Description
Biofilters or biofiltration systems are a suite of BMPs that use engineered media, such as Biosorption Activated Media (BAM), to enhance nutrient removal when native soils cannot provide adequate pollutant removal. Another use is where the native soils do not have an adequate infiltration rate.
They are also used in Sensitive Karst Areas to remove nitrates where sandy soils (without clay) allow nitrate movement into the groundwater. Another use is along coastal areas where protection of estuary waters is needed.
Commonly used BAM is located at the bottom of retention basins or in filters after wet detention ponds. It has been shown that retention basins and detention ponds may use filters on the discharge from the ponds to further remove nutrients, usually the nitrate form of Nitrogen and Phosphorus (Chang, N., Wen, D., Valencia, A., Colona, W., and Wanielista, M. (2019). Comparison of Biological Nutrient Removal via Two Biosorption Activated Media between Laboratory-scale and Field-scale Linear Ditch for Stormwater and Groundwater Co-Treatment. Water, Air and Soil Pollution Journal).
Small catchment areas can discharge into depression areas, rain gardens, and tree wells that incorporate BAM. Typically, these are either offline retention BMPs or online stormwater detention BMPs serving small drainage areas.
The annual water quality effectiveness is based on the volume that passes through the filter. Not all runoff water (particularly from high rainfall events) will pass through the filter; therefore, the annual removal is less than the maximum filter removal effectiveness.
Biofiltration systems with BAM can incorporate natural soils, recycled materials, cellulose, or other engineered media. For the removal of Phosphorus, BAM must possess Phosphorus sorption capacity. For removal of nitrate, the BAM must retain moisture to promote a near-zero oxygen zone. Planted vegetation to facilitate treatment for removal of nutrients is common. The use of BAM can also reduce toxic compounds and organisms in discharge waters.
There are many opportunities for biofiltration systems (Wanielista et al., 2014), making them a superior choice for onsite treatment in urban development, particularly in redevelopment areas (Wanielista, M.P., and Flint, M. (2014a). Up-flow Filtration for Wet Detention Pond. Florida Stormwater Association Annual Meeting, Sanibel, FL).
The USEPA (United States Environmental Protection Agency (2023). Green Infrastructure, April 24. https://www.epa.gov/green-Infrastructure/what-green-infrastruture) defines on-site filters as Green Stormwater Infrastructure devices (GSIs). They may include underdrains for surface discharge and are classified as detention systems; however, they are frequently designed to function as retention systems.
Applications and BMP Types
BMPs and GSIs that may incorporate BAM include:
- Detention rain gardens
- Retention rain gardens
- Landscape planter boxes
- Tree wells or tree box filters
- Side-bank filters after wet detention
- Up-flow filters after wet detention
- Up-flow filters after baffle box chambers
- Regional retention basins
- Exfiltration systems (vaults, pipe and rock)
- Vegetated filter strips
Before a BAM is used within the State of Florida, acceptance must be granted by the review or regulatory agency. Media currently used (January 2026) are listed within the BMPFast software. If not listed, a “User Defined” option is available.
Users are encouraged to confirm design parameters such as treatment rate and acceptable removal procedures with the regulatory agency prior to finalizing annual removal documentation.
Example Cross Sections
Depths and types of media will differ with applications. See regulatory requirements.
Additional Information and Performance Considerations
As with any BMP, a filter only removes the pollutants that it was certified as measured to remove. An example is a filter that removes 80% of the particulate in TP and 25% of the dissolved TP when the input is runoff. If that same filter is used downstream of a detention pond which removes most of the particulate, the TP removal is reduced and frequently estimated for the dissolved fraction of TP. In many cases, it is recommended to conduct field measurements to determine the removal effectiveness.
The removal of TN and TP within BMPFast includes currently (January 2026) used BAM mixes with specified authenticity and field sample performance. It is understood additional media mixes will be developed or are being used on a limited basis; thus, there are options to add other BAMs, identified as User Defined Media (UDM). All media mixes should be certified for removal performance.
Once permits are issued, the media type is included in software by name as has been the practice. If design changes or performance numbers are changed after the permit is issued, they can be made within the software at the recommendation of the user and acceptance by the review agency.
Usually, the media is mixed in a central facility. If not mixed in a central facility, then samples to verify the mix percentages during on-site mixing must be available. The pollution removal now in use assumes design criteria that must be used. Thus, follow the examples and design and maintenance information within this manual.
