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Enhanced anaerobic gas production of waste activated sludge pretreated by pulse power technique
Bioresource Technology 97 (2006) 198–203
Enhanced anaerobic gas production of waste activated sludge pretreated by pulse power technique Hanna Choi a, Seung-Woo Jeong
b,*
, Youn-jin Chung
c
a
b
Suwon Development Institute, Ajou University, Suwon 443-749, Korea School of Civil and Environmental Engineering, Kunsan National University, Kunsan 573-701, Korea c Division of Environmental and Urban Engineering, Ajou University, Suwon 443-749, Korea Received 18 August 2004; received in revised form 22 February 2005; accepted 22 February 2005 Available online 9 April 2005
Abstract An electric pulse-power reactor consisting of one coaxial electrode and multiple ring electrodes was developed to solubilize waste activated sludge (WAS) prior to anaerobic digestion. By pretreatment of WAS, the soluble chemical oxygen demand (SCOD)/total chemical oxygen demand (TCOD) ratio and exocelluar polymers (ECP) content of WAS increased 4.5 times and 6.5 times, respectively. SEM images clearly showed that pulse-power pretreatment of WAS was found to result in destruction of sludge cells. Batchanaerobic digestion of pulse-power treated sludge showed 2.5 times higher gas production than that of untreated sludge. Solubilized sludge cells by pulse-power pretreatment would be readily utilized for anaerobic microorganisms to produce anaerobically-digested gas. Slow or lagged gas production in the initial anaerobic digestion stage of pulse-power pretreated sludge implied that the methane-forming stage of anaerobic digestion would be the rate-limiting step for anaerobic digestion of pulse-power pretreated sludge. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Waste activated sludge; Anaerobic digestion; Pretreatment; Solubilization; Gas production; Pulse-power; Shockwave
1. Introduction Waste activated sludge (WAS) is generated by operation of the conventional wastewater treatment system. The disposal of excess sludge poses a significant challenge to wastewater treatment because the London agreement on the prevention of marine pollution by dumping of wastes prohibits sludge disposal into the sea (AGPS, 1996). Although anaerobic digestion has been chosen as economical volume reduction and stabilization methods for WAS, it presents some challenges for enhancing digestibility of sludge and reducing the final volume.
*
Corresponding author. Tel.: +82 11 9075 3595; fax: +82 63 469 4964. E-mail address: [email protected] (S.-W. Jeong). 0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.02.023
The three stages of anaerobic digestion process include hydrolysis, acetogenesis and methanogenesis. Among the three stages, hydrolysis is known as the rate limiting stage for the anaerobic digestion process and plays an important role to determine anaerobic digestibility (Eastman and Ferguson, 1981). Many attempts involving thermal and physical pretreatments have been made to increase substrate solubilization prior to anaerobic digestion. The primary objective of the attempts was to disrupt sludge floc structure. Vlyssides and Karlis (2004) achieved 46% reduction of the initial VSS and 0.281 l methane production per kg of the initial VSS by thermal-alkaline pretreatment. Weemaes et al. (2000) used ozone oxidation for pretreatment of WAS to obtain an increased methane production by a factor of 1.8. Kim et al. (2003) evaluated four pretreatment methods (thermal, chemical, ultrasonic, and thermo-chemical methods) on COD solubilization and gas production.
H. Choi et al. / Bioresource Technology 97 (2006) 198–203
They showed that a thermo-chemical method involving alkaline (7 g NaOH/l of WAS) addition and thermal condition at 121 °C for 30 min gave the highest solubilization and gas production. This study employed an electric pulse-power technology to disrupt sludge floc structure and to enhance digestibility of WAS. Although the pulse-power technology is known to be effective in destructing living cells and is widely utilized for the food industry (Mizuno and Hori, 1988; Devlieghere et al., 2004), no studies have been done to confirm that it may disrupt sludge floc structure and contribute anaerobic digestion of WAS. The electric pulse-power technology generates a pulsed high voltage discharge in water, which induces arc discharge. Electric power of the arc discharge in water would be spent to generate shockwave, intense ultraviolet radiation, strong electric field, and various radicals (Sunka, 2001). This study intended to use these generated impacts to destruct the cell wall of sludge. The objectives of this study were to utilize the electric pulse power technology for pretreatment of WAS by developing a pulse-power reactor, and to evaluate anaerobic digestibility of the pretreated WAS.
