Study On Removal Of Humic Acid And Protein From Water By Particle Coagulation Pre-Deposition And Membrane Filtration

Surface water is extremely susceptible to be polluted by natural organic matter such as humic acids and proteins.Membrane filtration technology is widely used in drinking water treatment projects,but there are still many problems in improving the removal effect of natural organic matter and controlling membrane pollution.In this study,water samples containing humic acid and protein were treated by a combined coagulation-ultrafiltration process.The pre-deposition layer formed on the surface of the ultrafiltration membrane by the particulate matter improves the removal rate of organic matter and slows down membrane fouling.In this study,two different aluminum forms of coagulant,AlCl₃and Al₁₃,were first used to treat the water body containing particulate matter,the turbidity and humic acid content of the effluent were measured.According to the advantages of Al₁₃coagulant,combined with organic polymer coagulant-polydimethyl diallyl ammonium chloride（PDADMAC）to obtain a composite coagulant according to different mass ratios,through the composite coagulant to coagulate the particulate matter pre-deposited on the ultrafiltration membrane,ultrafiltration of humic acid and protein solution.Three-dimensional fluorescence（EEM）,high performance liquid exclusion chromatography（HPSEC）,total organic carbon analyzer（TOC-VCPH）was used to measure the effluent quality,and the properties of the particulate flocs formed by different coagulants were analyzed online by laser particle size analyzer（Including particle size and fractal dimension）,and using water contact angle measuring instrument,scanning electron microscope（SEM）and atomic force microscope（AFM）to characterize,analyze and determine the microtopography of ultrafiltration membrane surface,the results show that:Both humic acid and protein can cause a certain degree of membrane fouling.The removal of humic acid using AlCl₃and Al₁₃coagulants has different results during the coagulation pre-deposition ultrafiltration of particulate matter.Al₁₃,which has a higher positive charge,can act as an"electrostatic clusters"at a low dosage,so it can remove organic matter.The pre-deposition to reduce the pollution of the ultrafiltration membrane is mainly affected by the removal rate of organic matter and the performance of floc.The floc formed by Al₁₃has a larger fractal dimension,but its floc particle size is smaller(Al₁₃floc particle size 300μm,AlCl₃floc particle size 600μm),resulting in the pre-deposited layer on the surface of the ultrafiltration membrane is relatively dense.At the same time,the surface of Al₁₃floc has a large positive charge(Al₁₃charge is 17.93 m V,AlCl₃charge is-4.93 m V),and its electrostatic adsorption ability is stronger.Therefore,the organic matter in the water body can be removed effectively,and the degree of improving membrane flux can be improved.Al₁₃can effectively alleviate the pollution of ultrafiltration membrane.For the composite coagulant,when the mass ratio of PDADMAC and Al₁₃with a large molecular weight（40～50k Da）is 2%,the coagulation and pre-deposition of particles has a better removal effect on humic acid.However,when the weight ratio of PDADMAC to Al₁₃with a low molecular weight（<10 k Da）is 2%,the protein removal effect is better when it is used as a coagulant.Among them,macromolecular organics are more easily trapped/adsorbed on the pre-deposited layer,and small molecular organics pass through the deposited layer and enter the effluent.In the process of organic-inorganic compounding coagulant,low content of PDADMAC can effectively enhance the coagulant’s electric neutralization ability and net capture sweeping ability with its higher positive charge and larger molecular weight.At that time,the charge reversal phenomenon caused by its excessively high positive charge,and at the same time,it will enter the effluent to increase the effluent organic content.