Science of the Total Environment 518–519 (2015) 586–594
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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Applicability of a novel osmotic membrane bioreactor using a specific draw solution in wastewater treatment Nguyen Cong Nguyen a, Shiao-Shing Chen a,⁎, Hau Thi Nguyen a, Huu Hao Ngo b,⁎, Wenshan Guo b, Chan Wen Hao a, Po-Hsun Lin c a b c
Institute of Environmental Engineering and Management, National Taipei University of Technology, No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 106, Taiwan, ROC School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology Sydney, Broadway, NSW 2007, Australia New Materials Research and Development Dept., China Steel Corporation, Taiwan, ROC
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A novel osmotic membrane bioreactor (MBBR–OsMBR) using a novel draw solution (DS) was developed. • The MBBR–OsMBR system successfully reduced membrane fouling. • EDTA sodium coupled with Triton X100 as novel DS resulted in low salt accumulation. • Nitrification and denitrification were well performed in a biocarrier. • The MBBR–OsMBR could remarkably remove phosphorus.
a r t i c l e
i n f o
Article history: Received 12 January 2015 Received in revised form 3 March 2015 Accepted 3 March 2015 Available online 16 March 2015 Editor: D. Barcelo Keywords: Osmotic membrane bioreactor Forward osmosis Draw solution Carrier Moving bed biofilm
a b s t r a c t This study aims to develop a new osmotic membrane bioreactor by combining a moving bed biofilm reactor (MBBR) with forward osmosis membrane bioreactor (FOMBR) to treat wastewater. Ethylenediaminetetraacetic acid disodium salt coupled with polyethylene glycol tert-octylphenyl ether was used as an innovative draw solution in this membrane hybrid system (MBBR–OsMBR) for minimizing the reverse salt flux and maintaining a healthy environment for the microorganism community. The results showed that the hybrid system achieved a stable water flux of 6.94 L/m2 h and low salt accumulation in the bioreactor for 68 days of operation. At a filling 3− rate of 40% (by volume of the bioreactor) of the polyethylene balls used as carriers, NH+ 4 -N and PO4 -P were al− − most removed (N 99%) while producing relatively low NO3 -N and NO2 -N in the effluent (e.g. b 0.56 and 0.96 mg/L, respectively). Furthermore, from analysis based on scanning electron microscopy, Fourier transform infrared spectroscopy, and fluorescence emission–excitation matrix spectrophotometry, there was a thin gellike fouling layer on the FO membrane, which composed of bacteria as well as biopolymers and protein-like substances. Nonetheless, the formation of these fouling layers of the FO membrane in MBBR–OsMBR was reversible and removed by a physical cleaning technique. © 2015 Elsevier B.V. All rights reserved.
⁎ Corresponding authors. E-mail addresses: [email protected] (S.-S. Chen), [email protected] (H.H. Ngo).
http://dx.doi.org/10.1016/j.scitotenv.2015.03.011 0048-9697/© 2015 Elsevier B.V. All rights reserved.
N.C. Nguyen et al. / Science of the Total Environment 518–519 (2015) 586–594
1. Introduction An innovative membrane bioreactor, known as osmotic membrane bioreactor (OsMBR), has recently been investigated (Achilli et al., 2009; Cornelissen et al., 2008). OsMBR is based on an ideal multibarrier technology combining biological process with forward osmosis (FO) membrane separation that can be used for indirect and direct potable reuse applications (Alturki et al., 2012; Nawaz et al., 2013). In an OsMBR, water is drawn by osmosis force from activated sludge through an FO membrane into a draw solution (DS), which offers several excellent advantages such as low energy consumption, high rejection rate for a wide range of contaminants, and low fouling propensity (Achilli et al., 2009; Lay et al., 2011; Nawaz et al., 2013; Qiu and Ting, 2013; Yin Tang and Ng, 2014) Hence, the OsMBR has been considered a promising alternative for wastewater treatment and reclamation, especially for removing emerging trace organic compounds (Alturki et al., 2012). However, one of the most important challenges in further developing current OsMBR systems is the lack of appropriate DSs to reduce salt accumulation within the bioreactor. The reverse diffusion of salts from the DS into the activated sludge and salt accumulation due to high retention property of the FO membrane can increase salt concentration in the bioreactor (Ge et al., 2012; Kim, 2014), resulting in adverse effect on microbial activity as some functional bacteria are more sensitive to elevated salinity conditions (Moussa et al., 2006; Osaka et al., 2008). In addition, the increase of total dissolved solid (TDS) concentration in activated sludge could also reduce the osmotic pressure difference across the FO membrane, thereby inducing rapid water flux decline (Uygur, 2006; Ye et al., 2009). For instance, Holloway et al. (2014) used NaCl salt as the DS for an OsMBR system and achieved 96% and N 99% removal of chemical oxygen demand (COD) and phosphate with high water flux (5.72 L/m2 h), respectively. Nevertheless, as monovalent ions (Na+ with hydrated radius of 0.18 nm and Cl− with hydrated radius of 0.19 nm (Kiriukhin and Collins, 2002)) could easily pass through the FO membrane (membrane pore of 0.37 nm) (Xie et al., 2012a), TDS concentration in the bioreactor increased approximately 8 g/L after 40 days (Holloway et al., 2014). To reduce the reverse salt flux, Qiu and Ting (2013) demonstrated that the use of divalent salt such as MgCl2 (Mg2+ with hydrated radius of 0.3 nm (Kiriukhin and Collins, 2002)) as the DS in a submerged OsMBR could facilitate organic matter removal up to 98% and reduce salt leakage compared with NaCl DS. However, the mixed liquor conductivity in OsMBR was still high ranging from 2 to 17 mS/cm within 80 days of operation, due to both the reverse transport of the MgCl2 from the DS and the rejection of dissolved solutes in the feed by the FO membrane. Therefore, to minimize the reverse salt flux, the present study proposes a novel DS in OsMBR consisting of ethylenediaminetetraacetic acid disodium salt (EDTA sodium) coupled with polyethylene glycol tert-octylphenyl ether (Triton X-100). An additional benefit of using highly charged EDTA coupled with a surfactant as the DS is to enlarge the molecular size of the draw solute so as to recover it easily using nanofiltration (NF) membrane (Archer et al., 1999; Hau et al., 2014; Kaya et al., 2006). Apart from salt accumulation in the bioreactor, the accumulation of − NO− 2 -N and NO3 -N is also a big challenge for current OsMBR system. − It was found that the concentrations of NO− 2 -N and NO3 -N in an OsMBR increased rapidly after 30 days (the concentrations of NO− 2 -N and NO− 3 -N reached 60 mg/L and 4 mg/L, respectively) (Qiu and Ting, − 2013). The reason was that NH+ 4 -N was converted to NO2 -N and NO− -N under nitrification conditions during the OsMBR operation, 3 − leading to accumulation of NO− 2 -N and NO3 -N in the bioreactor. − Moreover the FO membrane rejection rate of NO− 2 -N and NO3 -N in the bioreactor was low (approximately 70%), which resulted in a high − concentration of NO− 2 -N and NO3 -N in the DS (Holloway et al., 2014; Qiu and Ting, 2013). As an advanced treatment technology, moving bed biofilm reactor (MBBR) basically relies on the use of small plastic or sponge carrier elements for biofilm growth and removal of organic matter or other
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harmful constituents in wastewater (Guo et al., 2008; Ngo et al., 2006; Odegaard, 2006; Odegaard et al., 1999). As attached growth biofilm within carriers can form aerobic zone and anoxic zone along the direction of mass transfer, simultaneous nitrification and denitrification can occur in a single tank (Guo et al., 2008; Yang et al., 2010). Although previous studies on combined MBBR–MBR system exhibited complete nitrification (nitrification rates of 1.2 g NH4-N/m2 d) and high denitrification rate (denitrification rates of 3.5 g NO3-N/m2 d) (Odegaard, 2006), this is the first time report made by our research on MBBR–OsMBR. In this study, an integrated small plastic carrier based moving bedOsMBR hybrid system for enhanced nutrient removal and membrane fouling reduction was proposed. The performance of the laboratoryscale MBBR–OsMBR using EDTA sodium coupled with Triton X-100 as the innovative DS was evaluated. The membrane fouling characteristics were also investigated using a scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and fluorescence excitation–emission matrix (FEEM) spectrophotometry. 