Page 196 - ro membanes

8.7 PLANNING AND DESIGN CONSIDERATIONS 179 Average backwash rate 45e55 m3/m2 h (18e22 gpm/ft2)
Duration (total air & water) 20e30 min (includes filter cell draining & fill up).
All other filter design parameters are the same as those of single-stage dual-media filters, which were described in the previous section. It should be noted that usually the second stage (polishing filter) is typically designed as a dual-media shallow filter, unless enhanced organics removal is needed.
8.7.3 Design Examples
The design examples provided below are developed for a hypothetical 50,000 m3/day (13.2 MGD) seawater desalination plant designed for a recovery of 43%. The pretreatment filtration system has to be designed to produce the same quality of filtered water presented in Table 8.1.
8.7.3.1 Example of Single-Stage Dual-Media Gravity Filter
In this example, the source water quality has turbidity of 0.3e10 NTU (TSS of 0.5e15 mg/ L) with occasional spikes of turbidity to 15 NTU (TSS 1⁄4 20 mg/L) during rain events. The source water SDI5 is in a range of 6e12. Maximum algal count is <20,000 cells/L and total hydrocarbon levels are below 0.04 mg/L at all times. This water quality is typical for an open ocean intake with medium depth (8e10 m/26e33 ft from the water surface).
Since open-ocean water does not contain elevated content of silica, iron, and manganese, these fouling compounds will not have impact on filter design. Because of the relatively high level of turbidity in the water, the backwash volume is expected to be 5% of the intake source water quality. Since the plant is designed for a total 43% recovery, the total volume of filtered water that will need to be produced for the RO system operations is 50,000 m3/day/ 0.43 1⁄4 116,279 m3/day. In addition, the filtration system will have to be designed to produce backwash water for the filters of volume, which is approximately 5% of the source water flow. As a result, the total plant intake flow for which the filters will need to be designed is 116,279 m3/day  1.05 1⁄4 122,093 m3/day.
Table 8.4 presents a summary of the key design criteria for the dual-media (sand and anthracite) gravity filtration system for seawater pretreatment. The system is designed for airewater backwash and filtration cycles of 24e48h depending on the source-water turbidity. The desalination plant source water is preconditioned using ferric chloride for coagulation and polymer for flocculation. The chemical source water-conditioning system also includes addition of sulfuric acid to maintain optimum pH for the coagulation process. The filtration system is designed with rinse to waste provisions, which allows to discharge the first 10e15 min of the flow produced of individual filtration cells immediately after their backwash to avoid sending this higher turbidity water to the RO-membrane system.
8.7.3.2 Example of Single-Stage Dual-Media Pressure Filter
Pressure filters are less costly than gravity filters in terms of construction expenditures. However, they apply significantly higher pressures to the algae in the source water and therefore, as single-stage filters they are preferable to be used when the source water is collected by very