A floating dome digester is designed in such a way that it has a dome (normally made of steel) that fits in the neck of the digester and basically floats on the gas produced in the belly of the digester below. As gas is produced in the digester it collects in the dome, eventually creating enough pressure for the dome to in effect "float" on the gas.
The floating drum normally has a central steel pipe which facilitates its vertical up and down movement and also has metal guides built into the sides of the digester. The weight of the floating steel cylinder ensures that the gas produced is under constant pressure, which gives this type of digester its main advantage. Unfortunately this advantage also results in this type of digesters main disadvantage as the sliding mechanism requires specialized design and construction, is costly and further requires constant maintenance. Similar to the fixed dome digester, a high level of skilled labour is also required to construct this type of digester, which in turn becomes time consuming.