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Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion
Bioresource Technology 91 (2004) 201–206
Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion A.G. Vlyssides *, P.K. Karlis Laboratory of Organic and Chemical Technology, Department of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Strasse, Zographou, 157 00 Athens, Greece Received 10 February 2003; received in revised form 20 May 2003; accepted 26 May 2003
Abstract This work studied the hydrolysis kinetics and the solubilization of waste activated sludge under a medium range temperature (50– 90 °C) and pH in the alkaline region (8–11), as a pretreatment stage for anaerobic digestion. The hydrolysis rate for the solubilization of volatile suspended solids (VSS) followed a first-order rate. A linear polynomial hydrolysis model was derived from the experimental results leading to a satisfactory correlation between the hydrolysis rate coefficient, pH, and temperature. At pH 11 and a temperature of 90 °C the concentration of the VSS was 6.82%, the VSS reduction reached 45% within ten hours and at the same time the soluble COD was 70.000 mg/l and the total efficiency for methane production 0.28 l of CH4 per g of VSS loading. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Waste activated sludge; Thermal; Pretreatment; Alkaline; Lime; Solubilization; Hydrolysis
1. Introduction Sludge handling represents 30–40% of the capital cost and about 50% of the operating cost of many wastewater treatment facilities. About two-thirds of the sludge in Greek secondary treatment plants is waste activated sludge (WAS). The WAS has to be stabilized sufficiently to reduce its organic content, odor problems, and pathogen contamination before ultimate disposal. Anaerobic digestion is used as a common method for primary and secondary sludge (PS and WAS) stabilization leading to an energy recovery bonus in the form of methane gas production. The demands for higher efficiency processes, have prompted the need for pretreatment methods in order to improve substrate solubilization and digestibility. More studies focus on thermal or thermochemical process as a pretreatment stage of WAS. These studies include thermal pretreatment in the moderate temperature range of 60–100 °C (Hiraoka, 1984; Li and Noike, 1997), in the medium temperature range of 100–175 °C (Haug et al., 1978), and in a high temperature range of 175–225 °C (Haug, * Corresponding author. Tel.: +30-210-7723-268; fax: +30-210-7723163. E-mail address: [email protected] (A.G. Vlyssides).
0960-8524/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0960-8524(03)00176-7
1983). Also, thermochemical pretreatment is employed at ambient temperature and alkaline conditions (Lin et al., 1997) or acidic (Karlsson and Goransson, 1993) conditions. The most important parameter for controlling anaerobic digestion is the concentration of bicarbonate alkalinity which has to be as high as possible. The volatile acids produced during digestion can inhibit the methane production due to the fact that the pH can be decreased towards the acidic region. The bicarbonates are in equilibrium with the soluble CO2 producing a strong buffering at neutral pH. Carbon dioxide is a digestion product, so the strength of the buffering depends on the presence of cations. Hence, in many anaerobic digestion cases, it is necessary to add appropriate amounts of alkali in the form of lime or sodium or potassium hydroxide in order to keep the pH in the neutral region (Mukherjee and Levine, 1992). In addition, alkaline treatment of organic material was reported to induce swelling of particulate organic, making the cellular substances more susceptible to enzymatic attack (Baccay and Hashimoto, 1984). As a result the biodegradability of the solid phase may be enhanced significantly. The pretreatment of WAS at moderate temperature (50–90 °C) in alkaline conditions, by adding lime was
202
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
examined systematically regarding their effect on WAS solubilization and anaerobic biodegradation.
2. Methods 2.1. Methodology The WAS used for the experimental work was obtained from the secondary stage of a soft drinks production industry wastewater treatment plant, operated at a sludge retention time of 10 days. About 200 l of 5% WAS content was transferred to the laboratory, filtered, dried, mixed and stored in an airproof box at 4 °C immediately. Solid suspensions of 10% (w/v) in 250 ml of water were prepared by using the above described stored raw material. The total volatile suspended solids (VSS) content of the sludge used was 6.82% (w/v). In order to study the hydrolysis process of the activated sludge, a series of experiments were performed under various temperature and pH values. A total of 20 experimental sets were carried at 50, 60, 70, 80 and 90 °C with pH at 8, 9, 10, 11. About 10 batch reactors of 250-ml active volume each, were used in each set of experiments. The application of 10 batch reactors instead of one had to do with the accuracy of the intermediate results. The batch hydrolysis process took over 10 h and by terminating every one-hour the process of one reactor, more accurate results were obtained. Soluble COD and VSS were measured for the total content of the reactors every hour by terminating one reactor at a time. The anaerobic biodegradability of the final soluble products was evaluated by using a modified biochemical methane potential (BMP) test as described previously by Wang and Latchaw (1990). According to the BMP test, each assay bottle was prepared by placing 50 ml of a thermophilic microbial inoculum and 50 ml of testing sample in a 125-ml serum bottle. The seed inocula were obtained from a continuous flow 5 l thermophilic fermentor (55 °C), operated on a synthetic feed containing a nutrient solution. Prior to introduction of the seed inocula and reaction samples, the serum bottles were gassed with nitrogen at a flow rate of approximately 0.5 l/min for 10 min. Sodium sulfide (0.5 g/l) and L -cystine hydrochloride (0.5 g/l) were added to each bottle to provide a reducing environment. All samples were buffered at pH 7.0 with sodium bicarbonate. The serum bottles were then sealed and placed on a continuous shaker in a 55 °C incubator. After equilibration for 30 min at the incubation temperature, gas volumes were zeroed to ambient pressure with a syringe and the bottles were ready for test starting. After 15 days, total methane production and COD reduction were measured. Duplicates were run for all samples, including the seed blanks to which no sample was added.
