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close this bookBioconversion of Organic Residues for Rural Communities (UNU, 1979)
close this folderBiogas generation: developments. Problems, and tasks - an overview
View the document(introduction...)
View the documentIntroduction
View the documentWhat is biogas?
View the documentMicrobiology of CH4, or bio-methanogenesis
View the documentThe biogas plant-some technical considerations
View the documentEnvironmental and operational considerations
View the documentDevelopments and processes for rural areas
View the documentCost-benefit analyses
View the documentHealth hazards
View the documentBottlenecks, considerations, and research and development
View the documentReferences
View the documentDiscussion summary

The biogas plant-some technical considerations

The biogas plant consists of two components: a digester (or fermentation tank) and a gas holder. The digester is a cube-shaped or cylindrical waterproof container with an inlet into which the fermentable mixture is introduced in the form of a liquid slurry. The gas holder is normally an airproof steel container that, by floating like a ball on the fermentation mix, cuts off air to the digester (anaerobiosis) and collects the gas generated. In one of the most widely used designs (Figure 2), the gas holder is equipped with a gas outlet, while the digester is provided with an overflow pipe to lead the sludge out into a drainage pit.

Figure. 2. Diagram of Gobar-Gas Plant Used to Obtain Methane from Dung by Anaerobic Fermentation (After Prasad et al. [20]1

The construction, design, and economics of biogas plants have been dealt with in the literature (13 - 21). For biogas plant construction, important criteria are: (a) the amount of gas required for a specific use or uses, and lb) the amount of waste material available for processing. Fry (17)

Singh (21), and others (1, 3) have documented several guidelines for consideration in the designing of batch (periodic feeding) and continuous (daily feeding) compartmentalized and non-compartmentalized biogas plants that are of either the vertical or horizontal type. In addition, Loll (18) has recently dealt with the scientific principles, process engineering, and shapes of digestion reactors, and with the economics of the technology.

Digester reactors are constucted from brick, cement, concrete, and steel. In Indonesia, where rural skills in brick making, brick laying, plastering, and bamboo craft are well established, clay bricks have successfully replaced cement blocks and concrete. In areas where the cost is high, the "sausage" or bag digester (14) appears to be ideal (Figure 3). The digester is constructed of 0.55 mm thick Hypalon laminated with Neoprene and reinforced with nylon. The bag is fitted with an inlet and an outlet made from PVC. Even if imported from the United States, the cost of the digester and the gas holder (both combined in one bag) is only 10 per cent of that for a concrete-steel digester. Another advantage is that it can be mass produced and is easily mailed. In rural areas, the whole installation is completed in a matter of minutes. A hole in the ground accommodates the bag, which is filled two-thirds full with waste water. Gas production fully inflates the bag, which is weighted down and fitted with a compressor to increase gas pressure.

Figure. 3. Diagrammatic Sketch of the "Sausage" Bag Digester Made of Hypalon Laminated with Neoprene