Anaerobic digestion (AD) of source-separated municipal solid waste (MSW) and use of the digestate is presented from a global warming (GW) point of view by providing ranges of greenhouse gas (GHG) emissions that are useful for calculation of global warming factors (GWFs), i.e. the contribution to GW measured in CO(2)-equivalents per tonne of wet waste. The GHG accounting was done by distinguishing between direct contributions at the AD facility and indirect upstream or downstream contributions. GHG accounting for a generic AD facility with either biogas utilization at the facility or upgrading of the gas for vehicle fuel resulted in a GWF from -375 (a saving) to 111 (a load) kg CO(2)-eq. tonne(-1) wet waste. In both cases the digestate was used for fertilizer substitution. This large range was a result of the variation found for a number of key parameters: energy substitution by biogas, N(2)O-emission from digestate in soil, fugitive emission of CH( 4), unburned CH(4), carbon bound in soil and fertilizer substitution. GWF for a specific type of AD facility was in the range -95 to -4 kg CO(2)-eq. tonne(-1) wet waste. The ranges of uncertainty, especially of fugitive losses of CH(4) and carbon sequestration highly influenced the result. In comparison with the few published GWFs for AD, the range of our data was much larger demonstrating the need to use a consistent and robust approach to GHG accounting and simultaneously accept that some key parameters are highly uncertain.
Global warming potential (GWP) is an important impact category in life-cycle-assessment modelling of waste management systems. However, accounting of biogenic CO(2) emissions and sequestered biogenic carbon in landfills and in soils, amended with compost, is carried out in different ways in reported studies. A simplified model of carbon flows is presented for the waste management system and the surrounding industries, represented by the pulp and paper manufacturing industry, the forestry industry and the energy industry. The model calculated the load of C to the atmosphere, under ideal conditions, for 14 different waste management scenarios under a range of system boundary conditions and a constant consumption of C-product (here assumed to be paper) and energy production within the combined system. Five sets of criteria for assigning GWP indices to waste management systems were applied to the same 14 scenarios and tested for their ability to rank the waste management alternatives reflecting the resulting CO(2) load to the atmosphere. Two complete criteria sets were identified yielding fully consistent results; one set considers biogenic CO(2) as neutral, the other one did not. The results showed that criteria for assigning global warming contributions are partly linked to the system boundary conditions. While the boundary to the paper industry and the energy industry usually is specified in LCA studies, the boundary to the forestry industry and the interaction between forestry and the energy industry should also be specified and accounted for.
The collection, transfer and transport of waste are basic activities of waste management systems all over the world. These activities all use energy and fuels, primarily of fossil origin. Electricity and fuel consumptions of the individual processes were reviewed and greenhouse gases (GHG) emissions were quantified. The emission factors were assigned a global warming potential (GWP) and aggregated into global warming factors (GWFs), which express the potential contribution to global warming from collection, transport and transfer of 1 tonne of wet waste. Six examples involving collection, transfer and transport of waste were assessed in terms of GHG emissions, including both provision and use of energy. (GHG emissions related to production, maintenance and disposal of vehicles, equipment, infrastructure and buildings were excluded.) The estimated GWFs varied from 9.4 to 368 kg CO(2)-equivalent (kg CO(2)-eq.) per tonne of waste, depending on method of collection, capacity and choice of transport equipment, and travel distances. The GHG emissions can be reduced primarily by avoiding transport of waste in private cars and by optimization of long distance transport, for example, considering transport by rail and waterways.
Greenhouse gas (GHG) emissions related to composting of organic waste and the use of compost were assessed from a waste management perspective. The GHG accounting for composting includes use of electricity and fuels, emissions of methane and nitrous oxide from the composting process, and savings obtained by the use of the compost. The GHG account depends on waste type and composition (kitchen organics, garden waste), technology type (open systems, closed systems, home composting), the efficiency of off-gas cleaning at enclosed composting systems, and the use of the compost. The latter is an important issue and is related to the long-term binding of carbon in the soil, to related effects in terms of soil improvement and to what the compost substitutes; this could be fertilizer and peat for soil improvement or for growth media production. The overall global warming factor (GWF) for composting therefore varies between significant savings (-900 kg CO(2)-equivalents tonne(-1) wet waste (ww)) and a net load (300 kg CO(2)-equivalents tonne( -1) ww). The major savings are obtained by use of compost as a substitute for peat in the production of growth media. However, it may be difficult for a specific composting plant to document how the compost is used and what it actually substitutes for. Two cases representing various technologies were assessed showing how GHG accounting can be done when specific information and data are available.
