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88 4. SALINE WATER INTAKES AND PRETREATMENT
such configuration could also be applied for inland brackish water desalination plants where the host power plant uses brackish intake and discharge wells for once-though cooling.
Sharing intake infrastructure also has environmental benefits because it avoids the need for new construction in the open water body area near the desalination plant. The con- struction of a separate new open-intake structure and pipeline for the desalination plant could cause significant disturbance of the benthic marine organisms inhabiting the water body.
Another clear environmental benefit of the colocation of power and desalination plants is the reduced overall impingement and entrapment of marine organisms as compared to con- struction of two separate open-intake structuresdone for the power plant and one for the desalination plant. This benefit stems from the fact that total biomass of the impacted marine organisms is typically proportional to the volume of the intake saline source water. By using the same intake saline water twice (once for cooling and second time for desalination) the net intake inflow of saline water and aquatic organisms is minimized.
The colocation configuration has a number of discharge benefits: (1) the construction of a separate desalination plant outfall structure is avoided, thereby decreasing the overall costs for seawater desalination; (2) the salinity of the desalination plant discharge is reduced as a result of the mixing and dilution of the membrane concentrate with the power plant discharge, which has ambient seawater salinity; (3) because a portion of the discharge water is converted to potable water, the total amount of the power plant thermal discharge is reduced, which in turn lessens the negative effect of the power plant thermal discharge on the aquatic environment; (4) the blending of the desalination plant and the power plant discharges results in accelerated dissipation of both the salinity and the thermal discharges.
4.3.4 Selection of Open-Intake Type
Onshore intakes have one key advantage, that is, they are usually the lowest cost type of intake, especially for large desalination plants. However, such intakes typically produce the worst source water quality because in most cases they are designed to collect water from the entire depth of the water column and because they are located in the surf zone where breaking waves continuously lift particles from the bottom into suspension, thereby signifi- cantly increasing water turbidity as compared to deeper waters.
The first 8e10 m (26e33 ft) of the water column in the surf zone typically have several times higher levels of turbidity, algae, hydrocarbon contamination, silt, and organics than deeper waters. The quality of water collected from this upper zone of the water body could vary in a wide range because of the influence from wind, tides, currents, ship traffic, storms, and surface freshwater runoff. Onshore intakes could also be exposed to beach erosion and direct wave action with irreversibly damaging consequences.
Because onshore open intakes usually yield source water of worse quality than offshore intakes, desalination plants with such onshore intakes have complex multistage pretreatment system. Practical experience shows that in most full-scale desalination projects, the savings of lower cost onshore open intakes are negated by the higher expenditures needed for more elaborated source water pretreatment and more conservative RO-system design.