Here’s an idea: Let’s use the ocean to create an endless supply of pure water, no matter the amount of rain and snow that falls (or doesn’t) on California.

If it sounds like something out of the future, consider: As of today, seven ocean desalination plants are under consideration along the coast from Dana Point to Monterey Bay. By the mid 2020s, those plants could be using the Pacific to produce about 10 percent of the fresh water needed in parts of Los Angeles and Orange counties. Another project, in Carlsbad, opened about a year ago and is on track to produce about 8 percent of San Diego’s water.

Desalination, long considered something out of “The Jetsons,” is real.

But also consider this: Though the promise of desalination is appealing – fresh, clean water that can outlast any drought – critics and water experts have many questions.

Some believe the desalination process can inflict permanent environmental harm on the ocean and sea life. Others argue that the technology is relatively new and expensive, and that investing in new plants today will mean sunk costs that might look wasteful in the near future. Still others believe enough isn’t being done to conserve water we already have.

Among the local projects under consideration, two face particularly stiff opposition – a $380 million plant in El Segundo that could churn out 20 million gallons each day and a $1 billion plant in Huntington Beach that would generate 50 million gallons a day.

By summer, the State Lands Commission could issue a new environmental report on the Huntington Beach plant and renew a lease with the company behind the plant, Poseidon Water.

If state and regional approvals come this year, the project could go online by 2021. The El Segundo project also needs various approvals and could open by 2023.

Here are some commonly asked questions about desalination.

1. How does it work?

Seawater is drawn from the ocean through intake pipes set out in the open ocean or below the seafloor. That water is filtered for bigger contaminants (and some essential tiny sea creatures) before it is sent through a reverse osmosis system that pressurizes the water through membranes, separating out the salt.

The remaining product is drinkable water, though minerals are added to reduce its corrosive qualities and for taste.

Another desalination technology involves freezing water, but that’s not being considered for Southern California.

2. I hear water produced through desalination is more expensive than groundwater or imported water from the Colorado, Sacramento and San Joaquin rivers. Is that true?

At current prices, yes. According to John Kennedy, the executive director of engineering and water resources at the Orange County Water District, groundwater costs $402 per acre-foot (an acre-foot is about 326,000 gallons, about enough for two average American families per year). Imported water costs $1,059 per acre-foot in Southern California. Desalinated ocean water costs $1,900 to $2,100 per acre-foot.

If proposed desalination projects go forward, the average household bill is expected to increase by $3 to $6 each month.

So what’s the upshot? Well, as the state’s population grows and natural resources dwindle, the cost of locally sourced groundwater or imported water could rise, while the cost of making desalted water with improving technology could decline so that the difference might not be as staggering as it appears now.

“Proponents are quick to note that over time, those other sources will become more expensive as well,” said David Feldman, the director of Water UCI, which analyzes water science and policy. “Risks or cost of other sources – it may be the kind of thing where the differentials aren’t as significant as they first appear.”

3. Does it hurt the environment?

The jury is still out on this one.

One of the biggest criticisms of desalination is that it’s energy-intensive. That’s also what makes it expensive.

Backers of desalination point to mitigation measures they take to lessen a plant’s carbon footprint, and they say the process of moving water from sources as distant as the Colorado River to Southern California also is energy-intensive.

The company behind the proposed plant in Huntington Beach, Poseidon Water, has pledged to be carbon-neutral by buying carbon offsets and investing in projects such as reforestation.

“It is energy-intensive, but so is the alternative,” said Scott Maloni, Poseidon Water’s vice president of project development.

Other concerns for the environment are focused on the discharged water that returns to the ocean, which is twice as salty as the ocean water drawn into the plant. Seawater is naturally about 3 percent salt, and the brine – or salty discharge – is about 6 percent salt.

“No marine life can live in that area,” said Ray Hiemstra, the associate director of programs for Orange County Coastkeeper, an environmental group that has opposed the Poseidon plant.

“If we have this desal plant (all day, every day) there’s going to be this salt plume.”

