Research On Treatment Of Farmland Drainage By Adsorption-Microfiltration Process

Agricultural non-point source pollution is one of the main causes of water eutrophication in China.As the main carrier of agricultural non-point source pollution,farmland tail water often faced with emergency situation of large amount of discharge,which is difficult to handle by traditional technology.In this study,the adsorption materials for removing NH₄⁺-N,NO₃^--N and TP were made by modifying zeolite.The adsorption mechanism of modified zeolite was clarified through the investigation of its properties and adsorption characteristics.The treatment effect of the combination process of modified zeolite adsorption and microfiltration on farmland tail water were investigated,and the characteristics of membrane pollution were explored.The results show that:For NH₄⁺-N,TP and NO₃^--N adsorption,the optimal preparation conditions for modified zeolites were 0.2 mol·L^-1 Na Cl solution with solid-liquid ratio of 1:10,modifying time should be 2 h;0.01 mol·L^-1La Cl₃ solution with solid-liquid ratio of 1:10,modifying time should be 2h;0.02 mol·L^-1 CPB solution with solid-liquid ratio of 1:20,and modifying time should be 24 h.The modified zeolite for adsorbing NH₄⁺-N（MZ-Na）has the enhanced ability to adsorb NH₄⁺-N is attribute to the replacement of Mg²⁺,Ca²⁺and other metal ions in natural zeolite（NZ）by Na⁺through ion exchange,and to form sodium zeolite;La³⁺is mainly loaded on NZ in the form of La（OH）₃ and La₂O₃,forming the adsorption of TP modified zeolites（MZ-LA）and the adsorption of TP is through complexation action;CPB is loaded on NZ with two-tier structure to form NO₃^--N-modified zeolite（MZ-CPB）,On the one hand,MZ-CPB surface has positive electric charge to produce electrostatic attraction,on the other hand,the anion pollutant will also exchange with Br^-in CPB.The TP adsorption of MZ-La is fitting well with Freundlich adsorption isotherm model（R²>0.99）;the NO₃^--N adsorption of MZ-CPB has higher fitting degree with Langmuir adsorption isotherm model（R²>0.97）.The adsorption process is in accordance with the quasi second-order kinetic model.15 g·L^-1 MZ-CPB and 8 g·L^-1 MZ-La were used as combined adsorbents to treat farmland tail water.The concentrations of NH₄⁺-N and TP decreased from 5.11 mg·L^-1,0.53 mg·L^-1 to 0.19 mg·L^-1,0.015 mg·L^-1,and met the ClassⅡStandard of Surface Water,the concentration of NO₃^--N in the effluent decreased from 10.12 mg·L^-1 to 2.18 mg·L^-1,the removal rate was 78.21%,and demonstrate satisfying treatment effect.Within 10～60 min,HRT had significant effect on the treatment of NH₄⁺-N and NO₃^--N by adsorption microfiltration process,but had little effect on TN and TP;considering the treatment effect for all the pollutants,the optimal HRT of adsorption microfiltration process was set to 10 min.Stainless steel membrane filtration can significantly improve the removal of TP and SS.TP concentration can be reduced by 62.7%after stainless steel membrane filtration.Compared with natural sedimentation,SS can be reduced by 75%after stainless steel membrane filtration.However,stainless steel membrane filtration has little effect on NH₄⁺-N,NO₃^--N and TN concentration.Under different HRT,the changes of specific flux of membrane with time is similar.At the beginning of the reaction,the specific flux of membrane decreases slowly,and after a certain time,the specific flux of membrane decreases rapidly.The specific flux of stainless steel membrane can be recovered to 92.44%of that of original membrane by physical and chemical cleaning.Based on the above results,15 g·L^-1 MZ-CPB and 8 g·L^-1 MZ-La were used as combined adsorbents,while stainless steel membrane was used as filter membrane.the adsorption microfiltration process could be used as an emergency measure for treating farmland tail water quickly and efficiently.