Waste Management 34 (2014) 411–420
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Comparative assessment of municipal sewage sludge incineration, gasification and pyrolysis for a sustainable sludge-to-energy management in Greece M.C. Samolada b, A.A. Zabaniotou a,⇑ a b
Aristotle University of Thessaloniki, Dept. of Chemical Engineering, University Box 455, University Campus, 541 24 Thessaloniki, Greece Dept. Secretariat of Environmental and Urban Planning – Decentralized Area Macedonian Thrace, Taki Oikonomidi 1, 54008 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 14 February 2013 Accepted 4 November 2013 Available online 27 November 2013 Keywords: Municipal sewage sludge Pyrolysis Gasification Incineration SWOT Legislation Sustainable management
a b s t r a c t For a sustainable municipal sewage sludge management, not only the available technology, but also other parameters, such as policy regulations and socio-economic issues should be taken in account. In this study, the current status of both European and Greek Legislation on waste management, with a special insight in municipal sewage sludge, is presented. A SWOT analysis was further developed for comparison of pyrolysis with incineration and gasification and results are presented. Pyrolysis seems to be the optimal thermochemical treatment option compared to incineration and gasification. Sewage sludge pyrolysis is favorable for energy savings, material recovery and high added materials production, providing a ‘zero waste’ solution. Finally, identification of challenges and barriers for sewage sludge pyrolysis deployment in Greece was investigated. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Municipal sewage sludge (MSS) disposal faces significant environmental problems related to air emissions, threat to public health and contamination of soil and water resources, requiring therefore an appropriate treatment and careful management (Aggelakis et al., 2005). While world’s sludge production is on a relentless growth curve, environmental quality requirements for sludge are becoming increasingly stringent, disposal outlets are decreasing and economic pressures require low-cost solutions (EU, 2012). The amount of MSS production in the EU27 was estimated at 11.5 million tons of dry solids for 2010 and it is expected to rise to 13.0 million tons in 2020 (EC, 2008). Disposing sewage sludge to landfills is considered a beneficial use only when such disposal includes methane recovery for energy production. However, methane operations are relatively rare in most existing WWTPs (Waste Water Treatment Plants). However, due to o the limited capacity of available landfills, alternative beneficial uses are receiving greater attention. Valorisation of the nutrient components of MSS by considering the soil conditioning and fertilization is a beneficial use of sludge, especially in the case of forests and energy crops (Meeroff and Bloetscher, 1999; Wang
⇑ Corresponding author. Tel.: +30 2310 99 62 74; fax: +30 2310 996209. E-mail address: [email protected] (A.A. Zabaniotou). 0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.11.003
et al., 2008; O’Connor, 1996). However, use of sludge on land in the EU will not change dramatically in the future due to legislative restrictions. The proportion of recycled sludge to agriculture will remain almost constant across EU (42% in 2010) expecting to reach 44% in 2020 (Kelessidis and Stasinakis, 2012) as shown in Table 1. Looking forward to adoption of an efficient municipal sewage sludge management, to energy recovery should be considered by thermal routes. Although, high cost of power generated from sludge is a major barrier for the implementation of thermal routes, however, investments for sludge to power can become attractive if one considers the increase of energy prices in the international market by 20%. Thermal treatment methods include combustion/incineration and the ‘advanced’ or ‘emerging’ pyrolysis and gasification methods. The incineration share will raise slightly, (EC, 2008). Dewatered MSS has been successfully used for producing building materials (e.g. concrete, bituminous mixtures) and also in road construction (Aziz and Koe, 1990; Tay and Show, 1991; Anderson et al., 1996). Incineration ash residues can be used to produce road construction materials or concrete aggregates (Takeda et al., 1989; Lisk, 1989). MSS can be extensively used in cement manufacturing as a cheap alternative energy resource (Fytili and Zabaniotou, 2008) with substantial energy and environmental savings due to reduced CO2 emissions. However, selection of a particular, stand-alone sludge thermal treatment system should not be based primarily on technical insight, but it should also integrate all related social and
412
M.C. Samolada, A.A. Zabaniotou / Waste Management 34 (2014) 411–420
Table 1 Annual MSS production, disposal routes in EU27 countries (EC, 2008).
a b
Member state
Sludgea (Ktds/a)
Recyclea (%)
Incin/a (%)
Landfilla (%)
Othera (%)
Sludgeb (tds/a)
Recycleb (%)
Incinb (%)
Landfillb (%)
Otherb (%)
Greece Bulgaria Ireland Cyprus Latvia Estonia Lithuania Finland Malta Luxemburg Hungary Poland Romania Slovakia Spain France Italy UK Chech Rep. Slovenia Portugal Austria Denmark Germany Belgium Netherlands EU27 total
260 47 135 10 30 33 80 155 10 10 175 520 165 55 1,280 1,300 1,500 1,640 260 25 420 273 140 2,000 170 560 11,564
5 50 75 50 30 15 30 5 0 90 75 40 0 50 65 65 25 70 55 5 50 15 50 30 10 0 42
0 0 0 0 0 0 0 0 0 5 5 5 5 5 10 15 20 20 25 25 30 40 45 50 90 100 27
95 30 15 40 40 – 5 – 100 – 10 45 95 5 20 5 25 – 10 40 20 >1 – 0 – – 14
20 10 10 30 85 65 95 – 5 5 10 – 40 5 15 30 10 25 30 – 45 5 20 – – 16
260 151 135 17.62 50 33 80 155 10 10 200 950 520 135 1,280 1,400 1,500 1,640 260 50 750 280 140 2,000 170 560 13,047
5 60 75 50 30 15 55 5 10 80 60 25 20 50 70 75 35 65 75 15 50 5 50 25 10 – 44
40 10 10 10 10 0 15 0 0 20 30 10 10 40 25 15 30 25 20 70 40 85 45 50 90 100 32
55 10 5 30 30 – 5 – –0 – 5 20 30 5 5 5 5 – 5 10 5 >1 –
10 20 10 10 30 85 25 95 – – 5 45 40 5 – 5 30 10 5 5
– – 7
10 5 25 – – 16
Reflects statistical data for 2010. Reflects to predictions for 2020.
