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.     

Optimizing an Intermediate Ozone System Used for Primary Disinfection at a 55 MGD Surface Water Treatment Plant

The Henrico County VA Water Treatment Facility has been in operation since April 2004, making use of intermediate ozone for primary disinfection to take advantage of both the disinfection and DBP benefits. Since the summer of 2007 the staff at the Henrico WTF has embarked on an optimizing journey with their ozone system. This paper overviews the significant milestones accomplished on this journey as well as ongoing improvements. Optimizing efforts to reduce operating costs while achieving disinfection goals included: Staff training workshops; Automating ozone data collection and access; Operating system at optimum ozone concentration; Selecting optimum contactor sampling locations and number of contactors; Overcoming minimum gas flow limitations; Solenoid contactor sampling system issues and improvements; Disinfection calculation constraints; Excessive buildup of liquid oxygen (LOX) tank pressure. Each of these items is discussed following an overview of this 55 MGD surface water treatment facility.     

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.     

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.     

Finite Element Analysis And T10 Optimization Of Ozone Contactors

A finite element analysis (FEA) computer program was used to model the hydraulic characteristics and to predict the corresponding residence time distribution (RTD) curves for four full-scale ozone contactors with hydraulic capacities ranging from 14.5 to 394 m³/min (5.5 to 150 mgd). On average, model predictions for contactor efficiency (T₁₀/HDT) were within 7.9% of actual test values. The Metropolitan Water District of Southern California (Metropolitan) is using the finite element program as a design tool to optimize the hydraulics of future ozone contactors. For one template model, results from several optimization runs are presented which confirm that T₁₀/HDT increases with increasing depth-to-length (D/L) ratio and which indicate that by adding two features, vanes or wall foils, the T₁₀/HDT is improved from 0.63 to 0.68 or to 0.66, respectively. Results from the template model also indicate that the T₁₀/HDT is unchanged with the addition of a single configuration of comer fillets.     

Retention capacities and flow patterns of vortex contactors

The hydrodynamic features of vortex contactors, such as vortex chambers and contactors with swirled spouted beds, designed for the thermal treatment of dispersed materials, such as drying, when the material comes into direct contact with the swirling flow of the heat carrier gas are considered. The retention capacity of vortex chambers with vertical and horizontal axes is calculated and the flow patterns in vortex contactors are analyzed.     

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.     

Designing High Concentration Ozone Contactors for Drinking Water Treatment Plants

The design of high concentration ozone contacting systems requires special attention to ensure that mass transfer, mixing and disinfection objectives are met when low volumetric gas flow rates are used. This becomes especially critical when the ozone contactor is designed for primary disinfection and must meet CT (ozone concentration multiplied by contact time] disinfection values. This paper presents design guidelines for high concentration ozone contactors. These guidelines then are applied to the design of a new 55-mgd two-stage ozonation potable water plant for Henrico County, Virginia. Finally, a new type of in–line ozone injection contactor is presented as an alternative to fine-bubble diffusion contactors to provide primary disinfection.     

Computational fluid dynamics applied to high temperature hydrogen separation membranes

This work reviews the development of computational fluid dynamics (CFD) modeling for hydrogen separation, with a focus on high temperature membranes to address industrial requirements in terms of membrane systems as contactors, or in membrane reactor arrangements. CFD modeling of membranes attracts interesting challenges as the membrane provides a discontinuity of flow, and therefore cannot be solved by the Navier-Stokes equations. To address this problem, the concept of source has been introduced to understand gas flows on both sides or domains (feed and permeate) of the membrane. This is an important solution, as the gas flow and concentrations in the permeate domain are intrinsically affected by the gas flow and concentrations in the feed domain and vice-versa. In turn, the source term will depend on the membrane used, as different membrane materials comply with different transport mechanisms, in addition to varying gas selectivity and fluxes. This work also addresses concentration polarization, a common effect in membrane systems, though its significance is dependent upon the performance of the membrane coupled with the operating conditions. Finally, CFD modeling is shifting from simplified single gas simulation to industrial gas mixtures, when the mathematical treatment becomes more complex.     

