General Overview of Sewer Constraint Issue

What is the Problem?
How and Why Were the Problem Areas Identified?
Where are the Problem Areas Located?
How is the City Working Towards Resolving the Problem?
How Will This Problem Affect Builders and Developers?
What Can Builders and Developers Do to Help Resolve the Problem?
What is Being Done to Avoid This Problem in the Future?

What is the Problem?

The City of Phoenix is currently engaged in a major effort to reduce Sanitary Sewer Overflows (SSOs) and to ensure that all parts of the City’s extensive wastewater collection system are able to deal with flows in a variety of anticipated circumstances. Although the City has always been committed to providing a safe and efficient level of sewer service, and has been very successful in this regard, regulatory agencies at the federal and state levels have now introduced extremely rigorous guidelines for the prevention and management of SSOs. The City has responded aggressively to ensure that the City and its personnel comply with the law. As part of this response, the City’s Water Services Department (WSD) initiated a program to identify and eliminate any sewer capacity issues that could result in SSOs.

SSOs tend to occur when certain relatively unusual events happen in combination to cause sewers to overflow, which could back up into homes, businesses or into streets or watercourses. Sewers are designed to provide a certain amount of extra capacity to deal with these occurrences, such as significant rain events or simultaneous loadings from multiple residences or businesses. However, in some cases this extra capacity has been gradually used up to accommodate more development than was anticipated when the sewers were installed. Although City and private sector engineers have historically worked together effectively in Phoenix to design a wastewater network that in most cases has successfully dealt with changes in land use and sewer use, decades of redevelopment, incremental connections to existing sewers and other changes have left particular segments of the system with little extra capacity to deal with unusual events. The result is that some areas now have limited sewer capacity and are predisposed to SSO events. The City has had to initiate the construction of new sewer capacity projects and restrict new taps to the system until those projects are completed.

The WSD is currently preparing to initiate a large number of relief projects that will provide additional sewer capacity to all of the areas in the City that have been identified as being potentially subject to SSO events. In some cases, where a relief sewer project can be designed and constructed quickly, restrictions on new development will be removed within a year; in worst case scenarios it is anticipated that without any private sector assistance, restrictions on new development could last as long as up to three years. In all cases the City will try to prioritize and expedite projects in such a way as to provide sewer access to as many developers and builders, as quickly as possible, as can be accommodated. The Development Services Department (DSD), will work with WSD to ensure that all new developments, whether at the site plan, building permit or sewer tap stage, are not allowed to proceed in affected areas until relief projects have been completed. Although in a few limited cases other options exist (as discussed below), for the most part the development community will have to temporarily put new construction on hold or participate in relief projects in areas where potential SSO problems have been identified.

Even though potential problems have been identified in specific areas across the City, the vast majority of the City’s sewers have been reviewed and found to be operating with ample capacity. Of the approximately 4,200 miles of sewer in the collection system, approximately 15 miles of sewers have been identified as operating at or above design capacity, which is less than one half of a percent of the sewage collection system. The City readily acknowledges that addressing the SSO problem will result in some builders, developers and citizens being impacted. As a result the WSD is making the quick resolution of this problem one of its highest priorities in terms of both staff time and budgetary resources, and is spending in excess of $60 million on remedial projects over a three year time-frame.

How and Why Were the Problem Areas Identified?

The WSD routinely master plans and conducts assessments of the sewer system as part of its planning process, in an effort to ensure that all parts of the City are provided with adequate sewer capacity. A key tool used in the master planning of the sewer system is a hydraulic model. The hydraulic model is used to determine the remaining capacity of existing sewers and to size new sewers to accommodate the population growth. This is updated about every five years. The WSD also routinely conducts assessments of the sewers by televising the inside of the pipes and evaluating relevant information, such as pipe material and age. The sewer pipes are then prioritized for rehabilitation based on the seriousness of the defects found.

In addition to regular City efforts to monitor and assess the capacity of the City’s wastewater network, additional SSO regulations are being developed by the U.S. Environmental Protection Agency (EPA) that requires the development of a Capacity, Management, Operation, and Maintenance (CMOM) program. Although the SSO regulations have not yet been implemented by the EPA, the Arizona Department of Environmental Quality (ADEQ) has proceeded to implement the State’s version of these rules. It is anticipated that, once implemented, the new Federal and State requirements will likely be strictly enforced – similar precedents in the regulation of wastewater operations in the past indicate that any infractions could result in severe financial penalties for the City and both civil and criminal proceedings against City management and personnel.

