- Email: [email protected]
Thermodynamic analysis of the energy recovery from the aerobic bioconversion of solid urban waste organic fraction
Available online at www.sciencedirect.com
Waste Management 28 (2008) 805–812 www.elsevier.com/locate/wasman
Thermodynamic analysis of the energy recovery from the aerobic bioconversion of solid urban waste organic fraction Francesco Di Maria *, Manuel Benavoli, Mirco Zoppitelli Dipartimento di Ingegneria Industriale, Universita degli Studi di Perugia, Via G. Duranti 1A/4, 06125 Perugia, Italy Accepted 28 March 2007 Available online 18 May 2007
Abstract Waste management is of the utmost importance for many countries and especially for highly developed ones due to its implications on society. In particular, proper treatment before disposal of the solid urban waste organic fraction is one of the main issues that is addressed in waste management. In fact, the organic fraction is particularly reactive and if disposed in sanitary landfills without previous adequate treatment, a large amount of dangerous and polluting gaseous, liquid and solid substances can be produced. Some waste treatment processes can also present an opportunity to produce other by-products like energy, recycled materials and other products with both economic and environmental benefits. In this paper, the aerobic treatment of the organic fraction of solid urban waste, performed in a biocell plant with the possibility of recovering heat for civil or industrial needs, was examined from the thermodynamic point of view. A theoretical model was proposed both for the biological process of the organic fraction, as well as for the heat recovery system. The most significant results are represented and discussed. Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that the higher the well-being of a population, the more the population consumes and therefore the higher the waste production. On the other hand, the increase in well-being generally runs parallel to an increased attention to environmental issues. This phenomenon is of primary importance when a waste management system has to be implemented and operated. In fact many substances present in wastes produced during everyday activities can be polluting and hazardous for the environment and for human health, if not adequately treated and managed. Social, political and economic aspects involved in a modern waste management system (Di Maria and Saetta, 2000) are not to be underestimated either. In particular, wastes produced during civil and domestic activities repre*
Corresponding author. Tel.: +39 0755853738; fax: +39 0755853736. E-mail address: [email protected] (F. Di Maria).
0956-053X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2007.03.021
sent an important fraction of the entire waste production that has to be managed by local communities inside the town or region. In this case, one of the main problems is represented by the composition of solid urban waste (SUW) (Di Maria and Pavesi, 2006), because it contains different substances that have to be treated in a special way, before final disposal, to attain an acceptable environmental protection level. About 30% of the mass of the SUW produced in Europe and in more developed countries is represented by the organic fraction. The large amount of organic waste, i.e., more than 40 million tons per year in the European Community, represents a serious problem due to its high reactivity if not adequately treated before final disposal in a sanitary landfill. In fact here the organic fraction is naturally bioconverted into polluting liquid, solid and gaseous (Desideri et al., 2003) substances that also represent a high health risk factor. The solid substances, along with rain water, cause the formation of leachate, which is a pollutant for the water due to its high content of organic substances
806
F. Di Maria et al. / Waste Management 28 (2008) 805–812
Nomenclature BOD c C CH4 CO2 COP cp h HP K OF M m_ Q R S s SUW T W X a D k l
biological oxygen demand oxidate to total OF ratio specific heat methane carbon dioxide coefficient of performance specific heat at constant pressure vapour enthalpy heat pump convective heat transfer coefficient organic fraction mass mass rate heat power thermal resistance area thickness solid urban waste temperature power specific air humidity excess air difference thermal conductivity real to theoretical COP ratio
(i.e., BOD). On the other hand, the gas produced during anaerobic reactions taking place inside the landfill is constituted mainly by CO2 and CH4, representing a high risk of flammability and air and soil pollution. Different solutions have been proposed to reduce the risk arising from the organic fraction of waste (Di Maria and Saetta, 2004; Di Maria et al., 2003; Adani et al., 2004), including aerobic degradation of the organic content before its final disposal. In fact, this solution is more efficient in organic carbon reduction, if compared to other treatments (i.e., anaerobic) (Tuesseau-Vuillemin et al., 2003), and has less complicated plant requirements. Furthermore, the gas production is represented mainly by CO2 and vapour. In addition, the aerobic reduction of the organic carbon content produces a substance known as compost that in some cases can be utilized in agriculture to improve the quality of soil. Another aspect of aerobic treatment is represented by the large amount of heat generated during the decomposition process (Themelis and Kim, 2002; Liwarska-Bizukojc and Ledakowicz, 2003); this causes an increase in organic fraction temperature which positively relates to disinfection of the more diffused pathogenic bacteria. The temperature level is maintained below 70–75 °C, generally at about 55–65 °C, to avoid biological process inhibition, with the help of high thermal loss mainly due to convection and heat transfer by the large amount of
Subscripts air Air d dry el electrical ev evaporated in input ins insulate OF organic fraction out out ox oxidate pr produced re reinjected real real s stochiometric t time theo theoretical tot total w water 1 biocell walls 2 convection with ambient air 3 water evaporation 4 injected water 5 organic matter
fresh air necessary to supply oxygen to the aerobic bacteria. It is possible to build a special plant to reduce thermal loss and to introduce an adequate amount of air, in which the excess heat can be extracted in a different way, by thermal equipment. In such a way it is possible to recover energy from the process. This can yield environmental benefits due to the possibility of reducing fossil fuel demands, making the entire process environmentally sound. The system proposed in this paper consists of a bioconversion cell (Themelis and Kim, 2002), in which the aerobic bioconversion of the SUW organic fraction takes place, coupled with a heat pump to extract excess heat. A thermodynamic model was implemented to simulate the system’s behaviour and performance. By varying some main system parameters, both the biological process and the energy recovery process were analyzed and discussed. 2. Model description and main assumptions The proposed plant is plotted in Fig. 1. The biocell is surrounded by concrete walls in the shape of a parallelepiped (cube) inside of which the organic fraction is stored. The walls can be insulated to reduce heat loss during the process. The biomass can be introduced in batches by removing the biocell roof or walls; it occupies about 70% of the entire available volume, to allow for water and air injection. The
F. Di Maria et al. / Waste Management 28 (2008) 805–812
807
Fig. 1. Schematic of biocell with heat pump.