2. Materials and methods 2.1. The characteristics of membrane, feed and draw solutions The cellulose triacetate with embedded polyester screen support (CTA-ES) FO membranes used in this study were supplied by Hydration Technology Innovations (HTIs OsMem™ CTA Membrane 130806, Albany, OR, USA). The overall thickness of the membrane was approximately 50 μm, and the FO membrane was negatively charged at pH N 4.5 (Xie et al., 2012b). The contact angle of the CTA-ES FO membrane was determined to be 60–80°, indicating that the membrane was also moderately hydrophobic (Jin et al., 2012; Xie et al., 2012a). NF-TS80 membrane (molecular weight cut-off of 150, TriSep) was used to recover the diluted draw solution. A synthetic wastewater to simulate domestic wastewater was used as the feed solution (FS) as shown in Table 1. pH in bioreactor was adjusted to a value of 7.2 ± 0.5 using NaHCO3 or H2SO4. In addition, deionized (DI) water was also used as the FS to determine the reverse salt flux. EDTA sodium was purchased from Imperial Chemical Corp, Taiwan. Triton X-100 with an average molecular weight of 646.37 g/mol and a critical micelle concentration (CMC) of 0.2–0.9 mM was supplied by Scharlau Chemie, Spain. The DS was prepared using EDTA sodium coupled with surfactants at a mole ratio of 800:1. These mixtures were maintained at pH 8, and then continuously stirred for 48 h before performing FO tests. 2.2. Carrier acclimatization The acclimatization of polyethylene ball carriers is one of the essential components to provide a preferably active biomass growth on the carriers so that this biomass can perform well in the wastewater treatment process. Therefore, before commencing the operation of the MBBR–OsMBR system, the polyethylene ball carriers (Table 2) were acclimatized in a separate aeration tank (20 L) filled with synthetic wastewater and activated sludge (MLSS of 5 g/L) from a wastewater treatment plant in Taipei, Taiwan. Everyday, 8 L synthetic wastewater was added in the aeration tank and pH was maintained to 7 by adding Table 1 Composition of the synthetic wastewater. Composition
Unit
Concentration
Glucose (C6H12O6) Ammonium chloride (NH4Cl) Potassium phosphate (KH2PO4) Calcium chloride (CaCl2·2H2O) Magnesium sulfate (MgSO4·7H2O) Ferric chloride (FeCl3) Cobalt chloride (CoCl2·6H2O)
(mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L)
450 116 25.34 0.42 4.82 1.52 0.38
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Table 2 Specifications of media used in the bioreactor. Factor
Unit
Value/material
Media Shape Diameter Specific surface area Weight Biomass attached on media (after 60 days)
– – cm (cm2/g) g (mg/g carrier)
Polyethylene Circular 2.5 12.3 4.1 57.2
sulfuric acid or sodium carbonate anhydrous to support the microbial growth. The polyethylene ball carriers were acclimatized until the removal of COD, total organic carbon, total nitrogen, and PO34 −-P was stable. 2.3. Description of MBBR–OsMBR system A schematic of the laboratory-scale MBBR–OsMBR system is shown in Fig. 1. The FO module with an effective membrane area of 120 cm2 was fabricated with a tube configuration wrapped with OsMem™ CTA-ES flat sheet membranes (Hydration Technologies Inc., Albany, OR), and it was immersed in the bioreactor tank (6 L) in the vertical position with the active layer of the membrane facing the FS. The biocarriers after acclimatization, with a filling rate of 40% (by volume of the bioreactor), were added into the bioreactor tank. The air diffusers were installed at the bottom of the bioreactor for moving biocarriers and reducing membrane fouling. In MBBR–OsMBR system, synthetic wastewater supplied from a feed tank (6 L) was continuously pumped into the bioreactor tank and the liquid level in the bioreactor tank was maintained at a constant level by using the overflow returning to the feed tank. Meanwhile, the DS was pumped into an FO membrane tube, whereby water from FS will permeate through membrane to dilute draw solution. Constant DS concentration was maintained by a conductivity controller linked to a concentrated DS reservoir. The feed tank was placed on a digital scale (BW12KH, Shimadzu, Japan) and the water flux was calculated based on changes in the feed tank weight. Salt accumulation in the bioreactor was determined by monitoring the conductivity of the mixed liquor with the aid of a conductivity meter (Oakton Instruments, USA). The fluctuation in the room temperature during the experiment was in the range of 27–29 °C. Samples were collected
from the bioreactor and DS tank for measuring dissolved organic carbon, − − 3− NH+ 4 -N, NO3 -N, NO2 -N, and PO4 -P. During the entire MBBR–OsMBR operation, 300 mL mixed liquor was withdrawn daily (after 24 h) from the bioreactor and settled for 30 min, and the clarified supernatant (old biomass) was then discarded. The water of the mixed liquor was used as the samples for analysis. 2.4. Recovery process The NF test was carried out using a laboratory-scale cross-flow NF membrane cell (CF042 Delrin Acetal Crossflow Cell, USA) to recover diluted draw solution. The feed water was circulated into the membrane cell by a diaphragm pump (Triwin, Taiwan) under operating hydraulic pressure of 8 bar. All experimental data were collected after 2 h to avoid the influence of adsorption of ions on the membrane surface to the rejection. All data were obtained from three repeated tests of three fresh membranes. 2.5. Measurement of water flux and reverse salt flux The experimental water flux Jw (L/m2 h) was calculated by measuring the change in the feed container mass with time as follows:
Jw ¼
ΔV AΔt
ð1Þ
where, ΔV is the total increase in the volume of the permeate water (L) collected over a predetermined period, Δt (h) and A is the effective FO membrane area (m2). The reverse salt flux Js (g/m2 h) of the DS was determined according to the amount of salt accumulated in the feed tank:
Js ¼
Vt Ct −V0 C0 At
ð2Þ
where, Ct and Vt are the concentration and volume of the FS measured at time t, respectively, and C0 and V0 are the initial concentration and initial volume of the FS.
Fig. 1. A schematic of the laboratory scale MBBR–OsMBR system.
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2.6. Analytical methods − + The concentrations of -Na+, PO34 −-P, NO− 3 -N, NO2 -N and NH4 -N were analyzed using Ion Chromatography (Dionex ICS-90) and a UV– Vis spectrophotometer (HACH Model DR-4000, Japan). Samples for DOC analysis were first filtered with 0.45 μm filter paper, and analyzed using a TOC Analyzer (Aurora 1010C, O.I. Analytical Corporation, USA). − The method detection level (MDL) of DOC, PO34-P, NH+ 4 -N, NO3 -N, + NO− -N and Na was 0.1, 0.002, 0.01, 0.01, 0.001 and 0.3 mg/L, respec2 tively. The spiked recoveries for Na+, nutrient and DOC were 80–120% according to the requirement of Standard Methods for the Examination of Water and Wastewater (APHA et al., 2005). pH and DO of the bioreactor were measured everyday using a pH meter (HANNA instrument, model no. HI 9025) and DO meter (HORIBA Ltd. Japan, model no. OM51E), respectively. The viscosity was measured by a viscometer (Vibro Viscometer, AD Company, Japan) and the osmolality of solutions was measured using an osmometer (Model 3320, Advanced Instruments, Inc., USA). The fouled membranes were observed and examined using SEM (JEOL JSM-5600, Tokyo, Japan) and FTIR spectrophotometer (BioRad, Philadelphia, PA) at a resolution of 4 cm−1. FEEM Spectrophotometry analyses were also performed on DS samples before and after the
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experiments, and the feed in bioreactor. Polysaccharides and proteins were measured by a method established by Dubois et al. (1956) and by the Bradford (1976), respectively. In addition, Mineql + software (Version 4.6) was used to determine the speciation of complex and charged EDTA in DS at different pH values based on chemical equilibrium model from the thermodynamic database. 3. Results and discussions 3.1. Water flux and reverse salt flux of novel draw solution Fig. 2a presents variations in the water flux and reverse salt flux for different DSs when DI water is used as the FS. The results show that the reverse salt flux for 0.8 M EDTA sodium coupled with 1 mM Triton X100 was considerably lower than that for 0.8 M NaCl (84 folds) and 0.8 M MgCl2 (49 folds) because of the following reasons. Firstly, EDTA used in this study contained highly charged compounds at pH 8, such as 74.8% HEDTA3− and 23.7% Na[EDTA]3− (Fig. 3), which resulted in increased electrostatic repulsion between the negatively charged FO membrane and negatively charged H[EDTA]3−; this electrostatic repulsion reduced the reverse salt flux. Furthermore, the size of H[EDTA]3−
Fig. 2. (a) Comparison of water flux and reverse salt flux for different DSs. (b) Viscosity and osmolality of different draw solutions. Feed solution: DI water; Membrane orientation: active layer facing the feed solution; cross flow rate of DS: 6 cm/s; pH of the DSs: 8. Error bars were based on the standard deviations of three replicate tests of three independent membranes.
