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4.3 OPEN INTAKES 87
FIGURE 4.12 Colocation of desalination plant and power generation facility.
cooling water discharged from the condensers typically is 5e15C warmer than the source ocean water and is usually conveyed to the ocean via a separate discharge outfall.
Under the collocation concept, the intake of the seawater desalination plant is connected to the discharge outfall of the power plant to collect a portion of the cooling water for desalina- tion. After the desalination plant source seawater is pretreated, it is processed in an RO- membrane desalination system, which produces two key streamsdlow salinity permeate, which after conditioning is conveyed for potable water supply, and concentrate, whose salinity is typically two times higher than the source seawater. The desalination plant concen- trate is conveyed to the power plant discharge-outfall downstream of the point of desalina- tion plant intake connection. As a result the desalination plant does not have its own direct intake of ambient seawater and outfall to discharge concentrate; it uses the existing intake and outfall of the power plant with which it is collocated.
Usually, coastal power plants with once-trough cooling systems use large volumes of seawater. Because the power plant intake seawater has to pass through the small diameter tubes [typically 10-mm/(3/8-in.) or less] of the plant condensers to cool them, the plant discharge cooling water is already screened through bar racks and fine screens similar to these used at surface water intake desalination plants. Therefore, a desalination plant whose intake is connected to the discharge outfall of a power plant usually does not require the con- struction of a separate intake structure, intake pipeline, and screening facilities (bar-racks and fine screens). Since the construction cost of a new surface water intake structure and discharge outfall for a desalination plant is typically 20%e30% of the total plant construction expenditure, power plant colocation could yield significant construction-cost savings.
At present, this type of intake configuration has found application mainly for seawater desa- lination facilities co-sited with once-though cooling coastal power generation stations. However,