Filamentous fungi assisted bio-flocculation: A novel alternative technique for harvesting heterotrophic and autotrophic microalgal cells

The energy intensive harvesting of tiny microalgae cells (1-70 μm) from culture broth accounts for at least 20-30% of total costs of algal biomass production. The aim of this study was to develop an alternative fungus pelletization assisted bioflocculation method for harvesting microalgae using pellet-forming fungal strain (Aspergillus oryzae) isolated from municipal wastewater sludge. Facultative heterotrophic microalga Chlorella vulgaris UMN235 was used as a model species. Under heterotrophic growth condition, the key factors including spore inoculums, organic carbon concentration in medium as well as pH variation had significantly positive effects on fungus-algae pellet formation. The process parameters of 1.2 × 10⁴ spores/mL, 20 g/L glucose, and pH ranged from 4.0 to 5.0 were found optimal for efficient fungus-algae pellet formation. For autotrophic growth, when pH of culture broth was adjusted to 4.0-5.0 with organic carbon addition (10 g/L glucose), almost 100% harvesting efficiency of microalgae was obtained. Moreover, it was observed that diameter and the concentration of fungus-algae pellets were affected by the shaker rotation. The novel harvesting technology developed in this study might reduce the microalgae harvesting cost and will have potential to be applied to all types of microalgae species as alternative to other traditional harvesting methods.