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Soluble metal ions migration and distribution in sludge electro-dewatering
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Soluble metal ions migration and distribution in sludge electro-dewatering Hang Lva, Siqi Xinga, Daoguang Liub,c, Fang Wanga, Wenbiao Zhangb, Gangfan Sunb, Xu Wua,∗ a b c
School of Environmental Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, 430074, China Shanghai Techase Environment Protection Co., Ltd, 1121 North Zhongshan No. 2 Road, Shanghai, 200092, China College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Highly-salty sludge Electro-dewatering Ions migration and distribution Model simulation
In this study, the effects of electro-dewatering technology applied to high-salt industrial sludge dewatering performance were investigated, in terms of ions migrations and distributions by model simulation and layered tests. The simulation results of Na+ and K+ migrations were consistent with layered experiments during electrodewatering, where Na+ ions migrated faster than K+ ions. More than 80% Na+ ions were removed by electromigration, which would be useful in subsequent sludge utilization. The mass specific energy consumption was reduced from 350.08 to 295.88 kWh per ton sludge by means of piecewise voltage electro-dewatering method. This study provided insights into the soluble ions migration and distribution mechanism in electro-dewatering process, and a method to improve commercial application performance of high-salt industrial sludge electrodewatering.
1. Introduction Large quantities of industrial sludge from mechanical compression usually contains over 80% moisture content, which is difficult to deep dewatering (Lv et al., 2019b; Yang et al., 2005). In China, 40 million tons industrial sludge pollutants were released in 2018, which commonly exhibit more than 80% water content and are lack of deep dewatering and ultimate disposal (Lv, 2019; Seminar, 2019). Electro-dewatering (EDW) technology has been recently developed for deep dewatering of a wide variety of sludge pollutants (Lv et al., 2018; Wu et al., 2018; Yang et al., 2011). However, there are very few reports on industrial sludge electro-dewatering, especially for highly-salty industrial sludge electro-dewatering. EDW is effective in removing a great percentage of water from sludge that can hardly be removed by conventional mechanical dewatering method (Iwata et al., 2013). High efficiency, deep degree of dewatering and no additional chemical agents are their main advantages. For EDW machine commercial application, it was reported that the main key performances indicators were (1) dewatering degree (solid content or water content), (2) energy consumption and (3) space time yield (Lv et al., 2019b; Wu et al., 2018). Consequently, many studies about optimizing experimental operation and influencing factors of sludge EDW were conducted to improve dewatering performance. Pressure (Yuan and Weng, 2003), electric field intensity (Weng et al., 2013), current density (Olivier et al., 2015), pH (Navab-
∗
Daneshmand et al., 2015), temperature (Lv et al., 2019b; NavabDaneshmand et al., 2015), particle size (Sprute and Kelsh, 1981), solid content (Li et al., 2007). In general, high voltage, high current, and high pressure may improve deep dewatering. Our recent research shows that the comprehensive apparent factor affecting the electroosmotic flow velocity is the conductivity (Lv et al., 2018). Higher current density is derived from higher conductivity, and the higher current density contributes to deeper degree of dewatering. Lv et al. established a mathematical model to predict the sludge EDW process through the curve of the relationship between conductivity and water content (Lv et al., 2018). However, these theories and conclusions are basically based on the process of municipal sludge EDW. These pre-existing process parameters maybe not directly applicable to industrial sludge. As demonstrated in a number of literature researches, the sludge EDW performance was also usually affected by sludge properties (Iwata et al., 2013; Sheng et al., 2010). The organic matter in municipal sludge is mainly composed of extracellular polymeric substances (EPS) of microbial aggregates, while industrial sludge generally contains no microorganisms and high salinity content (over 4 g/kg dry sludge). There may be some differences in the mechanism and process of industrial sludge EDW. So, it's necessary to develop corresponding process parameter routes based on specific sludge. Sludge electro-dewatering was mainly carried out by the principle of directional movement of protons and some cations with water molecules attached to them under electric
Corresponding author. E-mail address: [email protected] (X. Wu).
https://doi.org/10.1016/j.envres.2019.108862 Received 12 August 2019; Received in revised form 12 October 2019; Accepted 25 October 2019 0013-9351/ © 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Hang Lv, et al., Environmental Research, https://doi.org/10.1016/j.envres.2019.108862
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field (Lv et al., 2018). The high-salt industrial sludge generally contains a large amount of free ions, and these ions may also move under electric field. The movement of these ions and their effects on sludge dewatering are worth to be studied. To study the dynamic variation of sludge characteristics, displacement of water in EDW process has been examined thoroughly in model systems and on a microscopic scale (Curvers et al., 2007; Hill, 2006; Iwata et al., 1991; Lv et al., 2018; Wu et al., 2018). These works shed light on the movement of water in electro-osmosis process and are of great value for optimizing EDW performance. In addition to the displacement of water, there is also ion migration in EDW process. Tuan and Sillanpaa used electro-osmosis to remove salt from anaerobically digested sludge (Tuan and Sillanpaa, 2010). Xiao et al. reported that (Xiao et al., 2017) less than 12.5 g Na2SO4 kg−1 DS (dry sludge) salt addition in municipal sludge showed positive impacts on EDW process. Previous research results indicated that the effects of ion migration on EDW performance cannot be ignored. Layered experiments were used as a good method to better understand the migration and distribution of various substances or components (Navab-Daneshmand et al., 2015). Conrardy et al. (2016) studied the electric resistance distribution in sludge cake on EDW process by layered tests, and the results manifested the electric resistance nonlinear decreased across the cake from the anode to the cathode, and this phenomenon can be explained by the water migration through layered tests based on the reference works (Navab-Daneshmand et al., 2015; Xiao et al., 2017). Therefore, the layered experiments and mathematical simulation method were used to investigate the migration and distribution of soluble metal ions in high-salt industrial sludge EDW process, so as to better regulate ions migration during EDW process. And trying to achieve better sludge dewatering effect by adjusting the operating parameters appropriately. Thereby providing some guidance for the commercial application of industrial sludge electro-dewatering machine.
