Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Effect of Thermal Disintegration of Excess Sludge on the Effectiveness of Hydrolysis Process in Anaerobic Stabilization

Effect of Thermal Disintegration of Excess Sludge on the Effectiveness of Hydrolysis Process in... The factor which essentially affects sludge biodegradation rate is the degree of fluidization of insoluble organic polymers to the solved form, which is a precondition for availability of nutrients for microorganisms. The phases which substantially limit the rate of anaerobic decomposition include hydrolytic and methanogenic phase. Subjecting excess sludge to the process of initial disintegration substantially affects the effectiveness of the process of anaerobic stabilization. As a result of intensification of the process of hydrolysis, which manifests itself in the increase in the value and rate of generating volatile fatty acids (VFA), elongation of methanogenic phase of the process and increase in the degree of fermentation of modified sludge can be observed. Use of initial treatment of sewage sludge i.e. thermal disintegration is aimed at breaking microorganisms' cells and release of intracellular organic matter to the liquid phase. As a result of thermal hydrolysis in the sludge, the volatile fatty acids (VFA) are generated as early as at the stage of the process of conditioning. The obtained value of VFA determines the course of biological hydrolysis which is the first phase of anaerobic stabilization. The aim of the present study was to determine the effect of thermal disintegration of excess sludge on the effectiveness of the process of hydrolysis in anaerobic stabilization i.e. the rate of production of volatile fatty acids, changes in the level of chemical oxygen demand (COD) and increase in the degree of reduction in organic matter. During the first stage of the investigations, the most favourable conditions of thermal disintegration of excess sludge were identified using the temperatures of 50°C, 70°C, 90°C and heating times of 1.5 h - 6 h. The sludge was placed in laboratory flasks secured with a glass plug with liquid-column gauge and subjected to thermal treatment in water bath with shaker option. Another stage involved 8-day process of anaerobic stabilization of raw and thermally disintegrated excess sludge. Stabilization was carried out in mesophilic temperature regime i.e. at 37°C, under periodical conditions. In the case of the process of anaerobic stabilization of thermally disintegrated excess sludge at the temperature of 50°C and heating time of 6 h (mixture B) and 70°C and heating time of 4.5% (mixture C), the degree of fermentation of 30.67% and 33.63%, respectively, was obtained. For the studied sludge, i.e. mixture B and mixture C, maximal level of volatile fatty acids i.e. 874.29 mg CH3COOH/dm3 and 1131.43 mg CH3COOH/dm3 was found on the 2nd day of the process. The maximal obtained value of VFA was correlated on this day with maximal COD level, which was 1344 mg O2/dm3 for mixture B and 1778 mg O2/dm3 for mixture C. INTRODUCTION The effect of initial treatment of sewage sludge on improved efficiency of methane fermentation is a promising research problem, both in terms of economics of the stabilization process and potential benefits resulting from fermentation technology i.e. increase in the degree of conversion of organic compounds to gaseous form (biogas). The degree of fluidization of insoluble organic polymers to a soluble form, available for microorganisms, has essential effect on the speed of sludge biodegradation. The phases which substantially limit the rate of anaerobic decomposition are hydrolytic and methanogenic phase. In order to facilitate the process of anaerobic stabilization, excess sludge should be subjected to the process of initial disintegration, which is a precondition for an increase in the effectiveness of the process [1, 2, 3, 4]. Waste activated sludge (WAS) is the main by-product in the process of biological waste treatment [5]. As a result of intensification of the process of hydrolysis, the methanogenic phase is elongated, which, in consequence, causes substantial increase in biogas production. The obtained biogas is a valuable energy carrier with high calorific value which allows for meeting the energy demand in the wastewater treatment plant, generation of heat for central heating system and electricity used for direct power supply to the equipment. The use of initial treatment of sewage sludge, e.g. thermal modification of the sludge, is aimed at breaking microorganisms' cells in the excess sludge, which is a precondition for release of intracellular organic matter to the liquid phase. As a result of thermal hydrolysis in the sludge, the volatile fatty acids (VFA) are generated as early as at the stage of the process of conditioning. The obtained value of VFA determines the course of biological hydrolysis, which is the first phase of anaerobic stabilization. Increase in the rate of generation of VFAs and increase in their values on consecutive days of acid fermentation directly affect the effectiveness of biogas production [6, 7, 8]. During the process of anaerobic stabilization, initiation of thermal hydrolysis allows for obtaining higher degree of transformation of volatile fatty acids (VFA) into methane. Shortening