Production of biodiesel from wet activated sludge

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1 Research Article Received: 7 May 2010 Revised: 12 July 2010 Accepted: 15 July 2010 Published online in Wiley Online Library: 13 September 2010 (wileyonlinelibrary.com) DOI /jctb.2491 Production of biodiesel from wet activated sludge Emmanuel Revellame, a Rafael Hernandez, a William French, a William Holmes, b Earl Alley b and Robert Callahan II a Abstract BACKGROUND: The production of biodiesel from activated sludge obtained from Tuscaloosa, AL was optimized based on the yield of fatty acid methyl esters (FAMEs) using an in situ transesterification process. An orthogonal central composite response surface design was considered to investigate the main and interaction effects of temperature, methanol to sludge ratio, and catalyst concentration. RESULTS: The biodiesel yield can be satisfactorily described by the quadratic response surface model with R 2 of and a statistically not significant lack of fit (p = 0.254). Coded regression coefficients, main effect plots and surface plots indicated that maximum biodiesel yield may be obtained at 75 C, 30 ml g 1 (methanol/sludge) and 10% volume (catalyst concentration). Numerical optimization showed that at this reaction condition, a biodiesel yield of 3.78% (weight) can be obtained. Experimental verification gave a biodiesel yield of 3.93 ± 0.15% (weight) giving a model error of 7.35%. This indicates high reliability of the model. CONCLUSIONS: The economic analysis showed that the in situ transesterification of wet activated sludge (84.5% weight moisture) is less economical than the in situ transesterification of dried sludge (5% weight moisture). However, sensitivity analysis indicated that the process can be made more economical by reduction of water to 50% (weight). At this level of moisture, a biodiesel break-even price of around $7.00 per gallon is attainable, which is still more expensive than petroleum-based diesel ( $2.95 per gallon). For the biodiesel from activated sludge to be economically competitive, a biodiesel yield of at least 10% (weight) is necessary. c 2010 Society of Chemical Industry Keywords: wet in situ transesterification; biodiesel; activated sludge; fatty acid methyl ester (FAME) INTRODUCTION Fatty acid alkyl esters, also known as biodiesel, is a fast growing renewable fuel technology that is commonly produced by the reaction of refined pre-extracted vegetable oil and a simple aliphatic alcohol in the presence of a base or acid catalyst. 1,2 Commonly, the alcohol used is methanol, thus producing fatty acid methyl esters (FAMEs). 3 The utilization of biodiesel as fuel offers a reduction of most harmful emissions, domestic sourcing, inherent lubricity, renewability and biodegradability. 1,4 6 One of the challenges to the adoption of biodiesel as an alternative to petroleum-based fuels is its economic noncompetitiveness. 2,7 The most commonly used oil feedstock are animal fats and soybean oil (USA) and rapeseed and sunflower oils (European Union). 8 The high cost of these feedstocks accounts for the high price of biodiesel. An economic study of biodiesel production showed that a soybean price of $0.8 per lb constitutes more than 95% of the operating cost of a biodiesel plant. 9 Furthermore, the use of feedstock like soybean oil for fuel production can compete with food uses that could result in unattractive increases in both fuel and food costs. 1 Efforts to reduce biodiesel cost include the use of cheap, non-food sources of oil (i.e. from non-food crops such as jatropha, castor, neem, and karanja, used frying oil, microalgae, microbial biomass and activated sludge) and process modifications to combine the oil extraction and fuel conversion steps (i.e. in situ transesterification). 2,3,10 15 Previous studies showed that sufficient yield of biodiesel can be obtained from in situ transesterification of activated sludge. Dufreche et al. 13 obtained a yield of 6.23% (based on dry weight of activated sludge). They estimated that for a biodiesel yield of 7% (weight), the cost of biodiesel is around $3.11 per gallon. In a related study, Mondala et al. 14 did an economic analysis of in situ transesterification of primary and secondary (activated) sludges. Their calculations were based on methanol to sludge mass ratio of 12 : 1 and an assumed biodiesel yield of 10% (weight). They Correspondence to: Rafael Hernandez, Renewable Fuels and Chemicals Laboratory, Dave C. Swalm School of Chemical Engineering, Mail Stop 9595, Mississippi State University, Mississippi State, MS 39762, USA. rhernandez@che.msstate.edu Dr. Earl Alley, passed away on May 26, His contributions to this work are greatly appreciated. a Renewable Fuels and Chemicals Laboratory, Dave C. Swalm School of Chemical Engineering, Mail Stop 9595, Mississippi State University, Mississippi State, MS 39762, USA b Mississippi State Chemical Laboratory, P.O. Box CR, Mississippi State, MS 39762, USA 61 J Chem Technol Biotechnol 2011; 86: c 2010 Society of Chemical Industry

