Integration of bioenergetics in the ADM1 and its impact on model predictions

Thermally enhanced solid-liquid separation process in food waste biorefinery: modelling the anaerobic digestion of solid residues

The biochemical valorization potential of food waste (FW) could be exploited by extracting decreasing added-value bio-based products and converting the final residues into energy. In this context, multi-purpose and versatile schemes integrating thermal and biochemical conversion processes will play a key role. An upstream thermal pretreatment + solid-liquid separation unit was here proposed to optimize the conversion of the liquid fraction of FW into valuable chemicals through semi-continuous fermentation process, and the conversion of the residual solid fraction into biomethane through anaerobic digestion. The solid residues obtained after thermal pretreatment presented a higher soluble COD fraction, which resulted in higher methane production with respect to the raw residues (0.33 vs. 0.29 Nm3CH4 kg-1VSfed) and higher risk of acidification and failure of methanogenesis when operating at lower HRT (20d). On the contrary, at HRT = 40 d, the pretreatment did not affect the methane conversion rates and both tests evidenced similar methane productions of 0.33 Nm3CH4 kg-1VSfed. In the reactor fed with pretreated residue, the association of hydrogenotrophic methanogens with syntrophic bacteria prevented the acidification of the system. Modelling proved the eligibility of the FW solid residues as substrates for anaerobic digestion, given their small inert fractions that ranged between 0% and 30% of the total COD content.

Conductive materials as fantastic toolkits to stimulate direct interspecies electron transfer in anaerobic digestion: new insights into methanogenesis contribution, characterization technology, and downstream treatment

Direct interspecies electron transfer (DIET) stimulated by conductive materials (CMs) enables intercellular metabolic coupling that can address the unfavorable thermodynamical dilemma inherent in anaerobic digestion (AD). Although the DIET mechanism and stimulation have been extensively summarized, the methanogenesis contribution, characterization techniques, and downstream processes of CMs-led DIET in AD are surprisingly under-reviewed. Therefore, this review aimed to address these gaps. First, the contribution of CMs-led DIET to methanogenesis was re-evaluated by comparing the effect of various factors, including volatile fatty acids, free ammonia, and functional enzymes. It was revealed that AD systems are usually intricate and cannot allow the methanogenesis stimulation to be singularly attributed to the establishment of DIET. Additionally, considerable attention has been attached to the characterization of DIET occurrence, involving species identification, gene expression, electrical properties, cellular features, and syntrophic metabolism, suggesting the significance of accurate characterization methods for identifying the syntrophic metabolism interactions. Moreover, the type of CMs has a significant impact on AD downstream processes involving biogas purity, sludge dewaterability, and biosolids management. Finally, the central bottleneck consists in building a mathematical model of DIET to explain the mechanism of DIET in a deeper level from kinetics and thermodynamics.

Modeling a propionate-oxidizing syntrophic coculture using thermodynamic principles

A coculture of Syntrophobacter fumaroxidans and Methanospirillum hungatei was modeled using four biokinetic models, which only differed by the functions used to describe the growth yields (dynamic or constant) and the hydrogen inhibition function (noncompetitive or based on thermodynamics). First, a batch experiment was used to train the model and analyze the fitted parameters. Two fitting procedures were followed by minimizing the error on different indicators. Second, a chemostat experiment was used as a test data set to assess the predictive power of the models. Overall, the four models were able to accurately fit the train data set following both fitting procedures. However, some parameters fitted with the ADM1-like model differed significantly from values reported in the literature and were dependent on the fitting procedure. When applied to the test data set it systematically resulted in positive Gibbs free energy changes values for propionate oxidation, in contradiction with the second law of thermodynamics. On the opposite, the parameters fitted with model including both a thermodynamic-based inhibition function and a dynamic computation of growth yields were more consistent with values reported in the literature and repeatable whatever the fitting procedure. The results highlight the potential of implementing thermodynamic-based functions in biokinetic models.

Modeling of interspecies electron transfer in anaerobic microbial communities

Interspecies electron transfer (IET) is a key phenomenon in anaerobic ecosystems, which is traditionally modeled as hydrogen transfer. Recently discovered alternative mediated IET (MIET) or direct IET (DIET) offer exciting alternative mechanisms of microbial partnerships that could lead to new strategies for the improvement of biotechnologies. Here, we analyze mathematical modeling of DIET and MIET in anaerobic ecosystems. Bioenergetics approaches already enable the evaluation of different energy sharing scenarios between microorganisms and give interesting clues on redox mediators and on possible ways of driving microbial communities relying on IET. The modeling of DIET kinetics however is currently only in its infancy. Recent concepts introduced for the modeling of electroactive biofilms should be further exploited. Recent modeling examples confirms the potential of DIET to increase the IET rates compared to H₂-MIET, but also point out the need for additional characterizations of biological components supporting IET to improve predictions.