Water Treatment

Water Supply, Treatment, and Distribution

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Drinking Water Supply 

The Ann Arbor water supply is comprised of both surface and groundwater sources. Approximately 85% of the water comes from the Huron River. The remaining 15% comes from multiple wells located south of Ann Arbor. 

Water Treatment

Soften, Dis​​​infect and Filter

Your water is treated through three main processes, lime softening, ozone disinfection and activated carbon filtration.   Lime softening is the process of adding lime to the water, which causes a chemical reaction to remove excess calcium and magnesium.  

Disinfection is the process of destroying harmufl bacteria.  Ann Arbor’s Water Treatment Plant uses ozone as the primary disinfectant and chloramines as a secondary disinfectant.  The City of Ann Arbor destroys the bacetria by adding ozone which reacts with the bacteria's cell wall to inactivate and harm it.  As a second layer of disinfection, chloramines (or “combined chlorine” a compound made up of chlorine and ammonia) is added before being filtered. These disinfectants are used as an alternative to chlorine treatment because they have fewer by-products, and are a longer lasting disinfectant than free chlorine. In addition, chloramines have less taste and odor than chlorine, and ozone is odorless. Ozone disinfection kills pathogens, improves water taste and reduces odors commonly detected in tap water.

The final step is filtration.  Using granular activated carbon and silica, filtration is the final process to give you excellent tasting and safe water for consumption.

Dis​tribution

When treatment is complete, the water from the Water Treatment Plant (WTP) is pumped to homes, schools, and businesses in Ann Arbor as well as to Ann Arbor and Scio townships for resale to their customers. The city supplies water to approximately 125,000 people and has an average day demand of 14 million gallons per day (approximately 5 billion gallons of water a year).

The WTP manages the city’s water distribution system, which is comprised of five pressure districts within the city. The main reservoir, three outlying reservoirs, four remote pump stations and two elevated tanks supply these districts.

Pressure Districts Map 

919 Sunset Road
Ann Arbor, MI 48103
Phone: (734) 994-2840
or (734) 794-6426


Business Hours: M-F 8 a.m. to 5 p.m. (except city holidays)

After hours water emergencies
(734) 994-2840