By Carl Linvill, John Candelaria and Catherine Elder
Aspen Environmental Group
February 2013
by S. Kawazoe (Tempes) and J. Combs (Geo Hills Associates)
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by L.Y. Bronicki
Encyclopedia of Physical Science & Technology - December 2003
by R. Sanchez Velasco, M. Lopez-Diaz, H. Mendoza, R. Tello-Hinojosa (CFE, Morelia, Mexico) 
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by J. Makower, R. Pernick, C. Wilder
Clean Edge - March 2008