Application of ultrafiltration in a paper mill: Process water reuse and membrane fouling analysis

Chen, C., Mao, S., Wang, J., Bao, J., Xu, H., Su, W., and Dai, H. (2015). "Application of ultrafiltration in a paper mill: Process water reuse and membrane fouling analysis," BioRes. 10(2), 2376-2391.

Abstract

High water consumption is a major environmental problem that the pulp and paper industry is facing. Ultrafiltration (UF) can be used to remove the dissolved and colloidal substances (DCS) concentrated during the recycling of white water (the process water) to facilitate the reuse of white water and reduce fresh water consumption. However, membrane fouling limits the application of UF in this industry. In this study, super-clear filtrate obtained from a fine paper mill was purified with a polyethersulfone (PES) ultrafiltration membrane to evaluate the reuse performance of the ultrafiltrate. The membrane foulants were characterized by scanning electron microscopy, energy-dispersive spectrophotometry, attenuated total reﬂection-fourier transform infrared spectroscopy, and gas chromatography-mass spectrometry. The results indicate that the retention rate of stock and the strength properties of paper increased when the ultrafiltrate was reused in the papermaking process compared to when super-clear filtrate was used. The reversible membrane foulants during ultrafiltration accounted for 85.52% of the total foulants and primarily originated from retention aids, drainage aids, and wet strength resins, while the irreversible adsorptive foulants accounted for 14.48% and mostly came from sizing agents, coating chemicals, and others. Moreover, the presence of dissolved multivalent metal ions, especially Ca2+, accelerated membrane fouling.

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Application of Ultrafiltration in a Paper Mill: Process Water Reuse and Membrane Fouling Analysis

Chen Chen,^a Shengtao Mao,^a Jingjing Wang,^a Jinfeng Bao,^b Hui Xu,^a Wenpeng Su,^a and Hongqi Dai ^a,*

Keywords: Paper mill white water; Reuse performance; Dissolved and colloidal substances (DCS); Foulant characterization; Foulant identification; Ultrafiltration

Contact information: a: Department of Pulp and Paper Science and Technology, Nanjing Forestry University, Nanjing Jiangsu, 210037, P. R. China; b: Zhejiang Creation Environment Technology Co. Ltd., Hangzhou Zhejiang, 310000, P. R. China; *Corresponding author: hgdhq@njfu.edu.cn

INTRODUCTION

The exploding world population and industrial establishment has accelerated economic growth in the 21st century, creating global water pollution and water shortage issues. The pulp and paper industry is water use-intensive (Buyukkamaci and Koken 2010), ranking third after the metal and chemical industries in terms of worldwide fresh water consumption (Kaya et al. 2010). Water consumption by this industry reaches 15 to 60 m³ per finished tonne of paper. High fresh water consumption and the accumulation of contaminants during the paper manufacturing process contribute to the large volume of effluent from paper mills. It has been estimated that the effluent of the pulp and paper industry is responsible for 50% of all industrial wastes (Zhang et al. 2009), and that the effluent volume and chemical oxygen demand (COD) discharged by this industry can reach 38.5 m³ and 7.5 kg per tonne of paper, respectively. Recently, the pulp and paper industry has been moving toward more intensive recycling of white water to reduce fresh water uptake and satisfy tightened discharge standards. However, white water cannot be recycled easily because dissolved and colloidal substances (DCS) and electrolytes become enriched with greater white water recycling (Yuan et al. 2011; Hubbe et al. 2012), adversely affecting paper machine runnability and paper quality (Francis and Ouchi 2001; Whipple and Maltesh 2002; Zhang et al. 2007).

The DCS in white water mainly comes from fiber extractives and the chemical additives consumed during manufacturing operations (Hubbe et al. 2012). They are also called “anionic trash” because of their negative charge (Miao et al. 2012). The DCS are generally stably dispersed in the pulp suspension or white water, easily gathering together and forming non-ionic stickies when operating conditions change; they can also react with electrolytes, especially Ca²⁺ and Mg²⁺ (Ordonez et al. 2010; Hubbe et al. 2012), which interferes with papermaking because the stickies cannot be effectively controlled by normal chemical methods. Most paper mills employ disc filters to recycle white water fines as much as possible, gaining a certain amount of super-clear filtrate to replace fresh water during paper production. However, DCS and electrolytes cannot be eliminated by disc filters (Ordonez et al. 2010). Thus, advanced water treatment technologies are required.

