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K2MnO4 as a pre-treatment for controlling UF membrane fouling in drinking water treatment
Journal of Membrane Science 473 (2015) 283–291
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Application of Fe(II)/K2MnO4 as a pre-treatment for controlling UF membrane fouling in drinking water treatment Wenzheng Yu, Nigel. J.D. Graham n Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2014 Received in revised form 27 August 2014 Accepted 28 August 2014 Available online 28 September 2014
This paper describes some results of mini-pilot-scale tests concerning the performance of potassium manganate (K2MnO4) as a pre-treatment chemical prior to ultrafiltration. Manganate is an intermediate in the commercial preparation of permanganate and in aqueous reactions MnO24 can act as both an oxidant and a coagulant/adsorbent arising from the formation of insoluble MnO2. In addition, the combination of ferrous sulfate and manganate (Fe/Mn), offers a potentially cheaper and effective combination of pre-oxidant and coagulant compared to the chemicals used currently in water treatment (e.g. ozone, chlorine, ferric sulfate). In comparative tests with conventional ferric sulfate and using simulated raw water, the results showed that Fe/Mn pre-treatment substantially reduced membrane fouling in terms of the rate of trans-membrane pressure development (arising from both external and internal fouling). Fe/Mn pre-treatment was effective in reducing bacterial activity, changing the characteristics of organic matter and decreasing the production of extracellular polymeric substances (EPS) by bacteria. The external fouling in this process was determined by the EPS concentration, and the internal fouling mainly determined by the adsorption of lower MW organic matter to the membrane pores. Fe/Mn pre-treatment reduced the amounts of both types of fouling material within the cake layer and membrane pores in comparison to conventional pre-treatment with ferrous sulfate, most likely through the formation of solid-phase Fe(III) and MnO2 and by MnO24 oxidation, thereby leading to a substantial increase in membrane run time. & 2014 Elsevier B.V. All rights reserved.
Keywords: Manganate Pre-treatment Ultrafiltration Membrane fouling
1. Introduction In recent years, membrane technology has been used increasingly in drinking water treatment. However, membrane fouling remains a major barrier limiting its application in treating surface water [1], and the major cause of fouling is the accumulation of a complex mixture of humic and fulvic acids, proteins, and carbohydrates [2–4]. These substances can either bind to the membrane surface or support the development of microbiological fouling, as extracellular polymeric substances (EPS) have a high affinity with sludge flocs [5]. Of particular importance is the biopolymer fraction with high molecular size which can be significantly retained on UF membrane surfaces [6]; this has a strong correlation with fouling resistance [7] and can be employed as a universal indicator for predicting membrane fouling potential in UF processes [8]. The protein concentrations of extracellular polymeric substances in conjunction with the carbohydrates of soluble microbial products have been reported to be the main factors that accelerate the
n
Corresponding author. Tel.: þ 44 2075946121; fax: þ 44 2075945934. E-mail addresses: [email protected], [email protected] (W. Yu), [email protected] (Nigel.J.D. Graham). http://dx.doi.org/10.1016/j.memsci.2014.08.060 0376-7388/& 2014 Elsevier B.V. All rights reserved.
membrane fouling [9]. Other researchers have also found biopolymer-type substances, together with humic acid and lower molecular weight neutral and acid compounds, to be responsible for membrane fouling [10–12]. In order to reduce fouling, therefore, decreasing the EPS or bacteria concentration in the membrane system may be the most effective option, together with removing humic substances. Quorum quenching enzyme (acylase) and fungal inoculation on the membrane surface can mitigate membrane biofouling [13,14], as well as modification of the membrane surface with antiadhesion and anti-bacteria properties [15,16]. Some other methods have also been considered for mitigating membrane fouling, such as adding cationic polymeric material [17], adding moving media [18] and providing a sufficient supply of oxygen [19,20]. The application of an oxidant or disinfectant can also prevent membrane fouling such as the use of pre-ozonation [21,22]. As well as the above methods, chemical coagulation or coagulation–hydraulic flocculation, has been shown to be an effective and cheap approach, not only to improve general water quality [1,23,24], but also to control membrane fouling [25–28]. For example, the removal of biopolymers is possible with iron and alumina salts [29], and integrated MIEX (magnetic ion exchange) and coagulation is potentially a viable pre-treatment approach to
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reduce membrane fouling [30]. However, many experimental studies and practical operations have indicated that cake layer formation is the main cause of membrane fouling [31–34]. Therefore, reducing the EPS concentration in the cake layer can decrease membrane fouling, as well as increase the porosity of the cake layer. As mentioned previously, a pre-oxidation process can also contribute to mitigating membrane fouling, such as by hypochlorite, ozone or permanganate (KMnO4) dosing. Examples of the latter are the combination of KMnO4 pre-oxidation with coagulation [35,36] and the pre-treatment of sand filter effluent water by KMnO4 [37]. A novel variant of these previous studies is the use of manganate (MnO2 4 ) in combination with a ferrous salt. Manganate is an intermediate in the commercial preparation of permanganate, and has a slightly greater oxidation potential than permanganate (viz: Eo ¼ þ1.74 V for MnO2 4 , versus þ1.51 V for MnO4 ; pHo7) [38]. Like permanganate, MnO2 can act as both an oxidant and a coagulant/adsorbent arising 4 from the formation of insoluble manganese dioxide [39]. In this paper we summarize the results of applying manganate (as K2MnO4) in combination with an Fe(II) salt (FeSO4) as the pretreatment for ultrafiltration. This novel arrangement is economically advantageous since the cost of Fe(II) salts may be considerably lower than Fe(III) salts (e.g. 50%, as in the UK), and K2MnO4 should be cheaper (as an intermediate) than KMnO4. The results obtained in this study are expected to provide a better understanding of the effectiveness of in-situ formed Fe(III) and MnO2, and the MnO2 oxidant, for 4 controlling membrane fouling and improving organic matter removal.
2. Materials and methods 2.1. Synthetic raw water and coagulant A synthetic raw surface water was chosen for the tests in order to simplify the study to provide sample consistency and reproducibility. Domestic sewage was added to the local (London, United Kingdom) tap water with a volumetric ratio of 1:50 to simulate a surface water supply slightly polluted by sewage discharges. In addition, 5 mg/L of humic acid (HA, sodium salt, Aldrich, Cat: H1, 675-2) was added to the raw water. Humic acid and domestic sewage represented types of organic matter found in raw waters that are relatively easy and difficult, respectively, to remove by coagulation. Prior to mixing with domestic sewage and humic acid solution, the tap water was left overnight to ensure the complete decay of residual chlorine. The characteristics of the synthetic raw water are listed in Table 1. During the course of the experimental program the temperature of the water was 207 2 1C. 2.2. Coagulation-UF processes A schematic illustration of the experimental set-up involving the coagulation-UF processes of Fe2(SO4)3 (CUF-Fe) and FeSO4 and
K2MnO4 (CUF-Fe/Mn), operated in parallel, is given in Fig. 1. Synthetic raw water was fed into a constant-level tank to maintain the water head for the membrane tanks. Certain doses of Fe2(SO4)3 (0.1 mM, calculated as Fe) or FeSO4 (0.1 mM) and K2MnO4 (0.05 mM) were continuously added into the rapid mixing units (#9 Fig. 1). The Fe dose was deliberately chosen to be the same in both membrane systems, with the Fe2(SO4)3 representing a typical Fe(III) pretreatment coagulant, and the FeSO4/K2MnO4 representing in-situ formation of Fe(III) from the MnO2 oxidation. In both cases, the 4 0.1 mM Fe dose corresponded to near zero zeta potential and the largest size of resulting flocs. The K2MnO4 dose was the theoretical, stoichiometric quantity required for the Fe(II) to Fe(III) conversion 2þ (viz: MnO2 þ4H2O¼MnO2 þ2Fe(OH)3 þ2H þ ). The rapid 4 þ 2Fe mix speed was 200 rpm (184 s 1) in the mixing units with a hydraulic retention time (HRT) of 1 min, which then reduced to 50 rpm (23 s 1) in the three flocculation tanks, each having a HRT of 5 min. After the flocculation tanks the flow passed directly into the membrane tanks. The tanks have a sludge storage volume at the bottom which was not disturbed by air blowing during membrane cleaning events. Each tank contained a submerged polyvinylidene fluoride (PVDF) hollow-fiber UF membrane module (Tianjin Motimo Membrane Technology Co., Ltd, China) with a nominal pore size of 0.03 μm and a surface area of 0.025 m2. UF permeate was continuously collected by a suction pump at a constant flux of 20 L/(m2 h), operated in a cycle of 30 min filtration and 1 min backwash (40 L/(m2 h)). Each backwash operation also involved the application of air to each reactor immediately below the membrane unit at 100 L/h (air: water ¼200:1) in order to physically disturb the membrane surfaces. The trans-membrane pressure (TMP) was continuously monitored by pressure gauges. The HRT of the membrane tanks was maintained at 0.5 h and accumulated sludge was released every 3 days.
