以鹽度追蹤劑試驗探討小型組合式人工溼地之水力特性

Hydraulic Characteristics of Small Scale Combined Constructed Wetland by Salt Tracer Experiments

由於人工溼地之水力特性對於非溶解性污染物及營養鹽之去除有其一定之關聯與影響，本研究乃應用鹽度對不同構型之人工溼地進行追蹤劑實驗，並藉由停留時間分佈函數、平均停留時間、正規停留時間、攪拌器數、有效體積比、短流度等參數評估小型表面流式（free water surface;FWS）與潛流式（subsurface flow, SSF）人工溼地之水力特性，由相關實驗結果發現追蹤劑以不同方式入流所形成之濃度停留歷線分佈型態亦有所不同，低長寬比FWS單元之空槽平均停留時間幾與正規停留時間相同，其主要原因係低長寬比及進流原水溫差所形成大型環流所致，當植栽生成後，其平均停留時間均較低，僅為正規停留時間之56%~63%，顯示此一流場結構遭植栽破壞，更甚者，亦衍生大量無用空間，降低溼地有效體積，而FWS與SSF單元組合後，其整體平均停留時間約為正規停留時間之54%~63%，與FWS單元相近。而由空槽FWS單元之短流度與植栽生成後結果相較，其短流度由0.42降至0.20~0.31，無用空間增加將導致短流現象更趨明顯，惟與其他處理單元以束縮管結合後，短流度可升至0.55~0.70，二者組合可降低短流現象。此外，植栽亦會造成攪拌器數值上升，降低FWS溼地單元之完全混合反應器特性，而將處理單元組合後，人工溼地之N值更由0.54升至2.53~2.88，顯示流場呈現較明顯之柱塞流特性。然試驗用之小型人工溼地之水力特性明顯有別於實場特性，於進行污染去除機制與溼地水力特性關聯性較高相關研究時，對此部份所造成之差異應有適當之考量。

Due to hydraulic characteristics of constructed wetland （CW） is closely connected to the removal of particulate pollutant and nutrient, in the present study, the tracer test by salt was conducted to assess the mentioned topics by hydraulic parameters; such as, residence time distribution function （RTD）, mean detention time （MDT）, nominal detention time （NMT）, number of stirred tanks, effective volume ratio, extent of short-circuiting. The vegetated combined CW was composed of free water surface （FWS） constructed wetland and subsurface flow （SSF） constructed wetland. According to the results obtained, the patterns of RTD are different with the ways of tracer injection. For a FWS without vegetation, the MDT is almost the same as the NDT, which is induced by the formation of large scale circulation. It is mainly resulted from the temperature difference between inflow water and wetland. When planted, the former decreases and is 56%~63% of the latter. It implies that the plant breaks down the circulation and result in more dead space. As a result, the effective volume and pollutant removal also decrease. The combined wetland of FWS and SSF shows a similar range of MDT decrease, 54%~63%. When compare the result of FWS without vegetation with planted FWS, the extent of short-circuiting decreases from 0.42 to 0.20~0.31. It shows that the vegetation increases the short-circuiting and decreases the contact between the pollutant and CW. However, the defect can be overcome by the combination of FWS and SSF since the extent of short-circuiting will increase to 0.55~0.70. The combination also influences the number of stirred tanks which increases from 0.54 to 2.53~2.88. The flow shows a feature more close to the plug flow instead of the complete mixing. As a matter of a factor, the vegetation possesses the same ability to increase the number of stirred tanks.