5 Things to Know About Water Desalination

Photo credit: Desalination Pipeline Nuweiba by prilfish

1. It’s growing in popularity

Worldwide use of water desalination has been growing in the past years. Countries in the Middle East (such as Saudi Arabia and Israel) have made impressive advances and significant investments in desalination technology in the last decade. Israel opened its first large-scale desalination plant in 2005. Ten years later,  desalination plants produce 40% of the country’s water supply.

In the U.S., the largest desalination plant came online in Carlsbad, San Diego County this past December— along with a billion-dollar price tag. Although smaller plants have been operating since the 1970s, this is the first plant at this scale that is projected to produce enough potable water to meet 8-10% of San Diego County’s water demands. It will supply 25 times more water than all the other small-scale desalination plants in California combined.  Poseidon Water, the same company that developed the Carlsbad plant, is proposing a similarly-sized plant in a location in Huntington Beach, though there has been significant pushback.

450px-Reverse_osmosis_desalination_plant
Reverse osmosis desalination plant in Barcelona, Spain by James Grellier

2. It’s expensive, though increasingly less so

One reason for why desalination was not widely implemented in the past is that it was prohibitively expensive. It is still expensive, but technological advances and the fact that other alternative potable water sources have become more scarce (and therefore driven up prices) is making desalination more attractive for certain places. The water produced by the Carlsbad plant costs about $2,000 per acre foot, which is twice as expensive as other alternative sources, such as the imported water the county currently buys from the Metropolitan Water District of Southern California. However, the investment is becoming increasingly attractive to locations that have seemingly run out of options and want a more reliable local source of water.

3. It’s energy intensive

The main reason for why desalination is so expensive is that it requires a lot of energy. The most common method of desalination in newer plants is that of reverse-osmosis—where salt water is put through filters at great  pressures to filter out the salt. This process is energy-intensive, more so than any other type of water treatment process.  The Carlsbad plant will use about 38 MW a day to produce about 50 million gallons of water.  As USC Professor of Environmental Studies Lisa Collins mentioned in our April Bedrosian Bookclub Podcast, adding to the energy used filtering the water through reverse osmosis is the energy required to pump water from sea level to a desalination plant.

4. It can have harmful environmental effects

Part of the problem of the process being energy-intensive is that, if the energy used is not renewable, desalination becomes a significant contributor of greenhouse gases. We then enter into a vicious cycle: we resort to these mechanisms because water is becoming scarcer, in part because of climate change, so we desalinate it generating greenhouse gases that contribute to that climate change. Of course, an obvious solution to this quandary is to advance renewable energy technology so that solar or other renewable energy can be used to desalinate water.  In the long-run, this should lower both the cost and the negative environmental impacts of desalination.

Other than energy usage, there are other environmental concerns related to water desalination. The main two concerns are that drawing water from the ocean could be harmful to marine wildlife, and that it produces heavy saline brine that needs to be properly disposed of as it can harm wildlife if it is released back into the ocean as-is. The first concern has to do with the intakes that draw the water out of the ocean and into the treatment plants. Although nets and filters are usually used, these filters don’t normally catch small fish, fish eggs, larvae, or plankton. These organisms are drawn in by the water intakes and killed as the water is further filtered and treated.

As for the second concern: for every gallon of fresh water that is produced, two gallons of seawater are used and a gallon of doubly salty water (or “brine”) is produced. If that brine is dumped back into the ocean, it doesn’t mix well with the less saline seawater and can have harmful effects on marine life and the coastal habitat.

5. Location, location, location

Both the plant location as well the different methods used to both take in ocean water and dispose of the brine are very important. In Huntington Beach, the California Coastal Commission has been pushing for the proposed plant to be built with a sub-surface intake. With this type of intake, water is drawn from below the sea floor at a slower rate and sand works as a natural filter that prevents organisms from being sucked into the treatment plant and killed. However, findings from a report published by a panel of experts show that it might make the project too expensive in the proposed location—to the point it would be unfeasible. As previously mentioned, desalination is generally already considered an expensive option as it is.

Desalination facility by Jon Wiley https://flic.kr/p/5Xpqkd
Desalination facility by Jon Wiley

Another important consideration is how to dispose of the saline waste product or “brine” so that it dissolves in the ocean, or how to dispose of it somewhere else where it can’t contaminate other ecosystems or freshwater sources. A method used by the Carlsbad plant is mixing water that an adjacent power plant uses as coolant with the brine—a process that makes the resulting water only about 20% more saline than normal seawater. However, going forward, power plants in California are not going to be allowed to draw seawater to cool their plants due to the same sort of environmental concerns raised by desalination plant intakes. This means that, in order to keep mixing seawater in, the Carlsbad plant would need to take in even more water which would exacerbate the problems caused by the water intakes.

Because desalination is so expensive and could have serious harmful effects on the environment, it should only be undertaken in places where there are no other feasible alternatives that are more environmentally and economically sustainable. Significant planning needs to happen before large-scale plants are built to ensure that the best methods to prevent environmental damages are used for that location. In many places, there are still cheaper and more sustainable approaches to be explored and invested in, such as storm water capture, wastewater recycling, conservation, etc.

 

If you are interested in issues of water management, don’t forget to check out our April Bedrosian Bookclub Podcast. This edition features The Water Knife by Paolo Bacigalupi, a post-apocalyptic novel that depicts a U.S. Southwest decimated by climate change—in particular by water scarcity. Don’t miss it!

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