Calcium sulfate precipitation studies with scale inhibitors for reverse osmosis desalination

• Development and performance evaluation of scale inhibitors for brackish water reverse osmosis desalination plant.
• Precipitation and inhibition studies of calcium sulfate (gypsum) scale.
• Characterization of gypsum scale.

This study is focused on precipitation and inhibition studies of calcium sulfate (gypsum) scale in reverse osmosis (RO) desalination plants. The temperature and concentration range encountered in brackish water RO desalination process has been considered in conducting precipitation and inhibition experiments. Initial screening of a number of additives has been carried out. Formation of gypsum scale has been verified by XRD and SEM analysis. Successful additives were evaluated by measuring induction times using conductivity of solutions and their performance was compared with that of blank supersaturated calcium sulfate solution. The induction time of STD100, SHMP, STD101 was found to be 70, 690 and 1200 min respectively at dose level of 10 ppm, whereas a blended formulation FM101 (10 ppm) did not show any precipitation till 3000 min. The SEM micrographs of precipitate collected when FM101 was added as a scale inhibitor with that of a blank solution and other inhibitors indicate that, the formulation FM101 has significantly modified the crystal morphology and is probably responsible for increased induction time.