Fig. 1. Sketch of 2D geometric model. Table 1 Parameter values and initial values for model simulations. Parameter
Value
Description
Rc Hc DNa DK ZNa ZK i0 Eeq
38 mm 10 mm 3.3 × 10−10 m2/s 1.96 × 10−10 m2/s 1 1 2.88 × 10−4 A/cm2 0.5 V
radius of sludge cake height of sludge cake Diffusion coefficient of sodium ion Diffusion coefficient of potassium ion Charge number of sodium ion Charge number of potassium ion exchange current density of electrode equilibrium potential
(Lv et al., 2018), and the content of organic matters of sludge samples were tested by 550 °C burned method (burned in muffle furnace at 550 °C for 2 h) (Xiao et al., 2017). The sludge water content was reduced by electro-dewatering to 70%, 65%, and 60%, which respectively exhibited time-space yield Y70, Y65 and Y60 and energy consumption E70, E65, and E60. The content of organic matters is equal to the ratio of volatile solids (VS) to total solids (TS). To detect the content of soluble metal ions in each layer, dried sludge cake were digested with nitric acid-hydrofluoric acid-perchloric acid (HNO3–HF–HClO4) system at 180–200 °C, and the Na+ and K+ concentrations were measured by inductive coupled plasma (ICP) (Optima 8300DV, PerkinElmer, USA) (Tang et al., 2017). Three layers EDW sludge pH values was analyzed in a 1:5 (sludge: water) suspension mixed for 30 min using a pH meter (Leici pHS-3C, Shanghai, China). The obtained pH value was considered as wet EDW sludge pH (Tang et al., 2018; Xiao et al., 2017).
2. Material and methods 2.1. Sludge sample Sludge sample was drawn from the mechanically dewatering unit of Riyou Chemical Co., Ltd in Changshu, China. The collected sludge sample was stored in a refrigerator at 4 °C before tests. The initial pH, dry solids content, organic content, weight of sodium and potassium content of sludge sample were 7.98 ± 0.1, 12.34 ± 0.21 wt%, 81.56 ± 0.41 wt%, 26.68 ± 0.13 g/kg dry sludge and 26.83 ± 0.12 g/kg dry sludge respectively. Before EDW experiments, sludge sample was kept out of refrigerator to reach the room temperature. In this work, there were no chemical additives in all sludge electro-dewatering tests.
2.4. Numerical modelling A simplified EDW transient state 2D model was set up with Comsol® Multiphysics simulation software (9401017) to simulate soluble metal ions migration. A pie-like 2D geometry (Fig. 1) was built to represent the three layers sludge cake in the model, which includes both cathode and anode electrodes and two thick steel meshes. The size of model is equal to the actual sludge cake. The initial parameters are listed in Table 1. The material balance for the species i in electrolyte is given by Eq. (1).
2.2. Experimental set-up EDW experimental apparatus was basically referenced from previous research (Lv et al., 2018). For the part of layered experiments, two 0.5 mm thick steel meshes (pore size 74 μm) were parallelly inserted in about 50.00 g sludge samples with about 76 mm diameter, which divided the sludge cake into trisection layers (NavabDaneshmand et al., 2015). To ensure the reliability of experimental data, old pieces of the steel mesh filters were replaced by the new ones for every EDW experiments. To investigate the dynamic variation of water content, Na+, K+, and pH in three layers vs. time, three parallel repeated interval tests were conducted in this paper, with relative standard deviation no more than 10%.
∂ci + ∇ (−Di ∇ci − Zi ui Fci ∇φi ) + u∇ci = Ri ∂t
(1) 3
where ci is the concentration (SI unit: mol/m ), Di give the diffusivities (SI unit: m2/s) (Furini et al., 2006; Jurinak et al., 1987), zi equals the charge, ui represents the mobility (SI unit: (mol m2)/(J s)), and Ri is the production term for species i (SI unit: mol/(m3 s)), and in this model Ri = 0, F denotes Faraday's constant (SI unit: C/mol), and ϕl is the
2.3. Analysis methods The cake dryness was determined according to 105 °C dry method 2
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Fig. 2. (a) Current, (b) filtrate, (c) temperature and (d) percentage of removed water, water evaporation and remaining in dewatered sludge.
The electrolyte current density (il) is calculated using Faraday's law by summing up the contributions from the molar fluxes, multiplied by the species charges.
Table 2 Energy consumption, water content and time-space yield.
5V 10 V 15 V 5 V–15 V
DC
Y70
Y65
Y60
E70
E65
E60
19.91 30.39 43.48 41.35
– 15.16 33.25 28.88
– – 27.85 23.61
– – 21.89 18.21
– 215.77 295.91 231.05
– – 320.16 260.94
– – 350.08 295.88
il = F ∑ (−Di ∇ci − Zi ui Fci ∇∅i )
The expressions for the local current density of electrode, iloc (SI unit: A/cm2), is based on the Butler−Volmer equation. So the electrolyte-electrode boundary interface governing equations that solve for local current densities are as Eq. (5). The overpotential (η) is the difference between of electrode potential and equilibrium potential (Eq. (6)).
DC means dry solid content at 3000 s, unit: %. Y70, Y65,Y60 were represent timespace yield when water content reached 70%, 65% and 60% respectively, unit: kg m2h−1. E70, E65, E60 were represent electric energy consumption per ton sludge (about 81.56% water content) when water content was reduced to 70%, 65% and 60% respectively, unit: kwh/ton.
electrolyte potential (SI unit: V). The outflow of soluble ions at the cathode is described by the following equation (Eq. (2)), and the mobility (ui) can be expressed as Nernst-Einstein equation (Eq. (3)):
− nDi ∇ci = ci (t ) ui =
Di RT
(4)
i
βFη ⎞ −(1 − β ) Fη ⎞ ⎞ iloc = i 0 ⎛ exp ⎛ − exp ⎛ RT RT ⎝ ⎠ ⎠⎠ ⎝ ⎝
(5)
η = Eelectrode − Eeq
(6)
n·il = iloc
(7)
⎜
(2) (3)
⎟
where i0 is exchange current density of electrode (Lv et al., 2018), β is coefficient of transmission which value is set as 0.5, Eelectrode is electrode potential, Eeq is equilibrium potential and T is temperature which is applied with real experimental data.
where T is temperature, R is Malar gas constant, n is the direction vector perpendicular to the electrode and ci(t) represent the quantity of outflow from cathode which is obtained from experimental data. 3
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Fig. 3. Percentage of (a) Na and (b) K content in each layer and filtration.
3. Results and discussion
content in sludge sample, however the removal of K was 74.20 mg/50 g sludge (account for 44.82% removal) which was less than that of Na.
