Department of Urban and Environmental Engineering(Environmental Science And Engineering)The harmful algal blooms (HABs) often involve serious economic loss of marine resources and contribute to pollution of coastal areas by causing the mass mortality of natural and aquaculture fish and shellfish. While spraying activated clay is a common practice to mitigate damage from red tide, it is not strong enough to completely control the red tide bloom. The use of chemical oxidants, frequently employed for disinfection in water treatment processes, is a potential method to reduce the risk from red tide for inland fish farms. In this study, chemical disinfectants were investigated as alternative technologies to control red tide in terms of the removal efficiency of C. polykrikoides, formation of byproduct, and toxicity to fish.
The first part of this study investigated the removal of C. polykrikoides by various oxidants including ozone (O3), permanganate (MnO4-), chlorine (Cl2) and hydrogen peroxide (H2O2) according to cell density and oxidant dose. O3 showed a much higher efficiency for the C. polykrikoides removal than the other oxidants under identical experimental conditions. Disinfectants effectively inactivate the C. polykrikoides in the order of O3 > MnO4- > Cl2 > H2O2. TRO produced from O3 and Cl2 mostly converted into HOBr due to the presence of bromide ion (Br-) in seawater. The seawater in the presence of C. polykrikoides decayed faster than in the absence of C. polykrikoides, indicating that TRO decay and HOBr production can be affected by the characteristics of water quality. Ozonation results in a much higher formation of bromate (BrO3-) than treatments with Cl2, MnO4-, and H2O2. The acute toxicity of MnO4-, Cl2, and H2O2 to juvenile red sea bream was reached at 72-h LC50 values of 0.39, 1.32, and 102.61 ppm, respectively.
Secondly, the aim of this study to evaluate the removal efficiency of C. polykrikoides by O3 according to various factors affecting the efficiency such as cell density, oxidant dose, humic acid concentration, carbonate concentration, pH, and temperature. The removal of C. polykrikoides by O3 was rapid, and appeared to be caused by initial ozonation, and then slow reaction, in which Br- was oxidized to HOBr. Low pH and temperature increased the removal efficiency. Thus, the C. polykrikoides removal efficiency with a low pH, and low temperature, and a high O3 dose increased as the in exposure to O3 increased. The BrO3- concentration of less than 10.0 ppb (WHO, 2011) can only be maintained at the O3 dose of less than 1.0 ppm.ope
