Advanced treatment of biologically pretreated coal gasification wastewater by a novel integration of heterogeneous catalytic ozonation and biological process

Bioresource Technology xxx (2014) xxx–xxx

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Short Communication

Advanced treatment of biologically pretreated coal gasification wastewater by a novel integration of heterogeneous catalytic ozonation and biological process Haifeng Zhuang, Hongjun Han ⇑, Shengyong Jia, Baolin Hou, Qian Zhao 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  Sewage sludge was converted into sludge based activated carbon (SBAC).  MnOx were loaded on SBAC to serve as catalyst (MnOx/SBAC) for catalytic ozonation.  MnOx/SBAC significantly improved the performance of pollutants removal in ozonation.  The catalytic ozonation process (COP) effluent was more biodegradable and less toxic.  The integration of COP and ANMBBR–BAF had efficient capacity of pollutants removal.

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Article history: Received 16 March 2014 Received in revised form 5 May 2014 Accepted 7 May 2014 Available online xxxx Keywords: Biologically pretreated coal gasification wastewater Heterogeneous catalytic ozonation Catalyst Mechanism discussion Biological process

a b s t r a c t Advanced treatment of biologically pretreated coal gasification wastewater (CGW) was investigated employing heterogeneous catalytic ozonation integrated with anoxic moving bed biofilm reactor (ANMBBR) and biological aerated filter (BAF) process. The results indicated that catalytic ozonation with the prepared catalyst (i.e. MnOx/SBAC, sewage sludge was converted into sludge based activated carbon (SBAC) which loaded manganese oxides) significantly enhanced performance of pollutants removal by generated hydroxyl radicals. The effluent of catalytic ozonation process was more biodegradable and less toxic than that in ozonation alone. Meanwhile, ANMBBR–BAF showed efficient capacity of pollutants removal in treatment of the effluent of catalytic ozonation at a shorter reaction time, allowing the discharge limits to be met. Therefore, the integrated process with efficient, economical and sustainable advantages was suitable for advanced treatment of real biologically pretreated CGW. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The biologically pretreated coal gasification wastewater (CGW) contains a large number of toxic and refractory compounds, such as phenolic compounds, polynuclear aromatic hydrocarbons and nitrogenous heterocyclic compounds, long-chain hydrocarbons, ammonia, and so on along with low biodegradability and unsatisfactory effluent quality (Zhuang et al., 2014). This wastewater control task has become a bottleneck for the development of coal gasification industry in China which has played a key role in new clean and renewable energy market in recent years (Wang and Han, 2012). Thus, it is very urgent to find an efficient and ⇑ Corresponding author. Address: School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China. Tel.: +86 451 87649777; fax: +86 451 86283082. E-mail address: [email protected] (H. Han).

cost-effective process for advanced treatment of biologically pretreated CGW. Heterogeneous catalytic ozonation process has attracted more and more attentions in recent years due to its efficient capacity in the degradation and mineralization of toxic and refractory compounds (Kasprzyk-Hordern et al., 2003). It was developed to overcome the limitations of ozonation process by catalysts which can promote the decomposition of aqueous ozone to generate hydroxyl radicals (OH). However, these efficient catalysts all have challenges in the technical complexity and high cost of production which limit their full-scale practical application. Meanwhile, the previous studies showed sewage sludge based activated carbon (SBAC) as a efficient catalyst for catalytic wet air oxidation of phenolic compounds and ozonation of oxalic acid (Marques et al., 2011; Wen et al., 2012), which was not only a environmentally beneficial and sustainable sewage sludge disposal method but also reduced the cost of production of catalyst. Furthermore, there is a

http://dx.doi.org/10.1016/j.biortech.2014.05.061 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Zhuang, H., et al. Advanced treatment of biologically pretreated coal gasification wastewater by a novel integration of heterogeneous catalytic ozonation and biological process. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.05.061

