Microalgal biomass and its cellular components are used as substrates for the production of fuels. A valuable group among the components of microalgal biomass is lipids, which act as a precursor for the production of biodiesel in the transesterification process. Some methods, including the creation of stressful conditions, are applied to increase the accumulation of lipids. This study aimed to determine the effect of limited nutrient access on the growth and development of the microalga Chlorella vulgaris and the amount of lipids stored in its cells. Aquaculture wastewater (AWW) was used in the study as a source of nutrients at doses of 20%, 40%, 60%, 80% and 100%. The amount of microalgal biomass, optical density, lipid content after extraction of the biomass in Soxhlet apparatus and chlorophyll a content were determined. It was observed that the microalgae efficiently used the nutrients contained in the AWW. The largest amount of biomass was obtained in AWW₈₀ (727 ± 19.64 mg·L⁻¹). The OD₆₈₀ (0.492 ± 0.00) determined under the same conditions was almost five times higher in AWW than in the synthetic medium. Under nutrient-stress conditions, the content of lipids in biomass ranged from 5.75% (AWW₈₀) to 11.81% (AWW₂₀). The highest content of chlorophyll a in microalgal cells was obtained in AWW₂₀ (206 ± 11.33 mg∙m⁻³).

Microalgae are considered to be potentially attractive feedstocks for biodiesel production, mainly due to their fast growth rate and high oil content accumulated in their cells. In this study, the suitability for biofuel production was tested for Chlorella vulgaris, Chlorella fusca, Oocystis submarina, and Monoraphidium strain. The effect of nutrient limitation on microalgae biomass growth, lipid accumulation, ash content, fatty acid profile, and selected physico-chemical parameters of algal biodiesel were analysed. The study was carried out in vertical tubular photobioreactors of 100 L capacity. The highest biomass content at 100% medium dose was found for Monoraphidium 525 ± 29 mg·L⁻¹. A 50% reduction of nutrients in the culture medium decreased the biomass content by 23% for O. submarina, 19% for Monoraphidium, 13% for C. vulgaris and 9% for C. fusca strain. Nutrient limitation increased lipid production and reduced ash content in microalgal cells. The highest values were observed for Oocystis submarina, with a 90% increase in lipids and a 45% decrease in ash content in the biomass under stress conditions. The fatty acid profile of particular microalgae strains was dominated by palmitic, oleic, linoleic, and linoleic acids. Nutrient stress increased the amount of saturated and unsaturated fatty acids affecting the quality of biodiesel, but this was determined by the type of strain.

Microalgae are one of the most promising sources of renewable substrates used for energy purposes. Biomass and components accumulated in their cells can be used to produce a wide range of biofuels, but the profitability of their production is still not at a sufficient level. Significant costs are generated, i.a., during the cultivation of microalgae, and are connected with providing suitable culture conditions. This study aims to evaluate the possibility of using sodium bicarbonate as an inexpensive alternative CO₂ source in the culture of Chlorella vulgaris, promoting not only the increase of microalgae biomass production but also lipid accumulation. The study was carried out at technical scale using 100 L photobioreactors. Gravimetric and spectrophotometric methods were used to evaluate biomass growth. Lipid content was determined using a mixture of chloroform and methanol according to the Blight and Dyer method, while the carbon content and CO₂ fixation rate were measured according to the Walkley and Black method. In batch culture, even a small addition of bicarbonate resulted in a significant (p ≤ 0.05) increase in the amount of biomass, productivity and optical density compared to non-bicarbonate cultures. At 2.0 g∙L^–1, biomass content was 572 ± 4 mg·L⁻¹, the maximum productivity was 7.0 ± 1.0 mg·L^–1·d^–1, and the optical density was 0.181 ± 0.00. There was also an increase in the lipid content (26 ± 4%) and the carbon content in the biomass (1322 ± 0.062 g∙dw^–1), as well as a higher rate of carbon dioxide fixation (0.925 ± 0.073 g·L^–1·d^–1). The cultivation of microalgae in enlarged scale photobioreactors provides a significant technological challenge. The obtained results can be useful to evaluate the efficiency of biomass and valuable cellular components production in closed systems realized at industrial scale.

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 Chlorella vulgaris 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⁻¹ dry weight under sodium lighting and 543 ± 1.92 mg·L⁻¹ dry weight under light-emitting diode, with maximum biomass productivity of 27.08 ± 7.80 and 25.00 ± 5.1 mg·L⁻¹∙d⁻¹, 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⁻³) under light-emitting diode and 18/6 h, pH 7 (135 ± 6.17 mg∙m⁻³) under sodium light, with maximum productivity of 26.34 ± 2.01 mg·m⁻³∙d⁻¹ (light-emitting diode) and 24.21 ± 8.89 mg·m⁻³∙d⁻¹ (sodium light). The largest number of microalgae cells (2.1 × 10⁶) 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.

The production of microalgal biomass on a commercial scale remains a significant challenge. Despite the positive results obtained in the laboratory, there are difficulties in obtaining similar results in industrial photobioreactors. Changing the cultivation conditions can affect not only the growth of microalgae but also their metabolism. This is of particular importance for the use of biomass for bioenergy production, including biofuel production. The aim of this study was to determine the biomass production efficiency of selected microalgal strains, depending on the capacity of the photobioreactor. The lipid and ash content of the biomass were also taken into account. It was found that as the scale of production increased, the amount of biomass decreased, irrespective of the type of strain. The change in scale also affected the lipid content of the biomass. The highest values were found in 2.5 L photobioreactors (ranging from 26.3 ± 2.2% for Monoraphidium to 13.9 ± 0.3% for Chlorella vulgaris). The least favourable conditions were found with industrial photobioreactors, where the lipid content of the microalgal biomass ranged from 7.1 ± 0.6% for Oocycstis submarina to 10.2 ± 1.2% for Chlorella fusca. The increase in photobioreactor capacity had a negative effect on the ash content.

Biogas production by fermentation is a relatively low-cost and simple method for the transformation of a substrate into an energy carrier with a wide range of possible applications. The aim of this study was to determine the potential of Arthrospira platensis biomass as a source of bioenergy produced during anaerobic digestion (AD). The studies were carried out on a fractional-technical scale. Biogas yield and composition were analyzed as a function of the amount of biomass subjected to anaerobic digestion, the substrate dosing frequency in the digester and the use of biomass pre-hydrolysis in the mixing compartment. The energy efficiency of the process was also compared for each sample. In addition, a biomass conversion power index was developed and determined. It was found that A. platensis biomass had significant energy potential, and the amount of biogas obtained and its calorific value changed depending on the applied treatments. The maximum cumulative biogas production was 505 L kg⁻¹ volatile solids (VS), while the maximum average methane (CH₄) content was 67.32%. A two-fold increase in the organic loading rate from 1 g VS·L⁻¹ volatile solids (VS) to 2 g VS·L⁻¹ had a positive effect on methane concentration. The highest energy efficiency of the AD process was obtained for 2 g VS·L⁻¹, with a single feedstock input into the digester, in a single-stage process (2/s/-), while the highest conversion power ratio was for a feedstock of 1 g VS·L⁻¹, under the same process conditions (1/s/-). Moreover, the energy efficiency of the microalgae fermentation process obtained in the study is higher compared to conventional substrates used in biogas plants. This energy analysis can support the selection of cogeneration power engines in a biogas plant and help to determine the potential output of the biogas plant, especially with varying energy and heat demand.