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Fig. 3. The speciation of complex and charged EDTA in draw solution at different pH values of 0.8 M EDTA sodium and 0.75 M NaOH (reproduced from the data of Mineql + software, Version 4.6). At pH 8, there is 74.8% H[EDTA]3− and 23.7% of complexion (Na[EDTA]3−) between EDTA and Na.
anions in EDTA sodium is considerably greater than that of Cl− anions in the NaCl and MgCl2 DS. Secondly, the adsorption of Triton X-100 on the FO membrane caused by the hydrophobic interaction between the tail of Triton X-100 and the membrane constricted membrane pores, appreciably reducing the reverse salt diffusion of Na+, H[EDTA]3 −, and Na[EDTA]3− (Jin et al., 2012; Kiso, 1986). Among DSs, MgCl2 achieved the highest water flux (8.55 L/m2 h) because of the effect of osmolality and viscosity (Fig. 2b). During the measurement of the osmolality of the DSs, the osmolality of 0.8 M MgCl2 reached as high as 2188 mOsm/kg, whereas the osmolality of 0.8 M EDTA sodium coupled with 1 mM Triton X-100 was 2088 mOsm/kg and that of 0.8 M NaCl was 1753 mOsm/kg. However, the difference in the water fluxes of these DSs was insubstantial. Moreover, the specific reverse salt flux (Js/Jw) of 0.8 M EDTA sodium coupled with 1 mM Triton X-100 was the lowest (0.01 g/L). Thus, among draw solutes, EDTA sodium coupled with Triton X-100 is expected to show the highest performance for the MBBR–OsMBR processes. Furthermore, 0.07 M EDTA sodium coupled with 1 mM Triton X-100 was used as a diluted draw solution in the regeneration process under an 8-bar pressure to demonstrate the applicability of NF technology. The efficiency of this regeneration method is validated by the high rejection with 96% of TDS removal. The water permeability of the NF-TS80 membrane was 3.52 ± 0.8 L/m2 h with DOC = 0.2 ± 0.1 mg/L, Na+ = 402.3 ± 9.6 mg/L, and TDS = 426.1 ± 6.5 mg/L. 3.2. Salt accumulation and water flux during MBBR–OsMBR operation The salt accumulation and water flux profiles of the MBBR– OsMBR system are shown in Fig. 4a, b. The result shows that the mixed liquor conductivity in the bioreactor increased gradually from 264 to 537 μS/cm after 68 days of operation because of the high rejection of dissolved solutes in the feed by the FO membrane (Fig. 4a). However, the concentration of the accumulated salt in the bioreactor was relatively low (b0.4 g/L) after 68 days of operation. It is due to the low specific reverse salt flux from the novel DS, which allowed normal growth of the microbial community. In practice, to prevent the inhibition of the activities of the microbial community, the maximum bioreactor tank salinity must not exceed 2 g/L (Holloway et al., 2014). Fig. 4b shows the water flux as a function of time for the period of testing the MBBR-OsMBR hybrid system. The water flux decreased gradually because of an increase in the mixed liquor salinity and membrane fouling. However, a difference of approximately 11% was observed between the water flux measured on the first day (7.20 L/m2 h) and that
measured on the 68th day (6.39 L/m2 h). The mild water flux decline suggested that membrane fouling in the MBBR–OsMBR is insubstantial as most of the microorganisms are attached to the carriers instead of the membrane, thus preventing membrane fouling (Fig. 5a and b). Moreover, because the MBBR–OsMBR hybrid system was operated with the membrane active layer against the mixed liquor (FO mode), foulant deposition occurred on the active layer, where it could be removed by the hydraulic shear force (Mi and Elimelech, 2008). Although the water flux appeared to be stable during operating the MBBR–OsMBR, there were low fluctuations that appeared to be related to changes in the DS and feed temperature. The temperature had a strong effect on the flux across the semipermeable membranes as it affected the water viscosity and thus the water diffusivity through the membrane (Cornelissen et al., 2011). 3.3. Nutrient removal efficiency In the activated sludge-OsMBR process, nutrient removal can be improved by recirculating the mixed liquor to the anoxic tank, which may lead to higher operation cost. Thus, the main advantage of adding a biocarrier to the bioreactor in the MBBR–OsMBR hybrid system is that the biocarrier successfully facilitates not only removing nitrogen and phosphorus but also reducing membrane fouling. Fig. 6a shows that the MBBR–OsMBR hybrid system consistently achieved complete + NH+ 4 -N removal (approximately 100%); the average NH4 -N concentration of the effluent was 0.08 mg/L. High ammonium removal has been observed in the present study and other researches (Achilli et al., 2009; Holloway et al., 2014; Qiu and Ting, 2013). This can be explained by most of the ammonium being converted to nitrite and nitrate in the nitrification process. Additionally, the high rejection by the FO membrane with unconverted NH+ 4 -N also enhanced the ammonium removal − efficiency. Fig. 6b, c shows that the concentrations of NO− 2 -N and NO3 -N in the bioreactor and DS increased slightly during the 68 days of the MBBR–OsMBR hybrid system operation. The concentrations of NO− 2 -N and NO− 3 -N in the bioreactor increased from 1.33 to 1.89 mg/L and 2.33 to 3.44 mg/L, respectively, indicating a high level of denitrification in the anoxic zone of the biofilm formed on the carrier. The entire system eliminated more than 76% NO− 3 -N (the effluent concentration in − the DS was 0.56 mg NO− 3 -N/L) and 75% NO2 -N (the effluent concentra− tion in the DS was 0.96 mg NO2 -N/L). Fig. 6d shows the PO3− 4 -P concentration in the influent, DS, and the overall removal efficiency. The MBBR–OsMBR system removed over 99% of PO3− 4 -P, which is higher than the removal efficiency of conventional MBRs (typically 93%) (Guo et al., 2011). A possible reason for
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Fig. 4. (a) Salt accumulation in the bioreactor during the operation of the MBBR–OsMBR hybrid system. (b) Water flux versus time. Draw solution: 0.8 M EDTA sodium coupled with 1 mM Triton X-100; feed solution: synthetic wastewater; cross flow rate of DS: 6 cm/s; membrane orientation: active layer facing the feed solution.
Fig. 5. (a) Microbial community attached to the carrier. (b) Fouling on the FO membrane during the testing of the MBBR–OsMBR hybrid system.
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− − 3− Fig. 6. Removal of (a) NH+ 4 -N, (b) NO2 -N, (c) NO3 -N, and (d) PO4 -P in the MBBR–OsMBR hybrid system.
the difference is that because the pore radius of the FO membrane was small (0.37 nm), all contaminants were rejected because of the steric effect and electrostatic repulsion of the FO membrane. For example, the hydrated radius of PO34 − was large (0.34 nm) (Kiriukhin and Collins, 2002), and the negatively charged FO membrane repulsed negatively charged phosphate because of the electrostatic force leading to increased PO34 −-P removal. Moreover, PO34 −-P removal was also enhanced because of biological phosphorus removal for a long SRT (Bao et al., 2007; Guo et al., 2008). During the 68 days of the MBBR–OsMBR hybrid system operation, the presence of phosphorus-accumulating organisms in forms of attached growth on biocarriers led to increased removal of phosphorus. This phenomenon was recorded by Guo et al (2011) as the total phosphorus removal in anoxic zone of attached growth on media was kept around 40–50%.