oxygen required by the process is introduced into the biocell by the ambient air that is fan-pumped under the organic fraction supported by a grid system. In this preliminary analysis the energy necessary to pump air was not considered. The air circuit can operate in two modes: (1) open way, by the continuous introduction of fresh ambient air; (2) closed way, by the circulation of the same air rate more times. A water injection system is necessary to keep the humidity of the organic fraction around 50–60% by mass. The water is pumped and injected on the top of the mass and then recovered and, to reduce water consumption, circulated back from the bottom of the biocell. The temperature of the organic fraction can be controlled during the process by the rate of injected air and by the heat extracted by a heat pump. In this second case it is possible, as shown in the following, to maximize the HP heat extraction operating in the closed way, related to the air injection system. The analysis of the system was performed by implementing a simulation code able to take into account both the biological process and the HP characteristic. In order to do this, some assumptions were made for each system component. 3. Biological process
Component
C (%)
H (%)
O (%)
N (%)
S (%)
Mixed food wastes Fruit wastes Meat wastes Mixed paper Yard wastes
48 48.5 59.6 43.4 50.1
6.4 6.2 9.4 5.8 6.4
37.6 39.5 24.7 44.3 42.3
2.6 1.4 1.2 0.3 0.1
0.4 0.2 0.2 0.2 0.1
The aerobic bioconversion of the organic substances can be schematically considered as an oxidation process with the main by-products of carbon dioxide, water and heat. Considering the reaction represented in (2), utilizing the heat of formation of each reactant and product, it is possible to estimate that the amount of heat released per mole of oxidised organic substances is about 616 kcal. ðC6 H10 O4 Þx þ 6:5O2 ¼ ðC6 H10 O4 Þx1 þ 6CO2 þ 5H2 O þ 616 kcal
ð1Þ
ð2Þ
Starting from Eq. (2), it is also possible to quantify that the stoichiometric air required by the process, is about 6.9 kg per kg of organic substance. To allow a better oxidation process and temperature control, higher amounts of air are introduced into the biocell. The higher amount is expressed by the parameter ‘‘a’’ given by the ratio of the introduced air mass rate related to the stoichiometric one (3). a¼
The SUW organic fraction consists mainly of food and paper waste. Once the mass fraction of each component is known, it is possible to establish the chemical composition of the organic fraction (Themelis and Kim, 2002) (Table 1). Starting from this data, the molecular formula of the considered substance was established (1), whereas the chemical components with lower mass fraction (i.e., N and S) were not considered. C6 H10 O4
Table 1 Ultimate analysis of MSW organic fraction (% by weight)
m_ air m_ air;s
ð3Þ
Another important aspect to consider is the rate at which the oxidative reaction takes place. According to the available models proposed in literature (Keener et al., 1998), the amount of the oxidised to total oxidable mass ratio is related to time and temperature as shown in (4). ct ¼ 1 eð0:006321:066
ðT 20Þ
Þt kg =kg ox tot
ð4Þ
This means that the time required for a complete oxidation process is theoretically infinite. For this reason, considering that real aerobic treatment requires generally 15–
808
F. Di Maria et al. / Waste Management 28 (2008) 805–812
20 days, computations were halted when the amount of bioconverted organic fraction was no less than 80%. Obviously, the time unit shown in (4) is 1 day.
Q4 ¼ m_ w C w DT
4. Thermal loss As previously illustrated, the biological process taking place inside the organic fraction produces heat that causes an increase in the temperature of the entire mass. This temperature has to be kept under 55–65 °C, as in real aerobic treatment. In this way the biological process is preserved along with the disinfection process, due to the quite high temperature level, for the destruction of many important pathogenic bacteria. This means that the heat generated inside the proposed biocell system has to be discharged to the environment to avoid an excessive temperature increase. The heat transfer process involved can be summarized in the following: (1) (2) (3) (4) (5)
heat heat heat heat heat
transfer by the biocell walls; exchanged by the process ambient air; absorbed by water evaporation; exchanged by injected water; and absorbed by the OF.
1 K OF
Rair
þ
sconc kconc
1 m2 K=W 1 þ ksins þ K air ins
1 ¼ 1 m2 K=W sins sconc 1 þ þ þ K air K air kconc kins
Q5 ¼ M OF C OF DT
ð10Þ
The entire amount of heat transferred is quantified as the sum of the different heat transfer processes mentioned previously (11). ð11Þ
5. Heat pump
ð5Þ
The heat pump represents the second important component of the proposed plant. In fact this system has to provide heat extraction under particular operating conditions of the system, and releases this heat for the different demands of users. Due to the temperature levels and to the achievable performance levels, a vapour compression heat pump was chosen. The HP behaviour was simulated by the definition of the theoretical coefficients of performance (12). COPteo ¼
ð6Þ
Once the different surfaces involved by the heat exchange are known, it is possible to evaluate the global heat exchanged in this first way (7). Q1 ¼ ROF S OF DT þ Rair S air DT kW
ð9Þ
The water injected from the top of the mass can produce a reduction in mass temperature due to the lower value of the water temperature. If this occurs, a fraction of the heat produced by the biological process is absorbed by the injected water. The final heat transfer process considered regards the heat absorbed by the organic fraction itself during the starting period, or due to some mass temperature reduction that can occur in different phases of the process (10).
Qtot ¼ Q1 þ Q2 þ Q3 þ Q4 þ Q5 kW
Considering the mean temperatures levels, the heat exchanged by radiation was not considered. Referring to the schematic reported in Fig. 1, it is possible to note that the heat exchanged by the biocell walls is due both to the hot organic fraction mass (5) and the heated air occupying the internal volume at the biocell top (6). ROF ¼
The fourth heat transfer process relates to the need of keeping the moisture content of the organic fraction quite constant during the entire treatment process (9).
ð7Þ
The second and third heat transfer processes happen in conjunction (8) and are represented by the one that occurs between the organic fraction mass and the process air. Q2 þ Q3 ¼ m_ air;d cp DT þ m_ air;d ðX out T out þ X in T in Þ ð8Þ The ambient air, injected form the bottom of the biocell, passes through the hot organic fraction extracting heat both in the convective and in the evaporative manner. In particular, the process of evaporation of moisture, that causes saturation of the ambient air at increasing temperatures, is the one that involves the higher amount of extracted heat. This phenomenon is very important only if the biocell system operates in an open manner, with the continuous injection of fresh air.