environmental activities. The ‘‘sludge-to-energy’’ approach is feasible with substantial benefits similar of those that any renewable energy source presents: decreasing the energy dependency of the WWTP and greenhouse gas emissions. Sludge-to-energy is technically feasible if the recovered energy could be directly used for operating the WWTP, resulting in reduction of conventional electricity consumption (Manara and Zabaniotou, 2012). Another approach called ‘‘sludge-to-fuel’’ (STF) involves a process that converts the organic matter of sludge into a combustion oil using a solvent, under atmospheric pressure and mild temperatures in the range of 200–300 °C (Millot et al., 1989) or alternatively, at high pressures (10 MPa) combined with high temperatures (Itoh et al., 1994). The produced oil is characterised by a high heating value (90% of common diesel fuel) and can be sold to offsite users or refineries (Hun, 1998). In Greece, according to recent statistical data (YPEKA, 2010), the current use of sludge in agriculture is very limited (0.15%) and estimations predict an increase up to 5% by 2020, as shown in Table 1. Due to the absence of established limits concerning water and pathogens content of MSS, local farmers are opposed and skeptical about the extensive use of sludge in agriculture. (Aggelakis et al., 2005). MSS use in agriculture is limited due to another important reason which is related to the use of CaO for the destruction of the pathogens which in addition to the destruction has a parallel negative impact on soil composition and fertility. In Greece, MSS is mostly used as an alternative fuel in existing cement kilns. Composting is particularly encouraged in Greece, reaching the share of 45.78% (YPEKA, 2010) as it is shown in Table 1. In this study, the selection of the most promising sewage sludge-to-energy method, that meets the goal for a ‘‘sustainable development’’, was attempted. The aim of the present study was the comparative assessment of 3 energy recovery options (incineration, pyrolysis, gasification) for municipal sewage sludge (MSS) in Greece through SWOT analysis, taking into account their current status of development. SWOT analysis tool, initially invented by Albert S. Humphrey, designed to be used in the preliminary stages
of decision-making; technologies and methods are compared in the base of economic, environmental and social metrics (Siomkos, 2004; Samolada and Zabaniotou, 2012). It was selected for application, since it has been proved to be a useful planning tool to understand the Strengths, Weaknesses, Opportunities and Threats of both processes and plans (UNEP 2009; Siomkos, 2004). 2. Legislation and sustainable integrated MSS management Municipal sewage sludge (MSS) is defined as the final solid residue produced during municipal waste water treatment. It is classified as a solid waste with the code of 19 08 05 according to the European Catalogue of Wastes (EEL 47/16-2-2001; Directive 2000/532/EK). MSS is also considered as a ‘‘specific stream’’ of non-dangerous solid wastes, which has to be treated according to a National Strategic Approach [Ministerial Order 50910/2003] with the objective of landfills minimization. MSS environmental management has to meet all the basic principles of the Wastes Framework Directive applied since December 12th 2010 [EU, Directive 2008/98/EC]. The European Catalogue of Wastes was introduced in the Greek law by the Ministerial Order 50910/2003 along with the Producers Responsibility (PR) principle according to which the ‘‘waste producer’’ is responsible for its effective and environmental discharge. [Presidential Decree 148/2009]. The Sewage Sludge Directive 86/ 278/EEC which was adopted with the Ministerial Order 80568/ 4225/1991 in the Greek law, seeks to encourage the use of sewage sludge in agriculture and to regulate its use in such a way as to prevent harmful effects on soil, vegetation, animals and man. To this end, it prohibits the use of untreated sludge on agricultural land unless it is injected or incorporated into the soil. MSS has been utilized in agricultural applications for several years, while it is restricted to prevent health risks to humans and livestock due to potentially toxic components, heavy metals, pathogens, and persistent organic pollutants and to the high amounts of soluble salts, which may affect the soil properties negatively. The presence of human pathogens in sewage sludge has led
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M.C. Samolada, A.A. Zabaniotou / Waste Management 34 (2014) 411–420
to a considerable amount of research to assess the health risks associated with the land applications of sludge. Significant environmental or health risks linked to the use of sewage sludge on land in the EU have not been widely demonstrated in the recent scientific literature since the 86/278/EEC Directive has taken effect, although there continue to be authoritative studies that identify and assess concerns. It is difficult to establish if the lack of evidence for adverse effects is because the provisions of the Directive are sufficient or is due to the more stringent national requirements in most Member States and Greece as well (EC, 2008). Epidemiological and risk assessment studies on the risks to health from microbial pathogens in sewage sludge for workers and populations in the vicinity of sludge operations have not generally found the risks to be significantly greater than background risks (Tanner, 2008; Kanakari, 2009). Overall health risks from the indirect exposure to pathogens have also been found to be low, with no clearly identified public infections from the use of food grown on land where sludge was applied in accordance with the provisions in the Directive (Gale et al., 2003). Environmental issues related to the recycling of sewage sludge on land include the risk of nutrient leaching, impact on soil biodiversity and greenhouse gas emissions (e.g. CH4 and N2O). In global warming potential (GWP) assessments of the different treatment, recycling or disposal routes, efficient treatment and recycling to agricultural land can usually be demonstrated to have a lower GWP effect compared to other disposal processes. There are some local circumstances, such as the location of the land or the nature of the sludge, in which the overall environmental impacts, either in terms of greenhouse gas emissions alone or in conjunction with other environmental factors, result in assessments that suggest non-agricultural routes may be more beneficial (EC, 2008). Treated sludge is defined as having undergone biological, chemical or heat treatment, long-term storage or any other appropriate process so as significantly to reduce its fermentability and the health hazards resulting from its use. The Directive also requires that sludge should be used in such a way that account is taken of the plants’ nutrient requirements of and that the quality of the soil and of the surface and groundwater is not impaired. It sets out requirements for keeping detailed records of the