Wet Oxidation—An Option for Enhancing Biodegradability of Leachate

Ind. Eng. Chem. Res. 2010, 49, 5575–5582

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Wet OxidationsAn Option for Enhancing Biodegradability of Leachate Derived From Municipal Solid Waste (MSW) Landfill Anurag Garg* and Alok Mishra Centre for EnVironmental Science and Engineering, Indian Institute of Technology Bombay, Mumbai, India, 400076

The present study reports the treatment of municipal solid waste (MSW) derived landfill leachate (initial pH ) 7.82 and chemical oxygen demand (COD) ) 6400 mg L-1) using catalytic wet oxidation (WO) process. The reaction was performed under moderate conditions (110-150 °C temperature and 0.7 MPa total pressures) in the presence of a CuSO4 catalyst. The effect of the promoter (Na2SO3) was also seen on the COD removal. The results were compared with that obtained for Fe2+ and H2O2 combination. Among all tested combinations, (CuSO4 + Na2SO3 + air) exhibited the best performance with ∼90% COD reduction at 150 °C temperature and 0.7 MPa pressures. Under these conditions, the biodegradability of the treated effluent was improved to 0.66 (from an initial value of 0.38). The average oxidation state of carbon (AOSC) value and Fourier transform infrared (FTIR) spectroscopy confirmed the presence of carboxylic acids in the treated effluent. 1. Introduction Leachate is an aqueous liquid stream generated from a solid waste landfill site due to the percolation of rainwater through the waste, inherent moisture, and biochemical reactions occurring within the disposed waste. A wide range of pollutants may be present in the wastewater, including nitrogenous compounds, heavy metals, organic matters (biodegradable as well as refractory), and inorganic salts. The age of the landfill and solid waste characteristics are the two major factors influencing the composition of leachate.1,2 Prior to safe disposal of the generated wastewater, the pollution load needs to be reduced up to the acceptable point. For India, the upper limits for various parameters to discharge the treated leachate either in inland surface water and public sewers or on land are provided in the Municipal Solid Wastes (Management and Handling) Rules.3 If the leachate is not handled properly, then it may pose major threats to the natural surface and groundwater sources.4 Conventional treatment methods suggested for landfill leachate include leachate recirculation into the disposed waste at landfills and traditional biological and physical/chemical treatment processes.2 Recirculation is not suitable in situations where the wastewater generation rates as well as the quantities of persistent pollutants are high. Often, leachate is diverted to municipal wastewater treatment plants to treat with sewage in various physical/chemical and biological unit processes. This method is feasible if the leachate is biodegradable in nature, generated from young landfills (age < 5 years) and the sewage treatment facility is located near the landfill site. However, the overall capacity of the sewage treatment plant will have to be increased according to the quantity of leachate generated. Among new treatment methods, membrane processes (such as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis) either alone or with other conventional processes enhance the removal of pollutants from leachate.2 The major drawback with membrane processes is the high cost and durability of the membrane itself. The present study investigated the efficacy of noncatalytic and catalytic wet oxidation (WO) processes (a chemical treatment method) for the removal of chemical oxygen demand (COD) from leachate (generated from a municipal solid waste (MSW) landfill site). * To whom correspondence should be addressed. Tel.: 91-2225767861. Fax: 91-22-25764650. E-mail: [email protected].

The WO process is an exothermic hydrothermal reaction that occurs at elevated temperatures (up to 320 °C) and pressures (e20 MPa) in the presence of oxygen.5,6 After the process, organic compounds are converted into stable end products like CO2 and H2O. Several oxidants (like air, molecular oxygen, hydrogen peroxide, and ozone) can be used as a source of oxygen. The use of a suitable catalyst (homogeneous or heterogeneous) may bring a significant drop in the operating conditions, thus resulting in an overall cost (capital and operating both) reduction of the process.7 This process is preferred for wastewaters having low biodegradability, i.e., a five- day biochemical oxygen demand (BOD5) to COD ratio less than 0.5. However, an easily biodegradable compound, CH3COOH, is formed as a major intermediate product that is simply not decomposed by the WO process under moderate operating conditions. Therefore, WO is also suggested as a pretreatment option for enhancing the biodegradability of the wastewater.8 This can be seen from the literature that most of the research studies employed a combination of chemical and biological methods for leachate treatment.9-13 Chemical methods include coagulation and/or advanced oxidation processes. Mainly, H2O2 and O3 have been used as oxidants during the oxidation process. Only a few research studies explored the WO reaction using air as an oxidant.11,12,14 Rivas et al.11 performed a noncatalytic WO study on two leachate samples (having COD ≈ 3300 and 6600 mg L-1) in the presence of oxone (a sulfate radical promoter). It was found that COD removal was increased to 60-90% from a conversion of only 20-30% obtained in the absence of a promoter.11 The above reductions were achieved at very high operating conditions (180-270 °C temperatures and 4-7 MPa pressures) during the reaction. Rivas and coworkers12 conducted another noncatalytic WO study on leachate having an initial COD of 9500 mg L-1 using H2O2 as an oxidant and oxone as promoters. Very low reduction in COD (40 ( 12% with 95% confidence) could be achieved after a reaction time of 180 min at a temperature of 250 °C and 5 MPa pressures. In another study, the degradation of landfill leachate (initial total organic carbon (TOC) ) 430 mg L-1) was investigated using catalytic WO.14 Two heterogeneous catalysts (Mn/Ce and Co/ Bi) were used for the reaction. The Co/Bi catalyst exhibited

