620007
research-article2016
WMR0010.1177/0734242X15620007Waste Management & ResearchSafar et al.
Original Article
Energy recovery from organic fractions of municipal solid waste: A case study of Hyderabad city, Pakistan
Waste Management & Research 1–10 © The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X15620007 wmr.sagepub.com
Korai M Safar1, Bux R Mahar2, Uqaili M Aslam3, Memon S Ahmed4 and Lashari I Ahmed1
Abstract Non-renewable energy sources have remained the choice of the world for centuries. Rapid growth in population and industrialisation have caused their shortage and environmental degradation by using them. Thus, at the present rate of consumption, they will not last very long. In this prospective, this study has been conducted. The estimation of energy in terms of biogas and heat from various organic fractions of municipal solid waste is presented and discussed. The results show that organic fractions of municipal solid waste possess methane potential in the range of 3%–22% and their heat capacity ranges from 3007 to 20,099 kJ kg−1. Also, theoretical biogas potential of different individual fruit as well as vegetable components and mixed food waste are analysed and estimated in the range of 608–1244 m3 t−1. Further, the share of bioenergy from municipal solid waste in the total primary energy supply in Pakistan has been estimated to be 1.82%. About 8.43% of present energy demand of the country could be met from municipal solid waste. The study leads us to the conclusion that the share of imported energy (i.e. 0.1% of total energy supply) and reduction in the amount of energy from fossil fuels can be achieved by adopting a waste-to-energy system in the country. Keywords Organic fractions, biogas potential, biodegradability, combustibility, solid waste to energy
Introduction Increasing population, urbanisation and rapid economic growth have caused resource consumption to increase. Consequently, the huge quantity of waste going into the environment is increasing (Jagdeep Singh et al., 2014). Globally, most of countries are facing socio-economic as well as environmental problems because of the improper disposal of municipal solid waste (MSW). It has become a very challenging issue to sustain the standard pattern of life. Moreover, the day-to-day urban environment is depreciating because of mismanagement of MSW (Vilas, 2012). At present, time is required to manage MSW in order to recover the energy from it (Olaleye and Richard, 2013). The energy recovery from waste is globally gaining more attraction, because extraction of energy from fossil fuels has left many environmental issues, such as greenhouse gas emissions from plant working upon nonrenewable energy sources (Richa Kothari et al., 2010; Teodor et al., 2012). It has been concluded by IEA (2008) that about 71% of fossil fuels and two-third of the world’s primary energy consumption are globally responsible for direct greenhouse gas emissions. In waste-to-energy treatment, a reduction of about 80% to 87% of the average greenhouse gas emissions per capita can be achieved over the gas boiler case (Keirstead, 2012). The curiosity of the exploitation of energy from biomass is increasing globally owing to various reasons, including the contribution of bioenergy
to sustainable development (Vanden, 2000). Therefore, there is an urgent need to develop such a type of urban energy system, which should maintain the socio-economic as well as environmental friendly conditions of cities. One way is to replace exhaustible energy sources with renewable resources, such as energy from wind, hydro energy, energy from waste, etc. Like other developing countries, the major cities of the Pakistan are in the worst conditions owing to mismanagement of MSW. A major portion of MSW is generally made up of organic waste (Muhammad and Zakariya, 2013). Population, socio-economic development and income level generally affect the MSW generation (Alabaster et al., 2012; Wilson, 2007). A huge quantity of
1Institute
of Environmental Engineering and Management, MUET, Jamshoro, Pakistan 2US-Pakistan Centers for Advanced Studies in Water, Meharn UET, Jamshoro, Sindh, Pakistan 3Electrical Engineering Department, MUET, Jamshoro, Sindh, Pakistan 4Department of Metallurgy and Mining Engineering, MUET, Jamshoro, Pakistan Corresponding author: Korai M Safar, Institute of Environmental Engineering & Management, MUET, Jamshoro, Sindh, Pakistan. Email: [email protected]
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MSW is generated in every city of Pakistan, with a 2.4% annual growth rate (Adeel et al., 2012; Environmental Protection Agency, 2005). Open dumping is observed as the only option for disposal of MSW. On the other hand, the country is seriously facing an energy shortage. Because of this, most of the non-renewable energy sources of energy have remained the choice of Pakistan, like other countries (Pakistan Economic Survey, 2013– 14). In this connection, waste-to-energy would be the most feasible way that could globally function as a more attractive method. Energy from MSW can be obtained in the form of biogas as well as heat energy. Major portion of biogas from biomass is contributed by CH4 and CO2, along with minor gases as nitrogen, hydrogen, carbon monoxide, etc. (Hilkiah et al., 2008). Theoretical biogas yield is dependent upon the contents of organic constituents of biomass (Braun, 2007). A total of 50% to 60% of biogas is composed of methane, which is odourless, colourless and has an energy value of 37.7 MJ m−3 (Yud-Ren et al., 1980). By Anaerobic digestion (AD) of organic fraction of municipal solid waste (OFMSW), not only is biogas generated, but a digestate product is also produced. This is rich in micro and macro nutrients, which are very useful for the improvement of soil fertility (Teodorita et al., 2008). The AD for conversion of OFMSW can also provide job opportunities associated with the collection and transport of feedstock, maintenance, operation and construction of the plant. There are sufficient numbers of biogas plants, where OFMSW is co-digested along with other organic substrate in developed countries. Not only are biogas plant properly working for power generation from MSW in the developed nations, but also varieties of thermal treatment processes. Currently, usage of different disposal/treatment methods for energy recovery from MSW is commonly observed in Asian countries (UN, 2010). But in Pakistan, neither engineered landfill nor any other waste-to-energy plant is functioning successfully in any city. Currently, many domestic biogas plants are functioning in Pakistan by using only animal dung as their feedstock (Sahito, 2010). But no such type of biogas plant is working where OFMSW is digested or co-digested with other organic substances. In this regard, the current study has been carried out with two main objectives. One is to find out the opportunity of disposal and/or treatment methods for OFMSW, by estimating the theoretical energy recovery potential from it. Second is to speed up the use of economically and environmently friendly bioenergy from MSW that would definitely contribute to a low carbon energy demand in the future for the Pakistan.
Methods Physical composition and quantity of MSW A heterogeneous mixture of various unwanted organic as well as inorganic fractions is termed as municipal solid waste. Heterogeneity of MSW is owing to various factors, including seasons, economic conditions of the area, tourist places, celebration days within the year, etc. Seasonal variations and generation source mostly affect the physical composition of MSW. For
examples, fruit and vegetable wastes are seasonal and for those institutions mostly packaging material containing paper, plastics, etc., these are recognised in an abundant quantity. On the basis of the physical composition of MSW (Korai, 2009), further the present quantity of MSW was estimated by equation (1) at a 2.4% growth rate of MSW generation (Adeel, 2012; Environmental Protection Agency, 2005; Therivel and Brown, 1999):
EQ( msw) = GR( msw) x AQ( msw) x N + AQ( msw) (1)
where, EQ(msw) is the estimated quantity of MSW in kg day−1, AQ(msw) is the available quantity of MSW in kg day−1; GR(msw) is the growth rate of MSW generation in percentage and N is the number of years.
Preparation of samples for analysis Using a sampling methodology (ASTM International, 2011; Giovanni et al., 2012), approximately 50 kg of MSW was collected from commercial as well as residential areas of the study area. Inorganic and organic fractions of MSW were separated manually. Samples of OFMSW (composition shown in Figure 1) were prepared according to the quartering method (Korai et al., 2014; Prasada et al., 2010) and analysed in the laboratory for proximate and elemental tests.
