Phosphorus credits for bioretention systems with an underdrain.
Link to this table

Particulate phosphorus Dissolved phosphorus
Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, particulate credit = 80% of the particulate fraction (assumed to be 55% of total P)
  • If no or unknown, particulate credit = 0%


TP removal credit

  • Particulate fraction (55% of TP) * removal rate for that fraction (80%) = 0.55 * 0.80 = 0.44 or 44%
1. Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, credit as a % (up to a maximum of 20%) = 20 * (depth of media above underdrain, in feet/2)
  • If no or unknown, credit = 0%

2. Does the system include approved P-sorbing soil amendments2?

  • If yes, additional 40% credit


TP removal credit

  • TP removal if dissolved credit is 20% = Dissolved fraction (45%) * removal rate for that fraction (20%) = 0.09 or 9 percent
  • Adjust TP removal if depth is less than 2 feet
  • Adjust TP removal if dissolved credit is higher due to use of P-sorbing soil amendments

1Other widely accepted soil P tests may be used. Note: a basic conversion of test results may be necessary
2Acceptable P sorption amendments include

  • 5% by volume elemental iron filings above IWS or elevated underdrain
  • minimum 5% by volume sorptive media above IWS or elevated underdrain
  • minimum 5% by weight water treatment residuals (WTR) to a depth of at least 10 cm
  • other P sorptive amendments with supporting third party research results showing P reduction for at least 20 year lifespan, P credit commensurate with research results


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This page was last edited on 15 April 2020, at 17:15.

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