4. Can sea creatures get sucked into the intake pipes?

They might, though not the biggest marine animals, such as dolphins, turtles, seals or whales. Instead, the sea life that could be affected by desalination is microscopic – think fish larvae and plankton.

“When we think of ocean life, we think of all that big stuff, but the reality is the vast majority of ocean life is really tiny,” Hiemstra said.

The State Water Resources Control Board established rules for intake pipes in the ocean, calling for them to be outfitted with screens that have openings as thin as a credit card and suck in water at a half foot per second so sea life doesn’t get stuck on the pipes.

But environmental groups argue those measures aren’t enough.

“Those screens won’t protect it at all,” Hiemstra said. “Ninety-nine percent of everything out there is smaller than 1 millimeter.”

Poseidon has said that its intake pipe will minimize the impact on ocean life and that the screens reduce the harm even more.

“There will be some minimizing effects,” Maloni said. “Our impacts are already very manageable and mitigable without the screens.”

5. Where is desalination popular?

Desalination is concentrated mostly in arid, drought-stricken countries, with more than 18,000 desalting plants in 150 countries, according to the trade organization International Desalination Association.

Saudi Arabia is the biggest producer of fresh water through desalination, with 27 plants, many near the Red Sea and Persian Gulf.

In the early 2000s, Israel turned to desalination in response to severe drought conditions, according to Feldman. Now the country is flush with fresh water.

Other places that use desal include Singapore and Australia.

6. Why are some plants more controversial than others?

A lot of it comes down to how the water gets into the plant.

In south Orange County, a desalination plant is proposed near San Juan Creek in Dana Point. So far, environmental groups haven’t opposed it mainly because the Doheny project would use slant wells far below the ocean floor to extract saltwater.

This process makes the likelihood of hurting marine life virtually nonexistent. Most small marine life to larger animals and even contaminants are filtered through the sand and don’t wind up getting sucked into the plant.

That being said, less water can go through these wells than can be extracted through an underwater pipe.

“The big argument for it is that there is less of an impact on marine life,” Hiemstra said. “They can also reduce the need for pretreatment, so there can be some savings there.”

Poseidon has investigated using slant wells in Huntington Beach but says it can’t use that technology without affecting the huge aquifer under north Orange County, which provides north Orange County with about 75 percent of its drinkable water. South Orange County doesn’t have such an aquifer and imports about 90 percent of its drinkable water.

Environmental groups argue that the technology is feasible for the Huntington Beach plant.

7. Can these plants adapt with new technology?

Much of the advances in desalination are in membrane technology, which is becoming more efficient, lasting longer and requiring less energy to create fresh water, Maloni said.

Poseidon’s plants are modular and automated, making it easier – and cost-effective – to switch in new membrane technology when it becomes available.

“We have an incentive to keep the plant modern and operational,” Maloni said.

8. Where does the state stand on all of this?

Overall, the State Water Resources Control Board, one of three regulatory agencies that oversee desalination plants, supports desalination as a complement to a broader portfolio of clean water resources.

However, there are more affordable and energy-free alternatives to desal, such as conservation, said board Vice Chairwoman Frances Spivy-Weber.

Desalination, Spivy-Weber said, should be considered an alternative when other options have been exhausted.

“If you’re a water agency or a community that is considering these things, you would want to make sure your agency looks at cost-effectiveness and reliability before they take that leap, before they go straight to desal.”

9. Would desalination ensure clean water during droughts?

Yes. From government agencies to environmental groups, there seems to be some consensus that some desalination is needed, though when such plants should come online and what technology they should use are at the heart of most of the controversy.

“Desal is going to be part of our mix,” said Hiemstra. “It’s a matter of doing it right.”

10. How much of our water supply will come from desal if it’s approved?

About 8 percent of Orange County’s water supply could come from Poseidon. In southwest Los Angeles County, the proposed desalination plant in El Segundo could provide 10 percent of the water supply to West Basin water district’s 1 million customers across 17 cities, from Inglewood to Malibu.

Staff writers Megan Barnes and Sandy Mazza contributed to this report.