2.3. Extraction and measurements of EPS from cake layer and sludge At the end of the test period, the fouled membrane modules were taken out from the membrane tanks. The external foulant materials on the membrane surface (cake layer) were carefully scraped off with a plastic sheet, and analyzed by the following methods to characterize the external membrane fouling. The extraction of internal fouling is described later. A heating and extraction method [40] was modified to extract the loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) from the sludge and cake layers, and make sure the EPS was not released from bacterial cells. Sludge suspension and cake layers were first dewatered by centrifugation (Model 5417C, Eppendorf, Germany) in a 20-mL tube at 3000g for 5 min. The concentrate liquor was recovered for water quality analysis. The sludge pellet in the tube was re-suspended in 10 mL phosphate buffer saline (PBS) solution, and the sludge suspension was then ultrasonically
Table 1 Water qualitya of raw water and UF influentsb/ filtrates. Parameter
Raw water
CUF-Fe influent
CUF-Fe/Mn influent
CUF-Fe filtrate
CUF-Fe/Mn filtrate
UV254 (cm 1) DOC (mg/L) Turbidity (NTU) P (mg/L) Fe (mg/L) Mn (mg/L) NH4þ –N (mg/L) NO3 –N (mg/L) pH
0.1037 0.006 4.7157 0.424 6.78 7 0.16 0.052 7 0.005 0.029 7 0.010 0.0067 0.008 0.3167 0.103 6.147 0.19 8.157 0.06
0.055 7 0.002 3.392 7 0.317 2.357 0.18 0.0167 0.004 0.026 7 0.012 0.0037 0.006 0.0697 0.026 6.447 0.07 7.89 7 0.09
0.054 7 0.002 3.022 7 0.215 1.75 7 0.21 0.0097 0.002 0.023 7 0.008 0.0697 0.022 0.098 7 0.032 6.38 7 0.06 7.917 0.04
0.0517 0.003 2.9417 0.238 0.077 0.02 0.0137 0.001 0.032 7 0.022 0.0107 0.003 0.050 7 0.011 6.477 0.09 7.917 0.05
0.050 7 0.002 2.655 7 0.313 0.067 0.03 0.0087 0.003 0.020 7 0.008 0.023 7 0.005 0.0717 0.009 6.40 7 0.13 7.92 7 0.04
a b
The values in Table 1 are averages for all the measurements every 7 days (7 times). Influent – within membrane tank, immediately after flocculation units.
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285
Fig. 1. Schematic diagram of the experimental set-up.
treated (Nusonics, USA) for 1 min. Without any delay, the suspension was then sheared by a vortex mixer (Vortex- Genies 2, Mo Bio laboratories, Inc., USA) for 15 min, followed by centrifugation at 3000g for 10 min. The organic matter in the supernatant was readily extractable EPS, and was regarded as the LB-EPS of the biomass. For extracting the TB-EPS, the sludge pellet left in the centrifuge tube was re-suspended in PBS solution to its original volume (10 mL), ultrasonically treated for 3 min, and heated to 80 1C in a water bath for 30 min. The mixture was centrifuged at 10,000g for 15 min. The supernatant collected was regarded as the TB-EPS. After the membrane surface was wiped with a sponge, 0.01 mol/L NaOH was used for extraction of internal foulants and the fibers were soaked for 24 h at 20 1C in the alkaline solution according to the method described by Kimura et al. [41] and Liu et al. [22]. The extracted organic matter was then subjected to the following chemical analyses. EPS extracted from the various samples was analyzed by threedimensional excitation-emission matrix (EEM) fluorescence and size exclusion chromatography (SEC). Fluorescence measurements were conducted using a spectrofluorometer (Perkin-Elmer, LS 55, USA) at an ambient temperature of 25 1C. Further details of the method can be found in the previous research reported elsewhere [22,42]. SEC was carried out to determine the apparent molecular weight (MW) distribution of UV-active substances in the waters from the CUF-Fe and CUF-Fe/Mn systems, as well as the organic matter (EPS) extracted from the cake layers and sludges. SEC was performed using a BIOSEP-SEC-S3000 column (Phenomenex, UK) (7.8 mm 300 mm) as well as the Security Guard column fixed with a GFC-3000 disc 4 mm (ID). 10 mM sodium acetate (Aldrich, USA) was used as the mobile phase. Analysis using High Performance Size Exclusion Chromatography (HPSEC) was achieved on the HPLC system (Perkin-Elmer, USA) using the following instrumentation; Series 200 pump, UV/vis detector operated at a wavelength of 254 nm and autosampler. The flow rate was set at 1 mL/min, and the injection volume of water samples was 100 μL. Prior to operation, the mobile phase was purged at a volumetric flow rate of 2 ml/min in order to clear any residual and wash out the column of any contaminants. Polystyrene sulfonate (PSS) standards (American Polymer Standard Corp., U.S.) of molecular weights 33,500, 14,900, 6530, and 1100 Da. were employed to calibrate the relationship between the MW and the retention time. The absolute polysaccharide content in the bound EPS was measured by the phenol–sulfuric acid method with glucose as the
standard [43]. A modification of the Bradford method [44] called the Coomassie procedure (Pierce Chemical), was used to quantify the absolute concentration of proteins, with bovine serum albumin (Sigma) as the standard. 2.4. Other analytical methods Fouled membrane fibers were cut from the two membrane modules, and the foulant layer attached on the membrane surface was retained on the membrane surface. The new and fouled membrane samples were then platinum-coated by a sputter and observed under a high resolution field emission gun scanning electron microscope (FEGSEM, LEO Gemini 1525, Germany). The UV absorbance at 254 nm (UV254) of 0.45 μm filtered solutions was determined by an ultraviolet/visible spectrophotometer (U-3010, Hitachi High Technologies Co., Japan). Dissolved organic carbon (DOC) was determined with a total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan). Residual iron, manganese and total P after 0.45 μm membrane filtration were measured by inductivity coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, Perkin-Elmer, USA). Residual turbidity measurements (Hach 2100, USA) were made for samples in the two membrane tanks. The concentration of NH4þ and NO3 was determined by the colorimetric method using a spectrometer by APHA standard methods [45]. The concentration of bacteria was determined as the Heterotrophic Plate Count (HPC) by the recommended method involving the use of yeast extract agar [46].
