Cyclic Processes Of Carbon, Nitrogen And Phosphorus In Constructed Wetlands And Its Environmental Effects

At present there are generally severe environmental problems especially water pollution and ecological degradation of the river’s basin in the developing countries, due to serious pollution and destruction caused by human beings. Constructed wetlands (CWs) is a reasonable option for treating wastewater and has been widely used in the developing countries, because of their lower cost, less operation and maintenance requirements, and lack of reliance on energy inputs. The design, operation and application of CWs is very important in pollution control, therefore, it is necessary to study the cyclic processes of carbon, nitrogen and phosphorus in CWs and its environmental impacts for the further treatment of polluted river water.In this study, the longterm treatment performance in the pilot-scale CWs for treating polluted river water was studied, and the contaminant transformation and degradation characteristics during removal process were also investigated. The main components of CWs, such as wetland plants and substrates, were systemically studied, and their function and role in wetland succession have been evaluated. The greenhouse gases fluxes and characteristics in CWs were studied through the method of static chamber-gas chromatography, furthermore, the correlations between greenhouse gases emission and its key influence factors was analysed. By building material cycling models in CWs, the contribution of different pollutant removal pathways in CWs during the experimental period quantified, and the dominant removal pathways for different pollutants in CWs were also determined. The environmental effects of CWs were assessed based on the in-situ monitoring in the full-scale surface CWs. The main research conclusions are as follows:(1) The CW was found to be suitable for treatming polluted river water and could achieve an excellent long-term removal performance. The average effluent concentration of COD and NH4+-N met Grade-III of national surface water standards in China (GB3838-2002), and the average removal efficiency of COD, NH4+-N, TN and TP was65.63%-76.69%,83.61%-94.43%,44.78%-82.77%and36.65%-70.77%. The calculated first-order removal rate constants for COD, NH4+-N, TN and TP removal in CWs were0.39d"1,0.26d-10.11d-1and0.01d-1, which illustrates that COD degraded more rapidly than NH4+-N, but TN and TP had the lower biodegradated rates. The removal process of organic matter in CWs mainly occurred in the front of the interface layer between sediment and water. Nitrogen in CWs degraded slowly in the interface layer between sediment and water, and its removal depended on plant uptake and microbial transformation processes in the sediment. However, phosphorus was mainly removed in the the front of the sediment in CWs.(2) All of plants in CWs grew well with as wetland systems operated and developed sustainably. The highest total biomasses of plants per year were0.5459-1.6841kg/m2. The rates of oxygen release of different plants were0.24-0.36gO2/FWg/d or126.67-297.78gO2/m2/d, and oxygen release rates in the budding, elongation, maturation and dormancy phases were higher than values obtained in other stages of plant growing. The rates of organic carbon excretion by roots were0.18-0.52mg TOC/gFW/d or120.48-431.31mg TOC/m2/d, and the maximum and minimum were obtained in the elongation phase and in the dormancy phase respectively. The organic carbon, nitrogen and phosphorus assimilated in plants were151.52-878.29g/m2,9.22-42.51g/m2and1.89-4.29g/m2, accordingly, the accumulated organic carbon, nitrogen and phosphorus accumulated in the sediment of various wetland systems at the end were23.88-37.81g/m2,10.94-14.13g/m2and3.98-4.17g/m2. The harvest of plants in different periods would influence the growth of plants and removal performance of CWs, and wetland plants should be harvest in November or December timely.(3) The CW was the sources of atmosphere N2O and CH4as a whole, on the contrary, it appeared to be the sink of atmosphere CO2. The mean N2O, CH4and CO2fluxes in wetland systems were215.39-514.3μg/m2/h、2.17-145.23μg/m2/h and-592.83-553.91mg/m2/h in this study, and the values in summer were higher than in other seasons. The mean N2O flux in this study was higher than the values reported in the literature for ecosystems e.g. farmland, forest and natural wetlands, but similar to the values in the paddy field. The mean CH4flux in this study was higher than the values in paddy fields, and similar to the values in forests, but lower than other ecosystems e.g. natural wetlands and reservoirs. The growth of wetland plants in CWs promoted the emission of N2O and CH4, however, reduced the emission of CO2. The high N2O, CH4and CO2fluxes were obtained as influent concentration in CWs increased. But CWs exhibited a decrease of N2O emission when having too high influent concentration. In general, regulating and stabilizing the influent of wetland systems properly, as well as selecting and harvesting plants timely, would be probable measures for controlling the greenhouse gas emission in CWs.(4) The dominant removal pathways for organic matter in CWs were aerobic biological metabolism besides sediment storage, but metabolising methane had little contribution. Microbial nitrification and denitrification processes were the main nitrogen removal pathway besides plant uptake and sediment storage, and ammonia volatilization reduced less nitrogen. Sediment storage was the key factor limiting phosphorous removal in CWs and plant uptake could also remove a portion of phosphorus. Based on the mass balance in CWs for treating polluted river water in this study, the emission of CO2by aerobic biological metabolism accounted for46.78～54.01%of the total carbon input, and sediment storage contributed10.15～16.08%. Plant uptake accounted for5.44～25.07%of the total nitrogen input, while nitrogen removal by sediment storage and N2O emission contributed6.45～8.32%and1.1～2.63%, respectively. However, the percentage of NO and N2emission due to nitrification and denitrification and other nitrogen loss was estimated to be35.89～48.92%. It was also shown that sediment storage accounted for28.27～33.92%of the total phosphorous input, while plant uptake accounted for12.83～32.9%.(5) Based on the full-scale study in Wu River CW, it was indicated that the CW improved the quality of the river water. The mean concentration of COD and NH4+-N in effluent met Chinese Grade-Ⅲ national surface water standards, and COD and NH4+-N removal capacity of the CW was estimated to be3.95t/hm2/y and0.75t/hm2/y. Wu River CW exhibited a certain risk of greenhouse gas emission. It was the sources of atmosphere N2O and CH4, but the emission flux in this study was lower than the values reported in the literature for sewage treatment plants. In addition, the riverside ecological ecosystem was also remediated by building Wu River CW, and the CW also played an important role in water conservation, biodiversity conservation, education and ecology landscape.