Membrane technologies such as microﬁltration (MF), ultraﬁltration (UF), nanoﬁltration (NF), and reverse osmosis (RO) are becoming more widely applied in process water and efﬂuent puriﬁcation with the aim of water recycling in the pulp and paper industry (Mänttäri et al. 2006; Jönsson et al. 2008; Beril Gönder et al. 2012; Vergili et al. 2012) because such technologies offer a high level of contaminant removal with relatively little energy consumption. Moreover, such technologies require small ﬂoor space and are easy to ﬁt into existing mill water circuits (Zhang et al. 2009). Currently, UF can be used to remove DCS from white water and render the permeate clean enough to replace fresh water as paper machine shower water (Nuortila-Jokinen et al. 2004). The permeate can otherwise be reused if it is RO-treated to remove salts (Zhang et al. 2009). However, membrane fouling limits the use of UF at a large scale in pulp and paper mills (Puro et al. 2011). According to some studies, DCS (especially fatty and resin acids) (Carlsson et al. 1998; Puro et al. 2002), Ca²⁺(Li and Elimelech 2004; Abrahamse et al. 2008), and Mg²⁺(Uyak et al. 2014) are enriched in process water and can complex, contributing to pore blocking, cake layer formation, and concentration polarization during ultrafiltration (Wang et al. 2013), ultimately reducing filtration efficiency. Currently, UF technology can only be used to filter paper mill effluent that has been pre-treated and meets discharge standards. It would be difficult for pulp and paper mills to afford such great expense in treating such large effluent volumes in the long run.

Research has demonstrated the membrane fouling mechanism (Nuortila-Jokinen and Nyström 1996; Costa et al. 2006; Kaya et al. 2010; Puro et al. 2010; 2011; Peter-Varbanets et al. 2011; Zhang et al. 2012a) and prevention methods (Costa et al. 2006; Pinelo et al. 2009; Park et al. 2010; Zhang et al. 2012a), but the characterization and identification of specific membrane foulants is relatively rare because of the complex composition of white water (Cho et al. 1998; Puro et al. 2002; Her et al. 2007). Furthermore, most existing studies have involved ﬁltering of simulated white water (Elefsiniotis et al. 1997) or model foulants (Ramamurthy et al. 1995) and evaluated the fouling behavior only by ﬂux and retention measurements. Hence, it is desirable to determine the specific white water fractions responsible for membrane fouling so that measures can be taken to remove them. For example, some anionic components of DCS can be bound to fibers and removed from process water together with the wet sheet when using fixatives with relatively low molecular mass and high cationic charge density (Miao et al. 2012), and dissolved multivalent metal ions such as Ca²⁺ can be removed by a modified natural marcomolecular substance called anionized cationic starch (ACS) (Ye et al. 2009; Yuan et al. 2012). Such work could make it possible for pulp and paper mills to use UF and RO modules, which up to this point mainly have been used after the wastewater treatment plant. By using the modules earlier in the manufacturing process, it may be possible to benefit the safe and stable operation of a papermaking operation. Reverse osmosis permeate can be reused in the papermaking process because of the relatively low electrical conductivity, and the concentrated retentates of UF may be of value (Puro et al. 2011). Numerous environmental and economic benefits may be realized (Pizzichini et al. 2005) by reducing fresh water consumption and diminishing organic load for external water treatment (Kaya et al. 2010; Beril Gönder et al. 2012).