2 E Revellame et al. concluded a break-even price of biodiesel at $3.23 per gallon. 14 Revellame et al. 2 presented a study on the optimization of in situ transesterification of activated sludge obtained from a municipal wastewater treatment facility. They obtained an optimum yield of 4.79 ± 0.02% (weight of dry sludge) at reaction temperature of 55 C, methanol to sludge ratio of 25 (ml g 1 ) and sulfuric acid concentration of 4% (volume). 2 The optimum biodiesel yield obtained was far below the value that Dufreche et al. 13 and Mondala, et al. 14 had assumed. This could result in a dramatic increase in the cost of biodiesel from in situ transesterification of activated sludge. All previous studies on the in situ transesterification of activated sludge were conducted using nearly dried sludges ( 5% weight moisture). The reduction of water content of the activated sludge from 98% to 5% (weight) could add up to 55% of the biodiesel cost. 13 Using a feedstock for in situ transesterification with as near its natural moisture content as possible could reduce the drying cost but may require relatively large amount of methanol. 1,7 On a study conducted by Haas and co-workers 1 on the in situ transesterification of distillers dried grains with solubles (DDGS), they found that the removal of 20% moisture from the sample had no effect on the methanol requirement of the reaction. They further concluded that more complete drying (2.62% weight moisture) reduces the methanol requirement of the process for high reaction conversion. 1 For the case of substrate with relatively low moisture, such as DDGS (8.7% weight natural moisture) 1 reduction of the moisture content to reduce the methanol requirement may be the best option to reduce the overall cost of the process, hence the product. However, for substrates with high moisture content, such as activated sludge (98% weight), 13 increasing the methanol loading might be more economical than reducing the water to a very low level so as to obtain high reaction conversion. For substrates with this high moisture level, reduction of water to a level that will result in an acceptable yield might also be necessary. This might jeopardize the yield of biodiesel but could result in a remarkable reduction of the production cost and hence the cost of biodiesel. In this study, the in situ transesterification process was applied to activated sludge with 84.5% weight moisture. Optimization of the process was conducted by varying process parameters (temperature, methanol to sludge ratio, and catalyst concentration) to determine the combination that gave the maximum/optimum yield of biodiesel based on FAMEs. The in situ transesterification process utilizes either acid or base liquid catalyst, depending on the nature of the lipids present in the substrate. 1,2,7,14,16 Owing to the possible presence of free fatty acids in the activated sludge, an acid catalyst, specifically sulfuric acid, was chosen. This is to maximize biodiesel yield and avoid soap formation as for the case of base catalysts. Among possible acids (sulfuric, hydrochloric, formic, acetic, and nitric acids) that can be used as catalyst for transesterification process, only sulfuric acid showed significant activity. 17 Sulfuric acid has been shown to be an effective catalyst for the in situ esterification of rice bran oil in the presence of significant amounts of moisture (13.40% weight). 16 The result of the optimization was then used to estimate the reduction of biodiesel cost using this process. Table 1. Orthogonal central composite response surface design for the in situ transesterification of wet activated sludge Experimental run Temperature ( C) Factors Methanol to sludge ratio (ml g 1 solid) Sulfuric acid (% volume) FAMEs standard mixture (14-component) of saturated, monounsaturated and poly-unsaturated fatty acids was purchased from Supelco (Bellefonte, PA, USA). The gases used (He, H 2,N 2 and Air) were obtained from nexair (Columbus, MS, USA). All chemicals, standards, and gases were used as received. Sample preparation The activated sludge sample was collected from a municipal wastewater treatment plant in Tuscaloosa, AL (City of Tuscaloosa, AL, USA ( The water content of the sample was reduced by gravity-settling overnight, followed by centrifugation using an IEC Centra GP6 centrifuge (Thermo Electron Corp., Milford, MA, USA) operated at 3000 rpm for 20 min. The solid content of the concentrated sludge was determined using an Ohaus MB45 infrared heater (Ohaus, Pine Brook, NJ, USA) and it was found to contain an average of 15.50% (weight) solid. The concentrated sludge sample was stored below 0 C until further use. Experimental design Temperatures from 45 to 75 C, methanol to sludge (solids) ratios from 5 to 30 ml g 1 and catalyst concentrations from 1 to 10% (based on volume of methanol) were studied. Orthogonal central composite response surface design with nine center points was used as experimental design, giving a total of 23 treatment combinations. The treatment combinations are presented in Table 1. Triplicate runs were conducted for all treatment combinations. 