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4.3 OPEN INTAKES 91
which it is collocated, and the power plant outfall configuration has to be adequate to avoid entrainment and recirculation of concentrate into the desalination plant intake. It is preferable that the length of the power plant-outfall downstream of the point of connection of the desalination plant discharge is adequate to achieve complete mixing prior to the point of entrance into the ocean.
A special consideration has to be given to the effect of the power plant operations on the cooling water quality, since this discharge is used as source water for the desalination plant. For example, if the power plant discharge contains levels of copper, nickel, or iron signifi- cantly higher than these of the ambient seawater, this power plant discharge may not be suit- able for collocation because these metals may cause irreversible fouling of the membrane elements. In such cases, the pretreatment costs for colocated desalination plant are likely to be higher than those for plant with separate open-ocean intake.
Another potential problem could be the location of the disposal of the power plant intake screenings. In most power plants the screening debris are removed from the intake cooling water and disposed off-site. However, this disposal practice may change during the course of the power plant and desalination plant operations.
For example, in the case of the Tampa Bay seawater desalination plant, during the final phase of the desalination plant construction, the power plant decided to change their screen- ings’ disposal practices and to discharge their intake screenings just upstream of the desali- nation plant intake rather than to continue disposing them off-site. This change in power plant operations had a dramatic effect on the Tampa Bay Water desalination plant start-up and operations, and especially on the pretreatment system performance.
Since the desalination plant was pilot tested and designed around the original method of power plant operations under which all screenings were removed from the cooling water, the desalination plant was not built with its own separate intake screening facilities. The presence of power plant-waste screenings in the desalination plant intake water had a detrimental effect of the pretreatment filter operations because the screening debris frequently clogged the filter distribution piping, airlifts, and sand media.
Although this problem has a significant effect on the desalination plant operations, it also has relatively straightforward solutionsdeither installing separate fine screening facilities for the desalination plant or moving the point of the power plant screening debris-discharge downstream of the desalination plant intake. A summary of key advantages and disadvan- tages of the collocation approach is presented in Table 4.2.
4.3.7 Open IntakesdEnvironmental Impacts and Mitigation Measures
Use of open-ocean intake for collecting saline source water for the desalination plant would result in some entrainment of aquatic organisms (WRA, 2011) as compared to subsur- face intakes because they take saline source water directly from the water column of the saline water body (e.g., ocean, sea), rather than source water which is prefiltered through the soils of the coastal aquifer. In addition, some aquatic organisms may be impinged on the screening facilities.
Impingement and entrainment of aquatic organisms is not an exception for seawater intakes. The same phenomena and extend of environmental impact may be observed at the intakes of conventional water treatment plants collecting fresh surface water from rivers, lakes, and