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Why Desalination Isn’t the Solution to Water Woes

Stanford University recently hosted a diverse roundtable discussion to explore whether seawater desalination can resolve California’s water problems. Leon Szeptycki, one of the organizers, helps us understand why the potential is limited.

Written by Matt Weiser Published on Read time Approx. 8 minutes
California desalination1
A worker passes giant tubes at the Carlsbad Desalination Project in Carlsbad, Calif. A new report shows that desalinated seawater will have limited impact on California's water supply.Gregory Bull, AP

It is easy to assume seawater desalination is the answer to California’s long-term water woes. All you have to do is look west, and the vast Pacific Ocean simply glimmers with opportunity.

But as with so many things involving water, desalination is not that simple. Converting seawater into drinking water is very expensive, it consumes a lot of electricity and it comes with a host of potentially unsavory environmental impacts.

To quantify the opportunities and the risks, the Woods Institute for the Environment at Stanford University hosted a workshop in January on the subject. Over two days, it brought together about 40 experts – scientists, water industry officials, environmentalists and government officials – to discuss and debate the technology’s true potential in California.

In May, the institute published a white paper based on the event to share the results with the world, and it isn’t particularly encouraging. The consensus was that desalination is unlikely to ever contribute significantly to California’s water supply, largely because of the costs and environmental impacts. On the other hand, it could become the primary source of water for some coastal communities.

To better understand the findings, Water Deeply spoke with Leon Szeptycki, a Stanford law professor attached to the Woods Institute who helped organize the workshop and coauthored the white paper.

Water Deeply: Your group concludes desalination “will not significantly reduce demand on freshwater resources,” a gloomy statement for anyone who believes in desal. How can that be, in a state with more than 800mi (1,287km) of ocean shoreline?

Leon Szeptycki: It’s only gloomy if you have unrealistically high hopes for desal. If you look at existing and planned ocean desal in the state, even if it’s all built at maximum capacity, it’s a small fraction of the state’s overall water demand. It adds up to just over 600,000 acre-feet (740 million cubic meters) a year, compared to overall freshwater withdrawals of something like 34 million acre-feet (41,938 million cubic meters) a year. Even if you’re bullish on desal, it really has a very limited role to play.

First of all, it’s very expensive. Second, it takes a lot of energy, and it’s hard to find places where you can site desal facilities that are acceptable to the local community and people who are concerned about marine resources.

The other point is that a lot of the places that really need desal don’t have access to the State Water Project. Because of where it’s going to happen – places like Monterey and Carmel or Moss Landing – it’s not going to reduce demand on the State Water Project or Central Valley Project appreciably, if at all. There’s maybe a specific location where it reduces pressure on a specific river or stream. And again, Carmel is a great example of that. The Monterey Peninsula desal facility is being built as a result of the need to reduce withdrawals from the Carmel River.

That 600,000 acre-foot number is if every currently planned plant gets built at its proposed capacity. Even an ambitious desal agenda is not a huge piece. It could be important in coastal communities. But in terms of the state’s overall water budget, it’s just not going to fundamentally shift the budget in the foreseeable future.

A positive thing that’s come out of the drought is a lot of communities are really focusing on all the possible sources of water they have available to them, including recycled wastewater and stormwater. It turns out those are, in most cases, cheaper than desal. So the ultimate footprint of desal may not be as large a some anticipate because those others sources are being looked at a lot more closely.

Water Deeply: Are there any environmental factors that might alter your conclusions? Like a megadrought, perhaps?

Szeptycki: I think it could. It’s certainly possible the scale could increase. But it’s really limited to the coast. That’s one of the issues with desal. We’ve got all these cities on the coast that need water, but also great water demand in the Central Valley, where groundwater is being depleted and most of the water use is agriculture. The state’s going to have to come up with a solution to water scarcity in that area, which might involve desalinating groundwater.

There are still lots of sources of water that we haven’t tapped into. We’ve seen in this drought that California does a good job of shrinking its water footprint when it really has to. A great example is the California American Water Co. facility planned in Monterey and Carmel. That area is really maxing out its use of other sources of water, including recycled water. That’s an example of where there’s a clear need for desal. But the size of the facility has gradually gone down as the community has tapped into other sources of water.

One driving issue will just be economics. What we have to do is make sure, with every decision, that we’re really putting on the table all the options and assess both the economic costs and then the environmental impacts.

That was something else that was discussed extensively. There just are no good quantification tools to really compare choices. For instance, comparing marine impacts to CO2 (climate change) impacts. There’s no ready way to compare those in California.

