Reactive Dye House Wastewater Treatment. Use of Hybrid Technology

After process parameters were fixed, studies were conducted with the actual dye waste stream. The actual waste stream was found to be refractory for w...
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Ind. Eng. Chem. Res. 1999, 38, 2058-2064

Reactive Dye House Wastewater Treatment. Use of Hybrid Technology: Membrane, Sonication Followed by Wet Oxidation Atul D. Dhale and Vijaykumar V. Mahajani* Chemical Engineering Division, Department of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India

To address problems associated with treatment of an aqueous waste stream from a reactive dye house, a model dye, turquoise blue CI25, was studied. A hybrid technology, membrane separation followed by sonication and wet oxidation, has been demonstrated to treat the wastewater for reuse and discharge. Experiments were first performed with the reactive dye solution in water. A nanofiltration membrane (MPT 30) was found to be suitable to concentrate the dye. The concentrate was then treated with a wet oxidation process. Kinetics studies were performed with and without catalyst, in the temperature range of 170-215 °C. The color destruction achieved was >99%. After process parameters were fixed, studies were conducted with the actual dye waste stream. The actual waste stream was found to be refractory for wet oxidation under the above conditions. Sonication of the concentrate obtained after membrane filtration, in the presence of CuSO4, made the waste stream amenable to wet oxidation. Sonication followed by wet oxidation was found to be more effective at near neutral conditions as compared to basic conditions. Introduction Treatment of a highly colored aqueous effluent stream from the textile dye house industry has attracted the attention of environmentalists, technologists, and entrepreneurs because of its socio-economic and political dimensions. Water consumption in the textile dye house is very high. Water as a utility is becoming costlier and costlier, and its availability is becoming more and more scarce. Water conservation can be achieved by means of the following: (a) Utilizing a smaller amount of water, thereby resulting in a highly concentrated waste stream. (The quantity of water is ultimately decided by the dye fixation process.) (b) Treating the waste stream to recycle water and chemicals. The effluent from the reactive dye bath is a highly colored stream containing an unutilized, hydrolyzed dye along with salts, auxiliary chemicals such as emulsifying agents, etc. Conventional bioprocesses are likely to be unattractive because streams would now be concentrated in new complex molecules which may not be amenable to biodestruction. There are several other technologies available for the removal of color and chemical oxygen demand (COD).1-4 We envisage hybrid technology (a blend of various technologies) would be the answer to water conservation via recycle. In the present research investigation, we have focused our attention on treatment of a reactive dye bath waste by “membrane-sonication-wet oxidation” (MEMSONIWO) technology. The membrane unit would allow concentration of the waste, and then the permeate (mostly water) could be recycled. The concentrate from the membrane unit could be treated by * To whom correspondence should be addressed. Phone: 9122-4145 616. Fax: 91-22-4145 614. E-mail: [email protected].

sonication to make it suitable for wet oxidation. After wet oxidation, the waste can be discharged or recycled. Membrane technologies, namely, reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF), are well accepted throughout the world because of the ease of operation.5 Although they are capable of treating a large volume of wastewater, the major drawback associated with the systems is that concentrated residue remains for disposal. Shende and Mahajani6 have successfully demonstrated the use of wet oxidation technology to treat anthraquinone and phthalocyanine classes of reactive dyes. Donlagic and Levec7 have also applied this technology to treat azo dye pollutants. Therefore, wet oxidation was chosen to treat the concentrate of the dye bath waste obtained from the membrane unit. Wet oxidation (WO) involves oxidation of organics and inorganics in the aqueous phase at elevated temperature (150-350 °C) and a pressure ranging from 1 to 30 MPa using air or oxygen as an oxidizing agent. Wet oxidation technology can be successfully applied to treat various types of waste streams including hazardous, toxic, and nonbiodegradable. Mishra et al.8 have reviewed and discussed various industrial applications of wet oxidation. Recently, Metatov-Meytal and Sheintuch9 have reviewed the heterogeneously catalyzed oxidation of wastewater. In wet oxidation, high molecular weight compounds are degraded to lower molecular weight acids and subsequently to harmless products such as CO2, water, and nitrogen, sulfur to sulfate, and phosphorus to phosphates. Wet oxidation has several advantages such as it can treat a high COD/BOD waste stream, requires less space, and has low operating cost and the reactor can be deep shaft and, finally, energy can be recovered in the form of steam and power through hydraulic turbines. Hybrid systems are becoming popular in the treatment of waste streams, which are otherwise difficult to handle. The powder-activated carbon-activated sludge

