Studies on the anaerobic decomposition of plant materials

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Abstract
I. Weighed amts. of rice straw chaff, packed into bottles of known vol. with specified amts. of mineral nutrients, soil inoculant and distilled water, were anaerobically incubated at the required temp. until (usually at stoppage of gas evolution) ready for analysis. About 60% of the hemicelluloses, 45% of cellulose and 25-30% of lignin are decomposed, the ultimate loss of ash-free dry matter being ca. 40-45%. Acetic and butyric acids, CO2, and CH4 are the main products obtained. From 100 gm. of straw, either ca. 20 gm. of organic acids or over 20 liters of gases containing about 50% CH4, equivalent to ca. 7200 cu. ft. of gas per ton of straw, are obtainable. The decomposition appears to proceed in 2 stages, the 1st involving the formation of organic acids and the 2d, the conversion of these into gaseous products. A slightly alkaline reaction (ca. pH 7.5), found opt. for the 2d stage, could best be secured by addition of (NH4)2CO3 in sufficient amt. to neutralize the acids formed. In contrast with aerobic decomposition, the N requirement for anaerobic decomposition is very low, 100 parts of straw requiring ca. 0.1 part or less.[long dash]II. For the anaerobic digestion of rice straw by mesophilic organisms, a temp. range of 30-35[degree] is found to be opt. The pH of the medium exerts a controlling influence on the course of the fermentation. The preliminary stage of acid formation is more tolerant to changes of reaction but gas formation is greatly impeded outside the range of pH 7.5-8. The water requirements for anaerobic digestion are much higher than for aerobic decomposition. A proportion of straw to water of 1:10 is found opt. for the former. The organisms required to decompose the straw are present on the straw itself and further inoculation is unnecessary and does not shorten the preliminary lag period. The P requirement of anaerobic digestion is satisfied by the amt. contained in the straw itself (0.42%) ; additions of phosphate have no appreciable stimulating effect. Excess of phosphate actually depresses the rate of decomposition. Finely ground straw does not decompose quicker than chaffed straw; the rate of gas production, is in fact slower. Of the different reagents tried for neutralising the acids formed during anaerobic digestion, NH4HCO3 and KHCO3 are the best; they could be used singly or better still in admixture. It is possible also to control the reaction of the medium by additions of alkali or acid from time to time. Urea or cyanamide can replace NH4HCO3, proteins cannot. The amt. of CO2 and CH4 produced is approximately the same irrespective of the vol. of the free space above the culture medium, provided the accumulated gases are removed from time to time so as not to produce high conc. of CO2 or lower the pH of the medium.

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