Most of the drinkable water in San Diego is a mix of desalinated water and metropolitan water. Desalination plants are an up and coming trend for water treatment in the United States, especially in coastal areas, like California. A worker from the Carlsbad Treatment Plant, Michelle Powelson came to talk to our class about desalination plants like the Carlsbad Treatment Plant. The Carlsbad Desalination Water Treatment Plant originated in 1998, but was not fully completed until December 2015. The water plant was built in consensus with the existing power plant in Carlsbad, which was using the ocean water in the lagoon to cool the power plant machinery. 

The treatment plant has an 8 step process: primary pretreatment, second pretreatment, reverse osmosis, energy recovery, post-treatment, product tank, and chemical area. In the primary pretreatment step, the intake water is filtered through to get rid of particles in the water. Then, in the secondary pretreatment step, the water is further filtered to remove fine particles in the water. In the reverse osmosis step, the water is separated into a highly concentrated, briny water, and the fresh drinkable water. 

This happens by pushing the source water through membranes that act like microscopic strainers that only allow water molecules through. In order to be more environmentally friendly, the plant then recovers some of the energy lost in the desalination process by transferring the hydraulic energy to the incoming seawater. The plant has also reduced their environmental impact by changing the  size of their intake filters to 1 mm to avoid taking in marine life. They also provide fish elevators to help marine life escape the area to avoid the desalination process. Additionally, the power plant has been shut down, since water is no longer needed to cool machinery with new technology, so this has reduced emissions in the area, and the plant promoted the planting of a few thousand trees in the Cuyamaca canyon. 

The next step in post-treatment which includes remineralization by passing the water through limestone chips. Reverse osmosis is very good at removing all dissolved salts and minerals, and need some in the water so the water does not corrode the transport pipes. The pH is also adjusted to be optimal for drinking, in this case the pH is increased. Then, the water goes to the product water tank, which is where the last treatment step takes place. The water is disinfected via chloramines, and the water is fluorinated. The water is then transported to the San Diego County water authority where it is then dispersed throughout San Diego county. Desalination plants, like the Carlsbad plant, are interesting, new ways to bring drinkable water to coastal areas.