The Stormwater Blog 

Contech Stormwater experts discussing Low Impact Development, Onsite Water Management, Rainwater Harvesting and all things Stormwater.

24 Rainwater Harvesting Design Questions

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 24 questions asked about Rainwater Harvesting Design:

  1. Q: Are there any issues with rainwater harvesting and water rights issues?
    A: Yes, water law can come into play and limit or prohibit rainwater harvesting and it is important to understand the legality in your state.

    There tend to be three views at the state level – restricted, passively or partially approved, and allowed or encouraged. Colorado and Nevada severely limit rainwater harvesting and it is not an accepted practice for commercial scale projects. Most other western states allow harvesting or provide clarification on what is acceptable. Washington State, for example, allows collection from a rooftop. Water rights are not an issue for many states in the eastern portion of the country. Many states, Texas and Georgia for example, encourage rainwater harvesting.
     
  2. Q: For large building areas, say 9 acres, does the engineer need to account for roof leader capacity?
    A: The building designer typically follows code requirements for draining the roof so the roof leader, roof drains and downspouts should be sized appropriately with or without a rainwater harvesting system.
     
  3. Q: How intensive are plumbing codes for non-potable water for indoor uses such as toilets for buildings?
    A: They can vary widely. The first priority is to follow guidelines and requirements to prevent cross contamination. We recommend chlorination to provide residual disinfection capability for this application.
     
  4. Q: If sprinkler irrigation water is from a pond with no cross connection to public water, do you need purple pipe and or backflow preventer? If a backflow preventer is required, what type?
    A: Local codes may vary on this issue. It is good practice to label any fixtures that connect to a non-potable supply. If there is no physical connection between the irrigation system and municipal supply than the cross connection is eliminated. It would still be good practice to place a typical irrigation backflow preventer to keep the water from flowing back into the pond. In this case though, there would be no way to irrigate if the pond is empty, except filling the pond with a backup supply.
     
  5. Q: Is it practical to retrofit a rainwater harvesting system into an existing building and if so, what might be the complications?
    A: Yes, it is possible. While practical for irrigation, installing the non-potable piping system within the walls may be cost prohibitive if you are trying to connect numerous bathrooms to the retrofit system.

    If a major renovation is underway and the walls or open, it would be more practical. Or, if you were supplying one source like a large HVAC unit, installing a single non-potable line may not be prohibitive. Some designers and owners are thinking ahead and adding the dual plumbing to new buildings and holding off on the actual RWH system in anticipation of adding one in the future.
     
  6. Q: On the balanced approach – why not have two storage calculations – one for water conservation and another for disposal and sum value for design storage?
    A: Our RWH calculator provides the designer both the volume and percentage of runoff reduction, and in addition, the volume and percentage of water savings. In some cases, you can provide a very high percentage of runoff reduction but low percentage of water savings, which is good if your goal is to meet stormwater requirements, but not as good if you are trying maximize conservation.

    More often, though, to increase runoff reduction, designers need to find more ways to use the water and increase the conservation than they may have otherwise. It is also common to combine a RWH system with detention where the cistern volume is optimized and housed in a water-tight cistern and the overflow is directed to a traditional detention system.
     
  7. Q: Where can I find local rainfall data for various areas, say Kentucky?
    A: NOAA (National Oceanic and Atmospheric Administration) has vast amounts of rainfall data from thousands of gages all over the country, many of them for 50 years on more. You can download smaller data sets or purchase large amounts of data for a modest fee. Our RWH model has included data from approximately 8,500 gauges from 1980-2000. We rejected data from thousands of other gauges because there were significant gaps in coverage where the data was deemed faulty or the gauge was out of commission.

    Whatever your source, it is important to ensure the data is complete and accurate. One participant in our webinar provided the following comment on this subject: “In the Midwest, regional governments such as Sewer Districts, City DPW-Water Depts, and Regional Planning Commissions collect accurate and continuous rainfall data. I can find water data with a one mile range of a project in the Milwaukee area from our Sewer District.”
     
