| 摘要: | 人工溼地係為自然處理系統之一，且具有省能源、低成本、無二次污染與易操作維護之綠色生態工程技術。人工溼地依水流動型式有表面流動式(FWS)與表面下流動式人工溼地(SSF)。本研究以試驗規模人工溼地去除受污染地下水之硝酸鹽氮(NO3¯-N)，在固定水力負荷為0.05 m d-1與平均進流NO3¯-N濃度約24.3 mg L-1之操作條件下，進行十一個月(自2004/7-2005/5期間)之水質監測。結果顯示，季節變化顯著影響人工溼地對NO3¯-N的去除效能，FWS人工溼地之NO3¯-N去除效率變化介於72-99%，而SSF人工溼地介於41-91%。此結果歸因於季節之變化導致溫度對於細菌性脫硝作用之影響。
本研究也探討了兩種人工溼地水質之日夜間動態變化。結果顯示光照度呈現規律的動態變化，早晨約6:00後光照度逐漸增加，最大強度約出現在中午12:00至午後3:00間，隨後光照度逐漸降低直到傍晚約6:00-7:00後偵測不到光度，日間光照度大小則依天氣月份而變。人工溼地系統的日夜水溫變化也相當明顯；FWS濕地約在早晨6:00時水溫開始提高，到了中午12:00至午後3:00達到最高溫，隨後水溫逐漸下降直到次日早晨；FWS的水溫變化轉折點與光照度變化轉折點頗為一致；與之比較SSF溼地溫度變化的轉折點約延遲3-6小時才發生，且SSF的日夜間溫差較FWS溼地小，此現象可能歸因於SSF溼地礫石層的熱傳導阻隔作用。本研究亦發現，水中pH、DO及ORP在早晨時有逐漸增加趨勢，到了下午時間達到最高值，隨後逐漸下降直到次日早晨。並且FWS溼地中pH、DO及ORP的日夜變化較SSF來的規律，可能是因為FWS人工溼地中含有藻類日間光合作用可增強溼地氧化狀態及增加水中pH，而夜間時恰可加速DO的消耗及降低pH值。大體上，FWS溼地日間的DO大小亦明顯高於SSF溼地。而FWS與SSF溼地系統pH值之動態變化仍介於弱酸性至中性範圍之間。人工溼地系統中的TOC濃度會受到日夜間交替顯著變化，在早晨期間(約6:00或更早)，人工溼地TOC濃度開始逐漸累積，推論是因光合作用的開始且TOC形成速率大於TOC消耗速率所導致；到了大約下午3:00-6:00時，光合作用活性下降甚至停止後使得TOC消耗速率大於TOC形成速率，因而造成TOC濃度逐漸的減少；此變化趨勢在FWS與SFF溼地中均明顯發生。再者，在FWS及SSF溼地中均發現NO3—-N濃度在早晨時(約6:00或更早)即開始下降，到了午後期間(3:00-6:00 p.m.) 濃度達到最低值，隨後NO3—-N濃度逐漸增加直到次日的早晨，NO3—-N濃度的日夜間變化呈規律性動態變化。本研究歸納出NO3—-N濃度的日夜間規律變化結果可能由於TOC的消長及水溫的升降，影響細菌脫硝作用的進形，進一步導致NO3—-N濃度的變化。
The constructed wetland is one of the natural treatment systems and possesses Green Ecological Engineering Technology of low energy requirement, low cost, no secondary pollution and ease of operation. In terms of water flow style, it can be classified into free water surface flow (FWS) and subsurface flow (SSF). This study employed both pilot-scale FWS and SSF constructed wetlands to remove nitrate nitrogen (NO3¯-N) from contaminated groundwater, under a regulated hydraulic load rate (0.05 m d-1) and an average influent NO3¯-N concentration around 24.3 mg L-1. Water quality was monitored for 11 months (2004/7 to 2005/5). The result shows that seasonal variation affected the performance of the constructed wetlands in removing NO3¯-N. During the period of the study, NO3—-N removal efficiency of the FWS wetland varied from 72 to 99%, and that of the SSF wetland varied from 41 to 91%. The effect resulted from seasonal variation may be caused by the temperature effect on bacterial denitrification.
This study also investigated the day/night dynamic variation of water quality in constructed wetlands. The illuminance began to increase at dawn (around 6:00 a.m.) and reached a greatest level at noon (between 12:00 to 3:00 p.m.), afterward the illuminance decreased gradually to an undetected level when night fell. The illuminance changed regularly with day and night. However, the levels of illuminance vary with weather condition and seasonal change. Water temperature in the FWS wetland rose after the early morning (around 6:00 a.m.), reached a maximum temperature at noon (between 12:00 to 3:00 p.m.) and then decreased until the next early morning. Similar dynamic variation of water temperature was found in the SSF wetland but with a lag time of around 6 hr. Moreover, the maximum temperature difference between the day and the night in the SSF wetland was lower than that in the FWS wetland. This founding may be due to the presence of gravel that can act as thermal insulation for the SSF wetland. This study also demonstrates that DO, ORP, and pH increased at the early morning, reached the maximum values at the afternoon time and then decreased gradually until the next morning. These cyclic variations in the FWS wetland were more regular than those in the SSF wetland; this phenomenon maybe because the FWS contains algae which can enhance oxidation status in wetland and elevate pH in the day-time and which can consume DO and reduce pH in the night-time. On the whole, DO in the day time of the FWS wetland were also obviously higher than that of the SSF wetland. The dynamic variation of pH value is still within weak acid to neutral ranges for the FWS or SSF wetland. TOC concentration was influenced by the alternation of day/night in constructed wetland systems. TOC concentration began to accumulate at the early morning (around 6:00 a.m. or earlier) and gradually achieved a maximum value at the afternoon time (between 3:00 and 6:00 p.m.). This TOC accumulation is probably because of the beginning of photosynthesis, which can lead to TOC forming speed exceeding TOC consuming speed. Afterward, TOC concentration reduced continuously until next early morning possibly because photosynthesis activity declines or even ceases, resulting in TOC consuming speed greater than TOC forming speed. The cyclic variation of TOC concentration was regularly observed in both FWS and SSF wetlands. In both FWS and SSF wetlands, NO3—-N level began to decrease at the early morning (around 6:00 a.m. or earlier), gradually reached a minimum level at the afternoon time (between 3:00 and 6:00 p.m.), and then increased gradually until the next early morning. This resulted in a regular day/night dynamic variation. Since the NO3—-N dynamic was highly consistent with the TOC dynamic or temperature dynamic, it is suggested that the day/night variation of NO3—-N was caused by the variation of TOC and/or temperature, which can control the rate of bacterial denitrification, thus determining the NO3—-N level in a nitrate treatment wetland. |
