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Fate of NaClO and membrane foulants during in-situ cleaning of membrane bioreactors: Combined effect on thermodynamic properties of sludge
Biochemical Engineering Journal 147 (2019) 146–152
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Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej
Regular article
Fate of NaClO and membrane foulants during in-situ cleaning of membrane bioreactors: Combined effect on thermodynamic properties of sludge Haifeng Zhanga,b,1, Ming Suna,1, Lianfa Songb, Jingbo Guoc, Lanhe Zhanga,
T
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a
School of Chemistry Engineering, Northeast Electric Power University, Jilin, 132012, PR China Department of Civil, Environmental, and Construction Engineering, Texas Tech University, Lubbock, TX, 79409-1023, USA c School of Civil and Architecture Engineering, Northeast Electric Power University, Jilin, 132012, PR China b
H I GH L IG H T S
high membrane fouling induced by in-situ NaClO cleaning. • ANaClO-treated had a higher adhesive and cohesive propensity. • The EPS of thesludge detached foulants contained a high PN level. • The detached foulants potentially aggregated with treated sludge. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane bioreactor Membrane foulants In-situ cleaning Interactions energy Extracellular polymeric substances
In this study, the fate of sodium hypochlorite (NaClO) and detached membrane foulants and their impacts on the thermodynamic properties of raw sludge were investigated in a membrane bioreactor (MBR). It was found that about 20.3% of the total NaClO was potentially released into the bulk sludge, which significantly altered the raw sludge surface properties, such as a reduction in the number of surface electron donor components (γ − ) and an increase in surface hydrophobicity. After exposure to NaClO, the treated sludge had a higher adhesive and cohesive propensity compared to the raw sludge. Based on the extended Derjaguin-Landau-Verwey- Overbeek (XDLVO) theory and the batch filtration test results, the detached foulants potentially aggregated with the treated sludge and form the reconstructed sludge. The reconstructed sludge had a high protein (PN) level in the extracellular polymeric substances (EPS), with a higher membrane fouling potential compared with the raw sludge.
1. Introduction Membrane fouling is a major hindrance to the faster commercialization of membrane bioreactors (MBRs). Thus, to overcome this hindrance, considerable efforts have been made to develop effective fouling control strategies in the last decades [1–3]. Although numerous approaches have been developed to control membrane fouling, chemical cleaning with sodium hypochlorite (NaClO) is still a major effective method for removing reversible and irreversible membrane fouling in MBRs [4,5]. Compared to ex-situ chemical cleaning, in-situ cleaning is generally preferred due to its simplicity, low cost and effectiveness [6,7]. Therefore, the periodic NaClO in-situ cleaning has been extensively employed for membrane fouling control in the actual MBR [8].
During in-situ membrane cleaning, the NaClO solution is injected into the membrane in reverse to normal flow filtration [8,9], and thus the raw sludge is inevitably exposed to NaClO. The exposure to NaClO has been previously reported to have significant negative impacts on various bulk sludge properties, such as sludge deflocculation, lysis of bacterial cells, release of dissolved matter, and sludge filterability deterioration, etc [7,9,10]. These adverse effects of NaClO on bulk sludge have raised serious concerns about membrane fouling induced by in-situ cleaning. In general, membrane fouling is determined by the thermodynamic properties of the membrane and activated sludge, which can be described by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory [1,11,12]. However, the information about the effect of NaClO diffusion on the thermodynamic properties of the sludge is still very
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Corresponding author. E-mail address: [email protected] (L. Zhang). 1 Equal contribution and co-first author. https://doi.org/10.1016/j.bej.2019.04.016 Received 29 September 2018; Received in revised form 13 April 2019; Accepted 17 April 2019 Available online 17 April 2019 1369-703X/ © 2019 Elsevier B.V. All rights reserved.
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limited. Moreover, the fate of the detached foulants after in-situ cleaning of the MBR tank is not clear. For example, the detached foulants could be returned into the MBR tank, and they may adhere onto the membrane surface and become membrane foulants again when membrane filtration restarts. On the other hand, the detached foulants may aggregate with the treated sludge after exposure to NaClO and form the reconstructed sludge. Accordingly, the objective of this study was to investigate the fate of NaClO and membrane foulants and their effects on the thermodynamic properties of bulk sludge. The concentration of the diffused NaClO was separately measured in the cleaning reactor and actual MBR with the cleaning time, and the detached foulants were collected from the cleaning reactor. The detached foulants, raw sludge and reconstructed sludge were characterized in terms of protein (PN), polysaccharide (PS) and extracellular DNA (eDNA) content. The interfacial interactions of the membrane with different types of sludge, and sludge themselves, were assessed using the XDLVO model. Additionally, the batch filtration test was employed to evaluate the fouling potential of the treated sludge and detached foulants.
Table 1 Compositions of the synthetic municipal wastewater. Main nutrients
Concentration (mg/L)
Trace elements
Concentration (mg/L)
Glucose NH4Cl KH2PO4 K2HPO4 Yeast extract NaHCO3
280 100 11 10 50 172
FeCl3 ZnCl2 MnCl2·4H2O CoCl2·6H2O CuSO4 CaCl2 MgSO4·7H2O
0.38 0.06 0.06 0.13 0.06 10 50
2.2. Membrane cleaning process and NaClO shock experiment The whole operation time was divided into four runs. When the trans-membrane pressure (TMP) reached about 30 kPa during Run I and Run II, the fouled membrane was transferred to the cleaning reactor (with the same volume as the MBR tank). A volume of 7.3 L of ultrapure water was added into the cleaning reactor prior to cleaning the membrane. NaClO (Sigma-Aldrich, Saint Louis, MO, USA) was used as cleaning reagent. According to the protocol provided by manufacturer, a volume of 200 mL of NaClO with an effective chlorine concentration of 3600 mg/L was fed into membrane with a vertical distance of 0.5 m. After 30 min, normal aeration (200 L/h) was applied for 150 min. During the 3-hour cleaning process, the leakage was determined by measuring the concentration of NaClO by the N, N-diethyl-p- phenylenediamine method using a Hach DR 1900 spectrometer (Hach Company, Loveland, CO, USA) at 15-min intervals. At the end of the membrane cleaning process, the detached foulants were collected from the reactor to performed further measurements. Specially, at the end of Run III, the in-situ cleaning was performed using the same cleaning conditions as those used in ex-situ cleaning (Run I and II). After membrane cleaning for 3 h, the sludge samples were collected and used as the reconstructed sludge. According to the NaClO diffusion level after the ex-situ cleaning, the batch test was independently conducted at the end of Run I and II. In the batch test, a sludge sample of 200 mL was collected from the MBR tank and divided into 2 types of samples of 100 mL each. One of the 100-mL sludge sample was stirred at 80 r/min and used as the control sample, referred to as the raw sludge. The other 100-mL sludge sample was combined with a pre-determined amount of NaClO (21.7 and 23.9 mg/L for Run I and II, respectively), stirred at 80 r/min and used as the treated sludge sample type. After 30 min, the two types of sludge samples were obtained and further analyzed.
