|摘要: ||利用人工溼地淨化水質，季節變化及日夜間交替所引起之環境因子的改變常常會影響溼地中微生物的活性，進而影響人工溼地的處理效能。本研究構築自由水層流動式(FWS)與表層下流動式(SSF)之試驗規模(pilot scale)人工溼地(長5m、寬1m、高0.8m)，探討溼地環境條件變化(包括季節變化及日夜間變化) 對溼地中硝酸鹽濃度及硝酸鹽去除效能的影響。
試驗規模人工溼地在固定水力負荷為0.05m d-1、進流平均硝酸鹽濃度約22.61 mg N L-1的操作下，經超過一年的水質監測。結果顯示，季節變化顯著影響人工溼地對NO3-N的處理效能。FWS溼地在春夏季的4～9月的處理效能介於72～97％，明顯高於秋冬季10～3月間的20～71％；SSF溼地在春夏季4～9月的處理效能則介於79～98％，進入到秋冬季10～3月後即下降至39～90％。整個實驗期間SFF之NO3-N去除效能(39~98%)略高於FWS(20~97%)。硝酸鹽去除效能的季節性變化主要是因為水溫的變化影響細菌脫硝作用所導致。由溼地的硝酸鹽去除效能與溼地中的硝酸鹽濃度變化可分別估算出一次動力學去除速率常數。以Modified Arrhenius Equation模擬一次動力學去除速率常數與溼地水溫的相關性，進而獲得溫度校正係數(θ)及20℃時之一階去除速率常數(k20)。FWS溼地及SSF溼地的θ分別為1.08~1.13及1.06~1.07，k20分別為0.132~0.192 d-1及0.135~0.319 d-1。FWS溼地NO3-N去除效能受溫度的影響似乎較SSF溼地顯著。
日夜間交替對濕地弁鉏v響的實驗結果顯示， DO及ORP的日夜間變化與NO3-N濃度變化所呈現的消長趨勢，剛好與細菌脫硝作用需低溶氧及低ORP條件的理論呈現矛盾。但是，溼地水溫的日夜間變化與NO3-N濃度變化呈消長趨勢，與Modified Arrhenius Equation所述溫度影響溼地硝酸鹽去除效能的結果相吻合。另外，不少文獻均報導溼地中進行脫硝作用及硝酸鹽去除的另一個環境限制因子為碳源(脫硝所需的電子提供者)，溼地脫硝所需碳源為水生植物生產力所貢獻。實驗結果顯示，溼地TOC濃度的日夜間變化與NO3-N濃度變化呈消長趨勢也與文獻所述碳源為溼地脫硝作用的限制因子結果相吻合。因此，判斷人工溼地中進行脫硝作用及硝酸鹽去除的環境限制因子，主要為溫度及碳源而不是DO及ORP。儘管日間溼地的外部環境(bulk environment)呈現較氧化的狀態，但是溼地中不乏低溶氧及低ORP的微環境(microenvironment)，在較高溫的條件下有利於脫硝的進行；相反地，到了夜間溼地的外部環境雖然呈現較還原狀態，但是因夜間溫度降低及碳源的減少卻阻礙脫硝的進行。
此外，由於公共工程的建設需求大量砂石原料，砂石價格日益增加，也進而提高表層下流動式(SSF)溼地之建設成本。本研究另外建立了數個小型SSF人工溼地(長0.7m、寬0.5m、高0.6m)，探討以蠔殼（Oyster shell）與礫石（Gravel）作為介質之水質淨化效能比較。水質監測結果顯示，蠔殼介質之蘆葦溼地的啟動適應期比礫石介質之蘆葦溼地短。並且由於溼地基材的不同，使蠔殼介質溼地在出流水NO3-N濃度均較礫石介質低。在相同水力負荷試驗中，蠔殼作為介質之蘆葦溼地其NO3-N去除速率均高於礫石作為介質之蘆葦溼地。在水力負荷為0.12 m d-1、NO3-N污染負荷為2.71g N m-2 d-1時，蠔殼作為介質之蘆葦溼地的去除速率為2.49 g N m-2 d-1，礫石作為介質之蘆葦溼地去除速率為2.35 g N m-2 d-1。若進一步增加負荷，NO3-N去除速率反而下降。回收蠔殼廢棄物作為表層下流動式人工溼地之介質是技術可行，不僅在NO3-N之去除效能的表現上，使用蠔殼介質的溼地比使用礫石介質還優越，並且可顯著降低SSF溼地的建]成本。
The variation of environmental factors resulted from seasonal variation and day-night sequences could affect the microbial activity, thus influencing the performance of constructed wetland for wastewater treatment. The primary goal of the study was to investigate the effect of seasonal variation and day-night sequences on nitrate removal from a nitrate-contaminated groundwater in free water surface flow (FWS) and subsurface flow (SSF) wetlands. Pilot-scale FWS and SSF wetlands, measuring 5m (length) × 1m (width) and planted with common reed (Phragmites australis), were set up to achieve the study purpose.
