Reverse Osmosis at the Carlsbad Desalination Plant

Reverse Osmosis at the Carlsbad Desalination Plant

Along the Pacific coast north of San Diego, a treatment plant in Carlsbad, California does something most water treatment plants do not attempt: it turns seawater into drinking water. The plant produces about 50 million gallons of finished water a day, supplying roughly 10 percent of San Diego County's total water use. To understand how a plant removes salt from seawater at that scale, it helps to start with a simpler process that happens constantly in nature: osmosis.

Osmosis occurs whenever a semipermeable membrane separates two solutions with different concentrations. This membrane is a barrier with pores too small for large dissolved particles to pass through. Water molecules move naturally through the membrane, from the side with fewer dissolved particles toward the side with more. This movement evens out the concentration on both sides. The flow requires no pump or outside energy. It happens because of the random motion of water molecules and the membrane's selective pores. Reverse osmosis, as its name suggests, runs this process backward. Engineers apply mechanical pressure to the seawater side, instead of letting water flow naturally toward the saltier side. That pressure must be greater than the water's natural osmotic pressure. The applied pressure forces water molecules through the membrane, away from the dissolved salt. The salt is left behind in an increasingly concentrated solution.

Carlsbad's treatment train carries out this idea in four stages. Seawater first passes through pretreatment. Screens and filters remove suspended sand, algae, and small marine organisms in this stage. These particles would otherwise clog or damage the delicate membranes downstream. Next, high-pressure pumps raise the filtered seawater to a very high pressure. This pressure is far higher than any pump used elsewhere in a conventional treatment plant, and it must exceed the ocean water's natural osmotic pressure. The pressurized water then enters the membrane separation stage. Here it passes through thousands of tightly wound membrane modules. These membranes are built from a thin polyamide layer, with pores sized at the nanometer scale. A dissolved salt ion travels through water surrounded by a cluster of attached water molecules, called a hydration shell. That hydration shell makes the ion too large to fit through the tiny pores. A single water molecule is small enough to pass through the same pores one at a time. Finally, the water that makes it through the membranes enters post-treatment. The plant adds back a small amount of minerals here for taste and stability, then disinfects the water before it enters the regional distribution system.

Not every drop of incoming seawater becomes finished drinking water. The membranes leave behind a concentrated salty stream called brine. Brine contains most of the dissolved salt that was originally in the seawater, along with whatever the pretreatment stage removed. Carlsbad returns this brine to the ocean through a specially designed outfall. The outfall mixes and dilutes the brine quickly, reducing its effect on marine life near the discharge point.

None of this comes cheap in energy terms. Desalinating seawater through reverse osmosis typically uses roughly 11 to 15 kilowatt-hours of electricity for every 1,000 gallons produced. Conventional treatment and delivery of water from a river or reservoir uses only about 1 to 2 kilowatt-hours for that same amount. High-pressure pumping is the main reason for the difference. Pushing water through membranes against its natural osmotic pressure takes far more energy than simply filtering and disinfecting fresh water that arrives at ordinary pressure. That energy cost shows up directly on water bills. It is the main reason desalination remains one tool among several, alongside conservation, water recycling, and imported supply, rather than a community's only source of water.

Even so, a coastal region facing drought and competition for imported water has reasons to accept that added cost. Unlike a river or a mountain snowpack, the ocean does not run dry. This makes seawater desalination a supply that a community can rely on even in the driest years. The chemistry that makes reverse osmosis possible is a membrane precise enough to sort individual water molecules from dissolved ions. Engineers now apply that same principle at plants like Carlsbad, turning one of the world's largest bodies of water into a dependable part of a region's water supply.

1. According to the passage, about how many gallons of drinking water does the Carlsbad plant produce each day, and roughly what share of San Diego County's water supply does that represent?

2. In ordinary osmosis, which direction does water move across a semipermeable membrane, and what causes that movement?

3. List, in order, the four stages of Carlsbad's treatment process described in the passage.

4. Why must the pressure applied during reverse osmosis be greater than seawater's natural osmotic pressure? Use the passage's explanation of ordinary osmosis to support your answer.

5. What is brine, according to the passage, and what happens to it after it leaves the membranes?

6. Based on the passage's discussion of energy use, explain why a coastal region might build a desalination plant even though it costs more energy than conventional treatment.

7. The passage explains that the membrane's pores block salt ions, which carry a hydration shell of attached water molecules, while letting individual water molecules pass through. Based on this detail, why does the size difference between a bare water molecule and a hydrated ion make reverse osmosis possible?

Writing task. In one paragraph, explain how reverse osmosis reverses the natural direction of ordinary osmosis. Use at least two details from the passage.