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Intermittent osmotic relaxation: A strategy for organic fouling mitigation in a forward osmosis system treating landfill leachate
Desalination 482 (2020) 114406
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Intermittent osmotic relaxation: A strategy for organic fouling mitigation in a forward osmosis system treating landfill leachate
T
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Bilal Aftab, Jinwoo Cho, Jin Hur
Department of Environment and Energy, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006, South Korea
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Forward osmosis Intermittent osmotic backwashing Physical cleaning Membrane fouling EEM-PARAFAC
In this study, the potential of osmotic backwashing was explored for the alleviation of the forward osmosis (FO) membrane fouling induced by landfill leachate. The normal osmotic backwashing employing NaCl as a draw solute resulted in an improved membrane flux (~ 4 LMH) and a reduction (45%) of the membrane resistance. We proposed an alternative way of osmotic backwashing based on the inherent osmotic potential of landfill leachate itself for the FO system. For the effort, intermittent osmotic relaxation (IOR) was newly introduced to the FO system to enhance the performance. This operation showed a comparable performance with fouling mitigation to the previous normal osmotic backwashing using NaCl as a draw solute. Under an optimized condition (4 h of filtration intervals and 20 min of backwashing time), the integration of the IOR with the FO system led to a 30% reduction in membrane resistance, a complete flux recovery, and 26% increment in the filtered volume. The IOR process does not require additional energy and/or chemical dosage. Therefore, it can be suggested as an innovative, practical, and energy-efficient osmotic backwashing strategy for a stable and the long-term operation of FO systems treating saline wastewater.
1. Introduction Over the past decades, forward osmosis (FO) has established its status as a promising green technology for desalination and wastewater treatment due to its benefit of naturally driven osmotic pressure by a
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solute intensity difference [1,2]. It has shown its superiority with its performance over other conventional membrane processes, which include nanofiltration (NF) and reverse osmosis (RO), with a higher organic rejection, a lower energy demand, and a greater propensity of reversible versus irreversible fouling [3,4]. Nevertheless, the
Corresponding author. E-mail address: [email protected] (J. Hur).
https://doi.org/10.1016/j.desal.2020.114406 Received 25 September 2019; Received in revised form 19 February 2020; Accepted 22 February 2020 0011-9164/ © 2020 Elsevier B.V. All rights reserved.
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showed a substantial reduction of membrane biofouling and high permeate fluxes [41,42]. Similarly, the maintenance of an average stable flux has been demonstrated in the ultrafiltration of river water for a prolonged operation that lasted 30 days via an intermittent operation [43]. This approach could be applied to the osmotic backwashing of the FO systems to treat landfill leachate with the expectation of a long-term filterability and an effective alleviation of organic fouling along with continued and stable operation. This option can be costeffective, because electrical, hydraulic, or ultrasonic assistance is not required. Recently, fluorescence excitations emissions matrix combined with parallel factor analysis (EEM-PARAFAC) has been successfully applied to probe the compositional variations of different fluorescent organic foulants in conventional membrane filtration systems [44–46]. On the contrary, only a limited amount of information is available in the literature to keep track of the different constituents in landfill leachate in organic de-fouling of an FO membrane via osmotic backwashing. This study aimed to (1) test the applicability of the osmotic relaxation for an FO system to treat landfill leachate, and (2) further explore the effectiveness of intermittent osmotic relaxation (IOR) with respect to the filterability and organic fouling behavior for an FO system. The EEMPARAFAC was utilized to track the changes of organic foulants and to identify the detached foulants after osmotic backwashing. For this study, a crossflow FO setup was operated to treat landfill leachate under different IOR conditions.
performance of a FO membrane is hampered by the fundamental operation problems, such as organic fouling. Due to the lack of pressurized forces, the foulant layer in an FO membrane is less prone to hydraulic compactions, it is comparatively looser, and it is more likely to be partially appressed compared to those in conventional membrane filtration, which makes it easy to become detached via simple physical cleaning practices [5–8]. This characteristic may improve the reliability of FO systems to treat complex wastewater. There were many previous examples to demonstrate the potential of FO systems for the treatment of treat sewage [9], textile wastewater [10], high strength wastewater [11,12], digested sludge [13], and landfill leachate [14–16]. Landfill leachate is regarded as the most hazardous form of wastewater due to the abundance of the range of persistent pollutants [17,18]. In particular, mature leachate from old landfill sites contains numerous refractory organics, which are resilient to biological treatments [19,20]. Various on-site treatment options, which includes membrane filtration, have been adopted to protect surrounding environments from their hazards [21]. Several obstacles hindering efficient FO systems have been identified [8,22,23], and membrane fouling is considered as one of the critical challenges that affect the operational cost and the quality/quantity of the treated water recovered from feed solutions [2]. In previous studies, several in-line physical cleaning strategies were proposed to alleviate the membrane fouling in FO systems, which included the use of ultrasound waves [7], the application of an electric current to a modified conductive FO membrane [24], the creation of high crossflow velocities [25], and extended crossflow flushing combined with ultrasonication [26]. However, these approaches usually include additional operating energy and costs, which overshadow the inherent benefit of FO as an economic operation. Thus, it is highly required to suggest an alternative low-cost physical cleaning technique for the fouling mitigation in FO systems. Osmotic backwashing refers to the osmotic scouring of a fouled layer by reversing the direction of the osmosis across the membrane and has been conventionally practiced for RO membranes [27]. It was previously reported that RO membranes were potentially backwashed with the osmotic pressure without any external pressure [28,29]. The osmotic backwash can be effective with respect to the cross flow velocity and the backwashing period [30]. However, the requirement of a very high concentration of draw solutes may hamper the performance of osmotic backwashing in RO membranes [31]. Many previous studies have proved the applicability of osmotic backwashing in FO systems for improved flux recovery and fouling reversibility [32–34]. In a comparison between the RO and the FO membranes subjected to wastewater reuse, it was observed that FO membrane was less prone to membrane fouling and could be cleaned by osmotic backwashing more easily as compared to RO membrane [35]. It has also been pointed out that using additional draw solute for backwashing may increase the operational cost, which is a drawback. Regarding this issue, it is worth noting that a characteristic of landfill leachate encompasses a variety of dissolved salts and organics with electric conductivity that ranges from 10 to 400 mS/cm [36–39]. Therefore, it is reasonably suggested that the inherent osmotic potential of feed leachate could be utilized to backwash fouled FO membrane by switching the draw solute with the feed solution possessing a lower osmotic potential, such as distilled deionized water (DDW). This setup may substantially reduce the operation cost that might be taken for the osmotic backwashing of an FO membrane for the treatment of landfill leachate. In previous reports, osmotic backwashing has been practiced as a cleaning protocol only after the completion of the entire filtration cycle [32,40]. On the other hand, the inline or intermittent relaxation via the osmotic backwashing of an FO membrane has not been tested to date even though this approach has evidenced its effectiveness as a fouling mitigation practice in conventional membrane systems. For example, it was previously reported that the intermittent scouring of ultrafiltration membrane foulants using pressure relaxation in membrane bioreactors
2. Materials and methods 2.1. Feed leachate sampling and its chemical properties A sample of stabilized landfill leachate (40 L) was collected from an old landfill site (Sudokwon management Corp.), situated in the vicinity of Incheon city, South Korea. Solid waste had been dumped into the landfill for 8 years, before the site was closed in 2000. Sampling was conducted in June 2018 using an acid washed 20 L- polypropylene plastic container. The sample was filtered through a pre-ashed Whatman GF/F filter with a pore size of 0.7 μm. The filtered landfill leachate sample was stored in a refrigerator at 4 °C. The basic water quality of the sample is listed in Table S1. The ion composition of the landfill leachate is shown in Table. S2. 2.2. Forward osmosis system configuration and operation A crossflow FO system with a working volume of 400 mL was set up to treat the raw filtered landfill leachate, which is shown in Fig. 1. A flat sheet cellulose triacetate (CTA) FO membrane, which was purchased from Fluid Technology Solution, Inc. (FTSH2O) (Albany, USA), was installed in an acrylic external FO membrane module with an effective filtration area of 22 cm2. The feed solution (FS) and the draw solution (DS) were circulated across the membrane module in concurrent directions with peristaltic pumps. Sodium chloride (NaCl) was used as a draw solute, and the solute concentration was maintained by intermittently adding a concentrated draw solution, which was 5 M, NaCl, into the DS tank. A pump for the DS tank was connected to an online conductivity controller to monitor and maintain a constant osmotic potential for the entire filtration run. Volumetric changes in the feed tank were regularly checked on a data logging scale, and the information was sent to a computer to calculate the membrane flux (Jw) in liters per minute per hour (LMH). The estimation was based on the following Eq. (1):
jw =
1 ΔVt · Am Δt
(1)
where Am is the working membrane area, and ΔVt is the volumetric change recorded by the logger at a time interval of Δt. A reference run of the FO filtration was carried out for 24 h to 2
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Fig. 1. Schematic diagrams of the forward osmosis and the IOR settings used in this study.
