EP A/600/ JA-02/226
2002
Wastewater collection system infrastructure research needs
Anthony N. Tafuri, P.E.*D
Ariamalar Selvakumar, Ph.D., P.E.**D
U.S. Environmental Protection Agency, D
National Risk Management Research laboratory, D
Urban Watershed Management Branch, D
2890 Woodbridge Avenue, Edison, NJ 08837. D
Abstract
Many of the wastewater collection systems in this country were developed in the early
part of this century. Maintenance, retrofits, and rehabilitations since then have resulted in
patchwork systems consisting of technologies from different eras. More advanced and cost-
effective methods to properly rehabilitate these systems must be considered to guarantee
sustainability into the future. Achieving sustainable development presents a challenge to deliver
new and innovative infrastructure and facilities needed to serve society while protecting the
environment. In the context of this paper, sustainable development would provide new and
improved solutions to existing and emerging problems associated with wastewater collection
system infrastructure. Such solutions would, for example, include consideration of innovative
approaches and practices for identifying and rehabilitating problems in existing systems and
ways of preventing these problems in new construction. The paper focuses on technical issues
ID
-------
and research needs in three major areas: (1) assessment of system integrity; (2) operation,
maintenance, and rehabilitation; and (3) new construction. Many of the issues and needs
discussed were identified at a USEPA sponsored experts workshop on infrastructure problems
associated with wastewater collection systems.
Keywords: Wastewater collection system, system integrity, flow monitoring, condition
assessment, rehabilitation, pipelines, manholes, infiltration and inflow, new construction.
Corresponding Author. Tel: (732) 321 6604; Fax: (732) 321 6640.
E-mail address: tafuri.anthony@epa.gov
1. Introduction
The aging condition of our cities and an associated deterioration of infrastructure
(buildings, highways, utility systems, water distribution systems, sewerage systems) leads to an
emerging research area addressing how best to design, construct, maintain and repair both
existing and new infrastructure. The costs associated with maintaining infrastructure are
staggering; the national investment in sewers alone approaches $1.8 trillion (ASCE, 1996).
Sewer pipeline stoppages and collapses are increasing at a rate of approximately 3% per year.
Roots that puncture and grow inside pipes cause over 50% of the stoppages, while a combination
of roots, corrosion, soil movement and inadequate construction are the cause of most structural
2D
-------
failures (ASCE, 1994). There are an estimated six to eight hundred thousand miles of sewer
pipes in the United States. According to a study conducted by the Urban Institute (1981),
approximately 50 major main breaks and 500 stoppages occur per 1000 miles per year,
amounting to an estimated 30,000 breaks and 300,000 stoppages annually. Deterioration of
jointing materials, pressure surges, disturbance by construction or direct tapping, and seismic
activity also contribute to collection line failures. These problems result in approximately 75%
of the nation's piping systems functioning at 50% of capacity or less (ASCE, 1994).
Besides stoppages and collapses, infiltration and inflow (I/I) can rob capacity in a
sanitary sewer system and negatively affect operation of the entire sewerage system. I/I are
extraneous flows that enter the sewer system. Inflow is generally defined as stormwater that
enters the collection system through direct connections (e.g., roof leaders, cellar and yard area
drains, foundation drains, commercial and industrial so-called "clean water" discharges, drains
from springs and swampy areas, etc.). Infiltration is defined as water that enters from the
ground. In the late 1980s, the term "rainfall-induced infiltration" was first used to describe
infiltration with flow characteristics resembling inflow (i.e., a rapid increase in flow which
coincides with a rain event). Rainfall-induced infiltration results when stormwater runoff causes
a rapid groundwater recharge around sewers, including manholes and building connections,
which then enters the system through defective pipes, pipe joints, or manhole walls.
I/I can greatly increase flows and cause unnecessary burdens on the treatment plant. In
some systems, sewerlines become overloaded and uncontrolled sanitary sewer overflows (SSOs)
occur at unspecified locations. Since most sanitary sewers do not have overflow structures
designed into the system, SSOs often occur through manholes and defective lines in residential
3D
-------
neighborhoods causing backups into homes and streets. I/I also causes surcharging of
wastewater treatment plants and pumping stations. In comparison to combined sewer overflows,
SSOs generally contain higher percentages of raw sanitary sewage (which can contain high
levels of infectious (pathogenic) microorganisms, suspended solids, toxic pollutants, floatables,
nutrients, oxygen-demanding organic compounds and fecal matter, oil and grease, and other
pollutants) and a lower level percentage of stormwater. SSOs represent a human health risk
because of pathogenic organisms. In San Diego, CA, SSOs have threatened drinking water
supplies, creating the potential for serious adverse public health impacts (Golden, 1996). The
oxygen-demanding compounds associated with SSOs can also affect the aquatic environment. In
Alabama, SSOs have had an adverse impact on the number and range of species found in the
Cahaba River (Cahaba River Society, 1993). They also reported low levels of dissolved oxygen
and algal blooms.
