AWWA ACE68911

Eastern Municipal Water District (EMWD), located in southern California, recently completed two brine treatment studies, one for the California Department of Water Resources (DWR) and the other for the United States Department of Interior, Bureau of Reclamation (USBR). These studies investigated the cost of different alternative process treatment trains to recover additional usable water from primary reverse osmosis (RO) brine, and either reduce the volume of brine significantly, or go to complete zero-liquid discharge (ZLD). The studies included pilot scale, bench scale and desktop evaluations of both established and emerging technologies for brine treatment. Chemical softening of primary RO brine followed by either a second RO process or electrodialysis reversal (EDR) is an example of so-called established technologies; emerging technologies investigated included membrane distillation (MD), forward osmosis (FO) and seeded reverse osmosis. A 50-gpm chemical softening process was operated for 6-months and provided feed water to RO and EDR pilot units. MD and FO were tested at bench scale, and seeded RO was operated in a batch pilot plant. Capital and operation and maintenance (O&M) cost estimates were established for several treatment configurations that also included brine concentration, crystallization and pond evaporation. These were then compared. Overall, total annual costs for 14 different treatment train combinations were generally within about 10-percent of each other. Treatment for removal and recovery of salable salt byproducts was, however, more costly, probably because of the capital costs associated with selective precipitation and washing and processing of the salts. Treatment trains that included a third concentration step (using the seeded RO process) ahead of thermal mechanical evaporation appeared to have slightly lower overall costs. This is attributed to the reduced size of the thermal evaporation process. Additional work on MD, FO and the seeded RO processes is needed. Includes 4 references, tables, figure.