Combined Filtration and Flocculation for Chlorella vulgaris Harvesting

The present study aims to explore the feasibility of using flocculation combined with filtration for microalgae harvesting, i.e., Chlorella vulgaris. This is important because microalgae have small sizes and its broth is stable, which makes it difficult to be harvested. The aforementioned facts cause the harvesting cost to be relatively high and become the bottleneck of microalgae processes. The objective of this research is to find the relation between microalgae concentration, chitosan dosing as a flocculant, and its filterability on membranes. Research was performed by first cultivating the microalgae in a lab-scale photobioreactor, followed by jar test, flocculation, and filtration experiment. Jar test flocculation was performed using chitosan and microalgae with different concentrations, by simply mixing it in a 100-mL bottle and analyzing the results with UV Vis Spectroscopy. Filtration experiments were performed using lab-made polyvinylidene fluoride membrane, in a 100-mL dead-end filtration cell and in a 5-L tank for submerged filtration. During both filtration tests, filtration flux and fouling were monitored and compared. Results showed that the chitosan concentration needed as a flocculant depends on the microalgae biomass concentration. For the filterability tests, the results proved that flocculation with chitosan enhanced the filterability of the microalgae broth both in dead-end and submerged filtration mode. For the used biomass concentration of around 400 mg/L, the filterability test showed an optimum concentration of chitosan at 7.5 to 10 mg/L, which resulted in a higher filtration flux and lower irreversible fouling in the dead-end filtration and a higher critical flux in the submerged filtration set-up. This increased filterability allowed higher fluxes to be operated, thus resulting in a more efficient harvesting process.