3.4. Membrane fouling Although the fouling potential in the MBBR–OsMBR hybrid system was considered to be low, fouling still occurred. As compared to original membrane, SEM observations of fouled membrane showed that there was a thin gel-like fouling layer composed of bacteria cells embedded within the extracellular polymeric substances (EPS) matrix attached to the membrane of the active layer (Fig. 7a, b). This phenomenon is in agreement with Zhang et al. (2012) who confirmed that EPS could be an important factor governing membrane fouling. The EPS content in the fouling layer on the FO membrane (105 mg/g MLVSS) was found to be much lower than that in the biofilm layer on a biocarrier (152 mg/g MLVSS) as showed in Fig. 8a. This indicates that MBBR–OsMBR hybrid system improved membrane fouling behaviors significantly as compared
Fig. 7. SEM micrographs of the FO membrane: (a) active layer of the original membrane, (b) active layer of the used membrane. Draw solution: 0.8 M EDTA sodium coupled with 1 mM Triton X-100; feed solution: synthetic wastewater; cross flow rate of the DS: 6 cm/s; membrane orientation: active layer facing the feed solution.
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Fig. 8. (a) The EPS content, (b) FTIR spectra of the biofilm layer on a biocarrier and the fouling layer on the FO membrane. Flow rate of the DS: 6 cm/s; membrane orientation: active layer facing the feed solution.
with previous study of activated sludge-OsMBR (Qiu and Ting, 2014). An explanation for this phenomenon would be that freely moving biocarrier and hydrodynamic shear force in MBBR–OsMBR system enhanced clearing FO tube membrane, which resulted in reduced membrane fouling. The composition of the fouling layer on the FO membrane and the biofilm layer on the carriers of the MBBR–OsMBR hybrid system was also analyzed using FTIR spectroscopy with a resolution of 4 cm− 1 (Fig. 8b). In the spectra, both samples showed high absorbance at 1632 and 1540 cm−1, which may be assigned to amides I and II in protein (Kimura et al., 2005; Qiu and Ting, 2014; Ramesh et al., 2006). Fig. 8b indicates that the broad peaks present at 1024 cm−1 in both the fouling layer and biofilm layer were due to symmetric and asymmetric C–O
stretching in polysaccharides (Kimura et al., 2005; Ramesh et al., 2006). N–H stretching was observed at the peak at the wavelength of 3284 cm−1 (Zularisam et al., 2006). Fig. 9 shows a comparison of the FEEM for the feed in the bioreactor, the DS, and the diluted DS on the same fluorescence intensity scale. The results based on fluorescence intensity confirm that the membrane rejects the soluble microbial by product-like and humic acid-like in the feed from being transported to the DS. The FEEM for the DS and diluted DS showed similar peaks, which may be attributed to Triton X-100. The results from the FTIR analysis combined with the FEEM spectrophotometry observations suggest that proteins and polysaccharides accumulated on the active layer of the used membrane, leading to the fouling of the FO membrane. These
Fig. 9. FEEM of (a) the feed in bioreactor, (b) the draw solution, and (c) the diluted draw solution.
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foulants have been identified as essential agents in MBR and OsMBR systems (Valladares Linares et al., 2012; Wang and Li, 2008). 4. Conclusions The developed MBBR–OsMBR hybrid system was successfully applied for synthetic wastewater treatment using EDTA sodium coupled with Triton X-100 as the ideal DS to achieve a stable water flux, low salt accumulation, and high nutrient removal. The results showed that using 0.8 M EDTA sodium coupled with 1 mM Triton as DS, negligible reverse salt flux was achieved (Js of 0.09 g/m2 h), which was an interesting exploration for the practical use of the OsMBR system. During the 68-day operation, the MBBR–OsMBR hybrid system achieved a stable water flux of 6.94 L/m2 h. Moreover, nutrient removal efficiency of proposed system were close to 100% indicating the high performance of simultaneous nitrification and denitrification processes in the biofilm layer on the carriers. These findings provide a valuable result for the design of advanced FO membrane and MBBR–OsMBR and can be a promising technology for wastewater treatment. Acknowledgment This work was supported by the Ministry of Science and Technology of the Republic of China under the grant number of 101-2221-E-027061-MY3. References Achilli, A., Cath, T.Y., Marchand, E.A., Childress, A.E., 2009. The forward osmosis membrane bioreactor: a low fouling alternative to MBR processes. Desalination 239, 10–21. Alturki, A., McDonald, J., Khan, S.J., Hai, F.I., Price, W.E., Nghiem, L.D., 2012. Performance of a novel osmotic membrane bioreactor (OMBR) system: flux stability and removal of trace organics. Bioresour. Technol. 113, 201–206. APHA, A, WEF, 2005. Standard methods for the examination of waters and wastewaters (Washington DC). Archer, A.C., Mendes, A.M., Boaventura, R.A.R., 1999. Separation of an anionic surfactant by nanofiltration. Environ. Sci. Technol. 33, 2758–2764. Bao, L.l., Li, D., Li, X.k., Huang, R.x., Zhang, J., Lv, Y., et al., 2007. Phosphorus accumulation by bacteria isolated from a continuous-flow two-sludge system. J. Environ. Sci. 19, 391–395. Bradford, M.M., 1976. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein–dye binding. Anal. Biochem. 72, 248–254. Cornelissen, E.R., Harmsen, D., de Korte, K.F., Ruiken, C.J., Qin, J.-J., Oo, H., et al., 2008. Membrane fouling and process performance of forward osmosis membranes on activated sludge. J. Membr. Sci. 319, 158–168. Cornelissen, E.R., DH, Beerendonk, E.F., Qin, J.J., Oo, H., de Korte, K.F., Kappelhof, J.W.M.N., 2011. The innovative Osmotic Membrane Bioreactor (OMBR) for reuse of wastewater. Water Sci. Technol. 63, 1557–1565. DuBois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Anal. Chem. 28, 350–356. Ge, Q., Su, J., Amy, G.L., Chung, T.-S., 2012. Exploration of polyelectrolytes as draw solutes in forward osmosis processes. Water Res. 46, 1318–1326. Guo, W., Ngo, H.H., Vigneswaran, S., Xing, W., Goteti, P., 2008. A novel Sponge Submerged Membrane Bioreactor (SSMBR) for wastewater treatment and reuse. Sep. Sci. Technol. 43, 273–285. Guo, W., Ngo, H.H., Wu, Z., Hu, A.Y.J., Listowski, A., 2011. Application of bioflocculant and nonwoven supporting media for better biological nutrient removal and fouling control in a submerged MBR. Sustain. Environ. Res. 21, 53–58. Hau, N.T., Chen, S.-S., Nguyen, N.C., Huang, K.Z., Ngo, H.H., Guo, W., 2014. Exploration of EDTA sodium salt as novel draw solution in forward osmosis process for dewatering of high nutrient sludge. J. Membr. Sci. 455, 305–311. Holloway, R.W., Wait, A.S., Fernandes da Silva, A., Herron, J., Schutter, M.D., Lampi, K., et al., 2014. Long-term pilot scale investigation of novel hybrid ultrafiltration-osmotic membrane bioreactors. Desalination. http://dx.doi.org/10.1016/j.desal.2014.05.040.
