Hierarchy of Sustainable Waste Management

Refuse Derived Fuel (RDF)

Refuse Derived Fuel refers to the segregated high calorific fraction of processed MSW. RDF can be defined as the final product from waste materials which have been processed to fulfill guidelines, regulatory or industry specifications mainly to achieve a high calorific value to be useful as secondary/substitute fuels in the solid fuel industry. RDF is mainly used as a substitute to coal (a fossil fuel) in high-energy industrial processes like power production, cement kilns, steel manufacturing, etc., where RDF’s use can be optimised to enhance economic performance.

The organic fraction (including paper) in RDF is considered to be a bio-fuel and is thus renewable. Since the carbon dioxide released by burning the organic fraction of RDF arises from plant and animal material, the net greenhouse gas (GHG) emissions are zero. The overall greenhouse emissions from RDF are however not zero. This is due to carbon emissions from burning the plastics fraction left in RDF. The amount of GHG emissions from RDF depends upon the composition of organics and plastics in the MSW stream it is being processed from. Using RDF prevents GHG emissions from landfills, displaces fossil fuels, and reduces the volume of waste that needs to be landfilled, thus increasing their operating life.

On the hierarchy of waste management, RDF is placed below aerobic composting, as a waste to energy technology. It is a slight variant of the waste-to-energy combustion (WTE) technology, which combusts MSW (processed or as it is) to generate electricity. RDF is different because the objective is to increase the calorific value by processing the fuel.

Waste-to-Energy Combustion (WTE)

Waste-to-Energy combustion (WTE) is defined as a process of controlled combustion, using an enclosed device to thermally breakdown combustible solid waste to an ash residue that contains little or no combustible material and that produces electricity, steam or other energy as a result. Even though both WTE combustion and RDF combust MSW, the objective of WTE combustion is treating MSW to reduce its volume. Generating energy and electricity only adds value to this process.

Combusting the organic fraction of MSW (a bio-fuel) and releasing carbon dioxide as the end product is a net zero emissions process. Due to the dominance of organic waste in MSW, MSW is considered as a bio-fuel which can be replenished by agriculture, which makes it renewable. In India, urban MSW contains as much as 60% organic fraction and 10% paper. Therefore, potentially, 70% of energy from WTE plants is renewable energy. Therefore, WTE is recognized as a renewable energy technology by the Government of India (GOI).

WTE combustion decreases the volume of wastes by up to 90%. Such reduction in volume would prolong the life of a 20 years landfill to 200 years. However, MSW should be combusted after all possible recycling and composting has been done. The input to WTE plants should be the rejects from material recovery and/or composting facilities. Such an integrated system can decrease the amount of wastes landfilled and prolong the life of landfills further. Therefore, WTE combustion is placed below recycling, aerobic and anaerobic digestion on the hierarchy of sustainable waste management.

Sanitary Landfilling

Sanitary landfills (SLFs) are built to isolate wastes from the environment and render them innocuous through the biological, chemical and physical processes of nature.

On the hierarchy of waste management, sanitary landfilling is expanded into three different categories

1. SLFs recovering and using methane (CH4)
2. SLFs recovering and flaring CH4
3. SLFs without any CH4 recovery

SLFs are categorised depending upon their ability to control and prevent negative impacts on environment, from a climate change perspective. They occupy the three positions after WTE technologies on the hierarchy of waste management. Handling CH4 generated during anaerobic digestion of organics dictates where each type of landfill is placed on the hierarchy of waste management.

Organic waste in landfills undergoes both aerobic and anaerobic digestion depending upon oxygen availability. Majority of the waste on the top undergoes aerobic digestion due to greater oxygen availability. Waste which is inside SLFs undergoes anaerobic digestion due to reduced oxygen availability. The final gaseous product of aerobic digestion is CO2, which results in a net zero emission. However, the final gaseous product of anaerobic digestion is CH4, which if captured can be used as a fuel, generating renewable energy and converting the carbon in CH4 to CO2 , thus resulting in net zero emissions.

In a business as usual scenario (BAU) in India and elsewhere, the CH4 is let out into the atmosphere and not captured. Capturing and flaring CH4 is environmentally preferred to sanitary landfilling without capturing CH4.

However, landfilling of materials should be the last option considered for disposing wastes in an integrated waste management system. Also, “currently, the implementation and practice of sanitary landfilling are severely constrained in economically developing countries (like India) by the lack of reliable information specific to these countries”.

Unsanitary Landfilling and Open Dumping

Unsanitary landfilling is generally characterized by open dumping of wastes, lack of monitoring of the site, stray animals and birds feeding on the wastes, absence of leachate or methane collection systems and wastes exposed to natural elements.

The direct implications of landfilling include burying materials which were extracted by energy and infrastructure intensive, and in most cases environmentally harmful methods and in turn depleting earth’s natural resources. From an energy recovery perspective, landfilling is equivalent to burying barrels of oil. Unmonitored landfills catch fires due to methane generation and heat, and result in uncontrolled combustion of wastes, releasing harmful gases like carbon monoxide, hydrocarbons and particulate matter into the lower level atmosphere. They also contribute to Climate Change by releasing methane, a greenhouse gas (GHG) with 21 times more global warming potential than carbon dioxide (in the first year of release, methane is 71 times more potent than carbon dioxide as a GHG). Unsanitary landfilling is not considered an option to treat wastes. It is a desperate attempt to get wastes out of each home or neighbourhood in an unplanned, unmonitored and an irresponsible manner.