Best Waste Disposal Methods to Reduce Greenhouse Gas Emissions

As the third-largest global methane producer, the waste sector can drastically reduce its emissions by moving away from dumps and landfills and adopting recovery methods that turn waste into value.

Methane is one of the most potent greenhouse gases (GHGs) responsible for global warming. Although it is short-lived and only stays in the atmosphere for 12 years, methane has 82 times the warming potential of carbon dioxide (CO2) in the first 20 years after it is emitted. According to the new IPCC Climate Change 2021 report, the impact of short-lived gases on climate warming in the next 10 to 20 years is at least as large as CO2's. Therefore reducing short-lived gases, the report continues, "would, in the short term, contribute significantly to the efforts of limiting warming to 1.5ºC."

If reducing short-lived gases as methane is essential for global warming mitigation, the waste sector with its 18% share of total global anthropogenic methane emissions has a big role to play. Only agriculture, accounting for 40% of methane emissions, and fossil fuel production and distribution - 34% - have larger roles.

The waste sector generates methane when waste decomposes in disposal sites. Dumps and landfills are the sector's main emitters, and the most common disposal method used for 70% of the world's waste (figure 1). The remaining 30% of global waste is recovered through recycling and composting (19%); and other processes (11%).

Reducing the sector emissions means to phase out dumps and landfills, and while this is the trend in some countries, any improvement in waste disposal would reduce environmental degradation and GHG emissions. The below points list the current waste disposal and recovery methods, their impact on emissions and their potential improvements.

0. Unmanaged dumps (figure 2). One-third of the world's waste is still not managed safely and is disposed with methods such as open dumping and burning. Although unmanaged shallow dumps (<5m waste) produce 60% less methane than managed disposal sites, due to more waste decomposing aerobically, air pollution from open burning still occurs concurrently with odours, public health and safety problems, and environmental degradation.

Policies / measures / instruments to reduce unmanaged dumping:

- Regulation aimed at restricting uncontrolled dumping of waste

- Implementation of controlled disposal and recycling practices

- Government support where user fee models can't be implemented

- International support to bridge the waste management gap

1. Controlled dump, or anaerobically managed solid waste disposal site (SWDS, figure 3), is a basic way to dispose waste but a massive improvement from previous point. These SWDS -disposing 29% of global waste- must have controlled placement of waste in specific deposition areas and either cover material; mechanical compacting; or levelling of the waste. Anaerobically managed SWDS emit leachate that penetrates the soil and contaminates the surrounding soil and aquifers, but first and foremost they are the largest methane emitters in the waste sector, its emissions being highest the first few years after deposition. Interestingly, they are also long-term carbon sinks since >50% of the carbon disposed is not converted to biogas but remains in the disposal site.

Policies / measures / instruments to reduce emissions:

- Use of cover soils (biocover) to significantly reduce methane via oxidation

- Regulation for reducing biodegradable waste going to SWDS

- Standards for landfill performance to reduce landfill

- Implementation of waste minimisation policies

- Government support / economic incentives to implement gas monitoring and control

2. Recycling & composting (figure 4). Together they recover 19% of global waste. Material recovery reduces the fraction sent to SWDS as well as GHG emissions through lower energy demand for production (avoided fossil fuel) and by substitution of recycled feedstocks for virgin materials. Efficient use of materials also reduces waste. The optimal recycling rate is calculated at 50% (Huhtala, 1997) while the technical maximum is estimated at 60%, assumed to be reached in 2050 (CEPI, 2003).

Composting decomposes source-separated organic fraction of waste aerobically into CO2 and a humid fraction rich in carbon and minerals such as phosphorus (P), aspects that make the outcome product a valuable natural fertiliser for agriculture or soil remediation. Composting avoids methane emissions by removing degradable organic matter (DOCm) from SWDS, and brings the much valued P back into the food cycle.

Compost, however, adds value to the soil in extractive agriculture practices, and not in the regenerative agriculture that seeks soil equilibrium with cover crops and other practices. When the world fully switches to regenerative agriculture - and this is the trend - compost as a product will be hard to place.

Policies / measures / instruments to promote recycling & composting:

- Implementation of policies like Extended Producer Responsibility (EPR), unit pricing (or PAYT/Pay As You Throw) and landfill taxes

- Implementation of source separation

- Regulation aimed at promoting the use of recycled products

- Regulation aimed at improving material efficiency

- Regulation aimed at promoting natural fertilisers

- Economic incentives to support the above regulations

- Government support to separate collection and recover specific fractions

3. Sanitary landfill with gas recovery (figure 5) or aerobically managed SWDS, is used to dispose 8% of global waste. These SWDS must have controlled placement of waste, as well as permeable cover material; leachate drainage system; regulating pondage; and gas ventilation system. Aerobically managed SWDS can recover >90% of methane to, either flare it or, use it to fuel industrial boilers; generate electricity; or to produce a substitute natural gas. In developing countries, this captured methane emissions can be turned into CERs (Certified Emissions Reductions) that are traded and sold to industrialised countries to meet their emission reduction targets. CERs, however, sold cheaply (€0.30/CER in 2019) and are no longer a compliance unit in the EU.

Aerobically managed SWDS equally avoid leachate leakage, although liners can be damaged beyond repair and leachate can still pollute the soil. Additionally, SWDS's land requirements are proving difficult to meet by land-strapped municipalities. Many countries are therefore moving away from landfills; the EU, for instance, is targeting a maximum landfilling rate of 10% by 2030.

Policies / measures / instruments to move forward:

- Regulation aimed at restricting the amount of landfilled waste

- Economic instruments to discourage landfilling

- Regulation aimed at promoting landfill alternatives such as recycling and energy recovery

4. Energy recovery (figure 6) is an alternative to landfilling for GHG emissions avoidance. It turns post-recycling waste into energy -be it heat, electricity or biofuels- using thermal (combustion), thermochemical (pyrolysis) or biological (anaerobic digestion) processes.

Combustion, the more common process in energy recovery, reduces GHG emissions, despite being a source of CO2. This is because only CO2 emissions resulting from oxidation of fossil carbon in waste (plastics, rubber etc) are considered net emissions, whereas CO2 emissions from biogenic carbon (ie paper, food and wood waste) are not. Methane emissions from combustion processes are the result of incomplete combustion (like in open burning) and hardly occur in controlled processes where combustion efficiency is v ery high. There are other emissions from combustion that don't affect global heating but air quality and they are trapped to nearly 99% with air pollution control.

Pyrolysis avoids GHG and polluting emissions since no combustion takes place, being the external energy-supplying system the only emission source of GHG in the pyrolysis plant. This makes pyrolysis the most environmentally friendly energy recovery technology, since anaerobic digestion still generates methane, and a residue that needs to be landfilled.

Energy recovery is a key step towards zero landfill. The IPCC calculates that "the maximum rate of incineration that could be implemented was 85% of the waste generated". This means that energy recovery alone cannot deliver a zero landfill solution - it's not a target either - and needs to work in conjunction with other waste management practices. Countries successfully implementing zero landfill are using a combination of recycling and energy recovery (figure 7), whereby the recycling rate is at 50+%.

Policies / measures / instruments to promote energy recovery:

- Regulation aimed at improving air quality

- Regulation aimed at promoting energy recovery

- Economic incentives to support waste-generated electricity

- Government support for construction of energy recovery plants combined with standards for energy efficiency

IN SHORT

GHG emissions and environmental degradation arising from current waste disposal methods can be avoided by introducing waste management practices and zero landfill policies achieved by means of energy recovery and recycling (figure 8).