[bunster]

Do you need to treat the water in any way to keep it drinkable for humans? Or does the closed system keep it in good condition?

[IrishCatholic]

The water is not potable. The water was collected from run off from the roof and gutter. How much bird poop, squirrel toe jam, air pollution particulate or dust, etc is in the water?
It is fine for garden use, in fact, the lack of chlorine makes it preferable for plants. However, for direct human consumption you need a filter/purification system.
Frankly I like tap water. My local water department here is well run and I have no concerns there.