This study investigated the efficacy of using micro-flocculation as a pretreatment approach in alleviating ultrafiltration (UF) membrane fouling caused by organic matter in treated wastewater. Three typical model dissolved organic matters (DOM), humic acid, fulvic acid, and sodium alginate, were employed to simulate membrane fouling. The results showed that micro-flocculation using poly aluminum chloride (PAC) or polymerized ferric sulfate (PFS) as flocculant could effectively enhance the treatment performance of the UF process on DOM. With 6 mg/L PAC, the removal efficiency of humic acid, fulvic acid, and sodium alginate by micro-flocculation combined UF process reached 79.95%, 63.25%, and 51.14%, respectively. Specifically, after micro-flocculation, micromolecular hydrophilic organic matter (e.g., fulvic acid) tended to form a compact cake layer. The macromolecular hydrophobic organic matter (e.g., humic acid) and macromolecular hydrophilic organic matter (e.g., sodium alginate) generally led to a loose cake layer. At PAC dosage of 6 mg/L, the membrane specific flux (J/J0) at the end was improved by 11.71%, 10.27%, and 2.2% for humic acid, sodium alginate and fulvic acid solutions, respectively, compared with the UF process alone. It could be inferred that micro-flocculation pretreatment can effectively mitigate the membrane fouling when treating wastewater containing humic acid, sodium alginate, or fulvic acid.
Micro-flocculation could enhance the capability of the UF process in removing DOM.
Micro-flocculation pretreatment effectively alleviated UF membrane fouling.
Cake layer played an important role in mitigating UF membrane fouling.
The compactness of cake layer was related to the molecular weight, hydrophilicity or hydrophobicity of organic matters.
Ultrafiltration (UF) is a reliable and promising technology owing to its guaranteed removal of suspended particles, colloids, and part of macromolecules for water treatment (Du et al. 2019; Takeuchi & Tanaka 2020). However, long-term operation of UF will cause membrane fouling (Ahmad et al. 2020), which is induced commonly by formation of a cake layer, concentration polarization, and membrane pore blocking (Wang et al. 2018a, 2018b; Bu et al. 2019). The UF membrane fouling is attributed to organic fouling and inorganic fouling (Wang & Tang 2011). Compared to inorganic membrane fouling, which is reversible and can be eliminated through permeate backwashing, the organic fouling resulting from dissolved organic matter (DOM) is a fatal inducement to UF membrane, which is tightly bounded to the membrane as irreversible fouling (Schulz et al. 2016; Wu et al. 2021).
The characteristics of DOM include hydrophobicity, molecular size, charge and solution chemistry, etc. which are likely to influence the formation of UF membrane fouling (Michael-Kordatou et al. 2015). However, there is a lack of consensus on the contribution of DOM fractions towards UF membrane fouling. Liu et al. (2014) found that the foulants larger than 1.2 μm had little effect on flux decline, but the molecular weight fraction between 100 kDa and 0.45 μm caused the most serious fouling. Some studies demonstrated that stronger hydrophobic parts were more responsible for UF membrane fouling than hydrophilic parts, but others reported that hydrophilic substances had a higher fouling potential (Shon et al. 2006; Tang et al. 2010). Tian et al. (2018) reported that reducing the proportion of macromolecular compound and improving the hydrophilic fractions can be effective approaches to mitigate membrane fouling.
A variety of pre-treatment technologies have been proposed and investigated to overcome the fouling issues during the UF treatment process, including coagulation (Bu et al. 2019), adsorption, and pre-oxidation (Cheng et al. 2016; Yu et al. 2018). Due to low cost, efficient colloidal removal, and reliable operation, the coagulation pretreatment is a widely used process (Yu et al. 2019). However, the coagulation process often involves a large number of coagulant dosages, a stirring process and a sedimentation process. Specifically, micro-flocculation is a kind of on-line coagulation pretreatment, with few dosages of flocculants, formation of micro-flocs (adsorbed to organic matter and retained by membrane) and without sedimentation, which can be directly combined with UF technology to remove organic matter and improve the membrane flux (Wang et al. 2018a, 2018b; Lee et al. 2021). In addition, micro-flocculation combined with UF process also has the advantages of removing low molecular weight organic matter and trace organic matter (Wang et al. 2018a, 2018b; Li et al. 2020). However, the difference of floc properties can lead to different degrees of membrane fouling (Feng et al. 2015). Also, the type of flocculants and operational parameters could affect the floc properties (Du et al. 2019).
