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5.3 MICROSCREENS 105
FIGURE 5.10 Disk filters.
of the filtration units. Source water debris and aquatic organisms (mainly plankton) of sizes smaller than the size of the microscreens are retained and accumulated in the cavity between the filter disks and the outer shell of the filters, thereby increasing the head loss through the filters. Once the filter head-loss reaches a preset maximum level (typically 0.35 bars/5 psi or less), the filters enter backwash mode. All debris retained on the outer side of the filters are then flushed by tangential water jets of filtered seawater flow under 0.15e0.2bars (2.0e3.0 psi) of pressure and the flush water is directed to a pipe, which returns the debris and marine organisms retained on the filters back to the ocean.
Because of the relatively low differential pressure the filters operate at, these filters are likely to minimize impingement of the marine organisms in the source water. Since the disk filtration system is equipped with an organism return pipe, the entrained marine organ- isms are returned back to the source water body, thereby reducing their entrainment.
One of the key issues associated with using membrane pretreatment is the puncture of membrane fibers by sharp objects contained in the source water (e.g., broken shells or sharp sand particles). In addition, seawater contains barnacles, which in their embryonic phase of development are 130e180 mm in size and can pass through the screen openings unless these openings are 120-mm or smaller.
If barnacle plankton passes the screens, it could attach to the walls of downstream pretreat- ment facilities, grow on these walls, and ultimately could interfere with pretreatment system operations. Once barnacles establish colonies in the pretreatment facilities and equipment, they are very difficult to remove and can withstand chlorination, which is otherwise very effective biocide treatment for most other marine organisms. Therefore, the use of fine micro- screens or disk filters (80e120-mm size) is essential for reliable operation of the entire desali- nation plant using membrane pretreatment. Microscreens or disk filters are not needed for pretreatment systems using granular media filtration because these systems effectively remove fine particulates and barnacles in all phases of their development.