Keeping a LID on Onsite Treatment

Aug. 14, 2012

Low-impact development (LID) for stormwater management becomes a bigger challenge when a site has limited space. Such is the case at a high-profile sports project in California’s San Francisco Bay Area.

The site of a new stadium for the National Football League’s San Francisco 49ers in Santa Clara, CA, used LID. The stadium will be built on a highly developed, constrained 32-acre site near the California’s Great America theme park in Santa Clara, among other developments.

The $1.02 billion, 68,500-seat facility, which is being designed by HNTB and built by a Turner Construction/Devcon Joint Venture, is scheduled to open in time for the 49ers’ 2014 season. In January 2012, before work began on the stadium itself, the 49ers’ engineering and environmental consulting team member, GHD, the Santa Clara Stadium Authority, and the Santa Clara Public Works Department issued construction documents to Ghilotti Construction Company of Santa Rosa, CA, to relocate utilities that were in the way of the future stadium footprint. In addition to the relocation, other existing utility structures were augmented to provide required infrastructure for the stadium. This remainder of the horizontal infrastructure–sanitary sewer, storm drainage, domestic water, recycled water, joint trench underground infrastructure–is scheduled to be completed in late 2012.

The stadium project is an example of LID projects that are being successfully implemented with the aid of the latest stormwater management systems. In many cases, these systems are designed to be installed in space-constrained sites, giving project owners a sustainable, space-efficient solution to stormwater management challenges.
The stadium design and construction team is attempting to have the facility Silver- and possibly Gold-certified according to US Green Building Council Leadership in Energy and Environmental Design (LEED) criteria using the LEED NC (New Construction) rating system. LEED design features include the use of native or adapted landscaping plants, high-efficiency irrigation equipment, water reclamation for non-potable uses, photovoltaic panels, water-conserving plumbing fixtures, and recycled materials.
Onsite stormwater treatment is addressed, too. California Regional Water Quality Control Board San Francisco Bay Region C.3 requirements dictate that the design of the treatment soil within the bioretention system have a maximum infiltration rate of 5 inches per hour. A C.3 application, City of Santa Clara Stormwater Pollution Prevention Data Form, was filed for the project and has received the city’s C.3 approval.

“The main thing we’re trying to do here is recognize an 80% or higher reduction in total suspended solids [TSS] and provide full trash capture prior to discharge to the city storm drainage system–that’s the main water quality goal we have to meet” per LEED 6.2 requirements and the San Francisco Bay Regional Water Quality Control Board, says Adrian Clarke, civil engineer for GHD.

Mike Kincaid, P.E., PLS, senior project director for GHD, says the firm’s team determined that having bioretention facilities installed on the site was the right course of action for stormwater pretreatment. “There really weren’t too many viable options,” Kincaid says of limited space on the site alongside roadways, parking lots, and a large pedestrian plaza. After determining that the first choice–open vegetated swales–was not really possible on the constrained site, the team began to investigate bioretention, namely KriStar Enterprises’ Bio-Mod Filter Cell Module. The system suits non-proprietary low- or high-flow soils, and the site gets two-tenths-of-an-inch-per-hour rainfall intensity about 85% of the time, according to Kincaid. Significantly, the Bio-Mod system is designed to meet a maximum 5-inch-per-hour infiltration rate for the project.

The system uses a precast concrete biofiltration cell system that utilizes a piping system and various stormwater pretreatment modules. The system collects stormwater at ground level, and the effluent is diverted to these modules via the piping system. The system’s main module, the Bio-Mod Filter Cell Module, is filled with gravel, soil, and mulch, and it may be specified with open or closed bottom to accommodate infiltration or closed systems. The curb cut along the roadway directs treatment flows to the bioretention cell surface and bypasses high flow to the swales guard/swale overflow filter to the storm drain piping. The Grated Pre-Filter Module is designed like the Curb Inlet Pre-Filter Module but also allows for pedestrian access over the Bio-Mod System. The Swale Guard Overflow filter is one of the pre-filter modules that remove and retain gross pollutants such as trash, debris, and coarse sediment that can inhibit performance of soil-based filter systems. Trash Module and Light & Tree Modules are also scheduled for use later in the project.