quantities of sludge produced, the quantities used in agriculture, the composition and properties of the sludge, the type of treatment and the sites where the sludge is used. Limits for heavy metals concentrations in both MSS and sludge-treated soils are introduced. Availability of land is an important issue to be taken in account for the selection of the appropriate strategy in sludge management. However, taken in account that lack of the available land space in combination with increasingly stringent regulations governing the design and operation of new landfills (EU Landfill Directive 99/31), have caused the construction and operating cost of new landfills to rise sharply. In addition, the increasingly restrictive targets for the continuous reduction of biodegradable wastes sent to landfills make land application of MSS an unattractive disposal option. The current global climate change drives societies to think about more sustainable ways of using resources and waste management (Zaman, 2009). MSS management should be greatly related to economic, environmental and social aspects in order to meet the goal of sustainable management (WCED, 1987). Decision makers should combine in an optimum way the available alternative MSS handling routes considering all available information on technical, economic and environmental issues. The selection of the most appropriate sludge treatment technology is a key factor in the application of integrated sewage sludge management (ISSM) system. In combination with economic and social considerations, this approach would help sludge managers to design more sustainable management systems (UNEP, 2009). MSS characteristics are the most important parameter to be taken
in account for the selection of the appropriate technology. A plausible solution in wastewater management should include collective management of sewage sludge and implementation of the 3R (Reduce, Reuse and Recycle) policies and strategies. In Fig. 1, the ISSM system is depicted (UNEP, 2009). Both European and Greek policy promotes including energy recovery from non-dangerous solid wastes and MSS as well. Integrated sewage sludge management (ISSM) refers to a strategic initiative for the sustained management of MSS through the use of comprehensive integrated format generated through sustained preventive approach to the complementary use of a variety of practices to handle sewage sludge in a safe and effective manner (UNEP, 2009). ISSM is based on the concept that all aspects of the sludge management systems (technical and non-technical) should be analyzed together, since they are in fact interrelated and developments in one area frequently affect practices or activities in another area. ISSM proposes to take a comprehensive approach across all types of sewage sludge streams and involves the use of a range of different options.
3. Materials and methods 3.1. Sludge composition and important characteristics Sewage sludge is a complex heterogeneous mixture of microorganisms, undigested organics such as paper, plant residues, oils, or Cities are facing an increasing growth in population resulting in increased amounts of wastewater and MSS production
Complexity, costs and
Industrialization
coordination of
and economic
wastewater management
growth has
NEED
generated increased
multi stakeholder
FOR
amounts of
involvement in every
ISSM
wastewater
stage of the waste stream
Wastewater treatment towards environmental protection and
Local Governments are now looking at wastewater as a business opportunity
sustainability
Fig. 1. Scheme of sludge integrated management (ISSM).
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M.C. Samolada, A.A. Zabaniotou / Waste Management 34 (2014) 411–420
fecal material, inorganic materials and moisture. The undigested organic materials contain a highly complex mixture of molecules coming from proteins and peptides, lipids, polysaccharides, plant macromolecules with phenolic structures (e.g. lignins or tannins) or aliphatic structures (e.g. cutins or suberins), along with organic micropollutants such as polycyclic aromatic hydrocarbons or dibenzofurans (Fonts et al., 2012). The inorganic materials present in the liquids come mainly from soil but also from synthetic polymers of anthropogenic origin. Sewage sludge has considerably higher nitrogen content. The nitrogen in sewage sludge comes mainly from the protein fraction of this material, which has its origin in the microorganisms used for water purification. Typical composition of untreated and digested sludge is reported in Table 2. The amount of sludge produced is affected in a limited scale by the treatment efficiency, while the sludge quality is strongly dependent on the original pollution load of the treated effluent and also, on the technical and design features of the WWTP process (Fytili and Zabaniotou, 2008; UNEP, 2009). An important issue in sludge treatment is the accumulation of most heavy metals of the original wastewater. Their high concentration may inhibit MSS’ use in agriculture without further treatment (Hsiau and Lo, 1998; EU, 2012). The ash from sewage sludge contains mainly minerals such as quartz, calcite or microline. These minerals are formed by elements such as Fe, Ca, K and Mg that can catalyze some pyrolysis reactions. Furthermore, some heavy metals can also be found in the sludge (Cr, Ni, Cu, Zn, Pb, Cd, Hg).Heavy metals such as zinc (Zn), copper (Cu), nickel (Ni), cadmium (Cd), lead (Pb), mercury (Hg) and chromium (Cr) are mostly met in MSS. Their potential accumulation in human tissues and biomagnifications through the food-chain create both human health and environmental concerns (Krogmann et al., 1999; Kanakari, 2009). The concentration of heavy metals in Greek MSS is below the limits that were set by EU and Greek legislation (Table 3). 3.2. Sludge-to-energy processes Potential advantages of thermal processes include reduction of volume and weight, destruction of toxic organic compounds, and recovery of energy, but economics need to be carefully evaluated. Incineration involves the complete oxidation of the volatile matter Table 2 Typical characteristics of sludge originating from various treatment methods (Manara and Zabaniotou, 2012). Characteristic
A
B1
B2
C
D
Dry matter (DM), % g/l Volatile matte (VM), % DM pH C, % VM H, % VM O, % VM N, % VM S, % VM C/N P, % DM Cl, % DM K, % DM Al, % DM Ca, % DM Fe, % DM Mg, % DM Fat, % DM Protein, % DM Fibres, % DM Calorific value, kW h/t DM
12 65 6 51.5 7 35.5 4.5 1.5 11.4 2 0.8 0.3 0.2 10 2 0.6 18 24 16 4200
9 67 7 52.5 6 33 7.5 1 7 2 0.8 0.3 0.2 10 2 0.6 8 36 17 4100
7 77 7 53 6.7 33 6.3 1 8.7 2 0.8 0.3 0.2 10 2 0.6 10 34 10 4800
10 72 6.5 51 7.4 33 7.1 1.5 7.2 2 0.8 0.3 0.2 10 2 0.6 14 30 13 4600
30 50 7 49 7.7 35 6.2 2.1 7.9 2 0.8 0.3 0.2 10 2 0.6 10 18 10 3000
A: primary sludge, primary sludge with physical/chemical treatment or high pollution load; B1: biological sludge (low load); B2: biological sludge from clarified water (low and middle load); C: mixed sludge (mix A and B2 types); D: digested sludge.
