Key Points:
- Anaerobic digestion is a process that breaks down organic waste in the absence of oxygen to produce biogas.
- The process involves the degradation of different organic compounds present in the waste, including lipids, lignin, cellulose, and hemi-cellulose.
- Among these compounds, lignin is the most difficult to degrade, and therefore, it is often left undergraded during anaerobic digestion.

Explanation:
Anaerobic digestion is a natural biological process that occurs in the absence of oxygen. It involves the breakdown of organic waste by microorganisms, which convert the waste into biogas, a mixture of methane and carbon dioxide. This process is widely used in biogas production for energy generation and waste management.

During anaerobic digestion, different organic compounds present in the waste are degraded by specific groups of microorganisms. These compounds include lipids (fats and oils), lignin, cellulose, and hemi-cellulose. However, the degradation of these compounds is not equal, and some are more difficult to break down than others.

Lignin:
One of the most challenging compounds to degrade during anaerobic digestion is lignin. Lignin is a complex polymer found in the cell walls of plants, and it provides structural support. Its complex structure and high resistance to microbial degradation make it difficult for the microorganisms involved in anaerobic digestion to break it down completely.

Cellulose and Hemi-cellulose:
Cellulose and hemi-cellulose are polysaccharides found in plant cell walls. They are composed of long chains of glucose molecules. These compounds are more easily degraded by microorganisms during anaerobic digestion compared to lignin. The microbial community involved in this process includes cellulolytic bacteria and fungi that produce enzymes called cellulases and hemicellulases, respectively. These enzymes break down cellulose and hemi-cellulose into simpler sugars, which can then be fermented by other microorganisms to produce biogas.

Lipids:
Lipids, such as fats and oils, are another organic compound present in organic waste. They can be easily hydrolyzed by lipase enzymes secreted by microorganisms during anaerobic digestion. The hydrolysis of lipids releases fatty acids, which can be further metabolized by other microorganisms to produce biogas.

In summary, while all the mentioned compounds (lipids, lignin, cellulose, and hemi-cellulose) are degraded during anaerobic digestion, lignin is the most difficult to break down completely. Therefore, it is often left undergraded, leading to its accumulation in the digested material.

During biogas formation, digestion of cellulose is slow ( rate- limiting) and most of the lignin is not decomposed. After cellulose lignin is the most abundant plant polymer. It form 20 to 30% of the wood tree, lignin is a complex polymeric molecule, made up of phenyl Propanoid units. Cellulose is a large chain polymer of glucose molecule which she linked with each other by glycosidic bond. Hemicellulose are branched polymers of glucose, xylose, galactose, mannose and arabinose.