Thermochemical Precipitation as a Pretreatment Step for the Chemical

In the present work, the removal of chemical oxygen demand (COD) and color of paper mill wastewater due to the thermochemical precipitation of dissolv...
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2016

Ind. Eng. Chem. Res. 2005, 44, 2016-2026

Thermochemical Precipitation as a Pretreatment Step for the Chemical Oxygen Demand and Color Removal from Pulp and Paper Mill Effluent Anurag Garg, I. M. Mishra,* and Shri Chand Department of Chemical Engineering, Indian Institute of Technology, Roorkee, Roorkee 247667, Uttaranchal, India

In the present work, the removal of chemical oxygen demand (COD) and color of paper mill wastewater due to the thermochemical precipitation of dissolved solids was studied in the temperature range from 20 to 95 °C using different catalysts/chemicals. The homogeneous CuSO4 catalyst was found to be the most active in comparison to the other heterogeneous catalysts under similar operating conditions. The pH value showed a pronounced effect on the precipitation process. At an optimum initial pH of 5.0, a maximum COD reduction of 63.3% was obtained with a catalyst concentration of 5 kg m-3, although the maximum color removal was 92.5% using a CuSO4 concentration of 2 kg m-3. The residual copper in the supernatant works as a good catalyst for wet air oxidation of the supernatant. Thermogravimetric analysis showed that the thermal oxidation of the solid residue follows an one-way transport diffusion model with first-order irreversible reaction kinetics. The heating value of the precipitate was found to be comparable (19.72 MJ/kg) to that of the Indian coal (20.90 MJ/kg). Introduction The wastewater generated from integrated pulp and paper mills is highly polluting because it contains high biochemical and chemical oxygen demands (BOD and COD), toxic substances, recalcitrant organics, and intense color. The effluent originating from the chemical pulping stage is called black liquor, and it contains lignin, organic acids, phenolic compounds, sulfur compounds, terpenes, resins, etc.1 Wood extractives from the cooking operation have poor solubility and high toxicity and therefore are not easily amenable to biological treatment.2 In general, most of the integrated mills use multiple effect evaporators for the concentration of the wastewater, followed by the incineration of the concentrated black liquor in a recovery furnace to recover the chemicals and heat energy in the form of steam; even though the recovery is not efficient.3 Catalytic wet (air) oxidation of the black liquor at high temperature and pressure ensures high efficiency of chemicals and energy recovery,4 but it is still not the preferred mode of treatment due to cost considerations. To make the secondary treatment cost effective as well as efficient in the removal of toxic organic compounds and color, coagulation/flocculation can be used as an effective primary treatment method. In this method, a maximum of colloids and dissolved solids present in wastewater can be removed by hydrolyzing metal salts using different coagulants.5 The precipitation of dissolved lignin and other matters by using different coagulants such as alum, ferric chloride, polyaluminum chloride (PAC), and lime has been reported by several researchers.5-12 Lime coagulation remains a popular method of color reduction although methods such as membrane processes and * To whom correspondence should be addressed. Tel.: +911332-285715. Fax: +91-1332-273560. E-mail: imishfch@ iitr.ernet.in.

activated carbon adsorption are also used. The pH of the wastewater has a significant effect on the coagulation/precipitation.13 The optimum pH range for different coagulants is different. The use of a single coagulant has been preferred over a combination of coagulants due to complexity involved in the coagulant recovery, regeneration, and reuse.5 The chemical salts as coagulants have some drawbacks, such as the high cost of chemicals for precipitation as well as for pH adjustment, problems associated with sludge management, i.e., bulkiness, dewatering, and disposal, and the residual concentration of metal cations, which leaves its imprint on the color of the supernatant.5,14 Sundin15 has studied the influence of several metal ions on the lignin removal and reported that only Ca2+ and Mg2+ ions showed appreciable lignin precipitation in the pH range of 1113. The industry generally uses primary physicochemical treatment followed by secondary treatment, such as wet air oxidation, incineration, or biological treatment. Several researchers have shown that the thermal pretreatment of high-strength wastewaters followed by the wet oxidation of the solution may be a better treatment strategy.16-19 A two-stage coagulation/precipitation (with PAC)-adsorption (with sugar cane bagasse fly ash) process has been found to be very effective to treat effluents of small agri-based pulp and paper mills.11 Garg et al.19 have observed that copper sulfate (CuSO4‚5H2O) is a very effective chemical in the thermochemical precipitation of the non-biodegradable organics present in the effluent of an integrated pulp and paper mill. The residual copper present in the supernatant of the pretreatment step was found to work as a very effective catalyst in the following wet oxidation step. To the best of our knowledge, no report is available in the literature that uses copper sulfate as the chemical coagulant for the coagulation/chemical precipitation of dissolved organics in wastewaters.

