Manure coverage is the practice of covering the surface of manure with materials of certain thickness instead of the traditional method of piling up manure to be exposed to air. Manure coverage changes the amount of manure surface in contact with air. Due to some reactions, i.e., a series of physical, biological and chemical reactions, it can reduce GHG emissions.
By covering manure with materials of a certain thickness (such as plastic sheeting, organic matter and expanded clay), the manure’s surface in contact with air is altered. This method can reduce the emission of GHGs and store nutrients in the manure.
Generally, covers are classified as impermeable or permeable. Impermeable covers do not allow gases coming from the manure to be emitted to the atmosphere. Permeable covers permit the transmission of some gases. Permeable covers usually include straw, geotextile, expanded clay, corn stalk, etc. The impermeable covers include floating plastic, suspended plastic, concrete, etc. Impermeable covers offer the opportunity to collect and use methane gas for fuel and power generation. A covered lagoon is a good example of a manure storage basin with an impermeable cover. It’s a large anaerobic lagoon, which can stably digest manure, reduce odour, and supply nutrient-rich effluent for application on fields and crops. Pathogens and weed seeds are reduced and biogas can be produced for use on the farm.
The effects on GHG emissions reduction vary for different covering materials and techniques. The principles of emission reduction are also different. For instance, impermeable materials such as plastic sheets can isolate manure from the external environment, thereby preventing loss of volatilised gases into the air. An anaerobic environment is also created within the manure. Since the first stage of N2O generation is the aerobic nitrification reaction of ammonia nitrogen, the adoption of manure covering technology prevents exposure to oxygen. By stopping this first reaction, N2O emissions are lowered.
Factors, such as temperature, moisture content, and pH of the manure also have a significant impact on the mitigation effect of storage covering technologies. The moisture content of manure greatly affects the generation of CH4. When the moisture content is high, anaerobic fermentation dominates, with greater production of CH4 and less production of CO2. When the moisture content is low, aerobic fermentation dominates, with CO2 generated as the major fermentative products and basically no CH4 is generated. The moisture content also affects nitrification and denitrification of manure. Neither extremely good nor poor permeability is conducive to the generation of N2O in nitrification or de-nitrification processes. Therefore, in
both cases of very low moisture content of animal manure and long-time submergence under water, N2O emissions are very low. However, the dry-wet alternation of manure promotes the generation and emission of N2O. Suitable pH environments vary for different microorganisms. In this sense, adjusting the pH value of liquid manure to affect the process of biochemical reaction and then lower the GHG emissions is another approach for emission mitigation.
So far, research has been carried out in covering manure storage technology yielding results relating to covering materials, external temperature, and composition of manure (Berg and Pazsiczki, 2006). In China, several experiments have been performed, which utilised straw and expanded clay to cover beef manure (Lu et al., 2007) and swine waste water (Li, 2008). In practice, however, due to a limited area of farms, and a lack of storage facilities for manure, the manure is discharged directly, digested by biogas plants, or applied to farmland.
In developed countries, the regulations concerning the management of animal manure and odour emissions are very rigid. With complete storing facilities, manure storage, and a large number of storage pools in animal farms, the loss of manure nutrients is prevented and odour emissions are controlled by extensive application of covering measures.
Although impermeable materials such as concretes and thin films are relatively stable and long-lasting, high initial investment expenses are a barrier to their widespread adoption. On the other hand, although permeable covering materials such as straw are inexpensive, they are not stable and have short service lives which makes their use seem futile and therefore also a barrier to adoption. Moreover, some covering materials, including straw, decompose when they come in contact with manure slurry, and then they themselves become an emission source.
Advantages of covering manure
- The advantages are low cost, simplicity of operation, and ease of implementation.
- Commonly used materials such as straws, expanded clay, thin films, etc. are low-cost and readily available. This makes it possible for animal farms to change the storing method of manure easily and conveniently.
Disadvantages of covering manure
- Covering and compacting manure creates an anaerobic environment within manure, which increases methane emissions although the generation of nitrous oxide is inhibited, i.e., a case of swapping one form of pollutant for another (Monteny, 2006).
- The potential for emission reductions is greatly affected by manure properties, temperature, and other factors for which there is currently limited understanding. Different covering materials should be selected for solid and liquid manure. Many experimental results indicate that covering liquid manure with organic matter, including straw, will greatly increase the amount of methane emissions, generating more methane in anaerobic fermentation of straws instead of reducing emissions. To adapt to the differences in climatic types (temperature, precipitation), manure properties and covering materials, experiments should be conducted to analyse and test the potentials of various combinations of these parameters to reduce greenhouse gas emissions.
Chadwick (2005) conducted an experiment to test the impact of compaction and covering methods of cattle manure on GHG emissions. Experimental results showed that compaction and covering with plastic film can reduce emissions of ammonia and N2O from manure by 90% and 30%, respectively. However, compaction and coverage created an anaerobic environment inside the manure, increasing the amount of methane emissions (Chadwick, 2005).
