Free Your Water: Fundamentals of a Rainwater Harvesting System

Posted on Tuesday, January 13th, 2009 at 1:46 pm by admin

In many areas of the country, a water-conserving household can provide for all its water needs from what it can catch off its roof. If the graywater and potentially the blackwater/humanure is also recycled for landscaping, each home can become an independent and sustainable part of the local ecology. We often speak of living off our annual income of solar energy, so it makes sense that we should try to live off our annual income of rainwater as well.

Fundamental Components of a Rainwater Harvesting System

Typically, the roof of your home will be the main rainwater-catchment surface. Supplemental water can be captured from other outbuildings if necessary. The gutter system captures the water that flows off the catchment surface. Leaf screens, first-flush diverters, and roof washers remove debris, such as bird droppings, dust, and leaves from the water before it is directed to the cistern. The water must be brought up to household water pressure through a delivery system, which is typically an electric booster pump and a pressure bladder, but in some places the water can be gravity-fed. Potable water systems will need to have some or all of the water filtered by some kind of treatment system.

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Catchment surface. The first required element for your rainwater harvesting system is a proper roofing material for your catchment system. Acceptable roofing materials are slate, terra-cotta tile, copper, untreated wood shingles, concrete, and metal painted (or prepainted during manufacture) with an epoxy paint. All of these materials provide a reasonably stable, nontoxic surface for collecting rainwater. Unacceptable materials are asphalt shingles, metal without epoxy paint, older concrete tiles (which can contain asbestos), tar, or treated wood shingles.

Asphalt shingles are by far the most common roofing material. Unfortunately, they leach toxins into the water that runs off them. Since this water is going into the ground around your home anyway, if you’re considering rainwater collection strictly for landscaping purposes the roofing material isn’t so important. But for bathing and especially for drinking, you’ll need one of the acceptable materials listed above. See page 163 for how to install a prepainted 5-V metal roof over an existing asphalt shingle roof.

Gutter system. Gutter installation is straightforward and can be accomplished by following the directions specific to the manufacturer. There are issues to consider before deciding whether to use your existing gutter system or whether to install a new system, especially if you will be drinking your rainwater.

PVC (polyvinyl chloride) gutter is probably the easiest to install, but there are serious issues with toxins produced in its manufacture and the potential for leaching. Where possible, avoid PVC that has been dyed other than white or that may contain other chemicals such as fungicides. Aluminum and painted steel gutters are slightly more difficult to install. Painted steel is the preferred choice in our opinion for highest water quality. Often companies that sell prepainted roofing metal also sell matching gutters.

The best way to ensure good water quality from your gutters is make sure that there is a continuous downslope for each run. Where water puddles, the potential for leaching of any chemical is much greater, and the opportunity exists for organic matter to collect and decay, potentially breeding microbes and imparting an off taste to your entire water supply.

Existing metal gutter systems could be suspect, since the solder at the joints could contain lead, which again could leach into your water and contaminate the entire supply. If you are unsure, effective lead test kits can be purchased for a few dollars each from hardware stores, from water quality departments, or over the Internet. Painting existing gutters with epoxy paint should eliminate this risk if lead is determined to be present.

Half-round gutters are much more complicated to install than standard ogee style (the more squarish variety) and are prone to spillage. Although the halfrounds have a great look, they should be avoided if you’ve never installed gutters before. Occasionally half-round gutters are available in greater widths (up to 10 inches), which may be worthwhile in areas of high flow.

One thing you’ll need to keep in mind if you are adding a metal roof onto an existing shingle roof is that rainwater could now potentially spill past your current gutters. This is because of the increase in height of the roof as well as the velocity of the water as it comes off your new metal roof. These two changes can cause much of the water to be projected beyond the gutter. This problem could be rectified by adding metal flashing above the current rim of the gutter to increase the gutter’s height and hence its water-catching capabilities. If you’re adding new gutters, always buy the largest size possible to ensure that most of the water can be caught during large downpours.

Screens, first-flush diverters, and roof washers. Between rains, undesirable material such as dust, bird poop, and leaves collects on the surface of your roof. Keeping this material out of your cistern will help reduce bacterial buildup and make sure your water is clean before it even gets to your filter.