2. Methods 2.1. Pretreatment An electric pulse-power reactor was developed for pretreatment of WAS. Pulse power of the reactor was generated by a system consisting of a high voltage DC power supply (20 kV), a capacitor (25 kV–100 nF), a thyratron switch (35 kV–15 kA), and a pulse transformer. The system provided a pulsed power of 1.2 kW under a capacitance of 30 nF. Details of the pulsepower generation system were shown in Lee et al. (2003). This study developed a new ring-type pulsepower reactor. The reactor and electrodes were made of stainless steel and the void volume of the reactor was about 20 ml. The reactor produced an arc discharge by the electric gap between a coaxial electrode and ring electrodes. The gap between the coaxial electrode and ring electrodes was about 3 mm. The length of the coaxial electrode exposed to the liquid was
11 cm. Fig. 1 shows a diagram of the pulse-power reactor consisting of a coaxial electrode and multiple ring electrodes. An arc discharge under a pulse power field was verified by waveform measurement and was also visualized by light generation. Fig. 2 shows experimental waveforms of natural and arc discharges. This study used two kinds of WAS. Feed sludges were collected from the thickeners of WAS at two wastewater treatment plants (Tanchun and Anyang, Korea) and then chilled to 4 °C for experiments. Each sludge of two plants was separately prepared. Tanchun sludge was used for evaluation of WAS solubilization by pulse-power treatment. The sludge was introduced into the pulse-power reactor by using a peristaltic pump at a rate of 800 ml/min. The hydraulic retention time of sludge in the pretreatment reactor was approximately 1.5 s. To pretreat WAS, 7 rings of the outer electrode were used with one coaxial electrode, and pulse-power at the conditions of a voltage of 19 kV and a frequency of 110 Hz was applied to the reactor. Anyang sludge was used for evaluation of digestibility of pretreated sludge. The sludge was treated by pulse-power technology prior to anaerobic digestion. The sludge was introduced into the pulse-power reactor by using a peristaltic pump at a rate of 600 ml/min. To pretreat WAS, 7 rings of the outer electrode were used with one coaxial electrode, and pulse-power at the condition of a voltage of 17 kV and a frequency of 150 Hz was applied to the reactor. The pretreated sludge was stored in a 20-l tank and then utilized for anaerobic digestion experiments. 2.2. Anaerobic digestion A laboratory-scale anaerobic batch digester was used for anaerobic digestion of solubilized sludge. It consists of a 1 l bottle, a 50 ml-glass syringe, and a three-way glass valve. The produced gas was collected in the syringe and then vented by three-way valve after the amount of gas was recorded. The reactor was seeded with digester sludge taken from the waster water treatment plant of Tanchun, Korea. The seed sludge and the pretreated sludge were poured into the digester with a ratio of 1:1 to 1:2, depending on the organic load rate (OLR) to be evaluated. The total sludge volume of the Water out Outer electrode
Inter electrode (5mm)
Water in
199
Insulator
Fig. 1. Pulse-power reactor consisting one coaxial electrode and 5-ring outer electrode.
200
H. Choi et al. / Bioresource Technology 97 (2006) 198–203
Fig. 2. Experimental waveforms of natural discharge (a) and arc discharge (b).
digester was approximately 600 ml. To prevent a decrease in pH during digestion, the initial pH of the sludge was adjusted to 7 by 1 N NaHCO3. After location of sludge in the digester, air of the digester was flushed by a gas mixture of methane and carbon dioxide. The digester was placed in a water bath and was maintained at a mesophilic digestion temperature of about 35 °C. A magnetic stirrer was used to mix the sludge inside the digester. After 30 min of the complete setting-up, gas produced from the digester was vent by the exit valve and the time was then set as the start. The volume of produced gas was measured for 20–30 days by displaced volume of the syringe.
understand the extent of solubilization of substrate by pulse-power pretreatment. Protein and carbohydrate, the main components of ECP, were also measured according to the Beadford method and the phenol-sulfuric acid method, respectively. The pH, conductivity, alkalinity, TS (total solids), VS (volatile solids), TCOD (total chemical oxygen demand), nitrogen, and phosphorus were measured according to Standard Methods.
