Organic Acids as a Decisive Limitation to Process Dynamics During Composting of Organic Matter

Abstract

Composting in the presence of oxygen is generally defined to be an aerobic and thermophilic microbiological process. A delay in the development of temperature on a level of 40 – 50°C during the initial stages of the process is frequently reported and explained as a change in the bacterial population from mesophilic to thermophilic. In the course of the experimental trials presented here this effect is accompanied by a significant drop in pH due to the accumulation of organic acids. Even acetic acid can be found at concentrations hindering bacterial metabolism nonspecifically and completely, thermophilic populations seem to be affected in particular. Since acetic acid is a common product of various bacterial metabolic pathways, numerous explanations for its accumulation can be given. Recent experimental results support the anaerobic formation in terms of the mixed-acid fermentation as well as strictly anaerobic fermentations by members of the genus Clostridia i.e.; anaerobic conditions are likely to develop inside even very small particles due to limited transport capacities for oxygen from gaseous to liquid phases. Another reasonable explanation is given by the so called bacterial crabtree-effect as a response to excessive substrate supply.