9.1 Design of Decentralized Rainwater Harvesting Systems
Decentralized rainwater harvesting systems collect and supply water on-site and reduce reliance on
potable water supplies.
Decentralized rainwater harvesting systems can also have an impact on energy demands. Efforts to
reduce our energy consumption have overlooked the cost of energy required to supply water to end
users. Treating and distributing drinking water in the U.S. accounts for 4% of the total energy used.
Almost 80 cents of every dollar spent by municipalities on water treatment and distribution is for
electricity (Electrical Power Research Institute, 2002). Much of this potable water is used for nonpotable water needs. An alternative to this energy-intensive process of supplying people with water is
falling from the sky: rainwater.
Benefits of Rain Water Harvesting:
 Save money on utility bills
 Manage storm water runoff
 Reduce non-point source pollution
 Reduce reliance on aging infrastructures
 Attain points for LEED certification
 Recharge local aquifers with tank overflows
 Reduce detention pond collection requirements
 Increase building resale value
One inch of rain on a 1,000-sq.-ft. roof is 623 gallons of water. Non-potable uses of water include:
Toilet flushing, irrigation, water features, laundry washing, vehicle washing, cooling towers, fire
suppression, pool/pond filling and manufacturing processes. When considering a rainwater harvesting
system, local authorities should be consulted regarding regulations or codes pertaining to rainwater.
Rainwater systems can be applied to single-family and multi-family residential, commercial and industrial
projects. When designing a system for new construction, incorporating a rainwater harvesting system can
reduce or eliminate the requirements for detention ponds. Civil and plumbing engineers should work
together during this process to accomplish their goals and meet regulatory requirements.
When designing a rainwater harvesting system, four basic steps should be followed and implemented.
The results of this design will be a low-maintenance, high water quality system with reduced risk of
failure, decreasing the need for additional treatment and concern from the plumbing designer.
Systems should be designed to protect and enhance the naturally occurring biofilm in the tank. The
biofilm is similar to biofilms used in water treatment, and it maintains water quality through the
breakdown of organic matter and trapping metals. If the four steps are followed, the tank will not require
cleaning or emptying, which could destroy the healthy biofilm layer.
Step 1
A pre-filter should be used to remove debris washed from the roof surface during rainfall events. There
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are many variations of pre-filters available. For an effective system, the filter should not require
maintenance after every rainfall event, and the first flush should be diverted away from the tank.
When a rain event begins, the first water from the roof surface carries much of the debris.
Fig. 42 Vortex Filter
Diverting this “first flush” away from the storage vessel helps maintain high water quality in the storage
vessel. These devices are normally placed in the system after the gutters and before water enter the
storage vessel. We have found the WISY filters to be a low maintenance and high performance first flush
diverter and filter. A WISY filter will help oxygenate the filtered water before the water enters the storage
vessel, preventing stagnation. The filter also separates debris from clean rainwater and protects water
quality before storage. Maintenance of the WISY stainless steel filter insert is as simple as rinsing the
filter with a hose or in a dishwasher.
Step 2
How water enters the storage vessel is very important. By using a smoothing inlet, sediment and biofilm
on the bottom of the storage vessel will not be stirred. The smoothing inlet also helps to oxygenate the
water and exchanges the old water with new pre-filtered water. New water enters the bottom of the vessel
in an upward direction and forces the older water upward and out the overflow.
Step 3
How water is extracted from the storage vessel is equally important in supplying high quality water.
Water extracted just below the surface is normally the highest quality. The use of a floating filter allows
for the extraction just below the water surface. Water extracted from the surface or tank floor contains
small particles and can cause problems with plumbing seals, seats, and valves.
Step 4
In order to maintain a healthy water system, some overflow should occur. The overflow will help
remove fine floating debris through a surface skimming action. Protection from small vermin entering
the storage vessel is needed to maintain water quality. The overflow is directed to the sewer or storm
water line, or day lighted to a pervious area like a bioswale and/or rain garden. If the overflow system is
connected to sewer or storm water lines, it may be necessary to protect the system from gases entering
the storage vessel. This can be accomplished with a multifunction overflow device or proper P-Trap.
