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Catalytic Thermal Treatment (Catalytic Thermolysis) of a Biodigester Effluent of an Alcohol Distillery Plant Parmesh Kumar Chaudhari, I. M. Mishra,* and Shri Chand Department of Chemical Engineering, Indian Institute of Technology, Roorkee, Roorkee 247667, Uttaranchal, India
The catalytic thermal treatment (catalytic thermolysis) of biodigester effluent from an alcohol distillery unit was studied in the presence of CuO catalyst in batch mode in the temperature range of 100-140 °C and the pressure range of 1-9 bar. The catalyst mass loading, Cw, was varied between 2 and 5 kg/m3. Thermal treatment at 140 °C with a Cw of 3 kg/m3 gave a maximum chemical oxygen demand (COD) reduction of about 70% from its initial value of 34 kg/m3. The biochemical oxygen demand reduction was found to be 83% from its initial value of 6.3 kg/m3. The COD reduction-time profiles show two clearly distinct steps: a fast process followed by a slower process. A good amount of charred solid residue is obtained, which is enriched with carbon, giving a C/H atomic ratio of 1:0.947, versus a C/H atomic ratio of 1:1.146 for the effluent. The charred residue is a good fuel material with a high heating value (∼17.92 MJ/kg) representing 42-47% energy recovery from the digester effluent. During the thermolysis, copper gets leached to the aqueous phase. The leached out copper has its lowest concentration of 91 mg/dm3 when the initial pH value of the biodigester effluent was adjusted to 4. The residue obtained after the treatment may be combusted and/or incinerated, and the ash, which is rich in copper, may be blended with organic manure for use in agricultural fields. Introduction The distilleries producing alcohol from the fermentation of sugar cane molasses generate about 12-17 dm3 of wastewater/dm3 of ethyl alcohol. The distillery wastewater (DWW), which is also called the distillery spent wash (DSW), slop, vinasse, stillage, etc., is dark brown and has a very high chemical oxygen demand (COD: 60-200 kg/m3) and a very high biochemical oxygen demand (BOD: 50-75 kg/m3). High COD and BOD are due to the presence of high concentrations of carbohydrates, reduced sugars, dissolved lignin, proteins, etc. Most of the distilleries in India use anaerobic treatment systems installed to recover the maximum amount of energy through biomethanation reactors (biodigesters). These reactors are either biphasic reactors or upflow anaerobic sludge blanket (UASB) reactors and their different variants. These reactors operate at around 8090% BOD removal efficiency and around 70% COD removal efficiency.1 Thus, the effluents of these treatment systems still contain very high COD (∼30-45 kg/ m3) and very high BOD (∼4.5-7 kg/m3).1,2 Because of very stringent discharge water quality standards for release into surface waters (BOD < 0.03 kg/m3; COD < 0.10 kg/m3) and sewers (BOD < 0.10 kg/ m3; COD < 0.30 kg/m3) applicable in India, the effluent from the biomethanation (biodigester) plant is to be treated further. Most of the distillery units use aerobic treatment systems for the biodigester plant effluent. However, aerobic treatment requires a high efficiency of oxygenation through either surface aerators or submerged high-pressure bubblers. The cost of operation of the oxygenation system is very high, and even then they are unable to meet the standards for the discharge * To whom correspondence should be addressed. Tel.: +91-1332-285715. Fax: +91-1332-273560. E-mail: imishfch@ iitr.ernet.in.
