Evaluating the Feasibility and Sustainability of Direct Potable Reuse in Las Vegas, NV
Creators: name: Dow, Cory 
Publisher: name: University of Nevada, Las Vegas
Year: 2018

description: Community water sources are becoming more and more strained due to several factors including severe drought, population growth, urbanization, and climate change. This has spurred several water agencies to evaluate alternative water supply options to extend their resources. One potential alternative is potable reuse. Potable reuse comes in two forms: indirect potable reuse (IPR) and direct potable reuse (DPR). IPR is the advanced treatment of wastewater effluent before discharging into an environmental buffer that is a drinking water supply such as a lake, river, or groundwater aquifer before extraction and use. DPR is the advanced treatment of wastewater effluent that is directly introduced into a drinking water supply without entering an environmental buffer.
         The Southern Nevada Water Authority (SNWA) provides water to the Las Vegas Valley. Ninety percent of the valley's supply comes from the Colorado River in Lake Mead, with an allocation of 300,000 acre-feet per year (ac-ft/y) (SNWA, 2015). All of the water used indoors (approximately 44% of overall use) makes its way to the sewer system and generally flows to one of four wastewater treatment plants (WWTPs). The WWTPs discharge their effluent into the Las Vegas Wash and the water is returned to Lake Mead (i.e., IPR). For every drop of water SNWA returns to the lake, the agency can withdraw an equivalent amount beyond their base allocation. This is referred to as return flow credits (RFCs) and currently provides an additional 200,000 ac-ft/y of supply (approximately). However, the elevation change from Lake Mead to the River Mountain Water Treatment Facility (RMWTF) (one of two major drinking water treatment facilities) is approximately 1,200 feet. Therefore, large amounts of energy and cost are expended to pump water into the Las Vegas Valley, which suggests that this current IPR configuration may not be the most ideal option considering the implications of the energy-water-environment nexus.
         This thesis concerns the feasibility of DPR for the Las Vegas water system and provides a sustainability comparison with the current IPR configuration (or status quo) and other supply alternatives. A system dynamics model was developed using Stella 10.1 for the Las Vegas Valley water system. Two DPR treatment trains were evaluated. DPR 1 alternative included microfiltration (MF), reverse osmosis (RO), and ultraviolet light disinfection with advanced oxidation (UV/AOP). DPR 2 alternative included ultrafiltration (UF), ozone (O_), biological filtration (BAF), and UV/AOP. The status quo, DPR 1, and DPR 2 alternatives were evaluated over a 50-year period from 2016 to 2066 based on metrics of energy to pump and treat the water, energy cost, and greenhouse gas (GHG) emissions. Additionally, water quality metrics of total dissolved solids (TDS) and eutrophication potential were projected. Model simulations for 25%, 50%, 75%, 90%, and 100% of RFCs for DPR were completed. Also, conceptual level capital costs were developed for each flow scenario. DPR 1 had higher costs for every flow alternative due to RO treatment and brine disposal.
         The alternatives were screened down to three final alternatives for triple bottom line (TBL) analysis: status quo, DPR 1 with 50% RFCs, and DPR 2 with 50% RFCs. Criteria and sub-criteria were established and weighted for economic, social, and environmental conditions. Status quo was ranked as the highest alternative. It was concluded that the amount of energy and cost saved from reduced pumping to implement DPR did not outweigh the DPR cost of pumping from the Las Vegas Wash to the RMWTF and additional treatment.