but the number of skilled technicians will get smaller the more intensive the training has to
be. In addition, training costs compete with actual construction costs for scarce (project)
resources. Higher technical sophistication also requires more expensive supervision and,
possibly, higher maintenance costs. To which extent prefabricated designs are suitable
depends largely on the cost of labor and transport.
Standardization
For larger biogas programs, especially when aiming at a self-supporting dissemination
process, standards in dimensions, quality and pricing are essential. Standard procedures,
standard drawings and forms and standardized contracts between the constructor, the
planner, the provider of material and the customer avoid mistakes and misunderstandings
and save time. There is, however a trade-off between the benefits of standardization and the
necessity of individual, appropriate solutions.
Types of plants
There are various types of plants. Concerning the feed method, three different forms can be
distinguished:
• Batch plants
• Continuous plants
• Semi-batch plants
Batch plants are filled and then emptied completely after a fixed retention time. Each design
and each fermentation material is suitable for batch filling, but batch plants require high labor
input. As a major disadvantage, their gas-output is not steady.
Continuous plants are fed and emptied continuously. They empty automatically through the
overflow whenever new material is filled in. Therefore, the substrate must be fluid and
homogeneous. Continuous plants are suitable for rural households as the necessary work
fits well into the daily routine. Gas production is constant, and higher than in batch plants.
Today, nearly all biogas plants are operating on a continuous mode.
If straw and dung are to be digested together, a biogas plant can be operated on a semi-
batch basis. The slowly digested straw-type material is fed in about twice a year as a batch
load. The dung is added and removed regularly.
Concerning the construction, two main types of simple biogas plants can be distinguished:
• fixed-dome plants
• floating-drum plants
But also other types of plants play a role, especially in past developments. In developing
countries, the selection of appropriate design is determined largely by the prevailing design
in the region. Typical design criteria are space, existing structures, cost minimization and
substrate availability. The designs of biogas plants in industrialized countries reflect a
different set of conditions.
Parts of a biogas plant
The feed material is mixed with water in the influent collecting tank. The fermentation slurry
flows through the inlet into the digester. The bacteria from the fermentation slurry are
intended to produce biogas in the digester. For this purpose, they need time. Time to multiply
and to spread throughout the slurry. The digester must be designed in a way that only fully
digested slurry can leave it. The bacteria are distributed in the slurry by stirring (with a stick
or stirring facilities). The fully digested slurry leaves the digester through the outlet into the
slurry storage.
The biogas is collected and stored until the time of consumption in the gasholder. The gas
pipe carries the biogas to the place where it is consumed by gas appliances. Condensation
collecting in the gas pipe is removed by a water trap.
Depending on the available building material and type of plant under construction, different
variants of the individual components are possible. The following (optional) components of a
5