of the duration of hydrolytic phase causes intensification and acceleration of the processes which take place in individual stages of anaerobic stabilization. The methods of thermal disintegration of sewage sludge include in particular [7, 9]: ­ low-temperature methods, at the temperatures below 100°C, ­ high-temperature methods, at the temperatures over 100°C. The literature [10] reports that thermal sludge treatment impacts significantly on the increase in biogas acquisition. Thermal hydrolysis (20 min at the temperature of 170°C) caused over 70% reduction in the content of dry matter at over 3-time increase in the load in the chamber with organic mass. The process of hydrolysis at lower temperatures (60÷80°C) favourably affects the composition of the obtained hydrolysate, but it requires longer times of reaction (60÷120 min) [11, 12]. In the result of disintegration followed stimulation of the inert part of excess sludge which significantly affect the intensification of the methane fermentation [13]. Moreover, the investigations carried out by Dbrowska et al. [14] confirmed that after the process of thermophilic digestion of the mixture of primary and fermenting sludge, the total concentration of seven PCBs was reduced by 84%, which suggests that thermophilic digestion affects reduction in PCBs positively. MATERIALS AND METHODS Characterisation of the Study Substrate The substrate in the study was excess sludge (90%), and fermented sludge (10%) which was used for inoculation. The sludge was sampled from the Warta Central Sewage Treatment Plant in Czstochowa. Excess activated sludge samples used for the investigations were taken immediately before mechanical densification. Excess sludge which is generated at the biological level is supplied to mechanical concentrators and, after densification to ca. 5-6% of dry matter, is pumped to WKFz. The effluent waters which are generated in the process are discharged in front of primary settling tanks. Raw and excess sludges, accumulated in WKFz, are subjected to methane fermentation at the temperature of 33°C. The following mixtures of sludge were subjected to methane fermentation: - Mixture A ­ raw excess sludge + fermented sludge - Mixture B ­ excess sludge, thermally disintegrated at the temperature of 50°C for 6 h + fermented sludge; - Mixture C ­ excess sludge, thermally disintegrated at the temperature of 70°C for 4.5 h + fermented sludge. Studies of anaerobic stabilization process do not include mixture D, pre-tested at the stage of selecting the most favorable conditions for thermal disintegration. In the case of mixture D, despite the increase in pretreatment temperature by 20°C in reference to mixture C, a satisfactory increase in the value of COD was not obtained, due to the nature of the physico-chemical properties of organic substances contained in the sludge with increasing preparation time, while watching the decline in VFA. Table 1 presents the characteristics of raw excess sludge and fermented sludge used in the study. Conditions of Thermal Disintegration of Excess Sludge In order to determine the most favourable conditions of thermal modification of excess sludge, the temperatures of 50°C, 70°C and 90°C were selected for heating for the period of 1.5 h; 3 h; 4.5 h and 6 h. The sludge poured to laboratory flasks was secured from air access with a glass plug with liquid-column gauge and placed in water bath. The sample volume was 500 ml. Conditions for the Process of 8-day Anaerobic Stabilization The laboratory experiments were aimed at the determination of the impact of thermal disintegration of excess sludge on the efficiency of hydrolysis process in anaerobic stabilization, i.e. intensity of generation of volatile fatty acids (VFA). The process of methane fermentation was carried out in eight glass laboratory flasks, which performed the role of fermentation chambers. Before the process, laboratory flasks with the volume of 0.5 dm3 were secured from air access with plugs with diameter of 33 mm and glass liquid-column gauges, which allowed for discharge of the generated biogas. They were placed in laboratory thermostat at the temperature of 37°C. The flasks were mixed manually, once a day, in order to mix the whole volume of the sludge and prevent from generation of the skin on the surface and creation of the areas overloaded with contaminants. The following physical and chemical determinations were carried out: ­ pH using pH meter by Cole Palmer 59002 ­ 00, according to (PN-9/C-04540/05) [15], Table 1. Selected physical and chemical parameters of sludge for the mixtures A, B and C VFAs mgCH3COOH/dm3 FERMENTED SLUDGE (inoculum) 823 RAW EXCESS SLUDGE 103 FERMENTED SLUDGE (inoculum) 617 1021 2700 240 25 165 220 140 28 1358 3300 200 28 mgO2/dm3 mgCaCO3/dm3 mgCaCO3/dm3 COD Alkalinity Acidity Ammonium nitrogen mgN-NH4+/dm3 g/dm3 g/dm3 g/dm3 MIXTURE A, MIXTURE B MIXTURE C RAW EXCESS SLUDGE 86 70 120 20 27 31 7.26 ­ dry matter, organic dry matter, mineral dry matter according to (PN-EN-12879) [16], ­ volatile fatty acids (VFA) by means of distillation with water vapour according to (PN-75/C-04616/04) [17], ­ alkalinity according to (PN-91/C-04540/05) [18], ­ acidity according to (PN-91/C-04540/05) [19], ­ chemical oxygen demand using dichromate method according to (ISO 7027) [20], ­ total