Principal

 

Water Disinfection with Ozone

by Rob Dunham, Hong He & Ken Woodard


Table of Contents

General Information

Chemistry

Generation

Advantages and Disadvantages

Links

References

General Information

Van Mauten first reported the existence of ozone in 1785. Schlobein was the first to isolate it, in 1840. Ozone was used as early as 1893 for disinfection of drinking water in Holland. Nice, France has employed ozonation of drinking water since 1906 (Nickols,et al,1992). Europeans used ozone to treat swimming pools during the 1950s to eliminate the irritated eyes and skin caused by chlorine. By 1980, there were over 1100 water treatment facilities utilizing ozone, mostly in Europe (Coate, 1997). Today, more than 2,000 water treatment plants throughout the world use ozonation for disinfection (Nickols,et al,1992).

Disinfection practice in the United States has developed principally around the use of chlorine. Because of concerns about byproduct formation, ozonation has become an alternative method to avoid the formation of halogenated organics caused by chlorination (Nickols,et al,1992). Prior to World War II, several water treatment plants in New York, Pennsylvania, and Indiana experimented with ozone. As of 1990, there are close to 40 U.S. water treatment plants that are equipped with ozonation facilities, including the third largest plant in the world, which is located in Los Angeles. About 40 percent of U.S. ozone plants use lake water as a source, and the remaining plants are evenly split between river, reservoir, and groundwater sources (Brink,et al,1991).

This drawing illustrates a typical ozone treatment unit.
Drawing courtesy of Ozonair Corporation

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Chemistry

Schlobein named ozone (chemical formula O3) after the Greek word meaning "smell." Ozone has a very distinctive smell, which is the "fresh and invigorating" odor (Towles, 1997, a) that can be detected in the air after a flash of lightning. It has been referred to as "nature's air purifier" (Towles, 1997, a). Ozone is highly unstable, which is why it must be generated on-site at the water treatment plant.

Ozone (molecular weight 48) is colorless at room temperature and has pungent odor. It is generally encountered in dilute form in a mixture with oxygen or air. Liquid O3 is very unstable and will readily explode. Ozone is 12.5 times more soluble in water than is oxygen.

As stated in General Information, ozone is very unstable, which is why is must be generated at the point of use. It can quickly change back to atmospheric oxygen. Molecular oxygen is composed of two atoms of the element oxygen. Ozone is comprised of three atoms of oxygen and is formed from molecular oxygen. The transformation from atmospheric oxygen to ozone is triggered by a high voltage, low amperage electrical charge (see Generation) or by the addition of ultraviolet (UV) radiation. The electrical charge breaks the bond between the oxygen atoms and causes them to form in groups of three as ozone.

Ozone is 12.5 times more soluble in water than is oxygen, leading to better mixing in water treatment (Towles, 1997, b). Also, the products of its reaction with organics are oxygen, carbon dioxide, and water. This prevents the incomplete disinfection products that could lead to trihalomethanes (THMs) in drinking water. Ozone is effective against odor-producers because it can easily oxidize these unsaturated compounds.

The reason that ozone is so effective at disinfecting is because of its oxidation potential, which is -2.07 V. For comparison, the oxidation potentials of hypochlorite and chlorine are -1.49 V and -1.36 V, respectively (Towles, 1997, b). The only element with a higher value is fluorine (Coate, 1997). Oxidation potential is important because it indicates the expected degree of chemical transformation. However, it does not indicate the speed or completeness of oxidation.

Heat can accelerate oxidation. Another way to raise this rate is by the addition of ultraviolet (UV) radiation. This combination forms a highly-reactive hydroxyl ion as the ozone and UV "destroy" each other (Coate, 1997). The free radicals produced can quickly oxidize a number of organic compounds. Ozone may not completely oxidize some organic compounds, such as those found in some industrial wastewaters, but "no other commonly employed and less powerful water treatment oxidant...will oxidize any organic material completely to carbon dioxide and water if ozone will not" (Towles, 1997, b).

Although the precise mechanisms of ozone disinfection are not firmly established, the most acceptable theory is that ozone can disrupt the function of the bacterial cell membrane by exerting its strong oxidizing capability. It is an effective disinfectant for a wide range of pathogens and is applicable for achieving the primary disinfection goal for the pathogen categories regulated in the EPA Surface Water Treatment Rule (Bryant,et al,1992).

The above drawing represents a typical ozonating water treatment plant.
Drawing courtesy of Ozonair Corporation

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Generation

Ozone can be generated from one of three sources, with varying concentrations of ozone obtained (Towles, 1997, a):

  1. Ambient air - 0.15 to 1.0% ozone
  2. Dried air to a dew point of -40 to -65 C - 1.0 to 3.0% ozone
  3. Oxygen as feed - 4.0 to 6.0% ozone

As one can surmise, a higher concentration requires more effort and ergo higher cost. However, the largest expense in an ozone treatment plant is usually the electricity usage, sometimes accounting for 75% of the cost of running the plant (EPRI, 1996). Using oxygen as the feed gas will double or triple the efficiency of ozone production per kWh (Towles, 1997, a).