quality of wastewater because the residual COD is still 8-15 kg/m3 and the residual BOD is in the range of 3-7 kg/m3. Therefore, most of the distilleries resort to dilution of the effluent from the aerobic treatment system, thus making a very large volume of dilution water polluted. This is a matter of great environmental concern. For this reason, a number of distilleries in China are concentrating the DWW in multieffect evaporators and the concentrated DWW is incinerated to recover its total energy content. A similar system is also in vogue in most of the kraft pulp and paper mills in India that have chemical recovery units. Wet oxidation of DWW at high temperature and pressure with the energy recovery in the form of steam3,4 and catalytic wet oxidation5,6 are also being investigated as alternatives to anaerobic-aerobic treatment and the concentrationincineration system. Noncatalytic thermal treatment (hydrolysis or thermolysis) has also been reported as a possible step for treatment, which is to be followed by wet oxidation3,7 or biological treatment. Wet oxidation (using oxygen, air, hydrogen peroxide, etc.), ozonation, flocculation, and thermolysis are some of the options for the treatment of a biodigester effluent and/or the effluent from the anaerobic biomethanation plant. Wet oxidation has been found to be an effective process for the treatment of various industrial effluents.8,9 The flocculation process for the treatment of the biodigester effluent using various flocculants has also been studied.10 A three-step treatment process for the biodigester effluent has been suggested by Dhale and Mahajani.11 In this process, the digester effluent was thermally treated, followed by flocculation and wet oxidation. During thermal treatment, a significant amount of COD reduction was achieved for highly concentrated DWW, with the initial COD (COD0) being 30-120 kg/m3 at temperatures of 150-250 °C. Wet
10.1021/ie048861u CCC: $30.25 © 2005 American Chemical Society Published on Web 06/24/2005
Ind. Eng. Chem. Res., Vol. 44, No. 15, 2005 5519 Table 1. Typical Composition of the Biodigester Effluent before and after Catalytic Thermal Treatment (at T ) 140 °C, pH0 ) 1, Cw ) 3 kgm-3, and tR ) 6 h)a
parameter
biodigester effluent
COD BOD organic carbon inorganic carbon total carbon reduced carbohydrates dissolved lignin protein NH4-N organic nitrogen total Kjeldahl nitrogen PO4K2+ SO4ClFe2+ Ca2+ Cu total dissolved solids total suspended solids pH color
34 000 6300 11612 1252 12864 32500 16100 8940 687 190 877 157 5170 7120 3000 10 558 8 28 320 12 180 7.8 blackish brown
a
biodigester effluent after thermal treatment 10 200 1075 3168 145 3313 6550 1230 640 140 73 213 54 3840 4230 2820 8 170 210 25 710 1.54 orange
All of the values except pH are in mg/dm3.
oxidation of the thermally treated DWW is more economical than wet oxidation alone. The aim of the present paper is to study the feasibility of augmenting the COD (and, consequently, also BOD) removal efficiency of the treatment system comprising of a high-efficiency UASB technology or its variants followed by catalytic thermal treatment (or catalytic thermolysis) at moderate temperatures and autogenous pressures with subsequent catalytic wet air oxidation or anaerobic-aerobic treatment. During some preliminary experimental runs, it was found that the efficiency of thermal treatment in reducing the COD of the effluent can be increased if the process was carried out in the presence of a suitable catalyst. Therefore, studies were carried out to determine the effect of different catalysts on the thermal treatment of the biodigester effluent. Different oxides of metals, viz., Cu, Mn, and Zn, either singly or in mixed forms were used as catalysts. However, CuO was found to be the best among them and, therefore, the CuO catalyst was chosen for further studies. The thermal treatment in the absence of oxygen was performed at temperatures ranging from 100 to 140 °C. Because thermolysis of the biodigester effluent produces a solid residue, the residue separation through filtration, the copper complexation of the residue, and the leaching of copper into the filtrate have also been studied. Experimental Section Materials. The biodigester effluent was obtained from Sir Sadi Lal Chemicals and Distillery Ltd., Pilkhani, U.P., India. A typical analysis of the effluent, before and after thermal treatment, is presented in Table 1. Laboratory reagent grade chemicals obtained from S.D. Fine-Chem Ltd., Mumbai, India, were used in the experiments. The CuO catalyst was prepared in the laboratory from cupric nitrate by alkali precipitation, followed by drying and calcination. To prepare 10 g of the CuO catalyst, a 30.40 mg/L copper nitrate solution