T out Q ¼ out T out T in W el;in
ð12Þ
Then to evaluate more realistic plant performances, the theoretical COP was corrected by the aid of an empiric coefficient l, (13), that was estimated considering the theoretical and real COP for a wide range of real heat pumps available on the market, of similar size to the one utilized in the present analysis. l¼
COPreal COPtheo
ð13Þ
When simulating the system behaviour under different operating conditions, some assumptions were made for the main system parameters. The evaluation of heat exchange is possible only if the geometry and the components of the biocell are defined (Table 2) and if ambient air inlet conditions are assumed (Table 3). The possibility of recovering heat is influenced both by biocell operating conditions and by heat pump features (Table 4). In particular, related to the HP, heat loss due to hot fluid transport inside dedicated pipes, and with minimum temperature differences between the hot organic fraction and the HP operating fluid, have been assumed.
F. Di Maria et al. / Waste Management 28 (2008) 805–812 Table 2 Biocell geometry and organic fraction characteristics
809 100
100
Parameter
Value
Unit
Width Length Height Wall thickness Insulate thickness Internal volume Moisture volume Moisture water content Moisture weight
4 10 4 0.2 0.01 160 120 65 96,000
m m m m m m2 m2 % by weight kg
Tair=20 [°C] sins=0,01 [m]
80
80
60
[%]
[°C]
60
40
40
bio conversion end
20
α=7 α=11
20
α=9 α=13
0
0 0
4
8
12
16
20
[day]
Table 3 Ambient air characteristics Parameter
Value
Unit
Pressure Temperature Relative humidity
1,013,325 20 50
Pa °C %
Fig. 2. Bioconverted fraction and temperature of the organic substance versus time, for different values of excess air.
140000 120000 100000
Table 4 HP main characteristics
Inlet air Global consumed air Stoichiometric air
Value
Unit
l Heat loss DT
40 5 5
% % °C
3
Parameter
[Nm ]
80000 60000 Tair=20 [°C] sins=0,01 [m] α=10
40000 20000
6. Main results
0
0
4
8
12
16
20
[day]
The simulation began from the analysis of the biological process behaviour concerning main parameters such as process temperature, ambient temperature and a. After this first phase, the evaluation of the amount of recoverable heat by the heat pump was performed. 7. Biological process The analysis of the biological process was conducted with the biocell operating in an open way, with continuous introduction of fresh air. In this condition, an increase in a yields a reduction of process temperature and a reduction of the bioconverted fraction (Fig. 2). In particular, for a = 7 the process achieves very fast results, in only two days, a temperature of 70 °C and, in less than 15 days, about 90% of bioconverted organic fraction, as shown in Fig. 2. As previously assumed, when the bioconverted OF becomes higher than 85%, the process ends and the mass temperature decreases (Figs. 2 and 6). Lower a values cause an excessive increase of the system temperature, higher than 70–75 °C in a few days, which is not compatible with the bacteria that produce the biological conversion. Anyway, it is possible to note (Fig. 2) that a higher than 11 causes longer process periods, mainly due to the lower temperature achieved by the system (less than 60 °C). In fact, after 20 days, the bioconverted fraction results are
Fig. 3. Inlet, global consumed and stoichiometric air versus time, for given excess air.
about 80%. This phenomenon reduces process efficiency with a reduction of the bioconversion rate per unit of time. The amount of stoichiometric and injected air (Fig. 3) concerns both process temperature and the amount of bioconverted fraction. In fact, the higher the process temperature the higher the amount of bioconverted organic fraction per unit of time. On the other hand, after the first few days, there is a high reduction of non-bioconverted organic fraction. This phenomenon causes a reduction of stoichiometric and injected air required by the process (Fig. 3). Obviously, the heat production process is also strongly influenced by the phenomenon previously described. It is possible to note (Fig. 4) how the shape of the curves, related to time, of the different heat exchanged by the system is very similar to the injected air ones. The larger fraction of heat (i.e., Q3) is represented by the water evaporation process. These effects can be better evaluated referring to Fig. 5 where the system water balance, related to different process days, is represented. It can be noted that the evaporated water results are more than 90% of the total of the water evolving in the system. More than 85% of the water lost by this process is reintroduced
810
F. Di Maria et al. / Waste Management 28 (2008) 805–812
50000000 Q1 Q2 Q5
40000000
[kJ]
30000000
Q3 Q4 Qtot
Tair=20 [°C] sins=0,01 [m] α=10
20000000 10000000 0 0
4
8
12
16
20
[day]
Fig. 4. Different heat transfer values versus time, for a = 10.
18000 16000 14000 watertot waterev waterpr waterre
[kg]
12000 10000 8000
8. Heat recovery
Tair=20 [°C] sins=0,01 [m] α=10
6000 4000 2000 0
0
4
8
12
16
20
[day]
Fig. 5. Amount of evaporated, produced, reinjected and total water for a = 10.
bio conversion end
70
α=7 α=9 α=11
60
[°C]
50 40 Tair=20 [°C] sins=0,01 [m]
30
When the bioconversion process ends, if no fresh organic fraction is introduced into the biocell, the bioconverted substances release heat to the environment, reducing its temperature. Fig. 6 shows this phenomenon related to time for different values of excess air. The lower the amount of excess air (a), the higher the results of the cooling curve. This is mainly due to the higher temperature levels achieved by the treated organic fraction. It is important to note how an increase in a value causes a noticeable increase in the process period. At a = 11, the process takes place in 30 days whereas at a = 9 less than 20 days are needed. Thus, even if lower a values seems to allow more interesting energy recovery levels, the risk of anaerobic condition, in some areas of the organic fraction mass, has to be carefully considered. In fact, a reduction in a reduces the probability of adequate oxygen in all of the treated mass, enhancing the beginning of an anaerobic process. For these reasons acceptable a values, related mainly to process efficiency, seem to range between 9 and 11.
Heat recovery can be utilized only if the biocell air circuit operates in a closed condition. In this operating condition, the heat extracted by the water evaporation process can be absorbed in the evaporator of a vapour compression heat pump. The fresh air, initially introduced with a certain a value, reduces its oxygen content due to the biological process. When the oxygen content of the circulating air becomes less or equal to the oxygen related to an a = 2, new fresh air is introduced inside the cell whereas the consumed one is discharged. During this air substitution period, no heat is recoverable by the HP (Fig. 8). The amount of introduced air and the interval period between two consecutive consumed air substitutions depend mainly on the initial a value and on the bioconverted fraction (Fig. 7). At lower air excess values the process temperature achieves higher values and so also the bioconversion rate are higher (Fig. 2), mainly in the first
20 0
10
20
30
40
120000
50
[day] 100000
Tair=20 [˚C] sins=0,01 [m]
Fig. 6. Process temperature versus time for different values of excess air.
