Simulation Study On Removal Of Contaminants From Rainfall Runoff Of Hard Revetment By Ecological Mats

With the development of urbanization in China,a large number of river banks,including urban or rural areas,have been transformed into hard revetments with concrete and masonry blocks.This resulted in huge damages to ecosystem of riparian zone and the loss of the ability to intercept and purify the contaminants in runoff,and thus intensified the eutrophication in river.In order to solve the issues of the vast urban hard revetments,in the present study the ecological mats technology was applied to improve the intercepting capacity on contaminants in runoff.Firstly the suitable culture medium for the ecological mats combined with plant cultivation were screened.Then the simulation experiment was conducted under different slopes（45°,60°and 90°）and different rainfall intensities（light rain intensity of 10 mm/d,medium rain intensity of 20 mm/d and heavy rain intensity of 40 mm/d）to remove the contaminants of different concentrations in runoff,in spring and summer,respectively.The associated mechanisms were analyzed as well.Aiming to explain the effects of ecological mats application on the restoration of hard revetment.The main results are shown as following:（1）Based on experiment of screening the suitable medium for the ecological mats combined with plant cultivation,the ryegrass was selected because of its higher seedling rate,chlorophyll content and root biomass on the ecological mats;the coir and pelelith were selected as growing medium because they promoted the plant growth on ecological mats with the highest effects.（2）The results from short-term rainfall simulation experiments in spring and summer showed that the removal efficiency of TSS in simulated runoff is the highest,and the removal efficiency of86%-95%and 90%-99%was found in spring and in summer,respectively.The removal efficiency of TSS by pelelith mats was not significant difference in two seasons.The removal efficiency of TN and NH₄⁺-N decreased with the increase of slope,rainfall intensity and rainfall duration,whereas it increased with the increase of contaminants concentration.The removal efficiency of TN and NH₄⁺-N by coir mats in summer was 11.95%-35.62%and 24.51%-59.21%,which was higher for pelelith mats and in spring.However,the removal capacity of P in runoff by ecological mats was weak.The removal efficiency of P was relatively higher under smaller slope（45°and 60°）and moderate or light rainfall intensity（20 or 10 mm/d）.The maximum removal rate of P was approximate 33%,which could be improved with the increase of contaminants concentration.The removal efficiency of N and P was higher in summer than in spring,respectively,for different ecological mats,which might be attributed to increase the biomass production of shoot and root under higher temperature in summer.The uptake of N and P by plants in summer was approximate1 fold than that in spring,resulting to increase the removal efficiency of contaminants in runoff by ecological mats.（3）The results from one-month culture experiment showed that the plant growth in coir-ryegrass ecological mats varied depending on different slopes.The growth of plant shoot and root increased with the decreasing slope.The N removal by the ecological mats was mainly attributed to plant N absorption and microbial nitrification/denitrification.With the increasing slope,the nitrogen removal ratefrom microbial nitrification/denitrification process significantly decreased from 62.8%at 45°to 34.9%at 90°.The contribution of plant absorption significantly increased with the increase of slope,which could be obtain from 28.1%at 45°to 60.5%at 90°.The N interception by substrate accounting for only 4.7%-12.1%due to its smaller porosity.The P removal by the ecological mats was predominantly attributed to plant absorption.Coir was a more stable substrate because its main components such as cellulose,hemicellulose and lignin,was not change remarkably,despite a slight loss of phosphorus.The ecological mats technology is cost-effective and easily operated,and is able to intercept and high efficiency of remove the TSS,TN and NH₄⁺-N in runoff.Therefore,it can be used as a novel ecological restoration technique to improve the ecological landscape of hard revetment and the interception and purification of contaminants in runoff.However,the large-scale application needs to be further investigated to optimize and improve the ecological mats technology gradually.