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On the degradability of printing and dyeing wastewater by wet air oxidation
PII: S0043-1354(00)00481-4
Wat. Res. Vol. 35, No. 8, pp. 2078–2080, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
RESEARCH NOTE ON THE DEGRADABILITY OF PRINTING AND DYEING WASTEWATER BY WET AIR OXIDATION XIJUN HU*, LECHENG LEI, GUOHUA CHEN and PO LOCK YUE Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong (First received 13 January 2000; accepted in revised form 6 September 2000) Abstract}A modified first-order kinetics model was used to study the wet air oxidation of printing and dyeing wastewater. The model simulations are in good agreement with experimental data. The results indicate that a certain fraction of organic pollutants in the printing and dyeing wastewater could not be removed even at elevated temperature and prolonged reaction time. The ratio of degradable organic matter is found independent of temperature and can be improved by using a catalyst. # 2001 Elsevier Science Ltd. All rights reserved Key words}wet oxidation, wastewater treatment, catalyst
INTRODUCTION
The textile wastewater discharged from printing and dyeing processes is characterized by high chemical oxygen demand (COD), low biochemical oxygen demand (BOD), and heavy colour. It is one of the major sources of pollutants in the textile industry. In particular, the COD and colour of the wastewater are resistant to conventional wastewater treatment. Wet air oxidation (WAO) has been shown to be a feasible method to convert the organic pollutants into water and carbon dioxide at elevated temperatures and pressures (Randall and knopp, 1980; Skaates et al., 1981; Dietrich et al., 1985; Levec, 1990; Mantzavinos et al., 1996). Since it can achieve very high conversion rates, the wet air oxidation process typically requires much less space. Furthermore, no additional sludge or concentrated waste is produced as in the case of biological processes. WAO has been demonstrated to be a viable process for the treatment of desizing, scouring, dyeing and printing wastewater from the textile industry (Lei et al., 1997, 1998, 2000). WAO requires high temperatures (about 3008C) and high pressures (over 10 MPa), to achieve a high COD removal within a reasonable time scale. A suitable catalyst can be added to reduce the reaction temperature and pressure (Chu et al., 1998; Hu et al., 1999). Because of the very stable structure of dyes, the first-order reaction kinetics model commonly used in *Author to whom all correspondence should be addressed. Tel.: +852-2358-7134; fax: +852-2358-0054; e-mail: [email protected]
the literature (Mishra et al., 1995; Ingale et al., 1996; Lei et al., 2000) does not fit our experimental data with the printing and dyeing wastewater. In this note, the first-order reaction kinetics model is modified by adding a fraction of non-oxidizable organics and used to study the WAO of printing and dyeing wastewater. THEORY
There are many different organic compounds in the printing and dyeing wastewater including various dyestuffs. In this study, the total organic carbon (TOC) is used to represent the organic concentration in the printing and dyeing wastewater. The mass transfer resistance is negligible because oxygen is in excess, as demonstrated by Mantzavinos et al. (1996) and Lei et al. (2000). By assuming that the WAO follows the first-order reaction, we have dY ¼ kY dt with the initial condition as
t ¼ 0;
Y ¼ Y0 ;
ð1Þ
ð2Þ
where Y and Y0 are the oxidizable organic concentrations at reaction time t and zero, t is reaction time and k is the rate constant, which follows the Arrhenius equation: k ¼ k0 eE=RT
ð3Þ
with k0 being the pre-exponential factor, E the activated energy, R the gas constant and T the temperature.
2078
Degradability of wastewater by wet oxidation
The solution for equation (1) is kt
Y ¼ Y0 e
:
ð4Þ
There are some components in the printing and dyeing wastewater which are non-oxidizable by WAO. Let a be the fraction of oxidizable organic among TOC, we have a ¼ Y0 =TOC0 ;
ð5Þ
where TOC0 is the TOC value at reaction time zero. The TOC value at any time is TOC ¼ TOC0 ð1 aÞ þ TOC0 aekt :
ð6Þ
The total removal of TOC at any time is TOC TOC TOC0 ¼1 TOCi TOC0 TOCi TOC0 ¼ 1 1 a þ aekt ; TOCi
Z¼1
ð7Þ
where the initial TOC value of the wastewater, TOCi, is different from TOC0 because of the thermal decomposition of wastewater during the heating up period (Lei et al., 2000).
