Pulp and Paper Industry

Transcription

1 ST2B-1-2: Ozonization of Dyeing Water of the Paper Mulberry Pulp and Paper Industry Tarnkamol Tarvornpanich 1 and Penjit Srinoppakun 1 ABSTRACT A study of ozonization dyeing using two different dimensional reactors of 300 ml was undertaken. The reactors were different with respect to their height one was two times higher than the other. In order to mimic the conditions in a factory, only synthetic wastewater was used. Dyeing color of 2.5% (w/v) green B (green color), 0.15% black EX (black color) and 0.2% G100 (yellow color) was used. COD, color, turbidity, suspended solid (SS), conductivity, total dissolved solid (TDS) and ph were investigated. Results show that the removal efficiency directly depends on the treatment period, ozone production rate, temperature and the retention time. At fixed rate of 100 mg/h of ozone production, the removal efficiency of color, COD, turbidity and SS are 99.56, 67.25, 94.7 and 97.22% whereas conductivity and TDS are slightly increased around 30.42% and 30.13% within 12 h of treatment in reactor 1. However, in the shorter reactor (reactor 2) removal of color, COD, turbidity and SS are 97.45, 67.5, and 96.49% but conductivity and TDS increased to and 39.18% in 24 h at the same ozone production rate. System temperature on removal efficiency when high led to an enhancement of 30 % the cost of ozone treatment is approximately baht/liter of wastewater. Comparing ozone and the chemical treatment, ozone treatment is the better process as can be removed by 80% at its best condition. 1 Department of Chemical Engineering Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand

2 INTRODUCTION Environmental problems are an important concern for society, especially those caused by wastes produced from industrial activity. In the pulp and paper industry the dyeing process generates effluents which are high in organic matter (about 2000 to 3000 mg/l as COD). This effluent is also colored due to the presence of polycyclic organic compounds known as chromophores. Discharge of colored water, even if it is not toxic, should be regulated for aesthetic reasons and because sunlight is blocked, which interferes with the plant and animal life cycle. Coloration in water from the pulp and paper industry is difficult to remove by conventional sedimentation and biological treatment. The current practice removes only 80-90% of the coloration from Kraft pulp and bleach wastes even after massive lime treatment. This method has a number of associated problems such as low ph and a large amount of sludge which needs to be eliminated (Doshi et al., 1980). In order to solve this problem, other methods are required to provide greater decolorization. Ozonization is a new technique, which according to the literature may present a potential alternative for decolorization (Beszedits, 1980, Green and Sokol, 1985, Gould and Groff, 1987, Lin and Lin, 1993 and Lin and Liu, 1994). Ozone is a potential decolorizing agent due to its high reactivity towards carbon-carbon double bonds. Compared with chlorine and hydrogen peroxide, ozone has a higher oxidation potential. Therefore, ozone is widely used for disinfection in drinking water and color, taste, odor and organic/inorganic compound removal. MATERIALS AND METHODS In the research reported in this paper removal efficiency of ozone production rates on the treatment, reactor dimension and system temperature were established. The efficiency of this method as a treatment for chemical coagulation (alum and ferric chloride) was also evaluated. A synthetic wastewater was developed for the purpose of the research, containing 2.5% (w/v) green B (green color), 0.15% black Ex (black color) and 0.2% G100 (yellow color). The schematic representation of the experimental apparatus is shown in Figure 1. The apparatus consisted of an ozone generator, gas pump, cooling water and two reactors (contactors). 622

3 Jacketed reactors were used. One reactor was 2.5 cm in diameter and 100 cm long, half the diameter and twice the length of the other reactor. Outlet gas from the reactor discharged to atmosphere Ozone generator Reactor Gas pump Water bath Electrical transformer Figure 1. Schematic diagram of the ozonation system In the first part of the study experimental runs were performed in a semi-batch system (liquid in : batch and gas in: continuous flow). Ozone was produced by a generator (corona discharge type) with air input. A fine bubble of ozone entered the reactor through a ceramic diffuser. During the treatment, parameters relating to water quality were measured as indicated in Table

4 Table 1. Measured parameters and analytical methods Parameters Analytical methods Chemical oxygen demand (COD) Dicromate method Absorbance UV-Visible spectrophotometer (UV-160A) Color Spectrophotometer DR/2010 Turbidity Spectrophotometer DR/2010 Suspended solid Spectrophotometer DR/2010 Conductivity Conductivity meter Total dissolved solid Conductivity meter ph ph meter Dissolved ozone Ozone test kid Ozone gas outlet Gastec Decolorization was spectrophotometrically monitored in the visible region and at maximum wavelength, using a UV-Visible spectrophotometer (UV-160A). The maximum wavelength of Green B, Black EX, G100 and all mix were 617 nm, 487 nm, 394 nm and 618 nm, respectively. Ozone concentration in the gas stream at the reactor outlet was analyzed using Gastec. 624

