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Minimization of hazardous sludge production using a bioelectrochemical system supplied by an alternating current electric field
Journal Pre-proofs Minimization of Hazardous Sludge Production Using a Bioelectrochemical System Supplied by an Alternating Current Electric Field Zohreh Moghiseh, Abbas Rezaee, Somayyeh Dehghani PII: DOI: Reference:
S1567-5394(19)30634-6 https://doi.org/10.1016/j.bioelechem.2019.107446 BIOJEC 107446
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Bioelectrochemistry
Received Date: Revised Date: Accepted Date:
12 September 2019 10 December 2019 15 December 2019
Please cite this article as: Z. Moghiseh, A. Rezaee, S. Dehghani, Minimization of Hazardous Sludge Production Using a Bioelectrochemical System Supplied by an Alternating Current Electric Field, Bioelectrochemistry (2019), doi: https://doi.org/10.1016/j.bioelechem.2019.107446
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Minimization of Hazardous Sludge Production Using a Bioelectrochemical System Supplied by an Alternating Current Electric Field
Zohreh Moghiseha, Abbas Rezaeea*, Somayyeh Dehghania,b
a
Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
b
Department of Environmental Health Engineering, Sirjan School of Medical Sciences, Sirjan, Iran
∗ Corresponding Author: A. Rezaee, Email: [email protected]
Abstract
In the present study, minimization of hazardous bio-sludge production was investigated using a bioelectrochemical system supplied by an alternating current electric field and supplemented with phenol as a cabon source. The experiments were conducted in an air-conditioned bioreactor and at neutral pH value. Moreover, steel wool and carbon cloth were utilized as electrodes in the bioelectrochemical system. The experiments were operated in an air-conditioned bioreactor at 25℃ and a neutral pH value with carbon to nitrogen (C/N) ratio of 0.5-6. The results obtained showed that complete phenol electro-biodegradation occurred at a C/N ratio of a frequency of 5 Hz, and 0.4 peak-to-peak voltage (Vpp) over 2 h. Besides, sludge production and sludge yield were obtained at the C/N ratio of 0.5-6 by 200-382 mg VSS/g COD and 82-89.4 mg TSS/g COD, respectively. Ultimately, the C/N ratio of 1 seemed to be optimum for microbial
1
growth with the phenol biodegradation efficiency of 99.9% as well as the lowest sludge production. These results demonstrated that the proposed bioelectrochemical system supplied by low-frequency and low-voltage electric current could reduce hazardous sludge production.
Keywords: Bioelectrochemical system; Alternating electric field; Sludge; C/N ratio; Phenol
1. Introduction
Bio-sludge production is known as one of the critical factors in conventional bio-treatment processes such as activated sludge process. Annual average bio-sludge production is 240 million30 wet tons in Europe [1]. Sludge management costs include economic, environmental, and social that will increase with time [2]. These costs are related to sludge disposal and energy consumption in sludge processing units [3]. In addition, sludge treatment cost are reported to be more than 50% of operational costs in a wastewater treatment plants [4-5]. The biological aerobic processes produce excess quantities of sludge [6]. In comparison, anaerobic treatment could reduce the amount of sludge, because they would biodegradate only 30-35% of organic contaminants [7]. Moreover, a decrease in sludge production occurs in modern processes such as sulfate reduction, autotrophic-denitrification, and nitrification-integrated processes which can decrease sludge at a high level, although some drawbacks such as long-term operation and use of special microbial populations have been thus far reported [8]. One of the proposed methods to mitigate sludge volume is utilization of bioelectrochemical system that can decline microbial growth yield. In bioelectrochemical system, microbial yield and energy production are related to pH value, electric current, and electron transfer efficiency [9]. Use of direct current (DC) has been reported for bio-
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remediation of pollutants in bioelectrochemical systems. However, they have consumed high current densities or voltages have been applied following an increase in initial pollutant concentrations [10, 11]. Moreover, it has been reported that bioelectrochemical systems can degrade some low biodegradable organic components such as dyes and polymers via oxidation or reduction processes [12, 13]. For example, aminophenol has been introduced as a byproduct of pnitrophenol reduction at a bioelectrochemical system[14] or reduction of phenol to acetate [15]. On the other hand, alternating current (AC) has been proposed for some electrochemical systems [16, 17]. In more recent studies by our team, the findings demonstrated that bioelectrochemical processes supplied by AC could result in a decrease of sludge production compared with conventional processes [18-20]. One of the advantages of AC is no electrode corrosion. In a bioreactor comprised of charged types, AC leads to dipole-dipole reactions through making many changes in polarity [16]. The electro-hydrodynamic force generated by AC can be thus a reason for microbial electro-stimulation in bio-electrodes [21]. In addition, a study had confirmed that AC could increase permeability of exopolysaccharide matrix due to charged molecular vibrations [22]. It can further affect carbon to nitrogen (C/N) ratio as nutrients required for microbial growth or biomass production [23]. Generally, the mass of the produced biomass depends on biodegradation of organic matters. According to scientific reports, addition of high C/N ratio could represent high biomass production [24]. Under this condition, biomass decomposition processes could be slowed down, leading to accumulation of less degraded substrate [25]. Therefore, applying low C/N ratio is a way of reducing biomass production in biological processes. To the best of authors’ knowledge, few studies have been so far conducted on sludge management using AC in a bioelectrochemical process. Hence, the main objective of this study was to investigate the effect
3
of AC on phenol removal efficiency under different C/N ratios, chemical oxygen demand (COD), and total suspended solids (TSS), and consequently to minimize hazardous sludge production.
2. Materials and Methods
2.1. Experimental Setup and Reactor Configuration
A Plexiglas cylindrical single-chambered bioelectrochemical system was operated in batch mode with the working volume of 1 liter, the height of 30 cm, and the diameter of 8 cm. Steel wool and carbon cloth were selected as anode and cathode, respectively. The steel wool electrode was then placed cylindrically in the bioreactor (Schematic 1). The carbon cloth electrode had the width and the effective height of 2 cm and 21 cm, respectively. The diameter of the steel wool electrode was by 5 cm. Moreover, the material of the reactor was subsequently mixed using a magnetic stirrer with 300 rpm (Heidolph, Model MR3001). A function generator (FG-5000, Taiwan) was utilized to provide an AC in the bioelectrochemical system. It should be noted that the experiments were conducted in an air-conditioned bioreactor maintained at 25℃ and a neutral pH value.
