The Effect Study On Nitrogen Removal In Horizontal Subsurface Flow Constructed Wetland Under Different Hydraulic Conditions

Through its natural eco-hydrological process, physical, chemical and biological functions are combined in the constructed wetland sewage treatment system, thereby CWs' ability is enhanced to degrade and deal with pollutants. The effluent water of CWs is of high quality and security and it enjoys low infrastructure and operating costs as well as a simple management. But on the other hand, there are usually too many affecting factors on the water quality including: internal factors like wetland fillers, wetland plants and microbes, load factors like hydraulic loading, pollutant load and its composition, water quality factors like water temperature, dissolved oxygen, pH, oxidation-reduction potential etc., external environmental factors like light, air temperature and humidity. The lack of research on these factors may not be conducive to the efficient operations and effective regulations of the constructed wetland sewage treatment systems.A study was launched with a self-built wetland simulation device in order to improve the nitrogen removal effect in the subsurface flow constructed wetlands, which included (1) the studies of the hydraulic and pollutant load impact on nitrogen removal; (2) the study on the relationship between the nitrogenous pollutants concentration and the water quality parameters of inflow, outflow and water sampled along the wetland; (3) the principal component analysis of the nitrogen removal in CWs; (4) the study on strengthened operational measures of the nitrogen removal in CWs; (5) photosynthesis and transpiration effects of wetland plants on the nitrogen removal in CWs and so on. The whole experiment was carefully designed with a comprehensive, long-term monitoring and recording. All these experimental data is used to make a more comprehensive analysis using regression analysis, orthogonal analysis and principal factor analysis.The experimental results show that the upper and front part is the main degradation site of COD and ammonia nitrogen, while the back-end and the lower part is the main transformation site of nitrate nitrogen. The upper part is suitable for the conduct of nitration, while the lower part is suitable for the conduct of denitrification; the optimal temperature for nitrogen removal should be around 20～25℃in CWs. If the hydraulic loading of wetlands takes the best operating parameters, the water surface will be just 0～5 cm below the substrate surface in the experimental device. This result shows that to improve the level of nitrogen removal effect in subsurface flow constructed wetland, its "subsurface" characteristic must be firstly maintained. The results of the orthogonal experiment of three kinds of pollutants show that the order of the affecting pollutants in TN removal is: ammonia nitrogen, nitrate nitrogen, organic matter. The C / N ratio is 3.33; accounting for 76.9% of them is COD, ammonia nitrogen, nitrate nitrogen 11.55% each. The mass ratio of ammonia nitrogen and nitrate nitrogen is about 1:1, indicating pollutant load balance may lead to nitrification and denitrification in balance so as to enhance the wetland nitrogen removal effectiveness. The enhanced oxygenation and reflux operation modes in the constructed wetlands have positive significance for the removal of the nitrogen-containing pollutants, but first of all the applicability the inflow water quality should be considered in order to select the most appropriate method. The water use efficiency is the main factor in the photosynthesis tests. An appropriate reduction of water use efficiency of the wetland plants in constructed wetlands can significantly improve nitrogen removal efficiency. The result of the principal factors analysis of outflow water quality shows that dissolved oxygen (DO) and water temperature is of significantly importance than that of the other factors. In addition, due to the inherent volatility of wetlands the order of importance among the factors is not constant.This study was designed to identify the efficient denitrification control parameters and effective modes of operation in suitable levels of subsurface flow constructed wetland system. It was also designed to identify the major influencing factors in different mode of operation and different hydraulic conditions. All these results are a reference while understanding the mechanics of nitrogen removal and engineering design of CWs.