Role of pre-oxidation, using potassium permanganate, for mitigating membrane fouling by natural organic matter in an ultrafiltration system

Role of pre-oxidation, using potassium permanganate, for mitigating membrane fouling by natural organic matter in an ultrafiltration system

Chemical Engineering Journal 223 (2013) 487–496 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage: ww...

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Chemical Engineering Journal 223 (2013) 487–496

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Role of pre-oxidation, using potassium permanganate, for mitigating membrane fouling by natural organic matter in an ultrafiltration system Tao Lin a,b, Shaolin Pan b, Wei Chen a,b,⇑, Shen Bin b a b

Ministry of Education Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Hohai University, Nanjing 210098, PR China College of Environment, Hohai University, Nanjing 210098, PR China

h i g h l i g h t s  The KMnO4 as a preoxidant for sand filter effluent pretreatment.  Significant improvements to the efficiency of natural organic matter removal.  We examine the mechanism of membrane fouling with preoxidation by KMnO4.  The KMnO4 oxidation changed the characteristics of organic pollutants.  The KMnO4 pretreatment had potential for mitigating transmembrane pressures.

a r t i c l e

i n f o

Article history: Received 13 September 2012 Received in revised form 4 March 2013 Accepted 6 March 2013 Available online 13 March 2013 Keywords: Ultrafiltration Preoxidation Potassium permanganate Membrane fouling control

a b s t r a c t An ultrafiltration (UF) system was used as an advanced treatment, following the conventional sand filter process, to purify raw water from the Yangtze River. Potassium permanganate was used as a pre-oxidant to oxidize the UF system influent for controlling the membrane fouling. The process was implemented by directly dosing KMnO4 into the UF system influent, i.e. the sand filter effluent. The optimal dosage of KMnO4 was 0.4 mg/L in terms of both the trans-membrane pressure (TMP) and product water quality. Compared to the results obtained when using a UF system without KMnO4, the preoxidation of the feed water by KMnO4 was found to result in a lower cake layer resistance. The pretreatment of the feed water by KMnO4 also indicated that some advantages in terms of mitigating TMPs were achieved by transforming molecular weight distribution of organic pollutants, which were mainly from hydrophilic and hydrophobic organic matter. The analysis of attenuated total reflection-Fourier transform infrared spectra of the cake layer showed that KMnO4 oxidized macromolecules of hydrophobic natural organic matter (NOM) to lower-molecular-weight hydrophilic organic matter, which mitigated the membrane fouling caused by the organic matter. Scanning electron microscopy indicated that loose fragments were formed on the filtration cake in the KMnO4/UF system, which was easily removed by hydraulic washing. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Membrane technology has been used worldwide, to facilitate compliance with current and anticipated water quality regulations. Low-pressure ultrafiltration (UF) membrane techniques have attracted considerable attention, due to their capacity to remove particulates by size exclusion, a process that usually produces a low-turbidity, pathogen-free filtrate from rivers, lakes and underground water supplies [1–4]. The use of membrane technology for drinking water treatment is increasing because it is more effective at removing pathogens than the case for conventional ⇑ Corresponding author at: Ministry of Education Key Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Hohai University, Nanjing 210098, PR China. Tel.: +86 13913899869; fax: +86 02583787618. E-mail address: [email protected] (W. Chen). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.024

filtration processes [5]. Nevertheless, membrane fouling is an important issue that presents a major impediment to the progress of this technology. Membrane fouling, primarily due to natural organic matter (NOM) such as humic substances, polysaccharides and proteins, is a major cause of flux declines in UF treatment processes [6,7]. NOM is the major precursor of disinfection by-products and it is also responsible for undesirable organoleptic properties such as tastes, odors and coloration, as well as increased levels of bacteria in distribution systems. NOM is partially trapped by UF, so can become a major contributor to membrane fouling. The approach, to alleviate membrane fouling by NOM, is an important factor determining the success of membrane technology in water treatment processes [8]. Membrane fouling is inevitable, due to the existence of NOM in the feed water of membrane systems. The development of effective pretreatment methods is therefore essential for the removal and/or

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Table 1 Water quality of feed water. Category

Characteristics

Values

Bacteria amount

CFU/mL

11–16

Sensory traits and physical indicators

Turbidity (NTU) Color (Pt–Co) pH Manganese (mg/L) Iron (mg/L) Aluminum (mg/L) Calcium (mg/L) Hardness (as CaCO3 mg/L) CODMn (mg/L)

0.26–1.28 <5 6.72–7.78 0.013–0.021 <0.06 0.013–0.017 24.1–27.8 98–112 1.36–3.16

Other values

Temperature (°C) Conductivity (ls/cm) UV254 (cm1) TOC (mg/L) SUVA (L/mg1 m1) DO (mg/L)

7.6–15.4 230.12–282.39 0.016–0.054 1.74–3.13 0.92–2.87 5.64–7.31

Table 2 Characteristics of UF membrane. Average molecular weight cut-off (Da)

50,000

Internal diameter (mm) External diameter (mm) Fiber length (m) Effective surface area (m2) Type

