Recycling, Composting, and the Hierarchy of Sustainable Waste Management

Contents
1 The Hierarchy
2. Material Recovery: Recycling
3. Material Recovery: Aerobic Composting


1. The Hierarchy
The Hierarchy of Sustainable Waste Management (Figure 10) developed by the Earth Engineering Center at Columbia University is widely used as a reference to sustainable solid waste management and disposal. This report is presented in reference to this hierarchy. For the specific purpose of this study, “Unsanitary Landfilling and Open Burning” has been added to the original hierarchy of waste management which ends with sanitary landfills (SLFs). Unsanitary landfilling and open burning will represent the indiscriminate dumping and burning of MSW and represents the general situation of SWM in India and other developing countries.

Figure 10, The Hierarchy of Sustainable Waste Management for India and Other Developing Nations

The hierarchy of waste management recognizes that reducing the use of materials and reusing them to be the most environmental friendly. Source reduction begins with reducing the amount of waste generated and reusing materials to prevent them from entering the waste stream (15). 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 recognized 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 (20). A sustainable solution to handle non-recyclable waste is energy recovery. Energy recovery from wastes falls below material recovery. Landfilling of MSW is equivalent to burying natural resources which could be used as secondary raw materials or as sources of energy. However, 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 and is not at all recommended.

2. Material Recovery: Recycling

Reducing and reusing are the most effective ways to prevent generation of wastes. Once the wastes are generated and collected, the best alternative to handle them would be recycling where the materials generally undergo a chemical transformation. Sometimes, reusing can also happen after collection, in cases where informal traders collect materials of no use from households, reshape or repair them and sell in second-hand markets. Unlike reusing a used material, recycling involves using the waste as raw material to make new products. Recycling thus offsets the use of virgin raw materials.

Figure 23, Sorted metal for further processing and eventual recycling

It is known that as much as 95% of a product’s environmental impact occurs before its discarded (21), most of it during its manufacturing and extraction of virgin raw materials. Thus, recycling is pivotal in reducing the overall life cycle 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 the limitations for source separation (See Section 5.6), wastes are collected in a mixed form which is referred to as municipal solid waste (MSW). Once the 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 marginalized urban populations in low and middle income countries. High income countries use machines to do the same but they would need the recyclables to be collected as a separate dry stream without mixing with organic food wastes.

The separated stocks of paper, plastic, glass and metal can then be recycled. A hundred percent 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.

3. Material Recovery: Aerobic Composting

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 on agricultural fields. Organic compost is rich in plant macro nutrients like Nitrogen, Phosphorous and Potassium, and other essential micro nutrients. Advantages of using organic manure in agriculture are well established and are a part of public knowledge.

The final product from a MSW composting (or mechanical biological treatment) facility 

United Nations Environment Program (UNEP) defines composting as the biological decomposition of biodegradable solid waste under predominantly aerobic conditions to a state that is sufficiently stable for nuisance-free storage and handling and is satisfactorily matured for safe use in agriculture. Composting can also be defined as human intervention into the natural process of decomposition as noted by Cornell Waste Management Institute. 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. Anaerobic composting recovers both energy and compost. Life cycle 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. However, 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. Contamination of MSW compost by heavy metals can cause harm to public health and environment and is the major concern leading to its restricted agricultural use (22).  Mixed waste composting is therefore not an option for sustainable waste management, but this issue is not a part of public knowledge. Mixed waste composting is widely practiced and is considered better (if not best) (8) 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.

Sources of urban organic wastes

●      Household kitchen waste

●      Food waste from restaurants, hotels and food joints

●      Vegetable market & slaughterhouse waste

●      Livestock & poultry waste

●   Sewage sludge