3.1. Sludge electro-dewatering and desalination performance 3.2. Sludge property variation in EDW process
In the light of previous researches, higher electric field strength contributed to the larger current (as Fig. 2a), and obtained more finally filtrate removal (as Fig. 2b and Table 2) (Wu et al., 2018; Yu et al., 2017). It can be seen from Fig. 2 that the high-salt sludge EDW phenomenon was consistent with existing research conclusions. However, the current variation phenomenon in the initial stage was slightly different form previous researches. The current rapidly increased in the initial few minutes and reached a peak before starting to fall. Yu's work indicated that the current magnitude was proportional to the osmotic velocity (Yu et al., 2017). Fig. 2a and b two images show that the EDW process could be divided into three stage. In the initial stage, it is a slow dewatering stage, in which the amount of filtrate increased slowly, and the current continuously increased and reached a peak. The second stage was the rapid dewatering stage, and the current decreased rapidly. The third stage was the dewatering limit stage, in which the current slowly decreased and gradually reached stable, and the amount of filtrate slowly increased and gradually reached the limit. At the same time, it can be seen that the higher the electric field strength, the shorter time for the initial slow dewatering stage (slow dewatering time of 5 V, 10 V, 15 V were about 450s, 300s, 120s) and faster to reach the dewatering limit (The time to reach the dewatering limit at 10 V and 15 V respectively was about 2000s, 1500s, and 5 V conditions was not reached the dewatering limit within 3000s.). Fig. 2c showed that the temperature rose rapidly and then gradually decreased. Higher electric field strength led to more joule heat and higher temperature (the maximum temperature of 5 V, 10 V, 15 V were 16.3, 32.1 and 63.5 °C respectively), which was related to current variations and environment heat dissipation (Lv et al., 2019b). The amount of evaporation was related to temperature. In Fig. 2d, average water evaporation of 5 V, 10 V, 15 V condition was 1.32, 2.88 and 4.16 g, respectively. It can be seen from Fig. 3 that some of the soluble ions (Na+ and + K ) flowed into the filtrate during sludge EDW, and more content of Na+ and K+ was detected in the filtrate for higher electric field strength condition. In the sludge samples of this study, the contents difference between Na+ and K+ (Na:164.06 mg/50 g sludge and K: 165.56mg/ 50 g sludge) was small, but comparing the results in Fig. 3a and b, it can be found that the removal amount of Na+ was significantly higher than that of K+. For example with 15 V, the weight of Na+ removed from sludge was 138.87 mg/50 g sludge which account for 84.65% of total
Layered experiments were performed to further investigate substances migration in sludge EDW process. Fig. 4 showed the dry solids content and pH variations in different layers of sludge. In the first few minutes, dry solid contents differences between each layer of sludge samples was unobservable. Such as with 5 V, at even 420 s, dry solid contents in bottom and top layers were 11.83% and 13.66%, respectively. The initial few minutes can be considered as a slow dewatering stage. After that, dry solid contents of middle and top layers raised rapidly, and dry solid content of the bottom layer was initially decreased and then increased. The dry solid content differences between top and bottom increased dramatically in fast dewatering stage. Then, the mass of filtrate per unit time gradually decreased and reached dewaterability limit stage. It should be noted that higher electric field intensity contributed to more dramatically solid content difference between top and bottom layers. It was reported that the existence of the dewaterability limit mainly owing to the high electrical resistance of the top dry layer (Tuan et al., 2012; Xiao et al., 2017). For the pH variation in three layers, it can be seen from Fig. 4 that the pH of the bottom layer fast increased (alkalinity enhancement from 7.98 to 11.00 at 10 V condition) within the first few minutes, with the pH of the top layer decreased (acidity enhancement from 7.98 to 6.32 at 10 V condition). The differences between top and bottom layers in acidity and alkalinity gradually increased. Then, the acidity of top layer and alkalinity of bottom layer basically remained stable. The pH of the intermediate layer varied little ( ± 0.5) throughout the EDW process. At the same time, applying higher electric field strength in EDW resulted in greater differences between the top and bottom layers. The distribution of pH was mainly related to the reaction of water electrolysis on the electrode surface, and the directional of hydroxide ions and protons movements under a certain electric field (Iwata et al., 2013). This indicated that during sludge EDW process, an acid-base gradient field was formed along the direction of the electric field. To observe the apparent morphology changes of industrial sludge electro-dewatering, the SEM images of industrial sludge EDW under different electric intensity were investigated, as shown in Fig. 5. It was clearly observed that elevated electric intensity caused substantial changes in industrial sludge surface morphology. As shown in Fig. 5a, the original industrial sludge surface after directly dried was tight 4
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Fig. 4. Solid content and pH variation vs. time at 5 V, 10 V and 15 V condition. (a) 5 V-solid content, (b) 10 V-solid content, (c) 15 V-solid content and (d) 5 V-pH, (e) 10 V-pH, (f) 15 V-pH.
improving the dewaterability (Ding et al., 2018; Stabnikova et al., 2005).
morphology. Compare to initial sludge, a great deal of cracks and micropores were found at the EDW sludge surface (as Fig. 5b, c and d), and the surface cracking degree increased with electric intensity elevated. It was reported that electric field tend to form porosity structures in sludge EDW process (Lv et al., 2019b). At the same time, these transformations of cracking and porosity structures was conductive to the reduction of water holding capacity in industrial sludge, finally
3.3. Soluble metal ions migration and distribution for different electric intensity EDW In the sludge EDW process, in addition to the migration of water 5
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Fig. 5. SEM micrographs of industrial sludge under different electric intensity EDW. with magnification of 150 × , 300 × and 500 × . (a) Initial sludge, (b) 5 V, (c) 10 V and (d) 15 V.
dry solid at 300s under 5 V condition. Meanwhile, it can be found that the concentration gradient was enhanced with increasing electric field strength. In order to verify the model, the sodium and potassium ion contents were detected in the layered experiment and the results were shown in Fig. 8. It can be seen that sodium ions and potassium ions moved toward the cathode side in the first few minutes, resulting in a rapid decrease in the top layer ion content and an increase in the bottom layer ion content. For example, under 15 V conditions (Fig. 8e), the top layer sodium ion content was reduced from the initial 26.67 g/kg dry sludge to 7.46 g/kg dry sludge in 300 s, while the bottom layer sodium ion
molecules, there were soluble ions migrations (Tuan and Sillanpaa, 2010). As mentioned earlier, the amount of filtrate was relatively less in the first few minutes during EDW. In order to better understand the migration and distribution of ions in sludge EDW, a migration model of Na+ and K+ during the initial 300 s was established by Comsol® software. The simulation results were shown in Figs. 6 and 7. It showed that sodium ions and potassium ions migrated from the anode to the cathode under the electric field, and finally concentrated on the cathode, forming a concentration gradient along the direction of the electric field. Such as the Na+ and K+ concentration differences between top layer and bottom layers were about 18.4 g/kg dry solid and 9.98 g/kg 6
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Fig. 6. Simulated Na+ migration map. (a) 5 V, (b) 10 V and (c) 15 V.
