To put it simply, it is often said that all waste that is organic matter can be treated by methanization. This statement should be taken with a pinch of salt to ensure the proper functioning of a methanizer.

It is true that organic matter, when not consumed, is intended to degrade by producing biogas. But depending on the product concerned, this degradation occurs more or less easily/quickly and sometimes with an external impact to be taken into account.

The aim of this article is to list the organic materials that deserve additional consideration before deciding to integrate them into a methanizer.

To begin with, all organic products that are packaged or that may contain significant traces of plastics or metals should not be introduced because they could contaminate the digestate or even damage the equipment. The rule also applies to glass or rubber that do not decompose and can cause mechanical damage.

Nor should we take the risk of integrating inputs that may contain chemicals since they could interfere with the fermentation process and pollute the biogas and digestate. In particular, we must be vigilant about rinsing water and ensure that the organic matter has been diluted in water without any addition of washing product.

Heavy oils and greases, such as engine oil, should not be added to a methane digester as they can cause phase separation problems and affect the quality of the biogas.

Manure from carnivorous animals should be considered hazardous, since the excrement of carnivorous animals can contain parasites and pathogenic bacteria that do not decompose well. It can be dangerous for the quality of the digestate and therefore for the agricultural land on which it would be spread.

Large green waste such as large pieces of wood can clog the feeding systems and disrupt the process. In addition, the entire wood part of plants contains lignin which is not well digested by bacteria and which would therefore have to be crushed or reduced to dust in order to be integrated into methanization.

Paper and cardboard are biodegradable but avoid putting them in large quantities in a methanizer, which could cause floating mattresses to form and disrupt the mixing of materials.

Finally, some products should be used in moderation in a methanizer due to their difficult decomposition and their texture. This is the case, for example, with citrus fruits whose skin is very thick and which are therefore better crushed or cut into small pieces before incorporation. They contain organic acids (such as citric acid) which can disrupt the pH balance.
Shellfish, and more specifically the shell part (such as oyster shells), require vigilance to avoid creating problems of accumulation or sedimentation in the digester. It is preferable to re-use shells in another way whenever possible, since they are now used for many other applications.

The choice of inputs is also influenced by the type of methanizer and the equipment available. It is therefore the responsibility of each site to judge the impact of each product on the balance of its fermentation process and on the maintenance of its machines.

For more advice on the subject, do not hesitate to contact us! contact@methappro.fr