Sample analysis Proximate and elemental analysis of samples included determination of its moisture content, volatile solids, total solids and ash content, carbon content, hydrogen content, nitrogen content, oxygen content and sulphur content, respectively. About 50 g of the food and yard waste sample and 100 g of the other waste sample were taken for proximate analysis (Lemma, 2007). The moisture content of all samples was determined according to ASTMD 3173 (2008) and Amin and Gosu (2012). Food and yard waste samples were dried in oven at 105 ºC for 24 h; whereas for the other fraction the temperature was same but the time was reduced to 1 h owing to their dryness (Lemma, 2007). The volatile solids and ash content of the samples was calculated according to the American Public Health Association (1995, 1998). After oven drying, samples, in triplicate form, were ignited at 550 ºC for 2 h, and volatile as well as ash contents were determined on a dry basis. For elemental analysis, first samples were dried at room temperature and their size was reduced to 250 µm by grinding and shredding with the help of scissors (Stewart, 1989). Then, according to the requirement of the elemental analyser, 2.5 mg of each sample was processed by standard procedure of the BBOT 23122013 method.
Theoretical biogas potential and heat capacity of OFMSW The amount of methane was estimated theoretically by using elemental analysis of OFMSW according to equation (2) (Mahar
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Figure 1. Composition of sample for analysis.
et al., 2012; Tchobanoglous and Kreith, 2002). This was used to show the biologically transformation of OFMSW by assuming the completely stabilisation of organic fraction:
Ca H bOc N d +
( 4a − b − 2c + 3d ) H O → 4
( 4a + b − 2c − 3d ) CH 8
4
2
( 4a − b + 2c + 3d ) CO +
2
8
(2) + dNH 3
First, the molar concentration of the element was estimated by using ultimate analysis of OFMSW. After that, the molecular formulas of each component were derived. Then, the theoretical methane potential (TMp) of OFMSW was determined with the help of equation (3) (Mahar et al., 2012), by using the derived co-efficient of their chemical formula. Finally, values were converted into km3 per year, by multiplying the total quantity of OFMSW, and their heat capacity was theoretically estimated using equation (4):
( 4a + b − 2c − 3d ) / 8 X molecular mass of CH 4 X TS % (1 − Ash % ) X 1000 TMp =
(3)
Ca H bOc N d X SpecificWeight of CH 4
Heat Capacity = 337C + 1420 ( H − 0 8 ) + 93S + 23 N kj kg
(4)
From above equations, the biodegradability and combustibility nature of OFMSW was determined, and provided results on
which one had the highest potential regarding biomethanisation and thermal treatment. Further theoretical biogas potential (BPT) of biodegradable waste samples was estimated by using equation (5), in which Sp.Wt. stands for specific weight at standard temperature pressure: 4a + b − 2c − 3d CH 4 X Sp.Wt. of CO2 + 8 4a − b + 2c + d CO2 X Sp.Wt. of CH 4 8 X 1000 (5) BPT = Ca H bOc N d X Sp.Wt. of Biogas
Experimental biogas potential of biodegradable wastes Observations from the literature review show that various researchers have produced works regarding biogas production from biodegradable wastes, such as mixed (OFMSW) and food waste (FW), at a laboratory scale, known as experimental biogas potential (BPE). In the literature, the unit of BPE is different, as either a total quantity or volatile solid, which are not similar. Therefore, values are changed into one unit (total quantity) for uniformity. Then the BPE of the mixed OFMSW and FW is estimated in metre cube per tonne (t) of each waste.
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Figure 2. Estimated quantiy of MSW.
Figure 3. Physical composition of MSW.
Results and discussion Quantity and composition of OFMSW Before taking the decision regarding the selection of suitable treatment and proper disposal of MSW, nobody can deny the significant role that quantification and composition has on MSW. There are two categories of composition of MSW. One is termed as chemical composition and the second is known as physical composition. The first has great importance in evaluating alternating processing and recovery options. The second plays an important role for examining the resources feasibility, recovery of energy and the disposal facility design. The estimated quantity of organic as well as inorganic fractions of MSW, along with total quantity, is represented in the Figure 2. It can be observed that the quantity of MSW is continuously increasing owing to rapid population growth. If generating MSW is not properly managed, it will become the responsible driver for unsustainability in economic development along with environment deprivation. Figure 2 shows that there is great potential to produce biogas and heat energy, leading to power generation from putrescible (i.e. food and yard waste) and combustible waste components
(i.e. cardboard, leather, paper, plastic, rubber, textile, wood, etc.), respectively. In this regard, it is time to analyse various disposal and/or treatment methods for power generation from MSW. The MSW generated in Hyderabad city, Pakistan, is composed of approximately 41% inorganic wastes and 59% organic wastes. The organic waste is further divided into various components of organic nature, as shown in Figure 3. There is variation in the different organic portions of MSW because of various parameters, such as seasons of a year, location to location, economic conditions of people, etc. Food and garden wastes are the first and second highest, respectively. Various samples of MSW were collected in three seasons (winter, summer and monsoon) of a year. In the summer season, mostly from June to August, huge quantities of mango peel and watermelons were found in the samples of selected areas, which contributed to a larger portion of FW in the composition of OFMSW. Samples were also collected from schools and colleges, where mostly paper and packaging material were observed. Because of that, the amount of plastic, cardboard and paper were observed at the third, fourth and fifth positions, respectively, in the composition of OFMSW after food and garden wastes (Figure 3).
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Safar et al. Table 1. Proximate and ultimate analysis of OFMSW. S.No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Components
Cardboard Food Waste Leather Paper Plastic Rubber Cloth waste Wood Garden waste Mixed OFMSW Banana peels Watermelon residue Mango peels Orange peels Grey fruits Pomegranate residue Garma residue Maize comb residue Lady finger residue Bitter gourd residue
Ultimate analysis
Proximate analysis
C%
H%
N%
O%
20.86 35.56 38.86 40.98 71.92 58.34 49.5 44.73 45.36 29.42 34.42 28.85 38.72 41.60 40.50 33.35 44.04 39.00 38.92 36.16
0.0 6.43 5.48 7.44 0.0 0.0 6.45 2.92 8.03 6.48 4.17 3.89 5.29 5.85 6.80 3.98 5.46 5.24 5.66 5.06
0.0 1.53 0.15 0.0 0.0 0.0 0.0 0.0 1.04 0.28 0.69 0.34 0.21 0.85 0.55 0.02 0.41 0.05 1.96 1.53
56.02 46.49 33.89 38.5 23.02 14.06 37.92 49.77 36.32 56.28 20.7 56.41 51.44 47.06 40.90 49.03 38.69 15.35 33.58 11.51
S% 0.1 0.18 0.26 0.08 0.00 0.17 0.00 0.15 0.12 0.06 0.01 0.08 0.02 0.08 0.05 0.10 0.03 0.07 0.14 0.16
MC%
TS%
VS%
AC%
3.91 75.02 0.71 4.82 0.24 0.18 1.34 4.71 30.38 35.79 90.35 92.85 71.02 73.44 69.63 46.15 93.41 63.54 86.87 89.62
96.09 24.98 99.29 95.18 99.76 99.82 98.66 95.29 69.62 64.21 9.66 7.15 28.92 26.56 30.38 53.85 6.60 36.46 13.13 10.38
73.86 82.43 70.40 85.97 95.03 84.13 88.04 96.51 88.85 82.82 60.00 89.57 95.45 95.44 89.28 86.48 88.26 59.88 80.26 54.42
23.02 9.81 21.36 13.00 5.06 27.43 6.13 3.06 8.13 17.18 40.02 10.43 4.55 4.56 10.72 13.52 11.75 40.12 19.74 45.58
OFMSW: organic fractions of municipal solid waste. C%, H%, N%, O%, S%, MC%, VS% and AC% represent percentages of carbon, hydrogen, nitrogen, oxygen, sulphur, moisture content, volatile solids and ash content, respectively.
