Page 113 - ro membanes

96 5. PRETREATMENT BY SCREENING
aquatic organisms, as well as microscreens that aim to retain finer solids such as silt, plankton, sand, shell particles, and other debris in the saline source water.
Open ocean intakes are typically equipped with coarse bar screens followed by smaller size (“fine”) screens with openings of 1e10 mm (0.04e0.4 in.), which preclude the majority of the adult and juvenile aquatic organisms (fish, crabs, etc.) from entering the desalination plant. While coarse screens are always stationary, fine screens could be two typesdstationary (passive) and periodically moving (i.e., rotating) screens.
The saline source water collected by open intakes always contains solid particles and aquatic organisms, which naturally exist in the ambient aquatic environment. Most of the particles and organisms larger than 20 mm are removed by screening and downstream filtra- tion before this water enters the RO desalination membranes for salt separation. After screening, the source water is typically processed by finer pretreatment filters, which usually have sizes of the filtration media openings (pores) between 0.01 and 0.04 mm for membrane ultra- and microfilters and 0.25e1.2 mm for granular media filters.
If the saline source water is collected using subsurface intake, the screening of most of the coarse particles and aquatic organisms is completed by the natural soil layer of the intake sys- tem and, therefore, the desalination plant screening facilities are simplified and often only incorporate cartridge or bag filters located upstream of the RO membrane system. It should be pointed out, however, that subsurface intakes are not an absolute screening barrier, and their ability to provide efficient screening is dependent on the type and thickness of the soil formations through which the saline source water travels before it reaches the intake collection system (e.g., well screens).
5.2 BAR, BAND, AND DRUM SCREENS
A typical surface water intake system for medium and large membrane desalination plants with open intakes includes a set of manually cleaned bar racks followed by automated trav- eling fine bar screens or fine-mesh band or drum screens.
5.2.1 Coarse Bar Screens (Bar Racks)
Bar racks usually have a distance between bars of 50e150 mm (2e6-in.) and their purpose is to retain large-size debris and aquatic life from entering the plant intake. For offshore in- takes, the screens are installed on the intake’s vertical tower (see Fig. 5.1).
The design flow-through velocity for clean screens is usually 0.10 m/s (0.33 fps). Such low design velocity is not only selected to minimize impingement of aquatic life on the screens but to also account for the loss of flow-through surface as a result of growth on shellfish and accumulation of debris on the surface of the coarse bars.
Typically, the screen bars are manufactured of super duplex steel or copper-nickel alloys (the latter are preferred) to suppress marine growth. Shellfish growth on the screens could narrow the open space between the bars with over 50% and as a result either the bars have to be cleaned manually every several years, or the through-screen velocity is increased from 0.10 cm/s (0.33 fps) to between 0.12 and 0.15 m/s (0.40e0.50 fps) within 1 to 2 years from installation (cleaning).