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United States Office of EPA 832-F-99-063 Environmental Protection Washington, D.C. September 1999 Agency Wastewater Technology Fact Sheet Disinfection

DESCRIPTION • Breakage of - bonds leading to depolymerization. Disinfection is considered to be the primary mechanism for the inactivation/destruction of TABLE 1 INFECTIOUS AGENTS pathogenic to prevent the spread of POTENTIALLY PRESENT IN UNTREATED to downstream users and the DOMESTIC WASTEWATER environment. It is important that wastewater be adequately treated prior to disinfection in order for Disease Caused any to be effective. Table 1 lists some common found in domestic Bacteria wastewater and the diseases associated with them. Escherichia coli Gastroenteritis (enterotoxigenic) Leptospira (spp.) Leptospirosis Ozone is produced when (O2) are dissociated by an source into oxygen typhi Typhoid fever and subsequently collide with an oxygen to Salmonella (=2,100 serotypes) Salmonellosis form an unstable , ozone (O3), which is used to Shigella (4 spp.) Shigellosis (bacillary disinfect wastewater. Most dysentery) generate ozone by imposing a high cholerae alternating (6 to 20 kilovolts) across a Protozoa gap that contains an Balantidium coli Balantidiasis oxygen-bearing gas. Ozone is generated onsite parvum because it is unstable and decomposes to elemental Entamoeba histolytica Amebiasis (amoebic oxygen in a short amount of time after generation. dysentery) Giardia lamblia Giardiasis Ozone is a very strong oxidant and virucide. The Helminths mechanisms of disinfection using ozone include: Ascaris lumbricoides Ascariasis • Direct oxidation/destruction of the cell wall T. solium Taeniasis with leakage of cellular constituents outside Trichuris trichiura Trichuriasis of the cell. Viruses Enteroviruses (72 types, e.g., Gastroenteritis, • Reactions with by-products of ozone polio, echo, and coxsackie heart anomalies, . viruses) meningitis Hepatitis A virus Infectious hepatitis • Damage to the constituents of the nucleic Norwalk agent Gastroenteritis ( and ). Rotavirus Gastroenteritis

Source: Adapted from Crites and Tchobanoglous, 1998. When ozone decomposes in water, the free radicals • The name of the inventor. peroxy (HO2) and hydroxyl (OH) that are formed have great oxidizing capacity and play an However, generators manufactured by different active role in the disinfection process. It is generally companies have unique characteristics but also have believed that the bacteria are destroyed because of some common configurations. protoplasmic oxidation resulting in cell wall disintegration (cell ). The electrical discharge method is the most common energy source used to produce ozone. Extremely The effectiveness of disinfection depends on the dry or pure oxygen is exposed to a controlled, susceptibility of the target organisms, the contact uniform high-voltage discharge at a high or low time, and the of the ozone. A line frequency. The point of the feed gas must be diagram of the ozonation process is shown in Figure -60EC (-76EF) or lower. The gas stream generated 1. The components of an ozone disinfection system from air will contain about 0.5 to 3.0% ozone by include feed-gas preparation, ozone generation, weight, whereas pure oxygen will form ozone contacting, and ozone destruction. approximately two to four times that concentration.

Air or pure oxygen is used as the feed-gas source After generation, ozone is fed into a down-flow and is passed to the ozone generator at a set flow contact chamber containing the wastewater to be rate. The energy source for production is generated disinfected. The main purpose of the contactor is to by electrical discharge in a gas that contains oxygen. transfer ozone from the gas bubble into the bulk Ozone generators are typically classified by: while providing sufficient contact time for disinfection. The commonly used contactor types • The control mechanism (either a voltage or diffused bubble (co-current and counter-current) are frequency unit). positive pressure injection, negative pressure (Venturi), mechanically agitated, and packed tower. • The cooling mechanism (either water, air, or Because ozone is consumed quickly, it must be water plus ). contacted uniformly in a near plug flow contactor.

• The physical arrangement of the The off- from the contact chamber must be (either vertical or horizontal). treated to destroy any remaining ozone before

Ozone Destruction Off-Gases

Recycle

Feed Gas Preparation •Oxygen Production Ozone Generation Discharge •Oxygen Storage Ozone Contact Basin •Air/Oxygen Treatment

Wastewater In

Source: U.S. EPA, 1986.