Proximate and ultimate analysis The elemental and proximate analyses of various OFMSW were carried out and their values are given in Table 1. It is very significant to characterise MSW through proximate and ultimate analysis. Carbon, hydrogen, oxygen, nitrogen and sulphur contents of organic components of MSW were experimentally calculated, whereas values regarding elemental analysis of inorganic components were taken from the literature. According to Table 1, the moisture content of OFMSW lies between 0.18% and 93.41%, and their total solids were observed from 6.60% to 99.82%. The highest and lowest values regarding MC% and TS% stands for garma residue and rubber, and vice versa respectively. Wood waste possesses larger and lower results in terms of volatile solids and ash content (i.e. VS = 96.51% and AC = 3.06%), respectively. The VS% of maize comb residue is smaller (i.e. 59.88%), whereas the highest result of AC% is also in favour of maize comb residue (i.e. 40.12%) than the rest of the components.
Estimation of theoretical heat capacity of OFMSW Combustible fractions of MSW play a vital role, upon which the estimation of energy potential depends. There are various parameters, including temperature, pressure, air–fuel ratio and therma properties (heat and caloricity) of substances, which affect the combustibility of MSW. Heat capacity of different waste
components of MSW is represented in Table 2 ,along with their chemical formula. From Table 2, it can be observed that there is variation in the heat capacity and chemical formula of OFMSW between the present study and from the literature. This variation is because of the composition of MSW. It varies from area to area and location to location. Other factors such as climate change, seasonal variation, weather conditions, etc., make MSW a heterogeneous mixture of various organic as well as inorganic components, generated from different sources. Moreover Table 2 shows that FW and garden waste possess a lesser amount of heat, whereas other components have more heat capacity. In this regard, it can be stated that food and garden waste can be useful for the biological transformation process, whereas others (i.e. cardboard, leather, paper, plastic, rubber, cloth waste and wood waste) for thermal treatment owing to their high combustibility nature in terms of heat capacity.
Theoretical and BPE of biodegradable wastes First, the theoretical methane potential (TMP) of various OFMSW was estimated, as given in Table 3, along with their chemical equations. This was estimated for the present year and after one and two decades. Table 3 indicates the maximum and minimum TMP of various OFMSW, which are 2936 km3 Y−1 and 32 km3 Y−1, respectively. The maximum value stands for FW, whereas the minimum result is for rubber waste. Similarly,
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Table 2. Heat capacity of OFMSW. Organic fraction
Food waste Paper Plastic Rubber Cloth waste Wood waste Garden waste Cardboard Leather Mixed OFMSW
Chemical formula
Heat capacity (kj kg−1)
This study
(Armisheva et al., 2010; Sliusar and Armisheva, 2013)
This study
(Armisheva et al., 2010; Sliusar and Armisheva, 2013)
C485H6463O3213N18S C1430H3315O112S C227HO55 C1003H4O184S C2H3O C883H814O802S C1118H3772O1390N22S C630H160O1353S C440H750O290NS C1289H3378O1851N10S
C3203H5709O1884N149S C5806H9523O4408N349S C35H50OS C4549H694NS C9788H1396O4168N702S C1321H1904O8556N46S C4248H6359O2538N641S — — —
3007 16,697 20,099 19,630 18,842 9853 1066 15,465 14,806 9139
1715 22,471 27,876 26,728 23,576 18,898 1360 27,876 26,728 —
OFMSW: organic fractions of municipal solid waste.
Table 3. Theoretical methane potential from OFMSW. Organic fraction
Cardboard Food waste Leather Paper Plastic Rubber Cloth waste Wood waste Garden waste Mixed OFMSW
Chemical equation
C4H1O8 – 0.25H2O 0.125CH4 + 3.875CO2 13.5CH4 + 13.5CO2 + NH3 C27H353O175N − 148H2O C2H3O1 − 0.75H2O 1.125CH4 + 0.875CO2 C2H3O1 – 0.75H2O 1.125CH4 + 0.875CO2 C227H1O55 + 199.25H2O 99.875CH4 + 127.125CO2 C334H1O61 – 303.25H2O 151.875CH4 + 182.125CO2 C2H3O1 – 0.75H2O 1.125CH4 + 0.875CO2 C1H1O − 0.25H2O 0.375CH4 + 0.625CO2 C50H168O62N − 22.25H2O 30.125CH4 + 19.875CO2 + NH3 C124H326O179N – 46.25H2O 57.625CH4 + 66.375CO2 + NH3
TMP
TMP (km3 Y−1)
(%)
2014
2024
2034
13.97 21.84 10.35 11.49 6.89 4.59 8.05 3.45 19.54 13.97
534 2936 48 345 358 32 70 26 1133 57,256
662 3640 60 428 444 40 87 33 1405 70,997
791 4345 71 511 530 48 103 39 1677 84,738
OFMSW: organic fractions of municipal solid waste.
a trend corresponding to maximum and minimum values of TMP for the same components was observed after one and two decades. Also, the value regarding TMP of mixed OFMSW was estimated as 57,256 km3 Y−1, 70,997 km3 Y−1 and 3 −1 84,738 km Y for the years of 2014, 2024 and 2034, respectively. This indicates that the mixed OFMSW can be physiochemically and biologically converted into methane production by adopting a biochemical reactor for its treatment. It has also been generally observed from this table, that TMP of FW is the highest by more than 20% as compared with other capabilities to generate methane. Yard waste has the second largest value, regarding TMP as greater than 15%. In addition, this tendency of food and garden waste to generate methane ensures that both are more suitable for anaerobic digestion treatment or any other biological conversion process. While remaining components including plastic, paper, rubber, cloth waste, leather, cardboard and wood waste have a lesser biodegradability property of less than 15%. Therefore, these can be thermally treated rather than biologically in order to yield heat energy. Low methane potential and biodegradability of plastic, rubber, leather, cloth waste and wood waste, are known to
be non-biodegradable. The result of methane potential of these components is not considered from the component itself, but because of biodegradable organic matter, which comes from other components during discharge, collection and transport, as reported (Jeon et al., 2007). The same authors estimated the TMP of plastic, wood, cloth waste and leather as 1149, 486, 511, 618 and 1024 mL gmVS, respectively. The biogas production potential of FW is higher than other substances (Figure 4) (Aragundy et al., 2008; Final Report, 2008; House, 2006; Nathan and Pragasen, 2012; Zaher et al., 2009). In the light of this fact regarding higher biogas potential of FW, BPT of OFMSW, FW and various other constituents of FW (i.e. banana peels, discarded watermelon, discarded mango, discarded orange, discarded grey fruits, discarded pomegranate, discarded garma, maize comb, discarded lady finger and discarded bitter gourd) was estimated as given in Table 4, along with their co-efficient chemical formula. Theoretical biogas prediction from FW (Nathan and Pragasen, 2012) and OFMSW (Gupta, 2010) give an amount of 797 m3 t−1 and 626 m3 t−1, respectively. Both values are slightly higher than the BPT of FW
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Safar et al. (i.e 741m3t–1) and OFMSW (i.e 589 m3t–1) of present study (Table 4). If there is little variation between both values, this is because of the heterogeneity of FW generated in different areas with varying climatically conditions, seasonal variation, weather conditions and many other factors.
Figure 4. Biogas yield of various substrate (m3 tVS−1).