Consequently, this paper aims to investigate the effects of micro-flocculation pretreatment on the fouling of UF membrane induced by different features of DOM. Three model DOM fractions, namely, humic acid (i.e., macromolecular and hydrophobic component), fulvic acid (i.e., micromolecular and hydrophilic component), and sodium alginate (i.e., macromolecular and hydrophilic component) were used to simulate the different influent quality of the micro-flocculation combined UF process. The looseness or closeness of the flocs have been analyzed in terms of different particle size distribution. Afterwards, UF membrane fouling behavior was evaluated through the measurement of the membrane permeate flux.
MATERIALS AND METHODS
Three model DOM in water samples were simulated. These were humic acid (81.7%, Sinopharm Chemical Reagent Co., Ltd), fulvic acid (85%, Shanghai Aladdin Biochemical Technology Co., Ltd), and sodium alginate (Shanghai yuanye Bio-Technology Co., Ltd). Humic acid and fulvic acid stock solutions were prepared by 0.1 mol/L NaOH solution and then filtered with 0.22 μm membrane to remove insoluble ash. Sodium alginate stock solution (1 g/L) was prepared by ultra-pure water. The stock solutions were diluted as needed. The turbidity in solution was regulated by Kaolin (CNBM Geological Engineering Exploration Academy Co., Ltd), controlled in 10.0 ± 0.5 NTU. The pH was adjusted to 7.0 ± 0.2 by 0.1 mol/L HCl or NaOH. All the water used in the experiment was ultra-pure water. The water samples were stored at 4 °C in dark conditions.
Polymerized ferric sulfate (PFS) and poly aluminum chloride (PAC) (≥98%, Aladdin Industrial Corporation, Shanghai, China) were used as the flocculants. The micro-flocculation experiment was conducted in a ZR4-6 coagulation experimental mixer (Zhongrun Water Industry Technology Development Co., Shenzhen, China) at 25 °C. The experimental procedure was as follows: mixing at fast speed of 200 r/min for 30 s after adding the flocculant (PAC or PFS), and mixing at slow speed of 40 r/min for 15 min. Afterwards, the experiments were conducted using two approaches: (1) immediately taking water samples for UF process (shown in Section 2.3) or floc size test; (2) settling for 30 min and taking the appropriate volume of water samples to measure the dissolved organic carbon (DOC).
UF process test
The UF process was simulated using a cup ultrafilter (Amicon 8400, Millipore, USA) with a capacity of 400 mL. The pressure was provided by nitrogen and was maintained at 0.10 MPa. A circular regenerated cellulose membrane (Millipore, plgc07610, USA) was used. The diameter was 76 mm, the effective membrane area was 41.8 cm2, and the molecular weight cutoff was 10 kDa. The electronic balance was connected to the computer, and the mass of accumulated filtrated water was recorded in real time at intervals of 60 s. The membrane flux (J) was calculated by cumulative water output. The new membrane was immersed in ultra-pure water for at least 24 h, in which the water was changed every 8 h to remove the protective substances on the membrane surface.
The ultra-pure water was filtrated under 0.10 MPa until the effluent flux was stable, which was the initial flux of membrane (J0), then the water samples after micro-flocculation were filtrated under the same conditions, which was the permeate flux of membrane (J). The normalized specific flux J/J0 was used to represent the membrane performance as a function of time. After the UF experiment, the permeate was collected and the DOC, turbidity and pH in permeate were further measured.
Turbidity and pH were measured using a 2100Q turbid meter and PHC101 pH meter (Hash, USA), respectively. DOC was measured using a total organic carbon analyzer (Vario-TOC, GER). The Zeta potential of the solution was tested by nano sizer and Zeta-potential tester (Malvern Zetasizer Nano ZS90, UK).
