Before a new water main or one that has recently under gone repair can be placed back into service, it must be flushed and disinfected. The conventional approach is to use high concentrations of chlorine for an extended period of time to kill the bacteria living in the biofilms attached to the water main surface. The disadvantage of this approach is that hypochlorite solutions need to be transported, stored and handled. These solutions are considered hazardous in some jurisdictions. In addition, the treated water must be dechlorianted prior to discharge, which requires another set of chemicals.
Ozone has been used in drinking water treatment for over 100 years and is a proven drinking water disinfectant. It has not been used extensively for the disinfection of water mains. The advantage of ozone use is that it can be generated on site from air and does not require any treatment prior to discharge. A question regarding the use of ozone for treating biofilms on water main surfaces is whether the ozone will penetrate the biofilm to inactivate all of the bacteria present.
Stantec Consulting and EPCOR Water Service Inc. of Edmonton, alberta, Canada conducted a laboraotry study to see if ozone would be effective against these biolfilms (Ozone Science and Engineering, 34: 243-251, Li Chang and Steve Craik). The study looked at HPC bacteria grown on concrete mortar substrates to simulate water main surfaces.
Ozone appears to be able to achieve a 1.7 log reduction of the biofilm at a CT value of 120 mg-min/l. Higher CT values did not appear to improve teh log reduction significantly. the study concluded that log reductions of less than 2.0 should be expected with ozone.
In the September 2006 issue of Opflow magazine published by the AWWA, ozone was also studied in treating water mains in Denver. In these tests the CT values were only 10-24 mg-min/l and the results were not as effective.
So, it appears that ozone may be an effective treatment for water mains that offers easier and safer application, but the ozone CT probably needs to be in the 120-240 mg-min/l range.