BioCycle February 2006, Vol. 47, No. 2, p. 21
Last month, we reported that EPA’s storm water office was about to include compost blankets, filter berms and filter socks as best management practices for storm water management. In late January, it became official. Part II
IT’S official! In Part I of this article in the January issue, “Compost on EPA Menu for Managing Storm Water,” we noted that an announcement was forthcoming from the U.S. EPA’s Office of Water regarding the inclusion of three compost-based tools for storm water management on its national menu of best management practices (BMPs). On January 23, 2006, the Office of Water officially posted fact sheets for compost blankets, compost filter berms and compost socks for Construction Site Storm Water Runoff Control. Links to the three BMPs are noted in the accompanying box.
Part I presented highlights of the new BMP fact sheets, including descriptions of blankets, berms and socks, where they typically are used and what standard BMP they can replace. Part II covers other parts of the fact sheets, including effectiveness of the BMPs, limitations and maintenance. Research references referred to in the next section can be found in the fact sheets.
EFFECTIVENESS OF BMPS
Blankets: Numerous studies conducted by a variety of universities and state Departments of Transportation (DOT) have reported the effectiveness of compost blankets; the fact sheets cite only a few of the more recent studies. For example, a University of Georgia research trial found that correctly applied compost blankets provide almost 100 percent soil surface coverage, while other methods (e.g., straw mats and mulches) provide only 70 to 75 percent coverage. Uniform soil cover by the compost blanket is a key component to effective erosion and sediment control because it helps maintain sheet flow and prevents storm water from forming rills under the blanket. Compost blankets also help protect the structural stability of the slope, particularly when vegetated. An Iowa State University study, sponsored by the Iowa Department of Natural Resources and Iowa DOT, compared compost-treated road embankments to conventionally treated embankments (i.e., topsoil added to surface). The study exposed the test plots to high intensity rainfall (4 inches/hour) lasting at least 30 minutes. The results showed that the 2- and 4-inch thick compost blankets reduced runoff from the embankment by 80 percent. The erosion rate from the compost blanket was less than one percent of that from the noncomposted areas, and weed growth on compost-treated areas was approximately 25 percent of that on untreated areas.
Filter Berms: Numerous studies have reported the effectiveness of compost filter berms in removing settleable solids, total suspended solids, and various organic and inorganic contaminants from storm water. These studies have consistently shown that compost filter berms are at least as effective as other traditional erosion and sediment control BMPs in controlling sediment; however, the results of the studies varied depending upon the site conditions. One example is a study conducted in Portland, Oregon in the early 1990s, which compared the effectiveness of a silt fence and a mixed yard debris compost filter berm to a control plot during five storm events. The study found that the filter berm was over 90 percent effective in removing settleable and total suspended solids when compared to the control plot and was approximately 66 percent more effective than the silt fence. A study performed by the Snohomish County, Washington, Department of Planning and Development Services showed no decrease in turbidity with a silt fence but a 67 percent reduction in turbidity using a compost filter berm.
Filter Socks: A large number of qualitative studies have reported the effectiveness of compost filter socks in removing settleable solids and total suspended solids from storm water. These studies have consistently shown that compost filter socks are at least as effective as traditional erosion and sediment control BMPs and often are more effective. Compost filter socks are often used in conjunction with compost blankets to form a storm water management system. Together, these two BMPs retain a very high volume of storm water, sediment, and other pollutants. The compost in the filter sock can also improve water quality by absorbing various organic and inorganic contaminants from storm water, including motor oil. In 2005, Filtrexx International conducted a laboratory test using 13 types of compost in filter socks. They found that half of the compost filter socks removed 100 percent of the motor oil introduced into the simulated storm water (at concentrations of 1,000 to 10,000 milligrams per liter [mg/L]) and the remaining compost filter socks removed over 85 percent of the motor oil from the storm water.
LIMITATIONS AND MAINTENANCE
The compost blanket limitations cited in the fact sheet are dependent on the site specifications, e.g., they are not generally used on slopes greater than 2:1 or in areas where concentrated runoff or water flow will occur. They can, however, be used on steeper slopes (1:1) if netting or confinement systems are used in conjunction with the compost blanket to further stabilize the compost and the slope or if the compost particle size and compost depth are specially designed for the application. With regard to maintenance, if areas of the compost blanket have washed out, another layer of compost should be applied. In some cases, it may be necessary to add another storm water BMP, such as a compost filter sock or silt fence. On slopes greater than 2:1, establishing thick, permanent vegetation as soon as possible is the key to successful erosion and sediment control.
Filter berms are not suitable for areas where large amounts of concentrated runoff are likely, such as streams, ditches, or waterways, unless the drainage is small and the flow rate is relatively low. Compost filter berms should be inspected regularly, as well as after each rainfall event, to ensure that they are intact and the area behind the berm is not filled with silt. Accumulated sediments should be removed from behind the berm when the sediments reach approximately one-third the height of the berm. Any areas that have been washed away should be replaced. If the berm has experienced significant washout, a filter berm alone may not be the appropriate BMP for this area. Depending upon the site-specific conditions, the site operator could remedy the problem by increasing the size of the filter berm or adding another BMP in this area, such as an additional compost filter berm or compost filter sock, a compost blanket, or a silt fence.
To ensure optimum performance of compost filter socks, heavy vegetation should be cut down or removed, and extremely uneven surfaces should be leveled to ensure that the sock uniformly contacts the ground surface. Filter socks can be installed perpendicular to flow in areas where a large volume of storm water runoff is likely, but should not be installed perpendicular to flow in perennial waterways and large streams. Compost filter socks should be inspected regularly, as well as after each rainfall event, to ensure that they are intact and the area behind the sock is not filled with sediment.If there is excessive ponding behind the filter sock or accumulated sediments reach the top of the sock, an additional sock should be added on top or in front of the existing filter sock in these areas, without disturbing the soil or accumulated sediment. If the filter sock was overtopped during a storm event, the operator should consider installing an additional filter sock on top of the original, placing an additional filter sock further up the slope, or using an additional BMP, such as a compost blanket in conjunction with the sock(s).
Some additional information on compost filter socks that was not presented in Part I of this article include the following: Socks are flexible and can be filled in place or filled and moved into position, making them especially useful on steep or rocky slopes where installation of other erosion control tools is not feasible. Since they do not have to be trenched into the ground, the socks can be installed on frozen ground or cement. Filter socks used for erosion control are usually 12 inches in diameter, although 8-inch, 18-inch, and 24-inch diameter socks are used in some applications. The smaller, 8-inch diameter filter socks are commonly used as storm water inlet protection. The size sock needed on a slope depends upon the degree of slope and slope length. For example, on slopes in the range of 10:1 to 50:1, with length of 125 feet, a 12-inch diameter sock is adequate, notes the fact sheet. Alternatively, on 2:1 to 3:1 slopes that are 50-feet in length, an 18-inch sock is recommended. Finally, the preferred anchoring method is to drive stakes through the center of the sock at regular intervals; alternatively, stakes can be placed on the downstream side of the sock.
THE U.S. Environmental Protection Agency’s Office of Water officially posted fact sheets for compost blankets, compost filter berms and compost socks for Construction Site Storm Water Runoff Control. Links to the BMPs are:
February 17, 2006 | General
It's Official! Compost Tools On EPA List As Stormwater BMPS
BioCycle February 2006, Vol. 47, No. 2, p. 21