Combination of Exhaust Gas Fermentation Effluent and Dairy Wastewater for Microalgae Production: Effect on Growth and FAME Composition of Chlorella sorokiniana

Abstract

Microalgae cultivation in wastewater is a promising strategy for reducing nutrient loads and generating biomass that can be further exploited. Although microalgae grown under such conditions are not suitable for high-value applications, the resulting biomass can still be valuable for uses such as biofuels, biofertilizers, or animal feed. In this study, Chlorella sorokiniana was cultivated in dairy wastewater and, to the best of our knowledge, for the first time in a spent effluent from gas fermentation, to assess its potential as a sustainable growth medium. Growth kinetics and biomass productivity were evaluated at different dilution ratios, and it was found that high concentrations of ammonium and hexanol in undiluted effluents were inhibitory, while an optimized 50:50 dilution led to the highest biomass accumulation (1.96 g L^-1) and productivity (0.5 g L^-1 d^-1) of C. sorokiniana. This strategy significantly reduced the nitrogen (100%), phosphate (100%), sulfate (68%), and carbon (61%) contents, demonstrating effective bioremediation activity. Furthermore, the fatty acid profile revealed an increased polyunsaturated fatty acid fraction, enhancing the potential of C. sorokiniana biomass as a feed supplement. Overall, contributing to the circular bioeconomy, this approach is scalable and cost-effective, reducing freshwater and chemical dependency in microalgae biomass production.

Keywords: FAME; dairy; gas fermentation; lipids; microalgae; mixotrophy; wastewater.

Figure 1

Comparison of the three main cultivation conditions for C. sorokiniana. (A) Biomass production, expressed as g of dry cell weight per liter of culture; (B) Growth rate, as day⁻¹, and biomass productivity, as gDW L⁻¹ per day; (C) Acetate consumption of both mixo- and heterotrophic conditions, as g L⁻¹; (D) Light regime and normalized light exposure in phototrophic and mixotrophic cultures. Light intensity (µmol s⁻¹ m⁻²) during C. sorokiniana cultivation in phototrophy (red) and mixotrophy (blue), and normalized light exposure (µmol s⁻¹ m⁻² /OD₇₅₀) are represented on the right and left Y axes, respectively. All tests were carried out in triplicate. Error bars represent the SD.

Figure 2

Ammonium tolerance of C. sorokiniana from 50 to 500 mg L⁻¹. (A) Growth curves. (B) Biomass productivity expressed as gDW L⁻¹ per day. Ammonium was tested at 50, 100, 200, and 500 mg L⁻¹. Cultures grown in 3N BBM were used as the control. All tests were carried out in triplicate. Error bars represent SD. Asterisks indicate statistical significance. ***, p-value ≤ 0.0002; ****, p-value ≤ 0.0001.

Figure 3

Alcohol tolerance of C. sorokiniana. Ethanol, butanol, and hexanol were assessed at the same concentration in the GFE (1×) or doubled (2×). The growth curves are expressed as biomass production (gDW L⁻¹) over time. All experiments were conducted in triplicate. Error bars indicate SD.

Figure 4

Growth performance of C. sorokiniana on acetate-, butyrate-, and caproate-supplemented media. (A) Biomass accumulation (gDW L⁻¹); (B) VFAs consumption, expressed as a percentage; (C) Consumption profile of acetate, butyrate, and caproate. All tests were conducted with 50% DWW in 3NBBM medium, while the control conditions at pH 7 and 8 had no external organic carbon source. All tests were carried out in triplicate. Error bars represent SD.

Figure 5

Screening of both DWW and GFE. (A) Growth curves of C. sorokiniana grown in 50–100% DWW. (B) Biomass productivity, expressed as gDW L⁻¹ per day, in DWW. (C) Growth curves of C. sorokiniana grown on 50–100% GFE. (D) Biomass productivity on GFE. Cultures grown on 3N BBM were adopted as control condition. All tests were carried out in triplicate. Error bars represent SD. Asterisks refer to statistical significance. ***, p-value ≤ 0.0002; ****, p-value ≤ 0.0001.

Figure 6

Analysis of C. sorokiniana performance in a mixture of 50% DWW and 50% GFE. (A) Biomass production, expressed as gDW L⁻¹, in comparison to 3N BBM supplemented with 1 g/L acetate. (B) Light exposure of biomass over time. (C) Volatile fatty acid consumption, i.e., acetate, butyrate, and caproate. (D) Depletion, expressed as a percentage, of the main components present in the mixture. (E) Fatty acid methyl ester profile of the biomass grown in 50% DWW and 50% GFE. (F) Fatty acid profile comparison between mixo and phototrophy cultivation conditions, by means of saturated (SFA), monosaturated (MUFA), and polysaturated fatty acids (PUFA) percentage among all the fatty acids detected in the biomasses. Error bars represent SD. Asterisks refer to statistical significance. *, p-value ≤ 0.05; ***, p-value ≤ 0.0002; ****, p-value ≤ 0.0001.