In the Danish emission inventories system emissions from waste are divided into five main groups of sources
Solid waste disposal sites (SWDS) has historically been the dominant source of greenhouse gases in the waste sector, but since 2020 the dominant source has been Biological treatment of solid waste; i.e. composting and anaerobic digestion at biogas facilities. The following figure presents the time series for greenhouse gas emissions from the waste sector divided in the five sub-sectors.
Greenhouse gas emissions from solid waste disposal on land have decreased from 54.5 kt CH4 (1526 kt CO2 eq) in 1990 to 15.5 kt CH4 (434 kt CO2 eq) in 2021, corresponding to a decrease of 72 %. While emissions from biological treatment of solid waste have increased from 1.3 kt CH4 and 0.1 kt N2O (56 kt CO2 eq) in 1990 to 17.7 kt CH4 and 0.2 kt N2O (562 kt CO2 eq) in 2021, corresponding to an increase of 905 %.
In addition to greenhouse gasses, the emissions inventories for the waste sector includes emissions of SO2, NOx, NMVOC, CO, NH3, particles (TSP, PM10 and PM2.5), heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Se, Zn) and POPs (PCDD/F, benzo(b)flouranthene, benzo(k)flouranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, HCB and PCBs). This section focuses on SO2, NMVOC, PM2.5, Pb, Hg and PCDD/F. For more information on the other pollutants, please refer to the UNECE report (IIR report).
Figure 2a-f Time series for SO2, NMVOC, PM2.5, Pb, Hg and PCDD/F for the waste sector.
Other waste (i.e. accidental fires) is the dominant source for several pollutants. While many other source from all sectors show a decreasing emission trend, this is not the case for accidental fires where emissions are rather constant throughout the time series. Because of this, accidental fires are becoming increasingly relevant on a national emission level. While an emission of 0.85 kt SO2 from the waste sector in 1980 only amounted to 0.2 % of the national SO2 emission, a similar emission (0.88 kt) in 2021 corresponds to 10.2 % of national emissions of SO2. The same picture is seen for emissions of Pb and PCDD/F, and to a lesser degree also PM2.5.
For NMVOC emissions from SWDS dominate the time series, but as the trend has been decreasing, accidental fires became the dominant NMVOC emission source from 2013 forward. The strong decrease in Hg emissions from cremations from 2011 is due to the installation of flue gas cleaning equipment at all Danish crematoria.
NH3 emissions from the waste sector stem from composting, biogas facilities and cremation. The vast majority of NH3 emissions in Denmark originate from the agricultural sector and emissions from the waste sector are below 1 % of the national total for all years in the time series.
Solid waste disposal sites constitutes a sub-sector in the waste sector. In the Danish emission inventory system CH4 emissions from the degradation of organic waste at solid waste disposal sites are calculated with use of a first order decay as recommended by the IPCC in their guidelines.
Figure 3 presents a time series of the annual deposited methane potential and the resulting annual net methane emission. The net emission is excluding methane collected for energy production and methane oxidised in the top soil layer (deposited waste is covered with soil to reduce emissions).
The general development for solid waste at disposal sites is a result of action plans by the Danish government. Since the "Action plan for Waste and Recycling 1993-1997", a series of action plans have been developed, resulting in continuous development in the reduction of depositing of degradable waste. The decrease in the methane emission throughout the time-series is much less than the general decrease in the amount of waste deposited. This is due to the time involved in the processes generating the CH4, which is reflected in the first order decay (FOD) model used for calculating the annual methane emission from solid waste disposal sites in Denmark. Figure 4 and Figure 5 presents activity data for deposited waste amounts since 1990 – The FOD model uses activity data back to the year 1940.
As may be observed from the figures showing the time trend in inert and degradable waste types, the amounts of deposited inert waste types is significantly higher than the deposited amounts of biodegradable waste types. The decrease in the amount of deposited degradable waste is reflected in the resulting time-series visualised in Figure 3.
