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June 2019

Continuing Education from the American Society of Plumbing Engineers ASPE.ORG/ReadLearnEarn

CEU 272

Rainwater and Stormwater Harvesting Systems

READ, LEARN, EARN: Rainwater and Stormwater Harvesting Systems

Note: In determining your answers to the CE questions, use only the material presented in the corresponding continuing education article. Using information from other materials may result in a wrong answer.

Rainwater and Stormwater Harvesting Systems Reprint from ASPE Plumbing Engineering Design Handbook, Volume 2, 2018. All rights reserved.

Reclaimed rainwater is defined as atmospheric precipitation that is captured off a building’s roof surface. Rainwater has some very useful characteristics. Due to its lack of contact with the minerals found in surface bodies of water and groundwater, it has virtually no hardness and very low total suspended solids (TSS). The total dissolved solids (TDS) in rainwater is generally around 20 parts per million (ppm), compared to city water with levels as high as 800 ppm. As a downside, the majority of rainwater in the United States is acidic in nature due to its lack of contact with the neutralization minerals found in the ground as well as its capacity for dissolving carbon and sulfate molecules in the atmosphere. Reclaimed stormwater is defined as atmospheric precipitation that is captured from any non-roof surface, such as pedestrian walkways and grass surfaces. Stormwater is differentiated from rainwater due to the increased amount of contamination collected in the water from its contact with the catchment surface. In general, rainwater is more useful than stormwater as a water supply because it requires substantially less treatment for reuse. The most common catchment surface for rainwater is a roof area, and the roof material affects the quality of the captured rainwater. Table 13-1 lists a few common types of roof surfaces and their effect on rainwater catchment. Stormwater catchment surfaces in general are not differentiated. None of them are ideal due to the amount of contamination they impart on the captured stormwater.

RULES AND REGULATIONS GOVERNING THE USE OF RECLAIMED RAINWATER The rules and regulations for the collection, storage, treatment, redistribution, and general use of rainwater and stormwater vary widely among the various authorities having jurisdiction (AHJs). The designer of a reclaimed system is encouraged to research the applicable federal, state, and local rules and regulations accommodating such designs. In some cases, federal rules may not permit such use of rainwater and stormwater without prior approval from the holder of the water rights contracts in certain locations. As an example, the Colorado River Compact agreement among seven states, originally signed in 1922, is made up of the Upper Division (Colorado, New Mexico, Utah, and Wyoming) and the Lower Division (Nevada, Arizona, and California). The water rights divided among these states has been attributed to many legal cases regarding the Table 13-1  Roof Catchment Surfaces Surface




• Readily available • Provides good flow with less evaporation

• Copper usage and flashing on the gutter system can cause discoloration

Clay and concrete

• Inert material

• Increased loss of water due to texture and porosity

Composite and asphalt shingles

• None

• Possible toxicity due to leaching • Increased loss of water due to texture and porosity


• None

• Majority of the surface cannot be utilized for rainwater catchment • Higher levels of particulate contamination possible

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READ, LEARN, EARN: Rainwater and Stormwater Harvesting Systems property and use of those water rights. As such, designers of reclaim systems in Western U.S. drainage basins and elsewhere shall be forewarned of potential pushback from local AHJs. Also, with recent state-wide droughts in the United States and several other countries, the use of rainwater and stormwater by building owners in lieu of allowing the water to drain downstream where originally intended could have legal consequences.

USES OF RECLAIMED WATER In commercial buildings, the three predominant uses of reclaimed water are: • Fixture flushing • Irrigation • Cooling tower/HVAC makeup Of the three water uses, irrigation and cooling tower makeup are the simplest to implement in a building. Due to the minimal points of connection when utilizing reclaimed water for these uses, the piping infrastructure is not heavily affected. Fixture flushing has the advantage of having the most consistent seasonal use. However, it also has the greatest impact on the piping infrastructure due to the number of connections required, and this additional infrastructure has a large financial impact on implementing a reclaim system. It’s important during the plumbing design to look at all of the potable and nonpotable uses in the building. These lines need to be segregated at all times and at all potential connections. The municipal water makeup, which is discussed in more detail later, is the only allowable connection between a reclaim water system and potable piping. As a result, utilizing reclaimed water in areas that also require potable water effectively doubles the piping infrastructure required in those areas. In large commercial buildings, this can result in a significant increase in the plumbing and construction costs in the building. In addition to these uses, a few less-common uses for reclaimed water include: • Building washing/power washing • Fire suppression • Pool/pond/water fixture filling • Vehicle washing These water uses utilize significantly less water than the other reclaimed water usages previously discussed.

