Study On Removal Efficiency And Mechanism Of Microaerated Constructed Wetland

Water dissolved oxygen level can directly affect the degradation of organic matter,nitrogen nitrification-denitrification,etc,which has great importance for pollution control in constructed wetland.Constructed wetland,with micro aeration condition is becoming more and more attention in recent studies.However,so far,there is limited understanding of the transformation characteristics and process mechanism of the microaeration in this constructed wetland.In this study,the performance of pollution removal in a micro-aerated comstructed wetland demonstration project with long successful operation was investigated.Besides,the optimal operation conditions and pollutants removal mechanism were investigated by running a pilot micto-aerated constructed wetland.It aims to optimize operation conditions and reveal the possible mechanisms of pollutants removal in the micro-aerated constructed wetland,which has great practical significance for the popularization and application of the micro aeration constructed wetland.The main conclusions that obtained in this paper are as follows:(1)Micro aeration was significantly improved the dissolved oxygen level in constructed wetland.DO concentrations reached 3-5 mg/L,which effectively relieved the short of oxygen supply in constructed wetland and strengthened the microorganism aerobic decomposition ability.(2)The average removal efficiency of turbidity,COD,TP and NH4~+-N and TN in the micro aeration constructed wetland demonstration project was 89.2%,89.1%,71.6%,71.5%,64.3%,respectively.The effluent concentration of COD and NH4~+-N reached level 1 A of GB18918-2002.The effluent concentration of TN and TP reached level 1 B of“Sewage Treatment Plant Pollutant Discharge Standard for Towns”.(3)Within the range of experimental carbon nitrogen ratio(C/N=3,6,9),the total nitrogen removal was increased with the increase of C/N to 6,while further increased the C/N to 9 decreased the TN removal.The optimal C/N was found to be 6 in this study.Higher organic matter content could inhibit ammonia oxidation,resulting in the reduction of TN removal by insufficient denitrifying nitrate concentration.(4)The suitable concentration of ammonia nitrogen has a positive effect on removal of total nitrogen,while excessive concentration of ammonia nitrogen has negative impact on TN removal.Under the condition of this experiment,the concentration of ammonia nitrogen in the micro-aerated constructed wetland can not exceed 140mg/L.The change of nitrate concentration has no effect on the removal of pollutants.(5)It was found that the reduction of COD and TN ratio was 1.1 in the third unit of the wetlands,which was less than those in the literatures for the theoretical ratio of denitrifying process.The experiment data also showed that COD reduction was insufficient for denitrification even if it was completely used for denitrification,which indicated other nitrogen removal pathways with low C/N ratio,could be exist in the pilot micro-aerated artificial wetland,such as anaerobic ammonia oxidation,sulfur autotrophic denitrification and shortcut nitrification denitrification,etc.(6)C/N ratio and nitrogen concentration has little effect on TP removal.The TP removal is mainly dominated by abiotic process in the micro-aerated constructed wetland.