Design information includes filtration (treatment) rates. These rates are determined by laboratory measures and validated from field operation under strict maintenance criteria. The rates used in the software have a safety factor of two, meaning the minimum field or lab rates are reduced by a factor of two to allow for operation difficulties.
| Media1 | Minimum | Treatment2 | Treatment Efficiency3 | Sustain | OP Removal5 | Density6 | |
| Depth (in) | Rate (GPM/SF) | TN (%) | TP(%) | Void %4 | Rate (mg/g) | Lbs/CF | |
| B&G ECT3 or ECT | 24 | 1 | 45/257 | 45/257 | 30 | 0.2 | 43 |
| ICS | 24 | 0.104 | 80 | 95 | 30 | 0.6 | 105 |
| User Defined | 24 | ||||||
| B&G CTS12 | 12 | 0.052 | 60 | 90 | 30 | 0.2 | 95 |
| B&G CTS24 | 24 | 0.052 | 75 | 95 | 30 | 0.2 | 95 |
| Per Pave8 | 3-12 | 0.052 | 60 | 90 | 30 | 0.2 | 95 |
| SAT9 | 24 | 0.02 | 20 | 45 | 30 | 0 | 100 |
2. Treatment Rate including a factor of safety equal to 2.
3. Average annual concentration reduction;
mass reduction is the product of concentration and water passing through filter.
4. Percent of water in the media after loading with stormwater equal to 10 years of operation.
5. Measured dissolved fraction usually as Ortho-Phosphate (OP) in (mg OP/g of media).
6. Dry densities at operating density, will vary with source materials.
7. 45/25 (stand-alone BMP/downstream of another BMP) average annual efficiencies.
8. Limited to the bottom of a 3 to 12-inch-deep pervious pavement reservoir.
9. Limited to a stand-alone BMP or first in a series of BMPs, reflecting particulate removal.
Notes: 1. Average annual concentration reduction values are for a 50% mix of particulates.
2. Efficiencies are for stormwater runoff events with dry periods (no rainfall).
3. Continuous application of runoff may affect the concentration reduction.
4. Species mix for TN has DON less than 20% of the TN.
Filter media used to remove a pollutant will have a design life expectancy, called service life. The service life for nutrient removal is based on the capacity to remove OP that includes the media volume and treatment rate for OP in a year. Service life can be calculated for all of the media used within this manual.
It is also noted that BAM can be used for removal of other pollutants such as metals, solids, toxic compounds, bacteria including pathogens, and emerging contaminants. There are no suggested data on removal other than Total Nitrogen and Total Phosphorus within the BMPFast software.
In Florida, the CTS12 media has been tested with VFS and in filters under pavements. The BAM Per Pave media is composed of the same media mix as CTS but has been used at 2-, 3-, and 12-inch depths in the reservoir of permeable pavement.
Also note the removal of nutrients by some media filters after a wet detention pond is less than the removal if the media is used as a stand-alone BMP. For example, ECT3 media has a stand-alone removal percentage of 45%, but when used in combination with a wet detention pond, the removal percentage is 25% (Wanielista and Flint, 2014a).
Caution
As with any BMP, the input water quality must be stormwater related within the EMC concentration values and species distribution. The amount of dissolved organic nitrogen (DON) must be less than about 10% of the total. For DON greater than 10%, consult the supplier of the BAM for removal effectiveness.
Input Data
Data to define the type and characteristics of a BAM media are to define the media type within the software menu.
The type of media is selected and the TN and TP maximum reduction percentage is noted. Not all of the annual runoff water will go through the filter; thus, the maximum percentage reduction is not attained. Only the water passing through the filter receives the maximum reduction. If 80% of the water passes through the filter in a year, then the pollutant reduction equals 80% of the maximum.
An example selection of B&G CTS24 media is shown. The removal time is greater than 2 hours, allowing maximum removal of TN. Removal time is calculated as:
Residence Time = Filter Depth (inches) ÷ Treatment Rate (inches/hour)
The CTS treatment rate is 5 inches per hour (0.052 GPM/SF). For 24 inches of BAM:
24 ÷ 5 = 4.8 hours
The ECT mixes have a faster treatment rate and residence time less than 2 hours, resulting in an efficiency of 25%.
An example of using a BAM B&G24 filter in a side drain after a wet pond shows approximately 82% of the annual runoff passing through the filter, resulting in a treatment depth of 1 inch. This 1 inch can be determined using the retention BMP worksheet. The average annual removal is shown as 61% for TN and 78% for TP.
