A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET Ozone Summary New water treatment goals for disinfection byproducts (DBP) and for microbial inactivation will increase the need to consider new disinfection technologies. Ozone is an attractive alternative. This technology has evolved and improved in recent years, thereby increasing its potential for successful application. It is important to note that ozone, like other technologies, has its own set of advantages and disadvantages that show up in differing degrees from one location to the next. What is ozone? • Decays rapidly in water, avoiding any undesir Ozone is a colorless gas that has an odor most able residual effects; often described as the smell of air after a spring • Removes color, taste, and odor; and electrical thunderstorm. Some people also refer • Aids coagulation. to the odor as similar to the smell of watermel- ons. Ozone (O3) is an extremely unstable gas. Limitations Consequently, it must be manufactured and used The use of ozone to treat water has some onsite. It is the strongest oxidant of the common limitations: oxidizing agents. Ozone is manufactured by pass- ing air or oxygen through two electrodes with high, • Toxic (toxicity is proportional to concentration alternating potential difference. and exposure time); • Cost of ozonation is high compared with What regulations govern ozone? chlorination; In the next century, small water systems will • Installation can be complicated; need to use disinfection methods that are effective • Ozone-destroying device is needed at the for killing pathogens without forming excessive exhaust of the ozone-reactor to prevent toxicity DBP. Disinfection byproduct compliance is more and fire hazards; likely to be a problem for small water systems • May produce undesirable aldehydes and treating surface water than for those treating ketones by reacting with certain organics; groundwater because surface water sources tend • No residual effect is present in the distribution to contain more natural organic matter that forms system, thus postchlorination is required; byproducts when mixed with disinfectants. • Much less soluble in water than chlorine; thus In August 1997, the U.S. Environmental Protect- special mixing devices are necessary; and ion Agency (EPA) listed ozone as a “compliance’’ • It will not oxidize some refractory organics in the requirements of the Surface Water Treatment or will oxidize too slowly to be of practical Rule for all three sizes of small drinking significance. water systems. Disinfection Advantages Design of an ozone system as primary treatment Using ozone to treat water has many advantages, should be based on simple criteria, including including the following: ozone contact concentrations, competing ozone demands, and a minimum contact time (CT) to • Possesses strong oxidizing power and requires meet the required cyst and viral inactivation short reaction time, which enables the germs, requirements, in combination with EPA recom- including viruses, to be killed within a few mendations. OFPAGE FOUR seconds; • Produces no taste or odor; Systems that need to provide CT to comply with one • Provides oxygen to the water after disinfecting; the Ground Water Disinfection Rule, but are also • Requires no chemicals; having problems with DBP or maintaining distri- • Oxidizes iron and manganese; bution system residuals, should consider using • Destroys and removes algae; ozone as the primary disinfectant and then chlo- • Reacts with and removes all organic matter; ramines for distribution system protection. NATIONAL DRINKING WATER CLEARINGHOUSE Ozone Ozone has been observed to be capable of disin- of these molecules into toxic or nontoxic byprod- fecting Cryptosporidium, and there is significant ucts. Such transformation can theoretically lead interest in this aspect of its application. Available to complete oxidation into carbon dioxide (CO2); data indicate that a significant increase in ozone however, this is rarely the case in water treatment. dose and CT may be required as compared with Any observable reduction in total organic carbon past practices. Therefore, these needs should be (TOC) is due either to a small degree of CO2 for- considered in planning. mation (for example, decarboxylation of amino acids) or the formation and loss of volatile com- Iron and Manganese Removal pounds through stripping. The standard oxidation-reduction potential and reaction rate of ozone is such that it can readily Effects on Coagulation oxidize iron and manganese in groundwater and It is important to understand that the coagulating in water with low organic content. Groundwater effects of ozone go beyond any direct oxidative systems that have iron levels above 0.1 milligrams effects on organic macro-pollutants. For