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Temporal variation of decomposition gases from baled municipal solid wastes
Bioresource Technology 112 (2012) 105–110
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Temporal variation of decomposition gases from baled municipal solid wastes Ismail Ozbay, Ertan Durmusoglu ⇑ University of Kocaeli, Department of Environmental Engineering, Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 16 February 2012 Accepted 17 February 2012 Available online 1 March 2012 Keywords: Municipal solid waste Baling technology Landfill gases Correlation Regression
a b s t r a c t In this study, after nine cylindrical bales containing a mix of different waste materials were constructed, they were stored in the open air and temporal variations of CO2, CH4, O2, and N2 were monitored over 10 months. In each bale, different waste fractions were considered in order to represent different moisture contents. The results showed that CO2 increased within very few days to approximately 80% and stabilized later in the range of between 10% and 35% in a month. The O2 levels dropped from approximately 15% to significantly less than 1%. There was no significant anaerobic decomposition since CH4 did not exceed 5% during the whole test period. N2 exhibited an opposite pattern with CO2. In addition, relationships between waste species in the bales and gas formations were determined by a bivariate correlation analysis. An empirical prediction model for the maximum CO2 production was also developed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Baling of municipal solid waste (MSW) has been extensively used in several countries for temporary waste storage, especially for incineration (Robles-Martinez and Gourdon, 2000; Nammari et al., 2007; Wagner and Bilitewski, 2009). The baling system uses a baling machine, which compacts the waste materials together in order to produce large bales completely sealed in plastic films. Compacting MSW into bales makes the material significantly easier to handle and store, when compared to loose material. Limited literature exists on the performance of waste bales. The most extensive studies have been carried out on environmental performance (Robles-Martinez and Gourdon, 2000; Baldasano et al., 2003; Wagner and Bilitewski, 2009), leachate quality (El-Fadel et al., 2002) and combustion emissions of bales (Hogland et al., 2001; Nammari et al., 2003, 2007). Robles-Martinez and Gourdon (2000) carried out laboratory experiments in order to assess the long-term behavior of baled household wastes. Several MSW bales were monitored mainly by biogas analysis and temperature measurement over 8 months. The results showed that the biodegradation of baled household waste is very low when the envelope is in good condition. In addition, no CH4 formation was observed. El-Fadel et al. (2002) investigated the temporal variation of leachate quality from baled MSW characterized with high organic and moisture content. They concluded that baling of waste did not hinder waste stabilization that can be attributed to the high ⇑ Corresponding author. Address: University of Kocaeli, Department of Environmental Engineering, Umuttepe Main Campus, Izmit, Kocaeli 41380, Turkey. Tel./fax: +90 262 303 3193. E-mail address: [email protected] (E. Durmusoglu). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.02.087
organic fraction and an initial moisture content that equals or exceeds field capacity. Baldasano et al. (2003) reviewed the chemical, physical, and biological processes, and the environmental performance of MSW bales. They pointed out that material inside the bales was stabilized without producing CH4 with a relatively high concentration of CO2. They also indicated based on the cost model that a landfill using rectangular plastic-wrapped bales represents an economically competitive option compared to a conventional landfill. Nammari et al. (2003) measured the temperature and emissions from cylindrical and rectangular bales. The bales were kept at two different ranges of temperatures (30–35 and 20– 25 °C). The results showed that all the bales exhibited aerobic decomposition and the temperature inside the bales did not increase than the ambient air temperature. Nammari et al. (2007) presented a methodological approach for the study of volatile organic compounds (VOCs) in air emitted during storage of MSW in bales. Differences in VOC concentrations in air were found between wastes stored in cylindrical or rectangular bales. They concluded that concentrations of VOCs after storing for cylindrical bales were higher than for rectangular bales. Wagner and Bilitewski (2009) reported different approaches (including baling technology) for interim storage of wastes and related problems. They indicated that further investigations are required to ensure the long-term safety of interim storage operations. Earlier researchers have reported several contradictory results on the waste decomposition inside the bales. In some investigations, bales produced and in others did not produce gaseous emission, methane was present in some cases and in others not. Moreover, studies performed monitoring the decomposition of baled wastes stored in open air are lacking. The effect of waste composition on the gas formation in the bales has not been studied
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samples were taken from the piles and sent to the laboratory for moisture content analysis. The initial moisture contents of the bales were presented in Table 1. The test bales were produced by the Roll-Press-Pack (RPP) system (Schuster Engineering, Germany). The RPP system is mounted in a 12.2 m container and includes a round bale press, a foil wrapper, a conveyor and a control system. The whole baling process lasts 4–5 min and is fully computerized. In order to prepare the individual test bales, the mixed waste materials were fed into the bale chamber until full pressure was reached. While the bale was wrapped, the baler arm returned to start position for new bale formation. The shape of the bales was stabilized by means of a polyethylene (PE)-net, and the bales were enclosed in multiple layers of wrapping material. A 750 mm wide low density polyethylene (LDPE) with a thickness of 25 lm (Duo Plast AG Optima-750, Germany) was used as the wrapping material. Approximately 1000 g of wrapping material per bale was used and the wrapping material constitutes only about 0.1% of the total weight of the bale. After the bales were constructed, they were transported by a front loader and stored in the open air on the premises of the baling site. The test bales were covered with a protective cover. The cylindrical test bales were 120 cm in diameter and 120 cm in height. Their mass depends on the type of waste material in them and their moisture content. The initial weights of the bales were ranged from 800 to 1100 kg (Table 1). The densities of the bales were between 0.6 and 0.8 t/m3. After the bales were stored, CO2, CH4, O2 and N2 were monitored over 10 months. Gas measurements were carried out using a Gas Data LMSxi G3 Portable Multi-function Landfill Gas Analyzer (Ashtead Technology, UK). Gas sampling tube was inserted 60 ± 5 cm at the center of the bales. As soon as the reading stabilized, a single value was recorded. The values were expressed in volume%. The typical accuracy of the analyzer with regard to methane detection is 0.2% at 5% CH4 and 3% at 100% CH4; with regard to carbon dioxide, the accuracy figures are 0.1% at 10% CO2 and 3% at 100% CO2; with regard to oxygen typical accuracy is 0.5% at 25% O2. A flow range of between 0.1 and 20 l/h can be recorded. The instrument calibration was verified regularly in order to minimize the risk of false measurement. Stability of the analyzer calibration for CH4, CO2 and O2 was checked using 103 l gas canister containing 5% CH4, 5% CO2 and 6% O2. The average air temperature was 10.7 °C during the first 5 months of storage (winter and spring). While the average temperature in the later 3 months (summer) was 24.4 °C, the average temperature was 17.8 °C in the last 2 months (autumn). The maximum temperature was recorded as 29 °C at the 7th month of the storage period. Majority of the precipitation was observed in the first 80 days of the storage. While the maximum precipitation was 70 mm at the 180th day of the storage, the average precipitation was 3 mm during the whole storage period. In addition, the wind speed recorded during the study was between 0.8 and 6.2 m/s.