10.1021/ie100003q  2010 American Chemical Society Published on Web 05/20/2010

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better TOC removal (∼70%) in comparison to the Mn/Ce catalyst showing only 50% TOC reduction at a 220 °C reaction temperature. This experimental study examined the performance of the catalytic WO process for the treatment of leachate generated from an MSW landfill site. The reaction was carried out under moderate conditions (up to 150 °C temperature and 0.7 MPa total pressures) using air as an oxidant. Since homogeneous CuSO4 has emerged as a very effective WO catalyst for different wastewaters in previous studies,15-17 it was used as a catalyst in the present experimental study. Besides, the influence of sodium sulfite (Na2SO3) as a promoter on the catalytic oxidation process was also studied. The efficiency of the catalytic oxidation reaction was compared with Fenton and noncatalytic reactions. Useful information regarding the biodegradability of treated water, color removal, and average oxidation state of carbon (AOSC) were deduced from the results. Sophisticated analytical techniques such as Fourier transform infrared spectroscopy and X-ray diffraction (XRD) were also used to characterize wastewater and solid residue, respectively. 2. Materials and Methods 2.1. Materials. Leachate used for the experimental study was collected from around an 8-year-old MSW landfill site situated in the state of Maharashtra, India. The wastewater was stored in a plastic container at temperatures below 4 °C to preserve its original characteristics. CuSO4, FeSO4, and H2O2 were purchased from Merck Chemicals, India, and Na2SO3 was supplied by BDH, India. All chemicals were of analytical reagent (A.R.) grade. An air compressor was used for the supply of air required for the oxidation reaction. 2.2. Methods. 2.2.1. WO Experimental Set up and Reaction Conditions. WO experimental runs were performed in a 700 mL capacity stainless steel (SS 316) closed reactor. The reactor was equipped with a six blade mechanical stirrer, safety valve, air inlet, and liquid sampling port. A heating jacket was used to raise the temperature of the reactor to the desired level. The temperature was controlled by means of a proportional integral derivative (PID) temperature controller. The reactor contents were cooled down rapidly by flowing tap water through a cooling coil. The operating parameters (like temperature, pressure, and stirrer speed) were directly recorded from a digital display panel attached to the reactor. For each experimental run, 300 mL of leachate was introduced into the reactor with predetermined amounts of the catalyst and/ or promoter and H2O2 (if required). The reaction mixture was then heated to the predetermined temperature. It took approximately 45 min to reach the temperature from ambient levels to the desired level. As soon as the reaction vessel attained the reaction temperature, a sample was withdrawn. Just after this, compressed air was introduced into the reactor from an air inlet port to increase the total system pressure up to 0.7 MPa (compressed air was also used for reactions using H2O2 as the oxidant). The WO reaction was considered started once the system achieved the desired temperature and pressure. This moment was termed “zero time”. More samples were withdrawn periodically during the reaction and analyzed for pH, COD, and TOC values. The noncatalytic and catalytic WO reactions were performed at moderate operating conditions (temperatures ranging from 110-150 °C and 0.7 MPa total pressures) for a duration of 4 h. All the runs were started with a pH of 7.82 (original pH of the wastewater). Homogeneous CuSO4 was used as a catalyst with

