Fig 1. Balloon Digester as seen in Costa Rica
A balloon digester consists of a plastic or rubber digester bag, in the upper part of which the gas is stored. The inlet and outlet are attached directly to the skin of the balloon. The gas is moved from the balloon to where it will be used by the pressure build up inside the balloon and can be enhanced by placing weights on the balloon. The fermentation slurry is agitated slightly by the movement of the balloon skin. This is favourable to the digestion process. Even difficult feed materials, such as water hyacinths, can be used in a balloon plant. The balloon material must be UV-resistant. Materials which have been used successfully include RMP (red mud plastic), Trevira and butyl.
Fig 2. Balloon Digester, Tumwesige (2013).
Advantages: Low cost, ease of transportation, low construction and installation (important if the water table is high), high digester temperatures, uncomplicated cleaning, emptying and maintenance. An ideal cheap system to generate methane for cooking stoves.
Disadvantages: Short life (about five years), easily damaged, does not create employment locally, little scope for self-help. Balloon plants can be recommended wherever the balloon skin is not likely to be damaged and where the temperature is even and high. In addition, the low gas pressure may require gas pumps and scum or sludge inside the balloon cannot be removed during operation. One variant of the balloon plant is the channel-type digester with folia and sunshade as seen in Figure 1 above. Whilst the sun shade can extend the life of balloons that don’t have in built UV protection, the shading can also slow the process of fermentation unless the ambient temperature is relatively high.