Effectiveness of ozone nanobubble treatments on high biomass cyanobacterial blooms: A mesocosm experiment and field trial

Cyanobacterial harmful algae blooms (cyanoHABs) are a global threat to water resources, and lake managers need effective strategies to suppress or control them. Algaecides may have negative environmental impacts, and their use is becoming restricted. Nanobubble ozone technology (NBOT) is an emerging water treatment option with potentially fewer negative impacts. We assessed the effectiveness of NBOT in treating Planktothrix cyanoHAB from Grand Lake St Marys (GLSM, Ohio USA) in a mesocosm (2,000L) experiment and two 4-week trials in a GLSM embayment (Sunset Beach, SBE; ∼4.7∗10⁷ L). In mesocosms, the medium (1.21 ± 0.08 ozone to dissolved organic carbon ratio, O₃:DOC) and high (2.04 ± 0.07 O₃:DOC) doses decreased both chlorophyll a (chl-a) and phycocyanin by 98-99% and microcystins by 62% and 92%, respectively. The low dose (0.68 ± 0.05 O₃:DOC) decreased chl-a and phycocyanin by over 70%. No effect was observed for chl-a nor microcystins in both oxygen-only nanobubble mesocosm treatments and the SBE NBOT trial. The average O₃:DOC at SBE was less than the low NBOT mesocosm experiment dose, and the percentage of water treated was lower. DOC chemistry, as indicated by SUVA_254, was more oxidized at the NBOT outlet than the inlet in the SBE trial, suggesting interaction with ozone. However, no differences were observed 3m from the outlet, indicating minimal treatment reach. The mesocosm experiment highlighted NBOT's ability to control cyanoHABs, but the limited effectiveness of NBOT at SBE was likely due to high cyanobacteria biomass and DOC at the onset of treatment, low O₃:DOC, and low percentage of lake water instantaneously treated.