Comparing Static Mixer Performances at Pilot and Full Scale for Ozonation,Inactivation of Bacillus subtilis,and Bromate Formation in Water Treatment

The potential benefits of using a static mixer for ozone dissolution was evaluated through comprehensive pilot- and full-scale studies under a variety of operating conditions and source waters. The static mixer pilot unit was operated side-by-side to a full-scale plant which also employed static mixers for ozonation. Based on the results obtained from this pilot study (and at other sites), it appears that an optimal ozone dose (≤0.5mgO₃/mgC) applied through a static mixer dissolution system integrated with a well-designed downstream contactor can result in enhanced microbial inactivation while keeping bromate formation below 10μg/L.     

Ozone Disinfection of Sewage in a Static Mixer Contacting Reactor System on a Plant Scale

Results of our earlier laboratory study on ozone contacting systems in a continuous flow mode identified that the ozone disinfection process is limited by the mass transfer rate (7). The main controlling factor is the mass transfer efficiency rather than the contact time of the contactor in determining the effect of disinfection. By applying these concepts, we suggested a new ozone disinfection technique of using a static mixer as the contactor to substitute for a conventional bubble column designed with contact time.     

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.     

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.     

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.     

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.     

Comparison of Mass Transfer Efficiency and Energy Consumption in Static Mixers

The aim of this study was to compare the pressure drop (Δp) generated by a static mixer with sieve plates in two-phase downflow (water as a continuous phase), and the mass transfer efficiency (k_La, a) with the performance of other static mixers (Sulzer, Kenics, Karman, etc.). The relationships for Δp, k_La and interfacial area (a) calculation depending on liquid and gas phase velocities and geometry of the plates (sieves) in this static mixer are presented. k_La was found to be strictly proportional to the power consumption (P/V) and its values were quite close to those obtained in Sulzer & Kenics mixers with an 8-element mesh. Enhancement factors for oxygen absorption in the sodium sulphite solution and for ozone absorption in Lake Ülemiste water were calculated and the plausible values of the interfacial area (a) were estimated.     

Effect of Ozone Injection on the Catalytic Reduction of Nitrogen Oxides

The improvement in the catalytic reduction of nitrogen oxides (NO_x) by means of the ozone injection into the exhaust gas was investigated. Nitric oxide (NO) in the exhaust gas was first oxidized to nitrogen dioxide (NO₂) by ozone, and then the exhaust gas containing the mixture of NO and NO₂ was directed to the catalytic reactor where both NO and NO₂ were reduced to nitrogen. The ozone injection method was very efficient for the oxidation of NO to NO₂ in a wide range of temperatures, and the increase in the content of NO₂ by the ozone injection remarkably improved the performance of the catalytic reactor.     

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.     

Influence of Geometrical Characteristics and Operating Conditions on the Effectiveness of Ozone Contacting in Fine-Bubbles Conventional Diffusion Reactors

Ozone contactor hydrodynamics are strongly dependent on the geometry and the operating conditions of the system. In this paper results obtained on a pilot scale reactor showing the relationship between the geometrical characteristics of an ozonation reactor and its hydrodynamic behavior are presented. The validity of the proposed models has been checked on several full-scale reactors for which data were available in the literature.     

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.     

Reduction of Excess Sludge Produced by Biological Treatment Processes: Effect of Ozonation on Biomass and on Sludge∗

The excess sludge produced during biological treatment of wastewater can be reduced by treating this sludge with ozone in a specific reactor and recycling it to the biological facility. This increases the biodegradability of the inert fractions of the sludge without deteriorating the activity of the microorganisms. Ozone reacts only within the film zone near the gas/liquid interface: it is assumed that the size of the microflocs of active microorganisms is greater than the effective thickness of the film, thus protecting them from ozone. This coupled treatment produces treated water having satisfactory characteristics and a residual excess sludge that has an extremely high settling capability.     

