Combined Absorption and Self-Decomposition of Ozone in Aqueous Solutions with Interfacial Resistance

A theoretical analysis is performed employing the film model for the isothermal absorption and self-decomposition of ozone in aqueous solutions with interfacial resistance, which is inversely proportional to the interfacial mass transfer coefficient k_s. A closed-form solution has been obtained. The effects of system parameters on the ozone mass transfer rate are examined. These parameters include the interfacial resistance (1/k_s), the acidic and basic self-decomposition reaction rate parameters (M_m ^0.5, M_n ^0.5.; M_m = [2D_Ak_mC_Ai ^m-1/(m+1)]/(k_L ⁰)², M_n=(2D_Ak_nC_Ai ^n-1/(n+1))/(k_L ⁰)², the reaction orders (m,n), the pH value of solution, and the liquid-phase mass transfer coefficient (k_L ⁰). The results indicate that the reduction effect of the interfacial resistance on the absorption rate is most significant for the situation with the larger values of M_m and M_n as well as with higher pH values. Also, for any particular finite value of k_L ⁰/k_s, the reduction effect encountered is greater for a gas liquid contactor with a lower k_L ⁰. The reduction effect should be avoided in order to maintain a higher mass transfer rate of ozone in aqueous solution. This analysis is of importance for the efficient use of ozone in water/wastewater treatment processes in the presence of interfacial resistance substances such as surface active agents. For some known special cases (for example, cases with no interfacial resistance), the present solution reduces to the previous works of other investigators.     

Axial Dispersion Model for Estimation of Ozone Self-Decomposition

A new method using the axial dispersion model for estimation of ozone self-decomposition kinetics in a semibatch bubble column reactor was developed. The reaction rate coefficients for literature equations of ozone decomposition and the gas phase dispersion coefficient were estimated and compared with literature data. The reaction order in the pH range 7–10 with respect to ozone 1.12 and 0.51 the hydroxyl ion were obtained, which is in good agreement with literature. The model parameters were determined by parameter estimation using a nonlinear optimization method. A sensitivity analysis was conducted to obtain information on reliability and identifiability of the estimated parameters.     

Modification of the Standard Neutral Ozone Decomposition Model

The modified Staehelin, Buhler, and Hoigné model for aqueous ozone decomposition was tested over a wide range of hydroxyl radical scavenger concentrations at a pH of 7.1–7.2. Results from these experiments showed that the modified model appeared to underpredict the residual ozone concentration and overpredict the residual hydroxyl radical probe compound, tetrachloroethylene, concentration. The modified Staehelin, Buhler, and Hoigné model was recalibrated and two rate constants, the rate constant of the initiation reaction of ozone decomposition of hydroxide ion and the rate constant of the promotion reaction of ozone decomposition by hydroxyl radical, were reestimated. The new estimates of these rate constants are 1.8 × 10² M^?1s^?1 (initiation reaction) and 2 × 10⁸ M^?1s^?1 (promotion reaction), while the values estimated by Staehelin, Buhler, and Hoigné for these rate constants are 70 M^?1s^?1 (initiation reaction) and 2 × 10⁹ M^?1s^?1 (promotion reaction). The recalibrated-modified model was tested and validated by conducting experiments at different pH values and hydroxyl radical scavenger concentrations. Also, the effect of phosphate buffer as a hydroxyl radical scavenger was investigated at phosphate buffer concentrations of 10 mM and 1 mM.     

Practical Design Model for Calculating Bubble Diffuser Contactor Ozone Transfer Efficiency

Ozone transfer to water or wastewater is necessary before desirable, effective ozone reactions occur. Several factors affect ozone transfer efficiency, including water quality characteristics, contactor configuration, and applied ozone characteristics. The design model presented in this paper addresses all factors affecting ozone transfer. The model was used to compare measured transfer efficiency with predicted transfer efficiency at four full-scale wastewater ozone disinfection facilities. A relatively good prediction was obtained at each plant.

The paper presents an example calculation of ozone transfer efficiency. Also, the effect of changes to some of the factors affecting transfer efficiency is presented, such as changes in diffuser depth, plant elevation, ozone concentration, water quality (i.e., ozone demand), pH, detention time, temperature, and acombination of factors. The design model may be used to evaluate the effect of changes in plant design on transfer efficiency, but cannot provide an absolute value for transfer efficiency until difficult-to-measure parameters, such as bubble diameter, are known.     