2. Material and methods 2.1. MBR systems A laboratory-scale submerged MBR with a working volume of 7.5 L was operated for about 100 days. The schematic illustration of the submerged MBR is depicted in Fig. 1. A flat-sheet polyvinylidene fluoride (PVDF) membrane (Shanghai SINAP Membrane Tech Co., Ltd., Shanghai, China) with a pore size of 0.22 μm and a filtration area of 0.1 m2 was submerged in the reactor. The filtrate through the submerged membrane module was continuously withdrawn using a peristaltic pump model BT-100 (Baoding Longer Precision Pump Co., Ltd., Baoding, Hebei, China) at a constant flux of 12.5 L/m2 h, operated in the mode of 13 min on and 2 min off. The hydraulic retention time (HRT) and sludge retention time (SRT) were about 6 h and 30 d, respectively. Fine bubble aeration (200 L/h) was provided by an air pump to maintain an appropriate concentration of dissolved oxygen. The MBR was fed with the synthetic municipal wastewater (Table 1), which had same compositions to that used in previous studies [3,13]. The influent concentrations of chemical oxygen demand (COD), ammonium nitrogen (NH4+-N), total phosphorus (TP) were 310.5 ± 16.5, 28.3 ± 1.5, and 6.3 ± 0.7 mg/L, respectively. Water quality parameters including COD, NH4+-N, TP, and mixed liquor suspended solids (MLSS) were measured according to the standard methods [14].
Fig. 1. Schematic illustration of the submerged MBR. 147
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2.3. Batch filtration test
method. A Zeta 90 Plus Zeta Potential Analyzer (Brookhaven Instruments Corp., Holtsville, NY, USA) was used to analyze the zeta potential of the cleaned membrane surface. The raw sludge was collected before the ex-situ cleaning in Run I and II, treated sludge and detached foulants were collected after the ex-situ cleaning at the end of Run I and II, and the reconstructed sludge was collected after in-situ cleaning in Run III. At least seven contact angles and zeta potentials were determined for each sludge sample with the highest and lowest values discarded. Average value was taken as the contact angle and zeta potential to calculate the surface tensions based on the Young’s equations [18].
A crossflow filtration equipment was used to evaluate the fouling potential of the treated sludge and detached foulants. A flat-sheet PVDF membrane with an effective area of 42 cm2, which was the same as that of the membrane in the actual MBR, was embedded in the crossflow cell (CF042 Membrane Cell, Sterlitech, Kent, WA, USA). A 250-mL sample was collected from the cleaning reactor, freeze-dried and used as the detached foulants. In order to simulate in-situ cleaning, the detached foulants were added into 250 mL of treated sludge in the feed chamber. The cross-flow rate was controlled at 0.3 L/min by a peristaltic pump. After 30 min of filtration, the fouled membrane was taken out and the initial membrane foulants were collected from the membrane surface.
2.6. XDLVO theory The surface tension parameters (γ LW , γ + and γ −) of the cleaned membrane, different types of sludge and detached foulants are calculated by solving a set of three Young’s equations [18]. According to the values of γ LW , γ + and γ −, the γ AB and γTot were additionally calculated. The ΔGadh is related to the adhesion of the loose fraction on the membrane surface, while ΔGcoh is the cohesion energy per unit area between the loose fraction and strong fraction. It has been established that the more negative the values of ΔGadh and ΔGcoh are, the stronger the adhesion and cohesion are [1]. The surface hydrophobicity/hydrophilicity ( ΔGsws ) was also evaluated using the free energy of the interaction between two identical surfaces immersed in water, the surface can be considered to be hydrophobic if ΔGsws < 0, and vice versa [11]. In addition, the total interaction energy between the sludge floc and membrane in an aqueous solution at a separation distance is equal to the sum of three interaction energy components (Lewis acidbase, Lifshitz-van der Waals and electrostatic double layer) of the interaction energy. The detailed calculation process is described in the supporting information.
2.4. Determination of extracellular polymeric substances (EPS) The stratification structure and extraction protocol for the activated sludge in this study were modified according to a previous study [3]. Briefly, a sludge suspension was first dewatered by centrifugation at 4000 g for 10 min at 4 ℃, and the bulk solution was collected and is referred to soluble EPS (S-EPS). The bottom sediment was resuspended to its original volume with 0.9% NaCl and sheared with a vortex mixer model G-560 (Scientific Industries, Inc., Bohemia, NY, USA) for 1 min. Then, it was centrifuged at 5000 g for 15 min at 4 ℃, and the bulk solution was collected as the loose bound EPS (LB-EPS). The residual sludge pellet in the centrifuge tube was resuspended again to its original volume and then extracted by ultrasonication at 20 kHz and 480 W for 10 min. The extracted solution was centrifuged at 20,000 g for 20 min to collect the tight bound EPS (TB-EPS), and the remainder was the pellet. Protein was determined by the Bradford (Coomassie brilliant blue) method [15] using bovine serum albumin (BSA) as the standard and the polysaccharide content was determined according to the Antrone’s method [16] using glucose as the standard. The eDNA was measured using the diphenylamine colorimetric method [17] using calf thymus DNA as the standard. The excitation and emission matrix (EEM) spectra of EPS were measured using a RF-6000 luminescence spectrometer (Shimadzu Co., Kyoto, Japan) and detailed information is provided in the supporting materials.
