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160 8. GRANULAR MEDIA FILTRATION
The size of the media and uniformity coefficient should always be configured to decrease while the specific density should increase in the direction of the flow. This configuration allows to prevent the intermixing of the different types of media materials during backwash. Intermix- ing of the media results in shorter filter cycles and the need for more frequent backwashing.
The depth of the filter bed is typically a function of the media size and follows the general rule thumb that the ratio between the depth of the filter bed (Ldin mm) and the effective size of the filter media (dedin mm), L/de, should be in a range of 1000e1500. For example, if the effective size of the anthracite media is selected to be 0.65 mm, the depth of the anthracite bed should preferably be approximately 1.0 m/3.3 ft (0.65  1500 mm 1⁄4 0.975 mm).
The depth of the GAC media is estimated based on the average contact time in this media, which is recommended to be 10e15 min. For example, if a filter is designed for a surface loading rate of 9 m3/m2 h (4 gpm/ft2), the depth of the GAC media should be at least 1.5 m/4.9 ft (9 m3/m2 h  10 min/60 min per hour 1⁄4 1.5 m).
When each of the filter media layers is first placed in the filter cells, and additional 3e5 cm (1.2e2.0 in.) of media should added to the design depth of the layer to account for the removal/loss of fine particles from the newly installed bed after backwashing.
It should also be pointed out that if the filters are designed to achieve TOC removal by biofiltration, it would take at least 4e6 weeks for the filters to create sustainable biofilm on the surface of the filter media grains that can yield steady and consistent filter performance and TOC removal. If the source water temperature is relatively low (i.e., below 20C), than biofilm formation process may take several weeks longer.
8.3.3 Media Support Layer and Undertrain System
The filtration media is typically supported by a layer of gravel bed that has depth of 0.3 m (12 in.) and is preferably graded in six layers e 1/6e3/4 in., with depth of 0.05 m (2 in.) each (Kawamura, 2000). The gravel bed in located on the top of a filter underdrain system. There are two types of filter underdrain systems widely used at present: underdrain blocks or false bottom with nozzles.
The nozzle type of underdrain has found a wider application for desalination applications. In this system, nozzles penetrate the bottom of the underdrain and allow to collect filtered water uniformly, and to distribute the backwash water more evenly during the backwash cycle. Typically, the flow velocity in the channel, pipe and false bottom below the underdrain system is designed to be relatively low [0.6 m/s (2 fps)] to provide uniform flow pattern distribution. The ratio between the nozzle area and the bed area varies from 0.2% to 1.5% depending on the type of underdrain system, backwash rate applied and type of the filter.
8.3.4 Service Facilities and Equipment
At present, most filters used for saline water pretreatment have air and water backwash systems. As a result each filtration system is equipped with air blowers (usually one or two duty and one standby) and water backwash pumps (usually two to three duty and one standby). As a rule of thumb, the backwash pumps should be designed to deliver water allowing to wash the filter at a surface loading rate, which is three to five times higher than the design filtration rate.