Performance Of Tidal-Flow Constructed Wetland For Polluted River Water Treatment

Along with the fast urbanization, rapid economic growth and continuous water-environment deterioration, the imbalance between water supply and demand has increasingly become prominent, which seriously hampers comprehensive, coordinated and sustainable development of our economy and society. It is effective to improve water environment and alleviate water shortage by enhancing reclamation and utilizing wastewater treatment technology. The constructed wetland takes advantage of interactions of soil-plant-microbe to achieve pollutant removal. Compared with other wastewater treatment technologies, it has many advantages, including low constructing and operating cost, high capability of pollutants removal such as nitrogen and phosphorus, simple management and perfect carrying capability. Therefore, it is widely applied to treat various polluted surface water and domestic wastewater in rural areas.Currently most used wetlands were subsurface flow constructed wetlands. However, the low DO in the subsurface flow constructed wetlands restrained the degradation rate of organic matters, which caused the accumulating of a great number of organic matters on gravel surface, and then the decreasing of hydraulic loading. Finally, the bad treatment effect of the constructed wetlands became inevitable. Therefore, sufficient DO is vital for the performance of the constructed wetlands and the improvement of the pollutant removal.The tidal-flow constructed wetland is a new type of constructed wetland which takes advantage of the rhythmical input and output of wastewater and air, so it forms the circumstance of aerobic and anaerobic to realize and enhance pollutant removal.This study focused on the difference of purification capacities of tidal-flow constructed wetlands and traditional continuous-flow constructed wetland at low temperature (9^-13 oC), as well as the treatment performances under different operation conditions. At the same time, the micro-organism activities were investigated in different reactors. The results showed that, under the hydraulic load of 0.2 m3·m-2·d^-1, the effluent DO value in the three-cycles·d^-1 tidal-flow constructed wetland was 0.61 mg.L^-1, which was higher than that in continuous-flow constructed wetland (0.22 mg.L^-1), the removal rates for NH₄⁺-N, COD and PO₄^3--P in the three-cycles·d^-1 tidal-flow constructed wetland could reach 48.57%, 68.96% and 29.02%, respectively, which were about 30%, 20% and 20% higher than those in the continuous-flow wetland, correspondingly. Meanwhile, the microbial respiration and dehydrogenase activity results suggested that the micro-organism activity in the three-cycle.d^-1 tidal-flow constructed wetland was higher than others.For further study of the NH₄⁺-N removal behavior in the three-cycles.d^-1 of tidal-flow constructed wetland, analysis and confirmation of the key factors which affected the NH₄⁺-N removal effect and then to further optimize the NH₄⁺-N removal process in the reactor, this study took the NH₄⁺-N removal in the three-cycles d^-1 tidal-flow constructed wetland as the research object, made dynamic analysis by the adoption of apparent dynamic model, multiple regression model and Monod-mechanism model and built up a mathematical model to make sure the key influential factors for the NH₄⁺-N removal effect, simulating and predicting the NH₄⁺-N removal effect in the tidal-flow constructed wetland. The experimental results showed that, the NH₄⁺-N removal fit through the use of apparent dynamic model, multiple regression models and Monod-mechanism model, which proved that it was efficiently fit by the Monod-mechanism model with the fitting degree R2 of 0.9031. At the same time, the local sensitivity was analyzed by modified Morris Screening method and it showed that, in the Monod-mechanism model, （NH₄⁺-N）in was a highly sensitive parameter, the average water temperature （T） was a sensitive parameter and DO was a medium sensitive parameter.The influence of vegetation and vegetation was also investigated in this study, the results showed that removal efficiencies of COD and NH₄⁺-N were greatly impacted by the temperature: for COD, the temperature dependence was significant （ANOVA, p<0.05）, and the average removal rate of COD in summer period could reach 19.64 g·m-2·d^-1, which was 30.9% higher than that in winter period; similarly, the temperature dependence was also significant （ANOVA, p<0.05） for NH₄⁺-N, the average removal rate of NH₄⁺-N in summer period could reach 2.92 g·m-2·d^-1, which was 74.85% higher than that in winter period. However, temperature didn’t show great influence on PO₄^3--P removal. Furthermore, it could be seen that N and P Removal effect in the tidal-flow constructed wetland was not focused on the adsorption and sedimentation of the grave, and treatment performance of the planted wetland for COD and NH₄⁺-N was generally better than the unplanted one.