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Biogas for households Mariska Ronteltap, PhD Lecturer Sanitary Engineering and Resource Oriented Sanitation Department of Urban Water Supply and Sanitation, UNESCO-IHE Biogas systems • Not so suitable for sole households without animals • Very suitable for block of households / in combination with other wastes • Relatively cheap; requires experience but no high tech skills / equipment Size of the system • The size of the plant depends on the substrate available • Volume chosen for number of cattle/ pigs • Larger plants = longer HRT = more gas • But: amount of daily fed substrate more important for gas production than digester volume Standard sizes • In some countries where biogas systems are important, standard sizes are chosen • This simplifies construction, operation and maintenance; makes material cheaper Example: • 16 m³ for gas for cooking and lighting for a normal family • 12 m³-plant for places of little gas demand • 30 and 50 m³-plants: fuel for big institutional kitchens and special appliances (refrigerators, power engines) Construction The plant consists of • non-load bearing bottom (A), • lower slurry-tight digester (B), • the upper gas-tight gas storage part (C), • neck (D) and gas-tight lid (E). • Gas storage part and digester are separated by the weak ring (10) in order to allow free reaction of the strong-ring (3) and to prevent cracks which have developed in the lower part of the digester to "grow" into the gas storage space. Construction • Plant rests on foundation ring (1), bearing mainly the vertical loads of' the construction and the soil cover (7). • The surrounding soil supports the construction to resist gas pressure (5) and slurry pressure (6). • Concrete at the outside of the lower layer of bricks (2) helps to reduce tangential forces at the foot point (9). • The ring forces of the upper part are absorbed by the strong-ring (3). Inlet and outlet • Inlet and outlet pipe must be placed in connection with bricklaying (Breaking holes spoils the whole structure) • The pipe rests below on a brick projecting 2 cm to the inside. • Above, it is kept in position at the rim of the excavation. • Inlet: 10 cm diameter; in line with top of weak ring • Outlet: 15 cm; starts at 4th brick layer