PHOTO 1
Schematic: Process flow schematic detailing Denver Water's trailer-mounted ozone disinfection system.

PHOTO 2
Clean pipeline: The interior of this pipeline has been cleaned with the high-pressure spray wash. It is now ready for ozone disinfection.

PHOTO 3
Ozone disinfection system: The ozone used for disinfection is generated onsite by this trailer-mounted electrical ozone generation system. The inlet and outlet fire hose connections are located at opposite sides of the trailer floor.

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Project Description
Pipe Disinfection Gets a Green Makeover

After a water main is installed or rehabilitated, it must be disinfected to protect against microbial contamination of the public water supply system and waterborne disease outbreaks. The industry standard of using a chlorine-based disinfectant is time consuming, expensive, and potentially harmful to the environment. Chlorination often requires onsite preparation of hazardous chemicals (typically calcium hypochlorite and sodium metabisulfite), or transport of chlorine gas cylinders, creating risk to workers and the community. Also, chlorinated water must be dechlorinated before it is discharged to prevent harm to the environment and aquatic life. In search of a green alternative to the traditional chlorine method, Denver Water and CDM have worked jointly on a 3-year study researching ozone as a disinfectant. Results showed that when combined with a high-pressure spray wash pre-cleaning system, ozone provides an economical, environment-friendly, and socially responsible method of disinfecting water mains. Based on these results, Denver Water has incorporated a trailer-mounted high-pressure spray wash pre- cleaning and ozone disinfection system into their standard distribution system disinfection processes.

INTEGRATED APPROACH

Environment-friendly ozone quickly degrades to oxygen. Disinfection with ozone is accomplished without affecting any other environmental media. Ozone naturally converts into oxygen typically in less than 1 hour inside a closed pipeline. Ozonated water poses little risk of environmental harm, and can be discharged safely to streets and sewers. Any ozone residuals released into the environment are quickly consumed upon contact with pavement, dirt, or sunlight. As an extra precaution, Denver Water uses ascorbic acid tablets to remove any ozone residuals prior to discharging the ozonated water–a process that is complete within seconds.

QUALITY

Saves time. Ozonation and high-pressure pre-cleaning achieves disinfection in 20 to 30 minutes, significantly faster than the typical 24-hour holding time required for chlorination. Chlorination and dechlorination solutions must be manually prepared onsite, requiring the transportation of chemicals. Both trips to the disinfection site take around 4 hours. In addition to the 24-hour holding time in the pipes to fully dissolve chlorine tablets, dechlorination requires a minimum of 24 hours.

Eliminates hazardous chemical risks. Chlorination and dechlorination involve storage, transportation, and mixing of the hazardous chemicals that are required to make the disinfection solutions, putting the workers and local environment at risk. By contrast, ozone is generated onsite and used immediately, as it is pumped directly into the pipe that is to be disinfected.

Reduces labor. Ozone disinfection reduces labor requirements by 50 percent by eliminating the need for preparing chemical solutions for chlorination and dechlorination, as well as multiple trips to the site.

Simple. Ozone is generated onsite by a trailer-mounted electrical ozone generation system, unlike the chlorine method, which requires tedious preparation of stock solutions. Ozone disinfection also eliminates the need for dechlorination.

ORIGINALITY and INNOVATION

Ground-breaking. The mobile high-pressure spray wash pre-cleaning system and ozonation unit developed by Denver Water is the first of its kind. While ozone is commonly used for drinking water, this is the first time it has been applied to the disinfection of water mains.

Best practice. Although no federal regulations currently exist for the disinfection of water mains, trends focusing on water quality indicate that it is only a matter of time. CDM is working with the American Water Works Association and the U.S. Environmental Protection Agency to establish ozone disinfection of water mains as a best practice.

Industry trend setter. The high-pressure pre-wash and ozone disinfection system has been generating interest among industry members. Other water utilities looking to implement green technologies, including the major water utility serving the San Francisco metropolitan area, have contacted and visited Denver Water to learn more about the innovative system. In addition, CDM and Denver Water have applied for a patent for the disinfection system, which is being commercialized by an ozone equipment provider for nationwide municipal water pipeline disinfection applications.

COMPLEXITY

Testing stages. The 3-year research study was conducted in three stages: laboratory, pipe-loop testing, and full- scale field trials. Chlorine and ozone were compared in the pipe-loop study, which used 2,000 feet of 6-inch piping in an abandoned facility. A trailer-mounted ozone system was designed and constructed by Denver Water for the field trials.

Effective. Ozone is about 100 times more powerful as a disinfectant than chlorine. Also, the high-pressure spray wash pre- cleaning system is effective in dislodging bacteria that harbor on pipe walls, the prime target of disinfection.

Disinfection simulation. Laboratory testing by the University of Alberta used concrete samples to recreate the pipeline disinfection environment for testing. Bacteria that are attached to pipe walls were accurately represented to evaluate the efficacy of ozone as a disinfectant.

SOCIAL/ECONOMIC ADVANCEMENT

Cost savings. Ozone disinfection requires 50 percent of the labor needed for the equivalent chlorine method. This labor reduction results in an estimated cost savings of $1,000 by Denver Water per pipeline disinfection event.

Customer service. Ozonation facilitates rapid restoration of water service to customers following pipe repair or rehabilitation.

Distribution system expansion. Disinfecting water mains more quickly, efficiently, and economically facilitates Denver Water's ability to extend the distribution system to meet growth demands and enhance availability of water to the city.

Water quality. The green approach to disinfecting water mains protects against the risk of waterborne disease outbreaks while eliminating the safety and environmental risks of chlorine use.

Sophisticated trailer. Positioned in an outdoor environment, the trailer-mounted ozone system is vulnerable to vibration and temperature. To accommodate these factors, the system has protective equipment including an air-cooled ozone generator and vibration isolation dampeners. Disinfection can be completed with a system using skid-mounted equipment easily transported by a small truck. In addition, dielectric tubes/plates can be made of ceramic materials for improved durability.