Study On Oxygen Environment Continuously Switching Modular Plant Filter Bed In Rural Non-point Source Pollution Control

Aiming at the problems of existing rural non-point source pollution control technology,such as inefficiency of nitrogen removal and complexity of construction,plant filter bed was combined with multilevel A/O process to construct oxygen environment continuously switching modular plant filter bed for pollution interception.In this study,the characteristics of pollutant absorption and removal effects of wetland plants were investigated from the aspects of radial oxygen loss,nitrogen and phosphorus absorption kinetics,as well as pollutant removal under different ratios of NH₄⁺-N/NO₃^--N.The influence of oxygen environmental conditions on the pollutant removal of modular filter bed was studied.On this basis,the filter beds without plants were connected in series to study the pollutant removal effects under different processing level.According to the pollution characteristics of each filter bed,plants were selected for optimizing the operating condition.The pollutant removal effects was monitored continuously from winter to spring.The microbial community in the filter bed and bioenzyme activities was analyzed.The experimental results of radial oxygen loss showed that the ROL of 5 wetland plants（Iris pseudacorus,Canna indica,Cyperus alternifolius,Ipomoea aquatica,Sagittaria sagittifolia）ranged from 13.90 to 22.46μmol O₂·plant^-1·h^-1,and capacity of radial oxygen loss was significantly correlated with root morphology.In practical application,fibrous root type or aerenchyma developed plants（such as Iris pseudacorus,Ipomoea aquatica,etc.）can be selected to improve the anoxic environment.According to the experimental results of pollutant removal effects of plants,if nitrogen removal is considered,Cyperus alternifolius is suited when main nitrogen form is NH₄⁺-N and Canna indica when it is NO₃^--N.When phosphorus removal is considered,Cyperus alternifolius is recommended.When landscape is considered,Iris pseudacorus is suggested with reasonable planting density.If economic benefit is considered,Sagittaria sagittifolia is suited in spring and summer.The experimental results of the influence of oxygen conditions on removal of pollutants in plant-free modular filter beds showed that the removal efficiency of NH₄⁺-N,TN and TP under oxic/anoxic/oxic（OAO）operation was significantly better than anoxic/oxic/anoxic（AOA）operation,but the removal efficiency of COD had no significant difference.The 6-levels filter bed should adopt OAOAOA operation mode.After stable operation without plants,the removal rates of COD,NH₄⁺-N,TN and TP were 65.1%,85.61%,82.20%and 62.71%,respectively,and the 1st oxic filter bed always maintained the best pollutant removal ability.According to the plant selecting results,the 6-levels modular filter beds were optimized.The 1st oxic filter bed was planted with Iris pseudacorus and Cyperus alternifolius which had strong oxygen secreting ability.The 3rd and 5th oxic filter beds were planted with Canna indica with strong nitrogen removal capacity and Cyperus alternifolius with strong phosphorus removal capacity.In winter,the removal rates of COD,NH₄⁺-N,TN and TP were 66.0%,35.46%,40.68%and 52.08%,respectively.In spring,the removal rates of the above 4 pollutants by the combined process were 73.4%,45.97%,47.02%and 33.73%,respectively,and the nitrogen and phosphorus retained in winter were released.The results showed that the total level of filter bed should not be too much in practical application,and the last level should be set as oxic.Plants could improve the microbial diversity and change the dominant species in oxic filter beds.The installation of anoxic filter bed was helpful to improve the denitrifying enzyme activity of the system,thus improving the denitrification effect of the process.