Process for high recovery treatment of brackish water reverse osmosis concentrate

A brackish water reverse osmosis concentrate consisting of 4170 mg/L total dissolved solids (TDS) and a gypsum saturation ratio (SR) of 1.52, was treated to a volumetric recovery of 96.5% in a total recycle process utilising hydrated lime and CO2 induced precipitation reactors and nanofiltration (NF) separation units. A chemical model that incorporates PHREEQC COM v3 and the pitzer.dat database was developed to verify the achieved volumetric recovery in terms of the experimental process parameters. The model showed that a NF concentrate purge of 1.2% would have been required to sustain the cycled up mass balance. The NF feed and concentrate streams cycled up to 16,750 and 32,900 mg/L TDS respectively. The experimentally achieved SR in the secondary NF (SNF) concentrate was 2.37. The generally accepted gypsum SR limit for effective scale suppression through antiscalant addition is <2.5-4.0. A high volumetric recovery at an acceptable gypsum SR in the SNF was thus attained.