EQUIPMENT AND CONTROLS A rainwater harvesting system involves several key components: a catchment surface, conveyance system, prefiltration system, collection cistern for storage, and treatment system. The treatment system at a minimum consists of pressurization pumps, recirculation pumps, filtration, sanitization, dye injection as required by local codes, and additional treatment depending on the quality of the water required at the point of use.

CALCULATING DEMAND As mentioned above, most commercial rainwater systems are designed to feed three main nonpotable water demands: fixture flushing, irrigation, and cooling tower makeup. Fixture flushing water demand is based on both the occupancy of the building as well as the types of fixtures in use and typically is calculated based on a minimum of two days of demand. It also includes transient occupation. Figure 13-1 illustrates how to calculate the fixture flushing demand. To calculate the water use, multiply the number of occupants by the average daily use and the consumption of the flushing fixture. Do this for both the normal and the transient occupants. Summing these values will provide the total daily use. Multiplying the total daily use by the number of occupied days per month provides the monthly total. Figure 13-1  Calculating Building Flushing Fixture Water Usage Fixture Type

Consumption Examples

Average Daily Uses (Transient)

Average Daily Occupants Uses (8 Hours) (Transient)

Toilet (female)

1.28 gpf



Toilet (male)

1.28 gpf



Urinal (male)

0.128 gpf




Water Use (Gallons)

Daily Total Days per Month Monthly Total

For irrigation, it is necessary to have an understanding of the total irrigated area and the type of irrigation as well as the different types of plants that will be used. For each area of different plants, a calculation must be made utilizing the plant water use factor to determine its irrigation requirements, as shown in Figure 13-2. Cooling tower demand is based on the cooling requirements and the size of the cooling tower. It is worth noting that rainwater is by its nature a good source of water for cooling towers, as rainwater has properties that will extend its lifetime in the cooling tower. This is a direct result of the relatively low total dissolved solids found in collected rainwater. 3  Read, Learn, Earn 

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READ, LEARN, EARN: Rainwater and Stormwater Harvesting Systems Figure 13-2  Calculating Irrigation Demand

Plant Type

Plant Factor

Low water use


Medium water use


High water use


CALCULATING SUPPLY The roof catchment surface determines the overall volume of rainwater that can be collected. The general rule of thumb for determining the volume of the water collected is to first calculate the square footage of all flat and slightly angled roof surfaces, and then calculate the square footage of all vertical surfaces on the roof and multiply that number by a factor of 0.5. (Water can only be collected from two vertical sides on a four-sided vertical catchment surface during any given precipitation event.) Adding these two numbers provides the total collection surface. Multiply this value by the average rainfall in feet in one month. Then multiply by 0.65, which is a correction factor that accounts for approximately 35 percent of rainfall loss due to runoff, evaporation, catchment surface filtration, and freezing. Depending on the location, this value can be as high as 42 percent in areas with very high hourly rainfall totals (4 to 8 inches per hour). Finally, multiply by the runoff coefficient, which accounts for additional rainwater loss due to the catchment material (see Figure 13-3). Clay and concrete roofs, as well as ceramic and asphalt tile roofs, can add an additional 10 percent of rainwater loss due to their porosity and texture. Runoff coefficients for various roof surfaces can be found in Chapter 4: Storm Drainage Systems, Table 4-1. It is important to note that most rainwater system designs require a review of five- and 100-year rainfall events to determine a maximum single precipitation event. The purpose for this is not to calculate the rainwater capacity, but to size the storm drain piping. This is important when sizing roof or conveyance system prefiltration. These events, however, are not useful for determining the water supply. Rainwater supply in feet of rainfall per month should be based on average precipitation in a given month. This information is readily available. It is recommended to utilize at least 10 years’ worth of rainfall data to ensure the proper sizing. Figure 13-3  Determining Rainwater Supply

