783
© IWA Publishing 2014 Water Science & Technology
|
69.4
|
2014
Environmental assessment of different dewatering and drying methods on the basis of life cycle assessment J. Stefaniak, A. Z˙ elazna and A. Pawłowski
ABSTRACT Sewage sludge is an inevitable product of wastewater treatment in municipal wastewater plants and its amount has increased dramatically due to the growing number of sewage systems users. This sludge needs to be adequately treated in order to decrease its hazardous properties and any negative influence on the environment. In this paper, gate to gate analysis, on the basis of life cycle assessment (LCA), was carried out in order to compare the environmental impact of alternative ways of sludge processing employing a dewatering press and three different kinds of dryers – belt dryer,
J. Stefaniak (corresponding author) A. Z˙ elazna A. Pawłowski Faculty of Environmental Engineering, Lublin University of Technology, Nadbystrzycka 40B 20-618, Lublin, Poland E-mail: [email protected]
container dryer and batch dryer. SimaPro 7.2 software and Ecoinvent 2.2 database were used to estimate the carbon footprint and energy balance of these processes. The main energy consumption in the scenarios analyzed is caused by the drying process. The solution based on application of the batch dryer allows a saving of 39.6% of energy compared with the most energy-consuming solution using a belt dryer. Sludge processing using belt and container dryers cause greater environmental burdens. Key words
| LCA, life cycle assessment, sewage dewatering, sewage drying, sewage sludge
INTRODUCTION Sewage sludge management is a current and ongoing problematic issue. This situation is connected with many technical and economic factors, since sewage sludge is a complex environment that requires complex treatment (Bien´ ). The sewage sludge had a total solids concentration of about 1–3%, depending on the treatment process, and its production represents 1–2% of the treated wastewater volume. Nevertheless, its management accounts for 20– 60% of the total operating costs of wastewater treatment plants (Cleverson et al. ; Uggetti et al. ). Due to the fast growth in world population and industrialization, the amount of wastewater and sludge has increased dramatically all over the world (Hong et al. ). In Poland this growth is caused mainly by improvements in living conditions and the increasing percentage of households connected to central treatment plants. Over the last 10 years, the total sewage sludge generated annually has increased by almost 200 thousand tons of dry sludge – Table 1. Moreover, according to some predictions the amount of sewage sludge will continue to increase and it could reach 706.6 thousand tons of dry solids in 2018 (Werle & Wilk ). Complex and expensive sludge processing as well as doi: 10.2166/wst.2013.778
increasing sludge production make sludge management a matter of concern in many countries. Sludge treatment begins with the separation of wastewater solids from liquid whereby the required level of wastewater purification can be obtained. In recent years, membrane based techniques have gained considerable attention in cases of wastewater and drinking water (Nataraj et al. b; Harisha et al. ). In the context of wastewater treatment, these techniques are being applied to a wider range of industrial applications and are also used in municipal wastewater treatment plants according to reports in several papers (Visvanathan et al. ; Nataraj et al. a, Nataraj et al., a, b; Ahmed et al. ). Employment of membrane based techniques for solids separation (e.g. microfiltration or ultrafiltration), can overcome the disadvantages of conventional techniques based on sedimentation tank and biological treatment methods. The main advantages of these techniques are: high quality of treated streams, flexibility and simplicity in operation, small footprint size of the treatment plant and a significantly reduced amount of sludge. When the separation stage is finished, sewage sludge must be subjected to another process so it can reach the end-of-life phase.
J. Stefaniak et al.
784
Table 1
|
|
Environmental assessment of different dewatering methods based on LCA
Sewage sludge from Polish municipal wastewater treatment plants (CSO 2011)
Total sewage sludge generated during the year in thousand tons of DS
2000
2005
2008
2009
2010
359.8
486.1
567.3
563.1
526.7
So far, the most popular end-of-life treatments in Poland have been landfilling, agricultural and land reclamation applications (Figure 1). However, European Union legislation limits the final treated sludge applications and disposal. Therefore, different methods must be implemented in sewage sludge processing. According to the National, Waste Management Plan (Polish Environment Ministry ), thermal methods of sludge utilization should be introduced in order to meet EU requirements. However, before sludge is subjected to thermal treatment it must be adequately prepared. Therefore, dewatering and sludge drying are key processes, leading to a reduction in sludge volume and an increase in its calorific value expressed in kJ/kg of dry solids that can be achieved through dewatering and drying processes (Bien´ ). Dewatering is an operation used to reduce the moisture content of sludge to the level of 60–85% water content, reducing sludge volume at the same time (Bien´ ). The commonly used sludge dewatering techniques are based on mechanical processes using filtration, squeezing, centrifugal sedimentation and compaction (Metcalf & Eddy ). The advantages of mechanical dewatering are simplicity, universality and relatively low operational costs. To improve dewatering properties some assisted mechanical dewatering methods can be used (e.g. electrical
Figure 1
|
Different end-of-life sewage treatments in Poland over the years (CSO 2011).
