| 摘要: | 本研究自2007年12月至2010年5月期間監測嘉南藥理科技大學人工濕地的進出流水質、CO2及CH4的釋放通量、濕地植物的淨生產量及濕地底泥的碳含量，並以人工濕地單元作為生態系統進行碳通量的質量收支平衡，以進一步了解人工濕地是CO2的儲場或產生源，及估算其淨儲存或產生通量。人工濕地由兩種不同類型濕地：SSF濕地(1450 m2)及FWS濕地(2200 m2)串連組成，進流經二級處理後的校園污水，研究期間平均進流流量為316 m3/d，平均水力負荷0.087 m/d，平均水力停留時間3.14 d)。進流水SS濃度介於4~70 mg/L、BOD5濃度介於1.7~33.2 mg/L，整體人工溼地的SS及BOD5平均去除效率分別達80.5 %及76.4 %，出流水質相當穩定，95%的水樣樣本數可達到SS < 10 mg/L、100%的水樣樣本數可達到BOD5 < 10 mg/L。估計進流水的TOC濃度介於3.68~21.33 mg/L (12.41±5.54 mg/L)，人工濕地系統的平均去除效率達68.3 %，濕地系統進流水帶入的TOC通量平均為379.7 g C/m2/year、出流水帶出的TOC通量平均為56.3 g C/m2/year。
以靜置箱採氣及氣體層析法監測濕地因異營性呼吸作用產生的CO2及厭氧分解產生CH4的釋放通量，結果顯示SSF濕地的CO2釋放通量介於-182.5~777.18 mg /m2/h，FWS濕地介於12.8~402.9 mg /m2/h。SSF濕地、FWS濕地及整個SSF-FWS濕地的整年平均CO2釋放通量分別為157.77 ±152.08、147.55±103.07及153.39±132.98 mg CO2/m2/hr。SSF濕地的CH4釋放通量範圍介於-1.09~55.36 mg CH4/m2/h，FWS濕地介於-4.17~28.86 mg /m2/h。由於CH4通量的日夜間變化及有無植物存在的監測值差異頗大，因此透過α, β, γ校正值估算出實際的平均通量，SSF濕地、FWS濕地及整個SSF-FWS濕地的整年平均CH4釋放通量分別為111.14、79.70及92.19 g C/m2/year。
本研究直接測量地面上植體淨初級生產力(ANPP)進而推估濕地的淨初級生產力(NPP)，結果顯示，SSF濕地、FWS濕地及整個SSF-FWS濕地的淨初級生產量(CNPP)分為1712、1056及1317 g C/m2/year。碳通量收支計算的結果顯示，本研究濕地場址為CO2的儲場而非產生源。濕地的淨生態系統CO2交換通量(CNEE)，SSF濕地、FWS濕地及整個SSF-FWS系統分別為1336，704，1182 (g C/m2/year)。SSF濕地、FWS濕地及整個SSF-FWS系統的碳累積通量或稱碳匯通量分別為2000.1、649.8、1175.3 (g C/m2/year)，相當於每年可累積CO2質量分別為10450、8521及18971 kg CO2。經計算人工濕地溫室氣體(CH4+N2O)釋放的全球暖化潛勢(GWP，以g CO2-C equivalent/m2/year單位表示)，發現濕地單元的碳匯量明顯超過或接近溫室氣體的GWP，此意味人工濕地可能成為氣候變遷的中和者(climate-change neutral)，不會增加全球暖化的負擔。惟人工濕地所累積的碳大多是以水生植物的生質量碳存在，而非積存於濕地底泥或土壤中。而濕地植物每年均會進行定期採收，因此如何運用這些採收的水生植物生質量，避免立即形成CO2又釋放回大氣中，才能達到真正碳匯功能。
關鍵詞：人工濕地、溫室氣體、二氧化碳、甲烷、碳匯、淨初級生產量、全球暖化潛勢、廢水處理
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 |
