EP A/600/ JA-02/226

          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


       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


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

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

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

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

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

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

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

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.

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

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


       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

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

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


       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


       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

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


 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


       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

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


       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

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.

       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.).

       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

                                           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


 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

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.


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                                          23 D

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       http://owww.cecer.armv.mil/facts/sheets/fll3.htm/. October.

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                                          24 D

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      Cincinnati. OH. Ongoing.

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      and Improving Performance. USEPA Office of Wastewater Management, Washington,

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      Optimization of Collection System Maintenance Frequencies and System Performance.

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