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Low temperature conversion of some Brazilian municipal and industrial sludges
Bioresource Technology 74 (2000) 103±107
Low temperature conversion of some Brazilian municipal and industrial sludges H. Lutz a, G.A. Romeiro b, R.N. Damasceno c, M. Kutubuddin a, E. Bayer a,* b
a Institute of Organic Chemistry, University of T ubingen, D-72076 T ubingen, Germany Universidade Federal Fluminense, Departamento de Quõmica Org^ anica, Niter oi, CEP 24210-150 RJ, Brazil c Universidade Federal Fluminense, Departamento de Geoquõmica, Niter oi, CEP 24210-150 RJ, Brazil
Received 20 May 1997; received in revised form 20 July 1999; accepted 8 January 2000
Abstract Three Brazilian municipal and industrial sludges were subjected to the Low Temperature Conversion (LTC) process. They include activated, digested and lacquer sludges. The activated sludge recorded the highest yield of LTC oil (31.4%), followed by lacquer sludge (14.0%) and the digested sludge 11%. 1 H-NMR studies of the oils indicated that they consisted mainly of aliphatic and ole®nic compounds, while the concentration of aromatics was below 2.5%. The major hydrocarbons in the oils were pentadecane and heptadecane. The distribution pattern of hydrocarbons present in the oils was similar to what is known from the conversion of other sludges. In addition the LTC oil from activated sludge contained 26% fatty acids, while the oils from digested and lacquer sludge contained only about 3% fatty acids. Recovery studies on the fate of heavy metals in the sludges indicated that they were accumulated in the char. Partial gasi®cation studies of the LTC chars resulted in active carbons with quite low iodine and methylene blue numbers. However, even if their use is limited the production of active carbon together with the recovery of LTC oil constitutes a complete disposal of the sludges. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: LTC; Sludges; Oil; Char
1. Introduction Wastewater treatments have recently found renewed attention in Brazil. This is accompanied by the problem of sludge management. Low Temperature Conversion (LTC), a thermocatalytic method simulating the natural process of oil formation developed by Bayer and Kutubuddin (1981, 1982, 1988), has been used to recover raw materials in the form of oil and char from these kinds of wastes. The process is carried out at 300±420°C in the absence of oxygen and can be applied to various kinds of organic waste materials. The products can be used as chemical and energetic feedstocks which are both easy to handle and store (Bridle and Hertle, 1988). In general the oil contains aliphatic and ole®nic hydrocarbons and fatty acids, together with lesser amounts of a number of other compounds. The residual char can be deposited, burned or activated by partial gasi®cation. This paper, therefore, considers the LTC of three Brazilian sludges from municipal and industrial waste* Corresponding author. Tel.: +49-7071-292-437; fax: +49-7071-295246.
water treatment plants, including the chemical characterization of the conversion products. 2. Methods 2.1. Collection of sludges Activated and digested sludges were collected from a wastewater treatment plant in Rio de Janeiro, Brazil, while the third sludge, called `lacquer sludgeÕ, was collected from a treatment plant of the printing industry. These were stored in plastic bags at 4°C and dried to constant weight prior to analysis and conversion. 2.2. Low temperature conversion Low temperature conversion was carried out batchwise in triplicate on 250±500 g samples in a laboratory reactor at 380°C for 3 h (Fig. 1). The sludge samples (2) were ®xed in the tube by glasswool (3), put into the furnace (1), and heated under nitrogen (5) (heating rate about 10 K minÿ1 ) to the ®nal temperature. The volatized compounds were condensed (6) into a separa-
0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 1 1 - 0
104
H. Lutz et al. / Bioresource Technology 74 (2000) 103±107 Table 1 Elemental analyses and total ashes of the sludges and LTC products (all weight percent) Samples
Activated sludgea Oil Char Water
Fig. 1. Schematic diagram of the LTC reactor.
