Ozonation and Advanced Oxidation of Wastewater: Effect of O3 Dose,pH, DOM and HO•-Scavengers on Ozone Decomposition and HO• Generation

The decomposition of ozone in wastewater is observed starting 350 milliseconds after ozone addition. It seems not to be controlled by the autocatalytic chain reaction, but rather by direct reactions with reactive moieties of the dissolved organic matter (DOM). A larger ozone dose increases ozone consumption prior to 350 milliseconds but decreases the rate of ozone decomposition later on; this effect is predicted by a second-order kinetic model. Transferred Ozone Dose (TOD) is poorly correlated with ozone exposure (= ∫[O₃]dt) indicating that TOD is not a suitable parameter for the prediction of disinfection or oxidation in wastewater. HO^? concentrations (> 10^?10 M) and R_ct (=∫[HO^?]dt/∫[O₃]dt > 10^?6) are larger than in most advanced oxidation processes (AOP) in natural waters, but rapidly decrease over time. R_ct also decreases with increasing pre-ozonation doses. An increase in pH accelerates ozone decomposition and HO^? generation; this effect is predicted by a kinetic model taking into account deprotonation of reactive moieties of the DOM. DOC emerges as a crucial water quality parameter that might be of use to normalize ozone doses when comparing ozonation in different wastewaters. A rapid drop of absorbance in the water matrix—with a maximum between 255–285 nm—is noticeable in the first 350 milliseconds and is directly proportional to ozone consumption. The rate of absorbance decrease at 285 nm is first order with respect to ozone concentration. A kinetic model is introduced to explore ozone decomposition induced by distributions of reactive moieties at sub-stoichiometric ozone concentrations. The model helps visualize and comprehend the operationally-defined “instantaneous ozone demand” observed during ozone batch experiments with DOM-containing waters.     

Process Control For Ozonation Systems: A Novel Real-Time Approach

For real-time control of ozonation processes in water works, a sequencing batch reactor was constructed to measure the ozone decay rate constant (k_O3) in short time intervals of about 15 min. The batch reactor is filled during the production process, immediately after dissolving ozone in water by a static mixer. On the basis of k_O3 and the initial ozone concentration ([O₃]₀), and the experimentally determined ratio of the concentrations of ^?OH radicals to ozone (R_ct), the degradation of micropollutants in ozone reactors (modeled as Continuously Stirred Tank Reactors - CSTRs) were calculated for compounds with known reaction rate constants with ozone and ^?OH radicals. Calculated degradation of atrazine, iopromide, benzotriazole and acesulfame are in good agreement with measured data. For acesulfame the following rate constants were determined in this study at 20 ^oC: reaction rate constant with ozone = 88 M^?1s^?1, reaction rate constant with ^?OH radical = 4.55?×?10⁹ M^?1s^?1. For the ozone reaction an activation energy of 35 kJ/mol was determined. Similarly to micropollutants, the relative inactivation of microorganisms (N/N₀) can be calculated based on the inactivation rate constant for ozone and if applicable the lag phase. The pI-value (=??logN/N₀) was introduced and implemented in the process management system to calculate online the log inactivation of reference microorganisms such as B. subtilis spores. The system was tested for variation of pH (6.5–8.5), DOC (1.2–4.2 mg/L) flowrate 3.2–12 m³/h and temperature (5.7–9 ^oC). Furthermore, a given pI-value, e.g. 1 for a 1-log inactivation of B. subtilis spores, can be set as control parameter in the process management system. The ozone gas flow is then adjusted until the set pI-value is reached. The process control concept was validated with B. subtilis spores. Generally, a good agreement was found between calculated and measured inactivation data. It was also demonstrated, that a constant ozone residual may lead to insufficient disinfection or overdosing of ozone. The new process control concept for ozonations based on onsite measurement of the ozone decay rate constant and the pI-value allows to assess disinfection and degradation processes quantitatively in real-time.     

Cooling Water Treatment with Ozone

Small scale tests oruan open recirculating cooling system with a cooling water flow of 10 m³/h conducted for a period of two years have shown that ozone could be a viable alternative to chlorine and other commonly used biocides. An average ozone dosage of 0.05 mg/L was applied continuously to the cooling water. Corrosion rates of copper alloy samples immersed in ozonized water were lower than the rates of samples in non-ozonized water. No corrosion was detected for the Cr-Ni steel alloys DIN 1.4306 and DIN 1.4404 as well as for titanium either in the presence or absence of ozone in water. This is ascribed to the formation of a thin protective layer in both cases.

