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Advanced treatment of biologically treated coking wastewater by membrane distillation coupled with pre-coagulation
Desalination 380 (2016) 43–51
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Advanced treatment of biologically treated coking wastewater by membrane distillation coupled with pre-coagulation Jianfeng Li a, Jing Wu a, Huifang Sun a, Fangqin Cheng a,⁎, Yu Liu b a
Institute of Resources and Environmental Engineering, State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi University, Taiyuan 030006, China School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore
b
H I G H L I G H T S • • • • •
Membrane distillation was developed for advanced treatment of coking wastewater. Pre-coagulation with PACl significantly reduces the solids and organic loadings. Small amount of low molecular weight aromatic substance presents in the MD permeate. PACl coagulation prevents the formation of calcium carbonate on membrane surface. PAM increases organic removal but causes aggregate accumulation and severer fouling.
a r t i c l e
i n f o
Article history: Received 23 June 2015 Received in revised form 17 November 2015 Accepted 20 November 2015 Available online xxxx Keywords: Membrane distillation Biologically treated coking wastewater Advanced treatment Pre-coagulation Membrane fouling
a b s t r a c t In this study, a laboratory-scale membrane distillation (MD) system was developed for advanced treatment of biologically treated coking wastewater (BTCW), while the effect of pre-coagulation was also investigated. Results showed that membrane distillation could effectively reject the salts (N99.1%) and organic pollutants in BTCW (N96.2%) and no membrane wetting was observed. The remaining organics in distillate was largely determined by the amount of volatile substances in the feed. Pre-coagulation with poly-aluminum chloride (PACl) was found to be effective for significantly reducing the contaminant level in BTCW. This in turn significantly reduces the propensity of membrane fouling. Use of polyacrylamide (PAM) as a coagulant aid could further decrease the contaminant level in BTCW, but it may lead to even severer membrane fouling. Scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and fluorescence spectra analyses revealed that the interaction between PACl and remaining organics could prevent the formation of calcium carbonate on the membrane surface, whereas the addition of PAM facilitated the accumulation of aggregates on membrane surface, leading to serious membrane fouling. This study shows that membrane distillation coupled with pre-coagulation could serve as a potential alternative for advanced treatment of biologically treated coking wastewater. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In China, pollution caused by coking wastewater remains a severe problem as China produces about 470 million tons of coke per year [1]. Coking wastewater is often generated during high-temperature carbonation, coal gas purification, and chemical refining in coke plants. It is a complex industrial wastewater comprising hundreds of organic pollutants and inorganic pollutants, which usually contains ammonia, phenol, benzene, nitrogen heterocyclic compounds (e.g., quinoline, pyridine, and indole), and polycyclic aromatic hydrocarbons [2–4]. ⁎ Corresponding author. E-mail address: [email protected] (F. Cheng).
http://dx.doi.org/10.1016/j.desal.2015.11.020 0011-9164/© 2015 Elsevier B.V. All rights reserved.
Many of these compounds are refractory, toxic, mutative, and/or carcinogenic [5,6]. Conventional treatment of coking wastewater includes physiochemical technologies (e.g. solvent extraction of phenolic compounds and steam stripping of ammonia), and biological treatment prior to discharge to the receiving water bodies. Activated sludge system, such as anoxic/oxic (A/O) or anaerobic/anoxic/oxic (A2/O) processes, is the dominant biological process in China for coking wastewater treatment because of its low cost, simple operation and maintenance [5,7,8]. Unfortunately, due to the presence of refractory and inhibitory contaminants, the effluents from these systems still contain relatively high concentration of non-biodegradable organic pollutants [2,3,9]. With the raised concerns of these hazardous effluents, China has recently
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strengthened the national discharge standard of coking wastewater (GB16171-2012). In addition, a new revised regulation on coking industry access requirements issued by Chinese government states that the discharge of production wastewater from any newly built coking plants is completely prohibited [8]. As a result, advanced treatment and zero discharge of the biologically treated coking wastewater (BTCW) are becoming increasingly important. Currently, various technologies have been proposed for advanced treatment of BTCW, such as advanced oxidation processes, adsorption technologies and membrane filtrations. The advanced oxidation processes like Fenton, ozone and electrochemical oxidation have the ability to remove or even mineralize the organics by generating hydroxyl radicals, but the cost is usually high due to the large consumption of oxidants and/or the use of energy intensive equipments [2,4,10,11]. Adsorption is a promising method, but the activated carbon or other polymers have difficulties in regeneration, and the disposal of the exhausted adsorbents remains a challenge [3,12,13]. Membrane filtration including microfiltration and ultrafiltration is effective in removing the biorefractory organics, however membrane performance can dramatically decrease as a consequence of membrane fouling due to the presence of the refractory organics in BTCWs [8,14,15]. In addition, the effluent of abovementioned systems often requires further desalination process such as nano-filtration (NF) and reverse osmosis (RO) to meet the reusing water standard [3,8]. Membrane distillation (MD) is a technology with the hydrophobic membrane which itself acts as a barrier to hold the liquid/vapor interfaces at the entrance of the pores, and the driving force is a vapor pressure difference across the membrane. It has the advantages of theoretically 100% salt rejection, lower operating temperature than conventional distillation processes, less requirements of membrane mechanical strength, and lower operating pressure compared to conventional pressure-driven membrane processes such as RO [16]. The first patents on MD were granted in the late 1960s, but it wasn't technologically feasible until ultrafiltration membranes in recent years enabled sufficiently high trans-membrane fluxes [17]. Since then, MD has regained extensive interests on membrane development, configuration design and application exploring [16–19]. Due to the effectiveness on rejection of non-volatile substances, membrane distillation has also been applied in advanced treatment of industrial wastewaters including petrochemical wastewater, olive mill wastewater, as well as radioactive wastewater [20–22]. In these systems, MD has demonstrated good distillate water quality and less fouling propensity than pressure-driven membrane processes. Therefore, it is suggested that MD will be a potential alternative for advanced treatment of BTCW, and it also could significantly reduce the volume of RO concentrates or other wastes. In addition, the coking plant often has a large amount of waste heat, which makes the MD process competitive in practice. However it should be noted that although MD requires less intensive pretreatment as compared to pressure-driven membrane processes, the importance of pretreatment in MD cannot be underestimated as BTCW remains containing a relatively high concentration of pollutants. In this regard, the present paper aims to determine the performance of advanced treatment of BTCWs with a laboratory direct contact membrane distillation (DCMD) system for a relatively long period (72 h). Furthermore, the effect of the different coagulation pretreatment methods on distillate quality and membrane fouling was investigated comprehensively. It is expected that this study would promote the application of MD in BTCW treatment. 2. Materials and methods 2.1. Wastewater characteristics The biologically treated coking wastewater (BTCW) was collected from the effluents of a coking wastewater treatment plant based on
