A rapid, low-cost flocculation technology for enhanced microalgae harvesting

A rapid, low-cost, scalable flocculation technology, using either potash alum (for culture pH < 8.5), or potash alum with hydrochloric acid (for culture pH > 8.5), is employed to harvest algal biomass in less than thirty minutes. The gaseous CO2 (0.04–5%) and acetic acid glacial added during mixotrophic cultivation produce hydrogen and acetate ions that react with hydrolysed potash alum to form charged species capable of electrostatic interactions with the charged microalgal cell surface, leading to flocculation and gravity settling. The post-cultivation and post-harvesting pH, as well as the biomass settling time, decrease exponentially with increasing inlet gas-phase CO2 concentration. The biomass settles from 10 L to 2 L in 13–29 min, with a harvesting efficiency of 95–98.7%, for Tris, ammonium chloride, and urea-mediated algal growth. Rapid chemical flocculation with multiple recycling reduces the harvesting cost to 4.48–30.77 INR (0.06$-0.42$) per kg of dry algal biomass, making it suitable for commercialization. © 2021 Elsevier Ltd