The effect of hydrodynamic cavitation on Microcystis aeruginosa: Physical and chemical factors

The various effects of hydrodynamic cavitation (HC) on algal growth inhibition were investigated. The gas-vacuolate species Microcystis aeruginosa responded differently to the gas-vacuole-negative alga Chlorella sp. When M. aeruginosa was subjected to HC, both its cell density and photosynthetic activity were subsequently reduced by nearly 90% after three days culture. However, the cell density of Chlorella sp. was reduced by only 63%, and its final photosynthetic activity was unaffected. Electron microscopy confirmed that HC had a minimal impact on algal cells that lack gas vacuoles. Shear stress during recirculation only modestly inhibited the growth of M. aeruginosa. The relative malondialdehyde (MDA) content, a quantitative indicator of lipid peroxidation, increased significantly during HC treatment, indicating the production of free radicals. Accordingly, the addition of H2O2 to the HC process promoted the production of free radicals, which also improved algal reduction. A comparison of the outcomes and energy efficiency of HC and ultrasonic cavitation indicated that HC gives the best performance: under 10 min cavitation treatment, the algal removal rate of HC could reach 88% while that of sonication was only 39%.