- Email: [email protected]
Thermochemical liquidization and anaerobic treatment of dewatered sewage sludge
JOURNALOP FEXMBNTATION ANDBIOENGINEERING Vol. 79, No. 3, 300-302. 1995
Thermochemical Liquidization and Anaerobic Treatment of Dewatered Sewage Sludge SHIGEKI SAWAYAMA,* SEIICHI INOUE, TATSUO YAGISHITA, TOMOKO OGI, AND SHIN-YA YOKOYAMA Biomass Laboratory, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305, Japan Received 26 September 1994/Accepted 22 December 1994
Dewatered sewage sludge was tbermocbemicaUy liquidized at 175°C and the liquidized sludge was separated by centrifugation to 57.7% (w/w) supernatant [moisture, 92.3%; volatile solid (VS), 7.0%1 and 42.3% precipitate (moisture, 71.6%; VS, 18.9%). The supematant was successfully anaerobicalJy digested. Biogas yield from the superuatant at organic loading concentrations of 1.9-2.2 g VS/I during 9 days’ incubation was 440 ml/g-added VS and digestion ratio was 66% (w/w). Biogas yield in the case of dewatered sewage sludge was 257ml/g-added VS and digestion ratio was 45%. Digestion of the supernatant resulted in high biogas productivity and a high digestion ratio compared with that of the dewatered sewage sludge. Moreover, it was suggested that the precipitate can be incinerated without the need for any supplemental fuel. [Key words: anaerobic digestion,
dewatered
sewage sludge, liquidization,
Treatment of sewage sludge has been an important environmental issue to resolve. Anaerobic digestion of sewage sludge is commonly used for treatment of sewage sludge and energy is recovered in the form of methane (1). A thermal pretreatment system for anaerobic digestion of concentrated sewage sludge with 2-3% volatile solid (VS) has been studied to improve anaerobic digestibility and dewatering properties (2-5). The volume of concentrated sewage sludge for thermal pretreatment is huge, and therefore this process consumes a large amount of energy. Dewatered sewage sludge (VS of 12%) is a clay like solid and is therefore not suitable for anaerobic digestion. An attempt at liquidization of dewatered sewage sludge by a thermochemical process was made by Dote et al. (6). The volume of dewatered sewage sludge decreased markedly from that of concentrated sewage sludge (VS of 2-3x), suggesting that heat treatment of dewatered sewage sludge would require less energy than in the case of concentrated sewage sludge. Methanogenesis tanks for sludge treatment are generally required to be very large; however, anaerobic digestion of liquidized sludge from dewatered sewage sludge or its supernatant could contribute to downsizing of digestion tanks. In this paper batch anaerobic treatments of liquidized sludge and its supernatant were examined. Dewatered sewage sludge used in the experiment was a mixture of primary and waste activated sludges from an aerobic sewage treatment facility in Ibaraki, Japan. Liquidization of dewatered sewage sludge was conducted as described by Dote et al. (6). After purging with nitrogen gas, the dewatered sewage sludge was heated at 175°C with an electric furnace and held at 4 MPa for 1 h in a 1 I autoclave. The liquidized sludge was separated by centrifugation (1,000 g, 10 min) to supernatant and precipitate. Seed sludge for batch anaerobic digestion was obtained from a sewage sludge treatment facility. Two liters of the seed sludge (VS 1 .l%, w/w) was incubated in a 2 I conical flask at 35 “C. Samples were added to the seed sludge at organic loading concentrations of 1.8* Corresponding author.
thermal treatment]
2.2 g-VS/l and air in the digester was purged with nitrogen gas. Digested samples were dewatered sewage sludge, liquidized sludge and its supernatant. Biogas production from the digester was monitored by displacement of water saturated with sodium chloride. Biogas composition was determined by gas chromatography (Model GC-8A, Shimadzu, Kyoto) with a column of Porapak Q (GL Sciences, Tokyo) at 9O’C. An analysis of elemental composition (carbon, hydrogen and nitrogen) for VS of sludge was conducted with a 2400 CHN elemental analyzer (Perkin-Elmer, Norwalk, CT, USA). Ammonium ion concentration was measured by calorimetry with sodium nitroprusside dihydrate solution and sodium hypochlorite (7). Moisture content was determined by heating the sample at 105°C for 24 h before weighing, and VS and ash content were determined by heating at 600°C for 1 h. Digestion ratio was expressed as the ratio of the amount of VS decrease during 4 or 9days’ digestion to the amount of VS added. High heating value (He, MJ/kg) for VS of sludges is calculated using to Dulong’s formula: HQ=O.3383 Cf 1.442(H--O/8) where C, H and 0 are weight percentages of carbon, hydrogen and oxygen, respectively. Low heating value (LQ, MJ/kg) is calculated using the following equation: LQ=HQ
V-O.0251
(9H+M)
where V is the ratio of volatile solid and M is the weight percentage of moisture. TABLE 1.
