BioCycle Magazine

Compost in the mix for storm water management

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Bioretention areas, also known as rain gardens, are increasingly being used to capture and treat storm water runoff from impervious surfaces such as roofs, sidewalks and parking lots. These landscaped areas often feature native plants installed in an engineered soil mixture that includes compost. “Bioretention areas or rain gardens are an attractive storm water Best Management Practice (BMP) because they can improve the environmental quality of water while meeting landscape requirements,” says Wayne King, Sr., owner of ERTH Products, LLC in Peachtree, Georgia. According to the 2001 Georgia Storm Water Management Manual, a properly-sized bioretention area with three to five percent organic content can remove 80 percent of total suspended solids, 60 percent of total phosphorus, 50 percent of total nitrogen and 80 percent of heavy metals.

The engineered soils and plant material use absorption, microbial action, plant uptake, sedimentation and filtration to reduce peak flows and volumes of storm water and control pollutants. “Once the water slows down, it loses its ability to carry pollutants such as sediment,” explains King. “The suspended pollutant particles settle to the bottom of the bioretention cell where microorganisms sequester and break down the pollutants.”

ERTH (Environmental Resource and Technology for Humanity) Products manufactures and engineers specialty soils using compost and lightweight aggregates. Its product, ERTH Food Compost, is made from biosolids and peanut hulls composted in aerated static piles at the company's plant outside of Plains, Georgia. Compost is sold in both bagged and bulk form.

King says his company has provided engineered soil mixes for more than 50 bioretention/rain garden projects over the past five years. Ecos Environmental Design, Inc. was the landscape architect for two recent projects - the Christopher W. Klaus Advanced Computing Building at Georgia Tech in Athens and the Southface Eco Office in Atlanta. Both projects were seeking LEED certification. (LEED is the Leadership in Energy and Environmental Design green building rating system developed by the U.S. Green Building Council.)

Ecos selected ERTH Products to provide approximately 500 cubic yards of engineered soil mix to the Klaus site and 30 cubic yards to the Southface site. The two projects originally had specifications of 20 to 30 percent leaf litter compost for the engineered soil mix, but ERTH Food Compost was substituted instead. “That type of leaf compost is very seasonal in Georgia and there wasn't enough available locally,” explains King. “With LEED projects, you try to do everything close to the site and not bring trucks in from out of state.”

Stephen Brooks, Vice-President of Ecos, adds that when the Klaus building was being designed, there were few companies providing any type of engineered soil mixes, especially at the volumes needed for the project. “We had previous experience with ERTH Products and were really pleased with their ability to obtain such a quality product in such large volumes. It only made sense to carry forward that relationship into the Klaus project.”

In November 2004, the Georgia Department of Community Affairs received grant funds from the U.S. Environmental Protection Agency (EPA) to showcase use of engineered soils for storm water management and develop Best Management Practices. “We needed to do more outreach and education and have some models to spotlight,” says King. “We wanted to educate the development community, architectural, engineering and landscape design firms, university researchers and educators, state agencies that establish BMPs, municipal and commercial compost producers, and other interested parties about how compost and engineered soils can successfully be used in bioretention areas. We targeted the Klaus Building as a demonstration site.”

The 414,000-sq.ft. Klaus Building contains the Schools of Computing, Engineering and Electrical Engineering and has 70 laboratories and a three-story parking deck. The building is on a 6-acre urban campus site where over 50 percent has been preserved as greenspace.

One of Georgia Tech's requirements for the Klaus Building was that 100 percent of the first flush of a storm event (the first one inch of rainfall) be contained on the site. (Most pollutants in storm water are carried off-site during the first flush of a storm.) “There is a 30-foot grade change from the rear to the front of the building,” says Brooks. “The design challenges for us were how to handle the grade change and the erosive properties of storm water runoff.”

Ecos' solution was to construct nine stone walls and install six bioretention cells between the stone walls. The walls are made of native granite, and have an interior chamber. They are connected to the building with rain scuffers that collect water coming off of the Klaus building's roof. The water travels into the stone wall chamber, and is dispersed into the bioretention cells through openings along the walls. The cells have an engineered soil mix and native perennial plants that store, infiltrate, evaporate, filter and slow the velocity of storm water runoff.

