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Investigating the Collection System

Ann Arbor's sanitary sewer system accepts wastewater from individual homes and businesses and conveys it to the wastewater treatment plant, where it is treated before discharge to the Huron River. This system comprises sections of sewer pipe typically situated under streets. Homes and businesses are connected directly to this pipe or to access points at manholes along the sewer line. The sanitary sewers are normally designed to convey flows from homes to the wastewater treatment plant by gravity, reducing both the operating costs of the system and the chance of backup caused by loss of power at pumping stations. The flows that are discharged into the sanitary sewer system vary throughout the day, and this is accounted for so that the level in the sewer pipes always stays below the top of the pipe. The sewers are also designed so that the velocity of the flow keeps solid matter in suspension until it reaches the wastewater treatment plant.

The stormwater collection system is also located below the streets and collects surface runoff during storms - the flows that typically come from rooftops, driveways, streets, and parking lots. Homes and businesses generally are constructed so that stormwater flows are directed toward the street. The stormwater collection system pipes are much larger than sanitary sewer system pipes because they are designed to handle peak flows generated during rainstorms. Because treatment of these flows is not required, the pipes discharge the water directly to a stream or a river. During large storms, flows received at the Ann Arbor wastewater treatment plants increase dramatically, and even when the treatment plants are operating at maximum capacity, the storage facility can fill to such a level that partially treated flows are discharged to the Huron River. Although this is a permitted discharge, it may be classified as an SSO under future regulations and discharge permits.

When it rains, a portion of the runoff finds it way into the sanitary sewer system, instead of into the stormwater collection system, through inflow and infiltration (I/I). Typically I/I is groundwater that leaks into the sanitary sewer system though cracks in pipes or joints between pipes, but it also can enter the sanitary sewer system through the manhole structures or even through manhole covers in streets that have standing water during storms. A final major source of collection system I/I is private residential and business connections and pipes that are on private property, including foundation footing drains. These drains are a common feature in homes with basements, and they usually are connected to the sanitary sewer system in homes built before 1980 because it was assumed that the wet-weather contribution from the drains was low and that the rate at which these flows entered the sanitary system was low. It was subsequently found throughout the country that these foundation footing drains could result in a significant wet-weather contribution to SSOs and basement backups, and the building codes were changed to exclude their use.

Although the sanitary sewer system is constructed to account for a certain amount of I/I, the pipes and connections can deteriorate over time, and flows increase as the collection system ages. When the capacity of the sanitary sewer system is exceeded, the levels in the sewer can rise above the top of the pipes and flows can back up into the basements of homes and other buildings. Homes with basements that are only slightly above the elevation of the tops of these sewers have a greater potential for basement backups. Water that enters the basements of homes might do so with considerable force, and the levels can be substantial - from a few inches to several feet in extreme cases. Although the fluid entering the basements is primarily rainwater, it can contain significant amounts of sewage, presenting a health hazard to residents and damaging household items in the basement. Finished basements present the greatest potential for damage and property loss.

Surveying the Problem

To develop a better understanding of the nature and extent of the basement backup problem, the project team conducted a survey and field monitoring program in several neighborhoods where backups were known to occur. After analyzing the city's collection system, the project team monitored flows and developed models. Through field investigations, the team determined basement floor elevations for homes in the areas that had experienced basement backups and collected wet-weather response rates and dry-weather flow rates of the sanitary collection systems serving the neighborhoods. The elevation and rainfall data were used in a model simulation calibrated to field-measured peak sewage levels to identify which homes had the potential for basement backups. The team also used the data to understand in detail the flows discharging from each of the neighborhoods and to calibrate both a trunk sewer model and individual neighborhood models.

Use of directional drilling equipment minimized disruption from construction to install the shallow drainage system, which accepts flows from the new sump pumps.

After the modeling and field investigations, the team found that the most significant source of I/I into the sanitary sewer system during storms was from the building footing drains, with their direct connection to the sanitary sewers. Previous studies in Ann Arbor had shown that the footing drain connection from a single home could contribute as much as 10 gpm during a storm. The project team estimated the flows produced from individual homes by measuring the flows generated during rainfall events and estimated similarly high rates of production.

