Acute and chronic toxicity of chlorine dioxide (ClO2) and chlorite (ClO2-) to rainbow trout (Oncorhynchus mykiss)

Effect of oral administration of chlorine dioxide on hematological, physiological parameters and intestinal microbiota in a murine model

Chlorine dioxide (CD) is a broad-spectrum disinfectant agent used to disinfect food products and, more recently, to treat diseases such as avian influenza and COVID-19 due to its potent antimicrobial properties. However, excessive use of chlorine dioxide can cause adverse health effects. While numerous in vivo studies have evaluated its toxicity CD's toxicity, few have investigated its impact on the intestinal microbiota. This study assesses the effects of oral administration of CD on hematological and physiological parameters, and the intestinal microbiota in a murine model. CD was produced at a concentration of 16578.62 mg/L after 7 days of reaction. Results showed a significant antimicrobial effect on dietary yeasts compared to probiotics. There were significant changes in the percentages of Firmicutes (CD 81.9-87.1%, control 63.05%) and Bacteroidetes (DC 3.4-4.5%, control 22.5%) in the CD administered groups compared to the control group. CD exposure showed toxicity in hematological parameters. Additionally, consuming CD for 90 days at 10 mg/kg body weight caused colon and cecum damage and decreased the rats' weight. These findings indicate than even low doses of CD can negative effect on the microbiota, the morphology of the cecum and colon, and body weight, suggesting that prolonged consumption should be avoided.

Insight mechanism of magnetic activated catalyst derived from recycled steel residue for black liquor degradation

The present work deals with developing a method for revalorizing steel residues to create sunlight-active photocatalysts based on iron oxides. Commercial-grade steel leftovers are oxidized under different combinations of pH and temperature (50-90 °C and 3 ≥ pH ≤ 5) in a low energy-intensive setup. The material with the highest production efficiency (yield > 12%) and magnetic susceptibility (χm = 387 × 10-6 m3/kg) was further explored and modified by diffusion of M2+ (Zn and Co) ions within the structure of the oxide using a hydrothermal method to create ZnFe2O4, CoFe2O4 and combined Co-Zn ferrite. (Co-Zn)Fe2O4 displayed a bandgap of 2.02 eV and can be activated under sunlight irradiation. Electron microscopy studies show that (Co-Zn)Fe2O4 consists of particles with diameters between 400 and 700 nm, homogeneous size, even distribution, and good dispersibility. Application of the developed materials in the sunlight catalysis of black liquors from cellulose extraction resulted in a reduction of the Chemical Oxygen Demand (- 15% on average) and an enhancement in biodegradability (> 0.57 BOD/COD) after 180 min of reaction. Since the presented process employs direct solar light, it opens the possibility to large-scale water treatment and chemical upgrading applications.

The toxicity of chlorine dioxide to clownfish and its bactericidal capability to pathogenic strains of vibrio spp

Global ornamental fish transportation ranging from hours to days can produce multiple stress factors impact fish health and cause mortality. Clownfish, particularly Amphiprion ocellaris, are among the most traded saltwater ornamental fish. Vibrio includes several pathogenic strains that affect aquatic animals. Consequently, prophylactic treatment of the water or fish is recommended. In this study, six Vibrio strains including V. alginolyticus, V. parahaemolyticus and V. harveyi isolated from sick A. ocellaris and one V. harveyi strain from a sick East Asian fourfinger threadfin (Eleutheronema rhadinum) were tested for their sensitivity to a popular disinfectant, chlorine dioxide (ClO2). The results showed that 0.25 ppm ClO2 effectively suppressed five of the seven tested Vibrio strains for 24 h; however, 0.1 ppm ClO2 is safer for A. ocellaris. Meanwhile, ClO2 2.5 ppm reduced the bacterial counts to below 3.3 × 105 CFU/mL for 24 hours. The LC50 of ClO2 for A. ocellaris was 0.87 ppm at 10 min and 0.72 ppm at 24 h post treatment. Mild changes in water quality, including dissolved oxygen (DO), temperature and pH, were recorded during the trial. More research is necessary to understand the sensitivity of various aquatic animal pathogens to ClO2 and its toxicity to different aquatic animals.