Kjeldahl nitrogen according to (PN-73/C-04576/10) [21], ­ amone nitrogen according to (PN-73/C-04576/02) [21]. RESULTS AND DISCUSSION Choice of the Most Favourable Parameters of Thermal Disintegration of Excess Sludge In order to choose the most favourable temperature and conditioning time, excess sludge was subjected to three test cycles in the range of low temperatures, i.e. the temperature of 50°C, 70°C and 90°C and conditioning time of 1.5; 3; 4.5; 6h. The criterion for selection of parameters was the increase in concentration of organic substance in sludge liquor of excess sludge measured by means of COD and VFA indexes. The content of organic dry matter in the excess sludge before the process of thermal conditioning amounted to 9.68 g/dm3. Table 2 presents changes in COD levels in sludge liquor after the process of thermal conditioning, whereas changes in VFA levels are presented in Fig. 1. Table 2. Changes in COD levels in sludge liquor in excess sludge subjected to thermal conditioning recorded depending on time and temperature of preparation. COD, mgO2/dm3 Temp. oC 50 70 90 Time, h 1.5 1315 1816 1970 3 1398 2104 2062 4.5 1489 2398 2158 6 1566 2374 2366 The obtained results confirm the theory of Wójtowicz [13] concerning a positive effect of disintegration on fluidization of organic matter contained in excess sludge, with its direct manifestation being an increase in COD and VFAs in sludge liquor. The lowest value of COD in sludge liquor was found for the temperature of 50°C, with heating time of 1.5 h (1315 mgO2/dm3), whereas its higher level of 2398 mgO2/dm3 was observed for the temperature of 70°C and heating time of 4.5 h. It was found during the process that the most favourable value of volatile fatty acids during 6-hour conditioning was obtained for the temperature of 70°C i.e. 428.57 mg CH3COOH/dm3. In the case of 6-hour conditioning, the value of VFA for all the temperatures rose with respect to the initial value. Downward trend for VFA level after 1.5 h of heating was observed for the temperature of 90°C. For the heating time of 1.5 h the level of VFAs reached 197.14 mg CH3COOH/dm3, whereas for the time of 6 h, this value was 154.27 mg CH3COOH/dm3. 500 400 300 200 100 0 0 1,5 3 VFAs, mgCH3COOH/dm3 4,5 Time of thermal disintegration Fig. 1. Changes in the VFA level observed in sludge liquor as a function of exposure time and disintegration temperature Process of Methane Fermentation in Raw Excess Sludge The process of 8-day anaerobic stabilization was carried out for the mixture of non-conditioned excess sludge (90%) and fermented sludge (10%) (mixture A). Degree of fermentation of the sludge for mixture A amounted to 16.88%. The content of organic dry matter before the process amounted to 5.39 g/dm3, whereas this value was 4.48 g/dm3 on the last day of the process. The reduction in the content of dry matter from 9.44 g/dm3 before the process to 6.92 g/dm3 after the process was observed. Reduction in dry matter of the sludge amounted to 26.70%. Value of volatile fatty acids before the process of anaerobic stabilization amounted to 111.43 mg CH3COOH/dm3; its maximal level was observed on the 3rd day of the process, i.e. 771.43 mg CH3COOH/dm3. A decline in the level of VFAs was initiated on the 4th day and, on the 8th day, VFA level amounted to 377.14 mg CH3COOH/dm3. Value of COD for sludge water before the process amounted to 288 mg O2/dm3, the highest value was reached on the third day and amounted to 1123 mg O2/dm3; a decline in the level of COD was observed from the fourth day, and, on the last day of the process this value amounted to 454 mg O2/dm3. Table 3 presents comparison of the results obtained for the course of the process of anaerobic stabilization in raw excess sludge (mixture A). Process of Methane Fermentation of Thermally Disintegrated Excess Sludge The process of 8-day anaerobic stabilization was employed for the mixture of excess sludge (90%) thermally conditioned at the temperature of 50°C for 6 h and fermented sludge (mixture B) and the mixture of the sludge thermally conditioned at the temperature of 70°C for 4.5 h and fermented sludge (10%) (mixture C). Table 3. Selected physical and chemical parameters of the mixture A (raw excess sludge + fermented sludge) subjected to the process of 8-day anaerobic stabilization Time of methane fermentation VFAs mgCH3COOH/dm3 111.43 445.71 548.57 771.43 651.43 514.29 437.14 394.29 377.14 454 606 737 824 1180 1250 1320 1240 937 1100 1123 1100 948 880 80 140 136 124 100 128 180 688 620 60 288 500 20 mgO2/dm3 mgCaCO3/dm3 mg/dm3 COD Alkalinity Acidity Ammonium nitrogen g/dm3 g/dm3 g/dm3 mgN-NH4+/dm3 56.00 112.00 165.20 218.40 221.20 218.40 215.60 224.00 280.00 The degree of fermentation of the sludge for mixtures B and C amounted to 30.67% and 33.63%, respectively. In the case of mixture B, the content of organic dry matter before the process was 6.37 g/dm3, whereas on the last day of the process this value was 4.41 g/dm3. The content of dry matter before the process of fermentation amounted to 9.30 g/dm3, whereas after the process it reached 5.43 g/dm3. Reduction in dry matter of the sludge amounted to 41.62%. For mixture C, the content of organic dry matter before the process amounted to 7.24 g/dm3, whereas on the last day of the process this value was 5.37 g/dm3. The content