Using Synthetic Covers to Reduce Odor Emissions
by Jeff Lorimor, ISU Extension agricultural and biosystems engineer
Synthetic covers are among the many products livestock producers can use to minimize odor emissions from manure storages. During the Iowa Odor Control Demonstration Project several producers installed synthetic covers to evaluate their effectiveness.
Synthetic covers usually are made of plastic. Other covers are made of wood or concrete or biodegradable materials such as hay or straw. Manure storage covers work by interrupting the flow of gases up from the liquid surface into the atmosphere. Iowa law recognizes the effectiveness of covers by reducing the separation distance between covered pits and neighbors.
In the odor demonstration project, synthetic covers significantly reduced odors when 100 percent of the storage surface was covered. To be effective, covers must be firmly attached to prevent the wind from catching and whipping them, and they must cover as much of the storage structure as possible. When less than 100 percent of the surface is covered, the odor reduction is much less dramatic. Figure 1 shows how field day attendees evaluated a covered and uncovered storage at one of the demonstration sites.
Floating plastic covers are the most popular type of synthetic cover. However, one demonstration site used a cover supported by cables above the stored liquid, giving the appearance of a tent, Figure 2. Figure 3 shows a floating cover on an earthen basin.
Notice the rain water holding the floating cover down, out of potential wind, in Figure 3. The edges are trenched 4 feet into the soil all around the pit, and gas vent lines are installed beneath the cover. Without the vent lines, large bubbles would form under the cover during warm weather, causing problems. The cover has a pumpout opening next to the buildings.
Some methane gas will be generated under synthetic covers. Unless the pit is extremely large, however, it wont be enough to harvest. Gas from the pits shown above is simply burned off or vented to the atmosphere.
Minnesota researcher are testing a new geotextile cover. The cover is made of a porous plastic fabric that floats on the pit surface. Bubbles beneath the cover are not a problem because the geotextile is porous. Initial data indicate that geotextile covers significantly reduce gas emission. An added benefit is that geotextile covers cost much less than the solid plastic covers currently used in Iowa.
The cost of synthetic covers ranges from $0.25 per square foot of surface area to more than $1 per square foot installed. The larger the storage, the less the cost per square foot, but the greater the total cost. Due to cost considerations and the larger size of anaerobic lagoons, synthetic covers are feasible for pits, but not for lagoons. The geotextile being tested in Minnesota cost $1.25 per square yard installed, significantly less than the solid covers. Because synthetic covers last for many years, their annual costs are comparable to those for biocovers.
Overall, synthetic covers are an excellent odor control technology, but they do require some up front investment, and some additional management.
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Page last updated October 5, 2004
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