Household food waste management – Evaluations of current status and potential improvements using life-cycle assessment methodology
Abstract: An increased understanding of the world’s limited resources and the negative environmental impacts connected to solid waste management has in recent years increased interest in separate collection and treatment of organic waste in many parts of the world. As an example, increased biological treatment of organic waste is an explicit goal within the Swedish national solid waste management strategy (SEPA, 2006) and the national environmental objectives state that 35% of all organic household waste should be treated biologically by 2010 (SEPA, 2007). Several alternatives are currently being used and developed in the areas of collection/transportation, treatment and final use of the goods recovered from organic solid waste. Household food waste makes up around 30-40 mass-% of the total household waste in Sweden – bulky household waste excluded – or roughly 100kg per person a year. The objective of this thesis is to explore possible alternatives for collection and treatment of this waste fraction with the aim of maximising the environmental benefits, using the life-cycle assessment (LCA) methodology. The LCA of four different systems for separate collection of household food waste for later anaerobic digestion showed that the conservation of nutrients and readily biodegradable carbon throughout the collection chain is of great importance to the overall environmental impacts of the collection alternatives studied. Thus, a higher input of energy and resources in the collection chain can be justified when resulting in higher potential energy and nutrient recovery later in the treatment process. This work also indicates that apart from information on households regarding waste recycling, also the practical arrangements of separate collection of food waste can have a large impact in the amount and quality of separately collected food waste. Hence, there is a need to gain a better understanding of household behaviour in relation to food waste disposal and further develop the infrastructure for separate collection of this waste, to assure convenient and accessible solutions for households where separate collection of food waste is introduced. The results also show that physical pre-treatment of separately collected household food waste in paper or plastic bags – currently the most common system for household food waste collection in Sweden – can result in large losses of biodegradable material and thus a reduction of the potential energy and nutrient recovery from this waste fraction. In the four pre-treatment plants included in this work, 13-39% of the potential methane production and 13-32% of N-total in incoming material were found in material separated in the pre-treatment process. The comparative LCA of different treatment alternatives for household food waste based on a case study from a residential area in southern Sweden shows that separate collection of household food waste for later anaerobic digestion with production of biogas to replace petrol as vehicle fuel was more beneficial than incineration with energy recovery (heat and power), composting in decentralised reactors without cleaning of emissions or biogas production with substitution of heat and power in relation to global warming potential (GWP), photochemical ozone depletion (ODP) and acidification potential (AP), while incineration was preferable in relation to eutrophication potential (EP). The energy balance performed also shows that the anaerobic treatment alternative is preferable from a net-energy production perspective, but that incineration is preferable when comparing the scenarios on a primary energy basis, as this credits the higher net-production of electricity from the incineration alternative. The results obtained in the comparative study were to a large extent dependent on several assumptions and methodological choices. Some of the more influential assumptions on the overall results were the environmental profile of energy carriers, peat and chemical fertilisers potentially substituted by waste treatment alternatives, emissions from on-land use of bio-fertilisers and emission control from waste treatment facilities. Factors that are more visual for households participating in separate collection of food waste – such as the use of bags for collection of food waste and transport to treatment facilities – were seen to be of lesser importance to the overall results from the treatment systems compared. The potentially large variations in results depending on choices and assumptions were reflected also in the outcomes from a review of 25 previously performed LCAs of food waste management. The review shows that different conclusions often are drawn regarding the most environmentally beneficial treatment alternative for food waste and that the GWP from collection and treatment of this waste fraction can vary greatly. Calculations of GWP from incineration, landfill disposal, anaerobic digestion and composting of food waste result in anything between large net avoidance to substantial contribution to GWP according to the reviewed studies (incineration: 640 to -305kg CO2-eq/ton; landfill: 1200 to 302kg CO2-eq/tonne; anaerobic digestion: 440 to -375kg CO2-eq/tonne; and compost: 1000 to -900kg CO2-eq/tonne). However, these differences are not found to be related mainly to actual differences in the environmental impacts from the systems studied, but rather to differences in system boundary settings, methodological choices (for example the view on biogenic carbon emissions and carbon sequestration as well as the eco-profile of substituted goods) and, to a lesser extent, to variations in the input data. Due to lack of consistency in system boundary setting, processes and parameters seen as highly relevant for the outcome in some studies are not considered at all in others, making comparisons between different studies difficult. Nevertheless, comparisons are readily made between results from studies, independently of the often differing frameworks and assumptions made in the studies. Also, several cases of internal inconsistencies were identified in the studies, generally due to differences in system boundary setting between scenarios. Assumptions related to the quality of input material (treated food waste), losses and emissions of carbon, nutrients and other compounds during collection, potential storage and physical pre-treatment, potential energy recovery through incineration, treatment of incineration residues, emissions from composting, emissions from storage and on-land use of bio-fertilisers and chemical fertilisers and eco-profiles of substituted goods were all identified as highly relevant for the outcomes in this type of comparison. Mass-flows of carbon, nutrients and heavy metals are in many cases not respected due to cut-offs and use of literature values rather than transfer coefficients throughout the treatment chain. Based on the results from the review and previously mentioned results from this work, more considerations should be given to: - The quantitative and qualitative impacts of different collection systems for source-separated food waste on the collected material - The quantitative and qualitative effects on separately collected food waste associated with physical pre-treatment processes - Environmental impacts from downstream processes in the treatment chain, such as storage and final use/disposal of goods/secondary waste stream resulting from different treatment processes - The potential substitution of goods with high negative environmental impact with those gained through food waste treatment In many cases there is a need for further research and improvement of input data. Unlike management systems for many other solid waste fractions, the food waste management scale can be assumed to be more influenced by geographical differences. Although the reviewed studies covered waste systems in 12 countries on three continents, similar input data found in the literature was commonly used in the studies, without further discussion of the need for adjustments to fit the local context. Six LCA-guidelines were also consulted in this work. These guidelines often did not address several of the identified key issues or give different recommendations in relation to these questions. Thus, establishment and use of more detailed guidelines within this field in order to increase both the general quality of the assessments performed as well as to increase the possibilities to compare different studies would be welcome. Increasing separate collection of food waste is very important to attain the environmental benefits viable through AD with energy and nutrient recovery of food waste. The results indicate that apart from information on households regarding waste recycling, the practical arrangements of separate collection of food waste also can have a large impact on the amount and quality of separately collected food waste. Thus, there is a need to further develop the infrastructure for separate collection of food waste, to assure convenient and accessible solutions in the households where separate collection of food waste is introduced. This poses an organisational challenge, as this can require closer collaboration between municipalities and facility owners to reach municipal and national goals on food waste collection and biological treatment. Swedish regulations on solid waste management provide a monopoly on collection and treatment of household waste. However, this work shows that many of the factors with great effect on the overall environmental impact of the food waste treatment chain are located outside of what is controlled by the decision-makers of municipal waste management strategies. This indicates the importance of a holistic approach when assessing different treatment alternatives and a need for extensive collaboration between the often different actors involved in the food waste management chain in order to maximize the potential environmental benefits.
This dissertation MIGHT be available in PDF-format. Check this page to see if it is available for download.