or without Na2SO3 as a promoter in air oxidized reactions. In the Fenton reaction, (FeSO4 + H2O2) was also used alone and in combination with Na2SO3. The reactor contents were mixed at a high impeller speed of 1200 rpm (rpm) to ensure good mixing among the reactants and eliminate any mass transfer resistances. 2.2.2. Analytical Methods. To characterize the leachate before and after treatment, the tests determining pH, COD, BOD5, and TOC were performed. The pH of the wastewater was monitored with a calibrated digital pH meter (model, LP139S; make, POLMON). COD and BOD5 were measured using the closed reflux method and modified Winkler’s method as per standard procedures laid down in the American Public Health Association (APHA) Handbook.18 For COD determination, the digestion of the samples was done in a COD digester (model, DRB-200; make, HACH, USA). TOC analyzer (VCSH; Shimadzu, Japan) was used to measure the TOC of the untreated and treated wastewater samples. Apart from this, the original wastewater was also analyzed for total solids (suspended and dissolved) and inorganic components. Total suspended solids (TSS) and total dissolved solids (TDS) were measured by filtering the wastewater through 1.2 µm pore size filter paper followed by oven drying of the filter paper and filtrate, respectively. Other parameters include alkalinity, inorganic macrocomponents (Na, K, Mg, Cl-, and SO42-) and metals (such as Cu, Fe, Pb, Zn, and Ni). Alkalinity was measured using the standard titration method.18 Ion chromatography (792 Basic IC; make, Metrohome, Switzerland) was performed to determine the anion concentration in the untreated leachate. Heavy metals were measured using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Cations such as Na+ and K+ were determined using a flame photometer (model, 128 Microcontroller; make, Systronics, India). An ultraviolet (UV)-visible spectrophotometer (GENESYS 20; make, Thermoelectron Corporation, USA) was used for measuring the absorbance (at 254 nm) of untreated and treated wastewater. FTIR spectroscopy was performed for untreated and treated liquid samples using the FTIR system (Spectrum - BX; PERKIN ELMER, Switzerland). A solid residue formed after the catalytic WO reaction was subjected to XRD analysis in a PANalytical “XpertPro” X-ray diffractometer. 3. Results and Discussion 3.1. Leachate Characterization. The leachate stored in a cool place was subjected to a determination of various parameters after thorough mixing of the plastic container. The physical and chemical characteristics of the leachate are presented in Table 1. The leachate was slightly alkaline with a pH of 7.82. The main organic compounds dissolved in the leachate may consist of fatty, humic, and fulvic acids. Among these compounds, humic and fulvic acids are difficult to biotreat and drop the biodegradability of the wastewater.4 This fact can be confirmed by looking at the original BOD5/COD ratio () 0.38) of the leachate. The value of the BOD5/COD ratio also indicates the presence of a transient phase (between the acid and methanogenic phases) in the landfill.19 During this phase, the BOD5/ COD ratio of leachate varies between 0.2 and 0.4. Since the landfill site is around 8 years old, it can be classified as an “intermediate” age landfill.2 However, the pH and BOD5/COD values were slightly higher than those reported for intermediate age landfills, though the COD was within the prescribed range of 4000-10 000 mg L-1. It was observed that the wastewater contains around 98% of the total solids that are in dissolved form (TDS ≈ 751 mg L-1).

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Table 1. Physical and Chemical Characteristics of the Leachate and Prescribed Discharge Limits in India discharge limits3 s. no.

parametera

leachate characteristics

inland surface water

public sewers

1 2 3 4 5 5 6 7 8 9 10 11 12 13 14 15 16 17 18

pH color COD BOD5 TOC TDS TSS alkalinityc Na+ K+ Ca2+ Mg2+ ClSO42Cu Fe Ni Pb Zn

7.82 pale yellow 6400 2400 2484 751 18 6200 760.7 169.7 544.1 264.2 51.98 1.98 6.3 3.7 0.7 0.1 0.5

5.5-9.0

5.5-9.0

250 30b

350b

1000

1000

3.0

3.0

3.0 0.1 5.0

3.0 1.0 15.0

All values are in mg L-1, except pH and color. These two parameters are unitless. b These are BOD3 values (3 days at 27 °C temperature) that is equivalent to corresponding BOD5 values (5 days at 20 °C temperature). c Alkalinity was measured in terms of mg L-1 as CaCO3. a

