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.     

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.     

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.     

Improvement of Ozone Oxidation and Disinfection Design

For a long timei the aim of ozonation has been the improvement of the organoleptic qualities of treated water. Disinfection! and particularly virus inactivationi also has been a reason for introducing ozonation steps in the treatment line of a potable water treatment plant. Other purposes of ozonation then have been developed in different fields of application, such as iron and manganese removal, micropollutants oxidation, urban wastewater disinfection! coagulant and flocculant aids, and improvement of biological treatment. During the last few years, numerous studies have been concerned either with the kinetics of ozonation reactions, or the modelling of contact systems. A better fit between these two factors should be found.     

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.     

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.     

Ozone in the United States of America -- State-Of-The-Art

Applications for ozone in the United States have evolved through a lengthy maturation process, which began with drinking water treatment (taste/odor/color removal) in the early 1900s, and grew slowly until acceleration began in the mid-1980s. Although deodorization became a stable market in the 1960s-1970s, these applications were small, for the most part. One of the largest uses for ozone is oxidation of process chemicals in the chemical industry, which began in the USA about the 1940s, and subsequently has spread worldwide. Today, thanks primarily to environmental regulatory pressures which began to impact ozone in the mid-1980s, ozone now is used increasingly in the USA for drinking water treatment and for some municipal and industrial wastewater applications. The U.S. Environmental Protection Agency (EPA) has recognized the growing importance of ozone (> 200 drinking water plants use ozone today), and has appointed IOA representatives to two of its regulatory development committees as stakeholders. Several U.S. cities have installed or are installing wastewater treatment processes for potable reuse purposes, which include the use of ozone. Three full-scale U.S. pulp bleaching plants use tons/day quantities of ozone. Smaller applications for ozone include water treatment for cooling tower waters (biofouling control), swimming pools and spas, marine aquaria, bottled water disinfection and maintenance of high purity waters in the pharmaceuticals and electronics industries. A new application for ozone is in commercial laundries to reduce energy costs and replace chemicals. In mid-1997, a public declaration was made by an expert panel that ozone is Generally Recognized As Safe (GRAS) for contact with foods. This declaration opens the door for ozone to be used in U.S. food processing industries. U.S. research scientists and engineers are at the forefront in studies which define the technical aspects of ozone technologies in a variety of applications employing advanced oxidation, including the treatment of hazardous wastes, groundwater remediation, and process water recovery and reuse in the semi-conductor industry.     

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.     

Principles of Ozone Oxidation and Disinfection Design

The efficiency of ozone for different purposes can be determined by different laboratory tests. It is necessary to use a specific test for each aim of ozonation. Consequently, the design of commercial ozone–water contact systems can be determined in relation with the purpose of ozonation: virucide effect, oxidation of pollutant, clarification improvement or wastewater disinfection.     

Destruction of Volatile Organic Contaminants in Drinking Water by Ozone Treatment

Controlled, pilot-plant ozone treatment tests were conducted on twenty-nine volatile organic contaminants in distilled water and groundwater. Results show that aromatic compounds and alkenes are well removed by ozone treatment, but that alkanes are poorly removed. Also, efficiency of destruction improved for the alkenes and aromatic compounds with increasing applied ozone dosage and, for some alkanes, with increasing pH. For most compounds, the efficacy of ozone was not severely affected by the background water matrix. Generally, information gathered from the literature regarding rate constants for the ozone treatment of compounds in the gaseous phase or in organic solution predicted, to a useful degree, the effectiveness of ozone in treating aqueous solutions in the present study.

Several of the test conditions selected for this preliminary study may be similar to those found in drinking water treatment plants. Consequently the findings of this research may help guide utilities in their choice of alternative treatments to meet Maximum Contaminant Levels for volatile organic contaminants such as trichloroethylene and benzene.     

