of tertiary
treatment are again assumed to be 100 percent of primary-secondary treatment.
The capital costs of primary and secondary facilities were computed
on a per capita basis. These were converted to gallons per day' by assuming
an average flow of 100 gal/day/person (379 I/day/person). Operation and
maintenance costs also were computed on a per capita basis. Regardless of
the sources of flow to the plant, the analysis gives a basis for making
decisions as to the effect of I/I volume on increased plant costs.
Capital Costs
As stated previously, the capital costs were based on a Public Health
Service study. Data on activated sludge plants were available from 133
construction projects in all parts of the country. These projects represented
design populations up to 100,000. The expected costs were estimated by5
regression analysis from the formula:
log lOy = 3.6533024 - 0.2782395 log X
y = expected per capita cost
X = design population
r = coefficient of correlation = (0.73)
The resulting values represent contract construction costs. Not in-
cluding engineering, legal, and administrative costs, and land acquisition
cost. The study points out that the nonconstruction costs, exclusive of land,
could add 20 percent to the expected costs. These costs have been added, as
well as 10 percent for land acquisition. All study capital costs were stated
in 1957-59 dollars. The costs presented herein were inflated to 1975 levels
using the USEPA treatment plant United States cost ind«x (1975 = 232).
Present worths of capital costs are based on an average useful life
of 20 years and a 7 percent interest rate, despite the fact that the officially
established interest rate for water resources projects is considerably lower
at present 5.875 percent in fiscal year 1975.
The capital cost of expanding the activated sludge plant is calculated
as the difference between the capital costs of the two sizes as estimated
by the equation described previously. Added to this are engineering costs
(20 percent)and land acquisition costs (10 percent), a total of 30 percent of
the expansion costs. The salvage value of the land, calculated as the present
23
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worth of the incremental land costs associated with plant expansion, is
deducted to arrive at the total present worth of the capital costs. No
other salvage values are claimed.
The following is an illustration of the calculations involved in
estimating the present worth of capital costs associated with expansion
of an activated sludge plant from 10 mgd to 17 mgd (438 1/s to 745 1/s)
design flow.
For a 17 mgd plant (745 1/s); equivalent population = 170,000
log 10 y = 3.6533024 - 0.2782395 log(170,000)
y - 15.776 = per capita cost (1957 - 1959 dollars)
Capital cost (1957 - 1959 dollars) = 15.776($170,000) = $2,682,000.
Capital cost (1975 dollars) = 2.32($2,682,000) = $6,222,000.
For a 10 mgd plant (438 1/s); equivalent population = 100,000
Per capita cost = $18.285 (1957-1959 dollars)
Capital cost (1975 dollars) = 2.32($18.285)(100,000) - $4,242,000.
Capital cost of expansion
$6,222,000 - $4,242,000
Engineering and legal costs (20%) =
Land costs @ 10%
Total Capital Costs of Expansion =
Salvage value of land
$1,980,000
396,000
198,000
$2,574,000
The salvage value of the land required for plant expansion is,
at the end of the 20-year planning period, taken to be $198,000,
the same as its current value. The present worth of this salvage
value is calculated by multiplying the salvage values by a single-
payment present worth factor,
P_ 1
F
n
(1 + i)
where: P = present worth
F = future (salvage) value
i = interest (discount) rate
n = planning period
In short, this calculation identifies the sum of money that, if
placed at interest for n years, would yield an amount equal to the
salvage value. Procedures for calculating present worths and'tabu-
lations of appropriate factors may be found in a variety of references (5),
For i «= 0.07 and n = 20 years, the present worth of the land salvage
value is: / . \ 20
$198,000 f 11()7 j = $198,000(0.253429) = $51,000
24
-------
Net present worth of capital costs (secondary treatment)
Since other capital costs are for current expenditures, they
do not require discounting; and the net present worth of the capital
costs for plant expansion is:
Present worth of new capital costs = $2,574,000 - $51,000
= $2,523,000
The capital costs associated with the addition of tertiary treatment
in 10 years (1985) is taken as equal to the capital costs of an activated
sludge plant of the same design flow. Since these costs would not be
incurred until 10 years into the planning period, their magnitude must be
reduced to a present worth by multiplying by the single-payment present
worth factor P/F = l/(l-i)n, with n = 10 years.
The service life of the tertiary facility is taken to be 20 years
from the time of construction, or 10 years beyond the planning period. As
a result, a salvage value is claimed for the tertiary facilities, based on
straight-line depreciation of the capital costs exclusive of engineering,
legal, and land costs. To this is added the salvage value of the land costs
related to tertiary treatment, taken as equal to the land cost at the time
of construction. The sum of the two salvage values is then converted to
a present worth by multiplying by the single payment present worth factor
(n - 20).
The calculations are illustrated below for a 17 mgd (745 1/s) design
flow plant capacity.
Capital cost = $6,222,000
(taken from earlier illustration, 17 mgd (745 1/s) plant)
Engineering and legal (20% of capital cost) = 1,244,000
Land ciosts (10% of capital cost) 622,000
Total Capital Cost $8,088,000
Present worth, total capital cost:
$8,088,000 . $8,Q88,00(0.50835) ,
(1.07)10
Salvage value, tertiary facilities:
/ \
[Service life - period used
$4,112,000
Service Life
capital cost = salvage value
2°
20
($6,222,000) =
Salvage Value, Land Costs =
Total Salvage Value =
Present Worth, Salvage Value = $3'733^°.0 =
.20
$3,111,000
622^000
$3,733,000
$ 965,000
(1.07)'
Net present worth, capital costs = $4,112,000 - $965,000 = $3,147,000
25
-------
It should be noted that the projects included in the Public Health
Service study were limited to those with design populations of 100,000 or
less. Therefore, the values obtained for plants over 10 mgd (438 1/s)
capacity may or may not reflect the actual costs of larger treatment
facilities. However, the resulting values appear reasonable for illus-
trative purposes.
Operating Costs
Plant operation and maintenance costs are also based on a Public
Health Service survey. Included in this cost evaluation were operating and
maintenance costs for 60 activated sludge treatment facilities. In this
study the valid design population range was up to 200,000. As before,
extension of the curve may or may not accurately reflect these costs for
larger plants. The expected values were estimated from the formula:
log y = I/[(0.50927) + 0.13791 log x]
y - the expected annual per capita cost x 10
X = population served x 0.01 (no r is given)
All costs were stated in 1957-1959 dollars. They have been brought
up to the 1975 level by use of the Bureau of Economic Affairs' Implicit
Price Deflator (BEA-IPD) of state and local government purchases of goods
and services (1975 = 227). Included in the operation and maintenance costs
are all costs other than central administration and capital maintenance.
The annual operating cost for secondary treatment is converted to
present worth by multiplying the annual cost by the series present worth
factor:
p_
A 1(1 + i)
where:
n
(1 + 1) -1
n
P = present worth
A = annual cost
i = interest (discount) rate
n = planning period, years
The annual operating cost for tertiary treatment is assumed to equal
that for secondary treatment. Since tertiary treatment is assumed to begin
after 10 years, it contributes to the operating cost only for the last half
of the planning period. Calculation of the present worth of this operating
cost requires two steps: conversion of the annual costs to a single 1985
value using the series present worth factor mentioned above with n = 10; and
the discounting of the 1985 value to a 1975 amount using the single-payment
present worth factor described earlier (again n = 10). These calculations
are illustrated as follows:
26
-------
Operating costs, 17 mgd (745 1/s) design flow activated sludge.plant:
log y = 1/(0.50927 + 0.13791 log 1,700) = 1.0474
Annual per capita cost (1957-1959)dollars = 0.1 y = 0.1(11.152) = $1.1152
Annual operating cost (1975 dollars) = 2.27($170,000)(1.1152) = $430,400
Present worth, annual operating costs = ($430,400)
|-(1.07)'"- 1 I
|_0.07 (1.07)2°J
.07 (1.07)'
= $430,400 (10.5940)
= $4,560,000
Operating costs, 17 mgd (745 1/s) Tertiary Treatment Facility
(assumed to equal annual operating costs of activated sludge plant)
Present worth, Tertiary Treatment Facility Operating Costs =
1
($430,400) ra.07)10-l 1
[p.07(1.07rU J (1.
($430,400) (7.70236) (0.50835) =
07)
10"
$1,537,000
The net present worth for several different sizes of treatment
plants, calculated in accordance with these illustrations, is summarized in
Table 7. Also shown in the table are the incremental costs of changing
the design capacity.
Collection System Costs
The capital or construction costs of a sewer system show wide varia-
tions among cities. Similar variations appear in sewer repair costs, due
to differences in climate, topography, soil, and groundwater conditions. In
the examples that follow, the sewer repair and replacement costs reflect
approximate values taken from several sources. While other sections of this
evaluation provide some guidance with respect to such costs, each community's
own experience provides the best estimate of their magnitude. Thus, the
numerical values contained in the examples are offered solely to illustrate
the procedure for analysis. It is assumed that the values presented include
all costs, such as landscaping and street cut repairs, and other -phases of
construction and reconstruction.
Among the costs associated with excessive infiltration and inflow are
estimates for emergency repairs resulting from street cave-ins and for
pumping and clean up due to wet-weather flooding of local areas. The annual
savings for emergency street repair costs have been assumed as $.250/yr/mi
($155/yr/km) of sewer repaired, or at a present worth of 7%, for 20 years, of
$2,650/mi ($l,646/km). Savings of pumping and clean up costs are estimated
at $5,000/yr/mgd ($114/yr/l/s) of I/I removed or excluded, or a present worth
of $53,000/mgd ($l,210/l/s). These figures were taken from approximations
and, as before, the community's own cost experiences should be used in making
the cost-effectiveness analyses. They enter the analysis either as additions
to the treatment costs, reflecting further costs associated with the I/I, or
as deductions from the costs of removing or excluding I/I. The most effective
27
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combination of sewer repair and I/I treatment is the same by either approach.
The former presents more completely the total costs of I/I, while the latter
is somewhat simpler to apply to the example problem as structured here.
As stated earlier, a two step process is involved in the identifica-
tion and quantification of the infiltration/inflow problem. The first step
is an I/I analysis based on currently available information on flows, pop-
ulation served, age of system, and other factors. Estimates are made of
the probable quantities of I/I in each, section or the sewer system and of
the expenditures needed to eliminate the I/I or reduce it to an acceptable
limit. In general, the results are compiled in terms' of a number of in-
dividual, separable projects that can be feasibly and rationally under-
taken separately and independently. For example, if different portions of
a sewer line require different types of repair and, therefore, have different
associated costs, each portion is identified as a separate project. Once
all the projects are identified, each is ranked according to its cost per
quantity of I/I reduction gained, with the lowest cost per unit I/I first.
Cumulative listings, in order of ranking of the projects' costs and I/I
quantities are prepared. Following the procedure described earlier, the
savings related to reductions in street cave-ins or costs of flooding
cleanups are incorporated in these cumulative lists. They may be added
to the treatment costs, if preferred.
For each of the projects as ranked above, the cumulative cost of
collection system repair/replacement is added to the present worth of
treatment costs required to treat the remaining flow not removed by repair
or replacement of the collection'system. The most .cost-effective solution
is that one for which the total costs expressed as present worth are least,
assuming all projects meet the same federal, state, and local standards as
a minimum and that none has overriding adverse social or environmental
nonmonetary consequences. The procedure is illustrated in the following
example.
Examples of Cost—Effectiveness Analyses
Infiltration/Inflow Analysis - On the basis of existing information,
an I/I analysis of the example community profiled in Table 6 revealed two
sectors of the community wastewater collection system that had substantial
I/I. Table 8 refers to Sector 1 and Sector 2, respectively. Sector 1 in-
volves about five percent of all building connections and smaller sewer
sizes of the example community, and a lesser proportion of the larger sewer
sizes. Sector 2 involves about two percent of all building connections and
smaller sewer sizes and again a lesser proportion of the larger sewer sizes.
The table identifies the probable sources of infiltration by size
and length of sewer, number of manholes or building connections involved;
the probable quantities of infiltration for each source; and the probable
costs of removing or eliminating I/I. For purposes of- this example, the
quantities of infiltration shown are taken to be in excess of any remaining
infiltration to be expected in the sewer line after remedial measures are
taken.
29
-------
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Each potential source identified in the table is regarded as a
separate project, and the total costs for remedial action on that project
are calculated. Deducted from these costs are the present worths of savings
anticipated from the reductions in numbers of street cave-ins and costs of
flood cleanups attributable to the infiltration. This yields a net cost
of remedial action for each project and provides a basis for calculating
the net cost per unit of flow of infiltration reduced or eliminated.
All projects are then ranked in order of increasing costs per mgd
of infiltration removed, as illustrated in Table 8. The costs incurred
and quantities of infiltration removed by undertaking each of the remedial
projects successively in order of their ranking are accumulated.
If infiltration at the rates shown in the table persists over long
periods of time, possibly caused by persistently high groundwater tables,
the treatment facility must be sized to accommodate the infiltration as
part of the design flow. This is assumed to be the case for the present
example, and so a reduction in quantity of infiltration produces an equal
reduction in the design capacity of the treatment facility. The costs
associated with each successive level of I/I reduction (Table 8) are added
to the net present worths of treatment costs (Table 7) for plant sizes
appropriate to the level of I/I reduction to obtain the overall cost of
treatment and I/I reduction. The most cost-effective combination is that
for which the total cost is least, assuming all other requirements discussed
earlier are met. The procedure is illustrated graphically in Figure 1, for
primary and secondary treatment only. Figure 2 includes tertiary treatment
and follows a similar pattern.
If only primary and secondary treatment is needed to meet all fed-
eral, state, and local requirements, the most cost-effective combination of
infiltration reduction and wastewater treatment is indicated by the lowest
point on the total cost curve C of Figure 1. This corresponds to a reduc-
tion in infiltration of 3.58 mgd (157 1/s) and a treatment plant capacity
of 13.42 mgd (588 1/s). Thus, the first seven infiltration reduction proj-
ects in order of ranking should be undertaken.
If tertiary treatment is required to meet water quality requirements,
the most cost-effective combination of infiltration reduction and wastewater
treatment is obtained from Figure 2. The least total cost is attained by
removing 4.08 mgd (179 1/s) of infiltration, corresponding to the eleventh-
ranked project in terms of accumulated infiltration reduction.
The most cost-effective level of infiltration reduction could also
be determined approximately by comparing the cost per mgd of each project
in ranked order as shown in Table 8 with the incremental unit flow cost of
treatment shown in parentheses in Table 7 for the appropriate size of
treatment facility.
It should be noted that the present worths of net costs for removal
of inflows itemized at the bottom of Table 8 are negative, thus indicating
31
-------
(U2I/I)
II
1526 !/>>
12
t Capacity, Rifd (Onion Flow)
(5701/1) (613IA) ' (6571/1)
15
(700 I/I)
(7*51/1)
17
(218 I/.) "75 I/O
Accumulated I/I Reduction, mgd
Cu'« A IntiHralion reduction ««ti (bottom *nd ntfn tcaleil
Gun** B Tieatment cotts (top and left scdks)
Total coils hop and left .coles)
Now The ariowi pointing to Curvei A and C indicate the potitiorw ol the eleventh tanked
mtiltuiion reduction project
Figure 1. Cost-effectiveness solutions for primary
and secondary treatment.
Tmtrmnt Cftpadty. rnfd (D*tign flow)
ISTDI/i) (8131/1) (6571/tt
15
(7011/l) (74SI/i)
(219 l/il
(175 l/i! (1311/1) (881/
Accumutittd I/I Rtduction, mgd
Cut** A Inliliration (eduction com Ibottom and nqht icjlei)
Cuive B Ticjlmtvit eoilt (lop ami left icita)
Cu«w C Tot*t coin (ton and (*H «a!«(
Figure 2.
Cost-effectiveness solutions for primary,
secondary, and tertiary treatment.
32
-------
net savings rather than costs. The savings result from the reduction in
flood clean-up costs that are attributed to the exclusion of these inflows.
As a result, these inflows merit elimination from the collection system
without consideration of treatment costs.
No specific consideration has been given thus far in the analysis
to the cost of transporting the infiltration and/or inflow to the treatment
facilities. Such costs may involve construction and operation of relief
sewers and/or pumping stations. These costs do not add to the infiltration
reduction costs, but to the treatment costs. To the extent that such costs
can be identified for each separate project shown in Tables 7 and 8, they
should be tabulated and accumulated in the reverse order of the rankings
shown in Table 8. That is, an extra column should be added to Table 8
which identifies the accumulated cost of transporting to the treatment
facility the remaining infiltration not removed by the infiltration
reduction projects listed through that point in the table. The relation-
ship of accumulated transportation costs to accumulated infiltration
reduction costs, and the incorporation of transportation costs in the cost-
effectiveness analysis are illustrated in Figure 3.
Note: The curves shown are not derived from the previous numerical
analyses and are merely suggestive of their form.
Since the transportation costs add to the treatment costs, it may
be desirable to postpone evaluation until rehabilitation projects have been
screened for preliminary cost-effectiveness in comparison with treatment
costs alone. Any projects not found to be cost-effective in this screening
will not be cost-effective when transportation costs are included, unless
the remedial action is needed to fulfill a transportation function as well.
The purpose of an infiltration/inflow analysis is to ascertain
whether the collection system is likely to be subject to excessive I/I.
Excessive I/I is defined as that which is more costly to transport and
treat than to exclude from the system. If the analysis identifies the
possible existence of excessive I/I, a sewer system evaluation survey
must be performed.
Sewer System Evaluation Survey
The sewer system evaluation survey requires a detailed, systematic
examination of the collection system to determine the specific location,
estimated flow rate, and method and cost of rehabilitation versus the cost
of transportation and treatment for each identified source of I/I. The
results are summarized in a report that includes, in addition to the infor-
mation previously described, a justification for each sewer section cleaned
and internally inspected, and a proposed rehabilitation program for the
sewer system to eliminate all excessive I/I.
For this example, it is assumed that a survey was performed and
that the sewer system infiltration data presented in Tables 7 and 8 were con-
firmed. However, the infiltration was found to be much less persistent than
33
-------
Tnitmtnt Capacity (increasing • I
Infiltration Reduction (decreasing - )
Figure 3. Incorporation of transportation costs in cost-effective analysis.
previously assumed, contributing mainly to the peak flow at the treatment
plant. As a consequence, the cost-effectiveness analysis requires that the
costs of infiltration reduction be added to those of treatment facilities,
expressed in terms of peak flow capacities rather than nominal or design flow
capacities. For this example, a peak flow treatment plant capacity of 34 mgd
(1,489 1/s) is assumed to be needed if no infiltration is excluded from the
collection system.
Since most plants are designed for peak capacities of two to three
times their design capacity, the' capital and operating costs of the treatment
facilities used in this example are those calculated for plants whose design
capacities are one-half of the peak capacities shown. Thus, the capital
and operating cost for a 30 mgd (1,314 1/s) peak capacity primary and second-
ary plant are estimated to be $5,938,000 as a present worth (7 percent,
20 years), the same value as shown in Table 7 for a 15 mgd (657 1/s) design
flow plant of that type. This procedure is appropriate for capital costs
but may overestimate the operating costs properly allocated to the treatment
of infiltration.
