EPA/600/2-85/020
March 1985
SELECTED TOPICS RELATED TO INFILTRATION AND INFLOW IN SEWER SYSTEMS
Richard H. Sullivan
James W. Ewing II
American Public Works Association
Research Foundation
Chicago, Illinois 60637
Cooperative Agreement No. CR 808934-01
Project Officer
Carl A. Brunner
Wastewater Research Division
Water Engineering Researcn Laboratory
Ci nci nnati , Ohi o 45268
WATER ENGINEERING RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI. OHIO 45?fi8
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DISCLAIMER
The information in this document has been funded wolly or in part by the
United States Environmental Protection Agency under assistance agreement
number CR808934 to the American Public Works Association. It has been subject
to the Agency's peer and administrative review, and it nas been approved for
publication as an EPA document. Mention of trade names or commercial products
does not constitute endoresement or recommendation for use.
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FOREWORD
The U.S. Environmental Protection Agency is charged by Congress witn
protecting the Nation's land, air, and water systems. Under a mandate of
national environmental laws, the agency strives to formulate and imple-
ment actions leading to a compatible balance between human activities and
the ability of natural systems to support and nurture life. The Clean
Water Act, the Safe Drinking Water Act, and tne Toxics Substances Control
Act are three of the major congressional laws that provide tne framework
for restoring and maintaining the integrity of our Nation's water, for
preserving and enhancing the water we drink, and for protecting the
environment from toxic substances. These laws direct the EPA to perform
research to define our environmental problems, measure the impacts, and
search for solutions.
The Water Engineering Research Laboratory is that component of EPA's
Research and Development program concerned with preventing, treating and
managing municipal and industrial wastewater discharges; establishing
practices to control and remove contaminants from drinking water and to
prevent its deterioration during storage and distribution; and assessing
the nature and controllability of releases of toxic substances to the
air, water, and land from manufacturing processes and subsequent product
uses. This publication is one of tne products of that research and
provides a vital communication link between the researcher and the user
communi ty.
Control of Infiltration/Inflow has become a major early step in
reducing the amount of untreated or poorly treated discharges of munici-
pal sewage to receiving waters. This report has been prepared to provide
information for those concerned with identifying and controlling Infiltra-
tion/Inflow.
Francis T. Mayo, Director
Water Engineering Research Laboratory
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ABSTRACT
This project was undertaken as a review of the current state-of-the-art
in infiltration/inflow control and to present information not included in
earlier manuals and reports on this subject. A series of nine regional sem-
inars was conducted to explore local problems and approaches for solution.
Chapters in this report respond to problem areas discussed at these seminars.
Besides an overview, the report includes information on problem determi nati or.
as approached by the Washington Suburban Sanitary Commission, methods for
flow determination including a discussion of accuracy, economics of sewer
rehabilitation, methods of rehabi1itation, long-term rehabilitation programs,
and long-term flow monitoring. A brief discussion is included of tne major
problems discussed at the regional seminars.
This report was submitted in fulfillment of Cooperative Agreement No.
CR808934-01 by the American Public Works Association Research Foundation
under the sponsorship of the U.S. Environmental Protection Agency. This
report covers a period from June 1982 to November 1984, and work was com-
pleted as of November 1984.
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TABLE OF CONTENTS
Foreword iii
Abstract iv
Chapter 1 Overview 1
Chapter 2 Threshold of Problem Determination 5
Flow Monitoring 6
Physical Survey 8
Internal Inspection 9
Cost-Effective Analysis 9
Summary 10
Migration 11
Single Point vs Key Manhole Monitoring 12
Monitor Spacing 14
Intensified Smoke Testing 14
Illegal Connections to House Laterals 17
Chapter 3 Flow Determination 22
Methods 22
Direct Measurement 22
The Dipper Monitor 22
The Bubbler 23
Locations 24
Dye Di 1 uti on 24
Electronic Devices 25
Velocity Meters 26
House Laterals 26
Pilot Program Rehabilitating House Laterals 27
Accuracy 29
Summary Comments 36
Chapter 4 Rehabilitation Methods 37
Spot Repair 37
In Place Replacement 39
Alternate Location Replacement 39
Chemical Grouting 40
Sliplining 41
Inversion Lining 42
Chapter 5 Economics of Rehabilitation 45
General Conditions 45
Rehabi1itation Techniques 48
Chapter 6 Long-Term Rehabi1itation Program 61
Inspection 61
Maintenance 62
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Chapter 7 Long-Term Flow Monitoring 63
Pre-Post Comparison 64
Long-Term Flow Monitoring After Sanitary
Sewer Rehabilitation 64
Flow Monitoring Location Selection 65
Flow Monitoring 65
Chapter 8 Regional Site Visits 69
120 GPCD as Guideline 69
House Lateral Construction Problems 69
House Lateral Contributions 70
House Lateral Rehabi1itation Tecnniques 70
Single Point vs Multi-Point Monitoring 71
Illegal Connections 72
Cross Connection 72
Mannole Inflow 73
Migration of Groundwater 73
Cost Effective I/I Removal 73
New Development Standards 74
Improve Building Lateral Access 74
Bibliography 75
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CHAPTER 1
OVERVIEW
The United States Environmental Protection Agency (USEPA), in the
interest of providing timely information to public agencies, awarded a
grant to the American Public Works Association (APWA) to review the
state-of-the-art of Infiltration /Inflow (i/l) detection and control as
well as sewer rehabilitation. Previous manuals prepared by the APWA and
Conklin detailed the state-of-the-art as of the time of this preparation.
However, in the years intervening, public agencies and consultants have
developed a body of information concerning problems and practices
associated with I/l control which have not been widely reported.
At the same time, the USEPA has acted to place the responsibility for-
conducting, implementing, and obtaining the results from i/l studies upon
local public agencies. Thus, this report is aimed at providing local
officials with developments since previous manuals were published.
A key concept inherent in this report is the need for local agencies to
initiate an ongoing preventive maintenance program. This is necessary to
maintain the minimum level of extraneous water flows. A sewer system
cannot be rehabilitated on a one-time basis and be expected not to develop
additional points of infiltration or inflow. One must keep in mind that
multiple factors may be responsible for various types of defects which
allow i/l to enter the system.
In using this report, it is essential to define the terms associated with
i/l. For easy reference the terms are defined as follows:
"INFILTRATION" — the volume of groundwater entering sewers and
building sewer connections from the soil, through defective
joints, broken or cracked pipe, improper connections, manhole
walls, etc.
"INFLOW" — the volume of any kind of water discharged into
sewer lines from such sources as roof leaders, cellar and yard
area drains, foundation drains, commercial and industrial "clean
water" discharges, drains from springs and swampy areas, etc.
It is distinguished from infiltration.
"INFILTRATION/INFLOW" (i/l) — the volume of both infiltration
water and inflow water found in existing sewer systems. Since
the two sources of extraneous waters are practically
indistinguishable, it is impossible to determine the amounts of
either.
"SEWER SYSTEMS EVALUATION STUDY" (SSES) — a process which
follows the analysis stage and the recommendations based on the
analysis process. The purpose is confirmation of overall
findings gleaned from analyses of the program and conversion of
preliminary diagnoses into firm conclusions as to the presence,
location, and degree of I/l. SSES also determines what i/l
intrusion is excessive in keeping conformity with criteria
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stated in PL 92-500 and USEPA rules and regulations.
Over the past years, several manuals and reports have been written
concerning I/I problems in sanitary sewer lines. This report does not
replace earlier literature, but rather should be used in conjunction with
the previous writings since it attempts to update the state-of-the-art
and to introduce some newer techniques being used in the field, not only
to rehabilitate the sewer systems, but also to evaluate systems and to
certify the thoroughness of rehabilitation.
The report presents a review of the current state-of-the-art. Since only
a review has been conducted, the major part of this report is intended
for the practitioner with knowledge and experience of the subject area.
To attain an overview of the national status of I/I detection, a series
of nine regional seminars was conducted. Local experts and interested
practitioners were invited to one-day sessions to explore local experi-
ence and practice. Participants discussed the solutions to problems
they were experiencing. APWA then used those discussions as the
starting point for the publication of the report.
At these regional meetings, APWA attempted to glean from participants
what they considered to be their major regional problems, their best
regional solutions and similar information on local I/I problems. Along
with the regional problems, specific areas such as house laterals, illegal
connections, groundwater migration, and similar problems were addressed.
Although these problems have been discussed many times in the past, little
research has been done in these areas nor has material been published on
them.
The nine regional meetings highlighted the diversity of problems
encountered by public agencies in various parts of the country. Because
of physical conditions which exist, such as age of the system, local
practices, soil and groundwater conditions it is apparent that national
guidelines must be broad to allow for local needs.
In addition, the regional seminars provided an extensive review of the
USEPA regulations governing I/I studies (0S-82). The review determined
many potential trouble spots for local agencies in adhering to the
guideline. Detailed comments were offered to USEPA.
Those who attended the seminars also identified two agencies which are
conducting extensive local programs for i/l control: Washington Suburban
Sanitary Commission (WSSC) of the City of Hyattsville, MD and the City of
Salem, OR.
This report is based on comments made at the regional meetings. Chapters
were developed to cover the major points discussed. For example, Chapter
2 describes the program of the Washington Suburban Sanitary Commission, a
very comprehensive attempt to overcome many of the identifiable failings
of the standard Sewer System Evaluation Study program. Under the
program, internal inspection is scheduled to allow final rehabilitation
recommendations by Autumn, 1985* The approach being followed is
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designated as the "Systems Approach To Sewer System Evaluation." The
"systems" approach in turn is based on the concept that collection
systems have "individual personalities." Since certain system
personalities behave similarly, personalities can generally be compared
and defined where pipe type, joint material, and other accepted grantee
compliance requirements are similar.
An accelerated pilot study of the "systems" approach to subarea
rehabilitation has been approved by the local regulatory agency. The
results of the study should be available at approximately the time the
Sewer System Evaluation is completed.
Chapter 2 contains an analytical listing of the various methods,
including smoke testing, used to determine if i/l should be considered
excessive.
Chapter 3 describes the difficulties in determining precise flow in
leaking house laterals. Such difficulties are the result of the
inaccessibility of the point of discharge from the house lateral to the
street sewer. The care to be taken to minimize the error inherent in
each item of measurement is discussed in detail. In addition the chapter
reviews interpretation of monitoring data and the potential range of
error in discharge data. The chapter also reviews the range of error
experienced in flow monitoring together with an error analysis scheme for
estimating energy slope. The error in the parameters for various flow
estimating formulae is also covered.
Chapter 4- reviews the economics of rehabilitation with the aim of aiding
the investigator in making decisions by discussing the problems
associated with current rehabilitation practices. One of the easiest of
the rehabilitation techniques available on the market — sealing and
grouting — is discussed. Associated problems arising from this
technique, traffic control, mobilization, and cleaning
are presented in detail. Similarly, the additional techniques are
discussed with the view of what is needed to economically evaluate a
rehabilitation project.
Chapter 5 is an evaluation of six basic techniques involved in
rehabilitation processes for sewer mains. A decision tree outlining
steps required for various methods of sewer rehabilitation is included.
This chapter examines briefly these techniques and discusses the positive
and negative aspects of the use of each. Processes reviewed are grouped
under the following classifications:
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1) Spot Repair
2) Reconstruction in Place
3) Construction of a Bypass
4) Grouting
5) Sliplining
6) Inversion Lining
The chapter also includes a flowchart which presents guidelines for the
use of each technique.
Chapter 6 lists the materials and workmanship utilized in the inspection
and maintenance processes of sewer systems. The inspection process
discussed consists of two phases: cataloging through inventory and
inspection by television surveillance. Also discussed are the two phases
comprising the maintenance process: the repair of deteriorated sections
and cleaning of the lines to allow free flow.
Chapter 7 separates the phases of long-term flow monitoring into three
sections: preparation of location, purchase and installation of
equipment, and evaluation of the project before and after completion.
The chapter further discusses how the evaluation of the success or
failure of a rehabilitation project should consider several factors:
prior moisture conditions, flow vs. rainfall, flow vs. intensity, flow
vs. duration, and existing conditions.
Chapter 8 summarizes comments of the participants at the various meetings
held by APWA for the project. Topics summaried:
House Lateral Construction Problems
House Lateral Contributions
House Lateral Rehabilitation Techniques
Single Point Vs. Multi-Point Monitoring
Illegal Connections
Cross Connection
Manhole Inflow
Groundwater Migration
Cost Effective i/l Removal
New Development Standards
Improved Building Lateral Access
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CHAPTER 2
THRESHOLD OF PROBLEM DETERMINATION
"Systems" Approach To Sewer System Evaluation
Surveys and Rehabilitation
From its inception in the Clean Water Act in 1972, the Infiltration/inflow
(i/l) program has been developing. Studies conducted during its infancy
produced unreasonable expectations of rehabilitation effectiveness and i/l
reductions. Increasing awareness of the limitations of the original
requirements have led to minor revisions in the format, i.e., PRM 78-10,
permitting concurrent television and joint testing and sealing, in 1983
publishing of interim final Construction Grant rules followed in 1984 by
final rules resulted in simplification of the approach dealing with I/I,
and the imposing upon Construction Grant recipients of a requirement
that effectiveness of control be certified.
Upon reviewing a number of completed Sewer System Evaluation Surveys
(SSES), Conklin and others found that problems inherent in the
surveys included:little recognition of the building sewer i/l
contribution; inadequate flow measurements and assessmentsj unrealistic
reduction expectations; imprecise cost-effective elements; impractical
flow peaking factorsj and minimal understanding of apparent ground water
migration effects. Despite the problems, surveys have been continued,
many in school book fashion, to satisfy i/l study requirements and to
maintain other funding eligibilities.
In an effort to salvage what is perceived as a less than satisfactory situation,
EPA has formally shifted responsibility for success to the grant recipient. The
1981 Clean Water Act amendments specify that i/l analyses and SSES work
fall lander the performance certification requirements which call for
postrehabilitation compliance monitoring. The impact and interpretation
of these requirements are still unclear. However, one fact is
unmistakable; studies cannot be performed the same way they were done
prior to the revised regulations. Modifications must be made to fulfill
the certification requirements.
Many factors are involved for an agency to determine that a rate of i/l is
excessive for their system. At present there are three methods for
dealing with I/l.
The first is total removal of all I/l sources within the system, which in
most cases will result in excessive costs. The second method is to define
only those areas which contribute the major portion of I/l and limit cost
effective repairs to those areas. The third method would be to disregard
completely all I/l and simply transport and treat, at the wastewater
treatment plant, all materials entering the sewer system.
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In Construction Grant, 1982 Interim Final Report (CG-82), EPA suggests
that a flow greater than 120 gal./capita/day (454 1/c/d) for the entire
system would be that flow which EPA would consider to be cost effective
in further evaluating the economics of removal of the major I/I sources.
The 120 GPCD (454 l/c/d) figure used in CG-82 is based on an average
sewage flow of 70 GPCD (265 1/c/d), and an average infiltration
rate of 50 GPCD (189 1/c/d). The EPA flow rate is based on a single
point measurement of the system at the entrance to the treatment plant
and thus, reflects averaging of the overall system as to both infiltration and
flow contribution of sanitary waste. One limitation of this technique is
that it does not differentiate the intact areas of the sewer system which
require no renovation and rehabilitation from the extremely poor
areas which would benefit from rehabilitation.
The only way to accomplish such a differentiation would be to use the area
monitoring system. Here, the entire sewage system is divided into
subsystems. Subsystems are discrete contributing areas of the entire
sewer system network and can be isolated and adequately monitored by
placing monitoring devices in key manholes. The subsystem could be an
entire drainage area or it could be nothing more than a subdrainage area
of a larger drainage basin. The size of the area would not be as
important as the fact that a flow measurement can be obtained for an area
that has a reasonably consistent structural condition throughout.
Several years ago, the Washington Suburban Sanitary Commission (WSSC)
recognized the shortcomings inherent in the standard method for SSES's and
proposed a "systems" approach of study for the last significant SSES in
the WSSC service area. A variety of departures from the established
procedures were necessary to combat what they felt were existing
deficiencies. The procedure followed included:
o Flow Monitoring
Subarea Long-Term Monitoring
Minisystem Temporary Monitoring
Subsystem Instantaneous Monitoring
o Physical Survey by Subarea
o Internal Inspection by Subsystem
o Cost-Effective Analysis by Subarea
FLOW MONITORING
Under current regulations the primary emphasis is for certification of the
i/l reduction following sewerline rehabilitation. Host current studies
cannot meet this requirement because of the limited amount of flow
monitoring during the SSES. Any comparison of the system before ajid after
rehabilitation would be apples and oranges and therefore not truly
representative because of fluctuating water tables, differing
precipitation activity, continued system deterioration, and many other
factors.
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Treatment plant monitoring has been emphasized, both in the past and
presently, in the current compliance requirements. In many cases, this
requirement is both impractical and imprudent. Like other large
utilities, e part of the WSSC system discharges to pump stations of 0.1
to 105 MGD (0.3 - 397 mLD) capacity and is eventually conveyed to local
and regional treatment plants of 1 to 300 MGD (3-7 to 1136 MLD) capacity.
The rest of the system is collected by gravity to the plant with only few
intermediate flow monitoring points in operation prior to 1981. Trunk
sewer and treatment plant flow monitoring were found to be too gross a
scale to be a valid indicator and recorder of i/l subarea problems.
Subarea Long-Term Monitoring
In the current SSES study by WSSC, long-term monitors have been
established on 29 subareas within the 284 sq mi (456 sq km) of the study
area. These subareas range in size from approximately 2.84 (4-5 km)
to 16.1 mi (25.8 km) of sewer line. The range of collection system sizes
stems partly from topographical subarea definition and from experimental
pursuit of the ideal study size. Monitors are placed on the subarea
outlet sewers prior to their junction with the interceptors. This
permits more accurate monitoring ar.d greater control of the flow
calibrations than in trunk sewer metering.
The subarea monitors were in place a full eight months prior to
subdividing the collection system into smaller minisystems, thus
providing a more complete data base upon which excessive flow
determinations could be made. This is the initial step in computing base
flows for the postrehabilitation compliance requirement, a significant
deviation from the "snapshot" approach to flow monitoring of earlier
studies.
Such monitors are vital to the execution of the work after the initial
determination of excessive i/l. Since they remain in place throughout the
SSES and rehabilitation, these monitors provide a pulse for the system and
give the earliest symptoms of significant changes in the subarea. Thus,
they are ideal in checking the rehabilitation reductions and supplying a
means of tracking system deterioration after the SSES. The WSSC subarea
monitors will remain in place in those systems that do not have excessive
i/l to serve as experimental controls. Ideally, after establishing the
points of data collection, the monitors would be mobile from system to
system for some extended period — for instance, biannually, for use in
an ongoing maintenance program.
