Prediction of silica scale formation in RO systems

Abstract

The prediction of silica scale formation in reverse osmosis (RO) systems has been based on the solubility of amorphous silica at equilibrium. Recommendations for preventing scaling by silica have required maintaining the concentration of silica in the brine stream below its maximum solubility. Because the precipitation of silica from solution occurs by the polymerization of the soluble monomeric form, however, the reaction is one of finite kinetics. In a RO device, silica is at its maximum brine concentration for a very short time. Therefore, the rate of polymerization at the prevailing conditions of temperature, pH, and concentration, is the controlling factor determining whether silica scale will form, rather than the equilibrium solubility. A complicating factor, however, is the presence of nucleating agents, such as suspended polyvalent metallic oxides, which adsorb monomeric silica from solution. Scaling under such conditions is dependent on neither polymerization kinetics nor equilibrium solubility. This paper presents the results of experiments with hollow fiber permeators where brine silica concentrations were in excess of solubility limits, yet no scaling occurred. The paper also discusses the effect of suspended metal oxides and the necessity for their rigorous exclusion in the prevention of silica scale formation.