2.3. Analysis
In Table 1, the properties of raw sludge were compared with those of pulse-power treated sludge. The property values shown in Table 1 were average over five experiments. Pulse-power treatment of WAS
SCOD (soluble chemical oxygen demand), VA (volatile acid), ECP (exocellular polymers) were measured to
3. Results and discussion 3.1. Pretreatment
Table 1 Changes in the sludge property by pulse-power treatment of waste activated sludge Properties unit, mg/l
pH (unitless) Conductivity (ls/cm) VA TS VS TCOD SCOD SCOD/TCOD Soluble-N Soluble-P ECP Protein Carbohydrate COD:N:P a b
WAS of Tanchun wastewater plant
Ratio
Raw (range)a
Treated (range)a,b
6.1 (5.9–6.4) 2270 350 (99–870) 27,938 (26,080–30,510) 18,706 (15,420–21,080) 32,328 (30,800–34,000) 1312 (440–2740) 0.040 (0.013–0.085) 200 (71–347) 143 65 (49–80) 13 (10–18) 52 (39–62) 100:0.62:0.44
5.9 (5.5–6.4) 2970 1083 (530–2260) 28,193 (25,730–30,655) 18,390 (14,860–22,410) 32,464 (31,600–34,000) 5746 (4680–7040) 0.180 (0.146–0.223) 664 (554–841) 332 420 (206–637) 68 (19–190) 352 (187–602) 100:2.05:1.02
All values are average (except for conductivity and soluble-P). Pretreatment conditions: 7 ring electrodes, a voltage of 19 kV, a frequency of 110 Hz, a flow rate of 800 ml/min.
3.1
4.4 4.5 3.3 2.3 6.5 5.3 6.7
H. Choi et al. / Bioresource Technology 97 (2006) 198–203
dramatically changed the properties of WAS. The property results of pulse-power treated sludge clearly showed substantial increases compared with the raw sludge data for SCOD, ECP, protein, and carbohydrate. Additionally, the pulse-power treatment of WAS resulted in a 1 °C temperature increase of WAS. Fig. 3 shows SEM images of raw and pulse-power sludge cells. The SEM images show distinct difference in cell appearance. The surface of sludge cell (Fig. 3(a)) is
201
relatively smooth and round, but the surface of pulsepower-treated sludge cell is rough and deformed. The images indicate that the sludge cell was broken or solubilized by pulse-power treatment and cell contents would be then leached into the solution. Solubilization of sludge cell would be attributed to the combined effects of shock waves, chemically active radicals, and generated hydrogen peroxide molecules and UV light. Solubilized cells would be readily utilized by anaerobic microorganisms (Vlyssides and Karlis, 2004; Weemaes et al., 2000). Therefore, the application of pulse-power pretreatment to WAS was found to result in destruction of sludge cells. 3.2. Anaerobic digestion
Fig. 3. SEM images of sludge cells: (a) image of raw activated sludge cell; (b) image of pulse-power treated sludge cell.
Increases in the SCOD/TCOD ratio of WAS by pulse-power treatment were also found in Table 2. Table 2 shows the properties of WAS used for evaluation of anaerobic digestibility. Note here that the sludges of Table 2 included digester seed sludges, which diluted the SCOD concentration of pretreated WAS. The gas production is depicted in Fig. 4 along with the anaerobic digestion time. Pulse-power treated sludges gave higher SCOD/ TCOD ratios than raw sludges. Gas production rates were also much higher for pulse-power treated sludge than for raw (untreated) sludge. Anyang 2 showed two times higher gas production rate for pulse-power treated sludge than for raw sludge. The results indicate that cells of WAS were solubilized by pulse-power pretreatment, increasing the SCOD/TCOD ratio. The solubilized cells were more readily utilized for anaerobic microorganisms to produce anaerobically-digested gas. Anaerobic digestion experiments of Anyang sludge were conducted by changing the organic loading rate (OLR). Anyang 1, 2, and 3 corresponded to low, medium, and high OLRs, respectively. As OLR increased,
Table 2 Anaerobic digestibility of raw and pulse-power treated sludges Unit, mg/l
pH VA TS VS TCOD SCOD SCOD/TCOD Soluble-N OLR (kg VS/m3)b GPR (m3/kg VSadded)c a b c
Anyang 1
Anyang 2 a
Anyang 3 a
Raw
Pretreated
Raw
Pretreated
Raw
Pretreateda
7.0 83 30,375 14,730 22,740 1086 0.048 638 2.91 0.161
7.0 179 29,242 14,318 22,431 2154 0.096 753 2.58 0.245
7.2 40 18,660 10,770 16,800 600 0.036 678 4.97 0.052
7.2 231 18,270 9840 15,800 1800 0.114 1347 4.81 0.129
7.2 221 21,970 11,640 26,100 1840 0.070 321 7.7 0.056
7.1 263 22,390 11,270 26,000 2860 0.110 615 7.4 0.089
Pretreatment conditions: 7 ring electrodes, a voltage of 19 kV, a frequency of 150 Hz, a flow rate of 600 ml/min. OLR: organic loading rate. GPR: gas production rate.