0.85 1.45 0.500 4.5  103 Outside-in

the transformation of NOM in feed waters, so as to improve the cost-effectiveness of the membrane system and broaden the application of membrane technology in water treatment systems [9,10]. The efficiency of pretreatment for the purpose of mitigating membrane fouling is strongly dependent on the type of agent (coagulant, adsorbent or oxidant), dosage, dosing mode (continuous or intermittent), dosing point, mixing method, temperature regimes, NOM properties (hydrophobicity, charge density, molecular weight and molecular size), solution environment (pH and ion strength) and characteristics of the membrane (membrane charge, hydrophobicity and surface morphology) [11]. The characteristics of UF membranes have a significant influence on the removal of organic matter in water samples. Some studies have shown that strongly hydrophilic UF membranes have a greater rejection capacity for membrane fouling than less hydrophilic membranes [12]. The hydrophobic factions of organic compounds also have a greater impact on membrane fouling [13]. The investigation of potential pretreatment options for UF membrane fouling control has been the

subject of several studies. Preoxidation is often adopted as part of the pretreatment for UF membrane fouling control and a number of studies have focused on a search for suitable preoxidants. There are numerous reports that oxidants, such as ozone, chlorine and permanganate, can be used for the preoxidation of raw water, so as to enhance the removal of pollutants during the coagulation and filtration processes [14,15]. Less information is, however, available on the direct preoxidation of UF influents, which may be an alternative for mitigating membrane fouling by transforming NOM molecular characteristics, due to the oxidization reaction. The Yangtze River, an important source of raw water for many cities in China, has a low NOM concentration (mostly less than 3 mg/L) and so the preoxidation of raw water to remove NOM is not usually adopted in most of the waterworks supplied from this source. The UF technique has attracted considerable attention in the Yangtze Delta area of China as an advanced treatment process to follow the conventional sand filter treatment processes. Oxidation pretreatment of UF influent has been advocated as a mechanism for controlling membrane fouling in preference to preoxidation of raw water. Compared to other oxidation processes, such as chlorination, potassium permanganate (KMnO4) preoxidation produces fewer by-products [16]. There is some evidence to indicate that KMnO4 preoxidation of raw water may remove NOM, ion and algae [17]. However, few studies refer to KMnO4 preoxidation of sand filter effluent prior to entry into the UF system. Because of the complex and variable nature of NOM present in river water, the fouling mechanism of the UF membrane system is poorly understood [18]. It is therefore not clear whether KMnO4 preoxidation of the sand filter effluent can mitigate membrane fouling of UF, particularly when using Yangtze River water as a source. In this paper, a bench-scale UF system was used as an advanced treatment process following conventional sand filter treatment, so as to purify raw water from the Yangtze River. The effect and mechanism of membrane fouling control were investigated in a UF system combined with KMnO4 pretreatment. The objectives of this research were to perform preoxidation before membrane filtration, to investigate permeate flux variation and filtrate qualities in the UF process, and to evaluate the potential for mitigating membrane fouling using KMnO4 pretreatment.

2. Material and methods 2.1. Feed water Water samples in this study were obtained from the sand filter effluent of the Beihe Water Plant in Nanjing, which treats raw water from the Yangtze River. The key water quality characteristics are given in Table 1.

Fig. 1. Schematic diagram of the KMnO4/UF system.

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1.000 0.900

P1/P0

0.800 0.700 0.600

0mg/L

0.1mg/L

0.2mg/L

0.500

0.3mg/L

0.4mg/L

0.5mg/L

0.400

0

20

40

60

80

100

Time (min) Fig. 2. Effect of the KMnO4 dosage on the value of P1/P0.

Table 3 Water quality parameters after UF with different dosage of KMnO4 (mg/L). Category

Characteristics

Feed water

0

0.1

0.2

0.3

0.4

0.5

Sensory traits and physical indicators

Turbidity (NTU) Color (Pt–Co) pH Manganese (mg/L) Iron (mg/L) Aluminum (mg/L) Calcium (mg/L) Hardness (as CaCO3 mg/L)

0.8 <5 7.1 0.016 0.05 0.012 24.0 102

0.1 <5 7.1 0.015 0.05 0.012 23.9 101

0.1 <5 7.2 0.029 0.04 0.012 23.7 105

0.1 <5 7.0 0.041 0.04 0.012 23.7 103

0.08 <10 7.3 0.059 0.03 0.01 23.1 105

0.08 <5 7.3 0.076 0.02 0.01 22.8 102

0.08 <10 7.4 0.11 0.02 0.009 22.2 104

Other values

CODMn (mg/L) Temperature (°C) Conductivity (ls/cm) UV254 (cm1) TOC (mg/L) SUVA (L/mg1 m1) DO (mg/L)

