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4.3 OPEN INTAKES 89
Due to the lower quality source water they collect, onshore open intakes have found very limited application for RO desalination plants, and unless the specific site location or costs dictate the need to use this type of intake, they are not used for such plants. However, onshore intakes are often the prime choice for thermal desalination plants, where source water quality in terms of suspended solids, organic, and algal content is of secondary impor- tance and has minimal influence on the desalination process.
4.3.5 Feasibility Considerations for Open Intakes
4.3.5.1 Intake Site Location and Configuration
Key factors that determine the selection of the location of open intakes are: potential for beach erosion in the intake area; location and direction of underwater currents; presence and location of active seismic faults; topography and geology of the floor of the water body; location of environmentally sensitive habitats along the intake pipe route and near the intake inlet; location and size of municipal and industrial wastewater discharges within 1 km (0.6 miles) radius from the intake; size of waves and depth of wave impacts; ship and boat traffic; tide and wind characteristics in the intake area.
Open intakes should be located away from coastal areas of active beach erosion and seismic faults; high waves; strong underwater currents carrying debris, silt, plankton, sea grass, weeds, and other stringy materials; and locations with heavy ship and boat traffic. If sensitive marine habitats are encountered, the intake route should either be modified or the intake conduit/s should be installed via directional drilling or tunneling under the sen- sitive area instead of using open trench construction or laying the conduit on the bottom of the water body.
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 SWRO 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).
The exact location and depth of the offshore intake structure must be determined based on a hydrological study to ensure that the intake will be adequately submerged at low tide; pro- tected from the damaging orbital storm wave motion; and will be far enough offshore to avoid the near-shore sediment transport zone where storms can cause suspension or deposi- tion of large quantities of silt and sediment, and can ultimately damage the intake structure and the interconnecting piping.
Diurnal and seasonal source water quality fluctuations should also be considered when determining the location of the intake structure. At minimum low tide conditions, inlet mouth should be submerged at least 3 m (10 ft) below the water surface. In addition, the distance between the inlet mouth and the ocean floor should be no less than 3 m (10 ft) to prevent excessive sand carryover into the downstream intake facilities. The intake water supply can be protected against large aquatic organisms and large floating debris by instal- lation of wire net across the intake mouth.