“In the course of design, which was several months later, we found these bioretention facilities with the intent that we could get curbs in place and control storm flow and trash more effectively into these facilities,” says Kincaid. He adds that, along a curved road at the training facility parking lot, a standard Bio-Mod unit was installed along with a horizontal offset at the joint. Although the Bio-Mod system cannot have a vertical offset within a unit, it can be designed to separate one long Bio-Mod unit into multiple units and have a vertical offset between the units. This flexible solution is another example of the system’s fit to the site.
The system is expected to contribute one point according to the LEED 6.2 system. Bioretention within the project site was designed to capture 90% or more of the average annual rainfall and, per California Stormwater Quality Association (CASQA) guidelines, bioretention is capable of removing 90% of the average annual post-development TSS load. Bioretention can contribute one point rating per LEED 6.2 credit.

“I think it really came down to the site constraints,” says Clarke. “We have numerous other pipe networks under the ground; we have storm, sewer, recycled water and domestic water, and joint trench systems. When you’re trying to combine all of those systems and maintain the separation from each system, it became pretty difficult to go with any other system because of the unique site constraints.”

For example, vegetated swales were considered for installation next to the roadway at the Southern Access Road, but they would have required a 3:1 side slope or flatter with additional buffer zone for transition between the roadway and the swale. On this site, where only a 3-foot-wide strip is available for the treatment, though, not enough space was available for vegetated swales. In contrast, the Bio-Mod system is a good fit because it does not require side slopes and a transition zone. Furthermore, the treatment area can be maximized by doweling the roadway curb directly to the top of the Bio-Mod wall.

By the end of the first quarter of 2012, Ghilotti had finished installing two of four Bio-Mod Systems, along with more than 20 KriStar FloGard Media Filter Manhole, Media Filter Catch Basin, and Media Filter Vault media filtration devices at the stadium discharge storm drains. One Bio-Mod System was installed north of a parking lot for a team training facility and another was installed near the Southern Access Road. The other two systems were to be completed on the east and west sides of a south parking lot and on the west side of a pedestrian plaza by fall 2012.

Kincaid notes that all of the bioretention facilities were on track to be operational by winter 2012. He has good reason to believe that the installations would be a great fit for the site when they were up and running. “What we learned here is that we can get bioretention installed on a very constrained site and make it blend in aesthetically with what’s going on in a major facility,” he says.

Protection for an Impaired River
Civil engineers who work for the city of Chula Vista, CA, recently implemented an LID best management practice (BMP) on two space-constrained sites: along a roadway and at a small commercial development.

The roadway project in the city of 230,000-plus located about 8 miles south of San Diego involved the installation of a 22-foot Modular Wetlands System from Bio Clean Environmental Services along North Broadway Street, about a block away from the Sweetwater River Estuary. The Sweetwater watershed is listed as a 303(d) impaired body of water according to Clean Water Act standards due to high levels of nitrogen, dissolved solids, phosphorus, and other pollutants.

A plateau and steep slope lie between the street and the river, so runoff flows rapidly from the pavement to the river. Besides high pollutant levels, the runoff caused chronic flooding in the area. In 2008, the city of Chula Vista began working on the North Broadway Basin Project, which involved construction of two new storm drain systems located about a block apart to eliminate flooding upstream and convey runoff downstream. Because the pavement was being completely reconstructed, the project was designated a priority project by the San Diego Regional Water Quality Control Board. That meant that the city had to pretreat stormwater runoff into the river.

The pretreatment requirement resulted from the fact this was a priority project, defined as such by the amount of pavement reconstruction (more than 5,000 square feet); its adjacency and discharge to an environmentally sensitive area, the historic Sweetwater River channel/estuary; and direct and indirect discharge to the Sweetwater River and San Diego Bay, which are 303(d) beneficial-use impaired water bodies.

