Rainwater Harvesting: Guidance for Homeowners
Contents
1. erways e Rainwater Harvesting Guidance for Homeowners Although rainwater harvesting has been practiced for thousands of years recent concerns over water supplies and the environment have prompted many homeowners to consider using rainwater harvesting systems While advanced systems are available from consultants and vendors a homeowner can construct a simple system for home use with a basic understanding of its components and function A reliable water supply has always been an important part of daily life Increased development and the 2007 2008 drought have focused our attention on water Most homeowners use potable water treated drinking water to satisfy all of their water needs But in many instances treated drinking water can be replaced by captured rainwater For example harvested rainwater can be used for irrigation vehicle washing and flushing toilets With special treatment and plumbing it is even possible for harvested rainwater to become the primary water supply for a home or business A rainwater harvesting system captures stormwater runoff often from a rooftop and stores the water for later use By using harvested rainwater for purposes that don t require treated drinking water we reduce the demand on municipal water supplies and increase the sustainability of drinking water supplies In some cases a rainwater harvesting system can be used to wash cars or water gardens even2. Standard gutter systems are designed to efficiently remove water from the rooftop and gutter through numerous downspouts Any modifications to the gutter system without careful consideration can result in overflows or structural failure In North Carolina gutters are designed to carry water for the 100 year 1 hour storm event the same event used for sizing the cistern overflow Carefully consider Tables 2 and 3 before modifying gutters to direct water into a cistern Table 2 lists the maximum rooftop area that can be safely drained by various gutter configurations while Table 3 lists the maximum roof area that can be drained by a downspout WATER SUPPLY PLUMBING CODES Because widespread public interest in rainwater harvesting systems 1s a relatively recent development many plumbing codes do not address the use of harvested rainwater Plumbing codes are constantly evolving Consult someone supplied with harvested rainwater and what specific regulations apply Many of the restrictions associated with plumbing codes can be avoided by maintaining entirely separate plumbing networks and using harvested rainwater for outdoor purposes only Regardless of the plumbing network any faucet or fixture supplied by the rainwater harvesting system must be properly labeled with its source Harvested rainwater contains pathogens and other pollutants at levels that pose health concerns if consumed Installing signs or labels that warn of these health
3. concerns familiar with local codes to determine which plumbing is important to prevent any potential hazards Figure 8 fixtures such as tubs toilets and sinks can legally be Table 2 Maximum roof area square feet drained by specified gutter diameters inches and slopes for select N C cities Maximum Roof Area ft by Gutter Slope and Diameter in Y2 Gutter Slope 1 Gutter Slope Diameter Asheville Boone Charlotte Elizabeth City Fayetteville Franklin Greenville Raleigh Rocky Mount Wilmington Winston Salem Table 3 Maximum roof area square feet drained by specified downspout diameters inches for select N C cities O Maximum Roof Area f by Gutter Diameter in Wilmington 652 Winston Salem RAINWATER HARVESTING SYSTEM MAINTENANCE With routine maintenance rainwater harvesting systems should provide a sustainable nonpotable water supply for many years Over time sediment carried by incoming runoff will be deposited within the bottom of the cistern and may require removal Using a first flush diverter or sediment trap will reduce sediment inputs into the cistern Figure 9 In most cases a 6 inch sediment storage Figure 9 First flush diverter installed for a residential rainwater harvesting system zone at the bottom of the cistern should provide years of sediment storage If sediment buildup is a concern i
4. E Assistant Professor and Extension Specialist Department of Biological and Agricultural Engineering North Carolina State University Published by North Carolina Cooperative Extension Service COELEEEGE Or AGRICULTURE V LIFE SCIENCES ACADEMICS RESEARCH 4 EXTENSION E cs A i E UNIVERSITY AGW 588 11 6 08 BS SSS E08 50318 Distributed in furtherance of the acts of Congress of May 8 and June 30 1914 North Carolina State University and North Carolina A amp T State University commit themselves to positive action to secure equal opportunity regardless of race color creed national origin religion sex age veteran status or disability In addition the two Universities welcome all persons without regard to sexual orientation North Carolina State University North Carolina A amp T State University U S Department of Agriculture and local governments cooperating
5. ases some type of gutter screen or filter is desirable to prevent debris from entering the system Although more expensive screens installed across the length of the gutter are less likely to Figure 6 A B C Basic overflow configurations 5 Underground cisterns offer some flexibility as to installation location because runoff can be piped underground to the cistern In fact piping runoff away from the house is preferred because installing an underground cistern immediately adjacent to a house may cause damage to the house foundation during excavation Because of the potential structural and safety concerns it is important to comply with all underground cistern installation instructions and regulations Before beginning an underground cistern installation contact your local utilities companies to locate any underground pipes or cables Depending upon the specific underground cistern being installed sand pea gravel or crushed stone backfill material may be required Consult the product literature to determine the required specifications for backfill material excavation depths and the depth of soil required over the cistern In areas where the water table can rise above the bottom of the underground cistern special consideration is required to ensure that the cistern is properly anchored against any potential buoyancy and is structurally suited to handle these additional forces An underground tank must be properly ve
