This study determined the effects of temperature and time on oil extraction from indigenous freshwater microalgae: Dictyosphaerium, Chlorella, Desmodesmus and Cosmarium species, cultured in 15 2-L column photobioreactors (PBR) (three per species). Growth and specific growth rates for the mixotrophic>autotrophic cultivation of the species were determined. Oil was extracted from the dried microalgae species at temperatures ranging from 40 to 120°C at 20°C intervals and times ranging from 30 to 210 min at 30 min intervals, by accelerated solvent extraction method. Extraction temperature, time and type of microalgae species had significant effect (p < 0.05) on oil yield (temperature > time > type of species). As extraction temperature and time increased, Desmodesmus armatus gave the optimum oil yield (72.6% at 92.5°C), whereas, Cosmarium spp. produced the least (45.5% at 91.7°C). Optimal oil yield and temperatures of Desmodesmus subspicatus, Chlorella lewinii and Dictyosphaerium spp. were 68.2% and 92.5°C; 72.3% and 91.9°C; and 66.7% and 92.5°C, respectively. The optimization result showed that oil extraction from microalgae should be conducted at about 80°C and at the first 30 min of heating for oil extraction. These findings will reduce extraction wastages of time, cost, energy, resources and chemicals.

Keywords: Oil yield, Dictyosphaerium spp., Desmodesmus subspicatus, Chlorella lewinii, Cosmarium spp., growth rate.