Gravity Granular Media Pretreatment Filters Design Calculations
Gravity granular media filters are typically reinforced concrete structures that operate at water pressure drop through the media of between 1.8 m and 2.4 m. 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. Downflow filters are preferred because they allow to retain algal biomass contained in the source seawater at the upper layer of the filter media and to minimize algal cell breakage which could cause release of soluble biodegradable organics in the filtered seawater.
Gravity Single Stage Dual Media Filters – Key Design Criteria and Considerations
Key design criteria for single-stage dual media gravity filters for medium and large size desalination plants are presented below:
| Filter type | Dual media, downflow, air-water backwash |
| Average Filter Cell Run Length | 24 h (between two filter cell backwashes) |
| Flow Distribution to individual cells | Pipe (if concrete channel used, channel depth should be tapered to keep velocity in the distribution channel above 2 m/sec at all times). |
| Number of Filter Cells | 8 to 16 (typically 12) |
| Filter Cell Width | 3 to 6 m |
| Filter Cell Depth | 4.5 to 7.5 m (typically 5 m) |
| Filter Cell Length-to-width ratio | 2:1 to 4:1 (typically 3:1) |
| Individual Filter Cell Area | 25 to 100 m2 |
| Maximum Water Depth Above Filter Bed | 2.5 m (should be equal or slightly higher than filter bed head loss which usually is 1.8 to 2.4m), |
| Filtration Rate (at Desalination Plant Intake Design Flow) | |
| With All Filters in Service | 8 to 10 m3/m2/h |
| With Two Filters Out of Service | 15 m3/m2/h |
| Filter Media | |
| Top Layer | Anthracite |
| Anthracite Layer – Depth | 1.5 m – 1.8 m (Deep Bed Filters – Recommended) |
| Anthracite Layer – Depth | 0.4 m – 0.8 m (Shallow Bed Filters – Used for Seawater of Low Turbidity (< 5 NTU) and Low Organics (TOC < 2 mg/l) |
| Anthracite – Effective Size | 0.8 to 2 (typical 1.5 mm) |
| Anthracite – Uniformity Coefficient | 1 .3 to 1. 7 (preferable < 1.4) |
| Anthracite – Specific Gravity | 1.5 to 1.6 tons/m3 |
| Anthracite – Bulk Density | 0.8 to 0.85 tons/m3 |
| Bottom Layer | Sand |
| Sand Layer – Depth | 1.0 to 2.0 m (Deep Bed Filters – Recommended) |
| Sand Layer – Depth | 0.4 to 0.6 m (Shallow Bed Filters). |
| Sand – Effective Size | 0.5mm |
| Sand – Uniformity coefficient | < 1.4 |
| Sand – Specific Gravity | 2.65 tons/m3 |
| Sand – Bulk Density | 1.5 to 1.9 tons/m3 |
| Filter Backwash System | Air – Water |
| Maximum Backwash Rate | 55 m3/m2/h |
| Average Backwash Rate | 40 – 45 m3/m2/h |
| Duration (Air + Water) | 40 – 60 min (includes filter cell draining & fill up). |
The design criteria presented above are guidelines only – media 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 (rains of intensity higher than 15 mm), dredging near the intake area, red tide events, etc.). Table 3 provides examples of key design criteria for
desalination plants of various size and water quality.
| Desalination Plant Location and Capacity | Pretreatment System Configuration | Average and Maximum Filter Loading Rate | Notes |
| Glen Rocky SWRO Plant, Gibraltar – 1,400 m3/day | 4 single-stage media vertical pressure filters 90cm sand 30cm anthracite | 11 m3/m2/h (avg.) 16 m3/m2/h (max) |
Intake type – 2/3 of volume from wells and 1/3 from open intake |
| Ashkelon SWRO Plant, Israel – 325,000 m3/day | 40 single-stage dual
media gravity filters. |
8 m3/m2/h (avg.) 12 m3/m2/h (max) |
Offshore Submersed Open intake – 1000 m from shore. |
| Tuas Desalination Plant – 136,000 m3/day | Combined DAF and sand media filtration 110 cm-sand. | 10 m3/m2/h (avg.) 14 m3/m2/h (max\ |
Near Shore Open Intake in Singapore Industrial Port. |
| El Coloso SWRO Plant, Chile – 45,400 m3/day | DAF followed by Two stage Dual Media Horizontal Pressure Filters | DAF Surface Loading Rate – 22 to 33m3/m2/ h. Filtration Rate – 25 m3/m2/h (avg.) | Open Intake In Industrial Port Frequently Plagued by Red Tide Events. |
| Fujairah SWRO Plant, UAE – 1 70,500 m3/day | 14 Single-stage Dual Media Gravity Filters | Filtration Rate 8.5 m3/m2/h (avg.) 9.5 m3/m2/h (max) |
Open Intake – High Hydrocarbon Content of Seawater |
| Kwinana SWRO Plant, Perth, Australia – 160,000 m3/day | 24 single-stage dual media pressure filters | 14.0 m3/m’/h (avg.) 18.0 m3/m2/h (max) |
Offshore Submersed Open Intake |
| Carboneras SWRO Plant, Spain – 120,000 m3/day | 40 single-stage dual media pressure filters | 12.0 m3/m’/h (avg.) 14.0 m3/m2/h (max) |
Offshore Submersed Open Intake |
Some of the largest SWRO desalination plants in the world in operation today such as the 325,000 m3/day Ashkelon plant in Israel (Figure 1) are equipped with dual-media single-stage gravity filters.
Figure 1 – Ashkelon Seawater Desalination Plant Showing Pretreatment Filters
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