Monitoring lipids profile, CO₂ fixation, and water recyclability for the economic viability of microalgae Chlorella vulgaris cultivation at different initial nitrogen

The economic viability of microalgae as a bioenergy source depends on many factors. High CO₂ fixing rate, improved lipids yield, and minimum water footprint are few key parameters. This study investigates the effect of four initial nitrogen concentrations (1-, 2-, 6- and 10-mM as nitrate) on lipids yield, their classification and composition, CO₂ fixation rate, and water quality for further reuse after first cultivation. The initial 6 mM nitrate was found optimum for the growth and overall lipid productivity of Chlorella vulgaris. The maximum quantum efficiency (as Fv/Fm ratio) for algae decreases along with the cell growth profile and depletion of the initial nitrate concentration. CO₂ fixation rate increased initially and peaked during exponential growth and then declined for the rest of the cultivation period. A higher CO₂ fixation rate was recorded at 6 mM, and an overall fixation rate of CO₂ was high at 6 mM. A higher total organic carbon (TOC) is produced in recycled water at a low nitrogen concentration of 1 and 2 mM. TOC changes during the cultivation period and with each reuse of water. Water was recycled twice successfully, while growth was inhibited during the 3rd cycle. Based on all these investigations, 6 mM of initial nitrogen was found optimal at given growth conditions.