Total Residual Oxidant Research Articles

Evaluating protein skimmer performance in a commercial seawater recirculating aquaculture system (RAS)

Combining protein skimming with ozone (O3) is a common method for removing microparticles in recirculating aquaculture systems (RAS). Nevertheless, there is a limited number of studies that have validated protein skimming's performance at a commercial scale. Additionally, variations in protein skimmer designs and operational variables may yield different performance outcomes. In the present study, the performance of two types of full-scale protein skimmer (S1 and S2) were compared and evaluated under two levels of hydraulic retention time (HRT) (1.8 and 2.2 min) and three levels of O3 doses (0, 7, and 14 g O3/kg feed) in a commercial seawater RAS facility. Samples from the inlet and outlet of the protein skimmers were collected at each combination of operational variables. They were analysed for several relevant water quality parameters to quantify the treatment efficiency. O3 dose significantly improved water quality and reduced the numbers of microparticles and bacterial activity in a single pass. Besides that, doses as high as 14 g O3/kg feed significantly increased total residual oxidant (TRO) concentration. Additionally, an increase in HRT exerted a moderate effect on removing microparticles and a strong effect on redox potential (ORP) and TRO. Finally, the type of protein skimmer only affected the ORP, causing no significant changes to other water quality metrics. The correlations between the investigated water quality parameters defined a clear pattern of the ongoing processes and particle characteristics. Overall, the results demonstrated that protein skimming combined with carefully selected O3 doses can improve general water quality and control critical factors such as bacterial activity and microparticles under commercial operations.

Elucidating the relation between residual oxidants formation behavior and quinolone antibiotic removal efficiency on catalytic ozonation in seawater-based aquaculture wastewater

Owing to the recent developments in aquaculture, the discharge of antibiotic-laden wastewater has become a growing concern. Quinolone antibiotics, which cause central nervous system and cardiac conduction disorders, are commonly detected in aquaculture wastewater. To remove antibiotics in wastewater, the ozone process is a commonly utilized method. In the process of seawater ozone treatment, bromide ions (Br−) produce total residual oxidant, such as hypobromous acid (HOBr). However, the oxidation potential of the total residual oxidant (TRO) is lower than that of ozone, and the presence of inhibitors, such as ammonium, and dimethyl sulfide can reduce the residual oxidant concentration. Therefore, the seawater ozone process requires a higher ozone injection than freshwater. In this study, various factors affecting TRO production were evaluated to determine the optimal ozone-catalytic process using gamma-alumina and elucidate the relationship between TRO production and antibiotic removal efficiency for efficient seawater-based wastewater treatment. The removal rates of representative quinolone antibiotics, such as enrofloxacin, ciprofloxacin, and oxolinic acid, were evaluated using liquid chromatography-fluorescence detection. The addition of a catalyst enabled the reaction to maintain a higher TRO concentration and improves antibiotic removal efficiency, even in the presence of ammonium in the aquaculture wastewater with the same ozone dosage applied. The results reveal that the ozone-catalyst process contributes to increased TRO production to suppress bacterial growth and effectively remove non-degradable organic matter, such as antibiotics, in seawater-based wastewater treatment.

Study on total residual oxidant decay in long-distance seawater intake pipeline of cooling system

Electrochlorination is often used for biofouling control along the water intake pipeline of seawater cooling system, but with the increasing of pipeline length, this process needs to be further improved. In this study, the dynamic circulation and field pilot test were used to simulate the long-distance seawater intake pipeline, investigating total residual oxidant (TRO) decay and its influencing factors by comparing the bench test. The results showed that intermediate dosing could increase terminal TRO, but also reduce the CT value, resulting in decline of local inactivation effect. The initial concentration of dynamic cycle test was higher than that of bench test under the same terminal TRO, and the difference value between the two was affected by holding time. When the initial concentration was greater than 8.5 mg L−1, TRO decay rate was proportional to the seawater flow rate and inversely proportional to the initial concentration. The initial concentration of 8.5–10 mg L−1 could meet TRO decay requirement under 3 h holding time, and the dosing concentration could be reduced to 6 mg L−1 when the temperature was low. The results provided important guidance for the actual operation of biofouling control in long-distance water intake pipelines of cooling system.

