The magnetic enhanced coagulation membrane filtration (MECMF) process was introduced into micro-polluted surface water treatment. The process was conducted by adding magnetic powder (MP) for enhancing coagulation. By contrasting the water quality parameters including dissolved organic carbon (DOC), UV254 and turbidity of permeates of MECMF and coagulation/flocculation membrane filtration (CFMF), results showed that the removal efficiency in the MECMF process was higher than those in the CFMF process. According to normalized flux and resistance analysis, membrane flux declined relatively slow and irreversible fouling resistance was lesser in the MECMF process. By analyzing the three-dimensional fluorescence of permeates and raw water, it was clearly shown that the permeate quality of the MECMF process was superior to that of the CFMF process apparently, which highlighted the removal of protein-like substances was more effectively in MECMF. Furthermore, the addition of MP could provide Lorentz and magnetic dipole forces between colloidal matters during coagulation, leading to the increase of collision frequency and efficiency and the formation of large size flocs with high fractal dimension. Large and high fractal dimension flocs could form a porous cake layer, which could increase water permeability. On the basic of the above findings, it was suggested that magnetic-enhanced coagulation that promoted flocs to develop could regulate cake layer structure and mitigate membrane fouling to some extent.

INTRODUCTION

With the rapid development of industry and agriculture, water quality security has been put on the agenda. Because of the deterioration of aquatic environment as well as increasingly stringent water quality standards (Feng et al. 2012). Therefore, there is an urgent need to protect water resources and develop water treatment technologies. Membrane filtration for its high quality permeate is a promising technology in the field of drinking water treatment (El Rayess et al. 2011; Hiroshi et al. 2014). Fouling is one of the main disadvantages with the membrane filtration process, which can cause flux decline, and result in higher water treatment cost and even deterioration of membranes. Generally, membrane fouling is mainly caused by soluble microbial products (SMP) and extracellular polymeric substances (EPS) (Park et al. 2005; Kim & Dempsey 2013). In terms of membrane fouling, numerous studies have focused on the addition of various chemical compounds in coagulation in order to mitigate subsequent membrane fouling, such as powder-activated carbon or magnetic ion exchange resin (MIEX) (Zhang et al. 2007; Hsu & Singer 2010; Remy et al. 2010; Jamal Khan et al. 2012). Chemical coagulation has been widely used as a method to mitigate membrane fouling in microfiltration/ultrafiltration (MF/UF) membranes used for drinking water treatment. Thus, a combination of the addition of magnetic powder and coagulation was studied in the literature.

Magnetic-enhanced coagulation (MEC) has applied in water treatment and purification, it is an effective enhanced coagulation technology (Gokon et al. 2002; Hu et al. 2012; Wang et al. 2013a, b; Yong et al. 2013). The mechanism of MEC is described briefly in Figure 1. The diminutive floc formed in the conventional coagulation process is gradually accrescent with the addition of magnetic powder (MP). With the introduction of MP, particulate content and collision efficiency increases. Flocs that took MP formed a core. By magnetic cohesion, adsorption and bridging, charged particles and slightly dissolved contaminants could be adsorbed effectively, leading to the further generation of larger magnetic flocs with stronger moving ability (Lipus et al. 2001; Tauxe et al. 2006; Stolarski et al. 2007).
Figure 1

The forming of magnetic coagulation.

Figure 1

The forming of magnetic coagulation.

Guo et al. (2012) adopted the magnetic coagulation process in treating high-turbidity mine water, showing that magnetic coagulation was one of the most efficient methods to remove chemical oxygen demand (COD) and suspended solids (SS). Effects of magnetic nanoparticles on inorganic coagulants and their coagulation performances were studied by Zhang et al. (2012). They found that magnetic poly-aluminium chloride (PAC) of basicity 2.0 (MPCl2.0) performed better than normal PAC on the removals of turbidity and dissolved organic carbon (DOC). Large, loose and weak flocs were produced by MPACl2.0s, which were preferred in magnetic powder recycling. Hu et al. (2011) and Li (2012) adopted a magnetic coagulation process in treating micro-polluted water, showing that the technique not only improved water quality but also modified flocs. It was found that MEF was efficient to remove COD (54.17%), turbidity (99.28%) and TP (75.82%) in treating micro-polluted water (Li 2014).

