Modeling Hydrodynamics And Mass Transfer Parameters In A Continuous Ozone Bubble Column

A computer model based on the establishment of mass balance equations and on the model of fluids flow “stirred tank in series” was developed in order to calculate the ozone transfer coefficient k_La and kinetic constant k_c of ozone consumption by water. On the basis of experimental data, the correlation for gas holdup ε_g and bubble diameter d_vs, were proposed and used to calculate the specific interfacial area a. The liquid-phase mass transfer coefficient k_L for ozone was evaluated from a and the k_La data.     

Mass Balance Analysis of Ozone in Conventional Bubble Contactors

Ozone transfer into potable water was studied in a commercial scale contactor. Ozone mass balances have been calculated to determine ozone utilization in the contactor. Gas and liquid flowrates, as well as inlet ozone concentrations in the gas were varied. From these data it was possible to determine the mass transfer coefficient, the ozone consumption and the transfer efficiency. Procedures for calculating the design of contactors and the optimal operating conditions are proposed.     

Effect of Micro-Mixing Conditions on Predictions of Cryptosporidium Inactivation in an Ozone Contactor

The U.S. EPA is considering segregated flow analysis as an alternative calculation method to determine Cryptosporidium parvum inactivation credit in continuous-flow ozone contactors in drinking water treatment facilities. A computer method is presented in which C. parvum inactivation in the reactive flow segment of a hypothetical ozone contactor with a pre-determined residence time distribution is calculated based on the assumption of either completely segregated or completely micro-mixed flow. In a series of computer simulations using typical ozonation conditions in a water treatment facility, inactivation predicted assuming complete segregation was 0.3 to more than 2.0 log greater than that predicted assuming complete micro-mixing, depending on the level of back-mixing, ozone decomposition rate and inactivation level. CFSTR-in-series model predictions of inactivation were between those of segregated flow analysis and micro-mixed analysis. It was concluded that segregated flow analysis calculations may result in significant over-prediction of C. parvum inactivation credit in ozone contactors and should be used with caution.     

Mass Balance Analysis of Ozone in a Conventional Bubble Column

Ozone transfer into potable water was studied in a conventional bubble column, and ozone mass balances have been calculated to determine ozone utilization efficiencies. Liquid and gas flow rates, as well as inlet ozone concentrations in the gas phase were varied. Using these data, it was possible to determine the ozone mass transfer coefficient, ozone transfer efficiency, and ozone consumption. A model of ozone transfer was established, and procedures for calculating the optimum design parameters and operating conditions are proposed.     

Efficient Inactivation of Cryptosporidium Parvum in a Static Mixer Ozone Contactor

Inactivation of C. parvum oocysts was measured in a small-scale static mixer ozone contacting system in series of challenge experiments. Measured inactivation ranged from 1.4 to 3.0 log-units, depending on the dissolved ozone by contact time product (C _avg t¯) in the contactor, and was equivalent to or slightly better than that predicted for a perfect plug flow contactor with the same dissolved ozone profile. Efficient and predictable inactivation of C. parvum in drinking water may be achieved in a two-stage, continuous-flow ozone contacting system composed of a gas dissolution system employing a static mixer, and followed by a liquid phase contactor, at least at small-scale.     

Pilot Studies of Ozonation for Inactivation of Artemia salina Nauplii in Ballast Water

A pilot-plant study was conducted in the Republic of Croatia to determine the applicability of ozonation for inactivation of non-indigenous species and to provide necessary information regarding use of ozone as a ballast water treatment option. Nauplii of the brine shrimp Artemia salina were used as model organisms to investigate the efficacy of ozonation at three different ozone dosages (2.4, 3.7 and 10.9 mg L^?1). Mortality of Artemia nauplii at 98.6%, was achieved after 3 h of exposure in ozone-treated water with the highest ozone dosage. Our results indicated that ozonation is a promising treatment for controlling non-indigenous and potentially invasive species; however, to draw more general conclusions, several species with higher level of resistance to ozone are required and will be studied in the future.     

Mass Transfer of Ozone Using a Microporous Diffuser Reactor System

An ozone reactor was constructed using a tubular gas diffuser made of microporous stainless steel to significantly reduce gas bubble size and increase overall mass transfer area. Overall mass transfer coefficient, K_La [s ^?1], was correlated with gas (G) and liquid (L) flow rates using K_La = AL^αG^β , with A = 3.96 × 10 ⁸ [s^?1], α = 1.53, and β = 0.40, with L and G in [m ³s^?1]. The reactor is essentially plug flow at high G or L. This system achieves one of the highest ozone mass transfer rates observed in the literature.     