Investigating the impact of chlorine dioxide in shrimp-rearing water on the stomach microbiome, gill transcriptome, and infection-related mortality in shrimp

AIMS

This study aimed to assess the effects of chlorine dioxide (ClO2) in water on whiteleg shrimp Penaeus vannamei, evaluating its impact on the stomach microbiota, gill transcriptome, and pathogens.

METHODS AND RESULTS

ClO2 was added to the aquarium tanks containing the shrimp. The application of ClO2 to rearing water was lethal to shrimp at concentrations above 1.2 ppm. On the other hand, most of the shrimp survived at 1.0 ppm of ClO2. Microbiome analysis showed that ClO2 administration at 1.0 ppm significantly reduced the α-diversity of bacterial community composition in the shrimp stomach, and this condition persisted for at least 7 days. Transcriptome analysis of shrimp gill revealed that ClO2 treatment caused massive change of the gene expression profile, including stress response genes. However, after 7 days of the treatment, the gene expression profile was similar to that of shrimp in the untreated control group, suggesting a recovery to the normal state. This 1.0-ppm ClO2 significantly reduced shrimp mortality in artificial challenges with an acute hepatopancreatic necrosis disease-causing Vibrio parahaemolyticus and white spot syndrome virus, which were added to rearing water.

CONCLUSIONS

The use of ClO2 at appropriate concentrations effectively eliminates a significant portion of the bacteria in the shrimp stomach and pathogens in the water. The results of this study provide fundamental knowledge on the disinfection of pathogens in water using ClO2 and the creation of semi germ-free shrimp, which has significantly decreased microbiome in the stomach.

Toxicity assays and in silico approach to assess the impacts of chlorine dioxide on survival, respiration and some biochemical markers of marine zooplankton

Chlorination is the common antifouling method in desalination and power plant water intake structures to control microbial and macrofouling growth. In this study, the impacts of chlorine dioxide on toxicity, metabolic activity and biochemical markers like glutathione S-transferase and catalase enzyme activity were tested using four zooplankton species (Centropages sp., Acartia sp., Oncaea sp., and Calanus sp.) collected from the Red Sea. The zooplankton species were treated with different concentrations (0.02, 0.05, 0.1, 0.2, and 0.5 mg L-1) of chlorine dioxide. Further, chlorite, the main decomposition product of chlorine dioxide, was used for molecular docking studies against glutathione S-transferase and catalase enzymes. The results indicated the LC50 range of 0.552-1.643 mg L-1 for the studied zooplankton species. The respiration rate of the zooplankton increased due to the chlorine dioxide treatment with a maximum of 0.562 μg O2 copepod h-1 in Acartia. The glutathione S-transferase and catalase enzyme activities showed elevated values in zooplankton treated with chlorine dioxide. Molecular docking of chlorite with enzymes involved in antioxidant defense activity, such as glutathione S-transferase and catalase enzyme showed weak interactions. Overall, this study yielded significant insights for understanding the effects of chlorine dioxide on the survival, metabolism, and biochemical composition of marine zooplankton.

Transcriptome and computational approaches highlighting the molecular regulation of Zacco platypus induced by mesocosm exposure to common disinfectant chlorine