The proportion of TDS and TSS also highly depends upon the pore diameter used for filtering the sample. To determine dissolved and suspended solids, a filter size between 0.45 and 2.0 µm can be used.20 In the present study, a filter paper with a pore size of 1.2 µm was used. The use of a smaller pore diameter will give a much higher TSS and vice versa. Probably, dissolved organic and inorganic compounds (metals, anions, and cations) are responsible for high TDS in the wastewater. High alkalinity of the wastewater (6200 mg L-1 as CaCO3) indicates the presence of larger fractions of bicarbonate and carbonate species in comparison to hydroxide ions. The metals concentration (Cu, Fe, Zn, Pb, and Ni) in the leachate sample was found to be approximately 11.3 mg L-1. All of these metals have been reported as very good WO catalysts for the treatment of different wastewaters due to their high redox potential and a good degree of stability.7,21 Hence, the presence of the metals encourages catalytic oxidation type processes for the treatment of such waste streams so that these metals could be utilized and the requirement of the catalysts could be reduced, thus improving the overall economy of the process. However, a downstream metal elimination step is required for the removal of existing as well as additional amounts. In addition, the concentration of other inorganic macrocomponents (like Ca2+, Mg2+, Na+, Cl-, and SO42-) depends on the age of the landfill. Chloride and sulfate ion concentrations were found to be quite low and within the acceptable limits.3 Scott et al.22 reviewed the characteristics of the leachate from different landfill sites and presented a wide variation in all the parameters. 3.2. WO Experimental Results. 3.2.1. Effect of Catalysts and Promoters. Figure 1 shows the reduction in COD of the leachate with time in the presence of different catalysts and promoters. The results were compared with that obtained from Fenton’s reagent (Fe2+ + H2O2). The runs were performed at 150 °C and 0.7 MPa of total pressure. For the reaction with Fenton’s reagent, 15 mL of H2O2 per liter of wastewater was added to the reactor. It can be seen from the curves that no change in COD was found during the initial heating period () 45 min), and the COD reduction started immediately after

Figure 1. Comparison of COD removals obtained from different combinations used for WO of leachate. CuSO4 concentration ) 3 g L-1, FeSO4 ) 3 g L-1, H2O2 concentration ) 15 mL L-1, total pressure ) 0.7 MPa, stirring speed ) 1200 rpm, total reaction time ) 4 h.

the introduction of O2 (i.e., at “zero time”). This suggests the rapid generation of free radicals (mainly hydroxyl (HO•) radicals), causing the initialization of the reaction. Apart from this, hydrolysis of organic compounds may also have triggered the production of free radicals. For a fast reaction, HO• radical formation is necessary since these radicals have a very high oxidation potential (E°) in comparison to H2O2 and molecular oxygen (EHO•° ) 2.80 eV; EH2O2° ) 1.78 eV; EO2° ) 1.23 eV).13 A significant COD reduction (∼34%) was observed during noncatalytic WO after 4 h of reaction time; probably the presence of various metals (total concentration ≈ 11.3 mg L-1) in the wastewater catalyzed the oxidation reaction. Use of a promoter (Na2SO3 concentration ) 3 g L-1) enhanced the overall COD reduction to 42% (an increase of 8% from the COD reduction obtained with noncatalytic oxidation) after the same reaction time (i.e., 4 h). However, the combination of CuSO4 (3 g L-1) and Na2SO3 (3 g L-1) raised the COD reduction to ∼90% (initial COD ) 6400 mg L-1; final COD ) 650 mg L-1). In the absence of a promoter (Na2SO3), the overall COD reduction with the homogeneous CuSO4 catalyst was around 76%. The presence of copper ions in the solution increases the rate of free radical formation, resulting in fast degradation of organic compounds present in the wastewater. The addition of sodium sulfite raised the overall removal efficiency of pollutants due to the generation of several free radicals such as O2•-, SO3•-, SO4•- and SO5•-.15 These radicals act as chain carriers during the catalytic WO reaction. Among the generated free radicals, the oxidizing power of SO5•- has been reported to be much higher than that of SO3•- 23 and may be responsible for the enhanced reduction in COD. A requirement of a longer duration (4 h) reaction may be due to the high alkalinity of the wastewater since anions like HCO32- and CO32- scavenge the free radicals and retard the reaction.6,24 FeSO4 (Fe2+ concentration ) 0.6 g L-1) and H2O2 () 15 mL L-1) combination showed ∼62% COD reduction under similar operating conditions, and the reduction was increased to ∼84% (final COD ) 1030 mg L-1 from an initial value of 6400 mg L-1) after the addition of Na2SO3 (concentration ) 3 g L-1). Nevertheless, H2O2 is a more effective oxidant in comparison to air; it showed lower reduction than the CuSO4 and air combination. This may be attributed to the presence of fewer Fe2+ ions in comparison to Cu2+ ions in the solution despite equal concentrations of the metal salts () 3 g L-1).