The BPT of banana peel, discarded watermelon, discarded mango, discarded orange, discarded grey fruits, discarded pomegranate, discarded garma, maize comb, discarded lady finger, and discarded bitter gourd was estimated to be 1063 m3 t−1, 608 m3 t−1, 750 m3 t−1, 811 m3 t−1, 842 m3 t−1, 718 m3 t−1, 926 m3 t−1, 1214 m3 t−1, 911 m3 t−1, 1244 m3 t−1, respectively (Table 4). This table shows the BPT of different substances, where the position of discarded bitter gourd and maize comb are the top one and two, respectively. The difference between biogas yields of various substrates is because of their chemical composition and volatile solids, as the yield of biogas is the function of volatile solids. Moreover, their tendency to yield biogas shows that these can be treated biologically, either by anaerobic digestion or aerobic composting treatment technology. Further, BPE of OFMSW and FW is represented in the Table 5, along with references. The average BPE of OFMWS and FW was obtained as 314 m3 t−1 and 330 m3 t−1, respectively (Table 5). It has been observed generally from the theoretical and experimental biogas potential of OFMSW and FW that BPT is higher than BPE, as mentioned in Tables 4 and 5, respectively. Completely destruction of volatile solids during BPT gives
Table 4. BPT of OFMSW and FW and its constitutes. S.No.
1 2 3 4 5 6 7 8 9 10 11 12
Waste type
Co-efficient of molecular formula
OFMSW Food waste (mixed) Banana peels Watermelon (discarded) Mango (discarded) Orange (discarded) Grey fruits (discarded) Pomegranate (discarded) Garma (discarded) Maize comb Lady finger residue Bitter gourd (discarded)
BPT
a
b
c
d
m3 t−1
124 27 55 175 233 57 85 1499 121 182 22 29
326 57 79 297 371 99 173 2079 198 1882 36 45
179 26 25 250 233 48 65 1650 78 350 14 7
1 1 1 1 1 1 1 1 1 1 1 1
589 741 1063 608 750 811 842 718 926 1214 911 1244
OFMSW: organic fractions of municipal solid waste.
Table 5. BPE of OFMSW and FW. Waste type
Biogas on the basis of VS/TQ (m3 t−1); BPꞌE
Reference
Correction factor, BPE/BPꞌE
BPE (m3 t−1)
Food waste
392
TQ
1.000
392
= = = Average BPE food waste OFMSW = = =
367 288 472
VS = =
(Abu-Qudais and Abu-Qdais, 2000; Muhammad and Zakariya, 2013) (Final Report, 2008) (Mohan and Bindu, 2008) (Chojae and Parksoon, 1995)
0.826 0.823 0.824
400 350 375 390
VS = = =
303 237 389 330 331 290 311 323
(Contractor’s Report, 2008) (Davidson et al., 2007b) (Davidson et al., 2007a) (Karnchanawong and Uparawanna, 2006)
Average BPE of OFMSW
0.828 0.828 0.829 0.828
314
TQ: total quantity; VS: volatile solids in total solids.
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Figure 5. Primary energy supply.
higher results of BPT than BPE (Nathan and Pragasen, 2012; Gupta, 2010; Muhammad and Zakariya, 2013). Whereas according to Mata-Alvarez (2003), Agriculture Ministry Ontario (2008) and Liu et al. (2009), an experimentally incomplete breakdown (i.e. 45%–65%) of organic material results in a lower amount of BPE as compared with BPT. In this regard, the average BPE of OFMSW and FW of the present study was observed to be 314 m3 t−1 and 330 m3 t−1, with 47% and 55% difference compared with the BPT of 589 m3 t−1 and 741 m3 t−1, respectively. Pakistan has been facing severe energy shortages for many years. Most of the cities of the country are gripping electricity cuts for up to 15–20 hours a day. Zofeen (2015) reported that the energy demand has risen to 19,000 MW. According to the Pakistan Year Energy Book (2013), the primary energy consumption has grown at the annual compound growth rate (ACGR) of 0.5% and stood at 64.58 million tonne (t) oil equivalent (MTOE) in the year 2013. At an ACGR of 0.5%, the primary total energy consumption has been estimated as 65.22 MTOE up to the year 2015 (Figure 5(b)), along with a percentage of different sources (Figure 5(a)). The contribution of bioenergy from MSW in the total primary energy supply of the country has been estimated and would be about 66.42652 MTOE for the year 2015 (Figure 6). During the present year a total of 47.82% (31.76 MTOE) of energy needs were met with indigenous gas, while oil accounted for 31.91% (21.2 MTOE), imported electric 0.098% (0.065 MTOE), nuclear electric 1.669% (1.109 MTOE), hydroelectric 10.8% (7.17 MTOE), coal 5.89% (3.91 MTOE) and MSW 1.82% (1.21 MTOE). This realises that bioenergy from MSW would be enough to replace import energy (i.e. 0.1%). Not only this, but also the burden on the energy supply from fossil fuels could be reduced to some extent by adopting the waste-to-energy concept. Additionally, based upon the theoretical energy potential of MSW, about 8.43% of present energy demand (i.e. 19,000 MW) can be obtained by conversion of MSW into energy. This would be surely beneficial for the better environment as well as economic growth of the country.
Conclusions Over consumption of fossil fuels owing to population growth and environmental degradation owing to emissions releasing from
Figure 6. Contribution of bioenergy from MSW in total supply of energy.
systems based upon them have raised many challenges globally for future economical development. In this regard, the present study was carried out to exploit energy from OFMSW, which is a growing world opinion in favour of looking for an alternative to non-renewable energy sources. The theoretical methane potential (TMP) of cardboard, FW, garden waste, paper, cloth waste, plastic, rubber, leather and wood waste was estimated to be 14%, 22%, 20%, 11%, 8%, 7%, 5%, 10% and 3%, respectively. The theoretical heat energy of plastic, leather, rubber, wood, cloth waste, paper, cardboard, garden waste, FW and mixed OFMSW was estimated as 20,099 kj kg−1, 14,806 kj kg−1, 19,630 kj kg−1, 9853 kj kg−1, 18,842 kj kg−1, 16,697 kj kg−1, 15,465 kj kg−1, 1066 kj kg−1, 3007 kj kg−1 and 9139 kj kg−1, respectively. These results indicate that the TMP of FW, garden waste, cardboard and paper are higher than others. Whereas, remaining components (i.e. plastic, rubber, cloth waste, leather and wood waste) show their excellence in terms of producing heat energy. Additionally, the BPT of banana peels, discarded watermelon, discarded mango, discarded orange, discarded grey fruits, discarded pomegranate, discarded garma, maize comb, discarded lady finger, discarded bitter gourd, mixed FW and mixed OFMSW was estimated as 1063 m3 t−1, 608 m3 t−1, 750 m3 t−1, 811 m3 t−1, 842 m3 t−1, 718 m3 t−1, 926 m3 t−1, 1214 m3 t−1, 911 m3 t−1, 1244 m3 t−1, 741 m3 t−1 and 589 m3 t−1, respectively. Moreover, the contribution of bioenergy from MSW in the total primary energy supply has
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Safar et al. been estimated as 1.82%. Consequently, about 8.43% of the present energy demand of the country can be met by conversion of MSW into energy. Thus from the results, it can be realised that there is an excellent energy potential of MSW in Pakistan. Therefore, it is recommended that different waste-to-energy technologies should be analysed before taking decision of converting MSW into energy.
Acknowledgements The authors are highly thankful to the Institute of Environmental Engineering and Management, Mehran UET, Jamshoro, for providing a sound environment to conduct experimental work for this research study.
Declaration of conflicting interests The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding The authors received no financial support for the research, authorship, and/or publication of this article.
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Richa K, Tyagi VV and Ashish P (2010) Waste to energy: A way from renewable energy sources to sustainable development. Renewable & Sustainable Energy Review 14: 3164–3170. Sliusar N and Armisheva G (2013) Energy resources recovery on MSW disposal. In: Sinks a vital element of modern waste management 2nd international conference on final sinks, Espoo, Finland, 16–18 May 2013. Sahito AR (2010) Characterization and quantification of waste agricultural biomass and its energy potential in the district Sanghar, Sindh, Pakistan. Master’s Thesis, IEEM, Mehran UET, Jamshoro, Sindh, Pakistan. Stewart EA (1989) Chemical analysis of ecological material, 2nd edition. London: Blackwell Scientific Publication. Teodorita AS, Dominik R, Heinz P, et al. (2008) Biogas handbook. Denmark: University of Southern Denmark. Teodor V, Simina N and Cornelia V (2012) Biogas production potential from waste in Timis Country. Analytical Science and Biotechnology 45: 366–373. Therivel R and Brown A (2000) Principles to guide the development of strategic environmental assessment methodology. Impact Assessment and Project Appraisal 18(3): 183–189.