anaerobic/anoxic/oxic (A2/O) process in Shanxi, China. The effluent quality of the A2/O process is given in Table 1. 2.2. Coagulation pretreatment The coagulation experiments were conducted in 500 mL beakers using conventional Jar-test apparatus at room temperature. In the jar tests, analytical-grades of poly-aluminum chloride (PACl, hydrogen ratio of 40%) and nonionic polyacrylamide (PAM) (N 300 kDa) were applied as coagulant and coagulant aid respectively. Samples were mixed at 250 r/min initially for 30 s before PACl or PACl/PAM was added. Then they were mixed with the coagulants at 200 r/min for another 90 s before reduced to 40 r/min for 30 min. When the coagulation was completed, the samples taken from each jar tests were centrifuged for 15 min and stored in a 4 °C refrigerator before conducting an analysis. The optimum dosages of PACl (175 mg/L) and PAM (2 mg/L) were determined by the removal of pollutants in the coagulated wastewater. In the pretreatment of PACl/PAM, PAM was added 10 s after dosing of PACl. The optimal dosage was used in all subsequent tests involving PACl and PAM. 2.3. DCMD experiment A microporous hydrophobic flat membrane was used in this work, which was provided by Sumitomo Electric Industry Ltd Corp (Japan). The membrane consists of a thin porous polytetrafluoroethylene (PTFE) active layer (35 μm) and a polypropylene non-woven fabric net support layer (152 μm). The active layer has a 0.22 μm nominal pore size and 82% porosity. The membrane module was made of nylon fiberboard and the effective membrane area is 14.4 cm2. The details of the membrane module can be found elsewhere [23]. The experiment set-up is shown schematically in Fig. 1. BTCWs were heated to 50 °C in a 1 L three-neck flask by a thermostatic watercirculator bath (HH-501A, Jiangsu, ±0.1 °C). MilliQ water was used as the cooling agent of DCMD, which was maintained at 20 °C in the entire experimental period by a precise low-temperature thermostat bath (DS-2006, Ningbo, ±0.1 °C). The BTCW and MillQ water were circulated at 0.3 m/s by two independent peristaltic pumps (WT600-2J, Baoding, China) in countercurrent directions. The water overflowed from the distillate reservoir (0.8 L) to a conical flask, which was continuously weighed by an electronic balance (DJ-1000J, Shanghai) and recorded every 10 min. The membrane flux was calculated by dividing the weight of overflow water and membrane effective area. The flow rate was measured with rotometers on each side of the membrane. The inlet and outlet temperatures of feed and permeate streams were measured using mercury thermometers. Conductivity in the BTCW and distillate was continuously examined by a conductivity meter (SevenMulti, Mettler Toledo, Germany). Concentration factor (CF) of feed was calculated by dividing the initial weight by the final weight of the feed as the distillates only contain little contaminants. 2.4. Analytical methods COD, ammonia nitrogen (NH4-N) and color were measured using standard method [24]. Turbidity was analyzed by a turbidity meter Table 1 Characteristics of biologically treated coking wastewater. Parameter
Unit
BTCW
pH BOD5 COD NH3-N Turbidity Color Conductivity
mg/L mg/L mg/L NTU ° μs/cm
7.8 ± 0.3 35 ± 5 315 ± 20 7.1 ± 0.5 33.3 ± 3.8 361 ± 7 6092 ± 23
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Fig. 1. Schematic set-up of the DCMD experiment.
(Hach 2100N, USA). UV absorption scan in the range of 220–800 nm was conducted by a UV–vis spectrophotometer (Ruili UV-1601, China). UV254 (UV absorbance at 254 nm) was measured on the spectrophotometer at 254 nm. The 3-dimensional excitation emission matrix (3D-EEM) spectra measurements were recorded on a spectrofluorometer (Cary Eclipse, Agilent, USA). EEM spectra were collected with the scanning emission (Em) spectra from 250 to 550 nm at 1 nm increases by varying the excitation wavelength (Ex) from 200 to 450 nm at 10 nm sampling intervals. All samples were diluted to a final dissolved total organic carbon (DOC) concentration of 1 mg/L with MilliQ-water before analysis to avoid the inner-filter effects. The EEM fluorescence spectra were plotted as graphs of contours using Origin 8.0. Chemical alteration of the membranes after 12 h of DCMD operation was investigated using an FTIR spectrometer (Frontier, PerkinElmer, USA), with an attenuated total reflection (ATR) attachment. The foulants on membrane surface were examined by using a Field Emission Scanning Electron Microscope (FESEM) (JSM-7001F, JEOL) coupled with an energydispersive X-ray spectroscopy (EDS, QX200, Bruker, Germany).
that a portion of the volatile organic compounds were removed in the coagulation process. Fig. 3a shows the respective effect of PACl and PACl/PAM on the removal of turbidity, color, COD, UV254 and NH4-N from the biologically treated coking wastewater. PACl was found to be effective for removing the suspended solids and colloids. As a result, the turbidity of feed wastewater was reduced from 33.6 NTU to below 2.0 NTU. Meanwhile, the removal of color, COD and UV254 by pre-coagulation with PACl was 39.5%, 40.1% and 32.2%, respectively. These results suggested that 30– 40% of organic matter removal was achievable with the precoagulation of BTCW. The changes in color of the BTCW are presented in Fig. 3b. Coking wastewater with a brown color is considered a highly hazardous industrial wastewater. PACl coagulation has been reported to be effective for removing some aromatic substances from this kind of wastewater [9]. It should also be noted that the addition of PAM together with PACl further enhanced the removal of COD and color of about 10%. This can be explained by the long chain molecular structure of PAM, which can bridge particles together [25]. However, regardless of
3. Results and discussion 3.1. Membrane distillation performance and effect of pre-coagulation Fig. 2 shows the conductivity of distillate and concentration factor (CF) of feed for raw, PACl, and PACl/PAM pretreated BTCW samples during 72 h of DCMD operation. The conductivity in the distillate of all three samples falls in a narrow range of 25–60 μs/cm even when they were concentrated up to 2.0–4.5 times. This indicated that the effect of PACl and PAM at current dosages on membrane wetting is negligible. It was also observed that the conductivity of the permeate streams increased in the initial stage of the MD process but decreased gradually in all the processes. This could be a result of penetration of the volatile organic compounds from feed to the distillate. Once the diffusion of volatile organic compounds was complete, the conductivity of the permeate stream began to decrease over the remaining period of operation time. The slightly lower maximal conductivity and final conductivity at the end of the MD operation for pre-coagulated samples suggested
Fig. 2. Conductivity in distillate and concentration factor of feed in the experiment.
