Turning Seawater into Drinking Water: The Carlsbad Desalination Plant

Most of Earth's water is not available to drink. About 97 percent sits in the oceans. That water is far too salty for humans or crops. Much of the remaining fresh water is locked away in ice caps and glaciers. For a coastal region facing drought, ocean water is not entirely off-limits, though. It can be treated to remove the salt. That process is called desalination, and it produces drinking water.

The largest desalination plant in the United States sits north of San Diego, in Carlsbad, California. The Carlsbad Desalination Plant produces about 50 million gallons of drinking water each day. That is roughly 10 percent of San Diego County's total water supply. The plant draws seawater from the Pacific Ocean and pushes it through a technology called reverse osmosis.

Reverse osmosis works at the scale of individual molecules. Seawater is pressurized and forced against a membrane. A membrane is a thin sheet of material. Its pores are small enough to let water molecules pass through while blocking most of the dissolved salt in the water. Only part of the seawater entering a reverse-osmosis plant becomes finished drinking water. The rest leaves as a concentrated brine, carrying away the salt the membrane rejected. The finished water still needs more treatment steps, such as adjusting its mineral balance and disinfecting it, before it enters the drinking water system.

The process is complex and expensive. Seawater reverse osmosis uses roughly 11 to 15 kilowatt-hours of electricity for every 1,000 gallons produced (about 3 to 4 kilowatt-hours per cubic meter). Conventional treatment of a fresh water source, such as a river or reservoir, uses only about 1 to 2 kilowatt-hours for the same volume. Building the high-pressure pumps, the membrane systems, and the pipelines to move finished water inland also takes a large upfront investment. A water utility and its ratepayers must be willing to fund that investment for years before the plant pays for itself in supply reliability.

Desalination is not the only way a water-stressed region can add to its supply. Well north of Carlsbad, Orange County takes a different approach. Its Groundwater Replenishment System is the largest facility of its kind in the world. It produces about 130 million gallons per day, more than double what Carlsbad produces from seawater. Instead of treating ocean water, the Orange County system treats wastewater that has already been used once. It purifies that water to drinking water standards and returns it to the region's groundwater basin, where it is later pumped back out and used again. This approach is often called potable reuse, because water that once went down a drain is treated until it is safe to drink again.

Both approaches share a common goal: adding a reliable new source of water that does not depend on rainfall or snowpack in a given year. Both also share a common challenge, cost. Desalination and advanced water reuse plants are expensive to build and to run. Their water typically costs more per gallon than water drawn from a traditional river or reservoir, at least while the plants are new and still paying down construction debt. A water utility choosing between these strategies has to weigh that added cost against the value of a water supply that keeps flowing even during a multi-year drought.

Location matters, too. Desalination only makes sense for a coastal community with ocean access nearby. An inland city has no seawater to treat. Potable reuse depends on something different: enough wastewater already flowing through a sewer system to treat and recycle. That usually means a large enough population to generate it. Neither strategy works everywhere. That is part of why different regions facing the same underlying problem, not enough reliable fresh water, sometimes choose very different solutions.

Both plants show a broader pattern in how water utilities respond to drought and growing populations. Instead of only building new dams or drawing more from rivers already stretched thin, more utilities now treat water sources once considered unusable, the ocean or wastewater, as sources worth the added engineering and cost. A community might follow Carlsbad's path, Orange County's path, or its own combination of strategies. Either way, the underlying decision is the same one water planners have always faced: how to keep a reliable supply of drinking water flowing to homes and businesses when the traditional sources can no longer be counted on alone.

1. According to the passage, what does the Carlsbad Desalination Plant produce each day? Roughly what share of San Diego County's water supply is that?
2. Describe how reverse osmosis separates fresh water from seawater at the membrane level. What happens to the salt the membrane rejects?
3. According to the passage, why does desalination generally use more energy than conventional treatment of a fresh water source?
4. How does Orange County's approach to adding water supply differ from Carlsbad's approach, according to the passage?
5. Using details from at least two paragraphs, explain why a water utility might weigh cost as heavily as reliability when choosing whether to build a desalination or reuse plant.
6. Based on the passage, why might an inland city be unable to use desalination, even if it wanted to?