Variation in organic matter characteristics and mitigation of ultrafiltration membrane fouling by chlorine and UV/chlorine pre-oxidation in secondary effluent treatment

Article information

Environmental Engineering Research. 2024;29(5)

Publication date (electronic) : 2024 January 2

doi : https://doi.org/10.4491/eer.2023.624

Zhihua Sun ¹^,²^,^*, Lei Gao ¹^,^*, Dong Wang ¹^,²^,

, Lingjie Liu ¹^,², Chensheng Qiu ¹^,², Shaopo Wang ¹^,²

¹Tianjin Key Laboratory of Aquatic Science and Technology, Tianjin 300384, China

²School of Environmental and Municipal Engineering, Tianjin Chengjian University, Tianjin 300384, China

^†Corresponding author: E-mail: wangdong06@tcu.edu.cn, Tel: +86 18202679521, Fax: +86-022-23085117

These authors contributed equally to this study

Received 2023 October 10; Revised 2023 December 13; Accepted 2023 January 1.

Abstract

Membrane fouling caused by effluent organic matter (EfOM) limits its further application in wastewater reuse. In this study, the effect of UV/chlorine pretreatment on membrane fouling was investigated in treating secondary effluent by ultrafiltration (UF) process. The relation between organic matter changes and fouling alleviation after UV/chlorine pretreatment was also studied according to the molecular weight (MW) changes in various resin fractions derived from EfOM. Results showed that UV/chlorine pretreatment effectively alleviated irreversible fouling, whereas chlorine pre-oxidation primarily mitigated reversible fouling. UV/chlorine pre-oxidation reduced 18% of reversible membrane fouling and 38% of irreversible membrane fouling at a chlorine dosage of 8 mg/L, indicating better performance in membrane fouling mitigation than chlorine pre-oxidation. UV/chlorine pre-oxidation also decreased dissolved organic matter in the UF permeate. The hydrophobic acidic (HPO-A) fraction caused dominant membrane fouling, while the hydrophilic (HPI) fraction contained most of high MW organic matter. Pre-oxidation changed the polarity of organic matter in the HPO-A fraction and decomposed the organics of high MW in the HPI fraction, which alleviated membrane fouling. These results showed that UV/chlorine pre-oxidation was a prospective pretreatment process prior to UF in wastewater reclamation.

Abstract

Graphical Abstract

Keywords: Fractionation; Membrane fouling; Molecular weight; Pre-oxidation; Ultrafiltration; UV/chlorine

1. Introduction

Wastewater reclamation and reuse is an essential approach to solving the water shortage problem. The secondary effluent of wastewater (as well as brine) is still in poor water quality and cannot be directly used for potable water and industrial applications [1–5]. Ultrafiltration (UF) is extensively applied in the advanced and reclamation treatment of wastewater [6]. However, membrane fouling causes increased energy consumption, reduced filtration productivity, and shortened membrane lifetime in the advanced and reclamation treatment processes of wastewater [3]. Fouling limits the further application of UF process in wastewater reclamation [7].

Effluent organic matter (EfOM) is considered as the dominant membrane foulant in wastewater reclamation processes. EfOM is a complex mixture of dissolved organic matter with diverse functional groups, wide molecular weight distribution, and complex physicochemical characteristics [8]. The process of membrane fouling by EfOM primarily consists of surface adsorption, pore blocking, and cake formation [9]. Surface adsorption is related to surface interaction forces between foulant and membrane surface, such as electrostatic, hydrophobic, and van der Waals forces. Pore blocking depends on the size of organic matter. Cake formation is the process of cake building up layer by layer on the membrane surface.

Various methods are applied to avoid or mitigate membrane fouling, such as backwashing, chemical cleaning, pretreatment of feed water, and membrane material modification [9,10]. The pretreatment processes extensively used prior to the UF process include coagulation, adsorption, and oxidation [11,12]. These pretreatment methods effectively mitigate membrane fouling. However, some drawbacks cannot be ignored in their application. Coagulation effectively removes colloids and macromolecular organics, which may lead to reversible fouling in UF processes. Irreversible fouling caused by organics of low and medium molecular weight (MW) cannot be alleviated by coagulation [13]. Moreover, nanofiltration (NF) and reverse osmosis (RO) are usually applied after the UF process in wastewater reclamation treatment. The presence of metal ions from coagulants in the UF permeate water aggravates the fouling of the NF and RO processes [12]. Powdered activated carbon (PAC) adsorption shows effective fouling migration. PAC may aggravate membrane fouling due to the combined foulant of PAC and dissolved organic matter (DOM) [14].