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Fig. 3. Effect of PACl and PACl/PAM coagulation on pollutant removal, (a) removal of pollutants and (b) photos of BTCWs.
Fig. 4. Effect of PACl and PACl/PAM coagulation on DCMD permeate quality, (a) water quality of DCMD permeate and (b) photos of DCMD permeate.
Fig. 5. UV–vis scan of organic pollutant in the BTCW samples, (a) wavelength from 200 nm to 800 nm and (b) wavelength from 200 nm to 300 nm.
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strong absorption peaks between 240 and 500 nm were well removed by pre-coagulation with PACl and PACl/PAM. However, the pollutants with absorption from 200 nm to 240 nm remain in the wastewater, and it also appeared in the MD permeate. The absorbance in this range is often correlated to the presence of low molecular weight of aromatic substances [9,17]. Therefore, the residual organics in distillate is likely attributed to these volatile substances. However, it should be noted in this study that the rejection of the salts and organic pollutants by membrane distillation was as high as 96.1–99.7% (Figs. 2–5). With such a high rejection, the distillate of DCMD process can be used directly for cooling purpose in coking industry. But it may require posttreatment to remove the remaining salts and organics prior to its application as the boiler feedwater. 3.2. Flux decline and membrane foulant analysis
Fig. 6. Normalized flux of raw and coagulation pretreated BTCW samples.
the pretreatment, the turbidity, color, COD, NH4-N and UV254 in the distillate all fell into a narrow range (Fig. 4). These seem to indicate that the quality of distillate from membrane distillation was not correlated to the pretreatment of BTCW. Fig. 5 presents the UV–vis spectra of the raw, and PACl- and PACl/ PAM-pretreated BTCW samples. It was found that the pollutants with
The initial membrane fluxes for the raw, PACl-Tr and PACl/PAM-Tr samples were 18.5, 20.1 and 18.4 kg/m2·h, respectively. The difference in initial membrane fluxes may be attributed to the change of diffusivity of BTCW samples as pollutant levels in them varied widely. The normalized flux J/J0 is often used to describe the membrane fouling rate in membrane distillation process and the flux decline is often owing to the influence of membrane foulants [26,27]. As can be seen in Fig. 6, PACl treated BTCW demonstrates the best performance in terms of fouling mitigation, e.g. the normalized flux remained as high as 0.85 after 12 h of operation. A rapid flux decline was observed with raw BTCW in the first 4 h, and reached less than 0.65 after 12 h of operation.
Fig. 7. SEM images of virgin and fouled membranes: (a) virgin membrane, (b) fouled membrane of raw, (c) fouled membrane of PACl-Tr and (d) fouled membrane of PACl/PAM-Tr.
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Whereas the J/J0 of PACl/PAM pretreated sample tended to decrease quickly over the entire period. These suggest that the membrane fouling rate with PACl pretreatment was the least among all the samples studied. Fig. 7 shows respective SEM images of virgin and fouled membranes. The virgin membrane surface had a large number of open pores and a relatively uniform surface structure. The different types of wastewaters (raw or pretreated BTCWs) fed to the DCMD process exhibited various membrane fouling patterns. Many membrane pores still remained open after the filtration of the PACl-Tr sample, while the effective membrane area was noticeably reduced due to deposition of particulate matters on the membrane surface. In the cases of raw and PACl/PAM-Tr samples, almost all membrane pores were blocked by a thick cake
layer formed on the surface of membranes. These results indicate that the pretreatment by coagulation with PACl was effective to avert cake formation during membrane distillation of BTCW. However, the addition of PAM could lead to the deposition or accumulation of relatively large aggregates on the membrane surface. The composition of membrane foulants was analyzed by the SEM– EDS method (Fig. 8). It revealed that apart from a significant amount of Ca, the deposit on raw BTCW membrane also contained Na, Mg, Si and S (Fig. 8a). The major component is likely to be CaCO3 according to the atomic percentage of each element. This is inconsistent with the previous results that CaCO3 scaling was often found to be the major component of fouling in the MD process for industrial wastewaters [17,22,28]. As shown in Fig. 8, the main inorganic component detected
Fig. 8. The result of SEM–EDS analysis of sparingly soluble salt deposit on the membrane surface: (a) raw BTCW, (b) PACl treated BTCW and (c) PACl/PAM treated BTCW.
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Fig. 9. FTIR spectra of membrane foulant for raw and pretreated BTCWs (the non-adsorption break was from 1800 cm−1 to 2700 cm−1): (a) virgin membrane, (b) fouled by raw BTCW, (c) fouled by PACl treated BTCW and (d) fouled by PACl/PAM treated BTCW.
