Modelling Hydraulics of Ozone Contactors

The U.S. EPA published the surface water treatment rule which imposed stringent requirements for disinfection. The rule is based on the C × T concept. The detention time T₁₀ must be determined for each type of contactor by performing tracer tests. Those tests have been performed for various ozone contactors. The results were evaluated using two types of models. For a classical diffuser-bubble contactor the ratio T₁₀/τ is about 0.5. For an industrial deep U-tube this ratio is equal to 0.55, but it reaches 0.9 for a pilot system. These results are indicating that a deep U–tube can provide a greater T₁₀/τ ratio than a diffuser–bubble contactor.     

Multistage Ozone Treatment of Dye Waste

An ozonation technology to treat lightly loaded effluents from dye manufacturing processes has been developed. The process uses airgenerated ozone and countercurrent contactors. An intermediate step is included to eliminate OH-radical scavengers and slowly reacting oxidized species from the liquid phase.

A study of ozone efficiency and yield concerning the reduction of TOC and COD levels is presented. The biological degradability.of treated water is discussed. A design and rating calculation procedure for countercurrent contactors is presented. Various waste treatment strategies using separation processes and ozone are discussed briefly. A cost estimate to treat a typical dye effluent in two stages on industrial scale is included.     

The Development Of An Ozone Contact Tank Simulation Model

Anglian Water (AW) operates ozone facilities as a treatment stage at all its surface water sources. The main ozone treatment stage takes place after filtration in multi-compartment concrete ozone contactors. These have two stages of ozone gas injection via ceramic bubble diffusers and two decay stages.

To enable AW to gain a better understanding of the contact tank performance, a computer simulation model incorporating Computational Fluid Dynamics and ozone process modeling was developed in conjunction with AEA Harwell. The model was calibrated using site data from Alton WTW. The results were used to improve the tank design and to optimize the ozone dose; this resulted in improved performance and a significant reduction in ozone dose.     

Ozone Plant Operations Under Drought Conditions

The Metropolitan Water District of Southern California (Metropolitan) owns and operates five potable water treatment plants. Currently four of the treatment plants utilize ozone as the primary disinfectant. The Metropolitan ozone contactors are over-under baffled and include 10 chambers, an afterbay, and an effluent channel. Ozone is generally added in either the first countercurrent gas/liquid flow chamber (Chamber 1) or the first and second (co-current gas/liquid flow) chambers. Disinfection credit is received from ozone residuals in Chambers 2 through 6. An ozone destruct system collects and destroys ozone off-gases from Chambers 1 through 9 and an ozone quenching system is used to minimize dissolved ozone from leaving the contactors. Severe drought in California, and resulting water supply limitations, have created significant operational issues ranging from reduced plant flows to source water changes and water quality challenges. The combination of reduced flow and different source water conditions resulted in the ozone systems being operated occasionally under unfamiliar conditions. This article discusses some of the challenges faced in operating the ozone plants at low flows and low ozone production rates. Further, water quality issues required prudent management of operations to meet regulatory requirements. From the lessons learned, several recommendations are offered in preparation for similar conditions in the future.     

Reactivity Studies In The Ozonolysis Of Pollutants Using A Perforated Spinning Disc Reactor To Enhance Mass Transfer

A perforated spinning disc ozone contactor is described with reference to its use as an absorber with simultaneous chemical reaction.

Greatly enhanced mass transfer coefficients k_L are measured whilst simultaneously maintaining low ozone loss. Comparisons of k_L and volumetric coefficient, k_L a values, are made with more conventional packed or bubble columns.

Acetic acid, 2-propanol and 4-nitrophenol, representing a wide reactivity range, are used to elucidate the applicability of rotating contactors in effluent treatment. It has been possible to study the effects of surface activity on mass transfer with subsequent reaction and to generate design data for the next generation of rotating contactors.     

Pilot-Scale Evaluation Of The Effects Of Mixing On Ozone Disinfection Of Escherichia Coli In A Semi-Batch,Stirred Tank Reactor

A research program was undertaken to examine the effects of mixing intensity and post-ozonation conditions on the survival of Escherichia coli ATCC 11775 in a high quality secondary wastewater effluent with a total organic carbon content of 8 mg/L and a chemical oxygen demand of 26 mg/L. The study was conducted using a pilot-scale, semi-batch stirred tank reactor with a 6-blade Rushton turbine. Two power conditions were investigated: 255 W/m³ and 870 W/m³. Two post-ozonation conditions also were investigated. In the first, a headspaceless sample was withdrawn and stored in the dark, in a quiescent condition. For the second, the gas flow to the reactor was stopped, but mixing was continued.