Survey of Seawater Intrusion in the Pearl River Basin and Modeling Bromate Formation during Ozonation of the Raw Water

Bromate formation has been identified as a significant barrier in the application of ozone during water treatment the downstream region of the Pearl River Basin that contains high levels of bromide. Seawater intrusion will increase bromide concentration in the inshore surface water. In this study, seawater intrusion in the Pearl River Basin was surveyed and modeling bromate formation during ozonation of the raw water affected by seawater intrusion was studied. Bromate formation models were developed to simulate the effects of the characteristics of water quality and the operating parameters of treatment processes on bromate formation during preozonation process and postozonation process. The results show that the downstream of the Pearl River Basin is affected seriously by seawater intrusion and the bromide mainly comes from seawater. Some empirical models were developed to estimate the concentration of bromate in ozonated surface raw water affected by seawater intrusion during the treatment process.     

Ozonation of Drinking Water: A Design Methodology

The formation of potentially carcinogenic organic halides has been shown to result from drinking water disinfection with chlorine. xidative treatment of organic halide precursors with ozone prior to chlorination has surfaced as an attractive technique for reducing the formation of these compounds. In addition to reduction of precursor levels, preozonation has been reported to effect other beneficial results in water treatment. This paper presents design methodologies to optimize the implementation of the ozonation process for water treatment applications. Pre-design considerations common to all ozonation design processes are discussed. Subsequently, design procedures for the ozone generation and contacting systems are reviewed.     

Mathematical Modeling and Simulation of Ozonation Processes in a Downstream Static Mixer with Sieve Plates

New standards for drinking water disinfection require better optimization of the ozonation stage on the basis of the concentration×contact time (CT) concept, and production of ozone from pure oxygen at higher concentrations presumes application of the new type of contactors operating efficiently at lower gas/liquid volumetric ratios. One possible construction to meet these requirements is a downstream static mixer with sieve plates. At higher flow rates of liquid in this mixer, the interfacial area may reach 10,000m²/m³ at energy dissipation 1–5kW/m³. Due to the very intensive hydrodynamic regime the ozone utilization degree in the gas phase reaches 98–100% in natural lake water ozonation. Mathematical simulation of lake water ozonation in this mixer indicated that the process proceeds mostly in the diffusion or kinetic regime depending on the operating parameters. The dominating parameters besides the sieve geometry are the liquid flow rate in the holes of the sieves and the volumetric liquid/gas ratio.     

The Role of Mixing in Ozone Dissolution Systems

The mixing efficiency of four alternative ozone dissolution systems, including conventional bubble diffusion and pipeline injection/diffusion reactors, were compared by measuring the variability of ozone residual measurements at the outlet of each reactor. The standard deviation and coefficient of variation of a time series of residual measurements were used to provide a quantitative measure of the level of mixing in the reactor. The results indicate that properly designed pipeline dissolution systems provide significantly improved mixing over oxygen-fed or air-fed bubble diffusion systems. Minimum mixing criteria for bubble diffusion systems cited in the literature may underestimate the level of mixing required to achieve stable ozone residuals downstream of the dissolution chamber of conventional bubble diffusion chambers.     

Nitrophenols As Model Compounds in the Design of Ozone Contacting and Reacting Systems

The apparent stoichiometry observed, the ozone to phenolic compound oxidized is equal to 7 (8)/1, 5/1, 4/1 respectively for tri–, di– and mononitrophenols. All nitrogroups are found to be in the form of nitrate in the medium after reaction. In a buffered medium uith NaHCO3; (pH 7.5–B), the results are consistent uith the bicarbonate – carbonate competition in indirect ozonization.     

Ozone Mass Transfer in the Mixing Zone of a Confined Plunging Liquid Jet Contactor

