Studies On Purification Characteristics Of Polluted River Water And Plants Mechanisms By Using Constructed Wetland

In order to improve the water quality and restore the ecological functions of the polluted river effectively, the purification characteristics of the constructed wetlands(CWs) for polluted river water treatment were studied with the idea of adjust measures to local conditions, eco-friendly and low-energy consumption. The pollutants removal mechanisms in different CWs combing free water surface flow(FWS) and subsurface flow(SSF) wetlands for polluted river treatment were also studied for improve the performance of the CWs, and the most suitable CWs combining form was determined during the experiment. Furthermore, combing with the effect of the design and operation conditions on the performance of CWs, and the removal pathways of suspended and dissolved pollutants, the functional mechanism of the components of CW in pollutants removal were also revealed. Finally, the growth characteristics and mechanisms of plants in different CW types, and the effect of interspecific competition and harvesting on the growth and roles of plants in CWs were analyzed. The selection and management strategies of plants in CWs were also optimized. The main research conclusions are as follows:(1) The multivariate CWs brought about an effective improvement of the river water quality. During the four years’ experiment, the water quality of the polluted river was improvement gradually. At the end of the experiments, the removal efficiencies of suspended solids(SS), chemical oxygen demand(COD), 5-day biochemical oxygen demand(BOD5), total phosphorus(TP), total nitrogen(TN) and Ammonia(NH3-N) were reached about 95%, 75%, 90%, 75%, 70% and 80%, respectively. Furthermore, the removal rates of SS, COD, BOD5 and TP were approximate linear positive correlation with the pollutant loading rate, and the removal of SS and TP were scarcely fluctuated with the seasonal variations. However, the removal rates of TN, NH3-N and COD, BOD5 in the CWs were highly impacted by the temperature and the growth status of the plants, respectively.(2) The results indicated that the removal rates of SS, COD and BOD5 in CWs with different combing forms were not different significantly(P>0.05), and the concentration of DO was the main limiting factor for the nitrogen removal in CWs. Furthermore, the nitrogen removal could be improved effectively while the clogging of the wetlands could be prevented in the FWS-SSF CW, in which the pathways of the pollutants removal were optimized. Finally, the removal of suspended pollutants was effectively in both of FWS and SSF CWs, which were set as the first-stage unit in the hybrid CWs. However, the DO concentration was only increased in the FWS CW. The removal of dissolved nitrogen and phosphorus could be enhanced in both of FWS and SSF CWs, which were set as the second-stage unit in the hybrid CWs. However, it was difficult to improve the removal of dissolved COD in FWS CW.(3) Compared with local gravel, using local slag as the substrate, the pollutants removal could be much improved. The FWS CWs showed better NH3-N and TN removal while SSF CWs showed better COD and TP removal. The results also indicated that wetland types mainly affected the removal of dissolved COD, TN and TP, as the differences was 13.2%, 12.5% and 55.6%, respectively. The effect of the substrates was mainly on the removal of dissolved pollutants. Furthermore, in the SSF unit of the FWS-SSF CW, the removal of dissolved nitrogen and phosphorus were higher in the planted wetland, and about 55.5% and 51.8% of the difference was caused by the plants direct uptake, respectively. Moreover, the contributions of plants in nitrification, denitrification and phosphorus removal were increased gradually. The combination of the substrates size grading and the plants could improve the conditions within the wetlands for nitrogen and phosphorous removal and kept the internal space of the wetland much more uniformity. In addition, SSF CWs had better treatment performance under lower hydraulic loading. However, compared with hydraulic loading, the effect of pollution loading on nitrogen removal was much greatly in the FWS CW.(4) Both in the surface flow and subsurface flow CWs, the plants density were increased exponential in the first year when there were more space for the plant growth(about 20 times higher than initial value). From the second year, the interspecific competition between the wetland plants species would be happened, and the P. australis began to encroach the standing zones of T. orientalis, which was much more serious in the SSF wetland. Furthermore, the results showed that surface flow CWs were much more suitable for the plants growth especially for T. orientalis. The plant biomass increased from 1.7 kg/m~2 to 5 kg/m~2 during the experiment period, which also enhanced the nutrient removal by plants year by year(52.5 g N/m~2 and 5.5 g P/m~2-190.6 g N/m~2 and 18.8 g P/m~2). Additionally, the effects of interspecific competition on the height, density and dry matter of plants and the performance of the CWs were limited. However, the standing zone of the plants was highly affected by interspecific competition, which resulted in a worse growth status and a lower nutrient uptake ability. Moreover, annual harvesting provided better space and light conditions for plants regeneration. Therefore, the height, density and dry matter of plants in harvest wetland were higher than that in unharvest wetland, and resulted in better nutrient removal capacity. The interspecific competition and harvest manner were two important factors for the growth and nutrient removal ability of the plants in CWs.