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Impact of aeration rate and dielectrophoretic force on fouling suppression in submerged membrane bioreactors
Chemical Engineering & Processing: Process Intensification 142 (2019) 107565
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Impact of aeration rate and dielectrophoretic force on fouling suppression in submerged membrane bioreactors
T
⁎
A.H. Hawaria, , B. Larbia, A. Alkhatiba, Ahmed T. Yasira, F. Dub, M. Bauneb, J. Thömingb a b
Department of Civil and Architectural Engineering, College of Engineering, Qatar University, 2713 Doha, Qatar Center for Environmental Research and Sustainable Technology, University of Bremen, Leobener Str. 6, D 28359 Bremen, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane bioreactor Dielectrophoresis Activated sludge Aeration Fouling
Membrane bioreactors (MBRs) present a unique opportunity in waste water treatment by combining the activated sludge process with membrane filtration. Although MBRs require less area, produce less volume of sludge and have high purification efficiency, they suffer from high membrane fouling. Membrane fouling can be reduced by applying aeration and Dep voltage. However, application of these two anti-fouling mechanisms would make the process expensive. Thus, a new aeration and DEP scheme of 5 min on – 5 min off is being suggested in this study. With this scheme, it was found that through application of DEP at 100 V and 0.3 L/min.m2 aeration flowrate and reduced lumen size of 6 mm, the membrane permeate flux can be increased by 58% compared to MBR without DEP and aeration. However, through energy analysis the optimum operating parameters were found to be at 0.3 L/min.m2 aeration flowrate and 50 V DEP using the suggested 5 min on-5 min off scheme. At the optimum operating condition, the permeability flux is 54% higher than MBR without DEP and aeration.
1. Introduction Membrane bioreactors (MBRs) combine the activated sludge process with micro/ultra filtration. This results in an enhanced treatment efficiency compared to a conventional aeration process [1]. The organic pollutants are degraded in the bio-reactor and the membrane filters the waste water and produces water free of suspended solid, bacteria and viruses [2]. Compared to the conventional activated sludge treatment process, MBRs are smaller in size, produces less sludge volume, have high COD removal efficiency and can handle high concentration of suspended solids [3]. However, MBRs suffer from high propensity to membrane fouling [3]. Fouling can be reduced in MBRs through aeration scouring and backwashing, chemical cleaning, electrically assisted fouling mitigation and ultrasound application [4–7]. These methods have their own drawbacks owing to membrane damage, secondary contamination and high energy demand [8,9]. However, it is possible to overcome these limitations using electrically assisted fouling mitigation technologies; such as, electrophoresis (EP) and dielectrophoresis (DEP) [8]. However, the DC current used in electrophoresis induces electrochemical reaction at the membrane surface and causes high energy consumption [10,11]. This limitation can be solved by dielectrophoresis [10,11]. In dielectrophoresis (DEP), the particles move away from membrane due to lower dielectric polarization compared to that of medium in ⁎
inhomogeneous electric field. When a strong electric field is applied, the particles will be repelled from the membrane surface as they have lower permittivity compared to water [11,12]. In order to prevent electro chemical reactions on membrane surface, AC current and insulated electrodes have been used successfully [11]. Moreover, by using cylindrical interdigitated electrodes (IDE), the membrane lifetime was increases by 9 times [12]. Due to quadratic dependence of DEP velocity on voltage, the fouling suppressant performance of DEP depends on the applied voltage [13]. Along with DEP, the flux through the membrane can be sustained through increasing the shear force on the surface of the membrane. The shear force can be enhanced by increasing the aeration rate inside the MBR [14,15]. Therefore, apart from providing oxygen for the biomass, aeration can be used to reduce fouling. Although the increase of aeration velocity positively impacts fouling suppression, the degree of such a positive influence was substantially reduced as MLSS increases due to the increased viscosity by the MLSS (Iorhemen et al., 2016). Ueda et al. concluded that aeration helps to remove cake from the membrane surface through creating turbulence in front of the membrane [16]. Gui et al showed that, aeration is more effective for sustaining flux at when the concentration of SS is higher in the waste water [17]. Martinelli et al. found low aeration rate to be responsible for accelerated fouling [18]. Although flux can be sustained through increasing aeration rate, Ueda et al. found that after reaching the critical aeration rate, cake
Corresponding author. E-mail address: [email protected] (A.H. Hawari).
https://doi.org/10.1016/j.cep.2019.107565 Received 17 April 2019; Received in revised form 13 June 2019; Accepted 19 June 2019 Available online 28 June 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering & Processing: Process Intensification 142 (2019) 107565
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filters used chlorinated polyethylene (CP-E) membranes with an average pore size of 0.2 μm and a maximum pore size of 0.4 μm. The interdigitated energized electrodes diameter was 2 mm and the distance between the powered electrodes was 10 mm and was isolated by a thin film of TiO2 to refrain from corrosion on their surface and lower the high pass filter effect influence. Between energized electrodes, acrylonitrile styrene acrylate (ASA) with square section 2 mm on all sides was mounted. Two 6 mm plastic spacers were used to sustain the active membrane and two barriers on each side of the module as seen in Fig. 1(b). With the experimental setup depicted in Fig. 2, the MBR system consisted of a bioparticle suspension in a 4-L bioreactor in which the DEP membrane module was immersed. An AC voltage generator by Variac® was connected to the module to generate a voltage of 50 V and a frequency of 50 Hz. The current and voltage were measured and monitored during the experiment using an oscilloscope (MDO 3024, Tektronix). The oscilloscope was calibrated using a standard resistor and capacitor. The bioparticles were acquired from a local wastewater treatment plant (Doha South Wastewater Treatment Plant in Qatar). A suction pump (Gear Pump Drive, Model 75211-15, Cole Parmar Instrument) was used to draw the permeate. A pressure transducer (PT) was utilized for measuring the pressure. To ensure that the membrane remained submerged in solution and to maintain a constant concentration, the water level was observed and controlled inside the MBR system. The permeate flux was obtained by measuring the weight over time. The permeate flux was measured for three different settings: without DEP and barrier, with DEP at 50 V & 100 V and aeration flow rate of 0.1 L/ min.m2 & 0.3 L/min.m2 without barrier, and with DEP at 50 V & 100 V and aeration flow rate of 0.1 L/min.m2 & 0.3 L/min.m2 with barrier. The pulsed aeration was supplied at an intermittent time of 5 min on and 5 min off. To reduce the power consumption of the DEP system, a pulsed DEP with 10 s with electric field and 15 s without electric field was applied. A diffuser was used for aeration inside the MBR. The aeration rate was chosen to maintain a minimum DO concentration of 5 mg/L. According to previous studies the DO concentration in aerated MBRs should be maintained between 3–6.5 mg/L. It was found that this DO concentration would increase the lifespan of the membrane [28,29]. Moreover, it was found that a DO concentration of 5 mg/L was sufficient to sustain the required MLSS concentration and microbial activity in the MBR system [30]. After each process, the membrane was backwashed by pumping pure water from the membrane substrate side (from inside the module to the outside).