of dry matter before the process of fermentation amounted to 10.93 g/dm3, whereas after the process it was 8.55 g/dm3. Reduction in dry matter of the sludge was 21.78%. In the case of mixtures B and C, the level of volatile fatty acids before the process of anaerobic stabilization amounted to 188.57 mg CH3COOH/dm3 and 342.86 mg CH3COOH/dm3, respectively. Maximal values for both mixture B and mixture C was observed on the 2nd day of the process and they reached 874.29 mg CH3COOH/dm3 and 1131.43 mg CH3COOH/dm3, respectively. A decline in the VFA level was observed on the third day of the process and, on the 8th day, this value was 171.43 mg CH3COOH/dm3 in mixture B and 377.14 mg CH3COOH/dm3 in mixture C. In the case of mixture B, the value of COD in sludge water before the process was 640 mg O2/dm3; its highest level (1344 mg O2/dm3) was found on the 2nd day and a decline in COD level in the sludge liquor was observed from the third day and it continued until it was 280 mg O2/dm3 on the last day. In mixture C, value of COD before the process of stabilization amounted to 1826 mg O2/dm3 and it was the highest level reached during the study. A decline in the value of COD in sludge liquor was observed from the first day and dropped to 800 mg O2/dm3 on the last day of the process. Tables 4 and 5 present comparison of the results obtained for the process of anaerobic stabilization of thermally disintegrated excess sludge (mixtures B and C). The results concerning the course of the process of anaerobic stabilization of thermally modified excess sludge confirm the effectiveness of this method as a disintegrating factor, which was also confirmed by the studied by other researchers [10, 11, 12]. CONCLUSIONS The aim of the present study was to identify the effect of thermal method of disintegration of excess sludge on its biodegradability and to assess the effectiveness of modification of the sludge in the aspect of increase in the value of VFAs, as well as in the value of COD and in the degree of disintegration of the modified sludge. Increase in the susceptibility of excess sludge on biochemical decomposition under anaerobic conditions occurs through the destruction of cell walls in microorganisms and the release of the organic substances they contain before they are used by the bacteria which exist under anaerobic conditions during the process of anaerobic stabilization. The obtained results allow for drawing the following conclusions: 1. In the case of thermal disintegration of excess sludge, the most favourable temperature was 70°C with the heating time 4.5 h, for which the highest level of COD among all the temperatures and times of thermal treatment was obtained (2398 mg O2/dm3). 2. As a result of subjecting excess sludge (mixture A) to the process of anaerobic stabilization of raw excess sludge, the degree of fermentation of 16.88% was obtained Table 4. Selected physical and chemical parameters of the mixture B (excess sludge, thermally disintegrated at the temperature of 50oC for 6 h + fermented sludge) subjected to the process of 8-day anaerobic stabilization Time of methane fermentation VFAs mgCH3COOH/dm3 188.57 788.57 874.29 840.00 617.14 522.86 360.00 188.57 171.43 280 621 576 892 1150 1120 1170 1240 1380 1440 1320 1060 1344 956 1179 860 80 100 110 70 70 72 80 66 640 450 4 mgO2/dm3 mgCaCO3/dm3 mg/dm3 COD Alkalinity Acidity Ammonium nitrogen g/dm3 g/dm3 g/dm3 mgN-NH4+/dm3 22.4 140 142.8 226.8 170.8 224 249.2 254.8 252 Table 5. Selected physical and chemical parameters of the mixture C (excess sludge, thermally disintegrated at the temperature of 70°C for 4.5 h + fermented sludge) subjected to the process of 8-day anaerobic stabilization Time of methane fermentation VFAs mgCH3COOH/dm3 342.86 891.43 1131.43 1062.86 891.43 737.14 411.43 685.71 377.14 800 882 578 997 1160 1300 1380 1420 1325 1020 1616 1000 1778 900 1680 840 60 154 180 200 200 180 220 240 1826 550 40 mgO2/dm3 mgCaCO3/dm3 mg/dm3 COD Alkalinity Acidity Ammonium nitrogen g/dm3 g/dm3 g/dm3 mgN-NH4+/dm3 168 196 238 260.4 282.8 280 280 308 322 after 8 days of fermentation. The maximal values of volatile fatty acids and COD (771.43 mg CH3COOH/dm3 and 1123 mg O2/dm3, respectively) were observed in the mixture on the 3rd day of the process. 3. In the case of the process of anaerobic stabilization of thermally disintegrated excess sludge, using the temperature of 50°C and heating time of 6h (Mixture B), and 70°C and heating time 4.5 h (mixture C), the following degrees of fermentation were obtained: 30.67% and 33.63%, respectively. Maximal levels of volatile fatty acids in mixtures B and C (874.29 and 1131.43 mg CH3COOH/dm3, respectively) were found on the 2nd day of the process. The obtained maximal level of VFA correlated with the maximal value of COD index found on the same (second) day of the process, which was 1344 mg O2/dm3 for mixture B and 1778 mg O2/dm3 for mixture C. Acknowledgments Scientific work founded by grant BG 401/402/10 resource sponsored by the State Committee for Scientific Research (KBN) in the year 2010-2012. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Archives of Environmental Protection de Gruyter

Effect of Thermal Disintegration of Excess Sludge on the Effectiveness of Hydrolysis Process in Anaerobic Stabilization

Download PDF
12 pages

Loading next page...
