Memories of my childhood come by reminding me of how I used to swim in the Danube river, until the age of 6 when after a walk along the coast I saw bodies of various animals rotting by the river, cows, pigs and many dead fish floating, I do not know to this day what caused their death but I've never swam again from that day.
This memory made me wonder about how medieval people drank their water, considering that rivers and lakes are and will always be nasty water sources. It is theorized that humans are so used and evolved to drink polluted water that our needs for B12 arisen from thousands of generations of drinking water contaminated with fecal bacteria, and in fact river and lake water is incredibly rich in B12 vitamin, a vitamin which naturally only forms in the gut of some animals and expelled with defecation.
and certainly a lil bit of feces in your water are not so deadly, I'm pretty sure some of that Danube water went in to my nose and some was absorbed by my skin, but I'm alive still, probably because I'm the result of the few humans who could tolerate a little fecal intoxication from swimming in polluted water. Billions of people swim every summer, and a lot of stuff is being dumped into beaches, not just feces but also t substances or plastic which transform into toxic stuff, Also the famous Hepatitis B happened because of a flood in England which discharged a lot of fecal matter into the sea, which was the absorbed by the world.
But imagine a creature which is way more sensitive than humans, a creature which needs a lot of purified water, what would be the least expensive and most natural way to clean their water? is it absurd to imagine a river covered in reeds and other pollution absorbing plants or algae? maybe reeds bred for the sole purpose to absorb as much toxic stuff as possible?
Probably taking down forests to burn some wood to boil water is not a good idea, and often burning bacteria only kills them but doesn't get rid of the toxic agents, people still get sick from contaminated meat even after roasting or boiling it completely and many parasites and viruses can withstand temperatures that would turn a steak into drywall.