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Improving material and energy recovery from the sewage sludge and biomass residues
Waste Management xxx (2014) xxx–xxx
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Improving material and energy recovery from the sewage sludge and biomass residues Irina Kliopova ⇑, Kristina Makarskiene˙ Kaunas University of Technology (KTU), Institute of Environmental Engineering (APINI), Lithuania1
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted 29 October 2014 Available online xxxx Keywords: AT: Biomass residues Fertiliser Pre-composting Sewage sludge Solid recovered fuel
a b s t r a c t Sewage sludge management is a big problem all over the world because of its large quantities and harmful impact on the environment. Energy conversion through fermentation, compost production from treated sludge for agriculture, especially for growing energetic plants, and treated sludge use for soil remediation are widely used alternatives of sewage sludge management. Recently, in many EU countries the popularity of these methods has decreased due to the sewage sludge content (heavy metals, organic pollutions and other hazards materials). This paper presents research results where the possibility of solid recovered fuel (SRF) production from the separate fraction (10–40 mm) of pre-composted materials – sewage sludge from municipal waste water treatment plant and biomass residues has been evaluated. The remaining fractions of pre-composted materials can be successfully used for compost or fertiliser production, as the concentration of heavy metals in the analysed composition is reduced in comparison with sewage sludge. During the experiment presented in this paper the volume of analysed biodegradable waste was reduced by 96%: about 20% of input biodegradable waste was recovered to SRF in the form of pellets with 14.25 MJ kg1 of the net calorific value, about 23% were composted, the rest – evaporated and discharged in a wastewater. The methods of material-energy balances and comparison analysis of experiment data have been chosen for the environmental impact assessment of this biodegradable waste management alternative. Results of the efficiency of energy recovery from sewage sludge by SRF production and burning, comparison analysis with widely used bio-fuel–sawdust and conclusions made are presented. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The amount of sewage sludge differentiates significantly among various countries in the European Union (EU). According to the data of Eurostat and the Lithuanian National Strategic Waste Management Plan (2014–2020), the minimal amount of sewage sludge per year is generated in Luxemburg – about 10 thousand tonnes, the maximal amount in Germany – about 1050 thousand tonnes year1, in Lithuania – approx. 50 thousand tonnes year1. Undoubtedly these amounts of sewage sludge bear direct relation to the population. In the case of relative indicators, the situation changes: minimal amount in Germany – 12.8 kg per capita, in Luxembourg – 19.9 kg per capita, in Lithuania – 16.6 kg per capita. According to the data of Eurostat, 21% of generated sewage sludge
⇑ Corresponding author at: Kaunas, K. Donelaicio str. 20, LT-44239, Lithuania. Tel./fax: +370 5 2649174. E-mail address: [email protected] (I. Kliopova). 1 http://ktu.edu/apini/en/.
was landfilled, 10% – incinerated, 45% – applied to land and remaining 24% – treated in other ways in the EU-27 in 2010. Unfortunately, so far in Lithuania only 40% of sewage sludge is processed by means of anaerobic or aerobic treatment methods, the rest of it is dewatered and exported, landfilled, or accumulated in the special sites – polygons. Besides, the treated and dewatered sewage sludge is stored too. At present, in Lithuania about 200 thousand tonnes of old sewage sludge is stored in special sites. In the EU-27 the amount of sewage sludge has a growing tendency due to the development of wastewater collection systems, particularly in urban areas, and to improvement of wastewater treatment technologies. Actually, the quality of sewage sludge is improving because of more strict requirements for the supply of hazardous substances into a sewerage collection system and the better pollution control. According to the Lithuanian National Strategic Waste Management Plan (2014–2020), sewage sludge accumulation in polygons has to be discontinued by 2015. Therefore, the optimisation of material and energy recovery from the sewage sludge for
http://dx.doi.org/10.1016/j.wasman.2014.10.030 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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minimising the waste volume with minimal environmental costs is an issue of great importance. According to Article 14 of the Council Directive 98/15/EEC amending Directive 91/271/EEC concerning the urban wastewater treatment ‘‘sludge arising from wastewater treatment shall be re-used whenever appropriate’’. Sewage sludge contains the components of agricultural value e.g., organic matter, the nutrients such as nitrogen, phosphorus, potassium, and lower concentrations of calcium, sulphur and magnesium, which make the sludge interesting as fertiliser, but heavy metals, toxins, viruses, pharmaceuticals and hormones, GMOs and dioxins are also present in it (Grob et al., 2008). Several authors in the field of this research have stated that the concentrations of total heavy metals can be reduced by composting sewage sludge with biomass (Sabiene et al., 2008; Obernberger et al., 2009; Albrecht et al., 2010; Cukjati et al., 2012). Sewage sludge has a low C:N ratio (5–10:1). The composting process begins when the C:N ratio is 20–30:1 (optimal ratio). Therefore, biodegradable waste, which is rich in carbon (e.g.in biomass residues when the C:N ratio is 40–750:1), is to be mixed to improve the analysed ratio (Albrecht et a1., 2010, Cukjati et al., 2012; te˙, 2013). There are other positive aspects Kliopova and Stankevicˇiu of this mixing, such as minimisation of the moisture content from 70–90% to optimal 50–60% maintaining the optimal oxygen content (15–20%). If the moisture content is higher than 70%, anaerobic process begins. The recommended mixing ratio of sewage sludge and biomass depends on the C:N ratio of bulking agents, for example, when the sawdust and aerobic sludge mixture is of 1:1 proportion. However in the case of more problematic sludge, such as the anaerobic sludge mixture studied, with higher conductivity and the presence of other toxic and phytotoxic substances, a 1:3 proportion is to be recommended because of the dilution effect on harmful parameters (Banegas et al., 2007). The EU Sewage Sludge Directive 86/278/EEC and the Lithuanian normative document LAND 20:2005 Requirements for Sewage Sludge Use for Fertilisation and Remediation prohibit the use of untreated sewage sludge on agricultural land, unless it is injected or incorporated into the soil. This EU Directive also sets the limit values for heavy metals and organic compounds to avoid bioaccumulation in plants and animals, and likewise the use of sewage sludge in agriculture is prohibited where the concentrations of heavy metals in the soil exceed both the indicated limits and the limits of sewage sludge load quantity (kg ha1 year1). For this reason application of sewage sludge and sewage sludge compost to agriculture is being decreased (kg ha1 year1), i.e. the existing soil contamination is hindering its usage. Sewage sludge can be employed as a renewable energy source, since it is generated in large amounts and has considerable energy content (Cao and Pawlowski, 2013). Due to high moisture content in primary sewage sludge its net calorific value is significantly lower, e.g. when moisture content is about 80%, its net calorific value is less than 1.5 MJ kg1, when sewage sludge dries up to 5– 6% of the moisture content, the net calorific value is increased up to 12–13 MJ kg1 (Cukjati et al., 2012; Urciuolo et al., 2012; Magdziarz and Wilk, 2013). Rather big ash content (from 35% to 45%) is one of the negative characteristics of sewage sludge minimizing its usage as a fuel (Cukjati et al., 2012). Volume and mass reduction, thermal destruction of toxic organics pollutions and energy recovery from organic matter are the main advantages of thermal processes (Ye et al., 2012; Magdziarz and Werle, 2014). There are fermentation and several thermal technologies such as pyrolysis, gasification, combustion and cocombustion for energy conversion from sewage sludge. Pyrolysis and gasification are rather complex and costly technologies. Some authors propose the technique of hydrothermal conversion, which involves the application of heat and pressure to treat biomass in
