Best Management Practice #14: Alternative Water Sources

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Federal facilities may have water uses that can be met with non-potable water from alternative water sources. Alternative water sources are sustainable sources of water, not supplied from fresh surface water or groundwater, that offset the demand for freshwater. Examples of alternative water sources include: 

  • Harvested rainwater from roofs
  • Onsite stormwater
  • Graywater
  • Discharged water from water purification processes
  • On-site reclaimed wastewater
  • Captured condensate from air handling units.


Alternative water is often treated to non-potable standards, meaning it is not safe for human consumption. Common uses of alternative water include landscape irrigation, ornamental pond and fountain filling, cooling tower make-up, and toilet and urinal flushing.

In general, the following should be considered when implementing an alternative water project:

  • Potential alternative uses available  
  • Treatment needed for the alternative water source and the water quality requirements of the application that will consume the alternative water (see table below)
  • Design of infrastructure requirements such as piping, storage, and pumps
  • Required permits from local or state government entities and the timing to secure them
  • Backflow prevention requirements
  • Cost-effectiveness of alternative water use (alternative water use is generally most cost effective when it is included in the design of new facilities).
Considerations for Alternative Water Sources
Alternative Water SourceWater Quality ConcernsPotential TreatmentPotential ApplicationsConsiderations
RainwaterSuspended solids, pathogensFiltration, possible sedimentation and disinfectionIrrigation, toilet and urinal flushingMinimal treatment is needed for irrigation
StormwaterSediments, organics, pathogensFiltration, possible sedimentation, disinfection, and biological treatmentIrrigation, cooling tower make-up, industrial usesFoundation drain water can be reused similarly to stormwater
Onsite Reclaimed WastewaterPathogens, sediments, organics, dissolved solids, hardnessFiltration, disinfection, biological treatmentIrrigation, cooling tower make-up, industrial uses 
GraywaterPathogens, sediments, organics dissolved solids, hardnessPossible sedimentation and biological treatmentToilet and urinal flushing, irrigationSubsurface irrigation is most appropriate unless water is disinfected
Air Handling CondensateHeavy metals, bacterial growthFiltration, disinfectionCooling tower make-up, industrial usesCondensed water can be corrosive to metals because condensate can be slightly acidic; water may absorb copper from cooling coils
Purified Water System DischargeDissolved solids, hardnessFiltrationIndustrial usesHigh dissolved solids can pose issues for cooling towers and landscape
Note: This table is adapted from the U.S. Environmental Protection Agency (EPA) WaterSense at Work: Best Management Practices for Commercial and Institutional Facilities, table 8-1 and table 8-2.


The rest of this best management practice describes alternative water sources and considerations to take into account when planning for implementation.

Rainwater Harvesting

Rainwater harvesting is the collection of rainwater from rooftops that is then diverted and stored for later use. Captured rainwater is commonly used for non-potable applications and is often used to irrigate landscaping because the water is free of salts and other harmful minerals and typically requires minimal treatment. Other uses include ornamental pond and fountain filling, cooling tower make-up, and toilet and urinal flushing. 

Rainwater harvesting can help to manage stormwater by reducing the amount of runoff, which eases flooding and erosion by slowing runoff and allowing it to soak into the ground, turning stormwater problems into water supply assets. Less runoff also means less contamination of surface water from sediment, fertilizers, pesticides, and other pollutants that might be transported in rainfall runoff.

The major components of a rainwater harvesting system include:

  • Roof surface
  • Gutters and downspouts to carry the water to storage
  • Leaf screens to remove debris
  • First-flush diverter that prevents the system from collecting the initial flow of rainwater
  • Cisterns/storage tanks to store the harvested rainwater
  • Conveyances to deliver the stored water either by gravity or pump
  • Water treatment system to settle, filter, and disinfect the water, if required.

The level of treatment required for harvested rainwater depends on how the water will be used. Minimal treatment is required for irrigation, while potable uses will require comprehensive treatment to meet safe drinking water standards. At a minimum, a rainwater harvesting system should have a leaf screen and a method to settle out suspended solids. 

