Molar ratios as a useful tool for prediction of scaling potential inside RO systems

Scaling is a major problem facing salt separation from water by reverse osmosis (RO) systems. The sophisticated saturation indices such as LSI (1936), RSI (1944), and S&DSI (1952), are widely used for the prediction of scaling potential in RO systems, although these indices were specifically developed for thermal heating equipment. The aim of this work is to provide a simplified tool to help in understanding the chemical nature of brine stream and its expected scaling potential under RO processing. This approach, based on close field observation, analyzes and investigates the actual formed scales, rather than relaying on inherited theories. Thirty-three cases were selected from RO plants that show common problems in scale formation and shortening of the membrane lifetime. In all of these cases, the production rate declined down to less than 50% after several months, and the membrane life shortened to about 1 year. It was found that the proposed molar ratios of the brine could be used as chemical indicators for the prediction of scale potential and its chemical nature as well under the RO processing. Positive relations and trends were found among the ion associations under the prevailing conditions of sudden pressure/dehydration on the membrane surface. The comparison of the saturation indices and the proposed molar-ratios concept shows that the validity of the latter is better than the saturation indices. The use of molar ratios could be of economic value in properly designing RO plants and optimizing subsequent operation and maintenance costs as well.

• The proposed molar ratios concept could be used to predict the scale chemical type and its excepted potential inside an RO system. • Positive relationships and trends were seen between molar ratios and the investigated scales. • The interrelationships among the different molar species could be useful in understanding the relative behavior of associated ions under such critical conditions of sudden dehydration under pressure that prevail on the membrane surface.