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Also, Cu2+ ions have been found to be much more efficient WO catalysts than Fe2+ for the treatment of other streams.7 The effect of various catalysts and promoters on the activity of the WO reaction was found to be as follows (in increasing order): (no catalyst + air) < (no catalyst + Na2SO3 + air) < (FeSO4 + H2O2) < (CuSO4 + air) < (FeSO4 + Na2SO3 + H2O2) < (CuSO4 + Na2SO3 + air) The characterization of the oxidation process can also be done by using the AOSC or mean oxidation number of carbon (MOC).25 AOSC or MOC gives the average oxidation state of carbon atoms in the mixture. Using this parameter, one can predict the compounds present in the mixture. In numerical form, it is defined as (eq 1) AOSC or MOC ) 4 - 1.5(CODO /TOC)

(1)

Here, CODO is the COD due to organic compounds only. The COD value calculated using the closed reflux method may also include the oxygen demand exerted by inorganic species. In our calculation, we assumed that the COD of the wastewater was mainly due to organic compounds. The assumption seems to be correct due to the presence of small concentrations of inorganic species that generally contribute to the COD value (such as Fe2+, SO32-). The AOSC values at the end of different noncatalytic and catalytic WO reactions (after 4 h reaction) were in the range of -0.298 to +0.065. These values were found to be lower than the initial AOSC value () +0.135) of the leachate. It has been reported that an AOSC value equal to 0.0 indicates the presence of acetic acid, formaldehyde, and benzoquinone.25,26 For acetaldehyde and maleic acids, the value of the parameter is -1 and +1, respectively. So it can be anticipated that the treated wastewater is a mixture of various low molecular weight carboxylic acids and aldehydes. The AOSC value depends on the composition of the wastewater. Hence, this can be ascribed to the fact that the parameter does not provide information about the extent of the oxidation reaction. The formation of such compounds could also be ascertained by recording the pH of the wastewater during the treatment that is discussed in the subsequent paragraph. During the reaction, the pH of the treated wastewater was observed with time. The addition of CuSO4 reduced the initial wastewater pH by 1 unit (from 7.82 to 6.82). However, the overall change in pH during the noncatalytic and catalytic WO reactions (from “zero time” to the end of the reaction) was small (0.01 to 0.71 units). The minimum variation was found for the noncatalytic reaction, whereas the maximum change in pH value was observed for the (FeSO4 + H2O2) mixture. In all the runs, pH values were found to decrease (except for noncatalytic run) from the initial value. During the noncatalytic run, the pH of the wastewater did not show any significant change. For the (CuSO4 + Na2SO3 + air) combination, the pH was dropped from 6.82 to 6.41 in the first 60 min and then increased to 6.56. A reduction in pH can be expected due to the formation of low molecular weight acidic compounds. Later, the conversion of these compounds in other less acidic components or CO2 gas may increase the solution pH. A narrow range of pH during the reaction may also be attributed to the presence of high alkalinity in the original wastewater. Due to high COD reduction capability, further studies were conducted with the (CuSO4 + Na2SO3 + air) combination. The effects of CuSO4 and Na2SO3 concentrations and temperature on the COD of the wastewater were observed.

Figure 2. Effect of CuSO4 and Na2SO3 concentrations on the COD removal of leachate. a ) CuSO4 (3 g L-1) + Na2SO3 (3 g L-1), b ) CuSO4 (2 g L-1) + Na2SO3 (3 g L-1), c ) CuSO4 (1 g l-1) + Na2SO3 (3 g L-1), d ) CuSO4 (3 g L-1) + Na2SO3 (1 g L-1). Temperature ) 150 °C, total pressure ) 0.7 MPa, stirring speed ) 1200 rpm, reaction time ) 4 h.

