A compost sock is a mesh tube filled with composted materials placed perpendicular to sheet-flow runoff to control erosion and retain sediment in disturbed areas. Depending on the particle size of the filter medium within the mesh tube, landscapers and contractors can either use them to reduce heavy flows and trap sediment or to remove pollutants from urban runoff.
Compost filter socks are proven effective at removing gasoline, diesel, and oil residues from runoff. Filter socks come in 5-inch, 8-inch, and 12-inch diameters and can either be purchased on pallets or made on site using the mesh tube and a blower truck.
Where to Use Treatment
Use compost socks to manage stormwater, control erosion, and remove pollutants from runoff. Place on erodible slope faces at regular intervals, at the slope top and toe, and at grade breaks. Align compost socks with the slope contour. Biodegradable socks are available in both 8-inch and 12-inch diameters in cotton, jute, sisal, burlap, wood-based yarn, or coir. Biodegradable compost socks will likely function up to 12 months, depending on climate and weather conditions. 8-inch diameter socks work well on slopes up to 2:1 (H:V). 12-inch diameter compost socks are too heavy for the middle of 2:1 slopes; however, they work well at the toe. Compost socks are typically spaced as follows:
- 10 feet apart for slopes steeper than 2:1 (horizontal:vertical)
- 15 feet apart for slopes from 2:1 to 4:1 (horizontal:vertical)
- 20 feet apart for slopes from 4:1 to 10:1 (horizontal:vertical)
- 50 feet apart for slopes flatter than 10:1 (horizontal:vertical)
For additional guidance, see the Storm Water Quality Handbook–Project Planning and Design Guide.
Benefits of Compost Filter Socks
- Reduces stormwater runoff velocity so that sediments can settle and be trapped
- Filters sediments out of stormwater, reducing runoff of sediments and heavy metals
- Very cost effective per volume of runoff treated
- Adsorbs contaminants, including oil and gasoline
- Acts as barrier against development of erosion rills and gullies
Limitations
- Some Regional Water Quality Boards may require compost socks to be setback from a water body 303d listed for nutrients.
- Sock must uniformly contact ground surface for maximum effectiveness.
- Difficult to install securely on steep or rocky slopes.
- 12-inch diameter compost socks are limited to being placed at the toe of 2:1 (H:V) slopes.
Plans and Details/Specifications
- Federal Specifications for Compost Filter Socks for Sediment and Erosion Control as specified by the American Association of State Highway and Transportation Officials, US Army Corps of Engineers, USDA Natural Resources Conservation Service, and USEPA
- Caltrans Erosion Control Toolbox webpage on Compost Sock
- USDA Agronomy Technical Note No. 4 – Utilization of Compost Filter Socks
- American Society of State Highway and Transportation Officials, U.S. Army Corps of Engineers, USDA-NRCS, USEPA: Federal Specifications for Compost Filter Socks for Sediment and Erosion Control
- California Stormwater Quality Association (CASQA) – Compost socks and berms (SE 13)
- Filtrexx Design Manual – Filter Media
Research
- Faucette, L.B. et al (2009): Storm Water Removal Performance of Filter Socks, Journal of Environmental Quality. 38(3): 1233-1239.
- Faucette, Britt et al. Performance of Compost Filtration Practice for Green Infrastructure Stormwater Applications. Water Environment Research. 85(9): 806-814.
- Keener, Harold M. et al (2007): Flow-Through Rates and Evaluation of Solids Separation of Compost Filter Socks versus Silt Fence in Sediment Control Applications. Journal of Environmental Quality. 36:742-752.
- Shipitalo, M.J. et al. (2012). Sorbent-amended compost filter socks in grassed waterways reduce nutrient losses in surface runoff from corn fields. Journal of Soil and Water Conservation. 67 (5): 433-441. Impact of Grassed Waterways and Compost Filter Socks on the Quality of Surface Runoff from Corn Fields
Resources
For more information contact: Organic Materials, organics@calrecycle.ca.gov