3. Results 3.1. TMP developments in CUF-Fe and CUF-Fe/Mn systems The dosage of coagulants for the two pretreatments was determined by the development of floc size, which was assessed quantitatively by means of the jar tests and in-line monitoring by PDA 3000. Fe(II)/Mn flocs were much greater in size than those from the dosed Fe(III) coagulant (Figure S1). It was found that a dose of 0.1 mM (as Fe) of both coagulants (Fe(II) and Fe(III)) corresponded to the peak floc size and this dose was chosen for both membrane systems for the comparative test runs. In the UF filtration experiments, the membrane flux of the CUF-Fe and CUF-Fe/Mn streams were both set at a constant value of 20 L/(m2 h) in a cycle of 30 min filtration/ min water backwash (40 L/(m2 h)), and membrane fouling was indicated by the increase
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in TMP. The comparative increase in TMP for the CUF-Fe and CUF-Fe/Mn streams are shown in Fig. 2. Comparing the two types of pretreatment, the combination of Fe(II) and K2MnO4 (Fe/Mn) produced a membrane fouling rate that was substantially lower than that of the dosed Fe(III). In particular, the initial (first 6 days) increase in TMP of the CUF-Fe membrane was rapid (1–5 kPa), while there was no measurable increase for the CUF-Fe/Mn membrane. Subsequently, after membrane washing (day 24), the rate of TMP increase of the CUF-Fe membrane was nearly double that of the CUF-Fe/Mn membrane. After physical cleaning of the membranes (cleaned by high pressure tap water) at day 24, it was evident that the TMP of the CUF-Fe/Mn membrane was nearly the same as the initial TMP for the new membrane (1 kPa). In sharp contrast, for the CUF-Fe membrane, a much greater initial TMP was found (4.5 kPa), which may be related to the difficulty of removing the cake layer on the CUF-Fe membrane. Furthermore, in addition to some residual cake layer on the surface of membrane, there may be blockage of the pores by other materials, causing the greater degree of irreversible membrane fouling. For the CUF-Fe/Mn membrane we can conclude that the membrane fouling was mainly determined by the removable cake layer on the surface of the membrane, although it is possible that internal fouling may develop after a long operation time. In summary, it can be confirmed that the internal and external membrane fouling in the CUF-Fe process was much greater than the CUF-Fe/Mn process. In view of this enhanced performance
19 18
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17 8
16
Fe (SO ) FeSO +K MnO Temperature
6
15 14
4
13
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10
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In order to explain the reason for the different TMP performances by the two pretreatments, differences in the presence of organic matter in the two systems were explored. The results summarized in Table 1 show that the removal of organic matter in the CUF-Fe/Mn stream, as indicated by UV254 and TOC concentrations, was slightly greater than in the CUF-Fe stream. Since biofouling of water treatment membranes has been reported previously to be an important mechanism [48], it is reasonable to assume that biopolymers may contribute significantly to the fouling of the ultrafiltration process after long operational periods. In most cases, as reported in many studies previously, deposits of the foulant material have been found both on the external membrane surface and to some extent inside membrane pores [49]. Chabalina et al. [50] used the EEM and SEC methods to evaluate the presence of LB-EPS and TB-EPS and their role in membrane fouling. Hence, in
3000
7.0
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6.5
Concentration (mg/L)
Bacteria concentration (mL )
From our previous studies, we observed that it is not the size and fractal dimension of flocs, but the size of nano-scale primary particles that determines the porosity of the cake layer if there is no EPS formed during the membrane filtration process. However, if EPS is present in the flocs, their properties can be linked to the microbial community and its activity [47]. The presence of viable bacteria (HPC) was investigated for the two membrane systems here and was found to increase in both membrane tanks as the operation process increased. Although the total bacteria concentration increased in both membrane tanks, it was much lower in the CUF-Fe/Mn tank, as a consequence of the effects of oxidation by K2MnO4 (Fig. 3). The different levels of bacterial presence in the two systems were in agreement with the relative conversion of NH4þ to NO3 observed in the membrane tanks. Thus, less NH4þ appeared to be converted to NO3 in the CUF-Fe/Mn tank (reduced bacterial concentration) than in the CUF-Fe tank, as indicated by the higher NH4þ concentrations and slightly lower NO3 concentrations. The lower TMP increase in the CUF-Fe/Mn tank is believed to be related to the lower concentration of bacteria, and the corresponding lower presence of EPS produced, as described subsequently. 3.3. EEM spectra of EPS in the two membrane systems
20 12
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3.2. Total bacteria concentration and transformation of NH4þ and NO3
21
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arising from Fe/Mn pre-treatment, further detailed investigation was undertaken concerning the nature of the flocs formed and the extent of bacteria activity in the UF influent solution and resulting floc cake.
2000
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NO3 + NH4
6.0
0.4 0.3 0.2
500 0.1 0
0.0 Raw water
Influent-Fe Influent-Fe/Mn effluent-Fe Effluent-Fe/Mn
Fig. 3. The bacteria concentration (HPC) (a) and
NH4þ
and
NO3
Raw water
Influent-Fe Influent-Fe/Mn Effluent-Fe Fffluent-Fe/Mn
concentrations (b) in the raw water and two membrane systems.
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our tests samples of both the cake layer (external foulant) and internal material in the membrane pores (internal foulant) were extracted from the fouled membranes and analyzed by EEM
spectroscopy and SEC at the end of the test run (Figs. 4 and 5, respectively). Equivalent information about the EPS from sludge samples can also be seen in Fig. S2.
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Fig. 4. EEM fluorescence spectra for: LB-EPS of cake layer for CUF-Fe (a) and CUF-Fe/Mn (b); TB-EPS of cake layer for CUF-Fe (c) and CUF-Fe/Mn (d); inner membrane fouling for CUF-Fe (e) and CUF-Fe/Mn (f).
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1.4 Raw water effluent-CUF-Fe effluent-CUF-Fe/Mn
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Fig. 5. MW distributions of DOM from CUF-Fe and CUF-Fe/Mn system: (a) pre-treated water, (b) LB-EPS in cake layer, (c) TB-EPS in cake layer, (d) inner membrane fouling.
