Effects of C/N Ratio on Nitrate Removal from PU Synthetic Leather Wastewater Treated by Anoxic Moving Bed Bio-film Reactors

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1 Effects of C/N Ratio on Nitrate Removal from PU Synthetic Leather Wastewater Treated by Anoxic Moving Bed Bio-film Reactors YUANHONG DING 1, *, QING WANG 2, JUN ZHOU 1, HONGQIANG REN 2, *, YANG SONG 1 and YUNJUN YANG 2 1 School of Environment, Qingdao University of Science and Technology, Qingdao China 2 State Key Lab. of pollution control and resources reuse, Nanjing University, Nanjing ,China ABSTRACT: One sample of PU synthetic wastewater was analyzed, its Total Nitrogen (TN) was mainly composed of about 300 mg/l nitrate, and three pieces of anoxic Moving Bed Bio-film Reactors (MBBR) were applied to treat it, one MBBR was run with raw wastewater, the second and third were with synthetic wastewater, and the results were as followings: TN could be removed off completely under the condition of relative low C/N ratio; the C/N ratio of removed pollutants was liner correlation with C/N ratio of influent, but the C/N ratio of effluent was increase violently, the above results demonstrated that, the external carbon source, necessary to the heterotrophic bacteria, was consumed more rapidly than nitrate nitrogen in initial denitrifying stage, then the nitrogen was utilized much more rapidly at later stage, and the ratio of C/N denitrification behaved as a chamfer linear. INTRODUCTION There are more than 2,000 synthetic leather factories located in Southeast China, discharging off 10,000 thousands tons of wastewater per year, in which N,N-Dimethylformamide (DMF) was usually found, it was used for surface modification of PU (poly urethane) synthetic leather as a kind of solvent, and toxic and inhibitive to bacteria s activities [1,2,3]. Many kinds of physic-chemical or biological processes, such as Extraction, Sequencing Batch Reactor, Integrated Anoxic-aerobic Biochemical Process and etc, are applied to treat synthetic leather wastewater [4,5], and these experimental results shows that, some main water quality index parameters, such as CODCr (chemical oxygen demand), SS (suspended solid), Cr and sulfide in effluent, are usually measured with the National Wastewater Discharge Standard (NWDS), but the residual total nitrogen (TN) is hardly up to standard [6]. When Pollutants Disposal Standard of Synthetic Leather Industrial Wastewater (PDSSLIW) GB was carried out since 2008, the demand for residual total nitrogen in effluent is much more stricter than ever before. At present, many old wastewater *Authors to whom correspondence should be addressed. (Ding) (Ren) Tel: treatment process of municipal sewage plants and industrial factories were renovated or updated to satisfy the objective. In theory, the main components of total nitrogen in wastewater treated by anoxic-aerobic biological treatment process, is mainly nitrate nitrogen, then there is little utilizing organic compounds remained in it, therefore, many kinds of physical-chemical ways are carried out to treat it effectively, such as Fe-Mg reduction [7,8], Ion exchange, Reverse Osmosis and Electrolysis [9,10], but all these physical-chemical ways also have some disadvantages on engineering application, such as high cost, secondary pollution and complicated operation control. Then, some innovated biological ways, such as deep-bed filters and etc., are continuously investigated to be used for deep nitrogen removal that is much more preferred than physical-chemical ways for some advantages, such as low operating cost and no secondary pollution [11]. Moving-Bed Biofilm Reactors (MBBR), composed of moving carriers, is regarded as one kind of effective biological method to treat refractory organic industrial wastewater [12], in which the microorganism attached on carriers, does not fall off easily, contacts and degrades with pollutants completely [13,14], it seems also effective to applied anoxic MBBR reactor to treat the synthetic leather wastewater as a kind of refractory Journal of Residuals Science & Technology, Vol. 14, No. 1 January /17/ DEStech Publications, Inc. doi: /issn /14/1/35 305

