If the water is pure, nothing will grow in it. But even if it's not perfectly pure, anything that may grow isn't likely to cause too much of a problem. Well, assuming it doesn't grow too long/ grow too big.

Thanks for always lending a hand grizz.

Hopefully with these low level ppm's the "assuming it doesn't grow too long/big" is not a concern which is really the worry here. I've burnt myself out trying to get a better answer, mainly because it is very convenient for me to collect water on an ongoing basis in small amounts and just put it in water bottles. The tap here apparently has 80-100 ppm [Ca + Mg type-hardness] and is in the top three worst US cities for failure in water quality measurements, so it's a pretty big deal.

On the bright side I did decide (with help from the CDC helpline) as long as I don't drink it directly, any Botulism, Salmonella and E. coli bugs will be killed within hours once the DWC is operating.

Just to help anyone else considering rainwater for a small hydro setup like me, let me share my approach:

I bought a couple of 17 gallon round PP wash tubs with handles for $5 each at Walmart and scrubbed them down well first. These tubs are around 21 inches in diameter at the top. That means for every inch of rainfall I get, they will get a gallon and a half of water. In July and August, we get about 7 inches a month (11-gal per tub). I bought a pack af large coffee filters for $1 and pour the water from the tub into the bottles using them in a funnel. The tubs get the rain directly from the sky, so there is no roof bird droppings and other crap, and I only put them out after it starts raining so it misses the initial dirt getting cleared from the air.

Sterilization would be the safest method, but it is not cost effective or convenient. Average US utility rates are around 10 cents per gallon to bring water to a boil on the stove. Alternately, if you have preferably a food safe hydrogen peroxide source, that could be convenient, not really for me. You won't find a recommendation anywhere for how much peroxide to use, but I can provide that: You need at least 0.15% in your water when you bottle it. There's a guy selling it for about $100 shipped food grade on eBay for six gallons of 17.5%, which actually comes out around the same as boiling in the long run.