The formation of bacterial biofilms and their removal by disinfection is important for water quality management of swimming pools, food processing lines, industrial water systems etc.
Microorganisms in a biofilm are protected by matrices of extracellular polymeric substances, and are more tolerant to antibiotics and biocides than plankton. Therefore tests using a biofilm system are important for evaluation of disinfection efficacy as a biofilm disinfectant.
Researchers at Nihon University in Japan have studied synergistic disinfection and removal of biofilms by ozone (O3) water in combination with hydrogen peroxide (H2O2) solution. They determined disinfection rates and observed changes in the biofilm structure in situ by confocal laser scanning microscopy (CLSM) using an established biofilm of Pseudomonas fluorescence.
Sequential treatment with O3 (1.0–1.7 mg/L) followed by H2O2 (0.8–1.1%) showed synergistic disinfection effects, while the reversed treatment, i.e. H2O2 followed by O3, showed only an additive effect.
Addition of methanol (CH3OH), a scavenger of hydroxyl radicals (·OH), into the H2O2 solution reduces the synergistic disinfection effect. This suggests the generation of hydroxyl radicals on or in the biofilm by sequential treatment with O3 followed by H2O2.
The primary treatment with O3 increased disinfection rates of H2O2 in the secondary treatment, and increased O3 concentration enhanced the rates. The cold temperature of O3 water (14°C and 8°C) increased the synergistic effect, suggesting increased O3 adsorption and hydroxyl radical generation in the biofilm.
CLSM observation showed that sequential treatment with O3 followed by H2O2 loosened the cell connections and thinned the extracellular polysaccharides (EPS) in the biofilm. Hydroxyl radical generation in the biofilm may affect the EPS and biofilm structure, and may induce effective disinfection with H2O2.
This sequential treatment method suggests a new practical procedure for disinfection and removal of biofilms by inorganic oxidants such as O3 and H2O2.
Water Research, Volume 64, 1 November 2014, Pages 94–101.