9.3 Storage Vessels/Tanks: Selection and Design
Storage tanks for rainwater come in many forms and sizes. These tanks are made of a variety of materials
including polyethylene, fiber glass, concrete, steel and modular cells Selecting the appropriate tank size
can be one of the keys to a successful, efficient and cost-effective rainwater harvesting system. In larger
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commercial and educational facilities, it is often advantageous to utilize a multi-tank rainwater harvesting
system. While the sizing of both tanks depends on the expected water demand, the thought process for
sizing each tank is different. Once the correct tank sizes are selected, tank materials and specific locations
can be determined.
While design of the large rainwater storage tank often falls to the civil engineer, cooperation between the
plumbing engineer and the civil engineer is crucial. For a cost- effective design, this tank should be
included in the storm water management plan. However, sizing the tank requires more than a typical
detention calculation.
Sizing a detention area is based on the roof area and a single event (for example, 5” of rainfall in one
hour). As long as the rainwater harvesting system will be backed up by municipal or well water, the
rainwater system only needs to meet average demand, not the peak or maximum demand. For example, in
an office building with 60 offices, three conference rooms that each hold 20 people and fitness centre
with 15-person occupancy, the true expected demand is likely the water use of 60 people, not 135 people.
Designing the rainwater harvesting system to meet the water demands of 135 people would result in an
over-sized and more costly system. Once this demand is determined, the best way to calculate the storage
tank size is modelling the system using daily rainfall data. Daily water level in the tank can be calculated
from input (rainfall x roof area x filter and runoff efficiency x 0.62) and the daily demand. A conversion
factor of 0.62 is used to convert from square feet-inches to gallons.
Fig.43 Treatment & Storage of Water Collected by rain water harvesting
A reasonable starting assumption would be that the crowd is half men and half women and 20% of the
men use the water closet. Allowing a little over 1 minute per water closet use and 30 seconds per urinal
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use, this whole process takes 30 minutes. Based on a 1.28-gpf water closet and the pint flush toilets, this
comes to 925 gallons of water used over 30 minutes. Using a pump in the larger rainwater tank and a
treatment system with a capacity of 30 gpm, only 25 gallons would need to be stored at the beginning of
the event to handle the water demand.
While the design load based on fixture units is much higher than 30 gpm for this system, with a multitank design a 30-gpm pump and treatment system from the rainwater storage tank can adequately meet
the expected demand. Decreasing the size of the pump in the rainwater tank can save both money and
energy because the storage tank is often located far from the end use.
While tanks can be optimally sized, backup water must be supplied either from a municipal source or
an approved well. The above described, multi-tank system, lends itself to using an air gap in order to
protect the backup water source. While it may appear that it would be more expensive to have a system
that requires two pump systems, multiple applications have led us to believe that this is a more cost
effective approach because the pumps and filtration equipment that provide the treated water to the
"day tank" are much smaller, as are all valves and fittings. Plus, the pump that sends the water from the
day tank to its end use is also kept comparatively small.
9.4 Design and Installation Requirements
9.4.1 Collection Parameters.
All piping and plumbing component materials used in the installation of a rainwater harvesting
system shall be as approved for the specific use per local plumbing code, or be listed by an
ANSI accredited product certification program as available.
A. Collection roofing, gutters, piping, fittings, valves, screens, down spouts, leaders,
flushing devices,
tanks, and liners, shall be approved for the intended use.
B. All tank interior surfaces, and equipment shall be washed clean before they are put into
C. For water storage volumes less than 360 gallons (1,363 litres), or intended for minor
utility, irrigation and garden use, no treatment is required.
D. Water level control devices that control pumps, makeup water valves, etc, in contact with
the water supply, shall be mercury free devices.
E. Overhanging vegetation and proximity to air borne pollution sources are to be avoided.
These standards do not apply to the collection of rainwater from vehicular parking or other
similar surfaces.
For non-potable water applications,
The collection surface may be constructed of any above-ground, hard surface,
impervious material.
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B. Harvested rainwater must be filtered or treated to an appropriate quality suitable for
intended use. No treatment is required for sub surface irrigation, agricultural, or garden use.
For above surface Irrigation, the local authority having jurisdiction should be consulted
regarding required water quality.
