Description
Vegetated Filter Strips are used adjacent to highways and parking areas. They have selected bio-sorption activated media (BAM) beneath the surface.
Runoff water enters the VFS as overland flow. Then the water appears after passing through the BAM and is discharged to a surface water transport nearby, usually a ditch or swale.
The seasonal high-water table is usually greater than one foot below the ground at the point of surface discharge. Typically, 12 inches of media is used, and the removal effectiveness is based on this depth. The minimum width in the direction of flow to obtain capture and pollution control is 15 feet. Usually, for the first 15 feet, the water table intercepts the flow, or an impermeable membrane is used to redirect the flow horizontally.
There are various BAM media that must be used to get credit for removal (Chang, N, Dan Wen, Andrea Valencia, William Colona, and Martin Wanielista. (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).
A vegetated cover must be used as well as a minimum of 3 inches of topsoil with vegetation. The topsoil must infiltrate at a rate greater than the infiltration rate of the BAM. Typical values are 5-20 inches/hour. The storage volume of the BAM is dependent on the type of BAM used. If native soils are used, they must be homogeneously mixed and certified. Details on removal using BAM mixes in Florida have been documented (Shokri, M, et.al. (2021). Hydraulic and Nutrient Removal Performance of Vegetated Filter Strips with Engineering Infiltration Media for Treatment of Roadway Runoff. Journal of Environmental Management. Volume 300. Dec 15,113747.
A schematic of the VFS is shown as:
Required Parameters
Annual capture effectiveness is a function of:
- Vegetated filter strip width
- Width of the impervious catchment
- Type of BAM
- Slope of the VFS
- Rainfall zone
Important:
The VFS area must equal the catchment area.
Example calculation:
3000 ft × (18.56 + 25 ft) = 130,680 SF = 4 acres
Removal effectiveness is defined for:
- Width ≤ 30 feet
- Slope ≤ 20%
If slope is less than 2%, set slope = 2% for analysis.
Example Input data:
Using the Media Button, a 12-inch-deep B&G filter is selected. The time for treatment or passage through the media is usually greater than 2 hours. This is because the filter media rate of treatment is about 5 inches per hour.
The removal effectiveness using a 12 deep BAM filter for this site is 67% N and 80% P (which meets the 55% N and 80% P performance standard specific removal criteria).
Frequently Asked Questions
Q: Is a media required for pollutant removal? And what one is typically used?
A: Yes, the media is required, and the media button is used for input. Typically, the B&G CTS12 media is used.
Q: What is the residence time in the media?
A: The residence time is the minimum contact time, and for all the specified media, it is greater than 2 hours.
Q: What is the typical depth of media?
A: The typical depth to achieve pollutant removal is 1 foot.
Q: Can I use a VFS width less than 15 feet?
A: No, the minimum width is 15 feet. Research showed that the minimum width is required to achieve the removal.
Q: What if my width is greater than 30 feet? What do I do?
A: Any width greater than 30 feet will produce no additional removal.
Q: Does the catchment area equal the watershed area?
A: Yes, it is stated in the VFS input worksheet. After input data are entered, there is a printout under watershed characteristics to help you make it equal.
Q: My slope is 1%. What should I do?
A: If outside the lower limit for slope, use the boundary data, for example, if the slope is 1%, use the minimum of 2%.
Q: My slope is equal to 30%, so can I evaluate this VFS?
A: No, the limit on slope is 20%.