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The key process stages of anaerobic digestion .There are four key biological and chemical stages of anaerobic digestion

•  Hydrolysis
• Acidogenesis
• Acetogenesis
• Methanogenesis

In most cases, biomass is made up of large organic polymers. For the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts, or monomers, such as sugars, are readily available to other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore, hydrolysis of these high-molecular-weight polymeric components is the necessary first step in anaerobic digestion. Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids. Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules, such as volatile fatty acids (VFAs) with a chain length greater than that of acetate must first be catabolised into compounds that can be directly used by methanogens. The biological process of acidogenesis results in further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here, VFAs are created, along with ammonia, carbon dioxide, and hydrogen sulfide, as well as other by-products. The process of acidogenesis is similar to the way milk sours. The third stage of anaerobic digestion is acetogenesis. Here, simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid, as well as carbon dioxide and hydrogen. The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here, methanogens use the intermediate products of the preceding stages and convert them into methane, carbon dioxide, and water. These components make up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low PHS and occurs between pH 6.5 and pH 8. The remaining, indigestible material the microbes cannot use and any dead bacterial remains constitute the dig estate. A simplified generic chemical equation for the overall processes outlined above is as follows:C6H12O6 ? 3CO2 + 3CH4.

There are a number of things that can affect the biogas production in a biodigester.

• Biogas leaks 
  If there is very little biogas, there may be a leak somewhere. The biogas balloon and biogas pipeline should be checked for leakage. A simple soap solution can be used to detect the leakage.
• Temperature problems 
  As stated before, if temperatures reach below 20°C, you will experience a drastic decrease in biogas production. If this is the case, look to adapt a heating system to your biodigester. 
• Problems with the biodigester’s pH 
  The pH in the biodigester tank should be as close to neutral (7) as possible. Since the anaerobic processes in a biodigester produce acids, the most common pH problem is one of acidity. If you do a simple litmus test on the biodigester’s contents and the result is below 7, you should add a small amount of lime or grounded lime stone to normalize the tank’s pH. Since excessive amounts of lime will not be soluble in the mixture and may harm the bacteria, you should never exceed a lime concentration of 500mg for every litre of mixture in the biodigester tank. 
• Other problems 
  There are a number of other problems that can arise during the life of a biodigester. To investigate problems, it is best to think back to the basics of what makes a biodigester work (organic material, strong seals, warmth) and eliminate anything else that could possibly harm its functioning. For example be careful not to introduce unnecessary chemicals into the tank, and try not to use livestock that has recently been given antibiotics or other medications, for these chemicals present in the manure may cause damage to the bacteria in the biodigester tank. Also, make sure to use non-corrosive materials for handling the gas and water. Cement and plastic cause no harm to the mixture in the tank, but metals should be avoided for use in the tank, or any of the tubing through which the biogas travels.