10.1021/ie980615t CCC: $18.00 © 1999 American Chemical Society Published on Web 04/03/1999

Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2059

Figure 1. Reactive turquoise blue CI25 (procion turquoise blue H5G, ICI). Class: phthalocyanine. Reactive system: vinyl sulfone-tetraammonium salt.

system (PACT system by Zimpro Environmental, Inc.) is a classic example of such system.10 Hellenbrand et al.11 have recently demonstrated a hybrid process, OXYMEM, where wet oxidation and nanofiltartion were employed together for the treatment of bioresistance industrial wastewater [poly(ethylene glycol)]. Nanofiltration was used to retain big molecules in a wet oxidation unit, until they were oxidized to the desired extent. The permeate was subjected to further oxidation in a bioreactor. Ingale and Mahajani12 have also demonstrated hybrid technology, sonication followed by wet oxidation (SONIWO), to treat otherwise refractory waste. The present study, consisting of membrane filtration followed by sonication and wet oxidation (MEMSONIWO) to treat a segregated dye bath waste stream, is focused on a reactive dyesprocion turquoise blue CI25 (refer to Figure 1)sand its dye bath waste. The membrane used was a nanofiltration membrane. To gain insight into various processes, initial experiments were performed using a pure dye solution for both membrane and wet oxidation. On the basis of this study, an actual sample from the dye bath of CI25 dye was taken for evaluation of the process concept. Experimental Section Apparatus. Membrane Filtration System. A benchscale membrane filter system manufactured by Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India, was used for the membrane separation experiment. Operating volumes used were from 20 to 50 L. The system consisted of a high-pressure triplex plunger pump which allowed a flow of 0.8 m3 h-1 and a pressure of up to 0.710.6 MPa. The 1/2 in. single tubular membrane module (Membrane Product of Kiryat Weizmann Ltd., Israel) having a total filter surface area of 0.025 m2 was used. The rotameter and heat exchanger were provided at the outlet of the concentrate to measure its flow rate and to maintain a constant temperature in the dye bath solution tank. Flow control valves on the panel consisted of a bypass valve, an open-close valve, and a backpressure controlling valve. A temperature indicator and pressure gauges were provided to measure the inlet and outlet pressures along with controller switches. Wet Oxidation Reactor. The oxidation studies were carried out in a 300 mL SS-316 Parr high-pressure reactor equipped with a Parr 4842 temperature controller (Parr Instruments Co., Moline, IL). The reactor (i.d. 65 mm) had a four-blade turbine-type impeller (diameter 35 mm) and was equipped with a pressure indicator, a gas sparging tube into the liquid phase beneath the impeller, and a sample collection outlet with a condenser. The impeller speed was varied between 0 and 26.6 rps with a variable-speed motor. The reactor was