2.2. Initial Biomass Preparation
Inoculating sludge was obtained from a municipal wastewater treatment plant, Tehran, Iran. The concentration of the sludge was 2000 mg L-1 as mixed liquor suspended solids (MLSS). It was perculated in the bioreactor with a 20% volumetric ratio. The sludge was then acclimated through co-metabolism with phenol for 1 month. The culture medium had some growth components, used
4
to provide the required nutrients for microorganisms (Table 1). Besides, phenol was selected as carbon source for biomass production whose initial concentration was by 25-300 mg L-1 in the synthetic wastewater, based on the required C/N. The C/N ratio of 0.5-6 was correspondingly examined for selecting lower biomass production in the bioelectrochemical system.
2.3. Analytic Methods
The COD, NH4+, NO2-, NO3-, MLSS, as well as mixed liquor volatile suspended solids (MLVSS) were analyzed according to the Standard Methods [26]. The phenol concentration was determined using ultraviolet-visible (UV/Vis) spectrophotometer (Ray 9200, China) based on the colorimetric method at 510 nm [26]. The microbial growth was further measured by optical density (OD600) analysis through a spectrophotometer. Moreover, the pH value was analyzed on an EZDO7011pH meter. It should be noted that all the experiments were made in triplicate.
2.4. Sludge Production Analysis
To determine the bio-sludge, the produced MLSS was filtered and dried at 103-105℃ based on the Standard Methods for water and wastewater examinations [26]. The amount of MLSS was measured at the end of each stable batch cycle. The sludge production was further estimated by the following equations [27]:
TSS Excess= TSS Reactor + ΣTSS Effluent
(1)
5
Σ∆COD= Σ (COD Influent - COD Effluent)
Sludge production=
(2)
TSS Excess
(3)
Σ∆𝐶𝑂𝐷
Moreover, the yield coefficient (Y) for phenol-utilizing sludge was determined by the following equation [28]:
Y=
∆X
(4)
∆𝑆
Where ΔX is biomass concentration variations and ΔS represents substrate concentration variations.
3. Results and Discussion
3.1. Sludge Production
During the start-up period, the MLSS concentration increased from 11 g L-1 to 50 g L-1 in the bioelectrochemical system during 4 months (Fig. 1A and 1B). As shown in Fig 1A, the MLSS of the bioreactor became 50 g L-1 within 120 days. The MLSS in the membrane bioreactor, as one of the advanced processes in wastewater treatment, was reported by 20 g L-1 at SRT of 20 days. Sludge could be thus more produced by the membrane bioreactor during a long operation. As observed in Fig 1B, the steel wool electrode as bioanode had more MLSS compared with the carbon cloth as biocathode attributable to its high surface area. In the control reactor, higher MLSS 6
was produced due to the effect of electric current on sludge production and cell yield at the bioreactor [9]. Increasing C/N ratio can produce high amount of sludge [29]. At the 0.5:6 C/N ratio, sludge production and sludge yield were thus obtained by 200-382 mg VSS/g COD and 8289.4 mg TSS/g COD, respectively. Fig. 2 illustrates sludge production and sludge yield at various C/N ratios. As depicted in Fig. 2, sludge production and sludge yield have been increased as the C/N ratio is added. According to the related literature, the amount of TSS effluent in the bioelectrochemical system is lower than aerobic and anaerobic bioreactors [9, 27], while the COD removal can be higher. The high sludge production can bring about various problems such as sludge fouling during operation and decrease removal efficiency. The optimum sludge production and sludge yield were accordingly obtained at the C/N ratio of 1. It meant the produced biomass might become equal to the amount of substrate consumed in the electrochemical bioreactor. We have reported in our previous study, the best phenol removal efficiency was obtained at the C/N ratio of 1 [20]. Sludge production and sludge yield in the present study were however reported lower than similar processes. In the activated sludge process, sludge production and sludge yield were reported by 190 mg TSS/g ∆COD and 600 mg VSS/g COD, respectively [9, 27]. Moreover, the sludge yield was reported 0.39 mg MLSS/mg TOC in membrane bioreactors, lower than the bioelectrochemical system [30]. The presence, growth, and diversity of high electro-active microorganisms along with optimum redox condition can be the reason for sludge minimization in the bio-electroreactors [9, 27]. It should be noted that, one of the main factors affecting biomass production is C/N ratio. Previous studies have reported that variations in carbon and nitrogen contents of microorganisms can result in limited cell growth [31]. An initial decrease in the substrate declines growth yield. Moreover, a high C/N ratio in batch culture can reduce biomass production. High C/N ratio provides energy metabolism uncoupling for decreasing biomass
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formation because of inducing a metabolic reaction pathway. Energy uncoupling further separates catabolism from anabolism at high C/N ratios [4]. Hence, it can be deduced that the substrate consumption rate is increased by microbial mass, but without any change in microbial mass [30]. In this regard, Hsien et al. described a linearized plot slop for increased concentration of suspended biomass vs. decreased phenol concentration as the yield coefficient [28]. The microbial yield obtained from the experiment was by 0.207 mg VSS/mg phenol. Another factor affecting sludge reduction is acidity constant (pKa) value, indicating an inverse relationship between sludge reduction and energy decomposition [33]. The pKa value of phenol is 9.8, but various concentrations of phenol can affect it. In this study, the effect of C/N ratio of 0.5-6 was investigated by adding appropriate amounts of initial concentrations of phenol affected by the pKa value. By increasing the phenol concentration, the pKa value may increase followed by an improved sludge reduction. As a result, there is a relationship between produced cell yield i.e. sludge production and added concentrations in the batch culture, for the reason that transport rate depends on concentration [33]. Furthermore, chemical agents including antibiotics and aromatic compounds such as phenolic compounds can play roles in reducing sludge production [34]. The phenolic compounds include nitrophenol, chlorophenol, 2,4-dinitrophenol, para-nitrophenol, and pentachlorophenol which can be toxic for some microorganisms [35]. The concentration of 20 mg L-1 p-chlorophenol can also lead to sludge reduction by 60% compared with sludge reduction of 57.8% by o-chlorophenol with the same concentration [36]. This study concluded that phenol was the best substrate for degradation after acclimation. The decrease in biomass production can be due to more degradable phenol in the system. Hence, phenol degradation was more at the C/N ratio of 1. The bioavailable C/N ratio is additionally important for biological degradation of organic matters [37]. The value of bioavailability can thus decrease with a rise in carbon content. For
8
example, wood has various compounds such as cellulose, hemicellulose, and lignin that make it a low biodegradable material. The biodegradability of newspaper is also lower because of the increased carbon. Adding to the low degradable carbon source such as phenol can also provide toxicity for biomass [38]. The pH of bioreactor has no significant variation using an AC in the bioelectrochemical system. Since neutral pH is appropriate for a minimum sludge production and sludge yield [9], pH adjustment does not require the effluent in the bioelectrochemical system.
