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10.11 CONCLUDING REMARKS 237
two systems differ significantly are the costs of filtration media, the pretreatment system, the RO system, and the solids handling facilities. The intake costs for the desalination plant with membrane pretreatment system are higher because this system would require the collection of 8% more source seawater than the conventional pretreatment system. As explained previ- ously, this additional intake water is needed for the washing of the microscreens and the backwash water of the pretreatment membranes.
The costs associated with the RO system are lower for the membrane pretreatment system because this system is designed at a 12.5% higher flux (15.3 vs. 13.6 L/m2 h). The high design flux allowance for RO systems with membrane pretreatment stems from the expectation that membrane filtration would provide superior pretreatment. Explanation for the cost differ- ences of the other items could be found in the previous sections of this chapter.
The main differences of the O&M costs of the two desalination plants are related to the higher use of power for the membrane pretreatment process (0.35 vs. 0.04 kWh/m3) and to the pretreatment system maintenance and membrane replacement costs. It should be pointed out that depending on the applied membrane pretreatment technology, the annual cost for replacement of pretreatment membranes could be comparable to these for replacement of RO membrane elements. On the other hand, the use of membrane pretreatment is expected to eliminate or significantly minimize the sludge disposal costs and to decrease RO mem- brane replacement rate, cleaning frequency, and costs.
Although the design assumptions used in Table 10.2 favor membrane pretreatment, in many cases, not all of the benefits of this type of pretreatment may be applicable to the site-specific conditions of the a given RO project and the cost of water difference between membrane and conventional pretreatment could exceed 10% in favor of conventional pre- treatment. As membrane filtration technologies evolve and next generations of membrane products are more closely tailored to fit the specific challenges of saline water pretreatment, it is very likely that membrane pretreatment would become cost competitive for the majority of saline source water conditions.
10.11 CONCLUDING REMARKS
Granular media filtration is currently the dominating technology for saline water pretreat- ment worldwide. However, since year 2000, membrane pretreatment has been gaining wider acceptance due to its superior removal of particulate and colloidal foulants, simplicity of plant operation, performance stability, and more compact footprint. In addition, membrane pretreatment is easier to monitor, operate, and automate. These advantages are likely to pro- pel membrane filtration into the pretreatment technology of choice in the future.
At present, two key impediments for the wider application of MF and UF membrane tech- nology are the lack of compatibility between various membrane products and configurationsd as well as the relatively higher costs of the membrane pretreatment systems. These challenges are likely to be solved in the near future by the convergence and commoditization of various UF and MF membrane technologies. Many of the key membrane manufacturers such as Dow, Toray, Hydranautics, Pall, GE Water and Power Technologies, and Memcor (Evoqua) are currently working on the development of commoditized products and systems, which will have pressure-driven UF membranes made of polyvinylidene difluoride (PVDF) material,