Permeable covers have been successfully constructed from a variety of materials including straw, cornstalks, Light Weight Expanded Clay Aggregate (EXPANDED CLAY AGGREGATE (ECA), ground rubber, plastic beads and geotextile materials. Other materials such as Vermiculite, Perlite and various oils have been tested, but found to provide unsatisfactory performance. Odor control effectiveness ranging from 40 – 90% has been reported for permeable covers made from various materials. Straw permeable covers are reported to have an odor control effectiveness of 40% when applied in a 4 inch depth and 90% at a 12 inch depth. Geotextile covers are reported to have a odor control effectiveness ranging from 40 – 60%, while a floating EXPANDED CLAY AGGREGATE (ECA) cover is reported to have a 90% odor control effectiveness. Ammonia control effectiveness is typically reported to be lower than odor control while hydrogen sulfide control is usually higher than odor control when comparing the same materials. Permeable covers mitigate air emissions from manure storages through two mechanisms. First, permeable covers provide a physical barrier that prevents direct contact of air to the stored manure’s surface. This reduces the transfer of gaseous compounds into the atmosphere additionally, many permeable cover materials provide an environment that promotes the growth of aerobic bacteria in the floating cover material. These aerobic bacteria can oxidize many of the odorous compounds released from the stored manure as they pass through this aerobic interface.
Materials such as straw, EXPANDED CLAY AGGREGATE (ECA), or other materials that allow air to penetrate the floating cover layer and provide adequate moisture and surface area to allow a population of aerobic bacteria to develop can provide this increased odor reduction effect. Floating permeable covers can be used to cover manure slurries of any species. It should be noted however that because these covers float on top of the manure’s surface, they will usually have a longer working life when used with manure slurries that have a relatively high total solids content. Research conducted by the Silsoe Research Institute in the United Kingdom directly addressed the impact of both rainfall and manure total solids content on cover longevity. The results of this research show that dry straw floats very well on manure slurries, but that when rained on, the rate at which the straw sinks dramatically increases, and that the rate at which the straw sinks increases as the manure slurry total solids content decreases. While straw covers are reported as being impractical on manure storages with over two acres of surface area, examples of straw covers on manure storage lagoons as large as four acres can be found. The authors agree however that the larger the manure storage structure the more difficult a uniform application of materials such as straw becomes. While a straw cover has a low initial cost, it also has a short lifespan as a cover. Straw covers floated on manure slurries can be expected to remain floating on the manures surface for two to six months. The length of time that a straw cover will continue to float is a function of rainfall and total solids content of the manure slurry the cover is floated on. Research has demonstrated that straw floated on water and exposed to rainfall sank in less than seven days, while straw floated on an eight percent total solids cattle slurry and exposed to rainfall maintained 80 percent surface coverage for 40 days. Straw floated on a beef manure slurry with four percent total solids and no exposure to rainfall provided an 80 percent surface coverage for 40 days, while straw floated on a beef manure slurry with eight percent total solids and no exposure to rainfall provided a 100 percent surface coverage for the full length of the 77 day test period (MAFF, 2000). At some point however, a straw cover will sink, and when this occurs it increases the difficulty of agitating and land applying the stored manure. Given the short-lived nature of a straw cover and the increased agitation and pumping difficulties that must be dealt with, straw covers are best used as a short-term odor control measure that can be implemented to control an unexpected or severe odor problem associated with a manure storage system. If long-term odor or emissions control is needed, a material with a longer lifespan than straw should be selected. A large round bale of straw can be expected to cover approximately 500 square feet of surface area when chopped and applied to a manure storage structure at a 12 inch depth (Nicolai, et. al, 2004). The cost of such a cover can be easily calculated based on the current price of straw in the region where the cover will be installed. For instance, if a 6 foot diameter roll of straw costs $40 per roll and 500 square feet could be covered per roll, the material cost for the cover would be $0.08 per square foot. Permeable straw covers are typically chopped and blown onto the manure storage surface. Figure 2 shows a straw cover one week after installation on an earthen manure storage structure. Lightweight Expanded Clay Aggregate, or EXPANDED CLAY AGGREGATE (ECA), is produced by heating a pre-treated clay with high plasticity in a rotary kiln, followed by burning the material at 1100 degrees C to produce the final EXPANDED CLAY AGGREGATE (ECA) product. EXPANDED CLAY AGGREGATE (ECA) is typically used as a lightweight aggregate construction material, but has been demonstrated to work very well as a permeable manure cover for both odor and ammonia control. Research has shown EXPANDED CLAY AGGREGATE (ECA) to provide 90% odor control and ammonia control ranging from 65-95% (MAFF, 2000, Nicolai, et. al, 2004). EXPANDED CLAY AGGREGATE (ECA) applied two to four inches deep on swine manure storage tanks in Iowa has been demonstrated to have a more than a 10 year lifespan, and has provided excellent odor control. While EXPANDED CLAY AGGREGATE (ECA) makes an excellent permeable cover, the initial capital cost is more expensive than many other permeable cover materials, with a reported cost of $1.75 per square foot installed (Nicolai, et. al, 2004). Availability within the United States is also a potential issue with EXPANDED CLAY AGGREGATE (ECA). While EXPANDED CLAY AGGREGATE (ECA) is produced in many areas of the world including India, Denmark, the United Kingdom, Iran, Italy and Spain, no producer in the United States is currently known to supply EXPANDED CLAY AGGREGATE (ECA) as a permeable manure cover material. Because of this, a large portion of the cost reported with EXPANDED CLAY AGGREGATE (ECA) use in the United States as a permeable manure cover is derived from the shipping cost to import the EXPANDED CLAY AGGREGATE (ECA). Figure 3 shows a EXPANDED CLAY AGGREGATE (ECA) permeable cover in-place on a swine manure storage tank. For more details click here