Biogas Plants (GTZ, 1988)
 4. Scaling of biogas plants
 4.1 Definitions 4.2 Scaling of the digester 4.3 Scaling of gasholder 4.4 Digester/gasholder ratio 4.5 Measuring and test programmes

### 4.5 Measuring and test programmes

The aim of a measuring and test programme is to determine the specific gas production obtained at specific retention times.

Since digester temperature affects gas production, the latter should be measured at both the coldest and hottest time of the year.

The programme consists of a set of at least four biogas plants of different sizes. A given filling volume results in different retention times, in turn yielding different amounts of gas production for one and the same filling volume.

Example (Figure 14):

Fig. 14: Biogas plants for a test programme for determinantion of gas production The length of the retention time (RT) has the greatest effect on digester size (VD). Test plants may have any shape. However' they should all be identical and should preferably conform to the type to be used later in a biogas programme. The test plants must be filled regularly for at least three months before gas production is measured. The gasholders must be all the larger, the longer the time between tests. Safe spanning of the after-dark hours must be ensured.

Filling volume: 30 kg manure and 30 lwater; 60 l/day
Retention times (RT) chosen: 30, 45, 60 and 90 days

Required digester volume:
RT(30): VD = 30 x 60 = 1800 l (1.8 m³)
RT(45): VD = 45 x 60 = 2700 l (2.7 m³)
RT(60): VD = 60 x 60 = 3600 l (3.6 m³)
RT(90): VD = 90 X 60 = 5400 l (5.4 m³)

Specific gas production is determined by dividing the daily volume of gas measured by the amount of slurry loaded into the plant (30 kg).

The results are plotted in a curve(like Figure 7 and 8) and are used for the scaling and calculation of the digester and gasholder volumes.

If a test programme is too expensive or complicated, the actual gas production values can also be derived from the results of measurement of a number of existing plants. For this purpose, the volume of gas stored must be measured before and after each consumption (Figure 15). Measurements must be effected for at least three consecutive days and nights.

Fig. 15: Measuring gas production on the plant In a floating-drum plant, the height of the gasholder is measured (top left), In a fixed-dome plant, the height of the slurry level is measured (top right). The manure is either weighed or measured in litres before introduction to the plant. Containers whose shape is easy to calculate are more accurate. If the lengths are measured in dm (1 dm = 10 cm), the volume in litres is obtained directly.