A Texas-Size Toast to Pure Water

Located in Dallas County, the largest ozonation facility in the world uses the ozone process to successfully control tastes and odors in drinking water

Currently, the process of ozonation is being implemented in drinking water purification in treatment plants around the globe. Ozone is a potent agent in the disinfection of raw water. When raw water is pumped into the treatment plant, it carries many bacteria and viruses. The use of ozone eradicates these harmful pathogens. There are many advantages to using ozone in the treatment process. The City of Dallas Elm Fork Water Treatment Plant, located in Carrollton, Texas, houses the world's largest ozonation treatment facility.

The Elm Fork Water Treatment Plant (EFWTP) was built in 1952. As part of the $100 million expansion program in 1992, the world's largest ozonation facility was added to the plant. EFWTP supplies water to parts of Dallas and 26 customer cities out of which 21 are supplied treated water. The plant has a treatment capacity of 330 million gallons per day (mgd). In the Dallas/Fort Worth (D/FW) Metroplex, which is one of the fasting growing areas in the country, it is important for the plants to plan for the future. For example, the water requirement for the City of Dallas system is presently 850 mgd. This requirement is satisfied by three main treatment plants: Elm Fork, Eastside, and Bachman. In 10 years, the water requirement is forecasted to reach one billion gallons per day. Because of this growth, the plant managers and city planners are forced to look for more sources of raw water in order to satisfy the rising demand in water that parallels population growth.

Water Treatment
The treatment process at Elm Fork is a careful and calculated process. First, the raw water is pumped into the Elm Fork Trinity River with low lift pumps. From there, water flows by gravity into the ozonation facility. This process will be later discussed in detail. After the raw water has been treated with ozone, it then flows to the chemical treatment facility.

EFWTP uses ferric sulfate and polymer as coagulants. These chemicals cause small particles in the water to coagulate into larger masses, which eventually settle out of the water. This process is called flocculation. This helps to remove mineral elements as well as organic materials from the water. Also, fluosilicic acid is added in the chemical treatment facility. Fluosilicic acid, a source of fluoride, is added because it prevents tooth decay. The most important chemical that is added at the chemical treatment stage is chloramine. Chloramines are formed when ammonia (NH3) reacts with chlorine in water. Chloramine is used instead of free chlorine for two reasons: 1) It leaves a longer-lasting residual in the water so that once the water leaves the plant, it is still protected from bacteria and other harmful microorganisms; and 2) Chloramine does not produce harmful disinfection byproducts that are associated with the use of free chlorine. Finally, lime is added to the water in order to soften the water. This softening agent lowers the relatively high mineral concentration characteristic of this region. Lime is slurried with water by adding lime pellets to heated tap water. This lime slurry is then mixed in with the treated water. Following these chemical additions, the water then continues on to the sedimentation basins.

The sedimentation basins are arranged in three trains, each containing two 10 million gallon basins each. Six of these basins hold a total of 60 million gallons at any one time. As the water slowly flows through these enormous basins, the sediment is gradually removed. The turbidity of the water slowly decreases as the sediment sinks to the bottom of the basins. The solids are sent to backwash lagoons through a valve located at the center of each basin. After the majority of the solids are removed, the water is sent to the filtration gallery, which contains 24 filters.

The filters use a filter media composed of a 40 inch depth of anthracite carbon, which adsorbs the solids from the water. Usually, after 25 hours to 30 hours of operation, each filter is backwashed to remove the solids collected by the filters. The water containing the solids is then sent to the backwash lagoon. After filtration, the turbidity of the water is approximately 0.06 nephelometric turbidity units (ntu), which is substantially lower than the government standard of 0.3 ntu. The water is then sent to the clearwells from where it is pumped to the customers.

The Ozonation Process
The ozonation process is a relatively new technology in the field of water treatment. The positive effects of ozone have been researched, and the use of ozone has been considered by many scientists to be beneficial in water treatment. The use of ozone at EFWTP has provided a significant improvement in the quality of the treated water. Ozone provides a strong control of the taste and odor of the water.

