Research On Enhanced Filtration Performance Of Chitosan And Its Compound Filter Aids

At present,China's economic and social development was in an accelerated rising stage,while the water environment has not been effectively improved.Surface water sources were still polluted to varying degrees,and water quality violations of drinking water sources have occurred frequently.In order to achieve high-quality drinking water supply and ensure the safety of drinking water,a variety of new water treatment technologies have emerged at the historic moment.Among them,the dual micro-flocculation enhanced filtration technology had the advantages of strong adaptability to water sources,good turbidity reduction effect,high removal rate of pathogenic microorganisms,low dosage of filter aids,low operation cost,etc.It was not only suitable for the strengthening of conventional process water treatment plants,but also could further improve the effluent quality of deep water treatment plants mainly based on ozone-activated carbon process.In the dual micro-flocculation enhanced filtration process,the selection of filter aids was essential for the enhanced filtration effect.Conventional aluminum salts and polyacrylamide filter aids had the risk of secondary pollution.Chitosan(CTS),as a natural macromolecule water treatment agent,has attracted extensive attention because of its wide source,harmless to human body and abundant active amino and hydroxyl groups in molecular chains.But the use of chitosan in drinking water treatment was limited,due to the shortcomings of chitosan,such as cations with less positive charge,poor water solubility,and poor treatment effect of organic matter.In this study,chitosan was used as the basic material to introduce inorganic metal ions into the chitosan molecular chain,enriching the cationic groups in the chitosan molecular chain,and three new inorganic metal salt-chitosan complex filter aids were prepared.The dual micro-flocculation enhanced filtration with three filter aids was studied,which taked the effluent from sedimentation tank of surface water treatment plant as the test raw water.By comparing the strengthening filtration effect of four kinds of chitosan and its derivatives commonly used in water treatment,the chitosan agent suitable for enhanced filtration was selected.BBD model in response surface design was used to optimize the operation parameters of dual micro-flocculation enhanced filtration of chitosan,and the dual micro-flocculation enhanced filtration mechanism was discussed.The results indicated that the protonated chitosan with hydrochloric acid(HCTS)had better enhanced filtration effect.When the dosage of HCTS was 0.60 mg/L,the micro-flocculation time was 2 min,and the micro-flocculation intensity(velocity gradient G value)was 110s-1,the dual micro-flocculation enhanced filtration performance was the best.The micro-flocs formed by HCTS was in contact with filter material surface,which was mainly affected by gravity precipitation and interception,while there was a good linear negative correlation between floc size and filtration period.The adhesion mechanism mainly relied on reducing the absolute value of colloid Zeta potential in water,enhancing the interfacial chemical interaction between colloid particles and filter material surface,and the cohesion of polymer adsorption bridging.On the basis of the preferred chitosan,three composite filter aids,aluminum sulfate-chitosan composite filter aid(AS-CTS),ferric chloride?chitosan composite filter aid(FC-CTS),titanium sulfate-chitosan composite filter aid(TS-CTS),were prepared respectively with the new water treatment agent titanium sulfate and the traditional water treatment agent aluminum sulfate and ferric chloride.The appearance morohology,microstructure and functional group changes of three complex filter aids were studied by UV-VlS spectrum,Fourier transform infrared spectrum,X-ray diffraction and scanning electron microscope.Fourier transform infrared spectroscopy showed that O-Al,N-Al and other group bonds were formed in AS-CTS filter aid,-Fe-O-Fe-bond and polymerized Fe-OH-Fe in FC-CTS,while the titanium ion hydrolyzed products reacted with the amino and hydroxyl groups of chitosan.The combination of these inorganic metal salts with chitosan enriched the cation and functional groups on the molecular chain of chitosan,improved the Zeta potential on the surface of colloidal particles and filter materials,and enhanced the contact and adhesion ability between colloidal particles and the surface of filter materials.X-ray diffraction analysis showed that some AS were still embedded in AS-CTS filter aid in the form of crystals,FC-CTS filter aid formed a new phase structure and existed in amorphous form,and TS-CTS filter aid formed a new amorphous crystal structure.The results of UV-VIS spectroscopy indicated that the inorganic metal salt reacted with chitosan.All the above results indicated that hydroxyl hydrolyzed products of inorganic metal salts were