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.
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.
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 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.
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.