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The Role of Shear Conditions on Floc Characteristics and Membrane RSC Advances The role of shear conditions on floc characteristics and membrane fouling in coagulation/ultrafiltration hybrid process- The effect of flocculation duration and slow shear force Journal: RSC Advances Manuscript ID RA-ART-09-2015-018328.R1 Article Type: Paper Date Submitted by the Author: 18-Nov-2015 Complete List of Authors: nan, jun; Harbin institute of technology, Yao, Meng; Harbin institute of technology, School of Municipal and Environmental Engineering Li, qinggui; Harbin institute of technology, School of Municipal and Environmental Engineering Zhan, Dan; Harbin institute of technology, School of Municipal and Environmental Engineering Chen, Ting; Harbin institute of technology, School of Municipal and Environmental Engineering Wang, Zhenbei; Harbin institute of technology, School of Municipal and Environmental Engineering Li, haoyu; Tianjin University, Subject area & keyword: Water < Environmental Page 1 of 16 RSC Advances The role of shear conditions on floc characteristics and membrane fouling in coagulation/ultrafiltration hybrid process- The effect of flocculation duration and slow shear force a, ∗ a a a a a b Jun Nan , Meng Yao , Qinggui Li , Dan Zhan , Ting Chen , Zhenbei Wang , Haoyu Li Abstract: Impacts of shear conditions during coagulation on ultrafiltration permeate flux in coagulation-ultrafiltration (C-UF) process were investigated. Different flocculation durations and slow shear forces were applied to explore the impact of shear condition on the C-UF process. Floc characteristics, including floc average size, fractal dimension and flocs size distribution under different coagulation conditions were studied. Besides, the normalized flux and reversibility of membrane fouling were also analyzed to investigate the membrane fouling. The results indicated that flocs formed under shorter flocculation duration with small fractal dimension, which easily developed cake layer with larger pores and fluffy structure on membrane surface. But, the higher frequency of small size flocs still remained at 5 min flocculation time attributing to the inadequate collision among particles, which could cause severe membrane fouling. As a result, appropriately shorten slow stirring time (for 10 min) and reduction of low-shear force (G=8 s -1) are conducive to form flocs with loose and porosity structure without the occurrence of more small particles in water, which seemed to be effectively improved the membrane permeability. Keywords: Flocculation duration; Slow shear force; Floc characteristic; Cake layer; Membrane fouling; 1. Introduction In drinking water treatment process, membrane technology is an attractive alternative for providing high quality water that can satisfy the requirements of more strict water quality regulations. 1 However, fouling is one of the main disadvantages with membrane ultrafiltration process, attributing that the decline of flux resulted in higher water treatment cost and the deterioration of membrane. 2Different pre-treatments before ultrafiltration have been widely applied as methods to lower membrane fouling, such as coagulation, adsorption and oxidation. The combination of powder active carbon (PAC) adsorption and UF has been reported to more effectively mitigate the membrane fouling. 3 The oxidation before ultrafiltration can decrease the density of organic matter of effluents and remove oxidation intermediates, which could reduce the severity of membrane fouling by the removal of foulants on membrane surface. 4, 5 Pre-coagulation before UF, especially by “in-line” chemical coagulation, can significantly improve membrane fouling. 6, 7 The effects of pre-coagulation on membrane permeability are mainly related the coagulation conditions, including coagulants types, dosages and mixing mode. 8, 9 Pikkarainen et al 10 has stated that the Fe- and Al-based coagulants caused similar hydraulic resistance properties in membrane filtration, but Fe-based coagulant would result in strongly colored on surface water. Choi and Dempsey 11 investigated the influence of alum concentration a State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, PR China, Tel.:+86 451 86084169; Fax: +86 451 86283001; Email: [email protected]. b School of Science & Technology, Tianjin University, Taijin 300072,PR China 1 RSC Advances Page 2 of 16 on polyethersulfone flat-sheet ultrafiltration membrane fouling, and found “in-line” coagulation can improve hydraulic removal of filter cake. Flocs formed under charge neutralization lead to lower hydraulic resistance than that of sweep floc condition. 12 Significantly, the properties of aggregates formed under different coagulations could affect the membrane fouling. 