About the Tree Box Filter System Performance

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Tree box filters are mini bioretention systems that combine the versa- POLLUTANT REMOVAL: 2004–2006 tility of manufactured devices with the water quality treatment of vegetated systems. They serve as attractive landscaping and drainage catchbasins. Unlike many other forms of urban landscaping, they are 100 not isolated behind curbs and deprived of water and nutrients in runoff. 90 Their water quality treatment performance is high, often equivalent 80 to other bioretention systems, particularly when well distributed 70 throughout a site. 60 50 40 Where to Use It Implementation 30 20 These systems are a relatively recent inno- Tree box filters can be used throughout the Efficiency % Removal 10 United States, and are especially useful vation that is growing in usage, especially 0 TSS TPH-D DIN Zn TP in settings where available space is at a in urban areas. The cost to install a tree sediments total dissolved metals total premium. They can be installed in open- or box filter to replace a catchbasin is $2,500. petroleum inorganic phosphorus hydrocarbons nitrogen closed-bottomed chambers where infiltration This does not include maintenance. UNHSC is undesirable or not possible, such as clay observations thus far reinforce stormwater Median Annual InfluentE vent Mean Concentrations (EMC) in mg/L 55.5 894.0 .49 .06 .124 soils, sites with high groundwater, and manual assessments that maintenance require- areas with highly contaminated runoff. ments for these systems are generally minimal.

Tree box filters are often installed along In general, tree box filters are sized and HYDRAULIC PERFORMANCE urban sidewalks, but they are highly adapt- spaced much like catchbasin inlets, and able and can be used in most development design variations for these systems are ––– Influent ––– Effluent scenarios. In urban areas, tree filters can be abundant. The system evaluated at UNHSC used in the design of an integrated street was designed by center researchers. A similar 70 landscape—a choice that transforms isolated patented design made by AmeriCast, the 60 street trees into stormwater filtration Filterra, is also available. Contact the 50 Unlike many other forms of urban landscaping, tree devices. They also can be used in designs UNHSC for more information about the 40 filters are not isolated behind curbs and deprived of that seek to convert entire non-functional design of the tree box filter. 30

water and nutrients from runoff. Instead, they receive streetscapes into large stormwater or (GPM) Flow 20 runoff through breaks in the curbing, and demonstrate combined sewer flow filtration systems. 10 strong water quality treatment. 0 050 100 150 200 250 300 350 400 Minutes

Annual Winter Summer Average Fast Facts Average Peak Flow Reduction 37% 0% 0% Average Lag Time (minutes) 14 26 19 Category Type How the System Works Filtration, Infiltration, Urban Retrofit

BMP Type Low Impact Development Design Design Design Source The tree box filter’s basic design is a Vegetation selected for these systems UNHSC concrete vault filled with a bioretention should consist of native, drought- and soil mix (BSM), planted with vegetation, salt-tolerant species. Plants with aggres- Basic Dimensions and underlain with a subdrain. The system sive root growth may clog the subdrain, Diameter: 6 ft evaluated at the UNHSC field site is a and therefore may not be suitable for this Depth: 4 ft six-foot diameter, concrete vault with type of system. an internal bypass. It is underlain by Specifications This tree box filter was sized for the water Catchment Area: 0.1 acre a subdrain that discharges to existing quality volume (WQ ), and should allow for Peak Flow: 0.1 cfs stormwater drainage. The vault is open- v four to six inches of ponding. Larger storm Water Quality Volume: 425 cf bottomed to enhance infiltration. events will be bypassed. The system’s filter Tree: Two-inch Caliper Ash The filter media is three feet deep, and media accommodates a high infiltration Water quality composed of 80 percent sand and 20 rate of 120 feet per day. treatment process percent compost. The mix was designed Physical, Chemical, & Biological to maximize permeability while providing minimum organic content (at least 10 Installation COST percent) to sustain vegetation. $2,500 Per Unit ($22,000 Per Acre Treated)

MAINTENANCE Maintenance Sensitivity: Low Inspections: Medium Sediment Removal: Low

18 About the Tree Box Filter System Performance

POLLUTANT REMOVAL: 2004–2006 Water Quality Treatment Maintenance The tree box filter does a good job of No maintenance has been performed on removing many of the pollutants commonly the tree box filter since it was installed 100 associated with stormwater treatment in fall 2005, and the system continues to 90 performance assessment. It consistently function well. Generally speaking, these 80 exceeded EPA’s recommended level of systems are designed to minimize mainte- 70 removal for total suspended solids and nance. Aside from routine trash removal, 60 meets regional ambient water quality the highest maintenance burden generally 50 criteria for petroleum products, nitrogen, coincides with the establishment of vege- 40 and total zinc. However, UNHSC research tation over the first several months after 30 demonstrates that water quality treatment installation. Once vegetation is established, 20 effectiveness can be negatively influenced the maintenance demand decreases. The

% Removal Efficiency % Removal 10 0 by an increased hydraulic loading rate, i.e., tree may need to be replaced, depending TSS TPH-D DIN Zn TP the filtration of a large surface area by a on hardiness of the selected species. sediments total dissolved metals total petroleum inorganic phosphorus small filter area. The system does not Adaptations to design can prevent root hydrocarbons nitrogen remove chloride, but does exhibit an ability constriction in the planting vault. Median Annual InfluentE vent Mean Concentrations (EMC) in mg/L to dampen chloride peaks. 55.5 894.0 .49 .06 .124 The chart at top left reflects system Cold Climate performance in removing total suspended The tree box filter’s ability to treat water solids, total petroleum hydrocarbons, quality remained relatively stable in all HYDRAULIC PERFORMANCE dissolved inorganic nitrogen, total seasons. This is consistent with UNHSC phosphorus, and zinc. Values represent observations of most LID stormwater ––– Influent ––– Effluent results recorded over two years, with systems—when they are properly designed 70 data further divided into summer and and installed, they are not dramatically 60 winter components. impacted by seasonal fluctuations. While 50 some seasonal variation in infiltration 40 Water Quantity Control capacity and nitrogen removal does occur, 30 cold conditions do not seem to warrant

Flow (GPM) Flow 20 Unlike other filtration systems, the tree significant design alterations. 10 box filter does not reduce peak flows unless 0 sited in appropriate soils, such as those in 050 100 150 200 250 300 350 400 groups “A” (sand, loamy sand, or sandy loam Minutes with high infiltration rates) and “B” (silt loams or loams with moderate infiltration Annual Winter Summer Average rates). In the figure at bottom left, the tree Average Peak Flow Reduction 37% 0% 0% box filter displays no significant peak flow Average Lag Time (minutes) 14 26 19 reduction or lag time for the range of seasons monitored.

Water Quality Treatment Process

1. Runoff flows into the tree filter 3. Filtered runoff is collected basin from the street and in a perforated subdrain and passes into the filter media. Vegetation returned to a storm drain centered in system, infiltrated into the 2. In the filter media, biological treatment subgrade, or discharged to treatment occurs through the the surface. uptake of pollutants, such as Mound 6” berm nitrogen and petroleum hydro- Impervious surface around tree filter rim carbons, by vegetation and soil Q v Conveyance microorganisms. Physical and protection bypass chemical treatment also occurs within the soil media. Other 1 Cross section of treatment unit processes 72” diameter concrete vault include sedimentation and sorption with organic matter 12” Overflow pipe and mineral complexes. Native soils

Bioretention soil mix 12” Perforated 2 80% sand, 20% compost subdrain 3

Crushed stone 12” Overflow outlet, discharges to existing storm drain or the Existing subgrade surface

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