In this work, the super-clear filtrate obtained from a disc filter in fine paper mill was investigated to simulate the actual production situation. An ultrafiltration process was used to separate DCS from the white water, and the reuse performance of the ultrafiltrate was evaluated. Scanning electron microscopy (SEM), attenuated total reﬂection-fourier transform infrared (ATR-FTIR) spectroscopy, and SEM-energy-dispersive spectrophotometry (EDS) were employed to analyze the membrane fouling characteristics, and gas chromatography-mass spectrometry (GC-MS) was carried out to identify the composition and possible origin of membrane foulants. The aim of the present study was to offer ways to prevent the membrane fouling caused by white water from the fountainhead.

EXPERIMENTAL

Materials

Paper mill super-clear filtrate (obtained from a disc ﬁlter fed with white water) used as a feed for the ultrafiltration experiments was supplied by UPM (Changshu) Co. Ltd., China. The main properties of the feed are 412.76 mg/LCOD, pH 7.3, 1268 μS/cmelectrical conductivity, and 648 NTU turbidity. The feed was stored at 5 °C to preserve its characteristics. The pulp used in the experiment was obtained from the machine chest (UPM, China). The proportion of recycled, coated broke was 17%. A stirred, dead-end UF module (MSC-300 Mosu, China) with a temperature controller was used in this study. The membrane area of this ﬁlter was 0.00332 m², the volume of the feed vessel was 350 mL, and the pressure range was 0 to 3 bar. The UF membranes (Mosu, China) used were made of polyethersulphone (PES) with a nominal cut-off value of 10 kDa.

Methods

Filtration and adsorption procedures

The membranes were pretreated by rinsing three times in an ultrasonic bath with deionized water for 10 min to remove preservatives (Puro et al. 2011). A new piece of membrane was used in each ﬁltration. The deionized water and the super-clear filtrate were heated to 40 °C, a common temperature of paper mill waters.

The feed batch was 300 mL, and 225 mL of permeate was collected during the ﬁltration. Thus, 75 mL remained as concentrated liquor. The experiments were performed at pressures of 1 bar. In the adsorption tests, the UF membranes were soaked in the super-clear filtrate at 40 °C and no pressure was used, and the adsorption time was the same as in the ﬁltrations at 1 bar. Each experiment was replicated three times.

Water analysis and evaluation of ultrafiltrate reuse performance

The fresh water, super-clear filtrate, and ultrafiltrate samples were analyzed. The analysis equipment and methods are shown in Table 1.

Table 1. Equipment and Methods Used in Analysis of Water Samples

The drainage value and retention rate of the stock can be determined with a dynamic drainage jar (DDJ) (Mütek TM DFR-05 BTG, Germany). The pulp (1% oven-dry consistency, 1000 mL) was diluted with fresh water, super-clear filtrate, or ultrafiltrate samples. Chemicals including 1% cationic starch (CS; National Starch, USA), 0.025% cationic polyacrylamide (CPAM; Ciba, Switzerland) (1000 rpm, 1 min), and 0.05% colloidal silica (Eka Chemicals, Holland) (750 rpm, 20 s) were added, in the order listed, and the drainage value was recorded after stirring for 20 s. The temperature of the pulp was maintained at 20 °C. The retention rate was determined under the same conditions as above.

The moisture content of pulp was diluted to 0.2% with three water samples and handsheets were made according to TAPPI standard T205 sp-02 (2002) using a sheet-making apparatus (RK-2A PTI, Austria) with the same experimental conditions and doses of chemicals as above and 20% added ground calcium carbonate (GCC; Daheng, China) as filler. The tensile index, tearing resistance, folding endurance, Cobb value, and whiteness were tested following TAPPI standards T494 om-01 (2001), T414 om-98 (1998), T511 om-02 (2002), T441 om-98 (1998), and T560 wd-03 (2003), respectively.

SEM and SEM-EDS analysis of UF membranes

The SEM analysis was carried out with a Quanta 200 (FEI, USA) both before and after ﬁltration to provide detailed structural information regarding the membranes. The dried membrane samples were fractured under liquid nitrogen, coated with a thin layer of gold prior to analysis (Gönder et al. 2011), and examined at an accelerating voltage of 15 to 20 kV. The EDS was used to determine the inorganic foulants present on the membrane surface.

