A method for evaluating antiscalant retardation of crystal nucleation and growth on RO membranes

An experimental method was developed for quantifying mineral scale nucleation and growth on RO membranes enabling comparison of the effectiveness of antiscalants in retarding mineral scale formation. Mineral scale growth and inhibition was evaluated, for calcium sulfate dihydrate (gypsum) as a model scalant, from direct visual observations of crystal growth on the membrane surface in a plate-and-frame RO cell. Comparison of membrane scaling with and without the use of antiscalants was carried out based on RO scaling tests conducted under identical operating conditions with respect to the initial flux and crossflow velocity, at identical initial concentration fields within the RO membrane channel and at the membrane surface. The range of gypsum saturation index (SIg) at the membrane surface was 1.3–2.3 enabling determination of gypsum scaling for the above SIg range from a single RO scaling test, given direct observation of crystal nucleation and growth on the membrane surface. The present work demonstrates that it is possible to distinguish antiscalants modes of scale inhibition and corresponding scale suppression effectiveness, over a reasonable range of solution supersaturation. The current approach introduces antiscalant crystal nucleation and growth retardation factors that can aid in the development, selection and dose optimization of antiscalants for use in RO desalination.