3.2.2. Effect of Catalyst and Promoter Doses on COD Reduction. Figure 2 illustrates the effect of CuSO4 and Na2SO3 concentrations on COD removal of the wastewater during the WO reaction performed at 150 °C and 0.7 MPa of total pressure. The CuSO4 concentration was varied from 1 g L-1 to 3 g L-1 (Cu2+ concentration ) 0.255-0.764 g L-1), and the Na2SO3 concentration was fixed at 3 g L-1. To observe the effect of Na2SO3, two WO runs with Na2SO3 concentrations of 1 and 3 g L-1 were carried out. During these runs, the CuSO4 catalyst concentration was kept at 3 g L-1 (Cu2+ concentration ) 0.764 g L-1). The results showed that the presence of copper had much more influence on the overall COD reduction than Na2SO3. The decrease in Cu2+ concentration from 0.764 to 0.255 g L-1 reduced the overall COD removal from 90% to 43% (a depletion of 47%). The result suggested that a Cu2+ concentration of 0.255 g L-1 was not enough to accelerate the generation of active free radicals, and the reduction was almost equal to that obtained with a noncatalytic Na2SO3 promoted WO reaction (COD reduction ≈ 42%). By increasing the CuSO4 concentration from 1 g L-1 to 2 g L-1, COD reduction was enhanced from 43% to 54% (11% more). With a further increase in CuSO4 concentration to 3 g L-1 (corresponding Cu2+ concentration ) 0.764 g L-1), the COD reduction was found to increase very rapidly to ∼90%. The curve suggests that the reaction occurred in two segments: A fast step followed by a slower one. During the first fast step, easily degradable compounds were either converted into CO2 and H2O or transformed into low molecular weight organic compounds.7 Further degradation of the intermediates into final end products or other highly resistant monocarboxylic acids (such as CH3COOH) took place at a slower pace in the second reaction zone. However, the duration of the first fast step was much longer for catalyst concentrations of 1 and 2 g L-1 (∼150 min in comparison to only 60 min for the reaction performed with 3 g L-1 of the CuSO4 catalyst). During the second slower step, COD reductions were almost negligible or very little in following 90 min after a fast reaction step of 150 min for low catalyst concentrations (∼3.6 and 7.0% for 1 and 2 g L-1 CuSO4 concentrations, respectively). The results suggest that, for a lower CuSO4 concentration, the reaction was almost complete in the first 150 min, and later no significant reduction in COD was obtained. Conversely, COD

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Figure 3. Effect of temperature on the WO reaction. CuSO4 concentration ) 3 g L-1, Na2SO3 concentration ) 3 g L-1, total pressure ) 0.7 MPa, stirring speed ) 1200 rpm.

reductions were found to be ∼51% (in first 60 min) and ∼38% (during last 180 min reaction) in two steps with a CuSO4 concentration of 3 g L-1. The profiles showing the effect of the Na2SO3 concentration suggests that the optimum amount of promoter will be toward the higher dose used (i.e., 3 g L-1) since the addition of 1 g L-1 of Na2SO3 marginally increases the COD removal to ∼77% from 76% (achieved without the addition of any promoter; Figure 2). During these runs, the CuSO4 concentration was maintained at 3 g L-1. Hence, it can be deduced that CuSO4 and Na2SO3 concentrations of 3 g L-1 are the optimum values under the reaction conditions adopted in the present research study. 3.2.3. Effect of Temperature. In order to investigate the influence of temperature on the COD removal, experimental runs (4 h duration each) were conducted at 110, 130, 140, and 150 °C. The total pressure of the reactor vessel was maintained at 0.7 MPa during the runs. CuSO4 and Na2SO3 concentrations were kept at 3 g L-1 for all the runs. The results from the experimental study are shown in Figure 3. As expected, an increase in temperature also enhanced the overall COD reduction. COD of treated wastewater were found to be 1980, 1480, 960, and 710 mg L-1 at temperatures of 110, 130, 140, and 150 °C, respectively after 4 h of reaction. It can be seen from the figure that, by increasing the temperature from 110 to 150 °C, the overall COD removal was raised from ∼70% to ∼90%. The nature of all the curves (obtained at different temperatures) was similar, i.e., fast reaction for the initial 60 min duration without any induction period followed by slow reaction for the next 180 min. The kinetic parameters (such as specific rate constant and activation energy) were determined assuming a first order reaction with respect to organic concentration (COD) in both the steps. This assumption was made on the basis of previously reported findings using the WO process for different wastewater streams.27-29 The simplified rate expression for the reaction can be expressed as shown in eq 2: -ln

C ) kt C0

(2)

where C represents the COD of the leachate at time t, C0 is the initial COD of the wastewater, and k is the rate of the reaction. In order to determine the rate of reaction for both steps at all tested temperatures, a semilog plot was drawn between the C/C0

