The floating drum digester was first time constructed by Khadi and Village Industries Commission and this model was developed in 1962 (Figure 1.12). Although the model is old, it is one of the most extensively used designs for household purposes in India. This design includes a movable inverted drum placed on a well­shaped digester. The inverted steel drum acts as a stor­age tank, which can move up and down depending on the quantity of accumulated biogas at the top of the digester. The weight of this inverted drum applies the

pressure needed for biogas flow through the pipeline (Singh and Sooch, 2004).

Floating drum digesters manufacture biogas at a sta­ble pressure with variable volume. In floating drum reactor, by position of the drum, the amount of biogas accumulated under the drum is easily noticeable. How­ever, the floating drum needs to be coated with paint at regular intervals to avoid rusting. Additionally, fibrous materials in biomass will block the movement of the digester. Hence, their accumulation must be avoided if possible (Adeoti et al., 2000). In Thailand, the floating dome has been customized with two cement jars on each side of the floating drum. The average size of these digesters is around 1.2 m3 (Gosling, 1982). For small and medium-size farms the size varies from around 5 to 15 m3. Singh and Singh (1991) compared 14 different biogas plants with a floating drum model and optimized the various parameters for maximum biogas production.