Journal of Membrane Science 301. High-recovery reverse osmosis desalination using intermediate chemical demineralization, страница 3

requiringthattheCaSO₄ saturationindexofthePROconcentrate stream is maintained at unity. For example, such analysis suggested that 60–65% Ca²⁺ removal from the PRO concentrate, generated from 83% water recovery of CRW (Table 1) would enable SRO stage to achieve a total product water recovery of up to 95%, while ensuring that the concentration of CaSO₄ in the SRO concentrate would not exceed saturation.

EffectiveprecipitationsofteningforCa²⁺ removalviaCaCO₃ precipitation can be achieved by pH adjustment with alkaline solutions of caustic (NaOH), lime (Ca(OH)₂), sodium bicarbonate (NaHCO₃), sodium carbonate (Na₂CO₃), or through combinations of the above chemical reagents. The choice of alkaline reagent would depend on the concentrate composition and chemical cost. For example, in a recent laboratory RO desalting study with CRW [12], it was reported that the PRO concentrate had a molar ratio of total carbonate (CO₃²−) to total Ca²⁺ of approximately 1:1. Therefore, in the above system, sufficient CO₃²− ions were available to precipitate >99% of the Ca²⁺ ions as CaCO₃. For the above concentrate, Ca(OH)₂ dosing would not be optimal since it would require the addition of Na₂CO₃ to deplete the additionally introduced Ca²⁺. For the CRW PRO concentrate chemistry, following the analysis of Rahardianto et al. [12], NaOH dosing was determined to require the minimal dosing (on a molar basis) to induce CaCO₃ precipitation. Therefore, the above process was deemed feasible for field applications, which was evaluated in the present study at the pilot scale.