removal percentage cannot be improved [16]. The aeration performance in MBR is quantified by superficial gas velocity (Ug ) , specific aeration demand in membrane area (SADm) or specific aeration demand in permeate product SADP [14]. Aeration intensity or superficial gas velocity (Ug ) can only be used for submerged MBRs and it is calculated as [14]:
Ug =
Qg (1)
Sr
Here, Qg and Sr are the flowrate of air and cross-sectional area of the membrane module, respectively. SADp is primarily used in the industry as it can be used directly to estimate the energy performance of the MBR and it is expressed as [19]:
SADP =
Qg Qp
(2)
Here, QP is the permeate flowrate. As seen from Eq.s (1) and (2), Ug and SADp depends mainly on the air flow rate. Chua et al. concluded that membrane fouling increases exponentially when Ug decreases [20]. Germain et al. found that in order to maintain the flux above 16.5 Lm−2 h-1, high aeration rate is required [21]. The size of lumen (or channel) through which the air flows also impacts the fouling rate [22]. Ozaki et al. found that, the lag phase decreased from 180 min to 30 min when the channel width was reduced from 15 mm to 5 mm [23]. This is because the flow becomes turbulent when the flowrate increases [24]. Although aeration helps to reduce fouling in MBR, the aeration consumes more than 50% of the net power demand and turns out to be between 45 and 75% of the operational cost in [25]. However, the usage of oxygen can be reduced by pulsed aeration. In pulsed aeration, the aeration frequency is higher than general intermittent aeration [26]. Pulse aeration provides better mixture and maintains sufficient DO inside the MBR [27]. Pulsed aeration can reduce the energy demand of MBR by 29.5%–48.5% [26]. This paper describes the design of a bench-scale pulsed DEP and pulsed aeration assisted MBR system with a lumen size of 6 mm. An optimum aeration scheme for operating cost reduction has been presented. Moreover, using the optimum aeration scheme, the impact of aeration volume and DEP on the permeate flux in MBR was established. The optimum aeration volume and DEP voltage was determined by energy analysis. 2. Materials and method 2.1. Experimental setup The membrane module (Fig. 1) was fabricated according to the method we used before [3]. The DEP membrane modules used had a filtration area of 90 cm2 (10 cm length and 9 cm width) and incorporated two acrylonitrile butadiene styrene (ABS) frames with a length of 16 cm, width of 14 cm, and thickness of 0.5 cm. The medium
2.2. Waste water characterization The characteristics of the wastewater used in this study are summarized Table 1. A WTW Multi3430 (Weilheim, Germany) multi-meter was used to continuously monitor the pH, temperature and dissolved
Fig. 1. (a) Dielectrophoretic (DEP) membrane module with square acrylonitrile styrene acrylate (ASA) obstacles installed in between interdigitatedly arrayed excited (IDE) energized electrodes (revised from the figure in Du et. al. 2018), (b) Membrane module and membrane barriers. 2
Chemical Engineering & Processing: Process Intensification 142 (2019) 107565
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Fig. 2. Experimental setup of the MBR process a) schematic diagram, b) actual setup.
3. Results and discussion
Table 1 Initial activated sludge characteristics. Parameter
Value
pH Initial temperature Mix Liquor Suspended Solid (MLSS) Mix Liquor Volatile Suspended Solid MLVSS Total Organic Carbon (TOC) Chemical Oxygen Demand (COD) Dissolved Oxygen (DO) Oxygen Uptake Rate (OUR) Specific Oxygen Uptake rate (SOUR)
6.9 ± 0.1 @ 21.9 °C 25 ± 2 °C 3,550 ± 410.1 mg/L 31.5 ± 2.6 mg/L 29.2 ± 0.5 mg/L 73.9 ± 1.5 mg/L 6.9 ± 0.5 mg/L 0.4 mg/L.min 7.6 ± 0.09 mg/g.h
In our previous investigation on fouling suppression in MBRs, a single membrane module was used, where it was found that, by applying 200 V DEP the permeate flux can be enhanced by 118% [3]. However, the investigation was more of a fundamental research with a lab-scale experimental setup which was carried out using a single membrane module and no aeration was provided. In an industrial MBR process, aeration works both as an oxygen supply for the biomass and as a fouling reducer. In order to study the effect of DEP on an actual MBR setup, aeration was provided and two membranes were used on both sides of the active membrane which would act as barriers and provide the impact of the lumen (or channel) size. Hence, the influence of process parameters, such as voltage, aeration pulsation, aeration flux, and space between membranes on the permeate performance were investigated.
oxygen (DO) of the wastewater sample during the course of the experiments. MLSS was measured by filtering 50 mL of the sample through a 1.5 μm glass micro fiber filter. After drying the filtrate at 105 °C for 24 h, gravimetric analysis was carried out on the filtrate to determine the solid pellet fraction. By incinerating the residual at 550 °C until a constant weight is obtained, the MLFSS was determined. MLVSS was determined from the difference in weight between MLSS and MLFSS. Based on the standard dichromate digestion method the concentration of COD in the filtrate was determined. The orion AquaMate UV-VIS Spectrometer (Waltham, Massachusetts) was used for the COD measurements. The Spectrometer was calibrated using potassium hydrogen phthalate (KHP) [31]. For the COD test, a wavelength of 600 nm was 600 nm. The filtrate was again used to determine TOC the Aurora 1030 W TOC Analyzer was used to determine TOC concentration. In order to determine the oxygen uptake rate (OUR), 200 mL of the biological suspension was withdrawn from the MBR after 20 min of each voltage application. The sample was stirred continuously to avoid settling of the sludge and was tightly sealed to avoid entrance of external oxygen into the sample. After the DO reading has stabilized, the DO reading was taken for 10 min at a 1 min interval. The DO value was plotted against time. The slope of the curve represents the OUR in milligrams of Oxygen/ L.min. The specific oxygen uptake rate (SOUR) was obtained by diving the OUR by the MLVSS value.