2.5. Membrane flux recovery and the resistance analysis
obtain the fouled membrane for further experiments. The reference run was operated under an optimized operating condition following Aftab et al. [36], in which 1.5 M NaCl was used for the DS, and the flow velocity for DS and FS was kept at 0.02 m/s.
For the membrane flux recovery and the resistance analysis, an average flux for the virgin membrane (Jv) was first calculated for 60 min under a normal FO setting using 0.5 M NaCl as a draw solute and DDW as feed water. For either the reference or the IOR run, the membrane flux for the fouled membrane was calculated for 60 min after each filtration run, which was denoted as the membrane flux after filtration (Jo). The membrane was then set for normal osmotic backwashing by circulating the NaCl solution on the feed side and DDW on the draw side for 20 min, while the loosely bounded organic fractions were extracted from the FO membrane. This extracted fraction was called reversible foulants [47]. The average membrane flux was then calculated for 60 min and denoted as a membrane flux after the physical cleaning (Jp). Next, the membrane was detached from the assembly and soaked into a 0.1 M NaOH solution (50 mL) for 30 min to chemically extract the irreversible foulants (IR) from the membrane. This procedure was adopted only to extract the membrane foulants from the membrane. The membrane was rinsed with DDW, and the average membrane flux was calculated again for 60 min, which was denoted as a membrane flux after the chemical cleaning (Jc). Detailed explanations for the flux recovery are available in the literature [9,36]. The osmotic pressure (Pa) of the landfill leachate was measured using the following Eq. (2):
2.3. Experimental conditions for osmotic backwashing of FO membrane Two types of osmotic backwash experiments were conducted using the fouled FO membrane obtained from the reference run. For the first case, the feed side of the module was replaced with a NaCl solution with an EC value the same as raw landfill leachate (~10 mS/cm), and the DS was replaced with distilled deionized water (DDW) to reverse the direction of the permeate flux across the membrane (Fig. S1). The samples were collected from the feed side every 5 min throughout a full period of 40 min and sonicated for 20 min to completely dissolve the extracted fouling layer. The samples were then passed through a 0.45 μm pre-rinsed CTA filter (47 mm, Advantac) for the dissolved organic matter (DOM) analyses. For the second case, to activate the inherent de-fouling potential of the landfill leachate, in which the backflow was induced by the osmotic potential of landfill leachate, the DS was replaced with DDW on the draw side of the module and no change was made on the feed side that contained landfill leachate (Fig. S1). For both de-fouling tests, a membrane flux was calculated for a 40 min of filtration run.
π=i×ϕ×C×R×T
(2)
where i, ϕ, and C are the number of ions, the osmotic coefficient, the sum of the concentrations of all solutes, respectively. R is the universal gas constant and T is the absolute temperature. The value for the solute concentrations in landfill leachate was taken from the information in Table S2. The FO membrane resistance (m−1) was calculated using the following Eqs. (3)–(5) based on membrane fouling in series model:
2.4. Intermittent osmotic relaxation (IOR) experiments The configuration for the IOR is shown in Fig. 1. The IOR was performed by intermittently replacing the DS in the draw tank with DW and keeping all other operational conditions the same as the reference run during the filtration run. This setup induces the osmotic backwashing of the FO membrane to be driven by the innate osmotic pressure of the landfill leachate. To evaluate the effect of the backwashing intervals, the IOR was performed such that the FO system was run for a certain period of time that included 2, 4, or 6 h, which was followed by different times of backwashing (i.e., 10, 20, or 40 min) for the whole 24 h-operation. After each FO run was completed under individual IOR conditions, the membrane resistance, the membrane flux recovery, and the final filtered volume were measured, and they were compared with those of the reference run. Meanwhile, small amounts (12 to 22 mL) of water in the backflow moving to the feed side during the backwash intervals were not considered for the calculation of the final filtered volume after the entire filtration run. The amounts of the DDW required for the IOR experiments are shown in Table S3.
J=
Rt =
πd − πf 1 dV × = A dt μ × Rt
(3)
πd − πf J×μ
(4)
Rt = Rm + Rre + Rir
(5)
where πd and πf are the osmotic pressure of DS and FS, respectively, which is the driving osmotic force of the FO operation. μ (Pa s) is the dynamic viscosity. Rm, Rre, and Rir refer to the intrinsic membrane resistance, the reversible membrane resistance, and the irreversible membrane resistance to filtration, respectively. Rm was measured by the flux Jv obtained for the virgin membrane. Rre was obtained using the flux Jp after the membrane was physically cleaned using osmotic 3
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3.2. Membrane filtration behavior and subsequent membrane fouling
backwash. Rir was calculated using the flux Jc after the membrane was chemically using 0.1 M NaOH.
Water recovery from raw landfill leachate was observed for the FO system that operated using NaCl as a draw solute at 0.02 m/s of flow velocity. It led to a concentration of raw leachate by a factor of 2.6 with a final recovered volume of 250 mL (versus the working volume of 400 mL). During the operation, the DOC of the leachate increased from 180 mg C/L to 310 mg C/L on the feed side. The membrane flux declined gradually from 8.3 to 2.2 LMH with filtration time (Fig. 3a). This trend was previously explained by the decrease in the osmotic pressure between the DS and the FS, which resulted from the concentration of the leachate and/or from the organic membrane fouling followed by the filtration of the landfill leachate [16]. In this study, the former was not taken into account as the major mechanism that caused the flux decline, because a larger difference in the conductivity between the DS (120 mS/cm) and the FS was observed even after the full concentration (~40 mS/cm) of the feed leachate. Thus, the latter process seems to play significant roles in the decline of the flux. Substantial membrane fouling was obvious as indicated by a decrease in the flux recovery of the FO membrane to 71% and a total membrane fouling resistance of 1/ m. On the other hand, a simple physical cleaning using osmotic backwashing resulted in the reduction of the membrane fouling resistance to a value of 680 × 105 1/m along with a flux recovery of 90.1%, which suggests that the physical cleaning is effective enough to recover the FO performance. Similar findings were previously reported in several previous studies of FO systems to treat landfill leachate or other types of feed solutions [26,36,60]. In this study, chemical cleaning led to a full recovery of the membrane flux with the reduction of the membrane fouling resistance to 620 × 105 1/m, which was nearly equals to the intrinsic resistance of the membrane. The organic foulants extracted from the FO membrane showed the supportive results that were evidence of a greater contribution of physical versus chemical cleaning to the fouling mitigation. The bulk DOC of the RE was 15 mg C/L, which was higher than that of the IR (8 mg C/L). Further probing the chemical composition of the membrane foulants via the EEM-PARAFAC revealed that C1 was the main fluorescent component responsible for both the RE and the IR with the percent contributions of 50% and 60%, respectively. The LMW characteristic of C1 likely made the fraction (i.e., C1) easier to penetrate the membrane pores. The relatively hydrophilic nature of the fraction could also have contributed to its binding with the hydrophilic membrane more strongly than the C2 or the C3 fractions via the dipole-dipole interactions or the hydrogen bonding [61]. The initial deposition of the C1 fraction on the membrane may cause the reduction of the repulsive charges on the membrane surface [47], which may facilitate the further attachment of relative hydrophobic fractions (C2 and C3). The much greater contribution of C3 fraction to the RE versus the IR (Fig. 3) indicates that the C3 component or a relatively hydrophobic fraction might be loosely bound to the membrane. The results in conjunction imply that reversible fouling contributed more to the decline of the flux and also that the physical cleaning using membrane backwashing was sufficient to recover the membrane flux to a satisfactory level. It was also found from the PARAFAC results that the C1 component was the main contributor to the membrane fouling. These results encouraged us to further explore the de-fouling performance of the simple backwashing method for fouled FO membrane at different settings and compare the applicability of different strategies for the successful removal of organic foulant fractions from a membrane surface. One promising strategy to be proposed here is benefitted by the high ionic strength of landfill leachate (EC: 10.2 mS/cm in this study) associated with a high osmotic potential [62,63]. It is also supported by the previous studies that showed using organic draw solutes were more effective in terms of osmotic pressure and membrane flux than inorganic draw solutes [64–66].