Oxidation processes especially advanced oxidation processes (AOP) effectively alleviate membrane fouling. Oxidation processes can transform the DOM of high MW into DOM of low MW [15]. These processes can also change the hydrophilicity/hydrophobicity characteristics of DOM [16]. These changes in DOM affect the membrane fouling of UF. Ozone pre-oxidation has a limited effect on alleviating the membrane fouling of UF when the O₃ dosing is less than 0.05 mg O₃/mg dissolved organic carbon (DOC). Conversely, ozone pre-oxidation is effective in increasing membrane flux and reducing irreversible fouling when the O₃ dosing is more than 0.1 mg O₃/mg DOC [17]. KMnO₄ pre-oxidation can alleviate membrane fouling by improving the hydrophilicity of DOM and decomposing the DOM of high MW [18]. However, pre-ozone oxidation can aggravate membrane fouling in some cases [19]. The oxidation of KMnO₄ narrows the pore size of the UF membrane, which adversely affects the membrane flux at the initial stage of filtration [20].

UV photolyzes oxidants when UV is coupled with AOP (UV-AOP). UV-AOP has various advantages, including the presence of abundant reactive radicals, easy operation, and the absence of a required catalyst. The oxidants in UV-AOP include H₂O₂, O₃, persulfate, and chlorine. UV/chlorine attracts more research interest than UV/H₂O₂, UV/O₃, and UV/persulfate because of the low cost and high photolysis rate of chlorine [21]. In the UV/chlorine process, hydroxyl radical and reactive chlorine species (RCS) are generated [22]. Although RCS are selective reactive species in UV/chlorine oxidation reactions [23], some RCS react with specific pollutants more quickly than hydroxyl radical [24]. Thus, the UV/chlorine process can be applied to remove micro-organic pollutants and DOM and to play a role in the pre-oxidation process for alleviating membrane fouling [25].

Various studies have focused on the effect of UV/chlorine pre-oxidation on membrane fouling mitigation and natural organic matter (NOM) changes in drinking water treatment [26,27]. UV/chlorine pre-oxidation changes the mechanism of membrane fouling. The cake filtration, intermediate pore blocking, and complete blocking of untreated humic acid, sodium alginate, and bovine serum albumin transform into pore blocking after UV/chlorine pre-oxidation [28]. UV/chlorine pre-oxidation also exhibits excellent removal performance of DOM, trace organic pollutants, and fluorescent organic matter [27,29].

However, the compositions of EfOM are complex and relatively different from those of NOM. EfOM is difficult to be simply classified as humic acids, proteins, and polysaccharides [28]. Hence, the fouling mechanism of EfOM remains complicated, and the effect of UV/chlorine pre-oxidation on fouling should be further investigated. Resin fractionation is a commonly used method to reveal the hydrophilicity/hydrophobicity characteristics of EfOM in secondary effluent [30]. Hydrophilicity/hydrophobicity characteristics are important physicochemical properties in membrane fouling processes [31]. Various hydrophilicity/hydrophobicity fractions from the same water sample have been proven to exhibit different fouling potentials. Hydrophobic and hydrophilic acids account for nearly all of the membrane fouling compared with base and neutral fractions [32]. Thus, the fouling characteristics of various fractions should be further explored to understand the fouling mechanism and alleviation with UV/chlorine pre-oxidation. The alleviation of membrane fouling after pre-oxidation is reportedly related to the reduction in MW of DOM, whereas the UF permeate is exacerbated by the decomposition of DOM [29]. Changes in the MW of various fractions of EfOM can reveal the details of fouling alleviation mechanism with UV/chlorine pre-oxidation.

In the present study, the effects of UV/chlorine pre-oxidation on UF membrane fouling mitigation and EfOM changes were explored in advanced treatment of real wastewater with different chlorine dosages. The possible mechanism of membrane fouling alleviation by UV/chlorine pre-oxidation was revealed by analyzing MW changes in various resin fractions of EfOM. Changes in the DOM, UV₂₅₄, and fluorescent characteristics of EfOM fractions were also examined.

2. Materials and Methods

2.1. Secondary Effluent Source and Agents

Secondary effluent was obtained from the wastewater treatment plant of an economic development park in Tianjin, China (2022). The activated sludge process with sequencing batch reactor was used in the wastewater treatment plant. The water quality details of the secondary effluent are shown in Table S1.

N,N-diethyl-p-phenylenediamine sulfate (DPD) was purchased from Sigma–Aldrich (USA). Sodium hypochlorite, NaOH, and HCl were obtained from Energy Chemicals (China). Methanol (guaranteed reagent) and acetonitrile (guaranteed reagent) were purchased from Jiangtian (China). Sodium hypochlorite stock solution (2 g/L) was freshly prepared by sodium hypochlorite (active chlorine content ≥ 10%) in ultrapure water. The stock solution was standardized by DPD spectrophotometric method and then stored in darkness. The secondary effluent samples used in the experiment were adjusted to pH 7.0 ± 0.1, and the temperature of the water samples was maintained at 20 ± 2°C throughout the experiment.