the aqueous medium. This method was evaluated as an effective means for energy conversion from moist biomass without prior drying (Yu et al., 2012). Combustion is a very simple technique for energy recovery from sewage sludge. In this treatment high water content in sewage sludge (70–90%) is fairly big deficiency as great quantities of energy have to be used for mass drying before heat conversion. To solve this problem the co-combustion with municipal solid waste, forest biomass and coal is recommended since the process can generate sufficient heat energy for drying sludge without additional usage of fossil fuel (Werther and Ogada, 1999; Xiao et al., 2010). In addition, sewage sludge is suitable for production of solid recovered fuel (SRF) by mixing it together with the other properly selected waste, such as under-grade-sized coal, waste from animal waste utilisation plants, wood waste (Wzorek, 2012). SRF is a solid fuel produced out of non-hazardous waste in order to be utilised for energy recovery in incineration and co-incineration plants. Properly processed, homogenised and up-graded to a certain quality SRF has to satisfy the classification and specification requirements presented in CEN/TR 15359:2011 (European Committee for Standardization, 2011). Fossil CO2 emissions decrease when renewable secondary fuel is co-combusted. This is one of the primary goals and achievements of co-combustion of biomass (European Commission (2006)). The assessment of the possibilities of producing SRF from the pre-composting of input materials, stabilised sewage sludge, municipal green waste (grass, branches, etc.) and bark is one of the goals of the PF7 program project ‘‘Polygeneration of energy, fuels and fertilisers from biomass residues and sewage sludge (ENERCOM)’’ (No TREN/FP7/EN/218916). In this case the pre-composting process is applying of a sewage sludge bio drying method with minimal energy demand. Results of the laboratory analysis show that 10–40 mm fraction (approx. 45% of the total amount) of pre-composted biodegradable waste (sewage sludge, green waste and biomass residuals) can be used for SRF production, <10 mm – for compost production (Obernberger et al., 2009; Kliopova and Makarskiene˙, 2013). It is evaluated that in this case 82% of biodegradable waste is convertible to energy and compost; the other mass becomes waste (ash, bottom ash and remains). The developed methodology of SRF production was applied by the compost production company Soil-Concept S.A. in Luxembourg. Depending on the mixing ratio, a net calorific value of produced SRF in pellets containing 10–15% of moisture amounts to 13–14 MJ kg1. The efficiency of energy recovery of SRF production and combustion is sufficiently high – up to 0.76 (Kliopova and Makarskiene˙, 2013). For example, the minimal energy recovery efficiency from municipal solid waste incineration facilities has to be 0.65 (Quiroga et al., 2010). Results of the environmental impact assessment of produced SRF burning show that in case of a suitable choice of SRF combustion technologies, the concentrations of air emissions do not exceed the maximal air emission limit volume according to the requirement of the Directive 2000/76/EC of the European Parliament and of the Council of December 4, 2000 on the incineration of waste (Kliopova and Makarskiene˙, 2013). The evaluation of the possibilities of a new method for the sewage sludge management was a primary objective of the research presented in this paper. Sewage sludge samples are taken from an ordinary municipal wastewater treatment plant in Lithuania (pilot plant). Generated sewage sludge is dewatered up to 20% of dry matter by mechanical press and accumulated in the special storage sites in the pilot company. An average amount of sewage sludge is 1500 tonnes (with 75–80% of the moisture content). Currently, a big amount of sewage sludge (about 9 thousand tonnes) has been collected in the storage sites. The environmental audit determines that sewage sludge can be treated by means of two
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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methods: aerobic treatment (composting) and SRF production from separate fraction (approx. 45% of the total amount) of precomposted materials. Such biomass residues as green waste generated in public territories (leaves, straw, hay, cutting waste, inc. a small amount of sawdust, etc.) with about 40% of the moisture content have been used for upgrading both technical conditions of the composting process and the characteristics of produced SRF. The results of the ENERCOM project were used for improving biodegradable waste management and process optimisation. 2. Materials and methods The experiment was carried out in the above-mentioned pilot plant. Sewage sludge with 75–80% of the moisture content was pre-composted with the biomass residues (leaves, straw, hay, cutting waste, inc. a small amount of sawdust, etc.) with about 40% of the moisture content. Traditional windrow composting method was applied. Mixing ratio of 1:1 was chosen for the increasing of C:N ratio of sewage sludge (from 7:1 up to 25–30:1). According to the results of the ENERCOM project, the pre-composting materials were screened by fractions: <10 mm, 10–40 mm, and >40 mm after 3 weeks from the start of the experiment (Kliopova and Makarskiene˙, 2013). The separated 10–40 mm fraction was used for SRF production and further analysis. The remaining fractions were used for compost for energetic plants production. Main steps of the feasible analysis were: – Determination of heavy metals concentrations in sewage sludge from the pilot plant. – Formation of material and energy balance of compost and SRF production from sewage sludge and biomass residues. – Determination of physical and chemical characteristics of the 10–40 mm fraction of pre-composted sewage sludge and biomass residues. – Classification of the produced SRF according to the Solid Recovered Fuel Classificatory CEN/TC 343 (European Committee for Standardization, 2011). – Formation of the fuel-energy balance of SRF production and combustion for 100 MWh of the heat energy production. Heavy metals concentrations in sewage sludge of the pilot plant and other chemical characteristics of pre-composted materials have been determined in the laboratory of Agrochemical Research in the Centre of Lithuanian Agrarian and Forest Science using: Multiwave sample digestion system (Multiwave 3000) and Microwave-Assisted Synthesis (Synthos 3000) for sample preparation. Mercury Analyser (Perkin Elmer FIMS 100) for evaluation of Hg content. Elemental analysis equipment (Elementar Vario EL cube (modification: CNS)) for C, N, S tests. Graphite Furnace Atomic Absorption Spectrometry (PE (Perkin Elmer) AAnalyst 600 (2008), GF-AAS) for Cd, Pb tests. Flame atomic absorption (AA) spectrophotometer system (AAnalyst 200 (2008), Perkin Elmer, USA) with computer software (WinLab 32 TM for ICP, V3.3.1) and with automatics sample input equipment (Perkin Elmer (PE)) for determination of other parameters. The method applied to evaluation of calorific value of SRF: dry Q dry net ¼ Q gross 2441 ð9 H=100Þ
Qdry net
ð1Þ 1
Qdry gross
where is the net calorific value in dry matter, kJ kg ; is higher calorific value (gross) in dry matter was measures with a bomb calorimeter, kJ kg1.
received Q as ¼ ½Q;dry net ð100 WÞ 2441 W=100 net received Qas net
ð2Þ 1
where is the net calorific value in as received, kJ kg ; H is hydrogen content in dry matter, % (H = 6%); W is moisture content, %. Heavy metals concentrations of analysed sewage sludge were compared with the EU average, with the requirements of the EU Sewage Sludge Directive 86/278/EEC, and with the requirements for sewage sludge usage for fertilisation and remediation (Lithuanian normative document LAND 20:2005). The material and energy balance of compost and SRF production from sewage sludge and biomass residues was formed for evaluating relative environmental indicators (EI) i.e. the quantity of one of the input or output flows of these technological processes (e.g. raw material, energy, water, compost, SRF, waste, etc. expressed by a unit per year) for processing one tonne of sewage sludge (tonne1). Physical and chemical characteristics of 10–40 mm fraction from pre-composted sewage sludge and biomass residues were compared with the following data: – Physical and chemical characteristics of sawdust, as a widely used renewable energy source in Lithuania and other EU countries. – Data of the ENERCOM project (Kliopova and Makarskiene˙, 2013). In pilot plant produced SRF was classified by the Solid Recovered Fuel Classificatory CEN/TC 343 (European Committee for Standardization, 2011). The fuel-energy balance of SRF production from the separate fraction (10–40 mm) of pre-composted materials, such as sewage sludge and biomass residues and the produced SRF combustion was formed and compared with sawdust combustion for 100 MW h of the heat energy production. The efficiency of energy recovery was calculated according to the energy balance. The analysis results are presented and discussed in this paper. 3. Results and discussion 3.1. Heavy metals concentrations in sewage sludge The elementary composition of sewage sludge and the content of heavy metals depend on many factors, such as wastewater composition and on sewage sludge treatment methods. Heavy metals concentrations in sewage sludge of the pilot plant are determined and compared with the EU average as well as with the limit values for the usage in agriculture as fertilisers for either growing energetic plants or land remediation (see Table 1). The concentrations of heavy metals in sewage sludge of the pilot plant are significantly lower than the EU average and they meet the requirements of EU Sewage Sludge Directive 86/278/EEC and the Lithuanian normative document LAND 20:2005 about the sewage sludge use for fertilisation and remediation. Therefore, sewage sludge from the pilot plant is suitable for application to the land, i.e. for soil remediation and fertilisation or compost production. 3.2. The material and energy balance of SRF and compost production from sewage sludge and biomass residues Traditional windrow composting (using a turner) in 1.6 m high and 3.6 m wide piles has been used in the experiment. The territory for this aerobic treatment process was covered by the light structured awning. Preparation of composting mixture of sewage sludge and biomass residues with the ratio of 1:1 (1 tonne of sewage sludge with 75–80% of the moisture content with 1 tonne of biomass residuals with about 40% of the moisture content) is the
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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Table 1 Concentrations of heavy metals in sewage sludge. Analysed parameters
Cadmium (Cd) Copper (Cu) Lead (Pb) Nickel (Ni) Chromium (Cr) Hydrargyrum (Hg) Zinc (Zn)
Concentration of heavy metals in sewage sludge, mg kg1 dm Case of the pilot plant
EU average
1–4 10–125 15–24 12–23 18–24 0.2–0.4 800–1000
1.68 377 57 80 150 2 1157
a
Limit concentration, mg kg1 dm 86/278/EEC
LAND 20-2005
20–40 1000–1750 750–1200 300–400 b – 16–25 2500–4000
1.5–20 75–1000 140–750 50–300 140–400 1.0–8.0 300–2500
Comments: a Database (Obernberger et al., 2009). b Each EU country determines its own limit concentration for Cr.