3.1. Impact of aeration and DEP scheme Identification of an optimum aeration scheme would highly reduce the operating cost of the MBR. Fig. 3 shows the normalized permeate flux over time for 6 different running conditions. In the first operating condition no aeration and no DEP were applied to the MBR system. According to Fig. 3, when no aeration and no DEP force are applied, the normalized permeate flux throughout the experimental run was the lowest and reached 38.2% after 90 min of operation time. In the subsequent operating conditions different aeration and DEP schemes were applied. Four different schemes were studied: 5 min on – 15 min off, 1 min on – 5 min off, 1 min on – 1 min off and 5 min on – 5 min off for both aeration and DEP. It should be mentioned that both aeration and DEP were turned on and off simultaneously, with and aeration flow rate of 0.1 L/min.m2 and 50 V DEP. The residence time for each operating condition was 90 min.. As seen from Fig. 3, a 5 min on- 15 min off scheme increases the final normalized permeate flux to 60.1%. Additionally, when 1 min on 5 min off scheme is used, the normalized permeate flux improves by 5.8% and a final normalized permeate flux of 65.9% is obtained. Furthermore, the flux improves by 5.8% when scheme of 1 min on - 1 min off is used. The final normalized flux at this operating condition is 71.7%. Finally, the best normalized permeate flux is obtained when the scheme of 5 min on - 5 min off is used. At this condition, the flux improves by 14% and a final normalized permeate flux of 73.7% is obtained. Since, 5 min on - 5 min off scheme gave the best result, an alternate setup, where DEP and aeration were not used simultaneously, was studied. In this scheme (marked as alt in Fig. 3), DEP was on for 5 min and no aeration was provided during this period. When DEP was turned off for 5 min., aeration was turned on (for 5 min.). After 90 min. of operation, this scheme resulted in a normalized permeate flux of 74.9%. Hence, both schemes showed very similar performance.
2.3. Normalized permeate flux calculation The membrane permeate flux is presented as the normalized permeate flux. In order to obtain the normalized permeate flux, the membrane was washed with distilled water after each experimental run then pure water permeate flux was measured until equilibrium was reached (V˙P . N ). The permeate flux of the original run in the MBR system (V˙P.0 ) was divided by V˙P . N to obtain the normalized permeate flux (ΦN ):
ΦN =
V˙P .0 V˙P . N
(3) 3
Chemical Engineering & Processing: Process Intensification 142 (2019) 107565
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Fig. 3. Comparison of normalized permeate flux in different processes with varied process parameters, such as aeration pulsation, DEP pulsation, and aeration flux.
Observing the trend of the normalized permeate flux and aeration & DEP schemes, it can be noticed that; schemes with longer activated aeration and DEP application give higher normalized permeate flux. Moreover, the inactivity of aeration and DEP has a negative impact on the normalized permeate flux. The 5 min on - 15 min off scheme gives the lowest normalized permeate flux. Whereas, the scheme with 5 min on-5 min off gives the highest normalized permeate flux. This is because, when aeration and DEP are active, anti-fouling/cleaning mechanism of the MBR is active [14]. Keeping DEP and aeration on for longer period, makes the membrane surface cleaner by removing more foulant material from the membrane surface. When these forces are inactive, particles would accumulate on the surface of the membrane [14]. Hence, a long aeration and DEP inactivity period (e.g. 5 min on15 min off) results in lower normalized permeate flux. Since, a 5 min on-5 min off scheme gives best normalized permeate flux throughout the 90 min run, for investigating the impact of DEP, aeration volume and lumen size, this scheme would be used.
voltages were applied. An aeration rate of 0.1 LPM and 0.3 LPM were studied at different DEP voltages of 50 V and 100 V. All tests were done with 5 min on – 5 min off aeration and DEP simultaneous application. As seen in Fig. 4, when an aeration rate of 0.1 L/min.m2 is provided, the normalized permeate flux improves and reaches to 45% after 90 min of operation. The normalized permeate flux can be further improved by applying 50 V DEP voltage along with aeration. After 90 min. of operation, the normalized permeate flux increased to 55%. The influence of space between membranes on permeate flux was examined by decreasing the lumen size from 145 mm to 6 mm. The reduced space between membranes further enhanced the normalized permeate flux to 64% at the end of the run. Thus, by applying aeration and DEP voltage, and by reducing lumen size, the permeate flux can be enhanced by 190% (from 22% to 64%). The normalized permeate flux was further enhanced by increasing aeration rate and DEP voltage. For, 0.1 L/ min.m2 aeration rate and 6 mm lumen size, application of 100 V DEP will result in permeate flux of 75%. Keeping the DEP voltage at 50 V and lumen size at 6 mm, increasing the aeration rate to 0.3 L/min.m2 increases the normalized permeate flux to 76%. With a lumen size, aeration rate and DEP voltage of 6 mm, 0.3 L/min.m2 and 100 V, respectively, the normalized permeate flux after 90 min of operation was found to be 80%. With the pulsation scheme presented in section 3.1, normalized permeate flux has been enhanced by three factors, as seen in Fig. 4. Firstly, by applying 0.1 L/min.m2 aeration rate, the permeate flux was enhanced by 104% (22% to 45%). Moreover, increasing the aeration rate to 0.3 L/min.m2 enhances the normalized permeate flux by 8.6%
3.2. Impact of aeration rate, DEP voltage and lumen size The performance of the selected aeration scheme was studied with variable aeration rates and different DEP voltages. Moreover, the effect of reduced lumen size was studied, where multiple membrane modules were installed in the MBR system with a lumen size of 6 mm from each side. Fig. 4 shows that without the application of DEP and aeration the normalized permeate flux was 22% after 90 min of operation. In the subsequent operating conditions different aeration rates and DEP
Fig. 4. Dependence of normalized permeate flux on pulsed aeration rate (0.1 L/min.m² and 0.3 L/min.m²), voltage (50 V and 100 V) and space between membranes (6 mm and 145 mm).
4
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Regarding the DEP impact, the DEP force is being provided by electrodes embedded under the full area of the membrane. Thus, it is acting uniformly along the surface of the membrane as seen in Fig. 5 (c). As seen from the profile of the fouling and anti-fouling factors of the proposed MBR, the fouling of the membrane will not be uniform. At the top, the suction force is inducing highest fouling rate and the aeration is inducing lowest anti-fouling rate. So, the permeation flux will drop rapidly at the top section of the membrane. A reverse phenomenon has been observed at the lower part of the membrane because the lower part has lower permeation rate and higher turbulence. The anti-fouling effect of DEP force is acting uniformly along the axis of the membrane. It is apparent that applying DEP voltage can enhance anti-fouling properties of the MBR. However, applying higher DEP force would increase the required energy. Therefore, the process has to be also optimized according to energy consumption.