tion funnel (7) with an exhaust for non-condensable gases (8). a
2.3. Analysis of oil The fatty acid and hydrocarbon contents were determined by gas chromatography. The water content of the oils were also determined and some thermogravimetric studies were done, both as described by Lutz et al. (1998). 2.4. Heavy metal analysis Heavy metals were analysed in triplicate with an AAS Phillips PU-9200, after re¯uxing 1±4 g dry sludge for 3 h in a mixture of concentrated HCl:HNO3 (3:1). 2.5. Activation of char The chars were activated in a rotary furnace at 850°C. A nitrogen/steam mixture (about 1:1 v/v) was supplied by bubbling nitrogen through a hot water bath kept at constant temperature. Char samples of 10±20 g were used and activated between 0 and 90 min. Details of the procedure have been described in earlier publications (Bayer et al., 1995a, b; Lutz et al., 1998). The methylene blue number is de®ned as the amount of methylene blue decolorized per unit weight of active carbon, (Barton, 1987) while the iodine number was determined according to the AWWA procedure (AWWA, 1974). 3. Results and discussion The results of the elemental analyses of the various sludges are given in Table 1. Activated sludge recorded the highest carbon content and the lowest ash, while the lacquer sludge contained the lowest carbon but the
Elemental composition C (%)
H (%)
N (%)
Total ash (%)
40.9
5.9
2.4
28.2
77.4 23.8 5.3
12.0 1.8 10.4
2.1 2.1 1.9
± 58.6 ±
Digested sludgea Oil Char Water
26.3
3.3
2.6
51.1
76.1 20.9 11.4
10.6 1.6 9.7
2.9 2.3 4.5
± 71.4 ±
Lacquer sludgea Oil Char Water
21.2 79.3 11.5 4.3
3.4 11.2 1.4 11.4
0.57 1.6 0.82 0.65
68.8 ± 81.5 ±
Dry sludges.
highest ash. After the LTC the product distribution as given in Table 2 shows that relatively high amounts of char (50.1±69.0%) were found, in keeping with the high initial ash contents. Activated sludge recorded the highest oil yield (31.4%), followed by lacquer sludge (14.0%). Even digested sludge, in which the organic matter should have already been converted to methane and CO2 gave an oil yield of 11%. Activated sludges are most suitable for utilization in the LTC process. Here, more than two thirds (Table 2) of the initial carbon, and with this in good extrapolation, two thirds of the energy present in the sludge can be recovered in the form of an oil, which is easy to store and to handle and which can be used as fuel or chemical raw material. LTC oils from lacquer and digested sludge contained 52.9% and 31.0% of the initial carbon, respectively, while the reaction water contained 1±4% and the product gases less than 10% of the primary energy content. The yields of reaction water and gases were both about 10%. 3.1. Analysis of oil The LTC oils obtained were soluble in CH2 Cl2 . The carbon content was between 76% and 79% and the calori®c values were between 35 and 38 kJ molÿ1 . Investigations with thermogravimetry showed a volatilisation higher than 99% below 450°C. The water content of the oils were between 5% and 8%. The results of 1 H-NMR studies indicated that the hydrogens in oils from activated, digested and lacquer sludges were mainly aliphatic (93.1%, 88.6%, 94.8%, respectively), with minor amounts of ole®nic (4.3%±9.0%) and aromatic protons (below 2.5%). This
H. Lutz et al. / Bioresource Technology 74 (2000) 103±107
105
Table 2 LTC yields, carbon recovery and comparison of the LTC oil yields with representative values from other sludges (all dry sludges) Char
Oil
Water
Gas
LTC yields (%) Activated sludge Digested sludge Lacquer sludge
50:1 1:5 69:4 0:5 68:0 0:4
31:4 1:8 11:0 0:5 14:2 0:6
6:8 0:7 10:2 0:5 10:1 0:4
11:7 0:3 9:4 0:4 7:7 0:2
