Efficiency And Mechanism Of Treating The Polluted River Water With Constructed Wetland

The water environment has been contaminated seriously with the development of urbanization in China. The water pollution not only destroyed the landscape but also did harm to the health of the citizen. At present, to restore the polluted river hasbecome an important task. Combining the "863" project------Improvement andEcological Restoration of Water Environment in Zhenjiang City, the efficiency and mechanism of treating the polluted river water with constructed wetland were studied in this paper. The results were as follow:(1) Compared with the sewage, the polluted river water has much different characteristics. For example, the nitrogen and phosphorus concentration was high, the organism concentration was low, and the water contained lots of suspended pollutant. The removal efficiency was good when treating the polluted water in the Ancient Canal which was worse than Grade V with the integrated vertical-flow constructed wetland. When the hydraulic loading(HL) was about 750mm/d, the removal rate of the COD_Mn, TN and TP were respectively from 50% to 60%, from 40% to 50%, and from 30% to 40%, and the quality of the effluent could reach Grade IVor even III.(2) When treating the polluted river water by the integrated vertical-flow constructed wetland, the pollutant was removed mainly by the down-flow chamber, especially the surface layer (0-30cm), while the function of the up-flow chamber was less. At the first step, the suspended pollutant was hold back and adsorbed by the padding and plant roots. The dissolved pollutant was transformed into stable material(such as microorganisms CO₂ N2, H₂O and so on ) by the microorganism on the padding and plant roots. At the second step, the suspended pollutant was transformed into dissolved pollutant which was flowing with the water and degraded by the microorganism.(3) The reason of low efficiency of up-flow chamber could be explained by the flowing: On the one hand, the suspended pollutant could be transformed into dissolved pollutant in the surface layer, and the concentration of the dissolvedpollutant was increased. So the removal efficiency of the up-flow chamber was decreased. On the other hand, the microorganism cost lots of the oxygen and organic material in the surface layer, so the concentration of the oxygen and the ratio of C/N were low in the water of the up-flow chamber, which decreased the degradation velocity of the organism and nitrogen.(4) The removal rate of the TN was improved by the method of adding carbon source to the nitrate wastewater. Despite of adding carbon source to the ammonia wastewater, the removal rate of the TN was not improved as the C/N improved. DO was low in the down-flow chamber, so the nitrobacteria was restricted and the removal rate of the TN decreased as the C/N improved.(5) If carbon source(C/N>8) in the influent was abundant, the TN removal rate of the nitrate wastewater was decreased as the HL increased. Especially when the HL>750mm/d, the removal rate of the TN was decreased prominently as the HL increased.(6) By analyzing the material balance of contaminant in the integrated vertical-flow constructed wetland, the organic contaminant and nitrogen was mainly removed by the microorganism and the respective removal rate was 97.3% and 85.2%. Phosphorus in the water was removed by chemical aggradations of the gravel padding. Intensifying the activity of microorganism could increase the removal rate of organism and nitrogen, and choosing the padding which adsorbed and aggradated phosphorus effectively could increase the removal rate of phosphorus.