on PACl-Tr membrane surface was NaCl, which is also a common inorganic foulant in membrane distillation [17]. It is interesting to note that no CaCO3 deposit on membrane surface occurs after the BTCW was pretreated with PACl or PACl/PAM. And the PACl/PAM-Tr membrane demonstrated a similar SEM–EDS profile with PACl-Tr but a distinct peak of N element is present, which indicates a significant amount of N element in the membrane foulant. This is probably caused by either the newly formed nitrogen-containing aggregates or residual PAM deposited on membrane surface [17,25]. As illustrated in Fig. 9, FTIR-ATR analysis was employed to investigate membrane organic fouling in this study. The FTIR spectra of the clean PTFE membrane show five typical transmittance bands in the region of 1205 cm−1, 1148 cm−1, 638 cm−1, 554 cm−1, and 499 cm−1. And the infrared spectrum of composition of foulant after membrane distillation shows bands at 3659 cm− 1 (weak), 3397 cm− 1 (weak), 2991 cm− 1 (medium), 2914 cm−1 (medium), 1645 cm− 1 (weak), 1402 cm−1 (medium), and 1083 cm− 1 (medium). According to the FTIR standard spectra [29], the band at 3659 cm−1 is assigned to O–H stretching mode of phenolic hydroxyl and alcoholic hydroxyl groups and the band at 3397 cm−1 belongs to N–H stretching mode of amine group, whereas the bands at 2991 cm− 1 and 2914 cm−1 (medium) are due to C–H stretching mode by alkanes. In addition, the bands at 1645 cm−1, 1574 cm−1, and 1402 cm−1 could be attributed to aromatic skeleton vibration that related to pyridine, furan, quinoline and benzene as well as the C–H deformation mode of substituted benzene groups [30]. The results demonstrate that organic fouling is present in all the three membrane surfaces after 12 h of DCMD operation. And the peaks observed in the FTIR spectra confirm the presence of aromatic organics and nitrogen containing organics in the membrane foulants, which originates from coking wastewater.
3.3. Interactions of organics and membrane fouling The biologically treated coking wastewater remains containing a relatively high concentration of organic pollutants. These pollutants are potential membrane foulants causing flux decline. However, it should be noted that the remaining organics in BTCWs may have interactions with inorganic ions/coagulant residuals and tend to form new type of composite and/or aggregates. Therefore, membrane fouling is a result of interactions between all the potential foulants and membranes.
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In order to further characterize the organic pollutants in the raw, pretreated BTCWs and distillate, 3D-EEM fluorescence spectra of various samples are illustrated in Fig. 10. Four fluorescence peaks, i.e. I, II, III and IV had often been detected in BTCWs [3,5,31]. Peaks I and III are related to aromatic protein-like substances and soluble microbial by product-like materials respectively, whereas peaks II and IV can be assigned to humic acid- and fulvic acid-like substances [5,31]. In this study, the intensities of peaks I and III were found to be weak in all the cases, showing that microbe-associated substances were not dominant in the BTCWs. In contrast, strong intensity of fluorescence at regions II and IV was observed, indicating that the BTCWs were rich in humic acid-and fulvic acid-like substances [3,5]. In addition, the fluorescence peak intensities of humic acid-like and fulvic acid-like substances were in the order of PACl/PAM-Tr N PACl-Tr N raw. However, it should be noted that the organic pollutants were significantly reduced after PACl and PACl/PAM pretreatments (Fig. 3), therefore the above 3D-EEM results only reflect the composition change of the analyzed samples. It had been reported that fulvic acids have a higher charge density, thus are less amenable to coagulation than humic acid. As a result, fulvic acid-like substances were continually being concentrated by PACl and PACl/PAM coagulation. It should be noted that peak I remains in the distillate of all the samples, although the intensity is low. This indicates that the humic acid-like and fulvic acid-like substances were completely rejected by the membranes, but volatile substances e.g. aromatic protein-like substances could pass through the membrane. It shows in Section 3.2 that membrane foulant of raw BTCW contains organics and CaCO3, whereas NaCl scaling and organic fouling are dominant in the cases of PACl-Tr and PACl/PAM-Tr. However, the PACl pretreated BTCW demonstrates the best performance in terms of fouling mitigation. This is largely attributed to the interaction between the humic acid-like and fulvic acid-like substances and Ca2+ as these macromolecules could act as an inhibitor to calcium carbonate precipitation by blocking the active growth sites through adsorption reaction [17]. However it should be noted that these macromolecules themselves are potential membrane foulant and may cause organic fouling. Therefore, the low fouling tendency in this case might be explained by the binding effect of Ca2+ (as well as other inorganic ions) to the carboxyl functional groups of the “humic-like and fulvic like substances”. This effect helps to form new type of composite which is less amenable to attach on membrane surface [32,33]. In the case of PACl/PAM pretreated BTCW, although it could achieve a better coagulation performance, the membrane fouling was even more severe than raw BTCW. The concentrated humic acid-like and fulvic acid-like substances probably have strong interaction with Ca2+ and/or PAM residuals, and thereby lead to the formation of relatively large aggregates (Fig. 7). These aggregates are more favorable to be adsorbed by the hydrophobic membranes, which leads to a severer fouling. The above results indicated that the PAM dosage in this study may not be optimized for membrane fouling mitigation, which should be examined in the future. 4. Conclusions This paper shows that membrane distillation could effectively reject the salts and organic pollutants in bio-treated coking wastewater without membrane wetting. The pre-coagulation with PACl and PAM aid was found to be effective for significantly reducing the pollutant level in biologically treated coking wastewater. The conductivity in distillate was maintained in a range of 25–60 μs/cm, and the remaining organics was largely determined by the amount of volatile substances in the feed. It was demonstrated that the pre-coagulation of BTCW by PACl could improve the permeate flux, whereas PAM aid might deteriorate the membrane performance. Membrane foulant analysis by FTIR and SEM–EDS indicates that organic fouling was found for all the samples, but calcium carbonates deposit is present only on the membrane surface when treating raw BTCW. PACl/PAM pre-coagulation leads to the concentration of humic-like and fulvic like substances, which could act as an
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Fig. 10. 3D-EEM results of DOM in the distillate and raw, pretreated BTCW samples. (a) Raw BTCW, (b) PACl-Tr BTCW, (c) PACl/PAM-Tr BTCW, (d) DCMD permeate of raw, (e) DCMD permeate of PACl-Tr and (f) DCMD permeate of PACl/PAM-Tr.
inhibitor to calcium carbonate precipitation. However, use PAM as a coagulation aid may lead to the formation of aggregates, which facilitates the formation of a dense cake layer on membrane surface.
Coal Based Key Scientific and Technological Project (Grant No. MJH201404), and Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province (Grant No. 2013-215).