first step of sewage sludge management. Given that the length of used biomass residuals must be less than 5 cm (optimal size – 2.5–5 cm) and thickness should be less than 5 mm, part of the biomass residuals (lop, bark, hay, etc.) was milled by chopper. Aerobic process will not start, if the size of the fraction of biodegradable te˙, 2013). waste is too big (Kliopova and Stankevicˇiu Main parameters of the composting process were taken into account during the technical assessment, namely, the temperature within a pile (more than 70 °C during 1 h or about 55 °C during 2 weeks), optimal volume of the moisture content (50–60%), oxygen content (15–20%), pH volume (6–8). When biodegradable waste is not sufficiently wet, it must be moisturized. When biodegradable waste is too wet, it must be mixed with the dry waste te˙, 2013). The number of turnings (Kliopova and Stankevicˇiu depends on air conditions (temperature, moisture), for example, in case of experiment, 1 time per week during pre-composting period and up to 2 times per week during the further composting process. Main aim is to maintain the above-mentioned parameters of the composting process. During the whole composting process the mass of input materials has decreased twice (e.g. about 80% – due to evaporation and discharged waste water, about 20% – due to losses of dry mate te˙, 2013). During the experiment rials) (Kliopova and Stankevicˇiu the moisture of the mixture was reduced from 57% to 37% after 3 weeks pre-composting. There are two main benefits of the precomposting of sewage sludge and biomass residues: – Reduction of sewage sludge moisture content by approx. 50% with minimal energy (diesel fuel) consumption for a compost turner. – Reduction in mass of input materials by approx. 34%. During the experiment, part of the generated wastewater was reused for further composting process; the other part was sent to the wastewater treatment plant or evaporated. The material and energy balance of processing sewage sludge with biomass residues (pre-composting, screening and further composting processes) is presented in Fig. 1(a). During one-year period 2.500 tonnes of sewage sludge and the same volume of biomass residuals have been processed. Three cycles of composting process were carried out within Lithuanian air conditions (since the end of March to the end of November). In a second step, the pre-composted materials were screened by fractions in a cylindrical separator (a drum screen): <10 mm (about 50% of total amount), 10–40 mm (approx. 40–45% of the total amount), and >40 mm. The separated 10–40 mm fraction of the pre-composted materials was used for SRF production and further analysis. The remaining fractions, inc.>40 mm (to maintain optimal oxygen content), were sent for further composting (inc. maturation).
The required moisture content of the mixture for SRF production in pellets has to be about 12–15%, and the moisture of SRF for efficient combustion has to be 10–15% (Kliopova and Makarskiene˙, 2013; Kwon et al., 2013). Therefore, in the third step, the belt filter press with capacity of 3–5 m3 h1 and 6.6 kW of installed electrical capacity was used for minimisation of the moisture content to 15%. Such method was chosen specially for this pilot plant after feasibility analysis due to two main aspects: (1) the pilot plant does not have its own combustion plant for heat production, (2) electricity savings in consumption to other dewatering and drying technologies. In the case of ENERCOM project thermal drying is carried out in dryer, since the heat energy is produced on own biomass boiler (Kliopova and Makarskiene˙, 2013). The pellets production process was a final technological step of SRF production. The capacity of the press used for pellets is 250 kg h1. Main part of standard technological line of pellets production: mixing equipment, transporter with metal separator, mill, dust aspirator, silo with scraper floor, press feeder, pellet-press, cooler, vibrating sieve, packaging machine (Kliopova and Makarskiene˙, 2013). The material and energy balance of SRF production from 10 to 40 mm fraction of pre-composting materials (dewatering and pelleting processes) is presented in Fig. 1(b). During this experiment about one thousand tonnes of SRF were produced. The formed material and energy balances were used for calculation of relative environmental indicators (see Table 2), which can be used for evaluation of the environmental performance and comparison to other sewage sludge management methods. The total amount of electricity for the processing of 1 tonne of sewage sludge is 67 kW h, total direct air emissions – 8 kg tonne1. The volume of produced SRF makes 20% of the total mass of input raw materials; the volume of produced compost makes approx. 23% of input raw materials. In comparison with fossil fuel (for example, natural gas) CO2 emissions to the air producing heat energy from produced SRF (988.4 tonnes) will be reduced by 642 tonnes year1. 3.3. Physical and chemical characteristics of raw materials and classification of produced SRF Physical and chemical characteristics of the 10–40 mm fraction of pre-composted materials were evaluated in the laboratory (see materials and methods). The results were compared with the data of the ENERCOM project and with the physical and chemical characteristics of sawdust (see Table 3). Characteristics of the pre–composted materials are directly related to the variety of compost composition (green waste, sewage sludge, etc). The net calorific volume of fuel has direct relationship with carbon content (Kliopova and Makarskiene˙, 2013). In the experiment the carbon content was greater than 37%. This result is better than in the case
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Outputs, units year -1
Inputs, units year -1 Sewage sludge (75-80% of the moisture content) 2 500 tonnes Biomass residues (39-40% of the moisture content) 2 500 tonnes Diesel fuel for biomass transportation 0.12 tonnes Diesel fuel for residues milling, composting and screening 2.43 tonnes Water 491 m3 Industrial oil 45 kg
Compost 1 139 tonnes
Precomposting (biological drying); Screening;
10-40 mm fraction (37% of the moisture content) 1 425 tonnes to SRF production >40 mm fraction 141 tonnes Dry matter loss 459 tonnes
Further composting, inc. maturation (fractions: <10 mm, >40mm)
Wastewater, inc. evaporation 1 836 m3 Air emissions from mobile sources 0.62 tonnes Waste 45 kg
(a) SRF (pellets) 988.4 tonnes
Pre-composted 10-40 mm fraction (37% of the moisture content) 1 425 t Electricity for filter press 5.4 MWh Industrial oil 22.5 kg Electricity for pelleting 162.1 MWh
Dewatering (up to 15% of the moisture content);
Matrix 2.4 units
Water evaporation 408 m3 Indirect environmental impact due to electricity consumption, in case of natural gas burning: CO2 – 34.31 tonnes PM emissions (from pelleting and cyclones) 19.6 tonnes
Pelleting Waste (metal and etc.) 20.02 tonnes
Water for filter press washing 500 m3
Wastewater 500 m3
(b) Fig. 1. The material and energy balance of compost production (a) and SRF production (b) from sewage sludge and biomass residues.
Table 2 Relative environmental indicators of sewage sludge processing. Inputs and flows of sewage sludge processing Inputs Sewage sludge Biomass residuesa Fuel (diesel) Electricity Water Industrial oil Outputs Compost SRF Wastewater Evaporated water Air emissions from mobile sources Air emissions from stationary sources (PM filter) Indirect environmental impact – CO2 emissions Waste (metal, from filters, etc.)
Dimension
Relative EI Units tonne1 of sewage sludge
tonnes tonnes tonnes MWh m3 kg
1.000 1.000 0.001 0.067 0.200 0.027
tonnes tonnes m3 m3 kg kg
0.456 0.395 0.934 0.163 0.248 7.800
tonnes
0.0137
tonnes
0.008
a
Biomass residues: green waste generated in public territories (leaves, straw, hay, cutting waste, inc. a small amount of sawdust, etc.).
of the ENERCOM project, but approx. 15% lower in comparison with sawdust. The pre-composting of sewage sludge with biomass residues has one more clear advantage – reduction of ash content in sewage sludge by half. In the case of the experiment, according to the data of an elementary composition of 10–40 mm fraction, the ash content in dry matter was 20.57%; that is approx. 1.5 times lower in
comparison with ENERCOM, but 20 times higher in comparison with sawdust, which is very low (up to 1%). The chlorine content was significantly (approx. 8.6 times) lower in comparison with the data of the ENERCOM project, but twice higher in comparison with sawdust. As it has been mentioned above the metal concentrations in sewage sludge of the pilot plant correspond with the requirements of the Sewage Sludge Directive 86/278/EEC and LAND 20:2005, but according to these legislations, sewage sludge application to land shall be prohibited where the concentrations of heavy metals in the soil exceed the indicated limits and the limits of the sewage sludge load quantity (kg ha1 year1) as well. Therefore, due to composting of sewage sludge with biomass residues the concentrations of heavy metals in produced compost are reduced and the possibility of sewage sludge application to land is expanded. The majority of heavy metals contained in SRF will be removed with ashes (treatment plant, etc.) during SRF combustion. In addition, the amount of ash after SRF combustion is approx. 0.04 tonnes per tonne of composting mixture (sewage sludge and biomass residues). Produced SRF has to be classified by the Solid Recovered Fuel Classificatory CEN/TC 343 (European Committee for Standardization, 2011). The major goal of standard classification is a high level of the environment protection. On the base of SRF class users can quickly assess SRF type and quality: (1) a net calorific value describes its value as a fuel, (2) chlorine contributes to corrosion, thus high chlorine content means a lower market value and (3) mercury, as heavy metal, is selected as an indicator of the environmental quality. Data of the laboratory analysis of SRF were compared with CEN/TC 343 classification (see Table 4). The comparison shows that the produced SRF of pre-composted materials
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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Table 3 Physical and chemical parameters of 10–40 mm fraction of pre-composted materials in comparison with sawdust. Analysed parameters
Ash content Hydrogen (H) Carbon (C) Nitrogen (N) Sulphur (S) Chlorine (Cl) Cadmium (Cd) Copper (Cu) Lead (Pb) Nickel (Ni) Chromium (Cr) Hydrargyrum (Hg) Zinc (Zn) Manganese (Mn) Iron (Fe) Calcium (Ca) Aluminium (Al)
Content of chemicals in dry matter, % Pre-composted materials (ENERCOM)a
Pre-composted materials (the pilot plant)b
Sawdustc
30.770 4.340 36.320 1.860 0.573 0.138 1.000 104 1.2 102 3 103 5 103 9 103 0.12 103 6.5 102 5.2 102 1.070 2.580 1.760
20.570 4.580 37.380 2.040 0.430 1.6 102 1.100 104 0.8 102 1.300 103 0.490 103 0.8 103 0.07 103 4.9 102 4.5 102 0.570 3.630 0.610
1.000 5.400 43.760 0.500 0.042 0.7 102 0.400 104 1.000 103 0.300 103 1 103 1 103 0.02 105 2 103 3 103 0.757 0.470 0.378
a 10–40 mm fraction of pre-composted materials (about 50% of digested and dewatered sewage sludge, about 26% of green waste and other biomass residuals) (Obernberger et al., 2009; Soil-Concept, 2010). b 10–40 mm fraction of pre-composted materials (about 50% of dewatered sewage sludge, about 50% of green waste public territories in municipality). c Sawdust: 30% of deciduous trees sawdust and 70% of softwood (Kliopova et al., 2013).
