- Email: [email protected]
Extended Dewatering of Sewage Sludge in Solar Drying Plants
Desalination 248 (2009) 733–739
Extended Dewatering of Sewage Sludge in Solar Drying Plants V.L. Mathioudakis*, A.G. Kapagiannidis, E. Athanasoulia, V.I. Diamantis, P. Melidis, A. Aivasidis Laboratory of Wastewater Management & Treatment Technologies, Department of Environmental Engineering, Democritus University of Thrace, Vas. Sofias 12, 67100, Xanthi, Greece Tel/Fax: +302541079376; email: [email protected], [email protected] Received 19 October 2008; accepted 9 January 2009
Abstract A solar drying process was implemented for extended dewatering, volume reduction and partial pathogen control of sewage sludge. The average sludge moisture content decreased from 85% to 6% within 7–12 d during summer, and to 10% within 9–33 days in autumn, resulting to a total volume reduction of 80–85%. Total and fecal coliforms were reduced during summer conditions by two orders of magnitude, resulting to a dry product with 2104 CFU/g DS and 103 CFU/g DS, respectively. Incorporating a solar water heater, with water recirculation through the bottom of the plant, the drying process was accelerated by 1–9 d during winter conditions. Solar sludge drying was proved to be efficient for regions which receive high annual solar radiation such as Greece. Keywords: Activated sludge; Dewatering; Sewage sludge; Solar collector; Solar drying; Solar water heater; Greenhouse
1. Introduction Aerobic wastewater treatment processes, besides the treated effluent, produce a semisolid, nutrient rich by-product called sewage sludge. Several aspects, like odors and pathogens, appoint sludge an undesirable material from an environmental and hygienic point of *Corresponding author. Presented at the Conference on Protection and Restoration of the Environment IX, Kefalonia Greece, June 30–July 3, 2008
view. Furthermore, sludge is undesirable in economic terms, as the management costs approximate 40–50% of the overall costs of a wastewater treatment plant, even though the quantity produced is about 1% of the quantity of treated wastewater [1]. This cost originates from the sophisticated equipment and the relative long treatment period required. In general, sludge management should not only meet the requirements of the imposed regulations, for the protection of the environment and public health, but also be cost effective.
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi: 10.1016/j.desal.2009.01.011
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V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
Since the quantity of sludge and its hygienic characteristics, are closely related to the overall management costs and the disposal or reuse options respectively, the development of novel treatment processes for volume reduction and pathogen control is always on the scene. Conventional aerobic or anaerobic digestion has been widely used for waste stabilization. Aerobic digestion is usually applied in small sewage plants, due to relatively low investment costs. However, it is characterized by high operational expenses, mainly due to the consumption of electrical energy (aeration). On the other hand, anaerobic digestion entails higher capital expenses, thus it is applicable in larger treatment plants. The major advantage of anaerobic digestion processes is the recovery of energy, in form of methane, and its low energy requirements, which approximates 5–10% of the respective aerobic process [2]. Both aerobic and anaerobic digestion focuses on the reduction of the organic content and the control of microbial activity. An alternative technology which requires low capital investment is the alkaline stabilization. The latter provides inhibition of the microbial processes and decrease of the pathogen content through controlled increase of the pH [3]. However, alkaline stabilization may lead to sludge volume increase and the generation of odors, while the removal of organic matter is limited. Regarding sludge volume reduction and dewatering, mechanical processes like centrifugation, belt and filter pressing are usually employed [4]. These processes lead to a dewatered sludge with a moisture content of no less than 75–80%. The remaining moisture can only be removed by thermal processes (extended dewatering), resulting in a dry sludge-product, however the capital and operational expenses may increase considerably [5,6]. Alternatively, extended dewatering of sewage sludge is possible in solar plants, which provide higher evaporation rates com-
pared to conventional drying beds/ponds, thus achieving significant sludge mass and volume reduction in shorter time periods. Solar sludge drying is also characterized by rather low capital investment and operational economic demands [7]. Moreover, the reduction of sludge mass through extended dewatering is closely related to a decrease of handling, transport and disposal costs, while the final product is suitable for energy production (combustion) or appropriate reuse. Solar sludge drying has acquired significant interest over the last years, and both pilot- and full-scale facilities have been reported [7–10]. However, data concerning process performance under typical Mediterranean conditions, which are characterized by high annual solar radiation and temperature, are not extensively documented. The current study aims to provide experimental data about the efficiency of a solar drying process for sewage sludge management in the region of northern Greece. 2. Materials and methods 2.1. Sewage sludge characteristics Secondary sewage sludge from the wastewater treatment plant of Komotini (Greece) was used for the study. Several batches of sludge were sampled after thickening and dewatering (belt filter pressing). Physical, chemical and biological characteristics of the dewatered sludge are presented in Table 1. The total and fecal coliform content varied considerably, between 2105– 6106 and 4103–8105 CFU/g DS, respectively. 2.2. Experimental set-up The dewatered sludge was dried using two pilot-scale solar drying plants of approximate volume 2.5m3 each, made of polycarbonate (Fig. 1). The first plant was equipped with a gravel floor, where hot water, generated by a commercial solar water heater, was circulated.
