Study On The Effect Of Different Substrate Types On Nitrogen And Phosphorus Removal In Baffle Constructed Wetland System

In order to solve the pollution problem of decentralized rural domestic sewage in northwest China,this study combined the temperature characteristics of northwest China and took the packing of the constructed wetland as the research object.The adsorption kinetics and thermodynamics of ammonia nitrogen?NH₄⁺-N?and phosphate(PO₄^3--P)were tested by gravel,concrete slag,red brick,ceramic tile,anthracite slag and biochar.The adsorption mechanism and law of nitrogen and phosphorus by fillers were revealed.Gravel,concrete slag and biochar were used to construct 3 groups of baffled constructed wetlands respectively.After stable running,the influence of hydraulic retention time?HRT?and the along range on sewage treatment of constructed wetlands was explored,providing references for the design and operation of constructed wetlands.Biochar baffled constructed wetland was operated for12 months to evaluate its stability and analyze the environmental and economic benefits of wetland application in rural sewage treatment.It provides a scientific basis for the selection of constructed wetland fillers in northwest China and has a good reference value for the application and promotion of constructed wetland fillers.The experimental results are as follows:?1?The adsorption equilibrium time of gravel,concrete slag,red brick,ceramic tile,anthracite slag and biochar on NH₄⁺-N was 12 h,12 h,17 h,12 h,8 h and 24 h,respectively.The maximum adsorption capacity was biochar>gravel>anthracite slag>tile>concrete slag>red brick.The adsorption equilibrium time of gravel,concrete slag,red brick,ceramic tile,anthracite slag and biochar on PO₄^3--P was 12 h,24 h,24 h,17 h,24 h and 24 h,respectively.The maximum adsorption capacity was concrete slag>biochar>anthracite slag>red brick>ceramic tile>gravel.The kinetic adsorption process of nitrogen and phosphorus by fillers is more in line with the quasi-secondary kinetic model,which indicates that the adsorption process of nitrogen and phosphorus by fillers includes surface adsorption,internal particle diffusion and external liquid film diffusion,and not rate control alone.Fillers on the isothermal adsorption of nitrogen and phosphorus more accord with the Freundlich adsorption model,the enthalpy change of?H^?theta,gibbs free energy change?G^?theta,entropy value?S^?theta were greater than zero.The results showed that the adsorption process of nitrogen and phosphorus by fillers was endothermic,multi-molecular layer adsorption and the adsorption surface was not uniform,and the enthalpy promotion was not spontaneous.?2?Under the condition of water temperature of 8?12?,the removal rate of chemical oxygen demand?COD?,NH₄⁺-N,PO₄^3--P and suspended matter?SS?in the baffled constructed wetland showed a significant positive correlation with the increase of HRT and along range?P<0.01?.The optimal HRT of gravel and concrete slag wetlands was 4.5 d,and that of biochar wetlands was 2.5 d,and the optimal along range was 3.0 m.Under these conditions,average removal rates of COD,NH₄⁺-N,PO₄^3--P and SS in gravel wetlands were74.24%,53.58%,74.48%and 89.12%,respectively.The average removal rates of COD,NH₄⁺-N,PO₄^3--P and SS in concrete slag wetlands were 65.53%,52.75%,95.89%and90.77%,respectively.The average removal rates of COD,NH₄⁺-N,PO₄^3--P and SS in biochar wetlands were 90.26%,70.91%,88.49%,95.44%.The baffled constructed wetland has obvious microbial degradation effect on the removal of COD,NH₄⁺-N,PO₄^3--P and SS.Gravel,concrete slag and biochar as wetland filler had the highest degradation rates for SS,PO₄^3--P and SS respectively.?3?Under the optimal HRT and along range conditions of biochar wetlands,the operation of reverted constructed wetlands for 12 months resulted in the removal rate of various pollutants in summer>autumn>spring>winter.The average annual removal rates of COD,NH₄⁺-N,PO₄^3--P,and SS in wetlands were 88.16%,83.16%,92.55%and 96.30%,respectively,and the effluent water quality was stable.Comprehensive baffled contacted wetland for sewage removal effect and biochemical degradation results,selection of biochar as baffled contacted wetland packing,not only can reduce the operation cycle and filler plug,also improve the efficiency of nitrogen and phosphorus,also can achieve effective processing northwest decentralized rural domestic sewage.In addition,in order to truly apply the constructed wetland in practice,it is necessary to evaluate the pollutant removal capacity,nitrogen and phosphorus recovery efficiency and design parameters of the whole process.