α=9 α=11
3
by the water injection system, whereas the remaining is supplied both by the one produced in the biological process and by the one introduced with the fresh ambient air. Fig. 6 shows the temperature shapes, related to time, for three different a values utilized by the process. The lower the a, the higher the temperature achieved by the process and lower the time required. As a rises, the process becomes slower and a greater number of days is required for achieving more than 80% of bioconverted fraction.
[Nm ]
80000 60000 40000 20000
0
0
5
10
15
20
[day]
Fig. 7. Fresh air introduction versus time for different a values.
F. Di Maria et al. / Waste Management 28 (2008) 805–812
60000000 α=9 α=11
Tair=20 [°C] sins=0,01 [m]
50000000 40000000
[kJ]
30000000 20000000 10000000 0
5
10
15
8 Tair=15 [°C] sins=0,03 [m] ΔT=22,5 [°C]
6
COPreal
Tair=15 [°C] sins=0,03 [m]
100
α=9 α=11
Heat
80 60 40 Electricity
20 0
0
4
8
12
16
20
[day]
Fig. 10. Heat production and electrical energy used by the heat pump under different operating conditions.
between 3.5 and 6 and remains quite constant during the entire process period. Once the COP has been determined, it is possible to assess the amount of electrical energy required by the heat pump compressor and the amount of heat available at the HP outlet, on different days of the process and in different operating conditions (Fig. 10). The amount of heat available by the proposed system can achieve peaks higher than 120 GJ per day, especially in the first few days of the process. After this period the heat released by the HP decreases; nevertheless it is usually higher than 30–40 GJ per day, and the difference between the heat available with higher and lower a values is significantly reduced. The results obtained are related to the plant size analyzed (Table 2), for given organic fraction and heat pump features (Table1). A more consistent heat production can be achieved if more biocells, working in parallel, are utilized instead of a single one as considered in this paper. This solution can reduce the heat production rate peaks but can be more useful in meeting user demands.
20
Fig. 8. Recoverable heat versus time for different a values.
ΔT=27,5 [°C]
5
ΔT=32,5 [°C]
4 ΔT=37,5 [°C]
α=9 α=11
3 2
120
9. Conclusion 0
[day]
7
140
[GJ]
few days. This means that during this period the amount of air required by the bioconversion process is higher at a lower a, whereas it decreases as the fraction of bioconverted organic fraction increases. Furthermore, due to the lower mass rate, lower air excess values need a higher frequency in consumed air substitution process. For these reasons, mainly in the first 10–20 days, the heat recoverable by the HP (Fig. 8) is higher when the process operates at lower air excess values. After 10–15 days, depending on process temperature, it is possible to note that the amount of heat recoverable by the higher a values becomes higher. This phenomenon, related to the last period of the process, is due to the higher fraction of non-bioconverted organic substance remaining in the biocell that produces a higher heat rate even if the temperature remains lower. As shown in Fig. 9, the COP of the pump is strongly dependent on absorbed heat temperature increase (i.e., DT) required by the users. The analyzed scenario ranges between 75 and 100 °C of temperature at which the HP heat is released. It is also interesting to note that the COP at lower air excess values are slightly higher than the one related to higher air excess. Anyway the obtainable COP ranges
811
0
4
8
12
16
20
[day]
Fig. 9. COPreal of the HP under different operating conditions.
The aerobic bioconversion of the solid urban waste organic fraction is one of the most suitable solutions for treating such waste, before final disposal, mainly due to its simplicity and efficiency in reducing harmful effects on the environment. By the aid of the plant proposed in this work, it is possible to couple the bioconversion process with the energy recovery process. In fact, the heat released by the aerobic bacteria activity, generally at 55–65 °C, can be utilized by a heat pump and increased both in quality, achieving about 80–90 °C, and in quantity, achieving about 4000–5000 kJ per kg of treated organic fraction. The amount of recoverable heat rate, and its temperature level, is influenced by many parameters, such as ambient air humidity and temperature, biocell insulation thickness, and the amount of organic fraction processed, but the greatest influence on this process is given by the air excess parameter.
812
F. Di Maria et al. / Waste Management 28 (2008) 805–812
The main result of the analysis shows that the most suitable values for this parameter range between 9 and 11. Lower values causes an increase in heat power peaks, mainly in the first days, and a reduction of the period required for the organic fraction oxidation, less than 15 days. The coefficient of performance of the heat pump is slightly influenced by the aforementioned air excess parameter, but are strongly dependent on the output heat temperature increase. As the temperature difference between the treated organic fraction and the released heat rises, the COP value decreases. With a temperature increase between 30–35 °C, the COP ranges between 4 and 3, high enough to make this an attractive solution. References Adani, F., Tambone, F., Gotti, A., 2004. Biostabilization of municipal solid waste. Waste Management 24 (8), 775–783. Desideri, U., Di Maria, F., Leonardi, D., Proietti, S., 2003. Sanitary landfill energetic potential analysis: a real case study. Energy Conversion and Management 44 (12), 1969–1981.
Di Maria, F., Pavesi, G., 2006. RDF to energy plant for a central Italia region SUW management system: Energetic and economical analysis. Applied Thermal Engineering 26, 1291–1300. Di Maria, F., Saetta, S., 2000. Life cycle assessment analysis for solid urban waste management: the case of a 100.000 inhabitants Italian town. In: The 16th International Conference on Solid Waste Technology and Management. Philadelphia, PA, December, pp. 10–13. Di Maria, F., Saetta, S., 2004. Life cycle assessment for municipal solid waste management improvements: the case of a 100.000 inhabitant Italian town. Journal od Solid Waste Technology and Management 30 (1), 53–65. Di Maria, F., Saetta, S., Leonardi, D., 2003. Life Cycle Assessment of a PPV plant applies to an existing SUW management system. International Journal of Energy Research 27 (5), 481–494. Keener, H.M., Elwell, D.L., et al., 1998. Specifying design/operating of composting system using pilot scale data. Applied Engineering in Agriculture 13 (3), 377–384. Liwarska-Bizukojc, E., Ledakowicz, S., 2003. Stoichiometry of the aerobic biodegradation of the organic fraction of municipal solid waste (MWS). Biodegradation 14 (1), 51–56. Themelis, J.N., Kim, Y.H., 2002. Material and energy balances in a largescale aerobic bioconversion cell. Waste Management and Research 20, 234–242. Tuesseau-Vuillemin, M.H., Dispan, J., Mouchel, J.M., Servais, P., 2003. Biodegradable fraction of organic carbon estimated under oxic and anoxic conditions. Water Research 37 (9), 2242–2247.