10.1021/ie048990a CCC: $30.25 © 2005 American Chemical Society Published on Web 03/05/2005

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2017 Table 1. Characteristics of Synthetic Wastewater (Diluted Black Liquor) parameters

values

chemical oxygen demand (COD) biochemical oxygen demand (BOD) pH total solids (TS) total dissolved solids (TDS) total suspended solids (TSS) sodium (Na+) potassium (K+) S as SO42chloride conductivity color

7 kg m-3 1.4 kg m-3 10.8 7.24 kg m-3 6.68 kg m-3 0.56 kg m-3 15.61 kg m-3 nil 1.43 kg m-3 0.05 kg m-3 51 760 µm hos cm-1 dark brown

The present work was basically aimed to study the catalytic (or chemical) precipitation/coagulation of the soluble non-biodegradable organic matter present in the black liquor by using different chemicals to serve as coagulant/catalyst/adsorbent, i.e., CuSO4‚5H2O, 5%CuO/ 95% activated carbon, 60% CuO/40% CeO2, and 60% CuO/40% MnO2. The effect of different parameters, such as initial pH, chemical (coagulant/adsorbent) concentration, and temperature, on the COD removal were also studied. Besides, the settling characteristics of the precipitated sludge and filterability of the effluent after the pretreatment at different precipitation conditions were also studied. The energy recovery from the wastewater in the form of the solid residue was computed, and the elemental analysis of the effluent and the precipitate were also done. Thermogravimetric analysis (TGA) of the precipitate formed after the coagulation/ precipitation was performed to understand its thermal degradation characteristics. Experimental Section Effluent from a Kraft Pulp Mill. Black liquor was obtained from a local integrated pulp and paper mill, which had a COD of ∼700 kg m-3. Synthetic wastewaters of different strengths were prepared by diluting this black liquor with tap water. The characteristics of such a synthetic wastewater are given in Table 1. Chemicals. All of the chemicals used for the preparation of various coagulants/catalysts/adsorbents were analytical reagent grade. CuSO4‚5H2O and Cu(NO3)3‚ 3H2O were purchased from s.d. Fine Chemicals Ltd., Mumbai, India, whereas Ce(NO3)3‚6H2O and activated charcoal were purchased from Loba Chemie and E. Merck Pvt. Ltd., Bombay, India, respectively. Ammonia solution was purchased from Ranbaxy Fine Chemicals Ltd., Mumbai, India. Coagulant/Catalyst Preparation. The preparation method of the coagulants/catalysts, i.e., 5% CuO/95% activated carbon and 60% CuO/40% CeO2, are given below. The 60% CuO/40% CeO2 catalyst was prepared by coprecipitation of the stoichiometric amounts of copper nitrate and cerium nitrate in aqueous ammonia solution for the corresponding catalysts. The pH was maintained at about 8.0. The mixture was then filtered and thoroughly washed with distilled water. The precipitate was dried in an oven at 378-383 K for 24 h, then calcined stepwise at 623 K for 2 h, and used for the precipitation experiments. The 5% CuO/95% activated carbon was prepared by precipitating copper hydroxide onto activated carbon. For this, copper nitrate was dissolved in distilled water,

and ammonia solution was added dropwise to precipitate it into copper hydroxide, and then the required amount of activated carbon was added to the solution. The solution was stirred for 1 h by means of a magnetic stirrer to allow good mixing. The solution was, thereafter, filtered, washed, and dried at 378-383 K for 24 h in an oven. The resulting mixture was heated stepwise at 473 K for 2 h and then used for the coagulation/ precipitation experiments. Analytical Methods. Na+ and K+ ion concentrations in dilute effluent were measured using a digital flame photometer (model CL 22D), purchased from Elico Pvt. Ltd., Hyderabad, India. Sulfate concentration in the paper mill effluent was determined by a gravimetric method (APHA, 1989). Electrical conductivity was measured using a conductivity bridge (Philips PR 9500). The treated wastewater samples were centrifuged (model R 24, Remi Instruments Pvt. Ltd., Mumbai, India) to obtain the clear supernatant and the precipitate. The CODs of the effluent before and after the treatment were determined by the dichromate open reflux as per the standard method.20 The color of the initial and final effluent after the treatment was measured at a wavelength of 363 nm using a UV-vis spectrophotometer (model Lambda 35, Perkin-Elmer, Switzerland). Thermal analysis (TGA/DTGA/DTA) of the residue left after the treatment was carried out at the Institute Instrumentation Centre, IIT Roorkee, India, using a TG analyzer (Pyris Diamond, PerkinElmer). The concentrations of the copper ion in the sludge and the solution were determined using an atomic absorption spectrometer (AAS) (Avanta GBC, Australia). The elemental analysis for C, H, N, and S of the black liquor and the precipitate was carried out by using an Elementar Vario EL III (Elementar Analyzensysteme GmbH, Germany). The heating value of the precipitated sludge was determined by preparing a suitable pellet and combusting it in a standard adiabatic bomb calorimeter.21 The proximate analysis of the sludge and the black liquor was carried out as per procedure in ref 22. Experimental Procedure. The experimental studies at temperatures higher than ambient temperature were carried out in a 500 mL three-neck glass reactor. Initially, the pH of the wastewater was adjusted by adding H2SO4 or ammonia solution, and then the wastewater was shifted to the three-neck glass reactor. Thereafter, the catalyst/coagulant/chemical was added to the solution. The temperature of the reaction mixture was raised using a hot plate to the desired value by a P. I. D. temperature controller, which was fitted in one of the necks through the thermocouple. The raising of the temperature of the wastewater from ambient to 95 °C took about 30 min (th). A vertical water-cooled condenser was attached to the middle neck of the reactor to prevent any loss of vapor. The time taken to attain the desired temperature is the heating time, th. Further heating is done at the desired temperature, and the time is measured by subtracting th from the total time. Thus, th is taken as zero for further heating and reaction. The reaction mixture was agitated using a magnetic stirrer. Initial experimental runs with different chemicals were conducted for 4 h at 95 °C, and the reactor samples were taken at periodic intervals for the measurement of the COD and pH. The samples were centrifuged to decant the supernatant. Then, the supernatant was tested for the COD and color. Some amount of color/COD gets