The current landfill gas (LFG) management (based on flaring and utilization for heat generation of the collected gas) and three potential future gas management options (LFG flaring, heat generation and combined heat and power generation) for the Old Ammässuo landfill (Espoo, Finland) were evaluated by life-cycle assessment modeling. The evaluation accounts for all resource utilization and emissions to the environment related to the gas generation and management for a life-cycle time horizon of 100 yr. The assessment criteria comprise standard impact categories (global warming, photo-chemical ozone formation, stratospheric ozone depletion, acidification and nutrient enrichment) and toxicity-related impact categories (human toxicity via soil, via water and via air, eco-toxicity in soil and in water chronic). The results of the life-cycle impact assessment show that disperse emissions of LFG from the landfill surface determine the highest potential impacts in terms of global warming, stratospheric ozone depletion, and human toxicity via soil. Conversely, the impact potentials estimated for other categories are numerically-negative when the collected LFG is utilized for energy generation, demonstrating that net environmental savings can be obtained. Such savings are proportional to the amount of gas utilized for energy generation and the gas energy recovery efficiency achieved, which thus have to be regarded as key parameters. As a result, the overall best performance is found for the heat generation option - as it has the highest LFG utilization/energy recovery rates - whereas the worst performance is estimated for the LFG flaring option, as no LFG is here utilized for energy generation. Therefore, to reduce the environmental burdens caused by the current gas management strategy, more LFG should be used for energy generation. This inherently requires a superior LFG capture rate that, in addition, would reduce fugitive emissions of LFG from the landfill surface, bringing further environmental benefits.
By using life cycle assessment (LCA) modeling, this paper compares the environmental performance of six landfilling technologies (open dump, conventional landfill with flares, conventional landfill with energy recovery, standard bioreactor landfill, flushing bioreactor landfill and semi-aerobic landfill) and assesses the influence of the active operations practiced on these performances. The environmental assessments have been performed by means of the LCA-based tool EASEWASTE, whereby the functional unit utilized for the LCA is "landfilling of 1ton of wet household waste in a 10m deep landfill for 100 years". The assessment criteria include standard categories (global warming, nutrient enrichment, ozone depletion, photo-chemical ozone formation and acidification), toxicity-related categories (human toxicity and ecotoxicity) and impact on spoiled groundwater resources. Results demonstrate that it is crucially important to ensure the highest collection efficiency of landfill gas and leachate since a poor capture compromises the overall environmental performance. Once gas and leachate are collected and treated, the potential impacts in the standard environmental categories and on spoiled groundwater resources significantly decrease, although at the same time specific emissions from gas treatment lead to increased impact potentials in the toxicity-related categories. Gas utilization for energy recovery leads to saved emissions and avoided impact potentials in several environmental categories. Measures should be taken to prevent leachate infiltration to groundwater and it is essential to collect and treat the generated leachate. The bioreactor technologies recirculate the collected leachate to enhance the waste degradation process. This allows the gas collection period to be reduced from 40 to 15 years, although it does not lead to noticeable environmental benefits when considering a 100 years LCA-perspective. In order to more comprehensively understand the influence of the active operations (i.e., leachate recirculation, waste flushing and air injection) on the environmental performance, the time horizon of the assessment has been split into two time periods: years 0-15 and 16-100. Results show that if these operations are combined with gas utilization and leachate treatment, they are able to shorten the time frame that emissions lead to environmental impacts of concern.