13-15 It should be worthwhile to note that the structure of particle aggregates also closely bonds to the flocculation duration and shear conditions involved in the coagulation/flocculation process. In various mechanical aquatic systems, particles and flocs in suspension are highly dependent on the varying flocculation durations and flow conditions. In pre-coagulation/ultrafiltration process, the circulation of suspension induced by different coagulation conditions most likely influence the characteristics of flocs and further the structure of cake layer on membrane surface. Rossini et al investigates the influence of the rapid mixing time on final coagulation efficiency, and found that a short duration of rapid mixing was helpful to produce the lower residual turbidity 16 . Eric J. Wahlberg 17 have shown in his work that the activated sludge flocculated by mechanical means rapidly grow by 99% within 10 minutes under batch conditions. With same condition, the properties and configuration of flocs formed in the whole flocculation process could intensively change under different flocculation time. Nevertheless, few attentions have been paid for the flocculation duration on the performance of coagulation-UF process. Biggs and Lant 18 considered that the large shear force, the smaller average size of flocs aggregated under steady-state conditions. Other literatures have reported that the morphology of flocs was strongly depended on how the shear rate (G) was applied when a polymeric flocculant was used. 19, 20 Weipeng has investigated the evolution of floc size and structure in low-shear force. 21 Klimpel and Hogg 22 thought that higher mixing speed caused large fractal dimension of floc structure. Up to now, previous studies were mainly focus on the relationship between flocs characteristics and conventional coagulation/flocculation process. But, there was lack of insufficiently comprehensive and thorough understandings of the relationship between floc characteristics formed under different pre-coagulation conditions and the subsequent membrane behaviors and fouling mechanisms in coagulation-ultrafiltration process. Our previous paper investigated the influence of flocs breakage process on membrane fouling in coagulation/ultrafiltration process, which was concentrated on the effect of additional coagulant of poly-aluminum chloride and polyacrylamide. Other articles are specialized on the comparisons of flocs characteristics formed by different coagulants as well as their distinct effects on membrane fouling. 23, 24 Little research has been done on the effects of different flocculation durations and low-shear force during coagulation and membrane performances. This work aimed to investigate the impact of different flocculation duration and low-shear force on coagulation process, in order to give a better mechanism understanding of the change of aggregates properties on membrane fouling in the subsequent ultrafiltration process. A series of modified flocculation tests were conducted under different flocculation durations and slow mixing speeds prior to UF. The main investigation focused on flocs size, structure and surface characteristic in pre-coagulation combined with UF membrane process. The reversibility of membrane fouling was still investigated. 2. Methods and materials 2.1 Suspension Kaolin clay (Tianjin, China) and humic aicd (shanghai, China) were used as testing water sample. Kaolin clay was prepared by dissolving 50.0 g of kaolin clay in 1 L deionized water with continuous 2 Page 3 of 16 RSC Advances magnetic stirring for 24 h. After sedimentation for 1 h, 800 ml supernatant was the stock suspension of kaolin clay. 10 g of humic acid (Shanghai, Jufeng, China) was dispersed in 0.1 mM/L NaOH, and mixed for 24 h by a magnetic stirrer. The suspension was filtered by a 0.45 μm fiber filter membrane. The pH of the filtered solution was adjusted to 7.5 using 0.01 mol/L NaOH or HCl and the solution diluted to 1000 mL in a measuring flask. 25 The solution was stored in the dark. Harbin tap water, China, has alkalinity of 15 mg/L as CaCO 3 and a pH of around 7.2. A small dosage of humic acid was added in the testing water to diminish the disturbance of divalent metal ions, like Ca 2+ and Mg 2+ in tap water. 26 For flocculation tests, the stock solution was diluted in the tap water, containing 100 mg/L of clay and 2 mg/L of humic acid. The initial turbidity was about 90 NTU. During the experimental program, the temperature of the solution was 22±2℃. 2.2 Coagulants Polyaluminum chloride (PACl) (28% quality calculated as Al 2O3), the basicity of which was 72.3%, was prepared as a concentration of 1 % by dissolving 5 g reagent in 500 mL deionized water. 27 This coagulant solution renewed 24h, for the flocculation
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