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 8 questions asked about Rainwater Harvesting Treatment:
- Q: With harvesting off of parking areas, how do you handle oil and grease?
A: Some pretreatment systems can baffle a large portion of the oil and your cistern should include a floating outlet or have a submersible pump. In either case, the inlet to the pump is both above the very bottom of the cistern and below the surface. So most of the oil would be trapped at the surface and can be removed during maintenance.
- Q: Please explain how the first flush containing undesirable contaminants is scalped away to avoid entering the cistern.
A: Opinions vary on the contaminant loading of “first flush” and the term itself has different meanings in a conventional RWH context versus a stormwater context. Some stormwater research shows the potential for a seasonal first flush, but suggest pollutant loading during a storm is associated more with rainfall intensity, not the timing.
Many RWH guides recommend bypassing the first portion of a storm event, in the range of 0.05”. Depending on the rainfall patterns, 0.05” can easily equate to more than 10% of the annual runoff. This is likely a low estimate, because it is based on total daily rainfall. However, rainfall often occurs multiple times per day, and can create multiple first flush volumes.
Any water bypassed by a first flush diverter still needs to be managed according to the SW requirements. We recommend capturing the entire storm if possible when the primary goal is runoff reduction; otherwise we’d let 10%+ of the runoff go past before the first drop is captured.
- Q: Are there any strategies you can recommend for runoff from parking areas where snow melt chemicals are used so this salted runoff can be saved and reused for irrigation?
A: Dissolved salts and chemicals are not able to be removed from the water with just filtration. You most likely will need an advanced treatment process, like reverse osmosis, to treat the water which may or may not be feasible.
- Q: Is the CDS pretreatment filter you presented a Contech product? Where can I find more information?
A: Yes, the CDS is a Contech product. Learn more about the CDS pretreatment unit here.
- Q: How would the engineer develop the mechanical control panel for these systems? Is a turnkey solution provided by the product vendor? Who are some typical control panel suppliers?
A: Contech offers UrbanGreen Rainwater Harvesting Mechanical Systems which are turn-key, fully integrated systems consisting of the
pumps, filters, disinfection, make up water valves, PLC controls and enclosures necessary. Engineers can specify the
equipment and rely upon the winning contractor to procure and assemble the components. No matter how the system is
designed and built, it is important to ensure a robust solution that will last for many years into the future.
- Q: Does the polyethylene or other plastic cisterns have any environmental concerns with chemicals leaching out the plastic? Even with non-potable uses, some people may be concerned because of the bph scare in water bottles, this may be a concern with clients who are nervous of plastic chemical leaching (for instance, when using harvested rainwater to water veggie gardens).
A: The resin used in Contech’s Steel Reinforced cisterns does have NSF 61 certification for potable use, but this standard may not fully address the concerns of some consumers. For more sensitive applications, like gardening, the local rain chemistry and materials used in the catchment area, piping, and cistern should be vetted against the goals of the user. 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; 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 pretreatment and treatment could be recommended.
- Q: Is it wise to separate storage systems for the varying types of water sources ie rainwater separate from HVAC and so on?
A: It can make sense to store HVAC condensate in a separate cistern from your roof or hardscape rainwater runoff if you have various demand applications that need different levels of treatment.
An example would be if you collect your HVAC condensate and recycle it back with very little treatment as the make-up water to the HVAC. This may allow you to reduce your treatment level if you can use the roof and hardscape rainwater that has additional pollutants for irrigation or another demand with lower treatment requirements.
- Q: What about building code restrictions on how long water can be stored for grey water use?
A: Over time, greywater will turn into black water, so many regulations limit maximum holding time to 24 hours. Greywater requires a high level of pretreatment to prevent lint, debris, and floatables from entering the cistern, and a high level of final treatment to ensure proper water quality.