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H. Choi et al. / Bioresource Technology 97 (2006) 198–203
Anyang 3 imply that although solubilized cells at relatively high organic loads would be readily used by the acid-forming bacteria, the formed organic acids were too much enough for the methane-forming bacteria to utilize for biogas production. As time elapsed, biogas production increased in either Anyang 2 or Anyang 3. The gradual increase indicates that the methane forming bacteria was inactive due to the high content of organic acids in the initial stage of anaerobic digestion, but the microorganism gradually adjusted to the environment and eventually used the organic acids for gas production. The solubilized sludge may be so effective in being converted to organic acids in the acid-forming stage, but a high content of organic acid may affect activity of the methane forming bacteria. It may take time for the methane-forming bacteria to convert the organic acids to methane. Although it is known that hydrolysis of sludge cells is the rate-limiting step for anaerobic digestion, the methane-forming process may be the rate-limiting step for anaerobic digestion of pulse-power treated sludge.
4. Conclusions Fig. 4. Anaerobic gas production rates of pulse-power pretreated sludge and untreated sludge by changing organic loads.
the gas production rate (GPR) was decreased for either raw sludge or pulse- power treated sludge (see Table 2). The low OLR (Anyang 1) represented the highest and fastest gas production during anaerobic digestion. Fig. 4 displays changes in anaerobic gas production at three different OLRs. Anaerobic gas production of pulse-power pretreated sludge was always higher for the low OLR than the raw sludge (Anyang 1), while anaerobic gas production of pretreated sludge was smaller for the medium OLR than raw sludge (Anyang 2) and was similar for the high OLR to raw sludge (Anyang 3) in an initial period of anaerobic digestion. The results indicate an importance of organic acid utilization by the methane-forming bacteria. The high OLR experiment introduced a high content of solubilized sludge into the anaerobic digester. The solubilized sludge would be readily utilized by anaerobic microorganisms and then would result in fast organic acid formation during the initial stage of anaerobic digestion. If the formed organic acid is sufficiently utilized by the methane-forming bacteria, biogas is proportionally produced as time elapsed. However, accumulation of organic acids affects activity of the methane-forming bacteria and may decrease methane formation during anaerobic digestion (Chyi and Dague, 1994). The gas production results of Anyang 2 and
This study conducted pretreatment of WAS by using a ring-type pulse-power pretreatment reactor, which was first developed for sludge pretreatment. By pretreatment of WAS, the SCOD/TCOD ratio and ECP of WAS increased 4.5 times and 6.5 times, respectively. SEM images clearly showed that sludge cells were ruptured by shockwave of pulse-power treatment system and by additional impacts. The application of pulse-power technique to WAS was found to result in destruction of sludge cells. Batch-anaerobic digestion of pulse-power treated sludge showed 2.5 times higher gas production than that of untreated sludge. The gas production of pulse-power treated sludge was dependent on organic load applied to the anaerobic digester. The low OLR represented the highest and fastest gas production during anaerobic digestion. However, the relatively high OLR gave a lagged gas production of pulse-power treated sludge. The gradual increase in the gas production of pulsepower treated sludge indicated that solubilized cells of WAS would be readily used for acid-forming bacteria to produce organic acids, but the produced organic acids would be partly utilized by methane-forming bacteria in the initial stage of anaerobic digestion. It seems that the methane-forming stage of anaerobic digestion would be the rate-limiting step for anaerobic digestion of pulse-power pretreated sludge. Therefore, further studies were warranted to determine the rate-limiting step for anaerobic digestion of pulse-power pretreated
H. Choi et al. / Bioresource Technology 97 (2006) 198–203
sludge, and to enhance biogas production during the limiting step. Acknowledgements Support for this research from Korean Energy Management Corporation is greatly appreciated.
References AGPS (Australian Government Publishing Service), 1996. Protocol to the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter of 29 December 1972 (London, 7 November 1996). Available from:. Chyi, Y.T., Dague, R.R., 1994. Effects of particulate size in anaerobic acidogenesis using cellulose as a sole carbon source. Water Environ. Res. 66, 670–678.