Selection and Design Considerations of Diffusers,Piping Systems,and Basin Configurations Producing Optimum Performance for the Ozone Treatment of Drinking Water

The performance of ozone diffusion systems is determined by the conversion of the engineered treatment theory to the hardware needed to accomplish the task. The correct design, selection, and fabrication of the ceramic gas piping systems which interface with the overall treatment scheme configuration achieves this end. Design details such as selection of proper stainless steel, ceramic, gasket materials, diffuser type, diffuser anchoring systems, and fabrication techniques assure the optimum performance of the treatment system.     

Wastewater Disinfection With Ozone - Process Control And Operating Results

The City of Indianapolis, Indiana operates two 125 mgd advanced wastewater treatment plants with ozone disinfection. The rated capacity of the oxygen-fed ozone generators is 6,380 Ib/day, which is used to meet geometric mean weekly and monthly disinfection permit limits for decal conforms of 400 and 200 per 100 mL, respectively. Since 1989, a disciplined process monitoring and control program was initiated. Records indicate a significant effect on process performance due to wastewater flow, contactor influent fecal coliform concentration, and ozone demand. Previously, ozone demand information was unknown. Several tasks/studies were performed in order to better control the ozone disinfection process. These include the recent installation of a pilot-scale ozone contactor to allow the plant staff to measure ozone demand on a daily basis. Also, tracer tests were conducted to measure contactor short-circuiting potential. Results demonstrated a noticeable benefit of adding additional baffles. Results also indicated operating strategies that could maximize fecal coliform removal, such as reducing the number of contactors in service at low and moderate flow conditions.     

Flocculation Effects of Ozone

There are numerous reports on coagulation/flocculation effects of preozonation, which cannot be interpreted by one single mechanism. About 7 basic processes are presently discussed. This paper presents results from drinking and wastewater treatment, indicating that preozonation effects can be explained with algae flocculation, with the destabilization of particles, and with polymerization of the dissolved organics. In view of the complexity of the systems, it is anticipated that several processes can be responsible, leading to an optimum ozone dosage.     

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.     

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.     

Combination of Ozone with Activated Carbon as an Alternative to Conventional Advanced Oxidation Processes

The objective of this study was to compare the efficiency of O₃/granular activated carbon (GAC) to enhance ozone transformation into ·OH radicals, with the common advanced oxidation processes (O₃/OH^?, O₃/H₂O₂). The results obtained with model systems under the given experimental conditions showed that the system O₃/OH^? (pH 9) and O₃/H₂O₂ (pH 7, [H₂O₂] = 1·10^?5 M) are more efficient than O₃/GAC (pH 7, [GAC] = 0.5 g/L) to enhance ozone transformation into ·OH radicals. However, in Lake Zurich water the O₃/GAC process has a similar efficiency as O₃/H₂O₂ for ozone transformation into ·OH radicals. The results also show that the presence of GAC during Lake Zurich water ozonation leads to (i) removal of hydrophilic and hydrophobic micropollutants, (ii) reduction of the concentration of CO₃ ^2?/HCO₃ ^?, and (iii) decrease of the concentration of dissolved organic carbon (DOC) present in the system.     

Hydrodynamic Evaluation of a Turbine Ozone Contactor

Detailed tracer and ozone profile evaluations in a full-scale turbine ozone contactor were conducted to support the development of a hydrodynamic model. The tracer results combined with ozone concentration profiles suggested that the flow through the contactor is very complex and cannot be described as a single CSTR as stipulated by current regulations. A multistage mathematical flow model was developed and calibrated. Cocurrent flow of water and gas in a central part of the reactor was postulated to explain significant ozone concentration gradients. The developed model was suitable for use as a framework for calculations of ozone mass transfer and residual concentrations.     

Applications of Ozone in Japan for Water and Wastewater Treatment

In Japan, ozone is used for potable water treatment, night soil and industrial wastewater treatment, and for offensive gas treatment at sewage and night soil treatment plants. This paper describes its implementation, and presents experimental findings of the combined ozone and ultraviolet radiation as a new ozone utilization technology.