Effects of antiscalants to mitigate membrane scaling by direct contact membrane distillation

The effects of antiscalants on mitigating the potential for membrane scaling by calcite and gypsum, respectively, were investigated during the direct contact membrane distillation process (DCMD) implemented with porous hydrophobic polypropylene (PP) hollow fibers having a porous fluorosilicone coating on the fiber outside surface. The surface tension and the membrane breakthrough pressure were tested for different kinds of antiscalants. At room temperature, antiscalant solutions behave like tap water. Based on this result, DCMD scaling experiments with CaSO4 or CaCO3 as a scaling salt were conducted. The supersaturation indices of the scaling salts used correspond to sea water concentrated 5 times for CaSO4 (∼75 °C) or half of the maximum saturation index (SI) reached during the concentration of sea water to 10 times for CaCO3 (∼73 °C). The results show that antiscalants K752 and GHR could dramatically extend the induction period for the nucleation of gypsum and calcite, respectively; further they slow down the precipitation rate of crystals, even at a dosage of only 0.6 mg/L. By comparison, a larger amount of antiscalant could further slow down the precipitation and also extend the induction period for both calcite and gypsum systems. There was no sign of any drop in the water vapor flux nor any increase in the distillate conductivity. Concentrates or reject streams from reverse osmosis desalination processes containing antiscalants may therefore be conveniently concentrated further by DCMD.