As far as the ball for the first flush system goes, you have two options. One is to use the ball. The other is to put two 90 degree bends near the top of the diversion pipe so turbulence doesn't stir debris back up into the flow toward the tank. I like the second because I like to use fewer specialized moving parts, but I could see the ball inspiring a little more confidence, and diverting less water to accomplish the same task.
I would guess quite a few people would swear by chlorination, and others would think any sort of treatment beyond first flush and settling out debris is unnecessary. That's probably a matter of personal comfort level with the hazards in your situation. I think it could be useful to look at the pros and cons of three scenarios here:
option 1: Water flows directly from your small tank under the gutter to your main tank. Let's call the first tank a settling tank, because that's what it will very effectively accomplish.
pros: Very simple, requires very little maintenance, just emptying the first flush between rain events and cleaning the bottom of the tank every ten years or so as John C Daley pointed out in a different post. If you design your tank well this can be done very easily. This topic is covered extensively in the book I mentioned earlier. It seems like many people live in large communities that use this type of system without ever hearing a story of someone getting sick from their water.(Australia and New Zealand) I can't say that for people who live off
wells near my parents' house. It seems like the first flush combined with settling leaves very little in terms of food for pathogens to multiply, especially if your main tank stays at a cooler temperature.
cons: Some question of safety. Where I live we have raccoons that climb all over the place. They are known to carry a roundworm which can cause permanent organ damage including blindness at very small exposures. The thought of such a pathogen making it into my water would keep me up at night, even though it is not terribly likely.
option 2: Chlorinate your water and filter it before you drink it.
pros: assuming your system is maintained and stays working correctly, there is little chance of pathogens growing in water which is essentially poisonous. Where I live this inspires more confidence because it is how virtually all municipal water supplies operate.
cons: depending on the concentration, chlorine almost certainly needs to be filtered out if it can't be given a chance to evaporate. These filters will need to be replaced. If you only filter drinking water this might not be too much of a burden, but you might want to consider a whole house filter as chlorine is no good for the skin. Most people I know
shower in chlorinated water, but I believe this water spends time in a tank where some chlorine can evaporate first. Furthermore you will need to buy and store chlorine, as well as monitor it's levels.
option 3: Run the water from your settling tank through a slow sand filter before it goes to the main tank The water slowly percolates through a sand medium which intentionally grows bacteria in a thick film. This traps pathogens and consumes most of the nutrients that other pathogens could use to multiply in the main tank.
pros: You can test your water before and after the filter and see e. coli before and no bacteria at all after the filter. When I see this I feel fairly certain that if a bacterium can't make it through, a much larger worm egg couldn't either. This type of filter requires very little maintenance, as it actually filters better with age, and only suffers from slower flow. If you oversize it, keep up with your first flush and settling tank, and design well, you may also be able to go a decade or more without messing with it. Buying chemicals and filter elements is unnecessary. Some municipal water systems in London have been using this technology for a very long time, and the CDC recommends it for emergency water treatment.
cons: It requires a little more infrastructure to build, I think perhaps 3 square feet for each 50 gal per day of water use. You would need to find a place for it which is sheltered from creatures and the elements in the same way your tanks require. It requires a little time, perhaps a week, to "mature" before it is fully functional, and will need a similar amount of time with water cycling back to the settling tank when you maintain it. It might not filter out more than 95% of viral pathogens but this seems to be adequate in most real world scenarios.
I have to admit I am highly biased toward slow sand filters, as they seem to carry all the hallmarks of an essential
appropriate technology. I wish I could see them discussed on the Permies with the same sort of priority as
rocket mass heaters, forest gardens, and natural building materials. Being able to treat water, an essential for staying alive, with as few inputs, particularly chemical ones as possible seems very important to me. Whether this is an appropriate solution for you or not, I hope some good discussion on potable water from the sky can happen. Perhaps that might help someone further realize the potential of this technology for their homestead.
I hope this was helpful, Happy designing!
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