To sum up, magnetic-enhanced coagulation was introduced before membrane filtration process in this study. We investigated the influence of magnetization on coagulation effect. The main subject of the combination technology was to improve pollutant removal rates, achieve permeate of higher quality and mitigate membrane fouling.

METHODS

Experimental material

The PVDF hollow fiber membranes with a mean pore size of 0.1 μm and the effective surface area of 0.04 m2 were used (Tianjin Motimo Membrane Technology Co., Ltd, China). Magnetic powder (Fe3O4, 99%) (see Figure 2) with sizes ranging from 10 to 35 μm was used with FeCl3 as magnetic coagulant for enhancing coagulation. The mass ratio between MP and FeCl3 was 1:4. The raw water was sampled from Luan River. The characteristics of feed water were detected on the spot. Characteristics of the water samples are presented in Table 1.
Table 1

Characteristics of natural surface micro-pollutant water

ParameterUV254(abs) cm−1DOC mg/LSUVA* L/(mg m)Turbidity NTUTemperature °C
Value 0.074 ± 0.008 6.65 ± 0.38 1.11 ± 0.17 3.64 ± 0.44 18 ± 1.7 
ParameterUV254(abs) cm−1DOC mg/LSUVA* L/(mg m)Turbidity NTUTemperature °C
Value 0.074 ± 0.008 6.65 ± 0.38 1.11 ± 0.17 3.64 ± 0.44 18 ± 1.7 

*Specific UV absorbance (SUVA) can reflect the ingredients of natural organic matter (NOM) and be calculated from the ratio of UV254 to DOC.

Figure 2

(μm) SEM analyses of magnetic powder.

Figure 2

(μm) SEM analyses of magnetic powder.

Magnetic enhanced coagulation and filtration experiment

The surface micro-pollutant water was coagulated immediately after sampling by using magnetic coagulant. The coagulant solutions were added to 1 L suspension and mixed rapidly at 200 rpm for 3 min, followed by mixing slowly at 25 rpm for 25 min. The solution was allowed to settle for 30 min. The entire coagulated suspension after settling for 30 min were collected. The membrane module was used to filtrate supernatant under a constant pressure of 100 kPa. Turbidity, DOC and UV254 absorbance of permeate samples were measured. Another coagulation membrane filtration experiment was conducted with conventional coagulant as a control group. In order to investigate the effect of direct coagulation treatment of surface water, no pH adjustment was made during the experiment.

Analytical method

DOC, UV254 absorbance, turbidity and pH were measured with TOC-Vcph analyzer (TOC-VCPH, Shimadzu, Japan), ultraviolet spectrophotometer (UV2550, Shimadzu, Japan), turbidimeter (2100P, Hach, USA) and pH meter (PHS-25, Leici, China), respectively. Image analysis software was applied to analyze the acquisition of flocs picture and calculate the average particle size distribution and fractal dimension. Flux decline was recorded by electronic scales (CN-SE, Sakura, China). Scanning electron microscopy (SEM) (XL30ESEM, Phillips, The Netherlands) was used to analysis cake layers. The excitation–emission matrix (EEM) spectra of permeates were determined by using luminescence spectrometry (F7000, Hitachi, Japan). The EEM spectra were obtained by scanning a sample for both excitation and emission wavelengths at 250–500 nm. The blank EEM spectrum acquired for double-distilled water was utilized to ensure accuracy of scanned spectra. For all measurements, excitation and emission slits were maintained at 5 nm. The EEM spectra are the elliptical shapes of contours, where the X-axis represents emission spectra at 250–500 nm and the Y-axis is the excitation wavelength at 200–400 nm. All instruments were calibrated as instructed by the manufacturers to ensure the quality of analytical results.

RESULTS AND DISCUSSION

Optimization of magnetizing time

On the basis of our previous study (Wang et al. 2013a, b), the efficiency was closely related with magnetizing time in magnetic enhanced coagulation process. A proper optimization of the factor could significantly increase treatment efficiency.