Considering the Alternatives

Ann Arbor reviewed what other communities had done to reduce SSOs and, in particular, to correct conditions that caused chronic basement backups. The communities they selected for review had collection systems and conditions similar to those of Ann Arbor and were used to provide a frame of reference for developing corrective alternatives in Ann Arbor. In developing its own plan of action, the city took into account the costs, methods, institutional hurdles, and successes of the other communities' approaches and evaluated four basic solutions in each of its own neighborhoods.

The first option the team reviewed was the use of relief sewers, which involves placing a pipe parallel to one that is already in the ground, enabling the flow in one pipe to be divided between two. The advantages of this method are that the required construction is performed in existing rights of way and easements and that contractors are familiar with this method. The disadvantages are that the construction causes disruption in the streets and that this method does not prevent the trunk sewer system from being affected by larger peak flows, meaning that additional pipe relief or other construction might be required farther downstream in the sewer system.

The team's second option consisted of bursting an existing pipe with a larger-diameter pipe, replacing the old pipe with the larger one, which can be up to two diameter sizes larger than the old one. The pipe-bursting construction method was considered because it normally disrupts less traffic than does relief pipe construction and can be completed relatively quickly. However, the increased-capacity method requires access pits to connect homes to the new pipe, and a specialized contractor would be needed for pipe-bursting work. Moreover, this method is also not immune to the effects of larger peak flows on the trunk sewer capacity, again possibly requiring increased construction and costs downstream in the sewer system.

The third option was the placement of storage basins in the system to temporarily store sewage during heavy rainfall events. This method would enable peak flows to be reduced downstream from the basins by storing the flows until the rainfall intensity has lessened. The collection system storage option has several advantages, the most important of which is that it does not move peak flows downstream into the trunk sewer system. It also does not require additional downstream construction, offsetting its cost. The disadvantages of this method, the project team found, are that the storage basins must be properly maintained, which requires access to the storage basins and incurs maintenance costs, and that there is potential for odor problems in the storage basins.

These three options are successful only to the extent of the design criteria and will not work if a large storm exceeds this design capacity. These options also incur the cost of treating the stormwater and groundwater added by footing drains.

The final option considered by the team was footing drain disconnection. The team knew that I/I from foundation footing drains connected to sanitary sewer systems add rainfall and groundwater to sanitary sewers and that disconnecting the drains would prevent this clean water from reaching the sanitary sewer system. Disconnection would involve installing a sump pump in each home with a disconnected foundation footing drain. The sump pump would then direct stormwater and groundwater into the storm sewer system, thereby reducing the volume of water entering the sewer system. The advantages of footing drain disconnection are that it requires only limited construction in the street, with pipe construction taking place between the sidewalk and the street, and that flows to the trunk sewer system are reduced, as are the costs associated with the unnecessary treatment of rainwater. A disadvantage is that it requires construction in basements and on lawns on private property, which can be difficult. Also, homeowners would be responsible for maintaining the sump pumps, and the sump pump discharges would have to be connected to the storm sewer system to prevent nuisance water around homes and on sidewalks and streets. Power failures during large storms and provision of alternative-powered backup sump pumps were of notable concern to residents.

Choosing the Solution

The project team used a monitoring program in private residences to establish the peak flows that the footing drain disconnection program removed.

The project team reviewed the four solutions according to a variety of selection criteria, including quality of life, cost, and the effects of construction. The evaluation showed that, in most cases, storage basins and footing drain disconnection were closely ranked as the best alternatives. After soliciting public feedback, the task force found that the public's paramount priorities were protection of natural features and elimination of long-term impact on the environment from SSOs.

With the public's input in mind, the project team determined that a comprehensive, citywide foundation footing drain disconnection program would be the best and most cost-effective solution, eliminating the cost of treating stormwater flow, a treatment that is needed only when the footing drains are connected to the sanitary sewer system. It also would save money on the cost of both anticipated wastewater treatment expansion and regulatory penalties for sanitary sewer discharges to the environment. The team also decided that a disconnection program would provide the greatest level of protection against future large rainstorms and would not move the problem downstream to previously unaffected neighborhoods or require extensive construction on downstream trunk sewers. Moreover, disconnecting the footing drains would preserve the natural features and protect the watershed by minimizing undesirable discharges to the nearby Huron River and its tributaries. Finally, the team decided, this solution would be compatible with the regulatory trend toward disconnection of footing drains from sanitary sewer systems.

