Evaluation of SHMP and advanced scale inhibitors for control of CaSO₄, SrSO₄, and CaCO₃ scales in RO desalination

Sodium hexa-meta-phosphate (SHMP), inhibitor-A (acrylic polymer), and inhibitor-B (polyacrylate+ phosphonate), each at a dose level of 1 ppm (on active basis) were evaluated at 25°C for their efficacy against CaSO₄, SrSO₄, and CaCO₃ scales in a synthesized brine which corresponded to 80% recovery from the locally available aquifer brackish water. At periodic time intervals brine pH was measured and samples were withdrawn and analyzed for Na⁺, Ca²⁺, Sr²⁺, SO^2-₄, and total alkalinity (TA). The total time of equilibration was 168h (7 days). The criterion adopted in comparing efficacy of the inhibitors was that if an inhibitor could achieve an induction period of at least 15 min, it would be considered an effective inhibitor at that dose level against that particular sealant. In the control experiment (no inhibitor added), there was an immediate drop in Ca²⁺, Sr²⁺, and TA levels and the solution became turbid much earlier than 15 min, suggesting that the brine was highly supersaturated and scale-forming with respect to CaCO₃ and SrCO₃ scales and would require injection of an inhibitor. In 168h, there was no drop in SO^2-₄ concentration suggesting that the brine possessed no potential to form CaSO₄.2H₂O and SrSO₄ scales. SHMP was able to keep the scale-forming constituents (Ca²⁺, Sr²⁺, and TA) in solution for 91h, inhibitor-A for 50h, and inhibitor-B for all of 168h, suggesting that the inhibitors at 1 ppm gave completely scale-free operation up to 80% product recovery and that performance of inhibitor-B was the best.