(70% to 76%). When aeration is applied, turbulence is created in front of the membrane surface [32]. This results in scouring of biomass in the membrane vicinity. At higher aeration rate, the turbulence created is higher. Thus, better flux is sustained at 0.3 L/min.m2 aeration rate. Moreover, higher aeration rate increased the Specific Aeration Demand (SAD) and the Specific Aeration Rate (Ug ) as predicted by Eq.s (1) and (2). Higher SAD and Ug helps to sustain flux for longer and reduces fouling [14]. Secondly, by applying DEP voltage of 50 V, the normalized permeate flux can be enhanced by 22% (45% to 55%). When a DEP voltage of 100 V is applied, the normalized permeate flux increases by 5% (76% to 80%). This is because the fouling materials are repelled from the membrane surface due to the presence of higher DEP force [11]. The quadratic DEP force is proportional to the applied voltage [3,11]. At higher voltage, the DEP force is higher, thus resulting in higher permeate flux. Finally, the normalized permeate flux enhanced by 27% (60% to 75%) when the lumen size was reduced to 6 mm. Reducing the lumen size to 6 mm would increase the shear stress through the channel, which would result in less accumulation of bioparticles on the membrane surface and hence, higher permeate flux [33]. The application of 100 V DEP force is not harmful for the biomass. According to Larbi et al. the main reason for the MLSS concentration reduction in a DEP-MBR system is the increase of temperature abve 40 °C [30]. In this study, a bigger reactor was used, aeration was supplied to the reactor and pulsed DEP was applied, hence, the temperature of the reactor was maintained around room temperature (i.e. 25 °C). Thus, with applying 100 V DEP the concentration of MLSS could be sustained. The main factor that enhances fouling is the suction force present at the top of the membrane module. Therefore, the pressure gradient at the top is higher compared to the lower section of the membrane. This creates a non-uniform flow inside the membrane as shown in Fig. 5 (a). The top section will have higher flow rate as it is directly affected by the suction force. The middle section will have lower flowrate because the flowrate in this section is induced by the pressure drop at the top section. The lower section will have the lowest flowrate because the flow at this section is induced by pressure drop in the middle section. Because of higher flowrate at the top section, more water will cross the membrane boundary at this section and cause more fouling. On the other hand, lower flowrate at the bottom will result in lower water permeation through this section and result in reduced fouling. The antifouling mechanism for the proposed set up comes from two factors; aeration and DEP force. The aeration is being provided by an aerator from the bottom of the MBR. This set up will create more air bubbles at the bottom of the tank compared to the top of the tank as shown in Fig. 5 (b). As a result, the turbulence created by the air bubbles will be more near the bottom of the MBR compared to the top. This will result in better anti-fouling performance at the bottom of the membrane compared to the top. The turbulence profile can be seen in Fig. 5 (b).
3.3. Energy analysis The energy requirement of the fouling suppression system mainly depends on electric parameters (Voltage and frequency), hydrodynamic parameters (aeration rate) and application period. As presented in Fig. 4, the increase of voltage and/or flow rate of aeration results in enhanced normalized permeate flux. Interestingly, normalized permeate fluxes in the process with an aeration rate of 0.3 L/min.m2 and 50 V voltage application are very similar to those in the process with applied voltage of 100 V but lower aeration flow rate of 0.1 L/ min.m2. This due to different magnitude of applied force from each force. To clearly present the performance of the different antifouling forces in the process, the intensification factors were calculated and shown in Fig. 6. The intensification factor in the process with 0.3 L/ min.m2 and 50 V is about 1.2 time higher than that of the process with lower aeration flow rate (0.1 L/min.m2) but doubled applied voltage (100 V). If we assume that the energy consumption in the process with 0.1 L/min.m2 aeration and 50 V applied voltage is considered as E, and the energy consumed by aeration is linearly dependent on the air volume flux. Then the energy consumption in the process with 0.3 L/ min.m2 aeration and 100 V is 12E, due to the quadratic dependence of electric energy on voltage. On the other hand, the process with parameters of 0.1 L/min.m2 and 100 V consumes energy 4E. While the energy consumption in the process with 0.3 L/min.m2 aeration and 50 V voltage is 3E. Therefore, depending on energy consumption the process with 0.3 L/min.m2 aeration and 50 V voltage has much higher intensification factor. In addition, for a certain amount of biological particles in the MBR, the feed of oxygen by aeration is constant. It means that the energy consumption by aeration is fixed and independent of the numbers of membrane modules. However, the electrical energy induced by electrodes on membrane modules is dependent on the membrane modules number and voltage applied when the frequency is constant. Therefore,
Fig. 5. Profile for (a) suction force, (b) shear force and (c) DEP force along the axis of the membrane. 5
Chemical Engineering & Processing: Process Intensification 142 (2019) 107565
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Fig. 6. Intensification factors of different processes with varied applied voltage and aeration flow rate. Table 2 Intensification factor of various published electrically enhanced membrane bio reactors. Enhancement method
Type of water
Intensification factor
Applied parameter
Source
Electrophoretic force Fe electrode Fe anode Ti anode Electrically conductive membrane
Wastewater Wastewater Domestic wastewater Domestic wastewater Synthetic wastewater
1.1 2 1.4 1.3 2.5
20 V/cm2 1V/cm2 2.72 V/cm2 7.79 V/cm2 10 mA
Chen et al. (2007) [34] Khaled et al. (2011) [35] Zhang et al. (2015) [36] Zhang et al. (2015) [36] Liu et al. (2012) [37]
089-2-044). The statements made herein are solely the responsibility of the authors. The authors would also like to acknowledge the help provided by Qatar Works Authority (Ashghal) for providing the wastewater and sludge samples.
with more membrane modules employed in the MBR, the energy consumption will be linearly increased depending on the number of membrane modules. Hence for optimally minimizing the energy demand of the MBR system, it is crucial to control or keep the applied voltage to minimum, 50 V in this case. Table 2 shows results from several studies that were able to improve the flux of MBR by applying electric field. The highest possible intensification factor was 2.5 using electrically conductive membranes by applying 10 mA current. In this study, the optimum operating condition of 50 V DEP and 0.3 L/min.m2 showed an intensification factor of 4. Thus, the results of this study show an improvement of the electrically assisted MBR process.