Minisystem Temporary Monitoring
After a subarea has been found to have excessive I/l conditions, temporary
monitoring of the minisystem is used to narrow the search for localized
defects.
Depending on topographic constraints, these minisystems have averaged
between 3,000 and 6,000 ft (914 - 1829 m) in size. When deployed in a
minisystem, the long-term subarea outlet meter provides a checks and
balances control against which the sum total of the temporary monitors
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can be evaluated. The meters were used for 21 days during the highest
ground water historical month. Since an extended data base exists at the
subarea long-term monitor, ground water activity can be reviewed as well
as the infiltration levels recorded at these sites to provide further
indications of optimal deployment timeframes.
Subsystem Instantaneous Metering
In the traditional SSES, instantaneous measurements took the form of
selectively dissecting a minisystem into line sections and performing
night isolation and measurement on each. Line sections exceeding a
minimum flow value were recommended for internal inspection. This
process has led to limited focus of the problem and fragmented the field
work to the point where it invites migration effects. Migration occurs when
groundwater which is excluded from a sewer by repair at one point flows outside
the pipe to an unrepaired location and enters the sewer at the new location.
From all indications, migration will be a factor in adjacent line sections but
will not transfer from one minisystem to another, i.e., hydraulic gradients
governing the ground water flow will not be shifted dramatically enough to make
system transfer possible. Any analysis to combat migration must then be made at
this subsystem stage of the survey.
To try to prevent this migration phenomenon, subsystem instantaneous
measurements for WSSC took the form of outlet and junctions (O&Js), so
designated because they are typically performed at outlet and junction
manholes within the minisystem. The working parameter has been
approximately 1,000 ft (305 m) of sewer, 4 average line sections,
measured per 0&J. This subsystem block, when evaluated for I/I
excessiveness is the basis for the internal inspection recommendation.
By inspecting and evaluating the subsystem blocks rather than unrelated,
fragmented, individual line sections, the impact of adjacent, but
nonleaking, defects can better be evaluated to preclude migration in any
rehabilitation recommendations.
PHYSICAL SURVEY
Under this system, manhole inspection and visual lamping are pursued for
all subareas considered excessive infiltration/inflow. The subarea size
was selected for this activity for several reasons. Manhole inspection
is an extremely efficient way to evaluate the visible and accessible part
of the collection system. Cost-efficiencies permit this activity to be
done on a larger scale than other evaluation techniques. It also has
been shown that manhole grouting for infiltration reduction and manhole
frame sealing for inflow elimination are two of the most cost-effective
corrections that can be made.
Rainfall simulation, smoke testing and selective dyed water flooding are
utilized in the traditional sense. Improved testing techniques are
employed, but the approach is essentially unchanged. Euilding
inspections may be employed when other activities do not fully account
for the level of inflow measured.
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INTERNAL INSPECTION
The only significant change used by WSSC to internal inspection is that
subsystem blocks, rather than isolated line sections, are inspected during
the high ground water period. All defects are inventoried and evaluated
for their impact on effects of migration. Leaking and nonleaking defects
will carry similar weight when reviewing these areas for rehabilitation.
COST-EFFECTIVE ANALYSIS
Of all the components of the SSES, this step is the most abstract and,
therefore, perhaps the hardest to properly assess. The cost-effective
analysis (CEA) combines treatment plant costs, collection system transport
costs and relief/replacement sewer options, to form a present worth
analysis. The CEA is then computed for specific point source repairs
taking into account the infiltration quantity at the source and a
percentage of effectiveness of the repair. This tunnel vision system of
correcting only significantly leaking defects found during an internal
inspection on one day of one year may result only in perpetuating the
failure of i/l programs.
All rehabilitation determinations in the innovative SSES by WSSC will be
made with respect to the subarea meter. The subarea meter is the focal
point for the amount of infiltration to be used in the subarea CEA. The
subarea meter becomes the control for evaluating flows and making
rehabilitation recommendations. The subarea meter also becomes the means
of postrehabilitation compliance certification.
Reviews of past i/l studies contain overly optimistic estimates of flow
reduction. Apparently, past estimates for I/l reduction have been made
on the basis of 60-100 percent reductions in the areas which are being
repaired. The investigation would pinpoint areas of severe infiltration
and a project would be implemented to correct these problems. No thought
was given to the phenomenon of migration and its effect on adjacent
sections. As a result, large leaks were plugged and small leaks ignored.
Migration of ground water allowed the infiltration to enter through the
small leaks. The net result was a very small decrease in overall I/l.
These same reviews indicate that a more realistic assessment might be in
the 30 percent reduction range.
Keeping this in mind, the proposed CEA will utilize a 30 percent
reduction as a target factor for use in the cost-effective equation.
Infiltration will be measured at the subarea meter. Treatment, transport,
and relief components will be factored into the analysis and then the
overall effectiveness of the rehabilitation program will be projected.
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The results were used to reveal the estimated amount of money which could
be cost-effectively spent to make the system corrections necessary for a
30 percent reduction in the excessive infiltration and inflow.
If the expenditure needed to correct those defects in the subarea to
effect a 30 percent infiltration reduction is less than the amount
computed, then repairs can begin.
Judgment is a factor in applying this procedure. The progression
(thread) of flow measurements from subarea metering to minisystem
temporary monitoring to subsystem outlet and junctions provides the flow
measurements necessary to form a judgment. Internal inspection of
subsystem blocks rather than isolated line sections provides the
inventory of leaking and nonleaking defects needed to properly assess the
rehabilitation potential to help preclude migration.
This concept has immense potential. In past studies at WSSC, it's been
found that collection systems have individual "personalities." Certain
system personalities behave in similar ways. Personalities can generally
be defined where pipe type, joint material, and other accepted grantee
compliance requirements are similar.
Test and seal pilot studies in the WSSC service area found, for instance,
that standard strength vitrified clay with jute and mortar/GK compound
joints built before 1945 had an extremely high air test failure rate.
When these collection systems are encountered, air testing is eliminated
and sealing proceeds on each joint of the affected line section. By
extrapolating such system responses and evaluations to all the components
involved (pipe, construction practices, types of connection, building
sewer material, joint material, age, location in system, topography,
infiltration rates existing and potential, manhole construction, etc.),
it is possible to make a judgment on the CEA. These judgments must be
used wisely as the subarea monitoring also serves as the
postrehabilitation flow reduction compliance check. Greater flexibility
in the CZA must be tempered with the responsibility to effect verifiable
reductions required of the grantee.
SUMMARY
The WSSC is involved in one of the most comprehensive attempts to overcome
many of the identified shortfalls of the standard SSES program. Subarea
monitoring began in the Spring, 1992. Minisystem and subsystem flow
activities were completed in the Spring of 1983. Inflow identification
was completed during the Summer and Fall, 1983. Internal inspection will
proceed during Spring, 1984 with the final rehabilitation recommendations
due in Fall, 1935.
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An accelerated pilot study of the "systems" approach to subarea
rehabilitation has been approved by the local regulatory agency. Several
subareas will be selected from the SSES study area and rehabilitation
will be implemented on the subarea basis. Results are due at
approximately the time of completion for the rest of the SSES.
This innovative "systems" approach to executing the SSES is not the last
step. Already, improvements and modifications have been suggested.
Perhaps the most valuable aspect of the systems "approach" is in
suggesting an alternative procedure which meets the grantee compliance
aspects required in current legislation and which provides the
flexibility to achieve the desired reductions.
MIGRATION
Service laterals typically contribute a good deal of both infiltration and
inflow to a sanitary sewer system. A service lateral source may be
located in the connection of the lateral to the sewer main and along the
lateral's pipe barrel and joints. Service laterals also transport
virtually all of the inflow that originates in the private sector.
Overview
The determination of the rate of infiltration from service laterals is
generally done in the same way as for sources in the sewer main by using
visual measurement based on internal inspection. The determination of
inflow rates is more direct using internal inspection together with
dye-water flooding with flow measurements at the downstream manhole. The
inflow measured for the segment is assigned only to sources that result
in a positive test. Interpretation of data from the internal inspection
of service laterals is more difficult than from other sources because it
is often not obvious whether the source is located at the sewer main
connection or whether it originates along the service lateral pipeline.
The uncertainty regarding the location of the infiltration or inflow
source in a service lateral, combined with the inherent cost of
excavation, have made repairs to service laterals generally less
cost-effective than rehabilitation of other sources. Even when
rehabilitation is successful at the source, migration to other defects in
the sewer main and laterals has resulted in limited overall flow
reduction. Findings from a recent migration study seem to indicate that
more effective rehabilitation of service laterals can be achieved.
Migration from rehabilitated sources was recently studied in detail for
two typical residential sites in a combined effort by the Washington
Suburban Sanitary Commission and RJN Environmental Associates, Inc.
Segment by segment infiltration was measured by plugging and weiring.
Ground water was monitored from observation wells installed for the
study. Each site contained approximately twelve segments and twenty
wells. Data was collected semiweekly for four months.
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The rehabilitation results obtained during the monitoring period indicate
that localized "mounding" of ground water in the sewer trench occurring at
rehabilitated points can increase but can be partially relieved by
unrehabilitated sources. If there are none nearby, then ground water
relief is achieved by dissipation to the surrounding soil.
Service lateral sources tend to be at a higher point than sources
impacting only the main. Migration trends, therefore, cause an
infiltrating sewer main defect to flow more due to the increase in
hydrostatic head. Where there is a significant rise in ground water
level, a normally dry service lateral may begin to contribute I/I after
local rehabilitation occurs. As a result, after rehabilitation has been
performed nearby, the possibility of a service lateral contributing i/l
cannot be ruled out simply because it was inactive during the internal
inspection prior to rehabilitation of the main.
Migration may be reduced and rehabilitation effectiveness increased if the
unrehabilitated sources are not near those being rehabilitated, and if the
permeability level of the surrounding soil is reasonably near to the
permeability level of the trench. If this is not the case, "mounded"
ground water will more easily travel in the trench and migrate farther to
unrehabilitated sources, as opposed to seeking relief through immediate
dissipation in the surrounding soil.
The potential for infiltration from seeminclv "drv" service laterals should
determined based on the condition of the connection and other
considerations, such as jointing, pipe materials, methods of
construction, or how similar laterals in the area that are known to be at
or below the water table behave.
SINGLE POINT VS. KEY MANHOLE MONITORING
In general, economics will dictate when single point or key manhole
monitoring should be used.
Single Point Monitoring.
Single point flow monitoring defines the hydraulic characteristics of a
collection system utilizing flow data obtained from a single point,
usually a pump station or treatment facility at the downstream end of the
system. Single point flow monitoring is relatively inexpensive and has
been accepted by the USEPA for preliminary evaluation as to whether the
tributary system has excessive i/l.
If single point monitoring determines that flow is less than 120 GPCD (454
lpcd), the system, by USEPA definition, does not have excessive I/l and no
further study is necessary. If I/l is found to be excessive, additional
evidence is required to show that it is still more expensive to transport
and treat it than remedy the I/l. If this can be shown, then the I/l may
be eligible for USEPA grant money to remove it.
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Single point flow monitoring has been used effectively in relatively small
(less than 100,000 linear feet (30,480 m) collection systems.. In these
systems, where i/l is found to be "excessive," it is generally more
economical to smoke test the entire system and complete the related
follow-up SSES tasks than to break the system into a series of subsystems
by flow monitoring.
The smoke is valuable since it is reasonably inexpensive and easy to use.
The crew to be trained in the operation requires only a technician level
supervisor to obtain credible results in a short period of time.
Key Manhole Monitoring.
Key manhole monitoring breaks the collection system into a series of
definable subsystems. There is some controversy as to length of pipe
within each subsystem to maximize the effects of this type of monitoring
which will be discussed later. The advantages are:
1) Flow monitoring may help to eliminate some of the subsystems
from further study.
2) This type of flow monitoring can set priorities for the
subsystems relative to i/l severity.
3) More data is available to identify hydraulic restrictions.
4) Flow monitoring, if performed regularly, can build a
foundation for a data base which can be used in the future to
evaluate the sewer system as the population increases and the
sewers become older.
5) SSES completion time may be reduced by eliminating subareas
from further study.
6) Better correlation of flow, e.g., leaks found vs. monitored
flow may be obtained.
7) The reaction of each subsystem to the entire system for a
particular storm can be identified; this reaction is
particularly important when rainfall varies in intensity
across the collection system or monitoring area.
8) Peak wet-weather flows are less travel-time dependent, making
the hydrograph easier to interpolate.
9) A more effective maintenance program can be implemented; the
program places emphasis on the subsystems, where most of the
i/l originates.
Some of the disadvantages to key manhole monitoring are:
1) Multipoint monitoring is initially more expensive.
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2) Resources required to monitor and maintain flow monitors
increase in direct proportion to the number of monitors used.
3) Some difficulty can be expected in balancing flows where one
subsystem flows through another, due to travel time, monitor
accuracy, and varying rainfall patterns.
4) Eliminating some subsystems from further study overlooks i/l
point sources in these subsystems that could be eliminated
cost-effectively.
5) Unexpected surcharging of sewers makes flow depth data
useless. Anticipated surcharging can be handled by the use of
appropriate equipment.
Although initially much more expensive than single point flow monitoring,
in most cases key manhole monitoring will be the most economical approach
because time will not have been wasted looking for leaks which do not
exist during a rehabilitation project.
MONITOR SPACING.
There is considerable controversy about appropriate spacing for multiple
monitoring locations. One school recommends intensive real-time
monitoring, claiming that only through detailed, time-dependent
comparison of data from adjacent monitors can field conditions be
correctly interpreted. In addition, when surcharging occurs, two
adjacent depth monitors can be used to establish the difference in water
level between the sewer inverts at monitoring manholes. The head loss
can be used to calculate flow, using a pipe full formula. The
justification for intensive monitoring is that savings in cost of
cleaning and TV inspection in those reaches found to have low i/l more
than offset monitoring costs. The problem is that i/l usually occurs
throughout in varying intensities. One monitoring station for each
10,000 to 20,000 linear feet (3048 - 6096 m) of main and lateral sewers
(excluding service connections) is reasonable for the preliminary survey,
but the final survey should allow about 4,000 feet (1219 m) per monitoring
station.
INTENSIFIED SMOKE TESTING
Like most i/l identification and correction procedures, smoke testing of
sanitary sewers is more complex than originally anticipated. Weather and
ground water conditions are only two of the factors which affect the
results obtained from a smoke testing program. In addition, smoke
testing procedures and the results obtained from it have been found to
vary with the number of blowers, blower spacing, and blower air output.
These items can have significant impact on the identification of ground
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water infiltration and stormwater sources in a sanitary sewer system.
Smoke testing of sanitary sewers has been used as a relatively quick and
inexpensive way to locate inflow sources in a sanitary sewer system.
Smoke is generally blown into the sanitary sewer by a specially designed
gasoline-powered blower. Smoke for the test is generated by either a fuse
lit smoke "bomb" or a smoke canister. Since smoke testing procedures have
generally not been standardized, field procedures have been developed over
the years.
Smoke testing is a reliable method of identifying "direct" inflow sources
such as downspouts, area drains, driveway drains, loading dock drains, and
direct cross-connections between the storm sewers and the sanitary sewer.
Such inflow sources generally have a direct path to the sanitary sewer.
Ideal field procedures and weather conditions are usually not required to
detect these sources.
The ability of smoke testing to identify "indirect" i/l, sources such as
foundation drains, service lateral defects, and indirect storm sewer to
sanitary sewer cross-connections, is not as well documented. These I/l
sources do not always have a "direct" path from the surface to the
sanitary sewer so the smoke must pass through seams in the soil to reach
the surface before it can be detected.
Controlled Field Tests
A section of the City of Des Plaines, IL sanitary sewer system known to
contain several I/l sources was smoke tested by RJN Environmental
Associates, Inc. to evaluate various smoke test procedures. The test
section consisted of nine manholes and approximately 2,100 linear feet
(640 m) of sanitary sewer. All tests were conducted on the same day in
October 1981, when the ground water table was low and the ground surface
was dry. Because the tests were conducted on the same day and also for
the same sewer lines, results could be attributed to each of the field
procedures that was evaluated, including:
1) Number of smoke blowers
2) Spacing of smoke blowers
3) Blower air output
Five different test procedures were evaluated, as shown in Table 1, SMOKE
TEST PROCEDURES EVALUATED. Upstream and downstream lines were sandbagged
and five minute smoke bombs were used during all tests. Test One
involved a single smoke blower set up on the center manhole of two
adjacent sewer line reaches. Smoke was blown both upstream and
downstream from the center manhole with the blower running at full
throttle. Test Two also involved the use of a single blower running at
full throttle, but only one sanitary sewer line reach was tested at a
time with the smoke blown towards the downstream manhole. Test Three
used two blowers with the blowers set up at the upstream and downstrean
manholes of two adjacent sewer line reaches. Smoke was blown upstream
and downstream towards the center manhole with blowers at full throttle.
Test Four used the same set up, but both blowers were set at
approximately half throttle for this test. Test Five was performed with
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the blowers set at full throttle on each manhole so that each individual
sewer segment had smoke blown simultaneously from the upstream and the
downstream manholes.
The location and intensity of smoke reaching the surface was recorded on
field forms for all tests. Results showed that the number of I/I sources
identified depends on the field procedure. Test procedure No. 5
identified most i/l sources and resulted in the most intense smoke
reaching the ground surface, as shown in Table 2,.
I/I sources with a direct flow path to the sanitary sewer, such as
stairway drains, driveway drains, roof leaders, and area drains, were
generally identified in all the tests, although Test No. 1 and Test No. 4
failed to detect one source each. Both of these tests were made with
smoke blowers set up on every other manhole.
i/l sources which have no direct flow path to the sanitary sewer include
direct cross-connections with storm sewers, foundation drains, and
defective building service laterals. Smoke must generally pass through
soil seams to reach the surface and be detected. Test results showed
that the number of "indirect" i/l sources varied significantly from one
testing technique to another. Best results were obtained when only one
sewer line segment was tested at a time and when both blowers were run at
full throttle. Almost as good results werc ot-tainec v.itn Ci,:- i/.o-cr ; ~r it
was run full t.irottle v;ith the smoke being dirsctec ir. or.lv or.', liireccv.