Screens along the length of the gutters have been touted as a way of keeping leaves out, but they do nothing for the smaller particles like soot or bird poop that you also want to keep out. Many gutter systems do not slope downward enough from the edge of the roof for these screens to be effective. The debris simply collects on top of them, gets wet, and eventually collapses the screen or at a minimum keeps the rainwater from being collected.

Our cultural obsession with “maintenance-free” has a downside. Vinyl siding, city water, coal-fired electricity—all these things supposedly remove the “hassle” of having to take care of our environment, on the micro (home) and macro (global) scale. The unintended consequences of prolonged maintenance deferral range from rotten sills to lead-poisoned drinking water to acid rain. Maintenance-free living is a myth. We recommend that you forget about adding screens and accept the fact that once or twice a year you’re going to need to clean out your gutters.

Much more effective in keeping the undesirables out of your cistern is a first-flush diverter. The principle is simple: the first flush of rainwater, which contains most of the junk, flows into a large-diameter pipe, just like a downspout but with a screw cap plumbed in at ground level. Once the pipe is full, any additional water flows past this pipe and continues on to the downspout of the cistern. Sometime in between rainfalls, you must empty the diversion pipe by unscrewing the cap at ground level. The diverted water then flows out. Using 4-inch PVC or larger for the diverter allows whatever was flushed off the roof to funnel through. Rich in organic matter, this water is great for thirsty garden plants.

Recommended quantities of water to divert are around 10 gallons per 1,000 square feet of roof area. If you live in an area with less frequent rainfalls, you may want to consider diverting more because additional debris will have accumulated on your roof between rainfalls. Ten gallons can be held in approximately 12 feet of 4-inch PVC. You’ll likely need several shorter sections rather than one long section depending on the height of your gutter and cistern (6- or 8-inch PVC can be used for shorter sections with potential equivalent volume). Make sure to empty your diverter quickly in cold weather; otherwise the pipe can freeze solid and potentially burst.

Potable-water-catchment systems (to supply household drinking water) must filter water more thoroughly than with just a first-flush diverter. There are various methods for this secondary filtering, sometimes called roof washers. Essentially the incoming water is prefiltered through a 5- or 10-micron filter or its equivalent before it enters the cistern, greatly reducing the quantity of suspended particles and consequent water turbidity and cistern sludge buildup. This can be accomplished either with sand and gravel filters in 55- gallon drums or with commercially available filters. In addition to this prefiltering, water intended to be potable should be further purified (more information on purification options in The Carbon-Free Home).

Cisterns. By far the largest item in terms of both expense and size will be your cistern. Finding a location for the 5,000 or more gallons of water you’ll potentially need can be a serious challenge. The top of the tank needs to be below the roofline, so that the water entering the tank is gravity-fed. While it may seem like a good idea to have an elevated tank to provide your home’s water pressure, this is rarely practical. To achieve the typical household 30 psi requires a tank 70 feet up in the air. Making 50,000 pounds of liquid stay 70 feet up in the air is no mean engineering feat. Having a more down-to-earth tank and a pump and pressure bladder will almost always be easier.

The first things to determine are the type and size of the tank. All tanks need to meet some fundamental criteria for long-term use. They must

  1. Be opaque to inhibit algae growth
  2. Not leach toxic materials
  3. Be covered to prevent drowning and accumulation of debris
  4. Be accessible for cleaning

There are a range of choices depending on budget, aesthetics, and time. The basic options are fiberglass, polypropylene, metal, concrete, ferrocement, plastered tire, or wood. Above-ground swimming pools are sometimes used, but we believe they should be avoided due to their long-term structural weakness and potential for leaching toxins from their linings. Table 8.1 summarizes the options available.

Ferrocement tanks are a good option for the do-it-yourselfer. These can be freestanding or buried and built by those with a limited amount of masonry experience. They’ve been field-tested for decades and hold up well, with some limited repair possible.

Plastered-tire cisterns are another homemade option, although less tested. Essentially these are mimicking the cheapest way to store rainwater, the pond, but building up bermed tires allows the creation of an aboveground cistern without large earth-moving equipment. Check the resources section of The Carbon-Free Home for leads on where to find more information on how to build your own.

This is an excerpt from The Carbon-Free Home: 36 Remodeling Projects to Help Kick the Fossil-Fuel Habit by Rebekah and Stephen Hren. It has been adapted for the web.

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