As you read this post, if it sounds like I am an inexperienced idealist it is because I am I am continuing my search for land in the SW of the US, which means scenic but dry land, and I want to learn more about what I am getting myself into.
As rainwater is caught, it first would be stored in a tank, or cistern or pond, and then pumped into a pond containing water hyacinths (or some other plant that does water filtering) to provide the first state of filtering. Many of these plants are good are removing not only biological matter but also other impurities. After a predetermined time, the water would be pumped through a sand or diatomaceous earth filter, polished in a charcoal filter (if necessary) and then pumped to be used. Off the top of my head, the biggest cost for this kind of system would be an established pond or tank of water plants, that would have to be big enough to accommodate changing water levels. This would also address the issue of how to store water without concerns for algae or other critters getting into it. If you have well water that has some issues, the well water water could be filtered through the same system.
My experience with rainwater is limited to some tinkering with rainwater collection & storage on a rather small scale (= tanks around the house amounting to a few thousand litres, used mainly for watering the vegetable garden and topping up an ornamental pond).
I'm also slowly planning a rainwater collection & storage system for my future house. The intention is to be self-sufficient in terms of water for domestic & garden use (the new house is also still at planning stage).
Based on such (limited) experience as well as on what I've read and learned from others in this research process, I believe some of the filtering steps that you describe could be an overkill and hence potentially wasteful.
I would not send rainwater from a tank / cistern into a pond with water plants if the intent is to filter it for domestic use. Assuming that water from the roof first passes through a coarse filter before being directed it into the tank / cistern, and is then allowed to settle for a while - at that point you should already have reasonably clean water in your cistern. From that point you could use a pump to get your water through a finer filter (say, 20...10 microns) - which would give you water suitable for household use, though not necessarily drinking water.
If water is scarce and hence a precious resource - as it is likely to be in dry regions -, it's better if you store your rainwater in a closed cistern, where evaporation is limited, rather than sending it into an open pond.
Also: I would only use even finer filtration (micro filtration) to obtain potable water. That would be done as-needed just above the point of use (e.g., by a filter at the tap of your kitchen sink). I don't think that the rest of the water used in the household has to be of potable quality.
If you're talking about in-home drinking/cooking water, you may consider skipping all the phases you're planning and buy Pritchard gerry can filters. Here's his TED presentation, but you can go to his website and investigate more. The deal with this filter is that it filters both bacteria and virus. At least worth reading about.
I live in Australia and everyone i know has rainwater tanks. I do not know anyone who filters their rainwater. My whole house except for the toilet is on rainwater and we drink straight out of the tap, no filtering. Unless you have acid rain or its raining dogs and cats I can't see the point in filtering it. It's just an extra cost running pumps and changing filters......
I would think that you would lose a lot of water via evaporation if you have an open pond in a hot, arid environment. Plus personally I don't see the need for such an elaborate filtration system.
I live with catchment water, that is...rainwater. We collect rain off our metal roof, running it through a sieve as it enters the storage tank. That removes any coarse debris, like leaves, twigs, etc. While the water sits in to catchment tank, finer debris, such as volcanic ash and dust, settles to the bottom. There isn't much. The water intact pipe sits 6 inches above the tank bottom, thus it doesn't suck up any of that fine sentiment. We have a filter installed between the tank and the water pump, in order to protect the pump.
Monthly we check the water's pH and adjust it as needed using baking soda. We use bleach to chlorinate the water, using a swimming pool test kit to monitor the chlorine level. We use this water without further filtration or treatment for everything except consumption. If I wanted to drink our catchment water, then I would install a UV lamp. I don't do that for two reasons. We have access to good tasting, free drinking water close by. I happen to pass the water station 6 days a week. And being off grid in solar electric, running a UV lamp 24 hours a day, 365 days a year isn't acceptable. We need to sanitize our drinking water because of the constant presence of roof rats. I live next door to a large macnut orchard, so zero rat population is impossible. The cats do a good job of eliminating rats, but some still manage to get past the cats occasionally. Hawaii has a very high incidence of leptospirosis, thus those rats pose a serious health hazard when it comes to untreated catchment water.
It's never too late to start! I retired to homestead on the slopes of Mauna Loa, an active volcano. I relate snippets of my endeavor on my blog : www.kaufarmer.blogspot.com
I agree with the others, you will loose water to evaporation, which is one method of wasting water, by doing the filtering system you describe. Better to use closed tanks for water catchment, keep sunlight off them to reduce microbe and algae growth then place in line filters designed to filter what needs to be filtered out for each purpose of that water. If you're using the collected water for gardens and toilet flushing, you really don't need to filter much if any at all. If you are using it for potable (drinking) uses, then you need a proper filter close to the faucets where that use will be.
We currently can save only 350 gallons U.S., but we have plans to have a catchment volume of 900-1500 gallons U.S. This water currently is for animals, gardens and everything except for potable water use. Once we have installed the complete system we will place a cartridge filter system in the line to the faucets in the house, the other uses will remain un-filtered. We use the metal roof to gutter collection method. The gutters have screens to keep large trash out, the downspout to the tanks is fitted with a smaller mesh screen, the tank opening is also covered with an even finer mesh screen. The set up flushes the first few gallons of water to the ground, this removes any collected dust and other junk that may have gotten through the gutter screens, the result is that our tanks have very little in the way of sediments to settle out, the tanks are stepped so gravity is used instead of pumps to move the water from the tanks to where it is used. Simple and easy is the best way to use caught water.
Your ideas seem to be more along the line of what I would do if I had a stream or spring that was my supply of water.