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8.7 PLANNING AND DESIGN CONSIDERATIONS 181
deep open-ocean intake (typically deeper than 15 m/50 ft), which collects water with limited algal content or by subsurface intakes (i.e., beach wells), which have already prefiltered algae contained in the water via slow-sand filtration.
In this example, the source-water quality has turbidity of 0.2e2 NTU (TSS of 0.5e5 mg/L) with consistent source-water quality, which is typically not affected significantly by algal blooms or rain events. The source-water SDI5 is in a range of 3e6. Maximum algal count is <1000 cells/L and total hydrocarbon levels are below 0.04 mg/L at all times.
For the purposes of this example, it is assumed that the source water does not contain elevated content of silica, iron, and manganese and, therefore, these fouling compounds will not have impact on filter design. Because of the relatively low level of turbidity in the water, the average backwash volume is expected to be only 3% 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 50,000m3/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 3% 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  1.03 1⁄4 119,767 m3/day (31.6 MGD).
Key design criteria for the dual-media (sand and anthracite) pressure filtration system for seawater pretreatment are provided in Table 8.5. Similar to the previous example, the source water is preconditioned using ferric chloride for coagulation and polymer for flocculation. However, the chemical dosages are smaller and reflective of the better quality source water. The chemical source water conditioning system also includes addition of sulfuric acid to maintain optimum pH for the coagulation process.
8.7.3.3 Example of Two-Stage Gravity/Pressure Filter System
Two-stage filtration systems are usually applied when the source water is collected by a relatively shallow open intake (4e8 m/13e26 ft) from the water surface or by onshore open intake, which collects water from entire depth of the water column. In this case, the pur- pose of the first-stage granular gravity media filters is to remove the larger-size particles captured from the surface water column, such as large algae, silt, and solids.
Typically, the first-stage filters produce filtrate of turbidity below 5 NTU and SDI5 in a range of 6e8 or lower. The main reason why the first-stage filters are selected to be gravity- rather than pressure-driven is to minimize the breakage of algae biomass and release of organics associated with it. The gravity filters operate at hydrostatic pressure, which practically elimi- nates such breakage by not allowing the algae to penetrate deep into the filters and by gently retaining them on the surface of the top layer of the filtration media.
The second-stage filters receive filtrate from the first stage and are designed to polish this filtrate to levels acceptable for its processing through the RO system (see Table 8.1). Because most of the algal mass in the source water is already removed by the coarser first-stage filters, the possibility for algal breakage and release of easily biodegradable organics in the second-stage filters is reduced significantly. This allows to design the second-stage filters as pressure-driven units and to benefit from the lower costs associated with the use of this type of filters.
The source water in this example is of worse quality than that of the previous two examples and has turbidity of 2e30 NTU (TSS of 5e50 mg/L) with occasional turbidity spikes of up to 50 NTU during the most severe period of algal blooms occurring periodically in the intake area.