Monday, October 13, 2014

What's that smell? LECA as a Permeable Cover for Manure Odor Reduction

Manure - we all know that it provides some great nutrients that can help support crop production, but even its most adamant supporters will recognize that in some situations is can have a smell. Here I'm going to provide a short discussion on the science of manure odor and then discuss one specific management practice, the use of LECA as a permeable cover on a manure storage, that can be used to mitigate odor in some situations. The use of LECA is by no means  the only option, it only receives highlighting here due to a recent question I received about its use. More information on the use of LECA as well as other permeable covers can be found at http://www.agronext.iastate.edu/ampat/storagehandling/pc/homepage.html, while information about other odor and greenhouse gas mitigation options for animal production systems can be found at http://www.agronext.iastate.edu/ampat/homepage.html.

Manure odor chemistry is complex, there are more than 400 gases that contribute to the odor, but generally the majority of the odor comes from just a few gases, including ammonia, hydrogen sulfide, cresols, volatile organic acids, and amines. These compounds all can result from the partial decomposition of organic matter in the manure. These compounds are natural byproducts of anaerobic breakdown of the organic material in the manure. To be perceptible as odor, these compounds must escape from the liquid phase and be present as a gas. The release of these compounds is impacted by factors such as the compounds vapor pressure, solubility, the manure's pH, or even if a  crust is present on the manure preventing release of these compounds. Most of our developed mitigation techniques try to adjust one or more of these properties to prevent the release of the odorous compounds.

One option that is sometimes used for odor control is permeable covers. 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 and Macrolite). The success of permeable covers depends on achieving season-long floatation and continuous 100% coverage of the manure storage structure, in this post we'll focus on LECA.

LECA is a light-weight expanded clay aggregate. It’s basically a ceramic shell with a honeycomb core produced by firing natural clays at 1100-1200C in a rotating kiln. The bulk density of 8-12 mm diameter LECA is about 325 kg/m3, allowing it to float. It is highly durable to both chemicals and frost giving it a long life (10 + years). LECA covers are typically applied in a 2 - 4 inch layer to provide good coverage of the manure. LECA is one of the highest cost of permeable covers and has been hard to obtain. (The LECA that I can generally find is still from European companies.) Below, is a picture of LECA.


Below is an example of an in-ground circular concrete tank swine manure storage that has a LECA cover. This LECA had been in use for over 10 years and provided a coverage depth of about 3-4” on the manure surface. Occasionally the LECA would all drift to one side of the manure storage from wind blowing from the same direction continuously, but it would generally fix itself after the wind direction switched. One potential issue with expanded clays is that agitation and pump out must be conducted in a way that ensures the floating cover material is not removed from the storage. This can generally be performed with minimal changes to standard agitation and pumping practices.  Such as corralling the LECA away from pump inlets or providing screening so the LECA cannot go through.


LECA works in a manner to any permeable cover (like chopped straw) in that it provides a barrier that helps reduce wind disturbance of the manure surface and in doing so lessens the transfer of ammonia, hydrogen sulfide, and other odors into the air. That is the permeable cover shields the manure surface from contact with the air. The major advantage of LECA over a material like straw is that the LECA has a long life, it is basically inert, so it lasts for a long time. When you are using LECA as a cover you are generally trying to get a 2- to 4- inch thick layer on the surface, and it generally is management free except when you are trying to pump the manure.

As far as studies of its performance there have been a few, but they mainly come from Denmark as this is where most of the LECA is made, but they generally show somewhere around a 50-90% reduction in odor, 60-80% reduction in hydrogen sulfide, and 70-90% reduction in ammonia, with performance varying a bit based on how well the manure stays covered and the thickness of the LECA cover. Thicker covers generally due a little better, but performance doesn’t increase that much as long as about 1.5-2 inches are floating on the surface. The problem has generally been that every square foot of manure storage you are covering costs something like $1-3, which adds up pretty quickly (this price is actually pretty comparable with what some impermeable covers would cost to purchase and install).

Although I haven’t seen in agricultural settings, a similar in concept and has me intrigued is the use of circular plastic balls. The balls float on the surface and act very similar to the LECA in covering the surface. There are a few companies settling these types of plastic module balls to municipal wastewater treatment plants, the challenge would again be how do they hold up in agricultural manure storages and do they interfere with our typical agitation and pumping practices?