Water Science & Technology
|
69.4
|
2014
mechanical dewatering and acoustic mechanical dewatering) which allow an increase in the final dry solids content and an accelerated dewatering process (Yuan & Weng ; Tuan & Sillanpää ; Mahmoud et al. ; Qi et al. ). Results achieved in the dewatering process, influenced the drying process. The higher the moisture content in the sludge, the greater the energy demand needed for drying. To dry 1 ton of sludge with 20% of dry solids, 60–70% more energy is required than to dry 1 ton of sludge with 35% of dry solids (Bien´ ). Drying is a process that is based on reducing water content in sludge (which has not been removed during the dewatering process) and involves the application of external heat (Bennamoun et al. ). After this process, the moisture content of sludge is at a level of 5–10% (Bien´ ). The most commonly used devices in Poland are belt dryers, batch dryers and container dryers (see Figure 2). The calorific value of raw sludge and digested sludge is 16–20 and 10–15 kJ/kg of dry solids, respectively. After dewatering and drying processes, the calorific value of sludge is comparable to brown coal (Werther & Ogada ). Therefore, the introduction of these dewatering and drying processes is required before thermal utilization such as incineration or co-combustion. Nevertheless, reducing the moisture content of sludge is one of the most problematic and challenging tasks in the field of wastewater treatment (Tuan & Sillanpää ). In this context there is a need to evaluate different methods of wastewater sludge processing in order to find the optimal solution from both economic and environmental points of view. Therefore, life cycle assessment (LCA) methodology, which is widely used around the world, can be
785
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
Water Science & Technology
|
69.4
|
2014
METHODS The analysis was carried out using the LCA method. LCA is a process of evaluating the effects that a product has on the environment over its life in order to increase resource-use efficiency and to decrease liabilities. It can be used to assess the environmental burdens connected with either a product or its function. LCA is commonly referred to as a ‘cradle-to-grave’ analysis (Zbicinski et al. ). The LCA of sludge processing includes four phases described in ISO 14040:2009 standards. In this work, SimaPro 7.2 software and Ecoinvent 2.2. database were used in order to establish environmental burdens and energetic performance. Goal and scope definition The goal of analysis was to estimate the carbon footprint and energy balance of three alternative ways of sludge processing based on the use of one kind of dewatering press and three different kinds of dryers. Figure 2
|
Total solids concentration after the individual processes of sewage sludge
•
treatment.
introduced to improve the decision making process (Beavis & Lundie ). Several studies have been published in the literature about the application of LCA methodology to evaluate different sewage sludge treatment scenarios. Suh & Rousseausx () compared the environmental impact of different sewage sludge treatment scenarios including incineration, composting, anaerobic digestion, landfilling and land application. Houillon & Jolliet () applied LCA methodology to assess six sewage sludge treatments: agricultural spreading, fluidized bed incineration, wet oxidation, pyrolysis, incineration in cement kilns and landfill. Hong et al. () conducted a more detailed analysis for the most common solutions in Japan. The environmental impacts of six single processes with or without digestion (dewatering, composting, drying, incineration, incinerated ash melting and dewatered sludge melting) and three end-oflife treatments (landfilling, agricultural application and building material application) were estimated and compared. However, it is important to emphasize that the proper selection of a single device can influence the LCA result. In this paper, gate to gate analysis has been conducted in order to assess the performance of the most commonly used devices in Poland. Three alternative ways of sludge processing connected with sludge dewatering and drying have been analyzed.
• •
Scenario 1: dewatering to 20% of dry solids by the use of a belt filter press and drying to 98% of dry solids by the use of a belt dryer. Scenario 2: dewatering to 25% of dry solids by the use of a belt filter press and drying to 98% of dry solids by the use of a container dryer. Scenario 3: dewatering to 30% of dry solids by the use of a belt filter press and drying to 98% of dry solids by the use of a batch dryer.
The extent of analysis includes the operational stage of the life cycle – materials and energy needed for the operation of the sludge processes considered. For calculation purposes, the Polish energy mix (based on the Ecoinvent 2.2 database) was used. The functional unit is 1 ton of sludge before dewatering (6% of dry solids). System boundaries are presented at Figure 3. Inventory analysis The data used for analysis cover the materials and energy consumption in the aforementioned life cycle stage. The amounts of electric power consumption for press and dryer operation, as well as for pumping (transportation) and washing were estimated according to the literature, producers and exploitation data. The use of washing water, flocculants and production of sewage were included. The Ecoinvent database was used for determining partial process characteristics.
786
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
Water Science & Technology
|
69.4
|
2014
balancing products and services according to detailed energy consumption throughout the life cycle. Environmental burdens are identified by energy inputs: direct – referring to primary energy for manufacture, transportation, etc, and indirect – referring to other purposes like infrastructure. CED analysis results are presented as MJeq. Interpretation Comparison of the results obtained indicates the most economical and environmentally-friendly process of sludge dewatering and drying. In addition to standard results, the calorific values calculation was presented against the energy demands for processes to define the energy balance of a system.
