Solutions Document

Subsurface Intakes

            Subsurface intakes’ water is naturally pretreated by passing through the sandy ocean floor. This water traveling through the bottom sediment contains low levels of solids, silt, and contaminants that are filtered by the sediment. However, because of the location, subsurface, these intakes are not only invasive in the marine environment because of the requirement that the bottom sediment be dredged, but also pose a large threat of entrainment. Therefore, subsurface intakes should be used in areas with a low risk of entrainment and are a solution to areas with a high level of impingement. Some methods of intake offer a simple solution to this problem, depending on the size of the treatment plant.

Vertical and Horizontal Wells

            Vertical wells can be implemented to supply water for small desalination plants of 1 million gallons per day or less. For larger plants, horizontal wells that have a higher capacity can be used. Radial Ranney wells of this type have intake pipes radiating out of a center point to intake maximum amounts of water. The wells also implement directionally drilled collectors that fan out under the seabed. The method of using wells can effectively alleviate the risk of impingement and lower the effects subsurface intakes have on marine organisms.

Offshore Intakes

            More economically feasible and less environmentally invasive than subsurface intakes are offshore intakes, which contain an inlet structure, placed underwater offshore, connected to a submerged pipe, which extends to the onshore concrete structure. While a more economically savvy than subsurface intakes, offshore intakes have a much greater risk for entrapment and impingement. However, several solutions to these problems are plausible.

Passive Wedge-Wire Screens

            Passive wedge-wire screens minimize impingement and entrainment because of their unique shape and eliminate the need for coarse and fine screens on the onshore concrete structure. The wedge-wire screens use cylindrical screens with trapezoidal shaped slots with openings from 0.5 mm to 10 mm, following data that states that the openings should be 3/8 inch or less to prevent entrapment of adult and juvenile fish. The screens also form low flow-through velocities to minimize impingement and entrainment. In addition, passive wedge-wire screens can be placed in locations where high natural cross flow in the current exists; the natural flow of water will prevent aquatic life from being impinged on the screen.

Active Traveling Screens

            To reduce the intake force on the surrounding organisms, active traveling screens can be implemented. They are placed after course bar screens, and the screens’ segments move in a rotational pattern. This rotating motion aids in preventing the impingement of fish by creating a cross flow that lessens the pull on the organisms around the area of intake.

Collocated Intakes

            In situations where desalination plants are located near power plants, collocated intakes can be used, reducing the number of intake valves needed for both plant. Because existing power plants require water intake for cooling purposes, new desalination plants can connect to the discharge water from these plants to use in the treatment process. This allows two plants to use one intake valve, therefore minimizing the amount of entrapment and impingement.