507311
research-article2013
WMR32110.1177/0734242X13507311Waste Management & ResearchDong et al.
Original Article
Comparison of municipal solid waste treatment technologies from a life cycle perspective in China
Waste Management & Research 2014, Vol 32(1) 13–23 © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X13507311 wmr.sagepub.com
Jun Dong, Yong Chi, Daoan Zou, Chao Fu, Qunxing Huang and Mingjiang Ni
Abstract China has endured the increasing generation of municipal solid waste; hence, environmental analysis of current waste management systems is of crucial importance. This article presents a comprehensive life cycle assessment of three waste treatment technologies practiced in Hangzhou, China: landfill with and without energy recovery, and incineration with waste-to-energy. Adopting region-specific data, the study covers various environmental impacts, such as global warming, acidification, nutrient enrichment, photochemical ozone formation, human toxicity and ecotoxicity. The results show that energy recovery poses a positive effect in environmental savings. Environmental impacts decrease significantly in landfill with the utilization of biogas owing to combined effects by emission reduction and electricity generation. Incineration is preferable to landfill, but toxicity-related impacts also need to be improved. Furthermore, sensitivity analysis shows that the benefit of carbon sequestration will noticeably decrease global warming potential of both landfill scenarios. Gas collection efficiency is also a key parameter influencing the performance of landfill. Based on the results, improvement methods are proposed. Energy recovery is recommended both in landfill and incineration. For landfill, gas collection systems should be upgraded effectively; for incineration, great efforts should be made to reduce heavy metals and dioxin emissions. Keywords Municipal solid waste, life cycle assessment, landfill gas to energy, waste to energy, heavy metals, improvement measurements
Introduction Continuous industrial development and urbanization has been accompanied by daily growth of municipal solid waste (MSW) around the world. According to the World Bank (Hoornweg et al., 2005), China has become the world’s largest MSW generator since 2005. The total generated MSW is approximately 118.19 and 158.05 million tons in 2000 and 2010, respectively, with an annual growth rate of 4% (CSYCC, 2001, 2011). Meanwhile, landfill is predominant, representing 79.4% of MSW management up to 2010, followed by incineration (18.8%) and composting (1.8%). How to manage MSW in an optimal way has been a highly controversial issue. Open dumping pose serious environmental burdens and should be prohibited. In response, the Chinese government has adopted two feasible alternatives: landfill with gas to electricity (LFGTE); and incineration with waste-to-energy (WTE). Landfill has been widely applied for its simple operation and low operation cost. Accompanied by the use of a highdensity polyethylene (HDPE) top cover and gas collection system, energy recovery becomes possible. However, disadvantages also exist: a large amount of land resources are occupied and the environmental burden remains high owing to methane leakage (Bove and Lunghi, 2006). Meanwhile, as another mainstream technology, incineration has also received increased attention owing to its great effects on waste volume reduction
and complete disinfection. With the installation of state-of-theart flue gas purification, atmospheric emissions can be reduced effectively. However, high investment costs are required, and the potential risk of heavy metal and dioxin emissions also exists. As a consequence, environmental performance of different treatment technologies should be re-examined and evaluated quantitatively. Life cycle assessment (LCA) is a systematic and scientific methodology that considers the entire life cycle of a specified system from cradle to grave. Guided by ISO 14040 (2006), LCA becomes a useful tool to examine the system’s environmental impacts, including up- and downstream activities. In waste management fields, LCA is able to compare the environmental performance of different waste management systems (Buttol et al., 2007; Kaplan et al., 2009), and to be used as a decision support tool (Cherubini et al., 2009; Mohareb et al., 2008). State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, PR China Corresponding author: Yong Chi, State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, China. Email: [email protected]
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
14
Waste Management & Research 32(1)
Table 1. Municipal solid waste composition and characteristics in Hangzhou. Composition and characteristics
Kitchen waste Paper Plastic and rubber Textiles Glass Metal Timber Ceramic and dust Moisture Low heating value
58.15% 13.27% 18.81% 1.47% 2.73% 0.96% 2.61% 2.00% 53.30% 4306 kJ kg-1
Using LCA, there have been some studies aimed at evaluating different MSW treatment technologies. Kaazke et al. (2013) analyzed the environmental impacts of mechanical–biological treatment, recycling, landfill and incineration in Russia. Kong et al. (2012) modeled greenhouse gas emissions of organic waste landfill, composting and anaerobic digestion in the USA. Dahlbo et al. (2005) conducted a comparison of treatment options for newspaper, including material recycling, energy recovery and landfill. However, LCA studies of MSW treatment technologies are rarely performed in developing countries like China. Moreover, any comparison within a same geographic region has never been reported. As pointed out by McDougall et al. (2001), there is no widely accepted optimal waste management system as a result of geographic differences in waste characteristics, energy sources or availability of treatment schemes. From this standpoint, it is worth extending LCA research to one given region in China. In addition, attempts should be made to complete the database, as there has been a noticeable lack of information on toxicity-related emissions; a comprehensive comparison between different impact categories is also limited. Accordingly, the goal of the present study was to provide a detailed life cycle comparison of different MSW treatment technologies in one specified region of China. To complete the LCA database in China, conventional pollutants and toxicity emissions of heavy metals and dioxins are also evaluated. Environmental performance of each technology in each treatment stage is examined to better obtain improvement measurements. Effective and realistic judgments are identified for further developing appropriate waste management strategies and policies.
Methodology ISO 14040 (2006) has established a four-step technical framework for LCA: (1) goal and scope definition; (2) life cycle inventory analysis (LCI); (3) life cycle impact assessment (LCIA); and (4) interpretation.
Goal and scope definition The goal of present study is to provide an environmental assessment of different MSW treatment technologies. Landfill
and incineration are considered, as they are representative of MSW treatment status in China. Meanwhile, as gas collection was started recently in China, both landfill with and without LFGTE exists and should be considered. Therefore, three types of representative treatment technologies in China are compared: 1. Scenario 0—landfill without energy recovery. MSW is sanitary landfilled, but landfill gas is not collected 2. Scenario 1—landfill with LFGTE. Biogas generated from MSW anaerobic decomposition is collected for energy recovery 3. Scenario 2—incineration with WTE. MSW is directly combusted, with the recovered heat used to generate electricity. Scenario 0 is set as the baseline. Hence, environmental improvement brought by LFGTE and WTE can be obtained quantitatively to better understand the emissions mitigation potential. Selected study area. Hangzhou, one of the most developed cities in China, is selected for this study. Approximately 6850 tons of MSW is generated per day. Landfill and incineration are adopted for MSW treatment in the city. As of the end of 2010, 51% of the MSW was landfilled and 49% incinerated. Information on MSW composition is shown in Table 1, which represents the yearly average value investigated by Hangzhou Municipal Solid Waste Disposal Supervision Center (2011). System boundary. The LCA system boundary is shown in Figure 1. Energy and material flows as inputs and outputs are shown with arrows. In general, processes of waste treatment, electricity generation and leachate treatment are considered. MSW collection and transportation is not investigated because it is same for all scenarios, but the transportation of incineration residues (bottom/fly ash) to landfill has to be considered. Waste management system and background system are defined to distinguish direct and indirect environmental burdens. Relevant upstream and downstream processes interacting with the system are also included. Production of diesel, lime, electricity, etc. is all considered, apart from their consumption phase. Thus, the calculation type of ‘cradle to grave’ is achieved. Emissions from infrastructure construction are not considered, as they are very small and could be ignored compared with those released during usage (McDougall et al., 2001). Functional unit. To compare different scenarios on a same basis, functional equivalence should be established. The functional unit in this study is defined as one ton of MSW, i.e. all input and output data should be converted to this basis. Allocation method. As scenario 1 and 2 provide two functional units—treatment of MSW and generation of electricity— allocation of the total environmental impacts becomes essential. In the present study, substitution by system expansion is used. The ‘avoided’ emissions generated by conventional electricity production are subtracted from those produced during waste
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
15
Dong et al.
Figure 1. System boundary of all scenarios.
treatment. Therefore, credits are provided to displace the same amount of electricity produced by fossil fuel consumption.
