Covers reduce or mitigate gas and odor release from outdoor manure storages or lagoons. They create a barrier between the stored manure and wastewater and the airflow above. Covers can be permeable or impermeable. There are many materials used to create covers, which vary in cost, effectiveness, design, and management and operation considerations. Here’s a look at cover options and factors to consider when selecting a cover.
How Covers Work
Manure decomposition produces gas and odor. The release or emission of gas and odor from the manure to the surrounding air depends on two main factors.
- The difference in how much odor/gas is in the manure relative to the surrounding air (the potential).
- The resistance (or lack thereof) created by the temperature, airspeed conditions above the manure surface (the flow), and the manure surface.
Manure composition and pH influence the amount of a gas or odorous compound at the manure surface. Each gas and compound behaves differently in manure and with respect to pH.
Covers reduce the effect of wind blowing across the surface of a manure storage facility. Higher wind increases the rate that gas molecules push away from the manure surface, increasing the potential.
Covers also can reduce wave action or rippling at the manure surface. Waves and ripples can decrease the resistance of naturally forming films or covers on manure.
Because the manure emits fewer volatile compounds, covers increase dissolved gas concentrations. The air space under a cover is limited, so gas concentrations build up quickly to the point where more molecules stay in the manure and fewer are emitted. This increased gas concentration in the manure needs to be managed to avoid excessive emissions during agitation, pump out, and land application.
Permeability decreases as resistance increases. Physical factors that affect cover gas transfer resistance include thickness and porosity. Cover resistance increases as cover thickness increases and as porosity decreases. Thin natural films can increase gas transfer resistance and reduce gas and odor emissions. Thick permeable covers and impermeable covers have large gas transfer resistances that reduce gas emissions.
Because of decreased gas emission, covers are likely to increase the nitrogen and sulfur content in the stored manure. Phosphorus content does not change due to a cover because there is no volatile form to be emitted. An increase in nitrogen or sulfur affects the manure’s fertilizer value.
Types of Covers
Covers are either permeable or impermeable. Gas molecules and water can pass through permeable materials. Permeable covers reduce emissions by making it harder for the gaseous molecules to escape. Impermeable covers trap almost all gases and water. Few gas molecules escape through holes in and around the edges of impermeable covers. Table 1 describes the effectiveness, lifetime, and cost of various covers. Some cover systems combine two materials, such as geotextile and straw. Impermeable cover systems need to deal with the emitted gases under the cover, which are highly concentrated, probably toxic, potentially explosive, and capable of creating large amounts of pressure. The gases may be released untreated into the atmosphere through a vent or treated with a gas-phase biofilter, flared for a boiler, or genset to generate electricity, which can offset cover costs. Methane is a potent greenhouse gas that if not flared or used to produce electricity, can contribute to global climate change. If possible, impermeable covers should have a low profile to minimize wind effects.
Table 1. Attributes of various permeable and impermeable covers.
|Type||Material||Percent Reduction - Odor||Percent Reduction - H2S||Percent Reduction - NH3||Life Expectancy||Capital Costs|
|Permeable Cover||Natural Crust||56-78% (a, b)||81% (b)||(-11)-37% (b, c)||3 months (b)||0.00 $/yd2|
|Permeable Cover||Straw||45-83% (a, b, e, f)||86-100% (d, e, f)||79-86% (c, d)||2-6 months (g)||0.25-0.90$/yd2 (g)|
|Permeable Cover||Straw (8 in) + Geotextile (2.4 mm)||76-83% (e)||85-98% (e)||79-86% (e)||--||--|
|Permeable Cover||Geotextile (2.4 mm)||51-63% (d, e, g)||59-71% (d, e, g)||(-15)-37% (e, g)||3-5 years (g)||1.25 2.00$/yd2 (g)|
|Permeable Cover||Leca®||69-89% (f)||64-75% (f)||83-95% (h)||10 years (i)||15.45$/yd2 (i)|
|Permeable Cover||Macrolite®||56-62% (d)||64-84% (d)||--||10 years (i)||15.45$/yd2 (i)|
|Impermeable Cover||Concrete||95-100% (a)||--||--||20 years (a)||--|
|Impermeable Cover||Wood Lid||75-95% (a)||--||98% (h)||--||--|
|Impermeable Cover||Positive Air Pressure||95% (g)||95-99% (j, k, l)||95% (j)||10 years (j, l)||6.75-12.60$/yd2 (j, k)|
|Impermeable Cover||Negative Air Pressure||95-99% (j, m)||95% (j)||95% (j)||10 years (j, n)||3.15-3.60$/yd2 (j, n)|
|Impermeable Cover||Floating||39-95% (a, b, l, j)||90-95% (d, j)||74-98% (c, h, j)||10 years (a, j)||4.5-9.00$/yd2 (j)|
Factors to Consider When Choosing a Cover
Many factors need to be considered when selecting a cover to reduce odor and gas emissions from manure storage facilities. Table 2 shows some considerations for permeable and impermeable systems.
Table 2. General considerations when considering permeable or impermeable covers for manure storages.
|Permeable Cover||Impermeable Cover|
|Cover Purpose||Odor and gas reduction||Odor and gas reduction Can be used to capture and use methane|
|Level of Effectiveness||~ up to 60-90% reduction in ammonia and hydrogen sulfide (Table 1)||~up to 95% reduction in ammonia and hydrogen sulfide (Table 1)|
|Service Life or Useful Life||Weather and material dependent 2 months to 10 years (Table 1)||5 to 15 years|
|Manure or Sludge Accumulation and Removal||Breaking up natural crusts and straw covers requires additional time and energy for manure agitation Synthetic cover material needs to be removed or kept separate during agitation and pumping||Synthetic cover material needs to be removed or kept separate during agitation and pumping Access ports may be necessary|
|Precipitation||Permeable covers allow water in and probably reduce evaporation, which may require an increase in storage capacity.||Snow, rain, debris, and silt may accumulate on top of impermeable covers Small, manually operated pumps are commonly used to remove liquid from the surface|
Covers generally are not designed to have animals or people walk on them. Surround manure storage facilities by fences to prevent animals or people from accidentally walking onto and falling through a cover that appears to be solid.
Concentrations of hazardous dissolved gases such as ammonia and hydrogen sulfide accumulate in the manure under covers. These gases can volatilize quickly if the cover is removed for agitation and pumping or inspection. Covers can create conditions with potentially high toxic gas concentrations near the inspection and agitation openings. Use extreme caution should be used when accessing manure under an impermeable cover (i.e., through access flaps or a cover lift system).
People should never enter a covered manure storage unit (i.e., inflated dome or covered tank) without personal breathing protection and adequate safety lines and personnel.
Reviewed in 2018