This document introduces the P-Trap software for designing and evaluating phosphorus removal structures. P-Trap was developed by the USDA-ARS to help conservationists, designers, and engineers design and evaluate the performance of P removal structures. It can be used to design structures to meet phosphorus removal and flow rate goals or evaluate existing or hypothetical structures. The document provides examples of using P-Trap to design a bed structure and evaluate an existing blind inlet structure. Key inputs for P-Trap include site characteristics, PSM properties, and structure configuration. The output provides structure specifications and predicted phosphorus and flow rate performance.

1. An introduction to P-Trap
software for designing and
evaluating phosphorus
removal structures
Chad Penn
Jim Frankenberger

2. Purpose and intended audience
• Conservationists, designers and engineers
interested in:
– Designing a P removal structure for a specific
site
– Evaluating how a theoretical or previously
constructed structure will perform
– Exploring potential options and economics
• Part of a series of modules intended to train
people how to design and construct PRS

3. Training Modules Nearly
Completed: 2022

4. Purpose and intended audience
• This will not provide detail about each input
variable or how to obtain a proper value for
each input variable
• Several other modules exist that provide
detailed information on how to obtain inputs
and basic design philosophy

5. Required Inputs for Design
Input
Surface runoff
confined bed
Ditch
filter
Blind inlet or bio-
retention cell P filter
Subsurface
drain filter
Annual flow volume X X X X
Target min peak flow rate X X X X
Dissolved P concentration X X X X
Available area X X X
Available ditch length X
Depth of ditch X X
Ditch survey X
Distance between ditch bottom and tile
outlet
X X
Target max loss in ditch flow capacity X
Slope X
Target RT X X X X
P removal goal and lifetime X X X X
PSM design curve and physical
properties
X X X X
Manifold pipe diameter X X X X

6. P-Trap software
• “Phosphorus Transport Reduction App”
• Developed by USDA-ARS and freely
available
• https://www.ars.usda.gov/nserl/ptrap

7. Two types of generic structures:
• P-Trap will provide a design for two
different categories of structures:
– Ditch filter
• PSM placed directly into a ditch where a dam is
placed on the downstream side of a PSM layer

8. Two types of generic structures:
• P-Trap will provide a design for two different
categories of structures:
– Bed structure
• Uniform and symmetrical layer of PSMs placed in any location
other than a ditch
• Ditch filters have unique design algorithms due to additional
hydrological constraints

9. Design or Evaluate
• Design to meet a specific P removal and
flow rate goal
• Evaluate a real or hypothetical structure
for P removal and flow rate

10. Information/Help Buttons
• Utilize the help buttons
when necessary
– Describes the input variable
in more detail
– Some of the information
links will provide suggested
values for input and basic
information about how to
obtain the inputs

11. Example:Design a Bed
Structure

12. • Load or save projects
• Brief description and location
• Choose to design or evaluate
structure
• Choose bed or ditch structure
Input PSM characteristics
• Site inputs for P and
hydrology
• Optional inputs on
sediment and total P
Input site constraints
and flow rate goal
Input structure
preferences and P
removal goal
“Run” button

13. Design a Bed Structure
• In this example we
are designing a
bed-style structure.
• Here you can also
save or load
previous plans,
including any
comments and
location

14. Design a Bed Structure
• The following example is for designing a
subsurface tile drain filter at a site in W.
Michigan.

15. Input site information on P
concentrations and hydrology
Required for conducting a design
• Not required
• Design is based on meeting DP
removal goals, but total P and
particulate P can be estimated after-
the-fact

16. PSM Characteristics
Physical characteristics
• Click on information/ help button to see a
table of suggested values for different
materials
• It is always better to measure these
parameters rather than assume a
value

17. PSM
Characteristics

18. PSM Characteristics
P removal design curve
• Measured by flow-through method
• Input exponential slope and intercept
• Click, “Graph Curve” to produce a visual design curve to the
right, possessing those specific parameters

19. PSM Characteristics: “Graph
Curve”
• Visual depiction of the input design curve
• Also provides calculation of CPaddmax
– Maximum cumulative P added until the PSM is no longer
able to remove any P

20. PSM Characteristics
If parameters for the design curve are not available, use the database of
design curves and choose one that corresponds best to your specific PSM
• If there is nothing available, contact Dr. Chad Penn at the National Soil
Erosion Laboratory about possibly testing your PSM.

