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.
A recent study in the Journal Ozone Science and Enginnering has shown that electrolysis followed by volatilization can remove Br from drinking water and as a result lower the formation of bromate from ozone water treatment.
This is an important issue since a large number of water sources contain Br ion. Ozone can react under the proper conditions of pH, Br ion concnetration and other factors to form bromate. Bromate is regulated by the EPA as a possible carcinigen and it is limited in driking water to less than 10 ppb. Recent studies have indicated that the correlation between bromate and cancer may be weaker than thought, but the regulatory limitation remains in palce. As a result, ozone use has been limited to water sources with low Br levels.
A complication with the process is that electrolysis can produce chlorine whcih also reacts with ozone. Dechlorinating agents need to be added and the amount of ozone required adjusted to compensate for the ozone demand of the water. When this was done, the study showed that the reductions in bromate formation due to ozone were proportional to the reduction in the Br ion levels in the water.
Theoretically at least, it should be possibel to mitigate bromate formation due to ozonation by pretreating the water using electrolysis to reduce Br levels. This would expand the range of source water that could benenfit from ozone drinking water treatment.
The US EPA has launched a new app and website that let’s users check on the health of waterways anywhere in the US. It provides information on thousands of lakes, rivers and streams across the US. The release of the website corresponds to the 40th Anniversity of the Clean Water Act. The app uses GPS and user entered zip code to provide information on local waterways. This technology gives citizens a way to participate in the stewardship of the nations waterways.
Source water quality directly impacts the challenges of converting source water to high quality drinking water or the necessity to treat water prior to discharge into waterways. It also impacts the usefulness of the waterway for commerce and recreation. Citizens will be able to make moe informed decisions about the recommendations for water treatment options and the associated costs if they better understand the existing quality of local waterways.
This knowledge becomes increasing important when it comes to more advanced water treatment technologies such as ozone, UV, and membrane filtration
The City of Santa Barbara’s Cater Water Treatment Plant is being upgraded
in response to stricter federal drinking water regulations. The $20 million ozone water treatment project is about 80 percent complete. The work should be completed by the end of January.
Ozone will replace chlorine in the pre-treatment of water at the plant. Besides meeting stricter federal regulations the city’s water taste should also improve. Ozone is finding increasing use by municipalities to meet increasingly stringent regulations for both pathogen control as well as a reduction in disinfection byproducts that can be associated with teh use of chlorine, especially at the pretreatment stage of the process.
Dallas Water Utilities experienced an outage at the Elm Fork Drinking Water Treatment Plant on December 10. As a result, the facility failed to meet the minimum treatment technique requirements that day when its water system failed to properly disinfect the drinking water for a period of more than four hours. DWU has indicated that the water was safe to drink. No adverse chemical compounds or pathogens were found in the water.
The failure was caused by a problem with the ozone system. The plant still produced and delivered water with a chlorine-based disinfectant.
As recommended by the US EPA, Dallas uses a multi-barrier system that protects the water should any one element of the system fail. Thus by combining filtration, ozone and chlorine, even with a failure of one element, the water produced was still safe to drink.
Ozone can react with bromide ion to form bromate. Currently, the USEPA has a limit of 10 micro grams per liter for ozone in drinking water. This level has been extended to other water treatment applications, for example, ground water remediation. The USEPA is considering lowering the allowable limit for bromate to 5 micro grams per liter. The concern about bromate is that it might be a possible carcinigen.
While there are countermeasures for minimizing the formation of bromate where ozone is applied, in applications with high doses of ozone and high levels of bromide bromate formation can be a problem.
Recent studies sponsored by the International Ozone Association and the American Water Works Association Research Foundation have indicated that bromate might not be as serious a concern as previous thought. Dr. Joseph Cotruvo presented an update on this research at the recent IOA conference in Milwaukee.
It appears that when bromate enters the digestive track it is broken down by processes in the stomach and intestines such that it does not reach target organs of concern. The research suggests that the bromate levels may in fact be increased to as high as 20 micro grams per liter.
A final decision on the matter is not expected for another 6-8 years.