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high expectation on SBAC modification by loaded Mn oxides (MnOx, the most reactive metal oxides catalyst) for further improving the catalytic activity and stability of SBAC. However, few studies on using this type of catalyst to enhance the catalytic activity in ozonation of real industrial wastewater have been published. It is noteworthy that catalytic ozonation process (COP) for complete eliminating pollutants is expensive because the oxidation intermediates formed during treatment tend to be more and more resistant to their chemical degradation (Muñoz et al., 2005). However, the oxidation intermediates are generally more biodegradable than the original molecules. Therefore, there is a great advantageous of integrating COP with biological process to attain a more efficient and cost-effective process for treating low biodegradability and high toxicity wastewater. Especially, nitrogen compounds, which were difficult to remove in COP, even the concentration increased by oxidation intermediates formed (Yang et al., 2011), were more suitable for biological treatment. It was reported that anoxic moving bed biofilm reactor (ANMBBR) and biological aerated filter (BAF) process (ANMBBR–BAF) with shortcut biological nitrogen removal (SBNR) was successfully applied to advanced treatment of real biologically pretreated CGW (Zhuang et al., 2014). But, the system still had several problems to be solved, such as overlong hydraulic residence times (12 h of HRT) and interference of toxic compounds (total phenols in excess of 100 mg/L restricted the biodegradation). Thus, the novel integration of COP and ANMBBR–BAF has substantial advantages to solve the difficult problems between them. In the present study, the catalytic activity of MnOx/SBAC in ozonation of raw wastewater was investigated, and then MnOx/SBAC dose and reaction time were optimized. Meanwhile, effects of initial pH and tert-butanol on pollutants removal were examined during COP. Furthermore, the performance of pollutants removal of the integrated COP with ANMBBR–BAF process was evaluated.

2. Methods 2.1. Materials Real biologically pretreated CGW was obtained from the effluent of an upflow anaerobic sludge bed reactor followed by anoxic–aerobic process after ammonia stripping and phenols solvent extraction in the Lurgi coal gasification wastewater treatment plant (China Coal Longhua Harbin Coal Chemical Industry Co., Ltd). The concentrations of the main pollutants of raw wastewater were as follows: 300–350 mg/L of COD, 0.05–0.07 of BOD5/COD value, 120–180 mg/L of total phenols (TP), 100–140 mg/L of total organic carbon (TOC), 60–80 mg/L of total nitrogen (TN) and 30–50 mg/L of NH+4-N. The pH ranged between 6.5 and 7.5. The dewatered sewage sludge was collected from the biological wastewater treatment plant (Harbin, China). The preparation process of SBAC followed the method developed by Wen et al. (2012). Briefly, the sewage sludge was dried at 105 °C for 24 h then ground and sieved into a uniform size of <0.1 mm. Then, a 10 g of sample was impregnated into a 75 ml of 3 mol/L ZnCl2 solution as an activation agent for 24 h at room temperature. When the supernatant liquid was completely removed, the sample was dried at 105 °C and subsequently was pyrolyzed in a muffle furnace where high pure N2 was in-poured for producing the absence of oxygen condition. The furnace temperature was gradually increased at a rate of 18 °C/min and the final temperature of 700 °C maintained for 1 h, prior to cooling in nitrogen gas. After being pyrolyzed, the products were washed with 3.0 mol/L HCl to remove inorganic impurities, then the products were washed with Milli-Q water until constant pH and dried. MnOx/SBAC was prepared by a simple wet impregnation improved technique