A pilot plant has been set up at the EVS Heilbronn coal-fired power station in West Germany in order to confirm the results obtained on small scale. The open recirculating cooling system has a cooling water flow of 1,000 m³/h and is fed by conditioned Neckar river water. An air-fed ozone generator is used to ozonize a 10% side stream of the cooling water. A bubble diffuser contactor is used to introduce ozone into the water and an air heater is operated to decompose excess ozone leaving the contactor. The plant can be run to yield either a constant ozone residual concentration in the water or a constant ozone dosage to the water leaving the contactor. All relevant data are registered continuously and the plant presently is operated automatically with a constant ozone dosage in the side stream. Corrosion experiments are performed using laboratory heat exchangers, and all physical, chemical, and microbiological data of the cooling water are acquired. The operation started February 1, 1989 and is scheduled to be terminated by the end of 1990.     

A Study of Ozone Formation on the Surfaces of Electrodes

It had been previously thought that ozone production occurred in gaseous space, especially the space between electrodes. However, based on our research, we believe that may only be one of the ozone-producing processes. In this study, we aimed to confirm that a third body, which is present at the interface between oxygen gas and a metal electrode, works to compose ozone. Ozone was not observed in pure oxygen (400x10^?6 Nm³/min flow rate) when electrical discharge was supplied after approximately 6 months. The concentration of ozone increased (approximately 0.07 ppm) when nitrogen (approximately 20x10^?6 Nm³/min flow rate) was added to a gas-mixing chamber. A third body was required to produce ozone when an oxygen molecule and an oxygen atom collided. The same phenomenon was observed on the surface of a copper anode. A simulation confirmed this. Using an industrial ozone generator which utilized ceramic dielectrics and expanded metal electrodes, an increase in the temperature of the cooling water led to a proportional decrease in ozone concentration. After changing from the titanium electrode to a nickel electrode and an antimony electrode, we observed the difference in the enthalpy changes which were calculated using van't Hoff's formula. The antimony electrode increases the efficiency of the ozone generator to produce ozone. We have come to believe that ozone can be composed on the surface of a metal electrode.     

Ozone Side-Stream Design Options and Operating Considerations

Disinfection with ozone is achieved by CT product, where “C” is residual ozone concentration and “T” is reaction time. Ozone residual is developed after ozone gas is dissolved into the water. Ozone dissolution is most often achieved by either bubble-diffuser or side-stream ozone systems. Side-stream ozone systems are different from bubble-diffuser ozone systems in many ways: 1) Ozone addition occurs outside of the ozone contactor; 2) additional energy is used due to side-stream pump operation; 3) equipment maintenance can be completed independent of ozone contactor operation; 4) ozone contactor size and shape can have unique design features, such as pipeline contacting or shallow-depth basins. Side-stream ozonation is a treatment technique that is gaining in popularity at larger and larger plants. Ozone generators can now operate at elevated ozone concentration, which improves the economics for installing and operating side-stream processes. Two different side-stream arrangements are discussed in this paper. The arrangements differ with the presence or absence of a de-gas vessel. The de-gas vessel option is more expensive in terms of capital and operating cost, but might be the chosen option for particular contactor configurations. Issues and considerations for installing a de-gas vessel are discussed in this paper.     

Compromise Between Bromate Ion Formation and Pesticides Degradation and/or Manganese Removal

Ozonation is a widely used technology within the water industry. Bromate ion formed by oxidation of water containing bromide ion was studied with the Gas Ozone Test and Pilot Scale Ozonation. Bromate ion formation was investigated along with the removal of triazines and/or manganese. Under identical conditions of ozonation, BrO₃ ^? formation is specific for each water and depends on parameters such as Total Organic Carbon, UV absorbance at 254 nm, applied ozone and ozone residual. Pesticides degradation by ozonation alone cannot be achieved without the formation of BrO₃ ^? at a high concentration. Hydrogen peroxide, at a constant ozone dose, reduces the BrO₃ ^? formation. However, even with the use of hydrogen peroxide, the concentration of BrO₃ ^? can remain in excess of the provisional Maximum Contaminant Level (10 μg/L). For certain types of water, pesticide degradation is difficult to achieve if the MCL for BrO₃ ^? has to be met. Manganese oxidation by ozone appears to be achieved without high bromate formation; indeed the presence of manganese hinders BrO₃ ^? formation.     

