Sedimentation of algae flocculated using naturally-available, magnesium-based flocculants

Abstract Algal flocculation using naturally-available ions in brackish water was evaluated as a technique to rapidly settle algae while minimizing flocculant costs. Basicity-induced flocculation using naturally-available ions with NaOH addition was used as surrogate for an autoflocculation process at photosynthetically-elevated pH. Flocculation was attempted with a variety of precipitating ions, including Mg 2 + , Ca 2 + , and CO 3 2 − . Satisfactory flocculation was only achieved in the presence of Mg 2 + , while extensive CaCO 3 precipitation yielded only partial, presumably sweep, flocculation. The flocculation was characterized by the settling-rate distribution, measured via light attenuation. Using Mg 2 + -dependent flocculation, an optimum pH was observed at which rapid settling was achieved with minimal precipitation. Further base addition caused extensive Mg(OH) 2 precipitation, formation of loose macroflocs, and a reduction of settling rate. Experiments demonstrated that both increased Mg 2 + concentration during growth and the presence of extracellular organic matter necessitated a higher flocculation pH and greater Mg 2 + depletion. A proof-of-concept flocculation was demonstrated in surrogate media previously developed to mimic brackish waters of the southwestern United States (Barclay et al., 1987). These flocculations achieved median settling rates of at least 100 cm/h, more than 100-fold greater than the settling rate of unflocculated cells.