Investigation of Ozone Reaction in River Waters Causing Instantaneous Ozone Demand

Ozone consumption by water can be characterized by the instantaneous ozone demand (IOD) and a pseudo first order decay constant. Utilizing the flow injection analytical system for measuring IOD, the instantaneous ozone demand characteristics of two river waters (Korea) were investigated, utilizing a ?OH probe compound and ?OH scavenger, and were compared with those of two commercial humic acids (the Suwannee River humic acid and Aldrich humic acid). The major findings were as follows; (1) The IOD in river waters was found to be mainly due to the reaction of the ozone with natural organic matter (NOM), which constituted approximately 0.26–0.29?mg/mg DOC, and was responsible for the consumption of more than 40% of the applied ozone. Whereas, the IOD of the two commercial humic acids were three times more than those of the river waters. (2) The IOD in the river waters was mainly caused by the direct ozone reaction with dissolved organics, not from the ?OH mediated ozone reaction. However, for the two commercial humic acids, more than 40% of the IOD came from the ?OH mediated ozone reaction. (3) The hydrophobic fractions of the dissolved organics in the river waters were mainly responsible for the IOD. The IOD of the hydrophobic organics was approximately ten times larger than that of the hydrophilic organics. Although the exact magnitude of the IOD, and the relative importance of the direct/indirect ozone reaction with river water may vary greatly depending upon the source of the NOM, the characteristics of the IOD compromise a significant fraction of the ozone dose need (especially in achieving good ozone disinfection) in water treatment plants.     

Investigation on the Kinetics of Heterogeneous Catalytic Ozone Decomposition in Aqueous Solution over Composite Iron-Manganese Silicate Oxide

The kinetics of heterogeneous catalytic ozone decomposition in aqueous solution over composite iron-manganese silicate oxide (FMSO) was investigated. Results showed that the presence of FMSO significantly accelerated the ozone decomposition rate from 0.022 (without FMSO) to 0.101 min^?1. The effects of inorganic anions and solution pH indicated that surface hydroxyl groups on FMSO were the active sites for catalyzing ozone decomposition and neutral charge surface seemed to show the highest catalytic performance. Tert-butanol inhibition experiments demonstrated that FMSO effectively accelerated the transformation rate of ozone into hydroxyl radicals. The contribution of hydroxyl radicals on ozone decomposition with and without FMSO was subsequently determined.     

Ozone kinetics and diesel decomposition by ozonation in groundwater

The ozone kinetics (ozone auto-decomposition; effects of pH and solubility) and diesel/TCE/PCE decomposition (effects of hydroxyl radical scavenger, pH, and ozone/H₂O₂) by ozonation process were investigated in aqueous phase using deionized water, simulated groundwater, and actual groundwater. Reactions with deionized water and groundwater both showed the second-order reaction rates: the reaction rate was much higher in groundwater (half-life of 14.7 min) than in deionized water (half-life of 37.5 min). It was accelerated at high pH condition in both waters. The use of ozone showed high oxidation rates of TCE, PCE, and diesel. Hydroxyl radical scavengers acted as inhibitors for diesel decomposition, and high pH condition and addition of hydrogen peroxide could promote to degrade diesel in groundwater indicating ozone oxidation process could be effectively applied to treating diesel contaminated-groundwater.     

Effect of pH and Importance of Ozone initiated Radical Reactions In Inactivating Bacillus subtilis Spore

The effect of pH on the inactivation of Bacillus subtilis spores with ozone in a batch reactor was examined in association with the role of OH radicals. The exact effect OH radicals have on ozone inactivation of microorganism is not well understood, although a direct reaction of molecular ozone has sometimes been emphasized. This study reports a novel observation that the presence of OH radicals plays a significant role in microbial inactivation. Considering the dependence of the ozone decay rate on pH, the observed C¯ T values for achieving a 2 log inactivation using the modified Chick-Watson model was 40 % lower at pH 8.2 compared to that at pH 5.6. In the presence of OH radical scavengers (tert-butanol), the observed change of C¯ T with pH was within 10%. This difference could be explained by the significant role of OH radicals.     

Kinetic Study and Optimum Control of the Ozone/UV Process Measuring Hydrogen Peroxide Formed In-situ

This study was conducted to develop a kinetic model of the ozone/UV process by monitoring the trend of in-situ hydrogen peroxide formation. A specifically devised setup, which could continuously measure the concentration of hydrogen peroxide as low as 10 μg/L, was used. The kinetic equations, comprised of several intrinsic constants with semi-empirical parameters (k_chain and k_R3) were developed to predict the time varied residual ozone and hydrogen peroxide formed in situ along with the hydroxyl radical concentration at steady state,[OH°]_ss, in the ozone/UV process. The optimum ozone dose was also investigated at a fixed UV dose using the removal rate of UV absorbance at 254 nm (A₂₅₄) in raw drinking water. The result showed that the continuous monitoring of hydrogen peroxide formed in situ in an ozone/UV process could be used as an important tool to optimize the operation of the process.     

