This page provides a summary of MPCA-approved mechanisms and values for removal of dissolved phosphorus from stormwater runoff. The discussion includes requirements for receiving the credit from the MPCA.
Information: Credits may be modified based on additional analysis and information.
Iron in iron enhanced sand filters
- No amendment added to attenuate P; DP removal = 0%
Two tiers for crediting dissolved phosphorus exist and conditions needed to meet the criteria are described.
- Tier 1: Amendment added and meets the following; DP removal = 40%
- Iron enhanced system, and
- Iron 5-8% by weight, high purity iron (90%+ elemental iron) with little or no toxic impurities (e.g., copper, cadmium, lead, etc.), and reactive with phosphate, and
- Inspect IESFs at least once annually, with more frequent inspections (monthly or quarterly) during the first 1-2 years if needed to ensure construction was successful (see Assessing the performance of iron enhanced sand filter), and
- Annually or as needed remove vegetation and accumulated sediment (as needed, typically after a storm greater than 1 in of precipitation), and
- Rake surface with a steel-tipped rake to break up oxidized iron clumps or crust that may form, and
- System designed to allow drying (e.g. exposed to sunlight and access to air through drain tile)
- Level spreader, modified pond inlets, or other method or design for spreading water across the system, and
- Enhanced pretreatment designed to remove organic material (see Pretreatment Practice Selection Tool to identify practices effective at removing debris and solids), and
- Designed to be offline such that flows beyond the water quality volume flows are diverted around the system, and
- Inspection and maintenance agreement established that identifies the entity responsible for inspection and maintenance.
- Tier 2: Determined from water quality monitoring. See Assessing the performance of iron enhanced sand filter.
Iron in iron enhanced wet pond benches
An iron-enhanced sand filtration bench in a wet pond is essentially a wet extended detention pond with a permanent pool and a flood pool. The dissolved phosphorus credit for this practice is 40% if the following design features are implemented.
The outlet structure of the pond is designed such that the water in the flood pool during and after a storm event is held above the elevation of the iron-enhanced sand filter bench, thereby allowing water to filter through the bench. The basic design elements of an iron-enhanced sand filter basin include:
- iron-enhanced sand filter of desired width and length sited along the perimeter of the wet pond (iron-enhanced sands filters should be no less than 5 percent but no greater than 8 percent iron by weight to prevent clogging, see Erickson et al., 2010, 2012, and presentation by John Gulliver, September 2012). The 5 to 8 percent range is based upon iron filing material that is approximately 90 percent elemental iron with a size distribution approximately equal to that of C33 sand;
- outlet structure that controls the flood pool elevation and can receive the filter bed drain;
- subsurface drains at the filter bed bottom to drain the bed. The outlet of these subsurface drains should be exposed to the atmosphere and above the downstream high water level to allow the filter to fully drain;
- impervious barrier (typically geotextile liner, for example HDPE) between the pond and the trench to minimize seepage from the pond into the trench;
- filter draw down within 48 hours of storm completion to avoid filter fouling and to prepare the filter for next storm event; and
- the underdrain may consist of corrugated polyethylene pipe with slits not holes to prevent loss of sand and minimize clogging. If holes are used, the pipe should be covered with pea gravel.
Best management practice with demonstrated mechanism for removing dissolved phosphorus
Pollutant removal credits may be given to a best management practice based on one or both of the following, at the discretion of the MPCA.
- Sufficient documentation in the literature. Typically this will be peer-reviewed literature or published studies that follow acceptable protocol (see below).
- Monitoring that follows acceptable protocol (see below). Note that site-specific monitoring may result in credits for only that specific site rather than a specific class of best management practice. MPCA discretion will be used in determining applicability.
Monitoring protocol for establishing credits must be consistent with the protocol described here, or other MPCA-approved protocol.
For an example of dissolved phosphorus credits given to a best management practice, see here.