Flotation of microalgal particles without chemical coagulant and role of extracellular polymeric substances under auto-flocculation

To effectively separate and collect microalgal particles from a water body without using a chemical coagulant, a series of flotation separation experiments were conducted during the auto-flocculation life cycle phase addressing a problem in separated algal biomass recycling. Based on the analysis of extracellular polymeric substances (EPS), which participate in algae particle auto-flocculation, we investigated the role of EPS and evaluated its applicability as a chemical coagulant substitute in the flotation process. The sources of the main algal growth factors, carbon, nitrogen, phosphorus, and light intensity, were controlled to compare the differences in EPS components and separation efficiency among the cultivation samples. Comparative analysis of proteins and carbohydrates revealed no qualitative difference in the EPS affecting the auto-flocculation and flotation separation of particles. However, there was a slight difference in the quantitative EPS concentration by culture condition, and under high EPS carbohydrate concentration, a high separation efficiency was achieved under nutrient-deficient conditions. Moreover, the supply of CO2 gas in a nutrient-deficient state promoted EPS production and increased the carbohydrate content, which induced algae particle hydrophobicity and promoted increased flotation efficiency. The study results suggested that microalgae particles can be separated in an eco-friendly manner using only EPS emitted by microalgae during auto-flocculation without a chemical coagulant during flotation.