廢水處理型人工濕地降低溫室氣體排放之研究－溫室氣體減量最佳化之分析

濕地藉由有機碎屑沉積物及厭氧的環境，因而具有碳吸存的能力，為重要的生態系服務功能之一。但濕地因長時間處於被水淹沒的狀態，導致土壤與底泥沉積物長時間保持厭氧狀態，致使土壤與底泥沉積物中的厭氧性微生物，藉由甲烷發酵作用及不完全脫硝作用，因而亦會分別釋出溫室效應分別較CO_2強28倍及256倍的CH_4與N_2O等溫室氣體，致使濕地的碳匯能力降低，甚至轉為碳源。尤其是對於有廢污水排入而遭受到污染的自然濕地，抑或是廢水處理型的人工濕地，由於該種濕地內所含的有機物及含氮污染物等成分的增加，致使CH_4與N_2O等溫室氣體的排放釋出量，將更為增加，也因此對濕地碳匯功能的影響也將更為加劇。因此，本研究的目的將針對廢水處理型的人工濕地，在不同的環境因子控制條件之下，進行溫室氣體減量釋出的可行性分析研究。本研究中將建置四組人工模濕地模槽，並根據不同的菌種污泥添加及植被（紅海欖）條件，設計出無菌體及無植被（C槽）、添加含厭氧氨氧化菌污泥及有植被（PA槽）、添加含硝化／脫硝菌污泥及無植被（S槽）和添加硝化／脫硝菌污泥及有植被（PS槽）等，分別進行三種主要溫室氣體（CO_2、CH_4與N_2O）排放濃度的連續監測，以討論在六種不同環境因子之操作水體參數條件下，不同模槽系統內溫室氣體排放的變化趨勢，分析出最能讓溫室氣體減量釋出的環境條件。研究中對於溫室氣體的監測，係使用開放式動態浮動氣罩（open dynamic floating chamber），並以非分散性紅外線光譜儀（NDIR），來進行24小時的連續偵測，同時計算出在不同水體參數條件下，不同溫室氣體的排放通量，並換算為二氧化碳當量（CO_2-eq）表示之。結果顯示，研究中所採用的人工濕地模槽在所有水體參數操作條件下，對於溫室氣體之排放濃度均有明顯的日夜變化趨勢；CO_2的部分因植物的光合作用呈現日低夜高的情形，而CH_4與N_2O則皆為日高夜低。此外，研究結果亦發現各模槽系統內，三種溫室氣體中仍屬N_2O具有最高的CO_2-eq，CH_4次之；也因此對於提高濕地模槽系統內水位的操作因子，由於有利於厭氧的甲烷菌及脫氮菌增長，因而分別釋出大量甲烷發酵及不完全脫硝作用的CH_4及N_2O等產物，而呈現出最高的CO_2-eq碳排放量（384 g CO_2 m^(-2) day^(-1)）。對於能讓脫硝菌進行完全脫硝的進流水添加有機碳源的參數條件，以及能增加厭氧氨氧化菌作用效率的進流水添加亞硝酸氮之參數條件，由於均能讓N_2O的排放量降低，因此濕地模槽系統呈現出較低的溫室氣體CO_2-eq排放量。由於實際廢水的水質條件及人工濕地系統內的環境條件均不利於對環境敏感性極高的厭氧氨氧化菌的生長，因此對於廢水處理型人工濕地，建議以減低濕地系統厭氧的環境條件，以及適量的增加濕地系統內有機碳的含量，為濕地系統溫室氣體排放減量之較為可行的環境參數條件。

關鍵字

人工濕地；溫室氣體；甲烷發酵作用；不完全脫硝作用；厭氧氨氧化作用

並列摘要

Wetlands have ability for carbon sequestration, which is one of the ecological service functions for wetland systems. Since wetlands are submerged in water for a long time, which keeps the soils and sediments in wetlands be anaerobic conditions constantly. Thus, the anaerobic microorganisms in wetland soils and sediments achieve methane fermentation and incomplete denitrification, and hence to let greenhouse gases (GHGs), CH_4 and N_2O, presenting 28 times and 265 times stronger greenhouse effects than CO_2 emitted out of the wetland systems, respectively, especially for those natural wetland polluted by discharging wastewaters, and constructed wetlands treating wastewaters, which contain more organic and nitrogenous pollutants. The emissions of CH_4 and N_2O may result in decreasing the carbon sink effects of wetland systems, and even further cause the systems become carbon sources. The purpose of this study is to investigate the feasibility of decreasing the emissions of GHGs from wastewater treatment constructed wetlands by controlling different environmental conditions, and then to select an optimal one. Four constructed wetland microcosms were built in this study set by four conditions of w/o additions of bacterial seeding sludge and w/o vegetation (Rhizophora stylosa) as followings: C tank (without sludge and vegetation), PA tank (with ANAMMOX bacterial sludge and vegetation), S tank (with anaerobic bacterial sludge and without vegetation), PS tank (with anaerobic bacterial sludge and vegetation). Six environmental condition factors of influent water quality and water levels were conducted in this study for analyzing optimal one to decrease GHG emissions. GHG emitted from the wetland microcosms were continuously collected and monitored concentrations by open dynamic floating chambers and NDIR facility, respectively. According the results, it was found that the concentrations of GHGs emitted from all wetland microcosms presented obvious day-night trends. The concentrations of CO_2 showed higher values in day than at night due to the photosynthesis of plants during day-time, while CH_4 and N_2O concentrations were both lower during day-time than at night-time. Among the three GHGs, N_2O exhibited the highest CO_2-eq emission flux, and then the values of CH4 emissions were after. Thus, for the controlling factor of increasing the water levels in the wetland microcosms, which was conducive to the growth of anaerobic methanogens and denitrifying bacteria, large amounts of the products from methane fermentation and incomplete denitrification of CH_4 and N_2O were emitted, respectively, with the highest emission flux of CO_2-eq (384 g CO_2 m^(-2) day^(-1)). Regarding the factors of influent quality of additions of organic carbon sources, which can allow denitrifying bacteria to achieve complete denitrificaiton, and of additions of NO_2^--N, which can increase the efficiency of anaerobic ammonia oxidation by ANAMMOX bacteria, since both can reduce the N_2O emission, the wetland microcosms presented lower GHG emission flux of CO_2-eq. However, the wastewater quality conditions and the environmental conditions in actual constructed wetland systems are not conducive to the cultivation and survival of ANAMMOX bacteria. Hence, the factors of either reducing anaerobic environmental conditions by lowering water levels, or suitably increasing organic carbon contents in wastewater treatment constructed wetland systems, might be the more feasible environmental parameter condition for the reduction of GHG emissions in the wetland systems.

並列關鍵字

Anaerobic ammonium oxidation ； constructed wetlands ； incomplete denitrification ； greenhouse gases (GHGs) ； methane fermentation

國際替代計量

廢水處理型人工濕地降低溫室氣體排放之研究－溫室氣體減量最佳化之分析

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