3.2. Effect of C/N Ratio
In this study, C/N ratio of 0.5/6 was investigated by adding appropriate amounts of the initial phenol concentrations, by 25-300 mg L-1. As shown in Fig 4, the average phenol removal efficiencies at C/N ratios of 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, and 6 were 94.48%, 100%, 81.85%, 78.53%, 61.87%, 58.40%, 51.85%, 55.82%, and 50.29%, respectively. The average removal efficiency of phenol was 99.9% in the C/N ratio of 1. Moreover, the ratio represented that phenol, as a slowly biodegradable carbon source, can be degraded by 99.9% under optimum conditions. It should be noted that high removal efficiency of phenol can be due to the electrohydrodynamic effect of AC on the permeability of cell membranes. Moreover, the phenol was more bioavailable in the bioelectrochemical system because of secretion of degrading enzymes. The C/N ratio of 1 causes high removal of phenol, whereas the average removal efficiency of phenol was 46% for the biological control reactor without AC, and <5% for the electrochemical control reactor applying AC using the C/N ratio of 1. The C/N ratio obtained in this study was lower than that in the literature. Under the optimum conditions, the maximum OD600 was obtained at the C/N ratio of 1. By enhancing the C/N ratio in the bioelectrochemical system from 0.5 to 6, biomass production
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consequently reached 0.284, 1.457, 1.11, 0.7, 0.85, 0.63, 0.51, 0.38, and 0.35, respectively. The maximum phenol efficiency and ammonium had been obtained at high C/N ratio of 16/1 in hybrid biological processes [39, 40]. Moreover, the optimum conditions of a system with batch suspended growth were observed at the C/N ratio of 30/1 within 20 h. This can be due to lower phenol biodegradation as a biomass carbon source [41]. A hybrid UASB reactor was also applied to evaluate the COD/NO3-N ratio by which a ratio of 6.36 under maximum removal of 98% phenol was obtained [42]. Likewise, the C/N ratio 0.5/2 may be attributed to the nitrifying or autotrophic denitrifying bacteria. The optimum conditions for this purpose included very low AC voltage (0.4 Vpp), the current of 5 mA, and the frequency of 5 Hz. Increasing organic matter could also enhance the denitrification process [43]. Besides, some heterotrophic bacteria might degrade organic chemicals to inorganic carbon found in the bioelectrochemical system [24]. Therefore, phenol as an electron source for the microorganism fed into the bioelectrochemical process was degraded easier by a microorganism. In this way, phenol would be more biodegradable in the bioelectrochemical process. In this regard, Feng et al. suggested that a current of 11 mA was capable using easier complex carbon such as starch at C/N ratio of 3.5 [43]. Meanwhile, it was reported that the membrane bio-reactor had the highest rate constant of phenolic compounds at a C/N ratio of 6 [44, 45]. They attributed the growth of ammonia-oxidizing bacteria with heterotrophic bacteria in sludge to increase in biodegradation of phenol by producing certain enzymes such as phenol hydroxylase, esterase, phthalate dioxygenase, and laccase. It was observed that the enzyme activity at the C/N ratio of 6 was higher than the C/N ratio of 1 [44, 45]. It was further shown that the high glucose removal )94.93-100%( could be obtained, when the C/N ratio ranged from 0.75 to 1. The heterotrophic and autotrophic denitrifying bacteria can identically participate in the mentioned range [46]. Among the phenolic compounds, chlorophenol could be
10
easily decomposed by the organism at the C/N ratio of 3/1 [47]. As a result, the bioavailable C/N ratio is an important factor for biological treatment of organic matters [37]. Lower C/N ratio can enhance anode transformation rate in bioelectrochemical systems [24]. Fig. 4 shows variation of energy consumption and removal efficiency of phenol versus volume ratio of sludge. Accordingly, energy consumption increased as the volume ratio of sludge was produced. On the other hand, the removal efficiency of phenol decreased in the anode of bioelectrochemical system. The removal efficiency or the consumed phenol can be also increased in the sludge. Therefore, the C/N ratio of 1 was obtained as the best value for sludge minimization and high phenol removal efficiency in the bioelectrochemical system.
3.3. Effect of AC on Phenol Biodegradation The bioelectrochemical process employing AC yielded 99.9% phenol removal efficiency under optimum conditions during 2 h. It should be noted that the biodegradation of phenol at a short time can be due to stimulation of microorganisms producing enzymes. These findings can be attributed to phenol degradation, based on COD analysis and organic acid formation, such as oxalic acid, acetic acid, and especially propionic acid, based on GC-MS analysis at 2 h. According to the most recently reported study, the propionic acid was identified at a lower concentration than acetic and oxalic acids, but it could play an important role in decomposition and mineralization of phenol because of color change, since phenol concentration was evident gradually after 60 min. The pathway of propionic acid formation was presented at our last study [20]. It was reported that the biodegradation of 100 mg L-1 phenol could occur in the bioelectrochemical system during 48 h [48]. In a similar study, bacterial cell growth was enhanced at 125 mV. Under this condition, phenol concentration was decreased from 100 mg L-1 to 25 mg L-1 during 100 h [49]. The residual
11
of phenol was higher than the standard concentration in the bioreactor. In the present study, phenol was completely removed. It was reported that phenol was biodegraded in maximum efficiency during 10 h via inducing 2 mA DC [50]. Therefore, phenol biodegradation occurred within higher reaction time compared with that our obtained results. Moreover, color removal efficiency in the bioelectrochemical system induced with the DC was reported by 20% degradation during 120 min [51]. The higher phenol efficiency in the lower reaction time could be due to increase in enzymatic activity.
