Effect Of Coagulation Pretreatment For Nanofiltration To Remove Sulfadiazine And Optimal Regulation

The presence of micropollutants such as sulfadiazine and other antibiotics in water bodies affects the quality of water environment,is not conducive to the recycling of water resources,and threatens human health and ecological balance.The current removal processes for sulfadiazine are mostly focused on biodegradation,oxidation,and adsorption,while nanofiltration membrane processes are relatively less used.Nanofiltration membranes can remove a wider range of contaminants than microfiltration and ultrafiltration,and have a lower energy consumption than reverse osmosis membranes,making them a promising application.Membrane fouling still need to be addressed by other means.Compared with the complex process and expensive cost of membrane surface modification,the combination with coagulation,a commonly used traditional treatment process,is an effective measure to improve the effluent quality and save cost,and has a greater research and application value.Therefore,in this paper,the combined coagulation-nanofiltration process was selected for the removal of typical micropollutants sulfadiazine,and the effects and mechanisms of pretreatment with two coagulants,traditional aluminum salts and emerging zirconium salts,on the removal of sulfadiazine by nanofiltration were investigated,and the main research contents and conclusions are as follows:（1）Different concentrations of Al₂（SO₄）₃coagulant were injected into the simulated water samples,and the supernatant was taken for nanofiltration experiments,while humic acid was selected as a coexisting pollutant to study the phenomenon and mechanism under composite pollution conditions.Among them,coagulation to remove Sulfadiazine（SDZ）only had a removal rate of less than 5%,while the combined process could enhance the removal rate,reaching 67.13%at 0.25 mg/L.However,the removal effect was related to the dosage,and the effect was better at low dosage.Humic acid could improve the removal of SDZ in coagulation at higher dosage with 17.23%removal,while the nanofiltration process would improve the removal rate due to the enhanced electrostatic rejection of the membrane with a maximum of58.84%.The residual Al in the supernatant in the insoluble state will cause a decrease in membrane flux,and the membrane contamination is most serious at low dosing levels,and irreversible contamination is predominant,with the most serious at 0.25 mg/L in a single water sample of SDZ,with a 51.5%decrease in flux.It is mainly due to the small volume floc particles with low hydrolysis degree can enter the pore size.（2）The emerging coagulant ZrCl₄ was selected for the coagulation-nanofiltration experiment.The removal effect of SDZ in a single water sample during coagulation was higher than that of aluminum salt,about 20%,and the removal rates of nanofiltration effluent were higher than that of raw water direct nanofiltration,but the enhancement effect was not significant,about 40%.The removal rates of both coagulation and nanofiltration effluent did not show a significant trend with the amount of dosage.In the composite water samples,the removal rate of SDZ decreased during coagulation due to the presence of HA macromolecules,with a maximum removal rate of only 9.8%,and the removal rate increased at the end of nanofiltration,and the effect was lower at low and medium dosing amounts than when the water samples were directly nanofiltrated,reaching a maximum removal of 60.86%when the dosing amount was 16.0 mg/L.In addition,the overall effect of ZrCl₄ on the change of nanofiltration membrane flux was small,in which coagulation pretreatment in a single system could improve the membrane flux,and only high dosage could be achieved in the compound contamination system,with a high percentage of irreversible contamination at low dosage,mainly caused by the insoluble state residual Zr in the coagulation effluent.（3）The same concentration of SDZ was added to the midstream water of the actual water body Jiazi Lake,and Al₂（SO₄）₃ and ZrCl₄ were injected for coagulation-nanofiltration experiments,respectively.The coagulation process still could not effectively remove SDZ,and the removal of natural organic matter was better,with the highest removal rates of 64.02%and51.91%when using aluminum and zirconium salts,respectively.The nanofiltration process can partially remove SDZ that cannot be removed by coagulation,with the best removal rates of56.50%and 58.75%at 4.0 mg/L Al₂（SO₄）₃ and 8.0 mg/L ZrCl₄,respectively.Nanofiltration almost completely removed the macromolecular organics from the actual water,and at the same time,the macromolecular organics promoted the removal of SDZ by enhancing the electrostatic repulsion on the membrane surface.The coagulant dosage was too high to facilitate the retention of SDZ by the nanofiltration membrane,and the removal rate of SDZ was reduced compared with the direct nanofiltration of raw water,but the pretreatment of both coagulants effectively reduced membrane contamination and membrane resistance,especially the irreversible resistance.In addition,the residual metals in the coagulation process could be effectively retained in front of the nanofiltration membrane,and the concentration of residual metals in the effluent was below 0.05 mg/L.Therefore,coagulation pretreatment could effectively promote the removal of nanofiltration water.Therefore,coagulation pretreatment can effectively promote more long-term and stable operation of nanofiltration process and save water treatment cost.