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100 5. PRETREATMENT BY SCREENING
Despite the benefits of the center-flow and dual-flow screen patterns, the most widely used configuration is that of flow-through pattern, mainly because of the higher construction and installation complexity of the two other configurations.
Newer installations are more commonly designed in dual-flow in-to-out configuration. An example of the application of such screens is the 300,000 m3/day Adelaide desalination plant, where three units (each capable to treat 100% of the intake flow of 624,600m3/ day
165 MGD) are installed in individual channels. The screens are of effective width of 2.8 m (9.2 ft) and have mesh openings of 3.0 mm (0.1 in.) and 50 mesh panels. The screen ma- terial is super duplex stainless steel.
Fine-mesh screens are modified band screens, which use finer screening panelsdwith screen openings of 0.5e1.0 mm (0.02e0.04 in.) and which sometimes are equipped with buckets, that allow capturing fish and other aquatic organisms. Such screens have been found to reduce significantly entrainment of aquatic organisms and sometimes are installed down- stream of the conventional size band screens. Such is the case of the Tampa Electric Power Company power-plant intake, which also serves as a cold-water intake for the Tampa Bay seawater desalination plant. The fish and other organisms captured in the screen buckets are conveyed to the source water through a low pressure/low speed pump system.
Besides the fine-mesh screens, there are other modified traveling band screens that have been specifically designed to reduce impingement and entrainment of marine species. Such screening technologies are discussed in greater detail in the following publication (Mackey et al., 2011).
5.2.2.3 Drum Screens
Drum screens have found wide application for intakes of large seawater desalination plants in Australia, the Middle East, and Europe. These screens consist of rotating cylindrical frame covered with wire-mesh fabric. This frame is located in a screen structure and sup- ported by a horizontal center-shaft, which rotates slowly on roller bearings. The screens are rotated by a drive located at shaft level.
The most commonly used water pattern for such screens is “in-to-out” (also referred to as “double-entry”)dthe source water enters the inner side of the cylinder and moves radially out, creating hydraulically beneficial converging flow pattern. Debris deposited on the inner surface of the screens are removed by a jet water spray located on the top of the screen and collected in a water troughdsee Fig. 5.5.
Drum screens have unit capacities of up to 3,000,000 m3/day (270 MGD). Similar to band screens, they are also available in single entry, double exit out-to-in and in-to-out configurations, as well as double entry-single exit (out-to-in) flow pattern (Rogers, 2009).
Drum screens are more advantageous for applications where the source water debris and materials may fluctuate significantly because they are less susceptible to overtorque due to the large influx of solids to the screen over short period of time, which could occur in on- shore open intakes or shallow off-shore intakes. In addition, drum screens typically create lower flow-through head-losses at the same flow.
Besides hydraulic loading, drum- and band screens are also frequently designed based on solids’ loaddespecially for jellyfish outbreaksdwhen the amount of these marine organisms in the water could exceed 300 tons/h. When jellyfish outbreaks occur, they can completely blind the screens and the removal of the jellyfish from the screen mesh is very cumbersome. From that prospective, the manual removal (scrubbing) of jellyfish from the screens is usually