2.6. Organic carbon and the spectroscopic analyses for the samples Dissolved organic carbon (DOC) concentrations of the pre-filtered raw leachate and the membrane foulant samples were determined using a TOC analyzer (Shimadzu V-series, TOC-CPH). A UV–Vis spectrophotometer (UV-1800, Shimadzu, Japan) was employed using a 1-cm cuvette to measure the ultraviolent absorption at a wavelength of 254 nm (UVA254). The fluorescence features of the DOM samples were examined via the excitation emissions matrices (EEMs) measured by a fluorescence spectrophotometer (Hitachi, F7000, Japan) at a scan speed of 12,000 nm/min. The slit widths were set at 10 nm for both the excitation and the emission scans with wavelength intervals of 1 nm and 5 nm, respectively, which were in the ranges of 200–450 nm and 300–500 nm. The inner filter correction was omitted by diluting the samples with DDW to a value of UVA254 < 0.05 cm−1 [36]. The Raleigh scattering was avoided using a 290 nm cut-off glass filter [48]. The EEM of the DDW was taken into account for the background signal from DDW [49]. 2.7. PARAFAC modeling on fluorescence data Parallel factor analysis (PARAFAC) modeling was executed on the fluorescence EEM dataset for all the samples using MATLAB 13.0 (Mathworks, Natick, MA) and a freely downloaded DOMFluor toolbox (http://www.models.life.du.dk/). Following the established stepwise procedure [50], the EEMs were processed for an outlier test and a sensitivity analysis. Split half validation and a residual analysis were used to confirm the fluorescent components computed from the modeling [51,52]. The fluorescence intensity was reported in Raman units (R.U.), and the maximum fluorescence intensity (Fmax) of the prominent peak was adopted to represent the relative concentrations of the individual fluorescent components. 3. Results and discussion 3.1. Identification of fluorescent fractions from landfill leachate Modeling the entire EEMs via PARAFAC showed that combinations of three distinguished fluorescent components could describe all the DOM samples obtained from this study (Fig. 2). Component 1 (C1) displays its primary peak at the excitation wavelength (Ex)/emission wavelength (Em) range of 225/340 nm and a secondary peak at 275/ 340 nm. The primary and the secondary peaks of component 2 (C2) are positioned at the Ex/Em range of 240/410 nm and 325/410 nm, respectively. On the other hand, component 3 (C3) presents three fluorescence peaks that include one primary peak at Ex/Em of 250/456 nm and two secondary peaks at Ex/Em of 305/456 nm and 365/456 nm. The three identified fluorescent components were also reported in previous studies using stabilized landfill leachate [53–56]. Based on the peak locations and the assignments described in the literature, C1 exhibits the properties similar to tryptophan-like fluorescence [39,56], and C2 and C3 can be assigned to the fulvic-like and the humic-like fluorescent components of landfill leachate, respectively [55,57]. The molecular weight (MW) distribution of the same landfill leachate previously showed that the MW of the fluorescent components tended to increase in the order of C1 < C2 < C3 [36]. It was also reported that fluorescence peaks at a higher Em were closely associated with the features of the organic matter with more hydrophobic properties [58,59]. Thus, the humic-like component (C3) can be presumed to be a high MW (HMW) fraction with a relatively high degree of hydrophobicity, while the tryptophan-like component (C1) as a low MW (LMW) fraction with relatively hydrophilic nature. 4
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Fig. 2. Three identified fluorescent components of landfill leachate samples in this study (a) tryptophan- like component - C1 (b) fulvic- like component - 2 (c) humiclike component - C3, and the corresponding loading plots (below) for (d) C1, (e) C2 and (f) C3. Fig. 3. (a) FO membrane flux of landfill leachate. (b) Membrane flux recovery after filtration run with landfill leachate, after osmotic backwash, and after chemically cleaning. (c) DOC concentrations and the intensities of three fluorescent components from extracted membrane foulants. (d) Membrane resistance of the FO system operated for 24 h using 0.5 M NaCl draw solute and at 0.02 m/s of the flow velocity (Rir, Rre, and Rin refer to the irreversible, reversible, and intrinsic membrane resistances to the FO membrane, respectively).
C/L to 45.8 mg C/L during the osmotic backwash duration from 5 to 40 min. This drastic increment in the bulk organic matter concentrations of the draw solution clearly indicates the substantial detachment of the fouling layer from the FO membrane caused by the backwashing. It also showed that the FO membrane fouling is highly reversible, which was in a good agreement with the previous reports that used the FO
3.3. Osmotic backwashing for FO membrane For osmotic backwash experiments, in which the flux direction was reversed using NaCl on the feed side, the DOC concentrations of extracted foulants on the feed side showed an increasing trend with the backwashing duration (Fig. 4a). For example, the DOC rose from 4.7 mg 5
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Fig. 4. Variations of DOC concentrations and the intensities of three fluorescent components of the detached membrane foulants with increasing osmotic backwash time (a) and the pictures of the fouled FO membranes at different backwash intervals. (b) Draw solute: NaCl (EC: 10 mS/cm), Feed solute: DDW, and flow velocity: 0.02 m/s.
3.4. Optimized condition for intermittent osmotic relaxation (IOR)
systems treating different types of organics [4,67,68]. The membrane de-fouling seems to mostly occur at an early phase of the backwash, which is shown by the initial sharp increase of the DOC in the organic foulants for the first 20 min followed by a slowly increasing or a steady trend for the rest of the period (Fig. 4). Based on the results, it can be presumed that 20 min might be the optimum backwash duration for an FO system. This trend was also visually observed from the actual membrane foulant layers pictured at different time intervals (Fig. 4b), in which there was no dramatic detachment of the fouled membrane layer after 20 min of backwashing time. Further investigation of the detached foulants via EEM-PARAFAC showed no major changes in the percent relative contributions of three different fluorescence fractions with an increasing backwashing duration (Fig. 4). The highest abundance (~60%) of C1 fraction was consistent with the previous observation of its dominant presence both in the RE and the IR foulants. The invariant trend of the chemical composition suggests that there is no preferential detachment property among the different foulant fractions with osmotic backwashing. Similar findings were observed from a study by Lotif et al. [60], which demonstrated that osmotic backwashing completely cleaned a fouled FO membrane by loosening the organic layer on/in the membrane followed by a detachment with water flushing. In another backwashing setting using raw landfill leachate on the feed side (Fig. 1), it was interesting to observe that the water flux was similar (~4 LMH) to the level obtained at the previous normal backwash mode using an NaCl solution as the draw solute and also that the membrane fouling resistance was considerably reduced by 45% (Fig. S2). Furthermore, the simple physical rinsing of the fouled FO membrane for 40 min did not result in a significant alleviation of membrane fouling (only 4% decrease in Rt) (Fig. S2), suggesting the necessity of osmotic backwashing. These results reveal the great possibility of using the inherent osmotic potential of landfill leachate for the osmotic backwashing of a fouled FO membrane. Moreover, the alternative option of backwashing requires no additional draw solutes and energy. Tu et al. [35] reported the superior cleaning efficiency of the osmotic backwashing over other physical methods, such as air scouring, and a high crossflow rate for an FO system treating wastewater effluents from activated sludge biological treatment processes. However, this approach may have limitations in practice when it is applied with a full cycle like the previous studies, because the membrane was fully fouled, and the attachment of the fouling layer can be strengthened throughout the full filtration period. Therefore, a strategy should be established for a more efficient operation. In this context, IOR can be an option to heighten the practical value.
