Experimental Study On Nitrate Migration And Transformation Rules In Constructed Wetland With Different Solid Carbon Source

Recent years, due to many inappropriate human activities, the nitrate pollution in groundwater has become a difficult problem in worldwide. At precent, the more effective way of control nitrate pollution is biological denitrification, but this process was effectted by many environmental factors. In which, how to choose carbon source type and the amount use was one of the limiting factors. Meanwhile, denitrification efficiency of constructed wetland was effectted by hydraulic loading and pollution loading.In this paper, by means of sand column experiment, choose the more effectice carbon source for denitrification from rice straw and reed marsh, then construct constructed wetland, find its flow characteristics, further, add the result of sand column experiment to the wetland model, study the nitrate removal efficiency of the model under different hydraulic loading and pollution loadingthe, the results showed that:(1)Nitrate in effluent of reed reactor was lower, its color was less, and reed marsh degraded more slowly, so considered it was more suitable for denitrification.(2)Build constructed wetland, first to master the flow characterictics in the wetland by tracer experiment, found that: average HRT was respectively 30.36h,65.22h,203.61h when hydraulic loading was respectively 106.56Ld^-1,53.28Ld^-1,26.64Ld^-1, theσ2 corresponding was 0.11,0.36,0.08. So when hydraulic loading was 53.28Ld^-1, the flow mixed most; when hydraulic loading was 26.64Ld^-1, the flow was closer to plug flow, but its HRT was too long to practical application.(3)When hydraulic loading was 106.56Ld^-1, a few of sample points set in short flow area, and there may be a vertical flow from below near the outlet; Along with the flow, the conductivity value of every sample point decrease. When hydraulic loading was 53.28Ld^-1, only one point set in short flow area; the same as former, there may be a vertical flow from below near the outlet. When hydraulic loading was 106.56Ld^-1, more area set in short flow area, the flow only flowing through one of the sample points in carbon layer. The conductivity peak value of every sample point was decrease by the hydraulic loading increase. (4)Add reed marsh in the wetland as carbon source of denitrification, comparedthe denitrification efficiency in different hydraulic loading, simulating nature wetland denitrification efficiencu in wet and dry season by this. When hydraylic loading was 106.56Ld^-1, 53.28Ld^-1, 26.64Ld^-1, nitrogen remocal rate was 41.13%, 76.31%, 67.78%.(5)When nitrate concentration was 50mgL^-1, hydraulic loading was 106.56Ld^-1, 53.28Ld^-1, 26.64Ld^-1, nitrite accumulation was about 2mgL^-1, <2mgL^-1 and <1.4mgL^-1. In this model, nitrite concentration of the fires layer was lower than the other layers. But in most case, nitrite concentration of the second layer was increase, this suggest that this layer was recharge by the first layer.(6) When nitrate concentration was 50mgL^-1, hydraulic loading was 106.56Ld^-1, 53.28Ld^-1, 26.64Ld^-1, ammonia nitrogen accumulation was <1.06mgL^-1, <4.01mgL^-1, <3.05mgL^-1. Under the three different hydraulic loading, ammonia nitrogen of the first layer was higher than the others, because DO in this layer was high, this will lead to ammoniation.(7)In most case, concentration of HCO₃^- was increase with NO₃^--N concentration decrease; Although denitrification was a anaerobic process, NO₂^--N will accumulated when DO concentration was too high or too small; Along with NH₄⁺-N concentration increase, pH increase.