Ozone. However, new concerns have arisen about the ability of chlorine to inactivate. TechCommentary,/Vol. 1/No. 4 1

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1 kisswglkil~u Wlll G llul dllul dvu UWS identified, including the formation of disinfection byproducts (DBPs) and harmful effects on fishand other aquatic species. As water and wastewater disinfection issues become more challenging, research has demonstrated that two electrotechnologies might prove useful in meeting these challenges: ozonation and ultraviolet (UV) light. The emergence of these two energyintensive electrotechnologies as viable alternatives to chlorination could have a significant effect on the electrical energy used at water and wastewater facilities. This Techcommentary discusses the advantages, disadvantages, costs, and energy requirements of ozone and UV systems. A Mixed Track Record for Chlorine Disinfection Since the early 19OOs, chlorine has been the disinfectant of choice in both the water and wastewater industries in North America. This widely-used oxidant has done an effective job in killing diseasecausing bacteria and at a relatively low cost. n the case of drinking water, not only does chlorine kill pathogens when it is added to water, but maintenance of a small chlorine residual in a community's drinking water distribution system is a regulatory requirement to prevent aftergrowth of bacteria. However, new concerns have arisen about the ability of chlorine to inactivate Ozone two microorganisms which can be found in the source water supplies for some drinking water systems: giardia lamblia and cryptosporidium. Giardia causes flulike, intestinal symptoms. Cryptosporidum causes severe nausea and diarrhea, particularly in patients with depressed immunity due to disease (Le. HV infection and chemotherapy). Both microorganisms form cysts, which are resistant to disinfection. Carefully controlled dosages of chlorine can inactivate giardia, but are ineffective for cryptosporidium. Simultaneously, the development of better analytical equipment has resulted in the identification of a new class of drinking water contaminants: disinfection byproducts (DBPs). These are halogenated organic compounds that fon when chlorine reacts with naturally occurring organic uv materials contained in drinking water. A few of the chlorinated byproducts, including trihalomethanes (THMs), have been shown to be carcinogenic and are strictly regulated at the present time. Newly-proposed regulations will result in more stringent standards for THMs and new standards for other DBPs. As disinfection and filtration regulations for pathogen inactivation and removal escalate, the obvious response would be to add more chlorine. Unfortunately, increased chlorine application and contact time with drinking water often results in higher production of DBPs and presents a potential conflict in meeting standards for them. A similar conflict exists for wastewater disinfection. Most wastewater discharges in North America are chlorinated. DBPs TechCommentary,/Vol. 1/No. 4 1

2 may be a concern. Also, chlorine not only kills pathogens, it is toxic to fish and other aquatic species. Therefore, standards have been set to limit or eliminate the concentration of chlorine in discharged wastewater. To comply, most wastewater utilities dechlorinate their effluent, usually by adding sulfur dioxide, which reacts instantaneously to destroy the free chlorine and produce chloride, which is nontoxic. n addition to water quality, public health, and aquatic protection issues associated with chlorine disinfection, water and wastewater utilities are concerned about worker safety and chemical handling. This is especially true for chlorine gas, where potential toxic leaks could occur, endangering employees and nearby residents. Leaks also are a concern with sulfur dioxide, which combines with moisture in the air to form sulfurous acid. Fire Code provisions require the addition of scrubbing systems for new facilities housing gaseous chlorine and/or sulfur dioxide. Thus, water and wastewater treatment plants are caught between standards requiring disinfection and other standards which make the use of chlorine gas problematic. Electrotechnologies are an option to resolve this conflict. Alternatives to Chlorine n recent years, several alternatives to chlorination have been evaluated and implemented throughout North America, and two dectrotechnologies are becoming increasingly viable: ozone and UV. Ozone, a strong oxidizing agent, is produced by passing an electric current through oxygen gas or dried air. As will be discussed later, the electricity requirements differ significantly depending upon which of these two feed gases the ozone system uses. UV disinfection is a physical process in which energy is used to damage the DNA of microorganisms, thus preventing their reproduction. When water is exposed to light generated by