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Wastewater Technology Fact Sheet: Ozone Disinfection

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Wastewater Technology Fact Sheet: Ozone Disinfection United States Office of Water EPA 832-F-99-063 Environmental Protection Washington, D.C. September 1999 Agency Wastewater Technology Fact Sheet Ozone Disinfection DESCRIPTION • Breakage of carbon-nitrogen bonds leading to depolymerization. Disinfection is considered to be the primary mechanism for the inactivation/destruction of TABLE 1 INFECTIOUS AGENTS pathogenic organisms to prevent the spread of POTENTIALLY PRESENT IN UNTREATED waterborne diseases to downstream users and the DOMESTIC WASTEWATER environment. It is important that wastewater be adequately treated prior to disinfection in order for Organism Disease Caused any disinfectant to be effective. Table 1 lists some common microorganisms found in domestic Bacteria wastewater and the diseases associated with them. Escherichia coli Gastroenteritis (enterotoxigenic) Leptospira (spp.) Leptospirosis Ozone is produced when oxygen (O2) molecules are dissociated by an energy source into oxygen atoms Salmonella typhi Typhoid fever and subsequently collide with an oxygen molecule to Salmonella (=2,100 serotypes) Salmonellosis form an unstable gas, ozone (O3), which is used to Shigella (4 spp.) Shigellosis (bacillary disinfect wastewater. Most wastewater treatment dysentery) plants generate ozone by imposing a high voltage Vibrio cholerae Cholera alternating current (6 to 20 kilovolts) across a Protozoa dielectric discharge gap that contains an Balantidium coli Balantidiasis oxygen-bearing gas. Ozone is generated onsite Cryptosporidium parvum Cryptosporidiosis 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 radical by-products of ozone polio, echo, and coxsackie heart anomalies, decomposition. viruses) meningitis Hepatitis A virus Infectious hepatitis • Damage to the constituents of the nucleic Norwalk agent Gastroenteritis acids (purines and pyrimidines). Rotavirus Gastroenteritis Source: Adapted from Crites and Tchobanoglous, 1998. When ozone decomposes in water, the free radicals • The name of the inventor. hydrogen 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 lysis). The electrical discharge method is the most common energy source used to produce ozone. Extremely The effectiveness of disinfection depends on the dry air 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 concentration of the ozone. A line frequency. The dew 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: liquid 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 oil). contacted uniformly in a near plug flow contactor. • The physical arrangement of the dielectrics The off-gases 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 atmosphere. 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 reuse 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 particulates 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 solubility 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, spores, and cysts. their particular sites. • Ozonation is a more complex technology APPLICABILITY than is chlorine 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 odor control. requiring corrosion-resistant material such as stainless steel. 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 solids (SS), has the ability to achieve higher levels of disinfection biochemical oxygen demand (BOD), than either chlorine or UV, however, the capital chemical oxygen demand, or total organic costs as well 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). PERFORMANCE 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 City 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 concentrations. 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
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