Now that's an interesting challenge.

Also timely, since my sump pump has been running regularly for a couple of weeks now. A fast snowmelt and a slowly receding frostline means that, in this old farmhouse, it's pump or swim.

Anyway, trying to think this through ...

Water from a sump pump probably comes from 6-10' below ground level, which means it's pretty grungy stuff.

I suppose it would have both biological and chemical contaminants in it. Everything from dog-leavings to weed-and-feed residue.

Filter it through another 30' or so of hard-packed clay, of course, and it becomes well water.

To do it yourself is a tall order, IMHO. You could build a pre-filter of sand, fabric and charcoal to start the process. And then, give it a good long boil. It would still be pretty foul stuff to drink, I think.

As far as your gutters go, there may of course be bird droppings and particulates from your local chemical plant. Still, a lot easier to treat (by far) than sump water. A particulate filter and quick boil would probably do it. Better also if you let the first hour of rain rinse the system out a little. If you make coffee from this, invite me over, unless you're downwind of Chernobyl. I'll pass on the sump water :-).