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After the first part of our four-part Rainwater Harvesting webinar series, Rainwater Harvesting as a Runoff Reduction Tool, participants had the opportunity to ask our Rainwater Harvesting experts questions.

We didn't have enough time in the hour to answer all of the questions, so we are posting them here so that they are available to all.

These were the top 9 questions asked about Rainwater Harvesting Applications:

  1. Q: If using only rooftop runoff for irrigation and toilet purposes, are the treatment requirements different than that of parking/overland flow? Where can I see data to explain this?
    A: Pollutant loading conditions will vary based on the type of roof, parking lot material, type of development, and maintenance practices. In almost all cases, though, it is likely the rooftop water will be “cleaner” than the parking runoff. Even though the rooftop water may be cleaner, the same level of treatment would be required for both water sources for the same application because you must remove pollutants of a given size from either source.

    Because loading is very likely to be higher for water from the parking lot and finer filtration is recommended for toilet flushing, designers should utilize an automated filtration system if the source water is from the parking lot or the usage is toilet flushing. This will help ensure lower maintenance and more uptime, while providing an appropriate level of water quality for the application. For either source, the water should be disinfected for the toilet flushing application.
  2. Q: Is there an issue of mixing condensate water with rainwater during cistern overflow events?
    A: This could be an issue if there are metals in the condensate water and your SW system does not enter a combined sewer (where the condensate would have ended up anyway). We suggest designing the system so that the stormwater overflow bypasses before the cistern and overflow does not go through the system. The condensate could be designed the same way, except bypass to the sanitary sewer. This way, the only exit from the cistern is through the treatment system and intended application.
  3. Q: Can cistern water be used for fire suppression?
    A: Technically yes, and we do provide cisterns for fire suppression. Fire suppression requires a permanent reservoir, so it would be separate from the harvesting system to ensure there is always water available in case of a fire. Ideally you would fill it once and never need it.
  4. Q: There are chemicals from roofing materials and mechanical systems on the roof. How do you treat the water in cisterns if you want to use the water to water vegetable or fruits grown on the roof top?
    A: There are many advocates of harvesting water for gardens, livestock and commercial agriculture. We are not aware of any guidelines that cover this issue specifically, but agencies would likely be conservative, and require more testing or treatment. There are studies that show rain, before it hits the roof or ground, can be sufficiently contaminated by particulates in the atmosphere to cause concern. This is usually near heavy traffic or industrial locations. The roofing material can also contaminate water as you noted, especially asphalt shingles and some metal roofs.

    There are also many examples of harvested water with very high water quality, which provides the advantage of a non-chlorinated source which is favorable for crops. For this application, we would recommend testing the runoff from the catchment before deciding on using the water for this application. Based on the results, appropriate treatment could be recommended.
  5. Q: I’m also looking at trying to recycle pool discharge and backwash water instead of discharging it off-site. Can elements of these systems be applied to this source?
    A: Pool water would be relatively clean when only dealing with overflow or pump down flows. Backwash water could have a high degree of biological and particulate content and would not be appropriate for a RWH system. That may be managed under greywater regulation and plumbing codes.
  6. Q: What about water from a dehumidifier?
    A: Essentially this is condensate that forms within the dehumidifier that drips off to a drain. So it would be similar to HVAC condensate supply.
  7. Q: What is subsurface irrigation?
    A: Subsurface irrigation consists of drip irrigation with drip lines buried below the surface.
  8. Q: Is there any risk with washing clothes or dishes with non-potable water?
    A: Non-potable rainwater should not be used for dishwashing unless the water is treated to potable standards and this is allowed in your jurisdiction. Laundry is a great choice for non-potable water, but final filtration and UV disinfection should be used to ensure proper water quality.
  9. Q: What kind of client/developer is installing these kinds of systems? Military? Commercial? Etc.?
    A: We have worked on RWH designs for a wide range of project types including retail, commercial office, multi-family housing, single-family housing, hospitals, libraries, universities, schools, parks, military bases and more. In addition to runoff reduction and water conservation, many designers are targeting LEED certification.

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