Fig. 7. Simulated K+ migration map. (a) 5 V, (b) 10 V and (c) 15 V.
7
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Fig. 8. The content of sodium-potassium ions in each layer changed with time. (a) Na–5V, (b) K–5V, (c) Na–10 V, (d) K–10 V, (e) Na–15 V and (f) K–15 V.
flowed out with the water osmosis. So the bottom ion content began to decrease rapidly after 300 s (for example, Fig. 8c), and ion contents in top and middle layer were also gradually decreased. There were also limits for the reduction of sodium and potassium ion content, which may be related to the mutual adsorption of ions and solid particles. Comparing the distribution of sodium and potassium ions in the same electric field intensity, it can be found that the concentration gradient of sodium ions was significantly more intensive than that of potassium ions. For example, at 300 s under 5 V condition, the difference in
content was increased to 39.71 g/kg dry sludge. This phenomenon was in agreement with the results of model simulation in Figs. 6 and 7. In the slow dewatering stage, the migration of soluble ions was dominant process, while the osmotic of water was relatively inconspicuous, so a large amount of soluble ions were enriched in the bottom layer. It was also because of the migration of soluble ions in the early stage, which caused the phenomenon of higher current in first few minutes and less filtrate output. Subsequently, the osmotic of water began to play a dominant role, and the soluble ions enriched in the bottom layer and 8
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Fig. 9. Piecewise voltage electro-dewatering effect. (a) Current and (b) filtrate.
utilization. The simulated results for migration of Na+ and K+ in EDW process were consistent with layered experiments phenomenon. Sludge electro-dewatering extent (dry solid content limit) and concentration gradient was increased with enhanced electric field intensity. Owing to the high content of soluble salts in the sludge, it takes excessive energy to conduct sludge electro-dewatering. And the energy consumption can be further reduced from 350.08 to 295.88 kWh per ton sludge by means of piecewise voltage method. Electrochemical reactions and physicochemical properties may have important effects on sludge electro-dewatering performance, which can be investigated in the future.
sodium ion content between the top and bottom layers was 19.12 g/kg dry sludge, while the value of potassium was 4.91 g/kg dry sludge. This indicates that perhaps the sodium ion migrated faster than potassium ion during the electro-dewatering of high-salt industrial sludge. 3.4. Piecewise voltage electro-dewatering effects How to reduce the energy consumption of sludge electro-dewatering is one of the common pursuit goals in this research field. It was well known that high voltages tend to result in high energy consumption. The layered experiment and model simulation results demonstrated that the excessive energy consumption was due to the migration of soluble ions in the slow dewatering stage. Then, if the lower voltage is used to concentrate the soluble ions in the bottom layer in the initial few minutes, and then the higher voltage was applied to dewatering maybe reduce the water mass specific energy consumption. The results of the piecewise voltage experiments were shown in Fig. 9. In the initial 300s, 5 V voltage was applied to migrate ions. During this time, the current was maintained at a lower level range (from 1.12A to 1.61A, Fig. 9a), and then the electro-dewatering was performed at a higher voltage of 15 V for 2700s. Compared to the 15 V constant voltage dewatering mode (filtrate about 31.72 g), the filtrate output also reached approximately 31.27 g. It can be seen from Table 2 that stronger electric field strength resulted in deeper dewatering degree (dewatering limit) and the dry solid content of 5 V–15 V method also reached 41.35 wt%, which was nearly to the same as that of 15 V method. Although the time-space yield was inevitably reduced (the E30 yield was reduced from 21.89 kg m2h−1 of 15 V condition to 18.21 kg m2h−1 of 5 V–15 V condition), but the energy consumption was reduced from 350.08 to 295.88 kWh per ton sludge. This result was better than the existing sludge thermal drying technology (the space-time yield much less than 18.21 kg m2h−1 and energy consumption over 600 kWh per ton sludge counted with 80 wt% water content (Mahmoud et al., 2017)). So, piecewise voltage mode might provide well guidance for high-salt industrial sludge electro-dewatering engineering application to reduced energy consumption.
Acknowledgements This study is sponsored by The Thousands Youth Talent Project, Natural Science Foundation of China (NSFC, No. 51704122), and Urban Sewage Sludge Safeguarding, Disposing and Recycling Full Chain Technology Improvement and Engineering Demonstration (NO. 2017ZX07403002). References Curvers, D., et al., 2007. Modelling the electro-osmotically enhanced pressure dewatering of activated sludge. Chem. Eng. Sci. 62, 2267–2276. Ding, N., et al., 2018. Improving the dewaterability of citric acid wastewater sludge by Fenton treatment. J. Clean. Prod. 196, 739–746. Furini, S., et al., 2006. Application of the Poisson-Nernst-Planck theory with space-dependent diffusion coefficients to KcsA. Biophys. J. 91, 3162–3169. Hill, R.J., 2006. Electric-field-induced force on a charged spherical colloid embedded in an electrolyte-saturated Brinkman medium. Phys. Fluids 18. Iwata, M., et al., 1991. Analysis of electroosmotic dewatering. J. Chem. Eng. Jpn. 24, 45–50. Iwata, M., et al., 2013. Application of electroosmosis for sludge dewatering-A review. Dry. Technol. 31, 170–184. Jurinak, J.J., et al., 1987. Ionic diffusion coefficients as predicted by conductometric techniques. Soil Sci. Soc. Am. J. 51, 625–630. Li, F.D., et al., 2007. Effect of different electric fields on temperature rise, energy efficiency ratio, and solids content during electro-osmotic dewatering of tofu residue (okara). Rep. Natl. Food Res. Inst. 71, 15–26. Lv, H., et al., 2018. Effects of piecewise electric field operation on sludge dewatering: phenomena and mathematical model. Ind. Eng. Chem. Res. 57, 12468–12477. Lv, H., et al., 2019. The effects of aging for improving wastewater sludge electro-dewatering performances. J. Ind. Eng. Chem. https://doi.org/10.1016/j.jiec.2019.08.049. In press. Lv, H., et al., 2019b. Effects of temperature variation on wastewater sludge electro-dewatering. J. Clean. Prod. 214, 873–880. Mahmoud, A., et al., 2017. A comparative study of electro-dewatering process performance for activated and digested wastewater sludge. Water Res. 129, 66–82. Navab-Daneshmand, T., et al., 2015. Impact of joule heating and pH on biosolids electrodewatering. Environ. Sci. Technol. 49, 5417–5424. Olivier, J., et al., 2015. Electro-dewatering of wastewater sludge: an investigation of the relationship between filtrate flow rate and electric current. Water Res. 82, 66–77. China Urban Sludge Treatment and Disposal Technology and Application Advanced