Studies had shown that the larger size of flocs deposited on the membrane surface made the affinity of membrane surface and flocs weaker which helped to mitigate membrane fouling (Tran et al. 2006; Matos et al. 2011). Besides, it has been generally believed that the coagulation effect has certain correlation with fractal dimension. The larger fractal dimension, the better coagulation effect. Therefore, in order to get optimal magnetizing time, we calculated floc size and fractal dimension under different magnetizing time. The experimental results were shown in Figure 3. From it, an optimal magnetizing time (6 min) was selected, through which peaks of floc size and fractal dimension were obtained, 397.06 μm and 1.08, respectively.
Figure 3

Impact of magnetizing time on the floc parameters: floc size and fractal dimension.

Figure 3

Impact of magnetizing time on the floc parameters: floc size and fractal dimension.

When MP reached saturated magnetic field intensity, the residual magnetization was greatest. In this investigation, magnetic induction of magnet was changeless, hysteresis effect of MP affected the acceleration of coagulation, while magnetization time influenced hysteresis effect and then impacted the effect of coagulation. Magnetization for too long may cause MP agglomeration, poor dispersion and is not conducive to enhance coagulation. Therefore, magnetization time should be controlled in a proper range.

Analysis of permeate quality

The permeate water quality parameters were contrasted under different coagulant dosage, for quantifying MECMF and CFMF processes. Figure 4 shows the removal of DOC, turbidity and UV254 under different dosage of coagulant. It is obviously showed that permeate in MECMF can achieve better quality with magnetic coagulant dosage of 15 mg/L, and the removal rates of DOC, turbidity and UV254 were 81.3%, 95.4% and 82.1%, respectively. With the increase of coagulant dosage, the removal rates of these parameters did not increased. While, those in CFMF with coagulant dosage of 20 mg/L reached maximums, but were still lower than those in MECMF.
Figure 4

The removal of DOC (a), turbidity (b), UV254 (c), under different dosages of coagulants.

Figure 4

The removal of DOC (a), turbidity (b), UV254 (c), under different dosages of coagulants.

CFMF removed 59–68% DOC in raw water, while MECMF removed a maximum of 69–81.3% DOC (Figure 4(a)). It is obviously shown that MECMF can be more efficient in DOC removal. MECMF showed the maximum turbidity removal at 15 mg/L, whereas, that of CFMF showed at 20 mg/L in Figure 4(b). Higher UV254 absorbing removal was observed with the increase of coagulant dosage. When the dosage was 15 mg/L, the rate of UV254 removal reached its maximum in MECMF. With the successive increase of coagulant dosage, the removal showed a reduction trend. Overall, a maximum of 72% of the UV254 was removed by MECMF at the tested dosage of 15 mg/L. The removal rate of UV254 in MECMF was significantly better than that in CFMF.

From above analysis, it is found that magnetic coagulation membrane filtration can achieve the best water quality at the coagulant dosage of 15 mg/L, while the preferred coagulant amount of the CFMF process was 20 mg/l. It is clear that the MECMF process can not only significantly reduce coagulant dosage but also improve permeate quality.

Membrane filtration performance

The changes of the normalized flux during filtration are showed in Figure 5(a). MP addition led to a diminution of flux decline velocity compared to the conventional coagulation alone. At the end of the filtration test, flux declines of 56.4% and 46.3% were observed in CFMF and MECMF, respectively. Consequently, flux in MECMF was 10.1% higher in presence of magnetic particles than that in CFMF.
Figure 5

Influence of MP addition on flux and resistance component.

Figure 5

Influence of MP addition on flux and resistance component.

Due to the addition of magnetic powder, increasing particle content and collision probability, magnetic flocs with stronger moving ability were formed with MP at the core. Besides, floc structure turned denser and the nanoscale particles in the surface water were agglomerated to form relatively larger size particles. Thus porous cake layer was formed to intercept colloidal particle and prevent colloidal particle from depositing on membrane surface and in membrane pores, increase permeability of water, and make the decrease of flux become slow.