(fraction – 10–40 mm) corresponds to class 4 by the net calorific value (14.25 MJ kg1), class 1 by the chlorine content in dry matter (0.016%) and class 3 by the mercury content (0.042 mg MJ1).
Table 4 Comparison of SRF produced from pre-composted materials with the Solid Recovered Fuel classification system (CEN/TC 343). SRF form
Net calorific value as receives, MJ kg1a Chlorine (Cl) content in dry matter, % Hydrargyrum (Hg) content, mg MJ1 (median) a
3.4. The efficiency of energy recovery
SRF of pre-composted materials (10–40 mm fraction) of the pilot plant Value Pellets
Class
14.25 0.016 0.042
4 (P10) 1 (60.2) 3 (60.08)
Energy consumption for 1 kg of SRF production was 0.1845 kWh kg1, including (see Fig. 1(a) and (b)): – 0.015 kW h – consumption of diesel fuel for green waste transportation, milling, for pre-composting, screening, loading, etc. (47–50% of diesel fuel (with 0.04307 TJ tonne1 of net calorific value), which was used for these processes).
Moisture content of SRF – 15%.
Table 5 The fuel-energy balance of SRF production and combustion in comparison with sawdust combustion for 100 MWh of the heat energy production. Dimensions
Fuel characteristics Moisture content Fuel lower calorific value Volume for 100 MW h of the heat energy production Main characteristics of combustion plants Capacity Efficiency Technology
a
100 MW h of the heat energy production, burning Sawdusta
SRF pellets
% MJ kg1 tonnes
50 12 31.600
15 14.25 29.721
MW %
<35 95 Grate firing with condenser economizer
35 85 FBC
Main inputs Raw materials moisture content before drying Raw materials for fuel production: Before drying After drying Electricity consumption for dewatering Electricity consumption for the main technological process (pellets production) Electricity consumption in combustion plant (in case of sawdust – in combustion plant, and in economizer) Diesel fuel consumption, e.g. for pre-composted materials mixing, compost turner, screening, compost and fuel loading
%
Sawdust 50
Pre-composted materials 37
tonnes tonnes MW h MW h MW h
31.600 31.600 – – 1.9 (18.74 kW h MW h1)
42.85 31.75 0.165 4.874 (0.164 kW h kg1) 1.8 (18 kW h MW h1)
MWh
0.2
0.456
Total energy consumption:
MWh
2.1
7.3
Main outputs Heat energy production Heat energy losses during production
MWh MWh
100 5.33
100 17.65
Information sources (Kliopova and Makarskiene˙, 2013).
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
I. Kliopova, K. Makarskiene˙ / Waste Management xxx (2014) xxx–xxx
– 0.0055 kW h – electricity consumption for dewatering of the pre-composted materials. – 0.164 kW h – for pellet production. Diesel fuel consumption for transportation of biomass residues depends on distance between biomass collection site and composting territory. In case of experiment this distance is less than 5 km. Approx. 3% of the total energy was used for dewatering of precomposted materials. This volume can be changed – by using other dewatering equipment. The energy balance of SRF production and combustion for 100 MW h of the heat energy production in comparison with sawdust combustion is presented in Table 5. Technologies of widely used sawdust and SRF combustion have been analysed: – For sawdust combustion – grate firing (the oldest and widely applied solid biofuel combustion technology). – For SRF combustion – fluidised bed combustion. In sawdust combustion, 95% of the thermal efficiency was evaluated by means of a condenser economizer. This is the most popular technique used for increasing the primary biofuel energy efficiency in combustion plants. According to the energy balance, obtained during SRF production and combustion, 7.3 MW h of energy is used for 100 MW h of energy production, thus the efficiency of energy recovery is much higher – 0.927, but it is 5.2% lower than in the case of sawdust. Due to the high efficiency of energy recovery SRF has to be used as a renewable energy resource.
7
– Biodegradable waste is converted to energy (waste-to-energy) and compost for energetic plants. In the case of the experiment during one-year period 988.4 tonnes of SRF (about 20% of input materials) and 1.139 tonnes of compost (about 23% of input materials) was produced from 2.5 thousand tonnes of sewage sludge and the same volume of biomass residues. – Produced SRF can be successfully used in the incineration and co-incineration plants due to the following reasons: SRF meets to necessary requirements and according to the SRF Classificatory CEN/TC 343 by the net calorific value (14.25 MJ kg1) contributes to class 4, by the chlorine content in dry matter (0.016%) to class 1 and by mercury content (0.042 mg MJ1) to class 3; the energy consumption for 1 kg of SRF production (inc. all technological processes) is less than 0.2 kWh; the efficiency of energy recovery from SRF production and combustion is very high – 0.927 due to pre-composting of input materials before SRF production for the reduction of moisture content and increase of carbon content. – SRF usage for energy production reduces both fossil fuel consumption and fossil fuel based GHG emissions, as renewable energy sources are accepted as ‘‘climate neutral’’ fuel. – Due to mixing of sewage sludge with biomass residues the concentration of heavy metals in the mixture is decreased and the application of sewage sludge as fertiliser can be extended. Moreover due to SRF production from the 10–40 mm fraction of pre-composted materials major part of heavy metals from biodegradable waste (more than 57%) will be supplied to the dust treatment equipment during SRF combustion in incineration plants and will not be composted.
4. Conclusions This paper has focused on SRF production from 10 to 40 mm fraction of pre-composted sewage sludge and biomass residues and compost production from the remainder fractions for energetic plants growing. The chemical analysis of sewage sludge from municipal waste water treatment plant selected for this experiment has determined that heavy metal concentrations of this sewage sludge are 1.8 times lower in comparison to the EU average (Obernberger et al., 2009) and 4.6 times lower than EU requirements for composting (86/278/EEC). The concentration of heavy metals in the sewage sludge of other countries can be significantly higher, but has to be less than limit concentrations (86/278/EEC). Results of the laboratory analysis of the 10–40 mm fraction of pre-composted materials were compared with the physical and chemical characteristics of sawdust and the data of the ENERCOM project. It can be stated that in comparison with the ENERCOM data the characteristics of the analysed fraction of pre-composted materials is better. In comparison with sawdust, carbon content (37.4% in dry matter) is 1.2 times lower, ash content (20.6% in dry matter) is 20 times higher and chlorine content (0.016% in dry matter) is twice higher. Nevertheless, the net calorific volume of SRF, produced from 10–40 mm fraction of pre-composted sewage sludge and biomass residues with 15% of moisture content is 1.2 times higher than in sawdust with 50% of moisture content widely used in Lithuania. Main advantages of proposed sewage sludge and biomass residues treatment method: – Minimizing of biodegradable waste by more than 96% with minimum energy consumption (0.08 kWh kg1 of sewage sludge or 0.04 kWh kg1 of all biodegradable waste); rest 4% of waste: PM from cyclones during SRF production and ashes during SRF combustion.
