Necessity: The mother of renovation
This is the first in a two-part series on the growing trend of using trenchless technology to upgrade or replace municipal collection systems. Part I describes how inflow and infiltration can deteriorate sewage systems. Part II, which will appear in our November '00 issue, will deal with the types of trenchless technology available and their advantages over conventional methods of sewer system installation.
Aging infrastructure and recent action by state and federal regulatory agencies have reinvigorated sewer system rehabilitation across the United States. Municipalities and industry across the United States are faced with storm water and wastewater collection systems sorely in need of repair. They have deteriorated over the years due to age, inadequate materials and construction methods and in some cases, inconsistent maintenance. Governments and industry are seeking cost-effective ways to renovate or replace sewer systems. Many are turning to trenchless technology to meet this challenge.
The need for the renovation or replacement of this infrastructure is driven by new efforts from state regulatory agencies and the U.S. Environmental Protection Agency (EPA). This past year, EPA has taken proactive steps to encourage and monitor sewer rehabilitation in U.S. Region 4 -- Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina and Tennessee. The organization has initiated a program entitled Publicly Owned Treatment Works Management, Operation and Maintenance (POTW MOM). According to EPA, "the goal of the POTW MOM is to bring 100 percent of the publicly owned treatment works handling domestic wastewater in Region 4 into compliance with the 'proper operation and maintenance' provision in their associated National Pollutant and Discharge Elimination System (NPDES) permits by the year 2011." EPA may expand its audit to the remaining regions across the country in the next several years.
Under this program, selected municipalities within a specific watershed are required to perform a detailed audit of their sewer facilities. Participants then submit a report detailing necessary improvements and a schedule for completion. Municipalities that self-disclose the need for rehabilitation work will be eligible for significantly less or in some cases, total elimination of possible civil penalties.
The aging of America's sewer systems
The adage "out of sight, out of mind" was commonly applied to sewer systems early this century. This is not surprising, for 100 years ago the general public did not recognize the correlation between health and pollution. Many of the sewer systems constructed during this time were designed to contain storm and wastewater. Called combined sewers, many of these are still in operation. They push treatment facilities to capacity by bringing storm water that does not need to be treated. In instances of significant precipitation, these systems can direct raw sewage into streams and rivers.
As recently as 50 years ago, the lone treatment method for polluted water was limited to the physical removal of waste solids. During this time, the country was celebrating victory in World War II. It was a time of unprecedented growth and expansion. Automobiles made it possible to live outside of town and commute to work. The rapid development of suburban areas required the creation of new sewer systems and utilities to dispose of waste. Many of these systems have never been upgraded and remain in use today.
Steps for effective sewer system rehabilitation
|
| Action step |
Description |
| Flow monitoring and isolation |
Flow meters are set-up at key manholes to isolate the sewer system into smaller sub-basins, typically for 30-60 days, to identify high flow areas of the system. This task is most effective when implemented during the rainy season. |
| Records review |
Water usage records are compared to flow rates of sewage treated. Typically, 85 percent of water purchased will become sewage and be treated. If sewer flows are close to water usage, the sewer system is in excellent condition. |
| Smoke testing |
Smoke is forced into the sewer system with smoke bombs and blowers. It will surface at shallow defects, typically service lines, roof leaders, defective manhole castings and storm sewer catch basins. The defects identified are usually inflow defects. |
| Internal inspection (televising) |
A small television camera is pulled through the sewer system to identify defective pipe - defective joints, cracks, roots, holes and sags. The exact location of each defect and service is indicated for accurate design and construction.
|
| Sewer rehabilitation design and construction(Phase 1) | The defects are prioritized and ranked in terms of providing the most benefit and the lowest cost to perform. A good practice is to start rehabilitation in the upper sewer reaches, which can be Phase 1. A set of construction documents for Phase 1 will be prepared which will identify the cost-effective defects to be rehabilitated in the upper reaches of the sewer system.A contract is awarded to a qualified contractor to correct the identified defects. The work may consist of line replacement; pipe lining, point repairs, manhole repairs and service line repairs. Often times it may involve multiple contracts dividing the work into basins or into types of work.
|
| Performance evaluation
(Phase 1)
|
Reinstall flow meters at the original key manholes to evaluate the effectiveness of the rehabilitation by comparing flows after construction with before construction. The sewer system operator shall observe the influent flow meter for lower system flows. Sometimes, if systems are susceptible to overflows the flows do not significantly drop according to the meter, but by eliminating the overflow, the project can be considered successful.
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| Sewer rehabilitation design and construction (Phase 2) |
A sizing check of the existing interceptor sewer for adequacy of carrying system flows should be undertaken based on post construction flow monitoring factoring in future growth. Often, the second phase of sewer rehabilitation includes hydraulic improvements such as parallel sewers and/or equalization basins as it may be more cost effective to transport and/or store the excessive flows than remove it.
