Synergistic effect of pressurized carbon dioxide and sodium hypochlorite on the inactivation of Enterococcus sp. in seawater

Abstract

This study investigated the effect of combined treatments using pressurized carbon dioxide (PCD) and sodium hypochlorite (NaOCl) on the inactivation of Enterococcus sp. in artificial seawater. Bacterial inactivation was conducted in a liquid-film-forming apparatus with various pressure conditions, CO₂ supply rates, and chlorine dosages. Combined PCD/chlorine treatments resulted in greater disinfection efficiency than those for the two individual treatments. Synergy values were correlated with pressure and CO₂ concentrations (p < 0.001). Combination of 0.9 MPa PCD (various CO₂ supply rates: 25% CO₂ + 75% N₂, 50% CO₂ + 50% N₂, and 100% CO₂) and chlorine (0.20 mg L^-1) yielded average synergy values of 4.9, 5.2, and 4.4 log, respectively, within 3 min. Combined treatment with PCD (100% CO₂, 0.3 MPa, and 20 °C) and chlorine (0.20-0.22 mg L^-1) achieved an average synergy value of 4.6 log and complete inactivation (5.2-5.5 log reductions) of Enterococcus sp. within 4 min. In contrast, when the two individual treatments (PCD and chlorine) were used, only 3.7 and 1.8-2.3 log reductions, respectively, were achieved after 25 min. These findings suggest that the combined PCD/chlorine treatment has synergistic benefits and provides a promising method for the disinfection of ballast water.

Keywords: Bactericidal inactivation; Ballast water disinfection; Enterococcus sp.; Pressurized carbon dioxide; Sodium hypochlorite; Synergy.