A new computer based life cycle assessment model (EASEWASTE) was used to evaluate a municipal solid waste system with the purpose of identifying environmental benefits and disadvantages by anaerobic digestion of source-separated household waste and incineration. The most important processes that were included in the study are optical sorting and pre-treatment, anaerobic digestion with heat and power recovery, incineration with heat and power recovery, use of digested biomass on arable soils and finally, an estimated surplus consumption of plastic in order to achieve a higher quality and quantity of organic waste to the biogas plant. Results showed that there were no significant differences in most of the assessed environmental impacts for the two scenarios. However, the use of digested biomass may cause a potential toxicity impact on human health due to the heavy metal content of the organic waste. A sensitivity analysis showed that the results are sensitive to the energy recovery efficiencies, to the extra plastic consumption for waste bags and to the content of heavy metals in the waste. A model such as EASEWASTE is very suitable for evaluating the overall environmental consequences of different waste management strategies and technologies, and can be used for most waste material fractions existing in household waste.
The global warming factor (GWF; CO(2)-eq. tonne(-1) waste) performance of municipal waste management has been investigated for six representative European Member States: Denmark, France, Germany, Greece, Poland and the United Kingdom. The study integrated European waste statistical data for 2007 in a life-cycle assessment modelling perspective. It is shown that significant GWF benefit was achieved due to the high level of energy and material recovery substituting fossil energy and raw materials production, especially in Denmark and Germany. The study showed that, despite strong regulation of waste management at European level, there are major differences in GWF performance among the member states, due to the relative differences of waste composition, type of waste management technologies available nationally, and the average performance of these technologies. It has been demonstrated through a number of sensitivity analyses that, within the national framework, key waste management technology parameters can influence drastically the national GWF performance of waste management.
Global warming factors (kg CO(2)-eq.-tonne(-1) of waste) have been modelled for 40 different municipal waste management scenarios involving a variety of recycling systems (paper, glass, plastic and organics) and residual waste management by landfilling, incineration or mechanical-biological waste treatment. For average European waste composition most waste management scenarios provided negative global warming factors and hence overall savings in greenhouse gas emissions: Scenarios with landfilling saved 0-400, scenarios with incineration saved 200-700, and scenarios with mechanical-biological treatment saved 200- 750 kg CO(2)-eq. tonne(- 1) municipal waste depending on recycling scheme and energy recovery. Key parameters were the amount of paper recycled (it was assumed that wood made excessive by paper recycling substituted for fossil fuel), the crediting of the waste management system for the amount of energy recovered (hard-coal-based energy was substituted), and binding of biogenic carbon in landfills. Most other processes were of less importance. Rational waste management can provide significant savings in society's emission of greenhouse gas depending on waste composition and efficient utilization of the energy recovered.
Accounting of emissions of greenhouse gas (GHG) is a major focus within waste management. This paper analyses and compares the four main types of GHG accounting in waste management including their special features and approaches: the national accounting, with reference to the Intergovernmental Panel on Climate Change (IPCC), the corporate level, as part of the annual reporting on environmental issues and social responsibility, life-cycle assessment (LCA), as an environmental basis for assessing waste management systems and technologies, and finally, the carbon trading methodology, and more specifically, the clean development mechanism (CDM) methodology, introduced to support cost-effective reduction in GHG emissions. These types of GHG accounting, in principle, have a common starting point in technical data on GHG emissions from specific waste technologies and plants, but the limited availability of data and, moreover, the different scopes of the accounting lead to many ways of quantifying emissions and producing the accounts. The importance of transparency in GHG accounting is emphasised regarding waste type, waste composition, time period considered, GHGs included, global warming potential (GWP) assigned to the GHGs, counting of biogenic carbon dioxide, choice of system boundaries, interactions with the energy system, and generic emissions factors. In order to enhance transparency and consistency, a format called the upstream-operating-downstream framework (UOD) is proposed for reporting basic technology-related data regarding GHG issues including a clear distinction between direct emissions from waste management technologies, indirect upstream (use of energy and materials) and indirect downstream (production of energy, delivery of secondary materials) activities.