Minneapolis Department of Public Works

Public Works

350 South 5th Street
RM 203 City Hall
Minneapolis, MN  55415-1390

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Fun Facts About the Ultrafiltration Plant

A lot of water from some tiny fibers

The ultrafiltration plant uses hollow fibers to take particulate matter out of the water. The walls of the fibers are porous, letting water through and keeping the particles behind.


A cross section of an ultrafiltration module shows the thousands of hollow fibers packed inside.


A powerful microscope shows the tiny holes in the walls of a fiber. Like a kitchen sieve, they let water through and leave larger material behind.

While these fibers are tiny, there are plenty of them. About 9,600 fibers are in one vessel, and there are four vessels in one long module. The plant has 28 vessels on each ultrafiltration unit, and there are 40 of those units in the new plant. That means there are about 43,008,000 fibers cleaning the city’s water.

Ultrafiltration Units

One of the 40 filtration units inside the Ultrafiltration Plant at Columbia Heights.

Altogether, these fibers create a surface area of 1,669,000 square feet. That’s larger than the area the Pentagon building covers. If put end-to-end, these fibers would stretch more than 40,000 miles, or about 1.6 times the circumference of the earth at the equator.

All of these fibers together produce a lot of water. In just one day, the ultrafiltration plant can produce up to 70 million gallons of clean, drinkable water. At that rate, it could fill Lake of the Isles in a little more than four days.

The new water plant will take impurities as small as 0.03 micrometer (µm) out of the water you drink. To give you an idea of how small that is, the table below lists the size in microns of a number of familiar small items:

Ultrafiltration Plant will remove microns as small as 0.03 micrometers (µm)


One inch

25,400 µm

One millimeter

1000 µm

Thickness of a dime

1250 µm

Thickness of a U.S. dollar bill

200 µm

Particle of beach sand

100 to 2000 µm

Thickness of human hair

30 to 200 µm


11 to 120 µm

Red blood cells

4 to 10 µm

Milled flour

1 to 110 µm

Harmful bacteria

0.2 to 2 µm


3 to 6 µm


1 to 3 µm


0.02 to 0.09 µm

Tobacco smoke

0.02 to 0.08 µm

Dissolved salts

0.0003 to 0.002 µm

Dissolved minerals (calcium, etc.)

0.0002 to 0.0003 µm


Although ultrafiltration removes nearly all viruses from the water supply, a fraction of them cannot be completely screened out. However, the threats posed by these pathogens are mitigated by chlorine treatment.

Dissolved salts and minerals are also small enough to remain in the water after ultrafiltration. These compounds are desirable, since they contribute to the taste of our water. The natural minerals in the water help make the water stable, to prevent pipe corrosion that would otherwise take place as the water is delivered to customers.

Last updated Sep. 9, 2013