Rainwater harvesting is not regulated by the federal government.  It is up to individual states to regulate the collection and use of rainwater. FEMP has assembled an interactive map that visually represents the general types of rainwater harvesting policies across the country and offers general information on the applicable state programs. You can use this map to quickly discern where rainwater harvesting is supported and regulated by the state.

Implementation Considerations

Rainwater collection and distribution systems can be incorporated into almost any site although it is easier to incorporate them into new construction.

Rainwater harvesting systems may require a permit from local or state government. Plan for this requirement early.

According to The Texas Manual on Rainwater Harvesting, 620 gallons of water can be collected per inch of rain per 1,000 square feet of catchment area. Use this equation to determine how much rain can be captured in one month.

Monthly Water Collection (gallons per month) = Roof Catchment (square feet) × Monthly Rainfall (inches) × 0.62 × Collection Factor

  • Roof catchment is the area of the roof in square feet
  • Monthly rainfall is the location’s average rainfall (in inches), which can be found on the National Oceanic and Atmospheric Administration website
  • The conversion factor of 0.62 is measured in gallons per square foot per inch of rainfall
  • Collection factor is the system efficiency, which accounts for water losses. For example, a system efficiency of 80% (or 0.8) means that for every 10 gallons of water that hits the roof surface, 8 gallons can be harvested; this represents a typical system efficiency.

Rainwater systems require maintenance. Make sure to institute a comprehensive operation and maintenance program that includes:

  • Monitoring tank levels
  • Cleaning system parts including gutters and first-flush diverter
  • Monitoring for leaks
  • Maintaining treatment systems including filter replacement and disinfection equipment maintenance
  • Testing water quality. 

Stormwater is precipitation runoff over ground-level surfaces that does not soak into the ground but has not entered a waterway such as a stream or lake. Stormwater can be harvested and reused for irrigation, wash applications, cooling tower make-up or process water, dust suppression, backup fire protection, vehicle washing, and other non-potable uses. 

Stormwater harvesting differs from rainwater harvesting in that runoff is collected from ground-level hard surfaces rather than from roofs. Groundwater that is pumped away from a building foundation is considered alternative water and can be reused similar to stormwater. Benefits of stormwater harvesting include reduction of pollutants and potential flooding from large water events that flow to surface water. Other benefits include reduction of stream bank erosion, sewer overflows, and infrastructure damage.

Stormwater is generally collected onsite from hard surfaces such as sidewalks, streets, and parking lots before it enters a waterway. After being diverted, it is stored temporarily in dams or tanks awaiting use in non-potable applications. The characteristics of stormwater harvesting and reuse systems vary considerably by project, but most include collection, storage, treatment, and distribution. 

Captured stormwater normally requires more treatment than captured rainwater because it is exposed to additional pollutants from drainage systems and surfaces that may have hydrocarbons or other miscellaneous debris. Treatment options to reduce pathogens and pollution levels include the use of constructed wetlands, sand filters and membrane filters, and disinfection techniques including chlorination and ultraviolet radiation. The degree of treatment required depends on the proposed use and the level of public exposure.

Implementation Considerations

  • Successful stormwater harvesting and reuse plans need specialist input from a number of areas, including stormwater management, water supply management, environmental management, and public health. 

  • There may be local limitations on the storage and reuse of stormwater and/or there may be permit requirements from local or state governments. Plan early for these types of requirements.

  • Stormwater systems require maintenance. Make sure to institute a comprehensive operation and maintenance program that includes:

    • Monitoring of storage 
    • Monitoring for leaks
    • Maintaining treatment systems including filter replacement and disinfection equipment maintenance
    • Testing water quality.

Consider carefully the potential limitations and disadvantages of stormwater harvesting and reuse, which include variable rainfall patterns, environmental impacts of storages, and potential health risks.