Carbon recovery (%)b Activated sludge Digested sludge Lacquer sludge
29.2 55.2 36.9
68.4 31.0 52.9
1.0 4.4 2.0
1.4 8.6 8.2
a
a
Mean S.D. of three determinations. Carbon recovery is used for the energy recovery and is calculated by {yield of LTC product (%) ´ carbon content of the LTC product (%)/carbon content of the sludge (%)}. b
low aromatic content would lead to a low toxicity of the LTC oil compared to, e.g., condensates from pyrolysis processes. The fatty acid compositions of the LTC oils are presented in Table 3. Oils from the activated sludge contained 26% fatty acids, of which 46% (11.9% of total oil) was palmitic. These fatty acids might represent an economical raw material for the chemical industry depending on the fatty acid distribution (Bayer and Kutubuddin, 1988). On the other hand oil from digested and lacquer sludges contained only about 3% fatty acids, making their utilization uneconomical. The hydrocarbons in the oils were also determined by GC. The GC of the hydrocarbon fraction of oil from activated sludge showed the typical distribution of hydrocarbons in LTC oils known from other wastewatertreatment sludges. The major n-alkane in the oils from activated and digested sludge was pentadecane (1.30%, 0.78%, respectively), while heptadecane was the dominant hydrocarbon in the oil from lacquer sludge (0.82%). Steroid skeletons were not detected in the oil from lacquer sludge, while their concentrations in the oils from activated and digested sludges were between 1.2% and 5.9%. LTC oils are characterized by high heating values, low toxicity and a hydrocarbon distribution comparable to diesel fuel. The yield of LTC oil, especially from the activated sludge, was at a very high level and this could increase the pro®tability of the process.
3.2. Analysis of LTC char Heavy metal contents (Cd, Co, Cr, Cu, Fe, Ni, Pb, and Zn) were investigated in the LTC chars and compared with their concentrations in the original sludges. In the activated and digested sludges the concentration of Cd and Co was less than 10 mg kgÿ1 , Ni less than 100 mg kgÿ1 , Cu, Pb and Zn were between 200 and 2000 mg kgÿ1 and Fe between 10 and 25 g kgÿ1 (Table 4). The industrial lacquer sludge showed a dierent pattern of the heavy metal distribution. The concentrations of Fe, Cd and Zn were very low, while the Co content was high. More than 90% of all these heavy metals were recovered in the LTC chars. This enrichment of heavy metals is consistent with what has been reported in similar studies (Schuller and Brat, 1993; Wenning, 1989). Bridle et al. (1990) reported a quantitative accumulation of all heavy metals, with the exception of mercury and arsenic, during the LTC process. The slightly lower recovery of 90% of heavy metals in the present work might have been due to incomplete acidic digestion with methods used. Leaching experiments showed that this LTC char could be easily deposited or also burned and the resulting ashes were classi®ed as not dangerous by the USEPA toxic leaching procedure (Bridle et al., 1990). Another utilization of LTC chars is the production of active carbon. In the case of agricultural wastes and biomasses this usage has been proved to be very prof-
Table 3 Fatty acid compositions of the LTC oils from dierent sludges
a b
Fatty acidsa (%)
Oil from activated sludge
Oil from digested sludge
Oil from lacquer sludge
Myristic acid Isomers of C16:1 Palmitic acid Isomers of C18:2 Isomers of C18:1 Stearic acid
1.71% 3.60% 11.8% 0.70% 3.75% 4.45%
0.18% 0.26% 1.25% <0.05% 0.41% 0.40%
n.d.b 0.14% 0.67% 1.59% 0.45% 0.57%
Total of the fatty acids
26.0%
2.5%
3.4%
Determined as methyl esters. Not detected.