2.2. Chlorine and UV/chlorine Pre-oxidation

A process schematic of pre-chlorine and UV/chlorine pretreatment following UF process is shown in Fig. S1. A 6 W UV lamp (ZW6S15Y-Z211, Cnlight, China) with an emission wavelength of 254 nm was used in the UV/chlorine oxidation pretreatment experiment. The UV lamp was fixed in the centerline of the glass reactor. The glass reactor was 50 mm wide and 212 mm long, consistent with the UV lamp length. The depth of the water sample in the glass reactor was 30 mm. The irradiation intensity of the UV lamp was measured with a UVC radiometer (UVC-254A, Lutron, China), and the UV irradiance was calibrated weekly. The average UV irradiation intensity of the sample surface was 1.8 mW/cm², as determined by adjusting the effective path length (12 cm) according to previous studies [27,28]. The sodium hypochlorite stock solution was injected into the water samples for chlorine and UV/chlorine pre-oxidation with active chlorine concentrations of 2.0, 4.0, 6.0, and 8.0 mg/L. Chlorine pre-oxidation was maintained for 30 min without UV irradiation, and UV/Cl pre-oxidation was performed with UV irradiation for 30 min. The UV lamp was turned on for 30 min to stabilize light output before pretreatment began. The samples after chlorine and UV/chlorine pre-oxidation were stored in darkness for 24 h [33] and deemed usable in experiments when no residual active chlorine was detected.

2.3. Dead-end UF System

Polyvinylidene fluoride (PVDF) flat plate UF membranes (Mosu, China) with a MW cut-off of 100 kDa were used in the experiment. For each experiment, the membrane was soaked in ultrapure water for over 24 h. The ultrapure water was renewed every 1 h to remove the glycerol protective layer on the surface.

UF membrane fouling experiments were conducted in dead-end filtration mode (Fig. S1). A water sample was added to the UF cell (MSC300, Mosu, China) and storage tank. The effective filtration area of the UF cell was 0.0038 m². A nitrogen cylinder provided consistent pressure into the UF cell. A balance connected to a computer recorded the weight of UF effluent per minute automatically.

2.4. Membrane Fouling Analysis

The characteristics of membrane fouling were investigated by analyzing the change curve of specific flux against filtration time and fouling resistances. Before the filtration experiment, the initial average flux (J₀) was determined with ultrapure water under a pressure of 0.10 MPa. Every filtration experiment comprised three filtration cycles, which included filtration and backwashing. Filtration of water samples was maintained for 1 h under a pressure of 0.10 MPa, and the average flux (J_i, i = 1, 2, 3) was measured per min. Backwashing was performed with 100 mL of ultrapure water under a pressure of 0.02 MPa. The procedure schematic of membrane fouling analysis is shown in Fig. S2.

In this experiment, the specific flux (J/J₀) was used to measure the membrane flux change [34]. The membrane flux was calculated by Eq. (1) [35]:

(1) J=36000Δmρ•A•Δt

where J is the membrane flux (L·m⁻²·h⁻¹), Δm is the difference between two readings of the balance (g), ρ is the density of water (g·cm⁻³), A is the effective area of UF membrane (cm²), and Δt is the time interval of balance reading, which is 60 s.

The membrane resistance includes the inherent resistance of the membrane (R_m), the reversible resistance (R_re), and the irreversible resistance (R_ir) [36]. The reversible resistance is used to characterize the contamination of the filter cake layer in membrane fouling, and the irreversible resistance is used to characterize the membrane contamination caused by membrane pore blockage. Each resistance value can be calculated by applying Eqs. (2)–(6) [35,37]:

(2) Rm=3.6×1011μ×J0

(3) Ri=3.6×1011μ×Ji

(4) Ri′=3.6×1011μ×Ji′

(5) Ri=Rm+Rrei+Riri

(6) Ri′=Rm+Riri

where μ is the viscosity coefficient (Pa·s), R_i, and J_i are the membrane resistance and average flux, respectively, after the i-th cycle of fouling (m⁻¹), and R_i′ and J_i′ are the membrane resistance and average flux, respectively, after the i-th cycle of backwashing (m⁻¹).

2.5. Resin Fractionation of EfOM

The classification of organics by their hydrophilic properties is usually achieved using macroporous resins. In this experiment, two types of macroporous adsorbent resins, namely, XAD-8 and XAD-4 (Amberlite, USA), were used [38]. The Soxhlet extraction method was utilized to clean the resins with methanol and acetonitrile for 24 h. After cleaning, the resin was loaded into the chromatography column and compacted. The resin was washed with ultrapure water until the DOC of column influent and effluent was consistent.