3.4.1. Optimum filtration frequency for IOR To find the optimum filtration intervals for IOR, the flux recoveries and the membrane fouling resistances were compared with three different filtration intervals, which included 2, 4, and 6 h, prior to the 20 min of long-backwashing. The flux behavior of the FO system after the IOR was not the same for the different filtration intervals, which is illustrated in Fig. 5. For the IOR run that operated at 6 h of the interval, the membrane flux showed a general decreasing trend during the operation time with an incomplete recovery of the flux shown after each backwash cycle. For example, the recovered fluxes after each backwash corresponded to 7.5, 7.0, and 6.7 LMH on the second, the third, and the fourth cycle, respectively (Fig. 5a). At a filtration interval of 4 h, the relative difference between the initial flux and the recovered flux immediately after the backwash became less pronounced with the fluxes after the fourth, the fifth, and the sixth cycles of the backwash corresponding to 7.9, 7.6, and 7.5 LMH, respectively. For 2 h of the filtration interval, the recovered membrane flux immediately after the backwash remained the same as the original initial flux throughout the first 7 cycles, and only a slight decrease was observed for the final recovered flux (Fig. 5a). The final membrane fluxes after 24 h of filtration under the IOR with the 2, 4, and 6 h filtration intervals were 6.4, 6.0, and 5.2 LMH, respectively, which corresponded to 372.4, 361.6, and 337.7 mL of the filtered volumes (Fig. 5b). These results showed that all the tested conditions significantly improved the performance in terms of membrane fouling mitigation compared to the reference run. However, the more frequent osmotic backwash was found to be more beneficial to achieve a higher flux or an extended filtered volume. The membrane flux recovery after 2, 4, and 6 h of the filtration intervals reached 94%, 90%, and 86%, respectively. Moreover, the fluxes of the membrane under all the IOR conditions were fully recovered after a 24-h run with a simple physical cleaning without further chemical cleaning (Fig. 5c). Such a successful performance of the IOR for membrane fouling mitigation was also proven with the substantial reduction in total membrane fouling resistance from 3358 × 105 1/m for the reference run to 2360 × 105, 2544 × 105, and 2586 × 105 1/m for 2, 4, and 6 h of the filtration intervals, respectively. These results were supported by the DOC concentrations of the extracted foulants from the membrane filtered at the different filtration intervals that included 2, 4, and 6 h, which corresponded to 24.6 ± 3.8, 28.3 ± 4.1, and 37.6 ± 9.4 mg C/L, respectively. They were much lower than the DOC concentration (52 mg C/L) of the reversible foulants from the reference run. The more frequent cycles of the IOR also contributed to the reduction of the irreversible fouling, which is shown by the DOC of the irreversible 6
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Fig. 5. Comparison of membrane flux (a) membrane resistance and filtered volume, (b) and flux recovery, and (c) at different filtration intervals for the FO system operated under intermittent osmotic relaxation. (Draw solute concentration: 0.5 M NaCl, flow velocity of the draw and feed solute: 0.02 m/s).
Fig. 6. DOC concentrations and the intensities of three fluorescent components of the (a) reversible and the (b) irreversible membrane foulants extracted from the membrane at different filtration intervals for the FO system operated under intermittent osmotic relaxation. (Draw solute concentration: 0.5 M NaCl, flow velocity of the draw and feed solute: 0.02 m/s).
membrane resistance and more complete flux recovery shown after the physical cleaning of the FO membrane.
foulants equivalent to 8.7 ± 0.8, 9.6 ± 1.6, and 12.4 ± 1.4 mg/L for 2, 4, and 6 h of the filtration intervals, respectively (Fig. 6b). Under the three IOR conditions, C1 was the most dominant fraction in both the RE and the IR foulants with a general trend of having less presence with a shorter filtration interval (Fig. 6). Meanwhile, the C2 and C3 components were completely absent in the irreversible foulants for the IOR runs, which indicates the loosen attachment of the two fractions onto the membrane during the relatively short filtration times (i.e., < 6 vs. 24 h). No substantial decrease in the reverse salt accumulation during the optimized condition of IOR (i.e. 4H 20 M) also suggests that salinity is not the governing factor for the improved membrane fluxes and low membrane fouling (Fig. S3b). The overall results suggest that shorter filtration intervals in the IOR led to the higher membrane fluxes, the higher water recovery, and the greater extent of the mitigation of membrane fouling with less
3.4.2. Optimum backwashing duration for IOR The improved performances were compared for the three different backwash durations of 10, 20, and 40 min at a fixed filtration cycle of 4 h (Fig. S4). For 10 min of backwashing time, only a slight improvement was observed for the membrane fouling. For example, the reduction of the total membrane resistance reached only 6.6% (Fig. S4b), and there were no statistical differences in the DOC removals and the flux recoveries between the IOR and the reference run (Student t-test, p > 0.05) (Figs. S4 and S5). These results demonstrated that the IOR strategy with 10 min of the backwashing was not sufficient for a satisfactory performance in terms of FO membrane fouling mitigation. In contrast, the longer backwashing times, which included 20 and 40 min, 7
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Appendix A. Supplementary data
resulted in substantial improvements in the performance. For example, a < 30% reduction in the total membrane fouling resistance, a complete flux recovery of the membrane after physical cleaning, an up to 26% increase in the filtered volume, and ~ 48% and ~65% reduction in DOC of the RE and IR membrane foulants, respectively, were observed for 20 min of backwashing time. A further increase of the backwashing time interval from 20 to 40 min did not significantly improved the filtered volume (Student t-test, p= 0.82) and the mitigation of membrane fouling as measured by the DOC concentrations of the extracted foulants (Student t-test, p= 0.74). These results suggest that a 20 min long backwashing would be the optimized condition for the IOR run of the FO system to treat landfill leachate. The findings of this study withhold the environmental significances with respect to FO processes targeted at the removal of landfill leachate, because the proposed IOR strategy does not require any additional energy or chemical dosages. Also, it ensures a stable long-term operation with alleviated membrane fouling. The option can be applied for other wastewater types with a high ionic strength, and it can also be extended into full-scale FO plants.
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4. Conclusions FO membrane subjected to landfill leachate treatment was considerably fouled with a total membrane resistance of (Rt) of 3360 × 105 1/m during 24 h of operation. Characterizing the membrane foulants of an FO membrane with EEM-PARAFAC revealed that a tryptophan-like component was the main fluorescence fraction responsible for both the RE and the IR. The FO membrane fouling was highly reversible, which could be significantly reduced by osmotic backwashing. Interestingly, the observed membrane flux (~4 LMH) and the reduction of Rt (45%) after osmotic backwashing were comparable to those of a novel option where the inherent osmotic potential of landfill leachate was utilized to clean the fouled membrane. The IOR strategy became more effective for a shorter filtration interval and a longer backwashing time. Under an optimized IOR condition, such as a 4 h of filtration interval and 20 min of backwashing, a significant reduction (30%) in Rt, a complete flux recovery after the physical cleaning, > 45% of DOC reduction of the extracted membrane foulants, and a 26% increment in filtered volume were observed. The results demonstrated that the newly proposed IOR strategy could be successfully applied for an economical and long-term operation of an FO membrane treatment for landfill leachate. This study presents a practicable and innovative strategy for a long-term operation of an FO system targeted at the treatment of high-salts containing wastewater to have an alleviated membrane fouling without additional energy and chemical cost. CRediT authorship contribution statement Bilal Aftab:Methodology, Formal analysis, Writing - original draft.Jinwoo Cho:Writing - review & editing.Jin Hur:Project administration. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by grants from the National Research Foundation of Korea (NRF) funded by the Korean government (2020R1A4A2002823). We are thankful to Mr. Tae Jun Park (Sejong University) for providing the ion composition of landfill leachate. 8
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Intermittent osmotic relaxation: A strategy for organic fouling mitigation in a forward osmosis system treating landfill leachate
T
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Bilal Aftab, Jinwoo Cho, Jin Hur
Department of Environment and Energy, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006, South Korea
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Forward osmosis Intermittent osmotic backwashing Physical cleaning Membrane fouling EEM-PARAFAC
In this study, the potential of osmotic backwashing was explored for the alleviation of the forward osmosis (FO) membrane fouling induced by landfill leachate. The normal osmotic backwashing employing NaCl as a draw solute resulted in an improved membrane flux (~ 4 LMH) and a reduction (45%) of the membrane resistance. We proposed an alternative way of osmotic backwashing based on the inherent osmotic potential of landfill leachate itself for the FO system. For the effort, intermittent osmotic relaxation (IOR) was newly introduced to the FO system to enhance the performance. This operation showed a comparable performance with fouling mitigation to the previous normal osmotic backwashing using NaCl as a draw solute. Under an optimized condition (4 h of filtration intervals and 20 min of backwashing time), the integration of the IOR with the FO system led to a 30% reduction in membrane resistance, a complete flux recovery, and 26% increment in the filtered volume. The IOR process does not require additional energy and/or chemical dosage. Therefore, it can be suggested as an innovative, practical, and energy-efficient osmotic backwashing strategy for a stable and the long-term operation of FO systems treating saline wastewater.
