Anaerobic biodegradation tests

What is anaerobic biodegradation?

Sustainability and environmental pollution are growing concerns in society. In certain cases, they make it necessary to replace conventional plastic materials with biodegradable plastics to help alleviate the pollution of different ecosystems by applying better waste management techniques.

The biodegradation of plastic materials is determined by the medium in which biodegradation takes place, i.e. a material may be biodegradable in a medium such as compost, but may not be biodegradable in a medium such as soil. This is due to the different conditions that exist in each medium, such as temperature, the absence or presence of oxygen, and the density of microorganisms.

Anaerobic digesters are widely used in sectors such as the livestock, agriculture and wastewater treatment industries. Plastic materials may be introduced in these digesters, which makes it necessary to know how these materials will behave under representative anaerobic digestion conditions.

Anaerobic digestion

Anaerobic digestion involves the degradation of organic matter through the combined action of different microorganisms in the absence of oxygen.

Anaerobic digesters can operate under mesophilic conditions (37°C) or thermophilic conditions (52°C). Under thermophilic conditions, digesters are more efficient at degrading organic matter and producing biogas.

The degradation products generated include new biomass and biogas, a mixture of methane (CH₄), carbon dioxide (CO₂) and traces of hydrogen sulphide (H₂S), which has a high potential for producing electricity and heat.

Stages of anaerobic digestion

The process itself is divided into four stages:

1. Hydrolysis, in which microorganisms secrete extracellular enzymes into the environment that help break down long-chain molecules into molecules that can be assimilated by the microorganisms.
2. Acidogenesis, in which acidogenic microorganisms transform these shorter chain molecules into volatile fatty acids such as propionic, acetic and valeric acid.
3. Acetogenesis, where volatile fatty acids are converted to acetic acid.
4. Methanogenesis, where methanogenic microorganisms use acetic acid, H₂ and CO₂ to obtain CH₄ and CO₂ as end products.

Standards for assessing anaerobic biodegradation

Different standards can be used to evaluate the behaviour of plastic materials under anaerobic biodegradation conditions, namely, ASTM D5511 and its European counterpart UNE-EN ISO 15985, where biodegradation of plastics under optimal anaerobic conditions is tested.

The test is designed to be carried out in at least 60 days in order to assure a complete biodegradation of the polymer in anaerobic digestors. Our essays can be extended until 240 days depending of the material tested.

Biodegradation is monitored by measuring biogas production during the entire test period. When the amount of biogas produced is known, the organic carbon released in the form of biogas can be calculated, along with the percentage of biodegradation over the course of the test.