The peaks obtained for EPS (LB-EPS and TB-EPS) were associated with soluble microbial by-product-like substances (predominantly protein-derived compounds) (peaks B, T1 and T2) and aromatic proteins, also associated with humic and fulvic acids (peaks A and C) [50]. It was evident that humic-like substances were among the external foulants in the cake layer of the two systems, as represented by peak C, and the results indicated the accumulation of humic acid-like substances on both membrane surfaces. In contrast, there were significant differences with respect to peak T1 of LB-EPS in the two membrane systems with a T1 peak clearly visible in the LB-EPS of the CUF-Fe cake layer, but not for the CUF-Fe/Mn membrane. A similar result was found for the TB-EPS samples, although the fluorescence intensity of the T1 peak was much lower, and there was only humic-like material evident (peaks A and C) in the CUF-Fe/Mn cake layer. Proteins have been reported to induce severe membrane fouling when present as a major component of the foulant material [19,20]. A significant difference between LB-EPS and TB-EPS for CUF-Fe (Fig. 4a and c) is the appearance of peak A in the latter, suggesting a modification of the structure of humic-like substances (principally peak C) caused by complexation and microorganism degradation. For the CUF-Fe/Mn membrane (Fig. 4b and d), the intensification of peak C is believed to be because of the accumulation of humic-like substances in the cake layer. As shown in Fig. S1, there was little evidence of protein-like materials in the sludge, which meant that these protein-like materials in the cake layer were not from the raw water, but from subsequent bacterial
growth. Their apparent absence in the cake layer of the Fe/Mn process highlighted the beneficial impact of MnO24 oxidation on reducing bacterial growth and the associated EPS concentration. The characteristics of EEM fluorescence spectra of internal membrane foulants were slightly different from those of the external foulants. By comparing their EEM fluorescence spectra (Fig. 4e and f), there was a very low intensity of peak T1 evident in both inner membrane fouling materials as compared to the humic-like substance peaks (C and A). The fluorescence spectra of inner fouling was similar to the TB-EPS, which suggested that humic acid-like materials separated from the cake layer and adsorbed on to the inner membrane with increasing operation time. 3.4. SEC of EPS in the two membrane systems To complement the EEM spectra, size exclusion chromatograms of the corresponding samples were produced to further characterize the dissolved organic matter (DOM) (Fig. 5). The MW distributions obtained from the chromatograms displayed significant differences in the feed and permeate samples (there was little difference between the samples from membrane tanks and their effluents), particularly in the range of large molecules identified as biopolymers and humic acid for all waters and EPS extractions. Comparing the SEC and EEM results, it is believed that the large MW (104–105) fraction evident from the SEC (highlighted in Fig. 5) corresponded to peak T1 (protein-like materials), and peaks A and C corresponded to the smaller MW (102–104) humic-like materials
W. Yu, Nigel.J.D. Graham / Journal of Membrane Science 473 (2015) 283–291
fraction. Thus, the SEC and EEM results confirmed that membrane fouling is highly related to the biopolymer and humic acid-like materials. The SEC results showed that the pre-treatment of Fe/Mn was associated with much lower levels of organic matter in general (100oMWo105), and especially the large molecular fraction (EPS) (MW 104–105) in the CUF-Fe/Mn tank. The organic matter from the CUF-Fe/Mn cake layer was much less than from the CUF-Fe (Fig. 5b and c), especially for the large molecular weight fraction, as well as from the corresponding sludges of the two membrane systems (Fig. S2c). Comparing the LB-EPS and TB-EPS of samples from the cake layers in the two membrane tanks, there was little difference in the fractions of organic matter with MW o10,000 Da for the two pretreatments, but there was a much greater presence of high molecular weight organic matter (4 10,000 Da) in the cake layer of the CUF-Fe membrane. These results clearly indicated that a higher concentration of EPS was produced by greater bacterial activity in the CUF-Fe tank. It was clear that less organic matter had deposited or was adsorbed in the membrane pores in the CUF-Fe/Mn system, compared to the CUF-Fe system, with a lower concentration of EPS, and most of the adsorbed organic matter was smaller than 10,000 Da (Fig. 5d). The higher concentration of humic-like materials in CUF-Fe, as well as EPS, was consistent with the observed higher internal fouling resistances, indicated by the TMP after the high pressure water wash (Fig. 2). These results confirmed that the adsorption of low molecular weight compounds to the membrane was attributed to hydraulically irreversible fouling, which was also found by Ayache et al. [11] in waste water treatment.
289
3.6. SEM In order to support the results indicating that membrane fouling can be mitigated by controlling EPS and humic-like substances in the cake layer and pores, SEM images were obtained (Fig. 7). The SEM images provided qualitative information that was consistent with the physical results discussed previously which showed that the concentration of EPS in the CUF-Fe was much greater than in the CUF-Fe/Mn, including in their respective cake layers and membrane pores. Thus, the presence of bacteria and more EPS in the CUF-Fe cake layer (Fig. 7a) was visible compared to that in the CUF-Fe/Mn cake layer (Fig. 7b). The higher concentration of EPS in the cake layer is believed to induce greater external membrane fouling, partly by decreasing the porosity between nano-scale primary particles. After the cake layer was washed and the membrane cleaned by sponge, the internal fouling of the membrane was mainly determined by the pore blockage in the cake layer. As seen from the SEM images (Fig. 7c and d), there were few primary particles resident on the surface of both membranes, but more pores were blocked by organic matter in the case of the CUF-Fe membrane. The statistical number of open pores of the two fouled membranes as quantified by the Scion Image software was dramatically different, as shown in Fig. 7e, with more pores of a wide range of sizes present in the case of the CUF-Fe/Mn membrane. These results further confirmed that more organic matter was adsorbed in the CUF-Fe membrane pores, which is consistent with the SEC results discussed previously. In view of this it is recommended that future studies should focus further on ways of decreasing bacteria and EPS concentrations and controlling the release of humic-like substances from the cake layer to the membrane pores.
3.5. Absolute EPS concentrations in cake layer and sludge Comparing the EPS content in the sludge and cake layer of the two systems, a significantly higher EPS concentration was found in the CUF-Fe cake layer than in the CUF-Fe/Mn cake layer (Fig. 6). The amount of total protein in the CUF-Fe cake layer was much higher than that in the CUF-Fe/Mn system (around 3 times) and increased throughout the membrane operating period. The variation of polysaccharide extracted from the cake layer and sludge in the two membrane systems was very similar to that of protein, with much greater quantities in the CUF-Fe system. Overall, the results further confirmed that the higher EPS concentration (proteins and polysaccharides) in the CUF-Fe cake layer corresponded directly with the greater extent of external membrane fouling.
1. The combination of Fe(II) and K2MnO4 (Fe/Mn) provides a potentially more cost-effective pre-treatment of raw waters prior to ultrafiltration, via the effects of in-situ coagulation and microorganism inactivation, most likely through the formation of solid-phase Fe(III) and MnO2 and by MnO24 oxidation. 2. In comparison to conventional pre-treatment with ferric sulfate (Fe), Fe/Mn pre-treatment substantially reduced overall membrane fouling in terms of the rate of development of transmembrane pressure. 3. Fe/Mn pre-treatment increased the removal of organic matter and substantially reduced the accumulation of bacteria in the
0.04
Polysaccharides concentration (g/g SS)
0.005
Protein concentration (g/g SS)
4. Conclusions
0.004
0.003
0.002
0.001
0.000
0.03
0.02
0.01
0.00 Fe(II)/K2MnO4 Fe(III) cake layer cake layer
Fe(III) sludge
Fe(II)/K2MnO4 sludge
Fe(III) cake layer
Fe(II)/K2MnO4 cake layer
Fe(III) sludge
Fe(II)/K2MnO4 sludge
Fig. 6. The concentration of protein (a) and polysaccharide (b) in the cake layers and sludges for the two membrane systems.
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600 550 500
CUF-Fe CUF-Fe/Mn
Number of pores
450 400 350 300 250 200 150 100 50 0 <15
15-25
25-35
35-50
50-70
70-100
>100
Size (nm) Fig. 7. SEM images of membranes with different pretreatments: CUF-Fe membrane without (a) and with (c) wash, CUF-Fe/Mn membrane without (b) and with (d) wash, and statistical details of pores for washed membranes (e).
UF influent water, which induced lower concentrations of extracellular polymeric substances (EPS) in the CUF-Fe/Mn cake layer and less organic matter adsorbed in the membrane pores, thereby causing reduced external and internal membrane fouling. 4. The EEM fluorescence results confirmed the presence of protein-like materials in the CUF-Fe cake layer, but not in the CUF-Fe/Mn cake layer or in the sludges of both membrane tanks, indicating that these protein-like materials were not from the raw water, but from bacterial growth. 5. Significantly less organic matter was found to have deposited or adsorbed in the pores of the CUF-Fe/Mn membrane than the CUF-Fe membrane, and most of the adsorbed organic matter
was of lower molecular weight (o 10,000 Da), corresponding to humic-like materials rather than EPS.
