Phosphorus Treatment Factors

1
Phosphorus Treatment Factors
The volume of runoff to be treated from the
contributing watershed for a given treatment
factor (VTF) or the buffer flow path length for a
given treatment factor (LTF) may be calculated as
follows VTF 0.35 VSZ / TF or LTF
0.35 LSZ / TF where VTF / LTF volume of
runoff to be treated or buffer flow path length
for desired treatment factor VSZ / LTF volume
of runoff to be treated or length/flow path
length for standard sizing of the BMP TF
desired treatment factor
2
Example 1 bioretention cell desired treatment
factor 0.25 catchment - 0.6 acre imp and 0.3
acre lawn VTF 0.35 VSZ / TF TF
0.25 standard sizing runoff depth impervious
1.0 in standard sizing runoff depth lawn 0.4
in VSZ (0.6 x 1.0) (0.3 x 0.4) x
43560/12 VSZ 2614 cu ft VTF 0.35 x 2614 /
0.25 3659 cu ft
3
Example 2 wet pond permanent pool desired
treatment factor 0.45 catchment 4.0 acre
imp and 2.0 acre lawn VTF 0.35 VSZ / TF
TF 0.45 standard sizing runoff depth
impervious 1.5 in standard sizing runoff depth
lawn 0.6 in VSZ (4.0 x 1.5) (2.0 x
0.6) x 43560/12 VSZ 26136 cu ft VTF 0.35
x 26136 / 0.45 20328 cu ft
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Example 3 meadow buffer, 5 slope, HSG B
Desired treatment factor 0.5 LTF 0.35 LSZ
/ TF TF 0.5 standard flow path length 85
ft LTF 0.35 x 85 / 0.5 60 ft Note Flow
path lengths may not exceed 150 ft
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Phosphorus Treatment Factors
Alternatively, the treatment factor for a given
sized BMP may be calculated as follows (same
method may be used to adjust buffer lengths) TF
0.35 VSZ / VBMP where TF treatment
factor of BMP VSZ volume of runoff to be
treated for standard sizing of the BMP VBMP
volume of runoff to be treated for the given BMP
sizing
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Example 4 bioretention cell treatment volume
(VBMP) 3050 cu ft catchment - 0.6 acre imp
and 0.3 acre lawn TF 0.35 VSZ / VBMP
standard sizing runoff depth impervious 1.0
in standard sizing runoff depth lawn 0.4
in VSZ (0.6 x 1.0) (0.3 x 0.4) x
43560/12 VSZ 2614 cu ft TF 0.35 X 2614 /
3050 0.30