The idea of collecting and storing rainwater for use is hardly new. It has been relied upon for centuries—even millennia—in many parts of the world. Rainwater is used extensively today in Australia, Bermuda, the U.S. Virgin Islands, and Hawaii. Even skyscrapers in Hong Kong are collecting and using rainwater. Is this practice a good idea? Is it something we should be promoting in green buildings?
This article takes a look at rainwater harvesting, examining appropriate uses and benefits, as well as potential concerns that arise when rainwater is used for drinking.
Rainwater Harvesting in the United States
Dr. Dennis Lye, a microbiologist with the U.S. Environmental Protection Agency in Cincinnati, a professor at Northern Kentucky University, and currently president of the American Rainwater Catchment Systems Association, surveyed state public health agencies a few years ago and concluded that there are roughly a quarter-million rainwater harvesting systems in use in the United States. In his Appalachian area of northern Kentucky, southern Ohio, and West Virginia, Lye believes there to be 40,000 rainwater harvesting systems, most on houses of lower-income, older people.
Interest in rainwater harvesting is growing in a number of regions, including central Texas and parts of Arizona and Colorado. Bill Hoffman of the Texas Water Development Board told
EBN that hundreds of systems have been installed in the past few years by at least a half-dozen companies in central Texas. That growth could quickly accelerate if Hays County Commissioners approve a proposed measure to permit greater development density when rainwater harvesting is used—the minimum lot size would be reduced from five acres to two acres on land above the Edwards Aquifer.
For rainwater harvesting systems to be practical as the sole water source, average rainfall of at least 24 inches (600 mm) is recommended, according to Gail Vittori of the Center for Maximum Potential Building Systems, in Austin, Texas. The entire eastern half of the U.S., from the southern tip of Texas to northwestern Minnesota, meets this requirement, as does much of California, western Oregon and Washington, significant pockets throughout the Rocky Mountains, and even areas in Arizona.
Rainwater harvesting systems can be used for potable water (drinking, cooking, bathing) or nonpotable uses such as landscape irrigation, livestock watering, washing, cooling towers in commercial buildings, and industrial process needs. There are numerous benefits of collecting and using rainwater, ranging from improved water quality to reduced stress on underground aquifers. There are also a number of concerns, primarily related to health concerns with the collected rainwater. The most significant of these pros and cons are described below:
•Rainwater can provide higher-quality water for drinking, washing, and gardening because of the absence of dissolved minerals, salts, and ground contaminants that may be present in surface and underground water. Rainwater typically has very low hardness levels. The softer water reduces the use of soaps and detergents and eliminates the need for a water softener. People on municipal water sometimes switch to harvested rainwater as a way to avoid chlorination and fluoridation treatments.
• Other water supplies might not be available, might not be dependable, or might be prohibitively expensive. On Caribbean islands and Hawaii, adequate aquifers often do not exist or are extremely deep, particularly on the dry sides of islands. Collected rainwater is sometimes the least expensive option and can easily be less expensive than bottled water.
• Reduced demand on ecologically sensitive or threatened aquifers: Falling aquifer levels in large areas of the West are threatening delicate wetland ecosystems. With rainwater harvesting, water extraction from aquifers is reduced.
•Reduced erosion and flooding by collecting some of the rainwater falling on impervious surfaces: On some lots, as much as 50% of the land area is covered by roof surface.
•Cost: Rainwater harvesting systems can be expensive. Adequately sized for a typical family and with sophisticated filtering and purification components, these systems can easily cost $15,000 to $20,000, and some cost considerably more, even when the cost of the roof is not factored in. Systems for non-potable uses are usually less expensive, because the purity require- ments are lower.
•Energy use: Some of the treatment methods for harvested rainwater are energy-intensive.
•Potential toxicity of harvested rainwater: Depending on the roofing material, the location of the system, and how the harvested water is treated and stored, there are potential health concerns. The only studies of water quality of rainwater runoff from roofs that
EBN could locate (three papers from the technical journals
Water Science Technology and
Water Research) reported elevated levels of lead, copper, zinc, bacteria, and suspended solids. (It should be noted that two of these studies were from locations where leaded gasoline was still in use—Australia and Malaysia—and nearly all of the roofs studied were old.)
How a Rainwater Harvesting System Works
A state-of-the-art rainwater harvesting system designed for potable water includes six primary components: catchment area; roof-wash system; rainwater conveyance system; cistern; delivery system; and water treatment systems. Each of these is discussed below.
The most common rainwater catchment system is a roof. Typically, this is the roof of the building where the water will be used, though it may be a separate building designed expressly for rainwater harvesting (a “water barn”). Nearly all types of roofs have been—and are being—used for rainwater collection, but some are better than others. There are three major health issues relating to the roofing material:
The first issue is whether the surface can support biological growth. Mold, algae, bacteria, and moss can all grow on some roofs, potentially contaminating water supplies. Among the most problematic roofing materials in terms of supporting biological growth are wood shakes, concrete or clay tiles, and asphalt shingles.