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Figure 2. COD reduction of paper mill effluent using various catalysts. Conditions: COD0 ) 7 kg m-3, tR ) 4 h, Cw0 ) 5 kg m-3, T ) 95 °C, P ) 1 atm, pH ) 8.0. Table 2. Final pH Values of the Effluent after Thermochemical Pretreatmenta sample no.

chemical

pH of the treated effluent

1 2 3 4 5

no chemical CuSO4 60% CuO/40% CeO2 activated carbon 5% CuO/95% activated carbon

9.27 4.50 4.80 7.75 7.87

a T ) 95 °C, P ) 1 atm, chemical mass loading ) 5 kg m-3, treatment time ) 4 h, initial pH ) 8.0, initial COD ) 7.0 kg m-3.

Figure 1. Flowchart of the experimental procedure.

reduced from the time the chemical/coagulant/adsorbent is added to the wastewater up to zero time. However, for brevity, this reduction in COD/color is shown at zero time in this paper. The final pH of the solution after the reaction was also observed. Except for initial runs, the rest of the experiments, at temperatures higher than the ambient temperature were carried out for a 30 min reaction period, after attaining the desired temperature and then allowed to settle for 2 h. Then, the treated effluent including sludge was rapidly mixed, and the slurry formed was used to study the settling and filterability characteristics of the sludge. The runs at ambient temperature were taken in a 500 mL beaker. The wastewater was put into the beaker after adjusting to the desired pH, and at the same time the coagulant/chemical was added to the beaker. The reaction mixture was agitated for 30 min using a magnetic stirrer, and then the wastewater was allowed to settle for 2 h. Thereafter, a small amount of the sample was taken out to determine the pH and COD of the treated sample. The solution left was again mixed rapidly for 5 min to make it a homogeneous solution, and then it was examined for the settling and filterability characteristics. The flow diagram for the experimental procedure is shown in Figure 1. Results and Discussion Effects of Different Chemicals. Figure 2 shows the effect of different chemicals at Cw0 ) 5 kg m-3 on the COD reduction of the effluent when heated to 95 °C at 1 atm at an initial pH 8.0 for 4 h. It is observed that

the thermochemical treatment shows its efficacy in the reduction of the COD of the effluent. CuSO4‚5H2O showed the best activity among all of the catalysts. A COD reduction of 61.43% was obtained with copper sulfate versus only 7.14% COD reduction when no chemical was used. The COD value was thus reduced to 2.7 kg m-3 from the initial 7.0 kg m-3 in the presence of a copper salt. Final pH values after the 4 h reaction period were also observed. These are shown in Table 2. It is found that the thermochemical treatment results possibly in the hydrolysis of the organics and other compounds, transforming them into lower carboxylic acids thereby reducing the pH of the treated wastewater. The presence of activated carbon either alone or in conjunction with CuO leads to adsorption of acids and the dissolved smaller molecule organics, and therefore, the pH is reduced only marginally. Heating to 95 °C without any chemical drives off the carbon dioxide and other acid gases such as H2S, which may result in a pH increase. At such mild temperatures, hydrolysis may also be very insignificant. It is also seen that activated carbon alone is more efficient than 60% CuO/40% CeO2 in the reduction of COD. However, CuSO4‚5H2O shows phenomenally high-COD reduction at zero time, which does not change with the increase in the reaction time. This leads to the conclusion that the thermochemical precipitation is a very fast (instantaneous) process and would need a very small reactor vessel in comparison to those for other catalysts/adsorbents. Therefore, only CuSO4‚5H2O was used for further studies. Effect of pH. Figure 3 shows the effect of initial pH at 25 °C on the COD reduction of the synthetic wastewater having an initial COD of 14.5 kg m-3 using CuSO4‚5H2O at a mass loading of 3 kg m-3. The runs

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2019

Figure 3. Effects of pH on the COD reduction with CuSO4. Conditions: COD0) 14.5 kg m-3, Cw0) 3 kg m-3, T ) 25 °C.