also provided with a rupture disk as well as a nonreturn valve at the gas inlet. The liquid samples were collected through a chilled condenser with cooler mounted on the reactor. Sonication. Sonication was carried out in a glass reactor of capacity 250 mL mounted in a conventional ultrasonic cleaning bath (manufactured by Toshniwal Brothers Pvt. Ltd., Bombay, India) equipped with a thermostated jacket. The system was operated at 30 °C, having 150/350 W average/peak at a frequency of 40 kHz. Analytical Techniques. Chemical Oxygen Demand. The samples collected during the course of the experiments were analyzed for their COD content by a standard dichromate reflux method described by Snell and Ettre.13 Color Measurement. The samples were also analyzed for color on a UV/vis spectrometer (Lambda 3B; PerkinElmer, Norwalk, CT). The light absorbance was measured spectrophotometrically, and the absorbance was used to characterize the percentage reduction in color. Gas Chromatography. To detect low molecular weight acids formed during wet oxidation gas chromatography (GC), Chemito 3865 was used (manufactured by Toshniwal Instruments Ltd., Mumbai, India). The glass column packed with Carbopack BD-A 4% and Carbowax 20M was used along with a flame ionization detector. GC conditions were as follows: N2 carrier gas, 40 mL/ min; detector and injector temperature, 190 °C; oven temperature (isothermal), 140 °C. Salt Determination. To see the salt rejection, chloride salt was determined by Volhard’s standard titrimetric method whereas sodium carbonate was determined by an alkalinity-titrimetric method using a phenolphthaline indicator.14,15 Materials. The reactive turquoise blue CI25 and its dye bath effluent samples were obtained from a progressive textile mill in Mumbai, India. The structure of the dye CI25 is elucidated in Figure 1. The reagents for analysis and other chemicals used were of A.R. grades and were obtained from S.D. Fine Chem Ltd., Mumbai, India. Oxygen with a minimum purity of 99.5% was used for wet oxidation. A.R. grade cupric sulfate was used as a catalyst. A standard nanofiltration polymeric membrane (Sel RO MPT 30) was procured from Membrane Product Kirayat Weizmann Ltd., Israel. For prefiltration, a polypropylene cartridge filter was used. Experimental Procedure. Membrane Filtration System. The membrane was characterized based on the rejection of the dye and the permeability of salts. The simulated dye solution was made of dye and salts with concentrations comparable to those of a real situation, and then results were compared with those of the actual waste. The system was operated in a batch mode. For performance evaluation, experiments were carried out in the total recycle mode as well as in the concentrate mode. In the total recycle mode, the permeate along with the concentrate was recycled back into the feed tank. On the other hand, in the concentrate mode the permeate was separated out. The actual waste was prefiltered through a micron membrane before experiments. Pressure at the inlet and outlet of the module was continuously monitored and kept constant. The permeate (flux) was measured by measuring the volume in a specific time while the concentrate was measured by a rotameter. The temperature was maintained with the help of a double-pipe

2060 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 Table 1. Typical Characteristics of a Dye Bath Waste (Reactive Turquoise Blue CI25) content

actual waste

simulated solution

COD NaCl Na2CO3 total dissolved solids (TDS) pH absorbance at 615 nm (100 times dilution)

∼1200 mg/L 2.4% (w/v) 1.1% (w/v) 3.34% (w/v) ∼11 0.146

∼1500 mg/L 2.2% (w/v) 0.9% (w/v) 3.74% (w/v) ∼10.5 0.575

heat exchanger provided at the outlet of the concentrate. The membrane was cleaned periodically with an aqueous solution of sodium salt of ethylenediaminetetraacetic acid (EDTA). Wet Oxidation. The experiments were carried out by taking a dye solution of known concentration in the reactor. In both cases, a catalytic, as well as a noncatalytic, reaction mixture was heated to the desired temperature, and once the temperature was attained, the sample was withdrawn through a sample condenser/ cooler. This time was considered to be “zero time” for the reaction. In the catalytic reaction, the catalyst was added while charging the reaction mixture in a highpressure reactor. A temperature controller (TIC) controlled the reaction temperature, and oxygen was sparged in the vessel to a predetermined level and maintained in order to keep a constant pressure while collecting the sample for COD and color analysis. The total pressure is the sum of the oxygen pressure and vapor pressure. The experiments were carried out at different temperatures and oxygen pressures and at various catalyst concentrations for the kinetics study. The experimental variation in COD measurement was less than 5%. Sonication. Sonication treatment was given to the actual waste to make the stream amenable for wet oxidation. The sonication was carried out in the presence CuSO4 catalyst. Results and Discussion The reactive dye house effluent contains spillage, exhausted and hydrolyzed dye exhibiting high COD as well as other auxiliaries such as surfactants and salts such as NaCl and Na2CO3. Here, our objective is to recycle the dye bath wastewater by separating dye molecules using the membrane. The salts goes into the permeate, and therefore, concentrate can be treated by wet oxidation. In doing so, with the effluent stream being smaller and highly concentrated, the size of the wet oxidation plant reduces. Also, because the concentrate possesses high COD, the efficiency of the wet oxidation is enhanced, thereby gaining an economical advantage. A. Studies with a Pure Dye Solution. Membrane Study. The polysulfone membrane, MPT 30, was selected from the process engineering viewpoint, because it showed almost complete rejection of reactive dye CI25. Also it has more permeability toward the salts and can tolerate a wide range of pHs (2-12). Typical characteristics of the dye bath waste are shown in Table 1. The performance of the membrane was evaluated in total recycle and concentrate modes using pure dye and simulated solutions and was compared with pure water permeability. The rejection coefficients of salts, dye compound, and COD were measured. The effluent stream contains many other organic compounds such as surfactants. It, therefore, exhibits finite COD. Hence,