UV lamps, it can disinfect in seconds, provided that the microorganisms are exposed. The lamps are placed in the stream of water to insure that all of the stream is exposed. From applications of ozone and UV technologies at municipal water and wastewater treatment plants, two trends have materialized: Ozone is becoming increasingly common in water treatment, both for disinfection and other treatment. benefits. (See Table 1.) UV has emerged as a potentially attractive option to chlorinationdechlorination at wastewater treatment plants. (See Table 2.) Water Treatment Ozone use is escalating at water treatment plants because of its disinfection effectiveness and because of its ability to oxidize organic compounds. UV use is declining because of its ineffectiveness in inactivating cysts and its lack of residual. Escalatincr Ozone Use Althoug; ozone has been used in end of European water treatment for over 100 years, its use in North America is relatively new. Compared to the more than 2,000 water treatment plants using ozone worldwide, as of June 1990, approximately 20 ozone plants were in operation in North America. Today, 74 water treatment plants with ozone are in operation, and this number should reach 100 by the Table 1: Comparison of Ozone to Chlorine Gas in Water Treatment Parameter Chlorine Gas Ozone Disinfection Effectiveness Residual Disinfection Propetties Disinfection Bypmducts EnerFNln-W capital- Operating Cost Safety ssues Water Utility Bqerience Future Potential as Disinfectant of Choice Good Yes THMs, HAAs LOW Low Moderate Chlorine gas is toxic and requires containment and neutralizing facilities. Extensive Extensive, but somewhat rgduced Greater No Bromates High Up to 8 times higher than chlorine gas More Toxic, but gas can be detected at concentrations less than harmful levels. Offgas control required. Moderate Table 2:Comparison of W to Chlorine Gas in Parameter Chlorine Gas UV Light Disinfection Effectiveness Residual Disinfection Properties Energy Use Capital Cost Operating Cost Safety ssues Wastewater Utili Experience Future Short-term Potential as Wastewater Disinfectant Good Yes Low Low High if dechlorination required Chlorine gas leaks Extensive Will continue to be principal disinfectant for wastewater ncreasing, but mainly for medium-to-large facilities Comparable to chlorine if facilities are well maintained No High Up to 2 times higher Comparable to chlorination-dechlorination Bums Limited Applications will increase TechCommentary/Vol. 1/No. 4 2

3 As shown in Figure 1, total ozone production capacity should rise from about 28,000 b/day in 1989 to nearly 92,000 b/day in 1994, a 440 percent increase in five years. When ozone is generated from air, the power consumption required is approximately 10 kwh/lb of ozone generated. When ozone is generated from oxygen, some systems operate as low as 6.5 kwh/lb of ozone generated. Based on these rates, assuming that two-thirds of the plants generate ozone from air, the 1994 electric energy requirements for ozone production are about 800,000 kwh/day. Figure 2 illustrates the process of ozonation for water disinfection, along with the important factors affecting each stage in the process. There are many different ways to design an ozone system, depending upon the following: System capacity. Generator cell design.. Frequency of operation. Type of feed gas (air or oxygen).. Method of air preparation. Type of contactor. Method of ozone off-gas destruction Year Source: Rip G. Rice, July 1993 Figure 7. Evolution of Ozone use in U.S. drinking water plants L- Air Feed Disinfection Effectiveness Disinfection effectiveness for drinking water is measured by concentrationtime (CT). This represents the product of a disinfectant s concentration (C), measured in milligrams per liter, and the contact time 0, measured in minutes. CT requirements are set forth in the regulations for the federal Surface Water Treatment Rule and vary depending upon the effectiveness of the disinfectant and water temperature. Ozone is a powerful disinfectant. At any given concentration, the time required to kill pathogens is short. Similarly, at any given time of contact, the required dose of ozone is small. Using the CT measure, ozone is from 100 to 300 times as effective as chlorine at killing giardia cysts. t also disinfects without forming THMs. Although powerful, ozone is unstable and reactive and has a half life of approximately 20 minutes. This short half life requires on-site ozone generation and application. Because ozone is so reactive, no residual can be maintained in the water distribution system. Ozone is often used in combination with chlorine for drinking water disinfection. For example, ozone can be applied early in the water treatment process as the primary disinfectant. Then, late in the Alternative LOW Medium High power generator Liquid feed stream + Figure 2. The general process of Ozonation f Gas Compression Gas Dtying Ozone Generation Ozone Contactor - Steps required for - ozone systems using air feed Alternative feed gases: Hgh-purity pw@, Oxygen ennched air +.mum ozone contact options: TihCommentary/Vol. l/no. 4 3