For equivalent contact times, it was found that two orders of magnitude more E. coli survived under the second post-ozonation condition when compared with the first condition. There was also a significant difference associated with the mixing intensity in the contactor, with the higher power input resulting in less efficient inactivation of E. coli.

The results confirm the importance of designing an ozone contactor to promote the maintenance of aqueous ozone in the contactor. In addition, optimum ozone mass-transfer may require different contacting conditions than those required for optimum disinfection performance. It was concluded that the design of ozone contactors should consider the use of at least two-stages: one optimized for ozone mass-transfer and one optimized for disinfection contacting.     

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.     

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.     

Ozone Contactor Hydrodynamics

Tracer tests were conducted at the 6,000 pounds of ozone per day Tucson, CAP Water Treatment Plant in Tucson, Arizona. The tests were designed to determine T₁₀ values through the contactors at various operating conditions. The tests were modeled using three techniques. Peclet Number was calculated for each of the runs, which would indicate the hydrodynamic conditions inside the ozone contactor. The results indicated that the increase in water flow rate and the number of cells with gas flow increased Peclet Number. The flow rate of liquid seemed to impact the Peclet Number more than gas flow. The headloss in each cell appeared to be important in controlling the distribution of liquid and gas through the cell. A correlation was developed between the product of gas and liquid phase Reynolds Number and Peclet Number.     

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.     

In-Line Ozone Dissolution Demonstration-Scale Evaluation

A demonstration-scale, in-line ozone dissolution and contacting system was constructed and operated for the Long Beach Water Department and the Metropolitan Water District of Southern California. The in-line process was assessed to determine its suitability for ozone oxidation of color in local groundwater. Results indicate that ozone utilization for color oxidation with an in-line system is more efficient than with a conventional bubble contactor.     

Modelisation Of Micropollutant Removal In Drinking Water Treatment By Ozonation Or Advanced Oxidation Processes (O3/H2O2)

A simulation program is described, tested and used, to predict micropollutant removal in an ozonation bubble tower with or without hydrogen peroxide addition. To compute the removal efficiency, we need to know the chemical reactivity between organic compounds and oxidant species (molecular ozone and hydroxyl radicals), the ozone mass transfer from the gaseous phase to the liquid phase (k_La) and the hydrodynamic model describing the reactor. In this case, we divide the reactor into three parts (water arrival, air arrival and intermediate zones). Each part is modelled using completely stirred tank reactors in series (CSTR).

In each CSTR, the calculation of oxidant concentrations (O₃, H₂O₂) is made through mass balance equations and a semi-empirical formula which gives hydroxyl radical concentrations as a function both of ozone concentration and the main characteristics of the water to be treated (pH, TOC, alkalinity). Another semi-empirical formula links ozone consumption to the same characteristics.     

Kinetic Modeling of Ozone Generation via Dielectric Barrier Discharges

A mathematical model combining chemical kinetic and reactor geometry is developed for ozone synthesis in dry O₂ streams with a wire-tube dielectric barrier discharge (DBD) reactor. Good agreement is found between the predicted ozone concentrations and experimental data. Sensitivity analysis is conducted to elucidate the relative importance of individual reactions. Results indicate that the ground-state oxygen atom is the most important species for O₃ generation; however, ozone generation will be inhibited if the O atom is overdosed. The excited species, that is, O(¹ D) and O₂(b ¹Σ), can decompose O₃ and suppress ozone synthesis. The model developed is then applied to modify the original DBD reactor design for the enhancement of ozone yield. With a thinner dielectric thickness, more than 10% increase of ozone concentration is achieved.     

Fluidized Bed GAC Filtration for the Control of Ozone Residuals

A laboratory scale study was conducted in order to investigate upflow granular activated carbon (GAC) filtration for the removal of aqueous ozone. The experimental results showed that fluidized bed GAC filtration operated at hydraulic loadings of 60 — 100 m/h is a promising process for the safe destruction of ozone residuals at the end of ozone contactors. The key parameters in terms of ozone destruction are empty bed contact time, the specific ozone destruction capability of the different GACs and the specific surface of the carbon. Ozone removal in such filters was well described with first order kinetics and external mass transfer was not limiting. Under given conditions in terms of ozone influent concentration, GAC granulometry, GAC density and bed depth, hydraulic loading and available headloss have to be traded off in order to design a well adapted and robust system.     

