Solid Waste Disposal Sites constitutes a sub-sector in the waste sector. In the Danish emission inventory system CH4 emissions from solid waste disposal sites are calculated with use of a first order decay as recommended by the IPCC in their guidelines.
From 1994 to 2005, the number of registered solid waste disposal sites (SWDSs) landfill sites in Denmark has decreased from 176 to 134. There are 56 active disposal sites (SWDS) existing today, reporting to the new waste data system. Methane collections from 29 of these SWDS are reported to be used at energy-producing installations in the Energy statistics in 2019.
Solid Waste Disposal on Land is the dominant source in the waste sector with contributions in the time series varying from 81 % (1990) to 44 % (2020) of the total emission, given in CO2 equivalents. Throughout the time series, the emissions are decreasing due to a reduction in the amount of degradable waste deposited. Comparing 2020 with 1990, the emissions from SWDS have decreased with 65 %.
Figure 1 Time-series for waste deposited for the years 1990-2020.
The general development for solid waste at disposal sites is a result of action plans by the Danish government called the "Action plan for Waste and Recycling 1993-1997" and "Waste 21 1998-2004". The latter plan had, inter alia, the goal to recycle 64 %, incinerate 24 % and deposit 12 % of all waste. The goal for deposited waste was met in 2000. Further, in 1996 a municipal obligation to assign combustible waste to incineration was introduced. In 2003, the Danish Government set up targets for the year 2008 for waste handling in a “Waste Strategy 2005-2008” report. According to this strategy, the target for 2008 was a maximum of 9 % of the total waste to be deposited; a target was already met in 2004 where 7.7 % of total waste was deposited according to Danish waste statistics. Waste Strategy 2009-12, part I was the sixth waste management plan or strategy adopted by the successive governments dating back to 1986. Waste Strategy 2009-12 set up targets for 2012 according to which a maximum of 6 % of the total waste produced were to be deposited. It appears that this target was already been met in 2009 where only 5.6 % of all produced waste was deposited. Waste Strategy 2009-2012, Part II includes goals of continued decrease in the amount of waste being deposited in Denmark and an increase in reuse, recycling and recovery (Danish Ministry of Environment, 2010). This report includes an evaluation of the capacity of Danish solid waste disposal sites divided into waste classes: inert, mineral, mixed and hazardous waste. The same waste classes are defined in the new Statutory Order for Landfill (Statutory Order no. 719, 24/06/2011), which refers to the Statutory Order for Waste (Statutory Order no. 1309, 18/12/2012) regarding characterisation of the waste according to the European waste code system; the EWC-code list included in Annex 2 of the statutory Order no. 1319. The New Danish Waste Reporting System (www.mst.dk) is based on the EWC-code system. Based on the new waste reporting system, waste amounts have been identified and allocated according to 18 waste types, characterised according to individual content of degradable organic matter and half-life.
The decrease in the 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 yearly methane emission from solid waste disposal sites in Denmark.
Figure 2 Time-series for inert waste deposited for the years 2019-2020.
Figure 3 Time-series for degradable organic waste deposited for the years 1990-2020.
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 1.
In Denmark, 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. In addition to emissions from composting, CH4 emissions from anaerobic digestion at biogas facilities are part of the Biological Treatment of Solid Waste subsector.
Figure 4 Trends in the national amount of composted waste.
Figure 5 Trends in the annual emission of CH4 from anaerobical digestion at biogas facilities.
Composting contributes with CH4 and N2O emissions; where CH4 contributes the most (51-55 %). Composting constitutes 35 % (2020) to 90 % (1990) of the subsector. Emissions from anaerobic digestion have increased 11.3 kt CH4 (283 kt CO2e ) since 1990; i.e. 5068 %, while emissions from composting have increased 109 kt CO2e (i.e. 223 %).
Biological treatment of solid waste constitutes 3 % (1990) to 37 % (2020) of the total CO2 equivalent emission from the Waste 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.
Figure 6 Trends in the emission from animal (AC) and human cremations (HC).
While emissions from human cremations have been steady for 1990-2010, emissions from animal cremations have increased. During the last decade however, emissions from human cremation have been slightly increasing while emissions from animal cremation have been decreasing.
In 1990, animal cremations represented 5 % of the total emission of CO2e from cremations. In 2020 this number has increased to 25 %.
Wastewater treatment and discharge is a sub-sector in the waste sector. The Danish wastewater treatment system is characterised by few big and advanced wastewater treatment plants (WWTPs) and many smaller WWTPs. From 1993 to 2017 the amount of wastewater treated at the most advanced technological WWTPs in Denmark has increased from 53 % to more than 90 %. This source category includes an estimation of the emission of CH4 and N2O from wastewater handling; i.e. wastewater collection and treatment.
The methane emission from the sewer system and WWTP processes (including anaerobic treatment) has increased by 33 % from 1990 to 20207. Regarding the fraction of the population not connected to the collective sewer system (CH4 emitted from septic tanks) an increase of 13 % is observed from 1990 to 2020.
Figure 7 Time-series of CH4 emissions due to venting (CH4,AD,net), mechanical and biological treatment (CH4,sewer+MB), scattered houses (CH4,st) and the total methane emission from wastewater treatment for the years 1990-2020.
The total methane emissions has increased from 1.64 kt in 1990 to 2.12 kt methane in 2020 corresponding to an increase in net methane emissions from wastewater handling of 29 %.
The emission of N2O from wastewater handling is calculated as the sum of contributions from wastewater treatment processes at the WWTPs (i.e. direct N2O emissions) and from sewage effluents (i.e. indirect emissions). In addition to sewage effluents from the Danish WWTPs, indirect N2O emissions, includes separate industrial discharges, rainwater-conditioned effluents, effluents from scattered houses, from mariculture and fish farming.
Figure 8 Time-series of N2O emissions from wastewater treatment and discharge.
Regarding the time trend, the indirect N2O emission has decreased 75 % N2O from 1990 to 2020, while the direct N2O emission has decreased 28 %. A net reduction in the total N2O emission from wastewater treatment and discharge of 39 % in 2020 compared to 1990 is observed.
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 subsector Other is comprised by the subcategory accidental fires grouped into accidental building and vehicle fires.
Figure 9 Time-series of CO2 and CH4 emissions from accidental building and vehicle fires.
The total emission in CO2 equivalents has increased 5 % in 2020 compared to 1990.