LCI analysis The aim of LCI is to quantify the environmental emissions related to the system. Thus, all inputs and outputs within the system boundary should be obtained and listed. Data collection. Data used in the present study are mainly derived from on-site investigation. Measurement data on one landfill site and one incineration plant in the city are used (HJIP, 2009; HTLS, 2003). To complete the emission database, different kinds of pollutants are collected, not only for conventional pollutants, but also for toxicity-related emissions. For dioxin emission, yearly average monitoring data are used. For heavy metals, both airborne and aquatic emissions are considered and obtained. For the background system, an inventory of conventional electricity production is obtained based on the national’s average condition (75.9% coal, 3% oil, 2% natural gas, 17.6% hydropower, and 1.5% nuclear power) (Di et al., 2007). Other data relating to raw materials production, if lacking, are obtained from GaBi 4.0 software database. Scenario 0: Landfill. MSW is sanitarily buried. HDPE membranes are used as the top and bottom covers to prevent leachate leakage to ambient soil. Owing to the decomposition of organic fractions, anaerobically-generated landfill gas, if uncollected, will emit to the atmosphere. In scenario 0, landfill gas
is not collected. It is actually a supposed scenario because the landfill site in Hangzhou is already equipped with a gas collection system. This situation will be defined later as scenario 1. The total landfill gas potential is 125.6 m3 per ton of waste according to the site-specific measurement, with CH4 54.4% and CO2 34.1%. Statistical data have shown that the average landfill gas potential in China is around 110–140 m3 t-1 MSW (Zheng et al., 2009), which is in well accordance with this practical data. Some 10% of CH4 oxidation is considered according to IPCC (2007). Meanwhile, a leachate treatment plant is installed for waste water purification. It will operate for 40 years. Afterwards, leachate quality is assumed to be in accordance with MSW landfill pollution standard in China (GB 16889-2008), and will be directly discharged to the city’s sewage treatment plant. Scenario 1: LFGTE. In scenario 1, MSW is disposed at the same landfill site as scenario 0, yet with supplementary of gas collection facilities. According to one year’s field measurements, the biogas is captured at the efficiency of 70 ± 4%, with an energy content of 19.5 MJ m-3. Landfill gas will be collected for 30 years. Collected gas is sent to gas turbine and electric generator producing electricity with 39.1% efficiency. Scenario 2: WTE. The selected incineration plant is a fluidized bed type, with a treatment capacity of 1200 ton day-1. Because the MSW’s heating value (4.31 MJ kg-1) is too low to keep stable burning, auxiliary coal, with a heating value of 17.4 MJ kg-1, is co-fired at a ratio of 50 kg per ton of fed MSW. The
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
16
Waste Management & Research 32(1)
energy recovery efficiency is 27%, with 20% of the generated electricity used as self-consumption. Bottom ash and fly ash, occupying about 22% of the weight, are disposed off in a hazardous waste landfill site 45.8 km far away. Key assumptions. To measure the emissions, some key assumptions are made. 1. One important difference between landfill and incineration in LCA is the time horizon. Emissions from landfill may prevail for thousands of years or more. In order to make potential emissions from landfill comparable, a ‘surveyable time period’ is chosen, which corresponds to approximately one century. It is the end of the biogas formation phase, and no landfill gas is assumed to produce beyond this time period. 2. Biogenic CO2 emission is considered to be carbon neutral and does not contribute to global warming (GW). Only fossilderived CO2 emission is considered, but biogenic carbon released as CH4 should be calculated. For landfill, the benefit of biogenic carbon sequestration is not considered, but will be further analyzed in the ‘Discussion’. 3. Chemical additives used for leachate treatment are not considered owing to the lack of data. This assumption is assumed to be acceptable because of the small amount used. 4. Emissions to the soil are not contained in present study. Longterm investigation should be conducted to measure this damage. Life cycle inventory. Table 2 summarizes key parameters and assumptions applied for both landfill and incineration. A detailed LCI for each scenario is given in Table 3.
LCIA Results from LCI are used for characterization in LCIA phase. The Danish EDIP 97 (Hauschild and Wentzel, 1998) impact assessment method is employed. It is in compliance with the ISO framework and is widely recognized by LCA experts. Equivalent factors are used to translate the emissions into some environmental impacts. Results can be given in three different levels: a characterized value, a normalized value or a weighted normalized value. The considered impact categories are shown in Table 4. Besides standard impact categories [GW, acidification (AC), nutrient enrichment (NE) and photochemical ozone formation (POF)], toxicity-related categories including human toxicity via water/air (HTw/HTa) and ecotoxicity in water chronic (ETwc) are also considered so that the evaluation of heavy metals and dioxin emissions can be obtained quantitatively. Results based on characterized values are used. Normalized references are used to convert the results into person equivalence (Kirkeby et al., 2006), so that the relative magnitude of each impact that contributes to one person by 1 year can be observed. For normalization, European standards are used owing to the lack of information in China. However, this spatial difference can be ignored because all scenarios share the same reference basis.
Table 2. Summary of key parameters and assumptions. Category Landfill Landfill gas generation Landfill gas components CH4 CO2 H2S NH3 Average landfill gas collection efficiency Leachate generation Main leachate pollutants after treatment NH3-N T-P Incineration Fossil-C content in MSW Fossil-C content in auxiliary coal Leachate generation Main leachate pollutants after treatment NH3-N T-P Incineration residues Ash transportation distance, round-trip
Unit
Value
m3 t-1 MSW Volume %
125.6 54.4 34.1 0.2 0.2 70
% kg t-1 MSW mg L-1
200 664 11.9
% %
10.6 44.5
kg t-1 MSW mg L-1
60
% MSW km
5.45 0.27 22 91.6
NH3-N: ammonia nitrogen; MSW: municipal solid waste; T-P: total phosphorus.
Interpretation Finally, output data from LCI and LCIA are summarized and discussed. Meanwhile, the environmental performance of each technology in each treatment stage is also investigated, and corresponding environmental weak points are then identified. Based on the assessment results, improvement measurements can be proposed to satisfy further developing appropriate waste management strategies.
Results Result from LCI analysis Emissions of harmful gases, acid gases and heavy metals are included in Table 3. As can be seen, CH4 in scenarios 0 and 1 is very high, but is not detected in scenario 2. Incomplete gas collection is responsible for the CH4 emitted in scenario 1, but the emissions are much less than that of scenario 0. CO2 emitted in scenario 2 is significantly higher than landfill. Two reasons can explain the result. First, a large amount of fossilderived CO2 is produced during waste combustion, but does not generate in the landfill. Second, coal is co-fired as auxiliary fuel in scenario 2 and becomes another contributor to CO2 emission. Landfill scenarios produce higher NH3, H2S and volatile organic compound (VOC) emissions due to the release of landfill gas, especially without the installation of gas collection system.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
17
Dong et al.
1.96
Conversely, incineration produces higher CO, acid gases and airborne heavy metals. Water pollution caused by landfill is generally higher than incineration owing to not only the larger leachate generation amount, but also higher pollutant concentration. Meanwhile, more energy is recovered by incineration than landfill, at the expense of more input fuels. Therefore, an indepth study into the corresponding environmental impacts should be performed and discussed in LCIA.
0.71
Results from LCIA
Table 3. Life cycle inventory of each scenario for the treatment of 1 ton of municipal solid waste. Activity parameter
Scenario
0
1
2
Inputs Energy Diesel for 0.16 1.26 mechanical operation (L) Diesel for ash 0 0 transportation (L) Electricity 0.42 1.76 (kWh) Materials and ancillary substances HDPE for 0.33 0.45 landfill walls and pipes (kg) Coal (kg) 0 0 Limestone (kg) 0 0 Outputs Direct emissions to air (kg) 0 0 CO2, fossil CO 0 0.07 43.91 17.86 CH4 0.19 0.08 NH3 0.38 0.17 H2S HCl 0 4.46E-02 NOx 0 0.06 0 0.03 SO2 VOCs 0.12 0.05 7.74E-05 3.44E-04 PM10 PCDD/DFs 0 2.40E-10 Mercury 8.36E-08 3.41E-08 Lead 0 0 Cadmium 0 0 Direct emissions to water (kg)a NH3-N 0.20 0.20 T-P 2.38E-03 2.38E-03 Cadmium 6.00E-06 6.00E-06 Copper 4.90E-05 4.90E-05 Chromium 2.00E-05 2.00E-05 Lead 1.00E-04 1.00E-04 Zinc 4.50E-05 4.50E-05 Nickel 1.15E-04 1.15E-04 Mercury 7.22E-06 7.22E-06 Solid residues Fly ash and 0 0 bottom ash (kg) Recovered materials Recovered 0 157.31 electricity (kWh)
73.58b 0 50 9.60 470.25 0.40 0 0 0 0.19 0.83 1.04 0 0.13 6.29E-10 5.03E-04 8.09E-04 6.29E-05 3.27E-04 1.62E-05 3.00E-06 1.74E-05 2.94E-06 8.40E-06 6.42E-06 2.38E-05 1.14E-07 220 367.91b
HDPE: high-density polyethylene; VOCs: volatile organic compounds; PM10: particulate matter of diameter ≤ 10 μm; PCDDs: polychlorinated dibenzo-p-dioxins; PCDFs: polychlorinated dibenzop-dioxins; NH3-N: ammonia nitrogen; T-P: total phosphorus. aFor landfill, due to the use of HDPE membrane, it is assumed no leakage to underground water. bFor incineration, electricity recovered output is used to meet the input energy demand.
