Flocculation of Microalgae via pH Change in a Turbulent Medium and Subsequent Filtration

Traditional microalgae harvesting techniques consume a lot of energy. Flocculation, or the formation of aggregates, is an energetically favorable process to collect biomass. Flocculation is normally carried in tanks to allow the formation of the aggregates after stirring, however, this consumes time and physical resources. In this work, flocculation of Chlorella vulgaris and Scenedesmus sp. by acidification and alkalization was compared to chemical flocculation in a turbulent medium for short periods of time (30 seconds and 2 minutes). Flocculation with potassium hydroxide at pH=10 showed to be nearly as efficient as traditional flocculation by using ferric sulphatum after two minutes. Acid flocculation with nitric acid was not as effective, even at values of pH=4. Flocculation by pH does not generate toxic wastes and the remaining added flocculants turn into nutrients after harvesting. After flocculation and neutralization, the remaining cells in the medium were viable to recultivate. Since pH-driven flocculation does not allow harvesting the total microalgae culture, remaining cells can be used to keep growing. Based on these results a semi continuous harvesting method incorporated in the microalgae growing phase seems promising.