Ozone Breakthroughs in Pipeline Disinfection
Denver Water and CDM Smith have collaborated to develop significant innovations that help ensure drinking water pipelines are safe for public health. Research and development (R&D) projects using ozone have established the potential for faster, cleaner and easier disinfection. Using these new methods, the team has discovered a safe and effective way to drastically reduce the time needed to complete most pipeline disinfection tasks, while at the same time uncovering a more effective treatment that can kill even the most persistent bacteria.
Working with progressive utilities like Denver Water provides the opportunity to try something new and potentially improve upon the traditional way of doing things.
After a water main is installed or rehabilitated, it must be disinfected before being used. For decades, disinfection with chlorine compounds has been the standard practice. This entails filling the pipe with highly-chlorinated water or calcium hypochlorite tablets and letting it stand for 24 hours before dechlorinating the pipe. While effective, this method is slow and extremely labor-intensive. It involves the use of potentially hazardous chemicals and must be carefully monitored for environmental impacts. And, if the disinfection is not successful, the entire process must be repeated, or the pipe must be replaced.
In the early 2000s, water quality staff at Denver Water noted the increasing implementation of ozone disinfection as a replacement for chlorine disinfection at water treatment facilities. They wondered if a similar technology application could provide a better means of water main disinfection. To find out, Denver Water formed an R&D team with CDM Smith senior vice president and environmental engineer Christopher R. Schulz, PE, who holds 11 patents for water treatment technologies.
The team devised a trailer-mounted ozone system that injects ozonated water into one end of a pipe segment filled with finished water, monitors the ozone residual at the other end, and then neutralizes the ozonated water as it is discharged to the environment. The trailer-mounted ozone system quickly became a useful tool in Denver Water’s arsenal, and an award-winner for research innovation. Schulz says of the results, “The ozone system significantly saves time and money by reducing a 24-hour process to a 4-hour trip.”
The minds of water innovators never stop working, however. Because ozone in the liquid phase is more unstable than chlorine and decays in several minutes to oxygen, the liquid-phase ozone system’s effectiveness is limited to 1,000 feet of 12” diameter pipe. Schulz says, “I continually look at technology and processes and see where the limitations are. I’m always considering if there’s a better way to do the things we’re doing.” Schulz kept thinking through Denver Water’s ozone breakthrough. What if ozone in the gas phase—a more stable condition than liquid—could be used to achieve the same results as the previous project, while opening up the possibility of disinfecting larger pipelines? Schulz and Denver Water soon developed a second R&D project to explore whether ozone gas could offer further improvements.
After Schulz recruited Clearwater Tech, a California-based vendor with extensive ozone experience in the food, beverage and swimming pool industries, the team developed a system using two portable carts. One cart generates high-concentration ozone gas that is diluted with a low-pressure air flow stream. The blended ozone gas stream is then forced into the pipe at a reasonable velocity until the gas can be detected at the other end. Once the ozone gas has filled the pipe, the pipe is pressurized. The ozone gas “pushes in” to the pipe wall, disinfecting it thoroughly. Then a blower is used to displace the ozone with air and the ozone is destroyed at the other end of the pipeline by the second cart. The entire process takes approximately 20 minutes.
Thus far, the ozone gas system has been tested at bench scale and at demonstration scale. Limited field testing has shown promising results, particularly in cases where chlorination failed to achieve successful results; under normal conditions, this would require a complete pipe replacement. “It takes some pretty tenacious bacteria to survive 24 hours of exposure to super-chlorinated water, but they couldn’t survive 20 minutes of the ozone gas,” says Schulz.
While field tests continue, the potential of a larger role for ozone gas in pipe disinfection appears significant. “I’ve worked on many research and development projects, and not all can be commercially realized, but I think this one has a real chance,” says Schulz. “Working with progressive utilities like Denver Water provides the opportunity to try something new and potentially improve upon the traditional way of doing things.”