Tree Box Filter

Sizing Guidelines

The sizing guidelines for a tree box filter are provided below.  This example is derived from a proprietary version of a tree box filter system known as Filterra™, which is sold by Americast Inc.1  

In general, tree box filters require low maintenance and a small drainage area with respect to surface area. 
Moreover, a specially designed filter media and sizing strategy allows treatment of very high volumes of runoff with high pollutant removals.  Based on the tested flow characteristics of a manufactured tree box filter (Filterra™), and the rainfall distribution / frequency data from the mid-Atlantic region, the optimum filter surface area to contributing drainage area is 0.33% (36 ft2) of filter surface for every ¼ acre of contributing impervious surface area.  This relationship was established during a two-year study at the University of Virginia demonstrating that a 36 ft2 filter had a flow rate of 0.01 cfs, treated approximately 90% of the annual volume of runoff, and resulted in pollutant removals of 85% total suspended solids, 74% total phosphorous, 82% heavy metals and 68% total nitrogen. 

Summary Table 1

 

 

 

 

 

Control Area: ¼ Acre

 

 

 

 

 

 

 

 

 

 

 

 

Tree Box Filter Size

Area
Ratio

Annual Volume Filtered

TP Removal

TN Removal

TSS Removal

Metal Removal

 

%

%

%

%

%

%

9 x 16

1.32%

99.48%

81.57%

75.70%

94.50%

90.52%

9 x 12

0.99%

98.55%

80.81%

74.99%

93.62%

89.68%

6 x 12

0.66%

96.58%

79.19%

73.40%

91.75%

87.88%

*6 x 6

0.33%

89.93%

73.74%

68.43%

85.43%

81.83%

3 x 6

0.16%

69.26%

56.79%

52.70%

65.79%

63.02%

3 x 3

0.08%

60.88%

49.75%

46.32%

57.83%

55.40%

* One 6’ x 6’ tree box filter per ¼ Acre is the optimum size for pollutant removal and cost effectiveness.

Sizing and Pollutant Removal

Basically all you need to know is how much of the annual volume you need to control and the size of the drainage area.  Then follow the step-by-step procedure to use the sizing chart below to find the total overall tree box filter surface area for your application and needs. 

The sizing strategy is based on optimizing the filter surface area / drainage area relationship to filter most of the annual rainfall intensities in your area.  For the Mid-Atlantic region 30 years of rainfall data was collected from Reagan National Airport (in/hr) from which the probability and frequencies of all rainfall intensities were determined.   Knowing this and the flow characteristics of the filter media you can determine the annual volume of runoff that can be treated and the optimum size system for any given drainage area.  From this information a sizing curve was developed as a function of annual rainfall volume and the tree box filter surface area to drainage area ratio, see sizing chart below.  

Pollutant removal calculations are even simpler.  Multiply the maximum pollutant removal from the tree box filter test data shown in Table 1 by the percent of the annual volume treated to get the percent annual pollution reduction.

I.  Sizing Procedure:  

  1. Select desired percentage of annual rainfall volume to be treated (y axis).
  2. From that point on the y-axis draw a horizontal line to the curve and a vertical line to x-axis to find Filter Surface Area (FSA) / Drainage Area (DA) ratio percent.
  3. Calculate total tree box filter surface area by using the following formula:
   
(FSA/DA %) (DA in acres) x (43,560 sq. ft.)   = 

(Tree box filter area required)

 100    
 

Example:

Desired Annual Volume Treated = 90% (Recommended %)

Drainage Area to be treated =  0.5 Acres

From Sizing Chart the FSA / DA % =  0.33

 

Total tree box filter surface area needed = (0.33) * (0.5) * (43,560) / 100 = 71.9 sq. ft.

Therefore two 36-ft2   (6’ x 6’) boxes are recommended.

 

II. Determine Annual Pollutant Removal  

Simply multiply the percent annual volume treated by the maximum pollutant removal percentage for each pollutant from Table 1 above.   

Example: 

Annual volume treated =  95 %

Maximum TSS Removal =  95% (From Table 1)

Annual TSS Removal =  (95)*(95) / 100 = 90.25%  

Repeat for each desired pollutant.  

The designer must verify that the tree box filter system has been sized and installed in accordance with the manufacturer's specifications. The distribution and sizing of tree box filters should be in accordance with the manufacturer’s recommendations to achieve the most cost-effective treatment practicable while satisfying the performance-based water quality criteria.

References


1 Filterra™ Stormwater Bioretention Filtration System, Americast, Ashland, VA. http://www.americastusa.com

2
Virginia Dept of Conservation and Recreation Stormwater Management Program http://www.dcr.virginia.gov/soil_&_water/stormwat.shtml

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