Analysis of environmental factors affecting methane production from high-solids organic waste

Abstract

A simple model developed from the Gompertz equation was used to describe the cumulative methane production curve in the batch culture. By using this model, three key parameters, namely methane production rate, potential and lagphase time, in a cumulative methane production curve were exactly estimated based on the experimental data. The results indicate that each gram of dry organic waste of a sludge cake, meat, carrot, rice, potato and cabbage had a methane production potential of 450, 424, 269, 214, 203 and 96 mL, respectively. The methanogenic activity of these digesters decreased with a decrease in the moisture content. The moisture content threshold limit, at which the methanogenic activity dropped to zero, was found to be 56.6% for the sludge cake, but greater than 80% for meat, carrot and cabbage. In the high-solids sludge digestion, the relative methanogenic activity dropped from 100% to 53% when the moisture content decreased from 96% to 90%. The rate of methane production at moisture contents of 90% to 96% functioned in a pH range between 6.6 and 7.8, but optimally at pH 6.8, and the process may fail if the pH was lower than 6.1 or higher than 8.3. On the other hand, the methanogenic activity was dependent on the level of ammonium, NH4+, but not free ammonia, NH3, indicating that the NH4+ was the more significant factor rather than the NH3 in affecting the methanogenic activity of a well-acclimatized bacterial system. In the wide pH range of 6.5 to 8.5, the methanogenic activity decreased with the increase in the NH4+; dropped 10% at the NH4+-N concentration of 1670-3720 mg·L−1, 50% at 4090-5550 mg·L−1 and dropped to zero at 5880-6600 mg·L−1. However, the lagphase time was dependent on the NH3 level, but not on NH4+, and when NH3-N was higher than 500 mg·L−1, a notable shock was observed. This suggests that the NH3 level was the more sensitive factor than the NH4+ level for an unacclimatized bacterial system.