Figure 2. Effect of the transmembrane pressure on the membrane flux.

it is important to characterize the waste stream in terms of COD, from the environmental process engineering point of view where engineers are mainly concerned about monitoring the COD of the stream. Water fluxes were checked after cleaning and washing to verify the fouling and degradation of the membrane. The various parameters such as transmembrane pressure, flow rates, and pH of the solution were studied to see its performance. The rejection of dye was studied in terms of the total organic load, i.e., COD and color. The rejection coefficient is measured according to

Rc ) 1 - (Cp/Cf)

(1)

and the concentration factor

Cv )

(

)

V0 V0 - Vp

(2)

The membrane flux is mainly governed by transmembrane pressure. Transmembrane pressure is the mean of the inlet and outlet pressures across the membrane. Figure 2 shows that when the pressure is increased, the flux was increased. An increase in the flow rate had little effect on the flux; however, high-velocity parallel flow produces shear and lift forces on the particles attached to the surface of the membrane filter, thus controlling the particle and solute concentration polarization and resulting in a more controllable flux decline. The performance of the membrane was evaluated by keeping process parameters constant. The system was operated at a transmembrane pressure of 1.5 MPa, a cross-flow rate of 0.5 m3 h-1, a temperature of 35 ((2) °C, and pH ∼ 11 of the dye bath waste. Table 1 listed the typical characteristics of the actual and simulated wastes. Table 2 shows the flux and rejection of dye and salts with respect to the simulated bath solution and pure water permeability in the total recycle mode. The flux was measured as 0.085 m3 m-2 h-1 whereas the pure water permeability was found to be 0.115 m3 m-2 h-1. Color rejection was found to be beyond 98%, and COD rejection was more than 90%. Permeations of NaCl and Na2CO3 were about 90% and 75% respectively. The nanofiltration membrane has a molecular weight cutoff

Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2061 Table 2. Membrane Performance in the Total Recycle Modea % rejection

a

solution type

COD

color

NaCl

Na2CO3

flux × 103 (m3 m-2 h-1)

simulated solution actual waste pure water

90.3 ((1.4)

98.7 ((0.6)

11.5 ((1.4)

26.8 ((3.1)

84.81 ((2.20)

63.4 ((1.2)

98.2 ((0.2)

34.3 ((1.2)

45.5 ((2.9)

38.34 ((0.1) 115.1 ((0.11)

Values in parentheses indicate the standard deviations (n ) 6).