4. Conclusions Electro-dewatering is a suitable technology for highly-salty industrial sludge reduction treatment, and in this study the dry solid content can achieve over 40 wt% without additional chemical addition. It was found that sodium ions migrate faster than potassium ions in EDW process, and more than 80% Na+ ions were removed by electromigration, which would be useful in subsequent dried sludge 9
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using an electrokinetic-assisted process: effects of electrical gradient. Separ. Purif. Technol. 117, 35–40. Wu, P., et al., 2018. Effect of electric field strength on electro-dewatering efficiency for river sediments by horizontal electric field. Sci. Total Environ. 647, 1333–1343. Xiao, J., et al., 2017. Migration and distribution of sodium ions and organic matters during electro-dewatering of waste activated sludge at different dosages of sodium sulfate. Chemosphere 189, 67–75. Yang, L., et al., 2005. Electro-kinetic dewatering of oily sludges. J. Hazard Mater. 125, 130–140. Yang, G.C.C., et al., 2011. Dewatering of a biological industrial sludge by electrokineticsassisted filter press. Separ. Purif. Technol. 79, 177–182. Yu, W., et al., 2017. Study on dewaterability limit and energy consumption in sewage sludge electro-dewatering by in-situ linear sweep voltammetry analysis. Chem. Eng. J. 317, 980–987. Yuan, C., Weng, C.-h., 2003. Sludge dewatering by electrokinetic technique: effect of processing time and potential gradient. Adv. Environ. Res. 7, 727–732.
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Environmental Research journal homepage: www.elsevier.com/locate/envres
Soluble metal ions migration and distribution in sludge electro-dewatering Hang Lva, Siqi Xinga, Daoguang Liub,c, Fang Wanga, Wenbiao Zhangb, Gangfan Sunb, Xu Wua,∗ a b c
School of Environmental Science and Engineering, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, 430074, China Shanghai Techase Environment Protection Co., Ltd, 1121 North Zhongshan No. 2 Road, Shanghai, 200092, China College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Highly-salty sludge Electro-dewatering Ions migration and distribution Model simulation
In this study, the effects of electro-dewatering technology applied to high-salt industrial sludge dewatering performance were investigated, in terms of ions migrations and distributions by model simulation and layered tests. The simulation results of Na+ and K+ migrations were consistent with layered experiments during electrodewatering, where Na+ ions migrated faster than K+ ions. More than 80% Na+ ions were removed by electromigration, which would be useful in subsequent sludge utilization. The mass specific energy consumption was reduced from 350.08 to 295.88 kWh per ton sludge by means of piecewise voltage electro-dewatering method. This study provided insights into the soluble ions migration and distribution mechanism in electro-dewatering process, and a method to improve commercial application performance of high-salt industrial sludge electrodewatering.
1. Introduction Large quantities of industrial sludge from mechanical compression usually contains over 80% moisture content, which is difficult to deep dewatering (Lv et al., 2019b; Yang et al., 2005). In China, 40 million tons industrial sludge pollutants were released in 2018, which commonly exhibit more than 80% water content and are lack of deep dewatering and ultimate disposal (Lv, 2019; Seminar, 2019). Electro-dewatering (EDW) technology has been recently developed for deep dewatering of a wide variety of sludge pollutants (Lv et al., 2018; Wu et al., 2018; Yang et al., 2011). However, there are very few reports on industrial sludge electro-dewatering, especially for highly-salty industrial sludge electro-dewatering. EDW is effective in removing a great percentage of water from sludge that can hardly be removed by conventional mechanical dewatering method (Iwata et al., 2013). High efficiency, deep degree of dewatering and no additional chemical agents are their main advantages. For EDW machine commercial application, it was reported that the main key performances indicators were (1) dewatering degree (solid content or water content), (2) energy consumption and (3) space time yield (Lv et al., 2019b; Wu et al., 2018). Consequently, many studies about optimizing experimental operation and influencing factors of sludge EDW were conducted to improve dewatering performance. Pressure (Yuan and Weng, 2003), electric field intensity (Weng et al., 2013), current density (Olivier et al., 2015), pH (Navab-
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Daneshmand et al., 2015), temperature (Lv et al., 2019b; NavabDaneshmand et al., 2015), particle size (Sprute and Kelsh, 1981), solid content (Li et al., 2007). In general, high voltage, high current, and high pressure may improve deep dewatering. Our recent research shows that the comprehensive apparent factor affecting the electroosmotic flow velocity is the conductivity (Lv et al., 2018). Higher current density is derived from higher conductivity, and the higher current density contributes to deeper degree of dewatering. Lv et al. established a mathematical model to predict the sludge EDW process through the curve of the relationship between conductivity and water content (Lv et al., 2018). However, these theories and conclusions are basically based on the process of municipal sludge EDW. These pre-existing process parameters maybe not directly applicable to industrial sludge. As demonstrated in a number of literature researches, the sludge EDW performance was also usually affected by sludge properties (Iwata et al., 2013; Sheng et al., 2010). The organic matter in municipal sludge is mainly composed of extracellular polymeric substances (EPS) of microbial aggregates, while industrial sludge generally contains no microorganisms and high salinity content (over 4 g/kg dry sludge). There may be some differences in the mechanism and process of industrial sludge EDW. So, it's necessary to develop corresponding process parameter routes based on specific sludge. Sludge electro-dewatering was mainly carried out by the principle of directional movement of protons and some cations with water molecules attached to them under electric
Corresponding author. E-mail address: [email protected] (X. Wu).
https://doi.org/10.1016/j.envres.2019.108862 Received 12 August 2019; Received in revised form 12 October 2019; Accepted 25 October 2019 0013-9351/ © 2019 Elsevier Inc. All rights reserved.