Isolation and characterization of membrane foulants

The membrane foulants were isolated from the fouled membranes by extraction. The UF membrane sample of 0.00332 m² was placed in a Soxhlet extractor and extracted with 50 mL of acetone-water solution (1:9, v/v) for 30 min. After extraction, all of the acetone solution was collected to be concentrated by evaporation and then dried in a nitrogen stream and further vacuum-dried at 40 °C. The samples were then weighed to determine the amount of foulants present in the extracted membrane area.

The samples described above may contain polymeric material from the membranes, potentially harming the gas chromatograph (GC) column. Therefore, the samples were diluted in tert-butylmethylether (MTBE) and extracted a second time with liquid-liquid extraction as described by Örså and Holmbom (1994) to remove any polymeric membrane material (Puro et al. 2011). A 1-uL MTBE solution obtained was injected into the GC-MS apparatus (Agilent 6890/5793, USA, equipped with NIST05 mass spectral libraries). The column (HP-5MS Agilent, USA) was a 25 m/0.25 mm I.D. wide-bore capillary with a nonpolar phase ﬁlm thickness 0.25 μm. The injector temperature was 260 °C. The initial column temperature was 150 °C and was programmed to increase to 230 °C at a rate of 7 °C/min starting 0.5 min after injection. The column then heated to 290 °C at 10 °C/min and maintained at that temperature for 10 min. The detector temperature was 290 °C.

To characterize the functional groups of the foulants, an ATR-FTIR apparatus (FTIR-650, China) was used to analyze the surface of both pristine and fouled membrane samples.

RESULTS AND DISCUSSION

Reuse Performance of Process Water Treated by Ultrafiltration

The ultrafiltrate of white water is clean enough to meet the demands of most positions of usage in the papermaking process because of the effective separation of macromolecular solutes and colloids. This study analyzed the quality of fresh water, super-clear filtrate, and its ultrafiltrate and determined the impacts of each water sample on stock and paper properties to evaluate the reuse performance of ultrafiltrate of super-clear filtrate. The results of the water analysis are shown in Table 2.

Table 2. Parameters of Fresh Water and Super-clear Filtrate Before and After Ultrafiltration

The quality of ultrafiltrate improved dramatically relative to the initial super-clear filtrate, as shown in Table 2. The COD of the ultrafiltrate decreased by 71.5% as compared to that of the super-clear filtrate, indicating that most organic contaminants in the feed were removed by UF treatment, although the value is still higher than that of fresh water. The turbidity and total solid content of super-clear filtrate were dramatically decreased by UF, which demonstrated the effective removal of macromolecular colloids (Yuan et al. 2011). Zhang et al. (2012b) found that the paper machine, the application of paper additives, and paper properties performed well when reusing white water with DCS below 900 mg/L. According to the results of this experiment (Table 2), the cationic demand of the ultrafiltrate decreased by 92.7% and the DCS content of the ultrafiltrate can be controlled to within this effective scope. However, the UF process cannot remove electrolytes; the slight drop in the conductivity of the ultrafiltrate may be due to the polyelectrolyte complexes of DCS with a few electrolytes, which can be removed by UF membranes. High conductivity limits the reuse of ultrafiltrate to some extent (Yuan et al. 2011), but it is possible for the ultrafiltrate to be reused at a position with higher demand if further treated by NF and RO.

Table 3. Impact of Fresh Water and Super-clear Filtrate Before and After UF on Stock and Paper Properties

The impact of each of the water samples on stock and paper properties are shown in Table 3. The retention rate of stock was increased greatly when the ultrafiltrate was reused to dilute pulp compared with the direct reuse of super-clear filtrate, and the value was close to that when fresh water was used. A recent study demonstrated that DCS adsorb onto the surface of cationic additives before fibers and can form deposits with electrolytes (especially Ca²⁺ or other multivalent ions), hindering connections between fibers and adsorption between fibers and additives (Yuan et al. 2011) and contributing to the loss of fines from the wire. The UF treatment can remove a large amount of the DCS present in super-clear filtrate, ultimately increasing the stock retention rate, the total retention rate, and the ash retention rate by 19.7 and 33.7%, respectively. The DCS have adverse effects on stock drainage, to some degree (Dai et al. 2008), but the reuse of ultrafiltrate resulted in only a minor increase in drainage value.