Conclusions
Close attention to field procedures is important when conducting a smoke
testing program. Each sewer line segment should be tested with a smoke
blower at both the upstream and the downstream manholes. Blowers should
be run at full throttle, if the intention is to identify I/l sources
other than "direct" inflow sources, such as roof leaders, driveway
drains, stairway drains or directly connected storm sewer inlets.
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Table 1
SMOKE TEST PROCEDURES EVALUATED
Number of Smoke Blower Smoke Blower
Test Smoke Blowers Spacing Throttling
1 1 Every Other Manhole Full Throttle
2 1 Every Manhole Full Throttle
3 2 Every Other Manhole Full Throttle
4 2 Every Other Manhole Half Throttle
5 2 Every Manhole Full Throttle
TABLE 2
INFILTRATION/INFLOW SOURCES IDENTIFIED
"Direct" I/I Sources
Stairway Driveway Roof Area
Test Drains Drains Leaders Drains
15 111
2 5 11 2
3 5 112
4 4 112
5 5 112
"Indirect" i/l Sources
Storm Defective Total i/l
Sewer Foundation Service Sources
Test Inlets Drains Laterals Identified
1
3
13
3
27
2
4
21
8
42
3
4
17
8
38
4
4
10
7
29
5
4
22
8
43
ILLEGAL CONNECTIONS TO HOUSE LATERALS
All sanitary sewers are subject to infiltration and inflow. But if the
research done by Seattle, Washington and WSSC is representative of the
prevailing conditions of house laterals, inflow from these may be the
greatest source of extraneous water entering the system. In the
individual house lateral (infiltration is not the same problem as is
encountered in main or trunk sewers) the length and diameter of the pipe
is much smaller and the lateral is generally above the main pipe and
often above the normal ground water level.
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In older sections of an urban area, roof leaders, floor drains, foundation
drains, etc. often were connected to the sanitary sewer which may have
started as a combined sewer. When separate storm drains were installed in
the streets, these flow sources remained and were not transferred to the
new storm drains because of the high costs involved in the relocation.
Some cities were aware of this and condoned the retention of these
connections. Small lots and dense urban development left little area for
surface disposal of roof water. Dry wells had limited life due to the
accumulation of solids entering them from roof leaders. As to floor and
foundation drains; storm drains, when existing, were usually above
basement elevations.
Originally, inflow in the sanitary system may not have been a problem,
since the older pipe design methods employ several safety factors which
provide excess initial flow capacities. High estimates of per capita
flows, a 25 percent reserve capacity for expansion, and the philosophy of
"make it big, costs are not critical," generally resulted in overdesigned
systems capable of accommodating some inflow. Treatment plants, if in
existence, were usually primary treatment only, and the increased
hydraulic loadings due to the temporary influence of inflow only moved
through the plant faster. In addition, to keep the cost of service to
the consumer at a minimum, the calculated costs of treatment in these
early plants were artificially low.
Inflow from house laterals is not limited to older sections of a sewered
area. Newer developments have, and will continue to have, illegal
connections. Builders and developers are interested in getting rid of the
water as quickly and inexpensively as possible. If adequate stormwater
drainage facilities are not provided, such flows are very apt to enter the
sanitary sewer system.
Usually, when a building develops a water problem, sometimes years after
it is built, underdrains and/or sump pumps are installed and routinely
connected to the sanitary sewer. Another longstanding practice is to
install a cleanout in the sanitary sewer line with its top flush with the
basement floor. The homeowner experiencing water in the basement opens
the cleanout plug and flushes the water into the sanitary sewer.
Monitoring flows in the system can indicate the type of inflow being
experienced. A sudden rise of flow at the treatment plant and in
interceptors during and shortly after a heavy rain which peaks and recedes
rapidly, can indicate surface inflow components. The source may be from
roof drains, yard drains, catch basins, and open direct sources such as
cleanouts. After a long period of rainy weather, usually in the spring or
fall, a slow increase in peak I/I may be observed. This peak lasting
sometimes for a long period of time and receding relatively slowly,
usually indicates ground water entering the system through foundation
drains, floor drains, and leaking service laterals.
Inspection and detection of illegal connections is very difficult and time
consuming. For many years it was the standard to install a trap at the
point just outside of the building to eliminate the intrusion of gas and
odors. Then what is now considered an illegal drain—the footing drain—
was connected on the building side of the trap. Presently a contractor or
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homeowner who wants to make an illegal connection may install a trap in
the line to reduce the possibility of detection.
Those installing sumps and/or underdrains in existing buildings may not
secure permits. Builders on new construction or major renovation will
have the sewer line inspected and approved and may then tie in a drainage
tile. The use of ball valves or back flow preventors is common and
recommended for many locations such as overloaded sewers, flat grades,
low topography, etc. But the use of traps and back flow preventors can
retard or make smoke testing ineffective, which is the most common,
quickest, and least costly method of checking for illegal connections.
The solution lies in education, inspection, surveillance, enforcement, and
legal commitment. Education is the heart of the program. The homeowner
may eliminate the illegal connection on his own, if convinced that it is
in his best interest. Citizens need to be informed that illegal
connections add many gallons of water to the sanitary system which can
cause the system to fail or discharge polluted water to the streams,
streets, and ground. An overloaded system may in turn need to be
enlarged; new sewer mains, pump stations, and treatment plants may be
required to convey the additional flows. In most cases, the stornwater
could be discharged to receiving waters without treatment. The cost of
treatment for clean water and polluted water in a wastewater treatment
plant is the same. Newspapers, radio reports, public information
meetings, and information pamphlets distributed by service clubs or
attached to utility bills, can be effective educational tools.
As the program is prepared, the municipality must decide who will pay for
the removal of documented illegal connections. At this point the
cooperation of the property owners is very important. If the City assumes
the cost of disconnection, much of the potential hostility of property
owners can be avoided. Some of the cost could be assumed by the City,
such as the cost of disconnection, while any costs for reconnection to
storm drains, dry wells, or streams could be assumed by the property
owners. Or, as an alternative, the total cost could be assumed by the
City with refunding to the City by the owner over a specified time period
as is done with sewer assessments.
The preceding ideas are in operation in various parts of the country. The
City of Dallas, TX requires the property owner to assume all
responsibility toward reduction of inflow from his lateral. If the
property owner is stubborn, the city terminates water service until the
owner has repaired his service. Salem, OR has a scale for which property
owners are surcharged depending on the type of inflow problem they have.
Joint assumption of responsibility is practiced in Oregon too.
Inspection covers identifying the areas of greatest and least inflow and
then setting priorities and programs to reduce or eliminate the I/l. Flow
measurements of sewer lines in dry and wet weather can identify the areas
of highest concern. Televising sewer mains will provide visual
comparisons of laterals discharging at higher rates than normal sanitary
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flow. House-to-house inspections can be made where smoke testing has been
found to be ineffective, but where flow measurements, visual inspection
of sewer mains, and flow calculations indicate excessive inflow exists.
An inspection of a basement and its exposed plumbing may show that
illegal connections are present. An inspection of the outside of the
building may reveal whether yard drains are present and roof leaders are
piped into the ground. If a yard drain has no apparent discharge to a
storm drain or surface outlet, it can be suspected of being connected to
the sanitary sewer. Proceeding with the investigation, the property
owner or occupant should be interviewed to obtain any additional
information regarding the illegal connections. Each suspected unit must
be checked. The best way to do this is to introduce water and a bright
colored dye (special dyes are made for this purpose) into the unit and
then check the appearance of the immediate downstream manhole or
cleanout.
This is a long process as sufficient time must elapse between tests to
remove all traces of the dye from previous tests. However, it provides
positive identification if carefully performed. Testing of roof leaders
and drains suspected of being in a dry well can sometimes be confirmed by
using a smoke test through the inlet. If the dry well is not deep and
the ground not hard packed, smoke will be released through the ground.
During the removal process, sewer lateral cleanouts should be secured
tightly and raised as much as possible above the basement floor.
Once illegal sewer connections have been removed, steps must be taken to
prevent their recurrence. Builders and owners should certify in writing
that no illegal connections are present in new construction or major
remodeling. The property owners should periodically be reminded of the
negative effects of illegal connections.
The surveillance part of the Program requires a continuing effort.
Periodic checks of flow in local sewers should be made, sudden changes in
flows at treatment plant and pump stations noted, and random spot checks
of buildings in a suspected area instituted. Inspectors of new
construction or renovation should make sure that all sewer trenches are
backfilled immediately after inspection, that individual plumbing permits
for sanitary piping in buildings are checked, that a final inspection is
made of subfloor piping just prior to the concrete floor being poured,
and that new house laterals replacing old subsurface disposal systems are
checked for their full length and backfilled before any illegal taps can
be made.
Enforcement and legal commitment are the least desirable parts of the
program but are perhaps the most important. An ordinance should be drawn
specifically making the installation and/or continued use of a connection
to the sanitary sewer of any wastewater other than sanitary, a civil
violation punishable by fine and the fine should be great enough to act
as a deterrent. The amount should vary with the higher fines being
assessed against a builder or contractor who makes an illegal connection
in new construction or major renovations, while the lower fines are
assessed against an owner who goes along with the contractor's suggestion
to make the connections. A method of collection should be set without
resorting to the already overcrowded court system. A reduction of a
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property owner's sewer use fee if he has proven that no illegal
connections exist and if he continues to maintain that status, might also
serve as an incentive.
As was discussed earlier, Oregon uses a surcharge for discovered but not
corrected inflow presently in operation. Once the property owner has
corrected the problem, and the city has inspected and approved the
repairs, the surcharge is eliminated from the bill.
Inflow and infiltration studies have shown that more then 50 percent of
the extraneous sewer water is inflow which comes primarily from service
laterals. The detection of this inflow is time consuming, but the removal
can be imaginative, and the results rewarding in terms of a task
accomplished and tax dollars saved.
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CHAPTER 3
FLOW DETERMINATION
METHODS
Several methods to measure flow in a sewer line are commonly used. Probably
the most common is the measurement of depth of flow in a length of pipe and
calculation of flow rate with a relationship such as the Manning Equation.
The next level of complexity involves insertion of a weir or a flume with
measurement of water level upstream of the flow restricting device. All of
these require a method for measuring liquid levels accurately. The most
commonly used, at present, appears to be the dipper monitor and its close
competitor, the bubbler. With the advent of solid-state technology, several
others have corae into use recently: (the electronic level detector and the
electronic velocity-level detector.) Still another is dye dilution, a tech-
nique which uses only a laboratory and sampling technique.
DIRECT MEASUREMENT OF FLOW DEPTH
The direct measurement method, in its simplest terms, is the placement of
some kind of measuring device, that is, 6 ft rule (1.8 m) yardstick or
some similarly marked stick, into the flow as close to the invert of the
pipe as one can estimate. The observer then reads the flow level on the
rule as closely as he can. Normally, this falls within 0.10 ft (3.05 cm)
to 0.05 ft (1.52 cm) of actual flow depth. This is generally how
calibration checks are made on electronic and mechanical measuring
equipment.
THE DIPPER MONITOR
The next simplest method is the use of a dipper or float. A known channel
cross section with calculated flow characteristics is inserted into the
sewer line at an accessible location. Generally, a Parschall flume is
used. This device allows a direct correlation between depth and quantity
of flow since the flow cross section is rectangular. The Pars ".311 flurae
utilizes a float which is connected through a series of levers or gears
to a pen. The pen in turn marks either a strip chart or a circle chart.
After reading the marks on the chart, a technician can convert those into
a calculated flow through the flume over the period of time measured on
the chart.
Several problems are involved in this method, however: one being that
because of the nature of the discharge the dipper is measuring, foreign
material can cling to the float causing inaccuracies due to the float's
reaction to changes in flow depths, and therefore, flow volumes. Second,
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should the sewer line surcharge, and the float reach its point of maximum
travel, the flow could exceed the calibrated limits of the Pars!iall
flurae. In this case, only a rough estimate of the total flow can be
surmised from the chart by extrapolating the intersection point of the
ascending and descending legs. Besides, Parshall flumes are calibrated
for certain flow ranges. A small Parshall flume is accurate for very
small flows, but overflows the benches rapidly. A large Parshall flume,
on the other hand, while having a greater range of flows than it can
measure, can not differentiate between small variations in those flows.
Although the Parshall flume, theoretically, is directly converting depth
of flow to quantity of flow, there are times when it will be impossible
to obtain accurate flow data from the depth of flow. The Parshall flume
measures depth of flow only; it cannot measure velocity through it.
A variation of the flow monitor utilizing a float involves a level sensor.
This device has an electrically operated probe with two electrical
contacts at the end. The probe is lowered until the electrical contacts
enter the flow, which completes an electrical circuit. The closed
circuit results in the sensor being withdrawn a short distance and the
elevation of the sensor head recorded at that point. After a
predetermined interval, the process is repeated.
Fouling of the head, resulting in either a continuous electric current or
no current at all, can cause problems using this technique. It does
however offer an advantage in that the sensor and its associated strip
charts are reasonably inexpensive to purchase and maintain.
Most of the problems associated with fouling of the recording instrument
are present in any kind of recording device utilizing mechanical reading
of either depth of flow or velocity of flow, such as dippers, floats,
propeller-meters, etc.
THE BUBELEP.
The next step up in technology is the bubbler. This device also uses a
known cross section for measuring the flow, commonly using a Parshall
flume. In place of an instrument which measures the top level of the
flow, however, it has a small airway tube which exits at the bottom of
the Parshall flume. A low volume, low pressure pump pressurizes the
airway until the pressure in the airway is marginally greater than the
pressure exerted at the bottom of the flume and a bubble is formed.
Hence the name, bubbler.
Because pressure at the bottom of the flow is directly proportional to the
depth of flow, the air pressure needed to cause a bubble can be directly
equated to the depth of flow. The more pressure needed to create the
bubble, the greater the depth of flow. The pressure required to force a
bubble can be indicated on a strip chart or a circle chart. In much the
same way that the float movement is charted, a barometric measuring
device will record variations in air pressure levels on a strip chart.
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Host of the problems with this particular device occur if a piece of
material becomes lodged in the airway. This can result in erroneous
readings because of the increase in pressure required to either blow out
or seep around the obstruction.
Since both of these methods are essentially mechanical, it should be
apparent that considerable maintenance is required to assure that meters
are functioning correctly.
LOCATION'S
Regardless of the kind of flow monitoring device being used in the sewer
line, it should be obvious that one major requirement for the site must be
met: the monitoring device must be able to record a representative
measurement of the flow in the reach of pipe. Unless the monitoring site
offers the ability to obtain a representative measurement, any data
collected by that monitor is useless since it obviously will feed
erroneous information in calculating the flow.
When siting a flow monitoring device, the investigation of the site should
reveal that there are no obstructions, either immediately upstream or
downstream of the monitor location. Obstructions upstream could result in
either a reduced or increased level because of the phenomenon of hydraulic
jump and the location of the meter with respect to the jump. An
obstruction downstream could result in an increased level measurement
because of backwater. Besides the possibility of one of the two
phenomena being present because of constant fluctuation of the flow in
the sewer line, a partial obstruction could cause an even more pronounced
surging condition than that which actually transpires in the sewer line
because of fluctuations in natural flow.
In addition, to have a clear flow way there must be no major sags or
bellies in the line immediately upstream or downstream from the
monitoring point. The change in velocity due to the bump or belly will
influence the apparent level of flow in the line. If the monitor is one
of the new level velocity meters, the velocity recorded will be either
higher or lower than the average velocity of the line because of the
effect of the bump or belly.
DYE DILUTION
Another method of determining flows in sanitary sewers is a technique
known as dye dilution. Here, either a dye or a chemical compound is fed
at a known rate into the waste stream. At a predetermined point
downstream, samples are taken and allowed to sit for approximately thirty
minutes so that the solids can settle out. Then, in the case of dye, the
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sample is run through a chemograph and flow is determined from the
dilution of the dye with respect to the original concentration fed into
the waste stream and the flow rate of the dye.
In the case of a chemical introduced in a known concentration into the
waste stream, two kinds of grab samples must be obtained. The first
involves analyzing the waste stream to determine the concentration of the
known chemical compound prior to injection of the tracer into the waste
stream. The second sample is tested to determine the concentration of
the waste stream at a point downstream from the injection point. The
variations in concentration of the tracer chemical can be equated
directly to the volume of flow during the injection period.
ELECTRONIC DEVICES
Finally, there are the electronic devices which, although not as
susceptible to the solids present in the flow, do present other problems
which are in some cases just as bad. The simplest
of the electronic measurement devices record depth of flow only.
Generally, a transducer is mounted on the bottom of a calibrated flow
way. The transducer is connected to a recording media, either an
electronic or strip chart recording. The voltage which travels through
the transducer is directly related to the depth of flow much like the
float relaying depth of flow to the strip chart.
In the same way, ultrasonic sound waves can be used to gauge depth of
flow by the reflection of the sound wave from the interface of air and
water. Because of the electronic and electrical nature of these devices,
extreme variations in voltage can result in variations between the
calibrated calculated depth of flow and the actual flow. In addition,
because most electronic components are very susceptible to voltage
surges, a sharp voltage spike could burn out the equipment resulting in
complete loss of flow data.
The first electronic measuring devices experienced a considerable number
of problems in obtaining accurate and consistent flow measurements.
Three years of on-site experience has eliminated, for the most part, the
majority of these problems. But as with all recording devices,
electronic or mechanical, the user should determine for himself,
periodically, the relationship between a measured flow through the sewer
line versus what the recording instrument indicates. If a significant
variation is revealed during that check, then the instrument should be
observed and, if necessary, recalibrated.
While depth of flow can indicate flow in a sanitary sewer line, there are
situations where depth of flow is not an adequate "known" for calculating
actual flow quantities in the sewer. Often, because of physical
limitations of the system, sanitary sewers are found under high flow
conditions, actually surcharging; that is, the water surface in a manhole
is actually higher than the crown of the transport pipe. In situations
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where small pipes enter larger collectors at their invert, flow often can
backup up the smaller pipe because of differences of hydraulic head
between the two lines. Under certain surcharge conditions, the velocity
in the pipe may very well be zero, while in the condition of differential
pressure heads, the flow in the smaller pipe may actually be in the
opposite direction. If only a depth-of-flow meter is utilized to record
flows, the variance between the calculated and actual flow rates could be
extreme and 100 percent off if the flow is actually going in a reverse
direction.
Because of this problem, it is widely acknowledged that the only accurate
method of determining flows in a sewer line which is susceptible to
surcharge and backflow, is to use not only a depth of flow meter, but
also a velocity meter. The velocity meter uses a screw, or propeller
within the flow coupled with a depth-of-flow meter, to show the
relationship between the depth-of-flow being measured and the actual
velocity of the flow. These two factors plugged into an equation will
give the best evaluation of actual flow in the line. Again, because of
the necessity of using mechanical devices in the flow, the velocity meter
must be maintained regularly so that results are consistent.
