What specifically makes oceanic desalination so expensive?
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1$\begingroup$ Welcome to chemistry.SE! If you had any questions about the policies of our community, please visit the help center. $\endgroup$– M.A.R.Jul 19, 2015 at 21:07
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2$\begingroup$ The biggest expense is not necessarily technology related, but rather policy. US Coastal Commissions, legal hearings and environmental impact studies consume a large portion of the budget to get to a completed working facility. $\endgroup$– docscienceJul 22, 2015 at 17:26
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1$\begingroup$ @docscience, what is your evidence to support that claim? $\endgroup$– Lora WesleyJul 22, 2015 at 19:28
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2$\begingroup$ I live in Carlsbad, Ca: home of the first desalination plant in California. And this project has been delayed for almost 15 years due to approvals. Time is money. $\endgroup$– docscienceJul 22, 2015 at 21:03
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3 Answers
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Reason is salt dissolve very easily with water, forming very strong bond. Not only salt there are also other minerals. To break this bond we need technology as well as energy and both of them are very expensive(today).
There are two methods by which we can break this bond:
- Thermal distillation: Thermal distillation involves heat: Boiling water turns it into vapor—leaving the salt behind—that is collected and condensed back into water by cooling it down
- Membrane distillation: The most common type of membrane separation is called reverse osmosis. Seawater is forced through a semipermeable membrane that separates salt from water. Because the technology typically requires less energy than thermal distillation, most new plants, now use reverse osmosis.
Today about 10 to 13 billion gallon water are desalinated worldwide per day. That's only about 0.2% of global water consumption.
Membrane distillation is bit more cheaper than thermal distillation. Even though large amounts of energy are needed to generate the high pressure that forces the water through the filter. Current methods require about 14 kilowatt-hours of energy to produce 1,000 gallons of desalinated seawater.
So we can conclude that cost of distillation is high because we need notable large amount of electricity to heat water in thermal plant and generate high pressure. Also other technology(like say burner, pumps, etc) are also expensive.
Source: ScientificAmerica and LiveScience
Also note we are trying to decrease the cost of distillation and day by day number of seawater distillation are increasing.
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$\begingroup$ What I meant by "specifically" was something like "electricity" or "replacement filters," etc. Is that what you're saying? That is, electricity for boiling and cost of filter replacement? Oh, and you misspelled "Scientific." $\endgroup$ Jul 22, 2015 at 19:32
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$\begingroup$ I have edited question and tried my best to be specific. $\endgroup$– FreddyJul 23, 2015 at 3:37
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Seawater is very corrosive so the materials of construction to resist corrosion are very expensive, this is amplified for thermal / distillation methods. Basically nickel alloys and titanium; and Ti has high fabrication expenses. Polymers have good corrosion resistance but have special fabrication requirements and lack robustness for a unit that will have a life measured in decades.
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To answer this question we must consider the huge amount of water consumed. And that the average seawater has $\pu{30g}$ of salt per $\pu{1000g}$ of water; along with the additional $\pu{60g}$ of impurities, sand, mud, dirt, etc.
Reverse Osmosis This is probably the same technology that you use to filter your water. Unless you use carbon filters that work by passing water through carbon candles and filtering it. If you however have an RO filter, then you may have realized the water passing back through a tube in to your sink, or the water that you may think "bad". If you switch on the filter faucet to take some water you may have realized how slowly it drips, this is because $90~\%$ of the water is being sent out through the reject stream.
A desalination plant works just like this but with a filter nearly 100 times bigger than yours. This entire processes take about 35 million dollars a day!