Removal of the red tide dinoflagellate Cochlodinium polykrikoides using chemical disinfectants

For decades, red tide control has been recognized as necessary for mitigating financial damage to fish farms. Chemical disinfectants, frequently used for water disinfection, can reduce the risk of red tides on inland fish farms. This study systematically evaluated four different chemical disinfectants (ozone (O3), permanganate (MnO4−), sodium hypochlorite (NaOCl), and hydrogen peroxide (H2O2)) for their potential use in inland fish farms to control red tides by investigating their (i) inactivation efficacy regarding C. polykrikoides, (ii) total residual oxidant and byproduct formation, and (iii) toxicity to fish. The inactivation efficacy of C. polykrikoides cells by chemical disinfectants from highest to lowest followed the order of O3 > MnO4− > NaOCl > H2O2 for different cell density conditions and disinfectant doses. The O3 and NaOCl treatments generated bromate as an oxidation byproduct by reacting with bromide ions in seawater. The acute toxicity tests of the disinfectants for juvenile red sea bream (Pagrus major) showed that 72-h LC50 values were 1.35 (estimated), 0.39, 1.32, and 102.61 mg/L for O3, MnO4−, NaOCl, and H2O2, respectively. Considering the inactivation efficacy, exposure time of residual oxidants, byproduct formation, and toxicity toward fish, H2O2 is suggested as the most practical disinfectant for controlling red tides in inland fish farms.

Improvement in compliance of ships' ballast water discharges during commissioning tests

The number of ships installing ballast water management systems (BWMS) has risen steeply since the Ballast Water Management Convention entered into force. Since June 2022, biological testing is required during commissioning to verify compliance with the Convention. Data from 676 tests (from 2019 to 2022) show substantial improvement over time: the failure rate decreased from ~20 % to ~6 %. Notably, nearly all failures occurred in the largest size class of organisms (≥50 μm). Interestingly, proxy measurements suggest that high concentrations of living organisms in uptake water did not cause the failures. Also, failures determined using “indicative” analysis (here, adenosine triphosphate, ATP) were typically not confirmed by “detailed” analysis (microscopy), suggesting that ATP limits are over-precautionary. Finally, discharges containing high levels of Total Residual Oxidants (TRO) decreased over time. These data highlight the need for ongoing testing—focusing at least on organisms ≥50 μm—to minimize environmental risks from organisms transported in ships' ballast water.

Ballast water treatment by ozone nanobubbles

AbstractBACKGROUNDThe uncontrolled discharge of untreated ballast water, which is essential for a ship's optimal operation, is one of the major causes of sea environmental pollution by shipping. Among the disinfection methods available for ballast water treatment, ozonation is a commonly used method, but its performance is limited by the rapid ozone auto‐decomposition rate. Nanobubbles (NBs) technology has attracted much scientific interest as it is characterized by a long residence time in the aqueous phase and a high surface area; therefore, ozone nanobubbles (OzNBs) are expected to enhance disinfection capacity and residual activity. The present study was designed to provide further insight into the inactivation of heterotrophic bacteria in saline water and to examine the use of OzNBs for disinfection of saline water.RESULTSThe survival rate of Escherichia coli (E. coli), which was used as indicator microorganism, along with the ozone consumption at different salinities (1.5, 4, 8 and 15 PSU) and bacterial concentrations (107, 106, and 105 CFU mL−1) with and without supplementation of OzNBs were investigated. The results indicated a statistical difference in the residual concentration of total residual oxidants (TRO) with the presence of OzNBs at salinity level 1.5 PSU and at 4 PSU only at the lowest bacterial content. At a low salinity and high bacterial concentration, the concentration of TRO was 6‐fold higher in the presence of OzNBs.CONCLUSIONThe salinity of water has a strong impact on the residual concentration of ozone. When salinity is increased, ozone reacts more rapidly with the bromide and chloride ions. The use of OzNBs exhibited a greater disinfection performance and higher residual activity. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Toxicity evaluation of chlorinated natural water using Photobacterium phosphoreum: Implications for ballast water management

