(1) Fixed cover. This cover is usually made of concrete. It may be flat,
with or without space for gas above the liquid level, or it may be dome-shaped with
space for gas. It is used for the primary stage of a two-stage digestion process. In lieu
of floating covers, and in cold regions, this cover may be used provided a gas collection
dome is installed in the top of the cover. At least two access manholes will be provided
in the tank roofs. Figure 2-25 shows a fixed-cover and a gas-holder cover digester
installed in series.
(2) Floating cover. Tanks with floating covers (see Figure 2-26) are similar
to the fixed-cover type, except that the cover floats on the tank's contents. These
covers are used for the secondary stage of a two-stage process. Floating covers are on
separate digestors. They are used in cold regions where freezing ice and snow are a
Figure 2-26. Digester with floating cover.