earlier. In this study, after nine cylindrical bales containing a mix of different waste materials were constructed, temporal variations of CO2, CH4, O2 and N2 were monitored over 10 months. In each bale, different waste fractions were considered in order to represent different moisture contents. In addition, relationships between waste species and moisture content in the bales and gas formations were investigated by a bivariate correlation analysis considering maximum CO2 levels during degradation process. An empirical prediction model for maximum CO2 was also developed using a multi linear regression analysis. 2. Methods A total of nine cylindrical bales containing a mix of different waste materials were constructed in January 2010 at the Gebze MSW Transfer Station in Kocaeli, Turkey. The test bales were constructed based on the compositions provided in Table 1. In each bale, different waste fractions were considered in order to represent different moisture contents since the composition of MSW varies substantially with socio-economic conditions, location, season, sampling and sorting procedures, etc. Eight types of fractions were considered: food wastes, paper-cardboard, wood, textiles, plastics, yard wastes, metals and glass. Food wastes were collected from a marketplace. Paper-cardboard, plastics, glass and metals were obtained from a waste recovery plant in Kocaeli. Cloth clippings supplied from a local textile factory were used as textile wastes. Sawdust, leaves, grass and wood were provided by the Metropolitan Municipality of Kocaeli. After waste materials were stored separately, the moisture contents (wet basis) of a representative sample of the waste materials were determined. The moisture contents of the waste materials in% wet basis were 80%, 30%, 2%, 3%, 52%, 40%, 1% and 0.5%, respectively, for food wastes, paper-cardboard, plastics, textiles, wood, yard wastes, metal and glass. It is well known that while low-income areas usually produce more organic materials, which generally mean high moisture content (60–75%), high-income areas produce relatively more inorganic wastes, which generally mean low moisture content (20–30%). In this study, the compositions of the test bales were decided in a way that the moisture contents of the mixed materials inside the bales would be between 25% and 65%. Hence, the test bales represent different locations generating waste with different moisture contents. Before baling, representative samples taken from each waste material were weighted based on the percents provided in Table 1, and they were piled separately. Hence, a total of nine piles having different waste fractions were prepared. Then, the individual piles were mixed thoroughly with the help of a front loader. Finally, the mixed materials for each pile were fed into a baler chamber via a feed conveyor. In total, 10 tons of waste materials were used to create nine cylindrical bales. Prior to baling, representative
Table 1 Compositions, weights and moisture contents of the test bales. Bale No.
1 2 3 4 5 6 7 8 9
Waste categories (% wet weight)
Weight (kg)
Moisture content (% wet basis)
Food wastes
Paper-cardboard
Plastics
Textiles
Wood
Yard wastes
Metal
Glass
Initial
Final
Initial
Final
30 40 30 15 37 59 75 40 20
15 15 9 35 25 13 14 18 32
17 18 19 37 24 9 5 25 30
17 6 30 6 6 10 6 10 16
10 15 – 7 8 9 – 7 2
11 6 4 – – – – – –
– – 5 – – – – – –
– – 3 – – – – – –
800 1100 900 1000 900 900 1000 1100 850
780 1060 880 980 880 850 920 1050 840
31 39 38 22 35 66 56 42 18
40 49 40 35 49 69 65 48 34
I. Ozbay, E. Durmusoglu / Bioresource Technology 112 (2012) 105–110
The results were evaluated by two statistical techniques; bivariate correlation analysis and multiple linear regression (MLR) analysis. These techniques have been employed for analyzing numerous environmental data, especially in the area of air pollution research (Statheropoulos et al., 1998). The analysis of the data was carried out using SPSS statistical software (version 17.0). Bivariate correlation analysis tests the degree of relationship between two quantitative variables and enables the determination of relations between a dependent variable (Y) and an independent variable (X) individually. Linear association between variables can be measured with the Pearson Correlation Coefficient that measures the strength and direction (decreasing or increasing, depending on the sign) of a linear relationship between two variables. The Pearson Correlation Coefficient (r) is defined as (Ahlgren et al., 2003)
P ðX XÞðY YÞ r ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P P 2 2 ðY YÞ ðX XÞ
ð1Þ
and Y are the means of the where X and Y are the variables and X variables. MLR analysis is a method used to model the linear relationship between a dependent variable (predictand) and several independent variables (predictors). MLR identifies the best combination of predictors of the dependent variable. A model of the relationship is hypothesized, and estimates of the parameter values are used to
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develop an estimated regression equation. MLR is expressed according to the following equation (Kovac-Andric et al., 2009).
y ¼ b0 þ b1 x1 þ b2 x2 þ þ bk xk þ e
ð2Þ
where, y is the predictand; b0 is the regression constant; bi are the regression coefficients; xi are the predictrors and e is the stochastic error associated with the regression. The model is estimated by least squares, which yields parameter estimates such that sum of squares of errors is minimized. More detailed information about bivariate correlation analysis and multiple linear regression (MLR) analysis can be found in many standard textbooks or references. 3. Results and discussion 3.1. Temporal variation of decomposition gases Fig. 1 shows the temporal variation of CO2, CH4, O2 and N2. The air was confined in the pores of the waste materials inside the bales when they were formed. Therefore, the bales were biologically active due to the initial quantity of air. Hence, an aerobic decomposition took place as soon as the bales were formed. CO2 exhibited a rapid increase in the first few days of baling depending on the amount of oxygen present inside the bales. The very high CO2 content at the beginning was due to the hydrolysis and acidogenesis stages occurred in the bales. The hydrolysis initiates the non-methanogenic stage by reducing complex organic
Fig. 1. Temporal variation of decomposition gases.
