Covers for manure storage units
Richard Nicolai and Steve Pohl, South Dakota State University; David Schmidt, University of Minnesota
Every livestock operation needs some kind of manure storage. Storage allows the nutrients in the manure to be applied to cropland at appropriate times in the growing cycle. Storage also allows the manure to be held when fields are frozen or snow covered when application might result in runoff that would pollute surface water. Recommended storage capacity for manure is six months or more, depending on the moisture content of the manure and whether it is liquid or solid. Solid manure piles typically emit very little odor due to crusting of the pile surfaces. However, liquid manure storages can be a significant source of odor and hydrogen sulfide emissions.
Odor emissions from manure storage are typically the leading cause of nuisance complaints. Liquid manure storage tends to give off odor and gas emissions when the surface is disturbed during windy conditions or during agitation and pumping prior to land application. Spring turnover, a phenomenon that occurs when the storage warms, also increases odor and gas emissions.
Covers over the lagoons significantly reduce both odor and hydrogen sulfide emissions. Covers create a physical barrier at the liquid-air-interface, which helps retain more volatile chemical compounds in the liquid phase and minimizes emissions to the atmosphere. However, there is limited design information, and it has been difficult to evaluate performance of covers in field conditions.
How covers work
When a cover is placed directly over the manure surface, the following processes take place:
1. Resistance to the transfer of gases is increased because of the physical barrier between the liquid and the air.
2. Gas concentrations build up under the cover.
3. The rate at which a gas diffuses out of the manure is reduced (because the concentration gradient has decreased).
4. Hydrogen sulfide, ammonia, and other volatile odorous compounds may be kept in solution, increasing the emissions of these gases when the cover is removed for land application of manure.
Most widely used covers float on the surface of the manure and are made of straw, geotextile or a combination of both. Other types of covers on farms include impermeable plastic covers; rigid covers made with concrete, wood and PVC material; and air-filled clay balls like Leca® and Macrolite®. Inflatable plastic covers have also been popular in Canada.
Covers are usually classified as impermeable or permeable. Impermeable covers do not allow any gases coming off the manure to be emitted to the atmosphere, while permeable covers permit transmission of some gases.
Impermeable covers
Because impermeable covers trap and hold gases coming off the manure, a vent must be provided to prevent build-up of excessive pressure in the headspace and to ward off a possible explosion. Odorous gases under covers are extremely corrosive or toxic. Flat, low-profile covers should be specified whenever possible to minimize headspace. Minimal headspace reduces air exchange volumes, reducing the need for odor control equipment.
Impermeable covers need to be designed for easy access for operation or maintenance, have a minimum number of joints and seals at all joints. The corrosive action of sulfides and sulfuric acid must be considered when selecting cover materials and concrete coatings. Overhangs, ledges, or lips on the underside of covers where condensate may collect should be avoided.
Odorous gases in a covered storage tank must be vented to the atmosphere to avoid pressure build-up inside the cover from the production of manure gases. Collecting and removing biogas can be done through installing perforated gas collection pipes or exhaust fans. Methods to reduce odor include the burning or flaring of these gases or some form of gas treatment as biofiltration or ozonation before discharge to the atmosphere.
Flexible membrane covers
Flexible membrane covers are constructed of high-density polyethylene. They have effectively controlled odor from industrial and municipal sites. The membranes are 20 mil minimum thickness and must be UV stabilized. Membrane covers, either permeable or impermeable, when used on an earthen basin manure storage system, are typically anchored to the manure storage perimeter with an anchor trench. The cover floats on the surface of the manure and partially inflates with manure gases.
If the cover is impermeable the gases must be vented to the atmosphere. This is accomplished by a variety of techniques; often, a perimeter tile is placed under the cover near the top of the berm of the manure storage and then vented through the cover. The gases are either flared or treated using some other gas treatment system.
Access to the manure is typically through a large flap that can be folded back. The flap must be large enough to allow for pumping and agitation equipment. Typical life of the covers is anticipated to be 10 to 15 years.