FIGURE 1 OZONE PROCESS SCHEMATIC DIAGRAM release into the . Therefore, it is • There are no harmful residuals that need to essential to maintain an optimal ozone dosage for be removed after ozonation because ozone better efficiency. When pure oxygen is used as the decomposes rapidly. feed-gas, the off-gases from the contact chamber can be recycled to generate ozone or for in the • After ozonation, there is no regrowth of aeration tank. The ozone off-gases that are not microorganisms, except for those protected used are sent to the ozone destruction unit or are by the in the wastewater stream. recycled. • Ozone is generated onsite, and thus, there The key process control parameters are dose, are fewer safety problems associated with mixing, and contact time. An ozone disinfection shipping and handling. system strives for the maximum of ozone in wastewater, as disinfection depends on the • Ozonation elevates the dissolved oxygen transfer of ozone to the wastewater. The amount of (DO) concentration of the effluent. The ozone that will dissolve in wastewater at a constant increase in DO can eliminate the need for temperature is a function of the partial pressure of reaeration and also raise the level of DO in the gaseous ozone above the water or in the gas the receiving stream. feed stream. Disadvantages It is critical that all ozone disinfection systems be pilot tested and calibrated prior to installation to • Low dosage may not effectively inactivate ensure they meet discharge permit requirements for some viruses, , and cysts. their particular sites. • Ozonation is a more complex technology APPLICABILITY than is or UV disinfection, requiring complicated equipment and efficient Ozone disinfection is generally used at medium to contacting systems. large sized plants after at least secondary treatment. In addition to disinfection, another common use for • Ozone is very reactive and corrosive, thus ozone in wastewater treatment is control. requiring -resistant material such as . Ozone disinfection is the least used method in the U.S. although this technology has been widely • Ozonation is not economical for wastewater accepted in Europe for decades. Ozone treatment with high levels of suspended (SS), has the ability to achieve higher levels of disinfection biochemical oxygen demand (BOD), than either chlorine or UV, however, the capital , or total organic costs as as maintenance expenditures are not carbon. competitive with available alternatives. Ozone is therefore used only sparingly, primarily in special • Ozone is extremely irritating and possibly cases where alternatives are not effective. toxic, so off-gases from the contactor must be destroyed to prevent worker exposure. ADVANTAGES AND DISADVANTAGES • The cost of treatment can be relatively high Advantages in capital and in power intensiveness.

C Ozone is more effective than chlorine in destroying viruses and bacteria.

C The ozonation process utilizes a short contact time (approximately 10 to 30 minutes). OPERATION AND MAINTENANCE

Belmont and Southport Wastewater Ozone generation uses a significant amount of Treatment Plants in Indianapolis, Indiana electrical power. Thus, constant attention must be given to the system to ensure that power is In 1985, the of Indianapolis, Indiana, operated optimized for controlled disinfection performance. two-125 million gallons per day (mgd) advanced wastewater treatment plants at Belmont and There must be no leaking connections in or Southport using ozone disinfection. The rated surrounding the ozone generator. The operator capacity of the oxygen-fed ozone generators was must on a regular basis monitor the appropriate 6,380 pounds per day, which was used to meet subunits to ensure that they are not overheated. geometric mean weekly and monthly disinfection Therefore, the operator must check for leaks permit limits for fecal coliforms of 400 and 200 per routinely, since a very small leak can cause 100 ml, respectively. unacceptable ambient ozone . The ozone monitoring equipment must be tested and Disinfection was required at both Indianapolis calibrated as recommended by the equipment treatment plants from April 1 through October 31, manufacturer. 1985. Equipment performance characteristics were evaluated during the 1985 disinfection season and Like oxygen, ozone has limited solubility and consequently, disinfection performance was decomposes more rapidly in water than in air. This optimized during the 1986 season. The capital cost factor, along with ozone reactivity, requires that the of both ozone systems represented about 8% of the ozone contactor be well covered and that the ozone plants' total construction cost. The ozone system’s diffuses into the wastewater as effectively as Operation and Maintenance (O&M) cost possible. represented about 1.9% and 3.7% of the total O&M costs at the Belmont and Southport plants, Ozone in gaseous form is once it reaches respectively. a concentration of 240 g/m3. Since most ozonation systems never exceed a gaseous ozone In 1989, a disciplined process monitoring and concentration of 50 to 200 g/m3, this is generally not control program was initiated. Records indicated a a problem. However, ozone in gaseous form will significant effect on process performance due to remain hazardous for a significant amount of time changes in wastewater flow, contactor influent fecal thus, extreme caution is needed when operating the coliform concentration, and ozone demand. ozone gas systems.

Previously, ozone demand information was It is important that the ozone generator, unknown. Several studies were conducted to enable distribution, contacting, off-gas, and ozone better control of the ozone disinfection process. destructor inlet be purged before opening the These included the recent installation of a pilot-scale various systems or subsystems. When entering the ozone contactor to allow the plant staff to measure ozone contactor, personnel must recognize the ozone demand on a daily basis. Also, tracer tests potential for oxygen deficiencies or trapped ozone were conducted to measure contactor gas in spite of best efforts to purge the system. The short-circuiting potential. Results demonstrated a operator should be aware of all emergency noticeable benefit of adding additional baffles. operating procedures required if a problem occurs. Results also indicated operating strategies that could All safety equipment should be available for maximize removal, such as reducing operators to use in case of an emergency. Key the number of contactors in service at low and O&M parameters include: moderate flow conditions. • Clean feed gas with a of -60EC (-76EF), or lower, must be delivered to the ozone generator. If the supply gas is moist, the reaction of the ozone and the moisture TABLE 2 TYPICAL COST ESTIMATE OF will yield a very corrosive condensate on the AN OZONE DISINFECTION inside of the ozonator. The output of the generator could be lowered by the formation of nitrogen (such as nitric ). Items Costs