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Factors affecting adsorption of organic solutes on cellulose acetate in an aqueous solution system. Chromatographia 22, 55–58. Lay, W.C.L., Zhang, Q., Zhang, J., McDougald, D., Tang, C., Wang, R., et al., 2011. Study of integration of forward osmosis and biological process: membrane performance under elevated salt environment. Desalination 283, 123–130. Mi, B., Elimelech, M., 2008. Chemical and physical aspects of organic fouling of forward osmosis membranes. J. Membr. Sci. 320, 292–302. Moussa, M.S., Sumanasekera, D.U., Ibrahim, S.H., Lubberding, H.J., Hooijmans, C.M., Gijzen, H.J., et al., 2006. Long term effects of salt on activity, population structure and floc characteristics in enriched bacterial cultures of nitrifiers. Water Res. 40, 1377–1388. Nawaz, M.S., Gadelha, G., Khan, S.J., Hankins, N., 2013. Microbial toxicity effects of reverse transported draw solute in the forward osmosis membrane bioreactor (FO-MBR). J. Membr. Sci. 429, 323–329. Ngo, H.H., Nguyen, M.C., Sangvikar, N.G., Hoang, T.T.L., Guo, W.S., 2006. Simple approach towards a design of an attached-growth sponge bioreactor (AGSB) for wastewater treatment and reuse. Water Sci. Technol. 54, 191–197. Odegaard, H., 2006. Innovations in wastewater treatment: the moving bed biofilm process. Water Sci. Technol. 53, 17–33. Odegaard, H., Igarashi, T., Watanabe, Y., Asano, T., Tambo, N., 1999. The moving bed biofilm reactor. Water environmental engineering and reuse of water. Hokkaido Press, pp. 250–305. Osaka, T., Shirotani, K., Yoshie, S., Tsuneda, S., 2008. Effects of carbon source on denitrification efficiency and microbial community structure in a saline wastewater treatment process. Water Res. 42, 3709–3718. Qiu, G., Ting, Y.P., 2013. Osmotic membrane bioreactor for wastewater treatment and the effect of salt accumulation on system performance and microbial community dynamics. Bioresour. Technol. 150, 287–297. Qiu, G., Ting, Y.-P., 2014. 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Contents lists available at ScienceDirect
Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv
Applicability of a novel osmotic membrane bioreactor using a specific draw solution in wastewater treatment Nguyen Cong Nguyen a, Shiao-Shing Chen a,⁎, Hau Thi Nguyen a, Huu Hao Ngo b,⁎, Wenshan Guo b, Chan Wen Hao a, Po-Hsun Lin c a b c
Institute of Environmental Engineering and Management, National Taipei University of Technology, No. 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 106, Taiwan, ROC School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology Sydney, Broadway, NSW 2007, Australia New Materials Research and Development Dept., China Steel Corporation, Taiwan, ROC
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• A novel osmotic membrane bioreactor (MBBR–OsMBR) using a novel draw solution (DS) was developed. • The MBBR–OsMBR system successfully reduced membrane fouling. • EDTA sodium coupled with Triton X100 as novel DS resulted in low salt accumulation. • Nitrification and denitrification were well performed in a biocarrier. • The MBBR–OsMBR could remarkably remove phosphorus.
a r t i c l e
i n f o
Article history: Received 12 January 2015 Received in revised form 3 March 2015 Accepted 3 March 2015 Available online 16 March 2015 Editor: D. Barcelo Keywords: Osmotic membrane bioreactor Forward osmosis Draw solution Carrier Moving bed biofilm
a b s t r a c t This study aims to develop a new osmotic membrane bioreactor by combining a moving bed biofilm reactor (MBBR) with forward osmosis membrane bioreactor (FOMBR) to treat wastewater. Ethylenediaminetetraacetic acid disodium salt coupled with polyethylene glycol tert-octylphenyl ether was used as an innovative draw solution in this membrane hybrid system (MBBR–OsMBR) for minimizing the reverse salt flux and maintaining a healthy environment for the microorganism community. The results showed that the hybrid system achieved a stable water flux of 6.94 L/m2 h and low salt accumulation in the bioreactor for 68 days of operation. At a filling 3− rate of 40% (by volume of the bioreactor) of the polyethylene balls used as carriers, NH+ 4 -N and PO4 -P were al− − most removed (N 99%) while producing relatively low NO3 -N and NO2 -N in the effluent (e.g. b 0.56 and 0.96 mg/L, respectively). Furthermore, from analysis based on scanning electron microscopy, Fourier transform infrared spectroscopy, and fluorescence emission–excitation matrix spectrophotometry, there was a thin gellike fouling layer on the FO membrane, which composed of bacteria as well as biopolymers and protein-like substances. Nonetheless, the formation of these fouling layers of the FO membrane in MBBR–OsMBR was reversible and removed by a physical cleaning technique. © 2015 Elsevier B.V. All rights reserved.
⁎ Corresponding authors. E-mail addresses: [email protected] (S.-S. Chen), [email protected] (H.H. Ngo).
http://dx.doi.org/10.1016/j.scitotenv.2015.03.011 0048-9697/© 2015 Elsevier B.V. All rights reserved.
N.C. Nguyen et al. / Science of the Total Environment 518–519 (2015) 586–594
1. Introduction An innovative membrane bioreactor, known as osmotic membrane bioreactor (OsMBR), has recently been investigated (Achilli et al., 2009; Cornelissen et al., 2008). OsMBR is based on an ideal multibarrier technology combining biological process with forward osmosis (FO) membrane separation that can be used for indirect and direct potable reuse applications (Alturki et al., 2012; Nawaz et al., 2013). In an OsMBR, water is drawn by osmosis force from activated sludge through an FO membrane into a draw solution (DS), which offers several excellent advantages such as low energy consumption, high rejection rate for a wide range of contaminants, and low fouling propensity (Achilli et al., 2009; Lay et al., 2011; Nawaz et al., 2013; Qiu and Ting, 2013; Yin Tang and Ng, 2014) Hence, the OsMBR has been considered a promising alternative for wastewater treatment and reclamation, especially for removing emerging trace organic compounds (Alturki et al., 2012). However, one of the most important challenges in further developing current OsMBR systems is the lack of appropriate DSs to reduce salt accumulation within the bioreactor. The reverse diffusion of salts from the DS into the activated sludge and salt accumulation due to high retention property of the FO membrane can increase salt concentration in the bioreactor (Ge et al., 2012; Kim, 2014), resulting in adverse effect on microbial activity as some functional bacteria are more sensitive to elevated salinity conditions (Moussa et al., 2006; Osaka et al., 2008). In addition, the increase of total dissolved solid (TDS) concentration in activated sludge could also reduce the osmotic pressure difference across the FO membrane, thereby inducing rapid water flux decline (Uygur, 2006; Ye et al., 2009). For instance, Holloway et al. (2014) used NaCl salt as the DS for an OsMBR system and achieved 96% and N 99% removal of chemical oxygen demand (COD) and phosphate with high water flux (5.72 L/m2 h), respectively. Nevertheless, as monovalent ions (Na+ with hydrated radius of 0.18 nm and Cl− with hydrated radius of 0.19 nm (Kiriukhin and Collins, 2002)) could easily pass through the FO membrane (membrane pore of 0.37 nm) (Xie et al., 2012a), TDS concentration in the bioreactor increased approximately 8 g/L after 40 days (Holloway et al., 2014). To reduce the reverse salt flux, Qiu and Ting (2013) demonstrated that the use of divalent salt such as MgCl2 (Mg2+ with hydrated radius of 0.3 nm (Kiriukhin and Collins, 2002)) as the DS in a submerged OsMBR could facilitate organic matter removal up to 98% and reduce salt leakage compared with NaCl DS. However, the mixed liquor conductivity in OsMBR was still high ranging from 2 to 17 mS/cm within 80 days of operation, due to both the reverse transport of the MgCl2 from the DS and the rejection of dissolved solutes in the feed by the FO membrane. Therefore, to minimize the reverse salt flux, the present study proposes a novel DS in OsMBR consisting of ethylenediaminetetraacetic acid disodium salt (EDTA sodium) coupled with polyethylene glycol tert-octylphenyl ether (Triton X-100). An additional benefit of using highly charged EDTA coupled with a surfactant as the DS is to enlarge the molecular size of the draw solute so as to recover it easily using nanofiltration (NF) membrane (Archer et al., 1999; Hau et al., 2014; Kaya et al., 2006). Apart from salt accumulation in the bioreactor, the accumulation of − NO− 2 -N and NO3 -N is also a big challenge for current OsMBR system. − It was found that the concentrations of NO− 2 -N and NO3 -N in an OsMBR increased rapidly after 30 days (the concentrations of NO− 2 -N and NO− 3 -N reached 60 mg/L and 4 mg/L, respectively) (Qiu and Ting, − 2013). The reason was that NH+ 4 -N was converted to NO2 -N and NO− -N under nitrification conditions during the OsMBR operation, 3 − leading to accumulation of NO− 2 -N and NO3 -N in the bioreactor. − Moreover the FO membrane rejection rate of NO− 2 -N and NO3 -N in the bioreactor was low (approximately 70%), which resulted in a high − concentration of NO− 2 -N and NO3 -N in the DS (Holloway et al., 2014; Qiu and Ting, 2013). As an advanced treatment technology, moving bed biofilm reactor (MBBR) basically relies on the use of small plastic or sponge carrier elements for biofilm growth and removal of organic matter or other
