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

► We evaluate brackish water ions as naturally-available flocculants and choose Mg2 +. ► Flocculation is assessed by ion use and settling-rate distribution via spectrometry. ► An optimum pH exists that yields flocculation without excessive precipitation. ► With extracellular organic matter present, the optimum pH and Mg2 + use are higher. ► Median settling-rate increased 100 × in proof-of-concept, brackish-water flocculation.