V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
735
Table 1 Characteristics of the dewatered secondary sludge used in the experimental campaign Parameter
Mean value
STDEV
# samples
Moisture content (%) Volatile solids (% DS) Total coliform (CFU/gDS) Fecal coliform (CFU/gDS) Cu (mg/Kg DS) Mn (mg/Kg DS) Cr (mg/Kg DS) Ni (mg/Kg DS) Zn(mg/Kg DS) Fe (mg/Kg DS) Cd (mg/Kg DS) Pb (mg/Kg DS)
85,0
0,8
10
72,9
2,4
10
4106
3106
5
3105
3105
5
154,4 144,4 26,7 21,0 616,3 5128,6 1,5 43,3
70,2 37,5 9,2 5,0 309,9 182,5 0,8 16,6
5 5 5 5 5 5 5 5
DS: Dry Solids. CFU: Colony Forming Units.
By this technique, further utilization of the solar energy was possible, additionally to the energy recovered by the greenhouse effect. Hot water circulation was controlled by a thermostat, and it was performed only when the water temperature was higher than the plant indoor temperature. A commercial fan was installed inside both plants in order to provide a turbulent air stream able to remove the moisture from the surface layer of the sludge. Additionally, two axial fans were constantly replacing the saturated indoor air with fresh ambient air. Approximately 8 Kg of dewatered sludge were placed into crates of ~ 10L volume, 0,1m2 surface, giving a sludge depth equal to 20–25cm. The sludge in each crate was mixed manually once per day. Every day, one crate was placed into each plant. The weight of each crate was determined periodically (3 times per week) and as soon as it was found to be constant for two consecutive
Fig. 1. Solar drying plants (left) and plant indoor view (right).
measurements, it was considered that the drying process was completed. The crate with the dried sludge was then replaced by a new one with fresh dewatered material. 2.3. Analytical procedures The experimental campaign included the monitoring of the sludge drying process during summer and autumn period, in order to obtain data about process efficiency throughout different weather conditions. For this purpose, meteorological data, such as air temperature, solar radiation, relative humidity, rainfall height, wind speed and direction, were collected using an onsite meteorological station. Plant indoor air temperature and relative humidity were also recorded daily. The moisture content of the sludge was determined by weight loss and the surface drying rate was calculated accordingly. Moreover, sludge Dry Solids (DS) content, volatile (organic) fraction of DS, heavy metals concentration, total and fecal coliform content were determined on representative samples from the dewatered sludge and the dry material. All parameters were measured according to APHA [11]. 3. Results and discussion 3.1. Experimental campaign during summer period During summer conditions, the operation of the plants was performed between 21/6/2007
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Fig. 3. Dewatered sludge and the corresponding sludge volume after drying.
Fig. 2. Daily average, maximum and minimum outdoor air temperature, relative humidity and daily average and maximum solar energy per surface unit.