62 MATERIALS AND METHODS Chemicals and gases All chemicals (methanol, sulfuric acid, n-hexane, toluene, anhydrous sodium sulfate, 1, 3-dichlorobenzene (1, 3-DCB), and butylated hydroxytoluene (BHT)) were purchased from Fisher Scientific (Pittsburgh, USA) at the highest purity available. The In situ transesterification The in situ transesterification and FAMEs analysis were conducted based on the procedure presented by Revellame et al. 2 All reactions were done using an Instatherm block system (Ace Glass Inc., Vineland, NJ, USA) for 24 h. One gram equivalent solid (6.45 g of concentrated sludge sample) was weighed into screw-capped (Teflon-lined) glass reaction vials. Treatments were assigned to wileyonlinelibrary.com/jctb c 2010 Society of Chemical Industry J Chem Technol Biotechnol 2011; 86:61 68

3 Production of biodiesel from wet activated sludge the vials and the assigned volume of methanolic sulfuric acid was added. The mixture was then heated to the assigned temperature at ambient pressure. Mixing during reaction was accomplished using a magnetic stirring bar. After reaction completion, the mixture was allowed to cool to room temperature. The supernatant was recovered into 60 ml glassvialsbycentrifugationat3000 rpmfor5 min.thesolidresidue was washed twice with 5 ml methanol, vortex-mixed for 2 min and centrifuged at 3000 rpm for 5 min. The supernatants were pooled and the volume was reduced to 6mLusingaTurboVap LV (Caliper Life Sciences, Hopkinton, MA, USA) at 45 Cunder15psi streamofn 2. The FAMEs were then extracted four times with n- hexane (20 ml total) and the extract was washed three times with 5 ml distilled water. During water washings, emulsions formed were broken down by centrifugation at 1500 rpm for 5 min. The residual water in the extract was removed by passing it through 2 g of anhydrous sodium sulfate. The hexane was removed using a TurboVap LV (Caliper Life Sciences) as described above and the FAMEs were re-dissolved in 5 ml of GC-diluent (toluene containing 200 µgml 1 1, 3-DCB and 100 µgml 1 BHT). FAMEs analysis The FAME analysis was conducted using an Agilent 6890N gas chromatograph equipped with flame ionization detector (GC- FID) (Agilent, Santa Clara, CA, USA) with 1, 3-DCB as internal standard. The BHT was added as an antioxidant. Prior to injection, the FAME samples were diluted (1 : 1) with the GC-diluent. The GC-FID used was equipped with a Restek Stabilwax-DA capillary column (Restek, Bellefonte, PA, USA) having dimensions of 30 m 0.25mmIDand0.25µm film thickness. Analyses were conducted using helium as carrier gas with a constant injector temperature of 260 C in splitless mode. The FID was set at 260 Cforthe duration of the analysis. The temperature program of the GC-FID oven was as follows: 50 C initial temperature for 2 min, ramped to 250 Cat10 Cmin 1, and held at 250 C for 18 min, giving a total of 40 min analysis time. A 14-component FAMEs standard mixture containing C 8 C 24 fatty acids was used for instrument calibration. Results of the GC-FID runs were used to calculate the percentage biodiesel yield (based on solid). Only compounds with concentration higher than 1% (weight) were included in the calculations. is the quadratic coefficient of factor i, β ij is the interactive effect coefficient for factor i and factor j, andε is the random error. 19 For the three factors investigated, expansion of the quadratic response surface model gives: Y = β 0 + (β 1 t + β 2 m + β 3 a) + (β 11 t 2 + β 22 m 2 + β 33 a 2 ) + (β 12 tm + β 13 ta + β 23 ma) + ε (2) for biodiesel yield, Y, temperature, t, methanol to sludge ratio, m, and catalyst concentration, a. 2 Data regression was done using the SAS software s ADX interface and the model representing only the significant effects (p < 0.05) was found to be Y = ( t m a) t tm (3) This model gave R 2 of and a not significant lack of fit (p = 0.254). This indicates good agreement between the model and the data. Furthermore, this implies that the reliability of the model is reasonably high. 1 The model presented in Equation (3) corresponds to the uncoded model of the data, which can be used to generate predicted values of biodiesel yield. This model is dependent on the unit of measure, and thus cannot be used to interpret how the response is influenced by the different effects. 