6. d galvanized metal and a plastic liner is installed inside the cistern Figure 2 Because a liner is needed it may be difficult for an inexperienced installer to construct a metal cistern If the rainwater harvesting system will be used to supply drinking water special cistern materials are required If cistern stored water is to be used for drinking purposes please consult a water harvesting vendor and 2 local drinking water ordinances Only certain resins are approved by the FDA for storing potable water Use of other materials may leach harmful chemicals into the stored water supply that cannot be easily removed by subsequent water treatment CISTERN COLOR is also important for plastic tanks Translucent or light colored materials may permit algae growth within the cistern Plastic tanks are relatively easy to paint or decorate and paint or other coatings can be used to prevent algae growth Check with your local paint store to make sure you are purchasing paint that will adhere long term to the type of plastic you will be coating CISTERN SIZE The size of the rainwater cistern can have the greatest impact on system cost and performance Several factors must be considered including contributing rooftop area rainfall patterns and anticipated usage Researchers at N C State University have developed a simulation tool that can help homeowners determine the ideal cistern volume to meet the goals of a rainwater harvesting syste
7. e to their increased structural requirements and potential floatation concerns below ground cisterns are generally more expensive than their above ground counterparts Do not place a plastic cistern manufactured for above ground use in the ground it will collapse Unlike an above ground cistern where some low pressure needs and casual uses can be satisfied without a pump the use of a pump is required to obtain water from an underground cistern Additionally some pumps may not be appropriate for an underground cistern because the pump must be installed below the water level or be capable of providing adequate suction lift A submersible pump or jet pump designed for shallow wells is a suitable option for pulling water from an underground cistern Pump selection is discussed in detail in Choosing a Pump for Rainwater Harvesting AG 588 08 The main advantage of below ground cisterns is that they do not occupy valuable space in a yard Below ground cisterns also offer flexibility as to cistern location because rooftop runoff can be routed through buried pipes CISTERN LOCATION The primary constraint in selecting a cistern location 1s the position of the gutter downspouts Although there is some flexibility in routing gutter pipes to various locations it is generally easiest and most cost effective to place the cistern near an existing downspout Figure 5 When possible locate the cistern near the site where water will be used Transp
8. ecially for large diameters Because an overflow connection at the top of the tank will not be submerged under pressure and a perfect seal with the cistern Calculation of cistern load to determine need for structural support Cistern Load Capacity x 8 35 lb gal Cistern Weight Footprint Area Capacity Cistern volume in gallons Cistern Weight Weight of the empty cistern in pounds The weight of an empty plastic cistern can be approximated as 0 3 b gal if the specific weight is unknown Footprint Area Area of the cistern that will be in contact with the ground in square feet EXAMPLE A 1 000 gallon plastic cistern has a diameter of 7 25 feet Determine if additional structural support is required Cistern Load 1 000 gal x 8 35 Ib gal 1 000 gal x 0 3 Ib gal 210 lb ft Tx 7 25 ft 20 The cistern load of 210 pounds per square foot is well below the soil load bearing capacity of 2 000 pounds per square foot therefore the cistern does not need additional structural support Table 1 Maximum roof area square feet drained by specified overflow diameters inches for select N C cities Maximum Roof Area ft by Overflow Diameter in sm 6 6 5 6 A Asheville 1 170 1 740 3 320 Boone Charlotte os w m mo wall is not necessary a variety of PVC fittings can be adapted to connect the overflow pipe If the overflow is routed through the bottom of the tank as shown in Figure 6A a bulkhead f
9. ic waterharvesting RAIN BARRELS Rain barrels are less expensive alternatives to large rainwater harvesting systems and can be used to meet small outdoor water demands A rain barrel is typically constructed from a 55 gallon container and has the same main components as a large rainwater harvesting system including a gutter connection overflow and outlet valve or faucet Figure 4 Because 55 gallon barrels are used to store and transport a variety of materials used barrels are inexpensive to obtain But they may require special cleaning depending on their previous use The relatively small size of a rain barrel usually does not merit the cost of installing a pump because gravity flow is generally adequate to fill a hand watering can Although a rain barrel does not provide enough water to irrigate a lawn it can be used to store water for hand watering plants ina small garden area ABove GROUND oR BELow Several factors should be considered when deciding whether a rainwater cistern will be installed above or below ground Above ground cisterns are typically easier to install than below ground because excavation for the cistern and plumbing network are not necessary And below ground cisterns have limited accessibility after gt Fas erty as fae ie be SS ee a J el Figure 4 A manufactured rain barrel installed for watering flowerbeds installation making it difficult to repair leaks or other problems Du