The surface morphology of UF membrane was observed with a scanning electron microscope (SEM, ZEISS Gemini 300, GER). Before SEM observation, samples were dried in a desiccator (45 °C) for 72 h. The surface roughness and morphology of floc deposition layer were measured by atomic force microscope (AFM, Olympus LEXT OLS4500 JPK NW4, JPN). The hydrophilicity of the fouled membranes was measured by a dynamic contact angle tensiometer (Biolin Theta Flex, SWE).
RESULTS AND DISCUSSION
Performance of the micro-flocculation combined UF process
The effects of different flocculants of PAC and PFS in the micro-flocculation combined UF process on the removal of organic matter in water samples are shown in Figure 1(a) and 1(b), respectively. The changes of pH in the micro-flocculation combined UF process with different flocculants were evaluated, as shown in Figure 2. The removal rates of humic acid, fulvic acid, and sodium alginate were 83.52%, 56.67%, and 43.66%, respectively, with direct UF process. As for PAC-based micro-flocculation combined UF process, the highest DOC removal efficiency was found at PAC dosage of 1 mg/L, and the removal efficiency on humic acid, sodium alginate, and fulvic acid was 86.39%, 86.75%, and 64.90%, respectively. However, with the further increase of PAC dosage (2–6 mg/L), the removal efficiency of the three model DOM was reduced and the pH of the solution had a gentle decrease after the reaction, which could be explained by the charge neutralization between negatively charged natural organic matter and positively charged aluminum hydrolysates (Wang et al. 2020). Particularly, at PAC dosage of 6 mg/L, the removal efficiency of humic acid, sodium alginate, and fulvic acid was 79.95%, 63.25%, and 51.14%, respectively. Deng et al. (2019) discovered that pretreatment is ineffective in eliminating micromolecular or hydrophilic components at the PAC dosage of 10 mg/L. Likewise, the addition of PFS as flocculant presents a similar tendency on the removal of the three model DOM. At the PFS dosage of 1 mg/L, the DOC removal efficiency of humic acid, sodium alginate, and fulvic acid was 85.55%, 70.18%, and 63.43%, respectively, while at the PFS dosage of 6 mg/L, the removal efficiency of the humic acid, sodium alginate, and fulvic acid was 84.31%, 59.82%, and 54.66%, respectively. After adding PFS as flocculant, the pH of the solution also showed a trend, similar to the effect of PAC.
Figure 1(c) and 1(d) show the changes of the effluent turbidity after the micro-flocculation combined UF process. With the increase of the flocculant dosage, the turbidity removal tended to be stable. Compared with the direct UF process, micro-flocculation prior to UF contributed slightly to the improvement on removal of turbidity of the effluent. The effects of PFS and PAC on turbidity removal were similar. Specifically, the effluent turbidity of water samples was lower than 0.8 NTU. Nevertheless, when the flocculant was added continuously, negative impacts on water turbidity may occur, since the flocs with large and the same charge were difficult to settle (Shen et al. 2020).
In summary, when the flocculants were added, the UF process could have a better removal efficiency on the three model DOM, which indicated that the micro-flocculation facilitated the removal of organic pollutants and hydrophobic ones were more easily removed by micro-flocculation UF than hydrophilic ones. However, the higher flocculant dose could not further improve removal efficiency. Moreover, the regenerated cellulose membrane is a hydrophilic membrane which is beneficial to the DOC removal in terms of hydrophobic organic matter (Yu et al. 2018).
In this study, it was found that the PAC with a dosage of 6 mg/L had the worst removal effects on three model DOM with the change of flocculants and its dosages, where there may have serious membrane fouling. Thus, PAC with a dosage of 6 mg/L was chosen to further study the influence of micro-flocculation on the pollution of the UF membrane.
Effect of micro-flocculation on the UF membrane flux
The influence of the flocs on membrane filtration behavior, membrane specific flux of suspensions with filtration time under constant pressure is shown in Figure 3.
The membrane-specific flux for humic acid is shown in Figure 3(a), in which a gentle decrease from 0.8405 to 0.8126 was observed within 540 min UF cycle and the attenuation rate was 3.32%, which suggested that the humic acid was more likely to block the membrane pores at the beginning of UF. When the dosage of PAC was 6 mg/L, initial and ending membrane-specific flux was 0.9466 and 0.9204, respectively, and the attenuation rate was 2.8%, showing a gentle decrease. In addition, the membrane-specific flux at the end was improved by 11.71% compared with UF alone, which indicated that the micro-flocculation had a positive effect on alleviating the membrane fouling. This may be caused by the cake layer produced by the combination of flocculant and the organic matters, then sedimentation on the surface of the UF membrane effectively prevented direct contact between the pollutants and the UF membrane (Wang et al. 2019; Wang et al. 2020), which slowed down the fouling caused by the absorption and blockage of membrane pores (Yu et al. 2018).