of around 300, retains the solutelike sugars and multivalent salts, and passes substantial amounts of monovalent salts. The rejection of salts and neutral species is mainly due to electrostatic interaction and size exclusion of the membrane, respectively. Wet Oxidation. Wet oxidation of a pure reactive dye was studied to have better insight into the process while treating the actual waste. In wet oxidation due to formation of highly refractory lower molecular weight carboxylic acids such as formic, acetic, glyoxalic, oxalic, etc.,8 the final effluent stream exhibits finite COD and BOD. Hence, severe oxidation conditions are required to have complete oxidation of these acids. The use of catalyst could reduce the severity of oxidation. Shende and Mahajani16,17 have observed a homogeneous copper sulfate catalyst to be very effective in destroying the above acids. Cheaply and readily available copper catalyst in the form of CuSO4 was chosen for this investigation to study the performance of wet oxidation for the present reactive dye. Because sulfate ions are more docile than chloride ions in the presence of oxygen toward material of construction, copper sulfate was preferred over copper chloride as the catalyst. The remaining residual copper after wet oxidation can be removed from the aqueous stream by a conventional ion-exchange technique or a precipitation technique. (1) Kinetic Study. WO is a heterogeneous gas-liquid reaction involving a series of steps taking place at the macroscopic level. In the present investigation, the gasside resistance was estimated to be negligible, for transfer of oxygen from the gas phase to the gas-liquid interface due to the higher diffusivity of oxygen in the gas phase and its low solubility in water, in the range of operating temperature and pressure used for oxidation studies. There was no effect on the rate of destruction of COD when the speed of agitation was varied between 6.7 and 25 rps; thereby diffusional resistances in liquid phase, for transfer of O2 from the gas-liquid interface to the bulk liquid phase were deemed to be absent. All further experiments were carried out at 20 rps. In the absence of solubility data for our system and with solutions being very dilute in the given substrate (COD ∼ 500-700 mg/L), we have used the solubility of oxygen in water under our experimental conditions taking data from Crammer.18 The wet oxidation organics substrate (RH) in the aqueous phase involves a free-radical mechanism,19-21 where strong oxidizing hydroxyl radicals are formed. The organic substrates are subjected to free-radical attack by hydroxyl radicals, and the reaction propagates. The pH of the solution has an effect on the activity of OH- ions, and therefore under alkaline conditions, reaction rates are expected to be lower than those at near neutral condition. This will also have an effect on the intermediate formed. The waste consists of a mixture of various organic species contributing to COD. Our preference to charac-

Figure 3. Kinetic plot for noncatalytic wet oxidation (effect of temperature at 0.69 MPa O2 pressure).

terize the waste in terms of COD over the total organic content (TOC) lies in the fact that TOC analysis fails to yield COD when compounds contain oxygen in their molecule such as acetic acid. Also contribution of other atomic species to COD like H, S, Cl, etc., is neglected in TOC analysis. The next important aspect is that TOC analysis needs an expensive instrument while COD can be measured by conventional cheaper techniques. We, therefore, thought that it is better to have a lumped kinetic model for various species in terms of COD. The reaction can be presented as k1

k2

(COD)mixture 98 (COD)lower mol wt compounds 98 CO2 + H2O (3) The reaction continues until CO2 and H2O are formed. Thus, in wet oxidation, the reaction would exhibit two different kinetic behaviors, namely, the fast oxidation of organic substrate followed by the slower oxidation of low molecular weight compounds formed such as acetic acid. The kinetic data were interpreted as studied previously17 and obtained the rate equations for catalytic and noncatalytic wet oxidation. The kinetics of wet oxidation of reactive dye CI25 was studied in the temperature range of 170-215 °C and oxygen partial pressure range of 0.414-1.380 MPa at near neutral pH. For catalytic oxidation, catalyst CuSO4 loading was varied in the range 7.515 × 10-5-3.757 × 10-4 kmol m-3. It was observed that in the absence of catalyst and at 0.69 MPa oxygen pressure at 200 °C about 85% COD reduction was achieved in 120 min. The kinetic plot shown in Figure 3 indicates that there exists an induction period at lower temperature. However, efficiency of wet oxidation was enhanced by the catalytic oxidation. In the presence of catalyst (3.757 × 104 kmol m-3), at 200 °C and 0.69 MPa oxygen pressure, more

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First step r1 ) 1.70 × 109 exp[-9605/T][COD][O2]0.4[CuSO4]0.24 (5a) Second step r2 ) 0.749 exp[-858/T][COD][O2]0.16[CuSO4]0.22 (5b)

Figure 4. Kinetic plot for catalytic wet oxidation (effect of temperature at 0.69 MPa O2 pressure and catalyst 3.757 × 10-4 kmol m-3).