3. Results and discussion 3.1. Performance of MBR The conventional pollutant removal efficiencies in all runs are shown in Table 2. The average COD and NH4+-N were both steadily removed in Run I, II and III (under ex-situ cleaning condition), keeping an average removal rate over 91 and 94%, respectively. In Run IV (under in-situ cleaning condition), the COD, NH4+-N and TP removal rates were slightly decreased compared with those of ex-situ cleaning condition, indicating a negative impact on microbial activity due to the diffusion of NaClO into the bulk sludge. The inhibition of the degradation of activated sludge has also been previously reported to occur during in-situ cleaning with NaClO in MBRs [6,9]. Typical TMP profiles depicting the fouling trends and the corresponding rates of increases in all runs are shown in Fig. 2. The profiles clearly reveal a sharp rise in the TMP rate of increase at the beginning of Run IV. For example, the TMP rate of increase during Run IV was 23.5 kPa/d (at the initial day 1), achieving a rate increase nearly 16.8, 18.1 and 13.1-fold higher than that of Run I, II and III (1.4, 1.3 and 1.8 kPa/d), respectively, indicating that in-situ cleaning with NaClO dramatically deteriorated the filterability of activated sludge and started a severe membrane fouling. Evidence of a faster membrane
2.5. Contact angle and zeta potential analysis The contact angles of the samples (including raw sludge, treated sludge, detached foulants and reconstructed sludge) were measured according to the method described by Hong et al. [2]. In brief, the samples were firstly filtered through a membrane with a 0.45 μm pore size, and the retained fraction was pressed with two slides to form a flat surface. Then, the samples were dried in a desiccator for 24 h to remove surplus water. Three probe liquids, including ultrapure water, diiodomethane and glycerol, were used for contact angle measurements. The static contact angles of the probe liquids on the prepared samples were measured using a JC2000D1 contact angle meter (Shanghai Powereach Co., Ltd., Shanghai, China) according to the sessile drop method. The zeta potentials of all samples were measured with a JS94H Zetasizer (Shanghai Powereach Co., Ltd.) using the electrophoretic mobility Table 2 A summary of the MBR performance in all runs. Item
COD removal (%) NH4+-N removal (%) TP removal (%) MLSS (mg/L)
Ex-situ cleaning
In-situ cleaning
Run I (0-25 d)
Run II (26-50d)
Run III (51-75d)
Run IV (76-96d)
92.5 ± 4.6 94.4 ± 5.0 23.7 ± 9.8 4043.0 ± 194.9
91.7 ± 3.0 95.6 ± 4.4 25.0 ± 6.3 4049.8 ± 237.8
93.1 ± 2.9 94.2 ± 4.9 26.4 ± 8.1 3971.3 ± 302.4
88.9 ± 5.1 90.2 ± 3.4 20.2 ± 2.4 3969.6 ± 258.6
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Fig. 2. Variation of the TMP profiles and membrane fouling rates with the operation time in all runs: (a) TMP changes, and (b) membrane fouling rate.z.
NaClO level in the mixed liquor can induce the changes in the surface properties of sludge [13], which may in turn enhance the post-development of membrane biofouling in the MBR. The data of the surface properties of activated sludge before and after exposure to NaClO in the batch tests are shown in Table 3, and were used to calculate the surface thermodynamic parameters shown in Table 4. The data presented in Table 4 reveal that the exposure to NaClO significantly changed the surface thermodynamic parameters of the treated sludge, for example, the electron donor components (γ −) of the treated sludge (7.1 mJ/m2) were decreased by about 230% compared with those of the raw sludge (23.4 mJ/m2). A reduction in the surface γ − indicated that the fouling layer formation by the treated sludge would favor the subsequent fouling development [2]. Compared with the raw sludge (−7.6 mJ/m2), the treated sludge had a higher hydrophobicity (−60.6 mJ/m2), indicating that more hydrophobic components were exposed by treatment with NaClO. The surface hydrophobicity is an important indicator of the fouling propensity of the sludge, thus a higher level of hydrophobicity leads to a higher membrane fouling in the MBR [3]. The interaction energy between the membrane and sludge is divided into two types. One is the attachment strength between sludge and membrane (Adhesion) and the other is the cohesive strength between the sludge themselves (Cohesion) [1,20]. The comparison of the
fouling rate after chemical cleaning with NaClO in the MBR has been reported in previous studies [6,9,13,19]. Compared to the ex-situ cleaning, the significant increase in the initial membrane fouling rate could be attributed to both the NaClO and detached foulants in the bulk sludge, leading to the changes in the membrane fouling potential of activated sludge.
3.2. Release of NaClO and its impact on the surface properties of raw sludge In order to investigate the release behavior of NaClO, the NaClO concentration was measured under the ex-situ and in-situ cleaning conditions (Fig. 3). The results shown in Fig. 3 reveal a linear increase in the NaClO concentration with increasing cleaning time up to 105 min during the ex-situ cleaning, with the maximal NaClO concentration in the cleaning reactor averaging 22.8 mg/L. Comparatively, an evidently lower NaClO content in the MBR was found during the in-situ cleaning (ranged from 0.5 to 3.4 mg/L). The lower level of NaClO in the MBR was mainly due to the fast oxidizing reaction between the microbes and NaClO [7], while the diffused NaClO was rarely consumed under the exsitu cleaning condition. Based on the results of the ex-situ cleaning tests, the potential proportion of NaClO utilization was approximately 73.2, 20.3, and 6.5% in the cleaning membrane, diffusion into the bulk, and residual in the membrane module, respectively. In particular, a high 149
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Fig. 3. Changes in the NaClO concentration with increasing cleaning time during the ex-situ and in-situ cleaning operation.
interaction energies between the raw sludge and treated sludge is shown in Fig. 4. The data clearly show that the strength of the adhesive interaction energy ( ΔGadh ) of the treated sludge was significantly increased by about 3.2 times with the membrane compared with that of the raw sludge. The higher negative ΔGadh indicated that the treated sludge had a higher membrane fouling potential than the raw sludge. Moreover, the higher ΔGadh also strongly suggested that the detachment of the treated sludge from the membrane surface was more difficult to achieve by shearing. In addition, the treated sludge exhibited higher self-cohesive strength (-48.6 mJ/m2) compared with the raw sludge (−5.9 mJ/m2). Apparently, the treated sludge exhibited a higher fouling potential due to its higher adhesive and cohesive ability than that of the raw sludge, which would explain why many researchers have observed a faster membrane fouling rate after chemical cleaning in MBRs [6,19,21].
Table 4 Calculated surface energy parameters (mJ/m2) for sludge samples and membrane. Materials
γLW
γ+
γ−
γAB
γTot
ΔGsws
Cleaned membrane Raw sludge Treated sludge Detached foulants Reconstructed sludge
28.32 31.17 26.94 34.12 29.78
1.07 4.08 1.58 0.14 0.82
12.88 23.41 7.12 22.02 8.81
−7.44 −19.54 −6.71 −3.50 −5.38
20.88 11.63 20.22 30.62 24.40
−36.41 −7.64 −60.63 −10.51 −50.84
a high PN content in the LB-EPS of detached foulants will have a negative impact on the sludge filterability [3,5]. The interaction energies of detached foulants, including adhesion 1 with membrane ( ΔGadh ), cohesion with themselves ( ΔG coh ), and cohe2 coh sion with treated sludge ( ΔG ), are each estimated from Fig. 6. All three interaction energies were negative and their absolute values 1 2 follow the order: ΔGcoh > ΔGadh > ΔG coh , The self-cohesive strength 2 of the detached foulants (-9.9 mJ/m ) was the weakest among these interactions, implying that the detached foulants did not preferentially aggregate with each other. Compared with the ΔGadh (-18.6 mJ/m2), the detached foulants had a stronger attraction with the treated sludge (-27.6 mJ/m2), showing a greater tendency to aggregate with the treated sludge. The batch crossflow filtration test was conducted with mixed samples as described in Section 2.3. The three-dimensional EEM spectroscopy was applied to identify the EPS components of the initial membrane foulants, treated sludge, and detached foulants shown in Fig S1.