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harmful constituents in wastewater (Guo et al., 2008; Ngo et al., 2006; Odegaard, 2006; Odegaard et al., 1999). As attached growth biofilm within carriers can form aerobic zone and anoxic zone along the direction of mass transfer, simultaneous nitrification and denitrification can occur in a single tank (Guo et al., 2008; Yang et al., 2010). Although previous studies on combined MBBR–MBR system exhibited complete nitrification (nitrification rates of 1.2 g NH4-N/m2 d) and high denitrification rate (denitrification rates of 3.5 g NO3-N/m2 d) (Odegaard, 2006), this is the first time report made by our research on MBBR–OsMBR. In this study, an integrated small plastic carrier based moving bedOsMBR hybrid system for enhanced nutrient removal and membrane fouling reduction was proposed. The performance of the laboratoryscale MBBR–OsMBR using EDTA sodium coupled with Triton X-100 as the innovative DS was evaluated. The membrane fouling characteristics were also investigated using a scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and fluorescence excitation–emission matrix (FEEM) spectrophotometry. 2. Materials and methods 2.1. The characteristics of membrane, feed and draw solutions The cellulose triacetate with embedded polyester screen support (CTA-ES) FO membranes used in this study were supplied by Hydration Technology Innovations (HTIs OsMem™ CTA Membrane 130806, Albany, OR, USA). The overall thickness of the membrane was approximately 50 μm, and the FO membrane was negatively charged at pH N 4.5 (Xie et al., 2012b). The contact angle of the CTA-ES FO membrane was determined to be 60–80°, indicating that the membrane was also moderately hydrophobic (Jin et al., 2012; Xie et al., 2012a). NF-TS80 membrane (molecular weight cut-off of 150, TriSep) was used to recover the diluted draw solution. A synthetic wastewater to simulate domestic wastewater was used as the feed solution (FS) as shown in Table 1. pH in bioreactor was adjusted to a value of 7.2 ± 0.5 using NaHCO3 or H2SO4. In addition, deionized (DI) water was also used as the FS to determine the reverse salt flux. EDTA sodium was purchased from Imperial Chemical Corp, Taiwan. Triton X-100 with an average molecular weight of 646.37 g/mol and a critical micelle concentration (CMC) of 0.2–0.9 mM was supplied by Scharlau Chemie, Spain. The DS was prepared using EDTA sodium coupled with surfactants at a mole ratio of 800:1. These mixtures were maintained at pH 8, and then continuously stirred for 48 h before performing FO tests. 2.2. Carrier acclimatization The acclimatization of polyethylene ball carriers is one of the essential components to provide a preferably active biomass growth on the carriers so that this biomass can perform well in the wastewater treatment process. Therefore, before commencing the operation of the MBBR–OsMBR system, the polyethylene ball carriers (Table 2) were acclimatized in a separate aeration tank (20 L) filled with synthetic wastewater and activated sludge (MLSS of 5 g/L) from a wastewater treatment plant in Taipei, Taiwan. Everyday, 8 L synthetic wastewater was added in the aeration tank and pH was maintained to 7 by adding Table 1 Composition of the synthetic wastewater. Composition
Unit
Concentration
Glucose (C6H12O6) Ammonium chloride (NH4Cl) Potassium phosphate (KH2PO4) Calcium chloride (CaCl2·2H2O) Magnesium sulfate (MgSO4·7H2O) Ferric chloride (FeCl3) Cobalt chloride (CoCl2·6H2O)
(mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L)
450 116 25.34 0.42 4.82 1.52 0.38
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Table 2 Specifications of media used in the bioreactor. Factor
Unit
Value/material
Media Shape Diameter Specific surface area Weight Biomass attached on media (after 60 days)
– – cm (cm2/g) g (mg/g carrier)
Polyethylene Circular 2.5 12.3 4.1 57.2
sulfuric acid or sodium carbonate anhydrous to support the microbial growth. The polyethylene ball carriers were acclimatized until the removal of COD, total organic carbon, total nitrogen, and PO34 −-P was stable. 2.3. Description of MBBR–OsMBR system A schematic of the laboratory-scale MBBR–OsMBR system is shown in Fig. 1. The FO module with an effective membrane area of 120 cm2 was fabricated with a tube configuration wrapped with OsMem™ CTA-ES flat sheet membranes (Hydration Technologies Inc., Albany, OR), and it was immersed in the bioreactor tank (6 L) in the vertical position with the active layer of the membrane facing the FS. The biocarriers after acclimatization, with a filling rate of 40% (by volume of the bioreactor), were added into the bioreactor tank. The air diffusers were installed at the bottom of the bioreactor for moving biocarriers and reducing membrane fouling. In MBBR–OsMBR system, synthetic wastewater supplied from a feed tank (6 L) was continuously pumped into the bioreactor tank and the liquid level in the bioreactor tank was maintained at a constant level by using the overflow returning to the feed tank. Meanwhile, the DS was pumped into an FO membrane tube, whereby water from FS will permeate through membrane to dilute draw solution. Constant DS concentration was maintained by a conductivity controller linked to a concentrated DS reservoir. The feed tank was placed on a digital scale (BW12KH, Shimadzu, Japan) and the water flux was calculated based on changes in the feed tank weight. Salt accumulation in the bioreactor was determined by monitoring the conductivity of the mixed liquor with the aid of a conductivity meter (Oakton Instruments, USA). The fluctuation in the room temperature during the experiment was in the range of 27–29 °C. Samples were collected
from the bioreactor and DS tank for measuring dissolved organic carbon, − − 3− NH+ 4 -N, NO3 -N, NO2 -N, and PO4 -P. During the entire MBBR–OsMBR operation, 300 mL mixed liquor was withdrawn daily (after 24 h) from the bioreactor and settled for 30 min, and the clarified supernatant (old biomass) was then discarded. The water of the mixed liquor was used as the samples for analysis. 2.4. Recovery process The NF test was carried out using a laboratory-scale cross-flow NF membrane cell (CF042 Delrin Acetal Crossflow Cell, USA) to recover diluted draw solution. The feed water was circulated into the membrane cell by a diaphragm pump (Triwin, Taiwan) under operating hydraulic pressure of 8 bar. All experimental data were collected after 2 h to avoid the influence of adsorption of ions on the membrane surface to the rejection. All data were obtained from three repeated tests of three fresh membranes. 2.5. Measurement of water flux and reverse salt flux The experimental water flux Jw (L/m2 h) was calculated by measuring the change in the feed container mass with time as follows:
Jw ¼
ΔV AΔt
ð1Þ
where, ΔV is the total increase in the volume of the permeate water (L) collected over a predetermined period, Δt (h) and A is the effective FO membrane area (m2). The reverse salt flux Js (g/m2 h) of the DS was determined according to the amount of salt accumulated in the feed tank:
Js ¼
Vt Ct −V0 C0 At
ð2Þ
where, Ct and Vt are the concentration and volume of the FS measured at time t, respectively, and C0 and V0 are the initial concentration and initial volume of the FS.
Fig. 1. A schematic of the laboratory scale MBBR–OsMBR system.