Because the site was space-constrained, MJC Construction of Bonita, CA, installed the Modular Wetlands Systems upstream of a catch basin. The incoming low flows are treated in four stages–screening, filtering, settlement, and bioretention–before entering the catch basin via a pipe. High flows either bypass the system and enter the catch basin or enter the system and travel through an internal bypass.

The pretreatment system is designed to act as a hybrid, using a combination of methods to remove trash, floatables, oil and grease, sediments, heavy metals, nutrients, and bacteria. A standard 41- by-24-inch grate-type traffic-rated catch basin opening directs stormwater into the system. A catch basin filter provides the first stage of treatment by capturing trash and litter, gross solids, and sediment. A settling chamber provides the second stage of treatment by separating larger suspended solids from the effluent. The third stage, a perimeter filter, utilizes a blend of biofiltration media in lieu of plants; beneficial bacteria and microflora within the media physically and chemically capture fine suspended solids, metals, nutrients, and bacteria. The final treatment stage, biological remediation, consists of a wetland chamber subsurface flow system employing a combination of physical, chemical, and biological processes. Other system features include a discharge chamber that controls flow rates with adjustable valves and contains a drain down filter that eliminates any standing water, and multi-level flow-control valves for primary and secondary treatment levels.

Kirk Ammerman, civil engineer in charge of design surveying and engineering on capital infrastructure projects for the city, says his staff completes a water-quality technical report that identifies pollutants of concern and water bodies into which runoff is discharged. He notes that the city also has an LID requirement mandating exploration of natural solutions, including ones in which runoff is contained in the system and treated with assimilative media prior to discharge. Ammerman’s group references the Countywide Model Standard Urban Stormwater Mitigation Plan to determine which BMPs remove the highest volume of certain pollutants; if site constraints prohibit the use of a particular BMP, the group selects the next most effective BMP that meets site requirements.

“We decided that this type of BMP was good for our type of project,” says Ammerman of the Modular Wetlands System. “Also, we usually rely on third-party testing for approving the BMP. We also take into account the experiences of other agencies,” he adds, referring to the Washington State Department of Ecology’s and city of Oceanside, CA’s use of the Modular Wetlands System as an example, “so we felt that this solution would be good for our project.”

Khosro Aminpour, senior civil engineer for the city’s Stormwater Management Section, points out that the design of the Modular Wetlands System made it a good fit for the North Broadway project, too. “Broadway is located in a large drainage area, and we get all kind of pollution: trash, debris, heavy metals, and things like that, so we were looking for a product that has a treatment train,” he says. “We needed a first stage to capture the pollutants such as plastic bottles and leaves and make them flow into the next stage and treat the water a little more, and eventually the final treatment is on fine particles and chemicals.” The Modular Wetlands System is categorized as a medium-efficiency BMP for pollutants that adhere to soil particles such as oil, grease, and metals and a high-efficiency BMP for gross pollutants like leaves, bottles, and cigarette butts.

Maintenance is also a key consideration, Ammerman adds. “We work closely with our Department of Public Works maintenance people, and a lot of our decisions on the selection of these products are based on their ability to maintain it in-house. Some agencies farm out the maintenance. We have the equipment to do maintenance, and we want to make sure that the maintenance is doable and not difficult for our staff. Our preference is not to enter the system, to be able to do it from the outside. In the past, we’ve had solutions that require manned entry–this system does not, and it’s a huge plus for us.”

Since February 2010, the systems have been operational; the most noticeable change has been the fact that seasonal flooding has decreased. In October 2010, the first treatment chamber was nearly two-thirds full of sediment and trash and the surfaces of the third-stage filter cartridges were covered with significant amounts of oil and grease. The city has not yet conducted its own testing to determine what percentage of pollutants the system removes; the manufacturer’s recommendation is that this occurs every five years.