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Quiroga, G., Castrillon, L., Fernandex-Nava, Y., Maranon, E., 2010. Physico-chemical analysis and calorific values of poultry manure. Waste Manage. 5 (30), 880–884. Sabiene, N., Paulauskas, V., Shen-Yi, Ch., Jih-Gaw, L., 2008. Variations of metal distribution in sewage sludge composting. Waste Manage. 9 (28), 1637–1644. Soil-Concept, KTU APINI, 2010. Generation of fuel: pelleting press monitoring report. Publishable summary. ENERCOM, pp. 17. Urciuolo, M., Solimene, R., Chirone, R., Salatino, P., 2012. Fluidized bed combustion and fragmentation of wet sewage sludge. Exp. Therm. Fluid Sci. 43, 97–104. Werther, J., Ogada, T., 1999. Sewage sludge combustion. Prog. Energy Combust. Sci. 25, 55–116.
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Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Improving material and energy recovery from the sewage sludge and biomass residues Irina Kliopova ⇑, Kristina Makarskiene˙ Kaunas University of Technology (KTU), Institute of Environmental Engineering (APINI), Lithuania1
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted 29 October 2014 Available online xxxx Keywords: AT: Biomass residues Fertiliser Pre-composting Sewage sludge Solid recovered fuel
a b s t r a c t Sewage sludge management is a big problem all over the world because of its large quantities and harmful impact on the environment. Energy conversion through fermentation, compost production from treated sludge for agriculture, especially for growing energetic plants, and treated sludge use for soil remediation are widely used alternatives of sewage sludge management. Recently, in many EU countries the popularity of these methods has decreased due to the sewage sludge content (heavy metals, organic pollutions and other hazards materials). This paper presents research results where the possibility of solid recovered fuel (SRF) production from the separate fraction (10–40 mm) of pre-composted materials – sewage sludge from municipal waste water treatment plant and biomass residues has been evaluated. The remaining fractions of pre-composted materials can be successfully used for compost or fertiliser production, as the concentration of heavy metals in the analysed composition is reduced in comparison with sewage sludge. During the experiment presented in this paper the volume of analysed biodegradable waste was reduced by 96%: about 20% of input biodegradable waste was recovered to SRF in the form of pellets with 14.25 MJ kg1 of the net calorific value, about 23% were composted, the rest – evaporated and discharged in a wastewater. The methods of material-energy balances and comparison analysis of experiment data have been chosen for the environmental impact assessment of this biodegradable waste management alternative. Results of the efficiency of energy recovery from sewage sludge by SRF production and burning, comparison analysis with widely used bio-fuel–sawdust and conclusions made are presented. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The amount of sewage sludge differentiates significantly among various countries in the European Union (EU). According to the data of Eurostat and the Lithuanian National Strategic Waste Management Plan (2014–2020), the minimal amount of sewage sludge per year is generated in Luxemburg – about 10 thousand tonnes, the maximal amount in Germany – about 1050 thousand tonnes year1, in Lithuania – approx. 50 thousand tonnes year1. Undoubtedly these amounts of sewage sludge bear direct relation to the population. In the case of relative indicators, the situation changes: minimal amount in Germany – 12.8 kg per capita, in Luxembourg – 19.9 kg per capita, in Lithuania – 16.6 kg per capita. According to the data of Eurostat, 21% of generated sewage sludge
⇑ Corresponding author at: Kaunas, K. Donelaicio str. 20, LT-44239, Lithuania. Tel./fax: +370 5 2649174. E-mail address: [email protected] (I. Kliopova). 1 http://ktu.edu/apini/en/.
was landfilled, 10% – incinerated, 45% – applied to land and remaining 24% – treated in other ways in the EU-27 in 2010. Unfortunately, so far in Lithuania only 40% of sewage sludge is processed by means of anaerobic or aerobic treatment methods, the rest of it is dewatered and exported, landfilled, or accumulated in the special sites – polygons. Besides, the treated and dewatered sewage sludge is stored too. At present, in Lithuania about 200 thousand tonnes of old sewage sludge is stored in special sites. In the EU-27 the amount of sewage sludge has a growing tendency due to the development of wastewater collection systems, particularly in urban areas, and to improvement of wastewater treatment technologies. Actually, the quality of sewage sludge is improving because of more strict requirements for the supply of hazardous substances into a sewerage collection system and the better pollution control. According to the Lithuanian National Strategic Waste Management Plan (2014–2020), sewage sludge accumulation in polygons has to be discontinued by 2015. Therefore, the optimisation of material and energy recovery from the sewage sludge for
http://dx.doi.org/10.1016/j.wasman.2014.10.030 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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minimising the waste volume with minimal environmental costs is an issue of great importance. According to Article 14 of the Council Directive 98/15/EEC amending Directive 91/271/EEC concerning the urban wastewater treatment ‘‘sludge arising from wastewater treatment shall be re-used whenever appropriate’’. Sewage sludge contains the components of agricultural value e.g., organic matter, the nutrients such as nitrogen, phosphorus, potassium, and lower concentrations of calcium, sulphur and magnesium, which make the sludge interesting as fertiliser, but heavy metals, toxins, viruses, pharmaceuticals and hormones, GMOs and dioxins are also present in it (Grob et al., 2008). Several authors in the field of this research have stated that the concentrations of total heavy metals can be reduced by composting sewage sludge with biomass (Sabiene et al., 2008; Obernberger et al., 2009; Albrecht et al., 2010; Cukjati et al., 2012). Sewage sludge has a low C:N ratio (5–10:1). The composting process begins when the C:N ratio is 20–30:1 (optimal ratio). Therefore, biodegradable waste, which is rich in carbon (e.g.in biomass residues when the C:N ratio is 40–750:1), is to be mixed to improve the analysed ratio (Albrecht et a1., 2010, Cukjati et al., 2012; te˙, 2013). There are other positive aspects Kliopova and Stankevicˇiu of this mixing, such as minimisation of the moisture content from 70–90% to optimal 50–60% maintaining the optimal oxygen content (15–20%). If the moisture content is higher than 70%, anaerobic process begins. The recommended mixing ratio of sewage sludge and biomass depends on the C:N ratio of bulking agents, for example, when the sawdust and aerobic sludge mixture is of 1:1 proportion. However in the case of more problematic sludge, such as the anaerobic sludge mixture studied, with higher conductivity and the presence of other toxic and phytotoxic substances, a 1:3 proportion is to be recommended because of the dilution effect on harmful parameters (Banegas et al., 2007). The EU Sewage Sludge Directive 86/278/EEC and the Lithuanian normative document LAND 20:2005 Requirements for Sewage Sludge Use for Fertilisation and Remediation prohibit the use of untreated sewage sludge on agricultural land, unless it is injected or incorporated into the soil. This EU Directive also sets the limit values for heavy metals and organic compounds to avoid bioaccumulation in plants and animals, and likewise the use of sewage sludge in agriculture is prohibited where the concentrations of heavy metals in the soil exceed both the indicated limits and the limits of sewage sludge load quantity (kg ha1 year1). For this reason application of sewage sludge and sewage sludge compost to agriculture is being decreased (kg ha1 year1), i.e. the existing soil contamination is hindering its usage. Sewage sludge can be employed as a renewable energy source, since it is generated in large amounts and has considerable energy content (Cao and Pawlowski, 2013). Due to high moisture content in primary sewage sludge its net calorific value is significantly lower, e.g. when moisture content is about 80%, its net calorific value is less than 1.5 MJ kg1, when sewage sludge dries up to 5– 6% of the moisture content, the net calorific value is increased up to 12–13 MJ kg1 (Cukjati et al., 2012; Urciuolo et al., 2012; Magdziarz and Wilk, 2013). Rather big ash content (from 35% to 45%) is one of the negative characteristics of sewage sludge minimizing its usage as a fuel (Cukjati et al., 2012). Volume and mass reduction, thermal destruction of toxic organics pollutions and energy recovery from organic matter are the main advantages of thermal processes (Ye et al., 2012; Magdziarz and Werle, 2014). There are fermentation and several thermal technologies such as pyrolysis, gasification, combustion and cocombustion for energy conversion from sewage sludge. Pyrolysis and gasification are rather complex and costly technologies. Some authors propose the technique of hydrothermal conversion, which involves the application of heat and pressure to treat biomass in
