Four years ago, the city of Toledo told more than 450,000 residents to immediately stop drinking water out of the tap.
That's after a toxin called microcystin was detected in the water. The toxin came from a bloom of cyanobacteria that had surrounded the city's water intake in Lake Erie.
The incident caused panic among some residents, hoarding of water, even fights at bottled water distribution sites - along with a lot of unwanted national media attention.
And it taught the city some hard lessons.
What happens to the water Toledo takes from Lake Erie
Municipal water treatment is a painstaking and complicated process.
Toledo's current water treatment plant was built in 1941, with multiple additions and updates over the years.
Inside the plant, it's humid and dark. Plant administrator Andrew McClure peers over the concrete side of a 20-foot deep channel, where 50,000 gallons of water a minute are flowing into the plant.
He says even at this early stage, the water has been partially treated, starting at the intake crib in Lake Erie.
"The water going through this channel has had potassium permanganate added, primarily for zebra mussel control, and powdered activated carbon for taste and odor and microcystin," McClure says.
Another chemical, aluminum sulfate, is added at this point. That gets all the microbes and other unwanted particles to coagulate or stick together.
"And then we go through a flocculation process to make floc and that's larger particles that actually settle out."
Flocculation involves passing the water through a fast and then slower churning. The chemicals mix thoroughly with the water, and the clumps of floc get bigger and heavier, so they can be removed later as sediment.
After flocculation, the plant adds lime, for softening, then sends the water to vast pools to pass through filters filled with sand and other materials.
Then the water's treated with chlorine and fluoride. Only after all this does the water leave the treatment plant.
Microcystin is detected and all heck breaks loose
Toledo had been testing for microcystin, a toxic by-product of cyanobacteria, only since about 2012. The U.S. EPA does not require the testing.
On August 1st, 2014, a routine weekly test detected a tiny amount of microcystin in the treated water -- 1.6 parts per billion, about the equivalent of a blade of grass in a football field. The microcystin came from a huge bloom of slimy green cyanobacteria that had surrounded the water intake in Lake Erie.
Janet Schroder is an analyst for Toledo public utilities. She says the detection put the city in a quandary. The World Health Organization says a microcystin level of one part per billion is unsafe - but that's a lifetime exposure level. There's been little reseach on the risk to human health from acute or temporary exposures.
And the U.S. Environmental Protection Agency provided no guidance.
"Microcystin was not a regulated contaminant," she says. "It still is not a regulated contaminant."
Out of an abundance of caution, the city told residents to stop drinking the water. Administrator McClure says Toledo wasn't well prepared for what happened next.
"People fighting over bottled water, and the distribution of bottled water... the emergency response wasn't what it could have been."
At the time, it seemed like the right thing to do
After the Toledo crisis, the U.S. EPA decided 1.6 parts per billion of microcystin is not a public health crisis. The agency now gives utilities that are screening for microcystin more than a week to eliminate the toxin from treated water. The Ohio EPA has a shorter timeline, but the agency still does not require a do not drink advisory to be issued right away.
"Had rules been in place then, that are in place now, there would not have been a do not drink advisory," says McClure.
That's not to say the crisis was all for naught.
Water systems all across the nation now have some idea what to do if it happens to them. In Michigan, communities that are monitoring for microcystin have had no incidents of detection in tap water.
And Toledo is determined to avoid a repeat.
The water treatment plant tests for microcystin once a day during HAB (harmful algal bloom) season, typically from July through September.
New water quality monitoring buoys around the intake crib in Lake Erie send hourly reports of changing conditions in the water, including increasing levels of cyanobacteria.
The plant also built a new carbon storage system and increased its ability to treat the water with chemicals by a factor of four.
Utilities analyst Janet Schroeder says the city has also developed a better emergency response plan, including better public outreach. That includes a dashboard on the internet to show people the water is safe.
"In the event we have an emergency, everyone knows what's going on," she says.
Toledo's microcystin levels have been undetectable since 2014. But the work to restore trust is ongoing. In the first couple of years after the incident, people would start rumors on Facebook, sparking runs on bottled water. McClure says that behavior seems to be reducing in frequency.
A few people are still buying bottled water to drink, four years after the crisis, says Schroeder. She says it's not uncommon for residents who go on public tours of the water treatment plant to express a willingness afterwards to start drinking the water from the tap.
So it could be a long time before people in Toledo take their drinking water for granted.
And maybe that's a good thing.