4.2 Determining the biogas production

The quantity, quality and type of biomass available for use in the biogas plant constitutes the basic factor of biogas generation. The biogas incidence can and should also be calculated according to different methods applied in parallel.

Measuring the biomass incidence (quantities of excrement and green substrate)

This is a time-consuming, somewhat tedious approach, but it is also a necessary means of adapting values from pertinent literature to unknown regions. The method is rather inaccurate if no total-solids measuring is included. Direct measurement can, however, provide indication of seasonal or fodder-related variance if sufficiently long series of measurements are conducted.

Determining the biomass supply via pertinent-literature data
(cf. tables 3.2/3.3)

According to this method, the biomass incidence can be determined at once on the basis of the livestock inventory. Data concerning how much manure is produced by different species and per liveweight of the livestock unit are considered preferable.

Dung yield = liveweight (kg) x no. of animals x specific quantity of excrements (in % of liveweight per day, in the form of moist mass, TS or VS).

Determining the biomass incidence via regional reference data

This approach leads to relatively accurate information, as long as other biogas plants are already in operation within the area in question.

Determining biomass incidence via user survey

This approach is necessary if green matter is to be included as substrate.
It should be kept in mind that the various methods of calculation can yield quite disparate results that not only require averaging by the planner, but which are also subject to seasonal variation.

The biomass supply should be divided into two categories:

Category 1: quick and easy to procure,
Category 2: procurement difficult, involving a substantial amount of extra work.

Table 4.2: Outline for determining biomass incidence (Source: OEKOTOP)