2.2. Apparatus and methods of analysis The 10 reactors were placed in an oil heating bath in order to maintain the required temperature and avoid fluctuations. Stirring of the reactors by magnetic devices was on a continuous basis. At the beginning, as well as during the course of each experiment, the pH was regulated in order to keep it at the desirable level by adding appropriate amounts of lime. For the estimation of VSS, COD, CODs ‘‘Standard Methods’’ of analysis (APHA, 1995) was employed. Biogas volume were measured by the displacement of the plunger in an appropriate sized wetted glass syringe. Methane content in biogas was determined using a gas chromatograph (Perkin Elmer Model 2000). The statistical and correlation analysis of the results was carried out utilizing the techniques given in Taylor (1990). 2.3. Hydrolysis kinetics At constant temperature and pH, the rate of hydrolysis is a first-order function for the conversion of particulate biomass to utilizable soluble substrate (Rajan et al., 1989). In this study a first-order rate expression of the degradable particulate COD, was tested, according to the following expression: dðCODÞ ¼ kðCOD1 CODs Þ dt
ð1Þ
where CODs ¼ soluble COD concentration, mg/l; CODp ¼ particulate COD concentration, mg/l; CODnh ¼ nonhydrolyzable CODp concentration, mg/l; COD1 ¼ maximum theoretical soluble COD, mg/l ¼ CODp ) CODnh ; k ¼ first-order hydrolysis rate constant, h1 . In the above equation, the two unknown parameters, k and COD1 can be determined by the method of nonlinear curve fitting of experimental data especially for this type of differential equation described by Thomas (1950) and proved by Vlyssides et al. (1997). According to this method the experimental quantities of (t) 1=3 1=3 and (t=CODs Þ have linear relationship: ðt=CODs Þ ¼ A þ BðtÞ. The constants A and B are determined from experimental data using a conventional linear regression method. After this, the COD1 and k are estimated according to the following equations: COD1 ¼ 1=ð6A2 BÞ and k ¼ 4:8387B=A. The correlation between soluble COD production and VSS reduction can be in accordance with Eq. (2) which is derived from Eq. (1) after defining the correlation constant ‘‘a’’. dðVSSÞ 1 dðCODÞ k ¼ ¼ ðCOD1 CODs Þ dt a dt a
ð2Þ
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
where a ¼ correlation parameter ¼ dðCODÞ=dðVSSÞ VSS ¼ volatile suspended solids concentration; mg=l ð3Þ The correlation parameter ‘‘a’’ depends on the hydrolyzed material. For hydrocarbons the ‘‘a’’ value is about 1.2 while for proteins and aminoacids it is about 2.0 and for fats and lipids 2.5 (Vlyssides, 1987). So the values obtained for ‘‘a’’ will indicate the type of the hydrolyzed materials.
