Study On Rainwater Purification Of Bioretention With Improved Collapsible Loess Substrate

The prevention and control of urban floods and the control and utilization of stormwater have become a hot issue in current urban construction.Bioretention is one of the great stormwater control measures.Collapsible loess is widely distributed in the northeast,northwest,parts of central and eastern of China in arid,semi-arid,and semi-humid areas.Collapsible loess will be collapsed suddenly when it is soaked with the water,resulting in the clogging of bioretention and reduction of pollutant removal efficiency,which will affect the life cycle of the facility.In order to solve the clogging problem caused by the collapsing of collapsible loess inside the bioretention,collapsible loess is promoted under the condition of not replacing the site soil with low cost and five groups of bioretention were constructed to study the adsorption effect of construction waste and collapsible loess,the purification effect of pollutants by bioretention with improved loess as a filler and the causes of clogging of bioretention.The study may provide reference for the promotion and application of bioretention technology in collapsible loess regions.The main achievements in this study are as follow:（1）The chemical improvement and physical improvement methods can increase the hydraulic conductivity of loess.The physical improvement methods using materials such as construction waste,sand and sawdust can make bioretention meet the requirements of the hydraulic conductivity which is more than 2cm/h.and have a smaller amount of settlement of the layer at the meantime.（2）Loess have a smaller maximum adsorption amount of PO₄^3--P than construction waste but a faster adsorption rate under PO₄^3--P concentration of 2 mg/L.Loess have a bigger maximum adsorption amount of NH₄⁺-N than construction waste and a faster adsorption rate under NH₄⁺-N concentration of 8 mg/L.The adsorption process of PO₄^3--P and NH₄⁺-N in runoff rainwater by two kinds of fillers is an important process of purifying runoff rainwater by bioretention.（3）The change of influent hydraulic load has a significant effect on the purification of NO₃^--N and COD in each bioretention.The removal rates of NO₃^--N and COD are highest in bioretention HT with the effluent COD concentrations below 45mg/L.Influent hydraulic load is not the main factor which affects the purification rates of NH₄⁺-N and PO₄^3--P by bioretention and each bioretention has a good purification rates of NH₄⁺-N and PO₄^3--P.（4）Influent NO₃^--N and PO₄^3--P loadings are important factors influencing the purification rates of NO₃^--N and PO₄^3--P in bioretention.Bioretention HT has the best removal efficiency for NO₃^--N and PO₄^3--P.Influent NH₄⁺-N load have little effect on the NH₄⁺-N purification performance of each bioretention;COD load changes have an impact on the COD purification efficiency of each bioretention and bioretention HT and HMT have a good effect on COD purification.（5）The addition of anaerobic zone can improve the removal efficiency of NO₃^--N、NH₄⁺-N、PO₄^3--P in various bioretention with the concentration of effluent below 1mg/L,0.3mg/L,and 0.15mg/L;The height of anaerobic zone can affect the COD removal efficiency of bioretention significantly and the removal efficiency for COD is best at the height of 300mm.The purification of the COD by bioretention HMT under the addition of anaerobic zone is always the best.（6）The alternation run of wet and dry has a significant effect on the purification effect of NO₃^--N in each bioretention.The effluent NO₃^--N can reach a minimum of0.5mg/L or less;The alternation run of wet and dry is not the main factor which affects the purification rates of NH₄⁺-N and PO₄^3--P by bioretention.The alternation run of wet and dry do have effects on the purification effect of COD in each bioretention and the minimum concentration of effluent can reach below 40mg/L.（7）The microbial extracellular polymeric substances（EPS）produced by the microorganisms in the bio-bedding layer inside the bioretention is an important factor that causes clogging which may decrease the COD purification rates by bioretention.The alternation run of wet and dry can ease the clogging of bioretention and increase the COD purification rates.