RESULTS AND DISCUSSION Figure 3
|
System boundaries of analyzed processes.
Impact assessment For the life cycle impact assessment, the global warming potential (GWP) method was used. It allows for the expression of greenhouse gases emissions in the mass unit – CO2eq (Cel et al. ). Carbon dioxide equivalent is a unit used to estimate the emissions of various greenhouse gases based upon their global warming potential. As a complementary method, the cumulative energy demand (CED) method was used. CED analysis aims at
Figure 4
|
Cumulative energy demand for individual processes.
Results of CED analysis are presented in Figure 4. As can be clearly seen, the main energy consumption is connected with the process of drying, therefore Scenario 1 (dewatering to 25% DS and drying to 98% DS) is the most energy intensive. The difference between Scenario 1 and the most energy-efficient Scenario 3 equals 531,5 MJeq, which means 39,6% less energy was used during Scenario 3 – dewatering to 35% and drying. Results of GWP analysis are presented in Figure 5. As most carbon dioxide and other greenhouse gas emissions (GHG) are connected with electric power consumption, the situation is comparable to CED analysis. Scenario 1,
787
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
Table 2
|
Water Science & Technology
|
69.4
|
2014
Energy demand for sludge processing, MJ/kg of dry solids
Energy demand per 1 kg of dry solids [MJeq]/kg
Scenario 1
23.37
Scenario 2
17.3
Scenario 3
13.52
CONCLUSIONS
Figure 5
|
Comparison of GHG emission for different scenarios.
as the most energy consuming, is responsible for the highest GHG emission, while Scenario 3 causes the smallest. If we analyze individual processes, GHG emission is mostly connected with drying (from 86% – Scenario 3, to 94% – Scenario 1), which is consistent with other studies (Hong et al. ). Sludge transportation, estimated according to the literature data, is not significant for the process assessment (see Figure 6). The energy demand for total sludge processing of 1 kg of dry solids differs for the processes, as shown in Table 2. In all cases, energy needs for sludge dewatering and drying are higher than the potential gains from its burning. However according to results presented in Houillon & Jolliet () the incineration process is characterized by low non-renewable primary energy consumption, compare with the other processes other studied. In addition, according to Hong et al. () the global warming potential generated from incineration can be significantly reduced through the reuse of waste heat for electricity or heat generation.
Figure 6
|
Due to increasingly stringent legal regulations common to both Poland and the European Union, thermal methods of sludge utilization are becoming a more popular method for end-of-life treatment. However, thermal utilization has to be preceded by water removal from sludge to the level of at least 90% of dry solids. In this paper, an analysis of the selected devices for sludge dewatering and drying is presented. Results of the conducted analysis show that the most favorable energy balance is achieved in Scenario 3 in which a batch dryer was used. In this case dewatering took longer because dry solids content in the sludge at the inlet of the batch dryer should be higher compared to other devices. Nevertheless, Scenario 3 is the least energy intensive, which results from the fact that the energy needs of dewatering are insignificant in comparison with drying. The drying process using belt and container dryers requires much more energy and involves greater environmental burdens, because dry solids at the inlet of these dryers are smaller and their concentration equals 20 and 25%, respectively. Generally, drying is a process that is connected with significant energy demand and is not introduced into the whole treatment in order to gain energy profits. Energy balances of the processes analyzed are negative – the amount of energy
Total carbon emission from sludge dewatering, drying and transportation for individual scenarios (in kg CO2eq).
788
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
spent on water removal is higher than energy potentially recovered from sludge burning. However, in the case of sewage sludge – an undesired product of wastewater treatment – drying allows its stabilization and easier, less expensive end-of-life treatment (including transportation) in accordance with EU guidelines.
