Rainwater harvesting is a highly customizable technology. Myriad solutions exist across a global context. Rainwater harvesting systems can usually be categorized as either passive or active types. A passive type uses almost no mechanical means to capture, convey, or treat the caught rainwater. An active type uses mechanical and/or electrical means to capture, convey, and/or treat rainwater.
Best Rainwater Harvesting Systems
I often avoid using the terms passive versus active and instead refer to landscape versus built types. Landscape type systems of rainwater harvesting use landscape features to slow, absorb, and/or store rainwater. Landscape type systems are usually considered passive.
A built type system uses mechanical and/or electrical means to capture, convey, and/or treat rainwater. Built type systems are usually considered active. Landscape types have the advantage of lower cost per volume of water. Built types have the advantage of providing cleaner water.
For the modern homestead, the built type offers some significant advantages. And most homesteads can take advantage of both types of systems… or even combining systems so that the overflow of a built system becomes the source for a landscape system.
Rainwater Harvesting Systems Components
Rainwater harvesting systems use various components to best meet needs. These components can be broken down into catchment surface, conveyance (gutters and downspouts), screens, first-flush, storage, water purification, and end use.
- Catchment surface – area that the rainwater falls on to be captured.
- Conveyance (gutters and downspouts) – transports the water from catchment to storage or use.
- Screens – separates debris from the water.
- First-flush – diverts the first, and dirtiest, portion of rainwater.
- Storage – holds water for later use.
- Purification – cleans the water to the needed level.
- End use – gives purpose to the system!
You know your homestead better than anyone… often you can conduct a ‘water audit’ just on the back of an envelope. If you are using the water for drinking, you will need more treatment than if you are using it for gardens. If you are using it for cleaning (tools, grounds, equipment, etc.), then you won’t need any treatment. It all depends on your needs and resources (e.g. do you only have seasonal need due to a seasonal creek).
The catchment volume is calculated from the precipitation falling on the collection area with some loss due to the efficiency of the collection materials (and leaks). In addition, conversion factors are used to yield the desired units of volume. Typically, monthly catchment values are calculated based on monthly average precipitation data. The collection volume for any period of time is calculated using the following formula (mnemonic device Vrake):
Equation: Vrake – Rainwater harvesting potential.
- V = Volume of collection in gal/time or m3/time or liters/time
- Note that time is usually in months.
- Use this to help determine potential yield and tank size.
- R = Precipitation in inches/time or mm/time
- Collect this data or find it from existing climate data.
- A = Footprint of collection surface in ft2 or m2
- This is the vertical projected area of the collection surface. For a rectangular house, use length times width.
- k = Needed conversion factors, such as 7.48 gal/ft3 or easier SI units
- Can also combine the 1ft/12in conversion for the precipitation data here.
- e = Efficiency of collection surface (which is unitless)
- 75 soil, 0.8 average, 0.95 metal
Another consideration is pressure. Pressure is critical to moving water from where it is gathered or stored to where it will be used. In a rainwater harvesting system, the water must be able to flow from catchment through the filters and conveyance into the first-flush and storage or end use. This pressure can be provided by gravity from vertical height difference or by a pump.
As an idea of how much pressure you will need: typical US residential water pressure is between 40 to 80 psi (pounds per square inch); typical drip irrigation systems (and some micro-sprinklers) need between 15 to 25 psi; and some appropriate technology drip irrigation systems need only 4 to 10 psi.
In addition to flow from catchment to storage, the flow from storage to end use is critical. Using the existing topography and/or platforms can often yield enough pressure for end use. If necessary, a pump can be implemented to add sufficient pressure. While utilizing a pump increases the pressure, it also increases the upfront and operational costs.
Gravity acting on the vertical height of the water column is what produces the pressure, which is also referred to as head. Make sure not to confuse volume with pressure (head). For instance, a 20-foot-tall water tower of 8000 gallons has the same water pressure as a 20-foot-tall pipe of 80 gallons.
If your homestead has varying topography, you want to catch and store the water at the high spots, and then use it in the low spots to provide enough pressure. Otherwise a pump can be used to generate the needed pressure.
In a system we built in Eureka, California, we were able to change the existing local law to allow us to place the catchment tank where it made topological sense, without being restrained by set-back limits.
The Book – To Catch The Rain
All of this and much more is included in a new book on Rainwater Harvesting, titled To Catch the Rain. It is available digitally for any donation (including $0) or as a physical copy at http://www.tocatchtherain.org. All of the proceeds come back to the book and to the Appropedia Foundation (a non-profit).