Hello there! Living off rainwater surely sounds like living the dream to me. We've been living on rainwater for almost a year now after not being sure it could be done, and hope you can do it too. What sort of climate, site topography and use/pressure needs do you have? That would help narrow down your options to figure out a good design. There are many considerations and it can seem overwhelming, but potable water from the sky is well worth it.
First and foremost, you probably need to start learning how to limit your water consumption, unless you want to keep massive tanks around. A typical house might use more water in a day than ours does in a month. Composting toilets are a great way to save water and build soil too.
We started by reading Art Ludwig's book called "Water Storage". This is a great book on the subject of safely working with rainwater, and provides an overview which really helped with our confidence. A few of the safety topics covered are not pulling from the bottom of the tank, keeping critters and mosquitos out, and proper materials.
First flush systems are a great idea, as are filters, and water testing kits. We replaced our shingle roof with metal (it was leaking anyway.) and use a first flush system to keep the majority of debris out of the tank. We made a slow sand filter out of a 55 gallon plastic barrel with information I found on the CDC's website. I like this type of filter because it can be maintained without buying replacement parts. We also use a biological testing kit called coliscan easygel, after learning about it on oasisdesign.net (Art Ludwig's website which you should also check out.) It makes it easy to figure out whether your water is likely to contain harmful bacteria or not. We boiled our water until we had proof it was safe and still use a Berkey filter for our drinking water.
To give you some examples of the sort of things you might want to look into relating to your specific site, we mainly pull from one side of our roof as the other side collects spruce needles which fill the water with tannins so it doesn't work well with the filter. There are tanks under the the gutter on each side to catch the water, buried in leaves to prevent freezing and light leading to toxic blue-green algae. We only pull from the spruce side for irrigation and drought emergencies. A small aquarium pump slowly moves the water up into the attic, where the slow sand filter leads to a final clean water tank. This all sits on a drained pond liner in case of leaks, all directly over a post so the ceiling structure isn't overworked by the extra ton of weight.
Make sure to do plenty of reading before you start, as this is by no means an exhaustive list of the considerations involved in a safe rainwater system. The best design for you might involve something totally different. Height considerations and functional needs determine what gravity can do and where pumps are necessary. We like the feel of low pressure water at our kitchen sink but it does require wider plumbing and sometimes lead to annoyances with our HE washing machine, which is necessary to keep our water consumption down.
Hope this helps. May you have a happy and fluid day!
Thank you for all your kind replies. We have already built an undergroundconcrete tank which has a capacity of 50 tons (50000 liters). We have around 140 sqm roof space which we will use to collects the water.
There will be a first flush system in the form of a T pipe where the first flush side ( a 6 meter long 4 inch pipe) will fill and then carry on to a 2000 liter water tank which will be filled from the bottom up. Only the water that reaches the top will continue on to the main 50 ton tank.
Now I'm not sure how to treat the water in our main tank. The main tank will be connected to our home for showers, washing dishes etc. Everything other than drinking. Drinking water will be filtered with reverse osmosis.
Should I put chlorine in the main 50 ton tank to make it safe?
How should I design the first filter? Should I put a ball in the pipe so it can block of the pipe going down so when it fills it can continue to the other 2 ton tank?
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.
Yes the CDC is the american center for disease control. I apologize for the lack of clarity. Here is their information on the topic that I used, though I must admit it is not perfect as a diy construction manual, you might search the youtube for plumbing details. All the diagrams in the slow sand filter links make it seem a bit more complicated than it is, the filter itself is incredibly easy, but some details about siphon breaks so it doesn't go dry yet still has pressure moving water through need to be observed.
CDC information on slow sand filters Here's a pamphlet where I saw the two bend method used instead of a ball. I think they actually used 45's now that I look again. The picture is on the bottom left of page 2.
first flush brochure list of brochures where that came from also here is a particularly relevant part of brad lancaster's website which has many great resources
rainwater harvesting resources and another from art ludwig's website
water central which links to a few particularly useful sections
slow sand filter water quality testing
Certainly rainwater is very soft and the best thing for washing clothes and showers. A simple gravity fed Berkey type filter would make rainwater potable for drinking although the OP doesn't list that as one of the intended uses.
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