2.8 11.6 267.3 0.034 2.93 1.16 6.5

2.4 11.6 266.5 0.032 2.74 1.17 6.3

2.4 11.6 268.3 0.032 2.62 1.22 5.8

2.5 11.6 268.9 0.030 2.48 1.19 5.8

2.5 11.7 273.1 0.029 2.32 1.19 5.8

2.2 11.6 273.3 0.026 2.18 1.22 5.9

2.1 11.7 276.8 0.022 2.13 1.03 5.9

2.2. UF experiments Hollow fiber UF membrane made of polyvinyl chloride (PVC) was provided by Litree Purifying Technology Co., Ltd. (SuZhou, China), for use in this study. The UF membrane has a molecular weight cut-off (MWCO) of 50,000 Da and an effective surface area of 0.0045 m2. Detailed characteristics of the membrane are summarized in Table 2. A schematic of the bench-scale KMnO4/UF system is shown in Fig. 1. The water level in the reaction tank was maintained at a constant level by an overflow pipe. The feed water was collected from the sand filter effluent of the water treatment facility and introduced into the feed water tank (capactiy 100 L). A peristaltic pump with a constant flux of 3 L/h, was used to pump feed water into the reaction tank in which the KMnO4 was dosed, so as to mix with the feed water using an electric stirring device of stirring speed at 60 r/min, to the designed concentration required for preoxidation. The mixed flow into the collecting tank was kept at a constant flux of 3 L/h by a feed pump. A circulating water tank was used to pump water, the same water quality as the mixed flow in the reaction tank, into the collecting tank by using a circulating pump with a constant flux of 1 L/h. An overflow tube was installed on the collecting tank to maintain the water level and the overflow rate was 1 L/h. The overflowing water was back to the reaction tank and then to the circulating tank. Water from the collecting tank flowed into the UF membrane pool by gravity. The flux of the UF membrane module in the membrane pool was maintained at 3 L/h (30 L/m2h) by means of a effluent pump. The UF experiments were designed as an outside-in type of submerged membrane reactor. During the experiment the UF membrane system operated at a filtration cycle of 45 min, which was determined by the spot bench-scale trial. The following processes were performed at the end of each filtration cycle: first wash at

6 m3/h for 15 s, backwash at 8 m3/h for 30 s, and a second wash at 6 m3/h for 15 s. Pure water was used as a backwash in this experiment and the effluent pump was controlled by a timer with an on/off time sequence of 45 min: 60 s. Aeration was used to maintain suitable hydraulic conditions for the filtration process, and was intermittently supplied through a diffuser at a flow rate of 20 m3/h at the bottom of the membrane tank. This operated during feed filtration by the UF membrane and stopped during the backwash. The membrane was cleaned at a dosage of 100 mg/L (as Cl2) sodium hypochlorite solution. Chemical cleaning, which was periodically undertaken at room temperature, involved 20 min back-flushing with 10 mg/L chlorine solution (pH 9.9) and 20 min back-flushing with deionized (DI) water. Prior to the experiment, virgin membranes were soaked in pure water overnight and then pretreated with pure water to achieve a stable permeate flux. After 2 h of pure water filtration, the effluent from the sand filter was pumped into the UF system and KMnO4 preoxidation was used to control membrane fouling. The influence of KMnO4 dosage on membrane fouling was first investigated to determine the feasible control effect. Various dosing levels of KMnO4 were investigated to ensure the optimal dosage of KMnO4 pretreatment. An electric stirrer was used to mix the KMnO4 solution and feed water so as to maintain a consistent level in terms of each pre-oxidation condition. The contact time for the oxidation process was 10 min. Prior to the commencement of continuous UF, the trans-membrane pressure (TMP) (P0) was measured by the filtration of pure water. The TMP of the sample (P1) was compared with the pure permeate TMP (P0), to provide a comparison between the different pretreatment conditions. TMP may increase in the course of filtration, which reflects the resistance of the UF membrane due to membrane fouling. The resistance-in-series model was used to

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determine the resistance [19]. The constant-flux component of this model [20] can be expressed as follows:



DP

lR

ð1Þ

where J is the permeate flux (L m2 h1), D P is the trans-membrane pressure (Pa), l is the viscosity of permeate (Pa s), and R is the total (overall) filtration resistance (m1). In the case of filtration the following equation was used:

R ¼ Rm þ Rf ¼ Rm þ Rrev þ Rirr

ð2Þ

where Rm is the intrinsic membrane resistance (1/m) which is determined from the experimental data of R in the filtration of pure water, Rf is the total fouling resistance, which includes the physically reversible fouling resistance (Rrev, 1/m) and the physically irreversible fouling resistance (Rirr, 1/m). The value of experimental resistance, R1, is determined from the experimental data at the end of filtration. The value of experimental resistance, R2, is determined from the experimental data at the commencement of next filtration cycle after hydraulic cleaning. We can obtain the formula for calculating resistance of Rirr and Rrevr:

Rirr ¼ R2  Rm

ð3Þ

Rrev ¼ R1  R2

ð4Þ

From Eqs. (1) and (2) we can obtain the formula for calculating resistance:



DP Jl

ð5Þ

where J is 30 L/m2 h and l is 1.005  103N S m2 at the atmospheric temperature. Pure water was filtered by the UF system and the value of TMP was recorded. The feed water, in the presence or the absence of KMnO4 pretreatment, was introduced into the UF system, so as to investigate the TMP variations. At the end of the experiment, ATR-FTIR (Attenuated total reflection-Fourier transform infrared spectroscopy) technology was used to explain the variation of functional groups in organic matter attachment on membrane surface, which was a result of KMnO4 pretreatment. A microscopic observation of the membrane surface was performed simultaneously, using scanning electron microscopy, and elementary analysis was performed, using energy dispersive X-ray spectroscopy. 2.3. Analytical methods To ensure the reliability of experimental results, the effluent of a UF system in a filtration cycle should be measured 15 times, so 15 replicate measurements of each sample were taken and the average value determined (P < 0.05). Turbidity was measured using a Turbidimeter (2100 N, Hach, USA). Samples were filtered through a 0.45-lm membrane (Millipore, USA) to remove particles prior to measuring organic matter. Water samples and fractionated fractions were stored at 4 °C until further analysis for UV254 and DOC. NOM was measured as ultraviolet absorbance at 254 nm (UV254), using a UV/VIS spectrophotometer (EV300, Thermo Fisher, USA). Dissolved organic carbon (DOC) was determined by a TOC analyzer (1030 W, OI, USA). The resins of Superlite DAX-8 (Supelco, USA), Amberlite XAD-4 (Rohm and Hass, Germany) and Amberlite IRA-958 (Rohm and Haas, Germany) were used to fractionate NOM into four groups [21]: hydrophobic (DAX-8 adsorbable), transphilic (XAD-4 adsorbable), polarity hydrophilic (IRA-958 adsorbable), and neutral hydrophilic organic (neither adsorbable) fractions. The resins were washed with methanol and deionized water prior to use in frac-