Ozone Contactor Hydraulic Considerations in Meeting “CT” Disinfection Requirements

Tracer studies were performed in bench and pilot scale ozone diffusion contactors to determine actual contact times for the bench and pilot scale units and to characterize the flow pattern through these reactors. It was recognized that the flow pattern through the ozone contactors approached plug flow conditions. Due to the short circuiting and dispersion, the actual detention time in the reactors was one-half of the theoretically calculated detention time. Actual values of observed detention time were used in calculations of the “CT” values for ozone in the water. Optimization of ozone dose and contact time for “CT” calculations was performed in the pilot plant. Operational guidelines for the application of ozone in Union Hills pilot plant were developed based on the data obtained in the pilot plant runs.     

Cooling Conditions of Ozone Generators

The efficiency of ozone generators is determined by many factors. Operating conditions such as feed gas quality and especially cooling conditions are of utmost importance. Cooling of ozone generators is absolutely necessary, since ozone destruction reactions increase exponentially with temperature. The most common way to cool an ozone generator is water flowing in close contact to the electrodes. The heat removal out of the discharge gap depends on different parameters. Electrical input power, cooling water flow conditions, electrode geometry and material properties are some of them. Simultaneously lowering cooling water temperature, applied power density and gap width, leads to a lower gas temperature in the discharge gap and thus to increased ozone production efficiency. Minimizing the temperature difference between the cooling water inlet and outlet improves the ozone production efficiency as well. This measure, however, results in high cooling water flows and requires additional cooling water chilling, resulting in higher operational costs and capital expenses. Cooling associated costs rise disproportionally with increasing cooling water flow. Simultaneously, energy consumption of ozone generators decreases as the average cooling water temperature goes down. As a result, there exists an optimum between the operational and capital expenses for the combination of ozone generator and cooling water system related expenses, offering significant cost savings for the customer.     

The “Big Switch”: Los Angeles Aqueduct Filtration Plant Treatment Of California State Project Water

Plant-scale tests were run in October 1987 and October 1988 to evaluate the ability of the 26.3-m³/s (600-mgd) Los Angeles Aqueduct Filtration Plant (LAAFP) to treat one of the source waters -- California State project water (SPW) -- used by the Metropolitan Water District of Southern California and to determine CT^a values for its ozone contact basins. During the test period, a large amount of water quality and operating data was collected, providing insight into the effectiveness of the preozonation/direct filtration process (followed by chlorination or chloramination) on SPW. This paper describes the results of the full-scale testing.     

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.     

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.     

Drinking water denitrification in membrane bioreactor/membrane contactor systems

The results of an experimental study, developed on a membrane bioreactor/membrane contactor pilot plant, aimed at drinking water denitrification are presented and discussed. In the adopted configuration the water, contaminated by nitrates, flows inside the fibres of a membrane unit. Due to the existing concentration gradient, nitrates migrate through the membrane and are reduced to nitrogen gas by the autotrophic biomass attached on the exterior of the fibres, and fed with an external source of organic carbon. Data obtained varying influent flow values and nitrate influent concentrations, confirm the potentiality of the system and show the possibility of full-scale applications. A new mathematical model, useful for both simulation and design of the system is also presented. The model is based on simple mass balances in the flow direction, and through the membrane. Each fibre is considered a plug-flow reactor, and nitrate concentration outside the fibres is assumed to be always zero. To obtain an explicit expression useful for simulation and design of membrane bioreactors/membrane contactors, steady-state conditions are supposed. Experimental data are in good agreement with the model's results, and confirm its applicability.     