REFERENCES Ahmed, S., Rasul, M. G., Martens, W. N., Brown, R. & Hashib, M. A. Advances in heterogeneous photocatalytic degradation of phenols and dyes in wastewater: a review. Water, Air, & Soil Pollution 215, 3–29. Beavis, P. & Lundie, S. Integrated environmental assessment of tertiary and residuals treatment – LCA in the wastewater industry. Water Science and Technology 47, 109–116. Bennamoun, L., Arlabosse, P. & Léonard, A. Review on fundamental aspect of application of drying process to wastewater sludge. Renewable and Sustainable Energy Reviews 28, 29–43. Bien´, J. B. Osady s´ciekowe teoria i praktyka (Sewage sludge theory and practice). Wydawnictwo Politechniki Cze˛ stochowskiej, Cze˛ stochowa, Poland. Cel, W., Pawłowski, A. & Cholewa, T. S´lad we˛ glowy jako miara zrównowaz˙ onos´ci odnawialnych z´ródeł energii (Carbon footprint as a measure of sustainability of renewable energy sources). In: Diagnozowanie Stanu S´rodowiska. Metody Badawcze – Prognozy (J. K. Garbacz, ed.), Vol. 4, Prace Komisji Ekologii i Ochrony S´rodowiska, Bydgoskie Towarzystwo Naukowe, Bydgoszcz, Poland, pp. 15–22. Cleverson, V. A., Von Speling, M. & Fernandez, F. Sludge Treatment and Disposal. Biological Wastewater Treatment Vol. 6. IWA Publishing, London. CSO Environment. Central Statistical Office. Statistical information, Warsaw, Poland (www.stat.gov.pl). Harisha, R. S., Hosamani, K. M., Keri, R. S., Nataraj, S. K. & Aminabhavi, T. M. Arsenic removal from drinking water using thin film composite nanofiltration membrane. Desalination 252, 75–80. Hong, J., Hong, J., Otaki, M. & Jolliet, O. Environmental and economic life cycle assessment for sewage sludge treatment processes in Japan. Waste Management 29, 696–703. Houillon, G. & Jolliet, O. Life cycle assessment of processes for the treatment of wastewater urban sludge: energy and global warming analysis. Journal of Cleaner Production 13, 287–299. Mahmoud, A., Olivier, J., Vaxelaire, J. & Hoadley, A. F. A. Electro-dewatering of wastewater sludge: Influence of the operating conditions and their interactions effects. Water Research 45, 2795–2810. Metcalf & Eddy Wastewater Engineering: Treatment and Reuse, Fourth Edition. McGraw-Hill Higher Education, New York.
Water Science & Technology
|
69.4
|
2014
Nataraj, S. K., Hosamani, K. M. & Aminabhavi, T. M. a Distillery wastewater treatment by the membrane-based nanofiltration and reverse osmosis processes. Water Research 40, 2349–2356. Nataraj, S. K., Hosamani, K. M. & Aminabhavi, T. M. b Electrodialytic removal of nitrates and hardness from simulated mixtures using ion-exchange membranes. Journal of Applied Polymer Science 99, 1788–1794. Nataraj, S. K., Hosamani, K. M. & Aminabhavi, T. M. a Potential application of an electrodialysis pilot plant containing ion-exchange membranes in chromium removal. Desalination 217, 181–190. Nataraj, S. K., Sridhar, S., Shaikha, I. N., Reddy, D. S. & Aminabhavi, T. M. b Membrane-based microfiltration/ electrodialysis hybrid process for the treatment of paper industry wastewater. Separation and Purification Technology 57, 185–192. Polish Environment Ministry National Waste Management Plan, available online at: http://www.mos.gov.pl/artykul/ 3340_krajowy_plan_gospodarki_odpadami_2014/ 21693_national_waste_management_plan_2014.html. Qi, Y., Thapa, K. B. & Hoadley, A. F. A. Application of filtration aids for improving sludge dewatering properties – A review. Chemical Engineering Journal 171, 373–384. Suh, Y.-J. & Rousseaux, P. An LCA of alternative wastewater sludge treatment scenarios. Resources, Conservation and Recycling 35, 191–200. Tuan, P.-A. & Sillanpää, M. Migration of ions and organic matter during electro-dewatering of anaerobic sludge. Journal of Hazardous Materials 173, 54–61. Uggetti, E., Llorens, E., Pedescoll, A., Ferrer, I., Castellnou, R. & García, J. Sludge dewatering and stabilization in drying reed beds: characterization of three full-scale systems in Catalonia, Spain. Bioresource Technology 100, 3882–3890. Visvanathan, C., Aim, R. B. & Parameshwaran, K. Membrane separation bioreactors for wastewater treatment. Environmental Science and Technology 30, 1–48. Werle, S. & Wilk, R. K. Energetyczne wykorzystanie osadów s´ciekowych (Thermal utilization of sewage sludge). In: Polska Inz˙ ynieria S´rodowiska pie˛ c´ lat po wsta˛ pieniu do Unii Europejskiej (M. R. Dudziń ska & A. Pawłowski, eds), Vol. 1, Monografie Komitetu Inz˙ ynierii S´rodowiska PAN vol. 58, Komitet Inz˙ ynierii S´rodowiska, Lublin, Poland, pp. 339–346. Werther, J. & Ogada, T. Sewage sludge combustion. Progress in Energy and Combustion Science 25, 55–116. Yuan, C. & Weng, C.-H. Sludge dewatering by electrokinetic technique: effect of processing time and potential gradient. Advances in Environmental Research 7, 727–732. Zbicinski, I., Stavenuiter, J., Kozlowska, B. & van de Coevering, H. Product Design and Life Cycle Assessment. The Baltic University Press, pp. 88–89.
