Efficient Inactivation Of Microorganisms By Photocatalytic Ozonation Process And Its Application For Ballast Water Treatment

Ballast water is water carried by ships to ensure the stability and maneuverability during transit. However, ships transporting ballast water between geographically isolated ports contribute to the spread of many aquatic species beyond their natural range limits, and it has been regarded as one of the four major risk factors that threaten global marine environmental safety. To prevent the introduction of potentially invasive species, ballast water management and treatment are essential.There are several established and under developing technologies available to be used for ballast water treatment. Unfortunately there is not a single method can reach the standard quality due to its own disadvantages. A combination of several methods result in high treatment efficiency compared with individual treatment, and has become the main direction of research to solve the problem of ballast water. The objective of this study was to evaluate the effectiveness of UV/Ag-TiO₂/O₃ treatment in preventing introduction of invasive species during the ballasting/deballasting process. The combination of photocatalyst with ozone system was established. The inactivation efficiency of UV/Ag-TiO₂/O₃ on bacteria and microalgae were studied, and their death mechanisms were investigated.To evaluate the feasibility of the UV/Ag-TiO₂/O₃ process, the bactericidal activities of ozone, UV irradiation and UV/O₃ process were researched firstly. Results indicated that both ozone and UV irradiation were effective for bacteria inactivation, and a significant enhancement for E. coli inactivation efficiency was obtained in the combined UV/O₃ process as compared with each individual unit process, especially in the initial phase of the reaction. In the ozonation under UV irradiation, the ozone was accelerated decomposed, and a larger number of hydrogen peroxide and hydroxyl radicals were produced, thereby accelerated the bactericidal reaction, repressing the bacteria photoreactivation after treatment.Secondly, an efficient and impact resistance photocatalytic nano-porous titanium dioxide film was prepared by anodic oxidation method in sulfuric acid solution, and was loaded sliver by photodeposition method. The effect of anodizing parameters (anodizing voltage, current density and electrolyte concentration) on the photocatalytic bactericidal activity of TiO₂ films was investigated. It showed that the titanium oxide film was mainly composed of anatase and rutile TiO₂. With the increasing of anodizing voltage, the content of rutile TiO₂ increased, and the average diameters of nano-holes enlarged. Propore increase of current density and suitable Ag loading could enhance the photocatalytic activity of TiO₂ film. When prepared at 140 V, 150 mA/cm2 in 1 mol/L sulfuric acid solution, and loaded silver in 3 g/L silver nitrate solution, the Ag-TiO₂ film had the best photocatalytic bactericidal activity. Compared with UV-C irradiation alone, the inactivation of E. coli by the UV/Ag-TiO₂ process was enhanced and the photoreactivation of the bacteria was much repressed.Then, different filtration and ozone dosing modes, light sources, and the photocatalytic reactors were analyzed, and the combination of photocatalyst with ozone system and equipment, which were used for ballast water treatment, were established. The effectiveness of filtration-UV/Ag-TiO₂/O₃ treatment in preventing introduction of bacteria (Escherichia coli, Enterococcus faecalis and vibrio alginolyticus) and microalga (Dunaliella salina, Amphidinium sp. and Phacodactylum tricornutum) in artificial seawater during the ballasting/deballasting process were evaluated.The effects of UV light intensity, ozone dose and raw water quality, were examined in this system, and the formation and decay of the initial total residual oxidant (TRO) was investigated.The results indicated that both UV/Ag-TiO₂ irradiation and ozonation were effective for bacteria inactivation. However, UV/Ag-TiO₂ irradiation was not effective for microalga inactivation and chlorophyll a degradation. Compared with individual unit processes with ozone or UV/Ag-TiO₂, the inactivation of bacteria and microalga by the combined UV/Ag-TiO₂/O₃ process were enhanced, especially in the initial phase of the reaction. With an initial ozone dose of 9.84 mg/L, a UV fluence rate of 6.5 mW/cm2, the required bacteria inactivation ratio would be obtained within a hydraulic residence time (HRT) of 3.0 s. When the HRT increased to 3.0 s, the detected inactivation efficiency of Dunaliella salina, Amphidinium sp. and Phacodactylum tricornutum were 0.92, 0.60 and 0.57 log, respectively. The ?OH that produced in the UV/Ag-TiO₂/O₃ system contributed partly to the disinfection of microorganisms. The ozone dose and UV light intensity had a significant influence on the disinfection efficiency. With the UV intensity and ozone dose increasing, the disinfection efficiency was improved. When the ammonia concentration ranged from 0.1 to 1.0 mg/L, the E. coli inactivation efficiency changed little. When the pH value ranged from 8.0 to 8.6, the disinfection efficiency increased with the pH value, while the temperature increased from 5℃to 20℃, the efficiency of disinfection decreased. Compared with the raw water (pH 8.0), the pH value of the effluent that after treatment by UV/Ag-TiO₂/O₃ process changed little. The initial total residual oxidant (TRO) concentration was positively correlated with ozone dose, and resulted in faster decay rate for lower initial concentration. All TRO concentrations reduced rapidly at first and attenuate slowly as the duration increased. When the initial TRO concentration increased above 2.89 mg/L by adjusting the ozone dose, complete inactivation of E. coli was achieved within 0.5 h.In order to further investigate the feasibility and practicality of the UV/Ag- TiO₂/O₃ process for ballast water treatment, the death mechanism of bacteria and microalga during UV/O₃, UV/Ag-TiO₂ and UV/Ag-TiO₂/O₃ disinfection were studied. The results implied that UV irradiation was not effective for the destruction of bacteria cell structure. Exposed to the UV/Ag-TiO₂ irradiation, the E. coli cell wall and cell membrane were gradually decomposed, and resulted in the leakage of intracellular potassium ion (K+) and protein, however comparable slowly. Contrastly, the ozonation could make the microorganism's cell structure destructed seriously, and the combination of photocatalyst with ozone enhanced this effect, the formation of malondialdehyde (MDA) and leakage of intracellular potassium ion (K+) and protein was notable, and the chlorophyll a concentration decreased immediately. Therefore, in the UV/Ag-TiO₂/O₃ system, the UV-C disinfection mainly via destroy the cell DNA, and the decomposition of cell structure, which is irreparable, is one of the important causes for microorganism death during ozonation. Moreover, the combination of ozonation and photocatalysis with Ag-TiO₂ thin film under UV-C irradiation may not only destroy the cell structure, but also induce the cell DNA damage, and resulted in the enhancement of disinfection efficiency.The combination of photocatalyst with ozone system that established in this paper is a new advanced oxidation process system for ballast water treatment. It has high efficiency for removal the target organisms, and the effluent has continuous and extended sterility effect. The size of the equipment is small, and easy to operate. Thus, it can be used to solve the bottleneck problem of the ballast water treatment, and has the theoretical significance as well as the bright application prospect.