and 20/7/2007. Daily meteorological parameters of interest are given in Fig. 2. The indoor temperature in both plants, ranged due to the artificial greenhouse effect between 35–60 oC, significantly higher compared to the ambient temperature. The relative humidity values were
equal inside and outside of the plants, suggesting that the ventilation system was operating efficiently. In general, there were no significant differences in the average temperature and relative humidity inside both pilot-plants, despite the circulation of hot water in the first case. The drying process reduced sludge weight up to 86% leading to major sludge volume reduction (Fig. 3). The moisture content decreased from 85% to only 6% and the period required was between 7–12 days. This period was significantly lower compared to 64–83 days reported in Northern Europe [8]. The characteristics of the final dry-product are presented in Table 2. A decrease by two orders of magnitude, for total and fecal coliforms was observed. This partial sludge disinfection may be attributed to the exposure of
Table 2 Dried sludge characteristics during the summer experimental campaign
Moisture Content (%) Volatile solids (% dry solids) Total coliform (CFU/gDS) Fecal coliform (CFU/gDS)
Dewatered sludge
Dry sludge
Mean value
Mean value
STDEV
# samples
85,0 72,9 4106 3105
6,3 71,8 2104 103
1,2 2,7 6103 0
10 10 8 8
V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
737
Fig. 4. Typical profile of surface dewatering rate as a function of the moisture content. The symbols in the graph illustrate different batches of dewatered sludge used in the continuous process.
microbes to solar UV radiation and to partial pasteurization performed at elevated indoor temperatures. While the fecal coliform content in the dried sludge was almost constant, the total coliform content ranged between 103–2104 CFU/gDS. In Fig. 4, the dewatering rate per surface area of the plant, as a function of the water content is given. From the data presented it is evident that the surface dewatering rate display a typical saturation curve, i.e. there is a decline of the dewatering rate with deceasing sludge moisture content. The initial maximum dewatering rate was between 12–16 Kg H2O/m2d. 3.2. Experimental campaign during autumn period During the autumn campaign, the plants were monitored between 10/9/2007 and 4/12/2007. The daily meteorological parameters are given in Fig. 5. Until 4/10/2007, the solar water heater was not in operation, due to technical reasons. During this period the time required for complete sludge drying was similar in both plants (between 10 and 16 days), slightly longer compared to the summer conditions. However,
Fig. 5. Daily average, maximum and minimum outdoor air temperature, relative humidity and daily average and maximum solar energy per surface unit, for the autumn period.
decrease of the ambient temperature (even to 2oC) at the end of the autumn period (November–December) resulted in a considerable increase of the time required for sludge drying (13–32 days). Re-operation of the solar heater in the first plant significantly accelerated the drying process, which was completed from 1 up to 9 days earlier, compared to the second plant. The maximum sludge dewatering rate in this case, was determined equal to 6 and 4 Kg H2O/m2d with and without the use of the solar collector, respectively, following the same kinetic profile as in the summer period. It was thus concluded that the variations in weather conditions should be taken seriously into consideration when designing a full-scale installation.
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Table 3 Dried sludge characteristics for the autumn experimental campaign
Moisture Content (%) Volatile solids (% dry solids) Total coliform (CFU/gDS) Fecal coliform (CFU/gDS)
Dewatered sludge
Dry sludge
Mean value
Mean value
STDEV
# samples
85,0 72,9 4106 3105
10,2 69,5 2106 8105
3,0 1,8 3106 106
11 11 12 12
During the autumn season, the average moisture content of the final product (dry sludge) was approximately 10% (Table 3), slightly higher compared to the summer conditions. The total and fecal coliform contents were equal to 3.103–5.106 and 2.103–4.106 CFU/gDS respectively. The pathogen content was not affected during prolonged periods of low solar radiation, low temperature and high relative humidity. The organic content of the sludge was not reduced by solar drying, which was also the case during the summer campaign. The heavy metal concentrations of the final sludge during the whole study period are given in Table 4. As it was expected, the heavy metals in the dry product were not significantly different compared to the dewatered sludge. Obviously, no heavy metal removal mechanisms are active during the drying process.