5 Effect of the ozone production rate RESULTS AND DISCUSSION Table 2. Effect of ozone production rate on measured parameters in reactor 1 at 25 C for 12 hrs Analytical results Ozone production rate (mg/h) COD (mg/l) Color (PtCo) Absorbance Turbidity (FAU) Suspended solid (mg/l) Conductivity (ms/cm) Total dissolved solid (mg/l) ph COD, color, absorbance, turbidity and suspended solid decreased when ozone production rate was increased. In contrast, conductivity, total dissolved solid and ph increased, though only slightly. This indicated the higher effective treatment at a high ozone production rate. Ozonation experiments at different ozone production rates show that ozone is capable of rapid disruption of the dye molecule. In addition to decolorization, added benefits of elimination of COD, suspended solids, turbidity were obtained. COD removal efficiency increased when ozone production rate increased. At the ozone production rate of 20 and 60 mg/hr COD sharply decreased from an initial level of 2,000 mg/l to 1,382 and 760 mg/l or 30.9% and 62% removal efficiency. However, COD removal efficiency slowly increased during mg ozone/h. This might result from the remaining organic/inorganic compounds which are tolerant to ozone treatment. 625

6 Significant reduction in color was observed. Organic dyestuffs generally are polycyclic, highly conjugated organic materials which are easily decolorized by a powerful oxidizing agent such as ozone. Ozone is particularly reactive with unsaturated groups, cleaving the carbon-carbon double bonds to produce ketones, aldehydes or acids, depending on the substituent at the carbon chain, the ozone dose and contact condition applied. As soon as the conjugation has been disrupted by oxidation, color disappears. The ph also decreased after 12 h ozonation the initial ph 6.6 decreased to 2 with 100 mg/h of ozone (Table 2). This indicates the possible formation of acidic by-products during ozonation or solubility of ozone in water, ozone decomposes to OH (hydroxyl radical) and HO 2 - (Hydrogenperoxide ion) which will further decompose to H+ (hydrogen ion) when it unreacted with dissolved organic compounds. Effect of reactor dimension COD reduction The COD was found to decrease, it decreased from 2,000 mg/l before treatment to 655 mg/l and 740 mg/l after treated in reactor 1 and reactor 2, respectively, assuring the removal ratio of 67.25% and 63% within a treatment time of 12 h. Generally, the COD values decreased by decolorization, the remaining COD values was resulted of organic matter that can not treat by ozone. Decolorization Removal of color (as decrease in absorbance at 618 nm) was observed to be much faster than organic matter. The faster rate of color removal compared to organic matter removal indicates the preference of ozone to attack the chromophoric groups of the waste, rather than complete oxidation of organics to carbon dioxide. The decolorization rate was fastest during the first 6 h, but this tapered-off with time. The color of wastewater was 20,200 PtCo before ozone treatment and decreased to 88 PtCo and 1,235 PtCo demonstrating overall decolorization after 12 h were about 99.56% and 60.61% after treated in reactor 1 and reactor 2, respectively. 626

7 In order to improve the rate and efficiency of decolorization of ozone, addition of H 2 O 2, UV, dilution of the wastewater and different ozonization temperatures were further investigated. Suspended solid removal Small, colloidal particles with charged surfaces are a cause of turbidity in effluent water. The strength of surface charges keeps particles in suspension because of the repulsive surface forces coupled with the small particle sizes. Combination of strong color and high dissolved solid content results in high turbidity of waste effluent from the dyeing process of pulp and paper industry. Ozone changes the nature and/or extent of these surface charges, thus allowing charged particles to agglomerates and precipitates to the bottom of the reactor. Similar to color removal, treatment in reactor 1 gave better results in terms of suspended solid removal compared with reactor 2, the fast rate of removal during the first 6 h in reactor 1 and 8 h in reactor 2. Total dissolved solid removal Change in conductivity and total dissolved solid in ozonated dyestuff water followed the same trend (Table 3), total dissolved solid slightly increased by about 30% in reactor 1 and 39% in reactor