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2.6. Analytical methods − + The concentrations of -Na+, PO34 −-P, NO− 3 -N, NO2 -N and NH4 -N were analyzed using Ion Chromatography (Dionex ICS-90) and a UV– Vis spectrophotometer (HACH Model DR-4000, Japan). Samples for DOC analysis were first filtered with 0.45 μm filter paper, and analyzed using a TOC Analyzer (Aurora 1010C, O.I. Analytical Corporation, USA). − The method detection level (MDL) of DOC, PO34-P, NH+ 4 -N, NO3 -N, + NO− -N and Na was 0.1, 0.002, 0.01, 0.01, 0.001 and 0.3 mg/L, respec2 tively. The spiked recoveries for Na+, nutrient and DOC were 80–120% according to the requirement of Standard Methods for the Examination of Water and Wastewater (APHA et al., 2005). pH and DO of the bioreactor were measured everyday using a pH meter (HANNA instrument, model no. HI 9025) and DO meter (HORIBA Ltd. Japan, model no. OM51E), respectively. The viscosity was measured by a viscometer (Vibro Viscometer, AD Company, Japan) and the osmolality of solutions was measured using an osmometer (Model 3320, Advanced Instruments, Inc., USA). The fouled membranes were observed and examined using SEM (JEOL JSM-5600, Tokyo, Japan) and FTIR spectrophotometer (BioRad, Philadelphia, PA) at a resolution of 4 cm−1. FEEM Spectrophotometry analyses were also performed on DS samples before and after the
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experiments, and the feed in bioreactor. Polysaccharides and proteins were measured by a method established by Dubois et al. (1956) and by the Bradford (1976), respectively. In addition, Mineql + software (Version 4.6) was used to determine the speciation of complex and charged EDTA in DS at different pH values based on chemical equilibrium model from the thermodynamic database. 3. Results and discussions 3.1. Water flux and reverse salt flux of novel draw solution Fig. 2a presents variations in the water flux and reverse salt flux for different DSs when DI water is used as the FS. The results show that the reverse salt flux for 0.8 M EDTA sodium coupled with 1 mM Triton X100 was considerably lower than that for 0.8 M NaCl (84 folds) and 0.8 M MgCl2 (49 folds) because of the following reasons. Firstly, EDTA used in this study contained highly charged compounds at pH 8, such as 74.8% HEDTA3− and 23.7% Na[EDTA]3− (Fig. 3), which resulted in increased electrostatic repulsion between the negatively charged FO membrane and negatively charged H[EDTA]3−; this electrostatic repulsion reduced the reverse salt flux. Furthermore, the size of H[EDTA]3−
Fig. 2. (a) Comparison of water flux and reverse salt flux for different DSs. (b) Viscosity and osmolality of different draw solutions. Feed solution: DI water; Membrane orientation: active layer facing the feed solution; cross flow rate of DS: 6 cm/s; pH of the DSs: 8. Error bars were based on the standard deviations of three replicate tests of three independent membranes.
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Fig. 3. The speciation of complex and charged EDTA in draw solution at different pH values of 0.8 M EDTA sodium and 0.75 M NaOH (reproduced from the data of Mineql + software, Version 4.6). At pH 8, there is 74.8% H[EDTA]3− and 23.7% of complexion (Na[EDTA]3−) between EDTA and Na.
anions in EDTA sodium is considerably greater than that of Cl− anions in the NaCl and MgCl2 DS. Secondly, the adsorption of Triton X-100 on the FO membrane caused by the hydrophobic interaction between the tail of Triton X-100 and the membrane constricted membrane pores, appreciably reducing the reverse salt diffusion of Na+, H[EDTA]3 −, and Na[EDTA]3− (Jin et al., 2012; Kiso, 1986). Among DSs, MgCl2 achieved the highest water flux (8.55 L/m2 h) because of the effect of osmolality and viscosity (Fig. 2b). During the measurement of the osmolality of the DSs, the osmolality of 0.8 M MgCl2 reached as high as 2188 mOsm/kg, whereas the osmolality of 0.8 M EDTA sodium coupled with 1 mM Triton X-100 was 2088 mOsm/kg and that of 0.8 M NaCl was 1753 mOsm/kg. However, the difference in the water fluxes of these DSs was insubstantial. Moreover, the specific reverse salt flux (Js/Jw) of 0.8 M EDTA sodium coupled with 1 mM Triton X-100 was the lowest (0.01 g/L). Thus, among draw solutes, EDTA sodium coupled with Triton X-100 is expected to show the highest performance for the MBBR–OsMBR processes. Furthermore, 0.07 M EDTA sodium coupled with 1 mM Triton X-100 was used as a diluted draw solution in the regeneration process under an 8-bar pressure to demonstrate the applicability of NF technology. The efficiency of this regeneration method is validated by the high rejection with 96% of TDS removal. The water permeability of the NF-TS80 membrane was 3.52 ± 0.8 L/m2 h with DOC = 0.2 ± 0.1 mg/L, Na+ = 402.3 ± 9.6 mg/L, and TDS = 426.1 ± 6.5 mg/L. 3.2. Salt accumulation and water flux during MBBR–OsMBR operation The salt accumulation and water flux profiles of the MBBR– OsMBR system are shown in Fig. 4a, b. The result shows that the mixed liquor conductivity in the bioreactor increased gradually from 264 to 537 μS/cm after 68 days of operation because of the high rejection of dissolved solutes in the feed by the FO membrane (Fig. 4a). However, the concentration of the accumulated salt in the bioreactor was relatively low (b0.4 g/L) after 68 days of operation. It is due to the low specific reverse salt flux from the novel DS, which allowed normal growth of the microbial community. In practice, to prevent the inhibition of the activities of the microbial community, the maximum bioreactor tank salinity must not exceed 2 g/L (Holloway et al., 2014). Fig. 4b shows the water flux as a function of time for the period of testing the MBBR-OsMBR hybrid system. The water flux decreased gradually because of an increase in the mixed liquor salinity and membrane fouling. However, a difference of approximately 11% was observed between the water flux measured on the first day (7.20 L/m2 h) and that
measured on the 68th day (6.39 L/m2 h). The mild water flux decline suggested that membrane fouling in the MBBR–OsMBR is insubstantial as most of the microorganisms are attached to the carriers instead of the membrane, thus preventing membrane fouling (Fig. 5a and b). Moreover, because the MBBR–OsMBR hybrid system was operated with the membrane active layer against the mixed liquor (FO mode), foulant deposition occurred on the active layer, where it could be removed by the hydraulic shear force (Mi and Elimelech, 2008). Although the water flux appeared to be stable during operating the MBBR–OsMBR, there were low fluctuations that appeared to be related to changes in the DS and feed temperature. The temperature had a strong effect on the flux across the semipermeable membranes as it affected the water viscosity and thus the water diffusivity through the membrane (Cornelissen et al., 2011). 3.3. Nutrient removal efficiency In the activated sludge-OsMBR process, nutrient removal can be improved by recirculating the mixed liquor to the anoxic tank, which may lead to higher operation cost. Thus, the main advantage of adding a biocarrier to the bioreactor in the MBBR–OsMBR hybrid system is that the biocarrier successfully facilitates not only removing nitrogen and phosphorus but also reducing membrane fouling. Fig. 6a shows that the MBBR–OsMBR hybrid system consistently achieved complete + NH+ 4 -N removal (approximately 100%); the average NH4 -N concentration of the effluent was 0.08 mg/L. High ammonium removal has been observed in the present study and other researches (Achilli et al., 2009; Holloway et al., 2014; Qiu and Ting, 2013). This can be explained by most of the ammonium being converted to nitrite and nitrate in the nitrification process. Additionally, the high rejection by the FO membrane with unconverted NH+ 4 -N also enhanced the ammonium removal − efficiency. Fig. 6b, c shows that the concentrations of NO− 2 -N and NO3 -N in the bioreactor and DS increased slightly during the 68 days of the MBBR–OsMBR hybrid system operation. The concentrations of NO− 2 -N and NO− 3 -N in the bioreactor increased from 1.33 to 1.89 mg/L and 2.33 to 3.44 mg/L, respectively, indicating a high level of denitrification in the anoxic zone of the biofilm formed on the carrier. The entire system eliminated more than 76% NO− 3 -N (the effluent concentration in − the DS was 0.56 mg NO− 3 -N/L) and 75% NO2 -N (the effluent concentra− tion in the DS was 0.96 mg NO2 -N/L). Fig. 6d shows the PO3− 4 -P concentration in the influent, DS, and the overall removal efficiency. The MBBR–OsMBR system removed over 99% of PO3− 4 -P, which is higher than the removal efficiency of conventional MBRs (typically 93%) (Guo et al., 2011). A possible reason for
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Fig. 4. (a) Salt accumulation in the bioreactor during the operation of the MBBR–OsMBR hybrid system. (b) Water flux versus time. Draw solution: 0.8 M EDTA sodium coupled with 1 mM Triton X-100; feed solution: synthetic wastewater; cross flow rate of DS: 6 cm/s; membrane orientation: active layer facing the feed solution.
Fig. 5. (a) Microbial community attached to the carrier. (b) Fouling on the FO membrane during the testing of the MBBR–OsMBR hybrid system.
