Effects of combined ozone and PAC pretreatment on ultrafiltration membrane fouling control and mechanisms

Author’s Accepted Manuscript Effects of combined ozone and PAC pretreatment on ultrafiltration membrane fouling control and mechanisms Weiwei Huang, Mengliu Hu, Wenzong Zhou, Weiguang Lv, Huaqiang Chu, Bingzhi Dong www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(16)32621-7 http://dx.doi.org/10.1016/j.memsci.2017.03.044 MEMSCI15155

To appear in: Journal of Membrane Science Received date: 23 December 2016 Revised date: 27 March 2017 Accepted date: 28 March 2017 Cite this article as: Weiwei Huang, Mengliu Hu, Wenzong Zhou, Weiguang Lv, Huaqiang Chu and Bingzhi Dong, Effects of combined ozone and PAC pretreatment on ultrafiltration membrane fouling control and mechanisms, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2017.03.044 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effects of combined ozone and PAC pretreatment on ultrafiltration membrane fouling control and mechanisms

Weiwei Huang*, Mengliu Hu, Wenzong Zhou, Weiguang Lv, Huaqiang Chu, Bingzhi Dong

1

Shanghai Academy of Agricultural Sciences, Shanghai 201403, China;

2

School of Environmental Science and Engineering, Tongji University, Shanghai

200092 *

Corresponding author phone: +86-21-65982691; fax: +86-21-65982691; e-mail:

[email protected]

Abstract This work investigates the effects of combined ozone and powered activated carbon (PAC) pretreatment on ultrafiltration (UF) performance. High performance size exclusion chromatography (HPSEC) combined with peak-fitting prediction and fluorescence excitation-emission matrix spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) were used to analyze membrane fouling and organic removal. We conclude that combined ozone and PAC pretreatment can significantly inhibit an increase in transmembrane pressure (TMP) whereby ozone plays a predominant role in delaying TMP increases. Ozone and activated carbon can greatly 1

improve the removal efficiency of organic matter, especially that of macromolecular biopolymers, medium molecules of humic-like substances and small molecular acids and neutrals; however, the removal efficiency of aromatic proteins containing tyrosine is limited. Macromolecular biopolymers show high correlations with TMP/TMP0, followed by that of building blocks of low molecular acids and neutrals, while humic-like substances have a minor influence on UF membrane fouling. Fluorescence EEM is not valid for use as an indicator of membrane fouling after combined ozone and PAC treatment according to this study.

Key Words: Ultrafiltration, Ozone, Powered activated carbon, Peak-fitting, Parallel factor analysis

1. Introduction Ultrafiltration is one of the most promising and attractive drinking water and wastewater treatment techniques. Despite the excellent retention of microorganisms, suspended particles and colloids through size exclusion for water treatment, problems still slow the progression of this technology. The most widely known problem is associated with membrane fouling, which governs the performance of the filtration process. In this respect, natural organic matter (NOM) is considered highly problematic, as it exists ubiquitously in natural water and can cause serious, irreversible membrane fouling [1]. 2

NOM is a heterogeneous organic consisting of various organic matters such as colloids and polysaccharides, humic acid, fatty acids, proteins, etc. Some researchers focused on the characterization of NOM fractions from the pool of NOM that caused fouling, such as colloidal organics, polysaccharides, humic substances, biopolymers, proteins, etc, whereas some others focused on the fractionation of NOM and its effect on fouling, i.e. hydrophobicity/hydrophilicity and molecular weight (MW) distribution [2, 3]. The composition of NOM is complex, and a more in-depth understanding of the role of NOM characteristics in the membrane fouling mechanism is needed. Conventional NOM fouling characterization techniques have involved the use of protein or polysaccharide determination, molecular weight distribution, fluorescence excitation emission matrices (EEMs), attenuated total reflection-Fourier infrared spectroscopy (ATR-FTIR), and scanning electron microscopy (SEM) [4，5]. More recent evaluations of organic matter fouling in UF based on advanced organic characterization techniques seem promising. High performance size exclusion chromatography (HPSEC) combined with peak-fitting prediction is one of the most promising techniques, as it allows NOMs to be analyzed more efficiently and thus provides qualitative information based on a comparison between raw water and different treatments and quantitative information on dissolved organic carbon (DOC) removal [6, 7]. Xing et al. studied the dissolved organic matter removal of low specific UV absorbance surface water predictions and found that HPSEC combined with peak-fitting approaches can be successfully used to identify removable DOC components [8]. Other studies have shown that this approach can better characterize 3

aquatic humic and non-humic matter [9]. Moreover, three-dimensional EEM fluorescence spectroscopy has proven to be a sensitive and rapid detection method for NOM fractions. Researchers have utilized EEMs to characterize fluorescent NOM, including protein-like and humic-like compositions [10]. EEMs only examine major peaks, i.e., a limited number of excitation-emission coordinated EEM pairs; therefore, accurate quantitative EEM information is needed. An EEM coupled with parallel factor analysis (PARAFAC) involves using a three-dimensional matrix decomposition algorithm based on the principle of alternating least squares iterative types [11]. PARAFAC is a commonly used analytical method that can be used to decompose complex EEMs into independent fluorescent components representing groups of similar fluorophores [12]. Researchers have used this fluorescence-based approach in membrane fouling investigations and have revealed its potential for monitoring on-line membrane fouling and creating early warning systems [13]. Shao et al. found that the EEM-PARAFAC method coupled with adsorption pretreatment can be used to predict UF membrane fouling patterns [14]. Yu et al. showed that the proposed EEM-PARAFAC-based approach is suitable for major foulant identification and mechanism determination [15]. As ways to overcome fouling issues during the UF process, several methods have been proposed, including membrane cleaning and the pretreatment of raw water [16, 17]. Ozone is one potential alternative pretreatment material that can be used to enhance NOM rejections [18]. Nonetheless, to summarize previous research findings, 4

