Harvesting rainwater sometimes makes sense for a variety of economic and environmental reasons. Rainwater is an economical alternative to public water, especially for exterior water uses such as landscape irrigation that require minimal filtration. Although, initial equipment installation can be significant.
Rainwater can supplement limited ground water resources. With reduced extraction rates, low-yield ground water wells and springs can last indefinitely. Also, rainwater can supplement surface water resources threatened by rapidly growing municipal water use. In addition, rainwater harvesting could significantly reduce water extraction rates from rivers during critical summer months, ensuring adequate water remains to support native ecosystems.
Rainwater is often the only viable water source in arid regions or on islands where other water sources may be high in salt, limited in availability, or very expensive.
Rainwater is low in minerals, so it is ideal for laundry, dishwashing, hair washing, and car washing. Since it contains no chlorine, rainwater is also ideal for filling garden ponds and irrigating sensitive plants.
Rainwater is not regulated by municipal water restrictions. During periods of drought, rainwater can protect investments in landscaping, garden ponds, and swimming pools.
Rainwater can cause leaky basements, eroded foundations, overflowing sewers, soil erosion, and water pollution. Harvesting rainwater can eliminate these problems while eliminating the need for expensive storm water controls.
Although rainwater can be harvested from virtually any surface, bare rooftops generally yield the best quality rainwater with the least treatment. Not all of the rainwater that strikes a roof can be captured: water is lost from evaporation, blowing wind, overflowing gutters, leaky collection pipes, first-flush devices, and self-cleaning filters. The net available rainwater from a bare roof can be roughly estimated as follows:
Available rainwater (gallons) = 0.5 x rainfall (inches) x area (square feet)
In terms of roof area, the available annual rainfall would be available rainwater, eastern states = 15 – 25 gallons per square foot available rainwater, central states= 10-20 gallons per square foot available rainwater, western states = 5 – 15 gallons per square foot.
For eastern states, rainfall is relatively evenly distributed throughout the year. In the western states rainfall is concentrated in the winter months. And, in the central states rainfall is concentrated in the summer months. This has important consequences for rainwater system sizing.
Sizing a Harvesting System
On average, Americans use 70 gallons per person per day to operate toilets, showers, clothes washers, sinks, and other water-using fixtures and appliances. By replacing fixtures and appliances with modern water-efficient versions and repairing leaks, water usage can be reduced to less than 50 gallons per person per day. Comparing demand for water with the availability and pattern of rainwater yields the following very rough “rules of thumb.”
For each person, eastern states: 500 square feet of roof+ 1000 gallons of storage for each person, central states: 750 square feet of roof+ 2000 gallons of storage for person, western states: 1000 square feet of roof+ 4000 gallons of storage
Determining requirements for irrigation is more complex because irrigation water usage can be greatly reduced by selecting native plants, or plants that thrive in regions with similar climates. In general, dry-climate plants thrive with one-half inch of rainfall per week, temperate-climate plants with one-inch of rainfall per week, and wet-climate plants with one and one-half inches of rainfall per week. Converting this to gallons:
- For irrigation of dry-climate plants (gallons/week)= 0.3 x area (square feet)
- Irrigation of temperate-climate plants (gallons/week)= 0.6 x area (square feet)
- For irrigation of wet-climate plants (gallons/week)= 0.9 x area (square feet)
As examples, for temperate climate plants such as typical vegetables, ornamentals, and lawn grasses grown in the eastern states, a 10ft x 10ft vegetable garden would do well with 0.6 x 100 square feet= 60 gallons per week which could be supplied by a small section of roof feeding a few rain barrels; a 10 ft x 100ft strip of ornamentals might need 0.6 x 1000 square feet=600 gallons per week which could be supplied by a typical house roof feeding a 1000 to 2000 gallon tank, but a quarter-acre (10,000 square feet) of lawn grass could use 0.6 x 10,000 square feet= 6,000 gallons of water per week, a quantity that is beyond the capacity of most rainwater harvesting systems.
It’s possible to harvest rainwater from roofs, parking areas, pavement, lawns, and almost any other surface, but roofs typically yield the best quality water at the lowest cost. The type of roof surface is of little consequence when rainwater is to be used for irrigation or other exterior water uses, but when rainwater is to be used for interior water uses, it is preferable to use relatively inert materials such as painted metal, terra cotta tile, cement tile, stone, and elastomeric membranes instead of composite shingles, bituminous membranes, and asphalt coatings. Nevertheless, rooftop debris usually poses a greater water-quality problem than the roofing material, and water from any roof can be treated to drinking-water quality without great expense.