Chlorination of ballast water could produce harmful disinfection by-products (DBPs) and total residual oxidants. The International Maritime Organization calls for toxicity testing of discharged ballast water with fish, crustacea and algae to reduce the risk, but it is difficult to evaluate the toxicity of treated ballast water in a short time. Therefore, the purpose of this study was to analyze the applicability of luminescent bacteria to the assessment of residual toxicity of chlorinated ballast water. The toxicity unit for all treated samples were higher for Photobacterium phosphoreum than for microalgae (Selenastrum capricornutum and Chlorella pyrenoidosa), after adding neutralizer, all samples showed little effect on the luminescent bacteria and microalgae. For the DBPs, except for 2,4,6-Tribromophenol, Photobacterium phosphoreum could produce more sensitive and rapid test results than other species, the results in Photobacterium phosphoreum showed that the toxicity of DBPs in order of: 2,4-Dibromophenol > 2,6-Dibromophenol > 2,4,6-Tribromophenol > Monobromoacetic acid > Dibromoacetic acid > Tribromoacetic acid, and most binary mixtures (aromatic DBPs and aliphatic DBPs) presented synergistic effects based on the CA model. The aromatic DBPs in ballast water deserve more attention. In general, for ballast water management, the use of luminescent bacteria to evaluate the toxicity of treated ballast water and DBPs is desirable, this study could provide beneficial information for enhancing ballast water management.

The effect of filtration and electrolysis on ballast water treatment

Ballast water is required to be treated to reach D-2 standard according to the International Maritime Organization (IMO). Filtration and electrolysis are important technologies for ballast water management systems (BWMS). However, the clogging of filters during the operation of BWMS leads to significant system trips and downtime. In this paper, a mesoscale experiment system was set up and the effects of filtration on ballast water treatment using electrolysis were investigated. The results showed pore size of filter had significant effect on filtration efficiency, total residual oxidant (TRO) decay and inactivation efficacy. The filtration efficiency of filter with 50 μm pore size was over 80% for ≥50 μm organisms. For 200–2000 μm pore sizes (200 μm, 500 μm, 1000 μm and 2000 μm), the filtration efficiency was less than 10%. The effect of filtration on TRO depended on pore size and holding time. Our results indicated that increasing pore size of electrolysis may not a good way for new BWMS design. If electrolysis was applied alone to treat ballast water, higher initial TRO concentration was needed. In our investigation, the initial TRO was more than 12 mg/L.

Toxicity effects of ballast water treated by electrolysis process under different salinity conditions on plankton

The impacts of total residual oxidant (TRO) on ecological health have been of great concern due to their widespread ubiquity in disinfection process. However, little is known about the effects of discharge water produced by electrolysis-based Ballast Water Management System (BWMS) on the toxicity of plankton. The present study was conducted to investigate the effects of TRO at discharge-relevant concentrations on plankton under different salinity conditions. The results showed that brackish water always gave the highest TRO concentrations, which is related to the natural organic matter (NOM) content. TRO exposure significantly (p<0.01) inactivated ≥ 50 μm and 10–50 μm organisms. Biodiversity dropped significantly after TRO exposure, especially in brackish water conditions. Meanwhile, the discharge water still has an impact on plankton after neutralization. Therefore, proper disposal of TRO generated by BWMS is significant to maintain ecological balance and protect native biodiversity.

Synergistic effect of total residual oxidants and microplastics on seawater disinfection: An ecotoxicity study

As fishery products are increasingly imported, a treatment system is required to prevent the introduction of invasive pathogens into domestic marine environments. In this study, disinfection efficiency was estimated through ozone treatment of Edwardsiella tarda, Streptococcus mitis, and Vibrio harveyi in seawater. The ecotoxicity of total residual oxidants (TROs) generated during the treatment process was evaluated based on the degree of bioluminescence variation in Aliivibrio fischeri. The interaction with microplastics according to their type and weight was compared in terms of bacterial inactivation and ecotoxicity, along with the level of TROs. In addition, morphological and chemical changes in the ozonated microplastics were analyzed through scanning electron microscopy and Fourier transform infrared spectroscopy. The optimal ozone doses for the inactivation rate of &gt; 99.5% for E. tarda, S. mitis, V. harveyi, and the mixed bacteria were 3, 4, 4, and 5 mg O3/min, while the bioluminescence inhibition rates by TROs were 59.7%, 58.8%, 49.7%, and 54.4%, respectively. Furthermore, there was no considerable difference in bacterial disinfection in the presence of microplastics, but the concentrations of TROs and bioluminescence inhibition rates were slightly lowered owing to the consumption of oxidants.