the aqueous medium. This method was evaluated as an effective means for energy conversion from moist biomass without prior drying (Yu et al., 2012). Combustion is a very simple technique for energy recovery from sewage sludge. In this treatment high water content in sewage sludge (70–90%) is fairly big deficiency as great quantities of energy have to be used for mass drying before heat conversion. To solve this problem the co-combustion with municipal solid waste, forest biomass and coal is recommended since the process can generate sufficient heat energy for drying sludge without additional usage of fossil fuel (Werther and Ogada, 1999; Xiao et al., 2010). In addition, sewage sludge is suitable for production of solid recovered fuel (SRF) by mixing it together with the other properly selected waste, such as under-grade-sized coal, waste from animal waste utilisation plants, wood waste (Wzorek, 2012). SRF is a solid fuel produced out of non-hazardous waste in order to be utilised for energy recovery in incineration and co-incineration plants. Properly processed, homogenised and up-graded to a certain quality SRF has to satisfy the classification and specification requirements presented in CEN/TR 15359:2011 (European Committee for Standardization, 2011). Fossil CO2 emissions decrease when renewable secondary fuel is co-combusted. This is one of the primary goals and achievements of co-combustion of biomass (European Commission (2006)). The assessment of the possibilities of producing SRF from the pre-composting of input materials, stabilised sewage sludge, municipal green waste (grass, branches, etc.) and bark is one of the goals of the PF7 program project ‘‘Polygeneration of energy, fuels and fertilisers from biomass residues and sewage sludge (ENERCOM)’’ (No TREN/FP7/EN/218916). In this case the pre-composting process is applying of a sewage sludge bio drying method with minimal energy demand. Results of the laboratory analysis show that 10–40 mm fraction (approx. 45% of the total amount) of pre-composted biodegradable waste (sewage sludge, green waste and biomass residuals) can be used for SRF production, <10 mm – for compost production (Obernberger et al., 2009; Kliopova and Makarskiene˙, 2013). It is evaluated that in this case 82% of biodegradable waste is convertible to energy and compost; the other mass becomes waste (ash, bottom ash and remains). The developed methodology of SRF production was applied by the compost production company Soil-Concept S.A. in Luxembourg. Depending on the mixing ratio, a net calorific value of produced SRF in pellets containing 10–15% of moisture amounts to 13–14 MJ kg1. The efficiency of energy recovery of SRF production and combustion is sufficiently high – up to 0.76 (Kliopova and Makarskiene˙, 2013). For example, the minimal energy recovery efficiency from municipal solid waste incineration facilities has to be 0.65 (Quiroga et al., 2010). Results of the environmental impact assessment of produced SRF burning show that in case of a suitable choice of SRF combustion technologies, the concentrations of air emissions do not exceed the maximal air emission limit volume according to the requirement of the Directive 2000/76/EC of the European Parliament and of the Council of December 4, 2000 on the incineration of waste (Kliopova and Makarskiene˙, 2013). The evaluation of the possibilities of a new method for the sewage sludge management was a primary objective of the research presented in this paper. Sewage sludge samples are taken from an ordinary municipal wastewater treatment plant in Lithuania (pilot plant). Generated sewage sludge is dewatered up to 20% of dry matter by mechanical press and accumulated in the special storage sites in the pilot company. An average amount of sewage sludge is 1500 tonnes (with 75–80% of the moisture content). Currently, a big amount of sewage sludge (about 9 thousand tonnes) has been collected in the storage sites. The environmental audit determines that sewage sludge can be treated by means of two
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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methods: aerobic treatment (composting) and SRF production from separate fraction (approx. 45% of the total amount) of precomposted materials. Such biomass residues as green waste generated in public territories (leaves, straw, hay, cutting waste, inc. a small amount of sawdust, etc.) with about 40% of the moisture content have been used for upgrading both technical conditions of the composting process and the characteristics of produced SRF. The results of the ENERCOM project were used for improving biodegradable waste management and process optimisation. 2. Materials and methods The experiment was carried out in the above-mentioned pilot plant. Sewage sludge with 75–80% of the moisture content was pre-composted with the biomass residues (leaves, straw, hay, cutting waste, inc. a small amount of sawdust, etc.) with about 40% of the moisture content. Traditional windrow composting method was applied. Mixing ratio of 1:1 was chosen for the increasing of C:N ratio of sewage sludge (from 7:1 up to 25–30:1). According to the results of the ENERCOM project, the pre-composting materials were screened by fractions: <10 mm, 10–40 mm, and >40 mm after 3 weeks from the start of the experiment (Kliopova and Makarskiene˙, 2013). The separated 10–40 mm fraction was used for SRF production and further analysis. The remaining fractions were used for compost for energetic plants production. Main steps of the feasible analysis were: – Determination of heavy metals concentrations in sewage sludge from the pilot plant. – Formation of material and energy balance of compost and SRF production from sewage sludge and biomass residues. – Determination of physical and chemical characteristics of the 10–40 mm fraction of pre-composted sewage sludge and biomass residues. – Classification of the produced SRF according to the Solid Recovered Fuel Classificatory CEN/TC 343 (European Committee for Standardization, 2011). – Formation of the fuel-energy balance of SRF production and combustion for 100 MWh of the heat energy production. Heavy metals concentrations in sewage sludge of the pilot plant and other chemical characteristics of pre-composted materials have been determined in the laboratory of Agrochemical Research in the Centre of Lithuanian Agrarian and Forest Science using: Multiwave sample digestion system (Multiwave 3000) and Microwave-Assisted Synthesis (Synthos 3000) for sample preparation. Mercury Analyser (Perkin Elmer FIMS 100) for evaluation of Hg content. Elemental analysis equipment (Elementar Vario EL cube (modification: CNS)) for C, N, S tests. Graphite Furnace Atomic Absorption Spectrometry (PE (Perkin Elmer) AAnalyst 600 (2008), GF-AAS) for Cd, Pb tests. Flame atomic absorption (AA) spectrophotometer system (AAnalyst 200 (2008), Perkin Elmer, USA) with computer software (WinLab 32 TM for ICP, V3.3.1) and with automatics sample input equipment (Perkin Elmer (PE)) for determination of other parameters. The method applied to evaluation of calorific value of SRF: dry Q dry net ¼ Q gross 2441 ð9 H=100Þ
Qdry net
ð1Þ 1
Qdry gross
where is the net calorific value in dry matter, kJ kg ; is higher calorific value (gross) in dry matter was measures with a bomb calorimeter, kJ kg1.
received Q as ¼ ½Q;dry net ð100 WÞ 2441 W=100 net received Qas net
ð2Þ 1
where is the net calorific value in as received, kJ kg ; H is hydrogen content in dry matter, % (H = 6%); W is moisture content, %. Heavy metals concentrations of analysed sewage sludge were compared with the EU average, with the requirements of the EU Sewage Sludge Directive 86/278/EEC, and with the requirements for sewage sludge usage for fertilisation and remediation (Lithuanian normative document LAND 20:2005). The material and energy balance of compost and SRF production from sewage sludge and biomass residues was formed for evaluating relative environmental indicators (EI) i.e. the quantity of one of the input or output flows of these technological processes (e.g. raw material, energy, water, compost, SRF, waste, etc. expressed by a unit per year) for processing one tonne of sewage sludge (tonne1). Physical and chemical characteristics of 10–40 mm fraction from pre-composted sewage sludge and biomass residues were compared with the following data: – Physical and chemical characteristics of sawdust, as a widely used renewable energy source in Lithuania and other EU countries. – Data of the ENERCOM project (Kliopova and Makarskiene˙, 2013). In pilot plant produced SRF was classified by the Solid Recovered Fuel Classificatory CEN/TC 343 (European Committee for Standardization, 2011). The fuel-energy balance of SRF production from the separate fraction (10–40 mm) of pre-composted materials, such as sewage sludge and biomass residues and the produced SRF combustion was formed and compared with sawdust combustion for 100 MW h of the heat energy production. The efficiency of energy recovery was calculated according to the energy balance. The analysis results are presented and discussed in this paper. 3. Results and discussion 3.1. Heavy metals concentrations in sewage sludge The elementary composition of sewage sludge and the content of heavy metals depend on many factors, such as wastewater composition and on sewage sludge treatment methods. Heavy metals concentrations in sewage sludge of the pilot plant are determined and compared with the EU average as well as with the limit values for the usage in agriculture as fertilisers for either growing energetic plants or land remediation (see Table 1). The concentrations of heavy metals in sewage sludge of the pilot plant are significantly lower than the EU average and they meet the requirements of EU Sewage Sludge Directive 86/278/EEC and the Lithuanian normative document LAND 20:2005 about the sewage sludge use for fertilisation and remediation. Therefore, sewage sludge from the pilot plant is suitable for application to the land, i.e. for soil remediation and fertilisation or compost production. 3.2. The material and energy balance of SRF and compost production from sewage sludge and biomass residues Traditional windrow composting (using a turner) in 1.6 m high and 3.6 m wide piles has been used in the experiment. The territory for this aerobic treatment process was covered by the light structured awning. Preparation of composting mixture of sewage sludge and biomass residues with the ratio of 1:1 (1 tonne of sewage sludge with 75–80% of the moisture content with 1 tonne of biomass residuals with about 40% of the moisture content) is the
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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Table 1 Concentrations of heavy metals in sewage sludge. Analysed parameters
Cadmium (Cd) Copper (Cu) Lead (Pb) Nickel (Ni) Chromium (Cr) Hydrargyrum (Hg) Zinc (Zn)
Concentration of heavy metals in sewage sludge, mg kg1 dm Case of the pilot plant
EU average
1–4 10–125 15–24 12–23 18–24 0.2–0.4 800–1000
1.68 377 57 80 150 2 1157
a
Limit concentration, mg kg1 dm 86/278/EEC
LAND 20-2005
20–40 1000–1750 750–1200 300–400 b – 16–25 2500–4000
1.5–20 75–1000 140–750 50–300 140–400 1.0–8.0 300–2500
Comments: a Database (Obernberger et al., 2009). b Each EU country determines its own limit concentration for Cr.