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matter to smaller and soluble components by means of extra cellular enzymes (Farquhar and Rovers, 1973). In the acidogenesis stage, organic materials are further modified producing ammonia, water, H2 and CO2. Furthermore, several types of organic acids are produced during this stage (Farquhar and Rovers, 1973). During the whole period of 10 months, the bales showed a continuous production of CO2. In general, after a rapid increase at the beginning, CO2 reached the maximum levels after several days. Then, the CO2 levels decreased and the CO2 production stabilized in the range of between 10% and 35% approximately in a month. Consequently, the variation of O2 showed a decreasing pattern. The baling process reduced the air gaps between the materials, thus reducing the amount of available oxygen. Moreover, the aerobic decomposition, took place in the first few days, consumed almost all of the available oxygen inside the bales, leaving just trace levels. O2 showed a rapid decrease in the first days of storage. The increase observed between the 30th and 120th days of baling was probably due to the acute O2 entrance during the measurements. After that, some control measures were taken and the O2 levels were decreased to expected levels. On the other hand, the O2 increase at the end of the 250 days was resulted from the possible defects occurred during the storage of the bales. In general, the test bales exhibited good stability during the storage. There was no sign of leachate discharge from the bales and the waste remained contained. No birds were observed during the entire observation period, no significant houseflies were observed, and no shifting or movement of the bales was observed. There was no significant settlement visibly noticeable. However, few small openings on the wrapping material were observed in some bales at the end of the test period. These openings were either caused by animal diggings or the loader during transportation, loading and unloading of the bales. There was practically no CH4 production (below 0.5% in all bales) in the first 3 months. Although CH4 production was later observed, the levels were below 1% in the first 5 months. Moreover, CH4 did not exceed 5% during the whole test period. As a logical consequence, once the oxygen has been consumed, the aerobic process should stop and an anaerobic phase should begin. However, there was no significant anaerobic decomposition in the test bales. Different reasons beside the lack of oxygen may explain the inhibited anaerobic decomposition. One reason would be the acidic environment produced inside the bales. This is explained by the fact that organic acids were formed during the first phase of decomposition. The high concentration of acids reduced the pH and hence the activity of methanogenic bacteria, which normally decompose the organic acids and produce CO2. In this study, the pH values of random measurements were between 5.6 and 7.4 with an average of 6.6. The pH measurements taken at the first several weeks of baling gave acidic results. However, the neutral pH values were observed after 6 months of storage. The other reason for the inhibition of anaerobic decomposition would be the moisture content of the waste materials inside the bales. Since anaerobic decomposition takes place in relatively wet conditions, moisture contents of the bales would not be high enough for bacterial support. Wastes that are too dry (less than 40% moisture content) decay slowly because they lack sufficient water for survival of bacteria. At moisture levels above 60%, small pore spaces that allow oxygen to move into the waste become filled with water. In this study, the initial moisture contents of six bales (1–5 and 9) were below the optimum moisture content for bacterial decomposition (<40%). While the initial moisture contents of two bales (7 and 8) were within the optimum moisture content (40–60%), bale 6 had the initial moisture content above 60%. The initial moisture contents were higher in the bales having higher food wastes. At the end of the test period, the average increase in moisture content was 31.4%. As seen from Table 1, the moisture increase was much
higher in the bales having higher paper-cardboard content. Papercardboard soaks more water than other materials inside the bales. Since the bales were wrapped with LDPE, moisture due to decomposition evaporated and condensed on the plastic. Some of the moisture trickled down onto the waste or was guided by the plastic to a collection point at the bottom. Since the bales were covered during the test period, they had some degree of resistance to perforation and tearing that prevented large quantities of water percolation into the bales. However, we have noticed that strong wind blew the cover several times during the study period. Therefore, few holes or cracks in the wrapping material would allow rainwater flux into the bales although LDPE merely reduces the flux for a period of time. Therefore, several bales would have been influenced by rainwater even in a short period of time. CH4 production (over 1%) observed at the end of 3 months would be caused by an increase in pH (>6.5) or moisture content (>40%). In MSW landfills, while N2 decreases at the beginning, CO2 increases. Then, the production of N2 ends, and the CO2 production stabilizes as soon as the anaerobic phase starts. In this study, N2 exhibited an opposite pattern with CO2 that should not be generally observed in MSW landfills. Robles-Martinez and Gourdon (2000) observed a similar N2 pattern and concluded that certain microbial activities, such as CO2 consumption to synthesize acetic acid or maybe N2 production through respiration with nitrate under anoxic conditions could explain the observed phenomenon. The test bales lost weight (3.5% in average) over 10 months due to the biodegradation took place especially in the early period of storage (Table 1). The weight losses were higher in the bales having higher food content. For moisture contents above 40%, average weight losses for the four bales (bales 2, 6, 7 and 8) ranged from 3.7% to 8.0%. Lower weight losses were measured in the drier bales, 1.2–2.5%. The decomposition gases from each test bale were presented in Fig. 2. In general, similar patterns were observed in all the bales. Moreover, gas levels measured especially after 40 days of test period were quite identical. This is due to the rapid decomposition of readily decomposable portion of the organic content in the early stage of the study. After that, the decomposition gases followed similar patterns since the remaining contents within the bales were similar characteristics. This shows that the content of the bales becomes less important after a certain period of time. On the other hand, in the early stage of storage, the temporal variations of decomposition gases were different in each bale. Therefore, the behavior of the bales is directly related to the material baled at the early stages of the storage. At the end of the test period, the productions of CO2 and N2 in all the bales stabilized in the range of 10–15% and 75–85%, respectively. On the other hand, the O2 levels were less than 1% in all the bales except in bales 6 and 8. This suggested that air infiltrated into bales 6 and 8 either through the actual sampling point (most likely) or through the plastics then into the waste after approximately 8 months of storage.
3.2. Bivariate correlation analysis In this study, the inorganic waste materials (i.e., glass and metal) were excluded in the correlation analysis since they are nonbiodegradable. CO2 evolution rates are generally used to indicate the rate of degradation of organic wastes in aerobic composting processes. The rate of CO2 evolution is expressed as the quantity of CO2 evolved per unit reaction time and per unit biodegradable fraction of the organic wastes (Kwon and Lee, 2004). Relationships between the maximum CO2 formations and bale components and moisture content were evaluated using the bivariate correlation analysis. Pearson Correlation Coefficients describing the relations were 0.89, 0.69, 0.80, 0.28, 0.15, 0.05 and 0.89, respectively,
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Fig. 2. Decomposition gases from different test bales.