Characteristics of dewatered sewage sludge, liquidized sludge and its separated fractions
PH Dewatered sludge Liquidized sludge Supernatant Precipitate
83.8 84.4 92.3 71.6
12.4 11.5 7.0 18.9
a Measured in a suspended form. b Not measured.
3.8 4.1 0.7 9.5
3.2” 3.7 4.0 4
6.5’ 5.4 5.4 -_b
VOL.
NOTES
19, 1995
301
TABLE 2. Elemental analyses for volatile solid of dewatered sewage sludge, liquidized sludge and its separated fractions Carbon content (%, w/w) 52.1 57.2 56.8 56.8
Dewatered sludge Liquidized sludge Supernatant Precipitate
Hydrogen content (%, w/w) 1.4 6.3 7.4 a.2
Nitrogen content (%, w/w)
5.8 7.0 9.2 5.1
Oxygen content* (%,w/w) 34.1 29.5 26.6 29.9
s r, 200 2 2 a100 * g .P m 0
a Calculated by difference (0=100-C-H-N). TABLE 3. Digestion ratios of dewatered sewage sludge, liquidized sludge and its supernatant during 4 or 9 days’ incubation 4 d digestion (%, w/w)
9 d digestion (%, w/w)
17 (1.0) 26 (3.6) 38 (3.2)
45 (2.3) 49 (4.7) 66 (1.3)
Dewatered sludge Liquidized sludge Supernatant
Values in parentheses show standard deviation.
Dewatered sewage sludge (moisture, 83.8%, w/w; VS, 12.4%, w/w) was successfully liquidized by thermochemical reaction at 175°C with holding time of 1 h. Liquidized sludge was separated by centrifugation to supernatant (57.7%. w/w) and precipitate (42.3%, w/w). Characteristics of dewatered sewage sludge, liquidized sludge and its separated fractions are presented in Table 1 and elemental compositions of carbon, hydrogen and nitrogen for VS of the samples are listed in Table 2. VS content in the supernatant (7.0%) was lower than that in the liquidized sludge (11.5%). Figure 1 shows biogas yield in anaerobic digestion of dewatered sewage sludge, liquidized sludge and its supernatant at the organic loading concentrations of 1.8-2.2 g VS/l during 9 days’ incubation and Fig. 2 shows methane yield. Total biogas yield from the supernatant during 9 days’ incubation was 440 ml/g-added VS. Anaerobic digestion of the supernatant showed the highest biogas and methane productivities among the samples tested. Biogas productivities in anaerobic treatments of the liquidized sludge and the supernatant were improved by thermochemical treatment compared with that of the
>soo-
G-
0
2
4
6
8
10
Days FIG. 2. Methane yield from anaerobic treatments of dewatered sewage sludge, liquidized sludge and its supernatant. Open triangles, dewatered sewage sludge; open circles, liquidized sludge; closed circles, supernatant.
dewatered sewage sludge. There was no difference in methane concentrations of biogas (75-77%) among the three anaerobic treatments. Table 3 shows digestion ratios for anaerobic treatments at the organic loading concentrations of 1.8-2.2 g VS/I. Digestion ratio of the supernatant during 9 days’ incubation was 66%. Anaerobic digestion of the supernatant resulted in the highest digestion ratio among the samples tested for both 4 and 9 days’ incubation, Eastman and Ferguson reported that the rate-limiting step in anaerobic digestion is the hydrolysis of particulate to soluble substrates (8). It is an advantage of anaerobic digestion of the supernatant that substrates are almost soluble. As shown in Fig. 1 and Table 3, the biogas productivity and the digestion ratio of the supernatant were higher than those of the liquidized sludge. These data suggest that this supernatant could be useful for rapid and highly efficient digestion. Moreover, the volume of the separated supernatant used for anaerobic treatment decreased by 42% from that of the liquidized sludge. Precipitate separated from the liquidized sludge was composed of 71.6% moisture, 18.9% VS and 9.5% ash. Calculated low heating value of the precipitate was 1.2 MJ/kg and that of the dewatered sewage sludge was - 1.0 MJ/kg. The precipitate has a positive heating value, suggesting that it could be incinerated without the need for any supplemental fuel. Based on these results, thermochemical liquidization of dewatered sewage sludge and anaerobic digestion of supernatant was shown to be an effective treatment for sewage sludge. We are grateful to the Wastewater Treatment Office of Ibaraki prefecture for preparation of sewage sludge and to Dr. Yutaka Dote of the Department of Civil and Environmental Engineering, Miyazaki University for useful discussion. REFERENCES
.,-
0
2
4
6
8
10
Days FIG. 1. Biogas yield from anaerobic treatments of dewatered sewage sludge, liquidized sludge and its supernatant. Open triangles, dewatered sewage sludge; open circles, liquidized sludge; closed circles, supernatant.