3.3. Composition of detached foulants and its fate in MBR The detached foulants were collected in the cleaning reactor (26 th and 51 th) and their EPS were quantified as shown in Fig. 5. The results revealed that PN was the major component in the different EPS layers of detached foulants. In particular, the detached foulants were found to contain high S-EPS level, which mainly originated from soluble foulants, cell lysis and LB-EPS released by NaClO oxidation [5,9]. In addition, a high eDNA level (59.7 mg/g SS on average) was detected in the S-EPS, confirming that a substantial amount of biomass within the fouling layer was chemically solubilized to dissolved organic matter by strong oxidation with NaClO. Overall, a significant amount of S-EPS and Table 3 Contact angle and zeta potential data for the membrane and sludge samples.a Materials
Cleaned membrane Raw sludge Treated sludge Detached foulants Reconstructed sludge a
Ultrapure water
87.81 83.84 97.97 69.61 90.80
± ± ± ± ±
1.87 1.72 5.73 2.18 0.13
Contact angle (°)
Zeta potential (mV)
Glycerol
Diiodomethane
93.16 ± 1.62 99.27 ± 1.96 100.75 ± 2.04 74.51 ± 10.65 92.05 ± 2.45
59.16 55.48 62.85 50.27 57.91
Each data was measured at least seven times. 150
± ± ± ± ±
1.68 4.29 4.63 8.37 1.68
−31.30 −42.88 −43.26 −43.54 −31.77
± ± ± ± ±
0.13 0.04 1.61 5.06 0.75
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Fig. 4. Comparison of the interaction energy of adhesion (ΔGadh) of both types of sludge with the membrane and cohesion energy (ΔGcoh) between the sludge themselves.
Fig. 7. Distributions of the EPS fractions in the raw sludge and reconstructed sludge.
the initial fouling layer. Due to NaClO oxidation, the detached foulants became more hydrophilic. Thus, the indicator of hydrophocity/hydrophylicy could be used to trace the detached foulants. As revealed by the data in Table 4, the hydrophobicity of the detached foulants, treated sludge, and reconstructed sludge was −10.5, −60.6 and −50.8 mJ/ m2, respectively, thus a significantly lower hydrophobicity was observed for the detached foulants. Especially, the reconstructed sludge exhibited a lower hydrophobicity than that of treated sludge, implying that the detached foulants participated in the reconstitution with treated sludge, thereby reducing the hydrophobicity of reconstructed sludge.
3.4. Characteristics of the reconstructed sludge In Run IV, the sludge samples were collected before and after in-situ cleaning and their EPS composition was measured as shown in Fig. 7. The data shown in Fig. 7, reveal that the PN content in the S-EPS and LB-EPS was about 1.7-fold and 2.1-fold higher, respectively, in the reconstructed sludge compared with the raw sludge. The high PN content in the EPS outer layer was likely mainly due to the exposure to NaClO in the raw sludge. Meanwhile, a substantial amount of PN in the EPS outer layer of detached foulants (Fig. 5) could be due to a different reason. Evidence has shown that a higher PN content induces higher membrane fouling in MBRs [22–24]. It is worth noting that the PN and PS content clearly increased in the TB-EPS of the reconstructed sludge compared with that in the raw sludge, which is consistent with a previous report in which the over-production of EPS was considered to be due to a bacterial response to stress [5]. A comparison of the interaction energy profile of the raw sludge with that of the reconstructed sludge when they approached the membrane surface is presented in Fig. 8. The interaction energy showed a two-stage profile characterized by a positive energy stage followed by a rapid energy decline as the separation distance decreased for both types of sludge. A relatively high energy barrier existed between the raw sludge-membrane combination (4,240.7 kT), whereas a relatively low energy barrier existed between the reconstructed sludge and the membrane (2,292.7 kT), strongly suggesting that the reconstructed sludge had a higher membrane fouling potential than that of the raw sludge. Therefore, the combined effects of the exposure to NaClO and detached foulants on activated sludge started a severe membrane fouling, especially in the initial stage (Fig. 2). In this work, the synthetic wastewater was used in order to avoid difficulties in quantification of metabolic products due to variation in the composition and concentration of organic pollutants in the real wastewater. In fact, the membrane fouling is a complex, dynamic process in MBRs, which is closely related to the composition of feedwater and change of organic-
Fig. 5. Composition of the EPS within the detached foulants collected from the cleaning reactor.
Fig. 6. The adhesion energy ( ΔGadh ) of the detached foulants with the mem1
brane, the cohesion energy ( ΔG coh ) of the detached foulants with themselves and cohesion energy (
2 ΔG coh
) of the detached foulants with the treated sludge.
Even though the EPS from the three samples exhibited similar composition, the intensities of the detected peaks corresponding to various organics were different. Clearly, other than the detached foulants, the initial foulants had a similar peak intensity with the treated sludge. According to the fluorescence regional integration (FRI) distribution (Fig. S1), the percent fluorescence response of each region in the initial membrane foulants showed a similar distribution as the treated sludge, suggesting that the detached foulants were not the main contributors to
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Fig. 8. Comparison of the interaction energy between the membrane-raw sludge combination and the membrane-reconstructed sludge combination as a function of the separation distance.
loading [25,26]. Based on the results of this study, therefore, the further work need focus on the verification investigation using the real wastewater in the future. 4. Conclusions In the present study, the fate of NaClO and detached foulants, and their combined effect on the thermodynamic properties of raw sludge were systematically investigated. About a 20.3% of the total NaClO potentially diffused into the bulk solution, and caused a higher initial membrane fouling rate. The NaClO altered the surface properties of the raw sludge and enhanced both the adhesive and cohesive propensity of the treated sludge. The detached foulants contained a high PN level within the different EPS layers, potentially aggregated with the treated sludge and formed the reconstructed sludge. The reconstructed sludge had a high PN content within the EPS, and exhibited a higher membrane fouling potential compared with the raw sludge. Acknowledgments The authors wish to thank and acknowledge the support of the National Natural Science Foundation of China (No. 51478093 & 51678119), the Jilin Province Scientific and the Technological Planning Project of China (No. 20170519013JH). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.bej.2019.04.016. References [1] Y. Tian, Z. Li, Y. Ding, Y. Lu, Identification of the change in fouling potential of soluble microbial products (SMP) in membrane bioreactor coupled with worm reactor, Water Res. 47 (6) (2013) 2015–2024. [2] H. Hong, M. Zhang, Y. He, J. Chen, H. Lin, Fouling mechanisms of gel layer in a submerged membrane bioreactor, Bioresour. Technol. 166 (2014) 295–302.