Acknowledgments References This work was financially supported by the National Natural Science Foundation of China (Grant No. 51408351), the Specialized Research Fund for the Doctoral Program of Higher Education (20121401120013), Shanxi
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Desalination journal homepage: www.elsevier.com/locate/desal
Advanced treatment of biologically treated coking wastewater by membrane distillation coupled with pre-coagulation Jianfeng Li a, Jing Wu a, Huifang Sun a, Fangqin Cheng a,⁎, Yu Liu b a
Institute of Resources and Environmental Engineering, State Environmental Protection Key Laboratory of Efficient Utilization Technology of Coal Waste Resources, Shanxi University, Taiyuan 030006, China School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore
b
H I G H L I G H T S • • • • •
Membrane distillation was developed for advanced treatment of coking wastewater. Pre-coagulation with PACl significantly reduces the solids and organic loadings. Small amount of low molecular weight aromatic substance presents in the MD permeate. PACl coagulation prevents the formation of calcium carbonate on membrane surface. PAM increases organic removal but causes aggregate accumulation and severer fouling.
a r t i c l e
i n f o
Article history: Received 23 June 2015 Received in revised form 17 November 2015 Accepted 20 November 2015 Available online xxxx Keywords: Membrane distillation Biologically treated coking wastewater Advanced treatment Pre-coagulation Membrane fouling
a b s t r a c t In this study, a laboratory-scale membrane distillation (MD) system was developed for advanced treatment of biologically treated coking wastewater (BTCW), while the effect of pre-coagulation was also investigated. Results showed that membrane distillation could effectively reject the salts (N99.1%) and organic pollutants in BTCW (N96.2%) and no membrane wetting was observed. The remaining organics in distillate was largely determined by the amount of volatile substances in the feed. Pre-coagulation with poly-aluminum chloride (PACl) was found to be effective for significantly reducing the contaminant level in BTCW. This in turn significantly reduces the propensity of membrane fouling. Use of polyacrylamide (PAM) as a coagulant aid could further decrease the contaminant level in BTCW, but it may lead to even severer membrane fouling. Scanning electron microscopy (SEM), energy-dispersive spectrometer (EDS) and fluorescence spectra analyses revealed that the interaction between PACl and remaining organics could prevent the formation of calcium carbonate on the membrane surface, whereas the addition of PAM facilitated the accumulation of aggregates on membrane surface, leading to serious membrane fouling. This study shows that membrane distillation coupled with pre-coagulation could serve as a potential alternative for advanced treatment of biologically treated coking wastewater. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In China, pollution caused by coking wastewater remains a severe problem as China produces about 470 million tons of coke per year [1]. Coking wastewater is often generated during high-temperature carbonation, coal gas purification, and chemical refining in coke plants. It is a complex industrial wastewater comprising hundreds of organic pollutants and inorganic pollutants, which usually contains ammonia, phenol, benzene, nitrogen heterocyclic compounds (e.g., quinoline, pyridine, and indole), and polycyclic aromatic hydrocarbons [2–4]. ⁎ Corresponding author. E-mail address: [email protected] (F. Cheng).
http://dx.doi.org/10.1016/j.desal.2015.11.020 0011-9164/© 2015 Elsevier B.V. All rights reserved.
Many of these compounds are refractory, toxic, mutative, and/or carcinogenic [5,6]. Conventional treatment of coking wastewater includes physiochemical technologies (e.g. solvent extraction of phenolic compounds and steam stripping of ammonia), and biological treatment prior to discharge to the receiving water bodies. Activated sludge system, such as anoxic/oxic (A/O) or anaerobic/anoxic/oxic (A2/O) processes, is the dominant biological process in China for coking wastewater treatment because of its low cost, simple operation and maintenance [5,7,8]. Unfortunately, due to the presence of refractory and inhibitory contaminants, the effluents from these systems still contain relatively high concentration of non-biodegradable organic pollutants [2,3,9]. With the raised concerns of these hazardous effluents, China has recently
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strengthened the national discharge standard of coking wastewater (GB16171-2012). In addition, a new revised regulation on coking industry access requirements issued by Chinese government states that the discharge of production wastewater from any newly built coking plants is completely prohibited [8]. As a result, advanced treatment and zero discharge of the biologically treated coking wastewater (BTCW) are becoming increasingly important. Currently, various technologies have been proposed for advanced treatment of BTCW, such as advanced oxidation processes, adsorption technologies and membrane filtrations. The advanced oxidation processes like Fenton, ozone and electrochemical oxidation have the ability to remove or even mineralize the organics by generating hydroxyl radicals, but the cost is usually high due to the large consumption of oxidants and/or the use of energy intensive equipments [2,4,10,11]. Adsorption is a promising method, but the activated carbon or other polymers have difficulties in regeneration, and the disposal of the exhausted adsorbents remains a challenge [3,12,13]. Membrane filtration including microfiltration and ultrafiltration is effective in removing the biorefractory organics, however membrane performance can dramatically decrease as a consequence of membrane fouling due to the presence of the refractory organics in BTCWs [8,14,15]. In addition, the effluent of abovementioned systems often requires further desalination process such as nano-filtration (NF) and reverse osmosis (RO) to meet the reusing water standard [3,8]. Membrane distillation (MD) is a technology with the hydrophobic membrane which itself acts as a barrier to hold the liquid/vapor interfaces at the entrance of the pores, and the driving force is a vapor pressure difference across the membrane. It has the advantages of theoretically 100% salt rejection, lower operating temperature than conventional distillation processes, less requirements of membrane mechanical strength, and lower operating pressure compared to conventional pressure-driven membrane processes such as RO [16]. The first patents on MD were granted in the late 1960s, but it wasn't technologically feasible until ultrafiltration membranes in recent years enabled sufficiently high trans-membrane fluxes [17]. Since then, MD has regained extensive interests on membrane development, configuration design and application exploring [16–19]. Due to the effectiveness on rejection of non-volatile substances, membrane distillation has also been applied in advanced treatment of industrial wastewaters including petrochemical wastewater, olive mill wastewater, as well as radioactive wastewater [20–22]. In these systems, MD has demonstrated good distillate water quality and less fouling propensity than pressure-driven membrane processes. Therefore, it is suggested that MD will be a potential alternative for advanced treatment of BTCW, and it also could significantly reduce the volume of RO concentrates or other wastes. In addition, the coking plant often has a large amount of waste heat, which makes the MD process competitive in practice. However it should be noted that although MD requires less intensive pretreatment as compared to pressure-driven membrane processes, the importance of pretreatment in MD cannot be underestimated as BTCW remains containing a relatively high concentration of pollutants. In this regard, the present paper aims to determine the performance of advanced treatment of BTCWs with a laboratory direct contact membrane distillation (DCMD) system for a relatively long period (72 h). Furthermore, the effect of the different coagulation pretreatment methods on distillate quality and membrane fouling was investigated comprehensively. It is expected that this study would promote the application of MD in BTCW treatment. 2. Materials and methods 2.1. Wastewater characteristics The biologically treated coking wastewater (BTCW) was collected from the effluents of a coking wastewater treatment plant based on