Gutters and Piping
Gutter and downspout sizing for rainwater harvesting can follow standard practice, although it is advisable to oversize components in order to minimize the potential for overflow due to improper installation or settling. Gutter cap systems can be used to reduce the maintenance of pre-filters, but should not be considered as substitutes for pre-filters.
Rainwater systems are most economical when all the rainwater is conveyed to a central site for pre-filtration, storage, and pumping. Piping should be sized using conventional storm water practice which means 4″ pipe will suffice for most residential systems but 6″ or larger pipe will be required for most commercial systems. A pitch of one-eighth to one-quarter inch per foot is recommended, but this sometimes poses a design challenge because the allowable burial depths of pre-filters and underground tanks are limited. Pipe connections should be watertight to prevent both water loss and infiltration.
A rainwater system should deliver clean water by simply opening a faucet or activating an irrigation valve, just like any other water supply system. In order to be reliable and effective, each component of a rainwater system must be specifically engineered for rainwater harvesting since off-the-shelf water system components are rarely suitable. Rainwater system components can be functionally classified as pre-filtration, storage, pumping, treatment, backup integration, and measurement and control.
Rainwater captured from rooftops contains significant quantities of plant debris, soil, eroded roof materials, and other solids that can clog pumps, valves, and pipes. Mineral solids collect as sediment at the bottom of storage tanks, reducing tank storage capacity. Organic solids remain in suspension and decompose, depleting oxygen and generating hydrogen sulfide and other noxious by-products.
One way to improve the quality of rainwater is to install a “roof washer” or “first-flush diverter”, a device that discards the initial runoff from a roof before it reaches the storage tank. While this technique has some value in regions with extended dry seasons and short but intense rain storms, it is not very effective in regions where rainfall is distributed throughout the year or where rain is often an all day event. Regardless of whether a first-flush diverter is installed, it is essential to filter all of the rainwater with low-maintenance, high-rate, mechanical filters specifically developed for rainwater harvesting.
First -Flush Option
A First-Flush Diverter retains the initial runoff from a roof in a length of pipe that is capped at the end. When the pipe is filled, a ball or flapper shuts off the top of the pipe so that additional rainfall flows directly into the rainwater storage tank. The pipe cap has a small-diameter outlet that slowly releases the “first-flush” water so that by the next rain the pipe is empty and is ready to receive more water.
Pot Filter Option
A Pot Filter is the simplest rainwater pre-filters, simply a flanged plastic tray with a perforated bottom that covers the top of a large basin with a side outlet. A filter pad is placed over the first-flush diverters perforations, the pad is covered with gravel, and the outlet is piped to a rainwater tank. Water from a downspout dumps onto the gravel which strains out leaves and coarse debris and then flows through the filter mat which retains solid particles as small as 1/64″. With minimal maintenance, a pot filter can capture and filter 100% of the rainwater from a single residential downspout. Normally Pot Filters are buried so that the top is flush with the ground surface, but they can be used above ground.
Basket Filter Option
A Basket Filter consists of a large screened filter basket that fits within a plastic filter body. Water flows in through a top port, down through the basket, and out through a bottom port. A second port is provided at the top to allow overflow should the filter basket become full.
Cascade Filter Options
Cascade Filters do not collect debris, but rather allow it to wash through the filter in order to minimize maintenance. This is achieved at the penalty of lower recovery rates, typically 95% depending on average rainfall intensity. Rainwater flows in through the top port and cascades over a curved, multi-level screened filter element positioned horizontally within a plastic filter body. Filtered water exits through one bottom port; debris is washed down the surface of the filter element and exits through a second bottom port.
Vortex Filter Option
Vortex Filters do not collect debris, but rather allow it to wash through the filter in order to minimize maintenance. Instead of a horizontal filter element, they utilize a vertical filter element. Rainwater flows in through the top port, spins around the circumference of the filter body, and spills into the top of the filter element. A capillary effect draws water through the side walls of the filter element and this filtered water exits through the upper side port. Debris washes down, passes through the open bottom of the filter element, and exits through the lower bottom port. This design requires very little maintenance, but at the penalty of reduced capture efficiency, typically 85% to 90% depending on average rainfall intensity.
Storage is usually the most expensive component of a rainwater system and often determines the type of filtration and pumping system. We provide a wide range of storage solutions from a 75 gallon consumer tank to custom-designed modular systems capable of storing millions of gallons to run large commercial or multi-family structures or irrigation systems.