1. Introduction Over the past decades, forward osmosis (FO) has established its status as a promising green technology for desalination and wastewater treatment due to its benefit of naturally driven osmotic pressure by a
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solute intensity difference [1,2]. It has shown its superiority with its performance over other conventional membrane processes, which include nanofiltration (NF) and reverse osmosis (RO), with a higher organic rejection, a lower energy demand, and a greater propensity of reversible versus irreversible fouling [3,4]. Nevertheless, the
Corresponding author. E-mail address: [email protected] (J. Hur).
https://doi.org/10.1016/j.desal.2020.114406 Received 25 September 2019; Received in revised form 19 February 2020; Accepted 22 February 2020 0011-9164/ © 2020 Elsevier B.V. All rights reserved.
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showed a substantial reduction of membrane biofouling and high permeate fluxes [41,42]. Similarly, the maintenance of an average stable flux has been demonstrated in the ultrafiltration of river water for a prolonged operation that lasted 30 days via an intermittent operation [43]. This approach could be applied to the osmotic backwashing of the FO systems to treat landfill leachate with the expectation of a long-term filterability and an effective alleviation of organic fouling along with continued and stable operation. This option can be costeffective, because electrical, hydraulic, or ultrasonic assistance is not required. Recently, fluorescence excitations emissions matrix combined with parallel factor analysis (EEM-PARAFAC) has been successfully applied to probe the compositional variations of different fluorescent organic foulants in conventional membrane filtration systems [44–46]. On the contrary, only a limited amount of information is available in the literature to keep track of the different constituents in landfill leachate in organic de-fouling of an FO membrane via osmotic backwashing. This study aimed to (1) test the applicability of the osmotic relaxation for an FO system to treat landfill leachate, and (2) further explore the effectiveness of intermittent osmotic relaxation (IOR) with respect to the filterability and organic fouling behavior for an FO system. The EEMPARAFAC was utilized to track the changes of organic foulants and to identify the detached foulants after osmotic backwashing. For this study, a crossflow FO setup was operated to treat landfill leachate under different IOR conditions.
performance of a FO membrane is hampered by the fundamental operation problems, such as organic fouling. Due to the lack of pressurized forces, the foulant layer in an FO membrane is less prone to hydraulic compactions, it is comparatively looser, and it is more likely to be partially appressed compared to those in conventional membrane filtration, which makes it easy to become detached via simple physical cleaning practices [5–8]. This characteristic may improve the reliability of FO systems to treat complex wastewater. There were many previous examples to demonstrate the potential of FO systems for the treatment of treat sewage [9], textile wastewater [10], high strength wastewater [11,12], digested sludge [13], and landfill leachate [14–16]. Landfill leachate is regarded as the most hazardous form of wastewater due to the abundance of the range of persistent pollutants [17,18]. In particular, mature leachate from old landfill sites contains numerous refractory organics, which are resilient to biological treatments [19,20]. Various on-site treatment options, which includes membrane filtration, have been adopted to protect surrounding environments from their hazards [21]. Several obstacles hindering efficient FO systems have been identified [8,22,23], and membrane fouling is considered as one of the critical challenges that affect the operational cost and the quality/quantity of the treated water recovered from feed solutions [2]. In previous studies, several in-line physical cleaning strategies were proposed to alleviate the membrane fouling in FO systems, which included the use of ultrasound waves [7], the application of an electric current to a modified conductive FO membrane [24], the creation of high crossflow velocities [25], and extended crossflow flushing combined with ultrasonication [26]. However, these approaches usually include additional operating energy and costs, which overshadow the inherent benefit of FO as an economic operation. Thus, it is highly required to suggest an alternative low-cost physical cleaning technique for the fouling mitigation in FO systems. Osmotic backwashing refers to the osmotic scouring of a fouled layer by reversing the direction of the osmosis across the membrane and has been conventionally practiced for RO membranes [27]. It was previously reported that RO membranes were potentially backwashed with the osmotic pressure without any external pressure [28,29]. The osmotic backwash can be effective with respect to the cross flow velocity and the backwashing period [30]. However, the requirement of a very high concentration of draw solutes may hamper the performance of osmotic backwashing in RO membranes [31]. Many previous studies have proved the applicability of osmotic backwashing in FO systems for improved flux recovery and fouling reversibility [32–34]. In a comparison between the RO and the FO membranes subjected to wastewater reuse, it was observed that FO membrane was less prone to membrane fouling and could be cleaned by osmotic backwashing more easily as compared to RO membrane [35]. It has also been pointed out that using additional draw solute for backwashing may increase the operational cost, which is a drawback. Regarding this issue, it is worth noting that a characteristic of landfill leachate encompasses a variety of dissolved salts and organics with electric conductivity that ranges from 10 to 400 mS/cm [36–39]. Therefore, it is reasonably suggested that the inherent osmotic potential of feed leachate could be utilized to backwash fouled FO membrane by switching the draw solute with the feed solution possessing a lower osmotic potential, such as distilled deionized water (DDW). This setup may substantially reduce the operation cost that might be taken for the osmotic backwashing of an FO membrane for the treatment of landfill leachate. In previous reports, osmotic backwashing has been practiced as a cleaning protocol only after the completion of the entire filtration cycle [32,40]. On the other hand, the inline or intermittent relaxation via the osmotic backwashing of an FO membrane has not been tested to date even though this approach has evidenced its effectiveness as a fouling mitigation practice in conventional membrane systems. For example, it was previously reported that the intermittent scouring of ultrafiltration membrane foulants using pressure relaxation in membrane bioreactors
2. Materials and methods 2.1. Feed leachate sampling and its chemical properties A sample of stabilized landfill leachate (40 L) was collected from an old landfill site (Sudokwon management Corp.), situated in the vicinity of Incheon city, South Korea. Solid waste had been dumped into the landfill for 8 years, before the site was closed in 2000. Sampling was conducted in June 2018 using an acid washed 20 L- polypropylene plastic container. The sample was filtered through a pre-ashed Whatman GF/F filter with a pore size of 0.7 μm. The filtered landfill leachate sample was stored in a refrigerator at 4 °C. The basic water quality of the sample is listed in Table S1. The ion composition of the landfill leachate is shown in Table. S2. 2.2. Forward osmosis system configuration and operation A crossflow FO system with a working volume of 400 mL was set up to treat the raw filtered landfill leachate, which is shown in Fig. 1. A flat sheet cellulose triacetate (CTA) FO membrane, which was purchased from Fluid Technology Solution, Inc. (FTSH2O) (Albany, USA), was installed in an acrylic external FO membrane module with an effective filtration area of 22 cm2. The feed solution (FS) and the draw solution (DS) were circulated across the membrane module in concurrent directions with peristaltic pumps. Sodium chloride (NaCl) was used as a draw solute, and the solute concentration was maintained by intermittently adding a concentrated draw solution, which was 5 M, NaCl, into the DS tank. A pump for the DS tank was connected to an online conductivity controller to monitor and maintain a constant osmotic potential for the entire filtration run. Volumetric changes in the feed tank were regularly checked on a data logging scale, and the information was sent to a computer to calculate the membrane flux (Jw) in liters per minute per hour (LMH). The estimation was based on the following Eq. (1):
jw =
1 ΔVt · Am Δt
(1)
where Am is the working membrane area, and ΔVt is the volumetric change recorded by the logger at a time interval of Δt. A reference run of the FO filtration was carried out for 24 h to 2
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Fig. 1. Schematic diagrams of the forward osmosis and the IOR settings used in this study.
