AskPablo: Desalination and the Water-Energy Relationship

desal.jpgThis week Gary writes “my city is considering a desalination plant to provide drinking water. I have heard that desalination is very energy intensive. How does it compare to delivering water by pipe?” Luckily I recently came across a report that will help me answer this question.

The average American uses 70 gallons of water per day in toilets, showers, sinks, and appliances. Industrial and agricultural water use increase this number several fold. While many people in parts of Africa live with less than a gallon per day we use treated drinking water as a solid waste conveyance from our toilets to the wastewater treatment facility. Many communities have begun using reclaimed “graywater” to irrigate public green spaces and some communities are beginning to allow domestic graywater use for flushing toilets. While we have a long way to go to a dual-sewer system for graywater and blackwater these are all steps in the right direction.
According to Richard Atwater from the Inland Empire Utilities Agency (IEUA), citing a 2005 California Energy Commission report, 19% of California’s electricity use, and 32% of its natural gas use is for pumping water and wastewater! Since reclaimed water does not travel very far it is quite efficient at 500 kWh/Acre-foot (0.405 Wh/liter).
Since a great deal of Northern California’s water comes from the Sierras, it is conveyed mostly by gravity, even generating some electricity on the way. But the California Water Project’s aqueducts and pipes that deliver Southern California’s water require multiple pumping stations and use 3,000 kWh/Acre-foot (2.432 Wh/liter). At the per capita water use of 70 gallons per day (265 liter), this requires 0.65 kWh per person, per day, or 237 kWh per year. This results in 121 kg of CO2 emissions and does not take into account per capita industrial and agricultural water use.
So, what is sillier than piping water 1000 miles to keep the fountains at Disneyland flowing? Well, desalination, of course… Desalination uses even more energy than piping water from Northern to Southern California, 4,000 kWh/Acre-foot (3.24 Wh/liter). This is because desalination either uses reverse-osmosis filtration, essentially filtering ocean water by pumping water, at high pressure, through a membrane, or distillation, where water is evaporated and condensed. Both of these methods require a lot of energy for pumping and/or heating the water.
But innovative solutions are under development. A new concept desalination plant utilizes ocean breezes and cold ocean water. It does this by piping cold ocean water through pipes. The water is then sprayed on a screen where some of it is picked up by the breeze in the form of humidity. The humid air then passes over the cooled pipes, where the water condenses and is collected, salt free.
This shows that stepping away from the post-industrial revolution thinking and observing the way plants and animals survive in arid Mediterranean climates by using natural condensation can help address some of our biggest problems. Life on this planet has been evolving and learning from its mistakes over millions of years. What makes us think that we can out-engineer a natural system with brute force, toxic materials, and abundant waste?
Pablo Päster, MBA
Sustainability Engineer

8 responses

  1. To make the process less energy intensive, I’m imagining geothermal + large solar concentrators to heat the water, recapturing the heat, and optimizing pumps for maximum efficiency and powering them with wind.

  2. To make the process less energy intensive, I’m imagining geothermal + large solar concentrators to heat the water, recapturing the heat, and optimizing pumps for maximum efficiency and powering them with wind.

  3. I have precisely zero sympathy for people in places like Phoenix, El Paso, or even Southern California who complain about water supplies. If you live in an overpopulated desert suburb you need to alter your lifestyle to atone for that fact. Golf courses, lawns, and a care-free attitude toward water are absurdities in those climates and people need to accept that. Frankly, I think the best way to make that happen, perhaps the only way, is to STOP subsidizing the price of water in desert climes. Right now massive government projects had delivered unrealistically cheap water. Triple or quadruple the price of the stuff and you will start to see behavioral change that are needed…

  4. Desalination is indeed energy-intensive, but it definitely has its place in the world.
    The fact is that water supplies are being stretched to their limits, prompting communities to look at less and less attractive source waters to fulfill their needs. In Australia, people will soon be drinking recycled wastewater, and in El Paso, Texas, the water district has begun desalinating brackish well water. The shell and tube condenser idea is novel, especially the way it’s employed in Southern India, but it’s not applicable in colder climes such as the Bay Area. Reverse osmosis membrane technology is ~50 years old, and there are essentially just a few ways to desalinate water. Instead of knocking traditional desalination as being too energy intensive, why not concentrate on using innovative, renewable energy sources to power the pumps and recover as much energy as possible within the system to both save energy and increase the overall recovery of clean water through the system.

  5. It appears that the siting of a desalinization facility will determine which of these excellent plans will produce the energy to remove salt from ocean water. Wind power might be the choice at a site with a constant sea breeze. Or the solution might be geo-thermal, or wave generators, solar, tidal, or some other energy source at sites with other characteristics.
    Will there be desalinization siting firms who can do the cost/benefit analysis of several completely different energy systems systems and find the cheapest or most robust or select by some other criteria?

  6. I've been studying California's water system and concluded that it requires over 3000 kWh/acre-foot just to lift water from the Central Valley to the reservoirs in the hills above the L.A. basin, PLUS the energy required to move that water sideways 400+ miles from the pumping stations near Tracy to the uphill pumps. This puts the total energy required on par with Pablo's figure for desalination, which is high compared to some of the latest desalination plants. So desalination uses less energy than the current water system and offers a potentially unlimited water source with no susceptibility to drought or supply/demand pressures, and at competitive prices for urban users. Conservation and water recycling are obviously desirable, but if we want an unlimited water source there are trillions of gallons sitting off the California coast just waiting to be tapped. All the fuss over water here represents a lack of will and vision, not an actual lack of water.

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