How Rainwater From Roof Is Captured, Reused By Leon County Sustainable Demonstration Building
Another key element of the project was the creation of a system to capture and reuse rainwater from the roof of the Sustainable Demonstration Building.
The rainwater system uses four repurposed, 10,000-gallon storage tanks that are buried on the site and connected with a roof collection system that can convey water at 1 cubic foot per second, so that roof drainage is not jeopardized even in the heaviest downpour. The cisterns were salvaged, single-wall fiberglass underground gasoline storage tanks that were thoroughly cleaned, repaired, and customized to serve their new purpose. The cisterns operate in pairs, and each pair has a 1.5 horsepower pump and an air bladder to pressurize it to 35 to 85 psi, so that it can be used by the microjet irrigation system.
Rainwater is filtered by gutter screens and a series of debris filters before it enters the cisterns. When water is pumped from the tanks, it passes through a hydrodynamic separator and polishing filters to prevent debris from clogging the irrigation system. Moisture sensors and “smart” monitoring devices wired into irrigation controllers further reduce the amount of water needed to keep the plants healthy.
An inch of rainfall falling on the roof is capable of generating 9,050 gallons of water stored in the cistern tanks. /the project effectively removes 11,614 square feet of impervious surface (roof) from the property – and reduces the velocity of runoff – reducing the need for traditional stormwater management at the site. The system retains the ability to use potable water from the city when the storage tanks are not able to maintain sufficient water levels.
A teaching center on the property displays an 8-foot diameter “slice” taken from an underground storage tank. The on-lookers are also shown how to gauge tank contents making use of the graduated “gauging stick.” In future, float gauges will be added to automate this process, perhaps even integrating them electrically with the tank pumping controls.
The rainwater system was designed entirely by facilities management staff and installed with assistance received from other county departments. Private industry partners were essential to developing certain water components, including HDPE “Smart Boxes” that were used to screen and remove rooftop particulates from the water stream, also to safely direct the flow of water away from the tanks once these were filled. These components were designed by facility management engineers, then made and donated by a plastics firm located in North Georgia. Leon County is currently seeking a patent for the water reclamation, storage, and re-use system that the facility staff developed.
Historical water use at the Extension Center averaged approximately 570,000 gallons a year of potable city water, but individual years have been as high as 890,000 gallons. Based on average Tallahassee rainfall of 63.21-inches per year, the rooftop collection system was expected to reduce demand for city water to just over 100,000 gallons in an average year. In 2013, the Extension Center purchased about 240,000 gallons of water. But that is not to say that the water conservation goals were not met. The Master Gardner programs at the Center were expanded, and extensive replanting of trees on the property taxed the onsite irrigation system. Although water from the cisterns is not metered, a review of water bills from suggests that Leon County purchased at least 300,000 gallons less water for the Extension Center than it would have without the rainwater system.
Because water in Tallahassee is available at low prices, the impetus for this rainwater capture project was an ethical concern for stewardship, rather than a financial motive. The system helps to protect the underground water budget of the Wakulla Springshed that underlies Tallahasse. What’s more, the city’s stormwater has a very large environmental footprint, and it is costly to taxpayers to maintain the city’s drainage infrastructure to prevent flooding. In the future, facilities that install cisterns could gain stormwater treatment credits if the role of rainwater harvesting in attenuating stormwater runoff is recognized and duly accounted for. Rainwater harvesting also presents an alternative to construction of stormwater facilities.
Tom Brantley is director of facilities management and construction for Leon County, Fla. The Leon County department of facilities management was an FMXcellence honoree for delivery of a new, 100,000-square-foot, mission-critical public safety complex that consolidated five emergency service functions of the city of Tallahassee and Leon County.