Many U.S. Army and Department of Defense (DoD) installations are located in water-stressed locations. Rainwater harvesting is an excellent means of supplementing water for use on site if the process is permitted by state and/or local laws. Using rainwater, where appropriate, conserves potable water consumption.
Rainwater harvesting describes processes in which precipitation that falls on a site is diverted, captured, and stored for use on-site, as opposed to allowing it to run off, evaporate, or infiltrate into the soil. Depending on its intended use, the captured precipitation may require treatment. In a broad sense, rainwater harvesting can also include capture from surface water runoff; however, in the U.S., runoff is subject to surface water regulations. This document will confine discussion of rainwater harvesting to roof-top capture.
Applications:
Rainwater harvesting can be useful for installations with the following
issues:
Installations in areas with water stress due to drought and arid environments.
Installations with on-site ground water wells that may require significant energy to use such as deep wells and challenging treatment.
This could include groundwater, which has high solids or natural contaminants (arsenic and fluoride are common issues).
Installations striving to increase water resilience by implementing net zero water goals.
Areas with regulatory requirements to reduce peak flows, lessening the erosive forces of storm water runoff due to sensitive environmental receptors, such as the Chesapeake Bay area.
Situations in which ponded surface water contributes to insect or algae problems. Capturing rainwater will reduce surface water available for ponding.
Rainwater can be harvested for a number of uses:
Non-potable uses, such as toilet flushing, irrigation, dust control, and
vehicle washing. Some of these applications may require specialized plumbing.
Potable water uses including drinking water, food preparation, showering, and washing. These applications would likely require treatment to ensure that the water meets state and federal requirements for potable uses.
Rainwater Harvesting Components:
Rooftop collection systems are common, taking advantage of drainage and gutter systems. Metal roofs are ideal for rainwater collection.
They are easy to keep clean and maintain a high level of rainwater quality. Some roof materials, such as asphalt, may limit water uses to
non-potable ones.
Conveyance systems via gutters, channels, and pipe systems are used to carry collected water to storage and areas of use.
Storage systems keep collected rainwater for later use. These are typically tanks, either on the surface or below ground. Open ponds may also be used, particularly for decorative effect.
Treatment will be required for most potable uses and possibly for some non-potable uses. Treatment typically includes filtration to remove particulate matter in the collection and conveyance of the rainwater. Simple disinfection (chlorination, ultraviolet - UV, solar) may be required to control microbial growth in various systems, including storage systems.
Distribution of water stored to its intended use may require a system of pumps, pipes, and controls.
Elements of a rainwater harvesting system, as outlined in the section above. This system shows both rooftop collection, along with subsurface storage tank and a pump to a treatment system.
Water quality, first flush diverters:
Rainwater is usually considered to be of high water quality, but it can
contain contaminants.
During precipitation, rain can dissolve airborne contaminants. This
effect is most prominent during the early stages of rainfall.
Roofs will accumulate sediment, plant material, animal droppings and
materials, and other contaminants. The initial rainfall will have the highest concentrations of these contaminants, but as rainfall continues, these contaminants will be washed off.
Systems are typically designed to reject the initial runoff, which will improve the water collected and stored.
3 Overall, rooftop catchment provides much less contamination, and therefore, much higher water quality than ground surface catchment.
Water capture/sizing storage tanks:
Storage is based on rainfall and expected usage.
Several methods can be used, but a rule of thumb is to provide for three
months of anticipated use.
4 A volume of 0.62 gal can be collected per square foot of collection
surface (roof or impervious surface) per inch of rainfall.
Collected Volume (gal) = 0.62 Area (ft2
) x Design Rainfall event
(precipitation in inches calculated from intensity for design event from
hydrology intensity-duration-frequency curves. (See reference 2, Army LID Technical User Guide, for more information).
Efficiency will vary. Rooftops are generally the best surfaces. A customdesigned metal roof can achieve close to 100% water capture, provided gutters are working efficiently. Retrofitted roofs are somewhat less efficient: 80 to 90%. Surface collection is generally less effective still.
Storage vessels can become affected by algae or insect larvae. A simple solution regarding algae is to use storage vessels impervious to light. Seals and screens can limit insect access to tanks. Chemical treatments
(such as low concentrations of bleach) can also be used to remove and destroy algae and insects.
Water treatment:
The extent of treatment depends on the water’s intended use. Potable use requires more intensive treatment; alternatively, use for toilet
flushing may require treatment only to limit color and sediment. In contrast, water used for irrigation purposes may require simple sediment removal — perhaps even no treatment would be necessary.
Typical systems consist of three components:
1. Sediment removal - Settling tanks/ponds, sand filters, cyclone filters, canister filters, or cartridge filters.
2. Organic treatment - Activated carbon filters/canisters, if needed.
3. Disinfection - Ultraviolet or chlorination, if needed.
Regional issues:
Rainwater harvesting can be applied to a wide range of climates.
For hot, dry climates, the focus is on efficient collection from large, occasional events. These may use large, underground storage units to provide water between events and to reduce evaporative losses. The focus should be on maintenance of collection surface, to allow minimal
waste during actual rain events.
Wet, temperate environments may rely on first flush for cleaning collection surface. Some losses over time may be acceptable, so storage can be smaller and use outdoor storage units, which are easier to inspect and maintain, and are more common.
Environments with frequent freezing may use underground storage.
Energy Savings Collection and use of rainwater:
Rainwater collection systems are typically designed to be gravity fed.
Storage areas are typically lower elevation, so a pumping system is generally required for intended reuse.
Since collected rainwater is generally used close to its capture, the energy needed to convey the water tends to be minimal.
Compared to conveyance from other supplied sources of water (groundwater, conveyed surface water, shipped water), rainwater harvesting would likely have lower energy costs.
Maintenance:
Systems are simple and tend to be robust. Monitoring and maintenance require additional effort. This includes maintenance of roofs used as collection areas, cleaning of gutters, maintenance of storage tanks and vaults, pumps and maintenance of water treatment systems (when installed).
Water Savings How much can rainwater harvesting provide?
A reasonable goal for an aggressive rooftop collection for a custom
building could be to reduce domestic water supply by 40 to 50% for that
building; this would then allow the building to meet LEED goals for sewage reduction (see reference [1]). A lower level may be expected for a retrofitted system.
An installation could be thought of as a small city. Liaw and Chaing5 estimated a maximized rooftop collection approach could supply about 32% of needs. However, they also estimated a 10% goal would be reasonable from an economic standpoint.
Environmental Impacts Army Directive 2014-02 Net Zero Installations Policy
A net zero water installation recharges as much water back into a local water
supply (aquifer) as it withdraws. Rainwater harvesting projects contribute to this goal by utilizing reclaimed water for reuse, shifting from the use of potable water as much as possible.
Reduce overall water use regardless of source; increasing use of technology that uses water more efficiently; recycling and reusing water, shifting from the use of potable water to non-potable water sources as much as possible; and minimizing interbasin transfers of any type of water, potable or non-potable.
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