Chlorine (Cl₂) is a disinfectant often used in swimming pools and water treatment facilities. However, it is released into aquatic ecosystems, where it may harm non-targeted organisms. Here, we performed a mesocosm experiment exposing Zacco platypus (Z. platypus) to biocide Cl₂ for 30 days (30 d) at two days' time points 15 days (15 d) and 30 d samples were collected. Here, Z. platypus was exposed to a sublethal concentration (0.1 mg/L) of Cl₂, and comparative transcriptomics analyses were performed to determine their response mechanisms at the molecular level. According to RNA sequencing of the whole-body transcriptome, 860 and 1189 differentially expressed genes (DEGs) were identified from the 15 d and 30 d responses to Cl₂, respectively. After enrichment analysis of GO (Gene Ontology) functions and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways, identified DEGs were demonstrated to be associated with a variety of functions, including "ion binding and transmembrane transporters". Cl₂ also induced oxidative stress in Z. platypus by increasing the levels of reactive oxygen species (ROS) while decreasing the catalase (CAT) content and the levels of solute carrier family 22 member 11 (slc22a11), Caspase-8 (casp-8), inducible nitric oxide synthase (NOS2), cytosolic phospholipase A2 gamma (PLA2G4). However, Z. platypus still allows recovery during stress suspension by increasing the expression levels of solute carrier family proteins. The GO and KEGG annotation results revealed that the expression of DEGs were related to the detoxification process, immune response, and antioxidant mechanism. Additionally, protein-protein interaction networks (PPI) and cytoHubba analyses identified sixteen hub genes and their interaction. These findings elucidate the regulation of various DEGs and signaling pathways in response to Cl₂ exposure, which will improve our knowledge and laid foundation for further investigation of the toxicity of Cl₂ to Z. platypus.

Development of benchmark datasets for text mining and sentiment analysis to accelerate regulatory literature review

In the field of regulatory science, reviewing literature is an essential and important step, which most of the time is conducted by manually reading hundreds of articles. Although this process is highly time-consuming and labor-intensive, most output of this process is not well transformed into machine-readable format. The limited availability of data has largely constrained the artificial intelligence (AI) system development to facilitate this literature reviewing in the regulatory process. In the past decade, AI has revolutionized the area of text mining as many deep learning approaches have been developed to search, annotate, and classify relevant documents. After the great advancement of AI algorithms, a lack of high-quality data instead of the algorithms has recently become the bottleneck of AI system development. Herein, we constructed two large benchmark datasets, Chlorine Efficacy dataset (CHE) and Chlorine Safety dataset (CHS), under a regulatory scenario that sought to assess the antiseptic efficacy and toxicity of chlorine. For each dataset, ∼10,000 scientific articles were initially collected, manually reviewed, and their relevance to the review task were labeled. To ensure high data quality, each paper was labeled by a consensus among multiple experienced reviewers. The overall relevance rate was 27.21% (2,663 of 9,788) for CHE and 7.50% (761 of 10,153) for CHS, respectively. Furthermore, the relevant articles were categorized into five subgroups based on the focus of their content. Next, we developed an attention-based classification language model using these two datasets. The proposed classification model yielded 0.857 and 0.908 of Area Under the Curve (AUC) for CHE and CHS dataset, respectively. This performance was significantly better than permutation test (p < 10E-9), demonstrating that the labeling processes were valid. To conclude, our datasets can be used as benchmark to develop AI systems, which can further facilitate the literature review process in regulatory science.

Superfast degradation of micropollutants in water by reactive species generated from the reaction between chlorine dioxide and sulfite

Chlorine dioxide (ClO₂) is used as an oxidant or disinfectant in (waste)water treatment, whereas sulfite is a prevalent reducing agent to quench the excess ClO₂. This study demonstrated that seven micropollutants with structural diversity could be rapidly degraded in the reaction between ClO₂ and sulfite under environmentally relevant conditions in synthetic and real drinking water. For example, carbamazepine, which is recalcitrant to standalone ClO₂ or sulfite, was degraded by 55%-80% in 10 s in the ClO₂/sulfite process at 30-µM ClO₂ and 30-µM sulfite concentrations within a pH range of 6.0-11.0. Results from experiments and a kinetic model supported that chlorine monoxide (ClO^·) and sulfate radicals (SO₄^·-) were generated in the ClO₂/sulfite process, while hydroxyl radical generation was insignificant. Apart from radicals, dichlorine trioxide (Cl₂O₃) was generated and largely contributed to micropollutant degradation, supported by experimental results using stopped-flow spectrometry and quantum chemical calculations. The impacts of pH, sulfite dosage, and water matrix components (chloride, bicarbonate, and natural organic matter) on micropollutant abatement in the ClO₂/sulfite process were evaluated and discussed. When treating the real potable water, the concentrations of organic (five regulated disinfection byproducts) and inorganic byproducts (chlorite and chlorate) formed in the ClO₂/sulfite process were all below the drinking water standards. This study disclosed fundamental knowledge advancements relevant to the reaction mechanisms between ClO₂ and sulfite, and highlighed a novel process to abate micropollutants in water and wastewater.

