The Impacts of New CT Requirements for Designing Ozone Systems for Cryptosporidium Inactivation

The impacts of new CT requirements for ozone disinfection of Cryptosporidium are evaluated using a desktop CT disinfection analysis. The analysis is applied to the design of new and upgraded ozone systems for two water treatment plants in Virginia. The results indicate that the feasibility of ozone disinfection of Cryptosporidium is dependent on ozone demand and decay characteristics of the water supply and may not be feasible for water supplies with moderate to high ozone decay rates. The CT analysis is a useful design procedure to evaluate the benefit-cost tradeoffs of using ozone to achieve alternative disinfection goals.     

Disinfecting the Coquitlam Water Supply: Ozone and UV Disinfection

The Greater Vancouver Water District (GVWD) is upgrading its unfiltered Coquitlam water treatment system to meet the updated Health Canada guidelines for providing 3-log reduction or inactivation of Cryptosporidium and Giardia. The existing Coquitlam system, which is designed for peak flows of 1200 ML/d (317 mgd), includes an ozonation facility providing 3 log Giardia inactivation, and a chlorination/corrosion control facility. To meet Health Canada guidelines, a UV disinfection facility will be constructed, which will provide 3-log inactivation of Cryptosporidium and Giardia, and the ozone dosage will be increased to reduce THM and HAA precursors as well as improve UV transmittance. This paper will focus on the conceptual design of the Coquitlam system and the preliminary bench-scale studies that were completed for GVWD as part of the predesign phase of the project.     

Modeling and Validating the Effective Hydraulic Detention Time for a 10 mgd Ozone Contactor at the Lake Washington Surface Water Treatment Plant,Melbourne, Florida

Ozonation has been added to the treatment process at the 20 mgd Lake Washington Surface Water Treatment Plant (SWTP), Melbourne, Florida to improve finished water quality, control taste and odor, and provide greater than 0.5-log credit for inactivation of Cryptosporidium. The new ozone contactors (2 parallel 10 mgd trains) are located between the existing Actiflo^TM clarifiers and the filters on a relatively congested site. Prior to construction, a Computational Fluid Dynamics (CFD) study indicated that the proposed contactor design was not optimized to provide the targeted Cryptosporidium log inactivation. Based on the CFD modeling, the design was modified to reduce the baffle heights (increasing end gaps at turns) and the baffle ends were chamfered to improve flow patterns within the contactor. Internal access platforms were rotated from parallel to the flow (along outside walls) to perpendicular to the flow (along baffle walls). These small design changes increased the modeled effective detention time (t₁₀) for disinfection and the corresponding hydraulic efficiency (baffling factor) by 22 %. In addition, the dosing location for the ozone quenching chemical (hydrogen peroxide) was modified to provide extra ozone detention. The ozone contactors were then constructed to incorporate the design changes optimized from the modeling. During the start up of the ozone system, fluoride tracer tests were completed to validate the effective detention time (and baffling factor) calculated by CFD. Furthermore, a series of modifications are planned for the ozonated water sampling regime to take greater advantage of Ct₁₀ credits and reduce the required ozone dose, thereby saving energy and operating costs. This paper discusses several calculation methods for Cryptosporidium inactivation Ct₁₀ reporting in compliance with the Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and the LT2ESWTR Pre-proposal Draft Regulatory Language for Stakeholder Review (USEPA, 2002). The selection of the method will depend upon ease of implementation, modifications required to the ozone-in-water residual sampling system, long-term results of bromate sampling and actual Cryptosporidium bin classification under the LT2ESWTR. The article describes the CFD studies, improvements made to the contactor design, the tracer validation of the design, Ct₁₀ reporting methods and modifications to the ozone residual sampling system that the City is considering.     

