Influence of Hydrodynamics on Giardia Cyst Inactivation by Ozone. Study by Kinetics and by “CT” Approach

This Research Note deals with the problem of providing appropriate disinfection from one of the most resistant microorganisms (the Giardia cysts) while trying to provide a more strict and logical approach to the “CT” concept. A further discussion of the U.S. Environmental Protection Agency (EPA) disinfection requirements in accordance with the kinetics and hydraulics of the ozonation process will be made in order to reconsider the definition of detention time. The main objective is the comparison of two possible approaches for determination of the disinfection conditions. The first implies consideration of the kinetics and hydraulics of the ozonation process. The second is based only on the hydrodynamics data and the use of an additional hypothesis known as the concept of the “CT” value.     

Wastewater Disinfection by Ozone: Influence of Water Quality and Kinetics Modeling

Wastewater disinfection by ozone was investigated at pilot scale on different effluents. The organic matter (COD and TOC) was shown to have the biggest influence on the ozone demand of the effluents. Disinfection of fecal indicators could be modeled as the reaction on a double population. The presence of more resistant microorganisms results in the need for higher treatment doses and a tertiary filtration when the effluent has to meet the stringent Title 22 standard. Eventually, the high virucidal power of ozone makes it very attractive when viruses are targeted.     

Pilot-Plant-Scale Ozone and PEROXONE Disinfection of Giardia muris Seeded into Surface Water Supplies

PEROXONE is an advanced oxidation process for water treatment which is generated by combining ozone and hydrogen peroxide. In this study, surface water supplies were seeded with viable Giardia muris cysts and disinfected by ozone and PEROXONE in the pretreatment columns of a 22.7 L/min pilot plant. Inactivation was examined in two source waters under conditions of varying applied ozone doses (0.5–4.0 mg/L), with and without the addition of hydrogen peroxide; at increasing contact times (3, 6, 9, and 12 min); and with and without added high turbidity (10 and 50 NTU). Approximately 10⁸ viable G. muris cysts were injected into the ozone contactor-column influent. The cysts were collected at the ozone contactor-column effluent and concentrated, and viability was then determined using in vitro excystation.     

Ozone Inactivation Of Biologically-Risky Wastewaters

Total inactivation of ten microorganisms inoculated in a model wastewater was achieved by ozone treatment at laboratory scale. Microorganisms selected included four viruses, three non-sporulated bacteria, two sporulated bacteria and one fungus, employed in vaccine preparation for animals. Viruses resulted in the most readily-inactivated microorganisms and sporulated bacteria the most resistant strains, although Salmonella typhimurium, a non-sporulated bacterium, showed an unusually high resistance. Inactivations followed a second-order kinetic law, depending on both ozone and microorganism concentrations. In each case, kinetic parameters which characterize the inactivation process were measured experimentally.     

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.     

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.     

Theoretical Analysis Of Ozone Disinfection Performance In A Bubble Column

Modelling the efficiency of ozone disinfection in a bubble column was completed using two kinetic approaches: an axial dispersion model (ADM) and a back-flow cell model (BFCM). Using these models, the effects of flow directions and mixing on disinfection performance were examined. Both models predicted the concentration profiles of dissolved ozone in water equally well. However, the BFCM could be solved much more easily than the ADM with regard to the degree of inactivation without the solution being plagued by divergence.     

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.     

Ozone Disinfection Efficiency for Indicator Microorganisms at Different pH Values and Temperatures

Disinfection efficiency of ozone was determined in various types of water at different pH (6, 7 and 8) values and temperatures (15, 25 and 35 ^°C) for E. coli and Salmonella. Three different applied ozone concentrations (1.5, 1.7, and 2 mg/L) in the gas phase were applied, and samples were taken at different time intervals to determine microbial survival using spread plate count (SPC) and ozone residual. Highest microbial inactivation was observed in distilled water with applied ozone concentration of 2 mg/L in the gas phase. Survival of E. coli was higher at pH 8 and 15 ^°C as compared to lower pH values and temperatures as depicted by the inactivation kinetics of the test microbes used in the study. Salmonella showed 5 and 6 log removal after contact time of 45 and 60 sec, respectively, at 2 mg/L. Disinfection of mixed culture showed relatively more survival of E. coli; as 3 and 4 log removal of E. coli and 4 and 5 log removal of Salmonella was observed after 45 and 60 sec.     

Advanced Treatment For Municipal Wastewater Reuse In Agriculture. III - Ozone Disinfection

Results of a pilot (100 m³/h) investigation on ozone disinfection of municipal tertiary effluents for reuse in agriculture carried out at West Bari (S. Italy) treatment plant are presented. Among dosages, contact times and advanced treatment schemes investigated it was demonstrated that ozone disinfection results in the achievement of the WHO microbial guideline (1,000 CFU/100ml for Fecal Coliforms) for unrestricted wastewater reuse in agriculture of both clarified and clarified-filtered municipal secondary effluents; it is very effective towards Pseudomonas aeruginosa, rather effective towards Giardia lamblia and substantially ineffective towards Cryptosporidium parvum and it forms limited amount of DBP (approx. 350 ppb of total aldehydes). O&M costs amount to 37 Euro/1000m³.     

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.     

