Biogas recovery plants are present in many industrial sectors such as in anaerobic treatment plants, in landfills, in purification plants.
If we take into consideration the landfill plants where waste for which recycling has not been possible is deposited, we see how in this place the residues of many wastes, above all organic solid urban waste (MSW), remain active for over 30 years and , through natural decomposition processes, they produce biogas and leachate.
- The first aerobic phase is rather short: reactions take place here thanks to the oxygen present at the time of waste collection. The oxidation of the organic material mainly produces: CO2 and energy in the form of heat. Aldehydes, ketones, alcohols and other substances are also produced which give the waste its characteristic odor;
- The second phase is anaerobic: acid fermentation reactions take place which develop ammonia, hydrogen, CO2 and partially degraded acidic organic compounds. This is where the production of CH4 methane begins. The leachate that is formed in this phase has high BOD and COD, is acidic and particularly aggressive to the point of making it possible to dissolve in it other organic and inorganic compounds. Anaerobic degradation reactions result in the release of products such as volatile carboxylic acids, esters and thioesters responsible for the nauseating smell of waste in the second phase of decomposition;
- The third phase is methanogenic anaerobic: it takes place after at least 6-12 months from the waste disposal. Anaerobic bacterial biodegradation proceeds slowly and decomposes organic acids and other products developed in the previous phase. The transformation of the acids causes an increase in the PH up to values close to neutrality or higher. The main products of the third phase of decomposition are methane CH4 and carbon dioxide CO2.
The formation of the "bad smell" occurs in all three of these phases.
If, through suitable and efficient systems for capturing and conveying the flows, the biogas produced are extracted from the mass of decomposing waste before dispersion into the atmosphere, not only can the "bad smell" be reduced, but energy recovery can also be performed of methane as fuel (for example by starting it in cogeneration engines, turbines, oxidizers).
Landfill biogas is a mixture of various types of gas and is mainly composed of methane (variable from 30-60%), carbon dioxide (variable from 20-40%) and nitrogen; to these three main components are added others in always variable and much lower percentages.
From the point of view of the atmospheric emissions of gases responsible for the intensification of the greenhouse effect, landfills for non-hazardous waste and those for hazardous waste are harmful if the waste is not previously treated and / or differentiated. It is scientifically proven by the international organization on climate change, IPCC (Intergovernmental Panel on Climate Change), that landfill waste causes high methane (CH4) and carbon dioxide (CO2) emissions, two very active "greenhouse gases" .
Methane is a greenhouse gas present in the earth's atmosphere in concentrations much lower than carbon dioxide but with a global warming potential 21 times higher (https://unfccc.int/process/transparency-and-reporting/greenhouse -gas-date / greenhouse-gas-date-UNFCCC / global-warming-potentials)
In particular, captured methane can be used as fuel rather than dispersed in the atmosphere; we want to remember that the CO2 produced by the combustion of methane thus obtained allows to balance the balance of carbon dioxide emitted into the atmosphere: in fact the CO2 emitted by the combustion of biogas is the same CO2 fixed by plants (or taken by animals indirectly through plants), as opposed to what happens for CO2 emitted ex novo from the combustion of fossil fuels.
A further ecological advantage in the collection and use of biogas is that of preventing the diffusion of methane in the troposphere: in fact, it should be remembered that the emission of 1 kg of CH4, in a time horizon of 100 years, is equivalent to emitting 25 kg of CO2 (IPCC 2007).
Furthermore, efficient biogas collection systems allow to reduce the problem related to bad odors emitted by the decomposition of waste.
For those who want more information on the topics discussed above, we recommend consulting the following documentation:
ISPRA environmental data yearbooks: http://www.isprambiente.it
Feedback on methane to the global climate system in a warmer world: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2017RG000559#rog20161-bib-0178
Information from the Sentinel-5 Precursor (Sentinel-5P) satellite developed by ESA as part of the Copernicus Earth observation program for air pollution monitoring: https://www.newence.com/2019/03 / 12 / copernicus-sentinel-5p-ha-unveiled-important-information-on-methano-and-ozone-in the atmosphere /
Assessment of methane emissions from the U.S. oil and gas supply chain - Alvarez et al., Science 361, 186–188 (2018) 13 July 2018