Page 260 - ro membanes

11.2 PRETREATMENT SELECTION GUIDELINES 243
the desalination plant intake. Therefore, even when the desalination plant has a deep intake, the source water quality in the vicinity of the intake has to be investigated year-around to capture worst-case scenario water conditions, which ultimately should be used for the selec- tion of the most viable pretreatment system configuration.
11.2.3 Plants With Open Intake Exposed to Moderate Algal Blooms
This type of desalination plants is most common worldwidedsuch plants usually have medium depth intakes where source water quality is periodically influenced by moderate algal blooms. The typical source water quality for such desalination plants is shown in Table 11.3.
The most efficient and cost-effective pretreatment alternatives for desalination plants with water quality influenced by moderate algal blooms are:
• Single-stage gravity granular dual media filters with a loading rate of 8e10 m3/m2 h (3e4 gpm/ft2) and cartridge filters;
• Pressure-driven UF/MF filters with a design flux of 50e60 Lmh (29e35 gfd);
• Vacuum-driven UF/MF filters with a design flux of 30e40 Lmh (18e23 gfd).
Although all three pretreatment configurations listed earlier have found full-scale imple- mentation worldwide, at present over 90% of the desalination plants with this source water quality use combination of gravity granular media filters and cartridge filters. Because of their relatively low-design loading rate, vacuum-driven UF or MF filters are usually the least cost competitive of the three pretreatment alternatives.
11.2.4 Plants With Open Intake Exposed to High-Intensity Algal Blooms
Desalination plants located in the equatorial and tropical zones of the world are most commonly exposed to high-intensity algal blooms and heavy rains, which could last for pro- longed periods of timed2e4 months per year. Such natural events have a profound impact on the source water quality of the desalination plants and their successful management involves
TABLE 11.3 Typical Source Water Quality of Plants With Intakes Exposed to Moderate Algal Blooms
Source Water Quality Parameter
Turbidity, NTU
SDI5
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
2e20 10e18 0.5e1.5 <0.02 <0.05 <0.02 50e40,000