Human Error.
"Well, you know, I was just taking my 12-gallon tank of polio for a walk down by the water treatment plant, and whoops! Butterfingers!"
I just. I don't. How.
My best guess is that a technician accidentally hit the "dump waste" button before the vessel was decontaminated/when it had the wrong solution in it/before the virus was deactivated, etc.
This incident is especially worrisome, but having seen the control boards for one of those giant reactors, I'm not surprised that mistakes are made on occasion.
>>Drake demonstrated that poliovirus is able to undergo multiplicity reactivation.[23] That is, when polioviruses were irradiated with UV light and allowed to undergo multiple infections of host cells, viable progeny could be formed even at UV doses that inactivated the virus in single infections.
It was likely some polio introduced into secondary and they didn't process it correctly in that area.
There was a case of a nurse who injected the wrong compound in a baby and killed it a decade ago (in a military hospital in France, one with a stellar reputation): two viles were strikingly similar (one containing vaccine, the other Potassium) even after being told what those were, you would confuse them.
In this case, I'm assuming concentrated virus is not meant to be near water release, therefore no one noticed until yesterday that there were stored in vats that look like possibly fat-digesting bacteria, or chlorine. 40L is a large quantity: the only container I can think of for those is either a metallic oil drum, or that omnipresent industrial blue plastic barrel (the one you see in that infamous scene in Breaking Bad).