Influence of elevated CO₂ concentrations on growth, nutrient removal, and CO₂ biofixation using Chlorella kessleri cultivation

In this study, the ability of microalgae Chlorella kessleri to remove nutrients, biofix CO₂, and generate valuable biomass was investigated. For this purpose, CO₂ at different concentrations was added to the synthetic tertiary municipal wastewater for cultivating C. kessleri in batch photobioreactors. The concentration of biomass increases gradually during the cultivation period for the tested CO₂ concentrations of 2%, 6%, and 10%, except 0% CO₂. The highest biomass concentration found was 607 mg/L, and the highest biomass productivity is 46 mg/L/day, at a CO₂ concentration of 2%. Monod growth kinetic model based on a single substrate factor was used, and the experimental findings agree well with the predictions by the model for all feed concentrations except 0% CO₂. Biofixation of CO₂ depends on the optimal CO₂ concentration supplied to the culture. The maximum biofixation rate of CO₂ achieved at 2% CO₂ is 83.88 mg/L/day. The maximum removal of total nitrogen of 99% was achieved for both 2% and 10% CO₂, while the total nitrogen removal is negligible by microalgae cultured with air without CO₂ enrichment, which confirms the beneficial effect of CO₂ on the removal of nutrients from wastewater media. These findings indicate the possibility of nutrient removal from tertiary municipal wastewater using microalgae C. kessleri along with CO₂ biofixation.