this rea- per liter (mg/L) may have iron complaints if ozon- son, one must be wary of studies claiming im- ation or chlorination is added. proved removal of organic matter when the data are based solely on color removal or ultraviolet (UV) Excessive doses of ozone will lead to the formation absorbtion. Also, when studying the removal of of permanganate, which gives water a pinkish DBP (for example, trihalomethanes), one must color. This soluble form of manganese (Mn) corre- be careful to incorporate controls permitting the sponds to a theoretical stoichiometry of 2.20 mg separate evaluation of ozone’s direct effects. O3/mg Mn. Stoichiometry is the determination of Finally, the coagulating effects of ozone may not the proportions in which chemical elements com- be observed with all water. Whenever considering bine or are produced and the weight relation in the use of ozone as a coagulant aid, the pre- a chemical reaction. ozonation effects should be critically evaluated in pilot studies incorporating the proper controls. Color Abatement Because humic substances are the primary cause Algae Removal of color in natural waters, it is useful to review Ozone, like any other oxidant, such as chlorine the reactions of ozone with humic and fulvic acids. or chlorine dioxide, has a lethal effect on some According to different authors, ozone doses of 1 to algae or limits its growth. Ozone is also capable 3 mg O3/mg C lead to almost complete color re- of inactivating certain zooplankton, e.g., mobile moval. The ozone dosages to be applied in order organisms, Notholca caudata. Such organisms to reach treatment goals for color can be very high. must first be inactivated before they are removed It is interesting to note that when the ozone dosage by flocculation and filtration. is sufficient, the organic structure is modified such that the final chlorine demand can decrease. Byproducts The alternative use of ozonation has generated Control of Taste and Odor much interest because of its ability to avoid the The National Secondary Drinking Water Regulat- formation of halogenated organics inherent in the ions recommend that the threshold odors number practice of chlorine treatment. However, raw water (TON) be 3 or less in finished water. It has been quality significantly affects ozonation results and shown that ozone can be effective in treating water could lead to the formation of other undesirable for taste and odor problems, especially when the byproducts. Brominated byproducts are a major water is relatively free from radical scavengers. concern in source waters containing bromide. Ozonation produces its own byproducts, such It has also been observed that ozone, in combina- as aldehydes, ketones, and carboxylic acids. tion with other downstream treatment processes, Assuming equivalent disinfection, benefit is especially granular activated carbon (GAC) filtra- achieved as long as the health concerns for tion, can greatly increase taste and odor treatment the new products are less than those for the efficiency and reliability. Again, the cause of taste chlorine byproducts. and odor compounds, as well as the source water to be treated, need to be carefully considered prior Personnel Requirements to designing a treatment system. Analysis and Personnel time requirements for system cleaning possibly pilot-scale experimentation may be re- may be fairly substantial. However, recent quired to determine the optimum choice of ozone advancements in ozonation technology include and downstream treatment. use of high purity oxygen feed systems, rather two than ambient air-feed systems. Ozonation treat- ment is therefore said to run cleaner and require Elimination of Synthetic Organic Chemicals less cleaning-related maintenance than had PAGE OFPAGE FOUR Ozone or advanced ozonation processes can remove the earlier versions of this technology. many synthetic organic chemicals (SOC). This removal leads to the chemical transformation TWELVE • DECEMBER 1999 No Residual Ozone contacting for disinfection has typically Ozone will not provide a disinfecting residual been accomplished in deep, multistage contac- that protects finished water in the distribution tors that employ fine bubble diffusers. Newer system. Therefore, the role of chlorine as a disin- alternatives have emerged that provide an option fecting agent is not entirely replaced, and its use for small systems through the use of side-stream in either the free chlorine or chloramine form will injection technologies that eliminate the need for be required for this purpose in many locations. fine bubble injection. Package units are available that include a gas separator that fuses and elimi- Process and Equipment nates excess gas that results from ozone addition The basic elements of an