Position of inlet and outlet
Both the inlet and the outlet pipe must be freely accessible and straight, so that a rod can be
pushed through to eliminate obstructions and agitate the digester contents. The pipes should
penetrate the digester wall at a point below the lowest slurry level (i.e. not through the gas
storage). The points of penetration should be sealed and reinforced with mortar.
The inlet pipe ends higher in the digester than the outlet pipe in order to promote more
uniform flow of the substrate. In a fixed-dome plant, the inlet pipe defines the bottom line of
the gas-holder, acting like a security valve to release over-pressure. In a floating-drum plant,
the end of the outlet pipe determines the digester’s (constant) slurry level.
Inlet and outlet pipe must be placed in connection with brick-laying. It is not advisable to
break holes into the spherical shell afterwards, this would weaken the masonry structure.
Digester
Requirements
No matter which design is chosen, the digester (fermentation tank) must meet the following
requirements:
• Water/gastightness - watertightness in order to prevent seepage and the resultant
threat to soil and groundwater quality; gastightness in order to ensure proper
containment of the entire biogas yield and to prevent air entering into the digester
(which could result in the formation of an explosive mixture).
• Insulation - if and to which extent depends on the required process temperature, the
local climate and the financial means; heat loss should be minimized if outside
temperatures are low, warming up of the digester should be facilitated when outside
temperatures are high.
• Minimum surface area - keeps cost of construction to a minimum and reduces heat
losses through the vessel walls. A spherical structure has the best ratio of volume
and surface area. For practical construction, a hemispherical construction with a
conical floor is close to the optimum.
• Structural stability - sufficient to withstand all static and dynamic loads, durable and
resistant to corrosion.
Internal and external forces
Two relevant forces act on the digester. The external active earth pressure causes
compressive forces within the masonry. The internal hydrostatic and gas pressures causes
tensile stress in the masonry. Thus, the external pressure applied by the surrounding earth
must be greater at all points than the internal forces. Round and spherical shapes are able to
accept the highest forces and distribute them uniformly. Edges and corners lead to peak
tensile stresses which can result in cracking.
Shapes of digesters
From the standpoint of fluid dynamics and structural strength, an egg-shaped vessel is about
the best possible solution. This type of construction, however, is comparatively expensive, so
that its use is usually restricted to large-scale sewage treatment plants. The Chinese fixed-
dome designs are of similar shape, but less expensive. The hemispherical CAMARTEC
design is optimized in structural strength, but does not make optimal use of the excavation
required.
Simplified versions of such digester designs include cylinders with conical covers and
bottoms. They are much easier to build and are sometimes available on the market as
prefabricated units. Their disadvantage lies in their less favorable surface-volume ratio. The
cylinder should have a height equal to its diameter. Prone cylinders have become quite
popular on farms, since they are frequently the more favorable solution for small-scale bio-
methanation. Cuboid digesters are often employed in batch-fed systems used primarily for
fermenting solid material, so that fluid dynamics are of little interest.
20