Updating from older blog
As a beginning for figuring how much rainwater can be captured, one inch of rainfall can produce 0.62 gallons per square foot of collection (roof) area.
A 1,000 square foot roof with a 1 inch rainfall would produce 620 gallons. That seems like quite a bit of water, and the weight of that much water is 2.5 tons.
However, most households go through that much water in only a few days to a week. Laundry, lawn & garden watering, baths, toilets use the largest amount of water, with potable water usage being much less.
In practice, the actual harvested amount will be lower depending on several factors. Heavy rain events can lead to water cascading over the rain gutters, or fail to drain into tanks because gutters and pipes can’t handle the amount. Some water is lost to first flush systems, or when tanks become full, water simply overflows the system.
As a more practical figure, 75% of the potential amount is a good estimate.
If I plug in our roof square footage, 1,072 square feet x 0.62 gallons per one inch of rain, by 0.75 to account for the loss factor, that comes out as 498.5 gallons.
While we currently don’t have sufficient storage for our water needs, this area averages 41 inches of moisture per year. That figures out to a potentional 20,438 gallons!
Our plans include using a number of 50 gallon food grade barrels with equalizing plumbing so they fill and empty in unison. Some will be installed under the house to protect them from temperature extremes.
With a good filtering system, we plan to be independent of the water companies.
How do you make sure the water you harvest and store is clean?
Make sure junk never gets into the storage.
Starts with the roof itself.
This trailer is old and the roof has quite a bit of rust. We were thinking of dealing with it with more filtration, but we came up with a better idea.
We’ll be coating the roof with three 18′ x 24′ tarps. At $99 each from Amazon, we’ll have several benefits.
The tarps will keep the rust from the roof from getting into the rainwater in the first place.
We can get tarps as we can afford them. The first one went over the north end of the house. Twenty foot gutter sections are up and leading into the storage tanks. The east side has a first flush filter, the west side needs one to be created.
The tarps are white, so we’ll have less heat gain during the summer heat.
The tarps will keep the roof from rusting out further, giving us needed breathing space until the roof needs to be replaced. It’s not leaking YET but I wouldn’t count on the roof metal lasting another year at this point. We’re estimating that the tarps will give us at least another two years.
The house is 67 feet long, so the second tarp will go over the south end of the house, leaving only 19 feet of the center to be covered by the third, giving plenty of overlap.
With the tarps keeping the rust out of the rainwater, the next part of keeping the water in the storage clean is what is called a “first flush” system. Basically, this diverts the first bit of water – including dust and bird droppings – into a side pipe and then lets the bulk of the rainwater into the storage tanks. It’s simple to build out of bits and pieces of PVC pipe.
After the first flush fills up, the water will come down the downspout. Before it hits the tank, It goes through a home made filter. Poly fiber, charcoal and gravel ensure that the water going into the storage is as clean as we can get it, and keeps mosquitoes from gaining access.
To prevent the growth of algae, we’re keeping sunlight from the barrels. Some of the barrels are the blue poly, and wouldn’t be susceptible to algae growth, but we do have a couple white barrels.
The barrels are sited on the north end of the house on an elevated stand, and there are plans to enclose them with a protective box. This will improve the appearance of the set up as well as keep the barrels from any weather damage.
The barrels will be connected with a manifold that lets them fill and drain in unison, and can be expanded with more barrels to keep our water storage ahead of our needs.
While our barrels fill quite well if we have a reasonable rain, reasonable rains can be a bit far apart, requiring us to go to town and get multiple five gallon bottles filled. The answer is going to involve getting at least ten more barrels and expanding our storage. I would like to get a couple more large IBC totes
There are a number of very good resources online.
This PDF is one of the best I’ve found. http://www.twdb.texas.gov/…/RainwaterHarvestingManual_3rded…
And this one from Texas A&M is also a big help.
It also includes links to quite a few useful publications.