10. itting is required to prevent leaks For underground cistern installations the overflow can be installed with a t fitting on the pipe carrying water from the gutter to the cistern For both above and below ground cisterns it is important to protect armor or reinforce the soil around the overflow outlet to prevent erosion Raleigh Rocky Mount Wilmington Winston Salem FAUCET OR OUTLET INSTALLATION To draw water from the cistern some type of faucet or outlet pipe must be installed Although a valve located near the bottom of the cistern can be used to pull water from the cistern for small uses such as filling watering cans a pump is necessary to supply adequate pressure for many demands Pump selection is discussed in detail in Choosing a Pump for Rainwater Harvesting AG 588 08 The outlet should be installed at least 6 inches above the bottom of the cistern to provide room for sediment storage Because the outlet connection will be subject to substantial water pressure a bulkhead fitting is necessary to prevent any leaks Install a bulkhead fitting by drilling a hole into the cistern and threading the fitting through the hole Figure 7 Because installing a bulkhead fitting requires access from the inside of the tank it is often worth the extra expense to have the bulkhead fitting installed by the cistern vendor GUTTER SYSTEM An existing gutter system can be easily modified to direct rainwater into a cistern In most c
11. m The free rainwater harvester model is available for download www bae ncsu edu topic waterharvesting DETERMINING CONTRIBUTING ROOFTOP AREA Examine the location of downspouts that will drain to the cistern to estimate the rooftop area Note the slope of the gutter and visualize what area of rooftop will drain to each downspout When measuring the rooftop area it is important to measure the roof s horizontal projection as if you were looking straight down on the roof from above ignoring the pitch of the rooftop Figure 3 Because the roof s slope is not a factor in the area measurements the contributing area can usually be estimated by measuring the area of the roof at the ground or foundation level ANTICIPATING WATER USAGE Estimating how much water will be used from the rainwater harvesting system is crucial to selecting an appropriate cistern size One of the best ways to estimate water usage is to use a simple garden Figure 2 Metal rainwater cistern being assembled Figure 3 Contributing rooftop area measurements water meter and record how much water is being used for tasks that will be replaced with harvested rainwater Garden water meters that measure the number of gallons flowing through a hose or other device are available from a variety of online retailers for about 10 Additional methods for estimating water usage are discussed online in the user manual for the rainwater harvesting model www bae ncsu edu top
12. nstall a valve at the bottom of the cistern to regularly drain the sediment laden water Due to the risk of drowning and exposure to toxic gases a homeowner should never enter a cistern for maintenance or any other purpose Check gutter connections every three to four months and after intense rainfall to check for any damage Clean gutters of leaves and debris as needed and at least seasonally Check mosquito screens periodically to ensure they are in place and not blocked by debris Maintain any pumps or filters used in the rainwater harvesting system according to the manufacturer s recommendations RESOURCES RELATED FACT SHEETS in the Urban Waterways series AG 588 North Carolina Cooperative Extension Service N C State University Hunt W F and L L Spizr 2006 Permeable Pavements Green Roofs and Cisterns Stormwater Treatment Practices for Low Impact Development AG 588 6 Online http www bae ncsu edu stormwater PublicationFiles BMPs4LID pdf Jones M P and Hunt W F 2006 Choosing a Pump for Rainwater Harvesting AG 588 8 Online http www bae ncsu edu stormwater PublicationFiles Pump4Cisterns2006 pdf RELATED Wes SITES Rainwater Harvesting at North Carolina State University http www bae ncsu edu topic waterharvesting N C State University BAE Stormwater Engineering Group http www bae ncsu edu stormwater Prepared by Matthew P Jones E l Graduate Research Assistant William F Hunt Ph D P
13. nted to the atmosphere to prevent the buildup of pressure or vacuum within the tank Vents are often incorporated into a tank or can be attached as a basic fitting clog and cause gutter overflows than filters located only at the downspout Metal gutter downspouts and fittings are available from home improvement stores and can be used to route rainwater into a nearby cistern Remember that metal downspouts are generally attached to a building and may require additional structural support when spanning any substantial distance to a cistern Corrugated plastic pipes are an alternative to metal downspouts that may ease installation and require less structural support To prevent mosquitoes from breeding within the cistern any open pathways to the captured water should be covered by 1 millimeter or smaller mesh screen Several approaches can be used to direct more water to a rainwater harvesting system The easiest and safest way to contribute more water is to pipe water from multiple Figure 8 Stickers and decals remind users that untreated cistern water is not safe for drinking existing downspouts into the cistern Using this approach provides water from a greater rooftop area without risk of exceeding the downspout or gutter capacity In locations where it is not practical to direct water from multiple downspouts into the cistern it may be possible to modify the gutter system so that more water is directed to a single downspout
