Anaerobic Digestion: optimizing the anaerobic digestion process

Bacteria marked by fluorescent probes in the digesters. © Irstea/O. Chapleur

For the past 30 years, Irstea has worked to optimize anaerobic digestion procedures. This research is now part of an integrated approach: combining environmental, economic and regional issues to support the development of the field. What are the latest innovations?

Irstea researchers have been working for 30 years to improve the production performance of anaerobic digestion procedures in order to maximize methane (biogas) yield in minimum time.

Good to know 

The ability of organic waste or effluent to produce biogas depends on its biodegradability, which in turn partly depends on its composition and structure.

Bacteria: a closely scrutinized diet

Researchers are focusing on microbial drivers, specifically the bacterial activity responsible for anaerobic digestion. In order to increase the production of biogas, scientists are combining organic resources to provide bacteria with a balanced diet. This additional waste is known as cosubstrate, and is chosen according to its ability to produce methane, which involves an optimal mix of water, fat, sugar, protein and vitamins. These are all necessary for bacterial activity. Some waste, such as fats, can produce a high level of methane and are easily biodegradable, while others, such as crop residues, do not break down easily.

Good to know

The growth of bacteria capable of transforming matter into energy is determined by a variety of factors, including temperature and the amount of dry matter.

Subsequently, disruptive elements are incorporated, such as temperature and the addition of complementary substrates, etc. The aim is to observe the impact of these on the microorganisms and on breakdown performance since improved breakdown leads to greater production of biogas.

This research is being used to better understand the way in which microbial activity functions as part of anaerobic digestion and thus to assess the possibility of optimizing waste breakdown stages.

Additional research

Through its research into functional microbiology and its applications to anaerobic digestion, particularly with regard to optimum temperatures, Irstea is contributing to improving the profitability of production units by promoting bacteria that work at colder temperatures (15-20°C compared to 35-40°C currently).

Furthermore, although dry anaerobic digestion processes are used in France, they are used empirically. The impact of operational parameters and the transfer of materials involved are currently unknown, which partly explains the small number of currently installed units.

Irstea is also evaluating the impact of biological pre-treatment for farming residue (wheat straw) using fungi or bacteria, which facilitate the production of methane, in order to consider their integration into an anaerobic digestion system.

Digestate: from waste to product?

Once the bacteria have digested the organic matter, a solid or liquid residue is obtained. This digestate, or leachate, containing nutrients and organic matter can be reused for agricultural applications as a fertilizer or soil enricher. This opportunity has untapped potential!

Irstea is also looking at the quality and sustainability of digestate management units. Currently, digestate is considered to be waste and as such, is subject to extremely rigid usage regulations, particularly in relation to spreading. In order for digestate to be approved as a fertilizer and prove its agricultural value, Irstea is working to characterize and transform it (through composting, for example).

What have we learned so far? The composition of digestate remains stable over time in a single unit, unless there is a significant change to the waste being processed. Analyses have provided data for farmers and public authorities showing their non-toxicity and value for farming. The aim is to update the regulatory framework so that the status of digestates changes from “waste” to “product”.

In addition to the technical and environmental issues, the institute is also studying the dynamics of pathogens during storage and spreading of these products.

Currently, there are few methods capable of recovering the inorganic nutrients such as nitrogen and phosphorous contained within the digestate without additional energy use. The waste treatment sector already grows specific algae (spirulina) to capture these nutrients. Initial studies carried out on the application of this technique on agricultural digestate revealed a 95% recovery rate. Research is continuing to optimize this technique and allow the recovery of nutrients in the form of usable and potentially exportable biofertilizers.