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9.4 FILTER TYPES AND CONFIGURATIONS 201
Usually, pressure-driven membrane systems are very suitable and cost competitive for source waters collected by deep open intakes and subsurface wells because such waters have relatively limited turbidity fluctuation span. Because pressure-driven systems can oper- ate at higher fluxes for the same TMP, these pretreatment systems are designed more aggres- sively and are more cost competitive in such applications.
9.4.3.2 Pretreatment System Footprint
If the source water quality is of high-solids content, which limits the design flux of the pressure systems, submerged membrane pretreatment systems are usually more space effi- cient than pressurized systems because they allow the installation of more membrane surface area per unit facility footprint area. Significant space reduction is achieved by the fact that the submerged membrane elements do not need to be installed in individual membrane vessels. In addition, the submerged systems typically have only one pipeline system for permeate collection. The distribution of the feed water and the collection and evacuation of the spent filter backwash water are completed at the tank level, which allows the system design to be simplified and the number of valves, piping, and auxiliary service facilities to be reduced.
The smaller footprint of submerged membrane pretreatment systems renders them more beneficial for large-size desalination plants. Typically, submerged membrane systems occupy 10%e20% less space than pressurized membrane installations for pretreatment systems with the same design flux. However, for colder source water of very high quality, which is typical for desalination plants with deep open intakes or subsurface intakes, because the pressurized systems can be designed at 20%e30% higher fluxes, pressure systems would yield pretreat- ment installations of smaller overall site footprint.
Submerged pretreatment systems have a clear advantage for retrofitting existing granular media filters into membrane pretreatment filters. Submerged membrane tank modules could be installed in the existing granular media filter cells, filter backwash tanks, disinfection con- tact tanks, or other existing structures, thereby reducing the construction costs for the retrofit. Typically, a conservatively designed granular media filter structure could house submerged membrane filtration system of 1.5e2 times higher production capacity than that of the orig- inal filtration system applying only moderate structural modifications.
9.4.3.3 Equipment and Construction Costs and Energy Requirements
Depending on the size of the system and the intake of saline water quality, the site-specific conditions may favor the use of either pressurized or submerged type membrane pretreat- ment system. Pressurized systems are typically very cost competitive for small and medium size installations because they can be manufactured and assembled in a factory off-site and shipped as packaged installations without the need of significant site preparation or construc- tion of separate structures.
As pressure-driven pretreatment technology evolves, the construction costs of these mem- brane systems are reduced, larger individual modules are becoming available on the market and most-recent projects indicate that pressure-driven membrane systems are becoming very cost-competitive for all plant sizes and currently they are the most commonly used type of membrane pretreatment systems in seawater desalination plants (Busch et al., 2009).
Equipment and construction costs of larger-size plants with more challenging saline source water quality may be lower when using submerged membrane systems, especially for plant