3.3. Characterization of dried sludge product in terms of beneficial reuse According to the U.S. EPA promulgated in 1993 at 40 Code of Federal Regulation Part 503, the requirements for beneficial use of treated sludge include three (3) parameters: pollutants (metals) limits, pathogen reduction and vector attraction [12]. Metal concentration and vector attraction requirements were met by the dried sludge product, since the heavy metal content was below the limits and the sludge was dried to more than 75% DS. The fecal coliform content however, was higher than the requirements for Class A Biosolids, rendering the sludge as a Class B Biosolid (reuse with restrictions). Concluding, a single solar drying process was not capable to comply with pathogen control, thus either aerobic or anaerobic digestion
Table 4 Heavy metals content in dried sludge Heavy metal content (mg/Kg dry solids)
Dewatered sludge
Dry sludge
Mean value
Mean value
STDEV
# samples
Cu Mn Cr Ni Zn Fe Cd Pb
154,4 144,4 26,7 21,0 616,3 5128,6 1,5 43,3
175,0 193,9 26,3 25,2 581,7 5855,0 1,8 34,9
74,6 91,2 9,6 11,0 278,8 1036,5 1,1 3,4
7 7 7 7 7 7 7 7
V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
processes, prior to the solar drying, should be considered. Potentially, limited alkaline stabilization as reported in the work of Salihoglu et al. [9] could be implemented in order to produce a material that meet Class A requirements. However, according to the Greek legislation, which is in agreement with the EU guidelines for sludge reuse in agriculture, dried sludge can be used without restrictions taking into consideration only the heavy metal content [13].
4. Conclusions The proposed solar drying process was efficient for extended dewatering and volume reduction of sewage sludge. The moisture content of the dried sludge decreased below 10% and the corresponding volume reduction was up to 85%. During summer conditions, drying was completed within 7–12 days, but in autumn conditions the duration increased up to 32 days. The organic content of the sludge was slightly affected by solar drying in general, however, the total and fecal coliform content were reduced by two orders of magnitude during the summer period. Incorporation of a solar water heater was beneficial to decreasing the required drying period, when ambient temperature was low.
739
References [1] I.S. Turonskiy and P.K. Mathai, (Eds.), Wastewater sludge processing, Wiley Interscience, New Jersey, 2006. [2] A.M. Bruce and J.W. Fisher, (Eds.), Sewage sludge stabilization and Disinfection, Ellis Horwood Limited, Chichester, 1984. [3] G. Tchobanoglous, F.L. Burton and H.D. Stensel, (Eds.), Wastewater engineering: Treatment and reuse, McGraw-Hill, New York, 2003. [4] G. Chen, P.L. Yue and S. Mujumdar., Dry. Technol., 20 (2002) 883–916. [5] B. Johnke and B. Wiebusch., Fuel and Energy Abstracts, 38 (1999) 350. [6] T. Kasakura and M. Hasatani, Dry. Technol., 14 (1996) 1389–1401. [7] U. Luboschik. Renew. Energy, 16 (1999) 785–88. [8] M. Bux, R. Baumann, S. Quadt, J. Pinnekamp and W. Muhlbauer, Dry. Technol., 20 (2002) 829–837. [9] N.K. Salihoglu, V. Pinarli and G. Salihoglu, Renew. Energy, 32 (2007) 1661–1675. [10] I. Seginer and M. Bux, Dry. Technol., 24 (2006) 1353–1363. [11] APHA, AWWA, WPCF, Standard methods for the examination of water and wastewater, Port City Press, 1998. [12] USEPA. Standards for use and disposal of sewage sludge - 40 Code of Federal Regulation, Part 503, 1993. [13] Council of European Communities. Directive on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture (86/278/EEC), 1986.
Extended Dewatering of Sewage Sludge in Solar Drying Plants V.L. Mathioudakis*, A.G. Kapagiannidis, E. Athanasoulia, V.I. Diamantis, P. Melidis, A. Aivasidis Laboratory of Wastewater Management & Treatment Technologies, Department of Environmental Engineering, Democritus University of Thrace, Vas. Sofias 12, 67100, Xanthi, Greece Tel/Fax: +302541079376; email: [email protected], [email protected] Received 19 October 2008; accepted 9 January 2009
Abstract A solar drying process was implemented for extended dewatering, volume reduction and partial pathogen control of sewage sludge. The average sludge moisture content decreased from 85% to 6% within 7–12 d during summer, and to 10% within 9–33 days in autumn, resulting to a total volume reduction of 80–85%. Total and fecal coliforms were reduced during summer conditions by two orders of magnitude, resulting to a dry product with 2104 CFU/g DS and 103 CFU/g DS, respectively. Incorporating a solar water heater, with water recirculation through the bottom of the plant, the drying process was accelerated by 1–9 d during winter conditions. Solar sludge drying was proved to be efficient for regions which receive high annual solar radiation such as Greece. Keywords: Activated sludge; Dewatering; Sewage sludge; Solar collector; Solar drying; Solar water heater; Greenhouse
1. Introduction Aerobic wastewater treatment processes, besides the treated effluent, produce a semisolid, nutrient rich by-product called sewage sludge. Several aspects, like odors and pathogens, appoint sludge an undesirable material from an environmental and hygienic point of *Corresponding author. Presented at the Conference on Protection and Restoration of the Environment IX, Kefalonia Greece, June 30–July 3, 2008
view. Furthermore, sludge is undesirable in economic terms, as the management costs approximate 40–50% of the overall costs of a wastewater treatment plant, even though the quantity produced is about 1% of the quantity of treated wastewater [1]. This cost originates from the sophisticated equipment and the relative long treatment period required. In general, sludge management should not only meet the requirements of the imposed regulations, for the protection of the environment and public health, but also be cost effective.