Please cite this article as: Hang Lv, et al., Environmental Research, https://doi.org/10.1016/j.envres.2019.108862
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field (Lv et al., 2018). The high-salt industrial sludge generally contains a large amount of free ions, and these ions may also move under electric field. The movement of these ions and their effects on sludge dewatering are worth to be studied. To study the dynamic variation of sludge characteristics, displacement of water in EDW process has been examined thoroughly in model systems and on a microscopic scale (Curvers et al., 2007; Hill, 2006; Iwata et al., 1991; Lv et al., 2018; Wu et al., 2018). These works shed light on the movement of water in electro-osmosis process and are of great value for optimizing EDW performance. In addition to the displacement of water, there is also ion migration in EDW process. Tuan and Sillanpaa used electro-osmosis to remove salt from anaerobically digested sludge (Tuan and Sillanpaa, 2010). Xiao et al. reported that (Xiao et al., 2017) less than 12.5 g Na2SO4 kg−1 DS (dry sludge) salt addition in municipal sludge showed positive impacts on EDW process. Previous research results indicated that the effects of ion migration on EDW performance cannot be ignored. Layered experiments were used as a good method to better understand the migration and distribution of various substances or components (Navab-Daneshmand et al., 2015). Conrardy et al. (2016) studied the electric resistance distribution in sludge cake on EDW process by layered tests, and the results manifested the electric resistance nonlinear decreased across the cake from the anode to the cathode, and this phenomenon can be explained by the water migration through layered tests based on the reference works (Navab-Daneshmand et al., 2015; Xiao et al., 2017). Therefore, the layered experiments and mathematical simulation method were used to investigate the migration and distribution of soluble metal ions in high-salt industrial sludge EDW process, so as to better regulate ions migration during EDW process. And trying to achieve better sludge dewatering effect by adjusting the operating parameters appropriately. Thereby providing some guidance for the commercial application of industrial sludge electro-dewatering machine.
Fig. 1. Sketch of 2D geometric model. Table 1 Parameter values and initial values for model simulations. Parameter
Value
Description
Rc Hc DNa DK ZNa ZK i0 Eeq
38 mm 10 mm 3.3 × 10−10 m2/s 1.96 × 10−10 m2/s 1 1 2.88 × 10−4 A/cm2 0.5 V
radius of sludge cake height of sludge cake Diffusion coefficient of sodium ion Diffusion coefficient of potassium ion Charge number of sodium ion Charge number of potassium ion exchange current density of electrode equilibrium potential
(Lv et al., 2018), and the content of organic matters of sludge samples were tested by 550 °C burned method (burned in muffle furnace at 550 °C for 2 h) (Xiao et al., 2017). The sludge water content was reduced by electro-dewatering to 70%, 65%, and 60%, which respectively exhibited time-space yield Y70, Y65 and Y60 and energy consumption E70, E65, and E60. The content of organic matters is equal to the ratio of volatile solids (VS) to total solids (TS). To detect the content of soluble metal ions in each layer, dried sludge cake were digested with nitric acid-hydrofluoric acid-perchloric acid (HNO3–HF–HClO4) system at 180–200 °C, and the Na+ and K+ concentrations were measured by inductive coupled plasma (ICP) (Optima 8300DV, PerkinElmer, USA) (Tang et al., 2017). Three layers EDW sludge pH values was analyzed in a 1:5 (sludge: water) suspension mixed for 30 min using a pH meter (Leici pHS-3C, Shanghai, China). The obtained pH value was considered as wet EDW sludge pH (Tang et al., 2018; Xiao et al., 2017).
2. Material and methods 2.1. Sludge sample Sludge sample was drawn from the mechanically dewatering unit of Riyou Chemical Co., Ltd in Changshu, China. The collected sludge sample was stored in a refrigerator at 4 °C before tests. The initial pH, dry solids content, organic content, weight of sodium and potassium content of sludge sample were 7.98 ± 0.1, 12.34 ± 0.21 wt%, 81.56 ± 0.41 wt%, 26.68 ± 0.13 g/kg dry sludge and 26.83 ± 0.12 g/kg dry sludge respectively. Before EDW experiments, sludge sample was kept out of refrigerator to reach the room temperature. In this work, there were no chemical additives in all sludge electro-dewatering tests.
2.4. Numerical modelling A simplified EDW transient state 2D model was set up with Comsol® Multiphysics simulation software (9401017) to simulate soluble metal ions migration. A pie-like 2D geometry (Fig. 1) was built to represent the three layers sludge cake in the model, which includes both cathode and anode electrodes and two thick steel meshes. The size of model is equal to the actual sludge cake. The initial parameters are listed in Table 1. The material balance for the species i in electrolyte is given by Eq. (1).
2.2. Experimental set-up EDW experimental apparatus was basically referenced from previous research (Lv et al., 2018). For the part of layered experiments, two 0.5 mm thick steel meshes (pore size 74 μm) were parallelly inserted in about 50.00 g sludge samples with about 76 mm diameter, which divided the sludge cake into trisection layers (NavabDaneshmand et al., 2015). To ensure the reliability of experimental data, old pieces of the steel mesh filters were replaced by the new ones for every EDW experiments. To investigate the dynamic variation of water content, Na+, K+, and pH in three layers vs. time, three parallel repeated interval tests were conducted in this paper, with relative standard deviation no more than 10%.
∂ci + ∇ (−Di ∇ci − Zi ui Fci ∇φi ) + u∇ci = Ri ∂t
(1) 3
where ci is the concentration (SI unit: mol/m ), Di give the diffusivities (SI unit: m2/s) (Furini et al., 2006; Jurinak et al., 1987), zi equals the charge, ui represents the mobility (SI unit: (mol m2)/(J s)), and Ri is the production term for species i (SI unit: mol/(m3 s)), and in this model Ri = 0, F denotes Faraday's constant (SI unit: C/mol), and ϕl is the
2.3. Analysis methods The cake dryness was determined according to 105 °C dry method 2
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Fig. 2. (a) Current, (b) filtrate, (c) temperature and (d) percentage of removed water, water evaporation and remaining in dewatered sludge.
The electrolyte current density (il) is calculated using Faraday's law by summing up the contributions from the molar fluxes, multiplied by the species charges.
Table 2 Energy consumption, water content and time-space yield.
5V 10 V 15 V 5 V–15 V
DC
Y70
Y65
Y60
E70
E65
E60
19.91 30.39 43.48 41.35
– 15.16 33.25 28.88
– – 27.85 23.61
– – 21.89 18.21
– 215.77 295.91 231.05
– – 320.16 260.94
– – 350.08 295.88
il = F ∑ (−Di ∇ci − Zi ui Fci ∇∅i )
The expressions for the local current density of electrode, iloc (SI unit: A/cm2), is based on the Butler−Volmer equation. So the electrolyte-electrode boundary interface governing equations that solve for local current densities are as Eq. (5). The overpotential (η) is the difference between of electrode potential and equilibrium potential (Eq. (6)).
