Effectiveness of antiscalants in preventing calcium phosphate scaling in reverse osmosis applications

Abstract

Antiscalants are well known to prevent the precipitation of carbonate and sulphate scales of calcium in reverse osmosis (RO) applications, but according to literature their inhibitory ability against calcium phosphate is not clear. The objective of this study was to investigate if antiscalants, without acid addition, can prevent calcium phosphate scaling in RO systems. Eight calcium phosphate antiscalants from different manufacturers spanning a range of concentrations were tested in batch (in glass reactors) experiments to inhibit the formation of calcium phosphate in synthetic concentrate corresponding to 85% recovery (Ca²⁺ = 765 mg/L, PO₄³⁻ = 13–15 mg/L and pH = 7.6) of a groundwater RO in the Netherlands. Additionally, once-through lab-scale RO tests were conducted where an RO element was fed with synthetic concentrate and the performance of antiscalants was evaluated from the rate of flux-decline in the RO element. Without antiscalant addition, a substantial flux-decline was observed due to the deposition of amorphous calcium phosphate (ACP) on the RO membrane. The tested antiscalants were unable to inhibit the formation of ACP and were incapable of preventing the deposition of the formed ACP particles, since with each antiscalant, the flux of the RO element decreased at least 15% in a 3-h period. Briefly, the available antiscalants, tested in this study, did not provide acceptable inhibition of calcium phosphate scaling in RO applications.