and the production of an inert residue (ash). Recent extensive reviews (Fytili and Zabaniotou, 2008; Manara and Zabaniotou, 2012) of the current literature on the effective sludge valorisation considered various available technologies including: anaerobic digestion, incineration, pyrolysis, gasification and wet oxidation. Thermochemical technologies found to be promising alternative valorisation routes of sewage sludge considering the decreasing availability and the increasing price of land for landfills. The principal goal of thermal processing of sewage sludge is the utilization of its energy content with minimizing the related environmental impact to meet the now increasingly stringent standards. It is well known that MSS is characterized by considerably high water (P50 wt%) content, thus consuming most of the thermal process energy (Aggelakis et al., 2005; Dennis et al., 2005; Fytili and Zabaniotou, 2008; Werle and Wilk, 2010; Manara and Zabaniotou, 2012). Thus, dewatered/dried prior to thermal processing is needed. However, the consequence of dewatering/drying prior to thermal processing is on energy balances and increasing the costs although, thermal valorisation processes are generally considered to be as energy self-sufficient (Khiari et al., 2004; Fytili and Zabaniotou, 2008; Manara and Zabaniotou, 2012). 3.2.1. Combustion/incineration Combustion is the currently used thermal treatment method for sludge energetic valorisation. The amount of sludge being incinerated in Denmark has already reached the percentage of 24% of the sludge produced, France the 20%, Belgium the 15%, Germany the 14% while in the USA and Japan the percentage has reached the 25% and 55% respectively (Lundin et al., 2004). Wet or dry sludge combustion (with a 41–65 wt% content of dry material) can be effectively introduced in fluid bed combustion reactors (Fytili and Zabaniotou, 2008). In most MSS thermal processes, a partial dehydration (85% dry matter) or a total dehydration (>85% dry matter) takes usually place (Oleszkiewicz and Reimers, 1998). Dry sewage sludge is characterized by a calorific value of 12–20 MJ/kg, lower than that of coal (14.6–26.7 MJ/kg) (Manara and Zabaniotou, 2012) but equivalent to that of lignite (11.7–15.8) MJ/kg (Samolada and Zabaniotou, 2012). Incineration technology is the controlled combustion of waste with the recovery of heat to produce steam that in turn produces power through steam turbines Combustion/incineration still remains the most attractive disposal method for MSS in Europe (EC, 2008), especially in most industrialized countries (Table 1). Having in mind the strict limitations concerning both sludge landfilling and agricultural reuse, combustion/incineration is expected to play a key role in the long term (Malerius and Werther, 2003). Modern fluidized bed incinerators have become more and more attractive both in terms of capital as well as operating costs, in comparison to the conventional multiple hearth type (Bartolo et al., 1997). The advantages of incineration can be summarized as: Large reduction of sludge volume. Thermal destruction of pathogens and odors minimization. Recovery of renewable energy. The drawback of incineration is that it is the route used for sludge minimization rather than for a complete disposal, since 30 wt% of the dry solids remain finally as ash. Combustion ash is a potential hazardous waste due to its content of heavy metals. Additional expenses are thus required for ash handling and disposal (Barbosa et al., 2009), although there are opportunities for ash utilization in the production of construction materials (Malerius and Werther, 2003). Another major constraint in the widespread use of incineration is the public concern about possible harmful emissions. However,
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M.C. Samolada, A.A. Zabaniotou / Waste Management 34 (2014) 411–420 Table 3 Greek MSS characteristics and limits for use in agriculture.
