- Email: [email protected]
A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater
Bioresource Technology xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater Baolin Hou, Hongjun Han ⇑, Haifeng Zhuang, Peng Xu, Shengyong Jia, Kun Li State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SAC-Fe developed from sewage/iron
sludge served as CPEs and catalyst in 3D EF. 3D EF exhibited excellent capacity in abating COLOR and toxicity and improving biodegradability. The enhancement of pollutants removal in 3D EF attributed to generating more H2O2 and OH. The total operating cost of the integrated process was 1.1 CNY/t. The integration of 3D EF and BAC was more efficient at shorter retention time.
a r t i c l e
i n f o
Article history: Received 13 June 2015 Received in revised form 19 July 2015 Accepted 20 July 2015 Available online xxxx Keywords: Biologically pretreated Lurgi coal gasification wastewater Three-dimensional electro-Fenton Catalytic particle electrodes Reaction mechanism discussion Biological activated carbon
a b s t r a c t A novel integrated process with three-dimensional electro-Fenton (3D EF) and biological activated carbon (BAC) was employed in advanced treatment of biologically pretreated Lurgi coal gasification wastewater. SAC-Fe (sludge deserved activated carbon from sewage and iron sludge) and SAC (sludge deserved activated carbon) were used in 3D EF as catalytic particle electrodes (CPEs) and in BAC as carriers respectively. Results indicated that 3D EF with SAC-Fe as CPEs represented excellent pollutants and COLOR removals as well as biodegradability improvement. The efficiency enhancement attributed to generating more H2O2 and OH. The integrated process exhibited efficient performance of COD, BOD5, total phenols, TOC, TN and COLOR removals at a much shorter retention time, with the corresponding concentrations in effluent of 31.18, 6.69, 4.29, 17.82, 13.88 mg/L and <20 times, allowing discharge criteria to be met. The integrated system was efficient, cost-effective and ecological sustainable and could be a promising technology for engineering applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Lurgi coal gasification wastewater (LCGW) is discharged in the process of coal gas purification, the composition of which is very complex. Although a series of options are employed, a large ⇑ Corresponding author.
number of toxic and refractory compounds as well as their derivatives are still residual in the effluent of biologically pretreated LCGW (Zhuang et al., 2014a). However, with the implementation of the increasingly stringent environmental regulations, the quality of secondary effluent is unable to satisfy the discharge standards, especially the requirement of zero liquid discharge, due to the high concentrations of organic matter, ammonia and COLOR. Hitherto,
E-mail address: [email protected] (H. Han). http://dx.doi.org/10.1016/j.biortech.2015.07.068 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
2
B. Hou et al. / Bioresource Technology xxx (2015) xxx–xxx
more efforts are essential to be focused on advanced treatment of biologically pretreated LCGW. Recently, advanced oxidation processes (AOPs) have drawn amazing attention for organic pollutants treatment and many efforts have been studied on the treatment of this wastewater (Jia et al., 2015; Xu et al., 2015). As one of AOPs, EF has drawn considerable attention as a promissory alternative technology by overcoming some drawbacks of traditional Fenton process. EF employs electrochemical reactions to generate in situ Fenton reagents from two-electron reduction of oxygen. Since OH production does not involve the use of harmful chemicals, this process is environmentally friendly. 3D EF significantly improved treatment efficiency and current efficiency by involving catalytic particle electrodes (CPEs) (Kong et al., 2006). Meanwhile, an increasing amount of sewage sludge generated from wastewater treatment plant has become an issue of particular concern. Recently, sludge derived carbon has been widely used as an adsorbent for organic pollutants or heavy metals (Phuengprasop et al., 2011). Previous studies have demonstrated that the sewage sludge based activated carbon could serve as an efficient and stable catalyst support for Fenton and catalytic ozonation (Zhuang et al., 2014b). Iron sludge generated in traditional Fenton process, Fe/C micro-electrolysis process and other physico-chemical methods, which mainly constituted of iron and organic compounds. The concept ‘‘using waste to treat waste’’ nowadays becomes more attractive in the field of environmental engineering, offering a promising strategy for the utilization of sewage sludge and iron sludge. Iron sludge can serve as the iron source for Fenton catalyst preparation. Additionally, although 3D EF oxidation has been proven to be an alternative method for the treatment of toxic and recalcitrant organic pollutions, it consumes a lot of energy and the general current efficiency is low (Anotai et al., 2006). Pretreatment of wastewater with poor biodegradability and high toxicity using 3D EF to improve biodegradability for further biological process may be more reasonable to avoid high energy consumption. Therefore, it is of great advantages to integrate 3D EF with biological process as a more efficient and cost-effective process. BAC involved activated carbon in biological reactor to combine adsorption and biodegradation (Reungoat et al., 2012). Hitherto, the integrated 3D EF–BAC system for the advanced treatment of biologically pretreated LCGW has not been reported in the literature yet. In the present study, a novel CPEs (SAC-Fe) developed for sewage sludge and iron sludge was applied in 3D EF oxidation of biologically pretreated LCGW and its electrocatalytic performance was investigated. The pollutants removal and biodegradability enhancement in 3D EF were evaluated. Meanwhile, the possible mechanism of SAC-Fe in 3D EF was discussed. Furthermore, the performance of the integrated process with 3D EF and BAC was evaluated and this novel integrated system exhibited substantial advantages in eliminating pollutants and shortening the hydraulic retention time.