3. Results and discussion From the experimental results for the dry solids before hydrolysis, the following composition was obtained: Total organic carbon 27.9 ± 0.25%, total volatile solids, 68.2 ± 0.35% and total nitrogen 4.92 ± 0.2%. Experimental data related to CODs and VSS concentrations during hydrolysis process are given in Figs. 1 and 2. An increase of soluble COD concentration as well as a decrease of VSS evaluated the hydrolysis process. For all the hydrolysis conditions, a rapid increase of CODs was observed during the first hour (Fig. 1). Then a decreasing rate on the CODs production was observed until the maximum value was reached. However, for more intensive hydrolysis conditions (pH P 10 and
T P 80 °C) the COD solubilization increased significantly until the 8th hour of hydrolysis, where about 80% of the solubilization had been achieved. At pH ¼ 11 and T ¼ 90 °C and after 10 h of hydrolysis, the CODs concentration was about 69 000 mg/l and the solubilization rate was still increasing significantly. In Fig. 2 the % VSS reduction is presented. The VSS solubilization during the first 2 h (pH ¼ 8, T ¼ 50 °C to pH ¼ 10, T ¼ 70 °C) was about 60–70% of the final value, obtained after 10 h of hydrolysis. The VSS concentration dropped below 58 mg/l (from an initial value of 68.2 mg/l) after 2 h of hydrolysis in all experimental conditions, with a slightly positive effect of higher alkalinity and temperature. Changing the temperature from 50 to 90 °C after the 2nd hour of operation only higher pH values (10–11) affected VSS by decreasing significantly their concentration. For each pair of pH–T conditions, the hydrolysis constant rate k and the maximum theoretical soluble COD1 were estimated according to Eq. (1). The good fit of the experimental data to Eq. (1) proved that VSS hydrolysis followed a first-order kinetic model to the remaining nonhydrolized COD. These results are presented in Table 1, where it is shown that COD1 was increased with a pH and temperature increase, while the hydrolysis rate constant k was decreased. Especially in the case of pH ¼ 11 the calculated values of k appeared to decrease rapidly as temperature was increased. Nonlinear analysis for COD1 and k calculated values led to the following relations:
pH=8
COD, mg/l
COD, mg/l
35000 30000 25000 20000 15000 10000
5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0
40000 35000 30000 25000 20000 15000 10000 5000 0
pH=10
COD, mg/l
COD, mg/l
5000 0 0 1 2 3 4 5 6 7 8 9 10 time, hours
0 1 2 3 4 5 6 7 8 9 10 time, hours
203
pH=9
0 1 2 3 4 5 6 7 8 9 10 time, hours
8000 7000 pH=11 6000 5000 4000 3000 2000 1000 0 0 1 2 3 4 5 6 7 8 9 10 time, hours
Fig. 1. Soluble COD production in relation to pH, temperature and hydrolysis time: (}) 50 °C, (h) 60 °C, (n) 70 °C, (s) 80 °C, ( ) 90 °C.
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
30
30
25
25
% VSS reduction
% VSS reduction
204
20 15 10
pH=8
20 15 10
5 0
0 0
2
4
6 hours
8
10
0
30
% VSS reduction
25 % VSS reduction
pH=9
5
20 15 10
pH=10
5 0 0
2
4
6
8
10
50 45 40 35 30 25 20 15 10 5 0
2
4
6 hours
8
10
pH=11
0
2
hours
4
6
8
10
hours
Fig. 2. % VSS reduction in relation to pH, temperature and hydrolysis time: (}) 50 °C, (h) 60 °C, (n) 70 °C, (s) 80 °C, ( ) 90 °C.
Table 1 Calculation of the hydrolysis rate constant k and the theoretical upper limit of soluble COD pH
Temperature (°C)
Hydrolysis rate k (h1 )
Maximum theoretical soluble COD (COD1 ) (mg/l)
Correlation coefficient of fitting (r2 )
8
50 60 70 80 90
0.207 0.203 0.216 0.202 0.199
22 128 24 723 27 075 28 656 33 218
0.98 0.96 0.97 0.94 0.97
9
50 60 70 80 90
0.207 0.201 0.202 0.203 0.199
23 565 26 587 27 114 29 546 33 872
0.95 0.97 0.96 0.96 0.95
10
50 60 70 80 90
0.186 0.021 0.200 0.202 0.102
24 311 26 352 32 821 33 010 48 330
0.94 0.95 0.93 0.92 0.88
11
50 60 70 80 90
0.221 0.204 0.093 0.100 0.077
26 341 27 768 44 425 45 221 72 925
0.87 0.88 0.80 0.81 0.75
COD1 ¼ 177 037:1 þ 18 517pH 2706:18T þ 33:7498pH T ðr2 ¼ 0:887Þ
k ¼ 0:3511722 þ 6:708757 102 pH þ 1:0653 102 T 0:01279pH T ðr2 ¼ 0:856Þ
"a" correlation factor
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
4. Conclusions
3 2.5 2 1.5 1 0.5 0 10000
205
30000
50000 70000 soluble COD, mg/l
90000
Fig. 3. Correlation factor (a) in relation to soluble COD concentration.