As shown in Table 3, the strength properties of the paper increased dramatically when the ultrafiltrate was used compared with the direct reuse of super-clear filtrate, mainly because the residual fillers and DCS in super-clear filtrate obstruct connections between fibers. Removing these contaminants with UF can increase the binding force between fibers and improve paper strength. The reuse of ultrafiltrate had a slight negative impact on the whiteness of the paper, and the whiteness of the paper made with super-clear filtrate was higher than that made with ultrafiltrate or fresh water. This is likely because the dyes absorbed on the surface of fillers and DCS remained in the super-clear filtrate.

SEM and EDS Analysis of Pristine and Fouled Membranes

To visualize the fouling on the UF membrane surface and pores caused by process waters, SEM images were generated before and after ﬁltration. Figure 1 shows the SEM images of both pristine and fouled membranes.

Fig. 1. SEM images of (a) pristine UF membrane surface; (b) cross-section of pristine membrane; (c) cross-section of pristine membrane; (d) fouled membrane surface fed with super-clear filtrate; (e) cross-section of fouled membrane fed with super-clear filtrate; and (f) cross-section of fouled membrane fed with super-clear filtrate

As can be seen from the SEM images in Fig. 1, the pristine UF membrane, with its asymmetric and compact cortical structure, had a clean, smooth surface (Fig. 1a). The sublayer inside the membrane had many finger-like holes (Fig. 1b) with homogeneous and compact pores on their walls (Fig. 1c); most macromolecular colloids can be retained by these pores. However, morphological variations occurred on the fouled membranes (Figs. 1d, 1e, and 1f). A thick fouling layer was formed on the surface of the membrane after ultrafiltration of the super-clear filtrate. Deposition can also be seen in the form of nodules and aggregates (Fig. 1d). The inside diameter of the finger-like holes visually decreased (Fig. 1e) and the size of pores clearly visible before ultrafiltration seemed to decrease, with some even disappearing as a result of pore blocking (Fig. 1f). Low-molecular weight materials entered into the holes during the earlier stage of ultrafiltration and adsorbed onto the hole walls or blocked the pores, inhibiting flux. Simultaneously, the macromolecular contaminants were retained on the surface of the membrane and gradually formed a cake layer as ultrafiltration continued. The results above agree with those of Kaya (2010) in the SEM measurements of a NF membrane fed with white water. According to recent studies, pore blocking can occur during the ﬁltration of pulp and paper mill wastewater by UF even at optimal operating conditions (Gönder et al. 2012).

Springer et al. (1985) reported that Na⁺, Mg²⁺, Ca²⁺, Cl^–, SO₄^2-, and some other ions accumulate during white water recirculation, and that these ions affect the ultrafiltration process. An EDS analysis was performed to detect the elements present on the surfaces and cross-sections of pristine and fouled PES membranes, in conjunction with the SEM observation, to explore the influence of metal ions on membrane fouling. The results and EDS spectra are presented in Table 4 and Fig. 2, respectively.

Table 4. Elemental Composition of Pristine and Fouled Membranes

Fig. 2. EDS spectra of (a) pristine membrane surface; (b) cross-section of pristine membrane; (c) fouled membrane surface; and (d) cross-section of fouled membrane

As can be seen in Table 4, carbon, oxygen, and sulfur were the main elements detected in the pristine PES membrane, which agrees with the elemental composition of the PES. However, the elemental proportion on the surface was different from that on the cross-section of the membrane, and trace amounts of sodium, calcium, and aluminum were detected in the pristine membrane, and these can be mainly attributed to the phase-inversion membrane preparation method. The carbon, oxygen, and sulfur contents in the fouled membrane fed with super-clear filtrate were different from those in the pristine PES membrane, and the sodium, calcium, magnesium, and iron contents increased, due to the coexistence of PES material and foulants including organic and inorganic materials.