The second issue is how effectively the surface holds pollutants that are deposited on it during dry periods. Porous or rough materials and shallow slopes are more likely to hold particulates, bird feces, and other contaminants than are smooth, impervious, and more steeply pitched surfaces. The latter surfaces are also rinsed off more effectively when roof-wash systems are used (see below).
The third issue is whether the material itself will leach chemicals or heavy metals that might be harmful. While galvanized steel and Galvalume roofing are generally considered safe for rainwater collection (despite leaching of zinc), this roofing is often installed using nails with lead washers, from which lead can leach. Lead solder was used in some roofing and often for joining gutter sections. “Terne,” an alloy of 80% lead and 20% tin that is fairly common as a coating on commercial stainless steel roofs, is of particular concern. Any existing roof being used for a potable water catchment system should be tested for lead; if lead is found, the roof should not be used for potable water. Treated wood shingles may leach toxic preservatives. Asphalt shingles may leach small amounts of petroleum compounds.
In addition to the health concerns, a porous or rough roof surface will hold back some of the water that would otherwise make it into the cistern. Asphalt roofing has a “collection efficiency” of about 85%, while enameled steel has a collection efficiency over 95%. This means that with the asphalt roofing, more of the rainwater stays on the roof in a typical rain event (i.e., the roof stays wet), though the actual percentage will depend on the duration of rainfall events. In general, the best roofing material for rainwater catchment is stainless steel or galvanized steel with a baked-enamel finish that is certified as lead-free.
To be most effective, the roof should be fully exposed and away from overhanging tree branches. This will reduce the risk of contamination from rotting leaves or droppings from birds and insects in the trees. Also, it may be advisable to avoid using roofs of buildings that rely on wood heat, as the smoke particles and soot deposited on the roof may contain polynuclear aromatic hydrocarbons and other hazardous materials.
Various pollutants settle out of the air onto roofs between rainfall events. Many rainwater harvesting systems incorporate a
roof-wash system to keep these contaminants out of the cistern. This idea is to capture and discard the first ten to twenty gallons (40–80 l) of rainwater during each storm event before conveying the rest to the cistern. A very simple roof-wash system can be made out of a 6- or 8-inch (150-200 mm) vertical PVC or polyethylene pipe installed beneath the gutter, with an inlet just above each downspout to the cistern (see figure). More sophisticated roof-wash systems are in use, but they tend to be unreliable and are generally discouraged by experts
Rainwater falling on the roof is captured and conveyed to the cistern via gutters and downspouts. These are usually constructed of roll-formed aluminum, galvanized steel, PVC (vinyl), or copper. As with the roofing, make sure that lead-based solder was not used in gutter or downspout connections. A common rule-of-thumb is that downspouts should be designed to handle 1.25 inches (32 mm) of rain in a ten-minute period. Depending on the cistern location, 4” (100 mm) PVC or polyethylene piping may be used to convey water around the building to the cistern. Leaves are usually kept out of the gutters with a continuous 1⁄4”-mesh (6 mm) screening and basket strainers at the downspouts.
The single largest investment with most rainwater harvesting systems is the cistern, or storage unit. A cistern can range from a recycled whiskey barrel under the eaves of a house (suitable for watering plants) to a large above-ground or buried tank holding 30,000 gallons or more. Cisterns are constructed out of a wide range of materials, the most common of which are described in the box to the right.
Most cisterns are cylindrical for optimal strength-to-weight ratios. A cistern with 10,000-gallon (38,000 l) capacity (a reasonable minimum for a family of four depending solely on rainwater) might be 12 feet in diameter and 12 feet deep (3.7 x 3.7 m), for example. No matter what the material, most experts recommend keeping the tank tightly closed to keep out sunlight (which will support algae growth), to keep out animals (insects, rodents, amphibians, etc.), and to prevent evaporation. Cisterns are often designed with settling compartments that keep sediment from mixing with the water. The cistern also needs an overflow pipe for additional rain that falls after the cistern is full. If the rainwater harvesting system is the only water source, it makes sense to locate the cistern so that it can be filled by a water tank truck if necessary.
In cold climates, the cistern needs to be protected from freezing. The easiest way to do this is to bury it underground or incorporate it into a basement. In northern climates where rainwater is the sole water source, it may be necessary to oversize the cistern to provide carryover during a significant portion of the winter when snow falls instead of rain.
If the rainwater collection and storage system is located uphill from where the water will be used, gravity-flow might be possible for delivery. Unfortunately, however, the vertical distance between storage and use is rarely enough to achieve adequate pressure for modern household plumbing. For most rainwater harvesting systems, a pump and pressure tank are required for water delivery.
Preliminary filtration and a roof-wash system (if used) provide the first line of defense against contamination. Rainwater harvesting systems supplying potable water should also include measures to treat water before use. There are several treatment options, including micro-filtration, UV sterilization, and ozonation.
Harley Rose, whose company Rainwater Collection Over Texas has installed or consulted on several hundred rainwater harvesting systems in recent years, recommends all three of these treatment systems. He installs an ozonation system in the cistern that automatically injects ozone from a corona-discharge ozone generator into the cistern and circulates the water for a six-hour period twice a day. Then, at the drinking/cooking water tap, Rose uses a combination microfiltration and UV treatment—water passes first through the filters, then through a UV light chamber, where high-energy ultraviolet light kills bacteria and other organisms. (Filtering before the UV treatment is important, because particulates in the water can shield organisms from the light.)
