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Biodigester- Anaerobic digestion free energy

May 1, 2012



Above is a sketch of the profile of a biodigester to better visualize the concept. In the picture, A represents the biodigester tank where the water and manure mixture is digested by the bacteria. When working with cow manure in a biodigester of this size (1.9 meters deep X 1.5 meters wide X 3 meters long), every day you need to add 10 gallons of water and 5 gallons of manure. When working with pig manure you work with a 1:1 ratio, or rather, 5 gallons of water for the same 5 gallons of manure. In Costa Rica they use twice as much water for the cow manure because the cattle grazes on grasses, making the manure more fibrous than that of pigs. So, keep in mind that grain-fed cattle may produce less-fibrous and easily-digested manure. In the picture B and C represent the entrance and exit tubes respectively. The entrance tube should enter the tank near the bottom and the exit tube should enter the tank just beneath the first row of cement block. D and E represent the mixing tub and the collection tub respectively. The mixing tub will ideally be more than 15 gallons in volume in order to mix the water and manure thoroughly. The mixture should have a uniform consistency to facilitate optimal digestion throughout the tank. Also, in the picture, the green circles represent the bottom support pins that catch the frame of the plastic in case the water level decreases drastically. The purple circles represent the top hangers against which the frame of the plastic rests as it tries to float up to the water surface. The curved tubes that enter the tank on each end are to hold the mixing rope. The mixing rope is to have 3-5 gallon jugs half-full with sand attached. When two people tug back-and-forth on this mixing rope for a few minutes daily, the partially submerged gallon jugs break up any thick film that may gather at the surface, suffocating the bacteria in the tank below. The dotted yellow line represents the liquid level. Notice that the level comes right up to the rim of the exit tube. This parity is important, as every day that you put in 15 gallons of mixture, the exit tube, in theory, will discard the same volume into the collection tub to be used for fertilizer. The black dome that hovers over the tank is the plastic frame that fits just under the top hangers (purple circles) and holds the plastic that balloons up when the biogas, represented by the upward arrows, bubbles up from the surface of the water/manure mixture. The biogas then escapes through the PVC tubing represented by the blue line that extends above the middle of the plastic. Through this tubing the biogas is transported to the kitchen to be burned for cooking.



Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen.[1] It is used for industrial or domestic purposes to manage waste and/or to release energy.

The digestion process begins with bacterial hydrolysis of the input materials to break down insoluble organic polymers, such as carbohydrates, and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.[2] The methanogenic archaea populations play an indispensable role in anaerobic wastewater treatments.[3]

It is used as part of the process to treat biodegradable waste and sewage sludge.[4] As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.[5]

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide and traces of other ‘contaminant’ gases.[1] This biogas can be used directly as cooking fuel, in combined heat and power gas engines[6] or upgraded to natural gas-quality biomethane. The use of biogas as a fuel helps to replace fossil fuels. The nutrient-rich digestate also produced can be used as fertilizer.

The technical expertise required to maintain industrial-scale anaerobic digesters, coupled with high capital costs and low process efficiencies, has so far been a limiting factor in its deployment as a waste treatment technology.[citation needed] Anaerobic digestion facilities have, however, been recognized by the United Nations Development Programme as one of the most useful decentralized sources of energy supply, as they are less capital-intensive than large power plants.[7] With increased focus on climate change mitigation, the re-use of waste as a resource and new technological approached which has lowered capital costs, anaerobic digestion has in recent years received increased attention among governments in a number of countries, among these the United Kingdom (2011),[8] Germany [9] and Denmark (2011).[10]




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