Rainwater harvesting

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Rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. is commonly employed on zero-lot-line projects like downtown condos. This cast in place concrete vault in one corner of the parking level is a typical design.

This article is about large, integrated rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. systems. For smaller, seasonal, outdoor, residential systems, see Rain barrels

Overview

Rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. (RWH) is the ancient practice of collecting and storing precipitationAny form of rain or snow. for later use. Although Ontario is a region with relatively abundant fresh water, RWH is increasing in popularity for a number of reasons:

  1. The simplicity of selecting and installing a system, owing to improvements in the technology and the development of a local industry,
  2. The ease of modelling RWH in a stormwater management (SWMStormwater Management) plan, owing to the fixed size of the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. and the cisternTank used to store rainwater (typically roof runoff) for later use.,
  3. Increasing transparency of storm sewer costs in some municipalities, and
  4. Increasing utility rates for potable water supply.

Rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. is an ideal technology for:

  • Sites which cannot infiltrate water owing to contaminated soils or shallow bedrock,
  • Zero-lot-line developments such as condos or dense urban infill, or conversely
  • Projects with extensive gardens and landscapes which would benefit from free irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 water.

The fundamental components of a rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. system are:

  • the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. area where the rain lands (e.g. rooftop),
  • a screen or filter to remove coarse debris (mostly leaves),
  • a cisternTank used to store rainwater (typically roof runoff) for later use. which will store the collected rainwater and preserve its quality,
  • the connecting pipe network including roof drains.

Additional components may include:

  • pumps to lift water to higher elevations, depending on the layout of the components,
  • additional water filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil. and treatment, depending on the intended use of the water.

Planning Considerations

An above-ground cisternTank used to store rainwater (typically roof runoff) for later use.. This system is not frost proof, and would have to be drained and closed during winter.
An above-ground cisternTank used to store rainwater (typically roof runoff) for later use. used for sub-surface irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47. A filter box is included on the roof drain, no further treatment is shown. The overflow is connected to an underground infiltration area.

Place in the Treatment Train

To maximize the volume of water available for reuse, a RWH system is usually located near to the front of a treatment trainStormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.A combination of lot level, conveyance, and end-of-pipe stormwater management practices.. i.e. Upstream of other LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. technologies. The most common exception would be where a site strategy employs a green roof. A simple warm-weather system may capture all of the rooftop runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. in an external tank above grade and use it for sub-surface irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47. In this scenario the tank can overflow via gravity to a sub-surface infiltration chamber or a bioretention cell. But, if the tank is located below the ground or inside a building the overflow may need a pump.

Catchments

The origin of the harvested water affects the permissible end uses. Some of the most common uses include irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 and flushing or toilets and urinals. As mixed source water can only be applied to the more limited end uses, selecting the catchments for a RWH system requires careful consideration.

  • The Ontario Building Code (OBC) regulates the use of harvested rainwater as one of many non-potable water sources. "Rainwater means storm sewageThe liquid waste from domestic, commercial and industrial establishments. City of Toronto Wet Weather Flow Management Guidelines November 2006 49 runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. that is collected from a roof or the ground, but not from accessible patios and driveways."
  • The CSA standard separates "roof runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface." from all other sources, including landscaped areas and green roofs. Collectively the green roofA thin layer of vegetation and growing medium installed on top of a conventional flat or sloped roof, also referred to as living roofs or rooftop gardens./landscaped and paved areas result in "stormwater runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.."

The current disparity between these two definitions affects all vegetated landscapes including green roofs. Confusion over terminology and regulation has been identified as a significant barrier to implementation of RWH since 2010[1].

CisternTank used to store rainwater (typically roof runoff) for later use. and Pipework

CisternsTank used to store rainwater (typically roof runoff) for later use. must be installed in locations where native soils or the building structure can support the load associated with the volume of stored water.
Expansion caused by freezing water will damage pipes, pumps and the cisternTank used to store rainwater (typically roof runoff) for later use.. There are two options for managing a RWH system in our climate:

  1. The entire system is drained and closed off ahead of sub-zero temperatures
  2. All pipework, pumps, filters and the cisternTank used to store rainwater (typically roof runoff) for later use. are protected from freezing during the winter

The first option may be suitable for systems optimized for exterior irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 only. But regulatory authorities may not permit the use of such seasonal systems as part of a storm water control strategy. Year round systems can be protected from freezing by locating the pipes, pumps and cisternTank used to store rainwater (typically roof runoff) for later use. indoors and/or below the frost penetration depth[2].