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4. Conclusion In this study, aeration and DEP schemes have been tasted in order to reduce the operating cost of the MBR operation. Experimentation showed that a 5 min on – 5 min off scheme gives the best normalized permeate flux after 90 min of operation. The scheme was applied to test the fouling suppression performance of DEP in aerated condition along with reduced lumen (or channel) size. The flux improved by 104%, 22% and 27% when aeration rate of 0.1 L/min.m2, DEP voltage of 50 V and lumen size of 6 mm was applied, respectively. At constant DEP voltage of 50 V, the permeate flux increased by 8.6% when aeration rate was increased from 0.1 L/min.m2 to 0.3 L/min.m2. At fixed aeration rate of 0.3 L/min.m2, the permeate flux increases by 5% when the DEP voltage is increase to 100 V from 50 V. Overall, it was possible to improve the flux by 58% by applying DEP voltage of 100 V, aeration rate of 0.3 L/ min.m2 and reduced lumen size of 6 mm. However, through energy analysis, the optimum DEP voltage and aeration rate was found to be 50 V and 0.3 L/min.m2 respectively and compared to conventional MBR, the permeate flux at this condition was found to be 54% higher. Thus, it can be confirmed that fouling can be reduced effectively by applying the developed aeration and DEP scheme. Acknowledgement This research project is made possible by NPRP award (NPRP7-0892-044) from Qatar National Research Fund (QNRF) (grant ID NPRP76
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7
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Impact of aeration rate and dielectrophoretic force on fouling suppression in submerged membrane bioreactors
T
⁎
A.H. Hawaria, , B. Larbia, A. Alkhatiba, Ahmed T. Yasira, F. Dub, M. Bauneb, J. Thömingb a b
Department of Civil and Architectural Engineering, College of Engineering, Qatar University, 2713 Doha, Qatar Center for Environmental Research and Sustainable Technology, University of Bremen, Leobener Str. 6, D 28359 Bremen, Germany
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane bioreactor Dielectrophoresis Activated sludge Aeration Fouling
Membrane bioreactors (MBRs) present a unique opportunity in waste water treatment by combining the activated sludge process with membrane filtration. Although MBRs require less area, produce less volume of sludge and have high purification efficiency, they suffer from high membrane fouling. Membrane fouling can be reduced by applying aeration and Dep voltage. However, application of these two anti-fouling mechanisms would make the process expensive. Thus, a new aeration and DEP scheme of 5 min on – 5 min off is being suggested in this study. With this scheme, it was found that through application of DEP at 100 V and 0.3 L/min.m2 aeration flowrate and reduced lumen size of 6 mm, the membrane permeate flux can be increased by 58% compared to MBR without DEP and aeration. However, through energy analysis the optimum operating parameters were found to be at 0.3 L/min.m2 aeration flowrate and 50 V DEP using the suggested 5 min on-5 min off scheme. At the optimum operating condition, the permeability flux is 54% higher than MBR without DEP and aeration.
1. Introduction Membrane bioreactors (MBRs) combine the activated sludge process with micro/ultra filtration. This results in an enhanced treatment efficiency compared to a conventional aeration process [1]. The organic pollutants are degraded in the bio-reactor and the membrane filters the waste water and produces water free of suspended solid, bacteria and viruses [2]. Compared to the conventional activated sludge treatment process, MBRs are smaller in size, produces less sludge volume, have high COD removal efficiency and can handle high concentration of suspended solids [3]. However, MBRs suffer from high propensity to membrane fouling [3]. Fouling can be reduced in MBRs through aeration scouring and backwashing, chemical cleaning, electrically assisted fouling mitigation and ultrasound application [4–7]. These methods have their own drawbacks owing to membrane damage, secondary contamination and high energy demand [8,9]. However, it is possible to overcome these limitations using electrically assisted fouling mitigation technologies; such as, electrophoresis (EP) and dielectrophoresis (DEP) [8]. However, the DC current used in electrophoresis induces electrochemical reaction at the membrane surface and causes high energy consumption [10,11]. This limitation can be solved by dielectrophoresis [10,11]. In dielectrophoresis (DEP), the particles move away from membrane due to lower dielectric polarization compared to that of medium in ⁎
inhomogeneous electric field. When a strong electric field is applied, the particles will be repelled from the membrane surface as they have lower permittivity compared to water [11,12]. In order to prevent electro chemical reactions on membrane surface, AC current and insulated electrodes have been used successfully [11]. Moreover, by using cylindrical interdigitated electrodes (IDE), the membrane lifetime was increases by 9 times [12]. Due to quadratic dependence of DEP velocity on voltage, the fouling suppressant performance of DEP depends on the applied voltage [13]. Along with DEP, the flux through the membrane can be sustained through increasing the shear force on the surface of the membrane. The shear force can be enhanced by increasing the aeration rate inside the MBR [14,15]. Therefore, apart from providing oxygen for the biomass, aeration can be used to reduce fouling. Although the increase of aeration velocity positively impacts fouling suppression, the degree of such a positive influence was substantially reduced as MLSS increases due to the increased viscosity by the MLSS (Iorhemen et al., 2016). Ueda et al. concluded that aeration helps to remove cake from the membrane surface through creating turbulence in front of the membrane [16]. Gui et al showed that, aeration is more effective for sustaining flux at when the concentration of SS is higher in the waste water [17]. Martinelli et al. found low aeration rate to be responsible for accelerated fouling [18]. Although flux can be sustained through increasing aeration rate, Ueda et al. found that after reaching the critical aeration rate, cake
Corresponding author. E-mail address: [email protected] (A.H. Hawari).
https://doi.org/10.1016/j.cep.2019.107565 Received 17 April 2019; Received in revised form 13 June 2019; Accepted 19 June 2019 Available online 28 June 2019 0255-2701/ © 2019 Elsevier B.V. All rights reserved.
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filters used chlorinated polyethylene (CP-E) membranes with an average pore size of 0.2 μm and a maximum pore size of 0.4 μm. The interdigitated energized electrodes diameter was 2 mm and the distance between the powered electrodes was 10 mm and was isolated by a thin film of TiO2 to refrain from corrosion on their surface and lower the high pass filter effect influence. Between energized electrodes, acrylonitrile styrene acrylate (ASA) with square section 2 mm on all sides was mounted. Two 6 mm plastic spacers were used to sustain the active membrane and two barriers on each side of the module as seen in Fig. 1(b). With the experimental setup depicted in Fig. 2, the MBR system consisted of a bioparticle suspension in a 4-L bioreactor in which the DEP membrane module was immersed. An AC voltage generator by Variac® was connected to the module to generate a voltage of 50 V and a frequency of 50 Hz. The current and voltage were measured and monitored during the experiment using an oscilloscope (MDO 3024, Tektronix). The oscilloscope was calibrated using a standard resistor and capacitor. The bioparticles were acquired from a local wastewater treatment plant (Doha South Wastewater Treatment Plant in Qatar). A suction pump (Gear Pump Drive, Model 75211-15, Cole Parmar Instrument) was used to draw the permeate. A pressure transducer (PT) was utilized for measuring the pressure. To ensure that the membrane remained submerged in solution and to maintain a constant concentration, the water level was observed and controlled inside the MBR system. The permeate flux was obtained by measuring the weight over time. The permeate flux was measured for three different settings: without DEP and barrier, with DEP at 50 V & 100 V and aeration flow rate of 0.1 L/ min.m2 & 0.3 L/min.m2 without barrier, and with DEP at 50 V & 100 V and aeration flow rate of 0.1 L/min.m2 & 0.3 L/min.m2 with barrier. The pulsed aeration was supplied at an intermittent time of 5 min on and 5 min off. To reduce the power consumption of the DEP system, a pulsed DEP with 10 s with electric field and 15 s without electric field was applied. A diffuser was used for aeration inside the MBR. The aeration rate was chosen to maintain a minimum DO concentration of 5 mg/L. According to previous studies the DO concentration in aerated MBRs should be maintained between 3–6.5 mg/L. It was found that this DO concentration would increase the lifespan of the membrane [28,29]. Moreover, it was found that a DO concentration of 5 mg/L was sufficient to sustain the required MLSS concentration and microbial activity in the MBR system [30]. After each process, the membrane was backwashed by pumping pure water from the membrane substrate side (from inside the module to the outside).