VELOCITY METERS
Just as electronics have invaded the area of depth-of-flow monitors,
several electronic velocity measuring devices are on the market. They
use the Doppler effect for measuring velocity; that is, they send a sound
wave into the flow. The sound wave is reflected back to the receiver and
the change in the wave pattern is measured and is related to the velocity
of the flow based on the characteristics of the returning wave as
compared with the characteristics of the transmitted wave.
HOUSS LATERALS
Precise flow determination from leaking house laterals is difficult and,
at best, an approximation due to the inaccessibility of the point of
discharge from the house lateral to the street sewer. Flows from these
sources can only be measured easily at some convenient point, generally
the nearest downstream manhole. Utilizing various kinds of measurement
devices, described in other sections of this manual, mass flow from a
sewer reach, which includes house connections or house laterals, can be
estimated by eliminating the upstream flow and then measuring the flow in
a reach bounded by two manholes. To eliminate upstream flow, it must be
temporarily blocked at an upstream manhole being careful not to create
flow backup into adjacent basements. Since house lateral connections to
the street sewer are inaccessible, one can only estimate initial overall
leakage of the reach. This is complicated by the fact that while total
flows suspected to be due to infiltration or flow during rain periods may
be measured in this way, a portion of the flow may in fact be running
wastewater or leaking plumbing fixtures feeding into to the system from
an adjacent building mistakenly considered part of the infiltration.
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Only when one can actually observe the house connection using television
techniques, can a subjective determination of flow from building sewers be
made. The flow attributed to the house connection, as compared to ground
water, visibly entering the street sewer in that reach, can also be
determined. But this is not always possible, particularly during low flow
periods. Experienced observers report they are able to estimate flows
precisely by visually observing a connecting lateral; however, there is
some doubt as to how precise this practice is since it is subjective at
best. While such a subjective analysis of service connections does have
some validity when evaluating sewer reaches with only one or two leaking
connections and serving single family residences, further complications
are seen in other circumstances. In some cases, internal inspection may
reveal more service connections than can be accounted for by surface
structures along the sewer reach being inspected. It must then be
determined if these unused connections (house laterals) are abandoned or
are merely stubs which were never connected to structures, and if in fact
they are leaking at any time. Frequently, riser connections are observed
leaking, but in densely populated areas it is hard to tell which building
or buildings are connected. Television inspection in sewers where
industrial, commercial, or apartment complexes are tributaries
complicates the task of visually defining leakage because process water,
cooling water, or plumbing fixture leakage may appear to be infiltration.
There is no exact way to measure house lateral flow (particularly if such
flow appears as a damp area rather than an observable stream of water
entering the sewer from the house lateral). This is further complicated
by the fact that at times ground water levels vary, depending on seasons
and the amount of rainfall, and may create entirely different hydrostatic
conditions about the piping system, resulting thereafter in changes in
the amount of measurable flow within the sewer system.
Various types of sewer infiltration/inflow control equipment, including
measuring and metering devices, are presented in U.S Environmental
Protection Agency, "Sewer Infiltration and Inflow Control Product and
Equipment Guide," produced by Municipal Environment Research Laboratory,
Office of Research and Development (EPA 600/2-77-017c,), U.S.E.P.A.,
Cincinnati, OH 45268, July 1977.
PILOT PROGRAM REHABILITATING HOUSE LATERALS
In 1982-83, Salem, Oregon, conducted a pilot rehabilitation project in an
area which had been constructed substantially between 1966-68. The
initial system evaluation was conducted by Westech Engineering, Inc., in
conjunction with the City of Salem. During the investigation, a
self-leveling weir box was built which could be positioned adjacent to
the lateral entrance into the sewer main, and which would rechannel the
entire flow from the lateral through the weir box, and subsequently
measure the total flow coming from the sewer lateral by observing a
notched weir with a television camera positioned downstream.
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During the investigation period, flows ranging from 10 gpm (37.8 l/min) to
more than 50 gpm (189 l/min) were measured using this method. Flows in
excess of 50 gpm (189 l/min) could not be measured adequately, as the weir
device used to measure flows had a capacity less than 50 gpm (189 l/min).
In addition, it was difficult to correlate the lateral flow measurements
because it took several days to monitor all of the laterals in the test
section. Some of the laterals which were monitored several times, several
days apart, showed a wide variance in the volume of flow coming from the
house lateral.
Another more commonly used method of determining flow from house laterals,
is that of blocking flow at an upstream manhole and measuring the flow at
the next downstream manhole. When the upstream manhole is totally
plugged it is obvious that the only flow being measured at the downstream
manhole would be that which infiltrates into the main line plus any
inflow and/or normal flow from the house laterals on that stretch of
line.
As imprecise a method as this is, it is better than no method at all. The
imprecision comes from the inability to adequately and correctly estimate
the flow coming from each individual house lateral so that a proper,
priorties-of-repair can be made by determining which laterals are serious
offenders and which not.
Compounding the problem are stretches of sewer line which are constructed
in residential areas. Televising the line could help determine that
there are more taps into the main line than adjacent structures. While
some of the newer television equipment has the capability of right angle
viewing, most equipment can only look straight ahead. That being the
case, it is impossible to determine whether the additional taps are
merely stubs that have never been connected and do not leak, or are
connections of old structures that have been broken off, and in fact,
leak substantially during wet weather.
It is an accepted premise that house laterals, probably are a major
source of inflow because of the nature of the construction techniques
that predominate in the building of house laterals. As a rule, house
laterals are constructed according to the standards of the building code
which are in effect in the municipality at the time; and not in
accordance with standard specifications for construction of main sewer
lines.
The building contractor will generally lay sewer laterals on the minimum
grade allowed by the building code until the property line is reached
adjacent to the main sewer. The contractor will determine at what
elevation the main sewer is and break the grade of the lateral at the
property line so that the lateral will intercept the main sewer line at
the proper elevation. The final run of the building lateral has been
known to achieve slopes greater than 1:1.
An additional problem in building sewers is the connection which many
contractors make with the main. If stub outs, either in the form of a
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tee or a wye are provided, the building contractor may or may not use
those particular appendages. In some cases building contractors have
been known to stub their building lateral into the main line within 4 or
5 ft ( 1.2 or 1.5 m) of an existing wye or tee because that was where
they intercepted the main sewer. This results in a makeshift connection
with the main sewer which after a short period generally is no longer
watertight. Also, because of the steep slope from the property line to
the main sewer, settlement of the building lateral can result in joint
displacement and even totally open joints due to line slippage. During
high water table periods, those open joints and taps will result in a
good deal of infiltration in the sanitary sewer.
Another problem in estimating flows from house laterals is that during
high ground water conditions, and increased flow in the sanitary sewer,
there is no easy way to observe adequately the flow coming from
individual house laterals. Direct observation through the use of
televising equipment is generally not possible either because during
those periods the sanitary sewer is normally flowing at least
three-quarters capacity making the television camera useless.
ACCURACY
Types of Monitoring Errors and Their Magnitude
Sewerage monitoring inaccuracies can be broken down into two categories:
error in human observation and imprecision in recording. An exanple of
the first is the estimation of a pipe's flow depth with an engineer's
ruler. An example of the latter is the monitor's recording of that
depth. Each of these contains several sources for error.
Error In Human Observation
The calibration of an open channel flow meter involves three sets of
measurements: measurement of the depth of flow of the sewage for the
purpose of calibrating a particular device, the measurement of the pipe's
diameter, assuming the pipe is round, and the measurement of friction,
slope, and Manning's "N" at that location (discharge calibration). Of
the three, the discharge calibration involves the most measurements and
is prone to the most errors. The other two, while seemingly
inconsequential in their execution, have great significance in flow
monitoring results.
The error inherent in measurement of the depth of flow (DOF) of sewage
in a pipe arises from the resolution of the device used to scale the
distance, the hydraulic conditions of the flow, and basic errors in human
judgment. The engineer's ruler has a stated resolution of 0.01 ft (0.3
cm). At velocities of greater than 2.00 fps (0.6 mps), a tail water
rises up the side of the ruler and causes a condition where the last few
hundredths of a ft of the measurement must be estimated. Further more, a
lapse of several minutes between the monitor's calibration adjustment and
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the observation will cause a time lag error of several hundredths of a
foot. Finally, the misplacement of the scale onto or around the pipe
invert will cause additional error. The value of the error of this
measurement has been observed to be approximately four percent of the
DOF. This percentage was estimated from observed adjustments to the
Washington Suburban Sanitary Commission's telemetering system. Once the
meter is considered "calibrated," that value of four percent of the
calibrated DOF will be assumed throughout the metered flows.
The measurement of the sewer pipe's diameter is generally accomplished
with the same scale as the DOF measurement. This error is somewhat
smaller than the DOF measurement as less judgment is involved. The value
of the error is estimated at the resolution of the ruler 0.01 ft (0.3
cm). Field conditions and manufacturer tolerances call for the
measurement of pipe diameter.
Flow conversion in open channel flow monitors is most often performed
using the familiar Manning equation:
Q = ((l.486)S-(l/2))(R*(2/3))A/(He)(N) (1)
Where
Q = Flow (cubic feet per second)
N = A roughness coefficient, or Manning's "N" (at full pipe)
He = Hydraulic elements adjustment factor for the variation of
Manning's "N" with depth
s =
= The friction slope
R =
= The hydraulic radius
(ft)
D =
= Pipe diameter (ft)
A =
= Cross sectional area
(square ft)
The unknown constants N and S can be combined into one constant (S/N) and
then determined using velocity measurements. A variety of techniques are
available for converting velocities into a calibration value; however,
three techniques will be mentioned here.
A very prevalent technique of estimating energy slope (S*(l/2))/lI of an
open channel meter is to obtain the velocity at four-tenths of the depth
of flow; assuming the obtained velocity is the average within the total
discharge. Another technique is to attempt to find maximum velocity
within a flow and convert it into an average velocity.
Using these single velocity techniques, errors may be as high as 35
30
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percent. Both techniques are extremely error-prone and should be avoided.
It is better to profile the flow and then average the obtained velocities
or fit the data with one of the techniques available in the literature.
After an average velocity is determined, equation (1) is either
manipulated algebraically to yield (S*(l/2))/N or a value for Manning's
"N" is assumed and S is derived.
The techniques have several built in Bources of error. First and foremost
is the error inherent in the velocity meter used. All velocity meters
will lose their accuracy in time unless they are calibrated regularly.
Calibration of electromagnetic (EM) velocity meters is generally done by
the manufacturer and can be very important. For example, in a three-month
period, Washington Suburban Sanitary Commission had to calibrate it's 3-1
meters three times after extensive use. The calibration drift encountered
was from 12-15 percent of full scale. Propeller-type meters can be
calibrated by the user, and this should be done regularly. The stated
accuracy of calibrated velocity meters (EM and propeller) is two percent
of scale.
The collection of quality velocity data is very difficult. Domestic
sewerage contains a large amount of suspended solids which will foul both
the propeller and EM velocity meters. Studies of the EM velocity meters
have shown that at least 8 seconds of uninterrupted flow are required to
obtain good readings. Propeller velocity meters require a somewhat longer
time—observed to be between 15-20 seconds—to respond, because the
revolutions of the propeller are processed into instantaneous velocities.
Erroneously low velocities are the norm, particularly in heavily
concentrated sewerage. A typical velocity profile will take from 10 to
15 minutes to collect. Assuming a good velocity is obtained, an
additional two percent in error may occur due to typical hydraulic
fluctuations during this period. The error calculated for the velocity
applies only to the estimation of the enerav slope's error. The error in
the energy slope will remain systemic throughout the conversion of the
DOF's to discharges.
Imprecisions in Recording Devices
The error inherent in the actual flow monitor can be broken down into two
categories: errors in recording and errors in recorded resolution. An example
of the first is the error of the device which records the depth of flow. An
example of the latter would be the precision of a circular chart for a dipper
or the amount of computer storage used to store a single DOF in a
microprocessor style flow meter. Not included in this discussion are the time
response errors of flow monitors or the errors in noncontinuous flow
monitoring.
The manufacturer's literature accompanying flow monitoring devices states the
error of the depth sensor for idealized situations and the resolution of the
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recording device. Just as an improperly installed flume will give erroneous
readings, a poorly installed flow monitor will yield poor results.
With the advent of computerized monitoring systems, errors in recording
resolution of these devices are negligible compared to the above-mentioned
sources of error. The same cannot be said of circular and strip recording
devices. In these, the error is that of the recording media or the minimal
divisions of this media. The reasoning behind this last statement is that the
width of the recording pen line together with the variations in the travel of
the recording media result in a line on the chart which only closely
approximates the actual conditions prevailing in the line at any time. This
information is then interpolated by eye to obtain the final readings used in
subsequent computations.
Error Propagation in Flow Monitoring
The results of the depth and velocity discussion are summarized in Table 3.
TABLE 3-
LISTING OF ERROR SOURCES AND THEIR VALUES
ERROR SOURC:
ESTIMATED VALUE
EQUIPMENT TYPE
DOF Calibration
Pipe Diameter
Velocity
Measurement
DOF Sensor
Recorded
Resolution
4% of (calibrated) DOF
0.01ft
7%
0.01ft
0.01ft
0.05ft
"0.0
0.1 in. - 0.5 in.
Pressure Transducers
Ultrasonic Dippers
Floats and Electric
Current Dippers
Monitors employing
ROM (Read Only Memory)
Storage
Strip Recording
Error is propagated throughout any system of equations in a probabilistic way.
Some of the errors mentioned above are random and may assume different values
throughout the collection of the data. An example would be the measurement of
the depth of flow. Each time this depth is measured, the error of the sensor
can either take on the higher or lower value. Once a meter has been
calibrated, the errors in the calibrated depth of flow and energy slope will be
constant. Thus, in order to estimate the error in flow monitoring, one must
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first estimate the "system errors," then calculate the random errors in the
data collection system. The four percent depth of flow error, a 0.01 ft (0.3
cm) pipe measurement, will propagate throughout the "energy slope" and the
depth of flow calculations.
One way to view the propagation of error is to use the coin toss analogy. In
estimating the energy slope, three error points are involved. To calculate an
error based on the higher and lower values of the error would negate the
possibility of one error being in apposition to another. Each error has an
equal possibility of assuming its higher or lower value. Thus, median error
must be calculated. Table A: ERROR ANALYSIS SCHEME FOR THE ESTIMATION OF
ENERGY SLOPE presents a scheme of estimating the error of energy or friction
slopes.
TABLE 4.
ERROR ANALYSIS SCHEME FOR THE ESTIMATION OF ENERGY SLOPE
VALUE ASSUMED BY ERROR TYPES
VELOCITY PIPE DIAMETER OOF
ERROR MEAS. ERROR ERROR
El E2 E3
"VALUE
H*
H
H
E(H,H,H,)
L*
L
L
E(L,L,L.)
L
H
H
E(L,H,H.)
H
L
H
E(H,L,H.)
H
H
L
E(H,H,L.)
L
L
H
E(L,L,H,)
H
L
L
E(H,L,L,)
L
H
L
E(L,H,L.)
*H = Upper bounds of error, L = Lower bounds of error.
**E (A,B,C), i.e., E(H,H,H) = the value of the estimate with
all of the errors assuming their high range minus the slope
estimate.
Table 4 is simply the combination of the three random errors in the estimate of
this parameter. In terms of combinations terms, we have 2x2x2=8 possible
combinations. A simple error, such as a series of measurements, should have an
approximately normal distribution of error and, thus, an equal higher or lower
range. The errors will assume, as in the toss of a coin, either higher or
lower ranges of their respective values. The calculations of the propagation
of error within a single depth of flow are rather simple, but should be
performed throughout the range of depths encountered in a sewer flow. Thus,
the problem is well suited for computer solution. A worked example is included
below to demonstrate the method of solution proposed.
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A monitoring site has been set up in a 24 in (0.6 m) pipe. Using a
profiling technique, the average velocity was estimated to be 3.5
fps (1.1 m/s ) (I-.075?) at a depth of flow of 14»3 in. (36.3 cm).
The pipe was remeasured at the point of data collection as 23.9 in.
(60.7 cm) (I-.1&). The monitor, a ROM-reading type with a pressure
transducer, was calibrated at a DOF of 13.60 in. (34-5 cm)
(l-.04%). What would be the energy slope and the upper and lower
bounds of the estimate?
A) Estimate the error of the energy slope, using values from Table 3
and the scheme in Table 4.
A1) The estimated central value of the energy slope is:
(S"(l/2))/N =
V/((R*(2/3))(1.486))=
((1.21)(3.5))/((.6731)(1.486))=4.234
A2) Calculate the error of the estimate with the upper bounds of
the stated errors:
V = 3-5+(0.07)(3-5) = 3.745 (upper value velocity)
D = 23.9+0.1 = 24 (upper value diameter)
DOF = 14.3+(0.04*14.3) (upper value flow depth) =
E(H,H,H)= ( (1.21) (3.745))/( (0.6827) (1.486))-4.234 = -0.233
A3) Calculate the error of the estimate with the lower bounds of
stated errors:
V = 3.5-(0.07)(3.5) = 3.255
D = 23-9-0.1 = 23.8
DOF = 14.3-0.04(14.3) = 13-728
E(L,L,L)=((1.21)(3,255))/((0.6627)(l.486)) -4.234 = -0.235
A4) Similarly the remaining error combinations are calculated:
E(L,H,H) = -0.351
E(H,L,H) = +0.305
E(H,H,L) = +0.355
E(L,L,H) = -0.288
E(H,L,L) = +0.367
E(L,H,L) = -0.244
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For this case, a better representation of the energy slope wou]d be:
((S"(l/2))/H = 4.2 (+)0.3
The error of a discharge would be estimated by repeating all of the possible
combinations of error (sensor resolution, DOF calibration error, pipe dianeter
error). Data recording error (16 combinations) throughout the (R(2/3))A
portion of the Manning equation then combining the resultant error with the
slope and respective error to achieve the resultant discharge and error.
Other Open Channel-Discharge Formulae
Research has been conducted into the development of improved open channel-flow
formulae.
Pomeroy proposed a manipulation of the Hazen-Williams equation which fits
existing field discharge-siope data more accurately. The equation is
expressed:
Q = (K"(1.316))(S*(0.54))(A~(1.316)) (2)
where
K = Constant of friction
A = Area
S = Slope
In examining the equation, the area of the flow is combined with the hydraulic
radius yielding, perhaps, a less sensitive equation. Table 5: FLOW ESTIMATES
OF TWO DIFFERENT FORMULAE, presents the results of a comparison of the result
of sensitivity analysis of a hypothetical flow monitoring situation as modeled
by equations 1 and 2 .