the effects of ozone on membrane fouling are inconsistent due to the heterogeneous and variable nature of NOM. Many studies have shown that ozone, when applied solely as a pretreatment, is in some cases able to reduce membrane fouling [19], while other studies have indicated that ozonation can cause significant biopolymer degradation, leading to minor filtration flux reduction for both UF and microfiltration (MF) [20]. The addition of powdered activated carbon (PAC) adsorption to UF is also widely employed to enhance contaminant rejection and to prevent membrane fouling [21]. The adsorption of membrane foulants by PAC helps to alleviate membrane fouling [22]. Organic removal by activated carbon is mainly dependent on microporous adsorption, and small molecular organics are preferentially adsorbed; however, oxidation preferably oxidizes electron-rich areas (aromatic p-delocalized systems) in target molecules and reduces the aromaticity of oxidized compounds [23]. Combined ozone and PAC technologies could better enhance the rejection of contaminants, which may alleviate UF fouling. Researchers have used combined ozone and PAC technologies to remove aromatic DOC and other organic matter from drinking water [23]. Nonetheless, consequent effects on membrane fouling and predominant membrane foulants resulting from the combined process have not been thoroughly investigated, especially when treating complicated organic matter in natural water by UF. Consequent effects on membrane fouling behavior must be clearly elucidated. Therefore, the objectives of this study are as follows: (1) to investigate treatment effects of UF fouling by the combined ozone and PAC treatment; (2) to 5

analyze the membrane fouling mechanism of the combined treatment; and (3) to utilize HPSEC combined with peak-fitting prediction and EEM-PARAFAC to analyze organic removal and membrane fouling outcomes. The results present practical suggestions regarding UF fouling control and membrane fouling monitoring during UF treatment. 2. Materials and Methods 2.1. Source Water The water used in this study was drawn from Lake Tai, Wuxi, China in August of 2014. Taihu Lake in China was studied because it is a large lake that is connected to the Yangtze River and that supplies drinking and irrigation water to 73 million people residing between Shanghai and Nanjing. The pH of the water sample was 8.2, the turbidity level was 41 NTU, and the DOC and specific UV absorbance (SUVA) values were approximately 3.808 mg/L and 2.074 L/(mg·m), respectively. 2.2 Pretreatments and UF experiment Ozonation was conducted using an ozone generator measuring 70 cm high and 5 cm in diameter. As shown in Fig. 1, oxygen flows were adjusted at 0.8 L/min, and the ozone concentration was set to 10%. After the ozone produced was introduced into the water sample and completely reacted for 10 min in confined conditions, residual ozone was stripped by high purity nitrogen (99.99%) for 20 min to terminate the reaction. The exhaust was routed into two scrubber bottles (4% KI solution) by the scrub vent at the top of the generator. Target ozone concentrations were set to 1 mg/L, 3 mg/L, 5 mg/L, and 7 mg/L using a compressed O2 tank, and the applied does of 6

ozone were 3 mg/L and 7 mg/L when conducting the combined ozone-PAC-UF experiment. The ozone concentration was consumed ozone concentration by deducting ozone remained in the exhausted gas. PAC adsorption was operated at doses of 20 mg/L, 50 mg/L, and 100 mg/L when the water was ozonized. The sequential steps used were as follows: 1 min of rapid stirring at 100 r/min after PAC was added, 30 min of slow stirring at 50 r/min, 30 min of precipitation, and supernatant use in subsequent UF tests. The characteristics of the PAC (Wood Activated Carbon 2, Tangxin Activated Carbon Co., Shanghai, China) were presented in Table S1. As detailed, the percentage of PAC micro pore size (<2 nm) was 59.18%, the transition pore size (2-50 nm) accounted for 48.82%, while the BET surface area was 1326.405 m2/g, the micro pore surface area was 705.901 m2/g, and the transition pore surface area was 620.504 m2/g. The adsorption abilities of PAC were evaluated from the equilibrium and kinetic studies, which can be found in the Supporting Information. UF experiments were carried out on a dead-end filtration unit that included a centrifugal pump (MGP-M256B220, IWAKI, Japan), an inlet pressure gauge (XP2i, Crystal), an outlet pressure gauge, and a hollow fiber membrane module. The pore size of the hollow fiber membrane was 0.02 μm. The outer diameter of the hollow fiber membrane was 1.3 mm. Sixteen cords of the hydrophilic PVDF (US Filter) membrane with a length of 40 cm for every fiber were assembled into a module with a total surface area of 0.18 m2. Peristaltic pumps (MGP-M256B220, IWAKI) were used to feed the water sample into the membrane module. Transmembrane pressure levels (TMP, gauge pressure in bar) were monitored based on pressure differences between 7

inlet and outlet pressure gauges (XP2i, Crystal, US), and data were recorded automatically every 15 s. The outlet pressure gauge was connected to an air source, and the pressure level was recorded as 0 kPa. The pressure level of the deionized water stable flux of the membrane module was 0.30 MPa. When conducting the filtration experiment, the filtration flux was set to 60 L  (m2  h)1 with a filtration flow rate of 175 mL/min and a filtration time of 1 h. Although the UF membrane filter was chemically cleaned after each filtration, initial transmembrane pressure (TMP0) levels still presented slight differences, and for convenience of comparison, the TMP0 at 0 kPa was used to compare differences in TMP under various pretreatment conditions. All UF tests were run in triplicate, and errors between TMPs of repeated experiments did not exceed 5%.