Fannie, K. F., Conrad, J. R., Srivastsva, V. J., Jerger, D.E., and Chynoweth, D. P.: Anaerobic processes. J. Water Poll. Control Fed., 55, 623-632 (1983). Haug, R. T.: Sludge processing to optimize digestibility and energy production. J. Water Poll. Control Fed., 49, 1713-1721 (1977). Haug,
R. T.,
Stuckey,
D. C.,
Gossett,
J. M.,
and McCarty,
302
SAWAYAMA ET AL.
P. L.: Effect of thermal pretreatment on digestibility and dewaterability of organic sludges. J. Water Poll. Control Fed., 50, 73-85 (1978). 4. Haug, R. T., LeBrun, T. J., and Tortorici, L. D.: Thermal pretreatment of sludges-a field demonstration. J. Water Poll. Control Fed., 55, 23-34 (1983). 5. Hiraoka, M., Takeda, N., Snkai, S., and Yasuda, A.: Highly efficient anaerobic digestion with thermal pretreatment. Water Sci. Technol., 17, 529-539 (1984). 6. Dote, Y., Yokoyama, S., Minowa, T., Masuta, T., Sato, K.,
J. FERMENT. BIOENG., Itoh, S., and Suzuki, A.: Thermochemical liquidization of dewatered sewage sludge. Biomass Bioenergy, 4, 243-248 (1993). 7. APHA, AWWA, and WPCF: Standard methods for the examination of water and wastewater, 17th edn. American Public Health Association, Washington, D.C. (1989). 8. Eastman, J. A. and Ferguson, J. F.: Solubilization of particulate organic carbon during the acid phase of anaerobic digestion. J. Water Poll. Control Fed., 53, 352-366 (1981).
Thermochemical Liquidization and Anaerobic Treatment of Dewatered Sewage Sludge SHIGEKI SAWAYAMA,* SEIICHI INOUE, TATSUO YAGISHITA, TOMOKO OGI, AND SHIN-YA YOKOYAMA Biomass Laboratory, National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305, Japan Received 26 September 1994/Accepted 22 December 1994
Dewatered sewage sludge was tbermocbemicaUy liquidized at 175°C and the liquidized sludge was separated by centrifugation to 57.7% (w/w) supernatant [moisture, 92.3%; volatile solid (VS), 7.0%1 and 42.3% precipitate (moisture, 71.6%; VS, 18.9%). The supematant was successfully anaerobicalJy digested. Biogas yield from the superuatant at organic loading concentrations of 1.9-2.2 g VS/I during 9 days’ incubation was 440 ml/g-added VS and digestion ratio was 66% (w/w). Biogas yield in the case of dewatered sewage sludge was 257ml/g-added VS and digestion ratio was 45%. Digestion of the supernatant resulted in high biogas productivity and a high digestion ratio compared with that of the dewatered sewage sludge. Moreover, it was suggested that the precipitate can be incinerated without the need for any supplemental fuel. [Key words: anaerobic digestion,
dewatered
sewage sludge, liquidization,
Treatment of sewage sludge has been an important environmental issue to resolve. Anaerobic digestion of sewage sludge is commonly used for treatment of sewage sludge and energy is recovered in the form of methane (1). A thermal pretreatment system for anaerobic digestion of concentrated sewage sludge with 2-3% volatile solid (VS) has been studied to improve anaerobic digestibility and dewatering properties (2-5). The volume of concentrated sewage sludge for thermal pretreatment is huge, and therefore this process consumes a large amount of energy. Dewatered sewage sludge (VS of 12%) is a clay like solid and is therefore not suitable for anaerobic digestion. An attempt at liquidization of dewatered sewage sludge by a thermochemical process was made by Dote et al. (6). The volume of dewatered sewage sludge decreased markedly from that of concentrated sewage sludge (VS of 2-3x), suggesting that heat treatment of dewatered sewage sludge would require less energy than in the case of concentrated sewage sludge. Methanogenesis tanks for sludge treatment are generally required to be very large; however, anaerobic digestion of liquidized sludge from dewatered sewage sludge or its supernatant could contribute to downsizing of digestion tanks. In this paper batch anaerobic treatments of liquidized sludge and its supernatant were examined. Dewatered sewage sludge used in the experiment was a mixture of primary and waste activated sludges from an aerobic sewage treatment facility in Ibaraki, Japan. Liquidization of dewatered sewage sludge was conducted as described by Dote et al. (6). After purging with nitrogen gas, the dewatered sewage sludge was heated at 175°C with an electric furnace and held at 4 MPa for 1 h in a 1 I autoclave. The liquidized sludge was separated by centrifugation (1,000 g, 10 min) to supernatant and precipitate. Seed sludge for batch anaerobic digestion was obtained from a sewage sludge treatment facility. Two liters of the seed sludge (VS 1 .l%, w/w) was incubated in a 2 I conical flask at 35 “C. Samples were added to the seed sludge at organic loading concentrations of 1.8* Corresponding author.