2. Methods 2.1. Materials The real biologically pretreated LCGW used in this study was collected from the effluent of secondary settling tank in the full-scale wastewater treatment facility in Harbin, China. The main characteristics of the wastewater were as follows: COD 140– 190 mg/L, BOD5/COD 0.05–0.09, COLOR 300–400 times, total phenols 80–120 mg/L, total organic carbon (TOC) 90–125 mg/L, NH+4-N 10–15 mg/L, total nitrogen (TN) 45–70 mg/L. The pH ranged between 7.0 and 8.0.
The dewatered sewage sludge sample used in this study was collected from the Wenchang wastewater treatment plant in Harbin, China. Iron sludge was obtained from a pilot industrial wastewater treatment plant (Harbin, China) setup with the combined process of Fe/C micro-electrolysis and Fenton oxidation. The CPEs of sludge deserved carbon with iron species (SAC-Fe) were synthesized from the pre-mixture of sewage sludge and iron sludge, followed by pyrolysis. The sludge deserved carbon (SAC) was prepared without the addition of iron sludge. The preparation processes were according to the methods in the previous reports (Gu et al., 2012; Zhuang et al., 2014b). Fe3O4 was the main component in SAC-Fe to be acted as catalyst (catalytic site) and chemical bonds were formed between Fe species and carbon matrix, according to the analytical results of XRD and FTIR (Figs. S1 and S2). The main characteristics of SAC-Fe were as follows: 351.6 m2/g of BET area, 0.258 cm3/g of macro and mesopores volumes, 3.614 nm of average pore size, 15.43% of Fe, 5.99% of Si and 2.86% of Al. The detail properties of the CPEs were listed in Table S1.
2.2. Experimental procedures 3D EF reaction was conducted in a one-compartment electrochemical cell (1.0 L) at room temperature. Ti/SnO2 and active carbon fiber (4 5 cm) were the anode and cathode, both electrodes were fixed on two plastic brackets with the distance between the electrodes of 5 cm. CPEs were filled into real biologically pretreated LCGW and shaken for 48 h to achieve adsorption equilibrium. Then the adsorption saturated CPEs were transferred into electrolysis cell between anode and cathode. The amount of CPEs varied from 2.5 to 10 g/L to ascertain the optimal CPEs dosage. The degradation reaction was initialed by switching on the DC current. Current was adjusted with the increasing current density of 5, 10, 15 and 20 mA/cm2. Air was bubbled from the bottom of the reactor (4 L/min) to provide oxygen and generate stirring in the solution. The supernatant of 3D EF was subsequently fed to the BAC system for the further purification. The prepared SAC was filled in BAC as carriers. The start-up and operational strategies of BAC system were described by previous literatures (Yapsakli and Çeçen, 2010; Kalkan et al., 2011) and has been operated for one month. The schematic diagram of the integrated process was shown in Fig. S3. The pH was adjusted with H2SO4 (1 mol/L) and NaOH (1 mol/L).
2.3. Analytical methods BET area was determined using Micromeritics ASAP 2020 via nitrogen adsorption. Microspores volume was calculated by t-plot method and the macro(meso)pores volume as well as pore diameter were calculated using the Barrett–Joyner–Halenda method. Elemental analysis was carried out on an X-ray fluorescence spectra (XRF) (1800, Shimadzu) and an Elemental Analyzer (Elementar Vario EL III). COD, BOD5, NH+4-N and total phenols were measured according to Standard Methods (APHA, 1998). COLOR was measured by dilution multiple method (Zhang et al., 2014). TOC and TN were measured with a TOC Analyzer (TOC-CPN, Shimadzu, Japan). The generation of hydroxyl radicals (OH) was monitored by means of terephthalic acid fluorescent probe method on RF-6500 fluorescence spectrometer. Hydrogen peroxide generated in the solution was measured with iodide method. The results were average of at least three measurements with an accuracy of ±5%.
