The effects of flow rate variation and vegetation ageing on the longitudinal mixing and residence time distribution (RTD) in a full-scale constructed wetland
The effects of flow rate variation and vegetation ageing on the longitudinal mixing and residence time distribution (RTD) in a full-scale constructed wetland
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DOI：
10.1016/j.ecoleng.2019.07.014
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发表时间：
2019-11
影响因子：
3.8
通讯作者：
V. Ioannidou;J. Pearson
中科院分区：
文献类型：
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作者：
V. Ioannidou;J. Pearson
在英国剑桥郡克纳普韦尔的一个全尺寸人工湿地内进行了一项基于现场的实验研究，该湿地旨在处理农田的径流。在8个月的时间里，研究了流量变化和自然植被老化对混合特性的影响。进行了详细的荧光测量，以检查湿地内溶质的纵向扩散。在英国11月的冬季和6月的夏季期间，使用每日自动注射示踪剂系统，对一系列干燥天气和风暴流动条件进行了125次示踪剂测试。纵向弥散结果表明，在低流量条件下，纵向弥散受流量的影响，而在较大流量时，纵向弥散与流量无关。每个试验月的停留时间分布(RTD)曲线通过一系列流动条件进行检验，范围从过渡流动(Re~2000)到湍流(Re~7000)。在测量的条件下，不同的流量会引起RTD的变化，表明较高的流量会导致较短的平均停留时间，从而主要产生平流流态。树龄对混合格局的影响显著。在植物年周期接近尾声的2/3月份，混合模式接近完全混合，由于RTDS上的长尾，纵向混合显著增加，平均流速变慢。这表明植物休眠期通常需要5-6个 月(10月至3月)，逐渐改变系统中的混合模式，使其与生长植物季节的混合模式显著不同。
A field-based experimental study has been undertaken within a full-scale constructed wetland, designed to treat runoff from agricultural land in Knapwell, Cambridgeshire, UK. The effects of flow rate variation and natural vegetation ageing on the mixing characteristics are investigated over a eight month period. Detailed fluorometric measurements were made to examine the longitudinal spreading of a solute within the wetland. Between a UK November winter period, and June summer period, 125 tracer tests were undertaken for a range of dry weather and storm flow conditions, using an automated daily injection tracer system. The longitudinal dispersion results show that the dispersion is influenced by the flow rate for low discharge conditions, however, for higher discharges, the longitudinal dispersion becomes independent of discharge. Residence Time Distribution (RTD) curves are examined through a series of flow conditions for each testing month, ranging from transitional (Re ~2000) to turbulent (Re ~7000) flow conditions. For the conditions measured, differing flow rates produce changes in the RTD, demonstrating that higher flow rates induce shorter mean residence times, generating predominantly an advective flow regime. The effects of plant age are prominent on the mixing pattern. Towards the end of the annual plant cycle, in February/March, mixing pattern approaches complete mixing, longitudinal mixing increases significantly due to long tails on the RTDs, and mean flow velocity is retarded. This indicates that the dormant plant period, which normally takes 5–6 months (October to March), alters progressively the mixing pattern in the system in such a way that it is significantly different from the mixing pattern during the growing plant season.