(Faria et al., 2009). An amount of SBAC was immersed in Mn nitrate solutions with a desired concentration and the suspension was stirred with 200 rpm for 24 h, and then evaporated in a rotary evaporator at 105 °C for 12 h. After that the MnOx/SBAC was calcined at 600 °C for 3 h in a muffle furnace in the absence of oxygen condition to obtain the required catalyst, and then washed with Milli-Q water to remove the loosely bonded metal irons and dried and stored. The main characteristics of MnOx/SBAC were as follows: 327.5 m2/g of BET area, 0.122 cm3/g of micropores volume, 0.204 cm3/g of macro and mesopores volumes, 3.318 nm of average pore size, 15.23% of Mn, 1.12% of Zn, 0.47% of Fe, 1.54% of Al and 6.58 of pHpzc. 2.2. Experimental procedures The raw wastewater was first added into COP reactor (1.2 L of the effective volume) followed by a continuous input of ozone gas. Ozone was generated using a corona discharge ozone generator with pure oxygen as feed gas (DHX-I, Harbin Jiujiu Electrochemistry Technology Co., Ltd., China). The flow rate of ozone gas was 500 ml/min and ozone gas concentration was 15 mg/L. The off-gas was absorbed by KI solution. In the adsorption test, air, instead of ozone, was introduced into the reactor with all other reaction conditions kept identical. Additionally, the added concentration of tert-butanol (TBA) as scavenger for OH was 100 mg/L. The COP effluent was poured into a sparger equipped for aeration 1 h to remove the remaining ozone and further treated in ANMBBR–BAF system. The composition of bioreactors, start-up and operational strategies of ANMBBR–BAF system were described by Zhuang et al. (2014). The pH was controlled by added NaOH (1 mol/L) and HCl (1 mol/L). 2.3. Analytical methods BET surface area and pore volume of MnOx/SBAC were measured using a surface area and porosity analyzer (ASAP 2020, Micromeritics). The pH at the point of zero charge (pHpzc) was measured with a mass titration method. Samples were gold-coated and observed under a scanning electron microscope (SEM, HITACHI S4800 HSD, Japan). The percentage content of major elements was determined by X-ray fluorescence (XRF) with X-ray spectrometer (AXIOS-PW4400, Holland). COD, BOD5, TP and NH+4-N were measured by Standard Methods (APHA, 1998). TOC and TN were determined with a total organic carbon analyzer (TOC-V, Shimadzu Corporation, Japan). PH values were determined with a pH meter (pHS-3C, Leici, China). The ozone gaseous concentration was measured using the iodometric titration method. Throughout experiments, the withdrawn samples were filtered using 0.45 lm acetic acid fiber filters to separate the catalyst particles prior to analysis and the results were average of at least three measurements with an accuracy of ±5%. 3. Results and discussion 3.1. Effects of catalyst on ozonation of biologically pretreated CGW The addition of MnOx/SBAC significantly enhanced the TP removal and biodegradability of treated wastewater in COP. Fig. 1 shows 83.5% of TP was degraded with ozonation alone in 60 min, the corresponding BOD5/COD value was improved to 0.152. With 1.0 g/L of MnOx/SBAC, the same removal efficiency and BOD5/COD value in COP reached within 30 min. When dosing MnOx/SBAC in the range from 0.0 to 1.0 g/L, the removal efficiency and BOD5/COD value increased faster than that when dosing in the range of 1.0–5.0 g/L. The removal efficiency and BOD5/COD value at 1.0 g/L of MnOx/SBAC were 13.4% and 286.8% higher than that in

Please cite this article in press as: Zhuang, H., et al. Advanced treatment of biologically pretreated coal gasification wastewater by a novel integration of heterogeneous catalytic ozonation and biological process. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.05.061

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ozonation alone. However, no significant differences were observed when the dose of MnOx/SBAC increased from 1.0 to 5.0 g/L. Thus, the optimal amount of catalyst of 1.0 g/L was a tradeoff between efficient catalytic activity and cost. Meanwhile, phenolic compounds had high toxicity over biodegradation which acted as the dominant toxic pollutants in real biologically pretreated CGW (around 51.6% of COD). It had been reported that TP concentration below 100 mg/L was selected as optimum conditions that organic compounds was commonly acceptable for biological treatment process (Zhuang et al., 2014). This level achieved within 30 min in COP. The corresponding BOD5/COD was 0.42 which was considered totally biodegradable (Esplugas et al., 2004). Under met requirements of the subsequent biological process, the reaction time was shortened to 30 min which saved 50% of energy consumption. Therefore, COP with MnOx/SBAC had a dual benefit in that it reduced the toxic pollutants and enhanced biodegradability in shorter reaction time. 3.2. Mechanism discussion As illustrated in Fig. 2, the catalytic activity of MnOx/SBAC significantly was improved with pH increased. When pH increased from 2 to 11, COD removal efficiency was improved by 32.4% and 40.1% in ozonation without and with MnOx/SBAC(1 g/L) in 30 min, respectively. Meanwhile, COD removal by adsorption of

Fig. 2. Effects of initial pH and TBA on the performance of COD and TOC removal. Error bars represent standard deviation of triplicate tests.