Influence of Carbonate on the Ozone/Hydrogen Peroxide Based Advanced Oxidation Process for Drinking Water Treatment

The influence of carbonate on the ozone/hydrogen peroxide process has been investigated. Carbonate radicals, which are formed from the reaction of bicarbonate/carbonate with OH radicals, act as a chain carrier for ozone decomposition due to their reaction with hydrogen peroxide. The efficiency of bicarbonate/carbonate as a promoter for the radical-based chain reaction in presence of hydrogen peroxide has been calibrated and compared to a well-known chain promoter (methanol) and an inhibitor (tert-butanol). Relative to tert-butanol, the hydrogen peroxide induced ozone decomposition is accelerated by bicarbonate/carbonate. Relative to methanol, bicarbonate/carbonate in presence of hydrogen peroxide is less effective as a promoter under comparable experimental conditions.     

A Two-Phase Computational Fluid Dynamics Model for Ozone Tank Design and Troubleshooting in Water Treatment

A computational fluid dynamics (CFD) tool was employed to design and study ozone contactors. The emphasis was to achieve the desired flow distribution. The Eulerian-Eulerian multiphase model was used with the standard k-? turbulent model. The water surface was slip wall boundary and was specified as a sink to remove ozone bubbles. For a single-column contactor with side entry, the flow pattern was found to be crucially dependent on both the direction and magnitude of the entry velocity from the inlet pipe. It was difficult to achieve uniform gas concentration over the contactor volume. In a multicompartment contactor, the countercurrent flow resulted in a mixed flow condition and the mixing increased with a higher gas rate. For the cocurrent flow, water was accelerated by the gas and the plug flow pattern was achieved. The flow distribution in each compartment can be significantly different even though the overall residence time distribution curves are similar.     

Applications of Ozone Decomposition Models

Simulated ozone decomposition profiles in “pure” water were made using two analytical kinetic ozone decomposition models and contrasted with experimental and literature data. Fundamental and applied applications of ozone consumption models are presented, demonstrating the importance of both direct and indirect oxidation of inorganic and organic species. A novel approach to simulating ozone decomposition in the presence of natural organic matter (NOM) is presented, concluding that NOM predominantly behaves as a direct consumer of ozone and promoter of ozone decomposition.     

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.     

Development Of An Ozone Disinfection Contactor Using A Physical Scale Model

This paper presents the design of a 1/5 scale model study of a five-stage counter-current ozone disinfection contactor. The selection of appropriate scaling laws is discussed and model test runs are presented and compared with the preliminary rule-of-thumb design. Simple modifications to the internal baffle design were tested leading to improved residence time characteristics.     

New Approach for Optimization of Ozone-Hydrogen Peroxide Water Treatment

A new radical reaction model for the ozone-hydrogen peroxide treatment, which was much simpler than current models such as SBH and TFG, has been developed by computer simulation in order to investigate a new reactor for this treatment process. It was revealed that the simulation results by using this new developed model were in good agreement with the experimental results in terms of the variation in the concentration of ozone in both gas and liquid phase, hydrogen peroxide and TOC, indicating the appropriateness of this new model. Thus, it was considered that this new model was very convenient for analysis of radical reactions because of its simplicity.     

Hydroxyl Radical/Ozone Ratios During Ozonation Processes. I. The Rct Concept

The ozonation of model systems and several natural waters was examined in bench-scale batch experiments. In addition to measuring the concentration of ozone (O₃), the rate of depletion of an in situ hydroxyl radical probe compound was monitored, thus providing information on the transient steady-state concentration of hydroxyl radicals (√OH). A new parameter, R_ct , representing the ratio of the √OH-exposure to the O₃-exposure was calculated as a function of reaction time. For most waters tested, including pH-buffered model systems and natural waters, R_ct was a constant value for the majority of the reaction. Therefore, R_ct corresponds to the ratio of the √OH concentration to the O₃ concentration in a given water (i.e. R_ct = [√OH]/[O₃]). For a given water source, the degradation of a micropollutant (e.g. atrazine) via O₃ and √OH reaction pathways can be predicted by the O₃ reaction kinetics and R_ct .