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
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B. Hou et al. / Bioresource Technology xxx (2015) xxx–xxx
3. Results and discussion 3.1. Effect of CPEs on 3D EF oxidation of biologically pretreated LCGW Fig. 1a reveals the effect of CPEs dosage on treatment performance in 3D EF. TOC removal efficiency increased significantly with the addition of SAC-Fe. TOC removal efficiencies were 13.32%, 41.28% and 60.23% for 120 min treatment when the amounts of SAC-Fe were 0, 2.5 and 5.0 g/L respectively. The addition of CPEs increased the electrode surface area by dozens of times and therefore accelerated and promoted the EF reactions and degradation processes. However, TOC removal efficiency only increased 4.08% when SAC-Fe dosage increased from 5.0 to 10 g/L, which was possibly related to the formation of short-circuit current caused by excessive SAC-Fe. Thus, CPEs amount of 5.0 g/L was reasonable and selected in the following tests. Current was the power of EF reactions and current density affected the TOC removal dramatically (Fig. 1b). TOC removal efficiency increased from 46.44% to 60.23% when current density rose from 5.0 to 10 mA/cm2. Treatment performance improved slightly when increasing the current density higher than 10 mA/cm2, causing lower current efficiency. From the viewpoint of economy and energy saving, current density of 10 mA/cm2 was a tradeoff between excellent treatment performance and cost. TOC removal efficiencies were 67.13%, 65.86%, 63.54% and 60.23% at pH 3.0, 4.5, 6.0 and 7.5 respectively. Effect of pH on TOC removal efficiency in this 3D EF was inconspicuous. TOC removal efficiency decreased
70
TOC removal efficiency (%)
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0 mg/L 2.5 mg/L 5 mg/L 7.5 mg/L 10 mg/L
50 40
3.2. Reaction mechanism discussion
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COD / total phenols (mg/L)
(b)
In order to understand the promotion effect of SAC-Fe in 3D EF, amounts of electro-generated H2O2 were measured. Fluor photometer was employed to detect the generation of OH to further investigate the possible role of OH in the 3D EF. As can be seen from Fig. 3a, with the addition of CPEs, amount of electro-generated H2O2 increased significantly. The corresponding amounts of H2O2 were 433.3 and 737.8 lmol/L in EF without and without SAC addition as CPEs (60 min), which demonstrated that the high efficient pollutants removal in 3D EF was mainly related to the more amount of H2O2 generation. As shown in Fig. 3b, 3D
0.5
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120 100
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BOD 5 /COD
(a)
only 6.9% when pH increased from 3.0 to 7.5. This behavior was related to the fact that effects of pH on Fenton reactants were attenuated in this 3D EF system. The precipitation of iron never occurred in this heterogeneous system, and the effect of H2O2 stability deterioration with increasing alkalinity was weakened since H2O2 was continuously electro-generated on the surface of CPEs (SAC-Fe) and maintained an almost constant concentration. The pH of the raw influent (7.0–8.0) was in the efficient pH range of 3D EF. Moreover, the adjustment of pH would cause extra cost in engineering practice. Thus, the pH of the raw influent was employed in this study. COD and total phenols were removed distinctly in the 3D EF (Fig. 2). More than a half of COD was removed in the 3D EF after 120 min. It was noteworthy that total phenols (dominant refractory pollutants in the wastewater) were degraded to a large extent. Total phenols removal efficiency achieved to 87.7% after 120 min EF oxidation and the concentration of total phenols declined to 50.67 mg/L after 60 min. It had been reported that total phenols concentration below 60 mg/L was beneficial to the further biological treatment process (Zhuang et al., 2014a). Notably, BOD5/COD rose dramatically after the treatment in 3D EF, demonstrating the significant improvement of biodegradability of the treated wastewater. It was commonly acknowledged that wastewater with BOD5/COD value higher than 0.4 was considered to be suitable for biological treatment (Esplugas et al., 2004). The BOD5/COD value increased from 0.08 to 0.45 for 60 min reaction and to 0.51 after 100 min treatment. Under met requirements of the subsequent biological process, the reaction time could be shortened to 60 min, saving 50% of energy consumption. Therefore, 3D EF with the addition of SAC-Fe as CPEs had a dual benefit to reduce the toxic pollutants and improve biodegradability in shorter reaction time. In addition, COLOR removal efficiency was higher than 90%, with COLOR in the effluent of about 20 times.
0.2
60 40
0.1
20 0
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Time (min) Fig. 1. Effects of CPEs dosage and current density on treatment performance. (Error bars represent standard deviation of triplicate tests.)
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0.0
Time (min) Fig. 2. Variations of COD, total phenols and BOD5/COD in 3D EF. (Error bars represent standard deviation of triplicate tests.)