EXPERIMENTAL
WAO experiments were carried out in a 2-l autoclave equipped with a cooling coil and a magnetic stirring system. The equipment description and its operation procedures were available in our previous work (Lei et al., 1997). The wastewater was collected directly from the printing and dyeing process of one textile company in Hong Kong. It has a COD value of 11100 mg/l, TOC of 3204 mg/l, BOD5 of 440 mg/l and pH of 6.6.
2079
of oxygen required to completely oxidize the organics in the wastewater. The experimental data are presented as symbols and the model fitting using the least-square method as solid lines. The TOC removal has a non-zero value at t ¼ 0. This is so because there is some thermal decomposition of wastewater during the heating up process (Lei et al., 2000). The thermal decomposition is more significant at a higher temperature. It can be seen in Fig. 1 that the model fits the experimental data well. The extracted parameters are listed in Table 1. The fraction of oxidizable organic in the printing and dyeing wastewater, a, is about 0.35 and nearly independent of temperature. This means that only about 35% of the organics can be oxidized after the thermal decomposition. Increasing the temperature can accelerate the oxidation speed but is unable to increase the fraction of organics oxidized. The extracted reaction rate constants at various temperatures are then used to obtain the activated energy of the oxidation reaction, which is found to be 43.7 kJ/mol (Fig. 2). The activated energy is much larger than 25 kJ/mol, a value where mass transfer resistance can be ignored (Satterfield, 1991; Shende and Mahajani, 1994). This together with the excess oxygen supplied fully support the model assumption that the WAO of printing and dyeing wastewater is under kinetics control. Because the conversion of organics in the printing and dyeing wastewater cannot be enhanced by increasing temperature (constant a), some catalysts are tried to improve the oxidation rate. Figure 3 shows the catalytic WAO experimental data (symbols) of printing and dyeing wastewater at 2008C and
RESULTS AND DISCUSSION
Figure 1 shows the TOC removal of printing and dyeing wastewater by WAO at four different temperatures, 150, 200, 250, and 3008C. The oxygen partial pressure is 2.65 MPa (at a reference temperature of 2008C), which is twice the theoretical amount
Fig. 1. TOC removal of printing and dyeing wastewater by WAO: (*) 1508C; (&) 2008C; (4) 2508C; (5) 3008C.
Table 1. Kinetic parameters of wet air oxidation at different temperatures T (8C)
150
200
250
300
a k (min1)
0.340 0.0020
0.340 0.0066
0.347 0.0255
0.369 0.0468
Fig. 2. Dependence of rate constant on temperature.
2080
Xijun Hu et al.
matter. However, the fraction of oxidizable organics of wastewater can be improved by adding a catalyst. Acknowledgements}The authors are grateful to the Industry and Technology Development Council of Hong Kong for its financial support for this research.
REFERENCES
Fig. 3. Effect of catalysts on the TOC removal of printing and dyeing wastewater by WAO.
an oxygen partial pressure of 2.65 MPa. The catalysts used are Cu(NO3)2 and CuO with a Cu2+ concentration of 200 mg/l. Both catalysts can significantly increase the oxidation rate and achieve better TOC removals. The model fitting is also plotted in Fig. 3 as lines. Again the model is in excellent agreement with experimental data. The kinetic parameters, a and k, are 0.434 and 0.0559/min for Cu(NO3)2, and 0.435 and 0.0492/min for CuO, respectively, compared with the values of 0.34 and 0.0066/min when no catalyst is present. The catalysts can enhance both the fraction of oxidizable organics and the reaction rate.