were taken at different initial pH values, i.e., at 5.0, 6.0, 7.0, 8.0, 9.0, and 10.5. pH adjustment was made with either H2SO4 or ammonium hydroxide solution. The initial pH value of 5.0 was found to be the best for the thermochemical pretreatment. A part of the resulting mixture after treatment was taken and centrifuged for 10 min at a speed of 10 000 rpm. Then, the supernatant was decanted off, and its COD was measured. A COD reduction of 57.24% (i.e., 6.2 kg m-3 from the initial value of 14.5 kg m-3) was obtained. The pH values after the treatment were also observed. The decrease in pH may be due to the dissociation of copper sulfate into Cu2+ and SO42- ions and also the formation of lower carboxylic acids. The sulfate ions after combining with H+ ions present in wastewater form H2SO4 that reduces the pH of the solution. The pH of the supernatant of the treated wastewater decreased from the initial pH 10.5 to 8.9 and from 5 to 4.6. Effect of Temperature. The effect of treatment temperature on the COD reduction was also observed for copper sulfate. The treatment temperature was varied from room temperature (i.e., 20 °C) to 95 °C, and it was found that an increase in temperature does not increase appreciably the reduction in COD, although the temperature does influence the filterability and settling characteristics of the treated wastewater. This aspect is discussed later. Effect of Copper Sulfate Mass Loading. The effect of copper sulfate mass loading on the COD reduction of the synthetic wastewater (initial COD ) 7.0 kg m-3) was observed at a temperature of 25 °C. The copper sulfate mass loading was varied from 1 to 8 kg m-3, while the initial pH of the diluted black liquor was adjusted to 5.0 for all of the experimental runs. With 1 kg m-3 copper sulfate mass loading, only a 20% reduction in the COD was observed. The solid precipitate also could not be observed. But at a mass loading of 2 kg m-3, a COD reduction of 58.92% was obtained with a substantial amount of precipitated solid residue. This chemical mass loading is termed as the critical chemical (or coagulant) concentration (ccc) at which the precipitation just starts. With an increase in the mass loading from the ccc up to 5 kg m-3, a maximum of 63.3% COD reduction was obtained. Beyond 5 kg m-3 chemical mass loading, no increase in COD reduction was observed. The final pH of the treated wastewater decreased from its initial value, and the decrease in pH was more pronounced as the chemical mass loading increased. This trend may be attributed to the enhanced formation of SO42- ions and/or the formation of carboxylic acids. However, no definite conclusions can be drawn on this

Figure 4. Effects of catalyst concentration on COD removal at 25 °C temperature.

aspect for the time being. At a chemical mass loading of 8 kg m-3, pH decreased from its initial value of 5.0 to 3.8. In another series of runs, the COD reduction for the synthetic wastewater having a COD of 35 kg m-3 was also observed with different chemical mass loadings varying from 0 to 35 kg m-3. The initial pH of the effluent was kept at 8.0. It was seen that a maximum reduction of 74.28% was obtained at a chemical mass loading of 25 kg m-3, after which no change in COD reduction was observed (Figure 4). The pH of the treated effluent also fell from 8.0 to 5.18 at a mass loading of 25 kg m-3, giving a ratio of 0.71 kg chemical per kilogram of initial COD. This is much higher than the 0.43 kg catalyst per kilogram of initial COD for the wastewater of the initial COD, 7 kg m-3. This shows that as the initial COD of the wastewater increases, comparatively larger chemical mass loading is required for optimum COD reduction. It is also found that the efficiency of COD reduction due to thermochemical precipitation increases as the initial COD of the effluent increases with optimum chemical mass loading. When the requirement of the chemical is calculated for each kilogram of COD removed, the difference in the chemical mass loading is observed. For example, for the initial CODs of 35.0 and 7 kg m-3, the optimum chemical requirements have been about 0.96 and 1.14 kg of copper sulfate per kilogram of COD removed. This also indicates that as the initial COD level of the effluent increases, the requirement of the chemical per unit removal of COD slightly decreases, reflecting a higher COD removal efficiency. The optimum coagulant/total carbon dosage for bleached kraft effluent has been recommended in the range of 0.5-2.5 kg/kg TOC for alum10 and 2 kg Fe/kg TOC for ferric chloride.23 Alrnemark and Ekengren24 recommended 3-6 mol of trivalent cation per gram of influent COD for different compounds. Compared to these values, the requirement for copper sulfate is very low ( 8). Charge neutralization takes place due to cationic surfaces of the coagulants, in this case copper. This lowers the ζ potential and results in the separation of dissolved compounds. This mechanism of separation may be termed as a perikinetic or electrokinetic mechanism.27 At lower pH values, negatively charged organic molecules form insoluble complexes by reacting directly with cations, whereas adsorption of organic species on the coagulant flocs followed by precipitation is the dominatant mechanism at higher pH values as well as high coagulant mass loading.5 Soluble organic matters are removed in the pH range of 5.3-5.7 using aluminum salts as a coagulant.10 With copper sulfate, maximum coagulation was obtained at pH 5.0. Actually, the presence of different compounds in the wastewater affects the pH range for the use for different coagulants. With the addition of copper sulfate, the pH of the wastewater decreases. This decrease may be due to diverse hydrolytic reactions, taking place during coagulation, forming multivalent charged hydrous oxide species and generating H3O+ ion during each step, thus reducing the pH value.28 Stephenson and Duff5 have also reported a decrease in pH after coagulant addition to highly acidic levels, because the coagulant dose is highly correlated with the pH value. The higher doses of coagulants without the need for minimization are reported due to two reasons, one for the increment in the aggregation rate and the other by enmeshing particulates into large aggregates.26

Figure 7. Effect of catalyst mass loading (Cw0) on color removal. Conditions: pH) 8.0, COD0) 7 kg m-3.