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
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3.3. Performance of integrated system of 3D EF and BAC in treating biologically pretreated LCGW
800
(a)
2D 3D with SAC
Considerable amounts of toxic and inhibitory compounds were residual in the biologically pretreated LCGW, which had adverse effect on the microorganism in the followed biological process. Furthermore, the biodegradability of biologically pretreated LCGW was poor which was difficult for further purification with biological options. Fig. 4a shows that effluent TOC was 43.27 mg/L in 24 h in BAC alone and further prolonging retention time had little promotion effect on pollutants removal. However, after the pretreatment with 3D EF for 60 min, pollutants were rapidly removed in the integrated process at a much shorter retention time of 9.0 h, with the corresponding TOC in the effluent of 17.82 mg/L. Additionally, as can be seen from Fig. 4b, the removal efficiencies of COD, total phenols, TN and COLOR were only 45.82%, 40.26%, 49.97% and 46.28% in BAC process alone, while the corresponding removal efficiencies promoted to 82.72%, 94.14%, 78.78% and 95.26% respectively. As for COD and total phenols removals, 3D EF accounted for 50.81% and 54.45% while followed BAC contributed to the rest 31.91% and 39.69% (data not shown). The high removal efficiencies of COD and COLOR in integrated process revealed that most toxic compounds and chromatic groups with N@N, C@O et al. could be oxided in 3D EF and further eliminated in the following biological process. The concentrations of COD, BOD5, NH+4-N, TN, total phenols and COLOR in the effluent were 31.18, 6.96, 1.16, 13.88, 4.29 mg/L and <20 times respectively, which all met class-I criteria of the Integrated Wastewater
C(H 2O2 ) (µmol/L)
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reaction system with SAC-Fe as CPEs generated more OH than with SAC and without CPEs, which was in consist with the observed degradation. Since the catalytic site of Fenton reaction was immobilized on the solid CPEs, decomposition of H2O2 was almost completely surface-catalyzed process. Based on the above study, the possible reaction mechanism involved in 3D EF with SAC-Fe was proposed. The reaction was possibility initialed by the carbon adsorption. Firstly an abundance of active O2 and pollutants were enriched in diverse size pores and kept a high local concentration. Then, active O2 was in situ reduced to H2O2 (Fig. S4). Meanwhile, the electro-generated H2O2 molecules would be catalytically decomposed to OH promptly by embedded iron oxide on the surface and internal of SAC-Fe. The pollutants were degraded by OH into intermediates or complete mineralization. Since pollutants were enriched and maintained at high concentration on SAC-Fe due to adsorption, these pollutants in situ captured OH and improved OH utilization. Abundant iron species embedded in porous SAC-Fe with high exposure endowed more catalytic active sites (Fig. S5). When the H2O2 was electro-generated on the CPEs, it was in situ decomposed promptly at the numerous catalytic sites, which resulted in the OH yield remarkably increasing. In addition, some inorganic components (SiO2 and Al2O3) in the catalyst support of sewage sludge-derived activated carbon exhibited co-catalytic effect for Fenton or photo-Fenton reaction (Tu et al., 2012). Thus, SAC-Fe in 3D EF promoted both the H2O2 generation and OH production.
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Fig. 3. H2O2 detection and fluorescence test for radicals measurement. (Error bars represent standard deviation of triplicate tests.)
BAC Integrated process
100
COD Total phenols TN COLOR
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0
BAC
Integrated process
Fig. 4. Comparison of the integrated process and BAC alone. (Error bars represent standard deviation of triplicate tests.)
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
B. Hou et al. / Bioresource Technology xxx (2015) xxx–xxx
Discharge Standard (GB18918-2002, China). The operating cost of the proposed advanced treatment principally comprised of the costs of electricity, reagents, utility and labor, which were 0.5, 0.2, 0.3 and 0.1 CNY/t respectively. The total operating cost was 1.1 CNY/t. More importantly, since SAC-Fe and SAC applied in 3D EF and BAC were developed from sewage sludge and iron sludge, this novel integrated process was efficient, cost-effective and ecological sustainable for wastewater treatment, catering to the concept of ‘‘using waste to treat waste’’.
4. Conclusions A novel process integrating 3D EF and BAC was successfully applied in advanced treatment of real biologically pretreated LCGW. 3D EF represented excellent capacity in abating pollutants, toxicity and COLOR as well as improving biodegradability by generating more H2O2 and OH. The integrated system was more effective to remove COD, TOC, TN, total phenols and COLOR at a much shorter retention time of 9.0 h (1.0 for 3D EF and 8.0 for BAC). Thus, the integrated process of 3D EF and BAC could serve as a promising method with efficient, economical and ecological sustainable advantages for advanced treatment of wastewater.
Acknowledgements This work was supported by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2015DX02).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.07. 068.
