Stages of Water Treatment in Ann Arbor
The Water Treatment Plant is actually two separate water plants.
Plant one was built in 1938 and 1949 and can process 22 million gallons of
water per day (MGD). Plant two, built in 1966 and 1975, can process 28 MGD.
Each plant has two stages, primary and secondary. The water is softened in the
primary stage and the water is recarbonated (pH
adjustment) in the secondary stage.
Primary and secondary stages each have three steps: rapid mixing, flocculation
and settling. Rapid mixing is for quick dispersion of the chemicals being
added. (The primary rapid mix adds lime to the water, while the secondary rapid
mix adds polymer.) Flocculation or slow mixing gives the chemical reaction the
time it needs to go to completion. Settling is the removal of solids from the
water by gravity.
After the settling stages, the water is disinfected with ozone and filtered through granular activated carbon (GAC). Because ozone deteriorates quickly, chloramine is added after filtration to keep the water disinfected through the distribution system to your homes. Fluoride is also added at this time.
Making high quality drinking water takes a team of licensed, educated,
caring personnel: operators, lab technicians, mechanics, engineers, scientists,
account clerks and administrators.
The Water Treatment Plant also 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.