DC means dry solid content at 3000 s, unit: %. Y70, Y65,Y60 were represent timespace yield when water content reached 70%, 65% and 60% respectively, unit: kg m2h−1. E70, E65, E60 were represent electric energy consumption per ton sludge (about 81.56% water content) when water content was reduced to 70%, 65% and 60% respectively, unit: kwh/ton.
electrolyte potential (SI unit: V). The outflow of soluble ions at the cathode is described by the following equation (Eq. (2)), and the mobility (ui) can be expressed as Nernst-Einstein equation (Eq. (3)):
− nDi ∇ci = ci (t ) ui =
Di RT
(4)
i
βFη ⎞ −(1 − β ) Fη ⎞ ⎞ iloc = i 0 ⎛ exp ⎛ − exp ⎛ RT RT ⎝ ⎠ ⎠⎠ ⎝ ⎝
(5)
η = Eelectrode − Eeq
(6)
n·il = iloc
(7)
⎜
(2) (3)
⎟
where i0 is exchange current density of electrode (Lv et al., 2018), β is coefficient of transmission which value is set as 0.5, Eelectrode is electrode potential, Eeq is equilibrium potential and T is temperature which is applied with real experimental data.
where T is temperature, R is Malar gas constant, n is the direction vector perpendicular to the electrode and ci(t) represent the quantity of outflow from cathode which is obtained from experimental data. 3
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Fig. 3. Percentage of (a) Na and (b) K content in each layer and filtration.
3. Results and discussion
content in sludge sample, however the removal of K was 74.20 mg/50 g sludge (account for 44.82% removal) which was less than that of Na.
3.1. Sludge electro-dewatering and desalination performance 3.2. Sludge property variation in EDW process
In the light of previous researches, higher electric field strength contributed to the larger current (as Fig. 2a), and obtained more finally filtrate removal (as Fig. 2b and Table 2) (Wu et al., 2018; Yu et al., 2017). It can be seen from Fig. 2 that the high-salt sludge EDW phenomenon was consistent with existing research conclusions. However, the current variation phenomenon in the initial stage was slightly different form previous researches. The current rapidly increased in the initial few minutes and reached a peak before starting to fall. Yu's work indicated that the current magnitude was proportional to the osmotic velocity (Yu et al., 2017). Fig. 2a and b two images show that the EDW process could be divided into three stage. In the initial stage, it is a slow dewatering stage, in which the amount of filtrate increased slowly, and the current continuously increased and reached a peak. The second stage was the rapid dewatering stage, and the current decreased rapidly. The third stage was the dewatering limit stage, in which the current slowly decreased and gradually reached stable, and the amount of filtrate slowly increased and gradually reached the limit. At the same time, it can be seen that the higher the electric field strength, the shorter time for the initial slow dewatering stage (slow dewatering time of 5 V, 10 V, 15 V were about 450s, 300s, 120s) and faster to reach the dewatering limit (The time to reach the dewatering limit at 10 V and 15 V respectively was about 2000s, 1500s, and 5 V conditions was not reached the dewatering limit within 3000s.). Fig. 2c showed that the temperature rose rapidly and then gradually decreased. Higher electric field strength led to more joule heat and higher temperature (the maximum temperature of 5 V, 10 V, 15 V were 16.3, 32.1 and 63.5 °C respectively), which was related to current variations and environment heat dissipation (Lv et al., 2019b). The amount of evaporation was related to temperature. In Fig. 2d, average water evaporation of 5 V, 10 V, 15 V condition was 1.32, 2.88 and 4.16 g, respectively. It can be seen from Fig. 3 that some of the soluble ions (Na+ and + K ) flowed into the filtrate during sludge EDW, and more content of Na+ and K+ was detected in the filtrate for higher electric field strength condition. In the sludge samples of this study, the contents difference between Na+ and K+ (Na:164.06 mg/50 g sludge and K: 165.56mg/ 50 g sludge) was small, but comparing the results in Fig. 3a and b, it can be found that the removal amount of Na+ was significantly higher than that of K+. For example with 15 V, the weight of Na+ removed from sludge was 138.87 mg/50 g sludge which account for 84.65% of total
Layered experiments were performed to further investigate substances migration in sludge EDW process. Fig. 4 showed the dry solids content and pH variations in different layers of sludge. In the first few minutes, dry solid contents differences between each layer of sludge samples was unobservable. Such as with 5 V, at even 420 s, dry solid contents in bottom and top layers were 11.83% and 13.66%, respectively. The initial few minutes can be considered as a slow dewatering stage. After that, dry solid contents of middle and top layers raised rapidly, and dry solid content of the bottom layer was initially decreased and then increased. The dry solid content differences between top and bottom increased dramatically in fast dewatering stage. Then, the mass of filtrate per unit time gradually decreased and reached dewaterability limit stage. It should be noted that higher electric field intensity contributed to more dramatically solid content difference between top and bottom layers. It was reported that the existence of the dewaterability limit mainly owing to the high electrical resistance of the top dry layer (Tuan et al., 2012; Xiao et al., 2017). For the pH variation in three layers, it can be seen from Fig. 4 that the pH of the bottom layer fast increased (alkalinity enhancement from 7.98 to 11.00 at 10 V condition) within the first few minutes, with the pH of the top layer decreased (acidity enhancement from 7.98 to 6.32 at 10 V condition). The differences between top and bottom layers in acidity and alkalinity gradually increased. Then, the acidity of top layer and alkalinity of bottom layer basically remained stable. The pH of the intermediate layer varied little ( ± 0.5) throughout the EDW process. At the same time, applying higher electric field strength in EDW resulted in greater differences between the top and bottom layers. The distribution of pH was mainly related to the reaction of water electrolysis on the electrode surface, and the directional of hydroxide ions and protons movements under a certain electric field (Iwata et al., 2013). This indicated that during sludge EDW process, an acid-base gradient field was formed along the direction of the electric field. To observe the apparent morphology changes of industrial sludge electro-dewatering, the SEM images of industrial sludge EDW under different electric intensity were investigated, as shown in Fig. 5. It was clearly observed that elevated electric intensity caused substantial changes in industrial sludge surface morphology. As shown in Fig. 5a, the original industrial sludge surface after directly dried was tight 4
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Fig. 4. Solid content and pH variation vs. time at 5 V, 10 V and 15 V condition. (a) 5 V-solid content, (b) 10 V-solid content, (c) 15 V-solid content and (d) 5 V-pH, (e) 10 V-pH, (f) 15 V-pH.
