Study On Anti-blocking Effect Of Earthworm In Vertical Flow Constructed Wetland And Its Mechanism

Constructed wetlands often have the problem of low operating efficiency caused by blocking backwater.In this paper,the effect of earthworms on pollutant purification efficiency and permeability of constructed wetland were studied.Through the application of finite element calculation software ANSYS and seepage calculation software GEOSEEPAGE,a two-dimensional seepage calculation model was established to simulate and analyze the influence of earthworms on the permeability of the surface soil of vertical flow wetland.The removal characteristics of COD and ammonia nitrogen in constructed wetland with the earthworm were analyzed in order to provide technical support for the design of vertical flow constructed wetland.The main research results and conclusions of this paper are as follows:The purification efficiency of vertical flow constructed wetland with or without earthworm were compared by experiments.It is clear that constructed wetlands with the earthworm could withstand the impact of higher hydraulic loads,and have higher removal efficiency of COD,ammonia nitrogen,total nitrogen and total phosphorus by wetlands than that without the earthworm.The introduction of earthworms into constructed wetlands not only improved the treatment efficiency of pollutants to a certain extent,but also effectively eliminates the backwater condition that is easy to occur when without earthworms.The constructed wetland without earthworms had backwater phenomenon after running for6 days and hardly recovered.However,the constructed wetland with earthworms could still be in a stable state after 200 days of operation and recover quickly by itself even if local temporary backwater occurred when the organic load of water inflow increased.The detection results of earthworm quantity showed that the earthworm quantity was 0.3665 g at the initial stage of operation and stabilized at 0.3018 g at the later stage of operation,and the soil surface porosity also stabilized from 31.11% to 26.02%,indicating a positive correlation between soil porosity and earthworm quantity.In order to explore the mechanism of earthworm's anti-clog effect on vertical-flow constructed wetland,the influence of earthworm on the permeability of wetland surface soil on the 45 th day of operation under the hydraulic load of 0.3m3/(m2·d)was investigated by modeling with ANSYS and other software based on the experimental data.The experimental results showed that the soil permeability coefficient of the earthworm free wetland decreased from 0.28m/d to 0.16m/d,indicating that the wetland was seriously affected by pollutant blockage and permeability decreased.For the wetland with earthworms,the permeability coefficient of soil increased from 0.28m/d to 8m/d,indicating that the wetland did not clog but improved permeability due to the action of earthworms.Therefore,the anti-clogging effect by earthworm on constructed wetland is related to its remarkable improvement of permeability of wetland.On the basis of the experimental data,the removal kinetics of COD,ammonia nitrogen,total nitrogen and total phosphorus in constructed wetlands with and without earthworms was discussed.Under low hydraulic load,the pollutant purification process of constructed wetland without earthworm and with earthworm can be well fitted by first-order dynamic equation,and the fitting degree is better with earthworm than without earthworm.However,under high hydraulic load,the first order dynamic equation does not accord with the removal process of COD.The dynamic equation established on the basis of experimental data can provide more reliable design parameters for the design of vertical flow constructed wetland.