Causes Document

Impingement

            Many shortcomings are evident in the large-scale process of converting seawater into fresh, clean water. Specifically, a detrimental outcome of the process is that marine organisms can be killed during the process. Through one cause, impingement, fish get stuck to the intake screen due to a high intake velocity. Impingement affects large aquatic and benthic organisms that are large enough to be caught by the intake screens, such as adult fish, crabs, and marine birds. Surface intake valves often have 150 mm outer layer bar screens followed by an inner fine screen with openings ranging from 1 mm to 20 mm. Although the majority of fish are prevented from entering and are removed by the screening and downstream filtration, many can get trapped against these screens due to the speed of the water being pulled inward.

Entrainment

            Another cause of harm to marine organisms is entrainment, through which organisms that are smaller than the screen are drawn into the intake, and end up going through the treatment process. This inevitably results in the organisms’ deaths. Entrainment affects smaller aquatic and benthic organisms that are not removed by the finer screens. When treatment processes utilize subsurface intake valves and intake wells, the water is filtered by ocean sediments above the intake, and organisms living in bottom sediment can be affected.

Entrapment

            Organisms can also be harmed during entrapment. Certain intake valves have an offshore pumping station that is connected to an onshore intake valve. The intake water is pumped from offshore stations through conveyance pipelines. If an organism travels through the pipeline to the onshore intake valve, it can be entrained into or impinged on the onshore intake valve’s retention screen.

Influences

            Ultimately, three factors largely influence whether or not impingement and entrainment will occur: intake velocity, screen opening sizes, and intake flow. If the speed of the intake is too fast, organisms will become caught against the screens meant to filter them out. Depending on the size of the screens, especially the finer ones used in the process, organisms may be able to pass through and killed during the treatment process. Finally, the larger the amount of water volume collected continuously, the greater the chance is that organisms will be negatively affected by the water treatment process.