removal percentage cannot be improved [16]. The aeration performance in MBR is quantified by superficial gas velocity (Ug ) , specific aeration demand in membrane area (SADm) or specific aeration demand in permeate product SADP [14]. Aeration intensity or superficial gas velocity (Ug ) can only be used for submerged MBRs and it is calculated as [14]:
Ug =
Qg (1)
Sr
Here, Qg and Sr are the flowrate of air and cross-sectional area of the membrane module, respectively. SADp is primarily used in the industry as it can be used directly to estimate the energy performance of the MBR and it is expressed as [19]:
SADP =
Qg Qp
(2)
Here, QP is the permeate flowrate. As seen from Eq.s (1) and (2), Ug and SADp depends mainly on the air flow rate. Chua et al. concluded that membrane fouling increases exponentially when Ug decreases [20]. Germain et al. found that in order to maintain the flux above 16.5 Lm−2 h-1, high aeration rate is required [21]. The size of lumen (or channel) through which the air flows also impacts the fouling rate [22]. Ozaki et al. found that, the lag phase decreased from 180 min to 30 min when the channel width was reduced from 15 mm to 5 mm [23]. This is because the flow becomes turbulent when the flowrate increases [24]. Although aeration helps to reduce fouling in MBR, the aeration consumes more than 50% of the net power demand and turns out to be between 45 and 75% of the operational cost in [25]. However, the usage of oxygen can be reduced by pulsed aeration. In pulsed aeration, the aeration frequency is higher than general intermittent aeration [26]. Pulse aeration provides better mixture and maintains sufficient DO inside the MBR [27]. Pulsed aeration can reduce the energy demand of MBR by 29.5%–48.5% [26]. This paper describes the design of a bench-scale pulsed DEP and pulsed aeration assisted MBR system with a lumen size of 6 mm. An optimum aeration scheme for operating cost reduction has been presented. Moreover, using the optimum aeration scheme, the impact of aeration volume and DEP on the permeate flux in MBR was established. The optimum aeration volume and DEP voltage was determined by energy analysis. 2. Materials and method 2.1. Experimental setup The membrane module (Fig. 1) was fabricated according to the method we used before [3]. The DEP membrane modules used had a filtration area of 90 cm2 (10 cm length and 9 cm width) and incorporated two acrylonitrile butadiene styrene (ABS) frames with a length of 16 cm, width of 14 cm, and thickness of 0.5 cm. The medium
2.2. Waste water characterization The characteristics of the wastewater used in this study are summarized Table 1. A WTW Multi3430 (Weilheim, Germany) multi-meter was used to continuously monitor the pH, temperature and dissolved
Fig. 1. (a) Dielectrophoretic (DEP) membrane module with square acrylonitrile styrene acrylate (ASA) obstacles installed in between interdigitatedly arrayed excited (IDE) energized electrodes (revised from the figure in Du et. al. 2018), (b) Membrane module and membrane barriers. 2
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Fig. 2. Experimental setup of the MBR process a) schematic diagram, b) actual setup.
3. Results and discussion
Table 1 Initial activated sludge characteristics. Parameter
Value
pH Initial temperature Mix Liquor Suspended Solid (MLSS) Mix Liquor Volatile Suspended Solid MLVSS Total Organic Carbon (TOC) Chemical Oxygen Demand (COD) Dissolved Oxygen (DO) Oxygen Uptake Rate (OUR) Specific Oxygen Uptake rate (SOUR)
6.9 ± 0.1 @ 21.9 °C 25 ± 2 °C 3,550 ± 410.1 mg/L 31.5 ± 2.6 mg/L 29.2 ± 0.5 mg/L 73.9 ± 1.5 mg/L 6.9 ± 0.5 mg/L 0.4 mg/L.min 7.6 ± 0.09 mg/g.h
In our previous investigation on fouling suppression in MBRs, a single membrane module was used, where it was found that, by applying 200 V DEP the permeate flux can be enhanced by 118% [3]. However, the investigation was more of a fundamental research with a lab-scale experimental setup which was carried out using a single membrane module and no aeration was provided. In an industrial MBR process, aeration works both as an oxygen supply for the biomass and as a fouling reducer. In order to study the effect of DEP on an actual MBR setup, aeration was provided and two membranes were used on both sides of the active membrane which would act as barriers and provide the impact of the lumen (or channel) size. Hence, the influence of process parameters, such as voltage, aeration pulsation, aeration flux, and space between membranes on the permeate performance were investigated.
oxygen (DO) of the wastewater sample during the course of the experiments. MLSS was measured by filtering 50 mL of the sample through a 1.5 μm glass micro fiber filter. After drying the filtrate at 105 °C for 24 h, gravimetric analysis was carried out on the filtrate to determine the solid pellet fraction. By incinerating the residual at 550 °C until a constant weight is obtained, the MLFSS was determined. MLVSS was determined from the difference in weight between MLSS and MLFSS. Based on the standard dichromate digestion method the concentration of COD in the filtrate was determined. The orion AquaMate UV-VIS Spectrometer (Waltham, Massachusetts) was used for the COD measurements. The Spectrometer was calibrated using potassium hydrogen phthalate (KHP) [31]. For the COD test, a wavelength of 600 nm was 600 nm. The filtrate was again used to determine TOC the Aurora 1030 W TOC Analyzer was used to determine TOC concentration. In order to determine the oxygen uptake rate (OUR), 200 mL of the biological suspension was withdrawn from the MBR after 20 min of each voltage application. The sample was stirred continuously to avoid settling of the sludge and was tightly sealed to avoid entrance of external oxygen into the sample. After the DO reading has stabilized, the DO reading was taken for 10 min at a 1 min interval. The DO value was plotted against time. The slope of the curve represents the OUR in milligrams of Oxygen/ L.min. The specific oxygen uptake rate (SOUR) was obtained by diving the OUR by the MLVSS value.