2.3 Analytical methods Dissolved organic carbon (DOC) was detected using a total organic carbon analyzer (TOC-VCPH, Shimadzu). The UV absorbance at 254 nm (UV254) was examined using a UV spectrophotometer (Hach-5000). The aqueous ozone was measured using a spectrophotometer via the indigo method [24]. The molecular weight (MW) of the organic matter was determined on a high-performance size exclusion chromatograph (Waters e2695, USA) coupled with a UV/visible detector (Waters 2489, USA)-total organic carbon analyzer (Sievers 900 Turbo, USA) system using a TSKgelG3000PWXL column (30 cm × 7.8 cm). The 8

pre-column of a TSK-GEL TSK guard column PWXL (6.0 mm × 4.0 cm) was used to protect the TSKgelG3000PWXL column. The HPSEC mobile phase was prepared with a phosphate buffer (0.05 mol/L KH2PO4+0.03 mol/L NaOH) and 0.02 mol/L Na2SO4, producing an ionic strength of 0.1 mol/L, and flow rate of 0.5 ml/min. The ionic strengths of samples were adjusted using a concentrated eluent solution to match the ionic strength of the HPSEC mobile phase (0.1 mol/L), and samples were neutralized to pH 7.0 for detection. Chromatograms were analyzed via peak-fitting techniques to determine the area under each peak. The PeakFit software package (Version 4.12 Systat Software Inc., CA, USA) was used for this analysis [6]. EEMs were prepared using a Fluorescence Spectrophotometer (F7000, Hitachi, Japan) at room temperature (21 ± 1 °C). Emissions (Em) and excitation (Ex) wavelengths were set at 200-600 nm and 200-400 nm, respectively, at 5 nm intervals. The scanning speed was set to 12,000 nm/min, and slit widths were 5 nm. To eliminate water Raman scattering and to reduce other background noise, an EEM of Milli-Q water was subtracted and utilized for Raman normalization. PARAFAC modeling procedures were conducted using 24 EEM fluorescence data via the DOMFluor Toolbox in Matlab® [12]. Fluorescence components were identified via a validation method that involved split half and residual analysis. The maximum fluorescence intensity (Fmax) of each component was determined. 3. Results and discussion 3.1 Characteristics of raw water 9

Fig. 2 presents the MW distribution of raw water measured by the DOC and UV detectors and the results of the DOC chromatography peak-fitting analysis. The main dissolved organic matter in the source water contains four peaks according to the DOC chromatography results (peak A, peak B, peak C, and peak D) with MW values of 1,000 K Da, 6,500 Da, 1,200 Da, and 440 Da, respectively. Peak A has high peaks according to the TOC chromatography results but no peaks according to the UV chromatography results and is likely associated with biopolymers (such as polysaccharides or amino sugars). Peak B has three small peaks according to the UV chromatography results that are considered to be humic-like substances (HS). Peaks C and D are considered the building blocks of low MW acids and humics as well as low MW neutrals according to previous studies [25].

3.2 TMP variations by combined oxidation and adsorption treatment Figure 3 shows variations in the TMP occurring during the ozone/PAC/UF process as a function of time. As shown in Fig. 3a, although the combination of ozone and PAC pretreatment significantly lowered the TMP increase rate relative to the direct filtration of raw water, the TMP values were relatively consistent when ozone was added in the same doses. For example, when ozone was at 3 mg/L, the TMP of the combined process was considerably reduced, by approximately 35%, relative to natural water regardless of how much PAC was added. The values were 53% lower when 7 mg/L ozone was added, indicating that ozone might play a major role in delaying TMP increases during the ozone/PAC/UF process; the higher the ozone 10

concentration, the more effective it was in improving TMP outcomes. In comparing the corresponding results from the separated PAC and ozone pretreatment (Fig. 3b), the TMP results were nearly consistent among treatments involving 3 mg/L ozone (38.29 kPa), combined 3 mg/L ozone + 20 mg/L PAC (37.22 kPa), and 50 mg/L PAC (36.97 kPa) used at the end of the filtration period; however, the results were slightly different for the procedure involving separated 20 mg/L PAC (33.13 kPa) treatment. This phenomenon can be explained by the fact that ozone exhibits organic matter oxidation selectivity, functioning well with benzene ring oxide organics and double carbon bond structures with UV absorbance [23]. When ozone is added at smaller doses (3 mg/L), some macro MW organics of biopolymers may be primarily oxidized into smaller organics, retarding membrane fouling, while meantime, the small MW organics might increase (Peak C concentration, see Table 1). Wang et al. found that macromolecular organics was the main substances that caused the rapid flux decline of membrane [26]. The other researcher indicated that the membrane fouling of low pressure membrane was collective effects by both macro and small MW organics [27]. Therefore, despite ozone oxidation lowed the concentration of macro MW organics, as the small molecule organic might increase, their effects by combined 3 mg/L ozone + 20 mg/L PAC on membrane fouling might not significant than that of separated 20 mg/L. No evident TMP discrepancies were found at the same ozone doses (Fig. 3a), and TMP levels at the end of the filtration period were consistent between the 3 + 20 mg/L, 3 + 50 mg/L, 3 + 100 mg/L, and 3 mg/L oxidation procedures. This phenomenon can be explained that although PAC 11