The pH of the water sample was adjusted to 2 with 6 M HCl. The water sample was passed through a 0.45 μm cellulose ester filter and then through chromatography columns containing XAD-8 and XAD-4 resins. The effluent was collected as the hydrophilic (HPI) fraction. The resins in the columns were rinsed with ultrapure water. The hydrophobic acidic (HPO-A) fraction was obtained from XAD-8 resin using a 0.1 mol/L NaOH solution (flow rate = 1 mL/min), and the transphilic acidic (TPI-A) fraction was collected from XAD-4 resin. The resin was rinsed again with ultrapure water. The hydrophobic neutral (HPO-N) fraction was obtained from XAD-8 resin with a 75% acetonitrile solution (flow rate = 1 mL/min), and the transphilic neutral (TPI-N) fraction was collected from XAD-4 resin with a 75% acetonitrile solution (flow rate = 1 mL/min). The water samples of different fractions were stored at 4°C before analysis. The procedure schematic of resin fractionation is shown in Fig. S3. The concentrations of the organic matter in various fractions were determined by DOC and UV₂₅₄. The MW distributions of organic matter in various fractions before and after pretreatment were analyzed by gel permeation chromatography (GPC). The membrane fouling analysis of the HPO-N and TPI-N fractions was not conducted in this experiment because they contained acetonitrile, which may affect sample polarity and membrane fouling characteristics.

2.6. Excitation–emission Matrix (EEM) Analysis

Fluorescence EEM spectroscopy was conducted to reveal the changes in EfOM during chlorine and UV/chlorine pre-oxidation [39]. A fluorescence spectrophotometer (LS55, PerkinElmer Inc., USA) was used to determine the EEM spectrum of water samples. Emission and excitation wavelengths were set from 250 nm to 550 nm with an increment of 5 nm. The slit widths for excitation and emission were 5 nm. The detector was set to high sensitivity, and the scanning speed was kept at 1200 nm/min.

2.7. GPC Analysis

GPC is commonly used for the MW analysis of organics. In this experiment, changes in the MW of EfOM in the samples before and after pre-oxidation were analyzed. The GPC system comprised a liquid chromatography system (Shimadzu, Japan) with a differential refractive index detector (RID-10A, Shimadzu, Japan) and a gel chromatography column (GF-510 HQ, Asahipak, Shodex, Japan). The column temperature was set at 40 °C. The mobile phase was ultrapure water, and flow rate was 0.5 mL/min. The injection volume of the sample was 20 μL.

2.8. Other Analytical Methods

A total organic carbon analyzer (TOC-V_CPH, Shimadzu, Japan) was applied to determine the DOC concentration. A UV–visible spectrophotometer (UV-2700, Shimadzu, Japan) was used to analyze UV₂₅₄.

3. Results and Discussion

3.1. Membrane Fouling Characteristics with Chlorine and UV/chlorine Pre-oxidation

The characteristics of membrane fouling were investigated through multi-cycle UF experiments in dead-end mode. The variations in specific flux and membrane fouling resistances during UF filtration are shown in Fig. 1. As depicted in Fig. 1a, the untreated secondary effluent caused a remarkable membrane flux decline. The final specific flux (J/J₀) in the third filtration cycle was 0.42. The samples after 2 mg/L chlorine pre-oxidation led to more serious fouling than the untreated sample. The final specific flux (J/J₀) of the samples with 2 mg/L chlorine pre-oxidation was 0.41 in the third filtration cycle. By contrast, chlorine pre-oxidation at chlorine dosages of 4, 6, and 8 mg/L alleviated membrane fouling. The final specific fluxes (J/J₀) of the samples with 4, 6, and 8 mg/L chlorine pre-oxidation were 0.44, 0.47, and 0.49, respectively, in the third filtration cycle. This observation illustrated that chlorine pre-oxidation with low dosages aggravated membrane fouling, whereas higher dosages of chlorine pre-oxidation alleviated it. This result was consistent with a previous study [33]. The reason was that chlorine pre-oxidation with low dosages converted EfOM into polychlorinated products and increased their hydrophobicity, whereas chlorine pre-oxidation with higher dosages decreased the MW and changed the hydrophilicity/hydrophobicity characteristics of EfOM [33].

Fig. 1

Membrane fouling characteristics with chlorine and UV/chlorine pre-oxidation: specific flux decline (a) and fouling resistance (b) with and without pre-oxidation (Cl: active chlorine).

In the case of samples treated with UV/chlorine pre-oxidation, the final specific fluxes (J/J₀) of samples with UV/chlorine pre-oxidation of 2, 4, 6, and 8 mg/L chlorine were 0.50, 0.47, 0.45, and 0.49, respectively, which were higher than that of the untreated water. This observation showed that membrane fouling was alleviated by UV/chlorine pre-oxidation. Notably, the final specific fluxes (J/J₀) decreased with increased chlorine dosage from 2 mg/L to 6 mg/L and then increased with increased chlorine dosage from 6 mg/L to 8 mg/L. This result illustrated that the alleviation of membrane fouling by UV/chlorine oxidation did not increase with increased chlorine dosage, as has been observed in a previous study [29]. The low value of the final specific fluxes (UV/chlorine, 6 mg/L chlorine) was related to the hydrophobicity increase and membrane pore blocking of proteins after UV/chlorine pre-oxidation (6 mg/L chlorine) [29,33].

