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Context

In the current context of energy and environmental crises, the transition towards greater use of biomass in a bioeconomy approach is a major challenge. Because of its biochemical composition and energy potential, biowaste is a highly attractive source for biological recovery. In particular, current treatment processes involve anaerobic digestion and aerobic composting, which can be used for energy (methane) and/or agricultural purposes (digestate or compost). However, the anaerobic/aerobic recovery cascade for this waste needs to be optimised. While bio-waste has significant methanogenic potential, its anaerobic digestion as a mono-substrate can present instabilities linked to the accumulation of volatile fatty acids or the mineralisation of organic nitrogen into ammoniacal nitrogen (NH3). These problems are particularly acute in the case of concentrated digestion processes (i.e. without diluting the substrate), even though these processes have the advantage of producing digestate that is easier to use afterwards, particularly as a soil improver or organic fertiliser. However, the latter, which can be composted, can also have negative impacts, for example through the emission of compounds such as ammoniacal nitrogen.

To provide solutions for reducing nitrogen losses in composting on the one hand and instabilities in anaerobic digestion on the other, the addition of biochar (produced by pyrolysis/gasification of lignocellulosic biomass) as an additive in each of these two biological processes has been studied increasingly over the last 10 years. However, the key physico-chemical and biological mechanisms that lead to these improvements are still at the level of scientific hypothesis. As a result, it is vital to improve our knowledge of the links between the physico-chemical characteristics of biochars and anaerobic/aerobic digestion processes (methanisation/composting), in order to propose the best operating conditions for the processes. Finally, the complementary nature of anaerobic digestion and composting, and the determination of the optimum biochar input stage(s) in this context, have not yet been addressed in the scientific literature. In this context, the proposed thesis aims to study the mechanisms of action of biochars and to propose selection criteria for their properties and operational parameters for their use in order to optimise anaerobic digestion, composting and the cascade of these two processes. It will draw on the combined skills of the INRAE OPAALE and LBE research units, as well as those of ENSCR.

PhD programme

The PhD will be structured around 4 areas of work:

1. Identification of interesting properties of biochars for the targeted applications (bibliographical study), and selection of a small number of biochars for the study.
2. Identification and quantification of the effects of biochar properties on methanisation in a simplified environment (substrate, small scale, short time) with a view to screening and validating the associated mechanisms. 
3. Identify and quantify the effects of their properties on composting and hypothesise mechanisms (small scale; fresh residues and digestate).
4. Evaluation of the influence of the change in scale of the process (reproduction of effects and mechanisms) and analysis of the methanisation-composting cascade as a function of the biochar input stage, in order to propose the best treatment combinations.

This work will be based on the experimental and analytical equipment available in the units:

• Analysis equipment for the physical and chemical characterisation of biochars (BET, porosimetry, zetametry, etc.) and the speciation of nitrogenous species (anionic and cationic chromatography) at ENSCR.
• Experimental equipment for anaerobic digestion (batch and continuous reactors from 100 mL to 10 L) and composting (10 L respirometers and 300 L pilots), as well as the associated protocols present and validated at LBE and OPAALE. Analytical equipment for physico-chemical and microbiological monitoring of bioreactors

Location

Rennes: UR INRAE OPAALE and ENSCR

Narbonne: UR INRAE LBE

Profile of the applicant

Master's degree or engineering degree with research experience. Sound knowledge of bioprocess engineering. Knowledge of analytical chemistry and/or materials would also be appreciated. The candidate must be autonomous, rigorous, able to work in a team, have good writing skills and a good command of English.

Application requirements:  CV, covering letter, transcript of records and letter(s) of support.

Contacts

Anne Trémier (anne.tremier@inrae.fr), Annabelle Couvert (annabelle.couvert@ensc-rennes.fr), Renaud Escudié (renaud.escudie@inrae.fr)