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Figure 4. Determination of reaction rate constants in the two stages of the WO reaction performed at different temperatures. Table 2. Reaction Rates Obtained at Different Temperatures after WO of Leachate step 1

step 2

temperature k value regression coeff. k value regression coeff. (R2) (min-1) (R2) s. no. (°C) (min-1) 1 2 3 4

110 130 140 150

0.006 0.007 0.011 0.012

0.987 0.989 0.989 0.993

0.004 0.005 0.007 0.008

0.991 0.988 0.993 0.991

ratio and time (Figure 4). The curves seemed to fit well in a straight line and validated the assumption. The slope of the curves directly gives the value of k in min-1. The reaction rates and regression coefficients obtained in the two steps are presented in Table 2. For the fast reaction step, the rate constant increased from 0.006 min-1 (at 110 °C) to 0.012 min-1 (150 °C), whereas the increase in k was from 0.004 (at 110 °C) to 0.008 min-1 (150 °C) for the slower step. It can be seen from the table that the k value was double at 150 °C that obtained at 110 °C in both steps. The values of the regression coefficient were in the range of 0.987 to 0.993. The reaction rate constants obtained at different temperatures were used to calculate the activation energy (Ea) and specific rate constant (k0) for both the steps during the WO reaction. The following relation (eq 3) was used to calculate the two parameters: ln k ) ln k0 -

Ea RT

(3)

where R is the universal gas constant () 8.314 J K-1 mol-1) and T is the temperature in K. Again, a semilog plot was drawn between -ln k and 1/T (Figure 5). The slope of the curve gave the value of Ea/R and the intercept was equal to -ln k0. The activation energy values were found to be 24.84 and 24.08 kJ mol-1 for fast and slow reaction steps, respectively. The specific rate constant for the fast step (0.135 min-1) was about 1.88 times that of the slow step (0.072 min-1). 3.3. Effect on Biodegradability of the Wastewater. The effect of the treatment temperature was seen on the biodegradability of the wastewater. The BOD5/COD ratio of the treated wastewater (at different temperatures) was compared with that of untreated wastewater. As mentioned earlier, the wastewater can be classified as not readily biodegradable due to a low BOD5/COD ratio (∼0.38). Figure 6 shows the bar chart having BOD5/COD values of the treated wastewater at different temperatures. The results reveal that the ratio was increased to

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Figure 5. Determination of activation energy and specific reaction rate constant using Arrhenius plot.

0.54, 0.60, 0.64, and 0.66 at 110, 130, 140, and 150 °C, respectively, in the presence of CuSO4 and Na2SO3. Hence, the leachate could be converted into a readily biodegradable stream even at a temperature of 110 °C. The BOD5 was reduced to 1070, 885, 615, and 470 mg L-1 at temperatures of 110, 130, 140, and 150 °C, respectively, from an initial BOD5 of 2400 mg L-1. Sometimes, the WO process can be used for changing the characteristics of wastewater or enhancing the biodegradability to 0.5 or more so that the pretreated wastewater becomes suitable for treatment in a conventional biological treatment plant. According to Indian standards for wastewater disposal in public sewers, the BOD3 of the wastewater should be less than 350 mg L-1 to treat it in a sewage treatment plant.3 Since the final BOD5 values were more than the prescribed value, the wastewater may require on-site biological treatment. Alternatively, the wastewater can either be treated at a higher temperature to meet the desired standard for BOD3 or be diverted to a closely located sewage treatment plant and mixed with sewage in an appropriate proportion to avoid any operational problems during the treatment of the mixed waste stream. 3.4. Color Removal. To determine the color removal from the wastewater after WO treatment at 150 °C and 0.7 MPa of total pressure, the absorbance of the treated and untreated samples was measured at 254 nm using a UV spectrophotometer (Figure 7). In the figure, the observed absorbance for the samples

Figure 7. Color removal from leachate after WO treatment.

