AskPablo: Desalination and the Water-Energy Relationship
This 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 PĂ¤ster, MBA