tionation. The surface water pH was adjusted to 2.0, prior to feeding into the DAX-8 absorption column (at a rate of 5 mL/min), the XAD-4 resin (at 15 mL/min) and Amberlite IRA-958 (at 10 mL/min). The absorbed organic fractions were then eluted from the resins of DAX-8, XAD-4 and IRA-958, using 0.1 mol/L NaOH, 1 mol/L NaOH and 1 mol/L NaCl, respectively. Molecular weight (MW) distributions were determined using high-pressure size exclusion chromatography (HPSEC). High performance liquid chromatography (HPLC, Lc-10ADVP, SHIMADZU, Japan) was used together with UV (SPD220A, 254 nm), which was set up as follows. Column: Shimadzu the Lc-10ADVP Type TSK gel; G4000PWXL, diameter of 8 mm  300 mm; mobile phase: high water, hydrodynamic injection; temperature 40 °C; flow rate: 10 mL min1. The separation range of the column was 100– 18,000 Da, based on polyethylene glycols (PEGs) and 500– 80,000 Da, with globular proteins. The SEC column separates compounds on the basis of hydrodynamic molecular size. The average retention time is affected by the effective size and structure of the molecules. Consequently, large, and linear-shaped, molecules are excluded earlier than small and globular shaped molecules [22]. PEGs were used to calibrate the relationship between MW and retention time. Concentrations of metal elements, including Mn, Al, Fe and Ca, were determined by inductively coupled plasma-atomic emission spectroscopy (Optima 2100 DV, PerkinElmer, USA). Membrane samples, including virgin and fouled membranes, were gently rinsed with deionized water and then dried overnight at room temperature. The ATR-FTIR technique (NEXUS870, Thermo Nicolet C, USA) was used at a resolution of 4 cm1 in the range of 500– 4000 cm1, to provide information on the functional groups of the foulants on the membrane. Before the FTIR analysis, the samples were totally air dried in clean plastic bags at room temperature. The dried samples were then cut to obtain a flat external surface. Samples exposed to infrared light absorbed energy corresponding to the vibration energy of atomic bonds. The characteristics of known functional groups that absorb energy at specific wavelengths are then used to identify such groups from samples. The surface of the membrane sample was observed by scanning electron microscopy (SEM) (Hitachi-3400N, Hitachi, Japan) coupled with an energy dispersive spectroscope (EDS, NORAN-VANTAGE) to investigate the structure of the filter cake. All samples were completely dried in clean plastic bags at room temperature and the membrane fibers were frozen with liquid nitrogen and fractured to obtain the plane. For SEM analysis all samples were then coated with gold (Au) and examined at a 30 kV accelerating voltage. EDS analysis of the structure of organic compounds on the membrane surface was also carried out [23]. 3. Results and discussion 3.1. Effect of KMnO4 dosage on the performance of the UF system The KMnO4 dosage was optimized to ensure the alleviation of membrane fouling. Water from the same source was used when Table 4 The concentration of organic matter in the water after sand filtration with different KMnO4 dosage. Dosage (mg/L)

TOC (mg/L)

UV254 (cm1)

SUVA (L/mg1 m1)

0 0.1 0.2 0.3 0.4 0.5

2.93 2.78 2.63 2.45 2.25 2.16

0.034 0.033 0.032 0.031 0.028 0.025

1.16 1.18 1.22 1.26 1.24 1.16

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0.10 UF influent UF effluent UF effluent/KM nO4 Feed water/KM no4

UV Response

0.08 0.06 0.04 0.02 0.00 0

1000

2000 3000 4000 M olecular weight (Da)

5000

6000

Fig. 3. Changes in the molecular weight of organic compounds for different conditions.

The average concentrations of effluent aluminum and iron in the KMnO4/UF system were also lower than those obtained after use of UF alone (the UF system). The effluent concentration of iron in the KMnO4/UF system was approximately 25.24% of that in the UF system. Iron in the presence of water is mainly divalent or trivalent. Oxidation of iron by KMnO4 to form insoluble complexes prior to removal by UF involves the transfer of electrons from iron, or other chemicals being treated by the oxidizing agent. Ferrous iron (Fe2+) is oxidized to ferric iron (Fe3+), which readily forms the insoluble iron hydroxide complex Fe (OH)3 [25]. The average concentration of Fe(II) in the feed water was 0.032 mg/L and it was lower than 0.005 mg/L in the effluent with KMnO4 preoxidation. The oxidation of ferrous iron to ferric iron by KMnO4 results in the formation of an iron hydroxide precipitate that is rejected by UF membranes. In conclusion, we found that in the KMnO4/UF system, a dosage of 0.4 mg/L KMnO4 was optimal in terms of both membrane fouling and product water quality.

