Floating Drum Biogas
Floating drum biogas plants consist of a digester and a moving gasholder. The gasholder floats either direct on the fermentation slurry or in a water jacket of its own. The gas collects in the gas drum, which then rises on this positive pressure. If gas is drawn off, the drum falls again. The gas drum is prevented from tilting by a guide frame.
Fig 1. Floating drum biogas plant. 1. Mixing tank with inlet pipe 2. Digester 3. Overflow on outlet pipe 4. Gasholder with braces for breaking up surface scum 5. Gas outlet with main cock 6. Gas drum guide structure 7. Difference in level = gas pressure in cm WC 8. Floating scum in the case of fibrous feed material 9. Accumulation of thick sludge 10. Accumulation of grit and stones 11. Water jacket with oil film.
Advantages: Simple, easily understood operation, constant gas pressure, volume of stored gas visible directly, few mistakes in construction.
Disadvantages: High construction cost of floating-drum, many steel parts liable to corrosion, resulting in short life (up to 15 years; in tropical coastal regions about five years for the drum), regular maintenance costs due to painting. In spite of these disadvantages, floating-drum plants are always to be recommended in cases of doubt. Water-jacket plants are universally applicable and especially easy to maintain. The drum won’t stick, even if the substrate has a high solids content. Floating-drums made of glass-fibre reinforced plastic and highdensity polyethylene have been used successfully, but the construction cost is higher than with steel.
In ground floating drum biogas plants made of wire-mesh-reinforced concrete are liable to hairline cracking and are intrinsically porous. They require a gastight, elastic internal coating. PVC drums are generally unsuitable because they are not resistant to UV. The floating gas drum can be replaced by a balloon above the digester. There are many ‘portable’ type home scale floating drum biogas kits now on the market. Like the ARTI unit these are often made of plastic, are light and easy to manoeuvre and very straightforward to operate.
The photo at the top of this page shows a floating drum biogas unit that is fully charged with biogas which is can be seen in that the drum is fully extended. This unit is located at Aman Bagh, a small farm that does not use pesticides or chemical fertilizers.