It is clear that the presence of most metal elements on the membrane surface, especially calcium, increased drastically following ultrafiltration and that most were higher than those of the cross-section. Multivalent metal ions, especially Ca²⁺,can easily disturb the stability of DCS via a Ca-DCS complex and aggregate formation,thus being retained on the membrane surface (Bobacka et al. 1998). Meanwhile, electrostatic repulsion between anionic hydrophobic colloids and negatively-charged, hydrophilic PES membrane prevented the colloids from entering into membrane pores, thus contributing to the fouling layer on the membrane surface formed by the complexing of rejected colloids with metal ions in the feed (Kim and Jang 2006). Sodium ions (Na⁺), present at relatively high concentration in the super-clear filtrate, existed in a free form so they cannot be removed by ultrafiltration and only an insignificant amount of sodium was detected in the fouled membrane. The results suggest that membrane fouling because of the instability and aggregation of DCS can theoretically be prevented by controlling the content of multivalent metal ions, especially Ca²⁺,in the white water.

ATR-FTIR Analysis of Pristine and Fouled Membranes

The major or representative functional groups of the pristine and fouled membrane samples were analyzed with ATR-FTIR spectroscopy and are compared in Fig. 3.

Fig. 3. ATR FTIR-spectra of pristine and fouled Fig. 4. Content of UF membrane PES membranes fed with super-clear filtrate foulants

As is shown in Fig. 3, the pristine membrane exhibited IR peaks representing aromatic double bonded carbons (3320 cm^-1), aromatic bands (1643 cm^-1), and the =C–H bonds of p-disubstituted benzene (841 cm^-1), suggesting the presence of an aromatic ring structure within the membrane material. The above-mentioned peaks, as well as the distinct absorption peaks ascribed to the stretching vibration of =C–O–C from aromatic ether (1040 cm^-1) and to the symmetric vibration of SO₂ groups (1272 cm^-1), all agree with the functional groups of PES material of the membrane (Cho et al. 1998). However, the peaks at 3320 and 1040 cm^-1 and the C–H bond peaks from methyl groups (around 2940, 2880, and 1400 cm^-1) disappeared in the spectra of the fouled membrane, resulting from the covering of the membrane surface with foulants. In addition, new peaks representing aromatic bands (around 1580 and 1490 cm^-1), the C–O bonds of hydroxy acids or the C–N bonds of amide group (1230 cm^-1), and the C-O vibrations of esters or acid anhydride (1160 cm^-1) appeared in the spectra of the fouled membrane, indicating trace residues of organic foulants from the super-clear filtrate, potentially originating from the polyacrylamide retention and drainage aids, internal sizing agents, coating latex, or elsewhere. The specific origin cannot be identified by ATR-FTIR analysis because of the complexity of the components in white water.

Quantitative Analysis and Identification of Membrane Foulants

Membrane fouling, consisting of both reversible and irreversible fouling, occurred during the ultrafiltration of white water. Reversible fouling was caused by pore blocking, concentration polarization, or a gel layer forming on the membrane surface and can be easily cleaned. Irreversible fouling resulted from adsorption of foulants onto membrane pores and can only be remedied by chemical cleaning, a key limitation of the UF membrane application.

Figure 4 illustrates that the total amount of membrane foulants present after filtration of super-clear filtrate was 1.506 g/m², of which 1.288 g/m² (85.52%)were reversible foulants and 0.128 g/m² were (14.48%) irreversible foulants.

The GC-MS can be used to determine the specific chemical components within the membrane foulants. Because the chemical additives consumed in UPM are already known, the possible origins of the component corresponding to each peak can be inferred from the GC-MS results and the amount of each component present can be calculated according to the relative peak area. According to the analysis of the main peaks in the total ion chromatograms (Figs. 5 and 6), the main components and possible origins of total foulants and adsorptive foulants are presented in Tables 5 and 6, respectively.