anaerobic/anoxic/oxic (A2/O) process in Shanxi, China. The effluent quality of the A2/O process is given in Table 1. 2.2. Coagulation pretreatment The coagulation experiments were conducted in 500 mL beakers using conventional Jar-test apparatus at room temperature. In the jar tests, analytical-grades of poly-aluminum chloride (PACl, hydrogen ratio of 40%) and nonionic polyacrylamide (PAM) (N 300 kDa) were applied as coagulant and coagulant aid respectively. Samples were mixed at 250 r/min initially for 30 s before PACl or PACl/PAM was added. Then they were mixed with the coagulants at 200 r/min for another 90 s before reduced to 40 r/min for 30 min. When the coagulation was completed, the samples taken from each jar tests were centrifuged for 15 min and stored in a 4 °C refrigerator before conducting an analysis. The optimum dosages of PACl (175 mg/L) and PAM (2 mg/L) were determined by the removal of pollutants in the coagulated wastewater. In the pretreatment of PACl/PAM, PAM was added 10 s after dosing of PACl. The optimal dosage was used in all subsequent tests involving PACl and PAM. 2.3. DCMD experiment A microporous hydrophobic flat membrane was used in this work, which was provided by Sumitomo Electric Industry Ltd Corp (Japan). The membrane consists of a thin porous polytetrafluoroethylene (PTFE) active layer (35 μm) and a polypropylene non-woven fabric net support layer (152 μm). The active layer has a 0.22 μm nominal pore size and 82% porosity. The membrane module was made of nylon fiberboard and the effective membrane area is 14.4 cm2. The details of the membrane module can be found elsewhere [23]. The experiment set-up is shown schematically in Fig. 1. BTCWs were heated to 50 °C in a 1 L three-neck flask by a thermostatic watercirculator bath (HH-501A, Jiangsu, ±0.1 °C). MilliQ water was used as the cooling agent of DCMD, which was maintained at 20 °C in the entire experimental period by a precise low-temperature thermostat bath (DS-2006, Ningbo, ±0.1 °C). The BTCW and MillQ water were circulated at 0.3 m/s by two independent peristaltic pumps (WT600-2J, Baoding, China) in countercurrent directions. The water overflowed from the distillate reservoir (0.8 L) to a conical flask, which was continuously weighed by an electronic balance (DJ-1000J, Shanghai) and recorded every 10 min. The membrane flux was calculated by dividing the weight of overflow water and membrane effective area. The flow rate was measured with rotometers on each side of the membrane. The inlet and outlet temperatures of feed and permeate streams were measured using mercury thermometers. Conductivity in the BTCW and distillate was continuously examined by a conductivity meter (SevenMulti, Mettler Toledo, Germany). Concentration factor (CF) of feed was calculated by dividing the initial weight by the final weight of the feed as the distillates only contain little contaminants. 2.4. Analytical methods COD, ammonia nitrogen (NH4-N) and color were measured using standard method [24]. Turbidity was analyzed by a turbidity meter Table 1 Characteristics of biologically treated coking wastewater. Parameter
Unit
BTCW
pH BOD5 COD NH3-N Turbidity Color Conductivity
mg/L mg/L mg/L NTU ° μs/cm
7.8 ± 0.3 35 ± 5 315 ± 20 7.1 ± 0.5 33.3 ± 3.8 361 ± 7 6092 ± 23
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Fig. 1. Schematic set-up of the DCMD experiment.
(Hach 2100N, USA). UV absorption scan in the range of 220–800 nm was conducted by a UV–vis spectrophotometer (Ruili UV-1601, China). UV254 (UV absorbance at 254 nm) was measured on the spectrophotometer at 254 nm. The 3-dimensional excitation emission matrix (3D-EEM) spectra measurements were recorded on a spectrofluorometer (Cary Eclipse, Agilent, USA). EEM spectra were collected with the scanning emission (Em) spectra from 250 to 550 nm at 1 nm increases by varying the excitation wavelength (Ex) from 200 to 450 nm at 10 nm sampling intervals. All samples were diluted to a final dissolved total organic carbon (DOC) concentration of 1 mg/L with MilliQ-water before analysis to avoid the inner-filter effects. The EEM fluorescence spectra were plotted as graphs of contours using Origin 8.0. Chemical alteration of the membranes after 12 h of DCMD operation was investigated using an FTIR spectrometer (Frontier, PerkinElmer, USA), with an attenuated total reflection (ATR) attachment. The foulants on membrane surface were examined by using a Field Emission Scanning Electron Microscope (FESEM) (JSM-7001F, JEOL) coupled with an energydispersive X-ray spectroscopy (EDS, QX200, Bruker, Germany).
that a portion of the volatile organic compounds were removed in the coagulation process. Fig. 3a shows the respective effect of PACl and PACl/PAM on the removal of turbidity, color, COD, UV254 and NH4-N from the biologically treated coking wastewater. PACl was found to be effective for removing the suspended solids and colloids. As a result, the turbidity of feed wastewater was reduced from 33.6 NTU to below 2.0 NTU. Meanwhile, the removal of color, COD and UV254 by pre-coagulation with PACl was 39.5%, 40.1% and 32.2%, respectively. These results suggested that 30– 40% of organic matter removal was achievable with the precoagulation of BTCW. The changes in color of the BTCW are presented in Fig. 3b. Coking wastewater with a brown color is considered a highly hazardous industrial wastewater. PACl coagulation has been reported to be effective for removing some aromatic substances from this kind of wastewater [9]. It should also be noted that the addition of PAM together with PACl further enhanced the removal of COD and color of about 10%. This can be explained by the long chain molecular structure of PAM, which can bridge particles together [25]. However, regardless of
3. Results and discussion 3.1. Membrane distillation performance and effect of pre-coagulation Fig. 2 shows the conductivity of distillate and concentration factor (CF) of feed for raw, PACl, and PACl/PAM pretreated BTCW samples during 72 h of DCMD operation. The conductivity in the distillate of all three samples falls in a narrow range of 25–60 μs/cm even when they were concentrated up to 2.0–4.5 times. This indicated that the effect of PACl and PAM at current dosages on membrane wetting is negligible. It was also observed that the conductivity of the permeate streams increased in the initial stage of the MD process but decreased gradually in all the processes. This could be a result of penetration of the volatile organic compounds from feed to the distillate. Once the diffusion of volatile organic compounds was complete, the conductivity of the permeate stream began to decrease over the remaining period of operation time. The slightly lower maximal conductivity and final conductivity at the end of the MD operation for pre-coagulated samples suggested
Fig. 2. Conductivity in distillate and concentration factor of feed in the experiment.