testing for the optimal concentrations of KMnO4. Comparative experiments were performed under different pretreatment conditions, in which the KMnO4 doses varied from 0.1 to 0. 5 mg/L. Fig. 2 shows the normalized filtrate TMP ratio (P1/P0 at the constant flux of 30 L/m2 h) when KMnO4 preoxidizes the sand filter effluent. The decline of permeate flux improved when the dosage of KMnO4 increased in the range of 0.1–0.5 mg/L. Results indicate that the use of KMnO4 in the UF system resulted in an improved permeate performance compared to the same system that had not undergone KMnO4 pretreatment. The results also imply that KMnO4 preoxidation reduces the rate and extent of membrane fouling. It was noteworthy that, in the dosage range of 0.1–0.5 mg/L, a greater concentration of KMnO4, in the pretreatment phase resulted in better control of membrane fouling. Details of effluent water quality from the UF system (Table 3) indicate that, at dosages of 0.5 mg/ L, the manganese element in the UF effluent sometimes exceeded the drinking water quality threshold standard of 0.1 mg/L. In terms of both TMP and product water quality in the KMnO4/UF system, a feasible recovery of permeate flux was obtained at a KMnO4 dosage of 0.4 mg/L, in which the concentration of manganese was below the water quality threshold level. The results indicate that KMnO4 pretreatment combined with UF (KMnO4/UF system) was more effective than direct UF for the removal of NOM. Results summarized in Table 4 indicates that, at a dosage of 0.4 mg/L KMnO4, the average concentration of TOC in the water sample had been reduced by 23.2% and that of UV254 by 17.6%, when compared to concentrations prior to pretreatment. The experimental results, summarized in Tables 3 and 4, indicate that KMnO4 treatment resulted in a steady improvement of the quality of influent into the UF membrane system, due to oxidation of organic matter. This result is in accordance with that of Liu et al. [24].

3.2. Effect of preoxidation by KMnO4 on molecular weight distribution The mechanisms associated with KMnO4 preoxidation on the mitigation of membrane fouling were assessed by measuring the molecular weight (MW) distribution of NOM (Fig. 3). Chromatograms of NOM in water samples usually indicate distinct UV response peaks. The results indicate that the molecular weight of NOM in water samples of UF influent, KMnO4 UF influent, the UF effluent and the KMnO4/UF effluent were generally less than 5000 Da. As shown in Fig. 3, the UV absorption peaks occurred at molecular weights of 400, 500, 1000, 1800, 2400 and 3500 Da. The changes in absorption peaks due to KMnO4 pretreatment were mainly at the 3500 D peak, which disappeared completely, while

Table 5 Effect of pretreatment conditions on change of compounds. Index

VHA

SHA

CHA

NEU

DOC (mg/L)

Feed water KMno4/feed water UF effluent KMno4/UF effluent

1.09 0.85 0.91 0.72

0.11 0.08 0.10 0.07

0.48 0.34 0.46 0.32

1.12 0.78 1.02 0.73

UV254 (cm1)

Feed water KMno4/feed water UF effluent KMno4/UF effluent

0.012 0.009 0.010 0.008

0.006 0.004 0.005 0.003

0.004 0.003 0.003 0.003

0.01 0.009 0.009 0.009

SUVA (L(mg1 m1))

Feed water KMno4/feed water UF effluent KMno4/UF effluent

1.10 1.06 1.05 1.06

5.45 5.00 5.00 3.00

0.83 0.88 0.70 0.88

0.90 1.15 0.90 1.19

Note: strong hydrophobic organic compounds: VHA; weak hydrophobic organic compounds: SHA; hydrophilic organic compounds: CHA; neutral organic compounds: NEU.

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the intensity of other absorption peaks also decreased. This indicates that KMnO4 pretreatment changed the molecular weight distribution of organic matter in the solution, indicating a breakdown of macromolecular organic matter to form substances of lower molecular weight, due to KMnO4 oxidation. The intensity of the absorption peak was further decreased in the KMnO4/UF system, due to the absorption, or trapping, of organic matter by the filter cake. Attachment of organic matter to membranes is the main cause of fouling. The large-MW peaks correspond to macromolecular compounds and colloidal organic matter, which contribute to significant organic fouling during low-pressure membrane filtration [22]. The low molecular organic matter (less than 3 kDa) – which mainly consists of humic substances that may be absorbed on the membrane surface and membrane pores – are not effectively removed by hydraulic washing [26]. Compared to the effluent from the UF system, the organic matter concentration of the effluent from the KMnO4/UF system decreased in the range of 1–3 kDa. The KMnO4/UF system displayed a capacity to remove organic matter that was concentrated in the range of 1–5 kDa (mostly in 1–3 kDa). This may be interpreted as being due to KMnO4 oxidation. In addition, the product (MnO2) from KMnO4 oxidation results in the absorption of some organic compounds prior to their attachment to the membrane [27]. The MnO2, together with organic compounds, therefore represents an important fraction of the loose fouling cake, which has a lower resistance than that of fouling cake formed by organic compounds that have not been preoxidized by KMnO4. 3.3. Effect of KMnO4 preoxidation on hydrophilic and hydrophobic fractions The characteristics of hydrophilic and hydrophobic fractions of organic matter are considered to influence membrane fouling. As shown in Table 5, the level of these fractions in the UF effluent,