Waste Management 28 (2008) 805–812 www.elsevier.com/locate/wasman
Thermodynamic analysis of the energy recovery from the aerobic bioconversion of solid urban waste organic fraction Francesco Di Maria *, Manuel Benavoli, Mirco Zoppitelli Dipartimento di Ingegneria Industriale, Universita degli Studi di Perugia, Via G. Duranti 1A/4, 06125 Perugia, Italy Accepted 28 March 2007 Available online 18 May 2007
Abstract Waste management is of the utmost importance for many countries and especially for highly developed ones due to its implications on society. In particular, proper treatment before disposal of the solid urban waste organic fraction is one of the main issues that is addressed in waste management. In fact, the organic fraction is particularly reactive and if disposed in sanitary landfills without previous adequate treatment, a large amount of dangerous and polluting gaseous, liquid and solid substances can be produced. Some waste treatment processes can also present an opportunity to produce other by-products like energy, recycled materials and other products with both economic and environmental benefits. In this paper, the aerobic treatment of the organic fraction of solid urban waste, performed in a biocell plant with the possibility of recovering heat for civil or industrial needs, was examined from the thermodynamic point of view. A theoretical model was proposed both for the biological process of the organic fraction, as well as for the heat recovery system. The most significant results are represented and discussed. Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction It is well known that the higher the well-being of a population, the more the population consumes and therefore the higher the waste production. On the other hand, the increase in well-being generally runs parallel to an increased attention to environmental issues. This phenomenon is of primary importance when a waste management system has to be implemented and operated. In fact many substances present in wastes produced during everyday activities can be polluting and hazardous for the environment and for human health, if not adequately treated and managed. Social, political and economic aspects involved in a modern waste management system (Di Maria and Saetta, 2000) are not to be underestimated either. In particular, wastes produced during civil and domestic activities repre*
Corresponding author. Tel.: +39 0755853738; fax: +39 0755853736. E-mail address: [email protected] (F. Di Maria).
0956-053X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2007.03.021
sent an important fraction of the entire waste production that has to be managed by local communities inside the town or region. In this case, one of the main problems is represented by the composition of solid urban waste (SUW) (Di Maria and Pavesi, 2006), because it contains different substances that have to be treated in a special way, before final disposal, to attain an acceptable environmental protection level. About 30% of the mass of the SUW produced in Europe and in more developed countries is represented by the organic fraction. The large amount of organic waste, i.e., more than 40 million tons per year in the European Community, represents a serious problem due to its high reactivity if not adequately treated before final disposal in a sanitary landfill. In fact here the organic fraction is naturally bioconverted into polluting liquid, solid and gaseous (Desideri et al., 2003) substances that also represent a high health risk factor. The solid substances, along with rain water, cause the formation of leachate, which is a pollutant for the water due to its high content of organic substances
806
F. Di Maria et al. / Waste Management 28 (2008) 805–812
Nomenclature BOD c C CH4 CO2 COP cp h HP K OF M m_ Q R S s SUW T W X a D k l
biological oxygen demand oxidate to total OF ratio specific heat methane carbon dioxide coefficient of performance specific heat at constant pressure vapour enthalpy heat pump convective heat transfer coefficient organic fraction mass mass rate heat power thermal resistance area thickness solid urban waste temperature power specific air humidity excess air difference thermal conductivity real to theoretical COP ratio
(i.e., BOD). On the other hand, the gas produced during anaerobic reactions taking place inside the landfill is constituted mainly by CO2 and CH4, representing a high risk of flammability and air and soil pollution. Different solutions have been proposed to reduce the risk arising from the organic fraction of waste (Di Maria and Saetta, 2004; Di Maria et al., 2003; Adani et al., 2004), including aerobic degradation of the organic content before its final disposal. In fact, this solution is more efficient in organic carbon reduction, if compared to other treatments (i.e., anaerobic) (Tuesseau-Vuillemin et al., 2003), and has less complicated plant requirements. Furthermore, the gas production is represented mainly by CO2 and vapour. In addition, the aerobic reduction of the organic carbon content produces a substance known as compost that in some cases can be utilized in agriculture to improve the quality of soil. Another aspect of aerobic treatment is represented by the large amount of heat generated during the decomposition process (Themelis and Kim, 2002; Liwarska-Bizukojc and Ledakowicz, 2003); this causes an increase in organic fraction temperature which positively relates to disinfection of the more diffused pathogenic bacteria. The temperature level is maintained below 70–75 °C, generally at about 55–65 °C, to avoid biological process inhibition, with the help of high thermal loss mainly due to convection and heat transfer by the large amount of
Subscripts air Air d dry el electrical ev evaporated in input ins insulate OF organic fraction out out ox oxidate pr produced re reinjected real real s stochiometric t time theo theoretical tot total w water 1 biocell walls 2 convection with ambient air 3 water evaporation 4 injected water 5 organic matter
fresh air necessary to supply oxygen to the aerobic bacteria. It is possible to build a special plant to reduce thermal loss and to introduce an adequate amount of air, in which the excess heat can be extracted in a different way, by thermal equipment. In such a way it is possible to recover energy from the process. This can yield environmental benefits due to the possibility of reducing fossil fuel demands, making the entire process environmentally sound. The system proposed in this paper consists of a bioconversion cell (Themelis and Kim, 2002), in which the aerobic bioconversion of the SUW organic fraction takes place, coupled with a heat pump to extract excess heat. A thermodynamic model was implemented to simulate the system’s behaviour and performance. By varying some main system parameters, both the biological process and the energy recovery process were analyzed and discussed. 2. Model description and main assumptions The proposed plant is plotted in Fig. 1. The biocell is surrounded by concrete walls in the shape of a parallelepiped (cube) inside of which the organic fraction is stored. The walls can be insulated to reduce heat loss during the process. The biomass can be introduced in batches by removing the biocell roof or walls; it occupies about 70% of the entire available volume, to allow for water and air injection. The
F. Di Maria et al. / Waste Management 28 (2008) 805–812
807
Fig. 1. Schematic of biocell with heat pump.