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research-article2016
WMR0010.1177/0734242X15620007Waste Management & ResearchSafar et al.
Original Article
Energy recovery from organic fractions of municipal solid waste: A case study of Hyderabad city, Pakistan
Waste Management & Research 1–10 © The Author(s) 2016 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X15620007 wmr.sagepub.com
Korai M Safar1, Bux R Mahar2, Uqaili M Aslam3, Memon S Ahmed4 and Lashari I Ahmed1
Abstract Non-renewable energy sources have remained the choice of the world for centuries. Rapid growth in population and industrialisation have caused their shortage and environmental degradation by using them. Thus, at the present rate of consumption, they will not last very long. In this prospective, this study has been conducted. The estimation of energy in terms of biogas and heat from various organic fractions of municipal solid waste is presented and discussed. The results show that organic fractions of municipal solid waste possess methane potential in the range of 3%–22% and their heat capacity ranges from 3007 to 20,099 kJ kg−1. Also, theoretical biogas potential of different individual fruit as well as vegetable components and mixed food waste are analysed and estimated in the range of 608–1244 m3 t−1. Further, the share of bioenergy from municipal solid waste in the total primary energy supply in Pakistan has been estimated to be 1.82%. About 8.43% of present energy demand of the country could be met from municipal solid waste. The study leads us to the conclusion that the share of imported energy (i.e. 0.1% of total energy supply) and reduction in the amount of energy from fossil fuels can be achieved by adopting a waste-to-energy system in the country. Keywords Organic fractions, biogas potential, biodegradability, combustibility, solid waste to energy
Introduction Increasing population, urbanisation and rapid economic growth have caused resource consumption to increase. Consequently, the huge quantity of waste going into the environment is increasing (Jagdeep Singh et al., 2014). Globally, most of countries are facing socio-economic as well as environmental problems because of the improper disposal of municipal solid waste (MSW). It has become a very challenging issue to sustain the standard pattern of life. Moreover, the day-to-day urban environment is depreciating because of mismanagement of MSW (Vilas, 2012). At present, time is required to manage MSW in order to recover the energy from it (Olaleye and Richard, 2013). The energy recovery from waste is globally gaining more attraction, because extraction of energy from fossil fuels has left many environmental issues, such as greenhouse gas emissions from plant working upon nonrenewable energy sources (Richa Kothari et al., 2010; Teodor et al., 2012). It has been concluded by IEA (2008) that about 71% of fossil fuels and two-third of the world’s primary energy consumption are globally responsible for direct greenhouse gas emissions. In waste-to-energy treatment, a reduction of about 80% to 87% of the average greenhouse gas emissions per capita can be achieved over the gas boiler case (Keirstead, 2012). The curiosity of the exploitation of energy from biomass is increasing globally owing to various reasons, including the contribution of bioenergy
to sustainable development (Vanden, 2000). Therefore, there is an urgent need to develop such a type of urban energy system, which should maintain the socio-economic as well as environmental friendly conditions of cities. One way is to replace exhaustible energy sources with renewable resources, such as energy from wind, hydro energy, energy from waste, etc. Like other developing countries, the major cities of the Pakistan are in the worst conditions owing to mismanagement of MSW. A major portion of MSW is generally made up of organic waste (Muhammad and Zakariya, 2013). Population, socio-economic development and income level generally affect the MSW generation (Alabaster et al., 2012; Wilson, 2007). A huge quantity of
1Institute
of Environmental Engineering and Management, MUET, Jamshoro, Pakistan 2US-Pakistan Centers for Advanced Studies in Water, Meharn UET, Jamshoro, Sindh, Pakistan 3Electrical Engineering Department, MUET, Jamshoro, Sindh, Pakistan 4Department of Metallurgy and Mining Engineering, MUET, Jamshoro, Pakistan Corresponding author: Korai M Safar, Institute of Environmental Engineering & Management, MUET, Jamshoro, Sindh, Pakistan. Email: [email protected]
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MSW is generated in every city of Pakistan, with a 2.4% annual growth rate (Adeel et al., 2012; Environmental Protection Agency, 2005). Open dumping is observed as the only option for disposal of MSW. On the other hand, the country is seriously facing an energy shortage. Because of this, most of the non-renewable energy sources of energy have remained the choice of Pakistan, like other countries (Pakistan Economic Survey, 2013– 14). In this connection, waste-to-energy would be the most feasible way that could globally function as a more attractive method. Energy from MSW can be obtained in the form of biogas as well as heat energy. Major portion of biogas from biomass is contributed by CH4 and CO2, along with minor gases as nitrogen, hydrogen, carbon monoxide, etc. (Hilkiah et al., 2008). Theoretical biogas yield is dependent upon the contents of organic constituents of biomass (Braun, 2007). A total of 50% to 60% of biogas is composed of methane, which is odourless, colourless and has an energy value of 37.7 MJ m−3 (Yud-Ren et al., 1980). By Anaerobic digestion (AD) of organic fraction of municipal solid waste (OFMSW), not only is biogas generated, but a digestate product is also produced. This is rich in micro and macro nutrients, which are very useful for the improvement of soil fertility (Teodorita et al., 2008). The AD for conversion of OFMSW can also provide job opportunities associated with the collection and transport of feedstock, maintenance, operation and construction of the plant. There are sufficient numbers of biogas plants, where OFMSW is co-digested along with other organic substrate in developed countries. Not only are biogas plant properly working for power generation from MSW in the developed nations, but also varieties of thermal treatment processes. Currently, usage of different disposal/treatment methods for energy recovery from MSW is commonly observed in Asian countries (UN, 2010). But in Pakistan, neither engineered landfill nor any other waste-to-energy plant is functioning successfully in any city. Currently, many domestic biogas plants are functioning in Pakistan by using only animal dung as their feedstock (Sahito, 2010). But no such type of biogas plant is working where OFMSW is digested or co-digested with other organic substances. In this regard, the current study has been carried out with two main objectives. One is to find out the opportunity of disposal and/or treatment methods for OFMSW, by estimating the theoretical energy recovery potential from it. Second is to speed up the use of economically and environmently friendly bioenergy from MSW that would definitely contribute to a low carbon energy demand in the future for the Pakistan.
Methods Physical composition and quantity of MSW A heterogeneous mixture of various unwanted organic as well as inorganic fractions is termed as municipal solid waste. Heterogeneity of MSW is owing to various factors, including seasons, economic conditions of the area, tourist places, celebration days within the year, etc. Seasonal variations and generation source mostly affect the physical composition of MSW. For
examples, fruit and vegetable wastes are seasonal and for those institutions mostly packaging material containing paper, plastics, etc., these are recognised in an abundant quantity. On the basis of the physical composition of MSW (Korai, 2009), further the present quantity of MSW was estimated by equation (1) at a 2.4% growth rate of MSW generation (Adeel, 2012; Environmental Protection Agency, 2005; Therivel and Brown, 1999):
EQ( msw) = GR( msw) x AQ( msw) x N + AQ( msw) (1)
where, EQ(msw) is the estimated quantity of MSW in kg day−1, AQ(msw) is the available quantity of MSW in kg day−1; GR(msw) is the growth rate of MSW generation in percentage and N is the number of years.
