Major 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, defined in Section 6.0). Section 7.2 provides figures (Figure 3 and 4) that can be used to determine percent of total runoff volume that can be treated given soil type, watershed imperviousness, and treatment volume capacity-VT.

Site Topography and Slopes: Sloped areas immediately adjacent to practice are RECOMMENDED to be less than 20% 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.

Depth to Water Table and Bedrock: 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: 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.

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.

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 (see Section 6.0).
  • 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.

Pre-treatment

The pre-treatment requirements for media filters apply to iron-enhanced sand filter basins. Pre-treatment 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 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. 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.


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, defined in Section 6.0). Section 7.2 provides figures (Figure 3 and 4) that can be used to determine percent of total runoff volume that can be treated given soil type, watershed imperviousness, and treatment volume capacity-VT.


Site Topography and Slopes: Sloped areas immediately adjacent to practice are RECOMMENDED to be less than 20% 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.


Depth to Water Table and Bedrock: 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: 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.


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.

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 (see Section 6.0).
  • 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.


Pre-treatment

The pre-treatment requirements for media filters apply to iron-enhanced sand filter basins. Pre-treatment 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 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. 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.