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi: 10.1016/j.desal.2009.01.011
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V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
Since the quantity of sludge and its hygienic characteristics, are closely related to the overall management costs and the disposal or reuse options respectively, the development of novel treatment processes for volume reduction and pathogen control is always on the scene. Conventional aerobic or anaerobic digestion has been widely used for waste stabilization. Aerobic digestion is usually applied in small sewage plants, due to relatively low investment costs. However, it is characterized by high operational expenses, mainly due to the consumption of electrical energy (aeration). On the other hand, anaerobic digestion entails higher capital expenses, thus it is applicable in larger treatment plants. The major advantage of anaerobic digestion processes is the recovery of energy, in form of methane, and its low energy requirements, which approximates 5–10% of the respective aerobic process [2]. Both aerobic and anaerobic digestion focuses on the reduction of the organic content and the control of microbial activity. An alternative technology which requires low capital investment is the alkaline stabilization. The latter provides inhibition of the microbial processes and decrease of the pathogen content through controlled increase of the pH [3]. However, alkaline stabilization may lead to sludge volume increase and the generation of odors, while the removal of organic matter is limited. Regarding sludge volume reduction and dewatering, mechanical processes like centrifugation, belt and filter pressing are usually employed [4]. These processes lead to a dewatered sludge with a moisture content of no less than 75–80%. The remaining moisture can only be removed by thermal processes (extended dewatering), resulting in a dry sludge-product, however the capital and operational expenses may increase considerably [5,6]. Alternatively, extended dewatering of sewage sludge is possible in solar plants, which provide higher evaporation rates com-
pared to conventional drying beds/ponds, thus achieving significant sludge mass and volume reduction in shorter time periods. Solar sludge drying is also characterized by rather low capital investment and operational economic demands [7]. Moreover, the reduction of sludge mass through extended dewatering is closely related to a decrease of handling, transport and disposal costs, while the final product is suitable for energy production (combustion) or appropriate reuse. Solar sludge drying has acquired significant interest over the last years, and both pilot- and full-scale facilities have been reported [7–10]. However, data concerning process performance under typical Mediterranean conditions, which are characterized by high annual solar radiation and temperature, are not extensively documented. The current study aims to provide experimental data about the efficiency of a solar drying process for sewage sludge management in the region of northern Greece. 2. Materials and methods 2.1. Sewage sludge characteristics Secondary sewage sludge from the wastewater treatment plant of Komotini (Greece) was used for the study. Several batches of sludge were sampled after thickening and dewatering (belt filter pressing). Physical, chemical and biological characteristics of the dewatered sludge are presented in Table 1. The total and fecal coliform content varied considerably, between 2105– 6106 and 4103–8105 CFU/g DS, respectively. 2.2. Experimental set-up The dewatered sludge was dried using two pilot-scale solar drying plants of approximate volume 2.5m3 each, made of polycarbonate (Fig. 1). The first plant was equipped with a gravel floor, where hot water, generated by a commercial solar water heater, was circulated.
V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
735
Table 1 Characteristics of the dewatered secondary sludge used in the experimental campaign Parameter
Mean value
STDEV
# samples
Moisture content (%) Volatile solids (% DS) Total coliform (CFU/gDS) Fecal coliform (CFU/gDS) Cu (mg/Kg DS) Mn (mg/Kg DS) Cr (mg/Kg DS) Ni (mg/Kg DS) Zn(mg/Kg DS) Fe (mg/Kg DS) Cd (mg/Kg DS) Pb (mg/Kg DS)
85,0
0,8
10
72,9
2,4
10
4106
3106
5
3105
3105
5
154,4 144,4 26,7 21,0 616,3 5128,6 1,5 43,3
70,2 37,5 9,2 5,0 309,9 182,5 0,8 16,6
5 5 5 5 5 5 5 5
DS: Dry Solids. CFU: Colony Forming Units.
