How do cities make Mississippi River water safe to drink?

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The Mississippi River has played a central role in the history of the Twin Cities, powering the industry that made the region into a national powerhouse. But our modern metropolis remains more intimately connected to the river than many people realize.

The Mighty Mississippi is both the destination of much of the region's purified wastewater, as well as the source of drinking water for most people living in the urban core.

The Star Tribune delved into the mechanics of purifying wastewater in a previous installment of Curious Minnesota, our community reporting project fueled by great reader questions. After seeing that story, high school teacher Doug Duea wanted to know about the process of treating drinking water.

"The river's pretty clean nowadays ... compared to what it used to be," Duea said. "But it's not something I would dip a cup in and drink. So I was wondering if that process is very similar to the cleaning of the wastewater."

Minneapolis and St. Paul both draw their water from the Mississippi, supplying their residents and neighboring suburbs with reliable, clean drinking water. That's different from most Twin Cities suburbs, which pull water from wells tapped into aquifers.

Compared with groundwater, however, river water is more challenging to purify because it contains a wide variety of runoff — such as whatever collects in streams and storm sewer systems. Minneapolis actually uses sensors attached to freshwater mussels as an early detection system that river conditions have changed.

The bad old days

The process for both Minneapolis and St. Paul begins, interestingly, in Fridley. That's because the river was historically so contaminated by raw sewage and industrial waste — before modern wastewater treatment — that engineers had to look upstream for a safe place to install their pumps.

"IT PUMPS DISEASE" read a 1901 headline in the Minneapolis Journal, blaming a pump station near downtown Minneapolis for rampant typhoid fever that killed 16 people in 15 days.

"They were repumping that untreated water and distributing it back to the customers," said George Kraynick, water quality manager for Minneapolis' water system. "So they within a short amount of time moved everything north of the city."

The two cities use similar methods to purify the water, but have slightly different infrastructure.

Minneapolis relies on four facilities located in Fridley and Columbia Heights: two for filtration, one for softening and another to handle all the stuff that's extracted from the water. St. Paul pumps its water through a series of lakes before treating it at a central facility in Maplewood.

Softening

The first treatment step is removing minerals like magnesium and calcium that accumulate in the water from the limestone that's common to the area. This "softening" is done by adding lime, which raises the pH and forces those minerals — as well as some harmful stuff — out of the water.

"We're basically just pushing the dissolved rock back into a rock form," said Jim Bode, production manager for St. Paul Regional Water Services. Minneapolis extracts about 40 tons of calcium and magnesium from the water every day.

The good news? Residents of Minneapolis and St. Paul don't need to own water softeners, which are needed in other communities to prevent "hard" water from leaving spots on glassware, shower stalls and clothing.

At this point, the water still has the tea-like color of river water, which the plants eliminate using aluminum sulfate.

"That brown is dissolved organic material, so it's not something you can just strain out of the water," Kraynick said. The organic material is not only unattractive, but can interfere with the disinfection process later on.

These steps remove a lot of the bacteria and viruses. But the water still isn't safe to drink on a daily basis.

Filtering

A filtration process removes remaining sediment and "bad guys" like cryptosporidium, giardia and algae. Cryptosporidium in Milwaukee's drinking water in the early 1990s led to an infamous outbreak that sickened more than 400,000 people.

The filter is several feet deep, comprised of layers of tightly packed sand and granulated carbon. Gravity pulls water from holding tanks through this physical barrier.

"The carbon itself is just there for [removing] taste and odors — and the bigger particles get on there," Kraynick said. "But that 12 inches of sand is what really filters the particles out of the water."

The water is now very clean, Bode said, but still could contain bacteria and viruses so small that they can't be physically removed.

"That's why you need the chlorine," Bode said.

Disinfection

Waterborne illnesses from drinking water were far more prevalent before treatment plants began disinfecting the water with chlorine more than a century ago. It remains arguably the most important step in the treatment process, and is required by state and federal law.

"Chlorination right around the turn of the century really basically eliminated getting typhoid and cholera and things like that from the drinking water supply," Bode said.

But highly concentrated chlorine — like bleach — is also a toxic chemical that is harmful to humans. So treatment plants add it after the water is largely cleaned, when only a relatively small and safe dose is needed to attack the remaining viruses and bacteria.

Those bacteria are a single cell surrounded by a thin protective wall, Bode said.

"That chlorine actually destroys that protective wall ... And he basically pops like a bubble," Bode said. "And at that point you've killed the bacteria."

The water is now nearly ready to head to our taps. First, however, the plants add some ammonia to extend the effects of the chlorine and remove the bleach smell.

Minneapolis then adds another chemical, ortho/polyphosphate, to protect residents from lead poisoning by creating a liner on the distribution pipes. Many homes still have privately owned lead pipes that connect to the water main in the street.

Finally, the cities add fluoride to the water to improve dental health, which has been required in Minnesota since 1970.

The plants don't just assume the water is totally pure at this point. The cities collect and analyze the water 180 times per month across their distribution network, looking for any problems. Laboratory staff in Minneapolis warm up the final product to smell and taste it every day, in addition to running it through a special machine that analyzes it for odor problems.

Both cities have minimal taste and odor complaints these days, which wasn't always the case.

"If you're getting any taste and odor issues in Minneapolis, you should be calling us to see what's going on," Kraynick said. "Our goal is to have zero taste and odor complaints out in our distribution system."

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