Reclaimed Wastewater

Reclaimed wastewater is water that is discharged from buildings and processes, treated at a wastewater treatment facility, and then reused in applications such as irrigation and industrial processes. Federal sites that treat wastewater onsite can potentially reclaim wastewater, and it is becoming more common for local municipalities to reclaim wastewater and sell it to customers to help lower the community’s demand for freshwater. This water is often available at a significantly lower cost than potable water. 

Reclaimed wastewater likely needs secondary treatment such as additional filtration and disinfection to further remove contaminants and particulates to ensure the water is safe for non-potable applications. 

An efficient and successful reclaimed water project requires a reliable source of wastewater of adequate quantity and quality to meet non-potable water needs. These projects may be more economically viable when the cost of freshwater is high and there is a lack of high-quality freshwater or there are future supply risks due to conditions such as drought.

Implementation Considerations

  • State and local governments regulate the use of reclaimed wastewater and the associated water quality requirements. Plan early for this requirement.

  • To minimize cross-connection problems, reclaimed water pipes must be color coded with purple tags or tape according to standards set by the American Water Works Association.

  • Signs should be used to indicate that reclaimed water is non-potable. Place these signs in public places such as in front of a fountain and on valves, meters, and fixtures.

  • To avoid accidental cross-connection, keep the pressure of reclaimed water 10 psi lower than potable water mains to prevent backflow and siphonage.

  • Run reclaimed water mains at least 12 inches lower in elevation than potable water mains and horizontally at least five feet away.

  • Review the quality of reclaimed water to minimize the potential for harmful effects from long-term use, such as salt buildup.


Graywater (also known as gray water, greywater, grey water) is lightly contaminated water that is generated by bathroom sinks, showers, and clothes washing machines. Graywater does not include wastewater from toilets, urinals, or kitchens. Graywater can be used to flush toilets and urinals, irrigate landscape, and supply water for ornamental ponds, and as make-up water in cooling towers.  Graywater use offers several benefits. It can reduce water withdrawn from freshwater sources, energy and chemicals used to treat water to potable standards, and the volume of wastewater being sent to wastewater treatment facilities.

In a graywater recycling system, water that would normally be discharged for municipal sewage treatment is collected, treated, and distributed for reuse, usually within the same building. Graywater often contains detergents and dissolved and suspended solids, and can contain pathogens. Basic graywater treatment consists of removing suspended solids from the water, while sophisticated treatment may consist of biological treatment with membrane filtration, activated carbon, and ultraviolet light or ozone disinfection to destroy pathogens. 

The major components of a graywater recycling system include:

  • Dual plumbing that collects graywater from sinks, showers, and laundry
  • Water storage tanks, which should be closed to minimize contact
  • Color-coding to identify piping as a graywater source
  • A treatment system to filter and disinfect water if required
  • Pumps to move the water
  • Controls.

When implementing a graywater system, ensure that it meets National Sanitation Foundation (NSF) standard NSF/ANSI 350, Onsite Residential and Commercial Reuse Treatment Systems, which establishes the minimum requirements for the materials, design, and performance of graywater systems. Graywater systems must also be installed in accordance with local plumbing codes and by professional, licensed plumbing contractors. Installing a graywater system requires retrofitting of existing plumbing. All alterations to the plumbing system must be approved by local authorities. Counties and cities that permit graywater recycling require building inspections to inspect sites and, after the installation, verify compliance and proper operation of the graywater system.

Local authorities may require that graywater supply systems be clearly distinguished from potable water supplies. Methods of doing so include extensive labeling of the system or the use of different piping materials for the different systems. All graywater outlets must be clearly labeled to indicate that they dispense non-potable water. Local codes may require graywater supplies be distinguished by adding biodegradable dye. Backflow preventers must also be installed to ensure the proper separation of potable water and graywater supply system.

Implementation Considerations

  • The use of on-site graywater recycling systems should be considered when constructing new buildings. Even though many of these systems are costly to purchase, the payback period in savings from discharging less wastewater can be 10 years or less.

