Centralized versus Decentralized Wastewater Reclamation in the Houghton Area of Tucson, Arizona

Abstract

Reclaimed wastewater is increasingly important to satisfaction of water-sustainability objectives in water-short municipalities throughout the United States and particularly in the Southwest. Water reclamation and reuse present new challenges for urban planners, who now tend to consider renewable freshwater and reclaimed wastewater as unique parts of a single water resources portfolio. Efficiency objectives in geographically dispersed communities lead planners to explore the relative merits of centralized versus decentralized wastewater-treatment capacity when new construction is required. However, the complexity of the planning landscape—in which existing water distribution and sewerage capacities; geographic factors; and uncertainty in growth projections, energy cost, and even the sustainability of existing freshwater supplies contribute to plan selection—suggests that decision support methods can usefully supplement engineering judgment to find a near-optimal level of decentralization in facilities planning. In this study, an existing decision support system (DSS) was modified to include costs attributable to infrastructure construction, operation, and maintenance for wastewater collection and transmission of both potable and reclaimed water at the regional (city or city subsection) level to aid water supply planning. The modified DSS was then applied to a study area in southeast Tucson, Arizona. Several scenarios are developed and compared on the basis of cost and energy consumption. A sensitivity analysis is provided. In general, increased peripheral demand, limited existing capacity, greater elevation differences, and lower discount rates favor decentralized design and construction.