9.4.2 Conveyance System
Roof Drainage System: Gutters and downspouts used to collect rainwater shall comply with the
A. All piping, plumbing components, and material used shall be manufactured of material
approved for the intended application, conforming to the standards meeting the intent of
applicable Building and Plumbing Codes.
B. Gutter and down spout systems leading to the cistern shall be fitted with debris excluder
or equivalent device.
Washers and Pre-filtration: All collected rainwater, for potable water application, shall pass
through a roof washer or pre-filtration system before the water enters the cistern(s). Roof washer
systems shall meet the following design requirements:
A sufficient amount of rainwater shall be wasted, and not allowed to enter the cistern,
to wash accumulated debris from collection surface. Approximate amount of rainfall to
be wasted shall be adjustable as necessary to minimize cistern water contamination.
B. The inlet to the roof washer shall be provided with a debris screen that protects the roof
washer from the intrusion of waste and vermin. The debris screen shall be corrosion
resistant and shall have openings no larger than 0.5 inches and no smaller than 0.25
inches nominal. Pre-filters which have a self-cleaning design are not required to have the
aforementioned debris screen.
Exception: This item is not required for pre-filters which provide their own method of
diverting the prescribed first flush.
Water drained from the first-flush diverter or pre-filter will be piped away from the
storage tank and terminate in a location which will not cause damage to property or cause
D. If more than one cistern is used a screen, roof washer or pre-filtration system shall be
provided for each cistern.
Exception: Where cisterns are interconnected to supply water in series, a single pre filter will
be permitted
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E. First flush diverters and pre-filters shall be provided with an automatic means of self
draining between rain events.
F. Roof washers shall be readily accessible for regular maintenance.
G. Pre-filtration screens or filters shall be maintained consistent with manufacturer=s
9.5 CISTERNS / STORAGE: The following are the minimum requirements for cisterns:
9.5.1 General:
A. Cisterns may be used as storm-water collection points that help to minimize flood damage,
while providing a reservoir for later use. Cisterns shall have access to allow inspection and
9.5.2 Installation:
A. Cisterns may be installed either above or below grade
B. Tank shall comply with the Administrative Authority having jurisdiction, local building codes
and ordinances, and / or as certified by a structural engineer.
C. Above grade plastic tanks shall be certified by the manufacturer for intended application.
D. Above grade cisterns shall be protected from direct sunlight and shall:
1. Be constructed using opaque, UV resistant, materials: i.e. heavily tinted plastic, lined
metal, concrete, wood, or painted to prevent algae growth,
2. Have specially constructed sun barriers e.g. installed in garages, crawlspaces, sheds, etc.
E. Below grade cisterns, located outside of the building, shall be provided with manhole risers a
minimum of 4 inches above surrounding grade and / or installed in such a way as to prevent
surface or ground water from entering through the top of any fittings.
F. Where the installation requires a foundation, the foundation shall be flat and shall be designed
to support the cistern weight when the cistern is full consistent with bearing capability of
adjacent soil.
G. In areas where sustained freezing temperatures occur, provisions will be made to keep cistern
and the related piping from freezing.
H. All cisterns shall be installed in accordance with the manufacturer’s installation instructions.
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1. Underground tanks shall comply with relevant construction Industry Standards, fall protection
rules and regulations and any local codes relating to excavation and backfill technique or
2. Above grade tanks shall be installed on a sturdy and level, foundation or platform, adequately
secured with adequate drainage
I. In a situation where the soil can become saturated, an underground tank shall be ballasted, or
otherwise secured, to prevent the tank from floating out of the ground when empty. The
combined weight of the tank and hold down ballast should meet or exceed the buoyancy force of
the tank, calculated as follows:
1. Buoyant force of Cistern (lbs) = Cistern Volume (cubic feet) x 62.4 (lbs / cubic foot) e.g.
For 1000 gallon tank, Buoyant force will be 1000 gallons x
(1 cubic foot / 7.48 gallons) x 62.4 ( lbs / cubic foot= 8342 lbs
2. If concrete used as ballast, the volume needed will be:
Volume (cubic feet) = 8342 lbs x cubic feet / 150 lbs = 55.6 cubic feet (2.1 cubic yards)
J. Cisterns shall be provided with a means for draining and cleaning.
K. All cistern openings shall be protected from unintentional entry by humans or vermin.
Manhole covers shall be provided and shall be secured to prevent tampering.