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Regular article
Fate of NaClO and membrane foulants during in-situ cleaning of membrane bioreactors: Combined effect on thermodynamic properties of sludge Haifeng Zhanga,b,1, Ming Suna,1, Lianfa Songb, Jingbo Guoc, Lanhe Zhanga,
T
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a
School of Chemistry Engineering, Northeast Electric Power University, Jilin, 132012, PR China Department of Civil, Environmental, and Construction Engineering, Texas Tech University, Lubbock, TX, 79409-1023, USA c School of Civil and Architecture Engineering, Northeast Electric Power University, Jilin, 132012, PR China b
H I GH L IG H T S
high membrane fouling induced by in-situ NaClO cleaning. • ANaClO-treated had a higher adhesive and cohesive propensity. • The EPS of thesludge detached foulants contained a high PN level. • The detached foulants potentially aggregated with treated sludge. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane bioreactor Membrane foulants In-situ cleaning Interactions energy Extracellular polymeric substances
In this study, the fate of sodium hypochlorite (NaClO) and detached membrane foulants and their impacts on the thermodynamic properties of raw sludge were investigated in a membrane bioreactor (MBR). It was found that about 20.3% of the total NaClO was potentially released into the bulk sludge, which significantly altered the raw sludge surface properties, such as a reduction in the number of surface electron donor components (γ − ) and an increase in surface hydrophobicity. After exposure to NaClO, the treated sludge had a higher adhesive and cohesive propensity compared to the raw sludge. Based on the extended Derjaguin-Landau-Verwey- Overbeek (XDLVO) theory and the batch filtration test results, the detached foulants potentially aggregated with the treated sludge and form the reconstructed sludge. The reconstructed sludge had a high protein (PN) level in the extracellular polymeric substances (EPS), with a higher membrane fouling potential compared with the raw sludge.
1. Introduction Membrane fouling is a major hindrance to the faster commercialization of membrane bioreactors (MBRs). Thus, to overcome this hindrance, considerable efforts have been made to develop effective fouling control strategies in the last decades [1–3]. Although numerous approaches have been developed to control membrane fouling, chemical cleaning with sodium hypochlorite (NaClO) is still a major effective method for removing reversible and irreversible membrane fouling in MBRs [4,5]. Compared to ex-situ chemical cleaning, in-situ cleaning is generally preferred due to its simplicity, low cost and effectiveness [6,7]. Therefore, the periodic NaClO in-situ cleaning has been extensively employed for membrane fouling control in the actual MBR [8].
During in-situ membrane cleaning, the NaClO solution is injected into the membrane in reverse to normal flow filtration [8,9], and thus the raw sludge is inevitably exposed to NaClO. The exposure to NaClO has been previously reported to have significant negative impacts on various bulk sludge properties, such as sludge deflocculation, lysis of bacterial cells, release of dissolved matter, and sludge filterability deterioration, etc [7,9,10]. These adverse effects of NaClO on bulk sludge have raised serious concerns about membrane fouling induced by in-situ cleaning. In general, membrane fouling is determined by the thermodynamic properties of the membrane and activated sludge, which can be described by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory [1,11,12]. However, the information about the effect of NaClO diffusion on the thermodynamic properties of the sludge is still very
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Corresponding author. E-mail address: [email protected] (L. Zhang). 1 Equal contribution and co-first author. https://doi.org/10.1016/j.bej.2019.04.016 Received 29 September 2018; Received in revised form 13 April 2019; Accepted 17 April 2019 Available online 17 April 2019 1369-703X/ © 2019 Elsevier B.V. All rights reserved.
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limited. Moreover, the fate of the detached foulants after in-situ cleaning of the MBR tank is not clear. For example, the detached foulants could be returned into the MBR tank, and they may adhere onto the membrane surface and become membrane foulants again when membrane filtration restarts. On the other hand, the detached foulants may aggregate with the treated sludge after exposure to NaClO and form the reconstructed sludge. Accordingly, the objective of this study was to investigate the fate of NaClO and membrane foulants and their effects on the thermodynamic properties of bulk sludge. The concentration of the diffused NaClO was separately measured in the cleaning reactor and actual MBR with the cleaning time, and the detached foulants were collected from the cleaning reactor. The detached foulants, raw sludge and reconstructed sludge were characterized in terms of protein (PN), polysaccharide (PS) and extracellular DNA (eDNA) content. The interfacial interactions of the membrane with different types of sludge, and sludge themselves, were assessed using the XDLVO model. Additionally, the batch filtration test was employed to evaluate the fouling potential of the treated sludge and detached foulants.
Table 1 Compositions of the synthetic municipal wastewater. Main nutrients
Concentration (mg/L)
Trace elements
Concentration (mg/L)
Glucose NH4Cl KH2PO4 K2HPO4 Yeast extract NaHCO3
280 100 11 10 50 172
FeCl3 ZnCl2 MnCl2·4H2O CoCl2·6H2O CuSO4 CaCl2 MgSO4·7H2O
0.38 0.06 0.06 0.13 0.06 10 50
2.2. Membrane cleaning process and NaClO shock experiment The whole operation time was divided into four runs. When the trans-membrane pressure (TMP) reached about 30 kPa during Run I and Run II, the fouled membrane was transferred to the cleaning reactor (with the same volume as the MBR tank). A volume of 7.3 L of ultrapure water was added into the cleaning reactor prior to cleaning the membrane. NaClO (Sigma-Aldrich, Saint Louis, MO, USA) was used as cleaning reagent. According to the protocol provided by manufacturer, a volume of 200 mL of NaClO with an effective chlorine concentration of 3600 mg/L was fed into membrane with a vertical distance of 0.5 m. After 30 min, normal aeration (200 L/h) was applied for 150 min. During the 3-hour cleaning process, the leakage was determined by measuring the concentration of NaClO by the N, N-diethyl-p- phenylenediamine method using a Hach DR 1900 spectrometer (Hach Company, Loveland, CO, USA) at 15-min intervals. At the end of the membrane cleaning process, the detached foulants were collected from the reactor to performed further measurements. Specially, at the end of Run III, the in-situ cleaning was performed using the same cleaning conditions as those used in ex-situ cleaning (Run I and II). After membrane cleaning for 3 h, the sludge samples were collected and used as the reconstructed sludge. According to the NaClO diffusion level after the ex-situ cleaning, the batch test was independently conducted at the end of Run I and II. In the batch test, a sludge sample of 200 mL was collected from the MBR tank and divided into 2 types of samples of 100 mL each. One of the 100-mL sludge sample was stirred at 80 r/min and used as the control sample, referred to as the raw sludge. The other 100-mL sludge sample was combined with a pre-determined amount of NaClO (21.7 and 23.9 mg/L for Run I and II, respectively), stirred at 80 r/min and used as the treated sludge sample type. After 30 min, the two types of sludge samples were obtained and further analyzed.