Furthermore, the addition of particles led to a less resistance (Loulergue et al. 2014), which can be quantified using the equation: 
formula
1
 
formula
2
with RT the total resistance (m−1), ΔP the pressure (Pa), μ the permeate viscosity (Pa s), J the permeate flux (m3 m−2 s−1), Rr reversible fouling (the part of remove by regular physical cleaning) (m−1), Ri irreversible fouling (the part of can not be removed by regular physical cleaning) (m−1), RF the summation of reversible fouling and irreversible fouling (m−1), Rm the clean membrane resistance (m−1), of which Ri equals the resistance after back washing minus the membrane resistance and Rr equals the resistance before back washing minus the membrane resistance.

The variation of resistance during filtration tests is shown in Figure 5(b). As shown in Figure 5(b), the total resistance for MECMF and CFMF process were 0.76 × 1012 m−1 and 1.09 × 1012 m−1, respectively, whose irreversible fouling were 17.07% and 24.38% after physical cleaning. Hiroshi et al. (2014) suggested that the major constituents causing irreversible fouling were hydrophilic organic matter including carbohydrates and proteins. The addition of MP may strengthen the removal of protein-like substances. To sum up, magnetic powder addition had a positive effect on mitigating membrane fouling.

Fluorescence EEMs

In order to investigate the effect of removing substances by adding MP, fluorescence EEM spectra of the dissolved natural organic matter (NOM) are presented in Figure 6. The organic matter (e.g., humic acid (HA) or protein) can be categorized by analyzing shapes and peaks of the contour map. The fluorescence EEM of surface water (Figure 6(a)) contained four regions or peaks representative of two major NOM fractions. There was a typical peak at excitation wavelength (Ex) 270–320 nm and emission wavelength (Em) 375–415 nm, which corresponds to the range reported for HA-like NOM (Coble et al. 1990; Sierra et al. 2005). A secondary peak, which also corresponds to HA (Sierra et al. 2005; Peiris et al. 2008) appears at Ex/Em 235–255 nm/425–445 nm. The high intensity of fluorescence EEM peak around Ex/Em 225–240 nm/325–350 nm is indicative of the presence of protein-like substances in the water. Another peak, which also corresponds to protein-like substances appears at Ex/Em260–280 nm/290–320 nm. The HA peak in the region is less clearly visible and would reflect the relatively low concentration levels of HA-like substances present in surface water compared to protein-like substances. Rayleigh light scattering regions observed in the fluorescence EEM have been shown to contain information related to colloidal/particulate matter present in water (Peiris et al. 2010).
Figure 6

Fluorescence EEMs of (a) surface water, (b) permeate in CFMF, (c) permeate in MECMF. For the CFMF processes, the coagulant dosage was 20 mg/L, and for MECMF, the magnetic coagulation was 15 mg/L.

Figure 6

Fluorescence EEMs of (a) surface water, (b) permeate in CFMF, (c) permeate in MECMF. For the CFMF processes, the coagulant dosage was 20 mg/L, and for MECMF, the magnetic coagulation was 15 mg/L.

Fluorescence EEMs of permeates acquired from the CFMF and MECMF are illustrated in Figure 6(b) and (c). After the coagulation membrane filtration process, the intensities of all NOMs from permeate were decreased compared with NOMs from raw water. The intense reduction of protein-like substances around Ex/Em 225–240 nm/325–350 nm treated with MEFMF processes observed in Figure 6(c) can be attributed to magnetic enhance coagulation process. In addition to the intensity reduction of protein-like substances, MECFMF processes also caused a peak reduction for HA-like substances around Ex/Em 235–255 nm/425–445 nm and 270–320 nm/375–415 nm from permeate, contrasting with the permeate in CFMF process.

The protein-like substances in permeate of MECFMF was removed more effectively than that in CFMF permeate. However, as for the HA-like substances, the removal rate of MECFMF was not as conspicuous higher as CFMF. Hence, the MECFMF process can provide an alternative technology for optimizing permeate quality.

Effect of MEC on floc morphology

The images of flocs collected from MECMF and CFMF processes are shown in Figure 7. The floc formed in CFMF process was looser and smaller. However, the size of magnetic floc was significantly larger and denser. A modicum of magnetic powder was evenly distributed in flocs.
Figure 7

Image of different flocs: (a) common floc in coagulation, (b) magnetic floc in MECFM.

Figure 7

Image of different flocs: (a) common floc in coagulation, (b) magnetic floc in MECFM.