addition could adsorb small MW organics to reduce membrane pore plugging and adsorption pollution, and alleviate membrane fouling (Fig. 3b), as PAC doses and filtration time are increased, the more organics absorbed by PAC might not the organics of membrane foulants, while those organics not adsorbed still can cause severe membrane fouling [22], and consequent retarding effects on TMP might not be significant. Membrane performance was improved through the combined 7 + 20 mg/L pretreatment followed by the 7 mg/L oxidation and 20 mg/L and 50 mg/L PAC pretreatments, with the TMP at the end of the filtration were 26.64, 29.4, 33.78, and 36.97 kPa, respectively; this may be attributed to the fact that ozone can decrease membrane fouling, as found by many researchers [28, 29], when ozone is present at high doses and more macro MW organics can be oxidized. Residual ozone can also be catalyzed through PAC [30], which may in turn improve the breakage of biopolymers and decrease membrane fouling. Therefore, as ozone might play a significant role in delaying TMP increases, the combined process at higher ozone doses may be more effective at minimizing TMP increases; however, organic removal and specific organic reduction methods require further investigation.

3.3. Organic removal by combined oxidation and adsorption treatment Fig. 4 shows organic removal results of the combined ozone and PAC treatment. The UV removal efficiency of the combined treatment was improved with increasing PAC and ozone doses. We obtained 39.24%, 50.63%, and 63.29% UV254 removal 12

results with 20 mg/L, 50 mg/L, and 100 mg/L PAC treatments, respectively, at an ozone level of 3 mg/L, and when ozone levels were increased to 7 mg/L, removal levels increased to 58.23%, 64.56%, and 70.89% at PAC 20 mg/L, 50 mg/L, and 100 mg/L, respectively. UV254 levels in the ozone/PAC/UF effluent were lower than those in ozone/PAC water, indicating that UF in the combined process plays a positive role in organic removal. This result shows that combined ozone and PAC pretreatment can significantly remove organics with UV absorbance and reduce the loads of UF membranes and of higher ozone and PAC additions as UV254 organics are removed.

The combined ozone/PAC/UF treatment also presented high levels of DOC removal efficiency (Fig. 5). DOC removal levels increased from 19.17% to 26.23% and then to 39.2% at PAC dosage increments of 20, 50, and 100 mg/L, respectively, at an ozone level of 3 mg/L and at 23.2%, 29.69%, and 41.94%, respectively, at an ozone level of 7 mg/L. The highest DOC removal efficiency also occurred at the highest PAC and ozone doses (7 mg/L + 100 mg/L); however, when comparing the results with the separated treatment, the percentage of DOC decline resulting from the separated ozone treatment was remarkably lower than that of the separated PAC treatment, ozone/PAC pretreatment, and ozone/PAC/UF. With 7 mg/L ozone, the DOC concentration after oxidation was even higher than that of raw water; however, it was greatly reduced due to the subsequent adsorption of PAC in effluent waters. This phenomenon can be explained by the fact that the water used in this study was algal-rich water in summer, when ozone is present in high doses (7 mg/L), the 13

intracellular organic matter (IOM) of algae might be released as a result of oxidation [31], further indicating that the addition of PAC or other adsorbents is needed after oxidation pretreatment to ensure organic matter concentrations in effluent waters. This result is consistent with the results of Hammes et al. who found that ozonation of algae caused a substantial increase in the concentration of DOC and assimilable organic carbon (AOC), notably nearly instantaneously upon exposure to ozone [31]. Notably, DOC removal by the separated ozone and PAC treatment showed some discrepancies with TMP increases. For example, when PAC was present at 20 mg/L, its DOC removal efficiency was higher than that of 7 mg/L ozone; however, its effect on TMP was comparatively lower (Fig. 3), illustrating that the removal of total organic matter might not have a direct relation with TMP increases but with specific organic removals.

3.4 Peak-fitting analysis of specific MW organic removal levels Figure 6 shows the MW distributions of source water before and after PAC and ozone treatment. The areas and concentrations of four peaks and the percent reduction at each concentration between the feed and permeate water under various ozone and PAC doses were also analyzed by peak-fitting technology to obtain specific MW organic removal levels (Table 1). Macro MW biopolymers were gradually decreased with increasing PAC and ozone doses, and the percent reduction of peak A concentrations between the source water and water after the combined treatment increased from 23.16% at 3 + 20 mg/L 14

to 30.62% at 3 + 50 mg/L and 53.0% at 3 + 100 mg/L and from 28.54% at 7 +20 mg/L to 41.76% at 7 + 50 mg/L and 56.61% at 7 + 100 mg/L. These results indicate that oxidation and PAC treatment have remarkable effects on the removal of macro MW biopolymers. In addition to high percent reductions found in peak A areas, peak B, which represents humic-acid organics, was also greatly reduced, illustrating that the combined ozone and PAC treatment also shows high levels of removal efficiency for humic-acid substances. Peak C, despite treating effects, was exceptionally lower than that of humic-acid organics, and the percent reduction also gradually increased with increasing PAC and ozone doses. The reduced medium and small MW organics found correspond to reports that PAC could better physically remove NOM and synthetic organic compounds (SOCs), reduce the direct loading of dissolved organic pollutants onto the membrane, and prevent membrane fouling [32]. For peak D, the organic removal of peak D fractions (low MW neutral) was much higher, and larger ozone and PAC doses caused a reduction in higher peak D organics.