a b c
Greek MSSa
Dir 86/278/EOK
MOb 80568/4225/91
Update of MO 80568/4225/91c
Physicochemical characteristics N (% TS) P (% TS) TS (%) VS (% TS) Pathogens (CFU/g dry solid)
2.4–11.0 0.9–8.8 14.0–49.930.0–69.9 47.0–11,997.0
– – – – –
– – – – –
18.0
Heavy metals (mg/kg dry solids) Ni Zn Cr Cd Cu Pb Hg
20–228 618–4140 21–981 0.8–4.1 76–580 83–450 No limit value
300–400 2500–4000 No limit value 20–40 1000–1750 750–1200 No limit value
300–400 2500–4000 No limit value 20–40 1000–1750 750–1200 No limit value
200 2500 500 5 800 500 5
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Comparative assessment of municipal sewage sludge incineration, gasification and pyrolysis for a sustainable sludge-to-energy management in Greece M.C. Samolada b, A.A. Zabaniotou a,⇑ a b
Aristotle University of Thessaloniki, Dept. of Chemical Engineering, University Box 455, University Campus, 541 24 Thessaloniki, Greece Dept. Secretariat of Environmental and Urban Planning – Decentralized Area Macedonian Thrace, Taki Oikonomidi 1, 54008 Thessaloniki, Greece
a r t i c l e
i n f o
Article history: Received 14 February 2013 Accepted 4 November 2013 Available online 27 November 2013 Keywords: Municipal sewage sludge Pyrolysis Gasification Incineration SWOT Legislation Sustainable management
a b s t r a c t For a sustainable municipal sewage sludge management, not only the available technology, but also other parameters, such as policy regulations and socio-economic issues should be taken in account. In this study, the current status of both European and Greek Legislation on waste management, with a special insight in municipal sewage sludge, is presented. A SWOT analysis was further developed for comparison of pyrolysis with incineration and gasification and results are presented. Pyrolysis seems to be the optimal thermochemical treatment option compared to incineration and gasification. Sewage sludge pyrolysis is favorable for energy savings, material recovery and high added materials production, providing a ‘zero waste’ solution. Finally, identification of challenges and barriers for sewage sludge pyrolysis deployment in Greece was investigated. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Municipal sewage sludge (MSS) disposal faces significant environmental problems related to air emissions, threat to public health and contamination of soil and water resources, requiring therefore an appropriate treatment and careful management (Aggelakis et al., 2005). While world’s sludge production is on a relentless growth curve, environmental quality requirements for sludge are becoming increasingly stringent, disposal outlets are decreasing and economic pressures require low-cost solutions (EU, 2012). The amount of MSS production in the EU27 was estimated at 11.5 million tons of dry solids for 2010 and it is expected to rise to 13.0 million tons in 2020 (EC, 2008). Disposing sewage sludge to landfills is considered a beneficial use only when such disposal includes methane recovery for energy production. However, methane operations are relatively rare in most existing WWTPs (Waste Water Treatment Plants). However, due to o the limited capacity of available landfills, alternative beneficial uses are receiving greater attention. Valorisation of the nutrient components of MSS by considering the soil conditioning and fertilization is a beneficial use of sludge, especially in the case of forests and energy crops (Meeroff and Bloetscher, 1999; Wang
⇑ Corresponding author. Tel.: +30 2310 99 62 74; fax: +30 2310 996209. E-mail address: [email protected] (A.A. Zabaniotou). 0956-053X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wasman.2013.11.003
et al., 2008; O’Connor, 1996). However, use of sludge on land in the EU will not change dramatically in the future due to legislative restrictions. The proportion of recycled sludge to agriculture will remain almost constant across EU (42% in 2010) expecting to reach 44% in 2020 (Kelessidis and Stasinakis, 2012) as shown in Table 1. Looking forward to adoption of an efficient municipal sewage sludge management, to energy recovery should be considered by thermal routes. Although, high cost of power generated from sludge is a major barrier for the implementation of thermal routes, however, investments for sludge to power can become attractive if one considers the increase of energy prices in the international market by 20%. Thermal treatment methods include combustion/incineration and the ‘advanced’ or ‘emerging’ pyrolysis and gasification methods. The incineration share will raise slightly, (EC, 2008). Dewatered MSS has been successfully used for producing building materials (e.g. concrete, bituminous mixtures) and also in road construction (Aziz and Koe, 1990; Tay and Show, 1991; Anderson et al., 1996). Incineration ash residues can be used to produce road construction materials or concrete aggregates (Takeda et al., 1989; Lisk, 1989). MSS can be extensively used in cement manufacturing as a cheap alternative energy resource (Fytili and Zabaniotou, 2008) with substantial energy and environmental savings due to reduced CO2 emissions. However, selection of a particular, stand-alone sludge thermal treatment system should not be based primarily on technical insight, but it should also integrate all related social and
412
M.C. Samolada, A.A. Zabaniotou / Waste Management 34 (2014) 411–420
Table 1 Annual MSS production, disposal routes in EU27 countries (EC, 2008).
a b
Member state
Sludgea (Ktds/a)
Recyclea (%)
Incin/a (%)
Landfilla (%)
Othera (%)
Sludgeb (tds/a)
Recycleb (%)
Incinb (%)
Landfillb (%)
Otherb (%)
Greece Bulgaria Ireland Cyprus Latvia Estonia Lithuania Finland Malta Luxemburg Hungary Poland Romania Slovakia Spain France Italy UK Chech Rep. Slovenia Portugal Austria Denmark Germany Belgium Netherlands EU27 total
260 47 135 10 30 33 80 155 10 10 175 520 165 55 1,280 1,300 1,500 1,640 260 25 420 273 140 2,000 170 560 11,564
5 50 75 50 30 15 30 5 0 90 75 40 0 50 65 65 25 70 55 5 50 15 50 30 10 0 42
0 0 0 0 0 0 0 0 0 5 5 5 5 5 10 15 20 20 25 25 30 40 45 50 90 100 27
95 30 15 40 40 – 5 – 100 – 10 45 95 5 20 5 25 – 10 40 20 >1 – 0 – – 14
20 10 10 30 85 65 95 – 5 5 10 – 40 5 15 30 10 25 30 – 45 5 20 – – 16
260 151 135 17.62 50 33 80 155 10 10 200 950 520 135 1,280 1,400 1,500 1,640 260 50 750 280 140 2,000 170 560 13,047
5 60 75 50 30 15 55 5 10 80 60 25 20 50 70 75 35 65 75 15 50 5 50 25 10 – 44
40 10 10 10 10 0 15 0 0 20 30 10 10 40 25 15 30 25 20 70 40 85 45 50 90 100 32
55 10 5 30 30 – 5 – –0 – 5 20 30 5 5 5 5 – 5 10 5 >1 –
10 20 10 10 30 85 25 95 – – 5 45 40 5 – 5 30 10 5 5
– – 7
10 5 25 – – 16
Reflects statistical data for 2010. Reflects to predictions for 2020.