Since the most cost-effective combination of infiltration reduction
and wastewater treatment is determined by the balancing of infiltration
reduction costs against the incremental costs associated with the change in
plant capacity required to accommodate the infiltration, the approach taken
here seems reasonable. In any event, actual costs for construction and
34
-------
operation should be developed on an item-by-item basis wherever possible.
Only those costs properly assignable to the treatment of infiltration
.should be used to determine a cost-effective solution.
The results of the cost-effectiveness analysis are shown in Figure 4.
The cost of the sewer system evaluation survey ($646,000) is included in the
determination of total costs. Its components and their costs are assumed as:
Physical Survey $100,000
Rainfall Simulation 120,000
Cleaning 20 percent of System 195,000
TV Inspection 181,000
Analysis 50.000
Total $646,000
The most cost-effective combination of infiltration reduction and
wastewater treatment appears to correspond to the elimination of 3 mgd
(131.4 1/s) of infiltration from the collection system. This is the point
at which the total cost curve D is at a minimum. Unlike the previous
illustrations, however, this point does not correspond to the reductions
obtained through completion of any of the individual projects. Rather, it
falls almost midway between the sixth and seventh ranked projects. Taken
individually, the total cost associated with implementation of the seventh
ranked project is less than that of the sixth; and so implementation of
the first seven infiltration reduction projects, in order of ranking, pro-
vides the most cost-effective combination. It is not likely that this
solution could be improved upon by completion of only the first six projects
and a portion of the seventh in an effort to reach the minimum point of the
curve since the costs would tend to follow a straight line connecting the
total costs associated with the sixth and seventh projects rather,than
following the curve shown.
As was stated previously, the costs of transporting the remaining
infiltration of approximately 0.85 mgd (37.2 1/s) to the treatment plant
should now be determined and incorporated in the analysis. If no relief
sewers or pumping station construction would be required and only incremental
pumping costs on existing facilities must be considered, the transportation
costs are estimated as follows:
Assume a total lift of 30 ft (7.9 m), overall efficiency of 65
percent, and energy cost of $0.015 per kilowatt-hour, and a flow
of 1.31 cfs (37.2 1/s).
•The power required is calculated from the equation:
P =' QTh
(1.31)(62.4)(30) = 5.11 kw
(7.376) (65)
7.376E
where:
P = power, kilowatts
Q = flow, cfs „
7 = unit weight of water, 62.4 Ibs/ft
h = pumping head, ft
E = overall pumping efficiency! %
(1 kg/1)
35
-------
Treatment Capacity, mgd (Peak Flow)
(123IM (1270IA) (1314IA) (1358 l/l) (14021/1)
-TT-
29
(1445 l/l)
33
(1489 l/i!
34
G
(219 l/l)
2
(88 IA)
Cum A:
CurvoB:
CurvaC:
Curvtt Dl
Note:
4 3
11751/1) (131 IA)
Accumulated I/I Reduction, mgd
Infiltration reduction costs (bottom and right scales).
Treatment costs (top and left scales).
Treatment plus survey costs (top and (eft scales)
Total costs (top and left scales).
The arrows pointing to Curves A and D indicate the positions
of the sixth and seventh-ranked infiltration reduction projects.
1
144 l/l)
Figure 4. Cost-effectiveness solutions for primary, secondary and tertiary
treatment based on peak flow capacity.
The energy cost for pumping 0.85 mgd (37.2 1/s) over a period of one
year is: (24 hr/day) (5.11 kw) (365 day/year) (0.15c/kw/hr) = $671/year.
Operation and maintenance costs, taken at 50 percent of the power
costs, make the total annual cost ($671) (1.50) = $1,007. The present
worth of this annual cost, ($1,007) (10.594) = $10,670. It is obvious that
this amount is not large enough to change the cost-effective solution.
USE OF HOUSEHOLD FLOW SEDUCTION DEVICES
In recent years, a variety of household flow reduction devices have
become commercially available. These devices restrict the flow rate of
household plumbing fixtures such as kitchen and lavatory faucets and showers,
or otherwise decrease the total amount of water used per cycle in fixtures
and appliances such as toilets and washing machines. Descriptions of the
devices and their costs and operating characteristics are reported in a
number of publications (6,7,8). While these devices were developed primarily
as a means for conserving water supplies, they also afford the opportunity
36
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for reducing wastewater collection system flows and, thereby, capacity
requirements for treatment plants as well. Consequently, the devices
merit consideration as a means for accomplishing wastewater flow reductions
in the development of wastewater facility plans.
From a water quality viewpoint, the effects of household flow
reductions are similar to those of I/I reductions. The principal difference
then, between flow reductions accomplished by sewer rehabilitation as con-
trasted to household flow reducing devices relates to the integrity of the
collection system itself. Use of the household devices makes no contribution
to the maintenance and/or improvement of the sewer system and may mask its
deterioration significantly. No benefits can be claimed with respect to
decreases in frequency of street cave-ins and savings in repair costs. The
household devices do, however, decrease water consumption and the savings
such decreases represent contributes substantially to the overall benefits
that may be attributed to them.
Many of the household flow reduction devices are not well suited for
installation in existing construction largely because of the installation
costs involved. Since a substantial proportion of the collection systems
subject to excessive infiltration are old systems, serving existing and
often old structures, such devices seem less suitable for incorporation in
an I/I analysis.
At lease three techniques afford sufficient ease of installation and
low cost to be included in an I/I analysis: dual-flush adapters for toilets,
insertion of bricks or other materials in toilet flush tanks, and devices
to limit flow in shower heads. Aerators for kitchen and lavatory faucets
also involve low cost and ease of installation, but the extent of their use
makes the estimation of the attainable flow reductions highly speculative.
Both the dual-flush devices and those limiting shower head flows have been
limited to only field testing in actual households for periods of about one
year. The published results (7, 8,) of these tests provide the primary source
of data for the following analyses.
The two types of dual-flush devices tested produced average water
savings of 5.42 gal (20.5/1) and.3.28 gal (12.4/1) per person-day, for an
overall average of 4.36 gal (16.5/1) per person-day. The cost, not including
installation cost, ranged from $4.00 to $14.00 per device.
For these calculations the installed cost is taken to be $10 per
device. As reported, the expected service life of the device is 10 years.
37
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For the example community detailed in Table 6:
Total flow reduction, 100,000 persons =
(106) (4.36 gal/d [16.5 1/d]) = (0.4 mgd [19.1 1/s])
Initial capital cost =
(28,570 housing units) (1.5 devices/unit [$10/device]) = $428,600
For a service life of 10 years, the present worth of
replacement cost =
($428,600)-
(1.07)
— = ($428,600) (0.5084)
Present worth of total capital cost =
($428,600) + ($217,900)
= $217,900
$646,500
Present worth of savings attributable to pumping cleanup
(0.436) ($53,000) (based on $5,000/yr/mgd [$114/yr/sec] )=$ 23,100
Present worth of net costs = ($646,500) - $23,100 = $623,400
$623.400
Net cost per unit flow reduction = — Q ^35
= ($832,646 1/s) = $l,429,900/mgd
The magnitude of this1 net cost per unit flow reduction is much
greater than any of the values calculated for the inflow reduction projects
listed in Table 8. As a result, use of the devices to accomplish flow
reductions would not be included in any of the most cost-effective combina-
tions of treatment and flow reduction in the previous examples.
However, if the additional benefits of household flow reductions
attributable to reduced costs of water purification and distribution are
included, the savings prospects change. The cost of water production and
distribution is assumed to be $348/mg ($0.0092/1). This is the wholesale
rate. charged to a neighboring community by a midwestern city of size
comparable to the example community.
Water supply savings = ($348/mgd) (0.436 mgd) (365 days/yr)
= $ 55,380/yr
Present worth (7 percent, 20 years) of water supply savings
($55,380) (10.594) == $586,700
If this value is deducted from the present worth of net costs
calculated previously, the revised value becomes:
Revised net present worth = ($623,400) - ($586,700) = $ 36,700
Revised net cost per unit flow reduction = $36,700 = $84,174/mgd
0.436 ($1,922 1/s)
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On this basis, the use of dual-flush devices for flow reduction would
rank third among the projects listed in Table 8, and would become part of all
of the most cost-effective combinations of the previous examples. Since the
incremental costs of wastewater treatment, in doliars/unit capacity, increase
slowly at about $20,000/mgd ($456 1/s) with decreases in treatment plant capa-
city, the most cost-effective combination of flow reductions and treatment
plant capacities in each of the previous examples would still include all of
the infiltration reduction projects of the previous solution, plus the house-
hold flow reduction project. Treatment plant capacity would decrease by 0.436
mgd (19 1/s), and total cost would decrease appropriately
Limiting Shower Head Flow
The field tests of limiting shower head flow revealed that a limiting
flow rate of about 3.5 gpm (0.22 1/s) produced an average water savings of
0.8 gal/person-day (3.028 I/person-day). Installed costs were reported as
equal to the material cost of $8 per device. The service life was reported
as 15 years.
For the example community:
Total flow reduction = (0.8 gal/day)(100,000 persons) = 0.08 mgd
Initial capital cost =
(28,500 housing units)(1 device/unit)($8/device) = $228,600
For a 15 year service life the present worth of
replacement cost ($228,600)(0.362) . = $ 82,800
Present worth of salvage value =
($228,600)(0.667)(0.2584) = $ 39,400
Net present worth of capital costs =
($228,600) + ($82,800) - ($39,'400) = $272,000
Present worth of savings attributable to
pumping and clean up = (0.08)($53,000) = $ 4,200
Water supply savings = ($348/mg)(0.08 mgd)(365 day/yr) = $ 10,200
Present worth of water supply savings =
($10,200)(10.594) = $108,100
Present worth of net costs =
($272,000) - ($4,200) - ($108,000)
($159,700)
(0.08)
Cost per mgd reduction
= $159,700
= $l,996,250/mgd=($45,576 1/s)
This cost per mgd of flow reduction is clearly in excess of any
reported in Table 8, and so the use of limiting shower head flow would not
become part of the most cost-effective combinations of flow reductions and
treatment in any of the previous examples. If the value of the water supply
savings was doubled, however, the cost per mgd flow reduction would be
$645,000/mgd ($14,726 1/s) and use of the device would become part of the
most cost-effective combinations of flow reduction and treatment for the
conditions represented by Figure 2.
39
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Need for further ClaTrifi.cdtj.on
In the development of the illustrations and examples presented in
this section, many questions arose which appear to be unanswered or, at best,
incompletely answered. In particular, two issues seem to merit further
consideration and study.
The first of the issues relates to the "clear water" characterization
of I/I. Is it reasonable to assume that infiltration is completely devoid
of suspended and/or dissolved constitutents that might impose increases in
the organic and/or solids loadings on treatment processes? If so, what is
the effect of "clear water" on treatment performance and on the operating
costs of treatment. Arguments can be made in support of either increases
or decreases in performance and in operating costs; further exploration
of the issue, especially analyses of actual case histories, would be helpful.
The second issue relates to the fate of the I/I excluded from the
collection system. Does it give rise to or otherwise magnify other environ-
mental problems in the community? How much of it finds its way to the
stormwater collection system? If treatment of stormwater is required of a
community, how does this affect the cost-effectiveness analysis with respect
to I/I reduction in the sanitary sewer system? Such questions as these
remain to be resolved.
Many other issues could be recited, some of which may not have sub-
stantial impact when considered on a community-by-community basis, but may
be important in the establishment of general policies relating to wastewater
management and planning. These issues should be identified and explored if
sound and rational policies and procedures for future wastewater management
and planning are sought.
40
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SECTION V
ROOT CONTROL PRACTICES
Relationship Between the Root Problem and Infiltration.
Public sewers are more than conduits for the collection and trans-
mission of wastewaters from homes, commercial and industrial structures and
urban surfaces. They are the 'well springs' of available moisture for plant-
ings with root structures which can reach down to the depth of street lines
and building connections to tap this unending source of fluid. The drainage
lines which serve as this source of water around the clock and over all
seasons are sanitary sewers and combined sewers. Of lesser dependability
and significance as an enclosure for root growths are separate storm sewers.
Plants, including trees and shrubs, depend upon moisture obtained from
the soil and other sources taken into the sap system by capillary action.
Plants fare best when the moisture is available, as interstitial fluid, not
in the form of water which when it inundates root structures of most plants
stunts or rots their growth.
The presence of such interstitial fluid in the soil is not assured
at all times because of unpredictable dry periods. The moisture-seeking
root growths of trees and shrubs reach out for sources of greater depend-
ability. Sewers which are laid sufficiently close to surface plantings
and at shallow enough depths to be reached by root structures provide this
dependability.
So great is the need and desire of tree and shrub growths for
moisture as well as for anchoring of aboveground growth structures into the
surrounding soil, that root growth, diametrically, equals the spread of a
tree's branch system. In fact, root spread is not limited by this width
factor.
Roots are composed of either tap or fibrous systems. The extent
of the system depends on whether the main or tap root persists as the
prominent structure or whether it is replaced by numerous secondary shoots
or root branches. The main root structure is composed of the central
tubular growth, or cortex, and the epidermis from which secondary roots
form in hair-line formations. Such water can set up new central systems
and their own capillaries. These hair root complexes increase the moisture-
absorbing surface of the root system and its spread into areas of potential
water and mineral sources. Roots are normally free of pith, beyond the
wood or xylem, but the "curtain" of subroot formations is relatively fibrous
and tough.
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The water^seeking function of the root formation, the ability of a
single root strand to become the parent of a new system of hair-like films,
and the toughness of the fibrous formations exacerbate the problem^of roots
in sewer systems. The vaporous atmosphere in sewer lines, together with the
potential plant nutrients contained in the sewage flpw and in the exuded
vapors, attract root intrusions.
Whether roots are the cause or effect of infiltration and exfiltration
is academic. Root intrusions and groundwater intrusion or extrusion are a
fact of life in sewer system operation and maintenance. If infiltration
occurs, groundwater is in contact with the exterior of the pipe. During dry-
conditions the sewer allows water vapor to be present in the surrounding
soil which may trigger root growth into such soil regions.
If groundwater does not surround all or parts of the sewer line during
the entire year, defective sewers can produce exfiltration of wastewater into
the soil structure. This moisture could readily become the inducement for
root growth into the wetted area. Once at this point, the roots seek the
source of the vapor - the inside of the sewer lines. In this case, just as
in the case of infiltration, root growths enter the sewer barrel and produce
the associated operation and maintenance problems.
Differences of opinion exist as to whether roots are the cause or
effect of infiltration and exfiltration: do roots find minor defects in
sewer lines which do not, in themselves, represent sufficient disunity in
pipes to cause either infiltration or exfiltration, and does the; prolif eratd.on
of the root intrusion result in a further opening of the minor opening which
may originally merely produce a moist outer surface on sewer pipe? Or,' does
infiltration or exfiltration exist through pipeline defects which are large
enough to permit the entry of root formations? And will roots, once they
intrude through such points of defect, cause rupture of pipe structures or
joints because of the growth of "wood" or xylem?
Those who support the belief that roots can produce progressive
sewer and joint defects point to the ability of tree roots to split or
fracture walls, foundations, and walkways. Whether this vegetative force
works in this manner or not, the fact remains that roots do enter sewer
lines, and do grow almost explosively in the sewer atmosphere, are not
readily discouraged, and do pose a problem. Without question, there is a
direct relationship between the conditions surrounding infiltration and
root intrusion, whether root growths create greater infiltration or whether
the dense root growths can actually fill in pipeline and joint defects and
temporarily "seal" leakages.
Another factor which may stimulate the growth of tree and shrub
roots into and through the soil surrounding sewer lines is the fact that when
lines are laid, they disrupt the virgin composition of the underground. The
backfill material, often carefully specified by construction requirements,
and though compacted, may provide easier access to root structures than the
surrounding indigenous soil. The composition of the pipe soil cushion and
the backfill up to the haunch of the pipeline may serve as a "French drain"
around the sewer line and provide flowing water or interstitial water
which roots seek.
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When sewers are laid along easements, trees may be placed along the
trench to provide a "fence" between properties, or because this is the
easiest place in which to plant.
The basic principle is clear that infiltration and the sewer root
problem are interlocked. For this reason a review of root control practices
was made a part of this study.
Root intrusion problems do not occur in the presence of minor vege-
tative or silvicultural growths. Grasses, small shrubs, flowering plants
and food, feed and small fiber growths do not produce, nor do they need,
far spreading, depth penetrating root structures to support the aboveground
growths. The main sources of root formations that can reach and enter sewer
lines are trees and large shrubs.
Proximity of such plantings to sewer lines is the key factor in
whether their root structures will reach sewer lines, enter them, and produce
operation and maintenance problems. This, then, limits the problem to the
urban environment and the juxtaposition of tree and shrub plantings, and
nearby sewer lines. It limits the problem, or at least the intensity of
the problem, to urban areas which are more heavily planted and where the
plantings are of sufficient age to have produced root growths of dimension
and depth to reach sewer lines.
The urban region can be divided into business and commercial areas
where tree planting and shrub growths are minimal, and residential sections
where street and property beautification has led to heavy growths. Even in
business areas recent aesthetic thrusts have produced curbside tree planting,
often by the transplanting of partially matured conifer and deciduous
forms which already have heavy root balls and which can rapidly spread to
seek moisture, nourishment, and anchorage.
Street beautification and lawn aesthetics are the major cause of
sewer rooting because they are located near house sewer connections with
street and easement sewers. In areas where no restrictions have been imposed
on the types of tree plantings and their locations by city ordinances,
property owners have often chosen fast-growing species which are known to
have greater affinity to moisture sources and to sewer intrusions. Where
municipal forestry agencies control tree plantings, they have greater
opportunity to stipulate types and locations that are less of a hazard
to sewer personnel in making tree planting decisions.
It is obvious that trees and yard shrubs place house connections in
greater root jeopardy than street sewers due to location and depth at which
house sewers are laid. Trees located along the front or rear of property
lines must extend root growths half the width of the street when sanitary
sewers are laid at the centerline of the right-of-way. Where sewers are
laid on both sides of the street, the distance from tree lines to sewer lines
is foreshortened and root problems may be intensified. The use of combined
sewers along both curb lines, as is the case in communities where short runs
are used to pick up stormwater inlets and catchbasins, results in tree
plantings that are located in overhanging position along the sewer lines.
43
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The incident of root intrusion into house laterals was reported more
often than for street sewers- a verification that root intrusion rates are
in inverse proportion to the distance of the sewer lines from tree and lawn
planting. Experience has shown many communities which types of trees are
the most troublesome and after-the-fact regulations have been enacted in
some cases to bar such so-called nuisance trees in future planting and
even order removal of existing stands.
It would be expected that high groundwater conditions might minimize
root intrusion into sewers because the "thirst" for underground moisture
could be met without drawing on sewer atmospheres and constant immersion
in groundwater could produce a rotting or stunting of root hair growth mats.
However, evidence of this phenomenon is not clear-cut. Nor is evidence
that certain types af soil are more prone to permit deep and widespread
root formations than other soils, such as heavy clays vis-a-vis sandy loams.