Laboratory Simulation of the Effect of Ozone and Monochloramine on Biofilms in Drinking Water Mains

Cultivated on cement surfaces to simulate growth on cement mortar lined drinking water mains was investigated in a laboratory biofilm reactor. The biofilms were subsequently exposed to ozone and monochloramine. Inactivation of biofilm bacteria by ozone was found to be a non-linear function of the ozone dose, and the results provided a more rational basis for determining conditions for ozone disinfection of water mains. Results further suggest that monochloramine might be an effective disinfectant for biofilms on pipe walls. Microbiological observations indicated that monochloramine is a non-selective disinfectant compared to ozone.     

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.     

Comparison of Ozone and Oxygen Mass Transfer in a Laboratory and Pilot Plant Operation

Mass transfer of ozone and oxygen to water was investigated both in pilot plant countercurrent bubble column and in a Rushton type laboratory stirred reactor supplied with a variable speed turbine agitator. A comparison was made for different hydrodynamic conditions with the main task of developing an engineering approach for determination of the physical volumetric mass transfer coefficient (K_L ^oa), specific interfacial area (a), and physical masstransfer coefficient (K_LO). The mass transfer characteristics of ozone and oxygen can be determined quickly in a pilot plant or laboratory apparatus, and employed to optimize the performance ofthe full scale water treatment plant.     

The Effect of SF6 on Ozone Generation using Dielectric Barrier Discharge

This paper reports the results of an experimental study of effect of SF₆ on ozone generation within a Dielectric Barrier Discharge (DBD) fed by both pure oxygen and dry air. The chemical reaction mechanisms relevant to the process of ozone generation (and destruction) are discussed. The experimental results show the oxygen source should avoid the presence of SF₆ but the addition of a small amount of SF₆ in an air discharge can improve ozone concentration and ozone produce efficiency.     

Determination of the Henry's Coefficient and Mass Transfer for Ozone in a Bubble Column at Different pH Values of Water

In this study, the ozone gas-liquid mass transfer into water in a bubble column was investigated for different pH values. The ozone volumetric mass transfer coefficient and the Henry's coefficient were determined simultaneously by parameter estimation using a nonlinear optimization method. A sensitivity analysis was conducted to obtain information on the reliability and identifiability of the estimated parameters. A minor dependence of the Henry's law constant on pH was detected at the pH range 4 to 9.     

Effect of Ozone Treatment on the Reduction of Chlorpyrifos Residues in Fresh Lychee Fruits

The effect of ozone on the reduction of chlorpyrifos residue in lychee cv. Chakapat (Litchi chinensis Sonn.) was studied. Lychee fruits were dipped in the solution of chlorpyrifos at a concentration of 10 mg L^?1 for 10 min. Then, they were exposed to ozone gas (O₃) at concentrations of 80, 160, 200, 240 mg L^?1 and dipped in ozone-containing water, at concentrations of 2.2, 2.4, 3.4 and 3.2 mg. L^?1 for 10, 20, 30 and 60 min, respectively. Both ozone gas and ozone-containing water reduced pesticide residue in lychee, but exposure to ozone gas for 60 min was most effective. When lychee fruits were stored at 25 °C for 6 days, both processes did not show significant differences in weight loss, total soluble solids (TSS) and titratable acidity (TA). However, ozone-containing water decreased the eating quality of lychees after storage, compared with the ozone-fumigated groups.     

Effect of Separation of Ozonation and Electrolysis on Effective Use of Ozone in Ozone-Electrolysis Process

For improving the ozone consumption efficiency (OCE) of the ozone-electrolysis process, an ozone-electrolysis system with an independent electrolytic cell from an ozone contactor (indirect ozone injection system) and a three-dimensional electrode were introduced in this study. The reactor successfully enhanced the OCE and the ratio of 1,4-dioxane degradation rate to ozone injection rate (RDO) in comparison with the previous reactor. The three-dimensional electrode also contributed the improvement of the OCE and the RDO due to lowering the current density on the cathode. As the indirect ozone injection system allows higher degree of freedom for the reactor design, it is useful for application of ozone-electrolysis to practical water treatments.     

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.