First received 29 May 2013; accepted in revised form 25 November 2013. Available online 10 December 2013
© IWA Publishing 2014 Water Science & Technology
|
69.4
|
2014
Environmental assessment of different dewatering and drying methods on the basis of life cycle assessment J. Stefaniak, A. Z˙ elazna and A. Pawłowski
ABSTRACT Sewage sludge is an inevitable product of wastewater treatment in municipal wastewater plants and its amount has increased dramatically due to the growing number of sewage systems users. This sludge needs to be adequately treated in order to decrease its hazardous properties and any negative influence on the environment. In this paper, gate to gate analysis, on the basis of life cycle assessment (LCA), was carried out in order to compare the environmental impact of alternative ways of sludge processing employing a dewatering press and three different kinds of dryers – belt dryer,
J. Stefaniak (corresponding author) A. Z˙ elazna A. Pawłowski Faculty of Environmental Engineering, Lublin University of Technology, Nadbystrzycka 40B 20-618, Lublin, Poland E-mail: [email protected]
container dryer and batch dryer. SimaPro 7.2 software and Ecoinvent 2.2 database were used to estimate the carbon footprint and energy balance of these processes. The main energy consumption in the scenarios analyzed is caused by the drying process. The solution based on application of the batch dryer allows a saving of 39.6% of energy compared with the most energy-consuming solution using a belt dryer. Sludge processing using belt and container dryers cause greater environmental burdens. Key words
| LCA, life cycle assessment, sewage dewatering, sewage drying, sewage sludge
INTRODUCTION Sewage sludge management is a current and ongoing problematic issue. This situation is connected with many technical and economic factors, since sewage sludge is a complex environment that requires complex treatment (Bien´ ). The sewage sludge had a total solids concentration of about 1–3%, depending on the treatment process, and its production represents 1–2% of the treated wastewater volume. Nevertheless, its management accounts for 20– 60% of the total operating costs of wastewater treatment plants (Cleverson et al. ; Uggetti et al. ). Due to the fast growth in world population and industrialization, the amount of wastewater and sludge has increased dramatically all over the world (Hong et al. ). In Poland this growth is caused mainly by improvements in living conditions and the increasing percentage of households connected to central treatment plants. Over the last 10 years, the total sewage sludge generated annually has increased by almost 200 thousand tons of dry sludge – Table 1. Moreover, according to some predictions the amount of sewage sludge will continue to increase and it could reach 706.6 thousand tons of dry solids in 2018 (Werle & Wilk ). Complex and expensive sludge processing as well as doi: 10.2166/wst.2013.778
increasing sludge production make sludge management a matter of concern in many countries. Sludge treatment begins with the separation of wastewater solids from liquid whereby the required level of wastewater purification can be obtained. In recent years, membrane based techniques have gained considerable attention in cases of wastewater and drinking water (Nataraj et al. b; Harisha et al. ). In the context of wastewater treatment, these techniques are being applied to a wider range of industrial applications and are also used in municipal wastewater treatment plants according to reports in several papers (Visvanathan et al. ; Nataraj et al. a, Nataraj et al., a, b; Ahmed et al. ). Employment of membrane based techniques for solids separation (e.g. microfiltration or ultrafiltration), can overcome the disadvantages of conventional techniques based on sedimentation tank and biological treatment methods. The main advantages of these techniques are: high quality of treated streams, flexibility and simplicity in operation, small footprint size of the treatment plant and a significantly reduced amount of sludge. When the separation stage is finished, sewage sludge must be subjected to another process so it can reach the end-of-life phase.
J. Stefaniak et al.
784
Table 1
|
|
Environmental assessment of different dewatering methods based on LCA
Sewage sludge from Polish municipal wastewater treatment plants (CSO 2011)
Total sewage sludge generated during the year in thousand tons of DS
2000
2005
2008
2009
2010
359.8
486.1
567.3
563.1
526.7
So far, the most popular end-of-life treatments in Poland have been landfilling, agricultural and land reclamation applications (Figure 1). However, European Union legislation limits the final treated sludge applications and disposal. Therefore, different methods must be implemented in sewage sludge processing. According to the National, Waste Management Plan (Polish Environment Ministry ), thermal methods of sludge utilization should be introduced in order to meet EU requirements. However, before sludge is subjected to thermal treatment it must be adequately prepared. Therefore, dewatering and sludge drying are key processes, leading to a reduction in sludge volume and an increase in its calorific value expressed in kJ/kg of dry solids that can be achieved through dewatering and drying processes (Bien´ ). Dewatering is an operation used to reduce the moisture content of sludge to the level of 60–85% water content, reducing sludge volume at the same time (Bien´ ). The commonly used sludge dewatering techniques are based on mechanical processes using filtration, squeezing, centrifugal sedimentation and compaction (Metcalf & Eddy ). The advantages of mechanical dewatering are simplicity, universality and relatively low operational costs. To improve dewatering properties some assisted mechanical dewatering methods can be used (e.g. electrical
Figure 1
|
Different end-of-life sewage treatments in Poland over the years (CSO 2011).