20 To determine how the significant effects influence the biodiesel yield, the factors were coded with a standard coding scale of 1 to +1 for the low versus high level, respectively. Factor coding removes the unit of measure and uses the intercept as a representative center of design. 2,21,22 The resulting coefficients of the coded model can then be compared with one another. The coded coefficient is also known as standard regression coefficient, which is a measure of the change in standard deviation units of the dependent variable for every standard deviation change in the independent variable with the other variables held constant. 2,23 The coded model is; Y = ( t m a) t tm (4) RESULTS AND DISCUSSION Statistical analysis and regression Statistical analyses, numerical optimizations and surface plots were done or generated using SAS software, a statistical analysis software package.,18 Regression analysis was done at a significance level of To determine the precision of the replicate runs, the relative standard deviation of biodiesel yield for all the treatments was determined and a maximum value of 8.20% was obtained. This indicates good agreement of the collected data. The significant effects (main effects and interaction effects) of the factors on the biodiesel yield were determined utilizing the quadratic response surface model which is given by Y = β 0 + k β i x i + i=1 k β ii xi 2 + βij x i x j + ε (1) i<j i=1 where Y is the response, k is the number of factors, β 0 is the constant term or intercept, β i is the linear coefficient of factor i, β ii with the coded coefficients plotted in Fig. 1. Results showed that all the significant effects except for the interaction between temperature and methanol to sludge ratio, have a positive influence on biodiesel yield. Moreover the methanol to sludge ratio or methanol loading has the greatest influence on the response. To better understand the effect of each of the factors on biodiesel yield, main effect plots were generated as shown in Fig. 2. The positive linear influence of temperature indicates that as the temperature was increased, biodiesel yield increased. The positive quadratic effect of temperature indicated that higher temperatures will result in a greater increase in biodiesel yield. It can be seen from Fig. 2 that the temperature where the biodiesel yield started to increase drastically is around 60 C. The positive linear influence for both methanol to sludge ratio, m, and acid concentration, a, indicated that biodiesel yield increases as the level of both factors increases. But since the SAS and all other SAS Institute Inc. product or service names are registered trademarks or trademarks of SAS Institute Inc. in the USA and other countries. indicates USA registration. 63 J Chem Technol Biotechnol 2011; 86:61 68 c 2010 Society of Chemical Industry wileyonlinelibrary.com/jctb

4 E Revellame et al. Figure 1. Coefficients for the coded model: t, temperature; m, methanol to sludge ratio; a, sulfuric acid concentration. coded coefficient of methanol to sludge ratio is higher than that of acid concentration (Fig. 1), it is expected that the increase in biodiesel yield would be greater for methanol to sludge ratio than for acid concentration. This is also evident on the main effect plots showing steeper slope for methanol to sludge ratio than for acid concentration (Fig. 2). Optimization The analyses done above imply that no optimum process condition can beattained forthe in situ transesterification of wet sludge. To see this more directly, surface plots were generated and are presented in Fig. 3. As can be verified from the figure, there is no optimum condition present within the experimental design. However, results indicated that there is a process condition that will give maximum yield of biodiesel. It is apparent from Figs 2 and 3 that this condition is at the high level of all the factors studied; i.e. a temperature of 75 C, methanol to sludge ratio of 30 ml g 1 and sulfuric acid concentration of 10% (volume). This specific combination of factors was not included in the design. Thus, to verify the findings and to further asses the reliability of the model obtained, the in situ transesterification experiment was conducted at this condition. A biodiesel yield of 3.93 ± 0.15% (weight) was obtained at this condition. Numerical optimization was conducted to determine the predicted yield of the model at the condition that gave maximum biodiesel yield. Since this condition was already established, optimization was conducted using discrete values of factors used in the experimental design. Result showed that a maximum biodiesel yield of 3.78% (weight) can be obtained at this condition, giving a model error of at most 7.35%. This indicates that the model is highly reliable. FAME analysis The GC chromatogram of the biodiesel obtained at the condition giving maximum yield is presented in Fig. 4. The FAME analysis was used to calculate the biodiesel yield for all the treatment Figure 2. Main effect plots of the factors investigated (T = temperature; M = methanol to sludge ratio; A = acid concentration), showing how the biodiesel yield, Y, changes with the change in the level of the factors relative to the center of design (β 0 = 1.51). 64 Figure 3. Predicted percentage biodiesel yield, Y: (a) as a function of methanol to sludge ratio, M and temperature, T at fixed level of acid concentration, A = 10% volume; (b) as a function of acid concentration, A and methanol to sludge ratio, M at fixed temperature, T = 75 C. wileyonlinelibrary.com/jctb c 2010 Society of Chemical Industry J Chem Technol Biotechnol 2011; 86:61 68

