DESALINATION PLANT

December 5, 2009

We have over 1100 people here in town.  Over 55,000 gallons of water is used daily for drinking, cooking, laundry, dishwashing, showers, etc.   The site manager has asked everyone to conserve as much as possible, but I was curious – how do they provide us with so much water?

So, I recently took a tour of the water plant at McMurdo.  We get our water from the ocean right here in McMurdo Sound.   To make it drinkable the salt, along with other impurities must be removed.  I will try to describe the process.

The seawater comes into the plant at 28 degrees.  If the salt were removed at that temperature the water would freeze.  So, the first step is to heat the water to above freezing.  I think they do that with a heat exchanger.  The water (temperature around 36 degrees) then goes into an 18,000 gallon storage tank.

The first step in the filtering process is a multimedia (or sand) filter.  These are tanks with layers of anthracite coal, sand, garnet and gravel.  The water then passes through a series of filters, the last of which is 5 microns.  At this point, none of the salt has been removed, just the larger stuff.  To remove the salt the water has to be pumped through reverse osmosis filters under 850 psi (pounds per square inch) of pressure.  After seeing and feeling the filters, it is easy to see why such a high pressure is needed to force the water through.  The filters are tubular, about 5 inches in diameter and 3 feet long.  Each molecule of water must pass through 24 separate membranes.  Surprisingly, the filters last about 5 years.  The “brine water” is returned to the ocean.

The “pure water” that remains is somewhat acidic.  So, they run it through a tank containing calcium carbonate crystals, which reduces the acidity.  The final step is to add small amounts of chlorine and soda ash to the water (to disinfect and re-adjust the ph level).   We were told that the water in McMurdo Sound is unusually clean and pure, so very little chlorine needs to be added.  A 5 gallon bucket of chlorine lasts them 3 months.  (Sometimes, it smells like the town of Berthoud goes through that much chlorine per hour).  The water is now safe for drinking and is stored in four 54,000 gallon tanks.

Writing this made me wonder:  How do they provide the South Pole with water?  The answer:  they have a special well, called a Rodriquez Well, that melts snow & ice with hot water.  The well is currently melting snow & ice from more than 400 feet below the surface, which has been there since around 500 A.D.  The residents on the South Pole are restricted to two minute showers, twice a week!  For an interesting article about the South Pole’s water situation go to the following article:

http://antarcticsun.usap.gov/features/contentHandler.cfm?id=1205

The light green tanks are the sand filters.

The white tubes contain the reverse osmosis filters – removing the salt.

One of four 54,000 gallon holding tanks. Mike & plant operator are on the left.