Comparison of the LCIA results is given in Figure 2; Figure 3 illustrates the contribution of each treatment stage. The stages are divided into main treatment, raw materials production and electricity generation. GW potential. With respect to GW, both landfill scenarios pose higher potentials than incineration. The dominant reason is the high amount of released CH4, which accounts for 25 times the GW potential than that of CO2. With the installation of a LFGTE system, the GW of scenario 1 has an obvious decrease, but is still higher than scenario 2 for the collection system’s low efficiency and short operation life. As shown in Figure 3, the GW potential essentially comes from the main treatment phase. Raw material production only contributes to a very small proportion for all scenarios. Meanwhile, the GW potential of scenario 2 generated during treatment is some higher than scenario 1. Plastic burning is the main reason, as it is present in a large amount in MSW and creates all fossil-derived CO2. However, more electricity can be recovered by incineration, and more avoided GW potential from fossil-fuel based electricity production can thus be obtained. This environmental saving is so significant that it leads to the lowest overall GW in incineration than both landfill scenarios. AC potential. The main contribution elements for AC are NH3, H2S, HCl, NOx and SO2. For AC, incineration has the best performance, with scenario 0 ranking last. Negative values appear in both scenarios 1 and 2, indicating that actual environmental savings can be achieved with the utilization of recovered energy. For scenario 0, 0.19 kg NH3 and 0.38 kg H2S are discharged, accounting for the most significant contributors to AC. But these emissions are reduced effectively in scenario 1 due to gas collection. If only the main treatment process is considered, incineration would have a much higher AC than landfill. This is mainly owing to high NOx and SO2 emissions generated during waste combustion, which are relatively low in scenario 0 and 1. However, this burden can be well counterbalanced by the effect of energy recovery, which offsets a large amount of acid gases emitted from conventional electricity production. NE potential. For NE, incineration has a negative value and is much lower than both landfill scenarios. Leachate pollutants are the main source causing NE. Landfill has not only a larger leachate generation amount, but also a higher concentration of
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
18
Waste Management & Research 32(1)
Table 4. Life cycle impact categories and normalization reference based on EDIP 97. Impact category
Characterization unit
Normalization reference
Scale
Global warming Acidification Nutrient enrichment Photochemical ozone formation Human toxicity, air Human toxicity, water Ecotoxicity, water chronic
kg CO2-eq. kg SO2-eq. kg NO3-eq. kg C2H4-eq. m3 air m3 water m3 water
8700 74 119 25 2,090,000,000 179,000 352,000
Global Regional Regional Regional Regional Regional Regional
leachate pollutants. In addition, the main compounds in leachate, NH3-N and T-P, both account for high levels of NE equivalent factors. NE of scenario 1 is less than scenario 0. There are two explanations. First, NH3 is another main contributor of NE, which is mainly released from landfill gas and can be reduced by gas collection. Second, as shown in Figure 3, part of the NE can be avoided by electricity generation, thus compensating the overall environmental loadings. POF potential. VOCs, CO and CH4 are main contributors for POF. In view of POF, scenario 0 has the worst performance, mainly owing to the release of VOCs and CH4. A significant decrease has been achieved in scenario 1 thanks to the gas collection and combustion. Both landfill scenarios exhibit higher POF than incineration. CH4 is the dominant reason, as its emission achieves two or three orders of magnitude more than other POF emissions. Although higher amount of CO is generated in incineration its effect on POF is insignificant owing to the low equivalent factor. HTa. For HTa, scenario 2 performs the best not only for the overall impact, but also for each individual stage. Scenario 0 has the worst performance mainly owing to H2S from biogas, which is release in large amounts, and has a high equivalent factor. Airborne dioxin and heavy metals are also main contributors to HTa for their high equivalent factors. However, their effects are not so significant, as the amount generated is relatively small compared with other acid gases. Meanwhile, the recovered electricity also plays an important role, as large amounst of NOx and SO2, which are also contributors to HTa, can be avoided. HTw and ETwc. Toxicity-related impacts via water considered here include HTw and ETwc, in which heavy metals and dioxin play a dominant role. Both HTw and ETwc of incineration are the highest among all scenarios. Incineration produces higher airborne heavy metals and dioxin than landfill, especially for dioxin emission, which is nearly three times than that of scenario 1. Although higher amount of aquatic heavy metals is generated from landfill, their equivalent factors can’t be comparable with that of dioxin. Scenario 1 has the best performance for both HTw and ETwc; even environmental loading has been turned to environmental savings with regard to HTw. When compared to scenario 0, the environmental improvement is mainly attributed to energy
recovery, as the utilization of biogas can be compensated for by the substitution of fossil fuels. Normalized impact potential. Figure 4 illustrates the normalized impact potential for all scenarios. It can be seen that GW and HTa are the principle contributors affected by landfill, while HTw is the most significant factor affected by incineration. Dramatic declines in all impact categories have been achieved when comparing scenario 1 to scenario 0, indicating the advantage brought by biogas collection and utilization. Incineration has the lowest environmental impacts in the majority of impacts, except for HTw and ETwc. The normalized impact potential of HTw in scenario 2 is the highest among all categories. Thus, effective measurements, especially for the reduction of dioxin and heavy metal emissions in the incinerator, become essential and inevitable.
Discussion Realizing the advantages and shortcomings of each technology, some key parameters and improvement measurements are discussed in the following.
Overview and recommendation of MSW treatment technologies In general, both landfills with LFGTE and incineration with WTE exhibit better environmental performance than the base scenario, i.e. landfill without energy recovery. Gas collection and energy recovery are responsible for the environmental savings. Their effects are so significant that they are recommended as an effective improvement method. Great efforts should be paid to reduce the direct emissions during treatment, which is the main source of pollution. Meanwhile, although more input energy and materials should be added for incineration, the environmental burden comes from raw material production is always insignificant. Incineration with WTE is favorable over landfill with LFGTE, as the majority of its environmental impacts are lower. An advanced flue gas cleaning system reduces the pollution level effectively. Meanwhile, energy recovery through WTE, which can generate a larger amount of electricity, is more efficient than LFGTE. However, the HTw and ETwc of incineration are higher, and requires a lot of attention for improvement.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
19
Dong et al.
Figure 2. Comparison of life cycle impact categories results.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
20
Waste Management & Research 32(1)
Figure 3. Environmental impact contribution to each stage.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
21
Dong et al.
Figure 4. Normalized impact potentials for each scenario. GW: global warming; AC: acidification; NE: nutrient enrichment; POF: photochemical ozone formation; HTa: human toxicity via air; HTw: human toxicity via water; ETwc: ecotoxicity in water chronic. Table 5. Biogenic carbon mass balance in landfill at the end of the life cycle assessment time horizon. Category (per ton of MSW)
Value
Input through MSW (kg bio-C) Output through gas (kg bio-C) Output through leachate (kg bio-C) Stored (%)
120.99 59.53 0.18 50.65
MSW: municipal solid waste; bio-C: biogenic carbon.
Figure 5. Global warming potential of each scenario considering carbon sequestration.
In conclusion, it can be stated that energy recovery brings significant advantages and should be adopted in landfill or incineration. Landfill without gas utilization is not recommended for its poor environmental performance. Incineration is a better choice than landfill with the utilization of recovered energy, but great efforts should be made to reduce toxicity-related emissions.
GW and carbon sequestration In some studies, GW is not considered owing to the fact that some emissions come from waste of biogenic origin. However,
the threat of global climate change has become one of the most important contemporary concerns, and greenhouse gas emissions have been required by Kyoto Protocols. Therefore, GW is considered in present study. To measure this indicator, carbon is divided into two parts: fossil-C and biogenic-C. Only fossilderived CO2 emission is considered;biogenic CO2 does not contribute to GW. When conducting LCA, carbon sequestration is another particularly important issue for landfill. Biogenic CO2 emission is considered neutral to GW because it originates from organic matter generated by an equivalent biological uptake of CO2 during plant growth. From this point of view, non-degraded organic carbon stored at the end of the considered time frame represents a ‘saved’ CO2 emission (carbon sequestration). When calculating GW, one of the key issues is associated with whether carbon sequestration should be considered. As carbon sequestration has been extensively discussed and supported by the Intergovernmental Panel (IPCC) (2007) and the US Environmental Protection Agency (USEPA, 2006), its effects are analyzed in this study. Table 5 lists the mass balance for biogenic carbon in landfill. A significant fraction (51%) of the biogenic carbon entering landfill system remains stored at the end of LCA time horizon. The value is consistent with IPCC’s estimate that at least 50% of biogenic carbon in landfill will remain sequestered (IPCC, 2007). In fact, the amount of carbon bound in landfill represents a CO2 reduction potential for approximately 225 kg t-1 MSW. The GW potential of each scenario is illustrated in Figure 5. The benefit of carbon sequestration highly influences the results. The GW potential of both landfill scenarios has a noticeable decrease, whereas incineration is unaffected by this assumption. The consideration of carbon sequestration leads to more preferable results for landfill, especially for scenario 1, which has surpassed incineration and obtained the best GW performance. Meanwhile, although the difference between landfill and incineration becomes smaller, scenario 2 is still preferable over scenario 0. In general, omitting carbon sequestration will significantly worsen the GW potential for landfill. Scenario 1 can achieve the best GW performance if carbon sink is taken into account. From this point of view, landfill with LFGTE is actually environmentally very friendly, and therefore effective measurements should be proposed to further improve the other environmental impacts.
