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Managing the Collection System to Balance Local Water Reclamation vs. Regional Treatment at the City of Scottsdale, Arizona

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The City of Scottsdale's wastewater is a valuable water resource that must be carefully managed to satisfy the conflicting constraints of pipeline capacity, reclaimed effluent supply contracts, and water resource portfolio goals. Changing constraints in recent years have caused the City to reevaluate strategies for managing wastewater flows.

Scottsdale is a member of the Sub-Regional Operating Group (SROG) consisting of the cities of Phoenix, Glendale, Mesa, Tempe, and Scottsdale that jointly own and maintain the 91st Avenue wastewater treatment plant and associated interceptors. Scottsdale discharges a portion of this wastewater into a SROG interceptor where the wastewater is conveyed to the 91st Avenue wastewater treatment plant (WWTP). Reclaimed water from this plant is sent to the Arizona Nuclear Power Plant (ANPP) for use in its cooling towers. All of the SROG cities are committed to providing reclaimed effluent to the ANPP. While meeting this commitment, especially during the summer, is a challenge, concerns have also been raised about the capacity of the regional interceptors in the SROG system, particularly following storm events. Historically Scottsdale, as well as other SROG member cities has used the SROG system to convey peak flows while base-loading WRPs owned within each City, potentially contributing to the concern about insufficient regional interceptor capacity. Flow equalization facilities in the SROG system have been proposed to handle peak storm flows to avoid increasing interceptor capacity, but the costs associated with this type of facility raises an increased concern and the need for such facilities have not been fully demonstrated. One disadvantage of utilizing the SROG system to treat wastewater is that the reclaimed effluent generated at the 91st Avenue WWTP beyond the amount needed to satisfy the ANPP commitment does not add to the City's water resources portfolio.

Besides conveying wastewater into a regional system, the City also has two water reclamation plants. The City's Water Campus and Gainey Ranch WRPs generate reclaimed effluent that is used by a consortium of golf courses for turf irrigation, thus reducing the use of potable water. The City has a commitment to provide water to these golf courses, either from reclaimed water or from Central Arizona Project (CAP) surface water supplies. Additional effluent from the WRPs is recharged into the aquifer to maintain water table levels and generate storage credits that are used to offset groundwater pumping. This practice, in combination with other overall strategies, has enabled the City to be one of the first cities in Arizona to achieve save yield in compliance with the 1980 Groundwater Management Act. Groundwater replenishment using reclaimed effluent has taken on additional significance in recent years as an extended drought has demonstrated the need to store water underground to mitigate the risk of reduced surface water supplies during extended droughts. Wastewater is increasingly becoming a valuable resource in the City's water resource portfolio.

The City manages the flows sent to SROG or to the Water Campus using five pump stations that can either operate to send wastewater to the Water Campus, or allow flow to bypass and go to SROG. Most of the flow into the Water Campus is through this “pumpback” system. By maximizing the flow of wastewater into the Water Campus, the City is able to augment its water resource portfolio to mitigate long-term drought effects and to meet safe yield requirements. To handle additional flows at the Water Campus, the pumpback system was operated differently, WRP treatment capacity is being increased, including the ability to handle peak flows, and recharge capacity was increased with additional vadose zone injection wells. With these strategies in place, the City's water resource portfolio and drought strategy have been refined so that the City has an updated plan to mitigate the effects of long-term drought on surface water supplies.

A final component of the City's plan to effectively manage wastewater flows has been to implement a wastewater flow-monitoring program. By more accurately measuring the flows in the collection system during both dry and wet weather conditions, the City will better understand how to operate the pumpback system to deliver only the required flows to the 91st Avenue WWTP and maximize use of the City's wastewater resources.

This paper explains how Scottsdale is effectively managing the collection system and operating the pumpback system to satisfy physical and contractual constraints, and to achieve water resource goals.

Keywords: Water reuse; advanced water treatment; aquifer storage; collection systems; pumpback systems; regional treatment; sustainability; vadose zone injection wells; wastewater treatment

Document Type: Research Article


Publication date: 2010-01-01

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