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

•Synergistic benefits of combined PCD/chlorine disinfection technology were studied.•Disinfection substantially increased with increased pressures and CO2 supply rates.•CO2 concentration of 1500 mg L−1 is optimal for the synergistic disinfection.•Chlorine dosages of 0.20-0.22 mg L−1 are optimal for the PCD/chlorine treatment.•The method inactivated Enterococcus sp. by 5.2-5.5 log in seawater within 4 min.

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.

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