CONCLUSIONS
A modified first-order kinetics model has been used to study the wet air oxidation of printing and dyeing wastewater. The model fits the experimental data well. Because of the highly stable structure of some dyes, a certain portion of organics in the wastewater cannot be oxidized even at elevated temperature and prolonged reaction time. Although increasing the temperature can accelerate the oxidation speed it cannot increase the ratio of degradable organic
Chu H. P., Lei L., Hu X. and Yue P. L. (1998) Metalloorganic chemical vapor deposition (MOCVD) for the development of heterogeneous catalyst. Energy Fuels 12, 1108–1113. Dietrich M. J., Randall T. L. and Canney P. J. (1985) Wet air oxidation of hazardous organics in wastewater. Environ. Prog. 4, 171–177. Hu X., Lei L., Chu H. P. and Yue P. L. (1999) Copper/ activated carbon as catalyst for organic wastewater treatment. Carbon 37, 631–637. Ingale M. N., Joshi J. B., Mahajani V. V. and Gada M. K. (1996) Waste treatment of an aqueous waste stream from a cyclohexane oxidation unit: a case study. Trans. IchemE 74, 265–272. Lei L., Hu X., Chen G., Porter J. F. and Yue P. L. (2000) Wet air oxidation of desizing wastewater from the textile industry. Ind. Eng. Chem. Res. 39, 2896–2901. Lei L., Hu X., Chu H. P. and Yue P. L. (1997) Catalytic wet air oxidation of dyeing and desizing wastewater. Water Sci. Technol. 65, 311–319. Lei L., Hu X. and Yue P. L. (1998) Improved wet oxidation for the treatment of dyeing wastewater concentrate from membrane separation process. Water Res. 32, 2753–2759. Levec J. (1990) Catalytic oxidation of toxic organic in aqueous solution. Appl. Catal. 63, L1–L5. Mantzavinos D., Livingston A. G., Hellenbrand R. and Metcalfe I. S. (1996) Wet air oxidation of polyethylene glycols: mechanisms, intermediates and implications for integrated chemical-biological wastewater treatment. Chem. Engng. Sci. 51, 4219–4236. Mishra V. S., Mahajani V. V. and Joshi J. B. (1995) Wet air oxidation. Ind. Engng. Chem. Res. 34, 2–48. Randall T. L. and Knopp P. V. (1980) Detoxification of specific organic substances by wet oxidation. J. WPCF 52, 2117–2129. Satterfield C. N. (1991) Heterogeneous Catalysis in Industrial Practice. McGraw-Hill, New York. Shende R. V. and Mahajani V. V. (1994) Kinetics of wet air oxidation of glyoxalic acid and oxalic acid. Ind. Engng. Chem. Res. 33, 3125–3130. Skaates J. M., Briggs B. A., Lamparter R. A. and Baillod C. R. (1981) Wet oxidation of glucose. Can. J. Chem. Engng. 59, 517–521.
Wat. Res. Vol. 35, No. 8, pp. 2078–2080, 2001 # 2001 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/01/$ - see front matter
RESEARCH NOTE ON THE DEGRADABILITY OF PRINTING AND DYEING WASTEWATER BY WET AIR OXIDATION XIJUN HU*, LECHENG LEI, GUOHUA CHEN and PO LOCK YUE Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong (First received 13 January 2000; accepted in revised form 6 September 2000) Abstract}A modified first-order kinetics model was used to study the wet air oxidation of printing and dyeing wastewater. The model simulations are in good agreement with experimental data. The results indicate that a certain fraction of organic pollutants in the printing and dyeing wastewater could not be removed even at elevated temperature and prolonged reaction time. The ratio of degradable organic matter is found independent of temperature and can be improved by using a catalyst. # 2001 Elsevier Science Ltd. All rights reserved Key words}wet oxidation, wastewater treatment, catalyst
INTRODUCTION
The textile wastewater discharged from printing and dyeing processes is characterized by high chemical oxygen demand (COD), low biochemical oxygen demand (BOD), and heavy colour. It is one of the major sources of pollutants in the textile industry. In particular, the COD and colour of the wastewater are resistant to conventional wastewater treatment. Wet air oxidation (WAO) has been shown to be a feasible method to convert the organic pollutants into water and carbon dioxide at elevated temperatures and pressures (Randall and knopp, 1980; Skaates et al., 1981; Dietrich et al., 1985; Levec, 1990; Mantzavinos et al., 1996). Since it can achieve very high conversion rates, the wet air oxidation process typically requires much less space. Furthermore, no additional sludge or concentrated waste is produced as in the case of biological processes. WAO has been demonstrated to be a viable process for the treatment of desizing, scouring, dyeing and printing wastewater from the textile industry (Lei et al., 1997, 1998, 2000). WAO requires high temperatures (about 3008C) and high pressures (over 10 MPa), to achieve a high COD removal within a reasonable time scale. A suitable catalyst can be added to reduce the reaction temperature and pressure (Chu et al., 1998; Hu et al., 1999). Because of the very stable structure of dyes, the first-order reaction kinetics model commonly used in *Author to whom all correspondence should be addressed. Tel.: +852-2358-7134; fax: +852-2358-0054; e-mail: [email protected]
