Writing Explanation II: Crafting a Well-Written Explanation

Returning The Water Is Tricky

With subsurface intakes, ocean water is drawn through perforated pipes buried in the seafloor. Mud and sand filter out nearly all lifeforms. However, such intakes are far more expensive to build.

The second major environmental concern with desalination is discharge water. That is water that's released back into the ocean. Most desalination plants take in two times more seawater than the fresh water they produce. To produce 50 million gallons per day of fresh water, Carlsbad will draw in 100 million gallons of seawater. The rest is returned to the ocean, but with its salt level doubled.

The discharge water is so salty that it does not dissolve well in the ocean.

“It’s like oil and vinegar — they stay separate,” said Victoria Whitney of the state water board. “You end up with these very large dead zones … where you have really salty water just sitting on the ocean bottom.”

Carlsbad will deal with this problem by mixing the salty water with water used to cool a nearby power plant. As a result, the discharge water will be only about 20 percent saltier than the ocean.

Another approach is to use spray nozzles to spread the discharge water under the ocean surface. This encourages mixing of the salty discharge. The water board currently recommends this approach. MacLaggan of Poseidon says, however, that the force of such sprayers is so great that it can kill some sea life.