| Invasion of non-indigenous species transported in ballast water by marine vessels is one of the most serious problems which would cause severe ecological and economical impacts all over the world. In order to resolve this problem, many solutions have been discovered and utilized for the inactivation of invasive microorganisms in ballast water. However, some of the existing treatment methods are not safe and some are not cost-effective and so more robust ballast water treatment methods have been investigating by the researchers. Recently, high-voltage pulse discharge plasma technology has become a very popular research area because of lower costs, higher treatment efficiency and occupying the smaller space volume. In this research, the gas-liquid hybrid discharge plasma technology for ballast water treatment has been carried out to investigate the killing feasibility of the liquid phase pulse discharge plasma upon algae.In this research work, the gas-liquid hybrid discharge using a multi-needle plate reactor was investigated to inactivate four kinds of algae (Chrysophyta spp.,Nitzschia closterium f.minutissima, Platymonas spp., and Chlorella spp.). The research experiments were carried out by four stages; (1) investigating the discharge characteristics of gas-liquid hybrid discharge with multi-needle plate reactor, (2) studying the properties of radicals produced by using the emission spectroscopy, (3) observing the hydrogen peroxide and ozone formation mechanisms, and (4) investigating the effects of discharge parameters upon the inactivation of target algae.First, the gas-liquid hybrid discharge characteristics were investigated by changing the discharge parameters; the pulse peak voltage, pulse frequency, the distance between water surface and needle tips (ds), and conductivity in order to know whether the reactor could discharge high conductivity sea water or not. Moreover, four different gases bubbling were introduced above the liquid into the reactor. The results found that the higher the pulse peak voltage or pulse frequency, or the closer the distance between the water surface and needle tips (ds), the greater the input energy and input power into the reactor. This was because the higher the pulse peak voltage or pulse frequency, or the narrower the ds, the greater the increase in the electric field strength, the larger the ionization energy and the more active free radicals (i.e.'OH, 'H,'O,'HO2 and O3) produced and these would favor the increase in the conductivity of the solution in the reactor. Furthermore, it was found that (1) both corona and spark discharge can occur alternatively and sometimes occur simultaneously and, (2) this reactor can discharge not only the lower conductivity solution but also the higher conductivity solution.Second, in order to study the properties of active species produced and their chemical reaction mechanisms in the high-voltage discharge, analysis of emission spectroscopy was carried out by using Photonic Multi-channel Spectral Analyzer (PMA-11) for the detection of'O,'H and OH radicals and the effects of pulse peak voltage upon the formation of these radicals. The influences of four different gases; oxygen, air, argon and nitrogen upon radical formations were also studied and compared. According to the observation of the effects of active species by emission spectroscopy, the radical emission intensity of'OH, Ha and atomic oxygen were higher in oxygen than in other gases. This was because oxygen injection included a very high concentration of pure oxygen molecules than other gases injection, and these oxygen molecules were ionized into atomic oxygen because of the sufficient thermal energy due to the electric field fluctuations in the gas phase. And these atomic oxygen radicals were readily reacted with water molecules to form the OH radicals, and then reacted again with OH radicals to form the atomic hydrogen. The OH radical and atomic oxygen emission intensity was increased when the pulse peak voltage was increased because of the larger discharge energy, larger discharge current and more streamer channels in the reactor at the higher pulse peak voltage.Third, the formation of hydrogen peroxide and ozone were investigated in order to know their mechanisms as they were the most important active species in the pulse discharge. Ozone concentration in liquid phase was determined by iodometric method and ozone concentration in the gas phase was measured by using O3 Gas Analyzer and hydrogen peroxide concentration was measured by a colorimetric method using a T6 UV-visible spectrophotometer. The effects of pulse peak voltage, treatment time, pulse frequency and solution pH on the formation of hydrogen peroxide and ozone were also investigated by the separate introduction of four different gases; oxygen, air, argon and nitrogen above the liquid in the reactor. Hydrogen peroxide