3.1. Impact of aeration and DEP scheme Identification of an optimum aeration scheme would highly reduce the operating cost of the MBR. Fig. 3 shows the normalized permeate flux over time for 6 different running conditions. In the first operating condition no aeration and no DEP were applied to the MBR system. According to Fig. 3, when no aeration and no DEP force are applied, the normalized permeate flux throughout the experimental run was the lowest and reached 38.2% after 90 min of operation time. In the subsequent operating conditions different aeration and DEP schemes were applied. Four different schemes were studied: 5 min on – 15 min off, 1 min on – 5 min off, 1 min on – 1 min off and 5 min on – 5 min off for both aeration and DEP. It should be mentioned that both aeration and DEP were turned on and off simultaneously, with and aeration flow rate of 0.1 L/min.m2 and 50 V DEP. The residence time for each operating condition was 90 min.. As seen from Fig. 3, a 5 min on- 15 min off scheme increases the final normalized permeate flux to 60.1%. Additionally, when 1 min on 5 min off scheme is used, the normalized permeate flux improves by 5.8% and a final normalized permeate flux of 65.9% is obtained. Furthermore, the flux improves by 5.8% when scheme of 1 min on - 1 min off is used. The final normalized flux at this operating condition is 71.7%. Finally, the best normalized permeate flux is obtained when the scheme of 5 min on - 5 min off is used. At this condition, the flux improves by 14% and a final normalized permeate flux of 73.7% is obtained. Since, 5 min on - 5 min off scheme gave the best result, an alternate setup, where DEP and aeration were not used simultaneously, was studied. In this scheme (marked as alt in Fig. 3), DEP was on for 5 min and no aeration was provided during this period. When DEP was turned off for 5 min., aeration was turned on (for 5 min.). After 90 min. of operation, this scheme resulted in a normalized permeate flux of 74.9%. Hence, both schemes showed very similar performance.
2.3. Normalized permeate flux calculation The membrane permeate flux is presented as the normalized permeate flux. In order to obtain the normalized permeate flux, the membrane was washed with distilled water after each experimental run then pure water permeate flux was measured until equilibrium was reached (V˙P . N ). The permeate flux of the original run in the MBR system (V˙P.0 ) was divided by V˙P . N to obtain the normalized permeate flux (ΦN ):
ΦN =
V˙P .0 V˙P . N
(3) 3
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Fig. 3. Comparison of normalized permeate flux in different processes with varied process parameters, such as aeration pulsation, DEP pulsation, and aeration flux.
Observing the trend of the normalized permeate flux and aeration & DEP schemes, it can be noticed that; schemes with longer activated aeration and DEP application give higher normalized permeate flux. Moreover, the inactivity of aeration and DEP has a negative impact on the normalized permeate flux. The 5 min on - 15 min off scheme gives the lowest normalized permeate flux. Whereas, the scheme with 5 min on-5 min off gives the highest normalized permeate flux. This is because, when aeration and DEP are active, anti-fouling/cleaning mechanism of the MBR is active [14]. Keeping DEP and aeration on for longer period, makes the membrane surface cleaner by removing more foulant material from the membrane surface. When these forces are inactive, particles would accumulate on the surface of the membrane [14]. Hence, a long aeration and DEP inactivity period (e.g. 5 min on15 min off) results in lower normalized permeate flux. Since, a 5 min on-5 min off scheme gives best normalized permeate flux throughout the 90 min run, for investigating the impact of DEP, aeration volume and lumen size, this scheme would be used.
voltages were applied. An aeration rate of 0.1 LPM and 0.3 LPM were studied at different DEP voltages of 50 V and 100 V. All tests were done with 5 min on – 5 min off aeration and DEP simultaneous application. As seen in Fig. 4, when an aeration rate of 0.1 L/min.m2 is provided, the normalized permeate flux improves and reaches to 45% after 90 min of operation. The normalized permeate flux can be further improved by applying 50 V DEP voltage along with aeration. After 90 min. of operation, the normalized permeate flux increased to 55%. The influence of space between membranes on permeate flux was examined by decreasing the lumen size from 145 mm to 6 mm. The reduced space between membranes further enhanced the normalized permeate flux to 64% at the end of the run. Thus, by applying aeration and DEP voltage, and by reducing lumen size, the permeate flux can be enhanced by 190% (from 22% to 64%). The normalized permeate flux was further enhanced by increasing aeration rate and DEP voltage. For, 0.1 L/ min.m2 aeration rate and 6 mm lumen size, application of 100 V DEP will result in permeate flux of 75%. Keeping the DEP voltage at 50 V and lumen size at 6 mm, increasing the aeration rate to 0.3 L/min.m2 increases the normalized permeate flux to 76%. With a lumen size, aeration rate and DEP voltage of 6 mm, 0.3 L/min.m2 and 100 V, respectively, the normalized permeate flux after 90 min of operation was found to be 80%. With the pulsation scheme presented in section 3.1, normalized permeate flux has been enhanced by three factors, as seen in Fig. 4. Firstly, by applying 0.1 L/min.m2 aeration rate, the permeate flux was enhanced by 104% (22% to 45%). Moreover, increasing the aeration rate to 0.3 L/min.m2 enhances the normalized permeate flux by 8.6%
3.2. Impact of aeration rate, DEP voltage and lumen size The performance of the selected aeration scheme was studied with variable aeration rates and different DEP voltages. Moreover, the effect of reduced lumen size was studied, where multiple membrane modules were installed in the MBR system with a lumen size of 6 mm from each side. Fig. 4 shows that without the application of DEP and aeration the normalized permeate flux was 22% after 90 min of operation. In the subsequent operating conditions different aeration rates and DEP
Fig. 4. Dependence of normalized permeate flux on pulsed aeration rate (0.1 L/min.m² and 0.3 L/min.m²), voltage (50 V and 100 V) and space between membranes (6 mm and 145 mm).
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Regarding the DEP impact, the DEP force is being provided by electrodes embedded under the full area of the membrane. Thus, it is acting uniformly along the surface of the membrane as seen in Fig. 5 (c). As seen from the profile of the fouling and anti-fouling factors of the proposed MBR, the fouling of the membrane will not be uniform. At the top, the suction force is inducing highest fouling rate and the aeration is inducing lowest anti-fouling rate. So, the permeation flux will drop rapidly at the top section of the membrane. A reverse phenomenon has been observed at the lower part of the membrane because the lower part has lower permeation rate and higher turbulence. The anti-fouling effect of DEP force is acting uniformly along the axis of the membrane. It is apparent that applying DEP voltage can enhance anti-fouling properties of the MBR. However, applying higher DEP force would increase the required energy. Therefore, the process has to be also optimized according to energy consumption.