than 90% reduction in COD was possible in 120 min. Figure 4 represents the kinetic plot for catalytic study. While treating the aqueous stream to meet the discharge standards, the destruction of color (colorforming bodies) is also equally important. The color concentration was determined by measuring the absorbance under various experimental conditions. It was observed that in absence of catalyst 99% color removal was possible at 215 °C in 120 min. However, the same color reduction can be achieved at 200 °C in catalytic oxidation after 120 min. The higher the temperature, oxygen partial pressure, and catalytic loading, the higher is the oxidation efficiency with respect to COD and color removal. We postulate that the oxidation of reactive turquoise blue CI25 proceeds via a two-step reaction. The first step corresponds to oxidation of dye into a low molecular weight compound, and the second step corresponds to the higher resistance provided by the lower molecular acids for further oxidation. In the present investigation the formation of acetic acid was observed on a gas chromatograph. The energy of activation for the first and second steps was found to be 16.80 and 6.27 kcal/ gmol, respectively. In the case of catalytic oxidation, similar kinetics was observed and activation energies were was found to be 19.02 and 1.70 kcal/gmol for the first and second steps, respectively. The overall reaction can be described by a two-step reaction. In both cases the observed rates could be fitted well assuming first order with respect to COD. The order with respect to O2 would vary between 0 and 1.0 depending upon the contribution of the various reactions involving O2. (2) Kinetic Equations. Noncatalytic oxidation of the reactive dye molecule is given by

First step r1 ) 1.72 × 107 exp[-8488/T][COD][O2]0.40

(4a)

Second step r2 ) 21.40 exp[-3167/T][COD][O2]0.25

(4b)

The catalytic oxidation of the reactive dye molecule under the above-mentioned conditions is given by

Because the dye bath waste has pH ∼ 11, typical experiments were carried out under an alkaline medium at similar operating conditions, where the catalytic system becomes heterogeneous. As is expected, the performance of wet oxidation decreased in alkaline conditions (Figure 5). The decrease in the reduction rate under alkaline conditions is attributed to the loss of •OH radicals in the alkaline medium21 as well as the heterogeneous nature of the catalyst. Thus, it is seen that the catalytic wet oxidation technique is more suitable for treating the aqueous solution of reactive dye CI25. It should be noted that no attempt is made to quantify contribution of the Cu atom present within the dye molecule itself. We have attributed the catalyst effect to Cu from CuSO4 only. At higher concentration of dye (very high COD obtained in the membrane concentration), the contribution of copper form where might not be neglected. B. Studies with Actual Waste. A progressive organization in Mumbai, India, supplied the sample of the segregated waste stream of CI25, generated at the dye bath. It was thought desirable to see how above studies based on pure dye CI25 can be used to treat the actual waste. Table 1 exhibits characteristics of the waste studied. Performance of the Membrane. The actual reactive dye bath waste stream contains chemicals such as surfactants, sizing agents, etc., along with inorganic salts. The membrane performance with the actual dye bath waste was studied under operating conditions similar to those used for the simulated solution. Table 2 shows the performance with the actual waste in the total recycle mode. To avoid clogging in the NF membrane, the actual waste was filtered through a 50 µm membrane filter to remove materials such as fabrics. Figure 6 represents the rejection and flux of an actual dye bath solution in the concentrate mode. Rejection of COD and salts permeability were comparatively less than those in the simulated solution. The flux started at a relatively high value (0.035 m3 m-2 h-1) and decreased continuously until it came to 0.022 m3 m-2 h-1 after 8 h of running time. The membrane cleaning was done after 8 h of running, and it regained about 90% of the original pure water permeability. So we can have an on-off mechanism to the system after 7 h, and cleaning can be done and again the membrane filtration continued. The cleaning was done by a 0.5% w/w EDTA salt solution. With the actual dye bath, flux declined very much. Probably, the actual waste contained other auxiliary chemicals, as well as some fabric materials at the submicron level, which results in the decrease in the flux. In the actual waste permeate showed the finite COD although color removal was more than 99%. The finite COD (∼300 ppm) was due to the fact that waste contains other organic material which can pass through the membrane. Hence, it was decided to give the activated charcoal treatment, so that the stream can be

Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999 2063

Figure 5. Effect of pH on the wet oxidation of a pure dye (temperature 200 °C, O2 pressure 0.69 MPa, catalyst 3.757 × 10-4 kmol m-3).

