BioCycle January 2006, Vol. 47, No. 1, p. 34
Compost blankets, berms and filter socks are now included on the U.S. EPA’s national menu of best management practices for storm water management. Part I
AS PART of the federal Clean Water Act, the U.S. Environmental Protection Agency promulgated rules for storm water management. The rules were implemented in two phases – Phase I was targeted at larger municipalities and construction sites, and Phase II, which came into effect in March 2003, was targeted at smaller municipalities and construction sites less than 5 acres and larger than one acre. Phase II has six minimum control measures that need to be implemented. On its Phase II website, EPA explains that these minimum control measures typically are implemented by applying one or more Best Management Practices (BMPs) appropriate to the source, location, and climate. The practices listed in its national menu of BMPs have been found by EPA to be representative of the types of practices that can be applied successfully to achieve the minimum control measures. The menu is intended to provide guidance as to the types of practices that can be used to develop and implement storm water management programs.
In a successful collaboration, staff in EPA’s Office of Solid Waste, Office of Water, and Regions 4 and 5 worked with contractor Booz, Allen and Hamilton to draft BMP fact sheets for compost-based tools to be included on the national menu. A technical review group was assembled to provide feedback on the drafts. Ultimately, three BMP fact sheets were written, covering compost blankets, compost filter berms and compost filter socks. In November 2005, the final drafts of these BMPs were submitted to EPA’s Office of Water for inclusion on the national menu of BMPs. The Office of Water expected to have the BMPs posted by the end of January (see http://cfpub.epa.gov/npdes/stormwater/menuofbmps/menu.cfm).
The compost BMPs drew heavily from specifications for compost blankets and filter berms developed for the American Association of State Highway Transportation Officials (AASHTO) in 2003, under a contract awarded to Ron Alexander of R. Alexander Associates by the Recycled Materials Resource Center. The compost use parameters in the new fact sheets were adjusted, where necessary, to reflect more current research data and field experiences.
The following categories are covered in the fact sheets: Description, Applicability, Siting and Design Considerations (which includes Compost Quality); Installation; Limitations; Maintenance Considerations; Effectiveness; Cost Considerations; and References. The compost parameters cover pH, soluble salt concentrations, moisture and organic matter content, stability and physical contaminants. Each fact sheet also states that the composts to be used can be made from a variety of feedstocks, including municipal yard trimmings, food residuals, separated municipal solid waste, biosolids, and manure. Part I of this article presents highlights of the new BMP fact sheets. Part II, to appear in the next issue, will provide more specific details regarding siting and design considerations as well as cost and background research.
A compost blanket is a layer of loosely applied compost or composted material that is placed on the soil in disturbed areas to control erosion and retain sediment resulting from sheet-flow runoff. It can be used in place of traditional sediment and erosion control tools such as mulch, netting, or chemical stabilization. When properly applied, the erosion control compost forms a blanket that completely covers the ground surface (making direct compost-to-surface contact). Compost blankets can be placed on any soil surface: rocky, frozen, flat, or steep. The method of application and the depth of the compost applied will vary depending upon slope and site conditions. The compost blanket can be vegetated by incorporating seeds into the compost before it is placed on the disturbed area or the seed can be broadcast onto the surface after installation.
Advantages of compost blankets over more traditional storm water BMPs such as geotextile blankets include: Retains a large volume of water, which helps reduce runoff, prevents or reduces sheet and rill erosion, and aids in establishing vegetation in the blanket; Acts as a buffer to absorb rainfall energy, which prevents soil compaction and crusting and facilitates rainfall infiltration; Facilitates plant growth by capturing and retaining moisture and providing a suitable microclimate and nutrients for seed germination. In addition, the compost improves the soil structure and removes pollutants from storm water, thus improving downstream water quality.
Specific site characteristics, such as existing vegetation, climate, structural attributes of the site, annual rainfall, and rainfall erosivity, are considered when determining the appropriate depth for the compost blanket (application should be uniform, with a thickness of between 1- and 3-inches). The fact sheet has sample compost blanket depths for various rainfall scenarios. Very coarse compost should be avoided on slopes that will be landscaped or seeded, as it will make planting and crop establishment more difficult. Thicker and/or coarser compost blankets are recommended for areas with higher annual precipitation or rainfall intensity, and for areas subject to wind erosion.
