Phosphorus credits for bioretention systems with an underdrain

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, credit = 80% If no or unknown, credit = 0% TP removal if credit is 80% = 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) test? 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 TP removal if credit is 20% = Dissolved fraction (45%) * removal rate for that fraction (20%) = 0.09 or 9 percent Adjust TP removal if credit is higher due to use of P-sorbing soil amendments

¹Other widely accepted soil P tests may be used.
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