 
/lp/de-gruyter/effect-of-thermal-disintegration-of-excess-sludge-on-the-effectiveness-aubFPHOgEE
Publisher
de Gruyter
Copyright
Copyright © 2012 by the
ISSN
2083-4772
eISSN
2083-4810
DOI
10.2478/v10265-012-0009-5
Publisher site
See Article on Publisher Site

Abstract

The factor which essentially affects sludge biodegradation rate is the degree of fluidization of insoluble organic polymers to the solved form, which is a precondition for availability of nutrients for microorganisms. The phases which substantially limit the rate of anaerobic decomposition include hydrolytic and methanogenic phase. Subjecting excess sludge to the process of initial disintegration substantially affects the effectiveness of the process of anaerobic stabilization. As a result of intensification of the process of hydrolysis, which manifests itself in the increase in the value and rate of generating volatile fatty acids (VFA), elongation of methanogenic phase of the process and increase in the degree of fermentation of modified sludge can be observed. Use of initial treatment of sewage sludge i.e. thermal disintegration is aimed at breaking microorganisms' cells and release of intracellular organic matter to the liquid phase. As a result of thermal hydrolysis in the sludge, the volatile fatty acids (VFA) are generated as early as at the stage of the process of conditioning. The obtained value of VFA determines the course of biological hydrolysis which is the first phase of anaerobic stabilization. The aim of the present study was to determine the effect of thermal disintegration of excess sludge on the effectiveness of the process of hydrolysis in anaerobic stabilization i.e. the rate of production of volatile fatty acids, changes in the level of chemical oxygen demand (COD) and increase in the degree of reduction in organic matter. During the first stage of the investigations, the most favourable conditions of thermal disintegration of excess sludge were identified using the temperatures of 50°C, 70°C, 90°C and heating times of 1.5 h - 6 h. The sludge was placed in laboratory flasks secured with a glass plug with liquid-column gauge and subjected to thermal treatment in water bath with shaker option. Another stage involved 8-day process of anaerobic stabilization of raw and thermally disintegrated excess sludge. Stabilization was carried out in mesophilic temperature regime i.e. at 37°C, under periodical conditions. In the case of the process of anaerobic stabilization of thermally disintegrated excess sludge at the temperature of 50°C and heating time of 6 h (mixture B) and 70°C and heating time of 4.5% (mixture C), the degree of fermentation of 30.67% and 33.63%, respectively, was obtained. For the studied sludge, i.e. mixture B and mixture C, maximal level of volatile fatty acids i.e. 874.29 mg CH3COOH/dm3 and 1131.43 mg CH3COOH/dm3 was found on the 2nd day of the process. The maximal obtained value of VFA was correlated on this day with maximal COD level, which was 1344 mg O2/dm3 for mixture B and 1778 mg O2/dm3 for mixture C. INTRODUCTION The effect of initial treatment of sewage sludge on improved efficiency of methane fermentation is a promising research problem, both in terms of economics of the stabilization process and potential benefits resulting from fermentation technology i.e. increase in the degree of conversion of organic compounds to gaseous form (biogas). The degree of fluidization of insoluble organic polymers to a soluble form, available for microorganisms, has essential effect on the speed of sludge biodegradation. The phases which substantially limit the rate of anaerobic decomposition are hydrolytic and methanogenic phase. In order to facilitate the process of anaerobic stabilization, excess sludge should be subjected to the process of initial disintegration, which is a precondition for an increase in the effectiveness of the process [1, 2, 3, 4]. Waste activated sludge (WAS) is the main by-product in the process of biological waste treatment [5]. As a result of intensification of the process of hydrolysis, the methanogenic phase is elongated, which, in consequence, causes substantial increase in biogas production. The obtained biogas is a valuable energy carrier with high calorific value which allows for meeting the energy demand in the wastewater treatment plant, generation of heat for central heating system and electricity used for direct power supply to the equipment. The use of initial treatment of sewage sludge, e.g. thermal modification of the sludge, is aimed at breaking microorganisms' cells in the excess sludge, which is a precondition for release of intracellular organic matter to the liquid phase. As a result of thermal hydrolysis in the sludge, the volatile fatty acids (VFA) are generated as early as at the stage of the process of conditioning. The obtained value of VFA determines the course of biological hydrolysis, which is the first phase of anaerobic stabilization. Increase in the rate of generation of VFAs and increase in their values on consecutive days of acid fermentation directly affect the effectiveness of biogas production [6, 7, 8]. During the process of anaerobic stabilization, initiation of thermal hydrolysis allows for obtaining higher degree of transformation of volatile fatty acids (VFA) into methane. Shortening of the duration of hydrolytic phase causes intensification and acceleration