Saturday, June 21, 2008

Albert’s Homebrew Recipe for Water

Here’s my chance to split three rounds in one swing. I was at The Farm’s Land Use Committee meeting last week, where the chairman, Alan Graf, is attempting to rope me in as regular attendee. The conversation turned to water, and Alan asked me to comment on rainwater collection, so I launched into an exegesis about what I had first observed in the Australian ecovillage, Crystal Waters, where rainwater collection exists on all homes and public buildings, and my good friend and ecovillage designer, Max Lindegger, authored an Owners Manual for residents, with basic lessons on composting, tree planting, doing without a car, and yes, home water storage. I pointed out that the alternative collecting and storing our own water at home, as I have been doing for 20 years, is adding more 25,000 gallon towers to the Farm water system every few years, along with the pumps to fill them.

The second round was a promise to Gwynelle Dismukes, the editor of the Free Press, our community’s weekly paper, that I would write something about cisterns and how to build them.

The third and most compelling incentive to get this done was my need to teach the 90-minute section on water storage as part of our annual Permaculture Design Course now on Day 7. One of the things I did was to take the class over to Earth Advocates Research Farm and show them Adam and Sue Turtle’s massive rainwater management system. When the exceptional drought last summer finally broke in October, they still had more than 30,000 gallons in reserve. One of their cisterns, collecting water from a large barn roof and taking it by pipe across their pathway, holds more than 40,000 gallons and was made from a recycled cement grain bin.

Here is my illustrated recipe for collecting, storing and cleaning rainwater. It is simplified for this space, but you’ll get the main ideas and then can go to more detailed downloads from my friends Art Ludwig at Oasis Design or Brad Lancaster at Sonoran Permaculture.

Rainwater Collection System

  • Some cement block or bricks
  • High-tensile steel banding
  • About a half dozen brackets
  • Portland cement, sand, and gravel
  • A few short sections of ¼-inch steel re-bar
  • A small amount of hog-wire fencing, preferably recycled
  • A pre-fab tank
  • Screws and nails
  • A whole bunch of plastic pipe (PDE is better than PVC), of various sizes, along with couplings and fittings as required.

You will notice that I am very unspecific about quantities in my ingredients list. That is because those will vary, depending on the size of your storage, distance to the home, run of pipe, number of tanks, etc.

There are lots of kinds of tanks, although the easiest are pre-made plastic ones from Tank Depot. I ordered two 750-gallon upright ones for my home, in green, that cost about $400 each, and arrived in 5 days. Fiberglass and polyethylene tanks are relatively inexpensive, lightweight and available in large sizes (up to about 10,000 gallons).

Galvanized steel is probably the most common cistern material. Off-the-shelf farm tanks are available up to about 3,000 gallons at your county Farmer’s Co-op or Tractor Supply Co. Because most farm tanks are not approved for potable water, some rainwater catchment system designers recommend coating the inside with an epoxy-based sealer. Above are some Brad Lancaster photographed in use on a rural house in Australia.

Concrete tanks are generally site-built using forms, though smaller pre-cast tanks are available. My friend Adam Turtle always integrates a cement tank into the basement of any new building, adding multiple benefits. At the ETC, we made a 5,000 gallon concrete tank using 4-inch block and high-tensile steel wire. It has worked flawlessly for more than a decade.

Ferrocement tanks are made by spraying or plastering a cement mortar over a wire mesh form. Wall thicknesses as thin as an inch can be produced, depending on the materials and the skill of the contractor. Because cracks can develop, some maintenance is required, and I usually build partly into the ground to help buttress the base and prevent a blow-out. Ferrocement is potentially one of the least expensive cistern materials. To the right is an example I saw at Eco-Centro in Brazil.

Mortared stone is traditionally used in some areas for cisterns. Construction cost is high, but they sure look good, and the water might even taste better. The 100,000 gallon community cistern at Ecoaldea Huehuecoytl in Mexico is mortared stone.

Durable wood, such as redwood or cypress, also can be used for tanks. If properly built, wood tanks can last 50 years or longer. Salvaged wood such as old wine vats and whiskey-aging casks can serve. At Findhorn they used the vats for houses, but they could have as easily used them for water tanks.

A polyethylene liner over a non-watertight frame is the least-cost alternative, but it also is the least permanent. Liners should be 20 or 30 mil. and made of a UV-stabilized, FDA-approved material. Another cheap alternative is culvert pipe. Brad Lancaster describes erection of modular culvert pipe cisterna in some detail on his website.

Sizing Your System

In the USA, per-person usage can range from 55 to 75 gallons per day. In Europe, 30 gpd is more common. Study your monthly water bills to get your average household usage, or meter it yourself.