By this technique, further utilization of the solar energy was possible, additionally to the energy recovered by the greenhouse effect. Hot water circulation was controlled by a thermostat, and it was performed only when the water temperature was higher than the plant indoor temperature. A commercial fan was installed inside both plants in order to provide a turbulent air stream able to remove the moisture from the surface layer of the sludge. Additionally, two axial fans were constantly replacing the saturated indoor air with fresh ambient air. Approximately 8 Kg of dewatered sludge were placed into crates of ~ 10L volume, 0,1m2 surface, giving a sludge depth equal to 20–25cm. The sludge in each crate was mixed manually once per day. Every day, one crate was placed into each plant. The weight of each crate was determined periodically (3 times per week) and as soon as it was found to be constant for two consecutive
Fig. 1. Solar drying plants (left) and plant indoor view (right).
measurements, it was considered that the drying process was completed. The crate with the dried sludge was then replaced by a new one with fresh dewatered material. 2.3. Analytical procedures The experimental campaign included the monitoring of the sludge drying process during summer and autumn period, in order to obtain data about process efficiency throughout different weather conditions. For this purpose, meteorological data, such as air temperature, solar radiation, relative humidity, rainfall height, wind speed and direction, were collected using an onsite meteorological station. Plant indoor air temperature and relative humidity were also recorded daily. The moisture content of the sludge was determined by weight loss and the surface drying rate was calculated accordingly. Moreover, sludge Dry Solids (DS) content, volatile (organic) fraction of DS, heavy metals concentration, total and fecal coliform content were determined on representative samples from the dewatered sludge and the dry material. All parameters were measured according to APHA [11]. 3. Results and discussion 3.1. Experimental campaign during summer period During summer conditions, the operation of the plants was performed between 21/6/2007
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Fig. 3. Dewatered sludge and the corresponding sludge volume after drying.
Fig. 2. Daily average, maximum and minimum outdoor air temperature, relative humidity and daily average and maximum solar energy per surface unit.
and 20/7/2007. Daily meteorological parameters of interest are given in Fig. 2. The indoor temperature in both plants, ranged due to the artificial greenhouse effect between 35–60 oC, significantly higher compared to the ambient temperature. The relative humidity values were
equal inside and outside of the plants, suggesting that the ventilation system was operating efficiently. In general, there were no significant differences in the average temperature and relative humidity inside both pilot-plants, despite the circulation of hot water in the first case. The drying process reduced sludge weight up to 86% leading to major sludge volume reduction (Fig. 3). The moisture content decreased from 85% to only 6% and the period required was between 7–12 days. This period was significantly lower compared to 64–83 days reported in Northern Europe [8]. The characteristics of the final dry-product are presented in Table 2. A decrease by two orders of magnitude, for total and fecal coliforms was observed. This partial sludge disinfection may be attributed to the exposure of
Table 2 Dried sludge characteristics during the summer experimental campaign
Moisture Content (%) Volatile solids (% dry solids) Total coliform (CFU/gDS) Fecal coliform (CFU/gDS)
Dewatered sludge
Dry sludge
Mean value
Mean value
STDEV
# samples
85,0 72,9 4106 3105
6,3 71,8 2104 103
1,2 2,7 6103 0
10 10 8 8
V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
737
Fig. 4. Typical profile of surface dewatering rate as a function of the moisture content. The symbols in the graph illustrate different batches of dewatered sludge used in the continuous process.
microbes to solar UV radiation and to partial pasteurization performed at elevated indoor temperatures. While the fecal coliform content in the dried sludge was almost constant, the total coliform content ranged between 103–2104 CFU/gDS. In Fig. 4, the dewatering rate per surface area of the plant, as a function of the water content is given. From the data presented it is evident that the surface dewatering rate display a typical saturation curve, i.e. there is a decline of the dewatering rate with deceasing sludge moisture content. The initial maximum dewatering rate was between 12–16 Kg H2O/m2d. 3.2. Experimental campaign during autumn period During the autumn campaign, the plants were monitored between 10/9/2007 and 4/12/2007. The daily meteorological parameters are given in Fig. 5. Until 4/10/2007, the solar water heater was not in operation, due to technical reasons. During this period the time required for complete sludge drying was similar in both plants (between 10 and 16 days), slightly longer compared to the summer conditions. However,
Fig. 5. Daily average, maximum and minimum outdoor air temperature, relative humidity and daily average and maximum solar energy per surface unit, for the autumn period.