Another rainwater system contractor
EBN spoke with, Rain Man Waterworks, along with Dr. Dennis Lye of Kentucky, who is president of the American Rainwater Catchment Systems Association, consider filtration and UV treatment to provide ade-quate protection, making the ozonation unnecessary.
Water treatment systems can add significantly to the initial cost as well as operating costs of a rainwater system. A UV sterilization system costs from a few hundred to $1,000 or more. Because the lamp generating the UV light needs to be left on all the time, annual electricity consumption can be significant—as much as 500 kWh or as little as 80 kWh with the 8.8-watt system Rose uses. An ozonation system can cost a lot more to install and operate—the system Rose installs for a 20,000-gallon cistern costs $1,200 to install and consumes as much as 6,000 kWh per year (primarily to operate a circulating pump that mixes the ozone in the cistern 12 hours per day).
A less expensive option is to treat water with chlorine or iodine, as is typically done with municipal water. The most common chemical added is chlorine in the form of sodium hypochlorite, which is available in liquid form. Household bleach, which is 5 percent sodium hypochlorite, is commonly used. The downsides to chlorination are the taste of the treated water and concern about harmful chemicals that could result from the added chlorine. In the presence of organic matter, chlorinated hydrocarbons may be formed. If the rainwater is acidic (low pH), it can be buffered using baking soda.
Most rainwater harvesting systems are not cheap, particularly those that are properly designed and built for potable water. A common rule-of-thumb is that the cost will be about $1.00 per gallon of storage ($0.26/l). Smaller systems and systems with expensive, custom-built components can cost significantly more. The large commercial system at the National Wildflower Research Center in Austin, with 17,000 square feet (1,580 m2) of galvanized steel roofing and 70,000 gallons (265,000 l) of storage, cost $250,000 to build. This rainwater harvesting system was designed as a prominent architectural element of the facility, however, and its educational role justifies the high cost.
At the other end of the scale, very simple and owner-built systems can be far less expensive. A simple whiskey barrel positioned underneath a roof downspout and used for watering gardens can be built at almost no cost. An owner-built potable-water rainwater collection system in Hidalgo County, Texas with a 600-gallon (2,300 l) ferrocement tank was built in 1992 at a cost of just $100.
To keep a rainwater harvesting system functioning properly, periodic maintenance is required—this is particularly important with systems used for potable water. Gutters and downspouts need to be kept free of leaves and other detritus. Roof-wash water may need draining after each rainfall event. The cistern may need periodic cleaning or repairs if cracks or leaks develop. Filters and UV lamps need replacement on a regular schedule. Pumps and ozonation systems may need occasional servicing. All piping and connections should be inspected periodically. Finally, occasional testing should be considered to ensure that suitable water quality is being achieved. Any rainwater harvesting system should be provided with complete maintenance instructions.
Rainwater harvesting systems offer several attractive environmental benefits and should be considered with many green building projects. When a building has access to municipal water or a developed well, the economic payback for a sole-use rainwater harvesting system is not likely to look good. In such situations, it may still make sense to put in a small, simple system for landscape/garden watering. When buildings are not on municipal water, the economics may be more favorable, especially when well water would require treatment (water softeners, for example, which require regular servicing), or when drilled wells are not dependable. Rainwater harvesting may also be a good solution in areas with dropping aquifer levels.
The biggest concern with rainwater harvesting—other than cost and energy consumption—is the quality of collected water. To reduce risk, never collect rainwater for potable uses from an existing roof unless careful inspection and/or testing is done to ensure that heavy metals, asbestos, or other hazards are not coming off the roof. Old metal roofs pose the greatest risk—from coatings as well as lead washers commonly used in nailing the roofing on, but even asphalt roofs can leach copper and other potentially hazardous materials.
When designing a building that will have a rainwater harvesting system, uncoated stainless steel and factory-enameled galvanized steel roofing are the safest options. With any metal coating, ask the manufacturer whether the coating contains heavy metals (red paint used on metal often contained lead in the past). Avoid porous and rough roof surfaces, because they collect contaminants more easily than smooth surfaces, may provide an environment for organisms to grow, and do not release accumulated pollutants as easily during the first rinse. Install a roof-wash system at each downspout and incorporate quality components for water purification. Because of the energy intensity of full-circulation ozona-tion, try to get by with careful multistage filtration and UV treatment.
If rainwater harvesting is to be used more extensively, it is imperative that additional testing be done on the safety of collected rainwater. Available data is very limited, but it suggests avoiding older roofs due to heavy metal contamination. The U.S. Environmental Protection Agency or state health agencies should take leadership roles in testing water quality from these systems and setting standards for safe use of rainwater.
– Alex Wilson
For more information:
Texas Guide to Rainwater Harvesting
Texas Water Development Board
Conservation: Attention Patsy Waters
P.O. Box 13231
Austin, TX 78711-3231
This excellent guide was written by the Cen-ter for Maximum Potential Building Systems.