Optimizing Mesa's Wastewater Treatment Capacity, Collection System Operations, and Reclaimed Water Utilization through Creative Modeling

The City of Mesa, AZ is faced with the challenge of operating its collection system to most effectively treat wastewater at multiple water reclamation plants while also maximizing the beneficial use of its reclaimed water resources. Wastewater generated in the City is conveyed to one of City's three water reclamation plants (WRPs) through the City's collection system or to the City of Phoenix 91st Avenue wastewater treatment plant (WWTP). Mesa has commitments to provide reclaimed wastewater to the Palo Verde Nuclear Generating Station as part of the Arizona Nuclear Power Project (ANPP) and to the Gila River Indian Community (GRIC) in exchange for potable surface water supplies. Reclaimed wastewater in excess of the amounts necessary for Mesa to meet its contractual obligations is recharged to the aquifer and “banked” as reclaimed water storage credits. Both the GRIC surface water exchange and the banked reclaimed water storage credits are critical components of Mesa's water resources portfolio and 100-year Assured Water Supply Designation. The location where reclaimed water is utilized strongly influences decisions regarding where wastewater needs to be treated. Because Mesa's water reclamation plants are located in geographically different areas, an analysis was conducted to optimize treatment plant capacity through collection system operations by considering the City's commitments to deliver reclaimed water for reuse, exchange, or groundwater recharge.Mesa's wastewater hydraulic model was used to develop collection system flow routing alternatives to quantify the amount of wastewater that could be sent to each treatment plant. Mesa's collection system has over 30 diversion structures that can be operated to deliver wastewater to the treatment plants. The model was used to identify hydraulic capacity limitations in the collection system between the diversion structures so that the amount of flow that could be sent to a particular outfall could be quantified. In the model, the diversion structures were configured to send flows to the various plants in order to meet reclaimed water resource objectives. Then, long term capital and operations and maintenance costs associated with the required routing of wastewater flow to the four treatment plants was calculated to determine optimal strategies to meet Mesa's goals.The preferred strategy included considerations for plant expansions, treatment process upgrades, collection system improvements, and the long-term utilization of reclaimed water resources. This approach provided the City with a framework for future wastewater collection system planning that integrates water reclamation plant expansions with reclaimed water utilization solutions. Utilizing this approach benefited the City by defining a collection system operations strategy that would satisfy both their ANPP and GRIC reclaimed water delivery commitments, maximize banked storage credits from reclaimed water, and define an expansion strategy for the City's WRPs. This paper presents the results of the collection system flow routing alternative analyses and how the optimal strategy was selected.