Municipal Solid Waste is the waste material typically discarded by households comprisingdry & wet waste materials. Solid waste is a significant contributor to the greenhouse gas (GHG) emissions through decomposition /anaerobic digestion. Anaerobic digestion (AD) is the process by which organic materials in the waste are broken down by micro-organisms in the absence of oxygen. This process releases methane (CH4)-a potent GHG. The majority of these emissions are a result of land filling –deposition of municipal solid waste materials on Solid Waste Disposal Sites (SWDS), which remains the primary waste disposal practice in the world.
As per the Intergovernmental Panel on Climate Change (IPCC), the estimated global annual emissions from SWDS are in the range of 20 to 40 million tonnes of methane, of which the most comes from industrialized countries. This contribution is estimated to be approximately 5-20% of the global anthropogenic methane, which is equal to about 1 to 4% of the total anthropogenic greenhouse gas (GHG) emissions.
India is one of the most populous countries with an increasing urban population. The rate of waste generation is directly proportional to the population increase. This creates an additional burden on the SWDS. Municipal corporations use SWDS to get rid of solid waste. Most of us might have seen huge hillocks of solid waste within the few kilometers periphery of our city. These hillocks of solid wastes are a result of decades of deposition with improper waste management practices.
Let us see how methane emission occurs from SWDS
As discussed, earlier, the bio-degradable waste materials on the SWDS develop anaerobicity in the absence of oxygen. It occurs due to the contentious deposition of waste materials on the disposal sites. For understanding how methane emission occurs from SWDC, first we need to understand the process of anaerobic digestion. Anaerobic digestion is a series of physical and chemical processes in which microorganisms i.e. methanogenic bacteria break down bio-degradable material in the absence of oxygen. The process goes through various stages. These are Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis.
|1||Hydrolysis||In this process, bacteria transform the organic substrate into liquefied monomers/polymers. Simply, the substrates like proteins, carbohydrates and fats are transformed to amino acids, mono-saccharides and fatty acids respectively.|
|2||Acidogenesis||In this stage, the acidogenic bacteria transform the products of the first reaction i.e. Hydrolysis into short chain volatile acids, alcohols, hydrogen and carbon dioxide.|
|3||Acetogenesis||In this stage, the products made in Acidogenesis process are transformed by acetogenic bacteria into hydrogen, carbon dioxide and acetic acid.|
|4||Methanogenesis||This is final stage where methanogenic bacteria convert the hydrogen and acetic acid formed in Acetogenesis process to methane and carbon dioxide gas.|
As the deposition of waste materials keeps on continuing on SWDS, the process of anaerobic digestion continues for longer period at these sites. The rate of anaerobic digestion in the SWDS is dependent on the physical conditions of the site and the time for which the waste materials are deposited. The process of anaerobic digestion mainly releases methane, carbon dioxide and heat.
In 21st century, we have developed technologies like controlled anaerobic digestion, aerobic composting and many others which can convert waste into commercially usable products. For example, bio-gas (i.e. Methane) can be captured with the help of technologically advanced anaerobic digesters and can be in thermal applications as well as for electricity generation. Aerobically treated waste materials are transformed into bio-compost that can be used for land application in agriculture and horticulture, etc.
Dust bin image credit: Tilo Hauke on Wikipedia