5
References Anotai, J., Lu, M.-C., Chewpreecha, P., 2006. Kinetics of aniline degradation by Fenton and electro-Fenton processes. Water Res. 40, 1841–1847. APHA, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, American Water Works Association, Water Environment Federation, Washington, DC. Esplugas, S., Contreras, S., Ollis, D., 2004. Engineering aspects of the integration of chemical and biological oxidation: simple mechanistic models for the oxidation treatment. J. Environ. Eng. 130, 967–974. Gu, L., Zhu, N., Zhou, P., 2012. Preparation of sludge derived magnetic porous carbon and their application in Fenton-like degradation of 1-diazo-2-naphthol-4sulfonic acid. Bioresour. Technol. 118, 638–642. Jia, S., Han, H., Zhuang, H., Xu, P., Hou, B., 2015. Advanced treatment of biologically pretreated coal gasification wastewater by a novel integration of catalytic ultrasound oxidation and membrane bioreactor. Bioresour. Technol. 189, 426– 429. Kalkan, Ç., Yapsakli, K., Mertoglu, B., Tufan, D., Saatci, A., 2011. Evaluation of Biological Activated Carbon (BAC) process in wastewater treatment secondary effluent for reclamation purposes. Desalination 265, 266–273. Kong, W., Wang, B., Ma, H., Gu, L., 2006. Electrochemical treatment of anionic surfactants in synthetic wastewater with three-dimensional electrodes. J. Hazard. Mater. B137, 1532–1537. Phuengprasop, T., Sittiwong, J., Unob, F., 2011. Removal of heavy metal ions by iron oxide coated sewage sludge. J. Hazard. Mater. 186, 502–507. Reungoat, J., Escher, B.I., Macova, M., Argaud, F.X., Gernjak, W., Keller, J., 2012. Ozonation and biological activated carbon filtration of wastewater treatment plant effluents. Water Res. 46, 863–872. Tu, Y., Tian, S., Kong, L., Xiong, Y., 2012. Co-catalytic effect of sewage sludge-derived char as the support of Fenton-like catalyst. Chem. Eng. J. 185&186, 44–51. Xu, P., Han, H., Hou, B., Zhuang, H., Jia, S., Wang, D., Li, K., Zhao, Q., 2015. The feasibility of using combined TiO2 photocatalysis oxidation and MBBR process for advanced treatment of biologically pretreated coal gasification wastewater. Bioresour. Technol. 189, 417–420. Yapsakli, K., Çeçen, F., 2010. Effect of type of granular activated carbon on DOC biodegradation in biological activated carbon filters. Process Biochem. 45, 355– 362. Zhang, S., Zheng, J., Chen, Z., 2014. Combination of ozonation and biological aerated filter (BAF) for bio-treated coking wastewater. Sep. Purif. Technol. 132, 610– 615. Zhuang, H., Han, H., Jia, S., Zhao, Q., Hou, B., 2014a. Advanced treatment of biologically pretreated coal gasification wastewater using a novel anoxic moving bed biofilm reactor. Bioresour. Technol. 157, 223–230. Zhuang, H., Han, H., Hou, B., Jia, S., Zhao, Q., 2014b. Heterogeneous catalytic ozonation of biologically pretreated Lurgi coal gasification wastewater using sewage sludge based activated carbon supported manganese and ferric oxides as catalysts. Bioresour. Technol. 166, 178–186.
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Short Communication
A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater Baolin Hou, Hongjun Han ⇑, Haifeng Zhuang, Peng Xu, Shengyong Jia, Kun Li State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
SAC-Fe developed from sewage/iron
sludge served as CPEs and catalyst in 3D EF. 3D EF exhibited excellent capacity in abating COLOR and toxicity and improving biodegradability. The enhancement of pollutants removal in 3D EF attributed to generating more H2O2 and OH. The total operating cost of the integrated process was 1.1 CNY/t. The integration of 3D EF and BAC was more efficient at shorter retention time.
a r t i c l e
i n f o
Article history: Received 13 June 2015 Received in revised form 19 July 2015 Accepted 20 July 2015 Available online xxxx Keywords: Biologically pretreated Lurgi coal gasification wastewater Three-dimensional electro-Fenton Catalytic particle electrodes Reaction mechanism discussion Biological activated carbon
a b s t r a c t A novel integrated process with three-dimensional electro-Fenton (3D EF) and biological activated carbon (BAC) was employed in advanced treatment of biologically pretreated Lurgi coal gasification wastewater. SAC-Fe (sludge deserved activated carbon from sewage and iron sludge) and SAC (sludge deserved activated carbon) were used in 3D EF as catalytic particle electrodes (CPEs) and in BAC as carriers respectively. Results indicated that 3D EF with SAC-Fe as CPEs represented excellent pollutants and COLOR removals as well as biodegradability improvement. The efficiency enhancement attributed to generating more H2O2 and OH. The integrated process exhibited efficient performance of COD, BOD5, total phenols, TOC, TN and COLOR removals at a much shorter retention time, with the corresponding concentrations in effluent of 31.18, 6.69, 4.29, 17.82, 13.88 mg/L and <20 times, allowing discharge criteria to be met. The integrated system was efficient, cost-effective and ecological sustainable and could be a promising technology for engineering applications. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Lurgi coal gasification wastewater (LCGW) is discharged in the process of coal gas purification, the composition of which is very complex. Although a series of options are employed, a large ⇑ Corresponding author.