Then use less.

Here is how to calculate the rainwater potential from your roof:
1. determine your region's annual rainfall, and minimum and maximum rainfall on a per-month basis, usually in inches, and convert that to feet. Notice we are not using metric, which is the world standard. Sorry, rest of the world, I assume you already know this.
2. determine the size of your roof catchment area – the part that is guttered and can be made to flow into your storage tank (area = length x width; for an angled roof, imagine a flat, two-dimensional shape through which rain travels downward onto the roof)
3. determine maximum runoff (catchment area (ft2) x rainfall (ft) x 7.48 gal/ ft3 = maximum runoff (gal)).
Lets take a roof that is 1,000 square feet. The average house sold in Nashville in 2007 was 1,832 square feet, roughly 61 square feet larger than the prior year, but we don’t need something that large to meet our water needs. There are 50 inches of rain annually, on average. Take the area (1,000 square feet) and multiply it by 4.17 (feet of rainfall). Take that number (4,170) and multiply it by 7.48. The amount of runoff in a year is 31,167 gallons. Or, in a dry year, that same area receiving 25 inches of rain will have 15,583 gallons of runoff. At 50 gal/person/day, the average runoff would last a person 622 days, and if you reduce your needs to 30/gal/person/day, a family of 4 could meet its needs for 260 days.

Wash your roof

The best roofing material for rainwater catchment is uncoated stainless steel or factory-enameled galvanized steel with a baked-enamel, certified lead-free finish. Living roofs are also fine, but retain much more of the water, depending on how dry it has been. Asphalt roofing has a "collection efficiency" of about 85 percent while enameled steel has a collection efficiency of more than 95 percent. With asphalt roofing, although more efficient than a living roof, more of the rainwater stays on the roof than for steel, though the actual percentage will depend on the duration of the storm.

Roofwashers capture and discard the first several gallons of rainwater before sending the rest to the cistern. A very simple roof-wash system can be made out of a 6- or 8-inch vertical PVC, PDE or polyethylene pipe installed beneath the gutter.

Ready-made roof washers: Rainwater System; Tank Town.

Here is a sketch of the roofwasher we built during the urban permaculture course in Nashville:

The roof in Nashville divided its guttering between three 300-gallon tanks, so we had to repeat constructing a roofwasher three times. Each time, we set the tank on a reinforced concrete pad (water is heavy — 8.34 pounds per gallon — so 300 gallons will weigh more than 2500 pounds) and fastened the tank to the building exterior with steel bands. Three 300 gallon tanks are only 900 total gallons, so in a rain event dropping more than 2 inches, storage capacity would be exceeded even if collection efficiency were only 80-percent. (1000 ft2 x 2/12 ft rain x 7.48 gal/ft2 x 80% = 997 gal.) Because of this, it is a good idea to have your overflow pipe on the tank be as large or larger than your intake pipe.

After the tank was up and the gutters of the house were attached to the roofwashers, we installed a drip irrigation system, with timers, at the base of the tanks. The irrigation ran through the terraced beds and cold frames of the urban garden that was formerly front and back lawn, sidewalk median, and alley margin.

A similar system, albeit much larger, is on my home, and also nearby at the Ecovillage Training Center on The Farm, where tens of thousands of gallons are caught and stored in the rainy months of the year. One of the important considerations is keeping nasty stuff from growing in your storage tank; keeping it sweet and smelling good. The lime in the cement seems to help this naturally, by neutralizing the pH of the rain, which tends to be more acid by the year.

We sometimes throw a big limestone rock into a cistern made of metal or plastic to accomplish the same purpose. Covering the lid tightly enough to keep out light is wise, because without light, algae cannot photosynthesize, and a tight seal will also keep out mosquitoes. The ancients also used to oxygenate their water before storage, so having flow-forms, rain chains, and drops from pipe to tank are all good additions.

If you are planning to use this water for drinking or preparing food, I recommend (a) not doing it from an asphalt-shingled roof, and (b) adding a ceramic filter system after it leaves the tank and comes to the house. We use an AquaStar filter, which is referenced in the water chapter of my book, but another good choice is the Doulton Jade Ceramic Gravity-Fed Water Filter
Jade Ceramic Gravity-Fed Water Filter Doulton
that removes 6 gallons per day, eliminating 95% of chlorine, pesticides, and heavy metals including iron, aluminum, lead and radioactive fallout, and 100% cryptosporidium, giardia and more typical roof sediment, like road dust and bird droppings.

1 comment:

Persa said...

"The ancients also used to oxygenate their water before storage, so having flow-forms, rain chains, and drops from pipe to tank are all good additions." Which ancient people used to oxygenate water? I'd like to read more about it and why they did it.





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