In response to these proposed rules, the WSD engaged engineering consultants to develop a CMOM program in January 2003. This included a “System Evaluation and Capacity Assurance Plan.” A system-wide sewer flow monitoring program was initiated in October 2003 to calibrate the City's hydraulic model. Over 100 meters with more accurate measurement technology than used for previous calibrations were installed at strategic locations within the sewage collection system. The meters gathered flow information during 2003 and 2004. Updated wastewater flow information from commercial and industrial dischargers, flow data from the City’s wastewater lift stations, updated and new collection system as-built information, and physical information collected by technical and field staff during sewage collection system condition assessments and rehabilitation projects were all used to improve the hydraulic model’s accuracy.

The CMOM Risk Assessment was finalized in June 2004 and the hydraulic model update was completed in July 2004. Although the WSD is currently working with consultants to prepare the associated CMOM plan and procedures, the newly calibrated and updated hydraulic model completed in July 2004 indicates that numerous sewer lines are at or above 80 percent of their design capacity. Since it is the WSDs practice to maintain sewer systems at levels under 80 percent to account for uncertainties of storm water infiltration, simultaneous loadings, and other unusual events to reduce the chances of SSOs occurring, these are the lines that are of particular concern to the WSD. All new development feeding into these particular sewers will be put on hold until relief projects can be completed, so that the additional loadings do not lead to a sewer overflow situation.

Where are the Problem Areas Located?

A preliminary map showing each capacity limited sewer segment based on the results of the hydraulic model was prepared by an engineering consultant in August 2004. Each of these segments was subsequently physically verified in the field. The verification process required the inspection of manholes, the survey of invert and manhole rim elevations, and adjustment of the hydraulic model for any changes that were found. The fieldwork was completed in January 2005. This field analysis reduced the number of sewers that had initially been identified as having capacity limitations. A map detailing the areas impacted by the sewer capacity constraint issue is available online.

As indicated on the map, areas currently threatened by potential SSO problems can be found throughout Phoenix, although they tend to be concentrated in particular areas such as the northeastern and central-western parts of the City. Because SSO problems tend to occur as the result of a gradual process involving many years of redevelopment and/or additions to the existing sewer network, effectively creating ‘bottlenecks’, most of the affected areas were largely built out ten years ago or more. As a result, as the map shows, relatively few parts of the City’s growth areas are affected by this problem. Interestingly, much of the central, older core of the City is also unaffected at this time. In many cases a large area will be shown as being affected by potential SSO problems, seemingly indicating that there are numerous sewers with capacity limitations. However, that is not always the case – in many of these situations one ‘bottleneck’ or capacity-restricted sewer is restricting capacity for all sewers feeding into it, and in cases like these one relief project will eliminate the restrictions associated with a large sewage collection area.

How is the City Working Towards Resolving the Problem?

To respond to this rapidly emerging and critical issue, the WSD has selected engineering consultants to design relief sewers for all of the known capacity constraints in the Phoenix sewage collection system. Two projects are currently under construction and two are being designed. The Department’s goal is to complete the construction of these relief sewers in three years. The City’s Five-Year Capital Improvement Program (CIP), which includes next year’s capital budget, has been amended to include over $60 million to pay for these projects. CIP funds will be used both to pay for City-managed projects and to contribute towards developer-managed projects that meet certain WSD criteria.

The projects are currently being prioritized using a variety of measures, including the degree of overcapacity, the size and number of redevelopment projects that would be removed from restrictions, the speed with which design and construction can take place, and the number and capacity of sewers that will no longer have severe capacity restrictions. In some cases the City’s engineering consultants are still doing preliminary design investigations to identify what types of relief sewer projects are possible (e.g. minor sewer by-pass projects versus major parallel line construction). This information will be used by the WSD to determine how the SSO problem can be most quickly and economically addressed.

Developers and builders working in areas where restrictions have been imposed are also being contacted so they are made aware of the SSO situation and the City’s efforts to remove the problems. In some cases contacts with these developers and builders will be critical to ensuring that private sector participation expedites the construction of relief projects. Conceptually, the problem associated with SSOs and associated temporary restrictions on development are very similar to the environment facing developers in growth areas currently not served by adequate water, sewer or streets – developers and builders must either arrange to have the facilities put in place to meet their schedules, or wait until the City’s CIP provides the necessary infrastructure. In these specific cases the adjacent sewer network is close to capacity and must be expanded.

How Will This Problem Affect Builders and Developers?

