Methane Emissions Driven Research From Integrated Vertical Flow Constructed Wetland

Recent years, the concentration of methane in the atmosphere are significantly increased which anthropogenic CH4emissions make up54～72%of the total global flux. According to the IPCC (2007) report, wetlands are a main natural source, with about20%contribution of the global methane source. Agricultural non-point source pollution has become one of the most serious environmental problems in China, resulting in a serious threat to the quality and safety of agricultural products, human health and the environment. Compared with other sewage treatment technology, constructed wetlands (CWs) belongs to the ecological wastewater treatment technology, and are especially suitable for the rural area where the economy is relatively backward and the non-point source pollution is serious in China. To date, the small courtyard constructed wetland treatment system were successfully applied in Yuantian Village and Anlong Village in Pixian County of Chengdu City in Sichuan Province, therefore it is significant to research on driving factors impacted on the methane emissions from this kind of constructed wetland.In this study, combining with one hydraulic tracer experiments, methane emissions driven by a kind of carbon source from the glucose (C6H12O6) were monitored, analysed and discussed from a physical model of Integrated Vertical-flow Constructed Wetland (IVCW), which composed of five different material layers in a laboratory level under the referred condition of the small courtyard constructed wetland system in Yutian Village. Five different material layers for IVCW with a total depth of600mm were, from top to bottom, compose of the sands from Min River (150mm depth), the coarse sand with diameters of1～2mm (100mm depth), the crushed gravel with diameters of2～10mm (100mm depth), the crushed gravel with diameters of10～30mm (100mm depth), cobble with the diameters of30～60mm (150mm depth).Firstly, the hydraulic infiltration situation of the IVCW in vertical direction were analysed and discussed through the sodium chloride (NaCl) tracer experiments.. Secondly, using the carbon source of glucose with different concentrations for IVCW, the methane emissions from root-water subsystem and stem-leaf subsystem at influent tank and effluent tank were collected and monitored within continuous four days with different monitoring frequencies, and analysed and discussed for the different relationship of methane emissions from the carbon source, index of water quality and different location of Cyperus alternifolius.Main research results of this study are as follows: (1) Due to the hydraulic diffuses were in inhomogeneous state within the different layers of IVCW, the actual hydraulic retention time was more longer than the theoretical residence time. The matrix porosity of I layer and II layer were too small, resulting in a serious phenomenon of stagnant water, even easily clogging. It finds that the water dynamic flow was very small, due to the unreasonable outlet position. The most serious retention phenomenon occurred in the V layer area. However, it has revealed that currently hydraulic conditions (more hydraulic retention time) had provided favorable conditions for producing methane. Also, it revealed that the optimizing of the hydraulic conditions is very important to control the produce of methane. The future research for the optimization of hydraulics are about to change the surface material in the IVCW, to design the optimal hydraulic parameters such as water distribution, hydraulic retention time and hydraulic loading, etc.(2) Methane emissions situations were accordingly relation to the driven carbon source of adding different concentrations of glucose. Main methane emissions were in the first48hours in the beginning of the driving experiment began. Then the methane emissions became decrease after48hours. But the results show that methane emissions driven by the high concentration (4mmol/L) of glucose were lower than the lower concentrations (0.5mmol/L,1mmol/L and2mmol/L). Importantly, the results presents that the methane emissions were significantly and positively associated with the concentrations of organic matter within the Ⅰ, Ⅱ and Ⅲ layers in the IVCW.(3) For the Cyperus alternifolius, methane emissions from the stem-leaf subsystem were higher than the root-water subsystem. Therefore, the main part of the Cyperus alternifolius releasing methane were the stems and leaves above20cm.(4) Methane emissions were some certain relationship with the index of water quality in this study. Firstly, the more lower value of the DO, redox potential, conductivity and total dissolved solids were, the more higher the methane emissions were. Secondly, the more higher the temperature and pH value, the more higher the methane emissions were. Thirdly, methane emissions had no significant relationship with the salinity and total nitrogen. Besides, methane emissions may be negatively related to phosphate concentrations. Next research will focus on the further quantitative analysis between more methane mass (gradient) and the mass (gradient) of the carbon source.