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228 10. COMPARISON OF GRANULAR MEDIA AND MEMBRANE PRETREATMENT 10.6 POWER USE
Granular media pretreatment systems use limited amount of power to separate particu- lates from the source water. As mentioned previously, large RO desalination plants typically include single-stage gravity granular media filtration pretreatment process, which has mini- mum power requirementsdtypically less than 0.05 kWh/m3 (0.2 kWh/1000 gal). On the other hand, depending on the type of the membrane system (pressure or vacuum-driven), membrane systems use approximately four to six times more powerd0.2 to 0.4 kWh/m3 (0.75e1.5 kWh/1000 gal) to remove particulates from the source water compared to gravity granular media filters. More power is not only used to create a flow-driving pressure through the membranes, but also for membrane backwash and source water pumping. The total po- wer use has to be taken into consideration when completing a life-cycle cost comparison of conventional versus membrane pretreatment system for a given application.
Although the power demand difference holds true for comparison of single-stage gravity granular media filters and single-stage pressure or vacuum-driven UF or MF filters, this dif- ference is negligible if pressure granular media filters are used for pretreatment. Single-stage pressure granular media filters operate at comparable feed pressures and power demand to membrane pretreatment systems. However, two-stage pressure filtration systems typically use more electricity than a single-stage MF or UF system producing comparable filtered wa- ter quality.
For example, a comparative cost analysis between a two-stage pressure filtration system and a single-stage pressure-driven membrane pretreatment system completed during the planning phase of the Perth II SWRO Project in Australia reveals that the two-stage pressure filter system would use 20% more electricity than the membrane pretreatment system (Molina et al., 2009). For this project, the need to consider two-stage granular media pretreat- ment was driven by the significant fluctuation in source water turbidity (5e50 mg/L of TSS) attributed to the relatively shallow plant intake.
10.7 ECONOMY OF SCALE
Membrane and granular media pretreatment systems may yield different economies of scale depending on the water treatment plant capacity. Usually, both technologies have a comparable economy of scale for plant capacity of up to 40,000 m3/day (10.6 MGD). For desalination plants with a capacity of 40,000e200,000 m3/day (10.6e52.8 MGD), the granular media filtration systems typically yield higher economy of scale benefits. The anticipated economy of scale reduction of construction costs for membrane pretreatment capacity in- crease from 40,000 to 200,000 m3/day (10.6e52.8 MGD) is in a range of 3%e5% only. For comparison, granular media filtration pretreatment systems in the same capacity range may yield 8%e10% economy of scale-related construction cost benefits.
The main reason for the smaller economy-of-scale benefits of membrane pretreatment technologies for large-capacity desalination plants is the maximum size of membrane modules currently available on the market. Typically, depending on the manufacturer and the membrane technology, the largest membrane modules available today are between 2,000 and 8,000 m3/day (0.5e2.1 MGD) of water production capacity, although recently