(506f) Application of Ozonation, Electrolysis, and UV for Seawater Treatment

APPLICATION
OF OZONATION, ELECTROLYSIS, AND UV

FOR
SEAWATER TREATMENT

Introduction

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

Ozonation
process

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^- (k_O3,Br– = 160 M^-1s^-1)
is
50,000 times faster than that with Cl^- (k_O3,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
process

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

UV
process

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.

Conclusion

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.

Reference

1. Jung, Y., Yoon,
Y., Hong, E., Kwon, M., and Kang, J. (2013), "Inactivation characteristics of
ozone and electrolysis process for ballast water treatment using B. subtilis
spores as a probe," Marine Pollution Bulletin 72, pp. 71-79

2. Jung, Y., Hong,
E., Yoon, Y., Kwon, M., and Kang, J. (2014), "Formation of bromate and chlorate
during ozonation and electrolysis in seawater for ballast water treatment,"
Ozone: Science & Engneering 36, pp. 515-525

3. Jung, Y., Yoon,
Y., Kwon, M., Roh, S., Hwang, T., and Kang, J. (2015), "Evaluation of energy
consumption for effective seawater electrolysis based on the electrodes and
salinity," Desalination and Water Treatment, in press

4. Cho, M., Chung,
H.M., Yoon, J. (2003), "Quantitative evaluation of the synergistic sequential
inactivation of Bacillus subtilis spores with ozone followed by chlorine,"
Environmental Science and Technology 37, pp. 2134–2138.

5. Jung, Y.J., Yoon,
Y., Pyo, T.S., Lee, S., Shin, K., and Kang, J. (2013), "Evaluation of
disinfection efficacy and chemical formation using MPUV ballast water treatment
system (GloEn-Patrol^TM)," Environmental Technology 33, pp.
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