Anaerobic digestion refers to the consumption of biomass by the nature's micro-organisms when exposed in an environment where oxygen is unavailable. This process leads to production of biogas energy that can be used in heating and lighting. The waste digestate can be used in the production of fertilizer for farming. This technology has been in use for some time but the current global environmental challenges have emphasized its importance in the modern times. This use of renewable energy has been seen as the most effective way of managing household wastes and is an appropriate technology for maintaining green house gases in the atmosphere. Different countries continue to give appropriate incentives to individuals and institutions to build this kind of alternative energy sources. Most of the biogas digester in developed economies are large and consumes huge amounts of capital to construct. However such facilities enjoy the economies of scale in the operating costs on the basis of their large output. Small scale biogas digesters that are simple to construct are gaining popularity in the developing world.
This paper focuses on a small scale project of biogas digester designed for electricity and heating purposes among the indigenous communities across the Latin America. The project is analyzed in a simple language and in a manner that it can be adopted for use in another region around the world. However, the complex analysis and calculation for the system are included in the appendix (Forst, 2002).
The design process
During the energy production carbon locked up in the system undergoes two transitional stages. In the first stage carbon is extracted from the biomass materials that are fed in the system and is converted to acids, this stage is commonly referred to as acidogenisis. In the second stage the acids are converted to gas in a process referred to as methanogenisis. In some common designs a lid or any other membrane is used to hold biogas generated and secure it to a plug constructed in a way that allows it to separate from water and the sludge. The substrate is usually pasteurized to reduce chances of hampering harmful pathogens. The biogas extracted this way contains about 60% of methane and 39% carbon dioxide. It can be used in this form or purified to eliminate carbon dioxide and achieve up to 99% methane. This gas can then be directed as appropriate depending on the use that is, to generate heat or light (Forst, 2002).
An inclined type of biogas digester is described here because it is easier to construct and cheap because it can utilize locally available materials. The actual size of the digester would depend on the energy requirement and the available capital. For general guidance a digester with 1M3 of gas can provide cooking heat for a household of 4 to 6 members per day (3 meals). The biomass required for such a digester can be supplied by 2 cows. The actual set up of the digester has a container in which the slurry is put and a scrubber to minimize the levels of carbon dioxide in the biogas.
The gas scrubber as shown above is made of an 80 liter drum having about 60 liters of water. The biogas generated from the digester system is connected to the scrubber through the lower pipe and is allowed to bubble through the water and get out through the upper pipe. The biogas is passed through water to minimize the content of carbon dioxide and sulphur dioxide that could be trapped in the biogas. The gas being getting out of the scrubber is then directed to a storage facility (Forst, 2002).
The biomass entered at the top end of the system gets through to the bottom. For instance when 2% of the total capacity is fed per day, it would take around 50 days for the biomass fed to pass through to the bottom. During the 50 days 50% of the carbon in the biomass is converted to acid and eventually to gas mainly methane and carbon dioxide at a temperature of about 800F. The ratio of carbon to nitrogen should be stabilized at about 30:1. The ratio from the cow dung is about 25:1 and addition of water at a ratio of 2:1 (Dung: Water) would make a good slurry mixer having around 20% solids that is ideal for charging the system. This system has the potential of producing about 27 cubic feet each day with slurry of five gallons (Forst, 2002).
2 cows can produce the daily requirements for slurry to be fed in the system daily. However this should vary depending on the size of the digester and the energy requirements. This kind of digester is mainly used for providing heat for cooking and the system is able to offer up to 33% efficiency. Other than cow dung any other organic biomass can be used successfully with the system. The cow dung is however essentially advantageous because of its average carbon to nitrate ratio. Other wastes especially from monogastric animals usually contain great under graded fiber which takes long to be digested. Crop based biomass is usable though it produces large potions of carbon dioxide in the expense of methane (Forst, 2002).
For a new digester it is advisable to put aside about five gallons of slurry and leave it to ferment before adding it to the digester. This will speed up the digestion process. If the digestion process slows down after using the digester for some time, some effluent can be ejected from the slurry exit re-entered at the charging tap. This action would effectively buffer the slurry operation and the process would soon be optimally restored. However if appropriate carbon nitrate ratios are maintained with the required viscosity of the slurry then the system should be able to operate indefinitely with regular charging.
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