Cation control through intermediate chemical demoralization ICD

Fig. 4 provides a conceptual schematic of a two-pass RO facility with integrated ICD [21]. The term ''chemical demineralizaton'' is a general term for a variety of technologies that have been proposed whereby precursor scalant ions are removed from the primary RO concentrate via chemical precipitation. A number of scoping studies using a variety of conceptual process schemes to achieve high-recovery RO desalting via ICD have been conducted [24-36]. More recently, Gabelich et al. [5,21] demonstrated that upwards of 95% total system water recovery was possible for CRW RO desalting using ICD at the pilot and demonstration scales. ICD was shown to be effective in reducing the concentrations of Ca2+ and other scaling precursors in the primary RO concentrate below saturation or to a metastable supersaturation range (i.e., very slow precipitation kinetics) so as to allow further RO desalting of this concentrate stream.

Chemical demineralization processes rely on CaCO3 precipitation (appropriate where calcium carbonate concentration is sufficiently high to enable calcite precipitation by pH adjustment) using alkaline solutions of caustic (NaOH), lime (Ca(OH)2), sodium carbonate (Na2CO3), or through

Pretreatment

Primary RO

Solids Contact Secondary Secondary Clarifier Filtration RO

Raw Water Feed

Permeate

Pretreatment

Primary RO

Permeate

Raw Water Feed

Acid/ Antiscalant

Solids Handling Facility

■ Solids Disposal

Permeate

Solids Handling Facility

Acid/ Antiscalant

Final Concentrate rr^r,

■ Solids Disposal

Figure 4 Conceptual schematic drawing for two-pass RO facility with integrated intermediate chemical demoralization.

combinations of each. The choice of alkaline reagent is dependent on the concentrate composition and chemical cost. For example, for CRW RO concentrate, the molar ratio of total carbonate to total calcium was approximately 1.1 [8]. Therefore, sufficient carbonate ions were available to precipitate >99% of the calcium ions as CaCO3. Thus, for this particular water chemistry, NaOH dosing required the minimal dosing (on a molar basis) to induce the precipitation of CaCO3 [9]. In contrast, Ca(OH)2 dosing would have required the addition of Na2CO3 to deplete the additionally introduced calcium. •••-

Example

Demonstration-scale studies were conducted by the Metropolitan Water District of Southern California at the U.S. Bureau of Reclamation's Water Quality Improvement Center in Yuma, Arizona [21]. These demonstration-scale studies were designed to provide process design criteria for a two-pass RO process. The demonstration studies focused on two portions of the two-stage RO process (Fig. 4), namely (1) maintaining proper pH control of the solids contact reactor (SCR) effluent and secondary RO units and (2) determining whether a 95% total system water recovery was possible at a near-production plant size. Optimizing operations of the primary RO and SCR was addressee in prior investigations [8,37] and these unit operations were run under a single, optimized operating condition for the duration of pilot testing.

For the duration of testing, the source water was CRW taken just prior to the Northern International Boundary Dam in Yuma, Arizona. The salinity of the Colorado River ranged from 700 to 900 mg/L. The primary RO concentrate stream (250 L/min) was split with approximately 190 L/min being sent to the SCR, and the remaining used for EDR testing (see Section 3.3.1). Chemical feed to the SCR included NaOH for pH control and NaHCO3 to increase alkalinity. Sulfuric acid was added to the SCR effluent to adjust the pH to 7.0. SCR effluent was processed through a microfiltration (MF) skid prior to being sent to a 20-22 L/min secondary RO unit. Antiscalant and H2SO4 (pH 7.0) were added to prevent membrane scaling in the secondary RO units.

Operating pH and Ca2+, Ba2+, and Sr2+ removal from the SCR are shown in Fig. 5. Calcium is removed from solution as CaCO3 solid and is the principal precipitate generated in the SCR. Over the range pH levels tested, Ca2+ removal varied from 54% to 83%. Removal of Ba2+ and Sr2+ exhibited similar pH dependence to Ca2+, with Ba2+ removal being slightly higher than that of Ca2+ and Sr2+ slightly lesser. scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) data of the SCR precipitate indicated that removal of Ba2+ and Sr2+ precipitated as BaSO4 and SrSO4, respectively, through either inclusion in the CaCO3 crystalline lattice or through adsorption on the freshly precipitated CaCO3 crystalline surface. Given the percentage of cation removal and MF effluent water quality, the performance of the secondary RO unit showed no signs of fouling over the 600 h of operation (Fig. 5, bottom), hence validating the ability of ICD to enhance RO recovery at practical scales. -•

100 200 300 400 Run Time (hr)

Figure 5 Intermediate chemical demoralization process data: (top) SCR pH data over time;(middle) percent cation removal;and (bottom) water transport coefficient for the terminal RO element.

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