An On-Site Cooling Tower Treated by Stand-Alone Low-Concentration Dissolved Ozone

We report on an on-site 500 RT cooling tower ozone treatment process, in which chemicals other than ozone itself were completely eliminated. Ozone in an amount leading to less than 0.1 ppm of dissolved ozone was continuously introduced via side-stream injection into the circulating water returning from the chiller. The ozonated water was initially made to flow from the distributor to the filler in order to eliminate the growth of algae, and then to the chiller to reduce the corrosion and the fouling in the water. Positive ions such as Ca⁺² and Mg⁺² in the circulating water were precipitated by chelating them with carboxylic acids formed by the oxidation of organic compounds. We observed that using an ozone dosage of 0.1 ppm resulted in a colony-forming unit (CFU) less than 2 × 10³ /mL. With that well-controlled CFU, corrosion controlling and scale reducing were achieved as well.     

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.     

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.     

Ozone Swimming Pool Water Treatment Under Tropical Conditions

This paper presents the first year operation report of an ozone swimming pool water treatment system under the severe conditions of tropical climate. The system installed in a 70 m³ pool with upflow hydraulics, comprises sand filtration and ozone/bromide ion treatment. Cupric sulfate was chosen as algaecide and pH was kept between 7.5 and 7.8 by adding adequate amounts of HC1. When required, a slight flocculation (aluminum sulfate, 5 mg/L) was applied weekly. Water physicochemical analyses performed twice a week throughout the year, including microbiological tests, clearly demonstrated the remarkable efficiency of the treatment, fulfilling all water quality standards, even at high bather loads and during the summer months (at water temperature higher than 31[ddot]C and an intense sunlight) being economically feasible as well. This experience will allow the extension of ozone water treatment to larger swimming pools in similar circumstances.     

Bactericidal Effectiveness of O3, O3/H2O2 and O3/TiO2 on Clostridium perfringens

The purpose of this research is to evaluate the bactericidal capacity of different Advanced Oxidation Treatments (AOTs) based on ozone: ozone, ozone/hydrogen peroxide and ozone/titanium dioxide on a wild strain of Clostridium perfringens, a fecal bacterial indicator in drinking water. The dose of ozone consumed ranges from 0.6 mg L^?1 min^?1 to 5.13 mg L^?1 min^?1 depending on the process and on the sample. In the treatments combined with O₃, H₂O₂ dose utilized is 0.04 mM and TiO₂ dose, 1 g L^?1. In order to evaluate the influence of natural organic matter and suspension solids over the disinfection rate, treatments are performed with two types of water – natural water from Ebro River (Zaragoza, Spain) and NaCl solution 0.9%. To achieve 4 log units of inactivation, 3.6 mg O₃ L^?1 is necessary in O₃ treatment, 4.25 mg O₃ L^?1 in O₃/TiO₂ system and 2.7 mg O₃ L^?1 in O₃/H₂O₂ after processing the natural water. In NaCl solution, to get the same inactivation, 0.42 mg O₃ L^?1 is necessary in O₃ treatment, 1.15 mg O₃ L^?1 in O₃/TiO₂ system and 0.06 mg O₃ L^?1 in O₃/H₂O₂ process. Even though the three treatments studied have a high bactericidal activity due to the number of surviving bacteria decreases to non-detectable levels, O₃/H₂O₂ is the most effective system for eliminating C. perfringens cells in a lower contact time, followed by O₃ and finally O₃/TiO₂ system.     

Power Evaluation of the Los Angeles Oxygen–Fed Ozone System

The Los Angeles Department of Water and Power (LADWP) operates a 600 mgd (2,270 ML/day) direct filtration water treatment plant which includes a 7,900 lb/day (149 kg/hr) ozonation system. Ozone is applied as a preoxidant for the purposes of disinfection and microflocculation. The ozonation system is unique in that high purity oxygen is generated on–site and is used as the feed–gas in a once–through system. The process was selected through competitive bids and evaluated for total present worth from 20–year life cycle costs which included capital plus energy expenses. Power consumption (and penalty) was valued at $6,500/kW. System power demand was measured at nine ozone production rates. A minimum specific energy of 6.5 kWh/lb (14.3 kWh/kg) of ozone was observed when generating ozone at a concentration between 5 and 6 % (wt) [65 and 80 g/m³; where the standard temperature and pressure are 70F (21.11 C) and 1 atm, respectively]. The test methodology and data assessment considerations were developed jointly by the owner and manufacturer, and produced results with practical significance beyond the performance testing objective.     