1. Where an opening is provided that could allow the entry of personnel, the opening shall be
9.5.3 Inlets, Outlets and Openings.
A. Cistern inlets shall be provided to permit water to enter tank with minimum turbulence.
B. The overflow outlet, or flap valve, shall be protected with a screen having openings no
greater than 0.125 inches, or as otherwise appropriate, for preventing entrance of insects or
vermin entering the cistern.
1. Overflow outlet shall be sized in accordance with prevailing gutter and down spout
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2. Water from the cistern overflow shall be discharged in a manner consistent with local storm
water runoff requirements and as approved by the local authority having jurisdiction, or may
be allowed to infiltrate excess collected water into the aquifer.
9.5.4 Pump
Where a pump is provided in conjunction with the rainwater harvesting system, the pump
shall meet the following provisions:
A. The pump and all other pump components shall be listed and approved for use with
potable water systems.
B. The pump shall be capable of delivering a minimum of 15 psig residual pressure at the
highest and / or most remote outlet served. Minimum pump pressure shall allow for friction and
other pressure losses. Maximum pressures shall not exceed 80 psig. A pressure reducing valve
shall be provided at water branch distribution piping if the pump is capable of exceeding 75 psig.
Filtration shall meet the following provisions
Where rainwater is used for non-potable use and for non critical operations, such as irrigation,
wash down, etc., a final stage filtration system is not required.
Where rainwater is used for non-potable use, interior to an occupied facility, for makeup for
laundry, toilets, process, etc.; the water is to be filtered as a safeguard against sediment or
discoloration, and for proper operation of valves or other devices.
There shall be no direct connection of any rainwater harvesting pipe system and a public utilityprovided domestic potable water pipe system without an approved back flow device.
Separation shall be maintained between potable and non potable water systems at all times.
Cross connections, without proper protection in accordance with local applicable plumbing code,
will not be permitted.
A. All material used as part of a rainwater harvesting system shall be as listed for the purpose
intended, as designated by local applicable code.
B. Where rainwater harvesting pipe and potable water pipe are installed in the same trench, wall
cavity, or other location, the potable water pipe shall be separated by a minimum distance of
twelve inches (12") above the rainwater -harvesting pipe. Both pipes shall be installed below
local frost depth.
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9.7.1 Piping Materials
A. Rainwater distribution water piping, fittings and other related system components shall be
suitable for domestic water application as indicated in the applicable local building and / or
plumbing code.
B. Plastic piping shall be protected from UV radiation by a factory apply protective coating, or
painted with a compatible latex paint. Piping and solvent cements shall be approved for the
intended use.
9.7.2 Labelling.
If a Rainwater Harvesting System is applied to any building, facility or residence, it shall be so
indicated as follows:
A. All rainwater supplied fixtures, not specifically treated for potable water use, shall be
prominently labelled
B. Non-potable water piping shall be designated by coloured bands and solid colour piping as
specified by the authority having jurisdiction or national code agencies, and labelled:
C. Outlets and fixtures served with harvested rainwater shall be easily recognizable by colour or
a symbol for non-potable water.
Inspections: Rainwater harvesting systems are considered a private water system under the
responsibility of the building owner / operator, and shall be minimally inspected according to the
following schedule:
A. Inspection of all elements before they are covered (rough-in inspection)
B. Final inspection including testing.
C. In addition to testing required by the code for plumbing systems, the following also apply:
1. Testing and Commissioning
Piping. A flow test shall be performed through the system to the point of water
distribution and disposal. In addition, the water distribution system shall be tested and
proved tight at the operating pressure. Where the manufacturer permits, a 50-psi
hydrostatic test may substitute for the test above. All lines and components shall be
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D. Other inspections as needed to assure proper system operation.
System Maintenance: It is the property owners’ responsibility to maintain the system
components according to manufactures written recommendations.
Rainwater harvesting systems shall be maintained in functioning order for the life of the system.
