Program Produces I/I Reductions in Wastewater Collection System
With less government funding available today, municipalities across the United States are looking for creative and cost-effective methods to provide utility services. This is certainly true in the wastewater field, and there is a continual search for ways to minimize capital costs and reduce operations and maintenance expenses at wastewater facilities.
Efforts to extend the useful lives of existing wastewater treatment systems, and maintain compliance with NPDES permit requirements, are receiving priority attention. Studies have shown that 50p;60 percent of annual flows treated at wastewater facilities can be attributed to inflow and infiltration as a result of miles of deteriorated old sewer lines that allow millions of gallons of stormwater, and sources other than wastewater, to enter the collection systems on a daily basis. It has been estimated that a minimum of $400 billion is needed nationwide to make up for years of neglected infrastructure.
The City of Crossville, Tennessee, like many other municipalities, has struggled for years to meet the challenge of providing cost-effective services within the restrictive limits of the city's NPDES permit. But efforts to improve wastewater collection and treatment operations in recent years have been successful. In fact, the municipality has been recognized by the Tennessee Department of Environment and Conservation for significantly reducing wastewater bypasses around the plant and for its commitment to environmental protection.
To identify specific inflow and infiltration problems (I/I), a sewer system evaluation survey (SSES) was conducted in the late 1970s to determine areas of critical need. With help from a 75-percent EPA grant, a wastewater treatment plant expansion was completed in 1983, increasing the design capacity. The expansion, along with a successful sliplining rehabilitation project carried out from '83 to '85, resulted in the removal of a state-imposed sewer moratorium that had lasted seven years. With its removal, the problems seemed to be in the past and high priority was not given to sewer system rehabilitation. However, with continued I/I contributions and residential growth, the wastewater facilities once again became overloaded, resulting in increasingly frequent bypasses and NPDES permit violations.
In 1990 the state imposed a second sewer moratorium on Crossville. Two years later construction began on a $3 million project to expand the wastewater treatment plant again and upgrade the collection system. The design consultant selected was GRW Engineering. Professional Services Group (PSG) was awarded a 5-year contract to operate and maintain the treatment plant and pump stations. The contract firm also was responsible for observing the project's engineering and construction phases. In 1993 the collection system rehabilitation was added to the O&M contract. The scope of services was broadened to include inspecting the entire collection system and updating the system map to include new sewer lines and storm drains. Tasks required were flow monitoring; line cleaning; smoke testing; dyed-water testing; night flow isolation inspection; TV inspection; updating the sewer use ordinance for privately-owned sewer I/I regulation; making recommendations for capital cost repairs and replacement; preparing monthly reports of progress; and post investigation of the rehab work.
Two-Phase Collection System Rehabilitation
The O&M contract firm implemented a two-phase collection system rehabilitation program. Phase one was to diagnose the condition of Crossville's system by drainage basins to identify inflow. Phase two was to carry out continuous I/I investigation, identification, and repair work. Two employees were dedicated full time to rehabilitation work with additional support as needed. The rehab crew began by locating and documenting approximately 74,000 ft of storm drains, intakes, and discharge points. The first objective was to identify direct inflow sources. The plan was to locate I/I by utilizing past flow and bypass data to determine priority areas, and to focus on inflow reduction in those areas first.
Since rehabilitation work was initiated in the dry weather summer months, smoke testing was conducted first. To inform the public, door hangers were printed and distributed in the areas where smoke testing would occur. Local radio, TV, and newspaper sources were asked to notify the public of the location of the smoke testing on a daily basis. The Fire Department was also notified. Records were developed to document smoke testing data. File notebooks were used to document findings and to attach photos taken during smoke testing. Several methods were used to locate storm drain cross connections, including taking fecal coliform tests at storm drains. The results from the smoke testing were documented under the following categories-residential, business/commercial, main line/lateral lines, storm drain cross connection, and prohibited connections.
After smoke testing was completed in the priority areas, the crew began televising lines and making point source repairs. While small repairs were being made, engineering plans were being developed to replace specific sections of old pipeline with new larger capacity lines, and extend lines to eliminate pump stations where possible. Two problem pump stations were removed from service by extending gravity lines and replacing an older pump station with a larger capacity facility.
Crossville also applied for and received a Community Development Block Grant for sewer rehabilitation. This grant was earmarked for the extension of another section of gravity line, the elimination of another problem pump station, and the upgrade of still another pump station. Also this grant is being used to remove approximately 3,000 ft of old sewer line that runs parallel to a newer trunk line and make new connections. Several manholes are to be restored with permanent seals against corrosion, infiltration, and exfiltration. Several manhole rehab methods are being studied.
