Effects of CO₂ Concentration and pH on mixotrophic growth of Nannochloropsis oculata

This communication reports an experimental investigation of integrated CO₂ bio-conversion, wastewater treatment, and biomass production by microalgae cultivation. In this regard, the effects of CO₂ concentrations on mixotrophic growth kinetics of a microalgae strain (Nannochloropsis oculata) are conducted in a semi-batch photobioreactor. The concentration of CO₂ in the feed stream is varied from 4 to 12 mol% by adjusting CO₂-to-air ratio. The variation of pH of the synthetic wastewater culture media and nutrient uptake by the microalgae are also monitored. The experimental evaluation shows that 8 % CO₂ gives the highest growth rate of N. oculata with a productivity of 0.088 g L⁻¹ day⁻¹. Under the studied conditions, the pH value of the culture media between 5.5 and 6.5 is favorable for the growth of N. oculata in mixotrophic condition. Among the nutrients available in the culture media, percentage of ammonia removal is found to be the highest (98.9 %) as compared that of other compounds such as nitrate (88.2 %) and phosphate (18.9 %). The thermochemical characteristics of the cultivated microalgae are assessed by thermogravimetric analysis in presence of air. The produced microalgae is thermally stable up to 200 °C. Following that, the microalgae biomass is sharply decomposed within 600 °C.