Water Treatment
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.
Our source waters quality and reliability is important to our operations, so we regularly update our Surface Water Intake Protection Plan and Wellhead Protection Plan to ensure our source water supply is protected, reliable, and that flow is adequate to the plant without sacrificing environmental quality, flow, or recreational uses downstream on the Huron River. Furthermore, our water treatment plant is designed to provide multiple barriers of treatment for each different class of contaminant that may be present in any of our source waters.
Ann Arbor also parterners with organizations like the Huron River Watershed Council to educate the community on the importance of the Huron River to our environment and drinking water supply. The City of Ann Arbor also tracks Huron River flow data that anyone can see.
Water treatment plant
Ann Arbor produces high-quality water by having multiple barriers of treatment against each class of contaminants.
Treatment process
Our three main processes are softening, disinfecting and filtering.
Softening
Softening of water refers to removing dissolved minerals that become solids and clog pipes, and it is accomplished through the use of lime - adding lime to the water causes a chemical reaction to remove excess calcium and magnesium.
Disinfection
Disinfection is the process of destroying harmful 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 bacteria by adding ozone which reacts with the bacteria's cell wall to inactivate and harm it. Ozone disinfection kills pathogens, improves water taste and reduces odors commonly detected in tap water. 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.
Filtration
Filtration removes particles, more microorganisms, and organic molecules such as PFAS. Our filter is similar to a giant Britta filter containing granular activated carbon. The granular activated carbon, also known as GAC, is a porous material that can strain out contaminants like a spaghetti strainer, can absorb contaminants like a sponge, or can support microogranisms that consume organic contaminants. (We also have more disinfection after filtration to destroy additional harmful bacteria!) Filtration provides excellent tasting that is safe water for consumption.
Definitions for other terms you may see on our site, because we employ them in the treatment of our water:
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Excess lime softening: We use calcium oxide, also known as lime, to remove minerals from the water
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Rapid Mixing: for quick dispersion of the chemicals being added
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Flocculation (slow mixing): gives the chemical reaction the time it needs to go to completion
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Settling: the removal of solids from the water by gravity.
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Filtration: to clean the water of physical, chemical and biological impurities. Because of the high level of hardness in Ann Arbor’s water supplies, Ann Arbor softens its water; lime softening uses calcium hydroxide to remove calcium, magnesium and iron. Lowering water hardness reduces build up and scaling on pipes, a factor that could extend the life of home hot water heaters, household pipes and other water-using appliances. Lower water hardness also reduces the amount of soap needed for effective cleaning and the potential for less spots remaining on utensils, glassware and dishware.
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Fluoridation: fluoride is added for dental protection (as recommended by the American Dental Association)
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Stabilizing: phosphate is added to stabilize the water after the softening process.
Plant location
The Water Treatment Plant is actually two separate water plants. The older plant was built in 1938 and 1949 and can process 22 million gallons of water per day (MGD). The newer plant was built in 1966 and 1975 and can process 28 MGD. The plants are upgraded continually to keep up with regulations, increase reliability and to improve water quality. Anyone can request a tour of the plant or visit staff at our annual Open House celebrated during National Drinking Water Week in May.
Water from both sources (the Huron River water and groundwater from local wells) is blended at the Water Treatment Plant. Because Ann Arbor uses surface water as a source for drinking water, we practice the rigorous process of complete treatment, outlined above.
Water distribution system
When water treatment is complete, the finished 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 City of Ann Arbor’s Public Works department 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.