Optimization of Integrated Electro-Bio Process for Bleaching Effluent

Article pubs.acs.org/IECR

Optimization of Integrated Electro-Bio Process for Bleaching Effluent Treatment Soloman Poopana Antony and Balasubramanian Natesan* Department of Chemical Engineering, A.C. Tech Campus, Anna University, Chennai-600 025, India ABSTRACT: The technical and economic feasibility of various electrochemical processes as pretreatment step for biological treatment for the management of kraft bagasse pulp bleach effluent has been studied, and the results are compared with that of the conventional chemical coagulation followed by biological treatment. A modeling methodology for integrating processes involving electrochemical and biological methods is proposed. Optimal combinations of operating variables giving a definite completion of pollutant removal are derived from the empirical models of each process. However, both the electrochemical steps as electrocoagulation and electro-oxidation were observed to perform technically better, and the former was noticeably more attractive based on the overall operating cost. The cost of electrocoagulation−biological (EC-Bio) treatment giving 80% COD removal was determined to be US $0.166/m3. A 41% reduction in the treatment cost was achieved when the conventional chemical coagulation−biological (CC-Bio) methodology replaced by an EC-Bio system. The research revealed the feasibility of electrochemical steps as a primer in the reduction of organic load and improvement in the biodegradability of pulp-bleaching effluent.

1. INTRODUCTION Pure water is fundamental to life and, presently, because of pollution, the availability of pure water has become a limiting resource in many countries. Several resources have been exhausted, and others are likely to be polluted, because of industrial activity, of special impact among the developed or developing countries. Historically, substantial pollution problems have been associated with pulp manufacturing operations. The bleach plant of kraft mills generates a substantial proportion of the total pollutants. Pulp and paper industry is quite old in India, with 406 registered pulp and paper mills, having an average installed capacity of 4.3 million tons per annum.1 Of these, only 34 mills have installed capacity more than 100 tpd (tons per day) and can be called large-scale paper mills; 120 mills have a capacity in the range of 30−100 tpd, and the remaining 252 mills have an installed capacity of 90% will drastically increase the overall operating cost. The performance of biological step is comparatively better than that of EO and EC. It is recommended to operate the system at an overall COD removal of ∼90%, with the conditions of the situation being as follows: current density-EC = 1 A/dm2, time-EC = 34.4 min, and initial pH-EC = 7.68, current density-EC = 1.5 A/dm2, time-EO = 90.5 min, and initial pH-EO = 7.7, durationbio = 521 min, and initial pH-bio = 7.1.

Figure 7. Variation of the performance terms for the optimal profile of operating variables of the combined chemical coagulation−biological system for various stages of completion of COD removal (pHi,Bio = 7.0).

contribution of operating cost of biological process is far less than that of chemical coagulation. It interesting to note that the 8217

dx.doi.org/10.1021/ie3009633 | Ind. Eng. Chem. Res. 2012, 51, 8211−8221

Industrial & Engineering Chemistry Research

Article

Table 4. Comparison of the Performance of the Four Individual Schemes at the Condition of the Maximum COD Removal in the Treatment Domain scheme

COD removal (%)

SSF (kg/(kg COD))

E1 (kWh/(kg COD))

operating cost ($/m3)

PT ($/(kg COD))

Bio EC EO CC

31.51 59.86 53.41 26.7

5.52 1.32 0 1.7

1.36 5.14 26.85 0

0.064 0.48 1.41 0.83

0.164 0.657 2.15 2.53

Table 5. Comparison of the Performance of the Four Combined Schemes at the Condition of the Maximum COD Removal in the Treatment Domain scheme

COD removal (%)

COD removed (mg/L)

SSF (kg/(kg COD))

E1 (kWh/(kg COD))


PT ($/(kg COD))

CC-Bio CC Bio af. CC EC-Bio EC Bio af. EC EO-Bio EO Bio af. EO EC-EO-Bio EC EO af. EC Bio af. EC-EO