Preparation of samples for analysis Using a sampling methodology (ASTM International, 2011; Giovanni et al., 2012), approximately 50 kg of MSW was collected from commercial as well as residential areas of the study area. Inorganic and organic fractions of MSW were separated manually. Samples of OFMSW (composition shown in Figure 1) were prepared according to the quartering method (Korai et al., 2014; Prasada et al., 2010) and analysed in the laboratory for proximate and elemental tests.
Sample analysis Proximate and elemental analysis of samples included determination of its moisture content, volatile solids, total solids and ash content, carbon content, hydrogen content, nitrogen content, oxygen content and sulphur content, respectively. About 50 g of the food and yard waste sample and 100 g of the other waste sample were taken for proximate analysis (Lemma, 2007). The moisture content of all samples was determined according to ASTMD 3173 (2008) and Amin and Gosu (2012). Food and yard waste samples were dried in oven at 105 ºC for 24 h; whereas for the other fraction the temperature was same but the time was reduced to 1 h owing to their dryness (Lemma, 2007). The volatile solids and ash content of the samples was calculated according to the American Public Health Association (1995, 1998). After oven drying, samples, in triplicate form, were ignited at 550 ºC for 2 h, and volatile as well as ash contents were determined on a dry basis. For elemental analysis, first samples were dried at room temperature and their size was reduced to 250 µm by grinding and shredding with the help of scissors (Stewart, 1989). Then, according to the requirement of the elemental analyser, 2.5 mg of each sample was processed by standard procedure of the BBOT 23122013 method.
Theoretical biogas potential and heat capacity of OFMSW The amount of methane was estimated theoretically by using elemental analysis of OFMSW according to equation (2) (Mahar
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Figure 1. Composition of sample for analysis.
et al., 2012; Tchobanoglous and Kreith, 2002). This was used to show the biologically transformation of OFMSW by assuming the completely stabilisation of organic fraction:
Ca H bOc N d +
( 4a − b − 2c + 3d ) H O → 4
( 4a + b − 2c − 3d ) CH 8
4
2
( 4a − b + 2c + 3d ) CO +
2
8
(2) + dNH 3
First, the molar concentration of the element was estimated by using ultimate analysis of OFMSW. After that, the molecular formulas of each component were derived. Then, the theoretical methane potential (TMp) of OFMSW was determined with the help of equation (3) (Mahar et al., 2012), by using the derived co-efficient of their chemical formula. Finally, values were converted into km3 per year, by multiplying the total quantity of OFMSW, and their heat capacity was theoretically estimated using equation (4):
( 4a + b − 2c − 3d ) / 8 X molecular mass of CH 4 X TS % (1 − Ash % ) X 1000 TMp =
(3)
Ca H bOc N d X SpecificWeight of CH 4
Heat Capacity = 337C + 1420 ( H − 0 8 ) + 93S + 23 N kj kg
(4)
From above equations, the biodegradability and combustibility nature of OFMSW was determined, and provided results on
which one had the highest potential regarding biomethanisation and thermal treatment. Further theoretical biogas potential (BPT) of biodegradable waste samples was estimated by using equation (5), in which Sp.Wt. stands for specific weight at standard temperature pressure: 4a + b − 2c − 3d CH 4 X Sp.Wt. of CO2 + 8 4a − b + 2c + d CO2 X Sp.Wt. of CH 4 8 X 1000 (5) BPT = Ca H bOc N d X Sp.Wt. of Biogas
Experimental biogas potential of biodegradable wastes Observations from the literature review show that various researchers have produced works regarding biogas production from biodegradable wastes, such as mixed (OFMSW) and food waste (FW), at a laboratory scale, known as experimental biogas potential (BPE). In the literature, the unit of BPE is different, as either a total quantity or volatile solid, which are not similar. Therefore, values are changed into one unit (total quantity) for uniformity. Then the BPE of the mixed OFMSW and FW is estimated in metre cube per tonne (t) of each waste.
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Figure 2. Estimated quantiy of MSW.
Figure 3. Physical composition of MSW.
Results and discussion Quantity and composition of OFMSW Before taking the decision regarding the selection of suitable treatment and proper disposal of MSW, nobody can deny the significant role that quantification and composition has on MSW. There are two categories of composition of MSW. One is termed as chemical composition and the second is known as physical composition. The first has great importance in evaluating alternating processing and recovery options. The second plays an important role for examining the resources feasibility, recovery of energy and the disposal facility design. The estimated quantity of organic as well as inorganic fractions of MSW, along with total quantity, is represented in the Figure 2. It can be observed that the quantity of MSW is continuously increasing owing to rapid population growth. If generating MSW is not properly managed, it will become the responsible driver for unsustainability in economic development along with environment deprivation. Figure 2 shows that there is great potential to produce biogas and heat energy, leading to power generation from putrescible (i.e. food and yard waste) and combustible waste components
(i.e. cardboard, leather, paper, plastic, rubber, textile, wood, etc.), respectively. In this regard, it is time to analyse various disposal and/or treatment methods for power generation from MSW. The MSW generated in Hyderabad city, Pakistan, is composed of approximately 41% inorganic wastes and 59% organic wastes. The organic waste is further divided into various components of organic nature, as shown in Figure 3. There is variation in the different organic portions of MSW because of various parameters, such as seasons of a year, location to location, economic conditions of people, etc. Food and garden wastes are the first and second highest, respectively. Various samples of MSW were collected in three seasons (winter, summer and monsoon) of a year. In the summer season, mostly from June to August, huge quantities of mango peel and watermelons were found in the samples of selected areas, which contributed to a larger portion of FW in the composition of OFMSW. Samples were also collected from schools and colleges, where mostly paper and packaging material were observed. Because of that, the amount of plastic, cardboard and paper were observed at the third, fourth and fifth positions, respectively, in the composition of OFMSW after food and garden wastes (Figure 3).
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Safar et al. Table 1. Proximate and ultimate analysis of OFMSW. S.No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Components
Cardboard Food Waste Leather Paper Plastic Rubber Cloth waste Wood Garden waste Mixed OFMSW Banana peels Watermelon residue Mango peels Orange peels Grey fruits Pomegranate residue Garma residue Maize comb residue Lady finger residue Bitter gourd residue
Ultimate analysis
Proximate analysis
C%
H%
N%
O%
20.86 35.56 38.86 40.98 71.92 58.34 49.5 44.73 45.36 29.42 34.42 28.85 38.72 41.60 40.50 33.35 44.04 39.00 38.92 36.16
0.0 6.43 5.48 7.44 0.0 0.0 6.45 2.92 8.03 6.48 4.17 3.89 5.29 5.85 6.80 3.98 5.46 5.24 5.66 5.06
0.0 1.53 0.15 0.0 0.0 0.0 0.0 0.0 1.04 0.28 0.69 0.34 0.21 0.85 0.55 0.02 0.41 0.05 1.96 1.53
56.02 46.49 33.89 38.5 23.02 14.06 37.92 49.77 36.32 56.28 20.7 56.41 51.44 47.06 40.90 49.03 38.69 15.35 33.58 11.51
S% 0.1 0.18 0.26 0.08 0.00 0.17 0.00 0.15 0.12 0.06 0.01 0.08 0.02 0.08 0.05 0.10 0.03 0.07 0.14 0.16
MC%
TS%
VS%
AC%
3.91 75.02 0.71 4.82 0.24 0.18 1.34 4.71 30.38 35.79 90.35 92.85 71.02 73.44 69.63 46.15 93.41 63.54 86.87 89.62
96.09 24.98 99.29 95.18 99.76 99.82 98.66 95.29 69.62 64.21 9.66 7.15 28.92 26.56 30.38 53.85 6.60 36.46 13.13 10.38
73.86 82.43 70.40 85.97 95.03 84.13 88.04 96.51 88.85 82.82 60.00 89.57 95.45 95.44 89.28 86.48 88.26 59.88 80.26 54.42
23.02 9.81 21.36 13.00 5.06 27.43 6.13 3.06 8.13 17.18 40.02 10.43 4.55 4.56 10.72 13.52 11.75 40.12 19.74 45.58
OFMSW: organic fractions of municipal solid waste. C%, H%, N%, O%, S%, MC%, VS% and AC% represent percentages of carbon, hydrogen, nitrogen, oxygen, sulphur, moisture content, volatile solids and ash content, respectively.