A systematic review on chlorine dioxide as a disinfectant

The COVID-19 pandemic has tremendously increased the production and sales of disinfectants. This study aimed to systematically review and analyze the efficacy and safety of chlorine dioxide as a disinfectant. The literature relating to the use of chlorine dioxide as a disinfectant was systematically reviewed in January 2021 using databases such as PubMed, Science Direct, and Google Scholar. Inclusion criteria were studies that investigated the use of chlorine dioxide to assess the efficacy, safety, and impact of chlorine dioxide as a disinfectant. Out of the 33 included studies, 14 studies focused on the disinfectant efficacy of chlorine dioxide, 8 studies expounded on the safety and toxicity in humans and animals, and 15 studies discussed the impact, such as water treatment disinfection using chlorine dioxide. Chlorine dioxide is a safe and effective disinfectant, even at concentrations as low as 20 to 30 mg/L. Moreover, the efficacy of chlorine dioxide is mostly independent of pH. Chlorine dioxide can be effectively used to disinfect drinking water without much alteration of palatability and can also be used to destroy pathogenic microbes, including viruses, bacteria, and fungi from vegetables and fruits. Our review confirms that chlorine dioxide is effective against the resistant Mycobacterium, H1N1, and other influenza viruses. Studies generally support the use of chlorine dioxide as a disinfectant. The concentration deemed safe for usage still needs to be determined on a case-by-case basis.

Risk of chlorine dioxide as emerging contaminant during SARS-CoV-2 pandemic: enzyme, cardiac, and behavior effects on amphibian tadpoles

Objective

The use of chlorine dioxide (ClO₂) increased in the last year to prevent SARS-CoV-2 infection due to its use as disinfectant and therapeutic human treatments against viral infections. The absence of toxicological studies and sanitary regulation of this contaminant represents a serious threat to human and environmental health worldwide. The aim of this study was to evaluate the acute toxicity and sublethal effects of ClO₂ on tadpoles of Trachycephalus typhonius, which is a common bioindicator species of contamination from aquatic ecosystems.

Materials and methods

Median lethal concentration (LC50), the lowest-observed effect concentration (LOEC), and the no-observed effect concentration (NOEC) were performed. Acetylcholinesterase (AChE) and glutathione-S-transferase (GST) activities, swimming behavior parameters, and cardiac rhythm were estimated on tadpoles of concentrations ≤ LOEC exposed at 24 and 96 h. ANOVA and Dunnett’s post-hoc comparisons were performed to define treatments significance (p ≤ 0.05).

Results

The LC50 of ClO₂ was 4.17 mg L⁻¹ (confidence limits: 3.73–4.66). In addition, NOEC and LOEC values were 1.56 and 3.12 mg L⁻¹ ClO₂, respectively, at 48 h. AChE and GST activities, swimming parameters, and heart rates increased in sublethal exposure of ClO₂ (0.78–1.56 mg L⁻¹) at 24 h. However, both enzyme activities and swimming parameters decreased, whereas heart rates increased at 96 h.

Conclusion

Overall, this study determined that sublethal concentrations of ClO₂ produced alterations on antioxidant systems, neurotoxicity reflected on swimming performances, and variations in cardiac rhythm on treated tadpoles. Thus, our findings highlighted the need for urgent monitoring of this chemical in the aquatic ecosystems.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13530-021-00116-3.