Impact of Water Temperature on Resolving the Challenge of Assuring Disinfection While Limiting Bromate Formation

Intensive pilot studies were performed to study the impact of ozone dose (0.6 to 3.5 mg/L), pH (6.0 to 7.5), and contact time (12 to 38 min) on bromate (BrO₃) formation, for different sand-filtered water qualities from the Neuilly-sur-Marne Treatment Plant (COT = 1.3 to 2.2 mg/L, TAC = 190 to 230 mg CaCO₃, [Br^?] = 25 to 50 μg/L, and T = 5°C to 26°C). Whatever the water quality studied, the main factors influencing bromate formation were ozone dose, pH, and a cross factor between them. Bromate formation was shown to be proportional to bromide concentration, and to increase only slightly with temperature, depending on the ozone dose and the pH. As on the contrary temperature has an important impact on disinfection, especially when considering Cryptosporidium inactivation, resolving the challenge of ensuring disinfection while limiting bromate formation was shown to be quite easily achievable, at intermediate temperature, and with more stringent conditions at high temperature (because of bromate formation) or at low temperature (because of disinfection).     

Influence of Hydrodynamics on Giardia Cyst Inactivation by Ozone. Study by Kinetics and by “CT” Approach

This Research Note deals with the problem of providing appropriate disinfection from one of the most resistant microorganisms (the Giardia cysts) while trying to provide a more strict and logical approach to the “CT” concept. A further discussion of the U.S. Environmental Protection Agency (EPA) disinfection requirements in accordance with the kinetics and hydraulics of the ozonation process will be made in order to reconsider the definition of detention time. The main objective is the comparison of two possible approaches for determination of the disinfection conditions. The first implies consideration of the kinetics and hydraulics of the ozonation process. The second is based only on the hydrodynamics data and the use of an additional hypothesis known as the concept of the “CT” value.     

Pilot-Scale Ozone Inactivation of Cryptosporidium and Other Microorganisms in Natural Water

A pilot-scale study was conducted to evaluate the inactivation by ozone against Cryptosporidium oocysts, Giardia cysts, poliovirus, and B. subtilis endospores spiked into Ohio River water. The indigenous Ohio River populations of total coliform bacteria, heterotrophic plate count bacteria and endospores of aerobic spore forming bacteria were also evaluated. Endospores were the only organisms found to be more resistant to ozone than Cryptosporidium oocysts. Endospores may serve as an indicator of microbial treatment efficiency. Cryptosporidium oocysts were more resistant than Giardia cysts or poliovirus. Although HPC bacteria were less resistant than Cryptosporidium oocysts, variability limits their usefulness as an indicator of treatment efficiency. Ozone inactivation data generated in a pilot-scale study employing natural surface waters were comparable to inactivation data derived from previously published bench-scale studies using laboratory waters. The ozone requirements for inactivation of Cryptosporidium oocysts may produce elevated levels of bromate and ozone byproducts.     

Mathematical Modeling of Theoretical Cryptosporidium Inactivation in Full-Scale Ozonation Reactors

Conditions for theoretical inactivation of Cryptosporidium by ozone could be achieved at full-scale facilities if their design is appropriate. To perform this task correctly the chemical engineer's approach for process design must be applied. This paper discusses the basic equations the estimation of the disinfection efficiency of different ozone reacting systems. Available kinetic data have been integrated in a global model accounting for the hydrodynamics and mass transfer performances of the ozonation reactor. Thus the proposed method allows one to predict Cryptosporidium inactivation level in a given ozonation system. However, if a specified disinfection goal is to be achieved for Cryptosporidium with the developed model it is also possible to choose and optimize the design of the ozone reactor.     

Evaluation of Process Control Alternatives for the Inactivation of Escherichia coli,MS2 Bacteriophage,and Bacillus subtilis Spores during Wastewater Ozonation

Dissolved ozone concentration integrated over time (CT) is a reliable indicator of disinfection efficacy in drinking water treatment. However, ozone CT may not be measurable in some wastewater ozone applications. In this study, alternative process control parameters, specifically ozone to total organic carbon (O₃:TOC) ratio, differential UV₂₅₄ absorbance (ΔUV₂₅₄), and differential total fluorescence (ΔTF), were correlated with the inactivation of Escherichia coli, the bacteriophage MS2, and Bacillus subtilis spores in five secondary wastewater effluents. CT values greater than 9 mg-min/L were generally required for measurable inactivation of B. subtilis spores, and CT values of 1 and 2 mg-min/L consistently achieved greater than 6- and 5-log inactivation of MS2 and E. coli, respectively. The O₃:TOC, ΔUV_254, and ΔTF correlations for MS2 and B. subtilis were useful for predicting inactivation, while those of E. coli were characterized by greater variability. [Supplementary materials are available for this article. Go to the publisher's online edition of Ozone: Science & Engineering for the following free supplemental resources: additional figures and data tables.]     