  8. Q: What computer applications are available for designing these systems and how flexible are they?
    A: We have used these three calculators that include runoff reduction and can be useful for designers:

    Contech’s UrbanGreen Runoff Reduction Calculator – web hosted, very flexible and includes rain data from 8,500 gauges across the United States.
    NC State – download program, very flexible, includes data from approximately 15 cities, mostly in NC.
    VA Department of Conservation and Recreation – Excel download, moderately flexible, includes rain data from 4 cities in VA. Used by VA DCR to calculate runoff reduction credits.
     
  9. Q: What happens at the end of the life of the system? Does this just mean that the container is no longer water tight?
    A: Cisterns should be inspected annually for solids accumulation, leaks, and structural integrity. As end of life approaches, structural analysis should be performed to determine if the cistern should be repaired, remediated or replaced. In most cases, it would be expected for leakage to occur before structural issues develop. We offer Steel Reinforced Polyethylene cisterns with a very long design life because the structural steel is protected by the polyethylene shell.
     
  10. Q: While I follow the big picture numbers, unless you had an oversized storage system, you would not be able to capture the full stormwater runoff, correct? It would be good to see realistic sizing of systems and the costs involved for that average 1 or 2 acre site.
    A: It is easy to capture the entire stormwater design storm – the challenge is making sure the cistern is drawn down by the next large storm event. This depends as much on the demand for non-potable water as the size of the catchment. We have seen small buildings that use 15,000 gallons per day and 100% runoff reduction is practical. On the other hand, large warehouses have a lot of roof and don’t use much water, so no matter how large your cistern it will fill up and overflow eventually.

    Our Runoff Reduction Calculator provides graphs that show the water volume inside the cistern for several years, so you can see how often it is full, empty and when overflows occur. Try it out for one of your typical projects to see if RWH meets your goals. We’re happy to provide a size and cost estimate that takes into account your project variables, so you have a much more accurate estimate than a very broad general cost guideline.
     
  11. Q: Do the mechanical system enclosures have built-in freeze protection?
    A: RWH Mechanical System enclosures can be outfitted with insulation and thermostatically operated heaters. But housing a RWH Mechanical System outdoors is not recommended in areas that have prolonged freezing temperatures. If that is the case, either shutting the system down and removing the water from the components during the winter or locating the RWH Mechanical System in a heated indoor mechanical room is recommended.
     
  12. Q: For an area such as Ohio with a lot of salts used on parking lots, would you have a separate system for the parking lot vs. rainwater off the roof?
    A: You would either have separate systems or manage the combined water based on the source with the lowest water quality. The frequency and type of salting, along with the ratio of parking lot to rooftop, could predict the types of concentrations to expect. There may be significant dilution when combing multiple sources. Depending on the usage application for the capture water, it may be okay. The most conservative approach would be to focus on the rooftop and management the parking lot with a different practice.
     
  13. Q: Is there a temperature requirement for outdoor mechanical system?
    A: Areas with prolonged cold temperatures should employ good design practices to prevent freezing of cisterns or mechanical systems. Housing the cisterns unground below the frost line or insulating over the tank would be recommended. RWH Mechanical Systems should be housed in a climate-controlled mechanical room. If not used during winter months, the RWH Mechanical System could be housed in an unheated outdoor enclosure as long as the system is blown out and all the water is removed from freeze prone components.
     
  14. Q: Are there any filtration requirements for non-drinking water building usages?
    A: Treatment requirements for indoor non-potable uses are determined by local plumbing code. Disinfection is recommended which requires filtration as a pretreatment. Plus, the filtration improves the clarity of the water and which ensures acceptance by users. Ultraviolet disinfection and possibly chlorine treatment are the most common disinfection techniques.
     
  15. Q: I was unable to attend the previous two precipitation levels due to changes in my schedule. I’m wondering if I’ll be able to eventually have access to the difference in runoff calculation/and those presentations?
    A: The recorded sessions and slides are no longer posted on our website.
     
  16. Q: It was indicated that the steel reinforced polyethylene cistern had a design life of 75 years. What is the design life of the concrete and metal systems you presented in that slide?
    A: RWH system design life should correspond with the expected design life of the site and there are two components: maintaining a watertight cistern and the structure integrity of a buried structure. Many cisterns rely upon a pond liner to create a watertight system. The most cost effective liners have a design life of 20-30 years, so the life of many cisterns is limited by the liner. Concrete cisterns can have structural design life of 75 years or more, but this depends on soil type, loading, and quality of the water stored. Similarly, the structural design life of Contech’s underground metal cisterns can exceed 75 years in the right conditions. We are currently conducting lifecycle testing to predict the life of the watertight sealing. Steel Reinforced Polyethylene has both a watertight and structural design life of 75 years.
     