was prepared in distilled water and an aqueous ammonia solution [25% (w/v)] was added to the solution gradually (drop by drop) while stirring the solution at a constant speed. The resultant precipitate was washed thoroughly with distilled water, and then it was dried in an oven at 105 °C for 18 h. For the calcination of the precipitate, the dried precipitate was kept in the furnace and the furnace temperature was raised to 400 °C in about 0.67 h. Finally, the precipitate was heated at 400 °C for 4 h. Thereafter, the furnace was cooled, and the calcined solid was ground in a laboratory grinder and sieved. The solid particles with an average size of 220 µm were used in the experiments. Experimental Setup and Procedure. The thermolysis experiments were performed in a 0.5 dm3 atmospheric pressure glass reactor (AGR) as well as a 1 dm3 high-pressure stainless steel (SS-316) reactor (SSR). The AGR was equipped with a vertical condenser. The SSR was equipped with electrical heating, a temperature indicator-cum-controller, a liquid sampling port, a pressure indicator, and a cooling coil. The SSR was loaded with 0.3 dm3 of the effluent for each experimental run. The reactor contents were agitated using a magnetic stirrer (the stirring speed cannot be determined, but the intensity of the stirring can be varied). After the start of an experimental run at a desired temperature, the effluent samples were withdrawn from the reactor at definite time intervals. The samples were filtered, and the filtrate was analyzed for its COD values.12 Each COD run was repeated to check the reproducibility of the results. Any run with a deviation of more than 3% was further repeated to check its reliability. The effects of variables such as the initial pH (pH0 ) 1-10), temperature (T ) 100-140 °C), and catalyst mass loading (Cw ) 2-5 kg/m3) on the COD removal efficiency were studied. The experiments at atmospheric pressure and up to 100 °C were carried out in the AGR, whereas high-temperature (100-140 °C) and self-generated high-pressure experiments were carried out in the SSR. The autogenous pressures of the solution at 100, 120, 130, and 140 °C were respectively 2.8, 5.1, 7.20, and 8.80 bar versus the vapor pressure of water, which were 1, 2, 2.7, and 3.6 bar at the corresponding temperatures.13 This pressure enhancement is due to the presence of organic matter in the effluent. The effluent was preheated from the ambient temperature to the treatment temperature, with the preheating period (θ) varying with the treatment temperature. The time of start of treatment was considered as the “zero time” when the treatment temperature was attained because of preheating of the wastewater from its ambient temperature. All of the experimental runs in the SSR were carried out for a duration of 6 h. The AGR was used to optimize the initial pH (pH0) of the effluent that gave the best performance for catalytic thermolysis. The treated effluent was filtered/centrifuged, and the solid residue was dried in an oven at 105 °C until its weight became constant. The filtration studies of the treated effluent were carried out on ordinary filter paper supported on a Bu¨chner funnel. Zero-haze grade A (pore size 7-11 µm) filter papers were procured from S.D. Fine-Chem Ltd., Mumbai, India. The oven-dried residue was analyzed for its C, H, N, S, and ash content. Analytical Procedure. COD was determined by the standard dichromate reflux method.12 BOD of a sample was determined by incubating the seed sample for 3 days at 27 °C.14 The chloride content was determined
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by a standard titrimetric Volhard method.15 The elemental (C, H, N, and S) analysis was done using an elemental analyzer (model Vario EL III; Elementar, Hanau, Germany). The ash content was evaluated by combustion in a muffle furnace at 925 °C for 7 min. The specific energy of the residue was determined by using the standard bomb calorimeter.16 The amounts of metal ions leached out in the solution and those fixed in the solid residue were determined by using an atomic absorption spectrometer (model Awanta; GBC, Dandenong, Victoria, Australia). The protein content was determined by the Bradford method.17 The reduced carbohydrates concentration was estimated by the Fehling method.18 Organic nitrogen and ammoniacal nitrogen were determined by using the standard Kjeldahl method12 and the Nesslerization method12 using a spectrometer (model DR/4000U; Hatch). Sulfates and phosphates were determined by using standard methods.12 The lignin content was determined by using the modified Klason method,19 by replacing sulfuric acid with hydrochloric acid. The lower carboxylic acids (viz., formic acid and acetic acid) and ethyl alcohol were determined by using a gas chromatograph (model HP 5890, with a flame ionization detector) with a HP 20M capillary column. The