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CONTEXT

Anaerobic digestion (AD) is a widely spread technology that consists in the degradation of organic matter by a highly coordinated chain of reactions,  eventually given a biogas principally form by CH₄ and CO₂ (¹). The produced CO₂ can be converted in CH₄ thanks to the addition of H₂. Methanation is a process based on the Sabatier reaction ( ), which takes place at high temperature and pressure. On the other hand, biological methanation (also known as biomethanation) allows this reaction to occur thanks to the action of methanogens at milder pressures and temperature. Several sustainable process can provide the necessary H₂ for the biomethanation reaction, such as water electrolysis (associated to the Power-to-Gas concept) and dark fermentation, among others (^1,2). There are two principal types of biomethanation: Ex situ biomethanation, which consist in the injection of H₂ and biogas into a reactor separated from the main digester and In situ biomethanation where H₂ is directly injected in the main digester, increasing the CH₄ content in the biogas through H₂ and CO₂ conversion by methanogens. Even though in situ biomethanation is at a lower Technological Readiness Level (TRL) than the ex situ biomethanation, this process has several advantages, such as a lower infrastructure and operating cost (²). Therefore, one of the principal objectives of the Metha-HYn (³) project is to develop and optimize the in situ biomethanation process.

However, in situ biomethanation process exhibits some limitations. The low dissolution rate of H₂ impedes its consumption by the microorganisms leading to low CH₄ yields. While an increase of the H₂ partial pressure in the reactor can provoke a AD process imbalance and an eventual process failure by the accumulation of Volatile Fatty Acids (VFA) (^1,2). Besides, in situ biomethanation process relies on the AD process, which can be inhibited by ammonia and VFA, among others factors (¹). In order to optimize this process, it is necessary to overcome these limitations.

Biochar, which is the porous carbonaceous solid residue produced during pyrolysis, and other additives have been widely used in order to improve AD (⁴). It has been reported that biochar in particular has a positive effect on AD performances. It can increase the buffer capacity of the system, mitigate some inhibitors, support and immobilize biomass and promote syntrophic interactions between microorganisms leading to better methane yields and digestate quality (⁴). However, the effects of biochar on in situ biomethanation have been poorly studied.

INTERNSHIP DESCRIPTION

The objective of the internship, of a 6-month duration, will consist in (i) evaluating the effect of different biochars supplementation on an in situ biomethanation process in batch system and (ii) determining the optimal supplementation strategy of the selected biochar(s) at higher scale (lab-scale semi-continuous bioreactors).

During experiments, the biogas production will be followed up and its composition will be determined by gas chromatography. HPLC and GC-FID analyses will allow to identify and quantify microbial metabolites. These date will be used to evaluate the in situ biomethanation performances with and without biochar supplementation (COD mass balance, productivity and yields calculations) to better understand the effect of biochar on the in situ biomethanation performances. Afterwards, microbial community, sampled at different times, will be analysed by sequencing and qPCR (metagenomics analysis). The obtained results will help to understand the influence of the studied biochar (based on biochar type, granulometry, its concentration) on the in situ biomethanation process.

LOCATION: Laboratoire de Biotechnologie de l’Environnement, INRAe, Narbonne, France.

BEGINNING OF THE INTERNSHIP: February/March 2022

CANDIDATE: This internship is intended for students at the Engineering or Master's level who have been trained in Microbiology, or Process Engineering. In addition, experimental rigour and fluency in English are required. A spirit of initiative and an aptitude for teamwork will also be appreciated.

HOW TO APPLY: CV and motivation letter until 05/12/2022

CONTACT: Renaud Escudié renaud.escudie_at_inrae.fr, Eric Trably eric.trably_at_inrae.fr, Charlotte Richard charlotte.richard_at_engie.com, Quentin Aemig quentin.aemig_at_engie.com, Margot Mahieux margot.mahieux_at_external.engie.com, Lucia Braga Nan lucia.braga-nan_at_inrae.fr.

REFERENCES

1.            Angelidaki, I. et al. Biogas upgrading and utilization: Current status and perspectives. Biotechnology Advances 36, 452–466 (2018).

2.            Rafrafi, Y., Laguillaumie, L. & Dumas, C. Biological Methanation of H2 and CO2 with Mixed Cultures: Current Advances, Hurdles and Challenges. Waste and Biomass Valorization 12, 5259–5282 (2021).

3.            Metha-HYn - Méthanation biologique In situ avec production d’hydrogène biologique. La librairie ADEME https://librairie.ademe.fr/dechets-economie-circulaire/5583-metha-hyn-methanation-biologique-in-situ-avec-production-d-hydrogene-biologique.html.

4.            Chiappero, M. et al. Review of biochar role as additive in anaerobic digestion processes. Renewable and Sustainable Energy Reviews 131, 110037 (2020).