Effect of the Initial COD of the Effluent on the COD Removal. Figure 5 shows the effect of initial COD of the effluent on COD removal at a constant catalyst mass loading of 5 kg m-3. The pH for all the experiments was kept at 8.0. It was seen that no removal in the COD was obtained, when the COD of the wastewater was 250 kg m-3. It can be seen from the figure that the reduction in COD was only 19.44% up to a COD of 36 kg m-3. The COD removal was 68% for an initial COD of 18 kg m-3, whereas, it was 61.43% for a wastewater having a COD value of 7 kg m-3. The reason for the retardation in COD removal is due to the increasing viscosity of the effluent as its COD increases and the requirement of increasing copper sulfate loading for effecting COD removal as was shown in an earlier subsection. Effect of pH and Copper Sulfate Mass Loading on the Color Removal. The effect of pH on the color removal for a diluted black liquor having a COD value of 7 kg m-3 is shown in Figure 6. The color removal of 88% was obtained at pH 8.0. The minimum color removal of 79.8% was found at pH 10.5. The color removal at the optimum pH 5.0 for COD removal was 80%, which was lower than the values obtained at higher pH values. It may be due to the increased concentration of copper ion present in the supernatant, thus affecting lower adsorption of color on the precipitate. Figure 7 shows the effect of copper sulfate mass loading on the color removal. It was found that the removal of color increases with the increase in copper sulfate mass loading up to the critical coagulant concentration (ccc). The maximum color removal of 92.5% was obtained at a copper sulfate mass loading of 2 kg m-3. Above 2 kg m-3, the color removal decreases. This reduction may be attributed to the enhancement of the dissolved copper concentration in the solution, which may interfere with color adsorption. Several researchers have reported a color removal in the range of 80-100%, depending upon the nature of the effluent and operating conditions using different coagulants.5,10,14,29-30 The results obtained in the present study are comparable to those that have been reported in the literature. Wanpen et al.31 reported only 24.42% color removal at an optimum pH of 8.0 using soil as a coagulant. The processes of chemical precipitation and the adsorption of color can be reasons for the removal of color. It is found that the ζ potential, expressed as electrophoretic mobility, is closely correlated with color removal. It is also found that the optimum pH for the removal of color may not be the optimum pH for the minimum solubility of the cation of the coagulant.28 The

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2021

Figure 8. Variation of the copper solubility with pH. (Initial copper sulfate concentration ) 5 kg m-3).

coagulant also imparts its own color in the pH range of minimum cation solubility. The maximum color removal was seen over a pH range of 6-9. The colored compounds present in paper mill effluent are basically high molecular weight compounds such as tannins, lignin, and lignin-derived and -associated compounds. These compounds are negatively charged, are, therefore, neutralized with the copper ions, and thus are removed leading to reduction in the color of the wastewater. The color reduction increases up to a breakpoint as the copper sulfate mass loading increases. After this critical mass loading, the coagulant contributes its own color to the effluent as the copper sulfate dosage increases. This is seen in Figure 7. This observation is in consonance with those observed by Stephenson and Duff5 and Ho et al.32 Concentration of Copper Ions in the Precipitated Sludge and the Supernatant Left after the Precipitation. With CuSO4‚5H2O as the chemical agent for the precipitation of dissolved solids, certain amounts of Cu2+ ions also remain in the solution or filtrate (supernatant). The amount of Cu2+ ions in the solution varies with the pH of the solution, as shown in Figure 8. The initial Cu2+ concentration added to the paper mill effluent was 1.272 kg m-3 (corresponding to 5 kg m-3 of CuSO4‚5H2O). The pH of the thermally pretreated solution was 4.5. A small amount (50 mL) of the solution was taken, and the pH values were adjusted to 6.0, 8.0, and 10.0. The solutions were allowed to remain for 2 h and then filtered, and the Cu2+ ion concentration in the filtrate was measured. The copper concentration was found to be almost the same at pH 6.0 and 8.0 (at around 0.59 kg m-3), but at pH 4.5 and 10.0, the majority of the copper remains in the solution (0.88 and 0.87 kg m-3, respectively). The presence of Cu2+ ions in the solution imparts toxicity to it. However, for the situations where the supernatant is to be taken for wet oxidation (secondary treatment), Cu2+ ion concentration in the supernatant may serve as the catalyst. Garg et al.19 have shown that the effluent obtained after the primary treatment (chemical precipitation) with CuSO4 can be directly taken for wet oxidation without any additional CuSO4‚5H2O. They found about 96% COD reduction from an initial COD of 7 kg m-3. Under alkaline environment, copper sulfate gets hydrolyzed and forms copper hydroxides, which are insoluble in water. The hydroxide is somewhat amphoteric, dissolving in excess sodium hydroxide solution to form trihydroxycuprate [Cu(OH)3] and tetrahydroxy cuprate [Cu(OH)4].33 Thus, in a basic environment, the

