Most simply an in-ground tube with no sun protection and effluent flow into collection pit. Most of the time called Salchicha or Sausage system. The materials move from one side of the tube (inlet) to the other side of the tube (outlet)
PE (poly ethylene) tube digesters are applied in Bolivia (Peru, Ecuador, Colombia and Mexico). The tube PE film is about 300 microns thick. It is bended at the ends and has PVC drainage pipe. PE tubes are very cheap but quite fragile.
Sausages can be produced using EPDM (ethylene polypropylene diene monomer (M-class).
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The Kenya based company Flexibiogas offers a system that includes the flexible digester bag as well as a foil greenhouse cover.1