Acknowledgments This research was supported by a Marie Curie International Incoming Fellowship (FP7-PEOPLE-2012-IIF-328867) within the 7th European Community Framework Programme. The authors wish to acknowledge the assistance of Thames Water Utilities Ltd (wastewater samples) and colleagues in Imperial College London,
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particularly Dr G. Fowler (assembly of the mini-pilot scale set-up) and Mrs E Ware (SEM analysis).
[24]
Appendix A. Supporting information
[25]
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2014.08.060.
[26]
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Application of Fe(II)/K2MnO4 as a pre-treatment for controlling UF membrane fouling in drinking water treatment Wenzheng Yu, Nigel. J.D. Graham n Department of Civil and Environmental Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
art ic l e i nf o
a b s t r a c t
Article history: Received 30 April 2014 Received in revised form 27 August 2014 Accepted 28 August 2014 Available online 28 September 2014
This paper describes some results of mini-pilot-scale tests concerning the performance of potassium manganate (K2MnO4) as a pre-treatment chemical prior to ultrafiltration. Manganate is an intermediate in the commercial preparation of permanganate and in aqueous reactions MnO24 can act as both an oxidant and a coagulant/adsorbent arising from the formation of insoluble MnO2. In addition, the combination of ferrous sulfate and manganate (Fe/Mn), offers a potentially cheaper and effective combination of pre-oxidant and coagulant compared to the chemicals used currently in water treatment (e.g. ozone, chlorine, ferric sulfate). In comparative tests with conventional ferric sulfate and using simulated raw water, the results showed that Fe/Mn pre-treatment substantially reduced membrane fouling in terms of the rate of trans-membrane pressure development (arising from both external and internal fouling). Fe/Mn pre-treatment was effective in reducing bacterial activity, changing the characteristics of organic matter and decreasing the production of extracellular polymeric substances (EPS) by bacteria. The external fouling in this process was determined by the EPS concentration, and the internal fouling mainly determined by the adsorption of lower MW organic matter to the membrane pores. Fe/Mn pre-treatment reduced the amounts of both types of fouling material within the cake layer and membrane pores in comparison to conventional pre-treatment with ferrous sulfate, most likely through the formation of solid-phase Fe(III) and MnO2 and by MnO24 oxidation, thereby leading to a substantial increase in membrane run time. & 2014 Elsevier B.V. All rights reserved.
Keywords: Manganate Pre-treatment Ultrafiltration Membrane fouling
1. Introduction In recent years, membrane technology has been used increasingly in drinking water treatment. However, membrane fouling remains a major barrier limiting its application in treating surface water [1], and the major cause of fouling is the accumulation of a complex mixture of humic and fulvic acids, proteins, and carbohydrates [2–4]. These substances can either bind to the membrane surface or support the development of microbiological fouling, as extracellular polymeric substances (EPS) have a high affinity with sludge flocs [5]. Of particular importance is the biopolymer fraction with high molecular size which can be significantly retained on UF membrane surfaces [6]; this has a strong correlation with fouling resistance [7] and can be employed as a universal indicator for predicting membrane fouling potential in UF processes [8]. The protein concentrations of extracellular polymeric substances in conjunction with the carbohydrates of soluble microbial products have been reported to be the main factors that accelerate the
n
Corresponding author. Tel.: þ 44 2075946121; fax: þ 44 2075945934. E-mail addresses: [email protected], [email protected] (W. Yu), [email protected] (Nigel.J.D. Graham). http://dx.doi.org/10.1016/j.memsci.2014.08.060 0376-7388/& 2014 Elsevier B.V. All rights reserved.
membrane fouling [9]. Other researchers have also found biopolymer-type substances, together with humic acid and lower molecular weight neutral and acid compounds, to be responsible for membrane fouling [10–12]. In order to reduce fouling, therefore, decreasing the EPS or bacteria concentration in the membrane system may be the most effective option, together with removing humic substances. Quorum quenching enzyme (acylase) and fungal inoculation on the membrane surface can mitigate membrane biofouling [13,14], as well as modification of the membrane surface with antiadhesion and anti-bacteria properties [15,16]. Some other methods have also been considered for mitigating membrane fouling, such as adding cationic polymeric material [17], adding moving media [18] and providing a sufficient supply of oxygen [19,20]. The application of an oxidant or disinfectant can also prevent membrane fouling such as the use of pre-ozonation [21,22]. As well as the above methods, chemical coagulation or coagulation–hydraulic flocculation, has been shown to be an effective and cheap approach, not only to improve general water quality [1,23,24], but also to control membrane fouling [25–28]. For example, the removal of biopolymers is possible with iron and alumina salts [29], and integrated MIEX (magnetic ion exchange) and coagulation is potentially a viable pre-treatment approach to
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reduce membrane fouling [30]. However, many experimental studies and practical operations have indicated that cake layer formation is the main cause of membrane fouling [31–34]. Therefore, reducing the EPS concentration in the cake layer can decrease membrane fouling, as well as increase the porosity of the cake layer. As mentioned previously, a pre-oxidation process can also contribute to mitigating membrane fouling, such as by hypochlorite, ozone or permanganate (KMnO4) dosing. Examples of the latter are the combination of KMnO4 pre-oxidation with coagulation [35,36] and the pre-treatment of sand filter effluent water by KMnO4 [37]. A novel variant of these previous studies is the use of manganate (MnO2 4 ) in combination with a ferrous salt. Manganate is an intermediate in the commercial preparation of permanganate, and has a slightly greater oxidation potential than permanganate (viz: Eo ¼ þ1.74 V for MnO2 4 , versus þ1.51 V for MnO4 ; pHo7) [38]. Like permanganate, MnO2 can act as both an oxidant and a coagulant/adsorbent arising 4 from the formation of insoluble manganese dioxide [39]. In this paper we summarize the results of applying manganate (as K2MnO4) in combination with an Fe(II) salt (FeSO4) as the pretreatment for ultrafiltration. This novel arrangement is economically advantageous since the cost of Fe(II) salts may be considerably lower than Fe(III) salts (e.g. 50%, as in the UK), and K2MnO4 should be cheaper (as an intermediate) than KMnO4. The results obtained in this study are expected to provide a better understanding of the effectiveness of in-situ formed Fe(III) and MnO2, and the MnO2 oxidant, for 4 controlling membrane fouling and improving organic matter removal.
