The presence of ferric iron promotes calcium sulphate scaling in reverse osmosis processes

Calcium sulphate is one of the most common components responsible for inorganic scaling in industrial heat exchangers and desalination plants. Insoluble scales cause problems like material degradation and a decrease of process efficiency. This is a major challenge for wastewater containing high concentrations of calcium sulphate, e.g. power plant ash deposit site leachates. Treatment of such water by means of reverse osmosis is considered in this study. It was found that iron ions present in the solution in low concentration can affect the scaling behaviour. If present, iron shortens both induction time and overall time of calcium sulphate crystallization from its supersaturated solution. Modelling, jar tests and RO separation tests were made to examine this phenomenon. Model solutions supersaturated by calcium sulphate were prepared in various conditions, always in two parallels: a solution containing iron ions and a solution without iron. The difference (induction time, crystal formation rate) is significant at lower supersaturation while at high supersaturation the results are very similar due to quick crystallization. When treating such supersaturated solution (or solution close to saturation) two main possible ways offer: removing iron prior to treatment (e.g. by aeration or coagulation) or preventing scale formation by adding antiscalants.