The ability to harvest microalgae at a commercial scale continues to be a major economic problem for the algal biomass industry. Due to their small cell size and relatively low concentration, large volumes of water are required to be removed to produce a commercially viable product, making the process both energy, and cost prohibitive. Among current commercially available harvesting methods flocculation has been identified as an efficient and cost-effective technique to concentrate algae biomass. For a flocculant to be suitable for commercial use it must be inexpensive, efficient, effective at low concentrations, and non-toxic for applications in food and feed. Additionally, the flocculant is expected to maintain the biochemical integrity of the harvested biomass.
Twenty commercially available polyelectrolyte flocculants were evaluated for their flocculation efficiency of high-density indoor cultures of Nannochloropsis oceanica (CS-702). Flocculation efficiency varied considerably ranging from 1 to 99% (OD750). Flocculants POLY SEPAR CFL 20 and CFL 40 showed the highest flocculation efficiency in the medium to high concentration range (50-100mg L-1) ranging from 91-99%.
POLY SEPAR CFL40 was chosen for subsequent scaled-up harvest trials of N. oceanica cultivated in 2300L outdoor raceways due to its high flocculation efficiency and low toxicity. Pilot-scale flocculation of 2300L of raceway cultures was trialled using CFL40 at concentrations of 80 and 120mg L-1 with a reluctant flocculation efficiency of 83 and 85%, respectively.
The biochemical composition of N. oceanica flocculated using CFL40 at concentrations of 80 and 120mg L-1 prior to centrifugation versus biomass harvested via centrifugation was evaluated. Among the various biochemical parameters monitored the addition of flocculant increased the ash content and decreased the lipid and energy content. However, flocculated algal biomass did not register any variation in protein and carbohydrates when compared to algal biomass harvested using a centrifuge.