A Simple Path to Responsible Water Use
(excerpt from Arcade Journal – Winter 2009 – guest editor Jason F.McLennan)
Ann and Gord Baird
Incorporating biomimicry into environmental design, whether commercial or residential, combines beauty, simplicity and function into responsible use. With that said, the natural world abounds with examples of simple rainwater harvesting. Lakes, ponds, wetlands and groundwater are excellent large-scale, natural examples of rainwater harvesting, while upturned leaves holding droplets serve as micro examples. Architectural expressions of natural systems for capturing and conveying the rain that falls on our sites can be inspiring and add significant meaning to any design. Storing rainwater for future use is a natural, life-supporting process that has been absent for much too long in modern building design.
However, times are changing, and we are increasingly cognizant of our abuses of the world’s water. By installing rainwater harvesting systems, the water that is industrially treated is reduced, contributing to individual and community water security and maintaining balance in the natural water cycles of surrounding ecosystems. The impacts of rainwater collection — a simple process — can be profound.
Rainwater can easily be used for many purposes including irrigation, flushing toilets (which accounts for 30% of indoor water use), mechanical systems, cleaning needs and even potable drinking water, but conservation should always be considered first so that water requirements are reduced.
A Guide to Rainwater Harvesting
A basic system collects rainfall from the roof, filters out debris and then stores it in cisterns; storing clean water results in less tank maintenance down the road. The stored water can then be drawn upon and filtered further as required depending upon the use. It is critical to match the appropriate quality of water to its intended use.
Roof materials, gutters and slope all have their pros and cons. Metal, clay tiles and slate are great, but better yet, consider a living roof, which among many other benefits also acts as a mini-watershed for primary filtration. A living roof also has built-in drainage layers, thus reducing or eliminating the need for exterior gutters.
A first flush diverter is required to discard the first seasonal rain that falls on a non-living roof to clean-up the pollen, bird poo, needles and leaves. There is usually a secondary simple filter (screen) for leaves and other sedimentary materials.
Tanks are literally a big topic, and their size is determined by three factors: roof size, monthly/annual rainfall and monthly/seasonal usage. Cisterns can be concrete with a sealer or membrane, wooden, metal with a membrane, HDPE plastic tanks, ponds or wetlands. Ideally, as with any system, it is important to incorporate biomimicry and consider all the household systems when designing. For example, a large, concrete cistern can be incorporated in the foundation of a home, simultaneously adding a temperature moderating thermal mass, providing a possible dump for solar thermal heat and a water reservoir for fire protection. Your climate, site, budget, other systems and environmental leanings will determine your choice.
Stored water can be used for potable or other uses; non-potable usage does not require any further filtering before it is used but should be marked as “not potable.”
Potable water needs to be filtered or sterilized to meet local code requirements. Usually rainwater will be filtered down to five microns before sterilization. Possible filters include sand, ceramic and disposable. Once adequately filtered, the water can be sterilized to kill any potential pathogenic bacteria via a UV sterilizer, a chemical sterilizer (such as bleach) or a membrane osmosis system; the choice depends on many factors, such as regulations and energy requirements.
What about code? Since policy can be lacking in many jurisdictions, common sense dictates having your water lab tested for potability. (For more on water regulation, please see Ben Gates’s article “Taking the Guerilla Out of Greywater” later in this issue .)
If energy conservation is vital when processing rainwater (as it is at our home), use mother nature’s pumps (gravity) wherever possible and then only special energy efficient pumps. Pump choice is based upon the desired pressure, the amount of head it has to travel (distance straight up) and the volume of flow.
Take for example our vegetable garden’s drip irrigation system. We have a 2,500 square-foot living roof that collects 1,300 gallons for every inch of rainfall; 1″ of rain on one square-foot of roof area produces 0.52 imperial gallons. Precipitation occurs during winter, so we store enough rainwater for four to six months of summer drought. Our storage capacity of 10,000 gallons consists of four tanks, all gravity fed to overflow to the next, thus eliminating our need for a separate stormwater system. Our domestic potable water comes from a deep well, and we utilize a no-flush toilet and low-flow fixtures to reduce our domestic water use. The rainwater system is designed for possible future upgrade to potable quality by installation of a sand filter between the living roof and the storage tanks and then through a five-micron ceramic-filter and UV sterilizer.
Larger scale examples consist of basically the same steps. Keeping systems simple and fully integrated into the building design is the key to water sustainability.
Nature can be our friend or our enemy. If we design with the natural environment, rainy, grey days are sources of joy and renewal, as we watch our vessels fill with the most precious of all resources: water.
For more information
Books to read
Rain Water Collection for the Mechanically Challenged, by Suzy Banks
Rain Water Harvesting for Drylands and Beyond, by Brad Lancaster