Old sewer systems, constructed before 1970 using mortar or mastic jointing materials,
can substantially contribute to exfiltration as well as to I/I. Exfiltration can occur when the
elevation of the sewer liquid level is above the groundwater table. This positive elevation head
can cause raw sewage to exfiltrate through open joints to ground water and other areas of the
environment. Stoppages and collapses can cause flows to exit the sewerline and concentrate in
the trenches dug for the sewerlines or other utility lines. The flow is then conveyed by gravity
along the trench to surface water or the flow can infiltrate into the ground and threaten drinking
water supplies. Loose joints and deteriorated pipes are the main source for the flows to exit the
sewer; house lateral connections to street sewers are other prime points of leakage. Exfiltration
from both combined and sanitary sewers can be a substantial source of pollution in terms of
4D
-------
impact on groundwater and surface water quality.
The Clean Water Act (CWA) prohibits point-source discharges of pollutants to waters of
the United States unless authorized by a National Pollutant Discharge Elimination System
permit. Thus, unpermitted discharges from sanitary sewer systems, i.e., SSOs, violate the CWA.
This is true whether the discharge is directly to surface water or indirectly through ground water
hydrologically connected to surface waters. Under the Federal Advisory Committee Act, the
Urban Wet Weather Flows Advisory Committee subcommittee was formed to provide
recommendations on how to address issues associated with SSOs, including deciding between
sewer rehabilitation and treatment options to control SSO pollution (USEPA, 1996).
The application of new and advanced technologies can provide more cost-efficient and
environmentally conscious solutions to the problems associated with deteriorating wastewater
collection systems. Research is critical to accomplish this. This paper identifies related
technical issues and research needs in three major areas: (1) assessment of system integrity; (2)
operation, maintenance, and rehabilitation; and (3) new construction. The material presented is
in part the results of an experts workshop on infrastructure problems in wastewater collection
systems sponsored by the Water Supply and Water Resources Division of EPA's National Risk
Management Research Laboratory (USEPA, 1998a).
2. Assessment of system integrity
In order for municipalities to cost-effectively plan, organize, and implement a
-------
maintenance and renewal effort, they will need improved information on the condition of the
pipe system. A thorough assessment is required to determine the extent of the problem and to
evaluate alternative approaches and costs for rehabilitation versus replacement versus storage-
treatment options. The process involves two broad, but closely related areas: flow monitoring
and physical condition assessment.
2.1. Flow monitoring
Flow monitoring in wastewater collection systems may be conducted for a variety of
purposes, including the determination of total system flows, customer billing, identification of
capacity problems, monitoring of system performance for operation and maintenance, detection
and quantification of overflows or bypasses, measurement of peak wet weather flows and I/I, and
calibration of flow models. One of the most widespread uses of flow monitoring in wastewater
collection systems is for quantifying wet weather flows (WWFs) and I/I. Two major types of
gravity flow meters are involved: area-velocity meters and critical depth meters. Critical depth
devices include flumes and weirs. Area-velocity devices utilize a number of different
technologies for depth and velocity measurement while critical depth devices just use depth
measurement. Commonly used depth measurement technologies include air bubblers, pressure
transducers, and ultrasonic transducers. Velocity measurement technologies include
electromagnetic sensors and acoustics devices.
The primary issues relating to flow monitoring technologies are accuracy and reliability.
Various physical and hydraulic site conditions such as pipe size, range of flow depths, flow
6D
-------
velocities, occurrence of surcharge and backwater conditions, turbulence and other nonuniform
flow conditions, pipeline sediment layers, etc., often affect the performance of flow meters. Use
of the flow monitoring data requires a good understanding of what accuracy and reliability can
be expected from the different types of monitoring technologies over the range of expected
monitoring conditions. Flow measurement during peak WWF conditions can vary from the
actual flow by 20% or more.