Improvement for landfill technology As mentioned above, pollutants from the main treatment phase contribute to the largest proportion of environmental burdens. The average gas collection efficiency is 70% for landfill in Hangzhou, which is also a representative value of modern landfill sites in China (Zhao et al., 2012). However, as pointed out by Kong et al. (2012), average gas collection efficiency is 75% in USA, and a collection efficiency higher than 82.5% can even be achieved in California. Since landfill gas collection was started in China in recent years, the key factor for improvement is to increase the gas collection efficiency. Figure 6 presents the changing in environmental impact for scenario 1 at different gas collection efficiencies. GW is
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
22
Waste Management & Research 32(1) fractions can increase the heating value of MSW, so that less input energy is needed in the incinerator. Currently, MSW source separated collection has already been attempted in Hangzhou and some other cities in China (Dong et al., 2013), but the application scale and the separation efficiency should be further improved.
Conclusions
Figure 6. Global warming potential of scenario 1 with the variation of gas collection efficiency.
investigated as it is a main impact suffered by landfill. Increasing the gas collection efficiency can significantly decrease GW. A 10% increase in collection efficiency can result in a decreased GW potential of 32.7%. If only a 5–10% increase is achieved, the GW potential of scenario 1 will also decrease and become less than scenario 2. Combined effects by emission reduction and increased electricity generation can explain the result. Although a collection efficiency of 100% cannot be achieved, the decreased GW trend can be obtained. As a consequence, an increase in the gas collection efficiency has proven to be one of the effective improvement measurements for landfill. Highly efficient landfill is actually environmentally very competitive for managing MSW with regard to GW, and is a vital part of an integrated waste management policy. Meanwhile, data from Table 3 show a high pollution level of aquatic emissions from landfill. Large amounts and high concentrations of leachate will generate from landfill. Thus, more effective and advanced leachate treatment technology should be adopted, and the operation life span of the leachate treatment facility should also be re-considered.
Improvement for incineration technology For incineration, the most concerning categories are toxicity-related impacts, such as HTw and ETwc. Dioxin and heavy metals are the main contributors; thus, a technology upgrade for the flue gas cleaning system is essential. Meanwhile, combustion technology should also be improved. As pointed out by McKay (2002), dioxin easily forms during the combustion process. Therefore, advanced combustion technology, such as good ‘3T+E’ (temperature, time, turbulence and excess air), should be further developed. Another the improvement method for incineration is the implementation of source separated collection. Some waste fractions that contain high concentrations of chloride and metals, such as polyvinylchloride and polychlorinated biphenyls, can be treated separately to decrease the possibility of the formation of toxicity-related emissions. In addition, separating out organic
A comprehensive LCA on different MSW treatment technologies commonly used in China has been conducted. Three scenarios were considered: landfill with (scenario 1) and without (scenario 0) energy recovery, and incineration with WTE (scenario 2). Environmental impacts investigated include not only the standard categories (GW, AC, NE, POF), but also toxicity-related categories (HTa, HTw, ETwc). The results show that scenario 0 presents the worst environmental performance. Once gas is collected and treated (scenario 1), all environmental impacts significantly decrease. Incineration (scenario 2) is preferable to landfill, except for toxicity-related impacts (HTw and ETwc). Energy recovery, whether through gas utilization or waste incineration, can lead to saved emissions to the environment. When examining the environmental contribution from each treatment stage, pollutants are mainly generated during treatment. Results from normalized impacts show that methane and other harmful gases contained in landfill gas are the main contributors affected by landfill, while dioxin and heavy metals are most influenced by incineration. Sensitivity analysis shows that if carbon sequestration is considered, the GW potential of both landfill scenarios would noticeably decrease. Scenario 1 will surpass incineration and obtained the best GW potential performance. Gas collection efficiency is also a key parameter influencing the performance of landfill, and highly efficient landfill is actually environmentally very competitive for managing MSW with regard to GW. Based on the results, some improvement methods are proposed. The positive effect of energy recovery is so great that is recommended in both landfill and incineration. For landfill, it is crucially important to ensure a high collection efficiency. For incineration, an advanced flue gas cleaning system and combustion technology should be developed, and a policy that promotes source separated collection should also be pursued. In conclusion, the research is helpful in supplementing the life cycle database in China. Meanwhile, results from the present study can provide scientific and systematic information for decision-makers regarding further developing waste management strategies.
Declaration of conflicting interests The authors do not have any potential conflicts of interest to declare.
Funding This project was supported by the National Basic Research Program of China (No. 2011CB201506) and the Program of Introducing Talents of Discipline to University (B08026).
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
23
Dong et al. References Bove R and Lunghi P (2006) Electric power generation from landfill gas using traditional and innovative technologies. Energy Conversion and Management 47: 1391–1401. Buttol P, Masoni P, Bonoli A, et al. (2007) LCA of integrated MSW management systems: Case study of the Bologna District. Waste Management 27: 1059–1070. Cherubini F, Bargigli S and Ulgiati S (2009) Life cycle assessment (LCA) of waste management strategies: Landfilling, sorting plant and incineration. Energy 34: 2116–2123. CSYCC (Chinese Statistics Yearbook Compiling Committee) (2001) [Chinese Statistics Yearbook 2000]. Beijing: Chinese Statistics Press [in Chinese]. CSYCC (Chinese Statistics Yearbook Compiling Committee) (2011) [Chinese Statistics Yearbook 2010]. Beijing: Chinese Statistics Press [in Chinese]. Dahlbo H, Koskela S, Laukka J, et al. (2005) Life cycle inventory analyses for five waste management options for discarded newspaper. Waste Management & Research 23: 291–303. Di X, Nie Z, Yuan B, et al. (2007) Life cycle inventory for electricity generation in China. International Journal of LCA 12: 217–224. Dong J, Ni M, Chi Y, et al. (2013) Life cycle and economic assessment of source-separated MSW collection with regard to greenhouse gas emissions: a case study in China. Environmental Science and Pollution Research 20: 5512–5524. GB 16889–2008 (2008) Municipal solid waste landfill pollution control standards in China [in Chinese]. Hangzhou Municipal Solid Waste Disposal Supervision Center (2011) [Hangzhou Municipal Solid Waste Physical Property Analysis and Disposal Method]. Hangzhou: Municipal Solid Waste Disposal Supervision Center [in Chinese]. Hauschild M and Wentzel H (1998) Environmental Assessment of Products. Volume 1: Methodology, Tools and Case. London: Chapman & Hall. HJIP (Hangzhou Jinjiang Incineration Plant) (2009) [Annual Operation Report 2009]. Hangzhou: Hangzhou Jinjiang Incineration Plant [in Chinese]. Hoornweg D, Lam P and Chaudhry M (2005) Waste Management in China: Issues and Recommendations. Washington, DC: East Asia Infrastructure Development, the World Bank.
HTLS (Hangzhou Tianziling Landfill Site) (2003) [Hangzhou Second Landfill Site Environmental Impact Reports]. Hangzhou: Hangzhou Tianziling Landfill Site [in Chinese]. IPCC (Intergovernmental Panel on Climate Change) (2007) Climate Change 2007: Mitigation, Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press. ISO 14040 (2006) Environmental management-Life cycle assessmentPrincipal and framework. Kaazke J, Meneses M, Wilke BM, et al. (2013) Environmental evaluation of waste treatment scenarios for the towns Khanty-Mansiysk and Surgut, Russia. Waste Management & Research 31: 315–326. Kaplan PO, Decarolis J and Thorneloe S (2009) Is it better to burn or bury waste for clean electricity generation. Environmental Science and Technology 43: 1711–1717. Kirkeby JT, Bhander GS, Birgisdottir H, et al. (2006) Environmental assessment of solid waste systems and technologies: EASEWASTE. Waste Management & Research 24: 3–15. Kong D, Shan J, Iacoboni M, et al. (2012) Evaluating greenhouse gas impacts of organic waste management options using life cycle assessment. Waste Management & Research 30: 800–812. McDougall F, White P, Franke M, et al. (2001) Integrated Solid Waste Management: A Life Cycle Inventory. London: Blackwell Science. McKay G (2002) Dioxin characterisation, formation and minimisation during municipal solid waste (MSW) incineration: review. Chemical Engineering Journal 86: 343–368. Mohareb AK, Warith MA and Diaz R (2008) Modelling greenhouse gas emissions for municipal solid waste management strategies in Ottawa, Ontario, Canada. Resources, Conservation and Recycling 52: 1241–1251. USEPA (US Environmental Protection Agency) (2006) Solid Waste Management and Greenhouse Gases: A Life-cycle Assessment of Emissions and Sinks. 3rd ed. Washington, DC: USEPA. Zhao C, Zhao L, Chen XM, et al. (2012) A study on the collection efficiency of methane in municipal solid waste landfill [in Chinese]. Acta Scientiae Circumstantiae 32: 954–959. Zheng X, Yang Y, Lei Y (2009) [Analysis of MSW landfill gas to energy in China]. Environmental protection 414: 19–22 [in Chinese].
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
research-article2013
WMR32110.1177/0734242X13507311Waste Management & ResearchDong et al.
