Global Environmental Benefits of Biogas Technology

With anaerobic digestion, a renewable source of energy is captured, which has an important climatic twin effect:

1. The use of renewable energy reduces the CO₂-emissions through a reduction of the demand for fossil fuels.
2. At the same time, by capturing uncontrolled methane emissions, the second most important greenhouse gas is reduced:

   1m³ cattle manure = 22,5 m³ biogas = 146 kWh gross = 36 kg CO₂- Emissions

Smaller agricultural units can additionally reduce the use of forest resources for household energy purposes and thus slow down deforestation (about 1 ha of forest per rural biogas plant), soil degradation and resulting natural catastrophes like flooding or desertification.

1 m³ biogas (up to 65% CH₄) = 0,5 l fuel oil = 1,6 kg CO₂

1 m³ biogas = 5,5 kg fire wood = 11 kg CO₂

When applied for industrial or municipal wastewater treatment, surface waters and other water resources (rivers, sea, ground and drinking water resources) are being protected. Often the purified wastewater can be reused, e.g. as process water in industry or as irrigation water in agriculture. Costs saved for providing additional water can be directly translated into benefits.

The introduction, promotion and broad-scale dissemination of anaerobic technology into agro-industrial, domestic and agricultural sector combined with efficient power and heat generation or household energy appliances allows by now an efficient and viable reduction of environmental pollutants.

The impact on the greenhouse effect

The greenhouse effect is caused by gases in the atmosphere (mainly carbon dioxide CO₂) which allow the sun's short wave radiation to reach the earth surface while they absorb, to a large degree, the long wave heat radiation from the earth's surface and from the atmosphere. Due to the "natural greenhouse effect" of the earth's atmosphere the average temperature on earth is 15°C and not minus 18°C.

The increase of the so called greenhouse gases which also include methane, ozone, nitrous oxide, etc. cause a rise of the earth's temperature. The World Bank Group expects a rise in sea levels until the year 2050 of up to 50 cm. Flooding, erosion of the coasts, salinization of ground water and loss of land are but a few of the consequences mentioned.

Until now, instruments to reduce the greenhouse effect considered primarily the reduction of CO₂-emissions, due to their high proportion in the atmosphere. Though other greenhouse gases appear to a smaller extend in the atmosphere, they cause much more harm to the climate. Methane is not only the second most important greenhouse gas (it contributes with 20% to the effect while carbon dioxide causes 62%), it has also a 25 times higher global warming potential compared with carbon dioxide in a time horizon of 100 years (Table 1).

Table 1: Relative climatic change potential caused through different greenhouse gases within a period of 100 years after the emission, data mass equivalent of CO₂
Source: 'Klimaänderung gefährdet globale Entwicklung'. Enquete-Comission "Schutz der Erdatmosphäre" of German Bundestag, 1992

Gas	Relative global warming potential 20 years after emission	Relative global warming potential 100 years after emission
CH4	63	24,5
N2O	270	320
FCKW12	n.	8.500
CF3Br (Halon 1301)		5.600
C2F6 (Perflourethan)		12.500

The reduction of 1 kg methane is equivalent to the reduction of 25 kg CO₂. The reduction of greenhouse gases with a high global warming potential can be more efficient compared with the reduction of CO₂.

Sources of methane emissions in the agricultural field

The amount of worldwide methane emissions from agricultural production comprises about 33 % of the global anthropogenic methane release. Animal husbandry alone comprises 16 %, followed by rice fields with 12 % and animal manure with 5 % . While methane released through digestion of ruminants (about 80 Mil t CH₄ per year) can rarely be reduced, methane emissions from animal waste can be captured and energetically used through anaerobic treatment. The amount of methane emission mainly depends on fodder, animal type and animal waste systems. For example: the methane emission potential from dairy cattle in industrialized countries is about 0,24 m³ CH₄/kg volatile solids (influence of fodder), in developing countries it is only about 0,13 m³ CH₄/kg volatile solids. But taking into account the aerobic condition of solid dung systems (only 5 % of the methane emission potential is released) it is mainly the liquid waste management systems which contribute through anaerobic conditions with a high methane release to the climate change (up to 90 % of the methane emission potential is released).

From the worldwide 30 Mil t of methane emissions per year generated from the different animal waste management systems like solid storage, anaerobic lagoon, liquid/slurry storage, pasture etc. half of the emissions could be reduced through anaerobic treatment.

Eastern Europe, Asia and Far East contribute with the highest amount of 6,2 Mil t methane emissions/year each. While in Eastern Europe the emissions are caused by anaerobic animal waste management system, in the Far East they are caused by the high numbers of livestock (Figure 1).

Fig. 1: Methane-emissions from different animal waste management systems Source: Cassada M. E., Safley L.M.Jr., 1990: "Global Methane Emissions from Livestock and Poultry Manure". EPA CX-816200-010.

Methane reduction potential through the application of biogas technology

Through anaerobic treatment of animal waste, respectively through controlled capture of methane and its energetic use, about 13,24 Mil t CH₄/year can be eliminated worldwide. This figure includes methane emissions resulting from incomplete burning of dung for cooking purposes. By replacing dung through biogas, these emissions are avoided. In total about 4 % of the global anthropogenic methane emissions could be reduced by biogas technology.

If fossil fuels and firewood is replaced by biogas additional CO₂-emissions can be avoided including a saving of forest resources which are a natural CO₂ sink. Including all these effects about 420 Mil t of CO₂-equivalents are avoidable (Table 2).

Table 2: CO₂-Reduction through biogas utilization, saving of fossil fuels and fire wood resources.

		CO2 Reduction [Mil t CO2/year]
CH4	13,24 Mil t/year CO2-equivalent: methane x 25	330,9
Biogas	33.321 m3/year
Substitution of fossil fuels		44,7-52,7
Fire wood savings		4,17 - 73,8
Total		388 - 449 = 418,5

Reduction potential of nitrous oxide emissions from agriculture

The relative climatic change potential of nitrous oxide is up to 320 times higher as that of CO₂ (Table 1). Nitrous oxide generation is a natural microbial process. It is produced during nitrification and de-nitrification processes in soils, stables and animal waste management systems. In general, nitrous oxides emissions appear in soils without anthropogenic influence. Fertilizing as well as special conditions during storage can immensely increase the emissions.

Little detailed information is available about the reduction potential of nitrous oxides through anaerobic digestion of animal waste. There is still a big need for further research. Nevertheless, ongoing research results indicate that anaerobic digestion of animal waste significantly reduces nitrous oxide emissions by:

1. avoiding of emissions during storage of animal waste,
2. avoiding of anaerobic conditions in soils,
3. reducing N₂O-emissions through increased nitrogen availability for plants and a faster nitrogen absorption through crop plants,
4. reducing application of inorganic nitrogen fertilizer by which N₂O-emissions are reduced during production of nitrogen fertilizer.

Considering all these effects a N₂O-reduction potential through anaerobic treatment of about 10 % can be assumed. This means that 49.000 t N₂O/year or 15,7 Mil t CO₂-equivalents could be reduced on average.

So far, the environmental costs of greenhouse gas emissions have not been calculated. One means, proposed by the US administration on the climate conference in Kyoto, is the introduction of emission rights which can be traded. In doing so, national economies could attribute a monetary benefit to the avoidance of greenhouse gas emissions.