Figure 3(b) presents the membrane-specific flux for fulvic acid after adding PAC as the flocculant. The flux attenuated from 0.9736 to 0.8766 and the attenuation rate was 9.96%. Compared with the UF alone, the initial membrane-specific flux increased by 0.0721 but the attenuation rate increased by 5.05%. Although the membrane-specific flux at the end was improved by 2.2% compared with UF alone, the degree of membrane fouling was exacerbated and the PAC may have a negative impact on mitigating the membrane fouling in fulvic acid solution. That could be explained by the fact that, owing to the compression of the electric double layer, the negatively charged fulvic acid becomes coiled (Shen et al. 2020), which induced formation of a cake layer with a dense structure, which could be confirmed from Table 1. Fulvic acid has the largest floc fractal dimension. In addition, the formed small flocs produced by micromolecular fulvic acid are likely to block the membrane pore or the pores of the cake layer, resulting in a faster rate of membrane fouling.
|Dissolved organic matter .||Floc fractal dimension .|
|Dissolved organic matter .||Floc fractal dimension .|
As shown in Figure 3(c), the membrane-specific flux of UF decayed from 0.8682 to 0.7656, where the attenuation rate was 11.82%. When the dosage was 6 mg/L, the membrane-specific flux attenuated from 0.9570 to 0.8532, and its attenuation rate was 10.85%. The membrane-specific flux at the end was improved by 10.3% compared with UF alone. This indicated that adding PAC could reduce membrane fouling that was the result of sodium alginate. This is related to the loose flocs formed by fulvic acid (Xiong et al. 2019), as shown in Table 1. However, the membrane-specific flux at the end was still at a relatively low level. This may be because the uniform gel-like substance with low permeability formed by the terminal carboxyl of sodium alginate preferentially formed coordination effect with metal ions. In addition, as a hydrophilic organic matter, it could be difficult to remove by a hydrophilic membrane, which was similar to fulvic acid.
For the three DOM, raw water without pretreatment may cause serious membrane fouling. The micro-flocculation as a pretreatment approach prior to UF membrane could, indeed, alleviate the membrane fouling issue, and the removal effects on macromolecular components (e.g., humic acid and sodium alginate) were better than those of micromolecular components (e.g., fulvic acid). Notably, the hydrophilic organics (e.g., fulvic acid and sodium alginate) were more likely to cause membrane fouling on hydrophilic membranes than hydrophobic organics (e.g., humic acid).
The floc properties on the UF membrane fouling
Some studies found that most flocs could accumulate on the membrane surface and lead to the formation of cake layer, which was beneficial to the removal of contaminants (Wang et al. 2019). However, flocs close to the membrane pores may cause serious membrane fouling due to pore clogging (Yu et al. 2017). Flocs that are significantly larger than the membrane pore size are unlikely to cause significant membrane fouling (Guo et al. 2012). Therefore, the floc size distribution and the floc fractal dimension after micro-flocculation were analyzed to indirectly reflect the nature of membrane fouling.
The analytical results are shown in Figure 4. After micro-flocculation, the floc size distribution formed by humic acid, fulvic acid, and sodium alginate was 0–32 μm (Figure 4(a)), 0–12 μm (Figure 4(b)), and 0–40 μm (Figure 4(c)), respectively. The average particle sizes of the formed flocs were sodium alginate (7.54 μm), humic acid (4.99 μm), and fulvic acid (3.12 μm) in descending order. The floc fractal dimension represents the degree of looseness and compactness (Yao et al. 2015; Wang et al. 2018a, 2018b). Also, the smaller the floc fractal dimension, the looser the flocs formed. As shown in Table 1, the floc fractal dimension produced by humic acid, fulvic acid and sodium alginate was 0.8786, 0.9765, and 0.8163, respectively, which showed that the flocs formed by fulvic acid were more compact than humic acid and sodium alginate.