A compost filter berm is a dike of compost or a compost product that is placed perpendicular to sheet flow runoff to control erosion in disturbed areas and retain sediment. It can be used in place of a traditional sediment and erosion control tool such as a silt fence or straw bale barrier. The compost filter berm, which is trapezoidal in cross section, provides a three-dimensional filter that retains sediment and other pollutants (e.g., suspended solids, metals, oil and grease) while allowing the cleaned water to flow through the berm. Compost filter berms are generally placed along the perimeter of a site, or at intervals along a slope, to capture and treat storm water that runs off as sheet flow. A filter berm also can be used as a check dam in small drainage ditches. The mix of particle sizes in the compost filter material retains as much or more sediment than traditional perimeter controls, such as silt fences or hay bale barriers, while allowing a larger volume of clear water to pass through the berm (silt fences often become clogged with sediment and form a dam that retains storm water). The berms can be vegetated or unvegetated. Vegetated filter berms are normally left in place and provide long-term filtration of storm water as a postconstruction BMP. Unvegetated berms are often broken down once construction is complete and the compost is spread around the site as a soil amendment or mulch.
Common industry practice is to use compost filter berms in drainage areas that do not exceed 0.25 acre per 100 feet of berm length and where flow does not typically exceed one cubic foot per second. The actual dimensions should be modified based on site-specific conditions. The precipitation and the rainfall erosivity index need to be taken into consideration as well. For sites in high rainfall areas or where there are severe grades or long slopes, larger dimension berms should be used. The project engineer may also consider placing berms at the top and base of the slope, constructing a series of berms down the profile of the slope (15 to 25 feet apart), or using filter berms in conjunction with other storm water BMPs such as compost blankets or silt fences.
The fact sheet includes the AASHTO filter berm specification, which covers the quality and particle size distribution of compost to be used. Other tables cover berm dimensions for various rainfall scenarios, as well as slope and slope length. Accumulated sediments should be removed when they reach approximately one-third the height of the berm. If the berm has experienced significant washout, increasing the size of the filter berm or adding another BMP, such as an additional compost filter berm or compost filter sock, a compost blanket, or a silt fence, could remedy the problem.
COMPOST FILTER SOCKS
A compost filter sock is a type of contained compost filter berm. It is a mesh tube filled with composted material that is placed perpendicular to sheet-flow runoff to control erosion and retain sediment in disturbed areas. The compost filter sock, which is oval to round in cross section, provides a three-dimensional filter that retains sediment and other pollutants (e.g., suspended solids, nutrients, and motor oil) while allowing the cleaned water to flow through. The filter sock can be used in place of a traditional sediment and erosion control tool such as a silt fence or straw bale barrier. They are generally placed along the perimeter of a site, or at intervals along a slope, to capture and treat storm water that runs off as sheet flow. There is greater surface area contact with soil than typical sediment control devices, thereby reducing the potential for runoff to create rills under the device and/or create channels carrying unfiltered sediment. Additionally, they can be laid adjacent to each other, perpendicular to storm water flow, to reduce flow velocity and soil erosion. Filter socks can also be used on pavement as inlet protection for storm drains and to slow water flow in small ditches.
Filter socks are assembled by tying a knot in one end of the mesh sock, filling the sock with the composted material (usually using a pneumatic blower), then knotting the other end once the desired length is reached. A filter sock the length of the slope is normally used to ensure that storm water does not break through at the intersection of socks placed end-to-end. In cases where this is not possible, the socks are placed end-to-end along a slope and the ends are interlocked. The diameter of the filter sock used will vary depending upon the steepness and length of the slope. Once the filter sock is filled and put in place, it should be anchored to the slope. The ends of the filter sock should be directed upslope, to prevent storm water from running around the end of the sock.
Compost filter socks can be vegetated or unvegetated. Vegetated filter socks can be left in place to provide long-term filtration of storm water as a postconstruction BMP. The vegetation grows into the slope, further anchoring the filter sock. Unvegetated filter socks are often cut open when the project is completed, and the compost is spread around the site as soil amendment or mulch. The mesh sock is then disposed of unless it is biodegradable. If there is excessive ponding behind the filter sock or accumulated sediments reaches the top of the sock, placement of an additional sock is recommended.
The compost specifications for vegetated filter berms developed for AASHTO are also applicable to vegetated compost filter socks. Unvegetated compost filter socks, however, have shown that they require a coarser compost than vegetated filter berms. The Minnesota DOT erosion control compost specifications for “compost logs” recommend 30 to 40 percent weed-free compost and 60 to 70 percent partially decomposed wood chips, and that 100 percent of the compost passes the 2-inch (51 mm) sieve and 30 percent passes the 3/8inch (10 mm) sieve. – N.G.