Ozone Applications in the Post-Harvest of Papaya (Carica papaya L.): An Alternative to Amistar Fungicide

The objective of this article was to evaluate ozone applications in the post-harvest of papaya (Carica papaya L) as an alternative to Amistar fungicide, taking into account the effect on the control of fungal pathogens growth, shelf-life, seed germination percentage and soluble solids content. Ozone doses were applied in vitro to strains of seven fungi species, which cause rot of papaya. Papayas cv. Maradol-red were harvested and treated with gaseous ozone (500 mg m^?3), ozone-containing water (1 mg L^?1) and a solution of Amistar fungicide (0.1 ml L^?1). They were stored for 10 days. Washing with ozone-containing water was found to be the most favorable alternative. The ozone use showed a delay in fruit ripening. The effect on seed germination percentage and soluble solids content was not significant. A methodology for ozone use in the post-harvest phase was proposed.     

Assessment of the Efficacy of Intermittent Ozone Disinfection

An example of intermittent disinfection occurs in dental-unit water systems (DUWS), which are disinfected only for a specified time per each day. The efficacy of intermittent ozonation was evaluated using a laboratory-scale, membrane-based ozone disinfection system (MBODS), which delivers bubbleless dissolved ozone to the DUWS. A new tool - the weighted Ct value, or C_w, - was applied to interpret heterotrophic plate counts (HPC) data. To achieve the American Dental Association's (ADA's) criterion (<200 CFU/mL), the required ozone dosage was C_w > 0.07 mg–O₃/L. However, even the highest ozone dosage (C_w > 0.130 mg/L) allowed biofilm HPC to persist at over 10⁴ CFU/cm². Although a higher C_w killed planktonic and biofilm bacteria more thoroughly, it also generated more biodegradable dissolved organic carbon (BDOC). Thus, this research illustrates the inherent trade-off of intermittent ozonation: a higher C_w kills more bacteria during the ozonation period, but creates more BDOC that fosters biofilm regrowth when ozonation is off.     

Uses of Ozone in a Three-Phase System for Water Treatment: Ozone Adsorption

A new technique of using ozone for water treatment is presented. This new technique consists of using a three-step-process composed firstly of ozone adsorption on an appropriate adsorbent, secondly water treatment, and thirdly regeneration of the adsorbent. Results regarding ozone adsorption (the first step) are presented in this paper. Different types of silica gel and a type of TiO₂ have been tested for ozone adsorption. It was found that the physical characteristics of the silica gel affect its capacity for ozone. Titanium dioxide has shown ozone decomposition instead of adsorption as it contains Lewis acid sites. An exponential decrease of the silica gel capacity with its moisture content has been found. Linear isotherms in the range of ozone concentrations less than 100?g/m³ NTP have been found. A particle diffusion model with linear equilibrium isotherm has been used to model the breakthrough curves in fixed bed columns.     

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.     

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.     

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.     

Obtaining Real-Time Kinetic Parameters on Ozone Decay from a Full-Scale Ozone Contactor Using the Axial Dispersion Reactor Model

Ozone decay kinetic parameters, including fast ozone demand ([D]₀), ozone decay rate constant (k_D), and rate constant for ozone reaction with ozone demand (k_R), are required for a numerical simulation targeting the design and operational optimization of an ozone contactor. The kinetic parameters of ozone decay and dispersion number were obtained from a full-scale ozone contactor for the axial dispersion reactor model simulation. The sensitivity analysis showed that the influence of k_R was minor and the constant 13 L mg^?1 min^?1 for k_R was suitable for carrying out simulations for sand-filtered raw water without measuring it. Curve fitting with on-site ozone concentrations and the ADR simulation results using a trial-and-error method could successfully provide kinetic parameters on ozone decay (i.e., k_D and [D]₀). Using these real-time kinetic parameters, we successfully predicted the CT, residual ozone, C. parvum log inactivation, and bromate formation. Compared to a method based on the CSTR in series, this method could provide more accurate CT and residual ozone for an ozone contactor with horizontal meandering flow and low dispersion number.