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11. GUIDELINES FOR PRETREATMENT SYSTEM SELECTION
TABLE 11.4 Typical Source Water Quality of Plants With Intakes Exposed to High Intensity
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
2e30 12e24 0.5e8.0 <0.02 <0.05 <0.02 100e60,000
the use of more complex, often multistage pretreatment systems. Table 11.4 presents the typical water quality of desalination plants, where intakes are of shallow (6e8 m/20e26 ft) or medium depth (8e15 m/26e50 ft) and exposed to high-intensity algal blooms.
Because of the high particulate and biofouling potentials of saline source waters exposed to high-intensity algal blooms or prolonged seasonal rain events (e.g., monsoons), the successful pretreatment of such waters requires the use of one of the following types of two-stage filtra- tion systems:
• Gravity granular dual media filters with a loading rate of 8e12 m3/m2 h (3e5 gpm/ft2); pressure granular dual media filters with a loading rate of 14e20 m3/m2 h (6e8 gpm/ft2) and cartridge filters;
• Dissolved air flotation (DAF) clarifiers, single-stage gravity granular dual media filters with a loading rate of 6e8 m3/m2 h (2.5e3.5 gpm/ft2) and cartridge filters;
• Gravity granular dual media filters with a loading rate of 10e12 m3/m2 h (4e5 gpm/ft2) and pressure-driven UF/MF filters with a design flux of 70e80 Lmh (41e47 gfd);
• Gravity granular dual media filters with loading rate of 10e12 m3/m2 h (4e5 gpm/ft2) and vacuum-driven UF/MF filters with a design flux of 50e60 Lmh (29e35 gfd).
• DAF clarifiers and pressure-driven UF/MF filters with a design flux of 50e60 Lmh (29e35 gfd);
• DAF clarifiers and vacuum-driven UF/MF filters with a design flux of 30e40 Lmh (18e23 gfd).
The most efficient and cost-effective configuration for the site-specific conditions of a given desalination project should be established based on lifecycle cost analysis of these alterna- tives. The pretreatment configuration that has found the widest application for such source waters at present is a combination of dual media gravity filters and dual media pressure fil- ters followed by cartridge filters.
The selection of DAF or gravity granular media filters as a first pretreatment stage will depend mainly on three factors: (1) length of periods when the source water turbidity exceeds