of the processes which take place in individual stages of anaerobic stabilization. The methods of thermal disintegration of sewage sludge include in particular [7, 9]: ­ low-temperature methods, at the temperatures below 100°C, ­ high-temperature methods, at the temperatures over 100°C. The literature [10] reports that thermal sludge treatment impacts significantly on the increase in biogas acquisition. Thermal hydrolysis (20 min at the temperature of 170°C) caused over 70% reduction in the content of dry matter at over 3-time increase in the load in the chamber with organic mass. The process of hydrolysis at lower temperatures (60÷80°C) favourably affects the composition of the obtained hydrolysate, but it requires longer times of reaction (60÷120 min) [11, 12]. In the result of disintegration followed stimulation of the inert part of excess sludge which significantly affect the intensification of the methane fermentation [13]. Moreover, the investigations carried out by Dbrowska et al. [14] confirmed that after the process of thermophilic digestion of the mixture of primary and fermenting sludge, the total concentration of seven PCBs was reduced by 84%, which suggests that thermophilic digestion affects reduction in PCBs positively. MATERIALS AND METHODS Characterisation of the Study Substrate The substrate in the study was excess sludge (90%), and fermented sludge (10%) which was used for inoculation. The sludge was sampled from the Warta Central Sewage Treatment Plant in Czstochowa. Excess activated sludge samples used for the investigations were taken immediately before mechanical densification. Excess sludge which is generated at the biological level is supplied to mechanical concentrators and, after densification to ca. 5-6% of dry matter, is pumped to WKFz. The effluent waters which are generated in the process are discharged in front of primary settling tanks. Raw and excess sludges, accumulated in WKFz, are subjected to methane fermentation at the temperature of 33°C. The following mixtures of sludge were subjected to methane fermentation: - Mixture A ­ raw excess sludge + fermented sludge - Mixture B ­ excess sludge, thermally disintegrated at the temperature of 50°C for 6 h + fermented sludge; - Mixture C ­ excess sludge, thermally disintegrated at the temperature of 70°C for 4.5 h + fermented sludge. Studies of anaerobic stabilization process do not include mixture D, pre-tested at the stage of selecting the most favorable conditions for thermal disintegration. In the case of mixture D, despite the increase in pretreatment temperature by 20°C in reference to mixture C, a satisfactory increase in the value of COD was not obtained, due to the nature of the physico-chemical properties of organic substances contained in the sludge with increasing preparation time, while watching the decline in VFA. Table 1 presents the characteristics of raw excess sludge and fermented sludge used in the study. Conditions of Thermal Disintegration of Excess Sludge In order to determine the most favourable conditions of thermal modification of excess sludge, the temperatures of 50°C, 70°C and 90°C were selected for heating for the period of 1.5 h; 3 h; 4.5 h and 6 h. The sludge poured to laboratory flasks was secured from air access with a glass plug with liquid-column gauge and placed in water bath. The sample volume was 500 ml. Conditions for the Process of 8-day Anaerobic Stabilization The laboratory experiments were aimed at the determination of the impact of thermal disintegration of excess sludge on the efficiency of hydrolysis process in anaerobic stabilization, i.e. intensity of generation of volatile fatty acids (VFA). The process of methane fermentation was carried out in eight glass laboratory flasks, which performed the role of fermentation chambers. Before the process, laboratory flasks with the volume of 0.5 dm3 were secured from air access with plugs with diameter of 33 mm and glass liquid-column gauges, which allowed for discharge of the generated biogas. They were placed in laboratory thermostat at the temperature of 37°C. The flasks were mixed manually, once a day, in order to mix the whole volume of the sludge and prevent from generation of the skin on the surface and creation of the areas overloaded with contaminants. The following physical and chemical determinations were carried out: ­ pH using pH meter by Cole Palmer 59002 ­ 00, according to (PN-9/C-04540/05) [15], Table 1. Selected physical and chemical parameters of sludge for the mixtures A, B and C VFAs mgCH3COOH/dm3 FERMENTED SLUDGE (inoculum) 823 RAW EXCESS SLUDGE 103 FERMENTED SLUDGE (inoculum) 617 1021 2700 240 25 165 220 140 28 1358 3300 200 28 mgO2/dm3 mgCaCO3/dm3 mgCaCO3/dm3 COD Alkalinity Acidity Ammonium nitrogen mgN-NH4+/dm3 g/dm3 g/dm3 g/dm3 MIXTURE A, MIXTURE B MIXTURE C RAW EXCESS SLUDGE 86 70 120 20 27 31 7.26 ­ dry matter, organic dry matter, mineral dry matter according to (PN-EN-12879) [16], ­ volatile fatty acids (VFA) by means of distillation with water vapour according to (PN-75/C-04616/04) [17], ­ alkalinity according to (PN-91/C-04540/05) [18], ­ acidity according to (PN-91/C-04540/05) [19], ­ chemical oxygen demand using dichromate method according to (ISO 7027) [20], ­ total Kjeldahl nitrogen according to (PN-73/C-04576/10) [21], ­ amone nitrogen according to (PN-73/C-04576/02) [21]. RESULTS AND DISCUSSION Choice of the