2. Material and methods 2.1. MBR systems A laboratory-scale submerged MBR with a working volume of 7.5 L was operated for about 100 days. The schematic illustration of the submerged MBR is depicted in Fig. 1. A flat-sheet polyvinylidene fluoride (PVDF) membrane (Shanghai SINAP Membrane Tech Co., Ltd., Shanghai, China) with a pore size of 0.22 μm and a filtration area of 0.1 m2 was submerged in the reactor. The filtrate through the submerged membrane module was continuously withdrawn using a peristaltic pump model BT-100 (Baoding Longer Precision Pump Co., Ltd., Baoding, Hebei, China) at a constant flux of 12.5 L/m2 h, operated in the mode of 13 min on and 2 min off. The hydraulic retention time (HRT) and sludge retention time (SRT) were about 6 h and 30 d, respectively. Fine bubble aeration (200 L/h) was provided by an air pump to maintain an appropriate concentration of dissolved oxygen. The MBR was fed with the synthetic municipal wastewater (Table 1), which had same compositions to that used in previous studies [3,13]. The influent concentrations of chemical oxygen demand (COD), ammonium nitrogen (NH4+-N), total phosphorus (TP) were 310.5 ± 16.5, 28.3 ± 1.5, and 6.3 ± 0.7 mg/L, respectively. Water quality parameters including COD, NH4+-N, TP, and mixed liquor suspended solids (MLSS) were measured according to the standard methods [14].
Fig. 1. Schematic illustration of the submerged MBR. 147
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2.3. Batch filtration test
method. A Zeta 90 Plus Zeta Potential Analyzer (Brookhaven Instruments Corp., Holtsville, NY, USA) was used to analyze the zeta potential of the cleaned membrane surface. The raw sludge was collected before the ex-situ cleaning in Run I and II, treated sludge and detached foulants were collected after the ex-situ cleaning at the end of Run I and II, and the reconstructed sludge was collected after in-situ cleaning in Run III. At least seven contact angles and zeta potentials were determined for each sludge sample with the highest and lowest values discarded. Average value was taken as the contact angle and zeta potential to calculate the surface tensions based on the Young’s equations [18].
A crossflow filtration equipment was used to evaluate the fouling potential of the treated sludge and detached foulants. A flat-sheet PVDF membrane with an effective area of 42 cm2, which was the same as that of the membrane in the actual MBR, was embedded in the crossflow cell (CF042 Membrane Cell, Sterlitech, Kent, WA, USA). A 250-mL sample was collected from the cleaning reactor, freeze-dried and used as the detached foulants. In order to simulate in-situ cleaning, the detached foulants were added into 250 mL of treated sludge in the feed chamber. The cross-flow rate was controlled at 0.3 L/min by a peristaltic pump. After 30 min of filtration, the fouled membrane was taken out and the initial membrane foulants were collected from the membrane surface.
2.6. XDLVO theory The surface tension parameters (γ LW , γ + and γ −) of the cleaned membrane, different types of sludge and detached foulants are calculated by solving a set of three Young’s equations [18]. According to the values of γ LW , γ + and γ −, the γ AB and γTot were additionally calculated. The ΔGadh is related to the adhesion of the loose fraction on the membrane surface, while ΔGcoh is the cohesion energy per unit area between the loose fraction and strong fraction. It has been established that the more negative the values of ΔGadh and ΔGcoh are, the stronger the adhesion and cohesion are [1]. The surface hydrophobicity/hydrophilicity ( ΔGsws ) was also evaluated using the free energy of the interaction between two identical surfaces immersed in water, the surface can be considered to be hydrophobic if ΔGsws < 0, and vice versa [11]. In addition, the total interaction energy between the sludge floc and membrane in an aqueous solution at a separation distance is equal to the sum of three interaction energy components (Lewis acidbase, Lifshitz-van der Waals and electrostatic double layer) of the interaction energy. The detailed calculation process is described in the supporting information.
2.4. Determination of extracellular polymeric substances (EPS) The stratification structure and extraction protocol for the activated sludge in this study were modified according to a previous study [3]. Briefly, a sludge suspension was first dewatered by centrifugation at 4000 g for 10 min at 4 ℃, and the bulk solution was collected and is referred to soluble EPS (S-EPS). The bottom sediment was resuspended to its original volume with 0.9% NaCl and sheared with a vortex mixer model G-560 (Scientific Industries, Inc., Bohemia, NY, USA) for 1 min. Then, it was centrifuged at 5000 g for 15 min at 4 ℃, and the bulk solution was collected as the loose bound EPS (LB-EPS). The residual sludge pellet in the centrifuge tube was resuspended again to its original volume and then extracted by ultrasonication at 20 kHz and 480 W for 10 min. The extracted solution was centrifuged at 20,000 g for 20 min to collect the tight bound EPS (TB-EPS), and the remainder was the pellet. Protein was determined by the Bradford (Coomassie brilliant blue) method [15] using bovine serum albumin (BSA) as the standard and the polysaccharide content was determined according to the Antrone’s method [16] using glucose as the standard. The eDNA was measured using the diphenylamine colorimetric method [17] using calf thymus DNA as the standard. The excitation and emission matrix (EEM) spectra of EPS were measured using a RF-6000 luminescence spectrometer (Shimadzu Co., Kyoto, Japan) and detailed information is provided in the supporting materials.
