The agricultural contribution to global non-CO2 greenhouse gas emissions is estimated to have been 5.1-6.1Gt CO2-equivalents (eq) per year in 2005. That is 10-12 per cent of the total global anthropogenic emission of greenhouse gases (Smith et al, 2007). The dominant gases emitted in agriculture are CH4 and N2O, contributing to about 47 per cent and 58 per cent of global anthropogenic emissions of CH4 and N2O, respectively. According to the IPCC, the main sources of agricultural emissions are soils (38 per cent), enteric fermentation (32 per cent), biomass burning (12 per cent), rice production (11 per cent) and manure handling (7 per cent) (Smith et al, 2007).
The United Nations Food and Agricultural Organization (FAO) reports global anthropogenic emissions of CH4 and N2O of 5.9 and 3.4Gt CO2-eq, respectively. Of these, some 2.2Gt CO2-eq of both CH4 and N2O are emitted by livestock (Steinfeld et al, 2006). These livestock emissions are expected to increase to 2.8Gt CO2-eq in 2020 (US EPA, 2006). Livestock represents about 80 per cent of agricultural methane emission. There are two main sources of CH4 from livestock: enteric fermentation and manure. Enteric fermentation represents around 80 per cent of the total emission by livestock, but the emissions are varying in time and between different regions in the world. Emissions from enteric fermentation are calculated by multiplying the number of animals with an emission factor, specific per category of animals and country. This emission factor can be determined at several levels of accuracy differing from country to country (VROM, 2008a). Methane emission is influenced by the milk production, the level of feed intake, the energy consumption, feed composition and rumen conditions (Monteny and Bannink, 2004).
Methods for mitigation of enteric fermentation focus on improving the productivity and efficiency of livestock production and on increasing the digestibility of feedstuff either by modifying feed composition or by interfering with the rumen digestive processes (Steinfeld et al, 2006). Although these strategies may be beneficial to emissions at regional or national level, they may increase the CH4 emissions per animal (US EPA, 2006). Enteric fermentation as a source of CH4 emission is discussed in more detail elsewhere in this book (see Chapter 9).
Biological processes are gaining more and more interest as a solution for environmental challenges. Among these processes is anaerobic digestion, which is increasingly applied throughout the world for the treatment of solid organic waste, sludge and wastewater and plays a central role in the treatment of waste and biomass (Figure 10.1). Organic compounds are converted into biogas, which is a mixture of CH4, CO2, hydrogen sulphide (H2S), H2 and in some cases N2O. In conventional aerobic wastewater treatment plants, organic matter is oxidized to CO2 combined with the removal of nitrogen and phosphorus. The production of CH4 during anaerobic processes is an advantage over other biological processes because of its possible reuse as an energy source. All applications that are fit for the use of natural gas are also able to handle biogas (after pretreatment). The energy content of 1kg chemical oxygen demand (COD) is similar to c. 1kWh (Aiyuk et al, 2006). Today, biogas is mostly converted into electricity and/or heat with electrical conversion efficiencies of up to 25 per cent (for less than 200kW combustion engines) or up to 30-35 per cent for larger (up to 1.5MW) engines. Combining
electricity generation with hot water recovery (via the exhaust gases of the combustion engine) may yield an efficiency as high as 65 to 85 per cent (IEA, 2001). However, care has to be taken that undesired emissions of greenhouse gases - CH4 (GWP 21) (US-EPA, 2002) and N2O (GWP 310) (US EPA, 2002) - are avoided.
The microbiology of the processes involved in this conversion is discussed in more detail elsewhere in this book. In short, the organic matter is converted to biogas in four subsequent reaction steps: hydrolysis, acidogenesis, acetogenesis and methanogenesis. Theoretically 0.35m3 (at standard temperature and pressure conditions; STP) CH4 can be produced per kg of converted COD. Together with the lower sludge yield, the formation of biogas is the most clear advantage of anaerobic treatment over aerobic treatment. By contrast, the effluent quality of aerobic treatment is better than of anaerobic treatment, in which nitrogen (N) and phosphorus (P) are not removed. When reuse of treated wastewater for fertilization and irrigation can be established, the latter becomes an advantage rather than a disadvantage. Methane emissions in aerobic treatment other than the CH4 that is produced during digestion of primary and secondary sludge appear to be negligible (El-Fadel and Massoud, 2001). Carbon dioxide is evidently emitted in open aerobic wastewater treatment systems, but this is 'short-cycle' CO2 not originating from fossil fuel. The CO2 emission related to the energy demand of an aerobic wastewater treatment plant also has to be taken into account. The formation of N2O seems to be more of a problem especially during nitrification and denitrification when oxygen levels are low, and when low C/N ratios in the wastewater lead to increased nitrite concentrations. The emission of N2O during nitrification is not related to the activity of nitrifying bacteria but is due to stripping of N2O formed in the denitrifying stage of the wastewater treatment plant. This is a very slow process, so N2O emission may even take place after discharge of the effluent to the surface water. Therefore the emission of N2O from aerobic wastewater treatment plants is small compared to total anthropogenic emissions each year (around 3 per cent), but much higher (26 per cent) when the total water chain is taken into account (Kampschreur et al, 2009). So far, it remains unclear which group of microorganisms is predominantly responsible for N2O formation in aerobic wastewater treatment plants. Since N2O is not the main focus of this book its emissions will not be discussed in more detail in this chapter.
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