In coastal areas where saline water tends to intrude into groundwater tables,
the presence of high chlorides could inhibit profuse root growths.
The relationship between infiltration and root intrusions is best
illustrated by the fact that defective joints are the major point of both
adverse conditions in sewer systems. The ability of root formations to grow
both in mass and in length once they gain a foothold in sewer lines is
well known and of concern to sewer maintenance personnel. The proliferation
of sewer mats or "lace curtains" in sewers exposes more root surface for
the absorption of sewer moisture thus indicating the favorable environment
represented by the sewer atmosphere. These long runs of roots can totally
clog a sewer barrel, as evidenced by growths removed from lines which have
assumed the circumference and diameter of the infested conduits.
It is obvious that roots may tend to enter points of sewer defects -
joints or line cracks or breaks - ne'ar the top of the pipe or above the side
springline of the pipe, rather than at the bottom of the sewer. The upper
section of the sewer is where roots are found in sanitary sewers and along
the pipe invert in storm sewers.
The propensity of roots to enter at these distinct locations supports
the theory that roots are seeking vapor rather than water which always
occupies the bottom sector of sanitary sewer lines.
Clogging of lines is the product of two conditions; the dense growths
which close off the free flow, and the straining or screening action of
root tentacles which are not dense enough to impede flows but become matted
with grease, sand, sewage particles such as paper, rags and fecal matter,
and storm runoff debris.
All of these conditions are translatable into sewer maintenance
problems which have been disclosed during the surveys of root experiences
and control measures carried out during the course of this study.
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ROOT CONTROL PRACTICES
Since roots go where wastewaters flow, it is essential that root
growths be "built out" of sewer systems. This again emphasizes the relation-
ship between infiltration and root infestation problems; if water can get
into or out of sewers, roots inevitably will find entry via the same points
of defect.
Until existing sewer lines are made tight by rehabilitation,
reconstruction or replacement, root growths will occur. The 1970 study by
the APWA Research Foundation (9) labelled I/I as "pirates" of sewer system
capacities, and "usurpers" of highly important carrying volumes. Root
infestations, in a similar manner, rob sewers of their flow-through
capabilities and create service failures which are costly and hazardous to
the sanitation and conveyance functions of modern sewer systems. If root
entry cannot be prevented in sewer lines, the challenge to system operation
and maintenance agencies is to conduct comprehensive and thorough maintenance
procedures.
Root formations and consequent line clogging and stoppages will occur
more frequently in building connection sewers than in street collector sewers.
Previous studies have demonstrated that the length of such house lines may
equal that of street sewers to which they are tributary, considering the runs
of such connection lines on both sides of any sewered street. The smaller
diameter of connection lines adds to the potential clogging and actual stop-
pages of service.
Part of the problem in root formations in house sewers is that once
the growth is established in these connections - frequently near the stub
connection to the street adjacent to the curb line - these root mats tend
to enter the street sewer in search of less restricted growth areas and the
uniform vapor atmospheres in the collector sewers.
Where house connection responsibilities are divided between the
municipal sewer agency and the private property owner, root problems may
involve both entities. Some municipalities maintain connections from the
property line to the street sewer, with property owners charged with main-
tenance from the house to the property line. Other agencies assume no
responsibility for stoppages in the entire connection line and still others
provide maintenance for the service line for its entire length. Root
control measures are influenced by these policies.
The principal root control measure used for house sewers is reaming
or cutting of the mats which cause clogging and impedence of service. An
entire industry has developed to provide house sewer stoppage corrections,
the bulk of which are due to root masses and the build-up of sewage solids
and greases caused by the straining action of filamentous growths. The
so-called "Roto Rooter" cleaning operations identify the work of the plumber
and drain cleaner regardless of the type of scrapper or cutter devices they
utilize.
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While some property owners use chemical herbicides to inhibit root
growth, e.g. as application of copper sulfate crystals periodically into
toilets or caustic drain compounds in sinks or other fixtures, the general
tendency is not to practice preventive measures. When a stoppage occurs,
the afflicted property owner often looks to the municipality for relief.
Diagnostic investigations are then initiated by sewer maintenance crews to
determine the location of the root infestation and to place responsibility
for clearance on the owner or the sewer agency.
Root control in street sewer systems involves what can be called
the four practices of build-out . . . block-out . . . clean-out . . . and
snuff-out (or kill-off).
° Build-out of root intrusions involves the construction of sewer
lines that provide water-tight and root-tight joints, and pipe that is free
from imperfections when installed and will not crack, break or deteriorate
during service. If a system is infiltration-free it will be root-free. The
types of pipe materials and joints now being manufactured are generally
capable of being installed to minimize root intrusion at joints.
o Block-out of roots in sewer systems involves the sealing or
closure of points of defects in pipe and joints through which roots now in-
trude and which to the point of this national study, are sources of infiltra-
tion. These block-out practices include chemical sealing practices, grouting
or repair of points of infiltration and root entry, insertion of internal
pipe or tube structures, and actual replacement of sections of sewer lines
which are deemed non-sealable or non-corrective by internal lining or which
may be more cost-effective than sealing or related methods. The methods,
materials and mechanisms used for chemical grouting, other sealing methods,
internal lining and pipe replacement are listed and described in the Product
and Equipment Guide and the Manual of Practice.
o Clean-out of roots in public sewers involves the tearing or cut-
ting of root formations and their physical removal by means of devices pulled,
dragged or rotated through sewer lines from manhole to manhole. The principle
involved is the same as that used in the "snaking" or cutting of root clogging
material in building sewer connections. The various types of tools for thread-
ing clean-out lines through sewers, devices for pulling or propelling root
clean-out equipment, and scraping, cutting and dragging root clumps from sewers
are described in the Product and Equipment Guide and the Manual of Practice.
Cleaning is a temporary and ineffective means of clearing sewers of
root growths. Roots are persistent in finding points of entry into sewer
lines. The first root formation is usually fine hair-line growths. When cut,
each hair-root can be expected to "twig" and resume growth after the abrading
or cutting operations have been completed. In addition, cleaning does not
remove all of the root within the sewer. Thus, sewer maintenance forces know
the locations of densest root intrusions and they expect the need to return
to these areas due to the recurrence of stoppage conditions. Root removal
procedures parallel those used for other sewer cleaning purposes such as
removal of sludge, sand, grease, debris and other deposited or congealed
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materials. In general, the same types of cleaning apparatus, with some
modifications to handle the tough fibrous root masses, are used for all
sewer maintenance purposes.
Root growth may be year-round but the general experience is that
roots intrude most heavily in the fall and winter season of general foliage
dormancy, except for conifer tree types, and in the spring before leafing
occurs. Surprisingly, root growth is less active during the summer season,
if general field experience is used as the gauge of the infestation problem.
o The snuff-out of sewer roots involves the use of inhibiting
chemicals which attack filamentous growths and disintegrate or decompose the
root structure. This procedure is likened to the use of herbicides to elimi-
nate unwanted vegetative growths by above-surface applications. It is more
than a trick of semantics to define sewer roots as the "crabgrass of drainage
systems." Chemical "snuff-out" may be more than the mere temporary removal
of in-sewer growths. The herbicide may infuse the root material and be
carried upward a short distance into the main root stem by capillary action.
If this occurs the chemical stunts regrowth beyond the time frame that would
be affected by the physical cutting or tearing of the root mat from the sewer
barrel by means described under "clean-out."
Chemical treatment for root control varies from the simple to the
sophisticated. Periodic dumping of dry chemicals into manholes upstream of
heavily infested sewers and the solution of this material in the sewage flow
during indeterminate time periods can be classified as a simple and generally
ineffective procedure. The introduction of root inhibitors into sewers in
the form of solutions which are blocked off and permitted to flood or foam
sewer lines and, in some circumstances, fill house laterals, have been
utilized in the past few years.
Advances in Root .Control with Chemicals
Much research has been devoted to what has been referred to here as
root control by so-called "snuff-out" methods - chemical herbicide kill -
under the direction of plant physiologists, botanists and other scientists
not directly involved in the wastewater field. Investigations of herbicidal
performance of various types of chemical formulations have been initiated by
such research agencies as the Connecticut Agricultural Station and the
University of California, using greenhouse and lathhouse screening methods to
evaluate kill levels. Such experimental work on various types of root struc-
tures of plants has been aimed at determining types of herbicides, strengths
of concentrations and methods of application which will inhibit or destroy
root growths without producing unwanted kills of the surface plant life itself,
The translation of research plant physiology and biology work into
workable methods for controlling root growths in sewer systems has resulted
from studies and definitive evaluations of performance at such locations as
the County of Sacramento, California.
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The Sacramento studies represented intensive examinations and
evaluations of many known herbicides (listed in Table 9).
A facet of chemical treatment that is also described in the Product
and Equipment Guide and the Manual of Practice involves the addition of herbi-
cidal material to chemical grouting compounds and becomes incorporated in the
stiff gels which either create a water seal around defective joints and pipe
defects or produce a closure in the joint itself or the pipe crack or fracture,
The terminology of "plus" is applied to the sealants which add chemical inhi-
bitors into their formulations. A commonly used material is metham; another
is dichlorobenzonitrile. AM 9-plus, and 3M elastometric sealants with a
similar chemical root retardant are described in these two volumes of the
project report. It is contended that the addition of such chemicals to the
sealant materials adds to the ability of the chemical grout to resist the
further intrusion of root growths for several years. Literature attributed
to the Connecticut Agricultural Experiment Station at Windsor, Connecticut,
describes the life and effect of these "plus" chemicals as toxicants to root
formations.
Table 9. Herbicides Tested in Root Control Experiments
Common Name
Bensulide
Dichlobenil
Dinoseb
Endothall
Chemical Name
0,0-diisopropyl phosphorodithioate
S-ester with /V-
(2-mercaptoethyl) =
benzenesulfonamide
2, 6-dichlorobenzonitrile
2-sec-butyl-4, 6-dinitrophenol
7-oxabicyclo (2.2.1) =
heptane-2, 3-dicarboxylic acid
Trade Name
Prefar
Metham (SMDC) sodium methyl=dithiocarbamate
Paraquat
Trifluralin
2, 4-D
2,4. 5-T
Copper sulfate
Chlorthiamid
1,1' -dirnethyI-4,4
bipyridinium ion
a, a, a, trifluoro-2,
6-dinitro-/V, N-
dipropyl-p-toluidine
(2,4-dichlorophenoxy)
acetic acid
(2,4, 5-trichloro = phenoxy)
acetic acid
copper sulfate 5H2O
2, 6-dichlorothiobenzamide
Casoron W50
Premerge
Dipotassium
eridothal
Hydrothol 191
Vapam
Vaporooter
Paraquat
Treflan
Dow Formula 40
Weedone LV4
Weedar 2, 4, 5-T
Bluestone
Prefix
Formulation
Emulsifiable concentrate
1% emulsion
50% wettable powder
Alkanolamine salt
Dipotassium salt
Cocoamine salt
Water soluble concentrate
Vapam + surfactant
Water soluble concentrate
Emulsifiable concentrate
Alkanolamine salts
Butoxyei:hanol ester
Triethylamine salt
94.3% pentahydrate
7.5% granules
48
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This information is included here, not with the intention of pre-
senting adequate details of chemical root control which is available else-
where in the current literature as well as to some degree 'in other portions
of this report, but to attest to the workability and feasibility of this
method of root control on a corrective and preventive basis. There is need
for such information to demonstrate to sewer authorities who have either used
chemicals ineffectively or who have abstained from 'utilization of herbicidal
treatment "until it has been proven effective," that this method of control
merits consideration.
The California studies were under the direction of professional
plant physiologists and biologists. Coupled with closed-circuit television
inspections of sewer root conditions and actual dig-up of sewer sections,
field studies resulted in a rating scale for root kill, ranging from one
for no-effect to ten for total kill. Rating of each joint allows follow-up
investigation of effectiveness to be made.
Immediate short-term and long-term effects on root growths in sewer
lines were determined. From information gained, Sacramento County estab-
lished its present chemical root control procedures, perhaps the most compre-
hensive program now in use in the United States. Other noteworthy investiga-
tions which have been translated into actual field practices by sewer main-
tenance agencies have occurred in California at the City of Los Angeles,
County of Los Angeles, and at St. Petersburg, Florida.
Soaking and foaming have been utilized in root control work, primarily
with two herbicidal chemicals — Metham and Dichlobenil, products now incor-
porated in commercially available sewer application materials and in the
major sewer sealants. Long-term root kills are reported, covering both
matured root growths in sewer barrels, and fine hair roots in pipe joints and
other points of entry before they are actually visible to inspection camera
equipment.
The Mail Survey
In order to explore the extent and nature of the root problem, a mail
survey of selected community sewer systems was conducted. Out of 402 inqui-
ries 133 sewer agencies responded. The information provided in these
responses is evaluated and summarized herewith.
A vast majority of communities reported that roots were a problem in
their sewer systems. The few systems with no root problems were geographi-
cally dissimilar, not located in any specific region of the nation.
Root intrusion problems are a part of overall sewer maintenance in
all community systems. These overall problems involve accumulations, depo-
sitions, growths and deteriorating debris which impede wastewater flow.
Grease, sand and other intruded materials, root growths, debris dumped into
manholes and other related conditions were all cited. The presence of such
conditions is presented graphically in Figure 5, with similar data given for
other facets of sewer maintenance problems-.
49
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O H
H U.
3 O
£x
a* ui
u X
ES
CO
I
O
H
P.
§
4J
CO
>%
0)
S3SNOJS3U dO U38Wf1N
S3SNOdS3U dO
M „,
r ci
O I-
1L Z
z <
O E
I- j
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50
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Figure 5 demonstrates that there is no single cause of the total
sewer maintenance problem.
When asked if they conduct root control programs, 83 cities
responded in the affirmative. This program in some cities resulted in
over 2,000 dig-ups per year due to root intrusion in sewer lines. The
highest rate of dig-ups per year was found in the southern half of the
United States. This is probably attributable to the warmer climate which
induces greater tree growth. The stoppages per year because of roots, in
all cases, were higher than the number of dig-ups per year.
The surveys disclosed that sanitary sewer house laterals are the
source of most problems with root intrusion. Root intrusion also was a
major problem in the combined sewer house lateral. Table-10 interprets
this problem in terms of type of sewer systems.
Table 10. Relative Importance of Root Problem by Types of Sewer
(Number of Yes Responses)
Collector
Interceptor
House Lateral
Sanitary
88
14
102
Combined
42
5
48
Source: 1975 APWA Survey
Thus, it can be concluded that roots are more often a problem in the
smaller, and often shallower lines, with the house lateral a key weak point.
In the course of this survey information was obtained on use of
chemicals: foam chemical grout sealing with herbicide additives and non-
herbicide grouting, and direct dumping of chemicals into manholes. A tabu-
lation of the responses to this question is given in Table 11.
Table 11. Nature of Chemical Maintenance Program
for Root Control
Chemicals
Foam
Dump in Manhole
Sealant
Source: 1975 APWA Survey
Number of Yes Responses
18
44
8
51
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The general extent of chemical control has therefore been to dump
liquid or dry chemicals in the manhole. Such procedures are often used to
attempt to open blockages in the line.
The use of rodding and sewer ball equipment, along with other methods
was explored. Rodding of sewers was the predominant method reported.
A total of 93 out of 133 cities reported that they are satisfied
with their maintenance techniques. The remaining were unhappy or dissatis-
fied with their procedures.
The mail survey included an inquiry concerning the size of collector
sewers affected by root intrusion. The majority of the respondents indicated
that the sizes of the sewer most affected by root intrusion are in the 8 to 12
in (20 to 30 cm) range (Table 12). This data again indicates the relative
importance of the root problems in small diameter pipe, where the presence of
I/I flows may have a major effect upon the available carrying capacity.
Table 12. Size of Collector Sewers Affected by Root Intrusion
Sewer
(in)
6
8-12
15-24
Over
24
size
(cm)
15
20-30
38-61
61
System
(mi)
4,
33,
5,
3,
161
003
828
113
length
(km)
6
55
9
5
,935
,005
,713
,190
Length affected
(mi) (km)
1,935
11,323
489
144
3,225
18,872
814
240
Percent of total
length affected
13
81
3
1
.9
.5
.5
.0
Source: 1975 APWA Study
Officials were asked what type of trees or shrubs produce the great-
est amount of root problems in their area. The majority responded that
willows caused the most problems, followed by elms, maples, oaks, poplars,
and pines. Shrubs such as hedges, lilacs and privet hedge plantings also
were reported to cause'root intrusion problems. Certain trees that are in-
digenous to specific climatic areas were reported to be root-intrusion prob-
lems but the types listed were reported for all parts of the country.
The points of root intrusion were identified as joints, house
laterals, wye connections to collectors, and line breaks, in relative order
as indicated in Table 13.
The importance of joints as a point of root intrusion can be attri-
buted to early pipe joint design such as use of mortar and jute joints, use
of early bituminous joint compounds, defective workmanship and;inspections
and inadequate replacement programs for sewers. Survey data indicated that
current compression-type-0-ring joints are serving to minimize infiltration
and root intrusion.
52
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Table 13. Point of Intrusion by Roots
Joints
House Laterals
Y on Collector
Other
Breaks
Source; 1975 APWA Survey
Median
60
20
15
15
5
Because of the importance of house laterals as a source of root
intrusion, the survey explored the question of who is responsible for
maintenance of the various lines. Sixty-nine of the 133 responding agencies
inspected the "Y" portion of the service connection, while 42 out of 133
respondents reported that they assumed responsibility for the house lateral
to the property line. Only a small portion, three out of 133, took
responsibility for the house lateral to the building line.
The use of sewer sealants by municipalities has become a more
general practice. The municipalities that have used sewer sealants reported
that root intrusion has been reduced. Subsequent root intrusions have had
minimal effect on chemically sealed joints. Some communities have modified
their sealing methods as a result of roots.
Many agencies reported that root intrusions have caused cracking,
opening or crushing of joints and pipe but a lesser number attributed
infiltration to such root actions. No clearly defined data were available
as to whether root growths preceded or followed infiltration conditions.
Relationship between the depth of groundwater and the extent of root
intrusion was explored; 46 cities reported that roots were not a problem in
shallow ground-areas while 61 stated that there was no such correlation.
Responding to the question of whether soil types had any relationship
to the extent of root intrusion, replies were evenly divided on the subject:
44 correlated soil types with root growth while 45 did not.
The survey indicated.the wide extent of the infiltration problem; 106
reported that infiltration did occur while only 21 said that it was not a
problem.
The findings of the mail survey lead to three major conclusions:
o Root intrusion is just one of many sewer maintenance problems.
It is not the total cause of system malfunctions. It is a
combination of a host of problems-and of the need for line
cleaning and rehabilitation. It follows from this that a
53
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complete sewer maintenance program is needed, based on principles
of preventive servicing before system failures and consequent
drainage functions are affected.
o The data base of the surveys was not complete in all respects.