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.     

Start-Up and Operation of the Indianapolis Ozone Disinfection Wastewater Systems

The two 125-mgd oxygen activated sludge wastewater treatment plants for the City of Indianapolis, Indiana use ozone disinfection. The process was constructed in 1983, and has maintained continuous, reliable operation since 1985. Disinfection is required at Indianapolis from April 1 through October 31. Equipment performance characteristics were evaluated during the 1985 disinfection season, and disinfection performance characteristics were optimized during the 1986 season. The evaluation documents the effect of cooling water temperatures on equipment performance, and the utilization of ozone contactor off-gas control of disinfection performance. The capital cost of both ozone systems represented about eight percent of the plants' total construction cost. The ozone system O & M cost represents about 1.9 percent and 3.7 percent of the total plant O & M cost at the Belmont and Southport plants, respectively.     

Action of Ozone on Trophozoites and Free Amoeba cysts,Whether Pathogenic or Not

Owing to the presence of free living amoeba – some ofwhich are pathogenic to man– in any aqueous medium (before or after traditional treatment processes in surface water, groundwater. swimming pools, etc.). our task wasto test the efficiency of ozone asa disinfecting agent. We began by studying the ozonation process, in bactericidal and virucidal conditions, on cysts of eleven free living amoeba strains. Most of thetested strains were destroyed under less drastic conditions than those arising from true ozonation (0.4 mg/L of dissolved ozone residual maintained for 4 minutes) and we attempted to find the ozonation parameters (dissolved ozone residual and contact time) Just necessary to obtain inactivation of the cystsunder laboratory conditions.     

Use of MS2 Coliphage as a Surrogate for Enteric Viruses in Surface Waters Disinfected with Ozone

The purpose of this study was to determine the relative inactivation of MS2 coliphage (American Type Culture Collection strain 15597-B1) and heterotrophic plate count bacteria using raw surface water under a variety of naturally occurring conditions. It was found that the applied ozone dose and dissolved organic carbon had the most impact on ozone disinfection of MS2 coliphage and HPC bacteria. The dissolved organic carbon was found to compete for the ozone and significantly reduce the inactivation of both the coliphage and the bacteria. Furthermore, it was observed that the presence of any ozone residual inactivated greater than 4 logs MS2 coliphage and 2 logs heterotrophic plate count bacteria within a 30 second contact time. Ozone residuals greater than 0.20 mg/L inactivated greater than 5 logs MS2 coliphage and 3 logs heterotrophic plate count bacteria also within 30 second contact time. Comparison of inactivation studies indicate that MS2 coliphage is probably more sensitive to ozone than enteric viruses. It was concluded that the regulatory agencies should reevaluate their recommendations for using MS2 coliphage as an ozone disinfection indicator of enteric viruses.     

Kinetics of Heterogeneous Catalytic Ozone Decomposition in Water on an Activated Carbon

Ozone decomposition in water in the presence of an activated carbon has been studied. Variables investigated were agitation speed, carbon particle size, temperature and pH. In all cases, the presence of activated carbon improved the ozone decomposition rate. Between pH 2 and 7 the ozone decomposition rate due to both the homogeneous and heterogeneous mechanisms hardly varied while a significant increase was noticed with increasing pH. A kinetic study based on a Langmuir-Hinselwood type mechanism for the heterogeneous surface reaction was undertaken. According to this mechanism the heterogeneous ozone decomposition kinetics can be simplified to follow a first order process. Fit of experimental results to the kinetic equations derived from the mechanism allowed for the determination of the apparent first order rate constants of the ozone surface heterogeneous reaction and adsorption equilibrium constants.     

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.     

Ozone Inactivation of Pseudomonas aeruginosa,Escherichia coli,Shigella sonnei and Salmonella typhimurium in Water.

Based on the properties of ozone as a strong germicidal agent, inactivation kinetics of Pseudomonas aeruginosa, Escherichia coli, Shigella sonnei and Salmonella typhimurium towards ozone in water were studied. The values of 90% inactivation (t₉₀) obtained varied from 0.20 minutes (2.4 mg/L, Escherichia coli ATCC 25922) to 8.33 minutes (0.39 mg/L, Pseudomonas aeruginosa wild strain). First order inactivation kinetics with respect to both the concentrations of ozone and microorganisms were found, resulting an overall second order inactivation kinetics. The ATCC strains showed to be the most sensitive toward ozone among all. Meanwhile, the environmental isolation of Pseudomonas aeruginosa was the most resistant and Escherichia coli the most sensitive wild strain. The longest time required to achieve total inactivation was 35 minutes.     

Impact of Ozone Treatment on Selected Microbiological Parameters

A pilot plant study was conducted to evaluate the effects of ozonation on the quality of Colorado River water delivered to the Phoenix (Arizona) Union Hills Water Treatment Plant through the Central Arizona Project canal. Raw and finished water were monitored for basic chemical water quality parameters and for microorganisms including total coliforms, heterotrophic plate count, enteric viruses, Giardia and Crvptosporidium cysts. Ozone, applied through diffusion contactors in dosages optimized to provide the required CT (concentration x contact time) value, was used as a primary disinfectant in two of the four 10 gpm (37.8 L/min) treatment trains.