58.47 26.69 43.35 85.97 59.85 65.07 87.27 53.08 72.87 94.91 59.21 46.19 76.83

719 328 391 1057 736 321 1073 653 421 1167 728 232 207

2.87 1.7 3.85 2.06 1.33 3.71 1.07 0 2.74 2.54 2.54 0 2.54

0.208 0 0.383 3.686 5.129 0.383 22.056 36.014 0.383 14.856 4.626 59.982 0.354

0.858 0.831 0.027 0.505 0.484 0.022 1.925 1.881 0.044 1.549 0.437 1.112 0.011

1.19 2.53 0.07 0.48 0.66 0.07 1.79 2.88 0.1 1.34 0.6 4.8 0.05

Table 6. Comparison of the Performance of the Four Combined Schemes at the Condition of 80% COD Removal (Outlet COD = 246 mg/L) in the Treatment Domain scheme

COD removal (%)

COD removed (mg/L)

SSF (kg/(kg COD))

E1 (kWh/(kg COD))


PT ($/(kg COD))

EC-Bio EC Bio af. EC EO-Bio EO Bio af. EO EC-EO-Bio EC EO af. EC Bio af. EC-EO

80 47.8 61.69 80 48.94 60.83 80 58.24 22.96 37.84

984 588 396 984 602 382 984 716 118 150

1.75 0.88 3.04 0.96 0 2.47 1.18 1.18 0 1.18

1.03 1.494 0.341 12.104 19.561 0.355 3.809 4.363 4.721 0.442

0.166 0.143 0.023 0.981 0.942 0.039 0.452 0.408 0.045 0.007

0.17 0.24 0.06 1 1.56 0.1 0.47 0.57 0.38 0.05

3.5. Comparison of the Treatment Schemes. The responses of the processes such as specific sludge formation, specific electrode consumption, specific energy consumption, etc. were largely dependent on the type of the treatment and on the completion of the process of pollutant removal. The specific sludge formation increases as the loading of COD removal increases for all the sludge-forming processes as electrocoagulation, chemical coagulation, and biological treatment. The range of variation of the term is 0.5−1.5 kg/(kg of COD removed) for EC, 0.6−2 kg/(kg of COD removed) for CC, and 1.2−6 kg/(kg of COD removed) for biological treatment. A similar phenomenon of dependence on the extent of completion of COD removal is noticed for other terms, such as specific energy consumption (in terms of kWh/(kg COD removed)) and anode dissolution (in terms of kg/(kg COD removed)). Both terms are lower at the early stage of the process, increase steadily, and later suddenly increase as the COD removal approaches a steady-state situation. The range of variation of the specific energy consumption is 0.3−1.4 kWh/ (kg COD removed) for biological process, 0.8−5 kWh/(kg COD removed) for EC, and 6−36 kWh/(kg COD removed) for EO. The dissolution of iron ranges from 0.2−0.8 kg/(kg

COD removed). A comparison of the performance of the four individual schemes at the condition of the maximum COD removal in the treatment domain is presented in Table 4. The completion of COD removal was also different for different types of combined processes. The comparison of the performance of the four combined schemes at the condition of the maximum COD removal in the treatment domain is presented in Table 5. As can be read from Table 5, the maximum COD removal achievable for combined schemes such as CC-bio, EC-bio, EO-bio, and EC-EO-bio are 58.5%, 86%, 87.3%, and 95%, respectively. The cost terms of EC-bio appeared to be the most attractive. In order for an exact comparison between the schemes, the performances of the schemes are derived using the model, for a given (80%) removal COD; the results are presented in Table 6. In the present investigation, 80% overall COD removal gives an outlet COD of 246 mg/L, which is below the tolerance limit for COD insisted by the Indian Pollution Control Board. The cost terms of the EC-bio system appear to be far better than those of the other schemes. This is basically due to the attractive SEC figures of the steps. Electricity cost and coagulant cost contribute >80% of the total operating costs 8218