3.4. Effect of AC on pH Variation The pH variations could affect the biodegradation of phenol. The obtained results showed that the pH value had no high variation in the bioelectrochemical system supplied by AC. The optimum pH values for aerobic biodegradation were reported by about 6-7. Moreover, the reaction solution did not require pH adjustment. The less consumed buffer and electrode corrosion in the proposed system could lead to higher operational costs. The AC can cause dipole-dipole interactions [52]. Generally, the electrolysis of water similarly occurs at pH 7. Accordingly, the produced OH− ions are transferred to the anode surface through the AC cycles that decrease with the lower frequency. The OH− ions produce •OH radicals and contribute to the oxidation process. As previously mentioned, steel wool and carbon cloth electrodes were applied in the present study. Based on Xray fluorescence (XRF) analysis, the highest ions released from steel wool are iron which can biologically and chemically affect the degradation mechanism of phenol [53]. In this study, the optimum frequency was 5 Hz in which phenol oxidation might be mostly carried out by electrode ionic content and then result in biostimulation. The pH value can be correspondingly influenced by production of some organic acids and other byproducts [48]. As depicted in Fig. 5, the obtained
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results with the AC revealed that the pH values are almost similar to the control pH values. Likewise, the pH values of phenol biodegradation induced by DC were higher than AC. Similar studies had further demonstrated that the pH variations were from 7.09 to 4.36 in an aerobic bioreactor induced by more than 10 mA DC. Applying phosphate-buffered saline solution was proposed to reduce the harmful effects of high DC [50]. The most recent study by authors had confirmed a slight increase in the pH value in the electrochemical process for RB19 dye removal using AC. It could be due to generation of •OH radicals [53]. Fig. 6 illustrates the pH value of the reaction solution under the following operating conditions: C/N ratio of 0.5-6, the applied EC of 1 mA, at 25℃. During the study of C/N ratio variations, the pH values were adjusted at 7. It was reported that the removal efficiency of p-nitrophenol, as one of the phenol derivatives, was considerable at pH 3 due to a decrease in oxygen evolution reaction at lower pH values [54]. The electrical conductivity values fluctuated between 2 and 3 mS/cm during various C/N ratios. The highest electrical conductivity values were similarly at high phenol concentrations, indicating that the ion concentration may increase electrical conductivity.
4. Conclusion
Sludge production could be controlled by several parameters. The C/N ratio was considered as one of the main parameters affecting biomass and subsequently sludge production. According to the results, a higher MLSS reduction could occur through applying an AC in the low C/N ratio. The low C/N ratio seemed to be more favorable for microbial growth limitation and better biodegradation of phenol as a carbon source. In addition, it was taken into account as one of the critical factors affecting sludge production minimization. In the present study, the minimization of
13
hazardous sludge production was achieved using the bioelectrochemical system supplied by an alternating current electric field. Moreover, sludge management including collection, transformation, disposal, as well as operational and treatment costs could become easier and more cost-effective in wastewater treatment plants.
Acknowledgement The authors gratefully acknowledge the financial and technical support provided by Tarbiat Modares University, Tehran, Iran.
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[39] Y. Wang, H. Chen, Y-X. Liu, R-P. Ren, Y-K. Lv, An adsorption-release-biodegradation system for simultaneous biodegradation of phenol and ammonium in phenol-rich wastewater, Bioresour Technol. 211 (2016) 711-719. [40] T. Wang, H. Zhao, H. Wang, B. Liu, C. Li, Research on degradation product and reaction kinetics of membrane electro-bioreactor (MEBR) with catalytic electrodes for high concentration phenol wastewater treatment. Chemosphere, 155 (2016) 94-99. [41] I. A. Vasiliadou, G. Tziotzios, D. V. Vayenas, A kinetic study of combined aerobic biological phenol and nitrate removal in batch suspended growth cultures, Int. Biodeterioration Biodegradation 61 (2008) 261-271. [42] A. Ramakrishnan, S. K. Gupta, Effect of COD/NO3−-N ratio on the performance of a hybrid UASB reactor treating phenolic wastewater, Desalination. 232 (2008) 128-138. [43] H. Feng, B. Huang, Y. Zou, N. Li, M. Wang, J.Yin, Y.Cong, D.Shen, The effect of carbon sources on nitrogen removal performance in bioelectrochemical systems, Bioresour Technol. 128 (2013) 565-570. [44] J. Boonnorat, S. Techkarnjanaruk, R. Honda, P. Prachanurak, Effects of hydraulic retention time and carbon to nitrogen ratio on micro-pollutant biodegradation in membrane bioreactor for leachate treatment. Bioresour. Technol. 219 (2016) 53-63. [45] J. Boonnorat, C.W. Chiemchaisri, K. Yamamoto, Kinetics of phenolic and phthalic acid esters biodegradation in membrane bioreactor (MBR) treating municipal landfill leachate. Chemosphere, 150 (2016) 639-649. [46] M. Tobajas, V. Verdugo, A.M. Polo, J.J. Rodriguez, A.F. Mohedano, Assessment of toxicity and biodegradability on activated sludge of priority and emerging pollutants, Environ. Technol., 37 (2016) 713-721.
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[47] A. Fakhruddin, M.A. Hossain, Optimisation of pH, Temperature and Carbon Nitrogen Ratio for the Degradation of m-Chlorophenol by Pseudomonas putida CP1. Bangladesh J.Microbiol., 23 (2006) 159-161. [48] L. Amor, M. Eiroa, C. Kennes, M.C. Veiga, Phenol biodegradation and its effect on the nitrification process. Water Res. 39(2005) 2915-2920. [49] H. Friman, A. Schechter, Y. Nitzan, R. Cahan, Phenol degradation in bio-electrochemical cells, In. Biodeterioration Biodegradation 84 (2013) 155-160. [50] N. Ailijiang, J. Chang, P. Liang, P. Li, Q. Wu, X. Zhang, X. Huang, Electrical stimulation on biodegradation of phenol and responses of microbial communities in conductive carriers supported biofilms of the bioelectrochemical reactor. Bioresour. Technol., 201(2016) 1-7. [51] S. Liu, H. Song, S. Wei, S.Liu, H-L. Song, S. Wei, Q. Liu, X.Li, X. Qian, Effect of direct electrical stimulation on decolorization and degradation of azo dye reactive brilliant red X3B in biofilm-electrode reactors. Biochem. Eng. J. 93 (2015) 294-302. [52] M. Eyvaz, Treatment of brewery wastewater with electrocoagulation: Improving the process performance by using alternating pulse current. Int. J. Electrochem. Sci., 11 (2016) 49885008. [53] E. Hoseinzadeh, A. Rezaee, Electrochemical degradation of RB19 dye using lowfrequency alternating current: effect of a square wave, RSC Adv. 5 (2015) 96918-96926. [54] F. Xie, Y. Xu, K. Xia, C. Jia, P. Zhang, Alternate pulses of ultrasound and electricity enhanced electrochemical process for p-nitrophenol degradation. Ultrason Sonochem 28 (2016) 199-206.