In a Confined Plunging Liquid Jet Contactor (CPLJC) a jet of liquid is introduced into an enclosed cylindrical column (downcomer) that generates fine gas bubbles that are contacted with the bulk liquid flow. The region where the liquid jet impinges the receiving liquid and expands to the wall of the downcomer is called the Mixing Zone (MZ). In the MZ, the energy of the liquid jet is dissipated by the breakup of the entrained gas into fine bubbles, and the intense recirculation of the two-phase mixture. The study presented here was undertaken to quantify the ozone-water mass transfer performance of the MZ through the determination of the volumetric mass transfer coefficient, k_La (s^?1), and to produce a model for predicting k_La based on the specific energy dissipation rate. It was found experimentally that k_La in the MZ increased with increasing superficial gas velocity. A maximum experimental k_La value of 0.84 s^?1 was achieved which compares well to other contactors used in water treatment. Such a large k_La value combined with the small volume of the reactor, favorable energy requirements and safety features of the system, suggests that the CPLJC provides an attractive alternative to conventional ozone contactors. The relatively large mass transfer rates were found to be a function of the high gas holdup and fine bubble size generated in the MZ, which results in an almost froth-like consistency. A model based on the specific energy dissipation rate of the water jet, E (kg · m^?1· s^?3), and MZ bubble size was used to predict k_La in the MZ. Using E, the number average bubble size was predicted which was then used to calculate the liquid phase mass transfer coefficient k_L. The bubble size was also used with the predicted mixing zone gas holdup to calculate the specific interfacial area, a (m^?1), which was then combined with k_L to determine a predicted value of k_La. The average deviation between experimental and predicted k_La was 6.2%.     

Application of Gas Absorption Theories to o-Cresol Ozonation in Water

The reaction rate between ozone and o–cresol in aqueous solution at pH 2 has been studied. According to the absorption theories, the kinetic regime of the process in the experimental conditions investigated, corresponds to an irreversible fast pseudo-first order reaction between non-dissociated o-cresol and ozone. Similar values for the rate constant are deduced from the different theories. Therefore the film theory, having a simple mathematical basis, seems to be the most appropriate model for this kinetic study.     

Study of the Catalytic Ozonation of Humic Substances in Water and Their Ozonation Byproducts

It is shown that using transition metals, especially Mn(II) and Ag(I), during ozonation of humic substances in water allows important reductions in the content of organic matter. Characterization of the organic compounds resulting from ozonation was made by concentrating the sample through liquid-liquid extraction or derivation with PFBOA.HCl, along with the GC/MS and GC/ECD techniques. In total, 110 different organic compounds were identified using GC/MS; mainly carboxylic acids, aromatics, hydrocarbons, aldehydes, ketones, and furan-carboxylic acids. The percentages of elimination or formation levels reached during ozonation are discussed.     

Ozonation Dynamics and Its Implication for Off-Gas Ozone Control in Treating Pulp Mill Wastewaters

Ozonation of biologically pretreated pulp mill wastewaters was studied using both bench and pilot scale fine bubble contactors to determine the oxidation efficiencies, mass transfer coefficients (k_La) and enhancement factors (E) due to the occurrence of chemical reactions. A sensitivity analysis based on the measured process parameters was then used to reveal the interrelated effects of key factors on off-gas ozone concentrations. It was shown that the removal efficiencies of color and AOX were simply related to the amount of utilized ozone, regardless of variation of other operating conditions. Furthermore, the rate of absorption fell within the fast or instantaneous kinetics regimes due to the occurrence of rapid chemical reactions. The Ek_La values were found to vary substantially during the course of ozonation, indicating that the enhancement factors were not only affected not only by operating conditions but also by wastewater characteristics. To effectively control the off-gas ozone emission, measures should be taken to minimize the backmixing, use a counter-current flow arrangement and provide adequate contact time.     

The Oxidation of Manganese and Disinfection By Ozonation in Water Purification Processing

The surface water of a river has been used as the raw water by the Waterworks Bureau of Osaka City. At present, the manganese contained in the raw water is oxidized by breakpoint chlorination and all oxides are removed by coagulation, sedimentation followed by rapid sand filtration, with chlorine being used as the final disinfectant.

Prechlorination was not conducted in the ongoing pilotplant experiment of an advanced water purification process with ozone and granular activated carbon. It is necessary, therefore, to oxidize manganese by the oxidative effect of ozone instead of prechlorination.

It is important for the treatment of manganese to adjust the ozone dosage because manganese is oxidized up to the soluble septavalent state by the surplus ozone. Since ozone does not continue to exist for very long in water, though its disinfecting power is high, final disinfection by chlorine is required.     

Kinetic Regime Changes in the Ozonation of 1,3-Cyclohexanedione in Aqueous Solutions

The reaction between ozone and 1,3-cyclohexanedione in aqueous solutions at different pH was investigated from the point of view of the film theory. The rate of ozone absorption is accompanied by a second order irreversible reaction with 1,3-cyclohexanedione. The kinetic regime of absorption was found to change from fast (the reaction being of pseudo first order) to instantaneous, according to the ozone partial pressure applied. The former kinetic regime allows the determination of the reaction rate constant, while the latter leads to the volumetric mass transfer coefficient of the system.