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 ft^{2}) 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 ft^{2
}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 FilterSize

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 filtermedia 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:

Select desired percentage of annual rainfall volume to be treated (y
axis).

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.

Calculate total tree box filter surface area by using the following
formula:

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

Therefore two 36-ft^{2 }(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.