Certain desirable data were not available because field experience
in some systems is too limited to permit a complete evaluation at:
this time.
o Infiltration is a major problem in a great majority of the systems
surveyed. This condition is caused by deteriorated lines, root
intrusion and lack of sewer system rehabilitation, repair and
replacement. The problem of infiltration must be given increased
attention. The USEPA "Needs Survey, 1974" estimated that almost
$5.3 billion will be needed nationwide to correct the infiltration
and inflow problem.
It was apparent that some of the sewer maintenance - root control -
infiltration relationships had never been evaluated by officials before this
survey was instituted. The mail survey findings were augmented by onsite
investigations of 21 selected community systems.
ON-SITE SURVEYS OF SEWER ROOTS PROBLEMS AND CONTROL
On the basis of the findings of the I/I study, information obtained
from a national mail survey questionnaire evaluation, and other guidance
means, 22 communities were selected for more intensive investigations of root
problems and control practices. On-site, in-depth investigations were carried
out in the selected systems by Research Foundation representatives with ex-
perience in this phase of sewer operation and maintenance.
This section of the report summarizes the on-site findings for the
investigated systems and evaluates and interprets the interview data from
those systems which had relevant data and experiences.
The community sewer systems investigated were chosen to represent a
cross section of sewer root problems and approaches. They covered large and
small systems in various parts of the United States; those with well developed
sewer maintenance and root control programs and those with less effective
practices; those which have used chemical control methods for root-kill and
those which have not utilized these means and have avowed objections to
their use; those with major root problems and those with only minor growth-
clogging conditions; those well versed and aware of the importance of root
control and those where root removal is considered only an unimportant part
of the sewer cleaning operation; those with highly sophisticated sewer main-
tenance equipment and those less prepared to keep their systems at peak
transmission capabilities.
The community systems covered by the on-site survey program were:
54
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The City of Los Angeles, California; Sacramento County, California; Los
Angeles County, California; City and County of Denver, Colorado; Charlotte-
Mecklenburg County, North Carolina; Chesapeake, Virginia; Buffalo Sewer.
District, New York; Shreveport, Louisiana; Dallas, Texas; Fort Worth, Texas;
Austin, Texas; Metropolitan St. Louis Sewer District, Missouri; North Little
Rock, Arkansas; Champaign, Illinois; Madison, Wisconsin; Yakima, Washington;
Seattle, Washington; Southwest Suburban Sewer District, Seattle,
Washington.
The questionnaire responses served as the springboard for the on-site
surveys. Therefore, a brief summary of each system is given prior to the
pertinent comments from its field interview.
Charlotte-Mecklenburg County. North Carolina
Overview - A system of separate sanitary sewers operated and main-
tained by a utility department serves a population of 300,000 in the City of
Charlotte, and urbanized parts of the county. Forty percent of sewer main-
tenance required for root contol - one-half of the total mileage of 1,082 mi
(1,803 km) of collectors and 200 mi (333 km) of interceptor sewer - is sub-
ject to root intrusion. Willows, maples, oaks, elms and poplars all produce
root problems. Cleaning must be repeated frequently because this procedure
does not remove all growths and regrowth occurs.
On-site comments - This system is an example of sewer conditions which
show a minimum relationship between infiltration and root intrusion. Root
problems affect about half of the city-county system of collector and inter-
ceptor lines but the groundwater table is relatively low in the service area
and most sewers are laid in the dry. Eighty-five percent of root intrusions
occur at defective joints, only three percent at pipe breaks and a very minor
amount at house laterals.
o Sewer construction practices have induced root entry at joints.
From 1927 to 1958, joints in vitrified clay sewers were made up of jute and
cement grout; from 1958 to 1961, bitumastic poured joints were used; since.
1961, 0-ring type compression joints have been installed and tight joints
have resulted.
o Ten percent of rooting .difficulties has occurred at the physical
connections between house sewers and street sewers, again due to poor con-
struction in older practice. In older connections, plumbers often broke out
a hole in the street sewer and inserted the end of the house lateral, with a
cement collar. Present practice is for the city-county agency to drill with
a diamond cutter and to make the connection with an ABS plastic sewer tap
saddle or with a cast Aluminum or cast iron saddle arrangement. Tight con-
nections have resulted. Connections for building sewers over 4 in. (10 cm)
size must be made via a manhole. Plumbers or owners are refused permission
to make connections; city work is billed. No pre-set wyes are provided in
street sewer construction. Root intrusion has been minimized in new
construction.
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o Root control is achieved by routine sewer maintenance methods,
with use of so-called conventional tools and equipment, including five rodding
machines and hydraulic jetting units. Root removal is done on both routine
maintenance schedules and emergency complaints. Officials consider a preven-
tive maintenance schedule of once-yearly de-rooting effective, but the present
average check of root-infested sewer sections is on an 18-month schedule.
o Officials consider root removal procedures as overly expensive for
the results accomplished because of regrowth of mat formations. No chemical
treatment has been tried; the county agency is waiting for proof of chemical
effectiveness and safety. It will consider chemical control if the current
USEPA-APWA study recommends it.
o Roots were reported to enter sewers in the upper quadrant of the
pipe; little root problem occurs in lines laid deeper than 8 ft (2.4 m).
Roots at the sewer crown are not cut off cleanly by cutting tools and hydra-
flushers. Regrowth is so rapid in some root-infested lines that bi-monthly
cleaning would be desirable.
o The clay-like soils in the Charlotte area do not inhibit root
growth. The opinion was expressed that root growths in joints and pipe de-
fect areas can produce cracking or progressive opening of points of entry.
o Roots are more dominant in older areas of the service territory^
where sewer joints are poorer and where tree growths are more mature than in
newer development areas. Willows are the worst offenders, with privet hedge
affecting building laterals. Maples, poplars, elms, all types of oaks, and
junipers, are found to be the most troublesome types of trees. No control
over street plantings was reported.
o The field coverage of root removal operations in a sewer section
that had not been cleaned for some six months, disclosed no root clogging even
though this was in an area with heavy tree overhang where sewer officials had
assumed they could show the investigator heavy infestation conditions.
Chesapeake, Virginia
Overview - Community of 100,000 population, with only 50,000 sewered.
System consists of 300 mi (500 km) of public sewers. Of 400 stoppages per
year due to roots, most are located in house laterals. No 'chemical treatment
has been attempted. System experiences heavy infiltration due to high ground-
water levels. Rooting in laterals is due to elms, maples, gums and quick
myrtles located close to shallow house lines.
On-site comments - This sewer system experiences high infiltration
but low root intrusion; it represents a paradox in this respect because .
sewer defects do not result in heavy rooting problems. The explanation, as
offered by sewer officials, is that groundwater is very high, sewers usually
inundated, and root growths impeded from seeking sewer entry in search of
either vapor or water. The groundwater table is affected by tides in the
56
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low-lying coastal area and intrusion of saline water into the groundwater is
reported to discourage root growths.
The major root problem occurs in house laterals, particularly at
connections to street sewers. Few basements are used in the city and the
laterals are laid at shallow depths - from a maximum of 42 in. (1.06 m) deep
to a minimum of only 12 in. (30.5 cm). Roots which enter building lines tend
to grow out into the street sewers, causing stoppages at the junction point
and in short stretches of collectors at these locations.
Chesapeake is one of a series of satellite communities around the
Norfolk, Virginia metro area. The Public Utility Department owns and
operates the sanitary sewers and the Public Works Department has responsi-
bility for storm sewers. The Hampton Roads Sanitation Commission, created
by a state act, provides interceptor and treatment services for Chesapeake
and other communities.
High infiltration is the concern of the commission which has prompted
the city and other communities to correct this condition. The commission is
in the unusual position of threatening to increase the sewer service charges
to cover the cost of high flows caused by infiltration if corrective actions
are not taken; a recent increase in service rates was imposed.
o The city assumes responsibility for building sewers from the
street sewer to the property line and, therefore, becomes involved in the
root problems in these lines. Building laterals are provided with clean-out
fixtures at the property line, which serve as entry points for root cutting
tools. It becomes a problem to check all lines to ascertain whether stop-
pages exist in the section between the building line and the property line.
o Of the 400 root stoppages per year, most are located in these
lines; about 50 dig-ups per year are required for stoppages which cannot
be cleared by rodding or flushing. Close proximity of tree and shrub growths
adds to root intrusions in house sewers.
o Street sewer root growths are usually limited to lines under
8 ft (2.4 m) deep. The soil is sandy and mud-structured, producing what
is known locally as "running sand." Infiltration carries this soil into
sewer lines, resulting in street washouts and pavement failures.
o Root control work has been limited to only about 5 mi (8.3 km)
of sewers per year, usually on a complaint basis, but some preventative
maintenance work is scheduled in areas where root infestation is known to
be high.
o Old sewers were laid with mortar joints which failed readily
later, bitumastic joints were poured; and more recently, compression-type
0-ring joints have been used, with reduced infiltration. Inflow is not a
problem because of the absence of basement and foundation drains and roof
leader connections.
57
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o Roots are reported to be more troublesome during dry seasons, thus
confirming the relationship between groundwater levels and the minimal root
conditions in street sewers.
o The city makes house sewer connections to street sewers, either
with wye fixtures and special water-tight saddle devices or plastic seals, or
by diamond-drilled openings, properly connected and made water and root-tight.
o While the computed frequency of root cleaning in the sewer, system
is many years between servicing incidents, those street sewers where rooting
is more troublesome average one cleaning per 18 months. So-called conven-
tional root removal procedures, rodding and high-pressure flooding, have been
employed. No chemicals have been used. If auger-type tools do not clear
lines, a scrapper known as "grandma" is pulled through the line in the form
of a wiper. Dig-ups are the final answer if underground methods fail.
o TV inspection of sewers has not been undertaken. No sewer sealing
by chemical grouting has been tried, and attempts to obtain funds for sewer
leakage control have failed. The city has advised the Hampton Roads agency
that it will progressively replace lines in poor condition, will install new
lines to meet the commission's standard infiltration of 350 gal/in-diam/mi/day
(0.33 m3/cm-diam/km/day), and will make street sewer connections without
plumber service but will require guarantees of tightness of all plumber s
work for one-year periods.
o As funds permit, concrete lines which failed by sulfating in tidal
groundwater are being replaced as part of the rehabilitation program.
o Trees which cause root problems primarily in house connections
are classified as: elms (the most troublesome), maples, gum trees and a
type known locally as "quick myrtles."
o Inflow can occur in manholes which are low-lying and subject to
inundation during storms. However, manhole covers are fitted with internal
"dust covers" which are set under the street cover and become relatively
watertight due to deposits of "dust" or debris that sift into the space
between the regular cover and the under-cover.
o It is again stated that infiltration is high in street sewers
and root intrusions relatively low. This must not be interpreted to mean
that no collector sewer root problems exist. They do, but in lesser degree
than could be expected from the condition of the city's sewer lines. The
relationship between high groundwater, saline water intrusion into the
fresh groundwater, and low root intrusion remains the highlight of this
on-site survey.
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Buffalo Sewer Authority. New York
aT-Q * A multi-community system serving the city of Buffalo and
and h- ^ nty< ^ P°Pulation of ^0,000 is served by both separate
and combmed sewers. The length of the separate sewers is 320 mi (533 km)
and of combined sewers 485 mi (808 km) . Fifty percent of the sewer malnSn-
ance problems are due to root growths. Roots are also a problem in storm
SGWG27S •
On-site Comments; The Buffalo Sewer Authority, created by state
legislative action in 1935, operates the sewer system for the city of Buffalo
but the bulk _ of the outlying sewer districts in the authority service area
maintain their own systems. The authority provides interceptor and treatment
service to the entire jurisdictional area. Buffalo is -sewered primarily by
combined sewers, in many cases as old as 100 years. Newer outlying areas
are served by separate sanitary sewers but interceptors are combined in nature
and overflows contribute to the pollution in international waters which is
the concern of the International Joint Commission representing the United
States and Canada.
Ahn,,i- fc subJect to ^filtration and heavy root intrusion.
About half of the lines are under the groundwater table at least during
Sat exmtraTr* ^ T °f ^ SyStem ±S lald *igh-and-dry; it is possible
that exfiltration occurs in such sewers. It was reported that 80 percent of
the city s sewer system is subject to root intrusion. Root control is
practiced on both a preventive basis, as time is available, and on complaint
o± sewer stoppages. While root intrusion problems were reported to cover
this major percentage of the city's system, the miles of lines cleaned per
year average only 40 to 50 mi (67 to 84 km); this would represent servicing
of the total system only once every six years but this schedule is
ameliorated by greater frequency of cleaning in areas where root difficulties
are known and anticipated.
Root control measures have been conventional rather than innovative
except for the use of chemicals which are periodically dumped into manholes'
in affected areas, reportedly with good results. No sophisticated flooding
foaming or vapor treatments have been used. Root removal has been by rodding
and use of routine cutting or pulling tools. The first high-pressure f lusher
purchased by the authority was delivered during the survey.
o The relationship between overall sewer maintenance and root
control in the Buffalo system is portrayed by the fact that 50 percent of the
maintenance problem is due to root growths - 30 percent to grease accumula-
tions and 20 percent to sand and debris accumulations. Industrial wastes
were reported to cause some sewer clogging difficulties.
o Collector sewers and house laterals are involved in both separate
and combined sewer maintenance problems, as well as in storm sewers. No
interceptor root problems were reported.
59
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o Roots enter sewer lines that are relatively deep - 50 percent of
the problem occurring in sewer lines 8 to 12 ft (2.4 to 3.7 m) deep. Even
sewers of greater depth are subject to root intrusions. Thirty percent of
the root problem is due to intrusions into poor joints and an equal amount
via sewer line breaks. House laterals, as usual, are the points of 40
percent of root entry, with most of the intrusion occurring at the connec-
tion of the house line to the street line.
o Root intrusions into street lines are induced by the fact that
combined sewers are laid near the curb line on both sides of the street,
adjacent to tree lines. These plantings are made by the City forestry De-
partment but there is no evidence that the authority is consulted about the
types of trees that will produce less root growth problems.
o Authority officials believe that root intrusions produce joint
deterioration and further breakage of line defects but there Is no evidence
on whether the openings into sewers existed prior to or after the root
intrusions occur.
o Root intrusion and infiltration are commensurate with age of
sewers. Three-quarters of the city's sewers are over 50 years old; 60 percent
were built prior to 1910 and only 7.8 percent installed since 1941. Trunks
built prior to 1930 were of brick, stone or segmented block. Street sewers
are mostly vitrified clay, with earlier joints made of mortar which were
subiect to openings for potential root intrusion and infiltration. Newer
lines have been built with compression joints, and root troubles and infil-
tration have been minimized.
o Sewer rodding is carried out with flat slat sticks strapped
together with disposable wire clips. The flexible and relatively unbreakable
slats which float in the sewer lines, are used to thread cleaning cables
through sewers. In many areas sewers are located under sidewalks, making
them vulnerable to tree root intrusions.
and elms.
Tree root problems in Buffalo are attributable to maples, poplars
Shreveport, Louisiana
Overview; This city of 225,000 is served by a 675 mi (1,125 km)
sanitary sewer system. Concrete and vitrified clay lines have been in-
stalled over the past 30 to 40 years, with grouted joints. Root stoppages
average 300 per year in the 25 percent of the sewer system which is subject
to root intrusion. About 15 percent of the 158 mi (263 km) of affected
sewers is cleaned annually, only five percent of the system requires main-
tenance per year because of rooting conditions, indicating that root con-
ditions are not a major problem in the city. Some parts of the system _
require as much as weekly root control servicing. Root clogging is limited
to street collectors and house laterals, with no difficulties experienced in
interceptor lines. Most of rooting occurs at joints - 60 percent of all
problems - with 25 percent at pipe breaks and 15 percent at wye connections
between house sewers and street lines. Root intrusion is greatest where
60
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groundwater table is low and sewers are laid at shallow depths in the sandy
loam soil.
On-site Comments; The Shreveport separate sanitary sewer system
experiences root intrusion problems, primarily due to poor joint construc-
tion in lines 30 to 50 years old. House lateral rooting represents 15
percent of the overall problem, located primarily at wye connections to
street sewers. Root intrusion is equated with low groundwater levels in the
system and shallow sewers. Chemical treatment has been tried with unsatis-
factory results.
o Root intrusion is minor into storm sewer lines. In sanitary
sewers root problems are located at points of line defects but the system
personnel do not believe that roots produce their own points of entry; in
other words, roots find existing openings rather than producing them.
o An ordinance enacted in June, 1975 requires property owners to
correct conditions in house laterals which produce system back-ups and
malfunctions.
Dallas, Texas
Overview; A population of approximately 900,000 is served by about
2,500 mi (4,167 km) of separate sanitary sewers and 500*ini (833 km) of inter-
ceptor lines. Roots are a major problem, judging from the 2,000 dig-ups per
year, 6,000 stoppages involving root growth and 600 mi (1,000 km) of sewers
cleaned per year. Forty percent of the sewer system was reported to be
affected by root intrusions, with a quarter of the system requiring annual
attention. Root intrusion in house laterals is "high." Shallow sewers,
under 5 ft (1.5 m) deep, are markedly affected by roots. Sewers deeper than
8 ft (2.4 m) are seldom affected. Joints are the major point of root entry;
30 percent of the total root problem occurs at wye connections to collector
sewers.
On-site Comments; Root growths are a problem and the investigation
provided an opportunity to obtain the views of wastewater collection
personnel on causes and effects of this condition.
o This system is constructed in soils with exceptionally low
groundwater levels. Root growths are induced by search for moisture; where
water tables are higher, root growth is impeded.
o Roots are not the cause of infiltration; they are the effect. In
fact, roots are thought to seal existing sewer line openings and thus reduce
infiltration. When roots enter through line breaks or joints their growth
is limited by the flow in the sewer because roots cannot grow into the flow.
Thus roots proliferate where flow rates do not cause sewer surcharges. Pipe
size has less influence on root growths than flow lines.
o Older sewer lines were of 4 to 8 in. (10 to 20 cm) sizes; newer
lines are being laid with 12 in. (30.5 cm) pipe, thus providing greater
capacity with lower flow lines.
61
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o Roots constitute 85 percent of the cause of sewer system stoppages
and dig-ups. Sewer lines fail due to improper backfill, pipe fatigue and
other contractor practices. Grease accumulations reduce flow-through capacity
and thus enhance root growths in the less-filled flow lines.
o Chemical treatment for root control is viewed with favor. Slip
lining of defective sewers with polyethylene pipe and heat fusion of pipe are
used successfully to correct infiltration and consequent root problems.
o Chemical grouting of defective sewers is practiced, such as in
concrete pipe which has failed due to corrosion. TV inspection is used to
confirm sewer conditions and benefits of sealing and other corrective measures.
o In arid areas, disturbed ground resulting from sewer construction
acts as a drain channel and creates infiltration. Root intrusions are greater
in clay and chalk-limestone soils than in sandy soils.
o House laterals are the cause of root intrusion, as well as of
infiltration. Defective laterals are checked by the city and the property
owner can be required to make repairs, subject to fines or shut-off of water
service. The city maintains lateral responsibility up to the property line at
which point a clean-out is installed at a cost of approximately $25 to the
property owner. This clean-out is used for clearing root growths and de-
termining the location1 and responsibility of stoppages. Officials believe
that house lateral infiltration and root intrusions cannot be solved by
conventional control measures instituted in the street sewer system.