number of toxic and refractory compounds as well as their derivatives are still residual in the effluent of biologically pretreated LCGW (Zhuang et al., 2014a). However, with the implementation of the increasingly stringent environmental regulations, the quality of secondary effluent is unable to satisfy the discharge standards, especially the requirement of zero liquid discharge, due to the high concentrations of organic matter, ammonia and COLOR. Hitherto,
E-mail address: [email protected] (H. Han). http://dx.doi.org/10.1016/j.biortech.2015.07.068 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
2
B. Hou et al. / Bioresource Technology xxx (2015) xxx–xxx
more efforts are essential to be focused on advanced treatment of biologically pretreated LCGW. Recently, advanced oxidation processes (AOPs) have drawn amazing attention for organic pollutants treatment and many efforts have been studied on the treatment of this wastewater (Jia et al., 2015; Xu et al., 2015). As one of AOPs, EF has drawn considerable attention as a promissory alternative technology by overcoming some drawbacks of traditional Fenton process. EF employs electrochemical reactions to generate in situ Fenton reagents from two-electron reduction of oxygen. Since OH production does not involve the use of harmful chemicals, this process is environmentally friendly. 3D EF significantly improved treatment efficiency and current efficiency by involving catalytic particle electrodes (CPEs) (Kong et al., 2006). Meanwhile, an increasing amount of sewage sludge generated from wastewater treatment plant has become an issue of particular concern. Recently, sludge derived carbon has been widely used as an adsorbent for organic pollutants or heavy metals (Phuengprasop et al., 2011). Previous studies have demonstrated that the sewage sludge based activated carbon could serve as an efficient and stable catalyst support for Fenton and catalytic ozonation (Zhuang et al., 2014b). Iron sludge generated in traditional Fenton process, Fe/C micro-electrolysis process and other physico-chemical methods, which mainly constituted of iron and organic compounds. The concept ‘‘using waste to treat waste’’ nowadays becomes more attractive in the field of environmental engineering, offering a promising strategy for the utilization of sewage sludge and iron sludge. Iron sludge can serve as the iron source for Fenton catalyst preparation. Additionally, although 3D EF oxidation has been proven to be an alternative method for the treatment of toxic and recalcitrant organic pollutions, it consumes a lot of energy and the general current efficiency is low (Anotai et al., 2006). Pretreatment of wastewater with poor biodegradability and high toxicity using 3D EF to improve biodegradability for further biological process may be more reasonable to avoid high energy consumption. Therefore, it is of great advantages to integrate 3D EF with biological process as a more efficient and cost-effective process. BAC involved activated carbon in biological reactor to combine adsorption and biodegradation (Reungoat et al., 2012). Hitherto, the integrated 3D EF–BAC system for the advanced treatment of biologically pretreated LCGW has not been reported in the literature yet. In the present study, a novel CPEs (SAC-Fe) developed for sewage sludge and iron sludge was applied in 3D EF oxidation of biologically pretreated LCGW and its electrocatalytic performance was investigated. The pollutants removal and biodegradability enhancement in 3D EF were evaluated. Meanwhile, the possible mechanism of SAC-Fe in 3D EF was discussed. Furthermore, the performance of the integrated process with 3D EF and BAC was evaluated and this novel integrated system exhibited substantial advantages in eliminating pollutants and shortening the hydraulic retention time.
2. Methods 2.1. Materials The real biologically pretreated LCGW used in this study was collected from the effluent of secondary settling tank in the full-scale wastewater treatment facility in Harbin, China. The main characteristics of the wastewater were as follows: COD 140– 190 mg/L, BOD5/COD 0.05–0.09, COLOR 300–400 times, total phenols 80–120 mg/L, total organic carbon (TOC) 90–125 mg/L, NH+4-N 10–15 mg/L, total nitrogen (TN) 45–70 mg/L. The pH ranged between 7.0 and 8.0.
The dewatered sewage sludge sample used in this study was collected from the Wenchang wastewater treatment plant in Harbin, China. Iron sludge was obtained from a pilot industrial wastewater treatment plant (Harbin, China) setup with the combined process of Fe/C micro-electrolysis and Fenton oxidation. The CPEs of sludge deserved carbon with iron species (SAC-Fe) were synthesized from the pre-mixture of sewage sludge and iron sludge, followed by pyrolysis. The sludge deserved carbon (SAC) was prepared without the addition of iron sludge. The preparation processes were according to the methods in the previous reports (Gu et al., 2012; Zhuang et al., 2014b). Fe3O4 was the main component in SAC-Fe to be acted as catalyst (catalytic site) and chemical bonds were formed between Fe species and carbon matrix, according to the analytical results of XRD and FTIR (Figs. S1 and S2). The main characteristics of SAC-Fe were as follows: 351.6 m2/g of BET area, 0.258 cm3/g of macro and mesopores volumes, 3.614 nm of average pore size, 15.43% of Fe, 5.99% of Si and 2.86% of Al. The detail properties of the CPEs were listed in Table S1.
2.2. Experimental procedures 3D EF reaction was conducted in a one-compartment electrochemical cell (1.0 L) at room temperature. Ti/SnO2 and active carbon fiber (4 5 cm) were the anode and cathode, both electrodes were fixed on two plastic brackets with the distance between the electrodes of 5 cm. CPEs were filled into real biologically pretreated LCGW and shaken for 48 h to achieve adsorption equilibrium. Then the adsorption saturated CPEs were transferred into electrolysis cell between anode and cathode. The amount of CPEs varied from 2.5 to 10 g/L to ascertain the optimal CPEs dosage. The degradation reaction was initialed by switching on the DC current. Current was adjusted with the increasing current density of 5, 10, 15 and 20 mA/cm2. Air was bubbled from the bottom of the reactor (4 L/min) to provide oxygen and generate stirring in the solution. The supernatant of 3D EF was subsequently fed to the BAC system for the further purification. The prepared SAC was filled in BAC as carriers. The start-up and operational strategies of BAC system were described by previous literatures (Yapsakli and Çeçen, 2010; Kalkan et al., 2011) and has been operated for one month. The schematic diagram of the integrated process was shown in Fig. S3. The pH was adjusted with H2SO4 (1 mol/L) and NaOH (1 mol/L).
