Study And Simulation On Constructed Wetland Residence Time Distribution Under The Unsteady State

As the point source water pollution control level was improved, non-point source pollution gradually stands out. Constructed wetland was gradually got the attention of people in urban and agricultural runoff, because of its advantages, for example, high treatment effeciency,low cost of operation and management, it can random intermittent operation and resist the loading fluctuation. The traditional theory of sewage treatment was usually based on steady state conditions of continuous inflow and relatively constant flow, this lead to the current theoretical, parameter, model show the obvious shortcomings when they are explaining, describing and simulating the unsteady state constructed wetland decontamination efficiency, so hinder constructed wetland technology extend from steady sewage treatment field to unsteady sewage treatment field. Non-point source pollution control in urban and agricultural runoff is the most prominent problem of sewage treatment under unsteady state. So, researching the flow law of constructed wetland under unsteady state has an important realistic meaning for the further development and application of constructed wetland technology, especially the application in the non-point source pollution control.This paper take horizontal flow constructed wetland and vertical flow constructed wetland as examples, based on the timeline of multipoint tracer test, obtained residence time distribution groups(RTDs),then compared it with the residence time distribution under steady state, analyses its flow characteristics and change rule, established residence time distribution model.Through the constant load steady-state tracer experiments of different nominal hydraulic retention time, it indicated when the inflow is constant, the flow regime of horizontal flow constructed wetland and vertical flow constructed wetland all straying from the traditional flow regime. There is different degree of mixing in the system, with the nominal hydraulic retention time increase, the mixing phenomenon is more obvious.Through the timeline of multipoint tracer test under shock load, the mean hydraulic retention time is smaller when the liquid flow into the constructed wetland more close to the shock moment, after the shock load, it tend to the mean hydraulic retention time of steady state, it indicate the inflow before the end of the shock are influenced by the shock load, and more closer the occur of the shock load, the influence are more obvious. During the entire process of shock load, the variance of residence time distribution has no significant increase or decrease trend, but it basically less than steady state flow’s, it indicate that shock load can reduce the internal longitudinal mixing and diffusion effect of wetland system in some degree, and Make the flow approach to the ideal flow.The residence time distribution characteristic of constructed wetland is greatly influenced by rate of inflow and outflow, the quantity of tracer, adding time of tracer, therefore, time variable f based on flow rate is used to normalize RTD, and the normalized RTD remove the effect of this factor, show the stability of the residence time distribution, so it more representative of the flow law of constructed wetland under unsteady state.CSTRs+PFD parallel model is used to simulate the RTD of unsteady state, this has four parameters, and they are the percentage of CSTRs volume to total volume, the percentage of CSTRs flow to total flow, the number of CSTR and distributed of PFD. This model has a good fitting with the RTD curve of experiments. Using this model to simulate the outflow concentration of horizontal flow constructed wetland under 8 times shock load, the simulation outflow concentration is very closed to the experiments, so the model can be used to outflow concentration of constructed wetland.