Fig. 1b shows the hydraulic reversible and irreversible fouling resistance of samples with and without pre-oxidation. For the untreated water sample, the average irreversible and reversible fouling resistances were 5.85 ± 1.58 × 10¹¹ and 5.24 ± 0.77 × 10¹¹ m⁻¹. For chlorine pre-oxidation, no notable difference in irreversible fouling resistances was observed between the raw and pre-chlorination samples. Chlorine pre-oxidation with a chlorine dosage of 6 mg/L resulted in the lowest hydraulic reversible fouling resistance, which was 3.59 ± 0.48 × 10¹¹ m⁻¹. Regarding UV/chlorine pre-oxidation, the hydraulic irreversible fouling resistances of samples with UV/chlorine pre-oxidation were 3.87 ± 1.02 × 10¹¹ m⁻¹ (2 mg/L of chlorine), 4.88 ± 1.58 × 10¹¹ m⁻¹ (4 mg/L of chlorine), 4.50 ± 0.87 × 10¹¹ m⁻¹ (6 mg/L of chlorine), and 3.62 ± 0.96 × 10¹¹ m⁻¹ (8 mg/L of chlorine), which were lower than those of the raw water and pre-chlorination samples. Conversely, the reversible fouling resistances with UV/chlorine pre-oxidation were higher than those with chlorine pre-oxidation but lower than those of raw water samples. This finding showed that UV/chlorine pre-oxidation primarily alleviated hydraulic irreversible fouling, whereas chlorine pre-oxidation trended to mitigate reversible fouling. This result was consistent with membrane fouling alleviation of NOM by UV/ClO₂ [29].

Compared with the results of secondary effluent pretreatment in previous studies, the specific flux (J/J₀) (0.50) of the samples with UV/chlorine pre-oxidation in the current work was similar to those with 10 mg/L of ozone (0.53), Fe (VI) (0.48), and 0.6 mmol/L of H₂O₂ coupled with UV (0.54) [13,25,40]; however, it was lower than that of 0.6 mmol/L of persulfate coupled with UV (0.69), coagulation followed by Fe (VI) oxidation (0.63) and TiCl₄ coagulation (0.86) [25,41,42]. UV/chlorine pre-oxidation showed less effective in membrane fouling alleviation than some pre-oxidation methods, such as UV/persulfate. Nevertheless, UV/chlorine pre-oxidation cost lower than these aforementioned methods.

3.2. Changes in EfOM after Chlorine and UV/chlorine Pre-oxidation

The DOC and UV₂₅₄ values in UF feed and permeate samples with chlorine and UV/chlorine pre-oxidation are shown in Fig. 2. As described in Fig. 2a, chlorine and UV/chlorine pre-oxidation decreased the DOC values with increased chlorine dosages from 0 mg/L to 8 mg/L. The removal efficiency of DOM by UV/chlorine pre-oxidation was also higher than that by chlorine pre-oxidation. The maximum efficiencies of DOC removal were 11.9% and 23.5% for chlorine pre-oxidation and UV/chlorine pre-oxidation, respectively. The substantial removal of DOC in the UV/chlorine pre-oxidation process was related to the decomposition of DOM by hydroxyl radicals (HO·) and chlorine radicals (Cl·) during UV/chlorine pre-oxidation [43].

Fig. 2

DOC (a) and UV₂₅₄ (b) of feed and permeate water samples with chlorine and UV/chlorine pre-oxidation (Cl: active chlorine).

As shown in Fig. 2a, the DOC values in permeate samples decreased due to membrane retention during UF filtration. A total of 6.3% of DOC was rejected by the UF membrane for the raw water sample. The UF membrane rejected 9.1% and 7.9% of the DOC with chlorine dosages of 2 and 4 mg/L, whereas the UF membrane rejected 5.9% and 6.1% of the DOC with chlorine dosages of 6 and 8 mg/L. Therefore, chlorine pre-oxidation with low dosages aggravated membrane fouling. This result coincided with the earlier result on the specific fluxes and fouling resistances of chlorine pre-oxidation with chlorine dosages of 2 and 4 mg/L. Moreover, the DOC rejected by UF for samples with UV/chlorine pre-oxidation was lower than those observed for chlorine pre-oxidation. This observation showed that UV/chlorine pre-oxidation performed better in membrane fouling alleviation than chlorine pre-oxidation. The reactive species from UV/chlorine pre-oxidation were effective in decomposing DOM into smaller fragments [28]. UV/chlorine pre-oxidation also caused lower DOC levels in the permeate samples than the untreated samples.

Fig. 2b shows that chlorine and UV/chlorine pre-oxidation caused a remarkable decrease in UV₂₅₄, and the removal efficiencies of UV₂₅₄ increased with increased chlorine dosage from 0 mg/L to 8 mg/L. For permeate samples, UF rejected 7.5% of the UV₂₅₄ for raw water. For samples with chlorine pre-oxidation, UF rejected 9.5%, 9.5%, 6.9%, and 6.6% of the UV₂₅₄ with chlorine dosages ranging from 0 mg/L to 8 mg/L. This result showed that UV₂₅₄ rejection increased with increased low chlorine dosage due to worsened membrane fouling. For samples with UV/chlorine pre-oxidation, 7.2%, 8.9%, 10.9%, and 10.4% of the UV₂₅₄ were rejected by UF with chlorine dosages ranging from 0 mg/L to 8 mg/L. This outcome suggested that UF tended to reject residual DOM with unsaturated bonds. This result was related to the hydrophilicity/hydrophobicity characteristics of residual DOM [27].