Emissions of NMVOC are estimated using the Tier 1 methodology (i.e. a default emission factor), while particulate matter (PM) emissions from waste handling at solid waste disposal sites are calculated according to the Tier 3 emission modelling and use of facility data as described in the EMEP/EEA air pollutant emissions inventory guidebook 2019. The Tier 3 calculations from which the PM emissions factors are derived, include parameters such as moisture content and national mean wind speed.
For the purpose of calculating PM emissions from operations associated with depositing waste at landfills, the annual amount of deposited waste (inert and organic) was applied, while for the calculation of the emissions of NMVOC, the total amount of organic waste deposited at the Danish landfills was applied. The activity data for these calculations are presented in Figure 4, Figure 5 and in their background data. Emissions are presented in Figure 2a-f.
Biological treatment of solid waste in Denmark, includes composting and anaerobic digestion at biogas facilities. Composting of solid biological waste includes composting of garden and park waste, organic waste from households and other sources, sludge and home composting of garden and vegetable food waste. Composting leads to emissions of CH4, N2O CO and NH3, while emissions from biogas facilities include CH4 and NH3. Emissions from composting have been calculated using both IPCC default emission factors and country-specific emission factors, corresponding to a hybrid Tier 1/Tier 2 methodology. Emissions of CH4 from biogas plants occur from stacks and ventilation during several stages of the process, e.g. ventilation in the receiving hall of the plant, from the emergency flare and from upgrading units. Emissions that are more significant occur from leakages in the production equipment and pipelines. These leakages are by nature very variable from plant to plant and as such difficult to quantify at a national level. Emissions from leakage is calculated using a country specific emission factor.Figure 6 and Figure 7 presents the activity data the two sources respectively.
Greenhouse gas emissions are presented in Figure 1. NH3 and CO emissions are miniscule on a national level.
Composting contributes with CH4 and N2O emissions, where CH4 contributes the most (57-60 % of CO2 equivalent compost emissions). Emissions from anaerobic digestion have increased from 0.2 kt CH4 (6.3 kt CO2e) in 1990 to 14.4 kt CH4 (402.5 kt CO2e ) in 2021; i.e. 6325 %, while emissions from composting have increased from 50 kt CO2e in 1990 to 159 kt CO2e in 2021 (i.e. 221 %). Due to the steep increase in energy production at biogas facilities, emissions from anaerobical digestion has increased from 11 % of CO2 equivalents from the Biological treatment of solid waste sub-sector in 1990 to 72 % in 2021.
Biological treatment of solid waste constitutes 3 % (1990) to 46 % (2021) of the total CO2 equivalent emission from the Waste sector.
Incineration of municipal, industrial, clinical and hazardous waste takes place with energy recovery and therefore the emissions are included in the relevant subsectors under the Energy sector. There is no open burning considered in the Danish inventories. The subsector of Incineration and Open Burning is comprised by emission from human and animal cremations. CO2 emissions from animal and human cremations are considered biogenic, leaving CH4 and N2O emissions as the only relevant greenhouse gasses. As seen from Figure 1, greenhouse gasses from this source is miniscule. However, the emissions of especially Hg caused by cremations contributes to a considerable part of the total national emissions. In addition to the most frequently discussed emissions of Hg and PCDD/Fs (dioxins and furans), are the emissions of compounds like SO2, NOx, NMVOC, CO, other heavy metals (As, Cd, Cr, Cu, Ni, Pb, Se, Zn), particulate matter, HCB, PAHs and PCBs. Emissions of the most relevant pollutants are presented in Figure 2.
The Danish wastewater treatment system is characterised by few big and advanced wastewater treatment plants (WWTPs) and many smaller WWTPs. For the part of the population, which is not connected to the collective sewer system (i.e. scattered houses), septic sludge is collected and transported for treatment at the centralised WWTPs. Municipal collection of sludge from septic tanks occurs at a frequency set by the local authorities and in general, septic tanks are emptied one time each year. This source category includes an estimation of the emission of CH4, N2O and NMVOC from wastewater handling; i.e. wastewater collection and treatment. CH4 is produced during anaerobic conditions and treatment processes, while N2O and NMVOC may be emitted as a by-product from nitrification and denitrification processes under anaerobic as well as aerobic conditions.
