AWWA MTC69669

Seawater desalination has been growing rapidly in the past five years, due in part to the many new membranes that are available to designers. These new membranes have significantly improved performance, which results in lower permeate salinity and lower operating pressure. Recently, new low pressure seawater elements have been developed, and the optimum design with these elements must be carefully considered. This paper analyzes the trade-offs which exist when choosing these membranes. In cases of lower feed temperatures, which are more common in the Pacific coastal area of the USA, and lower salinities, these lower energy seawater elements can provide sufficiently low permeate salinity, generally less than 500 mg/l. Alternatively, designers can use hybrid designs, where higher rejection, higher energy consumption elements are used in the front of the vessel and lower energy elements are used in the back of the vessel. This approach results in a feed pressure and permeate salinity between the two. Use of these new products can result in as much as 1 kwhr/kgal of energy savings. The advantage of this type of approach is that the lower permeable lead elements will have lower flux, resulting in a more balanced element flux distribution. Alternatively, these new membranes can be used in high area configurations, which have as much as 440 ft² of membrane area. The higher area can result in about $0.1/gpd in capital costs savings. Thus, these new designs offer a variety of advantages, but detailed analysis is needed to select the optimum element and configuration. Includes 4 references, tables, figures.