In Fig. 3 the relation between correlation parameter ‘‘a’’ calculated from equation (3) and the measured COD values is presented. From these results it can be concluded that there was a very good correlation. For CODs values less than 35 000 mg/l the ‘‘a’’ parameter is expressed as: a ¼ 0:806 þ 0:35 104 CODs While for higher CODs values the ‘‘a’’ parameter remains constant the value being: a ¼ 2:322. The ‘‘a’’ values obtained indicated (but not strong evidence) that the process of WAS solubilization started with the hydrolysis of carbohydrates followed by aminoacids and proteins and finally by fats and lipids. The anaerobic biodegradability of soluble COD that was produced after 10 h of hydrolysis in relation to temperature and pH was estimated According to BMP test. The anaerobic biodegradability, measured as per cent of soluble COD reduction, decreased as pH and temperature increased. According to the obtained results, for 50 °C temperature and pH ¼ 8, 90% of the soluble COD was decreased at thermophilic anaerobic digestion, while for 90 °C temperature and pH ¼ 11 only 80% of the soluble COD was decreased. On the other hand the total efficiency of the process (hydrolysis and anaerobic digestion) measured as m3 of methane production per kg of VSS loading, increased when temperature and pH were increased. For 50 °C temperature and pH ¼ 8, the total efficiency (for methane production) was only 0.07 l CH4 /g VSS while for 90 °C temperature and pH ¼ 11, the efficiency reached 0.28 l CH4 / g VSS. The corresponding efficiencies of a unit of anaerobic digestion of WAS, from a municipal wastewater treatment plant, without prehydrolysis with a retention time of WAS at 15 days inside the digester, were 19.5% VSS reduction and 0.11 l CH4 /g VSS methane production (Gossett and Belser, 1981).
There are two distinct phases in hydrolysis under medium temperature and alkaline conditions; a rapid initial phase of one hour duration time and a subsequent slower phase that follows a first-order kinetic model. The mean value of the rate constant k for the second phase is about 0.20 (0.19–0.22). The VSS reduction in the initial rapid hydrolysis phase, represents 64–85% of the total VSS reduction after 10 h of hydrolysis retention time. The solubilization rate after 10 h of hydrolysis was significantly increased (under pH ¼ 11 and T ¼ 90 °C). The interest for further hydrolysis was limited because the COD reached concentration of 69.000 mg/l, which was very close to the maximum COD of initial WAS, as well as higher retention hydrolysis time would lead to higher installation cost, because of high hydrolysis tank volumes. Prehydrolysis at 90 °C temperature and pH, 11, followed by anaerobic digestion of WAS at thermophilic region seems to have many advantages. Compared to the conventional method it provided a reduction of the initial VSS by about 46% and a methane production of 0.28 l of methane per kg of initial VSS loading.
References APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association Inc., Washington DC, USA. Baccay, R.A., Hashimoto, A.G., 1984. Acidogenic and methanogenic fermentation of causticized straw. Biotechnol. Bioeng. 26, 885–891. Gossett, M.J., Belser, L.R., 1981. Anaerobic Digestion of Waste Activated Sludge. J. Sanit. Eng. Div., ASCE 108 (EE6), 1101– 1120. Hiraoka, M., 1984. Highly efficient anaerobic digestion with thermal pretreatment. Water Sci. Technol. 17, 529. Haug, T.R., 1983. Thermal pretreatment of sludges––a field demonstration. J. Water Pollut. Control. Fed. 55, 23. Haug, T.R., Stuckey, D.C., Gossett, J.M., McCarty, P.L., 1978. Effect of thermal pretreatment on digestibility and dewaterability of organic sludges. J. Water Pollut. Control. Fed. 50, 73–85. Karlsson, I., Goransson, J., 1993. Thermal sludge treatment. Water. Sci. Technol. 27, 449–456. Li, Y.Y., Noike, T., 1992. Upgrading of anaerobic digestion of waste activated sludge by thermal pretreatment. Water Sci. Technol. 26, 857–866. Lin, J.G., Chang, C.N., Chang, S.C., 1997. Enhancement of anaerobic digestion of waste activated sludge by alkaline solubilization. Bioresour. Technol. 62 (3), 85–90. Mukherjee, R.S., Levine, A.D., 1992. Chemical solubilization of particulate organics as a pretreatment approach. Water. Sci. Technol. 26, 2289–2292. Rajan, V.R., Lin, J.G., Ray, B.T., 1989. Low-level chemical pretreatment for enhanced sludge solubilization. J. Water Pollut. Control. Fed. 61, 1678–1683.
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Taylor, K.J., 1990. Statistical Techniques for data analysis, first ed. Lewis Publishers, New York. Thomas, A.H., 1950. Graphical determination of BOD curve constants. Water Sewage Works 97, 123–124. Vlyssides, G.A., 1987. A mathematical model for anaerobic digestion in thermophilic region. Ph.D. Thesis, Department of Chemical Engineering, National Technical University of Athens.
Vlyssides, G.A., Bouranis, B.T., Loizidou, M., 1997. A mathematical approach for the evaluation of nutrient eluation kinetics from organic soil conditioners. Commun. Soil Sci. Plant Anal. 28 (6–8), 509–520. Wang, Y.T., Latchaw, L.J., 1990. Anaerobic biodegradability and toxicity of hydrogen peroxide oxidation products of phenols. J. Water Pollut. Control. Fed. 62, 234–238.
