Offshore open water intakes are common globally as they facilitate gathering large volumes of seawater. They also tend to be the most damaging to sea life. These intakes draw water directly from the marine environment, passing the water through mesh screens and piping it to the desalinization plant. Direct offshore intakes tend to have high rates of impingement, which refers to the number of fish and other creatures stuck to the intake screen due to high intake velocity. Entrainment, organisms drawn into the intake system, is also high. Environmental regulations in the United States discourage the use of offshore open water intakes due to relatively high mortality levels.
Impact on the marine environment can be improved if an open water intake utilizes a channel system to gather seawater. In this type of system, a channel reservoir is created to hold water before it passes into the intake system, a process that reduces impingement. However, because it draws surface water, this type of system takes in small sea life that is suspended in seawater, depleting plankton levels and potentially disrupting the ecology of fish reproduction.
Intakes that are located beneath the sea bed greatly reduce impact on the marine environment. Horizontal directional drains are created by drilling through the subsoil underneath the sea near the facility. Sites are tested beforehand to ensure adequate porosity of the sea bed, ensuring an accessible aquifer for drawing water. Numerous drains must be installed to supply adequate water for the plant. With this system, the sea bed acts as a filter, completely eliminating impingement and reducing entrainment to a low level.
Onshore indirect intakes are wells drilled from the beach that access water below the sea bed. Wells can be drilled vertically, horizontally or slanted, depending on circumstances related to the geology of the area. Beach wells function best when alluvial material is relatively coarse, as fine sands and clays can reduce intake volume. As with offshore sea bed filtration, beach wells are quite effective at eliminating impingement and entrainment.