the literature (Mishra et al., 1995; Ingale et al., 1996; Lei et al., 2000) does not fit our experimental data with the printing and dyeing wastewater. In this note, the first-order reaction kinetics model is modified by adding a fraction of non-oxidizable organics and used to study the WAO of printing and dyeing wastewater. THEORY
There are many different organic compounds in the printing and dyeing wastewater including various dyestuffs. In this study, the total organic carbon (TOC) is used to represent the organic concentration in the printing and dyeing wastewater. The mass transfer resistance is negligible because oxygen is in excess, as demonstrated by Mantzavinos et al. (1996) and Lei et al. (2000). By assuming that the WAO follows the first-order reaction, we have dY ¼ kY dt with the initial condition as
t ¼ 0;
Y ¼ Y0 ;
ð1Þ
ð2Þ
where Y and Y0 are the oxidizable organic concentrations at reaction time t and zero, t is reaction time and k is the rate constant, which follows the Arrhenius equation: k ¼ k0 eE=RT
ð3Þ
with k0 being the pre-exponential factor, E the activated energy, R the gas constant and T the temperature.
2078
Degradability of wastewater by wet oxidation
The solution for equation (1) is kt
Y ¼ Y0 e
:
ð4Þ
There are some components in the printing and dyeing wastewater which are non-oxidizable by WAO. Let a be the fraction of oxidizable organic among TOC, we have a ¼ Y0 =TOC0 ;
ð5Þ
where TOC0 is the TOC value at reaction time zero. The TOC value at any time is TOC ¼ TOC0 ð1 aÞ þ TOC0 aekt :
ð6Þ
The total removal of TOC at any time is TOC TOC TOC0 ¼1 TOCi TOC0 TOCi TOC0 ¼ 1 1 a þ aekt ; TOCi
Z¼1
ð7Þ
where the initial TOC value of the wastewater, TOCi, is different from TOC0 because of the thermal decomposition of wastewater during the heating up period (Lei et al., 2000).
EXPERIMENTAL
WAO experiments were carried out in a 2-l autoclave equipped with a cooling coil and a magnetic stirring system. The equipment description and its operation procedures were available in our previous work (Lei et al., 1997). The wastewater was collected directly from the printing and dyeing process of one textile company in Hong Kong. It has a COD value of 11100 mg/l, TOC of 3204 mg/l, BOD5 of 440 mg/l and pH of 6.6.
2079
of oxygen required to completely oxidize the organics in the wastewater. The experimental data are presented as symbols and the model fitting using the least-square method as solid lines. The TOC removal has a non-zero value at t ¼ 0. This is so because there is some thermal decomposition of wastewater during the heating up process (Lei et al., 2000). The thermal decomposition is more significant at a higher temperature. It can be seen in Fig. 1 that the model fits the experimental data well. The extracted parameters are listed in Table 1. The fraction of oxidizable organic in the printing and dyeing wastewater, a, is about 0.35 and nearly independent of temperature. This means that only about 35% of the organics can be oxidized after the thermal decomposition. Increasing the temperature can accelerate the oxidation speed but is unable to increase the fraction of organics oxidized. The extracted reaction rate constants at various temperatures are then used to obtain the activated energy of the oxidation reaction, which is found to be 43.7 kJ/mol (Fig. 2). The activated energy is much larger than 25 kJ/mol, a value where mass transfer resistance can be ignored (Satterfield, 1991; Shende and Mahajani, 1994). This together with the excess oxygen supplied fully support the model assumption that the WAO of printing and dyeing wastewater is under kinetics control. Because the conversion of organics in the printing and dyeing wastewater cannot be enhanced by increasing temperature (constant a), some catalysts are tried to improve the oxidation rate. Figure 3 shows the catalytic WAO experimental data (symbols) of printing and dyeing wastewater at 2008C and
RESULTS AND DISCUSSION
Figure 1 shows the TOC removal of printing and dyeing wastewater by WAO at four different temperatures, 150, 200, 250, and 3008C. The oxygen partial pressure is 2.65 MPa (at a reference temperature of 2008C), which is twice the theoretical amount
Fig. 1. TOC removal of printing and dyeing wastewater by WAO: (*) 1508C; (&) 2008C; (4) 2508C; (5) 3008C.