“When we’re working in developing or redeveloped areas, we’re very, very space-constrained, and this project was no different,” says Ammerman. “We determined pretty quickly that, given our space constraints, the Modular Wetlands System was probably one of our best options.” Aminpour adds that property owners should not be too loyal to one system. “You have to start with an open mind about products,” he says. “This technology is evolving all the time, and something that you like today might be second-best tomorrow. Each individual project must be considered on its own conditions and constraints.”

Another recent project in Chula Vista that used the same system was the site of a retail and medical office complex located at 1310 Third Avenue. In 2008, project engineering firm Joseph C. Truxaw & Associates Inc. of Orange, CA, was hired by the site developer to design a stormwater management system for the redeveloped 1.18-acre site with two main buildings totaling 14,360 square feet of multi-tenant use and more than 50 parking spots.

This was another site with space constraints, yet it had an LID pretreatment requirement for stormwater running off the roof. The only area available was about 5 feet back from the curb face near the back of the site, where construction of a bioswale was planned. There, runoff from the parking lot drains to a central ribbon gutter flowing from the back to the front side of the parking lot. The ribbon gutter drains to a catch basin that discharges to the street. A trench drain was installed in the bottom of the ribbon gutter upstream of the catch basin to catch low flows to the Modular Wetlands System-Linear Vault Type; high flows bypass the trench drain and enter the catch basin.

Truxaw needed to select a system that would not change the current drainage layout. Additionally, as at most small commercial/industrial sites, runoff at 1310 Third Avenue comes in contact with trash and debris, oil and grease, bacteria and viruses, and oxygen-demanding substances.

“Given the fact that they were looking for economic viability and making sure that they’re doing the right thing and cleaning up the stormwater that’s running over their site, this is a great solution because its footprint is minimal in comparison to other systems,” says Ammerman of the Modular Wetlands System. “This is a fairly compact bioretention system, which is an ideal situation for them and their site.” He added that minimizing maintenance was a major priority for the developer on this site as well.
The Modular Wetlands System-Linear Vault Type, which was installed on the site in December 2008, is 22 feet in length and can treat up to 1.4 acres, assuming a 0.95 impervious coefficient. Native plants were used to blend in with the surrounding landscape.

Aminpour notes that the city designates any commercial redevelopment over 5,000 square feet, like this one, a priority development project. “With commercial facilities, we always get more trash than normal, and because of the parking spaces, we get oil, grease, metals, sediment, and those kinds of pollutants,” he says. “In a private development, we don’t usually dictate to the developer what kind of BMP they should use–we just tell them to use as high-efficiency a BMP as they can. Every year, we send developers a form that they have to complete, and they tell us how frequently they clean and maintain the units. Once every year, we randomly go to 25% of the total projects that have these treatment BMPs and make sure they’re in good condition. We may require the owner to increase the frequency of maintenance or continue at the current level.”

Ammerman adds that the city gives developers a range of options in treatment systems. In this case, he says, “they might have been able to get a similar result with a trash-removal unit and a swale and some other system where they had a treatment train, but that would have taken up a much larger piece of property.” To date, the city has received one report from the developer verifying that the system is being maintained.

According to Ammerman, developers who own sites with stormwater pretreatment requirements no longer regard the systems as an afterthought. “There’s less of a thought that this is the last part of the development process but, rather, that this is a very integral part of the process,” he says. “Even when it’s an integral part of the process, they see that this system and other bioretention systems allow them to occupy a fairly small footprint, which allows them to maximize the space they have for commercial purposes.”

The third-party testing that manufacturers provide on the performance of these pretreatment systems makes the job of the city’s public works staff much easier, Ammerman concludes. “The nice thing about these systems is that we can go to Bio Clean or another manufacturer and say, this is the size box we need, this is the system we need, this is the space we have,” he says. “What we’ve found is that they’re a good resource for us to both fit these into our site and get their expertise on system effectiveness. What’s been good about working with this company and others we’ve worked with is that they’ll be upfront about whether or not it’s appropriate for the location that we’re putting it in. They’ll come out and tell us whether or not they feel we should be looking at another system.”