Modelling Industrial Wastewater Ozonation In Bubble Contactors. 2. Scale-up From Bench To Pilot Plant

A kinetic model constituted by ozone mol balance equations both in the gas and in the water phases and a total mole balance equation has been applied to predict concentrations of dissolved ozone, C_o3, ozone partial pressure at the reactor outlet, P_(o3)0, and remaining chemical oxygen demand, COD, for the ozonation of two industrial wastewaters released from distillery and tomato processing plants.

Kinetic equations for ozone absorption rate present in the model were derived from the application of film theory to an irreversible gas-liquid reaction. Parameters involved in the model, reaction rate and mass transfer coefficients, Henry's law constant, etc., were estimated from bench-scale experiments. The model was applied to ozonation in bubble contactors of height/diameter ratio equal to that of the bench scale contactor and to a pilot plant bubble column of a height/diameter ratio about 3.6 times higher.     

Comparing Different Designs and Scales of Bubble Columns for Their Effectiveness in Treating Kraft Pulp Mill Effluents

The experimental results obtained in three different types of ozone contactors were analyzed to study the effects of the ozone contactor design, configuration, operating conditions, and scale-up on the: (1) ozonation process induced reduction efficiencies of color, AOX, COD, and TOC from biologically treated Kraft pulp mill effluents; (2) the increase in biodegradability of this type of wastewater; and (3) the dynamics of the ozone gas absorption process. The three types of ozone contactors included: (1) an extra-coarse-bubble diffuser ozone contactor; (2) an impinging-jet ozone contactor; and (3) a fine-bubble diffuser ozone contactor. Similar treatment levels were achieved in those ozone contactors although the impinging-jet bubble column was more effective in treating Kraft pulp mill effluents due to its smaller reactor volume and lower off-gas ozone concentrations. Consequently, the operating costs of an ozonation process and ozone off-gas destruction facilities will be greatly reduced when using the impinging-jet bubble column design for treating Kraft pulp mill effluents.     

After a Decade of Development: A Profile of Municipal Drinking Water Plants Utilizing a Sidestream Injection Process

The technology of rapid ozone mass transfer followed by degasification, the GDT? Process, was first introduced in 1995 Mazzei, A.L., Meyer, R. and Bollyky, L.J. . Proceedings International Ozone Association Pan American Group Conference. “Mass Transfer of High Concentration Ozone with High Efficiency Injectors and Degassing Separators”, November, Cambridge, MA [Google Scholar] by Mazzei et al. At the time of introduction, municipal ozone installations utilized a low concentration, air-fed ozone gas as a disinfectant in atmospheric contact basins fitted with fine bubble diffusers (FBD). Over the past decade, air- fed ozone has given way to highly concentrated, oxygen-fed ozone. The change to concentrated oxygen feed gas has increased concerns about the corrosive effects of high finished water dissolved oxygen (DO). Water treatment plants using oxygen fed ozone have reported finished water DO levels in excess of 20 mg/L, with some plants resorting to air sparging at the back end of the contact basin to restore finished water to atmospheric gas levels.

However, the evolution to oxygen feed gas has also produced significant cost benefits. Operating an ozone generator on oxygen increases its ozone production; reducing the size and capital cost of the generator needed to meet ozone output requirements. The use of a concentrated gas stream has also led to the development of side stream injection systems, which move the gas mixing out of the atmospheric basin and into the upstream pipeline (Neemann, 2002 Neemann, J. . The Use of Injectors and Nozzles for Sidestream Ozone Addition. Proceedings AWWA Water Quality Technology Conference.  [Google Scholar]), resulting in a more compact contact basin design. Municipal water plants not having a CT requirement have streamlined one step further, by eliminating the ozone contact basin in favor of a sidestream injection Process. This paper reviews the technology of the GDT? sidestream injection process and introduces 2 municipal water treatments plant (WTP) installations utilizing this process to remediate taste and odor compounds and as a method to reduce finished water dissolved oxygen concentrations.     