Additionally, by decreasing the surface area of the manure heap and by timely transport of manure to an enclosed storage chamber, the amount of NH3 and CH4 emissions can be reduced effectively (Weiske et al., 2006).
Generally, reducing ammonia volatilisation and preventing odour can be achieved by covering liquid manure with straw, which may also increase methane emissions. Berg and Pazsiczki (2006) achieved GHG emission reductions by combining straw coverage with an acidising technique. Experimental results showed that methane emissions were reduced by 40% by adjusting the pH value of liquid manure to less than 6 with lactic acid and integrated covering with straws.
A hard crust is naturally formed during the storage of manure, which prevents ammonia produced by manure from escaping. An experiment by Smith et al. (2007) showed that ammonia emissions from manure with naturally formed crust can be reduced by over 60% compared to the emissions from manure without the crust. Besides slowing ammonia loss, the hard crust on manure slurry also reduces methane emissions. Petersen and Ambus (2006) proved that methane-oxidising bacteria exist in the hard crust of manure slurry, which oxidise methane into CO2, thus achieving an emission reduction because methane is a more potent greenhouse gas than CO2. When the concentration of methane is 500-50,000 ppmv, the amount of emission reduction by methane-oxidising bacteria is -1~-4.5 gCH4 m-2d-1) (Petersen and Ambus, 2006).
Permeable covers are less expensive than impermeable covers, but they do not last as long and are not as effective at reducing the emissions of odours and gases. However they can provide reductions in odour, ammonia and hydrogen sulfide emissions from manure storage facilities. A wide variety of organic and manmade materials have been utilised to construct permeable covers with variable results. Costs range from $1.10 to $18.80 per m2 installed. Straw shown in figure 1 is the least expensive permeable cover material with an approximate cost of $1.10 per m2 installed. The installed costs of longer lasting materials such as lightweight expanded clay aggregate (LECA) presented in fgure 2 can exceed US $10.80 per m2 installed (Burn and Moody, 2008). Impermeable covers may cost US $21.50 per m2 installed (Powers, 2006).
If impermeable covering materials are adopted, then the mass transfer between manure with the outside is cut off. Meanwhile, an anaerobic environment is created within the manure, promoting the generation of methane. Then gas collection devices can be installed to capture methane for cooking and heating purposes. In addition, the use of covering materials can effectively prevent the emission of nitrogencontaining gases such as ammonia, thereby retaining nutrients in the manure. After a period of storage, it can be applied onto farmland as organic fertiliser.
Although impermeable materials such as concretes and thin films are relatively stable and long-lasting, high initial investment expenses are a barrier to their widespread adoption. On the other hand, although permeable covering materials such as straw are inexpensive, they are not stable and have short service lives which makes their use seem futile and therefore also a barrier to adoption.
Berg W., and Pazsiczki I. (2006): Mitigation of methane emissions during manure storage. International Congress Series, 1293: 213-216.
Burn R. and Moody L. (2008): A Review of permeable cover options for manure storage. Retrieve April 2, 2011, from http://www.extension.org/pages/24017/ a-review-of-permeable-cover-options-for-manurestorage.
Chadwick D.R., (2005): Emissions of ammonia, nitrous oxide and methane from cattle manure heaps: effect of compaction and covering. Atmosphere Environment, 39: 787-799.
Li N. (2008): Study on greenhouse gas emission from slurry storage of swine farm. Dissertation. Beijing: China Academy of Agriculture Sciences (in Chinese).
Lu, R., Y. E. Li, Y. Wan, Y. Liu, and L. Jin (2007): Emission of greenhouse gases from stored dairy manure and influence factors. Transactions of CSAE, 23, 198-204 (in Chinese).
Powers W. (2006): Covering nutrients during manure storage. Retrieve April 2, 2011, from http://www.animalagteam.msu.edu/LandApplication/ManureandNutrientManagem...
Petersen, S.O. and Ambus, P. (2006): Methane oxidation in pig and cattle slurry storages, and effects of surface
crust moisture and methane availability. Nutrient cycling in agroecosystems, 74:1-11.
Smith, P., Martino, D., Cai., Z., Gwary, D., Janzen, HH., Kumar, P., McCarl, B., Ogle S., O’Mara, F., Rice, C., Schloes, B. and Sirotenko, O. (2007): Agriculture. In climate change 2007: Mitigation. Contribution of working group III to the fourth assessment report of the intergovernmental Panel on Climate Change [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Weiske A., V abitsch A. and Olesen J.E., (2006): Mitigation of greenhouse gas emissions in European conventional and organic dairy farming. Agriculture, Ecosystems and Environment, 112, 221-232.