Concentrate Production

permeate concentration), and RW is the fractional product water recovery (RW — Qp/Qf, where QP and QF are the permeate and feed volumetric flow rates, respectively). CF rises rapidly as product water recovery increases above about 80%, as illustrated in Fig. 1 for a salt rejection level of 97%. For example, for desalting at recovery levels of 90% and 98% (i.e., concentrate stream is 10% and 2% of the feed-flow rate, respectively) the retentate stream will be concentrated by about a factor of 10 and 48, respectively.

Evaluating the solubility limits of scalants of concern is vitally important when selecting a concentrate minimization technology. For example, for Colorado River water (CRW) desalting at 85% water recovery, the primary scalants of concern are BaSO4 and CaCO3 (Fig. 2). As the recovery is increased, CaSO4, and silica (SiO2) play increasing important roles in limiting product water recovery. Thermodynamic solubility calculations for the concentrate derived from 85% water recovery desalting of CRW water suggest that pH adjustment can be used to mitigate scale formation ofsome potential scalants (e.g., CaCO3, Mg(OH)2, and SiO2), though at differing pH ranges [9]. However, it is apparent that the saturation indices of CaSO4 and BaSO4 are relatively independent ofpH. Therefore, further desalting of

Water Recovery

Figure 1 Variation of concentrate volumetric flow rate and concentration factor (Qoncentrate/Cfeed) with product water recovery for a 760 x 103 m3/day membrane desalination plant with nominal salt rejection of 97%.

10000

0.01

----Saturation Line

Barium Sulfate

----Saturation Line

Barium Sulfate

0.01

Gypsum Silica

Figure 2 Saturation index of CaCO3, CaSO4, SiO2, and Mg(OH)2 for Colorado River water concentrate produced from RO desalting (85% water recovery at 97% salt rejection) [9].

Gypsum Silica

Figure 2 Saturation index of CaCO3, CaSO4, SiO2, and Mg(OH)2 for Colorado River water concentrate produced from RO desalting (85% water recovery at 97% salt rejection) [9].

the above CRW RO concentrate, even at low pH would be a challenge since the solution is nearly saturated with respect to CaSO4 and SiO2 and oversaturated with respect to BaSO4.

To achieve the goal of 95%, total system water recovery for CRW desalting, scaling thresholds for CaCO3, BaSO4, and CaSO4 must be overcome [9]. At the above conditions, desalting of even the relatively low-salinity CRW would result in the saturation indices, reaching values of 124, 141, 1.2, 1.2, and 1.0 at CF = 10, and 611, 1078, 5.7, 9.5, and 4.3 at CF — 48, for BaSO4, CaCO3, CaSO4, SrSO4, and SiO2, respectively. Saturation index are defined as,

sp,x where IAP is the ion activity product and Ksp,x is the solubility product for the mineral salt x. Clearly, the RO concentrate from 90% and 98% recovery desalination will either be saturated (SIx — 1) or oversaturated (SIx>1) with respect to the above minerals.

It is important to recognize that in cross-flow membrane desalting, as water permeates across an RO membrane, rejected salt ions accumulate near the membrane surface, resulting in the formation of a concentration boundary layer. As the concentration and osmotic pressure at the membrane surface gradually increase (from the RO channel entrance to the exit), the effective net driving force for permeation decreases, thus, the permeate flux decreases toward the exit region (Fig. 3). The concentration of salts at the membrane surface can be approximated using the simple film model [5]:

where Cm, Cb, and Cp are the solute concentrations at the membrane surface, in the bulk, and in the permeate, respectively, J is the permeate flux, and k is the solute feed-side mass transfer coefficient, Ro is the observed rejection (Ro — 1 —Cp/Cb), and CP is the concentration polarization modulus. CP increases along the RO membrane channel, reaching its highest value at the channel exit and thus the potential for mineral scaling correspondingly increases along the membrane surface toward the exit region of the membrane flow channel. The challenge in RO desalting is thus to enable one to reach sufficiently high recovery, while reducing mineral scaling propensity.

The following sections discuss the potential of various technologies in treating RO concentrate to minimize reject volume. Depending on the water quality, treatment goals, and site-specific characteristics, one or more

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