Process Analysis of Alkaline Flocculation Harvesting for Chaetoceros muelleri and Scenedesmus quadricauda

Abstract

Alkaline flocculation could be an attractive microalgae harvesting method, because it is low-cost, low energy consumption, and non-toxic to microalgal cells, and the high pH effectively sterilizes the microalgal biomass as well as the process water. In this paper, the mechanism of alkaline flocculation process and the difference between the two strains marine diatom Chaetoceros muelleri #862 and freshwater algae Scenedesmus quadricauda #507 were analyzed. The particle size of C. muelleri #862 cells increased linearly with the increasing pH value which increased dramatically by nearly fivefolds after alkaline flocculation. When pH was below 10.5, the absolute value of zeta potential on the surface of C. muelleri #862 increased rapidly, and also the conductivity of the solution declined quickly. The alkaline flocculation of C. muelleri #862 was mainly attributed to the production of Mg(OH)2 rather than calcium phosphate or calcium hydroxide. For the strain S. quadricauda #507, the cells still existed integrally as normal cells after alkaline flocculation. Firstly, with the increasing of pH in the solution, the absolute value of zeta potential had not declined. When pH was around 10.8, the absolute value of zeta potential had declined abruptly and the solution conductivity was increasing all the time. The cell wall of S. quadricauda #507 with the high content of cellulose Iα and low crystallization degree was good for the next step of saccharification or oil extraction.