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The mysterious journey of anaerobic digestion: How do microorganisms show their talents in an oxygen-free environment?
Anaerobic digestion is a surprising process in which microorganisms are able to efficiently degrade biodegradable materials in an oxygen-free environment. As an important biotechnology, anaerobic digestion can be used in industry or at home to help manage waste and produce fuel. Whether used in the fermentation of food and beverages or in home fermentations, anaerobic digestion is constantly performing its functions.
The process of anaerobic digestion can occur naturally in some soils and lake and ocean sediments, and is often referred to as "anaerobic activity."
This process goes through four main stages: hydrolysis, acidification, acetation and methane generation. The first step in anaerobic digestion is the hydrolysis of the input material by bacteria, converting insoluble organic polymers (such as carbohydrates) into soluble derivatives that can be used by other bacteria. Acid-generating bacteria then convert sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. In the subsequent acetogenesis phase, bacteria convert these organic acids back into acetic acid and again into other compounds such as ammonia, hydrogen and carbon dioxide. Finally, methanogenic archaea convert these products into methane and carbon dioxide.
This process not only converts waste into useful energy but also reduces landfill gas emissions.
Anaerobic digestion is particularly widely used in wastewater treatment and waste management. During the process, the biogas generated consists of methane, carbon dioxide and other trace "pollutant" gases. This biogas can be used directly as fuel, or fed into a gas generator that combines heat and electricity, or even upgraded to near-natural gas quality biomethane. As interest in waste reuse grows, governments in many countries, such as the UK and Germany, are also beginning to show greater interest in anaerobic digestion.
The process of anaerobic digestion
In anaerobic digestion, many microorganisms are involved, including acetic acid-producing bacteria and methane-producing archaea. These microorganisms facilitate a series of chemical reactions as they convert biomass into biogas. In anaerobic systems, the participation of gaseous oxygen is excluded under physical constraints, and anaerobic organisms use other electron acceptors besides oxygen. These receptors may come from the organic material itself, or may be supplied by inorganic oxides in the input material.
Four main stages
The four key stages of anaerobic digestion are hydrolysis, acidification, acetification and methane generation. Together, these stages biochemically degrade organic matter such as glucose into carbon dioxide and methane. The following is the detailed process of each stage:
Hydrolysis
Typically, biomass consists of large organic polymers. In order for the bacteria in anaerobic digestion to use the energy in these materials, these chains must first be broken down into smaller components. This process, called hydrolysis, breaks down complex organic molecules into simple sugars, amino acids, and fatty acids.
Acidification
Acidification is a biological process in which remaining components are further degraded by acid-producing bacteria. During this stage, by-products such as volatile fatty acids, ammonia, carbon dioxide and hydrogen sulfide are formed. This process is similar to how milk turns sour.
Acetation
During the acetification stage, simple molecules are further degraded by acetic acid-producing bacteria, ultimately producing the bulk of carbon dioxide and hydrogen.
Methane generation
The final stage of anaerobic digestion is methane production. In this process, the methanoarchaea use intermediates from previous stages to convert into methane and water, and these components make up the majority of the biogas produced in the system.
Digester design
Anaerobic digesters can be designed in many different configurations and classified according to different process modes such as batch or continuous. Continuous processes require more complex designs, but since batch processes require more initial construction costs, continuous processes may be more economical in terms of cost. Digesters can be divided into high solids and low solids processes depending on the solids content to be processed. Generally speaking, low solids processes can have solids contents below 15%, while high solids processes can have solids contents above this level.
As technology advances, some countries such as Germany and the United States have begun to explore new anaerobic digestion strategies, aiming to further improve the efficiency of the process and ultimately achieve a more sustainable way of managing waste.
These developments not only make it possible to recycle waste, but also promote the utilization and development of renewable energy.
Anaerobic digestion is a journey of subtle synergy of microorganisms. There are many mysteries and potentials hidden in this process, which are worthy of our more in-depth study and exploration. Can we use this wisdom more intelligently in the future of waste and energy?