310 6:0 620 19:3 480 7:3 730 17:1 1000 28:4 1350 17:4
870 1:1 1680 110 1250 2:7 2040 210 50 4:7 58 5:5
itable, whereas in the case of LTC chars from sludges the quality of the resulting active carbons is quite poor (Bayer et al., 1995a). This is a consequence of their very high inorganic content which does not contribute to the adsorption capacity. On the other hand the active carbons from sludges could be produced cheaply since the raw material costs are low or even negative. Thus, for less rigorous adsorption applications, this method could provide cheap active carbon while, at the same time, disposing of the sludge. After the activation (see methods) of LTC chars iodine numbers of 95 and 80 mg gÿ1 were measured for the chars from activated and digested sludge, respectively. The methylene blue numbers were below 20 mg gÿ1 . LTC char from lacquer sludge could not be activated since no carbon remained after reaching activation temperature. Although these values are not very high compared with the commercial high-grade active carbons, with iodine numbers of 500±1000 mg gÿ1 and methylene blue numbers up to 250 mg gÿ1 , however, it indicates the possibility of activation of some sludge chars. Due to the possible loss of heavy metals, their use should be limited to simple wastewater or gas cleaning.
Fe
13450 180 22400 3130 18850 475 22600 3240 70 0:57 50 9:6
Cu
49 0:03 86 21:3 370 29 550 112 5:8 0:31 7:4 2:89
4. Conclusions
a
Mean S.D. of three determinations.
4:9 0:39 7:5 0:29 6:5 0:33 6:7 0:75 290 4:4 410 22:1
Co Cd
2:4 0:05 5:3 0:28 4:3 0:66 5:6 0:38 0:16 0:05 0:64 0:08 Activated sludge LTC char Digested sludge LTC char Lacquer sludge LTC char
Concentration of metals (mg kgÿ1 )a Sample
Table 4 Concentrations of heavy metals in sludges and LTC chars (dry matter)
Cr
240 3:33 395 36:5 390 2:7 540 44:3 550 10:7 700 125
Ni
Pb
Zn
H. Lutz et al. / Bioresource Technology 74 (2000) 103±107
48 0:7 65 5:5 94 14:6 88 11:5 32 1:1 41 3:9
106
The LTC of dierent sludges from Brazilian wastewater treatment plants led to results comparable to those reported for sludges from other countries (Bridle and Hertle, 1988; Campbell and Bridle, 1985; Bayer and Kutubuddin, 1982). The recovery of LTC oil in excess of 310 kg per tonne from dry activated sludge is notable. The fatty acids in the oil from activated sludge would appear to be present in economically recoverable concentrations: 81 kg fatty acids could be produced from one tonne of dry sludge. Up to two thirds of the energy potential originally present in the sludges was recovered in the LTC oils. The quality of active carbons obtained from the LTC chars of the activated and digested sludges was poor and they could be used only for limited jobs. On the other hand the production costs would be very low since at the same time the process provides a means of complete sludge disposal. No active carbon was obtained from the lacquer sludge.
Acknowledgements We thank the German GKSS and the Brazilian CNPq for ®nancing the project and Dr. K.O. Esuoso for fruitful discussions.