3. Results and discussion 3.1. Performance of MBR The conventional pollutant removal efficiencies in all runs are shown in Table 2. The average COD and NH4+-N were both steadily removed in Run I, II and III (under ex-situ cleaning condition), keeping an average removal rate over 91 and 94%, respectively. In Run IV (under in-situ cleaning condition), the COD, NH4+-N and TP removal rates were slightly decreased compared with those of ex-situ cleaning condition, indicating a negative impact on microbial activity due to the diffusion of NaClO into the bulk sludge. The inhibition of the degradation of activated sludge has also been previously reported to occur during in-situ cleaning with NaClO in MBRs [6,9]. Typical TMP profiles depicting the fouling trends and the corresponding rates of increases in all runs are shown in Fig. 2. The profiles clearly reveal a sharp rise in the TMP rate of increase at the beginning of Run IV. For example, the TMP rate of increase during Run IV was 23.5 kPa/d (at the initial day 1), achieving a rate increase nearly 16.8, 18.1 and 13.1-fold higher than that of Run I, II and III (1.4, 1.3 and 1.8 kPa/d), respectively, indicating that in-situ cleaning with NaClO dramatically deteriorated the filterability of activated sludge and started a severe membrane fouling. Evidence of a faster membrane
2.5. Contact angle and zeta potential analysis The contact angles of the samples (including raw sludge, treated sludge, detached foulants and reconstructed sludge) were measured according to the method described by Hong et al. [2]. In brief, the samples were firstly filtered through a membrane with a 0.45 μm pore size, and the retained fraction was pressed with two slides to form a flat surface. Then, the samples were dried in a desiccator for 24 h to remove surplus water. Three probe liquids, including ultrapure water, diiodomethane and glycerol, were used for contact angle measurements. The static contact angles of the probe liquids on the prepared samples were measured using a JC2000D1 contact angle meter (Shanghai Powereach Co., Ltd., Shanghai, China) according to the sessile drop method. The zeta potentials of all samples were measured with a JS94H Zetasizer (Shanghai Powereach Co., Ltd.) using the electrophoretic mobility Table 2 A summary of the MBR performance in all runs. Item
COD removal (%) NH4+-N removal (%) TP removal (%) MLSS (mg/L)
Ex-situ cleaning
In-situ cleaning
Run I (0-25 d)
Run II (26-50d)
Run III (51-75d)
Run IV (76-96d)
92.5 ± 4.6 94.4 ± 5.0 23.7 ± 9.8 4043.0 ± 194.9
91.7 ± 3.0 95.6 ± 4.4 25.0 ± 6.3 4049.8 ± 237.8
93.1 ± 2.9 94.2 ± 4.9 26.4 ± 8.1 3971.3 ± 302.4
88.9 ± 5.1 90.2 ± 3.4 20.2 ± 2.4 3969.6 ± 258.6
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Fig. 2. Variation of the TMP profiles and membrane fouling rates with the operation time in all runs: (a) TMP changes, and (b) membrane fouling rate.z.
NaClO level in the mixed liquor can induce the changes in the surface properties of sludge [13], which may in turn enhance the post-development of membrane biofouling in the MBR. The data of the surface properties of activated sludge before and after exposure to NaClO in the batch tests are shown in Table 3, and were used to calculate the surface thermodynamic parameters shown in Table 4. The data presented in Table 4 reveal that the exposure to NaClO significantly changed the surface thermodynamic parameters of the treated sludge, for example, the electron donor components (γ −) of the treated sludge (7.1 mJ/m2) were decreased by about 230% compared with those of the raw sludge (23.4 mJ/m2). A reduction in the surface γ − indicated that the fouling layer formation by the treated sludge would favor the subsequent fouling development [2]. Compared with the raw sludge (−7.6 mJ/m2), the treated sludge had a higher hydrophobicity (−60.6 mJ/m2), indicating that more hydrophobic components were exposed by treatment with NaClO. The surface hydrophobicity is an important indicator of the fouling propensity of the sludge, thus a higher level of hydrophobicity leads to a higher membrane fouling in the MBR [3]. The interaction energy between the membrane and sludge is divided into two types. One is the attachment strength between sludge and membrane (Adhesion) and the other is the cohesive strength between the sludge themselves (Cohesion) [1,20]. The comparison of the
fouling rate after chemical cleaning with NaClO in the MBR has been reported in previous studies [6,9,13,19]. Compared to the ex-situ cleaning, the significant increase in the initial membrane fouling rate could be attributed to both the NaClO and detached foulants in the bulk sludge, leading to the changes in the membrane fouling potential of activated sludge.
3.2. Release of NaClO and its impact on the surface properties of raw sludge In order to investigate the release behavior of NaClO, the NaClO concentration was measured under the ex-situ and in-situ cleaning conditions (Fig. 3). The results shown in Fig. 3 reveal a linear increase in the NaClO concentration with increasing cleaning time up to 105 min during the ex-situ cleaning, with the maximal NaClO concentration in the cleaning reactor averaging 22.8 mg/L. Comparatively, an evidently lower NaClO content in the MBR was found during the in-situ cleaning (ranged from 0.5 to 3.4 mg/L). The lower level of NaClO in the MBR was mainly due to the fast oxidizing reaction between the microbes and NaClO [7], while the diffused NaClO was rarely consumed under the exsitu cleaning condition. Based on the results of the ex-situ cleaning tests, the potential proportion of NaClO utilization was approximately 73.2, 20.3, and 6.5% in the cleaning membrane, diffusion into the bulk, and residual in the membrane module, respectively. In particular, a high 149
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Fig. 3. Changes in the NaClO concentration with increasing cleaning time during the ex-situ and in-situ cleaning operation.
interaction energies between the raw sludge and treated sludge is shown in Fig. 4. The data clearly show that the strength of the adhesive interaction energy ( ΔGadh ) of the treated sludge was significantly increased by about 3.2 times with the membrane compared with that of the raw sludge. The higher negative ΔGadh indicated that the treated sludge had a higher membrane fouling potential than the raw sludge. Moreover, the higher ΔGadh also strongly suggested that the detachment of the treated sludge from the membrane surface was more difficult to achieve by shearing. In addition, the treated sludge exhibited higher self-cohesive strength (-48.6 mJ/m2) compared with the raw sludge (−5.9 mJ/m2). Apparently, the treated sludge exhibited a higher fouling potential due to its higher adhesive and cohesive ability than that of the raw sludge, which would explain why many researchers have observed a faster membrane fouling rate after chemical cleaning in MBRs [6,19,21].
Table 4 Calculated surface energy parameters (mJ/m2) for sludge samples and membrane. Materials
γLW
γ+
γ−
γAB
γTot
ΔGsws
Cleaned membrane Raw sludge Treated sludge Detached foulants Reconstructed sludge
28.32 31.17 26.94 34.12 29.78
1.07 4.08 1.58 0.14 0.82
12.88 23.41 7.12 22.02 8.81
−7.44 −19.54 −6.71 −3.50 −5.38
20.88 11.63 20.22 30.62 24.40
−36.41 −7.64 −60.63 −10.51 −50.84
a high PN content in the LB-EPS of detached foulants will have a negative impact on the sludge filterability [3,5]. The interaction energies of detached foulants, including adhesion 1 with membrane ( ΔGadh ), cohesion with themselves ( ΔG coh ), and cohe2 coh sion with treated sludge ( ΔG ), are each estimated from Fig. 6. All three interaction energies were negative and their absolute values 1 2 follow the order: ΔGcoh > ΔGadh > ΔG coh , The self-cohesive strength 2 of the detached foulants (-9.9 mJ/m ) was the weakest among these interactions, implying that the detached foulants did not preferentially aggregate with each other. Compared with the ΔGadh (-18.6 mJ/m2), the detached foulants had a stronger attraction with the treated sludge (-27.6 mJ/m2), showing a greater tendency to aggregate with the treated sludge. The batch crossflow filtration test was conducted with mixed samples as described in Section 2.3. The three-dimensional EEM spectroscopy was applied to identify the EPS components of the initial membrane foulants, treated sludge, and detached foulants shown in Fig S1.