Original Article
Comparison of municipal solid waste treatment technologies from a life cycle perspective in China
Waste Management & Research 2014, Vol 32(1) 13–23 © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0734242X13507311 wmr.sagepub.com
Jun Dong, Yong Chi, Daoan Zou, Chao Fu, Qunxing Huang and Mingjiang Ni
Abstract China has endured the increasing generation of municipal solid waste; hence, environmental analysis of current waste management systems is of crucial importance. This article presents a comprehensive life cycle assessment of three waste treatment technologies practiced in Hangzhou, China: landfill with and without energy recovery, and incineration with waste-to-energy. Adopting region-specific data, the study covers various environmental impacts, such as global warming, acidification, nutrient enrichment, photochemical ozone formation, human toxicity and ecotoxicity. The results show that energy recovery poses a positive effect in environmental savings. Environmental impacts decrease significantly in landfill with the utilization of biogas owing to combined effects by emission reduction and electricity generation. Incineration is preferable to landfill, but toxicity-related impacts also need to be improved. Furthermore, sensitivity analysis shows that the benefit of carbon sequestration will noticeably decrease global warming potential of both landfill scenarios. Gas collection efficiency is also a key parameter influencing the performance of landfill. Based on the results, improvement methods are proposed. Energy recovery is recommended both in landfill and incineration. For landfill, gas collection systems should be upgraded effectively; for incineration, great efforts should be made to reduce heavy metals and dioxin emissions. Keywords Municipal solid waste, life cycle assessment, landfill gas to energy, waste to energy, heavy metals, improvement measurements
Introduction Continuous industrial development and urbanization has been accompanied by daily growth of municipal solid waste (MSW) around the world. According to the World Bank (Hoornweg et al., 2005), China has become the world’s largest MSW generator since 2005. The total generated MSW is approximately 118.19 and 158.05 million tons in 2000 and 2010, respectively, with an annual growth rate of 4% (CSYCC, 2001, 2011). Meanwhile, landfill is predominant, representing 79.4% of MSW management up to 2010, followed by incineration (18.8%) and composting (1.8%). How to manage MSW in an optimal way has been a highly controversial issue. Open dumping pose serious environmental burdens and should be prohibited. In response, the Chinese government has adopted two feasible alternatives: landfill with gas to electricity (LFGTE); and incineration with waste-to-energy (WTE). Landfill has been widely applied for its simple operation and low operation cost. Accompanied by the use of a highdensity polyethylene (HDPE) top cover and gas collection system, energy recovery becomes possible. However, disadvantages also exist: a large amount of land resources are occupied and the environmental burden remains high owing to methane leakage (Bove and Lunghi, 2006). Meanwhile, as another mainstream technology, incineration has also received increased attention owing to its great effects on waste volume reduction
and complete disinfection. With the installation of state-of-theart flue gas purification, atmospheric emissions can be reduced effectively. However, high investment costs are required, and the potential risk of heavy metal and dioxin emissions also exists. As a consequence, environmental performance of different treatment technologies should be re-examined and evaluated quantitatively. Life cycle assessment (LCA) is a systematic and scientific methodology that considers the entire life cycle of a specified system from cradle to grave. Guided by ISO 14040 (2006), LCA becomes a useful tool to examine the system’s environmental impacts, including up- and downstream activities. In waste management fields, LCA is able to compare the environmental performance of different waste management systems (Buttol et al., 2007; Kaplan et al., 2009), and to be used as a decision support tool (Cherubini et al., 2009; Mohareb et al., 2008). State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, PR China Corresponding author: Yong Chi, State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, China. Email: [email protected]
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
14
Waste Management & Research 32(1)
Table 1. Municipal solid waste composition and characteristics in Hangzhou. Composition and characteristics
Kitchen waste Paper Plastic and rubber Textiles Glass Metal Timber Ceramic and dust Moisture Low heating value
58.15% 13.27% 18.81% 1.47% 2.73% 0.96% 2.61% 2.00% 53.30% 4306 kJ kg-1
Using LCA, there have been some studies aimed at evaluating different MSW treatment technologies. Kaazke et al. (2013) analyzed the environmental impacts of mechanical–biological treatment, recycling, landfill and incineration in Russia. Kong et al. (2012) modeled greenhouse gas emissions of organic waste landfill, composting and anaerobic digestion in the USA. Dahlbo et al. (2005) conducted a comparison of treatment options for newspaper, including material recycling, energy recovery and landfill. However, LCA studies of MSW treatment technologies are rarely performed in developing countries like China. Moreover, any comparison within a same geographic region has never been reported. As pointed out by McDougall et al. (2001), there is no widely accepted optimal waste management system as a result of geographic differences in waste characteristics, energy sources or availability of treatment schemes. From this standpoint, it is worth extending LCA research to one given region in China. In addition, attempts should be made to complete the database, as there has been a noticeable lack of information on toxicity-related emissions; a comprehensive comparison between different impact categories is also limited. Accordingly, the goal of the present study was to provide a detailed life cycle comparison of different MSW treatment technologies in one specified region of China. To complete the LCA database in China, conventional pollutants and toxicity emissions of heavy metals and dioxins are also evaluated. Environmental performance of each technology in each treatment stage is examined to better obtain improvement measurements. Effective and realistic judgments are identified for further developing appropriate waste management strategies and policies.
Methodology ISO 14040 (2006) has established a four-step technical framework for LCA: (1) goal and scope definition; (2) life cycle inventory analysis (LCI); (3) life cycle impact assessment (LCIA); and (4) interpretation.
Goal and scope definition The goal of present study is to provide an environmental assessment of different MSW treatment technologies. Landfill
and incineration are considered, as they are representative of MSW treatment status in China. Meanwhile, as gas collection was started recently in China, both landfill with and without LFGTE exists and should be considered. Therefore, three types of representative treatment technologies in China are compared: 1. Scenario 0—landfill without energy recovery. MSW is sanitary landfilled, but landfill gas is not collected 2. Scenario 1—landfill with LFGTE. Biogas generated from MSW anaerobic decomposition is collected for energy recovery 3. Scenario 2—incineration with WTE. MSW is directly combusted, with the recovered heat used to generate electricity. Scenario 0 is set as the baseline. Hence, environmental improvement brought by LFGTE and WTE can be obtained quantitatively to better understand the emissions mitigation potential. Selected study area. Hangzhou, one of the most developed cities in China, is selected for this study. Approximately 6850 tons of MSW is generated per day. Landfill and incineration are adopted for MSW treatment in the city. As of the end of 2010, 51% of the MSW was landfilled and 49% incinerated. Information on MSW composition is shown in Table 1, which represents the yearly average value investigated by Hangzhou Municipal Solid Waste Disposal Supervision Center (2011). System boundary. The LCA system boundary is shown in Figure 1. Energy and material flows as inputs and outputs are shown with arrows. In general, processes of waste treatment, electricity generation and leachate treatment are considered. MSW collection and transportation is not investigated because it is same for all scenarios, but the transportation of incineration residues (bottom/fly ash) to landfill has to be considered. Waste management system and background system are defined to distinguish direct and indirect environmental burdens. Relevant upstream and downstream processes interacting with the system are also included. Production of diesel, lime, electricity, etc. is all considered, apart from their consumption phase. Thus, the calculation type of ‘cradle to grave’ is achieved. Emissions from infrastructure construction are not considered, as they are very small and could be ignored compared with those released during usage (McDougall et al., 2001). Functional unit. To compare different scenarios on a same basis, functional equivalence should be established. The functional unit in this study is defined as one ton of MSW, i.e. all input and output data should be converted to this basis. Allocation method. As scenario 1 and 2 provide two functional units—treatment of MSW and generation of electricity— allocation of the total environmental impacts becomes essential. In the present study, substitution by system expansion is used. The ‘avoided’ emissions generated by conventional electricity production are subtracted from those produced during waste
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
15
Dong et al.
Figure 1. System boundary of all scenarios.
treatment. Therefore, credits are provided to displace the same amount of electricity produced by fossil fuel consumption.
