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Section 5 DESALTING TECHNOLOGY CONSIDERATIONS INTRODUCTION In order to meet the objective of determining the best desalting technique to use for each community, it has been necessary to evaluate a number of dif- ferent candidate desalting systems. The various candidate systems were analyz- ed to verify the technical feasibility of their use and to establish which sys- tem results in the lowest cost for product water in each city. System costs for the various techniques were derived using the Office of Saline Water manual of procedures for calculating costs.* CANDIDATE DESALTING TECHNIQUES In this study, primary consideration has been given to those desalting techniques which are currently considered the most economically favorable and which are desirable due to the feedwater qualities and plant capacities in- volved. Distillation processes were not examined in depth since they are most economical for systems with large plant capacities (much larger than the capa- cities required for the selected communities) and for feedwater with very large concentrations of total dissolved solids (up to 50,000 mg/l). The desalting techniques examined in most det~il were electrodialysis, reverse osmosis, and vacuum-freezing vapor-compression. A review of the water quality information for the communities of interest reveals that all of the cities have fairly high calcium and sulfate concentra- tions, expecially Las Animas. This fact is critical for distillation and mem- brane desalting techniques since special measures are needed to prevent calcium sulfate precipitation which will foul heat transfer surfaces in the case of distillation and membrane surfaces in the case of electrodialysis or reverse osmosis systems. Pretreatment of the feedwater by simple chemical addition I can alleviate this problem within limits. Where calcium concentrations are be- yond the limits of such pretreatments, two methods of avoiding scale formation are still possible. One method is to operate the system at low brine concentra- tion rates to avoid exceeding calcium sulfate solubility limits. The other me- thod is to lower feedwater calcium sulfate concentration by using an ion-ex- change pretreatment. The Office of Saline Water contracted separately with Bresler and Associates of .New York, New York to assist the Ken R. White Company and URS Research Company in examining the second method. Results of work per- formed by Bresler and Associates are included as Appendix A of this report. * Desalting Cost Calculating Procedures, U.S. Department of the Interior, Of- fice of Saline Water, Research and Development Progress Report No. 555. 29 ~\~\)