majority of copper is present in precipitated hydroxide form, and very little copper is present in a soluble form. TGA-DTA of the Sludge. Figure 9 shows the thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTGA), and differential thermal analysis (DTA) curves for the precipitated sludge at 10 °C min-1 heating rate with 200 mL min-1 air flushing rate. The nature of the TGA trace shows dehydration and volatilization (removal of volatiles) of the sample and the degradation of the lignin and other components up to a temperature of 363 °C, losing about 30.5% of its weight. In this regime, the precipitate is unstable. Between 363 and 435 °C, a narrow span of 72 °C, the precipitate oxidizes, losing about 50% of its original weight. The peak rate of weight loss of 21% min-1 is at a temperature Tmax ) 379 °C. The oxidation is found to be uniform and exothermic with a heat evolution of 7.87 MJ/kg, the peak of the exotherm being at a temperature of Tp ) 433 °C. The oxidation seems to be complete at 435 °C, and the traces of TG, DTG, and DTA reflect this fact. It is found that the organics of the precipitate, mainly lignin, protein, etc., get oxidized catalytically, leaving the ash fraction (∼17.4%) as the residual weight fraction. Figure 10 shows the TGA, DTGA, and DTA behaviors of the paper mill effluent under similar linear heating and air flushing rate conditions. In contrast to the TGA trace of the precipitate, the black liquor TG trace shows a gradual decrease in residual sample weight up to a temperature of 480 °C, shedding about 50% of the sample weight. Thereafter, the weight-loss rate is extremely slow, and up to 691 °C (over a temperature span of 211 °C), the weight loss is only 2.64%. This shows that the black liquor sample loses moisture at an almost steady rate along with volatilization of light volatiles up to 480 °C, and thereafter, the sample becomes dry and stable. At 691 °C, the oxidation of the dry sample started, and the sample lost weight quickly (∼16% weight loss) over a temperature range of 691726 °C (a temperature span of 35 °C). The maximum weight-loss rate was 26% min-1 at Tmax of 712 °C (see DTGA trace). The peak temperature for the exothermic reaction as exemplified by the DTA curve was at Tp ) 725 °C with heat release of 1.83 MJ/kg. Beyond 726 °C, the weight loss is steady but very slow, giving of ∼10% weight from 726 to 997 °C (over a temperature increase of 271 °C). A comparison of Figures 9 and 10 brings out clearly the fact that the precipitate having a copper ingredient gets oxidized at a much lower temperature range than that of the black liquor. This is due to the catalytic effect of copper in accelerating the oxidation of the precipitate, giving off a large amount of heat. The thermal degradation data (TGA, DTA, and DTGA) were analyzed using the kinetic models available in the literature.34-36 On the basis of the analysis of the kinetics, the overall thermal degradation characteristics are found to be well-represented by an one-way transport diffusion model assuming a first-order irreversible reaction of the organics in the precipitate. Through the use of the theory of the active complex (preceding state),34,37 the reaction rate constant, k, can be written as

k)

χekBT ∆S E exp exp h R RT

( ) (

)

(1)

where χ is the transmission coefficient (1.0 for mono-

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Figure 9. TGA-DTA analysis of the precipitate formed after thermal treatment. Conditions: atmosphere ) air, air flow rate ) 200 mL min-1, heating rate ) 10 °C min-1, sample weight ) 10.48 mg.

Figure 10. TGA-DTA analysis of the black liquor. Conditions: atmosphere ) air, air flow rate ) 200 mL min-1, heating rate ) 10 °C min-1, sample weight ) 10.57 mg.

molecular reactions), kB is Boltzmann’s constant, h is Plank’s constant, e is Neper’s number (2.7183), T is absolute temperature, R is the universal gas constant,

∆S is the change of entropy for the active complex formation from the reactant, and E is the activation energy.

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2023 Table 3. Kinetic Parameters Calculated for the Solid Residue Left after Treatment with Copper Sulfate from the One-Way Transport Diffusion Kinetics (D1) and Agarwal and Sivasubramanian (AGS) Models models sample no.

parameters

D1

AGS

1 2 3 4 5 6 7 8 9

n A (min-1) E (kJ mol-1) k (min-1) ∆S (J mol-1 K-1) ∆H (kJ mol-1) ∆G (kJ mol-1) P r2

1 36 313 73.68 0.05 -258.28 68.26 236.70 3.23 × 1014 0.981

0 1.11 × 10-6 31.22 3.52 × 10-9 -407.74 25.80 291.70 5.02 × 10-22 0.973

Through the use of the Arrhenius equation

(

k ) A exp -

E RT

)

(2)

where A is the preexponential or frequency factor. Combining the eqs 1 and 2 gives the preexponential factor A as

A)

χekBT ∆S exp h R

( )

(3)

Thus, ∆S can be calculated as

(

∆S ) R ln A - ln

)

χekBT h

E ) ∆H + RT

(4) (5)

The changes of the enthalpy ∆H and Gibbs free energy ∆G for the active complex formation can be calculated using the well-known thermodynamical equation

∆G ) ∆H - T∆S

(6)

∆S, ∆H, and ∆G may be calculated at T ) Tmax (Tmax is the DTG peak temperature), since this temperature characterizes the highest rate of the process and, therefore, is its most important parameter. The steric factor for a particular temperature zone of degradation of the precipitate may be given by P ) exp (∆S/R).34 This factor is necessary to determine whether the degradation taking place in the selected zone is slow or fast. If it is closer to unity for the selected zone than that for the other zone, then it can be said that the degradation in the selected zone is faster than that for the other zone. The best fit values of the kinetic parameters from the one-way transport diffusion kinetics (D1) and power law kinetic model with the Agarwal and Sivasubramanian (AGS) approximation for the exponential term34,36 are given in Table 3. Elemental and Compositional Characterization of the Sludge and Black Liquor. Tables 4 and 5 show the results of the C, H, N, S, and proximate analyses of the settled precipitate and the black liquor, respectively. The heating values of the precipitate and the black liquor are also given and compared with those of Indian coal. The elemental analysis shows that there are enhancements in carbon, nitrogen, hydrogen, and sulfur composition in the precipitate and that its heating value compares well with that of Indian coal. The supernatant left is much leaner in carbon and hydrogen composition, indicating clearly, as found