Thermal-alkaline solubilization of waste activated sludge as a pre-treatment stage for anaerobic digestion A.G. Vlyssides *, P.K. Karlis Laboratory of Organic and Chemical Technology, Department of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Strasse, Zographou, 157 00 Athens, Greece Received 10 February 2003; received in revised form 20 May 2003; accepted 26 May 2003
Abstract This work studied the hydrolysis kinetics and the solubilization of waste activated sludge under a medium range temperature (50– 90 °C) and pH in the alkaline region (8–11), as a pretreatment stage for anaerobic digestion. The hydrolysis rate for the solubilization of volatile suspended solids (VSS) followed a first-order rate. A linear polynomial hydrolysis model was derived from the experimental results leading to a satisfactory correlation between the hydrolysis rate coefficient, pH, and temperature. At pH 11 and a temperature of 90 °C the concentration of the VSS was 6.82%, the VSS reduction reached 45% within ten hours and at the same time the soluble COD was 70.000 mg/l and the total efficiency for methane production 0.28 l of CH4 per g of VSS loading. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Waste activated sludge; Thermal; Pretreatment; Alkaline; Lime; Solubilization; Hydrolysis
1. Introduction Sludge handling represents 30–40% of the capital cost and about 50% of the operating cost of many wastewater treatment facilities. About two-thirds of the sludge in Greek secondary treatment plants is waste activated sludge (WAS). The WAS has to be stabilized sufficiently to reduce its organic content, odor problems, and pathogen contamination before ultimate disposal. Anaerobic digestion is used as a common method for primary and secondary sludge (PS and WAS) stabilization leading to an energy recovery bonus in the form of methane gas production. The demands for higher efficiency processes, have prompted the need for pretreatment methods in order to improve substrate solubilization and digestibility. More studies focus on thermal or thermochemical process as a pretreatment stage of WAS. These studies include thermal pretreatment in the moderate temperature range of 60–100 °C (Hiraoka, 1984; Li and Noike, 1997), in the medium temperature range of 100–175 °C (Haug et al., 1978), and in a high temperature range of 175–225 °C (Haug, * Corresponding author. Tel.: +30-210-7723-268; fax: +30-210-7723163. E-mail address: [email protected] (A.G. Vlyssides).
0960-8524/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0960-8524(03)00176-7
1983). Also, thermochemical pretreatment is employed at ambient temperature and alkaline conditions (Lin et al., 1997) or acidic (Karlsson and Goransson, 1993) conditions. The most important parameter for controlling anaerobic digestion is the concentration of bicarbonate alkalinity which has to be as high as possible. The volatile acids produced during digestion can inhibit the methane production due to the fact that the pH can be decreased towards the acidic region. The bicarbonates are in equilibrium with the soluble CO2 producing a strong buffering at neutral pH. Carbon dioxide is a digestion product, so the strength of the buffering depends on the presence of cations. Hence, in many anaerobic digestion cases, it is necessary to add appropriate amounts of alkali in the form of lime or sodium or potassium hydroxide in order to keep the pH in the neutral region (Mukherjee and Levine, 1992). In addition, alkaline treatment of organic material was reported to induce swelling of particulate organic, making the cellular substances more susceptible to enzymatic attack (Baccay and Hashimoto, 1984). As a result the biodegradability of the solid phase may be enhanced significantly. The pretreatment of WAS at moderate temperature (50–90 °C) in alkaline conditions, by adding lime was
202
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
examined systematically regarding their effect on WAS solubilization and anaerobic biodegradation.
2. Methods 2.1. Methodology The WAS used for the experimental work was obtained from the secondary stage of a soft drinks production industry wastewater treatment plant, operated at a sludge retention time of 10 days. About 200 l of 5% WAS content was transferred to the laboratory, filtered, dried, mixed and stored in an airproof box at 4 °C immediately. Solid suspensions of 10% (w/v) in 250 ml of water were prepared by using the above described stored raw material. The total volatile suspended solids (VSS) content of the sludge used was 6.82% (w/v). In order to study the hydrolysis process of the activated sludge, a series of experiments were performed under various temperature and pH values. A total of 20 experimental sets were carried at 50, 60, 70, 80 and 90 °C with pH at 8, 9, 10, 11. About 10 batch reactors of 250-ml active volume each, were used in each set of experiments. The application of 10 batch reactors instead of one had to do with the accuracy of the intermediate results. The batch hydrolysis process took over 10 h and by terminating every one-hour the process of one reactor, more accurate results were obtained. Soluble COD and VSS were measured for the total content of the reactors every hour by terminating one reactor at a time. The anaerobic biodegradability of the final soluble products was evaluated by using a modified biochemical methane potential (BMP) test as described previously by Wang and Latchaw (1990). According to the BMP test, each assay bottle was prepared by placing 50 ml of a thermophilic microbial inoculum and 50 ml of testing sample in a 125-ml serum bottle. The seed inocula were obtained from a continuous flow 5 l thermophilic fermentor (55 °C), operated on a synthetic feed containing a nutrient solution. Prior to introduction of the seed inocula and reaction samples, the serum bottles were gassed with nitrogen at a flow rate of approximately 0.5 l/min for 10 min. Sodium sulfide (0.5 g/l) and L -cystine hydrochloride (0.5 g/l) were added to each bottle to provide a reducing environment. All samples were buffered at pH 7.0 with sodium bicarbonate. The serum bottles were then sealed and placed on a continuous shaker in a 55 °C incubator. After equilibration for 30 min at the incubation temperature, gas volumes were zeroed to ambient pressure with a syringe and the bottles were ready for test starting. After 15 days, total methane production and COD reduction were measured. Duplicates were run for all samples, including the seed blanks to which no sample was added.