Rainwater Harvesting at a Sustainable Facility
The new South Transfer Station in Seattle–a refuse transfer operation–is being designed as a sustainable facility in a sustainable industry that has incorporated the recycling of household materials for years. Actually, the entire 9.68-acre site is sustainable, as evidenced by the fact that the owner, Seattle Public Utilities, has designated the site for both LID and LEED–and a key contributor to both is a rainwater harvesting system.

In February 2011, KLB Construction of Mukilteo, WA, installed a 46,000-gallon underground UrbanGreen Rainwater Harvesting System from Contech Engineered Solutions as specified by engineering firm URS Corp Inc., which was part of a design/build team along with general contractor M.A. Mortenson Construction. The new $50 million two-story, 140,500-square-foot building is eventually replacing another transfer facility.

The site is being planted with various native drought-resistant plants that, while reducing irrigation water demand overall, still marginally increased the water demand compared with the site in its previous state. The plantings include 123 trees, 32,000 square feet of ground cover, and 81,000 square feet of grass cover. According to Becca Ochiltree, P.E., senior civil engineer for URS, the new vegetation will get established about a year after the facility becomes operational. At that point, the stored rainwater will be used for other applications: washing the transfer station tipping floor and for a wheel wash for trucks as they leave the tipping floor.

Ochiltree notes that the motivation for rainwater harvesting on the site came from the facility owner, SPU, which is required to design all projects for LID to the maximum extent practicable per the city’s Department of Planning and Development’s DR 17-2009, Vol. III Stormwater Flow Control and Water Quality. SPU also sought to achieve a LEED rating; the facility was on track for a Gold certification under the LEED NC rating system. Since the cistern water is not being used for flushing credit cannot be claimed under WEc3, Water Use Reduction, but it is being used for landscape irrigation and truck tire washing, resulting in an anticipated four LEED points. The facility is expected to receive a point each for SSc6.1, Stormwater Management: Quantity Control; SSc6.2, Stormwater Management: Quality Control; WEc1, Water Efficient Landscaping; and IDc1.4, Process Water Reuse. The combination of LID and LEED will allow SPU to showcase the transfer station’s sustainability.

The Contech UrbanGreen system uses a steel-reinforced polyethylene rainwater harvesting cistern. The system consists of two 84-inch-diameter, 75-foot-long cistern sections. The system also has a 12-inch rain leader for runoff collection, valve control outlets to drain the tanks for maintenance, four 36-inch access risers with integral ladders, and internal pumps connected to the irrigation system.

Mortenson favored the use of the system largely due to installation ease, Ochiltree recalls. “The system’s light weight allowed the use of an excavator to install it” rather than a large crane, she notes.

Generally speaking, rainwater harvesting was a sustainable choice on this project and the UrbanGreen system fit the bill. “The overarching feeling of Seattle Public Utilities was that they really wanted this to be an educational facility where they can showcase these new ideas that are practical and will pay off in the long run,” says Ochiltree.
The installation went smoothly, she notes. Contech conducted a preconstruction meeting to minimize operational snags, and the entire system was installed in only three days.

Ochiltree fully expects the system to serve its purpose when all is said and done. “The goal is definitely to reduce, if not eliminate completely, the amount of makeup water needed for irrigation and washdown,” she says. It helps that the site appeared to be a great fit for the cistern, she adds. “Just considering the physical site that we were given, it makes a lot of sense to do rainwater harvesting here. The transfer station has about a 100,000-square-foot roof that was almost begging for rainwater harvesting. We also have a lot of fill areas on the site, so we had a good location in which to install the cistern underground.”

Ochiltree primarily credits SPU for what should be a successful water-efficient LID project. “I would say that the most important thing is having an owner that’s really interested in low-impact design and gets behind it and is supportive of it,” she says. “That really makes a difference. Our owner really wanted to incorporate low-impact design, almost regardless of the cost–that made it easier than it would have been otherwise.”
About the Author

Don Talend

Don Talend specializes in covering sustainability, technology, and innovation.

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