INTRODUCTION Anaerobic biotransformations have been occurring naturally worldwide for millennia. Found in such diverse places as termite, rumen and human intestines, rice paddies and hot springs, heartwood of certain trees and river sediments, and swamps and deep ocean trenches,
the ancient methanogenic bacterial consortia have been continually carrying on their beneficial digestive process since the beginning of time. Adapting the complex characteristics of these bacteria by engineers to promote the controlled metabolism of wastewaters, however, has historically
been slow. Significant obstacles have prevented the easy harnessing of anaerobic features to wastewater treatment. Possibly the greatest obstacle may have been in apprehending the uniqueness of the anaerobic digestion process. The considerable differences between aerobic and anaerobic technologies
when applied to metabolizing acetate and propionate have had to be identified and accepted before progress could be made. In addition to complexities such as the critical need for close microbial proximity of the consortium in the final steps, one of the main barriers to successful applications
has been the mindset of professionals, which adheres to aerobic activated sludge process technology to the exclusion of envisioning different approaches when using anaerobic digestion. Even though many aerobic principles are not relevant, nevertheless aerobic concepts have not been easy to
set aside by many. It may be recalled that in the early 1960's, acetate as the sole substrate was deemed impossible to be metabolized anaerobically and only a few microbiologists could pure culture methanogens on acetate. Encouraging new research is now illuminating the enigmas of propionate
metabolism. In the same way that methanogens have been shown to be capable of metabolizing acetate (against theoretical predictions in the literature) so propionate is becoming amenable to higher rates of metabolism by the amazing anaerobic consortia.
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