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Devlieghere, F., Vermeiren, L., Debevere, J., 2004. New preservation technologies: possibilities and limitations. Int. Dairy J. 14, 273–285. Eastman, J.A., Ferguson, J.F., 1981. Solubilization of particulate organic carbon during the acid phase of anaerobic digestion. J. Water Pollut. Control Fed. 53, 352–366. Kim, J., Park, C., Kim, T-H., Lee, M., Kim, S., Kim, S-W., Lee, J., 2003. Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge. J. Biosci. Bioeng. 95 (3), 271– 275. Lee, H.Y., Uhm, H.S., Choi, H., Jung, Y.J., Kang, B.K., Yoo, H.C., 2003. Underwater discharge and cell destruction by shockwaves. J. Korean Phys. Soc. 42, S880–S884. Mizuno, A., Hori, Y., 1988. Destruction of living cells by pulsed highvoltage application. IEEE Trans. Industry Appl. 24, 387–394. Sunka, P., 2001. Pulse electric discharge in water and their applications. Phys. Plasmas 8, 2587–2594. Vlyssides, A.G., Karlis, P.K., 2004. Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion. Bioresour. Tech. 91, 201–206. Weemaes, M., Grootaerd, H., Simoens, F., Verstraete, W., 2000. Anaerobic digestion of ozonized biosolids. Water Res. 34, 2330– 2336.
Enhanced anaerobic gas production of waste activated sludge pretreated by pulse power technique Hanna Choi a, Seung-Woo Jeong
b,*
, Youn-jin Chung
c
a
b
Suwon Development Institute, Ajou University, Suwon 443-749, Korea School of Civil and Environmental Engineering, Kunsan National University, Kunsan 573-701, Korea c Division of Environmental and Urban Engineering, Ajou University, Suwon 443-749, Korea Received 18 August 2004; received in revised form 22 February 2005; accepted 22 February 2005 Available online 9 April 2005
Abstract An electric pulse-power reactor consisting of one coaxial electrode and multiple ring electrodes was developed to solubilize waste activated sludge (WAS) prior to anaerobic digestion. By pretreatment of WAS, the soluble chemical oxygen demand (SCOD)/total chemical oxygen demand (TCOD) ratio and exocelluar polymers (ECP) content of WAS increased 4.5 times and 6.5 times, respectively. SEM images clearly showed that pulse-power pretreatment of WAS was found to result in destruction of sludge cells. Batchanaerobic digestion of pulse-power treated sludge showed 2.5 times higher gas production than that of untreated sludge. Solubilized sludge cells by pulse-power pretreatment would be readily utilized for anaerobic microorganisms to produce anaerobically-digested gas. Slow or lagged gas production in the initial anaerobic digestion stage of pulse-power pretreated sludge implied that the methane-forming stage of anaerobic digestion would be the rate-limiting step for anaerobic digestion of pulse-power pretreated sludge. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Waste activated sludge; Anaerobic digestion; Pretreatment; Solubilization; Gas production; Pulse-power; Shockwave
1. Introduction Waste activated sludge (WAS) is generated by operation of the conventional wastewater treatment system. The disposal of excess sludge poses a significant challenge to wastewater treatment because the London agreement on the prevention of marine pollution by dumping of wastes prohibits sludge disposal into the sea (AGPS, 1996). Although anaerobic digestion has been chosen as economical volume reduction and stabilization methods for WAS, it presents some challenges for enhancing digestibility of sludge and reducing the final volume.
*
Corresponding author. Tel.: +82 11 9075 3595; fax: +82 63 469 4964. E-mail address: [email protected] (S.-W. Jeong). 0960-8524/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2005.02.023
The three stages of anaerobic digestion process include hydrolysis, acetogenesis and methanogenesis. Among the three stages, hydrolysis is known as the rate limiting stage for the anaerobic digestion process and plays an important role to determine anaerobic digestibility (Eastman and Ferguson, 1981). Many attempts involving thermal and physical pretreatments have been made to increase substrate solubilization prior to anaerobic digestion. The primary objective of the attempts was to disrupt sludge floc structure. Vlyssides and Karlis (2004) achieved 46% reduction of the initial VSS and 0.281 l methane production per kg of the initial VSS by thermal-alkaline pretreatment. Weemaes et al. (2000) used ozone oxidation for pretreatment of WAS to obtain an increased methane production by a factor of 1.8. Kim et al. (2003) evaluated four pretreatment methods (thermal, chemical, ultrasonic, and thermo-chemical methods) on COD solubilization and gas production.