The overall issues in flow monitoring are accuracy and reliability of the technology and
interpretation of results. Improved accuracy of measurements, or at least an indication of error
producing flow conditions, monitor fouling, etc., could be possible by installing multiple sensors
that provide redundant measurements of the same quantity and use different physical principles
and/or sensor placement. Miniaturization may be able to offset cost increases from the multiple
sensor approach. Wireless, miniaturized sensors would be able to be deployed in more locations
through a pipe network and hence provide a better picture of actual flow conditions and locations
of problems. To be able to use such sensors, internal data storage, wireless data transmission
and energy provision problems would need to be addressed.
2.2. Physical condition assessment
Increased flows at the treatment plant and SSOs indicate a problem, but do not isolate its
location. A thorough assessment of the physical condition of the collection system is critical for
repair or replacement and important to maintain the integrity of the system and to control I/I and
exfiltration. An assessment identifies structural features that may require correction and aids in
7D
-------
establishing priorities for rehabilitation or replacement of system components. The likelihood of
failure and the associated risk analysis is intrinsic in the evaluation when budgetary restraints
affect the work.
2.2.7. Gravity flow systems
Pipeline defects can be generated during installation (deflections, punctures, cracks,
rolled joints, poor bedding/backfill material, etc.) or overtime (corrosion, erosion, deterioration
of grouts and joints, root penetration, thermal and nonthermal stresses or strains, soil subsidence,
etc.). Inspection is usually done by smoke testing, man entry, flow isolation, dye-water flooding,
and closed circuit television (CCTV); results are very observer-subjective. Dye and smoke test
methods frequently cannot locate small leaks. Most currently available automated pipe
inspection systems have limited capabilities. Varying pipe diameters, materials (including brick,
concrete, ductile iron, and clay), odd shapes, sumps, and angle entries are not frequently suitable
for CCTV inspection systems. Physical inspection by workers is costly, disruptive to urban
commerce and travel, limited by pipe size, and potentially dangerous to personnel and
surrounding structures.
2.2.2. Pressure systems
Force mains do not get inspected regularly because they operate under pressure and are
generally critical sewers that do not experience significant periods of low/no flow. Inspection of
8D
-------
their physical condition often takes place during downtime repairs of adjacent pipe segments.
Inspection techniques are limited because of pipe size and accessability. In many cases, the only
access points are at either end of the main.
2.2.3. Appurtenances
Physical inspection of manholes and junction chambers is performed by either surface
inspection or by entering for internal inspection. Pump stations are typically inspected regularly
as part of routine maintenance. Because of the hazardous atmosphere and flow conditions near
siphons, only surface inspection of inlet and outlet structures is performed unless there are
known or suspected problems in the siphon.
The primary research issue associated with physical condition assessment is how to
effectively detect and locate defects/failures in wastewater collection systems to prevent I/I,
exfiltration, and collapses which can cause street surface hazards and pipeline blockages.
Although progress has been made in better defining inspection procedures and techniques since
passage of the CWA, there is still a need to standardize and improve approaches. Nearly all
inspection techniques depend on visual observation; defects may be missed or misinterpreted.
One of the greatest limitations is interpretation of defect severity.
Various technologies have been and are being developed in Europe, Japan, and the
United States to increase the reliability of information available from pipeline inspection. These
new technologies include: sewer scanner and evaluation technology (SSET), sonar, seismic
resurgence testing (SRT), acoustic testing, and infrared thermographic investigations. These
9D
-------
technologies are designed to provide additional information such as depth of corrosion, sludge
buildups, size and severity of cracks, wall thickness, ovality, etc., beyond what is available from
conventional CCTV inspection (Thomas and Laszczynski, 1997). There is an overall need to
evaluate these new technologies, along with CCTV, to document their performance and costs
under both controlled-condition testing and field testing for a variety of applications; e.g.,
gravity systems, pressurized systems, house/service laterals, manholes and pumping stations.
There is also a need to investigate the concept of "intelligent systems" for remote sensing and
monitoring of the structural integrity of systems. The results of these efforts will provide the
information and tools to move forward toward designing systems of a more sustainable nature.