Oxidation of 1,3,5‐trichlorobenzene using advanced oxidation processes

The oxidation of 1,3,5‐trichlorobenzene (TCB) by ozone, ozone/UV, ozone/H₂O₂ and ozone/UV/H₂O₂ was studied. All studies were conducted in a continuously‐flowing completely mixed reactor (CFCMR), operated at steady‐state conditions using a hydraulic retention time of 10 minutes. The greatest removal of TCB using ozone/H₂O₂ treatment was achieved using a H₂O₂ concentration of 60 μM. At low pH values (approx. 2) ozone/UV performed significantly better than either ozone alone or ozone/H₂O₂. However, at circumneutral pH, the removal efficiencies of TCB by ozone/UV and ozone/H₂O₂ and ozone/UV/H₂O₂ were essentially equal (～ 97% for TCB). The removal efficiency of ozone alone was ～93% for TCB. At high pH (> 9) there was no advantage in supplementing ozone with either UV or H₂O₂ as the removal efficiencies for all processes studied were essentially equal.

The effect of humic acid and bicarbonate on the removal of TCB was studied. At 1.6 mg/L humic acid, 92–95% of the TCB was oxidized by the processes studied. The removal of TCB by ozone alone was significantly affected by the presence of bicarbonate ion. For the other processes at 10 mM bicarbonate, approximately 80% of the TCB was oxidized.     

Use Of Static Mixer For Oxidation And Disinfection By Ozone

Ozone is used in drinking water treatment as a biocide, as an oxidant and as a pretreatment in order to improve the performance of subsequent processes. Increasing concern over the quality of drinking water has led to a number of new stringent regulations in the control of chemical and microbiological contaminants. Disinfection deals with the concept of “CT”, which is the need to maintain a certain minimum concentration for a given time. Under ideal laboratory conditions, it is 0.4 mg O₃/L for 4 min. In practice, since the method for the CT determination has not been finalized by the EPA, “T” can be the minimum detention time of 90% of total flow, and “C” can be a measured ozone residual at the outlet of cells of the contactor. New standards for micropollutants in drinking water imply an optimization of the ozonation step, by improving the ozone transfer from gas to water, and the control of the detention time as well as ozone residual within the contactor.

All these considerations have led us to use static mixers to transfer ozone into water. This process enables us to control the ozone concentration in water and detention time. It is a very simple system, with very low maintenance requirements due to the lack of moving parts. Civil engineering is minimized. A pilot scale study is presented here. It took place at the Méry-sur-Oise water treatment plant, on a pilot plant working at 8-12 m³/h. It is composed of a static mixer for the transfer of ozone from gas to liquid, linked to an air lift to separate gas from liquid, providing ozonated water.

The optimization of transfer was achieved by studying the impact of water flow, gas flow and ozone concentration in the gas. It is possible to reach 90% of transfer in less than 15 s. Headloss (ΔP) across the mixer is a function of gas and water flows and remains economically very acceptable as 0.15 bar for 12 m³/h.

Atrazine removal was studied using a static mixer, an air lift and a contact pipe 80-m long, providing an optimum contact time phase, working as a plug flow reactor. Ozone and H₂O₂/O₃ treatments were compared. The maximum reduction of atrazine concentrations (e.g., for an infinite contact time) is a function of the amount of transferred ozone, but H₂O₂ influences the kinetics of the reaction. In the presence of H₂O₂ with a ratio of H₂O₂ to O₃ of 0.4 w/w, maximum elimination is reached in 2 min 30 s.

The effect of such treatments on environmental bacteria also was followed. A counting of total germs at 20°C showed a decrease of 1- to 3-logs 10 after 1 min 30 s of contact time for about 2 mg/L of transferred ozone. No significant difference between treatments with or without H₂O₂ was shown. The same conclusions were obtained from heterotrophic plate counts (37°C) and epifluorescence countings.     

Modeling of the Reaction of Ozone with Dehydroabietic Acid

The oxidation of dehydroabietic acid (DHA) by ozone in solution was examined using a spectrophotometric method in combination with the computer program MATLAB to empirically fit the data. The reaction rate constant between the two compounds was determined to be 1.1×10² L/mol·s at 23 ºC. This value is in line with previous work on multiply-substituted aromatic compounds. The stoichiometric ratio of the ozone/DHA reaction was found to be 3 moles of ozone per mole of DHA consumed. Furthermore, a stoichiometric ratio of 3:1 H₂O₂ produced:DHA oxidized was determined to have the best fit with experimentally derived results.