Hydrodynamic Characterization and Mass Transfer Analysis of an In-Line Multi-Jets Ozone Contactor

This research study investigates mixing and ozone mass transfer characteristics of a pilot-scale in-line multi-jets ozone contacting system. The hydrodynamic characteristics of the contactor were studied using a two-dimensional laser flow map particle image velocimetry coupled with planar laser induced fluorescence (PIV/PLIF). The PIV/PLIF system provided a combination of simultaneous whole-field velocity and concentration data in two-phase flows for different operating conditions. All measurements were conducted under a total liquid flow rate of about 10 L/s with gas flow rate ranging from 0.05 to 0.4 L/s. The gas was introduced to the system through a series of side stream injectors. The side injectors were tested under opposing and alternating modes. A mass transfer study was also conducted to estimate the overall mass transfer coefficient under the same operational conditions used for the hydrodynamics study. It was found that for the same number of jets (i.e., same gas flow rate) the liquid dispersion (D_L) was higher when alternating jets were used. Higher ozone mass transfer rates were observed when using opposing jet compared to the same number of alternating jets.     

Durability Test Results of Construction and Process Materials Exposed to Liquid and Gas Phase Ozone

The Metropolitan Water District of Southern California performed a study to evaluate materials of construction for use in ozone contactor construction. The concept reactor is a baffled box conduit constructed of concrete with metallic valves, slide gates, etc. for flow control and metallic piping for ozone gas distribution and off-gas collection. The study concluded that ozone by itself has no impact on standard type 2/type 5 concrete and that pozzalonic admixtures could enhance the structural design. Metals exposed to ozone should be constrained to the austenitic grades of stainless steel. Plastics should be limited in their application, elastomers should be limited to Viton-A and EPDM, and none of the elastomers, protective coatings, structural adhesives or joint sealants tested show promise in long term performance.     

Design And Efficiency Of Ozone Contactors For Disinfection

One commercial contact chamber which is designed for ozone disinfection has been tested for the following : determination of residence time distribution (RTD), measurement of dissolved ozone concentrations in water, and measurement total plate counts. Disinfection is achieved but this type of reactor is not ideal, due to dispersion of the residence time distribution. As these bubble contactors do not behave like plug flow reactors, it is possible to improve the hydrodynamics of ozone contactors with the use of static mixers. Experiments with ozone static mixers have been conducted.     

Effects of baffles on separation of aqueous ethanol solution with hollow fibers

The effects of baffles on the operation and mass-transfer characteristics in novel hollow fiber membrane contactor used in distillation were investigated. Hollow fiber membranes, coated with a 7 μm polydimethyl-siloxane, were operated as structural packing in the separation of ethanol-water solutions. The parallel flow mode was chosen for separation due to the stronger driving force of the concentration difference, in which liquid flows through the lumens of the fibers and vapor flows countercurrent-wise outside the fibers. Two baffles were installed on the shell side of the membrane contactors to enhance separation, which had a round shape with a semi-lunar hole. The results show that both baffled and unbaffled membrane contactors gave better, more productive separations than traditional packing in distillation, such as the excellent Sulzer Gauze BX structured packing. The baffled membrane contactors performed better than unbaffled ones, especially at high vapor velocities. The minimal HTU of membrane contactor with baffles could reach as low as 4.5 cm, and almost all the contactors could work well above the limit where flooding normally occurs in conventional cases. Theoretical analysis predicted that baffles helped membrane module to obtain a higher masstransfer coefficient and a smaller mass-transfer resistance. Finally, theoretical mass-transfer coefficient and experimental value were compared as well as the contribution of each individual mass-transfer coefficients among liquid, gas and membrane.     

Enhancement of Wastewater Disinfection Efficiency in Full-Scale Ozone Bubble-Diffuser Contactors

Tracer and disinfection tests were performed with the ozone bubble-diffuser contactors at the Belmont and Southport Advanced Wastewater Treatment Plants operated by the City of Indianapolis, Department of Public Works. The objective of the study was to develop a better understanding for the role of hydrodynamics and contactor design on the disinfection efficiency achieved in these contactors. Tracer tests were performed at varying gas and wastewater flow rates. The results indicated that high backmixing occurred within each chamber of the over-under ozone bubble-diffuser contactor trains. The addition of three baffles to one of the contactor trains resulted in a decrease in overall contactor backmixing. Low contactor backmixing was observed at high wastewater flow rates combined with high or medium gas flow rates for both the modified and original trains. Monitoring of effluent fecal coliform concentrations for both the original and modified contactor trains revealed lower average concentrations in the modified train effluent as compared to the original train.