A current research project in this area involves the development of predictive tools or
performance indicators for measuring the degradation of sewer systems (Water Environment
Research Foundation, 1999). The results of this work will enable municipalities to identify
probable areas for rehabilitation prior to inspection based on a greater understanding of the
variables that lead to failure. Inspection and detection programs can then be strategically
focused in those areas that most likely need attention. Another project, which could have direct
application to reinforced concrete sewerlines, is developing a remote system that utilizes
electrochemical impedance techniques and electrochemical polarization decay for monitoring
corrosion in underground pipes, piles, and steel encased in concrete (US Army Corps of
Engineers, 1996a). The application of this technology could reduce/eliminate the need for costly
"dig-ups" presently required to determine the corrosion status of underground pipes. In addition,
application of the technology will provide valuable information on the actual mechanical process
of corrosion.
ion
-------
3. Operation, maintenance, and rehabilitation
The objective of pipeline system operation, maintenance, and rehabilitation is to ensure
the overall viability of the conveyance system by (1) maintaining structural integrity, (2) limiting
exfiltration and its potential for groundwater contamination and other negative environmental
impacts, and (3) reducing the amount of extraneous flows (I/I).
Effective maintenance and rehabilitation programs require a complete understanding of
the condition and performance of a system and any other contributing factors. Pipe age is a
factor; however, it is usually a combination of several factors that causes failures and influences
maintenance decisions, making the situation very complex. Soil conditions, stress conditions,
groundwater levels, sewage/soil acidity, dissolved oxygen levels, and electrical and magnetic
fields may negatively impact long-term performance of the system. Pipe materials, and bedding
and backfill materials are also factors. Decision factors should include an evaluation of both
internal conditions, based on some form of visual inspection, and conditions surrounding the
pipe. Decision support tools that incorporate pipe assessment methodologies are required to
quantify and rank the condition of a pipeline based on structural, hydraulic, water quality and
economic factors.
Pipeline rehabilitation procedures usually involve some form of cleaning to remove
foreign materials before other phases of rehabilitation. The removal of roots, sediments, and
debris is necessary for maintaining proper flow conditions and for reducing infiltration and
exfiltration and structural damage to the pipeline. Common rehabilitation methods of chemical
11D
-------
and cement grouting address problems associated with groundwater movement, washouts, soil
settlements, collapses, and soil voids. Grouting is effective in reducing or eliminating infiltration
but will not significantly improve the structural condition. It does, however, help stabilize the
surrounding soil mass. Other approaches include sliplining, spiral-wound pipe, segmented liner
pipes, cured-in-place pipe (CIPP), fold and form pipe, close-fit-pipe, pipe bursting, coatings,
mechanical sealing devices, spot repair, and replacement. Many rehabilitation methods (other
than grouting or sealing and pipe bursting) reduce pipe cross-sectional area and can reduce
hydraulic performance. Such reduction is frequently acceptable, but must be taken into account
in the rehabilitation evaluation (Water Environment Federation, 1994). New methods of sewer
sealing should be evaluated before major rehabilitation or replacement is undertaken.
Replacement is expensive, especially in older systems which tend to crack, crumble, deteriorate,
and erode. Newer "trenchless" rehabilitation technologies should be considered to reduce the
costs and inconvenience associated with traditional open-cut methods of pipe repair and
replacement.
Failures in force mains can result in the release of large amounts of sewage within a
short period. This can cause a rapid spread of contamination and potential health hazards. The
rehabilitated force main pipes must be structurally stable within the surrounding soil mass, must
not leak at operating pressures, must have adequate capacity to carry its design flows, and must
be serviceable under the other conditions it will experience (e.g., sulfide corrosion). The most
common rehabilitation method practiced on force mains is point repair of individual failed or
failing line segments.
Building connections to the street sewers (house or service laterals) can contribute as
12D
-------
much as 70 to 80% of the infiltration load (Field and Struzeski, 1972). Fluctuating ground
water, variable soil characteristics and conditions, traffic, erosion, washouts, etc., cause
enormous stresses on house/service lateral pipes and joints. Connectors and fittings in many
cases do not retain their watertight integrity while adjusting to these factors. Some connections
react to soil acid and may totally disintegrate in a few years. These conditions often result in
generating major points of infiltration at the connection of the house/service lateral to the street
lateral or main. With current technology, rehabilitating building connections may not be
economically feasible because of the shear number of connections. The problem is both critical
and sensitive because of private ownership and costs associated with disturbance to the occupant
and destruction of valuable landscaping. Because of this, municipalities are often reluctant to
address I/I problems from these sources. When performed, rehabilitation is done by point repair
or replacement; sliplining and pipe bursting are also in limited use. However, these approaches
do not overcome the private ownership problem or the problems associated with the location and
configuration of the line (i.e., sharp bends). In addition, it is significant to mention that past
studies have found that rehabilitation of sewerlines at the street alone does not completely solve
the I/I problem (USEPA, 1985). Successive rainfalls can elevate the groundwater table to levels
where entry occurs through the house laterals.
