APPLICATION OF OZONATION, ELECTROLYSIS, AND UV
FOR SEAWATER TREATMENT
Seawater treatment has been needed in various industries, including ballast water treatment, marine aquaculture, aquaria, and so on. Ballast water provides stability to oceangoing vessels when they have no cargo; however, it has been recognized as a major vector in the transfer of non-native marine microorganisms in the world's ocean, as such water contains various invasive marine organisms including plankton, bacteria, and viruses. The International Maritime Organization (IMO) has established international legislation for ballast water and sediment to protect the marine ecosystem. In case of marine aquaculture and aquaria, pathogenic organisms such as bacteria and viruses can cause disease in hatchery fish, aquarium fish, and so on. In fact, the main objective of seawater treatment is to eliminate pathogenic organisms or invasive species in seawater.
Ozonation, electrolysis, and UV have been used for seawater treatment as well as drinking water and wastewater. Since seawater contains a high volume of ions, especially Cl- and Br-, the mechanism of seawater treatment is very different to that of fresh water. The aim of this study is to evaluate the three technologies including ozonation, electrolysis, and UV for seawater treatment. Various factors were compared including the formation of residual oxidant, the inactivation of microorganisms, and the formation of by-products.
Materials and methods
Two types of seawater were used in this study: natural seawater collected from the East Sea, Gangneung, south of Korea and artificial seawater made by seasalt (Sigma Aldrich, USA). The ozone was generated with ozone generators (Ozonia¢ç, LAB2B, Switzerland) using highly pure oxygen. Electrolysis used two types of electrodes (i.e., grid-shaped Pt/Ti and IrO2/Ti electrodes [3 cm • 3 cm] SAMSUNG DSA, Korea). The anode and cathode were made of the same material and placed horizontally and parallel with a distance of 0.1 cm between the two in the reactor. For UV experiment, a collimated beam system was used with LP UV (254 nm).
The salinity and conductivity were measured using an Orion 115A+ (Thermo Electron Corporation, USA). Ozone was measured by indigo colorimetric method at 600 nm. The residual oxidants, including chlorine and bromine, were measured by a DPD (N,N-diethyl-p-phenylenediamine) colorimetric method with a DR2500 (HACH, USA) spectrophotometer in units of mg Cl2/L at 530 nm. The target microorganism used in this study was B. subtilis spores. The B. subtilis was obtained from the American Type Culture Collection (#6633). The spores were prepared according to Cho et al. (2003). The concentrations of inorganic by-products, BrO3- and ClO3-, were measured using an ion chromatography (IC) system (Dionex, USA).
Results and discussion
Although ozone is an effective disinfectant, its reaction mechanism in seawater is quite different from that in fresh water. The main difference being that seawater contains high levels of Br- which rapidly react with ozone to form a stable oxidant, bromine [HOBr/OBr-]. Since the rate constant of ozone with Br- (kO3,Br– = 160 M-1s-1) is 50,000 times faster than that with Cl- (kO3,Cl– = 0.003 M-1s-1), the bromine is the main oxidant in seawater ozonation. In fact, ozone is rapidly decomposed to generate bromine that can be active in the inactivation of marine organisms. Seawater ozonation generated bromine linearly at a rate of 0.61 mg as Cl2/L∙min at an ozone dosage of 1 mg/L∙min. For the inactivation of B. subtilis spores, the CT value (concentration of disinfectant x contact time) of bromine was 70 mg°¤min/L for 1 log inactivation. Regarding the formation of inorganic by-products, the BrO3- was produced after 5 mg/L of applied ozone dose despite the high concentration of Br- (63 mg/L). This was because of the low availability of residual ozone for the reaction of ozone with OBr-, since the majority of ozone was consumed by the high concentration of Br- in seawater. The ClO3- was not detected (MDL < 1.03 µg/L) due to the rather low reaction rate of Cl- and ozone.
Figure 1. Ozone decomposition in seawater with various salinities (5, 15, 32 PSU); [O3]0 = 2 mg/L, pH 8, Temp. = 20 °
Electrolysis can rapidly generate several reactive substances (e.g., chlorine [HOCl/OCl-], bromine [HOBr/OBr-], ozone molecules, hydroxyl radicals [OH•], etc.) due to the abundance of ions in seawater. Since these active substances can be simultaneously present, it is difficult to isolate all individual species accurately. Therefore, the term total residual oxidant (TRO) is used frequently to refer to residual chlorine and bromine. For the electrolysis process, two types of electrodes, Pt/Ti and IrO2/Ti, were tested for the formation of TRO and by-products (BrO3- and ClO3-). Electrolysis using Pt/Ti and IrO2/Ti produced TRO linearly with rates of 16.31 and 17.75 mg as Cl2/L∙min, respectively. The freshwater condition (Cl- = 100 mg/L) showed a different tendency, in which TRO formation rates were 5.23 and 0.56 mg as Cl2 /L for IrO2/Ti and Pt/Ti, respectively. The high-salinity water had low effect on electrodes for TRO formation by electrolysis. Even though the TRO formation rates were almost identical for the two electrodes, the formation of by-product was clearly different. The Pt/Ti electrode generated 5 times more BrO3- than IrO2/Ti electrode.
Figure 2. Formation of TRO by IrO2/Ti and Pt/Ti in 32 and 4 PSU of salinity
There are two types of UV for water treatment: LP-UV for 254 nm and MP-UV for multi wavelength. Neither UV processes produce any residual oxidant, unlike the ozonation and electrolysis. The UV can attack microorganism DNA directly as a physiochemical inactivation. By UV process, the inactivation efficiency of B. subtilis spores in seawater showed the same tendency as that in fresh water. Since the inactivation mechanism of the UV process in seawater is the same as that in fresh water, the UV transmittance is the main factor for an efficient UV process. The UV process should be used with a filter for increasing UV transmittance.
This study was performed to evaluate three technologies for seawater treatment: ozonation, electrolysis, and UV. The ozonation and electrolysis processes produced TRO, mainly bromine, while there was no formation of residual oxidant in the UV process. Despite the high concentration of bromide ion (63 mg/L), the ozonation process formed bromate after 5 mg/L of applied ozone dose. In the electrolysis process, the formation of TRO and by-products was affected by electrode and electrolysis conditions. For the UV process, the inactivation of B. subtilis spore in seawater was the same as in fresh water. In conclusion, there are different characteristics of ozonation, electrolysis, and UV technologies, and their application should be determined considering conditions and intended use of the water being treated.
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