H. Choi et al. / Bioresource Technology 97 (2006) 198–203
They showed that a thermo-chemical method involving alkaline (7 g NaOH/l of WAS) addition and thermal condition at 121 °C for 30 min gave the highest solubilization and gas production. This study employed an electric pulse-power technology to disrupt sludge floc structure and to enhance digestibility of WAS. Although the pulse-power technology is known to be effective in destructing living cells and is widely utilized for the food industry (Mizuno and Hori, 1988; Devlieghere et al., 2004), no studies have been done to confirm that it may disrupt sludge floc structure and contribute anaerobic digestion of WAS. The electric pulse-power technology generates a pulsed high voltage discharge in water, which induces arc discharge. Electric power of the arc discharge in water would be spent to generate shockwave, intense ultraviolet radiation, strong electric field, and various radicals (Sunka, 2001). This study intended to use these generated impacts to destruct the cell wall of sludge. The objectives of this study were to utilize the electric pulse power technology for pretreatment of WAS by developing a pulse-power reactor, and to evaluate anaerobic digestibility of the pretreated WAS.
2. Methods 2.1. Pretreatment An electric pulse-power reactor was developed for pretreatment of WAS. Pulse power of the reactor was generated by a system consisting of a high voltage DC power supply (20 kV), a capacitor (25 kV–100 nF), a thyratron switch (35 kV–15 kA), and a pulse transformer. The system provided a pulsed power of 1.2 kW under a capacitance of 30 nF. Details of the pulsepower generation system were shown in Lee et al. (2003). This study developed a new ring-type pulsepower reactor. The reactor and electrodes were made of stainless steel and the void volume of the reactor was about 20 ml. The reactor produced an arc discharge by the electric gap between a coaxial electrode and ring electrodes. The gap between the coaxial electrode and ring electrodes was about 3 mm. The length of the coaxial electrode exposed to the liquid was
11 cm. Fig. 1 shows a diagram of the pulse-power reactor consisting of a coaxial electrode and multiple ring electrodes. An arc discharge under a pulse power field was verified by waveform measurement and was also visualized by light generation. Fig. 2 shows experimental waveforms of natural and arc discharges. This study used two kinds of WAS. Feed sludges were collected from the thickeners of WAS at two wastewater treatment plants (Tanchun and Anyang, Korea) and then chilled to 4 °C for experiments. Each sludge of two plants was separately prepared. Tanchun sludge was used for evaluation of WAS solubilization by pulse-power treatment. The sludge was introduced into the pulse-power reactor by using a peristaltic pump at a rate of 800 ml/min. The hydraulic retention time of sludge in the pretreatment reactor was approximately 1.5 s. To pretreat WAS, 7 rings of the outer electrode were used with one coaxial electrode, and pulse-power at the conditions of a voltage of 19 kV and a frequency of 110 Hz was applied to the reactor. Anyang sludge was used for evaluation of digestibility of pretreated sludge. The sludge was treated by pulse-power technology prior to anaerobic digestion. The sludge was introduced into the pulse-power reactor by using a peristaltic pump at a rate of 600 ml/min. To pretreat WAS, 7 rings of the outer electrode were used with one coaxial electrode, and pulse-power at the condition of a voltage of 17 kV and a frequency of 150 Hz was applied to the reactor. The pretreated sludge was stored in a 20-l tank and then utilized for anaerobic digestion experiments. 2.2. Anaerobic digestion A laboratory-scale anaerobic batch digester was used for anaerobic digestion of solubilized sludge. It consists of a 1 l bottle, a 50 ml-glass syringe, and a three-way glass valve. The produced gas was collected in the syringe and then vented by three-way valve after the amount of gas was recorded. The reactor was seeded with digester sludge taken from the waster water treatment plant of Tanchun, Korea. The seed sludge and the pretreated sludge were poured into the digester with a ratio of 1:1 to 1:2, depending on the organic load rate (OLR) to be evaluated. The total sludge volume of the Water out Outer electrode
Inter electrode (5mm)
Water in
199
Insulator
Fig. 1. Pulse-power reactor consisting one coaxial electrode and 5-ring outer electrode.
200
H. Choi et al. / Bioresource Technology 97 (2006) 198–203
Fig. 2. Experimental waveforms of natural discharge (a) and arc discharge (b).
digester was approximately 600 ml. To prevent a decrease in pH during digestion, the initial pH of the sludge was adjusted to 7 by 1 N NaHCO3. After location of sludge in the digester, air of the digester was flushed by a gas mixture of methane and carbon dioxide. The digester was placed in a water bath and was maintained at a mesophilic digestion temperature of about 35 °C. A magnetic stirrer was used to mix the sludge inside the digester. After 30 min of the complete setting-up, gas produced from the digester was vent by the exit valve and the time was then set as the start. The volume of produced gas was measured for 20–30 days by displaced volume of the syringe.
understand the extent of solubilization of substrate by pulse-power pretreatment. Protein and carbohydrate, the main components of ECP, were also measured according to the Beadford method and the phenol-sulfuric acid method, respectively. The pH, conductivity, alkalinity, TS (total solids), VS (volatile solids), TCOD (total chemical oxygen demand), nitrogen, and phosphorus were measured according to Standard Methods.
