Environmental Impact Of Biogas
There are many environmental and sanitary benefits of using biogas technology. The most obvious sanitary benefit of installing an anaerobic digester system is the improvements to toilet facilities in the households. In most developing countries, where very few sewer systems are in place, toilet facilities are in simple shacks.
The toilet is generally a slot in the floor with either a pit underneath or alternatively a trough running to a storage pit behind the building. In the case of a pit toilet, the slurry in the pit is often literally moving with insect larvae, and in all cases the toilets are smelly and fly infested. For these reasons, toilets are generally located as far away from the other household buildings as practical.In comparison, toilets automatically feeding into an anaerobic digester are odourless and fly-less, and are as clean as toilets connected to sewer systems.
The toilets are located in the main housing building often directly adjacent to the kitchen.The other sanitary benefit of the digesters is that disease-causing organisms are killed during the digestion process. This is important in protecting the people consuming the farm produce, and also in protecting the workers in the fields, as they are in direct contact with the fertilizer while maintaining the crops.Throughout the world, a countless number of designs of biogas plants have been developed under specific climatic and socio-economic conditions.The performance of a biogas plant is dependent on the local conditions in terms of climate, soil conditions, the substrate for digestion and building material availability. The design must respond to these conditions.
In areas with generally low temperatures, insulation and heating devices may be important. If bedrock occurs frequently, the design must avoid deep excavation work. The amount and type of substrate to be digested have a bearing on size and design of the digester and the inlet and outlet construction. The choice of design will also be based on the building materials which are available reliably and at reasonable cost.CLIMATE CONDITIONS FOR BIOGAS DISSEMINATIONTemperaturesBiogas technology is feasible in principle under almost all climatic conditions. As a rule, however, it can be stated that costs increase for biogas production with sinking temperatures.
Either a heating system has to be installed, or a larger digester has to be built to increase the retention time. Unheated and un-insulated plants do not work satisfactory when the mean temperature is below 15 °C. Heating systems and insulation can provide optimal digestion temperatures even in cold climates and during winter, but the investment costs and the gas consumption for heating may render the biogas system not viable economically.Not only the mean temperature is important, also temperature changes affect the performance of a biogas plant adversely. This refers to day/night changes and seasonal variations. For household plants in rural areas, the planner should ensure that the gas production is sufficient even during the most unfavourable season of the year.
Within limits, low temperatures can be compensated with a longer retention time, i.e. a larger digester.Changes of temperature during the course of the day are rarely a problem as most simple biogas digesters are built underground.PrecipitationThe amount of seasonal and annual rainfall has mainly an indirect impact on anaerobic fermentation:Low rainfall or seasonal water scarcity may lead to insufficient mixture of the substrate with water. The negative flow characteristics of substrate can hamper digestion.Low precipitation generally leads to less intensive systems of animal husbandry. Less dung is available in central locations.High precipitation can lead to high groundwater levels, causing problems in construction and operation of biogas plants.Suitability of climatic zones
Tropical Rain Forest:
Annual rainfall above 1.500 mm,Mean temperatures between 24 and 28°C with little seasonal variation. These conditions are suitable for biogas production. However, animal husbandry is hampered by diseases like trypanosomiasis, leading to the virtual absence of substrate.
Rainfall between 1.000 and 2.000 mmMean temperatures between and 25°C (according to elevation). These conditions are suitable. Often agricultural systems highly suitable for biogas production (mixed farming, zero-grazing).
Rainfall between 800 and 1.500 mm,Moderate seasonal changes in temperature, mixed farming with night stables and day grazing favour biogas dissemination.
Seasonal water scarcitySeasonal changes in temperature.Pastoral systems of animal husbandry, therefore little availability of dung. Use of biogas possible near permanent water sources or on irrigated, integrated farms.
Thornbush Steppe and Desert:
Permanent scarcity of water.Considerable seasonal variations in temperature.Extremely mobile forms of animal keeping (nomadism).Conditions in these areas are unsuitable for biogas dissemination.GLOBAL ENVIRONMENTAL BENEFITS OF BIOGAS TECHNOLOGYWith anaerobic digestion, a renewable source of energy is captured which has an important climatic twin effect:The use of renewable energy reduces the CO2 emissions through a reduction of the demand for fossil fuelsAt the same time, by capturing uncontrolled methane emissions, the second most important greenhouse gas is reduced:Table 7.1 Amount of Human and animal waste discharged per day.KindDaily amount of excrement (kg)VS (%)Pig614Sheep1.520Chicken0.1016Human0.515Cow813Smaller agricultural units can additionally reducer the use of forest resources for household energy purposes and thus slow down deforestation (about 1hec. Forest per rural biogas plant), soil degradation and resulting natural catastrophes like flooding or desertificationWhen applied for industrial or municipal waste water treatment, surface waters and other water resources are being protected.