Fluorescence EEM was conducted to reveal the effect of chlorine and UV/chlorine pre-oxidation on the removal of fluorescence organic matter (Fig. 3). As shown in Fig. 3, the EEM of the raw water comprised four fluorescence peaks (A, C, T₁, and T₂), which are commonly observed in secondary effluent samples [44]. Peaks A and C were related to humic-like substances, whereas peaks T₁ and T₂ represented protein-like (including tryptophan-like) ones. Peak T₁ was also related to soluble microbial products [44]. The humic-like substance was the dominant fluorescence organic matter in the raw secondary effluent samples.

Fig. 3

Fluorescence EEM spectra of samples after chlorine and UV/chlorine pre-oxidation (Cl: active chlorine).

As shown in Fig. 3 and Table 1, the fluorescence intensities of all four peaks were reduced in the EEMs of samples after chlorine and UV/chlorine pre-oxidation. UV/chlorine pre-oxidation caused a more remarkable decrease in fluorescence intensity for all four peaks than did chlorine pre-oxidation. The fluorescence intensities of these peaks decreased gradually, and the fluorescence intensities of peak T₁ reduced more rapidly than those of peak A, C, and T₂ with increased chlorine dosages ranging 0 mg/L to 8 mg/L. It illustrated that peak T₁, related to protein-like substance and soluble microbial products [44], was decomposed rapidly by chlorine and UV/chlorine pre-oxidation. The maximum reduction in fluorescence intensity was found in the sample after UV/chlorine pre-oxidation with 8 mg/L Cl. The removal efficiencies of peaks A, C, T₁, and T₂ were 70.3%, 88.0%, 98.6%, and 72.6%, respectively. The decrease in fluorescence organic matter by chlorine and UV/chlorine pre-oxidation was related to the structural degradation of fluorophores in humic- and protein-like substances. Furthermore, the strong oxidative radicals from UV/chlorine pre-oxidation contributed to the deep decomposition of fluorescence organic matter [28].

Table 1

Fluorescence peak details of samples after chlorine and UV/chlorine pre-oxidation (Cl: active chlorine).

The excitation and emission wavelengths of peak C increased in the samples after chlorine and UV/chlorine pre-oxidation. This observation indicated a “red shift” in humic-like substances after pre-oxidation, which was related to the generation of hydrophilic groups during pre-oxidation [45]. The increase in hydrophilic characteristic of EfOM after UV/chlorine pre-oxidation was beneficial to membrane fouling alleviation [40].

3.3. MW Changes

The MW of EfOM in samples after chlorine and UV/chlorine pre-oxidation was also investigated, and results are shown in Fig. 4. The GPC results for the raw water sample comprised three peaks (peaks A, B, and C). The substances present in peaks A and B were related to humic-like materials with an MW in the range of 500–2000 Da [46]. The substance represented by peak C was associated with organic acids of low MW, amino acids of low MW, or simple sugars with MW < 500 Da [46].

Fig. 4

GPC results of samples (Cl: active chlorine): raw and permeate samples (a), samples after chlorine (b), and UV/chlorine (c) pretreatment.

Figs. 4b and 4c show that the response signal of the three peaks decreased in the samples after chlorine and UV/chlorine pre-oxidation with increased chlorine dosage from 0 mg/L to 8 mg/L. Peak B remarkably decreased in samples after chlorine pre-oxidation, whereas peaks A and B decreased notably in samples after UV/chlorine pre-oxidation. This result showed that humic-like substances can be decomposed by chlorine and UV/chlorine pre-oxidation. UV/chlorine pre-oxidation caused the further destruction of humic-like substances due to the presence of strong oxidative radicals. Peak C unremarkably decreased in samples after chlorine and UV/chlorine pre-oxidation with a chlorine dosage of 2 mg/L, whereas peak C notably decreased in samples after chlorine and UV/chlorine pre-oxidation with increased chlorine dosage from 4 mg/L to 8 mg/L. This observation suggested that substances of low MW were more resistant to decomposition by chlorine and UV/chlorine pre-oxidation when low chlorine dosages were used. Nevertheless, chlorine and UV/chlorine pre-oxidation can remove EfOM components with wide-ranging MWs, and UV/chlorine pre-oxidation showed better removal performance of EfOM than chlorine pre-oxidation. A similar result has been reported in a previous study [27].

