Drinking Water Treatment Process

Did you know that all of the water that comes into your home, whether it's for your sink, shower, toilet or outdoor faucet, is treated drinking water? To deliver safe drinking water to your taps, it must first go through a rigorous treatment process that includes Sedimentation (including coagulation and flocculation), Disinfection (including ozone and chlorine), Fluoridation, Filtration, ammoniation (or ammonia added) and pH adjustment after filtration. Once per year we perform the temporary change in disinfectant where ammonia is not added.

Coagulation

The treatment process begins when water from the lakes is brought into the treatment plant. Initially, the water contains various particles of sediment and other impurities that need to be removed.

These microscopic particles are too small to settle out by themselves, so we use a process called coagulation to bind them together for easier removal.

Since the particles carry negative charges, they naturally repel each other, similar to how magnets with the same charge push away from one another. To address this, we add coagulants like ferric sulfate, which have positive charges. This addition neutralizes the negative charges of the particles, allowing them to clump together, as opposite charges attract.

Flocculation

As the negatively charged particles and positively charged coagulants combine, they form larger particles known as floc.

Flocculation involves gently stirring the water, allowing smaller floc particles to collide and merge. This process causes the floc particles to continue growing larger and heavier as they interact and combine.

Sedimentation

The effects of coagulation and flocculation become evident during the sedimentation stage. Here, the floc particles, having grown large and heavy, settle to the bottom of a tank. The accumulated sludge is then removed, allowing the clearer water to pass on to the next stage.

Although the water is significantly cleaner after sedimentation, some particles may still remain.

Disinfection

After the water has been clarified, the water undergoes a two step disinfection process to prevent infectious disease 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

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 destroy taste and odor compounds as well. Since ozone dissipates rapidly, it leaves no harmful residual and is considered the best practice for water treatment. 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.

Chlorine Disinfection

Now that the water has been disinfected with ozone, a secondary disinfectant is added for additional disinfection ahead of the filtration process and for the formation of chloramines for distribution to cities.

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.

Chloramination

Once the water passes through the filter, ammonia is added to form chloramine. Chloramines are more stable and longer lasting than chlorine itself, maintaining required disinfectant levels as it travels through our transmission system and through cities distribution systems.

The treated water is then collected and flows into one of our pump stations at the water treatment plant, where it is pumped and delivered to our member and customer cities through 82 different delivery points. Once the water arrives, the cities also do their own water quality testing and may perform additional maintenance procedures to ensure the highest water quality.