8 Table 3. Analytical results of color wastewater treatment by ozone at ozone production rate 100 mg/h, 25 C Analytical results Before Reactor 1 (at 12 h) Reactor 2 (at 24 h) After Efficiency (%) After Efficiency (%) COD (mg/l) 2, Color (PtCo) 20, Absorbance (at 618 nm) Turbidity (FAU) Suspended solid (mg/l) Conductivity (ms/cm) Total disolved solid (mg/l) 1,437 1, , ph ph changes during ozonization Comparing ph results between ozonization in reactor 1 and in reactor 2 it was found that ozonated water from reactor 1 had lower ph than that from reactor 2. The ph decreased from 6.6 at the initial to 2 and 2.2 at reactor 1 and at reactor 2, respectively. This might be because of the better reaction of ozone and substances in reactor 1 due to the higher contact time in it. Making the formation of acidic by-product during ozonization resulted in ph decreased. Effect of system temperature Ozonization, with a cooling water system at 25 C, and without, were compared. Ozonization with a cooling water system gave higher removal efficiency of COD, color, turbidity and suspended solid than that without it (Table 4). These results support the fact that ozone is an unstable gas at temperatures above C. At higher temperatures ozone gas transform to oxygen and hydroxyl radicals, and that the solubility of ozone in water will decrease when temperature increase. 628

9 Table 4. Analytical results of color wastewater treatment by ozone in reactor 1 with cooling water (25 C) and without cooling water Analytical results Before With cooling water Without cooling water After Efficiency (%) After Efficiency (%) COD (mg/l) 2, , Color (PtCo) 20, , Absorbance Turbidity (FAU) Suspended solid (mg/l) Conductivity (ms/cm) Total disolved solid (mg/l) 1,437 1, , ph Chemical coagulation Chemical coagulation with alum and ferric chloride is commonly used to neutralize surface charges. When the surface charges are neutralized, particles coagulate which can be removed by sedimentation, filtration or flotation. Color (PtCo) color ph Alum (mg/l) ph Figure 1. Effect of the amount of alum on the decolorization and ph changed 629

10 Color (PtCo) color ph Ferric chloride (mg/l) ph Figure 2. Effect of the amount of ferric chloride on the decolorization and ph changed Ferric chloride gives best results for color removal than alum at the same dose (Figures 1 and 2). The optimum dose of alum and ferric chloride are the same, 2,500 mg/l, this can decrease color from 20,200 PtCo of initial wastewater to 3,070 PtCo (by alum) and 1,760 PtCo (by ferric chloride) or 84.5% and 91.11% of color removal. Demonstrating the ozonization is the better method of dyeing water treatment from the result of treatment 99.56% of decolorization. CONCLUSION Coloration contained in dyeing process wastewater can be removed by ozonization. The effect of operating parameters on the performance of ozonization can be summarized: 1. Higher removal efficiency can be achieved if ozone production rate increase. 2. Dimension of reactor has influence on the treatment efficiency. Reactor 1 (long reactor) improved quality of water more quickly than reactor 2 (short reactor). And the perfect time for wastewater treatment in reactor 1 and reactor 2 are 8 and 14 h, respectively with the color removal efficiency 95% and 95.5%. 3. Considering effect of system temperature on the removal efficiency showed that the higher removal efficiency was obtained for the system with the water cooling at 20 C. The color removal was 30% higher than the treatment system without the water cooling. 630

11 4. For chemical coagulation method, ferric chloride reduced color material more effective than alum with the same proper dose 2,500 mg/l. 5. The application of ozonation for color removal from wastewater derived from dyeing process of pulp and paper industry has proved to be successful. ACKNOWLEDGEMENTS This work was partly supported by the Kasetsart Agricultural and Agro Industrial Products Improvemnet Institute (KAPI) and Siamphromphrathan Co. Ltd. REFERENCES Beszidits Ozonation to decolor textile effluents. Am. Dye. Rep, 69: Gould, J.P. and K.A. Gross Kinetics of ozonolysis of synthetic dyes. Ozone Sci. Engng., 9: Green, J.M. and C. Sokol Using ozone to decolorize dyeing plant wastewater. Am. Dyes. Rep. 74: Lin, S.H. and C.H. Lin Treatment of textile wastewater by ozonation and chemical coagulation. Wat. Res. 27: Lin, S.H. and W.Y. Liu Continuous treatment of textile wastewater by ozonation and coagulation. J. Environ. Engng., ASCE. 120: Doshi, M.R., P.E. Parker and H.S. Dugal Color removal from neutral sulfite semichemical (NSSC) mill effluents by ultrafiltration. AIChE Symposium series, Water. 197(76):