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Fig. 3. Effect of PACl and PACl/PAM coagulation on pollutant removal, (a) removal of pollutants and (b) photos of BTCWs.
Fig. 4. Effect of PACl and PACl/PAM coagulation on DCMD permeate quality, (a) water quality of DCMD permeate and (b) photos of DCMD permeate.
Fig. 5. UV–vis scan of organic pollutant in the BTCW samples, (a) wavelength from 200 nm to 800 nm and (b) wavelength from 200 nm to 300 nm.
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strong absorption peaks between 240 and 500 nm were well removed by pre-coagulation with PACl and PACl/PAM. However, the pollutants with absorption from 200 nm to 240 nm remain in the wastewater, and it also appeared in the MD permeate. The absorbance in this range is often correlated to the presence of low molecular weight of aromatic substances [9,17]. Therefore, the residual organics in distillate is likely attributed to these volatile substances. However, it should be noted in this study that the rejection of the salts and organic pollutants by membrane distillation was as high as 96.1–99.7% (Figs. 2–5). With such a high rejection, the distillate of DCMD process can be used directly for cooling purpose in coking industry. But it may require posttreatment to remove the remaining salts and organics prior to its application as the boiler feedwater. 3.2. Flux decline and membrane foulant analysis
Fig. 6. Normalized flux of raw and coagulation pretreated BTCW samples.
the pretreatment, the turbidity, color, COD, NH4-N and UV254 in the distillate all fell into a narrow range (Fig. 4). These seem to indicate that the quality of distillate from membrane distillation was not correlated to the pretreatment of BTCW. Fig. 5 presents the UV–vis spectra of the raw, and PACl- and PACl/ PAM-pretreated BTCW samples. It was found that the pollutants with
The initial membrane fluxes for the raw, PACl-Tr and PACl/PAM-Tr samples were 18.5, 20.1 and 18.4 kg/m2·h, respectively. The difference in initial membrane fluxes may be attributed to the change of diffusivity of BTCW samples as pollutant levels in them varied widely. The normalized flux J/J0 is often used to describe the membrane fouling rate in membrane distillation process and the flux decline is often owing to the influence of membrane foulants [26,27]. As can be seen in Fig. 6, PACl treated BTCW demonstrates the best performance in terms of fouling mitigation, e.g. the normalized flux remained as high as 0.85 after 12 h of operation. A rapid flux decline was observed with raw BTCW in the first 4 h, and reached less than 0.65 after 12 h of operation.
Fig. 7. SEM images of virgin and fouled membranes: (a) virgin membrane, (b) fouled membrane of raw, (c) fouled membrane of PACl-Tr and (d) fouled membrane of PACl/PAM-Tr.
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Whereas the J/J0 of PACl/PAM pretreated sample tended to decrease quickly over the entire period. These suggest that the membrane fouling rate with PACl pretreatment was the least among all the samples studied. Fig. 7 shows respective SEM images of virgin and fouled membranes. The virgin membrane surface had a large number of open pores and a relatively uniform surface structure. The different types of wastewaters (raw or pretreated BTCWs) fed to the DCMD process exhibited various membrane fouling patterns. Many membrane pores still remained open after the filtration of the PACl-Tr sample, while the effective membrane area was noticeably reduced due to deposition of particulate matters on the membrane surface. In the cases of raw and PACl/PAM-Tr samples, almost all membrane pores were blocked by a thick cake
layer formed on the surface of membranes. These results indicate that the pretreatment by coagulation with PACl was effective to avert cake formation during membrane distillation of BTCW. However, the addition of PAM could lead to the deposition or accumulation of relatively large aggregates on the membrane surface. The composition of membrane foulants was analyzed by the SEM– EDS method (Fig. 8). It revealed that apart from a significant amount of Ca, the deposit on raw BTCW membrane also contained Na, Mg, Si and S (Fig. 8a). The major component is likely to be CaCO3 according to the atomic percentage of each element. This is inconsistent with the previous results that CaCO3 scaling was often found to be the major component of fouling in the MD process for industrial wastewaters [17,22,28]. As shown in Fig. 8, the main inorganic component detected
Fig. 8. The result of SEM–EDS analysis of sparingly soluble salt deposit on the membrane surface: (a) raw BTCW, (b) PACl treated BTCW and (c) PACl/PAM treated BTCW.
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Fig. 9. FTIR spectra of membrane foulant for raw and pretreated BTCWs (the non-adsorption break was from 1800 cm−1 to 2700 cm−1): (a) virgin membrane, (b) fouled by raw BTCW, (c) fouled by PACl treated BTCW and (d) fouled by PACl/PAM treated BTCW.