Austin, Texas
Overview: The separate sanitary sewers in this system, l,200^mi
(2,000 km) o£ collectors and interceptors serving 300,000 population is not
seriously affected overall by root problems. Forty percent of sewer main-
tenance is caused by grease accumulations, 30 percent by deteriorated lines,
20 percent by stoppages caused by material dumped into manholes, and only
10 percent by root growths, 6 percent of which requires annual maintenance.
On-site Comments; Only six percent of the separate sanitary sewer
system requires annual maintenance for root control. The main source of root
intrusion in the system was reported to be house laterals.
o A recent ordinance requires all house lateral construction to be
hydrostatically tested for joint and other defects with correction of such
defects due within 60 days of notification, subject to discontinuance of
water service.
o City officials estimate that 75 to 90 percent of the sewer system's
infiltration and root.problems are caused by defective laterals.
o The investigator gained the impression that roots cause a majority
of the problems although the proportion was not quantifiable.
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o Opposition was expressed to chemical control of roots because of
the danger to ground vegetation and protests from environmental groups.
o No rehabilitated lines have required repairs since 1955-1958,
due to use of 12 to 25 in. (30.5 to 63 cm) of crushed gravel backfill around
pipe, combined with the use of plastic pipe and expansion joints.
St. Louis Metropolitan Sewer District. Missouri
Overview; This metro system serves 900,000 population with separate
sanitary sewers and 700,000 population with combined sewers. Root growths
are a problem in the 2,300 mi (3,833 km) of sanitary and 1,200 mi (2,000 km)
of combined collectors but no root problems are reported in interceptors.
About 180 mi (300 km) of sewers are cleaned annually for root intrusion
control, or less than 10 percent of the 60 percent of the metro system affec-
ted by roots. Only four percent of the sewer system was reported to require
annual maintenance for root clearance, with the maximum cleaning frequency of
one year. Chemical treatment of roots has been tried, with unsatisfactory
results admittedly due to inability to apply properly because of back-ups
into basements. Roots represent 26 percent of the causes for sewer mainte-
nance; the greatest cause (42 percent) is rocks, rags and paper. Bucket
machine cleaning produces positive results but is reported to be too slow.
Sewers laid at depths of 8 to 12 ft (2.4 to 3.7 m) are reported to be
affected by root intrusions. Chemical grouting has reduced root growths.
"Plus" herbicides have been added to chemical grout to improve root control .
Willows, elms and sycamore trees were reported to produce the major root
problem.
On-site Comments; The District, while in control of the metro system,
does not have control of house services which represent about 10 percent of
the root intrusion problem. Eighty percent of system infiltration is thought
to occur from house services.
Officials believe that roots produce infiltration only when joint
bells are broken. When root entry is made through simple joint openings,
the belief is that exfiltration rather than infiltration will occur. The
extent to which root diameter growth would result in infiltration was not
given but growth is relatively slow.
Chemical control is practiced, but with some .skepticism. Several
trees and shrubs have been killed by root control chemicals and the District
has had to replace damaged vegetation. The main problem with chemical treat-
ment has been the need to block off house service lines when flooding is
practiced.
Metro is now experimenting with grouting with herbicide additives
because grout without additives has not stopped root intrusion.
o Cutting roots with a bucket with a blade is effective for small
roots but the District is seeking a more permanent method of root control.
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North Little Rock, Arkansas
Overview; This smaller system, serving a population of 60,000 with
separate sanitary sewers, 400 mi (667 km) of collectors and 50 mi (83 km) of
interceptors, has a root growth problem affecting 40 percent of the collectors.
Thirty percent of the sewer maintenance work is caused by root intrusions,
with other causes not exceeding 20 percent each, thus making root conditions
the main cause of stoppages. Fifty stoppages per year constitute the main-
tenance load, with 30 mi (50 km) of sewers cleaned annually. Ten percent of
the system requires root control maintenance per year, with a three-year
maximum frequency for root removal. Seven different brands of chemicals have
been tried for root control, methods of application unspecified, with un-
favorable results. Ninety-five percent of sewers of 6 in. (15 cm) size are
affected by roots and 90 percent of the 300 mi (500 km) of 8 to 12 in. (20
to 30.5 cm sewers are so affected; only minor percentages of 15 to 24 in.
(38 to 61 cm) sewers and two percent of over 24 in. (61 cm) lines are root
infested. The bulk of root intrusions occur at joints with house laterals
involved in 20 percent of the problem. Grouting of leaking lines has been
carried out, with a slight reduction in root growths; root intrusions have
not affected chemically sealed joints. Roots were reported to cause pipe
bell cracking and subsequent infiltration. Types of trees or shrubs causing
root problems were reported to be willows, oaks and shrubbery.
On-site Comments; The on-site survey helped to clarify the data
contained in the mail survey questionnaire.
o Seventy to 90 percent of infiltration in the system - and conse-
quent root intrusion - occurs in house laterals which the city does not main-
tain. Current requirements specify use of cast iron service lines but
earlier practice allowed fiber and concrete pipe. Plastic service lines have
been tried but officials could not control the quality of workmanship.
o A rise in river levels for navigation purposes by a Corps of
Engineers project resulted in higher groundwater levels and an increase in
infiltration. Other cities in the area have filed claims against the Corps
for similar conditions.
o Chemical control of roots has been relatively unsatisfactory.
o House services and wye connections are the responsibility of
property owners.
Champaign, Illinois
Overview; Fifty percent of the sewer maintenance problem in the
300 mi (500 km) system serving the 60,000 population of Champaign, Illinois,
results from root intrusion. The other major cause of sewer stoppages is
heavy grease accumulations. One hundred stoppages per year involve root
growths, with only five dig-ups required to augment the mechanical and
chemical control measures which are practiced "year round.." Root intrusions
occur in the separate sanitary sewer lines, with roots reported in separate
storm sewers as well. Chemical treatment, by means of herbicide materials
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dumped into manholes, was reported to kill bottom root formations and to make
mechanical cleaning with bucket machines and jet units more effective.
Eighty percent of sewers of 8 to 12 in. (20 to 30.5 cm) size are affected by
roots as well as 60 percent of sizes from 15 to 24 in. (38 to 61 cm). Sewers
laid at depths of 1 to 5 ft (0.3 to 1.5 m) and of 5 to 8 ft (1.5 to 2.4 m)
are most markedly rooted, 40 percent each. Eighty five percent of root
intrusions occur at joints, 10 percent at wye connections in house lateral
lines and only five percent at sewer line breaks. Roots were reported to
cause sewer bells to fail and induce infiltration,,
On-site Comments; The on-site survey report provided little addi-
tional information.
o Personnel of the sewer agency were not certain whether house
laterals are contributors of significant infiltration and root intrusions.
o Chemical control is minimally practiced, due possibly to the city's
experience with killing of surface trees and shrubs.
o No plans have been made to require property owners to make repairs
of defective house laterals.
Milwaukee, Wisconsin
Overview; This city of 700,000 population is served by 765 mi
(1,275 km) of sanitary sewers and 555 mi (925 km) of combined sewers. The
city has a root problem and a maintenance program. Chemical treatment by
Vapom has been tried, with reportedly unfavorable results, because the
Department of Public Works could not introduce chemical properly. Dig-ups
for root control are not required to augment rodding and jet root cutter
equipment. Only 25 sewer stoppages per year are directly attributed to root
infestation, with the overall maintenance program involving 160 mi (267 km)
per year. Twenty percent of the separate sanitary sewers is root-affected
and root problems are not reported in the combined sewer system. The
greatest root troubles occur in sewer lines of 8 to 12 in. (20 to 30.5 cm).
Eighty-five percent of the 1,450 mi (2,416 km) of this size range is affected
by root conditions. Ninety percent of root intrusions enter sewer lines of
5 to 8 ft (1.5 to 2.4 m) depth. Joints are the major source of root entry
accounting for 50 percent, with 30 percent at sewer breaks and 20 percent
through house laterals, which are also the source of a part of the infiltra-
tion problem in the system. Only elms were reported to cause root troubles.
On-site Comments; The survey provided confirmatory data as well as
augmentation of information on sewer cleaning and root control practices.
o Four basic types of equipment are used for sewer maintenance,
including root control: hydraulic jetting equipment, sewer rodding units,
sewer ball units and bucket machines and combinations of these methods. A
study made of cleaning techniques in 1974 showed that none of the methods
removed all or even substantial amounts of root growths.
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o Rodder cleaning, followed by jetting, was better than rodder alone.
Use of a root cutter and the jet nozzle gave better results than the jet
nozzle alone. Jet cleaning improved flow conditions, possibly by removing
greases and debris but jet nozzles did not remove roots effectively. Rodding
does not remove grease accumulations effectively. '
o The city maintains carefully scheduled preventive maintenance
programs which have reduced sewer stoppages from 160 per year in the 1960s
to under 60 per year in 1974. About 20 percent of stoppages occur in
combined sewers despite their relatively large sizes, with blockages due
more to debris than to root growths.
o Jets do an adequate job on light root growths. Cutting tools may
get hung up on bad joints and damage lines when used in conjunction with jets.
Rodding machines are used where root growths are heavy, with buckets called
into play under extreme root conditions. The value of jetting is attested
to by the recent purchase of a third machine and the planned elimination of
one bucket machine crew.
o Previous maintenance schedules were based on emergency calls;
today's schedules stress preventive cleaning.
o Elm trees are the major cause of rooting. An epidemic of Dutch
elm disease has thinned out these trees and they are being replaced gradually
with less troublesome types. The most active growth periods are spring and
fall and cleaning schedules are planned to meet these peak root: conditions.
Heavy rooted areas are serviced on a semi-annual basis. Roots seldom
intrude into sewers via gasketed joints.
o Roots enter sewers at the upper quadrant of mortar joints. No
cases of bell breakage due to root growths were reported.
o No program of chemical root control is in effect. A test of foam
application was made some three years ago, with little success,, possibly
due to improper or inadequate application methods. Evidences of foam in
downstream manholes from the point of application were not detected; the
test, though not conclusive of failure, discouraged further use of chemical
control at the time. „
Madison, Wisconsin
Overview; The 500 mi (833 km) separate sanitary sewer system which
serves a population of 162,000 has a root problem which represents 50 percent
of the overall sewer maintenance program. As in other communities, grease
accumulations are reported by Madison sewer officials to cause a major
sewer clogging threat, 30 percent of total maintenance needs. Stoppages
amount to 600 per year, 200 in street sewers and 400 in the collector lines
and laterals. Thirty percent of the system's mileage requires annual root
cleaning but some sections of the system require cleaning four times a year.
Two hundred twelve mi (353 km) of sewer are root-cleaned annually. Sewer
sizes affected range from 6 to 12 in. (15 to 30.5 cm) with the shallower
sewers most subject to root intrusions. Seventy percent of stoppages
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involve sewers under 8 ft (2.4 m) deep. The city does not assume responsi-
bility for house laterals from the building line to the street line, however,
wye connections are the responsibility of the city. Forty nine percent of
root entries occur at street sewer joints and an equal percentage in house
laterals. Elms, ash, oaks and some poplar trees are root-inducers. Roots
were reported to cause crushing of joints but this condition was not reported
to produce infiltration points.
On—site Comments; The on-site survey provided further information
on sewer maintenance, root control and internal grouting of defective sewers
to overcome infiltration.
o Sewer back-up complaints total 700 to 800 per year, with two-thirds
due to house lateral stoppages, thus indicating that the major source of root
infestation is in house connection lines.
o The city clears house laterals only on request and charges for the
service. Three rodding crews and two hydraulic jet crews handle 15 percent
of the affected sewers quarterly and the remainder are serviced twice
annually. The rest of the street sewers are cleaned as time permits.
o Roots enter the upper half of sewer joints, seeking moist air
rather than water. Shallow sewers are more affected than deeper lines. Root
growths are especially troublesome during dry years. Willow trees cause the
greatest root problems but elm tree roots have been known to grow in sewers
at least one year after the tree is cut down.
o A test of chemical control about ten years ago failed because the
herbicide could not be held in the line long enough.
o Madison owns its own TV inspection equipment and examines all new
construction for infiltration above the allowable 200 gals/in-diam/mi/day
(0-. 19/m /cm-diam/km/day) as well as for check-up on maintenance work. The
equipment is not used for chemical grouting; recent contracts for grouting
cost $23,000 for 133 joints, or $188 per joint. Reduction in flow to the
treatment plant fully compensated the city for these costs. City TV work
costs $0.22/ft ($0.73/m) with precleaning costs of $0.17/ft ($0.56/m).
Yakima, Washington
Overview: The 198 mi (330 km) separate sanitary sewer system which
serves 47,000 residents experiences root intrusion problems but overall
sewer maintenance involves only 20 percent for root control, as compared to
65 percent for grease removal. Forty-five stoppages per year are attributed
to root growths, with eight dig-ups required in cases where regular cleaning
does not clear the lines. One-hundred-twelve mi (187 km) of line are cleaned
for root control annually but only 18 percent of the city system is subject
to root problems. Some sections of the system are cleaned three or more
times annually, with such maintenance work limited to the 18 percent of the
mileage subject to root intrusions. Small size sewers are more affected
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than sewers from 15 in. (38 cm) to over 24 in. (61 cm). The bulk of root
intrusion occurs in lines laid at a depth of 5 to 8 ft (1.5 to 2.4 m).
Joints made of concrete and mastic prior to 1960 are the source of 50 percent
of the root problem. Forty percent of the problem is experienced in house
laterals.
While only 18 percent of the system is root-affected, some sections
are cleaned three or more times per year. Chemicals have been used with
unsatisfactory results; mechanical root removal is deemed the most effective
means of keeping lines functioning. The major points of intrusion are street
sewer joints and house laterals; the city maintains no authority over house
connections. The less the groundwater, the greater the root problem.
Cement mortar and mastic joints were used prior to 1960; rubber joints are
now used for asbestos-cement concrete and plastic sewers.
On-site Comments; One-half of the sewer system is laid in streets
and alleys, with the rest located in easements. Easement lines are subject
to unusual root intrusions, such as growths from an asparagus field which
plugged a four-foot-deep sewer. About one-third of sewers are laid below
groundwater levels at least for a portion of the year.
o About 5 mi (8.3 km) of the most heavily infested sewers get
special root control attention, ranging from weekly, or bi-weekly to
quarterly cleaning, including grease removal which produces 65 percent of the
overall maintenance problem. Rodding and jetting procedures are used, with
rodding applied for normal root conditions. Heavy roots which cannot be
removed by routine means are cut with a locally designed saw-like cutting
tool. This heavy tool has been used to cut off protruding house laterals
without damaging the lines. Two-man crews are used for both rodding and
jetting operations.
o The city has purchased its own TV inspection unit but it is not
equipped for internal grouting. Its function is to check new construction
for infiltration and existing lines for effectiveness of maintenance.
o The city does not accept responsibility for house laterals which
account for 75 percent of stoppages.
o Roots from troublesome trees, such as cottonwoods, sugar maples,
willows, weeping birches and even fruit trees tend to enter lines at the
lower quadrant of joints, rather than the upper portion. Some thumb-size
roots are more difficult to cut.
o Chemical treatment has been spasmodic and poorly controlled,
possibly explaining the unsatisfactory results of such materials. Caustic
soda, introduced into the soil around a root-infested line, apparently
entered the sewer and corrected clogging without affecting nearby trees.
No plans for chemical treatment were reported.
o Rooting can occur at any time of the year, rather than only
seasonally as reported in other community systems.
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Denver, Colorado
Overview; Of the 1,200,000 population of Denver, 1,100,000 are served
by 1,200 mi (2,000 km) of separate sanitary sewers and 200 mi (333 km) of
interceptors. Ten miles (167 km) of collector combined sewers serve only
100,000 people. Root intrusions are a problem in both street sewers and
house laterals but they pose no problem in combined sewers or separate
storm sewers. Dig-ups are not required to augment routine sewer maintenance
procedures. Only 12 stoppages due to rooting are experienced per year but
300 mi (500 km) of sewer lines are cleaned annually 'for root prevention.
Maximum frequency of cleaning is once yearly in the 30 to 35 percent of the
sewer system subject to root intrusions. Ninety percent of root problems
occur at joints and 10 percent in house laterals. The bulk (50 percent) of
root intrusions occur in lines laid at depths of 1 to 5 ft (0.3 to 1.5 m)
with an additional 20 percent in 5 to 8 ft (1.5 to 2.4 m) deep lines. Fifty
percent of 8 to 12 in. (20 to 30.5 cm) sewers are affected by roots, with
only 5 percent affecting lines of 15 to 24 in. (38 to 61 cm) depth and over
24 in. (61 cm) depth. Chinese elms were reported to be the most troublesome
type of trees in root-infested areas.
On-site Comments; Approximately 300 mi (500 km) of sewers are
checked for root growths per year. Root intrusions constitute 25 percent of
the total maintenance problem.
o While the mail survey indicated that roots are most troublesome in
shallower lines, the majority of the city system Is laid at depths of 9 to 12
ft (2.7 to 3.7 m) because of the predominance of basement construction in
residential areas.
o A carefully scheduled preventive maintenance program is used by
the city's four maintenance district crews, each equipped with a TV sealing
unit operated by a four-man crew. Prior to the scheduled program 60 back-ups
per month were experienced. The preventive program has reduced stoppages to
one or two a week. All reports of stoppages are followed up with TV inspec-
tions to determine the presence of roots or other impeding materials and the
success of the cleaning operations after corrective measures have been taken.
o The entire system is flushed
and jet cleaning averages once over two
serviced on a 30-day, 60-day and 90-day
greatest cause of sewer stoppages; when
hydrocleaner, a tag line is left on the
with final cleaning followed by another
and inspected once yearly; rodding
and a half years; some areas are
schedule for grease removal - the
crews cannot clear the line with a
job and a TV inspection is requested,
inspection.
o In 1975 the city could rod or jet at a cost of $0.05/ft ($0.17/m).
Five truck-mounted flexible rodding units, each with a two-man crew, are in
service; a similar number of hydrocleaners are used. Prior to 1965, crews
used hand rods, at which time Denver made a study of the operations of other
cities and modified its procedures.
o The city uses the 1 to 9 root infestation grading system estab-
lished at Sacramento County to record the intensity of root growths.
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o The city carries out a continuing sewer sealing program. If a TV
inspection shows joint offsets, root intrusion, radial breaks, poor joints
or other defects, these points are sealed. Sealing specifications are rigidly
established for time of grouting and setting of AM-9. From five to six joints
per 100 yds (90 m) of sewer length need sealing under the city's criteria
and 2 to 6 gal (7.6 to 22.7 1) of sealant are used per joint.
o Chemical root treatment has not been used. Copper sulfate has
been used with AM-9 for root herbicidal effect during the past five years
that sealing has been used.
o The city maintains control over inflow connections.
