Carbon balance in high rate dissimilation of acetic acid in semicontinuous methane fermentation

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Abstract
The nature of anaerobic methane fermentation has been studied extensively over a period of about three years by means of 10‐l. semicontinuous fermenters, with ethyl alcohol and calcium acetate as carbon and energy sources. The inoculum for the two systems was isolated by enrichment techniques from primary sewage sludge and used to initiate the processes. No subsequent seeding was necessary. The fermentation was started with a substrate carbon concentration of 50 mmole/l. which was gradually increased to 400 mmole/l.; this means that an increase of substrate level from 3000 ppm to 24,000 ppm of acetate and from 2300 to 18400 ppm of alcohol medium were effected during the study. Such high acetate tolerance of the methane organism does not appear to have been reported before; it was the results of adaptation by the organisms in a semicontinuous culture for a period of 30 months. Effect of supplementary available nitrogen on the rate of gas production at different carbon levels (acetate) was also studied. It was found from the gas analyses that CH4: CO2 ratios were always more than 2 to 3 times the theoretical values, when acetate was used as substrate carbon. At some stages of acetate breakdown the concentration of hydrogen in the gaseous products was found to be as high as 8–9%, a very significant feature of this fermentation. Although sulphate ions were carefully excluded from the constitutents of the media (Analytical reagent chemicals and double‐distilled water used throughout) during the period of study (30 months), contaminants later identified as Desulphovibrio desulphuricans were found throughout the study. Morphological study of the organisms reveal that they are short rods appearing in individual or chain form, highly motile, gram‐positive and nonflagellated, similar to Methanobacterium söhngenii Barker.