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

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 U.S. 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 and 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 10 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 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.

LEED Approved

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, LEED 2009, buildings can now earn up to 10 points for aggressive water use reductions. As for LEED’s upcoming version, LEED v4, 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 2009 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 like 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 and 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” 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 and drive the payback down 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., MBA, president, Design-Aire Engineering, Indianapolis, agrees that a straight payback against utility-provided water is not very attractive, generally speaking. But if the cost of utility impact and tapping fees could be included, in addition to credit for reduced stormwater runoff charges, then rainwater systems become more appealing.

In a similar vein, Van Giesen believes that defining ROI based upon the rainwater system costs versus 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 effect 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 price of water—at which point it will be easier to make a business case for rainwater harvesting.

Storage System Selection and Sizing

When selecting a storage tank, precast concrete is typically the least expensive option in terms of dollars per gallon, but the heavier tanks are most costly to transport. At the same time, concrete can also serve as part of the building structure. Alternatively, lighter-weight polyethylene and fiberglass tanks come at a higher first cost, but are more easily transported.

Generally speaking, Rattenbury often specifies plastic for smaller systems, segmented corrugated metal tanks with a liner for intermediate-sized systems and concrete for larger tanks.

If plastic is selected, the tank must be opaque since sunlight promotes algae growth in stored water. Also, if the harvesting water is intended for potable use, then the plastic tanks need to be manufactured from virgin plastic, as opposed to recycled plastic, to prevent chemicals from leaching into the water, says Boulware, former president of the Rainwater Catchment Association.

In addition, all storage tanks must be covered to keep away mosquitoes, sealed to prevent ground leaks, installed on stable foundations and securely covered to eliminate the threat of drowning incidents.

Another decision is whether to bury the tanks or keep them above ground. Practically speaking, underground tanks keep the water cool and out of the sunlight, thereby better protecting it from bacteria. And in the wintertime, the ground heat helps prevent freezing. On the other hand, excavation is an added expense, plus the fact that building owners sometimes prefer to display their rainwater harvesting system as a visual statement to showcase their commitment to sustainability, provide an educational opportunity and leverage the system as a PR opportunity.

“We’ve used everything from below-ground prefabricated cisterns to using unused space underneath parking garage ramps as storage space. I don’t think there’s a wrong answer and each project will be different,” says Jones. “For a 40,000-square-foot office building with no garage, a below-ground prefab system might make most sense. For a multiuse development with several parking decks, utilizing spaces within the deck could be a cost-effective way to achieve harvesting.”

In order to enable building owners to maximize their real estate, Interface Engineering is known for coming up with innovative locations for its clients’ storage tanks. For instance, Interface utilized an abandoned rifle range to function as a 167,000-gallon rainwater tank for the recent modernization of the U.S. General Administration’s Edith Green Wendell Wyatt Federal Building in Portland, Ore., and the bottom of a high-rise elevator tower was repurposed as a storage tank for another project.

“The elevator stopped at the ground level but was constructed through the structure’s three levels of underground parking, resulting in a 90,000-gallon rainwater tank and fire sprinkler onsite water supply combination,” explains Jonathan Gray CPD, principal, Interface Engineering, Portland, Ore.

Taking a unique approach to storage tank design, some engineers will even try to highlight the system as an architectural element.

For instance, the Lady Bird Johnson Wildflower Center at the University of Texas in Austin prominently displays its cistern on its site. Built from native sandstone, the tank blends seamlessly into the landscape.

“Immediately, water and the idea of water as a scarce resource enters your consciousness, and I think it changes your entire experience for the better,” offers Jones. “The more sustainable features become ‘design’ features, the more we will understand both their intrinsic benefit.” In terms of sizing the system, this is an important decision as oversizing can be an inefficient use of resources, whereas sizing the unit too small can compromise rainwater harvesting opportunities.

When determining the optimal size, predicted rainfall and project water uses are part of the equation, in addition to other water detection strategies that are part of the overall stormwater management plan.