environmental activities. The ‘‘sludge-to-energy’’ approach is feasible with substantial benefits similar of those that any renewable energy source presents: decreasing the energy dependency of the WWTP and greenhouse gas emissions. Sludge-to-energy is technically feasible if the recovered energy could be directly used for operating the WWTP, resulting in reduction of conventional electricity consumption (Manara and Zabaniotou, 2012). Another approach called ‘‘sludge-to-fuel’’ (STF) involves a process that converts the organic matter of sludge into a combustion oil using a solvent, under atmospheric pressure and mild temperatures in the range of 200–300 °C (Millot et al., 1989) or alternatively, at high pressures (10 MPa) combined with high temperatures (Itoh et al., 1994). The produced oil is characterised by a high heating value (90% of common diesel fuel) and can be sold to offsite users or refineries (Hun, 1998). In Greece, according to recent statistical data (YPEKA, 2010), the current use of sludge in agriculture is very limited (0.15%) and estimations predict an increase up to 5% by 2020, as shown in Table 1. Due to the absence of established limits concerning water and pathogens content of MSS, local farmers are opposed and skeptical about the extensive use of sludge in agriculture. (Aggelakis et al., 2005). MSS use in agriculture is limited due to another important reason which is related to the use of CaO for the destruction of the pathogens which in addition to the destruction has a parallel negative impact on soil composition and fertility. In Greece, MSS is mostly used as an alternative fuel in existing cement kilns. Composting is particularly encouraged in Greece, reaching the share of 45.78% (YPEKA, 2010) as it is shown in Table 1. In this study, the selection of the most promising sewage sludge-to-energy method, that meets the goal for a ‘‘sustainable development’’, was attempted. The aim of the present study was the comparative assessment of 3 energy recovery options (incineration, pyrolysis, gasification) for municipal sewage sludge (MSS) in Greece through SWOT analysis, taking into account their current status of development. SWOT analysis tool, initially invented by Albert S. Humphrey, designed to be used in the preliminary stages
of decision-making; technologies and methods are compared in the base of economic, environmental and social metrics (Siomkos, 2004; Samolada and Zabaniotou, 2012). It was selected for application, since it has been proved to be a useful planning tool to understand the Strengths, Weaknesses, Opportunities and Threats of both processes and plans (UNEP 2009; Siomkos, 2004). 2. Legislation and sustainable integrated MSS management Municipal sewage sludge (MSS) is defined as the final solid residue produced during municipal waste water treatment. It is classified as a solid waste with the code of 19 08 05 according to the European Catalogue of Wastes (EEL 47/16-2-2001; Directive 2000/532/EK). MSS is also considered as a ‘‘specific stream’’ of non-dangerous solid wastes, which has to be treated according to a National Strategic Approach [Ministerial Order 50910/2003] with the objective of landfills minimization. MSS environmental management has to meet all the basic principles of the Wastes Framework Directive applied since December 12th 2010 [EU, Directive 2008/98/EC]. The European Catalogue of Wastes was introduced in the Greek law by the Ministerial Order 50910/2003 along with the Producers Responsibility (PR) principle according to which the ‘‘waste producer’’ is responsible for its effective and environmental discharge. [Presidential Decree 148/2009]. The Sewage Sludge Directive 86/ 278/EEC which was adopted with the Ministerial Order 80568/ 4225/1991 in the Greek law, seeks to encourage the use of sewage sludge in agriculture and to regulate its use in such a way as to prevent harmful effects on soil, vegetation, animals and man. To this end, it prohibits the use of untreated sludge on agricultural land unless it is injected or incorporated into the soil. MSS has been utilized in agricultural applications for several years, while it is restricted to prevent health risks to humans and livestock due to potentially toxic components, heavy metals, pathogens, and persistent organic pollutants and to the high amounts of soluble salts, which may affect the soil properties negatively. The presence of human pathogens in sewage sludge has led
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to a considerable amount of research to assess the health risks associated with the land applications of sludge. Significant environmental or health risks linked to the use of sewage sludge on land in the EU have not been widely demonstrated in the recent scientific literature since the 86/278/EEC Directive has taken effect, although there continue to be authoritative studies that identify and assess concerns. It is difficult to establish if the lack of evidence for adverse effects is because the provisions of the Directive are sufficient or is due to the more stringent national requirements in most Member States and Greece as well (EC, 2008). Epidemiological and risk assessment studies on the risks to health from microbial pathogens in sewage sludge for workers and populations in the vicinity of sludge operations have not generally found the risks to be significantly greater than background risks (Tanner, 2008; Kanakari, 2009). Overall health risks from the indirect exposure to pathogens have also been found to be low, with no clearly identified public infections from the use of food grown on land where sludge was applied in accordance with the provisions in the Directive (Gale et al., 2003). Environmental issues related to the recycling of sewage sludge on land include the risk of nutrient leaching, impact on soil biodiversity and greenhouse gas emissions (e.g. CH4 and N2O). In global warming potential (GWP) assessments of the different treatment, recycling or disposal routes, efficient treatment and recycling to agricultural land can usually be demonstrated to have a lower GWP effect compared to other disposal processes. There are some local circumstances, such as the location of the land or the nature of the sludge, in which the overall environmental impacts, either in terms of greenhouse gas emissions alone or in conjunction with other environmental factors, result in assessments that suggest non-agricultural routes may be more beneficial (EC, 2008). Treated sludge is defined as having undergone biological, chemical or heat treatment, long-term storage or any other appropriate process so as significantly to reduce its fermentability and the health hazards resulting from its use. The Directive also requires that sludge should be used in such a way that account is taken of the plants’ nutrient requirements of and that the quality of the soil and of the surface and groundwater is not impaired. It sets out requirements for keeping detailed records of the