thermal treatment]
2.2 g-VS/l and air in the digester was purged with nitrogen gas. Digested samples were dewatered sewage sludge, liquidized sludge and its supernatant. Biogas production from the digester was monitored by displacement of water saturated with sodium chloride. Biogas composition was determined by gas chromatography (Model GC-8A, Shimadzu, Kyoto) with a column of Porapak Q (GL Sciences, Tokyo) at 9O’C. An analysis of elemental composition (carbon, hydrogen and nitrogen) for VS of sludge was conducted with a 2400 CHN elemental analyzer (Perkin-Elmer, Norwalk, CT, USA). Ammonium ion concentration was measured by calorimetry with sodium nitroprusside dihydrate solution and sodium hypochlorite (7). Moisture content was determined by heating the sample at 105°C for 24 h before weighing, and VS and ash content were determined by heating at 600°C for 1 h. Digestion ratio was expressed as the ratio of the amount of VS decrease during 4 or 9days’ digestion to the amount of VS added. High heating value (He, MJ/kg) for VS of sludges is calculated using to Dulong’s formula: HQ=O.3383 Cf 1.442(H--O/8) where C, H and 0 are weight percentages of carbon, hydrogen and oxygen, respectively. Low heating value (LQ, MJ/kg) is calculated using the following equation: LQ=HQ
V-O.0251
(9H+M)
where V is the ratio of volatile solid and M is the weight percentage of moisture. TABLE 1.
Characteristics of dewatered sewage sludge, liquidized sludge and its separated fractions
PH Dewatered sludge Liquidized sludge Supernatant Precipitate
83.8 84.4 92.3 71.6
12.4 11.5 7.0 18.9
a Measured in a suspended form. b Not measured.
3.8 4.1 0.7 9.5
3.2” 3.7 4.0 4
6.5’ 5.4 5.4 -_b
VOL.
NOTES
19, 1995
301
TABLE 2. Elemental analyses for volatile solid of dewatered sewage sludge, liquidized sludge and its separated fractions Carbon content (%, w/w) 52.1 57.2 56.8 56.8
Dewatered sludge Liquidized sludge Supernatant Precipitate
Hydrogen content (%, w/w) 1.4 6.3 7.4 a.2
Nitrogen content (%, w/w)
5.8 7.0 9.2 5.1
Oxygen content* (%,w/w) 34.1 29.5 26.6 29.9
s r, 200 2 2 a100 * g .P m 0
a Calculated by difference (0=100-C-H-N). TABLE 3. Digestion ratios of dewatered sewage sludge, liquidized sludge and its supernatant during 4 or 9 days’ incubation 4 d digestion (%, w/w)
9 d digestion (%, w/w)
17 (1.0) 26 (3.6) 38 (3.2)
45 (2.3) 49 (4.7) 66 (1.3)
Dewatered sludge Liquidized sludge Supernatant
Values in parentheses show standard deviation.