Water Science & Technology
|
69.4
|
2014
mechanical dewatering and acoustic mechanical dewatering) which allow an increase in the final dry solids content and an accelerated dewatering process (Yuan & Weng ; Tuan & Sillanpää ; Mahmoud et al. ; Qi et al. ). Results achieved in the dewatering process, influenced the drying process. The higher the moisture content in the sludge, the greater the energy demand needed for drying. To dry 1 ton of sludge with 20% of dry solids, 60–70% more energy is required than to dry 1 ton of sludge with 35% of dry solids (Bien´ ). Drying is a process that is based on reducing water content in sludge (which has not been removed during the dewatering process) and involves the application of external heat (Bennamoun et al. ). After this process, the moisture content of sludge is at a level of 5–10% (Bien´ ). The most commonly used devices in Poland are belt dryers, batch dryers and container dryers (see Figure 2). The calorific value of raw sludge and digested sludge is 16–20 and 10–15 kJ/kg of dry solids, respectively. After dewatering and drying processes, the calorific value of sludge is comparable to brown coal (Werther & Ogada ). Therefore, the introduction of these dewatering and drying processes is required before thermal utilization such as incineration or co-combustion. Nevertheless, reducing the moisture content of sludge is one of the most problematic and challenging tasks in the field of wastewater treatment (Tuan & Sillanpää ). In this context there is a need to evaluate different methods of wastewater sludge processing in order to find the optimal solution from both economic and environmental points of view. Therefore, life cycle assessment (LCA) methodology, which is widely used around the world, can be
785
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
Water Science & Technology
|
69.4
|
2014
METHODS The analysis was carried out using the LCA method. LCA is a process of evaluating the effects that a product has on the environment over its life in order to increase resource-use efficiency and to decrease liabilities. It can be used to assess the environmental burdens connected with either a product or its function. LCA is commonly referred to as a ‘cradle-to-grave’ analysis (Zbicinski et al. ). The LCA of sludge processing includes four phases described in ISO 14040:2009 standards. In this work, SimaPro 7.2 software and Ecoinvent 2.2. database were used in order to establish environmental burdens and energetic performance. Goal and scope definition The goal of analysis was to estimate the carbon footprint and energy balance of three alternative ways of sludge processing based on the use of one kind of dewatering press and three different kinds of dryers. Figure 2
|
Total solids concentration after the individual processes of sewage sludge
•
treatment.
introduced to improve the decision making process (Beavis & Lundie ). Several studies have been published in the literature about the application of LCA methodology to evaluate different sewage sludge treatment scenarios. Suh & Rousseausx () compared the environmental impact of different sewage sludge treatment scenarios including incineration, composting, anaerobic digestion, landfilling and land application. Houillon & Jolliet () applied LCA methodology to assess six sewage sludge treatments: agricultural spreading, fluidized bed incineration, wet oxidation, pyrolysis, incineration in cement kilns and landfill. Hong et al. () conducted a more detailed analysis for the most common solutions in Japan. The environmental impacts of six single processes with or without digestion (dewatering, composting, drying, incineration, incinerated ash melting and dewatered sludge melting) and three end-oflife treatments (landfilling, agricultural application and building material application) were estimated and compared. However, it is important to emphasize that the proper selection of a single device can influence the LCA result. In this paper, gate to gate analysis has been conducted in order to assess the performance of the most commonly used devices in Poland. Three alternative ways of sludge processing connected with sludge dewatering and drying have been analyzed.
• •
Scenario 1: dewatering to 20% of dry solids by the use of a belt filter press and drying to 98% of dry solids by the use of a belt dryer. Scenario 2: dewatering to 25% of dry solids by the use of a belt filter press and drying to 98% of dry solids by the use of a container dryer. Scenario 3: dewatering to 30% of dry solids by the use of a belt filter press and drying to 98% of dry solids by the use of a batch dryer.
The extent of analysis includes the operational stage of the life cycle – materials and energy needed for the operation of the sludge processes considered. For calculation purposes, the Polish energy mix (based on the Ecoinvent 2.2 database) was used. The functional unit is 1 ton of sludge before dewatering (6% of dry solids). System boundaries are presented at Figure 3. Inventory analysis The data used for analysis cover the materials and energy consumption in the aforementioned life cycle stage. The amounts of electric power consumption for press and dryer operation, as well as for pumping (transportation) and washing were estimated according to the literature, producers and exploitation data. The use of washing water, flocculants and production of sewage were included. The Ecoinvent database was used for determining partial process characteristics.
786
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
Water Science & Technology
|
69.4
|
2014
balancing products and services according to detailed energy consumption throughout the life cycle. Environmental burdens are identified by energy inputs: direct – referring to primary energy for manufacture, transportation, etc, and indirect – referring to other purposes like infrastructure. CED analysis results are presented as MJeq. Interpretation Comparison of the results obtained indicates the most economical and environmentally-friendly process of sludge dewatering and drying. In addition to standard results, the calorific values calculation was presented against the energy demands for processes to define the energy balance of a system.