Rehabilitation of appurtenances (e.g., manholes, pump stations, wet wells, siphons) is an
important component of both assuring sustainability of collection systems and of an I/I control
strategy. About 30 to 50% of system I/I is due to defects and conditions in or near
appurtenances, in particular, manholes (Perez, 1996). For example, manhole covers submerged
in one inch of water can allow as much as 75 gpm (4.73 1/s) to enter the system depending on the
13D
-------
number and size of holes in the cover (ASCE and WPCF, 1982). Rehabilitation of manholes,
pumping stations, and wet wells includes spray-on coatings, spot repairs, structural liners, and
replacement. Rehabilitation of siphons includes some of the options available for pipelines such
as grouting and lining. Many siphons have been rehabilitated using either CIPP or high density
polyethylene (HDPE) liners.
Selection of rehabilitation methods and materials suitable for various parts of the
wastewater collection system remains an issue. The issue is partly related to the lack of
understanding of the capabilities of each method relative to the problem. In addition, reliable
rehabilitation product performance under actual field conditions is lacking. Data on the
effectiveness and longevity of rehabilitation technologies and materials and life-cycle cost
information will be useful in determining whether rehabilitation or replacement is more cost
effective.
A range of trenchless technologies has been developed for rehabilitating most types of
pipelines. New processes and products come to the market continually. Many are proprietary
systems and the details of installation procedures and materials are trade secrets, limiting the
ability to compare and evaluate competing approaches. For some techniques, codes and
standards have been developed; however, because of the rapidly evolving technology in
rehabilitation, standards and codes often lag behind. Some of these technologies are over 20
years old; in some cases original patents have expired and a number of "look-alikes" have
appeared.
The fundamental research issues in this area relate to the selection of the most cost-
effective approach to operate, maintain, and manage existing sewer systems to reduce I/I and
14D
-------
exfiltration. Some specific needs include:
• D determining the longevity and performance of various rehabilitation methods and various
conditions that will provide comparative data on the cost effectiveness of each method;
• D evaluating new and improved repair and replacement methodologies;
• D evaluating approaches to optimize and assess O&M programs;
• D developing alternative technologies for rehabilitation of house/service laterals and
connections;
• D evaluating the performance of grouts and liners under various environmental conditions
such as humidity, temperature, pH, and wastewater chemistry;
• D evaluating alternatives to remove roots and prevent root growth; and
• D evaluating cost-performance tradeoffs between rehabilitation and replacement
alternatives.
To date very little effort and resources have been directed at assessing the problem of
exfiltration from sewerlines, mainly because the national concern has been in the area of direct
impacts from combined and sanitary sewer overflow. An ongoing project is attempting to
quantify on a national basis the magnitude of the exfiltration problem in wastewater collection
systems: identify contributory factors to the problem; document approaches for correcting the
problem; and identify technology gaps and research priorities (USEPA, 1998b).
Predictive methodologies are being developed to determine peak flows after sanitary
sewer rehabilitation (Water Environment Research Foundation, 1999). These methodologies
will provide information on the reductions in peak flows associated with approaches to I/I
reduction. This will enable agencies to more accurately assess the progress and results of current
15D
-------
I/I programs as well as to plan more cost-effective programs.
Methodologies are also being developed to evaluate and optimize the effectiveness of
sewer maintenance programs. One approach will provide information on how to: (1) establish an
effective O&M program to maintain functional and structural integrity in the collection system;
(2) evaluate the adequacy and effectiveness of an existing O&M program; and (3) prevent new
connect!ons/reconnections of inflow sources (USEPA, 1998c). Another will result in decision-
making methodologies which can be used by cities and agencies to evaluate maintenance costs
and system performance (USEPA, 1999a). This approach will provide information on how to:
(1) evaluate the effectiveness of maintenance and rehabilitation programs by reviewing
inspection activities; (2) review how maintenance and rehabilitation dollars are spent; and (3)
provide typical values for maintenance frequencies and system reinvestment expenses to serve as
benchmarks for local governments and agencies in evaluating their own programs.
A project, just initiated, will document European approaches for diagnosing and
analyzing wastewater collection systems (USEPA, 1999b). Information will be collected on new
and improved methodologies that assess the condition of a system and evaluate corrective action
alternatives. Best management practices for a well operated and maintained system will be
identified, including information on which pipe materials offer the best long-term behavior based
on performance, reliability and repairability. The use of performance indicators to compare and
improve O&M, and the need for related standard terminology, definitions and data collection
and storage will also be investigated. In addition, a document will be prepared on the
identification and assessment of existing "non-hydraulic" models for predicting failures in
wastewater collection systems and for otherwise managing and optimizing the O&M of these
16D
-------
systems.