Figure 8(a) shows the average particle size of both magnetic coagulation and coagulation flocs under different coagulant dosage. The average particle size of magnetic coagulation flocs are larger than that of coagulation flocs. Kim & Park (2002, 2005) showed that increasing particle size is an effective way to reduce cake formation. This suggests that enlarging particle size could promote the flow of fluid on membrane surface and reduce particle deposition.
Figure 8

Impact of coagulant dosage on the Flocs parameters (a) the average diameter and (b) fractal dimension. The data points represent the average of duplicate experiments.

Figure 8

Impact of coagulant dosage on the Flocs parameters (a) the average diameter and (b) fractal dimension. The data points represent the average of duplicate experiments.

In the fractal concept, the most important and powerful quantitative parameter is the fractal dimension (Zhou & Franks 2006), which indicates the space filling capacity (Thomas et al. 1999), that is, the compactness of the floc. Fractal dimension is an effective method to character floc properties (Adachi et al. 1998; Xiao et al. 2011), which is calculated by the following equation: 
formula
3
with A the projection area of floc; L maximum length of projection; α proportionality constant and Df fractal dimension of flocs in 2D space. Floc fractal dimension and coagulation effect has a certain relationship. In general, the higher floc fractal dimension, the better coagulation performance.

Higher fractal dimensions signify more compact flocs, which are usually preferred in most situations in water treatment to yield lower sludge volumes and easier sedimentation. Fractal dimension analysis results provided important information that its value relies on the coagulant dosage and the adding of magnetic powder. The fractal dimension of magnetic flocs and the dosage of coagulant have positive correlation.

As seen from Figure 8(b), magnetic coagulation has a significant effect on reducing the cake layer formed on the membrane surface. Findings showed that adding magnetic powder was more effective on increasing floc size and reducing cake layer formed on the membrane surface, which can be more effective on the mitigation of the flux decline of microfiltration membrane and more effectively decelerate the speed of the external membrane fouling.

Cake characteristics

Due to the change of floc characteristics, the cake layer will change accordingly. In order to better understand the influence of MP presence on cake characteristic, physical maps and SEM images were studied. These results are summarized in Figure 9.
Figure 9

Image and SEM analyses of membrane: (a) MECMF membrane image, (b) CFMF membrane image, (c) MECMF SEM, (d) CFMF SEM.

Figure 9

Image and SEM analyses of membrane: (a) MECMF membrane image, (b) CFMF membrane image, (c) MECMF SEM, (d) CFMF SEM.

It can be seen from Figure 9(a) and (b) that flocs deposited on the membrane surface formed a contamination layer. Meanwhile, comparing Figure 9(a) and (b), we see that the contamination layer of CFMF was denser while that of MECMF was loose and MP was uniformly distributed on membrane surface.

The variation of cake structure during these two different filtration experiments is shown in Figure 9(c) and (d). The differences of cake layers under different situations can be seen according to contrast analysis; the surface of cake layer is relatively smooth, dense structure, low porosity in CFMF process, while the cake layer formed in the presence of magnetic powder is rough, permeable and porous in MECMF process. Besides, previous research has shown that cake layer containing particles was less cohesive than that without particles (Loulergue et al. 2014). MECMF made a effective reduction of cake resistance and effective filtering area was relatively larger than that in CFMF process, therefore, permeate flux was more slowly than that without MP.

CONCLUSION

The permeate water characteristics such as DOC, turbidity and UV254 in MECMF process have been treated to a relatively lower level than those in CFMF. MECMF process could effectively mitigate membrane fouling. With the addition of MP, protein-like substances were removed availably, irreversible fouling resistance reduced effectively. Besides, magnetic force and magnetic dipole force in the process of magnetic enhanced coagulation had a marked impact on forming flocs of higher fractal dimension and larger size, thus porous cake layer was formed. In conclusion, MECFMF may provide a promising and environmental-friendly approach for treating micro-polluted surface water and control membrane fouling as well as optimize filtration performance.

ACKNOWLEDGEMENTS

This study is financially supported by the National Natural Science Foundation of China (No. 51378349, No. 51108314,) and Program for Changjiang Scholars and Innovative Research Team in University (IRT13084) and China Postdoctoral Science Foundation (2013M541184).

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