Agricultural lands (lands used for agricultural production, consisting of cropland, managed grassland and permanent crops including agro-forestry and bio-energy crops) occupy about 40- 50% of the Earth’s land surface. Agriculture accounted for an estimated emission of 5.1 to 6.1 GtCO2-eq/yr in 2005 (10-12% of total global anthropogenic emissions of greenhouse gases (GHGs)).
A variety of options exists for mitigation of GHG emissions in agriculture. The most prominent options are improved crop and grazing land management (e.g., improved agronomic practices, nutrient use, tillage, and residue management), restoration of organic soils that are drained for crop production and restoration of degraded lands.
- Reducing emissions (for instance by more efficient management of carbon and nitrogen flows in agricultural ecosystems, such as managing livestock for reduction of methane)
- Enhancing removals, such as storage of amounts of vegetative carbon in agro-forestry systems or other perennial plantings on agricultural lands and others).
- Displacing emissions: amounts of vegetative carbon can also be stored in agro-forestry systems or other perennial plantings on agricultural lands. The net benefit of these bio-energy sources to the atmosphere is equal to the fossil-derived emissions displaced, less any emissions from producing, transporting, and processing.
Livestock manures represent a valuable resource that, if used appropriately, can replace significant amounts of chemical fertilizers (van der Meer et al., 1987; Bremand and de Wit, 1987; van Boheemen, 1987). However, unless animal manure is managed carefully to minimize odour, nutrient losses and emissions, it becomes a source of pollution and a threat to aquifers and surface waters. The most known technology for manure management is the anaerobic digestion, which is a process in which organic matter from wet organic wastes (ie. liquid manure, food processing wastes, etc.) is converted into methane by bacteria in the absence of oxygen. The methane is then collected and may be used to generate electricity. In addition, the anaerobic digestions process creates potentially valuable by-products, such as the solids fraction - fiber, and liquid with available nutrients.
Figure 1: Anaerobic digestion (Source: AgCert)
Another common technique is the aerobic digestion, which is useful in treating liquid manure for odour reduction, chemical oxygen demand (COD) and biochemical oxygen demand (BOD) reduction, and pathogen control. Aerobic treatment is usually a batch process or, semi-continuous (batch feed). In a batch process, all of the treated material is removed from the facility before refilling with untreated slurry. In a batch feed or semi continuous process, some of the treated material is displaced by the addition of untreated material to the digestor.
Figure 2: Aerobic digestion (Source: Government of Alberta, Agriculture and Rural development)
A third method widely applied in the agricultural sector worldwide is the composting, which is an aerobic digestion process used for solid wastes. Slurries or separated solids can be composted if mixed with a carbon source such as straw, peat or wood shavings. However, composting a slurry without separating the solids requires a great deal of additional material to retain the liquid. This would be very impractical due to the cost of the material and the energy required to turn or aerate the compost. Composted manure is a premium organic fertilizer and holds some potential as a marketable product in the gardening and landscaping market. For some markets, and even some on-farm application techniques, the compost would have to be pelleted so that the nutrient content could be upgraded to a specific blend with commercial fertilizers.
Feasibility of technology and operational necessities
Cereal straw and other farm residues are already used in Denmark, Spain and Romania, for example, to produce significant heat and power production. In Brazil, Australia, South Africa and elsewhere, sugarcane bagasse is used for heat and power, both for use at the mill and for export to the grid. Vegetative grass crops such as Miscanthus and reed canary grass can be grown for combustion in commercial grate boilers for heat production. The co-firing of straw with coal is well demonstrated in fluidised-bed boilers. Small-scale (<500 kW) power generation plants based on the steam cycle have also been built, but they are relatively inefficient and hence have relatively high power production costs. Further RD&D in CHP would help reduce costs. In all cases, storing the biomass is important so that the bioenergy plant can be operated all year round, or at least for as long a season as possible, to spread the investment costs.
Some operational necessities are required for different types of manure management. For instance, in order for an anaerobic digester to operate properly a constant supply of “recoverable” manure is needed (Kubsch, 2003). Not all types of dairy manure are appropriate for anaerobic digestion purposes. Manure collected through a continuous scrape or other means from cows on cows kept on hard surfaces is better suited for use in an anaerobic digester. Manure which is left to dry in the pasture or drylot is not as useful for anaerobic digestion since drying reduces the methane producing properties.
The main barriers include fuel logistics, fuel quality fluctuations (due to variations in rainfall, for example) feedstock price fluctuations and delivery costs. Technical improvements in harvesting, storage, transport, fuel preparation and other measures are still possible for virtually all biomass feedstocks.
Status of the technology and its future market potential
- Grazing. Substantial losses through leaching may occur due to the uneven distribution of faeces and urine (urine patches may have a N load equal to 200-550 kg/ha). Volatilization of N may also be considerable (10-25%, see 3.3.5), but less important since part is deposited on nearby areas, though some of it on non-agricultural land.