LCI analysis The aim of LCI is to quantify the environmental emissions related to the system. Thus, all inputs and outputs within the system boundary should be obtained and listed. Data collection. Data used in the present study are mainly derived from on-site investigation. Measurement data on one landfill site and one incineration plant in the city are used (HJIP, 2009; HTLS, 2003). To complete the emission database, different kinds of pollutants are collected, not only for conventional pollutants, but also for toxicity-related emissions. For dioxin emission, yearly average monitoring data are used. For heavy metals, both airborne and aquatic emissions are considered and obtained. For the background system, an inventory of conventional electricity production is obtained based on the national’s average condition (75.9% coal, 3% oil, 2% natural gas, 17.6% hydropower, and 1.5% nuclear power) (Di et al., 2007). Other data relating to raw materials production, if lacking, are obtained from GaBi 4.0 software database. Scenario 0: Landfill. MSW is sanitarily buried. HDPE membranes are used as the top and bottom covers to prevent leachate leakage to ambient soil. Owing to the decomposition of organic fractions, anaerobically-generated landfill gas, if uncollected, will emit to the atmosphere. In scenario 0, landfill gas
is not collected. It is actually a supposed scenario because the landfill site in Hangzhou is already equipped with a gas collection system. This situation will be defined later as scenario 1. The total landfill gas potential is 125.6 m3 per ton of waste according to the site-specific measurement, with CH4 54.4% and CO2 34.1%. Statistical data have shown that the average landfill gas potential in China is around 110–140 m3 t-1 MSW (Zheng et al., 2009), which is in well accordance with this practical data. Some 10% of CH4 oxidation is considered according to IPCC (2007). Meanwhile, a leachate treatment plant is installed for waste water purification. It will operate for 40 years. Afterwards, leachate quality is assumed to be in accordance with MSW landfill pollution standard in China (GB 16889-2008), and will be directly discharged to the city’s sewage treatment plant. Scenario 1: LFGTE. In scenario 1, MSW is disposed at the same landfill site as scenario 0, yet with supplementary of gas collection facilities. According to one year’s field measurements, the biogas is captured at the efficiency of 70 ± 4%, with an energy content of 19.5 MJ m-3. Landfill gas will be collected for 30 years. Collected gas is sent to gas turbine and electric generator producing electricity with 39.1% efficiency. Scenario 2: WTE. The selected incineration plant is a fluidized bed type, with a treatment capacity of 1200 ton day-1. Because the MSW’s heating value (4.31 MJ kg-1) is too low to keep stable burning, auxiliary coal, with a heating value of 17.4 MJ kg-1, is co-fired at a ratio of 50 kg per ton of fed MSW. The
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
16
Waste Management & Research 32(1)
energy recovery efficiency is 27%, with 20% of the generated electricity used as self-consumption. Bottom ash and fly ash, occupying about 22% of the weight, are disposed off in a hazardous waste landfill site 45.8 km far away. Key assumptions. To measure the emissions, some key assumptions are made. 1. One important difference between landfill and incineration in LCA is the time horizon. Emissions from landfill may prevail for thousands of years or more. In order to make potential emissions from landfill comparable, a ‘surveyable time period’ is chosen, which corresponds to approximately one century. It is the end of the biogas formation phase, and no landfill gas is assumed to produce beyond this time period. 2. Biogenic CO2 emission is considered to be carbon neutral and does not contribute to global warming (GW). Only fossilderived CO2 emission is considered, but biogenic carbon released as CH4 should be calculated. For landfill, the benefit of biogenic carbon sequestration is not considered, but will be further analyzed in the ‘Discussion’. 3. Chemical additives used for leachate treatment are not considered owing to the lack of data. This assumption is assumed to be acceptable because of the small amount used. 4. Emissions to the soil are not contained in present study. Longterm investigation should be conducted to measure this damage. Life cycle inventory. Table 2 summarizes key parameters and assumptions applied for both landfill and incineration. A detailed LCI for each scenario is given in Table 3.
LCIA Results from LCI are used for characterization in LCIA phase. The Danish EDIP 97 (Hauschild and Wentzel, 1998) impact assessment method is employed. It is in compliance with the ISO framework and is widely recognized by LCA experts. Equivalent factors are used to translate the emissions into some environmental impacts. Results can be given in three different levels: a characterized value, a normalized value or a weighted normalized value. The considered impact categories are shown in Table 4. Besides standard impact categories [GW, acidification (AC), nutrient enrichment (NE) and photochemical ozone formation (POF)], toxicity-related categories including human toxicity via water/air (HTw/HTa) and ecotoxicity in water chronic (ETwc) are also considered so that the evaluation of heavy metals and dioxin emissions can be obtained quantitatively. Results based on characterized values are used. Normalized references are used to convert the results into person equivalence (Kirkeby et al., 2006), so that the relative magnitude of each impact that contributes to one person by 1 year can be observed. For normalization, European standards are used owing to the lack of information in China. However, this spatial difference can be ignored because all scenarios share the same reference basis.
Table 2. Summary of key parameters and assumptions. Category Landfill Landfill gas generation Landfill gas components CH4 CO2 H2S NH3 Average landfill gas collection efficiency Leachate generation Main leachate pollutants after treatment NH3-N T-P Incineration Fossil-C content in MSW Fossil-C content in auxiliary coal Leachate generation Main leachate pollutants after treatment NH3-N T-P Incineration residues Ash transportation distance, round-trip
Unit
Value
m3 t-1 MSW Volume %
125.6 54.4 34.1 0.2 0.2 70
% kg t-1 MSW mg L-1
200 664 11.9
% %
10.6 44.5
kg t-1 MSW mg L-1
60
% MSW km
5.45 0.27 22 91.6
NH3-N: ammonia nitrogen; MSW: municipal solid waste; T-P: total phosphorus.
Interpretation Finally, output data from LCI and LCIA are summarized and discussed. Meanwhile, the environmental performance of each technology in each treatment stage is also investigated, and corresponding environmental weak points are then identified. Based on the assessment results, improvement measurements can be proposed to satisfy further developing appropriate waste management strategies.
Results Result from LCI analysis Emissions of harmful gases, acid gases and heavy metals are included in Table 3. As can be seen, CH4 in scenarios 0 and 1 is very high, but is not detected in scenario 2. Incomplete gas collection is responsible for the CH4 emitted in scenario 1, but the emissions are much less than that of scenario 0. CO2 emitted in scenario 2 is significantly higher than landfill. Two reasons can explain the result. First, a large amount of fossilderived CO2 is produced during waste combustion, but does not generate in the landfill. Second, coal is co-fired as auxiliary fuel in scenario 2 and becomes another contributor to CO2 emission. Landfill scenarios produce higher NH3, H2S and volatile organic compound (VOC) emissions due to the release of landfill gas, especially without the installation of gas collection system.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
17
Dong et al.
1.96
Conversely, incineration produces higher CO, acid gases and airborne heavy metals. Water pollution caused by landfill is generally higher than incineration owing to not only the larger leachate generation amount, but also higher pollutant concentration. Meanwhile, more energy is recovered by incineration than landfill, at the expense of more input fuels. Therefore, an indepth study into the corresponding environmental impacts should be performed and discussed in LCIA.
0.71
Results from LCIA
Table 3. Life cycle inventory of each scenario for the treatment of 1 ton of municipal solid waste. Activity parameter
Scenario
0
1
2
Inputs Energy Diesel for 0.16 1.26 mechanical operation (L) Diesel for ash 0 0 transportation (L) Electricity 0.42 1.76 (kWh) Materials and ancillary substances HDPE for 0.33 0.45 landfill walls and pipes (kg) Coal (kg) 0 0 Limestone (kg) 0 0 Outputs Direct emissions to air (kg) 0 0 CO2, fossil CO 0 0.07 43.91 17.86 CH4 0.19 0.08 NH3 0.38 0.17 H2S HCl 0 4.46E-02 NOx 0 0.06 0 0.03 SO2 VOCs 0.12 0.05 7.74E-05 3.44E-04 PM10 PCDD/DFs 0 2.40E-10 Mercury 8.36E-08 3.41E-08 Lead 0 0 Cadmium 0 0 Direct emissions to water (kg)a NH3-N 0.20 0.20 T-P 2.38E-03 2.38E-03 Cadmium 6.00E-06 6.00E-06 Copper 4.90E-05 4.90E-05 Chromium 2.00E-05 2.00E-05 Lead 1.00E-04 1.00E-04 Zinc 4.50E-05 4.50E-05 Nickel 1.15E-04 1.15E-04 Mercury 7.22E-06 7.22E-06 Solid residues Fly ash and 0 0 bottom ash (kg) Recovered materials Recovered 0 157.31 electricity (kWh)
73.58b 0 50 9.60 470.25 0.40 0 0 0 0.19 0.83 1.04 0 0.13 6.29E-10 5.03E-04 8.09E-04 6.29E-05 3.27E-04 1.62E-05 3.00E-06 1.74E-05 2.94E-06 8.40E-06 6.42E-06 2.38E-05 1.14E-07 220 367.91b
HDPE: high-density polyethylene; VOCs: volatile organic compounds; PM10: particulate matter of diameter ≤ 10 μm; PCDDs: polychlorinated dibenzo-p-dioxins; PCDFs: polychlorinated dibenzop-dioxins; NH3-N: ammonia nitrogen; T-P: total phosphorus. aFor landfill, due to the use of HDPE membrane, it is assumed no leakage to underground water. bFor incineration, electricity recovered output is used to meet the input energy demand.