oxygen required by the process is introduced into the biocell by the ambient air that is fan-pumped under the organic fraction supported by a grid system. In this preliminary analysis the energy necessary to pump air was not considered. The air circuit can operate in two modes: (1) open way, by the continuous introduction of fresh ambient air; (2) closed way, by the circulation of the same air rate more times. A water injection system is necessary to keep the humidity of the organic fraction around 50–60% by mass. The water is pumped and injected on the top of the mass and then recovered and, to reduce water consumption, circulated back from the bottom of the biocell. The temperature of the organic fraction can be controlled during the process by the rate of injected air and by the heat extracted by a heat pump. In this second case it is possible, as shown in the following, to maximize the HP heat extraction operating in the closed way, related to the air injection system. The analysis of the system was performed by implementing a simulation code able to take into account both the biological process and the HP characteristic. In order to do this, some assumptions were made for each system component. 3. Biological process
Component
C (%)
H (%)
O (%)
N (%)
S (%)
Mixed food wastes Fruit wastes Meat wastes Mixed paper Yard wastes
48 48.5 59.6 43.4 50.1
6.4 6.2 9.4 5.8 6.4
37.6 39.5 24.7 44.3 42.3
2.6 1.4 1.2 0.3 0.1
0.4 0.2 0.2 0.2 0.1
The aerobic bioconversion of the organic substances can be schematically considered as an oxidation process with the main by-products of carbon dioxide, water and heat. Considering the reaction represented in (2), utilizing the heat of formation of each reactant and product, it is possible to estimate that the amount of heat released per mole of oxidised organic substances is about 616 kcal. ðC6 H10 O4 Þx þ 6:5O2 ¼ ðC6 H10 O4 Þx1 þ 6CO2 þ 5H2 O þ 616 kcal
ð1Þ
ð2Þ
Starting from Eq. (2), it is also possible to quantify that the stoichiometric air required by the process, is about 6.9 kg per kg of organic substance. To allow a better oxidation process and temperature control, higher amounts of air are introduced into the biocell. The higher amount is expressed by the parameter ‘‘a’’ given by the ratio of the introduced air mass rate related to the stoichiometric one (3). a¼
The SUW organic fraction consists mainly of food and paper waste. Once the mass fraction of each component is known, it is possible to establish the chemical composition of the organic fraction (Themelis and Kim, 2002) (Table 1). Starting from this data, the molecular formula of the considered substance was established (1), whereas the chemical components with lower mass fraction (i.e., N and S) were not considered. C6 H10 O4
Table 1 Ultimate analysis of MSW organic fraction (% by weight)
m_ air m_ air;s
ð3Þ
Another important aspect to consider is the rate at which the oxidative reaction takes place. According to the available models proposed in literature (Keener et al., 1998), the amount of the oxidised to total oxidable mass ratio is related to time and temperature as shown in (4). ct ¼ 1 eð0:006321:066
ðT 20Þ
Þt kg =kg ox tot
ð4Þ
This means that the time required for a complete oxidation process is theoretically infinite. For this reason, considering that real aerobic treatment requires generally 15–
808
F. Di Maria et al. / Waste Management 28 (2008) 805–812
20 days, computations were halted when the amount of bioconverted organic fraction was no less than 80%. Obviously, the time unit shown in (4) is 1 day.
Q4 ¼ m_ w C w DT
4. Thermal loss As previously illustrated, the biological process taking place inside the organic fraction produces heat that causes an increase in the temperature of the entire mass. This temperature has to be kept under 55–65 °C, as in real aerobic treatment. In this way the biological process is preserved along with the disinfection process, due to the quite high temperature level, for the destruction of many important pathogenic bacteria. This means that the heat generated inside the proposed biocell system has to be discharged to the environment to avoid an excessive temperature increase. The heat transfer process involved can be summarized in the following: (1) (2) (3) (4) (5)
heat heat heat heat heat
transfer by the biocell walls; exchanged by the process ambient air; absorbed by water evaporation; exchanged by injected water; and absorbed by the OF.
1 K OF
Rair
þ
sconc kconc
1 m2 K=W 1 þ ksins þ K air ins
1 ¼ 1 m2 K=W sins sconc 1 þ þ þ K air K air kconc kins
Q5 ¼ M OF C OF DT
ð10Þ
The entire amount of heat transferred is quantified as the sum of the different heat transfer processes mentioned previously (11). ð11Þ
5. Heat pump
ð5Þ
The heat pump represents the second important component of the proposed plant. In fact this system has to provide heat extraction under particular operating conditions of the system, and releases this heat for the different demands of users. Due to the temperature levels and to the achievable performance levels, a vapour compression heat pump was chosen. The HP behaviour was simulated by the definition of the theoretical coefficients of performance (12). COPteo ¼
ð6Þ
Once the different surfaces involved by the heat exchange are known, it is possible to evaluate the global heat exchanged in this first way (7). Q1 ¼ ROF S OF DT þ Rair S air DT kW
ð9Þ
The water injected from the top of the mass can produce a reduction in mass temperature due to the lower value of the water temperature. If this occurs, a fraction of the heat produced by the biological process is absorbed by the injected water. The final heat transfer process considered regards the heat absorbed by the organic fraction itself during the starting period, or due to some mass temperature reduction that can occur in different phases of the process (10).
Qtot ¼ Q1 þ Q2 þ Q3 þ Q4 þ Q5 kW
Considering the mean temperatures levels, the heat exchanged by radiation was not considered. Referring to the schematic reported in Fig. 1, it is possible to note that the heat exchanged by the biocell walls is due both to the hot organic fraction mass (5) and the heated air occupying the internal volume at the biocell top (6). ROF ¼
The fourth heat transfer process relates to the need of keeping the moisture content of the organic fraction quite constant during the entire treatment process (9).
ð7Þ
The second and third heat transfer processes happen in conjunction (8) and are represented by the one that occurs between the organic fraction mass and the process air. Q2 þ Q3 ¼ m_ air;d cp DT þ m_ air;d ðX out T out þ X in T in Þ ð8Þ The ambient air, injected form the bottom of the biocell, passes through the hot organic fraction extracting heat both in the convective and in the evaporative manner. In particular, the process of evaporation of moisture, that causes saturation of the ambient air at increasing temperatures, is the one that involves the higher amount of extracted heat. This phenomenon is very important only if the biocell system operates in an open manner, with the continuous injection of fresh air.
T out Q ¼ out T out T in W el;in
ð12Þ
Then to evaluate more realistic plant performances, the theoretical COP was corrected by the aid of an empiric coefficient l, (13), that was estimated considering the theoretical and real COP for a wide range of real heat pumps available on the market, of similar size to the one utilized in the present analysis. l¼
COPreal COPtheo
ð13Þ
When simulating the system behaviour under different operating conditions, some assumptions were made for the main system parameters. The evaluation of heat exchange is possible only if the geometry and the components of the biocell are defined (Table 2) and if ambient air inlet conditions are assumed (Table 3). The possibility of recovering heat is influenced both by biocell operating conditions and by heat pump features (Table 4). In particular, related to the HP, heat loss due to hot fluid transport inside dedicated pipes, and with minimum temperature differences between the hot organic fraction and the HP operating fluid, have been assumed.