2 306 C. DING, Q. WANG, J. ZHOU, H. REN, Y. SONG and Y. YANG organic industrial wastewater, and achieve effective denitrification simultaneously [15]. In this experiments, three pieces of anoxic MBBR was applied to treat synthetic leather wastewater with added external carbon source, and this paper was to investigate the relationship between the C/N ratio of wastewater and the removal efficiencies of nitrate, total nitrogen and organics in detail. EXPERIMENTAL Three pieces of anoxic Moving Bed Bio-film Reactors (MBBR) was operated continually in long run, named RUN1, RUN2 and RUN3 respectively, RUN1 and RUN2, were added with external carbon source in influent continuously, RUN3, was run with circulating influent, added with external carbon source at beginning, the specific operational parameters were shown in Table 1. All three MBBR reactors all are made of organic glass, with length 220 mm, breadth 200 mm and height 500 mm. The carriers is made of polyethylene materials, seemed like hollow cylinder, with height 7 mm and diameter 10 mm, its specific surface area is about 500m2/m3, the rate of carrier s filling in MBBR is about 40 percentage, and its tumbling force is supplied by an electric blender. The PU synthetic leather wastewater, sampled from China Hangzhou XINGYE Synthetic Leather Co. Ltd., was formerly biological treated, Table 2 showed the raw water quality parameters from one representative sample, and indicated that, some regular items, such as COD and ammonia could reach to the standard (PDSSLIW GB ), while total nitrogen (TN) did not meet the standards, and nitrate was the main component of TN, as the ratio of COD to TN (named C/N ratio) wasn t appropriate for denitrification of nitrate, then sodium acetate or glucose was added into it as external carbon source for experiments. The measurements of COD, ammonia and nitrate, total nitrogen were conducted according to Standard Methods, one spectrophotometer UV2500 (Shimadzu) was applied. Table 2. One PU Synthetic Wastewater Sample. No Component COD cr TN NO 2 NO 3 RESULTS AND DISCUSSIONS NH 4 + TN Denitrification of Raw Wastewater Cl BOD 5 mg/l ND In RUN1 experiment, the raw wastewater was used as influent, and its total nitrogen was mainly composed of nitrate nitrogen, shown on Table 2. RUN1 was added with sodium acetate and glucose as external carbon source continuously. In initial stage, the concentration of total nitrogen was too high to obtain 50% removal efficiencies, then its concentration was diluted and maintained on 100~300 mg/l, as Figure 1 shows, the removal efficiencies of total nitrogen was increased gradually up to 93.7%, this result indicated that, all kinds of nitrogen, including with nitrate, could be degraded off the wastewater by denitrification in anoxic MBBR. The TN of raw wastewater was mainly composed of nitrate nitrogen, its diluted concentration was maintained from 100 to 300 mg/l, about 72.7% of TN was removed off, and its ratio of C/N was varied from 0.5 to 3.5, about 2.1 averagely, shown on Figure 2. It seemed that denitrification in anoxic MBBR was able to work at lower C/N ratio than generally demanded value of 4~6. The results indicated that, the denitrifying bacteria attached on the carriers in anoxic MBBR, had a potential capacity to degrade nitrate nitrogen at lower C/N ratio. Table 1. Operational Parameters of RUN1, RUN2 and RUN3. Reactor Nitrogen Type External Carbon Source RUN1 nitrate, acetate sodium, glucose RUN2 nitrate, ammonia acetate sodium RUN3 nitrate acetate sodium Figure 1. The removal efficiency of total nitrogen by anoxic MBBR (RUN1).

