Process        Inputs & Outputs

Process
There are four key biological and chemical stages of anaerobic digestion:

1. Hydrolysis—Hydrolytic bacteria breaks down the organic material (feedstock) into simple sugars, amino acids and fatty acids.


2. Acidogenesis—the acidogenic bacteria breaks down the components of the previous step into volatile fatty acids, ammonia, carbon dioxide, hydrogen sulfide and other by-products.


3. Acetogenesis—the molecules created in the previous step are further digested by acetogenic bacteria to produce acetic acid, carbon dioxide and hydrogen.


4. Methanogenesis—the methanogenic bacteria convert the intermediate products into methane, carbon dioxide and water, making up the majority of the biogas output. This process is sensitive to pH levels, which must be between 6.5 and 8.0. The remaining non-digestable materials comprise the digestate.

The process can be run in batches or in a continuous flow system.
Batch—entire digestion process occurs in a single tank, making it the easiest and least capital intensive, but suffers from less control of the reactions in the system (acidogenic bacteria lowers pH prior to methanogenic bacteria working, which need a higher pH level), less efficient biogas production (it forms over a normal distribution pattern) and severe odor.
Continuous—the feedstock is constantly added to the digester with the byproducts continually moved. It also allows for more efficient digestion by the acidogenic and methanogenic bacteria in separate vessels as each operates best at different conditions; hydrolysis, acetogenesis and acidogenesis in the first vessel before being heated and moved into the second tank for methanogenesis.

The type of methanogens also determine how the system need to be heated:
Mesophilic—operate most efficiently between 98° and 106°F, but can work at temperatures between 68° and 113°F when mesophiles are the primary anaerobe. This wider temperature range makes mesophiles more stable than thermophiles.
Thermophilic—operate most efficiently between 122° and 126°F, but can work up to temperatures of 158°F when thermophiles are primary anaerobe. Thermophiles produce faster reactions and gas yields because of the higher temperature, but require higher energy costs to heat the system. It also leaves digestate with a lower bacterial content.