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8.4 FILTER TYPES AND CONFIGURATIONS 163
Under this configuration, granular media filters are typically designed as dual-media (anthracite and sand) downflow filters. The design surface loading rate of these filters is usually two to three times higher than that of single-stage dual-media filters (i.e., 15e35 m3/m2 h/6 to 14 gpm/ft2).
Since the O&M expenditures for he DAF clarifier are relatively high, the filtration portion of the pretreatment system is recommended to be designed for the lower end of that range 15e20 m3/m2 h (6e8 gpm/ft2), which would allow to operate only the filtration portion of the DAF-filter system, when the source water quality is good and the levels of turbidity and organics in the water are low.
8.4.5 Gravity and Pressure Filters
Depending on the driving force for water filtration, granular media filters are classified as gravity and pressure filters. The main differences between the two types of filters are the head required to convey the water through the media bed, the filtration rate, and the type of vessel used to contain the filter media.
Because of the high cost of constructing large pressure vessels with proper wetted surfaces for corrosion resistance, pressure filters are typically used for small- and medium-size capacity RO plants. Gravity pretreatment filters are used for both small and large RO-desalination plants.
8.4.5.1 Gravity FiltersdDescription
Typically, gravity filters are reinforced concrete structures that operate at water pressure drop through the media of between 1.8 and 3.0 m (6e10 ft). The hydrostatic pressure over the filter bed provides the force needed to overcome the headloss in the media. Single-stage dual-media downflow gravity filters are predominant type of filtration pretreatment technology used in desalination plants of capacity higher than 40,000 m3/day (10.6 MGD). Some of the largest SWRO desalination plants in the world in operation todaydsuch as the 200,000 m3/day Carlsbad desalination plant in California (Fig. 8.3), the largest SWRO desalination plant in the western hemisphere, and the 624,000 m3/day Sorek desalination plant (Fig. 8.4), the largest SWRO plant in the worlddare equipped with single-stage, dual-media gravity filters.
8.4.5.2 Gravity FilterseKey Advantages
8.4.5.2.1 BETTER REMOVAL OF ALGAL MATERIAL FROM THE SOURCE WATER
Surface saline water (i.e., seawater) always contains a measurable amount of algae, whose concentration usually increases several times during the summer period and may increase up to 10 times during periods of algal blooms.
There is a large variety of algal species in the seawater. Some algal species that occur during red-tide events have cells that are relatively easy to break under pressure as low as 0.4e0.6 bars. When the algal cells break, they release cytoplasm in the source water that has a very high content of easily biodegradable polysaccharides. When the amount of polysaccharides released by the broken algal cells exceeds certain level in the filtered seawater, they would typically trigger accelerated biofouling of the RO membranes.