IMG_20170503_105630crop.jpg

SECONDARY
DISINFECTION

York Water District is required to protect customers from potential issues such as main breaks within the distribution piping. This is accomplished not only through corrosion control but also by ensuring there is an adequate and persistent amount of chlorine available in the water to safely combat such issues. This is known as providing “secondary disinfection.” Secondary disinfectants must be a form of chlorine, whether free chlorine, monochloramine, or chlorine dioxide. Free chlorine and monochloramine are the most used forms and are accepted as safe and best available practices.

 

The use of chlorine to disinfect water has nearly eliminated waterborne illness. However, the trade-off with its use is that Disinfection Byproducts (DBPs) can form when any form of chlorine added to treated water during primary and secondary disinfection processes reacts with organic matter naturally present in surface waters and some groundwaters. Some DBPs are regulated by the US EPA as suspected carcinogens. The EPA has set limits or maximum levels in drinking water for the regulated DBPs Trihalomethanes (THMs) and Haloacetic Acids (HAAs).

 

Monochloramine disinfectant is most often used at mid-to-larger water utilities, and for those that use surface waters. This is because it is proven to significantly reduce the formation of regulated Disinfection Byproducts (DBPs) and provide a stable chlorine residual in larger distribution systems, thus playing an important dual role. Free chlorine, which is more reactive, can form DBPs faster and is more commonly used with groundwater supplies (utilities that use drilled or dug wells as a water supply) where DBP formation potential is less because groundwater tends to contain fewer natural organics which could form DBPs. In addition, most systems that use free chlorine are smaller in size, so typically can add less free chlorine and maintain lower water age due to a smaller piping area, also tending to lead to lower DBP formation. The type of secondary disinfectant used is tailored and dependent on many individual system conditions.

 

To reduce the formation of regulated DBPs, the York Water District, along with most other medium-to-large water providers using surface water supplies in the area, adds a small amount of ammonia (equivalent to four drops in a 55-gallon drum) to treated water to form monochloramine for secondary disinfection. Over 25 years ago, YWD made the switch from free chlorine to monochloramine and was able to reduce the formation levels of regulated DBPs by almost 70 percent. Monochloramine has also been proven to produce improved taste and odor over free chlorine.

 

York Water District currently maintains around 100 miles of underground pipe serving our customers, four pumping stations to boost pressure and provide adequate supply of water, as well as two large standpipe tanks containing a maximum of 5 million gallons which is available at any time. These tanks store millions of gallons so that if there were unusual circumstances such as excessive water demand, a large fire, or large pipe break, we would not run out of drinking water. We also maintain two emergency interconnections that can be opened to provide water in an emergency in either direction: one with Kennebunk, Kennebunkport, and Wells Water District to the north and the other with Kittery Water District to our south.