Qualitative benefits

Today in France, there is a strong impetus to reduce the storage of household and similar waste, and improve the way it is recovered in order to create a circular economy based on respect for the environment. The increasing amount of waste produced per capita, the pollution caused by storing it and the scarcity of raw materials are all major issues that are driving us to change our behaviour and the way we consume.

Recovering energy from household waste produced biologically is a promising field. The production of biohythane (composed of 5-20% of hydrogen and methane) from domestic waste via a two-stage process is a field of research that includes many avenues still to be developed in the long term with a view to using this process on an industrial scale. As far as the production of methane is concerned, the technology is a mature one, with many existing industrial processes. However more detailed examination of dark fermentation (to produce hydrogen) is required to better understand this process and find the optimal operating conditions for improving the hydrogen yield.

The move up from laboratory scale to a pilot and finally to industrial scale is based on a better understanding of the key mechanisms at work in this process in order to enable subsequent optimization. The key parameters related to Trifyl’s industrial context were mainly centred around resources available on the Trifyl site to carry out the dark fermentation process, namely the organic fraction of household waste collected by Trifyl and the leachate (wastewater) which both contributes to the microbial community producing hydrogen and provides an essential supply of water for producing a substantial amount of hydrogen. However, the use of these resources specific to Trifyl has led to several issues affecting this study. Hydrogen production can be limited by elements contained in the leachate which, by accumulating within the medium, can negatively impact the production of hydrogen. In this context, it was therefore necessary to first focus on the main limits of dark fermentation. When using organic waste made up partly of food waste (44%), a molecule called ammonia (NH3), resulting from the degradation of proteins, has regularly been reported as highly problematic. During this thesis it was therefore necessary to study the impact on our fermentation process. In addition, the leachate has a high concentration of ions which can also disturb the dark fermentation process. It was therefore essential to examine the consequences of this factor on the production of hydrogen.

In opposition to the parameters negatively impacting the production of hydrogen, an improvement in hydrogen yield was obtained by studying various parameters, either easily adjustable ones such as the amount of water in the medium or the temperature of the medium, or one that are difficult to modify at industrial level (pH, ammonia concentration and substrate characteristic). Overall knowledge of the values that help to intensify the productivity of hydrogen make the industrial-scale process more reliable. Overall, the hydrogen production optimization study has made it possible to improve the stability and robustness of the fermentation reactor in order to promote the economic credibility of such a process.

Finally, to be able to have a biohythane production process on an industrial scale, issues related to changes in scale and more particularly to the design of the reactor have also been carefully studied. The is because the configuration of the reactor can modify certain physico-chemical data such as hydrogen partial pressure and gas/liquid transfer which will potentially lead to a decrease in the hydrogen yield.

The results of these various studies are now used as a basis for the design of a two-stage process with a view to ultimately obtaining industrial production of biohythane.