improving the dewaterability (Ding et al., 2018; Stabnikova et al., 2005).
morphology. Compare to initial sludge, a great deal of cracks and micropores were found at the EDW sludge surface (as Fig. 5b, c and d), and the surface cracking degree increased with electric intensity elevated. It was reported that electric field tend to form porosity structures in sludge EDW process (Lv et al., 2019b). At the same time, these transformations of cracking and porosity structures was conductive to the reduction of water holding capacity in industrial sludge, finally
3.3. Soluble metal ions migration and distribution for different electric intensity EDW In the sludge EDW process, in addition to the migration of water 5
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Fig. 5. SEM micrographs of industrial sludge under different electric intensity EDW. with magnification of 150 × , 300 × and 500 × . (a) Initial sludge, (b) 5 V, (c) 10 V and (d) 15 V.
dry solid at 300s under 5 V condition. Meanwhile, it can be found that the concentration gradient was enhanced with increasing electric field strength. In order to verify the model, the sodium and potassium ion contents were detected in the layered experiment and the results were shown in Fig. 8. It can be seen that sodium ions and potassium ions moved toward the cathode side in the first few minutes, resulting in a rapid decrease in the top layer ion content and an increase in the bottom layer ion content. For example, under 15 V conditions (Fig. 8e), the top layer sodium ion content was reduced from the initial 26.67 g/kg dry sludge to 7.46 g/kg dry sludge in 300 s, while the bottom layer sodium ion
molecules, there were soluble ions migrations (Tuan and Sillanpaa, 2010). As mentioned earlier, the amount of filtrate was relatively less in the first few minutes during EDW. In order to better understand the migration and distribution of ions in sludge EDW, a migration model of Na+ and K+ during the initial 300 s was established by Comsol® software. The simulation results were shown in Figs. 6 and 7. It showed that sodium ions and potassium ions migrated from the anode to the cathode under the electric field, and finally concentrated on the cathode, forming a concentration gradient along the direction of the electric field. Such as the Na+ and K+ concentration differences between top layer and bottom layers were about 18.4 g/kg dry solid and 9.98 g/kg 6
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Fig. 6. Simulated Na+ migration map. (a) 5 V, (b) 10 V and (c) 15 V.
Fig. 7. Simulated K+ migration map. (a) 5 V, (b) 10 V and (c) 15 V.
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Fig. 8. The content of sodium-potassium ions in each layer changed with time. (a) Na–5V, (b) K–5V, (c) Na–10 V, (d) K–10 V, (e) Na–15 V and (f) K–15 V.
flowed out with the water osmosis. So the bottom ion content began to decrease rapidly after 300 s (for example, Fig. 8c), and ion contents in top and middle layer were also gradually decreased. There were also limits for the reduction of sodium and potassium ion content, which may be related to the mutual adsorption of ions and solid particles. Comparing the distribution of sodium and potassium ions in the same electric field intensity, it can be found that the concentration gradient of sodium ions was significantly more intensive than that of potassium ions. For example, at 300 s under 5 V condition, the difference in
content was increased to 39.71 g/kg dry sludge. This phenomenon was in agreement with the results of model simulation in Figs. 6 and 7. In the slow dewatering stage, the migration of soluble ions was dominant process, while the osmotic of water was relatively inconspicuous, so a large amount of soluble ions were enriched in the bottom layer. It was also because of the migration of soluble ions in the early stage, which caused the phenomenon of higher current in first few minutes and less filtrate output. Subsequently, the osmotic of water began to play a dominant role, and the soluble ions enriched in the bottom layer and 8
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Fig. 9. Piecewise voltage electro-dewatering effect. (a) Current and (b) filtrate.
utilization. The simulated results for migration of Na+ and K+ in EDW process were consistent with layered experiments phenomenon. Sludge electro-dewatering extent (dry solid content limit) and concentration gradient was increased with enhanced electric field intensity. Owing to the high content of soluble salts in the sludge, it takes excessive energy to conduct sludge electro-dewatering. And the energy consumption can be further reduced from 350.08 to 295.88 kWh per ton sludge by means of piecewise voltage method. Electrochemical reactions and physicochemical properties may have important effects on sludge electro-dewatering performance, which can be investigated in the future.
sodium ion content between the top and bottom layers was 19.12 g/kg dry sludge, while the value of potassium was 4.91 g/kg dry sludge. This indicates that perhaps the sodium ion migrated faster than potassium ion during the electro-dewatering of high-salt industrial sludge. 3.4. Piecewise voltage electro-dewatering effects How to reduce the energy consumption of sludge electro-dewatering is one of the common pursuit goals in this research field. It was well known that high voltages tend to result in high energy consumption. The layered experiment and model simulation results demonstrated that the excessive energy consumption was due to the migration of soluble ions in the slow dewatering stage. Then, if the lower voltage is used to concentrate the soluble ions in the bottom layer in the initial few minutes, and then the higher voltage was applied to dewatering maybe reduce the water mass specific energy consumption. The results of the piecewise voltage experiments were shown in Fig. 9. In the initial 300s, 5 V voltage was applied to migrate ions. During this time, the current was maintained at a lower level range (from 1.12A to 1.61A, Fig. 9a), and then the electro-dewatering was performed at a higher voltage of 15 V for 2700s. Compared to the 15 V constant voltage dewatering mode (filtrate about 31.72 g), the filtrate output also reached approximately 31.27 g. It can be seen from Table 2 that stronger electric field strength resulted in deeper dewatering degree (dewatering limit) and the dry solid content of 5 V–15 V method also reached 41.35 wt%, which was nearly to the same as that of 15 V method. Although the time-space yield was inevitably reduced (the E30 yield was reduced from 21.89 kg m2h−1 of 15 V condition to 18.21 kg m2h−1 of 5 V–15 V condition), but the energy consumption was reduced from 350.08 to 295.88 kWh per ton sludge. This result was better than the existing sludge thermal drying technology (the space-time yield much less than 18.21 kg m2h−1 and energy consumption over 600 kWh per ton sludge counted with 80 wt% water content (Mahmoud et al., 2017)). So, piecewise voltage mode might provide well guidance for high-salt industrial sludge electro-dewatering engineering application to reduced energy consumption.
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