Resilience of a microphytobenthos community from the Severn Estuary, UK, to chlorination: A mesocosm approach

The Severn Estuary is a large macrotidal estuary which includes an extensive mudflat with microphytobenthos (MPB) playing a key role in the ecosystem. This study evaluated the impact of chlorination at two different dosing levels (0.05 and 0.5 mg/l as total residual oxidants, TRO, representative of potential concentrations in the mixing zone and within the cooling water systems of a power station) on a MPB community representative of the Severn Estuary. Biomass and diversity were not negatively impacted while physiology was partially affected at the beginning of the experiment, and it recovered towards the end of the experiment. Further investigations for diversity are needed to consolidate our findings. In conclusion our results show that MPB is resilient to chlorination up to a concentration of 0.5 mg/l which is much higher (>10 times) than what might be expected near the chlorinated discharges for most coastal power stations.

Comparison of the bacterial viability assessments for the disinfected quarantined water along with an effect of total residual oxidants.

The water discarded from the quarantine station inspecting aquatic products can be served as an influx channel of invasive microorganisms to our own ecosystem. This study thus compared the viability of three different pathogenic bacteria (Escherichia coli, Vibrio harveyi, and Enterococcus faecalis) in either seawater or freshwater after their disinfection. For that, they were treated by ozonation (2.08mM of ozone), ultraviolet irradiation (UVC-254), or thermal treatment (90℃) for 10min, during which their resultant viability was monitored using colorimetric ATP assay, colony counting, and real-time quantitative RT-PCR. From this, ATP measurement and real-time quantitative RT-PCR have proved to be a much stronger correlation built in the fraction of each of their assays versus the colony counting, although they differed in the type of disinfection implemented. Especially, ATP assay was the most sensitively influenced by high levels of total residual oxidants(TRO) undesirably produced during the ozonation of V. harveyi and E. faecalis in seawater, although easily and shortly measured within 1h, with higher accuracy. Aside from that, the real-time quantitative RT-PCR had a stronger correlation versus either that of seawater ozonation or thermal treatment. It is decided referring to measurement time and convenience in the field that ATP assay can be more reliably used in bacterial cell viability measurement in the quarantine after the ozonation in seawater to specifically allow the bacterial deactivation, not to overwhelmingly produce TRO due to the residual ozone provided.

Ballast Water Treatment Performance Evaluation under Real Changing Conditions

We conducted a shipboard ballast water test using seawater of extreme turbidity collected from Shanghai Port (China) (&gt;300 mg total suspended solids (TSS)/L), and normal seawater collected in other ports (&lt;100 mg TSS/L). All three types of International Maritime Organization (IMO)-approved ballast water management system (BWMS) tested failed to properly operate because of filter clogging or insufficient generation of oxidants under near-fresh water conditions with extremely high concentration of suspended solid during ballasting. It was also found that the number of microorganisms increased with longer ballast water retention time, with higher numbers in the treated discharge water. The results suggest that when operating a BWMS involving a filter unit in areas with water having high concentrations of suspended solids, the filter unit should be used during ballast water discharge, rather than during ballasting. This method has the advantage of removing ≥50 µm organisms at discharge that could not be removed by a filter during ballasting. For ballast water retained for long storage times, the results suggest the use of BWMSs involving UV units or electrolysis during deballasting. In addition, BWMSs involving electrolysis units provide the opportunity to maintain residual total residual oxidant (TRO) levels, using a partial ballast tank. Although the BWMSs tested are a small subset of the large number of IMO-approved BWMSs, the results demonstrate that there is a significant gap between the technology currently available and capacity to meet IMO and US Coast Guard standards.

Formation of emerging iodinated disinfection by-products during ballast water treatment based on ozonation processes

Disinfection by-products (DBPs) generated by ballast water treatment pose a potential threat to marine environment which aroused widespread concern. In recent years, emerging iodinated DBPs have attracted widespread attention because of their stronger cytotoxicity and genotoxicity than brominated/chlorinated DBPs. In this study, the effects of different natural organic matter species, total residual oxidant (TRO) concentrations, storage time, temperature, pH, bromide and iodide concentrations on the generation of iodinated trihalomethanes (I-THMs) during ozonation process of ballast water were investigated. The results showed that bromochloroiodomethane and diiodochloromethane (DICM) were not detected under all conditions during ozonation of humaic acid (HA). Different kinds of precursors had a significantly effect on the formation of I-THMs. For algal cells as precursor, DICM were detected (1.22 μg/L), while DICM were not detected from oxidation of 1,3-etonedicarboxylic acid, fulvic acid (FA), phenol, resorcinol, hydroquinone and HA as precursors. The yields of I-THMs from oxidation of algal cells, FA and phenol were higher than other precursors. Linear relationships were observed between the formation of I-THMs and TRO concentrations. The yields of I-THMs reached a peak at 48 h (180 μg/L) after ozonation treatment of ballast water, and then decreased with storage time extension. An increase in temperature enhanced the formation of dibromoiodomethane and bromodiiodomethane, while wakened the formation of iodoform and dichloroiodomethane. The formation of I-THMs was complicatedly affected by different pH values in the range from 4 to 9. The more bromide concentrations, the more brominated I-THMs were formed. The concentrations of I-THMs increased with increasing iodide concentrations, and low concentrations of iodide had greater effect on the production of I-THMs than high concentrations of iodide.