Filtration and Disinfection systems shall be serviced in accordance with manufactures
B. System Abandonment. If the owner of a rainwater harvesting system elects to cease use of,
or fails to properly maintain such system, they shall abandon the system. To abandon the system,
the system owner shall minimally:
1. Remove or disable all system connecting piping to utility provided water system.
2. Replace the rainwater harvesting pipe system with an approved potable water supply pipe
system. Where an existing potable pipe system is already in place, fixtures may be reconnected to the existing system.
3. Secure cistern from accidental access by sealing or locking tank inlets and access points,
and / or filling with sand or equivalent.
9.8.1 Collection Surfaces for potable water applications shall be as noted in 23.1.1 above but
shall also be made of non-toxic material.
A. Painted surfaces are only acceptable if paint has been certified to ensure the toxicity level
of the paint is acceptable for drinking water contact. Lead, chromium or zinc based paints are
not permitted.
B. Enamelled Steel.
C. Flat Roofs: Roof products shall be certified to NSF Protocol P151.
D. Collection of water from vehicular parking surfaces is prohibited.
Not approved for potable water
E. Wood / Cedar shake roofing.
F. Copper roofing materials.
G. Lead flashing is not approved for potable water.
Not Recommended for Potable Water or to be used with caution.
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H. Bitumen / Composition roofing.
I. Galvanized, zinc-coated metal.
9.8.2 Cistern Inlets:
A. Methodology of water entering cistern shall be to maintain A quiet flow A in the cistern by
minimizing splashing and disturbance of sediment in bottom of cistern.
B. For potable water applications, and recommended for maintaining good water quality, the pipe
entering the cistern shall terminate in a return bend elbow pointed upward at the bottom of the
tank, or equivalent calming device.
Cistern outlets shall be provided with floating inlet to draw water from the cistern just below the
water surface.
A. Alternate: Cistern outlet to be located at least 4 inches above the bottom of the cistern.
Cisterns shall be intended for potable water use.
A. Cisterns shall be certified for use with potable water with NSF, or recognized equivalent.
Plastic tanks shall be constructed of virgin plastic and shall adhere to requirements of NSF /
ANSI Standard 61.
B. Cisterns shall not be connected directly to a public or community water supply without
approved back-flow protection. Make-up water to rainwater storage tanks, when provided,
may be made through a reverse pressure principle back flow device or an air gap per local
plumbing codes.
C. If installed below grade, cisterns shall be separated from sanitary waste piping a distance as
recommended by local authority having jurisdiction, or local plumbing codes, and up
gradient from septic field piping where applicable.
9.8.3 Filtration
A. Carbon filtration may be provided for reduction of taste, odour and organic chemicals.
B Filtration and Disinfection systems shall be located after water storage tank and as close to the
final point of use as possible.
C. All particulate filtration shall be installed upstream of disinfection systems.
D. Filters shall be adequate size to extend service time and must be comply with NSF / ANSI
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9.8.4 Water Disinfection
A. Chlorination: Chlorination may be used with an automated demand feed system, and if used,
shall enable adequate contact time and residual according to local health authorities.
B. Ozone: Ozone may be used with an approved ozone system ensuring adequate contact time
with the ozone. Provision must be made to off- gas ozone to a safe environment.
C. Ultra-violet disinfection may be used and shall be provided between final filtration (5 micron
maximum) and final point of use.
Operation and Maintenance
A. After several cycles of rain harvesting, a initial sample of the resultant accumulated water
shall be tested for compliance according to procedures listed in the latest edition of Standard
Methods for the Examination of Water and Wastewater (ALPHA).
B. For a Private System, prior to placing the water system into service, water quality testing, at a
minimum shall be performed for E-Coli, Total Coliform, and heterotrophic bacteria.
Subsequent periodic testing to assess the ongoing integrity of this system is recommended.
C. For a Public System (defined as a system where 25 different persons consume water from the
system over a 60 day period):
1. In addition to the above tests, water shall be tested for cryptosporidium.
2 Subsequent annual tests shall be made for Total Coliform, E Coli, Heterotrophic bacteria
and any chemicals of concern.
3 Records of test results shall be maintained for at least two (2) years.
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