Table 4. Elemental Analysis of Black Liquor and Precipitate Formed as a Result of Thermal Pretreatment with CuSO4 material

C (%)

H (%)

N (%)

S (%)

black liquor precipitate supernatant Indian coal

19.99 42.99 6.31 50.00

4.50 4.91 1.38 5.10

0.007 0.113 0.000 0.800

2.71 3.53 15.63 1.70

heating value of solids (MJ/kg) 12.14 19.72 20.90

Table 5. Proximate Analysis (Moisture-Free Basis) of Black Liquor and Precipitate Formed as a Result of Thermal Pretreatment with CuSO422 material

ash (%)

volatile matter (%)

fixed carbon (%)

black liquor precipitate

32.4 18.3

58.0 54.1

9.6 27.6

earlier in COD estimation, that the carbonaceous load of the treated wastewater after filtration has gone down considerably. The proximate analysis, as shown in Table 5, indicates a considerably lower ash content in the precipitate than that in the black liquor and a considerably higher fixed carbon content in the precipitate than that in the black liquor. The above analyses lead us to believe that the energy recovery from the black liquor in the form of combustible precipitate is a distinct possibility. Settling Characteristics of the Precipitate in the Treated Effluent. Although the average driving forces in batch and continuous sedimentation operations are different and the batch driving force varies with time, approximate methods are available for the calculation of the compression zone depth in continuous thickeners.38-40 Although a number of papers have appeared in the literature on the subject of calculating compression zone height in continuous thickeners from the batch sedimentation data,41-44 it is still preferable to use the method proposed by Richardson et al.38 to design a continuous thickener based on single batch sedimentation test. The method of Richardson et al.38 gives a conservative estimate with an inherently high safety limit due to the changing nature of the flocs and their settling and compression characteristics. The settling characteristics of the precipitate of the effluent after treatment with CuSO4 at two different temperatures, i.e., ambient temperature (20 °C) and 95 °C were observed in a 100 mL measuring cylinder. This was done to see the effects of the treatment temperature on the settling characteristics of the precipitate. The settling rate was observed to be higher for 95 °C than that of 20 °C, probably due to the bigger size and more compact aggregated flocs. Figure 11 shows the behavior of treated effluent during sedimentation. The Kynch theory was used to analyze the settling process.38 The parameters such as sedimentation velocity (uc), concentration C(t), and the sedimentation flux were calculated. The sedimentation velocity (uc) was found as the slope of the tangent at a given solid concentration, C. The concentration of sludge at a time t was determined by using the following formula

C) C0(total height)/(height of suspension after time t) The value of Cu, i.e., the concentration of the solids

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Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 Table 6. Effect of Treatment Temperature on Specific Cake Resistance and Resistance of Filter Medium

Figure 11. Settling characteristics of sludge in the treated effluent at 20 and 95 °C thermolysis temperatures using CuSO4. Conditions: COD0 ) 7 kg m-3, pH ) 8.

sample no.

temperature (°C)

specific cake resistance (m kg-1)

resistance of filter medium, Rm (m-1)

1 2 3

20 50 95

272.2 190.9 102.8

4.84 × 105 0.79 × 105 0.69 × 105

effluent was tested at 33, 50, 75, and 95 °C temperatures. The initial pH of the solution was 5.0, and it can be seen that the settling rate increases with temperature. Though, settling rates at 95 °C are lower than those obtained with treatment at pH 8.0. A PAC concentration of 2.5 kg m-3 was also added to see the effect of PAC on the settling characteristics of the sludge. However, the addition of PAC affects adversely the settling characteristics of the sludge. Filterability. The dewatering of the sludge obtained due to coagulation can be carried out by filtration using either a plate and frame filter or a rotary vacuum filter. The filtration characteristics can be determined assuming constant pressure filtration in a plate and frame filter. The force balance in the form of differential equation to evaluate the rate of filtration is given as45

∆t µ RCV ) + Rm ∆V A∆P A

(

Figure 12. Effects of temperature on the settling characteristics at pH 5.0.

required in the underflow, for the effluents treated at 20 and 95 °C were found to be 10 and 28 kg m-3, respectively. The maximum value of [{(1/C) - (1/Cu)}/ uc] can thus be determined. These were, respectively, 2.95 × 105 and 4.34 × 104 m2 kg-1s at 20 and 95 °C. Through the use of these values, the area of the sedimentation tank for any effluent flow rate can thus be calculated as

A ) vfC0

(

1 1 C Cu uc

)

where vf is the volumetric flow rate of the effluent (m3 s-1) and C0 is the initial solid concentration (kg m-3). The settling of the effluent treated at a higher temperature is faster than that treated at a lower temperature. Thus, an increase in the treatment temperature will bring down substantially the area of the sedimentation tank. From Figure 11, it can also be seen that the settling rate is very fast during the zone settling region. After about 5 h for 20 °C and 2 h for 95 °C, the settling rate becomes quite poor, because the solid settling enters the compression region. It is also seen that the compression region for the 95 °C settled sludge is much denser (more than twice) than that for the 20 °C settled sludge. Figure 12 shows the effects of temperature on the settling characteristics of the paper mill effluent. The