4 process, after filtration, when most of the organic matter has been removed from the water, chlorine can be applied as a secondary disinfectant to provide a residual in the distribution system. Because of the lower concentration of organic matter, fewer DBPs are formed from chlorine in this approach. Thus ozone is a partial, but not a complete, replacement for chlorine in the disinfection of drinking water. Other Advantages to Ozone Ozone has other uses besides disinfection. Because of its ability to oxidize organic compounds that can cause taste and odor or impart color to drinking water, ozone frequently is used to improve the aesthetic quality of drinking water. Ozone also can be used for pretreatment because of its ability to oxidize harmful organic compounds, such as pesticides, and troublesome inorganic compounds, such as iron and manganese, that cause discoloring of household plumbing fixtures. When used for pretreatment, ozone reduces the use of treatment chemicals. n addition, ozone can help optimize the water filtration process. On certain source waters, ozone has been demonstrated to be effective with deep bed monomedium filters, in increasing loading rates that can be applied to gravity filters. This enables more water to flow through the same filter per run, while maintaining very high quality finished water. (See Tucson Water Treatment Plant case study.) uv Laboratory testing using UV has shown high inactivation of enteric virus (99.99 percent), but poor inactivation of giardia lamblia cysts (as low as 80 percent). UV's ineffectiveness at killing cysts and its lack of residual limit this technology for drinking water disinfection, especially for surface water supplies. n 1984, there were 80 UV facilities in operation or under construction in North America for disinfection of wastewater. The growing number of projects described in the literature shows that the use of UV has become even more common since then. Ozone, on the other hand, has declined in use for wastewater disinfection, largely due to its high cost. UV Shows Promise Considerable research and demonstration has been conducted on large scale applications of UV to the disinfection of wastewater. Results show that UV can comply with discharge requirements and is cost-competitive when compared to chlorination-dechlorination systems. UV advantages are simple operation, absence of a harmful residual, operator safety, and absence of any intermediate chemical compounds. The process of UV for wastewater disinfection is shown in Figure 3. mportant factors in UV disinfection include: The dose, which is affected by the intensity of the lamps, the residence time in the contact chamber (a matter of seconds), and the clarity of the wastewater. The presence of interfering constituents in wastewater, especially iron, manganese, and hardness (calcium and magnesium). m Short-circuiting in the contact chamber, which must be minimized. The water in the contact chamber, which needs to be maintained at constant levels to ensure sufficient exposure to radiation. Various monitoring devices to ensure that effective disinfection occurs. Power supply - and control box - - Figure 3. Typical UV system Liquid feed stream f Effluent Ozone Ozone is seldom used for wastewater disinfection in North America. n 1989, a survey revealed that, although ozonation had been installed in 45 wastewater treatment plants, only about half of them were still using it. Wastewater contains organic matter that consumes ozone, necessitating large doses to achieve disinfection. As discussed below, the cost to generate large doses of ozone is often prohibitive. costs Cost information on ozone and UV disinfection is developing gradually. As shown in Tables 1 and 2, both altematives are more costly than chlorination, particularly in the case of ozone. Water Treatment Although project-specific, some general conclusions can be drawn for ozone system costs for water treatment. A recent EPRl study cited costs for a 270 million gallon/day (design flow) water treatment plant with an ozone dose of 2.0 parts per million (ppm). n 1992, cost estimates were about $1 1 million in construction costs and about $1 million/year in electricity costs, or $9.50 per million gallons of water treated. Data from the final year of operation of the 600 million gallon/day Los Angeles Aqueduct Filtration Plant indicate the electricity costs average about $5 to $6 per million gallons of water treated. As discussed above, the amount of electricity used in ozone generation depends on the feed gas, either air or oxygen. f air is used, more electricity is required in air preparation to remove moisture and particulate matter that interfere