Figure 6. Flux and rejection of salts, color, and COD (actual waste: concentrate mode).

made reusable in the process. The NORIT activated carbon in powder form (0.1% w/v) was tried. The COD was reduced to 150 ppm, and color reduction was 99.9% of its original value. Thus, the stream can be reused in the process, and the concentrate can be treated by wet oxidation. This membrane carbon polishing hybrid system can make the stream recyclable, thereby conserving the most precious utility, namely, water. Wet Oxidation. Wet oxidation performance was studied with the concentrate of the actual dye bath waste obtained from the membrane system and was compared with the pure dye compound under similar operating conditions. The concentrate from the membrane system contained practically the same chemicals as those in the waste. We have used dilute waste (COD 500-700 mg/ L) from the membrane concentration in wet oxidation studies for comparison purposes. It was interesting to note that in the case of waste only 30% reduction in COD was achieved in 120 min at higher operating temperature (225 °C) and O2 pressure (0.69 MPa) even in the presence of catalyst (3.757 × 10-4 kmol m-3), while 90% COD reduction was seen at 190 °C and oxygen partial pressure 0.69 MPa for the pure dye

Figure 7. Effect of sonication on the wet oxidation of an actual waste (temperature 225 °C, O2 pressure 0.69 MPa, catalyst 3.757 × 10-4 kmol m-3).

solution. This refractory nature of the waste at a given temperature could be attributed to the presence of refractory compounds in the waste from the dye bath. The organic surfactant such as alkyl benzenesulfonate and dye fixing agents present in the dye bath waste are difficult to be oxidized and may require more severe conditions. Ingale and Mahajani12 have demonstrated the destruction of refractory waste by the process of SONIWO. Hence, it was, therefore, decided to treat the dye bath waste in a similar fashion. The sonication treatment was given in the presence of catalyst before wet oxidation, in an agitated glass reactor for 30 min. The treatment was carried out at near neutral pH as well as for basic conditions (pH ∼ 11) of the concentrated waste. It was found that, after sonication, wet oxidation works smoothly and 80% COD reduction can be achieved in 120 min at similar conditions mentioned above for pH ∼ 7 (Figure 7). The wet oxidation was also studied at dye bath pH (∼11) where the system was heterogeneous with respect to the catalyst. However, under alkaline conditions, wet oxidation was not as effective as in near neutral conditions. Figure 7 shows the effect of pH on wet oxidation of the actual waste after sonication treatment. Thus, we see that sonication followed by wet oxidation treatment is very effective in treating the actual waste. The wet oxidation was found to be more effective in the presence of the CuSO4 catalyst and at neutral pH. While the stream is discharged, it is necessary to adjust pH 7-8 maximum. Therefore, it is recommended that the pH be adjusted to 7 before sonication followed by wet oxidation so that one can take advantage of the higher rates of COD reduction during wet oxidization due to homogeneously catalyzed wet oxidation. Conclusions It is always desirable to use the actual waste for treatability studies rather than using simulated waste prepared with major components. The auxiliary chemicals used in the process would hold the key to the treatability studies. A hybrid technology, MEMSONIWO, to treat and reuse the reactive dye bath wastewater

2064 Ind. Eng. Chem. Res., Vol. 38, No. 5, 1999

has been successfully demonstrated at the laboratory level for treating an aqueous stream containing reactive turquoise blue CI25 dye. Wet oxidation of phthalocyanine class reactive dye CI25 was found to be very effective to remove color and COD; however, the actual dye bath waste is relatively refractive because of other chemicals such as organic surfactant present in the waste. It is recommended that the pH of the concentrate be adjusted to 7 before catalytic sonication and wet oxidation. Acknowledgment A.D.D. thanks the University Grants Commission, Government of India, New Delhi, India, for the financial support of this work. We thank Mr. B. I. Motiwala, R&D Manager, The Hindoostan Spinning and Weaving Mills Ltd., Mumbai, India, for samples and useful discussion. Nomenclature COD ) chemical oxygen demand, mg/L BOD ) biological oxygen demand, mg/L [O2] ) oxygen concentration, kmol/m3 r ) rate of oxidation reaction with respect to COD reduction, mg/L‚min T ) temperature, K [COD]0 ) initial COD, mg/L [COD]f ) COD (mg/L) at time t (min) t ) time, min Rc ) rejection coefficient Cv ) concentration factor Cp ) concentration of the permeate Cf ) concentration of the feed V0 ) original feed volume Vp ) volume of the permeate collected i.d. ) internal diameter

