In-situ sterilization of microalgae photobioreactor via ozone-rich macrobubbles

Aseptic technique plays a critical role in determining high volumes of microalgae biomass during the cultivation stage as the presence of a contaminant is one of the limiting factors. This study focuses on developing an energy-efficient method in sterilizing the 2 L microalgae photobioreactor. Initially, the seawater bacterial contaminant was successfully screened, isolated and identified as Kurthia gibsonii. The photobioreactor sterilization using ozonolysis was optimized using one factor at a time (OFAT), with 3 parameters studied: ozone aeration rate (0.5 L/min, 1.0 L/min and 1.5 L/min), bacterial culture volume (1.0 L, 1.5 L, and 2.0 L) and ozonation time (maximum of 180 minutes with 30-minute intervals). The ozonation technique at optimum condition reduced the contaminants up to 7 log reductions. The initial cells concentration in 1 L culture containing 6.73×107 cells/ml was reduced to 7 cells/ml after being ozonated for 180 min. The morphology study of the bacteria before and after ozonation (at 1000X magnification) confirmed that ozonation shattered the cells into small pieces. This study reveals the possibility of replacing energy-intensive sterilization techniques such as the autoclave method with ozonolysis. Ozonation method is more energy efficient, with a total 1.26 kW energy used during the sterilization process as compared to autoclaved method with 3.565 kWh. The study proves that the ozonation method is energy efficient, cheaper and easily applied to a larger industrial scale as well.