3 Effects of C/N Ratio on Nitrate Removal from PU Synthetic Leather Wastewater 307 Figure 2. The relationship between the TN removal efficiency and C/N ratio in anoxic MBBR (RUN1). Effects of C/N Ratio on Denitrification of Synthetic Wastewater RUN2 was run to investigate the effects of C/N ratio on the denitrification process, its influent was synthetic wastewater, prepared with sodium nitrate as nitrate nitrogen. When C/N ratio of influent was enhanced from 1.0 to 3.1 (2.2, in average), the removal efficiencies of TN and COD were increased either, in which TN removal efficiency was increased from 74.5% to 97.5% (90.9%, in average) and that of COD was increased from 44.4% to 87.2% (77.0%, in average), Figure 3 showed that, a good performance of denitrification could be obtained by increasing C/N ratio of influent. Figure 4 showed that the C/N ratio of removed pollutants was linear to that of influent, which line slope coefficient was , however the C/N ratio of effluent was increased rapidly, which was increased from 2.8 to 17.1, shown in Figure 5, and bigger than normal range of 4 to 6, it seemed that the nitrogen in influent Figure 4. Relationship between C/N ratio of influent and C/N ratio in removed pollutant (RUN2). was removed effectively, which resulted in an increasing of C/N ratio in effluent. Shown in Figure 5, although the C/N ratio of effluent fluctuated violently, there was a liner relationship between C/N ratio of influent and C/N ratio of removed pollutants, when external carbon was enhanced, the averaged TN removal efficiencies of about 90.9% and removed TN load of mg/l was achieved, Figure 6 showed that, when C/N ratio was increased up to 3.1, the TN concentration of effluent was lower than 8 mg/l, these results indicated that, a good performance of denitrification could be achieved in an anoxic MBBR reactor under condition of lower C/N ratio of influent, it seemed enough for complete denitrificaiton in MBBR. Distribution of Nitrate, Nitrite, Ammonia in Influent and Effluent of Synthetic Wastewater Figure 7 and Table 3 showed that, all the concentrations of nitrate, nitrite and ammonia in effluent was de- Figure 3. Effects of C/N ratio on the removal efficiency of TN and COD (RUN2). Figure 5. C/N ratio of influent, effluent and removed pollutant (RUN2).

4 308 C. DING, Q. WANG, J. ZHOU, H. REN, Y. SONG and Y. YANG Table 3. Averaged Concentrations of Nitrate, Nitrite and Ammonia in Influent and Effluent. Percent of Concentration % Nitrate Nitrite Ammonia Influent Effluent Figure 6. Relationship between C/N ratio of influent and TN concentrations (RUN2). creased, nitrite was changed into gas completely, ammonia was degraded by assimilation in anoxic MBBR, however, nitrate was still the maximum residual component of TN in influent and effluent, for example, the maximum percentage of nitrate in influent and effluent was 78.9% and 64.3% respectively. When C/N ratio of influent was enhanced from 1.6 to 3.3, the removal efficiencies of TN and COD were increased either, Figure 8 showed that, the percentage of residual COD/ Total COD was decrease obviously from 55.6% to 12.7%, this results showed that, although an high C/N ratio demand much external carbon source to be added, the latter could be removed effectively off wastewater in the denitrification process. Variation of C/N and Nitrogen in One Circle Operation of Synthetic Wastewater Curve of C/N ratio of denitrification in one cycle operation was investigated by circularly operated RUN3, with sodium acetate added at beginning, in initial stage, the C/N ratio of mixed liquor was decreased slowly from 4.9 to a minimum ratio 1.9, then it was increased to large ratios 8.3 on tenth days, however the curve of nitrite nitrogen was opposite to that of C/N ratio, for example, on those corresponding point of C/N ratio, the concentration of nitrite was 95 mg/l and 0.9 mg/l respectively. as Figure 9 showed, on tenth days, the concentration of TN was maintained blow 8 mg/l with a maximum ratio of C/N 8.3, while the concentration of residual COD was decrease to a minimum ratio 73.6 mg/l, the results could be explained that, at beginning of denitrification, external carbon source was necessary to the heterotrophic bacteria, and was consumed more rapidly than nitrate nitrogen, so that ratio of C/N seemed high, however, at later stage, nitrogen was utilized much more rapidly than ever before, then the ratio of C/N seemed high either, or much more higher than that of beginning, so that the ratio of C/N denitrification was a chamfer line in this anoxic MBBR, and It take ten days to achieve a good performance of denitrification in one anoxic MBBR. CONCLUSIONS Nitrate was the main component of TN in PU synthetic wastewater, TN could be removed completely Figure 7. Concentrations of nitrate, nitrite and ammonia in influent and effluent (RUN2).