Implementing the Solution

Once the study and the recommendation process were complete, the project team moved to the task of developing and implementing a phased footing drain disconnection program. One of the reasons that footing drain disconnection is not a widespread practice is that much of the work needs to be performed on private property, using public funding. "The institutional hurdles can be daunting to some communities, but the payoff, however, is a solution that takes care of the root cause of system capacity issues, including basement backups, rather than moving the problem to other areas in the city," says Sue McCormick, water utilities director for the City of Ann Arbor.

The team undertook a pilot footing drain disconnection program to gain a better understanding of the technical and implementation challenges. They performed the pilot work on 11 homes, investigating the separation of the footing drain connections in basements, installing check valves for backflow prevention, disconnecting and rerouting footing drain flows to new sumps, and installing sump pumps to discharge this water from the homes. When installing the check valves for basement facilities, the project team made a point of disconnecting footing drain connections to the sanitary house leads, thereby preventing damage to the basement floor that could result from a buildup of water pressure underneath. At all of the 11 pilot locations, the team disconnected the footing drain from the sewer line and installed a sump at the disconnection point for pumping footing drain flows to a location in each yard that would keep the discharged water away from the house and not create a nuisance or safety hazard. The team also installed a standard sump pump, along with a water-powered backup pump that can operate in the event of a power outage, at each house.

Funding and Supporting the Program

The city decided that the funds to cover the cost of footing drain disconnection, including the work performed on private property, would come from sewage collection system user fees. Based on the pilot disconnection program, the project team calculated that the costs to complete such a program citywide would range between $80 million and $130 million, depending on the number of homes requiring disconnection. The team estimated that there were up to 20,000 Ann Arbor homes that required footing drain disconnection and that the approximate construction cost per home would be $8,000-$9,500 to disconnect the footing drain and provide a curbside collection system that would bring the rainwater and groundwater from the sump to the stormwater system. The pilot program demonstrated the necessity of the curb drainage system to prevent wetness problems on homeowner property and to prevent icing on sidewalks and roadways. The curbside collection system costs less and causes less disruption to the community than direct taps to the existing stormwater pipes beneath the street.

The team knew that effective implementation of the disconnection program - performed on private property and paid for with public funds - would require a construction management program to oversee the work. It also would be important for the city to foster public engagement in the program and demonstrate its commitment to a rapid implementation. The team helped the city implement a strong and comprehensive public engagement program that communicates why this work is needed, what the benefits are for both homeowners and the city, and what is included in the work. The city also collaborated with environmental groups and other stakeholders to engage citizens in supporting the disconnection program and developed an ordinance that provides a complete legal basis for the work.

The disconnection program is being implemented in four phases, providing a deliberate and well-planned approach that will prevent excessive expenditure of utility funds, overcommitment of the available contract work force, and creation of nuisances or hazards from inadequate control of sump pump discharges. The disconnection will be accomplished on a block-by-block basis in conjunction with construction of the sump discharge collection system using a priority sequence, in which homes in the five neighborhoods that have flooded most often are placed at the top, followed by homes adjacent to those five neighborhoods. The third level of priority encompasses homes that have not often flooded but that are in the basement backup-prone neighborhoods, and the fourth and final tier includes remaining homes in Ann Arbor with connected footing drains. The two highest-priority phases are expected to be complete by the end of 2004, and within the first few years of the program, it is estimated that sufficient flows will have been removed to eliminate the SSOs. The remaining work will focus on removing flows that have the highest potential for reducing other collection system capacity problems.

As the program moves forward, CDM is providing construction management services, which includes reviewing the work of prequalified contractors that are installing the new sumps and sump pumps in the basement of each home, constructing the discharge lines, installing backflow prevention devices in the basements, and disconnecting the footing drains from the sanitary piping. The company is also monitoring footing drain flows to better understand the variability in flow volume and peak rates from individual homes, which will enable the team to better estimate the total flows that will be removed once the footing drains are disconnected. This information will be useful in guiding future implementation phases of the program.

Ann Arbor's footing drain disconnection program has proven successful thus far with more long-term benefits than traditional methods. By addressing the cause of the problem, rather than applying a temporary solution, the Ann Arbor work shows that a total, successful, and cost-effective program to address basement backups from sanitary sewer overflows can be developed.