first step of sewage sludge management. Given that the length of used biomass residuals must be less than 5 cm (optimal size – 2.5–5 cm) and thickness should be less than 5 mm, part of the biomass residuals (lop, bark, hay, etc.) was milled by chopper. Aerobic process will not start, if the size of the fraction of biodegradable te˙, 2013). waste is too big (Kliopova and Stankevicˇiu Main parameters of the composting process were taken into account during the technical assessment, namely, the temperature within a pile (more than 70 °C during 1 h or about 55 °C during 2 weeks), optimal volume of the moisture content (50–60%), oxygen content (15–20%), pH volume (6–8). When biodegradable waste is not sufficiently wet, it must be moisturized. When biodegradable waste is too wet, it must be mixed with the dry waste te˙, 2013). The number of turnings (Kliopova and Stankevicˇiu depends on air conditions (temperature, moisture), for example, in case of experiment, 1 time per week during pre-composting period and up to 2 times per week during the further composting process. Main aim is to maintain the above-mentioned parameters of the composting process. During the whole composting process the mass of input materials has decreased twice (e.g. about 80% – due to evaporation and discharged waste water, about 20% – due to losses of dry mate te˙, 2013). During the experiment rials) (Kliopova and Stankevicˇiu the moisture of the mixture was reduced from 57% to 37% after 3 weeks pre-composting. There are two main benefits of the precomposting of sewage sludge and biomass residues: – Reduction of sewage sludge moisture content by approx. 50% with minimal energy (diesel fuel) consumption for a compost turner. – Reduction in mass of input materials by approx. 34%. During the experiment, part of the generated wastewater was reused for further composting process; the other part was sent to the wastewater treatment plant or evaporated. The material and energy balance of processing sewage sludge with biomass residues (pre-composting, screening and further composting processes) is presented in Fig. 1(a). During one-year period 2.500 tonnes of sewage sludge and the same volume of biomass residuals have been processed. Three cycles of composting process were carried out within Lithuanian air conditions (since the end of March to the end of November). In a second step, the pre-composted materials were screened by fractions in a cylindrical separator (a drum screen): <10 mm (about 50% of total amount), 10–40 mm (approx. 40–45% of the total amount), and >40 mm. The separated 10–40 mm fraction of the pre-composted materials was used for SRF production and further analysis. The remaining fractions, inc.>40 mm (to maintain optimal oxygen content), were sent for further composting (inc. maturation).
The required moisture content of the mixture for SRF production in pellets has to be about 12–15%, and the moisture of SRF for efficient combustion has to be 10–15% (Kliopova and Makarskiene˙, 2013; Kwon et al., 2013). Therefore, in the third step, the belt filter press with capacity of 3–5 m3 h1 and 6.6 kW of installed electrical capacity was used for minimisation of the moisture content to 15%. Such method was chosen specially for this pilot plant after feasibility analysis due to two main aspects: (1) the pilot plant does not have its own combustion plant for heat production, (2) electricity savings in consumption to other dewatering and drying technologies. In the case of ENERCOM project thermal drying is carried out in dryer, since the heat energy is produced on own biomass boiler (Kliopova and Makarskiene˙, 2013). The pellets production process was a final technological step of SRF production. The capacity of the press used for pellets is 250 kg h1. Main part of standard technological line of pellets production: mixing equipment, transporter with metal separator, mill, dust aspirator, silo with scraper floor, press feeder, pellet-press, cooler, vibrating sieve, packaging machine (Kliopova and Makarskiene˙, 2013). The material and energy balance of SRF production from 10 to 40 mm fraction of pre-composting materials (dewatering and pelleting processes) is presented in Fig. 1(b). During this experiment about one thousand tonnes of SRF were produced. The formed material and energy balances were used for calculation of relative environmental indicators (see Table 2), which can be used for evaluation of the environmental performance and comparison to other sewage sludge management methods. The total amount of electricity for the processing of 1 tonne of sewage sludge is 67 kW h, total direct air emissions – 8 kg tonne1. The volume of produced SRF makes 20% of the total mass of input raw materials; the volume of produced compost makes approx. 23% of input raw materials. In comparison with fossil fuel (for example, natural gas) CO2 emissions to the air producing heat energy from produced SRF (988.4 tonnes) will be reduced by 642 tonnes year1. 3.3. Physical and chemical characteristics of raw materials and classification of produced SRF Physical and chemical characteristics of the 10–40 mm fraction of pre-composted materials were evaluated in the laboratory (see materials and methods). The results were compared with the data of the ENERCOM project and with the physical and chemical characteristics of sawdust (see Table 3). Characteristics of the pre–composted materials are directly related to the variety of compost composition (green waste, sewage sludge, etc). The net calorific volume of fuel has direct relationship with carbon content (Kliopova and Makarskiene˙, 2013). In the experiment the carbon content was greater than 37%. This result is better than in the case
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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Outputs, units year -1
Inputs, units year -1 Sewage sludge (75-80% of the moisture content) 2 500 tonnes Biomass residues (39-40% of the moisture content) 2 500 tonnes Diesel fuel for biomass transportation 0.12 tonnes Diesel fuel for residues milling, composting and screening 2.43 tonnes Water 491 m3 Industrial oil 45 kg
Compost 1 139 tonnes
Precomposting (biological drying); Screening;
10-40 mm fraction (37% of the moisture content) 1 425 tonnes to SRF production >40 mm fraction 141 tonnes Dry matter loss 459 tonnes
Further composting, inc. maturation (fractions: <10 mm, >40mm)
Wastewater, inc. evaporation 1 836 m3 Air emissions from mobile sources 0.62 tonnes Waste 45 kg
(a) SRF (pellets) 988.4 tonnes
Pre-composted 10-40 mm fraction (37% of the moisture content) 1 425 t Electricity for filter press 5.4 MWh Industrial oil 22.5 kg Electricity for pelleting 162.1 MWh
Dewatering (up to 15% of the moisture content);
Matrix 2.4 units
Water evaporation 408 m3 Indirect environmental impact due to electricity consumption, in case of natural gas burning: CO2 – 34.31 tonnes PM emissions (from pelleting and cyclones) 19.6 tonnes
Pelleting Waste (metal and etc.) 20.02 tonnes
Water for filter press washing 500 m3
Wastewater 500 m3
(b) Fig. 1. The material and energy balance of compost production (a) and SRF production (b) from sewage sludge and biomass residues.
Table 2 Relative environmental indicators of sewage sludge processing. Inputs and flows of sewage sludge processing Inputs Sewage sludge Biomass residuesa Fuel (diesel) Electricity Water Industrial oil Outputs Compost SRF Wastewater Evaporated water Air emissions from mobile sources Air emissions from stationary sources (PM filter) Indirect environmental impact – CO2 emissions Waste (metal, from filters, etc.)
Dimension
Relative EI Units tonne1 of sewage sludge
tonnes tonnes tonnes MWh m3 kg
1.000 1.000 0.001 0.067 0.200 0.027
tonnes tonnes m3 m3 kg kg
0.456 0.395 0.934 0.163 0.248 7.800
tonnes
0.0137
tonnes
0.008
a
Biomass residues: green waste generated in public territories (leaves, straw, hay, cutting waste, inc. a small amount of sawdust, etc.).