Literature Cited (1) Porter, J. J. What You Should Know About Waste Treatment Process. Am. Dyest. Rep. 1971, 60 (8), 17. (2) Green, J. M.; Sokol, C. Using Ozone to Decolorize Dyeing Plant Wastewater. Am. Dyest. Rep. 1985, 74 (4), 50. (3) Balasubramaninan, M. R.; Murlisankar, I. Utilization of Fly Ash and Tea-Waste Ash as Decolorising Agents for Dye Effluents. Indian J. Technol. 1987, 25 (10), 471. (4) Scott, J. P.; Ollis, D. F. Integration of Chemical and Biological Oxidation Processes for Water Treatment: Review and Recommendations. Environ. Prog. 1995, 14, 88.

(5) Elliott, J. Membrane Filtration Techniques in Dyestuff Recovery. In Environmental Chemistry of Dyes and Pigments; Reife, A., Freeman, H. S., Eds.; John Wiley and Sons: New York, 1996. (6) Shende, R. V.; Mahajani, V. V. Wet Air Oxidation of Anthraquinone and Phthalocyanine Class Reactive Dyes. Am. Dyest. Rep. 1994, 6, 40. (7) Donlagic, J.; Levec, J. Oxidation of an Azo Dye in Subcritical Aqueous Solutions. Ind. Eng. Chem. Res. 1997, 36, 3480. (8) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (9) Matatov-Meytal, Y. I.; Sheintuch, M. Catalytic Abatement of Water Pollutants. Ind. Eng. Chem. Res. 1998, 37, 309. (10) Hutton, D. G. The PACT System for Wastewater Treatment. Environmental Chemistry of Dyes and Pigments; Reife, A., Freeman, H. S., Eds.; John Wiley and Sons: New York, 1996; p 215. (11) Hellenbrand, R.; Mantzavinos, D.; Metcalfe, I. S.; Livingston, A. G. Integration of Wet Oxidation and Nanofiltration for Treatment of Recalcitrant Organics in Wastewater. Ind. Eng. Chem. Res. 1997, 36, 5054. (12) Ingale, M. N.; Mahajani, V. V. A Novel Way to Treat Refractory Waste: Sonication Followed by Wet Oxidation (SONIWO). J. Chem. Technol. Biotechnol. 1995, 64, 80. (13) Snell, F. D.; Ettre, L. S. Encyclopedia of Industrial Chemical Analysis; John Wiley and Sons: New York, 1967; Vol. 17, p 215. (14) Vogel, A. I. A Text Book of Quantitative Inorganic Analysis, 3rd ed.; Langman Group Ltd.: London, 1975. (15) Manivasakam, N. Treatment of Textile Processing Effluents (Including Analysis); Sakthi Publications: Coimtore, India, 1995. (16) Shende, R. V.; Mahajani, V. V. Kinetics of Wet Air Oxidation of Glyoxalic Acid and Oxalic Acid. Ind. Eng. Chem. Res. 1994, 33, 3125. (17) Shende, R. V.; Mahajani, V. V. Kinetics of Wet Oxidation of Formic Acid and Acetic Acid. Ind. Eng. Chem. Res. 1997, 36, 4809. (18) Crammer, S. D. The Solubility of Oxygen in Brines from 0 to 300 °C. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 300. (19) Li, L.; Chen, P.; Gloyna, E. F. Generalized Kinetic Model for Wet Oxidation of Organic Compounds. AIChE J. 1991, 37, 1687. (20) Tufano, V. A. Multi-Step Kinetic Model for Phenol Oxidation in High-Pressure Water. Chem. Eng. Technol. 1993, 16, 186. (21) Thomsen, A. B. Degradation of Quinoline by Wet OxidationsKinetc Aspects and Reaction Mechanism. Water Res. 1998, 32 (1), 136.

Received for review September 24, 1998 Revised manuscript received January 26, 1999 Accepted February 7, 1999 IE980615T