Whichever source is used, the basic means of production is the same. An electrical charge of high voltage and low amperage is passed through the gas to form ozone from oxygen in the corona discharge. Ultraviolet radiation can also be used to generate ozone. This ozone gas must then be changed to the liquid phase, which is easy because ozone is 12.5 times more soluble than oxygen. A decrease in the water temperature correlates with an increase in the solubility of ozone.





Advantages/Disadvantages

Ozone for Treatment of Water:
ADVANTAGES DISADVANTAGES
More powerful disinfectant than chlorine compounds (more effective at Cryptospiridium removal) Tends to cost more than traditional chlorinated disinfection techniques.
Has no negative residuals such as trihalomethane production Does not produce a disinfection residual that would prevent bacterial regrowth.
Does not alter the pH of the water Forms nitric oxides and nitric acid which could lead to corrosion
Increases coagulation
Helps with the removal of iron and manganese
Has taste and odor control properties

Advantages

Ozone has been shown to be the second most powerful oxidizer, after fluorine. Ozone can disinfect roughly 3000 times faster for Cryptospiridium removal than can chlorine (DEL Industries, 1997). This allows for either a lower concentration of the disinfectant or a faster travel time through the treatment system, whichever is more advantageous to the designer.

Summary of Ct-Value Range for 99% Inactivation of Various Microorganisms by Disinfectants at 5oC

Organism Free Chlorine
pH 6-7		 Preformed Chloramine
pH 8-9		 Chlorine Dioxide
pH 6-7		 Ozone
E.coli 0.034-0.05 95-180 0.4-0.75 0.02
Pollo I 0.1-2.5 768-3740 0.2-6.7 0.1-0.2
Rotavirus 0.01-0.05 3806-6476 0.2-2.1 0.006-0.06
Phage F2 0.08-0.18 --- --- ---
Giardia Lamblla cysts 47-150 --- --- 0.5-0.6
Giardia muris cysts 36-630 1400 7.2-18.5 1.8-2.0
Cryptosporidium parvum 7200 7200 79 5-10
Source: Inactivation of Microbcarbon dioxide, and water.

As stated earlier, ozone reacts with almost anything it can. This provides ozone with strong taste and odor control as well as allowing it to oxidize many metals and organics. Lab results have shown that ozone can remove the following metals at 99.5% or above: aluminum, arsenic, cadmium, chromium, iron, nickel, cobalt, lead, zinc, copper, and manganese (Coate, 1997). Ozone can completely oxidize mercury at pH 4 as well (Coate, 1997). Ozonation changes nitrite ions to nitrate ions, and has also been shown to be effective in treating the following: acetic acid, butoxyethanol, isopropyl alcohol, methyl-ethyl ketone, acetone, cetyl alcohol, glycerol, propylene glycol, n-butyl acetate, formaldehyde, methacrylic acid, benzene benzyl alcohol, resorcinol, n-butyl phthalate camphor, para-phenylenediamine, styrene trecresyl-phosphate, xylene, butane, liquefied-petroleum-gas, mineral spirits, methylene-chloride, perchloroethylene, trichloroethylene, hydrogen cyanide, ammonium-hydroxide, ethanolamine, toluene, isobutane, propane, methyl-chloroform, amino-phenol, ammonia, ammonium-persulfate-phenacetin, ethylene tetracetic acid (EDTA), alkylated silicates, and non-ionic detergents (Coate, 1997).

Disadvantages

The primary drawback to the use of ozonation is the cost. Not only are capital costs significantly higher than its chlorinated alternatives, but the maintenance and daily operation costs also tend to be higher. Some have estimated that ozonation can cost up to eight times more than disinfection (EPRI, 1996). Although this may be true in certain cases if only disinfection is considered, the total cost benefit will largely depend on the water being treated. As stated earlier, ozone provides numerous other advantages besides disinfection: ozone can reduce treatment needed for taste, odor, manganese, and iron control, and aid in coagulation.

Costs of ozonation by Aquaflow Corporation

CAPITAL COSTS MAINTENANCE COSTS
$100,000 per MGD treated 0.3 kWh per 1000 gallons treated
Costs provided by the Aquaflow Corporation

Notice that the maintenance costs of ozonation are measured in kilowatt-hours. This is because the primary cost associated with ozone operation is the electricity needed to operate the ozone generators. Taking into consideration the lifetime operation of a treatment plant, some have estimated that up to 75% of all costs associated with ozonation are due to electricity needs (EPRI, 1996). Currently, many companies are working on ways to increase electrical efficiency in ozonators or create methods where less ozone is needed. Developments in these areas could reduce the cost of ozonation for the future.

The other major drawback to ozonation is the chance of bacterial regrowth in the system. Unlike chlorine which leaves a small disinfecting residual throughout the system, ozone leaves no such residual. Because there is no longer any disinfecting mechanism to follow the water throughout the distribution system, there is the possibility that bacteria will begin to regrow in the water. If this becomes a problem, the solution is to add a small amount of a chlorinated compound before the water leaves the plant.

Another drawback of ozone is that it can form nitric acid or nitric oxides which are very corrosive. The solution is to use non-corrosive materials, non-oxidizable materials in and downstream of the ozone generation and application system.

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Links to Related Sites

References



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Student Authors: Rob Dunham rdunham@vt.edu, Hong He hhe@vt.edu, and Ken Woodard kwoodard@vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1997 Daniel Gallagher
Last Modified: 2-5-1998