14. orting water over long distances will increase pump requirements Before deciding where to locate a cistern contact your local utilities companies to locate any underground pipes or cables that might be affected by a cistern or its structural support Because cisterns can be incredibly heavy when filled with water avoid placing the cistern over any buried pipes septic tanks or drain fields that may not support the load exerted by it Your local health department can provide assistance if you have a septic system and have difficulty locating it Do not locate an underground cistern immediately adjacent to a building because excavation and installation could damage or compromise the foundation CISTERN STRUCTURAL SUPPORT In some cases the weight exerted by the cistern and the water it holds may require more structural support than the in situ soil can provide North Carolina soils are generally presumed to support at least 2 000 pounds per square foot Based on the cistern s volume footprint the square footage it occupies and the density of water you can determine whether additional structural support for a cistern 1s necessary The load exerted by the cistern can be calculated using Equation 1 below If the calculated load is greater than the load bearing capacity of the soil 2 000 pounds per square foot some type of structural support will be required Structural support for an above ground cistern could consist of a concre
15. te or gravel pad installed using standard building design practices Alternatively a cistern with a larger footprint or smaller volume could be selected to reduce the cistern load Sand stone or gravel backfill and anchoring or even the use of a poured concrete pad may be required to support an underground cistern Be sure to follow the manufacturer s instructions and any local regulations when installing an underground cistern OVERFLOW SIZING AND INSTALLATION Some type of overflow or bypass is required to release water when the cistern has reached its capacity The cistern overflow must handle the same flow as the gutter system EQUATION 1 Figure 5 Above ground cistern located at the corner of a house near existing downspouts image courtesy of LSU Ag Center which is runoff from a 100 year 1 hour storm event The required overflow size can be determined by using Table 1 for cities throughout North Carolina Using the row for the city closest to the proposed cistern installation select an overflow diameter such that the maximum roof area in Table 1 is greater than the rooftop area contributing water to the cistern Install the overflow near the top of the tank leaving several inches between the overflow and top of the tank for storage during intense storms A bulkhead fitting installed near the top of the tank will provide a sturdy connection point for the overflow pipe however these fittings can be expensive esp
16. water harvesting system components CISTERN SELECTION For most systems the cistern will have to be ordered and shipped directly to the location where it will be installed Due to the large size and weight of many cisterns delivery charges can be substantial Many online retailers sell tanks that can be used for rainwater harvesting Some companies focus specifically on rainwater harvesting systems Homeowners can also find water tanks at local stores that sell agricultural lawn and garden or industrial supplies Select a cistern based on its material size whether it will be installed above or below ground and where it will be located CISTERN MATERIALS Cisterns can be constructed from a variety of materials The most commonly available cisterns are made of plastic fiberglass or galvanized metal PLASTIC CISTERNS are generally less expensive than other materials and don t require assembly but they may not be the best option when aesthetics are a priority Their appearance can be improved with wraps made of wood or other materials Plastic cisterns can be moved into place without much difficulty They are relatively lightweight and the plastic material can be easily cut or drilled with standard tools to install the necessary valves and fittings METAL CISTERNS are often adapted from grain bins and their basic appearance is generally preferred to plastic cisterns Metal cisterns are typically assembled from sections of corrugate
17. when water restrictions prevent the use of municipal water for those purposes Also because a rainwater harvesting system reduces potable water bills the system can pay for itself over time Rainwater harvesting systems can improve the environment by capturing nutrients and other pollutants from rooftop runoff preventing them from contaminating surface waters Moreover the nutrients in rooftop runoff such as nitrogen and phosphorus can help plants grow when the captured water is used for irrigation A rainwater harvesting system consists of four main components Figure 1 e The cistern stores runoff for later use e The gutter system collects runoff from the rooftop and directs it into the cistern e The overflow pipe allows excess runoff to leave the cistern in a controlled manner e The outlet pipe which is sometimes connected to a pump draws water from the bottom of the cistern for use Consider all of these components and how they work together before installing a rainwater harvesting system The cistern or water storage tank is the primary component so select and locate it based on anticipated water needs Remember that local plumbing codes might affect the installation and periodic maintenance will be required State University NC A amp T State University COOPERATIVE AEE EXTENSION fF SY Empowering People Providing Solutions Gutter Connection Cistern Overflow Figure 1 Basic rain
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