Design for Maintenance

Detailed inspection and maintenance advice can be found in Sustainable Technologies' LID I&M guide. The two primary operational concerns for RWH systems are:

  1. A leak developing,
  2. Debris obstructing some part of the plumbing.

Planning can help ensure that these are identified and fixed more easily and cheaply. Example questions:

  • Is the roof (catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.) readily accessible to sweep debris periodically?
  • Could the accumulation of debris on the roof be reduced by removing any overhanging branches?
  • Can the leaf screens accessed from the roof? Or from a maintenance room?
  • Will the cisternTank used to store rainwater (typically roof runoff) for later use. require entry for inspection in the future? How will this be accessed?

RWH systems producing higher quality water will have additional maintenance requirements. These will depend on the the treatment technologies being used.

Design

Large cisternTank used to store rainwater (typically roof runoff) for later use. being installed underground
Conceptual gif of an underground cisternTank used to store rainwater (typically roof runoff) for later use. being used for irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

Rainwater Harvesting: Sizing and Modelling

Catchments

Decisions need to be made about the selection and grading of catchments. If one catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. is very large, can it be regraded to drain to two or more outlets? Is it desirable to capture both rooftop water and other stormwaterSurface runoff from at-grade surfaces, resulting from rain or snowmelt events.? This will may determine the quality improvements required to use the water. See table below, which illustrates the higher treatment required for storm water (i.e. non-rooftop).

CisternsTank used to store rainwater (typically roof runoff) for later use.

Preformed above-ground tanks are usually constructed from polyethylene or galvanized steel. They are available with storage capacity up to around 50,000 L. Preformed below-ground tanks may be constructed from reinforced fiberglass or concrete. Fiberglass tanks are available up to around 150,000 L. Concrete vaults can be constructed in almost any size. Wooden tanks are less common but are also available and permitted in the regulations. As a standing body of freshwater, RWH cisternsTank used to store rainwater (typically roof runoff) for later use. present ideal habitat for mosquitoes. Mosquitoes should be prevented from entering by using a mesh screen on all openings. Larvicides may be added when the water is only to be used for irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 purposes. To prevent algal growth, the cisternTank used to store rainwater (typically roof runoff) for later use. must be opaque or otherwise protect the water from light.

Plumbing and Regulation

The current Ontario Building Code requires that rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. systems are designed, constructed and installed to conform to good engineering practice. References are made to ASHRAE, ASPE[3] and CSA [4] guides for plumbing detailing. These guides focus on ensuring that the rainwater does not contaminate or become mistaken for the municipal drinking water supply. Similarly, rainwater must be prevented from becoming contaminated from the sewer. In both cases, an air gap or a back-flow preventer is required.

Performance

Water Quantity

In theory a large enough RWH cisternTank used to store rainwater (typically roof runoff) for later use. could retain 100% of a single storm. However, sizing a stormwater cisternTank used to store rainwater (typically roof runoff) for later use. must account for regulatory requirements, available space, budget, and draw-down i.e. rate of use. If a RWH system is being employed for storm water control, the cisternTank used to store rainwater (typically roof runoff) for later use. size will typically be greater than that for optimized potable water use reduction.
In 2007-2010 STEP monitored and modelled three rainwater harvestingThe practice of intercepting, conveying and storing rainwater for future use. Captured rainwater is typically used for outdoor non-potable water uses such as irrigation, or in the building to flush toilets. systems in the Greater Toronto Area[5]. Each system was sized to balance stormwater management objectives with with potable water use reduction for irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 and toilet flushing. Around 18-20% of the precipitationAny form of rain or snow. was lost directly from the rooftop, and the annual stormwater capture varied between 18 and 42 %.

Water Quality

The important water quality parameters for harvested rainwater differ from other types of LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.. This is due to the potential for direct human contact, rather than environmental discharge. As of July 2017, the CSA and ICC are finalizing a standard[6] which specifies different water quality treatments according to the source (roof runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. versus stormwater runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface.) and the intended use.

Tier 1 end uses are most readily achievable, requiring only that larger particles are filtered out of the water. Removing the bulk of the solid particles reduces the nutrient concentration in the water and prevents clogging of the water distribution system. Toilet and urinal flushing are the next most popular use of harvested rainwater. If flushing or other higher tier end uses are desired, disinfection of some type is required and consideration may be given to colour and odour of the water. Technologies for achieving higher standards of water quality include:

  • Ultraviolet (UV) disinfection requires additional filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil. to remove particles so that the light can penetrate the water and destroy the viruses and bacteria,
  • Chlorine disinfection also requires additional filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil. to remove larger particles,
  • Micro- or Ultra- filtrationThe technique of removing pollutants from runoff as it infiltrates through the soil. uses such fine membranes that the vast majority of harmful viruses, bacteria etc. are excluded from the water directly.
All three water purification technologies require specialist design and consultation with a subject matter expert is recommended for higher tiers of water use.