Free-standing plastic tanks offer the least expensive means of rainwater storage, are available in a wide range of sizes, and .are relatively easy to install. On the other hand, they have many liabilities. Plumbing and pre-filtration can be problematic when surface tanks are used for large roofs with multiple downspouts. Without expensive insulation systems, surface tanks must be drained for the winter in cold climates. In hot humid climates where nighttime temperatures do not drop significantly, water stored in surface tanks can get quite warm, accelerating biological activity. Since surface tanks are exposed to the weather, they have finite life spans that must be factored into the cost evaluation. Large surface tanks are very difficult to conceal, although sometimes they can be incorporated as a dramatic building design element.
In contrast to surface tanks, underground tanks are invisible, are unaffected by freezing weather, and can last indefinitely. Plumbing and pre filtration is straightforward, even for large roofs with multiple downspouts. Since underground tanks provide a cool, dark environment inhospitable to algae and microbial growth, they are always preferred when rainwater is to be reused inside buildings. On the other hand, underground storage is usually two to three times as expensive as surface storage and involves significant excavation which can be problematic for sites with large rocks or high groundwater.
Typical underground plastic water storage tanks are simply septic tanks made with FDA grade plastics and re-labeled as cisterns. While these tanks may work well as holding tanks for low yield wells, or as holding tanks for fire control, most are not sufficiently strong to remain empty for any period of time and are not suitable for rainwater storage systems.
If rainwater is simply dumped into a storage tank, it will create turbulence that will suspend solids that have accumulated at the bottom and submerge debris floating on the surface. Until the water column has sufficient time to re stratify, often several days, the quality of extracted rainwater will be diminished. This problem can be largely avoided by using a diffuser at the bottom of the tank, a device that reduces the water velocity and re-directs the water upward and away from the sediment layer.
Depending on the filter and tank, it may also be appropriate to use a rainwater tank trap, a very large version of a sink trap. A properly designed trap will prevent insects and small animals from entering a rainwater tank through the overflow system.
Although gravity flow can be used for flood irrigation, most other rainwater uses require pumping. Submersible pumps are installed within a rainwater storage tank, but most require a pump controller that cannot be submerged or flooded. Surface pumps are typically installed within a nearby building or pump enclosure.
The Difference in the Pump
Submersible transfer pumps are low-pressure pumps designed to be used with open-ended piping systems.They are used for non-pressurized rainwater applications such as transferring rainwater between slave and master storage tanks, transferring rainwater from sump pits to remote storage tanks, pumping excess rainwater from storage tanks where gravity flow is not possible, operating first-flush tanks, and emptying rainwater storage tanks. Transfer pumps do not have sufficient pressure to directly operate irrigation or water supply systems.
In comparison with submersible transfer pumps, submersible pressure pumps deliver water at the considerably higher pressures required to operate irrigation or water supply systems.
Like submersible pressure pumps, surface pressure pumps are used when rainwater is to directly operate irrigation or water supply systems where water pressures of 40 psi to 70 psi are typically required.
Since sediment accumulates at the bottom of a rainwater tank, fine solids are concentrated just above the bottom, and organic debris floats at the water surface, the cleanest water in the tank is generally a few inches below the surface. A floating extractor is a screened intake attached to the end of a suction hose and suspended from a float. The extractor rises and falls with the tank water level so the screened intake always draws the cleanest water just under the surface.
Rainwater from a properly designed rainwater pre-filtration and storage system can be used without further treatment for landscape irrigation, garden ponds, and most exterior applications. When rainwater is used within buildings, supplemental filtration is essential and disinfection is recommended. For toilet flushing and clothes washing, a sediment filter will remove suspended solids which can clog and damage valves, and an activated-carbon filter will remove dissolved organic matter which can cause discoloration and odors.
For showering, hand washing, or drinking, use a high-intensity ultraviolet sterilizer to kill microorganisms that could cause illness. All filtration and disinfection components should be oversized in order to maximize performance and minimize maintenance.
The Issue with UV
The problem with using typical UV sterilizers for rainwater is that although rainwater is a relatively high quality water source, it can have elevated levels of dissolved organics, iron compounds, and phosphates that absorb UV energy. It is not uncommon to see the percent of UV energy transmission, called the UVT, drop below 70% to 75%, the minimum UVT required for ordinary UV sterilizers.
Over time, the quartz sleeves in UV sterilizers become coated with a mineral deposit that inhibits UV transmission. The simplest UV sterilizers require periodic manual disassembly and cleaning, a difficult and messy process that occasionally results in a broken sleeve. Better UV sterilizers utilize a sleeve wiping device, but these are not completely effective and still require a regular maintenance schedule. UV sterilizers with UV monitors are also prone to problems caused by mineral deposits on the sensors which are exposed to water.