2.3. Analytical methods BET area was determined using Micromeritics ASAP 2020 via nitrogen adsorption. Microspores volume was calculated by t-plot method and the macro(meso)pores volume as well as pore diameter were calculated using the Barrett–Joyner–Halenda method. Elemental analysis was carried out on an X-ray fluorescence spectra (XRF) (1800, Shimadzu) and an Elemental Analyzer (Elementar Vario EL III). COD, BOD5, NH+4-N and total phenols were measured according to Standard Methods (APHA, 1998). COLOR was measured by dilution multiple method (Zhang et al., 2014). TOC and TN were measured with a TOC Analyzer (TOC-CPN, Shimadzu, Japan). The generation of hydroxyl radicals (OH) was monitored by means of terephthalic acid fluorescent probe method on RF-6500 fluorescence spectrometer. Hydrogen peroxide generated in the solution was measured with iodide method. The results were average of at least three measurements with an accuracy of ±5%.
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
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3. Results and discussion 3.1. Effect of CPEs on 3D EF oxidation of biologically pretreated LCGW Fig. 1a reveals the effect of CPEs dosage on treatment performance in 3D EF. TOC removal efficiency increased significantly with the addition of SAC-Fe. TOC removal efficiencies were 13.32%, 41.28% and 60.23% for 120 min treatment when the amounts of SAC-Fe were 0, 2.5 and 5.0 g/L respectively. The addition of CPEs increased the electrode surface area by dozens of times and therefore accelerated and promoted the EF reactions and degradation processes. However, TOC removal efficiency only increased 4.08% when SAC-Fe dosage increased from 5.0 to 10 g/L, which was possibly related to the formation of short-circuit current caused by excessive SAC-Fe. Thus, CPEs amount of 5.0 g/L was reasonable and selected in the following tests. Current was the power of EF reactions and current density affected the TOC removal dramatically (Fig. 1b). TOC removal efficiency increased from 46.44% to 60.23% when current density rose from 5.0 to 10 mA/cm2. Treatment performance improved slightly when increasing the current density higher than 10 mA/cm2, causing lower current efficiency. From the viewpoint of economy and energy saving, current density of 10 mA/cm2 was a tradeoff between excellent treatment performance and cost. TOC removal efficiencies were 67.13%, 65.86%, 63.54% and 60.23% at pH 3.0, 4.5, 6.0 and 7.5 respectively. Effect of pH on TOC removal efficiency in this 3D EF was inconspicuous. TOC removal efficiency decreased
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In order to understand the promotion effect of SAC-Fe in 3D EF, amounts of electro-generated H2O2 were measured. Fluor photometer was employed to detect the generation of OH to further investigate the possible role of OH in the 3D EF. As can be seen from Fig. 3a, with the addition of CPEs, amount of electro-generated H2O2 increased significantly. The corresponding amounts of H2O2 were 433.3 and 737.8 lmol/L in EF without and without SAC addition as CPEs (60 min), which demonstrated that the high efficient pollutants removal in 3D EF was mainly related to the more amount of H2O2 generation. As shown in Fig. 3b, 3D
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only 6.9% when pH increased from 3.0 to 7.5. This behavior was related to the fact that effects of pH on Fenton reactants were attenuated in this 3D EF system. The precipitation of iron never occurred in this heterogeneous system, and the effect of H2O2 stability deterioration with increasing alkalinity was weakened since H2O2 was continuously electro-generated on the surface of CPEs (SAC-Fe) and maintained an almost constant concentration. The pH of the raw influent (7.0–8.0) was in the efficient pH range of 3D EF. Moreover, the adjustment of pH would cause extra cost in engineering practice. Thus, the pH of the raw influent was employed in this study. COD and total phenols were removed distinctly in the 3D EF (Fig. 2). More than a half of COD was removed in the 3D EF after 120 min. It was noteworthy that total phenols (dominant refractory pollutants in the wastewater) were degraded to a large extent. Total phenols removal efficiency achieved to 87.7% after 120 min EF oxidation and the concentration of total phenols declined to 50.67 mg/L after 60 min. It had been reported that total phenols concentration below 60 mg/L was beneficial to the further biological treatment process (Zhuang et al., 2014a). Notably, BOD5/COD rose dramatically after the treatment in 3D EF, demonstrating the significant improvement of biodegradability of the treated wastewater. It was commonly acknowledged that wastewater with BOD5/COD value higher than 0.4 was considered to be suitable for biological treatment (Esplugas et al., 2004). The BOD5/COD value increased from 0.08 to 0.45 for 60 min reaction and to 0.51 after 100 min treatment. Under met requirements of the subsequent biological process, the reaction time could be shortened to 60 min, saving 50% of energy consumption. Therefore, 3D EF with the addition of SAC-Fe as CPEs had a dual benefit to reduce the toxic pollutants and improve biodegradability in shorter reaction time. In addition, COLOR removal efficiency was higher than 90%, with COLOR in the effluent of about 20 times.
