AWWA ACE68912

Growing demand, concerns over droughts, over-allocation of surface water resources, and depletion of freshwater aquifers have all made desalination of brackish groundwater an increasingly important option for inland communities. However, these communities must find a means to dispose of the concentrated saline residual waste stream in an environmentally sound manner. Evaporation ponds are one of the primary options, but this technology has a large land requirement, which makes it costly. A concern for large facilities is that this technology is one of the few treatment methods that offers decreasing returns to scale due to increasing boundary layer resistance for larger ponds. This study evaluated a number of innovative options for improving the performance of evaporation ponds. Viable methods identified from the literature include the following: fabric evaporators; wetted boundary layer breakers; salt-tolerant plants; and, droplet spraying. Two cost models are developed, one for boundary layer breakers and one for droplet spraying. Incremental costs and incremental evaporation enhancements are compared with site-specific cost information for a wastewater treatment facility in California's Central Valley. Results indicate that both boundary layer breakers and spray technologies are cost-effective compared to a simple expansion of the pond area. Boundary layer breakers appear to be more cost-effective per gallon incremental capacity but have a lower evaporation enhancement capacity compared to droplet spraying (24% enhancement vs. 35% enhancement). For a new facility, an example calculation with preliminary cost information indicates that spray evaporation is more cost-effective because of avoided pond excavation and lining costs. Boundary layer breakers are preferred as a retrofit to an existing facility, if they provide sufficient additional capacity to avoid the need for an expansion of the pond. Includes 16 references, appendices.