  • The pathogenic organisms in sanitary graywater must not come into contact with either humans or animals. This can be done by treating the water to eliminate pathogens or avoiding their introduction into water by not mixing sanitary graywater with any potable water source. Human exposure can be prevented by not collecting or storing the graywater in an open container.

  • Sanitary graywater used for irrigation should not be applied through a spraying device, but rather injected directly into the soil through drip irrigation. Drip irrigation provides the benefits of graywater use without contaminating animals, humans, or edible plants.

  • If you install a graywater recycling system, consider using biodegradable cleaning products that do not contain sodium, chlorine, or boron. Cleaning products that contain high chemical levels may enter the graywater recycling system and could poison plants or damage soil through the buildup of inorganic salts.

  • When graywater is used for irrigation, rain or excessive irrigating could cause ground saturation and result in pools of graywater on the surface. To help eliminate this situation, turn the graywater system off and divert the graywater to the sanitary sewer line during rainy periods. 

  • For buildings with slab foundations, recoverable graywater may be limited to washing machine discharge because most drain pipes, such as for sinks, are buried beneath the slab and thus are not easily accessible without a significant expense.

  • For buildings with perimeter foundations, graywater may be recoverable from most sources by accessing piping from crawl spaces.

  • The most appropriate graywater treatment method (e.g., media filtration, collection and settling, biological treatment units, reverse osmosis, sedimentation/filtration, physical/chemical treatment) will depend on the graywater source, application, recycling scheme, and economics.

  • Maintenance programs for a graywater system must include the following steps, all of which must be performed regularly:

    • Inspecting the system for leaks and blockages
    • Cleaning and replacing the filter 
    • Replacing the disinfectant
    • Ensuring that controls operate properly
    • Periodically flushing the entire system.
Other Alternative Water Sources

Other sources of alternative water that should be investigated to offset the use of freshwater include captured air handling condensate and discharge water from water purification systems.

Captured Air Handling Condensate

Water condenses on air handling units (AHUs) and cooling coils when humid air contacts these cool surfaces. A large amount of condensate can form on cooling equipment in areas with hot, humid summers such as the southeastern United States. Water that collects on the AHUs and cooling coils must be drained to prevent damage to the equipment or building from water build-up. Typically, the condensate is collected in a central location and discharged to a sewer drain. In a condensate capturing system, the condensate is directed to a central storage tank or basin and then distributed for reuse. 

Make-up water for cooling towers can be an ideal use of captured air handler condensate. Cooling tower make-up water is needed the most during the hot summer months, when the largest amount of air handler condensate can be collected. By nature this water is very pure with very low dissolved mineral content, which is ideal for cooling towers. However, condensate can potentially grow bacteria during the storage phase, requiring disinfection to avoid introducing bacteria contaminated water to the cooling tower system. Condensate can also contain heavy metals because of contact with cooling coils. Treatment to remove these heavy metals may be required. To limit this contamination, when cleaning coils, make sure cleaning water does not enter the condensate capture system.

Read about a federal case study about a condensate capture project implemented by EPA.

Water Purification System Discharge Water

Water purification systems, such as reverse osmosis, remove impurities from a water supply for processes that require ultra-pure water. Some of the water supplied to the system is purified, while the remaining water containing the filtered impurities is rejected from the system. The ratio of purified water to the total supply water is called the recovery rate. A common recovery rate of a water purification system is between 50% and 75%. (Source:  EPA WaterSense at Work.) This equates to 50% to 25% of the total water supplied being rejected, which can be a significant amount of water discharged from the system. Discharge water, or reject water, from these systems can be recovered and reused. 

Appropriate uses of discharge water are toilet and urinal flushing, cooling tower make-up water, irrigation, and vehicle wash. The discharge water will likely be high in dissolved solids, since this is the process of the water purification system. Therefore, it is important to choose applications where elevated dissolved solids will not cause harm or are properly managed. For cooling tower make-up, the total dissolved solids (TDS) of the discharge water should be less than the TDS set point of the cooling tower. If discharge water is used for landscape irrigation, the landscape plants should have a high tolerance for salinity. Read about reverse osmosis optimization.

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