Research On The Seasonal Distribution Characteristics Of Nitrogen And Phosphorus In Typical Biological Retention System

In order to solve a series of urban water problems including urban waterlogging and runoff pollution caused by the rapid development of urbanization and the increase of impervious surface area,China has put forward the concept of"Sponge City",and the biological retention system is the preferred technical measure for the construction of sponge cities in my country For one,a large number of scholars have carried out model studies on the mechanism of removing nitrogen and phosphorus from rainwater runoff,but there are few reports on the operating effects of biological retention systems and the distribution characteristics of nitrogen and phosphorus in the system in actual engineering.This study takes the biological retention system of a sponge city pilot in Chongqing as the research object,analyzes the distribution characteristics of different forms of nitrogen and phosphorus in the biological retention system in different seasons and their denitrification and phosphorus removal efficiency under different rainwater runoff situations,and discusses the influence of nitrogen and phosphorus.The main factors of phosphorus transformation and distribution provide references for the construction of sponge cities and the operation and management of biological retention systems.The main conclusions are as follows:（1）The TN and TP of the four biological retention facilities in ecological tree ponds,rain gardens,biological retention zones and high flower beds are mainly distributed on the surface of the soil,and decrease with the increase of depth.TN and TP showed a trend of first decreasing and then increasing with the change of summer,autumn and winter.NH₄⁺-N and NO₃^--N have little change in vertical space,but the content of NH₄⁺-N is higher in summer and autumn,and NO₃^--N is higher in summer and winter.The available phosphorus in ecological tree ponds,rain gardens,and biological retention zones are mainly concentrated in the surface and middle and upper layers.There is little difference in vertical space for high flower beds.The AP of the four facilities in summer and winter are slightly higher.（2）In terms of the mean value,the TN content of the rainwater garden is the highest among the four facilities,with the highest value being 1058mg/kg,and the content of TN in high flower bed is the lowest,with the lowest value being 78mg/kg.Rain garden has the highest NH₄⁺-N content,the highest value is 31.2mg/kg,and the NH₄⁺-N content in high flowerbed is the lowest,the lowest value is 1.3mg/kg.The NO₃^--N content of the biological retention zone is the highest,the highest value is 7.26mg/kg,and the NO₃^--N content of the high flowerbed is the lowest,the lowest value is 0.14mg/kg.The TP content of the ecological tree pond is the highest,the highest value is 704mg/kg,the TP content of the rainwater garden and the biological retention zone are both low,the lowest value is 100mg/kg.The AP content in the biological retention zone is the highest,the highest value is 23.5mg/kg,and the AP content in the rain garden is the lowest,the lowest value is 0.3mg/kg.（3）pH mainly affects the distribution of NO₃^--N and AP in biological retention facilities,and the maximum Person correlation coefficients are-0.743 and-0.749,respectively;f mainly affects the distribution of NO₃^--N,and its Person correlation coefficient is 0.858,OM mainly affects the distribution of TN,TP and AP.The Person correlation coefficients are 0.937,0.903 and 0.930,respectively.NH₄⁺-N is less affected by pH,f and OM.（4）The concentration of rainfall runoff pollutants（TN,NH₄⁺-N,NO₃^--N,TP,COD,SS）in the four kinds of biological retention facilities in and out of the water under the three different rainfall intensities showed 0～30min fluctuations.It is large and reaches the peak value,gradually decreases with time and tends to be a gentle change law.When the rainfall intensity is heavy rain,the ecological tree pond has a better removal effect of pollutants,up to 50%removal rate.In moderate rain,high flowerbeds have better pollutant removal effects,up to 60%removal rate.When there is light rain,there is no runoff from the ecological tree ponds and high flower beds,and the rainwater gardens and biological retention zones have poor removal effects due to external factors or the facilities themselves.There is a significant liner correlation between SS and COD,and a weak correlation between SS and TN,NH₄⁺-N,NO₃^--N,TP.（5）In order to improve the decontamination effect of the biological retention system,several suggestions are put forward:1)Improve the filling of the biological retention system;2)Set up a flooded area;3)Clean the water inlet and outlet of the biological retention facility regularly;4)Increase appropriately the number of water inlet gaps in biological retention facilities.