Figure and Table captions
20
Schematic 1: The schematic of bioreactor; 1- Plexiglas bioreactor, 2- Steel wool electrode, 3Carbon cloth electrode, 4- Magnet, 5- Stirrer, 6- Alternating current (AC) function generator, 7Air pump. Fig. 1. (A) MLSS concentrations in the bioelectrochemical system during the experimental periods, and (B) comparison of the average of MLSS between bioelectrochemical system induced by alternating current and control bioreactor Fig. 2. Variations of sludge production and sludge yield during various utilized C/N ratios( Sludge production/ mg TSS/gr COD,
Sludge yield/mg VSS/gr COD)
Fig. 3. Carbon-to-nitrogen ratios for the average phenol removal efficiency during various times (frequency
=
5
Hz
and
applied
voltage
=
0.4
Vpp)
(
) Fig. 4. The variation of energy consumption and phenol removal efficiency versus volume ratio of sludge (
Removal Efficiency/%,
Energy consumption/kWh Kg-1,
Energy
consumption/kWh L-1 ) Fig. 5. The pH values and removal efficiency of the solution during the experiments
(
)
Fig. 6. The pH and EC variations with different C/N ratios at 25℃ and applied current of 1 mA (
C/N ratio,
EC/mS/cm,
pH)
Table 1. Components of culture medium
21
Table 1. The components of culture medium
Components
Concentration (g L-1)
CaCl2
0.3
KH2PO4
0.1
Na2HPO4
0.15
NH4Cl
0.1
NaHCO3
0.6
MgSO4. 7H2O
0.3
FeCl2
0.2
High lights - A new approach for sludge minimization has been presented. - Bioelectrochemical system supplied by alternating current was utilized for sludge minimization - Reduction of initially mixed liquor suspended solid was 99.5 mg L-1 - C/N ratio 1 has the lowest biomass production
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Zohreh Moghiseh, Abbas Rezaee, Somayyeh Dehghani
S1567-5394(19)30634-6 https://doi.org/10.1016/j.bioelechem.2019.107446 BIOJEC 107446
To appear in:
Bioelectrochemistry
Received Date: Revised Date: Accepted Date:
12 September 2019 10 December 2019 15 December 2019
Please cite this article as: Z. Moghiseh, A. Rezaee, S. Dehghani, Minimization of Hazardous Sludge Production Using a Bioelectrochemical System Supplied by an Alternating Current Electric Field, Bioelectrochemistry (2019), doi: https://doi.org/10.1016/j.bioelechem.2019.107446
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© 2019 Published by Elsevier B.V.
Minimization of Hazardous Sludge Production Using a Bioelectrochemical System Supplied by an Alternating Current Electric Field
Zohreh Moghiseha, Abbas Rezaeea*, Somayyeh Dehghania,b
a
Department of Environmental Health Engineering, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran
b
Department of Environmental Health Engineering, Sirjan School of Medical Sciences, Sirjan, Iran
∗ Corresponding Author: A. Rezaee, Email: [email protected]
Abstract
In the present study, minimization of hazardous bio-sludge production was investigated using a bioelectrochemical system supplied by an alternating current electric field and supplemented with phenol as a cabon source. The experiments were conducted in an air-conditioned bioreactor and at neutral pH value. Moreover, steel wool and carbon cloth were utilized as electrodes in the bioelectrochemical system. The experiments were operated in an air-conditioned bioreactor at 25℃ and a neutral pH value with carbon to nitrogen (C/N) ratio of 0.5-6. The results obtained showed that complete phenol electro-biodegradation occurred at a C/N ratio of a frequency of 5 Hz, and 0.4 peak-to-peak voltage (Vpp) over 2 h. Besides, sludge production and sludge yield were obtained at the C/N ratio of 0.5-6 by 200-382 mg VSS/g COD and 82-89.4 mg TSS/g COD, respectively. Ultimately, the C/N ratio of 1 seemed to be optimum for microbial
1
growth with the phenol biodegradation efficiency of 99.9% as well as the lowest sludge production. These results demonstrated that the proposed bioelectrochemical system supplied by low-frequency and low-voltage electric current could reduce hazardous sludge production.
Keywords: Bioelectrochemical system; Alternating electric field; Sludge; C/N ratio; Phenol
1. Introduction
Bio-sludge production is known as one of the critical factors in conventional bio-treatment processes such as activated sludge process. Annual average bio-sludge production is 240 million30 wet tons in Europe [1]. Sludge management costs include economic, environmental, and social that will increase with time [2]. These costs are related to sludge disposal and energy consumption in sludge processing units [3]. In addition, sludge treatment cost are reported to be more than 50% of operational costs in a wastewater treatment plants [4-5]. The biological aerobic processes produce excess quantities of sludge [6]. In comparison, anaerobic treatment could reduce the amount of sludge, because they would biodegradate only 30-35% of organic contaminants [7]. Moreover, a decrease in sludge production occurs in modern processes such as sulfate reduction, autotrophic-denitrification, and nitrification-integrated processes which can decrease sludge at a high level, although some drawbacks such as long-term operation and use of special microbial populations have been thus far reported [8]. One of the proposed methods to mitigate sludge volume is utilization of bioelectrochemical system that can decline microbial growth yield. In bioelectrochemical system, microbial yield and energy production are related to pH value, electric current, and electron transfer efficiency [9]. Use of direct current (DC) has been reported for bio-
2
remediation of pollutants in bioelectrochemical systems. However, they have consumed high current densities or voltages have been applied following an increase in initial pollutant concentrations [10, 11]. Moreover, it has been reported that bioelectrochemical systems can degrade some low biodegradable organic components such as dyes and polymers via oxidation or reduction processes [12, 13]. For example, aminophenol has been introduced as a byproduct of pnitrophenol reduction at a bioelectrochemical system[14] or reduction of phenol to acetate [15]. On the other hand, alternating current (AC) has been proposed for some electrochemical systems [16, 17]. In more recent studies by our team, the findings demonstrated that bioelectrochemical processes supplied by AC could result in a decrease of sludge production compared with conventional processes [18-20]. One of the advantages of AC is no electrode corrosion. In a bioreactor comprised of charged types, AC leads to dipole-dipole reactions through making many changes in polarity [16]. The electro-hydrodynamic force generated by AC can be thus a reason for microbial electro-stimulation in bio-electrodes [21]. In addition, a study had confirmed that AC could increase permeability of exopolysaccharide matrix due to charged molecular vibrations [22]. It can further affect carbon to nitrogen (C/N) ratio as nutrients required for microbial growth or biomass production [23]. Generally, the mass of the produced biomass depends on biodegradation of organic matters. According to scientific reports, addition of high C/N ratio could represent high biomass production [24]. Under this condition, biomass decomposition processes could be slowed down, leading to accumulation of less degraded substrate [25]. Therefore, applying low C/N ratio is a way of reducing biomass production in biological processes. To the best of authors’ knowledge, few studies have been so far conducted on sludge management using AC in a bioelectrochemical process. Hence, the main objective of this study was to investigate the effect
3
of AC on phenol removal efficiency under different C/N ratios, chemical oxygen demand (COD), and total suspended solids (TSS), and consequently to minimize hazardous sludge production.
