In order to make clear the characteristics of the nitrous oxide (N₂O) emission from a sewage treatment process, total amount of the N₂O emission including the direct emission, the indirect emission and the dissolved N₂O emission through the treated water was assessed based on the survey. Three daily surveys were conducted in the A-STP, which is run with Pseudo-AO process, and the river receiving its effluent in Sep.2013, Dec.2013 and Sep.2014, respectively. Based on the survey, the amounts of the N₂O emission were estimated by 868 kgCO₂/day in September and 3,897 kgCO₂/day in December, respectively. The percentages of each origin in September were as follows: the duct gathering the gases from the grit chamber, the first settling tank and sludge treatment units was 0.3% ; the direct emission from aeration tanks was 14.8% ; the gasification of the dissolved N₂O in the treated water was 0.9% ; the effluent to the river as dissolved N₂O in treated water was 5.8% ; indirect emission based on the IPCC guideline was 78.2% . On the other hand, the percentages of these origins in December were 0.7% , 66.4% , 8.9% , 7.1% and 16.9% , respectively. Based on the results, by accounting for the dissolved N₂O in treated water as well as the direct and the indirect emission, the correct amount of N₂O emission can be calculated. Since the dissolved N₂O concentrations in the aeration tanks were high in winter, the controlling of DO concentration in the aeration tanks and SRT with the monitoring of nitrite concentration, might be effective to reduce the N₂O emission, especially in winter.

In order to prevent the occurrence of road subsidence due to sewer systems, it is important to narrow down the sites where subsidence is likely to occur and then perform appropriate maintenance and management. Therefore, in this study we have extracted factors affecting road subsidence from a macro perspective using decision tree analysis. The highly influential factors extracted by this analysis are exclusion classifications, periods when maintenance was commenced, and ratios of unknow water. By constructing decision trees, the extracted factors are arranged systematically while taking into account the priority of the factors, to be suggested that they can be used in a practical sense in narrowing down the sites where subsidence is likely to occur.