Permeable Covers

Application: used to reduce odors and emissions from manure storages

Pros

• Relatively easy to implement
• Increase nitrogen retention in the manure
• Can be used with earthen, concrete, and steel manure slurry manure storages at swine, dairy, or beef animals.

Cons

• Only applicable to outdoor manures storages
• Some types of permeable covers (biomass covers) have short lifespans
• Can make agitation of the manure more difficult
• Effectiveness varies based on level of coverage
• Rainwater volume must be accounted for in the manure storage since direct precipitation will infiltration through cover and into the manure

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. Straw cover being installed on a 4-acre lagoon in Iowa. Baled straw is ground and blown onto provide an approximately 6 to 12" thick layer of straw. Higher levels of odor control are generally seen at the higher thickness.

Permeable covers are materials that lie directly on the surface of the stored manure and provide a physical barrier between the manure and the surrounding air. Examples of permeable covers include natural crusts, layers of natural vegetative materials (such as straw, corn stalks,ground corncobs, etc.), vegetable oils, permeable fabrics (geotextiles), as well expanded clays, ceramics, and ground rubbers (examples include LECA andMacrolite). The success of permeable covers depends on achieving season-long floatation and continuous 100% coverage of the manure storage structure.

The principle behind permeable covers is to increase the resistance of gas transfer from the liquid to air. Permeable covers do this by providing a layer of material on the manure surface that shields the manure from contact with moving air. This reduces transfer of hydrogen sulfide, ammonia, and other volatile odorous compounds from the liquid manure; however, higher emission rates may occur when the cover is removed or the manure is agitated. Due to the potential buildup of manure gasses, especially hydrogen sulfide, caution should be used when removing the cover and agitating the manure. In addition to reduction gas transfer, most permeable cover materials provide an aerobic zone above the manure. Aerobic microorganisms can consume many odorous compounds; therefore covers that provide aerobic layers at the storage surface may facilitate growth and development of these aerobic organisms and provide additional odor reduction.

Permeable covers do not require gas collection systems as gases produced by the decomposition of the manure will slowly migrate through the cover. Similarly, rainwater collection systems are not required, as rainfall onto the cover surface will infiltrate through the cover and into the storage. However, this means that rainfall must be accounted for when sizing the manure storage. Additionally, permeable covers tend to reduce evaporation from the manure storage, thus manure storages using permeable covers need to be sized to have sufficient storage when accounting for the reduced evaporation.

Figure 2. Geo-synthetic cover installed on a manure storage. Edge of the geo-membrane is typically trenched in. Note, these covers are not to be walked on and the storage should be surround by a fence.

Cover maintenance varies greatly with the type of cover utilized. Fabric (geotextile) cover maintenance includes repairing tears and punctures, 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. Straw and crop residue covers may break up or sink due to high winds and heavy rains. To be effective these biomaterial covers should be at least 8” thick, but 12” is recommended as thicker layers have been found to float longer and be more effective. Biomaterials that have waxy coatings and large open cross sections that facilitate floating tend to have longer cover lives as they resist absorbing water and are naturally more buoyant. If straw/biomaterial covers start to break up additional straw material should be added, but it is important to remember that sunk straw will have to be dealt with at the time of agitation and manure removal.Straw and biomaterial covers tend to provide short-term solutions (2 to 6 months) depending on manure solids content and rainfall amounts) for odor control before additional straw is required. Light weight expanded clay aggregates tend to have higher life expectancies (often in excess of ten years) and require less maintenance but do need to be segregated from agitators and pump inlets when removing manure.

One major concern with all covers is the impact it has on our ability to agitate and pump out the manure from the storage unit. Natural crusts and permeable covers made with other biomaterials can usually be broken up, mixed into the manure, and pumped from the manure storage when the manure is land applied but generally require the use of a chopper pump to handle the larger particles. If using custom manure haulers notify them prior to pumping that you have a biocover that will need to be broken up during agitation. Permeable covers made with geotextile fabrics or floating synthetic materials (expanded clays) may need to be moved out of the way to facilitate agitation and. In the case of geotextile covers this may involve removing the cover and then replacing it after pumping or site specific designs that facilitate agitation without cover removal. For expanded clays and ground rubbers,agitation and pump out must be conducted in a way that ensures the floating cover material is not removed.This can generally be performed with minimal changes to standard practices agitation practices by corralling the floating cover material away from agitation and pump out locations.

Effectiveness

Cost Considerations

Costs include the initial costs of purchasing the permeable cover material, the costs of applying it over the surface of the manure storage, and costs associated with extra difficulties in agitating and pumping out the manure storage.

A wide range of organic and manmade materials have been utilized to construct permeable covers with variable results and costs ranging from $0.10 to $1.75 per square foot installed. Straw/corn stover is one of the low cost options at around $0.10 per square foot to purchase material and $0.02 to 0.04 to apply, while longer lasting materials such as LECA may cost $1.00 to $3.00 per square foot.

References

Clanton, C.J., D.R. Schmidt, L.D. Jacobson, R.E. Nicolai, P.R. Goodrich, K.A. Janni. 1999. Swine manure storage covers for odor control. Applied Engineering in Agriculture 15(5): 567-572.

Clanton, C.J., D.R. Schmidt, R.E. Nicolai, L.D. Jacobson, P.R. Goodrich, K.A. Janni, and J.R. Bicudo. 2001. Geotextile fabric-straw manure storage covers for odor, hydrogen sulfide, and ammonia control. Applied Engineering in Agriculture 17(6): 849-858.

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:

A Review of Permeable Cover Options for Manure Storage
Permeable Covers for Odor and Air Pollution Mitigation in Animal Agriculture - A Technical Guide