In cases of overflow, water is routed around the end of the stone walls to the next lower bioretention cell and finally to an overflow drain at the lowest elevation at the end of the chain of bioretention cells. “By locating the overflow at the lowest point rather than along the way, we maximized the opportunity for infiltration throughout the entire chain of cells,” says Brooks. The storm water that the bioretention cells don't absorb is captured by a drain underneath the bioretention cells and sent down to two large, concrete cisterns - one in front of the building and the other behind the building. The rear cistern is linked to the front cistern, which in turn is linked to the municipal storm system as an overflow. Total capacity is 175,000 gallons. “Any water collected by the overflow is routed back to the cistern and only if the cisterns are at capacity does the storm water flow offsite to the municipal storm system,” he adds.

Condensate from the building's cooling tower and groundwater from the site are also fed into the cisterns. All of this water is used to irrigate the building's grounds during droughts through a series of irrigation pumps and sprinkler heads.

The earthwork for the bioretention cells began in early 2006. Ecos' specifications for the engineered soil mix - the same for the Klaus and Southface projects - included 20 to 30 percent ERTH Food Compost, 50 to 60 percent sand and 20 to 30 percent topsoil with a clay content of under 5 percent. “We try for environmental reasons to use the existing soil that is on the site,” says King. “We test it on site and determine its characteristics and then add the necessary soil amendments for stability, permeability and fertility. Because on-site soil was not available for the Klaus or Southface projects, we had to blend the mixes off-site, using sand mixed with topsoil taken from other land-disturbing activities, as well as the compost and ERTH-Hydrocks expanded clay,” says King. After blending, samples were sent to a laboratory for testing to ensure they met Ecos' standards and specifications and conformed to the Georgia Storm Water Management Manual. To build the bioretention cells, Ecos excavated four feet, laid down filter fabric and covered it with gravel. An underdrain pipe was placed within the gravel bed and then the entire assembly was wrapped with a filter fabric and connected to the cisterns. To maintain positive drainage within the bioretention area, the grade changes in front of the building were transitioned by installing large boulders - salvaged while excavating the foundation of the building -in the bioretention cells. The engineered soil mix was dropped into each bioretention cell using a front-end loader and then spread by hand. The final grade was done by hand application in 8 to 12-inch layers to achieve an even level of compaction. “Compaction is a real issue, so you don't want to dump the soil mix in all at one time,” says King. Each layer was lightly watered and then left to dry for two days to ensure good settlement of the soil mix. The process was repeated three times until the proper design elevation was reached. “Klaus was a great example of doing it right,” he adds.

Native river rock was placed by hand over the bioretention soil mix once it had settled. “Ecos specified nonmechanical compaction to ensure that the final product maintained a high infiltration rate,” explains Brooks. “The round shape of the rocks and their larger size helps to ensure the flow of the water is being continually divided into smaller flows during smaller rain events, thus reducing the velocity of the flow.”

A mixture of native Georgia plants was used, including hydrangea, rhododenrum, senecio and river oat. “The plant materials are suitable for this type of application and have the tolerance to withstand regular inundations of water,” says Brooks. The plantings began in fall 2006 and were finished in the spring of 2008. “Since certain varieties of the specified plants were unique Georgia native material, they had to be specifically grown for the project to achieve the quantities needed,” he adds.

The rear of the Klaus Building has the only lawn space on the site, which is used for social events at the university. The same ERTH engineered soil mix was used in this lawn space as was used in the bioretention cells in the front of the building. “This allows us to capture the water shooting off of the hardscapes and infiltrate it below ground,” says Brooks. A series of underdrains collect excess water and route it to the cisterns.

The Klaus Building received a Gold LEED certification (the second highest) in May 2008 because of the storm water collection system as well as its energy-efficient heating and cooling systems, waterless urinals, and use of recycled materials. Points are assigned in the LEED program, which ultimately decides the project ranking. “The bioretention areas certainly did contribute to the Gold LEED certification,” notes Brooks. “While it's tough to say if its contribution was 'the' point that made the difference between silver and gold, definitely every point does become very important in the collective process.”

