Journal of Membrane Science 301. High-recovery reverse osmosis desalination using intermediate chemical demineralization, страница 6

It is noted that, over the course of the 3-month test period, there were seasonal water quality variations (i.e., twice the standard deviation) in CRW PRO feed leading to variation of species concentration by up to 7% relative to the test period time-averaged concentration (Table 1).

Removal of Ca²⁺, Ba²⁺, Sr²⁺, Mg²⁺, SiO₂, and dissolved organic carbon (DOC) in the SCR generally increased with increasing pH, with percent removal approaching maximum steady-state values at pH above about 10 (Fig. 3). The increase in Ca²⁺ removal with pH increase was consistent with the rise in CaCO₃ supersaturation in the SCR (Fig. 1) and, as a result, the rising driving force for CaCO₃ precipitation. Above pH 10, Ca²⁺ removal, primarily as calcium carbonate, was in the range of 94–97%. The source water typically had low levels of noncarbonate Ca²⁺ hardness which therefore only required NaOH dosing to achieve high Ca²⁺ removal (>90%) [12]. Under conditions of reduced feed water carbonate-to-Ca²⁺ ratio (i.e., the ratio may fall below unity due to variability of feed chemistry), although not observed during Phase I pilot testing period, excessive non-carbonate Ca²⁺ hardness may lead to reduced Ca²⁺ removal. Therefore, in order to increase Ca²⁺ removal under such conditions, non-carbonate Ca²⁺ hardness would need to be reduced by the addition of soda ash. Such a contingency was not implemented in the present Pilot system due to (a) the complexity of automating soda ash addition, and (b) the fact that the overall percent Ca²⁺ removal above 60–65% was deemed sufficient to achieve 95% overall product water recovery while maintaining the final RO concentrate below saturation with respect to gypsum.

Removal of Ba²⁺ and Sr²⁺ exhibited similar pH dependence to Ca²⁺ removal with the highest removals generally above pH of 10 (Fig. 3). This behavior is attributed to increasing co-precipitationofBa²⁺ andSr²⁺ withincreasingCaCO₃ precipitationatelevatedpH,consistentwiththerecentlaboratorystudy of Rahardianto et al. [12]. It is plausible that co-precipitation of Ba²⁺ and Sr²⁺ may have occurred either via inclusion of these ions into the CaCO₃ crystalline lattice or through adsorption onto freshly precipitated CaCO₃ crystalline surface [20–22]. The precipitation of CaCO₃ may have also induced BaSO₄ precipitation in the SCR solution (which was supersaturated with respect to BaSO₄, at an average saturation index of ∼62) by acting as mineral seeds (e.g., [3,4]). When operating above about pH 10, Ba²⁺ and Sr²⁺removals were in the range of 97–98% and 88–95%, respectively. Mg²⁺ and SiO₂ removals (Fig. 3) also generally increased with pH and were in the range of 38–80% and 67–85%, respectively, at pH above 10. While Mg²⁺ removal at high pH (>9.7) was primarily through Mg(OH)₂ precipitation (e.g., Fig. 1), it is plausible that magnesium may have also co-precipitated with calcium carbonate [23].

GiventhatSiO₂ scalinghasbeenshowntobeapotentialproblemathighrecoverydesaltingofCRW[12–14],SiO₂ removalin the SCR (Fig. 3) was seen as highly beneficial to reducing membrane scaling propensity in the SRO stage. It has been reported that, at high pH (>10), SiO₂ removal is associated with magnesium removal, suggesting that SiO₂ removal may be due to association with Mg(OH)₂ flocs or by formation of magnesium silicates [24–27]. An extensive study by Mujeriego [27] has

dashed curves are best fit smoothed spline curves.

indicated that an optimum pH for maximum SiO₂ removal by Mg²⁺ (in industrial waters) exists at about pH 11, which was also consistent with the data shown in Fig. 3.