Friday, October 5, 2012

Reverse Osmosis: Most energy Efficient Desalination Process?

First developed during the heyday of NASA’s Apollo program, is reverse osmosis still the most energy efficient desalination process we have so far?

By: Ringo Bones

Back in the heyday of the Apollo program, reverse osmosis – due to lack of an efficient polymer filtering membrane – can only be able to desalinate or purify human urine into fresh drinkable water. After a few decades of development, polymer chemists had finally been able to develop a reverse osmosis membrane that can actually be able to turn the full-on salinity of sea water into potable fresh drinking water. Not only that, reverse osmosis has since more or less became the most energy efficient way to desalinate sea water for drinking purposes – dethroning its previously most energy-efficient desalination method called low-pressure flash distillation process.

A typical reverse osmosis desalination membrane – usually there are banks of them – turns salt water into fresh water when salty sea water is pressurized through it at 1,000 pounds per square inch. Only the smaller molecules of water can go through the structure of the “filtering fabric” in a typical reverse osmosis membrane while the larger molecules of sodium chloride and other salts are left behind. And what makes a typical reverse osmosis plant more efficient that its predecessors is that the highly pressurized salt water and used briny effluent can be reused to run an electric turbine en route to its release back into the normal prevailing atmospheric pressure.

Despite of energy efficiency figures, it still costs 17 million US dollars annually to run a typical large scale reverse osmosis plant that has the capacity to turn enough sea water to fresh drinking water to supply a typical metropolis – 10 million US dollars of which pays for the yearly electric bill. And compared to other sources of tap water, a reverse osmosis desalinated tap water typically costs around 3.38 US dollars per 1,000 gallons. While a river or lake sourced treated tap water costs around 2 US dollars per 1,000 gallons while subsurface groundwater sourced treated tap water costs around 1 US dollars per 1,000 gallons – something to think about when you decide which water utility company you chose to supply your household needs. 

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