After raw water has been filtered and clarified, NTMWD disinfects water in a two-step process to prevent infectious diseases from spreading and to ensure water is safe for people and the environment. The first step is ozone disinfection and the second step is chlorine disinfection.


Ozone disinfection is one of the highly effective ways we remove bacteria and viruses because it purifies water by removing harmful microorganisms. It inactivates harmful bacteria, including Giardia and Cryptosporidium, and helps kill taste and odor organisms as well. Since ozone dissipates rapidly, it leaves no harmful residual and is considered the best practice for water treatment.

DID YOU KNOW?  NTMWD operates the largest fully ozonated water treatment plant in the state and one of the largest in the country. 

This process has an added benefit of reducing taste and odor issues caused by seasonal changes in the reservoirs that are the source of our water supply. While taste and/or odor changes occur in drinking water occasionally, they pose no health threat. These changes can arise when warmer temperatures, nutrients and sunlight conditions align to form naturally occurring algae blooms in lake water. We regularly sample lake water for elevated levels of compounds caused by the blooms and treat water using ozone disinfection to minimize taste and odor issues.

Ozone Disinfection Water Treatment


After the water has been cleaned and disinfected with ozonation, we have one more step in the disinfection process – the addition of chlorine. Chlorine is a highly effective method of disinfection for killing germs in the water, but its effectiveness fades over time. Adding ammonia to the chlorinated water forms chloramines which provide a longer lasting residual disinfectant than chlorine alone to the water in the transmission and distribution systems extends the life of the chlorine disinfection and allows the water to stay safer longer. Once we make a few adjustments to the pH of the water, it is ready for the final steps.

Ozone Disinfection Water Treatment 2

Temporary Change in Water Disinfectant

Each spring for about one month, NTMWD temporarily changes our water disinfectant process. During the annual change, the cities we serve may help move the chlorine-disinfected water through the system by releasing water from fire hydrants. Learn more.