2.2. Apparatus and methods of analysis The 10 reactors were placed in an oil heating bath in order to maintain the required temperature and avoid fluctuations. Stirring of the reactors by magnetic devices was on a continuous basis. At the beginning, as well as during the course of each experiment, the pH was regulated in order to keep it at the desirable level by adding appropriate amounts of lime. For the estimation of VSS, COD, CODs ‘‘Standard Methods’’ of analysis (APHA, 1995) was employed. Biogas volume were measured by the displacement of the plunger in an appropriate sized wetted glass syringe. Methane content in biogas was determined using a gas chromatograph (Perkin Elmer Model 2000). The statistical and correlation analysis of the results was carried out utilizing the techniques given in Taylor (1990). 2.3. Hydrolysis kinetics At constant temperature and pH, the rate of hydrolysis is a first-order function for the conversion of particulate biomass to utilizable soluble substrate (Rajan et al., 1989). In this study a first-order rate expression of the degradable particulate COD, was tested, according to the following expression: dðCODÞ ¼ kðCOD1 CODs Þ dt
ð1Þ
where CODs ¼ soluble COD concentration, mg/l; CODp ¼ particulate COD concentration, mg/l; CODnh ¼ nonhydrolyzable CODp concentration, mg/l; COD1 ¼ maximum theoretical soluble COD, mg/l ¼ CODp ) CODnh ; k ¼ first-order hydrolysis rate constant, h1 . In the above equation, the two unknown parameters, k and COD1 can be determined by the method of nonlinear curve fitting of experimental data especially for this type of differential equation described by Thomas (1950) and proved by Vlyssides et al. (1997). According to this method the experimental quantities of (t) 1=3 1=3 and (t=CODs Þ have linear relationship: ðt=CODs Þ ¼ A þ BðtÞ. The constants A and B are determined from experimental data using a conventional linear regression method. After this, the COD1 and k are estimated according to the following equations: COD1 ¼ 1=ð6A2 BÞ and k ¼ 4:8387B=A. The correlation between soluble COD production and VSS reduction can be in accordance with Eq. (2) which is derived from Eq. (1) after defining the correlation constant ‘‘a’’. dðVSSÞ 1 dðCODÞ k ¼ ¼ ðCOD1 CODs Þ dt a dt a
ð2Þ
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
where a ¼ correlation parameter ¼ dðCODÞ=dðVSSÞ VSS ¼ volatile suspended solids concentration; mg=l ð3Þ The correlation parameter ‘‘a’’ depends on the hydrolyzed material. For hydrocarbons the ‘‘a’’ value is about 1.2 while for proteins and aminoacids it is about 2.0 and for fats and lipids 2.5 (Vlyssides, 1987). So the values obtained for ‘‘a’’ will indicate the type of the hydrolyzed materials.