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In the '60s and early '70s, Americans began to realize the dangers associated with water pollution. EPA and the State of Illinois issued the first municipal wastewater discharge permit in 1973. EPA also began a construction grants program to build wastewater treatment facilities. A significant stipulation for receiving a grant required the applicant to conduct and implement the recommendations of a sewer system evaluation study. This resulted in the birth of sewer rehabilitation in the United States.
During that decade, municipalities that wished to expand their water treatment facilities through EPA grants were required to investigate the condition of their sewer systems. On the surface, this appeared to be a positive step that would lead to the repair and replacement of inadequate sewer systems.
Millions of dollars were spent evaluating collection systems across the United States. Municipal surveys yielded large amounts of pertinent and useful information. However, rehabilitation efforts eventually fizzled. It was predicted that the rehabilitation technology of the day could significantly lessen the amount of storm and groundwater in sewer systems. This higher rate of removal of non-waste water indicated that less money could produce more significant outcomes than was realistically possible. Unfortunately, lasting and effective results were not realized due to limitations in technology.
In the past decade, several large municipalities have entered into agreed orders with state governments and EPA to rehabilitate their collection system. Billions of dollars are being spent to comply with these orders, and it is anticipated that many more municipalities will be subject to the same course of action.
Certain systems have deteriorated to the extent that governments have, at times, placed a moratorium on all residential, commercial and industrial growth within a municipality or county -- essentially shutting down economic development until the sewer system or treatment plant has undergone rehabilitation that meets EPA and state requirements.
Local and state governments are moving to address these challenges. The first step many are taking is to broaden their understanding of gravity sewer systems and ways to rehabilitate them.
Conventional gravity sewer systems and rehabilitation methods
For the majority of older sewer systems, infiltration (groundwater entering the sewer system) and inflow (surface water entering the sewer system) present the greatest threat to aging sewer lines and treatment facilities operating at full capacity.
Inflow is typically recognized during and after substantial rainfall. Common sources of inflow include runoff into defective or submerged manhole castings, broken or missing homeowners' cleanout caps, cracked pipe under ditches or streams and down spout connections. SignificantMunicipalities that self-disclose the need for rehabilitation work will be eligible for significantly less or in some cases, total elimination of possible civil penalties.
rainfall can result in sewers overflowing and bypassing wastewater treatment facilities and then being discharged into ditches and streams. It is common for flow rates to exceed design conditions by more than 10 times the rated capacity of the collection and treatment systems during wet weather events. The impact of inflow is usually short lived. It is felt during and immediately after the rain event.
Infiltration defects are pipe-related failings such as cracks from improper installation or root intrusions. Infiltration occurs when groundwater leaks into pipes. Typically, infiltration is a constant flow, yet of lesser magnitude than inflow. However, when quantified over a long period of time, infiltration may total more than inflow.
Inflow and infiltration can strain the operations of older sewer systems and wastewater treatment facilities. If storm and wastewater in the system exceed the capacity of the collection or treatment systems, the result can be basin, pump station and collection system overflows. In this instance, storm water and wastewater can be passed through the system untreated, overflowing into streams and rivers. Additionally, peak flows over long durations can wash vital solids from treatment basins causing permit violations.
Lessening inflow and infiltration into sewer systems is a goal of water and wastewater departments across the United States. Reducing storm water into the waste sewer systems allows treatment facilities to concentrate on treating wastewater, thus increasing efficiency.
It often costs less to correct inflow defects than infiltration. For example, an offset manhole casting in a low-lying area can allow surface water to pour in like a storm water catch basin. Correction of this problem is often as simple as replacing the casting with a water-tight one. Another example of infiltration is a missing cap on a private sewer. In a submerged area, this can allow as much water into the sewer as the service line can carry. Assuming a one percent slope, a submerged cleanout of a four-inch service can allow 83-gallons per minute or 120,000-gallons per day of rainwater to enter the system. Installing a cleanout cap, which can be purchased at a local hardware store for under $10, can stop this.
Infiltration defects are not as easily corrected. Traditionally, infiltration has been corrected by digging and replacing damaged line segments. While expensive and disruptive to homeowners and businesses, municipalities found it necessary to manage increased flows into sewer treatment plants.
Fortunately, trenchless technology allows for the replacement of line segments and the installation of lining systems that expand to the internal diameter of the existing pipe with little or no excavation. This type of sewer rehabilitation is gaining popularity in the gravity sewer industry for these reasons. Improvements in technology allow municipalities to more cost-effectively reduce infiltration and inflow into existing sewers. Today's practices result in greater reduction in flows and much less disturbance to the public.
e-sources
Insituform Technologies Inc. -- www.insituform.com
Thomas Technologies -- www.ttechnologies.com
POTW MOM Programs Project -- www.epa.gov/region
end notes
1Public Law 92-500, October 18, 1972 -- Water Pollution Control Act Amendments would not allow grant funding of a Wastewater Treatment Plant unless it is documented that each sewer system discharging into such treatment works is not subject to "excessive infiltration and inflow."
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This article appeared in Environmental Protection, Volume 11, Number 10, October 2000, Page 52.
This article originally appeared in the 10/01/2000 issue of Environmental Protection.