Effect of antiscalant degradation on salt precipitation and solid/liquid separation of RO concentrate

The key limitation to the application of reverse osmosis (RO) desalination on inland brackish waters is concentrate disposal. Due to precipitation of sparingly soluble salts (CaCO₃, CaSO₄, BaSO₄, SrSO₄), RO membrane recovery cannot be increased further; therefore, other strategies must be investigated. Antiscalants are often added to RO feed water to help prevent precipitation and increase RO recovery, but in concentrate treatment, antiscalants may prevent precipitation of problematic constituents. A three-stage process to treat brackish water RO concentrate was investigated; the stages include oxidation of antiscalants with ozone and hydrogen peroxide, precipitation at elevated pH, and solid/liquid separation. A model water concentrate was used to perform laboratory scale experiments for each treatment stage. Experimental results showed that the advanced oxidation process (AOP) of ozonation and hydrogen peroxide on phosphonate antiscalants allowed increased calcium precipitation as well as loss of the solubilizing effects of antiscalants as compared to precipitation without prior ozonation of the antiscalants. The AOP also removed the effect of antiscalant on precipitate particle size distribution and particle morphology. In some cases, the AOP also improved microfiltration performance for the solid/liquid separation stage. The concentrate treatment could increase overall recovery from 80% to 90% for non-ozonated, antiscalant-dosed concentrate and from 80% to 94% for ozonated, antiscalant-dosed concentrate.