2. Materials and Methods
2.1. Experimental Setup and Reactor Configuration
A Plexiglas cylindrical single-chambered bioelectrochemical system was operated in batch mode with the working volume of 1 liter, the height of 30 cm, and the diameter of 8 cm. Steel wool and carbon cloth were selected as anode and cathode, respectively. The steel wool electrode was then placed cylindrically in the bioreactor (Schematic 1). The carbon cloth electrode had the width and the effective height of 2 cm and 21 cm, respectively. The diameter of the steel wool electrode was by 5 cm. Moreover, the material of the reactor was subsequently mixed using a magnetic stirrer with 300 rpm (Heidolph, Model MR3001). A function generator (FG-5000, Taiwan) was utilized to provide an AC in the bioelectrochemical system. It should be noted that the experiments were conducted in an air-conditioned bioreactor maintained at 25℃ and a neutral pH value.
2.2. Initial Biomass Preparation
Inoculating sludge was obtained from a municipal wastewater treatment plant, Tehran, Iran. The concentration of the sludge was 2000 mg L-1 as mixed liquor suspended solids (MLSS). It was perculated in the bioreactor with a 20% volumetric ratio. The sludge was then acclimated through co-metabolism with phenol for 1 month. The culture medium had some growth components, used
4
to provide the required nutrients for microorganisms (Table 1). Besides, phenol was selected as carbon source for biomass production whose initial concentration was by 25-300 mg L-1 in the synthetic wastewater, based on the required C/N. The C/N ratio of 0.5-6 was correspondingly examined for selecting lower biomass production in the bioelectrochemical system.
2.3. Analytic Methods
The COD, NH4+, NO2-, NO3-, MLSS, as well as mixed liquor volatile suspended solids (MLVSS) were analyzed according to the Standard Methods [26]. The phenol concentration was determined using ultraviolet-visible (UV/Vis) spectrophotometer (Ray 9200, China) based on the colorimetric method at 510 nm [26]. The microbial growth was further measured by optical density (OD600) analysis through a spectrophotometer. Moreover, the pH value was analyzed on an EZDO7011pH meter. It should be noted that all the experiments were made in triplicate.
2.4. Sludge Production Analysis
To determine the bio-sludge, the produced MLSS was filtered and dried at 103-105℃ based on the Standard Methods for water and wastewater examinations [26]. The amount of MLSS was measured at the end of each stable batch cycle. The sludge production was further estimated by the following equations [27]:
TSS Excess= TSS Reactor + ΣTSS Effluent
(1)
5
Σ∆COD= Σ (COD Influent - COD Effluent)
Sludge production=
(2)
TSS Excess
(3)
Σ∆𝐶𝑂𝐷
Moreover, the yield coefficient (Y) for phenol-utilizing sludge was determined by the following equation [28]:
Y=
∆X
(4)
∆𝑆
Where ΔX is biomass concentration variations and ΔS represents substrate concentration variations.
3. Results and Discussion
3.1. Sludge Production
During the start-up period, the MLSS concentration increased from 11 g L-1 to 50 g L-1 in the bioelectrochemical system during 4 months (Fig. 1A and 1B). As shown in Fig 1A, the MLSS of the bioreactor became 50 g L-1 within 120 days. The MLSS in the membrane bioreactor, as one of the advanced processes in wastewater treatment, was reported by 20 g L-1 at SRT of 20 days. Sludge could be thus more produced by the membrane bioreactor during a long operation. As observed in Fig 1B, the steel wool electrode as bioanode had more MLSS compared with the carbon cloth as biocathode attributable to its high surface area. In the control reactor, higher MLSS 6
was produced due to the effect of electric current on sludge production and cell yield at the bioreactor [9]. Increasing C/N ratio can produce high amount of sludge [29]. At the 0.5:6 C/N ratio, sludge production and sludge yield were thus obtained by 200-382 mg VSS/g COD and 8289.4 mg TSS/g COD, respectively. Fig. 2 illustrates sludge production and sludge yield at various C/N ratios. As depicted in Fig. 2, sludge production and sludge yield have been increased as the C/N ratio is added. According to the related literature, the amount of TSS effluent in the bioelectrochemical system is lower than aerobic and anaerobic bioreactors [9, 27], while the COD removal can be higher. The high sludge production can bring about various problems such as sludge fouling during operation and decrease removal efficiency. The optimum sludge production and sludge yield were accordingly obtained at the C/N ratio of 1. It meant the produced biomass might become equal to the amount of substrate consumed in the electrochemical bioreactor. We have reported in our previous study, the best phenol removal efficiency was obtained at the C/N ratio of 1 [20]. Sludge production and sludge yield in the present study were however reported lower than similar processes. In the activated sludge process, sludge production and sludge yield were reported by 190 mg TSS/g ∆COD and 600 mg VSS/g COD, respectively [9, 27]. Moreover, the sludge yield was reported 0.39 mg MLSS/mg TOC in membrane bioreactors, lower than the bioelectrochemical system [30]. The presence, growth, and diversity of high electro-active microorganisms along with optimum redox condition can be the reason for sludge minimization in the bio-electroreactors [9, 27]. It should be noted that, one of the main factors affecting biomass production is C/N ratio. Previous studies have reported that variations in carbon and nitrogen contents of microorganisms can result in limited cell growth [31]. An initial decrease in the substrate declines growth yield. Moreover, a high C/N ratio in batch culture can reduce biomass production. High C/N ratio provides energy metabolism uncoupling for decreasing biomass
7