21. PSM Characteristics: PSM
design curve database
Click on any heading to sort
based on alphabetical and
numerical order
Other information is provided: by-product or manufactured
media, source, location, type of experiment used to
determine slope and intercept (lab, pilot, or field scale),
and most important: the RT and P concentration utilized

22. PSM Characteristics: PSM
design curve database
Click on any heading below to
see all groups within a single
category, and then select one
to view entries only within that
category

23. • Click on a table entry that you want to use in the design
• Then click, “Use These Values in Design”, and it will select the design curve slope and
intercept associated with it
• In this example, an 8% steel shavings-pea gravel mixture was chosen
• After this, the database window will close and will return to the P-Trap main page

24. PSM Characteristics: Design
Curve
• The slope and intercept will now be populated with your entry selection
• Also lists the material that associates RT and P concentration as a reminder
• This is very important

25. Design Curve Input
REMEMBER:
–Need to reasonably match retention
time and inflow concentration

26. Importance of using
representative design curve
0
20
40
60
80
100
0 100 200 300 400 500 600 700
P
removed
(%)
P added (mg kg-1)
10 min RT cumulative removal
0.5 min RT cumulative removal
i.e. Don’t input a 10 min RT curve and
then design a PRS with a 0.5 min RT !

27. Structure Preferences
Insert RT value at least as long as
RT used to produce design curve
Choose cumulative P removal
goal and lifetime
Choose the perforated pipe diameter
and slope that you plan to use for the
drainage manifold

28. Structure Preferences
Select only if you want to restrict flow
rate with a single orifice; this allows
you to use unlimited # drain pipe in
manifold: if selected, input diameter
and slope
Choose either top-down or bottom-
up flow regime; there are pros and
cons to both
If “top-down” is chosen, you have the option to design a bed that will not possess any
free water in between flow events; there are pros and cons to both

29. Site Constraints and Flow Rate
Goal
You want the structure to be able
to handle at least this flow rate
Input maximum possible
thickness of PSM for structure
Input maximum possible area
available for structure
Gravitational force available for pushing water through the PSM bed

30. “Run Calculation”

31. Output for Bed Design:
Figure corresponds
to a top-down bed
structure:

32. 1
2
3
4
1. Required depth of
PSM
2. Required length
and width
3. Required number
of pipe
4. Required mass of
PSM
5. Optional: total P
and particulate P
removal
6. Maximum flow rate
of structure
7. Actual retention
time at max flow
rate
8. Required orifice
size (optional)

33. Output: Bed Design Specs
Mass and thickness of PSM layer
required
Required area
Necessary # of drain pipes
Actual min RT and max flow
rate at this configuration
If the RT and flow rate goal are not met, then a suggestion is made for
increasing the target lifetime of structure. In this case, nothing appears
because both the RT and flow rate goal were met.
Necessary diameter of single orifice only
appears when this option is chosen.
No Red Flags Here!

34. Output: P Removal Table
• % cumulative DP
removed over
structure lifetime
• Cumulative DP
load removed
over structure
lifetime
• Optional: PP and
total P removal

35. Example: Evaluate an existing
bed structure
• Use this option to evaluate a real or
hypothetical P removal structure
– Predicts P removal and flow performance
• The following example is for a blind inlet
constructed using gravel-metal shavings
mixture as the PSM

36. treated water
outlet to tile or
ditch
runoff flow into
depression
downward
flow through
gravel-sand
treated water
collection
manifold
Modified
blind
Inlet

37. Input PSM characteristics
same as for design function
Input site
information same as
for design function
Instead of structure preferences and site constraints, you
will input the configurations of the real/hypothetical
structure
Select “Evaluate an
existing bed
structure”

38. P-Trap assumes that the entire
annual volume will flow through
the unit, regardless of the flow rate
that the structure is able to handle
Important Assumption !

39. Evaluate existing bed
• Input the real/hypothetical parameters:
– Drainage pipe diameter, slope, and number of
pipe found in PSM bed
– Length, width, and thickness of PSM bed
– Hydraulic head

40. Evaluate existing bed
• Input the RT that represents the design
curve
– Not used in a calculation, but for comparison to
the actual RT of field unit
• i.e. to know if the predicted results are valid or not

41. Output: Bed Evaluation
• Provides prediction of cumulative annual P
removal as before
• Prediction of PSM mass and flow
performance
PSM mass based on input
configuration
Predicted RT and flow rate
through structure
Highest possible flow rate
through pipes (“pipe flow”
and media (“Darcy flow”)
Structure flow rate is least
of the two

42. Output: Bed Evaluation
• Notice that the actual RT is greater than
the RT that corresponds to the PSM
design curve (10 min)
If the structure RT is less than P removal design
curve RT, then a red warning appears. No red
flags here!

43. Output: Bed Evaluation
• Recall the assumption: 100% of annual
flow volume will enter the bed
– Consider if this is reasonable or not based on
the predicted flow rate of structure compared
to the actual inflow rate at the site
• If reasonable, then P removal table is valid

44. Output: Bed
Evaluation

45. Questions?
Chad.penn@ars.usda.gov
Twitter Handle: House of Phos
Contact me if you need help with a design