TABLE 5.
FLOW ESTIMATES OF TWO DIFFERENT FORMULAE
MANNING MANNING POMEROY POMEROY
DEPTH OF FLOW DISCHARGE ERROR DISCHARGE ERROR
(FT)
(CFS)
(CFS)
(CFS)
(CFS)
0.18
0.36
0.54
0.72
0.90
1.07
1.25
1.43
1.61
1 .79
0.18
0.75
1.70
2.96
4-48
6.28
8.21
10.27
12.29
14.19
0.1
0.2
0.4
0.6
0.9
1.2
1 .4
1 .7
2.0
0.23
0.85
1 .83
3.10
4.61
6.30
8.10
9.95
11.77
13.45
0.1
0.3
0.4
0.6
0.9
1.1
1 .4
1.6
1.9
Notes: Pipe diameter: 2.15 ft. (+-)0.01 ft., V = 3.5(+-)0.25 FPS at 1.1 ft.,
calibrated DOF =1.2 ft.(+-)0.05 ft.
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Examination of each prediction shows that each term is within the range of the
other. Only better methods of attempting to quantify the required parameters
will answer the question of which equation provides a "better estimate" of the
flow in a sewer pipe.
SUMMARY COMMENTS
The most common error to occur using an open channel flow monitor is the
combination calculated as described in the schemes presented in Table 4.
Care must be taken to minimize the error inherent in each item of
measurement. The decisions made with flow monitoring data must consider the
range of error in the discharge data.
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CHAPTER A
REHABILITATION METHODS
The rehabilitation processes for sewer mains can be divided into several
basic techniques. The following is a discussion of six of these
techniques with a look at the positive and negative aspects of each, and
a discussion concerning the best use of each.
The six basic techniques to be examined here are:
1) Spot repair
2) Reconstruction in place
3) Construction of a bypass
4) Grouting
5) Sliplining
6) Inversion lining
Another would include:
7) Manhole rehabilitation
SPOT REPAIR
Here the object is to determine the location(s) of the problem areas in a
length of line, remove the overburden from the line, and take away and
replace the damaged areas to bring the line back to a reasonably leakproof
condition.
There are many methods of accomplishing spot repairs, some considerably
better than others. Many agencies will locate the distress point, uncover
the line at that point, wrap a sheet of tin around the pipe, and then
encase that section of pipe in concrete. As a rule, this will solve the
collapse problem but will not reduce the amount of infiltration/inflow
from that point significantly because the joint between the old pipe and
the new concrete cannot be made watertight. Groundwater will be able to
flow between the encased concrete and the sewer pipe until it reaches the
area of distress and enters the stream.
The proper way to do spot repair is to first find the area of distress and
uncover, not only the section of pipe which is broken, but also the
sections immediately upstream and downstream of that broken pipe section.
This is done because the proper method of correction is to completely
remove the damaged section of pipe and replace it with a new section,
especially if the sewer line which is being repaired is of the premium
joint variety. If additional money was spent to purchase that kind of
pipe initially, then it makes sense to spend slightly more money on the
spot repair to protect the integrity of line by replacing the damaged
section with like construction.
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Admittedly, it is more difficult to replace one section of the line than
it is to cap it with concrete. To do this, three sections, or possibly
merely two sections, of pipe must be removed and relaid to complete the
repairs. This is necessary because the line is already in place and in
order to replace only one section of pipe, the bell will have to be
broken on the downstream end. This again would destroy the integrity of
the line however watertight it is because of the need to cap the breakage
with concrete. To properly replace the damaged section, the free ends of
the sections of pipe immediately upstream and downstream must be lifted
where the new section of pipe is to be inserted. As the two free ends
are lifted, the distance between ends will increase so that a new section
of pipe can be slipped over the spigot end of the one and into the bell
of the other. Once this is accomplished, then all three pieces of pipe
can be lowered onto the original bedding of the trench. Since sewer line
generally comes in 4 ft or 8 ft (1.2 or 2.4 m) sections, this could mean
that as much as 25 ft (7.6 m) of trench would have to be excavated to
accomplish this operation. With sewer lines at depths greater than 4 ft
(1.2 m) there is another problem associated with either placing shoring
in the excavation, or sloping the sidewalls to prevent collapse into the
excavation.
It is obvious why the cheaper and quicker method is used more often, but
when the final condition of the sewer line is evaluated, comparing the
more time-consuming and expensive operation versus the "quick and dirty,"
it should be obvious that, in the long run, it is more cost-effective to
do it right the first time.
The problems related to this method vary with location. The biggest
problem involves the use of major equipment to uncover and recover the
damaged line. This requires a crew of several persons, most of whom are
idle during the major portion of the digging. Once the digging is
completed, the equipment operator then becomes the one who is idle.
Besides the problem of achieving efficient use of the crew, there is the
problem of the stability of the line itself. If the line has begin to
undergo distress, it's quite likely that the whole line is in less than
satisfactory condition. Because of various constraints, there may not be
enough time or resources immediately available to renovate all of the
damaged sections in a given line that need replacement. Later, the crew
often find themselves at nearby location on the sane line section
performing another repair job within a short period of tine. This could
lead to duplication of effort in a given area, and consequently to
excessive expenditures of both manpower and resources. There are also
the added problems of traffic control and interference with other
utilities which tend to slow down a job and increase costs.
The advantage is that if there is a section of line which has few problems
and only one or two major deficiencies, it may be more advantageous to fix
those isolated areas with the spot repair rather than to expend large
amounts of resources in hopes of only a small increase in function.
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IN-PLACE REPLACEMENT
This leads to the next method of line section rehabilitation, and this
involves total replacement in-place. This method is similar to spot
repair except that it entails total removal and replacement of a specific
run of line. While this technique does eliminate the need to return and
repair adjacent sections of line time after time, it does create problems
because of the need of maintaining service to the affected users during
actual construction. This may result in use of temporary lines, or
something of this nature, to maintain continuity of service.
As with spot repairs, the confrontations with traffic and other utilities
still exist; however, the problems are now compounded because of the
increased area of disturbed ground the project will encompass.
ALTERNATE LOCATION' REPLACEMENT
To eliminate the problem of maintaining service while the rehabilitation
project is being completed, an alternative to in-place replacement is
often used. This involves relocation of the line by constructing a new
line to replace the function of the damaged line section. While this may
reduce the severity of some of the problems of the two previous methods,
it does require the purchase of additional land in order to construct the
new line. In addition, it may be necessary to do minor repairs on the
existing line to maintain service while the new line is .being
constructed.
Unless the new easement is located outside of the right-of-way (R/W), the
problems of traffic control and utility confrontation will still have to
be overcome. Even if the new easement is not in the R/W of the street,
the possibility of utility confrontation could still exist.
The major advantage of this method is that the line is now new, and has an
expected life far greater than that of a renovated line using most of the
rehabilitation techniques being discussed. On the other hand, the first
cost of totally reconstructing the line is normally far more expensive
than that of renovating it.
Because some of the pipe rehabilitation techniques being discussed here
have existed only for a short period of time, there is not enough data to
ascertain what the life, and therefore the long-term costs of the
rehabilitation project would be. It will be only after enough data has
been collected that an accurate comparison can be made of the actual
long-term cost of total rehabilitation versus the cost of reconstruction
to determine which technique is actually most cost-effective for a given
case.
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CHEMICAL GROUTING
This leads to a discussion of the more familiar and specialized methods of
pipe rehabilitation, the most common of which is chemical grouting. This
is a process where some kind of a two-component gel is pumped into and
through the pipe joints in the line. Chemical grouting is good only when
the line is in a reasonably stable structural condition. It should be
recognized that more than a dozen brands of chemical grout are currently
available. The user also should be aware that most types are machine
dependent. That is, some manufacturers' grout types will not work in
other manufacturers' machines.
Chemical grouting begins with the investigation of the system to determine
the areas where excessive infiltration/inflow is occurring. Once this is
determined, the lines in question must be cleaned. Cleanine is neces?arv
so that the television camera operator can adequately observe the lines and
determine where the deficiencies are. The most efficient method available
for cleaning sewer lines is the self-propelled, high-pressure water jets.
After cleaning televising can then begin. Problem locations are logged on
the record using the footaee counter, which is coupled with the TV camera,
and which enables the operator to accurately locate problems with respect
to the distance from the entered manhole.
Within a reasonable time after that, before the line becomes too dirty
again, the grout packer is placed in the line, and positioned over the
problem area once it is relocated using the TV camera and the footage
counter.
The packer has two inflatable "doughnuts," one at each end of the unit,
which are inflated, partially sealing the line upstream and downsteam of
the pipe joint. The two-part grout is then pumped into the cavity
between the packer and the sewer line. Then additional pressure is
applied, forcing the grout through the joint and into the fill or any
cavity which may exist outside the line. The curing time is allowed to
lapse after reducing pressure. Once the curing time is completed (45 to
90 seconds, depending on the material mixture) the joint is air tested to
assure that the grout has completely sealed it. If the joint passes the
air test, the packer is moved to the next joint and the process is
repeated until the line is completely sealed.
If there are large voids outside the pipe, depending on the grout used and
its characteristics, a large amount of grout may have to be used before
the line is completely sealed. This could be very expensive. In
addition, because of the design of the packer, it is almost impossible to
seal joints that are immediately adjacent to a service lateral junction
because no pressure can be applied. The packer cannot work on a
longitudinal crack in the line which extends beyond the "doughnut" for
the same reason. Also, the packer cannot seal a joint with more than a
slight amount of lateral displacement because of the inability to apply
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pressure to the grout.
If the line being rehabilitated has severely distressed areas, these must
be corrected using some other method since the grout gives no structural
stability to the line and probably would not be usable anyway due to the
inability of the packer to pressurize the area.
SLIPLINING
Sliplining is a technique where the sewer line is rehabilitated by
placing a rigid liner inside the existing pipe, either by pushing or
pulling the liner into place. The liner is generally made of either
plastic or fiber glass, and is flexible enough to accommodate small
alignment changes in the existing line.
The major problem with this technique is the inability of the liner, as it
moves through the existing pipe, to surmount either major alignment
changes or obstacles, such as collapsed pipe or projecting service
laterals. Such problems must be removed or corrected before the lining
operation can take place.
Once the lining is in place, the service connections, (which were located
by televising the line before lining and logging their locations), must
be dug up and connected to the new lining. This can be accomplished
either by (1) digging the connections and "welding" a wye or a tee into
the liner and then connecting to the end of the existing service lateral
or (2) filling the annular space between the liner and existing pipe and
placing a saddle on the outside pipe and connecting the service lateral
to it. Regardless of the method used to join the existing service to the
new liner, each junction must be dug and treated as a spot repair.
After the service laterals have been reconnected, the problem of the void
between the outside of the liner and the inside of the old pipe must be
addressed. There are two opinions about what to do about this. One
theory says to fill the annular space completely; the other says that
only portions of the annular space need to be filled to prevent liner
movement. Since the lining technique is a relatively new process, there
is not enough data at present to determine which theory is correct.
One point can be made for grouting the entire annular space and that is
simply this: With the entire space filled, the structural capability of
the line will be increased. If a collapse occurred on a nonfilled liner,
there is a good chance that the liner would collapse also since the liner
is not as rigid as the original pipe.
This technique is best used where there is little structural instability
in the existing line and few service laterals to contend with. It is
effective where there is confined space to work in both from aboveground
and underground conditions. The lining technique does not have to
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contend with other adjacent utilities except for the pit which has to be
dug at one end of the line to allow the liner to enter into the existing
line.
INVERSION LINING
This technique uses a felt sleeve soaked with a thermo setting
reaction-based plastic material. The sleeve is inside out at the start.
One end of the sleeve is attached to the end of the pipe to be lined, and
cold water or air is pumped into the area of the sleeve, between the
sleeve which is inside out and the return part which is right side out.
The air or water in that space is then pressurized and forces the sleeve
down the line pulling the inside out portion through the center of the
right side out sleeve in a continuous operation. Because of the
inversion process, it is not necessary to have additional pulling or
pushing equipment to facilitate the placement of the sleeve. Beside the
sleeve being self-placing, the water pressure forcing the inner sleeve
down the line also forces the outer portion of the sleeve into continuous
contact with the interior of the existing sewer line.
As it travels down the line, the sleeve is turned right side out so the
thermo chemicals are facing the inside of the pipe. Once the sleeve has
traversed the entire length of line, the cold water is replaced with hot
water in the liner which sets off a reaction of the thermo reactive
chemicals, bonding the liner to the existing walls, and creating a solid
liner in the pipe.
The drawback here is that the line must be thoroughly cleaned prior to the
lining process, and the laterals must be located and logged.
The second problem involves the service lateral connection. While this
process has devised a method of cutting (routing actually) the liner from
inside the pipe, the accuracy of the cutter has not been sufficiently
refined to provide a clean cut into the service lateral and it is time
consuming. On the other hand, the total elimination of the need to
uncover the service lateral connections in order to reconnect thera is a
major advantage.
Inversion lining has still another advantage over sliplining. At the
commencement of the lining process, the sleeve is very flexible. The
sleeve can be introduced into the pipe to be lined through a manhole
large enough to allow entry by a man, and therefore, does not require a
large pit to introduce the liner as sliplining does. Since the sleeve is
flexible, it can also handle larger variations in alignment and
displacement than sliplining can.
This is a new process with no significant degree of historical data to
fall back on. The process is patented and therefore offered by only a
few select contractors. These two facts combined are why competitive
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bids will be hard to find if the particular rehabilitation technique has
to be bid.
In addition to the previous discussion describing the different
techniques of sewer rehabilitation, a flowchart, or "decision tree," is
included (Figure 1 ) with the objective of guiding the user through the
decision-making process in conjunction with the most effective materials.
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FLOW
<12C
FIGURE 1
SF«T?. RZ^AKLITATXCN
T>Z~ZSZCn T5ZZ
GPCUT
. 11V.Z
IBL!
CLASS,
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CHAPTER 5
ECONOMICS OF REHABILITATION
The purpose of this chapter is to discuss problems inherent
with rehabilitation techniques currently being used and how thev mav affect
the economics of rehabilitation selection and lead to a more precise cost
estimation. In spite of empirical information available from various
sources it is best to generate cost figures using a rational approach.
While the approach can be local or regional or based on general
guidelines, specific and historical cost information used should be as
up-to-date as possible. In this matter practitioners, engineers,
contractors, specialists, and state agencies can be helpful. However,
hard local information to verify actual conditions as they relate to
specific problems, schedules, approach, and required success to be
achieved should be considered in determining true "Bottom Line"
economics. What are some of the problems affecting the economics
of rehabilitation?
GENERAL CONDITIONS
Traffic Control
Most sewers can be found within the traveled rights-of-way such as
streets, roads, and highways. Therefore, entrance to sewers generally is
made via manholes which are also constructed within the rights-of-way.
Occasionally these structures can be found within an off the traveled
ways such as parkways or in parking areas. Since the grouting and
sealing is done from a self-contained van, and the technique requires
access from at least two manholes, provision must be made to protect
workers from traffic. This is particularly important in heavily traveled
ways. Regardless of how minimal the traffic is, for safety's sake there
must be some provision and therefore cost involved in providing traffic
control. Obviously, the heavier the volume of traffic anticipated the
more extensive the traffic control and personnel protection. Traffic
control on lightly traveled streets may only require one or two signs,
road cones, or such, while traffic control on a well traveled way may
require multiple signs, lane closures, barricades, flagpersons, or in
some instances, police. Many typical traffic control setups are detailed
in the APWA publication "Work Site Traffic Control".
Mobilization
Another factor influencing the cost of grouting rehabilitation is
mobilization. This cost relates to marshalling men, equipment, and
materials from a central location and transporting all that is necessary
to comply with contract requirements on-site and if necessary, removal
from the site to a centrally located storage facility. Included in this
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cost is the personnel training necessary to insure performance of the
tasks in a safe and workman-like manner. Since many grouting projects,
as well as other rehabilitation work, are awarded through competitive
bidding, the contractor will undoubtedly include other costs in the
mobilization costs, such as preparation, insurance, bonds, supplies and
inspection crews, etc. These costs are included in the price(s) bid or
as a separate item, if provided in his bid-documents. The mobilization
factor can have a significant impact on costs, particularly if the work
of grouting is relatively discontinuous. Major moves of labor and
equipment over wide areas of work can add significantly to overall costs.
Also, delays in rehabilitation sequencing requiring removal of idle
equipment can also increase costs significantly. Being forewarned of
rehabilitation problems and providing for them in contracts can save
money claims for extra work at the end of the project.
Line Cleaning
A major problem and cost factor is line cleaning. Sewers must be cleaned
prior to all internal operations in order to properly inspect, survey, and
subsequently repair the sewer. Cleaning is necessary to remove solids of
a wide range of sizes that have settled in the pipe invert and also to
remove root intrusion or obstructions that have occurred. Roots can be
removed by a number of recognized root removal procedure and control
actions. Chemical root control is usually used to prevent or reduce
growth of roots. The amount of effort necessary for root treatment is
largely dependent upon the amount of root intrusion and obstructions.
The remainder of the cleaning costs will be dependent upon the kind of
cleaning equipment used,personnel involved, whether it is a mechanical or
hydraulical operation and how effective or efficient it is. In either
case, a source of water must be available to complete the flushing
necessary to clean the line prior to grouting operations.
Sewer debris can vary greatly throughout the sewerage network. This
debris or deposition is impacted by rain related inflow from street
drainage, interconnected area drains, downspouts, manhole covers, and the
like. Interconnections can result in large amounts of transported sand,
gravel, or mud accumulating in the sewer. A sewer serving a residential
area can contain sediments of organic material, i.e., grease, grass
clippings, and food waste generated by garbage disposals. Each type of
deposition requires a particular cleaning technique to adequately and
efficiently remove them from the sewer. If the sewer run is short, e.g.,
less than 400 ft (120 m), with accessible manholes at each end of the
run, a hydraulically self-propelled cleaning machine often can be used
effectively to clean the pipe, washing debris to a downstream manhole
where vacuum equipment removes the debris from the system and prevents
downstream deposition. If the reach of sewer is longer than 400 ft (120
m) or if manholes are not available or accessible, cleaning may require
the more costly use of bucket machines. Whichever method is adopted the
debris will need to be removed via a manhole, transported, and then
disposed of in an approved location. The disposal of debris,
particularly in cases which involve priority pollutants or hazardous
materials, may present costly problems.