2.3. Analysis
In Table 1, the properties of raw sludge were compared with those of pulse-power treated sludge. The property values shown in Table 1 were average over five experiments. Pulse-power treatment of WAS
SCOD (soluble chemical oxygen demand), VA (volatile acid), ECP (exocellular polymers) were measured to
3. Results and discussion 3.1. Pretreatment
Table 1 Changes in the sludge property by pulse-power treatment of waste activated sludge Properties unit, mg/l
pH (unitless) Conductivity (ls/cm) VA TS VS TCOD SCOD SCOD/TCOD Soluble-N Soluble-P ECP Protein Carbohydrate COD:N:P a b
WAS of Tanchun wastewater plant
Ratio
Raw (range)a
Treated (range)a,b
6.1 (5.9–6.4) 2270 350 (99–870) 27,938 (26,080–30,510) 18,706 (15,420–21,080) 32,328 (30,800–34,000) 1312 (440–2740) 0.040 (0.013–0.085) 200 (71–347) 143 65 (49–80) 13 (10–18) 52 (39–62) 100:0.62:0.44
5.9 (5.5–6.4) 2970 1083 (530–2260) 28,193 (25,730–30,655) 18,390 (14,860–22,410) 32,464 (31,600–34,000) 5746 (4680–7040) 0.180 (0.146–0.223) 664 (554–841) 332 420 (206–637) 68 (19–190) 352 (187–602) 100:2.05:1.02
All values are average (except for conductivity and soluble-P). Pretreatment conditions: 7 ring electrodes, a voltage of 19 kV, a frequency of 110 Hz, a flow rate of 800 ml/min.
3.1
4.4 4.5 3.3 2.3 6.5 5.3 6.7
H. Choi et al. / Bioresource Technology 97 (2006) 198–203
dramatically changed the properties of WAS. The property results of pulse-power treated sludge clearly showed substantial increases compared with the raw sludge data for SCOD, ECP, protein, and carbohydrate. Additionally, the pulse-power treatment of WAS resulted in a 1 °C temperature increase of WAS. Fig. 3 shows SEM images of raw and pulse-power sludge cells. The SEM images show distinct difference in cell appearance. The surface of sludge cell (Fig. 3(a)) is
201
relatively smooth and round, but the surface of pulsepower-treated sludge cell is rough and deformed. The images indicate that the sludge cell was broken or solubilized by pulse-power treatment and cell contents would be then leached into the solution. Solubilization of sludge cell would be attributed to the combined effects of shock waves, chemically active radicals, and generated hydrogen peroxide molecules and UV light. Solubilized cells would be readily utilized by anaerobic microorganisms (Vlyssides and Karlis, 2004; Weemaes et al., 2000). Therefore, the application of pulse-power pretreatment to WAS was found to result in destruction of sludge cells. 3.2. Anaerobic digestion
Fig. 3. SEM images of sludge cells: (a) image of raw activated sludge cell; (b) image of pulse-power treated sludge cell.
Increases in the SCOD/TCOD ratio of WAS by pulse-power treatment were also found in Table 2. Table 2 shows the properties of WAS used for evaluation of anaerobic digestibility. Note here that the sludges of Table 2 included digester seed sludges, which diluted the SCOD concentration of pretreated WAS. The gas production is depicted in Fig. 4 along with the anaerobic digestion time. Pulse-power treated sludges gave higher SCOD/ TCOD ratios than raw sludges. Gas production rates were also much higher for pulse-power treated sludge than for raw (untreated) sludge. Anyang 2 showed two times higher gas production rate for pulse-power treated sludge than for raw sludge. The results indicate that cells of WAS were solubilized by pulse-power pretreatment, increasing the SCOD/TCOD ratio. The solubilized cells were more readily utilized for anaerobic microorganisms to produce anaerobically-digested gas. Anaerobic digestion experiments of Anyang sludge were conducted by changing the organic loading rate (OLR). Anyang 1, 2, and 3 corresponded to low, medium, and high OLRs, respectively. As OLR increased,
Table 2 Anaerobic digestibility of raw and pulse-power treated sludges Unit, mg/l
pH VA TS VS TCOD SCOD SCOD/TCOD Soluble-N OLR (kg VS/m3)b GPR (m3/kg VSadded)c a b c
Anyang 1
Anyang 2 a
Anyang 3 a
Raw
Pretreated
Raw
Pretreated
Raw
Pretreateda
7.0 83 30,375 14,730 22,740 1086 0.048 638 2.91 0.161
7.0 179 29,242 14,318 22,431 2154 0.096 753 2.58 0.245
7.2 40 18,660 10,770 16,800 600 0.036 678 4.97 0.052
7.2 231 18,270 9840 15,800 1800 0.114 1347 4.81 0.129
7.2 221 21,970 11,640 26,100 1840 0.070 321 7.7 0.056
7.1 263 22,390 11,270 26,000 2860 0.110 615 7.4 0.089
Pretreatment conditions: 7 ring electrodes, a voltage of 19 kV, a frequency of 150 Hz, a flow rate of 600 ml/min. OLR: organic loading rate. GPR: gas production rate.