The rehab crew has been innovative in locating I/I sources and making repairs. For instance, they have used hand held Weedeater blowers for smoke testing service lines. Within the first 6 months of the program, the storm drain system was mapped, over 125,000 ft of sewer line had been smoke tested, 30 vented manholes exhibiting inflow were sealed, 21 storm drain connections were located, and 12 of these repaired. Seven major pipe sections allowing inflow were located and repaired. Over 400 ft of sewer line had been replaced at point source I/I locations, and over 200 residential line inflow sources were documented. More than 1,500 extraneous flow sources were identified.
After smoke testing was complete and storm drains repaired, the contract crew began television inspection of sewer lines with a Cues TV van purchased for the rehabilitation project. To date over 40,000 ft of pipe has been televised. Workers did find that several lines with excessive root intrusion made televising them impossible. After several attempts to remove roots with a high pressure washer, dragging bob-wire, chain balls, and various innovative devices, other methods were investigated. Costs were obtained for chemical removal. This was an expensive alternative. Most companies charge by the foot of line treated regardless of the number of points where roots intrude. Many devices were tried but did not perform satisfactorily. So the crew set out to custom design and build a suitable piece of equipment.
First they located a heavy duty military surplus winch and mounted it on the back of an old tractor. Then they fabricated a root remover they called the "groundhog." This was made by placing two sections of PVC pipe over a steel frame with wire cable looped through the exterior of the PVC pipe. The device was pulled through the lines by the tractor mounted winch. It was successful in removing roots, rocks, and debris while the lines remained in service. And once cleared and opened, their hydraulic capacity was restored and upstream bypasses were virtually eliminated. Methods to prevent the roots from returning are now being investigated.
The BIG find (the one everyone in sewer rehab looks for) came when the O&M crew began inspecting 6,000 ft of a 24-in. main trunk line to the treatment plant. During night flow isolation and physical inspection, 20 joints were discovered which were allowing over one million gallons per day of direct measured inflow from the nearby Obed River. Considering the average daily influent to the plant is less than two million gallons, this was a very important find. A significant decrease in average wastewater treatment plant flow and a reduced number of bypass events have been documented since repair work on this section of the system was completed (Fig.1).
A number of approaches to reducing or eliminating this inflow were considered. Some would have been in the millions of dollars cost range. Miller Pipeline Corporation, which specializes in trenchless repair, scheduled a crew to come to Crossville to fix four leaking joints and determine if their method of repair would work. Custom seals were installed at the leaking joints, and the results were satisfactory. The city then contracted with the company to install 16 additional seals in the 24-in. lines at the leaking joints. Post nightflow isolation flow testing and televising documented the elimination of over one mgd of inflow. The repair method proved to be fast and cost effective.
As a result of the ongoing program the sewer moratorium, the second one for Crossville, was lifted in May of 1994, five years after the removal of the city's earlier seven year ban. This was an important day for the community. The efforts of the contract team had paid off and the requirements of the order had been met in a timely manner.
Crossville continues to move forward with its sewer rehabilitation project. A public participation program has been implemented, with information posters on display in public buildings, articles in the local newspaper, and distribution of self-inspection forms for sewer lines serving buildings. Privately-owned line repair orders generated from review of the smoke testing data are now being issued requiring homeowners to make repairs to faulty systems. And the sewer use ordinance has been revised to include enforcement measures to require such private lines to be repaired where inflow occurs. The city is purchasing a push rod sewer line TV camera to be used to inspect smaller private lines. Also, a 24-hour radio telemetry monitoring system has been installed on the major pump stations, and engineering is almost complete for replacement of additional problematic sewer lines and upgrade of pump stations.
Progressive successful sewer system rehabilitation is never a one-shot deal. It is imperative that a long range schedule and budget be implemented. Budgeting yearly is more cost-effective than incurring large capital expenditures with hydraulic expansions of wastewater treatment plants while risking sewer moratoriums and permit violations. At Crossville we have proved that successful on-going collection system rehab is achievable with proper planning and management. Conclusive data show a reduction in system bypass frequency of about 60 percent while rainfall in one year increased significantly (Fig. 2). Most of the '94 bypasses were the result of mechanical failure. The new telemetry system is expected to improve our response to pump station problems, which should further reduce bypass problems.
About the Authors:
Clark Annis is operations manager and Bill Davidson is project manager on the staff of Professional Services Group, the contract operations firm which runs the wastewater treatment facilities in Crossville, Tennessee.