“We also try to look at how to maximize the amount of water we harvest, reduce overall water use in the building onsite and look at innovative ways to fill the tank when there is no rain—such as water recycling and catchment from chilling units—to come up with the most appropriate size for the project,” says Jones.

At the same time, Rattenbury points out that at $2 to $5 per gallon—plus excavation costs, if applicable—the cistern is one of the most expensive components of the rainwater system. So, right-sizing is key.

Although at present there are no comprehensive guidelines available in the U.S. that establish a best practice, there is a European Standard called DIN 1989-1:2001-10 “Rainwater Harvesting Systems - Part 1” where the annual water use requirement is compared to the average annual yield of rainwater possible from the catchment area and the annual rainfall. According to Rattenbury, specifiers are instructed to choose the lesser value of the two and multiply it by 0.06 to establish the working volume of the cistern. This means that the cistern is sized for 6 percent of either the use or supply, which is approximately three weeks worth of water storage.

“However, I have found that this method can lead to oversized tanks in some applications. The same European standard also says that the tank size is best optimized by simulating the precipitation and the consumption in daily time steps. The standard recommends a 5- to 10-year simulation, but the question is: How does one simulate rainfall?”

Based upon historical rainfall data, Rattenbury has set up a sizing spreadsheet that uses rainfall data over a 50-year historical period as an analog to the anticipated rainfall behavior 50 years in the future. Using this methodology, he has found this to be the optimal way to size cisterns.

“For example, for the LEED WE 2 and WE 3 credits, only about a 10 to 15 percent additional reduction in water use is necessary to achieve the maximum points allowed,” he relates. “I have found cisterns of 5,000 to 7,500 gallons are usually sufficient to supply the quantity needed for an office building, and have the benefit of overflowing frequently so that old water is replenished with new.”

Drainage and Filtration

The most commonly used approach for directing rainwater to the rainwater system is traditional gravity roof drainage. However, designers are beginning to look at siphonic drainage systems, which are quite popular in Europe. In addition to allowing for smaller pipe sizes and draining large commercial roofs more efficiently, siphonic systems offer more flexibility in the realm of water storage as siphonic action is used to transport the water as opposed to relying on gravity and a pitched pipe, explains Boulware.

While Rattenbury sees the technology starting to catch on, he points out that a significant learning curve—which requires plumbing designers to perform the necessary hydraulic analysis—hinders its growth.

One approach which Jones often takes when designing the drainage systems is to look for interesting and innovative ways to shine a spotlight on the water—for example, via an exposed downspout—to really turn it into a design feature.

“By exposing water during a rain event, it can really show people how much water is actually coming off of the roof, which is really an amazing amount, and what we’re doing with the water,” he explains. “By really highlighting it, water becomes a feature and people begin to look at it differently.”

But regardless of the drainage system design, pre-filtering the water before it enters the cistern is a crucial part of the rainwater system.

As such, Hofmeister commonly specifies Vortex-type filters which separate any solids from the rainwater. The technology is much more efficient than the old-school approach, which involved using floating suction filters and baffles to remove the solids after they had already entered the tank.

Another strategy is only harvesting the rainwater after the “first flush,” which is the first part of a rain event where contaminants and dust are flushed away. The technology used to facilitate this is inexpensive and well-suited for small to medium-sized systems. However, for larger facilities, the first flush volume becomes too large to be practical, according to Boulware.

But Rattenbury is not a big fan of first-flush diverters. He says that they are prone to clogging and can require frequent maintenance. Instead, he prefers self-cleaning, non-clogging cistern inlet screens. A fairly recent innovation, the industry appears to quickly be adopting it.

In addition to the inlet screen, Rattenbury includes a few other filtering components including:

• An inlet diffuser that maintains a quiescent flow of water into the cistern to keep any silt or sediment from being disturbed in the cistern;
• A floating inlet or intake on the transfer pump to ensure the best quality water is used from the top of the tank; and
• A skimming overflow to allow floating contaminants like pollen to be purged from the tank.