2.5. Membrane flux recovery and the resistance analysis
obtain the fouled membrane for further experiments. The reference run was operated under an optimized operating condition following Aftab et al. [36], in which 1.5 M NaCl was used for the DS, and the flow velocity for DS and FS was kept at 0.02 m/s.
For the membrane flux recovery and the resistance analysis, an average flux for the virgin membrane (Jv) was first calculated for 60 min under a normal FO setting using 0.5 M NaCl as a draw solute and DDW as feed water. For either the reference or the IOR run, the membrane flux for the fouled membrane was calculated for 60 min after each filtration run, which was denoted as the membrane flux after filtration (Jo). The membrane was then set for normal osmotic backwashing by circulating the NaCl solution on the feed side and DDW on the draw side for 20 min, while the loosely bounded organic fractions were extracted from the FO membrane. This extracted fraction was called reversible foulants [47]. The average membrane flux was then calculated for 60 min and denoted as a membrane flux after the physical cleaning (Jp). Next, the membrane was detached from the assembly and soaked into a 0.1 M NaOH solution (50 mL) for 30 min to chemically extract the irreversible foulants (IR) from the membrane. This procedure was adopted only to extract the membrane foulants from the membrane. The membrane was rinsed with DDW, and the average membrane flux was calculated again for 60 min, which was denoted as a membrane flux after the chemical cleaning (Jc). Detailed explanations for the flux recovery are available in the literature [9,36]. The osmotic pressure (Pa) of the landfill leachate was measured using the following Eq. (2):
2.3. Experimental conditions for osmotic backwashing of FO membrane Two types of osmotic backwash experiments were conducted using the fouled FO membrane obtained from the reference run. For the first case, the feed side of the module was replaced with a NaCl solution with an EC value the same as raw landfill leachate (~10 mS/cm), and the DS was replaced with distilled deionized water (DDW) to reverse the direction of the permeate flux across the membrane (Fig. S1). The samples were collected from the feed side every 5 min throughout a full period of 40 min and sonicated for 20 min to completely dissolve the extracted fouling layer. The samples were then passed through a 0.45 μm pre-rinsed CTA filter (47 mm, Advantac) for the dissolved organic matter (DOM) analyses. For the second case, to activate the inherent de-fouling potential of the landfill leachate, in which the backflow was induced by the osmotic potential of landfill leachate, the DS was replaced with DDW on the draw side of the module and no change was made on the feed side that contained landfill leachate (Fig. S1). For both de-fouling tests, a membrane flux was calculated for a 40 min of filtration run.
π=i×ϕ×C×R×T
(2)
where i, ϕ, and C are the number of ions, the osmotic coefficient, the sum of the concentrations of all solutes, respectively. R is the universal gas constant and T is the absolute temperature. The value for the solute concentrations in landfill leachate was taken from the information in Table S2. The FO membrane resistance (m−1) was calculated using the following Eqs. (3)–(5) based on membrane fouling in series model:
2.4. Intermittent osmotic relaxation (IOR) experiments The configuration for the IOR is shown in Fig. 1. The IOR was performed by intermittently replacing the DS in the draw tank with DW and keeping all other operational conditions the same as the reference run during the filtration run. This setup induces the osmotic backwashing of the FO membrane to be driven by the innate osmotic pressure of the landfill leachate. To evaluate the effect of the backwashing intervals, the IOR was performed such that the FO system was run for a certain period of time that included 2, 4, or 6 h, which was followed by different times of backwashing (i.e., 10, 20, or 40 min) for the whole 24 h-operation. After each FO run was completed under individual IOR conditions, the membrane resistance, the membrane flux recovery, and the final filtered volume were measured, and they were compared with those of the reference run. Meanwhile, small amounts (12 to 22 mL) of water in the backflow moving to the feed side during the backwash intervals were not considered for the calculation of the final filtered volume after the entire filtration run. The amounts of the DDW required for the IOR experiments are shown in Table S3.
J=
Rt =
πd − πf 1 dV × = A dt μ × Rt
(3)
πd − πf J×μ
(4)
Rt = Rm + Rre + Rir
(5)
where πd and πf are the osmotic pressure of DS and FS, respectively, which is the driving osmotic force of the FO operation. μ (Pa s) is the dynamic viscosity. Rm, Rre, and Rir refer to the intrinsic membrane resistance, the reversible membrane resistance, and the irreversible membrane resistance to filtration, respectively. Rm was measured by the flux Jv obtained for the virgin membrane. Rre was obtained using the flux Jp after the membrane was physically cleaned using osmotic 3
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3.2. Membrane filtration behavior and subsequent membrane fouling
backwash. Rir was calculated using the flux Jc after the membrane was chemically using 0.1 M NaOH.
Water recovery from raw landfill leachate was observed for the FO system that operated using NaCl as a draw solute at 0.02 m/s of flow velocity. It led to a concentration of raw leachate by a factor of 2.6 with a final recovered volume of 250 mL (versus the working volume of 400 mL). During the operation, the DOC of the leachate increased from 180 mg C/L to 310 mg C/L on the feed side. The membrane flux declined gradually from 8.3 to 2.2 LMH with filtration time (Fig. 3a). This trend was previously explained by the decrease in the osmotic pressure between the DS and the FS, which resulted from the concentration of the leachate and/or from the organic membrane fouling followed by the filtration of the landfill leachate [16]. In this study, the former was not taken into account as the major mechanism that caused the flux decline, because a larger difference in the conductivity between the DS (120 mS/cm) and the FS was observed even after the full concentration (~40 mS/cm) of the feed leachate. Thus, the latter process seems to play significant roles in the decline of the flux. Substantial membrane fouling was obvious as indicated by a decrease in the flux recovery of the FO membrane to 71% and a total membrane fouling resistance of 1/ m. On the other hand, a simple physical cleaning using osmotic backwashing resulted in the reduction of the membrane fouling resistance to a value of 680 × 105 1/m along with a flux recovery of 90.1%, which suggests that the physical cleaning is effective enough to recover the FO performance. Similar findings were previously reported in several previous studies of FO systems to treat landfill leachate or other types of feed solutions [26,36,60]. In this study, chemical cleaning led to a full recovery of the membrane flux with the reduction of the membrane fouling resistance to 620 × 105 1/m, which was nearly equals to the intrinsic resistance of the membrane. The organic foulants extracted from the FO membrane showed the supportive results that were evidence of a greater contribution of physical versus chemical cleaning to the fouling mitigation. The bulk DOC of the RE was 15 mg C/L, which was higher than that of the IR (8 mg C/L). Further probing the chemical composition of the membrane foulants via the EEM-PARAFAC revealed that C1 was the main fluorescent component responsible for both the RE and the IR with the percent contributions of 50% and 60%, respectively. The LMW characteristic of C1 likely made the fraction (i.e., C1) easier to penetrate the membrane pores. The relatively hydrophilic nature of the fraction could also have contributed to its binding with the hydrophilic membrane more strongly than the C2 or the C3 fractions via the dipole-dipole interactions or the hydrogen bonding [61]. The initial deposition of the C1 fraction on the membrane may cause the reduction of the repulsive charges on the membrane surface [47], which may facilitate the further attachment of relative hydrophobic fractions (C2 and C3). The much greater contribution of C3 fraction to the RE versus the IR (Fig. 3) indicates that the C3 component or a relatively hydrophobic fraction might be loosely bound to the membrane. The results in conjunction imply that reversible fouling contributed more to the decline of the flux and also that the physical cleaning using membrane backwashing was sufficient to recover the membrane flux to a satisfactory level. It was also found from the PARAFAC results that the C1 component was the main contributor to the membrane fouling. These results encouraged us to further explore the de-fouling performance of the simple backwashing method for fouled FO membrane at different settings and compare the applicability of different strategies for the successful removal of organic foulant fractions from a membrane surface. One promising strategy to be proposed here is benefitted by the high ionic strength of landfill leachate (EC: 10.2 mS/cm in this study) associated with a high osmotic potential [62,63]. It is also supported by the previous studies that showed using organic draw solutes were more effective in terms of osmotic pressure and membrane flux than inorganic draw solutes [64–66].