Enhancement of ozonation of seawater-based wastewater containing pharmaceutical compounds by total residual oxidants: Salinity, ammonia, and organic matter

The ozonation process has recently been proposed as an effective treatment method for removing pharmaceutical compounds from seawater-based wastewater discharged from fish farms or hatcheries. Seawater ozonation can lead to the production of secondary stable oxidants such as bromine (HOBr/OBr−), which can be expressed as a total residual oxidants (TRO) owing to the high reaction rate constant with respect to ozone and bromide ion. TRO formation depends on water quality, in terms of aspects such as salinity, ammonia, and organic matter content; therefore, it is important to understand the variations in the ozone chemistry of bromide-containing water by considering the factors influencing TRO formation. In addition, the removal of pharmaceutical compounds in bromide-containing water should be evaluated owing to the different oxidation efficiency between TRO and pure aqueous ozone. This study aims to investigate the factors influencing the formation of TRO and the removal of pharmaceutical compounds during the ozonation of bromide-containing water. The results show that salinity increased the TRO formation rate by 6%, while ammonia and organic matter decreased the TRO formation rate by 51% and 39%, respectively. The removal efficiency of pharmaceutical compounds by TRO was higher compared with that by only ozone. Ammonia and organic matter inhibited the ozonation of bromide-containing water. These results indicate that TRO can be broadly used in the field of seawater ozonation; however, additional case studies on the removal of pharmaceutical compounds from seawater-based wastewater are needed to elucidate the oxidation mechanisms of various TRO.

Long-term Changes of Disinfection Byproducts in Treatment of Simulated Ballast Water.

This study investigated the changes in concentrations of haloacetic acids (HAAs) and haloacetonitriles (HANs) as disinfection byproducts (DBPs) for different storage times (as long as 20 days) and temperatures (5 to 20°C). A ship’s voyage after treatment of its ballast water with active substances was considered. The HAA showed a clear trend of increasing concentration only with storage time, especially for dibromoacetic acid (DBAA). Dissolved organic nitrogen concentration was increased by the decomposition of dead organisms at 10 days, and then reacted with the remaining total residual oxidants, resulting in increased concentration of DBPs. An environmental risk assessment indicated that DBAN and monochloroacetic acid (MCAA) could have a negative impact on the marine environment. This study suggests that, because all international vessels must have a ballast water management system installed by September, 2024, the conc e ntra tio ns of DBPs, especially DBAN, MCAA, and DBAA, should be monitored in the waters at major international ports.

Expression of circadian rhythm-related hormones in juvenile olive flounders (Paralichthys olivaceus) following exposure to total residual oxidant

ABSTRACT Some substances used in aquaculture may be toxic to fish depending on their concentration. Here, we investigated the changes in circadian rhythm in olive flounders (Paralichthys olivaceus) exposed to total residual oxidant (TRO), a substance produced by the reaction of O3 with Br− ions, at two different concentrations (20 and 40 μg/L) for different periods (2, 6, 10, 14, 18, 22, 30, and 42 h) to identify the optimum concentration of TRO for aquaculture. We analysed mRNA expression and the activity of circadian rhythm-related hormones (period 2 (Per2), cryptochrome 1 (Cry1), melatonin receptor 1 (MT-R1), and melatonin) in the diencephalon and plasma. The levels of circadian rhythm-related hormones in fish exposed to 40 μg/L TRO were significantly lower than those in the other groups. However, there were no significant changes in circadian rhythm-related hormone levels in fish exposed to 20 μg/L TRO. These results indicate that 20 μg/L TRO does not significantly affect the circadian rhythms in the olive flounder and that it is suitable for sterilisation purposes in aquaculture.