Comparison of the LCIA results is given in Figure 2; Figure 3 illustrates the contribution of each treatment stage. The stages are divided into main treatment, raw materials production and electricity generation. GW potential. With respect to GW, both landfill scenarios pose higher potentials than incineration. The dominant reason is the high amount of released CH4, which accounts for 25 times the GW potential than that of CO2. With the installation of a LFGTE system, the GW of scenario 1 has an obvious decrease, but is still higher than scenario 2 for the collection system’s low efficiency and short operation life. As shown in Figure 3, the GW potential essentially comes from the main treatment phase. Raw material production only contributes to a very small proportion for all scenarios. Meanwhile, the GW potential of scenario 2 generated during treatment is some higher than scenario 1. Plastic burning is the main reason, as it is present in a large amount in MSW and creates all fossil-derived CO2. However, more electricity can be recovered by incineration, and more avoided GW potential from fossil-fuel based electricity production can thus be obtained. This environmental saving is so significant that it leads to the lowest overall GW in incineration than both landfill scenarios. AC potential. The main contribution elements for AC are NH3, H2S, HCl, NOx and SO2. For AC, incineration has the best performance, with scenario 0 ranking last. Negative values appear in both scenarios 1 and 2, indicating that actual environmental savings can be achieved with the utilization of recovered energy. For scenario 0, 0.19 kg NH3 and 0.38 kg H2S are discharged, accounting for the most significant contributors to AC. But these emissions are reduced effectively in scenario 1 due to gas collection. If only the main treatment process is considered, incineration would have a much higher AC than landfill. This is mainly owing to high NOx and SO2 emissions generated during waste combustion, which are relatively low in scenario 0 and 1. However, this burden can be well counterbalanced by the effect of energy recovery, which offsets a large amount of acid gases emitted from conventional electricity production. NE potential. For NE, incineration has a negative value and is much lower than both landfill scenarios. Leachate pollutants are the main source causing NE. Landfill has not only a larger leachate generation amount, but also a higher concentration of
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
18
Waste Management & Research 32(1)
Table 4. Life cycle impact categories and normalization reference based on EDIP 97. Impact category
Characterization unit
Normalization reference
Scale
Global warming Acidification Nutrient enrichment Photochemical ozone formation Human toxicity, air Human toxicity, water Ecotoxicity, water chronic
kg CO2-eq. kg SO2-eq. kg NO3-eq. kg C2H4-eq. m3 air m3 water m3 water
8700 74 119 25 2,090,000,000 179,000 352,000
Global Regional Regional Regional Regional Regional Regional
leachate pollutants. In addition, the main compounds in leachate, NH3-N and T-P, both account for high levels of NE equivalent factors. NE of scenario 1 is less than scenario 0. There are two explanations. First, NH3 is another main contributor of NE, which is mainly released from landfill gas and can be reduced by gas collection. Second, as shown in Figure 3, part of the NE can be avoided by electricity generation, thus compensating the overall environmental loadings. POF potential. VOCs, CO and CH4 are main contributors for POF. In view of POF, scenario 0 has the worst performance, mainly owing to the release of VOCs and CH4. A significant decrease has been achieved in scenario 1 thanks to the gas collection and combustion. Both landfill scenarios exhibit higher POF than incineration. CH4 is the dominant reason, as its emission achieves two or three orders of magnitude more than other POF emissions. Although higher amount of CO is generated in incineration its effect on POF is insignificant owing to the low equivalent factor. HTa. For HTa, scenario 2 performs the best not only for the overall impact, but also for each individual stage. Scenario 0 has the worst performance mainly owing to H2S from biogas, which is release in large amounts, and has a high equivalent factor. Airborne dioxin and heavy metals are also main contributors to HTa for their high equivalent factors. However, their effects are not so significant, as the amount generated is relatively small compared with other acid gases. Meanwhile, the recovered electricity also plays an important role, as large amounst of NOx and SO2, which are also contributors to HTa, can be avoided. HTw and ETwc. Toxicity-related impacts via water considered here include HTw and ETwc, in which heavy metals and dioxin play a dominant role. Both HTw and ETwc of incineration are the highest among all scenarios. Incineration produces higher airborne heavy metals and dioxin than landfill, especially for dioxin emission, which is nearly three times than that of scenario 1. Although higher amount of aquatic heavy metals is generated from landfill, their equivalent factors can’t be comparable with that of dioxin. Scenario 1 has the best performance for both HTw and ETwc; even environmental loading has been turned to environmental savings with regard to HTw. When compared to scenario 0, the environmental improvement is mainly attributed to energy
recovery, as the utilization of biogas can be compensated for by the substitution of fossil fuels. Normalized impact potential. Figure 4 illustrates the normalized impact potential for all scenarios. It can be seen that GW and HTa are the principle contributors affected by landfill, while HTw is the most significant factor affected by incineration. Dramatic declines in all impact categories have been achieved when comparing scenario 1 to scenario 0, indicating the advantage brought by biogas collection and utilization. Incineration has the lowest environmental impacts in the majority of impacts, except for HTw and ETwc. The normalized impact potential of HTw in scenario 2 is the highest among all categories. Thus, effective measurements, especially for the reduction of dioxin and heavy metal emissions in the incinerator, become essential and inevitable.
Discussion Realizing the advantages and shortcomings of each technology, some key parameters and improvement measurements are discussed in the following.
Overview and recommendation of MSW treatment technologies In general, both landfills with LFGTE and incineration with WTE exhibit better environmental performance than the base scenario, i.e. landfill without energy recovery. Gas collection and energy recovery are responsible for the environmental savings. Their effects are so significant that they are recommended as an effective improvement method. Great efforts should be paid to reduce the direct emissions during treatment, which is the main source of pollution. Meanwhile, although more input energy and materials should be added for incineration, the environmental burden comes from raw material production is always insignificant. Incineration with WTE is favorable over landfill with LFGTE, as the majority of its environmental impacts are lower. An advanced flue gas cleaning system reduces the pollution level effectively. Meanwhile, energy recovery through WTE, which can generate a larger amount of electricity, is more efficient than LFGTE. However, the HTw and ETwc of incineration are higher, and requires a lot of attention for improvement.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
19
Dong et al.
Figure 2. Comparison of life cycle impact categories results.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
20
Waste Management & Research 32(1)
Figure 3. Environmental impact contribution to each stage.
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
21
Dong et al.
Figure 4. Normalized impact potentials for each scenario. GW: global warming; AC: acidification; NE: nutrient enrichment; POF: photochemical ozone formation; HTa: human toxicity via air; HTw: human toxicity via water; ETwc: ecotoxicity in water chronic. Table 5. Biogenic carbon mass balance in landfill at the end of the life cycle assessment time horizon. Category (per ton of MSW)
Value
Input through MSW (kg bio-C) Output through gas (kg bio-C) Output through leachate (kg bio-C) Stored (%)
120.99 59.53 0.18 50.65
MSW: municipal solid waste; bio-C: biogenic carbon.
Figure 5. Global warming potential of each scenario considering carbon sequestration.
In conclusion, it can be stated that energy recovery brings significant advantages and should be adopted in landfill or incineration. Landfill without gas utilization is not recommended for its poor environmental performance. Incineration is a better choice than landfill with the utilization of recovered energy, but great efforts should be made to reduce toxicity-related emissions.
GW and carbon sequestration In some studies, GW is not considered owing to the fact that some emissions come from waste of biogenic origin. However,
the threat of global climate change has become one of the most important contemporary concerns, and greenhouse gas emissions have been required by Kyoto Protocols. Therefore, GW is considered in present study. To measure this indicator, carbon is divided into two parts: fossil-C and biogenic-C. Only fossilderived CO2 emission is considered;biogenic CO2 does not contribute to GW. When conducting LCA, carbon sequestration is another particularly important issue for landfill. Biogenic CO2 emission is considered neutral to GW because it originates from organic matter generated by an equivalent biological uptake of CO2 during plant growth. From this point of view, non-degraded organic carbon stored at the end of the considered time frame represents a ‘saved’ CO2 emission (carbon sequestration). When calculating GW, one of the key issues is associated with whether carbon sequestration should be considered. As carbon sequestration has been extensively discussed and supported by the Intergovernmental Panel (IPCC) (2007) and the US Environmental Protection Agency (USEPA, 2006), its effects are analyzed in this study. Table 5 lists the mass balance for biogenic carbon in landfill. A significant fraction (51%) of the biogenic carbon entering landfill system remains stored at the end of LCA time horizon. The value is consistent with IPCC’s estimate that at least 50% of biogenic carbon in landfill will remain sequestered (IPCC, 2007). In fact, the amount of carbon bound in landfill represents a CO2 reduction potential for approximately 225 kg t-1 MSW. The GW potential of each scenario is illustrated in Figure 5. The benefit of carbon sequestration highly influences the results. The GW potential of both landfill scenarios has a noticeable decrease, whereas incineration is unaffected by this assumption. The consideration of carbon sequestration leads to more preferable results for landfill, especially for scenario 1, which has surpassed incineration and obtained the best GW performance. Meanwhile, although the difference between landfill and incineration becomes smaller, scenario 2 is still preferable over scenario 0. In general, omitting carbon sequestration will significantly worsen the GW potential for landfill. Scenario 1 can achieve the best GW performance if carbon sink is taken into account. From this point of view, landfill with LFGTE is actually environmentally very friendly, and therefore effective measurements should be proposed to further improve the other environmental impacts.