After membrane filtration (Table 2), peak A organics were greatly intercepted by the membrane, consistent with research by Henderson et al. showing that biopolymer fractions are the organic fractions most significantly reduced through UF treatment [13]. Additionally, the removal efficiencies of peak C organics were higher; however, the organic removal efficiencies of peaks B and D were limited. Kennedy et al. [33] investigated the interaction between fractional components of NOM in surface water 15

and a hydrophilic PES/PVC hollow fiber UF membrane. They found that among all of the organic fractions, the UF membrane mainly rejected polysaccharides. Zheng et al. reported that substances detected in the biopolymer peak (mostly macro polysaccharide-like and protein-like molecules) were almost completely retained by the UF membrane [34], consistent with the results of the present study.

Table 3 shows the organic fractions removed by separated ozone and PAC treatments. The MW distributions of these separated treatments were analyzed to better compare these treatment effects with the combined treatment. Macro MW was gradually decreased with increasing ozone doses, and the DOC removal efficiency levels of peak A organics (biopolymers) between raw water and water after 3 mg/L and 7 mg/L ozone additions were 25.23% and 31.66%, respectively. Furthermore, we found that the peak B humic substances caused the second highest reduction with increasing ozone doses. However, peaks C and D caused much lower peak reductions; in fact, their concentrations increased after ozonation, potentially due to the oxidation of macromolecular organics. Song et al. investigated Huangpu River water treatment by MF with ozone pretreatment and found that when ozone is present at low concentrations (0.5 mg/L or 1.0 mg/L), organic matter levels ranging between 2 k Da and 7 kDa decreased compared to raw water, whereas organic matter concentrations lower than 0.2 kDa increased [35]. PAC treatment could remove some macro MW biopolymers in raw water. However, the proportions of peak B and C removals were higher, especially when PAC was administered at 100 mg/L, the removal efficiency of 16

the peak C area was as high as 21.59%.

The above results of the two separated pretreatment techniques are compared with the combined PAC/ozone treatment in Table 1. The table first shows that the efficiency of peak A DOC removal was greatly increased through the combined treatment compared to that of the separated PAC and ozone pretreatment. However, the simple addition of the two pretreatments did not produce these results. For example, the DOC removal levels of peak A concentrations were 25.23% after 3 mg/L oxidation and 6.158%, 21.25%, and 18.87% with 20 mg/L, 50 mg/L, and 100 mg/L PAC treatments, respectively; however, peak A DOC removal levels reached 23.16%, 30.62%, and 53.00% at 3 +20 mg/L, 3 +50 mg/L, and 3 +100 mg/L, respectively, illustrating that the combined treatment has certain promotional effects on macro MW organic removal outcomes. Furthermore, as a result of ozone and/or PAC existence, the removal efficiencies of medium MW (peak B) organics were also greatly increased by the combined treatment when compared to the separated PAC or ozone treatment. When the combined process delivered was at 7 mg/L ozone + 100 mg/L PAC, the percent reduction level reached as high as 58.78%, which is substantially higher than those of the 7 mg/L ozone (8.37%) and 100 mg/L PAC (15.98%) treatments. Notably, small MW organics of peaks C and D were almost greatly reduced through the combined treatment, thus generating extremely different results from those of the separated ozone treatment. This may occur because ozone has certain degradation effects on macro and medium MW organics that transform them 17

into small MW organics; however, as a result of the subsequent PAC treatment, residual lower MW organics can be further adsorbed via PAC. Polanska et al. investigated assimilable organic carbon (AOC) in drinking water treatments and found that oxidation can decompose macromolecules in water into smaller particles and improve their biodegradability. The resulting biologically activated carbon can then biodegrade and remove 45% to 78% of the AOC related to low molecular organics [36]. Upon comparing the treatment effects of organic removal by UF between separated PAC and ozone treatments (Table 4) and combined treatments (Table 2), it was found that the removal efficiency of macro MW organics and of medium and small MW organics is greatly reduced through the combined treatment. The reduced organic rejections may be the main cause of TMP decline consistent with the results shown in Fig. 3. It is of note that although the DOC removal efficiency of 3 + 20 mg/L was higher than that of 20 mg/L PAC(Fig. 4), from the analysis of MW organic removals (Table 1 and Table 3), it was found that peak C of low MW acids increased after combined 3 + 20 mg/L, however, the treating effects of peak C was lower than that of separated PAC 20 mg/L, this result indicated that although ozone could oxidize macro MW organics into small MW organic matters, as the small MW organics cannot be fully adsorbed by subsequent PAC 20 mg/L, their treating effects might be not significant than separated PAC treatment due to the existence of extra organics generated by oxidation. The discrepancy of peak C removals between separated PAC 20 mg/L and combined 3 + 20 mg/L might be one of the causes of their TMPs discrepancies. Liu et al. found that building blocks of low MW acids and 18

humics were mainly responsible for the evolution of irreversible fouling by adsorbing into membrane pores [37].

