Isotopomer Analysis of Production and Consumption Mechanisms of N_2O and CH_4 in an Advanced Wastewater Treatment System

摘要

Wastewater treatment processes are believed to be anthropogenic sources of nitrous oxide (N_2O) and methane (CH_4). However, few studies have examined the mechanisms and controlling factors in production of these greenhouse gases in complex bacterial systems. To elucidate production and consumption mechanisms of N_2O and CH_4 in microbial consortia during wastewater treatment and to characterize human waste sources, we measured their concentrations and isotopomer ratios (elemental isotope ratios and site-specific N isotope ratios in asymmetric molecules of NNO) in water and gas samples collected by an advanced treatment system in Tokyo. Although the estimated emissions of N_2O and CH_4 from the system were found to be lower than those from the typical treatment systems reported before, water in biological reaction tanks was supersaturated with both gases. The concentration of N_2O, produced mainly by nrtrifier-denttrification as indicated by isotopomer ratios, was highest in the oxic tank (ca. 4000% saturation). The dissolved CH_4 concentration was highest in inflow water (ca. 3000% saturation). It decreased gradually during treatment Its carbon isotope ratio indicated that the decrease resulted from bacterial CH_4 oxidation and that microbial CK_4 production can occur in anaerobic and settling tanks.