Biogas from anaerobic digestion
Complex organic compounds –> Simple organic compounds
Simple organic compounds –> Organic acids and alcohols
Organic acids and alcohols –> Hydrogen, carbon dioxide and acetic acids
Hydrogen, carbon dioxide and acetic acids ––> Biogas (methane and carbon dioxide)
The flowchart above and its description below explain a general system setup of a large or medium-sized reactor.
Before the reactor
Before the organic waste can be turned into biogas its size needs to be reduced, a process called pretreatment. This can be done manually or for example with an industrial crusher, mixer or screw pump. Water is then added in order to achieve the particle concentration most suitable for the biological process. The resulting substrate, called slurry, is fed into the reactor manually, by machines or by pump-driven piping. Filling is made easier by the reactor being placed in a U-shaped ditch as deep as to 2/3 of its height. A small inclination from inlet to outlet maintains a good slurry flow. The daily flow out of the reactor (if the reactor is fed daily, as is optimal) is the same as the volume fed into it.
In the reactor
As soon as the slurry enters the reactor it starts taking part in the ongoing anaerobic digestion, i.e. the breakdown of organic material by bacteria (powered by nitrogen, phosphorous and other nutrients) and enzymes in an oxygen-free environment. Biogas is the end product of this process.
The anaerobic digestion can be divided into four phases. During hydrolysis, hydrolytic bacteria break down carbohydrates, protein and fats into their component parts: simple sugars, amino acids and fatty acids, respectively. During acidogenesis, a process similar to how milk turns sour, acidogenic (fermentative) bacteria further break down the compounds. This results in organic acids and alcohols as well as byproducts such as ammonia and hydrogen sulfide. Next, the acids and alcohols are converted by acetogenic bacteria into acetic acid, carbon dioxide (CO2) and hydrogen (H2). This phase is called acetogenesis. The last phase is methanogenesis. Here, so-called methanogenic bacteria finally produce the biogas, which consists mainly of methane (CH4) but also of carbon dioxide and water vapor. The biogas assembles in the top of the reactor and exits through a gas valve. The effluent after the digestion process is called digestate and exits through the outlet.
The temperature in the reactor varies: the bacteria perform well in psychrophilic mode (about 20 °C), mesophilic mode (about 37 °C) or thermophilic mode (about 55 °C). The process works at higher or lower temperatures as well, but then with lower efficiency (half-burying the reactor, as well as the properties of the material itself, somewhat helps with keeping temperatures optimal).
Anaerobic digestion, then, is a delicate process: stable conditions are necessary and the supply of micro- and macro-nutrients needs to be well balanced. Furthermore, any reactor material used for anaerobic digestion is presented with many challenges: fluctuating pH values, the aggressive byproducts hydrogen sulfide and ammonia, the described temperature differences, and the sun’s UV rays. (Our reactor material has been extensively stress-tested and proven to withstand these challenges very well.)
Only 1/3 of the organic material in the slurry exits the reactor through the digestate outlet. The rest has been turned into biogas and exits via the gas outlet.
After the reactor
The biogas is piped directly into houses, other buildings or a gas tank. It can also be converted into electricity by a generator or upgraded into biofuel.
The digestate, also called “black water”, easily exits the reactor since the outlet is placed lower than the inlet. No pump is necessary. The digestate contains a high amount of nitrogen, phosphates, and other nutrients. Some of the digestate is fed back into the reactor, thus saving water. The rest can be used as a high-quality organic fertilizer (which is another reason for why the nutrient supplies need to be well balanced).
Except for the parts about the reactor material and the support, little in the above account is unique for our biogas systems. What makes our reactors great choices for most system sizes and types of organic waste is their long lifetimes, their reliable production, their scalability and the fact that the local expert support is included in the investment. These factors combine to give you the best available cost effectiveness.