Advanced Treatment For Municipal Wastewater Reuse In Agriculture. III - Ozone Disinfection

Results of a pilot (100 m³/h) investigation on ozone disinfection of municipal tertiary effluents for reuse in agriculture carried out at West Bari (S. Italy) treatment plant are presented. Among dosages, contact times and advanced treatment schemes investigated it was demonstrated that ozone disinfection results in the achievement of the WHO microbial guideline (1,000 CFU/100ml for Fecal Coliforms) for unrestricted wastewater reuse in agriculture of both clarified and clarified-filtered municipal secondary effluents; it is very effective towards Pseudomonas aeruginosa, rather effective towards Giardia lamblia and substantially ineffective towards Cryptosporidium parvum and it forms limited amount of DBP (approx. 350 ppb of total aldehydes). O&M costs amount to 37 Euro/1000m³.     

Ozone and chlorine dioxide: Similar chemistry and measurement issues

The chemical reactions associated with ozone and chlorine dioxide can be complicated and involve numerous intermediates. When ozone is applied, the presence of reactive intermediate species (O₂ ^‐, O₃ ^‐, OH, HO₂, HO₂ ^‐, and H₂O₂) influence the extent of oxidation that takes place and determines the amount and types of by‐products formed. Similarly, when chlorine dioxide is applied the amount of intermediate (Cl₂O₂) formed determines whether chlorine dioxide producing reactions or chlorate ion forming reactions occur. Ozone and chlorine dioxide are excellent agents for inactivating Cryptosporidium and Giardia. Microbiologically, each of the agents are very reactive. In the case of ozone, typically each molecule undergoes a one‐electron change. The mechanism of chlorine dioxide inactivation involves a recycling process whereby chlorine dioxide is reduced to chlorite ion followed by the “regeneration” of chlorine dioxide that continues to react within the cell over and over again. Chlorite ion also has oxidizing power and in some cases, is a biocide. When ozone and chlorine dioxide are used in combination, it is important that the chlorine dioxide application follow the ozone treatment to prevent the formation of unwanted by‐products such as ClO₃ ^‐.     

Conditions Affecting Bromate Formation During Ozonation of Bottled Water

Bromate concentration, ozone lifetime and ozone exposure (CT value) measured in bottled water in full-scale runs, were in good agreement to those measured in laboratory experiments. Ozone lifetime in bottled water was high enough to result in a CT value greater than 5 even for ozone dose as low as 0.1?mgO₃/L, at a water pH of 7.6. Bromate was gradually formed during the ozone lifetime. Bormate formation and ozone exposure were significantly influenced by pH. In full-scale runs, an ozone dose of 0.15?mgO₃/L at pH=7.6 resulted in a CT of 10.3 and a bromate concentration of 13.5?µg/L, while at pH=7.25 the values of CT and BrO₃ ^? were 12.6 and 9.6?µg/L, respectively. By decreasing further the pH to 6.8, an increase of CT value to 15.8 and a reduction of bromate to 5.5?µg BrO₃ ^?/L were observed. In addition, results in full-scale runs showed that ozone exposure and bromate concentrations were linearly related to ozone dose in the working range of 0.1 to 0.25?mgO₃/L.     

