Learning Objectives - After reading this article, you should be able to:
- Gain perspective on how rainwater harvesting and vegetated roofs contribute to an overall stormwater management plan.
- Perform sound calculations for right-sizing rainwater cisterns.
- Identify good specification strategies for rainwater storage tanks, drainage, filtration and disinfection.
- Quantify green roof performance in terms of stormwater runoff rates and volumes.
|1 AIA LU/HSW|
|1 GBCI CE Hour|
Driven by LEED certification, net-zero building trends, and local codes and standards, stormwater management-traditionally falling under the local municipality’s realm of responsibility-is increasingly being relegated to facilities through their building and site designs.
In fact, stormwater management laws and regulations enacted at the state level, which incorporate language from the Environmental Protection Agency’s (EPA) National Storm Water Program, typically mandate that the post-development peak discharge rate and volume of direct storm runoff from a given site cannot exceed the pre-development peak rate and volume. Furthermore, the “first flush,” which is the initial flow from a rain event, must be collected within a certain amount of time in order for adequate chemical and biological mitigation to take place, according to John Rattenbury, P.E., LEED AP, associate, plumbing & piping group, R.G. Vanderweil Engineers, Boston.
On the one hand, some may view this as an additional burden, but on the other, sustainably-minded designers and owners see stormwater management as a great opportunity.
“People have long considered stormwater to be a waste stream that must be disposed of as soon as possible. But over the past ten to 15 years, there has been a significant shift in this attitude which now recognizes the value of stormwater for irrigation, groundwater recharge and other uses,” observes Rattenbury.
But not only that, treating and pumping water from a centralized water supply system, through a vast network of piping to its point of use, is a costly endeavor. Sourcing water onsite, via rainwater harvesting, is a much more efficient approach, says Edward G. Van Giesen, ARCSA, Masters in Landscape Architecture, policy coordinator, BRAE Rainwater Harvesting Systems, Athens, Ga.,
Bringing up another point, Van Giesen, a regional representative of American Rainwater Catchment Systems Association, explains that the water shedding off of roofs and parking surfaces carries all types of pollutants, which incidentally the U.S. EPA identifies as the number one cause of water pollution in the country. By redirecting stormwater into rainwater catchment systems, where it is then treated, the remaining runoff is generally of better quality.
PHOTOS COURTESY OF XERXES
By buying the rainwater storage tanks underground, the water stays cool in the summer and is protected from freezing in the winters.
Based on the water efficiencies it offers, LEED is very supportive of rainwater harvesting systems, as evidenced by the fact that each new version of the rating system has offered more points for achieving increasingly higher levels of water savings. For example, back in 2004, when Version 2.0 was released, water conservation measures and alternative water sources could earn a total of five points. While Version 2.2, released in 2005, still offered five points, the provisions were better defined.
Under the current Version 3.0, buildings can now earn up to 10 points for aggressive water use reductions. As for LEED’s upcoming Version 4.0, experts anticipate that the point system will give even more weight to water conservation.
Breaking down the current 2009 version, buildings can earn credits for the following:
- 20 percent building water use reduction prerequisite
- Water Efficiency (WE) Credit 1- 50 percent irrigation water use reduction, two points, or elimination of potable water for irrigation, four points
- WE Credit 2- 50 percent potable water reduction for sewage conveyance, two points
- WE Credit 3- 30 percent overall potable water use reduction, two points, or a 40 percent reduction, four points
Offering some commentary on how to achieve these levels, Rattenbury explains that the use of high efficiency fixtures can achieve close to 40 percent savings for Credit 2, “but to get up and over the 50 percent bar, rainwater harvesting has to be employed.”
Because LEED 3.0 lowered the baseline flow rates for public lavatories from 2.5 gpm to 0.5 gpm, this means that it is no longer possible to achieve an overall potable reduction of 40 percent for Credit 3. Consequently, rainwater harvesting must be utilized to reach a 40 percent reduction.
In addition to LEED, a number of other programs are also very supportive of rainwater harvesting including Sustainable Sites and the National Association of Home Builders’ National Green Building Standard.
“These programs all recognize the ecological and environmental importance of rainwater harvesting and offer substantial points for achieving important thresholds,” explains Richie Jones, RLA, LEED AP, partner with the Nashville-based landscape architecture firm, Hodgson & Douglas. However, “there are only so many ways to reach these reductions, and rainwater harvesting can be a crucial strategy for achieving these.”