treated with CuSO4 + air and CuSO4 + Na2SO3 + air is expressed as the percentage of the initial absorbance of untreated leachate. It can be seen that the absorbance was reduced by 50 and 70% for the leachate treated using (CuSO4 + air) and (CuSO4 + Na2SO3 + air), respectively. The absorbance at this wavelength is related to the presence of unsaturated organic compounds.11 The reduction in absorbance could be achieved due to the degradation of humic and fulvic acids (major color imparting compounds). Hence, the removal of these compounds reduced both the color and refractory nature of the wastewater. 3.5. FTIR Analysis. The change in intensity of various functional groups after WO treatment of the leachate was identified by performing FTIR spectroscopy (Figure 8). The refractory organic compounds were dissociated into new and simpler organic molecules. It is clear from the figure that the intensity of the two major functional groups (C-H and CdC) diminished, whereas no change could be found in the intensity of the O-H functional group. These results are in agreement with the previous findings.24 Apart from this, the C-O (acid) functional group that can be noticed at wavenumber 1342 cm-1 also reduced after the WO reaction. This indicates that the overall concentration of acids in the treated wastewater was decreased. The reason for this may be the complete conversion

Figure 6. Biodegradability enhancement of the leachate after WO treatment carried out at different temperatures in the presence of CuSO4 and Na2SO3.

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tion of CuO can be expected at a pH value of 6.56 since it has been reported previously that the optimum pH for copper precipitation lies between 6.0 and 8.0.31,32 The removal of copper may further be enhanced by raising the pH value to above neutral (i.e., 7.0). 4. Conclusions

Figure 8. FTIR spectra of leachate samples before and after treatment with CuSO4 + air and CuSO4 + Na2SO3 + air. Table 3. Identification of Functional Groups Present in the Leachate Based on FTIR Spectra s. no.

wave number (cm-1) of observed peaks

functional group

type of vibration

1 2 3

1342 1598 and 2500 3370 and 3750

C-O (acid) CdC (alkenes) O-H (alcohols)

stretch stretch stretch

of some compounds in CO2 and H2O during the reaction. The peaks matched with the standard absorption frequencies of different functional groups are shown in Table 3. 3.6. Characterization of the Precipitate Formed after Catalytic WO of the Leachate. Solid residue obtained after the catalytic WO of leachate treated with the CuSO4, Na2SO3, and air combination was characterized using the XRD technique (Figure 9). The XRD was performed over an oven-dried (at 105-110 °C temperature) solid powder to identify major precipitated components. Cu KR was used as a target, and the scanning angle (2θ) was set in the range of 20-70°. The pattern shows the presence of copper oxide in the residue. Prominent CuO peaks were formed at 35, 42, and 57° angles.30 Other peaks may be due to the presence of several other metal oxides present in the precipitate. It is difficult to identify individual peaks due to very low concentrations and similar 2θ values. The precipita-

Several conclusions can be elicited from the presented research study. The WO process promoted by Na2SO3 seems to be very effective for the enhancement of biodegradability of the leachate obtained from a medium age landfill. The BOD5/ COD ratio could be increased to 0.66 (from an initial value of 0.38) at 150 °C and a 0.7 MPa oxygen partial pressure in the presence of CuSO4 (catalyst) and Na2SO3 (promoter). The treated effluent was found amenable for biological treatment even at a lower temperature (BOD5/COD ) 0.54 at 110 °C temperature). The (CuSO4 + Na2SO3 + air) combination exhibited the best performance (overall COD reduction ∼90%) among all tested combinations such as (CuSO4 + air), (FeSO4 + H2O2), and (FeSO4 + Na2SO3 + H2O2) at the highest temperature used in the present study. The presence of heavy metals in the wastewater assisted in achieving a COD reduction of 34% in noncatalytic runs. The addition of a promoter (3 g L-1) in combination with CuSO4 and FeSO4 catalysts raised the COD removal significantly. Usage of homogeneous catalysts in the reaction necessitates metal removal in a post-treatment step, though the final pH of the treated effluent favored the precipitation of metals in the form of oxides (as shown in the XRD pattern). Finally, it is concluded on the basis of the above findings that catalytic WO in conjunction with biological methods can be a viable option for the treatment of leachate generated from medium and old age MSW landfills. Further studies should be aimed at the treatment of leachate (obtained from intermediate and old age landfills) with supported or unsupported heterogeneous catalysts. The durability and thermal stability of the active metal species and supports should also be investigated. Acknowledgment The authors are thankful to Industrial Research and Consultancy Centre (IRCC), Indian Institute of Technology (IIT), Bombay, India for the financial assistance to carry out the reported work. Literature Cited

Figure 9. XRD spectra of the precipitate obtained after the CuSO4 + Na2SO3 catalyzed WO reaction.

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ReceiVed for reView January 1, 2010 ReVised manuscript receiVed April 9, 2010 Accepted May 6, 2010 IE100003Q