Page 259 - ro membanes

242 11. GUIDELINES FOR PRETREATMENT SYSTEM SELECTION 11.2.2 Plants With Open Intake and Good Water Quality
Usually, desalination plants with deep open intakes (e.g., intakes with depth of 15 m (50 ft) or more below the water surface) produce excellent source water quality (see Table 11.2).
Although the source water quality produced by deep open intakes is good, such saline wa- ters usually require pretreatment mainly aiming at the reduction of particulate fouling. Biofouling and colloidal fouling potential of such waters are relatively low and typically hy- drocarbons cannot reach such depth at quantities that can create RO membrane fouling problems.
Practical experience shows that the most suitable pretreatment for desalination plants with deep intakes and good water quality are:
• Single-stage pressure granular dual media filters with a loading rate of 14e20 m3/m2 h (6e8 gpm/ft2) and cartridge filters;
• Pressure-driven UF/MF filters with a design flux of 70e80 Lmh (41e47 gfd);
• Vacuum-driven UF/MF filters with a design flux of 50e60 Lmh (29e35 gfd).
The most common pretreatment configuration for desalination plants with water quality shown in Table 11.2 to date has been a combination of pressure-driven granular media filters and cartridge filters. Although gravity granular media filters could be used instead of pres- sure filters, they are usually costlier for this water quality and do not offer the advantage of producing filtered water with a significantly lower biofouling potential because source water at depths higher than 15 m (50 ft) does not contain such large amounts of algae as that collected by shallow intakes.
It should be pointed out that not all deep open intakes do always yield source water qual- ity as good as that presented in Table 11.2. If the open intake is located on the pathway of an underwater current, near the point of confluence with a river, in the tidal zone or in an area with frequent naval traffic (e.g., port or ship channel), the source water column is mixed frequently and the surface source water contamination is spread through the entire water col- umn depth, which in turn results in the deterioration of the source water quality collected by
TABLE 11.2 Typical Source Water Quality of Plants With Deep Open Intakes
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
0.5e10.0 8e10 0.5e1.0 <0.01 <0.05 <0.02 100e20,000