Proximate and ultimate analysis The elemental and proximate analyses of various OFMSW were carried out and their values are given in Table 1. It is very significant to characterise MSW through proximate and ultimate analysis. Carbon, hydrogen, oxygen, nitrogen and sulphur contents of organic components of MSW were experimentally calculated, whereas values regarding elemental analysis of inorganic components were taken from the literature. According to Table 1, the moisture content of OFMSW lies between 0.18% and 93.41%, and their total solids were observed from 6.60% to 99.82%. The highest and lowest values regarding MC% and TS% stands for garma residue and rubber, and vice versa respectively. Wood waste possesses larger and lower results in terms of volatile solids and ash content (i.e. VS = 96.51% and AC = 3.06%), respectively. The VS% of maize comb residue is smaller (i.e. 59.88%), whereas the highest result of AC% is also in favour of maize comb residue (i.e. 40.12%) than the rest of the components.
Estimation of theoretical heat capacity of OFMSW Combustible fractions of MSW play a vital role, upon which the estimation of energy potential depends. There are various parameters, including temperature, pressure, air–fuel ratio and therma properties (heat and caloricity) of substances, which affect the combustibility of MSW. Heat capacity of different waste
components of MSW is represented in Table 2 ,along with their chemical formula. From Table 2, it can be observed that there is variation in the heat capacity and chemical formula of OFMSW between the present study and from the literature. This variation is because of the composition of MSW. It varies from area to area and location to location. Other factors such as climate change, seasonal variation, weather conditions, etc., make MSW a heterogeneous mixture of various organic as well as inorganic components, generated from different sources. Moreover Table 2 shows that FW and garden waste possess a lesser amount of heat, whereas other components have more heat capacity. In this regard, it can be stated that food and garden waste can be useful for the biological transformation process, whereas others (i.e. cardboard, leather, paper, plastic, rubber, cloth waste and wood waste) for thermal treatment owing to their high combustibility nature in terms of heat capacity.
Theoretical and BPE of biodegradable wastes First, the theoretical methane potential (TMP) of various OFMSW was estimated, as given in Table 3, along with their chemical equations. This was estimated for the present year and after one and two decades. Table 3 indicates the maximum and minimum TMP of various OFMSW, which are 2936 km3 Y−1 and 32 km3 Y−1, respectively. The maximum value stands for FW, whereas the minimum result is for rubber waste. Similarly,
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Table 2. Heat capacity of OFMSW. Organic fraction
Food waste Paper Plastic Rubber Cloth waste Wood waste Garden waste Cardboard Leather Mixed OFMSW
Chemical formula
Heat capacity (kj kg−1)
This study
(Armisheva et al., 2010; Sliusar and Armisheva, 2013)
This study
(Armisheva et al., 2010; Sliusar and Armisheva, 2013)
C485H6463O3213N18S C1430H3315O112S C227HO55 C1003H4O184S C2H3O C883H814O802S C1118H3772O1390N22S C630H160O1353S C440H750O290NS C1289H3378O1851N10S
C3203H5709O1884N149S C5806H9523O4408N349S C35H50OS C4549H694NS C9788H1396O4168N702S C1321H1904O8556N46S C4248H6359O2538N641S — — —
3007 16,697 20,099 19,630 18,842 9853 1066 15,465 14,806 9139
1715 22,471 27,876 26,728 23,576 18,898 1360 27,876 26,728 —
OFMSW: organic fractions of municipal solid waste.
Table 3. Theoretical methane potential from OFMSW. Organic fraction
Cardboard Food waste Leather Paper Plastic Rubber Cloth waste Wood waste Garden waste Mixed OFMSW
Chemical equation
C4H1O8 – 0.25H2O 0.125CH4 + 3.875CO2 13.5CH4 + 13.5CO2 + NH3 C27H353O175N − 148H2O C2H3O1 − 0.75H2O 1.125CH4 + 0.875CO2 C2H3O1 – 0.75H2O 1.125CH4 + 0.875CO2 C227H1O55 + 199.25H2O 99.875CH4 + 127.125CO2 C334H1O61 – 303.25H2O 151.875CH4 + 182.125CO2 C2H3O1 – 0.75H2O 1.125CH4 + 0.875CO2 C1H1O − 0.25H2O 0.375CH4 + 0.625CO2 C50H168O62N − 22.25H2O 30.125CH4 + 19.875CO2 + NH3 C124H326O179N – 46.25H2O 57.625CH4 + 66.375CO2 + NH3
TMP
TMP (km3 Y−1)
(%)
2014
2024
2034
13.97 21.84 10.35 11.49 6.89 4.59 8.05 3.45 19.54 13.97
534 2936 48 345 358 32 70 26 1133 57,256
662 3640 60 428 444 40 87 33 1405 70,997
791 4345 71 511 530 48 103 39 1677 84,738
OFMSW: organic fractions of municipal solid waste.
a trend corresponding to maximum and minimum values of TMP for the same components was observed after one and two decades. Also, the value regarding TMP of mixed OFMSW was estimated as 57,256 km3 Y−1, 70,997 km3 Y−1 and 3 −1 84,738 km Y for the years of 2014, 2024 and 2034, respectively. This indicates that the mixed OFMSW can be physiochemically and biologically converted into methane production by adopting a biochemical reactor for its treatment. It has also been generally observed from this table, that TMP of FW is the highest by more than 20% as compared with other capabilities to generate methane. Yard waste has the second largest value, regarding TMP as greater than 15%. In addition, this tendency of food and garden waste to generate methane ensures that both are more suitable for anaerobic digestion treatment or any other biological conversion process. While remaining components including plastic, paper, rubber, cloth waste, leather, cardboard and wood waste have a lesser biodegradability property of less than 15%. Therefore, these can be thermally treated rather than biologically in order to yield heat energy. Low methane potential and biodegradability of plastic, rubber, leather, cloth waste and wood waste, are known to
be non-biodegradable. The result of methane potential of these components is not considered from the component itself, but because of biodegradable organic matter, which comes from other components during discharge, collection and transport, as reported (Jeon et al., 2007). The same authors estimated the TMP of plastic, wood, cloth waste and leather as 1149, 486, 511, 618 and 1024 mL gmVS, respectively. The biogas production potential of FW is higher than other substances (Figure 4) (Aragundy et al., 2008; Final Report, 2008; House, 2006; Nathan and Pragasen, 2012; Zaher et al., 2009). In the light of this fact regarding higher biogas potential of FW, BPT of OFMSW, FW and various other constituents of FW (i.e. banana peels, discarded watermelon, discarded mango, discarded orange, discarded grey fruits, discarded pomegranate, discarded garma, maize comb, discarded lady finger and discarded bitter gourd) was estimated as given in Table 4, along with their co-efficient chemical formula. Theoretical biogas prediction from FW (Nathan and Pragasen, 2012) and OFMSW (Gupta, 2010) give an amount of 797 m3 t−1 and 626 m3 t−1, respectively. Both values are slightly higher than the BPT of FW
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Safar et al. (i.e 741m3t–1) and OFMSW (i.e 589 m3t–1) of present study (Table 4). If there is little variation between both values, this is because of the heterogeneity of FW generated in different areas with varying climatically conditions, seasonal variation, weather conditions and many other factors.