Chlorfenapyr containing anions uptake from industrial wastewater by ethylene glycol functionalized benzyl dimethyl tetradecyl ammonium bromide membrane

The preservation of water and wastewater treatment has become a global challenge. The concentration of anions such as chlorides, fluorides, cyanides, and perchlorates above the permitted levels in water is harmful to human and aquatic life. Chlorfenapyr is an insecticide that contains the aforesaid anions and is abundantly present in industrial wastewater. This research is focused on the removal of these anions from wastewater by ethylene glycol functionalized benzyl dimethyl tetradecyl ammonium bromide immobilized on soluble polymer anion exchange membrane. The real wastewater samples rich in chlorfenapyr from two different sources (industrial and pond) were analyzed. Membrane efficiency was more than 50 ppm for each anion in a single fold. The double folds of membrane showed enhanced uptake and separation efficiency for chloride, fluoride, and cyanide from wastewater samples between 0.01 and 0.02 ppm down to lethal concenetrations values (LD 50). The membrane shows maximum separation efficiency between the pH ranges of 6-7. The interference effect on membrane separation efficiency showed that the replacement ability of sample anions was in the order of fluoride > chloride > perchlorate > cyanide. This high replacement efficiency of fluoride and chloride is attributed to the more chemical interactions of these anions with membrane.

Colorimetric Quantification Methods for Peracetic Acid together with Hydrogen Peroxide for Water Disinfection Process Control

Peracetic acid (PAA) water solutions is applied for disinfection of industry systems, food products and non-potable water. Commercially available peracetic acid is always supplied mixed with hydrogen peroxide (H₂O₂). H₂O₂ degrade slower than the peracetic acid which creates a need to quantify both peroxides separately to gauge the disinfection power of the solution and the residuals. Two combinations of colorimetric reactions are presented that allows simultaneous quantification at the mg·L⁻¹ level used in disinfection liquids and water disinfection. The first dichromic reaction use titanium oxide oxalate (TiO-Ox) which only react with H₂O₂ followed by addition of N,N-diethyl-p-phenylenediamine with iodide (DPD/I⁻) and the concentrations are read by simultaneously measuring the absorbance at 400 and 515 nm. Limit of quantification (LOQ) and maximal concentration determined was 4.6 µg·L⁻¹ and 2.5 mg·L⁻¹ for PAA and 9.1 µg·L⁻¹ and 5 mg·L⁻¹ for H₂O₂. The two color reactions didn’t interfere with each other when the reagent addition was consecutive. Another combination of colorimetric reaction also used where TiO-Ox was used to first measure H₂O₂ at 400 nm, before addition of 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS)) and reading the absorbance at 405 nm. ABTS changes the absorbance at 405 nm necessitating the two measurements be done separately. LOQ and maximal concentration determined using ABTS colorimetric assay was 42.5 µg·L⁻¹ and 30 mg·L⁻¹ for PAA and for titanium oxide oxalate colorimetric assay was 12.7 µg·L⁻¹ and 75 mg·L⁻¹ for H₂O₂. Both methods tested satisfactory in typical water samples (Tap, sea, lake, and biological treated sewage) spiked with peracetic acid and H₂O₂, separately.

Natural or synthetic - how global trends in textile usage threaten freshwater environments

As the global demand for textiles increases, so to do the potential environmental impacts that stem from their production, use and disposal. Freshwater ecosystems are particularly at risk: rivers often act as the primary recipients of waste generated during the production of textiles and are subject to pollutants released during the broader lifecycle of a textile product. Here, we investigate how global technological and societal processes shape the way we produce, use and dispose of textiles, and what this means for the environmental quality and ecological health of freshwaters. We examine two predominant 'natural' and synthetic textiles (wool and Polyethylene terephthalate (PET), respectively), and find that risks to freshwater ecosystems vary throughout the lifecycle of these textiles; and across geographies, in-line with regulatory and economic landscapes. Woollen textiles pose most risk during the Production Phase, while PET textiles pose most risk during the Use and Disposal Phases. Our findings show that: (i) both 'natural' and synthetic textiles present substantial challenges for freshwater environments; and (ii) bespoke solutions are needed in areas of the world where the global division of labour and less stringent environmental regulations have concentrated textile production; but also in regions where high textile consumption combines with unsustainable disposal behaviours. Effective mitigation may combine technological advances with societal changes in market mechanisms, regulations, textile use and disposal.