5 Production of biodiesel from wet activated sludge Figure 4. FAME analysis of the biodiesel produced by in situ transesterification of wet activated sludge at temperature 75 C, methanol to sludge ratio 30 ml g 1 and sulfuric acid concentration 10% (volume). acid (C18 : 1), and linoleic acid (C18 : 2), indicating the suitability of the process used for converting the lipid component of the activated sludge into biodiesel. 2,13,14 Figure 5. FAME composition of the biodiesel produced by in situ transesterification of wet activated sludge at a temperature 75 C, methanol to sludge ratio 30 ml g 1 and sulfuric acid concentration 10% (volume). combinations included in the experimental design. The fatty acid profile of the biodiesel obtained was then calculated neglecting components with concentration less than 1% (weight) and is shown in Fig. 5. The activated sludge sample used in this study and the activated sludge sample used by previous workers 2,13,14 was obtained from the same wastewater treatment facility in Tuscaloosa, AL. For this study, the profile obtained was in agreement with those obtained by previous workers. The biodiesel obtained is predominantly composed of methyl esters of palmitic acid (C16 : 0), palmitoleic acid (C16 : 1), stearic acid (C18 : 0), oleic Economic analysis The in situ transesterification of wet activated sludge gave a fairly high biodiesel yield. Assuming that the biodiesel yield (4.79 ± 0.02% weight) obtained by Revellame et al. 2 is the highest biodiesel yield that can be obtained from activated sludge, the maximum yield that was obtained in this study is only 17.95% lower. To determine if the in situ transesterification of wet activated sludge is more economical than the in situ transesterification of dried sludge, an economic analysis involving costs associated with methanol and sulfuric acid requirements and equipment sizes was conducted. The results are presented in Table 2. A complete breakdown of estimated costs for in situ transesterification of dried sludge ( 5% moisture) obtained from Tuscaloosa, AL has already been done. 14 They estimated the annual production costs to be $992, for a biodiesel plant with an annual production of gallons. This resulted in a biodiesel break-even price of $ However, the methanol and catalyst requirements and biodiesel yield that they used in the analysis are different from those obtained in this study. Using the results of the optimization togetherwiththeresultsobtainedbyrevellame et al., 2 thepossible cost reduction associated with the process presented in this study was estimated. Results of the analysis showed that the in situ transesterification of dried sludge (5% moisture) is still more economical than that of wet sludge (84.5% moisture). Although the in situ transesterification of wet sludge eliminates the drying cost, the high methanol and catalyst requirements and large equipment sizes for the process resulted to higher break-even price of biodiesel. 65 J Chem Technol Biotechnol 2011; 86:61 68 c 2010 Society of Chemical Industry wileyonlinelibrary.com/jctb

6 E Revellame et al. Table 2. Cost estimates ($) for in situ transesterification of driedandwetactivatedsludges Dried sludge a (5% weight moisture) Wet sludge b (84.5% weight moisture) Annual biodiesel production, gal A. Feedstock preparation 1. Centrifugation 0.43/gal c 63, , Drying 1.29/gal c 189, $0.00 B. Methanol 0.08/gal d 169, , C. Catalyst 0.15/gal d 77, , Equipment cost 276, d 1,660,373.5 Total annual production cost d 1,091, ,747, Biodiesel price (break-even), $gal 1 a Revellame et al. 2 ; methanol requirement = 25 ml g 1 dried sludge, catalyst requirement = 4% (volume of methanol). b This study; methanol requirement = 30 ml g 1 dried sludge, catalyst requirement = 10% (volume of methanol). c Dufreche et al. 13 d Mondala et al. 14 Figure 6. The impact of moisture content of activated sludge on the break-even price of biodiesel. The in situ transesterification of wet activated sludge was conducted for 24 h. This was based on the study conducted by Mondala et al. 14 on the kinetics of in situ transesterification of activated sludge. The long reaction time might be due to mass transfer resistance of methanol and oil during the in situ transesterification process. 