H. Lutz et al. / Bioresource Technology 74 (2000) 103±107
References AWWA, 1974. Standard of Grannular Activated Carbon. J. American Water and Wastewater Association B607-674, 66, 672. Barton, S.S., 1987. Adsorption of methylene blue by active carbon. Carbon 25, 343±350. Bayer, E., Kutubuddin, M., 1981. Olaus M ull und Schlamm. bild der wissenschaft 9, 69. Bayer, E., Kutubuddin, M., 1982. Oil from sewage sludge. Korrespondenz Abwasser 29 (6), 377. Bayer, E., Kutubuddin, M., 1988. Thermocatalytic conversion of lipidrich biomass to oleochemicals and fuels, Research. In: Bridgwater, A.V., Kuester, J.L. (Eds.), Thermochemical Biomass Conversion, Elsevier Applied Science. Bayer, E., Maurer, A., Becker, G., Kutubuddin, M., 1995a. Recovery of activated carbons from wastes via Low Temperature Conversion, Part I: Preparation and determination of pore structure. Fresenius Env. Bull. 4, 533±538. Bayer, E., Maurer, A., Deyle, C.-J., Kutubuddin, M., 1995b. Recovery of activated carbons from wastes via Low Temperature Conver-
107
sion, Part II: Analysis and evaluation of applicability. Fresenius Env. Bull. 4, 539±544. Bridle, T.R., Hertle, C.K., 1988. Oil from sludge: A cost eective sludge management system. J. Australian Water Wastewater Assoc. 15 (3), 32. Bridle, T.R., Hammerton, I., Hertle, C.K. 1990. Control of heavy metals and organochlorines using the oil from sludge process presented at IAWPCR Sludge Management Conference, LA. Campbell, H.W., Bridle, T.R., 1985. Sludge management by thermal conversion to fuels presented at the conference on New Directions and Research in Waste Treatment and Residual Management, Vancouver, B.C. Lutz, H., Esuoso, K., Kutubuddin, M., Bayer, E., 1998. Low temperature conversion of sugar-cane by-products. Biomass and Bioenergy 15 (2), 155. Schuller, D., Brat, B., 1993. Kl arschlammpyrolyse mit R uckstandsvergasung. Chem.-Ing.-Tech. 65 (4), 401. Wenning, P.H., 1989. Verwertung von Kl arschlamm. Chem.-Ing.Tech. 61 (4), 277.
Low temperature conversion of some Brazilian municipal and industrial sludges H. Lutz a, G.A. Romeiro b, R.N. Damasceno c, M. Kutubuddin a, E. Bayer a,* b
a Institute of Organic Chemistry, University of T ubingen, D-72076 T ubingen, Germany Universidade Federal Fluminense, Departamento de Quõmica Org^ anica, Niter oi, CEP 24210-150 RJ, Brazil c Universidade Federal Fluminense, Departamento de Geoquõmica, Niter oi, CEP 24210-150 RJ, Brazil
Received 20 May 1997; received in revised form 20 July 1999; accepted 8 January 2000
Abstract Three Brazilian municipal and industrial sludges were subjected to the Low Temperature Conversion (LTC) process. They include activated, digested and lacquer sludges. The activated sludge recorded the highest yield of LTC oil (31.4%), followed by lacquer sludge (14.0%) and the digested sludge 11%. 1 H-NMR studies of the oils indicated that they consisted mainly of aliphatic and ole®nic compounds, while the concentration of aromatics was below 2.5%. The major hydrocarbons in the oils were pentadecane and heptadecane. The distribution pattern of hydrocarbons present in the oils was similar to what is known from the conversion of other sludges. In addition the LTC oil from activated sludge contained 26% fatty acids, while the oils from digested and lacquer sludge contained only about 3% fatty acids. Recovery studies on the fate of heavy metals in the sludges indicated that they were accumulated in the char. Partial gasi®cation studies of the LTC chars resulted in active carbons with quite low iodine and methylene blue numbers. However, even if their use is limited the production of active carbon together with the recovery of LTC oil constitutes a complete disposal of the sludges. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: LTC; Sludges; Oil; Char
1. Introduction Wastewater treatments have recently found renewed attention in Brazil. This is accompanied by the problem of sludge management. Low Temperature Conversion (LTC), a thermocatalytic method simulating the natural process of oil formation developed by Bayer and Kutubuddin (1981, 1982, 1988), has been used to recover raw materials in the form of oil and char from these kinds of wastes. The process is carried out at 300±420°C in the absence of oxygen and can be applied to various kinds of organic waste materials. The products can be used as chemical and energetic feedstocks which are both easy to handle and store (Bridle and Hertle, 1988). In general the oil contains aliphatic and ole®nic hydrocarbons and fatty acids, together with lesser amounts of a number of other compounds. The residual char can be deposited, burned or activated by partial gasi®cation. This paper, therefore, considers the LTC of three Brazilian sludges from municipal and industrial waste* Corresponding author. Tel.: +49-7071-292-437; fax: +49-7071-295246.