3.5 Fluorescent components identified by PARAFAC Fig. 7 shows the EEM contour and the Ex and Em loadings of the components of raw water according to the PARAFAC analysis. Four component models were detected according to the residual analysis results, a spectral properties examination, a split half analysis, and a random initialization analysis. Component 1 has an obvious Ex maximum at 220 and an Em maximum at 330 nm representing an aromatic protein (tyrosine). Component 2 has an Ex peak at 275 and an Em peak at 320 nm representing soluble microbial organics (tryptophan). Component 3 has primary and secondary Ex peaks occurring at 230 nm and 280 nm, respectively, and an Em wavelength of 290 - 460 nm that is believed to involve fulvic-like organics. Component 4 is composed of humic-like organics [38, 39].

The fluorescent component Fmax results of the raw water and water after ozone and PAC treatment are shown in Fig. 8. The Fmax of components 1, 2, and 3 of raw water were 137.5, 134.9, and 125.85 Raman units (R.U.), respectively, while component 4 Fmax was 48.6 R.U. The fluorescence intensity of protein-like organics of raw water was higher than that of humic-like organics, which is different from traditional NOM water. The phenomenon can be explained by the fact that the study was conducted in the summer (August) when algae are plentiful in Lake Tai. Huang et 19

al. monitored water quality changes in Taihu Lake for one and a half years and found that algae bloom outbreaks often occur in August and September, most likely due to high water temperatures and summer sunlight levels [40]. After the combined treatment, components 2, 3, and 4 were preferentially removed; however, the treatment did not exhibit any comparable removal efficiencies for component 1. In fact, the Fmax of component 1 was increased after the combined treatment, indicating that the combined ozone and PAC treatment performs better for the removal of soluble microbial, fulvic-like, and humic-like organics but not for tyrosine-like organics. For the six combined treated water samples, the Fmax changed dramatically across the different treatments. Fmax values in components 2, 3, and 4 decreased with increasing ozone and PAC doses. The combined 7 mg/L ozone + PAC 100 mg/L value reduced the Fmax of soluble microbial, fulvic-like, and humic-like organics from 134.9 to 14.59 R.U., 125.85 to 5.63 R.U., and 48.61 to 8.52 R.U., respectively. Component 1 tyrosine levels were greatly increased from 137.5 to 325.5, 763.7, and 249.2 R.U., respectively, after 3 + 20 mg/L, 3 + 50 mg/L, and 3 + 100 mg/L treatments and were then decreased at 7 + 20 mg/L, 7 + 50 mg/L, and 7 + 100 mg/L. This result shows that the combined ozone and PAC treatment can effectively modify the fluorescent NOM matrix in feed water consistent with the results of Shao et al. [15].

Fig. 9 shows the removal efficiency of Fmax by UF. Component 1 had the highest removal efficiencies, followed by components 2 and 3. Few particles were 20

removed in component 4, consistent with the result in Table 2 and previous studies showing that biopolymers and proteins rather than humic substances contribute to the reversible and irreversible fouling of low-pressure membranes [1]. Ye et al. also found that polysaccharide and proteins are the main organics responsible for membrane fouling [41].

3.6 Correlations between MW distribution peak concentrations, component Fmax values and membrane fouling To elucidate UF membrane fouling behavior, feed water characteristics, including MW distribution concentrations and the Fmax of each fluorescent component, were correlated with normalized transmembrane pressure levels (TPM/TPM0) through a linear regression. MW distribution concentrations correlated with TPM/TPM0 by separated PAC and ozone treatment were also analyzed for the comparing of combined ozone and PAC treating effects. As can be seen in Fig. 10, remarkable correlation was observed between peak A concentration and TMP/TMP0 during ozone treatment (R2 = 0.9021), however, no significant correlation was observed between peak B concentration and TMP/TMP0 (R2 = 0.204), and comparatively poor correlations existed between peak C, D and TMP/TMP0, indicating that peak A, which is associated with macro MW biopolymers such as polysaccharides, proteins and amino sugars, had significant effects on UF membrane fouling, while humic-like substances had minor effects on UF membrane fouling during ozone treatment, which can probably be explained by the fact that most humic-like substances passed through 21

the membrane and only a small portion was retained by UF [1]. Unlike the observed result by ozone treatment, the peak A concentration with the TMP/TMP0, yielding a low value of correlation (R2= 0.4449) during PAC treatment, whereas low MW acids greatly contributed to membrane fouling which is supported by the remarkable correlation of peak C with TPM/TMP0 (R2 = 0.7828). However, from the analysis of correlations between different MW distribution concentrations and TMP/TMP0 by combined ozone and PAC treatment, it was found that strong correlation still existed between the peak A concentration and TPM/TMP0, where R2 = 0.622, followed by peak C and peak D, with R2 = 0.4826 and 0.4142, respectively, yet there was poor correlation between peak B concentration and TPM/TMP0 (R2 = 0.2883), suggesting that the membrane fouling of NOM by combine ozone and PAC treatment was combined effects of macro MW and micro molecules organics. Previous research indicated that macromolecular organics, such as polysaccharides or amino sugar could be intercepted or adsorbed onto the membrane surface to form reversible fouling, while micro MW organics, such as building blocks of low MW acids and humics and low MW neutrals, could be adsorbed into membrane pores, which was responsible for the evolution of irreversible fouling [37]. The above results thus suggested that ozone could remove macro MW organics to alleviate reversible membrane fouling, while PAC could alleviate membrane irreversible fouling by lowering the peak C loading. It was of note that although ozone oxidation can significantly reduce the TMPs (higher correlation between the peak A concentration and TPM/TMP0), as the small molecular organics extra generated by ozone may also 22

affect the irreversible membrane fouling, the combined ozone and PAC treatment on reversible and irreversible fouling control still needed to be further investigated in the future work.