following organic removal is low, particularly for CHA. KMnO4 pretreatment plays an important role in the removal of DOC and UV254, which mainly depends on the breakdown of strong hydrophobic organic compounds. KMnO4 preoxidation facilitates the removal of organic compounds and a similar breakdown in VHA and NEU fractions was noted after KMnO4 pretreatment. The presence of the UF membrane (composed of PVC) means that the hydrophobic organic matter can be easily absorbed onto the membrane surface, while the hydrophilic organic compounds can easily pass through the membrane. This suggests that the hydrophobic organic compounds are associated with membrane fouling. As shown in the SEM examination results, KMnO4 pretreatment results in structural changes to the gel layer on the membrane surface, which improves the removal efficiency of organic matter in the combined process. The KMnO4 oxidation reduces the hydrophobicity of organic matter in the water and mitigates membrane fouling. MnO2, as an intermediate product, was formed during the oxidation by KMnO4 [27]. The intermediate MnO2 had a large surface area with strong adsorptive capacity, which absorbed neutral organic matter and formed a loose filter cake, as shown in Fig. 6c. The capacity of the combined process for neutral organic removal was therefore also improved. In this way, KMnO4 pretreatment not only mitigates membrane fouling but also improves the water quality of the effluent from the UF system, due to the removal of organic matter. 3.4. Variation of TMP The influence of KMnO4 pretreatment on the TMP was further explored over a longer period, at a KMnO4 dosage of 0.4 mg/L. Two membranes were run in parallel (with and without peroxidation) with the feed water taken from the same water sample. Fig. 4 shows that the use of KMnO4 resulted in a lower rate of increase of TMP than for the sample without pretreatment, which implies that the preoxidation of the UF influent by KMnO4 could

Fig. 6. Scanning electron micrographs of the UF hollow fiber membranes: (a) virgin membrane (magnified 20,000); (b) used membrane with KMnO4 (magnified 20,000); (c) used membrane without KMnO4 (magnified 20,000).

Fig. 4. TMP variations of UF process.

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simultaneously. The impacts of the temperature change on the UF performance were therefore alike, which would not confound the performance comparison. It was also observed that several significant variations of the TMP occurred in the combined KMnO4/UF system during filtration. TMP rapidly increased during the initial and final phases, but had a slow rate of increase during the mid-period of filtration. The rapid increase of TMP at the initial stage was probably due to the absorption of pollutants into the membrane pores [30], while its slow increase during the subsequent mid-period demonstrates the formation of a loose filter cake during the course of filtration. In the mid-period of filtration, during the continuous UF phase, an increase in TMP, due to the formation of filter cake, balances with TMP recovery as a result of hydraulic cleaning [31]. The TMP therefore slowly increases during the mid-period. The obvious increase of TMP at the end of filtration is mainly caused by membrane fouling due to accumulating hole blockage in membranes, which is not effectively mitigated by hydraulic cleaning. Irreversible membrane fouling results in an increase in filtration resistance, which contributes to an increase in TMP at the end of the filtration. The rate of TMP increase in the UF system during these three periods was 4.17 kPa/d (initial stage), 1.14 kPa/d (mid-period), and 4.89 kPa/d (final phase).

reduce membrane fouling. The value of TMP reached its limit (0.0800 MPa) on the fifth day in the UF system, but was present on Day 23 in the KMnO4/UF system. The UF system, without pretreatment, resulted in an increase of TMP from 0.0201 to 0.0800 MPa for about 5 days. When the combined KMnO4/UF system was run for 23 days, the TMP increased from 0.0203 to 0.0800 MPa. NOM particles are the main foulants contributing to UF fouling and KMnO4 can enhance organic matter removal by oxidation and absorption [28], which results in a slow increase of TMP in the KMnO4/UF system. The result ensures that KMnO4 pretreatment, as a way of mitigating membrane fouling, is effective in extending the operating period of UF. It can be seen that the decrease of TMP appeared after 3 days in the experiment performed with the UF system. The membrane fouling is slight at the beginning of this experiment and the variation of TMP is easily influenced by exterior factors. The TMP decrease associated with the UF system is not only due to the hydraulic cleaning, every 45 min, which resulted in a low degree of TMP recovery after a filtration cycle, but may also have been caused by factors such as raw water quality and temperature. According to experimental records, the organic matter concentration in the feed water decreased from 2.31 mg/L at the beginning of the experiment to 1.63 mg/L on the third day, due to rain in the water source. The UF system is sensitive to organic matter concentration in the feed water, while the combined system is less influenced by the feed water quality, due to the oxidation of organic matter by KMnO4. The influence of water temperature on TMP is determined by the variation in the range of temperature. Brandhuber indicated that the reduction of applied pressure with temperature was significant and approximately 30% less pressure was required to maintain an equal flux at 40 °C compared to that required at 20 °C [29]. In the present study, however, the main variation range of water temperature was from 8 °C to 12 °C. Therefore, a small variation in water temperature has less influence on the UF system operation. In addition, the two different UF systems (the UF system and the KMnO4/UF system) were carried out

3.5. Effect of KMnO4 preoxidation on cake layer resistance Table 6 shows the values of cake layer resistance (Rm, Rrev, Rirr) of the fouled membrane, with or without KMnO4 pretreatment. Overall, the R value of the UF system with KMnO4 pretreatment was lower than that without such treatment. The Rrev value of the KMnO4/UF system continuously increased at a slow rate, while that without pretreatment had a rapid rate of increase. The Rrev value of UF after 10, 60, 120 min was 0.003 (0.29%), 0.024 (2.31%), and 0.061 (4.7%), respectively, in the UF system. Under this condition, while in the KMnO4/UF system, the Rrev values at the same times were 0.002 (0.19%), 0.021 (1.93%), and 0.030

