Effect of medium recycle ratio on spirulina cultivation: Experiments and modelling

To improve water and nutrients management in microalgal cultivation, medium recycling should be implemented. However, a general accumulation of secondary inhibitory metabolites due to such operation should be properly handled, by optimising the proportions of medium to be recycled. In this paper, an experimental work was developed to describe Arthrospira maxima cultivation performances in continuous photobioreactors, assessing the effects of the operating residence time, and of different degrees of medium recycling. Working at a biomass residence time of 1.2 d and recycling 90 % of the cultivation medium resulted in about 444 g m-3 of biomass, equivalent to the value obtained without recycling, while higher recycle ratios caused growth inhibition. The results demonstrated that tuning the recycle ratio reduces exopolysaccharides accumulation and boosts biomass production, also improving the process water footprint. Additionally, a mathematical model was built to dynamically describe species behaviour under such process configuration, aiming at faithfully describing biomass growth and exopolysaccharides accumulation. With the latter standing as key indicators for the entire set of inhibitory compounds, a tailored data-driven mathematical term accounting for biomass growth inhibition was included, representing the major novelty of the model. Model fitting and validation was carried out on experimental data collected at different medium recycle degrees.