A Study of Silica Scaling in High Recovery Reverse Osmosis Systems

摘要

This project was specifically concerned with the treatment of water from the Wallton-Mohawk irrigation drainage return canal located in southwestern Arizona. Laboratory and field tests were conducted to ascertain incipient scaling concentration of silica in reverse osmosis systems operating at high recovery levels. Ensuring that other salt scale species would not precipitate out as interferance was a necessary condition. The project involved two related phases of study: corrosion protection, and potential for scale formation. Both stainless steel and some polymeric plastics stood up well against brackish Colorado River water, but the summertime temperatures and strong sunlight at Yuma were a serious problem for certain plastics. Calcium sulfate scale was greatly reduced through pretreatment with lime softening and filtration, and addition of up to 4 ppm of sodium hexametaphosphate (SHMP). When Welton-Mohawk brine was concentrated by evaporation, silica precipitation occurred at 100 ppm silica, which is less than saturation. However, in reverse osmosis tests with up to 90% salt rejection, no silica scaling was observed. Further testing was suggested to effect improvements for the 100-million-gallon per day membrane desalting plant planned for the lower Colorado River outside Yuma, Arizona.