echanais Fermne
n A. M. BUSWELL and H. F. MUELLEPI INZZRQZS S t a t e W a f e rSurvog, Urbana, IRL
T h i s paper discusses the chemical mechanisms b3 which methane may be produced by bacteria in nature. The early history and bacteriology are briefly reviewed. Jlethane formation has been found to occur both with and without the reduction of carbon dioxide. Hydrocarbons, ethers, and lignin have not been found to be fermentable. With these exceptions the fermentation appears to proceed smoothly and continuously regardless of the chemical composition of the substrate. Complete conversion of the
With a little care, it is possible to get 95 to 100% yields calculated from this equation.
ETHANE formation in nature has interested bacteriologists and chemists for more than sixty years. The studies in this field have progressed to a point where sufficient methane for fuel for power plants of several thousand horsepower is produced by anaerobic fermentation. The literature in t,his field was summarized some years ago b>McBeth and Scales ( 1 3 ) and by Stephenson (59). Briefly, it xms known that when moist organic matter \\-as allowed to dccompose under restricted oxygen conditions it yielded hydrogen, carbon dioxide, methane, and a variety of organic acids in greater or less amounts. Little or no quantitative data o n the yield of the various products were available, and little had been done with pure compounds although Sohngen (23) had shown that lovcer fatty acids with an even number of carbon atoms could he decomposed by mixed cultures, giving methane and carbon dioxide. Whether hydrogen and fatty acids were necessary intermediates in the process of methane formation was not known. The information on the susceptibility of various natural products to methane fermentation was conflicting. Cellulose vas known to be fermented to some extent (22, 65) but so-called lignocellulose was said to be resistant (16). One author held that grease nould decompose anaerobically (24j, while another held that it \?-odd not ferment to any substantial extent (33). The addition of lime to favor methane production was preferred hy one group of workers and opposed by another. Of the earlier systematic studies of this fermentation, those of Sohngen were the most extensive (29). More recent studies are those of Fowler and Joshi (15),Sen, Pal, and Ghosh ( U ) ,Fischer, Lieske, and Winzer (141, and Buswell et nl. (8). Engineering design and loading factors have been discussed previously ( 7 ) .
to be environment rather than flora which determines the result obtained. Continuous Process. This situation makes possible the colitinuous operation of the fermentation, a procedure which i p at, least very unusual in fermentology. It is possible t o carry on this process in apparatus arranged to allow the subst,rate to enter continuously at one point and the exhausted or inert residue t,o be discharged continuously at, another, while the products, methane and carbon dioxide, are given off at a steady rate. There is apparently n o limit to the size of apparatus which can be used. Large tanks yielding several hundred thousand cubic feet of gas a day operate as snioothly as laboratory-size flasks. Independent of Substrate. A third characterigtic of thw(1 fermentations is t'hat practically any sort or kind of organic matter may be used as a substrate. Nearly 100 different pur(' substances ( 1 , 3 4 , S b ) and some 30 or 40 natural plant and animal products ( 4 , 5 , Q), such as cornstalks, milk whey, etc., have been used successfully as fermentation material. There is apparently no decomposition of hydrocarbons, ethers are not fermented, and lignin, when isolated, is attacked not a t all or with difficulty ( 6 , 18, 21). Quantitative Yields. The nearly quantitative yields of the two simple products carbon dioxide and methane are somewhat unique. It is true that in the various commercial fermentat,ions of grains the starch is practically quantitatively recovered in the products, but, the fats, proteins, and fiber are not attacked a t all. The methane fermentation converts the entire grain, with the possible exception of a small amount of fiber, to carbon dioxide and methane within 24 to 7 2 hours. The reaction is of t,he oxidation-reduction type involving water, represented by the empirical equation
Characteristics of MothaKe Fernmenitation Mixed Culture. The anaerobic fermcntations as carried out for the production of methane differ in many respects from other types of fermentations. The most important difference is perhaps the fact that it is not necessary t o use a pure culture of organisms in order to obtain uniform results, nor is it necessary to maintain purified cultures for inoculation or reinoculation. The bacteria which are capable of producing methane are found almost universally in nature, although in preponderating numbers in black mud and decaying vegetable matter. Under proper conditions these bacteria can be cultivated to a high degree of activity within a few days. The culture can then be maintained a t this high degree of activity indefinitely, providing a few simple rules concerning chemical and physical environment are followed. In fact, microscopic and subculture studies of this ferment'at>ion reveal such a mixed and variable flora that it appears a t present, 550
substrate to the two simple products, methane and carbon dioxide, is a unique characteristic of the process. The reaction is an oxidation-reduction one involving water, represented by the empirical equation
Cn&Ob
CnH,Ob
+ ( n - a/4 - b/2)H?O + (n/2 - a/8 f b / 4 ) C O z + ( n / 2 + u / 8 - b/4)CHI
+ ( n - a / 4 - b/2)H20 ( n / 2 - a/8 + b/4)C02 + ( n / 2 + a/8 ---f
- h/4)CH4
With a little cme, it is possible to get 95 to 100% yields calculated from this e q u t i o n . The uniformity of the end products, carbon dioxide and methane, was a t first puzzling. A study of the energy (34) of the reaction of various compounds with water showed the maximum free energy when the final products were carbon dioxide and methane. In experience in this laboratory, neither carbon monoxide nor higher hydrocarbons havc bcen observed among the products. A few earlier reports in the literature stating that ethane was formed have not been verified and the amounts reported are within the limits of analytical error in the experiments referred to. In more recent investigations Grosse and Libby (16) found the gas from a sludge digestion tank to contain methane free from
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 3
-LZquZd Indus&Zal Wasteshigher hydrocarbons within the limits detectable by a mass spectrograph. Table I, furnished by Grosse but not included in the abstract referred to, shows a comparison of the composition of petroleum methane and sewage methane.
Table I. Composition of Baltimore Sewage Gas after Purification and Comparison with Petroleum Methane Constituent Methane Ethane Propylene Nitrogen Carbon dioxide Air
Petroleum Methane, VOl. % 98 0 0 0 0 0
6 1 7 4
0 2
Sewage Methane, Vol. % 99 2