MnOx/SBAC dropped (around 15.0%). Especially, at pH higher than 7.0 both catalyst and most of pollutants of raw wastewater were negatively charged (catalyst pHpzc was 6.28 and raw wastewater pH was approximately 7) occurring repulsive forces between them which would inhibit adsorption. In raw wastewater pH, COD removal efficiency was 62.8% with COP in 30 min. In contrast, the adsorption of COD on MnOx/SBAC only accounted for 33.7% of COD. Thus, the improvement of COD removal was attributed to stronger decomposition of ozone into OH rather than adsorption by the catalyst at alkaline conditions (Moussavi and Khosravi, 2012). However, the effect of initial pH on catalytic activity was limited, and COD removal slightly improved in COP (around 12.6%) when pH increased from 7 to 11. In order to further investigation the intervention of OH in COP, the effect of TBA as radical scavenger on TOC removal was performed. Fig. 2 exhibits the presence of TBA affected negatively the catalytic activity of catalyst. TOC removal efficiency was decreased from 26.1% (ozonation alone) and 52.5% (MnOx/SBAC) in 30 min without TBA to 15.2% and 27.8%, respectively, when 100 mg/L of TBA were added under raw wastewater pH. The results indicated the main reaction pathways of COP involved the participation of the highly reactive OH. Compared to COP with SBAC (1 g/L) as catalyst achieved around 30.2% reduction of TOC (data not shown) in raw wastewater, the high dispersion of Mn oxides on the SBAC (Fig. S1) and the multivalence oxidation states were

Please cite this article in press as: Zhuang, H., et al. Advanced treatment of biologically pretreated coal gasification wastewater by a novel integration of heterogeneous catalytic ozonation and biological process. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.05.061

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suggested to be responsible for the higher catalytic activity of MnOx/SBAC. Such as cobalt oxides enhanced mineralization of herbicide 2,4-D by generated OH with electron transferred, which had multivalence oxidation states of cobalt (Hu et al., 2008). Similarly, Mn oxides also existed in mixed valence (+2 and +3 valences). It can be assumed that the presence of such species had a catalytic effect to the generation of OH by electron transferred between loaded Mn oxides and ozone molecules in COP. As regards 1.12% of Zn was found in the bulk of MnOx/SBAC, Wen et al. (2012) had confirmed ZnO and ZnCl2 had no catalytic activity in ozonation of organic matter. Additionally, there were differences between the TP (81.2%) and TOC (52.5%) removal trend, which indicated the most of phenolic compounds were converted into intermediates rather than completely eliminated. 3.3. Biological treatment of the COP effluent The real biologically pretreated CGW consists considerable amounts of toxic and inhibitory compounds with low biodegradability (around 0.06 of BOD5/COD value), which are the most difficult to break down by microorganisms (Padoley et al., 2008). Fig. 3 shows it took about 12 h for the TN to be reduced to below 15 mg/L with raw wastewater in ANMBBR–BAF, which had efficient performance of TN removal with SBNR process, especially under the high toxic loading (around 160 mg/L of TP). But, it was difficult to decrease the TOC to below 20 mg/L in 24 h. In contrast, after pretreating of raw wastewater with COP in 30 min, the biological process reduced remaining TOC (around 57.0 mg/L) of the COP effluent to 17.4 mg/L at a much shorter time of 5.5 h, the corresponding TN decreased to 13.6 mg/L. Moreover, most of the TN (77.3%) was removed in the ANMBBR–BAF which consumed around 25.6% of COD and 17.8% of TP as carbon resource for denitrification. These observations clearly depicted that, COP could degrade the toxic and inhibitory compounds into simple intermediates or completely eliminate, thereby overcoming raw wastewater negative impact on the metabolism of microorganisms. Additionally, the optimum HRT of integrated process was 6 h, further prolonged time did not significantly enhanced pollutants removal. Table S1 summarizes the average removal efficiencies of COD, TOC, NH+4-N, TN and TP were 87.5%, 85.5%, 90.9%, 80.6% and 98.7%, the corresponding effluent concentrations of 37.5, 17.4, 3.9, 13.6 and 1.7 mg/L, respectively,