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Anyang 3 imply that although solubilized cells at relatively high organic loads would be readily used by the acid-forming bacteria, the formed organic acids were too much enough for the methane-forming bacteria to utilize for biogas production. As time elapsed, biogas production increased in either Anyang 2 or Anyang 3. The gradual increase indicates that the methane forming bacteria was inactive due to the high content of organic acids in the initial stage of anaerobic digestion, but the microorganism gradually adjusted to the environment and eventually used the organic acids for gas production. The solubilized sludge may be so effective in being converted to organic acids in the acid-forming stage, but a high content of organic acid may affect activity of the methane forming bacteria. It may take time for the methane-forming bacteria to convert the organic acids to methane. Although it is known that hydrolysis of sludge cells is the rate-limiting step for anaerobic digestion, the methane-forming process may be the rate-limiting step for anaerobic digestion of pulse-power treated sludge.
4. Conclusions Fig. 4. Anaerobic gas production rates of pulse-power pretreated sludge and untreated sludge by changing organic loads.
the gas production rate (GPR) was decreased for either raw sludge or pulse- power treated sludge (see Table 2). The low OLR (Anyang 1) represented the highest and fastest gas production during anaerobic digestion. Fig. 4 displays changes in anaerobic gas production at three different OLRs. Anaerobic gas production of pulse-power pretreated sludge was always higher for the low OLR than the raw sludge (Anyang 1), while anaerobic gas production of pretreated sludge was smaller for the medium OLR than raw sludge (Anyang 2) and was similar for the high OLR to raw sludge (Anyang 3) in an initial period of anaerobic digestion. The results indicate an importance of organic acid utilization by the methane-forming bacteria. The high OLR experiment introduced a high content of solubilized sludge into the anaerobic digester. The solubilized sludge would be readily utilized by anaerobic microorganisms and then would result in fast organic acid formation during the initial stage of anaerobic digestion. If the formed organic acid is sufficiently utilized by the methane-forming bacteria, biogas is proportionally produced as time elapsed. However, accumulation of organic acids affects activity of the methane-forming bacteria and may decrease methane formation during anaerobic digestion (Chyi and Dague, 1994). The gas production results of Anyang 2 and
This study conducted pretreatment of WAS by using a ring-type pulse-power pretreatment reactor, which was first developed for sludge pretreatment. By pretreatment of WAS, the SCOD/TCOD ratio and ECP of WAS increased 4.5 times and 6.5 times, respectively. SEM images clearly showed that sludge cells were ruptured by shockwave of pulse-power treatment system and by additional impacts. The application of pulse-power technique to WAS was found to result in destruction of sludge cells. Batch-anaerobic digestion of pulse-power treated sludge showed 2.5 times higher gas production than that of untreated sludge. The gas production of pulse-power treated sludge was dependent on organic load applied to the anaerobic digester. The low OLR represented the highest and fastest gas production during anaerobic digestion. However, the relatively high OLR gave a lagged gas production of pulse-power treated sludge. The gradual increase in the gas production of pulsepower treated sludge indicated that solubilized cells of WAS would be readily used for acid-forming bacteria to produce organic acids, but the produced organic acids would be partly utilized by methane-forming bacteria in the initial stage of anaerobic digestion. It seems that the methane-forming stage of anaerobic digestion would be the rate-limiting step for anaerobic digestion of pulse-power pretreated sludge. Therefore, further studies were warranted to determine the rate-limiting step for anaerobic digestion of pulse-power pretreated
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sludge, and to enhance biogas production during the limiting step. Acknowledgements Support for this research from Korean Energy Management Corporation is greatly appreciated.
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