Crayfish as Bioindicators for Monitoring ClO2: A Case Study from a Brewery Water Treatment Facility

This study focuses on the use of crayfish as bioindicators in the water treatment process during operating conditions. The crayfish physiological responses to water disinfected with chlorine dioxide (ClO2) was evaluated. Monitoring was conducted at the private commercial enterprise Protivín Brewery in Czech Republic under standard operating conditions. This brewery has a water treatment facility, where ClO2 is used for water purification. A total of 25 adult signal crayfish (Pacifastacus leniusculus) were kept in separate flow-through aquaria receiving the purified water with ClO2 concentrations ranging from 0.01 to 0.29 mg L−1. Diurnal rhythms of 32% of crayfish was disturbed even at lower concentrations of ClO2 (0.01–0.2 mg L−1), while higher concentrations (>0.2 mg L−1) affected all animals. A random decline and rise of heart rate was detected. In addition, the frequent occurrence of higher levels of ClO2 significantly increased mortality. On average, mortality of crayfish occurred three to four weeks after stocking into the experimental system. Crayfish mortality is estimated to occur at concentrations exceeding 0.2 mg L−1 of ClO2. Our results suggest that long-term exposure to ClO2 adversely affects crayfish physiology. In addition, the results of this study could contribute to the use of crayfish as bioindicators in long-term water quality monitoring under industrial conditions.

Acute toxicity and risk evaluation of the CSO disinfectants performic acid, peracetic acid, chlorine dioxide and their by-products hydrogen peroxide and chlorite

The ecotoxicological evaluation of combined sewer overflow (CSO) disinfectants, with their degradation products, is important for ensuring safe use. For this form of toxicity, data for organisms representing different trophic levels are needed. We studied the toxicity of the alternative disinfectants peracetic acid (PAA), performic acid (PFA) and chlorine dioxide (ClO₂) and their degradation products hydrogen peroxide (H₂O₂) and chlorite (ClO₂^-) on Vibrio fischeri and Daphnia magna. ClO₂ was more toxic to D. magna (EC₅₀ < 0.09 mg/L) and PFA was most toxic to V. fischeri (EC₅₀ 0.24 mg/L). EC₅₀ of PFA, PAA, ClO₂, H₂O₂ and ClO₂^- on D. magna were 0.85, 0.78, <0.09, 3.46 and 0.36 mg/L, respectively. Similarly, EC₅₀ of PFA, PAA, ClO₂, H₂O₂ and ClO₂^- on V. fischeri were 0.24, 0.42, 1.10, 5.67 and 30.93 mg/L, respectively. For both PFA and ClO₂, the degradation in water was faster than for PAA, H₂O₂ and chlorite. Using these data together with literature values, we derived environmental quality standards. By combining these with typical concentrations of disinfectants used for CSOs, we estimated the dilution required for discharging CSOs after disinfection, which can be used for quick assessment of the environmental feasibility of disinfection systems at specific CSO sites. Minimal dilutions in the receiving water, in the orders of 44, 70 or 138-fold, are needed for ClO₂, PFA and PAA, respectively. This highlights PFA as the most widely applicable disinfectant, taking into account both its efficiency and the lower risk of unwanted environmental effects.

Treatment of Arctic wastewater by chemical coagulation, UV and peracetic acid disinfection

Conventional wastewater treatment is challenging in the Arctic region due to the cold climate and scattered population. Thus, no wastewater treatment plant exists in Greenland, and raw wastewater is discharged directly to nearby waterbodies without treatment. We investigated the efficiency of physicochemical wastewater treatment, in Kangerlussuaq, Greenland. Raw wastewater from Kangerlussuaq was treated by chemical coagulation and UV disinfection. By applying 7.5 mg Al/L polyaluminium chloride (PAX XL100), 73% of turbidity and 28% phosphate was removed from raw wastewater. E. coli and Enterococcus were removed by 4 and 2.5 log, respectively, when UV irradiation of 0.70 kWh/m³ was applied to coagulated wastewater. Furthermore, coagulated raw wastewater in Denmark, which has a chemical quality similar to Greenlandic wastewater, was disinfected by peracetic acid or UV irradiation. Removal of heterotrophic bacteria by applying 6 and 12 mg/L peracetic acid was 2.8 and 3.1 log, respectively. Similarly, removal of heterotrophic bacteria by applying 0.21 and 2.10 kWh/m³ for UV irradiation was 2.1 and greater than 4 log, respectively. Physicochemical treatment of raw wastewater followed by UV irradiation and/or peracetic acid disinfection showed the potential for treatment of arctic wastewater.

