Efficient removal of cyanobacterial harmful algal blooms through chitosan-clay flocculation: Effects of dosage and application procedure

Cyanobacterial harmful algae blooms (cyanoHABs) are increasingly common in freshwater around the world, impairing freshwater ecosystems as well as access to water used for drinking, agriculture, fishing, and industry. A combined coagulant-clay flocculation strategy is a promising method to remove harmful algal cells, yet the optimally efficient application protocol and dosage remain unclear, making cost estimation difficult. Here we investigate how (1) the application procedure and (2) the relative dosages of clay and chitosan affect the removal efficiency of M. aeruginosa, a representative freshwater cyanobacterium. Bentonite was chosen for its abundance and high cation exchange capacity, and chitosan was chosen for its biodegradability and regulatory approval in the United States. We conducted lab-scale flocculation experiments that showed (i) a stepwise application of chitosan and bentonite, regardless of application order, results in a higher cell removal efficiency than a single-step chitosan-modified bentonite application (CMB); (ii) in all three approaches, there is an optimal chitosan dose that is sensitive to both cyanobacterial cell density and the bentonite dose; and (iii) upon optimization according to application procedure and dosage, the addition of chitosan at 2–4 mg/L can reduce the required bentonite dose to remove 80 % of cyanobacteria to <80 mg/L. Though experimental, our findings indicate a sensitivity of flocculation efficiency to chitosan dose and application procedure, suggesting that important care should be taken to optimize these two parameters in field campaigns that use the EPA-approved materials, chitosan and bentonite, to efficiently remove cyanoHABs-cells in freshwater ecosystems.