Upon analyzing the relationship between the Fmax of the components and TPM/TMP0 (Fig. 11), it was found that the Fmax of C3 has a higher correlation with TPM/TMP0 than the Fmax values of C1, C2, and C4 with an R2 value of 0.6146. This suggests that C3, which represents fulvic-like substances, shows a relatively high correlation with total fouling. This result contradicts the results of Peldszus et al. [1] and Peiris et al. [42], which show that protein-like substances are highly correlated with total fouling. While fulvic-like substances show a high correlation with total fouling, C3 may not be the key foulant of UF. C3 values may have varied due to a fouling culprit that may not be detected through fluorescence EEM, as not all NOM patterns in water show fluorescence. For instance, polysaccharides, which are known to contribute greatly to membrane fouling, do not exhibit fluorescence. Therefore, although fluorescence EEM was used to monitor membrane fouling by untreated or physically treated water [39], it was not valid for use as an indicator of membrane fouling by polysaccharides or protein-enriched water after combined ozone and PAC treatments in the present study.

3.7 Mechanisms of fouling mitigation by combined treatments Size exclusion is widely recognized as a core mechanism of UF membrane 23

filtration. Substances such as polysaccharides and proteins that are close to the membrane pore size can cause pore blockages that severely increase filtration resistance while substances much larger than membrane pores lead to cake formation [43]. According to research by Cheng et al., high MW fractions of humic acid and sodium alginate can be decomposed into smaller fragments pre-ozonation and can contribute to membrane fouling mitigation, while ozonated bovine serum albumin molecules remain close to the membrane pore size and cause severe pore blockages followed by cake formation [44]. According to previous studies and the results of the present study, it can be inferred that while the ozone doses applied in this study can ozonize high-MW organics (such as biopolymers and humic-like organics) into smaller fragments, that can pass through or adsorb into membrane pores, because the subsequent treatment of PAC adsorption can further remove micro MW organics that originally existing in NOM and after ozone oxidation, thus alleviating cake formation and pore blockage, and because combined ozone and PAC treatments have certain synergistic effects on macro organic removal outcomes, better membrane fouling control performance was observed. On the basis of the above results, schematics of the mechanisms of effects of combined ozone and PAC treatments on membrane fouling were created and are presented in Fig. 12.

4. Conclusions The purpose of this study was to investigate the membrane fouling mitigation of UF by combined ozone and PAC treatment. Organic removal and membrane fouling 24

mechanisms were also investigated by HPSEC combined with peak-fitting prediction and EEM-PARAFAC. The results obtained are summarized here. First, combined ozone and PAC pretreatment can significantly inhibit an increase in TMP, whereby ozone plays a major role in delaying TMP increases during ozone/PAC/UF processes. The inhibitory effects of TMP by combined 3 mg/L ozone + 20 mg/L PAC were generally consistent with those of 3 mg/L ozone and separated 50 mg/L PAC, while membrane performance levels were significantly mitigated through the combined 7 + 20 mg/L pretreatment followed by the 7 mg/L oxidation pretreatment and 20 mg/L and 50 mg/L PAC pretreatments. Second, ozone and PAC treatment can improve the removal efficiency of macro MW biopolymers, soluble microbial organics (tryptophan), medium MW humic-like substances and small MW acids and neutrals while aromatic proteins containing tyrosine increase after the combined treatment. Third, macro MW biopolymers show high correlations with TMP/TMP0 during combined ozone and PAC treatment, humic-like substances have minor effects on UF membrane fouling. Membrane fouling of NOM by combine ozone and PAC treatment was combined effects of macro MW and micro molecules organics. Fluorescence EEM was not found to be a valid indicator of membrane fouling after combined ozone and PAC treatment in this study. Acknowledgments We acknowledge funding received through the National Water Pollution Control and Treatment Key Technologies R&D Program (2012ZX07403-001-003) of China, 25

the Chinese Postdoctoral Funds (0400229109), and the Technological Innovation Action Plan (15391912200).

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Fig. 1 Schematic of the membrane filtration set-up. Fig. 2 MW distribution of raw water by the TOC and UV detector and the results of the TOC chromatography peak-fitting analysis. Fig. 3 Variations in the TMP during ozone/PAC/UF processes as a function of time, a by combined ozone and PAC treatment, b by separation ozone and/or PAC treatment and combined ozone and PAC treatment. Fig. 4 UV254 removal by the combined ozone/PAC/UF treatment, a 3 mg/L ozone, b 7 mg/L ozone. Fig. 5 TOC removal by the combined ozone/PAC/UF treatment, a 3 mg/L ozone, b 7 mg/L ozone. Fig. 6 MW distribution of source water before and after combined PAC and ozone treatment. Fig. 7 EEM contour (left) and Ex and Em loadings of the components (right) of raw 32

water according to the PARAFAC analysis. Fig. 8 The fluorescent component Fmax of the raw water and water after ozone and PAC treatment. Fig. 9 Removal efficiency of Fmax by UF. Fig.10 Correlation between different MW distribution peak concentrations and normalized transmembrane pressure levels, A by separated ozone treatment, B by separated PAC treatment, C by combined ozone and PAC treatment. Fig. 11 Relationships between the Fmax of the components and TPM/TMP0. Fig. 12 Schematic of mechanisms of combined ozone and PAC treatment effects on membrane fouling.