A simulated nitrate-contaminated groundwater containing around 22 mg N/l of nitrate were continuously fed into the constructed wetlands under a hydraulic loading rate (0.05 m d-1). The influent-effluent data of more than 19 months continuous operation showed that the FWS and SSF wetland respectively reduced 72~97％ and 79~98％ of influent nitrate during warm season from April to September, and these reductions decreased to 20~71％ and 39~90％ during cold season from October to March of a next year. Nitrate removal of the SSF (39~98％) was slightly better than that of the FWS (20~97％). The relationships between nitrate removal and water temperature in the FWS and SSF wetlands were found fitting the modified Arrhenius equation with correlation coefficients of 0.775 and 0.511, respectively. As a consequence, temperature correction factors（θs） of 1.08~1.128 and 1.06~1.07 and 20℃ first-order removal constants (k20) of 0.132~0.192 and 0.135~0.319 d-1 were obtained for the FWS and SSF system, respectively. Temperature effect on the FWS wetland was seemly more significant than the SSF wetland.
Several parameters of water column in the FWS and SSF wetlands were also monitored sequentially to examine the effect of temporal variation of wetland behaviors during night and day sequences. While dissolved oxygen (DO) and oxidation-reduction potential (ORP) consistently increased in the day and decreased in the night, nitrate were maintained at low level in the day and high level in the night. This phenomenon conflict the fact that nitrate removal due to bacterial denitrification requires an anoxic condition. Besides, water temperature increased due to sunshine in the day and decreased in the night, resulting in around 3~4 ℃ of temperature difference between day and night. Furthermore, dissolved total organic carbon (TOC) was found cyclically increased in the day and decreased in the night, which was probably because of the release of organic carbon due to photosynthesis of macrophyte in wetland that occurring in the day and ceasing in the night. Accordingly, both temperature and TOC in water column, rather than DO and ORP, were considered as the limiting factors that could affect the nitrate reduction in constructed wetlands.
Another essential goal of this study was to investigate the feasibility of recycling the wasted oyster shell as substrates or media using in an SSF constructed wetland, since the exploit of gravel, a normally used substrates, is restrained by the local government and its price is becoming more and more expensive. Four small-scale SSF wetland beds (each with dimensions of 0.6 m width and 0.7 m length), in which two beds were packed with oyster shell and another two with gravel, were used for treating the same nitrate-contaminated groundwater as abovementioned. One oyster shell bed and gravel bed were planted with common reed, whereas another oyster shell bed and gravel bed were unplanted. The oyster shell beds seemed to need shorter period to achieve a stable nitrate removal than gravel bed. Oyster shell beds, both planted and unplanted, generally produced lower effluent nitrate levels than gravel beds under the same hydraulic loading rate. Planted oyster shell bed and gravel bed exhibited a nitrate removal rate of 2.49 and 2.35 g N m-2d-1, respectively, when hydraulic loading rate retaining 0.12 m d-1and nitrate loading rate maintaining 2.71 g N m-2d-1. Recycling the oyster shell as substrate for SSF constructed wetland was found economically and technically feasible.