introduced into chitosan molecules.The optimal compound ratio and dosage of AS-CTS,FC-CTS and TS-CTS filter aids were determined by CCD model in response surface design.Under the optimal compounding condition,the enhanced filtration performance of three compounded filter aids was clarified,and the main strengthening filtration mechanism was investigated by investigating the changes of colloidal stability,floc size and fractal dimension.The optimization results showed that the AS-CTS filter aid mAS/mCTS was 4:1,the CTS dosage was 0.2 mg/L,the FC-CTS filter aid mFC/mCTS was 9:2,and the CTS dosage was 0.2 mg/L,while the TS-CTS filter aid mTS/mCTS was 2:1,and the CTS dosage was 0.3 mg/L.Under the optimal compounding condition,the three filter aids had enhanced the filtration performance index was AS-CTS>TS-CTS>FC-CTS,forming micro-flocs size was FC-CTS>AS-CTS>TS-CTS,fractal dimension was AS-CTS>FC-CTS>TS-CTS,and interface chemistry with Zeta potential as an indicator affects was AS-CTS>FC-CTS>TS-CTS.AS-CTS and FC-CTS filter aids formed micro-flocs were contacted with the surface of filter material,which was mainly affected by gravity precipitation and interception,and the migration mechanism of TS-CTS enhanced filtration was dominated by gravity precipitation and hydrodynamics.The AS-CTS enhanced filtration adhesion mechanism was mainly dominated by the interface chemistry of AS and CTS hydrolyzed cations and the adsorption bridging of CTS polymers,FC-CTS mainly relied on the adsorption and cohesion action of Fe(OH)3,Fe(OH)2+,Fe3(OH)45+ and the bridging of CTS,TS-CTS enhanced filtration mainly relied on the adsorption and cohesion of multi-core hydroxyl partial oxygen titanium and the bridging of macromolecule.Through the ultrafiltration membrane method,resin separation,the measures of the three dimensional fluorescence spectrum,from the molecular level,quantitative analysis and qualitative analysis of molecular weight,hydrophobic and hydrophobic distribution and fluorescent substances in influent and effluent of sand filtration were carried out.The removal efficiency of organic substances and precursors of disinfection by-products in water by micro-flocculation enhanced filtration with three kinds of composite filter aids was investigated.The results showed that AS-CTS mainly removed HOA(hydrophobic acidic organics)and HIN(hydrophilic neutral organics)with large molecular weight in water.FC-CTS enhanced filtration mainly removed HOA and HOB(hydrophobic alkaline organics)with molecular weight<3 KDa.TS-CTS enhanced filtration mainly removed HOA and HIC(hydrophilic charged organic matter)in molecular weight components of>30 KDa and<3 KDa.The enhanced filtration of FC-CTS had a good removal effect on trihalomethane precursors,with a removal rate of 53.92%.The enhanced filtration technology of TS-CTS filter aid was applied to treat the effluent of sedimentation tank during the high algal water period.The state and removal effect of the microbial cells were detected by flow cytometry,which were in the water before and after filtration.In view of the poor condition of removing algae by using TS-CTS filter aid alone,the combined process of potassium ferrate pre-oxidation and TS-CTS filter aid-enhanced filtration was adopted to treat the high algae water.The results showed that the TS-CTS filter aids has no damage to microbial cells in the process of enhanced filtration,and could effectively remove nucleic acid microorganisms such as bacteria in the water,and the microbial removal effect was better than algae.The results also indicated that the combined process had a better removal effect on algae and algae odorant 2-MIB,while the chlorophyll a removal rate was 91.6%,which was 18.1%higher than that without pre-oxidation.When the influent concentration of 2-MIB was 45 ng/L,the effluent concentration was 5 ng/L,which was lower than the standard limit of drinking water of 10 ng/L.The combined process could also effectively remove the algal source organic matter,wherein the removed EOM was mainly dissolved microbial metabolites and tryptophan proteins,while the IOM was mainly humic acid and fulvic acid organics.The combined process mainly relied on the strong oxidation of potassium ferrate and the adsorption electric neutralization of potassium ferrate reducing Fe(H2O)63+,the hydrolyzed product of new pig iron,to enhance the filter material's ability to trap algal pollutants.In summary,the inorganic metal salt and protonated chitosan compound filter aid materials had an ideal enhanced filtration effect on the actual water body,and the combined process with high iron pre-oxidation could be used to treat high algae water.Meanwhile,the morphology and stability of micro flocs formed by filter aid,could predict the operation state and filtration performance of the filter.The research results could provide a certain technical support for the improvement of the filtration process and the construction of the new water plant,which had broad application prospects.