Table 1. Kinetic parameters of wet air oxidation at different temperatures T (8C)
150
200
250
300
a k (min1)
0.340 0.0020
0.340 0.0066
0.347 0.0255
0.369 0.0468
Fig. 2. Dependence of rate constant on temperature.
2080
Xijun Hu et al.
matter. However, the fraction of oxidizable organics of wastewater can be improved by adding a catalyst. Acknowledgements}The authors are grateful to the Industry and Technology Development Council of Hong Kong for its financial support for this research.
REFERENCES
Fig. 3. Effect of catalysts on the TOC removal of printing and dyeing wastewater by WAO.
an oxygen partial pressure of 2.65 MPa. The catalysts used are Cu(NO3)2 and CuO with a Cu2+ concentration of 200 mg/l. Both catalysts can significantly increase the oxidation rate and achieve better TOC removals. The model fitting is also plotted in Fig. 3 as lines. Again the model is in excellent agreement with experimental data. The kinetic parameters, a and k, are 0.434 and 0.0559/min for Cu(NO3)2, and 0.435 and 0.0492/min for CuO, respectively, compared with the values of 0.34 and 0.0066/min when no catalyst is present. The catalysts can enhance both the fraction of oxidizable organics and the reaction rate.
CONCLUSIONS
A modified first-order kinetics model has been used to study the wet air oxidation of printing and dyeing wastewater. The model fits the experimental data well. Because of the highly stable structure of some dyes, a certain portion of organics in the wastewater cannot be oxidized even at elevated temperature and prolonged reaction time. Although increasing the temperature can accelerate the oxidation speed it cannot increase the ratio of degradable organic
Chu H. P., Lei L., Hu X. and Yue P. L. (1998) Metalloorganic chemical vapor deposition (MOCVD) for the development of heterogeneous catalyst. Energy Fuels 12, 1108–1113. Dietrich M. J., Randall T. L. and Canney P. J. (1985) Wet air oxidation of hazardous organics in wastewater. Environ. Prog. 4, 171–177. Hu X., Lei L., Chu H. P. and Yue P. L. (1999) Copper/ activated carbon as catalyst for organic wastewater treatment. Carbon 37, 631–637. Ingale M. N., Joshi J. B., Mahajani V. V. and Gada M. K. (1996) Waste treatment of an aqueous waste stream from a cyclohexane oxidation unit: a case study. Trans. IchemE 74, 265–272. Lei L., Hu X., Chen G., Porter J. F. and Yue P. L. (2000) Wet air oxidation of desizing wastewater from the textile industry. Ind. Eng. Chem. Res. 39, 2896–2901. Lei L., Hu X., Chu H. P. and Yue P. L. (1997) Catalytic wet air oxidation of dyeing and desizing wastewater. Water Sci. Technol. 65, 311–319. Lei L., Hu X. and Yue P. L. (1998) Improved wet oxidation for the treatment of dyeing wastewater concentrate from membrane separation process. Water Res. 32, 2753–2759. Levec J. (1990) Catalytic oxidation of toxic organic in aqueous solution. Appl. Catal. 63, L1–L5. Mantzavinos D., Livingston A. G., Hellenbrand R. and Metcalfe I. S. (1996) Wet air oxidation of polyethylene glycols: mechanisms, intermediates and implications for integrated chemical-biological wastewater treatment. Chem. Engng. Sci. 51, 4219–4236. Mishra V. S., Mahajani V. V. and Joshi J. B. (1995) Wet air oxidation. Ind. Engng. Chem. Res. 34, 2–48. Randall T. L. and Knopp P. V. (1980) Detoxification of specific organic substances by wet oxidation. J. WPCF 52, 2117–2129. Satterfield C. N. (1991) Heterogeneous Catalysis in Industrial Practice. McGraw-Hill, New York. Shende R. V. and Mahajani V. V. (1994) Kinetics of wet air oxidation of glyoxalic acid and oxalic acid. Ind. Engng. Chem. Res. 33, 3125–3130. Skaates J. M., Briggs B. A., Lamparter R. A. and Baillod C. R. (1981) Wet oxidation of glucose. Can. J. Chem. Engng. 59, 517–521.