Modified Red Mud Filter Media For The Removal Of Ammonia Nitrogen In Low-temperature Sewage

Urban sewage treatment plants treat a large number of human domestic sewage and industrial wastewater,including pollutants such as BOD,COD,SS,ammonia nitrogen,total phosphorus,among which the excessive discharge of ammonia nitrogen will lead to eutrophication of water and damage to the aquatic ecosystem.Ammonia nitrogen,total phosphorus and other pollutants in sewage are mostly degraded by activated sludge method.However,in cold areas,the sewage water temperature will be reduced to about5 degrees Celsius,which makes activated sludge process is seriously inhibited.Low temperature will increase the concentration of ammonia nitrogen（NH₄⁺）,exceeding the national emission standard of 15 mg/L（GB18918-2002）,which will further cause harm to the environment.Therefore,it is urgent to develop a filter material to remove ammonia nitrogen from low-temperature sewage and effectively reduce the subsequent biological treatment load.A modified filter material was developed to treat ammonia nitrogen at low temperature.First,the red mud and sifted straw powder were calcined and granulated at high temperature for use.Then the lignin was extracted from the straw,and the lignin was crosslinked and phenolized.The modified lignin and polyvinyl alcohol were mixed and sprayed on the surface of the red mud ball to prepare the modified filter.The actual domestic wastewater containing ammonia nitrogen was treated with filter material and its best performance was tested by changing the single factor.（1）The samples were tested by four characterization methods:energy dispersive X-ray spectroscopy（EDS）,scanning electron microscope（SEM）,fourier transform infrared spectroscopy（FT-IR）and Zeta potential.It can be seen from the SEM image that the surface of modified lignin becomes‘coral’,and the specific surface area becomes larger,which is suitable for water treatment applications.Besides,the surface will not change significantly before and after sewage treatment,and the performance is stable.FT-IR showed that lignin was successfully extracted and the phenolic hydroxyl content was increased by phenolic modification.Zeta potential shows that the isoelectric point of crosslinked lignin is,and the isoelectric point of crosslinked phenolic lignin is.After crosslinking,the isoelectric point becomes lower.When the p H value of crosslinked phenolic lignin is 7,the potential value is,with a large amount of negative charge.EDS shows that N element is increased on the surface of filter material after treatment than before treatment.（2）In this paper,the optimum conditions were determined by changing the amount of filter material added,treatment time of filter material,p H of low-temperature sewage,sewage temperature,initial concentration of low-temperature sewage and the influence of co-existing cation in low-temperature sewage on the removal rate.When the amount of modified red mud filter material was 100 g/L,the removal rate of ammonia nitrogen reached 73%,and the best removal effect was achieved at 2.5 h.When the p H value of water is 8,the removal rate reaches the maximum.The coexistence cation will affect the removal rate.The greater the charge,the greater the influence.（3）Preparation conditions optimization investigation can be seen that best use straw powder should be 6%,best with lignin phenol ratio is 4.5:1,the best catalyst dosing should account for 50%of the crosslinked lignin,best phenol modified time of2.0 h,best phenol modified temperature is 140℃,circulation experiment was carried out five times,the fifth can still reach 65.5%removal rate,very suitable for the actual production use.（4）The quasi-first-order kinetic model and the quasi-second-order kinetic model were used to fit the experimental results.The quasi-second-order kinetic model described the treatment process of the modified red mud filter material,and the correlation coefficient was 0.9990 higher than that of the quasi-first-order kinetic model.Langmuir adsorption isotherm and Freundlich adsorption isotherm were used to fit the results.At temperatures of 275 K,280 K and 285 K respectively,the Langmuir adsorption isotherm had higher correlation coefficient.The n value in the Freundlich model proved that the reaction was easy to occur and exotherm.ΔG andΔH⁰are less than zero prove electrostatic adsorption process is spontaneous exothermic process.