| Summary: | Biomass of microalgae and the components contained in their cells can be used for the production of heat, electricity, and biofuels. The aim of the presented study was to determine the optimal conditions that will be the most favorable for the production of large amounts of microalgae biomass intended for energy purposes. The study analyzed the effect of the type of lighting, the time of lighting culture, and the pH of the culture medium on the growth of <i>Chlorella vulgaris</i> biomass. The experiment was carried out in vertical tube photobioreactors in three photoperiods: 12/12, 18/6, and 24/0 h (light/dark). Two types of lighting were used in the work: high-pressure sodium light and light-emitting diode. The increase in biomass was determined by the gravimetric method, by the spectrophotometric method on the basis of chlorophyll a contained in the microalgae cells. The number of microalgae cells was also determined with the use of a hemocytometer. The optimal conditions for the production of biomass were recorded at a neutral pH, illuminating the cultures for 18 h a day. The obtained results were 546 ± 7.88 mg·L<sup>−1</sup> dry weight under sodium lighting and 543 ± 1.92 mg·L<sup>−1</sup> dry weight under light-emitting diode, with maximum biomass productivity of 27.08 ± 7.80 and 25.00 ± 5.1 mg·L<sup>−1</sup>∙d<sup>−1</sup>, respectively. The maximum content of chlorophyll a in cells was determined in the 12/12 h cycle and pH 6 (136 ± 14.13 mg∙m<sup>−3</sup>) under light-emitting diode and 18/6 h, pH 7 (135 ± 6.17 mg∙m<sup>−3</sup>) under sodium light, with maximum productivity of 26.34 ± 2.01 mg·m<sup>−3</sup>∙d<sup>−1</sup> (light-emitting diode) and 24.21 ± 8.89 mg·m<sup>−3</sup>∙d<sup>−1</sup> (sodium light). The largest number of microalgae cells (2.1 × 10<sup>6</sup>) was obtained at pH 7 and photoperiod of 18/6 h under sodium light, and 12/12 h under light-emitting diode. Based on the results, it can be concluded that the determination of the optimal parameters for the growth and development of microalgae determines the production of their biomass, and such research should be carried out before starting the large-scale production process. In quantifying the biomass during cultivation, it is advantageous to use direct measurement methods.
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