Studies on biological denitrification by methane utilizing mixed bacteria

Abstract

Experiments were directed towards the metabolic characteristics of biological denitrification in the culture of methane-utilizing mixed bacteria, and the investigation of the possibility of replacing methanol by methane as the reducing agent. Appreciable wastage of methane was prevented by gas-phase recirculation. In batch culture with continuous gas supply, the reduction of nitrite, an intermediate in denitrification, was slower than the nitrate reduction in the nitrate-reducing system using methane-utilizing mixed bacteria. The rate of nitrate reduction was almost constant irrespective of initial nitrate concentration. In the continous cultures, the rate of nitrogen evolution and nitrite accumulation were higher in the oxygen-limited culture than in the methane-limited culture. Growth yield on nitrate was higher in the methane-limited culture than in the oxygen-limited culture. Also from the nitrogen balance, the amount of nitrate incorporated into cellular organic nitrogen was higher in the methane-limited culture than in the oxygen-limited culture. From the above results, it is suggested that under the oxygen-limited culture the reduction of nitrate was consumed for energy source, which leads to dissimilatory reduction of nitrate, and under the methanelimited culture it was consumed for cell synthesis via ammonia. As the amount of gas-phase recirculation in this fermentor (N. B. S. C-30, 2 1) is increased, cell concentration becomes higher, but no increase in the specific denetrification rate is resulted. Specific denitrification rate in the oxygen- and methane-limited culture are 34.1 and 21.66 (mg NO$_3^-$-N/g cell, hr), respectively. By the comparision of denitrification with others experiments using methane or methanol as the carbon source, we may conclude that biological denitrification by methane-utilizing mixed bacteria used in this study may be, economically and efficiently, applicable to wastewater treatment.