Quantitative analysis of CO₂ impact on succinic acid fermentation process: an in-tegrated experimental and model approach

Succinic acid is one of the most promising chemicals for market growth and applications. Particularly, the fermentation process for its production caught the attention due its green impact and the possibility of fixing CO₂. While CO₂ has a well-known role during the fermentation, its behaviour and model approaches are not extensively explored.
Therefore, the thesis investigates the profile and the influence of CO₂ in multiple fermen-tation strategies using as organic carbon source a sugar rich industrial waste. The dis-solved CO₂ profile is monitored initially in batch conditions applying three different par-tial pressures to the headspace. A final titer of 25.5 ± 2.4 g/L is achieved with the fixation rate during the exponential phase and 82.7 ± 7.4 % biomethane purity.
Once the behaviour in the liquid broth was described, a mechanistic model is developed in continuous condition. A new specific production rate is introduced in the mathematical formulation to better estimate the distribution of carbon with several concentrations of CO₂ and control product over byproducts formation. The model is successfully validated at lab-scale, both using pure glucose and sugars-rich industrial waste. The highest titer and productivity reach respectively 15.0 ± 0.9 g/L and 1.18 g/Lh. Of particular interest, the fixation rate increases from batch operation achieving a maximum of 0.258 ± 0.041 g/Lh. Moreover, a shift between the two metabolic pathways is found below 0.18 g/L of dissolved CO₂ concentration. Lately, the specific production rate term is implemented in a mechanistic model to characterize the fermentation outcome in a packed bed reactor. The model, validated with two bioreactor datasets at different steady states, is used to predict the variables profile based on the reactor length, particularly testing the product and byproducts formation at high and low concentrations of CO₂. Last the model is used to predict the fixation rate of the bioreactors showing a possible uptake ranging between 5 and 7 g/Lh.
In conclusion, the thesis does not only elucidate the complex behaviour of CO₂ in the succinic acid fermentation process but also presents a novel mechanistic model concept that significantly enhances the understanding of the process dynamics. Thus, by showing the importance of the second substrate and several methods to predict its profile, this research pioneers sustainable pathways, introducing the carbon fixation potential of the system and providing valuable insights for future green bioprocesses.