quantities of sludge produced, the quantities used in agriculture, the composition and properties of the sludge, the type of treatment and the sites where the sludge is used. Limits for heavy metals concentrations in both MSS and sludge-treated soils are introduced. Availability of land is an important issue to be taken in account for the selection of the appropriate strategy in sludge management. However, taken in account that lack of the available land space in combination with increasingly stringent regulations governing the design and operation of new landfills (EU Landfill Directive 99/31), have caused the construction and operating cost of new landfills to rise sharply. In addition, the increasingly restrictive targets for the continuous reduction of biodegradable wastes sent to landfills make land application of MSS an unattractive disposal option. The current global climate change drives societies to think about more sustainable ways of using resources and waste management (Zaman, 2009). MSS management should be greatly related to economic, environmental and social aspects in order to meet the goal of sustainable management (WCED, 1987). Decision makers should combine in an optimum way the available alternative MSS handling routes considering all available information on technical, economic and environmental issues. The selection of the most appropriate sludge treatment technology is a key factor in the application of integrated sewage sludge management (ISSM) system. In combination with economic and social considerations, this approach would help sludge managers to design more sustainable management systems (UNEP, 2009). MSS characteristics are the most important parameter to be taken
in account for the selection of the appropriate technology. A plausible solution in wastewater management should include collective management of sewage sludge and implementation of the 3R (Reduce, Reuse and Recycle) policies and strategies. In Fig. 1, the ISSM system is depicted (UNEP, 2009). Both European and Greek policy promotes including energy recovery from non-dangerous solid wastes and MSS as well. Integrated sewage sludge management (ISSM) refers to a strategic initiative for the sustained management of MSS through the use of comprehensive integrated format generated through sustained preventive approach to the complementary use of a variety of practices to handle sewage sludge in a safe and effective manner (UNEP, 2009). ISSM is based on the concept that all aspects of the sludge management systems (technical and non-technical) should be analyzed together, since they are in fact interrelated and developments in one area frequently affect practices or activities in another area. ISSM proposes to take a comprehensive approach across all types of sewage sludge streams and involves the use of a range of different options.
3. Materials and methods 3.1. Sludge composition and important characteristics Sewage sludge is a complex heterogeneous mixture of microorganisms, undigested organics such as paper, plant residues, oils, or Cities are facing an increasing growth in population resulting in increased amounts of wastewater and MSS production
Complexity, costs and
Industrialization
coordination of
and economic
wastewater management
growth has
NEED
generated increased
multi stakeholder
FOR
amounts of
involvement in every
ISSM
wastewater
stage of the waste stream
Wastewater treatment towards environmental protection and
Local Governments are now looking at wastewater as a business opportunity
sustainability
Fig. 1. Scheme of sludge integrated management (ISSM).
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fecal material, inorganic materials and moisture. The undigested organic materials contain a highly complex mixture of molecules coming from proteins and peptides, lipids, polysaccharides, plant macromolecules with phenolic structures (e.g. lignins or tannins) or aliphatic structures (e.g. cutins or suberins), along with organic micropollutants such as polycyclic aromatic hydrocarbons or dibenzofurans (Fonts et al., 2012). The inorganic materials present in the liquids come mainly from soil but also from synthetic polymers of anthropogenic origin. Sewage sludge has considerably higher nitrogen content. The nitrogen in sewage sludge comes mainly from the protein fraction of this material, which has its origin in the microorganisms used for water purification. Typical composition of untreated and digested sludge is reported in Table 2. The amount of sludge produced is affected in a limited scale by the treatment efficiency, while the sludge quality is strongly dependent on the original pollution load of the treated effluent and also, on the technical and design features of the WWTP process (Fytili and Zabaniotou, 2008; UNEP, 2009). An important issue in sludge treatment is the accumulation of most heavy metals of the original wastewater. Their high concentration may inhibit MSS’ use in agriculture without further treatment (Hsiau and Lo, 1998; EU, 2012). The ash from sewage sludge contains mainly minerals such as quartz, calcite or microline. These minerals are formed by elements such as Fe, Ca, K and Mg that can catalyze some pyrolysis reactions. Furthermore, some heavy metals can also be found in the sludge (Cr, Ni, Cu, Zn, Pb, Cd, Hg).Heavy metals such as zinc (Zn), copper (Cu), nickel (Ni), cadmium (Cd), lead (Pb), mercury (Hg) and chromium (Cr) are mostly met in MSS. Their potential accumulation in human tissues and biomagnifications through the food-chain create both human health and environmental concerns (Krogmann et al., 1999; Kanakari, 2009). The concentration of heavy metals in Greek MSS is below the limits that were set by EU and Greek legislation (Table 3). 3.2. Sludge-to-energy processes Potential advantages of thermal processes include reduction of volume and weight, destruction of toxic organic compounds, and recovery of energy, but economics need to be carefully evaluated. Incineration involves the complete oxidation of the volatile matter Table 2 Typical characteristics of sludge originating from various treatment methods (Manara and Zabaniotou, 2012). Characteristic
A
B1
B2
C
D
Dry matter (DM), % g/l Volatile matte (VM), % DM pH C, % VM H, % VM O, % VM N, % VM S, % VM C/N P, % DM Cl, % DM K, % DM Al, % DM Ca, % DM Fe, % DM Mg, % DM Fat, % DM Protein, % DM Fibres, % DM Calorific value, kW h/t DM
12 65 6 51.5 7 35.5 4.5 1.5 11.4 2 0.8 0.3 0.2 10 2 0.6 18 24 16 4200
9 67 7 52.5 6 33 7.5 1 7 2 0.8 0.3 0.2 10 2 0.6 8 36 17 4100
7 77 7 53 6.7 33 6.3 1 8.7 2 0.8 0.3 0.2 10 2 0.6 10 34 10 4800
10 72 6.5 51 7.4 33 7.1 1.5 7.2 2 0.8 0.3 0.2 10 2 0.6 14 30 13 4600
30 50 7 49 7.7 35 6.2 2.1 7.9 2 0.8 0.3 0.2 10 2 0.6 10 18 10 3000
A: primary sludge, primary sludge with physical/chemical treatment or high pollution load; B1: biological sludge (low load); B2: biological sludge from clarified water (low and middle load); C: mixed sludge (mix A and B2 types); D: digested sludge.