Runoff Coefficients Catchment Surface



Roof: metal, gravel, asphalt, shingle, fiberglass, mineral paper 0.90


Paving: concrete, asphalt






Soil: Flat, bare



Soil: Flat, with vegetation



Lawn: Flat, sandy soil



Lawn: Flat, heavy soil



STORAGE TANKS A cistern tank is typically located outside, either aboveground or buried, but it is occasionally located indoors for smaller applications. These storage tanks are available in a wide variety of sizes and configurations. The most common styles are interior thermoplastic or fiberglass tanks (if the building footprint permits), exterior thermoplastic, fiberglass, concrete, or wood tanks, and buried fiberglass or concrete tanks. The metal and wood tanks have liners in them, generally. Metal tanks can also be wrapped in wood for aesthetics. Figure 13-4 highlights the advantages and disadvantages of the different tank materials. Tank sizing depends largely on the application. LEED and the plumbing codes require a minimum volume of two days of water use, but not less than 50 gallons. This is generally considered a fairly small volume of collected water. Sizing the tank volume for the water use for the longest drought period in the region will guarantee maximum use of the captured rainwater. Across the United States, drought periods vary from 10 days to 125 days. This can lead to large cisterns, so a balance must be struck between optimizing water recovery and the available space and budget for the application. Undersizing the tank leads to wasted rainfall and more city water use, while oversizing leads to stagnation and increased project cost. 4  Read, Learn, Earn 

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READ, LEARN, EARN: Rainwater and Stormwater Harvesting Systems Cistern features include level controls to monitor volume and transfer to the point of use, manways for access, and overflows in the event of a large precipitation event. Additional available features include floating filters that allow cistern pumps to draw off the middle layer of the tank (which mitigates pumping solids that build up at the bottom of the tank) and smoothing inlets to prevent agitating built-up debris in the tank.

Figure 13-4  Advantages and Disadvantages of Storage Tank Materials

WATER TREATMENT METHODS There are two overall types of water treatment schemes (see Figure 13-5). The designs include similar components but differ in how they interact with the cistern. The first type of system is referred to as the direct storage system. These systems pressurize the water for use directly from the cistern through the treatment system. They are used primarily for low-flow applications and tight footprint constraints. Since rainwater reclaim systems act as a second nonpotable water source for a building, they generally require high instantaneous flow rates for use, such as in the case of fixture flushing, but not sustained flow rates. Direct storage systems are not practical at high flow rates, since the entire treatment system must be sized for these instantaneous flow rates, which leads to increased cost, footprint, and utility requirements for the treatment system. To compensate for larger flow systems, a second design referred to as cistern storage is often utilized. The main difference between direct and cistern storage is that in a cistern storage system, water is pressurized from the cistern through the treatment system to a clean water tank, which serves as a break tank in the system. This clean water tank is considerably smaller than the cistern. The water from the clean water tank is then pressurized for use, and thus the treatment system is independent of the building use flow and pressure. Since building use is an instantaneous flow, the clean water storage tank can be constantly filled at a lower flow rate, thus reducing the size of the treatment system. Regardless of the system style, all treatment systems are designed to filter, sanitize, and dye the water if required with other treatment options available depending on use, degree of water contamination, and local codes. Figure 13-5  Water Treatment Schemes

Direct Storage

Cistern Storage

Filtration The treatment process for rainwater and stormwater reclamation typically begins at the catchment surface. Before the water is stored, it passes through a conveyance system that contains two types of pretreatment filters. The first is referred to as a first-flush device. These are mechanical float systems that divert approximately the first 5 percent of the collected rainwater directly to the storm drain. This is done because the majority of the contaminants found on the catchment surface are washed into the conveyance system during the first few minutes of a precipitation event. By diverting this water from the reclaim system, the load that the treatment system has to remove is reduced. The second pretreatment device is a gravity screen filter. These devices filter large sediment and either flush a continuous stream of sediment and a fraction of the reclaimed water (generally in the range of 15 percent) to the storm drain or collect the particulates for removal later. These devices are used primarily to prevent large particulates, such as leaves and feathers that can be flushed down the conveyance system over the course of a precipitation event, from entering the reclaim system. In the case of stormwater reclamation, these devices are replaced with an oil/water separator system to remove not only the particulate loading (which will be much higher in the case of stormwater treatment), but also the bulk of the oils and other chemicals that will be picked up from the catchment surface. Depending on the nature of these oils, it may be necessary to utilize emulsion breakers as well so the oil can be physically separated. 5  Read, Learn, Earn 