In addition to these passive technologies, sediment filters are also utilized to further process the water. Rated by particle sizes, these filters are selected based upon the end-use requirements, whether it’s toilet flushing, irrigation or some other use.

As for disinfection, chlorine injection is popular as it can treat large volumes of water at a relatively low cost. While chlorine does a good job with bacteria, it’s not as effective with viruses and spores, such as giardia and cryptosporidium, which are undesirable parasites.

“A popular alternative, particularly used on small systems, is ultraviolet light preceded by filtration to a minimum of 5 micron,” recommends Boulware. “This serves to immobilize the viruses to a better degree than chlorine, and the systems are safer to use, being essentially a device that allows water to pass through and be exposed to a UV light.”

One other strategy is ozonization, which can be generated on demand and offers an effective, chemical-free method of disinfection, according to Rattenbury. However, the ozone generation equipment does come at a cost.

Overall, with filtration, disinfection and pressurization available in a pre-packed skid with all the pre-fabricated components, Hofmeister appreciates the convenience of this plug-and-play type system.

Stormwater Management

Looking at the bigger picture, stormwater management, which rainwater harvesting can contribute to, is a larger issue that building owners are becoming increasingly responsible for. In fact, in New York City, a recently passed a regulation requires 97 percent of stormwater to be retained onsite.

Raising the bar even further, a high quality stormwater management plan can even achieve a net-zero hydrologic impact. In the case of a greenfield project, this means that the new building and site does not negatively affect the surrounding natural hydrological systems by adding runoff or pollutants, for example. As for a brownfield project, the stormwater management plan seeks to improve the site’s ecology, or at least repair it, according to Jones.

Of course, a basic principle in stormwater management is the fact that the ground naturally absorbs a good proportion of rainwater. However, once a structure is built on a site, that ability is compromised.

For instance, if a 6,000-square-foot building sits on a 10,000-square-foot site, this means that more than half of the site’s absorption capability has been compromised with an impervious surface.

“For obvious reasons, this new 6,000-square-foot building cannot percolate that rainwater into the ground, so the remaining 4,000 square feet is called upon to do that. Most often, it cannot. And in the old days, that 6,000-square-foot building was allowed to directly connect its downspouts into the storm sewer that ran down the street or alley,” explains Richard Hayden, ASLA, CLARB, national garden roof department manager, American Hydrotech, Chicago.

However, as urban development increased, permeable surfaces decreased. Eventually, many municipalities were forced to add restrictors—i.e., reduced openings—to the pipes that fed their sewer systems in an attempt to manage the volume of stormwater.

“This would deliberately create a backup situation for the building’s developers, forcing them to store the stormwater somewhere on the site before it was slowly released into the sewer through this small restrictor plate,” he explains. “This gave rise to storing the water in parking lots or underground in large pipes or cavernous concrete structures. In suburban areas, water was stored in ubiquitous detention/retention ponds that dot the landscape.”

With the goal of enabling as much stormwater as possible to be absorbed into the ground before entering the sewer system, the EPA is currently encouraging the following best management practices (BMP):

• Reduce the amount of stormwater generated;
• Delay the release of any stromwater generated to the stormwater sewer system;
• Increase the quality of stormwater generated to make the stormwater cleaner before it enters the system.

Consequently, strategies like bioswales, infiltration beds, rain gardens and permeable pavements are becoming more common. However, each of these requires at-grade land space, which is expensive real estate, particularly in urban areas.

This being the case, Hayden sees green, vegetated roofs as becoming an important stormwater BMP as they take advantage of this under-utilized space and offer good absorption levels, not to mention adding a beautiful aesthetic to the property.

“In urban environments such as Philadelphia, green roofs have become the stormwater management tool of choice,” reports Mark T. Celoni, P.E., vice president and office director of the Philadelphia-based civil engineering firm, Pennoni Associates. “Many sites in central Philadelphia have most, if not all, of their lot covered by their building footprint. So, the lack of site area makes implementation of traditional stormwater tools impractical.”