Fig. 4 also provided insights into the chlorine oxidation by-products. The signal response (41 790 A. U.) of peak A in the sample with 2 mg/L chlorine pre-oxidation was higher than that (39 328 A. U.) of raw water. The increase in peak A after pre-chlorination was related to the polychlorinated products, which decreased membrane flux [33]. This result was consistent with those of the specific-flux decline in this study.

3.4. Fractionation and Polarity Variation of EfOM after Chlorine and UV/chlorine Pre-oxidation

The fraction samples collected from raw water were treated with chlorine and UV/chlorine pre-oxidation. The membrane fouling characteristics of different fractions with chlorine and UV/chlorine pretreatment were investigated by analyzing the variation in membrane flux (Fig. 5). As illustrated in Fig. 5a, the membrane flux of HPO-A fraction in the raw water decreased more rapidly than that of the TPI-A and HPI fractions. This result illustrated that the HPO-A fraction in the raw water caused more critical membrane fouling than the TPI-A and HPI fractions. The HPO-A fraction frequently exhibited the strongest fouling potential [40,47]. The differences in membrane flux of the three fractions were related to the interaction between the membrane surface and organic matter within each fraction [48].

Fig. 5

Membrane fluxes of different fractions with chlorine and UV/chlorine pretreatment (Cl: active chlorine): raw water (a), chlorine (b), and UV/chlorine (c) pre-oxidation at a chlorine dosage of 6 mg/L.

As shown in Figs. 5b and 5c, the membrane fluxes of the three fractions after chlorine and UV/chlorine pre-oxidation were higher than those of raw water. Chlorine and UV/chlorine pre-oxidation caused a notable increase in the membrane flux of HPO-A and HPI, whereas the membrane flux of TPI-A increased unremarkably. The membrane flux of the fractions with UV/chlorine pre-oxidation was higher than that of those treated with chlorine pre-oxidation. This outcome suggested that fouling caused by the three fractions can be mitigated with chlorine and UV/chlorine pre-oxidation. Chlorine and UV/chlorine pre-oxidation alleviated the fouling of HPO-A and HPI more notably than that of the TPI-A fraction. Thus, the polarity or MW distribution was altered during chlorine and UV/chlorine processes. The organic matter in the TPI-A fraction showed more resistant capacity against chlorine and UV/chlorine pre-oxidation than that in HPO-A and HPI. The presence of strong oxidative radicals from UV/chlorine pre-oxidation was associated with the high membrane flux of the fractions after UV/chlorine pre-oxidation [28].

The effect of chlorine and UV/chlorine pre-oxidation on the organic matters in fraction samples was investigated. These fraction samples were directly treated by chlorine and UV/chlorine processes, and the variations in DOC and UV ₂₅₄ for the fraction samples are shown in Fig. 6. As described in Fig. 6a, the dominant fraction in the raw water sample of secondary effluent was HPI, followed by HPO-A and TPI-A. HPI and HPO-A are frequently found to be the dominant fractions in the secondary effluent samples [49]. The UV₂₅₄ of HPO-A had the highest value in the raw secondary effluent sample (Fig. 6b). This finding agreed with those of a previous study [50]. The HPI fraction also exhibited a higher UV₂₅₄ value than TPI-A. By contrast, the TPI-A fraction showed a low UV₂₅₄ value of 0.003 cm⁻¹. This result highlighted the abundance of organic matter with unsaturated structures in the HPO-A and HPI fractions.

Fig. 6

Changes in DOC (a) and UV₂₅₄ (b) in fraction samples with chlorine and UV/chlorine pre-oxidation (Cl: active chlorine).

Chlorine and UV/chlorine pre-oxidation partly degraded organic matter in the three fractions and caused a slight decrease in DOC. The UV₂₅₄ values of HPI and HPO-A fractions also decreased with chlorine and UV/chlorine pre-oxidation, whereas that of TPI-A changed unremarkably. This outcome suggested that a small portion of the organic matter in the three fractions was mineralized by chlorine and UV/chlorine pre-oxidation. The reduction in UV₂₅₄ for the HPO-A and HPI fractions was related to ring opening and substitution by pre-oxidation processes [33]. In the TPI-A fraction, the removed organic matter may be related to organic acids of low MW that lacked aromatic and other unsaturated carbon structures. According to the DOC and UV₂₅₄ changes in various fractions, UV/chlorine pre-oxidation showed better organic-matter removal and transformation than chlorine pre-oxidation in these fractions.

The MW distribution of the three fractions was further investigated to reveal the relationship between DOM changes and membrane fouling mitigation with chlorine and UV/chlorine pre-oxidation (Fig. 7). As illustrated in Fig. 7, low-MW organic matter was found in the all three fractions, whereas high-MW organic matter was found only in HPI. The signals of all peaks in the three fractions slightly decreased after chlorine and UV/chlorine pre-oxidation. According to the GPC results, UV/chlorine pre-oxidation also showed better DOM removal performance than chlorine pre-oxidation.

Fig. 7

GPC results of the HPI (a), HPO-A (b), and TPI-A (c) fractions with chlorine and UV/chlorine pre-oxidation (Cl: active chlorine).

