Applications of antiscalants to control calcium sulfate scaling in reverse osmosis systems

Abstract

In the desalination process involving reverse osmosis, the formation of mineral deposits on the membrane surface can severely reduce the performance and efficiency of the system. A commonly employed method for controlling the deposition of scale involves the use of antiscalants which influence the rate of crystal formation. In the work described here, a seeded growth technique has been used to study the influence of antiscalants on the crystallization of calcium sulfate dihydrate from stable supersaturated solution at 25°C. The antiscalants studied were: polyphosphates, phosphonates, polystyrene sulfonate, polyacrylamide, polyacrylate, and formulated polyelectrolytes. The results indicate that the crystallization in the presence of antiscalants is preceeded by an induction period following which CaSO 4·2H 2O crystallizes at a rate close to that in the pure (i.e. without antiscalant) supersaturated solution. Antiscalant concentrations, nature of the functional groups, and the molecular weight greatly affect, the duration of the induction periods. For all the antiscalants, the relative effectiveness is: Formulated Polyelectrolyte > Polyacrylate > Hexametophosphate > > Pyrophosphate ≈ Tripolyphosphate ≈ Polystyrene Sulfonate ≈ Polyacrylamide ≈ Control (No Antiscalant). A mechanism based upon surface adsorption model has been proposed to explain the marked effect on the initial induction period by the antiscalants.