2.6. Organic carbon and the spectroscopic analyses for the samples Dissolved organic carbon (DOC) concentrations of the pre-filtered raw leachate and the membrane foulant samples were determined using a TOC analyzer (Shimadzu V-series, TOC-CPH). A UV–Vis spectrophotometer (UV-1800, Shimadzu, Japan) was employed using a 1-cm cuvette to measure the ultraviolent absorption at a wavelength of 254 nm (UVA254). The fluorescence features of the DOM samples were examined via the excitation emissions matrices (EEMs) measured by a fluorescence spectrophotometer (Hitachi, F7000, Japan) at a scan speed of 12,000 nm/min. The slit widths were set at 10 nm for both the excitation and the emission scans with wavelength intervals of 1 nm and 5 nm, respectively, which were in the ranges of 200–450 nm and 300–500 nm. The inner filter correction was omitted by diluting the samples with DDW to a value of UVA254 < 0.05 cm−1 [36]. The Raleigh scattering was avoided using a 290 nm cut-off glass filter [48]. The EEM of the DDW was taken into account for the background signal from DDW [49]. 2.7. PARAFAC modeling on fluorescence data Parallel factor analysis (PARAFAC) modeling was executed on the fluorescence EEM dataset for all the samples using MATLAB 13.0 (Mathworks, Natick, MA) and a freely downloaded DOMFluor toolbox (http://www.models.life.du.dk/). Following the established stepwise procedure [50], the EEMs were processed for an outlier test and a sensitivity analysis. Split half validation and a residual analysis were used to confirm the fluorescent components computed from the modeling [51,52]. The fluorescence intensity was reported in Raman units (R.U.), and the maximum fluorescence intensity (Fmax) of the prominent peak was adopted to represent the relative concentrations of the individual fluorescent components. 3. Results and discussion 3.1. Identification of fluorescent fractions from landfill leachate Modeling the entire EEMs via PARAFAC showed that combinations of three distinguished fluorescent components could describe all the DOM samples obtained from this study (Fig. 2). Component 1 (C1) displays its primary peak at the excitation wavelength (Ex)/emission wavelength (Em) range of 225/340 nm and a secondary peak at 275/ 340 nm. The primary and the secondary peaks of component 2 (C2) are positioned at the Ex/Em range of 240/410 nm and 325/410 nm, respectively. On the other hand, component 3 (C3) presents three fluorescence peaks that include one primary peak at Ex/Em of 250/456 nm and two secondary peaks at Ex/Em of 305/456 nm and 365/456 nm. The three identified fluorescent components were also reported in previous studies using stabilized landfill leachate [53–56]. Based on the peak locations and the assignments described in the literature, C1 exhibits the properties similar to tryptophan-like fluorescence [39,56], and C2 and C3 can be assigned to the fulvic-like and the humic-like fluorescent components of landfill leachate, respectively [55,57]. The molecular weight (MW) distribution of the same landfill leachate previously showed that the MW of the fluorescent components tended to increase in the order of C1 < C2 < C3 [36]. It was also reported that fluorescence peaks at a higher Em were closely associated with the features of the organic matter with more hydrophobic properties [58,59]. Thus, the humic-like component (C3) can be presumed to be a high MW (HMW) fraction with a relatively high degree of hydrophobicity, while the tryptophan-like component (C1) as a low MW (LMW) fraction with relatively hydrophilic nature. 4
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Fig. 2. Three identified fluorescent components of landfill leachate samples in this study (a) tryptophan- like component - C1 (b) fulvic- like component - 2 (c) humiclike component - C3, and the corresponding loading plots (below) for (d) C1, (e) C2 and (f) C3. Fig. 3. (a) FO membrane flux of landfill leachate. (b) Membrane flux recovery after filtration run with landfill leachate, after osmotic backwash, and after chemically cleaning. (c) DOC concentrations and the intensities of three fluorescent components from extracted membrane foulants. (d) Membrane resistance of the FO system operated for 24 h using 0.5 M NaCl draw solute and at 0.02 m/s of the flow velocity (Rir, Rre, and Rin refer to the irreversible, reversible, and intrinsic membrane resistances to the FO membrane, respectively).
C/L to 45.8 mg C/L during the osmotic backwash duration from 5 to 40 min. This drastic increment in the bulk organic matter concentrations of the draw solution clearly indicates the substantial detachment of the fouling layer from the FO membrane caused by the backwashing. It also showed that the FO membrane fouling is highly reversible, which was in a good agreement with the previous reports that used the FO
3.3. Osmotic backwashing for FO membrane For osmotic backwash experiments, in which the flux direction was reversed using NaCl on the feed side, the DOC concentrations of extracted foulants on the feed side showed an increasing trend with the backwashing duration (Fig. 4a). For example, the DOC rose from 4.7 mg 5
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Fig. 4. Variations of DOC concentrations and the intensities of three fluorescent components of the detached membrane foulants with increasing osmotic backwash time (a) and the pictures of the fouled FO membranes at different backwash intervals. (b) Draw solute: NaCl (EC: 10 mS/cm), Feed solute: DDW, and flow velocity: 0.02 m/s.
3.4. Optimized condition for intermittent osmotic relaxation (IOR)
systems treating different types of organics [4,67,68]. The membrane de-fouling seems to mostly occur at an early phase of the backwash, which is shown by the initial sharp increase of the DOC in the organic foulants for the first 20 min followed by a slowly increasing or a steady trend for the rest of the period (Fig. 4). Based on the results, it can be presumed that 20 min might be the optimum backwash duration for an FO system. This trend was also visually observed from the actual membrane foulant layers pictured at different time intervals (Fig. 4b), in which there was no dramatic detachment of the fouled membrane layer after 20 min of backwashing time. Further investigation of the detached foulants via EEM-PARAFAC showed no major changes in the percent relative contributions of three different fluorescence fractions with an increasing backwashing duration (Fig. 4). The highest abundance (~60%) of C1 fraction was consistent with the previous observation of its dominant presence both in the RE and the IR foulants. The invariant trend of the chemical composition suggests that there is no preferential detachment property among the different foulant fractions with osmotic backwashing. Similar findings were observed from a study by Lotif et al. [60], which demonstrated that osmotic backwashing completely cleaned a fouled FO membrane by loosening the organic layer on/in the membrane followed by a detachment with water flushing. In another backwashing setting using raw landfill leachate on the feed side (Fig. 1), it was interesting to observe that the water flux was similar (~4 LMH) to the level obtained at the previous normal backwash mode using an NaCl solution as the draw solute and also that the membrane fouling resistance was considerably reduced by 45% (Fig. S2). Furthermore, the simple physical rinsing of the fouled FO membrane for 40 min did not result in a significant alleviation of membrane fouling (only 4% decrease in Rt) (Fig. S2), suggesting the necessity of osmotic backwashing. These results reveal the great possibility of using the inherent osmotic potential of landfill leachate for the osmotic backwashing of a fouled FO membrane. Moreover, the alternative option of backwashing requires no additional draw solutes and energy. Tu et al. [35] reported the superior cleaning efficiency of the osmotic backwashing over other physical methods, such as air scouring, and a high crossflow rate for an FO system treating wastewater effluents from activated sludge biological treatment processes. However, this approach may have limitations in practice when it is applied with a full cycle like the previous studies, because the membrane was fully fouled, and the attachment of the fouling layer can be strengthened throughout the full filtration period. Therefore, a strategy should be established for a more efficient operation. In this context, IOR can be an option to heighten the practical value.
