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90 4. SALINE WATER INTAKES AND PRETREATMENT
The proper design of open-ocean intakes requires the collection of detailed source water quality data from the proposed site of the intake, characterization of aquatic life in the vicinity of the intake and completion of detailed sanitary survey assessing the potential sources of RO-plant source water-quality contamination in the vicinity of the intake location (such as waste discharges of industrial and municipal wastewater plants, stormwater discharges or large industrial port or municipal marina activities, which may result in oil and gasoline spills and in other ocean water contamination).
4.3.5.2 Inlet Screens
Inlet screens are typically coarse bar screens with distance between the bars of 50e300 mm (2e12 in.). The bar length is usually between 1.0 and 3.0 m (3.3e9.9 ft) and is determined based on the selected maximum distance between the screen velocity cap and the water surface and the bottom.
Design maximum through screen velocity varies depending on the content of jellyfish in the source water and is usually in a range of 0.10e0.15 m/s (0.3e0.5 fps). It is important to note that the design through-screen velocity should be calculated for 50% of the total area between the screen bars to allow for growth of shellfish and accumulation of debris over time. It is anticipated that the inlet screening area will be reduced by 30%e50% every 18e24 months and, therefore, the screens would need to be cleaned periodically by divers.
A maximum through-screen velocity of 0.10 m/s or less is typically selected if the source water contains jellyfish load of 0.5 kg/m3 of source water or more. This lower flow through- screen velocity allows to minimize the intake of jellyfish into the plant and thereby to mini- mize their negative impact on downstream screening facilities. Jellyfish outbreaks are typical for warmer, shallower, and polluted waters, especially if the intake is located in the middle of an underwater current though which jellyfish travel most frequently.
Usually, small jellyfish specimen that can enter the intake weigh between 200 and 400 g (0.9 lbs) per individual and for large plants with few fine screens jellyfish could add significant load to the screens, which in extreme conditions could cause equipment damage. In addition, jellyfish are very difficult to clean from the screens and often standard debris jet sprays with which fine screens are equipped cannot remove them. From this perspective, drum screens are usually more difficult to clean once the jellyfish attach on the screens and therefore, reducing their intake by reducing the inlet through-screen velocity is of critical importance, especially for plants with fine drum screens. If large quantities of jellyfish enter the intake, they could render the fine screens inoperable or reduce significantly plant source water flow rate.
4.3.5.3 Inlet Materials
Inlet screens/towers are typically built from corrosion-resistant materials such as coppere nickel alloy, concrete, or stainless steel. The screen bars are either made of 90/10 Cu/Ni or super duplex stainless steel for seawater applications and duplex stainless steel for brackish water intakes.
4.3.6 Feasibility Considerations for Colocated Intakes
For the collocation concept to be cost-effective and possible to implement, the power plant cooling-water discharge flow has to be larger than the capacity of the desalination plant with