Chlorine dioxide as an alternative antifouling biocide for cooling water systems: Toxicity to larval barnacle Amphibalanus reticulatus (Utinomi)

Chlorine dioxide (ClO₂) is seen as an effective alternative to chlorine, which is widely used as an antifouling biocide. However, data on its efficacy against marine macrofoulants is scanty. In this study, acute toxicity of ClO₂ to larval forms of the fouling barnacle Amphibalanus reticulatus was investigated. ClO₂ treatment at 0.1mg/L for 20min elicited 45-63% reduction in naupliar metamorphosis, 70% inhibition of cyprid settlement and 80% inhibition of metamorphosis to juveniles. Increase in concentration to 0.2mg/L did not result in any significant difference in the settlement inhibition or metamorphosis. Treatment with 0.2mg/L of ClO₂ elicited substantial reduction in the settlement of barnacle larvae compared to control. The study indicates the possibility of using ClO₂ as an alternative antifouling biocide in power plant cooling water systems. However, more work needs to be done on the environmental effects of such switchover, which we are currently undertaking.

Comparative toxicological effects of two antifouling biocides on the marine diatom Chaetoceros lorenzianus: Damage and post-exposure recovery

Antifouling biocides are commonly used in coastal electric power stations to prevent biofouling in their condenser cooling systems. However, the environmental impact of the chemical biocides is less understood than the thermal stress effects caused by the condenser effluents. In this study, Chaetoceros lorenzianus, a representative marine diatom, was used to analyse the toxicity of two antifouling biocides, chlorine and chlorine dioxide. The diatom cells were subjected to a range of concentrations of the biocides (from 0.05 to 2mg/L, as total residual oxidants, TRO) for contact time of 30min. They were analysed for viability, genotoxicity, chlorophyll a and cell density endpoints. The cells were affected at all concentrations of the biocides (0.05-2mg/L), showing dose-dependent decrease in viability and increase in DNA damage. The treated cells were later incubated in filtered seawater devoid of biocide to check for recovery. The cells were able to recover in terms of overall viability and DNA damage, when they had been initially treated with low concentrations of the biocides (0.5mg/L of Cl₂ or 0.2mg/L of ClO₂). Chlorophyll a analysis showed irreparable damage at all concentrations, while cell density showed increasing trend of reduction, if treated above 0.5mg/L of Cl₂ and 0.2mg/L of ClO₂. The data indicated that in C. lorenzianus, cumulative toxic effects and recovery potential of ClO₂ up to 0.2mg/L were comparable with those of Cl₂, up to 0.5mg/L concentration in terms of the studied endpoints. The results indicate that at the biocide levels currently being used at power stations, recovery of the organism is feasible upon return to ambient environment. Similar studies should be carried out on other planktonic and benthic organisms, which will be helpful in the formulation of future guidelines for discharge of upcoming antifouling biocides such as chlorine dioxide.

Chemical disinfection of combined sewer overflow waters using performic acid or peracetic acids

We investigated the possibility of applying performic acid (PFA) and peracetic acid (PAA) for disinfection of combined sewer overflow (CSO) in existing CSO management infrastructures. The disinfection power of PFA and PAA towards Escherichia coli (E. coli) and Enterococcus was studied in batch-scale and pre-field experiments. In the batch-scale experiment, 2.5 mg L(-1) PAA removed approximately 4 log unit of E. coli and Enterococcus from CSO with a 360 min contact time. The removal of E. coli and Enterococcus from CSO was always around or above 3 log units using 2-4 mg L(-1) PFA; with a 20 min contact time in both batch-scale and pre-field experiments. There was no toxicological effect measured by Vibrio fischeri when CSO was disinfected with PFA; a slight toxic effect was observed on CSO disinfected with PAA. When the design for PFA based disinfection was applied to CSO collected from an authentic event, the disinfection efficiencies were confirmed and degradation rates were slightly higher than predicted in simulated CSO.