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A sewer vacuuia requires a holding tank, and once the holding tank is full,
the vacuum must be taken out of service to dispose of the material
collected. This, of course, requires stoppage of cleaning until the
disposal process is completed. The problem of costly delays can be
resolved by providing a storage tank truck to travel with the vacuum
(eductor) truck. The vacuum process need not stop as long as the mobile
tank truck is available to receive the sewer debris. Therefore, cleaning
can proceed uninterrupted. A bucket machine, on the other hand, can be
used to load a dump truck and then load another truck while the first
dump truck transports the waste to an approved disposal site. A bucket
machine, however, works much slower than hydraulic equipment and cost
adjustments are necessary. Removal of waste by dump trucks may require
trucker with special body structure to prevent drainage to streets during
transport from the site.
Depth of a sewer can have a tremendous impact on removal techniques of
sewer sediment. If the sewer, for example, is greater than 30 ft (9-1 m)
deep, a vacuum truck cannot physically lift the material from that depth.
Cleaning must revert either to using a bucket machine or a hydraulic
cleaner to move the sediment to the manhole where it is removed
mechanically.
Flow Diversion
Depth of flow can often impede the efficiency and progress of cleaning.
While a low flow permits line cleaning to take place, heavier flows may
require blockage and/or diversion of flow. A sewer flowing half to
three-quarters full would require flow diversion during the cleaning
activity. A sewer flowing three-quarters full will reduce the scouring
action of hydraulic equipment or impede bucket machinery from efficiently
removing debris due to boundary action. During low flow upstream sections
may be carefully blocked making certain that flows do not back up into
adjacent connectors. This type of flow blockage is relatively inexpensive
but may require work interruption because of upstream flow buildup. A
more costly but safer technique, particularly for heavier flow, is to
bypass pump. This can be costly if needed around the clock.
Consideration must be given to flow control under any condition to
improve efficiency of the cleaning operation.
Condition of the System
A heavily deteriorated sewer can be damaged further by hydraulic or
mechanical cleaning. If a sewer is found to be cracked or there are
cavities present in the pipe, the action of a hydraulic jet could further
displace portions of the sewer. Furthermore, older joints, particularly
grout could also be displaced and increase sewer
infiltration. The danger is potential collapse or at least, deposits of
surrounding backfill entering the sewer. If, for example, house
connections have been permitted and connected to the sewer by a practice
known as stabbing, any mechanical cleaning equipment used could be caught
causing damage to the line or requiring spot excavation to free it.
The difficulty inherent to cleaning a deteriorated sewer is that the
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actual deterioration is discovered too late. Since it is necessary to
clean a sewer prior to internal inspection (TV), the very act of cleaning
may cause further damage to the system. Any suspicion of structural
damage should be investigated carefully prior to committing the line to a
full-blown cleaning activity. This additional precaution can often
reduce contractors' claims and general arguments over cause of damage
during the actual rehabilitation work.
One approach occasionally used, particularly in larger systems, is use of
a testing program. A representative area of the entire system is
selected to be investigated in-depth to determine, among other
information, condition of the system, effective cleaning and
rehabilitation techniques, success of various methods, and most
important, a range of contractor prices for the various approaches. The
test program also provides an insight into special problems that may be
anticipated in preparing major rehabilitation contracts.
In smaller systems, of course this would not be practical and would
require very detailed investigation where pipe - deterioration may be
involved. In this case, attention to service records of pipe blockage or
chronic street settlement may provide some clues. A firm understanding
of the age and history of the system can result in substantial savings in
the cost of the rehabilitation phase.
Kiscellaneous Factors
Weather will always remain a variable factor in its effect on the work.
The problem is obvious: the worse the weather the longer it takes for
particular jobs to be performed and, therefore, the greater the cost.
Adequate time should be allowed for the work with due consideration given
to inclement weather.
After cleaning, scheduling of the inspection phase should closely follow.
All cost factors for cleaning must be taken into account while inspecting,
including the costs of safety. Personnel entering the sewer must be
protected and assured of ample fresh air and a safe nonexplosive
atmosphere. This means special ventilation, safety equipment and special
training for personnel. Personnel security in high crime area may be an
important factor and in certain circumstances can affect the economics of
a project. The need for night work can add to that problem too.
Finally, documentation required during inspection should be defined.
This can affect overall costs. The manner and degree of this
documentation/records should be clearly defined.
REHABILITATION TECHNIQUES
Sealing and Grouting
Sealing and grouting is perhaps one of the most convenient and relatively
cost-effective method of rehabilitation available today. Nevertheless,
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there are problems attending this approach which add to the costs.
General conditions and type of cleaning procedures employed can and do
affect the overall costs. Once the cleaning and inspection phases are
completed, the rehabilitation work should begin shortly thereafter. All
of the preceding factors should be taken into consideration in estimating
the cost of chemical grouting as well as other factors specifically
related to this technique, such as the type of grout to be used. Some
chemical grout is more expensive than others and, depending upon specific
problems and a application in a line, it may be necessary to use one type
of grout instead of another as discussed in Chapter Five, Rehabilitation
Methods. The cleanliness of a pipe has considerable impact upon the
progress and effectiveness of the grouting operation. Debris can impede
progress of T.V. camera and packer. Impound water can make clear
observation and precise judgment of interpreted conditions more
difficult. Grease, for example, on pipe walls can produce glare
impairing clear video observation. Furthermore, certain chemical grouts
are less apt to adhere to pipe that is dirty. Therefore, more effort
must be expended to guarantee seal of a joint.
Other internal conditions affect the efficiency of chemical grout
operations, such as, settled or misaligned pipe affecting internal
passage of grouting equipment. Of course, the degree of displacement is
important. A slightly displaced joint will not have the same economic
impact upon the grouting procedure as a severely offset joint. The more
severe the displacement the greater the difficulty in moving the packer
through the line, and, of course, the wider the joint the more difficult
it is to effect a seal, irrespective of the type of chemical grout
employed.
The number of service laterals, as well as the type of service, has a
major impact upon grouting costs. If the service laterals are stabbed
and sealed into the sewer, then depending upon the penetration and
interference of the service laterals it may be difficult, if not
impossible, to grout at or near the service lateral. Depending upon the
depth of stab, the grouting machine may have difficulty in proceeding
past the interference. The packer with its limited clearance between the
inside walls of the pipe is impeded from travel in the pipe by a
relatively small service lateral penetration. Care also must be
exercised to prevent damage to the service lateral so installed.
The structural condition of the pipe can most certainly impact the
grouting operation. A badly cracked or fractured pipe may not permit
chemical grout repair. Other methods may be more appropriated. Assuming
the structural condition is such that grouting has a good chance of
success, then the line must be isolated (by placing plugs at the
extremities of the pipe) in order that the grout be forced through the
cracks and allowed to seal. In other words, for chemical grout to work
the cracks must be isolated and sealed. Longitudinal cracks are difficult
to isolate, particularly if they are extensive and occur along several
pipe sections.
If the line is seriously defective structurally, the sealing rings on the
packer unit when expanded could create further damage by displacing the
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pipe, resulting in a future collapse. Prudence and caution in this
instance can be translated into increased cost of operation. This kind
of problem most probably would call for spot repairs (external excavation
and repair), performed on the severely deteriorated pipe sections prior
to internal grouting. Spot repair is one of the more expensive
techniques if evaluated against the amount of work accomplished. Where
there is evidence of a severe defective structural condition, an economic
analysis of all things considered nay logically lead to use of another
rehabilitation techniques.
Sliplining
This technique, as with other rehabilitation methods, requires
consideration of the common factors presented affecting the economics of
the operation which have been previously mentioned. Sliplining require? that
the line be clean, that traffic be controlled, and so on. Sliplining is a
technique which employs the insertion of another pipe, that passes
through an outer pipe unimpeded. There is, however, an additional
procedures peculiar to sliplining, that must be performed; "proving" the
line. Proving a line is accomplished by winching a dummy section with a
conical taper through the sewer to determine if any longitudinal or
transverse changes in the line are extensive enough to prohibit the
movement of the slipline along the sewer reach. In some cases, proving
may actually adjust some of the minor displacements enabling the slipline
to proceed unimpeded. On the other hand, if proving cannot correct small
structural problems, then spot repairs must be accomplished prior to the
slipline process. Proving a line will also delineate those service
laterals which have been stabbed into the line and cause interference.
Any horizontal or vertical misalignments or joint displacements which
will hamper sliplining also will be defined. Preliminary techniques
adding to the cost of this work may require internal cutting of
proturberances. Of course, careful preliminary evaluations should be made
to project the most effective rehabilitation method needed dependent upon
active conditions. A firm understanding of the evaluation process of
actual conditions can result in a considerable cost savings during
preparation of rehabilitation contracts.
The economics of sliplining is affected by the materials used or the
liner type. Rehabilitation should not reduce the sewer capacity to any
greater extent than is absolutely necessary. Since sliplining is
generally performed with either plastic or fiberglass, the diameter of
the liner should be as large as possible and still fit inside the existing
sewer. Depending upon the structural condition of the existing sewer
main, the design of the slipliner may incorporate an increased wall
thickness to compensate for potential stress be expected to be exerted on
it. The wall thickness of the slipliner could be increased where
structural stability is of major importance and infiltration is
excessive, although some of the effective area of conveyance may be
reduced by the introduction of the slipline. The relative smoothness and
continuity of the new pipe wall and its lower coefficient of friction can
allow similar, if not greater capacity, than the original impaired pipe
section. Keeping this in mind, the right combination must be found for
this job. There are various strengths of plastic liner available on the
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market, however, as the specified strength increases so does the cost.
Generally, as the strength of a liner increases, so does its weight.
This, of course, has an important effect upon the cost of shipment of the
pipe from the manufacturer or distributor to the job site; and to a lesser
degree, cost of handling on the job. Consideration must also be given to
the technique and cost of joining (welding) and trimming various types of
slipliners.
Prior to a slipline insertion, an insertion pit is necessary, since the
new pipe does not bend to any substantial degree. A ramp must be built
from ground level to the sewer elevation. As the sewer depth increases,
so must the depth of the insertion pit. Therefore, the volume of
excavated material, dewatering, soil stabilization, etc, increases. Deep
sewers present a special problem of long ramps to allow insertion of the
liner into the sewer main with a minimum of bending. The sewer depth
also impacts the kind of equipment necessary to dig the insertion pit. A
shallow line perhaps would require only a backhoe and two man crew.
An extremely deep line could require the use of a bulldozer or similar
earthmoving equipment to construct the required ramp, trucks to move
earth, pryloaders and other large excavating equipment with the
concurrent increase in crew size and substantial increase in the amount
of time and space necessary to prepare an insertion pit. At depths
greater than five feet there is the additional requirement to either
slope back the sides of the insertion pit to stablize grades; that is the
angle of repose for the excavated soil, or to brace or shore up the pit
so that there is no danger to workmen at the bottom of the pit from
sidewall collapse. The installation of shoring not only is time consuming
and costly, but could create delays due to bracing obstructions during the
sliplining process. Properly placed, however, the shoring should have
very little impact upon the sliplining operation other than its cost of
installation and removal.
Because there is considerable construction activity in the pit bottom and
ramp, temporary paving of the bottom could prevent excessive dirt or mud
from being picked up by the liner and carried into the sewer line. The
obvious reason to avoid large amounts of debris or material being carried
into the sewer is to remove the possibility of the liner binding and
causing costly delays.
Depending upon the groundwater condition, dewatering facilities should be
considered and available to keep the pit dewatered, reducing the
possibility of mud and other material being washed into the sewer. Care
should be exercised at night and over the weekend to maintain dewatered
conditions during cessation of construction activity.
Insertion liners are usually delivered in lengths of approximately 40 ft
(12.2 m). Equipment must be present at the the job site to weld the
sections of pipe together to make a continuous liner for insertion into
the sewer to reduce extraneous flows. Since sewer mains normally run for
300 to 600 ft (91.46 to 182.92 m) reaches (manhole to manhole), insertion
projects normally do three to five reaches or lengths at a time. This
continuous length is accomplished by fusing the end of the pipe together
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electrically. The type and thickness of the liner pipe determines the
power requirement necessary to perform the required heat weld.
The heavier the pipe liner the larger the crew necessary to handle the
work and the fewer the number of welds a crew can do in a day's time. As
the pipe size increases the required time and cost of the job increases.
The actual installation of the sliplined pipe has several factors worth
considering. As the pipe size increases and as the amount of horizontal
and vertical displacement increases, the size and power of the winches
required to pull the slipliner must also increase. Winches are generally
used since pulling tends to reduce the diameter of the front of the
liner, making it somewhat easier to pull the liner through the main.
Pushing the slipline may be successful for short runs. The particular
reach of sewer being rehabilitated should determine the cable length
necessary to execute the insertion and will also determine the number of
guide pulleys necessary to allow the insertion pipe to travel freely
along the reach.
Since the cost of an insertion pit is a major percentage of the total
cost of the job, a single manhole-to-manhole pull is normally avoided
unless there is only one section of pipe in need of rehabilitation. That
can be costly particularly in a deep sewer. The sliplining operation
generally is planned for several sewer reaches at a single pull. There
are some limitations to length however. As the length of pull increases
there is an increasing danger of stretching the pipe beyond its
deformation limit. Additional internal surface drag or friction can
cause "freezing up." The slipline pull is usually attempted as a
continuous effort to avoid the danger of "freezing." Costly freezing
time should even under the best conditions be anticipated. Proving the
line, as you can see, may appear to be costly at first but it can save
considerable time, effort and money in the long run. In some cases,
friction can be overcome by initially coating the outside of the liner
with lubricant during the insertion process. A continuous pull is
important, nevertheless in some cases of substantial pipe deformation, it
may not be possible to overcome the friction force and start the liner
moving again once the pull has been discontinued. The liner may have to
be withdrawn, if possible, or an even more costly approach from the other
direction tried with all the costs attendant to using an emergency
procedure.
Naturally, arrangements will have to be made for bypassing the normal
sewerage flow for the period of time necessary to accomplish the
sliplining. Furthermore, building service lines must be temporarily taken
out of service until sliplining is completed. In this regard, a short
period of time is required to allow initial conformation or expansion to
the original dimensions of the liner pipe. It is then and only then that
the individual service laterals must be dug up, cut, retapped into the
new slipline within the sewer, and reconnected for service. Sliplining
does not provide a way in the present state-of-the-art for reconnection
of the building sewers from inside the liner. The service laterals are
approached as spot repairs that are excavated from the outside.
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Once the liner pull is completed and the initial reconfonnation to
original dimensions has taken place, the liner pipe must be cut at the
manhole and the annular space between sewer and slipline must be sealed.
This prevents extraneous flows entering the old sewer and infiltrating
the new slipline at the manhole. In cases where the annular space is
larger, consideration should be given to filling the space with grout to
1) prevent transportation of infiltration and inflow, 2) reduce the
possibility of liner movement, and 3) provide increased structural
support to the deteriorated sewer.
The sealing of the liner at the manholes is a specialty job. The cost of
the seal plus the manpower necessary to perform that operation should be
included in the cost. Consideration should also be given to sealing the
space between the slipline and service laterals prior to reconnection.
Finally, the insertion pit must be refilled and must be restored to
pre-existing conditions. This includes the repair or rebuilding of the
manhole where insertion was made. Also, the manhole used for winching may
need repairs. All these extraneous costs should be accounted for in the
estimating process.
The cost of rehabilitating sewers by sliplining can vary greatly due to
the several factors discussed. These factors are concisely summarized in
the following Table No. 6.
Each of these factors, independently or in combination, can have a major
impact on the overall cost of a lining project. A thorough understanding
of each of these factors on an individual project basis is required
before a realistic cost estimate can be made. Table 7 is presented as an
"Estimating Guide." It identifies many of the cost elements that can be
segregated for cost determination purposes by sequence of task.
Each lining project is sufficiently different in its complexities,
combination of difficulty factors, and variables to make the presentation
of any general total cost information impossible. The intent of this
section is to provide the reader with a general understanding of the
various factors that may affect the cost of a project and the elements
that must be considered when developing a cost estimate.
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TA3LE 6.
FACTORS AFFECTING SLIPLINING COSTS
Mobilization distance
Access to the site of work and security
Storage area at the site for pipe materials
Storage area at the site for excavated materials
Extent of sewer cleaning required
The technique to "prove" or preinspect the sewer line
Availability and cost of labor
Availability of electrical power for fusing
Size of liner pipe to be handled
Length and location of sewer to be lined
Size and condition of pipe to be lined
Liner pipe wall thickness required
Number of service connections to be made
Ground water elevation and pumping requirements
Excavation and earth stabilization requirements
Grade and direction change of the sewer to be lined
Depth of flow in the sewer line and by passing requirements
Lining costs
Pipe transportation requirements
Type of manhole "seals" needed
Annular grouting requirements
Type of restoration required
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TABLE 7.
ESTIMATING GUILE (COST ELEMENTS)
A. PREPARATION
1. Sewer Line Cleaning
2. TV Inspection
3. "Proving" of Sewer Line
B. COST OF PIPE
1.
Diameter of Existing Sewer
2.
Diameter of Liner Pipe
3.
Depth of Sewer Line
4.
Wall Thickness and Strength Required
5.
Length of Pipe Required
6.
Cost Per lb (kg) of Liner
7.
Weight Required
8.
Freight to the Site
C. INSERTION PITS
1. Depth of Pit
2. Equipment Required
3. Crew Size Required
4. Crew Cost
5• Time Required
6. Special Conditions
a) Shoring Required
b) Dewatering Required
7. Cement for Curing Liner Pit
8. Paving Required
9. Manhole Repairs
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D. WELDING PIPE
1. Crew Size Required
2. Crew Rate
3. Number of Welds Per Day
4- Equipment Required - Handling and Fusion
5. Power Requirements
E. INSTALLING PIPE
1. Winch Equipment Required
2. Pipe Handling Equipment Required
3. Guide Pulleys Required
4. Cable
F. OTHER CONSIDERATIONS
1. Sewage Bypassing Requirements
2. Dewatering Requirements
3. Annular Grouting Requirements
G. CONNECTING BUILDING SEWERS
1.
Type of Connection to be Made
2.
Depth of Connection
3.
Crew Size Required
4.
Crew Rate
5.
Number of Connections Made Per Day
6.
Equipment Required
7.
Materials Required
8.