This could be explained by the fact that owing to the relatively loose cake layer produced by humic acid compared with fulvic acid, the binding sites were available on the humic acid molecules and on the surface of the PAC (Wang et al. 2018a, 2018b), which caused the adsorption of humic acid in the cake layer (Wang et al. 2019) and the high DOC removal efficiency of humic acid (Figure 1(b)). Cho et al. (2006) discovered that small flocs could aggravate membrane fouling. The compact flocs formed by fulvic acid could cause the cake layer to become less porous and highly resistant on the surface of UF membrane (Shen et al. 2020). This could explain the phenomenon that the tight cake layer was not conducive to DOC removal. According to previous research, the formation of loose and multi-branched flocs helps to form a porous and fluffy cake layer and alleviate membrane fouling produced by sodium alginate (Roth et al. 2015). This conclusion was consistent with the phenomenon shown in Figure 3(c).
In summary, the average particle size of the flocs formed by the macromolecular component was significantly larger than that of the micromolecular component. Moreover, the micromolecular components were more inclined to form compact flocs, while macromolecular components were more likely to form loose flocs.
The zeta potential of the flocs was examined to evaluate the ability of the flocculant to destabilize the particles (Dayarathne et al. 2020). According to Figure 5, within the selected PAC dosage (1–6 mg/L), the zeta potentials of the three model DOM were all negative, and reached the lowest at the PAC dosage of 6 mg/L. It was observed that the zeta potentials in humic acid, fulvic acid, and sodium alginate samples varied from −47.4 mV to −44.7 mV, −28.7 mV to −26.2 mV, and −61.0 mV to −47.9 mV, respectively. The decrease in negative zeta potential indicates a decrease in repulsive electrostatic forces (Chekli et al. 2017). This indicated that charge neutralization may be the main mechanism in the flocculation of DOM by PAC (Wang et al. 2019; Shen et al. 2020).
Characterizations of the cake layer on UF membranes
Membrane surface morphology
The surface topographies of pristine and fouled membranes were characterized using SEM, and the images are shown in Figure 6. It could be observed that the pristine membrane exhibited a clean and smooth surface without any foulants accumulated (Figure 6(a)). After the direct UF process of different DOM solutions, the membrane surface morphologies became different due to the accumulation of the foulants. However, as can be seen from Figure 6(b) and 6(f), the membrane fouled by humic acid and sodium alginate solutions without pretreatment did not show obvious variations. Comparatively, as shown in Figure 6(d), a dense and compact fouling layer was present on the membrane surface with fulvic acid solution. According to the results of membrane-specific flux of humic acid and sodium alginate, the formation of internal fouling in membrane was inferred (Wang et al. 2019; Wan et al. 2021).
After the micro-flocculation, humic acid was deposited and a relatively dense cake layer was formed on the membrane surface (Wu et al. 2020), where the obvious cracks appeared after the drying procedure, as shown in Figure 6(c). However, the membrane surface of the pretreated fulvic acid and sodium alginate showed a smoother, dense, and without obvious porous structure (Xiong et al. 2019; Long et al. 2021).
Combining the experimental results of floc fractal dimension and membrane flux, it could be inferred that sodium alginate forms a more permeable and loose cake layer on the membrane surface, which could effectively reduce membrane fouling (Zhao et al. 2020). In addition, the humic acid showed similar results. Although the SEM images of fulvic acid and sodium alginate were relatively similar, the cake layer produced by fulvic acid was more compact and uniform. This may be explained by the fact that fulvic acid, as micromolecular organic matter, easily led to the small size of the formed flocs. Combined with the results shown in Figure 3(b) that fulvic acid solution was associated with a larger membrane flux attenuation rate, it could be inferred that some small flocs blocked the membrane pores and hindered the removal of fulvic acid.
Contact angle of fouled membranes on different DOM
The contact angle is used to evaluate the hydrophilicity and hydrophobicity of the material. The small membrane contact angle corresponds to a high hydrophilicity (Thomas et al. 2014).
