Systems Analysis of Organic Waste Management in Denmark

Systems Analysis of Organic Waste Management in Denmark

Summary

Organic household waste is a resource that is not fully utilised in Denmark today through composting or anaerobic digestion. On a yearly basis about 1,7 million tons of household waste is collected. The organic part is about 700 000 tons. To get a better understanding of the consequences when increasing biological treatment of organic household waste, the Danish EPA is conducting a socio-economic survey of increased biological treatment of organic household waste. In order to do an socio-economic survey, there is need to quantify flows of material, use of resources, energy, emissions to air, water and soil. In this project use of resources, emissions to air, water and soil, use and generation of energy and environmental impacts are calculated and presented on a national level.

To describe the system for handling and treatment of organic household waste a system analytical approach is used. In order to calculate the flows, emissions and environmental impact from handling organic household waste a computer-based model ORWARE (ORganic WAste REsearch) is used for the systems analysis.

ORWARE is a tool for environmental systems analysis of waste management. It is a computer-based model for calculation of substance flows, environmental impacts, and costs of waste management. ORWARE consists of a number of separate sub models, which may be combined to design a waste management system for e.g. a city, a municipality or a company. ORWARE is a model primarily for material flow analysis (MFA). The material flows from different sources (wastes) through different methods for waste treatment (composting, anaerobic digestion etc) to different end uses (spreading of residues on agricultural soil or landfill). Emissions from transports, treatments etc are allocated as emissions to air, water and soil. Using methodology for impact analysis from life cycle assessment (LCA) different environmental impact categories are calculated.

The study consists of two sets of scenarios and a number of sensitivity analyses. The first set of scenarios, the multi-treatments scenarios, incorporates the different waste management technologies into five different scenarios that reflect the implementations of different treatment technologies available for treatment of organic household waste. The scenarios describe increased composting and anaerobic digestion in favour of incineration. This is made in a short-term timeframe and a long-term timeframe based on utilisation of organic household waste at a national level. The results from the multi-treatment scenarios are presented for core-, up-/ downstream and compensatory system levels.

The second scenarios are set up in order to compare specific emissions from three different treatment technologies. All organic waste is directed to one type of treatment; incineration, anaerobic digestion or composting. In each scenario, emissions are expressed for the different stages: collection, transport, treatment and downstream processes (gas utilisation and spreading of organic fertiliser). The results from the mono-treatment scenarios are presented as emissions to air and water. These scenarios only cover the core and upstream system, compensatory production is not accounted for.

In addition to the two sets of scenarios a number of sensitivity analysis were conducted in order to conclude the importance of different parameters and assumptions not covered by the scenarios:

No recycling of slag to road construction?
Losses from source-separation are changed from 35 % to 10 % for biogas and from 15 % to 5 % for compost.
A sensitivity analysis with a higher biogas production, 150 nm³instead of originally 125 nm³is carried out.
Arable land can be used as a carbon sink in order to decrease CO₂-emissions.
Utilising natural gas to be the marginal power production.
Heat from coal is replaced by heat from natural gas.

The study produced a large amount of results, which are incorporated in the socioeconomic study carried out by Danish EPA. In general, the results show that increased anaerobic digestion compared to the present situation has both positive and negative impacts on the environment.

Advantages and disadvantages depend on which environmental impacts that are considered. An increase in windrow and reactor composting in favour of incineration is with some exception worse than the present treatment situation, especially when the long-term compost scenario (No. 5) is compared to the reference scenario. In an overall perspective the differences is larger between long-term scenarios for composting than the long-term scenario for anaerobic digestion.

Different amount of functions is generated from each scenario. In this project following functions were identified:

kg Nitrogen (N-tot) recycled to agricultural soil
kg Phosphorus (P-tot) recycled to agricultural soil
kg Potassium (K-tot) recycled to agricultural soil
TJ electricity generated from waste treatment
TJ heat (as district heating) generated from waste treatment.

The amount of functions from each multi-treatment scenario is distributed as follows: Maximum amount of P and K is generated in scenario 5, 311 and 761 tonnes P and K respectively. Maximum amount of N is generated in the scenario 3, 1 162 tonnes, electricity in scenario 3 and heat in scenario 1, 483 TJ el. and 1 761 TJ heat.

The sensitivity analysis show some changes compared to the original scenarios. Valuing arable land as carbon sink affects solely on GWP. It is favourable for the long-term biogas and compost scenarios, because of a high degree of recycling of carbon to arable land.

Decreased losses from source-separation will lead to functional units’ increase in magnitude except for district heating which remains the same as in the original scenario-set. More waste sent to anaerobic digestion and composting results in higher electrical power generated and more nitrogen, phosphorus and potassium to soil. Global warming increases for all scenarios and the differences between the reference scenario and the biogas scenarios are levelled out. Composting becomes less favourable with respect to GWP than before. Acidification and eutrophication increase but the internal order remains the same. The same result is found for photochemical oxidants and consumption of primary energy carriers.

Higher biogas production increases the functional units’ electrical power and nitrogen increase. For GWP the differences are small but the biogas scenarios become better in relation to the reference and composting worse. For the other environmental impact categories, the differences are extremely small and the internal relation is not changed between the scenarios. The consumption of primary energy carriers goes through the same changes as GWP.

By shifting power generation from coal to natural gas decreases total emissions from power generation and changes the order between scenarios, concerning different environmental impact. Using natural gas has a positive effect on composting scenarios that uses large amount of compensatory electricity. The shift in energy carrier has therefore a negative impact in biogas scenarios because they generate most of their electricity from biogas.

The last sensitivity analysis is a change of fuel from coal to natural gas for compensatory heat. As the scenarios to a larger and larger extent include compensatory heat, the impact of fuel change affects the scenarios in correspondence to that. The changes are however never so large that the order between the scenarios is changed.