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176 8. GRANULAR MEDIA FILTRATION
offshore). Besides dredging the intake area, alternative solutions of this treatment challenge is the construction of pretreatment step such as lamella settler clarification or grit separation by microscreens. Additional information on granular media filter performance monitoring and troubleshooting is presented in the following publications (Beverly, 2005; Voutchkov, 2014).
8.6 SOURCE WATER PRETREATMENT PRIOR TO GRANULAR MEDIA FILTRATION
Most saline water particles and microorganisms have a slightly negative charge, which has to be neutralized by coagulation. In addition, these neutralized particles would need to be agglomerated in larger flocks that can be effectively retained within the filter media. Therefore, source-water conditioning by coagulation and subsequent flocculation are necessary prior to granular media filtration. Coagulation and flocculation processes and design criteria associated with source-water conditioning prior to filtration are discussed in Chapter 6.
Source water may need to undergo additional pretreatment prior to granular filtration (sand removal, sedimentation, DAF) depending on its quality. Alternative pretreatment processes and configurations are presented in Chapter 7.
8.7 PLANNING AND DESIGN CONSIDERATIONS
The design criteria presented herein should be used as guidelines onlydmedia size, depth and configuration, especially for medium and large desalination plants, are recommended to be selected based on pilot testing for the site-specific conditions and water quality associated with the project for a period that encompasses worst-case scenario water quality (i.e., significant rain events of intensity higher than 15 mm, dredging near the intake area, algal bloom events, pe- riods of strong seasonal winds, etc.).
8.7.1 Single-Stage Dual-Media Filters
8.7.1.1 Gravity FiltersdKey Design Criteria
Key design criteria for single-stage dual-media gravity filters for medium- and large-size desalination plants are presented below:
Filter type
Backwash
Average filter cell run length
Flow distribution to Individual cells
Number of filter cells Filter cell width
Dual-media, down-flow
Airewater
24 h
Pipe (if concrete channel used, channel depth should be tapered to keep velocity in the distribution channel above 2 m/sec at all times).
8e18
3e8 m (10e26 ft)