An investigation into the efficacy of ozone for inactivation of microalgae cells in photobioreactors

The sterilisation of photobioreactors is paramount to successful cultivation of microalgae. Due to the adhesive nature of microalgae, fouling of the light transmitting surfaces can be problematic. Traditional methods of sterilising photobioreactors between batches involve ultrasound, autoclaving or as seen in recent times, the photobioreactor incorporates a cleaning system. The objective of this project was to look at an alternative method of sterilisation. A preliminary study was carried out to determine the efficacy of ozone as treatment for the algae strain Dunaliella tertiolecta. Algae cultures at 104 cells CFU mL-1 were exposed to 3-51 μg mL-1 of ozone at different time periods (0-80 sec) and constant gas flow rates of 0.125 and 0.25 L min -1. Inactivation rates for total reduction of D. tertiolecta were found to increase significantly with an increase in ozone concentration and treatment time. This study supports the proposed mechanism of cell inactivation by ozone that causes cell membrane destruction and finally cell death. Therefore, it was found that ozone can be used to effectively eradicate microalgae in photobioreactors.