Effect of algogenic organic matter (AOM) and sodium chloride on Nannochloropsis salina flocculation efficiency

•Efficient polymer flocculation of marine algae requires highly charged polymers.•Ionic strength does not affect polymer flocculation of N. salina.•AOM increased flocculant requirement 7-fold when using synthetic cationic polymers.•Synthetic cationic polymers do not require pH adjustment like chitosan and AlCl3.•AOM affects flocculant dosage irrespectively of the flocculant type. This study evaluates the effect of polymer molecular weight and charge density, algogenic organic matter (AOM), and salt concentration on harvesting efficiency of marine microalgae. Aluminum chloride (AlCl3), chitosan, and five synthetic cationic polymers of different molecular weights and charge density levels were used as flocculation agents. Polymer flocculation of marine microalgae was most efficient when using the highest charge density polymer (FO4990). The flocculant dosage irrespectively of the agent chemistry and charge density was affected by the amount of AOM secreted into the culture media. The presence of AOM increased the amount of required flocculant 7-fold when using synthetic cationic polymers; 10-fold with chitosan; and ∼3-fold with AlCl3. Salt concentration of 5 or 35g/L NaCl alone did not significantly affect removal efficiency, indicating that AOM were the main cause for the increased flocculant dosage requirement. The synthetic cationic polymer (FO4990) was the least expensive flocculation agent.