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 brick- laying (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 4 th brick layer