Water quality treatment required for multi-residential and commercial applications (Abridged from CSA B805 draft)

Application Roof runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. pathogen reduction Stormwater pathogen reduction
End Use Tier Example uses Viruses Bacteria Protazoa Viruses Bacteria Protazoa
1
  • Sub-surface irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 (drip/bubbler)
  • Spray irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 (restricted access)
  • Fire suppression
  • Ice rinks
0 0 0 0 0 0
2
  • Toilet and urinal flushing
  • Clothes Washing
  • Rooftop cooling
0 99% 99% 99.99% 99.99% 99.9%
HVAC systems In accordance with ASHRAE 188
3
  • Hose bibbs
  • Pressure washing
  • Vehicle washing
  • Spray irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47 (non-restricted)
  • Decorative fountains
99.9% 99.9% 99.9% 99.99% 99.99% 99.99%
4
  • Human consumption
  • Oral care
  • Food preparation
  • Dish-washing
  • Bathing, showering, and hand washing
  • Pool/hot tubs/spas/splash pads
  • Misting stations
  • Swamp coolers
0 99.999% 99.999% Outside of the scope of the CSA standard

Source Water Quality

A study of many types of roof surfaces in Texas found:

  • 'Cool' membrane, concrete tile, and metal roofs all produced water of similar good quality for non-potable use,
  • The runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. water from asphaltA mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces. shingle and green roofs contained significantly more dissolved organic carbon (DOC). The DOC can add a yellow colour to the water. If the water is treated with chlorine, for drinking purposes, DOC can produce toxic compounds[1].

Research in Hamilton, ON assessed the water quality of rain collected from three highly reflective 'cool roof' membranes[2]. Key findings:

  • The water was free from significant contamination with by-products of plastic manufacture and did not show elevated levels of the five metals tested.
  • Increased microbiological contamination was found in runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from roof areas where ponding occurred.

Note: Increased microbiological contamination in roof runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. is also associated with warmer weather [3].

Incentives and Credits

In Ontario

City of Mississauga The City of Mississauga has a stormwater management credit program which includes RWH as one of their recommended site strategies[7].

LEED BD + C v. 4

Water Efficiency: Rainwater management (up to 3 points) Note that for lines 1. and 2. preference is given to LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting. that 'best replicates natural site hydrology':

  • Two points (or 1 point for Healthcare) will be awarded if the project manages "the runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from the developed site for the 95th percentile of regional or local rainfall events."
  • Three points (or 2 points for Healthcare) will be awarded if the project manages "the runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from the developed site for the 98th percentile of regional or local rainfall events."

OR
For zero-lot-line projects only, 3 points (or 2 points for Healthcare) will be awarded if the project manages "the runoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. from the developed site for the 85th percentile of regional or local rainfall events." This last clause relating to zero-lot-line projects is where RWH may prove most applicable compared to other LIDLow Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting..
Pilot Credits: Whole Project Water Use Reduction (up to 10 points) This pilot credit requires whole building water use modeling to demonstrate reduced water use compared to a baseline model. A sliding scale awards between 1 point for 10% reduction to 10 points for 65% reduction. Making this kind of water use reduction would typically require reuse of greywater as well as an optimized RWH plan.

See Also

External Links

Organizations

RWH Design guides

Proprietary systems

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.


  1. Mendez CB, Klenzendorf JB, Afshar BR, et al. The effect of roofing material on the quality of harvested rainwater. Water Res. 2011;45(5):2049-2059. doi:10.1016/j.watres.2010.12.015.
  2. Cupido, A., B. Baetz, Y. Guo, and A. Robertson. 2012. An evaluation of rainwater runoff quality from selected white roof membranes. doi: 10.2166/wqrjc.2012.011.
  3. Vialle C, Sablayrolles C, Lovera M, Jacob S, Huau MC, Montrejaud-Vignoles M. Monitoring of water quality from roof runoff: Interpretation using multivariate analysis. Water Res. 2011;45(12):3765-3775. doi:10.1016/j.watres.2011.04.029.