The methodology developed for this submission for estimating emission of methane and nitrous oxide from wastewater handling follows the IPCC Guidelines and the NMVOC calculations follow the default Tier 2 methodology of EMEP/EEA Guidebook.
Figure 8 presents greenhouse gas emissions from wastewater in detail.
Fugitive methane emissions from the municipal and private WWTPs originate from 1) the sewer system, primary settling tank and biological N and P removal processes, 2) from anaerobic treatment processes in closed systems with biogas recovery for energy production and 3) septic tanks
Regarding the time trend, the net CH4 emission from anaerobic treatment has increased 177 % from 1990 to 2021, while a less significant increase is observed in the CH4 emission from the sewer system (incl. mechanical and biological treatment) (12%). Lastly, the CH4 emission from scattered houses not connected to the collective sewer system has decreased with 28 % reflecting the increase in the number of people connected to the collective sewer system. In total CH4 emissions quantified as a sum of CH4 emissions from anaerobic treatment processes, the sewer system, mechanical and biological treatment, and scattered houses have increased by 31 % from 1990 to 2021.
N2O formation and releases, both during the treatment processes at the WWTPs and from discharged effluent wastewater, are included.
The emission of N2O from wastewater handling is calculated as the sum of contributions from wastewater treatment processes at the WWTPs (direct emissions) and from sewage effluents (indirect emissions). The emission from effluent wastewater, i.e. indirect emissions, includes separate industrial discharges, rainwater-conditioned effluents as well as effluents from scattered houses and from aquaculture.
Regarding the time trend, the indirect N2O emission has decreased 77 % from 1990 to 2021, the direct N2O emission from separate industries has decreased by 96 %, while the direct N2O emission from municipal wastewater treatment plants has increased by 107 %. The latter is mainly due to the fact that the fraction of industrial wastewater being treated at municipal WWTPs has increased to 40 % during the whole time series. In total, the N2O emission has decreased 58 % from 1990 to 2021.
The annual fluctuations are caused by several factors, e.g. climatic condition such as variations in precipitation and as a result varying contributions to the influent N and varying characteristics of especially the industrial contributions to the influent. Furthermore, infiltration of groundwater, as well as exfiltration of overload rainwater and wastewater contribute to the “noise” or fluctuation in the trend of the calculated N2O emission.
The sub-sector Other is comprised by the subcategory accidental fires grouped into accidental building and vehicle fires. Greenhouse gasses that are estimated from these processes are CH4 and CO2. No emission factors are available for N2O. Air pollutants that are emitted from fires include SO2, NOx, NMVOC, CO, particulate matter, heavy metals (As, Cd, Cr, Cu, Ni, Hg, Pb, Zn), PCDD/F and PAHs.
Emissions from building fires are calculated by multiplying the number of building fires with selected emission factors. Six types of buildings are distinguished with different emission factors: detached houses, undetached houses, apartment buildings, industrial buildings, additional buildings and containers. Emissions from vehicle fires are calculated by multiplying the mass of vehicle fires with selected emission factors. Emission factors are not available for different vehicle types, whereas it is assumed that all the different vehicle types lead to similar emissions. The activity data are calculated as an annual combusted mass by multiplying the number of different full scale vehicle fires with the Danish registered average weight of the given vehicle type.
Greenhouse gas emissions from this source are small, see Figure 1, but emissions of SO2, PM2.5, Pb and PCDD/F from accidental fires are significant compared to the national emissions. For 2021, SO2, PM2.5, Pb and PCDD/F emissions from accidental fires comprise 10 %, 2 %, 15 % and 26 % of the national emissions respectively. The trends for these emissions are presented in Figure 2.