for food wastes (Pf), paper-cardboard (Pc), plastics (Pp), textiles (Pt), yard wastes (Py), wood (Pw) and moisture content (M). The food wastes and moisture content exhibited highest correlations with the maximum CO2 formation rate. This can be explained with the readily biodegradable characteristic of food wastes as a result of high volatile fraction of almost 90% of total solids (Selvam et al., 2010). Similarly, moisture content is of great importance for microorganisms since the process takes place primarily in the liquid phase. There was a strong negative correlation between plastics and the maximum CO2 formation. The kinetic studies on the decomposition of various plastics introduced different and great temperature dependency of plastic wastes affecting decomposition rate (Ali and Siddiqui, 2005). So, plastics are resistant to environmental degradation (Buekens and Huang, 1998). The negative relationship between plastics and the maximum CO2 formation can be explained with hardly biodegradability characteristics of plastics. A negative correlation was obtained between paper-cardboard and the maximum CO2 formation. Paper-cardboards are among the major biodegradable organic fraction of MSWs. However, the content of biodegradable carbon of paper-cardboard is rather inaccessible for degradation since the main components are cellulose in the paper-cardboard. Textile wastes contain mixtures of natural and synthetic fibers such as cotton, wool, silk, nylon, olefin and polyester. Different types of fibers show different biodegradability characteristics. For instance, cotton is more degradable than polyester (Miranda et al., 2007). Therefore, the amounts and characteristics of fibers affect the biodegradability characteristics of textile wastes (Ryu et al., 2007). In this study low negative correlation was obtained for the relation between textile wastes and the maximum CO2 production. Digestion of woody biomass has not been considered techni-
cally feasible without pre-treatment. Many factors may influence the biodegradability of wood such as low moisture content, relative lignin, cellulose and hemicellulose content, proportion of structural and non-structural carbohydrates, cellulose crystallinity, particle size, wood-to-bark ratio and toxic components. Lignin contained in wood and yard wastes reduces the biodegradability of these components (Gunaseelan, 1997). In this study, a low positive correlation was obtained between the maximum CO2 and wood whereas yard wastes exhibited a low negative correlation with the maximum CO2 formation. 3.3. Multiple linear regression analysis In this study, MLR analysis was used to develop an empirical equation for the maximum CO2 production. Similar to the correlation analysis, the inorganic waste materials (i.e., glass and metal) were excluded in the MLR analysis. The dependent variable was the CO2-max. The independent variables were the moisture content and the weight percents of food wastes, paper-cardboard, wood, textiles, plastics and yard wastes. Hence, the model was run with 9 different bales and 6 groups of waste materials and moisture content (n = 63). After the multiple regression, the coefficients of the CO2-max empirical prediction model in Eq. (2) were determined and rewritten as (R2 = 0.98)
CO2max ¼ 337:83 2:22Pf 3:09P c 2:90Pp 3:35Pt 3:57P y 1:86Pw 0:23M
ð3Þ
where CO2-max is the maximum CO2 production (% volume); Pf is food wastes (% wet weight); Pc is paper-cardboard (% wet weight); Pp is plastics (% wet weight); Pt is textiles (% wet weight); Py is yard
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within the first few days. However, an anaerobic process was not initiated under a lack of oxygen since water content was not high enough and the bales were too acidic for methanogenic bacteria. Therefore, taking all in consideration, baling can be used as a temporary waste storage technique. Moreover, waste bales can be used for the storage of energy for thermal utilization since there was low mass loss in the bales during the storage.
85
Predicted CO2-max (%)
80 75
R2 = 0,98
70 65 60
Acknowledgement
55
The waste materials were provided by the Metropolitan Municipality of Kocaeli. The authors express their thanks to the General Directorate of the Izmit Waste and Residue Treatment, Incineration and Recycling Co. (IZAYDAS) for collaboration.
50 45 40 40
45
50
55
60
65
70
75
80
85
Measured CO2-max (%) Fig. 3. Relationship between the measured and predicted CO2-max.
wastes (% wet weight); Pw is wood (% wet weight) and M is moisture content (% wet basis). The relationship between the measured and predicted CO2-max is given in Fig. 3. Using Eq. (3), the maximum CO2 can be predicted based on the organic waste components and the moisture content. During the temporary storage prior to incineration, energy content of wastes inside the bales may show variations due to possible changes in moisture content and combustible fractions as a result of degradation of organic content. Before baling, waste components and the moisture content would be adjusted using Eq. (3) such that CO2-max is minimized, thus reducing the loss of energy content. Hence, bales showing low decomposition and having high energy content would be formed. 4. Conclusions The present study investigated the temporal variation of CO2, CH4, O2 and N2 in the test bales over 10 months. A total of nine cylindrical bales containing a mix of different waste materials were constructed. While the CO2 production exhibited a rapid increase in the first few days of baling, the CO2 levels decreased and stabilized approximately in a month. Consequently, the aerobic decomposition, took place in the first few days, consumed almost all of the available oxygen inside the bales, leaving just trace levels. There was practically no CH4 production in the first 3 months. Moreover, CH4 did not exceed the 5% during the whole test period. N2 exhibited an opposite pattern with CO2 that should not be generally observed in MSW landfills. In general, similar patterns were observed in all bales and the gas levels measured especially after 40 days of study period were quite identical. The results of this study showed that the content of the bales become less important after a certain period of time. However, the behavior of the bales is directly related to the material baled at the early stage of the storage. The highest positive correlation was obtained between the maximum CO2 and food wastes and moisture content whereas plastics exhibited the highest negative correlation with the maximum CO2. An empirical prediction model for the maximum CO2 production was also developed. The results showed that no significant biodegradation took place during the 10 months of storage. Oxygen was consumed by a rapid process of aerobic decomposition
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2012.02.087. References Ahlgren, P., Jarneving, B., Rousseau, R., 2003. Requirements for a cocitation similarity measure, with special reference to Pearson’s correlation coefficient. J. Am. Soc. Inf. Sci. Technol. 54, 550–560. Ali, M.F., Siddiqui, M.N., 2005. Thermal and catalytic decomposition behavior of PVC mixed plastic waste with petroleum residue. J. Anal. Appl. Pyrol. 74, 282–289. Baldasano, J.M., Gasso, S., Perez, C., 2003. Environmental performance review and cost analysis of MSW landfilling by baling-wrapping technology versus conventional system. Waste Manage. 23, 795–806. Buekens, A.G., Huang, H., 1998. Catalytic plastics cracking for recovery of gasolinerange hydrocarbons from municipal plastic wastes. Resour. Conserv. Recyl. 23, 163–181. El-Fadel, M., Bou-Zeid, B., Chahine, W., Alayli, B., 2002. Temporal variation of leachate quality from pre-sorted and baled municipal solid waste with high organic and moisture content. Waste Manage. 22, 269–282. Farquhar, G.J., Rovers, F.A., 1973. Gas production during refuse decomposition. Water Air Soil Pollut. 2, 483–495. Gunaseelan, V.N., 1997. Anaerobic digestion of biomass for methane production: a review. Biomass Bioenergy 13, 83–114. Hogland, W., Marques, M., Nimmermark, S., Nammari, D., Moutavtchi, V., 2001. Seasonal and Long-term Storage of Waste Fuels with Baling Technique. University of Kalmar, Department of Technology, Report No: 112, Sweden. Kovac-Andric, E., Brana, J., Gvozdic, V., 2009. Impact of meteorological factors on ozone concentrations modelled by time series analysis and multivariate statistical methods. Ecol. Inform. 4, 117–122. Kwon, S.H., Lee, D.G., 2004. Evaluation of Korean food waste composting with fedbatch operations I: using water extractable total organic carbon contents (TOC). Process Biochem. 39, 1183–1194. Miranda, R., Blanco, C.S., Martinez, D.B., Vasile, C., 2007. Pyrolysis of textile wastes-I: kinetics and yields. J. Anal. Appl. Pyrol. 80, 489–495. Nammari, D.R., Hogland, W., Moutavtchi, V., Marques, M., Nimmermark, S., 2003. Physical and chemical processes in baled waste fuel, with emphasis on gaseous emissions. Waste Manage. Res. 21, 309–317. Nammari, D.R., Marques, M., Thorneby, L., Hogland, W., Mathiasson, L., Martensson, L., 2007. Emissions from baled municipal solid waste: I. Methodological approach for investigation of gaseous emissions. Waste Manage. Res. 25, 39–48. Robles-Martinez, F., Gourdon, R., 2000. Long-term behaviour of baled household waste. Bioresour. Technol. 72, 125–130. Ryu, C., Phan, A.N., Sharifi, V.N., Swithenbank, J., 2007. Combustion of textile residues in a packed bed. Exp. Therm. Fluid Sci. 31, 887–895. Selvam, A., Xu, S.Y., Gu, X, Y., Wong, J.W.C., 2010. Food waste decomposition in leachbed reactor: role of neutralizing solutions on the leachate quality. Bioresour. Technol. 101, 1707–1714. Statheropoulos, M., Vassiliadis, N., Pappa, A., 1998. Principal component and canonical correlation analysis for examining air pollution and meteorological data. Atmos. Environ. 32, 1087–1095. Wagner, J., Bilitewski, B., 2009. The temporary storage of municipal solid wasterecommendations for a safe operation of interim storage facilities. Waste Manage. 29, 1693–1701.
