Impermeable Covers

Application: used to reduce odors and emissions from manure storages or to capture biogas for heat or power generation

Pros

• Increases nitrogen retention in the manure.
• Reduces dilution of manure due to rainwater.
• Can be used to capture methane gas produced by manure.
• Can be used with earthen, concrete, and steel slurry manure storages at swine, dairy, or beef sites.

Cons

• Only applicable to outdoor manures storages
• Can make agitation of the manure more difficult
• Can be expensive to implement
• Typically need a rainwater handling system to remove water from cover

Description

Storage of manure is an important component of manure management as it facilitates delaying manure application until appropriate times in the crop growing cycle,or to avoid manure application on frozen or snow-covered ground. However, slurry/liquid manure storages can be a significant source of odor and gaseous emissions. Liquid manure storages tend to give off odor and gas emissions when the surface is disturbed, such as windy conditions or during agitation. Covers have been demonstrated to provide effective odor and air emission control from manure storages. Two types, impermeable and permeable, cover options are available. Impermeable covers provide excellent odor and emission control, but have a high capital cost. Permeable covers generally are not as effective, but generally have a substantially lower capital cost.

Figure 1. Detail of a batten strip and fasters used to attach a plastic liner to a concrete storage in a horizontal plane This attachment is generally preferable to a vertical attachment as it is simpler to do and reduces stress on the liner as compared to a vertical-to-hortizontal transition.

Impermeable covers are made from materials that are impermeable, i.e., do not allow the transfer of liquids or gases through them. The impermeable covers are placed over the manure storage to reduce gas and odor emissions from the manure by creating a physical barrier that prevents the transfer of volatile chemical compounds from the manure storage headspace to the atmosphere. These covers work by trapping gases emitted from the manure, causing concentration of these gases in the storage headspace to increase, thereby slowing release of the gas from the manure. The level of success of an impermeable cover is dependent only on the level of sealing between the cover material and the manure storage.

Impermeable covers consist of two types, rigid and flexible. Examples of rigid covers include concrete, wood/metal roofed structures, and plastic coated fabrics stretched over framing. Rigid covers can be utilized on small concrete manure storages, but are not recommended over lined earthen storages. For safety and longer life expectancy, the cover materials, structural members, and fasteners must be able to withstand exposure to moisture, corrosive gases, and manure. Rainwater falling on the cover must be directed off the cover and away from the manure storage structure and equipment should be kept off of the cover unless cover was explicitly designed for such activities.

Flexible covers are generally plastics,including geomembranes and geosynthetic materials, are typically constructed of high-density polyethylene, linear low-density polyethylene, polyvinyl chloride, or similar. Thicknesses of these membranes is usually 10 to 60 mils and they often consist of multiple layers to reduce permeability, provide reinforcement and strength, or provide ultraviolet or chemical protection. There are three types of flexible impermeable cover systems, floating covers, positive air pressure covers, and negative air pressure covers. As the name implies, floating covers are stretched over the manure storage where the cover floats on the manure surface. Positive air pressure covers generally have central posts that hold the cover up. The cover is then inflated either from biogas production or with the use of air blowers that produce enough pressure to lift the plastic up and withstand forces from wind, rain, and snow (covers are deflated to agitate and pump). Finally negative air pressure covers use fans to pull air out from the cover and create a vacuum that helps hold the cover in place on the liquid surface.

Figure 2. This is an example of an anchor trench on an earthen storage. In this photo the anchor trench has been filed with concrete to hold the cover in place.

Covers need to be designed to withstand wind damage, ultraviolet radiation, and other environmental effects (rainfall, hail, wildlife,etc.) and their level of success is dependent on how well they are sealed around the storage edges. Cover sealing on earthen basins and lagoons is often done with an anchor trench or by using batten strips and anchors to fasten plastic covers to concrete walls. Impermeable covers require gas collection/extraction system to prevent over-pressurizing the cover and causing it to rip or tear. This can be done providing negative pressure ventilation of the manure storage headspace, installing check valves that will vent at a lower pressure than that required to tear the cover or a weld seam, or by installing perforated pipe to collect and utilize the biogas. Similarly, rainwater collection and removal systems are required when impermeable covers are used. As these covers do not allow infiltration of rainfall through the cover, a means of removing the clean rainwater from the storage is required. This can be accomplished by laying a series of perforated collection pipes (or ballasts) on the cover to collect and drain water to a pumping system.

Plastic cover maintenance includes repairing tears and punctures in the cover, removal of debris that accumulate on the surface of the fabric, and either removal and replacement of the cover for manure agitation and pumping or a design that includes access for agitation of the manure. Additional maintenance on the rainwater removal systems includes pump upkeep and ensuring that the drainage system stays in place on operating correctly. Plastic and fabric covers are usually not designed to support animals or people walking on them, so a fence should be used to keep animals and people off the cover.

Effectiveness

Cost Considerations

Typical lives of these covers is anticipated to be around 10 to 15 years with installation costs of approximately $3 to 10 per square yard covered (price includes rainfall and gas venting systems). The cost is dependent on both the size of the storage and the level of sophistication required in the gas collection/venting system and the rainwater removal system.

References

Nicolai, R., S. Pohl, and D. Schmidt. Covers for manure storage units. Livestock Development in South Dakota: Environment and Health. FS 925-D. South Dakota State University Cooperate Extension Service.

VanderZaag, A.C., R.J. Gordon, V.M. Glass, R.C. Jamieson. 2008. Floating covers to reduce gas emissions from liquid manure storages: a review. Applied Engineering in Agriculture 24(5): 657-671.

Livestock and Poultry Environmental Learning Community:

Impermeable Covers for Odor and Air Pollution Mitigation in Animal Agriculture: A Technical Guide
Covers for Mitigating Odor and Gas Emissions in Animal Agriculture: An Overview