Most Favourable Parameters of Thermal Disintegration of Excess Sludge In order to choose the most favourable temperature and conditioning time, excess sludge was subjected to three test cycles in the range of low temperatures, i.e. the temperature of 50°C, 70°C and 90°C and conditioning time of 1.5; 3; 4.5; 6h. The criterion for selection of parameters was the increase in concentration of organic substance in sludge liquor of excess sludge measured by means of COD and VFA indexes. The content of organic dry matter in the excess sludge before the process of thermal conditioning amounted to 9.68 g/dm3. Table 2 presents changes in COD levels in sludge liquor after the process of thermal conditioning, whereas changes in VFA levels are presented in Fig. 1. Table 2. Changes in COD levels in sludge liquor in excess sludge subjected to thermal conditioning recorded depending on time and temperature of preparation. COD, mgO2/dm3 Temp. oC 50 70 90 Time, h 1.5 1315 1816 1970 3 1398 2104 2062 4.5 1489 2398 2158 6 1566 2374 2366 The obtained results confirm the theory of Wójtowicz [13] concerning a positive effect of disintegration on fluidization of organic matter contained in excess sludge, with its direct manifestation being an increase in COD and VFAs in sludge liquor. The lowest value of COD in sludge liquor was found for the temperature of 50°C, with heating time of 1.5 h (1315 mgO2/dm3), whereas its higher level of 2398 mgO2/dm3 was observed for the temperature of 70°C and heating time of 4.5 h. It was found during the process that the most favourable value of volatile fatty acids during 6-hour conditioning was obtained for the temperature of 70°C i.e. 428.57 mg CH3COOH/dm3. In the case of 6-hour conditioning, the value of VFA for all the temperatures rose with respect to the initial value. Downward trend for VFA level after 1.5 h of heating was observed for the temperature of 90°C. For the heating time of 1.5 h the level of VFAs reached 197.14 mg CH3COOH/dm3, whereas for the time of 6 h, this value was 154.27 mg CH3COOH/dm3. 500 400 300 200 100 0 0 1,5 3 VFAs, mgCH3COOH/dm3 4,5 Time of thermal disintegration Fig. 1. Changes in the VFA level observed in sludge liquor as a function of exposure time and disintegration temperature Process of Methane Fermentation in Raw Excess Sludge The process of 8-day anaerobic stabilization was carried out for the mixture of non-conditioned excess sludge (90%) and fermented sludge (10%) (mixture A). Degree of fermentation of the sludge for mixture A amounted to 16.88%. The content of organic dry matter before the process amounted to 5.39 g/dm3, whereas this value was 4.48 g/dm3 on the last day of the process. The reduction in the content of dry matter from 9.44 g/dm3 before the process to 6.92 g/dm3 after the process was observed. Reduction in dry matter of the sludge amounted to 26.70%. Value of volatile fatty acids before the process of anaerobic stabilization amounted to 111.43 mg CH3COOH/dm3; its maximal level was observed on the 3rd day of the process, i.e. 771.43 mg CH3COOH/dm3. A decline in the level of VFAs was initiated on the 4th day and, on the 8th day, VFA level amounted to 377.14 mg CH3COOH/dm3. Value of COD for sludge water before the process amounted to 288 mg O2/dm3, the highest value was reached on the third day and amounted to 1123 mg O2/dm3; a decline in the level of COD was observed from the fourth day, and, on the last day of the process this value amounted to 454 mg O2/dm3. Table 3 presents comparison of the results obtained for the course of the process of anaerobic stabilization in raw excess sludge (mixture A). Process of Methane Fermentation of Thermally Disintegrated Excess Sludge The process of 8-day anaerobic stabilization was employed for the mixture of excess sludge (90%) thermally conditioned at the temperature of 50°C for 6 h and fermented sludge (mixture B) and the mixture of the sludge thermally conditioned at the temperature of 70°C for 4.5 h and fermented sludge (10%) (mixture C). Table 3. Selected physical and chemical parameters of the mixture A (raw excess sludge + fermented sludge) subjected to the process of 8-day anaerobic stabilization Time of methane fermentation VFAs mgCH3COOH/dm3 111.43 445.71 548.57 771.43 651.43 514.29 437.14 394.29 377.14 454 606 737 824 1180 1250 1320 1240 937 1100 1123 1100 948 880 80 140 136 124 100 128 180 688 620 60 288 500 20 mgO2/dm3 mgCaCO3/dm3 mg/dm3 COD Alkalinity Acidity Ammonium nitrogen g/dm3 g/dm3 g/dm3 mgN-NH4+/dm3 56.00 112.00 165.20 218.40 221.20 218.40 215.60 224.00 280.00 The degree of fermentation of the sludge for mixtures B and C amounted to 30.67% and 33.63%, respectively. In the case of mixture B, the content of organic dry matter before the process was 6.37 g/dm3, whereas on the last day of the process this value was 4.41 g/dm3. The content of dry matter before the process of fermentation amounted to 9.30 g/dm3, whereas after the process it reached 5.43 g/dm3. Reduction in dry matter of the sludge amounted to 41.62%. For mixture C, the content of organic dry matter before the process amounted to 7.24 g/dm3, whereas on the last day of the process this value was 5.37 g/dm3. The content of dry matter before the process of fermentation amounted to 10.93 g/dm3, whereas after the process it was 8.55 g/dm3. Reduction in dry matter of the sludge was 21.78%. In the case of mixtures B and C, the level of