The Nitrogen Removal Performance Of Horizontal Baffle Subsurface Flow Constructed Wetland Based On Slow Release Carbon Source

Nitrogen and phospho rus are two main sources of water nutrient pollution, especially in the urban and countryside domestic sewage. Meanwhile, constructed wetland has already been widely utilized to degrade rural sewage as it has advantages of low construction cost and convenience of maintenance. But the ammonium and nitrate nitrogen pollutants in the wastewater could not be effectively removed. Hence, this research built a horizontal baffle subsurface flow constructed wetland(HBSF) system, which has combined the merits of horizontal flow and vertical flow constructed wetland system, in order to improve the nitrogen removal performance. To be specific, slag, wood and gravel were selected as the matrix and combined into two different orders(A. gravel, wood and slag; B. wood, gravel and slag). The constructed wetland system also used canna and Lythrum salicaria as the wetland plants. The aims of the project were to analyze the main impact factors of nitrogen and phosphorus removal and summarize the removal mechanism through the analysis of nitrogen and phosphorus removal efficiency. The main conclusions are listed below.1. The nitrogen removal rate in unplanted HBSF increased with the extension of hydraulic retention time(HRT). And the phosphorus removal rate raised in the first stage, and then decrease with the extension of HRT. The optimal HRT for P removal was 3 days. In terms of the combination of matrix, order A and order B had better nitrogen and phosphorus removal performance respectively when the system ran steadily.2. Compared with the unplanted HBSFCW, when wood was the controlled-release carbon source, HBSF with plant had better performance in both nitrogen and phosphorus removal, especially in nitrate nitrogen removal which was much increased 22% to over 96%. The efficiency of ammonia nitrogen and total nitrogen removal rate were also enhanced 4.3% and 5% respectively.3. Aeration could promote the removal performance of ammonia nitrogen and total nitrogen and inhibit the treatment of nitrate nitrogen respectively. The total nitrogen removal rate were increased 17% compared with non-aerated system to over 70% when the air-water ratio reached 1:1, 2:1 and 4:1. Phosphorus removal rate was also increased under the condition of aeration. However, the total phosphorus removal performance was decrease with the increase of air-water ratio. Thus, the optimal air-water ratio for phosphorus treatment was 2:1.4. Effluent recycling could strongly promote the removal of ammonia nitrogen and total nitrogen. Especially when the recirculation ratio was 150%, the removal rate of ammonia nitrogen and total nitrogen were increased 13% and 11% respectively. Nevertheless, the removal performance of phosphorus was slightly increased and most of the phosphorus was adsorbed on the surface of matrix and taken into the wetland plant.5. Composite the study result of HBSF without plants, with plants and strengthen measures, the HBSF which matrix sequence is slag, wood and gravel and wentland plants are canna and Lythrum salicaria can remove the nitrogen pollution of domestic sewage under HRT is 3d. When the strengthen measures are used, the gas-water ratio is 2 and reflux ratio is 150% will be the better ratio to remove nitrogen pollution efficiently.