Quantification of environmental effects from anaerobic treatment of source-sorted organic household waste

Trine Lund Hansen

    Research output: Book/ReportPh.D. thesis

    159 Downloads (Pure)


    The Danish national waste strategy for 1998-2004 recommended increased biological treatment of municipal organic waste to improve the environmental profile of the Danish waste system (The Danish Government, 1999). However, data for a thorough environmental assessment of Danish systems for biological treatment of this waste fraction were not available. The results presented in this PhD thesis are based on a range of activities partly founded by the Danish Environmental Protection Agency in 2001-2003 to increase the knowledge within this field. The activities included a comprehensive field-sampling program including a range of existing large- and fullscale Danish systems for source-sorting and biological treatment of municipal organic waste, laboratory tests and analyses, literature studies, model simulations and construction of case studies. Municipal organic waste was defined as kitchen waste, in some cases including house plants with soil, cat litter and diapers, but no garden waste.

    Based on analyses of waste samples, the typical chemical composition of collected organic waste could be determined. The composition of the waste varied between cities due to differences in sorting instructions and choice of collection bag material (paper or plastic). The main differences were seen for degradability of the waste, and content of ash and plastic. Minor, but nevertheless significant differences were seen between “identical” waste samples from different seasons. The dwelling type (single- or multifamily houses) did not influence the composition of the waste. On average 80% of the dry matter in the waste samples was easily degradable (determined as enzyme degradable organic matter, EDOM), while 12% was inorganic (ash). The dry matter contained on average 2.5% nitrogen, 0.4% phosphorus and 0.9% potassium. The calorific value was 20 MJ/kg dry matter, while the plastic content depended on the choice of collection bag material (0-8% of dry matter).

    The methane potential for each waste sample was determined by laboratory batch tests (2-liter reactors). The average potential was 459 STPm3 CH4/t VS. Only minor variations with respect to city, pre-treatment technology, season and dwelling type were seen (428-489 STPm3 CH4/t VS). Pilot-scale digestion (35-liter reactors) of the same waste samples showed that 75-80% of the methane potential determined in the batch tests could be expected as methane yield on a larger scale (360 STPm3 CH4/t VS). The three investigated pre-treatment technologies, screw press, disc screen and shredder + magnet, routed on average 59, 66 and 98% (wet weight) of the collected organic waste to anaerobic digestion. Since 80-98% of the rejected material consisted of organic matter, the choice of pre-treatment technology was important for the methane potential per ton of collected municipal organic waste, ranging from 48 to 107 STPm3 CH4/t
    collected waste for the investigated systems.

    Methane emissions from storage of treated organic waste were investigated through a combination of sampling from full-scale storage tanks and laboratory work. Assuming average Danish conditions with respect to temperature and operational pattern of the tanks, the produced methane may decrease the global warming savings from anaerobic treatment of municipal organic waste by 3%. Higher temperatures or changed practice could increase the methane production significantly.

    The environmental effects of agricultural application of the treated organic waste are affected by many specific parameters and are thus difficult to generalize. Simulation of a range of typical Danish scenarios in the agro-ecosystem model Daisy showed wide intervals for the resulting nitrogen losses depending on the scenario: ammonia emissions, typically 15% of the applied ammonia; nitrous oxide emissions, typically 1.4-1.6% of the applied nitrogen; nitrate loss to surface waters, 0-30% of the applied nitrogen and nitrate loss to groundwater, 3-87% of the applied nitrogen. Carbon retention in the soil was estimated to 63-84, 17-37 and 2-16% of the applied carbon after 10, 50 and 100 years, respectively. Whether this effect contributes to the environmental assessment is a methodology question. The content of heavy metals and organic pollutants in the treated organic waste contributed to the environmental assessment through the toxicity impact categories. Substitution of commercial N, P and K fertilizers was based on nutrient content in the treated organic waste and the plant availability of organic waste compared to plant availability of commercial fertilizers, the mineral fertilizer equivalent (MFE) value. The MFE values were assessed to be maximally 0.3 for organic nitrogen, 0.8 for mineral nitrogen and 1 for phosphorus and potassium. Legal regulations and agricultural practice should be included in the MFE value in each specific scenario.

    Based on the aforementioned data and literature studies, modules concerning biological treatment were constructed for the life cycle assessment-based pc tool for environmental assessment of solid waste systems and technologies, EASEWASTE, developed at the Technical University of Denmark. The tool was used for environmental assessment of the system for source-sorting and anaerobic digestion of municipal organic waste in the Municipality of Århus, Denmark. The environmental effects of the anaerobic treatment system were strongly influenced by energy-related parameters, such as energy efficiency at the biogas and incineration plant, energy consumption in the system, efficiency of the pre-treatment plant, biogas potential and waste composition. The choice of the energy source substituted by the produced energy also influenced the results significantly. The potential toxicity effects from heavy metals in the treated organic waste applied to agricultural land had a large affect on the environmental assessment. These effects are, however, relatively uncertain due to methodology issues and varying heavy metal content in municipal organic waste. In most of the environmental impact categories assessed, the differences were only marginal between anaerobic digestion and incineration of the municipal organic waste fraction. Therefore, none of these treatment methods can be appointed as preferable to the other based on potential environmental impacts.

    The work presented has increased the knowledge about environmental effects from anaerobic digestion systems in general through thorough investigation of existing Danish systems and evaluation of previously performed work within the area. The developed LCA-based tool may support future decisions at different political levels regarding biological treatment of municipal organic waste by allowing comparison of environmental effects with e.g. economics and service in the waste management system.
    Original languageEnglish
    Place of PublicationKgs. Lyngby
    PublisherDTU Environment
    Number of pages42
    ISBN (Print)87-89220-90-0
    Publication statusPublished - Sept 2005


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