Methane Fermentation
A. M. BUSWELL, Consultant
Gainesville, Florida
INTRODUCTION
Anaerobic digestion as applied to waste treatment appears to date from the
Austin tank described by Metcalf (1) as first constructed in 1857. About the same
time (1860) Louis H. Mouras, a French engineer, built the "Mouras automatic
scavenger." Liquification of organic solids was the recognized effect of the action in these tanks and this was early attributed to anaerobic organisms by bacteriologists of the time.
Separation of sedimentation and digestion by means of two story tanks was
the next development. The Travis tank in England and the Imhoff tank built in
Germany about 1907 were the most successful examples. The Imhoff tank found
wide use in the U. S. largely because of its suitability to the hot summer climate.
The baffled separation of the lower or digestion compartments aided in keeping
septic liquor from mixing with the effluent. By the early 1920's the separate
digestion tank began to rise in favor. The cost of the separate structure was found
to be justified by the improved operation control which was possible, especially
when provided with Downs floating covers.
In the meantime, Imhoff and co-workers in Germany, and O'Schaugnessy in
England had developed much of the basic chemistry and bacteriology required for
starting and operating anaerobic sludge digestion in either two story or separate
tanks. The importance of thorough mixing of a considerable amount of well digested sludge with the incoming raw sludge for inoculation was well known and
loading limits in terms of weight per volume per time unit had been worked out
empirically to give practical operating conditions. Chemically it was known that
most organic substances found in domestic sewage were readily susceptible to
anaerobic fermentation,   lignin and hydrocarbons being the principal exceptions.
The chemical course of the process was supposed to involve a series of hy-
drolytic reactions resulting in the production of organic acids which were later
converted to CH4 and CO2 with some Ho. The varying amounts of N2 were assumed to be present in the raw sewage as dissolved air. Organic or proteinaceous
N2 was hydrolyzed to NH3. The fact that a considerable amount of the organic
matter was completely removed as gas, not known to Austin and Mouras, was an
attractive characteristic of the process. In the digestion the complex gelatinous
substances lost almost completely their water binding properties so that when
drawn they drained rapidly and dried to a granular consistency. Odorous compounds were greatly reduced in amount and dissolved in the liquor so that the
drained sludge had an inoffensive earthy odor and when rewetted did not imbibe
water nor become slimy nor odorous.
The lower fatty acids were early observed in the liquor and were found to
include acetic, propionic, and butyric acids, acetic being in preponderance.
Naturally these acids, especially acetic, were considered to be the precursors of
CH4 and CO2. The fermentation was considered as occurring in two stages; the
primary stage or acid stage, and the secondary or methane stage, sometimes
called the alkaline stage. The alkaline phase was so called from the fact that
the conversion of acetic acid (CH3COOH) to methane and carbon dioxide (CH4
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