If rainwater is to serve as the principal water supply for a building, or if it is the primary exterior water supply, then provision must be made to automatically switch to a backup water supply when there is insufficient rainwater to meet demand.
Least Expensive Option
The least expensive backup method is cistern backup. When the water level in the rainwater tank reaches a pre-set low level, a valve opens and water from the backup source is added directly to the rainwater tank. To prevent cross-contamination, the outlet of the backup water pipe in the tank must be several inches above the highest possible water level of the rainwater tank, or the backup water can flow through a funnel device that provides a similar air gap. Unfortunately, cistern backup creates the undesirable situation where potable-quality backup water is mixed with sediment-laden water at the tank bottom, producing a degraded mixture as the water supply.
The simplest cistern backup system utilizes a special mechanical valve with a weighted float at the end of a long cord. When the water level in the tank reaches the float, the valve opens and adds several inches of water. More sophisticated systems utilize a solenoid valve controlled by a low-voltage float switch or an electronic control device.
Most Reliable Option
The most reliable backup method is direct backup in which both the backup source and rainwater supply directly connect to the plumbing system through a motorized three-port valve. When there is sufficient water in the rainwater tank, the valve connects the rainwater supply to the plumbing system. When the water in the rainwater tank reaches a pre-set low level, the valve connects the backup water supply to the plumbing system. A reduced pressure backflow preventer is used between the backup supply and the valve to assure there is no cross-contamination.
The safest backup method is float tank backup, named for a small wall-mounted tank that contains a float valve. When there is sufficient rainwater, an external pump draws water from the rainwater tank by way of a three-port valve and delivers it to the plumbing system. When the rainwater tank is low, the three-port valve switches so that the pump draws water from the wall-mounted tank. As the water level in this small tank drops, a float valve inside the tank opens and refills the tank. The tank has a primary overflow that is directly plumbed through a trap, as well as an upper emergency overflow that simply dumps water on the floor if the primary overflow ever gets clogged. This assures there will always be an air gap below the float valve, providing absolute assurance that there will be no cross-contamination.
Measurement and Control
To effectively manage rainwater utilization, it is important to know how much water remains in storage. The more sophisticated systems can manage backup and other rainwater system functions by operating valves and pumps in response to the measured water level.
Mechanical Water Level Indicators mount directly into a hole in the top of surface storage tanks. A weighted float hung from a nylon cord rises and falls with the water level, causing a pointer to spin and accurately display the water level from 0- 100″. Calibration is quick and simple: the cover is opened and the indicator ring is rotated to set the zero point. Its ultra-low cost makes it practical for the smallest tanks.
Pneumatic Water Level Indicators is the least expensive remote-reading system. It can be mounted up to 150ft from the tank and will accurately display the water level from 0 to 100% by measuring water pressure. The dial gauge must be mounted in a dry location, calibrated to the tank depth (up to 8 feet of water), and zeroed. Pulling a knob at the bottom of the gauge operates an internal air pump that clears any water in the hose and the tank probe. The water in the tank then exerts pressure on the air within the hose and probe in proportion to the water depth, which is displayed on the gauge.
This device is inexpensive enough to be affordable for any rainwater system and can measure in spaces with internal obstructions, including modular underground tanks where electronic devices require expensive pressure sensors to give accurate measurements.
Wireless Ultrasonic Water Level Indicators use a directed ultrasonic beam to determine the water level in a rainwater tank and then transmits the data without wires to a display unit located in a building up to 1000 feet away. The water level is displayed as a series of eight lights, each representing approximately one-eighth of the tank height. Unlike more primitive ultrasonic devices, this unit features a high-power transmitter design that is unaffected by condensation plus sophisticated electronic circuitry to eliminate stray signals, preserve battery life, and warn of problems such as rapid water loss possibly caused by a leaking tank. The direction of the ultrasonic signal can also be adjusted to provide accurate results with domed tanks.
Digital Water Level Indicator is an electronic device that accurately displays the water level in a rainwater tank from 0 to 100% using a radio frequency cable sensor that hangs in any open plastic or concrete tank. Due to the possibility of radio-frequency interference, this device is not recommended for use in steel tanks or modular underground tanks.
When it’s not necessary to know exactly how much water is in a rainwater tank, float switches can be used to operate a pump, valve, or other electrical device at a pre-set level.
(Adapted from white paper by Conservation Technology)