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− − 3− Fig. 6. Removal of (a) NH+ 4 -N, (b) NO2 -N, (c) NO3 -N, and (d) PO4 -P in the MBBR–OsMBR hybrid system.
the difference is that because the pore radius of the FO membrane was small (0.37 nm), all contaminants were rejected because of the steric effect and electrostatic repulsion of the FO membrane. For example, the hydrated radius of PO34 − was large (0.34 nm) (Kiriukhin and Collins, 2002), and the negatively charged FO membrane repulsed negatively charged phosphate because of the electrostatic force leading to increased PO34 −-P removal. Moreover, PO34 −-P removal was also enhanced because of biological phosphorus removal for a long SRT (Bao et al., 2007; Guo et al., 2008). During the 68 days of the MBBR–OsMBR hybrid system operation, the presence of phosphorus-accumulating organisms in forms of attached growth on biocarriers led to increased removal of phosphorus. This phenomenon was recorded by Guo et al (2011) as the total phosphorus removal in anoxic zone of attached growth on media was kept around 40–50%.
3.4. Membrane fouling Although the fouling potential in the MBBR–OsMBR hybrid system was considered to be low, fouling still occurred. As compared to original membrane, SEM observations of fouled membrane showed that there was a thin gel-like fouling layer composed of bacteria cells embedded within the extracellular polymeric substances (EPS) matrix attached to the membrane of the active layer (Fig. 7a, b). This phenomenon is in agreement with Zhang et al. (2012) who confirmed that EPS could be an important factor governing membrane fouling. The EPS content in the fouling layer on the FO membrane (105 mg/g MLVSS) was found to be much lower than that in the biofilm layer on a biocarrier (152 mg/g MLVSS) as showed in Fig. 8a. This indicates that MBBR–OsMBR hybrid system improved membrane fouling behaviors significantly as compared
Fig. 7. SEM micrographs of the FO membrane: (a) active layer of the original membrane, (b) active layer of the used membrane. Draw solution: 0.8 M EDTA sodium coupled with 1 mM Triton X-100; feed solution: synthetic wastewater; cross flow rate of the DS: 6 cm/s; membrane orientation: active layer facing the feed solution.
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Fig. 8. (a) The EPS content, (b) FTIR spectra of the biofilm layer on a biocarrier and the fouling layer on the FO membrane. Flow rate of the DS: 6 cm/s; membrane orientation: active layer facing the feed solution.
with previous study of activated sludge-OsMBR (Qiu and Ting, 2014). An explanation for this phenomenon would be that freely moving biocarrier and hydrodynamic shear force in MBBR–OsMBR system enhanced clearing FO tube membrane, which resulted in reduced membrane fouling. The composition of the fouling layer on the FO membrane and the biofilm layer on the carriers of the MBBR–OsMBR hybrid system was also analyzed using FTIR spectroscopy with a resolution of 4 cm− 1 (Fig. 8b). In the spectra, both samples showed high absorbance at 1632 and 1540 cm−1, which may be assigned to amides I and II in protein (Kimura et al., 2005; Qiu and Ting, 2014; Ramesh et al., 2006). Fig. 8b indicates that the broad peaks present at 1024 cm−1 in both the fouling layer and biofilm layer were due to symmetric and asymmetric C–O
stretching in polysaccharides (Kimura et al., 2005; Ramesh et al., 2006). N–H stretching was observed at the peak at the wavelength of 3284 cm−1 (Zularisam et al., 2006). Fig. 9 shows a comparison of the FEEM for the feed in the bioreactor, the DS, and the diluted DS on the same fluorescence intensity scale. The results based on fluorescence intensity confirm that the membrane rejects the soluble microbial by product-like and humic acid-like in the feed from being transported to the DS. The FEEM for the DS and diluted DS showed similar peaks, which may be attributed to Triton X-100. The results from the FTIR analysis combined with the FEEM spectrophotometry observations suggest that proteins and polysaccharides accumulated on the active layer of the used membrane, leading to the fouling of the FO membrane. These
Fig. 9. FEEM of (a) the feed in bioreactor, (b) the draw solution, and (c) the diluted draw solution.
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foulants have been identified as essential agents in MBR and OsMBR systems (Valladares Linares et al., 2012; Wang and Li, 2008). 4. Conclusions The developed MBBR–OsMBR hybrid system was successfully applied for synthetic wastewater treatment using EDTA sodium coupled with Triton X-100 as the ideal DS to achieve a stable water flux, low salt accumulation, and high nutrient removal. The results showed that using 0.8 M EDTA sodium coupled with 1 mM Triton as DS, negligible reverse salt flux was achieved (Js of 0.09 g/m2 h), which was an interesting exploration for the practical use of the OsMBR system. During the 68-day operation, the MBBR–OsMBR hybrid system achieved a stable water flux of 6.94 L/m2 h. Moreover, nutrient removal efficiency of proposed system were close to 100% indicating the high performance of simultaneous nitrification and denitrification processes in the biofilm layer on the carriers. These findings provide a valuable result for the design of advanced FO membrane and MBBR–OsMBR and can be a promising technology for wastewater treatment. Acknowledgment This work was supported by the Ministry of Science and Technology of the Republic of China under the grant number of 101-2221-E-027061-MY3. References Achilli, A., Cath, T.Y., Marchand, E.A., Childress, A.E., 2009. The forward osmosis membrane bioreactor: a low fouling alternative to MBR processes. Desalination 239, 10–21. Alturki, A., McDonald, J., Khan, S.J., Hai, F.I., Price, W.E., Nghiem, L.D., 2012. Performance of a novel osmotic membrane bioreactor (OMBR) system: flux stability and removal of trace organics. Bioresour. Technol. 113, 201–206. APHA, A, WEF, 2005. Standard methods for the examination of waters and wastewaters (Washington DC). Archer, A.C., Mendes, A.M., Boaventura, R.A.R., 1999. Separation of an anionic surfactant by nanofiltration. Environ. Sci. Technol. 33, 2758–2764. Bao, L.l., Li, D., Li, X.k., Huang, R.x., Zhang, J., Lv, Y., et al., 2007. Phosphorus accumulation by bacteria isolated from a continuous-flow two-sludge system. J. Environ. Sci. 19, 391–395. Bradford, M.M., 1976. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein–dye binding. Anal. Biochem. 72, 248–254. Cornelissen, E.R., Harmsen, D., de Korte, K.F., Ruiken, C.J., Qin, J.-J., Oo, H., et al., 2008. Membrane fouling and process performance of forward osmosis membranes on activated sludge. J. Membr. Sci. 319, 158–168. Cornelissen, E.R., DH, Beerendonk, E.F., Qin, J.J., Oo, H., de Korte, K.F., Kappelhof, J.W.M.N., 2011. The innovative Osmotic Membrane Bioreactor (OMBR) for reuse of wastewater. Water Sci. Technol. 63, 1557–1565. DuBois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A., Smith, F., 1956. Colorimetric method for determination of sugars and related substances. Anal. Chem. 28, 350–356. Ge, Q., Su, J., Amy, G.L., Chung, T.-S., 2012. Exploration of polyelectrolytes as draw solutes in forward osmosis processes. Water Res. 46, 1318–1326. Guo, W., Ngo, H.H., Vigneswaran, S., Xing, W., Goteti, P., 2008. A novel Sponge Submerged Membrane Bioreactor (SSMBR) for wastewater treatment and reuse. Sep. Sci. Technol. 43, 273–285. Guo, W., Ngo, H.H., Wu, Z., Hu, A.Y.J., Listowski, A., 2011. Application of bioflocculant and nonwoven supporting media for better biological nutrient removal and fouling control in a submerged MBR. Sustain. Environ. Res. 21, 53–58. Hau, N.T., Chen, S.-S., Nguyen, N.C., Huang, K.Z., Ngo, H.H., Guo, W., 2014. Exploration of EDTA sodium salt as novel draw solution in forward osmosis process for dewatering of high nutrient sludge. J. Membr. Sci. 455, 305–311. Holloway, R.W., Wait, A.S., Fernandes da Silva, A., Herron, J., Schutter, M.D., Lampi, K., et al., 2014. Long-term pilot scale investigation of novel hybrid ultrafiltration-osmotic membrane bioreactors. Desalination. http://dx.doi.org/10.1016/j.desal.2014.05.040.
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