Examining a New Disinfection Method for Municipal Water Distribution Pipelines
Two years of research has produced a new disinfection method for drinking water distribution systems using ozone technology. This innovation is called Rapid Pipeline Disinfection and can disinfect a pipeline in about one hour as compared to traditional methods using chlorine that can take up to 24 hours or more.
Water quality leaving treatment plants in North American water utilities is exceptional and protects public health, thanks to the variety of treatment tools water treatment professionals have at their disposal. From carefully controlled water treatment plants, this high-quality water travels through miles and miles of water mains and distribution pipelines to reach homes, schools, parks, recreation centers and businesses. Sadly, on the way, this once-safe, high-quality potable water can be contaminated in the distribution system.
In a 1991 study, Barbara Herwaldt noted that microbial contamination in new or repaired water mains has been associated with several waterborne disease outbreaks in public water supply systems. In 2000, Gregory Kirkmeyer wrote that there were 35 reported cases of waterborne disease outbreaks associated with contamination of water mains between 1920 and 1984. That study also showed that pathogens and fecal coliform bacteria are present at detectable levels in soil and trench water at water main repair sites.
For these reasons, proper disinfection of water mains and pipelines is required by law and is an essential public health protection measure.
Investigating dissolved ozone
A few years ago, Denver Water and its consultant, Camp Dresser and McKee (CDM), investigated dissolved ozone as a pipeline disinfectant. Denver Water used the continuous feed or slug dose method for larger mains (12 to 108 in.) and the tablet method for smaller mains (2 to 12 in.) in accordance with AWWA Standard C651. Chlorine and sodium bisulfite solutions were fed from trailer-mounted storage tanks. The process was costly, time-consuming and considered inefficient.
An extended pilot system evaluation on 4- to 6-in. pipelines was conducted by Denver Water based on positive results performed in small-scale laboratory ozone disinfection studies. In 2003, they assembled a mobile, trailer-mounted ozone system for use on 6- to 12-in. pipelines for real-world operation.
The result was Rapid Pipeline Disinfection—the patent-pending innovation from Denver Water and CDM that uses patented pressurized mass transfer and degas technology from Mazzei Injector-GDT Corp. and patented floating plate air-cooled ozone generating from Pacific Ozone Technology.
How it works
The trailer-mounted ozone system receives water from a local fire hydrant using a fire hose. Ozone is produced from a pressurized oxygen cylinder using an air-cooled 180-g/h Pacific Ozone Technology ozone generator and is added to the hydrant water using a packaged pressurized ozone mass transfer system. More than 90% of the ozone gas is dissolved in the process with the off-gas removed and discharged through an ozone destruct unit. This allows only water with dissolved ozone to enter the pipeline and go near plug flow transit and disinfection.
Operators and the environment are not exposed to ozone due to the unique process design. The high ozone residual at the entrance to the pipeline reacts with organisms and decays as it moves down the pipeline before discharge to a sewer at approximately 0.1 ppm, which is neutralized by ascorbic acid for discharge. A pressure regulator controls inlet pressure to the trailer, resulting in ozonated water flows to a pipeline between 200 and 260 gal per minute, depending on line size with maximum applied dosages of 4 and 3 mg/L, respectively.
The concentration x time (Ct) value at the end of pipe is the monitored value for pipeline disinfection. By using the end of pipeline ozone residual value, it is assured that higher disinfection will occur earlier in the pipeline because it is exposed to a higher dissolved ozone level (C) for the same time (t). The ozone trailer is typically operated for 30 minutes with the residual ozone in the pipeline either allowed to naturally convert to oxygen (less than one hour), or is simply flushed with hydrant water over ascorbic acid tablets before sampling.
Denver Water has been tracking operational results of both the ozone and chlorination-dechlorination disinfection systems on six to 12 mains with runs of up to 6,000 ft for almost two years. They have found first-pass success rates to be virtually identical at 87%.
The summary of benefits recognized includes:
- No dechlorination step—chemical and labor cost reduction;
- No hazardous chemical storage or transportation—ozone generated onsite; and
- Effective and rapid disinfection—minutes, not hours/days before flush and sampling.
Current practices for water main disinfection depend on chlorine as the disinfectant, with its associated chemical handling, time-consuming application, measurement, detention and lengthy flush periods and problematical wastewater disposal requirements as summarized in the American Water Works Association Standard C651 (AWWA 1993).
Currently, the three widely used and generally effective methods are:
- Flush the pipeline with a high concentration chlorine solution (>100 ppm) flowed by pipeline fill at a minimum of 25 ppm, with final residual free chlorine greater than 10 ppm held for 24 hours. Dechlorinate and dispose of wastewater;
- Slow flush/fill with free chlorine at greater than 100 ppm for a minimum of three hours. Dechlorinate and dispose of wastewater; and
- Add calcium hypochlorite tablets to the pipeline and then fill with water, achieving a minimum of 25 ppm of free chlorine residual and hold for 24 hours. Dechlorinate and dispose of wastewater.
Each of these three options can be effective but has significant drawbacks:
- Dechlorination of these highly chlorinated water volumes is recommended prior to discharge to a sanitary or storm sewer, storage pond or flood control channel to prevent harm to wastewater treatment facilities and the environment;
- Chemical storage, transportation and handling of hazardous chemicals (hypochlorite and sodium bisulfite) are concerns of the utility or contractor employees and the area residents where mobile chemical trailers are used; and
- Labor-intense process for utility and/or contractor staff is limited by contracted time constraints to go to the site, conduct disinfectant addition, depart and allow the 24-hour hold time and return for dechlorination, flush and sample.
A 1998 AWWA survey by Charles Haas of 250 public water utilities found that most water utilities (87%) fail to practice dechlorination to prevent harm to wastewater treatment plant facilities, fish kills and other environmental hazards.
Ozone technology for pipeline disinfection
Ozone is recognized as the strongest commercially available oxidant and disinfectant and now can be applied for safe, efficient and thorough pipeline disinfection without the negative drawbacks of the outdated methods. The new method using safe and efficient ozone technology can be summarized as:
Flush pipeline using the Rapid Pipeline Disinfection ozonated water system for usually less than one hour. Ozone naturally decomposes and converts to oxygen. No disposition of wastewater required.
It sounds easy, better and more efficient because it is. Ozone technology has been successfully employed within drinking water treatment facilities for more than 100 years, with more than 400 municipal water treatment ozone systems operating in the U.S. alone. Ozone technology provides multiple benefits such as color reduction, taste and odor control, microflocculation, iron, manganese and hydrogen sulfide oxidation, and disinfection of bacteria, viruses and parasitic cysts. In addition, ozone has been used effectively for piping disinfection/clean-in-place in the food, beverage, pharmaceutical, cosmetics and electronics industries for many years.
Denver Water’s testing has shown trailer-mounted ozone treatment is a strong and viable alternative to chlorination-dechlorination practices for municipal main/pipeline disinfection.