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Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
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3.3. Performance of integrated system of 3D EF and BAC in treating biologically pretreated LCGW
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Considerable amounts of toxic and inhibitory compounds were residual in the biologically pretreated LCGW, which had adverse effect on the microorganism in the followed biological process. Furthermore, the biodegradability of biologically pretreated LCGW was poor which was difficult for further purification with biological options. Fig. 4a shows that effluent TOC was 43.27 mg/L in 24 h in BAC alone and further prolonging retention time had little promotion effect on pollutants removal. However, after the pretreatment with 3D EF for 60 min, pollutants were rapidly removed in the integrated process at a much shorter retention time of 9.0 h, with the corresponding TOC in the effluent of 17.82 mg/L. Additionally, as can be seen from Fig. 4b, the removal efficiencies of COD, total phenols, TN and COLOR were only 45.82%, 40.26%, 49.97% and 46.28% in BAC process alone, while the corresponding removal efficiencies promoted to 82.72%, 94.14%, 78.78% and 95.26% respectively. As for COD and total phenols removals, 3D EF accounted for 50.81% and 54.45% while followed BAC contributed to the rest 31.91% and 39.69% (data not shown). The high removal efficiencies of COD and COLOR in integrated process revealed that most toxic compounds and chromatic groups with N@N, C@O et al. could be oxided in 3D EF and further eliminated in the following biological process. The concentrations of COD, BOD5, NH+4-N, TN, total phenols and COLOR in the effluent were 31.18, 6.96, 1.16, 13.88, 4.29 mg/L and <20 times respectively, which all met class-I criteria of the Integrated Wastewater
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reaction system with SAC-Fe as CPEs generated more OH than with SAC and without CPEs, which was in consist with the observed degradation. Since the catalytic site of Fenton reaction was immobilized on the solid CPEs, decomposition of H2O2 was almost completely surface-catalyzed process. Based on the above study, the possible reaction mechanism involved in 3D EF with SAC-Fe was proposed. The reaction was possibility initialed by the carbon adsorption. Firstly an abundance of active O2 and pollutants were enriched in diverse size pores and kept a high local concentration. Then, active O2 was in situ reduced to H2O2 (Fig. S4). Meanwhile, the electro-generated H2O2 molecules would be catalytically decomposed to OH promptly by embedded iron oxide on the surface and internal of SAC-Fe. The pollutants were degraded by OH into intermediates or complete mineralization. Since pollutants were enriched and maintained at high concentration on SAC-Fe due to adsorption, these pollutants in situ captured OH and improved OH utilization. Abundant iron species embedded in porous SAC-Fe with high exposure endowed more catalytic active sites (Fig. S5). When the H2O2 was electro-generated on the CPEs, it was in situ decomposed promptly at the numerous catalytic sites, which resulted in the OH yield remarkably increasing. In addition, some inorganic components (SiO2 and Al2O3) in the catalyst support of sewage sludge-derived activated carbon exhibited co-catalytic effect for Fenton or photo-Fenton reaction (Tu et al., 2012). Thus, SAC-Fe in 3D EF promoted both the H2O2 generation and OH production.
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Fig. 4. Comparison of the integrated process and BAC alone. (Error bars represent standard deviation of triplicate tests.)
Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068
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Discharge Standard (GB18918-2002, China). The operating cost of the proposed advanced treatment principally comprised of the costs of electricity, reagents, utility and labor, which were 0.5, 0.2, 0.3 and 0.1 CNY/t respectively. The total operating cost was 1.1 CNY/t. More importantly, since SAC-Fe and SAC applied in 3D EF and BAC were developed from sewage sludge and iron sludge, this novel integrated process was efficient, cost-effective and ecological sustainable for wastewater treatment, catering to the concept of ‘‘using waste to treat waste’’.
4. Conclusions A novel process integrating 3D EF and BAC was successfully applied in advanced treatment of real biologically pretreated LCGW. 3D EF represented excellent capacity in abating pollutants, toxicity and COLOR as well as improving biodegradability by generating more H2O2 and OH. The integrated system was more effective to remove COD, TOC, TN, total phenols and COLOR at a much shorter retention time of 9.0 h (1.0 for 3D EF and 8.0 for BAC). Thus, the integrated process of 3D EF and BAC could serve as a promising method with efficient, economical and ecological sustainable advantages for advanced treatment of wastewater.
Acknowledgements This work was supported by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2015DX02).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.07. 068.
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Please cite this article in press as: Hou, B., et al. A novel integration of three-dimensional electro-Fenton and biological activated carbon and its application in the advanced treatment of biologically pretreated Lurgi coal gasification wastewater. Bioresour. Technol. (2015), http://dx.doi.org/10.1016/ j.biortech.2015.07.068