RESULTS AND DISCUSSION Figure 3
|
System boundaries of analyzed processes.
Impact assessment For the life cycle impact assessment, the global warming potential (GWP) method was used. It allows for the expression of greenhouse gases emissions in the mass unit – CO2eq (Cel et al. ). Carbon dioxide equivalent is a unit used to estimate the emissions of various greenhouse gases based upon their global warming potential. As a complementary method, the cumulative energy demand (CED) method was used. CED analysis aims at
Figure 4
|
Cumulative energy demand for individual processes.
Results of CED analysis are presented in Figure 4. As can be clearly seen, the main energy consumption is connected with the process of drying, therefore Scenario 1 (dewatering to 25% DS and drying to 98% DS) is the most energy intensive. The difference between Scenario 1 and the most energy-efficient Scenario 3 equals 531,5 MJeq, which means 39,6% less energy was used during Scenario 3 – dewatering to 35% and drying. Results of GWP analysis are presented in Figure 5. As most carbon dioxide and other greenhouse gas emissions (GHG) are connected with electric power consumption, the situation is comparable to CED analysis. Scenario 1,
787
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
Table 2
|
Water Science & Technology
|
69.4
|
2014
Energy demand for sludge processing, MJ/kg of dry solids
Energy demand per 1 kg of dry solids [MJeq]/kg
Scenario 1
23.37
Scenario 2
17.3
Scenario 3
13.52
CONCLUSIONS
Figure 5
|
Comparison of GHG emission for different scenarios.
as the most energy consuming, is responsible for the highest GHG emission, while Scenario 3 causes the smallest. If we analyze individual processes, GHG emission is mostly connected with drying (from 86% – Scenario 3, to 94% – Scenario 1), which is consistent with other studies (Hong et al. ). Sludge transportation, estimated according to the literature data, is not significant for the process assessment (see Figure 6). The energy demand for total sludge processing of 1 kg of dry solids differs for the processes, as shown in Table 2. In all cases, energy needs for sludge dewatering and drying are higher than the potential gains from its burning. However according to results presented in Houillon & Jolliet () the incineration process is characterized by low non-renewable primary energy consumption, compare with the other processes other studied. In addition, according to Hong et al. () the global warming potential generated from incineration can be significantly reduced through the reuse of waste heat for electricity or heat generation.
Figure 6
|
Due to increasingly stringent legal regulations common to both Poland and the European Union, thermal methods of sludge utilization are becoming a more popular method for end-of-life treatment. However, thermal utilization has to be preceded by water removal from sludge to the level of at least 90% of dry solids. In this paper, an analysis of the selected devices for sludge dewatering and drying is presented. Results of the conducted analysis show that the most favorable energy balance is achieved in Scenario 3 in which a batch dryer was used. In this case dewatering took longer because dry solids content in the sludge at the inlet of the batch dryer should be higher compared to other devices. Nevertheless, Scenario 3 is the least energy intensive, which results from the fact that the energy needs of dewatering are insignificant in comparison with drying. The drying process using belt and container dryers requires much more energy and involves greater environmental burdens, because dry solids at the inlet of these dryers are smaller and their concentration equals 20 and 25%, respectively. Generally, drying is a process that is connected with significant energy demand and is not introduced into the whole treatment in order to gain energy profits. Energy balances of the processes analyzed are negative – the amount of energy
Total carbon emission from sludge dewatering, drying and transportation for individual scenarios (in kg CO2eq).
788
J. Stefaniak et al.
|
Environmental assessment of different dewatering methods based on LCA
spent on water removal is higher than energy potentially recovered from sludge burning. However, in the case of sewage sludge – an undesired product of wastewater treatment – drying allows its stabilization and easier, less expensive end-of-life treatment (including transportation) in accordance with EU guidelines.