In the Real World
Despite LEED’s most favorable treatment of rainwater harvesting, at this point, market penetration is still somewhat limited and unlike other sustainable technologies, such as solar power, there are few incentives supporting it.
For example, while it’s quite common to see tax rebates or utility incentives for onsite power generation, such as the ability to sell excess power back to the grid, no such benefits currently exist for rainwater harvesting systems.
“Energy costs for buildings are much higher than water use costs, so the focus on alternative energy, together with the political objective to reduce foreign energy dependence, ends up getting much more attention than water, which has no foreign supply issue involved,” observes Tom Tietjen, vice president, sales & marketing, Xerxes, Minneapolis.
Jones also points out that water isn’t perceived as a scarce, limited resource. Although this line of thinking is slowly changing, it will take time for this perception to change.
Another limiting factor is the fact that unlike solar panels, which can be fairly easily installed on existing buildings, rainwater harvesting is not so retrofit-friendly and requires a certain amount of space for tank storage, not to mention changes to the roof drainage system and a new piping system to deliver the harvested water to the plumbing fixtures.
Furthermore, there is a lack of qualified engineers when it comes to designing rainwater systems, combined with a lack of guidance from the local codes, although a number of organizations are currently working on developing a rainwater standard (See sidebar “Coming Soon,” on pg. XX for more.)
Ultimately, Van Giesen predicts that as rainwater harvesting technology improves and wider adoption occurs, the system’s benefits will become more apparent and will result in louder calls for incentives.
Of course, “all of us in the rainwater harvesting industry would like to see the incentives arrive sooner rather than later,” he adds.
Working Out the ROI
Another tricky point is the fact that the return on investment doesn’t always work out so favorably for rainwater catchment systems.
For example, if a facility is paying $4 per 1,000 gallons of water consumed, and a small, $25,000 rainwater system would save 250,000 gallons per year, then this would equate to $1,000 in annual water savings. However, it would take 25 years to earn back the investment and this doesn’t include electricity and maintenance costs.
Take a larger $200,000 system as another example. If the rainwater could provide at least 750 gallons per day of flush valve supply, this would save close to 200,000 gallons per year. However, the payback would be more than 60 years. But if the rainwater system was designed to supply water to other building systems, such as cooling tower make-up water, then this would dramatically increase the water savings, driving down the payback to just seven years, according to David Hofmeister, P.E., plumbing/fire protection manager, Bala Consulting Engineers, King of Prussia, Penn.
E. W. Bob Boulware, P.E., M.B.A. president, Design-Aire Engineering, Indianapolis, agrees that generally speaking, a straight payback against utility provided water is not very attractive. But if the cost of utility impact and tapping fees could be included, in addition to credit for reduced stormwater run-off charges, then rainwater systems become more appealing.
In a similar vein, Van Giesen believes that defining ROI based upon the rainwater system costs vs. municipal water costs is only part of the picture. “Builders today are required to build stormwater drainage infrastructure with no regard to its ‘payback.’ The same applies to handicapped parking spaces, tempered glass, fire sprinklers, smoke alarms and fire hydrants. These are all parts of what comprises a civilized and safe built environment,” he argues.
That being said, Van Giesen views rainwater harvesting as a technology whose benefits have not been entirely valued for all that it accomplishes.
“Besides being an extra source of water for toilet flushing, cooling towers, chillers and outdoor irrigation, it provides reduction in pollution resulting from impervious surfaces, reduction in runoff volumes resulting from heavy storm events, and consequently fewer opportunities for raw sewerage to overflow into receiving waters when combined sewer overflows exceed their capacity,” he explains. “Harvesting rainwater also decreases the burden on both the supply as well the drainage side of the municipal infrastructure, and it increases the resiliency of the water supply, not only for the individual building parcel, but also for the community as a whole.”
Taken alone, rainwater harvesting does offer site-specific benefits, but Jones believes that the real value comes in when implemented over a large scale in urban areas where the sum affect is greater than the parts. “These individual projects then become integral solutions to some of our most pressing urban infrastructure problems,” he says. “As cities face ever-increasing financial strains, these strategies should be seen as innovative ways to reduce long-term government infrastructure costs.”
However, those capabilities are not rewarded, and at present, water is viewed as a relatively cheap commodity. Consequently, rainwater harvesting is merely embraced as a “best management” practice. On the other hand, Jones anticipates that over time the availability of water will decrease, as the global demand steadily increases, eventually leading to a market which will have to bear the trice price of water, at which point it will be easier to make a business case for rainwater harvesting.