- Kraals. These enclosures are often used as in-situ fertilization of arable land by moving the kraal regularly. Soil fertility of a larger area, used for grazing, is partially concentrated on the arable land, thus enabling crop production in resource-poor situations. Losses through leaching will be slightly higher than during grazing as equivalent N and K fertilization rates are increased.
- Dry lot storage. If urine is not collected and bedding is sparsely used, losses of N and K in particular will be high as most urine is lost. Depending on the storage facilities and storage time of the faeces part of the nutrients in faeces will also be lost through leaching and surface runoff, in the case of a precipitation surplus and uncovered manure heaps. Urine collection will minimize K losses but N losses will often remain high as volatilization will increase, though this is dependent on climatic conditions, storage time and storage method. Using bedding, with sufficient absorption capacity to capture urine, might reduce N losses with ca. 15% of the mineral N.
- Slurry storage. This system of manure storage, where faeces and urine are stored together, is the main system in intensive livestock systems in OECD countries, except for broilers. Volatilization losses are dependent on the level of ventilation, depth of storage tanks and storage time, but often range between 5 and 35% of the total N excreted.
- Lagoons. Lagoon systems are quite common at large livestock farms in Eastern European countries and, to a lesser extent, in Asia, while their importance is growing in the USA. Liquid manure, either before on after separating part of the solids, is treated in anaerobic lagoons. Organic material is decomposed, thereby mineralizing part of the nutrients. The liquid phase is either discharged into surface water or used for irrigation. The main problems are related to the discharge into surface water, leaching through the lagoon bottom, and odour. High NH3 emission will occur as a major part of the N in mineral form, while also high CH4 and N2O emissions are also common.
- Plastering material for house construction. This is particularly important in Africa, however the amount of manure involved on a global scale is considered to be too insignificant to be discussed here. In this system all nutrients are lost for agriculture.
- Fuel. In many developing countries, and particularly in India, manure is an appreciable fuel. If burnt directly, most of the C, and all the N and S will be lost; other nutrients may be recycled to arable land via the application of the ash. The production of biogas from manure is another method to valorize the energetic value of manure. The high water content of the slurry makes it more difficult to handle, and N losses via volatilization may be high, because most N in slurry is in mineral form. Though strongly promoted (e.g. in China) and applied to some extent in Asia, its present application is still limited mainly due to high investment costs (both for the digester and adjoining equipment) and technical problems (Henglian, 1994).
- Feed. Manure could be recycled by feeding it to animals, both livestock and fish (Müller, 1980), but this practice is limited. In addition to widespread reluctance to use manure as feed, probably originating from fear of health hazards, this can be explained by the low nutritive value of most types of manure, except for poultry manure as ruminant feed which is of a reasonable quality (ca. 55-60% TDN, 20-30% CP). Consequently, in intensive production systems where collectable manure is abundant, more economic feed is available, while in production systems where the use of low quality feeds is common, high collection costs and/or opportunity costs (manure as fertilizer or fuel) are prohibiting the use of manure as feed.
In addition, biogas produced from the anaerobic digestion of animal manure, green crops and other forms of organic waste can be used for heat and power generation as well as for transport fuels – after scrubbing to remove CO2 and H2S (IEA, 2008). Several research programs exist, which aim at diffusing information and best practices for manure management.
How the technology could contribute to socio-economic development and environmental protection
Contribution of the technology to economic development (including energy market support)
Contribution of the technology to protection of the environment
The effect on the environment of the manure produced in a particular agricultural system should be assessed by considering its role in the total nutrient management of the system. If the import and export of nutrients in the system is in balance, and animal manure is to play a positive role, it implies that losses from animal manure must be minimal. It also implies that efficient use is made of the manure in crop production, i.e. a large fraction of the nutrients from the manure is taken up by the crop. Another example is with the aerobic treatment, which can control dangerous bacteria such as Cryptosporidium and Salmonella and they cannot exist in the presence of oxygen. On the down side, aerobic treatment can cause excessive loss of nitrogen as nitrogen gas, nitrous oxide or even ammonia if excessive aeration rates are used. This loss of nitrogen to the atmosphere can create con cerns of acid rain in some instances. Another concern is the potential loss of the economic value as nitrogen fertilizer.
Land application of raw or composted manure can be tailored to reduce the emission of GHGs and their impact on the environment. Application of more nitrogen than a crop needs via manure will result in excess nitrogen accumulation in soil and will increase the release of nitrogen as nitrous oxide. Application of manure at the wrong time of year, for example in the very early spring, will also increase the release of nitrous oxide, as will applying raw manure during wet conditions. Researchers believe that timing manure application correctly and ensuring proper application amounts will contribute to an overall reduction in GHG emissions from agricultural operations.