Comparison of electrochemical method with ozonation, chlorination and monochloramination in drinking water disinfection

Electrochemical process in chloride-free electrolytes was proved to be powerful in disinfection due to the strong oxidants produced in the electrolysis and no formation of disinfection byproducts (DBPs). In this study, disinfection experiments were conducted by electrochemical treatment compared with ordinary and advanced methods (ozonation, chlorination and monochloramination), with Escherichia coli (E. coli) K-12, Staphylococcus aureus (S. aureus) A106, Bacillus subtilis (BST) and an isolated Bacillus as the representative microorganisms. Firstly, factor tests were performed on E. coli to obtain the optimal conditions of the four disinfection procedures. At their respective optimal condition, CT (concentration of disinfectant × contact time) value of a 4-log E. coli inactivation was 33.5, 1440, 1575, 1674 mg min L⁻¹ for electrochemical process, ozonation, chlorination and monochloramination, respectively. It was demonstrated that the disinfection availability was in the following order: electrochemical process > ozonation > chlorination > monochloramination, which could be attributed to the hydroxyl radical generated in the electrolysis, with strong oxidizing ability and non-selectivity compared with the other three disinfectants. Moreover, the disinfection efficacy of the four disinfection procedures was compared for four different bacteria. It was found that the disinfection efficacy was similar for the selected four bacteria in electrochemical process, while in the other three treatments inactivation of the two Bacillus was much slower than E. coli and S. aureus. As a result, the non-selectivity of electrochemical disinfection with BDD anode to different kinds of microorganisms was further proved, which was primarily controlled by the hydroxyl radicals existed in the free state. For each bacterium, the order of disinfection availability of the four processes was consistent. Finally, scanning electron microscopy (SEM) was implemented to observe the cell morphology. It was shown that cell surface damage was more obvious in electrochemical system with strong oxidants compared with that after ozone treatment, while the integrity of cells were not affected in weak oxidizing chlorine and monochloramine.     

Hydroxyl Radical/Ozone Ratios During Ozonation Processes. II. The Effect of Temperature,pH, Alkalinity,and DOM Properties

The influence of temperature, pH, alkalinity, and type and concentration of the dissolved organic matter (DOM) on the rate of ozone (O₃) decomposition, O₃-exposure, ?OH-exposure and the ratio R_ct of the concentrations of ?OH and O₃ has been studied. For a standardized single ozone dose of 1 mg/L in all experiments, considerable variations in O₃-exposure and ?OH-exposure were found. This has important implications for water treatment plants regarding the efficiency of oxidation and disinfection by O₃. In oligotrophic surface waters and groundwaters, minimal calibration experiments are needed to model and control the ozonation process, whereas in eutrophic surface waters more frequent measurements of O₃ kinetics and R_ct values are required to evaluate seasonal variations.     

Inactivation of Giardia muris Using Ozone and Ozone-Hydrogen Peroxide

The disinfection effects of the ozone molecule alone and that of ozone decomposition products when inactivating Giardia muris cysts were investigated at bench-scale using two different ozone demand-free laboratory buffer systems. The first water was a 0.05 M phosphate buffer with hydrogen peroxide added at a 10:1 weight ratio. The second water was a 0.05 M phosphate – 0.01 M bicarbonate buffer which quickly scavenged radical species from ozone decomposition. The C3H/HeN mouse model was used to assess the infectivity of ozone treated cysts.

The phosphate-bicarbonate buffer system had significantly greater (P ≤ 0.05) inactivation of G. muris cysts than that observed in the phosphate buffer – peroxide system where ozone was completely decomposed in less than 120 s. Consequently, the design of ozone disinfection processes should maintain ozone residual for disinfection prior to the addition of hydrogen peroxide for the oxidation of other compounds.     

The Mortality of Nematodes in Drinking Water in the Presence of Ozone,Chlorine Dioxide,and Chlorine

ABSTRACT

Nematodes are among the organisms most commonly found in treated water. Although they are believed to be harmless, colonization of nematodes by pathogenic bacteria may transform them into harmful contaminants of the drinking water. Thus, efficient disinfectants must be applied to inactivate and remove nematodes. In the present study, nematode inactivation using different concentrations of ozone, chlorine dioxide, and chlorine for different exposure times are described. Three different disinfection models were tested. Based on experimental data, the best statistically significant relationship between mortality of nematodes in probits (Y), concentrations of disinfectants mg/L (C), and the exposure times in minutes (t) was established as Y = a*log(C) + b*log(t) + c. The estimated values of regression coefficients a, b, and c were 6.89, 0.76, and 1.13 for ozone, 0.66, 1.23, and 5.28 for chlorine dioxide and 1.59, 0.92, and 3.06 for chlorine, respectively. Comparison of the predicted LC(90) molar concentrations of the investigated disinfectants, chlorine dioxide, ClO₂, was found to be the most effective disinfectant, followed by ozone, O₃, while chlorine, Cl₂, was the least efficient disinfectant for inactivation of nematodes.     