The U.S. Army Corps of Engineers has evaluated a trenchless pipe insertion method
developed by the gas industry in the United Kingdom for installing pipelines into existing, older
lines, with limited excavation (US Army Corps of Engineers, 1996b). The technology, known as
"pipe bursting", involves destroying the pipe in place and forcing it into the surrounding soil
with an impact mole. Equipment is then used to push or pull a new pipe into the cavity created
by the impact mole. The result is a pipe which follows the existing sewerage collection route
from manhole to manhole. This technology as well as other trenchless approaches have
experienced more widespread use and acceptance in the last seven years. The Trenchless
Technology Center of Louisiana Tech University, Rustin, LA is involved in extensive research
on materials and methods used in both trenchless rehabilitation and construction (Trenchless
Technology Center, 1999).
The Center for Innovative Grouting Materials & Technology at the University of
Houston, Houston, TX has constructed a test facility to evaluate coatings and liners for concrete
and clay brick wastewater sewers/appurtenances. New products are tested to evaluate their
ability to maintain a bond with the concrete surface under pressurized conditions (to simulate
groundwater conditions). Products are applied to the concrete media (wet/dry) and tested to 15
psi (equivalent to a pipe sitting in 32 ft of water). Over a period of five months of testing, two of
the coatings out of eight developed fine cracks. In addition, chemical resistance of the product is
tested with and without holidays (pinholes) by immersing a coated concrete sample in a 3%
sulfuric acid solution (to simulate worst possible sewer conditions) and a 30% sulfuric acid
solution (to simulate long-term performance). The impact of the pinholes and whether or not the
17D
-------
presence of a pinhole in a coating results in significant deterioration of the concrete surface
beyond the coating is evaluated. Specimens in 3% and 30% sulfuric acid showed an increase in
weight of 0.6 and 1%, respectively. With the increase in weight, blisters were formed around the
pinholes leading to cracking of some of the coatings. The program includes field testing in
actual collection systems (Vipulanandan et al., 1996).
4. New construction
Improved materials and construction techniques could reduce future rehabilitation needs
as sewer systems age. Concrete pipe (with or without a protective layer of clay tiles/bricks) and
vitrified clay pipe (VCP) have been used in the United States since initial sewerlines were
installed (early 1800s). Concrete can corrode from sulfuric acid formation, and VCP, virtually
inert to sulfuric acid, historically had problems due to leaking joints, short segment lengths, and
brittleness. These pipes are gradually being replaced with plastic materials such as HDPE,
polyvinyl chloride (PVC), reinforced plastic mortar (RPM), centrifugally cast fiberglass
reinforced plastic mortar (CCFRPM), polymer concrete, and acrylonitrile-butadiene-styrene
(ABS). Plastic materials are normally chemically resistant to domestic sewage; however, they
can also cause problems since they are not rigid and tend to creep over time. Newer pipe joints
provide watertight and root free service. Cement lined, coal tar/asphalt coated, and plastic lined
concrete pipes have been used for larger diameter pipelines. RPM, CCFRPM, and polymer
concrete pipes are gradually entering the market.
18D
-------
The physical impairment of pipeline systems can result from a wide assortment of causes.
Older pipes used jute, wax, cement or asphalt/tar packed or grouted joints, which over time,
deteriorate. Some joints are out-of-round or cracked or broken when installed, allowing for root
penetration and subsequent infiltration. Some pipes were installed without proper bedding or
backfill material, resulting in the common broken-back failure and/or pipeline sags.
The relationship between the chemistry of sewage to the pipe materials conveying it is of
primary concern in the selection of pipe materials. Normal domestic sewage has a pH of 6 to 8.
However, domestic sewage contains bacteria, sulfate, and organic matter which generates
sulfides. In the presence of water, sulfides immediately convert to hydrogen sulfide gas, which
is slightly heavier than air. The hydrogen sulfide gas finds its way into the sewer atmosphere
and comes into contact with the moist surface of the pipe crown, where it is rapidly oxidized by
bacteria into sulfuric acid. Sulfuric acid is highly corrosive to many piping materials.