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READ, LEARN, EARN: Rainwater and Stormwater Harvesting Systems Filtration integral to the treatment system is finer filtration than is accomplished in the conveyance system pretreatment filters. The filters on a treatment system are designed to filter to the 50- to 10-micron range depending on the application. The filters are a step-down filtration process—generally 100 micron and then 50 microns—and duplexed to allow the filters to be replaced without shutting down the system. The filters are monitored by means of a differential pressure switch. Filters are either cartridge or bag filters. On higher flow systems, using low-flow backwashable filters is common to avoid wasting water during the cleaning cycle. As such, multimedia filters are not common on water reclaim systems due to their high backwash requirements. Typically, backwashable filters are designed to be backwashed with reclaim water. If the water is going to be used for potable uses, a 1-micron NSF absolute filter is required. In addition to normal filtration, in some unique applications, membrane filtration systems are utilized. The systems are either ultra-filters or reverse osmosis filters. The membrane filters in these cases remove dissolved solids, organics, pyrogens, submicron colloidal matter, viruses, and bacteria. As an example, the only means of removing chemical discoloration from reclaimed rainwater and stormwater is by utilizing membrane filtration. Discoloration of the water can occur from either metal ions leaching from a metallic roof source or contact with certain stormwater containments. A membrane filter is utilized because it is the only practical means of targeting these contaminants.

Disinfection The second step in the treatment process is disinfection. The two types of disinfection most commonly used in water reclamation are chemical and radiation (UV light). Chemical treatment uses chlorine (liquid or pellet feed sodium hypochlorite) or ozone. The main advantage of chlorine disinfection is its residual disinfection after initial contact. One disadvantage of chlorine is its long residence time requirement before disinfection begins. It can often take 20 to 30 minutes or longer for chlorine to start disinfection as it depends on factors such as the pH of the water. It needs agitation due to its differences in specific gravity with water, and maintenance personnel is required to handle the chemical. Another detriment of chlorine is that it does not treat common parasites such as Legionella, Giardia, and Cryptosporidium. Further, it cannot be used on applications where the reclaimed water is used for irrigation or chemical-sensitive equipment (e.g., cooling towers). For these reasons, ozone has gained popularity over chlorine due to its increased potency as well as its easy removal with the use of UV destruct. UV is the common form of disinfection used in water reclamation systems. UV used in rainwater reclaim systems is in the 254-nanometer range. The intensities vary from 30,000 to 186,000 mW/cm2 depending on the water contamination. UV has the advantages of instantaneous treatment as well as no chemical handling by maintenance personnel or residual in the water to affect irrigation or chemical-sensitive equipment. Its only disadvantage is that it has no residual disinfection after initial contact. It is imperative that the quartz tubes containing the UV bulbs remain clean to prevent surface contamination on the tubes from affecting light transmittance and subsequent disinfection. This is generally not necessary if the water is filtered to 10 microns or better.

pH Adjustment In certain regions and applications, it is also necessary to adjust the pH of the rainwater. Due to contact with atmospheric gases such as carbon dioxide and sulfur dioxide, rainwater is predominantly acidic (in some regions). This effect will cause leaching problems in metal plumbing treatment systems as well as create problems in certain irrigation applications such as greenhouses. Generally, pH adjustment is a one-way treatment, either through contact with calcite-based filters or through chemical injection such as a weak base or bicarbonate.

DYE INJECTION AND PIPE COLORING In addition to treatment, these systems also utilize several technologies to identify the water as a nonpotable source (which is much more common than potable reclaim systems). This includes nonhazardous blue food dye injection, which gives a visual warning of the nature of the water. In addition, the system plumbing as well as field-installed plumbing is colored or painted purple to identify it as a nonpotable source for future occupants and contractors. The insulation should also be labeled accordingly. In more extreme cases as dictated by code, items that utilize nonpotable water are required to have warning signs. Not all of these features are used on every system, and these requirements vary by region and local codes. The designer is responsible for reviewing all local codes while designing the system.

MUNICIPAL MAKEUP All rainwater and stormwater systems have a source of municipal makeup water. This is a result of the unpredictability of precipitation events. The municipal backup source ensures that an adequate supply of water is always available. The implementation of the municipal makeup varies depending on the system. However, all municipal backup systems share one design feature: they must be implemented in a way that prevents the nonpotable water from entering the municipal feed (cross-connection control). There are two connection types for the municipal feed. The first is to tie the municipal feed directly into the piping downstream of the reclaim system. The water is fed via a control valve into the reclaim piping. In this case, at a minimum a double backflow preventer and an air gap are required to prevent cross-contamination of the municipal source. Additional preventions may be necessary depending on local codes. The second (recommended) type is to tie the municipal feed into the storage tank. This is controlled based on the water level of the tank. It can be tied into either the cistern or the clean water storage tank. Often, to prevent excessive use of municipal water, only a fraction of the tank is filled at any given time, as reclaim water may become available at any point. The reason to tie the municipal supply into the storage tank is to utilize the booster pumps on the reclaim water system if the municipal pressure is inadequate to supply the points of use. The municipal supply is fed to the tank via a control valve. The tank is atmospheric, so no additional backflow devices are necessary; however, a backflow 6  Read, Learn, Earn 

