Iron-enhanced sand filters are filtration Best Management Practices (BMPs) that incorporate filtration media mixed with iron. The iron removes several dissolved constituents, including phosphate, from stormwater. Iron-enhanced sand filters may be particularly useful for achieving low phosphorus levels needed to improve nutrient impaired waters. Iron-enhanced sand filters could potentially include a wide range of filtration BMPs with the addition of iron; however, iron is not appropriate for all filtration practices due to the potential for iron loss or plugging in low oxygen or persistently inundated filtration practices. Here iron-enhanced filtration is limited to two types:
Iron-enhanced sand filters may be applied in the same manner as other filtration practices and are more suited to urban land use with high imperviousness and moderate solids loads. Iron-enhanced sand filters are more suitable to conditions with minimal groundwater intrusion or tailwater effects. Because the primary treatment mechanisms are filtration and chemical binding and not volume reduction, vegetating the filter is not needed and may impair the filter function. 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. The exit drain from the iron-enhanced sand filter should be exposed to the atmosphere and above downstream high water levels in order to keep the filter bed aerated.
Iron-enhanced sand filters may be used in a treatment sequence, as a stand-alone BMP, or as a retrofit. If an iron-enhanced sand filter basin is used as a stand-alone BMP, an overflow diversion is recommended to control the volume of water, or more specifically, the inundation period in the BMP. As with all filters, it is important to have inflow be relatively free of solids or to have a pre-treatment practice in sequence.
The iron-enhanced sand filter basin may be used in conjunction with other structural controls. The iron-enhanced sand filter bench is constructed along the perimeter of a pond that provides pretreatment. Placement of a plunge pool or some sort of pre-treatment upstream of an iron-enhanced sand filter basin is recommended to extend the lifespan of the filter.
One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. These standards must be applied in all projects in which at least 1 acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.
For regulatory purposes, filtration practices fall under the “Infiltration / filtration category described in Part III.D.1 of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the filtration practice will be used in combination with other practices, standards are described for the case in which it is a stand alone practice.
The following terms are thus used in the text to distinguish various levels of filtration practice design guidance:
REQUIRED:Indicates design standards stipulated by the MPCA Permit (or other consistently applicable regulations).
HIGHLY RECOMMENDED: Indicates design guidance that is extremely beneficial or necessary for proper functioning of the filtration practice, but not specifically required by the MPCA permit.
RECOMMENDED: Indicates design guidance that is helpful for filtration practice performance but not critical to the design.
Of course, there are situations, particularly retrofit projects, in which an infiltration facility is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular infiltration facility, depending on where it is situated both jurisdictionally and within the surrounding landscape.
All of the iron-enhanced sand filters covered here are suitable as retrofits and may be best employed downstream or in conjunction with existing wet ponds or other settling basins. The iron-enhanced sand filter basin should not be placed downstream of a pond or wetland that delivers an unabated flow of stormwater to the filter. If the filter bed is not allowed to drain dry to promote bed aeration, it is possible that the bed may become anaerobic and cause filtration bed fouling or iron loss.
The following table provides guidance regarding the use of filtration practices in areas upstream of special receiving waters. The corresponding information about other BMPs is presented in the respective sections of this Manual.
Summary of design restrictions for special waters for iron enhanced filters.
Link to this table
|A Lakes||B Trout Waters||C Drinking Water||D Wetlands||E Impaired Waters|
|Iron enhanced filters||RECOMMENDED||RECOMMENDED||RECOMMENDED||RECOMMENDED||RECOMMENDED|
The iron-enhanced sand filter basin and the iron-enhanced sand filter bench in wet ponds are both suitable for cold climates.
Iron-enhanced sand filters do not provide water quantity control. (Currently, no volume reduction credit is given for iron-enhanced sand filtering systems. Volume losses through evapotranspiration and infiltration below an underdrain are being investigated for all BMPs and will be applied if it is deemed appropriate.)
Although iron-enhanced sand filters can remove solids, the primary water quality benefit of iron-filters is the removal of dissolved constituents. Limited solids and phosphorus removal data are available for full scale treatment systems. Available data are provided below for an iron-enhanced sand bench that was constructed for a wet pond in Prior Lake, Minnesota and an iron-enhanced sand filter basin constructed in Maplewood, Minnesota. The outflow concentrations can be used to assess how well a BMP is performing and the potential benefits to down-gradient receiving waters.
Iron-enhanced sand filters are not designed to discharge a part of the effluent to groundwater nor are they designed to treat all runoff events. The water quality benefit of the iron-enhanced sand filter should only be accrued based on the volume of water that is treated by the BMP.
Pollutant concentrations and removals for iron enhanced filters.
Link to this table
|Practice||TSS Out (mg/L)||TSS removal (%)||TP Out (mg/L)||TP removal (%)||Phosphate Out (mg/L)||Phosphate removal (%)|
1 Parallel iron-enhanced sand filter benches in a wet pond. Values are from an average of two parallel 7.2% and 10.7% iron by weight iron-enhanced sand benches reported by Erickson et al. (2010, 2012). Averages are from a total of five storms monitored from July through September 2010. Values reported as below the detection limit were set equal to one-half the detection limit when calculating average phosphorus at the outlet and percent removals.
2 Iron-enhanced sand filter basin. Values are from an average of 19, 19, and 11 (for TP, TSS, and phosphate, respectively) storm events monitored from April through September 2010. For phosphate, only storms with above detection limit data at the inlet were used to calculate removals. Phosphate data below detection limits at the outlet were set equal to one-half the detection limit (0.01 mg/L) when calculating an average and removal rates. Data were collected by the Ramsey-Washington Metro Watershed District and reported by Barr Engineering Company, December, 2010.
3 ND is "not determined"