Dewatered sewage sludge (moisture, 83.8%, w/w; VS, 12.4%, w/w) was successfully liquidized by thermochemical reaction at 175°C with holding time of 1 h. Liquidized sludge was separated by centrifugation to supernatant (57.7%. w/w) and precipitate (42.3%, w/w). Characteristics of dewatered sewage sludge, liquidized sludge and its separated fractions are presented in Table 1 and elemental compositions of carbon, hydrogen and nitrogen for VS of the samples are listed in Table 2. VS content in the supernatant (7.0%) was lower than that in the liquidized sludge (11.5%). Figure 1 shows biogas yield in anaerobic digestion of dewatered sewage sludge, liquidized sludge and its supernatant at the organic loading concentrations of 1.8-2.2 g VS/l during 9 days’ incubation and Fig. 2 shows methane yield. Total biogas yield from the supernatant during 9 days’ incubation was 440 ml/g-added VS. Anaerobic digestion of the supernatant showed the highest biogas and methane productivities among the samples tested. Biogas productivities in anaerobic treatments of the liquidized sludge and the supernatant were improved by thermochemical treatment compared with that of the
>soo-
G-
0
2
4
6
8
10
Days FIG. 2. Methane yield from anaerobic treatments of dewatered sewage sludge, liquidized sludge and its supernatant. Open triangles, dewatered sewage sludge; open circles, liquidized sludge; closed circles, supernatant.
dewatered sewage sludge. There was no difference in methane concentrations of biogas (75-77%) among the three anaerobic treatments. Table 3 shows digestion ratios for anaerobic treatments at the organic loading concentrations of 1.8-2.2 g VS/I. Digestion ratio of the supernatant during 9 days’ incubation was 66%. Anaerobic digestion of the supernatant resulted in the highest digestion ratio among the samples tested for both 4 and 9 days’ incubation, Eastman and Ferguson reported that the rate-limiting step in anaerobic digestion is the hydrolysis of particulate to soluble substrates (8). It is an advantage of anaerobic digestion of the supernatant that substrates are almost soluble. As shown in Fig. 1 and Table 3, the biogas productivity and the digestion ratio of the supernatant were higher than those of the liquidized sludge. These data suggest that this supernatant could be useful for rapid and highly efficient digestion. Moreover, the volume of the separated supernatant used for anaerobic treatment decreased by 42% from that of the liquidized sludge. Precipitate separated from the liquidized sludge was composed of 71.6% moisture, 18.9% VS and 9.5% ash. Calculated low heating value of the precipitate was 1.2 MJ/kg and that of the dewatered sewage sludge was - 1.0 MJ/kg. The precipitate has a positive heating value, suggesting that it could be incinerated without the need for any supplemental fuel. Based on these results, thermochemical liquidization of dewatered sewage sludge and anaerobic digestion of supernatant was shown to be an effective treatment for sewage sludge. We are grateful to the Wastewater Treatment Office of Ibaraki prefecture for preparation of sewage sludge and to Dr. Yutaka Dote of the Department of Civil and Environmental Engineering, Miyazaki University for useful discussion. REFERENCES
.,-
0
2
4
6
8
10
Days FIG. 1. Biogas yield from anaerobic treatments of dewatered sewage sludge, liquidized sludge and its supernatant. Open triangles, dewatered sewage sludge; open circles, liquidized sludge; closed circles, supernatant.
Fannie, K. F., Conrad, J. R., Srivastsva, V. J., Jerger, D.E., and Chynoweth, D. P.: Anaerobic processes. J. Water Poll. Control Fed., 55, 623-632 (1983). Haug, R. T.: Sludge processing to optimize digestibility and energy production. J. Water Poll. Control Fed., 49, 1713-1721 (1977). Haug,
R. T.,
Stuckey,
D. C.,
Gossett,
J. M.,
and McCarty,
302
SAWAYAMA ET AL.
P. L.: Effect of thermal pretreatment on digestibility and dewaterability of organic sludges. J. Water Poll. Control Fed., 50, 73-85 (1978). 4. Haug, R. T., LeBrun, T. J., and Tortorici, L. D.: Thermal pretreatment of sludges-a field demonstration. J. Water Poll. Control Fed., 55, 23-34 (1983). 5. Hiraoka, M., Takeda, N., Snkai, S., and Yasuda, A.: Highly efficient anaerobic digestion with thermal pretreatment. Water Sci. Technol., 17, 529-539 (1984). 6. Dote, Y., Yokoyama, S., Minowa, T., Masuta, T., Sato, K.,
J. FERMENT. BIOENG., Itoh, S., and Suzuki, A.: Thermochemical liquidization of dewatered sewage sludge. Biomass Bioenergy, 4, 243-248 (1993). 7. APHA, AWWA, and WPCF: Standard methods for the examination of water and wastewater, 17th edn. American Public Health Association, Washington, D.C. (1989). 8. Eastman, J. A. and Ferguson, J. F.: Solubilization of particulate organic carbon during the acid phase of anaerobic digestion. J. Water Poll. Control Fed., 53, 352-366 (1981).