The Evaluation Of Ozonation and Chlorination On Disinfection By-Product Formation for a High-Bromide Water

High-bromide raw water was ozonated or chlorinated with and without hydrogen peroxide to study the effect of the disinfectants on the disinfection by-product (DBP) formation. Less bromate was formed when ozonation was made at the ambient pH of 5.8 as compared to ozonation at pH 7, showing the effectiveness of pH reduction in controlling the bromate formation. When chlorine dose was 1 mg/L instead of 2.3 mg/L, the trihalomethane formation was 50 μg/L instead of >100 μg/L, and the proportional distribution of the trihalomethanes was similar. The use of ozone for this water could provide good results in respect of the DBP formation.     

Seawater Ozonation of Bacillus subtilis Spores: Implications for the Use of Ozone in Ballast Water Treatment

The potential of ozone for disinfection of ships’ ballast water was investigated using Bacillus subtilis spores as an indicator. The effects of pH, presence of iron, and bacterial strain on disinfection efficacy in seawater, under simulated ballast conditions, were investigated. Ozone dosages of 9 mg/L (pH 7) and 14 mg/L (pH 8.2) and 24 h contact achieved a 4-log inactivation with the various oxidant residuals formed. Iron surface at a ratio to water of 9 m²/m³ impaired the oxidant residuals and the disinfection of spores. Different strains of B. subtilis resulted in different CT values. Ozone does not seem to be a good choice for the control of spore-forming organisms in ballast water, but may be suitable for the control of other species.     

Disinfection of recycled red‐meat‐processing wastewater by ozone

Ozonation of a real red‐meat‐processing wastewater was conducted in a semi‐batch reactor to explore the possibility of the water reuse. The experimental results revealed that ozone was very effective in disinfection of the red‐meat‐processing wastewater. After 8 min of ozonation with an applied ozone dose of 23.09 mg min^?1 liter^?1 of wastewater, 99% of aerobic bacteria, total coliforms and Escherichia coli were inactivated. Empirical models were developed to predict the microbial inactivation efficacy of ozone from the CT values for the real red‐meat‐processing wastewater. A correlation was also derived to estimate the CT values from the applied ozone dose and the ozone contact time. The results also revealed that under the ozonation condition for 99% inactivation of aerobic bacteria, total coliforms and E coli, the decrease in the chemical oxygen demand and the 5‐day biological oxygen demand of the wastewater were 10.7% and 23.6%, respectively. However, ozonation under this condition neither improved the light transmission nor reduced the total suspended solids (TSS) despite of the decolorization of the wastewater after ozonation. Copyright © 2005 Society of Chemical Industry     

Minimizing Bromate Concentration by Controlling the Ozone Reaction Time in a Full-Scale Plant

The latest European Directive 98/83/CE (5 December 1998), concerning the quality of water intended for human consumption, has set a two-stage parametric value for bromate. Bromate concentration will comply with 25 μg/L after December 25, 2003, and with 10 μg/L after December 25, 2008. Bromate formation in water is generally due to bromide oxidation during the ozonation stage. Due to higher temperatures, this latter parametric value is often exceeded in summer. Minimizing bromate levels is thus a crucial problem for drinking water producers. A bromate-minimizing strategy consists of shortening the reaction time between ozone and water. This can be done by neutralizing dissolved ozone residual with bisulfite at the exit of the ozone reactor chamber and/or by managing the introduction of ozone in different chambers depending on the water flow rate. This is only possible if, in our case, the disinfection goal for ozone is respected toward bacteria and viruses. The CT value must comply with 1.6 mg/min/L. In our plants, this value could be very large due to high contact time in and after leaving the ozone reactors.