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tion of 71% of the organic matter available in the EO-treated effluent due to biological treatment for one day is the most attractive result noticed in the tables. The tables show that the untreated effluent is less biodegradable. Moreover, pretreatment due to electrochemical means gives a marked difference in biodegradation, compared to pretreatment by chemical coagulation. 3.7. Estimation of Biokinetic Coefficients. The four cases of pretreated effluents (Table 7) and the untreated effluent case are tested for the biokinetic coefficients using eq 4; the results are presented in Table 9. Moreover, the biodegradability index (BI) and color removal casued by these pretreatments are also presented in Table 9. 3.8. Average Oxidation State. The same samples (Table 7) are tested for total oxygen content (TOC) and, in turn, the average oxidation state (AOS) for further confirmation on the improvement in biodegradability; the results are presented in Table 10. The trend and magnitude of improvement in biodegradability is comparable in both of the confirmatory investigations.

for electro oxidation−biological and chemical coagulation− biological systems, respectively. In the present investigation, the cost of treatment of chemical coagulation is observed to almost double, in comparison to that of electrocoagulation. The performance terms of the CC-Bio system (US $1.19/(kg COD)) are much higher than those of the EC-Bio system (US $0.48/(kg COD)). For equal pollutant removal situations (COD removal: 80%), the cost of operation is lowest for the EC-Bio system (US $0.116/m3), in comparison with the EOBio (US $0.98/m3) and EC-EO-Bio (US $0.45/m3) systems. The performance terms are also in the same trend as that presented in Table 6. It has been reported38 that the cost of operation accounting for electrical power consumed, chemicals, and consumption of electrodes, during electrocoagulation of agro-industry wastewater ranges from US $0.95/m3 to US $4.93/m3. Bayramoglu et al. concluded that, in the case of the Fe electrode, the operating cost is approximately US $0.1/(kg COD removed), and for the Al electrode, it is US $0.3/(kg COD removed). Moreover, the contribution of electrode consumption cost reported ∼50% of the total cost for iron, and 80% of the total cost for aluminum. In a separate investigation,39,40 the total operating cost for electrocoagulation of poultry slaughterhouse wastewater using an Fe electrode was reported to be between US $0.3/m3 and US $0.4/m3, which is nearly half that of the Al electrode. In a recent work, Khansorthonga et al. reported the cost of operation of electrocoagulation in treating pulp and paper mill wastewater up to 77% COD removal to be US $0.29/m3. Upendra et al. reported that the operating cost of combined treatment (potash alum coagulation + electrochemical)3 of the raw pulp bleach effluent is US $0.7−0.9/m3. 3.6. Zhan−Wellens Test. The improvement in biodegradability due to pretreatment is confirmed by the Zhan−Wellens test. The pretreatment steps are run at the optimum conditions evolved during the investigation (see Table 7). The result of

4. CONCLUSION The feasibility of the idea of using various electrochemical processes as pretreatment steps for the biological treatment of pulp bleaching effluent is studied, and the results are compared with that of the conventional practice of chemical coagulation, followed by biological treatment. A methodology for modeling the overall performance of the integrated processes is proposed. Optimal combinations of operating variables giving minimum operating cost for a definite completion of pollutant removal are derived from the empirical models of each coupled system. Biological treatment of the raw effluent was found to be limited to ∼65% after 6 days of treatment. The electrotreated effluent was found to give ∼70% completion in COD removal by biological treatment for one day. Although both of the electrochemical steps (EC and EO) were observed to perform technically better as a pretreatment in achieving the abovementioned result, EC was noticeably more attractive in overall cost of operation. The cost of operation of the EC-Bio system (US $0.17/m3) was determined to be many fold less that that necessary for the conventional practice of the CC-Bio system (US $0.86/m3) for almost equal completion of pollutant removal. The completion of pollutant removal of the CC-Bio system was inferior to all three electro-bio cases studied. The cost of the coagulant contributed >95% of the total operating cost of the system. Electricity and electrode cost together contributed ∼50% of the operating cost of the EC-Bio system, while electricity cost contributed >95% of the operating cost of the EC-Bio system. Better electrode life and the absence of sludge were the attractive features of the EO-Bio system. However, the operating cost of the EO-Bio system (US $0.98/ m3) was determined to be considerably higher than that for the