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Temporal variation of decomposition gases from baled municipal solid wastes Ismail Ozbay, Ertan Durmusoglu ⇑ University of Kocaeli, Department of Environmental Engineering, Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 16 February 2012 Accepted 17 February 2012 Available online 1 March 2012 Keywords: Municipal solid waste Baling technology Landfill gases Correlation Regression
a b s t r a c t In this study, after nine cylindrical bales containing a mix of different waste materials were constructed, they were stored in the open air and temporal variations of CO2, CH4, O2, and N2 were monitored over 10 months. In each bale, different waste fractions were considered in order to represent different moisture contents. The results showed that CO2 increased within very few days to approximately 80% and stabilized later in the range of between 10% and 35% in a month. The O2 levels dropped from approximately 15% to significantly less than 1%. There was no significant anaerobic decomposition since CH4 did not exceed 5% during the whole test period. N2 exhibited an opposite pattern with CO2. In addition, relationships between waste species in the bales and gas formations were determined by a bivariate correlation analysis. An empirical prediction model for the maximum CO2 production was also developed. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Baling of municipal solid waste (MSW) has been extensively used in several countries for temporary waste storage, especially for incineration (Robles-Martinez and Gourdon, 2000; Nammari et al., 2007; Wagner and Bilitewski, 2009). The baling system uses a baling machine, which compacts the waste materials together in order to produce large bales completely sealed in plastic films. Compacting MSW into bales makes the material significantly easier to handle and store, when compared to loose material. Limited literature exists on the performance of waste bales. The most extensive studies have been carried out on environmental performance (Robles-Martinez and Gourdon, 2000; Baldasano et al., 2003; Wagner and Bilitewski, 2009), leachate quality (El-Fadel et al., 2002) and combustion emissions of bales (Hogland et al., 2001; Nammari et al., 2003, 2007). Robles-Martinez and Gourdon (2000) carried out laboratory experiments in order to assess the long-term behavior of baled household wastes. Several MSW bales were monitored mainly by biogas analysis and temperature measurement over 8 months. The results showed that the biodegradation of baled household waste is very low when the envelope is in good condition. In addition, no CH4 formation was observed. El-Fadel et al. (2002) investigated the temporal variation of leachate quality from baled MSW characterized with high organic and moisture content. They concluded that baling of waste did not hinder waste stabilization that can be attributed to the high ⇑ Corresponding author. Address: University of Kocaeli, Department of Environmental Engineering, Umuttepe Main Campus, Izmit, Kocaeli 41380, Turkey. Tel./fax: +90 262 303 3193. E-mail address: [email protected] (E. Durmusoglu). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2012.02.087
organic fraction and an initial moisture content that equals or exceeds field capacity. Baldasano et al. (2003) reviewed the chemical, physical, and biological processes, and the environmental performance of MSW bales. They pointed out that material inside the bales was stabilized without producing CH4 with a relatively high concentration of CO2. They also indicated based on the cost model that a landfill using rectangular plastic-wrapped bales represents an economically competitive option compared to a conventional landfill. Nammari et al. (2003) measured the temperature and emissions from cylindrical and rectangular bales. The bales were kept at two different ranges of temperatures (30–35 and 20– 25 °C). The results showed that all the bales exhibited aerobic decomposition and the temperature inside the bales did not increase than the ambient air temperature. Nammari et al. (2007) presented a methodological approach for the study of volatile organic compounds (VOCs) in air emitted during storage of MSW in bales. Differences in VOC concentrations in air were found between wastes stored in cylindrical or rectangular bales. They concluded that concentrations of VOCs after storing for cylindrical bales were higher than for rectangular bales. Wagner and Bilitewski (2009) reported different approaches (including baling technology) for interim storage of wastes and related problems. They indicated that further investigations are required to ensure the long-term safety of interim storage operations. Earlier researchers have reported several contradictory results on the waste decomposition inside the bales. In some investigations, bales produced and in others did not produce gaseous emission, methane was present in some cases and in others not. Moreover, studies performed monitoring the decomposition of baled wastes stored in open air are lacking. The effect of waste composition on the gas formation in the bales has not been studied
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samples were taken from the piles and sent to the laboratory for moisture content analysis. The initial moisture contents of the bales were presented in Table 1. The test bales were produced by the Roll-Press-Pack (RPP) system (Schuster Engineering, Germany). The RPP system is mounted in a 12.2 m container and includes a round bale press, a foil wrapper, a conveyor and a control system. The whole baling process lasts 4–5 min and is fully computerized. In order to prepare the individual test bales, the mixed waste materials were fed into the bale chamber until full pressure was reached. While the bale was wrapped, the baler arm returned to start position for new bale formation. The shape of the bales was stabilized by means of a polyethylene (PE)-net, and the bales were enclosed in multiple layers of wrapping material. A 750 mm wide low density polyethylene (LDPE) with a thickness of 25 lm (Duo Plast AG Optima-750, Germany) was used as the wrapping material. Approximately 1000 g of wrapping material per bale was used and the wrapping material constitutes only about 0.1% of the total weight of the bale. After the bales were constructed, they were transported by a front loader and stored in the open air on the premises of the baling site. The test bales were covered with a protective cover. The cylindrical test bales were 120 cm in diameter and 120 cm in height. Their mass depends on the type of waste material in them and their moisture content. The initial weights of the bales were ranged from 800 to 1100 kg (Table 1). The densities