5 Effects of C/N Ratio on Nitrate Removal from PU Synthetic Leather Wastewater 309 Figure 8. Relationship between percentage of residudal COD/Total COD and ratio of C/N (RUN2). Figure 9. Curve of C/N of denitrification in a separate anoxic MBBR (RUN3). with a relatively lower C/N ratio in anoxic MBBR; the C/N ratio of removed pollutants was liner correlation with C/N ratio of influent, but the C/N ratio of effluent was increase violently, it indicated that, in initial stage of denitrification, external carbon source was necessary to the heterotrophic bacteria, and the carbon was consumed more rapidly than nitrate nitrogen, then at later stage, nitrogen was utilized much more rapidly, so that the denitrifying curve of C/N ratio behaved as chamfer linear. ACKNOWLEDGEMENT Authors wish to thank the Foundation of Jiangsu Science and Technology Project (BE ) to support the research, focusing on deep nitrogen removal of synthetic leather wastewater. REFERENCE 1. G.H. Insel, E. Görgün, N. Artan, D. Orhon, Model based optimization of nitrogen removal in a full scale activated sludge plant, Environ. Eng. Sci., 26, 2009, https://doi.org/ /ees A. Mannucci, G. Munz, G. Mori, C. Lubello, Anoxic treatment of vegetable tannery wastewaters: A review, Desalination, 264, 2010, 1 8. https://doi.org/ /j.desal A. Zhang, R. Xu, P.W. Xu, R.Y. Chen,Q. He, J. Zhong, X. H. Gu, Performance study of ZrO 2 ceramic micro-filtration membranes used in pretreatment of DMF wastewater, Desalination, 346, 2014, 1 8. https://doi.org/ /j.desal M. Kolomaznik, I. Adamek, M. Andel, Leather waste potential threat to human health and a new technology of its treatment, J. Hazard. Mater., 160, 2008, https://doi.org/ /j. jhazmat M. Tamal, D. Dalia, M. Subhasis, D. Siddharth, Treatment of leather industry wastewater by aerobic biological and Fenton oxidation process, J. Hazard. Mater., 180, 2010, https://doi.org/ /j. jhazmat L. Giusy, M. Sureyya, E.Z. Gülsüm, O. Derin, Chemical and biological treatment technologies for leather tannery.chemicals and wastewaters: A review, Sci. Total. Environ., 461, 2013, L.N. Shi, J.H. Du, Z.L. Chen, M. Mallavarapu, N. Ravendra, Functional kaolinite supported Fe/Ni nanoparticles for simultaneous catalytic remediation of mixed contaminants (lead and nitrate) from wastewater, J. Colloid. Interf. Sci., 428, 2014, https://doi.org/ /j. jcis J.H. Luo, G.Y. Song, J.Y. Liu, J.R. Qian, Z.P. Xu, Mechanism of enhanced nitrate reduction via micro-electrolysis at the powdered zerovalent iron/activated carbon interface, J. Colloid. Interf. Sci., 435, 2014, https://doi.org/ /j.jcis M.L. Bosko, M.A. Rodrigues, Z.F. Jane, Nitrate reduction of brines from water desalination plants by membrane electrolysis, J. Membrane. Sci., 451, 2014, https://doi.org/ /j.memsci D.H. Xie, C.C. Li, R. Tang, Z.S. Lv, C.H. Wei, C.H. Feng, Ion-exchange membrane bioelectro-chemical reactor for removal of nitrate in the biological effluent from a coking wastewater treatment plant, Electrochem. Commun., 46, 2014, https://doi.org/ /j. elecom J.M. Park, S.H. Kim, D. Ronald, J.S. Cho, J.S. Heo, Enhancement of nitrate removal in constructed wetlands utilizing acombined autotrophic and heterotrophic denitrification technology for treating hydroponic wastewater containing high nitrate and low organic carbon concentrations, Agr. Water. Manage., 162, 2015, https://doi. org/ /j.agwat H. Odegaard, Innovations in wastewater treatment: The moving bed bioreactor, Water. Sci. Technol., 53, 2006, https://doi. org/ /wst T. Leiknes, H. Odegaard, The development of a biofilm membrane bioreactor, Desalination, 2007, 202: https://doi. org/ /j.desal S. Yang, F. Yang, Z. Fu, R. Lei, Comparison between a moving bed membrane bioreactor and a conventional membrane bioreactor on organic carbon and nitrogen removal, Bioresource. Technol., 100, 2009, https://doi.org/ /j.biortech J.C. Leyva, A. Gonzalez, J. Gonzalez, M.M. Munio, J.M. Poyatos, Kinetic modeling and microbiological study of two-step nitrification in a membrane bioreactor and hybrid moving bed biofilm reactor membrane bioreactor for wastewater treatment, Chem. Eng. J., 259, 2015, https://doi.org/ /j.cej