Often the purified waste water 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.Firewood Consumption and Soil ErosionA unique feature of biogas technology is that it simultaneously reduces the need for firewood and improves soil fertilization, thus substantially reducing the threat of soil erosion. Firewood consumption in rural households is one of the major factors contributing to deforestation in developing countries.
Most firewood is not acquired by actually cutting down trees, but rather by cutting off individual branches, so that the tree need not necessarily suffers permanent damage. Nonetheless, large amounts of firewood are also obtained by way of illegal felling.First, biogas could increasingly replace firewood as a source of energy.Rapid deforestation due to increasing wood consumption contributes heavily to the acceleration of soil erosion. This goes hand in hand with overgrazing which can cause irreparable damage to soils. In the future, investments aimed at soil preservation must be afforded a much higher priority.The Impact on the Greenhouse EffectThe greenhouse effect is caused by gases in the atmosphere mainly 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 15oC and not minus 18oC.The increase of the greenhouse gasses which also include methane (CH4), ozone (O3), nitrous oxide (NOx) 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 50cm. Flooding, erosion of the costs, 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 emission, due to their high proportion in the atmosphere.
Though other greenhouse gases appear to a smaller extent in the atmosphere, they cause much more harm to the climate. Methane is not only the second most important greenhouse gas (20% with CO2 being 62%), it has also 25 times higher global warming potential compared with carbon dioxide in a time horizon of 100 years.The reduction of 1kg methane is equivalent to the reduction of 25kg 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 fieldThe 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 can rarely be reduced, methane emissions from animal waste can be captured and energetically used through energetic 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, in the developing countries it is only about 0.13m3 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 contributes through anaerobic conditions with a high methane released to the climate change (up to 90% of the methane emission potential is released)From the worldwide 30million tons of methane emission per year generated from the different animal waste management systems like solids storage, anaerobic lagoons, liquid/slurry storage, pasture etc, half of the emission could be reduced through anaerobic treatment.Methane reduction potential through the application of biogas technologyThrough anaerobic treatment of animal waste through controlled capture of methane and its energetic use, about 13.24 million tonnes 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 total 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 420million tones of CO2 equivalents are avoidable.The case for tradable emission entitlements-international market based instrumentsThe major anthropogenic greenhouse gas is carbon dioxide which accounts for about 50% of the effects of trace gases relevant to the climate followed by methane and nitrous oxide.The search for a control mechanism that could be used in a broad based international agreement to limit greenhouse gas emissions must give high priority to the need to achieve such a limitation at the lowest possible economic costs.
Three main options have been advanced for achieving market based instruments: an international emission tax, external offsets and internationally tradable emission entitlements. The idea of external offsets imply that in the international context, countries with an agreed national emission target could either meet this by either curtailing their own emissions to this level, or is allowed to offset this by investing in measures to reduce emissions by an equivalent amount elsewhere.Clean Development Mechanism (CDM)The Clean Development Mechanism (CDM), a cooperative mechanism established under the Kyoto Protocol, has the potential to assist developing countries in achieving sustainable development by promoting environmentally friendly investment from industrialized country governments and businesses.The 1997 Kyoto Protocol, a milestone in global efforts to protect the environment and achieve sustainable development, marked the first time that governments accepted legally-binding constraints on their greenhouse gas emissions.Allows governments or private entities in industrialized countries to implement emission reduction projects in developing countries and receive credit in the form of "certified emission reductions," or CERs, which they may count against their national reduction targets. The CDM strives to promote sustainable development in developing countries, while allowing developed countries to contribute to the goal of reducing atmospheric concentrations of greenhouse gases.
The CDM will include projects in the following sectors:End-use energy efficiency improvementsSupply-side energy efficiency improvement
Renewable energy (e.g. Biogas Production and Utilization)
Fuel switchingAgriculture (reduction of CH4 and N2O emissions)Industrial processes (CO2 from Cement etc., HFCs, PFCs, SF6)Sinks projects (only afforestation and reforestation)PROBLEMSGive some conditions necessary for consideration for biogas disseminationGive the four climatic zones and state why they are suitable or not for biogas disseminationA farm with 100 sheep, 140 pigs and 200 fowls. Using Table 7.1, Table 1.6 (max. yield) and assuming global warming potential of methane is 21, estimate the amount of CO2 that would be released into the atmosphere in a week when the biogas is not harnessed.Explain how biogas affect firewood consumption and soil erosionHow does biogas dissemination impacts on greenhouse effect.Explain the CDM concept
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