Algal autoflocculation--verification and proposed mechanism.

Biomass autoflocculation in outdoor algal cultures was found to be associated with increases of culture pH levels, due to CO(2) consumption by the algal photosynthetic activity. Under these alkaline conditions, some medium chemical ions precipitated together with the algal biomass. The chemical substances involved with the process and its dependence on pH value were studied by simulation of autoflocculation in laboratory experiments. Proper concentrations of calcium and orthophosphate ions in the medium are important for autoflocculation and, in order to attain it within the pH range 8.5-9.0, the culture should contain 0.1mM-0.2mM orthophosphate and 1.5mM-2.5mM calcium prior to raising the pH level. Calcium phosphate precipitates are considered as the flocculating agent which reacts with the negatively charged surface of the algae and promotes aggregation and flocculation.