formation because of inducing a metabolic reaction pathway. Energy uncoupling further separates catabolism from anabolism at high C/N ratios [4]. Hence, it can be deduced that the substrate consumption rate is increased by microbial mass, but without any change in microbial mass [30]. In this regard, Hsien et al. described a linearized plot slop for increased concentration of suspended biomass vs. decreased phenol concentration as the yield coefficient [28]. The microbial yield obtained from the experiment was by 0.207 mg VSS/mg phenol. Another factor affecting sludge reduction is acidity constant (pKa) value, indicating an inverse relationship between sludge reduction and energy decomposition [33]. The pKa value of phenol is 9.8, but various concentrations of phenol can affect it. In this study, the effect of C/N ratio of 0.5-6 was investigated by adding appropriate amounts of initial concentrations of phenol affected by the pKa value. By increasing the phenol concentration, the pKa value may increase followed by an improved sludge reduction. As a result, there is a relationship between produced cell yield i.e. sludge production and added concentrations in the batch culture, for the reason that transport rate depends on concentration [33]. Furthermore, chemical agents including antibiotics and aromatic compounds such as phenolic compounds can play roles in reducing sludge production [34]. The phenolic compounds include nitrophenol, chlorophenol, 2,4-dinitrophenol, para-nitrophenol, and pentachlorophenol which can be toxic for some microorganisms [35]. The concentration of 20 mg L-1 p-chlorophenol can also lead to sludge reduction by 60% compared with sludge reduction of 57.8% by o-chlorophenol with the same concentration [36]. This study concluded that phenol was the best substrate for degradation after acclimation. The decrease in biomass production can be due to more degradable phenol in the system. Hence, phenol degradation was more at the C/N ratio of 1. The bioavailable C/N ratio is additionally important for biological degradation of organic matters [37]. The value of bioavailability can thus decrease with a rise in carbon content. For
8
example, wood has various compounds such as cellulose, hemicellulose, and lignin that make it a low biodegradable material. The biodegradability of newspaper is also lower because of the increased carbon. Adding to the low degradable carbon source such as phenol can also provide toxicity for biomass [38]. The pH of bioreactor has no significant variation using an AC in the bioelectrochemical system. Since neutral pH is appropriate for a minimum sludge production and sludge yield [9], pH adjustment does not require the effluent in the bioelectrochemical system.
3.2. Effect of C/N Ratio
In this study, C/N ratio of 0.5/6 was investigated by adding appropriate amounts of the initial phenol concentrations, by 25-300 mg L-1. As shown in Fig 4, the average phenol removal efficiencies at C/N ratios of 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, and 6 were 94.48%, 100%, 81.85%, 78.53%, 61.87%, 58.40%, 51.85%, 55.82%, and 50.29%, respectively. The average removal efficiency of phenol was 99.9% in the C/N ratio of 1. Moreover, the ratio represented that phenol, as a slowly biodegradable carbon source, can be degraded by 99.9% under optimum conditions. It should be noted that high removal efficiency of phenol can be due to the electrohydrodynamic effect of AC on the permeability of cell membranes. Moreover, the phenol was more bioavailable in the bioelectrochemical system because of secretion of degrading enzymes. The C/N ratio of 1 causes high removal of phenol, whereas the average removal efficiency of phenol was 46% for the biological control reactor without AC, and <5% for the electrochemical control reactor applying AC using the C/N ratio of 1. The C/N ratio obtained in this study was lower than that in the literature. Under the optimum conditions, the maximum OD600 was obtained at the C/N ratio of 1. By enhancing the C/N ratio in the bioelectrochemical system from 0.5 to 6, biomass production
9
consequently reached 0.284, 1.457, 1.11, 0.7, 0.85, 0.63, 0.51, 0.38, and 0.35, respectively. The maximum phenol efficiency and ammonium had been obtained at high C/N ratio of 16/1 in hybrid biological processes [39, 40]. Moreover, the optimum conditions of a system with batch suspended growth were observed at the C/N ratio of 30/1 within 20 h. This can be due to lower phenol biodegradation as a biomass carbon source [41]. A hybrid UASB reactor was also applied to evaluate the COD/NO3-N ratio by which a ratio of 6.36 under maximum removal of 98% phenol was obtained [42]. Likewise, the C/N ratio 0.5/2 may be attributed to the nitrifying or autotrophic denitrifying bacteria. The optimum conditions for this purpose included very low AC voltage (0.4 Vpp), the current of 5 mA, and the frequency of 5 Hz. Increasing organic matter could also enhance the denitrification process [43]. Besides, some heterotrophic bacteria might degrade organic chemicals to inorganic carbon found in the bioelectrochemical system [24]. Therefore, phenol as an electron source for the microorganism fed into the bioelectrochemical process was degraded easier by a microorganism. In this way, phenol would be more biodegradable in the bioelectrochemical process. In this regard, Feng et al. suggested that a current of 11 mA was capable using easier complex carbon such as starch at C/N ratio of 3.5 [43]. Meanwhile, it was reported that the membrane bio-reactor had the highest rate constant of phenolic compounds at a C/N ratio of 6 [44, 45]. They attributed the growth of ammonia-oxidizing bacteria with heterotrophic bacteria in sludge to increase in biodegradation of phenol by producing certain enzymes such as phenol hydroxylase, esterase, phthalate dioxygenase, and laccase. It was observed that the enzyme activity at the C/N ratio of 6 was higher than the C/N ratio of 1 [44, 45]. It was further shown that the high glucose removal )94.93-100%( could be obtained, when the C/N ratio ranged from 0.75 to 1. The heterotrophic and autotrophic denitrifying bacteria can identically participate in the mentioned range [46]. Among the phenolic compounds, chlorophenol could be
10
easily decomposed by the organism at the C/N ratio of 3/1 [47]. As a result, the bioavailable C/N ratio is an important factor for biological treatment of organic matters [37]. Lower C/N ratio can enhance anode transformation rate in bioelectrochemical systems [24]. Fig. 4 shows variation of energy consumption and removal efficiency of phenol versus volume ratio of sludge. Accordingly, energy consumption increased as the volume ratio of sludge was produced. On the other hand, the removal efficiency of phenol decreased in the anode of bioelectrochemical system. The removal efficiency or the consumed phenol can be also increased in the sludge. Therefore, the C/N ratio of 1 was obtained as the best value for sludge minimization and high phenol removal efficiency in the bioelectrochemical system.
