Desal 101

How does desalination work?

Desalination plants use a process called "reverse osmosis," which basically means using high pressure to force a liquid such as seawater through a semi-permeable membrane to separate out the water from any solids dissolved in it. On the other side you're left with "brine," which is the salt and any other minerals that were in the seawater (the "solution").

This process is called "reverse" osmosis because in the "normal" osmosis process, clear water would move through the membrane to dilute the solution (the water with dissolved solids in it). There's a great explanation in layman's terms here.

Process Overview

Here's an overview how the whole desalination process would work in the Cal-Am desal plant (paraphrased from Cal-Am's CPUC application, Appendix H.)

• Seawater pumped from the wells is delivered to elevated storage tanks. Using gravity, the water moves through pretreatment filters to remove larger contaminants (small particles that could damage more-sensitive equipment downstream).
• The filters are backwashed with chlorinated water once a day to clean and disinfect them.
• The "backwash water" that now contains the gunk removed from the filters goes into a storage basin with an impermeable liner, where the suspended solids are allowed to settle out.
• The "decanted water" is skimmed off the top; it will be combined with the brine that's left after desalination.
• Accumulated solids that settled to the bottom are removed periodically.
• The pretreated water now enters the desalinization plant. It is forced at very high pressures through semi-permeable membranes, and the water molecules, smaller than almost all impurities, pass through the membranes. The impurities and residual water left behind (the brine) are discharged. (We'll come back to this in a minute.)
• About 40-50% of the water that comes out of the "first pass" membrane array goes through a "second-pass" array. The second pass is designed to lower the concentration of three specific compounds: boron, chloride, and sodium. The resulting "second pass" water is then combined with the other half of the "first pass" water.
• All told, about 40-50% of the feedwater going in comes out as clean water (e.g., 20 million gallons of feed water produces about 9 million gallons of desalinated water).
• Remember that decanted "backwash water" that came out of the pretreatment filters? It joins up with the brine from the reverse osmosis membranes, and the mixture flows via gravity to the brine discharge pipe, which will join with MRWPCA's outfall from the wastewater treatment plant.
• Next, the desalinated water goes to post-treament, where hardness, alkalinity and pH are adjusted, and sodium hypochlorite is added for disinfection.
• Finally, the post-treatment water flows by gravity to storage tanks called "clear wells" for eventual conveyance into the delivery pipelines.

For more details, including a diagram of the facility, and very specific descriptions of all the elements, see the CEQA Memorandum attached as Appendix H to the CPUC application. It's a long document; Appendix H starts on page 68.

The DeepWater Desal plant design, while different in some details, uses basically the same process. Intake tank, pre-treatment through a granular filter to remove larger particles, high-pressure through reverse osmosis membranes, then post-treatment for water quality.