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154 8. GRANULAR MEDIA FILTRATION
8.7.1.1 Gravity FiltersdKey
Design Criteria 176
8.7.1.2 Pressure FiltersdKey
Design Criteria 178
8.7.2 Two-Stage Filters 178
8.7.2.1 Key Design Criteria 178
8.7.3 Design Examples 179
8.7.3.1 Example of Single-Stage Dual-Media Gravity
Filter 179
8.7.3.2 8.7.3.3
Example of Single-
Stage Dual-Media
Pressure Filter 179 Example of Two-Stage Gravity/Pressure Filter System 181
8.8 Construction Costs of Granular
Media Filtration Systems 183
References 186
8.1 INTRODUCTION
Granular media filtration, also often referred to as conventional filtration, is the most commonly used source water pretreatment process for reverse osmosis (RO) desalination plants today (other than cartridge filtration). This process includes filtration of the source water through one or more layers of granular media (e.g., anthracite coal, silica sand, garnet). Conventional filters used for saline water pretreatment are typically rapid single-stage dual- media (anthracite and sand) units (Fig. 8.1).
However, in some cases where the source water contains high levels of organics (TOC concentration higher than 6 mg/L) and suspended solids (monthly average turbidity exceeds 20 NTU), two-stage filtration systems are applied. Under this configuration, the first filtration stage is mainly designed to remove coarse solids and organics in suspended form. The second- stage filters are configured to retain fine solids and silt, and to remove a portion (20%e40%) of the soluble organics contained in the saline water by biofiltration. All pretreatment systems are designed to meet the filtered-water quality specifications listed in Table 8.1. These specifications
FIGURE 8.1 Typical configuration of single-stage dual-media filter.