2. Materials and methods 2.1. Synthetic raw water and coagulant A synthetic raw surface water was chosen for the tests in order to simplify the study to provide sample consistency and reproducibility. Domestic sewage was added to the local (London, United Kingdom) tap water with a volumetric ratio of 1:50 to simulate a surface water supply slightly polluted by sewage discharges. In addition, 5 mg/L of humic acid (HA, sodium salt, Aldrich, Cat: H1, 675-2) was added to the raw water. Humic acid and domestic sewage represented types of organic matter found in raw waters that are relatively easy and difficult, respectively, to remove by coagulation. Prior to mixing with domestic sewage and humic acid solution, the tap water was left overnight to ensure the complete decay of residual chlorine. The characteristics of the synthetic raw water are listed in Table 1. During the course of the experimental program the temperature of the water was 207 2 1C. 2.2. Coagulation-UF processes A schematic illustration of the experimental set-up involving the coagulation-UF processes of Fe2(SO4)3 (CUF-Fe) and FeSO4 and
K2MnO4 (CUF-Fe/Mn), operated in parallel, is given in Fig. 1. Synthetic raw water was fed into a constant-level tank to maintain the water head for the membrane tanks. Certain doses of Fe2(SO4)3 (0.1 mM, calculated as Fe) or FeSO4 (0.1 mM) and K2MnO4 (0.05 mM) were continuously added into the rapid mixing units (#9 Fig. 1). The Fe dose was deliberately chosen to be the same in both membrane systems, with the Fe2(SO4)3 representing a typical Fe(III) pretreatment coagulant, and the FeSO4/K2MnO4 representing in-situ formation of Fe(III) from the MnO2 oxidation. In both cases, the 4 0.1 mM Fe dose corresponded to near zero zeta potential and the largest size of resulting flocs. The K2MnO4 dose was the theoretical, stoichiometric quantity required for the Fe(II) to Fe(III) conversion 2þ (viz: MnO2 þ4H2O¼MnO2 þ2Fe(OH)3 þ2H þ ). The rapid 4 þ 2Fe mix speed was 200 rpm (184 s 1) in the mixing units with a hydraulic retention time (HRT) of 1 min, which then reduced to 50 rpm (23 s 1) in the three flocculation tanks, each having a HRT of 5 min. After the flocculation tanks the flow passed directly into the membrane tanks. The tanks have a sludge storage volume at the bottom which was not disturbed by air blowing during membrane cleaning events. Each tank contained a submerged polyvinylidene fluoride (PVDF) hollow-fiber UF membrane module (Tianjin Motimo Membrane Technology Co., Ltd, China) with a nominal pore size of 0.03 μm and a surface area of 0.025 m2. UF permeate was continuously collected by a suction pump at a constant flux of 20 L/(m2 h), operated in a cycle of 30 min filtration and 1 min backwash (40 L/(m2 h)). Each backwash operation also involved the application of air to each reactor immediately below the membrane unit at 100 L/h (air: water ¼200:1) in order to physically disturb the membrane surfaces. The trans-membrane pressure (TMP) was continuously monitored by pressure gauges. The HRT of the membrane tanks was maintained at 0.5 h and accumulated sludge was released every 3 days.
2.3. Extraction and measurements of EPS from cake layer and sludge At the end of the test period, the fouled membrane modules were taken out from the membrane tanks. The external foulant materials on the membrane surface (cake layer) were carefully scraped off with a plastic sheet, and analyzed by the following methods to characterize the external membrane fouling. The extraction of internal fouling is described later. A heating and extraction method [40] was modified to extract the loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) from the sludge and cake layers, and make sure the EPS was not released from bacterial cells. Sludge suspension and cake layers were first dewatered by centrifugation (Model 5417C, Eppendorf, Germany) in a 20-mL tube at 3000g for 5 min. The concentrate liquor was recovered for water quality analysis. The sludge pellet in the tube was re-suspended in 10 mL phosphate buffer saline (PBS) solution, and the sludge suspension was then ultrasonically
Table 1 Water qualitya of raw water and UF influentsb/ filtrates. Parameter
Raw water
CUF-Fe influent
CUF-Fe/Mn influent
CUF-Fe filtrate
CUF-Fe/Mn filtrate
UV254 (cm 1) DOC (mg/L) Turbidity (NTU) P (mg/L) Fe (mg/L) Mn (mg/L) NH4þ –N (mg/L) NO3 –N (mg/L) pH
0.1037 0.006 4.7157 0.424 6.78 7 0.16 0.052 7 0.005 0.029 7 0.010 0.0067 0.008 0.3167 0.103 6.147 0.19 8.157 0.06
0.055 7 0.002 3.392 7 0.317 2.357 0.18 0.0167 0.004 0.026 7 0.012 0.0037 0.006 0.0697 0.026 6.447 0.07 7.89 7 0.09
0.054 7 0.002 3.022 7 0.215 1.75 7 0.21 0.0097 0.002 0.023 7 0.008 0.0697 0.022 0.098 7 0.032 6.38 7 0.06 7.917 0.04
0.0517 0.003 2.9417 0.238 0.077 0.02 0.0137 0.001 0.032 7 0.022 0.0107 0.003 0.050 7 0.011 6.477 0.09 7.917 0.05
0.050 7 0.002 2.655 7 0.313 0.067 0.03 0.0087 0.003 0.020 7 0.008 0.023 7 0.005 0.0717 0.009 6.40 7 0.13 7.92 7 0.04
a b
The values in Table 1 are averages for all the measurements every 7 days (7 times). Influent – within membrane tank, immediately after flocculation units.
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285
Fig. 1. Schematic diagram of the experimental set-up.
treated (Nusonics, USA) for 1 min. Without any delay, the suspension was then sheared by a vortex mixer (Vortex- Genies 2, Mo Bio laboratories, Inc., USA) for 15 min, followed by centrifugation at 3000g for 10 min. The organic matter in the supernatant was readily extractable EPS, and was regarded as the LB-EPS of the biomass. For extracting the TB-EPS, the sludge pellet left in the centrifuge tube was re-suspended in PBS solution to its original volume (10 mL), ultrasonically treated for 3 min, and heated to 80 1C in a water bath for 30 min. The mixture was centrifuged at 10,000g for 15 min. The supernatant collected was regarded as the TB-EPS. After the membrane surface was wiped with a sponge, 0.01 mol/L NaOH was used for extraction of internal foulants and the fibers were soaked for 24 h at 20 1C in the alkaline solution according to the method described by Kimura et al. [41] and Liu et al. [22]. The extracted organic matter was then subjected to the following chemical analyses. EPS extracted from the various samples was analyzed by threedimensional excitation-emission matrix (EEM) fluorescence and size exclusion chromatography (SEC). Fluorescence measurements were conducted using a spectrofluorometer (Perkin-Elmer, LS 55, USA) at an ambient temperature of 25 1C. Further details of the method can be found in the previous research reported elsewhere [22,42]. SEC was carried out to determine the apparent molecular weight (MW) distribution of UV-active substances in the waters from the CUF-Fe and CUF-Fe/Mn systems, as well as the organic matter (EPS) extracted from the cake layers and sludges. SEC was performed using a BIOSEP-SEC-S3000 column (Phenomenex, UK) (7.8 mm 300 mm) as well as the Security Guard column fixed with a GFC-3000 disc 4 mm (ID). 10 mM sodium acetate (Aldrich, USA) was used as the mobile phase. Analysis using High Performance Size Exclusion Chromatography (HPSEC) was achieved on the HPLC system (Perkin-Elmer, USA) using the following instrumentation; Series 200 pump, UV/vis detector operated at a wavelength of 254 nm and autosampler. The flow rate was set at 1 mL/min, and the injection volume of water samples was 100 μL. Prior to operation, the mobile phase was purged at a volumetric flow rate of 2 ml/min in order to clear any residual and wash out the column of any contaminants. Polystyrene sulfonate (PSS) standards (American Polymer Standard Corp., U.S.) of molecular weights 33,500, 14,900, 6530, and 1100 Da. were employed to calibrate the relationship between the MW and the retention time. The absolute polysaccharide content in the bound EPS was measured by the phenol–sulfuric acid method with glucose as the
standard [43]. A modification of the Bradford method [44] called the Coomassie procedure (Pierce Chemical), was used to quantify the absolute concentration of proteins, with bovine serum albumin (Sigma) as the standard. 2.4. Other analytical methods Fouled membrane fibers were cut from the two membrane modules, and the foulant layer attached on the membrane surface was retained on the membrane surface. The new and fouled membrane samples were then platinum-coated by a sputter and observed under a high resolution field emission gun scanning electron microscope (FEGSEM, LEO Gemini 1525, Germany). The UV absorbance at 254 nm (UV254) of 0.45 μm filtered solutions was determined by an ultraviolet/visible spectrophotometer (U-3010, Hitachi High Technologies Co., Japan). Dissolved organic carbon (DOC) was determined with a total organic carbon (TOC) analyzer (TOC-VCPH, Shimadzu, Japan). Residual iron, manganese and total P after 0.45 μm membrane filtration were measured by inductivity coupled plasma optical emission spectrometer (ICP-OES, Optima 7300 DV, Perkin-Elmer, USA). Residual turbidity measurements (Hach 2100, USA) were made for samples in the two membrane tanks. The concentration of NH4þ and NO3 was determined by the colorimetric method using a spectrometer by APHA standard methods [45]. The concentration of bacteria was determined as the Heterotrophic Plate Count (HPC) by the recommended method involving the use of yeast extract agar [46].
