非稳态条件下人工湿地水流规律及其对水力停留时间的影响

由于降水的随机性，处理农业或城市径流的人工湿地通常在进水水量波动的非稳态条件下工作.为研究在非稳态条件下人工湿地的水流规律，在基于时间轴的多点示踪剂试验的基础上，利用RTD(水力停留时间分布)探讨了单峰和双峰两种水力冲击非稳态条件下水平潜流人工湿地中的水流规律.结果表明，非稳态过程的水量变化对不同时刻流入人工湿地的液体的RTD影响明显.用基于出水流量的非线性无量纲时间变量(φ)对原始RTD进行归一化处理，所得的RTD浓度曲线C'(φ)]在整个非稳态过程中相对稳定，具有相似的倾斜度、峰值时间(单峰冲击下为0.83~1.00，双峰冲击下为0.87~1.08)、重心位置(单峰冲击下为1.08~1.25，双峰冲击下为1.05~1.25)及相关统计学参数，表现出一种独立于流量变化之外的相对稳定的特征水流规律.与稳态基流量下的t_mean(平均停留时间)相比，水力冲击使人工湿地t_mean呈冲击前持续缩短、冲击后又快速回升的趋势(单峰、双峰冲击下最大值与最小值相差分别约为30和32 h)；而t_mean'(归一化平均停留时间)在非稳态过程中却相对稳定(单峰、双峰冲击下最大值与最小值相差分别约为0.15和0.20)，适合作为非稳态下表征人工湿地水流规律和停留时间的特征参数.研究显示，归一化RTD易于模拟，可用其描述非稳态下人工湿地的水流规律以及水流规律对污染物接触反应时间的影响. 

Flow Pattern of Constructed Wetland under Unsteady Inflow Conditions and its Effect on Residence Time

Constructed wetland treating urban or agriculture runoff usually runs under unsteady state with variable inflow. In order to study the flow pattern of the constructed wetland under unsteady state condition, the residence time distribution (RTD) of horizontal subsurface constructed wetland (CW) under unsteady inflow resulting by one-peak and double-peak shock loads was studied through multiple-time-injection tracer tests. The results indicated that the effect of flow variation under unsteady state condition on RTD of inflow at different time is significant. The original RTD was normalized by dimensionless nonlinear time viable φ based on the outflow rate of the CW. The concentration curve of normalized RTD presents a relatively stable shape with similar leaning, peak time (0.83-1.00 under one-peak shock load condition, and 0.87-1.08 under double-peak shock load condition), centroid (1.08-1.25 under one-peak shock load condition, and 1.05-1.25 under double-peak shock load condition) and statistics, which indicats a stable flow feature that was not related to the flow variation. Compared with the mean residence time (t_mean) of the CW of the steady base flow, t_mean decreased dramatically before the inflow shock loads and restored rapidly after the shock loads. (The differences between maximum and minimum values under one-peak and double-peak shock loads conditions are 30 hours and 32 hours, respectively) However, the normalized t_mean' remained steady relatively. (The differences between maximum and minimum values under one-peak and double-peak shock loads conditions are 0.15 and 0.20, respectively), which can be used as characteristic parameter for representing unsteady state flow and RTD of constructed wetland. The study shows that the normalized RTD was stable in unsteady inflow condition and modelled, thus it could be applied to characterize the flow pattern and its impact on reaction time during pollutant removal process.