Hydrodynamic cavitation for bacterial disinfection and medium recycling for sustainable Ettlia sp. cultivation

Open cultivation systems are the most economic option for microalgal biomass production but their harvesting cost, bacterial invasion, and water usage are still deterring large-scale applications. Therefore, in this study, low-cost techniques for harvesting, bacterial disinfection, and medium recycling were sequentially implemented for Ettlia sp. For harvesting, the employed auto-flocculation harvested 96% of algae but was not able to remove bacteria that invaded during cultivation. For this reason, hydrodynamic treatment was applied as a bacterial disinfection method for medium recycling, and treated up to 100% of the bacteria in the supernatant. Afterward, the supernatant was replenished with nutrients and reused to grow microalgae. In medium-recycled cultivation, auto-flocculation recycled (AR) medium showed the highest final biomass concentration of 3.84 g L−1, while the centrifugation recycled (CR) medium and the fresh medium (FM) resulted in 3.78 and 3.67 g L−1, respectively. In addition, the lipid content of the AR group was highly increased to 20.4%, resulting in the highest lipid productivity of 0.11 g L−1 d−1. Moreover, an analysis of fatty acid methyl ester compositions confirmed that 76.6% of lipid from the AR group were mainly composed of C16-C18, which are suitable for biodiesel. Consequently, the results showed that the implementation of low-cost techniques not only reduced process costs and water usage but also enhances both the biomass and lipid productivities of the species. [Display omitted] •Low-cost harvesting, bacteria control, and medium recycling were combined.•Auto-flocculation harvested microalgae but left bacteria in the supernatant.•Hydrodynamic cavitation disinfected bacteria with low-energy consumption.•Biomass productivity of auto-flocculation recycled medium was slightly increased.•Auto-flocculation recycled medium resulted in 1.6-fold higher lipid productivity.