Enhancement of biodiesel-promising microalgae Chlorella pyrenoidosa growth using stimulants in municipal sewage

Cultivation of high-lipid microalgae using municipal sewage can both treat polluted water and gain biodiesel. Rapid cultivation of microalgae is crucial for the entire process. In this study, four stimulants (phytohormone 2,4-d; Fe³⁺; photosynthetic bacteria—Rhodopseudomonas palustris; CO₂) were firstly chosen to investigate their effects on microalgae Chlorella pyrenoidosa cultivation in municipal sewage. Meanwhile, orthogonal experiments were designed to compare the enhancing effects on the growth of microalgae C. pyrenoidosa and lipid accumulation by different combinations of stimulants, which were also conducted for outdoor culture. The optimum stimulant combination was 1.0 mg/L 2,4-d; 10⁻⁷ mol/L Fe³⁺; 4.5 mL (10⁹ cells/mL) photosynthetic bacteria; and 8% CO₂. Among the four kinds of stimulants, phytohormone 2,4-d and photosynthetic bacteria played more important roles on the microalgae growth and lipid accumulation. The optimum combination of stimulants could also promote the microalgae biomass and lipid yield during outdoor cultivation. The maximum OD₆₈₁ of microalgae was 37.9% higher than that of the control group without addition of stimulants. The obtained microalgae dry weight and lipid production were 0.3 and 0.13 g /L, which were 80.0% and 85.7% higher than that of the control group, respectively. The promotion of microalgae photosynthetic rate was the main reason for the stimulant-enhanced microalgae cultivation. This study exerts positive effects on the development of microalgae biodiesel.