Table 7. Optimal Conditions for Repeated Pretreatment Runs for the Zhan−Wellens Test Conditions Sl. No.

treatment

i (A/dm2)

t (min)

pHi

CD (mg/L)

1 2 3 4 4

EC EO CC EC of EC-EO EO of EC-EO

0.25 0.5

34 96

1064

1 0.5

33 15

7.5 8.5 10 7.5 7.6

the Zhan−Wellens test of the untreated case and the four cases of pretreatments, as shown in Table 7, is presented in Table 8. As can be seen from the tables, there is considerable improvement in the biodegradability of the bleaching effluent due to pretreatment, especially electrochemical. The degrada-

Table 8. Biological Degradation of Pulp Bleach Effluent: Zhan−Wellens Test Degradation (%) treatment

0

6h

Day 1

Day 2

Day 3

Day 4

Day 5

Day 6

untreated EC-treated EO-treated CC-treated EC-EO-treated

0 0 0 0 0

4.2 12.2 13.1 5.8 10.7

29.7 65.3 71 46.6 58.7

42 73.5 77.9 55.8 73.3

51.7 81 89 63.9 89.3

60.8 85.7 95.9 68.3 96

64.3 87.8 96.6 70.7 97.3

65.4 87.8 95.9 71.6 97.3

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Table 9. Terms Representing the Improvement in Biodegradability and Color Removal due to Pretreatments pretreatment

ks (mg/L)

Cm (mg/(L h))

COD (mg/L)

BOD (mg/L)

BI

Abs.

color removal (%)