water treatment plants, including the chemical characterization of the conversion products. 2. Methods 2.1. Collection of sludges Activated and digested sludges were collected from a wastewater treatment plant in Rio de Janeiro, Brazil, while the third sludge, called `lacquer sludgeÕ, was collected from a treatment plant of the printing industry. These were stored in plastic bags at 4°C and dried to constant weight prior to analysis and conversion. 2.2. Low temperature conversion Low temperature conversion was carried out batchwise in triplicate on 250±500 g samples in a laboratory reactor at 380°C for 3 h (Fig. 1). The sludge samples (2) were ®xed in the tube by glasswool (3), put into the furnace (1), and heated under nitrogen (5) (heating rate about 10 K minÿ1 ) to the ®nal temperature. The volatized compounds were condensed (6) into a separa-
0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 1 1 - 0
104
H. Lutz et al. / Bioresource Technology 74 (2000) 103±107 Table 1 Elemental analyses and total ashes of the sludges and LTC products (all weight percent) Samples
Activated sludgea Oil Char Water
Fig. 1. Schematic diagram of the LTC reactor.
tion funnel (7) with an exhaust for non-condensable gases (8). a
2.3. Analysis of oil The fatty acid and hydrocarbon contents were determined by gas chromatography. The water content of the oils were also determined and some thermogravimetric studies were done, both as described by Lutz et al. (1998). 2.4. Heavy metal analysis Heavy metals were analysed in triplicate with an AAS Phillips PU-9200, after re¯uxing 1±4 g dry sludge for 3 h in a mixture of concentrated HCl:HNO3 (3:1). 2.5. Activation of char The chars were activated in a rotary furnace at 850°C. A nitrogen/steam mixture (about 1:1 v/v) was supplied by bubbling nitrogen through a hot water bath kept at constant temperature. Char samples of 10±20 g were used and activated between 0 and 90 min. Details of the procedure have been described in earlier publications (Bayer et al., 1995a, b; Lutz et al., 1998). The methylene blue number is de®ned as the amount of methylene blue decolorized per unit weight of active carbon, (Barton, 1987) while the iodine number was determined according to the AWWA procedure (AWWA, 1974). 3. Results and discussion The results of the elemental analyses of the various sludges are given in Table 1. Activated sludge recorded the highest carbon content and the lowest ash, while the lacquer sludge contained the lowest carbon but the
Elemental composition C (%)
H (%)
N (%)
Total ash (%)
40.9
5.9
2.4
28.2
77.4 23.8 5.3
12.0 1.8 10.4
2.1 2.1 1.9
± 58.6 ±
Digested sludgea Oil Char Water
26.3
3.3
2.6
51.1
76.1 20.9 11.4
10.6 1.6 9.7
2.9 2.3 4.5
± 71.4 ±
Lacquer sludgea Oil Char Water
21.2 79.3 11.5 4.3
3.4 11.2 1.4 11.4
0.57 1.6 0.82 0.65
68.8 ± 81.5 ±
Dry sludges.