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which all met class-I criteria of the Integrated Wastewater Discharge Standard (GB18918-2002, China). The results showed that integration of COP and ANMBBR–BAF was an efficient, costeffectively and sustainable process for treating real biologically pretreated CGW with a short retention time. 4. Conclusions A novel integration of COP and ANMBBR–BAF was successfully applied to advanced treatment of biologically pretreated CGW. The results indicated COP with MnOx/SBAC significantly improved performance of pollutants removal by generated OH and the effluent were more biodegradable and less toxic than that in ozonation alone. Meanwhile, ANMBBR–BAF showed efficient capacity of pollutants removal in COP effluent at short reaction time. The results showed the integrated process with efficient and economical advantages was beneficial to engineering application. Acknowledgements This work was supported by Sino-Dutch Research Program (SDRP: 2012–2016) and the independent subject sponsored by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. 2013DX10). 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.2014.05. 061. References 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. Faria, P.C.C., Órfão, J.J.M., Pereira, M.F.R., 2009. Activated carbon and ceria catalysts applied to the catalytic ozonation of dyes and textile effluents. Appl. Catal. B: Environ. 88, 341–350. Hu, C., Xing, S., Qu, J., He, H., 2008. Catalytic ozonation of herbicide 2,4-D over cobalt oxide supported on mesoporous zirconia. J. Phys. Chem. C 112, 5978–5983. Kasprzyk-Hordern, B., Ziólek, M., Nawrocki, J., 2003. Catalytic ozonation and methods of enhancing molecular ozone reactions in water treatment. Appl. Catal. B: Environ. 46, 639–669. Marques, R.R.N., Stüber, F., Smith, K.M., Fabregat, A., Bengoa, C., Font, J., Fortuny, A., Pullket, S., Fowler, G.D., Graham, N.J.D., 2011. Sewage sludge based catalysts for catalytic wet air oxidation of phenol: preparation, characterization and catalytic performance. Appl. Catal. B: Environ. 101, 306–316. Moussavi, G., Khosravi, R., 2012. Preparation and characterization of a biochar from pistachio hull biomass and its catalytic potential for ozonation of water recalcitrant contaminants. Bioresour. Technol. 119, 66–71. Muñoz, I., Rieradevall, J., Torrades, F., Peral, J., Doménech, X., 2005. Environmental assessment of different solar driven advanced oxidation processes. Sol. Energy 79, 369–375. Padoley, K.V., Mudliar, S.N., Pandey, R.A., 2008. Heterocyclic nitrogenous pollutants in the environment and their treatment options – an overview. Bioresour. Technol. 99, 4029–4043. Wang, W., Han, H.J., 2012. Recovery strategies for tackling the impact of phenolic compounds in a UASB reactor treating coal gasification wastewater. Bioresour. Technol. 103, 95–100. Wen, G., Pan, Z.H., Ma, J., Liu, Z.Q., Zhao, L., Li, J.J., 2012. Reuse of sewage sludge as a catalyst in ozonation – efficiency for the removal of oxalic acid and the control of bromate formation. J. Hazard. Mater. 239–240, 381–388. Yang, S., Wang, Q.H., Zhang, T., Li, P., Wu, C.F., 2011. Biological nitrogen removal using the supernatant of ozonized sludge as extra carbon source. Ozone Sci. Eng. 33, 410–416. Zhuang, H.F., Han, H.J., Jia, S.Y., Zhao, Q., Hou, B.L., 2014. Advanced treatment of biologically pretreated coal gasification wastewater using a novel anoxic moving bed biofilm reactor (ANMBBR)-biological aerated filter (BAF) system. Bioresour. Technol. 157, 223–230.

Please cite this article in press as: Zhuang, H., et al. Advanced treatment of biologically pretreated coal gasification wastewater by a novel integration of heterogeneous catalytic ozonation and biological process. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.05.061