untreated EC EO CC EC-EO

1385 534 554 789 444

5.23 3.37 5.79 2.49 4.84

1230 676 668 920 388

175 305 355 210 215

0.14 0.45 0.53 0.23 0.55

0.102 0.004 0.002 0.049 0.003

0 96.1 98 52 97.1

(8) Pokhrel, D.; Viraraghavan, T. Treatment of pulp and paper mill wastewaterA review. Sci. Total Environ. 2004, 333, 37. (9) Morais, J. L.; Zamora, P. P. Use of advanced oxidation processes to improve the biodegradability of mature landfill leachates. J. Hazard. Mater. 2005, 123, 181. (10) Chamarro, E.; Marco, A.; Esplugas, S. Use of fenton reagent to improve organic chemical bio-degradability. Water Res. 2001, 35, 1047. (11) Juttner, K.; Galla, U.; Schmieder, H. Electrochemical approaches to environmental problems in the process industry. Electrochim. Acta 2000, 45, 2575. (12) El-Ashtoukhy, E. S. Z.; Amin, N. K.; Abdelwahab, O. Treatment of paper mill effluents in a batch-stirred electrochemical tank reactor. Chem. Eng. J. 2009, 146, 205. (13) Khansorthonga, S.; Hunsoma, M. Remediation of wastewater from pulp and paper mill industry by the electrochemical technique. Chem. Eng. J. 2009, 151, 228. (14) Parama Kalyani, K. S.; Balasubramanian, N.; Srinivasakannan, C. Decolorization and COD reduction of paper industrial effluent using electrocoagulation. Chem. Eng. J. 2009, 151, 97. (15) Zaied, M.; Bellakhal, N. Electrocoagulation treatment of black liquor from paper industry. J. Hazard. Mater. 2009, 163, 995. (16) Patel, U. D.; Suresh, S. Electrochemical treatment of pentachlorophenol in water and pulp bleaching effluent. Sep. Purif. Technol. 2008, 61, 115. (17) Dominguez-Ramos, A.; Aldaco, R.; Irabien, A. Electrochemical Oxidation of Lignosulfonate: Total Organic Carbon Oxidation Kinetics. Ind. Eng. Chem. Res. 2008, 47, 9848. (18) Boroski, M.; Rodrigues, A. C.; Garcia, J. C.; Gerola, A. P.; Nozaki, J.; Hioka, N. The effect of operational parameters on electrocoagulation−flotation process followed by photocatalysis applied to the decontamination of water effluents from cellulose and paper factories. J. Hazard. Mater. 2008, 160, 135. (19) Zalęska-Chrost, B.; Smoczynski, L.; Wardzynska, R. Treatment of model pulp and paper wastewater by electrocoagulation. Pol. J. Nat. Sci. 2008, 23 (2), 450. (20) Ugurlu, M.; Gurses, A.; Dogar, C.; Yalcin, M. The removal of lignin and phenol from paper mill effluents by electrocoagulation. J. Environ. Manage. 2008, 87, 420. (21) Ma, H.; Wang, B.; Wang, Y. Application of molybdenum and phosphate modified kaolin in electrochemical treatment of paper mill wastewater. J. Hazard. Mater. 2007, 145, 417. (22) Wang, B.; Gu, L.; Ma, H. Electrochemical oxidation of pulp and paper making wastewater assisted by transition metal modified kaolin. J. Hazard. Mater. 2007, 143, 198. (23) Wang, B.; Kong, W.; Ma, Z. Electrochemical treatment of paper mill wastewater using three-dimensional electrodes with Ti/Co/ SnO2−Sb2O5 anode. J. Hazard. Mater. 2007, 146, 295. (24) Ben Mansour, L.; Ksentini, I.; Elleuch, B. Treatment of wastewaters of paper industry by coagulation−electroflotation. Desalination 2007, 208, 34. (25) Perng, Y.-S.; Wang, E. I-C.; Yu, S.-T.; Hsieh, Y.-C.; Chen, P.-R.; Chi, H.-Y. Treatment of a Specialty Paper Mill Wastewater Using a Pilot-scale Pulsed Electrocoagulation Unit. Taiwan J. For. Sci. 2007, 22 (3), 355. (26) Ugurlu, M.; Karaoglu, M. H.; Kula, I. Experimental Investigation of Chemical Oxygen Demand, Lignin and Phenol Removal from Paper Mill Effluents Using Three-Phase Three-Dimensional Electrode Reactor. Pol. J. Environ. Stud. 2006, 15 (4), 647.

Table 10. Total Oxygen Content (TOC) and Average Oxidation State (AOS) of the Selected Pretreatment Cases pretreatment

COD (mg/L)

TOC (mg/L)

AOS

untreated EC EO CC EC-EO

1232 676 678 922 392

414 308 330 358 197

−0.46 0.71 0.92 0.14 1.02

EC-Bio system for equal pollutant removal performances. This is primarily due to the consumption of a relatively greater amount of electrical energy. Thus, it is possible to conclude that the EC-Bio system is the best treatment strategy for the management of kraft bagasse pulp bleaching effluent. The performance term of the EC-Bio system was far superior to the other systems at equal pollutant removal condition of 80% COD removal, satisfying Indian pollution control norms. The performances of the pretreatment strategies are validated using other tests/terms such as Zhan− Wellens test, biodegradability index (BI), and average oxidation state (AOS). The trends of the result were consistent with the early-stage findings. The research not only establishes the technical feasibility of electrochemical steps in the reduction of organic load, color, and improvement in the biodegradability of the kraft pulp bleaching effluent, but also shows that the method is economically more attractive, in comparison with the existing practice of chemical coagulation, followed by biological treatment.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Optimization of Integrated Electro-Bio Process for Bleaching Effluent

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