Cost of Materials
H. SEALING OFF AT MANHOLES
1. Number of Manholes to Seal
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2. Type of Seal Required
3. Cost of Seals
4. Number of Seals Made Per Day
5. Crew Size Required
6. Crew Rate
I. TRANSPORTATION
1. Mobilization
2. Demobilization
3. Freight on Materials
Inversion Lining
This technique as with other rehabilitation methods requires
consideration of the common factors affecting the economics of the
operations which were previously described. Inversion lining also
requires that the sewer line be clean, internally inspected, and
"proved." Proving the sewer for inversion lining is not as critical as
proving the line for sliplining since the inversion liner is somewhat
more flexible than a slipline. It is capable of negotiating a wide range
of pipe alignments and deficiencies. The previous discussion of
sliplining material costing is very similar to the approach to costing of
materials for inversion lining with one notable exception, the inversion
lining requires a thermal reaction chemical saturating the felt lining
material used in the process. The common truth is that as the required
diameter and thickness of the liner material is increased, the cost of
necessary materials and chemicals also increases. One cost advantage of
inversion lining over sliplining is the avoidance of insertion pits.
The technique used in inversion lining is relatively simple.
The liner is usually inserted into the sewer main via a standard type
manhole from the ground surface. There is the requirement, however, of a
scaffold structure immediately above the ground to allow
liner to pass smoothly into the manhole and on into the sewer. Water, an
important part of the procedure, is introduced to carry the liner into the
sewer. The major difference in transportation and preparation costs for
inversion lining as compared to sliplining costs is that the thermal
plastic resin used in the inversion method is heat sensitive. The felt
liner is prepared at a factory and shipped to the rehabilitation site
location. Prior to the inversion or introduction of the liner into the
sewer, the liner is saturated with the thermal plastic resin normally
within 24-hours of use and is then placed in a refrigerated vehicle to
prevent the onset of a setting reaction. This process, depending upon
the location of the preparation plant in relation to the job site and
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weather, can increase the cost of inversion lining significantly because
of the need for refrigerated transportation. The inversion liner treated
with plastic resin is essentially flowed through the existing sewer to be
rehabilitated using the force of water within the inversion tube. It is
essential that the job site has the capability of producing and
delivering hot water to be introduced into the inversion liner. This
accelerates the reaction and causes the thermal plastic resin to set
against the existing sewer pipe. Estimates of the cost of this technique
must allowed for available sources and disposal points of water and
capability of bypassing sewage from street sewers as well as temporarily
halting discharge from service laterals.
The cost factors to be considered after inversion takes place are the time
and materials necessary to seal the inversion liner at the manholes and
the number of service lines that need to be reinstalled on the main line.
The process of reconnection of services is accomplished by positioning a
cutter adjacent to each building service line inside the main line and by
remote control and TV observation plus cutting out the solidified resin
material blocking the service laterals. Since the process is remote
controlled, an operator may have to position the internal cutter a number
of times to determine the exact position of the connector. It is not
always evident where the connection is made. The average time for the
cutting operation per lateral is about 15 to 20 minutes. As improved
techniques are devised, cost of this operation should decrease.
There is no need to grout an annular space since the set resin conforms to
the inside surface of the existing sewer. Two types of resin may be used.
If an epoxy resin is used, the line must be clean and kept that way or the
resin will not adhere well. On the other hand, if a vinyl resin is used,
absolute cleanliness is not essential since there is no actual bond
created between the liner and the inside of the pipe.
At the present time, inversion lining is a patented process and one which
is totally sole source. Therefore, the estimator should be alert to the
possibility of price fluctuations of material and equipment as well as an
absence of competitive bidding. The inversion lining process presently
has been franchised to single source representatives in various areas of
the country. Those areas do not overlap and the franchise does not allow
the dealer to operate outside of a specific sales territory, thereby
creating a single source negotiation type of pricing schedule. It is
pointless therefore, to summarize the factors affecting inversion lining
costs when you must defer to quotations from francished operations.
Typical costs for this type of rehabilitation have been reported (on the
East Coast) to be in the range of $50 to $60 per ft for pipe 10 in. (0.25
m) in diameter. The reported costs include line evaluation and
preparation before the inversion lining process.
There are additional considerations to a complete rehabilitation project
which should not be overlooked. They are sealing infiltration into
manholes and probably the most difficult of all, sealing leaking building
service laterals.
Manhole Rehabilitation
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Manhole rehabilitation costs must be approached from two distinctly
different points of view, 1) rehabilitation from infiltration, and 2)
rehabilitation from inflow.
Inflow can be estimated by evaluating the elimination of surface rain
water entering the manhold cover area and direct drainage connections to
the manhole. Costs attendant to this condition are relatively low and
inexpensive methods can be employed to eliminate inflow. Reducing this
important fraction of the extraneous flows to the Bewer can occur during
and immediately after rainy periods.
Infiltration on the other hand is somewhat more costly and difficult to
correct.
In that regard manhole rehabilitation cost estimates require an
understanding and evaluation of numerous interrelated factors. These
include type of manhole construction; type, cause, and extent of failure;
depth of manhole; type of soil surrounding the manhole; manhole location;
the rehabilitation techniques selected; rehabilitation material selected;
and other factors. A list of available manhole rehabilitation techniques
is provided.
Rehabilitation Techniques:
Internal Cement Grouting by Pressure Injection
Internal Chemical Sealant Grouting by Pressure Injection
External Chemical Sealant Grouting by Soil Injection
External Cement Grouting by Soil Injection
Internal Epoxy Mortar Resurfacing
Internal Cement Mortar Resurfacing or Guniting
Mortar Joint Replacement by Pressure Injection
"External Drop" Sealing
Resetting and Resealing of Manhole Frames
Replacement of Manhole
Each of the above techniques may be utilized separately or combined in
various manners depending on manhole condition and the extent, type, and
nature of rehabilitation required. The obvious difficulty is in
determining which technique or combination of techniques will be most
successful in a particular circumstance. The selection is generally
dictated by type of manhole construction in conjunction with the nature
of the physical failure involved.
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Service Lateral Rehabilitation
It is recognized that a substantial fraction of infiltration and in some
cases, inflow, can be traced to house connections (building service
laterals). Some of the extraneous flow can be observed (at time by TV
inspection) to come from the vicinity of a connection to the street sewer.
Whether or not the observed flow originates from the point of connection
or further up the service line is often unclear. The rehabilitation
barrier is the property line.
Costs of service line repairs to date have been relatively expensive due
to inaccessibility because of property lines and the incumbent
implications. Proprietary methods have been or are in the process of
being developed for repairing connections as well as using a chemical
grouting method a short distance up the service line. Other methods have
included spot repair using excavation methods. The essential problem
still remains; repair of relatively shallow house connections (service
laterals) on developed and private property. The most inexpensive method
would be investigation and repair from the house basement to the street»
but by whom, and at whose cost. A recent approach to this problem (on
the West Coast) has promise if universally adopted; that would be to make
the houseowner responsible for his drain by providing a certification of
inspection and rehabilitation of a service line each time a property
changes hand. Guaranteed in this manner, poorly installed service lines
or lines damaged by roots must be examined and repaired prior to transfer
of title. Whether or not this approach becomes popular must await
resolution and widespread adoption by governmental agencies.
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CHAPTER 6
LONG-TERM REHABILITATION' PROGRAM
Any rehabilitation program designed to upgrade sewer systems involves
time, personnel and material. The final success or failure of a
rehabilitation program depends on materials used, workmanship and
maintenance of the sewers once they have been renovated. Accordingly,
the administrator of the system should assure adequate inspection during
rehabilitation efforts and provide for routine inspection and repairs of
the system, that is, a preventive maintenance program.
INSPECTION
"Cataloging" is the first step in the inspection process. It involves
taking an inventory of manholes, determining invert elevations, and
mapping the direction of flow together with the number of inlets and
outlets. If the agency has had a Sewer System Evaluation Study (SSES),
most of the initial mapping will be available. Once the known manholes
are logged, dye or smoke testing will show the flow routes in the system.
When the dye disappears or when smoke appears unexpectedly, investigators
can be certain that an unknown manhole or other diversion structure is
diverting the flow; they can also estimate where overflows exist in the
system. If overflow structures exist, it is possible that they affect
inflow.
Next comes the inspection phase. The usual procedure is to insert a
remote control TV camera into the sewer line and then to log the
conditions observed.
To insert the camera, technicians allow a float with a light line
attached to it to float from an upstream manhole down to the next manhole
on the run. A cable winch is set over the lower manhole, and the line is
connected to a cable. After drawing the cable through the sewer, it is
fastened to a winch. The front end of the TV camera is attached to the
free end of the cable and a second cable to the rear. A second cable,
along with the TV transmission lines, is fastened to a second winch in
the survey van. The second winch has a built-in footage counter which is
synchronized with the camera's entry into the upstream end of the sewer
line. Filming is done in the downstream direction to prevent the flow
from fouling the camera lens. If the flow is excessive, the line must be
plugged and diverted during the investigation. Using two winches to move
the camera back and forth, the operator can locate problems in the line.
By keeping a log of observed conditions, the process of decay can be
evaluated and recommendations as to repairs required can be made.
For inspection purposes, the system should be separated into such units as
drainage areas and city sections. If the inspection team works a specific
area, it will be easier to keep track of progress and to ensure that all
the lines are inspected regularly.
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MAINTENANCE
Maintenance is as important as inspection for an uncorrected problem can
lead to the rapid deterioration of other parts of the system. Maintenance
consists not only of repairing the deteriorated sections, but also of
cleaning the lines to allow free flow. Cleaning is necessary because the
lines are seldom constructed on an even grade: Blight changes in grade can
cause solids to build up in eddies. Along with the solids found
naturally in the flow, any number of articles find their way into the
sewer line through manholes, catch basins, laterals, and cross
connections. These articles range from small stones and grass to auto
parts and building materials and they can cause stoppage which not only
inconveniences those served, but can cause #xfiltration to discharge to
the sewer backfill material. At some point in the process a cave-in may
occur which may in turn collapse the pipe.
Tree roots cause additional problems. Roots are drawn to moisture which
the sewer system provides in abundance. Where the ground water table is
well below the sewer system, the sewer is even more threatened.
One of the most widely used methods for protecting sewers from root
damage is augoring. Here the root system is cut off with a rotating
blade inserted in the line. Since roots grow from their tips, auaerina is
only a temporary solution: the roots begin to grow again at each cut; the
cuts are larger in area than the previous tips, and so the roots grow
back even larger and stronger.
Recently, several methods have been devised to stunt root growth without
killing the vegetation above ground. Two methods currently employed are
flooding and foaming the sewer line with a special herbicide. Both
methods require the sewer line to be filled with herbicide. Foaming is
especially helpful for lines with low flows because it does not require
the complete relocation or blocking of flows.
Another technique presently being employed is the mixing of herbicide with
sealing grout during rehabilitation of the line. This not only retards
root growth, but also seals minor cracks in the sewer line, thereby
reducing the amount of infiltration.
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CHAPTER 7
LONG-TERM FLOW MONITORING
The success of rehabilitation projects depends on the decrease of
infiltration/inflow in the test section. By conducting prerehabilitation
testing during the rainy season and postrehabilitation testing during the
dry season, a substantial decrease in infiltration flow can be effected.
Monitoring under these conditions, however, is worthless. The real test
comes from long-term monitoring occurring over several wet and dry
seasons; results can then be averaged over a reasonable period of time.
Testing should take several factors into account: prior moisture
conditions, flow versus rainfall, flow versus intensity, flow versus
duration, and existing conditions.
Upon completion of the rehabilitation project, inspectors should
re-evaluate the project area. The evaluation should include an analysis
of the extent of the rehabilitation project. Did financial or other
constraints limit the work or was there an effort to remove all
identified I/I? Were house laterals a part of the rehabilitation
program? Were manholes and other structures rehabilitated? If the
project area has been made sound, the presence of i/l will indicate
either overlooked points of entry, failure of rehabilitation, new sources
of i/l, or tranference of i/l from areas outside the project.
The next task, in comparing pre-and postrehabilitation performance, is to
evaluate moisture conditions in the soil, at the start of a measured
rainfall. If the area to be measured has just undergone a long hot dry
spell, the ground may have been baked sufficiently so that most of the
initial rainfall runs off and does not penetrate to the sewer line. If
the ground is dry but not baked, there will be a considerable delay
between the rainfall and a noticeable rise in flow through the sewer
line. When the rain has stopped, the flow increase should continue as
the ground water level recedes. If the ground is saturated at the time
of the rainfall, the flow in the sewer line should continue at a high
level and then decrease after it stops raining. Soil conditions at the
time that the original estimate of I/l balances were made should also be
considered for comparison purposes.
The intensity, duration, and timing of rainfall events are extremely
important as to their potential impact on i/l flows. When one rainfall
event occurs soon after another, the soil may already be completely
saturated; thus, there is almost total run off of the second rainfall,
regardless of intensity or duration. When the second rainfall occurs
substantially later, the soil has a chance to return to normal and to
accept additional rain.
If the soil has returned to normal, two more points must be considered:
rainfall intensity and duration. With a normal soil, that is, ground
water level at an elevation standard for the area, a light rain of short
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duration will create a flow condition for which a hydrograph of the
sanitary sewer flow closely approximates the rainfall histogram. The
flow will, however, lag behind the rain event because of the time
necessary for the water to percolate through the soil and be intercepted
by the sewer line. Discounting base line flow, the sewer hydrograph
should be less than the rainfall histogram because only some of the rain
hitting the ground is actually collected by the sewer line; the rest
remains within the soil.
Light rainfall of long duration will also produce a sewer line hydrograph
which will lag behind the rainfall histogram. Discounting the base line
flow, the sewer hydrograph will be less than the rainfall histogram by the
amount of surface runoff.
Heavy rainfall of short duration will have little effect on flow in the
sewer because most of the water that reaches the ground will run off,
since it has no time to soak in.
Heavy rainfall of long duration will have a greater impact on the sanitary
line.
PRE - POST COMPARISON
Fair evaluation of a rehabilitation program demands that inspectors
compare conditions existing before and after rehabilitation. Because no
two rainfalls have exactly the same characteristics, it is extremely
difficult to correlate prerehabilitation and postrehabilitation flows;
only through long-term monitoring can the flows be averaged to the point
where significant conclusions can be drawn. While agencies like to
evaluate projects immediately, inspectors can only give educated guesses
until their data is complete.
MONITORING AFTER SANITARY SEWER REHABILITATION
The primary objective of long-term flow monitoring is to measure sanitary
system performance against predicted performance, under a defined set of
circumstances. This will determine the effectiveness of rehabilitation
and act as a "flag" indicating when additional maintenance work is required.
Long-term flow monitoring consists of the development, installation, and
operation of a systemwide network following completion of rehabilitation.
USEPA now requires "after-action" flow monitoring of the system to
"certify the i/l conditions in the sewer system at the end of the first
year of operations..."
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Specific benefits to be derived from long-term flow monitoring include:
1. Allows the continuous evaluation of I/I conditions.
2. Provides a basis for implementing additional I/I removal, as
needed.
J. Provides a constant check on system deterioration, and thus
avoids a large "one time" expenditure to update the system.
4. Allows pricing of a system's requirements throughout its
life.
5. Identifies areas which are "top priority" for future
maintenance.
6. Help to determine if a project has met i/l removal
standards.
Long-term flow monitoring involves three phases: preparation of locations,
purchase and installation of equipment, and evaluation of the project
before and after completion. The first two phases are discussed below:
FLOW MONITORING LOCATION SELECTION
The first step in a long-term flow monitoring program is to select key
manholes. Key manholes (KKH) are those manholes selected for
installation of a flow meter; manholes with drop services, rough troughs,
significant sludge deposits, or sewerage buildup are not suitable.
Engineers should also check sewer conditions upstream and downstream from
a potential key manhole. This is important for future reference if
upstream and downstream lines require calibration. Key manholes located
immediately upstream from a highly fluctuating level in a pump wet well
will yield poor results. Key manholes Immediately downstream from a lift
station or force main will also yield poor results. This is also true of
key manholes immediately downstream from a lift station or force main
because of pump cycling.
Once a key manhole location has been selected, location and the diameter
of the line in which the meter is to be mounted should be recorded.
FLOW MONITORING
After selecting key manholes, monitoring equipment must be selected and
installed. A variety of long-term monitoring equipment is currently
available. Equipment can be as simple as a depth recording meter with
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on-site data retrieval or as sophisticated as a meter measuring both flow
depth and velocity, with telemetric tie-in to a central computer which
analyzes, sorts, and stores flow data.
Flow monitoring should coincide with the operation of a nearby
continuously-recording rain gauge. In this manner, engineers can
correlate sewerage flows and precipitation data. The "tipping bucket"
rain gauge, which is generally used, should be mounted in an area free
from buildings and trees-and if possible, at the wastewater treatment
plant, city sewer, water department, or some other readily accessible
location. Before starting the monitoring period, engineers should check
the rain gauge for calibration.
For some installations, level sensing flow meters may be adequate. Where
possible, flumes or weirs can serve as primary flow measuring devices. In
pump stations with constant speed pumps, an event recorder can monitor
flow. Occasionally, such meters will be inadequate and other kinds of
meters will have to be used.
Initially, meters should be serviced at least once a week and after major
rainfalls. After engineers have verified that meters are operating
properly, semiweekly or monthly maintenance may be all that is necessary.
Recorded meter depth should be checked against the measured depth and
adjusted accordingly. Velocity readings should also be taken to provide
calibration data. There should be a suitable site for the storage of
batteries and spare meters, and for battery charge.
For those gravity flow monitoring locations without a primary measuring
device, calibration is required to correlate flow rate and liquid depth.
This is best performed during an infiltration and inflow period when
fluctuations in flow are greatest.
Engineers should establish at least four calibration points covering a
wide range of depths, and should conduct both instantaneous calibration
and dye-travel calibration. The calibration done at the meter location
will establish the average velocity. The dye is placed either in the
manhole upstream from the key manhole or in the key manhole itself. By
using a stopwatch, the time that is required for the dye to reach the
downstream manhole can be estimated. Depth of flow measurement of the
key manhole at the point where the float will measure must be made,
noting any deposition. Instantaneous velocity probes may be used at the
point where flow was measured. But readings decrease in accuracy with
low depth of flow. To determine an average velocity using the time of
dye travel, the distance between manholes must be measured with a tape or
rollotape wheel. The invert elevation of the sewer must also be
determined.
After a satisfactory calibration curve for each location is established,
the depth data can be converted. When contractors are involved in
digitizing flow results, engineers should maintain a log of charts and
resulting printouts, together with mailing dates, dates they received
printouts, and the chart numbers for both the printouts and the charts.
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All digitized data must be analyzed promptly to assure that it is
satisfactory.