New site plans, site permits, and building permits will not be able to move forward until the relief sewer that solves that project’s capacity constraint is constructed by the City or the developer or the developer constructs a City and County approved temporary sewage collection system such as a septic tank and disposal system. Restrictions on development in areas with the potential for SSO will be very similar to that experienced in the growth areas of the City where development must wait until adequate water and sewer facilities are provided either by the development community or the City, except that in these cases unfortunately the City has been forced to put a hold on development approvals even at the building permit and sewer tap stage (development in newer areas tends to face restrictions with sewer capacity at the site plan review stage).

If building permits were issued for development projects prior to the identification of a SSO problem, the City will use reasonable judgment based on technical criteria to predict whether the project's proposed sewer flows will or will not create surcharge or overflow conditions in any of the affected downstream sewer lines prior to allowing a tap into the City's sewer system. If a tap is not allowed, the project will not be able to move forward until the relief sewer that solves that project’s capacity constraint is constructed by the City or the developer or the developer constructs a City and County approved temporary sewage collection system such as a septic tank and disposal system.

Building permits for single-family homes on single lots or single family building additions will not be impacted by the City’s new policies regarding SSOs. Renovations of existing residential or commercial structures will be permitted when it can be shown that the changes will result in the same or reduced loadings to the wastewater system (on both a peak and average flow basis).

What Can Builders and Developers Do to Help Resolve the Problem?

The City will reimburse the private sector for City approved permanent sewer relief projects that the private sector chooses to build if the projects are approved in the Wastewater Capital Improvement Program (CIP) and in the time frame of the relief sewers funding schedule. For projects that were issued building permits prior to implementation of this procedure, the City will also consider reimbursement for the incremental capital cost of City and County approved temporary or alternative sewer systems needed to serve the site. The City will not pay for any maintenance, operation or abandonment costs of such systems.

If a relief sewer effort in a particular area has not yet been initiated by the City, and one or more developers are anxious to proceed with construction as quickly as possible, one avenue that should be explored is that of the developer or developers retaining an engineering consultant to review different options and begin design work. After consultation with WSD engineers, if the proposed relief project proves to be technically and economically viable, the developer or developers may be allowed to proceed in an expeditious manner and still be partially or wholly compensated by the City, depending on the circumstances. Design and construction work managed by the private sector can often be undertaken more quickly than by the City, because of numerous requirements associated with the selection of consultants and bidding of projects. However, it should be noted that private-sector managed projects utilizing City funding will still involve certain public bidding and other procedural requirements.

Builders and developers who are being adversely impacted by the SSO problem, and who would like to pursue the option of completing needed sewer relief projects themselves, should contact (602)534-5813 to make an appointment with City staff to obtain information on the process that must be followed. Usually private-sector engineering consultants will have to be retained to undertake the necessary initial evaluation and design work in any case, so it may be advisable to retain an engineering consultant to deal with WSD staff from the start. Builders and developers seeking to ensure that their projects move forward as quickly as possible should follow the attached procedure.

What is Being Done to Avoid This Problem in the Future?

The Development Services and WSD have developed an improved process to assess sewer capacity at the onset of development. The development project’s wastewater flows are reviewed at the site plan stage or building permit stage (if no site plan is required) to assess if there is adequate capacity in the sewer system to serve that project. This process not only allows City staff to make an initial projection of flows to check for adequate capacity, but it also tracks approvals and corresponding flows. In this manner, a continuous record of proposed development flows is maintained in the hydraulic model for each sewer segment to avoid reaching its design capacity.

While the development of an accurate hydraulic model can be somewhat difficult since it is as much art as it is science, the WSD depends on the hydraulic model as a tool to help manage its resources effectively. Without this tool, future predictions of sewer flow growth and assessments of current conditions would be almost impossible short of having hundreds of crews constantly checking manholes for flow information. The WSD has used the model to prevent sewer flows from exceeding existing and planned sewer capacities by projecting future flows from planned and existing development and to assess the operational characteristics and performance of the existing sewer system.

Uncertainties related to estimating the amount of sewer flow from any project includes factors such as the variations in land use allowed under the zoning ordinance, number of occupants of a building during the day, family size, age, vacancy rates, type of fixtures and appliances such as low flow toilets and washing machines, and the changes that occur in these factors over time. Other uncertainties may be related to assessing the performance of the sewer system when peak flows occur, the rate of degradation of sewer lines, subsidence, how the line was built compared to its design intent (accuracy of as-built information), climate and the effects of infiltration during rain storms, and rate of wastewater flow from the land uses it serves. All of these factors can change over time.

Though the WSD has been implementing the systems and programs described above and will continue to improve planning and operations management as more information and tools become available, there is a possibility that sewer capacity constraints may arise in the future. As more information about the impacts of development on capacity and on the performance of the existing sewage collection system becomes available, the WSD will be able to make more accurate projections and assessments.