Photobioreactor production of microalgae for potential fuel oils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology at Massey University, Palmerston North, New Zealand
This work focussed on a detailed characterization of the freshwater microalga Chlorella vulgaris as a producer of potential fuel oils. Uniquely, growth and oil production of C. vulgaris were characterized in full strength seawater-based media, something that has not been previously reported. C. vulgaris was selected for a detailed study after a screening of six potential oil producing microalgae. For photoautotrophic growth, always under carbon sufficiency and at normal growth temperature, the characterization study covered: the biomass growth rate; lipid content in the biomass; productivities of the lipids and the biomass; the biomass loss in the dark; the lipid/biomass yields on macronutrients; and the energy content of the biomass. The above key production parameters were characterized in a purpose-built tubular photobioreactor (~80 L) and in stirred tank photobioreactors (~7.5 L) under conditions of nitrogen sufficiency and at various levels of nitrogen limitation. Production was evaluated in both batch and continuous cultures at various dilution rates using indoor light to mimic sunlight. The production temperature mimicked the relatively warm conditions that would be encountered in a potential production system located outdoors in a tropical climate.
In seawater media at 25–27 °C, C. vulgaris was shown to have a crude oil productivity of >37 mg L⁻¹ d⁻¹ and the energy content of the biomass could exceed 25 kJ g⁻¹, depending on the culture conditions. Both these values were high compared with the reported data for this alga in freshwater media. Compared with continuously illuminated culture, day–night cycling of irradiance reduced oil productivity by ~31%, but the energy content of the biomass were reduced by only about 8%. In seawater, the alga could be grown as rapidly and stably as in freshwater. The lipid content of the biomass commonly exceeded 30% by dry weight and in exceptional cases a lipid content of more than 50% (by weight)
was achieved. Biomass calorific values of ≥27 kJ g⁻¹ could be attained in some cases. Nitrogen starvation enhanced the lipid contents of the biomass by >3-fold relative to the lipid contents for the nonstarved case. Steady-state continuous cultures were shown to be possible. Both batch and continuous operations were feasible, especially in stirred tanks, but the culture was more failure prone, or relatively less productive, in the tubular photobioreactor.