Off the Grid Sustainable Housing Options
Common Technologies for Off-Grid Living: Cisterns and Rainwater Harvesting
Depending on the climate and location of your off-grid home many homeowners can use rainwater harvesting for most of your water supply needs for bathing, dish washing and general needs though you may want to supplement bottled water for your drinking needs.
Off-grid water collection systems typically consist of the following elements:
a) Rain catchment surface. A metal roof and metal gutter system is best for rainwater catchment . Calculate the amount of water the roof may harvest by getting your local monthly rainfall, and using the following formula: Harvested water= catchment area (ft2) x rainfall depth (inches) x .623
For example, for a 11×22 micro house in Washington DC (avg 3” rain/month), collection could be up to approximately 500 gallons for an average month.
b) Rainwater transport. When rain falls on the roof, it should flow through gutters and piping that allow a high water flow to the collection tank and should be at least 3-4” pipes for a 250 s.f. roof. If the pipe system can’t handle the times the rain really lets loose, you’ll be losing a high % of your monthly water collection to spillage so ideally water should simply gravity fall through pipes directly into your cistern (collection tank), without the need for pumping, switches and or active maintenance. A simple approach is to have a direct gravity ‘roof-to-tank’ system. Early settlers who built rain collection systems followed this method.
c) Collection tank/cistern. The tank is where the bulk of the rainwater is stored. In cold climates it must be insulated or placed underground to prevent water from freezing. In slightly more temperate climates, a bit of electric heat tape under the tank and around the pipes can keep enough water flowing during the coldest months. Sizing should be based on both rainfall patterns and expected water use. Just as for electricity, it is best to first minimize water use (rather than invest in larger collection/storage systems) through low flow faucets, shower heads, and foot pedal faucets. Just as a rule of thumb, with efficient fixtures estimate 30 gal/week/person with regular washing, showers, etc. When the collection tank is full, there should be a basic overflow mechanism that gutters the water far away from the house Tanks can be rigid plastic or flexible- the key design trait being a large enough inlet to accept high water flow. The flexible water pillow tanks are far cheaper to ship than the large rigid tanks, easy to unroll etc.
d) Water pumping. From the collection tank water is then pressurized to move through a potable water hose to the garden, or through the same hose to an onboard holding tank and water filter. A simple 12 or 120 volt pump and switch should work and make sure the pump is disconnected during freezing weather to avoid cracking. Placing the pump inside the home is a more preferable option.
e) Water filtration. For potable shower and sink water, a quality water filter is essential. The filter takes water from the collection tank/cistern or on-board tank and makes it drinkable and there are many kinds available. The requirements here are water efficiency, energy efficiency, maintenance simplicity, filtration efficacy (on bacteria/virus/chemicals), and cost. A pump with a ceramic filter has been proven effective for hundreds of years and is easily cleaned up with a scouring pad.
So to recap a now thoroughly tested/proven off grid water collection and
treatment system is:
Rooftop –> large gutters/pipes –> 250 gallon flexible cistern –>
water pump #1 –> potable water hose –> garden or 40 gal onboard tank –>
water pump #2 –> RIF-10 filter –> sinks/shower
For foundation built houses, check local codes on rainwater harvesting for
potable use as it is not allowed in many United States and counties.
Most experts will agree that a pre-filter on the collection tank is necessary to
filter out pollen, dust, etc from roof water and a basic leaf screen in line in the
gutter is needed as well. Note that when filling an sealed water tank, air must
be exhausted as water rises.
Depending on the climate and location of your off-grid home many homeowners
can use rainwater harvesting for most of your water supply needs.
Figure 1: (Right) Rainwater Harvesting Cistern Diagram with Pump. http://www.minimhomes.com/design/microhouse-off-grid-water/