Pipes are normally installed using open trenching methods. As mentioned earlier,
trenchless replacement and construction techniques have been gaining popularity. These
methods include: pipe bursting; micro tunneling; directional drilling; pipe jacking; plowing in;
and fluid jet cutting (Water Environment Federation, 1994).
Most I/I comes from faulty house/service laterals, especially from coupling between
house service and street laterals due to differential settling. Most service laterals installed today
are plastic pipes with either rubber-ring slip joints or solvent-welded joints. Sol vent-welded
joints are preferred because they resist root intrusion better than rubber-ring slip joints. The pipe
wall thickness must be great enough to resist damage by rodents and crushing or fracture from
heavy loads (automobiles, trucks, plows, tractors, etc.).
19D
-------
Current force main construction and replacement materials include ductile iron, steel, and
concrete (typically prestressed concrete) pipe. The use of plastic materials is increasing.
Corrosion continues to be the problem for all metallic, as well as prestressed concrete pipes.
Improved standards and materials of construction are required to realize quality improvements
and sustainability of products.
Current construction practices for manholes typically include precast lined or unlined
concrete sections with a formed joint sealed with a butyl-type rubber seal. Manholes are usually
placed about every 300 to 400 feet (90 to 120 meter), at changes in pipe diameter, direction and
elevation, or where two or more pipes have to be joined into one pipe. With the current
population growth rate, about 300,000 to 400,000 new manholes are constructed each year.
Corrosion of concrete remains an issue for all appurtenances and corrosion protection is required
in areas of high hydrogen sulfide. I/I occurs even in new manholes through defects at pipe seals,
wall joints, chimney joints, and covers. Because manholes are a large contributor of I/I, the
spacing of manholes is an issue. Increased spacing reduces the number of manholes, but
increases the lengths of pipes between manholes, frequently making maintenance more difficult.
Proper bedding and placement of manholes are critical to avoid differential settlement failures at
pipe-manhole connections. Design engineers must consider these issues when designing
wastewater collection systems. Removing covers during routine maintenance often results in
improperly placed covers and increased stormwater runoff entry. Depressed manhole covers
also provide a source for stormwater entry. Alternative designs for watertight manholes have
been developed; additional research into the cost effectiveness of these designs should be
performed.
20 D
-------
The primary research issues in this category are associated with the application of new
and improved design, construction, and installation technologies and materials to improve the
quality and the long-term effectiveness of the installed product. Research needs include:
• D identifying materials that could be incorporated into pipe materials and coatings that
could control corrosion and increase strength;
• D developing materials/sensors that could be incorporated into piping systems as "smart
materials" for which it is possible to track deterioration or structural performance over
time;
• D evaluating new designs for cost-effective watertight manholes;
• D evaluating the long-term performance of the various plastic pipes now being used for
force mains;
• D reviewing and evaluating current sewer design and installation practices (including
materials for improved structural performance);
• D evaluating new and improved coupling techniques and house service laterals to
significantly alleviate I/I and exfiltration problems;
• D determining whether solvent-welded pipe is better than rubber-gasketed pipe for long-
term I/I and root control in house/service laterals.
An ongoing research project is investigating innovative materials and techniques that
could be used to advance the state-of-the-art for new and replacement sewer collection systems.
The focus of the work is on: design and construction practices; techniques and materials
introduced or under research in the United States and other countries; new installation and
jointing techniques; and developments in design concepts, technologies and materials used for
21D
-------
sewer rehabilitation. The final document will provide a resource to review alternative
replacement technologies and new design, materials, and construction practices (Water
Environment Research Foundation, 1999).
5. Summary
The rehabilitation of sewers across the United States has experienced substantial growth
for many years, in response to both political pressures and technical developments. The
deteriorating underground sewer network threatens the environment, public health, and safety.
Over the past decade, many improvements have been made to the nation's wastewater piping
system; however, much more work still needs to be done. Leaking sewer systems can
contaminate water supplies, while SSOs may discharge into rivers and oceans. Each year
problem systems make the newspaper headlines when a road collapses due to a break or faulty
pipe section. The piping systems in many parts of the country are overused and overworked,
resulting in a higher frequency of breakdown or failure. The demand to maintain and upgrade
the wastewater piping network continues and new methods are required to improve their
performance more cost effectively. The sewer system is a national asset; the value of this asset
is more than one trillion dollars and it cannot be easily replaced. There are well-established
design methods and materials to deliver high-quality performance for both new pipe installation
and rehabilitation of existing networks. However, new thinking about pipe design, new
technologies and new materials, both in the United States and in other parts of the world, offer
22 D
-------
opportunities for improving upon traditional methods as well as providing alternative solutions.
