That design looks good, it should filter out most particulates, however it won't filter out any heavy metals, chemicals and pesticides that were blown into the upper atmosphere and got washed down in the rain. Boiling may work for steralizing creek water, however the rainwater has already been steralized through evaporation and boiling won't remove most of the chemicals and heavy metals that are in the water. You would only need to boil the water if your tank got contaminated somehow. Rain water is pretty clean of organisms and will not grow anything so long as you have a tank that does not let the light in. Oh and it needs to be pretty well sealed to keep out those pesky mozzies. Any parasites that are in bird droppings will probably be killed by the heat and UV from the sun, then flushed down your preflush.

If you want to filter out some of the chemicals you can add crushed charcoal to your filter stack.

As far as the preflush design goes, when the u section is full then some of the contaminants, especially the floating ones will wash into your good water, also your bad water will mix with your good water, I would put a floating ball in a cage at the overflow point, when the U section is full the ball will seal off the opening, then the water will only go where you want.

I have thought about the same problems for when I get my own house in the country. Most of the air is pretty clean over here in Southern Australia (so long as you aren't down wind of a major city like Melbourne), but I am still considering filtering our drinking water at least using a reverse osmosis filter and a pressure pump (they require 60 to 100 psi).

You also have to remember that the body adsorbs more of some chemicals through the skin and breathing in during showering, than through drinking. This is true for chlorine and fluride in city water. Filtering your drinking water does little or no good if you don't have a shower filter.... there isn't an effective one for fluoride.

That having been said, our city water here and almost everywhere else is not filtered at all. They throw in chlorine and then toxic waste from industry called fluoride and then pass it along to us. It has more of the nasties in it than you would get out in the country.

In the country you have fertalizers and pesticides and herbacides, that is the purpose of the preflush, to wash that stuff off the roof. But all in all the water that you get off the roof of your country house is much better than any city service can provide.

In the city... well filter it the best you can, get a big berkey or something like that and it will take most of the junk out. In a major emergency there may be fires which are pumping toxic chemicals into the air and they can eat the coating right off the inside of your tank, I wouldn't drink that even if I had the best filter in the world. It could be good to have a diverter on your tank for the rare occasion that you don't want to have your good water contaminated with toxic smoke residue.

Just a few thoughts to throw in the mix, I hope it helps someone.

Happy drinking,
Macgyver.