Table 1 Organic removal by combined ozone and PAC via HPSEC peak-fitting

Raw

3+20

3+50

3+100

7+20

7+50

7+100

Water

(mg/L)

(mg/L)

(mg/L)

(mg/L)

(mg/L)

(mg/L)

Peak A Area Concentration

0.00403 0.2050

0.00322 0.1575 (23.16*)

0.00319 0.1423 (30.62*)

0.00233 0.0964 (53.00*)

0.00305 0.1465 (28.54*)

0.00270 0.1194 (41.76*)

0.00229 0.0889 (56.61*)

Peak B Area Concentration

0.00658 0.3343

0.00456 0.2233 (33.2*)

0.00598 0.2671 (20.09*)

0.00303 0.1249 (62.62*)

0.00510 0.2662 (20.37*)

0.00519 0.2299 (31.21*)

0.00355 0.1378 (58.78*)

Peak C Area Concentration

0.05218 2.6515

0.0571 2.7946 (-5.39*)

0.04959 2.2124 (16.55*)

0.04860 2.0067 (24.32*)

0.03535 2.2248 (5.961*)

0.05046 2.2333 (15.78*)

0.04964 1.928 (27.28*)

33

Peak D Area Concentration

0.01214 0.6171

0.00451 0.2205 (64.26*)

0.00843 0.3762 (39.03*)

0.00681 0.2809 (54.47*)

0.01032 0.3314 (46.31*)

0.00634 0.2806 (54.53*)

0.00626 0.2431 (60.60*)

* Percent reduction in concentrations between source water and water after combined ozone and PAC treatment

Table 2 DOC removal of Peaks A to D of various combined ozone and PAC treatment water samples by UF using peak-fitting.

Sample

Parts of the HPSEC-OCD chromatogram Peak A

Peak B

Peak C

Peak D

Source

81.5*

-0.2*

10.18*

17.94*

3+20

80.54*

-12.3*

12.08*

-21.74*

3+50

76.21*

23.45*

3.29*

4.02*

3+100

62.93*

-3.27*

13.37*

-2.99*

7+20

59.50*

-5.97*

0.2*

25.27*

7+50

52.81*

-6.75*

5.32*

-1.9*

7+100

64.15*

-1.34*

19.4*

-4.1*

34

*Percent reduction in peak concentrations between feed and permeate.

Table 3 Organic removal by separated ozone and PAC doses via HPSEC peak-fitting Raw water

3

7

20

50

100

(mg/L)

(mg/L)

(mg/L)

(mg/L)

(mg/L)

Peak A area Concentration

0.00403 0.2050

0.00343 0.15339 (25.23*)

0.00260 0.1401 (31.66*)

0.00397 0.1924 (6.158*)

0.00357 0.1615 (21.25*)

0.003493 0.1663 (18.87*)

Peak B area Concentration

0.00658 0.3343

0.0057 0.2550 (23.73*)

0.0057 0.3063 (8.37*)

0.00647 0.3134 (6.256*)

0.00611 0.2763 (17.35*)

0.0063 0.2809 (15.98*)

Peak C area Concentration

0.05218 2.6515

0.06056 2.7034

0.05285 2.8459

0.04844 2.3475

0.04837 2.1859

0.04645 2.0791

35

Peak D area Concentration

0.01214 0.6171

(-1.956*)

(-7.329*)

(11.47*)

(17.56*)

(21.59*)

0.0143 0.6393 (-3.594*)

0.01238 0.6667 (-8.031*)

0.01200 0.5817 (5.74*)

0.01364 0.6163 (0.126*)

0.01351 0.6047 (2.02*)

* Percent reduction in concentrations between source water and water after separated ozone or PAC treatment

Table 4 The DOC removal of Peaks A to D of separated PAC and ozone treatments by UF using peak-fitting Sample

Parts of the HPSEC-OCD chromatogram Peak A

Peak B

Peak C

Peak D

Source

81.5*

-0.2*

10.18*

17.94*

3**

78.82*

13.66*

4.42*

36.4*

7**

57.47*

-27.8*

-4.7*

1.29*

20**

78.38*

31.33*

5.24*

18.68*

50**

82.99*

3.34*

0.035*

41.83*

100**

86.31*

28.62*

17.73*

23.44*

*Percent reduction in peak concentrations between feed and permeate. 36

** Separated PAC and ozone doses (mg/L) for pretreatment

Highlight 1. Membrane fouling by combined ozone and PAC pretreatment was evaluated. 2. The combined treatment at higher ozone doses gave a substantially potential on TMP control. 3. Macro molecules biopolymers had high correlations with TMP/TMP0 increase.

Graphichal Abstract

37

a

b

a

b

a

b

A

B

C

NOM

With Ozone

With Ozone +PAC

high MW humic-like organics protein Polysaccharide low MW organics low MW humic-like organics low MW organics low MW neutrals