highest ash. After the LTC the product distribution as given in Table 2 shows that relatively high amounts of char (50.1±69.0%) were found, in keeping with the high initial ash contents. Activated sludge recorded the highest oil yield (31.4%), followed by lacquer sludge (14.0%). Even digested sludge, in which the organic matter should have already been converted to methane and CO2 gave an oil yield of 11%. Activated sludges are most suitable for utilization in the LTC process. Here, more than two thirds (Table 2) of the initial carbon, and with this in good extrapolation, two thirds of the energy present in the sludge can be recovered in the form of an oil, which is easy to store and to handle and which can be used as fuel or chemical raw material. LTC oils from lacquer and digested sludge contained 52.9% and 31.0% of the initial carbon, respectively, while the reaction water contained 1±4% and the product gases less than 10% of the primary energy content. The yields of reaction water and gases were both about 10%. 3.1. Analysis of oil The LTC oils obtained were soluble in CH2 Cl2 . The carbon content was between 76% and 79% and the calori®c values were between 35 and 38 kJ molÿ1 . Investigations with thermogravimetry showed a volatilisation higher than 99% below 450°C. The water content of the oils were between 5% and 8%. The results of 1 H-NMR studies indicated that the hydrogens in oils from activated, digested and lacquer sludges were mainly aliphatic (93.1%, 88.6%, 94.8%, respectively), with minor amounts of ole®nic (4.3%±9.0%) and aromatic protons (below 2.5%). This
H. Lutz et al. / Bioresource Technology 74 (2000) 103±107
105
Table 2 LTC yields, carbon recovery and comparison of the LTC oil yields with representative values from other sludges (all dry sludges) Char
Oil
Water
Gas
LTC yields (%) Activated sludge Digested sludge Lacquer sludge
50:1 1:5 69:4 0:5 68:0 0:4
31:4 1:8 11:0 0:5 14:2 0:6
6:8 0:7 10:2 0:5 10:1 0:4
11:7 0:3 9:4 0:4 7:7 0:2
Carbon recovery (%)b Activated sludge Digested sludge Lacquer sludge
29.2 55.2 36.9
68.4 31.0 52.9
1.0 4.4 2.0
1.4 8.6 8.2
a
a
Mean S.D. of three determinations. Carbon recovery is used for the energy recovery and is calculated by {yield of LTC product (%) ´ carbon content of the LTC product (%)/carbon content of the sludge (%)}. b
low aromatic content would lead to a low toxicity of the LTC oil compared to, e.g., condensates from pyrolysis processes. The fatty acid compositions of the LTC oils are presented in Table 3. Oils from the activated sludge contained 26% fatty acids, of which 46% (11.9% of total oil) was palmitic. These fatty acids might represent an economical raw material for the chemical industry depending on the fatty acid distribution (Bayer and Kutubuddin, 1988). On the other hand oil from digested and lacquer sludges contained only about 3% fatty acids, making their utilization uneconomical. The hydrocarbons in the oils were also determined by GC. The GC of the hydrocarbon fraction of oil from activated sludge showed the typical distribution of hydrocarbons in LTC oils known from other wastewatertreatment sludges. The major n-alkane in the oils from activated and digested sludge was pentadecane (1.30%, 0.78%, respectively), while heptadecane was the dominant hydrocarbon in the oil from lacquer sludge (0.82%). Steroid skeletons were not detected in the oil from lacquer sludge, while their concentrations in the oils from activated and digested sludges were between 1.2% and 5.9%. LTC oils are characterized by high heating values, low toxicity and a hydrocarbon distribution comparable to diesel fuel. The yield of LTC oil, especially from the activated sludge, was at a very high level and this could increase the pro®tability of the process.
3.2. Analysis of LTC char Heavy metal contents (Cd, Co, Cr, Cu, Fe, Ni, Pb, and Zn) were investigated in the LTC chars and compared with their concentrations in the original sludges. In the activated and digested sludges the concentration of Cd and Co was less than 10 mg kgÿ1 , Ni less than 100 mg kgÿ1 , Cu, Pb and Zn were between 200 and 2000 mg kgÿ1 and Fe between 10 and 25 g kgÿ1 (Table 4). The industrial lacquer sludge showed a dierent pattern of the heavy metal distribution. The concentrations of Fe, Cd and Zn were very low, while the Co content was high. More than 90% of all these heavy metals were recovered in the LTC chars. This enrichment of heavy metals is consistent with what has been reported in similar studies (Schuller and Brat, 1993; Wenning, 1989). Bridle et al. (1990) reported a quantitative accumulation of all heavy metals, with the exception of mercury and arsenic, during the LTC process. The slightly lower recovery of 90% of heavy metals in the present work might have been due to incomplete acidic digestion with methods used. Leaching experiments showed that this LTC char could be easily deposited or also burned and the resulting ashes were classi®ed as not dangerous by the USEPA toxic leaching procedure (Bridle et al., 1990). Another utilization of LTC chars is the production of active carbon. In the case of agricultural wastes and biomasses this usage has been proved to be very prof-
Table 3 Fatty acid compositions of the LTC oils from dierent sludges
a b
Fatty acidsa (%)
Oil from activated sludge
Oil from digested sludge
Oil from lacquer sludge
Myristic acid Isomers of C16:1 Palmitic acid Isomers of C18:2 Isomers of C18:1 Stearic acid
1.71% 3.60% 11.8% 0.70% 3.75% 4.45%
0.18% 0.26% 1.25% <0.05% 0.41% 0.40%
n.d.b 0.14% 0.67% 1.59% 0.45% 0.57%
Total of the fatty acids
26.0%
2.5%
3.4%
Determined as methyl esters. Not detected.
