Study on the Emission of Methane and Nitrous Oxide from Constructed Wetland

• Main Authors: Chien-Chih Lai, 賴建志
• Other Authors: Ying-Feng Lin
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/08086966888734403789

碩士 === 嘉南藥理科技大學 === 環境工程與科學系碩士班 === 96 === Because global warming and climate changes are following upon an increase in atmospheric levels of greenhouse gases, there is intense concern with the sources and emissions of the gases. Constructed wetland technology is a natural treatment system for wastewater engineering and is characterized by the advantages of moderate capital costs and very low energy consumption and maintenance requirements. However, constructed wetlands are inherently the net source of greenhouse gases such as methane (CH4) and nitrous oxide (N2O). Wetland construction and the area covered by constructed wetlands are increasing domestically and globally, thus there is an urgent need to elucidate the impact of constructed wetlands on atmospheric burden of these gases. In a study period from December 2007 to June 2008, emission rates of nitrous oxide and methane as well as several parameters of water quality were simultaneously and monthly monitored at various sampling locations of a practical-scale constructed wetland system in Chia-Nan University of Pharmacy and Science, which has been operated for tertiary treatment of campus wastewater for near three years. This wetland system consists of a subsurface flow (subsurface flow, SSF) wetland and followed by a free water surface flow (free water surface, FWS) wetland with a total surface area of 3,800 m2. The objectives of the study were to: (1) investigate the temporal and spatial variations in emission rate of greenhouse gases; (2) examine the differences in greenhouse gas emissions between various types of constructed wetlands, and compare these results with those of domestic natural wetlands from literature; (3) investigate the relationship between gas emissions and water quality of the constructed wetland to elucidate possible factors that can affect greenhouse gas emission; (4) investigate the day-night dynamic in greenhouse gas emission. The results monitored in the first-half year study showed that emission rates ranged from -6.10 to 128.78 μg N2O m−2 h-1 and -4.17 ~ 44.4 mg CH4 m−2 h-1. These values fall within the emission rates reported by literature of constructed wetlands(−46.3 ~ 150 μg N2O m−2 h−1 and–15.63~ 72.46 mg CH4 m−2 h−1), but are slightly higher than that measured from domestic natural wetlands in other studies. The emission rate of either N2O or CH4 was found to be significantly different (p<0.05) between various sampling locations of FWS or SSF wetlands, significantly decreasing (p<0.05) along the flow path of either wetland. A variety of water quality parameters were further correlated with gas emission rates by using single variable linear regression method. TN (R=0.953，P＜0.05)、TKN(R=0.950，P＜0.05) of water was closely and significantly correlated with N2O emission in the SSF wetland. TN (R=0.957，P=0.04) of water was closely and significantly correlated with CH4 emission in the SSF wetland and BOD (R=0.999，P＜0.001) of water was closely and significantly correlated with CH4 emission in the FWS wetland.It imply the emissions of the greenhouse gases in constructed wetlands depend on various complex and interconnecting processes, in which their effects can not be simply and clearly elucidated by a statistic model and limited data. Although the FWS wetland is installed following the SSF wetland in this study, there was no significant difference (p<0.05) in N2O or CH4 emission between the two types of wetlands. In temporal variation study, emission rate of either N2O or CH4 was found to vary with the month as gas sampling. Up to now, the greatest emission rates of N2O were recorded in March, while the lowest rate were noted in February.The greatest emission rates of CH4 were recorded in June, while the lowest rate were noted in February.The possible reasons for this phenomenon is temperature variation caused by seasonal change lead affect the biological processes responsible for gas emission in wetlands, resulting in the change of emission rate. The relationship between emission rate and temperature (either ambient or water) were found following the modified Arrhenius equation (p<0.05，R>0.796)。The above results and conclusions were made only based on the preliminary study of the first-half year project. To make a consistent conclusion, it is necessary to sustain the long-term study further so as to obtain statistically meaningful data.