Biological and Chemical Stages of Anaerobic Digestion
Anaerobic digestion is a biological process that breaks down biodegradable materials in the absence of oxygen, resulting in the production of biogas, primarily composed of methane (CH₄) and carbon dioxide (CO₂).
This process involves four main stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
1. Hydrolysis: Hydrolysis is the first stage in anaerobic digestion, where complex organic materials such as carbohydrates, fats, and proteins are broken down into simpler soluble molecules like sugars, fatty acids, and amino acids.
This step is crucial as it converts large, insoluble organic matter into smaller, more manageable compounds that can be further processed by bacteria in the subsequent stages. Hydrogen and acetate products of first stage are directly used by methanogens.
2. Acidogenesis: During acidogenesis, the simpler molecules produced in hydrolysis are further broken down by fermentative (acidogenic) bacteria. This stage results in the production of volatile fatty acids, along with by-products like ammonia, carbon dioxide, and hydrogen sulphide.
3. Acetogenesis: In acetogenesis, the volatile fatty acids and other products from acidogenesis are converted into acetic acid, hydrogen, and carbon dioxide by acetogenic bacteria.
4. Methanogenesis: Methanogenesis is the final stage, where methanogenic bacteria convert the acetic acid, hydrogen, and carbon dioxide into methane (CH₄), carbon dioxide (CO₂), and water.
- Significance: This stage completes the anaerobic digestion process, resulting in the production of biogas, which can be captured and used as a renewable energy source. Methanogenesis is highly sensitive to pH levels, and maintaining an optimal pH is essential for efficient biogas production.
Overall Process:
- The overall process of anaerobic digestion can be summarized by the following simplified chemical equation:
- The indigestible residue left after digestion, known as digestate, consists of non-biodegradable material and microbial biomass.