3.3. Effect of AC on Phenol Biodegradation The bioelectrochemical process employing AC yielded 99.9% phenol removal efficiency under optimum conditions during 2 h. It should be noted that the biodegradation of phenol at a short time can be due to stimulation of microorganisms producing enzymes. These findings can be attributed to phenol degradation, based on COD analysis and organic acid formation, such as oxalic acid, acetic acid, and especially propionic acid, based on GC-MS analysis at 2 h. According to the most recently reported study, the propionic acid was identified at a lower concentration than acetic and oxalic acids, but it could play an important role in decomposition and mineralization of phenol because of color change, since phenol concentration was evident gradually after 60 min. The pathway of propionic acid formation was presented at our last study [20]. It was reported that the biodegradation of 100 mg L-1 phenol could occur in the bioelectrochemical system during 48 h [48]. In a similar study, bacterial cell growth was enhanced at 125 mV. Under this condition, phenol concentration was decreased from 100 mg L-1 to 25 mg L-1 during 100 h [49]. The residual
11
of phenol was higher than the standard concentration in the bioreactor. In the present study, phenol was completely removed. It was reported that phenol was biodegraded in maximum efficiency during 10 h via inducing 2 mA DC [50]. Therefore, phenol biodegradation occurred within higher reaction time compared with that our obtained results. Moreover, color removal efficiency in the bioelectrochemical system induced with the DC was reported by 20% degradation during 120 min [51]. The higher phenol efficiency in the lower reaction time could be due to increase in enzymatic activity.
3.4. Effect of AC on pH Variation The pH variations could affect the biodegradation of phenol. The obtained results showed that the pH value had no high variation in the bioelectrochemical system supplied by AC. The optimum pH values for aerobic biodegradation were reported by about 6-7. Moreover, the reaction solution did not require pH adjustment. The less consumed buffer and electrode corrosion in the proposed system could lead to higher operational costs. The AC can cause dipole-dipole interactions [52]. Generally, the electrolysis of water similarly occurs at pH 7. Accordingly, the produced OH− ions are transferred to the anode surface through the AC cycles that decrease with the lower frequency. The OH− ions produce •OH radicals and contribute to the oxidation process. As previously mentioned, steel wool and carbon cloth electrodes were applied in the present study. Based on Xray fluorescence (XRF) analysis, the highest ions released from steel wool are iron which can biologically and chemically affect the degradation mechanism of phenol [53]. In this study, the optimum frequency was 5 Hz in which phenol oxidation might be mostly carried out by electrode ionic content and then result in biostimulation. The pH value can be correspondingly influenced by production of some organic acids and other byproducts [48]. As depicted in Fig. 5, the obtained
12
results with the AC revealed that the pH values are almost similar to the control pH values. Likewise, the pH values of phenol biodegradation induced by DC were higher than AC. Similar studies had further demonstrated that the pH variations were from 7.09 to 4.36 in an aerobic bioreactor induced by more than 10 mA DC. Applying phosphate-buffered saline solution was proposed to reduce the harmful effects of high DC [50]. The most recent study by authors had confirmed a slight increase in the pH value in the electrochemical process for RB19 dye removal using AC. It could be due to generation of •OH radicals [53]. Fig. 6 illustrates the pH value of the reaction solution under the following operating conditions: C/N ratio of 0.5-6, the applied EC of 1 mA, at 25℃. During the study of C/N ratio variations, the pH values were adjusted at 7. It was reported that the removal efficiency of p-nitrophenol, as one of the phenol derivatives, was considerable at pH 3 due to a decrease in oxygen evolution reaction at lower pH values [54]. The electrical conductivity values fluctuated between 2 and 3 mS/cm during various C/N ratios. The highest electrical conductivity values were similarly at high phenol concentrations, indicating that the ion concentration may increase electrical conductivity.
4. Conclusion
Sludge production could be controlled by several parameters. The C/N ratio was considered as one of the main parameters affecting biomass and subsequently sludge production. According to the results, a higher MLSS reduction could occur through applying an AC in the low C/N ratio. The low C/N ratio seemed to be more favorable for microbial growth limitation and better biodegradation of phenol as a carbon source. In addition, it was taken into account as one of the critical factors affecting sludge production minimization. In the present study, the minimization of
13
hazardous sludge production was achieved using the bioelectrochemical system supplied by an alternating current electric field. Moreover, sludge management including collection, transformation, disposal, as well as operational and treatment costs could become easier and more cost-effective in wastewater treatment plants.
Acknowledgement The authors gratefully acknowledge the financial and technical support provided by Tarbiat Modares University, Tehran, Iran.
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Figure and Table captions
20
Schematic 1: The schematic of bioreactor; 1- Plexiglas bioreactor, 2- Steel wool electrode, 3Carbon cloth electrode, 4- Magnet, 5- Stirrer, 6- Alternating current (AC) function generator, 7Air pump. Fig. 1. (A) MLSS concentrations in the bioelectrochemical system during the experimental periods, and (B) comparison of the average of MLSS between bioelectrochemical system induced by alternating current and control bioreactor Fig. 2. Variations of sludge production and sludge yield during various utilized C/N ratios( Sludge production/ mg TSS/gr COD,
Sludge yield/mg VSS/gr COD)
Fig. 3. Carbon-to-nitrogen ratios for the average phenol removal efficiency during various times (frequency
=
5
Hz
and
applied
voltage
=
0.4
Vpp)
(
) Fig. 4. The variation of energy consumption and phenol removal efficiency versus volume ratio of sludge (
Removal Efficiency/%,
Energy consumption/kWh Kg-1,
Energy
consumption/kWh L-1 ) Fig. 5. The pH values and removal efficiency of the solution during the experiments
(
)
Fig. 6. The pH and EC variations with different C/N ratios at 25℃ and applied current of 1 mA (
C/N ratio,
EC/mS/cm,
pH)
Table 1. Components of culture medium
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Table 1. The components of culture medium
Components
Concentration (g L-1)
CaCl2
0.3
KH2PO4
0.1
Na2HPO4
0.15
NH4Cl
0.1
NaHCO3
0.6
MgSO4. 7H2O
0.3
FeCl2
0.2
High lights - A new approach for sludge minimization has been presented. - Bioelectrochemical system supplied by alternating current was utilized for sludge minimization - Reduction of initially mixed liquor suspended solid was 99.5 mg L-1 - C/N ratio 1 has the lowest biomass production
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
Zohreh Moghiseh, Abbas Rezaee, Somayyeh Dehghani