3.3. Composition of detached foulants and its fate in MBR The detached foulants were collected in the cleaning reactor (26 th and 51 th) and their EPS were quantified as shown in Fig. 5. The results revealed that PN was the major component in the different EPS layers of detached foulants. In particular, the detached foulants were found to contain high S-EPS level, which mainly originated from soluble foulants, cell lysis and LB-EPS released by NaClO oxidation [5,9]. In addition, a high eDNA level (59.7 mg/g SS on average) was detected in the S-EPS, confirming that a substantial amount of biomass within the fouling layer was chemically solubilized to dissolved organic matter by strong oxidation with NaClO. Overall, a significant amount of S-EPS and Table 3 Contact angle and zeta potential data for the membrane and sludge samples.a Materials
Cleaned membrane Raw sludge Treated sludge Detached foulants Reconstructed sludge a
Ultrapure water
87.81 83.84 97.97 69.61 90.80
± ± ± ± ±
1.87 1.72 5.73 2.18 0.13
Contact angle (°)
Zeta potential (mV)
Glycerol
Diiodomethane
93.16 ± 1.62 99.27 ± 1.96 100.75 ± 2.04 74.51 ± 10.65 92.05 ± 2.45
59.16 55.48 62.85 50.27 57.91
Each data was measured at least seven times. 150
± ± ± ± ±
1.68 4.29 4.63 8.37 1.68
−31.30 −42.88 −43.26 −43.54 −31.77
± ± ± ± ±
0.13 0.04 1.61 5.06 0.75
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Fig. 4. Comparison of the interaction energy of adhesion (ΔGadh) of both types of sludge with the membrane and cohesion energy (ΔGcoh) between the sludge themselves.
Fig. 7. Distributions of the EPS fractions in the raw sludge and reconstructed sludge.
the initial fouling layer. Due to NaClO oxidation, the detached foulants became more hydrophilic. Thus, the indicator of hydrophocity/hydrophylicy could be used to trace the detached foulants. As revealed by the data in Table 4, the hydrophobicity of the detached foulants, treated sludge, and reconstructed sludge was −10.5, −60.6 and −50.8 mJ/ m2, respectively, thus a significantly lower hydrophobicity was observed for the detached foulants. Especially, the reconstructed sludge exhibited a lower hydrophobicity than that of treated sludge, implying that the detached foulants participated in the reconstitution with treated sludge, thereby reducing the hydrophobicity of reconstructed sludge.
3.4. Characteristics of the reconstructed sludge In Run IV, the sludge samples were collected before and after in-situ cleaning and their EPS composition was measured as shown in Fig. 7. The data shown in Fig. 7, reveal that the PN content in the S-EPS and LB-EPS was about 1.7-fold and 2.1-fold higher, respectively, in the reconstructed sludge compared with the raw sludge. The high PN content in the EPS outer layer was likely mainly due to the exposure to NaClO in the raw sludge. Meanwhile, a substantial amount of PN in the EPS outer layer of detached foulants (Fig. 5) could be due to a different reason. Evidence has shown that a higher PN content induces higher membrane fouling in MBRs [22–24]. It is worth noting that the PN and PS content clearly increased in the TB-EPS of the reconstructed sludge compared with that in the raw sludge, which is consistent with a previous report in which the over-production of EPS was considered to be due to a bacterial response to stress [5]. A comparison of the interaction energy profile of the raw sludge with that of the reconstructed sludge when they approached the membrane surface is presented in Fig. 8. The interaction energy showed a two-stage profile characterized by a positive energy stage followed by a rapid energy decline as the separation distance decreased for both types of sludge. A relatively high energy barrier existed between the raw sludge-membrane combination (4,240.7 kT), whereas a relatively low energy barrier existed between the reconstructed sludge and the membrane (2,292.7 kT), strongly suggesting that the reconstructed sludge had a higher membrane fouling potential than that of the raw sludge. Therefore, the combined effects of the exposure to NaClO and detached foulants on activated sludge started a severe membrane fouling, especially in the initial stage (Fig. 2). In this work, the synthetic wastewater was used in order to avoid difficulties in quantification of metabolic products due to variation in the composition and concentration of organic pollutants in the real wastewater. In fact, the membrane fouling is a complex, dynamic process in MBRs, which is closely related to the composition of feedwater and change of organic-
Fig. 5. Composition of the EPS within the detached foulants collected from the cleaning reactor.
Fig. 6. The adhesion energy ( ΔGadh ) of the detached foulants with the mem1
brane, the cohesion energy ( ΔG coh ) of the detached foulants with themselves and cohesion energy (
2 ΔG coh
) of the detached foulants with the treated sludge.
Even though the EPS from the three samples exhibited similar composition, the intensities of the detected peaks corresponding to various organics were different. Clearly, other than the detached foulants, the initial foulants had a similar peak intensity with the treated sludge. According to the fluorescence regional integration (FRI) distribution (Fig. S1), the percent fluorescence response of each region in the initial membrane foulants showed a similar distribution as the treated sludge, suggesting that the detached foulants were not the main contributors to
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Fig. 8. Comparison of the interaction energy between the membrane-raw sludge combination and the membrane-reconstructed sludge combination as a function of the separation distance.
loading [25,26]. Based on the results of this study, therefore, the further work need focus on the verification investigation using the real wastewater in the future. 4. Conclusions In the present study, the fate of NaClO and detached foulants, and their combined effect on the thermodynamic properties of raw sludge were systematically investigated. About a 20.3% of the total NaClO potentially diffused into the bulk solution, and caused a higher initial membrane fouling rate. The NaClO altered the surface properties of the raw sludge and enhanced both the adhesive and cohesive propensity of the treated sludge. The detached foulants contained a high PN level within the different EPS layers, potentially aggregated with the treated sludge and formed the reconstructed sludge. The reconstructed sludge had a high PN content within the EPS, and exhibited a higher membrane fouling potential compared with the raw sludge. Acknowledgments The authors wish to thank and acknowledge the support of the National Natural Science Foundation of China (No. 51478093 & 51678119), the Jilin Province Scientific and the Technological Planning Project of China (No. 20170519013JH). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.bej.2019.04.016. References [1] Y. Tian, Z. Li, Y. Ding, Y. Lu, Identification of the change in fouling potential of soluble microbial products (SMP) in membrane bioreactor coupled with worm reactor, Water Res. 47 (6) (2013) 2015–2024. [2] H. Hong, M. Zhang, Y. He, J. Chen, H. Lin, Fouling mechanisms of gel layer in a submerged membrane bioreactor, Bioresour. Technol. 166 (2014) 295–302.
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