"In fact, when we had to close down the plant for a week due to renovations, we received many complaints from consumers about the water coming from other parts of the systen," said Kenneth DelRegno, Division Manager at EFWTP. According to DelRegno, this is mainly due to the fact that Dallas' other plants have not yet integrated an ozonation facility as part of their treatment plant. Construction is now under way to add ozone treatment at Dallas' other two water treatment plants.

Capture of Oxygen
EFWTP is home to the largest ozonation facility in the world. To produce ozone, air is captured from the atmosphere and stored in large gas storage tanks. The air in the earth's atmosphere has an oxygen concentration of approximately 21 percent. When the air is forced through a series of molecular sieves, oxygen is separated from the atmospheric nitrogen. The nitrogen is released back into the atmosphere. Pure oxygen is sent to the ozone generation room.

Ozone Generation
Inside the ozone generation room are large ozone generator tanks. Each of these generators contains hundreds of dielectric tubes with electrodes inside. Behind the ozone generation room, is a room full of power supply units (PSUs). These PSUs produce large voltages so that strong currents can be sent through the electrodes within the ozone generators. When the current is sent through the electrodes, a large potential difference is created between the electrode and the inside of the vessel. This high voltage causes the current to jump across an air gap between the dielectric and the vessel wall. When the pure oxygen flows through the air gaps, the spark causes the O2 to decompose into single oxygen atoms. These atoms then recombine to form ozone. The generators at EFWTP can produce up to six percent ozone in the oxygen mixture.

Ozone Treatment
The ozone that is generated must be immediately sent to the treatment facility. Ozone is a highly unstable gas. The half-life of ozone gas is extremely short, usually around 45 minutes. However, in high temperatures characteristic of North Texas in the summertime, the half-life can decrease all the way to 25 minutes. Moreover, in water, ozone has a half-life of only 30 seconds to 45 seconds.

Once the ozone is sent to the ozone contractors, it is vented into the raw water in a very sophisticated process. As the water flows down towards the bottom of the tank, a number of aerated tubes bubble ozone gas up towards the top. The process is repeated several times in order to achieve maximum contact between the gas and the raw water.

Why Ozone?
The question that comes to mind is why is ozone so beneficial to the water treatment process. There are numerous benefits of ozone that have made its integration into the treatment process so usefull. First of all, ozone is a characteristically bactericidal and viricidal chemical. There are many infectious agents that can be successfully eliminated by ozonation: Echerichia coli that causes gastroenteritis, Cryptosporidium parvum that causes cryptosporidiosis, as well as the Hepatitis A virus among numerous others. For this reason alone, the raw water is first treated with ozone so that all harmful pathogens can be successfully removed.

Also, ozone is a very potent oxidant, and executes the oxidation of manganese and iron as well as any organic materials. Ozone is also extremely helpful in controlling the physical qualities of water. Ozone provides for a control of the taste and odor of water.

"By reacting the unsaturated groups and cleaving the carbon-carbon double bonds within organic compounds, ozone produces aldehyde, ketones and acid depending on the groups affected, the ozone dosage and contact conditions. When oxidized, the resulting organics absorb light differently than the original material, resulting in a reduction of true color," according to Paul Overbeck of Osmonics Labs. The control of the color caused by organic materials in raw water is an important process.

The benefits of ozone in the actual treatment process are numerous. Water can be treated with ozone without the formation of disinfection byproducts, principally trihalomethanes (THMs). THMs are most commonly formed during disinfection of drinking water with free chlorine. THMs, are potential carcinogens that can be harmful to humans. Another important characteristic of ozone in the treatment process is the fact that it decomposes in 30 seconds to 45 seconds in water. Also, no harmful substances are formed in the decomposition of ozone. Ozone, O3, simply decomposes into elemental oxygen.

Since ozone leaves no residual in the water, it cannot be used without subsequent chloramine addition. If water was treated exclusively with ozone, it would be susceptible to the growth of many microorganisms in the distribution process. However, because of the many benefits of ozone, it is an important and highly useful process to implement before chlorination.

E-Sources

This article originally appeared in the 11/01/2002 issue of Environmental Protection.

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