Figure 4. Biogas yield of various substrate (m3 tVS−1).
The BPT of banana peel, discarded watermelon, discarded mango, discarded orange, discarded grey fruits, discarded pomegranate, discarded garma, maize comb, discarded lady finger, and discarded bitter gourd was estimated to be 1063 m3 t−1, 608 m3 t−1, 750 m3 t−1, 811 m3 t−1, 842 m3 t−1, 718 m3 t−1, 926 m3 t−1, 1214 m3 t−1, 911 m3 t−1, 1244 m3 t−1, respectively (Table 4). This table shows the BPT of different substances, where the position of discarded bitter gourd and maize comb are the top one and two, respectively. The difference between biogas yields of various substrates is because of their chemical composition and volatile solids, as the yield of biogas is the function of volatile solids. Moreover, their tendency to yield biogas shows that these can be treated biologically, either by anaerobic digestion or aerobic composting treatment technology. Further, BPE of OFMSW and FW is represented in the Table 5, along with references. The average BPE of OFMWS and FW was obtained as 314 m3 t−1 and 330 m3 t−1, respectively (Table 5). It has been observed generally from the theoretical and experimental biogas potential of OFMSW and FW that BPT is higher than BPE, as mentioned in Tables 4 and 5, respectively. Completely destruction of volatile solids during BPT gives
Table 4. BPT of OFMSW and FW and its constitutes. S.No.
1 2 3 4 5 6 7 8 9 10 11 12
Waste type
Co-efficient of molecular formula
OFMSW Food waste (mixed) Banana peels Watermelon (discarded) Mango (discarded) Orange (discarded) Grey fruits (discarded) Pomegranate (discarded) Garma (discarded) Maize comb Lady finger residue Bitter gourd (discarded)
BPT
a
b
c
d
m3 t−1
124 27 55 175 233 57 85 1499 121 182 22 29
326 57 79 297 371 99 173 2079 198 1882 36 45
179 26 25 250 233 48 65 1650 78 350 14 7
1 1 1 1 1 1 1 1 1 1 1 1
589 741 1063 608 750 811 842 718 926 1214 911 1244
OFMSW: organic fractions of municipal solid waste.
Table 5. BPE of OFMSW and FW. Waste type
Biogas on the basis of VS/TQ (m3 t−1); BPꞌE
Reference
Correction factor, BPE/BPꞌE
BPE (m3 t−1)
Food waste
392
TQ
1.000
392
= = = Average BPE food waste OFMSW = = =
367 288 472
VS = =
(Abu-Qudais and Abu-Qdais, 2000; Muhammad and Zakariya, 2013) (Final Report, 2008) (Mohan and Bindu, 2008) (Chojae and Parksoon, 1995)
0.826 0.823 0.824
400 350 375 390
VS = = =
303 237 389 330 331 290 311 323
(Contractor’s Report, 2008) (Davidson et al., 2007b) (Davidson et al., 2007a) (Karnchanawong and Uparawanna, 2006)
Average BPE of OFMSW
0.828 0.828 0.829 0.828
314
TQ: total quantity; VS: volatile solids in total solids.
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Waste Management & Research
Figure 5. Primary energy supply.
higher results of BPT than BPE (Nathan and Pragasen, 2012; Gupta, 2010; Muhammad and Zakariya, 2013). Whereas according to Mata-Alvarez (2003), Agriculture Ministry Ontario (2008) and Liu et al. (2009), an experimentally incomplete breakdown (i.e. 45%–65%) of organic material results in a lower amount of BPE as compared with BPT. In this regard, the average BPE of OFMSW and FW of the present study was observed to be 314 m3 t−1 and 330 m3 t−1, with 47% and 55% difference compared with the BPT of 589 m3 t−1 and 741 m3 t−1, respectively. Pakistan has been facing severe energy shortages for many years. Most of the cities of the country are gripping electricity cuts for up to 15–20 hours a day. Zofeen (2015) reported that the energy demand has risen to 19,000 MW. According to the Pakistan Year Energy Book (2013), the primary energy consumption has grown at the annual compound growth rate (ACGR) of 0.5% and stood at 64.58 million tonne (t) oil equivalent (MTOE) in the year 2013. At an ACGR of 0.5%, the primary total energy consumption has been estimated as 65.22 MTOE up to the year 2015 (Figure 5(b)), along with a percentage of different sources (Figure 5(a)). The contribution of bioenergy from MSW in the total primary energy supply of the country has been estimated and would be about 66.42652 MTOE for the year 2015 (Figure 6). During the present year a total of 47.82% (31.76 MTOE) of energy needs were met with indigenous gas, while oil accounted for 31.91% (21.2 MTOE), imported electric 0.098% (0.065 MTOE), nuclear electric 1.669% (1.109 MTOE), hydroelectric 10.8% (7.17 MTOE), coal 5.89% (3.91 MTOE) and MSW 1.82% (1.21 MTOE). This realises that bioenergy from MSW would be enough to replace import energy (i.e. 0.1%). Not only this, but also the burden on the energy supply from fossil fuels could be reduced to some extent by adopting the waste-to-energy concept. Additionally, based upon the theoretical energy potential of MSW, about 8.43% of present energy demand (i.e. 19,000 MW) can be obtained by conversion of MSW into energy. This would be surely beneficial for the better environment as well as economic growth of the country.
Conclusions Over consumption of fossil fuels owing to population growth and environmental degradation owing to emissions releasing from
Figure 6. Contribution of bioenergy from MSW in total supply of energy.
systems based upon them have raised many challenges globally for future economical development. In this regard, the present study was carried out to exploit energy from OFMSW, which is a growing world opinion in favour of looking for an alternative to non-renewable energy sources. The theoretical methane potential (TMP) of cardboard, FW, garden waste, paper, cloth waste, plastic, rubber, leather and wood waste was estimated to be 14%, 22%, 20%, 11%, 8%, 7%, 5%, 10% and 3%, respectively. The theoretical heat energy of plastic, leather, rubber, wood, cloth waste, paper, cardboard, garden waste, FW and mixed OFMSW was estimated as 20,099 kj kg−1, 14,806 kj kg−1, 19,630 kj kg−1, 9853 kj kg−1, 18,842 kj kg−1, 16,697 kj kg−1, 15,465 kj kg−1, 1066 kj kg−1, 3007 kj kg−1 and 9139 kj kg−1, respectively. These results indicate that the TMP of FW, garden waste, cardboard and paper are higher than others. Whereas, remaining components (i.e. plastic, rubber, cloth waste, leather and wood waste) show their excellence in terms of producing heat energy. Additionally, the BPT of banana peels, discarded watermelon, discarded mango, discarded orange, discarded grey fruits, discarded pomegranate, discarded garma, maize comb, discarded lady finger, discarded bitter gourd, mixed FW and mixed OFMSW was estimated as 1063 m3 t−1, 608 m3 t−1, 750 m3 t−1, 811 m3 t−1, 842 m3 t−1, 718 m3 t−1, 926 m3 t−1, 1214 m3 t−1, 911 m3 t−1, 1244 m3 t−1, 741 m3 t−1 and 589 m3 t−1, respectively. Moreover, the contribution of bioenergy from MSW in the total primary energy supply has
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Safar et al. been estimated as 1.82%. Consequently, about 8.43% of the present energy demand of the country can be met by conversion of MSW into energy. Thus from the results, it can be realised that there is an excellent energy potential of MSW in Pakistan. Therefore, it is recommended that different waste-to-energy technologies should be analysed before taking decision of converting MSW into energy.
Acknowledgements The authors are highly thankful to the Institute of Environmental Engineering and Management, Mehran UET, Jamshoro, for providing a sound environment to conduct experimental work for this research study.
Declaration of conflicting interests The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding The authors received no financial support for the research, authorship, and/or publication of this article.
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