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11. GUIDELINES FOR PRETREATMENT SYSTEM SELECTION
TABLE 11.5 Typical Source Water Quality of Plants With Intakes Exposed to Severe Algal Blooms
Source Water Quality Parameter
Turbidity, NTU
SDI2.5
Total organic carbon, mg/L
Total hydrocarbons, mg/L
Total iron in reduced form, mg/L
Total manganese in reduced form, mg/L Algae, cells/L
SDI, silt density index.
Value
5e40 16e30 0.5e16.0 <0.02 <0.05 <0.02 >60,000
acceptable levels the biofouling potential of the source water. The recommended pretreat- ment alternatives are as follows:
• Gravity granular trimedia filters with loading rate of 8e10 m3/m2 h (3e4 gpm/ft2), pressure granular dual media filters with a loading rate of 14e16 m3/m2 h (6e7 gpm/ft2), and cartridge filters;
• DAF clarifiers, single-stage gravity granular trimedia filters with a loading rate of 6e8 m3/m2 h (2.5e3.5 gpm/ft2), and cartridge filters;
• Gravity granular trimedia filters with a loading rate of 10e12 m3/m2 h (4e5 gpm/ft2) and pressure-driven UF/MF filters with a design flux of 60e70 Lmh (35e41 gfd);
• Gravity granular trimedia filters with a loading rate of 10e12 m3/m2 h (4e5 gpm/ft2) and vacuum-driven UF/MF filters with a design flux of 45e55 Lmh (26e32 gfd);
• DAF clarifiers and pressure-driven UF/MF filters with a design flux of 40e50 Lmh (23e29 gfd);
• DAF clarifiers and vacuum-driven UF/MF filters with a design flux of 25e35 Lmh (15e20 gfd).
To date, the most widely used pretreatment system configurations for desalination plants with saline source waters experiencing severe algal blooms are (1) combination of DAF clar- ifiers and deep gravity media filters and (2) of DAF clarifiers and pressure driven UF or MF membrane filters.
11.2.6 Plants With Open Intake Exposed to High Hydrocarbon Concentrations
Natural saline source waters typically do not contain hydrocarbons, oil, or grease. These colloidal foulants originate from manmade activities and are observed in industrial or recreational ports, ship channels, along oil tanker routes, or near wastewater treatment plant discharges. Content of THC over 0.02 mg/L in the water fed to the RO membranes will cause their irreversible fouling and destruction. Therefore, plants exposed to high THC concentra- tions will have to be equipped with robust technologies for the removal of hydrocarbons.