3. Results and discussion From the experimental results for the dry solids before hydrolysis, the following composition was obtained: Total organic carbon 27.9 ± 0.25%, total volatile solids, 68.2 ± 0.35% and total nitrogen 4.92 ± 0.2%. Experimental data related to CODs and VSS concentrations during hydrolysis process are given in Figs. 1 and 2. An increase of soluble COD concentration as well as a decrease of VSS evaluated the hydrolysis process. For all the hydrolysis conditions, a rapid increase of CODs was observed during the first hour (Fig. 1). Then a decreasing rate on the CODs production was observed until the maximum value was reached. However, for more intensive hydrolysis conditions (pH P 10 and
T P 80 °C) the COD solubilization increased significantly until the 8th hour of hydrolysis, where about 80% of the solubilization had been achieved. At pH ¼ 11 and T ¼ 90 °C and after 10 h of hydrolysis, the CODs concentration was about 69 000 mg/l and the solubilization rate was still increasing significantly. In Fig. 2 the % VSS reduction is presented. The VSS solubilization during the first 2 h (pH ¼ 8, T ¼ 50 °C to pH ¼ 10, T ¼ 70 °C) was about 60–70% of the final value, obtained after 10 h of hydrolysis. The VSS concentration dropped below 58 mg/l (from an initial value of 68.2 mg/l) after 2 h of hydrolysis in all experimental conditions, with a slightly positive effect of higher alkalinity and temperature. Changing the temperature from 50 to 90 °C after the 2nd hour of operation only higher pH values (10–11) affected VSS by decreasing significantly their concentration. For each pair of pH–T conditions, the hydrolysis constant rate k and the maximum theoretical soluble COD1 were estimated according to Eq. (1). The good fit of the experimental data to Eq. (1) proved that VSS hydrolysis followed a first-order kinetic model to the remaining nonhydrolized COD. These results are presented in Table 1, where it is shown that COD1 was increased with a pH and temperature increase, while the hydrolysis rate constant k was decreased. Especially in the case of pH ¼ 11 the calculated values of k appeared to decrease rapidly as temperature was increased. Nonlinear analysis for COD1 and k calculated values led to the following relations:
pH=8
COD, mg/l
COD, mg/l
35000 30000 25000 20000 15000 10000
5000 4500 4000 3500 3000 2500 2000 1500 1000 500 0
40000 35000 30000 25000 20000 15000 10000 5000 0
pH=10
COD, mg/l
COD, mg/l
5000 0 0 1 2 3 4 5 6 7 8 9 10 time, hours
0 1 2 3 4 5 6 7 8 9 10 time, hours
203
pH=9
0 1 2 3 4 5 6 7 8 9 10 time, hours
8000 7000 pH=11 6000 5000 4000 3000 2000 1000 0 0 1 2 3 4 5 6 7 8 9 10 time, hours
Fig. 1. Soluble COD production in relation to pH, temperature and hydrolysis time: (}) 50 °C, (h) 60 °C, (n) 70 °C, (s) 80 °C, ( ) 90 °C.
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
30
30
25
25
% VSS reduction
% VSS reduction
204
20 15 10
pH=8
20 15 10
5 0
0 0
2
4
6 hours
8
10
0
30
% VSS reduction
25 % VSS reduction
pH=9
5
20 15 10
pH=10
5 0 0
2
4
6
8
10
50 45 40 35 30 25 20 15 10 5 0
2
4
6 hours
8
10
pH=11
0
2
hours
4
6
8
10
hours
Fig. 2. % VSS reduction in relation to pH, temperature and hydrolysis time: (}) 50 °C, (h) 60 °C, (n) 70 °C, (s) 80 °C, ( ) 90 °C.
Table 1 Calculation of the hydrolysis rate constant k and the theoretical upper limit of soluble COD pH
Temperature (°C)
Hydrolysis rate k (h1 )
Maximum theoretical soluble COD (COD1 ) (mg/l)
Correlation coefficient of fitting (r2 )
8
50 60 70 80 90
0.207 0.203 0.216 0.202 0.199
22 128 24 723 27 075 28 656 33 218
0.98 0.96 0.97 0.94 0.97
9
50 60 70 80 90
0.207 0.201 0.202 0.203 0.199
23 565 26 587 27 114 29 546 33 872
0.95 0.97 0.96 0.96 0.95
10
50 60 70 80 90
0.186 0.021 0.200 0.202 0.102
24 311 26 352 32 821 33 010 48 330
0.94 0.95 0.93 0.92 0.88
11
50 60 70 80 90
0.221 0.204 0.093 0.100 0.077
26 341 27 768 44 425 45 221 72 925
0.87 0.88 0.80 0.81 0.75
COD1 ¼ 177 037:1 þ 18 517pH 2706:18T þ 33:7498pH T ðr2 ¼ 0:887Þ
k ¼ 0:3511722 þ 6:708757 102 pH þ 1:0653 102 T 0:01279pH T ðr2 ¼ 0:856Þ
"a" correlation factor
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
4. Conclusions
3 2.5 2 1.5 1 0.5 0 10000
205
30000
50000 70000 soluble COD, mg/l
90000
Fig. 3. Correlation factor (a) in relation to soluble COD concentration.