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READ, LEARN, EARN: Rainwater and Stormwater Harvesting Systems preventer or an air gap is still typically utilized. The tank itself acts as an air gap to prevent cross-connection. Additional preventions may be necessary depending on local codes.

PRESSURIZATION AND PUMPING Rainwater and stormwater pressurization in many ways operates similarly to city water booster pump applications. The system is sized based on the flow and pressure load dictated by the water use (i.e., fixture flushing, irrigation). The notable difference is that the pumping system is tied into the water treatment system. In applications with direct storage, the pressurization must pass though the treatment system and then carry through to the points of use utilizing a pressure control such as a variable-frequency drive or bladder tank. In the case of cistern storage, the pressurization to the point of use is separated from the treatment system utilizing the clean water break tank; however, a second set of pumps is needed to move the water from the cistern through the treatment system to the clean water break tank. Regardless of the system type or whether the pumping system is tied into the treatment system or not, they are typically redundant. It is worth noting that in many cases a third pumping system is utilized to recirculate the water through the treatment system. This is often the case in cistern storage systems as a means to maintain quality in the clean water storage tank.

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READ, LEARN, EARN: Rainwater and Stormwater Harvesting Systems

ASPE Read, Learn, Earn Continuing Education You may submit your answers to the following questions online at If you score 90 percent or higher on the test, you will be notified that you have earned 0.1 CEU, which can be applied toward CPD or CPDT recertification or numerous regulatory-agency CE programs. (Please note that it is your responsibility to determine the acceptance policy of a particular agency.) CEU information will be kept on file at the ASPE office for three years. Expiration date: Continuing education credit will be given for this examination through June 30, 2020. Thank you to Mark Girgenti of the New York City Chapter for authoring this month’s quiz. CE Questions — “Rainwater and Stormwater Harvesting Systems” (CEU 272) 1.

Rainwater is defined as _______. a. Atmospheric precipitation captured off any surface b. Atmospheric precipitation captured off a ground level surface c. Atmospheric precipitation caught off a roof surface d. None of the above


What is a beneficial chemical property of rainwater? a. It can be acidic b. It has low TDS c. It is very hard d. It has a high oxygen content


What is a primary use for reclaimed rainwater? a. Flushing fixtures b. Cooling towers c. Irrigation d. All of the above


What is the purpose of dye injection in rainwater systems? a. As a nonpotable marking b. To make the water more aesthetically pleasing c. To help remove biofilm d. To aid in filtration


When calculating the amount of rainwater that can be captured from a roof surface, what is the loss factor. a. 0.55 b. 0.95 c. 0.65 d. 0.25


What is reason to use cistern storage system as opposed to a direct storage system. a. You have too large a cistern tank. b. The need to recirculate the cistern c. You have a high flow requirement and a break tank would help minimize the flow of the treatment system. d. You have too little rainwater

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7. 8. 9. 10.

Why is prefiltration of the cistern critical? a. To minimize the solids loading on the down stream rainwater system b. To avoid particulate build up in the cistern c. To make it easier to transfer water from the cistern d. All of the above. Which of the following disinfection methods does leave a residual disinfectant.. a. UV b. Chlroine c. Ozone d. B and C What is the primary benefit of UV as a disinfection method? a. It does not leave a chemical which may be harmful to plants or downstream plumbing equipment b. It needs water without pigmentation c. It has great residual disinfection properties d. It works slowly to avoid damage to downstream plumbing equipment.

Why do rainwater systems need municipal make-up. a. Due to unpredictably of rainwater precipitation b. To ensure critical plumbing system stay in operation c. To allow owners to preform maintenance on the rainwater system d. All of the above.


What is a common point in the system for municipal make-up a. Into a clean water storage tank b. After the rainwater system. c. Into the cistern pump suction d. A and B

12. Rainwater system booster pump sizing to points of use is similar too? a. High purity water loop booster pumps b. Lab Waste Water booster c. City Water booster pumps d. Effluent sump pumps