)

where ∆t is the time interval of filtration (s), ∆V is the volume of the filtrate collected during ∆t (m3), V is the cumulative average volume of the filtrate collected up to that time interval (m3), C is the concentration of the solids in the wastewater, kg m-3, R is the specific cake resistance (m kg-1), µ is the viscosity of the filtrate (Pa s), ∆P is the pressure drop across the filter (Pa), A is the area of filtration (m2), and Rm is the resistance of the filter medium (m-1). In the present study, the filterability of the sludge (solids) obtained from the treatment of the paper mill effluent with copper sulfate at three temperatures, i.e., 20, 50, and 95 °C, was tested by using a gravimetric filter having pore size of ca. 11 µm (grade 1) supported on a ceramic Buchner funnel 0.15 m in diameter. The change in the hydrostatic head was assumed negligible, and the gravity filtration was considered as constant pressure filtration. The filtrate volume with time was observed, and a plot between ∆t/∆V and V was drawn for the effluents treated at different temperatures. The plot showed a linear relationship. It was found that the increase in the temperature of the treatment of the wastewater gives precipitate with reduced specific cake resistance as well as reduced resistance of filter medium. Thus, the filterability of the treated effluent gets improved with an increase in the treatment temperature. The effects of temperature on the filterability characteristics of the treated wastewater are shown in Table 6. This aspect has also been dealt with by Lele et al.17 during thermal pretreatment of the alcohol distillery effluent. The values of R and Rm reported by them were 5.46 × 10-10 to 9.36 × 10-10 m kg-1 and 4.4 × 10-8 to 10.15 × 10-8 m-1, respectively, which are much lower than those shown in Table 6. This difference can be ascribed to several factors, i.e., high treatment temperatures (160-250 °C) and pressures (2.02-7.69 MPa) used by Lele et al.,17 the morphological and floc characteristics of the sludge, which may be different for alcohol distillery effluent than that of the pulp and paper mill effluent, etc.

Ind. Eng. Chem. Res., Vol. 44, No. 7, 2005 2025

Conclusions Among several catalysts used for the thermochemical precipitation of dissolved organics from pulp and paper mill effluent, CuSO4 has shown maximum promise. pH plays a very important role in the thermochemical precipitation and thereby removal of COD in the form of solid residue; pH 5.0 shows the optimum removal efficiency. Critical catalyst concentration (ccc) was found to be 2 kg m-3 at which a sudden spurt in the COD removal from 20% to 58.9% is obtained, when catalyst concentration is increased from 1 to 2 kg m-3 and when precipitation was visible through naked eyes. An increase in temperature from 20 to 95 °C does not show an impact of significance on the precipitation, indicating that the COD removal is basically chemical precipitation due to chemical complexation and coagulation. As the COD value of the effluent increases, the requirement of CuSO4 also increases. For example, a 3 kg m-3 dosage of CuSO4 is sufficient to remove 63.3% COD from initial COD of 7 kg m-3 with pH being maintained at 5.0. If the initial COD of effluent is 35 kg m-3, a CuSO4 dosage of 25 kg m-3 may have to be used with pH being 8.0. Chemical precipitation reduces the pH of the solution. This indicates that the chemical precipitation leads to the formation of H3O+ ions and SO4- ions, which render the solution more acidic. For color removal, pH 8.0 is found to be the optimum at which about 88% color is removed. The elemental analysis of the sludge, black liquor, and the supernatant shows carbon, hydrogen, and sulfur enrichment in the sludge in relation to black liquor. The heating value of the sludge is around 19.72 MJ/kg, which is comparable to that of coal. Thus, a large part of the energy of the effluent could be recovered in the form of sludge with simultaneous reduction in its COD value. The settling and filtration characteristics of the treated effluent improve with the increase in the treatment temperature. The cake resistance and resistance of filter medium were found to be 102.8 m kg-1 and 6.92 × 104 m-1, respectively, at 95 °C in contrast to 272.2 m kg-1 and 4.84 × 105 m-1 at 20 °C. The residual copper concentration in the filtrate after the coagulation/ precipitation can be utilized as a catalyst in the secondary treatment, e.g., in wet oxidation. Copper can also be reduced by pH adjustment. Acknowledgment The authors thank the reviewers for their comments and suggestions. A.G. expresses thanks to the Council of Scientific and Industrial Research, New Delhi, India, for providing the financial support to carry out the present research work. Literature Cited (1) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (2) Stephenson, R. J.; Duff, S. J. B. Coagulation and precipitation of a mechanical pulping effluent-II. Toxicity removal and metal salt recovery. Wat. Res. 1996, 30, 793. (3) Flynn, B. L. Wet air oxidation of black liquor recovery. Chem. Eng. Prog. 1976, 72, 66. (4) Teletzke, G. H.; Pradt, L. A. Zimpro wet air oxidation of soda pulping liquors. Proceedings of 24th Purdue Industrial Waste Conference, Purdue University, Lafayette, IN, 1969; Ann Arbor Science: Ann Arbor, MI, Vol. 139, p 1195.

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Received for review October 17, 2004 Revised manuscript received January 15, 2005 Accepted January 21, 2005 IE048990A