| Summary: | 碩士 === 嘉南藥理科技大學 === 環境工程與科學系暨研究所 === 98 === This study monitored water quality of influent and effluent, gas emissions of CO2 and CH4, net primary production of wetland macrophytes, and carbon content of wetland soil in a constructed wetland system built in Chia Nan University of Pharmacy & Sciencae during the period December 2007 to May 2010. By considering the constructed wetland as ecosystem’s boundary, the mass flux of each flow stream in carbon budget was estimated so as to examine whether the constructed wetland is carbon sink or source, and calculate carbon sequestration flux and/or net exchange rate.
The constructed wetland system consisted of two wetland units with various water flow patterns: a subsurface flow (SSF) wetland (1450 m2) and a free water surface flow (FWS) wetland (2200 m2), in series connection. It was designed to receive and purify the secondary treated effluent from a wastewater treatment plant treating campus wastewater. The average influent flow was 316 m3/d, the average hydraulic loading was 0.087 m/d, and the average hydraulic retention time was 3.14 d during the period of the study. The SS and BOD5 concentrations of the influent ranged from 4 to 70 mg/L and 1.7 to 33.2 mg/L, respectively.
The efficiencies of SS and BOD5 removals were rather high, achieving 80.5 % and 76.4 %, respectively. The effluent water quality is very stable, with 95 % of water samples regarding the SS parameter were less than 10 mg/L, and 100 % of water samples in reference to BOD5 were below 10 mg/L. TOC concentration of influent was estimated between 3.68 to 21.33 mg/L (12.41±5.54 mg/L), with 68.3 % of the TOC removed by the constructed wetland system. Thus, the average annual iuput of TOC generated from influent was estimated to be 79.7 g C/m2/year, with average annual output being 56.3 g C/m2/year for the whole wetland system.
Gas emissions of CO2 and CH4 produced by heterotrophic respiration were measured by gas sampling with static chambers and gas analysis with gas chromatography. The results demonstrate that the monitored CO2 flux ranged from 182.5 to 777.18 mg /m2/h for the SSF wetland and from 12.8 to 402.9 mg /m2/h for the FWS wetland. The CO2 flux of SSF, FWS and SSF-FWS wetland were 157.77±152.08、147.55±103.07 and 153.39±132.98 mg CO2/m2/hr, respectively. Methane fluxes in the SSF and FWS wetland were in the range of 1.09 to 55.36 mg CH4/m2/hr and 4.17 to 28.86 mg CH4/m2/hr, respectively. Because a considerable difference in CH4 fluxes between day-time and night-time measurements, and between planted and unplanted area, we estimated the actual average flux by using α, β, γ correction coefficients. Consequently, the estimated average CH4 flux of the SSF, FWS and SSF-FWS wetland were 111.14, 79.70 and 92.19 g C/m2/year, respectively.
Additionally, we estimated the net primary production (NPP) by measuring the above-ground net primary production (ANPP). The results show that net primary productions of the SSF , FWS and SSF-FWS wetland were 1712, 1056 and 1317 g C/m2/year, respectively. The results of carbon budget calculation demonstrate that the constructed wetland in this study is the sink rather than source of CO2. While the net ecosystem CO2 exchange flux (CNEE) of SSF, FWS and SSF-FWS wetland were estimated to be 1336, 704 and 1182 (g C/m2/year), respectively, the carbon sequestration fluxes of the SSF , FWS and SSF-FWS wetland were 2000.1, 649.8 and 1175.3 (g C/m2/year), respectively.
This means the SSF, FWS, and SSF-FWS could accumulated approximately 10450, 8521, and 18971 kg CO2 annually. After further calculating global warming potential (GWP, in term of g CO2-C equivalent/m2/year) of greenhouse gas emissions of constructed wetlands, it is found that the mount of carbon sequestration was obviously greater than or close to the GWP of greenhouse gas emissions. This finding suggests that the constructed wetlands may act as the climate-change neutral, and won’t increase the burden of global warming. However, the CO2 sequestered by constructed wetlands was mostly converted into biomass organic carbon of macrophytes, and not into wetland’s soil. The macrophytes of wetland was harvested every year. Therefore, how to utilize these harvested macrophytes to avoid release of CO2 back into the atmosphere is essential and can really reach carbon sink.
Keywords: constructed wetlands, greenhouse gas, carbon dioxide, methane, carbon sequestration, net primary production, global warming potential, wastewater treatment
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