AbstractBecause of its reliance on simple indices for scale prediction, inhibitor selection, and dosage estimation, water chemistry design and treatment software currently used for membrane systems is not always adequate for the rigors of high-recovery systems. This article discusses applying an industry-standard ion association model calculation engine to predict scale in membrane systems and optimize control. The ion association model engine calculates the most likely distribution of species for a brine and uses free ion concentrations to calculate the driving scale force for minerals such as calcite, gypsum, barite, celestite, and silica, as well as calcium phosphate-based scales. Models for optimizing scale-inhibitor dosages are also described. The article draws on the concept of induction time as a basis for the mathematical models used to develop predictive models from actual data. The models are commercially available inhibitors that have been used successfully to select treatments and optimize dos...