F. Di Maria et al. / Waste Management 28 (2008) 805–812 Table 2 Biocell geometry and organic fraction characteristics
809 100
100
Parameter
Value
Unit
Width Length Height Wall thickness Insulate thickness Internal volume Moisture volume Moisture water content Moisture weight
4 10 4 0.2 0.01 160 120 65 96,000
m m m m m m2 m2 % by weight kg
Tair=20 [°C] sins=0,01 [m]
80
80
60
[%]
[°C]
60
40
40
bio conversion end
20
α=7 α=11
20
α=9 α=13
0
0 0
4
8
12
16
20
[day]
Table 3 Ambient air characteristics Parameter
Value
Unit
Pressure Temperature Relative humidity
1,013,325 20 50
Pa °C %
Fig. 2. Bioconverted fraction and temperature of the organic substance versus time, for different values of excess air.
140000 120000 100000
Table 4 HP main characteristics
Inlet air Global consumed air Stoichiometric air
Value
Unit
l Heat loss DT
40 5 5
% % °C
3
Parameter
[Nm ]
80000 60000 Tair=20 [°C] sins=0,01 [m] α=10
40000 20000
6. Main results
0
0
4
8
12
16
20
[day]
The simulation began from the analysis of the biological process behaviour concerning main parameters such as process temperature, ambient temperature and a. After this first phase, the evaluation of the amount of recoverable heat by the heat pump was performed. 7. Biological process The analysis of the biological process was conducted with the biocell operating in an open way, with continuous introduction of fresh air. In this condition, an increase in a yields a reduction of process temperature and a reduction of the bioconverted fraction (Fig. 2). In particular, for a = 7 the process achieves very fast results, in only two days, a temperature of 70 °C and, in less than 15 days, about 90% of bioconverted organic fraction, as shown in Fig. 2. As previously assumed, when the bioconverted OF becomes higher than 85%, the process ends and the mass temperature decreases (Figs. 2 and 6). Lower a values cause an excessive increase of the system temperature, higher than 70–75 °C in a few days, which is not compatible with the bacteria that produce the biological conversion. Anyway, it is possible to note (Fig. 2) that a higher than 11 causes longer process periods, mainly due to the lower temperature achieved by the system (less than 60 °C). In fact, after 20 days, the bioconverted fraction results are
Fig. 3. Inlet, global consumed and stoichiometric air versus time, for given excess air.
about 80%. This phenomenon reduces process efficiency with a reduction of the bioconversion rate per unit of time. The amount of stoichiometric and injected air (Fig. 3) concerns both process temperature and the amount of bioconverted fraction. In fact, the higher the process temperature the higher the amount of bioconverted organic fraction per unit of time. On the other hand, after the first few days, there is a high reduction of non-bioconverted organic fraction. This phenomenon causes a reduction of stoichiometric and injected air required by the process (Fig. 3). Obviously, the heat production process is also strongly influenced by the phenomenon previously described. It is possible to note (Fig. 4) how the shape of the curves, related to time, of the different heat exchanged by the system is very similar to the injected air ones. The larger fraction of heat (i.e., Q3) is represented by the water evaporation process. These effects can be better evaluated referring to Fig. 5 where the system water balance, related to different process days, is represented. It can be noted that the evaporated water results are more than 90% of the total of the water evolving in the system. More than 85% of the water lost by this process is reintroduced
810
F. Di Maria et al. / Waste Management 28 (2008) 805–812
50000000 Q1 Q2 Q5
40000000
[kJ]
30000000
Q3 Q4 Qtot
Tair=20 [°C] sins=0,01 [m] α=10
20000000 10000000 0 0
4
8
12
16
20
[day]
Fig. 4. Different heat transfer values versus time, for a = 10.
18000 16000 14000 watertot waterev waterpr waterre
[kg]
12000 10000 8000
8. Heat recovery
Tair=20 [°C] sins=0,01 [m] α=10
6000 4000 2000 0
0
4
8
12
16
20
[day]
Fig. 5. Amount of evaporated, produced, reinjected and total water for a = 10.
bio conversion end
70
α=7 α=9 α=11
60
[°C]
50 40 Tair=20 [°C] sins=0,01 [m]
30
When the bioconversion process ends, if no fresh organic fraction is introduced into the biocell, the bioconverted substances release heat to the environment, reducing its temperature. Fig. 6 shows this phenomenon related to time for different values of excess air. The lower the amount of excess air (a), the higher the results of the cooling curve. This is mainly due to the higher temperature levels achieved by the treated organic fraction. It is important to note how an increase in a value causes a noticeable increase in the process period. At a = 11, the process takes place in 30 days whereas at a = 9 less than 20 days are needed. Thus, even if lower a values seems to allow more interesting energy recovery levels, the risk of anaerobic condition, in some areas of the organic fraction mass, has to be carefully considered. In fact, a reduction in a reduces the probability of adequate oxygen in all of the treated mass, enhancing the beginning of an anaerobic process. For these reasons acceptable a values, related mainly to process efficiency, seem to range between 9 and 11.
Heat recovery can be utilized only if the biocell air circuit operates in a closed condition. In this operating condition, the heat extracted by the water evaporation process can be absorbed in the evaporator of a vapour compression heat pump. The fresh air, initially introduced with a certain a value, reduces its oxygen content due to the biological process. When the oxygen content of the circulating air becomes less or equal to the oxygen related to an a = 2, new fresh air is introduced inside the cell whereas the consumed one is discharged. During this air substitution period, no heat is recoverable by the HP (Fig. 8). The amount of introduced air and the interval period between two consecutive consumed air substitutions depend mainly on the initial a value and on the bioconverted fraction (Fig. 7). At lower air excess values the process temperature achieves higher values and so also the bioconversion rate are higher (Fig. 2), mainly in the first
20 0
10
20
30
40
120000
50
[day] 100000
Tair=20 [˚C] sins=0,01 [m]
Fig. 6. Process temperature versus time for different values of excess air.
