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Morehead City, NC, uses an ARIMA study to end a state moratorium on new construction.

Article Excerpt
Morehead City, North Carolina, faced a moratorium on new construction because of problems with its sewage system. We developed an autoregressive integrated moving average (ARIMA) transfer function-intervention model to help town officials explain the sources of variation in the volume of sewage-treatment-plant discharge to the state officials who had imposed the moratorium on extending the sewage system lines. The results convinced the state officials that the problems were temporary, not systemic, and that the town's efforts to rehabilitate the system were improving its operation. Consequently, the state officials cut the moratorium by at least a year, allowing development projects worth $50 million to proceed.

Key words: planning: community; forecasting: ARIMA processes.

History: This paper was refereed.

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Growing communities are challenged to manage problems with antiquated sewage-treatment systems and may face the prospect of a complete shutdown in new development. In Morehead City, North Carolina, a small coastal community of about 8,000 people, tourism is a growing component of the local economy. The deterioration of this community's sewage-collection system caused treatment-plant daily discharges that intermittently exceeded the maximum volume permitted by the state. North Carolina authorities imposed a moratorium on sewer extensions, halting all new residential, business, and industrial facilities construction. The total economic value of the projects being delayed was estimated at $50 million.

The root cause of the problem was inflow and infiltration: rainwater and groundwater entering the sewage-collection system at damaged or defective pipes, pipe joints, connections, and manhole structures. This infiltration reduced the system's capacity to carry actual sewage, caused excessive discharges, and overloaded the sewage-treatment plant. The Morehead City manager identified the problem and obtained a state grant to repair a part of the sewer system. The town began a nine-month rehabilitation project in June 2003 to rehabilitate or replace 254 manholes and about five miles of the town's oldest sewer lines. It expected the project to reduce the inflow enough to bring the facility into compliance with the state-authorized permit levels. Unfortunately, the discharge rates after the sewer improvements began on June 1, 2003, remained high and frequently exceeded the 1.7 million gallons per day (mgpd) level permitted by the state (Figure 1). Consequently, the state imposed a new construction moratorium in January 2004, with the sewer improvements about 80 percent complete. Town officials believed that a weather change masked the improvements expected from the sewer project. The prior year had been uncharacteristically dry, with unusually low groundwater levels, whereas 2003 had abnormally high rainfalls and groundwater levels that dramatically increased sewer inflow and infiltration.

[FIGURE 1 OMITTED]

Morehead City officials tried to negotiate the terms of the moratorium with state regulators but were unsuccessful because they had no benchmark against which to gauge the impact of sewer improvements. The state regulatory officials were understandably reluctant to relax the moratorium until Morehead City officials could document improvements from the rehabilitation project. The state preferred to wait at least a year after the March 1, 2004, expected completion date of the rehabilitation project while the plant measured actual flows. Documenting the behavior of the system for a year would delay several large development projects, some of which were already more than a year behind schedule.

Town officials recognized that the state might be willing to lift the moratorium if mathematical modeling could show that unusually large rainfall volumes had been the primary cause of the plant overflows and that the sewer rehabilitation project had resulted in substantial system improvements. In late January 2004, they asked us to help them analyze the sewage-treatment system to assess the impact of the ongoing improvements.

Modeling the Sewage-Collection-and-Treatment Process

We formed a project team consisting of the authors, the Morehead City manager, a representative of the town council, the manager of the sewage-treatment plant, the manager of the municipal water-delivery system, and a professional engineer who was serving as a consultant to the town. As a result of our initial meeting in early February 2004, we agreed to model the components of flow in the system to determine the effects of the unusually wet year. Other team members agreed to provide us with data and sewage-system expertise to support our model development. We agreed to deliver a report to the Morehead City manager before May 2006 on a model that would explain the components of sewage-treatment-plant discharge for the unusually wet year, including any reduction of infiltration and inflow to the system resulting from the rehabilitation project. If favorable, the Morehead City manager would present this report to state officials. Based on these discussions, we developed a model of the sewer-collection-and-treatment system (Figure 2).

[FIGURE 2 OMITTED]

The Physical Sewage-Collection-and-Treatment System

The discharge from the sewage-treatment facility comes from three major sources: water from the municipal sewer system, storm-water inflow, and groundwater infiltration. The municipal water comprises residential, business, and industrial water discharged into the town sewer system. Storm water enters the system when heavy rains create localized flooding and surface water enters at manhole stations along the sewer-collection system. Groundwater leaks into sewer pipes through cracks and joints as a result of pressure from rainfall and groundwater.

The simplest model for measuring...

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