Laboratory studies of sludge digestion. - Illinois State Water Survey

have shown the reaction to be thermodynamically possible, it having
a negative free energy change at 25° of 11,582 calories. On the other
hand, there are bacteria in soils and muds which can assimilate hydrogen
without methane production, 169 and we have found that only fresh,
fairly actively fermenting sludges show hydrogen absorption to any
extent:
It seems rather probable that hydrogen-utilizing organisms are
present in fresh sludge, but die off in more specialized laboratory cultures
and are absent in the highly specialized Sohngen cultures (built
up by decantation as described later). However, regardless of whether
hydrogen is absorbed by hydrogen acceptors, by union with CO2 or
by bacterial assimilation, its absence in sludge gases does not rule out
the possibility of beta-oxidation in the degradation of sewage grease.
The increasingly unsaturated nature of sewage grease, as shown
by the higher iodine number, during digestion may indicate dehydrogenation
of the carbon chains, or it may result from preferential digestion
of the saturated chains. At present then, we can only say that
while the absence of hydrogen in digestion gases does not prove the
absence of beta-oxidation in fat degradation in sludge digestion tanks,
another mechanism is not only possible but is proved to apply to methane
production from the lower fatty acids.
Lower Fatty Acids
Historical. Turning now to the question of lower fatty acid degradation,
more pertinent information is available. The pioneer work on
this subject is credited to Hoppe-Seyler 94 who obtained methane from
calcium acetate. Into a 600 cc. flask he put 50 cc. of river mud and
then filled the flask with river water containing 9.7677 grams of anhydrous
calcium acetate. The flask was closed by a rubber stopper,
surrounded externally by a mercury seal, and carrying a gas-delivery
tube leading to a mercury-filled reservoir; the gases were withdrawn
and analyzed at intervals. No hydrogen was found and only a little
nitrogen during the early part of the digestion; the ratio of CO2 :CH4
then became constant at 1:2, indicating the reaction Ca(C2H3O2)2 +
H2O=CaCO3 + CO2 + 2CH4. Determinations of the sludge solids
were made in triplicate on the initial sludge, but only once on the final
contents:
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