3.4.1. Optimum filtration frequency for IOR To find the optimum filtration intervals for IOR, the flux recoveries and the membrane fouling resistances were compared with three different filtration intervals, which included 2, 4, and 6 h, prior to the 20 min of long-backwashing. The flux behavior of the FO system after the IOR was not the same for the different filtration intervals, which is illustrated in Fig. 5. For the IOR run that operated at 6 h of the interval, the membrane flux showed a general decreasing trend during the operation time with an incomplete recovery of the flux shown after each backwash cycle. For example, the recovered fluxes after each backwash corresponded to 7.5, 7.0, and 6.7 LMH on the second, the third, and the fourth cycle, respectively (Fig. 5a). At a filtration interval of 4 h, the relative difference between the initial flux and the recovered flux immediately after the backwash became less pronounced with the fluxes after the fourth, the fifth, and the sixth cycles of the backwash corresponding to 7.9, 7.6, and 7.5 LMH, respectively. For 2 h of the filtration interval, the recovered membrane flux immediately after the backwash remained the same as the original initial flux throughout the first 7 cycles, and only a slight decrease was observed for the final recovered flux (Fig. 5a). The final membrane fluxes after 24 h of filtration under the IOR with the 2, 4, and 6 h filtration intervals were 6.4, 6.0, and 5.2 LMH, respectively, which corresponded to 372.4, 361.6, and 337.7 mL of the filtered volumes (Fig. 5b). These results showed that all the tested conditions significantly improved the performance in terms of membrane fouling mitigation compared to the reference run. However, the more frequent osmotic backwash was found to be more beneficial to achieve a higher flux or an extended filtered volume. The membrane flux recovery after 2, 4, and 6 h of the filtration intervals reached 94%, 90%, and 86%, respectively. Moreover, the fluxes of the membrane under all the IOR conditions were fully recovered after a 24-h run with a simple physical cleaning without further chemical cleaning (Fig. 5c). Such a successful performance of the IOR for membrane fouling mitigation was also proven with the substantial reduction in total membrane fouling resistance from 3358 × 105 1/m for the reference run to 2360 × 105, 2544 × 105, and 2586 × 105 1/m for 2, 4, and 6 h of the filtration intervals, respectively. These results were supported by the DOC concentrations of the extracted foulants from the membrane filtered at the different filtration intervals that included 2, 4, and 6 h, which corresponded to 24.6 ± 3.8, 28.3 ± 4.1, and 37.6 ± 9.4 mg C/L, respectively. They were much lower than the DOC concentration (52 mg C/L) of the reversible foulants from the reference run. The more frequent cycles of the IOR also contributed to the reduction of the irreversible fouling, which is shown by the DOC of the irreversible 6
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Fig. 5. Comparison of membrane flux (a) membrane resistance and filtered volume, (b) and flux recovery, and (c) at different filtration intervals for the FO system operated under intermittent osmotic relaxation. (Draw solute concentration: 0.5 M NaCl, flow velocity of the draw and feed solute: 0.02 m/s).
Fig. 6. DOC concentrations and the intensities of three fluorescent components of the (a) reversible and the (b) irreversible membrane foulants extracted from the membrane at different filtration intervals for the FO system operated under intermittent osmotic relaxation. (Draw solute concentration: 0.5 M NaCl, flow velocity of the draw and feed solute: 0.02 m/s).
membrane resistance and more complete flux recovery shown after the physical cleaning of the FO membrane.
foulants equivalent to 8.7 ± 0.8, 9.6 ± 1.6, and 12.4 ± 1.4 mg/L for 2, 4, and 6 h of the filtration intervals, respectively (Fig. 6b). Under the three IOR conditions, C1 was the most dominant fraction in both the RE and the IR foulants with a general trend of having less presence with a shorter filtration interval (Fig. 6). Meanwhile, the C2 and C3 components were completely absent in the irreversible foulants for the IOR runs, which indicates the loosen attachment of the two fractions onto the membrane during the relatively short filtration times (i.e., < 6 vs. 24 h). No substantial decrease in the reverse salt accumulation during the optimized condition of IOR (i.e. 4H 20 M) also suggests that salinity is not the governing factor for the improved membrane fluxes and low membrane fouling (Fig. S3b). The overall results suggest that shorter filtration intervals in the IOR led to the higher membrane fluxes, the higher water recovery, and the greater extent of the mitigation of membrane fouling with less
3.4.2. Optimum backwashing duration for IOR The improved performances were compared for the three different backwash durations of 10, 20, and 40 min at a fixed filtration cycle of 4 h (Fig. S4). For 10 min of backwashing time, only a slight improvement was observed for the membrane fouling. For example, the reduction of the total membrane resistance reached only 6.6% (Fig. S4b), and there were no statistical differences in the DOC removals and the flux recoveries between the IOR and the reference run (Student t-test, p > 0.05) (Figs. S4 and S5). These results demonstrated that the IOR strategy with 10 min of the backwashing was not sufficient for a satisfactory performance in terms of FO membrane fouling mitigation. In contrast, the longer backwashing times, which included 20 and 40 min, 7
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Appendix A. Supplementary data
resulted in substantial improvements in the performance. For example, a < 30% reduction in the total membrane fouling resistance, a complete flux recovery of the membrane after physical cleaning, an up to 26% increase in the filtered volume, and ~ 48% and ~65% reduction in DOC of the RE and IR membrane foulants, respectively, were observed for 20 min of backwashing time. A further increase of the backwashing time interval from 20 to 40 min did not significantly improved the filtered volume (Student t-test, p= 0.82) and the mitigation of membrane fouling as measured by the DOC concentrations of the extracted foulants (Student t-test, p= 0.74). These results suggest that a 20 min long backwashing would be the optimized condition for the IOR run of the FO system to treat landfill leachate. The findings of this study withhold the environmental significances with respect to FO processes targeted at the removal of landfill leachate, because the proposed IOR strategy does not require any additional energy or chemical dosages. Also, it ensures a stable long-term operation with alleviated membrane fouling. The option can be applied for other wastewater types with a high ionic strength, and it can also be extended into full-scale FO plants.
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4. Conclusions FO membrane subjected to landfill leachate treatment was considerably fouled with a total membrane resistance of (Rt) of 3360 × 105 1/m during 24 h of operation. Characterizing the membrane foulants of an FO membrane with EEM-PARAFAC revealed that a tryptophan-like component was the main fluorescence fraction responsible for both the RE and the IR. The FO membrane fouling was highly reversible, which could be significantly reduced by osmotic backwashing. Interestingly, the observed membrane flux (~4 LMH) and the reduction of Rt (45%) after osmotic backwashing were comparable to those of a novel option where the inherent osmotic potential of landfill leachate was utilized to clean the fouled membrane. The IOR strategy became more effective for a shorter filtration interval and a longer backwashing time. Under an optimized IOR condition, such as a 4 h of filtration interval and 20 min of backwashing, a significant reduction (30%) in Rt, a complete flux recovery after the physical cleaning, > 45% of DOC reduction of the extracted membrane foulants, and a 26% increment in filtered volume were observed. The results demonstrated that the newly proposed IOR strategy could be successfully applied for an economical and long-term operation of an FO membrane treatment for landfill leachate. This study presents a practicable and innovative strategy for a long-term operation of an FO system targeted at the treatment of high-salts containing wastewater to have an alleviated membrane fouling without additional energy and chemical cost. CRediT authorship contribution statement Bilal Aftab:Methodology, Formal analysis, Writing - original draft.Jinwoo Cho:Writing - review & editing.Jin Hur:Project administration. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This work was supported by grants from the National Research Foundation of Korea (NRF) funded by the Korean government (2020R1A4A2002823). We are thankful to Mr. Tae Jun Park (Sejong University) for providing the ion composition of landfill leachate. 8
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