Improvement for landfill technology As mentioned above, pollutants from the main treatment phase contribute to the largest proportion of environmental burdens. The average gas collection efficiency is 70% for landfill in Hangzhou, which is also a representative value of modern landfill sites in China (Zhao et al., 2012). However, as pointed out by Kong et al. (2012), average gas collection efficiency is 75% in USA, and a collection efficiency higher than 82.5% can even be achieved in California. Since landfill gas collection was started in China in recent years, the key factor for improvement is to increase the gas collection efficiency. Figure 6 presents the changing in environmental impact for scenario 1 at different gas collection efficiencies. GW is
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
22
Waste Management & Research 32(1) fractions can increase the heating value of MSW, so that less input energy is needed in the incinerator. Currently, MSW source separated collection has already been attempted in Hangzhou and some other cities in China (Dong et al., 2013), but the application scale and the separation efficiency should be further improved.
Conclusions
Figure 6. Global warming potential of scenario 1 with the variation of gas collection efficiency.
investigated as it is a main impact suffered by landfill. Increasing the gas collection efficiency can significantly decrease GW. A 10% increase in collection efficiency can result in a decreased GW potential of 32.7%. If only a 5–10% increase is achieved, the GW potential of scenario 1 will also decrease and become less than scenario 2. Combined effects by emission reduction and increased electricity generation can explain the result. Although a collection efficiency of 100% cannot be achieved, the decreased GW trend can be obtained. As a consequence, an increase in the gas collection efficiency has proven to be one of the effective improvement measurements for landfill. Highly efficient landfill is actually environmentally very competitive for managing MSW with regard to GW, and is a vital part of an integrated waste management policy. Meanwhile, data from Table 3 show a high pollution level of aquatic emissions from landfill. Large amounts and high concentrations of leachate will generate from landfill. Thus, more effective and advanced leachate treatment technology should be adopted, and the operation life span of the leachate treatment facility should also be re-considered.
Improvement for incineration technology For incineration, the most concerning categories are toxicity-related impacts, such as HTw and ETwc. Dioxin and heavy metals are the main contributors; thus, a technology upgrade for the flue gas cleaning system is essential. Meanwhile, combustion technology should also be improved. As pointed out by McKay (2002), dioxin easily forms during the combustion process. Therefore, advanced combustion technology, such as good ‘3T+E’ (temperature, time, turbulence and excess air), should be further developed. Another the improvement method for incineration is the implementation of source separated collection. Some waste fractions that contain high concentrations of chloride and metals, such as polyvinylchloride and polychlorinated biphenyls, can be treated separately to decrease the possibility of the formation of toxicity-related emissions. In addition, separating out organic
A comprehensive LCA on different MSW treatment technologies commonly used in China has been conducted. Three scenarios were considered: landfill with (scenario 1) and without (scenario 0) energy recovery, and incineration with WTE (scenario 2). Environmental impacts investigated include not only the standard categories (GW, AC, NE, POF), but also toxicity-related categories (HTa, HTw, ETwc). The results show that scenario 0 presents the worst environmental performance. Once gas is collected and treated (scenario 1), all environmental impacts significantly decrease. Incineration (scenario 2) is preferable to landfill, except for toxicity-related impacts (HTw and ETwc). Energy recovery, whether through gas utilization or waste incineration, can lead to saved emissions to the environment. When examining the environmental contribution from each treatment stage, pollutants are mainly generated during treatment. Results from normalized impacts show that methane and other harmful gases contained in landfill gas are the main contributors affected by landfill, while dioxin and heavy metals are most influenced by incineration. Sensitivity analysis shows that if carbon sequestration is considered, the GW potential of both landfill scenarios would noticeably decrease. Scenario 1 will surpass incineration and obtained the best GW potential performance. Gas collection efficiency is also a key parameter influencing the performance of landfill, and highly efficient landfill is actually environmentally very competitive for managing MSW with regard to GW. Based on the results, some improvement methods are proposed. The positive effect of energy recovery is so great that is recommended in both landfill and incineration. For landfill, it is crucially important to ensure a high collection efficiency. For incineration, an advanced flue gas cleaning system and combustion technology should be developed, and a policy that promotes source separated collection should also be pursued. In conclusion, the research is helpful in supplementing the life cycle database in China. Meanwhile, results from the present study can provide scientific and systematic information for decision-makers regarding further developing waste management strategies.
Declaration of conflicting interests The authors do not have any potential conflicts of interest to declare.
Funding This project was supported by the National Basic Research Program of China (No. 2011CB201506) and the Program of Introducing Talents of Discipline to University (B08026).
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
23
Dong et al. References Bove R and Lunghi P (2006) Electric power generation from landfill gas using traditional and innovative technologies. Energy Conversion and Management 47: 1391–1401. Buttol P, Masoni P, Bonoli A, et al. (2007) LCA of integrated MSW management systems: Case study of the Bologna District. Waste Management 27: 1059–1070. Cherubini F, Bargigli S and Ulgiati S (2009) Life cycle assessment (LCA) of waste management strategies: Landfilling, sorting plant and incineration. Energy 34: 2116–2123. CSYCC (Chinese Statistics Yearbook Compiling Committee) (2001) [Chinese Statistics Yearbook 2000]. Beijing: Chinese Statistics Press [in Chinese]. CSYCC (Chinese Statistics Yearbook Compiling Committee) (2011) [Chinese Statistics Yearbook 2010]. Beijing: Chinese Statistics Press [in Chinese]. Dahlbo H, Koskela S, Laukka J, et al. (2005) Life cycle inventory analyses for five waste management options for discarded newspaper. Waste Management & Research 23: 291–303. Di X, Nie Z, Yuan B, et al. (2007) Life cycle inventory for electricity generation in China. International Journal of LCA 12: 217–224. Dong J, Ni M, Chi Y, et al. (2013) Life cycle and economic assessment of source-separated MSW collection with regard to greenhouse gas emissions: a case study in China. Environmental Science and Pollution Research 20: 5512–5524. GB 16889–2008 (2008) Municipal solid waste landfill pollution control standards in China [in Chinese]. Hangzhou Municipal Solid Waste Disposal Supervision Center (2011) [Hangzhou Municipal Solid Waste Physical Property Analysis and Disposal Method]. Hangzhou: Municipal Solid Waste Disposal Supervision Center [in Chinese]. Hauschild M and Wentzel H (1998) Environmental Assessment of Products. Volume 1: Methodology, Tools and Case. London: Chapman & Hall. HJIP (Hangzhou Jinjiang Incineration Plant) (2009) [Annual Operation Report 2009]. Hangzhou: Hangzhou Jinjiang Incineration Plant [in Chinese]. Hoornweg D, Lam P and Chaudhry M (2005) Waste Management in China: Issues and Recommendations. Washington, DC: East Asia Infrastructure Development, the World Bank.
HTLS (Hangzhou Tianziling Landfill Site) (2003) [Hangzhou Second Landfill Site Environmental Impact Reports]. Hangzhou: Hangzhou Tianziling Landfill Site [in Chinese]. IPCC (Intergovernmental Panel on Climate Change) (2007) Climate Change 2007: Mitigation, Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press. ISO 14040 (2006) Environmental management-Life cycle assessmentPrincipal and framework. Kaazke J, Meneses M, Wilke BM, et al. (2013) Environmental evaluation of waste treatment scenarios for the towns Khanty-Mansiysk and Surgut, Russia. Waste Management & Research 31: 315–326. Kaplan PO, Decarolis J and Thorneloe S (2009) Is it better to burn or bury waste for clean electricity generation. Environmental Science and Technology 43: 1711–1717. Kirkeby JT, Bhander GS, Birgisdottir H, et al. (2006) Environmental assessment of solid waste systems and technologies: EASEWASTE. Waste Management & Research 24: 3–15. Kong D, Shan J, Iacoboni M, et al. (2012) Evaluating greenhouse gas impacts of organic waste management options using life cycle assessment. Waste Management & Research 30: 800–812. McDougall F, White P, Franke M, et al. (2001) Integrated Solid Waste Management: A Life Cycle Inventory. London: Blackwell Science. McKay G (2002) Dioxin characterisation, formation and minimisation during municipal solid waste (MSW) incineration: review. Chemical Engineering Journal 86: 343–368. Mohareb AK, Warith MA and Diaz R (2008) Modelling greenhouse gas emissions for municipal solid waste management strategies in Ottawa, Ontario, Canada. Resources, Conservation and Recycling 52: 1241–1251. USEPA (US Environmental Protection Agency) (2006) Solid Waste Management and Greenhouse Gases: A Life-cycle Assessment of Emissions and Sinks. 3rd ed. Washington, DC: USEPA. Zhao C, Zhao L, Chen XM, et al. (2012) A study on the collection efficiency of methane in municipal solid waste landfill [in Chinese]. Acta Scientiae Circumstantiae 32: 954–959. Zheng X, Yang Y, Lei Y (2009) [Analysis of MSW landfill gas to energy in China]. Environmental protection 414: 19–22 [in Chinese].
Downloaded from wmr.sagepub.com at UNIV CALIFORNIA SAN DIEGO on January 11, 2015
