What does a municipality do when its current wastewater outfall sewer is rapidly reaching its capacity and approaching the end of its life cycle? That was the challenge faced by the City of Center Line, Mich., in 2008.
In order to eliminate an existing gravity overflow weir (which constitutes a sanitary sewer overflow or SSO), the City and its engineering consultant, Anderson, Eckstein and Westrick Inc. (AEW), negotiated for additional treatment and transport of the sanitary sewer capacity with the Detroit Water and Sewerage Department (DWSD). The contract capacity with the regional treatment facility was increased from 6.4 cu ft per second (CFS) to 8.6 CFS. The increase allowed for elimination of the overflow weir during the required 25-year, 24-hour storm event without a large and costly storage facility.
For most communities, this scenario typically results in one solution: construction of a new interceptor, which can be constructed in either the open-cut or tunnel method. In 2009, the City of Center Line elected to modify and upgrade its existing outfall sewer — originally built in 1962 — to handle the increased flow capacity.
AEW considered a number of options in order to provide for the additional sanitary sewer capacity or to limit the flow. The options included: construction of a new outfall sewer by tunneling or open-cut, increasing the size of the sewer through pipe bursting or conversion to a forcemain. AEW recommended to the City of Center Line that the force main option be used to upgrade the outfall sewer. Several options for the conversion were considered including sliplining with a smaller PVC or HDPE pipe, an HDPE lining system or cured-in-place-pipe lining (CIPP). The conversion option would also dictate the modifications necessary at the existing pump station.
Based on AEW’s recommendations, the City determined that the most cost-effective option was to upgrade the existing gravity outfall pipe system into a forcemain sewer, using a CIPP reinforced liner. Construction plans and contract specifications for the required upgrade options were approved by the Michigan Department of Environmental Quality (MDEQ currently MDNRE) and the contract was put out for a public bid. Detroit-headquartered Inland Waters Pollution Control (IWPC) outbid six competitors and secured the contract. A member of the Inland Pipe Rehabilitation (IPR) family of companies, IWPC is a regional leader in sewer, water and other pipe rehabilitation.
While Center Line officials had previously determined that they wanted to go with a reinforced liner, there were a number of technical and procedural details that would need to be addressed during construction. Foremost was the issue of how to physically connect the new liner to a series of existing manhole covers in a way that would allow the gravity sewer to be transformed into a forcemain sewer. The actual conversion of a gravity sewer into a forcemain sewer requires a closed conduit, upgraded in a manner whereby the system can handle fluid being pumped under pressure. The big challenge in such an upgrade is getting the connections to the existing manholes to be watertight. In the past, this has been attempted in a limited fashion using various methods, but no one had ever been able to successfully execute such an upgrade in a way that enabled the new connection to be truly watertight.
IWPC also had to complete the project that was not only timely and cost-effective, but minimized disruption to the surrounding community. IWPC would be lining the outfall sewer for the 1.5-mile stretch where it connected to the Detroit system. During this time, IWPC would work to minimize road closures and construction delays along the major thoroughfares of Van Dyke Avenue and Eight Mile Road (two Michigan Department of Transportation or MDOT rights of way). IWPC assembled a quality construction team, including O’Laughlin Construction Co. (OCC), which was able to provide innovative options for the construction and execution of the project. IWPC’s team established a good working rapport with the Cities of Center Line and Warren, AEW and various regulatory agencies, including MDOT.
The system upgrade successfully converted Center Line’s existing 24-in. RCP gravity outfall pipe system into a 24-in. CIPP forcemain pipe system. The upgrade was achieved by installing a fiberglass reinforced CIPP liner — IWPC installed 8,069 lf of fiberglass reinforced CIPP liner spread out over 15 individual “shots,” the longest of which was 1,050 lf — between the existing manholes and the installation of site-specific stainless steel fittings at existing gravity manholes to convert them to either air release or cleanout structures. This upgrade also resulted in the elimination of existing manhole structures by lining through the existing flow channels with the reinforced CIPP liner. This type of conversion has been on the collective industry drawing board for quite some time, but this is the first time it has been effectively implemented. The reinforced liner and related air or cleanout manhole fittings allowed the gravity sewer to be transformed into a forcemain sewer. The increased capacity of the new forcemain sewer was complemented by the modernization and upgrade of the City’s lift station equipment.
The IWPC’s team’s ability to design and install the connection fittings in order to convert existing gravity manholes was a critically important component of the project and came about as the result of a series of innovative technical and design solutions. IWPC’s team established 27 individual connections, designing and fabricating entirely new connectors using a blend of techniques and technologies to establish this effective solution. Each manhole received type-specific flexible fittings to interface with and attach to the new liner; an extremely effective level of connective flexibility that made it possible for the manhole opening to be multi-functional. This was accomplished by installing specially designed stainless steel connectors with a removable plate at the top of the fitting large enough to use as a cleanout opening. A pipe nipple interface in the middle of the plate makes it possible to use the manhole opening as an air release valve as well. Conversion of a number of existing manholes to straight-through manholes was achieved using a form installed over existing flow channels.
IWPC developed a form technique to insulate the area around the straight-through manholes so that that section of the liner held the same temperature, avoiding rapid cooling and preventing cracking due to temperature changes. In the end, the finished project involved modifying five existing manholes to cleanout manholes, eight existing manholes to air release manholes and 14 existing manholes to straight-through manholes.
One of the most noteworthy aspects about the Center Line project is the fact that the outfall sewer conversion was accomplished while the existing sewer system remained in service during most of the project’s timeline. This is done by bypassing the sewage. The bypass for this project was achieved by establishing a single physical connection to a neighboring community’s sanitary system — the City of Warren — and isolating the section being upgraded from the main sewer system by isolating the entire outfall sewer while the project was under way. IWPC worked closely with the Department of Public Works in Center Line and the City of Warren Engineering Department, and AEW to execute an agreement whereby Center Line paid Warren (instead of DWSD) to treat the City’s sewage during the construction period.
By creating this temporary bypass system that tapped into the Warren interceptor, IWPC was able to avoid a traditional and more disruptive bypass system. The nature of the bypass pumping plan aided in the review and approval of the bypass pumping agreement between the two communities. The bypass connection as constructed can isolate the entire outfall sewer resulting in significant benefits to the City of Center Line including the use for future maintenance or outfall disruptions. This solution also saved the City a great deal of money, offsetting the cost of new, sophisticated monitoring equipment necessary to upgrade the existing pump station. The bypass was such a success that the temporary connection was retained as a permanent feature that could work both ways in the future and benefit both cities.
The project was deemed a technical, financial and logistical success. The City of Center Line was delighted with the result and the project’s success has resulted in no recorded SSOs since its completion. The elimination of the SSO weir has a significant and beneficial environmental effect. The reduced timeframe — the project was completed in a little under a year, despite close to three months of weather interruptions — was another added benefit; construction began in the spring 2009 and was the outfall sewer construction was completed and operational by the first week of February 2010.
IWPC was the prime contractor for this project and self-performed all of the reinforced CIPP liner installations. The lift station improvements were handled by project subcontractor OCC. Together, IWPC, OCC and the City of Center Line Design Engineer Anderson, Eckstein & Westrick Inc. collaborated to deliver a unique and cost-effective upgrade of an outdated sanitary sewer outfall system.
Joseph Fattore is a project manager with Inland Waters Pollution Control.