In fact, according to Green Roofs for Healthy Cities, a green roof can retain as much as 70 to 90 percent of precipitation, depending upon the plants and growing medium depth. As for the water that does run off, the green roof serves as a natural filter and also delays the runoff, thereby reducing stress on the sewer system at peak flow times.

At the same time, Jones points out if green roofs are solely considered as a stormwater management tool, the investment can be difficult to justify.

“Fortunately, they have many more benefits such as reducing building energy costs, extending the life of the roof membrane and reducing the urban heat island effect,” he reports. “When combining all of these benefits, they begin to make economic sense, although reducing stormwater infrastructure and upfront stormwater fees are a big part of their value.”

Quantifying Green Roof Performance

While green roofs can be an effective stormwater management strategy, one of the challenges associated with their application has been reliably predicting their performance capabilities.

For example, the civil engineering industry can accurately calculate storage capacities underneath permeable pavement systems, but growing medias in vegetated roofs are so varied—with different combinations of plants, aggregates, sand and composts—that it is difficult to quantify performance.

Toward the end, an ASTM testing standard was created to establish:

• The media’s permeability—how fast water moves through it;
• Total saturated density—how much the media weighs when totally saturated with water;
• Total pore space—the air volume within the media.

Taking the test results from its growing media, and combining it with civil engineering calculations based upon climatological data and stormwater modeling concepts, Hydrotech has created a special software tool that displays green roof performance data in a hydrographic format that is familiar to civil engineering professionals.1

In particular, this proprietary tool incorporates storm intensity, storm duration and stormwater flow data—which is available from U.S. government agencies—and selected area of watershed and release rates of that watershed.

“The Hydrotech tool is very effective, and there are some other independent calculators that can be used as well,” observes Jones. “Ultimately, one has to find numbers that the municipality you are working with will accept, because part of the goal is to reduce the amount of water one is required to detain and/or associated stormwater fees, and this is constantly evolving as municipalities begin to recognize green roofs as an acceptable stormwater runoff mitigation tool.”

In particular, Hydrotech’s Hydrology Tool (HHT) calculates how vegetated roofs impact stormwater runoff volumes and the rate at which runoff is slowed, demonstrates potential LEED compliance and performs long- and short-term simulations.

“The HHT software allows for the easy creation of ‘what-if scenarios’ where various combinations of media depths, vegetated roof areas and drainage/retention components can be modeled to optimize the building owner’s needs for stormwater storage,” Hayden explains.

To demonstrate the differences in total flow and runoff, HHT can be used to overlay a hydrograph curve for a standard ballasted roof, a vegetated roof and a combined roof. Although the curves will vary depending upon specific vegetated and non-vegetated roof areas, the media depth and any drainage and/or retention components used in the assembly, general estimates show that a garden roof will delay stormwater runoff by more than five hours, according to Hayden.

In addition, the garden roof reduces the runoff by more than a half, as compared to the ballasted roof.

“This combination of delayed and lower stormwater flows are very valuable to reducing the hydrological impact of a building on a storm sewer system,” notes Hayden. “The delays mean that the storm sewers can have a chance to empty enough to accept the new water flows and the flow reduction means that the storm sewers have less water to accept.”

The Outlook

Green roofs aside, experts predict that rainwater systems will continue to gain acceptance, albeit slowly, thanks to its recent addition to the national plumbing codes and support from LEED. Granted, it will take some time until the national codes are adopted by local municipalities, but Jones anticipates that in the long run, many cities, particularly major urban areas which are already leading the charge, will adopt green infrastructure master plans that incorporate rainwater harvesting, as well as green roofs and bioswales.

In addition, Rattenbury predicts that in the long term, the rising costs of water coupled with more frequent water restrictions and bans will make rainwater harvesting a much more attractive option.

“As an ever-increasing focus is directed to the dispelling of myths surrounding rainwater collection and the celebration of its virtues, rainwater harvesting as an industry will flourish and the built environment will be better for it,” concludes Van Giesen.

Take the quiz!