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Disinfection of marine ballast using persulfate and nanobubble technology

Pavlineris Nikolaos

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Summary

Shipping is responsible for the transportation of approximately 80% of the world’s exports. Over the past decade alone, the global commercial fleet has increased by 40%. This growth presents a range of environmental challenges that require immediate attention. A central issue among these is the management of ships’ ballast water, as its untreated discharge into the marine environment leads to serious ecological, economic, and social consequences.The International Maritime Organization (IMO) mandates the treatment of ballast water onboard ships, setting clear effluent quality criteria. The need to comply with Regulation D2, which has been fully enforced for all ships since 2024, has led to the development of increasingly efficient disinfection methods.The aim of this thesis is to examine the effectiveness of ballast water disinfection through the activation of sodium persulfate (Na₂S₂O₈), in combination with the application of nanobubble (NB) technology, as this specific type of activation has not been extensively studied. The use of persulfate salt following activation is considered an effective disinfection method. However, conventional activation techniques (such as thermal or chemical activation) have limitations in terms of efficiency and free radical generation. Nanobubbles have the potential to offer an innovative solution by achieving faster and more efficient activation due to their unique physicochemical properties. The microorganisms used for the inactivation process were Escherichia coli and Enterococcus faecalis, in varying concentrations, both of which are regulated under IMO Regulation D2. Additionally, the effect of thermally activated sodium persulfate in combination with nanobubble technology was investigated.The study’s results demonstrated that high concentrations of sodium persulfate (20 mM) combined with nanobubbles led to significant disinfection activity against the lower microbial concentration (10⁴ CFU/mL) used, within 180 minutes. In corresponding experiments where nanobubbles were not used, a slower and less effective disinfection action was observed. Furthermore, immediate and complete disinfection of higher microbial load concentrations (10⁶ CFU/mL) was observed when the seawater sample, even with the lowest concentration of sodium persulfate (3 mM), was preheated to 60°C before being used for disinfection. In corresponding experiments without the use of nanobubbles, no disinfection activity was observed. Therefore, it is concluded that the thermal activation of sodium persulfate in combination with nanobubble technology is of critical importance for effective disinfection. The temperature of 60°C, together with nanobubbles, appears to be sufficient for the decomposition of the persulfate ion and the generation of the necessary sulfate free radicals (SO₄•⁻), which are responsible for the disinfection action.

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