27/01/2024
Anaerobic Digestion is a "microbial-mediated process" (Kougias & Angelidaki, 2018), where bacteria break down organic matter such as manure, wastewater biosolids, and food waste in the absence of oxygen (EPA, 2022).
The Biogas Process
This process produces Biogas (composed of carbon dioxide and methane, however, it also includes small amounts of nitrogen, hydrogen, ammonia, and hydrogen sulfide) and Digestate (a liquid or solid material that is the process output). Biogas can be used for scrubbing (heating, cogenerating electricity) or upgraded to natural gas for fueling and injecting into a public grid (Abandes et al., 2021).
The AD process steps are listed as follows:
Hydrolysis:
This is where insoluble biopolymers break down into simple organics such as proteins, carbohydrates, and fats.
Acidogenesis:
Microbes break down glucose molecules, fatty acids & amino acids into volatile fatty acids & alcohols.
Acitogenesis:
Volatile fatty acids and alcohols are transformed into hydrogen, CO2 & Ammonia.
Methanogenesis:
Archaea transform hydrogen and acetic acids into methane and CO2.
One of the key points to successful AD operation is keeping a balance between the populations of the methanogenic microorganisms as well as the hydrolytic, acidogenic, and acetogenic microorganisms (aka) also known as heterotrophs, which means they consume complex organic substances.
This process had been known and used for a while, but this has increased ever since the rise of energy prices during the 1970s. Recently, the process attracted even more attention due to a demand for alternative energy sources to curb the reliance on fossil fuels. The process still remains though, due to the environmental benefits of anaerobic waste degradation.
Biogas was traditionally utilized for producing heat or combined heat and power (CHP), it can also be extensively used for fueling purposes and providing lighting although this highly depends on the size of the AD, and in this case, the biogas digester is household scaled. Farm-scaled or centralized digesters include a CHP unit, and engine and can transform gas into electrical energy, and heat.
Anaerobic Digestion and Climate Change
Anaerobic Digestion or AD systems can contribute towards mitigating the use and spread of harmful gas emissions, reaching sustainable development goals, saving the environment, economy, and society, and fighting climate change.
Climate Change is a global challenge of our time (UN, 2022), affecting everybody in the world and one of the largest contributors to Climate change/Global warming is fossil fuels (coal, oil, and gas). According to the United Nations (2022), fossil fuels are responsible for 75% of the world's greenhouse gas emissions and about 90% of all carbon emissions. Anaerobic digesters capture methane and assist in using it in a beneficial way (EPA, 2022).
They also produce renewable energy from bio-wastes (Agricultural, Domestic, Livestock, Industrial, Sludge) and manure which can be returned to the surface through farming. Fisher (2022) suggests that, according to an Oxford University study, changing from fossil fuels to renewable energy could save the globe as much as twelve trillion dollars by 2050. This is interesting, considering that if nothing is done, Flavelle (2021), climate change could reduce the world economy by $23 trillion (by 2025). Something has to be done.
Types of Anaerobic Digesters
There are various types of AD systems but two of the most common are:
1. Stand-Alone Digesters:
Accept and process feedstock from a single or more sources
Can be utilized as an organics recycling business and municipal waste management
They operate in the food and beverage industry because they are created to process industry-specific waste
The standard feedstock processed in stand-alone digesters is domestic waste while co-digesting yard waste, manures, and waste-water solids
Up-flow anaerobic sludge beds, attached sludge beds, induced bed reactors
Stand-alone digesters include the following components:
Waste pretreatment
Solid separation
Equipment for nutrient and water recovery.
2. On-Farm Digesters:
More targeted at farmers
Can accept outside food waste as a feedstock
types of on-farm digesters include plug flow, complete mix, and covered lagoons
On-Farm Digesters include the following components:
Includes waste reception and short-term storage
These are usually above-ground tanks or covered lagoons
Includes a mixing system, effluent/digestate storage, and processing system to exclude impurities.
Engine-generator
Feedstock
There is a large variety of organic materials that can be utilized in anaerobic digestion to produce biogas, these include:
Livestock manures
Waste feed
Food-processing wastes
Slaughter-house waste
Farm mortality
Corn Silage (energy crop)
Ethanol stillage
Glycerine as the product from biodiesel production
Milkhouse wash water
Fresh produce waste
Industrial wastes
Food cafeteria wastes
Sewage sludge
However, there are scientific boundaries, engineering, and legal limits to what can be included in a digester.
Conclusion
Anaerobic Digestion is a fundamental process to biogas production that can contribute to mitigating the spread of harmful gas emissions and producing renewable energy. The process is simple but can also get complicated as it includes other parts like purification, processing, compressing, etc. These systems are not new to the scene and have been used in farming for storing animal waste but the technology has since gained more popularity due to ongoing demand for renewable energy sources and technological advancements (e.g. purification systems, and power generators).
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