In Fig. 3 the relation between correlation parameter ‘‘a’’ calculated from equation (3) and the measured COD values is presented. From these results it can be concluded that there was a very good correlation. For CODs values less than 35 000 mg/l the ‘‘a’’ parameter is expressed as: a ¼ 0:806 þ 0:35 104 CODs While for higher CODs values the ‘‘a’’ parameter remains constant the value being: a ¼ 2:322. The ‘‘a’’ values obtained indicated (but not strong evidence) that the process of WAS solubilization started with the hydrolysis of carbohydrates followed by aminoacids and proteins and finally by fats and lipids. The anaerobic biodegradability of soluble COD that was produced after 10 h of hydrolysis in relation to temperature and pH was estimated According to BMP test. The anaerobic biodegradability, measured as per cent of soluble COD reduction, decreased as pH and temperature increased. According to the obtained results, for 50 °C temperature and pH ¼ 8, 90% of the soluble COD was decreased at thermophilic anaerobic digestion, while for 90 °C temperature and pH ¼ 11 only 80% of the soluble COD was decreased. On the other hand the total efficiency of the process (hydrolysis and anaerobic digestion) measured as m3 of methane production per kg of VSS loading, increased when temperature and pH were increased. For 50 °C temperature and pH ¼ 8, the total efficiency (for methane production) was only 0.07 l CH4 /g VSS while for 90 °C temperature and pH ¼ 11, the efficiency reached 0.28 l CH4 / g VSS. The corresponding efficiencies of a unit of anaerobic digestion of WAS, from a municipal wastewater treatment plant, without prehydrolysis with a retention time of WAS at 15 days inside the digester, were 19.5% VSS reduction and 0.11 l CH4 /g VSS methane production (Gossett and Belser, 1981).
There are two distinct phases in hydrolysis under medium temperature and alkaline conditions; a rapid initial phase of one hour duration time and a subsequent slower phase that follows a first-order kinetic model. The mean value of the rate constant k for the second phase is about 0.20 (0.19–0.22). The VSS reduction in the initial rapid hydrolysis phase, represents 64–85% of the total VSS reduction after 10 h of hydrolysis retention time. The solubilization rate after 10 h of hydrolysis was significantly increased (under pH ¼ 11 and T ¼ 90 °C). The interest for further hydrolysis was limited because the COD reached concentration of 69.000 mg/l, which was very close to the maximum COD of initial WAS, as well as higher retention hydrolysis time would lead to higher installation cost, because of high hydrolysis tank volumes. Prehydrolysis at 90 °C temperature and pH, 11, followed by anaerobic digestion of WAS at thermophilic region seems to have many advantages. Compared to the conventional method it provided a reduction of the initial VSS by about 46% and a methane production of 0.28 l of methane per kg of initial VSS loading.
References APHA, 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. American Public Health Association Inc., Washington DC, USA. Baccay, R.A., Hashimoto, A.G., 1984. Acidogenic and methanogenic fermentation of causticized straw. Biotechnol. Bioeng. 26, 885–891. Gossett, M.J., Belser, L.R., 1981. Anaerobic Digestion of Waste Activated Sludge. J. Sanit. Eng. Div., ASCE 108 (EE6), 1101– 1120. Hiraoka, M., 1984. Highly efficient anaerobic digestion with thermal pretreatment. Water Sci. Technol. 17, 529. Haug, T.R., 1983. Thermal pretreatment of sludges––a field demonstration. J. Water Pollut. Control. Fed. 55, 23. Haug, T.R., Stuckey, D.C., Gossett, J.M., McCarty, P.L., 1978. Effect of thermal pretreatment on digestibility and dewaterability of organic sludges. J. Water Pollut. Control. Fed. 50, 73–85. Karlsson, I., Goransson, J., 1993. Thermal sludge treatment. Water. Sci. Technol. 27, 449–456. Li, Y.Y., Noike, T., 1992. Upgrading of anaerobic digestion of waste activated sludge by thermal pretreatment. Water Sci. Technol. 26, 857–866. Lin, J.G., Chang, C.N., Chang, S.C., 1997. Enhancement of anaerobic digestion of waste activated sludge by alkaline solubilization. Bioresour. Technol. 62 (3), 85–90. Mukherjee, R.S., Levine, A.D., 1992. Chemical solubilization of particulate organics as a pretreatment approach. Water. Sci. Technol. 26, 2289–2292. Rajan, V.R., Lin, J.G., Ray, B.T., 1989. Low-level chemical pretreatment for enhanced sludge solubilization. J. Water Pollut. Control. Fed. 61, 1678–1683.
206
A.G. Vlyssides, P.K. Karlis / Bioresource Technology 91 (2004) 201–206
Taylor, K.J., 1990. Statistical Techniques for data analysis, first ed. Lewis Publishers, New York. Thomas, A.H., 1950. Graphical determination of BOD curve constants. Water Sewage Works 97, 123–124. Vlyssides, G.A., 1987. A mathematical model for anaerobic digestion in thermophilic region. Ph.D. Thesis, Department of Chemical Engineering, National Technical University of Athens.
Vlyssides, G.A., Bouranis, B.T., Loizidou, M., 1997. A mathematical approach for the evaluation of nutrient eluation kinetics from organic soil conditioners. Commun. Soil Sci. Plant Anal. 28 (6–8), 509–520. Wang, Y.T., Latchaw, L.J., 1990. Anaerobic biodegradability and toxicity of hydrogen peroxide oxidation products of phenols. J. Water Pollut. Control. Fed. 62, 234–238.