Antiscalants are well known for preventing the precipitation of sparingly soluble compounds such as calcium carbonate in reverse osmosis (RO) applications, but it is unclear whether they can also inhibit calcium phosphate scaling. Furthermore, a reliable method to determine the optimum antiscalant dose in RO is currently not available. The main objectives of this study were: i) to optimize the dosing of antiscalants and minimize antiscalant consumption in RO systems, and ii) to investigate the performance of antiscalants in preventing calcium phosphate scaling in RO processes. A dosing algorithm was investigated to minimize antiscalant consumption for calcium carbonate in different RO plants. Furthermore, the effectiveness of several commercial antiscalants (from different suppliers) in preventing calcium phosphate scaling was evaluated using pilot scale RO measurements and using a once-through laboratory scale RO system, which was developed in this research study.
The dosing algorithm proved to be a useful tool in identifying real-time optimum antiscalant doses required to prevent scaling for a given RO recovery. With the implementation of the dosing algorithm, the consumption of antiscalant in the RO plants was reduced by 85-90%. It was revealed that the feedwater chemistry, specifically the presence of phosphate and humic substances, plays a significant role in antiscalant dose reduction. For example, antiscalant was not required at all for a RO plant in the Netherlands as calcium carbonate scaling was prevented by the phosphate and humic substances naturally present in the RO feed. Furthermore, the amorphous phase of calcium phosphate was found to be responsible for flux decline in RO, for which the tested antiscalants were unable to provide acceptable inhibition, as flux decreased by at least 15% in less than 24 hours with each antiscalant. Consequently, further research is required, in collaboration with antiscalant suppliers, to develop and test antiscalants that are capable of preventing amorphous calcium phosphate scaling in RO systems.