Most generally available information for harvesting precipitation focuses on rainwater harvesting for individual homeowners or similarly small ventures, but techniques for harvest and reuse can be implemented at almost any scale, ranging from individual homes to large multi-story buildings and sprawling commercial developments. The demand of any individual installation depends on water quality, proposed end-use and any applicable regulations.
Define Objectives
Overarching objectives for implementing a stormwater harvesting are typically necessary to justify the time and expense in retrofitting or expanding beyond business-as-usual practices for handling stormwater. Examples of some valuable objectives include
- Reduce potable water use
- Reduce stormwater fees
- Reduce pollution levels in stormwater runoff
- Demonstrate environmental sensitivity
Adoption of specific goals that align with regional or corporate goals may help in procuring funding for the project.
Identify Volume and Exposure of Captured Stormwater
When evaluating the potential use of captured stormwater, the provenance should be carefully considered since the concentration and types of likely pollutants will determine water quality and the need for treatment before use. Estimating the volume of captured rainfall will help determine the necessary storage volume as well as provisions for overflow in the case of exceptionally severe storm events and will likely inform the most suitable end use.
Calculate Expected Quality of Captured Stormwater
In some cases, the season and timing of stormwater events can affect water quality. This should be accounted for since seasonal reduction in water quality may affect its suitability for certain end uses without additional treatment.
- In zones with seasonal rainfall, for example, the first storm event following a long, dry summer will carry particularly high levels of oil and grease from road surfaces.
- Snowmelt tends to have higher pollutant concentrations than runoff from the same surfaces at other times of the year.
- While green spaces such as parks and lawns generally produce higher quality runoff than other sources, they may reflect elevated levels of nutrients and pesticides in the spring and early summer. In these spaces, pets and wildlife (especially the ubiquitous Canadian Geese) also leave feces and other waste which can produce very high levels of pathogens.
Irrespective of season, certain surface types are typically associated with specific pollutants. Even relatively “clean” runoff from roofs is associated with certain types of pollution. Some metal roofs, for example, are associated with higher levels of toxic metals, while asphalt and composite shingles may have been treated for mold or with herbicides. Cedar shakes and wood shingles as a class are often treated with fire retardants or other chemicals to discourage mold, algae, and insects.
Green and brown roofs, in comparison, are associated with comparatively low concentrations of pollutants, since the vegetation does a good job of filtering sediment, but runoff still doesn’t meet strict potable water standards. Overall, regardless of the roofing material, runoff should also be considered to include potential pathogens from bird or animal feces and airborne pollutants that may have settled onto any roof surface.
Determine End Uses
Harvested stormwater can be appropriate for a wide range of uses, depending on quality, harvestable volume of the source water, expected level of human exposure, and any treatment processes employed.
Potential beneficial uses for rainwater and stormwater include:
Outdoors
- Sanitary sewer flushing
- Irrigation
- Outdoor washing (vehicles, buildings)
- Firefighting
- Water features such as fountains
- Street cleaning and dust control
Indoors
- Fire suppression
- Cooling
- Process/boiler water
- Flushing
- Washing (bathing, laundry, dishwashing)
- Drinking water
Ultimately, the quality of harvested rain and stormwater must be matched with the intended beneficial use. However, the specific quality requirements of a particular use may vary with circumstances, and any mismatch between the two must ultimately be rectified with appropriate treatment processes.
Some types of treatment are best suited to use before harvested water is stored. The effects of these steps are permanent and won’t erode over time when the water is stored for later use. Other treatment protocols are best initiated after storage, especially when the full effect is relatively short lived, as with disinfection. We’ll discuss the specifics of stormwater treatment in the next article.
Collecting and Storing Stormwater
At the most basic level, stormwater harvesting systems perform four primary functions: collection, storage, treatment, and distribution.
Collection systems may incorporate elements of an overall stormwater management system and may even incorporate existing infrastructure such as curbs, gutters and storm sewers. Roof drains and downspouts from buildings, parking lots and other structures could be an integral part of an existing design, or an external retrofit. Ground level surface runoff from the local catchment area may include excess water captured under pervious pavement and runoff from vegetated swales, typically conveyed using pipes and channels.
The design of your collection system will be influenced by the size and location of your storage tanks as well as the intended beneficial use of the collected water. When appropriate, it may be advantageous to incorporate treatment practices, such as filtering, directly into the collection sequence.
The size of the conveyance components is determined by the volume of water the collection system is expected to handle, with an eye on managing extreme storm events where both volume and flow rates will be elevated. If the conveyance components and backup overflows are undersized or poorly maintained, there is always a risk of damage and injury.
Stormwater collected from ground level surfaces is usually stored in stormwater ponds or underground tanks, while higher quality water collected from roofs and similar areas may be stored in cisterns, ready for disinfection and use.
It’s important, in rooftop collection, to avoid using materials that could contribute more pollutants to the equation, such as metal roofing that contains copper and lead. Other roofing materials that can lower water quality include asphalt shingles, wood shingles, and any materials that contain chemicals which are known to pose health risks.