310 6:0 620 19:3 480 7:3 730 17:1 1000 28:4 1350 17:4
870 1:1 1680 110 1250 2:7 2040 210 50 4:7 58 5:5
itable, whereas in the case of LTC chars from sludges the quality of the resulting active carbons is quite poor (Bayer et al., 1995a). This is a consequence of their very high inorganic content which does not contribute to the adsorption capacity. On the other hand the active carbons from sludges could be produced cheaply since the raw material costs are low or even negative. Thus, for less rigorous adsorption applications, this method could provide cheap active carbon while, at the same time, disposing of the sludge. After the activation (see methods) of LTC chars iodine numbers of 95 and 80 mg gÿ1 were measured for the chars from activated and digested sludge, respectively. The methylene blue numbers were below 20 mg gÿ1 . LTC char from lacquer sludge could not be activated since no carbon remained after reaching activation temperature. Although these values are not very high compared with the commercial high-grade active carbons, with iodine numbers of 500±1000 mg gÿ1 and methylene blue numbers up to 250 mg gÿ1 , however, it indicates the possibility of activation of some sludge chars. Due to the possible loss of heavy metals, their use should be limited to simple wastewater or gas cleaning.
Fe
13450 180 22400 3130 18850 475 22600 3240 70 0:57 50 9:6
Cu
49 0:03 86 21:3 370 29 550 112 5:8 0:31 7:4 2:89
4. Conclusions
a
Mean S.D. of three determinations.
4:9 0:39 7:5 0:29 6:5 0:33 6:7 0:75 290 4:4 410 22:1
Co Cd
2:4 0:05 5:3 0:28 4:3 0:66 5:6 0:38 0:16 0:05 0:64 0:08 Activated sludge LTC char Digested sludge LTC char Lacquer sludge LTC char
Concentration of metals (mg kgÿ1 )a Sample
Table 4 Concentrations of heavy metals in sludges and LTC chars (dry matter)
Cr
240 3:33 395 36:5 390 2:7 540 44:3 550 10:7 700 125
Ni
Pb
Zn
H. Lutz et al. / Bioresource Technology 74 (2000) 103±107
48 0:7 65 5:5 94 14:6 88 11:5 32 1:1 41 3:9
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The LTC of dierent sludges from Brazilian wastewater treatment plants led to results comparable to those reported for sludges from other countries (Bridle and Hertle, 1988; Campbell and Bridle, 1985; Bayer and Kutubuddin, 1982). The recovery of LTC oil in excess of 310 kg per tonne from dry activated sludge is notable. The fatty acids in the oil from activated sludge would appear to be present in economically recoverable concentrations: 81 kg fatty acids could be produced from one tonne of dry sludge. Up to two thirds of the energy potential originally present in the sludges was recovered in the LTC oils. The quality of active carbons obtained from the LTC chars of the activated and digested sludges was poor and they could be used only for limited jobs. On the other hand the production costs would be very low since at the same time the process provides a means of complete sludge disposal. No active carbon was obtained from the lacquer sludge.
Acknowledgements We thank the German GKSS and the Brazilian CNPq for ®nancing the project and Dr. K.O. Esuoso for fruitful discussions.
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