Figure 3: Technical potential of emissions reductions from agricultural technologies (Source: IPCC 2007)
For calculation of these GHG emission reductions, it is recommended to apply the approved methodologies for GHG emissions reductions from manure management systems (large scale), GHG mitigation from improved animal waste management systems in confined animal feeding operations (large scale), methane recovery in agricultural and agroindustrial activities (small scale), consolidated methodology for GHG emissions reductions from manure management systems (large scale), which have been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html.
Financial requirements and costs
For water quality aspects the costs of removing polluting compounds can be assessed. These costs vary according to the technology used, local prices for energy, labour, etc. Exploitation costs for removing N and P were estimated at 20-37 ECU (i.e. ca. US$ 27-50) per kg nutrient emission from point source pollution (like manure directly discharged into surface water). These values refer to Eastern European countries; they are based on 8% discount rate and are dependent on the level of nutrient reduction aimed at (Haskoning, 1994).
Information on costs of liquid manure treatment is still scarce. The few available estimates on total operational costs of purification plants vary between 5 and 15 ECU (i.e. 6.75 and 19 US$) per m3 (Ten Have and Chiappini, 1993), while the required effluent quality (10-15 mg N/l and 1-2 mg P/l; EEC-Council Directive 91/271/EEC, 1991) is still not attained, even if the plants work at optimal level. The conclusion is that large-scale treatment of liquid manure as sewage is not economically justified, even if management and infrastructural problems are solved. Second, all losses of nutrients via emission, leaching, direct discharge to the environment, etc., can be valued according to current costs of inorganic fertilizer, by way of replacement costs or foregone benefits. World market price (Feb 2010) for urea is about US$ $270/st.
Indicatively, Rausch and Sohngen (1999) conducted an economic comparison of three manure handling systems: (1) earthen holding pond using drag-line direct injection; (2) earthen holding pond using a liquid tanker and; (3) stack pad using a conventional spreader. Some figures of these costs as a reference are presented below.
Figure 4: Economic comparison of three manure handling techniques (Source: Rausch and Sohngen, 1999)
Clean Development Mechanism market status
Project developers of methane avoidance in manure and wastewater projects in the CDM pipeline mainly apply the following methodologies:
Figure 5: Overview of methane avoidance in manure and wastewater projects in the CDM (Source: UNEP Risoe CDM/JI Pipeline Analysis and Database, February 1st 2010)
Example CDM project
- Brandjes, P.J., de Wit, J. and Van der Meer, H.G., 1996. Environmental Impact of Animal Manure Management. Food and Agriculture Organization of the United Nations. Available at: http://www.fao.org/WAIRDOCS/LEAD/X6113E/x6113e00.htm#Contents
- Breman, H. and De Wit, C.T., 1983. Rangeland productivity and exploitation in the Sahel, Science 221, pp. 1341–1347.
- Gonzalez-Avalos, E. and Ruiz-Suarez, L.G., 2001. Methane emission factors from cattle in Mexico. Bioresource Technology, 80, 63-71.
- IEA, 2008. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.
- IPCC, 2007. Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).
- Kubsch, K., 2003. Cooperative Anaerobic Digestionas a Manure Management Alternativein Northeastern WisconsinA Research Study Summary. Focus on Energy report. Available at: http://www.focusonenergy.com/files/document_management_system/renewables/glacierlandresearchstudy_feasibilitystudy.pdf
- Rausch, J. and Sohngen, B., 1999. An economic comparison of three manure handling systems. Report Number AE-5-99. Ohio State University Extension, Agricultural Economics.
- US EPA (2006). Global Anthropogenic Non-CO2 Greenhouse GasEmissions: 19902020.United States Environmental Protection Agency,EPA 430-R-06-003, June 2006. Washington, D.C. Available at: http://www.epa.gov/nonco2/econinv/downloads/GlobalAnthroEmissionsReport....
- Van Beheemen, P.J.M., 1987. Extent, effects and tackling of a reginal manure surplus; acase-study for a Dutch region, Animal Manure on Grassland and Fodder Crops:Fertilizer or Waste? (H.G. VAN DER MEER, R.J. UNWIN, T.A. VAN DIJK, G.C.ENNIK, Eds.), Developments in Plant and Soil Sci. 30 (1987) 175–193, Martinus Nijhoff Publishers, Dordrecht, the Netherlands.
- Van der Meer, H.G., Unwin, R.J., Van Dijk, T.A. and Ennik, G.C., 1987. Animal Manure on Grassland and Fodder Crops: Fertilizer or Waste? Developments in PlantSoil Sci. 30, Martinus Nijhoff Publishers, Dordrecht, the Netherlands.