Hierarchy of Sustainable Waste Management

The original hierarchy of waste management addresses Reducing, Reusing, Recycling of waste, Aerobic and Anaerobic Composting, Waste-to-Energy and Sanitary Landfilling. For the specific purpose of this study, “Unsanitary Landfilling and Open Burning” has been added to represent the indiscriminate dumping and burning of MSW and the general situation of SWM in India and other developing countries.

The hierarchy of waste management recognises that source reduction begins with reducing the amount of waste generated and reusing materials to prevent them from entering the waste stream. Thus, waste is not generated until the end of “reuse” phase. Once the waste is generated, it needs to be collected. Material recovery from waste in the form of recycling and composting is recognised to be the most effective way of handling wastes. Due to technical and economic limitations of recycling, product design, inadequate source separation and lack of sufficient markets that can use all sorted materials, most of the MSW generated in India ends up in landfills. Local authorities should start working with their partners to promote source separation. While this is being achieved and recycling is increased, provisions should be made to handle the non-recyclable wastes that are and will be generated in the future.

A sustainable solution to handle non-recyclable waste is energy recovery from wastes. Energy recovery falls below material recovery on the hierarchy. Landfilling of MSW is equivalent to burying natural resources which could be used as secondary raw materials or as sources of energy. In the present society, landfills are required as a small fraction of wastes will have to be landfilled. However, unsanitary landfilling or open dumping of wastes is not considered as an option to handle MSW.

Material Recovery

Recycling

Reducing and reusing are the most effective ways to prevent generation of wastes. The best alternative to handle wastes would be recycling which involves using the waste as raw material to make new products. Recycling thus offsets the use of virgin raw materials.

It is known that as much as 95% of a product’s environmental impact occurs before it is discarded, most of it during the manufacturing and extraction of virgin raw materials. Thus, recycling is pivotal in reducing the overall lifecycle impacts of a material on environment and public health. Recycling however requires a separated stream of waste, whether source separated or separated later on (after collection).

Due to limitations in source separation, wastes are collected in a mixed form as municipal solid waste (MSW). Once wastes are mixed, it becomes difficult to separate them. Recyclables can still be separated manually to some extent. Such separation and sale of recyclables from mixed wastes provides livelihood to marginalised urban populations in low and middle income countries.

The separated stocks of paper, plastic, glass and metal can be recycled. A 100% separation of these materials from MSW is highly energy and time intensive and is generally not carried out. Therefore, mixing of waste will always result in a fraction of residues, which can neither be recycled nor composted and needs to be combusted in RDF or WTE plants to avoid landfilling, and generate energy.

Aerobic Composting

Sources of Urban Organic Wastes

• Household waste
• Food waste from restaurants, hotels and food joints
• Vegetable market & slaughterhouse waste
• Livestock & poultry waste
• Sewage sludge

Similar to the recycling of inorganic materials, source separated organic wastes can be composted and the compost obtained can be used as an organic fertilizer in agricultural fields. Organic compost is rich in plant macro nutrients like Nitrogen, Phosphorous and Potassium, and other essential micro nutrients.

The biological decomposition accomplished by microbes during the process involves oxidation of carbon present in the organic waste. Energy released during oxidation is the cause for rise in temperatures in windrows during composting. Due to this energy loss, aerobic composting falls below anaerobic composting on the hierarchy of waste management. Lifecycle impacts of extracting virgin raw materials and manufacturing make material recovery options like recycling and composting the most environment friendly methods to handle waste. They are positioned higher on the hierarchy compared to other beneficial waste handling options like energy recovery.

Quality of the compost product depends upon the quality of input waste. Composting mixed wastes results in low quality compost, which is less beneficial and has the potential to introduce heavy metals into human food chain.

Aerobic composting of mixed waste results in a compost contaminated by organic and inorganic materials, mainly heavy metals. However, mixed waste composting is widely practiced and is considered better (if not best) in countries like India where more than 91% of MSW is landfilled and there are no other alternatives. It is considered better probably because public health and environmental impacts of unsanitary landfilling are more firmly established by research than those impacts due to heavy metal contamination of MSW compost.

Energy Recovery

Energy recovery is a method of recovering the chemical energy in MSW. Chemical energy stored in wastes is a fraction of input energy expended in making those materials. Due to the difference in resources (materials/energy) that can be recovered, energy recovery falls below material recovery on the hierarchy of waste management.

Anaerobic Digestion

The USEPA defines Anaerobic Digestion (AD) as a process where microorganisms break down organic materials, such as food scraps, manure and sewage sludge, in the absence of oxygen. In the context of SWM, anaerobic digestion (also called Anaerobic Composting or Biomethanation) is a method to treat source separated organic waste to recover energy in the form of biogas, and compost in the form of a liquid residual. Biogas consists of methane and carbon dioxide and can be used as fuel or, by using a generator it can be converted to electricity on-site. The liquid slurry can be used as organic fertilizer. The ability to recover energy and compost from organics puts AD above aerobic composting on the hierarchy of waste management.

Similar to aerobic composting, AD needs a feed stream of source separated organic wastes. AD of mixed wastes is not recommended because contaminants in the feed can upset the process. Lack of source separated collection systems, and public awareness and involvement strike off large scale AD from feasible SWM options in India. However, AD on a small scale (called small scale biogas) has emerged as an efficient and decentralized method of renewable energy generation, and waste diversion from landfills.

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.

A joint report by Earth Engineering Center,

Wast-to-Energy Research & Technology Council,

Coloumbia University