(70% to 76%). When aeration is applied, turbulence is created in front of the membrane surface [32]. This results in scouring of biomass in the membrane vicinity. At higher aeration rate, the turbulence created is higher. Thus, better flux is sustained at 0.3 L/min.m2 aeration rate. Moreover, higher aeration rate increased the Specific Aeration Demand (SAD) and the Specific Aeration Rate (Ug ) as predicted by Eq.s (1) and (2). Higher SAD and Ug helps to sustain flux for longer and reduces fouling [14]. Secondly, by applying DEP voltage of 50 V, the normalized permeate flux can be enhanced by 22% (45% to 55%). When a DEP voltage of 100 V is applied, the normalized permeate flux increases by 5% (76% to 80%). This is because the fouling materials are repelled from the membrane surface due to the presence of higher DEP force [11]. The quadratic DEP force is proportional to the applied voltage [3,11]. At higher voltage, the DEP force is higher, thus resulting in higher permeate flux. Finally, the normalized permeate flux enhanced by 27% (60% to 75%) when the lumen size was reduced to 6 mm. Reducing the lumen size to 6 mm would increase the shear stress through the channel, which would result in less accumulation of bioparticles on the membrane surface and hence, higher permeate flux [33]. The application of 100 V DEP force is not harmful for the biomass. According to Larbi et al. the main reason for the MLSS concentration reduction in a DEP-MBR system is the increase of temperature abve 40 °C [30]. In this study, a bigger reactor was used, aeration was supplied to the reactor and pulsed DEP was applied, hence, the temperature of the reactor was maintained around room temperature (i.e. 25 °C). Thus, with applying 100 V DEP the concentration of MLSS could be sustained. The main factor that enhances fouling is the suction force present at the top of the membrane module. Therefore, the pressure gradient at the top is higher compared to the lower section of the membrane. This creates a non-uniform flow inside the membrane as shown in Fig. 5 (a). The top section will have higher flow rate as it is directly affected by the suction force. The middle section will have lower flowrate because the flowrate in this section is induced by the pressure drop at the top section. The lower section will have the lowest flowrate because the flow at this section is induced by pressure drop in the middle section. Because of higher flowrate at the top section, more water will cross the membrane boundary at this section and cause more fouling. On the other hand, lower flowrate at the bottom will result in lower water permeation through this section and result in reduced fouling. The antifouling mechanism for the proposed set up comes from two factors; aeration and DEP force. The aeration is being provided by an aerator from the bottom of the MBR. This set up will create more air bubbles at the bottom of the tank compared to the top of the tank as shown in Fig. 5 (b). As a result, the turbulence created by the air bubbles will be more near the bottom of the MBR compared to the top. This will result in better anti-fouling performance at the bottom of the membrane compared to the top. The turbulence profile can be seen in Fig. 5 (b).
3.3. Energy analysis The energy requirement of the fouling suppression system mainly depends on electric parameters (Voltage and frequency), hydrodynamic parameters (aeration rate) and application period. As presented in Fig. 4, the increase of voltage and/or flow rate of aeration results in enhanced normalized permeate flux. Interestingly, normalized permeate fluxes in the process with an aeration rate of 0.3 L/min.m2 and 50 V voltage application are very similar to those in the process with applied voltage of 100 V but lower aeration flow rate of 0.1 L/ min.m2. This due to different magnitude of applied force from each force. To clearly present the performance of the different antifouling forces in the process, the intensification factors were calculated and shown in Fig. 6. The intensification factor in the process with 0.3 L/ min.m2 and 50 V is about 1.2 time higher than that of the process with lower aeration flow rate (0.1 L/min.m2) but doubled applied voltage (100 V). If we assume that the energy consumption in the process with 0.1 L/min.m2 aeration and 50 V applied voltage is considered as E, and the energy consumed by aeration is linearly dependent on the air volume flux. Then the energy consumption in the process with 0.3 L/ min.m2 aeration and 100 V is 12E, due to the quadratic dependence of electric energy on voltage. On the other hand, the process with parameters of 0.1 L/min.m2 and 100 V consumes energy 4E. While the energy consumption in the process with 0.3 L/min.m2 aeration and 50 V voltage is 3E. Therefore, depending on energy consumption the process with 0.3 L/min.m2 aeration and 50 V voltage has much higher intensification factor. In addition, for a certain amount of biological particles in the MBR, the feed of oxygen by aeration is constant. It means that the energy consumption by aeration is fixed and independent of the numbers of membrane modules. However, the electrical energy induced by electrodes on membrane modules is dependent on the membrane modules number and voltage applied when the frequency is constant. Therefore,
Fig. 5. Profile for (a) suction force, (b) shear force and (c) DEP force along the axis of the membrane. 5
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Fig. 6. Intensification factors of different processes with varied applied voltage and aeration flow rate. Table 2 Intensification factor of various published electrically enhanced membrane bio reactors. Enhancement method
Type of water
Intensification factor
Applied parameter
Source
Electrophoretic force Fe electrode Fe anode Ti anode Electrically conductive membrane
Wastewater Wastewater Domestic wastewater Domestic wastewater Synthetic wastewater
1.1 2 1.4 1.3 2.5
20 V/cm2 1V/cm2 2.72 V/cm2 7.79 V/cm2 10 mA
Chen et al. (2007) [34] Khaled et al. (2011) [35] Zhang et al. (2015) [36] Zhang et al. (2015) [36] Liu et al. (2012) [37]
089-2-044). The statements made herein are solely the responsibility of the authors. The authors would also like to acknowledge the help provided by Qatar Works Authority (Ashghal) for providing the wastewater and sludge samples.
with more membrane modules employed in the MBR, the energy consumption will be linearly increased depending on the number of membrane modules. Hence for optimally minimizing the energy demand of the MBR system, it is crucial to control or keep the applied voltage to minimum, 50 V in this case. Table 2 shows results from several studies that were able to improve the flux of MBR by applying electric field. The highest possible intensification factor was 2.5 using electrically conductive membranes by applying 10 mA current. In this study, the optimum operating condition of 50 V DEP and 0.3 L/min.m2 showed an intensification factor of 4. Thus, the results of this study show an improvement of the electrically assisted MBR process.
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4. Conclusion In this study, aeration and DEP schemes have been tasted in order to reduce the operating cost of the MBR operation. Experimentation showed that a 5 min on – 5 min off scheme gives the best normalized permeate flux after 90 min of operation. The scheme was applied to test the fouling suppression performance of DEP in aerated condition along with reduced lumen (or channel) size. The flux improved by 104%, 22% and 27% when aeration rate of 0.1 L/min.m2, DEP voltage of 50 V and lumen size of 6 mm was applied, respectively. At constant DEP voltage of 50 V, the permeate flux increased by 8.6% when aeration rate was increased from 0.1 L/min.m2 to 0.3 L/min.m2. At fixed aeration rate of 0.3 L/min.m2, the permeate flux increases by 5% when the DEP voltage is increase to 100 V from 50 V. Overall, it was possible to improve the flux by 58% by applying DEP voltage of 100 V, aeration rate of 0.3 L/ min.m2 and reduced lumen size of 6 mm. However, through energy analysis, the optimum DEP voltage and aeration rate was found to be 50 V and 0.3 L/min.m2 respectively and compared to conventional MBR, the permeate flux at this condition was found to be 54% higher. Thus, it can be confirmed that fouling can be reduced effectively by applying the developed aeration and DEP scheme. Acknowledgement This research project is made possible by NPRP award (NPRP7-0892-044) from Qatar National Research Fund (QNRF) (grant ID NPRP76
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