3. Results 3.1. TMP developments in CUF-Fe and CUF-Fe/Mn systems The dosage of coagulants for the two pretreatments was determined by the development of floc size, which was assessed quantitatively by means of the jar tests and in-line monitoring by PDA 3000. Fe(II)/Mn flocs were much greater in size than those from the dosed Fe(III) coagulant (Figure S1). It was found that a dose of 0.1 mM (as Fe) of both coagulants (Fe(II) and Fe(III)) corresponded to the peak floc size and this dose was chosen for both membrane systems for the comparative test runs. In the UF filtration experiments, the membrane flux of the CUF-Fe and CUF-Fe/Mn streams were both set at a constant value of 20 L/(m2 h) in a cycle of 30 min filtration/ min water backwash (40 L/(m2 h)), and membrane fouling was indicated by the increase
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in TMP. The comparative increase in TMP for the CUF-Fe and CUF-Fe/Mn streams are shown in Fig. 2. Comparing the two types of pretreatment, the combination of Fe(II) and K2MnO4 (Fe/Mn) produced a membrane fouling rate that was substantially lower than that of the dosed Fe(III). In particular, the initial (first 6 days) increase in TMP of the CUF-Fe membrane was rapid (1–5 kPa), while there was no measurable increase for the CUF-Fe/Mn membrane. Subsequently, after membrane washing (day 24), the rate of TMP increase of the CUF-Fe membrane was nearly double that of the CUF-Fe/Mn membrane. After physical cleaning of the membranes (cleaned by high pressure tap water) at day 24, it was evident that the TMP of the CUF-Fe/Mn membrane was nearly the same as the initial TMP for the new membrane (1 kPa). In sharp contrast, for the CUF-Fe membrane, a much greater initial TMP was found (4.5 kPa), which may be related to the difficulty of removing the cake layer on the CUF-Fe membrane. Furthermore, in addition to some residual cake layer on the surface of membrane, there may be blockage of the pores by other materials, causing the greater degree of irreversible membrane fouling. For the CUF-Fe/Mn membrane we can conclude that the membrane fouling was mainly determined by the removable cake layer on the surface of the membrane, although it is possible that internal fouling may develop after a long operation time. In summary, it can be confirmed that the internal and external membrane fouling in the CUF-Fe process was much greater than the CUF-Fe/Mn process. In view of this enhanced performance
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In order to explain the reason for the different TMP performances by the two pretreatments, differences in the presence of organic matter in the two systems were explored. The results summarized in Table 1 show that the removal of organic matter in the CUF-Fe/Mn stream, as indicated by UV254 and TOC concentrations, was slightly greater than in the CUF-Fe stream. Since biofouling of water treatment membranes has been reported previously to be an important mechanism [48], it is reasonable to assume that biopolymers may contribute significantly to the fouling of the ultrafiltration process after long operational periods. In most cases, as reported in many studies previously, deposits of the foulant material have been found both on the external membrane surface and to some extent inside membrane pores [49]. Chabalina et al. [50] used the EEM and SEC methods to evaluate the presence of LB-EPS and TB-EPS and their role in membrane fouling. Hence, in
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From our previous studies, we observed that it is not the size and fractal dimension of flocs, but the size of nano-scale primary particles that determines the porosity of the cake layer if there is no EPS formed during the membrane filtration process. However, if EPS is present in the flocs, their properties can be linked to the microbial community and its activity [47]. The presence of viable bacteria (HPC) was investigated for the two membrane systems here and was found to increase in both membrane tanks as the operation process increased. Although the total bacteria concentration increased in both membrane tanks, it was much lower in the CUF-Fe/Mn tank, as a consequence of the effects of oxidation by K2MnO4 (Fig. 3). The different levels of bacterial presence in the two systems were in agreement with the relative conversion of NH4þ to NO3 observed in the membrane tanks. Thus, less NH4þ appeared to be converted to NO3 in the CUF-Fe/Mn tank (reduced bacterial concentration) than in the CUF-Fe tank, as indicated by the higher NH4þ concentrations and slightly lower NO3 concentrations. The lower TMP increase in the CUF-Fe/Mn tank is believed to be related to the lower concentration of bacteria, and the corresponding lower presence of EPS produced, as described subsequently. 3.3. EEM spectra of EPS in the two membrane systems
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our tests samples of both the cake layer (external foulant) and internal material in the membrane pores (internal foulant) were extracted from the fouled membranes and analyzed by EEM
spectroscopy and SEC at the end of the test run (Figs. 4 and 5, respectively). Equivalent information about the EPS from sludge samples can also be seen in Fig. S2.
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Fig. 4. EEM fluorescence spectra for: LB-EPS of cake layer for CUF-Fe (a) and CUF-Fe/Mn (b); TB-EPS of cake layer for CUF-Fe (c) and CUF-Fe/Mn (d); inner membrane fouling for CUF-Fe (e) and CUF-Fe/Mn (f).
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Fig. 5. MW distributions of DOM from CUF-Fe and CUF-Fe/Mn system: (a) pre-treated water, (b) LB-EPS in cake layer, (c) TB-EPS in cake layer, (d) inner membrane fouling.