Preliminary test of ultrasonically disinfection efficacy towards selected aquaculture pathogens

The disinfection potential of ultrasound was evaluated for fish production in aquaculture systems by investigating the sensitivity of a wide range of different model organisms representing different taxa of common fish pathogens such as bacteria or parasites and algae at a size range of a few micrometers to 1 mm. Therefore, dose-dependent inactivation rates depending on consumed energy (kJ/L) at low (20 kHz, LFUS) and high (850 kHz, HFUS) frequency ultrasound were determined in in-vitro tests using a laboratory-scaled set-up for three zooplankton and five algae species as well as for heterotrophic marine and fresh water bacteria separated from recirculating aquaculture system water. LFUS and HFUS were effective against most tested eukaryotic organisms even at low energy input and the dose-dependent inactivation could be well described by functions of an exponential decay. HFUS seemed to be more effective compared to LFUS regarding the inactivation of bigger-sized algae species (≥14 μm). Within the tested zooplankton species, no systematic correlation between the size of the organisms and the effect of ultrasound frequency on the inactivation efficiency became apparent, suggesting a strong species-dependency likely caused by additional relevant morphological and physiological factors. Our data also suggest ultrasound treatment alone to be insufficient for an efficient bacterial reduction as LFUS and HFUS did not significantly reduced total viable counts even at high energy inputs up to 144 kJ/L. Measurements of total residual oxidants (TRO) revealed a higher formation potential of sonochemically-produced oxidants for HFUS compared to LFUS as well as an increased TRO accumulation at saline waters most likely due to the formation of more persistent halogenated secondary oxidants. Hence, particularly when HFUS is applied in a marine process chain total residual oxidants should be monitored in order to avoid deleterious impacts on animal health.

Use of multiple regression models for predicting the formation of bromoform and dibromochloromethane during ballast water treatment based on an advanced oxidation process

Disinfection byproducts (DBPs) generated by ballast water treatment have become a concern worldwide because of their potential threat to the marine environment. Predicting the relative DBP concentrations after disinfection could enable better control of DBP formation. However, there is no appropriate method of evaluating DBP formation in a full-scale ballast water treatment system (BWTS). In this study, multiple regression models were developed for predicting the dibromochloromethane (DBCM) and bromoform (TBM) concentrations produced by an emergency BWTS using field experimental data from ballast water treatments conducted at Dalian Port, China. Six combinations of independent variables [including several water parameters and/or the total residual oxidant (TRO) concentration] were evaluated to construct mathematical prediction formulas based on a polynomial linear model and logarithmic regression model. Further, statistical analyses were performed to verify and determine the appropriate mathematical models for DBCM and TBM formation, which were ultimately validated using additional field experimental data. The polynomial linear model with four variables (temperature, salinity, chlorophyll, and TRO) and the logarithmic regression model with seven variables (temperature, salinity, dissolved oxygen, pH, turbidity, chlorophyll, and TRO) exhibited good reproducibility and could be used to predict the DBCM and TBM concentrations, respectively. The validation results indicated that the developed models could accurately predict DBP concentrations, with no significant statistical difference from the measured values. The results of this work could provide a theoretical basis and data reference for ballast water treatment control in engineering applications of emergency BWTSs.

Effects of Total Residual Oxidant on Oxidative Stress in Juvenile Olive Flounder Paralichthys Olivaceus

ABSTRACTWe investigated oxidative stress in olive flounder Paralichthys olivaceus induced by exposure to total residual oxidant (TRO) at three different concentrations (20, 40, and 60 μg/L) over 14 days to determine concentrations of TRO appropriate for aquaculture. We analyzed survival rate, antioxidant enzyme (superoxide dismutase [SOD] and catalase [CAT]) mRNA abundance, and heat shock protein 70 (HSP 70) expression (mRNA and protein) in the gills and liver, in addition to levels of hydrogen peroxide in plasma. Furthermore, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) was used to measure apoptosis in the gills. All flounder exposed to 40 and 60 μg/L TRO died by day 14 and day 7, respectively. Antioxidant enzyme mRNA and HSP 70 expression (mRNA and protein) in the gills and liver significantly increased with increasing TRO concentration but decreased on day 14 in the 20 μg/L TRO group. In the gills, TUNEL-positive cells increased, and histological modifications were observed at 40 and 60 μg/L TRO; however, these differences were not observed in the 20 μg/L TRO group. These results indicate that 20 μg/L TRO does not significantly affect olive flounder survival after 14 days or the indicators of oxidative stress between 7 and 14 days. This target concentration of TRO (TRO 20) may be suitable for water treatment in recirculating aquaculture systems containing olive flounder.