with the efficiencv of ozone generation. Approximately 6 kwh/lb of ozone generated is required for the air preparation step alone. Therefore, in evaluating power consumption for ozone use, the specifics of the ozone generation system have to be carefully considered. On the average, electricity costs amount to about 75 percent of the total operating costs of an ozone system. About one half of the construction cost would be attributable to ozone generation and about one half to ozone contact basins. UV lamp banks n wastewater disinfection, where dechlorination is required, UV systems are much cheaper to install than conventional chlorination-dechlorination systems, and their operating costs are comparable. An example of the costs of UV versus chlorine and ozone for wastewater treatment is shown in Figure 4, which compares both total cost and operating costs per million gallons treated. As indicated above, ozone costs usually are prohibitive for wastewater systems due to the large doses required to treat the organics. TihCommentary/Vol. 1 /No. 4 4

5 Tips for Reducing Energy Cost Although both ozone and UV systems are energy intensive, the following tips may help in reducing energy consumption and power costs: 7. Follow a good preventive maintenance program. Ozone generator tubes and UV lamp sleeves are subject to fouling and require periodic maintenance. As this equipment becomes dirty, it is less efficient. Periodic maintenance, as recommended by the manufacturers, will ensure that the tubes and lamp sleeves will always be operating near their peak efficiencies. 2. Use temperature control to maximize pmess efgciency. For water treatment, maintaining the cooling water in the ozone generator at 27 degrees Centigrade should effect a 95% efficiency factor. For wastewater treatment, the temperature should be about 40 degrees C; consequently, an insulating quartz sleeve is placed around the lamp. 3. n water treatment, use storage to control plant throughput Water treatment plants operate best when the flowrates through the plant do not vary widely. Ozone generators as well as other plant systems do not operate as efficiently when flowrates fluctuate -My. Storage reservoirs allow control of the plant throughput and permit increased plant operation during off-peak periods when electric rates are low. The new 150 mgd Tucson Water Treatment Plant, which went into operation in the fall of 1992, treats Colorado River Water delivered to Tucson via the Central Anzona Project (CAP). Key water treatment plant goals were to satisv disinfection CT requirements, to meet stringent local standards for THMs (five times stricter than existing federal standards), and to provide a good tasting water supply. To meet these goals, the plant uses ozone as the primary disinfectant and chloramines (a combination of chlorine and ammonia) as the secondary disinfectant. The treatment process was determined following an extensive, 11 -month pilot testing program. The award-winning pilot program indicated that ozone would best meet Tucson s treatment objectives based on several key factors: Regulatory Compliance Ozone, coupled with chloramines for residual disin- fection in the distribution system, will allow the plant to meet state and federal disinfection CT standards and the Ci of Tucson s stringent THM standards. Taste and odor improvements Tucson Water personnel conducted tests where consumers were asked to evaluate taste and odor of the finished water. Based on this testing, 98 percent liked the taste of the treated water and classified it as tastina clean. The odor characteristics of filtered CAP water were altered from grassy or swampy to pleasant or aromatic. Pretreatment Benefis Deep bed monomedia filters in conjunction with ozone pretreatment would equal or exceed the performance of dual media filtration. The process scheme also will.enable the City to easily and economically expand the plant to 225 mgd (an increase in capacity of more than 50 percent) without the need for additional filters. Data from initial months of operation suggest that the actual energy required to produce ozone for the Tucson CAP Water Treatment Plant has been about 10.0 kwnl b of ozone produced, which is appropriate for an air feed system. This energy consumption is based on the ozone system operating at 75 percent of design production capability, or 3,150 b/day of ozone. The energy usage is the total energy needed to operate the system, including the ozone generators as well as the compressors, dryers, chillers, and cooling water and ancillary systems. To maximize energy efficiency and allow generation capability to be matched to plant production needs, the plant. has six parallel units with the capabilitv to goto nine units in the future. Systems Total Cost 3.4 MGD 400 Dollars in 000 s Operating Costs Comparisons per Million Gallons Treated 4. n wastewater treatment, control emuent quality prior to UK The effectiveness of UV and the dosage required for disinfection are controlled by the amount of suspended solids in the effluent. A well-run treatment process and filtration prior to UV disinfection wil ensure low solids in the effluent, good disinfection, and energy-efficient operation of the UV system. 