and ozone concentrations were increased when the pulse peak voltage, pulse frequency and treatment time were increased. This was because when the pulse peak voltage was increased, the number of discharge plasma channels and the electric field strength were also increased, which led to increase free electron energy and speed, resulting in the generation of more strong oxidizing free radicals. When the pulse frequency was increased, the number of discharges per unit time was also increased producing more high-energy electrons, hydroxyl radicals, active oxygen and other strong oxidants. With the increase of treatment time, the concentrations of hydrogen peroxide and ozone were increased due to the continuous formation of more radicals for a longer treatment time leading to produce more hydrogen peroxide and ozone. Similarly, more hydrogen peroxide and ozone concentrations were formed in the higher pH solution than in the lower ones. This was because more H+ ions formed in the lower pH solution react with Cl- ions from the sea water and more H2O2 were decomposed into HOCl and H2O, leading to reduce the formation of hydrogen peroxide. For ozone formation, when the pH was increased, the ozone formation rate increased rapidly. This may be because of the significantly increased generation of OH radicals when the pH is raised. The concentrations of hydrogen peroxide and ozone were found to be increasing in the order of oxygen> air> argon> nitrogen. The concentrations of hydrogen peroxide and ozone were the highest when oxygen was introduced because the gas phase discharge generated not only a lot of'OH but also other active species such as ozone (O3) peroxide (O2-) and oxygen (1O2). These active species generated at the gas-liquid interface could help more formation of hydrogen peroxide.Fourth, in order to know the inactivation rates of different algae, many experiments have been carried out by using a multi-needle plate gas-liquid hybrid discharge reactor and changing the discharge parameters. When the pulse peak voltage, pulse frequency and treatment time were increased, the algae's survival rate was decreased and the inactivation rate of algae was increased to 100%. From the experimental results, it was found that the inactivation rates of Chrysophyta spp. was reached to 100% when the pulse peak voltage, pulse frequency and treatment times were at 28 kV,30 Hz and 7 min, Nitzschia closterium f. minutissima was reached to 100% at 28 kV,30 Hz and 9 min, and that of Chlorella spp. was reached almost 100% at 28 kV,30 Hz and 10 min. But the inactivation rate of Platymonas spp. was reached to 100% at 30 kV,30 Hz and 15 min. So, Platymonas spp. was the most difficult to inactivate, followed by Chlorella spp. as the second most difficult and Nitzschia closterium f.minutissima as the third. The Chrysophyta spp. was the easiest to inactivate. The differences in the difficulty of inactivation depend on cell species and cell volume of different algae. The inactivation rates of Chlorella spp. with the separate oxygen, air, argon and nitrogen gas bubbling were found to be 98.2%,93.4 %,85.5% and 76.8% respectively. Therefore, it can be concluded that the effectiveness in the inactivation rate was found to be in the order of oxygen> air> argon> nitrogen. This was because only a small extent of'O,'H, O3 and other active substances were produced in both gas and liquid phases while introducing air and therefore the inactivation rate was not as high as oxygen bubbling. On the other hand, the inactivation rate of algae was higher with argon than with nitrogen because the ionization energy of argon was lower than that of nitrogen leading to produce more high energy electrons when argon gas was used. In addition, argon is a noble gas so it is more difficult to capture an electron than the nitrogen gas. Therefore, the high energy electrons generated in the discharge can collide with water molecules to produce more hydroxyl radicals and hydrogen atoms.Finally, mechanisms of active substances and free radicals were investigated. On the basis of these mechanisms, algae inactivation mechanisms were also investigated under various pulse parameter conditions. It was found that the electric fields, ultra-violet radiations, OH radicals, ozone and hydrogen peroxide were the main factors for the inactivation of algae. Among these three algae inactivation factors, the amount of active substances and free radicals (OH radicals, O radicals, ozone and hydrogen peroxide) is the most important for the inactivation of algae. When the algae inactivation rate by nitrogen bubbling is assumed to be due to only electric field and ultraviolet radiation, the estimated algae inactivation rates due to only active substances and free radicals for oxygen, air and argon gases bubbling are 21.8%, 16.6% and 8.7% respectively.
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