of the bales were between 0.6 and 0.8 t/m3. After the bales were stored, CO2, CH4, O2 and N2 were monitored over 10 months. Gas measurements were carried out using a Gas Data LMSxi G3 Portable Multi-function Landfill Gas Analyzer (Ashtead Technology, UK). Gas sampling tube was inserted 60 ± 5 cm at the center of the bales. As soon as the reading stabilized, a single value was recorded. The values were expressed in volume%. The typical accuracy of the analyzer with regard to methane detection is 0.2% at 5% CH4 and 3% at 100% CH4; with regard to carbon dioxide, the accuracy figures are 0.1% at 10% CO2 and 3% at 100% CO2; with regard to oxygen typical accuracy is 0.5% at 25% O2. A flow range of between 0.1 and 20 l/h can be recorded. The instrument calibration was verified regularly in order to minimize the risk of false measurement. Stability of the analyzer calibration for CH4, CO2 and O2 was checked using 103 l gas canister containing 5% CH4, 5% CO2 and 6% O2. The average air temperature was 10.7 °C during the first 5 months of storage (winter and spring). While the average temperature in the later 3 months (summer) was 24.4 °C, the average temperature was 17.8 °C in the last 2 months (autumn). The maximum temperature was recorded as 29 °C at the 7th month of the storage period. Majority of the precipitation was observed in the first 80 days of the storage. While the maximum precipitation was 70 mm at the 180th day of the storage, the average precipitation was 3 mm during the whole storage period. In addition, the wind speed recorded during the study was between 0.8 and 6.2 m/s.
earlier. In this study, after nine cylindrical bales containing a mix of different waste materials were constructed, temporal variations of CO2, CH4, O2 and N2 were monitored over 10 months. In each bale, different waste fractions were considered in order to represent different moisture contents. In addition, relationships between waste species and moisture content in the bales and gas formations were investigated by a bivariate correlation analysis considering maximum CO2 levels during degradation process. An empirical prediction model for maximum CO2 was also developed using a multi linear regression analysis. 2. Methods A total of nine cylindrical bales containing a mix of different waste materials were constructed in January 2010 at the Gebze MSW Transfer Station in Kocaeli, Turkey. The test bales were constructed based on the compositions provided in Table 1. In each bale, different waste fractions were considered in order to represent different moisture contents since the composition of MSW varies substantially with socio-economic conditions, location, season, sampling and sorting procedures, etc. Eight types of fractions were considered: food wastes, paper-cardboard, wood, textiles, plastics, yard wastes, metals and glass. Food wastes were collected from a marketplace. Paper-cardboard, plastics, glass and metals were obtained from a waste recovery plant in Kocaeli. Cloth clippings supplied from a local textile factory were used as textile wastes. Sawdust, leaves, grass and wood were provided by the Metropolitan Municipality of Kocaeli. After waste materials were stored separately, the moisture contents (wet basis) of a representative sample of the waste materials were determined. The moisture contents of the waste materials in% wet basis were 80%, 30%, 2%, 3%, 52%, 40%, 1% and 0.5%, respectively, for food wastes, paper-cardboard, plastics, textiles, wood, yard wastes, metal and glass. It is well known that while low-income areas usually produce more organic materials, which generally mean high moisture content (60–75%), high-income areas produce relatively more inorganic wastes, which generally mean low moisture content (20–30%). In this study, the compositions of the test bales were decided in a way that the moisture contents of the mixed materials inside the bales would be between 25% and 65%. Hence, the test bales represent different locations generating waste with different moisture contents. Before baling, representative samples taken from each waste material were weighted based on the percents provided in Table 1, and they were piled separately. Hence, a total of nine piles having different waste fractions were prepared. Then, the individual piles were mixed thoroughly with the help of a front loader. Finally, the mixed materials for each pile were fed into a baler chamber via a feed conveyor. In total, 10 tons of waste materials were used to create nine cylindrical bales. Prior to baling, representative
Table 1 Compositions, weights and moisture contents of the test bales. Bale No.
1 2 3 4 5 6 7 8 9
Waste categories (% wet weight)
Weight (kg)
Moisture content (% wet basis)
Food wastes
Paper-cardboard
Plastics
Textiles
Wood
Yard wastes
Metal
Glass
Initial
Final
Initial
Final
30 40 30 15 37 59 75 40 20
15 15 9 35 25 13 14 18 32
17 18 19 37 24 9 5 25 30
17 6 30 6 6 10 6 10 16
10 15 – 7 8 9 – 7 2
11 6 4 – – – – – –
– – 5 – – – – – –
– – 3 – – – – – –
800 1100 900 1000 900 900 1000 1100 850
780 1060 880 980 880 850 920 1050 840
31 39 38 22 35 66 56 42 18
40 49 40 35 49 69 65 48 34
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The results were evaluated by two statistical techniques; bivariate correlation analysis and multiple linear regression (MLR) analysis. These techniques have been employed for analyzing numerous environmental data, especially in the area of air pollution research (Statheropoulos et al., 1998). The analysis of the data was carried out using SPSS statistical software (version 17.0). Bivariate correlation analysis tests the degree of relationship between two quantitative variables and enables the determination of relations between a dependent variable (Y) and an independent variable (X) individually. Linear association between variables can be measured with the Pearson Correlation Coefficient that measures the strength and direction (decreasing or increasing, depending on the sign) of a linear relationship between two variables. The Pearson Correlation Coefficient (r) is defined as (Ahlgren et al., 2003)
P ðX XÞðY YÞ r ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi P P 2 2 ðY YÞ ðX XÞ
ð1Þ
and Y are the means of the where X and Y are the variables and X variables. MLR analysis is a method used to model the linear relationship between a dependent variable (predictand) and several independent variables (predictors). MLR identifies the best combination of predictors of the dependent variable. A model of the relationship is hypothesized, and estimates of the parameter values are used to
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develop an estimated regression equation. MLR is expressed according to the following equation (Kovac-Andric et al., 2009).
