At the beginning of the desalination process, intake pipes collect seawater, which travels through a pre-treatment “train.” At the front end of the train is a series of screens that remove kelp, seaweed, and plastic debris as well as deny entry to juvenile and adult fish.
The next step is to run the seawater through sand filters similar to those used in swimming pools or settling ponds like those used in wastewater treatment facilities. These remove even smaller organisms, fine sediment, and organic particles, which join a discharge stream that is returned to the ocean. Chemicals called coagulants are sometimes used in small quantities at this stage of pre-treatment to accelerate the removal of these small particles. The reverse osmosis membranes that remove the salt from seawater to make it drinkable can remove viruses and sub-micron particles down to the molecular level.
Minimizing environmental damage in the process of extracting fresh water from seawater is an important part of the desalination process. The intake of plankton such as the larvae of fishes and marine invertebrates often destroys it during the filtering process. Some portion of the destroyed larvae would have gone on to become adult fishes and invertebrates, meaning that their loss could damage the ecosystems from which they were removed. The loss of the larvae could also have an economic impact as well because of the reduced stock available to commercial and sport fishermen.
Engineers are working to improve the process to ensure that as few organisms as possible enter the intake system. Currently a small number of pilot plants are experimenting with a device known as a wedge wire screen. The screen deflects small organisms away from intake pipes in a lateral motion to steer them clear from danger. Researchers are also refining screens that are used at a later step in the process called traveling micro-filtration screens. These screens function like a conveyor belt and remove more microorganisms from the stream that ultimately becomes drinking water.
-- Scott Jenkins, coastal engineer, Marine Physical Laboratory