Water Catchment - What It Is and How To Do It
AKA Water Harvesting
Water Catchment - What It Is and How To Do It
Water catchment (also called water harvesting) is how you catch rainwater from a surface (most likely your roof) and store it in barrels known as cisterns. The catchment water is then pumped or gravity fed into a filtration system that will address any contaminants before it is pressurized and fed into the various plumbing lines throughout your home.
So, basically, there are 4 main parts:
- Collection surface (roof)
- A storage facility (cisterns),
- A way to get the water from the roof to the cisterns (gutters)
- Filtration - you need to filter before the water hits the cisterns and when the water is making it's way from the cisterns into your home.
From there, the water will be pressurized and pumped through your home, just like a conventional system.
Our favorite water catchment system is the one employed by Earthship Biotecture because it is an elegant system that takes the rainwater harvest and uses it 4 times for 4 different reasons.
After the water is filtered, pumped and pressurized, it can be used for drinking, cooking, cleaning, bathing, and laundry. After the water is used for those reasons, it becomes known as “grey water” and is fed into planter cells for growing food. These cells range in depth and have several layers of rock, gravel, sand, etc. that filter the water as it travels from one cell to the next. So this ingenious design is watering your plants and filtering the water at the same time. By the time the water reaches the final planter cell, it is clean enough to flush your toilet! The used toilet water, known as “black water” is then piped outside into a black water botanical cell. This exterior botanical cell is perfect for fruit trees and berry brambles. Any human excrement eventually ends up in what is basically a traditional septic system. But unlike a traditional septic situation, you will be sending way less wastewater, so you won’t need to pump your septic as much. And, don’t worry, we don’t recycle the water from there. You will not be drinking poo or pee water. This final cell is the end of the line. (unlike in cities like Las Vegas that use waste water reclamation, but we won’t get into that for now...).
The Best Roofing Materials To Use
Every building has a roof. So every building has the potential to catch rainwater. BUT be careful! Not all roofs are created equal! There are appropriate and inappropriate roof systems for water catchment. Your roof will be the first thing the rain water hits, and, if you're not careful, it will take all the stuff on your roof with it along with any chemicals leaching from your roofing material strait into your supply. That’s why choosing the proper material for your roof is essential. Water collected on asphalt roofs is really only good for non-edible plants (like bushes and grass). Therefore, any kind of traditional asphalt will not do for in-home use water harvesting. You need a material that will catch clean water. You can use EPDM, but it will need to be coated with a special paint to keep the rubber membrane from leaching contaminants. EPDM is also very tricky as it can get a small hole or puncture and water will find a way into your house very easily, but you will have a very hard time finding that hole! Another option is TPO - this is a newer roofing system similar to EPDM. It could also be decent for water catchment as long as its coating is addressed, however, I have heard mixed reviews on this material - especially if you live in an extreme climate. Slate or tiles roof are an interesting choice if you have the money and your structure can take the weight, but then you need to consider all the little crevices and areas between the tiles where water can get caught and sit for long periods of time, creating the perfect breeding ground for bacteria. YUCK.
When all is said and done, we think the best roofing material is Propanel. This coated metal is relatively light-weight, durable, and has a coating that is safe for catchment. They are also extremely long-lasting and are very reasonably priced roofing systems. The only thing about Propanel is that your roof will need meet the minimum slope requirements (which should be fine because you need a decent slope to catch clean water anyway). Yes indeedy. Propanel is the best!
Cisterns
A cistern is a storage tank for water, and there's a huge array of sizes and material to choose from. No matter which one you choose, it needs to be made out of an inert material - food-grade heavy duty plastic, reinforced concrete, fiberglass, or even tires rammed with earth and lined with cement and rubber are suitable materials. Basically, it needs to be watertight and safe to drink out of while being strong enough to withstand being buried or standing out in the sun.
Your cistern can be buried, or free-standing - either close or further away from your house. Check with your local planning department to see if there are any guidelines for placement or material.
Your cistern doesn't really require a whole lot of supervision, but it will require occasional maintenance. The main concern, of course, being contamination of the water from dust, bird and animal poo, insects, and air pollution. Most of these things can be taken care of with clean materials, proper maintenance and cleaning of your gutters, and filtering the water before it ever hits your cisterns.
You should also look into a system that allows for “flushing” the first part of the rainfall so that it doesn’t go into the cistern funnel. You should do this because the first 10 minutes or so of rainfall are the dirtiest! That’s the time when all those contaminants will be the strongest.
You will also need to check and clean your tanks. It’s sucks, but all tanks need cleaning. Every few years you will need to scrub the inner walls and floors of your tanks, and you might even need to use a tiny amount of chlorine. Make sure when you design your tanks, that they are easy to get in and out of and that there is the least amount of sunlight getting into them (this will keep bacterial growth to a minimum).
Sizing Your Cisterns
A good calculation to use is this:
Harvested Water (gallons) = Catchment Area (sq ft) x rainfall depth (inches) x 0.623 (conversion factor)
A good rule of thumb is to oversize your roof catchment are and your cistern storage capacity if you live in a climate where the rains only come during one part of the year. For instance, in New Mexico, where Kirsten and I live, the majority of or rain comes in the summer. That rain water has to last us throughout the rest of the year. Even though we get snow in the winter, there is only 1 inch of water catchment for every foot of snow. So, we need to have a lot of storage to make sure we are capturing every inch during the summer months to last us for the rest of the year.
For More Information On Roof Water Catchment Check Out These Resources:
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