on PACl-Tr membrane surface was NaCl, which is also a common inorganic foulant in membrane distillation [17]. It is interesting to note that no CaCO3 deposit on membrane surface occurs after the BTCW was pretreated with PACl or PACl/PAM. And the PACl/PAM-Tr membrane demonstrated a similar SEM–EDS profile with PACl-Tr but a distinct peak of N element is present, which indicates a significant amount of N element in the membrane foulant. This is probably caused by either the newly formed nitrogen-containing aggregates or residual PAM deposited on membrane surface [17,25]. As illustrated in Fig. 9, FTIR-ATR analysis was employed to investigate membrane organic fouling in this study. The FTIR spectra of the clean PTFE membrane show five typical transmittance bands in the region of 1205 cm−1, 1148 cm−1, 638 cm−1, 554 cm−1, and 499 cm−1. And the infrared spectrum of composition of foulant after membrane distillation shows bands at 3659 cm− 1 (weak), 3397 cm− 1 (weak), 2991 cm− 1 (medium), 2914 cm−1 (medium), 1645 cm− 1 (weak), 1402 cm−1 (medium), and 1083 cm− 1 (medium). According to the FTIR standard spectra [29], the band at 3659 cm−1 is assigned to O–H stretching mode of phenolic hydroxyl and alcoholic hydroxyl groups and the band at 3397 cm−1 belongs to N–H stretching mode of amine group, whereas the bands at 2991 cm− 1 and 2914 cm−1 (medium) are due to C–H stretching mode by alkanes. In addition, the bands at 1645 cm−1, 1574 cm−1, and 1402 cm−1 could be attributed to aromatic skeleton vibration that related to pyridine, furan, quinoline and benzene as well as the C–H deformation mode of substituted benzene groups [30]. The results demonstrate that organic fouling is present in all the three membrane surfaces after 12 h of DCMD operation. And the peaks observed in the FTIR spectra confirm the presence of aromatic organics and nitrogen containing organics in the membrane foulants, which originates from coking wastewater.
3.3. Interactions of organics and membrane fouling The biologically treated coking wastewater remains containing a relatively high concentration of organic pollutants. These pollutants are potential membrane foulants causing flux decline. However, it should be noted that the remaining organics in BTCWs may have interactions with inorganic ions/coagulant residuals and tend to form new type of composite and/or aggregates. Therefore, membrane fouling is a result of interactions between all the potential foulants and membranes.
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In order to further characterize the organic pollutants in the raw, pretreated BTCWs and distillate, 3D-EEM fluorescence spectra of various samples are illustrated in Fig. 10. Four fluorescence peaks, i.e. I, II, III and IV had often been detected in BTCWs [3,5,31]. Peaks I and III are related to aromatic protein-like substances and soluble microbial by product-like materials respectively, whereas peaks II and IV can be assigned to humic acid- and fulvic acid-like substances [5,31]. In this study, the intensities of peaks I and III were found to be weak in all the cases, showing that microbe-associated substances were not dominant in the BTCWs. In contrast, strong intensity of fluorescence at regions II and IV was observed, indicating that the BTCWs were rich in humic acid-and fulvic acid-like substances [3,5]. In addition, the fluorescence peak intensities of humic acid-like and fulvic acid-like substances were in the order of PACl/PAM-Tr N PACl-Tr N raw. However, it should be noted that the organic pollutants were significantly reduced after PACl and PACl/PAM pretreatments (Fig. 3), therefore the above 3D-EEM results only reflect the composition change of the analyzed samples. It had been reported that fulvic acids have a higher charge density, thus are less amenable to coagulation than humic acid. As a result, fulvic acid-like substances were continually being concentrated by PACl and PACl/PAM coagulation. It should be noted that peak I remains in the distillate of all the samples, although the intensity is low. This indicates that the humic acid-like and fulvic acid-like substances were completely rejected by the membranes, but volatile substances e.g. aromatic protein-like substances could pass through the membrane. It shows in Section 3.2 that membrane foulant of raw BTCW contains organics and CaCO3, whereas NaCl scaling and organic fouling are dominant in the cases of PACl-Tr and PACl/PAM-Tr. However, the PACl pretreated BTCW demonstrates the best performance in terms of fouling mitigation. This is largely attributed to the interaction between the humic acid-like and fulvic acid-like substances and Ca2+ as these macromolecules could act as an inhibitor to calcium carbonate precipitation by blocking the active growth sites through adsorption reaction [17]. However it should be noted that these macromolecules themselves are potential membrane foulant and may cause organic fouling. Therefore, the low fouling tendency in this case might be explained by the binding effect of Ca2+ (as well as other inorganic ions) to the carboxyl functional groups of the “humic-like and fulvic like substances”. This effect helps to form new type of composite which is less amenable to attach on membrane surface [32,33]. In the case of PACl/PAM pretreated BTCW, although it could achieve a better coagulation performance, the membrane fouling was even more severe than raw BTCW. The concentrated humic acid-like and fulvic acid-like substances probably have strong interaction with Ca2+ and/or PAM residuals, and thereby lead to the formation of relatively large aggregates (Fig. 7). These aggregates are more favorable to be adsorbed by the hydrophobic membranes, which leads to a severer fouling. The above results indicated that the PAM dosage in this study may not be optimized for membrane fouling mitigation, which should be examined in the future. 4. Conclusions This paper shows that membrane distillation could effectively reject the salts and organic pollutants in bio-treated coking wastewater without membrane wetting. The pre-coagulation with PACl and PAM aid was found to be effective for significantly reducing the pollutant level in biologically treated coking wastewater. The conductivity in distillate was maintained in a range of 25–60 μs/cm, and the remaining organics was largely determined by the amount of volatile substances in the feed. It was demonstrated that the pre-coagulation of BTCW by PACl could improve the permeate flux, whereas PAM aid might deteriorate the membrane performance. Membrane foulant analysis by FTIR and SEM–EDS indicates that organic fouling was found for all the samples, but calcium carbonates deposit is present only on the membrane surface when treating raw BTCW. PACl/PAM pre-coagulation leads to the concentration of humic-like and fulvic like substances, which could act as an
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J. Li et al. / Desalination 380 (2016) 43–51
Fig. 10. 3D-EEM results of DOM in the distillate and raw, pretreated BTCW samples. (a) Raw BTCW, (b) PACl-Tr BTCW, (c) PACl/PAM-Tr BTCW, (d) DCMD permeate of raw, (e) DCMD permeate of PACl-Tr and (f) DCMD permeate of PACl/PAM-Tr.
inhibitor to calcium carbonate precipitation. However, use PAM as a coagulation aid may lead to the formation of aggregates, which facilitates the formation of a dense cake layer on membrane surface.
Coal Based Key Scientific and Technological Project (Grant No. MJH201404), and Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province (Grant No. 2013-215).
Acknowledgments References This work was financially supported by the National Natural Science Foundation of China (Grant No. 51408351), the Specialized Research Fund for the Doctoral Program of Higher Education (20121401120013), Shanxi
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