Table 6 Various membrane resistances (in terms of four parameters) subjected to only UF and UF combined with 0.4 mg/L KMnO4 pretreatment. System

60 min

120 min

Rm Rrev Rirr Rt

1.040a (99.05%)b 0.003 (0.29%) 0.007 (0.66%) 1.05 (100%)

1.040 0.024 0.040 1.107

(94.00%) (2.31%) (3.69%) (100%)

1.040 0.061 0.203 1.304

(79.8%) (4.7%) (15.5%) (100%)

KMnO4/UF

Rm Rrev Rirr Rt

1.040 0.002 0.004 1.046

1.040 0.021 0.028 1.089

(95.5%) (1.93%) (2.57%) (100%)

1.040 0.030 0.102 1.172

(88.74%) (2.56%) (8.7%) (100%)

(99.42%) (0.19%) (0.39%) (100%)

Resistance (1012 m1). Proportion of total resistance.

100 Reflectance(100%)

a b

10 min

Only UF

80 60 40

fouled membrane fouling membrane with KMnO4 new membrane

20 0 4500

4000

3500

3000 2500 2000 Wavenumbers(cm -1)

1500

1000

Fig. 5. The FTIR spectra of UF membranes at various conditions.

500

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(2.56%), respectively. This phenomenon also indicates that KMnO4 preoxidation reduced the reversible fouling that is mostly attributed to filter cake deposition. Reversible fouling of the membrane is mainly caused by the filter cake on the membrane surface and irreversible fouling is mainly caused by membrane pore blockage. This suggests that KMnO4 oxidizes and destroys certain types of organic matter, which may be absorbed on the membrane surface and inner pore. As shown in Table 6, the reversible resistance of different systems indicates that reversible fouling of the cake layer has been alleviated by KMnO4 pretreatment. The SEM morphology of the filter cake on the membrane surface (Fig. 6) indicates that the filter cake structure on the membrane surface was changed in the presence of KMnO4. The UF system, without KMnO4 pretreatment, had a dense and compact surface structure of the filter cake (Fig. 6c), while it was loose and particulate in UF system after KMnO4 pretreatment (Fig. 6b). The loose surface structure of the filter cake results in a low reversible resistance in the membrane fouling model. 3.6. ATR-FTIR FTIR analysis was performed to determine whether retained organic compounds were found on the membrane surface. The FTIR spectrum of virgin and fouled PVC hollow fiber membranes was examined to assess changes in functional groups (Fig. 5). All spectra absorption peaks at wave numbers of 1725.0 and 3368.3 cm1 are attributed to the membrane material. The FTIR analysis of the fouled membrane without KMnO4 pretreatment indicated a specific peak at around 1250 cm1. This corresponds to a C–O stretching vibration in alcohol and phenol (1260–1000 cm1). The fouled UF membrane, that directly filtered effluents from the sand filter, had strong peaks around the wave number of 1736.8 cm1, which signifies the presence of the [email protected] bonds of lipids and/or lipid-like material in the foulant layers [32]. This peak was not observed for the membrane that had been fouled with preoxidized influent. The peaks near 1141.2 and 1327.8 cm1 are characteristic of alkanes [33] and are thus more likely to be attributed to C–H bonds. The intensity of the spectral bands, peaks near 1141.2 and 1327.8 cm1, was higher for the membrane fouled with preoxidized feed water in comparison with that fouled with sand filter effluent. It indicates that the C–H bonds in the aromatic rings disappeared and more C–H bonds appeared in the alkanes after KMnO4 pretreatment. Since most intensities of spectral bands of retained organic compounds on membranes exposed to preoxidized influent were weaker than those on membranes exposed to feed water without KMnO4 preoxidation, these results indicate that, after preoxidation, NOM components in feed water were transformed into substances with a lower adsorbable capability.

3.7. Changes in membrane morphology To evaluate the effect of KMnO4 pretreatment on membrane fouling, SEM images were investigated in the hollow fiber PVC membrane sample, before and after UF (Fig. 6). SEM micrographs display a virtual structure of foulant layer on the fouled membrane surface. SEM micrographs of the virgin membrane (Fig. 6a) clearly show the membrane surface, while the micrographs of fouled membranes (Fig. 6b and c) exhibit different surface morphologies, due to clogging by pollutants. A comparison of SEM micrographs indicates that the surface of the membrane in the KMnO4/UF system had a sparse, loosely packed filtration cake (Fig. 6b). The membrane that had not been subjected to KMnO4 pretreatment appeared to have a relatively dense and compact filter cake (Fig. 6c). KMnO4 oxidizes organic matter and causes characteristic changes in the filter cake, due to uneven foulant deposition. The uneven structure of the foulant deposition is consistent with the high degree of surface roughness seen in the SEM images (Fig. 6b). The loose structure is easily disturbed by hydraulic washing. This indicates that the total resistance was reduced by KMnO4 pretreatment, resulting in an increase in the permeate flux. The sand filter effluent exhibited decreased surface roughness, due to NOM absorption onto depression areas of the accumulated foulant layer (as shown in Fig. 6c). The molecular weight distribution indicated that, in membranes that had been pretreated by KMnO4, the macromolecular organic matter was broken down to matter comprised of lower-molecular-weight particles. KMnO4 preoxidation facilitates the removal of organic compounds, resulting in the mitigation of membrane fouling. EDS analysis was used for the in-depth investigation of the foulant layer, following hydraulic washing. The virgin membrane was used as a blank control, which only involved the element composition of membrane materials in EDS analysis. A comparison of the EDS analysis of the filter cake (Table 7) shows that the weight proportions of each element, that had not undergone KMnO4 pretreatment, were C (32.76%), O (25.31%), Al (3.70%), Ca (0.34%), Fe (0.54%), Si (1.76%), Cu (0.78%), and Zn (0.52%). The proportions of elements that had undergone KMnO4 pretreatment were C (20.36%), O (22.0%), Al (3.30%), Mn (5.9%), Cu (0.54%), and Zn (0.34%). It is apparent that the KMnO4 pretreatment led to a decrease in calcium compounds on the membrane surface. Such compounds would be difficult to remove by means of hydraulic washing. MnO2, as an intermediate product, was formed during the oxidation by KMnO4. The particle size of MnO2 varies from a minimum of 20 nm to a maximum of 100 nm [27]. The intermediate MnO2 has a large surface area with strong adsorptive capacity, which absorbes both organic or inorganic compounds. This resulted in a decreased proportion of low-molecular-weight ele-

