Page 280 - ro membanes

12.4 ALTERNATIVE SWRO-MEMBRANE SYSTEMS AND PRETREATMENT 263
TABLE 12.3 Reverse Osmosis Permeate-Water Quality, Seawater SourcedAtlantic Ocean Permeate Water Quality
Water-Quality Parameter
Temperature, C pH
Ca2þ, mg/L Mg2þ, mg/L Naþ, mg/L
Kþ, mg/L CO32, mg/L HCO3, mg/L SO42, mg/L Cl, mg/L
F, mg/L NO3, mg/L B, mg/L
Br, mg/L TDS, mg/L
Atlantic Ocean-Source Seawater Quality
16e30 8.0 410 1,302 10,506 390 2.0
145
2,720
19,440 2.5 0.00 4.5
78 35,000
Single-Pass SWRO System
17e31 6.3e7.2 0.7e1.4 1.6e2.4 83e160 2.4e4.5 0.0
1.4e2.0
2.4e5.8
146e220 1.0e1.6 0.00 0.7e1.2 0.8e1.1 240e400
Split Partial Two-Pass RO System
18e32 7.7e8.0 0.3e0.5 0.35e0.8 11e25 0.35e0.50 0.0
0.5e0.8
1.1e1.2
20e34 0.8e1.2 0.00 0.3e0.5 0.3e0.5 35e65
The main advantage of such SWRO system configuration is that it allows achieving very high level of use (recovery) of the available source seawater and the energy used by the first- stage RO system. For example, while a single-stage SWRO system configuration typically al- lows turning 35%e50% of the source seawater to potable water, the two-stage SWRO system recovery may reach 60%e65%. Designing the SWRO plant around higher recovery allows minimizing the size of the plant intake and pretreatment facilities, and the capital expendi- tures for their construction and operation.
However, because of the high salinity of the first-stage concentrate (typically above 55,000 mg/L), the practical implementation of two-stage SWRO systems requires use of high-pressure SWRO membranes, membrane vessels, piping, and auxiliary equipment that can withstand and perform well at very high pressures (up to 98 bars/1400 psi). The cost of this equipment is usually higher than the cost of the same size and type of equipment built to operate at more “standard” pressures (i.e., below 70 bars/1000 psi). Therefore, the viability of using two-stage SWRO system has to be carefully assessed based on a comprehensive life- cycle cost analysis for the site specific conditions of a given project. To date, two-stage