  17. Q: Can we avoid the dual pluming by connecting the make-up water to the tank?
    A: No. You will still need a potable line to sinks, showers, and other potable sources, and a non-potable water line to the nonpotable applications, such as toilet flushing. The two common ways to provide a back-up water supply are using an air gap, where the back-up water enters the cistern or wet well and then must be pumped to the final source, or using a 3-way valve where municipal water directly pressurizes the existing non-potable line when the cistern does not have sufficient water. In either case, a non-potable would be required to service your non-potable applications.
     
  18. Q: I know many of Contech’s products can be modeled with HydroCAD. Are there any plans to work within Civil 3D’s application Storm and Sanitary Analysis?
    A: Currently we do not have plans to add our products to Civil 3-D’s storm and sanitary analysis. As modeling software continues to advance, we may look at adding our products to various programs.
     
  19. Q: All jurisdictions I have worked with in the stormwater codes require a reduction in the post-developed runoff for a specified storm event, which require large amounts of runoff storage. Is this amount of storage possible, and financially practical, within underground structures?
    A: Our calculator allows you to input a design storm depth you need to manage. Overflow up to this depth counts against your runoff reduction and overflow after this depth is discounted. We find cistern sizes are often similar to water quality volumes, which isn’t surprising because they are both trying to manage the majority of annual runoff.

    One option designers pursue is to optimize your cistern based on your runoff reduction or water savings goals and store the remaining volume in a detention system. Once your cistern volume is full, the overflow can be routed to a detention system. This ensures that your cistern is full and ready for use before spilling over to the detention portion. Contech has different cistern material choices including watertight metal, Steel Reinforced Polyethylene, and concrete. We also offer economical underground detention and infiltration systems that can save valuable land space.
     
  20. Q: Is the system at all beneficial for average single family home?
    A: We have explored several projects where the developer would provide small system for each house or provide a community system for irrigation. This type of approach may become common in the future.

    Small retrofit rainwater harvesting systems including rain barrels for watering plants can be beneficial, but currently Contech does not provide these small systems. Large home improvement stores now carry the necessary downspout connections, rain barrels, and small pumps for home owners to install.
     
  21. Q: Is there a way for the mechanicals to know when water in the cistern is low and automatically switch to the municipal system?
    A: Yes. Most systems incorporate a level sensor that relays the cistern water level to a controller that interfaces with the pump. When the cistern reaches a low set point, the pump will turn off and make-up water can directly pressurize the line feeding to your non-potable application via a 3-way valve and backflow preventer. In jurisdictions that do not allow direct pressurization of the non-potable line and require an air gap, the low level in the cistern can also control a valve to fill a small portion of the cistern or wet well to allow the pump to continue pumping water to the non-potable demand.

     
  22. Q: What’s the footprint on the mechanical system?
    A: The mechanical system footprint will vary depending on the final filtration, disinfection, and level of controls. Many systems using a final filter, UV disinfection, a submersible pump that is outside of the control skid, and a control panel will be roughly 3’W x 5’ L x 3’ H.
     
  23. Q: What is make-up water?
    A: Make-up water is a secondary water source to supplement the system when the cistern water level is low. For critical demands such as toilet flushing, it is vital to have a make-up or secondary source of water to ensure an uninterrupted flow in the event that the rainwater harvesting cistern does not have sufficient capacity. This is commonly municipal water or well water.
     
  24. Q: Do you offer any project design assistance?
    A: Yes. Contech has a team of Design Engineers that are knowledgeable about stormwater treatment, storage, and rainwater harvesting design. Please visit our local resources page to get in touch with your local Design Engineer.

Post Rating

Comments

There are currently no comments, be the first to post one!

Post Comment

Name (not required)

Email (not required)

CAPTCHA image
Enter the code shown above:

Stormwater Blog Update Hubspot subscription form

We respect your privacy,
your email will only be used to send
Stormwater blog updates.