Inflatable dome system
With inflatable cover systems, a tarp is fastened to the tank perimeter as tightly as possible and supported by a center column with radiating straps. Air is delivered through a low-pressure blower, and the cover is maintained at a constant operating pressure. For agitation and pumping, the structure is deflated, allowing the tarp to lie over the radiating supports. Access doors are then opened to introduce pumping equipment.
Permeable covers
Permeable covers, such as straw, geotextile or floating clay balls, are also effective alternatives for reducing odor from livestock manure facilities. Effectiveness of odor and gaseous emissions control is lower than with impermeable covers. A biofilm may develop at the interface of cover and liquid. Some of the odorous compounds that escape to the atmosphere are broken down within the aerobic layer that is established.
Straw
Both barley and wheat straw can be used as organic floating covers; there is no significant difference between them. The straw is applied to manure storage tanks using a straw chopper/blower. The degree of odor control is not affected by the type of straw but rather the ability of the straw to float on the surface. Thus, odor reduction will vary from 90 percent for a thick, newly applied cover to 40 percent or less depending on straw thickness and uniformity. Sometimes oil is added to the straw at the time of application to increase the time the straw floats.
Straw covers usually last between 2 and 6 months depending on the amount applied (depth of straw), uniformity of application, basin size and wind conditions during application. If the cover starts to break up or sink, additional straw must be added to retain effectiveness. Successful agitation and pumping of straw-covered storages can be accomplished by appropriate equipment (chopper pumps).
A 12-inch depth of straw is typically recommended, since this depth has been shown to float longer than lesser depths. The amount of straw needed depends on the area of the manure storage and desired depth of the straw layer. A single large round straw bale (6 foot diameter) can cover about 500 square feet of storage (12-inch layer).
Geotextile
Other floating permeable covers, such as geotextile materials (non-woven fabric composed of thermally bonded, continuous polypropylene filaments), may provide a better solution than straw alone for certain types of storage basins. Geotextile materials are self-floating and provide a physical barrier to mass transfer of gases from the liquid to the air.
There is also some possibility that the geotextile helps maintain an aerobic layer of microorganisms on the manure surface, but more research is needed to verify this process. This layer would reduce the odorous gases to carbon dioxide and water. Geotextile materials have higher initial cost than straw covers but all costs, such as installation and maintenance, must be included in the final evaluation of a cover.
One concern with geotextile or geotextile-straw covers is the ability to agitate the manure storage. This applies only in manure storage basins and tanks and not to lagoons, which are typically not agitated. Most types of agitation equipment pump manure over the surface to help with the stirring. This is not possible with the geotextile covers. To achieve any agitation, the cover is partially removed – typically from one corner of the basin – or the cover is lifted by a cable and winch system and the agitation/pumping equipment is positioned under the cover. Neither of these options allows for vigorous agitation.
Management of manure covers
After a cover is installed properly, there are additional technical and operational needs. Parts of the cover may need to be removed to permit agitation and pumping when the manure is removed for land application. A permanent opening may be installed that can be sealed between pumping intervals. Safety should always be considered during agitation and pumping of manure. There may be a high concentration of hydrogen sulfide or other gases under the fabric cover. Opening the flaps or lifting part of the cover must be done with caution.
Cover maintenance includes the repair of tears or punctures and removal of debris and silt accumulation on the cover surface. Geotextile covers without an additional float system may sink after the winter season and may take one or two months to float again on the manure surface. Disposing of plastic and geotextile covers after they are no longer usable may be difficult and costly depending on local hauling and landfill fees.
Straw covers may break up or sink due to high winds and heavy rain. If a straw cover starts to break up or sink, additional straw may be added to reestablish the cover’s original effectiveness. Agitation and pumping of straw-covered manure storages can be successful.
Manure nutrient concentration may increase after an impermeable cover is installed. More land is needed to achieve the same agronomic application rate if the manure concentration is increased. ANM
References omitted but are available upon request at editor@progressivedairy.com
—Excerpts from South Dakota State University Extension website