The alleviation in membrane fouling was associated with hydrophilicity/hydrophobicity changes and the removal of DOM in the fractions after chlorine and UV/chlorine pre-oxidation. In the HPI fraction, the reduction in membrane fouling by pre-oxidation may be related to the cleavage of DOM with high MW. In the HPO-A fraction, the mitigation of membrane fouling by pre-oxidation was related to the changes in the polarity of organic matter. In the TPI-A fraction, the alleviation in membrane fouling was unremarkable with chlorine and UV/chlorine pre-oxidation, which was possibly due to the refractory characteristics of DOM against oxidation in the TPI-A fraction.

4. Conclusions

The effects of chlorine and UV/chlorine pre-oxidation on membrane fouling were investigated during UF in wastewater reclamation. UV/chlorine pre-oxidation effectively reduced irreversible membrane fouling, whereas chlorine pre-oxidation primarily mitigated reversible membrane fouling. UV/chlorine pre-oxidation also decreased DOM in UF permeate. Among the various fractions, HPO-A contributed considerably to membrane fouling in the raw secondary effluent compared with HPI and TPI-A, and the EfOM with high MW was primarily found in HPI fraction. UV/chlorine pre-oxidation changed the polarity of EfOM in HPO-A and decomposed the EfOM with high MW in HPI, which mitigated membrane fouling. However, chlorine and UV/chlorine pre-oxidation exhibited limited efficiency in alleviating membrane fouling in the TPI-A fraction due to its refractory characteristics against pre-oxidation. This work highlighted the potential of UV/chlorine pre-oxidation as a pretreatment for UF membrane fouling mitigation in wastewater reclamation. It also provided a promising insight into the effect of UV/chlorine oxidation on EfOM removal in the advanced treatment of wastewater.

Supplementary Information

eer-2023-624-Supplementary.pdf

Acknowledgement

This study was supported by Tianjin Research innovation Project for Postgraduate Students (Grant No. 2022SKYZ174) and National Innovation and Entrepreneurship Training Program for college students (Grant No. 202310792013).

Nomenclature

Symbols Nomenclature

Δm

difference between two readings of the balance

Δt

time interval of balance reading, which is 60 s

viscosity coefficient

density of water

effective area of UF membrane

J/J₀

specific flux

membrane flux

J₀

initial membrane flux

J_i

membrane flux after the i-th cycle of fouling

J_i′

membrane flux after the i-th cycle of backwashing

R_re

inherent resistance of UF membrane

R_i

membrane resistance after the i-th cycle of fouling

R_i′

membrane resistance after the i-th cycle of backwashing

R_ir

irreversible resistance

R_re

reversible resistance

Notes

Conflict-of-Interest Statement

The authors declare that they have no conflict of interest.

^{Author contributions}

S.Z.H. (M.S. student) and G.L. (B. S. student) conducted all the experiments and wrote the first draft of the manuscript. Prof. W. D. contributed to the study design and funding support. Dr. L. L. J. provided material preparation, data collection and analysis. Prof. Q.C.S. and prof. W.S.P. commented on previous versions of the manuscript.

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Sample	Peak A		Peak C		Peak T₁		Peak T₂

	E_x/E_m	Intensity	E_x/E_m	Intensity	E_x/E_m	Intensity	E_x/E_m	Intensity
Raw water	240/425	40.06 ± 0.01	330/420	32.94 ± 0.02	290/345	9.17 ± 0.02	230/345	24.96 ± 0.07
2 mg/L Cl	240/425	32.26 ± 0.04	335/425	25.31 ± 0.01	290/345	6.99 ± 0.01	230/345	20.85 ± 0.01
4 mg/L Cl	240/425	21.17 ± 0.03	335/420	12.55 ± 0.03	290/345	1.42 ± 0.01	230/345	13.63 ± 0.03
6 mg/L Cl	235/420	15.86 ± 0.04	340/430	9.69 ± 0.01	290/345	1.38 ± 0.03	230/345	13.43 ± 0.02
8 mg/L Cl	240/425	10.66 ± 0.02	335/425	4.31 ± 0.05	290/345	0.88 ± 0.01	230/345	8.98 ± 0.04
UV/2 mg/L Cl	235/430	30.14 ± 0.07	335/425	18.54 ± 0.01	290/345	2.92 ± 0.04	230/345	18.15 ± 0.01
UV/4 mg/L Cl	240/420	18.55 ± 0.02	340/425	8.04 ± 0.04	290/345	1.28 ± 0.03	230/345	8.84 ± 0.06
UV/6 mg/L Cl	260/425	15.08 ± 0.02	335/420	4.70 ± 0.03	290/345	0.21 ± 0.04	230/345	8.43 ± 0.05
UV/8 mg/L Cl	260/430	11.91 ± 0.01	335/415	3.90 ± 0.04	290/345	0.11 ± 0.02	230/345	7.06 ± 0.08