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General design criteria for iron-enhanced filtration systems includes

  • water quality sizing for filtration applicable to iron-enhanced sand filters;
  • iron by weight no less than 5 percent but no greater than 8 percent (Erickson et al., 2012, and presentation by John Gulliver, September 2012) of iron-sand mixture. The iron weight range is based upon high surface area iron filing material that is approximately 90 percent elemental iron. Estimates of longevity and performance will need to be proportionately adjusted according to the percent composition of elemental iron for alternative iron materials. The alternative materials should not have elevated levels of other contaminants such as other metals and oils;
  • filter draw down within 48 hours of storm completion to avoid filter fouling and to prepare the filter for next storm event;
  • minimal tailwater to allow aeration of filter bed between storm events;
  • use of filter fabric around drain pipe or between media layers discouraged (e.g., no sock) due to clogging and aeration suppression effects;
  • head (top of filter to outlet invert) of 2 to 6 feet recommended depending upon application; and
Warning: For projects covered under a Construction Stormwater permit, pretreatment is Required for sand filtration beds.

Design phase maintenance considerations

Caution: Maintenance considerations are an important component of design

Implicit in the design guidance is the fact that many design elements of infiltration and filtration systems can minimize the maintenance burden and maintain pollutant removal efficiency. Key examples include

  • limiting drainage area;
  • providing easy site access (REQUIRED);
  • providing pretreatment (REQUIRED); and
  • utilizing native plantings (see Plants for Stormwater Design). NOTE this does not apply to the sand filter surface.
Caution: The MPCA Recommends that the iron-sand filter surface be maintained free of vegetation or grasses. Ground cover can be used to stabilize the banks of the live storage zone above the elevation of the sand filter surface. Shrubs or other woody plants can be planted above live storage.

For more information on design information for individual infiltration and filtration practices, link here.

Design elements

Physical feasibility initial check

Before deciding to use an iron-enhanced sand filter for stormwater management, the designer should consider several items that affect the feasibility of using such a practice at a given location. The following list of considerations will help in making an initial judgment as to whether a filtration device is the appropriate BMP for the site.

  • Drainage area: In addition to the practical size limitations of constructing a properly functioning (e.g., designing a filter than can drain properly and can be accessed for maintenance) iron-enhanced sand filter basin or sand bench in wet ponds, treatment objectives and credit needs will determine the drainage area that can be treated. Runoff volume is a function of soil type and watershed imperviousness. Given space availability or other factors that will dictate filter treatment volume capacity (Vt). For more information on contributing area, see Contributing drainage area to stormwater BMPs.
  • Site topography and slopes: Sloped areas immediately adjacent to practice are Recommended to be less than 20 percent but greater than 1 percent, to promote positive flow towards the practice. Iron-enhanced sand filters do not require any slope to promote treatment.
  • Soils: No restrictions or sizing adjustments are required by soil type.
  • Karst: There are no special requirements for iron-enhanced sand filters in karst terrain. All of the iron-enhanced sand filters require underdrains that serve to convey filtered and treated stormwater and to aerate the filter bed between storms. It is recommended that an impermeable liner along the bottom of the filtration media be used if the filtration system is located in active karst areas.
  • Depth to water table and bedrock:
Warning: The draft MPCA General Construction Stormwater Permit Requires that an impermeable liner be constructed for a filtration system with less than 3 feet of separation from the seasonally saturated soils or from bedrock.
  • Site location/minimum setbacks:
Warning: A minimum setback of 50 feet between a stormwater pond and a water supply well is Required by the Minnesota Department of Health Rule 4725.4350. For purposes of this guidance, a stormwater pond is assumed to include a stormwater filtration system.

Conveyance

Conveyance design elements for filtration are largely applicable to iron-enhanced filtration with the following exceptions:

  • When a flow splitter is not used, the maximum size of the contributing drainage area will be a function of filter size (filter surface area and live storage) for an iron-enhanced sand filter basin, and live storage and outlet design for an iron-enhanced sand bench in a wet pond.
  • Use of permeable filter fabric around the drain pipe or between media layers is Not Recommended as the filter fabric may clog, reduce infiltration into the drain pipe, or limit aeration of the filter media.

Pretreatment

The pretreatment requirements for media filters apply to iron-enhanced sand filter basins. Pretreatment is provided by the wet ponds associated with iron-enhanced sand filter benches.

Treatment

The treatment guidelines applicable to filtration are also applicable to iron-enhanced filtration.

Landscaping

The MPCA Recommends that the iron-sand filter surface be maintained free of vegetation or grasses. Ground cover can be used to stabilize the banks of the live storage zone above the elevation of the sand filter surface. Shrubs or other woody plants can be planted above live storage.

Warning: The MPCA Requires that impervious area construction be completed and pervious areas established with dense and healthy vegetation prior to introduction of stormwater into a filtration practice.

Design

Iron-enhanced sand filtration basins

photo of an iron ehanced sand filter basin
Iron enhanced sand filter basin, Maplewood, MN. Photo courtesy of Barr Engineering.
schematic of an iron enhanced sand filter basin
Iron-enhanced sand filter basin schematic.

Iron-enhanced sand filtration basins are analogous to media filters. The basic design elements of an iron-enhanced sand filter basin include:

  • inlet that directs and controls maximum flows to the basin;
  • basin that includes a storage volume above the filter bed;
  • iron-sand filtration bed (iron-enhanced sand filters should be no less than 5 percent but no greater than 8 percent iron by weight to prevent clogging, see Erickson et al., 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 sand;
  • the iron and sand need to be thoroughly mixed. If mixing with a rototiller, 20 passes may be required;
  • normal and flood control outlet;
  • 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;
  • 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;
  • filter draw down within 48 hours of storm completion to avoid filter fouling and to prepare the filter for next storm event; and
  • minimal tailwater to allow aeration of filter bed between storm events.

Iron-enhanced sand filtration bench in wet ponds

photo of an iron ehanced sand bench
Iron enhanced sand bench, Prior Lake, MN. Photo courtesy of Ross Bintner.
schematic of an iron enhanced sand filter bench
Iron-enhanced sand filter bench schematic.

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 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.


Related pages

This page was last modified on 12 February 2020, at 09:21.

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