Seattle. Washington
Overview! The combined sewer and separate sanitary sewer system of
Seattle, serving a total population of 523,000, are affected by root growths
but stoppages are minimal, 10 per year, and dig-ups are not required to
augment the Sewer Utility's manual and chemical root control procedures. The
city is generally served by combined sewers, 1,018 mi (1,697 km) of line, as
compared with 493 mi (822 km) of separate sanitary sewers. Fifty three per-
cent of sewer maintenance involves root growths, with 35 percent of clogging
attributable to sand deposits and 10 percent due to grease accumulation.
Some parts of the system require root removal every three months but only
20 percent of the overall system requires root maintenance annually. Chemi-
cals such as DPO, XL222 San Fax, copper sulfate and sewer solvent have been
used. The opinion was expressed that results were unsatisfactory because
the materials were effective for grease but not for roots. Root problems
involve street sewers, interceptors and house laterals, with no problems
reported in storm sewer lines. Cleaning operations involve use of rodding
equipment, high-pressure jet units and cable drag bucket machines. Sixty
percent of root intrusions occur at joints and 20 percent at house laterals.
Most sewers are 8 to 24 in. (20 to 61 cm) in size and these sizes are most
affected by root intrusions. Shallow sewers are most predominantly intruded,
with 50 percent of the problem in lines under 5 ft (1.5 m) deep. Wye con-
nections to street sewers are the location of 15 percent of the root intrusion
problem. Most troublesome tree types are willows, poplars, maples, cotton-
woods , laurels.
On-site Comments: Four rodding crews, two hydraulic jet crews, one
TV survey crew and one inspection crew are used, over and above the repair
crews. An additional TV unit has been purchased, with video taping equipment
but no grouting equipment is owned.
o Twenty to 25 percent of the sewer system has been color-coded on a
map for guidance of sewer maintenance schedules. The trouble areas are thus
designated. Root cutting is performed in larger sewers with buckets, and
with rodding machines and straight cutting blades in smaller lines. Attempts
to cut roots with a hydraulic jet fitted with a saw attachment were unsuccess-
ful and this method is not being used. Maximum attention for root clearing is
four times per year.
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o Sewer blockages are the cause of major concern because the city
pays claims for the resulting damages. The city has paid such claims on at
least six occasions due to root stoppages from trees in city streets.
o With annual rainfall of 35 in. (89 cm) roots apparently enter
sewers seeking food rather than moisture. Root entry at joints occurs near
the top of the pipe. In one case, roots substantially filled a 21 in. (53 cm)
pipe in "four years. Willows and poplars are the most troublesome. Roots
never intrude through gasket joints.
o Chemical treatment for root control is viewed with disfavor be-
cause of the steep grades of sewers which would produce unwanted back-ups of
chemicals into basements, and because the cold temperature of city water and
wastewater flows would impede such treatment. Dumping of copper sulfate into
manholes, with holding in the line, proved unsuccessful. Other trials with
blocked-off sewers, using unspecified chemicals and caustics, proved
indet erminat e.
Sacramento County, California
Overview; Sacramento was one of the first sewer agencies to use Vapo-
rooter for most control purposes. Studies of application methods and results
have been conducted with the University of California at Davis and numerous
reports have been prepared and published, covering various types of herbicides
and combinations thereof.
o County sewers are relatively small, mostly from 6 to 12 in. (15 to
30.5 cm) in size. Root problems predominate in the 6 to 8 in. (15 to 20 cm)
lines, seldom in lines larger than 12 in. (30.5 cm). Root intrusions are
heaviest in lines laid at depths of 7 to 15 ft (2.1 to 4.6 m) which constitute
70 percent of the system's mileage.
o Root intrusions occur primarily in lines that are at least 25 years
old because they were laid with joints which are considered undesirable under
today's standards. Asphalt fiber sewers have failed as house laterals. Ten
percent of root intrusions occur at connection points between house laterals
and street sewers; 10 percent enter poor joints in the house laterals; the
rest intrude through defective joints in street collectors. Root intrusion in
storm sewers is usually at joints which are merely butted together without a
joint material: here roots are "woody" in nature and enter the bottom of the
line. In sanitary sewers roots are more hair-like and intrude into the sides
and top of the line, but dig-ups have disclosed that root growths generally
encircle the total pipe circumference.
o On—site Comments; While the county's chemical control program is
of major significance and was the main thrust of the on-site survey, routine
sewer maintenance is standard practice in the system. Four ball-crews work
year-round; a high-pressure hydrocleaner crew of two men is used mostly for
grease removal, a major maintenance problem. Two coil-rodding crews work
throughout the year, and one TV inspection crew of three men carries out
inspections of root conditions and other surveillance work. These crews clean
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600 mi (1,000 km) of sewers per year, varying in frequency of service from
yearly to every six years depending on root and other intrusion and deposi-
tion problems in specific areas of the overall system. An effective preven-
tive maintenance program is guided by good records and scheduling.
o The highly publicized herbicide treatment program was initiated in
1969. One crew of three men works during the summer season when root growth
is reported to be most pronounced, as compared with other climates where
spring and fall seasons are considered the periods of prolific root growth.
The crew has treated 200 mi (333 km) of sewers at a rate of 50 mi (83 km) per
year, over and above the program of treating house laterals with chemical
foam. A two-year re-treatment cycle is used in the most troublesome sewer
sections.
o In order to interpret the degree of root intrusion into sewers and
the condition of roots after chemical treatment, a grading system has been
adopted. It involves rating root densities from 1 to 9 in terms of standard
internal photographs in the hands of TV evaluators. Reinspection of treated
lines shows a kill of 50 to 75 percent. A progressive reduction in root
densities occurs with continued treatment as compared to an increase in root
growths in untreated lines.
o The sections of sewers being chemically treated are located,
primarily, in rear easements where property owners utilize the softer backfill
trench area for planting shrubs to serve as backyard fences. The backfill
material tends to encourage the growth of tree and shrub roots, as compared to
the hardpan nature of the native soil.
o Herbicide treatment in collector lines is carried out by plugging
a downstream manhole, flooding the upstream section with herbicide, allowing
the flooding to remain for one hour, unplugging the downstream line, plugging
a manhole further downstream and allowing the herbicide waters to stand in the
succeeding section for an hour. Each batch is used in four successive
sections and the same crew can have four separate batches working
simultaneously.
o House laterals are plugged at the property line and foamed with
herbicide, using a portable unit. Twelve to 18 house laterals can be treated
per day. The property owners are not informed of the chemical treatment of
laterals or street sewers in order to prevent any fears or protests.
o Strict safety measures are invoked to prevent injury to crews or
damage to surface vegetation. During the entire program, only one tree has
been partially affected and one grape vine was temporarily stunted. No crew
difficulties were reported. Crews are given full indoctrination on chemical
handling procedures and they are clothed with clean uniforms and protective
footwear. Aprons and respirators are provided to protect against contact
with the chemicals and the breathing of fumes. Complete records are main-
tained of sewer sections treated, chemical usage and other pertinent facts.
Approximately $26,000 was spent on chemicals in 1974.
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Southwest Suburban Sewer District, Seattle. Washington
Overview; The oldest parts of the district sewer system are 30 years
old, although most of the system was installed in the mid-1950s. Sewer depths
range from 3 to 20 ft (0.9 to 6m); concrete pipe with mortar joints was laid
prior to 1956 and compression jacket joints are now standard practice. Sewers
are subject to root intrusion. The district installs house laterals from the
street sewer to the right-of-way line and plumbers extend the laterals to the
property, often with poor joints. Much of the root problem originates in
house laterals because property owners plant shrubs along the right-of-way in
close proximity to the house sewers, providing easy entry for roots. Roots
then run longitudinally along the house lateral and into the collector sewers
via the wye connections. Services have been dug up and found to be full of
roots.
On-site Comments; Two distinct types of roots cause troubles in the
district; those that enter sewers through the lower quadrant of joints and
grow longitudinally along the bottom of the sewer, such as poplars and
willows; and those like the honey locust, that enter all parts of the joint
and form a ring effect in the pipe but seldom extend downstream more than a
minimal distance from the point of entry.
o Power rodders equipped with a,three-prong cutting tool are used
to cut the ring-type roots. Six stoppages are experienced per year, mostly
stemming from root entry into house laterals which enter the street sewers.
Sewer sections with, root problems are cleaned every six months.
o Control of roots with dry chemicals has been tried on several
occasions, by adding materials into upstream manholes and allowing the plugged
line to stand full of wastewater for varying time periods; with a 60-minute
holding time considered optimum. Dye has been used as a tracer to show the
presence of the herbicide chemical. Hair roots are killed in the sewer and
exfiltered chemical solutions have been known to kill roots outside of the
pipe. Root growths have been killed for upwards of five years.
o Copper sulfate treatment has been used by placing the chemical in
a plastic bag hung in a manhole in contact with flowing wastewater. Crystals
have been replaced, as needed, over a period of several months. Growths have
been impeded for three years with no detrimental effect experienced at the
treatment plant.
o No ongoing chemical control program is presently in effect. A
ruling by an attorney representing the District has stated that the agency is
not responsible for damage to trees growing on private property if they are
damaged by chemical control measures.
Los Angeles County, California
Overview; The Department of Public Works maintains 3,500 mi (5,833
km) of collector sewers, with interceptor sewers operated by the County
Sanitary District. Over 90 percent of the system has 8 in. (20 cm) sewers
with 90 percent installed since 1945. Groundwater is low and sewers laid at
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depths of 7 to 9 ft (2.1 to 2.7 m) are not subject to infiltration.
On-site Comments; Roots in desert areas will go to any depth to enter
sewers, with elms, palms, tamaracks, oleanders and pepper trees producing the
greatest root intrusion problems. Roots affect sewers laid prior to 1964.
o Routine maintenance for grease removal, the greatest problem, and
for root control, involves the use of hydraulic cleaner crews, rodding crews
and an inspection crew which televises 300 to 400 mi (500 to 633 km) of sewers
per year. In 1974 cost of sewer maintenance was $2.50 per residence served.
o A county-wide crew of three to four men is used for chemical
control which was instituted in 1970 using Vapom experimentally. Early find-
ings showed that minor regrowth was experienced. Flooding was found inad-
visable because of the terrain conditions. Foaming is now used in plugged
sewer sections. A 200 gal (756 1) mix tank on a truck is used to make up
batches of the chemical for foam application.
o Employee safety measures were evaluated, including use of hoods,
cannister masks, full face shields and taped cuffs and coveralls. Crew
members do not enter manholes. At the present rate of treatment it will take
two and a half years to cover the whole system. The same crew works on rat
and roach control when not applying root chemicals.
City of Los Angeles. California
On-site Comments: Root intrusions are present in 20 percent of the
system's 6,000 mi (10,000 km) of sanitary sewers and 1,200 mi (2,000 km) of
combined sewers, including hillside lots, rear yard easements and streets.
Sewers are often shallow and have been affected by earth movement; during
seismic incidents.
o Groundwater levels are low and most sewers are laid iri-the-dry.
Minor inflow conditions exist but apparently infiltration is not a general
problem. Roots are a problem but grease accumulations are the primary cause
o'f sewer stoppages; about 300 grease stoppages are recorded per year.
o Mechanical rodding has been the accepted method of sewer mainte-
nance but recent budget reductions have resulted in the use of hand rodding
crews. Each of eight service districts has one hydrocleaner crew and one
inspection crew which handles partial stoppages. A TV inspection crew is
available and a city-wide bucket machine crew handles major stoppages.
o A herbicide treatment program with Vaporooter was initiated in
1974. Some 50 mi (83 km) of sewers were treated from October through March,
as contrasted with the Sacramento summer program. One three-man crew is
used.
o The initial program involved sewer flooding, paralleling the
Sacramento program, with one-hour retention of the herbicide in only one
sewer section at a time. A one-percent solution was used for flooding which
was made difficult by steep grades in the sewers. Preliminary findings
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showed herbicide treatment will control roots for a period of one to ten
years, with an average period of five years. Treated sections have less than
10 percent regrowth of roots. Root problems are not found in interceptor
lines or storm drains.
o Foaming has replaced flooding because of greater control of opera-
tions and apparently less hazard involved. The on-site report referred to
a few accidents occurring during flooding and to five accidents involving
foam getting into houses. Flooding involved the dumping of chemicals into
manholes by hand and then filling the line with a fire hose.
o Contrary to Sacramento practice, property owners are notified that
foaming will take place. Men enter manholes rather than working at the
surface; the county does not supply clothing or other protective devices.
For foaming, a five-percent solution is used. An experimental run with a
10-percent solution high-pressure spray required greater safety control and
other precautions which were considered drawbacks.
o Chemical expenditures amounted to $8,000 in 1975. If funds are
available, $80,000 per year will be expended on the program, with two full-
time crews.
o There has been little acceptance by the district crews of the
technique of chemical treatment and little change has been made in the manual-
mechanical maintenance procedures.
o Copper sulfate treatment of roots was attempted several years.ago,
with no tangible benefits.
St. Petersburg. Florida
On-site Comments! The St. Petersburg sewer system is laid, primarily,
under the groundwater table and has been subject to heavy infiltration. This
condition is being corrected by continuing TV inspections, sewer sealing and
repair procedures. Root intrusions are extremely heavy in shallow lines.
Dig-ups are very common, 1,200 per year, and stoppages due to root growths are
equally high, 1,500 annually. Roots represent 50 percent of sewer maintenance
problems, with deteriorated lines and sand deposits responsible for 20 percent
each; grease accumulations, which are high in other systems, represent only
five percent of maintenance problems. Thirty mi (50 km) of sewers are cleaned
annually for root removal but 60 percent of the overall system of 800 mi
(1,333 km) of collectors and 100 mi (167 km) of interceptors were reported as
subject to root intrusion. One-half of the root problems are located in
street sewer joints and half in house lateral growths usually located at the
first joint between the property line and the street connection wye, which is
the closest to hedge row trees. Chemical control with Vaporooter Plus,
Sanifoam, has produced results that varied from 0 to 100 percent effective.
A qualified applicator is required to gain the most effectiveness from the
chemical control. Only five percent of the system is reported to require
yearly cleaning, despite the apparent concentration of the root problem.
"Punk" trees and Australian pipe trees are the major root causers but
Vitex, Brazilian pepper, Jacaranda and Turks Cap are listed as troublesome.
Roots cause joint failures and consequent infiltration.
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SUMMARY
The excerpts from the individual survey reports are intended to add
further dimension to the types of problems and solutions to root control. The
thrust of the excerpts has been to screen out the points which add to the
consensus of root problems and solutions or, where significant, demonstrate
differences in procedures indigenous to variances in local conditions.
The agencies which were judged to have the most comprehensive root
control programs also appeared to have excellent overall preventative main-
tenance programs.
Mechanical cleaning offers a positive initial result but appears to
make root growth more of a problem over the long run. The work done by the
city of Milwaukee provides an excellent guide as to the capability of various
methods of mechanical cleaning.
Daily attempts at chemical control of roots were generally unsuccess-
ful as they involved placing chemicals in the manhole and letting the flow
carry the chemicals along the sewer. The use of the Vaporooter family of
herbicides appears to be the only generally acceptable method of chemical
control. Flooding as practiced by Sacramento County is not generally
feasible in other areas. Foam application as practiced by Los Angeles
County and St. Petersburg appears to offer an effective method.
The safety of Vaporooter to both employees and above-ground plant
life appears to be acceptable when used with recommended safety precautions,,
Although St. Petersburg was the only agency found to use licensed herbicidal
applicators transferred from the Parks Department, use of personnel with
such training should enhance the safety of a program.
Chemical control with Vaporooter is not 100 percent effective, but
repeated applications over several years approaches this limit. No other
method of control approaches the effectiveness found in the four major
agencies using Vaporooter.
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SECTION VI
TIDE AND BACKWATER GATES
Previous studies (10) and the initial APWA questionnaire for the
present study identified problems with tide and backwater flow gates as a
source of inflow into systems utilizing such devices.
A survey and field visits were made in order to obtain additional in-
formation. The firm of C. E. Maguire was retained to analyze the available
data and give an engineering review to the design problem.
For the purpose of the study, a tide gate, known also as a flap gate,
or backwater gate, is as defined in the 1969 APWA Glossary of Water and Waste-
water Control Engineering:
Tide Gate; A- gate with a flap suspended from a free-swinging
horizontal hinge, normally placed at the end of a conduit dis-
charging into a body of water having a fluctuating surface
elevation. The gate is usually closed because of external
hydraulic pressure, but will open when the internal head is
sufficient to overcome the external pressure, the weight of
the flap, and the friction of the hinge.
Note that a tide gate that conforms to this definition is "self-
acting;" that is, energy to actuate the gate comes directly from the water
systems in contact with the gate. Note, also, that the term tide gate is
used, even though the gate may be locally referred to by another name.
Other arrangements, designed to achieve the same result (i.e., allow flow in
one direction, block it in the other) amount to four percent of all gates
reported. Sluice gates, fabric dams, etc., are used, but these arrangements
are not examined in depth in this mail survey.
The kind of tide gate found to be in use throughout the U.S. and
Canada consists of a movable plate or disc, pivoted at the top end, hanging
against a slightly sloping opening so that gravity holds the gate shut in
the absence of a hydraulic head that can cause the gate to open. Large
gates 4 ft (12 m) and larger are often of timber, with corrosion-resistant
fittings. Gates are used to protect systems from frequent or extraordinary
high water conditions and are usually taken for granted. Their effectiveness
and economy is generally good and depends largely on the maintenance given
them. Only nine percent were reported as being installed at or with
mechanical regulators.
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Users are 80 to 90 percent satisfied with tide gates. Fouling and
sticking cause about 60 percent of malfunctions, with an average annual
maintenance expense per gate of $194. The annual labor required is 23 man
hours per gate.
The survey indicated that engineering standards for design and
application are available from many manufacturers, and that, like all other
waste treatment equipment, a certain amount of maintenance cannot be avoided.
Tide Gate Applications
The APWA survey elicited replies from 55 communities which used some.
1,300 gates. Nine percent of the installations were at mechanical regulator
locations, 62 percent protected sewerage systems from reverse flow, 25 percent
protected pump stations and treatment plants, and four percent protected open
streams. In other words, all tide gates are used to allow outflow in one
direction and preserve the system from inflow in the other. Figure 6 shows
the evolution for the need for tide gates.
An older application, now rare, used tide gates to lift water so that
it could be used to produce power. In Boston, for example, a system of two
ponds, two sets of tide gates and a water wheel were used to drive a mill.
Inflow gates on one pond kept it full from high tide, while outflow gates on
the second pond kept it empty by discharging to low tide. The system was irt
Use until 1866 (11).
The prevention of reverse flow in a sewer system is commonly accom-
plished by use of check valves in small systems and backwater, flood or tide
gates in larger systems.
Backflow prevention devices include check valves, self-acting tide
or flap gates, and power actuated gates used for backflow prevention. Of the
larger, self-acting variety, three general types have been commonly used:
o The side-pivoted tide gate, now obsolete
o The flap gate, with horizontal pivot above a swinging
flat plate or disc, and
o The flexible flap type occasionally used for gates less
than 12 in. (20.5 cm) diameter.
The common flap gate is manufactured in three common forms: as a
solid plate of cast iron or steel, as a thin hollow pontoon (usxaally of
welded steel), or as a timber gate (see Figures 7-12). The powered gate can
be of many types, the sluice gate and the newer pneumatic are examples.