α=9 α=11
3
by the water injection system, whereas the remaining is supplied both by the one produced in the biological process and by the one introduced with the fresh ambient air. Fig. 6 shows the temperature shapes, related to time, for three different a values utilized by the process. The lower the a, the higher the temperature achieved by the process and lower the time required. As a rises, the process becomes slower and a greater number of days is required for achieving more than 80% of bioconverted fraction.
[Nm ]
80000 60000 40000 20000
0
0
5
10
15
20
[day]
Fig. 7. Fresh air introduction versus time for different a values.
F. Di Maria et al. / Waste Management 28 (2008) 805–812
60000000 α=9 α=11
Tair=20 [°C] sins=0,01 [m]
50000000 40000000
[kJ]
30000000 20000000 10000000 0
5
10
15
8 Tair=15 [°C] sins=0,03 [m] ΔT=22,5 [°C]
6
COPreal
Tair=15 [°C] sins=0,03 [m]
100
α=9 α=11
Heat
80 60 40 Electricity
20 0
0
4
8
12
16
20
[day]
Fig. 10. Heat production and electrical energy used by the heat pump under different operating conditions.
between 3.5 and 6 and remains quite constant during the entire process period. Once the COP has been determined, it is possible to assess the amount of electrical energy required by the heat pump compressor and the amount of heat available at the HP outlet, on different days of the process and in different operating conditions (Fig. 10). The amount of heat available by the proposed system can achieve peaks higher than 120 GJ per day, especially in the first few days of the process. After this period the heat released by the HP decreases; nevertheless it is usually higher than 30–40 GJ per day, and the difference between the heat available with higher and lower a values is significantly reduced. The results obtained are related to the plant size analyzed (Table 2), for given organic fraction and heat pump features (Table1). A more consistent heat production can be achieved if more biocells, working in parallel, are utilized instead of a single one as considered in this paper. This solution can reduce the heat production rate peaks but can be more useful in meeting user demands.
20
Fig. 8. Recoverable heat versus time for different a values.
ΔT=27,5 [°C]
5
ΔT=32,5 [°C]
4 ΔT=37,5 [°C]
α=9 α=11
3 2
120
9. Conclusion 0
[day]
7
140
[GJ]
few days. This means that during this period the amount of air required by the bioconversion process is higher at a lower a, whereas it decreases as the fraction of bioconverted organic fraction increases. Furthermore, due to the lower mass rate, lower air excess values need a higher frequency in consumed air substitution process. For these reasons, mainly in the first 10–20 days, the heat recoverable by the HP (Fig. 8) is higher when the process operates at lower air excess values. After 10–15 days, depending on process temperature, it is possible to note that the amount of heat recoverable by the higher a values becomes higher. This phenomenon, related to the last period of the process, is due to the higher fraction of non-bioconverted organic substance remaining in the biocell that produces a higher heat rate even if the temperature remains lower. As shown in Fig. 9, the COP of the pump is strongly dependent on absorbed heat temperature increase (i.e., DT) required by the users. The analyzed scenario ranges between 75 and 100 °C of temperature at which the HP heat is released. It is also interesting to note that the COP at lower air excess values are slightly higher than the one related to higher air excess. Anyway the obtainable COP ranges
811
0
4
8
12
16
20
[day]
Fig. 9. COPreal of the HP under different operating conditions.
The aerobic bioconversion of the solid urban waste organic fraction is one of the most suitable solutions for treating such waste, before final disposal, mainly due to its simplicity and efficiency in reducing harmful effects on the environment. By the aid of the plant proposed in this work, it is possible to couple the bioconversion process with the energy recovery process. In fact, the heat released by the aerobic bacteria activity, generally at 55–65 °C, can be utilized by a heat pump and increased both in quality, achieving about 80–90 °C, and in quantity, achieving about 4000–5000 kJ per kg of treated organic fraction. The amount of recoverable heat rate, and its temperature level, is influenced by many parameters, such as ambient air humidity and temperature, biocell insulation thickness, and the amount of organic fraction processed, but the greatest influence on this process is given by the air excess parameter.
812
F. Di Maria et al. / Waste Management 28 (2008) 805–812
The main result of the analysis shows that the most suitable values for this parameter range between 9 and 11. Lower values causes an increase in heat power peaks, mainly in the first days, and a reduction of the period required for the organic fraction oxidation, less than 15 days. The coefficient of performance of the heat pump is slightly influenced by the aforementioned air excess parameter, but are strongly dependent on the output heat temperature increase. As the temperature difference between the treated organic fraction and the released heat rises, the COP value decreases. With a temperature increase between 30–35 °C, the COP ranges between 4 and 3, high enough to make this an attractive solution. References Adani, F., Tambone, F., Gotti, A., 2004. Biostabilization of municipal solid waste. Waste Management 24 (8), 775–783. Desideri, U., Di Maria, F., Leonardi, D., Proietti, S., 2003. Sanitary landfill energetic potential analysis: a real case study. Energy Conversion and Management 44 (12), 1969–1981.
Di Maria, F., Pavesi, G., 2006. RDF to energy plant for a central Italia region SUW management system: Energetic and economical analysis. Applied Thermal Engineering 26, 1291–1300. Di Maria, F., Saetta, S., 2000. Life cycle assessment analysis for solid urban waste management: the case of a 100.000 inhabitants Italian town. In: The 16th International Conference on Solid Waste Technology and Management. Philadelphia, PA, December, pp. 10–13. Di Maria, F., Saetta, S., 2004. Life cycle assessment for municipal solid waste management improvements: the case of a 100.000 inhabitant Italian town. Journal od Solid Waste Technology and Management 30 (1), 53–65. Di Maria, F., Saetta, S., Leonardi, D., 2003. Life Cycle Assessment of a PPV plant applies to an existing SUW management system. International Journal of Energy Research 27 (5), 481–494. Keener, H.M., Elwell, D.L., et al., 1998. Specifying design/operating of composting system using pilot scale data. Applied Engineering in Agriculture 13 (3), 377–384. Liwarska-Bizukojc, E., Ledakowicz, S., 2003. Stoichiometry of the aerobic biodegradation of the organic fraction of municipal solid waste (MWS). Biodegradation 14 (1), 51–56. Themelis, J.N., Kim, Y.H., 2002. Material and energy balances in a largescale aerobic bioconversion cell. Waste Management and Research 20, 234–242. Tuesseau-Vuillemin, M.H., Dispan, J., Mouchel, J.M., Servais, P., 2003. Biodegradable fraction of organic carbon estimated under oxic and anoxic conditions. Water Research 37 (9), 2242–2247.