Capital Costs • Maintain the required flow of generator (air, water, or other liquid). Oxygen feed gas and compressor $245,500 Contact vessel (500 gpm) $4,000-5,000 • Lubricate the compressor or blower in accordance with the manufacturer's Destruct unit: specifications. Ensure that all compressor Small (around 30 cfm) $800 sealing are in good condition. Large (around 120) $1,000-1,200

• Operate the ozone generator within its Non-component costs $35,000 design parameters. Regularly inspect and Engineering $12,000-15,000 clean the ozonator, air supply, and dielectric Contingencies 30% assemblies, and monitor the temperature of the ozone generator. Annual O&M Costs Labor $12,000 • Monitor the ozone gas-feed and distribution system to ensure that the necessary volume Power 90 kW comes into sufficient contact with the Other (filter replacements, $6,500 wastewater. compressor oil, spare dielectric, etc.) gpm = gallons per minute • Maintain ambient levels of ozone below the cfm = cubic feet per minute limits of applicable safety regulations. Source: Champion Technology, 1998.

COSTS to the liquid is a critical economic consideration. For this reason, the contact The cost of ozone disinfection systems is dependent chambers used are usually very deep and covered. on the manufacturer, the site, the capacity of the plant, and the characteristics of the wastewater to be The overall cost of an ozonation system is also disinfected. Ozonation costs are generally high in largely determined by the capital and O&M comparison with other disinfection techniques. expenses. The annual operating costs for ozone disinfection include power consumption, and Table 2 shows a typical cost estimate (low to supplies, miscellaneous equipment repairs, and medium) for ozone disinfection system used to staffing requirements. disinfect one mgd of wastewater. The costs are based on the wastewater having passed through Another consideration for the cost is that each both primary and secondary treatment processes of ozonation system is site specific, depending on the a properly designed system (the BOD content does plant’s effluent limitations. Chemical suppliers not exceed 30 milligrams per liter [mg/L] and the SS should be contacted for specific cost information. content is less than 30 mg/L). In general, costs are largely influenced by site-specific factors, and thus, the estimates that follow are typical values and can vary from site to site.

Because the concentration of ozone generated from either air or oxygen is so low, the transfer efficiency REFERENCES 9. Research and Development. Cincinnati, Ohio. EPA/625/1-86/021. 1. Crites, R. and G. Tchobanoglous. 1998. Small and Decentralized Wastewater 10. Water Environment Federation (WEF). Management Systems. The McGraw-Hill 1996. Operation of Municipal Wastewater Companies. New York, New York. Treatment Plants. Manual of Practice No. 11. 5th ed. vol. 2. WEF. Alexandria, 2. Martin, E. J. and E. T. Martin. 1991. . Technologies for Small Water and Wastewater Systems. Environmental 11. Rasmussen, Karen ( & Sullivan). Engineering Series. Van Nostrand Reinhold 1998. Engineering Online. (now acquired by John Wiley & Sons, Inc.). “Market Forecast: Wastewater Treatment New York, New York. pp. 209–213. Equipment Markets.” WEFTEC, Orlando, FL. 3. Metcalf & Eddy, Inc. 1991. Wastewater Engineering: Treatment, Disposal, and ADDITIONAL INFORMATION Reuse. 3d ed. The McGraw-Hill Companies. New York, New York. Mark Boner, P.E. WWETCO 4. Rakness, K. L.; K. M. Corsaro; G. Hale; and 1825 2nd Avenue B. D. Blank. 1993. "Wastewater Columbus, GA 31901 Disinfection with Ozone: Process Control and Operating Results." Ozone: Science and Roy F. Weston Inc. Engineering. vol. 15. no. 6. pp. 497–514. Peter J. Lau 1515 Market Street, Suite 1515 5. Rakness, K. L.; R. C. Renner; D. B. , PA 19102-1956 Vornehm; and J. R. Thaxton. 1988. "Start-Up and Operation of the Indianapolis The mention of trade names or commercial products Ozone Disinfection Wastewater Systems." does not constitute endorsement or recommendation Ozone: Science and Engineering. vol. 10. for use by the U.S. Environmental Protection no. 3. pp. 215–240. Agency.

6. Rudd, T. and L. M. Hopkinson. December 1989. "Comparison of Disinfection Techniques for and Sewage Effluents." Journal of International Water and Environmental Management. vol. 3. pp. 612–618.

7. Task Force on Wastewater Disinfection. 1986. Wastewater Disinfection. Manual of Practice No. FD-10. Control Federation. Alexandria, Virginia.

8. U.S. Environmental Protection Agency (EPA). 1986. Design Manual: Municipal Wastewater Disinfection. EPA Office of The technical content of this fact sheet was provided by the National Small Flows Clearinghouse and is gratefully acknowledged.