y ¼ b0 þ b1 x1 þ b2 x2 þ þ bk xk þ e
ð2Þ
where, y is the predictand; b0 is the regression constant; bi are the regression coefficients; xi are the predictrors and e is the stochastic error associated with the regression. The model is estimated by least squares, which yields parameter estimates such that sum of squares of errors is minimized. More detailed information about bivariate correlation analysis and multiple linear regression (MLR) analysis can be found in many standard textbooks or references. 3. Results and discussion 3.1. Temporal variation of decomposition gases Fig. 1 shows the temporal variation of CO2, CH4, O2 and N2. The air was confined in the pores of the waste materials inside the bales when they were formed. Therefore, the bales were biologically active due to the initial quantity of air. Hence, an aerobic decomposition took place as soon as the bales were formed. CO2 exhibited a rapid increase in the first few days of baling depending on the amount of oxygen present inside the bales. The very high CO2 content at the beginning was due to the hydrolysis and acidogenesis stages occurred in the bales. The hydrolysis initiates the non-methanogenic stage by reducing complex organic
Fig. 1. Temporal variation of decomposition gases.
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matter to smaller and soluble components by means of extra cellular enzymes (Farquhar and Rovers, 1973). In the acidogenesis stage, organic materials are further modified producing ammonia, water, H2 and CO2. Furthermore, several types of organic acids are produced during this stage (Farquhar and Rovers, 1973). During the whole period of 10 months, the bales showed a continuous production of CO2. In general, after a rapid increase at the beginning, CO2 reached the maximum levels after several days. Then, the CO2 levels decreased and the CO2 production stabilized in the range of between 10% and 35% approximately in a month. Consequently, the variation of O2 showed a decreasing pattern. The baling process reduced the air gaps between the materials, thus reducing the amount of available oxygen. Moreover, the aerobic decomposition, took place in the first few days, consumed almost all of the available oxygen inside the bales, leaving just trace levels. O2 showed a rapid decrease in the first days of storage. The increase observed between the 30th and 120th days of baling was probably due to the acute O2 entrance during the measurements. After that, some control measures were taken and the O2 levels were decreased to expected levels. On the other hand, the O2 increase at the end of the 250 days was resulted from the possible defects occurred during the storage of the bales. In general, the test bales exhibited good stability during the storage. There was no sign of leachate discharge from the bales and the waste remained contained. No birds were observed during the entire observation period, no significant houseflies were observed, and no shifting or movement of the bales was observed. There was no significant settlement visibly noticeable. However, few small openings on the wrapping material were observed in some bales at the end of the test period. These openings were either caused by animal diggings or the loader during transportation, loading and unloading of the bales. There was practically no CH4 production (below 0.5% in all bales) in the first 3 months. Although CH4 production was later observed, the levels were below 1% in the first 5 months. Moreover, CH4 did not exceed 5% during the whole test period. As a logical consequence, once the oxygen has been consumed, the aerobic process should stop and an anaerobic phase should begin. However, there was no significant anaerobic decomposition in the test bales. Different reasons beside the lack of oxygen may explain the inhibited anaerobic decomposition. One reason would be the acidic environment produced inside the bales. This is explained by the fact that organic acids were formed during the first phase of decomposition. The high concentration of acids reduced the pH and hence the activity of methanogenic bacteria, which normally decompose the organic acids and produce CO2. In this study, the pH values of random measurements were between 5.6 and 7.4 with an average of 6.6. The pH measurements taken at the first several weeks of baling gave acidic results. However, the neutral pH values were observed after 6 months of storage. The other reason for the inhibition of anaerobic decomposition would be the moisture content of the waste materials inside the bales. Since anaerobic decomposition takes place in relatively wet conditions, moisture contents of the bales would not be high enough for bacterial support. Wastes that are too dry (less than 40% moisture content) decay slowly because they lack sufficient water for survival of bacteria. At moisture levels above 60%, small pore spaces that allow oxygen to move into the waste become filled with water. In this study, the initial moisture contents of six bales (1–5 and 9) were below the optimum moisture content for bacterial decomposition (<40%). While the initial moisture contents of two bales (7 and 8) were within the optimum moisture content (40–60%), bale 6 had the initial moisture content above 60%. The initial moisture contents were higher in the bales having higher food wastes. At the end of the test period, the average increase in moisture content was 31.4%. As seen from Table 1, the moisture increase was much
higher in the bales having higher paper-cardboard content. Papercardboard soaks more water than other materials inside the bales. Since the bales were wrapped with LDPE, moisture due to decomposition evaporated and condensed on the plastic. Some of the moisture trickled down onto the waste or was guided by the plastic to a collection point at the bottom. Since the bales were covered during the test period, they had some degree of resistance to perforation and tearing that prevented large quantities of water percolation into the bales. However, we have noticed that strong wind blew the cover several times during the study period. Therefore, few holes or cracks in the wrapping material would allow rainwater flux into the bales although LDPE merely reduces the flux for a period of time. Therefore, several bales would have been influenced by rainwater even in a short period of time. CH4 production (over 1%) observed at the end of 3 months would be caused by an increase in pH (>6.5) or moisture content (>40%). In MSW landfills, while N2 decreases at the beginning, CO2 increases. Then, the production of N2 ends, and the CO2 production stabilizes as soon as the anaerobic phase starts. In this study, N2 exhibited an opposite pattern with CO2 that should not be generally observed in MSW landfills. Robles-Martinez and Gourdon (2000) observed a similar N2 pattern and concluded that certain microbial activities, such as CO2 consumption to synthesize acetic acid or maybe N2 production through respiration with nitrate under anoxic conditions could explain the observed phenomenon. The test bales lost weight (3.5% in average) over 10 months due to the biodegradation took place especially in the early period of storage (Table 1). The weight losses were higher in the bales having higher food content. For moisture contents above 40%, average weight losses for the four bales (bales 2, 6, 7 and 8) ranged from 3.7% to 8.0%. Lower weight losses were measured in the drier bales, 1.2–2.5%. The decomposition gases from each test bale were presented in Fig. 2. In general, similar patterns were observed in all the bales. Moreover, gas levels measured especially after 40 days of test period were quite identical. This is due to the rapid decomposition of readily decomposable portion of the organic content in the early stage of the study. After that, the decomposition gases followed similar patterns since the remaining contents within the bales were similar characteristics. This shows that the content of the bales becomes less important after a certain period of time. On the other hand, in the early stage of storage, the temporal variations of decomposition gases were different in each bale. Therefore, the behavior of the bales is directly related to the material baled at the early stages of the storage. At the end of the test period, the productions of CO2 and N2 in all the bales stabilized in the range of 10–15% and 75–85%, respectively. On the other hand, the O2 levels were less than 1% in all the bales except in bales 6 and 8. This suggested that air infiltrated into bales 6 and 8 either through the actual sampling point (most likely) or through the plastics then into the waste after approximately 8 months of storage.
