Stormwater Design Recommendations to Enhance Phosphorus Removal

Summary of stormwater design recommendations to enhance phosphorus removal.
Link to this table

BMP Design	Design recommendations
Infiltration	Infiltration BMPs are preferred practices Provide a minimum 3-foot separation from the seasonally-high water table, bedrock or impervious soil layer
Filtration (includes practices with an underdrain)	Organic filters are a source of soluble phosphorus and should not be used Practices with engineered media are acceptable when the appropriate filter media is used Employ finer-grained media in the filter bed with a small diameter (15 microns), or provide a finer-grained layer at mid-depth in the filter profile The process for pretreatment and/or filtration should extend from 36 to 48 hours, where possible Filters should be oriented to provide maximum solar exposure Wet swales are not recommended because they may export phosphorus Open channels should be designed to promote maximum sediment retention
Stormwater ponds1	Pond aeration is encouraged Design wet ponds with a depth no greater than 10 feet to prevent stratification and potential release of phosphorus from bottom sediments Avoid the use of dry or dry extended detention ponds Designers should consider the snowmelt runoff volume and design ponds for seasonal operation Use a surface or mid-depth release from the pond Landscape pond to discourage geese Add shallow benches and wetland areas to enhance the plankton community Pond sediments may be amended to retain phosphorus (e.g. iron, aluminum)
Constructed Stormwater wetlands	Pond/ wetland system is the preferred wetland design Use a surface or mid-depth release from the wetland Maximize surface micro-topography Landscape wetland to discourage geese

¹The recommendations for constructed ponds are from the original Minnesota Stormwater Manual. MPCA anticipates updating this information in the near future.