9-3A 9-38 Note: Electfical costs based on 5.5 cents/kwh Chlorine 8 22 centwlb Sulfur dioxide 841 centdlb Source: Katadyn Systems Figure 4. Typical wastewater treatment cost. UV vs Chlorine/Dechlorination & Ozone 5. nstall multiple units. Ozone generators have limited turndown capacity if they have to operate at low flowrates. nstalling a larger number of smaller capacity ozone generators instead of a few large units will provide greater operating flexibility if the plant has to operate under fluctuating flowrates. Similarly for UV systems, if multiple UV lamps are installed in series, some bat teries of lamps can be turned off wher flowrates decrease. TechCommentary/Vol. 1 /No. 4

6 Summary As disinfection and water quality requirements escalate, better disinfection is required. However, better disinfection can create problems, especially if the disinfectant is chlorine. These problems have prompted a search for other methods of disinfection instead of or in combination with chlorination. Two of these methods-azone and UV-are electrotechnologies. Ozone is the most powerful disinfectant currently in use. Although its use for wastewater is limited due to its high costs, ozone is an excellent disinfectant for drinking water. t also can be used for pretreatment, reducing chemical use at the plant. When used as a partial replace- ment for chlorine, ozone reduces the levels of carcinogens deriving from chlorine. However, ozone cannot completely replace chlorine because ozone, unlike chlorine, does not leave a residual of disinfectant in water distribution systems as required by drinking water regulations. Ozone costs are quite variable, depenckng on the particular type of application. Limited data indicates electricity costs for ozone systems are in the range of $5 to $1 0 per million gallons of water treated. Electricity consumption is in the range of 5 to 10 kwh/lb ozone produced. The number of ozone plants is increasing rapidly. Energy requirements for ozonation are predicted to reach about 800,000 kwh per day by the middle of this decade. UV is an effective disinfectant, but mainly for bacteria. Because it does not provide a residual, UV disinfection is used primarily for wastewater or for drinking water derived from groundwater, where cysts from giardia or cryptosporidium are not present. UV disinfectant systems 4 are relatively cheap to install, safe and easy to operate, and are comparable in operating costs to chlorinationdechlorination systems. The use of ozone for disinfection of drinking water and UV for disinfection of wastewater is expected to increase. The growing reliance on these two electrotechnologies will affect future energy and electricity demands at water and wastewater facilities. However, careful attention to facili design, maintenance, and operation may limit the increases in electricity use. For technical information contact: Community Environmental Center EPRl Community Environmental Center Washington University Campus Box One Brookings Drive St. Louis, MO (31 4) FAX: (31 4) For ordering information, please call EPRl s AMP Program AMP This issue of TechCommentaly was written and produced by EPRl s Community Environmental Center in association with Burton Environmental Engineering, Black & Veatch, and Design Productions. The information presented in this TechCommentary is intended to provide a basic understanding of the role of electrotechnolcgies in water and wastewater disinfection. For more information, contact your electric utility marketing representative. LEGAL NOTCE This Techcommentary was prepared and sponsored by EPR. Neither members of EPRl nor any person acting on their behalf: (a) makes any warranty expressed or implied with respect to the use of any information, apparatus, method, or process disclosed in this TechCommentary or that such use may not infringe privately owned rights; or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information apparatus, method, or process disclosed in this TechCommentary. Funding for this Techcommentary is provided by The Electric Power Research nstitute (EPR), a non-profit organization whose mission is to discover, develop, and deliver advances in science and technology for the benefit of EPR-member utilities, their customers, and society. Techcommentaries are one way the EPRl ndustrial Program assists in communicating information concerning energy-efficient, electric-based technologies. Applicable SC codes: 4941, Copyright 1993 Electric Power Research nstitute Palo Alto. California Printed on Recycled All rights reserved. TC Printed 9/93 TechCommentary/Vol. 1 /No. 4 6