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, lighterweight 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 as 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.

IMAGE COURTESY OF INTERFACE ENGINEERING For SERA Architect’s deep energy retrofit of the Edith Green Wendell Wyatt federal building in Portland, Ore., Interface Engineering took a former shooting range in the building’s basement and repurposed it as a 160,000-gallon storage tank where rainwater is collected and reused for toilet flushing, irrigation and cooling tower use

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 showcasing their commitment to sustainability, providing an educational opportunity and leveraging the system as a PR opportunity.

“We’ve used everything from below ground prefabricated cisterns, to using unused space underneath a 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 multi-use development with several parking decks, utilizing spaces within the deck could be a costeffective 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 their client’s 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, and for another project, the bottom of a high-rise elevator tower was repurposed as a storage tank.

“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.

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.”