of the ENERCOM project, but approx. 15% lower in comparison with sawdust. The pre-composting of sewage sludge with biomass residues has one more clear advantage – reduction of ash content in sewage sludge by half. In the case of the experiment, according to the data of an elementary composition of 10–40 mm fraction, the ash content in dry matter was 20.57%; that is approx. 1.5 times lower in
comparison with ENERCOM, but 20 times higher in comparison with sawdust, which is very low (up to 1%). The chlorine content was significantly (approx. 8.6 times) lower in comparison with the data of the ENERCOM project, but twice higher in comparison with sawdust. As it has been mentioned above the metal concentrations in sewage sludge of the pilot plant correspond with the requirements of the Sewage Sludge Directive 86/278/EEC and LAND 20:2005, but according to these legislations, sewage sludge application to land shall be prohibited where the concentrations of heavy metals in the soil exceed the indicated limits and the limits of the sewage sludge load quantity (kg ha1 year1) as well. Therefore, due to composting of sewage sludge with biomass residues the concentrations of heavy metals in produced compost are reduced and the possibility of sewage sludge application to land is expanded. The majority of heavy metals contained in SRF will be removed with ashes (treatment plant, etc.) during SRF combustion. In addition, the amount of ash after SRF combustion is approx. 0.04 tonnes per tonne of composting mixture (sewage sludge and biomass residues). Produced SRF has to be classified by the Solid Recovered Fuel Classificatory CEN/TC 343 (European Committee for Standardization, 2011). The major goal of standard classification is a high level of the environment protection. On the base of SRF class users can quickly assess SRF type and quality: (1) a net calorific value describes its value as a fuel, (2) chlorine contributes to corrosion, thus high chlorine content means a lower market value and (3) mercury, as heavy metal, is selected as an indicator of the environmental quality. Data of the laboratory analysis of SRF were compared with CEN/TC 343 classification (see Table 4). The comparison shows that the produced SRF of pre-composted materials
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Table 3 Physical and chemical parameters of 10–40 mm fraction of pre-composted materials in comparison with sawdust. Analysed parameters
Ash content Hydrogen (H) Carbon (C) Nitrogen (N) Sulphur (S) Chlorine (Cl) Cadmium (Cd) Copper (Cu) Lead (Pb) Nickel (Ni) Chromium (Cr) Hydrargyrum (Hg) Zinc (Zn) Manganese (Mn) Iron (Fe) Calcium (Ca) Aluminium (Al)
Content of chemicals in dry matter, % Pre-composted materials (ENERCOM)a
Pre-composted materials (the pilot plant)b
Sawdustc
30.770 4.340 36.320 1.860 0.573 0.138 1.000 104 1.2 102 3 103 5 103 9 103 0.12 103 6.5 102 5.2 102 1.070 2.580 1.760
20.570 4.580 37.380 2.040 0.430 1.6 102 1.100 104 0.8 102 1.300 103 0.490 103 0.8 103 0.07 103 4.9 102 4.5 102 0.570 3.630 0.610
1.000 5.400 43.760 0.500 0.042 0.7 102 0.400 104 1.000 103 0.300 103 1 103 1 103 0.02 105 2 103 3 103 0.757 0.470 0.378
a 10–40 mm fraction of pre-composted materials (about 50% of digested and dewatered sewage sludge, about 26% of green waste and other biomass residuals) (Obernberger et al., 2009; Soil-Concept, 2010). b 10–40 mm fraction of pre-composted materials (about 50% of dewatered sewage sludge, about 50% of green waste public territories in municipality). c Sawdust: 30% of deciduous trees sawdust and 70% of softwood (Kliopova et al., 2013).
(fraction – 10–40 mm) corresponds to class 4 by the net calorific value (14.25 MJ kg1), class 1 by the chlorine content in dry matter (0.016%) and class 3 by the mercury content (0.042 mg MJ1).
Table 4 Comparison of SRF produced from pre-composted materials with the Solid Recovered Fuel classification system (CEN/TC 343). SRF form
Net calorific value as receives, MJ kg1a Chlorine (Cl) content in dry matter, % Hydrargyrum (Hg) content, mg MJ1 (median) a
3.4. The efficiency of energy recovery
SRF of pre-composted materials (10–40 mm fraction) of the pilot plant Value Pellets
Class
14.25 0.016 0.042
4 (P10) 1 (60.2) 3 (60.08)
Energy consumption for 1 kg of SRF production was 0.1845 kWh kg1, including (see Fig. 1(a) and (b)): – 0.015 kW h – consumption of diesel fuel for green waste transportation, milling, for pre-composting, screening, loading, etc. (47–50% of diesel fuel (with 0.04307 TJ tonne1 of net calorific value), which was used for these processes).
Moisture content of SRF – 15%.
Table 5 The fuel-energy balance of SRF production and combustion in comparison with sawdust combustion for 100 MWh of the heat energy production. Dimensions
Fuel characteristics Moisture content Fuel lower calorific value Volume for 100 MW h of the heat energy production Main characteristics of combustion plants Capacity Efficiency Technology
a
100 MW h of the heat energy production, burning Sawdusta
SRF pellets
% MJ kg1 tonnes
50 12 31.600
15 14.25 29.721
MW %
<35 95 Grate firing with condenser economizer
35 85 FBC
Main inputs Raw materials moisture content before drying Raw materials for fuel production: Before drying After drying Electricity consumption for dewatering Electricity consumption for the main technological process (pellets production) Electricity consumption in combustion plant (in case of sawdust – in combustion plant, and in economizer) Diesel fuel consumption, e.g. for pre-composted materials mixing, compost turner, screening, compost and fuel loading
%
Sawdust 50
Pre-composted materials 37
tonnes tonnes MW h MW h MW h
31.600 31.600 – – 1.9 (18.74 kW h MW h1)
42.85 31.75 0.165 4.874 (0.164 kW h kg1) 1.8 (18 kW h MW h1)
MWh
0.2
0.456
Total energy consumption:
MWh
2.1
7.3
Main outputs Heat energy production Heat energy losses during production
MWh MWh
100 5.33
100 17.65
Information sources (Kliopova and Makarskiene˙, 2013).
Please cite this article in press as: Kliopova, I., Makarskiene˙, K. Improving material and energy recovery from the sewage sludge and biomass residues. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.030
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– 0.0055 kW h – electricity consumption for dewatering of the pre-composted materials. – 0.164 kW h – for pellet production. Diesel fuel consumption for transportation of biomass residues depends on distance between biomass collection site and composting territory. In case of experiment this distance is less than 5 km. Approx. 3% of the total energy was used for dewatering of precomposted materials. This volume can be changed – by using other dewatering equipment. The energy balance of SRF production and combustion for 100 MW h of the heat energy production in comparison with sawdust combustion is presented in Table 5. Technologies of widely used sawdust and SRF combustion have been analysed: – For sawdust combustion – grate firing (the oldest and widely applied solid biofuel combustion technology). – For SRF combustion – fluidised bed combustion. In sawdust combustion, 95% of the thermal efficiency was evaluated by means of a condenser economizer. This is the most popular technique used for increasing the primary biofuel energy efficiency in combustion plants. According to the energy balance, obtained during SRF production and combustion, 7.3 MW h of energy is used for 100 MW h of energy production, thus the efficiency of energy recovery is much higher – 0.927, but it is 5.2% lower than in the case of sawdust. Due to the high efficiency of energy recovery SRF has to be used as a renewable energy resource.
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– Biodegradable waste is converted to energy (waste-to-energy) and compost for energetic plants. In the case of the experiment during one-year period 988.4 tonnes of SRF (about 20% of input materials) and 1.139 tonnes of compost (about 23% of input materials) was produced from 2.5 thousand tonnes of sewage sludge and the same volume of biomass residues. – Produced SRF can be successfully used in the incineration and co-incineration plants due to the following reasons: SRF meets to necessary requirements and according to the SRF Classificatory CEN/TC 343 by the net calorific value (14.25 MJ kg1) contributes to class 4, by the chlorine content in dry matter (0.016%) to class 1 and by mercury content (0.042 mg MJ1) to class 3; the energy consumption for 1 kg of SRF production (inc. all technological processes) is less than 0.2 kWh; the efficiency of energy recovery from SRF production and combustion is very high – 0.927 due to pre-composting of input materials before SRF production for the reduction of moisture content and increase of carbon content. – SRF usage for energy production reduces both fossil fuel consumption and fossil fuel based GHG emissions, as renewable energy sources are accepted as ‘‘climate neutral’’ fuel. – Due to mixing of sewage sludge with biomass residues the concentration of heavy metals in the mixture is decreased and the application of sewage sludge as fertiliser can be extended. Moreover due to SRF production from the 10–40 mm fraction of pre-composted materials major part of heavy metals from biodegradable waste (more than 57%) will be supplied to the dust treatment equipment during SRF combustion in incineration plants and will not be composted.
4. Conclusions This paper has focused on SRF production from 10 to 40 mm fraction of pre-composted sewage sludge and biomass residues and compost production from the remainder fractions for energetic plants growing. The chemical analysis of sewage sludge from municipal waste water treatment plant selected for this experiment has determined that heavy metal concentrations of this sewage sludge are 1.8 times lower in comparison to the EU average (Obernberger et al., 2009) and 4.6 times lower than EU requirements for composting (86/278/EEC). The concentration of heavy metals in the sewage sludge of other countries can be significantly higher, but has to be less than limit concentrations (86/278/EEC). Results of the laboratory analysis of the 10–40 mm fraction of pre-composted materials were compared with the physical and chemical characteristics of sawdust and the data of the ENERCOM project. It can be stated that in comparison with the ENERCOM data the characteristics of the analysed fraction of pre-composted materials is better. In comparison with sawdust, carbon content (37.4% in dry matter) is 1.2 times lower, ash content (20.6% in dry matter) is 20 times higher and chlorine content (0.016% in dry matter) is twice higher. Nevertheless, the net calorific volume of SRF, produced from 10–40 mm fraction of pre-composted sewage sludge and biomass residues with 15% of moisture content is 1.2 times higher than in sawdust with 50% of moisture content widely used in Lithuania. Main advantages of proposed sewage sludge and biomass residues treatment method: – Minimizing of biodegradable waste by more than 96% with minimum energy consumption (0.08 kWh kg1 of sewage sludge or 0.04 kWh kg1 of all biodegradable waste); rest 4% of waste: PM from cyclones during SRF production and ashes during SRF combustion.
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