decrease of the ambient temperature (even to 2oC) at the end of the autumn period (November–December) resulted in a considerable increase of the time required for sludge drying (13–32 days). Re-operation of the solar heater in the first plant significantly accelerated the drying process, which was completed from 1 up to 9 days earlier, compared to the second plant. The maximum sludge dewatering rate in this case, was determined equal to 6 and 4 Kg H2O/m2d with and without the use of the solar collector, respectively, following the same kinetic profile as in the summer period. It was thus concluded that the variations in weather conditions should be taken seriously into consideration when designing a full-scale installation.
738
V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
Table 3 Dried sludge characteristics for the autumn experimental campaign
Moisture Content (%) Volatile solids (% dry solids) Total coliform (CFU/gDS) Fecal coliform (CFU/gDS)
Dewatered sludge
Dry sludge
Mean value
Mean value
STDEV
# samples
85,0 72,9 4106 3105
10,2 69,5 2106 8105
3,0 1,8 3106 106
11 11 12 12
During the autumn season, the average moisture content of the final product (dry sludge) was approximately 10% (Table 3), slightly higher compared to the summer conditions. The total and fecal coliform contents were equal to 3.103–5.106 and 2.103–4.106 CFU/gDS respectively. The pathogen content was not affected during prolonged periods of low solar radiation, low temperature and high relative humidity. The organic content of the sludge was not reduced by solar drying, which was also the case during the summer campaign. The heavy metal concentrations of the final sludge during the whole study period are given in Table 4. As it was expected, the heavy metals in the dry product were not significantly different compared to the dewatered sludge. Obviously, no heavy metal removal mechanisms are active during the drying process.
3.3. Characterization of dried sludge product in terms of beneficial reuse According to the U.S. EPA promulgated in 1993 at 40 Code of Federal Regulation Part 503, the requirements for beneficial use of treated sludge include three (3) parameters: pollutants (metals) limits, pathogen reduction and vector attraction [12]. Metal concentration and vector attraction requirements were met by the dried sludge product, since the heavy metal content was below the limits and the sludge was dried to more than 75% DS. The fecal coliform content however, was higher than the requirements for Class A Biosolids, rendering the sludge as a Class B Biosolid (reuse with restrictions). Concluding, a single solar drying process was not capable to comply with pathogen control, thus either aerobic or anaerobic digestion
Table 4 Heavy metals content in dried sludge Heavy metal content (mg/Kg dry solids)
Dewatered sludge
Dry sludge
Mean value
Mean value
STDEV
# samples
Cu Mn Cr Ni Zn Fe Cd Pb
154,4 144,4 26,7 21,0 616,3 5128,6 1,5 43,3
175,0 193,9 26,3 25,2 581,7 5855,0 1,8 34,9
74,6 91,2 9,6 11,0 278,8 1036,5 1,1 3,4
7 7 7 7 7 7 7 7
V.L. Mathioudakis et al. / Desalination 248 (2009) 733–739
processes, prior to the solar drying, should be considered. Potentially, limited alkaline stabilization as reported in the work of Salihoglu et al. [9] could be implemented in order to produce a material that meet Class A requirements. However, according to the Greek legislation, which is in agreement with the EU guidelines for sludge reuse in agriculture, dried sludge can be used without restrictions taking into consideration only the heavy metal content [13].
4. Conclusions The proposed solar drying process was efficient for extended dewatering and volume reduction of sewage sludge. The moisture content of the dried sludge decreased below 10% and the corresponding volume reduction was up to 85%. During summer conditions, drying was completed within 7–12 days, but in autumn conditions the duration increased up to 32 days. The organic content of the sludge was slightly affected by solar drying in general, however, the total and fecal coliform content were reduced by two orders of magnitude during the summer period. Incorporation of a solar water heater was beneficial to decreasing the required drying period, when ambient temperature was low.
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