Figure 13 shows a recent installation of a pneumatic device.
Users in 57 communities reported 88 percent (1,156 gates) were of
the flap type, the remainder (156) being power-actuated or miscellaneous.
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Original storm drain or combined sewer without reverse flow protection.
INTERCEPTOR
SEWER
REGULATING
SYSTEM
Modified sewer system. Tide gates are necessary at A or A1 to protect
interceptor.
Figure 6. Evolution of typical modern sewer system to divert
dry flow to treatment.
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Figure 7. Typical large check valve.
This type, usually of cast iron, is commercially available
in sizes up to 2 ft (61 cm) nominal pipe size.
Wtt^irxiOT
/ // \V/V-:--.
jfi\
7-6
Vertical Longitudinal Section
Horizontal Section
TIDE G*TE CHAMCLR TOR
DOUBLE BARN Doof\ TYPE: .
Figure 8. Side-pivoted tide gate.
Now obsolete, used in Boston about 1900.
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Figure 9. Typical flap gate or flap valve.
Commercially available in sizes up to 2.5 ft (75 cm)
Figure 10.
Source: Rodney Hunt
Typical cast tide gates.
Available in sizes to 8 x 8 ft (2.4 m x 2.4 m)
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Figure 11. Circular pontoon tide gate.
Available in sizes 10 ft (3 m)
Figure 12. Typical timber tide gate.
Available in sizes over 10 ft (3 m)
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Figure 13. Inflatable fabric dam.
Courtesy: Firestone Coated Fabrics Co.
General Nature of Tide Gate Design
Flap gates are shown schematically in Figure 14. The outlet from the
system which is to be protected from backflow terminates in a flat, sloping
seat against which the flap hangs. Major rotation is about the upper pivot;
the lower pivot provides flexibility to ensure better closure and is standard
with larger gates.
The tide gate is actuated by a difference in head across it. The
static forces on the flap consist of weight and external head tending to
close it, and buoyancy and internal head tending to open it. Friction
opposes motion in either direction. Sensitivity of the gate, that is the
gravitational force tending to hold it shut, can be adjusted by moving the
upper pivot in or out or by adding weights. Design factors are reviewed
below and Figure 15 shows the forces which are acting on the gate.
Dynamic forces are usually unimportant, although a sudden, extreme
reversal in net head across the flap can cause the flap to pop open and then
slam shut. This problem usually occurs on the outlet of large pump systems.
Designs are available to reduce the problem. Another area in which dynamic
forces can cause a problem is during opening, when in some cases the flap
may flutter (12). This situation may occur when flap gates are installed in
tandem with discharge to a river. The outer gate may be damaged by wave
action after the inner gate seats.
The lower pivot is restricted in its degree of rotation. Too great
a rotation about the lower pivot in either direction can cause the upper or
lower edge of the flap to hit or enter the seat opening at an angle, causing
the gate to wedge open.
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UPPER PIVOT
LOWER PIVOT
SEAT
FLAP
WALL OR
END OF PIPE
Figure 14. Schematic tide gate.
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FORWARD FLOW
HEAVY DUTY
RUBBER SEATED >.
MEDIUM DUTY
LIGHT
DUTY
-lor.Kb. .
10 /
/
5 ft / UNDEFINED AREA
' VC5-
50' 40' 30'
NET SEATING HEAD (FT.)
20 ^JO-^" 0 0.2 0.4 0.6
FLOWING HEAD LOSS (FT.)
METAL SEATED
.1 FP.S.
LEAKAGE FLOW
Figure 15. Theoretical tide gate behavior.
(Note changes in scale)
The standard seat is inclined from the vertical to utilize the seal-
ing force of the flap; typically, gates smaller than 18 in. (45 cm) for a
5 slope, intermediate sizes 3 and very large sizes 2°. The flap is
designed to withstand the expected seating heads. There is an upper practi-
cal size for tide gates because of the strength requirements and difficulties
in fabrication. Castings are usually limited to 8 ft (2.4 m) in the major
dimension, with pontoon and timber gates, or separate gates being more
practical for very large sizes.
Pivots must meet strength and low torque requirements and must also
be adjustable in the larger sizes. In most cases, a permanently lubricated
design is preferred. Seats can be of base metal, steel, bronze, rubber, etc.
They may be installed either on the flap or the frame, or (with bronze) on
both. An undamaged soft seat seals best. If damaged, it leaks more than
a metal seat.
Timber gates are preferably made from greenheart timber from British
Guiana, with suitable metal reinforcement and fittings, and with elastomeric
seats, if desired. Tropical woods have become very expensive, therefore
gates of southern pine with iron frames and the finished timber creosoted to
refusal have been used and are reported to give good service, especially
where constantly submerged in polluted water.
Flexible flap gates can be made in sizes up to 1 ft (30.5 cm). A
simple design consists of sections of flat conveyor or power belting, rein-
forced on both sides by iron discs and hung from the top. This design has
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successfully replaced cast iron flaps in a silting situation as will be des-
cribed later.
Of the 1,156 flap gates reported in the recent survey, 26 percent were
under 2 ft (61 cm), 31 percent 2 to 3.5 ft (61 to 105 cm), 30 percent 3.5 to
6 ft (105 to 108 cm), one percent of special alloy, and 25 percent of timber.
Forty percent had single pivots, 60 percent double. Twenty-three percent had
bronze seats, 19 percent had seats of rubber, and 58 percent had no special
seats. Nineteen percent of the gates were installed with counterweights.
Tide Gate Selection
The simple cast metal flap valve, shown in Figure 16, is suitable for
lines up to 2 or 2.5 ft (61 to 75 cm). It can be furnished with bronze or
rubber seats and with flange, collar or pipe (spigot) type attachment. There
is usually sufficient play in the assembly to allow good seating in spite ot
having only one pivot.
The larger circular or rectangular cast iron flap gate usually has
doubly-pivoted arms and is the common choice from 1 to 8 ft (30.5 to 240 cm).
They are usually provided with a flat back to bolt to a flat wall, wall thim-
ble or pipe flange. The main casting is available in standard cast iron
Courtesy: C. E. Maguire
u- ,1 + -i „* * *ian natP B. Shows the back side of the flap gate.
A. Shows hinge detail of a flap gate ^^ ^^ stuck g|ong c|osjng whjch
does not allow complete closure.
Figure 16. Typical urban tide gate Installation.
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or alloyed with nickel (Ni-resist), while the seats, faces, links, pivots, etc,
are available in a variety of materials. Special modifications are available
to restrict openings beyond 90° or to cushion slamming.
Pontoon and timber gates are used where available slopes are very
small because their weight is less than cast gates and less head is required
to open them or hold them open. They are commonly available in sizes from
3 to 10 ft (90 to 305 cm). Corrosion can destroy a pontoon gate'faster than a
cast gate, since there is less metal, consequently the base material must be
carefully selected. A timber gate must usually be larger than the equivalent
cast gate to have equal strength. Timber gates not heavily impregnated are
subject to rotting and attack by marine borers in relatively unpolluted water.
Those made from greenheart seem to last as long as any other type.
Flexible flap type gates apparently work better and longer than cast
flap gates where the water is very dirty or high in solids and there is in-
sufficient flushing action.
Most installations are made with the gate the size of the discharge
pipe, or its equivalent. Installing a smaller gate, if sufficient for the
flow, would generally be more expensive than to construct a transition section.
Selection Recommendations
The small, single-pivoted flap valve is suitable for most cases up to
2 ft (60 cm). Where leakage must be prevented, neoprene or similar soft
seats are available. Principal disadvantages are that the simple hinge pin
may bind or freeze, due to corrosion or fouling, and that the rigid flap may
fail to seat properly.
Cast flap gates, with doubly-pivoted hinge links, are the most -common
type. They are available with hardware of almost any desired material (steel,
Monel, stainless, bronze, etc) and with metal or resilient seats. They have
a certain minimum sensitivity, about 2.5 in. (6.2 cm), that can be decreased
by adding weights. In the large sizes, especially in heavy duty construction,
they become rather heavy and expensive. In certain instances, they can have
too little sensitivity (because of weight) or can flutter or slam.-
Timber and pontoon gates are preferred for large facilities but are
apt to suffer from corrosion or decay more rapidly than cast gates.
Flexible flap gates are reported to function longer between cleanings
than the cast type in heavy silting operations but they are not widely avail-
able and their practical service life is yet to be determined.
Receiving Water Considerations
Industrial wastes and saline waters pose special corrosion problems
that are best addressed on an individual basis. The largest user of pontoon
gates (New York City) is reported to have abandoned this type because of
their rapid corrosion, but the material specified for their fabrication
(wrought steel plate) was probably inferior. They are in use in Philadelphia
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and Omaha, to cite two examples. Timber gates of pine are reported to decay-
fairly rapidly, unless heavily creosoted. The use of tropical greenheart is
reportedly quite successful, although there may be a problem with marine worms
and the like when the receiving waters are freed of pollution.
Preferred Materials
Cast iron, galvanized steel, bronze and Neoprene rubber and green-
heart timber are the most popular materials and are generally suitable. The
APWA survey indicated that only seven percent of gate failures were reported
as being due to corrosion, and the mean service life is estimated at between
10 and 100 years. Nickel-bearing or Ni-resist cast iron is available, as is
type 316 SS where the cost can be justified.
Performance
Users are reported to be 80 to 90 percent satisfied with their
existing tide gate facilities. Fouling and sticking cause about 60 percent
of malfunctions. Gates are visited an average of 14 times a year at an
annual maintenance expense of $194 and 23 man hours per gate. In spite of
this, from half to two thirds of all agencies reporting stated that they
experienced problems due to malfunctioning tide gates, and that the resulting
system overloading occurred about 29 days each year. Such malfunctions may
contribute large amounts of inflow, depending upon receiving water levels.
Gate life is estimated at 35 years.
Modifications
Tide gates are modified:
1. By substituting materials, in whole or in part, for improved
corrosion resistance.
2. For better seating by using double pivots and soft seats.
3. To change sensitivity by moving the upper pivot horizontally
upstream or downstream in relation to the seat, or by adding
or removing weights, or, as in pontoon and timber gates, by
using different methods of construction.
4. To reduce friction by using better pivot lubrication. (The
complete removal of the pivot results in the flexible flap
type previously mentioned).
5. To limit opening overtravel by providing a flexible stop.
6. To prevent slamming, by furnishing shock-absorbing mechanisms.
7. For telemetering for remote monitoring by applying devices to
sense gate position.
Tide gate supporting structures are modified:
1, To provide better access, especially for maintenance.
2. With outer barriers, seal walls, dolphins etc. to protect the
tide gate from wave action, floating ice and debris in the
receiving waters.
3. With additional supports for tandem or parallel gates for
increased capacity or to ensure better sealing against backflow.
4. With provisions for telemetering devices.
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Maintenance Requirements
Nearly two thirds of all tide gate failures are due to fouling or
sticking. Since the quality of the water occurring at the gate can seldom
be improved, continual maintenance is the only answer. Many gates are applied
to emergency overflows so that they are not often fouled, but in many cases
gates will require cleaning after every actuation if they are to succeed in
preventing reverse flow.
Maintenance crews vary from one to five men, and may require special
equipment such as mobile cranes to open the large gates and air testing
equipment to check for flammable vapor or lack of oxygen before entering a
manhole. Fire hoses are one of the most used means of removing deposits
from gates.
USE OF REMOTE SENSING SYSTEMS
Open Gate Sensors; Proximity sensors have been used successfully to
monitor whether or not the gate is fully shut. This information can be used
to check for gate malfunction after high water subsides, whether a gate has
been caused.to open, or when a gate opens and closes in relation to other
events.
Level Sensors; Level switches can detect high water levels on the
upstream side to indicate when a tide gate may be open. Conductive type
probes must be used very carefully, since many may signal falsely due to
condensation or fouling.
Flow Rate Sensors; On-off devices, such as simple paddle switches,
can sense reverse flow and are desirable for situations in which the gate is
usually submerged. A suitable paddle switch, although inherently simple,
requires careful and well thought out construction and installation.
Remote sensing systems are expensive, but can be justified on the
basis of reduced maintenance and better system performance. The maintenance
of the sensing systems must also be carefully considered.
Other Instrumentation
The use. of quantitative flow rate sensors is unlikely to be justi-
fiable for the tide gate installation alone, especially since such a sensor
would be impaired by the same conditions that foul the gate. Such sensors
would also be costly to purchase and maintain.
Tide gate failure can be detected by a systems approach in which
inflow to various parts of a sewerage system is monitored for unusually
high values. A measure of influent salinity can also be used in certain
cases to detect saline water intrusion if a working salinity measuring
system is truly available. In any case, leakage of any one gate in a large
system is often impossible to detect.
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Recommendations for Design Improvements
The present style flap gate is considered good or at least acceptable
by 90 percent of users. The principal problems (fouling and sticking) might:
conceivably be better handled by changing the surface material or the shape,
and closure might be made more positive, but otherwise the design is satis-
factory. The mechanically actuated gates used by Metropolitan Seattle
offer a positive, controlled facility that prevents backflow.
Recommendations for Application Improvements
A major problem in present tide gate installations is accessibility,
as was shown in Figure 16. Many of the installations in heavily built-up
metropolitan areas and many older installations are accessible only by
manhole. Maintainability of a tide gate located under a city street,
especially where the gate is always submerged, is a particular problem.
Other considerations include protection of the gate from wave action and
debris that floats past the gate, and location of the gate so that there is
sufficient clearance and solids do not accumulate and prevent closure. Non-
corroding ceiling hooks and stop-log slots (on both sides) should be
considered.
Installations on pump discharges must be designed with consideration
of the possibility of slamming which has also been reported when gates are
installed in tandem. Slamming can be cushioned by soft seats.
Tide gate sizing data is scanty. The usual statement is that gates
start to open at about 2.5 in. (6.2 cm) differential, but there is no avail-
able data of the effect of submergence on head loss. Studies conducted in
Iowa in 1923 seem to offer the only reliable data (12). A sizing procedure
would be desirable including the effect of submergence, friction, and flap
weight on discharge head loss.
Operating Costs
Non-powered tide gates impose no operating costs other than those
for amortization and maintenance. Maintenance costs are generally site
specific as time for each gate varies with location conditions and past
experience. Some gates may be serviced too often, but the trend seems to
be to neglect maintenance until problems arise. There appears to be little
coordination with treatment plant operations and tide gate maintenance. In
some communities tide gates have actually been abandoned due solely to the
tide gate facility maintenance cost. Treatment plant operation costs do not
appear to have been a consideration in the decision.
Other Designs
Tide gates were defined in the survey as self-acting arid self-
contained. More sophisticated and more expensive are flood and stormwater
control systems using powered gates or dams. Such systems have been demon-
strated at Metropolitan Seattle and Minneapolis-St. PaulO). Fabric dams
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(.see Figure 13) have been extensively tested in Minneapolis and Cleveland(9).
Fabric dams are reported to work well when properly installed and so long as
water is not permitted to flow over them while they are inflated. Seattle
uses sluice gates.
The performance of powered sluice gates in place of tide gates in
centrally-controlled systems is considered to be indistinguishable from
sluice gates in common service and offers a positive control.
SUMMARY
"Standard" types of tide gates are available from several well-
established manufacturers. Users are 80 to 90 percent satisfied with the
present design of tide gates. Fouling and sticking cause about 60 percent
of malfunctions. The principal problems are of accessibility and maintenance.
Tide gates, by their simplicity, have received little attention over the
years in spite of their successful utilization in many of the larger munici-
pal systems. When properly applied, however, they can be relied upon to
operate successfully for their typical life of several decades with adequate
maintenance.
Methods for selection and sizing are very incomplete. For example,
data could not be found to indicate opening heads as a function of submergence
or friction, or flowing head loss as a function of flap weight. Design
manuals for tide gates and sluice gates would be helpful. Manuals of practice
or design recommendations for area-wide sensing and control systems would also
be desirable. Such manuals should discuss such areas as theory and present
practice, recommend standards, and demonstrate practical examples.
Tide gates should be integrated into area-wide control systems. Such
integration has been accumulated by only a few authorities, but is within the
existing technology. Remotely-actuated gate systems, to supplant tide gates,
have also demonstrated their feasibility. They represent a corresponding
increase in technology and cost. The positive control which such systems
represent offers major advantages and protection to wastewater treatment
facilities.
Field visits were made to six authorities.
investigator's reports follow.
Akron, Ohio
Highlights of the
The sewerage system of Akron consists of combined and separate
systems with sewer sizes ranging from 8 in. (20 cm) to 101 x 56 in. (2.6 x
1.4 cm) conduits. One treatment plant serves the area. The plant overflows
are not equipped with backwater gates. The system also includes 20 pumping
stations, 18 of which have emergency overflows. No mechanical regulators are
installed in the 38 combined sewer overflows; control is provided by set
weirs or dams.
The system has backwater gates at only five of the 18 pumping
stations with overflows. These gates are all flap type and prevent intrusion
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of local streams into the system. All the gates are cast iron, four under
2 ft (61 cm) and one between 2 and 3.5 ft (61 and 105 cm). First installa-
tion was made in 1939 and the latest in 1973. The useful life was estimated
at over 25 years and performance was reported as satisfactory despite
occasional fouling or sticking. Maintenance is performed on the flap gates
only when excessive inflow is observed at the pumping stations.
Akron uses many small, flexible flap gates made from flexible
belting of 1 ft (30.5 cm) size, to seal off street catch basins from combined
sewers. These gates are more successful than the original cast iron gates
that had a tendency to silt up rapidly.
Akron's telemetering system does not monitor backwater gate condition,
but only the level at overflows and pumping station operations. Gate mal-
functions can be deduced from excessive pumping and wetwell elevations. The
telemeter system uses leased telephone lines to transmit signals to a central
control station.
Detroit, Michigan
Detroit's wastewater collection system consists almost entirely of
combined sewers. Until 1930 the city collector sewers led directly into the
Detroit and Rouge Rivers. Then a primary wastewater treatment plant was
built and sewers were built to intercept the collector sewers near the out-
falls. Automatic regulator stations with outfall tide'gates were constructed
at connecting points between the interceptor and collector sewers. The back-
water gates protect the regulators and prevent high river levels from sur-
charging the system and overloading the treatment plant through the intercep-
tors. Forty eight of the 76 river outfalls are protected by backwater gates
using 108 gates, more than half of which are installed in tandem.
Most of the backwater gates were installed in the 1930s and are still
operating. All timber gates were constructed with vertical rather than
horizontal members. This design seems to be more economical and provide
better hinge connections at the top of the gate. Of the 108 tide gates in
the Detroit system, 30-35 are cast iron, the rest timber. Most of the metal
gates use brass seals. Some use neoprene.
The Detroit system utilizes proximity sensors on tide gates. The
sensors indicate at a central control office whether or not the gate is
closed. Other monitoring components consist of rain gauges, inflatable dams
and level sensors, etc. all of which are connected to the telemetry system.
Most problems with this monitoring system stem from damage to overhead
telephone lines during storms.