ERTH Products donated 30 yards of an engineered soil mix including compost for a rain garden/bioretention area designed by Ecos Environmental Design that collects rainwater on the site of the new Southface Eco Office in Atlanta. The rainwater is filtered by the rain garden and then collected in an underground cistern. “Everything, including the rain garden, is intended to serve as a demonstration of best practices from an environmental sustainability standpoint,” says Frank Burdette, project manager for Southface's Commercial Green Building Services. “We were looking for an engineered soil product that would serve to percolate rain water and capture it in the site's cistern. We relied on Ecos to specify the product and that's when Wayne King came in and donated it.”

The cistern and rain garden were designed to contain storm water from smaller events on the site, keeping it from emptying into Atlanta's overburdened sewer system. The harvested rainwater will be used to flush toilets, operate mechanical heat exchangers in the Eco Office building and for drip irrigation, drastically reducing reliance on the city of Atlanta's water supply, which has been suffering from a long-term drought.

“Our rainwater harvesting program, in conjunction with use of low-flow fixtures, will allow us to use 84 percent less water, approximately 130,000 gallons a year less, than if we were to install conventional fixtures and not capture rainwater,” says Burdette. “The use of drip irrigation to establish our drought-resistant landscaping is also a factor in that reduction.” The Eco Office is being built by The Southface Energy Institute, a nonprofit organization in Atlanta that promotes sustainable homes, workplaces and communities through education, research, advocacy and technical assistance. It works with architects, homebuilders and homeowners to incorporate green building practices. ERTH was one of 100 local businesses that donated materials and labor to construct the Southface Eco Office and landscape the site.

The 2,040 cubic foot cistern was buried under 16 inches of the ERTH soil mix. The cistern, which can store up to 14,500 gallons of rainwater, was constructed of 160 RainTank Modules manufactured by an Australian company. Each module is 13 cu.ft., and made from recycled plastic panels that snap together to form cubes resembling milk crates.

Rainwater from the green roof of the Eco Office and part of the adjacent Southface Energy and Environmental Resource Center flows through six-inch wide PVC drain pipes into the cistern. Roof runoff from the atrium connecting the two buildings is directed via granite runnels to a dry streambed planted with native vegetation, which flows to the rain garden. Storm water runoff from the entire site is also being collected through this streambed.

To install the bioretention cell, a 25-foot by 50-foot trench was dug and lined with a bed of 8-inch deep crushed recycled concrete stones. The trench was then lined with a 30 millimeter EPDM rubber membrane that was placed over the crushed stone to contain rain water. The RainTank Modules were placed on top of the rubber membrane, then covered with a layer of filter fabric to help prevent sediment from entering the top of the cistern. The filter fabric was covered with six to eight inches of recrete and another layer of filter fabric was placed over the recrete.

A blower truck then installed the 16 inches of engineered soil media over the top of the cistern assembly. “It's important that you make sure that you don't dig the hole and have it fill up with water before you get the engineered soil in,” notes King. “If there are several rain events, the hole can get filled up with sediment.” Ecos specified the native perennial plants and adapted species that would be planted in the ERTH soil mixture.

The area around the cistern was backfilled with crushed recrete and a temporary Type C silt fence made of fabric over woven wire was erected at the perimeter of the bioretention area. The silt fence will be removed once the plants are established. “It's the first time we've built a bioretention pond at Southface and worked with this type of an engineered soil mix,” says Burdette. “We were told to keep the soil matrix as clean as possible. It acts as a filter and if it became contaminated, it would affect performance.”

Filtrexx International socks were installed along the border of the bioretention pond to capture concentrated storm water flows as well as provide structural protection, erosion control, filtration, vegetation and vegetation reinforcement. “The Filtrexx socks will break down from UV rays after a number of years, at which point the plants will have stabilized the streambank,” notes Burdette.

A 70-foot by 30-foot concrete wall detention pond was constructed around the bioretention cell to slow the runoff of a 100-year storm event. “The detention pond will make us compliant with state regulations even if we have a full cistern,” he adds. Deep Root Tree Root Guides were installed around the perimeter of the cell to prevent tree roots from puncturing the cistern's membrane. The units are 24-inches tall and made from postconsumer recycled plastic.

The Southface Eco Office is in the final stages of construction. Documentation has been submitted for LEED Platinum certification, the highest level that can be achieved.

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