Table 7 Energy dispersive X-ray spectroscopy values obtained from various UF membranes (%). Element

C N O Al Si S Cl Ca Fe Mn Cu Zn Totals

Virgin membrane

Used membrane with KMnO4

Used membrane without KMnO4

Weight

Atomic

Weight

Atomic

Weight

Atomic

44.26 – 15.09 – – 0.73 38.02 – – – 0.73 1.17 100

64.06 – 16.39 – – 0.40 18.64 – – – 0.20 0.31 100

30.36 8.52 22.0 3.30 – 1.39 27.65 – – 5.90 0.54 0.34 100

45.27 10.82 24.58 2.24 – 0.77 13.88 – – 1.98 0.24 0.22 100

32.76 5.66 25.31 3.00 1.76 1.08 28.25 0.34 0.54 – 0.78 0.52 100

46.92 7.57 27.21 1.89 0.86 0.58 14.17 0.15 0.17 – 0.31 0.17 100

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ments, such as carbon, oxygen and other elememts, following KMnO4 preoxidation. The disappearance of Fe was attributed to the soluble ferrous iron being oxidized to ferric iron, which readily formed an insoluble iron hydroxide complex on the filter cake that was easily discharged with the backwash water. Similar findings by other researchers showed that metal oxide absorption contributed to a reduction in fouling [34]. In this study, the element Mn may have been derived from the KMnO4 dosing process. When dosing KMnO4, the chemical reaction generated an intermediate product (MnO2), which was retained on the membrane surface to form a filter cake. Although the filter cake was washed intermittently by water, a portion of the manganese remained on the membrane. Similar variation of Al element, before and after preoxidation, indicated that aluminum compounds were not significantly influenced by KMnO4 preoxidation because the aluminum element had a stable chemical valency. Filter cake, caused by the adherence of organic matter to the membrane surface and/or membrane pores, has a close structure and low permeability. The membrane fouling caused by metal ions is mostly irreversible, which may lead to a decrease in permeate flux. The intermediate MnO2 and metal oxide particles, a product of the oxidation of metal ions, can absorb organic matter, which more readily attached to the membrane surface other than to the membrane pores. Such a phenomenon may, to some extent, result in a loose structure and a high permeability of filter cake, in comparison with membranes that have not undergone KMnO4 pretreatment.

4. Summary The preoxidation of feed water by KMnO4 was investigated to mitigate the membrane fouling. The optimal dose of KMnO4 was decided in the KMnO4/UF system. The results showed that the KMnO4/UF system slowed down the rate of UF membrane fouling. The total resistance was significantly lower in the KMnO4/UF system than in the UF system. The effective mitigation of membrane fouling was achieved by KMnO4 pretreatment, as described below. (1) The performance of an ultrafiltration system was investigated by preoxidizing the feed water with KMnO4. The results of this investigation demonstrated that the pretreatment of the feed water by KMnO4 was effective for the mitigatiion of membrane fouling in the UF system when it was used as an advanced method for treating the effluent from sand filter of waterworks, to purify raw water from the Yangtze River. The KMnO4/UF system was a feasible method to ensure the removal of organic compounds and to control membrane fouling. (2) The optimal dose of KMnO4 was 0.4 mg/L, in terms of both membrane fouling control and product water quality in the KMnO4/UF system. The preoxidation of feed water by KMnO4, prior to UF, resulted in a change in the nature of organic matter, including an alteration to the characteristics of NOM, as well as to hydrophilic, hydrophobic, and molecular weight distributions. KMnO4 preoxidation resulted in the degradation of major NOM components responsible for membrane fouling. KMnO4 preoxidation facilitated the removal of hydrophobic and hydrophilic organic compounds, particularly strongly hydrophobic and neutral organic compounds, which significantly mitigated membrane fouling. (3) KMnO4 pretreatment resulted in the formation of a filtration cake with a loose structure and a high permeability. The sum of Rrev and Rirr due to membrane fouling was significantly lower in the UF systems with pretreated feed water than

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in those that have not been pretreated with KMnO4. Measurements of permeate flux and TMP of the UF provided further evidence that KMnO4 pretreatment improved the performance of UF membrane system. The variation of the filtration cake, on the membrane surface, resulted in the reversible resistance change of the membrane fouling model.

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