Engineers can calibrate pump stations by the "fill and draw" method or by
a sonic meter mounted on the force main; they can also take depth and
velocity readings to measure peak discharge at the end of the force main.
Information on servicing and calibration should be kept in a log book.
Each monitoring location should also be recorded. Exact diagrams or
drawings of each location should be made, noting the kind of meter, date
of installation, and any other distinguishing information.
When servicing a location, a record of problems encountered such as added
deposition in trough, debris on a float, recorder malfunction, or battery
failure, etc., should be made. If data appears to be missing or
conditions indicate that data may be unreliable, such observations also
should be recorded in the log book as well as on the strip chart, if
available.
Primary Measuring Devices
A precalibrated primary measuring device such as a weir or flume can
simplify meter installation and calibration. Meters which measure both
flow depth and flow velocity do not require a primary device.
Weirs and flumes each offer advantages and disadvantages. A weir is the
simplest device for measuring flow in open channels. It is cheap,
relatively easy to install, and quite accurate when used properly.
However, a weir normally operates with a rather significant head loss,
and variations in the approach velocity of flow can affect its accuracy.
Also, weirs require periodic cleaning to prevent sediment or solids
buildup on its upstream side, for this, too, reduces accuracy.
A flume tends to be self-cleansing since the flow through it has an
increased velocity and there is no actual "dam" across the channel. Then
too, flumes operate with a much smaller head loss than a weir, an
important factor in cases where the available head is limited. An added
benefit is that varying approach velocities do not affect a flume as much
as they do a weir.
Equipment Selection
Equipment selection involves consideration of initial cost, maintenance
cost, accuracy, system reliability, and ease of data acquisition and
reduction. To be effective, long-term flow monitoring systems require
low maintenance and easy data acquisition and reduction. Equipment with
telemetry and computer data retrieval and reduction is therefore
recommended; failing that, equipment should at least have a computer
summary of stored data, with on-site retrieval.
Several manufacturers offer sophisticated flow monitoring equipment:
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although the initial cost is more than that of simple short-term
monitoring systems, equipment with computer telemetry and/or data
reduction capabilities accomplishes more, requires less maintenance, and
is therefore more economical.
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CHAPTER 8
REGIONAL SITE VISITS
In the latter part of 1982, and early in 1983, the staff of APWA held ten
meetings with interested federal, state, local agencies, and consultants
regarding control of infiltration and inflow into sanitary sewers. While
all of the regions reported problems with inflow and infiltration into
their sanitary lines, some of those problems crossed regional boundaries.
The staff found that there were many divergent regional differences.
120 GPCD AS GUIDELINE
In general, all of the regions agreed that instituting a single
all-encompassing empirical value for excessive flow in a sewer system is
not the preferred way to attack the problem. The consensus was that
while an empirical number may well be a good starting point, the local
agency should have the opportunity to evaluate its system to determine if
the final evaluation number would be higher or lower than 120 GPCD (454),
based on local operating conditions. The evaluation should take into
account such factors as population density, climatic condition, age of
the system, kind of population, and certain other geographical
considerations which influence the groundwater table.
HOUSE LATERAL CONSTRUCTION PROBLEMS
A major source of inflow discussed at most site visits resulted from house
laterals constructed at a different time than the main sewer line and to
less rigorous inspection standards. Participating agencies indicated that
generally the main sewer line which lays in the right-of-way and is the
main transport vehicle for sewerage was constructed either with agency
forces or contracted out to private construction firms and inspected by
agency forces. Either way, this resulted in a product which generally was
constructed well enough to meet the standard critera for maintaining the
long-term integrity of the sewer line.
Service laterals, on the other hand, are generally laid at the time a
building is constructed. This could be years after the installation of
the sewer main. Also, this work is generally done by a plumbing
contractor or general building contractor. Usually, this work has had
little Inspection or testing and the sewer line is often laid on a poorly
prepared subgrade with little or no attention to proper sealing of
joints. Backfill of the sewer trench is often accomplished with only
readily available materials.
Because of these factors, the sewer line generally settles over time, the
joints separate, a rock in the fill may penetrate the sewer line, or the
alignment of the sewer line itself may shift and shear the pipe. All of
these situations result in significant defects which will allow
infiltration.
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To further complicate the problem, it is a general construction practice
to lay the building sewer on a minimum grade from the building line to the
property line. At the property line, the lateral sewer grade is increased
significantly to intercept the sanitary sewer below. This procedure is
followed because it is less costly to the contractor laying the line,
since most of the pipe run will be in a shallow trench, as compared to
the last 15 to 20 ft. run (4.6 - 6.1 m) of the service lateral. Because
of the steep grade and the other factors which have been noted, a
significant amount of stress is placed at the junction of the building
sewer and the main sewer line. If the contractor has been diligent, he
will intercept the sewer line at the location where a wye or tee has been
placed for access into the main sewer line. If not, a standard is to
stab a section of building sewer line into the side of the main sewer and
patch the area with mortar. Theoretically this seals that location from
infiltration or inflow. Over time, the stress at that point generally
results in the connection opening up and allowing a significant amount of
infiltration into the main sewer. Because of the current design of sewer
rehabilitation machinery, such a joint is almost impossible to seal.
HOUSE LATERAL CONTRIBUTIONS
While there is almost no concrete evidence to support the participants'
feeling, all intuitively indicated that apparently the house lateral or
service line is a major contributor to the i/l flow for reason cited.
One rehabilitation project in Washington State was staEed and the T/I
reductions achieved as a result during the project seemed to bear that
out. In a second project currently underway, the Washington Suburban Sanitary
Commission is studying both the effects of groundwater migration and the
effect of service laterals in l/l. The work accomplished to date with a
sewer sock appears to reinforce the idea that service laterals are a major
contributor to l/l. Finally, several areas in EPA Region 1, especially in New-
England have begun demonstration projects to place underdrain and clay
barriers adjacent to sanitary sewer lines in an effort to reduce the
infiltration and inflow into the sanitary sewers.
HOUSE LATERAL REHABILITATION TECHNIQUES
Agencies are currently using two techniques to try to solve such problems.
One is to position the packer over the building lateral and fill the lateral
with grout. Once the grout is set up, an auger is sent down the house.lateral
to ream the inside of the service lateral and allow usage again.
The second technique being used in Washington State and by the Washington
Suburban Sanitary Commission is what is referred to as a "sewer sock." This
is a specially-adapted grouter which has a 6 ft. (1.8 m) long sock that
can be projected into the house lateral for 6 ft. (1.3 m). A doughnut is
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then inflated at the end of the grouter, and grout is pumped along the
outside of the sock, sealing the house lateral. Once the grout has set
up, the sock is deflated, pulled back out of the house lateral,
eliminating the need to remount the lateral itself. Several problems are
yet to be worked out with this particular techniques One is extensive
wear and tear on the sock and the second is that it is almost impossible
to project the sock up into laterals entering the sewer directly from the
crown of the sewer main.
SINGLE POINT VS. MULTI POINT MONITORING
Considerable discussion was held at the site meeting regarding the
requirement in CG 82 regarding single-point monitoring of the system to
determine excessive I/I. Only in New England, where towns are small, did
participants indicate that single point monitoring at the plant would be
adequate. This was supported by the fact that, with a few exceptions,
most New England towns were very small and therefore, the sanitary
collection systems were very small too. Thus, they could approximate a
single sub area with a monitor at the key manhole. Representatives from
all other regions expressed some displeasure at the prospect of plant
monitoring. With few exceptions, the agencies have existing monitoring
points so that a reasonable division of the system could be accomplished.
They indicated that one reason plant monitoring might be inadequate is
that, with few exceptions, the agencies have existing bypass structures;
that is, a facility operating only during high flow periods which will
eject from the system those flows greater than the system is capable of
transporting. Such a facility generally would be quite prevalent in an
older system found normally in the East, and in some cases, in the
Northwest. These facilities are found in these systems because, during
their early development, prior to institution of treatment plants,
combined sewers were built and the flow discharged directly into a
receiving water.
With the advent of treatment plants for regional areas and individual
municipalities, a series of intercepter lines have been constructed which
have diverted the flow from individual collector lines prior to their
discharging to the receiving water.
In many cases, the intercepter was constructed to accommodate maximum
sanitary sewer flows regardless of stormwater surcharging. The overflow
was built to take care of those situations immediately following rain
events which tremendously surcharged the system. The overflow would
relieve the backwater condition and thereby reduce the amount of basement
flooding in the system caused by the backup.
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ILLEGAL CONNECTIONS
A similar condition of inflow occurs in the East and other older sections. In
many cases, where a combined sewer was constructed, the street drainage svsteni
was tied into the sewer as well as downspouts from the houses, sump pumps
in basements, and foundation drains. Often those agencies responsible
for the sanitary sewers have indicated existing inflow points generally
are cost effective to remove, with the exception of foundation drains.
Because foundation drains are almost as deep as the sanitary sewer as a
rule, storm sewers would not be located in places where the foundation
drain could be tapped into it. without a sump pump there is nowhere the water
which collects in the foundation drain can be put other than in the sanitary
sewer where it currently is collected. For these reasons, these agencies
say it is impractical , for the most part, to remove the foundation
drains. Conversely, while they have found that downspouts, sumps, area
drains, and other similar inflow points can be removed economically, they
are extremely difficult to maintain when disconnected. This requires
constant surveillance of systems and continual removal of the inflow
points as they are reconnected.
Generally, a property owner does not appreciate the downspout running
stormwater into his yard and forming a puddle which may take from five
minutes to several hours to drain back into the soil; thus, creating an
unpleasant soft soil condition with which he must contend. It is,
therefore, much easier for the owner to reconnect the downspouts, sump
pumps, and area drains to allow for quick disposal of the water from his
property. Several municipalities in a number of the regions have
considered suggesting a remedy of placing a surcharge on the sewer fee of
property owners with "illegal sources" connected to the sanitary system;
the surcharge would be adequate to pay for the cost of additional
transportation and treatment. Furthermore, that surcharge may in fact be
levied for varying amounts at various times of the year depending upon
the natural ground condition and maximum points of peak inflow which the
system experiences during the year.
CROSS CONNECTION
While a demonstration project for installation of underdrainsand clay
barriers to prevent groundwater migration and to lower the groundwater
table around sanitary lines has been discussed, several regions indicated
that many underground utilities lie adjacent to one another within the
street right-of-way. Sanitary sewers, potable water lines, and storm
sewers in particular often parallel each other and, in some cases, even
run in the same excavated trench.
Investigation of parallel storm and sanitary systems has shown in certain
cases, migration from the storm sewer when surcharged to the sanitary
sewer through leaking joints. In other cases, they have found potable
water in the sanitary sewer finding its way into the sanitary sewer
because of leaks in the pressurized water system.
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In the northern areas, a parallel to leaking water mains occurs during the
winter. Because of the prolonged cold experienced in those areas,
property owners will leave a trickle of water running to prevent their
potable water lines from freezing. This results obviously in raising the
domestic sewerage component of the total flow in the sanitary sewer line
during the winter. Parenthetically, this is another reason for not using
a hard and fast 120 GPCD (454) amount to determine excessive i/l in a
system.
MANHOLE INFLOW
In the same northern areas, a phenomenon known as "frostheave" creates
havoc on manholes in the winter. The upward
movement of the ground haB been known to completely separate manhole
frames and lids from the riser of the manhole by some 2 to 3 in. (5.1 to
7.6 cm). This breaks the mortar seal at that particular location.
If water is found running along the subgrade, which is especially common
under concrete streets, the broken seal between the frame and the corbel
is a perfect entry place for the inflow.
MIGRATION OF GROUNDWATER
One of the major discussions for which there is only a small smount of hard data
to complement the intuition of those agencies in the area, is the phenomenon
of groundwater migration. Here again, the Washington Suburban Sanitary
Commission currently has a research project underway to investigate
this phenomenon. But it is the intuition of those who work with the
sanitary system that when a visible inflow point in the sewer is fixed
with no regard to the condition of the line upstream and downstream, the
hydraulic head is increased at the point of repair. This forces the
groundwater out through smaller failures in the pipe, which did not leak
prior to the original repair. Therefore, while there is a 100 percent
reduction of infiltration from the point of repair, the total reduction of
infiltration in that particular reach is infinitesimal since the increased
hydraulic head is now forcing the water through many smaller holes.
Through the use of trace materials, groundwater gauges, and flow
monitoring in the sanitary sewer lines, the WSSC hopes to be able to
define empirically the effect of groundwater migration on a sanitary line
after rehabilitation. They hopu to make a better determination of the
extensiveness of rehabilitation required to effectively reduce the amount
of infiltration and inflow into a reach of line.
COST EFFECTIVE I/I REMOVAL
Finally, the agencies felt that removal of inflow is cost effective and
should be done positively to reduce transport and treat costs. If a
rehabilitation is planned for a sewer network, they feel that
contributors of major inflow should be removed first so that the ground
water migration effect can be maximized to pinpoint as many as possible
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of the smaller leaking joints. In this way, the rehabilitation
evaluation will not just identify the major contributors, but will give a
fair representation of the actual condition of the line prior to
rehabilitation. With full knowledge of the condition, perhaps a better
rehabilitation project can be instituted which will show a much more
dramatic reduction in l/l than past infiltration/inflow projects have
shown.
While the agencies acknowledged that they need to reduce inflow and
infiltration in their sanitary sewer systems, they recognize that
realistically the priority for l/l work is very low. As long as
downspouts and/or sump pumps and area drains are allowed to exist in the
sanitary system because local agencies do not have funding adequate for
this kind of work, l/l will continue to be a problem in the sanitary
sewer systems. At present, all of the local agency's money is being
spent to maintain a system which is rapidly deteriorating and in allowing
the system to transport the total flow from the points of input to the
treatment plant, no matter how much I/I it captures along the way.
NEW DEVELOPMENT STANDARDS
Many participants in the site visits indicated that better methods of
lowering groundwater tables should be investigated. They believe that, in
many areas the easily developed properties have already been utilized and
development must now occur in areas more prone to problems with respect to
construction techniques and drainage, to mention a few. Since developers
must now utilize property which does not drain well, both sanitary and
storm sewer systems will more often be subjected to high water tables
than was true of those constructed previously. While this makes no
difference in storm sewer, the sanitary sewer must be able to withstand
the increased potential for infiltration and inflow. As aging occurs in
the sewer lines, defects will begin to appear and infiltration will become
evident.
IMPROVE BUILDING LATERAL ACCESS
With the advent of the 2 in.(5.1 cm) self-propelled TV cameras, house
laterals can now be checked readily to determine their condition. In many
areas of the Northwest, it is required that the builder or property owner
provide an access point to the house lateral somewhere outside the
structure. This access point is normally a clean out.
This provides the utility with the ability to enter the lateral with
investigation equipment, and where necessary rehabilitation equipment,
without having to excavate the entire lateral in order to rehabilitate
it. In some cases, the utility has actually constructed the clean outs
on house laterals where the original construction occurred before the
requirement for an exterior access point.
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BIBLIOGRAPHY
Economic Analysis, Root Control, and Backwater Flow Control as Related to
Infiltration/Inflow Control, EPA-600/2-77-017a, Sullivan, R. H. , et al. ,
American Public Works Association, U.S. Environmental Protection Agency,
Cincinnati, Ohio, 1977.
Sewer Infiltration and Inflow Control Product and Equipment Guide,
EPA-600/2-77-017c, Sullivan, R. H., et al., American Public Works Associ-
ation, U.S. Environmental Protection Agency, Cincinnati, Ohio, 1977.
Sewer System Evaluation, Rehabilitation and New Construction - A Manual of
Practice, EPA-600/2-77-017d, Sullivan, R. H., et al., American Public
Works Association, U.S. Environmental Protection Agency, Cincinnati,
Ohio, 1977.
Evaluation of Infiltration/Inflow Program, Conklin, G. F., and P. W. Lewis,
Project No. 68-01-4913, U. S. Environmental Protection Agency, Washington,
D.C., 1980.
Assessment of Sewer Sealants, EPA-600/8-82-012, Sullivan, R. H. and W. B.
Thompson, American Public Works Association, U.S. Environmental Protection
Agency, Cincinnati, Ohio, 1982.
Existing Sewer Evaluation and Rehabilitation, ASCE Manuals and Reports on
Engineering Practice No. 62 and WPCF Manual of Practice No. FD-6,
American Society of Civil Engineers, 345 E. 47th St., New York, NY 10017
and Water Pollution Control Federation, 2626 Pennsylvania Avenue, N.W.,
Washington, D.C. 20037, 1983.
Sewerage Rehabilitation Manual, Water Research Center, WRC Environment,
Medraenham Laboratory, P.O. Box 16, Henley Rood, Medmenhara, Marlow,
Buckinghamshire SL7 2HD, England, 1984.
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TECHNICAL REPORT DATA
(Please read /uu/verions or. the reverse before com;
PRflR -1 770C\r>
1 . REPORT NO. 2.
EPA/600/2-85/n?n
i in inn inn
Mill
J. t.tle and SUBTITLE
Selected Topics Related to Infiltration and Inflow in
Sewer Systems
5. REPORT DATE
March 1985
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
Richard H. Sullivan and James W. Ewing, II
8. PERFORMING ORGANIZATION REPORT NO.
9 PERFORMING ORGANIZATION NAME ANO ADDRESS
American Public Works Association Research Foundation
1313 East 60th Street
Chicago, Illinois 60637
10. PROGRAM ELEMENT NO.
B 113
1 1. CONTRACT/GRANT NO.
No. CR808934-01
12. SPONSORING AGENCY NAME ANO ADDRESS
Water Engineering Research Laboratory-Cin., OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Carl A. Brunner 513-684-7655
16. ABSTRACT
Tnis project was undertaken as a review of the current state-of-trie-art
in infiltration/inflow control and to present information not included in
earlier manuals and reports on this subject. A series of nine regional sem-
inars was conducted to explore local problems and practices for solution.
Chapters in tnis report respond to problem areas discussed at these seminars.
Besides an overview, the report includes information on problem determination
as approached by the Washington Suburban Sanitary Commission, methods for
flow determination including a discussion of accuracy, economics of sewer
rehabilitation, methodsof rehabilitation, long-term rehabilitation programs,
and long-term flow monitoring. A brief discussion is included of the major
problems discussed at the regional seminars.
17. KEY WORDS ANO DOCUMENT ANALYSIS
a. DESCRIPTORS
b IDENTIFIERS'OPEN ENDED TERMS
c. COSati Fie:^ Group
13 OiS'RiBuT.ON STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS {This Reporl)
UNCLASSIFIED
21 NC OP PAGES
8?
20 security CLASS 'Tins page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
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