Study On Operational Parameters Optimization And Removal Mechanism In Constructed Wetland

In this paper, the multi-stage constructed wetland (MSCW) located in the outdoors wasstudied to optimize the operational parameters and analyze mechanism of pollutant removal ofthe MSCW. Through the investigation of the MSCW, the study provides technical informationand practical experience for the design, run and promotion of the MSCW.The optimization of the hydraulic loading rate (HLR) showed that the average removal ratesof CODcr (Chemical Oxygen Demand), NH4+-N (Ammonia Nitrogen), TN (Total Nitrogen) andTP (Total Phosphorus) for the HLR of6cm/d were69.1%,91.9%,50.5%and56.5%,respectively; and those for the HLR of12cm/d were50.8%,76.7%,46.3%and46.3%; and thosefor the HLR of24cm/d were60.2%,75.7%,38.0%, and40.4%; and those for the HLR of36cm/d were49.8%,65.4%,33.4%and28.8%, respectively. In conclusion, the HLR of6cm/d wasrecommended for the best comprehensive removal efficiency.The optimization of the hydraulic residence time (HRT) showed that the average removalrates of CODcr, NH4+-N, TN and TP for the HRT of2.5d were70.0%,67.0%,52.8%and47.5%,respectively; and those for the HRT of2d were87.4%,92.5%,58.5%and43.0%; and those forthe HRT of1d were61.0%,81.6%,56.1%and37.9%; and those for the HRT of0.5d were53.4%,80.9%,51.0%and28.0%, respectively. According to non-significant discrepancy (P>0.05) of theaverage removal rates of TP for the HRT of2.5d and2d, it is suggested that the HRT of2dshould be used in the MSCW.The mechanism of pollutant removal of the MSCW with the HRT of2d (equivalent to theHLR of10cm/d) was also studied, and the first result showed that the average concentrations ofthe four kinds of pollutants along the distance decreased from inflow to outflow, gradually, andthat the ability of removal rate of Vertical Flow Wetland (unit E) was best in comparison with theother wetland units. And the second result showed that the influent pollution loading was ofcritical importance in the way of water purifying rate. In addition, it was statistically analyzedthat there was some important correlation between the influent pollutant loading and effluent removal rate of each pollution index, such as highly significant positive correlation from NH4+-N,significant positive correlation from COD and TN, non-significant positive correlation frombetween ratio of NH4+-N/TN and removal of TN as well as TP. And the third result showed thatabout51.84%of nitrogen and76.30%of phosphors were removed by the soil substrate, butabout17.30%of nitrogen and21.78%of phosphors of removed by the plants. The adsorptionand uptake of soil (mainly microbial process) was the dominant way of nitrogen and phosphorsremoval, following by the uptake of plants and the other factors including the pH value ofwastewater and nitration-denitrification reaction.