Maintenance and inspection was performed every 1 or 2 months until
recently when trips were reduced to emergency situations only. Problems are
encountered after every rainfall large enough to cause gates to open. Debris
often prevents complete gate closure, indicating an open gate at the central
office.
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Included in Detroit's collection system are three inflatable dams,
designed to increase storage capacity within the system. The dams are
also monitored by central control for inflation pressures and sewage depths
retained by the dams.
New York, New York
New York City's sewer system consists primarily of combined sewers
and due to the low-lying area and wide tidal changes the system requires
extensive protection from flooding. The first tide gates were installed in
1937 when the Wards Island treatment plant was built. As a new treatment
plant was constructed, tide gates were also installed at interceptors to
protect the system.
The New York system has approximately 365 tide gates. The various
gates include 160 flap type, 100 pontoon and 105 timber gates. Flap gates
range from under 2 ft (61 cm) to 6 ft (1.8 m), pontoon type are used for 3.5
to 6 ft (1.05 to 1.8 m) gates, and timber gates are used for larger applica-
tions. A limited number of tide gates were reported to be installed in tan-
dem or in parallel with two or more units protecting the same regulator cham-
ber. Tide gates are located on the downstream side of regulator chambers and
approximately 10 gates are used between sanitary sewers and combined or sto.rm
sewer interconnections. The latest tide gate was installed in 1974. No
pontoon gates have been installed since 1960. In the future, New York plans
a new treatment plant and interceptors with tide gate protection.
The City's experience with tide gates shows that cast iron gates have
a long useful life, with a few cases of broken hinges and casting. The cast
iron gates have resisted corrosion, unlike the steel pontoon gates that have
rusted and deteriorated after 10 years. Because of the short useful life and
their lack of rigidity, the City no longer specifies pontoon gates. Most of
the pontoon gates in service are in poor condition. Original timber gates had
a reported life of approximately 10 years, but timber gates are now built
with vertical planks of tropic hardwood. This new design is expected to
extend the useful life. New York uses resilient seals on all their gates.
Most of their gates are usually partly or wholly submerged.
The performance of the iron flap and new timber gates was reported
good, but poor for the old yellow pine gates. Typical gate failures for all
three type gates included sticking or binding, fouling, corrosion and broken
hinges or frames. Tide gates and regulators are maintained by five crews
consisting of five men in each crew. Servicing and inspection varies from
once a month to once a week. Servicing trips are on a routine schedule,
with additional trips as required to answer complaints. The estimated
man-hours per year was 16,000 for 1974 and $100,000 per year for cost of
maintenance. The above estimated values yield 43 mhr/yr/gate and
$274/yr/gate.
Excessive flows observed at pumping stations, treatment plants or
interceptors are used to alert crews to possible tide gate malfunctions.
The system is under daily surveillance for salt water inflow. High salt
concentrations alert crews for additional check-ups.
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Metropolitan Seattle, Washington
The Metropolitan Seattle area sewerage system comprises 20 pumping
stations, 16 regulators and 5 wastewater treatment plants. The service area
is generally served by combined sewers. The 16 regulator stations are
typically constructed at intersections of collecting sewers and interceptors.
Each station consists of a chamber with a regulator gate to modulate wet
weather flow into the interceptor and an overflow/outfall protected by a
power actuated sluice gate. Monitoring allows operation of the sluice gate
only when there is sufficient upstream head to allow discharge.
Numerous sensing devices are used in the Seattle system. Level
sensors in interceptors, regulator stations and outfalls control the sluice
gates and prevent ocean water from entering the system. The entire sewerage
system is monitored and controlled through a central computer station and
two satellite stations located at the major treatment plants.
The Seattle systems use only four conventional tide gates, one is
over 6 ft (183 cm) and protects a lift station and three are between 2 and
3.5 ft (61 and 105 cm) and provide emergency high level relief at regulators.
All gates are top hinged and cast iron with monel seats. No monitoring
system is used on these tide gates and no information was available relative
to performance or maintenance other than all systems are checked quarterly.
No dissatisfaction was expressed with operation of the four flap gates.
Minneapolis-St. Paul, Minnesota
During the 1950s and 1960s studies of the Minneapolis-St. Paul
sewerage system revealed that combined sewer overflows occurred even at times
when interceptor sewers were operating at less than capacity. In 1966 a
program was initiated to modify the regulators and construct a central com-
puter monitoring and control station. Sixteen control and monitoring stations
were constructed to handle 80 percent of the system's combined sewage. In-
flatable dams were installed immediately downstream from the mechanical
regulator gates, which are operated by hydraulic cylinders. Gate position
is sensed by mechanical potentiometers for telemetry.
The central computer station monitors sewage elevations in inter-
ceptor and trunk sewers, positions of regulator gates, air pressure in the
inflatable dams, levels in rain gauges and river quality from sample
stations. The computer, however, does not operate the system.
Although the Minneapolis-St. Paul system has extensive overflow
facilities, at present it does not utilize conventional type tide gates.
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SECTION VII
RECENT RESEARCH AND DEVELOPMENT ACTIVITIES
Relatively limited research work has been carried out on methods or
practices aimed at reducing infiltration, other than product development
work by pipe manufacturers. This study has identified five recent or ongoing
research and development actions which merit consideration. The following
discussion is intended to indicate the nature of these specific fields of
research and development as examples of the types of studies which are needed
to improve I/I conditions in sewer systems.
PIPES AND JOINTS
Heat Shrinkable Joints
The usual location of entry of infiltration into otherwise structur-
ally sound sewer systems is at line joints, at the connection of the house
lateral to the collector sewer and at manholes. Some pipe systems have
minimized the number of joints by using long pipe sections where the weight
of the pipe is not so great as to impede handling ability.
Other pipe systems have used chemical welds to produce tight joints.
Heat shrinkable plastic tubing (HST) has been developed and researched
by the Western Company (13). Laboratory studies of materials and joints were
conducted to determine their characteristics and their operational and econo-
mic feasibility. A wide variety of HST materials and joints were tested in
addition to conventional joints for clay, concrete and asbestos-cement pipe.
The results of both small-scale tests and full-scale tests using
commercial 8 in. (20 cm) sewer pipe indicated that a polyolefin with a
polymeric base hot-melt adhesive produced the most durable, watertight joints
and were significantly superior in performance compared to existing pipe
joining methods. In addition, cost analyses indicated that HST joints are
economically feasible and compare favorably to conventional joints when
considering both material and installation costs. The HST joint does not
require a bell or other enlarged pipe end. Thus, the breakage of the pipe
in shipping and job site handling is reduced, cost of production is lowered,
and bridging between joints in the trench and structural failures are
minimized.
The use of HST joint has been accepted by manufacturers and can be
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purchased from many suppliers. For example, the joint system has been used
with smaller diameter clay pipe.
Impregnation of Concrete Pipe
The use of concrete pipe for sanitary and combined sewers has been
affected in many areas because of concern over corrosion of the concrete.
Concrete pipe is produced widely over the nation and has been used extensively
for many other systems, notably separate storm sewers.
The Southwest Research Institute conducted a study (14)concerning
the impregnation of concrete pipe. The study sought to provide a method to
increase corrosion resistance and strength, and reduce the permeability of
concrete used in sewer line applications by impregnating the pipe with
low-cost resins.
Figure 17 shows the area where sewer pipe is usually attacked.
The use of sulfur or five percent hydrofluoric acid resulted in a
decrease of 10 to 30 times the rate of corrosion loss. The cost of such
2 2
treatment was estimated to be from $0.05 to $0.15/ft ($0.54 to $1.61/m )
depending upon the material used. The cost of plastic lining, on the other
hand varies from $0.85 to $1.00/ft ($9.15 to $10.76/m ).
The study concluded that there is a possibility that lines could be
treated in place.
SEWER CONSTRUCTION
Trenchless Sewer
Plastic pipe, because of its light weight and flexibility, has been
used in relatively long lengths in Europe for some time. Some projects have
extended the pipe length at the job site to permit extremely long pipe
sections between chemical welds. However, these applications have generally
been used for water lines.
An adaptation of European practice to sewer construction is being
demonstrated in Sussex County, Delaware, by the Evanston Development
Corporation (15), where 1,500 ft (457 m) of sewer line has been "plowed in."
A contract for installation of the test sewer was awarded, in Septem-
ber 1974, to the Evanston Development Corporation, Huntington Valley,
Pennsylvania.
The trenchless pipe laying system was developed in England in the
early 1960s. It has been employed throughout Europe to "plow in" pipe,
cable and conduit. The equipment simultaneously forms a- tunnel and installs
a pipe in a continuous and rapid manner.
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Condensate with large aerobic
bacteria population which oxidizes
hydrogen sulfide to sulfuric acid
which attacks concrete.
Silt
Accumulation
Concrete
Slime accumulation with large
anaerobic bacteria population
which convert sulfur compounds
in sewage to hydrogen sulfide gas
which is released to the vapor space.
Figure 17. Cross section of concrete sewer pipe under
typical corrosion conditions.
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The sewer being installed is 8 in. (20 cm) diam. schedule 40 PVC
pipe. It is being placed at depths between 4 and 6 ft (1.2 and 1.8 m) and
below the water table. All chemically welded joints are made above ground
where they can be carefully controlled and given 24 hours to cure.
The trenchless machine uses an electro-optically-controlled blade
mounted vertically. As the blade is moved along the route, the blade forces
a passage in the ground. An expander on the lower trailing edge opens a
tunnel in the soil. The guidance system operates with a precision which sat-
isfies tolerances of 0.25 in. (0.6 cm) in grade and 1 in. (2.5 cm) in align-
ment, in reaches of 800 ft (244 m). The automation control method maintains
depth, gradient and alignment with high accuracy. The automatic control
feature reduces the possibility of human error. The nature of the guidance
system and the blade design provides for grade control without the minor
grade variation involved in conventional trenching. This grade control is
especially important when laying the more flexible plastic pipe.
In traditional trenching the pipe bedding may be rough and uneven
requiring additional work and materials for stabilization. Varying compac-
tion levels in the backfill may produce uneven loadings which could be com-
plicated by slippage loads, reversed arch effects, gravity and surface loads
transmitted directly to the pipe and concentrated shear and point loads. The
most careful trench bedding practices may be insufficient to insure proper
bedding in high water table areas or in unstable subsoils.
The trenchless system places the pipe in a close fitting circular bed.
The expander closes the top of the channel so that the surrounding soil
becomes homogeneous. Even distribution of the radial pressure results, elim-
inating excessive loads in any direction. During the operation, the tunnel
wall is thoroughly compacted and the prospect of abrasion from rock or stone
during installation is minimized.
The ground surface disturbance is minimal and no backfilling is re-
quired beyond restoring the single access trench. A "pull-in" is made, start-
ing from a short access trench. When possible, the "pull-in" is made toward
shallower grade to minimize the power required and strain on the pipe. While
the equipment is capable of handling lengths up to 1,200 ft (366 m), the
piping layout at the demonstration site used continuous lengths of approxi-
mately 800 ft (244 m). Thus, conventional trenching was required only at
manholes and lateral connections.
A short time is required for pipe laying. In one demonstration, 10
minutes was needed for the entire "plow-in" of 400 ft (122 m) of sewer. Setup
of the optical control system took about 15 to 25 minutes. Since virtually
no backfilling is required, the only time required is to run a bulldozer over
the trench path for compaction.
Upon approval of the test section by the Chief Construction Inspector,
an additional 4.7 mi (7.6 km) of sewer will be put in place using the trench-
less technique.
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It is claimed that the trenchless system should:
1. Reduce construction costs by an estimated 29 percent when
compared to conventional methods of construction.
2. Provide a system which includes better grade maintenance,
make-up of pipe joints above ground and curing for 24 hours
before being disturbed, better joints.
3. Reduce construction time, minimizing interference with the
residents and traffic flow.
4. Reduce areas of disturbance and costs of reconstruction and
paving.
5. Lower workers' exposure to hazards of trench construction.
6. Allow a longer construction season, especially in areas
where disruption cannot be tolerated.
Among the major limitations on the use of trenchless installations is
the size of the pipe which can be handled, and the need to be first in con-
struction, because other utilities must be placed after sewer installation.
SEWER BEDDING
The Gulf Coast area has many places with high water tables and deltas
or alluvial soils. Infiltration of sewer lines has presented particular prob-
lems in such areas.
Tulane University (16)conducted a study of sewer bedding practices
for the region. Infiltration studies were conducted in 1962, 1963 and 1970.
The results, when compared, varied from slight increase to decreases in
infiltration. The decreases were attributed to soil and grease clogging the
points of entry. Three and one-half percent of the manholes were found to
be experiencing infiltration at the time of inspection.
It was concluded that poor construction techniques were the major
cause of the infiltration problem. Bedding material has been generally used
only for providing uniform support for the pipe and distributing the load
over the entire length of the pipe in order to reduce the possibility of
structural failures. The Tulane report suggested that an additional function
should be to impede the flow of groundwater along the pipe trench.
The report recommended that in high groundwater areas where graded
materials are used for bedding, bentonite or other expanding materials, or
sand and Portland cement be mixed with the bedding material.
Major observations were made on the extent of line settlement. Field
findings could not be duplicated in the laboratory, emphasizing the need for
well-controlled field construction practices. A major problem in sewer lines
was found at points of infiltration where grease introduced from kitchen
food-waste grinders tended to coagulate upon contact with the cooler ground-
water.
The importance of flexible joints for use in areas with poor bedding
characteristics was also shown. A moulded joint sewer pipe was tested under
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a head of 30 ft (9.1 m) of water and a deflection angle of 10.9 degrees
before leakage was observed.
SEALING
Evaluation of Sealing
The Montgomery County Sanitary Department, Dayton, Ohio, conducted a
sealing program for small-diameter lines beginning in 1957. In 1968, a
program was initiated to investigate the effects of infiltration reduction
by joint sealing and to study closed-circuit TV techniques(17)• The report
documented the difficulties in performing an evaluation, the techniques used,
and the limited usefulness of the conclusions. Information concerning cost,
equipment, procedures and organization was given in detail.
For the County system it was concluded that inflow was the major
source of extraneous flow, so much so that the effect of joint sealing was
obscured.
The use of a flow-through packer design was found useful in elimina-
ting the pumping of sewage around the section being sealed.
Rapid rises in the groundwater table were observed after sufficient
precipitation had saturated the soil. Therefore, the flow from pipe line
leaks may- be much higher during periods of precipitation than noted during
the time of inspections.
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REFERENCES
1. American Public Works Association. Prevention and Correction of
Excessive Infiltration and Inflow into Sewer Systems - A Manual
of Practice. EPA Report No. 11022 EFF 01/71 (NTIS-PB 203 208),
U.S. Environmental Protection Agency, Cincinnati, Ohio, 1971.
131 pp.
2. Kaiser, Donald R. Modern Sewage Treatment Plants - How Much Do
They Cost? NTIS PHS No. 1229, Public Health Service, Division
of Water Supply and Pollution Control, U.S. Department of Health,
Education and Welfare, Washington, D.C., 1964. 43 pp.
3. Smith, R. and W. F. McMichael. Cost and Performance Estimates for
Tertiary Wastewater Treating Processes. Report No. TWRC-9, Federal
Water Pollution Control Administration, U.S. Department of the
Interior, Cincinnati, Ohio, June, 1969.
4. Rowan, P. P., K. L. Jenkins, and D. H. Howells. Estimating Sewage
Treatment Plant Operation and Maintenance Costs. Journal of Water
Pollution Control Federation, Vol. 23, Feb., 1961.
5. Grant, E. L., and W. G. Ireson. Principles of Engineering Economy.
Ronald Press, New York, 1970.
6. Bailey, James R., Richard J. Benoit, John L. Dodson, James M. Robb,
and Harold Wallman. A Study of Flow Reduction and Treatment of
Wastewater from Households. Water Pollution Control Research Series
11050 FKE 12/69, December, 1969. 168 pp. .
7. Cohen, Sheldon and Harold Wallman. Demonstration of Waste Flow
Reduction from Households. EPA-670/2-74-071, U.S. Environmental
Protection Agency, National Environmental Research Center, Office
of Research and Development, September, 1974. 103 pp.
8. Bostian, H. E., S. Cohen, and H. Wallman. Saving Water in the Home.
In: 7th International Water Quality Symposium, Washington, D.C.,
April, 1974.
9. Lager, J. A., and W. G. Smith. Urban Stormwater Management and
Technology: An Assessment. EPA-670/2-74-040 (NTIS No. PB 240 687),
U.S. Environmental Protection Agency, May, 1974.
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REFERENCES (Cont'd.)
10. American Public Works Association. Combined Sewer Regulator
Overflow Facilities. Water Pollution Control Research Series
11022 DMU 07/70 (NTIS No. PB 214 469), U.S. Environmental Protection
Agency, Cincinnati, Ohio, 1970. 139 pp.
11. Freeman Report. Types of Backflow Prevention Devices Used in Large-
Scale Water and Wastewater Systems. In: Massachusetts Committee on
Charles River Dam, Boston, Massachusetts, 1903. 190 pp.
12. Nagler, F. A. Hydraulic Tests of Calco Automatic Drainage Gates.
State University of Iowa, 1923. Excerpted in: Combined Sludge
Gate Catalog, USEPA 11022 DMU 08/70, Section 9, 1971.
13. The Western Company of North America. Heat Shrinkable Tubing as
Sexier Pipe Joints. Water Pollution Control Research Series 11024
FLY 06/71. Water Quality Office, U.S. Environmental Protection
Agency, June, 1971.
14. Southwest Research Institute. Impregnation of Concrete Pipe.
Water Pollution Control Research Series 11024 EQE 06/71. Water
Quality Office, U.S. Environmental Protection Agency, June, 1971.
15. Masters, Hugh E. Field Trip Report to Bethany Beach, Sussex County,
Delaware. USEPA Grant No. S-800690, September 27, 1974.
16. Mayer, John K., F. W. Macdonald, and S. E. Steimle. Sewer Bedding
and Infiltration Gulf Coast Area. EPA-11022 DEI 05/72, U.S. Environ-
mental Protection Agency, May, 1972.
17. Cronk, G. E. Ground Water Infiltration and Internal Sealings of
Sanitary Sewers, Montgomery County, Ohio. EPA-11020 DHO 06/72
(NTIS - PB 212 267), U.S. Environmental Protection Agency, June, 1972,
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing/
1. REPORT NO.
EPA-600/2-77-017a
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
ECONOMIC ANALYSIS, ROOT CONTROL, AND BACKWATER FLOW
CONTROL AS RELATED TO INFILTRATION/INFLOW CONTROL
5. REPORT DATE
December 1977
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S) '
Richard H. Sullivan, Robert S. Gemmell,
Lawrence A. Schafer, and William D. Hurst
8. PERFORMING ORGANIZATION REPORT NO
3. PERFORMING ORGANIZATION NAME AND ADDRE
American Public Works Association
1313 East 60th Street
Chicago, Illinois 60637
10. PROGRAM ELEMENT NO.
1BC611, SOS #1, TASK #33
1. CONTRACT/GRANT NO.
803151
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory—Gin.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final (1974 to July 1976)
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Anthony N. Tafuri
(201).321-6679
8-340-6679
. B TRACTA stu
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