71% of the earth is covered by oceans and weekly I get the same question – Why don’t we just use ocean water to solve the water problems you keep talking about? Removing salt from water, especially ocean water in coastal communities is an option for solving our water issues. Desalination is the process used to remove salt and minerals from seawater or brackish water (brackish water is less salty than sea water and more salty than fresh water and usually comes from combining the two). Sometimes this is called desalting and Alan likes to call it desal.
How Does It Work?
There are two popular technologies currently used for the desalination process. One is membrane technology. This process uses pressure to force water through a membrane that does not allow the salt particles to pass. You probably know this process by the term reverse osmosis. Sometimes instead of using pressure to drive the water through the membrane electricity is used and we call this a voltage-driven process.
A thermal process is when salt water is heated which produces vapor and then condensed and collected as fresh water. This is very similar to the evaporation process of ocean water and explains why rain water is not salty. When the ocean is heated by the sun the water evaporates and escapes as vapor the salt does not evaporate and is left behind.
Benefits of Desalination
97% of the world’s water is salt water. Desalination of salt water provides an unlimited supply of water to the world’s population. As demand for water grows and the population increases desalination provides a source of water that can meet our new demands for water.
The costs for desalination are moving lower, and the quality of the water is very high. Combining this with the availability for water makes desalination attractive to many people around the world.
So What’s the Problem With Desalination?
The first challenge is cost of the water produced. As mentioned above the cost is decreasing overall, but even in coastal communities we find the cost of desal water to be about twice the cost of traditional water supplies.
Another expense is the cost of energy needed to operate a desalination plant. According to a congressional research service report approximately 33% to 50% of the operating costs of a desalination plant are for electricity.
In addition to costs what we do with the brine created in the desalination process is a big issue. If the highly concentrated salt water is disposed back into the ocean there is chance the salt will sink to the ocean bottom and may have an adverse effect on marine organisms. What to do with the brine is one of the most volatile issues when discussing desal.
We are seeing some large projects under construction in the U.S. After twelve years of planning and over six years in the state’s permitting process, the Carlsbad $1 billion desalination project has received final approvals. The San Diego Water Authority has contracted with the plant to purchase all the water produced. It is going to be piped from the coast (where the desal process will take place) to San Diego County Water Authority Aqueduct in San Marcos. The plant is a reverse osmosis plant and should be producing water by 2016.
There is a need for additional sources of water. As the price of water increases these options look better and better. It also seems like a bet on desalination is a bet against conservation and the new water saving technologies like smart controllers for your landscape. The desalination plant in Carlsbad is an example of an investment in water supply for the future when traditional water is much more expensive (estimates are the year 2020 before this plant is competitive in pricing). Desalination could also be viewed as insurance for conservation. If we can’t conserve ourselves out of the problem we have a more expensive option to fall back on. One thing is for sure, we have to examine all the possibilities because the consequences are too dire and the answer I imagine will be a collection of ideas and not one specific solution.