Many Archaea are capable of growing in extreme environments, such as high temperatures (up to 90°C), high ionic strength, and highly reduced conditions. Consequently, members of this domain were first thought to be restricted to growth in such environments, although that proved to be incorrect. More recent studies have shown that Archaea are abundantly distributed in a wide variety of environments."1 As our knowledge of the Archaea expands it is likely that wastewater treatment engineers will find more applications for them. Currently, however, their major use in biological wastewater treatment is in anaerobic operations, where they play the important role of producing methane. Methane-producing Archaea, commonly called methanogens, are obligate anaerobes that bring about the removal of organic matter from the liquid phase by producing an energy rich gas of low solubility. This allows capture of the energy in the pollutants in a useful form. Because methanogens are very limited in the substrates they can use, they grow in complex microbial communities with Bacteria, which carry out the initial attack on the pollutants and release the methanogens' substrates as fermentation products.
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