Fig. 5. The total ion chromatogram of total foulants Fig. 6. The total ion chromatogram of adsorptive foulants

Table 5. Main Components and Possible Origins of Total Foulants of UF Membrane

As shown in Fig. 5 and Table 5, the main causes of filtration membrane fouling were N-isopropyl-acrylamide and 1,1,4,7,7-pentamethyl diethylenetriamine. These had contents of 0.922 and 0.242 g/m², respectively, calculated according to their peak areas.

Table 6. Main Components and Possible Origins of Adsorptive Foulants of UF Membrane

It can be inferred from knowledge of the paper additives used that the N-isopropyl-acrylamide may come from the polyacrylamide, widely used as retention and drainage aid or a reinforcing agent by the paper industry. Diethylenetriamine is a main component in the organic synthesis of the PAE resin used as wet strength agent (Obokata and Isogai 2004), so the 1,1,4,7,7-pentamethyl diethylenetriamine likely originates from the inefficient production of PAE resins.

It can be clearly seen from Fig. 6 and Table 6 that the adsorptive foulants were complex, though their absolute contents were relatively low. Butylated hydroxytoluene may come from an oxidant for a polyester or resin; the dicyclopentadiene may be derived from unsaturated polyester resins released mainly by surface sizing agents. In addition, the plasticizer shown in Table 6 may come from coating latexes released by the coated broke recovery process or from sizing agents. The hexadecanoic acid may originate from dispersants, emulsifying agents, or internal sizing agents and the phenanthrene may come from polyester resin surface sizing agents or dyes.

The specific components of ultrafiltration membrane foulants from fine paper mill white water mostly orginate from the chemical additives used in the paper manufacturing process and their ineffective manufacture and use, rather than from wood extractives. This finding disagrees with the preceding results of a study of ultrafiltration of ground wood mill (GWM) circulation water (Puro et al. 2002).

CONCLUSIONS

The quality of ultrafiltrates improved significantly following UF of paper mill super-clear filtrate by PES membranes with a cut-off value of 10 kDa.
The DCS content in ultrafiltrate can be controlled to within a stable range, with slight decreases in conductivity, using ultrafiltration.
When ultrafiltrate was reused in the papermaking process, the retention rate of stock increased greatly over the value achieved with the direct reuse of super-clear filtrate and was close to the value achieved when fresh water was used.
The increase in the drainage value of stock following ultrafiltration was slight, whereas the paper strength properties increased dramatically and the whiteness was slightly reduced.
Reversible fouling was the major form of fouling during ultrafiltration of super-clear filtrate and included deposits of foulants on the membrane surface and pore blocking. Reversible membrane foulants accounted for 85.52% (1.288 g/m²) of the total membrane foulants, while irreversible adsorptive foulants accounted for 14.48% (0.218 g/m²).
The membrane foulants during the filtration of super-clear filtrate mainly originated from the chemical additives used in paper manufacturing process and their ineffective use. The reversible membrane foulants came mostly from retention and drainage aids, reinforcing agents, and wet strengthening resins whereas the irreversible adsorptive foulants (which play a pivotal role in limiting UF technology application) came mainly from sizing agents, coating chemicals, dispersants, and other sources.
Dissolved multivalent metal ions, especially Ca²⁺, accelerated membrane fouling via the formation of a Ca-DCS complex and the formation of aggregates that were retained on the membrane surface.
The key to preventing membrane fouling in the ultrafiltration of paper mill process water is reducing the dissolved multivalent metal ion content (especially Ca²⁺) and the amount of paper additives lost in the white water during pretreatment. Another helpful technique would be to choose membrane cleaning protocols according to the characteristics of the membrane foulants.
The insights gained from this study can make it possible for the pulp and paper industry to reduce its fresh water consumption and use UF, currently used following the waste water treatment plant, earlier in the manufacturing process.

ACKNOWLEDGMENTS

The authors are grateful for the support of the National Natural Science Foundation of China (Grant No. 31270614) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Article submitted: December 29, 2014; Peer review completed: February 9, 2015; Revised version received and accepted: February 20, 2015; Published: February 25, 2015.

DOI: 10.15376/biores.10.2.2376-2391