injection temperature was 200 °C, and the detector temperature was maintained at 250 °C. Pure samples of formic acid and acetic acid [analytical reagent (AR) grade] from S.D. Fine-Chem Ltd., Mumbai, India, and ethyl alcohol (AR grade) from Hongtu Industries Corp., China, were used as reference compounds. Inorganic carbon, organic carbon, and total carbon were estimated using a total carbon analyzer (model TOC-VCSN; Shimadzu, Kyoto, Japan). Results and Discussion Effect of the pH. The effect of the pH on thermolysis of the digester effluent was studied in the AGR at 100 °C and atmospheric pressure with a condenser at the top and in the SSR at 140 °C temperature and effluent autogenous pressure. The initial pH (pH0) of the effluent batch was varied between 1 and 10. The pH0 was adjusted with either sulfuric acid or liquid ammonia. The results of thermolysis at 100 °C and atmospheric pressure, with and without catalyst, are shown in Figure 1a as COD reduction in the final filtrate versus the pH0 of the digester effluent, while keeping other parameters constant. All of the experiments were carried out for treatment time tR ) 3 h measured from zero time with the initial COD (COD0) kept at 34 kg/m3 and the catalyst mass loading (Cw) at 3 kg/m3. The highest COD reductions of 32% with catalyst and 22% without catalyst were obtained at pH0 ) 1. Figure 1b shows the effect of the pH0 on COD reduction at 140 °C and autogenous pressure for different tR values. Increases in the pressure and temperature result in increased COD reduction as compared to that reported in Figure 1a. Unlike the findings of several authors20-22 on the catalytic effect of the reactor wall material on the wet oxidation of organic acids, no such effect was observed in the present study. The experimental data at 100 °C in the AGR and SSR revealed that the reactor wall material does not show any appreciable difference in COD reduction. Although the SSR wall was not coated with any inert material to test the catalytic effect of the wall, the data at 100 °C do indicate the absence of the catalytic effect of the reactor wall on COD reduction under mild conditions
Figure 1. Effect of the pH0 on COD reduction of the biodigester effluent during thermolysis. COD0 ) 34 kg/m3: (a) T ) 100 °C; P ) 1 atm; tR ) 3 h; (b) T ) 140 °C; P ) autogenous pressure; Cw ) 3 kg/m3.
of temperature and pressure. When the SSR was opened after a batch experiment, the reactor wall was found to be coated with the precipitate. It is found from Figure 1a,b that COD reduction is a maximum at pH0 ) 1. Above pH0 ) 2-4, the COD reduction efficiency goes down drastically. Above pH0 ) 4 up to 8, COD reduction increases, although marginally, and then decreases (Figure 1a). During noncatalytic thermolysis at 100 °C, COD reduction shows a decreasing trend up to pH0 ) 6 and then it levels off. Figure 1a signifies the effect of the catalyst and the initial pH on COD reduction. It is seen from Figure 1a,b that the treatment at pH0 ) 8 is best in the range 4 e pH0 e 10. The role of the pH0 on the catalytic wet oxidation of DSW or the biodigester effluent has not been dealt with by the previous investigators.3,4,6,24 It has, however, been reported that thermal treatment of the biodigester effluent at 150 °C with a low molecular weight anionic synthetic flocculant can reduce its COD by about 37% and its dissolved solids by about 44% in the pH range of 5-12.11 Lele et al.7 have shown that pH0 ) 1 gives the best COD reduction during noncatalytic thermal pretreatment of alcohol DSW at 250 °C and autogenous pressure. The minimum COD reduction was reported at pH0 ) 7. The effect of the initial acidity on the wet oxidation of high-strength wastewater with different catalysts has been studied earlier.5 Lignin and carbohydrates contain several reactive groups including hydroxyl groups. Hydroxyl groups generally react at pH
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) 0-2 and 6-8.19 The presence of CuO in heterogeneous and homogeneous forms may catalyze the functional groups to react and get precipitated. However, it must be emphasized that the nature of the catalytically active sites is unknown and that the phenomenon of metal complexation with the reactive groups in the effluent and the effect of the pH on complexation are not explained at present. Treatment at low pH0 will require neutralization of the effluent before it is allowed to be discharged. However, the thermolysis step is only the first treatment step, and the effluent from this step has to be treated further to meet the effluent standards for discharge into sewers or receiving water bodies. Further studies in our laboratory revealed that the subsequent wet oxidation of the filtrate from the thermolysis step shows a maximum COD removal efficiency at low pH0 (