3.2. Bivariate correlation analysis In this study, the inorganic waste materials (i.e., glass and metal) were excluded in the correlation analysis since they are nonbiodegradable. CO2 evolution rates are generally used to indicate the rate of degradation of organic wastes in aerobic composting processes. The rate of CO2 evolution is expressed as the quantity of CO2 evolved per unit reaction time and per unit biodegradable fraction of the organic wastes (Kwon and Lee, 2004). Relationships between the maximum CO2 formations and bale components and moisture content were evaluated using the bivariate correlation analysis. Pearson Correlation Coefficients describing the relations were 0.89, 0.69, 0.80, 0.28, 0.15, 0.05 and 0.89, respectively,
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Fig. 2. Decomposition gases from different test bales.
for food wastes (Pf), paper-cardboard (Pc), plastics (Pp), textiles (Pt), yard wastes (Py), wood (Pw) and moisture content (M). The food wastes and moisture content exhibited highest correlations with the maximum CO2 formation rate. This can be explained with the readily biodegradable characteristic of food wastes as a result of high volatile fraction of almost 90% of total solids (Selvam et al., 2010). Similarly, moisture content is of great importance for microorganisms since the process takes place primarily in the liquid phase. There was a strong negative correlation between plastics and the maximum CO2 formation. The kinetic studies on the decomposition of various plastics introduced different and great temperature dependency of plastic wastes affecting decomposition rate (Ali and Siddiqui, 2005). So, plastics are resistant to environmental degradation (Buekens and Huang, 1998). The negative relationship between plastics and the maximum CO2 formation can be explained with hardly biodegradability characteristics of plastics. A negative correlation was obtained between paper-cardboard and the maximum CO2 formation. Paper-cardboards are among the major biodegradable organic fraction of MSWs. However, the content of biodegradable carbon of paper-cardboard is rather inaccessible for degradation since the main components are cellulose in the paper-cardboard. Textile wastes contain mixtures of natural and synthetic fibers such as cotton, wool, silk, nylon, olefin and polyester. Different types of fibers show different biodegradability characteristics. For instance, cotton is more degradable than polyester (Miranda et al., 2007). Therefore, the amounts and characteristics of fibers affect the biodegradability characteristics of textile wastes (Ryu et al., 2007). In this study low negative correlation was obtained for the relation between textile wastes and the maximum CO2 production. Digestion of woody biomass has not been considered techni-
cally feasible without pre-treatment. Many factors may influence the biodegradability of wood such as low moisture content, relative lignin, cellulose and hemicellulose content, proportion of structural and non-structural carbohydrates, cellulose crystallinity, particle size, wood-to-bark ratio and toxic components. Lignin contained in wood and yard wastes reduces the biodegradability of these components (Gunaseelan, 1997). In this study, a low positive correlation was obtained between the maximum CO2 and wood whereas yard wastes exhibited a low negative correlation with the maximum CO2 formation. 3.3. Multiple linear regression analysis In this study, MLR analysis was used to develop an empirical equation for the maximum CO2 production. Similar to the correlation analysis, the inorganic waste materials (i.e., glass and metal) were excluded in the MLR analysis. The dependent variable was the CO2-max. The independent variables were the moisture content and the weight percents of food wastes, paper-cardboard, wood, textiles, plastics and yard wastes. Hence, the model was run with 9 different bales and 6 groups of waste materials and moisture content (n = 63). After the multiple regression, the coefficients of the CO2-max empirical prediction model in Eq. (2) were determined and rewritten as (R2 = 0.98)
CO2max ¼ 337:83 2:22Pf 3:09P c 2:90Pp 3:35Pt 3:57P y 1:86Pw 0:23M
ð3Þ
where CO2-max is the maximum CO2 production (% volume); Pf is food wastes (% wet weight); Pc is paper-cardboard (% wet weight); Pp is plastics (% wet weight); Pt is textiles (% wet weight); Py is yard
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within the first few days. However, an anaerobic process was not initiated under a lack of oxygen since water content was not high enough and the bales were too acidic for methanogenic bacteria. Therefore, taking all in consideration, baling can be used as a temporary waste storage technique. Moreover, waste bales can be used for the storage of energy for thermal utilization since there was low mass loss in the bales during the storage.
85
Predicted CO2-max (%)
80 75
R2 = 0,98
70 65 60
Acknowledgement
55
The waste materials were provided by the Metropolitan Municipality of Kocaeli. The authors express their thanks to the General Directorate of the Izmit Waste and Residue Treatment, Incineration and Recycling Co. (IZAYDAS) for collaboration.
50 45 40 40
45
50
55
60
65
70
75
80
85
Measured CO2-max (%) Fig. 3. Relationship between the measured and predicted CO2-max.
wastes (% wet weight); Pw is wood (% wet weight) and M is moisture content (% wet basis). The relationship between the measured and predicted CO2-max is given in Fig. 3. Using Eq. (3), the maximum CO2 can be predicted based on the organic waste components and the moisture content. During the temporary storage prior to incineration, energy content of wastes inside the bales may show variations due to possible changes in moisture content and combustible fractions as a result of degradation of organic content. Before baling, waste components and the moisture content would be adjusted using Eq. (3) such that CO2-max is minimized, thus reducing the loss of energy content. Hence, bales showing low decomposition and having high energy content would be formed. 4. Conclusions The present study investigated the temporal variation of CO2, CH4, O2 and N2 in the test bales over 10 months. A total of nine cylindrical bales containing a mix of different waste materials were constructed. While the CO2 production exhibited a rapid increase in the first few days of baling, the CO2 levels decreased and stabilized approximately in a month. Consequently, the aerobic decomposition, took place in the first few days, consumed almost all of the available oxygen inside the bales, leaving just trace levels. There was practically no CH4 production in the first 3 months. Moreover, CH4 did not exceed the 5% during the whole test period. N2 exhibited an opposite pattern with CO2 that should not be generally observed in MSW landfills. In general, similar patterns were observed in all bales and the gas levels measured especially after 40 days of study period were quite identical. The results of this study showed that the content of the bales become less important after a certain period of time. However, the behavior of the bales is directly related to the material baled at the early stage of the storage. The highest positive correlation was obtained between the maximum CO2 and food wastes and moisture content whereas plastics exhibited the highest negative correlation with the maximum CO2. An empirical prediction model for the maximum CO2 production was also developed. The results showed that no significant biodegradation took place during the 10 months of storage. Oxygen was consumed by a rapid process of aerobic decomposition
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