The Preparation Of Fe-Ti Based Coagulant And Its Application In Algae Removal And Membrane Fouling Controlling

The water quality caused by algae blooms in source water has drawn great attention in recent years. Algae water pollution not only refers to algae cell itself, extracellular organic matter released by algal cell included. Coagulation-sedimentation process and ultrafiltration process is two main techniques in algae turbid water treatment. However, owing to low NOM removal efficiency and unsatisfactory coagulation performance, and the serious membrane fouling problems in ultrafiltration process, the traditional water treatment process faced serious troubles. Because of human-friendly property and fast floc sedimentation speed, Fe-based coagulation has attracted widely research and application. As a new metal coagulant, Ti-based coagulant has becoming research hotspot recently for the advantage of high NOM removal efficiency. The Ti4+ and PO43- were introduced as additive and complexing agent into Fe3+ solution, respectively, and the coagulant Polyferric titanium sulfate(PFTS) with positive, negative ion and high coagulation performance was obtained by adjusting the basicity of solution to promote the polymerization of coagulant. The optimal preparation parameters leading to high coagulation efficiency were acquired by Single-factor experiment and Response surface methodology(RSM) design experiment, and the main information of PFTS such as species distribution and phase structure were obtained by some characterization method such that Fourier transform infrared Spectrum(FTIR), X-ray diffraction analysis(XRD) and Fe-Ferron test. The lab-grown algae water was divided into original algal water sample, algae cell water sample, bound extracellular organic matter(bEOM) water sample and dissolved extracellular organic matter(dEOM) water sample through refrigerated centrifugation process. The main composition and molecular weight distribution of bEOM and dEOM was analysed by ultrafiltration and three-dimensional fluorescence spectrum. The above water samples were purified by PFTS, and the role of bEOM and dEOM in the removal of algae cell and the coagulation mechanism of PFTS in the removal of bEOM and dEOM were explored by analysing the variation of zeta potential, FTIR, and flocs properties. Algae cell and its extracellular organic matter is the main pollutant in Ultrafiltration process, by pre-coagulation process with PFTS, paper also studied the efficiency of membrane fouling control by pre-coagulation process in filtering of algae cell, bEOM and dEOM solution, and the mechanisms in controlling the membrane fouling were also discussed by zeta potential and flocs properties.The main results concluded were as follows:(1) A coagulant Polyferric titanium sulfate(PFTS) with superior coagulation efficiency was prepared by FeSO4·H2O, Ti(SO4)2 and Na2HPO4. Through Single-factor experiment and Response surface methodology(RSM) design experiment, the optimal preparation parameters resulting in superior coagulation efficiency in treating kaolin-fulvic acid water sample were obtained: hydration temperature: 0-25 ?; hydration time: 60 min; oxidizing agent adding rate: 4 ml/min; Ti/Fe molar ratio: 1/8; P/Fe molar ratio: 0.2; OH/Fe molar ratio: 0.3, and by which the lowest residual turbidity of 0.9 NTU, highest DOC removal of 53.4% and the lower than 0.1 mg/L of residual Fe concentration can be obtained. Compared with commercial PFS and PAC, the DOC removal efficiency of PFTS is higher than that of PFS and PAC.(2) PFTS is an inorganic polymer coagulant consisting of Fe3+?Ti4+ and PO43-, and the structure is complex with polynuclear hydroxy. FT-IR result suggest that the group bond such as-Fe-P-Fe-,-P-Fe-P-,-Ti-P-Ti-,-Fe-O-Fe-[P-Fe]n-Fe-,-(-P-O- [O-H]n-OFe)-,-(-P-O-[O-H]n-O-Ti)- were synthesized and repeated in the PFTS, and the generated group bond may promoted the polymerization of coagulant, and the absorption/bridge effect and sweep flocculation ability of coagulant was enhanced. XRD results indicates the main phase structure of PFTS at different Ti/Fe molar ratio is Fe4.67(SO4)6(OH)220H2O, and some new phase structure such that FeFe2(PO4)2(OH)24 H2O?TiH2(PO4)24H2O and FeTi(SO4)3 were discovered with the introduction of Ti4+ and PO43-. Feb is regard as most effective species in Fe-based coagulant, and more content of Feb were observed with the addition of Ti4+ and PO43-, and the comparatively steady distribution of Fe species of PFTS is more convenient in practical application.(3) The removal efficiency of microcystis aeruginosa, chlorella and scenedesmus in different conditions(with or without EOM) in coagulation-flocculation by PFTS were tested. The experiment results suggest that the removal rate of turbidity and Chl-a can be achieved when the dosage above 25 mg/L. bEOM and dEOM play an negative effect in coagulation-flocculation in the removal of algae cell, and dEOM play a more negative role.(4) The main composition of bEOM and dEOM is protein, polysaccharide and humus-like organic matter. The concentration of protein is higher than DOC in bEOM, while that of polysaccharide is lower than DOC. The relationship of protein, polysaccharide and DOC showed different trend. Molecular weight distribution test result suggest that the main composition of bEOM and dEOM were >100KDa and ?1KDa. Coagulation test experiment suggest the optimal PFTS dosage in the removal of bEOM and dEOM of microcystis aeruginosa, b EOM and dEOM of chlorella, bEOM and dEOM of scenedesmus is 24?40?16?32?16?32 mg Fe/L, respectively. It is worth noting that the floc size of bEOM is higher than that of dEOM in three algae, though DOC of bEOM is lower than that of dEOM. The result can be attributed to that the hydrophobic macromolecular organic matter of bEOM may facilitate the absorption/bridge and sweep flocculation effect, therefore the floc with larger size were produced.(5) Owing to the highest DOC, the membrane fouling caused by dEOM is most serious, followed by algae cell and bEOM. Whereas, bEOM caused the highest proportion of irreversible membrane fouling, and next is algae cell and bEOM. When the algal density is 0.84 billion/L, DOC of bEOM is 8.46 mg/L and dEOM is 24.64 mg/L, the optimal dosage of 18 mg/L, 24 mg/Land 25 mg/L in pre-coagulation process for three water samples, respectively, can retard the membrane flux decline and reduce the membrane fouling caused by algae cell, bEOM and dEOM effectively. The main mechanism in reducing the membrane fouling in pre-coagulation ultrafiltration process is that the small colloidal particle can gathered together to form the flocs with large size and irregular structure through adsorption/charge neutralization and sweep flocculation effect when the PFTS was added into, and the cake layer formed when the flocs accumulate on the surface of ultrafiltration membrane. The cake layer can acts as a filter and prevent the ultrafiltration membrane fouling from organic matter. The results in controlling membrane fouling from original algal water sample in pre-coagulation ultrafiltration process suggest that the proper dosage in pre-coagulation can reduce the membrane fouling effectively.