and the production of an inert residue (ash). Recent extensive reviews (Fytili and Zabaniotou, 2008; Manara and Zabaniotou, 2012) of the current literature on the effective sludge valorisation considered various available technologies including: anaerobic digestion, incineration, pyrolysis, gasification and wet oxidation. Thermochemical technologies found to be promising alternative valorisation routes of sewage sludge considering the decreasing availability and the increasing price of land for landfills. The principal goal of thermal processing of sewage sludge is the utilization of its energy content with minimizing the related environmental impact to meet the now increasingly stringent standards. It is well known that MSS is characterized by considerably high water (P50 wt%) content, thus consuming most of the thermal process energy (Aggelakis et al., 2005; Dennis et al., 2005; Fytili and Zabaniotou, 2008; Werle and Wilk, 2010; Manara and Zabaniotou, 2012). Thus, dewatered/dried prior to thermal processing is needed. However, the consequence of dewatering/drying prior to thermal processing is on energy balances and increasing the costs although, thermal valorisation processes are generally considered to be as energy self-sufficient (Khiari et al., 2004; Fytili and Zabaniotou, 2008; Manara and Zabaniotou, 2012). 3.2.1. Combustion/incineration Combustion is the currently used thermal treatment method for sludge energetic valorisation. The amount of sludge being incinerated in Denmark has already reached the percentage of 24% of the sludge produced, France the 20%, Belgium the 15%, Germany the 14% while in the USA and Japan the percentage has reached the 25% and 55% respectively (Lundin et al., 2004). Wet or dry sludge combustion (with a 41–65 wt% content of dry material) can be effectively introduced in fluid bed combustion reactors (Fytili and Zabaniotou, 2008). In most MSS thermal processes, a partial dehydration (85% dry matter) or a total dehydration (>85% dry matter) takes usually place (Oleszkiewicz and Reimers, 1998). Dry sewage sludge is characterized by a calorific value of 12–20 MJ/kg, lower than that of coal (14.6–26.7 MJ/kg) (Manara and Zabaniotou, 2012) but equivalent to that of lignite (11.7–15.8) MJ/kg (Samolada and Zabaniotou, 2012). Incineration technology is the controlled combustion of waste with the recovery of heat to produce steam that in turn produces power through steam turbines Combustion/incineration still remains the most attractive disposal method for MSS in Europe (EC, 2008), especially in most industrialized countries (Table 1). Having in mind the strict limitations concerning both sludge landfilling and agricultural reuse, combustion/incineration is expected to play a key role in the long term (Malerius and Werther, 2003). Modern fluidized bed incinerators have become more and more attractive both in terms of capital as well as operating costs, in comparison to the conventional multiple hearth type (Bartolo et al., 1997). The advantages of incineration can be summarized as: Large reduction of sludge volume. Thermal destruction of pathogens and odors minimization. Recovery of renewable energy. The drawback of incineration is that it is the route used for sludge minimization rather than for a complete disposal, since 30 wt% of the dry solids remain finally as ash. Combustion ash is a potential hazardous waste due to its content of heavy metals. Additional expenses are thus required for ash handling and disposal (Barbosa et al., 2009), although there are opportunities for ash utilization in the production of construction materials (Malerius and Werther, 2003). Another major constraint in the widespread use of incineration is the public concern about possible harmful emissions. However,
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a b c
Greek MSSa
Dir 86/278/EOK
MOb 80568/4225/91
Update of MO 80568/4225/91c
Physicochemical characteristics N (% TS) P (% TS) TS (%) VS (% TS) Pathogens (CFU/g dry solid)
2.4–11.0 0.9–8.8 14.0–49.930.0–69.9 47.0–11,997.0
– – – – –
– – – – –
18.0
Heavy metals (mg/kg dry solids) Ni Zn Cr Cd Cu Pb Hg
20–228 618–4140 21–981 0.8–4.1 76–580 83–450 No limit value
300–400 2500–4000 No limit value 20–40 1000–1750 750–1200 No limit value
300–400 2500–4000 No limit value 20–40 1000–1750 750–1200 No limit value
200 2500 500 5 800 500 5