You can assess what it costs to treat wastewater and desalinate seawater and import water from the State Water Project. But its much harder to put a number on the climate change impacts of a new desal facility versus the impacts of exporting more water from the delta. To go all the way from source to user and calculate not just the financial impacts to users but all the other impacts it may have – whether it’s fisheries or the communities involved – we just don’t do that very well and it’s hard to do. There are a lot of value judgments you have to make and a lot of potential sources of data.

Water Deeply: You highlight seawater intakes as a crucial issue. Is this a dealbreaker for many desal projects?

Szeptycki: The best way of handling it, and this is the way state policy is going, is to have subsurface intakes and that deals with the problem. The state policy really pushes people in that direction. The problem is, that really limits the size of your facility. You have to build bigger pump stations for subsurface intakes, so the terrestrial footprint of something like the Carlsbad facility would just be huge if you relied on subsurface intakes. It turns out it’s very, very hard to build a big plant with subsurface intakes.

Open-ocean intakes are the other option. And the state has a mitigation policy for open-ocean intakes. You have to replace habitat according to the amount of mortality (of fish and larval fish), and there’s a formula for that. But some people are saying we don’t really understand the effect of mortality. We know how many larvae are being killed, but we don’t really know how that translates to adult species.

I think one positive thing that’s developing now and into the future is a lot of these power plants with intakes in the ocean for their cooling water are being retired. And they have much larger water requirements than desal facilities. So even if desal were to increase over the next 15 to 30 years, the overall entrainment (of fish) by ocean water intakes is going to go down as these big power plants get retired.

I think a lot of the environmental concerns are very site specific. There’s a concern that there hasn’t been enough long-term monitoring of environments like these.

Water Deeply: What has driven the location choices so far? Is it primarily economic?

Szeptycki: In some cases, it’s economic concerns. In some cases, it’s something else. The current plants and planned plants are cited kind of opportunistically to be near some abandoned-but-useful industrial facility, or co-located with a power plant.

For example, the Carlsbad plant is co-located with a power plant to take advantage of the intakes at the power plant and to combine the brine discharge with the cooling water from the power plant. That was very strategic and designed to not add to the impacts of the intakes, and take advantage of the salty water already being discharged from the power plant. You mix that with the brine and you get an immediate dilution. It takes advantage of an existing facility to minimize the impacts of both the intake and the discharge.

Water Deeply: You found certain criteria make desal more feasible. What are they?

Szeptycki: We tried to identify a suite of attributes where you could say “that’s really sustainable desal.” That would include a smaller facility driven by local demand, powered by some percentage of renewable energy and, because they are smaller, able to rely on subsurface intakes.

There was a preference for subsurface seawater intakes, meaning underground, because of the impact of open-ocean intakes, where entrainment of sea life has to be mitigated. Under the new state policy, if you have subsurface intakes, there’s no requirement to mitigate those impacts.

The state itself or NGOs could try to develop policies to create a comprehensive vision for desal and come up with criteria for truly sustainable desal. That was one idea we talked about that needs further work.

Something that’s a great idea that the Nature Conservancy is currently working on is developing mapping technology to identify places where desal would be likely to have really minimal impacts on local communities and on marine and coastal resources. They’re undertaking a more comprehensive look at the whole coast to try to identify places that would be good spots for desal plants places where the nature of the marine environment, combined with the nature of local communities, could really minimize the impacts.

Water Deeply: Is it really feasible to power desalination entirely with renewable energy?

Szeptycki: The short answer is yes. The only question is whether that would shift other users away from renewables.

If a desal facility engaged in direct purchase agreements with a big solar plant, that solar is not going to be available for other people. I think there’s some of that going on in Perth (Australia), where a large portion of the desal is being run by renewables. That means the rest of the grid in Perth is not getting that renewable energy. As the overall grid shifts more to renewables, the climate change impacts of desal will certainly go down. But it’s a net positive thing, when you’re building a desal facility, to look at how you can provide it with some renewable power.

Water Deeply: What did you all decide is needed next?

Szeptycki: One thing is better public education and outreach just so people in California can understand both the potential footprint and also desal’s limitations, and fight this reflex that we can just desalinate our way out of this water problem, which is not true.

Also coming up with criteria to find places where, based on good science, we understand the effect of both seawater intakes and brine disposal will be minimal. Also integrated mapping and analysis to identify the least impactful places to locate desal facilities.

Another issue is finding better ways to compare the true costs of water so you can put desal on the table with imported surface water, or water conservation and water recycling, and really have a better understanding of which one has the fewest impacts and is the best choice for the community.

There is a lot of research going on to improve desal. There are some more recent desal facilities, like the ones built in Perth that have much more advanced brine disposal and controls on intakes. So there’s a need to look at the long-term monitoring results to make sure they are working the way they are intended and not having unforeseen impacts. There are places both in and outside of California where there is going to be several years of data that will help better understand how well the current menu of technologies is working to protect the ocean.

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