The peaks obtained for EPS (LB-EPS and TB-EPS) were associated with soluble microbial by-product-like substances (predominantly protein-derived compounds) (peaks B, T1 and T2) and aromatic proteins, also associated with humic and fulvic acids (peaks A and C) [50]. It was evident that humic-like substances were among the external foulants in the cake layer of the two systems, as represented by peak C, and the results indicated the accumulation of humic acid-like substances on both membrane surfaces. In contrast, there were significant differences with respect to peak T1 of LB-EPS in the two membrane systems with a T1 peak clearly visible in the LB-EPS of the CUF-Fe cake layer, but not for the CUF-Fe/Mn membrane. A similar result was found for the TB-EPS samples, although the fluorescence intensity of the T1 peak was much lower, and there was only humic-like material evident (peaks A and C) in the CUF-Fe/Mn cake layer. Proteins have been reported to induce severe membrane fouling when present as a major component of the foulant material [19,20]. A significant difference between LB-EPS and TB-EPS for CUF-Fe (Fig. 4a and c) is the appearance of peak A in the latter, suggesting a modification of the structure of humic-like substances (principally peak C) caused by complexation and microorganism degradation. For the CUF-Fe/Mn membrane (Fig. 4b and d), the intensification of peak C is believed to be because of the accumulation of humic-like substances in the cake layer. As shown in Fig. S1, there was little evidence of protein-like materials in the sludge, which meant that these protein-like materials in the cake layer were not from the raw water, but from subsequent bacterial
growth. Their apparent absence in the cake layer of the Fe/Mn process highlighted the beneficial impact of MnO24 oxidation on reducing bacterial growth and the associated EPS concentration. The characteristics of EEM fluorescence spectra of internal membrane foulants were slightly different from those of the external foulants. By comparing their EEM fluorescence spectra (Fig. 4e and f), there was a very low intensity of peak T1 evident in both inner membrane fouling materials as compared to the humic-like substance peaks (C and A). The fluorescence spectra of inner fouling was similar to the TB-EPS, which suggested that humic acid-like materials separated from the cake layer and adsorbed on to the inner membrane with increasing operation time. 3.4. SEC of EPS in the two membrane systems To complement the EEM spectra, size exclusion chromatograms of the corresponding samples were produced to further characterize the dissolved organic matter (DOM) (Fig. 5). The MW distributions obtained from the chromatograms displayed significant differences in the feed and permeate samples (there was little difference between the samples from membrane tanks and their effluents), particularly in the range of large molecules identified as biopolymers and humic acid for all waters and EPS extractions. Comparing the SEC and EEM results, it is believed that the large MW (104–105) fraction evident from the SEC (highlighted in Fig. 5) corresponded to peak T1 (protein-like materials), and peaks A and C corresponded to the smaller MW (102–104) humic-like materials
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fraction. Thus, the SEC and EEM results confirmed that membrane fouling is highly related to the biopolymer and humic acid-like materials. The SEC results showed that the pre-treatment of Fe/Mn was associated with much lower levels of organic matter in general (100oMWo105), and especially the large molecular fraction (EPS) (MW 104–105) in the CUF-Fe/Mn tank. The organic matter from the CUF-Fe/Mn cake layer was much less than from the CUF-Fe (Fig. 5b and c), especially for the large molecular weight fraction, as well as from the corresponding sludges of the two membrane systems (Fig. S2c). Comparing the LB-EPS and TB-EPS of samples from the cake layers in the two membrane tanks, there was little difference in the fractions of organic matter with MW o10,000 Da for the two pretreatments, but there was a much greater presence of high molecular weight organic matter (4 10,000 Da) in the cake layer of the CUF-Fe membrane. These results clearly indicated that a higher concentration of EPS was produced by greater bacterial activity in the CUF-Fe tank. It was clear that less organic matter had deposited or was adsorbed in the membrane pores in the CUF-Fe/Mn system, compared to the CUF-Fe system, with a lower concentration of EPS, and most of the adsorbed organic matter was smaller than 10,000 Da (Fig. 5d). The higher concentration of humic-like materials in CUF-Fe, as well as EPS, was consistent with the observed higher internal fouling resistances, indicated by the TMP after the high pressure water wash (Fig. 2). These results confirmed that the adsorption of low molecular weight compounds to the membrane was attributed to hydraulically irreversible fouling, which was also found by Ayache et al. [11] in waste water treatment.
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3.6. SEM In order to support the results indicating that membrane fouling can be mitigated by controlling EPS and humic-like substances in the cake layer and pores, SEM images were obtained (Fig. 7). The SEM images provided qualitative information that was consistent with the physical results discussed previously which showed that the concentration of EPS in the CUF-Fe was much greater than in the CUF-Fe/Mn, including in their respective cake layers and membrane pores. Thus, the presence of bacteria and more EPS in the CUF-Fe cake layer (Fig. 7a) was visible compared to that in the CUF-Fe/Mn cake layer (Fig. 7b). The higher concentration of EPS in the cake layer is believed to induce greater external membrane fouling, partly by decreasing the porosity between nano-scale primary particles. After the cake layer was washed and the membrane cleaned by sponge, the internal fouling of the membrane was mainly determined by the pore blockage in the cake layer. As seen from the SEM images (Fig. 7c and d), there were few primary particles resident on the surface of both membranes, but more pores were blocked by organic matter in the case of the CUF-Fe membrane. The statistical number of open pores of the two fouled membranes as quantified by the Scion Image software was dramatically different, as shown in Fig. 7e, with more pores of a wide range of sizes present in the case of the CUF-Fe/Mn membrane. These results further confirmed that more organic matter was adsorbed in the CUF-Fe membrane pores, which is consistent with the SEC results discussed previously. In view of this it is recommended that future studies should focus further on ways of decreasing bacteria and EPS concentrations and controlling the release of humic-like substances from the cake layer to the membrane pores.
3.5. Absolute EPS concentrations in cake layer and sludge Comparing the EPS content in the sludge and cake layer of the two systems, a significantly higher EPS concentration was found in the CUF-Fe cake layer than in the CUF-Fe/Mn cake layer (Fig. 6). The amount of total protein in the CUF-Fe cake layer was much higher than that in the CUF-Fe/Mn system (around 3 times) and increased throughout the membrane operating period. The variation of polysaccharide extracted from the cake layer and sludge in the two membrane systems was very similar to that of protein, with much greater quantities in the CUF-Fe system. Overall, the results further confirmed that the higher EPS concentration (proteins and polysaccharides) in the CUF-Fe cake layer corresponded directly with the greater extent of external membrane fouling.
1. The combination of Fe(II) and K2MnO4 (Fe/Mn) provides a potentially more cost-effective pre-treatment of raw waters prior to ultrafiltration, via the effects of in-situ coagulation and microorganism inactivation, most likely through the formation of solid-phase Fe(III) and MnO2 and by MnO24 oxidation. 2. In comparison to conventional pre-treatment with ferric sulfate (Fe), Fe/Mn pre-treatment substantially reduced overall membrane fouling in terms of the rate of development of transmembrane pressure. 3. Fe/Mn pre-treatment increased the removal of organic matter and substantially reduced the accumulation of bacteria in the
0.04
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4. Conclusions
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Fig. 6. The concentration of protein (a) and polysaccharide (b) in the cake layers and sludges for the two membrane systems.
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600 550 500
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Number of pores
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>100
Size (nm) Fig. 7. SEM images of membranes with different pretreatments: CUF-Fe membrane without (a) and with (c) wash, CUF-Fe/Mn membrane without (b) and with (d) wash, and statistical details of pores for washed membranes (e).
UF influent water, which induced lower concentrations of extracellular polymeric substances (EPS) in the CUF-Fe/Mn cake layer and less organic matter adsorbed in the membrane pores, thereby causing reduced external and internal membrane fouling. 4. The EEM fluorescence results confirmed the presence of protein-like materials in the CUF-Fe cake layer, but not in the CUF-Fe/Mn cake layer or in the sludges of both membrane tanks, indicating that these protein-like materials were not from the raw water, but from bacterial growth. 5. Significantly less organic matter was found to have deposited or adsorbed in the pores of the CUF-Fe/Mn membrane than the CUF-Fe membrane, and most of the adsorbed organic matter
was of lower molecular weight (o 10,000 Da), corresponding to humic-like materials rather than EPS.
Acknowledgments This research was supported by a Marie Curie International Incoming Fellowship (FP7-PEOPLE-2012-IIF-328867) within the 7th European Community Framework Programme. The authors wish to acknowledge the assistance of Thames Water Utilities Ltd (wastewater samples) and colleagues in Imperial College London,
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particularly Dr G. Fowler (assembly of the mini-pilot scale set-up) and Mrs E Ware (SEM analysis).
[24]
Appendix A. Supporting information
[25]
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2014.08.060.
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