References
American Society of Civil Engineers and Water Pollution Control Federation. (1982). Gravity
Sanitary Sewer Design and Construction. ASCE No. 60, WPCF No. FD-5.
American Society of Civil Engineers. (1996). Creating the 21st Century Through Innovation.
Civil Engineering Research Foundation. Report #96-5016.E.
American Society of Civil Engineers. (1994). State and Local Public Works Needs. Civil
Engineering Research Foundation. December.
Cahaba River Society. (1993). Cahaba River Society Special Report: Sewage and the Cahaba
River. Birmingham, AL. July.
Field, R., and EJ. Struzeski, Jr. (1972). Management and Control of Combined Sewer
Overflows. Journal of Water Pollution Control Federation, 22(7): 1393.
Golden, J.B. (1996). An Introduction to Sanitary Sewer Overflows. National Conference in
Sanitary Sewer Overflows, Seminar Publication. United States Environmental Protection
Agency. September. EPA/625/R-96/007.
23 D
-------
Perez, D. (1996). Advantages of Structurally Enhanced Cementitious Materials for Manhole
Rehabilitation. Presented at New England Water Environmental Association Specialty
Conference.
Thomas, A. and G. Laszczynski. (1997). New Technology for Enhanced Pipeline Evaluation.
Trenchless Pipeline Projects - Practical Applications, Proceedings of the conference
sponsored by the Pipeline Division of the American Society of Civil Engineers, Boston,
MA. June.
Trenchless Technology Center. (1999). College of Engineering and Science. Louisiana Tech
University, Rustin, LA.
US Army Corps of Engineers. (1996a). Fact Sheet: Remote Corrosion Monitoring Technology.
http://owww.cecer.armv.mil/facts/sheets/fll3.htm/. October.
US Army Corps of Engineers. (1996b). Fact Sheet: Trenchless Rehabilitation of Sewer Pipes.
http://owww.cecer.armv.mil/facts/sheets/FINAL7.htm/. April.
US Environmental Protection Agency. (1985). Demonstration of Service Lateral Testing and
Rehabilitation Techniques. USEPA Office of Research and Development. February.
EPA/600/2-85/131.
24 D
-------
US Environmental Protection Agency. (1996). Risk Management Research Plan for Wet
Weather Flows. USEP A Office of Research and Development. November. EPA/600/R-
96/140.
US Environmental Protection Agency. (1998a). Wastewater Collection Systems Workshop
Issue Paper. Preliminary Draft. USEPA Office of Research and Development,
Cincinnati, OH, August. EPA Contract No. 68-C7-0011, WA 0-04.
US Environmental Protection Agency. (1998b). Exfiltration in Sewer Systems. National Risk
Management Research Laboratory. USEPA Office of Research and Development,
Cincinnati. OH. Ongoing.
US Environmental Protection Agency. (1998c). Collection Systems: Methods for Evaluating
and Improving Performance. USEPA Office of Wastewater Management, Washington,
D.C. EPA Cooperative Agreement No. CX924908-01-0.
US Environmental Protection Agency and American Society of Civil Engineers. (1999).
Optimization of Collection System Maintenance Frequencies and System Performance.
USEPA Office of Water, Washington, D.C. February. EPA Cooperative Agreement No.
CX824902-01-0.
US Environmental Protection Agency. (1999b). Urban Infrastructure Pipeline Performance
25
-------
Improvement Practices. USEPA Office of Research and Development. GSA-MOBIS
Contract No. GS-23F-9737H. Logistics Management Institute. Ongoing.
Urban Institute. (1981). Capital Infrastructure Investment Needs and Financing Options. HUD-
0003404. Department of Housing and Urban Development, Washington, D.C.
Vipulanandan,C., H. Ponnekanti, D.N. Umrigar, and A.D. Kidder. (1996). Evaluating Coatings
for Concrete Wastewater Facilities. Proceedings of the Fourth Materials Conference,
Washington, D.C. American Society of Civil Engineers. November.
Water Environment Federation. (1994). Existing Sewer Evaluation and Rehabilitation. WEF
Manual Practice FD-6, ASCE Manuals and Reports on Engineering Practice No. 62.
Water Environment Research Foundation. (1999). Collection and Treatment Facilities: On-
Going and Planned for Year 1999 and 2000 WERF Projects.
http://216.32.70.181/research/ongoingl.htm/.
26
------- |