Effect of Initial Biomass Concentration on the Growth Kinetics of Chlorella vulgaris in Cylindrical Photobioreactors

Microalgae of the genus Chlorella are widely used in biotechnology for biofuel production, wastewater treatment, and biomass generation. This study examined the effect of initial biomass concentration on the growth kinetics of Chlorella vulgaris cultivated in cylindrical photobioreactors. Experiments were performed in identical 4 L reactors under constant illumination (~300 µmol·m−2·s−1), aeration (0.5 vvm), and atmospheric CO2 (~0.04%). Four initial biomass concentrations (0.012, 0.053, 0.110, and 0.530 g·L−1) were tested in duplicate. Growth curves were fitted using the logistic, Gompertz, and Baranyi–Roberts models. The best-performing Gompertz model was further extended by relating its kinetic parameters to the initial biomass concentration, allowing biomass productivity to be evaluated as a continuous function of cultivation time and inoculum level. Initial biomass strongly affected growth dynamics. Increasing the initial concentration from 0.012 to 0.110 g·L−1 reduced the lag phase from 53.9 ± 5.1 h to 4.0 ± 6.9 h, while no distinct lag phase was observed at 0.530 g·L−1. Meanwhile, the maximum specific growth rate decreased from 0.0507 to 0.0251 h−1. The model-based analysis indicated that the optimal initial biomass concentration is time-dependent: higher values are preferable for short cultivations, whereas lower values become advantageous during prolonged cultivation. Although the predicted optimum partly lies between experimentally tested values and should be interpreted as exploratory rather than predictive, the proposed approach demonstrates the potential of model-assisted optimization for future process design, pending experimental validation.