Investigation of antiscalant dosing influence on scaling process in reverse osmosis facilities and membrane surface adsorption

Introduction. Understanding of crystal growth mechanism enables to develop efficient tools to control scaling and improve the process of treatment using membranes and increasing the amount of filtrate output. This investigation is aimed at studying an antiscalant behaviour in reverse osmosis (RO) process when treating ground water. Experimental dependences of calcium carbonate scaling efficiency on antiscalant dosage were found. Rates of adsorption on crystal surface of scaling deposit and on membrane surfaces were compared. Dependences of rates of inhibitor adsorption on crystal surface versus scaling rates were determined. Inhibitor adsorption on RO membrane surfaces was studied. New approaches to studying crystal growth mechanism in the presence of polymeric inhibitors are presented. Materials and methods. In the course of experiments conducted with using inhibitor dissolved in distilled water, inhibitor sorption on membrane surface was observed in the absence of calcium ions. As to experiments with dosing the inhibitor in tap water, the inhibitor sorption on the membrane did not occur: the inhibitor was adsorbed on the surface of the scaling crystals. Results. Experimental relationships are obtained that show dependencies of calcium carbonate deposit growth rates versus RO facility filtrate output values in the presence of different antiscalants with their dose values of 3, 5 and 7 mg/l. The article shows that antiscalant dose value does not provide substantial influence on antiscalant efficiency when natural water with low hardness is treated in the RO facility. This permits substantial reduction of operational costs. It was also proved that inhibitor is not adsorbed on membrane surface during natural water treatment that also confirms efficiency of low antiscalant dosing. Conclusions. Low hardness values of natural water (3–5 mill equivalents per liter) demonstrate that antiscalant efficiencies do not depend on its dose. Rate of inhibitor adsorption on crystal surface during calcium carbonate deposition also increases with scaling rate increase. Rates of antiscalant consumption increase with antiscalant dose values. In natural water the dissolved antiscalant molecules are bonded with calcium ions therefore antiscalant does not react with membranes and is not adsorbed on membrane surface. Acknowledgements.The authors would like to express their gratitude to Russian Foundation of Basic Research that supported investigations on development of fluorescent antiscalants and visualization tools by research grant No. 17-08-00061. Research on evaluation of antiscalant efficiency and adsorption properties were supported by RFBR grant No. 19-08-00982.