As can be seen in Table 2, the contact angle of the pristine membrane was 30.4°, indicating that regenerated cellulose was hydrophilic membrane (Tlili & Alkanhal 2019). After filtering the humic acid, fulvic acid, and sodium alginate solution, the contact angle of fouling membrane increased apparently to 69.9°, 61.5°, and 41.2°, respectively (Figure S1 and Table 2). This happened because the surface of the membrane became more hydrophobic due to the formation of the fouling on UF membrane. According to previous studies, the hydrophobicity of membrane surface is closely related to the presence of aromatic compound, and high aromatic compound content could enhance the hydrophobicity (Mu et al. 2019). However, compared to the UF process alone, after employing micro-flocculation as pretreatment, the contact angles became smaller, with 57.6°, 39.3°, and 40.2° in humic acid, fulvic acid, and sodium alginate solutions, respectively. It can be inferred that the micro-flocculation pretreatment improved the hydrophilicity of cake layer. Some studies have suggested that the formation of the hydrophilic cake layer is beneficial to reduce membrane fouling (Sun et al. 2018).
|.||Ultra-pure water .||Humic acid .||Fulvic acid .||Sodium alginate .|
|PAC + UF||57.6||39.9||40.2|
|.||Ultra-pure water .||Humic acid .||Fulvic acid .||Sodium alginate .|
|PAC + UF||57.6||39.9||40.2|
Surface roughness and morphology of floc deposition layer
The roughness of surface is an important factor affecting the membrane fouling resistance, which is represented by Ra (Yu et al. 2019). The surface morphology of the pristine membrane and the contaminated film were analyzed by AFM (Li et al. 2021), as shown in Figure 7.
The convex structures on the surface of pristine membrane (Figure 7(a)) were uniformly distributed, with an Ra of 3.3. After UF with humic acid alone, the surface roughness of the membrane increased. When PAC was added, the protrusions on the surface area increased (Figure 7(c)), with an Ra of 27.7. According to Figure 3, the steady-state membrane-specific flux of raw water was lower than that with the addition of PAC, which indicated that the cake layer became more porous. The SEM results also confirmed the findings. After filtrating fulvic acid directly, the surface roughness of the membrane increased significantly, with an Ra of 54.7, and a large amount of sediment adhered to the membrane surface. With the addition of PAC, it was observed that the cake layer became tighter (Figure 7(e)), the Ra decreased to 30.5, and the steady-state membrane had a larger attenuation rate. This is consistent with the results obtained in Table 2. Interestingly, after filtrating the sodium alginate solution directly (Figure 7(f)), the surface roughness of the membrane was smaller than pristine membrane and the Ra was 1.5. Combined with SEM findings, it could be concluded that the convex structure on the pristine membrane surface was smoother and denser. When treating sodium alginate solution, despite the fact that the membrane surface was covered and wrapped by contaminants, little variation on surface roughness was observed since the sodium alginate solution is viscous.
Basically, after adding PAC, Ra of three DOMs increased significantly, and the steady-state membrane-specific flux was higher than raw water. This implied that the membrane-specific flux was related to the roughness of the membrane surface.
This study focused on the removal efficiency of DOM and the change of membrane performance induced by different organic matters through micro-flocculation pretreatment. The micro-flocculation pretreatment was found to be effective to enhance the capability of the UF process in removing DOM. The hydrophilic membranes were able to achieve better removal efficiency on hydrophobic organic matters over hydrophilic organic matter. During the sole UF process, the membrane pores were easily clogged by DOM, but after applying micro-flocculation as the pretreatment approach, the increase of membrane-specific flux was observed, suggesting that the cake layer could alleviate membrane fouling. In addition, feed solution with micromolecular hydrophilic organic matter (e.g., fulvic acid) was more inclined to induce a compact cake layer. Noticeably, a relatively loose cake layer was formed by macromolecular hydrophobic organic matter (e.g., humic acid) or macromolecular hydrophilic organic matter (e.g., sodium alginate). The hydrophilic organics (e.g., fulvic acid and sodium alginate) were more likely to cause membrane fouling on hydrophilic membranes over hydrophobic organics (e.g., humic acid). In addition, the micro-flocculation could improve the hydrophilicity of the cake layer. Overall, incorporating micro-flocculation as a pretreatment prior to UF plays an important role in alleviating the membrane fouling of the UF membrane and improving the removal of organic matter during the UF treatment process.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.