24 Increasing the agitation speed might shorten the reaction time by minimizing the mass transfer limitations. This has been proven true even for transesterification of pre-extracted oils. 25 Agitation speed could be a potential cost saving strategy for the in situ transesterification of wet activated sludge and might reduce the break-even price of biodiesel obtained using the process presented in this study Sensitivity analysis The results of the economic analysis indicated that the removal of 14% of the water in the activated sludge is not enough to reduce the cost of biodiesel. Thus, a sensitivity analysis was conducted to determine how the break-even biodiesel price changes with the moisture content of the feedstock. For this analysis a linear relationship among biodiesel yield, moisture content, and methanol and catalyst requirements was assumed. As shown in Fig. 6, a lowest biodiesel breakeven price of $7.00 per gallon can be obtained at 50% moisture content. However, this price is still not economically Figure 7. The influence of yield on the break-even price of biodiesel from wet activated sludge. competitive with that of petroleum-based diesel, which is around $2.95 per gallon (Energy Information Administration, Department of Energy ( March, 2010)). It is estimated that a biodiesel yield of more than 10% (weight) which corresponds to at least gallons per year biodiesel production (Fig. 7), will make this feedstock cheaper than petroleum-based diesel. wileyonlinelibrary.com/jctb c 2010 Society of Chemical Industry J Chem Technol Biotechnol 2011; 86:61 68

7 Production of biodiesel from wet activated sludge The yield of fuel that can be obtained from activated sludge can be increase significantly by conversion of other compounds that are also extracted during the in situ transesterification reaction into fuel. The extract may contain other compounds, such as sterols, waxes, alkyl benzene, hydrocarbons, polycyclic aromatic hydrocarbons, etc., resulting in a gravimetric extract yield of as high as 15% (weight of solid). 2,27 Analysis of relative concentrations of these compounds in the extract is necessary in order to determine suitable processes for fuel conversion (e.g. hydrocracking, hydrotreating). Biodiesel properties (i.e. low-temperature operability, oxidative and storage stability, viscosity, cetane number, exhaust emissions, and energy content) are strongly dictated by the presence of contaminants and other minor components. 28 In addition to the components mentioned above, the biodiesel from activated sludge may also contain contaminants such as metals, free fatty acids, triglycerides, diglycerides, monoglycerides, methanol, sulfuric acid (catalyst), and water. These compounds need to be minimized or removed from the activated sludge biodiesel for it to meet the ASTM D6751 or EN specifications. It was expected that additional processes might be necessary for the biodiesel from wet activated sludge to pass the ASTM or EN specifications. For this reason, the processing cost allotted in the economic analysis was at least twice the processing cost of biodiesel from soybean oil which is approximately $0.30 per gallon. 13,29 ASTM testing of biodiesel from wet activated sludge will be included in future work. CONCLUSIONS The in situ transesterification of wet activated sludge (84.5% weight moisture) was investigated to determine the economics of the process. Reaction temperature, methanol to sludge ratio and catalyst concentration were the variables considered in the investigation. Statistical analyses showed that within the experimental design there exists a condition where the yield of biodiesel is highest. This condition was at a temperature of 75 C, methanol to sludge ratio of 30 ml g 1 and catalyst concentration of 10% (weight). The model predicted a biodiesel yield of 3.78% (weight) at this condition. Experimental verification gave a yield of 3.93 ± 0.15% (weight) giving a model error of 7.35%. The result of FAME analysis showed that the fatty acid profile of the biodiesel obtained is similar to those obtained by previous workers. 2,13,14 This implies the ability and suitability of this process for converting the lipid component of activated sludge. Economic analysis was conducted to determine if the process used in this study can reduce the cost of biodiesel from activated sludge. The optimization data obtained by Revellame et al. 2 was also used for calculation of biodiesel cost from in situ transesterification of dried sludge. Results indicated that the breakeven price of biodiesel obtained using in situ transesterification of wet activated sludge is higher than that when the sludge was dried due to high methanol and catalyst requirements, and high equipment costs. 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