volatile fatty acids before the process of anaerobic stabilization amounted to 188.57 mg CH3COOH/dm3 and 342.86 mg CH3COOH/dm3, respectively. Maximal values for both mixture B and mixture C was observed on the 2nd day of the process and they reached 874.29 mg CH3COOH/dm3 and 1131.43 mg CH3COOH/dm3, respectively. A decline in the VFA level was observed on the third day of the process and, on the 8th day, this value was 171.43 mg CH3COOH/dm3 in mixture B and 377.14 mg CH3COOH/dm3 in mixture C. In the case of mixture B, the value of COD in sludge water before the process was 640 mg O2/dm3; its highest level (1344 mg O2/dm3) was found on the 2nd day and a decline in COD level in the sludge liquor was observed from the third day and it continued until it was 280 mg O2/dm3 on the last day. In mixture C, value of COD before the process of stabilization amounted to 1826 mg O2/dm3 and it was the highest level reached during the study. A decline in the value of COD in sludge liquor was observed from the first day and dropped to 800 mg O2/dm3 on the last day of the process. Tables 4 and 5 present comparison of the results obtained for the process of anaerobic stabilization of thermally disintegrated excess sludge (mixtures B and C). The results concerning the course of the process of anaerobic stabilization of thermally modified excess sludge confirm the effectiveness of this method as a disintegrating factor, which was also confirmed by the studied by other researchers [10, 11, 12]. CONCLUSIONS The aim of the present study was to identify the effect of thermal method of disintegration of excess sludge on its biodegradability and to assess the effectiveness of modification of the sludge in the aspect of increase in the value of VFAs, as well as in the value of COD and in the degree of disintegration of the modified sludge. Increase in the susceptibility of excess sludge on biochemical decomposition under anaerobic conditions occurs through the destruction of cell walls in microorganisms and the release of the organic substances they contain before they are used by the bacteria which exist under anaerobic conditions during the process of anaerobic stabilization. The obtained results allow for drawing the following conclusions: 1. In the case of thermal disintegration of excess sludge, the most favourable temperature was 70°C with the heating time 4.5 h, for which the highest level of COD among all the temperatures and times of thermal treatment was obtained (2398 mg O2/dm3). 2. As a result of subjecting excess sludge (mixture A) to the process of anaerobic stabilization of raw excess sludge, the degree of fermentation of 16.88% was obtained Table 4. Selected physical and chemical parameters of the mixture B (excess sludge, thermally disintegrated at the temperature of 50oC for 6 h + fermented sludge) subjected to the process of 8-day anaerobic stabilization Time of methane fermentation VFAs mgCH3COOH/dm3 188.57 788.57 874.29 840.00 617.14 522.86 360.00 188.57 171.43 280 621 576 892 1150 1120 1170 1240 1380 1440 1320 1060 1344 956 1179 860 80 100 110 70 70 72 80 66 640 450 4 mgO2/dm3 mgCaCO3/dm3 mg/dm3 COD Alkalinity Acidity Ammonium nitrogen g/dm3 g/dm3 g/dm3 mgN-NH4+/dm3 22.4 140 142.8 226.8 170.8 224 249.2 254.8 252 Table 5. Selected physical and chemical parameters of the mixture C (excess sludge, thermally disintegrated at the temperature of 70°C for 4.5 h + fermented sludge) subjected to the process of 8-day anaerobic stabilization Time of methane fermentation VFAs mgCH3COOH/dm3 342.86 891.43 1131.43 1062.86 891.43 737.14 411.43 685.71 377.14 800 882 578 997 1160 1300 1380 1420 1325 1020 1616 1000 1778 900 1680 840 60 154 180 200 200 180 220 240 1826 550 40 mgO2/dm3 mgCaCO3/dm3 mg/dm3 COD Alkalinity Acidity Ammonium nitrogen g/dm3 g/dm3 g/dm3 mgN-NH4+/dm3 168 196 238 260.4 282.8 280 280 308 322 after 8 days of fermentation. The maximal values of volatile fatty acids and COD (771.43 mg CH3COOH/dm3 and 1123 mg O2/dm3, respectively) were observed in the mixture on the 3rd day of the process. 3. In the case of the process of anaerobic stabilization of thermally disintegrated excess sludge, using the temperature of 50°C and heating time of 6h (Mixture B), and 70°C and heating time 4.5 h (mixture C), the following degrees of fermentation were obtained: 30.67% and 33.63%, respectively. Maximal levels of volatile fatty acids in mixtures B and C (874.29 and 1131.43 mg CH3COOH/dm3, respectively) were found on the 2nd day of the process. The obtained maximal level of VFA correlated with the maximal value of COD index found on the same (second) day of the process, which was 1344 mg O2/dm3 for mixture B and 1778 mg O2/dm3 for mixture C. Acknowledgments Scientific work founded by grant BG 401/402/10 resource sponsored by the State Committee for Scientific Research (KBN) in the year 2010-2012.

Journal

Archives of Environmental Protection – de Gruyter

Published: Jan 1, 2012

References

  • Efektywność fermentacji metanowej osadów
    J. Bień
  • Możliwości zwiększenia efektywności przeróbki i ostatecznego zagospodarowania osadów ściekowych
    J. Bień
  • Intensyfikacja produkcji biogazu w procesie stabilizacji beztlenowej osadów przemysłowych
    J. Bień
  • Wpływ ultradźwięków na strukturę osadów ściekowych w procesie ich kondycjonowania
    J. Bień
  • Energy-efficient sludge sonification: Power and sludge characteristics
    G. Zhang