REFERENCES Ahmed, S., Rasul, M. G., Martens, W. N., Brown, R. & Hashib, M. A. Advances in heterogeneous photocatalytic degradation of phenols and dyes in wastewater: a review. Water, Air, & Soil Pollution 215, 3–29. Beavis, P. & Lundie, S. Integrated environmental assessment of tertiary and residuals treatment – LCA in the wastewater industry. Water Science and Technology 47, 109–116. Bennamoun, L., Arlabosse, P. & Léonard, A. Review on fundamental aspect of application of drying process to wastewater sludge. Renewable and Sustainable Energy Reviews 28, 29–43. Bien´, J. B. Osady s´ciekowe teoria i praktyka (Sewage sludge theory and practice). Wydawnictwo Politechniki Cze˛ stochowskiej, Cze˛ stochowa, Poland. Cel, W., Pawłowski, A. & Cholewa, T. S´lad we˛ glowy jako miara zrównowaz˙ onos´ci odnawialnych z´ródeł energii (Carbon footprint as a measure of sustainability of renewable energy sources). In: Diagnozowanie Stanu S´rodowiska. Metody Badawcze – Prognozy (J. K. Garbacz, ed.), Vol. 4, Prace Komisji Ekologii i Ochrony S´rodowiska, Bydgoskie Towarzystwo Naukowe, Bydgoszcz, Poland, pp. 15–22. Cleverson, V. A., Von Speling, M. & Fernandez, F. Sludge Treatment and Disposal. Biological Wastewater Treatment Vol. 6. IWA Publishing, London. CSO Environment. Central Statistical Office. Statistical information, Warsaw, Poland (www.stat.gov.pl). Harisha, R. S., Hosamani, K. M., Keri, R. S., Nataraj, S. K. & Aminabhavi, T. M. Arsenic removal from drinking water using thin film composite nanofiltration membrane. Desalination 252, 75–80. Hong, J., Hong, J., Otaki, M. & Jolliet, O. Environmental and economic life cycle assessment for sewage sludge treatment processes in Japan. Waste Management 29, 696–703. Houillon, G. & Jolliet, O. Life cycle assessment of processes for the treatment of wastewater urban sludge: energy and global warming analysis. Journal of Cleaner Production 13, 287–299. Mahmoud, A., Olivier, J., Vaxelaire, J. & Hoadley, A. F. A. Electro-dewatering of wastewater sludge: Influence of the operating conditions and their interactions effects. Water Research 45, 2795–2810. Metcalf & Eddy Wastewater Engineering: Treatment and Reuse, Fourth Edition. McGraw-Hill Higher Education, New York.
Water Science & Technology
|
69.4
|
2014
Nataraj, S. K., Hosamani, K. M. & Aminabhavi, T. M. a Distillery wastewater treatment by the membrane-based nanofiltration and reverse osmosis processes. Water Research 40, 2349–2356. Nataraj, S. K., Hosamani, K. M. & Aminabhavi, T. M. b Electrodialytic removal of nitrates and hardness from simulated mixtures using ion-exchange membranes. Journal of Applied Polymer Science 99, 1788–1794. Nataraj, S. K., Hosamani, K. M. & Aminabhavi, T. M. a Potential application of an electrodialysis pilot plant containing ion-exchange membranes in chromium removal. Desalination 217, 181–190. Nataraj, S. K., Sridhar, S., Shaikha, I. N., Reddy, D. S. & Aminabhavi, T. M. b Membrane-based microfiltration/ electrodialysis hybrid process for the treatment of paper industry wastewater. Separation and Purification Technology 57, 185–192. Polish Environment Ministry National Waste Management Plan, available online at: http://www.mos.gov.pl/artykul/ 3340_krajowy_plan_gospodarki_odpadami_2014/ 21693_national_waste_management_plan_2014.html. Qi, Y., Thapa, K. B. & Hoadley, A. F. A. Application of filtration aids for improving sludge dewatering properties – A review. Chemical Engineering Journal 171, 373–384. Suh, Y.-J. & Rousseaux, P. An LCA of alternative wastewater sludge treatment scenarios. Resources, Conservation and Recycling 35, 191–200. Tuan, P.-A. & Sillanpää, M. Migration of ions and organic matter during electro-dewatering of anaerobic sludge. Journal of Hazardous Materials 173, 54–61. Uggetti, E., Llorens, E., Pedescoll, A., Ferrer, I., Castellnou, R. & García, J. Sludge dewatering and stabilization in drying reed beds: characterization of three full-scale systems in Catalonia, Spain. Bioresource Technology 100, 3882–3890. Visvanathan, C., Aim, R. B. & Parameshwaran, K. Membrane separation bioreactors for wastewater treatment. Environmental Science and Technology 30, 1–48. Werle, S. & Wilk, R. K. Energetyczne wykorzystanie osadów s´ciekowych (Thermal utilization of sewage sludge). In: Polska Inz˙ ynieria S´rodowiska pie˛ c´ lat po wsta˛ pieniu do Unii Europejskiej (M. R. Dudziń ska & A. Pawłowski, eds), Vol. 1, Monografie Komitetu Inz˙ ynierii S´rodowiska PAN vol. 58, Komitet Inz˙ ynierii S´rodowiska, Lublin, Poland, pp. 339–346. Werther, J. & Ogada, T. Sewage sludge combustion. Progress in Energy and Combustion Science 25, 55–116. Yuan, C. & Weng, C.-H. Sludge dewatering by electrokinetic technique: effect of processing time and potential gradient. Advances in Environmental Research 7, 727–732. Zbicinski, I., Stavenuiter, J., Kozlowska, B. & van de Coevering, H. Product Design and Life Cycle Assessment. The Baltic University Press, pp. 88–89.
First received 29 May 2013; accepted in revised form 25 November 2013. Available online 10 December 2013
