Phosphorus credits for bioretention systems with an underdrain.
Link to this table
|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 = 100% 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% for bioretention and tree trench and 68% for swale main channel) = 0.55 * 0.80 = 0.44 or 44% for bioretention and tree trench (with underdrain) and 0.55 * 0.68 or 36% for swale main channel (with underdrain)
|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